Elsevier

Thin Solid Films

Volume 605, 30 April 2016, Pages 283-288
Thin Solid Films

The chemical states of As 3d in highly doped ZnO grown by Molecular Beam Epitaxy and annealed in different atmospheres

https://doi.org/10.1016/j.tsf.2015.09.016Get rights and content

Highlights

  • Arsenic-doped zinc oxide has been grown by molecular beam epitaxy.

  • The annealing atmosphere strongly affects the properties of ZnO:As thin films.

  • Three As-derived components have been observed by X-ray Photoelectron Spectroscopy.

  • Photoluminescence measurements confirm complex nature of As-acceptors.

Abstract

Arsenic doped ZnO films were grown by plasma assisted molecular beam epitaxy and post-growth annealed at 700 °C in oxygen, nitrogen or argon atmosphere. The high resolution X-ray photoelectron spectroscopy (XPS) studies of the ZnO:As films revealed that the As3d core level spectra is formed by three components located at about 41 eV, 44.5 eV and 45.5 eV below the Fermi level which we ascribe to AsO, AsZn‐2VZn and AsZn, respectively. The relative intensity of the three XPS contributions strongly depends on an annealing atmosphere, but in any case none of the contributions clearly dominates, which is a fingerprint of complicated nature of arsenic states in ZnO. This conclusion is also confirmed by the temperature dependent photoluminescence (PL) studies. Differences in the dominant PL peak positions and in their relative intensities are present and suggest different acceptor states in the examined samples.

Introduction

Zinc oxide (ZnO) has attracted a lot of scientific interest because of the 60 meV exciton binding energy, much larger than in other wide band gap semiconductors, like GaN (25 meV) or ZnSe (20 meV). Its wide band gap of 3.37 eV at room temperature is useful for various applications, such as transparent conductive films [1], photodetectors of ultraviolet UV light [2], [3], [4], diodes [5], [6], [7], [8] and light sources.

The necessary condition for applications of ZnO in opto- and microelectronics is good control of n- and p-type doping. Obtaining p-type ZnO is not an easy task, because undoped, “as-grown” zinc oxide usually reveals n-type conductivity at different growth methods and conditions. Persistent n-type doping used to be ascribed to native defects, such as O vacancies (VO) [9], Zn interstitials (Zni) [10] and incorporation of unintentional impurities such as hydrogen [11]. All these defects introduce deep or shallow donor levels which effectively prevent achieving p-type conductivity in ZnO.

Group V elements such as N, As or Sb substituting O atoms are natural candidates for acceptor dopants. However, theory predicts that substitutional group V atoms on oxygen sites introduce deep acceptor levels [12]. On the other hand, Ryu et al. [13] presented evidence that As can be a good acceptor in ZnO. They suggested that in ZnO:As two energy levels located at 110 meV and 170 meV above the valence band maximum are related to arsenic. These levels give rise to free electron-to-acceptor (FA) recombination at 3.322 eV and 3.273 eV. Ryu et al. [13] observed also photoluminescence at 3.359 eV, which they assigned to recombination of excitons bound to neutral acceptors (AoX), but they did not propose a model for the As-related acceptor. Theoretical approach to the problem of the structure of the As-acceptor in ZnO was undertaken by Limpijumnong and co-workers [14]. Their model predicts that As atoms do not occupy O sites (AsO) as one could expect, but may form acceptor-like complexes, such as AsZn‐2VZn with two spontaneously induced Zn vacancies in the next nearest neighbor (NNN) positions compensating the electrical charge of the AsZn atom [14]. There are also newer theoretical models considering even more complex As-centers in ZnO, such as AsZn‐3VZn [15]. Wahl et al. [16], [17] confirmed experimentally that the majority of the implanted As atoms were located on Zn sites, however, after thermal annealing a few percent of As went onto O sites. They also showed that in As implanted ZnO annealed in vacuum ~ 30% of As atoms locate interstitially close to tetrahedral positions.

Realization of p-ZnO with As doping has been demonstrated by several groups for layers obtained with different growth techniques, such as thermal oxidation of ZnTe deposited on GaAs [18], radio frequency sputtering [19], [20], pulsed laser deposition [21], hybrid beam epitaxy [13], and ion implantation [22]. Although p-type conductivity of ZnO:As was obtained, the location of As dopant in the lattice of ZnO as well as the local atomic configuration around As is still under debate and needs clarification. Identification of the chemical state of As would be very helpful for the understanding of the behavior of As dopant and effectiveness of p-type doping. Specific chemical states of arsenic may depend on many factors, including growth methods and post-growth thermal treatment conditions [19], [23].

X-ray Photoelectron Spectroscopy (XPS) was used to investigate chemical states of arsenic in ZnO. The high-resolution XPS spectra of the As3d core level in ZnO:As films were measured by a few groups of authors [24], [25] and some conclusions about the chemical environment around arsenic were proposed. In particular, it was suggested that the As-related complexes, such as AsZn‐2VZn, contribute to the XPS spectra of the As3d state at the binding energy (BE) of 44–45 eV [24], [26], [27], whereas the binding energy of AsO and AsZn are about 41 and 48 eV, respectively.

In this paper we use high-resolution XPS to investigate the chemical state of arsenic in ZnO:As grown by Molecular Beam Epitaxy (MBE). The aim of the paper is the determination of the influence of growth conditions and post-growth annealing of ZnO:As in different atmospheres on the contribution of the As3d state of As into the XPS spectra. We also present correlations of the XPS spectra, with optical properties of ZnO:As layers.

Section snippets

Experimental conditions

Epitaxial zinc oxide layers doped with arsenic were grown on commercial GaN/Al2O3 substrates by plasma assisted MBE. Knudsen cells were used as sources of arsenic and zinc and a radio frequency rf plasma cell as a source of oxygen. The power of the O2 source was 350–400 W and the growth temperature was about 500 °C. The substrates were chemically cleaned before the growth and then out gassed at 700 °C under high vacuum. After the growth the samples were annealed by the rapid thermal annealing

Photoluminescence measurements

In Fig. 1 the near band gap PL for undoped and arsenic-doped ZnO samples on GaN substrates are presented. The big difference in PL between these two samples is observed. In the undoped ZnO, the main sharp peaks located at 3.358 and at 3.366 eV are related to donor bound exciton transitions (DoX) and the low intensity peak at 3.33 eV is probably related to two electron satellites TES. At room temperature emission at about 3.29 eV related to free excitons is observed (Fig. 1a).

In arsenic-doped

Discussion

Changes of the PL spectra after annealing in different atmospheres reflect changes in local environment around As atoms. This fact is confirmed by the HR XPS spectra which clearly show that annealing modifies the character of As-related centers. This one-to-one correspondence allows us to assign the optical transitions at 3.3 and 3.2 eV to two acceptor centers due to As. The PL measurements as a function of temperature suggest different nature of the PL peaks observed at low temperature at 3.24

Conclusions

Arsenic-doped ZnO films were obtained on GaN substrates by molecular beam epitaxy method and annealed after growth in argon, oxygen, and nitrogen atmosphere. The XPS studies reveal different chemical states of the As dopant, which relative contribution depends on the annealing atmosphere. The high-resolution As3d XPS spectra reveal three coexisting chemical states of arsenic. One of them is supposed to be derived from AsZn–2VZn complexes, which are commonly accepted as responsible for effective

Acknowledgments

We would like to acknowledge the support by the NCN project DEC-2013/09/D/ST3/03750.

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