Impurities and dopants in vanadium doped CdTe

Abstract

Vanadium doped CdTe (CdTe:V) behaves as an efficient photorefractive material due to its higher figure of merit (n3r41) than InP:Fe. It is a potential material for optoelectronic applications, in particular to manufacture telecommunication switching devices at 1.3–1.5 μm. Two laser beams interfering with an angle of 2 θ create an interference pattern of grating spacing Λ = λ/2 sin θ (λ the wavelength). Free carriers (electrons or holes) photoionized in the bright regions generate a spatially modulated electric field which, due to the electro-optic effect, gives a refractive index variation. The strength of the modulation depends on the quantity of redistributed charges according to the concentrations of deep levels, shallow carriers, and finally, the V3+V2+ ratio. So, the knowledge of the vanadium doping level and shallow carrier concentrations are of primary importance to understand the photorefractive behavior of CdTe:V. This paper describes the physico-chemical analyses of 27 different CdTe or CdZnTe centimeter-size crystals by secondary ion mass spectrometry (SIMS) which has been shown to be a powerful technique to quantify all kind of dopants in semiconductors. The low detection limits of the method, in the range of 1 × 1030 atom cm−3, for main p-type, n-type residual dopants, together with vanadium, in CdTe, allow to classify, qualitatively, the different crystals according to whether their V3+V2+ ratio is greater or lower than one. In particular, the high levels of lithium, copper, and phosphorus in certain crystals are clearly evidenced. Some hints about the origin of these residual dopants are given.