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The device is pumped with an electrical current injection. The magnitude of carrier density in QD is higher than in QW since it is the summation of carrier densities in all groups of dots. The corresponding output remains minimal until the carrier concentration in the QD states reaches threshold and then increases rapidly which reduces the carrier density and carrier density rises again through recovery of carriers from current injection. This cycle repeats several time until steady state is achieved. The probability of carrier concentration in the QD states is shown in Fig. 2. The carriers occupy the lower energy states first as their probability being highest then drops all the way with the increase of energy. Once the current reaches the threshold, we can observe a “hole” near central energy, which is known as the spectrum hole burning (SHB) and this SHB becomes more significant with more current injection.

In this work the current versus voltage data of a p-n+ junction is converted into minority carrier lifetime data. Space charge recombination currents dominate at modest reverse bias at 80K and taking the dominant recombination centers to be located at the intrinsic Fermi level, the lowest minority carrier lifetime τ0 is determined to be 35ns. This single Shockley-Read-Hall carrier recombination parameter provides an excellent fit to the data over temperature range 40K≤ T≤ 130K; the 35ns minority carrier lifetime also explains the quantum efficiency data. The transition from diffusion to space charge currents occurs for temperatures, T ≤ 100K. For T≤ 40K trap assisted tunneling is the dominant current component. Based on imaging system requirements to be near background limited for photon flux ≈ 1015 ph/cm2-s and detector temperature of 80K, the minority carrier lifetime will need to be increased by one order of magnitude.

Fluorescence Studies of Surface Recombinationin CdSe ElectrodesM.L. Shumaker, D. Burdeiski and D.H. WaldeckDepartment of Chemistry,University of Pittsburgh,Pittsburgh, PA 15260ABSTRACTFluorescence decay profiles are used to monitor carrier dynamics at theinterface between single crystalline CdSe electrodes and 0.5M KOH solution. Thefluorescence decay profiles are found to be independent of temperature over therange from 277K to 337K. It is also shown that a diffusion model of the carrierdynamics with a single rate constant for surface recombination does notadequately describe the observed dynamics.1. INTRODUCTIONWith extrinsic material under low level injection conditions and when selfabsorption is small the fluorescence decay provides a temporal profile of theminority carrier concentration. The fluorescence decay can be exploited tomonitor the dynamics of charge carrier relaxation and charge transfer at thesemiconductor/electrolyte interface. Time-resolved bandgap fluorescence hasbeen used to determine the surface recombination velocity of photogeneratedcarriers in semiconductor electrodes. Fluorescence studies of Il—VI compounds,in particular CdSe and CdS, in contact with electrolyte solutions is available[1,2]. The earlier work concentrates on the influence of surface preparationand electrolyte composition on the surface recombination velocity. In general,agreement with simple diffusion models of carrier motion has been observed, orassumed. The electrolyte composition, as well as surface preparation, is foundto strongly affect the recombination velocity. An asset of time-resolvedstudies is the ability to directly test the validity of diffusion models for thecarrier motion and recombination.The work reported here on the fluoresence decay of CdSe electrodes istwofold. First decay profiles are shown which are not adequately described by asimple diffusion model for the carrier dynamics. Second a temperature study ofthe carrier recombination reveals that the surface recombination velocity doesnot have a strong temperature dependence.2. MODEL OF CARRIER DYNAMICSAn appropriate model for the carrier motion probed by thesestudies is diffusion of minority carriers,6p(r,t) =