•A novel strategy for high-quality AlN epitaxy on sapphire substrates with sputtered buffer layers combined with a low- and high-temperature alteration technique is proposed.•Threading dislocations density was significantly decreased due to the alteration of growth mode.•The graded composition of AlON layer is believed to alleviate lattice mismatch between sapphire and AlN.In this work, a novel strategy for high-quality AlN templates epitaxy on sapphire substrates with sputtered buffer layers combined with a low- and high-temperature alteration technique is proposed. The best full width at half maximum values for (0002) and (11¯02) reflections are 207 and 377 arcsec, respectively. Investigations indicate the joint effect of growth mode control and sputtered buffer layer results in the improvement of AlN crystalline quality. Firstly, threading dislocations density can be significantly decreased due to the alteration from three-dimensional to two-dimensional growth mode. Moreover, the graded composition of AlON layer in the sputtered buffer layer is believed to alleviate lattice mismatch between sapphire substrates and AlN, which also contributes to low dislocations density in AlN templates.

•The high energetic hot electrons can assist vertical leakage and even accelerate vertical breakdown.•A model is proposed that high energetic electrons can detrap or ionize defect states.•The trap states which are responsible for the phenomenon are identified.We present a hot electron assisted vertical leakage/breakdown mechanism in AlGaN/GaN heterostructures on Si substrates by a combination of applying vertical and lateral bias conditions. Beyond a critical bias point, the vertical leakage current under the combined bias condition is larger than that under a pure vertical bias condition which results in a lower breakdown voltage. The critical bias has a positive temperature dependence. A model is proposed that highly energetic hot electrons can release trapped electrons from defects and even ionize them. The model is proved by investigating the detrapping and ionization mechanisms by changing hot electron energy.

Lattice-polarity-driven epitaxy of hexagonal semiconductor nanowires (NWs) is demonstrated on InN NWs. In-polarity InN NWs form typical hexagonal structure with pyramidal growth front, whereas N-polarity InN NWs slowly turn to the shape of hexagonal pyramid and then convert to an inverted pyramid growth, forming diagonal pyramids with flat surfaces and finally coalescence with each other. This contrary growth behavior driven by lattice-polarity is most likely due to the relatively lower growth rate of the (0001̅) plane, which results from the fact that the diffusion barriers of In and N adatoms on the (0001) plane (0.18 and 1.0 eV, respectively) are about 2-fold larger in magnitude than those on the (0001̅) plane (0.07 and 0.52 eV), as calculated by first-principles density functional theory (DFT). The formation of diagonal pyramids for the N-polarity hexagonal NWs affords a novel way to locate quantum dot in the kink position, suggesting a new recipe for the fabrication of dot-based devices.

•We grew a number of high-purity AlN films by molecular beam epitaxy.•The biaxial stress coefficient for E2(high) mode was determined, well in accord with the theoretical value.•The crystalline quality of MBE-grown AlN films was improved by increasing the Al/N ratio during AlN buffer layer growth.Residual stress in AlN films grown by molecular beam epitaxy (MBE) has been studied by Raman scattering spectroscopy. A strain-free Raman frequency and a biaxial stress coefficient for E2(high) mode are experimentally determined to be 657.8 ± 0.3 cm−1 and 2.4 ± 0.2 cm−1/GPacm−1/GPa, respectively. By using these parameters, the residual stress of a series of AlN layers grown under different buffer layer conditions has been investigated. The residual compressive stress is found to be obviously decreased by increasing the Al/N beam flux ratio of the buffer layer, indicating the generation of tensile stress due to stronger coalescence of AlN grains, as also confirmed by the in-situ reflection high energy electron diffraction (RHEED) monitoring observation. The stronger coalescence does lead to improved quality of AlN films as expected.

Understanding the semiconductor surface and its properties including surface stability, atomic morphologies, and even electronic states is of great importance not only for understanding surface growth kinetics but also for evaluating the degree to which they affect the devices’ performance. Here, we report studies on the nanoscale fissures related surface instability in AlGaN/GaN heterostructures. Experimental results reveal that edge dislocations are actually the root cause of the surface instability. The nanoscale fissures are initially triggered by the edge dislocations, and the subsequent evolution is associated with tensile lattice-mismatch stress and hydrogen etching. Our findings resolve a long-standing problem on the surface instability in AlGaN/GaN heterostructures and will also lead to new understandings of surface growth kinetics in other hexagonal semiconductor systems.Keywords: edge dislocations; hexagonal nitrides; nanoscale fissures; surface instability; TEM;

Spintronic devices rely on the spin degree of freedom (DOF), and spin orbit coupling (SOC) is the key to manipulate spin DOF. Quasi-one-dimensional structures, possessing marked anisotropy gives more choice for the manipulation of the spin DOF since the concrete SOC form varies along with crystallographic directions. The anisotropy of the Dresselhaus SOC in cadmium selenide (CdSe) nanobelt and nanowire was studied by circular photogalvanic effect. It was demonstrated that the Dresselhaus SOC parameter is zero along the [0001] crystallographic direction, which suppresses the spin relaxation and increases the spin diffusion length, and thus is beneficial to the spin manipulation. To achieve a device structure with Rashba SOC presence and Dresselhaus SOC absence for manipulating the spin DOF, an ionic liquid gate was produced on a nanowire grown along the [0001] crystallographic direction, and the Rashba SOC was induced by gating, as expected.

Epitaxial growth of AlN films on c-sapphire using a multilayer structure has been investigated by metal–organic chemical vapor deposition adopting multiple alternation cycles of low- and high-temperature (LT–HT) growth. It is found that the surface morphology and crystal quality can be greatly improved using three alternation cycles with X-ray diffraction ω-scan full width at half maximum values of 311 and 548 arcsec for the (0002) and (10−12) peaks, respectively, which are induced by the alternation of the three-dimensional (3D) and two-dimensional (2D) growth modes caused by the LT–HT process. The first 3D–2D cycle is found to play a major role in threading dislocation reduction, while the second and third cycles mainly account for tensile stress relaxation.

This paper presents the design for a heterojunction AlGaN solar-blind avalanche photodiode (APD) with improved noise performance. Increasing the Al composition of AlGaN in the multiplication layer from 0.40 to 0.45 was calculated to significantly reduce the excess noise factor of this heterojunction APD. The polarization electric field induced in the multiplication layer had the same direction as the applied reverse bias field, which helped lower the avalanche breakdown voltage. The calculated results demonstrated that the apparent spike in the electric field intensity at the i-Al0.4Ga0.6N/n-Al0.5Ga0.5N interface can be effectively suppressed by inserting a grading n-AlGaN layer, which helps reduce the dark current.

Gate-defined quantum point contacts (QPCs) were fabricated with Al0.25Ga0.75N/GaN heterostructures grown by metal–organic chemical vapor deposition (MOCVD). In the transport study of the Zeeman effect, greatly enhanced effective g factors (g*) were obtained. The in-plane g* is found to be 5.5 ± 0.6, 4.8 ± 0.4, and 4.2 ± 0.4 for the first to the third subband, respectively. Similarly, the out-of-plane g* is 8.3 ± 0.6, 6.7 ± 0.7, and 5.1 ± 0.7. Increasing g* with the population of odd-numbered spin-splitted subbands are obtained at 14 T. This portion of increase is assumed to arise from the exchange interaction in one-dimensional systems. A careful analysis shows that not only the exchange interaction but the spin–orbit interaction (SOI) in the strongly confined QPC contributes to the enhancement and anisotropy of g* in different subbands. An approach to distinguish the respective contributions from the SOI and exchange interaction is therefore proposed.

Electrically manipulating electron spins based on Rashba spin–orbit coupling (SOC) is a key pathway for applications of spintronics and spin-based quantum computation. Two-dimensional electron systems (2DESs) offer a particularly important SOC platform, where spin polarization can be tuned with an electric field perpendicular to the 2DES. Here, by measuring the tunable circular photogalvanic effect (CPGE), we present a room-temperature electric-field-modulated spin splitting of surface electrons on InN epitaxial thin films that is a good candidate to realize spin injection. The surface band bending and resulting CPGE current are successfully modulated by ionic liquid gating within an electric double-layer transistor configuration. The clear gate voltage dependence of CPGE current indicates that the spin splitting of the surface electron accumulation layer is effectively tuned, providing a way to modulate the injected spin polarization in potential spintronic devices.

The influence of the annealing pressure of the nucleation layer (NL) on the resistance of GaN films grown on sapphire by means of metal-organic chemical vapor deposition has been investigated. It is found that the sheet resistance of GaN increases significantly with decreasing the annealing pressure. When the annealing pressure is 10 kPa, GaN with sheet resistance higher than 1011 Ω/sq is achieved. It is determined that the edge-component threading dislocations (ETDs) rather than the screw-component ones are the dominant structural feature responsible for the resistance variation. Based on the analysis of in-situ reflectance and atomic force microscopy, it is believed that the island density and island size in the NL are the key factors accounting for the resistance variation in GaN. The high island density and small island size in the NL not only lead to more ETDs in GaN, which introduce more acceptors to compensate the background electrons, but also enhance the rapid coalescence in GaN and thus effectively limit the diffusion of oxygen impurities into GaN epilayers from GaN/sapphire interfaces.

•High-resistance of the GaN buffer is realized by eliminating oxygen impurities.•Using this technique, HR-GaN of the order of 1016 Ω/sq has been achieved.•Growth mode modification of GaN is performed on the AlN layer.•A 3D growth mode is introduced to improve the crystalline quality.•The density of threading dislocations are greatly reduced while maintaining HR.High-resistance GaN with low dislocation density adopting growth mode modification has been investigated by metalorganic chemical vapor deposition. The sheet resistance of the order of 1016 Ω/sq has been achieved at room temperature by diminishing the oxygen impurity level close to the substrate with an AlN blocking layer. Attributed to this method which offers more freedom to tailor the growth mode, a three-dimensional (3D) growth process is introduced by adjusting the growth pressure and temperature at the initial stage of the GaN epitaxy to improve the crystalline quality. The large 3D GaN grains formed during this period roughen the surface, and the following coalescence of the GaN grains causes threading dislocations bending, which finally remarkably reduces the dislocation density.

•InAlN/AlGaN bilayer barriers are adopted in GaN-based heterostructures.•It has higher 2DEG density than AlGaN/GaN, while maintaining high 2DEG mobility.•Gate leakage current density is 2 orders of magnitudes smaller than InAlN/GaN.•Temperature dependence of the 2DEG density is verified.Electrical properties of GaN-based heterostructures adopting InAlN/AlGaN bilayer barriers are investigated by Hall-effect and current–voltage measurements. It is found that this structure possesses both merits of high two-dimensional electron gas (2DEG) density and low gate leakage current density, while maintaining high 2DEG mobility. Furthermore, temperature dependence of the 2DEG density in this structure is verified to follow a combined tendency of InAlN/GaN (increase) and AlGaN/GaN (decrease) heterostructures with increasing temperature from 90 K to 400 K, which is mainly caused by superposition of the effects from carrier thermal activation induced by extrinsic factors in InAlN layer and the reduced conduction-band discontinuity.