Raman Studies of Carrier Activation in Laser Annealed GaAs Capped with Silicon Nitride
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RAMAN STUDIES OF CARRIER ACTIVATION IN LASER ANNEALED GaAs CAPPED WITH SILICON NITRIDE A. COMPAAN, S.C. ABBI, H.D. YAO, A. BHAT and F. HASHMI Department of Physics, Kansas State University, Manhattan, KS 66506 ABSTRACT Carrier concentrations exceeding 101 9/cm 3 in GaAs implanted with Si (2 x 1014/cmn @ 140 keV) have been obtained by pulsed laser annealing with either a dye laser (X = 728 mu)or a XeCl excimer laser (X = 308 nm). Carrier concentrations were measured by plasmon Raman scattering over a wide range of anneal energy densities. Compared with capless laser annealing, much higher carrier activations were achieved when the annealing laser pulse was incident through a Si 3 N4 cap. INTRODUCTION The preparation of heavily doped, n-type surface layers in GaAs without excessive substrate heating is important for a number of applications, including fabrication of ohmic contacts to n-type GaAs. 1 Activation of n-type dopants above 2x10 1 8 /cmn is very difficult by conventional diffusion or furnace annealing. Therefore a number of studies have focussed on the use of pulsed annealing and rapid thermal annealing of ion implanted layers-mostly with Se as the implant specie.' 2, 3 Pulsed laser or electron-beam annealing is potentially attractive because typically only the top -1pm of the wafer is strongly heated. However, early studies identified surface decomposition and other near-surface defects as major problems producing low activation and low mobility in the pulsed annealed layers. 4 ' 5 ' ' It has been shown that the use of caps such as Si N or SiO2 on the GaAs surface during furnace or lamp annealing can inhibit thlis decomposition;2,3,6,8 however, differential expansion appears to introduce extended defects near the surface. With pulsed laser annealing little lateral expansion occurs so this mode of defect production may not be a problem. In the present work we have studied the influence of a SiN 4 cap on pulsed laser annealing of Si-implanted GaAs with a side-by-side comparison of capped and uncapped samples. In addition, we have studied the annealing effectiveness of two lasers (excimer and dye) with similar pulse durations (-10 ns) but very different optical penetration depths (13 m and 520 nm). We find that excellent carrier activation can be achieved with either laser if the annealing is done through the cap and at pulse energies well below those used in previous laser annealing studies. -3 Raman scattering indicates that carrier concentrations of -2 x 1019/cm3 are possible. LASER ANNEALING Implantation of 28 Si at 140 keV was done on two-inch semi-insulating GaAs (100) substrates. The substrates were implanted through the -60 mu Si N. cap to avoid the partial recrystallization which occurs during the -7u0 C heating necessary for the pyrolitic deposition. Four different ion doses were used: 6 x 10 /cm , 2 x 10 /cm , 6 x 10 /cm and 2 x 10 /cn2 . After implantation the substrates were cleaved into smaller pieces for laser annealing. For studies of capless annealing, the cap was etched off with a concentrate
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