Thick CdTe Films Research Articles

Design issues in the fabrication of CdS–CdTe solar cells on molybdenum foil substrates

Abstract Investigations on CdTe–CdS solar cells on molybdenum foil substrates revealed that the depletion layer spans the entire CdS and CdTe film thickness and the cell should be conceived as a single junction device instead of the three separate junctions (Mo–CdTe, CdTe–CdS, CdS–TCO). Higher open circuit voltages were achieved when two CdS layers (separated by an air anneal) were used instead of a single CdS layer. The high series resistance of this solar cell continues to be the limiting factor in cell performance. Modeling and design issues for improving cell performance are presented.

Effect of CdTe thickness reduction in high efficiency CdS/CdTe solar cells

ABSTRACTHigh efficiency CdTe solar cells are typically grown with CdTe thicknesses from 3 to 15 μm, although the thickness required for 90% absorption of the incident irradiation at 800 nm is only ∼1 μm. In this paper, we present the effect of CdTe thickness reduction on the performance of CdS/CdTe solar cells in which both the CdS and CdTe films were grown by sputtering. We produced a series of cells with different CdTe thickness (from 0.5 to 3.0 μm), and held the CdS thickness and back-contact-processing constant. The effect of CdTe thickness reduction on the diffusion of CdS into CdTe was studied using optical absorption and x-ray diffraction techniques. Only slight decreases occur in open-circuit voltage, short-circuit current, and fill factor with decrease in CdTe film thickness to 1.0 μm. Almost 10% efficient cells were obtained with 1 μm CdTe. Below 1 μm, all cell parameters decrease more rapidly, including the red quantum efficiency.

Photorefractive Beam-Coupling Measurement of the Nonlinear Refractive Index of Semiconductor Films

The nonlinear refractive index n2 of short lengths (<25 m) of a 20 m silica fiber were measured by photorefractive beam-coupling of 53 ps, 1064 nm pulses that experienced self-phase modulation (SPM) in the fiber. The resultant Induced Grating Autocorrelation (IGA) response yields a value of n2 and a calibration standard for the fiber. By coupling the 1064 nm signal pulse into a semiconductor film and taking the output and coupling it into the calibrated fiber, the modified IGA response, with respect to the fiber alone, results in both the sign and the magnitude of n2 of the semiconductor film. The sign and magnitude of n2 for 1 mm thick GaAs, ZnTe, and CdTe films have been measured by noting the modified IGA response – the temporal shift of the photorefractive beam-coupling response due to SPM in the semiconductor.

Photorefractive Beam-Coupling Measurement of the Nonlinear Refractive Index of Semiconductor Films

The nonlinear refractive index n2 of short lengths (<25 m) of a 20 m silica fiber were measured by photorefractive beam-coupling of 53 ps, 1064 nm pulses that experienced self-phase modulation (SPM) in the fiber. The resultant Induced Grating Autocorrelation (IGA) response yields a value of n2 and a calibration standard for the fiber. By coupling the 1064 nm signal pulse into a semiconductor film and taking the output and coupling it into the calibrated fiber, the modified IGA response, with respect to the fiber alone, results in both the sign and the magnitude of n2 of the semiconductor film. The sign and magnitude of n2 for 1 mm thick GaAs, ZnTe, and CdTe films have been measured by noting the modified IGA response – the temporal shift of the photorefractive beam-coupling response due to SPM in the semiconductor.

Effect of lattice mismatch and thermal expansion on the strain of CdTe/GaAs heterostructures

Photoluminescence (PL) measurements on CdTe/GaAs heterostructures grown by molecular beam epitaxy (MBE) were carried out to investigate the effect of the lattice mismatch and the thermal expansion on the strain due to the CdTe epitaxial layer thickness in CdTe/GaAs heterostructures. The PL peak of the acceptor bound exciton shifts toward the higher-energy side with increasing CdTe film thickness. A new theoretical equation obtained from the strain on the lattice-mismatched heterostructure is proposed. The values of the strains determined from the PL measurements were in reasonable agreement with those determined from the new theoretical calculations taking into account the lattice mismatch together with the thermal expansion difference between the CdTe epilayers and the GaAs substrates. These results can help improve the understanding of the structural properties of CdTe/GaAs heterostructures.

Molecular-beam epitaxial growth of HgCdTe infrared focal-plane arrays on silicon substrates for midwave infrared applications

Molecular beam epitaxy has been employed to deposit HgCdTe infrared detector structures on Si(112) substrates with performance at 125K that is equivalent to detectors grown on conventional CdZnTe substrates. The detector structures are grown on Si via CdTe(112)B buffer layers, whose structural properties include x-ray rocking curve full width at half maximum of 63 arc-sec and near-surface etch pit density of 3–5 × 105 cm−2 for 9 µm thick CdTe films. HgCdTe p+-on-n device structures were grown by molecular beam epitaxy (MBE) on both bulk CdZnTe and Si with 125K cutoff wavelengths ranging from 3.5 to 5 µm. External quantum efficiencies of 70%, limited only by reflection loss at the uncoated Si-vacuum interface, were achieved for detectors on Si. The current-voltage (I-V) characteristics of MBE-grown detectors on CdZnTe and Si were found to be equivalent, with reverse breakdown voltages well in excess of 700 mV. The temperature dependences of the I-V characteristics of MBE-grown diodes on CdZnTe and Si were found to be essentially identical and in agreement with a diffusion-limited current model for temperatures down to 110K. The performance of MBE-grown diodes on Si is also equivalent to that of typical liquid phase epitaxy-grown devices on CdZnTe with R0A products in the 106–107 Θ-cm2 range for 3.6 µm cutoff at 125K and R0A products in the 104–105 Θ-cm2 range for 4.7 µm cutoff at 125K.

The Reduction of The Defect Density in CdTe Buffer Layers for The Growth of HgCdTe Infrared Photodiodes on Si (211) Substrates

CdTe epilayers were grown by molecular beam epitaxy on As-passivated nominal (211) Si substrates using thin interfacial ZnTe layers. By using thin recrystallized (initially amorphous) ZnTe buffei layers, we utilized migration enhanced epitaxy (MEE) in the ZnTe layer and overcome the tendency toward three dimensional nucleation. The threading dislocation densities in 8–9 tm thick CdTe films deposited on the recrystallized amorphous ZnTe films were in the range of 2 to 5 × 105 cm−2. In addition to the reduction of threading dislocation density, the interface between the ZnTe layers and the Si substrate is much smoother and the microtwin density is an order of magnitude lower than in regular MEE growth. In order to understand the initial nucleation mechanism of the ZnTe on the As precursor Si surface, we also grew ZnTe epilayers on Te precursor treated Si substrates. The growth mode, microtwin density, and threading dislocation density are compared for films grown on Si substrates with different surface precursors and grown by different growth methods.

The Reduction of The Defect Density in CdTe Buffer Layers for The Growth of HgCdTe Infrared Photodiodes on Si (211) Substrates

CdTe epilayers were grown by molecular beam epitaxy on As-passivated nominal (211) Si substrates using thin interfacial ZnTe layers. By using thin recrystallized (initially amorphous) ZnTe buffer layers, we utilized migration enhanced epitaxy (MEE) in the ZnTe layer and overcome the tendency toward three dimensional nucleation. The threading dislocation densities in 8–9 μm thick CdTe films deposited on the recrystallized amorphous ZnTe films were in the range of 2 to 5 × 105cm−2. In addition to the reduction of threading dislocation density, the interface between the ZnTe layers and the Si substrate is much smoother and the microtwin density is an order of magnitude lower than in regular MEE growth. In order to understand the initial nucleation mechanism of the ZnTe on the As precursor Si surface, we also grew ZnTe epilayers on Te precursor treated Si substrates. The growth mode, microtwin density, and threading dislocation density are compared for films grown on Si substrates with different surface precursors and grown by different growth methods.

Heteroepitaxy of CdTe on {211} Si using crystallized amorphous ZnTe templates

CdTe films were grown by molecular-beam epitaxy on As-passivated nominal {211} Si substrates using thin interfacial ZnTe buffer layers. ZnTe layers were grown by the following two growth methods: (1) migration enhanced epitaxy (MEE) directly on Si and (2) MEE on 80 Å templates of crystallized amorphous ZnTe buffers deposited directly on Si. CdTe films thicker than 8 μm had threading dislocation densities in the range of 2.5 to 4×106 cm−2 and 1 to 2×106 cm−2 for methods (1) and (2), respectively. Double crystal x-ray rocking curve peak widths decreased from 550 to 120 arcsec as CdTe film thickness increased from 3 μm to 9 μm for samples grown by method (1), and for samples grown by method (2), peak widths decreased from 300 to 84 arcsec in the same thickness range. CdTe film grown on vicinal {211} Si substrates misoriented by 5° toward [111] with ZnTe buffer layer prepared by method (2) had a dislocation density of 8×105 cm−2 and an x-ray peak width of 72 arcsec.

Real-time monitoring and control during MOVPE growth of CdTe using multiwavelength ellipsometry

New multi-wavelength in-situ ellipsometer hardware and data analysis software are described. The hardware can simultaneously acquire accurate ellipsometric data at 12 wavelengths in less than 1 s, is simple and compact and is well suited for in-situ monitoring. The data analysis software implements a “virtual interface” approach to determine in real time the characteristics (growth rate and composition) of the near-surface region of the film. These new tools were used to study the metal-organic vapor phase epitaxy (MOVPE) growth of CdTe on GaAs. From a post-deposition analysis of the in-situ data, dielectric constants of CdTe at growth temperature were obtained. Non-uniformity in the CdTe film thickness, and film nucleation during the initial stages of growth, were also observed in the post-deposition analysis. The determined CdTe dielectric constants were utilized in subsequent depositions to determine the growth rate of a CdTe film in real time. Feedback control of the CdTe growth rate was effected by connecting an analog control voltage line from the data acquisition/analysis computer to the Cd mass flow controller.

Photoassisted organometallic vapor-phase epitaxy of CdTe

A mathematical model has been developed which describes the transport phenomena and elementary reaction kinetics encountered during photoassisted organometallic vapor‐phase epitaxy of cadmium telluride. Dimethylcadmium and dimethyltellurium in helium carrier gas are fed to a horizontal, square duct containing a substrate heated to 165 °C. The organometallic compounds are photodissociated in the gas by illumination with 248 nm photons from a continuous‐wave laser. Average deposition rates predicted by the model agree to within 10% of those measured experimentally. The numerical simulations reveal that the growth rate is controlled by the photolysis rate of the organometallic compounds. This results in a linear dependence of the growth rate on dimethylcadmium pressure and laser power. The simulations further indicate that elemental tellurium deposits with cadmium telluride over a wide range of operating conditions. The elemental tellurium is avoided by operating below 20 Torr total pressure, by illuminating the gas directly above the substrate, and by feeding two–four times more dimethylcadmium than dimethyltellurium. Under these conditions, the flux of methyl radicals to the surface is sufficient to etch away the excess tellurium. The model has also been used to identify an illumination geometry suitable for obtaining uniformly thick CdTe films.

Metalorganic chemical vapor deposition growth of CdTe on GaAs in a vertical reactor with multi-nozzles

The metalorganic chemical vapor deposition (MOCVD) growth of CdTe films on 3 inch diameter (100) GaAs substrates using a vertical rotating-susceptor reactor with linearly alligned multi-nozzles was studied. Specular single ( 1 1 1 )B CdTe films over the entire 3 inch wafer were obtained by adjusting the VI/II ratio individually for each nozzle. The thickness variation for a 2.5 μm thick CdTe films was below 8%. The full-width at half-maximum (FWHM) of the (333) double-crystal X-ray rocking curve (DCRC) was about 250 arc sec. To evaluate its suitability as a substrate for HgCdTe epilayers, a Hg 0.7Cd 0.3Te layer 3 μm thick was grown on the CdTe/GaAs using the same MOCVD system. The HgCdTe layer also showed a specular surface. No macroscopic crystal defects such as cellular structures or antiphase domains were observed on the etched HgCdTe surface.

Laser-induced selected area epitaxy of CdTe and HgTe

The enhancement of growth rate of epitaxial CdTe on GaAs(100)2°→(110) substrates is investigated. Photolytic decomposition of the precursors dimethylcadmium with either dimethyl telluride or diethyl telluride is brought about by absorption of 257 nm radiation from an intra cavity doubled argon ion laser. Growth rate was studied as a function of ultraviolet (UV) intensity by the time resolved reflectivity method to monitor CdTe film thickness. Evidence for enhancement of growth rate from surface absorption of uv radiation is discussed together with the factors that can affect pattern definition. The photoenhancement factor depends primarily on the ratio of photolytic to pyrolytic growth rates, the more stable dimethyl telluride yielding higher photoenhancement factors than diethyl telluride. Results on photopatterned bar and mesa structures are presented with feature sizes down to 50 μm. Preliminary experiments on HgTe photopatterning indicate similar kinetic mechanisms to those for CdTe but slow desorption of reaction products might be responsible for lower growth rates. Possible surface reaction mechanisms are discussed and the prospects for selected area epitaxy of mercury cadmium telluride (MCT) considered.

Microstructural, optical and electrical studies of sputtered epitaxial CdTe films

Growth of thick (5 μm or more) epitaxial CdTe(100) films on KBr(100) substrates by planar r.f. magnetron sputtering is discussed. The films are characterized by means of X-ray diffraction, scanning and transmission electron microscopy, UV reflectance and visible and near-IR transmittance measurements.

Characterization of molecular-beam epitaxially grown CdTe surfaces by high-energy electron diffraction and synchrotron radiation photoemission spectroscopy

The homoepitaxial growth of CdTe films on CdTe(110) substrates was studied by reflection high-energy electron diffraction (RHEED) and synchrotron radiation photoemission spectroscopy. Deposition of 50 to 100 monolayer thick CdTe films at room temperature and subsequent annealing to 350 °C produced high structural quality surfaces. The RHEED pattern was well defined with sharp streaks and distinctly visible Kikuchi lines. The valence-band photoemission spectra developed sharp features characteristic for direct transitions from a highly ordered surface. We obtained similar high structural quality for the homoepitaxial growth of CdTe on a substrate held at 250 °C. The evolution of the RHEED pattern, the Cd 4d core level emission, and the Fermi level pinning position for growth temperatures ranging from 25 to 350 °C provide detailed information about homoepitaxial growth characteristics. We discuss the appearance of these spectral features in terms of the structural properties of the CdTe(110) surface.

Some electrical properties of AuxTe/CdTe heterojunctions on CdTe thick films

Measurements of I–U characteristics at several temperatures, the voltage and frequency dependences of capacitance, and the photocurrent of samples are used for studying some electrical properties of heterojunctions formed on sintered layers of n-type CdTe. According to the geometrical arrangement, two types of samples are investigated: group A on an insulating substrate with planar electrodes, group B on a conducting substrate with sandwich electrodes. The p–n junctions are formed in both cases by chemical reaction with AuCl3 solution. The transport of charge across the junction at both, A and B groups is controlled by some process of tunnelling through the barrier from one side and by thermal emission from the opposite side of the junction followed by recombination at interface states. The C–U characteristics of B-samples show the usual form typical for the abrupt junction, whereas the capacitance of A-samples is essentially influenced by the internal capacitance of the polycrystalline layer. An energy band model of the junction is suggested. [Russian Text Ignored]

Growth of CdTe films on sapphire by molecular beam epitaxy

Results of initial attempts to grow cubic-phase CdTe films on sapphire by molecular beam epitaxy are reported. Depositions have been completed on (11̄02) R-plane, (12̄10) A-plane, and (0001) basal plane substrates. Substrate temperatures in the range 260–350 °C were employed along with deposition rates of 1.5–7.5 Å/s. Depositions on (11̄02) sapphire generally produced films containing some elements of the hexagonal phase, as disclosed by x-ray diffraction and UV reflectance measurements. Sharp cubic-phase epitaxy was obtained for thick (∼5 μm) CdTe films grown on (12̄10) and (0001) sapphire substrates. The epitaxial films are smooth and mirrorlike in appearance. Nomarski micrographs show a featureless CdTe surface.

Explosive crystallization of rf-sputtered amorphous CdTe films

Explosive crystallization of about 2-μm thick amorphous CdTe films prepared by rf-sputtering in an Ar-N2 ambient has been observed. The transformation can be initiated with or without external thermal or mechanical impulses depending on the oxygen content of the films. The crystallization is accompanied by a light flash. X-ray diffraction, Auger analysis, and resistivity measurements were made on these films to arrive at a qualitative model to explain the observations.

Preparation and basic electrical properties of CdTe thick films

Results of I–U characteristics, C–U dependences, impedance and phase angle measurements, and microscopic measurements on n- and p-type CdTe films allow to construct the following model of the film. The film is composed of conductive grains separated by more resistive intergranular (IG) regions with barriers on interfaces. The space charge regions are formed in grains and in IG regions. The barriers are formed in consequence of localized states on grain–IG regions interfaces and the diffusion across the barriers is the dominating mechanism for transport of charge in the film at 2 to 5 V. The equivalent circuit of the film represents a series chain composed from elements containing parallel (RC) circuits with series resistances R0. R0's correpsond to the resistance of neutral bulk of grains and IG regions, parallel RC terms correspond to space charge regions. The frequency dependence of capacity of these regions is caused probably by inertial charge accomodation by localized states on grain---IG regions interfaces, or inside the IG regions. Also the form of Cole-Cole plots can be explained by presence of these states and corresponds to a symmetrical distribution of relaxation times of the interface states. [Russian Text Ignored]

Structure and electrical properties of CdS and CdTe thick films for solar cell applications

CdTe/CdS heterojunction solar cells show particular promise for large-scale terrestrial use. To evaluate these cells, thick films of CdTe and CdS have been grown and their structure and electrical properties determined. These films have been deposited on both single-crystal and fused-quartz substrates and on each other by close-spaced vapor deposition in H2 and by two-source vacuum evaporation. The influence of growth parameters on epitaxy and crystallite size was determined by scanning electron microscopy, and the electrical transport properties were investigated by conductivity and thermoelectric power measurements. The electrical mobility of the CdTe films on fused quartz shows an exponential dependence on E/kT, indicating that grain boundaries dominate the bulk mobility in the film. To achieve large crystallite sizes and high conductivity in reasonable deposition times, temperature/time profiles have been used to control nucleation and growth of the CdTe films. Post-deposition heat treatments in H2 have been used to increase the conductivity of the CdS films.