Luke Gordon

Distributed surface donor states and the two-dimensional electron gas at AlGaN/GaN heterojunctions

Surface donor states with distributed and finite density are implemented in Schrodinger–Poisson simulations of AlGaN/GaN high electron mobility transistors, with the goal of studying their effects on the two-dimensional electron gas. Our recent experimental observations of an increasing surface barrier height with increasing AlGaN thickness are fitted very well by simulations including surface donor levels represented by a constant density of states (DOS) with a density on the order of 1013 cm−2 eV−1. The highest occupied surface states are found to be around 1 eV below the conduction-band minimum, considerably higher in energy than previously reported single surface donor levels. These trends can be explained by the features of oxidized AlGaN surfaces. Furthermore, the surface DOS that fit the experimental results are found to be larger for samples with higher Al concentration.

Controlling the density of the two-dimensional electron gas at the SrTiO3/LaAlO3interface

The polar discontinuity at the SrTiO3/LaAlO3 interface (STO/LAO) can in principle sustain an electron density of 3.3E14 cm-2 (0.5 electrons per unit cell). However, experimentally observed densities are more than an order of magnitude lower. Using a combination of first-principles and Schrodinger-Poisson simulations we show that the problem lies in the asymmetric nature of the structure, i.e., the inability to form a second LAO/STO interface that is a mirror image of the first, or to fully passivate the LAO surface. Our insights apply to oxide interfaces in general, explaining for instance why the SrTiO3/GdTiO3 interface has been found to exhibit the full density of 3.3E14 cm-2.

BaSnO3 as a channel material in perovskite oxide heterostructures

BaSnO3 (BSO) is a transparent perovskite oxide with high room-temperature mobility, a property that is highly desirable for a channel material in transistors. However, its low density of states (DOS) makes it challenging to confine a high-density two-dimensional electron gas (2DEG). Using hybrid density functional theory, we calculate the band structure of BSO, its DOS, and its band offsets with candidate barrier materials, such as SrTiO3 (STO), LaInO3, and KTaO3. With the calculated material parameters as input, Schrodinger-Poisson simulations are then performed on BSO heterostructures to quantitatively address the issue of 2DEG confinement. The BSO/STO interface with a conduction-band offset of 1.14 eV limits the 2DEG density confined within BSO to 8×1013 cm−2. Strategies to improve the confinement via band-offset engineering are discussed.

Hydrogen bonds in Al2O3 as dissipative two-level systems in superconducting qubits

Dissipative two-level systems (TLS) have been a long-standing problem in glassy solids over the last fifty years, and have recently gained new relevance as sources of decoherence in quantum computing. Resonant absorption by TLSs in the dielectric poses a serious limitation to the performance of superconducting qubits; however, the microscopic nature of these systems has yet to be established. Based on first-principles calculations, we propose that hydrogen impurities in Al2O3 are the main source of TLS resonant absorption. Hydrogen is an ubiquitous impurity and can easily incorporate in Al2O3. We find that interstitial H in Al2O3 forms a hydrogen bond (O-H…O). At specific O-O distances, consistent with bond lengths found in amorphous Al2O3 or near Al2O3 surfaces or interfaces, the H atom feels a double well. Tunneling between two symmetric positions gives rise to resonant absorption in the range of 10 GHz, explaining the experimental observations. We also calculate the expected qubit-TLS coupling and find it to lie between 16 and 20 MHz, consistent with experimental measurements.

Impact of electric-field dependent dielectric constants on two-dimensional electron gases in complex oxides

High-density two-dimensional electron gas (2DEG) can be formed at complex oxide interfaces such as SrTiO3/GdTiO3 and SrTiO3/LaAlO3. The electric field in the vicinity of the interface depends on the dielectric properties of the material as well as on the electron distribution. However, it is known that electric fields can strongly modify the dielectric constant of SrTiO3 as well as other complex oxides. Solving the electrostatic problem thus requires a self-consistent approach in which the dielectric constant varies according to the local magnitude of the field. We have implemented the field dependence of the dielectric constant in a Schrodinger-Poisson solver in order to study its effect on the electron distribution in a 2DEG. Using the SrTiO3/GdTiO3 interface as an example, we demonstrate that including the field dependence results in the 2DEG being confined closer to the interface compared to assuming a single field-independent value for the dielectric constant. Our conclusions also apply to SrTiO3/LaAlO3 as well as other similar interfaces.

Identification of Microscopic Hole-Trapping Mechanisms in Nitride Semiconductors

Hole trapping has been observed in nitride heterostructure devices, where the Fermi level is in the vicinity of the valence-band maximum. Using hybrid density functional calculations, we examine microscopic mechanisms for hole trapping in GaN and AlN. In a defect-free material, hole trapping does not spontaneously occur, but trapping can occur in the vicinity of impurities, such as C-a common unintentional impurity in nitrides. Using Schrödinger-Poisson simulations, we assess the effects of C-derived hole traps on N-face high-electron mobility transistors, which we find to be more detrimental than the previously proposed interface traps.