Unintentional N-type Conductivity Research Articles

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 ε(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)].

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 attributed to nitrogen vacancies, but is due to unintentional incorporation of donor impurities. Native point defects may play a role in compensation and in phenomena such as the yellow luminescence, which can be attributed to gallium vacancies. In the section on impurities, specific attention will be focused on dopants. Oxygen, which is commonly present as a contaminant, is a shallow donor in GaN but becomes a deep level in AlGaN due to a DX transition. Magnesium is almost universally used as the p-type dopant, but hole concentrations are still limited. Reasons for this behavior are discussed, and alternative acceptors are examined. Hydrogen plays an important role in p-type GaN, and the mechanisms that underlie its behavior are explained. Incorporating hydrogen along with acceptors is an example of codoping; a critical discussion of codoping is presented. Most of the information available to date for defects and impurities in nitrides has been generated for GaN, but we will also discuss AlN and InN where appropriate. We conclude by summarizing the main points and looking towards the future.

Strategies for Controlling the Conductivity of Wide-Band-Gap Semiconductors

Wide-band-gap semiconductors have numerous applications in electronic and optoelectronic devices. Progress is currently hampered by a lack of control over electrical conductivity: for instance, 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. An investigation of likely donor impurities reveals that hydrogen acts as a shallow donor. In contrast, hydrogen acts as an amphoteric impurity in most other semiconductors; calculations for hydrogen in ZnSe are included here to highlight this difference. Experimental results are discussed in light of these new insights.

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.

Native defects and impurities in InN: First-principles studies using the local-density approximation and self-interaction and relaxation-corrected pseudopotentials

We perform first-principles density-functional calculations to investigate the electronic and atomic structure and formation energies of native defects and selected impurities ~O, Si, and Mg! in InN. For p-type material, the nitrogen vacancy has the lowest formation energy. In n-type material all defect formation energies are high. We discuss the effect of the band-gap underestimate in density functional theory ~DFT!, and compare the defect electronic structure obtained using DFT ~in the local-density approximation, LDA! with a recently developed self-interaction and relaxation-corrected ~SIRC! pseudopotential treatment. The SIRC calculations affect the positions of some of the defect states in the band gap, but the general conclusions obtained from the standard DFT-LDA calculations remain valid. Indium nitride is the least studied of the group-III-nitride materials, which are currently under intense investigation. Bulk InN is difficult to prepare due to its low thermal stability; 1 reliable experimental information about the properties of InN is therefore scarce. Indium-containing nitride alloys are an important constituent in devices: for example, the active layer in short-wavelength light-emitting diodes and laser diodes usually consists of In xGa12xN. Not intentionally doped InN has often been found to have very high electron densities—an observation similar to GaN before better doping control of that material was achieved. The unintentional n-type conductivity of InN has been attributed to the nitrogen vacancy (VN) or to the nitrogen antisite. 2 In order to control the material properties and ultimately the device characteristics, an understanding of native defects and impurities in the III nitrides and their alloys is essential. The calculations reported here show that neither vacancies nor antisites can explain the observed n-type conductivity of InN. We have therefore examined oxygen and silicon impurities, finding that they act as donors and that they can easily be incorporated during growth. First-principles calculations based on density functional theory ~DFT! within the local density approximation ~LDA! have produced important information about defects and impurities in semiconductors in general, and the nitrides in particular. 3,4 It is well known, however, that DFT-LDA produces band gaps significantly smaller than experiment. 5 Defects can introduce levels in the band gap; when these levels are occupied with electrons, they contribute to the total energy of the system. If the energetic position of the defect levels is incorrect due to the band-gap error, the resulting total energy may also be affected. The band-gap error results largely from a discontinuity in the exchange-correlation potential upon addition of an extra electron. 6 This discontinuity is inherent to the Kohn-Sham treatment of DFT; indeed, we have found that use of the generalized gradient approximation ~GGA! offers no improvement over LDA with respect to the band-gap problem. 7

Theory of doping and defects in III–V nitrides

Doping problems in GaN and in AlGaN alloys are addressed on the basis of state-of-the-art first-principles calculations. For n-type doping we find that nitrogen vacancies are too high in energy to be incorporated during growth, but silicon and oxygen readily form donors. The properties of oxygen, including DX-center formation, support it as the main cause of unintentional n-type conductivity. For p-type doping we find that the solubility of Mg is the main factor limiting the hole concentration in GaN. We discuss the beneficial effects of hydrogen during acceptor doping. Compensation of acceptors by nitrogen vacancies may occur, becoming increasingly severe as x increases in AlxGa1−xN alloys.

Ab initio study of oxygen point defects in GaAs, GaN, and AlN.

We have studied oxygen point defects with the plane-wave pseudopotential method in GaAs, GaN, and AlN. The calculations demonstrate a qualitatively different behavior of oxygen impurities in these materials. ${\mathrm{O}}_{\mathrm{As}}$ in GaAs acts as a deep center with an off-center displacement and negative-U behavior, in agreement with the experimental data. ${\mathrm{O}}_{\mathrm{N}}$ in GaN is found to be a shallow donor with a low formation energy, and is suggested to act as a partial source for the unintentional n-type conductivity commonly observed in GaN. O in AlN is also found to easily substitute for N, which is consistent with the experimentally observed large oxygen concentrations in AlN. However, ${\mathrm{O}}_{\mathrm{N}}$ in AlN is shown to be a deep center due to the wide band gap, in contrast with ${\mathrm{O}}_{\mathrm{N}}$ in GaN. Our calculations thus predict that isolated oxygen acts as a DX-type center in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$N alloys. Results for other oxygen point defect configurations and for the dominant native defects are also presented. \textcopyright{} 1996 The American Physical Society.