Pulsed Ruby Laser Annealing of Zn, Mg, Se and Si Ion Implants in Semiconducting GaAs
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PULSED RUBY LASER ANNEALING OF Zn, IN SEMICONDUCTING GaAs
223
Mg,
Se AND Si ION IMPLANTS
DOUGLAS H. LOWNDES, J. W. CLELAND, W. H. CHRISTIE,* AND R. E. EBY* Solid State Division, Oak Ridge National Laboratory** Oak Ridge, Tennessee 37830
ABSTRACT The properties of p+ and n+ layers formed by pulsed ruby laser annealing (PRLA) of shallow (Rp -u 320-680 A) implantations of Mg, Zn, Si and Se ions in both n- and p-type semiconducting GaAs have been evaluated using a combination of SIMS and electrical properties measurements. High activation (> 80%) was obtained for high dose (5 x 101' ions/cm2 ) implants of both Mg and Zn, within a pulsed laser energy 2 density "window" 0.5 ( EZ < 0.8 J/cm (FWHM pulse duration = 20-25 ns). SIMS measurements following PRLA show a wellbehaved increasing penetration of dopant ions into the GaAs substrate, with dopant ion concentrations well in excess of 20 3 10 ions/cm in the near-surface region. Measured hole mobilities are consistent with the values anticipated for these high concentrations of ionized impurity scattering centers.
INTRODUCTION Investigations of pulsed laser and electron beam annealing in GaAs during the past two years [1,2] have focused almost exclusively on the use of semiinsulating (Cr-doped) GaAs, with two principal applications in mind: (1) the production of conducting regions in an otherwise highly insulating substrate and (2) the formation of ohmic contacts. The experiments described in this paper differ from nearly all other work reported to date on ion implantation (II) and pulsed laser annealing (PLA) of GaAs, through our use of implantation directly into semiconducting, rather than semi-insulating, GaAs. The purpose of our study is to evaluate and optimize the II/PLA technique as a method for directly producing shallow p+-n and n+-p homojunctions in a semiconducting GaAs substrate for possible use in photovoltaic solar cells. The optimized laser processing techniques could be transferred later to thick film GaAs substrates produced by CVD. Some of the important laser processing parameters that must be provided by a study with this goal include (I) the determination of whether there exists an energy density (E.) "window" for pulsed ruby laser annealing (PRLA) that results in high electrical activation of implanted ions while remaining below the threshold for serious surface damage due to vaporization, (2) selection of implanted ion species which will provide the highest electrical activation and mobility in the near-surface PRLA region, and (3) determination of spatial homogeneity requirements on the pulsed laser beam, in order to obtain uniform annealing and junction depth. In this paper we report the results that we have obtained in these areas, using electrical properties *Analytical Chemistry Division, ORNL. **Operated by Union Carbide Corporation for the U.S. 7
under contract W- 405-eng-26.
Department of Energy
224 measurements, SIMS measurements of dopant redistribution profiles resulting from PLA and a variety of ion implantation conditions. EXPER
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