Detailed characterization of deep centers in CdTe: Photoionization and thermal ionization properties

Abstract

Electronic properties of four defect centers (designated as E7, E8, I, and IV) in A1-doped n-CdTe single crystals annealed under various Cd vapor pressures ( pCd) have been investigated by a combined use of photocapacitance, deep level transient spectroscopy (DLTS), and thermally stimulated capacitance techniques. Electron trap E7 at Ec−0.68 eV, a dominant deep defect center in samples quenched after annealing under high pCd, has shown a strong interaction with the lattice vibrations (S≳30, S: electron-phonon coupling parameter). The photoionization threshold energy for electrons E0n for the purely electronic transition has been estimated to be larger than 1.30 eV. Temperature dependence of the capture cross section for electrons σtn has been found to be very weak. The configuration coordinate diagram for E7 has suggested that E7 will nonradiatively capture the free electron. On the other hand, recombination center E8 at Ec−0.74 eV, a dominant deep defect center in samples annealed under low pCd, has shown only a negligible interaction with the lattice vibrations (S∼4) and has the photoionization threshold energies for electrons E0n ( = 0.80 eV) and for holes E0p ( = 0.95 eV) close to the thermal ionization energies for electrons Etn ( = 0.74 eV) and for holes Etp ( = 0.83 eV), respectively. The configuration coordinate diagram for E8 has revealed that E8 can radiatively capture the free electron and the free hole and can emit photons of ∼0.66 and ∼0.75 eV, respectively. E7 and E8 have been proposed to be identical with the levels previously reported as the doubly ionized Cd interstitial and the doubly ionized Cd vacancy, respectively. As to true origins of these centers, the data obtained in this study have suggested that both E7 and E8 may be donorlike complex centers including native defects, or impurities whose solubilities or stable sites in the lattice are strongly dependent on pCd. Hole trap IV at Ev +0.48 eV, which gradually decreases in concentration with decreasing pCd, has shown a weak interaction with the lattice vibrations (S∼6.7). It has been revealed that level IV can capture the free electron with emission of the photon of ∼1.0 eV. This level has been attributed to an impurity whose solubility in the crystal is moderately dependent on pCd. Electron trap I, which exists only in samples quenched after annealing under the highest pCd and have an electron-repelling barrier, has shown a very strong interaction with the lattice vibrations.