AVS2011 Session EM-WeA: Defects in Electronic Materials

The Mg_xZn_1-xO material system is very promising for ultraviolet emission and detection applications because of its direct band-gap, band-gap tunability, low lattice mismatch, and low polarization field. Using molecular beam epitaxy, a-plane 1.5 micron thick n-type Mg_xZn_1-xO films (x=0, 0.26, 0.36, 0.43, 0.52) were grown on r-plane sapphire. Semi-transparent 100 Å thick Au Schottky contacts were used for deep level optical spectroscopy measurements (DLOS). The Schottky diodes (SD) exhibited rectification with low leakage currents and barrier heights between 1.00-1.19 eV from Mg_0.52Zn_0.48O to ZnO. The frequency dependence of the capacitance caused by the high series resistance of the SDs required low frequency lock-in based capacitance measurements for accurate determination of capacitance. Lock-in 1kHz DLOS measurements revealed six deep levels in Mg_xZn_1-xO samples while only three deep levels were observed in ZnO. In all samples, the energy of the deep levels relative to the conduction or valence band was independent of Mg content. The ~Ec-1.4 eV, ~Ec-1.85 eV, and ~Ec-2.50 eV levels follow the conduction band while ~Ev+1.20 eV, ~Ev+0.60eV, and ~Ev+0.28 eV follow the valence band. Because of high concentration of two dominant deep levels, lighted capacitance-voltage measurements was applied on all the samples in order to obtain true values of the concentration of the traps. The concentration of the two dominant compensating centers at ~E_v+0.28 eV and ~E_v+0.60 eV are ~3x10¹⁶ cm^-3 and ~1x10¹⁶ cm^-3, respectively.

SrTiO₃ and related perovskite materials are increasingly being utilized in a wide range of electrical applications. Furthermore, SrTiO₃ is often used as a substrate for growth of thin film structures. Thus, knowledge of its defect structure and, in particular, their depth dependence is crucial for understanding their impact on conductivity and optical phenomena. In the present work we utilize the limited penetration depth of x-rays to study the near-surface properties of SrTiO₃ substrates and epilayers and compare the results to analogous measurements using electrons. For an incidence angle of 10 degrees, the x-ray penetration depth varies between 22 and 207 nm as the energy changes from 600 to 1500 eV. For electrons the mean penetration depth varies from 3 to 90 nm as the energy changes from 0.5 to 5 keV. Thus, by obtaining x-ray excited optical luminescence (XEOL) and cathodoluminescence (CL) spectra as a function of x-ray or electron energy, it is possible to probe the near-surface region with nm-scale resolution. We will present energy-dependent XEOL and CL data from several SrTiO₃ samples. The luminescence spectra show features that can be assigned to Ti³⁺ (~1.6 eV) and oxygen-related (1.9-2.9 eV) defects, but the relative intensities differ between XEOL and CL. We will discuss the results in terms of the respective mechanisms, previous CL studies [1,2] and theoretical models.

This work was performed at the Advanced Photon Source and was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357. We gratefully acknowledge

Ohio State University support from Army Research Office Grant

W911NF-10-1-0220

1. J. Zhang, et al, J. Vac. Sci. Technol. B 26, 1456 (2008).

2. J. Zhang, et al, Appl. Phys. Lett. 94, 092904 (2009).

The defect and surface properties of titania have been well-studied for its potential use in memory resistors and nanoelectronic gas sensors but the control of these properties in such fields has still not been fully realized. Such control of defect mobility and concentration in titania is essential for improving the operation of these technologies. Previous work in our research group has found that controlling the rutile (110) surface has opened up a new pathway for oxygen self-diffusion via an oxygen-interstitial mechanism in conditions where oxygen vacancies and titanium interstitials are thought to dominate. The present work models the detailed diffusion-reaction network of point defects in rutile to simulate interstitial-mediated self-diffusion of oxygen and to calculate a generation rate of bulk point defects at the surface. The model explicitly incorporates gas adsorption onto the (110) surface, interstitial generation at the surface/near surface, and diffusion of the oxygen in an interstitial-mediated mechanism through the bulk. In addition, the model employs techniques drawn from systems engineering to estimate the key kinetic parameters. Simulated diffusion profiles were compared to experimental profiles obtained by exposing annealing single-crystal rutile to isotopically labeled oxygen gas and then measuring isotopic oxygen concentration with secondary ion mass spectrometry. The simulated diffusion profiles matched experimental results, verifying that oxygen interstitials do indeed mediate oxygen self-diffusion in our experiment. More importantly, our model allows us to calculate the oxygen interstitial generation rate at the (110) surface, a first for titania.

The properties of polycrystalline graphene largely depend on the properties of the grain boundaries (GB) [1,2]. Classical molecular dynamics simulations coupled with density functional calculations of vacancies and self-interstitials at grain boundaries show that lower formation energies are induced by the local strain near the GB. The vacancies interact with each other as well as the interstitials. Vacancies and interstitials [3] can diffuse in graphene due to their low migration barrier. We investigated the time scale of vacancy and interstitial dynamics in pristine graphene and at grain boundaries at the distances where defect-defect and defect-GB interactions are found to be significant. GB-558 consists of pentagon and octagon pairs, whereas GB-575 is formed by repeated pentagon-heptagon pairs. We show that recombination of vacancies and interstitials can occur at grain boundaries resulting in efficient annealing of defects. The compression (stretch) for GB-558 is up to 3%, while in GB-575, the stretch reaches 9% at the C-C bonds between pentagon and heptagon or between hexagon and heptagon. We find that single vacancies can diffuse towards a grain boundary, attach to a pentagon, then merge into the grain boundary to release the compressive strain and cause local reconstruction. Molecular dynamics simulations show that if a single vacancy exits at 1nm from the grain boundary GB-558, it diffuses towards the grain boundary and attaches at the pentagon ring after 160 ps at 3000 K. It takes another 40 ps to merge into the grain boundary. Similar phenomena occur in GB-575. Stretched C-C bonds at the heptagon, on the other hand, accumulate interstitials. An interstitial that was initially positioned 7 Å above the graphene sheet, was adsorbed on the grain boundary. Once the vacancy coalesces into the grain boundary, the carbon adatom fills the reconstructed vacancy and recovers the original grain boundary structure. This recombination takes only ~ 0.5 ns at 2000 K. The results suggest a new mechanism of defect annealing with a two-step procedure, i.e. point defects (single vacancies and interstitials) diffuse and recombine locally at grain boundaries. This work was supported by DTRA Grant No. HDTRA1-10-1-0016, the US Department of Energy, Basic energy Sciences, and the William A. and Nancy F. McMinn Endowment at Vanderbilt University. The calculations were performed at ORNL's Center for Computational Sciences and the Air Force Research Laboratory DoD Supercomputing Resource Center

[1] P. Y. Huang et. al, Nature 469, 389-393, 2011.

[2] Q. X. Pei, Y. Z. Zhang, V. B. Shenoy, Carbon 48, 898-904, 2010.

[3] A. Hashimoto, et. al, Nature 430, 870-873, 2004.

Graphene is a truly two-dimensional material that consists of carbon atoms connected by sp² bonding and arranged in a honeycomb lattice. Due to its unique properties and potential applications in future electronic devices, graphene has received much attention recently from the scientific community. Initial studies of the effects of electron-beam and proton irradiation have been performed on graphene materials, but there remain significant questions about the nature of the conductivity and of the defects.

In this work, the responses of graphene materials to 10-keV x-ray radiation are evaluated with Raman spectroscopy. A defect related peak (D-peak) appears (~1345 cm^-1) after x-ray irradiation in air; the intensity of the D-peak increases with increasing total dose, and decreases with post-irradiation vacuum annealing at 350 °C. (See supplemental figures.) These results suggest that new defects can be created and/or that weak bonds at defect precursor sites can be scissioned and decorated with impurities (e.g., H, O) during x-ray exposure. The charge neutral point of graphene transistors made from similar material shifts positively with increasing x-ray dose, suggesting that similar defects may contribute to shifts in the Dirac point and degradation in the conductivity.

It has been shown previously that ozone can react with C-C bonds to degrade graphene layers. We have measured significant ozone generation during x-ray irradiation, suggesting that the x-rays may not directly create the defects sensed by Raman, but that x-ray generated ozone may lead to at least some of the observed defects. However, it is likely that there is an additional source of degradation, since in previous work the ozone reaction typically has been shown to lead to irreversible degradation, but we find that vacuum annealing at elevated temperature leads to partial recovery. Hydrogen annealing treatments of graphene have shown reversible degradation. Thus, it seems likely that hydrogen released by x-ray exposure in the SiO₂ substrate reacts with defect precursors (e.g., weak bond sites) in the graphene. Hydrogen reactions with C-C bonds can enhance the intervalley scattering, thereby increasing the D line intensity. Much of the hydrogen may be driven out by annealing in vacuum at 350 °C, leading to recovery of the degradation, consistent with our experimental results.

This work was supported by the DTRA Basic Research Program through Grants HDTRA1-10-1-0016 and HDTRA1-10-1-0041.

It has been suggested that alloy films composed of highly immiscible solute atoms in a solvent, termed “highly-mismatched alloys” (HMAs), are promising for energy conversion devices due to their ability to efficiently absorb light and heat, and to subsequently transport charge carriers. The properties of HMAs are often described with models focusing on the influence of individual solute atoms, assuming that all solute atoms “see” the same atomic environment. In the case of GaAsN alloys, the single local environment models predict a N composition-dependence of the energy band gap which agrees qualitatively with experiment. However, such models do not quantitatively explain several extraordinary electronic and optical properties. In this talk, I will discuss our investigations of the growth [1], structure [2-3], and properties [4-6] of GaAsN alloys. I will focus on correlations between the presence of N-N, N-As, and N-Si pairs [2,5-6] and extraordinary physical phenomena, including non-monotonic composition-dependent effective masses [7] and persistent photoconductivity [8]. I will also discuss our preliminary work on complementary alloys in the GaAsBi system.

[1] M. Reason, et al, J. Appl. Phys. 101, 083520 (2007).

[2] M. Reason, et al, Appl. Phys. Lett. 85, 1692 (2004).

[3] M. Reason, et al, J. Appl. Phys. 97, 103523 (2005).

[4] M. Reason, et al, J. Appl. Phys. 102, 103710 (2007).

[5] Y. Jin, et al, Appl. Phys. Lett. 95, 062109 (2009).

[6] Y. Jin, et al. Appl. Phys. Lett. 95, 092109 (2009).

[7] T. Dannecker, et al. Phys. Rev. B 82, 125203 (2010).

[8] R.L. Field III, et al., to be submitted (2011).