Effect of double AlN buffer layer on the qualities of GaN films grown by radio-frequency molecular beam epitaxy

This paper reports that the GaN thin films with Ga-polarity and high quality were grown by radio-frequency molecular beam epitaxy on sapphire （0001） substrate with a double A1N buffer layer. The buffer layer consists of a high-temperature （HT） A1N layer and a low-temperature （LT） A1N layer grown at 800℃ and 600℃, respectively. It is demonstrated that the HT-A1N layer can result in the growth of GaN epilayer in Ga-polarity and the LT-A1N layer is helpful for the improvement of the epilayer quality. It is observed that the carrier mobility of the GaN epilayer increases from 458 to 858cm^2/V.s at room temperature when the thickness of LT-A1N layer varies from 0 to 20nm. The full width at half maximum of x-ray rocking curves also demonstrates a substantial improvement in the quality of GaN epilavers by the utilization of LT-A1N layer.     

The influence of an SixNy interlayer on a GaN film grown on an Si（lll） substrate

<正>A method to drastically reduce dislocation density in a GaN film grown on an Si(111) substrate is newly developed. In this method,the Si_xN_y interlayer which is deposited on an A1N buffer layer in situ is introduced to grow the GaN film laterally.The crack-free GaN film with thickness over 1.7 micron is successfully grown on an Si(111) substrate. A synthesized GaN epilayer is characterized by X-ray diffraction(XRD),atomic force microscope(AFM),and Raman spectrum.The test results show that the GaN crystal reveals a wurtzite structure with the(0001) crystal orientation and the full width at half maximum of the X-ray diffraction curve in the(0002) plane is as low as 403 arcsec for the GaN film grown on the Si substrate with an Si_xN_y interlayer.In addition,Raman scattering is used to study the stress in the sample.The results indicate that the Si_xN_y interlayer can more effectively accommodate the strain energy.So the dislocation density can be reduced drastically,and the crystal quality of GaN film can be greatly improved by introducing an Si_xN_y interlayer.     

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.     

Influence of AlN buffer layer thickness on structural properties of GaN epilayer grown on Si (111) substrate with AlGaN interlayer

We present the growth of GaN epilayer on Si (111) substrate with a single AlGaN interlayer sandwiched between the GaN epilayer and AlN buffer layer by using the metalorganic chemical vapour deposition. The influence of the AlN buffer layer thickness on structural properties of the GaN epilayer has been investigated by scanning electron microscopy, atomic force microscopy, optical microscopy and high-resolution x-ray diffraction. It is found that an AlN buffer layer with the appropriate thickness plays an important role in increasing compressive strain and improving crystal quality during the growth of AlGaN interlayer, which can introduce a more compressive strain into the subsequent grown GaN layer, and reduce the crack density and threading dislocation density in GaN film.     

Effect of Ⅲ/Ⅴ Ratio of HT-AIN Buffer Layer on Polarity Selection and Electrical Quality of GaN Films Grown by Radio Frequency Molecular Beam Epitaxy

We investigate the effect of A/N ratio of the high temperature （HT） AIN buffer layer on polarity selection and electrical quality of GaN films grown by radio frequency molecular beam epitaxy. The results show that low Al/N ratio results in N-polarity GaN films and intermediate Al/N ratio leads to mixed-polarity GaN films with poor electrical quality. GaN films tend to grow with Ga polarity on Al-rich AIN buffer layers. GaN films with different polarities are confirmed by in-situ reflection high-energy electron diffraction during the growth process. Wet chemical etching, together with atomic force microscopy, also proves the polarity assignments. The optimum value for room-temperature Hall mobility of the Ga-polarity GaN film is 703cm^2/V.s, which is superior to the N-polarity and mixed-polarity GaN films.     

Stress Analysis of ZnO Film with a GaN Buffer Layer on Sapphire Substrate

A 5.35-μm-thick ZnO film is grown by chemical vapour deposition technique on a sapphire （0001） substrate with a GaN buffer layer. The surface of the ZnO film is smooth and shows many hexagonal features. The full width at half maximum of ZnO （0002） u-rocking curve is 161 arcsec, corresponding to a high crystal quality of the ZnO film. From the result of x-ray diffraction 0 - 20 scanning, the stress status in ZnO film is tensile, which is supported by Raman scattering measurement. The reason of the tensile stress in the ZnO film is analysed in detail. The lattice mismatch and thermal mismatch are excluded and the reason is attributed to the coalescence of grains or islands during the growth of the ZnO film.     

Study of depth-dependent tetragonal distortion of quaternary AIInGaN epilayer by Rutherford backscattering/channeling

<正>A 240-nm thick Al_0.4In_0.02Ga_0.58N layer is grown by metal organic chemical vapour deposition,with an over 1-μm thick GaN layer used as a buffer layer on a substrate of sapphire（0001）.Rutherford backscattering and channeling are used to characterize the microstructure of AlInGaN.The results show a good crystalline quality of AlInGaN(x_min= 1.5%) with GaN buffer layer.The channeling angular scan around an off-normal（1213） axis in the {1010} plane of the AlInGaN layer is used to determine tetragonal distortion e_T,which is caused by the elastic strain in the AlInGaN.The resulting AlInGaN is subjected to an elastic strain at interfacial layer,and the strain decreases gradually towards the near-surface layer.It is expected that an epitaxial AlInGaN thin film with a thickness of 850 nm will be fully relaxed （e_T=0）.     

Influence of double A1N buffer layers on the qualities of GaN films prepared by metal-organic chemical vapour deposition

<正>In this paper we report that the GaN thin film is grown by metal-organic chemical vapour deposition on a sapphire (0001) substrate with double AlN buffer layers.The buffer layer consists of a low-temperature(LT) AlN layer and a high-temperature(HT) AlN layer that are grown at 600℃and 1000℃,respectively.It is observed that the thickness of the LT-AlN layer drastically influences the quality of GaN thin film,and that the optimized 4.25-min-LT-AlN layer minimizes the dislocation density of GaN thin film.The reason for the improved properties is discussed in this paper.     

Growth of Semi-Insulating GaN by Using Two-Step AIN Buffer Layer

Semi-insulating GaN is grown by using a two-step A1N buffer layer by metalorganic chemical vapour deposition. The sheet resistance of as-grown semi-insulating GaN is dramatically increased to 10^13 Ω/sq by using two-step A1N buffer instead of the traditional low-temperature GaN buffer. The high sheet resistance of as-grown GaN over 10^13 Ω/sq is due to inserting an insulating buffer layer （two-step A1N buffer） between the high-temperature GaN layer and a sapphire substrate which blocks diffusion of oxygen and overcomes the weakness of generating high density carrier near interface of GaN and sapphire when a low-temperature GaN buffer is used. The result suggests that the high conductive feature of unintentionally doped GaN is mainly contributed from the highly conductive channel near interface between GaN and the sapphire substrate, which is indirectly manifested by room-temperature photoluminescence excited by an incident laser beam radiating on growth surface and on the substrate. The functions of the two-step A1N buffer layer in reducing screw dislocation and improving crystal quality of GaN are also discussed.     

Two-Step Growth of MgO Films on Sapphire （0001） Substrates by Radio Frequency Plasma-Assisted Molecular Beam Epitaxy

We report on the growth of rock salt MgO films on sapphire (0001) substrates by rf plasma-assisted molecular beam epitaxy.A two-step method,i.e.high temperature epilayer growth after low-temperature buffer layer growth,was adopted to obtain the single crystal MgO film.The epitaxial orientation between the MgO epilayer and the sapphire (0001) substrate was studied by using in situ reflection high energy electron diffraction and ex situ x-ray diffraction,and it is found that the MgO film grows with [111] orientation.The role of the low temperature buffer layer in the improvement of crystal quality of the MgO epilayer is discussed based on the cross-sectional scanning electron microscopy.     

Tetragonal Distortion of GaN Epilayer with Multiple Buffer Layers on Si （111） Studied by RBS/Channelling and HRXRD

Rutherford backscattering （RBS）/channelling and high resolution x-ray diffraction （HRXRD） have been used to characterize the tetragonal distortion of a GaN epilayer with four Alx Ga1-xN and single AIN buffer layers grown on a Si （111） substrate by metal-organic vapour phase epitaxy （MOVPE）. The results show that a 1000nm GaN epilayer with a perfect crystal quality （Xmin = 1.54%） can be grown on the Si （111） substrate in virtue of multiple buffer layers. Using the RBS/channelling angular scan around an off-normal （1213） axis in the （1010） plane and the conventional HRXRD θ - 20 scans normal to GaN （0002） and （1122） planes at the 0° and 180° azimuth angles, the tetragonal distortion eT, which is caused by the elastic strain in the epilayer and different buffer layers, can be obtained respectively. The two experiments are testified at one result, the tetragonal distortion of GaN epilayer is nearly to a fully relaxed （eT = 0）.     

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.     

GaP layers grown on GaN with and without buffer layers

The growth of GaP layer on GaN with and without buffer layers by metal-organic chemical vapour deposition (MOCVD) has been studied. Results indicate that the GaP low temperature buffer layer can provide a high density of nucleation sites for high temperature (HT) GaP growth. These sites can promote the two-dimensional (2D) growth of HT GaP and reduce the surface roughness. A GaP single crystal layer grown at 680°C is obtained using a 40-nm thick GaP buffer layer. The full-width at half-maximum (FWHM) of the (111) plane of GaP layer, measured by DCXRD, is 560 arcsec. The GaP layer grown on GaN without low temperature GaP buffer layer shows a rougher surface. However, the FWHM of the (111) plane is 408 arcsec, which is the indication of better crystal quality for the GaP layer grown on GaN without a low temperature buffer layer. Because it provides less nucleation sites grown at high growth temperature, the three-dimensional (3D) growth is prolonged. The crystalline quality of GaP is lightly improved when the surface of GaN substrate is pretreated by PH3 , while it turned to be polycrystalline when the substrate is pretreated by TEGa.     

High-Quality and Strain-Relaxation GaN Epilayer Grown on SiC Substrates Using AIN Buffer and AlGaN Interlayer

We study the effect of the AlGaN interlayer on structural quality and strain engineering of the GaN films grown on SiC substrates with an AIN buffer layer. Improved structural quality and tensile stress releasing are realized in unintentionally doped GaN thin films grown on 6 H—SiC substrates by metal organic chemical vapor deposition.Using the optimized AlGaN interlayer, we find that the full width at half maximum of x-ray diffraction peaks for GaN decreases dramatically, indicating an improved crystalline quality. Meanwhile, it is revealed that the biaxial tensile stress in the GaN film is significantly reduced from the Raman results. Photoluminescence spectra exhibit a shift of the peak position of the near-band-edge emission, as well as the integrated intensity ratio variation of the near-band-edge emission to the yellow luminescence band. Thus by optimizing the AlGaN interlayer,we could acquire the high-quality and strain-relaxation GaN epilayer with large thickness on SiC substrates.     

A Comparison of GaN Epilayers with Multiple Buffer Layers and with a Single Buffer Layer Grown on Si（111） Studied by HRXRD and RBS/Channeling

Two hexagonal GaN epilayers （samples A and B） with multiple buffer layers and single buffer layer are grown on Si （111） by metal-organic vapour phase epitaxy （MOVPE）. From the results of Rutherford backscattering （RBS）/channeling and high resolution x-ray diffraction （HRXRD）, we obtain the lattice constant （a and c） of two GaN epilayers （aA = 0.3190 nm, cA = 0.5184 nm and aB = 0.3192 nm, CB = 0.5179 nm）, the crystal quality of two GaN epilayers （ ХminA=4.87%, ХminB =7.35% along 〈1-↑213〉 axis） and the tetragonal distortion eT of the two samples along depth （sample A is nearly fully relaxed, sample B is not relaxed enough）. Comparing the results with the two samples, it is indicated that sample A with multiple buffer layers have better crystal quality than sample B with a single buffer layer, and it is a good way to grow GaN epilayer on Si （111） substrates using multiple buffer layers to improve crystal quality and to reduce lattice mismatch.     

Study of depth-dependent tetragonal distortion of quaternary AlInGaN epilayer by Rutherford backscattering/channeling

A 240-nm thick Al_0.4In_0.02Ga_0.58N layer is grown by metal organic chemical vapour deposition, with an over 1-μ m thick GaN layer used as a buffer layer on a substrate of sapphire (0001). Rutherford backscattering and channeling are used to characterize the microstructure of AlInGaN. The results show a good crystalline quality of AlInGaN (χ_min=1.5%) with GaN buffer layer. The channeling angular scan around an off-normal <12^{-13> axis in the {101-0} plane of the AlInGaN layer is used to determine tetragonal distortion e_T, which is caused by the elastic strain in the AlInGaN. The resulting AlInGaN is subjected to an elastic strain at interfacial layer, and the strain decreases gradually towards the near-surface layer. It is expected that an epitaxial AlInGaN thin film with a thickness of 850 nm will be fully relaxed (e_T = 0).}     

Investigation of β-Ga_2O_3 films and β-Ga_2O_3/GaN heterostructures grown by metal organic chemical vapor deposition

In this work,(-201) β-Ga_2O_3 films are grown on GaN substrate by metal organic chemical vapor deposition(MOCVD). It is revealed that the β-Ga_2O_3 film grown on GaN possesses superior crystal quality, material homogeneity and surface morphology than the results of common heteroepitaxial β-Ga_2O_3 film based on sapphire substrate. Further, the relevance between the crystal quality of epitaxial β-Ga_2O_3 film and the β-Ga_2O_3/GaN interface behavior is investigated. Transmission electron microscopy result indicates that the interface atom refactoring phenomenon is beneficial to relieve the mismatch strain and improve the crystal quality of subsequent β-Ga_2O_3 film. Moreover, the energy band structure of β-Ga_2O_3/GaN heterostructure grown by MOCVD is investigated by X-ray photoelectron spectroscopy and a large conduction band offset of 0.89 eV is obtained. The results in this work not only convincingly demonstrate the advantages of β-Ga_2O_3 films grown on GaN substrate, but also show the great application potential of MOCVD β-Ga_2O_3/GaN heterostructures in microelectronic applications.     

Microstructure and strain films using in-plane grazing analysis of GaN epitaxial incidence x-ray diffraction

This paper investigates the major structural parameters, such as crystal quality and strain state of （001）-oriented GaN thin films grown on sapphire substrates by metalorganic chemical vapour deposition, using an in-plane grazing incidence x-ray diffraction technique. The results are analysed and compared with a complementary out-of-plane x- ray diffraction technique. The twist of the GaN mosaic structure is determined through the direct grazing incidence t of （100） reflection which agrees well with the result obtained by extrapolation method. The method for directly determining the in-plane lattice parameters of the GaN layers is also presented. Combined with the biaxial strain model, it derives the lattice parameters corresponding to fully relaxed GaN films. The GaN epilayers show an increasing residual compressive stress with increasing layer thickness when the two dimensional growth stage is established, reaching to a maximum level of-0.89 GPa.     

Growth of Structured Non—crystalline Boron—Oxygen—Nitrogen Films and Measurement of Their Electrical Properties

The boron-oxygen-nitrogen(BON) films have been grown on Si wafer by the low-frequency rf-plasma-enhanced metal-organic chemical vapour deposition method.The homogeneous film structure of completely amorphous BON is first fabricated on a low-temperature-made buffer at 500℃ with N2 plasma and is observed with a highresolution-electron microscope by the transmission-electron diffraction.The results show that the interfaces among substrate/buffer/film are clear and straight in the structured film.A heterogeneous film containing nano-sized crystalline particles is also grown by a routine growth procedure as a referential structure,The C-V characteristic is measured on both the amprphous and crystal-containing films by using the metal-oxidesemiconductor structure,The dielectric constants of the films are,therefore,deduced to be 5.9 and 10.5 for the amorphous and crystal-containing films,respectively,The C-V results also indicate that more trapped charges exist in the amorphous film.The binding energy of the B,O.and N atoms in the amprphous film is higher than that in the crystal-containing one,and the N-content in the latter is found to be higher than that in the former by x-ray photo-electron spectroscopy.The different electrical Property of the films is thought to originate from the energy state of the covalent electrons.     

Growth of Strain Free GaN Layers on （0001） Oriented Sapphire by Using Quasi-Porous GaN Template

We report the reduced-strain gallium-nitride （GaN） epitaxial growth on （0001） oriented sapphire by using quasiporous GaN template. A GaN film in thickness of about 1 μm was initially grown on a （0001） sapphire substrate by molecular beam epitaxy. Then it was dealt by putting into 45% NaOH solution at 100℃ for lOmin. By this process a quasi-porous GaN film was formed. An epitaxial GaN layer was grown on the porous GaN layer at 1050℃ in the hydride vapour phase epitaxy reactor. The epitaxial layer grown on the porous GaN is found to have no cracks on the surface. That is much improved from many cracks on the surface of the GaN epitaxial layer grown on the sapphire as the same as on GaN buffer directly.