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The effect of substrate temperature on atomic layer deposited zinc tin oxide
Thin Solid Films 586 (2015) 82–87
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
The effect of substrate temperature on atomic layer deposited zinc tin oxide Johan Lindahl ⁎, Carl Hägglund, J. Timo Wätjen, Marika Edoff, Tobias Törndahl Ångström Solar Center, Division of Solid State Electronics, Uppsala University, P. O. Box 534, SE-75121 Uppsala, Sweden
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Article history: Received 5 November 2014 Received in revised form 27 March 2015 Accepted 10 April 2015 Available online 18 April 2015 Keywords: Zinc tin oxide (ZTO) Atomic layer deposition (ALD) Buffer layer Mixed oxide Thin film photovoltaics Optical band gap
a b s t r a c t Zinc tin oxide (ZTO) thin films were deposited on glass substrates by atomic layer deposition (ALD), and the film properties were investigated for varying deposition temperatures in the range of 90 to 180 °C. It was found that the [Sn]/([Sn] + [Zn]) composition is only slightly temperature dependent, while properties such as growth rate, film density, material structure and band gap are more strongly affected. The growth rate dependence on deposition temperature varies with the relative number of zinc or tin containing precursor pulses and it correlates with the growth rate behavior of pure ZnO and SnOx ALD. In contrast to the pure ZnO phase, the density of the mixed ZTO films is found to depend on the deposition temperature and it increases linearly with about 1 g/cm3 in total over the investigated range. Characterization by transmission electron microscopy suggests that zinc rich ZTO films contain small (~10 nm) ZnO or ZnO(Sn) crystallites embedded in an amorphous matrix, and that these crystallites increase in size with increasing zinc content and deposition temperature. These crystallites are small enough for quantum confinement effects to reduce the optical band gap of the ZTO films as they grow in size with increasing deposition temperature. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Zinc tin oxide, Zn1 − xSnxOy (ZTO) is a wide band gap semiconductor material that has a possible new application in photovoltaics, where it is used as a cheap, earth abundant and non-toxic buffer layer alternative for thin film Cu(In,Ga)Se2 (CIGS) [1–4], cadmium telluride (CdTe) [5,6], metal-oxide [7] and inverted organic solar cells [8]. One buffer layer deposition technique that has proven suitable for making highly efficient CIGS solar cells is atomic layer deposition (ALD) [9]. ALD is a chemical vapor deposition technique utilizing an alternation of selflimiting gas to solid reactions, which enables deposition of highly uniform and conformal films at relatively low temperatures [10]. An interesting feature of ALD deposited ZTO is that the structural and optical properties depend strongly on the relative amounts of tin and zinc in the films. ZTO can form three intermediate crystalline phases; two based on the metastable zinc stannate, ZnSnO3, which may either form the face-centered perovskite [11] or the ilmenite structure [12], and in addition the more stable zinc orthostannate, Zn2SnO4, which has a cubic spinel structure [11]. However, un-annealed ALD ZTO films deposited in a temperature range of 120–150 °C have in general been
found to be X-ray amorphous [2,7,13,14] for [Sn]/([Sn] + [Zn]) compositions above 0.1. An advantage of the ZTO material as a buffer layer is that its band gap and conduction band offset can be tuned by changing the [Sn]/([Zn] + [Sn]) ratio [7,13,15], which possibly enables an improved conduction band alignment to different CIGS surface compositions. Another advantage is that it only takes a ZTO film thickness of about 15 nm to fabricate highly efficient CIGS solar cells [3] with a demonstrated conversion efficiency of up to 18.2% for a 0.5 cm2 cell [4], which is comparable to CdS buffer layer reference cells. So far, the ALD ZTO process and its corresponding film properties have mostly been studied at 120 and 150 °C, whereas ZTO buffer layers for CIGS solar cells have been grown at a temperature of 120 °C. The material, optical and electrical properties of buffer layers for CIGS solar cells are also very important for the electrical performance of the devices. Therefore, the aim of this paper is to continue the development of the ZTO material by investigating how the substrate temperature during ALD deposition affects different important film properties in a wider temperature range that is suitable for buffer layer deposition for CIGS solar cells. 2. Experimental details
⁎ Corresponding author at: Uppsala University, Department of Engineering Sciences, Solid State Electronics, Box 534, SE-751 21 Uppsala, Sweden. Tel.: +46 18 471 72 39; fax.: +46 18 55 50 95. E-mail addresses: [email protected] (J. Lindahl), [email protected] (C. Hägglund), [email protected] (J.T. Wätjen), [email protected] (M. Edoff), [email protected] (T. Törndahl).
http://dx.doi.org/10.1016/j.tsf.2015.04.029 0040-6090/© 2015 Elsevier B.V. All rights reserved.
2.1. Sample fabrication The ZTO films were deposited on glass substrates in a Microchemistry F-120 ALD reactor, using process parameters as summarized in Table 1. The glass substrates used for material analysis of
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the ZTO films were 1 mm thick, but otherwise of the same low-iron soda-lime glass type used as the CIGS solar cell substrates in the ÅSC solar cell baseline [4]. The substrates were loaded into the ALD-chamber 30 min prior to film deposition for temperature stabilization. The Sn precursor was heated to 40 °C in a water bath, to achieve a suitable vapor pressure, whereas both the water and the zinc precursors were dosed from sources kept at room temperature. In the Zn1 − xSnxOy notation, y depends on x as in y = x + 1, with the assumptions that the oxidation states of zinc and tin are +2 and +4, respectively, and on the simplification that there is no hydrogen in the film. The index x corresponds to the [Sn]/ ([Sn] + [Zn]) composition of the films, which was controlled by the relative number of zinc or tin containing precursor pulses. As an example, a layer with a Zn:Sn pulse sequence of 3:2 was defined as one with three Zn precursor:N2:H2O:N2 cycles for every two Sn precursor:N2: H2O:N2 cycles. The result was a process with a Sn/(Sn + Zn) cycle fraction of 0.4. Due to differences in reactive sites as well as different sizes of the precursor molecules, the final film composition was not the same as the cycle fraction. A total of 1000 cycles were performed for each film deposition throughout this paper, unless otherwise stated. Properties due to deposition temperature were investigated for ZTO films grown by three Sn/(Sn + Zn) cycle fractions of 0.67, 0.4, and 0.25, along with pure ZnO and SnOx films. 2.2. Sample characterization To determine thickness and density of the ZTO films, X-ray reflectivity (XRR) measurements were done in a Philips X'pert MRD system (measurement errors are estimated to be within 5 relative percent). The same diffractometer was used for grazing incidence X-ray diffraction (GIXRD) measurements using Cu Kα radiation at diffractometer settings of 45 kV and 40 mA, respectively. The relative composition of the ZTO films when deposited on glass substrates were analyzed by X-ray fluorescence spectrometry (XRF) measurements in a PANalytical Epsilon 5 EDXRF spectrometer. To calibrate the XRF results to obtain accurate composition measurements, two ZTO samples were prepared and measured by Rutherford backscattering spectrometry (RBS) at the Uppsala Tandem Laboratory, using a 2 MeV He+-beam and a backscattering angle of 170°. These ZTO films were deposited on quartz glass substrates, using a total of 1000 cycles with a Sn/(Sn + Zn) cycle fraction of 0.4 at 120 °C and 150 °C, respectively. The RBS measurements resulted in a [Sn]/([Sn] + [Zn]) composition of 0.17 for the ZTO film deposited at 120 °C and 0.17 for the ZTO film deposited at 150 °C. From the two RBS reference measurements a correction factor was calculated and used for all XRF results (measurement errors are estimated to be within 10 relative percent). Ex-situ X-ray photoelectron spectroscopy (XPS) using monochromatic Al Kα for sample excitation was used to study the surface and bulk composition of the ZTO films by using a Quantum 2000 Phi XPS instrument. XPS depth profiles were obtained by removal of film material through argon ion sputtering. Optical properties of ZTO films grown on glass substrates by Sn/(Sn + Zn) cycle fractions of 0.67, 0.4 and 0.25 at different ALD deposition temperatures were characterized by spectroscopic ellipsometry. Standard measurements were performed using a Woollam VASE instrument for Table 1 ALD process parameters used for Zn1 − xSnxOy films. Parameters
Condition
Zinc precursor Tin precursor Oxygen precursor Carrier gas Substrate Substrate temperature Cycle sequence Cycle sequence times
Diethyl zinc (DEZn), Zn(C2H5)2 Tetrakis(dimethylamino) tin (TDMASn), Sn(N(CH3)2)4 Deionized water, H2O Nitrogen gas, N2 (99,9999%) 1 mm thick low-iron soda-lime glass 90–180 °C DEZn or TDMASn:N2:H2O:N2 400:800:400:800 ms
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wavelengths from 250 to 1700 nm, and angles of incidence of 65, 70 and 75°. Scotch tape was used to suppress reflections from the glass back surface. The ellipsometric raw data was analyzed in terms of a stratified model including the glass substrate, the ZTO layer and a mixed layer of 50% ZTO and 50% air representing surface roughness through the isotropic Bruggeman effective medium approximation [16]. Tabulated optical constants were used for the glass. The optical constants of the ZTO layer were represented by an oscillator model using two polynomial spline functions together with single poles in the ultraviolet and infrared parts of the spectrum outside the measured range. The parameters of the oscillator model was fitted along with the ZTO and roughness layer thicknesses, so as to minimize the root mean squared deviation of the model output from the measured data. The ZTO refractive index and absorption coefficient, α(hν), as a function of the photon energy, hν, were thereby obtained. To extract the optical band gap energy, the Tauc plot method was used. The function (αhν)r is then plotted versus hν [17] and the linear portion of the curve is extrapolated to zero. A band gap model with r = 0.5 is expected for amorphous phase, while a direct model with r = 2 applies to crystalline direct band gap type materials. In the present work, Tauc plots for amorphous type band gaps were of highest relevance, see further below. For high resolution transmission electron microscopy (TEM), the samples had to be coated with a conductive layer of Au. Subsequently, the TEM lamella were prepared using the lift-out technique on a FEI Strata DB235 focused ion beam (FIB) operated at 30 kV followed by a 5 kV final cleaning step. The TEM samples were characterized in a Tecnai F30 ST. 3. Result and discussion 3.1. Composition and growth rate The influence of deposition temperature on composition, thickness, density and band gap of SnOx, ZnO and ZTO films deposited on glass substrates are shown in Fig. 1(a), (b), (c) and (d), respectively. For different Zn:Sn pulse sequences the [Sn]/([Sn] + [Zn]) composition of the ZTO films varies with the deposition temperature, as illustrated in Fig. 1(a). The deposition temperature affects the [Sn]/([Sn] + [Zn]) composition in the 0.67 Sn/(Sn + Zn) cycle fraction films the most, where the composition decreases from 0.35 at 90 °C to 0.28 at 180 °C. The composition of both the 0.4 and 0.25 Sn/(Sn + Zn) cycle fraction films shows a small decrease in tin concentration when going from 90 to 105 °C, but remain fairly constant for higher temperatures, with tin contents of approximately 0.17 and 0.13 for the 0.4 and 0.25 Sn/ (Sn + Zn) cycle fraction processes, respectively. Fig. 1(b) shows how the thicknesses of the films by XRR are influenced by the deposition temperature. Similar results are obtained from the ellipsometry analysis. For the pure SnOx films the thickness, and thereby the growth rate, decreases with increasing deposition temperature, from ~1.3 Å/cycle at 90 °C to ~0.6 Å/cycle at 180 °C. This decrease in growth rate with increasing deposition temperature correlates well with the findings of [18], which also uses TDMASn and water as precursors, but Si(100) as substrates, and shows that our pure SnOx behaves as expected. The film thickness dependence on deposition temperature for the pure ZnO layers exhibits a growth rate, which initially increases with the substrate temperature. This behavior is typical for growth that is limited by reaction barriers and insufficient energy of the reactants at the sample surface (substrate temperature). At a temperature of around 135 °C, an ALD window of self-limited growth is reached, as is signified by a relatively temperature independent growth rate. Various ALD windows for ZnO growth are reported for different precursors, reactor designs and substrates, but generally the ALD window for typical ZnO deposition from DEZn and H2O is estimated to be around 110–170 °C [19]. The growth rate of this study is approximately 2 Å/cycle within the process window, which also corresponds well with the reported
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(a)
(b)
(c)
(d)
Fig. 1. Material properties for ZnO, ZTO and SnOx films with Sn/(Sn + Zn) cycle fractions of 1.00, 0.67, 0.40, 0.25 and 0.00 grown by ALD on glass substrates by the stated number of deposition cycles (#c). (a) [Sn]/([Sn] + [Zn]) composition as a function of ALD deposition temperature, measured by XRF and corrected by RBS. (b) Thickness as a function of ALD deposition temperature, measured by XRR. (c) Density as a function of ALD deposition temperature, measured by XRR. (d) Band gaps from indirect and direct Tauc plot models as a function of ALD deposition temperature, measured by ellipsometry.
values in [19], which shows that also our pure ZnO behaves as expected. In this study the upper limit in deposition temperature of the ALD window is not reached, but at higher temperatures the growth rate may either increase from non-saturated growth caused by thermal decomposition of DEZn [20], or decrease due to desorption of H2O from the film surface [21–23]. The growth rate for ZTO is reduced as compared to the growth rates of the binaries, especially in comparison with pure ZnO. Tanskanen et al. [24] shows that a SnOx cycle (TDMASn/H2O) on ZnO reduces the reaction site density, while subsequent ZnO cycles (DEZn/H2O) increase this density. This explains the lower growth rate of the ZTO films as compared to that of ZnO and why in general thicker layers are obtained for films with higher Zn:Sn pulse sequence. It is also likely that growth rate of SnOx on a ZnO terminated surface is lowered in a similar way. This has been observed by comparing two different 0.4 Sn/(Sn + Zn) cycle fraction recipes where the ZnO and SnOx cycles are mixed in different ways (ZnO:SnOx:ZnO:SnOx:ZnO as compared to ZnO:ZnO: ZnO:SnOx:SnOx). The size of the ligands is different for TDMASn and DEZn, which can also influence why the [Sn]/([Sn] + [Zn]) composition of the ZTO films is lower than the Sn/(Sn + Zn) cycle fraction. Furthermore, the thickness and growth rate dependence on deposition
temperature for the ZTO films seem to correlate well with pure ZnO and SnOx, since the temperature dependent growth rate of the 0.67 Sn/(Sn + Zn) cycle fraction process is similar to that of the SnOx and the 0.25 Sn/(Sn + Zn) cycle fraction process to the ZnO process. Due to the difference in ZTO thickness obtained at the different deposition temperatures samples were made with an adjusted number of pulse cycles to obtain films with similar thickness for the structural and optical measurement. The number of pulse cycles used for these sample films were 1600, 1100, 1000 and 1000 at 90 °C, 120 °C, 150 °C and 180 °C, which resulted in thicknesses of 53, 50, 55 and 56 nm, respectively. 3.2. Material structure Pure SnOx films as-deposited by TDMASn and water are found to be X-ray amorphous when deposited at 120 °C [2] and at 150 °C [13,18], while binary ZnO films as-deposited by DEZn and water crystallizes in the hexagonal wurtzite structure [19]. Furthermore, previous work on ALD ZTO films, as-deposited by TDMASn, DEZn and water demonstrates a crystalline-to-amorphous transition when mixing ZnO cycles with SnOx cycles [2,13], with films being X-ray amorphous above a [Sn]/
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(a)
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(b)
Fig. 2. GIXRD diffractograms of ZTO films as-deposited by (a) the 0.4 Sn/(Sn + Zn) cycle fraction process and (b) the 0.25 Sn/(Sn + Zn) cycle fraction process at deposition temperatures of 90, 120, 150 and 180 °C, respectively. The peak positions of powder ZnO are shown at the bottom.
([Sn] + [Zn]) composition of approximately 0.1 [13]. The results from the XRD measurements in this study demonstrate X-ray amorphous structures for ZTO films deposited with a Sn/(Sn + Zn) cycle fraction of 0.67 for all the deposition temperatures analyzed in this paper (not shown here). The ZTO films deposited with the 0.4 Sn/(Sn + Zn) cycle fraction process, shown in Fig. 2(a), also display X-ray amorphous structures when deposited at 90, 120 and 150 °C. However, the ZTO film deposited at 180 °C appears to be mostly amorphous but with a broad peak centered at a position of around 35° in 2θ. The latter probably corresponds to the high intensity peaks of crystalline ZnO at 2θ positions of 31.8°, 34.4° and 36.3° for the (100), (002) and (101) crystal planes, respectively. For ZTO films deposited with the 0.25 Sn/(Sn + Zn) cycle fraction, shown in Fig. 2(b), the same broad peak at 2θ position at around 35° appears already at 120 °C and at higher deposition temperatures, the characteristic peaks for ZnO are clearly distinguishable. In the high resolution TEM image of the sample deposited with the 0.25 Sn/(Sn + Zn) cycle fraction process at 90 °C, shown in Fig. 3(a), areas of parallel atomic planes are visible, which show that this sample has a polycrystalline character. The apparent grain size is approximated to be in the range of 10 nm. Due to the small grain size and physical extent of the TEM lamella along the direction of the electron beam, one has to assume that several of these grains can overlap. The Fourier transformation, Fig. 3(b), reveals the lattice planes as distinct points and they line up well with the positions for polycrystalline ZnO of (100) to (112) planes, which are depicted as red markers. The grains are randomly oriented and as the image excerpt is limited, not all expected planes have to be present, as is the case for the (103) planes. While this film appeared X-ray amorphous in the XRD measurement the
lattice fringes are clearly visible in the TEM images and prove the nanocrystalline nature of the ZTO layers. The reasons why the ZTO film deposited with the 0.25 Sn/(Sn + Zn) cycle fraction process at 90 °C looks amorphous in GIXRD in Fig. 2(b), while the TEM images demonstrate a nanocrystalline character, are probably a combination of too small crystallites with poor crystalline quality that give rise to a large peak broadening, and a too small diffraction volume for the XRD measurement. For smaller particle sizes weaker signals, peak broadening and overlaps have been observed in XRD measurements of ZnO nanoparticles [25]. Our interpretation of the XRD and TEM results is that the ZTO films contain some small ZnO or ZnO(Sn) crystallites of sizes smaller than 10 nm, possibly surrounded by an amorphous material. This is at least true for the most zinc rich films deposited with the 0.25 Sn/(Sn + Zn) cycle fraction process, which is confirmed by TEM. These crystallites are then likely to decrease in size with increasing tin content and decreasing deposition temperature. One can find support for this hypothesis in the literature, where the crystal size and quality are previously found to increase as a function of deposition temperature for ALD grown ZnO [20]. Furthermore, the formation of small crystallites after annealing is also suggested in the case of X-ray amorphous SnOx [18], and the formation of crystalline Zn2SnO4 is observed after annealing of ALD grown ZTO layers, where a phase separation occurs at lower annealing temperatures for more zinc rich films [14]. As can be seen in Fig. 1(c), the density of the X-ray amorphous SnOx and the ZTO films increases more or less linearly with temperature, by about 1 g/cm3 in total, when the deposition temperature increases from 90 to 180 °C. Meanwhile, the crystalline ZnO film stays at a
Fig. 3. (a) Excerpt of high resolution TEM image of a ZTO film deposited by the 0.25 Sn/(Sn + Zn) cycle fraction process at a deposition temperature of 90 °C. (b) Fourier transformation of the full image for the 0.25 Sn/(Sn + Zn) cycle fraction film deposited at 90 °C.
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constant value of about 5.5 g/cm3, which is close to the bulk value of 5.6 g/cm3 for ZnO. It is also obvious from Fig. 1(c) that the ternary ZTO films are less dense than the binary ZnO and SnOx films and that the density of the ZTO films increases with increasing Zn content in the films. If one looks at the data in Fig. 1(c) one can see that there are some variations within the series. These variations are most likely associated to measurement errors in both the ALD deposition temperature measurement and in the characterization methods. As an example, the reason that the density is similar for the 0.67 and 0.4 processes at 180 °C is most likely a coincidence explained by the measurement errors. The amorphous-to-semi-crystalline transition in the ZTO films, which seems to occur with increasing deposition temperature and zinc content, is one likely explanation to the density trends since more crystalline films are ordinarily more densely packed. Another possible explanation for the lower observed density at lower deposition temperatures may be related to the presence of hydrogen in the form of hydroxyl groups remaining in the deposited films. Growing metal oxide films using water as oxygen precursor at low temperatures by ALD is known to produce hydroxyl rich oxide materials. To investigate the possibility of –OH groups in the ZTO films, two samples deposited with a Sn/(Sn + Zn) cycle fraction of 0.4 at 90 and 180 °C, are analyzed by XPS. The XPS spectra collected from the surfaces of the two films reveal a split of the O1s peak with maxima at around 530.3 and 531.7 eV, indicating a presence of both metal oxide and hydroxide. However, depth profiles performed by low power sputtering with argon ions show no significant differences between the two samples, except that the sputtering rate is found to be higher for the 180 °C sample. 3.3. Optical properties Fig. 4(a) shows the refractive index, n, determined by spectroscopic ellipsometry for ZTO films deposited by the 0.4 Sn/(Sn + Zn) cycle fraction process at the different deposition temperatures. From Fig. 4(a) one can conclude that the refractive index increases with increasing deposition temperature. Similar trends are obtained for the 0.67 and 0.25 Sn/ (Sn + Zn) cycle fraction processes. Furthermore, a slight general increase of about 0.1 in refractive index, is observed between the wavelengths of 500 to 1700 nm with increasing zinc content between 0.67 and 0.25 Sn/(Sn + Zn) cycle fraction processes. In general, the refractive index increases with the density of a material, and this likely explains the main trends observed here since the density increases with both deposition temperature and increasing zinc content in the films, as illustrated by Fig. 1(c). Fig. 1(d) shows that band gap values extracted by the Tauc method, such as in Fig. 4(b), yield quite different values depending on if the amorphous phase model with r = 0.5 or if the direct gap model with r = 2 is used. The former is probably more relevant for most of the films in this study since they are X-ray amorphous, and since this
(a)
model generally produces a more linear portion of the curve. However, it is known that pure crystalline ZnO has a direct band gap, so in this case a model with r = 2 should rather be used. This makes it uncertain which model is the most appropriate for the ZTO films deposited at higher temperatures, since a two phase structure is observed and the measured optical properties are then the effective properties of the mixture of ZnO like crystallites and their amorphous surrounding. It should be noted that the extracted band gaps from Tauc plots are quite sensitive to the models used and how well they fit to the measured ellipsometry data, especially for amorphous type band gaps. Comparing the gap values in Fig. 1(d) with soft X-ray measured band gaps for ZTO films grown with similar composition and at a deposition temperature of 120 °C [15] shows that the obtained values are higher in this study. For example, a ZTO film grown with a 0.40 Sn/(Sn + Zn) cycle fraction process at 120 °C shows approximately 0.5 eV higher band gap as compared to the corresponding soft X-ray measurement. It is noted that factors such as film thickness, cycle sequence and timings, and substrate type may influence the average film properties. The Tauc method is also prone to some ambiguity. We therefore emphasize the trends rather than the absolute values of Fig. 1(d). Both models in Fig. 1(d) demonstrate a trend where the band gap of ZTO thin films decreases with deposition temperature. It has previously been shown that the band gap [13,15] and conduction band level [15] of ZTO thin films change with the [Zn]/([Zn] + [Sn]) composition. However, as shown in Fig. 1(a), the composition dependence on deposition temperature is very small and can therefore not explain the decreasing trend for the band gaps as a function of deposition temperature. Rather, this decreasing trend is probably related to the observed changes in material structure. If a semiconductor nanoparticle is small enough, such that the electronic wave functions of the excitons are confined below the Bohr radii in all three dimensions, the electronic and optical properties will change. This quantum confinement results in an expansion of the band gap from its bulk value, in inverse relation to the particle size. Similar changes of the band gap as in Fig. 1(d) are also attributed to quantum confinement effects for X-ray amorphous SnOx [18] and for Zn2SnO4 nanoparticles [11,26]. Furthermore, it is shown that films consisting of wurtzite isotropic shaped ZnO nanoparticles in size range up to 9 nm are small enough to display quantum confinement effects as well, and that the band gap, Eg (in eV), decreases with increasing particle diameter, d (in nm), according to the expression Eg = 3.30 + 0.293 / d + 3.94 / d2 [25,27]. The TEM images of ZTO films deposited with the 0.25 Sn/(Sn + Zn) cycle fraction process at 90 and 180 °C reveal that the films contain ZnO or ZnO(Sn) particles of sizes within this range. These, crystallites also show a broad size distribution, and are likely surrounded by an amorphous phase. A similar increase of the band gap value, as seen in [25,27] as a function of decreasing particle size, is not observed for the ZTO films in this study as a function of deposition temperature, which can be seen in Fig. 1(d). The main reason for
(b)
Fig. 4. (a) Refractive index measured by spectroscopic ellipsometry for ZTO films deposited by a Sn/(Sn + Zn) cycle fraction of 0.4 at deposition temperatures of 90 °C, 120 °C, 150 °C and 180 °C, respectively. (b) Tauc plots for several ZTO films, using r = 1/2 as for amorphous type band gaps. The films were deposited with a Sn/(Sn + Zn) cycle fraction of 0.4 at deposition temperatures of 90 °C, 120 °C, 150 °C and 180 °C, respectively.
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this observation may be that the ZTO films contain ZnO or ZnO(Sn) crystallites with a broad size distribution and that the average size of the crystalline grains in the ZTO films are found in the upper part of the examined range of the ZnO particle sizes in [25,27]. 4. Conclusions This study reports on the relation of changes in structural and optical properties of ALD deposited zinc tin oxide (ZTO) films when varying the deposition temperature between 90 up to 180 °C, and for varying the ALD pulse ratio. It is shown that the effect of the deposition temperature on the [Sn]/([Sn] + [Zn]) composition is small, while the growth rate, density and band gap of the ZTO films change significantly. The growth rate behavior of ZTO films as a function of deposition temperature depends on the relative number of zinc or tin containing precursor pulses and correlates with the growth rate behavior of pure ZnO and SnOx. The density of the ZTO films increases while the band gap decreases with increasing deposition temperature, which is related to microstructural changes of the ZTO films. Transmission electron microscopy measurements indicate that the ZTO films contain small ZnO or ZnO(Sn) crystallites imbedded in an amorphous matrix, and that these crystallites increase in size with increasing zinc content and deposition temperature. The crystallite sizes are approximated to be in the 10 nm range for a 0.25 Sn/(Sn + Zn) cycle fraction process deposited at 90 °C, which means that quantum confinement effects influence the ZTO optical properties. This can explain why the band gaps of the ZTO films decrease with increasing deposition temperature. Acknowledgments The authors gratefully acknowledge T. Ericsson and Dr. D. Primetzhofer for the RBS measurements carried out at Uppsala Tandem Laboratory (Ion Technology Center). The authors also wish thank the Swedish Energy Agency (32787-3) for the financial support and C. Hägglund is grateful for support from the Marcus and Amalia Wallenberg foundation. References [1] A. Hultqvist, M. Edoff, T. Törndahl, Evaluation of Zn–Sn–O buffer layers for CuIn0.5Ga0.5Se2 solar cells, Prog. Photovolt. Res. Appl. 19 (2011) 478. [2] A. Hultqvist, C. Platzer-björkman, U. Zimmermann, M. Edoff, T. Törndahl, Growth kinetics, properties, performance, and stability of atomic layer deposition Zn–Sn–O buffer layers for Cu(In, Ga)Se2 solar cells, Prog. Photovolt. Res. Appl. 20 (2012) 883. [3] J. Lindahl, J.T. Wätjen, A. Hultqvist, T. Ericson, M. Edoff, T. Törndahl, The effect of Zn1 − xSnxOy buffer layer thickness in 18.0% efficient Cd-free Cu(In, Ga)Se2 solar cells, Prog. Photovolt. Res. Appl. 21 (2013) 1588. [4] J. Lindahl, U. Zimmermann, P. Szaniawski, T. Törndahl, A. Hultqvist, P. Salomé, et al., Inline Cu(In, Ga)Se2 co-evaporation for high-efficiency solar cells and modules, IEEE J. Photovoltaics 3 (2013) 1100.
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[5] X. Wu, High-efficiency polycrystalline CdTe thin-film solar cells, Sol. Energy 77 (2004) 803. [6] R. Dhere, M. Bonnet-Eymard, E. Charlet, E. Peter, J. Duenow, H. Moutinho, et al., The effect of CdTe deposition temperature on device properties of different TCOs and glass substrates, Photovolt. Spec. Conf. (PVSC), 35th IEEE 2010, p. 340. [7] Y.S. Lee, J. Heo, S.C. Siah, J.P. Mailoa, R.E. Brandt, S.B. Kim, et al., Ultrathin amorphous zinc-tin-oxide buffer layer for enhancing heterojunction interface quality in metaloxide solar cells, Energy Environ. Sci. 6 (2013) 2112. [8] T. Zaw, R.D. Chandra, N. Yantara, R. Ramanujam, L.H. Wong, N. Mathews, et al., Zinc Tin Oxide (ZTO) electron transporting buffer layer in inverted organic solar cell, Org. Electron. 13 (2012) 870. [9] J.A. van Delft, D. Garcia-Alonso, W.M.M. Kessels, Atomic layer deposition for photovoltaics: applications and prospects for solar cell manufacturing, Semicond. Sci. Technol. 27 (2012) 074002. [10] R.L. Puurunen, Surface chemistry of atomic layer deposition: a case study for the trimethylaluminum/water process, J. Appl. Phys. 97 (2005) 121301. [11] S. Baruah, J. Dutta, Zinc stannate nanostructures: hydrothermal synthesis, Sci. Technol. Adv. Mater. 12 (2011) 013004. [12] D. Kovacheva, K. Petrov, Preparation of crystalline ZnSnO3 from Li2SnO3 by lowtemperature ion exchange, Solid State Ionics 109 (1998) 327. [13] M.N. Mullings, C. Hägglund, J.T. Tanskanen, Y. Yee, S. Geyer, S.F. Bent, Thin film characterization of zinc tin oxide deposited by thermal atomic layer deposition, Thin Solid Films 556 (2014) 186. [14] J. Heo, S. Bok Kim, R.G. Gordon, Atomic layer deposited zinc tin oxide channel for amorphous oxide thin film transistors, Appl. Phys. Lett. 101 (2012) 113507. [15] M. Kapilashrami, C.X. Kronawitter, T. Törndahl, J. Lindahl, A. Hultqvist, W.-C. Wang, et al., Soft X-ray characterization of Zn1− xSnxOy electronic structure for thin film photovoltaics, Phys. Chem. Chem. Phys. 14 (2012) 10154. [16] G.A. Niklasson, C.G. Granqvist, O. Hunderi, Effective medium models for the optical properties of inhomogeneous materials, Appl. Opt. 20 (1981) 26. [17] J. Tauc, Optical properties and electronic structure of amorphous Ge and Si, Mater. Res. Bull. 3 (1968) 37. [18] M.N. Mullings, C. Hägglund, S.F. Bent, Tin oxide atomic layer deposition from tetrakis(dimethylamino)tin and water, J. Vac. Sci. Technol. A 31 (2013) 061503. [19] T. Tynell, M. Karppinen, Atomic layer deposition of ZnO: a review, Semicond. Sci. Technol. 29 (2014) 043001. [20] J. Lim, C. Lee, Effects of substrate temperature on the microstructure and photoluminescence properties of ZnO thin films prepared by atomic layer deposition, Thin Solid Films 515 (2007) 3335. [21] E.B. Yousfi, B. Weinberger, F. Donsanti, Atomic layer deposition of zinc oxide and indium sulfide layers for Cu(In, Ga)Se2 thin-film solar cells, Thin Solid Films 387 (2001) 29. [22] A. Yamada, B. Sang, M. Konagai, Atomic layer deposition of ZnO transparent conducting oxides, Appl. Surf. Sci. 112 (1997) 216. [23] V. Lujala, J. Skarp, M. Tammenmaa, T. Suntola, Atomic layer epitaxy growth of doped zinc oxide thin films from organometals, Appl. Surf. Sci. 82 (83) (1994) 34. [24] J.T. Tanskanen, C. Hägglund, S.F. Bent, Correlating growth characteristics in atomic layer deposition with precursor molecular structure: the case of zinc tin oxide, Chem. Mater. 26 (2014) 2795. [25] J. Jacobsson, T. Edvinsson, Absorption and fluorescence spectroscopy of growing ZnO quantum dots: size and band gap correlation and evidence of mobile trap states, Inorg. Chem. 50 (2011) 9578. [26] J. Zeng, M. Xin, K. Li, H. Wang, H. Yan, W. Zhang, Transformation process and photocatalytic activities of hydrothermally synthesized Zn2SnO4 nanocrystals, J. Phys. Chem. C 112 (2008) 4159. [27] J. Jacobsson, T. Edvinsson, Photoelectrochemical determination of the absolute band edge positions as a function of particle size for ZnO quantum dots, J. Phys. Chem. C 116 (2012) 15692.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
The effect of substrate temperature on atomic layer deposited zinc tin oxide Johan Lindahl ⁎, Carl Hägglund, J. Timo Wätjen, Marika Edoff, Tobias Törndahl Ångström Solar Center, Division of Solid State Electronics, Uppsala University, P. O. Box 534, SE-75121 Uppsala, Sweden
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Article history: Received 5 November 2014 Received in revised form 27 March 2015 Accepted 10 April 2015 Available online 18 April 2015 Keywords: Zinc tin oxide (ZTO) Atomic layer deposition (ALD) Buffer layer Mixed oxide Thin film photovoltaics Optical band gap
a b s t r a c t Zinc tin oxide (ZTO) thin films were deposited on glass substrates by atomic layer deposition (ALD), and the film properties were investigated for varying deposition temperatures in the range of 90 to 180 °C. It was found that the [Sn]/([Sn] + [Zn]) composition is only slightly temperature dependent, while properties such as growth rate, film density, material structure and band gap are more strongly affected. The growth rate dependence on deposition temperature varies with the relative number of zinc or tin containing precursor pulses and it correlates with the growth rate behavior of pure ZnO and SnOx ALD. In contrast to the pure ZnO phase, the density of the mixed ZTO films is found to depend on the deposition temperature and it increases linearly with about 1 g/cm3 in total over the investigated range. Characterization by transmission electron microscopy suggests that zinc rich ZTO films contain small (~10 nm) ZnO or ZnO(Sn) crystallites embedded in an amorphous matrix, and that these crystallites increase in size with increasing zinc content and deposition temperature. These crystallites are small enough for quantum confinement effects to reduce the optical band gap of the ZTO films as they grow in size with increasing deposition temperature. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Zinc tin oxide, Zn1 − xSnxOy (ZTO) is a wide band gap semiconductor material that has a possible new application in photovoltaics, where it is used as a cheap, earth abundant and non-toxic buffer layer alternative for thin film Cu(In,Ga)Se2 (CIGS) [1–4], cadmium telluride (CdTe) [5,6], metal-oxide [7] and inverted organic solar cells [8]. One buffer layer deposition technique that has proven suitable for making highly efficient CIGS solar cells is atomic layer deposition (ALD) [9]. ALD is a chemical vapor deposition technique utilizing an alternation of selflimiting gas to solid reactions, which enables deposition of highly uniform and conformal films at relatively low temperatures [10]. An interesting feature of ALD deposited ZTO is that the structural and optical properties depend strongly on the relative amounts of tin and zinc in the films. ZTO can form three intermediate crystalline phases; two based on the metastable zinc stannate, ZnSnO3, which may either form the face-centered perovskite [11] or the ilmenite structure [12], and in addition the more stable zinc orthostannate, Zn2SnO4, which has a cubic spinel structure [11]. However, un-annealed ALD ZTO films deposited in a temperature range of 120–150 °C have in general been
found to be X-ray amorphous [2,7,13,14] for [Sn]/([Sn] + [Zn]) compositions above 0.1. An advantage of the ZTO material as a buffer layer is that its band gap and conduction band offset can be tuned by changing the [Sn]/([Zn] + [Sn]) ratio [7,13,15], which possibly enables an improved conduction band alignment to different CIGS surface compositions. Another advantage is that it only takes a ZTO film thickness of about 15 nm to fabricate highly efficient CIGS solar cells [3] with a demonstrated conversion efficiency of up to 18.2% for a 0.5 cm2 cell [4], which is comparable to CdS buffer layer reference cells. So far, the ALD ZTO process and its corresponding film properties have mostly been studied at 120 and 150 °C, whereas ZTO buffer layers for CIGS solar cells have been grown at a temperature of 120 °C. The material, optical and electrical properties of buffer layers for CIGS solar cells are also very important for the electrical performance of the devices. Therefore, the aim of this paper is to continue the development of the ZTO material by investigating how the substrate temperature during ALD deposition affects different important film properties in a wider temperature range that is suitable for buffer layer deposition for CIGS solar cells. 2. Experimental details
⁎ Corresponding author at: Uppsala University, Department of Engineering Sciences, Solid State Electronics, Box 534, SE-751 21 Uppsala, Sweden. Tel.: +46 18 471 72 39; fax.: +46 18 55 50 95. E-mail addresses: [email protected] (J. Lindahl), [email protected] (C. Hägglund), [email protected] (J.T. Wätjen), [email protected] (M. Edoff), [email protected] (T. Törndahl).
http://dx.doi.org/10.1016/j.tsf.2015.04.029 0040-6090/© 2015 Elsevier B.V. All rights reserved.
2.1. Sample fabrication The ZTO films were deposited on glass substrates in a Microchemistry F-120 ALD reactor, using process parameters as summarized in Table 1. The glass substrates used for material analysis of
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the ZTO films were 1 mm thick, but otherwise of the same low-iron soda-lime glass type used as the CIGS solar cell substrates in the ÅSC solar cell baseline [4]. The substrates were loaded into the ALD-chamber 30 min prior to film deposition for temperature stabilization. The Sn precursor was heated to 40 °C in a water bath, to achieve a suitable vapor pressure, whereas both the water and the zinc precursors were dosed from sources kept at room temperature. In the Zn1 − xSnxOy notation, y depends on x as in y = x + 1, with the assumptions that the oxidation states of zinc and tin are +2 and +4, respectively, and on the simplification that there is no hydrogen in the film. The index x corresponds to the [Sn]/ ([Sn] + [Zn]) composition of the films, which was controlled by the relative number of zinc or tin containing precursor pulses. As an example, a layer with a Zn:Sn pulse sequence of 3:2 was defined as one with three Zn precursor:N2:H2O:N2 cycles for every two Sn precursor:N2: H2O:N2 cycles. The result was a process with a Sn/(Sn + Zn) cycle fraction of 0.4. Due to differences in reactive sites as well as different sizes of the precursor molecules, the final film composition was not the same as the cycle fraction. A total of 1000 cycles were performed for each film deposition throughout this paper, unless otherwise stated. Properties due to deposition temperature were investigated for ZTO films grown by three Sn/(Sn + Zn) cycle fractions of 0.67, 0.4, and 0.25, along with pure ZnO and SnOx films. 2.2. Sample characterization To determine thickness and density of the ZTO films, X-ray reflectivity (XRR) measurements were done in a Philips X'pert MRD system (measurement errors are estimated to be within 5 relative percent). The same diffractometer was used for grazing incidence X-ray diffraction (GIXRD) measurements using Cu Kα radiation at diffractometer settings of 45 kV and 40 mA, respectively. The relative composition of the ZTO films when deposited on glass substrates were analyzed by X-ray fluorescence spectrometry (XRF) measurements in a PANalytical Epsilon 5 EDXRF spectrometer. To calibrate the XRF results to obtain accurate composition measurements, two ZTO samples were prepared and measured by Rutherford backscattering spectrometry (RBS) at the Uppsala Tandem Laboratory, using a 2 MeV He+-beam and a backscattering angle of 170°. These ZTO films were deposited on quartz glass substrates, using a total of 1000 cycles with a Sn/(Sn + Zn) cycle fraction of 0.4 at 120 °C and 150 °C, respectively. The RBS measurements resulted in a [Sn]/([Sn] + [Zn]) composition of 0.17 for the ZTO film deposited at 120 °C and 0.17 for the ZTO film deposited at 150 °C. From the two RBS reference measurements a correction factor was calculated and used for all XRF results (measurement errors are estimated to be within 10 relative percent). Ex-situ X-ray photoelectron spectroscopy (XPS) using monochromatic Al Kα for sample excitation was used to study the surface and bulk composition of the ZTO films by using a Quantum 2000 Phi XPS instrument. XPS depth profiles were obtained by removal of film material through argon ion sputtering. Optical properties of ZTO films grown on glass substrates by Sn/(Sn + Zn) cycle fractions of 0.67, 0.4 and 0.25 at different ALD deposition temperatures were characterized by spectroscopic ellipsometry. Standard measurements were performed using a Woollam VASE instrument for Table 1 ALD process parameters used for Zn1 − xSnxOy films. Parameters
Condition
Zinc precursor Tin precursor Oxygen precursor Carrier gas Substrate Substrate temperature Cycle sequence Cycle sequence times
Diethyl zinc (DEZn), Zn(C2H5)2 Tetrakis(dimethylamino) tin (TDMASn), Sn(N(CH3)2)4 Deionized water, H2O Nitrogen gas, N2 (99,9999%) 1 mm thick low-iron soda-lime glass 90–180 °C DEZn or TDMASn:N2:H2O:N2 400:800:400:800 ms
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wavelengths from 250 to 1700 nm, and angles of incidence of 65, 70 and 75°. Scotch tape was used to suppress reflections from the glass back surface. The ellipsometric raw data was analyzed in terms of a stratified model including the glass substrate, the ZTO layer and a mixed layer of 50% ZTO and 50% air representing surface roughness through the isotropic Bruggeman effective medium approximation [16]. Tabulated optical constants were used for the glass. The optical constants of the ZTO layer were represented by an oscillator model using two polynomial spline functions together with single poles in the ultraviolet and infrared parts of the spectrum outside the measured range. The parameters of the oscillator model was fitted along with the ZTO and roughness layer thicknesses, so as to minimize the root mean squared deviation of the model output from the measured data. The ZTO refractive index and absorption coefficient, α(hν), as a function of the photon energy, hν, were thereby obtained. To extract the optical band gap energy, the Tauc plot method was used. The function (αhν)r is then plotted versus hν [17] and the linear portion of the curve is extrapolated to zero. A band gap model with r = 0.5 is expected for amorphous phase, while a direct model with r = 2 applies to crystalline direct band gap type materials. In the present work, Tauc plots for amorphous type band gaps were of highest relevance, see further below. For high resolution transmission electron microscopy (TEM), the samples had to be coated with a conductive layer of Au. Subsequently, the TEM lamella were prepared using the lift-out technique on a FEI Strata DB235 focused ion beam (FIB) operated at 30 kV followed by a 5 kV final cleaning step. The TEM samples were characterized in a Tecnai F30 ST. 3. Result and discussion 3.1. Composition and growth rate The influence of deposition temperature on composition, thickness, density and band gap of SnOx, ZnO and ZTO films deposited on glass substrates are shown in Fig. 1(a), (b), (c) and (d), respectively. For different Zn:Sn pulse sequences the [Sn]/([Sn] + [Zn]) composition of the ZTO films varies with the deposition temperature, as illustrated in Fig. 1(a). The deposition temperature affects the [Sn]/([Sn] + [Zn]) composition in the 0.67 Sn/(Sn + Zn) cycle fraction films the most, where the composition decreases from 0.35 at 90 °C to 0.28 at 180 °C. The composition of both the 0.4 and 0.25 Sn/(Sn + Zn) cycle fraction films shows a small decrease in tin concentration when going from 90 to 105 °C, but remain fairly constant for higher temperatures, with tin contents of approximately 0.17 and 0.13 for the 0.4 and 0.25 Sn/ (Sn + Zn) cycle fraction processes, respectively. Fig. 1(b) shows how the thicknesses of the films by XRR are influenced by the deposition temperature. Similar results are obtained from the ellipsometry analysis. For the pure SnOx films the thickness, and thereby the growth rate, decreases with increasing deposition temperature, from ~1.3 Å/cycle at 90 °C to ~0.6 Å/cycle at 180 °C. This decrease in growth rate with increasing deposition temperature correlates well with the findings of [18], which also uses TDMASn and water as precursors, but Si(100) as substrates, and shows that our pure SnOx behaves as expected. The film thickness dependence on deposition temperature for the pure ZnO layers exhibits a growth rate, which initially increases with the substrate temperature. This behavior is typical for growth that is limited by reaction barriers and insufficient energy of the reactants at the sample surface (substrate temperature). At a temperature of around 135 °C, an ALD window of self-limited growth is reached, as is signified by a relatively temperature independent growth rate. Various ALD windows for ZnO growth are reported for different precursors, reactor designs and substrates, but generally the ALD window for typical ZnO deposition from DEZn and H2O is estimated to be around 110–170 °C [19]. The growth rate of this study is approximately 2 Å/cycle within the process window, which also corresponds well with the reported
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(a)
(b)
(c)
(d)
Fig. 1. Material properties for ZnO, ZTO and SnOx films with Sn/(Sn + Zn) cycle fractions of 1.00, 0.67, 0.40, 0.25 and 0.00 grown by ALD on glass substrates by the stated number of deposition cycles (#c). (a) [Sn]/([Sn] + [Zn]) composition as a function of ALD deposition temperature, measured by XRF and corrected by RBS. (b) Thickness as a function of ALD deposition temperature, measured by XRR. (c) Density as a function of ALD deposition temperature, measured by XRR. (d) Band gaps from indirect and direct Tauc plot models as a function of ALD deposition temperature, measured by ellipsometry.
values in [19], which shows that also our pure ZnO behaves as expected. In this study the upper limit in deposition temperature of the ALD window is not reached, but at higher temperatures the growth rate may either increase from non-saturated growth caused by thermal decomposition of DEZn [20], or decrease due to desorption of H2O from the film surface [21–23]. The growth rate for ZTO is reduced as compared to the growth rates of the binaries, especially in comparison with pure ZnO. Tanskanen et al. [24] shows that a SnOx cycle (TDMASn/H2O) on ZnO reduces the reaction site density, while subsequent ZnO cycles (DEZn/H2O) increase this density. This explains the lower growth rate of the ZTO films as compared to that of ZnO and why in general thicker layers are obtained for films with higher Zn:Sn pulse sequence. It is also likely that growth rate of SnOx on a ZnO terminated surface is lowered in a similar way. This has been observed by comparing two different 0.4 Sn/(Sn + Zn) cycle fraction recipes where the ZnO and SnOx cycles are mixed in different ways (ZnO:SnOx:ZnO:SnOx:ZnO as compared to ZnO:ZnO: ZnO:SnOx:SnOx). The size of the ligands is different for TDMASn and DEZn, which can also influence why the [Sn]/([Sn] + [Zn]) composition of the ZTO films is lower than the Sn/(Sn + Zn) cycle fraction. Furthermore, the thickness and growth rate dependence on deposition
temperature for the ZTO films seem to correlate well with pure ZnO and SnOx, since the temperature dependent growth rate of the 0.67 Sn/(Sn + Zn) cycle fraction process is similar to that of the SnOx and the 0.25 Sn/(Sn + Zn) cycle fraction process to the ZnO process. Due to the difference in ZTO thickness obtained at the different deposition temperatures samples were made with an adjusted number of pulse cycles to obtain films with similar thickness for the structural and optical measurement. The number of pulse cycles used for these sample films were 1600, 1100, 1000 and 1000 at 90 °C, 120 °C, 150 °C and 180 °C, which resulted in thicknesses of 53, 50, 55 and 56 nm, respectively. 3.2. Material structure Pure SnOx films as-deposited by TDMASn and water are found to be X-ray amorphous when deposited at 120 °C [2] and at 150 °C [13,18], while binary ZnO films as-deposited by DEZn and water crystallizes in the hexagonal wurtzite structure [19]. Furthermore, previous work on ALD ZTO films, as-deposited by TDMASn, DEZn and water demonstrates a crystalline-to-amorphous transition when mixing ZnO cycles with SnOx cycles [2,13], with films being X-ray amorphous above a [Sn]/
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(a)
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(b)
Fig. 2. GIXRD diffractograms of ZTO films as-deposited by (a) the 0.4 Sn/(Sn + Zn) cycle fraction process and (b) the 0.25 Sn/(Sn + Zn) cycle fraction process at deposition temperatures of 90, 120, 150 and 180 °C, respectively. The peak positions of powder ZnO are shown at the bottom.
([Sn] + [Zn]) composition of approximately 0.1 [13]. The results from the XRD measurements in this study demonstrate X-ray amorphous structures for ZTO films deposited with a Sn/(Sn + Zn) cycle fraction of 0.67 for all the deposition temperatures analyzed in this paper (not shown here). The ZTO films deposited with the 0.4 Sn/(Sn + Zn) cycle fraction process, shown in Fig. 2(a), also display X-ray amorphous structures when deposited at 90, 120 and 150 °C. However, the ZTO film deposited at 180 °C appears to be mostly amorphous but with a broad peak centered at a position of around 35° in 2θ. The latter probably corresponds to the high intensity peaks of crystalline ZnO at 2θ positions of 31.8°, 34.4° and 36.3° for the (100), (002) and (101) crystal planes, respectively. For ZTO films deposited with the 0.25 Sn/(Sn + Zn) cycle fraction, shown in Fig. 2(b), the same broad peak at 2θ position at around 35° appears already at 120 °C and at higher deposition temperatures, the characteristic peaks for ZnO are clearly distinguishable. In the high resolution TEM image of the sample deposited with the 0.25 Sn/(Sn + Zn) cycle fraction process at 90 °C, shown in Fig. 3(a), areas of parallel atomic planes are visible, which show that this sample has a polycrystalline character. The apparent grain size is approximated to be in the range of 10 nm. Due to the small grain size and physical extent of the TEM lamella along the direction of the electron beam, one has to assume that several of these grains can overlap. The Fourier transformation, Fig. 3(b), reveals the lattice planes as distinct points and they line up well with the positions for polycrystalline ZnO of (100) to (112) planes, which are depicted as red markers. The grains are randomly oriented and as the image excerpt is limited, not all expected planes have to be present, as is the case for the (103) planes. While this film appeared X-ray amorphous in the XRD measurement the
lattice fringes are clearly visible in the TEM images and prove the nanocrystalline nature of the ZTO layers. The reasons why the ZTO film deposited with the 0.25 Sn/(Sn + Zn) cycle fraction process at 90 °C looks amorphous in GIXRD in Fig. 2(b), while the TEM images demonstrate a nanocrystalline character, are probably a combination of too small crystallites with poor crystalline quality that give rise to a large peak broadening, and a too small diffraction volume for the XRD measurement. For smaller particle sizes weaker signals, peak broadening and overlaps have been observed in XRD measurements of ZnO nanoparticles [25]. Our interpretation of the XRD and TEM results is that the ZTO films contain some small ZnO or ZnO(Sn) crystallites of sizes smaller than 10 nm, possibly surrounded by an amorphous material. This is at least true for the most zinc rich films deposited with the 0.25 Sn/(Sn + Zn) cycle fraction process, which is confirmed by TEM. These crystallites are then likely to decrease in size with increasing tin content and decreasing deposition temperature. One can find support for this hypothesis in the literature, where the crystal size and quality are previously found to increase as a function of deposition temperature for ALD grown ZnO [20]. Furthermore, the formation of small crystallites after annealing is also suggested in the case of X-ray amorphous SnOx [18], and the formation of crystalline Zn2SnO4 is observed after annealing of ALD grown ZTO layers, where a phase separation occurs at lower annealing temperatures for more zinc rich films [14]. As can be seen in Fig. 1(c), the density of the X-ray amorphous SnOx and the ZTO films increases more or less linearly with temperature, by about 1 g/cm3 in total, when the deposition temperature increases from 90 to 180 °C. Meanwhile, the crystalline ZnO film stays at a
Fig. 3. (a) Excerpt of high resolution TEM image of a ZTO film deposited by the 0.25 Sn/(Sn + Zn) cycle fraction process at a deposition temperature of 90 °C. (b) Fourier transformation of the full image for the 0.25 Sn/(Sn + Zn) cycle fraction film deposited at 90 °C.
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constant value of about 5.5 g/cm3, which is close to the bulk value of 5.6 g/cm3 for ZnO. It is also obvious from Fig. 1(c) that the ternary ZTO films are less dense than the binary ZnO and SnOx films and that the density of the ZTO films increases with increasing Zn content in the films. If one looks at the data in Fig. 1(c) one can see that there are some variations within the series. These variations are most likely associated to measurement errors in both the ALD deposition temperature measurement and in the characterization methods. As an example, the reason that the density is similar for the 0.67 and 0.4 processes at 180 °C is most likely a coincidence explained by the measurement errors. The amorphous-to-semi-crystalline transition in the ZTO films, which seems to occur with increasing deposition temperature and zinc content, is one likely explanation to the density trends since more crystalline films are ordinarily more densely packed. Another possible explanation for the lower observed density at lower deposition temperatures may be related to the presence of hydrogen in the form of hydroxyl groups remaining in the deposited films. Growing metal oxide films using water as oxygen precursor at low temperatures by ALD is known to produce hydroxyl rich oxide materials. To investigate the possibility of –OH groups in the ZTO films, two samples deposited with a Sn/(Sn + Zn) cycle fraction of 0.4 at 90 and 180 °C, are analyzed by XPS. The XPS spectra collected from the surfaces of the two films reveal a split of the O1s peak with maxima at around 530.3 and 531.7 eV, indicating a presence of both metal oxide and hydroxide. However, depth profiles performed by low power sputtering with argon ions show no significant differences between the two samples, except that the sputtering rate is found to be higher for the 180 °C sample. 3.3. Optical properties Fig. 4(a) shows the refractive index, n, determined by spectroscopic ellipsometry for ZTO films deposited by the 0.4 Sn/(Sn + Zn) cycle fraction process at the different deposition temperatures. From Fig. 4(a) one can conclude that the refractive index increases with increasing deposition temperature. Similar trends are obtained for the 0.67 and 0.25 Sn/ (Sn + Zn) cycle fraction processes. Furthermore, a slight general increase of about 0.1 in refractive index, is observed between the wavelengths of 500 to 1700 nm with increasing zinc content between 0.67 and 0.25 Sn/(Sn + Zn) cycle fraction processes. In general, the refractive index increases with the density of a material, and this likely explains the main trends observed here since the density increases with both deposition temperature and increasing zinc content in the films, as illustrated by Fig. 1(c). Fig. 1(d) shows that band gap values extracted by the Tauc method, such as in Fig. 4(b), yield quite different values depending on if the amorphous phase model with r = 0.5 or if the direct gap model with r = 2 is used. The former is probably more relevant for most of the films in this study since they are X-ray amorphous, and since this
(a)
model generally produces a more linear portion of the curve. However, it is known that pure crystalline ZnO has a direct band gap, so in this case a model with r = 2 should rather be used. This makes it uncertain which model is the most appropriate for the ZTO films deposited at higher temperatures, since a two phase structure is observed and the measured optical properties are then the effective properties of the mixture of ZnO like crystallites and their amorphous surrounding. It should be noted that the extracted band gaps from Tauc plots are quite sensitive to the models used and how well they fit to the measured ellipsometry data, especially for amorphous type band gaps. Comparing the gap values in Fig. 1(d) with soft X-ray measured band gaps for ZTO films grown with similar composition and at a deposition temperature of 120 °C [15] shows that the obtained values are higher in this study. For example, a ZTO film grown with a 0.40 Sn/(Sn + Zn) cycle fraction process at 120 °C shows approximately 0.5 eV higher band gap as compared to the corresponding soft X-ray measurement. It is noted that factors such as film thickness, cycle sequence and timings, and substrate type may influence the average film properties. The Tauc method is also prone to some ambiguity. We therefore emphasize the trends rather than the absolute values of Fig. 1(d). Both models in Fig. 1(d) demonstrate a trend where the band gap of ZTO thin films decreases with deposition temperature. It has previously been shown that the band gap [13,15] and conduction band level [15] of ZTO thin films change with the [Zn]/([Zn] + [Sn]) composition. However, as shown in Fig. 1(a), the composition dependence on deposition temperature is very small and can therefore not explain the decreasing trend for the band gaps as a function of deposition temperature. Rather, this decreasing trend is probably related to the observed changes in material structure. If a semiconductor nanoparticle is small enough, such that the electronic wave functions of the excitons are confined below the Bohr radii in all three dimensions, the electronic and optical properties will change. This quantum confinement results in an expansion of the band gap from its bulk value, in inverse relation to the particle size. Similar changes of the band gap as in Fig. 1(d) are also attributed to quantum confinement effects for X-ray amorphous SnOx [18] and for Zn2SnO4 nanoparticles [11,26]. Furthermore, it is shown that films consisting of wurtzite isotropic shaped ZnO nanoparticles in size range up to 9 nm are small enough to display quantum confinement effects as well, and that the band gap, Eg (in eV), decreases with increasing particle diameter, d (in nm), according to the expression Eg = 3.30 + 0.293 / d + 3.94 / d2 [25,27]. The TEM images of ZTO films deposited with the 0.25 Sn/(Sn + Zn) cycle fraction process at 90 and 180 °C reveal that the films contain ZnO or ZnO(Sn) particles of sizes within this range. These, crystallites also show a broad size distribution, and are likely surrounded by an amorphous phase. A similar increase of the band gap value, as seen in [25,27] as a function of decreasing particle size, is not observed for the ZTO films in this study as a function of deposition temperature, which can be seen in Fig. 1(d). The main reason for
(b)
Fig. 4. (a) Refractive index measured by spectroscopic ellipsometry for ZTO films deposited by a Sn/(Sn + Zn) cycle fraction of 0.4 at deposition temperatures of 90 °C, 120 °C, 150 °C and 180 °C, respectively. (b) Tauc plots for several ZTO films, using r = 1/2 as for amorphous type band gaps. The films were deposited with a Sn/(Sn + Zn) cycle fraction of 0.4 at deposition temperatures of 90 °C, 120 °C, 150 °C and 180 °C, respectively.
J. Lindahl et al. / Thin Solid Films 586 (2015) 82–87
this observation may be that the ZTO films contain ZnO or ZnO(Sn) crystallites with a broad size distribution and that the average size of the crystalline grains in the ZTO films are found in the upper part of the examined range of the ZnO particle sizes in [25,27]. 4. Conclusions This study reports on the relation of changes in structural and optical properties of ALD deposited zinc tin oxide (ZTO) films when varying the deposition temperature between 90 up to 180 °C, and for varying the ALD pulse ratio. It is shown that the effect of the deposition temperature on the [Sn]/([Sn] + [Zn]) composition is small, while the growth rate, density and band gap of the ZTO films change significantly. The growth rate behavior of ZTO films as a function of deposition temperature depends on the relative number of zinc or tin containing precursor pulses and correlates with the growth rate behavior of pure ZnO and SnOx. The density of the ZTO films increases while the band gap decreases with increasing deposition temperature, which is related to microstructural changes of the ZTO films. Transmission electron microscopy measurements indicate that the ZTO films contain small ZnO or ZnO(Sn) crystallites imbedded in an amorphous matrix, and that these crystallites increase in size with increasing zinc content and deposition temperature. The crystallite sizes are approximated to be in the 10 nm range for a 0.25 Sn/(Sn + Zn) cycle fraction process deposited at 90 °C, which means that quantum confinement effects influence the ZTO optical properties. This can explain why the band gaps of the ZTO films decrease with increasing deposition temperature. Acknowledgments The authors gratefully acknowledge T. Ericsson and Dr. D. Primetzhofer for the RBS measurements carried out at Uppsala Tandem Laboratory (Ion Technology Center). The authors also wish thank the Swedish Energy Agency (32787-3) for the financial support and C. Hägglund is grateful for support from the Marcus and Amalia Wallenberg foundation. References [1] A. Hultqvist, M. Edoff, T. Törndahl, Evaluation of Zn–Sn–O buffer layers for CuIn0.5Ga0.5Se2 solar cells, Prog. Photovolt. Res. Appl. 19 (2011) 478. [2] A. Hultqvist, C. Platzer-björkman, U. Zimmermann, M. Edoff, T. Törndahl, Growth kinetics, properties, performance, and stability of atomic layer deposition Zn–Sn–O buffer layers for Cu(In, Ga)Se2 solar cells, Prog. Photovolt. Res. Appl. 20 (2012) 883. [3] J. Lindahl, J.T. Wätjen, A. Hultqvist, T. Ericson, M. Edoff, T. Törndahl, The effect of Zn1 − xSnxOy buffer layer thickness in 18.0% efficient Cd-free Cu(In, Ga)Se2 solar cells, Prog. Photovolt. Res. Appl. 21 (2013) 1588. [4] J. Lindahl, U. Zimmermann, P. Szaniawski, T. Törndahl, A. Hultqvist, P. Salomé, et al., Inline Cu(In, Ga)Se2 co-evaporation for high-efficiency solar cells and modules, IEEE J. Photovoltaics 3 (2013) 1100.
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