Improvement of Minority-Carrier Properties of GaAs on Si
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IMPROVEMENT OF MINORITY-CARRIER PROPERTIES OF GaAs ON Si S.M. Vernon,* R.K. Ahrenkiel,** M.M. Al-Jassim,** T.M. Dixon,* K.M. Jones** S.P. Tobin,* and N.H. Karam* *Spire Corporation, Patriots Park, Bedford, MA 01730, **Solar Energy Research Institute, 1617 Cole Blvd., Golden, CO 80401.
ABSTRACT GaAs-on-Si structures have been grown by atmospheric-pressure metalorganic chemical vapor deposition (MOCVD); in some samples, the GaAs nucleation layer was deposited by atomic layer epitaxy (ALE). Material quality has been characterized by Nomarski microscopy, time-resolved photoluminescence, transmission electron microscopy, and the performance of photovoltaic devices. The minority-carrier lifetime has been correlated with defect density and growth parameters. The use of a thermal-cycle-growth technique is seen to be a major factor in improving GaAs-on-Si material quality without resorting to the use of thick buffer layers. INTRODUCTION For many of the applications for which GaAs-on-Si structures are being considered, such as the monolithic integration of GaAs-based optoelectronic devices with Si integrated circuitry, the minority-carrier properties of the GaAs layers are critically important. While there have been numerous reports of the successful fabrication of majority-carrier devices (such as fieldeffect transistors) from GaAs-on-Si wafers [11, there are far fewer successes reported in the minority-carrier-device segment of the GaAs-on-Si field. This is due mainly to the fact that the presence of the high dislocation density typically found in GaAs-on-Si layers deleteriously affects the minoritycarrier material properties (e.g., lifetime) much more severely than the majority-carrier properties
(e.g., mobility).
EXPERIMENTAL PROCEDURE
In the present work, we study a number of ways to reduce the dislocation density, and thus improve the minority-carrier lifetime, in MOCVD-grown GaAson-Si structures. The examined variables include buffer region thickness, first-layer nucleation process, in situ thermal anneal, and usage of a strained layer. The test structures studied are shown schematically in Figure 1. The three top layers represent the GaAs-AlGaAs double heterostructure (DH) specifically designed for the measurement of lifetime by the photoluminescence (PL) technique described below. All samples are grown by atmospheric-pressure MOCVD in the SPI-MO CVDTM 450 reactor built by Spire. The GaAs-on-Si growth is accomplished by the now popular three-step method outlined here: The Si substrates (oriented 2* off (100) in the direction) are heated in hydrogen to over 1000 0 C to remove the oxide, and then the temperature is lowered to approximately 400*C for nucleation and deposition of -200 X of GaAs; finally, a thick GaAs layer (2 Žlm) is deposited at typical MOCVD conditions using a 700%C growth temperature, a 4 pm/hour growth rate, and a V-III ratio of 15:1. In all growths, the source chemicals used are trimethylgallium and arsine, and the n-type dopant gas is silane diluted in hydrogen. In some runs, the low-temperatur
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