InAs wetting layer evolution on GaAs(0 0 1)

The evolution of two-dimensional (2D) strained InAs wetting layers on GaAs(0 0 1), grown at different temperatures by molecular beam epitaxy, was studied by in situ high-resolution scanning tunneling microscopy. At low growth temperature (400 °C), the substrate exhibits a well-defined GaAs(0 0 1)-c(4 × 4) structure. For a disorientation of 0.7°, InAs grows in the step-flow mode and forms an unalloyed wetting layer mainly along steps, but also in part on the terrace. The wetting layer displays some local c(4 × 6) reconstruction, for which a model is proposed. 1.2 monolayer (ML) InAs deposition induces the formation of 3D islands. At a higher temperature (460 °C), the wetting layer is obviously alloyed even at low InAs coverage. The critical thickness of the wetting layer for the 2D-to-3D transition is shifted to 1.50 ML in this case presumably since the strain is reduced by alloying.     

GaN nucleation on (0 0 0 1)-sapphire via ion-induced nitridation of gallium

The growth of epitaxial GaN films on (0 0 0 1)-sapphire has been investigated using X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). In order to investigate the mechanism of the growth in detail, we have focused on the nitridation of pre-deposited Ga layers (droplets) using ion beam-assisted molecular beam epitaxy (IBA-MBE). Comparative analysis of XPS core-level spectra and LEED patterns reveals, that nitride films nucleate as epitaxial GaN islands. The wetting of the surface by GaN proceeds via reactive spreading of metallic Ga, supplied from the droplets. The discussed growth model confirms, that excess of metallic Ga is beneficial for GaN nucleation.     

XPS study of the formation of ultrathin GaN film on GaAs(1 0 0)

The nitridation of GaAs(1 0 0) surfaces has been studied using XPS spectroscopy, one of the best surface sensitive techniques. A glow discharge cell was used to produce a continuous plasma with a majority of N atomic species. We used the Ga3d and As3d core levels to monitor the chemical state of the surface and the coverage of the species. A theoretical model based on stacked layers allows to determine the optimal temperature of nitridation. Moreover, this model permits the determination of the thickness of the GaN layer. Varying time of nitridation from 10 min to 1 h, it is possible to obtain GaN layers with a thickness between 0.5 nm and 3 nm.     

HREELS of H/GaN(0001): evidence for Ga termination

Two issues relevant to the growth and processing of GaN are the termination of the GaN(0001) surface and its reaction with hydrogen. We have used high-resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES) to study the adsorption of hydrogen on MOCVD-grown GaN(0001). LEED of the sputtered and annealed surface shows evidence of facetting. No adsorbate vibrations are observed on the clean surface by HREELS, only Fuchs–Kliewer phonons at intervals of 700 cm⁻¹. Following exposure of the clean GaN surface to hydrogen atoms, HREEL spectra show adsorbate loss peaks at 2580, 3280, and 3980 cm⁻¹. The Ga–H stretching vibration at 1880 cm⁻¹ becomes evident when the HREEL spectrum is deconvoluted to remove the phonon multiple-loss peaks. We assign the 2580, 3280, and 3980 cm⁻¹ peaks to combination modes of the Ga–H stretch and phonon(s). Upon dosing with deuterium, the Ga–D bending mode is observed at 400 cm⁻¹. No vibrational peaks due to N–H (N–D) species are observed after H (D) exposure. We conclude that sputtered and annealed GaN(0001) is Ga-terminated.     

GaN grown on Si(1 1 1) with step-graded AlGaN intermediate layers

The authors report the growth of crack-free GaN on Si(1 1 1) substrate with step-graded AlGaN intermediate layers all grown at 1120 °C. By preparing all these layers at high-temperature, we can simplify the growth proceduce and minimize the growth time. Using X-ray diffraction and transmission electron microscopy, it was found that the high-temperature step-graded AlGaN intermediate layers can effectively reduce the tensile stress on GaN epitaxial layers. Photoluminescence and Raman measurements also indicate that we can improve the crystal quality of GaN by inserting the step-graded AlGaN intermediate layers.     

Characterization of GaN layers grown on silicon-on-insulator substrates

In this study, we report growth and characterization of GaN layers on (1 0 0)- and (1 1 1)-oriented silicon-on-insulator (SOI) substrates. Using metalorganic chemical vapor deposition (MOCVD) technique, GaN layers are grown on KOH treated Si (1 0 0) overlayers of thin SIMOX SOI substrates. Growth of GaN on such surface with an AlN buffer leads to c-axis orientated textured GaN. This is evident from high-resolution X-ray diffraction (HRXRD) measurement, which shows a much broader rocking curve linewidth. Significantly enhanced photoluminescence (PL) intensity and partial stress relaxation is observed in GaN layers grown on these SOI substrates. Furthermore, GaN grown on (1 1 1)-oriented bonded SOI substrates shows good surface morphology and improved optical quality. Micro-Raman, micro-PL, and HRXRD measurements reveal single crystalline hexagonal GaN oriented along (0 0 0 1) direction. We also report growth and characterization of InGaN/GaN multi-quantum well structures on (1 1 1)-oriented bonded SOI. Such an approach to realize nitride epilayers would be useful to fabricate GaN-based micro-opto-electromechanical systems (MOEMS) and sensors.     

Elemental steps in the growth of thin β-Ga2O3 films on CoGa(1 0 0)

The oxidation of CoGa(1 0 0) at 700 K was studied by means of high resolution electron energy loss spectroscopy (EELS), scanning tunneling microscopy, low energy electron diffraction and Auger electron spectroscopy (AES). At 700 K, thin well-ordered β-Ga₂O₃ films grow on CoGa(1 0 0). The EEL spectrum of the Ga-oxide films exhibit Fuchs–Kliewer phonons at 305, 455, 645, and 785 cm⁻¹. For low oxygen exposure (<0.2 L), the growth of oxide-islands starts at step edges and on defects. The oxide films have the shape of long, rectangular islands and are oriented in the [1 0 0] and [0 1 0] directions of the substrate. For higher oxygen exposure, islands of β-Ga₂O₃ are found also on the terraces. After an exposure of 200 L O₂ at 700 K, the CoGa(1 0 0) surface is homogeneously covered with a thin film of β-Ga₂O₃.     

High resolution X-ray diffraction of GaN grown on Si (1 1 1) by MOVPE

High temperature GaN layers have been grown on Si (1 1 1) substrate by metalorganic vapor phase epitaxy (MOVPE). AlN was used as a buffer layer and studied as a function of thickness and growth temperature. The growth was monitored by in situ laser reflectometry. High resolution X-ray diffraction (HRXRD) revealed that optimized monocrystalline GaN was obtained for a 40 nm AlN grown at 1080 °C. This is in good agreement with the results of morphological study by scanning electron microscopy (SEM) and also confirmed by atomic force microscopy (AFM) observations. The best morphology of AlN with columnar structure and lower rms surface roughness is greatly advantageous to the coalescence of the GaN epilayer. Symmetric and asymmetric GaN reflections were combined for twist and stress measurements in monocrystalline GaN. It was found that mosaicity and biaxial tensile stress are still high in 1.7 μm GaN. Curvature radius measurement was also done and correlated to the cracks observations over the GaN surface.     

高阻GaN的MOCVD外延生长

利用金属有机化合物气相沉积(MOCVD)在蓝宝石衬底上生长了高阻GaN薄膜。对GaN成核层生长的反应室压力、生长时间和载气类型对GaN缓冲层电学特性的影响进行了分析。实验结果表明,延长GaN成核层的生长时间,降低成核层生长时的反应室压力,载气由H₂换为N₂都会得到高阻的GaN缓冲层。样品的方块电阻R_s最高为2.49×10¹¹ Ω/□。以高阻GaN样品为衬底制备了AlGaN/AlN/GaN结构HEMT器件,迁移率最高达1 230 cm²/(V·s)。     

Theoretical study of Ni adsorption on the GaN(0 0 0 1) surface

First-principles pseudo-potential calculations within density-functional theory framework are performed in order to study the structural and electronic properties of nickel adsorption and diffusion on a GaN(0 0 0 1)-2×2 surface. The adsorption energies and potential energy surfaces are investigated for a Ni adatom on the Ga-terminated (0 0 0 1) surface of GaN. This surface is also used to study the effect of the nickel surface coverage. The results show that the most stable positions of a Ni adatom on GaN(0 0 0 1) are at the H₃ sites and T₄ sites, for low and high Ni coverage respectively. In addition, confirming previous experimental results, we have found that the growth of Ni monolayers on the GaN(0 0 0 1) surface is possible.     

Temperature-dependent photoluminescence of GaN grown on β-Si3N4/Si (1 1 1) by plasma-assisted MBE

Photoluminescence (PL) of high quality GaN epitaxial layer grown on β-Si₃N₄/Si (1 1 1) substrate using nitridation-annealing-nitridation method by plasma-assisted molecular beam epitaxy (PA-MBE) was investigated in the range of 5-300 K. Crystallinity of GaN epilayers was evaluated by high resolution X-ray diffraction (HRXRD) and surface morphology by Atomic Force Microscopy (AFM) and high resolution scanning electron microscopy (HRSEM). The temperature-dependent photoluminescence spectra showed an anomalous behaviour with an ‘S-like’ shape of free exciton (FX) emission peaks. Distant shallow donor-acceptor pair (DAP) line peak at approximately 3.285 eV was also observed at 5 K, followed by LO replica sidebands separated by 91 meV. The activation energy of the free exciton for GaN epilayers was also evaluated to be ∼27.8±0.7 meV from the temperature-dependent PL studies. Low carrier concentrations were observed ∼4.5±2×10¹⁷ cm⁻³ by measurements and it indicates the silicon nitride layer, which not only acts as a growth buffer layer, but also effectively prevents Si diffusion from the substrate to GaN epilayers. The absence of yellow band emission at around 2.2 eV signifies the high quality of film. The tensile stress in GaN film calculated by the thermal stress model agrees very well with that derived from Raman spectroscopy.     

Morphological anisotropy during migration enhanced epitaxy of GaAs(0 0 1)

The GaAs(0 0 1) surface morphology and structure during growth by migration enhanced epitaxy (MEE) has been studied by reflection high energy electron diffraction and scanning tunneling microscopy. Changes induced by varying the incident As/Ga flux ratio, growth temperature and the total amount of material deposited in each cycle have been studied and the results compared with GaAs(0 0 1) growth by conventional molecular beam epitaxy (MBE). Comparison of the surface morphology at the end of the Ga and As cycles indicates no clear evidence for any enhancement in the Ga adatom diffusion length during the Ga cycle. However, the morphological anisotropy of the growth front does change significantly and it is proposed that this changing anisotropy during MEE enables Ga adatom diffusion along both azimuths. The surface anisotropy during MEE growth is found to increase with the Ga/As ratio. Although there is a clear correlation between composition and morphology, we have also found that highly ordered and flat surfaces are not necessarily an indication of stoichiometric material. We also attempt to clarify a recent controversy on the structure of the c(4 × 4) reconstruction by studying the surface structure at the end of the As cycle as a function of the As/Ga ratio.     

Structural characterization of ultrathin Cr and Sc films for soft X-ray mirrors

The structure of multilayers of ultrathin scandium (Sc) and chromium (Cr) films has been characterized by means of transmission electron microscopy (TEM). Face centered cubic Sc was found both in magnetron sputtered thin Sc layers on Si(0 0 1) and in Cr/Sc multilayers for soft X-ray mirrors. The single Sc and Cr layers are polycrystalline with randomly oriented grains, while Sc and Cr within the Cr/Sc multilayer show a strong [0 0 1] texture in the deposition direction. From high-resolution images the orientation-relationship at the Cr/Sc interfaces could be deduced as: Sc_[110]//Cr_[100] and Sc_[010]//Cr_[110], which was confirmed by image simulations.     

A novel (8 × 4) superstructure as precursor to the c(4 × 4) phase during Sb/Si(0 0 1) desorption

The role of kinetics in the superstructure formation of the Sb/Si(0 0 1) system is studied using in situ surface sensitive techniques such as low energy electron diffraction, Auger electron spectroscopy and electron energy loss spectroscopy. Sb adsorbs epitaxially at room-temperature on a double-domain (DD) 2 × 1 reconstructed Si(0 0 1) surface at a flux rate of 0.06 ML/min. During desorption, multilayer Sb agglomerates on a stable Sb monolayer (ML) in a DD (2 × 1) phase before desorbing. The stable monolayer desorbs in the 600–850 °C temperature range, yielding DD (2 × 1), (8 × 4), c(4 × 4), DD (2 × 1) phases before retrieving the clean Si(0 0 1)-DD (2 × 1) surface. The stable 0.6-ML (8 × 4) phase here is a precursor phase to the recently reported 0.25-ML c(4 × 4) surface phase, and is reported for the first time.     

Nonpolar a-plane GaN grown on r-plane sapphire using multilayer AlN buffer by metalorganic chemical vapor deposition

Mirror-like and pit-free non-polar a-plane (1 1 −2 0) GaN films are grown on r-plane (1 −1 0 2) sapphire substrates using metalorganic chemical vapor deposition (MOCVD) with multilayer high-low-high temperature AlN buffer layers. The buffer layer structure and film quality are essential to the growth of a flat, crack-free and pit-free a-plane GaN film. The multilayer AlN buffer structure includes a thin low-temperature-deposited AlN (LT-AlN) layer inserted into the high-temperature-deposited AlN (HT-AlN) layer. The results demonstrate that the multilayer AlN buffer structure can improve the surface morphology of the upper a-plane GaN film. The grown multilayer AlN buffer structure reduced the tensile stress on the AlN buffer layers and increased the compressive stress on the a-plane GaN film. The multilayer AlN buffer structure markedly improves the surface morphology of the a-plane GaN film, as revealed by scanning electron microscopy. The effects of various growth V/III ratios was investigated to obtain a-plane GaN films with better surface morphology. The mean roughness of the surface was 1.02 nm, as revealed by atomic force microscopy. Accordingly, the multilayer AlN buffer structure improves the surface morphology and facilitates the complete coalescence of the a-plane GaN layer.     

Microstructural study of MBE-grown ZnO film on GaN/sapphire (0001) substrate

Single crystalline ZnO film is grown on GaN/sapphire (0001) substrate by molecular beam epitaxy. Ga₂O₃ is introduced into the ZnO/GaN heterostructure intentionally by oxygen-plasma pre-exposure on the GaN surface prior to ZnO growth. The crystalline orientation and interfacial microstructure are characterized by X-ray diffraction and transmission electron microscopy. X-ray diffraction analysis shows strong c-axis preferred orientation of the ZnO film. Cross-sectional transmission electron microscope images reveal that an additional phase is formed at the interface of ZnO/GaN. Through a comparison of diffraction patterns, we confirm that the interface layer is monoclinic Ga₂O₃ and the main epitaxial relationship should be and .      

Enhanced dispersion and stability of gold nanoparticles on stoichiometric and reduced TiO2(1 1 0) surface in the presence of molybdenum

Mo, Au and their coadsorbed layers were produced on nearly stoichiometric and oxygen-deficient titania surfaces by physical vapor deposition (PVD) and characterized by low energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and scanning tunnelling microscopy (STM). The behavior of Au/Mo bimetallic layers was studied at different relative metal coverages and sample temperatures.

STM data indicated clearly that the deposition of Au on the Mo-covered stoichiometric TiO₂(1 1 0) surface results in an enhanced dispersion of gold at 300 K. The mean size of the Au nanoparticles formed at 300 K on the Mo-covered TiO₂(1 1 0) was significantly less than on the Mo-free titania surface (2 ± 0.5 nm and 4 ± 1 nm, respectively). Interestingly, the deposition of Mo at 300 K onto the stoichiometric TiO₂(1 1 0) surface covered by Au nanoparticles of 3–4 nm (0.5 ML) also resulted in an increased dispersity of gold. The driving force for the enhanced wetting at 300 K is that the Au–Mo bond energy is larger than the Au–Au bond energy in 3D gold particles formed on stoichiometric titania. In contrast, 2D gold nanoparticles produced on ion-sputtered titania were not disrupted in the presence of Mo at 300 K, indicating a considerable kinetic hindrance for breaking of the strong Au-TiO_x bond.

The annealing of the coadsorbed layer formed on a strongly reduced surface to 740 K did not cause a decrease in the wetting of titania surface by gold. The preserved dispersion of Au at higher temperatures is attributed to the presence of the oxygen-deficient sites of titania, which were retained through the reaction of molybdenum with the substrate. Our results suggest that using a Mo-load to titania, Au nanoparticles can be produced with high dispersion and high thermal stability, which offers the fabrication of an effective Au catalyst.     

Molecular beam epitaxy of organic films investigated by high resolution low energy electron diffraction (SPA-LEED): 3,4,9,10-perylenetetracarboxylicacid-dianhydride (PTCDA) on Ag(1 1 1)

A detailed investigation of the multilayer growth of PTCDA on Ag(1 1 1) by high resolution LEED (SPA-LEED) is reported. The first two monolayers are closed and exhibit a structure, which is commensurate with respect to the underlying Ag(1 1 1) surface. The lattice parameters are close to those of the (1 0 2) plane of the β bulk phase of PTCDA, with deviations ?2%. The vertical stacking of the second layer with respect to the first monolayer (observed at 300 K) corresponds to that in the β bulk phase of PTCDA. At high growth temperatures (400 K), Stranski-Krastanov growth occurs from the third monolayer onward, and PTCDA clusters, preferentially with few well defined facets, grow. The structure of the clusters is that of the thermodynamically more stable α bulk phase of PTCDA. Contrary, at low growth temperatures (200 K), the growth proceeds in the Frank van der Merve mode, with several open layers. From slope selection there is evidence for an Ehrlich-Schwoebel barrier. The lateral packing of the PTCDA grown at low temperatures corresponds within error to that of the β bulk phase. The low temperature structure and morphology is meta-stable. Short annealing at 300 K flattens the PTCDA film, and prolonged annealing at 400 K causes the film to adapt the structure and morphology obtained directly at 400 K growth temperature. Presumably, the formation of layers with a β-phase-like lattice at low temperature is due to the better fit of the β phase, compared to the α phase lattice, to the underlying commensurate monolayer. However, at high growth temperatures, the thermodynamically more stable α phase grows, reducing the area of lattice misfit to the underlying commensurate first two layers by formation of clusters.     

Si(111)衬底上生长有多缓冲层的六方GaN晶格常数计算和应变分析

利用卢瑟福背散射/沟道技术和高分辨率X射线衍射技术对在Si(111)衬底上利用金属有机化合物气相外延技术(MOVPE)生长有多缓冲层的六方GaN外延膜进行结晶品质计算、晶格常数计算和应变分析. 实验结果表明：GaN外延膜的结晶品质为χ_min=1.54%，已达到完美晶体的结晶品质(χ_min=1%—2%)；GaN外延膜的水平方向和垂直方向晶格常数分别为：a_epi＝0.31903nm，c_epi=0.51837nm，基本达到G 关键词： GaN 高分辨X射线衍射 卢瑟福背散射/沟道 弹性应变     

AlGaN/GaN heterostructure grown on 1 -tilt sapphire substrate by MOCVD

AlGaN/GaN epitaxial layers were grown on 0^°-tilt and 1^°-tilt sapphire substrates by metalorganic chemical vapor deposition (MOCVD). With exactly the same growth conditions, it was found that dislocation density was smaller and crystal quality was better for the AlGaN/GaN epitaxial layers prepared on 1^°-tilt sapphire substrate. We also found that AlGaN/GaN epitaxial layers on 1^°-tilt sapphire substrate were grown with step growth mode while those on 0^°-tilt substrate were grown with two-dimensional island growth. From the temperature-dependent mobility, it was found that crystal quality of the AlGaN/GaN epitaxial layer prepared on 1^°-tilt sapphire substrate is better.