Identifying and designing physical systems for use as qubits, the basic units of quantum information, are critical steps in the development of a quantum computer. Among the possibilities in the solid state, a defect in diamond known as the nitrogen-vacancy (NV-1) center stands out for its robustness—its quantum state can be initialized, manipulated, and measured with high fidelity at room temperature. Here we describe how to systematically identify other deep center defects with similar quantum-mechanical properties. We present a list of physical criteria that these centers and their hosts should meet and explain how these requirements can be used in conjunction with electronic structure theory to intelligently sort through candidate defect systems. To illustrate these points in detail, we compare electronic structure calculations of the NV-1 center in diamond with those of several deep centers in 4H silicon carbide (SiC). We then discuss the proposed criteria for similar defects in other tetrahedrally coordinated semiconductors.

http://absimage.aps.org/image/MAR11/MWS_MAR11-2010-000614.pdf

Quantum computing with defects

Single crystalline bulk and epitaxially grown gallium oxide (β–Ga2O3) was irradiated by 0.6 and 1.9 MeV protons to doses ranging from 5 × 109 to 6 × 1014 cm−2 in order to study the impact on charge carrier concentration and electrically active defects. Samples irradiated to doses at or above 2 × 1013 cm−2 showed a complete removal of free charge carriers in their as-irradiated state, whereas little or no influence was observed below doses of 6 × 1012 cm−2. From measurements at elevated temperatures, a thermally activated recovery process is seen for the charge carriers, where the activation energy for recovery follow a second-order kinetics with an activation energy of ∼1.2 eV. Combining the experimental results with hybrid functional calculations, we propose that the charge carrier removal can be explained by Fermi-level pinning far from the conduction band minimum (CBM) due to gallium interstitials (Gai), vacancies (VGa), and antisites (GaO), while migration and subsequent passivation of VGa via hydrogen-derived or VO defects may be responsible for the recovery. Following the recovery, deep level transient spectroscopy (DLTS) reveals generation of two deep levels, with energy positions around 0.75 and 1.4 eV below the CBM. Of these two levels, the latter is observed to disappear after the initial DLTS measurements, while the concentration of the former increases. We discuss candidate possibilities and suggest that the origins of these levels are more likely due to a defect complex than an isolated point defect.

Impact of proton irradiation on conductivity and deep level defects in β-Ga2O3

The surface electronic properties of bulk-grown β-Ga2O3 (2¯01) single crystals are investigated. The band gap is found using optical transmission to be 4.68 eV. High-resolution x-ray photoemission coupled with hybrid density functional theory calculation of the valence band density of states provides insights into the surface band bending. Importantly, the standard linear extrapolation method for determining the surface valence band maximum (VBM) binding energy is found to underestimate the separation from the Fermi level by ∼0.5 eV. According to our interpretation, most reports of surface electron depletion and upward band bending based on photoemission spectroscopy actually provide evidence of surface electron accumulation. For uncleaned surfaces, the surface VBM to Fermi level separation is found to be 4.95 ± 0.10 eV, corresponding to downward band bending of ∼0.24 eV and an electron accumulation layer with a sheet density of ∼5 × 1012 cm−2. Uncleaned surfaces possess hydrogen termination which acts as surface donors, creating electron accumulation and downward band bending at the surface. In situ cleaning by thermal annealing removes H from the surface, resulting in a ∼0.5 eV shift of the surface VBM and formation of a surface electron depletion layer with upward band bending of ∼0.26 eV due to native acceptor surface states. These results are discussed in the context of the charge neutrality level, calculated bulk interstitial hydrogen transition levels, and related previous experimental findings.

Transition from electron accumulation to depletion at β-Ga2O3 surfaces: The role of hydrogen and the charge neutrality level

Results from hybrid density functional theory calculations on the thermodynamic stability and optical properties of the Zn vacancy ($V_{\text{Zn}}$) complexed with common donor impurities in ZnO are reported. Complexing $V_{\text{Zn}}$ with donors successively removes its charge-state transition levels in the band gap, starting from the most negative one. Interestingly, the presence of a donor leads only to modest shifts in the positions of the $V_{\text{Zn}}$ charge-state transition levels, the sign and magnitude of which can be interpreted from a polaron energetics model by taking hole-donor repulsion into account. By employing a one-dimensional configuration coordinate model, luminescence lineshapes and positions were calculated. Due to the aforementioned effects, the isolated $V_{\text{Zn}}$ gradually changes from a mainly non-radiative defect with transitions in the infrared region in \textit{n}-type material, to a radiative one with broad emission in the visible range when complexed with shallow donors.

Zn vacancy-donor impurity complexes in ZnO

The electronic and optical properties of (InxGa1–x)2O3 alloys are highly tunable, giving rise to a myriad of applications including transparent conductors, transparent electronics, and solar-blind ...

Indium Gallium Oxide Alloys: Electronic Structure, Optical Gap, Surface Space Charge, and Chemical Trends within Common-Cation Semiconductors.

Proton implantation is shown to increase the concentration of the so called and commonly observed E3 defect level in zinc oxide (ZnO). Box and single profiles of protons with doses ranging from 6×1010 cm−2 to 4.3×1012 cm−2 were implanted into hydrothermally grown ZnO samples with original concentrations of E3 below 5×1014 cm−3. Capacitance-Voltage profiling and junction spectroscopy measurements showed that the charge carrier concentration and absolute concentration of E3 centers increase by more than one order of magnitude compared to the as-grown samples as well as control samples implanted with He ions. The results provide strong evidence for the involvement of H in the formation of the E3 center, and a complex involving interstitial H and an oxygen sub-lattice primary defect are discussed.

The E3 center in zinc oxide: Evidence for involvement of hydrogen

Hydrothermally grown n-type ZnO bulk samples have been implanted with protons and deuterium ions to fluences in the range of 8 × 1010 to 8 × 1011 cm−2. The implantations were performed at the temperature of 285 K, and the samples were then analyzed in-situ by deep level transient spectroscopy (DLTS) using a setup connected to the implanter beam line. The concentration of the so-called E4 center, having an apparent energy level at ∼0.57 eV below the conduction edge, is found to increase linearly with the ion fluence and the generation rate is proportional to the elastic energy deposition, as expected for a primary defect. Isothermal annealing of the E4 center at temperatures between 290 and 315 K reveals first-order kinetics with the activation energy of ∼0.6 eV. The annealing rate is strongly enhanced with increasing hydrogen fluence, and a model invoking migration of interstitial hydrogen and subsequent reaction with E4 exhibits close agreement with the experimental data. The rate of electron capture by ...

/pdf/formation-and-annihilation-of-e4-centers-in-zno-influence-of-mx26xgqwq8.pdf

Formation and annihilation of E4 centers in ZnO: Influence of hydrogen

The evolution of sheet resistance of n-type In2O3 and Ga2O3 exposed to bombardment with MeV 12C and 28Si ions at 35 K is studied in situ. While the sheet resistance of Ga2O3 increased by more than eight orders of magnitude as a result of ion irradiation, In2O3 showed a more complex defect evolution and became more conductive when irradiated at the highest doses. Heating up to room temperature reduced the sheet resistivity somewhat, but Ga2O3 remained highly resistive, while In2O3 showed a lower resistance than as deposited samples. Thermal admittance spectroscopy and deep level transient spectroscopy did not reveal new defect levels for irradiation up to [Formula: see text] cm-2. A model where larger defect complexes preferentially produce donor like defects in In2O3 is proposed, and may reveal a microscopic view of a charge neutrality level within the conduction band, as previously proposed.

/pdf/electrical-conductivity-of-in2o3-and-ga2o3-after-low-2qjc8qu4nq.pdf

Electrical conductivity of In2O3 and Ga2O3 after low temperature ion irradiation; implications for instrinsic defect formation and charge neutrality level.

Hydrothermally grown $n$-type ZnO samples, implanted with helium $({\text{He}}^{+})$ at a sample temperature of $\ensuremath{\sim}40$ K and fluences of $5\ifmmode\times\else\texttimes\fi{}{10}^{9}$ and $5\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{4.pt}{0ex}}{\text{cm}}^{\ensuremath{-}2}$, have been studied in situ by capacitance voltage (CV) and junction spectroscopy measurements. The results are complemented by data from secondary ion mass spectrometry and Fourier transform infrared absorption measurements and first-principles calculations. Removal/passivation of an implantation-induced shallow donor center or alternatively growth of a deep acceptor defect are observed after annealing, monitored via charge carrier concentration $({N}_{d})$ versus depth profiles extracted from CV data. Isothermal anneals in the temperature range of 290--325 K were performed to study the evolution in ${N}_{d}$, revealing a first-order kinetics with an activation energy, ${E}_{a}\ensuremath{\approx}0.7\phantom{\rule{4.pt}{0ex}}\text{eV}$ and frequency factor, ${c}_{0}\ensuremath{\sim}{10}^{6}\phantom{\rule{4.pt}{0ex}}{\text{s}}^{\ensuremath{-}1}$. Two models are discussed in order to explain these annealing results. One relies on transition of oxygen interstitials $({\text{O}}_{i})$ from a split configuration (neutral state) to an octahedral configuration (deep double acceptor state) as a key feature. The other one is based on the migration of Zn interstitials (double donor) and trapping by neutral Zn-vacancy-hydrogen complexes as the core ingredient. In particular, the latter model exhibits good quantitative agreement with the experimental data and gives an activation energy of $\ensuremath{\sim}0.75$ eV for the migration of Zn interstitials.

/pdf/evolution-kinetics-of-elementary-point-defects-in-zno-22vno3e4j5.pdf

Evolution kinetics of elementary point defects in ZnO implanted with low fluences of helium at cryogenic temperature

TiO2 nanowires (NWs) can improve the advantageous photocatalytic properties of TiO2 by increasing the active surface area. Here we investigate the synthesis of TiO2 NWs by thermal oxidation, studying the role of temperature, annealing time, and gas flow rates. The optimal thermal growth conditions were found to be 800 °C for 4 h in a mixed gas flow of Ar and O2. Morphological and structural characterizations, carried out by scanning and transmission electron microscopy, and X-ray diffraction, indicated a TiO2 rutile monocrystalline structure of the nanowires. An in situ thermal growth analysis was performed by means of an environmental electron microscope, providing additional insights into the TiO2 NWs growth dynamics. The photocatalytic properties were studied by using the degradation rate of methylene blue under UV light. TiO2 NWs revealed a 70% improvement of the degradation rate compared to a reference TiO2 bulk sample. Moreover, NWs were additionally annealed in forming gas (5% H2 in N2) in order to promote the formation of oxygen vacancies and an increase of the carrier density. Indeed, the photocatalytic activity of the NWs treated in forming gas was 3 times higher than that of the reference TiO2 bulk sample. The photonic efficiency and the quantum efficiency, also showed an increase versus bulk TiO2 of about 20% in the presence of NWs, and of about 100% with the forming gas annealing. Moreover, this increase in the photocatalytic activity after the forming gas annealing also correlates with the disappearance of a deep level recombination center as observed by deep level transient spectroscopy.

/pdf/single-crystal-tio2-nanowires-by-seed-assisted-thermal-y6ps967jyb.pdf

Chidanand Bhoodoo

Papers

The E3 center in zinc oxide: Evidence for involvement of hydrogen

Formation and annihilation of E4 centers in ZnO: Influence of hydrogen

Electrical conductivity of In2O3 and Ga2O3 after low temperature ion irradiation; implications for instrinsic defect formation and charge neutrality level.

Evolution kinetics of elementary point defects in ZnO implanted with low fluences of helium at cryogenic temperature

Single-crystal TiO2 nanowires by seed assisted thermal oxidation of Ti foil: synthesis and photocatalytic properties