On the investigation of electronic defect states in ZnO thin films by space charge spectroscopy with optical excitation

Abstract

Electronic defect states in a n-type conducting zinc oxide thin film sample were investigated by means of space charge spectroscopy focussing on levels in the midgap region as well as on hole traps. To overcome the experimental difficulties arising from the wide bandgap and the lack of p-type conduction, optical excitation was employed to measure the emission of trapped charge carriers from these levels. Therefore – besides deep-level transient spectroscopy measurements – photo-capacitance, optically chopped photo-current, minority carrier transient spectroscopy, and optical capacitance–voltage experiments were conducted. In doing so, a midgap level labelled T4, and hole traps labelled TH1 and TH2 were detected. In the case of T4 and TH1 the photo-ionisation cross-section spectra were determined.

Introduction

Undoped zinc oxide (ZnO) is an n-type conducting semiconductor exhibiting a wide bandgap of ${\mathcal{E}}_{\mathsf{g}}\approx 3.3\mathrm{eV}$ at room-temperature [1]. Electronic defect states in this material have been studied since the late 1940s [2], [3], [4], [5]. Extensive knowledge especially on shallow donors was gained from temperature-dependent Hall effect measurements. The observation of green luminescence and photo-conductivity were first hints on defect levels in the midgap [6].

Space charge spectroscopic methods are highly sensitive tools for the investigation of electronic states introduced into the bandgap of semiconductors by defects [7]. First thermal admittance spectroscopy (TAS) and deep-level transient spectroscopy (DLTS) measurements were conducted on ZnO varistors in the 1980s [8], [9], [10], [11]. In the 1990s ZnO single crystals became commercially available, different techniques to grow n-ZnO thin films were established and the quality of Schottky contacts on these materials was enhanced [12], [13], [14], [15], [16], [17]. This promoted the application of space charge spectroscopy and a set of deep-levels in the upper third of the ZnO bandgap has been detected [18], [19], [20], [21], [22]. However, reports on midgap levels and shallow acceptor states are scarce. The main reasons are the large bandgap and the difficulties in producing p-type conducting ZnO. The problem arising from the wide bandgap is, that mostly only the thermal emission of trapped charge carriers is measured in space charge spectroscopy experiments. The thermal emission rate of a trapped charge carrier obeys [7]

$$e_{\mathsf{n}\text{,}\mathsf{p}}^{\mathsf{th}}(T)\propto T^2{\sigma }_{\mathsf{n}\text{,}\mathsf{p}}^{\infty }\exp \left(-\frac{{\mathcal{E}}_{\mathsf{a}}}{k_{\mathrm{B}}T}\right)\text{,}$$

where T is the temperature, k_B is Boltzmann’s constant, ${\mathcal{E}}_{\mathsf{a}}$ the thermal activation energy for the emission process, and ${\sigma }_{\mathsf{n}\text{,}\mathsf{p}}^{\infty }$ denotes the high-temperature limit for the capture cross-section of either electrons or holes. For typical capture cross-sections, ${\sigma }_{\mathsf{n}\text{,}\mathsf{p}}^{\infty }<{10}^{-12}{\mathrm{cm}}^2\text{,}{e}_{\mathsf{n}\text{,}\mathsf{p}}^{\mathsf{th}}$ of a trap with ${\mathcal{E}}_{\mathsf{a}}>1\mathrm{eV}$ is too low to be conveniently measured at temperatures below where degradation of the Schottky contacts sets in. Levels deep in the bandgap are therefore undetectable in these experiments. Furthermore, the detection of hole traps with electronic states in the vicinity of the valence band edge is difficult using Schottky contacts on n-ZnO, since the injection of holes from the metal into the space charge region is not possible for the reported Schottky barrier heights of ≈1 eV [14]. Data on defect states in the vicinity of the valence band edge has mostly been gained from Hall-effect measurements on p-ZnO, e.g. [23], or using space charge regions at pn-junctions [24], [25].

The aim of this paper is to demonstrate, how space charge spectroscopy can be applied to study midgap levels and hole traps in n-ZnO using Schottky contacts. This can be achieved, employing optical excitation. Two approaches are possible: The first is to illuminate the space charge region with photon energies larger than the bandgap, $h\nu >{\mathcal{E}}_{\mathsf{g}}$, which generates electron–hole pairs, and to measure the capture and thermal emission of the photo-generated holes by defect states. Experiments of this type have been demonstrated by Polyakov et al. [26] who detected hole traps in proton-irradiated ZnO single crystals using DLTS with optical injection pulses. For thin film samples Brunwin et al. [27] suggested an improved variant of this technique called minority carrier transient spectroscopy (MCTS). In Section 3.2 some experimental aspects of MCTS are outlined in brief and its suitability to study hole traps in ZnO thin film samples is demonstrated.

The second approach is to measure the photo-ionisation of defect states by space charge spectroscopy. A set of experiments was proposed by Sah et al. [28]. The rate for the optical emission of a trapped charge carrier into either the conduction or the valence band is given by

$$e_{\mathsf{n}\text{,}\mathsf{p}}^{\mathsf{o}}({\Phi }_{\mathsf{ph}}(h\nu ))={\sigma }_{\mathsf{n}\text{,}\mathsf{p}}^{\mathsf{o}}(h\nu ){\Phi }_{\mathsf{ph}}(h\nu )\text{.}$$

Φ_ph denotes the flux of monochromatic photons of energy hν. ${\sigma }_{\mathsf{n}\text{,}\mathsf{p}}^{\mathsf{o}}(h\nu )$ is the photo-ionisation cross-section spectrum of the particular defect state. Since hν can be tuned, in principle every defect state in the bandgap is detectable in case it exhibits a sufficiently large ${\sigma }_{\mathsf{n}\text{,}\mathsf{p}}^{\mathsf{o}}$.

We previously demonstrated the suitability of photo-capacitance and photo-current measurements [29], [30], optical DLTS [31], and optical capacitance–voltage spectroscopy (OCV) [32] for the investigation of electronic defect states in ZnO thin films. In this work we set out to scan the entire bandgap of a ZnO thin film sample for electronic defect states using space charge spectroscopic methods. Thereby the focus is on the midgap region and the vicinity of the valence band edge. It is shown that electronic defect states that are hidden in experiments in which merely the thermal ionisation is measured can easily be traced if optical excitation is employed. While from TAS and DLTS measurements often only ${\mathcal{E}}_{\mathsf{a}}$ and ${\sigma }_{\mathsf{n}}^{\infty }$ are gained, further electronic properties like the photo-ionisation cross-section spectra can be measured in the optical experiments.

Last but not least we intend to give a guideline for space charge spectroscopy experiments on wide-bandgap semiconductors in general. Often these also exhibit a doping asymmetry and thus the problems occurring in defect studies are similar to those described for ZnO.