Caracterization of AIN buffer layers on (0001)-sapphire substrates

It is now established that low temperature-grown buffer layers are needed to improve the structural and electronic properties of GaN layers grown on sapphire. Using X-ray diffraction, we have studied the dependency on temperature and annealing time of the recrystallization of AlN buffer layers grown by low pressure MOVPE. The Warren–Averbach method applied to the broadening and shape analysis of the (0002) and (0004) X-ray diffraction peaks has allowed us to separate grain size distribution from micro-strain effects. The obtained evolution of the relative frequency distribution of the grain sizes with annealing conditions is correlated with atomic force microscopy experiments (dynamic mode). The angular distribution of the c-axis of the grains is determined from X-ray rocking-curves experiments. X-ray reflectometry experiments and a simulation procedure have given us access to the roughness and the chemical composition of the sapphire-buffer layer interface and to the thickness and the roughness of the AlN grown.     

Transmission electron microscopy structural characterisation of GaN layers grown on (0001) sapphire

GaN layers grown by MOCVD or by MBE on (0001) sapphire have been characterised by transmission electron microscopy (TEM). We make a review of the different crystallographic structures found in theses GaN layers. We comment shortly on the nitridation of the sapphire and the structure of the buffer layer (BL). We point out that the roughness of the BL can be an important parameter for releasing the residual strain of the GaN layer. We compute the Keating energies of the main inversion domain boundaries (IDBs) and translation domains boundaries (TDBs) observed in some GaN layers. The observed structures correspond to the lowest energy models. Perfect dislocations have Burgers vectors equal to a, a+c and c. The dislocation lines are generally parallel to the c-axis. a-Edge dislocations are generally not dissociated and we propose an atomic model for them. Screw dislocations with a Burgers vector equal to c, can ‘open and close' during growth leaving holes (the so-called nanopipes) in the structure.     

Properties of GaN grown on sapphire substrates

Epitaxial growth of GaN on sapphire substrates using an open-tube growth furnace has been carried out to study the effects of substrate orientation and transfer gas upon the properties of the layers. It has been found that for the (0001) substrates, surface appearance was virtually independent of carrier gas and of doping levels. For the (1 ¯102) substrates surface faceting was greatly reduced when He was used as a transfer gas as opposed to H₂. Faceting was also reduced when the GaN was doped with Zn and the best surfaces for the (1 ¯102) substrates were obtained in a Zn-doped run using He as the transfer gas. The best sample in terms of electrical properties for the (1¯102) substrate had a mobility greater than 400 cm² V^–1 sec^–1 and a carrier concentration of about 2 × 10¹⁷ cm^–3. This sample was undoped and used He as the transfer gas. The best (0001) sample was also grown undoped with He as the transfer gas and had a mobility of 300cm²V^–1 sec^–1 and a carrier concentration of 1 × 10¹⁸ cm^–3.     

PbTe films grown by hot wall epitaxy on sapphire substrates

A ‘hot wall epitaxy’ is applied to grow PbTe thin films on sapphire substrates with BaF₂ buffer layer deposited by molecular beam epitaxy (MBE). The microstructural and strain state characteristics of PbTe layer were examined with high resolution X-ray diffraction techniques. The epilayer is composed of two (111)PbTe‖(0001)Al₂O₃ epitaxially oriented domain variants. The domains are azimuthally rotated and their interfacial directions relative to the substrate are [011?]PbTe‖Al₂O₃ and [1?54?]PbTe‖Al₂O₃, respectively. Another possible alignment of the domain variant corresponds to PbTe‖Al₂O₃ orientation. The strain state analysis of PbTe layer points to its relaxation via domain formation and high dislocation density generation in the lattice. Despite the domains formation the measured mobility of electron carriers is approximately 1600 cm²/V s and 30 000 cm²/V s at 300 K and 77 K, respectively. The theoretical analysis of the measured electrical properties indicates that the scattering by acoustic and optical phonons is the factor affecting the conduction process.     

半导体Si(111)上电沉积NiFe缓冲层薄膜

采用电化学沉积法,在半导体硅片上制备了具有纳米晶粒尺寸的NiFe缓冲层薄膜,并确定了获得Ni80Fe20合金的工艺条件.由SEM形貌观测分析,当薄膜名义厚度＞25 nm时,可形成连续性镀层.I-t暂态曲线及STM结果表明,NiFe薄膜在低过电位下以三维岛状模式生长,在高过电位下以二维层状模式生长,其RMS表面粗糙度最小值仅为0.5 nm.XRD结果表明,薄膜为面心立方Ni基固溶体结构,并具有明显的(111)晶面择优取向.当薄膜组成为Ni80Fe20时,各向异性磁电阻效应(AMR)最大,AMR值为1.8%.     

ZnO thin films grown on platinum (111) buffer layers by pulsed laser deposition

C-axis oriented ZnO layers were grown by pulsed-laser deposition on the surface of a platinum (111) epitaxial thin film supported by a c-sapphire substrate. The Pt bottom layer provides good in-plane lattice matching with c-ZnO, enabling epitaxial re-growth of the latter, as shown by X-ray diffraction data. Room- and low-temperature reflectance and photoluminescence measurements have been performed on such ZnO/Pt heterostructures for the first time. Intense resonances, corresponding to the A and B free excitons, are clearly evidenced in the reflectance measurements at 30 K, while the deconvolved full widths at half maximum of the bound excitonic lines, observed in the photoluminescence spectra at 28 K, range between 3 and 7 meV. This report clearly demonstrates that ZnO epitaxial thin films with very good structural and optical properties can be grown on a Pt bottom electrode and, thus, establishes the potential of this material system for use in ZnO-based optoelectronic devices.     

Strain evolution in heteroepitaxial ZnO/sapphire(0001) thin films grown by radio frequency magnetron sputtering

We present the strain evolution of ZnO/sapphire(0001) thin films studied by X-ray diffraction, scanning electron microscopy, and atomic force microscopy. The effect of input radio frequency (rf) sputtering power and growth temperature was examined. Initially the highly strained flat ZnO layers were grown. At samples deposited at higher temperatures than ∼ 500 °C, the strain was fully relaxed by nucleating the ZnO nano-grains as the film thickness increased. However, we found that the strain was partially relaxed at the samples grown at lower temperatures, which was attributed to the insufficient surface undulations. For the effect of rf input power, a consistent behavior of strain relaxation independent of the rf input power was observed. The ZnO nano-grains were enlarged and transformed to well-defined hexagonal nano-discs with film thickness.     

High quality epitaxial ZnO films grown on solid-phase crystallized buffer layers

High quality epitaxial ZnO films on sapphire (110) plane have been fabricated on ZnO homo-buffer layers crystallized via solid-phase epitaxially (SPE). The SPE-ZnO films are fabricated by annealing of amorphous ZnON (a-ZnON) films deposited by RF magnetron sputtering. During annealing, the a-ZnON films are oxidized and converted to ZnO crystal. X-ray diffraction (XRD) analysis shows that the resultant films are epitaxially grown on the sapphire substrates. By using the SPE-ZnO films as homo-buffer layers, the ZnO films with high crystallinity, which are deposited by RF magnetron sputtering, are fabricated. The full width at half-maximum of XRD patterns for 2θ-ω and ω scan of (002) plane are 0.094° and 0.12°, respectively, being significantly small compared with 0.24° and 0.55° for the films without buffer layers. Thus utilizing SPE buffer layers is very promising to obtain epitaxial ZnO films with high crystallinity.     

New buffer sublayers for heteroepitaxial III–V nitride films on sapphire substrates

It is suggested to deposit III–V nitride films onto sapphire substrates upon preliminary deposition of a buffer sublayer of a crystalline material with a cubic structure. It is shown experimentally that the deposition of a heteroepitaxial niobium sublayer onto a (0001)-oriented sapphire substrate or a niobium nitride sublayer onto a ( 11[`2]011\bar 20 )-oriented Al₂O₃ substrate eliminates a 30° rotation of the (0001)-oriented nitride film in the substrate plane. The elimination of this rotation provides considerable reduction of the lattice mismatch between the substrate and the nitride film, which, in turn, should increase the degree of crystal perfection of the film. In addition, the planes of semiconductor nitride films become parallel to the natural cleavage planes of the substrate. This fact provides for the possibility of manufacturing a heterolaser with a Fabry-Perot resonator, in which the role of the mirrors is played by natural cleavage planes of the film.     

Stress reduction and electric properties of InSb thin films grown by metalorganic vapor phase epitaxy on sapphire substrates with an InAs buffer layer

InSb thin films were grown by metalorganic vapor phase epitaxy using an InAs buffer layer on sapphire (0001) substrates. The stresses and strains in InSb were controlled by the thickness of the InAs buffer layer, and it was found that with decreasing compressive stress in InSb, the crystalline quality and the electrical properties improved. The thermoelectric properties of InSb were assessed and it was found that the power factor of InSb with a thickness of 5 μm reached as high as 5.8 × 10⁻³ W/mK² at 600 K.     

Effects of sapphire substrates surface treatment on epitaxial ZnO thin films grown by MOCVD

The surface treatment effects of sapphire substrate on the quality of epitaxial ZnO thin films grown by metal-organic chemical vapor deposition (MOCVD) were studied. The sapphire substrates have been investigated by means of atomic force microscopy (AFM) and X-ray diffraction rocking curves (XRCs). The results show that sapphire substrate surfaces have the best-quality by CMP with subsequent chemical etching. The surface treatment effects of sapphire substrate on the ZnO thin films were examined by X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL) measurements. Results show that the intensity of (002) diffraction peak of ZnO thin films on sapphire substrates treated by CMP with subsequent chemical etching is strongest. FWHM of (002) diffraction peak is narrowest and the intensity of UV peak of PL spectrum is strongest, indicating surface treatment on sapphire substrate preparation may improve ZnO thin films crystal quality and photoluminescent property.     

Epitaxial growth of MgFe2O4 (111) thin films on sapphire (0001) substrate

Pulsed laser deposition technique is used to fabricate thin films of MgFe₂O₄ on sapphire substrate. X-ray diffraction patterns show that films are epitaxial along (???) direction on c-axis oriented sapphire. The observation of six fold symmetry in phi scan for the film suggests the presence of twinned in-plane alignments. The presence of magnetic hysteresis loop at room temperature indicates the ferromagnetic behavior of the film. The coercive field of 742 Oe and remnant magnetization of 151 emu/cm² is observed at room temperature. The coercive field and remnant magnetization decrease with increase in temperature.     

A microscopic study of preferred orientation and adhesion of solid copper particles on (0001) sapphire substrates

The microscopic phenomena of solid copper particles equilibrating on (0001) sapphire surfaces have been investigated by scanning electron microscopy. Very sharp and clear silhouettes of the particles are developed. At temperatures between 900 and 1080° C, with the oxygen partial pressure lower than 10^–17 atm, the copper particles tend toward their equilibrium shape as the {111} and {110} preferred faces develop eventually into truncated octahedra or tetrakaidecahedra. Recrystallization of the particles originating at the particle-substrate interface by a heteroepitaxial mechanism is discussed. The {111} plane of the copper particles does not grow parallel to the (0001) sapphire surface as previously reported, but rather with about 20° of tilt, suggesting epitaxial growth of the Cu {112} plane on the (0001) sapphire surface. The surface energy of solid copper decreases with the increase of temperature and/or oxygen partial pressure while the work of adhesion between solid copper and (0001) sapphire can be enhanced by increasing the temperature and/or decreasing the oxygen partial pressure.     

MBE方法在GaAs(001)衬底上外延生长GaSb膜

利用分子束外延方法(MBE)在GaAs(001)衬底上外延生长了GaSb薄膜,利用高分辨透射电子显微镜(HRTEM)、原子力显微镜(AFM)、Hall效应(HallEffect)和低温光荧光谱(LTPL)等手段对薄膜的晶体质量、电学性能和光学质量进行了研究。发现直接生长的GaSb膜表面平整,空穴迁移率较高。引入GaSb/AlSb超晶格可有效阻断进入GaSb外延层的穿通位错,对应的PL谱强度增强,材料的光学质量变好。     

GaN films grown by vapor-phase epitaxy in a hydride-chloride system on Si(111) substrates with AlN buffer sublayers

Oriented GaN layers with a thickness of about 10 μm have been grown by hydride-chloride vaporphase epitaxy (HVPE) on Si(111) substrates with AlN buffer layers. The best samples are characterized by a halfwidth (FWHM) of the X-ray rocking curve of ω_θ = 3–4 mrad. The level of residual mechanical stresses in AlN buffer layers decreases with increasing temperature of epitaxial growth. The growth at 1080°C is accompanied by virtually complete relaxation of stresses caused by the lattice mismatch between AlN and Si.