Characterization of point defects in ZnO thin films by optical deep level transient spectroscopy

Abstract

We have applied a special variant of optical DLTS to deep levels found in ZnO thin films. The optical emission rates of charge carriers from deep levels into a band have been measured over a range of photon energies from 0.7 to 3.0 eV and photoionization cross-section spectra calculated from this data. Two of the defects -among these the well-known E3 -showed no optical emission in the investigated range of photon energies. The experimental photo cross-section spectra of two optically active defects were compared and discussed using a model [Chantre, Vincent, and Bois, Phys. Rev. B 23(10), 5335 (1981)] that yields information about the band structure and defect properties. DLTS signal simulations were compared to the experimental data for the case that E4 has two states within the bandgap. These simulations revealed that a two-state model where both states independently allow electron emission is in accordance with the experimental data. The thermal DLTS peak of E4 was observed to be broadened and has been simulated under the assumption of a distribution of the carrier capture cross-section. The agreement between the simulation and the measured DLTS peak possibly explains why one of the photocross section spectra appears more broadened than can be simulated by the model cited above.

On the T2 trap in zinc oxide thin films

“…In the meantime we found that annealing the samples can also significantly increase the T2 concentration whereupon the T2 generation is more efficient the lower the oxygen partial pressure is. In a second study we measured the T2 photo-ionisation cross-section by ODLTS [23]. It was found that T2 can be photo-ionised with photon energies above %700 meV.…”

“…A relation among these levels is supported by the fact that the published data pairs (E a , s The concentration of T2 in ZnO samples is often up to 10 16 cm À3 and does therefore significantly influence the electron concentration. Since this level can furthermore be photo-ionised [23], it can be expected that the performance of ZnO-based opto-electronic devices will, amongst other things, strongly depend on the T2 concentration. A profound understanding of formation and properties of T2 is therefore necessary.…”

“…The knowledge on acceptor states in the vicinity of the valence band edge is scarce, see for example [13][14][15][16]. Junction capacitance techniques like deep-level transient spectroscopy (DLTS) [7,11,[15][16][17][18][19][20][21][22], deep-level optical spectroscopy (DLOS) [15], optical DLTS (ODLTS) [23] and photo-capacitance [16,22] have been used to study the properties of deep-levels in the zinc oxide band-gap. In doing so, knowledge on deep-levels was gained especially in the upper third of the zinc oxide band-gap.…”

Introduction and annealing of primary defects in proton‐bombarded n‐GaN

“…The digital correlation of the capacitance transients with the correlation function used by Ellguth et al [28] yielded the DLTS spectra shown in Fig. 6a.…”

A study of the T2 defect and the emission properties of the E3 deep level in annealed melt grown ZnO single crystals

“…Schmidt et al 9 have demonstrated the photo-ionisation of a defect level T2 observed in Pulsed Laser deposited grown ZnO samples using optical deep level transient spectroscopy (ODLTS) while Ellguth et al 10 showed the photoionisation of the T2 and E4 defects in PLD ZnO thin films also using ODLTS. Such observation has not been reported in bulk single crystal ZnO and neither has T2 been observed in as-grown Cermet single crystal ZnO.…”