1834 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-26, NO. 11 , NOVEMBER 1979

varied from 0.2 J . cm-2 to 0.8 J . cm-'. No dielectric en- capsulating layers were deposited on the InP surface prior to electron pulsing.

Threshold pulse energies have been established for obtainin electrical activity and sheet carrier concentrations -3 X lo1 cm-* which compare favorably with thermal annealing have been achieved. Profiling through layer stripping and differen- tial Hall measurements indicate that although conduction oc- curs mainly at depths found in thermally annealed layers, an anamolous surface component is also present. The mobilities -600 cmz . V-' s-' are only about half of the thermally annealed values. No carrier freezeout occurs at 78 K as has been reported in one study on laser annealed InP' and so the conduction does not appear to be dominated by a defect deeper than the shallow dopant level.

While the annealed surfaces appear to retain their initial polish with some pitting, interference contrast microscopy re- veals a very fine regrowth ripple extending over the surfaces. Small isolated craters, indicating localized heating, account for the pitting. These, as well as the regrowth pattern, become more pronounced as the pulse energy increases and particularly beyond the threshold for electrical activation.

f

2A. G. Cullis, H. C. Webber, and D. S. Robertson, presented at Conf. on Laser-Solid Interactions and Laser Processing, Boston, MA, 1978.

TA-A5 Ion-Implanted, Laser-Annealed GaAs Solar Cells' - John C. C. Fan, Ralph L. Chapman, Joseph P. Donnelly, George W. Turner, and Carl 0. Bozler, Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02173.

Conversion efficiencies up to 12-percent AM1 have been obtained for ion-implanted, laser annealed (IILA) GaAs solar cells utilizing a shallow-homojunction n+-p-p+ structure with- out a GaAlAs window. To the best of our knowledge, this is the first time that good conversion efficiencies have been ob- tained for ion-implanted GaAs solar cells, and these cells represent the first GaAs devices that have been fabricated by ion implantation followed by laser annealing. The n+ layer in the structure was obtained by Se+ ion implantation into the p layer, which was grown epitaxially by chemical vapor deposi- tion (CVD) on a single-crystal p+ substrate. The implanted layer was annealed, without encapsulation, by scanning with a CW Nd:YAG laser. Cell metallization was electroplated, and an antireflection coating was formed by anodic oxidation of the n+ layer. Since these devices are at an early stage of de- velopment, optimizing the implantation and annealing param- eters should lead to a significant improvement in cell perfor- mance. At this time we see no obstacles that would prevent their efficiency from ultimately reaching at least 20 percent, which we have previously achieved for shallow-homojunction cells with CVD-grown n+ layers.

'This work was sponsored by the Department of the Air Force.

TA-A6 Effect of Laser Annealing on the Electrical Character- istics of Silicon on Sapphire Transistors-Giora Yaron, Newport Beach Research Center, Hughes Aircraft Company, Newport Beach, CA 92663, Laverne D. Hess and Ross A. McFarlane, Hughes Research Laboratories, Malibu, CA 90265.

Although laser processing is a rapidly expanding subject and is being studied by research laboratories throughout the world, only a few studies have been reported which are directly con- cerned with the application of this promising technique to the

fabrication of actual semiconductor devices. In the work re- ported here we have conducted an experimental study of the influence of ultraviolet and visible laser radiation on the elec- trical characteristics of transistors fabricated from silicon on sapphire (SOS).

Silicon islands were photolithographically defined and chem- ically etched (KOH) on standard SOS wafers which were subse- quently exposed to pulsed laser radiation (20 ns). Since sapphire is transparent to both visible and UV radiation, com- parative studies of the effect of front and back side (through the sapphire) irradiation of the silicon can be conveniently conducted. One of the objectives of this work is to evaluate the effect of laser radiation on the electrical behavior of the silicon-sapphire interface region. Using standard processing techniques MOS transistors were fabricated and electrically characterized after being exposed to radiation from either an excimer (X = 2490 a) or ruby ( X = 6943 A) laser; both edge- less and conventional transistor configurations were studied.

It was found that front side excimer irradiation at energy levels less than 0.5 J/cm2 produces essentially no changes in the device electrical characteristics. An increase in the laser energy density to -0.85 J/cm' causes a small negative shift in the transistor threshold voltage but does not cause any deg- radation in source-to-drain leakage current or degradation of channel mobility. However, back-side irradiation with the UV laser at 0.85 J/cm2 was found to cause an increase in the source-to-drain leakage current. Comparative studies con- ducted with the edgeless and standard device configurations indicate that this leakage current is due to degradation of the silicon-sapphire interface structural properties. Front ex- posure using a ruby laser at power levels below 0.65 J/cm' resulted in no changes in the device electrical characteristics. However, an increase in the laser energy density to -1 J/cm' caused a positive shift in the threshold voltage, an increase in the source-to-drain leakage current, and a reduction in channel mobility.

Thus we have determined experimentally that a range of energy densities from pulsed UV and visible lasers can be used to explore the possibility of utilizing laser processing for im- provements in SOS device performance. This initial work provides the basis for subsequent experimentation with this new technique which is expected to provide subtle changes in localized regions which will lead to improvements in device performance and reliability. Experiments are currently being conducted to assess the feasibility of utilizing pulsed laser pro- cessing techniques in the fabrication of advanced SOS devices with improved electrical performance characteristics.

TA-A7 Ga Implantation into Si at Ultra-High Dose Rates' - R. C. Kubena, R. R. Hart,' H. L. Dunlap, M. D. Clark, V. Wang, R. A. Jullen, C. L. Anderson, and R. L. Seliger, Hughes Research Laboratories, Malibu, CA.

The most elegant use of a submicrometer focused ion beam in semiconductor microfabrication is for masklession implanta- tion doping. The primary advantage is process simplification by means of a dramatic reduction in the number of process steps. However, high dose rates are necessary in order that the focused beam have a reasonable writing speed. This paper presents Rutherford Backscattering (RBS) and Hall measure- ments o f lattice damage and electrical activity for Si doped by a 59-keV focused Ga+ beam with a current density of 1.5 A/cm'. The current density was -1 O6 times greater than the

tored by NOSC, and N00014-78-C-0337 monitored by ONR. 'Supported by DARPA under Contracts NOOl23-78-C-0195, moni-

2Permanent address: Texas A&M University, College Station, TX.