A Study of Electroluminescent Emission from CVD Diamond
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Mat. Res. Soc. Symp. Proc. Vol. 423 01996 Materials Research Society
of diamond films. We give here a tentative explanation of the electronic behaviour of the diamond film in terms of the presence of a deep state in band gap which rules its main properties. EXPERIMENT Electroluminescence (EL) and photocurrent (PC) measurements have been performed on a CVD diamond sample supplied by Norton, Co., Northboro, MA, USA, prepared by the dc arc-jet technique and not intentionally doped. The film is about 400 ýtm thick with an area of 1 x 1 cm 2. The substrate and a part of the film (on the bottom) which contains graphite, amorphous carbon and very small crystallites have been cut off. Electric contacts with an area of about 8 x 8 mm2 on the polished surfaces have been deposited by subsequent evaporation of Ti/Pt/Au [7] EL has been excited by the application of voltages ranging from 850 to 1950 V. Emitted light, coming from one side of the sample, has been collected by a system consisting of a standard luminescence apparatus. Spectra have been taken ranging from 340 to 800 nm. The stability of the EL has been checked by operating the sample for one day continuously, and no changes in both intensity and spectra have been observed during operation. In order to study the dependence of EL signal from temperature, the sample has been placed in a cryostat at a pressure lower than 10-6 mbar. The temperature has been varied from 346 to 110 K. All the temperature-dependent measurements have been performed under a 800 V bias. Electronic properties have been studied by means of I-V characteristics and subgap PC. PC measurements have been performed under the excitation of the 442 nm (25 mW/cm 2 power per unit area) and 325 nm (12 mW/cm2) lines of a HeCd laser under a bias varying between 10 and 500 V; temperature has been varied from 110 to 346 K, in order to study the dependence of PC with respect to temperature.
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Energy (eV) Fig. 1: Electroluminescence spectra at different voltages at room temperature
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RESULTS AND DISCUSSION In Fig. 1 EL spectra at voltages between 850 and 1950 V, taken at room temperature, are shown. They present the characteristic band A feature [5] with a peak at a photon energy of 2.8 eV: all the bands show an asymmetric tail at low energies and a FWHM of about 0.4 eV. Fig. 2 shows IV characteristics measured at different temperatures (110-346 K): a log I-log V plot shows a power law dependence of current on voltage (I - V'). The exponent n, evaluated from a linear fit of log I vs. log V curves, ranges from 6.88 to 9.75; for T > 167 K it scales linearly with the inverse of temperature. For this range of temperatures this is consistent with a model based on the formation of space charge limited current (SCLC) explaining the behaviour of our film under an applied voltage [8]. -*--•T= 110 K---e--T= 123 K : 10-6
10-8
--- T = 140 K-T= 195 K--+
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