Determination of the ionization energy of vanadium levels in zinc selenide

Abstract

It is known that transition metals in wide gap semi conductors form deep impurity states, which in many respects determine the physical properties of these semiconductors [1]. In this case, determination of deep center parameters, the main one of which is the ionization energy E i , is important. Very often this energy is determined by comparing the experimental spectra of impurity absorption with those obtained using the known Lucovsky formula [2]. In this study, the problem is solved for the V impurity in ZnSe, the implantation of which in a crystal causes a significant increase in the band edge emission efficiency at 300 K [3].Doping was carried out by the diffusion of V into ZnSe single crystalline substrates, 4 × 4 × 1 mm in size, in a sealed quartz ampoule evacuated to 10 -4 Torr. It should be noted that the reasonably high diffusivity of vanadium in zinc selenide (D ≈ 5 × 10 -8 cm 2 /s at T = 1500 K [4]) ensures complete bulk doping of the sub strates over 2-4 h. In this case, the surface of samples remains smooth and requires no additional treatment for carrying out optical investigations.The absorption spectra were measured with the help of a SF 5 spectrophotometer, the entrance slit of which was illuminated with a 100 W incandescent lamp. For the photodetector, we used a Ge photo diode, the signal from which was fed to the currentvoltage converter input. To exclude the effect of opti cal recharging of the impurity, the samples were located behind the monochromator, and the spectrum was scanned from the long wavelength side.In Fig. 1, we show the typical spectral distribution of relative absorptivity α for the ZnSe:V samples at 300 K. It contains the low energy V band with the dif fuse peak បω V , the energy position of which is much lower than E g of zinc selenide. Since this band is absent in the absorption spectra of the base substrates, we may assume that it is caused by the presence of the V impu rity. According to [5], there are four basic types of impurity absorption spectra, the shape of which depends on the symmetry of the impurity center and the semiconductor band structure. Comparison of the experimental dependence α(បω) with the expressions cited in [5] showed that it can be described by the Lucovsky formula [2] (1) which is valid for electron transitions from the valence band to the local center. The energy of this center can be determined by constructing the V band in theAbstract-By comparing the experimental spectra of optical absorption and photoconductivity with those calculated using the Lucovsky formulas, it is established that the V impurity in ZnSe forms acceptor levels with the ionization energy 0.62 eV.