Spin orbit coupling in bulk ZnO and GaN

Using group theory and Kane-like $\mathbf{k\cdot p}$ model together with the L\"owdining partition method, we derive the expressions of spin-orbit coupling of electrons and holes, including the linear-$k$ Rashba term due to the intrinsic structure inversion asymmetry and the cubic-$k$ Dresselhaus term due to the bulk inversion asymmetry in wurtzite semiconductors. The coefficients of the electron and hole Dresselhaus terms of ZnO and GaN in wurtzite structure and GaN in zinc-blende structure are calculated using the nearest-neighbor $sp^3$ and $sp^3s^\ast$ tight-binding models separately.Comment: 9 pages, 6 figures, to be published in J. Appl. Phy

Large Bychkov-Rashba spin-orbit coupling in high-mobility GaN/AlGaN heterostructures

We present low temperature magnetoconductivity measurements of a density-tunable and high mobility two-dimensional electron gas confined in the wide bandgap GaN/AlGaN system. We observed pronounced anti-localization minima in the low-field conductivity, indicating the presence of strong spin-orbit coupling. Density dependent measurements of magnetoconductivity indicate that the coupling is mainly due to the Bychkov-Rashba mechanism. In addition, we have derived a closed-form expression for the magnetoconductivity, allowing us to extract reliable transport parameters for our devices. The Rashba spin-orbit coupling constant is $\alpha_{so}$ $\sim$ 6$\times$ 10$^{-13}$eVm, while the conduction band spin-orbit splitting energy amounts to $\Delta_{so}$ $\sim$ 0.3meV at n$_e$=1$\times10^{16}$m$^{-2}$.Comment: Accepted for publication in PR

Ground state of excitons and charged excitons in a quantum well

A variational calculation of the ground state of a neutral exciton and of positively and negatively charged excitons (trions) in single quantum well is presented. We study the dependance of the correlation energy and of the binding energy on the well width and on the hole mass. Our results are are compared with previous theoretical results and with avalaible experimental data.Comment: 8 pages, 5 figures presented to OECS

Dislocation density in GaN determined by photoelectrochemical and hot-wet etching

Defects in GaN layers grown by hydride vapor-phase epitaxy have been investigated by photoelectrochemical ~PEC! etching, and by wet etching in hot H3PO4 acid and molten potassium hydroxide ~KOH!. Threading vertical wires ~i.e., whiskers! and hexagonal-shaped etch pits are formed on the etched sample surfaces by PEC and wet etching, respectively. Using atomic-force microscopy, we find the density of ‘‘whisker-like’’ features to be 23109 cm22, the same value found for the etch-pit density on samples etched with both H3PO4 and molten KOH. This value is comparable to the dislocation density obtained in similar samples with tunneling electron microscopy, and is also consistent with the results of Youtsey and co-workers

On the origin of interface states at oxide/III-nitride heterojunction interfaces

The energy spectrum of interface state density, D-it(E), was determined at oxide/III-N heterojunction interfaces in the entire band gap, using two complementary photo-electric methods: (i) photo-assisted capacitance-voltage technique for the states distributed near the midgap and the conduction band (CB) and (ii) light intensity dependent photo-capacitance method for the states close to the valence band (VB). In addition, the Auger electron spectroscopy profiling was applied for the characterization of chemical composition of the interface region with the emphasis on carbon impurities, which can be responsible for the interface state creation. The studies were performed for the AlGaN/GaN metal-insulator-semiconductor heterostructures (MISH) with Al2O3 and SiO2 dielectric films and AlxGa1-x layers with x varying from 0.15 to 0.4 as well as for an Al2O3/InAlN/GaN MISH structure. For all structures, it was found that: (i) D-it(E) is an U-shaped continuum increasing from the midgap towards the CB and VB edges and (ii) interface states near the VB exhibit donor-like character. Furthermore, D-it(E) for SiO2/AlxGa1-x/GaN structures increased with rising x. It was also revealed that carbon impurities are not present in the oxide/III-N interface region, which indicates that probably the interface states are not related to carbon, as previously reported. Finally, it was proven that the obtained D-it(E) spectrum can be well fitted using a formula predicted by the disorder induced gap state model. This is an indication that the interface states at oxide/III-N interfaces can originate from the structural disorder of the interfacial region. Furthermore, at the oxide/barrier interface we revealed the presence of the positive fixed charge (Q(F)) which is not related to D-it(E) and which almost compensates the negative polarization charge (Q(pol)(-))

Growth and Investigation of GaN / AlN Quantum Dots

We have fabricated GaN quantum dots (QDs) in AlN confined layer structures by molecular beam epitaxy. The size distribution and density of the QDs have been estimated from an atomic force microscopy study. Very high quantum efficiency of photoluminescence (PL) has been obtained in some samples with QDs. Compared to the GaN bulk samples, it increased by orders of magnitude. In some samples the quantum size effect dominated, resulting in the blue-shift of the QD related PL peak, whereas in the samples with larger dots a red-shift up to 0.8 eV has been observed, which is related to strong polarization effects. We have observed a blue-shift of the PL peak with excitation intensity in the samples with large dots due to screening effect. The temperature-induced quenching of PL occurs at higher temperatures compared to bulk GaN due to the confinement of nonequilibrium carriers in the QDs. An excited state has been observed in some samples

Degradation and phase noise of InAlN/AlN/GaN heterojunction field effect transistors: Implications for hot electron/phonon effects

In15.7%Al84.3%N/AlN/GaN heterojunctionfield effect transistors have been electrically stressed under four different bias conditions: on-state-low-field stress, reverse-gate-bias stress, off-state-high-field stress, and on-state-high-field stress, in an effort to elaborate on hot electron/phonon and thermal effects. DC current and phase noise have been measured before and after the stress. The possible locations of the failures as well as their influence on the electrical properties have been identified. The reverse-gate-bias stress causes trap generation around the gate area near the surface which has indirect influence on the channel. The off-state-high-field stress and the on-state-high-field stress induce deterioration of the channel, reduce drain current and increase phase noise. The channel degradation is ascribed to the hot-electron and hot-phonon effects

Comparative studies of efficiency droop in polar and non-polar InGaN quantum wells

We report on a comparative study of efficiency droop in polar and non-polar InGaN quantum well structures at T = 10 K. To ensure that the experiments were carried out with identical carrier densities for any particular excitation power density, we used laser pulses of duration ∼100 fs at a repetition rate of 400 kHz. For both types of structures, efficiency droop was observed to occur for carrier densities of above 7 × 1011 cm−2 pulse−1 per quantum well; also both structures exhibited similar spectral broadening in the droop regime. These results show that efficiency droop is intrinsic in InGaN quantum wells, whether polar or non-polar, and is a function, specifically, of carrier density.Engineering and Physical Sciences Research Council (Grant IDs: EP\J001627\1 and EP\J003603\1)This is the final version of the article. It first appeared from AIP Publishing via http://dx.doi.org/10.1063/1.495423

Reliability characteristics and conduction mechanisms in resistive switching memory devices using ZnO thin films

In this work, bipolar resistive switching characteristics were demonstrated in the Pt/ZnO/Pt structure. Reliability tests show that ac cycling endurance level above 106 can be achieved. However, significant window closure takes place after about 102 dc cycles. Data retention characteristic exhibits no observed degradation after 168 h. Read durability shows stable resistance states after 106 read times. The current transportation in ZnO films is dominated by the hopping conduction and the ohmic conduction in high-resistance and low-resistance states, respectively. Therefore, the electrical parameters of trap energy level, trap spacing, Fermi level, electron mobility, and effective density of states in conduction band in ZnO were identified