Influence of Ion-Implantation on Characteristics of Picosecond Photoconductive Switches

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INFLUENCE OF ION-IMPLANTATION ON CHARACTERISTICS OF PICOSECOND PHOTOCONDUCTIVE SWITCHES JOHN F. KNUDSEN, ROBERT C. BOWMAN, JR., DUANE D. SMITH, AND STEVEN C. MOSS. The Aerospace Corporation, 2350 E. El Segundo Blvd., El Segundo, CA 90245 ABSTRACT Ion-implantation induced amorphization has been used to modify the linearity of response of ultrafast photoconductive switches fabricated on SOS. The extent of amorphization was determined using various materials characterization techniques. TRIM-86 Monte Carlo calculations were used to model the defect densities produced by ion implantation. Linearity of response is critically dependent upon the nature of the semiconductor region under metallic contacts and the character of the response is opposite to that expected from reports in the literature. INTRODUCTION We have characterized the ion-implantation induced amorphization of Silicon-onSapphire (SOS) using RBS, Raman spectroscopy, and Electron Beam-modulated ElectroReflectance (EBER) measurements and have related the extent of amorphization to the linearity of response of ultrafast photoconductive switches fabricated upon SOS. The use of RBS in conjunction with Raman spectroscopy and EBER measurements allowed us to investigate the disorder through the bulk as well as near the Si surface and the Si-sapphire interface. While photocarriers will be generated throughout the Si epilayer, the response of these switches will be affected by the nature of the metalsemiconductor contact and by the effects of trapping states near the Si-sapphire interface. Despite extensive recent studies [1] of ultrafast photoconductive switches produced by ion-implantation of SOS, no measurements have been reported which display the photoconductive response as a function of electrical bias. Nonetheless, picosecond optoelectronic sampling techniques based upon ultrafast photoconductive switches have been used both to generate and to sample ultrafast electrical waveforms [2], to measure the pulse-response of high speed electronic devices [3], to measure the frequency dependence of the scattering parameters of high speed microwave devices [41 as well as the transient electrical waveforms generated during optical excitation of high speed photosensitive devices [5]. Reliable extraction of the scattering parameters of microwave devices and determination of the temporal impulse response of photosensitive devices requires that the photoconductive response of these fast switches be linear with electrical bias and signal as well as incident optical power. It is commonly believed that contacts fabricated on crystalline semiconductors will display rectifying behavior [6] unless special processing such as sintering is used, and that contacts fabricated on semiconductors heavily damaged by ion-implantation will display ohmic response [6] without special processing steps. Our measurements show that ultrafast photoconductive switches fabricated on a Si epilayer heavily damaged by ionimplantation do not have a response which is linear with electrical bias, and that

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