Photoluminescence and photoconductivity in CdTe crystals doped with Bi

Abstract

Defect levels in CdTe doped with Bi are studied by low temperature photoluminescence, photoinduced current transient spectroscopy, photoconductivity measurements, and optical absorption. Two centers associated with the doping with Bi are reported. The first one, a deep level located at E v + 0.71 eV, only present at low dopant concentrations, has donor character and hole-trap properties, and is mainly responsible for the high resistivity and very high photoconductivity of the samples. The second one, an acceptor center located at E v + 0.30 eV, assigned to Bi Te species, is only present at high dopant concentrations and is mainly responsible for the low resistivity and poor photoconductivity of these samples.

Investigation of the origin of deep levels in CdTe doped with Bi

“…As was reported previously, 14 When the Bi concentration exceeds a certain limit, the dopant preferentialy goes to Te sites and the resistivity falls down. The amphotheric behavior of Bi is able to explain the dependence of the resistivity with the Bi concentration.…”

“…On the other hand, for samples Und and Bi4 the parameters of the ͑D 0 −X͒ emission agree with the values of impurities such as In or Cl, 15 commonly present as contaminants in CdTe:Bi. 14 In fact, using mass spectroscopy, we detect In and, using x-ray fluorescence, Cl in both samples.…”

“…Clearly, the energy position and shape of the band are not the same for both samples, implying a different origin or changes of their structure. In the case of Bi1, this center is responsible for the semi-insulating state, 14 having a donor character. Taking into account that Bi introduces a shallow donor level, it is hard to believe that this deep center is formed by a simple structure.…”

“…not able to explain completely the electrical properties of semi-insulating CdTe:Bi crystals, 13,14 being necessary to study the deep levels. 14 The other important emission in the spectra is a complex band centered at 1.55 eV, which is absent in sample Bi1 and shifted to higher energies in sample Bi4 with respect to the undoped ͓see Fig. 1͑b͔͒.…”

“…13,14 A Bi related center at E C − 0.71 eV was confirmed as the responsible for the semi-insulating state and high photosensitivity of the crystals. Nevertheless, the actual origin of the deep center is still unknown, but was suggested the posibility that large atomic displacements in large-size-mismatched doped materials ͑as is the case of Bi in CdTe͒, could be important in their formation.…”