G.B. Lush

A study of minority carrier lifetime versus doping concentration in n‐type GaAs grown by metalorganic chemical vapor deposition

Time‐resolved photoluminescence decay measurements are used to explore minority carrier recombination in n‐type GaAs grown by metalorganic chemical vapor deposition, and doped with selenium to produce electron concentrations from 1.3×1017 cm−3 to 3.8×1018 cm−3. For electron densities n 0<1018 cm−3, the lifetime is found to be controlled by radiative recombination and photon recycling with no evidence of Shockley–Read–Hall recombination. For higher electron densities, samples show evidence of Shockley–Read–Hall recombination as reflected in the intensity dependence of the photoluminescence decay. Still, we find that radiative recombination and photon recycling are important for all electron concentrations studied, and no evidence for Auger recombination was observed.

Minority‐carrier lifetime and photon recycling in n‐GaAs

We are reporting here the hole or minority‐carrier lifetime in n‐type GaAs over a concentration range from 1 × 1017 to 2 × 1018 cm−3. The lifetimes were resolved by means of time‐resolved photoluminescence using the time‐resolved single photon counting technique. Diagnostic isotype double heterostructures were grown by metalorganic chemical vapor deposition. The isotype double heterostructures had confinement layers of composition Al0.3Ga0.7As. At a fixed majority‐carrier concentration, active layer thicknesses were varied from ∼0.25 to 10.0 μm. Using the accepted B coefficient as 2 × 10−10 cm3/s, the photoluminescence lifetime is over an order of magnitude larger than the radiative lifetime. The data were interpreted in terms of photon recycling, and the experimental results agreed quite well with theory.

Electro-optical characterization and modeling of thin film CdS–CdTe heterojunction solar cells

Abstract Optoelectronic characteristics of thin film CdTe–CdS solar cells fabricated at four different laboratories were measured and analyzed. Current versus voltage measurements revealed that, under one sun illumination, tunneling was the dominant current flow mechanism in all cells. Tunneling was also the dominant current flow mechanism in the dark for all types except P3 which exhibited a generation-recombination type current flow process in the dark. A theoretical model involving bulk traps in CdTe and a charged thin layer (T-layer) near the junction under forward bias and/or illumination was developed. The model is able to explain all significant features in the experimental results obtained from current versus voltage, and capacitance.

Physical and electrical characterization of CdS films deposited by vacuum evaporation, solution growth and spray pyrolysis

The physical and electrical characteristics of CdS thin films deposited by vacuum evaporation, solution growth and spray pyrolysis were analysed. The effects of the common grain growth promoter CdCl2 and annealing were investigated. Grain size, bulk composition and surface composition were measured by energy-dispersive X-ray fluorescence, Auger spectroscopy and scanning electron microscopy. Schottky diode analysis was performed to study the electrical characteristics of the films, and energy band gap was measured by spectral transmission.

Microsecond lifetimes and low interface recombination velocities in moderately doped n‐GaAs thin films

We have observed lifetimes greater than 1 μs in moderately doped, thin film, n‐GaAs/Al0.3Ga0.7As double heterostructure membranes formed by etching away the substrate. We attribute these ultralong lifetimes to enhanced photon recycling caused by the removal of the substrate. Nonradiative recombination in the bulk and at the interfaces is very low; the upper limit of the interface recombination velocity is 25 cm/s. Such long lifetimes in GaAs doped at ND=1.3×1017 cm−3 suggest that thin‐film solar cells offer a potential option for achieving very high efficiencies.

Machine vision for solar cell characterization

An in-line, non-destructive process is being developed for characterizing polycrystalline thin-film and other large area electronic devices using computer vision based imaging of the manufacturing and inspection steps during the device fabrication process. This process is being applied specifically to Cadmium Telluride/Cadmium Sulfide (CdTe/CdS) thin film, polycrystalline solar cells. Our process involves the acquisition of reflective, transmission and electroluminescence (EL) intensity images for each device. The EL intensity images have been processed by use of a modified median cut segmentation. The processed images reveal different gray level regions corresponding to different intensities of EL originating from radiative recombination events occurring within a biased solar cell. Higher efficiency devices show a more uniform intensity distribution in contrast with lower efficiency devices. The uniform intensity regions are made up of gray level intensity values found near the mean of the histogram distribution these are identified as regions of good device performance and are attributed to better material quality and processing. Low intensity regions indicate either material defects or errors in processing. This novel characterization process and analysis are providing new insights into the causes of poor performance in CdTe-based solar cells.

Light and voltage dependence of the junction transport properties of CdTe/CdS photovoltaics

Abstract The J–V curve of CdTe/CdS photovoltaics does not consist of a simple superposition of a loss current and a light generated current with a considerable loss in conversion efficiency. This paper uses capacitance/voltage measurements and J–V measurements at a variety of temperatures and light levels to develop a model for this non-superposition. It was found that a light dependent tunneling mechanism dominates at low voltages. Moreover, the tunneling takes place from a trap level within the CdTe.

Compensation of interfacial states located inside the “buffer-free” GaSb/GaAs (001) heterojunction via δ-doping

We report the compensation of interfacial states formed by interfacial misfit dislocation (IMF) arrays via δ-doping. The IMF arrays are located inside the “buffer-free” heterojunction of GaSb/GaAs (001). The interfacial states are measured using surface photovoltage measurements and are positioned at 0.41, 0.49, and 0.61 eV. A higher reverse bias leakage current (IRB) was observed in the heterogeneous GaSb/GaAs IMF sample (73 μA at −5 V) compared to the homogeneous GaAs control sample (3.9 μA), which does not contain IMF. This increase in IRB is attributed to the interfacial states. Hence, the interfacial states are compensated by δ-doping the GaSb/GaAs interface using Te atoms. A low turn-on voltage of 0.85 V and a very low IRB of 0.1 nA were achieved for the δ-doped sample compared to the control and IMF samples. Hence, for optoelectronic applications, such as lasers, solar cells, and detectors, this compensated IMF technology is useful for integration of buffer-free III-Sb devices on an inexpensive GaA...

Thin-film GaAs solar cells by epitaxial lift-off

The authors have performed basic characterization studies of thin-film GaAs n/p and p/n solar cells made by epitaxial lift-off. They find that the internal quantum efficiency is enhanced by as much as 71% in these cells after lift-off due to the presence of a back reflector. An unalloyed, low series resistance back contact can be made with proper contacting techniques and using only van der Waals bonding to gold or palladium. These preliminary results show that thin-film solar cells can be processed without loss of film quality, and suggest that successful implementation of this design concept should lead to increased conversion efficiencies.<<ETX>>

Concentration‐dependent optical‐absorption coefficient in n‐type GaAs

The doping‐dependent, near‐band‐edge optical‐absorption coefficient α(hν) was deduced from optical transmission measurements in n‐type GaAs thin films. The selenium‐doped films were grown by metalorganic chemical‐vapor deposition and doped to produce room‐temperature electron concentrations from 1.3×1017 to 3.8×1018 cm−3. The transmission measurements covered photon energies between 1.35 and 1.7 eV and were performed on double heterostructures with the substrate removed by selective etching. The results show good qualitative agreement with previous studies and good quantitative agreement, except for the heavily doped samples. For n0=3.8×1018 cm−3, α(1.42 eV) is approximately four times that reported by previous workers. Secondary‐ion‐mass spectrometry measurements on films grown under differing conditions demonstrate that α(hν) is sensitive to electrically inactive dopants and supports the hypothesis that precipitates or compensation influenced previous measurements. These comprehensive results on high‐qu...