R. K. Ahrenkiel

Time-resolved photoluminescence studies of CdTe solar cells

We show that time-resolved photoluminescence measurements of completed polycrystalline CdTe solar cells provide a measure of recombination near the CdTe/CdS metallurgical interface that is strongly correlated to the open-circuit voltage in spite of complex carrier dynamics in the junction region. Oxygen in the growth ambient during close-spaced sublimation generally reduces this recombination rate; grain size does not have a strong effect.

Lattice-mismatched approaches for high-performance, III-V photovoltaic energy converters

We discuss lattice-mismatched (LMM) approaches utilizing compositionally step-graded layers and buffer layers that yield III-V photovoltaic devices with performance parameters equaling those of similar lattice-matched (LM) devices. Our progress in developing high-performance, LMM, InP-based GaInAs/InAsP materials and devices for thermophotovoltaic (TPV) energy conversion is highlighted. A novel, monolithic, multi-bandgap, tandem device for solar PV (SPV) conversion involving LMM materials is also presented along with promising preliminary performance results.

Recombination lifetime of In0.53Ga0.47As as a function of doping density

We have fabricated devices with the structure InP/In0.53Ga0.47As/InP, with a InGaAs doping range varying from 2×1014 to 2×1019 cm−3. These isotype double heterostructures were doped both n and p type and were used to measure the minority-carrier lifetime of InGaAs over this doping range. At the low doping end of the series, recombination is dominated by the Shockley–Read–Hall effect. At the intermediate doping levels, radiative recombination is dominant. At the highest doping levels, Auger recombination dominates as the lifetime varies with the inverse square of the doping concentration. From fitting these data, the radiative- and Auger-recombination coefficients are deduced.

Measurement of minority-carrier lifetime by time-resolved photoluminescence

Abstract Much of the current compound semiconductor device research is based on minority-carrier devices such as heterojunction bipolar transistors and diode lasers. The improvement of minority-carrier-parameters is the focal point of much ongoing materials research. The minority-carrier lifetimes of III-V compound semiconductors are most easily characterized by time-resolved photoluminescence. This is a quick and contactless technique which directly measures the excess minority-carrier carrier density. By using a focused laser beam as the excitation source, the required sample area may be very small. In special diagnostic structures which utilize a confinement or passivating layer, the interface recombination velocity can be determined. Here I will describe the measurement theory and measurement techniques which are used in our laboratory. Recent developments in the III-V materials technology will be reviewed.

INTENSITY-DEPENDENT MINORITY-CARRIER LIFETIME IN III-V SEMICONDUCTORS DUE TO SATURATION OF RECOMBINATION CENTERS

The minority‐carrier lifetime has been measured by time‐resolved photoluminescence in a variety of III‐V epitaxial material including GaAs and AlxGa1−xAs. In cases where Shockley–Read–Hall recombination is dominant, the measured lifetimes are dependent upon the intensity of the excitation source. These lifetime effects can be described by a Shockley–Read–Hall model that includes the injection dependence of the recombination. As the lifetimes increase with the injection level, we describe the effects as the saturation of recombination centers.

Ultralong minority-carrier lifetime epitaxial GaAs by photon recycling

The minority‐carrier lifetime has been measured by time‐resolved photoluminescence in epitaxial films of GaAs grown by metalorganic chemical vapor deposition. The measured lifetimes in thicker devices are 4 to 6 times the theoretical or radiative lifetime. These long lifetimes are the result of photon recycling or self‐generation of the self‐absorbed radiation.

Minority Carrier Lifetime Analysis in the Bulk of Thin-Film Absorbers Using Subbandgap (Two-Photon) Excitation

We describe a new time-resolved photoluminescence (TRPL) analysis method for the determination of minority carrier lifetime τB. This analysis is based on subbandgap excitation (two-photon excitation, or 2PE) and allows selective lifetime determination at the surface or in the bulk of semiconductor absorbers. We show that for single-crystal CdTe, τB could be determined even if surface recombination velocity is >105 cm s-1. Two-photon excitation TRPL measurements indicate that radiative lifetime in undoped CdTe is >>66 ns. We also compare one-photon excitation (1PE) and 2PE TRPL data for polycrystalline CdS/CdTe thin films.

Deep-level impurities in CdTe/CdS thin-film solar cells

We have studied deep-level impurities in CdTe/CdS thin-film solar cells by capacitance–voltage (C–V), deep-level transient spectroscopy (DLTS), and optical DLTS (ODLTS). CdTe devices were grown by close-spaced sublimation. Using DLTS, a dominant electron trap and two hole traps were observed. These traps are designated as E1 at EC−0.28 eV, H1 at EV+0.34 eV, and H2 at EV+0.45 eV. The presence of the E1 and H1 trap levels was confirmed by ODLTS. The H1 trap level is due to Cu-induced substitutional defects. The E1 trap level is believed to be a deep donor and is attributed to the doubly ionized interstitial Cu or a Cu complex. The E1 trap is an effective recombination center and is a lifetime killer.

Chapter 2 Minority-Carrier Lifetime in III–V Semiconductors

Publisher Summary This chapter emphasizes on the development of mathematical tools for analyzing time-resolved photoluminescence data. The chapter presents the development of these tools and provides an abbreviated discussion of the primary recombination mechanisms that are important in III-V minority-carrier physics. The chapter discusses the important aspects of light emission. It focuses on the PL decay analysis of various device structures and on high-injection effects. The present preferred method of lifetime measurement is time-resolved photoluminescence. However, this method has not proven to be applicable to weak light emitters and the smaller-bandgap materials. The latter problem is instrument-related and may well be solved by new technology in the near future. The chapter highlights the experimental data on lifetime in GaAs DH structures and reviews present data related to minority-carrier lifetime in AI x Ga 1- x As.

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.