Optical characterization of epitaxial single crystal CdTe thin films on Al2O3 (0001) substrates

    Optical Characterization of Epitaxial Single Crystal CdTe Thin Films on Al2 O3 (0001) Substrates S.M. Jovanovic, G.A. Devenyi, V.M. Jarvis, K. Meinander, C.M. Haapamaki, P. Kuyanov, M. Gerber, R.R. LaPierre, J.S. Preston PII: DOI: Reference:

S0040-6090(14)00905-5 doi: 10.1016/j.tsf.2014.09.027 TSF 33716

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

3 January 2014 14 September 2014 17 September 2014

Please cite this article as: S.M. Jovanovic, G.A. Devenyi, V.M. Jarvis, K. Meinander, C.M. Haapamaki, P. Kuyanov, M. Gerber, R.R. LaPierre, J.S. Preston, Optical Characterization of Epitaxial Single Crystal CdTe Thin Films on Al2 O3 (0001) Substrates, Thin Solid Films (2014), doi: 10.1016/j.tsf.2014.09.027

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ACCEPTED MANUSCRIPT Optical Characterization of Epitaxial Single Crystal CdTe Thin Films on

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Al2O3 (0001) Substrates

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S. M. Jovanovic, a G.A. Devenyi, a,1 V. M. Jarvis, a K. Meinander,a C.M. Haapamaki, a P.

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Kuyanov, a M. Gerber, a R.R. LaPierre, a and J. S. Prestona

Department of Engineering Physics, McMaster University, 1280 Main St. West, Hamilton,

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Ontario, Canada, L8S 4L8

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Abstract

The optoelectronic properties of single crystal CdTe thin films were investigated by

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photoluminescence spectroscopy, photoreflectance spectroscopy and variable angle

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spectroscopic ellipsometry. The room temperature bandgap was measured to be 1.51 eV and was consistent between spectroscopic measurements and previously reported values. Broadening of

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bandgap emission was consistent with high quality material. Low temperature photoluminescence spectra indicated a dominant emission consistent with bound excitons. Emissions corresponding to self-compensation defects and doping and contaminants were not found. Variable angle spectroscopic ellipsometry measurements over the near-UV to infrared range demonstrated sharp resonance peaks. All spectroscopic measurements indicate high quality thin film material of comparable or better quality than bulk CdTe. Keywords: Cadmium Telluride; Pulse Laser Deposition; Photoluminescence; Thin Films; Photoreflectance; Variable Angle Spectroscopic Ellipsometry

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Corresponding Author Email: [email protected]

ACCEPTED MANUSCRIPT 1. Introduction

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Cadmium telluride (CdTe) and its ternary alloys (e.g. HgCdTe) are of great industrial

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significance. The relatively high mass density of CdTe makes it an attractive choice for γ-ray detection, while the tunability of the bandgap in the HgxCd1-xTe system can be exploited for use

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in infrared detectors. The bandgap for CdTe is also well suited for use as an absorber in solar cell

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applications. While detectors are often fabricated from single crystals [1] , thin films are primarily used for commercial solar cells [2] due to processing costs and the lack of large

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diameter wafers of bulk CdTe. CdTe thin films have demonstrated a strong tendency towards polycrystalline growth that can lead to high defect densities, difficulty with doping, surface

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roughing, and an overall decrease in material quality, which limit all optoelectronic device

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efficiency [3]. Despite continued research efforts on polycrystalline CdTe solar cells, measured CdTe solar cell efficiencies are far below calculated theoretical efficiencies and are limited by

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grain sizes/boundaries, carrier mobility and carrier lifetime [4] , [5] . As such, there are

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considerable advantages to the growth and study of the optoelectronic properties of high quality CdTe single crystal thin films to promote and facilitate wider material utilization and efficiency in device applications.

CdTe is a zinc blende crystal with a tendency to form its (111) plane parallel to the substrate surface [6]. As such, its deposition upon {100} cubic surfaces gives rise to sixfold/four-fold symmetry mismatch at the interface allows multiple crystal orientations during growth by providing geometrically equivalent domains [6] . It is possible to suppress this domain equivalency using offcut silicon substrates [7], [8], [9]. Alternatively, we have overcome this challenge by deposition on sapphire (0001) substrates [10], which have been long recognized as

ACCEPTED MANUSCRIPT a potentially superior substrate for CdTe heteroepitaxy [11]. Further, like silicon, basal plane

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sapphire wafers are available commercially up to 100 mm in diameter for large area depositions.

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The most stable surface termination of Al2O3 (0001) in vacuum conditions has been shown to be an Al-O3 layer [12], [13], [14], [15], [16] illustrated from the top and side in Figure 1 a) and b),

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respectively. Geometrically, (111) CdTe has two domains that match the six-fold symmetry of

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the sapphire (0001) surface, separated by a 180º in-plane rotation, as shown in Figure 1 c) and d). Due to differences in the chemical environment of the surface-exposed Al atoms [17] ,

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however, one of these domains will be highly preferred over the other. Using two dimensional X-ray diffraction (XRD2) techniques, the structure of our films has been thoroughly

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characterized and determined to contain single domains due to the properties of this substrate

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surface termination [9]. Combining XRD2 stereographic projections of the (111) and scanning

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transmission electron microscopy (STEM) the films have also been found to be (111)A CdTe.

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In the work presented here the optical properties of CdTe thin films are reported for structurally well characterized crystals. The films are epitaxial and single crystalline, despite a 3.7% lattice mismatch in the heteroepitaxial system. The CdTe films exhibit strong photoluminescence and photoreflectance with narrow broadening parameters and sharp ellipsometric optical resonances. This is consistent with an accommodation of the tensile mismatch strain resulting in negligible presence of quenching defects such as dislocations or other extended defects. High quality, large area CdTe films represent a significant step towards the development of low-cost processes for the large scale production of a variety of devices including high efficiency single and tandem junction photovoltaics.

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2. Methodology

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CdTe thin films were deposited on 12 mm by 12 mm squares diced from single crystal c-plane Al2O3 wafers (MTI Corporation). Prior to deposition, substrates were solvent cleaned in an

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ultrasonic bath and plasma cleaned in an H2-O2 atmosphere. After cleaning, substrates were

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loaded into the growth chamber and annealed at 450ºC in 13 Pa of O2 for one hour. The chamber was then evacuated to a base pressure of 13 µPa before deposition. CdTe thin films were

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deposited by pulsed laser deposition using a GSI Lumonics IPEX-848 KrF excimer laser with a wavelength of 248 nm. Pulses from the laser were focused onto a rotating CdTe target at 0.5 Hz

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with a spot size of 4.5 mm2 and average energy density of 1.8 J/cm2. The CdTe 5N purity

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pressed powder target (Princeton Scientific) was stoichiometric and undoped. During growth samples were kept at a nominal temperature of 300°C as measured by a Pt-Rh thermocouple on

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the growth furnace surface.

Structural information was obtained using two dimensional X-ray diffraction techniques (XRD2). A Bruker SMART6000 Charge-coupled device (CCD) detector on a Bruker 3-circle D8 goniometer with a Rigaku RU-200 rotating anode X-ray generator and parallel-focusing mirror optics were used for the data collection. Photoluminescence (PL) spectroscopy was measured using a 0.55 m Horiba Jobin Yvon spectrometer and dispersed onto a LN2 cooled Silicon charge coupled array. Light collection from the film was achieved using a microscope with 60x objective with a numerical aperture of 0.7. A 130 mW Argon Ion laser at 488 nm was used as the excitation source and focused to a spot size of 2 μm through the microscope objective. Low

ACCEPTED MANUSCRIPT temperature PL was also taken using an open-cycle Helium cryostat. Variable angle spectroscopic ellipsometry (VASE) was performed using a J. A. Woollam M2000V ellipsometer

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with measurements taken at incident angles ranging from 50° to 66° in increments of 2°.

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Photoreflectance (PR) spectra were obtained using a Bruker Optics Vertex 80v FT-IR spectrometer with a Silicon detector, and Mercury arc lamp modulated by a 510 nm diode laser

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chopped at 210 Hz.

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3.1 Two Dimensional X-ray Diffraction

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3. Results & Discussion

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The central peak of the stereographic pole figure, seen in Figure 1 e), corresponds to CdTe (111) planes parallel to the sapphire substrate surface, while the three higher radius peaks correspond ,

and

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to the

planes. The faint central peak in the magnified portion of Figure

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1 e) is signal bleed-through of the Al2O3 (012) reflection tail indicating epitaxial alignment of the film to the substrate and domain orientation with reference to the substrate.

FIG. 1 HERE Fig. 1 a) Top-view (0001) and b) side-view ball models of the Al2O3 unit cell. Only unique atomic sites are coloured in the top-view image. c) and d) show the first layer alignment for CdTe in each of the two different growth domains that can be experimentally observed. Differences in the chemical nature of the surface Al account for preferential growth of only one of these domains, as can be seen in e) the (111) stereographic projection of XRD2 for a CdTe film grown on Al2O3 (0001). The sample crystallinity of CdTe (111) has a single domain with a relative micro-twin density of <0.1%, as seen in the inset region of the pole figure in Figure 1 e), near the noise floor of the

ACCEPTED MANUSCRIPT instrument. Texture analysis of these thin films is consistent with high structural quality material.

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A 1.0 nm root-mean-squared surface roughness was measured by atomic force microscopy.

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3.2 Photoluminescence Spectroscopy

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PL spectra obtained from films and a reference single crystal CdTe wafer (MTI Corporation) of

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the same crystallographic structure were in good agreement at 270K as seen in Figure 2. Etalon oscillations present in the raw spectra due to the CCD window were removed using power

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spectrum analysis and a low pass filter. Direct band-to-band transitions dominate PL emission near room temperature. Inhomogeneous extrinsic impurities, such as zinc, or intrinsic impurities,

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such as vacancies, have associated strains which can cause local changes in the lattice constant

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causing broadening of the PL emission. The films demonstrate higher homogeneity than the reference wafer despite heteroepitaxial tensile strain as evidenced by the smaller room

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temperature PL broadening.

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Fig. 2 Normalized PL spectrum at 270 K of a CdTe (111)/Al2O3 (0001) film and reference CdTe (111) wafer.

Low temperature PL was also used to evaluate the quality of these films. The PL spectrum in Figure 3 was taken at 8.5 K.

FIG. 3 HERE Fig. 3 Normalized PL spectrum at 8.5 K of a CdTe (111)/Al2O3 (0001) film.

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The spectrum at 8.5 K is dominated by a near band-edge emission. The dominant peak occurs at

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1.576 eV with an asymmetric shoulder at 1.597 eV. CdTe is reported as having a bandgap, Eg0.

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of 1.606 eV [18] and a optical phonon (LO) energy of 21.3 meV [19]. The shoulder of the dominant emission is attributed to free excitons as seen in other undoped single crystal CdTe

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[20]. A candidate for the main emission is the convolution of a free electron LO peak and a

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shallow bound exciton peak related to residual impurities in the undoped material. These features

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are consistent with high quality CdTe.

There exists an additional PL peak at1.47 eV, seen commonly in CdTe epitaxial material, which

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has been denoted the Y-band. The Y-band is purported to be caused by the radiative

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recombination of excitons bound to extended defects, emission from Te(g) type glide dislocations, and the associated LO replicas [21]. At 77 K, high quality CdTe films grown on c-plane sapphire

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by molecular beam epitaxy (MBE) exhibited a Y-band relative defect density of 0.07 [22] ,

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whereas, chemi-mechanical polished bulk CdTe had a relative defect density of 0.05 [22]. The relative defect density is the ratio of the defect band PL intensity to the highest near band-edge emission PL intensity. Films optimally grown here by pulsed laser deposition (PLD) have a relative Y-band defect density of 0.035 at 77 K. This is consistent with the narrower PL line widths of films grown on sapphire relative to a CdTe bulk wafer at 270 K.

3.3 Variable Angle Spectroscopic Ellipsometry

ACCEPTED MANUSCRIPT VASE optical measurements taken at incident angles ranging from 50° to 66° in increments of 2° for CdTe thin films and a reference (111)A CdTe wafer were taken at 300 K and fit with a Cody-

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Lorentz function to determine the values of the index of refraction (n) and extinction coefficient

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(k) as a function of photon energy, as seen in Figure 4. Mean squared error was minimized when the model included a 21.43 Å native oxide layer. Similar oxides on CdTe surfaces have been

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reported in the literature [23]. Films had a calculated root-mean-squared average roughness of

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14.98 Å as determined by the ellipsometry model, which is in reasonable agreement with 10 Å

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measured by atomic force microscopy.

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FIG. 4 HERE Fig. 4 n and k determined from VASE for CdTe (111) films and a reference CdTe (111) wafer.

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The optical resonances in Figure 4 closely align with that from reference wafer, but have sharper features. Particularly around the bandgap both n and k for the film exceed that of the wafer

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indicating a sharper direct-bandgap transition. Negligible free carrier absorption is observed in

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the extinction coefficient at energies below the bandgap, typical for low defect, intrinsic semiconductor materials.

3.4 Photoreflectance Spectroscopy

CdTe films were assumed to have negligible electric fields and hence the low-field regime for PR spectroscopy was chosen for analysis. The dielectric function was chosen to have a Lorentzian form as expected for band-to-band processes in semiconductors and the PR spectrum was described by the equation: [24]

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Where ħθ is the electro-optic energy, C is the amplitude, Φ is the phase of the complex dielectric

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function and accounts for the influence of non-uniform electric fields or electron-hole interaction effects, Eg is the bandgap energy and Γ is the broadening parameter. Assuming a Lorentzian

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form for three dimensional critical points, such as the direct bandgap of CdTe, the value of m is

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2.5 [25]. A Levenberg-Marquardt numerical method was used to obtain spectral fits. A single resonance Lorentzian fit adequately describes features with energy ≥ Eg, however, there was

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poor agreement with the spectrum at energies below the bandgap. Superimposing a second optical resonance, with an unconstrained value of m, slightly below the bandgap with Gaussian

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form upon the first resonance significantly reduces the mean square error of the fit to the data as

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seen in Figure 5. The addition of this second optical resonance is consistent with free exciton emission observed in PL. This superposition of two optical resonances in the fitting describes a

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system with both excitonic and continuum band-band radiative transitions [26], [27].

FIG. 5 HERE Fig. 5 Room temperature PR spectra of CdTe (111)/Al2O3 (0001) film with two resonances fit. The band to band optical resonance was fit with the parameters: Eg=1.51 eV, Γ1=39 meV, Φ1=6.7 rad, C1=5.4x10-6 and m1=2.5. The excitonic optical resonance was fit with the parameters: Ee=1.50 eV, Γ2=52 meV, Φ2=4.2 rad, C2=2.4x10-6 and m2=3.

ACCEPTED MANUSCRIPT The bandgap energy fit to the PR spectrum, 1.51 eV, is consistent with the bandgap measured by PL spectroscopy and VASE. Taking the difference between the energies of the radiative band to

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band and excitonic optical resonance contribution yields a binding energy of 10 meV for the

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room temperature exciton which is comparable to values reported in the literature [28]. The broadening parameter for the band to band contribution, Γ1, is small and consistent with

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broadening parameters for high quality materials [29].

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4. Conclusions

Using XRD2 texture analysis techniques for structural characterization, the growth of undoped

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CdTe (111)/Al2O3 (0001) from the PLD of a pressed powder target has been optimized to yield

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single crystal films. The normalized PL spectrum at 270 K of a high quality film and a reference single crystal wafer demonstrated agreement in peak emission location and emission about that

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peak also demonstrated less broadening. Low temperature PL spectra of CdTe films were

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dominated by excitonic contributions and had smaller Y-band emissions than those reported for CdTe films grown on sapphire by MBE and chemi-mechanically polished CdTe wafers. Overlapping optical resonance positions in ellipsometry indicated that radiative emissions are of the same energy for the film and wafer, optical resonances were sharper for the films suggesting higher quality material. The bandgap of the films at room temperature was found to be 1.51 eV, a value consistent across all optical measurements. Single crystal CdTe thin films present the advantages of longer carrier lifetimes, improved doping control and lowered cost compared to polycrystalline thin films and bulk wafers. High quality large area CdTe films combined with

ACCEPTED MANUSCRIPT lift-off techniques offer the ability to template and produce high quality photovoltaic and

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photosensitive devices on a large scale.

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Acknowledgements

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This work is funded by the Natural Sciences and Engineering Research Council of Canada

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(NSERC) through the NSERC CREATE program in Photovoltaics. X-ray work was carried out at the McMaster Analytical X-Ray Diffraction Facility (MAX), a facility supported by NSERC

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and McMaster University.

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High quality epitaxial CdTe thin films were grown Two dimensional x-ray diffraction characterization confirmed single crystal material Photoluminescence indicated low defect density when compared to bulk single crystals Optical characterization indicated the presence of room temperature excitons

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