DX-Center in Se-Doped Al x Ga 1-x As
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DX-CENTER IN Se-DOPED AIxGal-xAs Thomas R. Hanak*+, Assem M. Bakry**, Richard K. Ahrenkiel** and Michael L. Timmons*** University of Denver, Department of Physics, University Park, Denver, CO 80210 Solar Energy Research Institute, 1617 Cole Blvd., Golden, CO 80401 Research Triangle Institute, Research Triangle Park, NC, 27709-2194 ABSTRACT We report the measurement of the thermal activation energy for the DX- center in Se-doped AIxGaj-xAs grown by metal-organic chemical vapor deposition (MOCVD) for different alloy compositions (x=0.19, 0.23, 0.27, 0.31). The peaks obtained from conventional DLTS are often broad or asymmetric with shoulders on one or both sides. These phenomena often arise from two or more traps which are active in the same temperature range. The capacitive transients are recorded digitally and analyzed directly by applying a nonlinear double exponential fitting routine to the data. This fitting produces two Arrhenius plots and yields the densities of the defect states. From the Arrhenius plots, the capture cross sections at infinite temperature and the thermal activation energies are calculated. These results are then used to simulate the DLTS spectra. Excellent agreement between real and simulated spectra is shown. INTRODUCTION It is well known, that the conductivity in n-type AIxGalxAs is controlled by a deep donor, namely the DX-center. 1 It is therefore of main importance to understand the optical and electrical properties of these traps. The energy levels of deep centers are closely connected to the band structure of the material which is in turn an important aspect in determining the properties of the material. In this paper we present a study of the thermal ionization energies of the DX center for the alloy composition in the range between 0.19 and 0.31. The shapes of the DLTS peaks due to the DX center are often broad 1,2,3 as it is in our spectrum as well. The posibility of nonexponential transients observed for deep levels has been discussed by Omeling 4 et al. in terms of alloy broadening. The transient data in this study indicated a two trap model. All DLTS data were fit with the two level model. Thus, our data indicate DX center pairs with the two energy levels being nearly equal. A decrease of the activation energy values with increasing AlAs mole fraction was found. In addition, the capture cross-section of the DX-center-pairs strongly decreased with AlAs concentration. EXPERIMENTAL For the rate window spectra, we used the SULA Technologies Deep Level spectrometer. The details of the full transient acquisition apparatus are explained elsewhere 5 6, . For all capacitance transient signals a pulse width of 100 ms and a pulse height of 2 V was used. The applied reverse bias was 2 V and the pulse frequency 3 s-1.
+ Work performed at the Solar Energy Research Institute
Mat. Res. Soc. Symp. Proc. Vol. 163. c1990 Materials Research Society
782
THEORY An important characteristic for the DX center is the temperature activated capture cross section (1)
ae-a exp(-EB/ kBT)
where a. is the
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