Evolution of microstructure, stress and dislocation of AlN thick film on nanopatterned sapphire substrates by hydride vapor phase epitaxy

A crack-free AlN film with 4.5 μm thickness was grown on a 2-inch hole-type nano-patterned sapphire substrates(NPSSs) by hydride vapor phase epitaxy(HVPE). The coalescence, stress evolution, and dislocation annihilation mechanisms in the AlN layer have been investigated. The large voids located on the pattern region were caused by the undesirable parasitic crystallites grown on the sidewalls of the nano-pattern in the early growth stage. The coalescence of the c-plane AlN was hindered by these three-fold crystallites and the special triangle void appeared. The cross-sectional Raman line scan was used to characterize the change of stress with film thickness, which corresponds to the characteristics of different growth stages of AlN. Threading dislocations(TDs) mainly originate from the boundary between misaligned crystallites and the c-plane AlN and the coalescence of two adjacent c-plane AlN crystals, rather than the interface between sapphire and AlN.

Growth of β-Ga_2O_3 Films on Sapphire by Hydride Vapor Phase Epitaxy

Two-inch Ga_2O_3 films with（ˉ201）-orientation are grown on c-sapphire at 850–1050°C by hydride vapor phase epitaxy. High-resolution x-ray diffraction shows that pure β-Ga_2O_3 with a smooth surface has a higher crystal quality, and the Raman spectra reveal a very small residual strain in β-Ga_2O_3 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 e V.

The fabrication of AlN by hydride vapor phase epitaxy

Aluminum nitride(AlN)is the promising substrates material for the epitaxial growth ofⅢ-nitrides devices,such as high-power,high-frequency electronic,deep ultraviolet optoelectronics and acoustic devices.However,it is rather difficult to obtain the high quality and crack-free thick AlN wafers because of the low surface migration of Al adatoms and the large thermal and lattice mismatches between the foreign substrates and AlN.In this work,the fabrication of AlN material by hydride vapor phase epitaxy(HVPE)was summarized and discussed.At last,the outlook of the production of AlN by HVPE was prospected.

Influence comparison of N2 and NH3 nitrogen sources on AlN films grown by halide vapor phase epitaxy

A comparison of the nitrogen sources(N2 and NH3)influence on AlN films grown by high-temperature halide vapor phase epitaxy(HVPE)is reported.The x-ray rocking curves(XRCs)indicate that the full width at half maximum(FWHM)of(0002)plane for AlN films using N2 as nitrogen source is generally smaller than that using NH3.Optical microscope and atomic force microscope(AFM)results show that it is presently still more difficult to control the crack and surface morphology of AlN films with thicknesses of 5-10µm using N2 as the nitrogen source compared to that using NH3.Compared with one-step growth,two-step growth strategy has been proved more effective in stress control and reducing the density of threading dislocations for AlN epilayers using N2 as the nitrogen source.These investigations reveal that using N2 as nitrogen source in HVPE growth of AlN is immature at present,but exhibits great potential.

Porous AlN films grown on C-face SiC by hydride vapor phase epitaxy

We report the growth of porous AlN films on C-face SiC substrates by hydride vapor phase epitaxy(HⅤPE).The influences of growth condition on surface morphology,residual strain and crystalline quality of Al N films have been investigated.With the increase of theⅤ/Ⅲratio,the growth mode of Al N grown on C-face 6H-SiC substrates changes from step-flow to pit-hole morphology.Atomic force microscopy(AFM),scanning electron microscopy(SEM)and Raman analysis show that cracks appear due to tensile stress in the films with the lowestⅤ/Ⅲratio and the highestⅤ/Ⅲratio with a thickness of about 3μm.In contrast,under the mediumⅤ/Ⅲratio growth condition,the porous film can be obtained.Even when the thickness of the porous Al N film is further increased to 8μm,the film remains porous and crack-free,and the crystal quality is improved.

Growth of a-Plane GaN Films on r-Plane Sapphire by Combining Metal Organic Vapor Phase Epitaxy with the Hydride Vapor Phase Epitaxy

Hydride vapor phase epitaxy （HVPE） is utilized to grow nonpolar a-plane GaN layers on r-plane sapphire templates prepared by metal organic vapor phase epitaxy （MOVPE）. The surface morphology and microstructures of the samples are characterized by atomic force microscopy. The full width at half maximum （FWHM） of the HVPE sample shows a W-shape and that of the MOVPE sample shows an M-shape plane with the degree of 0 in the high-resolution x-ray diffraction （HRXRD） results. The surface morphology attributes to this significant anisotropic. HRXRD reveals that there is a significant reduction in the FWHM, both on-axis and off-axis for HVPE GaN are compared with the MOVPE template. The decrease of the FWHM of E2 （high） Raman scat tering spectra further indicates the improvement of crystal quality after HVPE. By comparing the results of secondary- ion-mass spectroscope and photoluminescence spectrum of the samples grown by HVPE and MOVPE, we propose that C-involved defects are originally responsible for the yellow luminescence.

Halide vapor phase epitaxy of monolayer molybdenum diselenide single crystals

Single-crystalline transition metal dichalcogenides(TMD)films are of potential application in future electronics and optoelectronics.In this work,a halide vapor phase epitaxy(HVPE)strategy was proposed and demonstrated for the epitaxy of molybdenum diselenide(MoSe_(2))single crystals,in which metal halide vapors were in-situ produced by the chlorination of molybdenum as sources for the TMD growth.Combined with the epitaxial sapphire substrate,unidirectional domain alignment was successfully achieved and monolayer single-crystal MoSe_(2) films have been demonstrated on a 2-inch wafer for the first time.A series of characterizations ranging from centimeter to nanometer scales have been implemented to demonstrate the high quality and uniformity of the MoSe_(2).This work provides a universal strategy for the growth of TMD single-crystal films.

Growth of Free-Standing AlN Nanocrystals on Nanorod ZnO Template

AlN film is deposited on a nanorod ZnO template by metalorganic chemical vapor deposition. Scanning electron microscopy measurements reveal that this film forms a lying nanorod surface. The grazing incidence X- ray diffraction further proves that it is entirely a wurtzite AIN structure, and the average size of the crystallite grains is about 12nm,which is near the ZnO nanorod diameter （30nm）. This means that the nanorod ZnO template can restrict the AlN lateral overgrowth. Additionally, by etching the ZnO template with H2 at high temperatures,we directly achieve epitaxial lift-off during the growth process. Eventually, free-standing AlN nanocrystals are achieved,and the undamaged area is near 1cm × 1cm. We define the growth mechanism as a ＂grow-etch- merge＂ process.

Progress in bulk GaN growth

Three main technologies for bulk GaN growth, i.e., hydride vapor phase epitaxy （HVPE）, Na-flux method, and am- monothermal method, are discussed. We report our recent work in HVPE growth of GaN substrate, including dislocation reduction, strain control, separation, and doping of GaN film. The growth mechanisms of GaN by Na-flux and ammonother- mal methods are compared with those of HVPE. The mechanical behaviors of dislocation in bulk GaN are investigated through nano-indentation and high-space resolution surface photo-voltage spectroscopy. In the last part, the progress in growing some devices on GaN substrate by homo-epitaxy is introduced.

Response time improvement of AlGaN photoconductive detectors by adjusting crystal-nuclei coalescence process in metal organic vapor phase epitaxy

AlGaN photoconductive ultraviolet detectors are fabricated to study their time response characteristics. Persistent photoconductivity, a deterring factor for the detector response time, is found to be strongly related to the grain boundary density in AlGaN epilayers. By improving the crystal-nuclei coalescence process in metal organic vapor phase epitaxy, the grain-boundary density can be reduced, resulting in an-order-of-magnitude decrease in response time.

Electrochemical liftoff of freestanding GaN by a thick highly conductive sacrificial layer grown by HVPE

Separation technology is an indispensable step in the preparation of freestanding GaN substrate. In this paper, a largearea freestanding GaN layer was separated from the substrate by an electrochemical liftoff process on a sandwich structure composed of an Fe-doped GaN substrate, a highly conductive Si-doped sacrificial layer and a top Fe-doped layer grown by hydride vapor phase epitaxy(HVPE). The large difference between the resistivity in the Si-doped layer and Fe-doped layer resulted in a sharp interface between the etched and unetched layer. It was found that the etching rate increased linearly with the applied voltage, while it continuously decreased with the electrochemical etching process as a result of the mass transport limitation. Flaky GaN pieces and nitrogen gas generated from the sacrificial layer by electrochemical etching were recognized as the main factors responsible for the blocking of the etching channel. Hence, a thick Si-doped layer grown by HVPE was used as the sacrificial layer to alleviate this problem. Moreover, high temperature and ultrasonic oscillation were also found to increase the etching rate. Based on the results above, we succeeded in the liftoff of ~ 1.5 inch GaN layer. This work could help reduce the cost of freestanding GaN substrate and identifies a new way for mass production.

GaN substrate and GaN homo-epitaxy for LEDs:Progress and challenges

After a brief review on the progresses in GaN substrates by ammonothermal method and Na-flux method and hydride vapor phase epitaxy （HVPE） technology, our research results of growing GaN thick layer by a gas fow-rnodulated HVPE, removing the GaN layer through an efficient self-separation process from sapphire substrate, and modifying the uniformity of multiple wafer growth are presented. The effects of surface morphology and defect behaviors on the GaN homo-epitaxial growth on free standing substrate are also discussed, and followed by the advances of LEDs on GaN substrates and prospects of their applications in solid state lighting.

Preparation of AlN film grown on sputter-deposited and annealed AlN buffer layer via HVPE

AlN films grown on sputter-deposited and annealed AlN buffer layer by high temperature hydride vapor phase epitaxy(HVPE)have been fabricated and structurally characterized.The crystalline quality and surface morphology of as-grown AlN films with various V/III ratios were studied and compared.The XRD results showed that the crystalline quality of the AlN film could be optimized when the growth V/III ratio was 150.At the same time,the full width at half-maximum(FWHM)values of(0002)-and(10¯12)-plane were 64 arcsec and 648 arcsec,respectively.As revealed by AFM,the AlN films grown with higher V/III ratios of 150 and 300 exhibited apparent hillock-like surface structure due to the low density of screw threading dislocation(TD).The defects microstructure and strain field around the HVPE-AlN/sputtered-AlN/sapphire interfaces have been investigated by transmission electron microscopy(TEM)technique combined with geometric phase analysis(GPA).It was found that the screw TDs within AlN films intend to turn into loops or half-loops after originating from the AlN/sapphire interface,while the edge ones would bend first and then reacted with others within a region of 400 nm above the interface.Consequently,part of the edge TDs propagated to the surface vertically.The GPA analysis indicated that the voids extending from sapphire to HVPE-AlN layer were beneficial to relax the interfacial strain of the best quality AlN film grown with a V/III ratio of 150.

Migration characterization of Ga and In adatoms on dielectric surface in selective MOVPE

Migration characterizations of Ga and In adatoms on the dielectric surface in selective metal organic vapor phase epitaxy （MOVPE） were investigated. In the typical MOVPE environment, the selectivity of growth is preserved for GaN, and the growth rate of GaN micro-pyramids is sensitive to the period of the patterned SiO2 mask. A surface migration induced model was adopted to figure out the effective migration length of Ga adatoms on the dielectric surface. Different from the growth of GaN, the selective area growth of InGaN on the patterned template would induce the deposition of InGaN polycrystalline particles on the patterned Si02 mask with a long period. It was demonstrated with a scanning electron microscope and energy dispersive spectroscopy that the In adatoms exhibit a shorter migration length on the dielectric surface.

Growth Front Evolution of GaN Thin Films on Sapphire Substrate During HVPE

The growth front evolution of GaN thin films deposited on sapphire substrate by hydride vapor phase epitaxity has been studied with atomic force microscope. The evolution of the surface morphology presents four features of stage with the growth process. In initial growth stage, the surface is granular, and the typical grain diameter is about 250 nm for t =0.1 min. 3D growth plays a key role before the films come up to full coalescence, which causes a rough surface. After 0. 1 min the growth dimension decreases with the increase of lateral over growth, the surface roughness obviously decreases. From 0.4 min to 3 rain, the growth front roughness increases gradually, and the evolution of the surface roughness exhibits the characteristics of self-affined fractal. Beyond 3 min, the root-mean-square decreases gradually, which means the deposition behavior from hyper-2D growth gradually turns into layer growth mode with the increase of growth time.

Kinetic Study on Uniformity of AlGaAs Grown by MOVPE

The growth kinetic factors affecting the uniformity of AlGaAs in a horizontal atmospheric MOVPE (AP-MOVPE) reactor with a horizontal susceptor were investigated. The decrease in the growth rate of AlGaAs (R-AlGaAs) in the now direction is related to both the gas depletion and the decreasing growth rate of GaAs (R-GaAs) with the increasing gas temperature in this direction. The change of x in the flow direction has relations not only to the different changes of R-GaAs and R-AlAs(the growth rate of AlAs) with the gas temperature, according to which x will increase in the now direction, but also to the higher depletion rate of Al containing species compared with that of Ga containing species, according to which x: will decrease. Due to the similar reason, the loss of symmetry about the length axis of the susceptor can also result in the monotone increase or decrease in thickness and x in the crosswise direction.

Studies on Growth of N-Polar InN Films by Pulsed Metal-organic Vapor Phase Epitaxy

We reported the growth of N-polar InN films on N-polar GaN/sapphire substrates by pulsed metal-organic vapor phase epitaxy. The crystalline quality, surface morphology, optical and electrical properties of N-polar InN films were investigated in details by varying the breaking time and trimethylindium（TMIn） duration of pulse cycle. It has been found that when the breaking time and the TMIn duration in each cycle remain at 30 and 60 s, respectively, the N-polar InN film obtained exhibits a better crystalline quality and greater optical properties. Meanwhile, the surface morphology and electrical properties of the N-polar InN films also greatly depend on the given growth conditions.

MOCVD Growth of Device-quality GaN on Sapphire

An AlN Layer grown by an ALE has been developed to improve the growth quality of GaN on Al 2O 3 substrate by low pressure metalorganic vapor phase epitaxy (LP MOVPE). An ALE AlN layer grown on Al 2O 3 substrate has a high quality and the structure is similar to GaN, this AlN layer can release the stress between Al 2O 3 substrate and GaN epilayer. By using this method, the orientation of substrate is extended to GaN epilayer, and the column tilt and the twist are improved, so as to obtain the device quality GaN.

A MOVPE method for improving InGaN growth quality by pre-introducing TMIn

We propose a metal organic vapor phase epitaxy(MOVPE) method of pre-introducing TMIn during the growth of uGa N to improve the subsequent growth of In Ga N and discuss the impact of this method in detail. Monitoring the MOVPE by the interference curve generated by the laser incident on the film surface, we found that this method avoided the problem of the excessive In Ga N growth rate. Further x-ray diffraction(XRD), photoluminescence(PL), and atomic force microscope(AFM) tests showed that the quality of In Ga N is improved. It is inferred that by introducing TMIn in advance, the indium atom can replace the gallium atom in the reactor walls, delivery pipes, and other corners. Hence the auto-incorporation of gallium can be reduced when In Ga N is grown, so as to improve the material quality.

Selective growth of wide band gap atomically thin Sb2O3 inorganic molecular crystal on WS2

Heterostructures combined by different individual two-dimensional(2D)materials are essential building blocks to realize unique electronic,optoelectronic properties and multifunctional applications.To date,the direct growth of 2D/2D atomic van der Waals heterostructures(vdWHs)have been extensively investigated.However,the heterostructures from 2D inorganic molecular crystals and atomic crystals have been rarely reported.Here we report two-step direct epitaxial growth of the inorganic molecular-atomic Sb2O3/WS2 vdWHs.The thickness of Sb2O3 nanosheets on WS2 nanosheets can be tuned by variable growth temperatures.Oriented growth behavior of Sb2O3 on WS2 was determined through statistics.Optical images,Raman spectra,Raman mappings and selected-area electron diffraction(SAED),etc.,reveal that Sb2O3/WS2 heterostructures are vertically stacked with high crystal quality.Electrical transport measurements demonstrate that the heterotransistors based on the heterostructures possess high current on/off ratio of 5 × 10^5,obvious gate-tunable and current rectification output characteristics.Optoelectronic characterizations show that the heterostructures have a clear photoresponse with high responsivity of 16.4 AW.The growth of vdWHs from 2D inorganic molecular-atomic crystals may open up new opportunities in 2D functional electronics and optoelectronics.