Observation of Retarded Recombination in Charge-Separation Structures
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Observation of Retarded Recombination in ChargeSeparation Structures R.K. Ahrenkiel, D. Friedman, W. K. Metzger, M. Page, and J. Dashdorj, National Renewable Energy Laboratory, Golden, CO 80401 ABSTRACT Measurement of recombination and minority-carrier lifetimes has become a very common activity in current semiconductor technology. The two primary measurement techniques are based on photoconductive decay (PCD) and time-resolved photoluminescence (TRPL). The measurement of the "true" lifetime depends on the carriers being confined to a given spatial region of a diagnostic device. When internal electric fields exist that separate the charges, the measured value does not represent the real minority-carrier lifetime. In these cases, the measured quantity is a function of the true lifetime and the measurement technique. INTRODUCTION The concept of charge separation dates to the early work of Samuelson et al [1] who observed charge separation in double heterostructures of indirect-GaAsP/GaAs and indirect AlGaAs/GaAs. The common structures that produce charge separation are p-n junctions and double heterostructures (DHs) with type II alignment. There is a large volume of literature describing the recombination and charge-separation processes in p-n junctions [2,3,4,5,6]. In a companion paper [7], we discuss the theory of carrier transport and recombination kinetics in p-n junctions. In this work, we will present PCD and TRPL data on a variety of such structures. EXPERIMENTAL DETAILS The TRPL measurements were performed using a high-repetition-rate pulsed dye laser. The repetition rate was typically 1 Mhz, and the pulse width is about 10 ps full width at half maximum (FWHM). The dye laser was tuneable from about 570 to 630 nm. The PCD measurements were performed using our resonant-coupled photoconductive system (RCPCD) [8]. The light source is a Coherent Infinity XPO optical parametric oscillator (OPO). The OPO was set up in the idler mode and was tuneable from about 710 nm to about 2200 nm. The pulse width is 5 ns, FWHM and the pulse energy was controlled by the insertion of calibrated neutral density filters. The measured recombination or minority-carrier lifetimes only represent the intrinsic or real lifetime when the excess carriers are confined to a volume of material. When there is current flow out of the excitation volume and into an adjacent volume, the measured lifetimes are altered. The TRPL used the radiative recombination rate to measure the density of excess minority carriers. The PCD techniques measure the density of excess carriers. The decay curves may be quite different when minority carriers diffuse to an adjacent volume and become majority carriers, such as in a p-n junction. The TRPL method will only measure the time that the given carriers exist as minority carriers,
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whereas the RCPCD will measure the excess carriers until they are finally annihilated. Another difference is that TRPL is observable when the rate of photon production is appreciably above the background rate. Thus, with T
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