Charge Carrier Recombination in AlGaAs Studied by Time-Resolved Microwave Conductivity Experiments
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CHARGE CARRIER RECOMBINATION IN AlGaAs STUDIED BY TIME-RESOLVED MICROWAVE CONDUCTIVITY EXPERIMENTS
A. WERNER*, A. M. AGARWAL*, T. D. MOUSTAKAS*, AND M. KUNST** * Boston University, College of Engineering, Boston, MA 02215, USA Hahn-Meitner-Institut, D-1000 Berlin 39, W. Germany
Abstract Al.GaliAs films, doped with silicon in the 1 x 1018 cm- 3 range, have been prepared in the composition range x = 0.20 - 0.46 by molecular beam epitaxy. The influence of defect states on excess carrier trapping and recombination processes is studied by a contactless transient photoconductivity technique in the microwave region. Effective electron lifetimes in the 1 x 10's range for the 20% Al film down to the 1 x 10-7s range for the 46% Al film were found, indicating a larger density of electron traps in more Al rich films. The decay kinetics in AlGaAs is compared with the decay kinetics in GaAs.
Introduction Much interest has been shown in the recent years to understand the defect structure of ntype AlGaAs since it is an important material for opto- electronic applications [1]. DLTS [2] and photo- Hall measurements [3,4,5] have been succesfully applied to estimate the concentration and energetic position of the DX center [2] which is believed to be related to the donor atom itself [6], usually silicon. Recently it has been reported [7] that the use of Se dopants can reduce the deep defect density. These deep defects are the cause for the persistent photoconductivity (ppc) effect observed in this material at low temperature.The increase of the electron concentration at low temperatures after illumination is related to the concentration of DX center, while the dark carrier concentration is related to the concentration of shallow donors. In this work the recombination and trapping processes of excess carriers in n-type Al.Gal_.As, with x varying between 0.20 and 0.46, have been investigated by the contactless time-resolved microwave conductivity technique (TRMC) [8,9], in order to study the influence of the Al-concentration on excess charge carrier decay processes. This technique, under rather general conditions, gives the photoconductivity of the investigated samples [9]. The findings of the time-resolved experiments will be related to dark and photo-Hall measurements performed on the same samples.The data show that an increase of the Al concentration results in a decrease of the electron lifetime indicating a larger density of electron traps in more Al-rich films.
Experimental Methods The Al,,Gal_,As films, n-type doped with Si, have been prepared by molecular beam epitaxy (MBE). All films were grown at substrate temperature of 630 0C. The temperatures of the Ga and Si-effusion cells were kept constant, while the temperature of the Al-effusion cell was adjusted to produce AlGal_.As films with x varying from 0.20 - 0.46. Simultaneously the temperature of the As- effusion cell was adjusted in order that the ratio of Mat. Res. Soc. Symp. Proc. Vol. 145. 01989 Materials Research Society
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beam equivalent pressures is BEP(As)/BEP
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