Midgap traps related to compensation processes in CdTe alloys

Abstract

We study, by cathodoluminescence and junction spectroscopy methods, the deep traps located near midgap in semiconducting and semi-insulating II-VI compounds, namely, undoped CdTe, CdTe:Cl, and Cd 0.8 Zn 0.2 Te. In order to understand the role such deep levels play in the control of the electrical properties of the material, it appears necessary to determine their character, donor, or acceptor, in addition to their activation energy and capture cross section. Photoinduced-current transient spectroscopy and photo deep-level transient spectroscopy are used to investigate the semi-insulating ͑SI͒ samples, and a comparison of the complementary results obtained allows us to identify an acceptor trap, labeled H, and an electron trap, labeled E. Level H is common to all investigated compounds, while E is present only in CdTe:Cl samples. This provides clear experimental evidence of the presence of a deep trap in CdTe:Cl, which could be a good candidate for the deep donor level needed to explain the compensation process of SI CdTe:Cl. ͓S0163-1829͑97͒08944-3͔The interest in Cd-based binary and ternary II-VI compounds arises from their promising applications as ␥-and x-ray detectors and in electro-optic devices. The high resistivity of the material ( у10 8 ⍀ cm) is one of the most stringent requirements, together with a high mobility-lifetime product. Semi-insulating ͑SI͒ CdTe can be obtained by growing impurity-free stoichiometric crystals or, more easily, by introducing during growth group-III or -VII donors. The dopants thus introduced act as shallow donors which compensate for the native acceptors, the cadmium vacancies (V Cd ), generated during the growth in Te-rich conditions. The resulting complex (V Cd -Don Te ) is the so-called center A, 1 which acts as a single acceptor located at E V ϩ0.15 eV. 2-4 Its energy level is too shallow to account for the pinning of the Fermi level observed in compensated Si II-VI compounds, and the existence of a deep donor has been suggested to explain the compensation process. 3,5 While the introduction of group-V impurities is known to directly introduce deep levels which play a significant role in the resulting semi-insulating behavior of the material, group-III impurities do not directly generate deep levels but only introduce shallow ones. 6 In order to study the electrical activity of the deep traps, we used electrical spectroscopy methods which can be applied to SI materials, namely, PICTS ͑photoinduced-current transient spectroscopy͒ and PDLTS ͑photo deep-level transient spectroscopy͒. 7-10 These techniques allow the analysis of the deep levels in a wide region of the forbidden gap, including those located near midgap, i.e., the traps which may intervene in the pinning process. Moreover, they can determine the hole or electron character of the deep levels. The characterization of the deep level optical activity has been carried out with cathodoluminescence ͑CL͒ measurements which also allow us to compare our results to those reported in the literature.We have investigated four ...

Thermoelectric Effect Spectroscopy and Photoluminescence of High-Resistivity CdTe:In

“…The origin of the levels (named P05, P5, and P50 in Table I) can be attributed to a complex of Cd vacancy with donors. 2 Complexes with different donors will result in slightly different defect energies, which is in agreement with our experiment. Ab initio calculations 10 also found a complex of a Cd vacancy and a Te antisite at this energy.…”

Defect structure of high resistive CdTe:In prepared by vertical gradient freeze method

“…This deep trap near the middle of the gap was reported by a number of authors being common for doped and intentionally undoped CdTe and CdZnTe. It was attributed to a deep acceptor complex involving acting as a recombination center (labeled in the [19]). In the same paper, however, another deep donor level (E) lying slightly above the level was found in CdTe:Cl samples.…”

Electrical activity of deep traps in high resistivity CdTe: Spectroscopic characterization

“…Only two transitions are detected, associated with transitions E V + 0.66 eV and E C − 0.78 eV, respectively, which can be assigned to the midgap trap accounting for the material's semi-insulating properties. 4,25 It is noteworthy that SPS identifies and distinguishes between transitions to both the conduction and the valence band. In the case of midgap traps, which often behave as recombination centers, this feature allows us to distinguish between transitions from the midgap center and either one of the bands.…”