Chris G. Van de Walle

Fundamentals of zinc oxide as a semiconductor

In the past ten years we have witnessed a revival of, and subsequent rapid expansion in, the research on zinc oxide (ZnO) as a semiconductor. Being initially considered as a substrate for GaN and related alloys, the availability of high-quality large bulk single crystals, the strong luminescence demonstrated in optically pumped lasers and the prospects of gaining control over its electrical conductivity have led a large number of groups to turn their research for electronic and photonic devices to ZnO in its own right. The high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy make ZnO suitable for a wide range of devices, including transparent thin-film transistors, photodetectors, light-emitting diodes and laser diodes that operate in the blue and ultraviolet region of the spectrum. In spite of the recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. While a number of research groups have reported achieving p-type ZnO, there are still problems concerning the reproducibility of the results and the stability of the p-type conductivity. Even the cause of the commonly observed unintentional n-type conductivity in as-grown ZnO is still under debate. One approach to address these issues consists of growing high-quality single crystalline bulk and thin films in which the concentrations of impurities and intrinsic defects are controlled. In this review we discuss the status of ZnO as a semiconductor. We first discuss the growth of bulk and epitaxial films, growth conditions and their influence on the incorporation of native defects and impurities. We then present the theory of doping and native defects in ZnO based on density-functional calculations, discussing the stability and electronic structure of native point defects and impurities and their influence on the electrical conductivity and optical properties of ZnO. We pay special attention to the possible causes of the unintentional n-type conductivity, emphasize the role of impurities, critically review the current status of p-type doping and address possible routes to controlling the electrical conductivity in ZnO. Finally, we discuss band-gap engineering using MgZnO and CdZnO alloys.

First-principles calculations for defects and impurities: Applications to III-nitrides

First-principles calculations have evolved from mere aids in explaining and supporting experiments to powerful tools for predicting new materials and their properties. In the first part of this review we describe the state-of-the-art computational methodology for calculating the structure and energetics of point defects and impurities in semiconductors. We will pay particular attention to computational aspects which are unique to defects or impurities, such as how to deal with charge states and how to describe and interpret transition levels. In the second part of the review we will illustrate these capabilities with examples for defects and impurities in nitride semiconductors. Point defects have traditionally been considered to play a major role in wide-band-gap semiconductors, and first-principles calculations have been particularly helpful in elucidating the issues. Specifically, calculations have shown that the unintentional n-type conductivity that has often been observed in as-grown GaN cannot be a...

Oxygen vacancies in ZnO

The electronic properties of ZnO have traditionally been explained by invoking intrinsic defects. In particular, the frequently observed unintentional n-type conductivity has often been attributed to oxygen vacancies. We report first-principles calculations showing that the oxygen vacancy VO is not a shallow donor, but has a deep e(2+∕0) level at ∼1.0eV below the conduction band. The negative-U behavior that causes the 1+charge state to be unstable is associated with large local lattice relaxations. We present a detailed configuration coordinate diagram, which allows us to provide a detailed interpretation of recently reported ODEPR (optically detected electron paramagnetic resonance) measurements [L. S. Vlasenko and G. D. Watkins, Phys. Rev. B 71, 125210 (2005)].

Gallium vacancies and the yellow luminescence in GaN

We have investigated native defects and native defect‐impurity complexes as candidate sources for the yellow luminescence in GaN. Using state‐of‐the‐art first‐principles calculations, we find strong evidence that the Ga vacancy (VGa) is responsible. The dependence of the VGa formation energy on Fermi level explains why the yellow luminescence is observed only in n‐type GaN. The VGa defect level is a deep acceptor state, consistent with recent pressure experiments. Finally we show that the formation of VGa is enhanced by the creation of complexes between VGa and donor impurities.

Indirect Auger recombination as a cause of efficiency droop in nitride light-emitting diodes

InGaN-based light-emitting diodes(LEDs) exhibit a significant efficiency loss (droop) when operating at high injected carrier densities, the origin of which remains an open issue. Using atomistic first-principles calculations, we show that this efficiency droop is caused by indirect Auger recombination, mediated by electron-phonon coupling and alloy scattering. By identifying the origin of the droop, our results provide a guide to addressing the efficiency issues in nitride LEDs and the development of efficient solid-state lighting.

Heterojunction band offset engineering

Abstract Control of band discontinuities in semiconductor heterostructures may introduce a new important degree of freedom in the design of heterojunction devices and allow independent optimization of carrier injection, carrier confinement and ionization thresholds in high speed and optoelectronic devices. We will review recently proposed methods to microscopically control heterojunction parameters by means of local interface dipoles introduced at the heterointerface during growth. A parallel survey of new theoretical models of semiconductor heterojunctions will illustrate our newfound ability to derive from first principles rules of heterojunction behavior. The combination of new empirical methods and theoretical models is establishing the new area of heterojunction engineering in surface and interface science.

ROLE OF HYDROGEN IN DOPING OF GAN

We investigate the interactions between hydrogen and dopant impurities in GaN, using state‐of‐the‐art first‐principles calculations. Our results for energetics and migration reveal a fundamental difference in the behavior of hydrogen between p‐type and n‐type material; in particular, we explain why hydrogen concentrations in n‐type GaN are low, and why hydrogen has a beneficial effect on acceptor incorporation in p‐type GaN. Our results identify the conditions under which hydrogen can be used to control doping in semiconductors in general.

Band bowing and band alignment in InGaN alloys

We use density functional theory calculations with the HSE06 hybrid exchange-correlation functional to investigate InGaN alloys and accurately determine band gaps and band alignments. We find a strong band-gap bowing at low In content. Band positions on an absolute energy scale are determined from surface calculations. The resulting GaN/InN valence-band offset is 0.62 eV. The dependence of InGaN valence-band alignment on In content is found to be almost linear. Based on the values of band gaps and band alignments, we conclude that InGaN fulfills the requirements for a photoelectrochemical electrode for In contents up to 50%.

Defect analysis and engineering in ZnO

Zinc oxide has numerous applications in electronic and optoelectronic devices. Progress is currently hampered by a lack of control over electrical conductivity: ZnO is typically n-type conductive, the cause of which has been widely debated. A first-principles investigation, based on density functional theory, shows that native defects are unlikely to be the cause of the unintentional n-type conductivity. Detailed results for the oxygen vacancy show that it is a deep donor, and that its paramagnetic state is metastable. An investigation of likely donor impurities reveals that hydrogen acts as a shallow donor. Experimental results are discussed in the light of these new insights.

Microscopic origins of surface states on nitride surfaces

We report a systematic and comprehensive computational study of the electronic structure of GaN and InN surfaces in various orientations, including the polar c plane, as well as the nonpolar a and m planes. Surface band structures and density-of-states plots show the energetic position of surface states, and by correlating the electronic structure with atomistic information we are able to identify the microscopic origins of each of these states. Fermi-level pinning positions are identified, depending on surface stoichiometry and surface polarity. For polar InN we find that all the surface states are located above the conduction-band minimum, and explain the source of the intrinsic electron accumulation that has been universally observed on InN surfaces.