J. R. Weber

Oxygen vacancies and donor impurities in β-Ga2O3

Using hybrid functionals we have investigated the role of oxygen vacancies and various impurities in the electrical and optical properties of the transparent conducting oxide β-Ga2O3. We find that oxygen vacancies are deep donors, and thus cannot explain the unintentional n-type conductivity. Instead, we attribute the conductivity to common background impurities such as silicon and hydrogen. Monatomic hydrogen has low formation energies and acts as a shallow donor in both interstitial and substitutional configurations. We also explore other dopants, where substitutional forms of Si, Ge, Sn, F, and Cl are shown to behave as shallow donors.

Quantum computing with defects

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.

Origin and passivation of fixed charge in atomic layer deposited aluminum oxide gate insulators on chemically treated InGaAs substrates

We report experimental and theoretical studies of defects producing fixed charge within Al2O3 layers grown by atomic layer deposition (ALD) on In0.53Ga0.47As(001) substrates and the effects of hydrogen passivation of these defects. Capacitance-voltage measurements of Pt/ALD-Al2O3/n-In0.53Ga0.47As suggested the presence of positive bulk fixed charge and negative interfacial fixed charge within ALD-Al2O3. We identified oxygen and aluminum dangling bonds (DBs) as the origin of the fixed charge. First-principles calculations predicted possible passivation of both O and Al DBs, which would neutralize fixed charge, and this prediction was confirmed experimentally; postmetallization forming gas anneal removed most of the fixed charge in ALD-Al2O3.

Native defects in Al2O3 and their impact on III-V/Al2O3 metal-oxide-semiconductor-based devices

Al2O3 is a promising material for use as a dielectric in metal-oxide-semiconductor devices based on III-V compound semiconductors. However, the presence of deep levels and fixed charge in the Al2O3 layer is still a concern, with native defects being a possible cause of traps, leakage, and fixed charge. We report hybrid density functional calculations for vacancies, self-interstitials, and antisites in Al2O3. The energetic positions of defect levels are discussed in terms of the calculated band alignment at the interface between the oxide and relevant III-V materials. We find that oxygen vacancies are the defects most likely to introduce gap levels that may induce border traps or leakage current in a gate stack. In addition, both self-interstitials and aluminum vacancies introduce fixed charge that leads to increased carrier scattering in the channel and shifts the threshold voltage of the device.

Dangling-bond defects and hydrogen passivation in germanium

The application of germanium in complementary metal-oxide semiconductor technology is hampered by high interface-state densities. Using first-principles calculations, we investigate the effects of dangling bonds (DBs) and their interaction with hydrogen. We find that Ge DBs give rise to electronic levels below the valence-band maximum. They therefore occur exclusively in the negative charge state, explaining why they cannot be observed with electron spin resonance. The associated fixed charge is likely responsible for threshold-voltage shifts and poor performance of n-channel transistors. We also find that passivation of DBs by hydrogen will be ineffective because interstitial hydrogen is also stable exclusively in the negative charge state.

Experimental electronic structure of In2O3 and Ga2O3

Transparent conducting oxides (TCOs) pose a number of serious challenges. In addition to the pursuit of high-quality single crystals and thin films, their application has to be preceded by a thorough understanding of their peculiar electronic structure. It is of fundamental interest to understand why these materials, transparent up to the UV spectral regime, behave also as conductors. Here we investigate In2O3 and Ga2O3, two binary oxides, which show the smallest and largest optical gaps among conventional n-type TCOs. The investigations on the electronic structure were performed on high-quality n-type single crystals showing carrier densities of ~1019?cm?3 (In2O3) and ~1017?cm?3 (Ga2O3). The subjects addressed for both materials are: the determination of the band structure along high-symmetry directions and fundamental gaps by angular resolved photoemission (ARPES). We also address the orbital character of the valence- and conduction-band regions by exploiting photoemission cross sections in x-ray photoemission (XPS) and by x-ray absorption (XAS). The observations are discussed with reference to calculations of the electronic structure and the experimental results on thin films.

Oxidation and the origin of the two-dimensional electron gas in AlGaN/GaN heterostructures

The surface of the AlGaN barrier layer in AlGaN/GaN high electron mobility transistors has strong and hitherto unexplained effects on transistor characteristics. Indeed, it has been cited as the source of the two-dimensional electron gas at the AlGaN/GaN interface. Using computational methods based on density functional theory, we investigate surface reconstructions on realistic GaN and AlN (0001) surfaces, which are invariably oxidized. Numerous structures with different oxide coverage and different stoichiometry are examined, and their stability is interpreted in terms of driving mechanisms such as the electron counting rule and oxide-stoichiometry matching. We discuss which structures are likely to form under a variety of oxidation conditions, and show that these structures explain the observed dependence of electron density on thickness and variations in surface barrier height.

The electronic structure of β-Ga2O3

β-Ga2O3 has the widest energy gap of the transparent conducting oxides. The interest in its electronic properties has recently increased because of its applications in various optoelectronic devices, semiconductor lasers, and ultrasensitive gas detecting systems. In contrast, information on the electronic structure of β-Ga2O3 is very scarce. Here, we present the experimental valence-band structure of β-Ga2O3 single crystals determined by high-resolution angle-resolved photoelectron spectroscopy utilizing synchrotron radiation. We find good matching of the experimental band structure with the advanced density functional theory calculations employing hybrid functionals and projector augmented wave potentials.

Defects in SiC for quantum computing

The successful implementation of the nitrogen-vacancy (NV) center in diamond as a qubit has spawned a great deal of interest in this defect. In principle, similar defects suitable for quantum computing should exist in other material systems; however, very little work has been done on identifying NV-like centers in other materials. We discuss the key properties of the NV center in diamond in the context of uncovering similar defects in other materials, with the specific example of SiC. Using first-principles calculations, we compare the properties of the NV center in diamond to the analogous defect in 4H-SiC. We also compare the properties of the bare vacancies. We calculate defect formation energies and charge-state transition levels to determine which defects are likely to form. Then, by analyzing the defect-induced electronic states, we determine whether stable defects in 4H-SiC may have properties similar to those of the NV center in diamond.

Intrinsic and extrinsic causes of electron accumulation layers on InAs surfaces

Using first-principles calculations we investigate the origins of electron accumulation on InAs surfaces. Among the possible intrinsic causes (i.e., involving only In and/or As), In adatoms are the only native defects that can induce a surface electron accumulation layer. As an extrinsic mechanism, we find that adsorption of hydrogen on the surface also leads to charge accumulation. Hydrogen is an ubiquitous impurity, present in all growth and processing environments and therefore likely to be present on InAs surfaces. Both indium and hydrogen adatoms create donor states above the InAs conduction-band minimum and can explain the observed electron accumulation.