Improvement of Atomic-Layer-Deposited Al2O3/GaAs Interface Property by Sulfuration and NH3 Thermal Nitridation

Fermi level pinning at the interface between high-h gate dielectric and GaAs induced by unstable native oxides is a major obstacle for high performance GaAs-based metal-oxide-semiconductor （MOS） devices. We demonstrate the improved Al2O3/GaAs interracial characteristics by （NH4）2S immersion and NH3 thermal pretreatment prior to A1203 deposition. X-ray photoelectron spectroscopy （XPS） analysis confirms that sulfuration of GaAs surface by （NH4 ）2S solution can effectively reduce As-O bonds while Ga-O bonds and elemental As still exist at Al2O3 /GaAs interface. However, it is found that N incorporation during the further thermal nitridation on sulfurated GaAs can effectively suppress the native oxides and elemental As in the sequent deposition of Al2O3. Atomic force microscopy （AFM） shows that the further thermal nitridation on sulfurated GaAs surface can also improve the surface roughness.     

Fabrication and Characterization of SmCo5/Fe65Co35 and SmCo5/Fe Composite Spring Exchange Magnets

SmCo5/Fe65Co35 and SmCo5/Fe spring exchange magnets are fabricated by dc magnetron sputtering on MgO substrates and 100-nm-thick Si3N4 membranes, respectively. The base pressure of sputtering chamber is kept below 10^-7 Tort, and Ar pressure is 3 to 8mTorr. The samples are characterized by an x-ray diffractometer, a superconducting quantum interference magnetometer, and high resolution magnetic soft x-ray microscopy. We obtain the complete exchange coupling and single phase behaviour of composite magnets. The （BH）max value achieved is 28.8 MGOe.     

High-Reflectivity A1GaN/A1N Distributed Bragg Reflector in Ultraviolet Region

Thirty-pair Alo.3 Gao.T N/A1N distributed Bragg reflectors centred at 32Ohm are designed and grown on sapphire substrates by metalorganic chemical vapour deposition. No cracks are observed in the main area of the 2-inch wafer except for about 4 mm margin under an optical microscope. Regular stack of alternating layers is shown by scanning electron microscopy. Clear two-dimensional growth steps and very low surface roughness are shown by atomic force microscopy （AFM）. Well-defined periodicity is shown by high resolution x-ray diffraction. High refiectivity of 93% at 313nm with a bandwidth of 13nm is obtained.     

Structural, Optical and Electrical Properties of Hydrogen-Doped Amorphous GaAs Thin Films

Amorphous CaAs films are deposited on substrates of quartz glass and sificon by rf magnetron sputtering technique in different gas ambient. First, the amorphous structure of the prepared samples is identified by x-ray diffraction. Second, analysis by radial distribution function and pair correlation function method is established to characterize the microstructure of the samples. Then, the content and bond type of hydrogen are analysed using Fourier transform infrared absorption spectroscopy. It is found that the bonded hydrogen content increases with increasing partial pressure PH of H2. However, the hydrogen content saturates at PH 〉 1 × 10^-1 Pa. Hydrogen addition shills the optical absorption edge to higher energy, decreases the dark conductivity and improves the photo-sensitivity. The optical gap, dark conductivity and photo-sensitivity of the films are dependent on the bonded hydrogen content. These results demonstrate that hydrogen has obvious passivation effects on rf sputtered amorphous GaAs thin films.     

Low Density Self-Assembled InAs/GaAs Quantum Dots Grown by Metal Organic Chemical Vapour Deposition

The self-assembled InAs quantum dots （QDs） on GaAs substrates with low density （5×10^8 cm^-2） are achieved using relatively higher growth temperature and low InAs coverage by low-pressure metal-organic chemical vapour deposition. The macro-PL spectra exhibit three emission peaks at 1361, 1280 and 1204nm, corresponding to the ground level （GS）, the first excited state （ES1） and the second excited state （ES2） of the QDs, respectively, which are obtained when the GaAs capping layer is grown using triethylgallium and tertiallybutylarsine. As a result of micro-PL, only a few peaks from individual dots have been observed. The exciton-biexciton behaviour was clearly observed at low temperature.     

Sb-mediated growth of high-density InAs quantum dots and GaAsSb embedding growth by MBE

Self-assembled InAs quantum dots (QDs) with high-density were grown on GaAs(0 0 1) substrates by antimony (Sb)-mediated molecular beam epitaxy technique using GaAsSb/GaAs buffer layer and InAsSb wetting layer (WL). In this Sb-mediated growth, many two-dimensional (2D) small islands were formed on those WL surfaces. These 2D islands provide high step density and suppress surface migration. As the results, high-density InAs QDs were achieved, and photoluminescence (PL) intensity increased. Furthermore, by introducing GaAsSb capping layer (CL), higher PL intensity at room temperature was obtained as compared with that InGaAs CL.     

GaAs surface passivation by ultra-thin epitaxial GaP layer and surface As-P exchange

The GaAs surface passivation effects of epitaxially grown ultra-thin GaP layers and surface As-P exchange have been investigated. Optical properties of passivated and unpassivated InGaAs/GaAs near-surface quantum wells (QWs) grown by metal organic vapor phase epitaxy (MOVPE) are studied by low-temperature continuous-wave and time-resolved photoluminescence (PL). By optimizing the growth conditions, smooth surface morphologies and significant improvement of optical properties were observed for both passivation methods. Passivation improved the PL intensity more than two orders of magnitude and notably increased the PL decay time.     

Low-Dimensional Forest-Like and Desert-Like Fractal Patterns Formed in a DDAN Molecular System

Two kinds of forest-like and desert-like patterns are formed by thermal evaporation of 4-dicyanovinyl-N, Ndimethylamino-1-naphthalene （DDAN） onto SiO2 substrates. Based on thermal kinetics of the molecules on the substrate the transformation between the forest and desert patterns is due to two factors. The first one is the diffusion length, which is related to the deposition rate, the diffusion potential energy barrier and the substrate temperature. The second one is the strong interaction between the two polarity chemical groups of the molecules, which is beneficial to the formation of branches. Totally different patterns are also found on mica substrates, and are attributed to the anisotropic diffusion and the stronger interaction between DDAN molecules and the mica surface.     

Growth, Antimony Incorporation Behaviour and Beryllium Doping of GaAs1-ySby Grown on GaAs by Molecular Beam Epitaxy

A series of GaAs1-ySby epilayers are grown on GaAs substrates under antimony compositions of samples with beryllium doping are obtained different growth conditions. Different A non-equilibrium thermodynamics model is used to calibrate and fit the Sb composition. Activation energy of 0.37 eV for the dissociation process of Sb4 molecules is obtained. Carrier mobility and concentration of samples are influenced by the Sb composition. Quasi-qualitative analysis of mobility is used to explain the relations among Sb composition, carrier mobility and concentration. High resolution x-ray diffraction （HRXRD） rocking curves and Hall effects measurements are used to determine the crystal quality, carrier mobility and concentration.     

Photoluminescence and secondary ion mass spectroscopy characterization of GaAs-AlGaAs quantum wells grown on GaAs (1 0 0) substrates with different surface treatments

GaAs (1 0 0) substrates prepared in a quartz chamber under a H₂/As₄ flux, and then exposed to air were used for the subsequent growth of GaAs-AlGaAs single quantum wells by molecular beam epitaxy. The substrates prepared by this method showed atomically flat surfaces corroborated by atomic force microscopy analysis. Quantum wells grown directly on these substrates without a GaAs buffer layer exhibited narrow and intense photoluminescence peaks, an indication of a high quality material. The secondary ion mass spectroscopy analysis showed oxygen and carbon traces on the first AlGaAs barrier layer grown after air exposure with no effects on the quantum wells optical emissions. From the results we conclude that the prepared GaAs surfaces are useful for the epitaxial growth of high quality quantum structures.     

Non-catalytic growth of high aspect-ratio ZnO nanowires by thermal evaporation

High-density and high aspect-ratio ZnO nanowires were grown on Si(100) substrates by the thermal evaporation of metallic zinc powder without the use of metal catalysts or additives. The as-grown nanowires had diameters in the range of 60-100 nm with lengths 5-15 μm. Detailed structural characterization indicated that the obtained nanowires are single-crystalline with a perfect hexagonal facet and surfaces. The room temperature PL spectrum exhibited strong UV emission, affirming that the as-grown products have good optical properties. The possible growth mechanism for the formation of hexagonal-faceted and perfect surface ZnO nanowires is also discussed.     

Interfaces in InAs/GaSb Superlattices Grown by Molecular Beam Epitaxy

Short period InAs（4ML）/GaSb（SML） superlattices （SLs） with InSb- and mixed-like （or Ga1-xInxAs1-ySby- like） interfaces （IFs） are grown by molecular-beam epitaxy （MBE） on （001） GaSh substrates at optimized growth temperature. Raman scattering reveals that two kinds of IFs can be formed by controlling shutter sequences. X-ray diffraction （XRD） and atomic force microscopy （AFM） demonstrate that SLs with mixed-like IFs are more sensitive to growth temperature than that with InSb-like IFs. The photoluminescence （PL） spectra of SLs with mixed-like IFs show a stronger intensity and narrower line width than with InSb-like IFs. It is concluded that InAs/GaSb SLs with mixed-like IFs have better crystalline and optical properties.     

Determination of Tungsten Layer Profiles in Bilayer Structures Using X-Ray Reflectivity Method

An effectual method is presented to determine the profiles of a tungsten （W） layer, such as the density, the thickness and the roughness in the multilayer structures, using the x-ray reflectivity technique. To avoid oxidation effects of tungsten, a B4 C capping layer is deposited onto to the W layer. To observe the profiles of the tungsten layer with different thicknesses, three groups of W/B4 C bilayers with different thicknesses are prepared by using ultra high vacuum dc magnetron sputtering and measured by an x-ray diffractometer. A type of genetic algorithm called the differential evolution is used to simulate the measurement data so as to obtain the parameters of bilayers. According to the simulation, it is shown that the W layer density varies from 95.26% to 97.51% compared to the bulk. In our experiment, the deposition rate is 0.044 nm/s, and the thickness is varied in the range of 9.8-19.4 nm.     

Influence of Different Substrates on Laser Induced Damage Thresholds at 1064nm of Ta2O5 Films

Ta2O5 films are prepared on Si, BK7, fused silica, antireflection （AR） and high reflector （HR） substrates by electron beam evaporation method, respectively. Both the optical property and laser induced damage thresholds （LIDTs） at 1064 nm of Ta2O5 films on different substrates are investigated before and after annealing at 673 K for 12 h. It is shown that annealing increases the refractive index and decreases the extinction index, and improves the O/Ta ratio of the Ta2O5 films from 2.42 to 2.50. Moreover, the results show that the LIDTs of the Ta2O5 films are mainly correlated with three parameters： substrate property, substoichiometry defect in the films and impurity defect at the interface between the substrate and the films. Details of the laser induced damage models in different cases are discussed.     

Structure and homoepitaxial growth of GaAs(6 3 1)

We have studied the surface atomic structure of GaAs(6 3 1), and the GaAs growth by molecular beam epitaxy (MBE) on this plane. After the oxide desorption process at 585 °C reflection high-energy electron diffraction (RHEED) showed along the [−1 2 0] direction a 2× surface reconstruction for GaAs(6 3 1)A, and a 1× pattern was observed for GaAs(6 3 1)B. By annealing the substrates for 60 min, we observed that on the A surface appeared small hilly-like features, while on GaAs(6 3 1)B surface pits were formed. For GaAs(6 3 1)A, 500 nm-thick GaAs layers were grown at 585 °C. The atomic force microscopy (AFM) images at the end of growth showed the self-formation of nanoscale structures with a pyramidal shape enlarged along the [5−9−3] direction. Transversal views of the bulk-truncated GaAs(6 3 1) surface model showed arrays of atomic grooves along this direction, which could influence the formation of the pyramidal structures.     

Atmospheric-pressure metal–organic vapor-phase epitaxy of GaAsBi alloys on high-index GaAs substrates

We investigated the growth characteristics and properties of GaAsBi layers grown by atmospheric-pressure metal–organic vapor-phase epitaxy on different GaAs substrate orientations. The surface morphology of GaAsBi alloys was investigated by means of scanning electron microscopy. The structural and optical properties of the alloys were examined using high-resolution X-ray diffraction (HRXRD) and photoreflectance spectroscopy, respectively. HRXRD results show that the GaAsBi growth rate was significantly lower on (1 1 5)A than on (0 0 1), (1 1 1)A and (1 1 4)A GaAs. The highest Bi content was obtained for GaAsBi layers grown on (1 1 5)A GaAs substrates.     

In situ characterization of MBE grown GaAs and Al x Ga1−x As films using RHEED,SIMS, and AES techniques

A combination of the surface diagnostic techniques Auger electron spectroscopy (AES), reflection high energy electron diffraction (RHEED), and secondary ion mass spectroscopy (SIMS) was used in order to get more detailed information on basic processes which lead to the formation of high quality monocrystalline GaAs and Al _x Ga_1−x As films by molecular beam epitaxy (MBE) under ultra-high vacuum conditions. The formation and changes of reconstructed surface structures on (100) GaAs as a function of growth parameters were observedduring growth by RHEED. AES was used to determine the relative ratio of Ga/As on the surface for different reconstructed structures, to investigate the impurity contamination on substrate surfaces and grown films, and to study the surface segregation of Sn in MBE GaAs during doping. Finally, intentional and unintentional impurities incorporated during the growth of GaAs and Al _x Ga_1−x As by MBE were detected by the SIMS technique immediately after growth within the reaction chamber.     

Metamorphic growth of InAlAs/InGaAs MQW and InAs HEMT structures on GaAs

The large electron mobility at room temperature and the absence of Schottky barrier to metals make InAs two-dimensional electron gas (2DEG) a good candidate for SPIN-FET (FET—field-effect-transistor) applications. So far the growth was done either on the InAlAs epilayers compositionally matched to InP substrates, or on Sb-based compound semiconductors. Here we aim to grow InAs 2DEG on GaAs substrates by using a strain-relaxing buffer layer. We introduce In_0.4Al_0.6As glue layer to the metamorphic structure and investigate the physical properties of InAlAs/InGaAs multi-quantum-well (MQW) structures via X-ray diffraction, transmission electron microscopy, and photoluminescence. We find that the use of As dimer instead of tetramer and the choice of proper growth temperature are essential for successful growth. InAs-inserted-channel InAlAs/InGaAs inverted high-electron-mobility transistor (HEMT) structures show promising results for SPIN-FET application.     

Influence of Ytterbia Content on Residual Stress and Microstructure of Y2O3--ZrO2 Thin Films Prepared by EB-PVD

Y2O3 stabilized ZrO2 （YSZ） thin films with different Y2O3 molar contents （0, 3, 7, and 12 mol%） are deposited on BK7 substrates by electron-beam evaporation technique. The effects of different Y2O3 contents on residual stresses and structures of YSZ thin films are studied. Residual stresses are investigated by means of two different techniques： the curvature measurement and x-ray diffraction method. It is found that the evolution of residual stresses of YSZ thin films by the two different methods is consistent. Residual stresses of films transform from compressive stress into tensile stress and the tensile stress increases monotonically with the increase of Y2O3 content. At the same time, the structures of these films change from the mixture of amorphous and monoclinic phases into high temperature cubic phase. The variations of residual stress correspond to the evolution of structures induced by adding of Y2O3 content.     

Transport Behaviour of La0.8Sr0.2AlO3 Thin Film on Oxygen Deficient SrTiO3 Substrate

La_0.8Sr_0.2AlO₃ (LSAO) thin films are grown on SrTiO₃ (STO) and MgO substrates by laser molecular beam epitaxy. The LSAO thin film on oxygen deficient STO substrate exhibits metallic behaviour over the temperature range of 80--340K. The optical transmittance spectrum indicates that theLSAO thin films on MgO substrate are insulating at room temperature. The transport properties of LSAO thin films on STO substrates deposited in different oxygen pressure are compared. Our results indicate that oxygen vacancies in STO substrates should be mainly responsible for the transport behaviour of LSAO thin films.