Cl-doping of Te-rich CdTe: Complex formation, self-compensation and self-purification from first principles

Abstract

The coexistence in Te-rich CdTe of substitutional Cl-dopants, ClTe, which act as donors, and Cd vacancies, VCd−1, which act as electron traps, was studied from first principles utilising the HSE06 hybrid functional. We find ClTe to preferably bind to VCd−1 and to form an acceptor complex, (ClTe–VCd)−1. The complex has a (0,-1) charge transfer level close to the valence band and shows no trap state (deep level) in the band gap. During the complex formation, the defect state of VCd−1 is annihilated and leaves the Cl-doped CdTe bandgap without any trap states (self-purification). We calculate Cl-doped CdTe to be semi-insulating with a Fermi energy close to midgap. We calculate the formation energy of the complex to be sufficiently low to allow for spontanous defect formation upon Cl-doping (self-compensation). In addition, we quantitatively analyse the geometries, DOS, binding energies and formation energies of the (ClTe–VCd) complexes.

Characterization of point defects in CdTe by positron annihilation spectroscopy

“…2 The Cl Te nearest neighbor is displaced away from the vacancy Cd site by 5.0%, while the three Te ions relax toward the site by 1.3% and the slight increase in the lifetime obtained (Table I), compared to the unrelaxed V Cd , is consistent with these relaxations. The recently calculated structure for the neutral charge state of the Te vacancy (Fig.…”

“…Characterization of point defects in CdTe by positron annihilation spectroscopy M. R. M. Elsharkawy, 1,2 G. S. Kanda, 1 E. E. Abdel-Hady, 2 Positron lifetime measurements on CdTe 0.15% Zn-doped by weight are presented, trapping to monovacancy defects is observed. At low temperatures, localization at shallow binding energy positron traps dominates.…”

“…27,30 Recent firstprinciples calculations of point defects in CdTe have provided further insight on local structure and stability. [1][2][3][4][5]31 Possible relaxed geometries of the two stable charge states, À1 and À2, of the Cd vacancy, 1,3 and for the À1 charge state of Cl-donor A-center, V Cd with a Cl Te nearest neighbor, 2 have been reported. The other point defect considered of particular importance in these materials is the Te antisite; this has also been studied and relaxed structures for the three charge states, Te þ2…”

Direct Thermal Growth of Large Scale Cl-doped CdTe Film for Low Voltage High Resolution X-ray Image Sensor

“…Although some of doped Cl atoms are located in grain boundaries, many Cl atoms would substitute Te atoms as shown in a schematic crystal lattice (inset of Fig. 3b ), donating electrons whose concentration could be in the order of ~10 15 cm −3 29 – 32 . As a result, those number electrons would compensate most of the holes in principle, depleting the charge carriers.…”

“…CdTe has cubic Zinc Blende structure and maintains p-type conduction after growth in general 27 , 28 . For low voltage operation, we applied Cl doping at all stages of CdTe film deposition to control or reduce the hole concentration of p-type CdTe, which then plays as Schottky diode material for Ti(or Al) electrode showing ohmic behavior for Au 29 – 32 . As a result, our CdTe Schottky diode/CMOS array demonstrates much higher resolution X-ray imaging in 7 × 9 cm 2 large scale as a direct conversion APS imager than that of CsI/CMOS array, an indirect conversion imager.…”