Photo-Induced Current Spectroscopy in Undoped Cvd Diamond Films
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A most sensitive technique for detecting trap levels in high resistivity materials as diamond is the Photo Induced Current Transient Spectroscopy (PICTS). The traps are primed with photons of energy higher than the bandgap and the current transients from the traps are analysed [4]. In the present work this technique is applied to undoped CVD diamond. Due to the high bandgap of intrinsic diamond the optical excitation of the current is obtained by a synchronised pulsed UV source. A comparison of combined TSC and PICTS measurements is discussed. TSC analysis has been also carried out with a 20 MeV electron beam to evaluate the trap concentrations of the principal deep levels. EXPERIMENT Two high quality "detector-grade" commercial CVD diamond films have been studied by means of PICTS and TSC. The samples have a thickness of about 650 gm and a 5x5 mm 2 size. Circular chromium and gold ohmic contacts have been coated onto the surfaces. The samples resistivity at room temperature is higher than 1014 Qcm. A vacuum chamber connected to a helium-gas closed-cycle Gifford-McMahon two-stage cryogenerator has been used for the TSC at low temperatures, between 120 K and 320 K. A second vacuum system has been used for the PICTS and TSC analyses from room temperature (RT) to 650 K [5]. During the measurements a 100 V bias has been used. The trap filling has been performed by use of a UV Xenon lamp with a 100 Hz repetition rate, 0.1 J per pulse and a spectral emission from 200 nm to 1000 nm with 277 Mat. Res. Soc. Symp. Proc. Vol. 588 ©2000 Materials Research Society
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slow analog signals analog signals ........ digital signals C_ E HV discharge -fast
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Figure 1. Schematic of the PICTS system. The photodiode has been used to measure the flash pulse waveform. a main contribution below 300 nm. The excitation was performed through a UV-grade quartz window a few centimeters above the sample. The TSC signals have been measured by means of a Keithley 6517A electrometer used also as the voltage source. The sensitivity of the TSC signal is about 0.1 pA. The experimental apparatus for PICTS measurements is described in Figure 1. The sample is biased by a voltage source (Keithley 237) and the current is measured across a 1 kn resistor. The voltage transients are read by a digital oscilloscope (Tektronix TDS520D) which performs a signal averaging over several periods to reduce noise, and send the data to the CPU. A low heating rate of 0.05 K/s was used to avoid spectral distortions. The UV light above bandgap in the excitation source penetrates about 100 gm in the samples. In order to investigate the uniformity of the samples we have performed TSC measurements with a 20 MeV electron beam from a linear accelerator, ensuring a uniform filling of the traps in the bulk.
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Figure 2. TSC signals obtained by filling the traps with 20 MeV electrons and UV Xenon lamp. 278
RESULTS The TSC and PICTS measurements have been analyzed usin
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