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Vertically aligned nanowire arrays, with high surface-to-volume ratio and efficient light-trapping absorption, have attracted much attention for photoelectric devices. In this paper, vertical β-Ga2O3 nanowire arrays with an average diameter/height of 110/450 nm have been fabricated by the inductively coupled plasma etching technique. Then a metal-semiconductor-metal structured solar-blind photodetector (PD) has been fabricated by depositing interdigital Ti/Au electrodes on the nanowire arrays. The fabricated β-Ga2O3 nanowire PD exhibits ∼10 times higher photocurrent and responsivity than the corresponding film PD. Moreover, it also possesses a high photocurrent to dark current ratio (I light/I dark) of ∼104 and a ultraviolet/visible rejection ratio (R 260 nm/R 400 nm) of 3.5 × 103 along with millisecond-level photoresponse times.

We report here the preparation of porous single-crystal GaN films by direct top-down high temperature nitridation of β-Ga2O3. It is found that, although the crystallographic orientations of the original β-Ga2O3 samples are different, the GaN films converted from both bulk (200) β-Ga2O3 single crystal and (−201) β-Ga2O3 film have the same (0002) orientation and show no obvious stress. And we also found that the crystal quality of converted GaN strongly depends on the crystal quality of β-Ga2O3. In particular, we have successfully converted β-Ga2O3 film entirely to single-crystal GaN porous film on sapphire, and the self-separation happens naturally between the porous GaN film and sapphire.

We have successfully obtained a hexagonal single crystal GaN layer with (002) orientation by nitridating β-Ga2O3 film under NH3 flow despite structural mismatch between β-Ga2O3 and hexagonal GaN. The conversion process of β-Ga2O3 to GaN has also been systematically investigated. The nitridated GaN layer shows a network structure without significant stress, which makes it very suitable to be used as a template for the epitaxial growth of high-quality GaN films. The GaN/Ga2O3 heterostructure can also be used to obtain free-standing GaN (FS-GaN) films by self-separation or chemical lift-off (CLO) process due to the selective etching of β-Ga2O3. Further investigation demonstrates the feasibility of the in situ growth of low-stress FS-GaN substrates by halide vapor phase epitaxy (HVPE).

Cubic silicon carbide (3C-SiC) has been grown on Si (111) substrate by chemical vapor deposition. The crystal quality of SiC with the SiCN buffer layer is obviously improved comparing with that grown on the SiC buffer layer. The SiCN film, composed of elemental Si and SiC1-xNx alloy, is formed by the constant-source diffusion of the C and N atoms into the Si substrate. During the 3C-SiC high temperature growth, this in-situ formed SiCN film evolves into a double sub-layered SiCN buffer, including the SiC1-xNx alloy and the SiCN composite sub-layers. The SiC1-xNx alloy layer (x from 0 to 0.09) results from the limited-source diffusion of N from the in-situ formed SiCN film to the subsequently deposited SiC. The SiCN composite layer, composed of elemental Si and SiC1-xNx alloy (x from 0.09 to 0.06), is caused by the constant-source diffusion of C from the SiC towards the substrate, as well as the outgoing diffusion of N in the in-situ formed SiCN film. The SiCN buffer layer can effectively accommodate the lattice mismatch between SiC and Si substrate.

We report here high-quality β-Ga2O3 nanorods (NRs) grown on sapphire substrates by hydrothermal method. Ammoniating the β-Ga2O3 NRs results in strain-free wurtzite gallium nitride (GaN) NRs. It was shown by XRD and Raman spectroscopy that β-Ga2O3 was partially converted to GaN/β-Ga2O3 at 1000 °C and then completely converted to GaN NRs at 1050 °C, as confirmed by high-resolution transmission electron microscopy (HRTEM). There is no band-edge emission of β-Ga2O3 in the cathodoluminescence spectrum, and only a deep-level broad emission observed at 3.68–3.73 eV. The band edge emission (3.39 eV) of GaN NRs converted from β-Ga2O3 can also be observed.

We have developed a simple top-down technique for preparing GaN porous templates with high single-crystallinity by high-temperature nitridation of β-Ga2O3 films. The GaN template converted from β-Ga2O3 film contains many voids and exhibits good-crystallinity with (0 0 2) orientation. The results confirm that the GaN porous template is stress-free, and the re-nitridation can effectively improve the crystal quality and luminescence properties of the GaN template. A ∼100 μm GaN thick film has been grown on the GaN porous template by halide vapor phase epitaxy, and self-separation of the GaN thick film from the sapphire substrate has been successfully achieved.

The SiCN film, which has the same crystal structure as 3C-SiC verified by X-ray diffraction and Raman scattering, has been formed on Si (111) substrate by constant-source diffusion using a chemical vapor deposition system. Via X-ray photoelectron spectroscopy, the film has been identified as the mixture of elemental Si and SiC 1 − x N x alloy, with the SiC 1 − x N x content decreases from 87.4% to 17.9% and the mole fraction x increases from 0.17 to 0.21, as the depth extends from the near surface to the interface. The measured composition distribution can be well explained by the constant-source diffusion model, confirms that the formation mechanism of the SiCN film is the diffusion of C and N into the Si substrate. This provides a new method for fabricating crystalline SiCN thin films.

GaN pyramid arrays have been successfully synthesized by selective photo-assisted chemical etching in a K2S2O8/KOH solution. A detailed analysis of time evolution of surface morphology has been conducted, which describes an etching process of GaN pyramids. Room temperature cathodoluminescence images indicate that these pyramids are composed of crystalline GaN surrounding dislocations, which is caused by the greater recombination rate of electrons and holes at dislocation than that of crystalline GaN. The Raman results show a stress relaxation in GaN pyramids compared with unetched GaN. The optical property of both unetched GaN and GaN pyramids has been studied by photoluminescence. The formation mechanism and feature of GaN pyramids are also rationally explained.

The InGaN/GaN multiple quantum wells (MQWs) with truncated pyramid structure have been successfully epitaxially grown on the GaN hexagonal pyramids template through a simple and low-cost etch-regrown process. GaN hexagonal pyramids template was contained by a convenient photo-assisted chemical (PAC) etching method. The truncated pyramids are composed of { 10 1 ¯ 1 ¯ } and { 11 2 ¯ 2 } semi-polar facets as well as (0001) polar facet. It was observed that the InGaN/GaN MQWs substantially emitted broad spectrum with multiple peaks by room temperature photoluminescence (PL). The cathodoluminescence of MQWs red-shifts as the location moves from bottom to top on the facets due to the indium diffusion mechanism.

Stress-free InN nanowires (NWs) have been grown on Au/graphene/GaN/sapphire substrate by atmospheric pressure metal-In sublimation method. The phonon E2(high) and A1(LO) mode in Raman spectra of InN grown on graphene is observed at 490.8 and 585 cm−1 respectively, which is similar to that of bulk InN. The high resolution TEM (HRTEM) image of InN NWs shows that the interplanar spacing of (0 0 0 2) and (1 1 −2 0) are 2.85 A and 1.77 A, which is according with the XRD result. All the results confirm that Au-catalyst assists the growth of InN NWs, and the stress is released by the aid of the graphene inter-layer.

The GaN thick films have been grown on porous GaN template and planar metal-organic chemical vapor deposition (MOCVD)-GaN template by halide vapor phase epitaxy (HVPE). The analysis results indicated that the GaN films grown on porous and planar GaN templates under the same growth conditions have similar structural, optical, and electrical properties. But the porous GaN templates could significantly reduce the stress in the HVPE-GaN epilayer and enhance the photoluminescence (PL) intensity. The voids in the porous template were critical for the strain relaxation in the GaN films and the increase of the PL intensity. Thus, the porous GaN converted from β-Ga2O3 film as a novel promising template is suitable for the growth of stress-free GaN films.

High-quality AlInGaN film grown by metal organic chemical vapor deposition is realized by fabricating the AlGaN/InGaN short-period superlattices, and then, Schottky contacts are fabricated on the AlInGaN superlattices. The crystal quality of AlInGaN superlattice samples is characterized by X-ray diffraction and scanning electron microscopy, and the correlation between the compositions of AlInGaN and growth condition is further clarified. The current transport mechanisms of the Pt/Au Schottky contacts to AlInGaN superlattice samples with different background carrier concentrations are studied by current–voltage (I–V) characteristics and theory analysis based on the thermionic emission (TE) and thermionic field emission (TFE) models. It is found that the results obtained from the TE and TFE models are almost identical for the Schottky contact to strain-balanced AlInGaN sample with low background carrier concentration, indicating that the thermionic emission is a dominant current transport mechanism. However, the Schottky contacts fabricated using high background carrier concentration AlInGaN samples demonstrate degraded barrier characteristics due to an occurrence of donor-like defects which result in the defect-assisted tunneling current, and thus, the combination of tunneling transport and thermionic emission constitutes the current transport mechanisms in Schottky contacts.

The nanoporous InGaN/GaN multiple quantum wells (MQWs) has been fabricated through rapid thermal annealing (RTA) and inductively coupled plasma (ICP) dry etching process using self-assembled Ni nanoporous masks. In comparison with the as-grown planar InGaN/GaN MQWs, both internal quantum efficiency and light extraction efficiency for nanoporous InGaN/GaN MQWs are increased, which can be concluded from the photoluminescence (PL) measurements. The thermal activation energy of nanoporous structure (107.44 meV) is significantly higher than that of the as-grown sample (33.02 meV) from temperature-dependent PL measurement, indicating that carriers are well confined and the non-radiative recombination caused by the dislocations and other defects has been reduced. Besides, enhanced light scattering in the disordered nanoporous system can further increase the output emission intensity. The enhanced performance of nanoporous InGaN/GaN MQWs reveals its promising applications for high-efficiency light-emitting devices.

Single-crystal GaN layers have been obtained by nitriding β-Ga2O3 films in NH3 atmosphere. The effect of the temperature and time on the nitridation and conversion of Ga2O3 films have been investigated. The nitridation process results in lots of holes in the surface of films. The higher nitridation temperature and longer time can promote the nitridation and improve the crystal quality of GaN films. The converted GaN porous films show the single-crystal structures and low-stress, which can be used as templates for the epitaxial growth of high-quality GaN.

Anodic aluminum oxide (AAO) films with different porosities were fabricated on the surfaces of InGaN/GaN multiple quantum wells (MQWs). We measured the photoluminescence spectra of these samples and then calculated the light extraction enhancement factors (E). It was found that the light extraction of the MQWs with the AAO films has been greatly enhanced compared to the area without AAO coverage. The maximum value of E reaches 3.5 at a certain wavelength and 2.29 in the integral intensity. The increase of E with the decrease of the porosities demonstrates that the nano-hole structure in the AAO films plays an important role in enhancing the light extraction efficiency.

InGaN/GaN single quantum well (SQW) structures under various InGaN growth temperatures have been grown by metal organic chemical vapor deposition (MOCVD), the surface morphologies and optical properties are investigated. The radius of the typical V-pits on the SQW surface is affected by the InGaN well-temperature, and the surface roughness decreased as the well-temperature reduced. Room-temperature photoluminescence (PL) and cathode luminescence (CL) shows the quantum well and quantum dot (QD)-like localized state light emission of the SQWs grown at 700 and 690 °C, respectively, whereas the samples grown at 670 and 650 °C present hybrid emission peaks. Excitation power dependent PL spectra indicates the QD-like localized state emission dominates at low excitation power and the quantum well emission starts to take over at high excitation power.

Uniform InGaN nanodots were successfully grown on SiO2 pretreated GaN surface. It was found that the InGaN nanodots were 20 nm in diameter and 5 nm in height, approximately. After the growth of two peri- ods of InGaN/GaN quantum wells on the surface of In- GaN nanodots, nanodot structure still formed in the In- GaN well layer caused by the enhanced phase separation phenomenon. Dual-color emissions with different behavior were observed from photoluminescence (PL) spectrum of InGaN nanodots hybrid with InGaN/GaN quantum wells. A significant blueshift and a linewidth broadening were mea- sured for the low-energy peak as the increase of PL excita- tion power, while a slight blueshift and a linewidth narrow- ing occurred for the high-energy peak. Accordingly, these two peaks were assigned to be from the In-rich nanodots and quantized state transition from the InGaN/GaN quan- tum wells with indium content, respectively.

Selective area epitaxy has been applied to grow GaN stripe templates with side facts, followed by InGaN/GaN multiple quantum well (MQW) structures. Stripes oriented along the direction of [1 1 −2 0] form smooth triangular {1 −1 0 1} facets, while stripes in the [1 −1 0 0] direction produce incomplete coalesced {1 1 −2 2} side facets with planar (0 0 0 1) surface under the same growth conditions. Room-temperature luminescence property is investigated by using photoluminescence (PL) and cathodeluminescence (CL) spectra. It is found that uniform emission at 400 nm has been obtained for triangular MQWs oriented the direction of [1 1 −2 0]. With respect to the MQWs grown along the [1 −1 0 0] direction, large redshift is achieved for the ridge area compared to the sidewall due to the lateral vapor diffusion and surface migration.

The m-plane InN (1 \(\bar 1\) 00) epilayers have been grown on a LiAlO2 (1 0 0) substrate by a two-step growth method using a metal-organic chemical vapor deposition (MOCVD) system. The low temperature InN buffer layer (LT-InN) is introduced to overcome the drawbacks of thermal instability of LiAlO2 (LAO) and to relieve the strains due to a large thermal mismatch between LAO and InN. Then the high temperature m-plane InN (1 \(\bar 1\) 00) epilayers (HT-InN) were grown. The results of X-ray diffraction (XRD) suggest that the m-plane InN (1 \(\bar 1\) 00) epilayer is a single crystal. The X-ray rocking curves (ω scans) (XRC) and atomic force microscopy (AFM) indicate that the m-plane InN (1 \(\bar 1\) 00) epilayer has anisotropic crystallographic properties. The PL studies of the materials reveal a remarkable energy band gap structure around 0.70 eV at 15 K.

GaN thin films were etched by inductively coupled plasma (ICP). The effects of BCl3 and Ar with different Cl2 fraction are studied and compared. The ICP power and RF power are also altered to investigate the different effects by using Cl2/BCl3 or Cl2/Ar as etching gases. The etch rate and surface morphology of the etched surface are characterized by using surface profiler, scanning electron microscopy and atomic force microscopy. The root-mean-square roughness values are systematically compared. It is found that the etch rates of Cl2/Ar are higher than that of the Cl2/BCl3 in the Cl2 fraction ranging from 10 to 90%. When the ICP power is increased, the RMS roughness of GaN surface after ICP etching shows reverse trend between Cl2/BCl3 and Cl2/Ar gas mixture. The results indicate quite different features using Cl2/BCl3 and Cl2/Ar for GaN ICP etcing under the same conditions.

Two-inch Ga2O3 films with ( )-orientation are grown on c-sapphire at 850–1050°C by hydride vapor phase epitaxy. High-resolution x-ray diffraction shows that pure β-Ga2O3 with a smooth surface has a higher crystal quality, and the Raman spectra reveal a very small residual strain in β-Ga2O3 grown by hydride vapor phase epitaxy compared with bulk single crystal. The optical transmittance is higher than 80% in the visible and near-UV regions, and the optical bandgap energy is calculated to be 4.9 eV.

InN films with electron concentration ranging from n similar to 10(17) to 10(20) cm(-3) grown by metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were investigated by variable-temperature photoluminescence and absorption measurements. The energy positions of absorption edge as well as photoluminescence peak of these InN samples with electron concentration above 10(18) cm(-3) show a distinct S-shape temperature dependence. With a model of potential fluctuations caused by electron-impurity interactions, the behavior can be quantitatively explained in terms of exciton freeze-out in local potential minima at sufficiently low temperatures, followed by thermal redistribution of the localized excitons when the band gap shrinks with increasing temperature. The exciton localization energy sigma (loc) is found to follow the n (5/12) power relation, which testifies to the observed strong localization effects in InN with high electron concentrations.

High-resolution X-ray diffraction has been used to analyze GaN(0001) epitaxial layers on sapphire substrates. Several structural properties of GaN, including the lattice constants, strains, and dislocation densities are revealed by the technique of X-ray dffraction (XRD). Lattice constants calculated from the omega/2theta scan are c=0.5185 nm and a=0.3157 nm. Also, the in-plane strain is −1.003%, while out of the plane, the epitaxial film is almost relaxed. Several methods are used to deduce the mosaicity and dislocation density of GaN, showing that the edge type dislocations are the overwhelming majority.

Uniformly distributed GaN nano-pillars have been successfully fabricated by inductively coupled plasma etching using self-organized Nickel nano-islands mask on GaN film. Ni nano-islands have been formed through the annealing under the ammonia. The size and distribution of GaN nano-pillars can be controlled by the thickness of Ni film and the NH 3 annealing time. The optimum diameter and the density of GaN pillars by analyzing the top-view SEM images are about 350 nm and 2.6×10 8 cm −2 , respectively. The photoluminescence (PL) measurements showed the improved optical properties and the strain relaxation in GaN nano-pillars compared to the bulk materials.

A new approach to fabricating high-quality AlInGaN film as a lattice-matched barrier layer in multiple quantum wells (MQWs) is presented. The high-quality AlInGaN film is realized by growing the AlGaN/InGaN short period superlattices through metalorganic chemical vapor deposition, and then being used as a barrier in the MQWs. The crystalline quality of the MQWs with the lattice-matched AlInGaN barrier and that of the conventional InGaN/GaN MQWs are characterized by x-ray diffraction and scanning electron microscopy. The photoluminescence (PL) properties of the InGaN/AlInGaN MQWs are investigated by varying the excitation power density and temperature through comparing with those of the InGaN/GaN MQWs. The integral PL intensity of InGaN/AlInGaN MQWs is over 3 times higher than that of InGaN/GaN MQWs at room temperature under the highest excitation power. Temperature-dependent PL further demonstrates that the internal quantum efficiency of InGaN/AlInGaN MQWs (76.1%) is much higher than that of InGaN/GaN MQWs (21%). The improved luminescence performance of InGaN/AlInGaN MQWs can be attributed to the distinct reduction of the barrier-well lattice mismatch and the strain-induced non-radiative recombination centers.

GaN nanorods are fabricated using inductively coupled plasma etching with Ni nano-island masks. The poly [2-methoxy-5-(2-ethyl)hexoxy-1,4-phenylenevinylene] (MEH-PPV)/GaN-nanorod hybrid structure is fabricated by depositing the MEH-PPV film on the GaN nanorods by using the spin-coating process. In the hybrid structure, the spatial separation is minimized to achieve high-efficiency non-radiative resonant energy transfer. Optical properties of a novel device consisting of MEH-PPV/GaN-nanorod hybrid structure is studied by analyzing photoluminescence (PL) spectra. Compared with the pure GaN nanorods, the PL intensity of the band edge emission of GaN in the MEH-PPV/GaN-nanorods is enhanced as much as three times, and the intensity of the yellow band is suppressed slightly. The obtained results are analyzed by energy transfer between the GaN nanorods and the MEH-PPV. An energy transfer model is proposed to explain the phenomenon.

The non-polar m-plane InGaN/GaN light-emitting diodes (LEDs) grown by metal-organic chemical vapor deposition (MOCVD) on LiAlO 2 (100) substrates were investigated. The structure of LEDs is composed by GaN p-n junction and five InGaN/GaN multiple quantum wells (MQWs). They emit blue or green light, respectively, with different indium composition of InGaN as active layers in the MQWs. Third-order satellite peaks are observed in high resolution X-ray diffraction spectrum, indicating good quantum well interfaces. For 800 × 800 μm 2 blue LED device, the output power achieves 3mW under 200 mA injection current. The electroluminescence (EL) spectra of both non-polar blue and green InGaN/ GaN LEDs show that the EL peak wavelength of LEDs almost saturates at certain position with increasing injection currents. This proves the absence of polarization fields in the active region present in c-plane structures.

The frequency dependence of capacitance-voltage (C-V) characteristics of Hg/GaN and Hg/InGaN/GaN Schottky contacts are investigated for 1KHz, 10KHz and 1MHz at room temperature. An anomalous peak in the C-V curves of Hg/GaN sample is observed but no peak for the Hg/InGaN/GaN sample occurs. The interface states, series resistance and minority-carrier injection would be the origin of this anomalous peak. The interface states density is calculated through C-V measurement of high and low frequencies, and a four-element circuit model is proposed to determine the series resistance. In addition, energy bind structures are taken into account. The three aspects above are analyzed for both two samples, and the origin of the anomalous peak is attributed to the series resistance and minority-carrier injection rather than the interface states.

GaN micro/nano-structures of different morphology were successfully preparated by the electrodeless photo-assisted chemical etching in KOH solution, with K2S2O8 as the oxidizing agent. The structure, crystalline quality, stress state and optical property of both normal GaN and etched GaN were provided by the combined results of scanning electron microscopy (SEM) images, cathodoluminescence mapping (CL mapping), high-resolution X-ray diffraction (HRXRD) detection, micro-Raman spectra and photoluminescence (PL), respectively. The results show that pits, micro/ nanopillars and nanowires of high crystalline quality are formed in the high KOH concentrations (1 mol/L) and the low light intensity; GaN pyramid arrays have been fabricated in 0.4 mol/L KOH solution with high UV light intensity, and CL mapping indicates that these pyramids are composed of crystalline GaN surrounding a dislocation; Whiskers can also be obtained in 0.1 mol/L KOH with high UV light intensity after etching. The whiskers are believed to be directly correlated with line-type defects of edge and mixed character. The formation mechanism and the feature of GaN different morphology were rationally explained. And the etching temperature and the polarity of GaN surface have an evident influence on the surface morphology of GaN.

A novel fishing rod-shaped GaN nanorod is successfully fabricated through a new method by using the two-step growth technology. This growth method is applicable to continuous synthesis and is able to produce a large number of single-crystalline GaN nanorods with a relatively high purity and at a low cost. X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy are used to characterize the as-synthesized nanorods. The results show that most of the nanorods consist of a main rod and a top curved thread. It is single-crystal GaN with hexagonal wurtzite structure. The representative photoluminescence spectrum at room temperature exhibits a strong UV light emission band centered at 370.8 nm. Furthermore, a possible two-stage growth mechanism of the fishing rod-shaped GaN nanorod is also briefly discussed.

Uniform GaN nanopillar arrays have been successfully fabricated by inductively coupled plasma etching using self-organized nickel nano-islands as the masks on GaN/sapphire. GaN nanopillars with diameters of 350 nm and densities of 2.6 × 108 cm−2 were demonstrated and controlled by the thickness of Ni film and the NH3 annealing time. These GaN nanopillars show improved optical properties and strain change compared to that of GaN film before ICP etching. Such structures with large-area uniformity and high density could provide additional advantages for light emission of light-emitting diodes, quality improvement of ELO regrowth, etc.

We report on the photoluminescent characteristics of InGaN/GaN multiple quantum well (MQW) nanorod arrays with high internal quantum efficiency. The InGaN/GaN MQWs are grown by metalorganic chemical vapor deposition on c-plane sapphire substrates, and then the MQW nanorod arrays are fabricated by using inductively coupled plasma etching with self-assembled Ni nanoparticle mask with low-damage etching technique. The typical diameter of the nanorods is from 200 nm to 300 nm and the length is around 800 nm, which almost is dislocation free. At room temperature, an enhancement of 3.1 times in total integrated photoluminescence intensity is achieved from the MQW nanorod arrays, in comparison to that of the as-grown MQW structure. Based on the temperature-dependent photoluminescence measurements, the internal quantum efficiency of the nanorod structure is 59.2%, i.e., 1.75 times of as-grown MQW structure (33.8%). Therefore, the nanorod structure with a significant reduction of defects can be a very promising candidate for highly efficient light emitting devices.