Theoretical Studies of ZnO and Related Mg x Zn 1-x O Alloy Band Structures
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ABSTRACT First principles calculations are carried out for ZnO, MgO and ZnMgO 2 in various crystal structures. The nature of the valence band ordering in ZnO is shown to depend strongly on the Zn3d band position. MgO in the wurtzitic form is found to gave an unusual 5-fold coordiated structure. The band gap dependence in the alloy system is found to be in fair agreement with experimental data and the band-offset is predicted to be type I. INTRODUCTION Recently, ZnO has attracted attention not only as a suitable closely lattice-matched substrate for GaN but also as a potentially useful active optoelectronic material in its own right [1, 2, 3, 4, 5] This raises new questions not only about the basic properties of ZnO but also about those of related materials which can be combined with ZnO in heteroepitaxial device structures. In fact, one would need a material with a higher band gap to use as barrier in order to make ZnO quantum wells. Ohtomo et al. [6] showed that it is possible to grow MgZnl- O alloy films with the wurtzite structure over a certain range of concentrations. This possibility was far from obvious because MgO has the rocksalt rather than the wurtzite structure. In this paper, we first re-examine the band structure of ZnO. Although this material has been studied extensively in the past, [7, 8, 9, 10] there are some remaining open questions about its band structure. Most notably, there has been a long-standing controversy over the nature of the valence band ordering. The question is: is it the usual F 9 above F 7 as proposed by Park et al. [13] or the inverse ordering as suggested by Thomas [12]. Recently, new results supporting Park
et al.'s point of view has been obtained by Reynolds et al. [14]. We show that this ordering depends crucially on the position of the Zn3d bands, which is not correctly obtained in the local density approximation (LDA). We also present results of our total energy results for ZnO in both wurtzite and zincblende structure. Next, we present results of first-principles calculations of the relative stability of the wurtzite, zincblende and rocksalt forms of MgO and obtain an estimate for the expected gap in wurtzitic MgO. We also present preliminary results for an ordered ZnMgO 2 structure, which allows us to obtain a first idea of the expected band gap dependence in this system. Using the dielectric midgap energy charge neutrality point model, [11] we obtain an estimate of the band-offset between ZnO and MgO. METHOD OF CALCULATION The computational method used in this work is the density functional theory in the local density approximation (LDA). The Kohn-Sham equations are solved using the full-potential linear muffin-tin orbital (LMTO) method developed by Methfessel [15]. Details of the computational procedure (sphere radii choice, angular momentum cut-off paramters, size of basis sets) are similar to those in recent work on GaN and related materials and were carefully checked for convergence
[17]. For the study of the alloys, we used zincblende derived structures as a model
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