Energetics and electronic structure of stacking faults in ZnO Yanfa Yan, G. M. Dalpian, M. M. Al-Jassim, and Su-Huai Wei National Renewable Energy Laboratory, Golden, Colorado 80401, USA (Received 7 May 2004; published 19 November 2004) The energetics and electronic structures of basal-plane stacking faults in wurtzite (WZ) ZnO are studied using first-principles density-functional total energy calculations. All the basal-plane stacking faults are found to have very low formations energies. They also introduce a downward shift at the conduction-band minimum (CBM). However, plane-averaged charge densities of the CBM state reveal that the CBM states are not very localized, indicating that these stacking faults should be electronically inert. The high concentration of these stacking faults can result in embedded zinc-blende (ZB) ZnO surrounded by WZ materials. The WZ/ZB interface exhibits a type-II lineup with DE V < 0.037 eV and DE C < 0.147 eV. DOI: 10.1103/PhysRevB.70.193206 PACS number(s): 61.72.Nn, 61.72.Yx, 61.72.Ji ZnO has long been recognized as a useful material for optically transparent conducting layers in displays and pho- tovoltaic devices. 1,2 Recently, it has attracted more attention because, like other wide-band-gap II-VI semiconductors, it could be an important material for next-generation short- wavelength optoelectronic devices such as low-cost light- emitting diodes (LEDs) and lasers, transparent p-n junctions, large-area flat-panel displays, and solar cells. 3–7 So far, most ZnO thin films are grown on mismatched substrates such as Al 2 O 3 or SiC and contain a high density of extended defects. 8 Extended defects are known to play an important role in electronic and mechanical properties of semiconduc- tors. For example, these defects may introduce electrically active energy levels in the energy gap. 9,10 In that case, the quantum efficiencies and device lifetime can be affected. Thus, it is important to know whether the extended defects are active or inert in ZnO thin films. So far, only the effects of inversion domain boundaries have been investigated. 11 In II–VI semiconductor compounds, basal-plane stacking faults are one of the main types of extended defects. High- resolution transmission electron microscopy has observed such stacking faults even in ZnO films epitaxially grown on ZnO substrates. 12 In this paper, we present first-principles total-energy calculations on the atomic and electronic struc- tures and formation energies of basal-plane stacking faults in WZ ZnO. We find that all basal-plane stacking faults have very low formation energies. The electronic structure calcu- lations reveal that the stacking faults introduce a downward shift at the conduction band minimum (CBM). However, the CBM states are not very localized, indicating that the stack- ing faults are electronically inert. The high concentration of these stacking faults can result in embedded ZB ZnO sur- rounded by WZ materials. We find that the WZ/ZB interface exhibits a type-II lineup with DE V < 0.037 eV and DE C < 0.147 eV. ZnO normally possesses a WZ structure, which can be described by the stacking of close-packed double layers of (0001) planes in the [0001] direction. The normal, perfect stacking sequence is …AaBbAaBb… . The perfect ZB structure can be described by the stacking sequence of …AaBbCcAaBbCc… . Here each letter represents a stacking plane. The upper-case and lower-case letters indicate Zn and O planes, respectively. The letters Aa, Bb, and Cc indicate three possible projected positions of the atoms. In the WZ structure, the two neighboring planes of every stacking plane are at the same position. In this case, Zn-O bonds in each stacking plane are called hexagonal bonds. In the ZB struc- ture, the two neighboring planes of every stacking plane are at different positions. In this case, Zn-O bonds are called cubic bonds. The WZ structure contains only hexagonal bonds, whereas the ZB structure contains cubic bonds only. A mistake induced to the perfect WZ stacking sequence will result in a basal-plane stacking fault in WZ ZnO. We study four types of basal-plane stacking faults proposed by Stampfl and Van de Walle 13 for stacking faults in WZ III-V nitrides. Figures 1(a)–1(c) show the structures for the so-called type-I, type-II, and type-III stacking faults. The type-I stacking faults contain one violation of the stacking rule, resulting in a stacking sequence: …AaBbAaBb u CcBbCcBb… . The symbol “u” indicates the position where the violation of the stacking rule starts. This stacking fault introduces one cubic bond in the stacking se- quence, as indicated by the white arrow. The type-II stacking faults contain two violations of the stacking rule, giving a stacking sequence as …AaBbAaBb u CcAaCcAa… . It intro- duces two connected cubic bonds, as indicated by two white arrows. The type-III stacking faults contain a double-layer at the wrong position, leading to a stacking sequence as …AaBbAaBbuCcuBbAaBb… . The two symbols “u” indicate the “wrong” double layer. It introduces two but separated cubic bonds at the interface. Figure 1(d) shows the structure of the extrinsic stacking faults, which contain an additional double layer inserted in the midst of the normal stacking sequence, resulting in a stacking sequence as …AaBbAaBbuCcuAaBbAaBb… . The two symbols “u” indi- cate the additional double layer. It introduces three connected cubic bonds. Our calculations on the total energy and electronic struc- ture of stacking faults are based on the density-functional theory, using the Vienna ab initio Simulation Package (VASP). 14 We used the local density approximation for the exchange correlation, and ultrasoft Vanderbilt-type pseudopotentials 15 as supplied by Kresse and Hafner. 16 Be- cause the formation energies for stacking faults are usually very small in most II-VI semiconductors, care must be taken to obtain accurate results. The Zn 3d electrons were treated PHYSICAL REVIEW B 70, 193206 (2004) 1098-0121/2004/70(19)/193206(4)/$22.50 ©2004 The American Physical Society 70 193206-1