Subnanosecond Scintillations in Diamond
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299 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
resolution of lns in the spectral range of 300 to 1I00nm. The kinetics of luminescence in the subnanosecond range has been measured through light filters by a streak camera "Agat-SF". RESULTS AND DISCUSSION Luminescence spectra and decay kinetics studies in natural and synthetic diamonds show that the fastest luminescence decay time is observed in the wide band with maximum near 400 nm (Fig. 1). These spectra were observed after excitation by 5 ns electron pulses. The decay time of the 400 nm luminescence band is less than the electron pulse duration. A second band of fast luminescence with maximum at 500 nm is observed in synthetic diamonds only. It dominates in the sample with very high concentration of nitrogen. Its decay time is about 200 ns at room temperature (RT). It could be seen from Fig. 1 this "inertial" emission band has maximum different from regular emission band observed under stationary X-ray excitation. In order to determine the decay time of the fast luminescence at 400 rum the samples have been irradiated by 50 ps electron pulses. The decay time of the fast luminescence in the spectral region near 400 nm is equal to -300 ps both for natural and synthetic diamonds at RT. Experimental investigations on the conductivity relaxation on natural diamond were carried recently [ 1,2]. Crystals were excited by picosecond laser pulses. The photon energy was 6.1 to 6.2 eV. At this energy, the photons can create carrier pairs across the 5.5 eV gap band. A comparison between the luminescence kinetics and the conductivity leads to conclusion about the mechanism of the processes responsible for the fast decaying luminescence. The kinetics of the fast luminescence and fast conductivity [1] is shown on Fig.2. These data were obtained for e-h pair concentrations 1017 cm- 3 [2] . It can be seen that the kinetics of luminescence and the conductivity are the same. This suggests a common mechanism for the processes of luminescence and conductivity. According [1] , the lifetime of the band holes does not depend on the excitation density and is less than the electron one. It is determined by trapping on uncharged centers connected with nitrogen. The calculations [1] give the band hole life time of 50 to 60 ps. The free electrons can only recombine with a few impurity centers that are already ionize or have captured holes. As a result the lifetime of a band electron ( t e= 300 ps) is much greater than the one for band holes. When the excitation density is increasing the electron lifetime is decreasing but it is increasing again when the e-h pair concentration exceeds 1017 cm- 3 because of carriercarrier scattering [1]. If the process of electron capture by a positively charged nitrogen center is radiative it is possible to observe the luminescence with maximum at 400 nm. From the result presented in Fig.2 we conclude that the luminescence decay time is connected with the band electron lifetime. This hypothesis is based on following : if decay time
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