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MOCVD for solar cells, a transition towards a chamberless inline process
Author's Accepted Manuscript
MOCVD for solar cells, a transition towards a chamberless inline process V. Barrioz, S. Monir, G. Kartopu, D.A. Lamb, W. Brooks, P. Siderfin, S. Jones, A.J. Clayton, S.J.C. Irvine
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PII: DOI: Reference:
S0022-0248(14)00773-8 http://dx.doi.org/10.1016/j.jcrysgro.2014.11.014 CRYS22536
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Journal of Crystal Growth
Cite this article as: V. Barrioz, S. Monir, G. Kartopu, D.A. Lamb, W. Brooks, P. Siderfin, S. Jones, A.J. Clayton, S.J.C. Irvine, MOCVD for solar cells, a transition towards a chamberless inline process, Journal of Crystal Growth, http://dx.doi.org/ 10.1016/j.jcrysgro.2014.11.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
MOCVD for solar cells, a transition towards a chamberless inline process V. Barrioz*, S. Monir, G. Kartopu, D.A. Lamb, W. Brooks, P. Siderfin, S. Jones, A.J. Clayton, S.J.C. Irvine Centre for Solar Energy Research, OpTIC Centre, Glyndŵr University, St Asaph, LL17 0JD * Corresponding author: [email protected]; 00 44 1745 535 159 Abstract MOCVD has been associated with batch processing of III-V opto-electronic devices for decades, with epitaxial structures deposited on up to 200 mm diameter wafers. Recent development in thin film PV has seen the gap in conversion efficiencies closing in on that of the commonly found multicrystalline Si wafer based PV. To further improve the conversion efficiency of thin film PV towards the theoretical limits of single junction solar cells requires a technique such as MOCVD with scalability potential. Preliminary results on the development of a chamberless inline process are reported for up to 15 cm wide float glass, progressively coating each layer in the CdTe solar cell as the heated substrate passes under each coating head in turn and entirely at atmospheric pressure. Emphasis is made on ensuring that the chamberless coating heads can be operated safely using a combination of nitrogen curtain flows and a balanced exhaust pressure system. Results are also presented on the exclusion of oxygen and moisture from the coating area, achieved using the same gas flow isolation process. This paper also reviews the achievements made to-date in the transfer of the high efficiency batch MOCVD produced CdTe solar cell to the chamberless inline process demonstrating device quality thin films deposition. Highlights: • • • • •
A chamberless coating head was designed for scale up of MOCVD as an inline process. Suitable containment was achieved using CFD modelling and leak testing tools, measuring leak rates as low as 2 × 10-10 mbar·l·s-1. Cadmium compounds were deposited with oxygen below detectable levels. Good transfer of layer quality for thin film PV were reached with good dopant and alloying capabilities. Set of chamberless coating heads can be placed sequentially to deposit the full structure of a CdTe thin film PV.
Keywords: Metalorganic chemical vapor deposition (A3); Semiconducting II-VI materials (B2); Cadmium Compounds (B1); Oxides(B1); Zinc Compounds(B1); Photovoltaic Solar Cells (B3) 1. Introduction Metal-organic chemical vapour deposition (MOCVD) has been gaining importance in opto-electronic device development, since the late 1970s [1]. This has predominantly occurred with the requirements for high controllability and uniformity of optical and/or electrical properties in stacks of thin epitaxial layers containing very low defects density, deposited onto single crystal wafers. Subsequently, the MOCVD technique has been known as a low-pressure batch process for epitaxial growth of semiconductors, with greater potential to be scaled up when compared with molecular beam epitaxy (MBE) [2]. The applications are generally based around III-V semiconductor structures from LEDs and transistors [2] to triple junction solar cells [3]. However, II-VI semiconductor work has also been important with the development of infrared detectors [4] and other opto-electronic applications [5, 6]. “High-throughput” batch reactors are commercially available to grow these epitaxial structures on up to 200 mm diameter wafers, therefore increasing the yielded number of devices per run over the area of the wafer. From a photovoltaic application
view point, MOCVD has enabled monolithically integrated InGaP/GaAs/InGaAs triple-junction solar cells to achieve a world record best efficiency of 44.4%, under concentrated sunlight [7]. For non-concentrated single junction solar cells, there have been recent developments with thin film PV CdTe and CIGS, matching record conversion efficiency with multicrystalline Si wafer based PV at ~20 % for laboratory cells and ~17 % for PV modules [7]. Considering that reducing cost per watt peak is the main driver for PV, these developments have great significance as thin film PV offers more potential for cost reduction through lower temperature processing steps and much lower material usage. The most successful thin film module production has used CdTe technology [8] thanks to its near optimum band gap for solar absorption and the scalable vapour transport deposition (VTD) technique employed [9]. However, getting closer to the theoretical limits for single junction solar cells would require a technique such as MOCVD to controllably achieve alloying and high doping concentrations, with low absorber thicknesses [10], whilst also maintaining compatibility with scalable inline continuous processes. MOCVD grown CdTe absorber layers for thin film PV was first reported in 1989 [11] where 9.4 % conversion efficiency was achieved for Tellurium rich absorber layers. The first full MOCVD CdS/CdTe:As structure reported was in 1998 [12]. Since then, recent work [13] resulted in a CdZnS/CdTe:As/CdTe:As+/CdCl2 structure, grown on glass/ITO sequentially in a batch horizontal MOCVD reactor at atmospheric pressure (AP), achieving > 16 % conversion efficiency. The best example for the scalability potential of chemical vapour deposition is in the deposition of oxide coatings on-line during float glass manufacturing where deposition up to 12 µm·min-1 are reported over a continuous glass substrate with a width of 4 m [14]. Xie et al. [15] also reported on MOCVD being developed for roll to roll processes to deposit YBCO as superconducting film on metal tape. More recently, Van Deelen et al. [16] reported on the deposition of aluminium doped zinc oxide (AZO), at atmospheric pressure where they achieved deposition rate up to 800 nm·min-1 on substrates moving up to 0.5 cm·min-1. To the authors’ knowledge, no AP-MOCVD system has been reported on moving substrate, as a chamberless inline process for non-oxide materials. Preliminary results in transferring the process from a horizontal batch reactor to an enclosed inline chamber showed promising results with ~40 % material utilisation [17]. The ability to control the semiconductor properties of the layers, from a p-n junction fully deposited in this inline reactor, was demonstrated with conversion efficiencies of ~12 % for a 1 µm thick CdTe absorber layer, at a substrate translation speed of 2 cm·min-1 [18]. This paper introduces the recent development of a chamberless inline process, with uniform gas injector delivery [19], for MOCVD, where both cadmium based chalcogenides (II-VI group) and oxides layers were successfully deposited with the demonstration of both alloying and dopant controllability. 2. Materials and Methods There are challenges for MOCVD to move to an inline process in order to deposit the full PV structure. These are different to that needed on the float line, for example, where chemical precursors are selected to react quickly for a given temperature gradient and high continuous glass translation speed. For a full thin film PV structure, the key is to achieve an end-to-end time within 2 hours from a bare glass to a finished PV module. This is the benchmark set by the existing CdTe PV manufacturers. Therefore, moving towards a high volume MOCVD process, a few design criteria need to be set for the development of the coating head units: -
Safe operation (containment of all precursors). Simple and flexible coating head arrangement. Chamberless design for easy retrofit onto different processes and substrates. Containment to avoid ingress of air and moisture into the deposition area. Uniformity of flow delivery to the substrate.
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High material utilisation.
Figure 1 – Top view schematic of a coating head unit [19] where only the injection slits and exhaust are represented. The dashed edges (a, b, c and d) represent the injection slits for the nitrogen curtain, the single solid line represents the precursor delivery, the hashed perimeter is the active exhaust surrounding the deposition area and the black dots represent the extraction points. The light grey band represents the substrate width while the central dotted arrow indicates its direction of travel. 2.1 The chamberless coating head unit In collaboration with Scanwel Ltd, a multi-chamber injector design has been developed [19] to allow premixing and uniform delivery of gases to the surface of the substrate via an injection slit. CFD modelling has also been used during the development prior to manufacture of the prototype. The injector design is versatile and multiple injectors can be used side by side to scale up to the width of the substrate to be deposited. Each injector has been used with gas flows from 0.5 – 4 slm. The injector layout is detailed in Fig. 1, forming the current chamberless coating head unit. Each coating head is a self-contained unit addressing the design criteria mentioned previously. The overall dimensions of the coating head are 43.4(L) × 22.8(W) × 2.2(H) cm3 and although the width of precursor delivery, within the current design, could be up to 30 cm, it has been limited to 15 cm for the initial feasibility tests. The nitrogen (N2) curtain provides a barrier to separate the deposition area from the ambient air surrounding the coating head and the total nitrogen curtain flow used can vary between 6 and 24 slm. Within the coating head, the deposition area is surrounded by the exhaust which is actively controlled by measuring the pressure differential, from 0 to 65 mbar, between the ambient air and the exhaust line manifold. Maintaining a positive pressure differential provides further containment to ensure the by-products and unreacted chemical precursors are disposed of safely. Furthermore, the plume of deposition can be adjusted by using a combination of restrictive flow orifices in the exhaust setup [20] (not shown here). The coating head to substrate distance is adjustable from 1 to 5 mm with the optimum distance for this reported work being 2 mm. During operation, the nitrogen curtain uses high purity nitrogen from a high pressure cryotank. In standby mode, the nitrogen is switched to 99.9 % purity nitrogen generated onsite from compressed air and used to flush the injectors to avoid the ingress of moisture. Finally, as the stainless steel body of the coating head is water-cooled, all injectors from the units have been helium leak tested at rate of < 10-11 mbar·l·s-1 to ensure that there will be no ingress of water from the water-cooled channels or gas from the surrounding air.
2.2 The chamberless inline system The coating head unit is versatile and can be configured to deposit various materials, the main requirement being that the precursors used need to be sufficiently volatile to reach the deposition area. A number of chamberless coating heads can be located in sequence to either deposit the same thin film layer (thus increasing growth rate) or different materials to realise the full PV device structure. The chemical precursors are housed in bubblers which are mounted on a modular gas delivery unit and can be independently connected to a coating head unit (not shown here). As the current aim is to demonstrate transferability of the PV structure used on the batch reactor to a chamberless inline system (i.e. ZnO:Al/ZnO/CdZnS/CdTe:As/CdTe:As+/CdCl2), a semi-automated inline system containing 6 individual coating heads has been built. The inline system is mounted within an extracted cabinet, with extraction ducting above each coating head. The extract ducting also includes a 0 – 1 % hydrogen (H2) sensor, supplied by Honeywell, linked to the safety interlock of the system. A schematic of the 6.3 m long inline system is shown in Fig. 2, where each of the 6 coating heads is allocated a layer sequentially from the PV structure. In a production scale version of this system, continuous heat control of each deposition zone would be envisaged however, for this demonstration system, an infrared heated graphite plate travels with the substrate. The temperature is ramped at 30 ˚C/min followed by a stabilisation time of 15 min under a watercooled radiation shield plate arrangement adjacent to each coating head. Once the surface temperature (Tsurf) is stable, the substrate is translated underneath the coating head for deposition with a current translation speed set at 1.8 cm·min-1.
ZnO:Al
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Figure 2 – Front view of the chamberless inline process (630(L) × 178(H) × 72(D) cm3). The inline process consists of six individually extracted deposition sections (including a radiation shield and a coating head) from left to right to deposit the ZnO:Al, ZnO, CdZnS, CdTe:As, CdTe:As+, and CdCl2 layers part of the structure. The last extracted section on the right is the loading bay containing the translating substrate heater arrangement. 2.3 Methods to test containment of the individual coating head Computational fluid dynamic (CFD) modelling was carried out prior to design and manufacture of the coating heads. All simulations were conducted using the commercial CFD software ANSYS 14.5, running on a workstation with a 3.3 GHz Quad-Core Intel Xenon and 16.0 GB of RAM. A simplified three-dimensional model based on the coating head designs schematically represented in Fig. 1, was developed employing a binary mixture of either hydrogen (H2) and nitrogen (N2) or H2 and oxygen (O2) as mass species. The concentration of the species is represented as a distribution profile of H2 mass fraction (MF) within the volume delimited by the perimeter and top walls of the coating head as well as the surface of the substrate. The models were defined by a combined mesh not exceeding 100,000 cells for two-dimensional and 700,000 cells for three-dimensional modelling [20]. In order to derive the differential equations to express conservation of mass, momentum and energy, some assumptions were made in the model: (1) the gas mixture is treated as a continuum; (2) laminar gas flow conditions exist; (3) steady-state conditions prevail; (4) the equation of state obeys the ideal gas law. As the coating head design is chamberless, it is paramount to ensure that suitable containment is achieved from the nitrogen curtain surrounding the deposition area for
both safe containment of reacting species within and avoid ambient air entering the deposition area as many metal-organic precursors are air sensitive. This was confirmed from the results found from CFD modelling. The key factor to test for adequate containment of the chamberless coating head is the leak rate where either ambient air penetrates the deposition area or reactive species escape. For safety, the latter was tested first using a standard method, routinely used on MOCVD equipment. It consists of a handheld hydrogen detector, model SP-205ASC from Riken Keiki with detectable leak rate as low as 5 × 10-6 mbar·l·s-1. Such a level of detection will allow turbulent and laminar flow leakage to be detected, using the nozzle arrangement of the detector. To carry out this test, the coating head was positioned above a flat substrate of equal or larger dimension than the coating head itself, separated from each other by a 2 mm gap. The total nitrogen curtain flow (Fcurt), around the coating head, was varied between 6 and 24 slm and the differential pressure (∆P) was varied between 6 and 33 mbar in the exhaust. A hydrogen flow (Fprec) of 0.5 slm or 1 slm was used through the precursor delivery injector. After allowing the setup to reach steady state, the nozzle of the handheld hydrogen detector was placed sequentially, close to the 2 mm gap, at regular intervals (i.e. 32 test points) around the perimeter of the coating head. The condition for passing this test is semi-quantitative by ensuring that the alarm of the detector does not trigger for safe containment of the hydrogen and therefore other reactive species within. To achieve a more quantitative result, a helium leak detector, model Modul-200 from Inficon, was used with a sniffer probe. The detector being in “Vacuum Mode” was able to detect leak rate <5×10-11 mbar·l·s-1. This type of detector is routinely used to check welds and seals in vacuum chambers. The setup was calibrated by placing the sniffer probe inside a 100% helium bag and reading a maximum leak rate of 10-3 mbar·l·s-1, (equal to 100% He leak rate). In the laboratory, background reading in the air near the test area (i.e. without the deliberate presence of He supplied through the injector) was recorded as 10-8 mbar·l·s-1. Therefore, helium containment is achieved if the measurements indicate helium containment for any leak rate readings at or below the latter value. Readings below 10-8 mbar·l·s-1are related to an increased concentration of the nitrogen curtain surrounding the coating head and diluting the other gas species sampled by the sniffer probe. Thereafter, the same setup used for the hydrogen leak test was employed, but flowing helium gas through the injector slit and using the sniffer probe around the perimeter of the coating head. Once conditions have been found for the safe containment of reactive species, an additional test was carried out to ensure that the ambient air was not passing through the nitrogen curtain, using an atmospheric residual gas analyser (RGA) based on quadrupole mass spectrometry and able to detect mass fragments from 0 to 300 atomic mass unit (amu). Once calibrated using nitrogen, the unit was used to detect partial pressure of gases over 5 decades (i.e. 1000 – 10-2 mbar). The RGA, model Cirrus 2 from MKS, can be connected either onto the exhaust port or the precursor injector inlet port of any coating head. The RGA could also be routinely used during the process phase. 2.4 Growth conditions and characterisation For simulating the CdTe growth on the surface of the substrate, a surface chemical reaction was applied to a streamlined two-dimensional model, in order to compare with experimental results. A simplified surface chemical reaction [21] on the substrate was considered which correlated well with the kinetic regime observed during the pyrolysis of CdTe in the temperature range studied. Further details on the numerical modelling can be found elsewhere [20, 21]. Detailed growth conditions for the deposition of ZnO:Al as a transparent conducting oxide (TCO), CdZnS as a n-type window layer and CdTe(:As) as a p-type absorber, within the PV structure have been described elsewhere [22, 23]. This study is concerned with the operation of the chamberless inline process and the resulting optical and physical quality of the layers deposited, without the presence of oxides, in the case of the cadmium compounds.
In all cases, the metal-organic precursors were delivered pre-mixed to the precursor injector of the coating head. The carrier gas was nitrogen for the oxides and hydrogen for the cadmium compounds. The precursors used were supplied by SAFC Hitech and included: diethylzinc (DEZn), tertiarybutanol (TBA), trimethylaluminium (TMAl), dimethylcadmium (DMCd), ditertiarybutylsulfide (DtBS), diisopropyltelluride (DiPTe) and tris(dimethylamino)arsenic (tDMAAs) for Zn, O, Al, Cd, S, Te and As respectively. The translation speed of the 15 × 15 cm2 substrate was fixed at 1.8 cm·min-1. Substrates used were NSG TECTM AB glass (i.e. bare float glass with sodium barrier layers) for TCO deposition and NSG TECTM C15 (i.e. SnO2:F coated NSG float glass product with nominal sheet resistance of 15 Ω/□ ) for the cadmium compounds. Precursor partial pressures, at the inlet of the injector, were in the range of 1 – 5 mbar (except TBA at 15 mbar and tDMAAs at 8 × 10-3 mbar). In the case of the oxide, the TBA precursor delivery lines were heated to 50 ˚C during the deposition process. The surface temperature (Tsurf) range used for growing the different layers was between 300 and 500 ˚C. The surface morphology and elemental composition of the layers were characterised using scanning electron microscopy (SEM) and an Oxford Instruments energy dispersive X-rays analyser (EDX) accessory of a Hitachi TM3000. Layer thicknesses were measured ex-situ using a Veeco Dektak 150 stylus profilometer. Total transmittance was referenced against air and measured using a Varian Cary 5000 with an integrating sphere over the spectral range 200 to 2000 nm. The band gap energy was thereafter calculated using the (αE)2 versus E relationship, where α is the absorption and E the photon energy. The sheet resistance map was measured using a four-point-probe from Jandel. The crystal structure of the layers was studied by X-ray diffraction (XRD) using a Phillips X’Pert with a CuKα source and scanned between 15˚ and 75˚2θ. The reference pattern used for zincblende CdTe structure was taken from the International Centre for Diffraction Data (ICDD) (Ref:00-015-0770). After measuring a bare TECTM C15 glass sample, the related peaks were allocated accordingly. 3. Results and Discussion 3.1 Containment from CFD modelling
Figure 3 – CFD modelling of the possible H2 containment conditions within the coating head arrangement as shown in Fig. 1. The hydrogen flow came from the precursor delivery injector at either 0.5 slm (left) or 1slm (right). In all cases the substrate was at Tsurf = 450 (˚C) and the pressure surrounding the coating head was atmospheric pressure. (a,b) H2 containment achieved: Fcurt = 24 slm and ∆P = 20 mbar; (c,d) H2 leaks through the exhaust containment: Fcurt = 24 slm and ∆P = 0 mbar; (e,f) O2 (representing ambient air) leaks through curtain containment: Fcurt = 0 slm and ∆P = 20 mbar. Note: scale bar shows a colour gradient for MF from 0 to 100 % H2, where 0 is equivalent to 100 % N2 (or O2).
CFD modelling was carried out using a range of conditions as described in section 2.3. Considering 2 gases at any one time, it was possible to model the mass fraction (MF) distribution of H2 (being one of the 2 gases) within the volume delimited by the edge of the contour of the coating head, the top walls of the coating head and the surface of the substrate. The 3 possible conditions for containment (or lack of) are summarised in Fig. 3 for 0.5 slm (left) and 1 slm (right) H2 flows coming from the precursor delivery injector. If a combination of a suitable positive ∆P, between the ambient air pressure and exhaust line pressure, and a suitable N2 curtain flow was used, then containment of the hydrogen was achieved within the boundaries of the exhaust, as shown in Fig. 3(a) and 3(b) (see Fig. 1 for schematic details of the coating head). In these conditions, doubling Fprec increases the space covered by the gases from the injector but it may result in a combination of laminar flow toward the front and turbulent flow at the back of the precursor injector slit (Fig. 3(b)). Further control of this plume can be achieved by adjusting the front and back flow in the active exhaust contour [20]. If ∆P is now 0 or negative (i.e. no active exhaust, blocked exhaust or backflow from the exhaust), then H2 will leak past the exhaust surrounding the deposition area towards the N2 curtain and beyond, as shown in Fig. 3(c) and 3(d). In these cases, as extraction to the exhaust was greatly reduced, the volume occupied by H2 extends to the N2 curtain slits from the short edges (d) and (b) of the coating head, referring to Fig. 1. Fig. 3(d) shows potential leakages routes in these conditions with H2 escaping through the corner where the nitrogen curtain injection slit was discontinued. In the event where Fcurt was reduced to 0, but ∆P was maintained and although the hydrogen is contained within the exhaust perimeter, there would be leakage of ambient air (represented by O2) into the deposition area. The ideal scenario for containment, as modelled by the CFD, is that of Fig. 3(a) and 3(b). Once the coating head prototype was produced, these containment conditions were taken forward for leak testing. 3.2 Hydrogen and helium leak testing With the chamberless coating head prototype designed and manufactured, the containment test was first made using the H2 handheld detector to confirm the test conditions observed in the CFD modelling of Fig. 3(a) and 3(b) and explore the flexibility in ∆P and Fcurt combinations. Safe containment was indeed achieved for different conditions with the flow setting of the precursor delivery injector at either 0.5 or 1 slm. Within experimental errors, a semi-quantitative plot was made, as shown in Fig. 4, where containment is shown to be in direct relation to both the differential pressure at the exhaust and the total flow from the nitrogen curtain. Indeed, it is assumed under balanced conditions that at any point along the vertical injection slits of the nitrogen curtain, the impinging flow of nitrogen splits 50:50 on impact to the flat surface of the substrate towards the deposition are and into the ambient air. As seen in Fig. 4, the conditions for hydrogen containment converge for high nitrogen curtain flow which can be understood by the reduced flow percentage of hydrogen (4 – 8 %) compared with Fcurt (12 slm) being extracted through the exhaust. Once the flow of nitrogen impinges on the substrate surface and, if the nitrogen flow vector travelling to the exhaust is sufficient, it will act as a barrier and contain the hydrogen within the coating head perimeter. If the nitrogen flow vector is weakened, then hydrogen is no longer contained. Therefore, to maintain hydrogen containment, any change in the exhaust pressure needs to coincide with a change of total flow of gas being extracted through it. Lower Fcurt and ∆P combinations for containment, as seen in Fig. 4, will be more sensitive to variation of both pressure fluctuations in the surrounding ambient air and/or the gap between the coating head and the substrate surface, as Fprec becomes a larger fraction of the total flow being extracted (i.e. up to 33%). At Fcurt =24 slm and ∆P = 20 mbar, it was found that containment was most stable with respect to the coating head to substrate gap, with a tolerance of ± 0.5 mm, hence this condition was set as constant thereafter in this study. It is noteworthy to say that although containment could be achieved at higher Fprec flow (not shown here), the current operation was limited to 1 slm to avoid the usage of high ∆P and high Fcurt. At this hydrogen flow, in the event of total containment failure, the percentage of hydrogen escaping would be below the self-ignition threshold of ~4 %.
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Figure 4 – Semi-quantitative results showing safe containment of hydrogen within the coating head perimeter for Fprec at 0.5 or 1 slm and for different containment condition varying Fcurt between 6 and 24 slm, and ∆P from 6 to 20 mbar. Note that the “Containment thresholds” are a guide to the eye indicating that, for a given Fprec, any further increase in ∆P would contain hydrogen as sensed by the handheld detector. For quantitative results of the leak rate coming from the precursor delivery injector slit, a helium leak detector was used. The results, using the procedure described in section 2.3, are shown in Fig. 5 for two different flows of helium at the precursor delivery injector slit. In both cases, containment was achieved with leak rates better than 10-8 mbar·l·s-1. Overall, it can be observed that the helium leak rate was dependent on where the measurement was made around the perimeter of the coating head. This was attributed to the extraction uniformity of the active exhaust surrounding the deposition area as well as to the distance between the nitrogen curtain injector slits and the closest extraction point. Referring to the nitrogen curtain injection slits of Fig. 1, troughs in leak rate are observed for edge (b) and (d) in Fig.5 where values as low as 2 × 10-10 mbar·l·s-1 were measured. Although the variations are amplified by the leak rate being on a logarithmic scale, it was not clear why the leak rates in the troughs area are higher for lower Fprec and may be due to measurement error as one would expect to have similar or stronger dilution of helium in these cases which should result in lower helium leak rates. The highest peak leak rates have values as high as 7 × 10-9 mbar·l·s1 . These occurred at positions moving away from the extraction points at the corners (position 0, 43.4, 66.2, 109.6 and 132.4 cm in Fig. 5) where there were gaps in the nitrogen curtain slits. Peaks were also observed in the middle of the long edges (a) and (b). Along these edges, the lowest leak rates were observed where the extraction points were situated with values as low as 10-9 mbar·l·s-1. It has now been established that suitable containment can be reached to ensure that the pyrophoric and/or toxic gases flowing from the precursor delivery injection slit stay within the perimeter of the coating head.
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Figure 5 - Helium leak test around the perimeter of the coating head over 32 test points. The containment was set constant, with ∆P = 20 mbar and Fcurt = 24 slm, for both Fprec of 0.5 and 1.0 slm. Note that leak rate values recorded below 10-7 mbar.l.s-1 means that containment is achieved within the coating head. 3.3 Result of containment from ambient air The next step was to ensure that the concentration of ambient air, surrounding the coating head, containing oxygen and water was sufficiently low not to react with the precursors in the deposition area, defined by the exhaust perimeter, as demonstrated by CFD modelling in Fig. 3(a) and 3(b). The test was carried out using the RGA connected to the exhaust line of a coating head, therefore sampling any gases which would be extracted from underneath the coating head. Thus, detection of for example water, oxygen or carbon dioxide in the exhaust gases would indicate ingress of ambient air in the deposition area. The fragments were correlated with a reference database [24]. Fig. 6(a) represents a real-time measurement of the gases in the exhaust. For the first 150 seconds (s), only the active exhaust is operating with a ∆P of 20 mbar. In this condition, the ambient air surrounding the coating head was directly extracted into the exhaust where a typical mass spectrum is shown in Fig. 6(b). Compared with reported data on air composition [25], there are some variations (i.e. 70 % N2 rather than 78 %), however, this measurement provides a reference point to identify whether containment is effective. After an elapsed time of 150 s, timeline (I) in Fig. 6(a), the 24 slm total flow of nitrogen curtain was started and the partial pressure of N2 consequently increases from 700 to 850 mbar (i.e. 85 % of the 1000 mbar total pressure). From this point, CO2 falls by nearly 2 decades below detection levels (i.e. ~10-2 mbar) after only 70 s while it takes a further 470 s for O2 to decrease its concentration by 4 decades. Once the oxygen was below 10-2 mbar, the water decreased to 10-1 mbar and steadily reduced thereafter. The mass spectrum provided in Fig. 6(c) provides a footprint of the expected conditions for achieving containment from the ambient air. Water does not follow the trend from air as it is more likely to be adsorbed on the stainless steel surfaces, within the coating head exhaust and would therefore take longer to purge. It is also observed that the concentration of hydrocarbon products from remnant metal-organics from previous growth runs (i.e. mass 15 amu (methyl radical) used to indicate hydrocarbon to avoid confusion with O fragments) remains constant throughout the measurement and reflects the amount of parasitic by-products in the exhaust lines and capillary of the RGA. Looking at the mass spectra in Fig. 6(b) and (c), other hydrocarbons such as propene C3H6 (mass 42 amu) are also present in the background and remain constant at 2 – 3 × 10-1 mbar. During this ~ 10 min containment phase, the substrate is static whilst being heated up underneath the radiation shield, adjacent to the coating head, and therefore both processes take place in parallel to allow for process shortcuts. Finally, once containment was achieved around the deposition area, its integrity was maintained until the nitrogen curtain and differential
pressure were switched off. If one wants to test the limit of the containment which would be via the observation of ambient air through the RGA, the nitrogen curtain flow can safely be reduced down to 12 slm, however, considering the results in section 3.2, 3.2 one should keep Fcurt to 24 slm.
Figure 6 – (a) Real-time mass fragment sampling of partial pressures for representative masses related to air, air using an atmospheric pressure RGA,, namely 15 (CH3), 18 (H2O), 28 (N2), 32 (O2), 40 (Ar), 44 (CO2). Mass spectra 0 – 50 amu, measured at timeline (I) and (II) in Fig. 6(a), correspond to gas present (b) before and (c) after the containment was reached. 3.4 Example of simulated and measured pyrolysis of DMCd and DiPTe It was shown [20] that good agreement could be found between the CFD modelling and experiment for CdTe layers grown from the pyrolysis of DMCd and DiPTe using the chamberless coating head; w where the first order kinetic regime gave way to mass transport limits limit above 420 ˚C. At this his temperature, the material utilisation was above 50 %, while this his decreased decrease to values as low as 6 % by decreasing the substrate surface temperature to 300 ˚C. Fig. 7(a) shows the two-dimensional two dimensional surface deposition profile across the surface of the substrate from CFD modelling, for two surface temperatures, namely 300 and 450 ˚C.. The VI:II precursor ratio was set at 0.3, which is generally the preferred condition to promote arsenic incorporation, from tDMAAs, DMAAs, as a shallow acceptor dopant. It is clear from Fig. 7(a) that the pyrolysis is less efficient at low temperature as one would predict.. In terms of deposition plume, it would not be expected to extend further than 5cm in this configuration. Knowing K the plume profile,, translation speed and final thickne thickness deposited, allows determination of the average growth rate of the layer. This can differ significantly from the peak growth rate observed in Fig 7(a). For example, using the chamberless inline process, a CdTe layer deposited at Tsurf = 450 ˚C on a moving g substrate at 1.8 cm·min-1, with a 5 cm plume depth of deposition results in an average thickness of 1.2 µm, which is equivalent to an a average growth rate of ~430 nm·min-1. Assuming that such uch absorber thickness is sufficient to be used in the PV structure,, at the current substrate translation speed, it would take 6 hours to deposit the full structure on the system described in Fig. 2. Therefore, the aim will be to further increase the translation speed and growth rate by a factor of 3 to reach the 2 hours end-to-end target.
(a)
(b)
Figure 7 – Pyrolysis of DMCd and DiPTe to grow CdTe for a Fprec = 1.0 slm, Fcurt =24 slm and ∆P = 20 mbar. (a) Simulated surface deposition rate of CdTe using a chamberless coating head head. The surface temperature of the substrate was varied from (i) ( 300 ˚C to (ii) 450 ˚C. (b) Partial pressures as measured from the RGA in 3 different conditions, while connected the exhaust of the coating head, for the following mass fragments: 2 amu (H2); 112 amu (Cd); 127 amu (Te or MCd); MCd) 143 amu (DMCd) and 214 amu (DiPTe). The mass fragments associated with the precursor and elemental species from the process can be monitored using the RGA connected to the exhaust of the coating head. Fig. 7(b) shows the partial partial pressure measured for the 5 mass fragment of interest, namely 2 amu (H2), 112 amu (Cd); 127 amu (Te or Met Methyl-cadmium (MCd)); 143 amu (DMCd) and 214 (DiPTe DiPTe) for 3 different conditions. In the first instance, the deposition area is at 30 ˚C without any precursor present. At a partial pressure of 10.6 mbar, the hydrogen alone represents 1% of the sampled mass fragment. Once precursors are introduced, both DMCd and DiPTe mass fragments are visible with a VI:II precursor ratio of 2.5, compared to that expected of 0.3. In effect, this is believed to be an artefact caused by the ionisation pprocess of the precursor molecule in the mass spectrometer. At mass 112 and 127, for or the elemental species Cd and Te, a 1:1 ratio is observed. Mass 127 can also be associated with methyl-cadmium cadmium radical likely to be present in the RGA from the ionisation of DMCd. At this temperature, no decomposition of the precursors is expected and indeed indeed no deposition was visible on the substrate. Once the temperature of the substrate was increased to 450 ˚C, before reaching the mass transport limit,, DiPTe and DMCd were reduced in concentration with 32 and 20 % remaining remaining, respectively. At this temperature, ature, only 17 and 8.9 % of mass 112 and 127 remained, confirming consumption and likely formation of CdTe, but also the Cd rich condition of the process. Decomposition of DMCd appeared to occur more readily and further investigation will be required to individually study the mass fragments pattern for each precursor molecule. There was however excess of precursor materials which are simply exhausted and further potential optimisation will be possible to further increase the he material utilisation. The crystal al structure, surface morphology and elemental composition of this CdTe layer, with a calculated band gap of 1.5 eV, are shown in Fig. 8. The texture coefficient (Chkl) was calculated from the XRD pattern (Fig. 8(a)), following the method employed by Zoppi et al. [26],, and it was found that a more random orientation was observed with a slight preferred orientation for (311) (i.e. C311 = 2.6 out of 6). This differs from previously reported (111) preferred orientation for MOCVD CdTe [18, [ 26]] and further work will be carried out to investigate the source of this change. Nevertheless, a good 1:1 stoichiometry is observed from the EDX spectra without the detectable presence of oxygen further proving that the deposition area achieved containment sufficient enough to deposit high quality CdTe materials.
(a) (b)
Figure 8 – (a) XRD pattern and (b) SEM picture (inset: EDX Spectra) of a 1.2 µm thick CdTe layer grown on TEC C15 at 450 ˚C in the chamberless inline system. In the XRD pattern, substrate peaks ((TECTM C15) are marked with an asterisk (*)) and CdTe peaks could be indexed with a cubic structure. 3.5 Other materials and thin film PV devices Using the chamberless inline system, it has been possible to control and transfer the incorporation of arsenic in the CdTe layer to ~2 × 1018 cm-3, as well as the alloying of CdS with zinc to widen its band gap [22] [ from the recipes used in the standard horizontal batch MOCVD reactor. reactor To demonstrate that there were no detectable levels of oxygen incorporation within with the window, an EDX spectrum is shown in Fig. 99(a) for a Cd0.55Zn0.45S layer with a calculated band gap of 2.65 eV (Fig. 9(b)) which correlates well with the band bowing relationship for Cd1-xZnxS given by Kartopu et al. [27]. [27] Individual layers deposited from the chamberless inline system have been independently tested as part of a series of hybrid devices where the remaining thin film layers of the device structure w were deposited in the batch reactor to test the device quality characteristics of the layers deposited in the chamberless system. system Conversion efficiencies ~13 % have been achieved which is very encouraging considering the current best platform device AM1.5 efficiency of > 16 % [13]. Aluminium doped zinc oxide has ha also been successfully transferred to the chamberless inline system with a controlled incorporation of aluminium (i.e. 1 – 2 %) reaching minimum resistivity of 4.4 × 10-4 Ω·cm and sheet resistance of 7 Ω/□ □ for 680 nm thick layers layers. An example of the thickness uniformity profile achieved with this system is provided in Fig. 10 for a ZnO:Al layer with average thickness of 432 ± 24 nm across the 15 cm width of the substrate.. The average sheet resistance for this ZnO:Al layer was 13 ± 5 Ω/□ over the 15 × 15 cm2 sample ple area, offering similar performance to commercially available TCOs such as TECTM C15 from NSG.
(a)
(b)
Figure 9 – (a) EDX spectra and (b) (αE) (α 2 vs E curve of a Cd0.55Zn0.45S layer deposited on TECTM C15 at 400˚C. ˚C. The band gap was determined to be 2.65 eV.
Figure 10 – Thickness profile of a ZnO:Al layer deposited using the chamberless inline system at 400 ˚C with a substrate translating at 1.8 cm·cm-1. The average thickness was 432 ± 24 nm. 4. Conclusions MOCVD is normally associated with batch process with limited opportunity to be scaled to an inline process process. In this study, a simple chamberless coating head design is introduced with potential for transferring a successful batch process atmospheric pressure MOCVD towards a large scale continuous inline processes. In order to help optimise the design and dictate the flow parameters for the chamberless coating head assembly, CFD modelling was used. The CFD modelling demonstrated the conditions conditions required for containment of the precursor flow within the deposition area and obstruct ambient air from entering. entering Experimental work supported the results from the CFD model, model using a range of leak rate testing tools where leak rates as low as 2 × 10-10 mbar·l·s-1 were measured from the combination of a nitrogen curtain and differential pressure balance between the exhaust and the ambient pressure. It was shown that suitable containment can be achieved isolating the deposition area from the ambient air and confirmed by real-time time monitoring of mass fragments using an RGA sampling the exhaust gases. gases Finally, thin film layers deposited, using the chamberless inline system, have been achieved achieve without detectable levels of atmospheric oxygen. It is noteworthy that there are various routes to increase the throughput of the system moving towards a continuous process to deposit the PV structure, to achieve the benchmark time of 2 hours end end-to-end. These engineering options may include: -
Reducing the he temperature gradient between layers, Multiple and sequential coating heads for layers with low growth rates or high thickness requirements, Increasing the growth rates by increasing the precursor concentrations, Increasing growth rates by utilising more reactive precursors.
The inline chamberless MOCVD system has the potential for depositing other compound semiconductors such as III-Vs Vs that may require high throughput for relatively simple device structures. structures
5. Acknowledgments The authors would like to thank A4B for funding the CIPAM project and ERDF/LCRI for funding the SPARC project. ESF and Scanwel Ltd. are also thanked for funding a Knowledge Economy Skills Scholarship (KESS). The Chemistry Department at Bangor University is also gratefully thanked for allowing access to their XRD. Paul Warren and Kieran Cheetham, from NSG, are also thanked for fruitful discussions and supplying the TEC glass.
6. References: [1] R.D. Dupuis and P.D. Dapkus, Appl. Phys. Lett. 32 (1978) 406. [2] I.M. Watson, Coordination Chemistry Reviews 257 (2013) 2120. [3] A.P. Kirk, Solar Energy Materials & Solar Cells 94 (2010) 2442. [4] S.J.C. Irvine, J.B. Mullin, J. Giess, J.S. Gough, A. Royle, G. Crimes, J. Cryst. Growth 93(1–4) (1988) 732. [5] J.C. Fan, K.M. Sreekanth, Z. Xie, S.L. Chang, K.V. Rao, Progress in Materials Science 58 (2013) 874. [6] W. Stutius, J. Cryst. Growth 59(1–2) (1982) 1. [7] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Prog. Photovolt: Res. Appl. 22 (2014) 701. [8] M. Marwede and A. Reller, Resources, Conservation and Recycling 69 (2012) 35. [9] G.M. Hanket, B.E. McCandless, W.A. Buchanan, S. Fields, R.W. Birkmire, Journal of Vacuum Science and Technolology A24 (2006) 1695. [10] M. Woodhouse, A. Goodrich, R. Margolis, T. James, R. Dhere, T. Gessert, T. Barnes, R. Eggert, D. Albin, Solar Energy Materials and Solar Cells 115 (2013) 199. [11] A. Nouhi, P.V. Meyers, R.J. Stirn, C.H. Liu, J. Vac. Sci. Technol. A 7(3) (1989) 833. [12] R.A. Berrigan, N. Maung, S.J.C. Irvine, D.J. Cole-Hamilton, D. Ellis, J. Cryst. Growth 195 (1998) 718. [13] G. Kartopu, V. Barrioz, S. Hodgson, E. Tedejor, D. Dupin, A.J. Clayton, S.J.C. Irvine, Proc. PVSAT10, Loughborough, 23rd-25th April 2014, p63-66. [14] R.J. McCurdy, Thin Solid Films, 351(1–2) (1999), 66. [15] Y.-Y. Xie, A. Knoll, Y. Chen, Y. Li, X. Xiong, Y. Qiao, P. Hou, J. Reeves, T. Salagaj, K. Lenseth, L. Civale, B. Maiorov, Y. Iwasa, V. Solovyov, M. Suenaga, N. Cheggour, C. Clickner, J.W. Ekin, C. Weber, V. Selvamanickam, Physica C 426–431 (2005) 849. [16] J. van Deelen, A. Illiberi, B. Kniknie, E.H.A. Beckers, P.J.P.M. Simons, A. Lankhorst, Thin Solid Films 555 (2014) 163. [17] V. Barrioz, G. Kartopu, S.J.C. Irvine, S. Monir, X. Yang, J. Cryst. Growth 354 (2012) 81. [18] G. Kartopu, V. Barrioz, S.J.C. Irvine, A.J. Clayton, S. Monir, D.A. Lamb, Thin Solid Films 558 (2014) 374. [19] V. Barrioz, D.A. Lamb, S. Monir, S. Trueman, G. Kartopu, I.W. Owen, S.J.C. Irvine, X. Yang, UK Patent Application (GB1302306.4); PCT patent application (PCT/GB2014/050386). [20] S. Monir, G. Kartopu, V. Barrioz, D.A. lamb, S.J.C. Irvine, X. Yang, J. Cryst. Growth (to be submitted). [21] Y. Wu, X. Yang, X. Huang, V. Barrioz, S. Monir, S.J.C. Irvine, G. Kartopu, Applied Mechanics and Materials 217-219 (2012) 1265. [22] G. Kartopu, V. Barrioz, S. Monir, D.A. Lamb, S.J.C. Irvine, Thin Solid Films (submitted June 2014). [23] V. Barrioz, D.A. Lamb, W. Brooks, P. Siderfin, S.J.C. Irvine, S. Monir, G. Kartopu, A.J. Clayton, Vacuum (submitted July 2014). [24] NIST Standard Reference Database 69: NIST Chemistry WebBook: http://webbook.nist.gov/chemistry/ ; last accessed: 2nd July 2014. [25] A. Picard, R.S. Davis, M. Gläser, K. Fujii, Metrologia 45 (2008) 149. [26] G. Zoppi, K. Durose, S.J.C. Irvine, V. Barrioz, Semicond. Sci. Technol. 21 (2006) 763. [27] G. Kartopu, A.J. Clayton, W.S.M. Brooks, S.D. Hodgson, V. Barrioz, A. Maertens, D.A. Lamb, S.J.C. Irvine, Prog. Photovolt: Res. Appl. 22 (2014) 18.
List of Figures Figure 1 – Top view schematic of a coating head unit [19] where only the injection slits and exhaust are represented. The dashed edges (a, b, c and d) represent the injection slits for the nitrogen curtain, the single solid line represents the precursor delivery, the hashed perimeter is the active exhaust surrounding the deposition area and the black dots represent the extraction points. The light grey band represents the substrate width while the central dotted arrow indicates its direction of travel. Figure 2 – Front view of the chamberless inline process (630(L) × 178(H) × 72(D) cm3). The inline process consists of six individually extracted deposition sections (including a radiation shield and a coating head) from left to right to deposit the ZnO:Al, ZnO, CdZnS, CdTe:As, CdTe:As+, and CdCl2 layers part of the structure. The last extracted section on the right is the loading bay containing the translating substrate heater arrangement. Figure 3 – CFD modelling of the possible H2 containment conditions within the coating head arrangement as shown in Fig. 1. The hydrogen flow came from the precursor delivery injector at either 0.5 slm (left) or 1slm (right). In all cases the substrate was at Tsurf = 450 (˚C) and the pressure surrounding the coating head was atmospheric pressure. (a,b) H2 containment achieved: Fcurt = 24 slm and ∆P = 20 mbar; (c,d) H2 leaks through the exhaust containment: Fcurt = 24 slm and ∆P = 0 mbar; (e,f) O2 (representing ambient air) leaks through curtain containment: Fcurt = 0 slm and ∆P = 20 mbar. Note: scale bar shows a colour gradient for MF from 0 to 100 % H2, where 0 is equivalent to 100 % N2 (or O2). Figure 4 – Semi-quantitative results showing safe containment of hydrogen within the coating head perimeter for Fprec at 0.5 or 1 slm and for different containment condition varying Fcurt between 6 and 24 slm, and ∆P from 6 to 20 mbar. Note that the “Containment thresholds” are a guide to the eye indicating that, for a given Fprec, any further increase in ∆P would contain hydrogen as sensed by the handheld detector. Figure 5 - Helium leak test around the perimeter of the coating head over 32 test points. The containment was set constant, with ∆P = 20 mbar and Fcurt = 24 slm, for both Fprec of 0.5 and 1.0 slm. Note that leak rate values recorded below 10-7 mbar.l.s-1 means that containment is achieved within the coating head. Figure 6 – (a) Real-time mass fragment sampling of partial pressures for representative masses related to air, using an atmospheric pressure RGA, namely 15 (CH3), 18 (H2O), 28 (N2), 32 (O2), 40 (Ar), 44 (CO2). Mass spectra 0 – 50 amu, measured at timeline (I) and (II) in Fig. 6(a), correspond to gas present (b) before and (c) after the containment was reached. Figure 7 – Pyrolysis of DMCd and DiPTe to grow CdTe for a Fprec = 1.0 slm, Fcurt =24 slm and ∆P = 20 mbar. (a) Simulated surface deposition rate of CdTe using a chamberless coating head. The surface temperature of the substrate was varied from (i) 300 ˚C to (ii) 450 ˚C. (b) Partial pressures as measured from the RGA in 3 different conditions, while connected the exhaust of the coating head, for the following mass fragments: 2 amu (H2); 112 amu (Cd); 127 amu (Te or MCd); 143 amu (DMCd) and 214 amu (DiPTe). Figure 8 – (a) XRD pattern and (b) SEM picture (inset: EDX Spectra) of a 1.2 µm thick CdTe layer grown on TEC C15 at 450 ˚C in the chamberless inline system. In the XRD pattern, substrate peaks (TECTM C15) are marked with an asterisk (*) and CdTe peaks could be indexed with a cubic structure.Figure 9 – (a) EDX spectra and (b) (αE)2 vs E curve of a Cd0.55Zn0.45S layer deposited on TECTM C15 at 400˚C. The band gap was determined to be 2.65 eV. Figure 10 – Thickness profile of a ZnO:Al layer deposited using the chamberless inline system at 400 ˚C with a substrate translating at 1.8 cm·cm-1. The average thickness was 432 ± 24 nm.
Highlights: • • • • •
A chamberless coating head was designed for scale up of MOCVD as an inline process. Suitable containment was achieved using CFD modelling and leak testing tools, measuring leak rates as low as 2 × 10-10 mbar·l·s-1. Cadmium compounds were deposited free from oxygen. Good transfer of layer quality for thin film PV were reached with good dopant and alloying capabilities. Set of chamberless coating heads can be placed sequentially to deposit the full structure of a CdTe thin film PV.
MOCVD for solar cells, a transition towards a chamberless inline process V. Barrioz, S. Monir, G. Kartopu, D.A. Lamb, W. Brooks, P. Siderfin, S. Jones, A.J. Clayton, S.J.C. Irvine
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S0022-0248(14)00773-8 http://dx.doi.org/10.1016/j.jcrysgro.2014.11.014 CRYS22536
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Journal of Crystal Growth
Cite this article as: V. Barrioz, S. Monir, G. Kartopu, D.A. Lamb, W. Brooks, P. Siderfin, S. Jones, A.J. Clayton, S.J.C. Irvine, MOCVD for solar cells, a transition towards a chamberless inline process, Journal of Crystal Growth, http://dx.doi.org/ 10.1016/j.jcrysgro.2014.11.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
MOCVD for solar cells, a transition towards a chamberless inline process V. Barrioz*, S. Monir, G. Kartopu, D.A. Lamb, W. Brooks, P. Siderfin, S. Jones, A.J. Clayton, S.J.C. Irvine Centre for Solar Energy Research, OpTIC Centre, Glyndŵr University, St Asaph, LL17 0JD * Corresponding author: [email protected]; 00 44 1745 535 159 Abstract MOCVD has been associated with batch processing of III-V opto-electronic devices for decades, with epitaxial structures deposited on up to 200 mm diameter wafers. Recent development in thin film PV has seen the gap in conversion efficiencies closing in on that of the commonly found multicrystalline Si wafer based PV. To further improve the conversion efficiency of thin film PV towards the theoretical limits of single junction solar cells requires a technique such as MOCVD with scalability potential. Preliminary results on the development of a chamberless inline process are reported for up to 15 cm wide float glass, progressively coating each layer in the CdTe solar cell as the heated substrate passes under each coating head in turn and entirely at atmospheric pressure. Emphasis is made on ensuring that the chamberless coating heads can be operated safely using a combination of nitrogen curtain flows and a balanced exhaust pressure system. Results are also presented on the exclusion of oxygen and moisture from the coating area, achieved using the same gas flow isolation process. This paper also reviews the achievements made to-date in the transfer of the high efficiency batch MOCVD produced CdTe solar cell to the chamberless inline process demonstrating device quality thin films deposition. Highlights: • • • • •
A chamberless coating head was designed for scale up of MOCVD as an inline process. Suitable containment was achieved using CFD modelling and leak testing tools, measuring leak rates as low as 2 × 10-10 mbar·l·s-1. Cadmium compounds were deposited with oxygen below detectable levels. Good transfer of layer quality for thin film PV were reached with good dopant and alloying capabilities. Set of chamberless coating heads can be placed sequentially to deposit the full structure of a CdTe thin film PV.
Keywords: Metalorganic chemical vapor deposition (A3); Semiconducting II-VI materials (B2); Cadmium Compounds (B1); Oxides(B1); Zinc Compounds(B1); Photovoltaic Solar Cells (B3) 1. Introduction Metal-organic chemical vapour deposition (MOCVD) has been gaining importance in opto-electronic device development, since the late 1970s [1]. This has predominantly occurred with the requirements for high controllability and uniformity of optical and/or electrical properties in stacks of thin epitaxial layers containing very low defects density, deposited onto single crystal wafers. Subsequently, the MOCVD technique has been known as a low-pressure batch process for epitaxial growth of semiconductors, with greater potential to be scaled up when compared with molecular beam epitaxy (MBE) [2]. The applications are generally based around III-V semiconductor structures from LEDs and transistors [2] to triple junction solar cells [3]. However, II-VI semiconductor work has also been important with the development of infrared detectors [4] and other opto-electronic applications [5, 6]. “High-throughput” batch reactors are commercially available to grow these epitaxial structures on up to 200 mm diameter wafers, therefore increasing the yielded number of devices per run over the area of the wafer. From a photovoltaic application
view point, MOCVD has enabled monolithically integrated InGaP/GaAs/InGaAs triple-junction solar cells to achieve a world record best efficiency of 44.4%, under concentrated sunlight [7]. For non-concentrated single junction solar cells, there have been recent developments with thin film PV CdTe and CIGS, matching record conversion efficiency with multicrystalline Si wafer based PV at ~20 % for laboratory cells and ~17 % for PV modules [7]. Considering that reducing cost per watt peak is the main driver for PV, these developments have great significance as thin film PV offers more potential for cost reduction through lower temperature processing steps and much lower material usage. The most successful thin film module production has used CdTe technology [8] thanks to its near optimum band gap for solar absorption and the scalable vapour transport deposition (VTD) technique employed [9]. However, getting closer to the theoretical limits for single junction solar cells would require a technique such as MOCVD to controllably achieve alloying and high doping concentrations, with low absorber thicknesses [10], whilst also maintaining compatibility with scalable inline continuous processes. MOCVD grown CdTe absorber layers for thin film PV was first reported in 1989 [11] where 9.4 % conversion efficiency was achieved for Tellurium rich absorber layers. The first full MOCVD CdS/CdTe:As structure reported was in 1998 [12]. Since then, recent work [13] resulted in a CdZnS/CdTe:As/CdTe:As+/CdCl2 structure, grown on glass/ITO sequentially in a batch horizontal MOCVD reactor at atmospheric pressure (AP), achieving > 16 % conversion efficiency. The best example for the scalability potential of chemical vapour deposition is in the deposition of oxide coatings on-line during float glass manufacturing where deposition up to 12 µm·min-1 are reported over a continuous glass substrate with a width of 4 m [14]. Xie et al. [15] also reported on MOCVD being developed for roll to roll processes to deposit YBCO as superconducting film on metal tape. More recently, Van Deelen et al. [16] reported on the deposition of aluminium doped zinc oxide (AZO), at atmospheric pressure where they achieved deposition rate up to 800 nm·min-1 on substrates moving up to 0.5 cm·min-1. To the authors’ knowledge, no AP-MOCVD system has been reported on moving substrate, as a chamberless inline process for non-oxide materials. Preliminary results in transferring the process from a horizontal batch reactor to an enclosed inline chamber showed promising results with ~40 % material utilisation [17]. The ability to control the semiconductor properties of the layers, from a p-n junction fully deposited in this inline reactor, was demonstrated with conversion efficiencies of ~12 % for a 1 µm thick CdTe absorber layer, at a substrate translation speed of 2 cm·min-1 [18]. This paper introduces the recent development of a chamberless inline process, with uniform gas injector delivery [19], for MOCVD, where both cadmium based chalcogenides (II-VI group) and oxides layers were successfully deposited with the demonstration of both alloying and dopant controllability. 2. Materials and Methods There are challenges for MOCVD to move to an inline process in order to deposit the full PV structure. These are different to that needed on the float line, for example, where chemical precursors are selected to react quickly for a given temperature gradient and high continuous glass translation speed. For a full thin film PV structure, the key is to achieve an end-to-end time within 2 hours from a bare glass to a finished PV module. This is the benchmark set by the existing CdTe PV manufacturers. Therefore, moving towards a high volume MOCVD process, a few design criteria need to be set for the development of the coating head units: -
Safe operation (containment of all precursors). Simple and flexible coating head arrangement. Chamberless design for easy retrofit onto different processes and substrates. Containment to avoid ingress of air and moisture into the deposition area. Uniformity of flow delivery to the substrate.
-
High material utilisation.
Figure 1 – Top view schematic of a coating head unit [19] where only the injection slits and exhaust are represented. The dashed edges (a, b, c and d) represent the injection slits for the nitrogen curtain, the single solid line represents the precursor delivery, the hashed perimeter is the active exhaust surrounding the deposition area and the black dots represent the extraction points. The light grey band represents the substrate width while the central dotted arrow indicates its direction of travel. 2.1 The chamberless coating head unit In collaboration with Scanwel Ltd, a multi-chamber injector design has been developed [19] to allow premixing and uniform delivery of gases to the surface of the substrate via an injection slit. CFD modelling has also been used during the development prior to manufacture of the prototype. The injector design is versatile and multiple injectors can be used side by side to scale up to the width of the substrate to be deposited. Each injector has been used with gas flows from 0.5 – 4 slm. The injector layout is detailed in Fig. 1, forming the current chamberless coating head unit. Each coating head is a self-contained unit addressing the design criteria mentioned previously. The overall dimensions of the coating head are 43.4(L) × 22.8(W) × 2.2(H) cm3 and although the width of precursor delivery, within the current design, could be up to 30 cm, it has been limited to 15 cm for the initial feasibility tests. The nitrogen (N2) curtain provides a barrier to separate the deposition area from the ambient air surrounding the coating head and the total nitrogen curtain flow used can vary between 6 and 24 slm. Within the coating head, the deposition area is surrounded by the exhaust which is actively controlled by measuring the pressure differential, from 0 to 65 mbar, between the ambient air and the exhaust line manifold. Maintaining a positive pressure differential provides further containment to ensure the by-products and unreacted chemical precursors are disposed of safely. Furthermore, the plume of deposition can be adjusted by using a combination of restrictive flow orifices in the exhaust setup [20] (not shown here). The coating head to substrate distance is adjustable from 1 to 5 mm with the optimum distance for this reported work being 2 mm. During operation, the nitrogen curtain uses high purity nitrogen from a high pressure cryotank. In standby mode, the nitrogen is switched to 99.9 % purity nitrogen generated onsite from compressed air and used to flush the injectors to avoid the ingress of moisture. Finally, as the stainless steel body of the coating head is water-cooled, all injectors from the units have been helium leak tested at rate of < 10-11 mbar·l·s-1 to ensure that there will be no ingress of water from the water-cooled channels or gas from the surrounding air.
2.2 The chamberless inline system The coating head unit is versatile and can be configured to deposit various materials, the main requirement being that the precursors used need to be sufficiently volatile to reach the deposition area. A number of chamberless coating heads can be located in sequence to either deposit the same thin film layer (thus increasing growth rate) or different materials to realise the full PV device structure. The chemical precursors are housed in bubblers which are mounted on a modular gas delivery unit and can be independently connected to a coating head unit (not shown here). As the current aim is to demonstrate transferability of the PV structure used on the batch reactor to a chamberless inline system (i.e. ZnO:Al/ZnO/CdZnS/CdTe:As/CdTe:As+/CdCl2), a semi-automated inline system containing 6 individual coating heads has been built. The inline system is mounted within an extracted cabinet, with extraction ducting above each coating head. The extract ducting also includes a 0 – 1 % hydrogen (H2) sensor, supplied by Honeywell, linked to the safety interlock of the system. A schematic of the 6.3 m long inline system is shown in Fig. 2, where each of the 6 coating heads is allocated a layer sequentially from the PV structure. In a production scale version of this system, continuous heat control of each deposition zone would be envisaged however, for this demonstration system, an infrared heated graphite plate travels with the substrate. The temperature is ramped at 30 ˚C/min followed by a stabilisation time of 15 min under a watercooled radiation shield plate arrangement adjacent to each coating head. Once the surface temperature (Tsurf) is stable, the substrate is translated underneath the coating head for deposition with a current translation speed set at 1.8 cm·min-1.
ZnO:Al
ZnO
CdZnS
CdTe:As
CdTe:As+
CdCl2
Figure 2 – Front view of the chamberless inline process (630(L) × 178(H) × 72(D) cm3). The inline process consists of six individually extracted deposition sections (including a radiation shield and a coating head) from left to right to deposit the ZnO:Al, ZnO, CdZnS, CdTe:As, CdTe:As+, and CdCl2 layers part of the structure. The last extracted section on the right is the loading bay containing the translating substrate heater arrangement. 2.3 Methods to test containment of the individual coating head Computational fluid dynamic (CFD) modelling was carried out prior to design and manufacture of the coating heads. All simulations were conducted using the commercial CFD software ANSYS 14.5, running on a workstation with a 3.3 GHz Quad-Core Intel Xenon and 16.0 GB of RAM. A simplified three-dimensional model based on the coating head designs schematically represented in Fig. 1, was developed employing a binary mixture of either hydrogen (H2) and nitrogen (N2) or H2 and oxygen (O2) as mass species. The concentration of the species is represented as a distribution profile of H2 mass fraction (MF) within the volume delimited by the perimeter and top walls of the coating head as well as the surface of the substrate. The models were defined by a combined mesh not exceeding 100,000 cells for two-dimensional and 700,000 cells for three-dimensional modelling [20]. In order to derive the differential equations to express conservation of mass, momentum and energy, some assumptions were made in the model: (1) the gas mixture is treated as a continuum; (2) laminar gas flow conditions exist; (3) steady-state conditions prevail; (4) the equation of state obeys the ideal gas law. As the coating head design is chamberless, it is paramount to ensure that suitable containment is achieved from the nitrogen curtain surrounding the deposition area for
both safe containment of reacting species within and avoid ambient air entering the deposition area as many metal-organic precursors are air sensitive. This was confirmed from the results found from CFD modelling. The key factor to test for adequate containment of the chamberless coating head is the leak rate where either ambient air penetrates the deposition area or reactive species escape. For safety, the latter was tested first using a standard method, routinely used on MOCVD equipment. It consists of a handheld hydrogen detector, model SP-205ASC from Riken Keiki with detectable leak rate as low as 5 × 10-6 mbar·l·s-1. Such a level of detection will allow turbulent and laminar flow leakage to be detected, using the nozzle arrangement of the detector. To carry out this test, the coating head was positioned above a flat substrate of equal or larger dimension than the coating head itself, separated from each other by a 2 mm gap. The total nitrogen curtain flow (Fcurt), around the coating head, was varied between 6 and 24 slm and the differential pressure (∆P) was varied between 6 and 33 mbar in the exhaust. A hydrogen flow (Fprec) of 0.5 slm or 1 slm was used through the precursor delivery injector. After allowing the setup to reach steady state, the nozzle of the handheld hydrogen detector was placed sequentially, close to the 2 mm gap, at regular intervals (i.e. 32 test points) around the perimeter of the coating head. The condition for passing this test is semi-quantitative by ensuring that the alarm of the detector does not trigger for safe containment of the hydrogen and therefore other reactive species within. To achieve a more quantitative result, a helium leak detector, model Modul-200 from Inficon, was used with a sniffer probe. The detector being in “Vacuum Mode” was able to detect leak rate <5×10-11 mbar·l·s-1. This type of detector is routinely used to check welds and seals in vacuum chambers. The setup was calibrated by placing the sniffer probe inside a 100% helium bag and reading a maximum leak rate of 10-3 mbar·l·s-1, (equal to 100% He leak rate). In the laboratory, background reading in the air near the test area (i.e. without the deliberate presence of He supplied through the injector) was recorded as 10-8 mbar·l·s-1. Therefore, helium containment is achieved if the measurements indicate helium containment for any leak rate readings at or below the latter value. Readings below 10-8 mbar·l·s-1are related to an increased concentration of the nitrogen curtain surrounding the coating head and diluting the other gas species sampled by the sniffer probe. Thereafter, the same setup used for the hydrogen leak test was employed, but flowing helium gas through the injector slit and using the sniffer probe around the perimeter of the coating head. Once conditions have been found for the safe containment of reactive species, an additional test was carried out to ensure that the ambient air was not passing through the nitrogen curtain, using an atmospheric residual gas analyser (RGA) based on quadrupole mass spectrometry and able to detect mass fragments from 0 to 300 atomic mass unit (amu). Once calibrated using nitrogen, the unit was used to detect partial pressure of gases over 5 decades (i.e. 1000 – 10-2 mbar). The RGA, model Cirrus 2 from MKS, can be connected either onto the exhaust port or the precursor injector inlet port of any coating head. The RGA could also be routinely used during the process phase. 2.4 Growth conditions and characterisation For simulating the CdTe growth on the surface of the substrate, a surface chemical reaction was applied to a streamlined two-dimensional model, in order to compare with experimental results. A simplified surface chemical reaction [21] on the substrate was considered which correlated well with the kinetic regime observed during the pyrolysis of CdTe in the temperature range studied. Further details on the numerical modelling can be found elsewhere [20, 21]. Detailed growth conditions for the deposition of ZnO:Al as a transparent conducting oxide (TCO), CdZnS as a n-type window layer and CdTe(:As) as a p-type absorber, within the PV structure have been described elsewhere [22, 23]. This study is concerned with the operation of the chamberless inline process and the resulting optical and physical quality of the layers deposited, without the presence of oxides, in the case of the cadmium compounds.
In all cases, the metal-organic precursors were delivered pre-mixed to the precursor injector of the coating head. The carrier gas was nitrogen for the oxides and hydrogen for the cadmium compounds. The precursors used were supplied by SAFC Hitech and included: diethylzinc (DEZn), tertiarybutanol (TBA), trimethylaluminium (TMAl), dimethylcadmium (DMCd), ditertiarybutylsulfide (DtBS), diisopropyltelluride (DiPTe) and tris(dimethylamino)arsenic (tDMAAs) for Zn, O, Al, Cd, S, Te and As respectively. The translation speed of the 15 × 15 cm2 substrate was fixed at 1.8 cm·min-1. Substrates used were NSG TECTM AB glass (i.e. bare float glass with sodium barrier layers) for TCO deposition and NSG TECTM C15 (i.e. SnO2:F coated NSG float glass product with nominal sheet resistance of 15 Ω/□ ) for the cadmium compounds. Precursor partial pressures, at the inlet of the injector, were in the range of 1 – 5 mbar (except TBA at 15 mbar and tDMAAs at 8 × 10-3 mbar). In the case of the oxide, the TBA precursor delivery lines were heated to 50 ˚C during the deposition process. The surface temperature (Tsurf) range used for growing the different layers was between 300 and 500 ˚C. The surface morphology and elemental composition of the layers were characterised using scanning electron microscopy (SEM) and an Oxford Instruments energy dispersive X-rays analyser (EDX) accessory of a Hitachi TM3000. Layer thicknesses were measured ex-situ using a Veeco Dektak 150 stylus profilometer. Total transmittance was referenced against air and measured using a Varian Cary 5000 with an integrating sphere over the spectral range 200 to 2000 nm. The band gap energy was thereafter calculated using the (αE)2 versus E relationship, where α is the absorption and E the photon energy. The sheet resistance map was measured using a four-point-probe from Jandel. The crystal structure of the layers was studied by X-ray diffraction (XRD) using a Phillips X’Pert with a CuKα source and scanned between 15˚ and 75˚2θ. The reference pattern used for zincblende CdTe structure was taken from the International Centre for Diffraction Data (ICDD) (Ref:00-015-0770). After measuring a bare TECTM C15 glass sample, the related peaks were allocated accordingly. 3. Results and Discussion 3.1 Containment from CFD modelling
Figure 3 – CFD modelling of the possible H2 containment conditions within the coating head arrangement as shown in Fig. 1. The hydrogen flow came from the precursor delivery injector at either 0.5 slm (left) or 1slm (right). In all cases the substrate was at Tsurf = 450 (˚C) and the pressure surrounding the coating head was atmospheric pressure. (a,b) H2 containment achieved: Fcurt = 24 slm and ∆P = 20 mbar; (c,d) H2 leaks through the exhaust containment: Fcurt = 24 slm and ∆P = 0 mbar; (e,f) O2 (representing ambient air) leaks through curtain containment: Fcurt = 0 slm and ∆P = 20 mbar. Note: scale bar shows a colour gradient for MF from 0 to 100 % H2, where 0 is equivalent to 100 % N2 (or O2).
CFD modelling was carried out using a range of conditions as described in section 2.3. Considering 2 gases at any one time, it was possible to model the mass fraction (MF) distribution of H2 (being one of the 2 gases) within the volume delimited by the edge of the contour of the coating head, the top walls of the coating head and the surface of the substrate. The 3 possible conditions for containment (or lack of) are summarised in Fig. 3 for 0.5 slm (left) and 1 slm (right) H2 flows coming from the precursor delivery injector. If a combination of a suitable positive ∆P, between the ambient air pressure and exhaust line pressure, and a suitable N2 curtain flow was used, then containment of the hydrogen was achieved within the boundaries of the exhaust, as shown in Fig. 3(a) and 3(b) (see Fig. 1 for schematic details of the coating head). In these conditions, doubling Fprec increases the space covered by the gases from the injector but it may result in a combination of laminar flow toward the front and turbulent flow at the back of the precursor injector slit (Fig. 3(b)). Further control of this plume can be achieved by adjusting the front and back flow in the active exhaust contour [20]. If ∆P is now 0 or negative (i.e. no active exhaust, blocked exhaust or backflow from the exhaust), then H2 will leak past the exhaust surrounding the deposition area towards the N2 curtain and beyond, as shown in Fig. 3(c) and 3(d). In these cases, as extraction to the exhaust was greatly reduced, the volume occupied by H2 extends to the N2 curtain slits from the short edges (d) and (b) of the coating head, referring to Fig. 1. Fig. 3(d) shows potential leakages routes in these conditions with H2 escaping through the corner where the nitrogen curtain injection slit was discontinued. In the event where Fcurt was reduced to 0, but ∆P was maintained and although the hydrogen is contained within the exhaust perimeter, there would be leakage of ambient air (represented by O2) into the deposition area. The ideal scenario for containment, as modelled by the CFD, is that of Fig. 3(a) and 3(b). Once the coating head prototype was produced, these containment conditions were taken forward for leak testing. 3.2 Hydrogen and helium leak testing With the chamberless coating head prototype designed and manufactured, the containment test was first made using the H2 handheld detector to confirm the test conditions observed in the CFD modelling of Fig. 3(a) and 3(b) and explore the flexibility in ∆P and Fcurt combinations. Safe containment was indeed achieved for different conditions with the flow setting of the precursor delivery injector at either 0.5 or 1 slm. Within experimental errors, a semi-quantitative plot was made, as shown in Fig. 4, where containment is shown to be in direct relation to both the differential pressure at the exhaust and the total flow from the nitrogen curtain. Indeed, it is assumed under balanced conditions that at any point along the vertical injection slits of the nitrogen curtain, the impinging flow of nitrogen splits 50:50 on impact to the flat surface of the substrate towards the deposition are and into the ambient air. As seen in Fig. 4, the conditions for hydrogen containment converge for high nitrogen curtain flow which can be understood by the reduced flow percentage of hydrogen (4 – 8 %) compared with Fcurt (12 slm) being extracted through the exhaust. Once the flow of nitrogen impinges on the substrate surface and, if the nitrogen flow vector travelling to the exhaust is sufficient, it will act as a barrier and contain the hydrogen within the coating head perimeter. If the nitrogen flow vector is weakened, then hydrogen is no longer contained. Therefore, to maintain hydrogen containment, any change in the exhaust pressure needs to coincide with a change of total flow of gas being extracted through it. Lower Fcurt and ∆P combinations for containment, as seen in Fig. 4, will be more sensitive to variation of both pressure fluctuations in the surrounding ambient air and/or the gap between the coating head and the substrate surface, as Fprec becomes a larger fraction of the total flow being extracted (i.e. up to 33%). At Fcurt =24 slm and ∆P = 20 mbar, it was found that containment was most stable with respect to the coating head to substrate gap, with a tolerance of ± 0.5 mm, hence this condition was set as constant thereafter in this study. It is noteworthy to say that although containment could be achieved at higher Fprec flow (not shown here), the current operation was limited to 1 slm to avoid the usage of high ∆P and high Fcurt. At this hydrogen flow, in the event of total containment failure, the percentage of hydrogen escaping would be below the self-ignition threshold of ~4 %.
Active Exhaust ∆P (mbar)
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Figure 4 – Semi-quantitative results showing safe containment of hydrogen within the coating head perimeter for Fprec at 0.5 or 1 slm and for different containment condition varying Fcurt between 6 and 24 slm, and ∆P from 6 to 20 mbar. Note that the “Containment thresholds” are a guide to the eye indicating that, for a given Fprec, any further increase in ∆P would contain hydrogen as sensed by the handheld detector. For quantitative results of the leak rate coming from the precursor delivery injector slit, a helium leak detector was used. The results, using the procedure described in section 2.3, are shown in Fig. 5 for two different flows of helium at the precursor delivery injector slit. In both cases, containment was achieved with leak rates better than 10-8 mbar·l·s-1. Overall, it can be observed that the helium leak rate was dependent on where the measurement was made around the perimeter of the coating head. This was attributed to the extraction uniformity of the active exhaust surrounding the deposition area as well as to the distance between the nitrogen curtain injector slits and the closest extraction point. Referring to the nitrogen curtain injection slits of Fig. 1, troughs in leak rate are observed for edge (b) and (d) in Fig.5 where values as low as 2 × 10-10 mbar·l·s-1 were measured. Although the variations are amplified by the leak rate being on a logarithmic scale, it was not clear why the leak rates in the troughs area are higher for lower Fprec and may be due to measurement error as one would expect to have similar or stronger dilution of helium in these cases which should result in lower helium leak rates. The highest peak leak rates have values as high as 7 × 10-9 mbar·l·s1 . These occurred at positions moving away from the extraction points at the corners (position 0, 43.4, 66.2, 109.6 and 132.4 cm in Fig. 5) where there were gaps in the nitrogen curtain slits. Peaks were also observed in the middle of the long edges (a) and (b). Along these edges, the lowest leak rates were observed where the extraction points were situated with values as low as 10-9 mbar·l·s-1. It has now been established that suitable containment can be reached to ensure that the pyrophoric and/or toxic gases flowing from the precursor delivery injection slit stay within the perimeter of the coating head.
Perimeter of the coating head from slit (a) to (d) (cm) 0
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Figure 5 - Helium leak test around the perimeter of the coating head over 32 test points. The containment was set constant, with ∆P = 20 mbar and Fcurt = 24 slm, for both Fprec of 0.5 and 1.0 slm. Note that leak rate values recorded below 10-7 mbar.l.s-1 means that containment is achieved within the coating head. 3.3 Result of containment from ambient air The next step was to ensure that the concentration of ambient air, surrounding the coating head, containing oxygen and water was sufficiently low not to react with the precursors in the deposition area, defined by the exhaust perimeter, as demonstrated by CFD modelling in Fig. 3(a) and 3(b). The test was carried out using the RGA connected to the exhaust line of a coating head, therefore sampling any gases which would be extracted from underneath the coating head. Thus, detection of for example water, oxygen or carbon dioxide in the exhaust gases would indicate ingress of ambient air in the deposition area. The fragments were correlated with a reference database [24]. Fig. 6(a) represents a real-time measurement of the gases in the exhaust. For the first 150 seconds (s), only the active exhaust is operating with a ∆P of 20 mbar. In this condition, the ambient air surrounding the coating head was directly extracted into the exhaust where a typical mass spectrum is shown in Fig. 6(b). Compared with reported data on air composition [25], there are some variations (i.e. 70 % N2 rather than 78 %), however, this measurement provides a reference point to identify whether containment is effective. After an elapsed time of 150 s, timeline (I) in Fig. 6(a), the 24 slm total flow of nitrogen curtain was started and the partial pressure of N2 consequently increases from 700 to 850 mbar (i.e. 85 % of the 1000 mbar total pressure). From this point, CO2 falls by nearly 2 decades below detection levels (i.e. ~10-2 mbar) after only 70 s while it takes a further 470 s for O2 to decrease its concentration by 4 decades. Once the oxygen was below 10-2 mbar, the water decreased to 10-1 mbar and steadily reduced thereafter. The mass spectrum provided in Fig. 6(c) provides a footprint of the expected conditions for achieving containment from the ambient air. Water does not follow the trend from air as it is more likely to be adsorbed on the stainless steel surfaces, within the coating head exhaust and would therefore take longer to purge. It is also observed that the concentration of hydrocarbon products from remnant metal-organics from previous growth runs (i.e. mass 15 amu (methyl radical) used to indicate hydrocarbon to avoid confusion with O fragments) remains constant throughout the measurement and reflects the amount of parasitic by-products in the exhaust lines and capillary of the RGA. Looking at the mass spectra in Fig. 6(b) and (c), other hydrocarbons such as propene C3H6 (mass 42 amu) are also present in the background and remain constant at 2 – 3 × 10-1 mbar. During this ~ 10 min containment phase, the substrate is static whilst being heated up underneath the radiation shield, adjacent to the coating head, and therefore both processes take place in parallel to allow for process shortcuts. Finally, once containment was achieved around the deposition area, its integrity was maintained until the nitrogen curtain and differential
pressure were switched off. If one wants to test the limit of the containment which would be via the observation of ambient air through the RGA, the nitrogen curtain flow can safely be reduced down to 12 slm, however, considering the results in section 3.2, 3.2 one should keep Fcurt to 24 slm.
Figure 6 – (a) Real-time mass fragment sampling of partial pressures for representative masses related to air, air using an atmospheric pressure RGA,, namely 15 (CH3), 18 (H2O), 28 (N2), 32 (O2), 40 (Ar), 44 (CO2). Mass spectra 0 – 50 amu, measured at timeline (I) and (II) in Fig. 6(a), correspond to gas present (b) before and (c) after the containment was reached. 3.4 Example of simulated and measured pyrolysis of DMCd and DiPTe It was shown [20] that good agreement could be found between the CFD modelling and experiment for CdTe layers grown from the pyrolysis of DMCd and DiPTe using the chamberless coating head; w where the first order kinetic regime gave way to mass transport limits limit above 420 ˚C. At this his temperature, the material utilisation was above 50 %, while this his decreased decrease to values as low as 6 % by decreasing the substrate surface temperature to 300 ˚C. Fig. 7(a) shows the two-dimensional two dimensional surface deposition profile across the surface of the substrate from CFD modelling, for two surface temperatures, namely 300 and 450 ˚C.. The VI:II precursor ratio was set at 0.3, which is generally the preferred condition to promote arsenic incorporation, from tDMAAs, DMAAs, as a shallow acceptor dopant. It is clear from Fig. 7(a) that the pyrolysis is less efficient at low temperature as one would predict.. In terms of deposition plume, it would not be expected to extend further than 5cm in this configuration. Knowing K the plume profile,, translation speed and final thickne thickness deposited, allows determination of the average growth rate of the layer. This can differ significantly from the peak growth rate observed in Fig 7(a). For example, using the chamberless inline process, a CdTe layer deposited at Tsurf = 450 ˚C on a moving g substrate at 1.8 cm·min-1, with a 5 cm plume depth of deposition results in an average thickness of 1.2 µm, which is equivalent to an a average growth rate of ~430 nm·min-1. Assuming that such uch absorber thickness is sufficient to be used in the PV structure,, at the current substrate translation speed, it would take 6 hours to deposit the full structure on the system described in Fig. 2. Therefore, the aim will be to further increase the translation speed and growth rate by a factor of 3 to reach the 2 hours end-to-end target.
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Figure 7 – Pyrolysis of DMCd and DiPTe to grow CdTe for a Fprec = 1.0 slm, Fcurt =24 slm and ∆P = 20 mbar. (a) Simulated surface deposition rate of CdTe using a chamberless coating head head. The surface temperature of the substrate was varied from (i) ( 300 ˚C to (ii) 450 ˚C. (b) Partial pressures as measured from the RGA in 3 different conditions, while connected the exhaust of the coating head, for the following mass fragments: 2 amu (H2); 112 amu (Cd); 127 amu (Te or MCd); MCd) 143 amu (DMCd) and 214 amu (DiPTe). The mass fragments associated with the precursor and elemental species from the process can be monitored using the RGA connected to the exhaust of the coating head. Fig. 7(b) shows the partial partial pressure measured for the 5 mass fragment of interest, namely 2 amu (H2), 112 amu (Cd); 127 amu (Te or Met Methyl-cadmium (MCd)); 143 amu (DMCd) and 214 (DiPTe DiPTe) for 3 different conditions. In the first instance, the deposition area is at 30 ˚C without any precursor present. At a partial pressure of 10.6 mbar, the hydrogen alone represents 1% of the sampled mass fragment. Once precursors are introduced, both DMCd and DiPTe mass fragments are visible with a VI:II precursor ratio of 2.5, compared to that expected of 0.3. In effect, this is believed to be an artefact caused by the ionisation pprocess of the precursor molecule in the mass spectrometer. At mass 112 and 127, for or the elemental species Cd and Te, a 1:1 ratio is observed. Mass 127 can also be associated with methyl-cadmium cadmium radical likely to be present in the RGA from the ionisation of DMCd. At this temperature, no decomposition of the precursors is expected and indeed indeed no deposition was visible on the substrate. Once the temperature of the substrate was increased to 450 ˚C, before reaching the mass transport limit,, DiPTe and DMCd were reduced in concentration with 32 and 20 % remaining remaining, respectively. At this temperature, ature, only 17 and 8.9 % of mass 112 and 127 remained, confirming consumption and likely formation of CdTe, but also the Cd rich condition of the process. Decomposition of DMCd appeared to occur more readily and further investigation will be required to individually study the mass fragments pattern for each precursor molecule. There was however excess of precursor materials which are simply exhausted and further potential optimisation will be possible to further increase the he material utilisation. The crystal al structure, surface morphology and elemental composition of this CdTe layer, with a calculated band gap of 1.5 eV, are shown in Fig. 8. The texture coefficient (Chkl) was calculated from the XRD pattern (Fig. 8(a)), following the method employed by Zoppi et al. [26],, and it was found that a more random orientation was observed with a slight preferred orientation for (311) (i.e. C311 = 2.6 out of 6). This differs from previously reported (111) preferred orientation for MOCVD CdTe [18, [ 26]] and further work will be carried out to investigate the source of this change. Nevertheless, a good 1:1 stoichiometry is observed from the EDX spectra without the detectable presence of oxygen further proving that the deposition area achieved containment sufficient enough to deposit high quality CdTe materials.
(a) (b)
Figure 8 – (a) XRD pattern and (b) SEM picture (inset: EDX Spectra) of a 1.2 µm thick CdTe layer grown on TEC C15 at 450 ˚C in the chamberless inline system. In the XRD pattern, substrate peaks ((TECTM C15) are marked with an asterisk (*)) and CdTe peaks could be indexed with a cubic structure. 3.5 Other materials and thin film PV devices Using the chamberless inline system, it has been possible to control and transfer the incorporation of arsenic in the CdTe layer to ~2 × 1018 cm-3, as well as the alloying of CdS with zinc to widen its band gap [22] [ from the recipes used in the standard horizontal batch MOCVD reactor. reactor To demonstrate that there were no detectable levels of oxygen incorporation within with the window, an EDX spectrum is shown in Fig. 99(a) for a Cd0.55Zn0.45S layer with a calculated band gap of 2.65 eV (Fig. 9(b)) which correlates well with the band bowing relationship for Cd1-xZnxS given by Kartopu et al. [27]. [27] Individual layers deposited from the chamberless inline system have been independently tested as part of a series of hybrid devices where the remaining thin film layers of the device structure w were deposited in the batch reactor to test the device quality characteristics of the layers deposited in the chamberless system. system Conversion efficiencies ~13 % have been achieved which is very encouraging considering the current best platform device AM1.5 efficiency of > 16 % [13]. Aluminium doped zinc oxide has ha also been successfully transferred to the chamberless inline system with a controlled incorporation of aluminium (i.e. 1 – 2 %) reaching minimum resistivity of 4.4 × 10-4 Ω·cm and sheet resistance of 7 Ω/□ □ for 680 nm thick layers layers. An example of the thickness uniformity profile achieved with this system is provided in Fig. 10 for a ZnO:Al layer with average thickness of 432 ± 24 nm across the 15 cm width of the substrate.. The average sheet resistance for this ZnO:Al layer was 13 ± 5 Ω/□ over the 15 × 15 cm2 sample ple area, offering similar performance to commercially available TCOs such as TECTM C15 from NSG.
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Figure 9 – (a) EDX spectra and (b) (αE) (α 2 vs E curve of a Cd0.55Zn0.45S layer deposited on TECTM C15 at 400˚C. ˚C. The band gap was determined to be 2.65 eV.
Figure 10 – Thickness profile of a ZnO:Al layer deposited using the chamberless inline system at 400 ˚C with a substrate translating at 1.8 cm·cm-1. The average thickness was 432 ± 24 nm. 4. Conclusions MOCVD is normally associated with batch process with limited opportunity to be scaled to an inline process process. In this study, a simple chamberless coating head design is introduced with potential for transferring a successful batch process atmospheric pressure MOCVD towards a large scale continuous inline processes. In order to help optimise the design and dictate the flow parameters for the chamberless coating head assembly, CFD modelling was used. The CFD modelling demonstrated the conditions conditions required for containment of the precursor flow within the deposition area and obstruct ambient air from entering. entering Experimental work supported the results from the CFD model, model using a range of leak rate testing tools where leak rates as low as 2 × 10-10 mbar·l·s-1 were measured from the combination of a nitrogen curtain and differential pressure balance between the exhaust and the ambient pressure. It was shown that suitable containment can be achieved isolating the deposition area from the ambient air and confirmed by real-time time monitoring of mass fragments using an RGA sampling the exhaust gases. gases Finally, thin film layers deposited, using the chamberless inline system, have been achieved achieve without detectable levels of atmospheric oxygen. It is noteworthy that there are various routes to increase the throughput of the system moving towards a continuous process to deposit the PV structure, to achieve the benchmark time of 2 hours end end-to-end. These engineering options may include: -
Reducing the he temperature gradient between layers, Multiple and sequential coating heads for layers with low growth rates or high thickness requirements, Increasing the growth rates by increasing the precursor concentrations, Increasing growth rates by utilising more reactive precursors.
The inline chamberless MOCVD system has the potential for depositing other compound semiconductors such as III-Vs Vs that may require high throughput for relatively simple device structures. structures
5. Acknowledgments The authors would like to thank A4B for funding the CIPAM project and ERDF/LCRI for funding the SPARC project. ESF and Scanwel Ltd. are also thanked for funding a Knowledge Economy Skills Scholarship (KESS). The Chemistry Department at Bangor University is also gratefully thanked for allowing access to their XRD. Paul Warren and Kieran Cheetham, from NSG, are also thanked for fruitful discussions and supplying the TEC glass.
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List of Figures Figure 1 – Top view schematic of a coating head unit [19] where only the injection slits and exhaust are represented. The dashed edges (a, b, c and d) represent the injection slits for the nitrogen curtain, the single solid line represents the precursor delivery, the hashed perimeter is the active exhaust surrounding the deposition area and the black dots represent the extraction points. The light grey band represents the substrate width while the central dotted arrow indicates its direction of travel. Figure 2 – Front view of the chamberless inline process (630(L) × 178(H) × 72(D) cm3). The inline process consists of six individually extracted deposition sections (including a radiation shield and a coating head) from left to right to deposit the ZnO:Al, ZnO, CdZnS, CdTe:As, CdTe:As+, and CdCl2 layers part of the structure. The last extracted section on the right is the loading bay containing the translating substrate heater arrangement. Figure 3 – CFD modelling of the possible H2 containment conditions within the coating head arrangement as shown in Fig. 1. The hydrogen flow came from the precursor delivery injector at either 0.5 slm (left) or 1slm (right). In all cases the substrate was at Tsurf = 450 (˚C) and the pressure surrounding the coating head was atmospheric pressure. (a,b) H2 containment achieved: Fcurt = 24 slm and ∆P = 20 mbar; (c,d) H2 leaks through the exhaust containment: Fcurt = 24 slm and ∆P = 0 mbar; (e,f) O2 (representing ambient air) leaks through curtain containment: Fcurt = 0 slm and ∆P = 20 mbar. Note: scale bar shows a colour gradient for MF from 0 to 100 % H2, where 0 is equivalent to 100 % N2 (or O2). Figure 4 – Semi-quantitative results showing safe containment of hydrogen within the coating head perimeter for Fprec at 0.5 or 1 slm and for different containment condition varying Fcurt between 6 and 24 slm, and ∆P from 6 to 20 mbar. Note that the “Containment thresholds” are a guide to the eye indicating that, for a given Fprec, any further increase in ∆P would contain hydrogen as sensed by the handheld detector. Figure 5 - Helium leak test around the perimeter of the coating head over 32 test points. The containment was set constant, with ∆P = 20 mbar and Fcurt = 24 slm, for both Fprec of 0.5 and 1.0 slm. Note that leak rate values recorded below 10-7 mbar.l.s-1 means that containment is achieved within the coating head. Figure 6 – (a) Real-time mass fragment sampling of partial pressures for representative masses related to air, using an atmospheric pressure RGA, namely 15 (CH3), 18 (H2O), 28 (N2), 32 (O2), 40 (Ar), 44 (CO2). Mass spectra 0 – 50 amu, measured at timeline (I) and (II) in Fig. 6(a), correspond to gas present (b) before and (c) after the containment was reached. Figure 7 – Pyrolysis of DMCd and DiPTe to grow CdTe for a Fprec = 1.0 slm, Fcurt =24 slm and ∆P = 20 mbar. (a) Simulated surface deposition rate of CdTe using a chamberless coating head. The surface temperature of the substrate was varied from (i) 300 ˚C to (ii) 450 ˚C. (b) Partial pressures as measured from the RGA in 3 different conditions, while connected the exhaust of the coating head, for the following mass fragments: 2 amu (H2); 112 amu (Cd); 127 amu (Te or MCd); 143 amu (DMCd) and 214 amu (DiPTe). Figure 8 – (a) XRD pattern and (b) SEM picture (inset: EDX Spectra) of a 1.2 µm thick CdTe layer grown on TEC C15 at 450 ˚C in the chamberless inline system. In the XRD pattern, substrate peaks (TECTM C15) are marked with an asterisk (*) and CdTe peaks could be indexed with a cubic structure.Figure 9 – (a) EDX spectra and (b) (αE)2 vs E curve of a Cd0.55Zn0.45S layer deposited on TECTM C15 at 400˚C. The band gap was determined to be 2.65 eV. Figure 10 – Thickness profile of a ZnO:Al layer deposited using the chamberless inline system at 400 ˚C with a substrate translating at 1.8 cm·cm-1. The average thickness was 432 ± 24 nm.
Highlights: • • • • •
A chamberless coating head was designed for scale up of MOCVD as an inline process. Suitable containment was achieved using CFD modelling and leak testing tools, measuring leak rates as low as 2 × 10-10 mbar·l·s-1. Cadmium compounds were deposited free from oxygen. Good transfer of layer quality for thin film PV were reached with good dopant and alloying capabilities. Set of chamberless coating heads can be placed sequentially to deposit the full structure of a CdTe thin film PV.