C.H. Champness

Anomalous inductive effect in selenium Schottky diodes

A region of anomalous negative capacitance has been observed with forward bias in Se‐Tl Schottky evaporated layer structures. The effect, which is more prevalent in diodes with lower series resistance, is due to an inductive contribution to the impedance that is believed to arise from high‐level injection of minority electrons into the bulk selenium.

Method of avoiding ampoule adhesion of ingots in Bridgman growth of CuInSe2

Abstract One of the main difficulties in growing CuInSe 2 is the adhesion of the material to the inner wall of the quartz ampoule after the growth. Carbon coating on the inner wall of the ampoule was found to eliminate the adhesion problem, not by forming a barrier layer between the CuInSe 2 and the ampoule, but by reducing the oxygen or water vapour inside the sealed ampoule. The carbon was only partially coated on the ampoule and had no direct contact with the CuInSe 2 throughout the experiment to avoid any contamination. Addition of other gettering agents of oxygen such as Ti, Mo and graphite were also able to eliminate the adhesion problem. By adding a suitable gettering agent into the ampoules, ingots without cracks and voids were grown, and monocrystals with dimensions as large as 2 cm on a side were obtained by the horizontal Bridgman method.

Boron nitride powder coating of ampoule for Bridgman-grown CuInSe2

Abstract It is found that CuInSe2 grown by the Bridgman method results in ingots which adhere strongly to the inner walls of the quartz ampoule container. However, by coating the inside of the ampoule with a film of boron nitride powder and flaming it into the quartz prior to the introduction of the starting elements, the ingots showed no sticking whatsoever after growth. As a result of this technique, it has been possible to grow ingots of CuInSe2 which are void-free, crack-free, and contain monocrystals of centimeter dimensions.

Czochralski growth of tellurium single crystals

Abstract It has been found that tellurium single crystals, with typical dimensions of 2 cm diameter and 10 cm length, can be grown routinely by the Czochralski method with essentially a 100% yield under appropriate conditions. One of the most important of these is the avoidance of thermal shock at the start of the growth, which can be achieved by having a high background temperature with a slow approach of the seed to the melt surface prior to dipping. Using this procedure, single crystals boules have been grown in the c -direction at pulling rates from 0.8–3.4 cm/h, without obvious deterioration of the crystal quality, as evidenced by visual inspection and Laue X-ray diffraction. It was found that the ratio of the three larger faces of a c -grown crystal (hexagonal cross-section) to that of the three smaller ones was increased with increase of pull rate. High background temperature was also found to be beneficial during the main part of the growth process. Growth with the pull direction at right angles to the c -direction was also carried out, yielding flat-shaped crystals, in which again, the ratio of the long and short axes was found to increase with pull rate.

Studies on monocrystalline CuInSe2 and CuIn3Se5

Crystalline ingots of composition CuIn 3 Se 5 corresponding to the reported ordered vacancy compound (OVC), were grown by a horizontal Bridgman method. X-ray powder diffraction measurements showed the characteristic peaks of this material: (002), (110), (200)/(004), (202) and (114). From these results, lattice constants were calculated to be a = 5.759 A and c = 11.524 A. Hall effect measurements were made on this material, indicating that it is n-type with a carrier concentration of about 10 9 cm -3 and a mobility of around 200 cm 2 /(V s). The CuIn 3 Se 5 material was found to be strongly photoconductive over the wavelength range from 600 to 1100 nm. Since the formation of the OVC is reported to exist only within the surface layer of the chalcopyrites, some studies were initiated on the surface composition determination of monocrystalline CuInSe 2 . In photovoltaic cells using monocrystalline CuInSe 2 as a substrate, it was found earlier, in this laboratory, that preannealing the substrate in argon at 350°C for about 2 h improved photovoltaic performance. Measurements of Auger profiles in the first 200 A or so of annealed and unannealed monocrystalline samples indicate no evidence of the formation of an OVC near the surface during the heat-treatment process.

Study of CuInSe2 thin films prepared by electrodeposition

Abstract Thin films of p-type CuInSe2 prepared by a one-step electrodeposition method have been studied by constructing CdS/CuInSe2 junctions. After the electrodeposition, the CuInSe2 films were treated either in vacuum or in Ar. Cells of the form CdS (high σ)/CdS (low σ)/CuInSe2 were then fabricated for studying the electrodeposited films. Measurements were specifically carried out to determine the diffusion length of minority carriers in the p-type CuInSe2. It was found that the minority carrier diffusion length in CuInSe2 films treated in Ar was generally greater than that for films treated in vacuum under similar conditions. A small area cell (active area 0.11 cm2) with a conversion efficiency of about 7% (under 125 mW/cm2 illumination) has been fabricated.

Preparation and characterization of tellurium surfaces

Abstract Abrasive methods of cutting and polishing tellurium single crystals are found to produce strong surface structural damage which cannot be entirely removed by annealing, even up to 400°C. Cutting and polishing by solvent methods, using chromic-hydrochloric and chromic-nitric acid mixtures, are readily carried out without producing surface defects. Structural defects can be indicated conveniently by etch pits produced using a chromic-phosphoric acid mixture. They can also be revealed by thermal etch pits, which have characteristically different shapes on the (1010) and (0001) surfaces.

Growth by directional freezing of CuInSe2 and diffused homojunctions in bulk material

After a brief review of previous work on the growth of bulk crystalline CuInSe2 by directional freezing, a description is given of the Bridgman and Stockbarger methods used by the present authors to prepare homogeneous, p-type, void-free and crack-free ingots of the compound containing single crystal grains up to 15 mm × 5 mm × 2 mm. Homojunctions prepared from such material by in-diffusion of indium are described, along with characterization of the diodes using dark current-voltage characteristics, photovoltaic response and capacitance-bias measurements, from which concentration profiles are deduced.

Relation between barrier height and work function in contacts to selenium

In Se–Tl, Se–Cd, Se–Au, and Se–Te contacts prepared by evaporation on a crystallized selenium layer, Schottky barrier heights were measured from junction capacitance. These were found to decrease systematically with the work function of the contacting metal. From this variation, the density of interface states was estimated to be about 1014 cm−2 eV−1, assuming an interfacial layer thickness of 10 A. The neutral level of the states was estimated to lie close to the valence band edge.

Some characteristics of In‐diffused CuInSe2 homojunctions

CuInSe2 homojunctions have been fabricated by diffusing indium (at 200 °C) into p‐type monocrystalline samples. The electrical and photovoltaic properties of the junctions were investigated at room temperature. The majority carrier concentration distribution in the p‐type side of the junction was determined from differential capacitance measurements.