Jack L. Boone

Deposition of CdTe by spray pyrolysis

Abstract We report the spray pyrolysis of thin films of CdTe which show promise as a base material for solar cell applications. The films predominately show a zinc blende structure and appear to be intrinsic. Microprobe analysis indicates an excess of cadmium but this is not detectable by X-ray diffraction or from electrical conduction measurements. Measurements indicate a nominal direct bandgap of 1.47 eV and an absorption coefficient of the order of 10 4 cm −1 . The room temperature electrical resistivity is of the order of 10 7 Ω cm.

Electrical and crystallographic characterization of CdTe grown by the vapor transport method

Crystallographic and electrical characterization techniques were performed on CdTe single crystal samples grown by the sublimation and physical vapor transport (SPVT) technique. The SPVT growth process described here has resulted in the routine growth of 45–50 mm diameter, 250–300 g boules of single crystal CdTe. As-grown material is p-type in the 5–10 ω cm range. Etch pit densities (EPD) are nominally 7×104 cm-2 along the [111] growth direction and 3×104 cm-2 along the [111] direction. X-ray full width at half maximum (FWHM) on recent samples is 8.6 arc sec compared to 8.5 arc sec theoretical. The as-grown p-type material displays room temperature mobility in the 80–90 cm2 V-1 s-1 range and displays acceptor levels due to Cd vacancies 0.045 eV above the valence band and due to Cd vacancy-donor complexes 0.16 eV above the valence band. The boules are a constant diameter over most of their length (∼5.5 cm) and generally display no visual or X-ray detectable twins or grain boundaries.

Solar-cell design based on a distributed diode analysis

The front surface of a p-n junction solar cell has resistive losses associated with the diffused layer, the metal-semiconductor contact, and the grid structure. These losses are analyzed by considering the spatially distributed nature of the p-n junction and the grid conductors. This distributed diode analysis is especially useful for solar cells operated under concentrated sunlight conditions. The results show the dependence of the V-I characteristics and the maximum power output per unit cell on the ratio of the diffused layer resistance to the junction dynamic resistance. This ratio can assist the designer in establishing proper grid structure geometries and should tpically be less than 0.1 if the power output per unit cell is to be within 3 percent of that for the lossless case. Experimental measurements are reported which confirm the theoretical calculations. An analysis of the grid conductor losses associated with multiple-connected unit cells shows the disastrous effect that the grid header resistance can have on the performance of a solar cell. The results indicate that the use of a tapered header conductor to decrease the metal coverage may actually worsen cell performance.

Photoluminescence and Raman studies of CdS films grown by metal-organic chemical vapor deposition on Si{111} substrates

Abstract CdS epitaxial films grown using metal-organic chemical vapor deposition on Si{111} substrates at growth temperatures ranging from 270 to 325 °C have been characterized using photoluminescence and Raman spectroscopy. Out of all the samples the best optical behavior is obtained from that grown at the lowest temperature. Samples grown at the lowest temperature exhibited increased intensity of the bound excitonic peaks and reduced emission at the defect-related luminescence region. The Raman spectra were compatible with the film surface being normal to the c axis, indicating a preferred growth orientation.

MOCVD of CdS onto (111) silicon substrates

Abstract Thin films of CdS were grown onto (111) silicon substrates (wafers 2 in in diameter) by employing a horizontal, low-pressure MOCVD system. The observed growth of heteroepitaxial films occurred in the temperature range of 275–325 ±5 °C. The growth of polycrystalline samples with a high degree of preferred orientation occurred at temperatures above 325 ± 5 °C. The later films exhibited irregularly shaped surface pits about 1 μm in depth. These pits were observed by sectioning the substrates and examining them with an SEM; no significant crystallinity was apparent. X-ray analysis of an heteroepitaxial and a polycrystalline film via a profile fitting algorithm revealed bulk strain values normal to the silicon substrate of 0.283% and 0.128% respectively.

Electrical properties of pure CdS

Electrical measurements on single‐crystal CdS are used to evaluate the various material parameters of vapor‐grown material. Hall measurement data is used to obtain a computer fit to a single donor level partially compensated model for the purpose of determining the donor density, acceptor density, and activation energy. Hall mobility data is fitted to a scattering model which includes lattice, neutral impurity, and ionized impurity scattering. The donor level in the low density doping limit is found to be approximately 32 meV below the conduction band. Over a temperature range of 100 k–300 K lattice scattering is predominate and the mobility variations have little impact on the electrical properties; the major variations are due to ionized donor variations and the band‐gap variations with donor densities. At low temperatures, impurity densities and the type of impurity scattering have a significant impact on the mobility. Pure CdS displays a very high mobility, i.e., μ>10 000 cm2/V sec at low temperatures...

Evidence of interaction between two DX centers in N-type AlGaAs from RDLTS and temperature dependent pulse-width DLTS measurements

Two well-separated electron traps with activation energies: Et1 ≊ 0.286 eV and Et2 ≊ 0.433 eV have been consistently detected in the n-type Al0.6Ga0.4As confinement layer of AlGaAs/GaAs single quantum well laser diodes. The physical characteristic parameters for these two traps, including capture cross section, emission time constant, and capture time constant, have been calculated. Reverse-bias pulsed deep level transient spectroscopy (RDLTS) results provide the evidence for the first time that these two traps have strong interaction during emission processes. This allows us to conclude that Et1 and Et2 are indeed both donor-unknown centers. Furthermore, using a temperature-dependent pulse-width method, DLTS signals from Et1 alone can be obtained. The corrected activation energy appears to be a little shallower at Et1 ≊ 0.265 eV.

Free carrier density profiling by scanning infrared absorption

A prototype instrument for making contactless measurements of free carrier concentration as a function of the surface coordinates has been developed. The instrument requires minimal sample preparation and the measurements are nondestructive. Measurements are based upon the free carrier absorption of photons at a wavelength of 10.6 μm. The measurement sensitivity is determined by the ability to resolve small variations in the transmitted wave. These variations can be significantly influenced by changes in sample thickness. A scheme has been devised which eliminates thickness variations by rotating the sample through small angles to achieve a transmission maximum at each data point. Preliminary results indicate that system drift limits the sensitivity of the prototype system to measurements of the order of 1015 free carriers per cm3. Data on CdS samples indicate accurate measurements for free electron densities greater that 1016 cm−3. Refinements in the instrument should reduce this level to 1014 cm−3 or less.

Growth of tungsten selenide films through pyrolytic conversion and anodic electrooxidation of ammonium selenotungstate ((NH4)2WSe4)

WxSey thin films have been prepared by the spray pyrolysis and anodic electrodeposition of solutions of ammonium selenotungstate, (NH4)2WSe4, with y/x usually between two and three for the sprayed films and yx>>1 for the electrodeposited films. The sprayed films indicated regions of crystalline WSe2 and have optical absorbance spectra consistent with a mixture of WSe2, WSe3, and selenium. The electrodeposited films are totally amorphous, highly photoconductive, and have optical absorbance spectra typical of amorphous or vitreous selenium. The sprayed films transform into WO3 at temperatures greater than 350°C in the presence of oxygen wile the electrode-posited films tend to evaporate and/or decompose upon annealing.

Free electron density measurements by IR absorption in CdS

A contactless method for determining the spatial variation of the free carrier density in CdS is presented. The method involves the correlation of IR transmission with the free carrier density via the IR absorption coefficient. Preliminary measurements indicate a free electron density sensitivity of less than 5×1014 cm−3 and a spatial resolution which is limited only by the spot size of a CO2 (λ=10.6 μm) laser; this should be of the order of 100 μm. Experimental results indicate that the predominate scattering is due to impurity interaction at room temperature; this is indicative of the high degree of compensation in CdS.