T. L. Chu

Photothermal and photoconductive determination of surface and bulk defect densities in amorphous silicon films

The sub‐band‐gap optical absorption spectra of high‐quality hydrogenated amorphous silicon (a‐Si:H) films are shown to be dominated by surface and interface state absorption when measured by photothermal deflection spectroscopy (PDS), while spectra determined using the constant photocurrent method (CPM) are not. For bulk defect states (both as‐deposited and light‐induced), the integrated subgap absorption is approximately twice as large for PDS as for CPM. Similarly, the conversion factor relating integrated subgap absorption with neutral dangling bond density is twice as large for CPM as PDS. This factor of 2 results from CPM seeing only transitions from below midgap into the conduction band while PDS sees transitions from the valence band into states above midgap as well.

Diffusion lengths in solar cells from short‐circuit current measurements

The minority‐carrier diffusion length in the base region of a silicon solar cell has been determined by measuring the short‐circuit current as a function of the wavelength of incident light. The incident light intensity required to produce a given short‐circuit current is a linear function of the reciprical absorption coefficient for each wavelength, and the extrapolation of this linear relation to zero intensity yields the diffusion length. This method is similar to the surface photovoltage and open‐circuit voltage methods; however, the accuracy for high light bias levels appears to be greatly improved.

Thin film cadmium telluride solar cells by two chemical vapor deposition techniques

Cadmium telluride (CdTe) has long been recognized as a promising thin film photovoltaic material. In this work, polycrystalline p-CdTe films have been deposited by two chemical vapor deposition techniques, namely the combination of vapors of elements (CVE) and close-spaced sublimation (CSS). The CVE technique is more flexible in controlling the composition of deposited films while the CSS technique can provide very high deposition rates. The resistivity of p-CdTe films deposited by the CVE and CSS techniques can be controlled by intrinsic (cadmium vacancies) or extrinsic (arsenic or antimony) doping, and the lowest resistivity obtainable is about 200 Ω cm. Both front-wall (CdTe/TCS/glass) and back-wall (TCS/CdTe/substrate) cells have been prepared. The back-wall cells are less efficient because of the high and irreproducible p-CdTe-substrate interface resistance. The CSS technique is superior to the CVE technique because of its simplicity and high deposition rates; however, the cleaning of the substrate in situ is more difficult. The interface cleanliness is an important factor determining the electrical and photovoltaic characteristics of the heterojunction. Heterojunction CdS/CdTe solar cells of area 1 cm2 with conversion efficiencies higher than 10% have been prepared and junction properties characterized.

Deposition and characterization of p‐type cadmium telluride films

The deposition of CdTe films on foreign substrates by the direct combination of the elements in a gasflow system has the flexibility that the conductivity type and electrical resistivity of the film can be controlled by adjusting the composition of the reaction mixture. The deposition and properties of p‐type CdTe films are emphasized in this paper because of its importance in thin‐film solar cells. Graphite, W/graphite, mullite, and Corning 7059 glass were used as substrates for the deposition process. While CdTe films deposited on W/graphite and mullite substrates could be n or p type, depending on the composition of the reaction mixture, all films deposited on graphite substrates were p type, irrespective of the reactant composition, substrate temperature, or the purification of graphite, suggesting that carbon is electrically active in CdTe. The resistivity of p‐type CdTe films on W/graphite and mullite substrates has been controlled for the first time by (1) using a Cd‐deficient reaction mixture, and...

Electrical properties of CdS/CdTe heterojunctions

The electrical properties of n‐CdS/p‐CdTe heterojunctions depend strongly on the cleanliness of the interface region. In this work, CdTe films were deposited on CdS/glass substrates by close‐spaced sublimation (CSS) under various conditions. The dark current‐voltage characteristics of the resulting heterojunctions were measured over a wide temperature range, and the capacitance‐voltage characteristics were measured in the dark and under illumination. When the CdS surface is in situ cleaned prior to the deposition of the CdTe film, the current transport across the junction is controlled by a thermally activated process. Tunneling makes an important contribution to the interface recombination at temperatures below room temperature when the in situ cleaning of CdS is not used. The dark capacitance of CdS/CdTe heterojunctions prepared with in situ etching is essentially independent of the reverse bias due to intrinsic interface states. Under white light illumination, the 1/C 2 vs V relation is nearly linear. ...

Deposition and properties of zinc telluride and cadmium zinc telluride films

Thin films of ZnTe and Cd1−xZnxTe (x<0.5) have been deposited by the direct combination of the elements on the surface of W/graphite, ceramic, and glass substrates at 550–600 °C in a hydrogen or helium atmosphere. Their microstructure, crystallographic, optical, and electrical properties were studied. Because of the greater stability of ZnTe than CdTe, the Zn/Cd molar ratio in Cd1−xZnxTe films is always less than that in the gas phase. The optical band gap of ZnTe has been determined to be 2.25 eV and that of Cd1−xZnxTe is a linear function of the composition. Preliminary work on Cd1−xZnxTe /CdS heterojunctions has also been carried out.

Surface passivation and oxidation of cadmium telluride and properties of metal‐oxide‐CdTe structures

The oxidation of single‐crystal p‐type CdTe of {111} orientation by thermal, wet chemical, and plasma techniques has been investigated. The C‐V measurements of metal‐oxide‐semiconductor (MOS) structures prepared from various oxides indicate that device quality thermal oxide can be prepared by hydrogen annealing of CdTe prior to oxidation. The thermal oxide consists mainly of TeO2. MOS structures prepared from oxidation of the Cd(111) or Te(111) face of CdTe show low oxide fixed charge density (1011/cm2) and low interface state density (1010/cm2 eV). MOS structures prepared from wet chemical oxide and plasma oxide have less desirable properties.

Deposition and properties of zinc phosphide films

Thin films of zinc phosphide have been deposited on tungsten‐coated steel substrates at 400–600 °C by the reaction of zinc and phosphine in a hydrogen atmosphere; tungsten was used as a barrier against the diffusion of iron from the substrate into zinc phosphide. By using a proper PH3/Zn molar ratio, the deposited films have been identified by electron microprobe and x‐ray diffraction techniques to be Zn3P2. The electrical resistivity and photovoltage of unintentionally doped zinc phosphide films were measured as a function of the composition of the reaction mixture. The effective intragrain minority carrier diffusion length in nearly stoichiometric films was measured by the scanned beam method using a Schottky barrier structure. The incorporation of dopants into zinc phosphide films was also explored.

Cadmium telluride films on foreign substrates

Thin films of cadmium telluride have been deposited on mullite and tungsten‐coated graphite substrates at 500–700 °C by the direct combination of cadmium and tellurium in a hydrogen atmosphere. Their microstructure and crystallographic properties were studied. The importance of controlling the Cd/Te molar ratio in the reaction mixture to obtain nearly stoichiometric films was demonstrated. The electrical properties of nonstoichiometric and nearly stoichiometric films on mullite substrates were measured by the van der Pauw technique. Schottky barriers were used to measure the electrical properties of cadmium telluride films on W/graphite substrates. The effective intragrain minority carrier diffusion length in n‐type films was measured by the scanned electron beam method using a Schottky barrier structure.

Deposition and photoconductivity of hydrogenated amorphous silicon films by the pyrolysis of disilane

The thermal decomposition of disilane (Si2H6) in a hydrogen or helium flow has been used for the deposition of hydrogenated amorphous‐silicon (a‐Si:H) films on the surface of Corning 7059 glass substrates at 450–500 °C. The reaction product consists of monosilane and trisilane in addition to the unreacted disilane and ethylsilane (the major impurity in commercial disilane). The concentration of Si2H6 in the reaction mixture has been found to strongly affect the deposition rate and the photoconductivity of a‐Si:H films. At a given Si2H6 concentration, the deposition rate of a‐Si:H films increases exponentially with temperature. At a given substrate temperature, the AM1 conductivity of a‐Si:H films increases with increasing Si2H6 concentration and approaches 10−5 Ω cm−1 at Si2H6 concentrations higher than about 4%, and the conductivity ratio in better films is about 105. The conductivities of CVD a‐Si:H films have been found to show negligible change under illumination over a period of several days. The opt...