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The characteristics of an AlGaN/GaN high-electron-mobility transistor buffer structure are studied and optimized by employing an AlN/GaN superlattice (SL) structure. Through vertical leakage analysis and back-gate measurement, combined with Silvaco-TCAD simulation, the influence of buffer trapson the carrier transport behaviors and electrical performance for SL buffer structures under a high electric field is analyzed. The AlN/GaN SL buffer structures are further optimized with various AlN/GaN thickness ratios and their total thickness through both simulation and experimental studies. As a result, a high breakdown voltage of up to 1.3 kV with a maximum breakdown electric field of 2.8 MV cm−1 has been achieved. Moreover, the buffer trapping effect is dramatically suppressed, leading to a minimum drop of channel current for the optimized sample, in which donor traps are found to play a positive role in the device dynamic characteristics.

The paramount issues facing commercialization of GaN-on-Si RF devices include the inevitable wafer bowing and large thermal resistance when incorporating the conventional AlN/AlGaN multilayer or superlattice buffer for stress control. The large thermal resistance due to the strong phonon scattering by the AlGaN alloy and multiple interfaces of the AlN/AlGaN multilayer or superlattice buffer must be carefully addressed, which can significantly affect the performance and reliability of GaN-on-Si RF devices. In this letter, we report a successful growth of crack-free 2 μm-thick GaN film on 6 inch high-resistivity (>10 kΩ cm) Si with a single-layer AlN buffer. By virtue of a quasi-2D GaN nucleation on the AlN buffer layer, the as-grown GaN-on-Si wafer presents a tremendous reduction in warpage from 800 to less than 8 μm, while preserving a high crystalline quality. The full width at half maximum of x-ray diffraction of GaN (0002) and (10 1 ˉ 2) planes are 457 and 509 arcsec, respectively, and the residual stress is as low as 0.2 GPa. The underlying physical mechanisms of the quasi-2D GaN nucleation layer involved in stress engineering and dislocation annihilation have been discussed in detail. This work paves the way for the fabrication of low-cost and high-performance GaN RF devices grown on HR-Si.

This letter has studied the gate reliability of p-GaN high electron mobility transistors (HEMTs) influenced by the Mg doping level at the initial growth stage of p-GaN layer. Normally-off HEMTs with p-GaN gate fabricated with relatively low and high Mg doping concentration have been analyzed and compared based on their performances at various statuses of as-fabricated, forward step-stressed, reverse step-stressed and long-term reverse stressed. It reveals that Mg over-doping has a detrimental effect on the p-GaN gate reliability through degrading the PIN diode formed by p-GaN/AlGaN/GaN structure. Severe drain current reduction, threshold voltage instability and gate leakage increase have been observed after being negatively stressed due to the high-density defects acting as apparent acceptor-like traps induced by Mg over-doping. The detailed studies on the p-GaN gate performance with various Mg concentrations give a greater depth of understanding to realize high gate reliability of p-GaN gate HEMTs.

This letter reports on the results of room-temperature electrically injected GaN-based distributed feed-back laser diodes (LDs) grown on Si. A hundred pairs of high-order sidewall gratings were prepared by dry-etching along the ridge to select only single mode, and tetramethyl ammonium hydroxide polishing technology was adopted to remove the etching damage and make the sidewall smooth and steep. As a result, we have successfully fabricated GaN-based distributed feedback LDs grown on Si with a side-mode suppression ratio of ∼10 dB. Further analysis revealed that the fabrication of gratings reduced the injection efficiency and increased the optical loss, which deteriorated the device performance. Further improvements of the laser material quality and device fabrication are underway.

In2S3 (β-In2S3), semiconducting chalcogenide with desirable physicochemical properties, has fascinated researchers in photoelectrochemistry. Because of its wide band gap, In2S3 can utilize solar energy below 600 nm. However, rapid photogenerated electron–hole recombination and low quantum efficiency have limited the practical application of In2S3 in this field. In a two-step in situ hydrothermal process we introduced a narrow band gap semiconductor (ReS2) below the In2S3 and constructed a direct Z-scheme heterostructure with nanoflower and honeycomb morphology. The formation of a direct Z-scheme heterostructure and coordination of the trap-like structure of the composite give a wider absorption range, higher migration and separation efficiency, and faster interfacial transfer speed than for pristine In2S3, and the photoelectrochemical performance is approximately three times better than that of pristine In2S3 at 1.23 V versus a reversible hydrogen electrode under sunlight. This method therefore provides a new prospect for optimizing the performance of In2S3 and applying the novel heterojunction.

The degradation characteristics of InGaN-based laser diodes (LDs) grown on Si substrate have been studied under an electrical stress with a pulsed current of 180 mA. After the electrical stress, the light output power, leakage current and capacitance of the LD decreased, while the operation voltage and threshold current increased, and the slope efficiency remained nearly unchanged after the pulsed electrical stress for 620 hours. Further analysis shows that the degradation was probably due to the newly generated group-III vacancies and/or related defects, which can not only act as acceptor-like defects to compensate the n-type donors, leading to the reduction of conductance and capacitance, but also work as non-radiative recombination centers affecting the internal quantum efficiency and emission intensity of the active region.

The increasing demand for complementary metal-oxide-semiconductor integrated GaN-based optoelectronics and power electronics calls for wafer-scale crack-free and high quality GaN thick film grown on Si. However, the huge mismatch in both lattice constant and coefficient of thermal expansion between GaN and Si often causes a high density of threading dislocations (TDD, typically 109–1010 cm−2) and the formation of micro-crack networks, producing a poor-quality GaN film with a limited thickness (

An abnormal aging phenomenon is reported for GaN-based near-ultraviolet laser diodes (LDs). Under an electrical stress for several minutes, the threshold current of the LDs decreased, while the light output power and the operation voltage increased. The amplitude of the abnormal aging phenomena was found to be mainly related to the excess Mg concentration in the p-AlGaN electron blocking layer (EBL). It almost disappeared when the Mg concentration in the AlGaN EBL was reduced to 2??×??1019 cm?3. We propose that this phenomenon was related to the Mg?VN?H complex defects formed in highly doped AlGaN EBL ([Mg]??>??3??×??1019 cm?3).

Thin AlN spacer layer is widely adopted in Al(In)GaN/AlN/GaN heterostructures to improve the mobility of two-dimensional electron gas (2DEG) by increasing the 2DEG confinement and reducing the remote alloy scattering. However, the nominal AlN spacer is found to be an AlGaN spacer mainly due to the unintentional incorporation of Ga from the decomposition of the underlying GaN channel during the growth of AlN spacer and the temperature ramping process from the GaN channel to the Al(In)GaN barrier. The growth conditions of AlN spacer layer were varied systematically to study their influences on the unintentional Ga incorporation and the 2DEG mobility of AlInGaN/AlN/GaN heterostructures. Interestingly, the nominal AlN spacer (or AlGaN spacer in fact) can still effectively reduce the remote alloy scattering and deliver a 2DEG mobility substantially higher than that of AlInGaN/GaN heterostructures with no spacer or an AlGaN spacer uniformly containing a Ga content similar to the average Ga composition of the nominal AlN spacer. A simplified microstructure of the nominal AlN spacer with an inhomogeneous Ga incorporation was proposed to discuss about the influence of the spacer on the 2DEG mobility.

We report the roles of island coalescence rate and strain relaxation in the development of anisotropic in-plane strains, striation feature, and subsequent degree of polarization in NH3-flow-rate-dependent m-plane GaN. In the high-NH3-flow-rate samples, the results of cathodoluminescence, polarized Raman and in situ optical reflectance measurements reveal that a slower coalescence and unrelieved lattice misfit strain lead to larger anisotropic in-plane strains, striated surface and luminescence patterns, and a lower density of basal-plane stacking fault (BSF) and prismatic stacking fault (PSF). In contrast, a lower NH3 flow rate leads to more rapid island coalescence and fully relaxed lattice misfit strain such that relaxed in-plane strains, a reduced striation surface, and a higher density of BSF and PSF are observed. It is suggested that the anisotropic in-plane strains, striation feature, and BSF and PSF density are consequences of how rapidly coalescence occurs and the degree of relaxation of lattice misfit strain. In addition, the simulation results of the k · p perturbation approach confirm a larger anisotropic strain results in a smaller degree of polarization. The research results provide important information for optimized growth of nonpolar III-nitrides.

Growth of cubic InN (c-InN) under N-rich condition was achieved by metalorganic chemical vapour deposition on GaP(1 0 0) substrates with a cubic GaN (c-GaN) buffer layer. Insertion of the c-GaN buffer layer suppressed In droplet formation in c-InN. X-ray diffraction and transmission electron microscopy investigations showed that the InN layers have zincblende structure with only a small fraction of oxide phase inclusions and no significant hexagonal InN is present. By systemically varying growth conditions, it was found that the c-InN growth is dominated mainly by In adatoms' surface diffusion and InN surface decomposition and three distinct regimes of c-InN growth are observed. The growth of c-InN on c-GaN/GaP(1 0 0) templates eventually followed a three-dimensional growth mode in the thermally activated growth regime and density and size distribution of c-InN dots significantly changed with substrate temperature and growth rate. These results provide a stronger understanding of the growth mechanism to design and engineer InN-based nanostructures with desired shapes for potential technological applications.