J.J. Loferski

Semiconducting Properties of Single Crystals of n‐ and p‐Type Tungsten Diselenide (WSe2)

The optical absorption, photoconductivity, contact photovoltage, electrical conductivity, and Hall coefficient of single crystals of WSe2 have been studied over the temperature range 77°–295°K. It was found that the forbidden energy gap Eg was 1.35 eV at 295°K and that the temperature dependence of Eg was given by dEg/dT=−4.6×10−4 eV/°K. The material as grown by iodine vapor transport in a sealed ampule is p‐type with hole mobility μh∼80 cm2/V·sec and p∼1016/cc at 295°K. The carrier concentration could be reduced by pumping out excess selenium. Doping with rhenium during the crystal growth process resulted in n‐WSe2 with a carrier concentration n∼1017/cc and electron mobility μn∼100 cm2/V·sec at 295°K.

RF-sputtered CuInSe2 thin films

Abstract Thin films of CuInSe 2 were prepared by rf-sputtering from targets which were fabricated from previously synthesized CuInSe 2 powders subjected to various combinations of annealing and cold pressing. By judicious selection of target properties and sputtering conditions, it was possible to obtain CuInSe 2 films of chalcopyrite structure, grain size up to 1 μm, resistivities in the 0.3 to 2.0 ω cm range and Hall mobilities up to about 6 cm 2 /V s. Solar cells were made by combining these films with thin CdS films fabricated by rf-sputtering without breaking the vacuum and by evaporation in a separate system. Better cells having efficiencies up to 5% were obtained by evaporating CdS onto rf-sputtered CuInSe 2 films.

Spray pyrolysis of silver indium sulfides

Abstract Two silver indium sulfide compounds, AgInS2 and AgIn5S8, were prepared by spray pyrolysis. AgInS2 always exhibited an orthorhombic structure, whereas AgIn5S8 was cubic. Electrical and optical properties of both compounds were measured.

CuInS2 films prepared by spray pyrolysis

Abstract The electrical, optical and structural properties of thin films of CuInS 2 prepared by spraying a solution of Cu 2 Cl 2 , InCl 3 and thiourea onto heated glass and alumina substrates are described. The polycrystalline films produced at prescribed spraying rates and substrate temperatures were nearly stoichiometric as shown by EDAX analysis, homogeneous, and had the sphalerite structure The films were p -type and their resistivity could be adjusted over the range 0.1 ω cm to very high values by controlling the temperature of the substrate during spraying. Heterojunction devices which exhibited a weak photovoltaic effect were prepared by combining these films with either single crystals of CdS or with sprayed films of CdS.

Spray pyrolysis of CuInSe2 thin films

Abstract The structural, electrical and optical properties of thin films of CuInSe 2 prepared by spray pyrolysis are described. The films were of the sphalerite or chalcopyrite structure, depending on the substrate temperature. Some effects of post-deposition heat-treatment are also presented.

Valence band structures of heavily doped strained GexSi1−x layers

Valence‐band structure and density‐of‐state (DOS) effective mass are calculated for heavily p‐type‐doped Si and strained GexSi1−x layers grown on (100) Si. At low doping values calculations have been made by earlier authors and the results agree with the published values of DOS mass by these authors. The Fermi energy EF measured from the valence‐band edge has been calculated using our values of DOS effective mass. At 1020 cm−3 doping level the value is 77.3 meV compared with 80 meV determined from luminescence experiments. The value of EF obtained by using the normally accepted value of 0.57 for the hole effective mass in Si is more than the experimental value by a factor about 2, i.e., the calculations reduce the discrepancy from 100% to less than 4%. The effective barrier height of (p+‐Ge0.42Si0.58)/(p‐Si) internal photoemission photo detector has also been calculated using our calculated value of the DOS hole mass. The calculated value is in reasonable agreement with the experimental value. The values ...

Cathodoluminescence characteristics of CuξS films produced by different methods

Abstract This paper shows how the cathodoluminescence (CL) spectrum of chalcocite (Cu 2,00 S) films like those in CuCdS solar cells can be used to develop information about the opto-electronic properties of the material. Comparison of the CL spectra of bulk polycrystalline samples known to be chalcocite with those of thin films produced by sulfurizing thin films of copper and of thin films produced by the chemical substitution processes commonly used to fabricate CuCdS cells, leads to the conclusion that the CL spectra of the latter may be affected by the presence of Cd as a dopant. This hypothesis was tested by observing the CL spectra of chalcocite samples which were doped with various amounts of cadmium. Comparison of these CL spectra suggest that chalcocite films derived from CdS contain 0.25 and 1.0 wt% Cd. The CL spectra of chalcocite films produced by dipping CdS crystals into aqueous solutions of cuprous chloride exhibit an “ageing” process whose cause has not been identified. Cathodoluminescence was also observed from thin film and sintered samples containing a tetragonal phase (not djurleite) having an approximate composition Cu 1,96 S.

Measurement of diffusion length in CuInSe2 and CdS by the electron beam induced current method

The minority‐carrier diffusion length Le for electrons in p‐CuInSe2 was determined by analyzing the magnitude of the electron beam induced current (EBIC) as a function of distance from the p/n junction in an n‐CdS/p‐CuInSe2 solar cell whose solar energy conversion efficiency was about 8%. Data obtained for electron beam energies of 10, 20, and 30 keV showed that Le was at least 0.92 μm. Analysis of the data in terms of a model in which the injection process is approximated by a point source results in a value of Le ∼2.5 μm and a surface recombination velocity of about 1.4×106 cm/sec on a as‐cut surface. The analysis leads to the inference that the ’’point source’’ is located at a depth equal to about 0.06 times the electron range below the surface on which the beam is incident.

Growth and properties of CuInNS2 epitaxial layers obtained by chemical vapour transport

Abstract Epitaxial CuInS 2 layers have been grown on GaP substrates by the chemical vapour transport technique. Suitable growth conditions have been determined. Thermodynamic calculations were used to interpret the vapor-growth of CuInS 2 and etching of GaP. Electron micrographs (SEM) showed different morphology for the CuInS 2 layers eptiaxially grown on (111) A-plane and (111) B-plane of GaP crystals. The as-grown facet of the epi-layers was determined to be (112). The as-grown CuInS 2 layers were p-type and had resistivities of the order of 10 4 μ cm. The i – V characteristics of the p/n heterostructures exhibited rectification and in some cases, a photovoltaic effect.

Cu(1−y)AgyInS2(1−x)Se2x as a prototype of the pentanary chalcopyrite semiconductor system for solar photovoltaic cells

Abstract Group III–V mixed alloy quarternary semiconductors, such as Ga (1− y ) In y As (1− x ) P x , have been extensively employed in lattice matching different semiconductor layers (at specified bandgaps) to form heterojunction electro-optical devices. The feasibility of employing the analogous pentenary alloys, consisting of the ternary chalcopyrites groups I–III–VI 2 and II–IV–V 2 , are being reported. As a prototype of such alloys, samples of the pentenary CU (1− y ) Ag y InS 2(1− x ) Se 2 x have been synthesized and studied. These were prepared by reacting stoichiometric powder mixtures at about 900°C. X-ray diffractometry tests suggest the compounds maintain complete solid solubility throughout the system in a chalcopyrite type crystal structure. The alloys intrinsic conductivity type appears to tend towards n-type for silver and sulfer rich compounds, while forming p-type for copper and selenium rich materials. Using cathodoluminescence spectra the samples bandgap energies were estimated at 300 and 77 K. These indicate that all the alloys synthesized were direct bandgap semiconductors. Using least square fits on the data, topological maps of the bandgap and lattice constants versus composition have been produced.