Why shallow defect levels alone do not cause high resistivity in CdTe

Abstract

It is usually assumed that deep defect levels are responsible for the high resistivity in detector-grade CdTe, however, it has been recently reported that shallow defects alone can explain high resistivity. In order to resolve this contradiction we analyze different high-temperature compensation regimes and we particularly show that donor-acceptor self-compensation is not a sufficient condition for high resistivity. We also analyze the dependence of the Fermi level on shallow donor concentration at both high temperature and room temperature using analytical solution of the charge neutrality equation and the graphical method based on the formation energy diagrams. We derive the limits of the high-temperature Fermi level values that lead to the high resistivity at room temperature in a system with shallow defect levels only and find that it is theoretically possible to obtain high resistivity using only shallow defects, but only under an extremely narrow range of physical parameters that is unlikely to occur in practice. Finally, we show that the transition levels of cadmium vacancy acceptor are not deep enough to provide high resistivity.

Energetics and Electronic Properties of Interstitial Chlorine in CdTe

“…In absence of native defects like V Cd , the lower formation energies of donor‐acceptor systems suggest a self‐compensation between ( BC ) and ( T 1 ), and between ( Spl ) and ( T 1 ), with (+/−) transition states at VBM + 0.65 and VBM + 0.75 eV, respectively. The self‐compensation is understood as the response of an active impurity system to the introduction of another active impurity, which tend to compensate their electrical activity through the formation of opposite charged defects . Self‐compensation by the same impurity has been also suggested in P‐doped CdTe …”

“…In addition of good carrier transport properties, high‐performance radiation detectors also require high electrical resistivity for a low leakage current . The latter property may be achieved by controlled compensation between donors and acceptors, but it is very challenging in practice . In the case of CdTe, native defects, grain boundaries, and impurities may introduce electron or hole traps, decreasing the concentration and mobility of free carriers.…”

“…Chlorine doping is another well‐established route to obtain high resistivity in CdTe . Studies on single‐crystal CdTe:Cl indicate a hole trap near midgap .…”

“…An explanation proposed by Biswas et al suggests that the observed high resistivity in CdTe:Cl is due to compensation between shallow donors (Cl Te ) and shallow acceptors ( V Cd ). However, it was argued by Krasikov et al that shallow defect levels alone, although theoretically possible, are unlikely to cause high resistivity.…”

“…[22][23][24] Chlorine doping is another well-established route to obtain high resistivity in CdTe. [13,14,[25][26][27] Studies on single-crystal CdTe:Cl indicate a hole trap near midgap. [28][29][30] However, the compensation mechanism responsible for the observed semi-insulating condition is still debated and remains as an open issue.…”