Effect of Misfit Dislocation Originated from Strained Layer on Photoluminescence Properties of InxGa1-xN／GaN Multiple Quantum Wells

Inx Ga1-x N/GaN multiple quantum well (MQW) samples with strain-layer thickness larger/less than the critical one are investigated by temperature-dependent photoluminescence and transmission electron microscopy, and double crystal x-ray diffraction. For the sample with the strained-layer thickness greater than the critical thickness, we observe a high density of threading dislocations generated at the MQW layers and extended to the cap layer. These dislocations result from relaxation of the strain layer when its thickness is beyond the critical thickness. For the sample with the strained-layer thickness greater than the critical thickness, temperature-dependent photoluminescence measurements give evidence that dislocations generated from the MQW layers due to strain relaxation are main reason of the poor photoluminescence property, and the dominating status change of the main peak with increasing temperature is attributed to the change of the radiative recombination from the areas including dislocations to the ones excluding dislocations.     

Effect of the V/Ⅲ ratio during buffer layer growth on the yellow and blue luminescence in undoped GaN epilayer

Effect of the V/III ratio during buffer layer growth on the yellow and blue luminescence in undoped GaN epilayer has been studied by means of photoluminescence spectroscopy and high resolution X-ray diffraction.It is found that the densities of screw and edge threading dislocations increase with the V/III ratio of the buffer layer,and the intensities of the yellow luminescence(YL) and blue luminescence(BL) emissions also increase dramatically.However,the density ratio of the edge threading dislocation to the screw threading dislocation remains invariant,as well as the intensity ratio of YL emission to BL emission.It can be concluded from these phenomena that the edge threading dislocation and screw threading dislocation can enhance the YL and BL emissions,respectively.     

Mosaic Structure Evolution in GaN Films with Annealing Time Grown by Metalorganic Chemical Vapour Deposition

We investigate mosaic structure evolution of GaN films annealed for a long time at 800℃ grown on sapphire substrates by metalorganic chemical vapour deposition by high-resolution x-ray diffraction. The result show that residual stress in GaN films is relaxed by generating edge-type threading dislocations （TDs） instead of screw-type TDs. Compared to as-grown GaN films, the annealed ones have larger mean twist angles corresponding to higher density of edge-type TDs but smaller mean tilt angles corresponding to lower density of screw-type TDs films. Due to the increased edge-type TD density, the lateral coherence lengths of the annealed GaN films also decrease. The results obtained from chemical etching experiment and grazing-incidence x-ray diffraction （GIXRD） also support the proposed structure evolution.     

Optical and Electronic Properties of InGaN／GaN Multi—Quantum—Wells Near—Ultraviolet Lighting—Emitting—Diodes Grown by Low—Pressure Metalorganic Vapour Phase Epitaxy

The near-ultraviolet lighting-emitting-diodes (UV-LEDs) with the InGaN/GaN multi-quantum-well (MQW) structure were grown by low-pressure metalorganic vapour phase epitaxy. The double crystal x-ray diffraction revealed a distinct second-order satellite peak. The near-ultraviolet InGaN/GaN MQW LEDs have been successfully fabricated to emit at 401.2nm with narrow FWHM of 14.3nm and the forward voltage of 3.6 V at 20 mA injection current at room temperature. With increasing forward current from l 0 mA to 50 mA, the redshift of the peak wavelength was observed due to the band-gap narrowing caused by heat generation.     

Dopant Effects on Defects in GaN Films Grown by Metal-Organic Chemical Vapour Deposition

The effects of dopants on the defects of GaN films were investigated by using different methods, such as wet,etching of pits, x-ray dit~action and photoluminescence (PL). Three kinds of the samples were prepared with different dopants, that is, nominally undoped, Si-doped and Mg-doped GaN films. It was found that the lowest density of the, etched pit was existed in the nominally undoped GaN, while the highest in the Mg-doped sample.The effects of the dopants on the, etching pits were discussed.     

The effect of single A1GaN interlayer on the structural properties of GaN epilayers grown on Si （111） substrates

High-quality and nearly crack-free GaN epitaxial layer was obtained by inserting a single AlGaN interlayer between GaN epilayer and high-temperature AlN buffer layer on Si (111) substrate by metalorganic chemical vapor deposition. This paper investigates the effect of AlGaN interlayer on the structural properties of the resulting GaN epilayer. It confirms from the optical microscopy and Raman scattering spectroscopy that the AlGaN interlayer has a remarkable effect on introducing relative compressive strain to the top GaN layer and preventing the formation of cracks. X-ray diffraction and transmission electron microscopy analysis reveal that a significant reduction in both screw and edge threading dislocations is achieved in GaN epilayer by the insertion of AlGaN interlayer. The process of threading dislocation reduction in both AlGaN interlayer and GaN epilayer is demonstrated.     

Improved crystal quality of GaN film with the in-plane lattice-matched Ino.17Alo.s3N interlayer grown on sapphire substrate using pulsed metal-organic chemical vapor deposition

We report on an improvement in the crystal quality of GaN film with an Ino.17Alo.83N interlayer grown by pulsed metal-organic chemical vapor deposition, which is in-plane lattice-matched to GaN films. The indium composition of about 17% and the reductions of both screw and edge threading dislocations （TDs） in GaN film with the InA1N interlayer are estimated by high resolution X-ray diffraction. Transmission electron microscopy （TEM） measurements are employed to understand the mechanism of reduction in TD density. Raman and photoluminescence measurements indicate that the InA1N interlayer can improve the crystal quality of GaN film, and verify that there is no additional residual stress induced into the GaN film with InA1N interlayer. Atomic force microscopy measurement shows that the InA1N interlayer brings in a smooth surface morphology of GaN film. All the results show that the insertion of the InA1N interlayer is a convenient method to achieve excellent crystal quality in GaN epitaxy.     

Improved crystal quality of GaN film with the in-plane lattice-matched In_(0.17)Al_(0.83)N interlayer grown on sapphire substrate using pulsed metal organic chemical vapor deposition

We report on an improvement in the crystal quality of GaN film with an In0.17Al0.83N interlayer grown by pulsed metal–organic chemical vapor deposition, which is in-plane lattice-matched to GaN films. The indium composition of about 17% and the reductions of both screw and edge threading dislocations(TDs) in GaN film with the InAlN interlayer are estimated by high resolution X-ray diffraction. Transmission electron microscopy(TEM) measurements are employed to understand the mechanism of reduction in TD density. Raman and photoluminescence measurements indicate that the InAlN interlayer can improve the crystal quality of GaN film, and verify that there is no additional residual stress induced into the GaN film with InAlN interlayer. Atomic force microscopy measurement shows that the InAlN interlayer brings in a smooth surface morphology of GaN film. All the results show that the insertion of the InAlN interlayer is a convenient method to achieve excellent crystal quality in GaN epitaxy.     

Improved Semipolar(11(2|-)2) GaN Quality Grown on m-Plane Sapphire Substrates by Metal Organic Chemical Vapor Deposition Using Self-Organized SiN_x Interlayer

The effect of a self-organized SiN_x interlayer on the defect density of(11(2|-)2) semipolar GaN grown on m-plane sapphire is studied by transmission electron microscopy,atomic force microscopy and high resolution x-ray diffraction.The SiN_x interlayer reduces the c-type dislocation density from 2.5 × 10~(10) cm~(-2) to 5 × 10~8 cm~(-2).The SiN_x interlayer produces regions that are free from basal plane stacking faults(BSFs) and dislocations.The overall BSF density is reduced from 2.1 × 10~5 cm~(-1) to 1.3 × 10~4 cm~(-1).The large dislocations and BSF reduction in semipolar(11(2|-)2) GaN with the SiN_x interlayer result from two primary mechanisms.The first mechanism is the direct dislocation blocking by the SiN_x interlayer,and the second mechanism is associated with the unique structure character of(11(2|-)2) semipolar GaN.     

Comparative study of different properties of GaN films grown on(0001) sapphire using high and low temperature AlN interlayers

Comparative study of high and low temperature AlN interlayers and their roles in the properties of GaN epilayers prepared by means of metal organic chemical vapour deposition on (0001) plane sapphire substrates is carried out by high resolution x-ray diffraction, photoluminescence and Raman spectroscopy. It is found that the crystalline quality of GaN epilayers is improved significantly by using the high temperature AlN interlayers, which prevent the threading dislocations from extending, especially for the edge type dislocation. The analysis results based on photoluminescence and Raman measurements demonstrate that there exist more compressive stress in GaN epilayers with high temperature AlN interlayers. The band edge emission energy increases from 3.423~eV to 3.438~eV and the frequency of Raman shift of $E_{2 }$(TO) moves from 571.3~cm$^{ - 1}$ to 572.9~cm$^{ - 1}$ when the temperature of AlN interlayers increases from 700~$^{\circ}$C to 1050~$^{\circ}$C. It is believed that the temperature of AlN interlayers effectively determines the size, the density and the coalescence rate of the islands, and the high temperature AlN interlayers provide large size and low density islands for GaN epilayer growth and the threading dislocations are bent and interactive easily. Due to the threading dislocation reduction in GaN epilayers with high temperature AlN interlayers, the approaches of strain relaxation reduce drastically, and thus the compressive stress in GaN epilayers with high temperature AlN interlayers is high compared with that in GaN epilayers with low temperature AlN interlayers.     

Growth and Optical Properties of Double Heterostructure GaN／InGaN／GaN Films with Large Composition

The double heterostructure GaN/InGaN/GaN films with different thicknesses of the InGaN layer were grown at 780℃ or 800℃ by metal-organic chemical vapour deposition.The samples were investigated using x-ray diffraction (XRD),room-temperature photoluminescence (PL) and Raman scattering.The dependences of the samples on both the growth temperature and the thickness of the InGaN layer were studied.The composition of InGaN was determined by the results of XRD,and the bowing parameter of InGaN was calculated in terms of the PL spectra.When the thickness of the InGaN layer was reduced,the phase separation of InGaN was found in some samples.The raman frequency of the A1(LO) and E2(low) modes in all the samples shifted and did not agree with Vegard‘s law.     

Different influences of Schottky metal on the strain and relative permittivity of barrier layer between AIN/GaN and AIGaN/GaN heterostructure Schottky diodes

Ni/Au Schottky contacts on AlN/GaN and AlGaN/GaN heterostructures are fabricated. Based on the measured current-voltage and capacitance-voltage curves, the polarization sheet charge density and relative permittivity are analyzed and calculated by self-consistently solving Schrodinger＇s and Poisson＇s equations. It is found that the values of relative permittivity and polarization sheet charge density of AlN/GaN diode are both much smaller than the ones of AlGaN/GaN diode, and also much lower than the theoretical values. Moreover, by fitting the measured forward 1-V curves, the extracted dislocations existing in the barrier layer of the AlN/GaN diode are found to be much more than those of the AlGaN/GaN diode. As a result, the conclusion can be made that compared with AlGaN/GaN diode the Schottky metal has an enhanced influence on the strain of the extremely thinner AlN barrier layer, which is attributed to the more dislocations.     

Different influences of Schottky metal on the strain and relative permittivity of barrier layer between AlN/GaN and AlGaN/GaN heterostructure Schottky diodes

Ni/Au Schottky contacts on AlN/GaN and AlGaN/GaN heterostructures are fabricated.Based on the measured current–voltage and capacitance-voltage curves,the polarization sheet charge density and relative permittivity are analyzed and calculated by self-consistently solving Schrdinger’s and Poisson’s equations.It is found that the values of relative permittivity and polarization sheet charge density of AlN/GaN diode are both much smaller than the ones of AlGaN/GaN diode,and also much lower than the theoretical values.Moreover,by fitting the measured forward I–V curves,the extracted dislocations existing in the barrier layer of the AlN/GaN diode are found to be much more than those of the AlGaN/GaN diode.As a result,the conclusion can be made that compared with AlGaN/GaN diode the Schottky metal has an enhanced influence on the strain of the extremely thinner AlN barrier layer,which is attributed to the more dislocations.     

Effects of AlGaN/AlN Stacked Interlayers on GaN Growth on Si （111）

We report the growth of high quality and crack-free GaN film on Si （111） substrate using Al0.2Ga0.8N/AlN stacked interlayers. Compared with the previously used single AlN interlayer, the AlGaN/AlN stacked interlayers can more effectively reduce the tensile stress inside the GaN layer. The cross-sectional TEM image reveals the bending and annihilation of threading dislocations （TDs） in the overgrown GaN film which leads to a decrease of TD density.     

Investigation of dislocations in 8° off-axis 4H-SiC epilayer

This paper reports that the etching morphology of dislocations in 8° off-axis 4H-SiC epilayer is observed by using a scanning electronic microscope.It is found that different types of dislocations correspond with different densities and basal plane dislcation (BPD) array and threading edge dislocation (TED) pileup group lie along some certain crystal directions in the epilayer.It is concluded that the elastic energy of threading screw dislocations (TSDs) is highest and TEDs is lowest among these dislocations,so the density of TSDs is lower than TEDs.The BPDs can convert to TEDs but TSDs can only propagate into the epilyer in spite of the higher elastic energy than TEDs.The reason of the form of BPDs array in epilayer is that the big step along the basal plane caused by face defects blocked the upstream atoms,and TEDs pileup group is that the dislocations slide is blocked by dislocation groups in epilayer.     

Epitaxial evolution on buried cracks in a strain-controlled AlN/GaN superlattice interlayer between AlGaN/GaN multiple quantum wells and a GaN template

Epitaxial evolution of buried cracks in a strain-controlled AlN/GaN superlattice interlayer(IL) grown on GaN template, resulting in crack-free AlGaN/GaN multiple quantum wells(MQW), was investigated. The processes of filling the buried cracks include crack formation in the IL, coalescence from both side walls of the crack, build-up of an MQW-layer hump above the cracks, lateral expansion and merging with the surrounding MQW, and two-dimensional step flow growth.It was confirmed that the filling content in the buried cracks is pure GaN, originating from the deposition of the GaN thin layer directly after the IL. Migration of Ga adatoms into the cracks plays a key role in the filling the buried cracks.     

The influence of AlN/GaN superlattice intermediate layer on the properties of GaN grown on Si（111） substrates

AlN/GaN superlattice buffer is inserted between GaN epitaxial layer and Si substrate before epitaxial growth of GaN layer. High-quality and crack-free GaN epitaxial layers can be obtained by inserting AlN/GaN superlattice buffer layer. The influence of AlN/GaN superlattice buffer layer on the properties of GaN films are investigated in this paper. One of the important roles of the superlattice is to release tensile strain between Si substrate and epilayer. Raman spectra show a substantial decrease of in-plane tensile strain in GaN layers by using AlN/GaN superlattice buffer layer. Moreover, TEM cross-sectional images show that the densities of both screw and edge dislocations are significantly reduced. The GaN films grown on Si with the superlattice buffer also have better surface morphology and optical properties.     

Effects of Dislocation on High Temperature Transport Characteristics of Unintentionally Doped GaN

High temperature transport characteristics of unintentionally doped GaN have been investigated by means of high temperature Hall measurements from room temperature to 500^o C. The increment of electron concentration from room temperature to 500^o C is found to vary largely for different samples. The dispersion of temperature dependence of electron concentration is found to be directly proportional to the density of dislocations in GaN layers calculated by fitting the FWHM of the rocking curves in x-ray diffraction measurements （XRD）. The buildup levels in persistent photoconductivity （PPC） are also shown to be directly proportionM to the density of dislocations. The correlation of XRD, Hall and PPC results indicate that the high temperature dependence of electron density in unintentional doped GaN is directly dislocation related.     

Highly Strained Si Films with Ultra-low Dislocation Density Grown on Virtual Substrates of Thin Thickness

By using compositionally graded SiGe films as virtual substrates, tensile strained Si films with the strain of 1.5% and the threading dislocation density less than 1.0 × 10^5 cm-2 are successfully grown in micron size windows by molecular beam epitaxy （MBE）. The thickness of the virtual substrates was only 33Onto. On the surface of the s-Si films no cross-hatched lines resulting from misfit dislocations could be observed. We attribute these results to the edge-induced strain relaxation of the epitaxial films in windows, and the patterned virtual substrates with compositionally graded SiGe films.     

Improved breakdown voltage of AlGaN/GaN HEMTs grown on Si substrates using partially Mg-doped GaN buffer layer by MOCVD

AlGaN/GaN high electron mobility transistors(HEMTs) were grown on Si substrates by MOCVD.In the HEMT structure,a 1 μm GaN buffer layer was partially doped with Mg in an attempt to increase the resistivity and minimize the buffer leakage.The AlGaN/GaN HEMTs grown on undoped and partially Mg-doped GaN buffer layers were processed and the DC characteristics of the devices were characterized for comparing the effect of Mg doping.For the device with the partially Mg-doped GaN buffer layer,a lower drain leakage current density of 55.8 nA/mm,a lower gate leakage current density of 2.73 μA/mm,and a higher off-state breakdown voltage of 104 V were achieved with device dimensions Lg/Wg/Lgs/Lgd=1/10/1/1 μm,better than the device with the undoped GaN buffer layer,which has a higher drain leakage current density of 9.2 μA/mm,a higher gate leakage current density of 91.8 μA/mm,and a lower off-state breakdown voltage of 87 V with the same device dimensions.