Electrical Mobility and Carrier Lifetime in Single-Crystal, Isotopically Pure Type IIa Synthetic Diamond
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ELECTRICAL MOBILITY AND CARRIER LIFETIME IN SINGLE-CRYSTAL, ISOTOPICALLY PURE TYPE IIA SYNTHETIC DIAMOND
L. S. PAN*, S. HAN*, D. R. KANIA*, AND W. BANHOLZER** * L-476, Lawrence Livermore National Laboratory, 7000 ** GE Superabrasives, 6325 Huntley Rd., P. 0. Box 568,
East Ave., Livermore, CA 94550 Worthington, OH 43085
ABSTRACT
Single-crystal type Iha diamonds were synthesized by applying high temperatures (1200'C) and high pressures (52000 atm) to powdered polycrystalline diamond grown using conventional CVD techniques. These samples were isotopically pure, consisting of approximately 99.93% 12 C, compared to roughly 98.96% in natural Ila diamonds. In addition, the dislocation density of the synthetic samples is significantly lower than in natural Ila diamonds, as indicated by birefringence measurements. Electrical properties of these samples were measured using transient photoconductivity, where a 3 ps pulse of ultraviolet light (6.1 eV) was used to excite free electron-hole pairs. Compared to natural Ila diamonds, the lifetime at low fields (200 V/cm) and low excitation densities (1015 cm- 3) in the synthetic sample was significantly longer (1 ns in the synthetic sample vs. 200 to 300 ps in natural diamond). At higher fields, a much longer decay component, exceeding 10 ns, was observed in the synthetic sample. Combined electron and hole mobilities were around 2500 cm 2/V-s in the synthetic diamond, compared to 3000 to 4000 cm 2/V-s in the best natural samples. At a field of 2 kV/cm, the drift distance in the synthetic sample was over 50 gim, considerably longer than that of natural diamond (10 gim). This is due primarily to the much longer carrier lifetimes. The longer lifetimes in the synthetic sample demonstrate that the properties of the best natural diamonds can be exceeded and are encouraging for the development of sensitive diamond radiation detectors. These longer lifetimes are likely due to the higher quality and lower defect density in the synthetic samples, rather than the isotopic purity.
INTRODUCTION AND EXPERIMENTAL SETUP 12 The synthetic sample was made starting with isotopically enriched methane (99.9% C) as the source gas for a standard chemical vapor deposition technique. The resulting polycrystalline diamond was crushed and powdered and converted to a single crystal using the high-temperature, high-pressure method.1 The resulting sample was approximately 99.93% 12 C, compared to roughly 98.96% in natural Ila diamonds. Thermal conductivity of such material has been described in several previous references.'.2,3 The thermal diffusivity was a factor of 50% larger than in diamonds with natural 13 C abundance. These crystals are known to have a high4 degree of crystalline perfection, as reflected by the narrowness of measured x-ray Bragg peaks, and by birefingence measurements. Raman spectroscopy on the sample used in this experiment placed the diamond peak at 1333.13 cm-1, with a full width at half maximum (FWHM) of 2.18 cm- 1 , compared to a natural Ila diamond at 1332.51 cm- 1, with a FWHM o
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