Molecular Beam Epitaxy of Low Temperature Grown GaAs Photoconductors
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MOLECULAR BEAM EPITAXY OF LOW TEMPERATURE GROWN GaAs PHOTOCONDUCTORS D.R. Dykaar, D.J. Eaglesham, U.D. Keil, B.I. Greene, P.N. Saeta, L.N. Pfeiffer, R.F. Kopf, S.B. Darack, and K.W. West AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974 ABSTRACT We report on the characterization of Low Temperature (LT) epitaxial growth of GaAs photoconductors. Samples were characterized using electro-optic sampling, transient femtosecond reflectivity, transmission electron microscopy, and pulsed terahertz spectroscopy as a function of growth temperature, As 4 flux, doping and anneal conditions. We find the strongest effect on pulsewidth to be the temperature of an ex-situ rapid thermal anneal. In addition we find evidence of a temperature threshold for As precipitation. For more than an order of magnitude change in As precipitate density we find no corresponding change in electrical pulsewidth. Doping to 101 7 /cm 3 also produces no change in the measured electrical response. INTRODUCTION Short carrier lifetime semiconductors are an important component of an ultrafast electrical characterization system. Intrinsic GaAs, when used in conjunction with a fast, sub hundred femtosecond FWHM (Full Width at Half Maximum) laser pulse, produces carrier populations with subpicosecond rise times, but hundred picosecond fall times. Usually, a transmission line structure is formed by metal electrodes with some sort of gap forming the switch, with one side biased, and the other acting as the output line. This structure, when illuminated by a short optical pulse transforms the optical pulse into an electrical pulse. The electrical pulse is limited, in general, not by the optical pulse, but by the carrier population which produces the electrical transient. The long fall time associated with intrinsic GaAs can introduce complicated electrical reflections. Ideally, one would wish for a delta function, large in amplitude, and with vanishing temporal width, so that spectral analysis would be clean and simple, and a device integrable with other active and passive components. While damaged Si on Sapphire (SOS) provides pulses with very narrow temporal width, satisfying the first requirement, their amplitude is very small, typically only a few mV. In addition SOS devices are not easily integrable with GaAs devices. The first LT grown GaAs photoconductors [1] produced pulses about 1.5 ps FWHM, but it was clear that this technology offered the possibility of both large amplitude, and short pulsewidth in an integrable technology. In this paper we present the first in depth study of LT GaAs photoconductors. While short pulsewidths have been realized [2], there has been no systematic study of pulsewidth with respect the myriad of variables involved in the MBE growth and subsequent annealing processes. GROWTH Sample histories, as well as a summary of some of the characterization results are shown in Table 1. Growth was carried out in a UHV MBE chamber with a base pressure of
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