Effect of thickness on the microstructure of GaN films on Al2O3 (0001) by laser molecular beam epitaxy

Heteroepitaxial GaN films are grown on sapphire (0001) substrates using laser molecular beam epitaxy. The growth processes are in-situ monitored by reflection high energy electron diffraction. It is revealed that the growth mode of GaN transformed from three-dimensional (3D) island mode to two-dimensional (2D) layer-by-layer mode with the increase of thickness. This paper investigates the interfacial strain relaxation of GaN films by analysing their diffraction patterns. Calculation shows that the strain is completely relaxed when the thickness reaches 15 nm. The surface morphology evolution indicates that island merging and reduction of the island-edge barrier provide an effective way to make GaN films follow a 2D layer-by-layer growth mode. The 110-nm GaN films with a 2D growth mode have smooth regular hexagonal shapes. The X-ray diffraction indicates that thickness has a significant effect on the crystallized quality of GaN thin films.     

Growth of wide band gap wurtzite ZnMgO layers on (0001) Al2O3 by radical-source molecular beam epitaxy

Wurtzite structure ZnMgO layers have been grown using radical-source molecular beam epitaxy on high-quality ZnO buffer layers grown on (0001) sapphire substrates. The thickness of the ZnO buffer layers is 300 nm, with full width at half maxim of the HR-XRD (0002) rocking curves as low as 25 arcsec. In-situ Reflection High-Energy Electron Diffraction (RHEED) was employed for the optimization of the ZnMgO growth. RHEED and X-Ray Diffractometry measurements did not reveal any phase change from the wurzite structure to the rocksalt structure. The C-lattice parameter of Zn_1−xMg_xO films decreased from 5.209 to 5.176 Å with increasing x to 0.2. The surface morphology of the samples was studied with atomic force microscopy. The root mean square roughness values of 200 nm thick ZnMgO (x=0.2) was less than 1 nm. The main photoluminescence peak of Zn_1−xMg_xO shifted to as high as 3.77 eV owing to the increasing Mg composition of up to x=0.2.     

AlGaN/GaN HEMTs grown on silicon (001) substrates by molecular beam epitaxy

We report the realization of an AlGaN/GaN HEMT on silicon (001) substrate with noticeably better transport and electrical characteristics than previously reported. The heterostructure has been grown by molecular beam epitaxy. The 2D electron gas formed at the AlGaN/GaN interface exhibits a sheet carrier density of 8×10¹² cm⁻² and a Hall mobility of 1800 cm²/V s at room temperature. High electron mobility transistors with a gate length of 4 μm have been processed and DC characteristics have been achieved. A maximum drain current of more than 500 mA/mm and a transconductance g_m of 120 mS/mm have been obtained. These results are promising and open the way for making efficient AlGaN/GaN HEMT devices on Si(001).     

Polarity control and residual strain in ZnO epilayers grown by molecular beam epitaxy on (0001) GaN/sapphire

We report on the polarity control of ZnO grown by plasma assisted molecular beam epitaxy on Ga polar (0001) GaN/sapphire templates simply via the oxygen‐to‐Zn (VI/II) ratio during the growth of a thin nucleation layer at 300 °C. Following Zn pre‐exposure, the ZnO layers nucleated with low VI/II ratios (<1.5) exhibited Zn‐polarity. Those nucleated with VI/II ratios above 1.5, exhibited O‐polarity. Supported by scanning transmission electron microscopic imaging, we have unequivocally demonstrated that polarity inversion takes place without formation of any vertical inversion domains and within one monolayer of presumably non‐stoichiometric GaO_x formed at the ZnO/GaN interface. A direct correlation between polarity and strain sign of ZnO layers has been found. The Zn‐polar ZnO layers were under tensile biaxial strain, whereas the O‐polar material exhibited compressive strain. Moreover, the amount of residual strain varied linearly with VI/II ratio used during the low‐temperature nucleation layer growth. Strain control with VI/II ratio has been explained by the potential formation of Zn interstitials.     

Growth of ZnO thin films on c-plane Al2O3 by molecular beam epitaxy using ozone as an oxygen source

The growth of c-axis oriented ZnO thin films on c-plane Al₂O₃ via molecular beam epitaxy (MBE) using dilute ozone (O₃) as an oxygen source was investigated. Four-circle X-ray diffraction (XRD) indicates that films grown at 350 °C are epitaxial with respect to the substrate, but with a broad in-plane and out-of-plane mosaic. The films were highly conductive and n-type. Epitaxial film growth required relatively high Zn flux and O₃/O₂ pressure. The growth rate decreased rapidly as growth temperature was increased above 350 °C. The drop in growth rate with temperature reflects the low sticking coefficient of Zn at moderately high temperatures and limited ozone flux for the oxidation of the Zn metal. Characterization of the films included atomic force microscopy (AFM), X-ray diffraction, photoluminescence, and Hall measurements. These results show that molecular beam epitaxy of ZnO using ozone is rate limited by the ozone flux for growth temperatures above 350 °C.     

Current transport in ZnO/Si heterostructure grown by laser molecular beam epitaxy

The n-ZnO/p-Si heterojunction was fabricated by depositing high quality single crystalline aluminium-doped n-type ZnO film on p-type Si using the laser molecular beam epitaxy technique. The heterojunction exhibited a good rectifying behavior. The electrical properties of the heterojunction were investigated by means of temperature dependence current density-voltage measurements. The mechanism of the current transport was proposed based on the band structure of the heterojunction. When the applied bias V is lower than 0.15 V, the current follows the Ohmic behavior. When 0.15V 0.6 V), the space charge limited effect becomes the main transport mechanism. The current-voltage characteristic under illumination was also investigated. The photovoltage and the short circuit current density of the heterojunction aproached 270 mV and 2.10 mA/cm 2 , respectively.     

Co on thin Al2O3 films grown on Ni3Al(1 0 0)

The growth of Co on thin Al₂O₃ layers on Ni₃Al(1 0 0) was investigated by Auger electron spectroscopy, high resolution electron energy loss spectroscopy (EELS), and scanning tunneling microscopy. At 300 K, Co grows in three-dimensional clusters on top of the Al₂O₃ layer. A defect structure of the alumina layer plays a crucial role during the early stage of Co growth. After deposition of 10 Å of Co, a complete screening of the dipoles of the Al₂O₃ layer due to the Co film is found in the EELS measurements. Annealing the Co film reveals a process of coalescence of Co clusters and, above 700 K, diffusion of the Co atoms through the oxide film into the substrate takes place.     

Structural and optical properties of GaN thin films grown on Al2O3 substrates by MOCVD at different reactor pressures

GaN thin films grown by MOCVD on (0 0 0 1) Al₂O₃ substrates at different growth pressures were characterized by field-emission scanning electron microscopy, atomic force microscopy, micro-Raman, and photoluminescence at room temperature. It was found that there is an optimum pressure of 76 Torr at which the structural and optical properties of the GaN samples are superior. On the other hand samples grown at higher pressure exhibited hexagonal surface pits and surface spirals. The results showed that the growth pressure strongly influences the morphology, and significantly affects the structural and optical properties of the GaN epilayers.     

The high quality ZnO growth on c-Al2O3 substrate with Cr2O3 buffer layer using plasma-assisted molecular beam epitaxy

High quality epitaxial ZnO films were grown on c-Al₂O₃ substrates with Cr₂O₃ buffer layer by plasma-assisted molecular beam epitaxy (P-MBE). The hexagonal crystalline Cr₂O₃ layer was formed by oxidation of the Cr-metal layer deposited on the c-Al₂O₃ substrate using oxygen plasma. The epitaxial relationship was determined to be ZnO//Cr₂O₃//Cr//Al₂O₃ and ZnO//Cr₂O₃//[0 0 1]Cr//Al₂O_3. The Cr₂O₃ buffer layer was very effective in improving the surface morphology and crystal quality of the ZnO films. The photoluminescence spectrum showed the strong near band-edge emissions with the weak deep-level emission, which implies high optical quality of the ZnO films grown on the Cr₂O₃ buffer.     

The role of a ZnO buffer layer in the growth of ZnO thin film on Al2O3 substrate

A ZnO buffer layer and ZnO thin film have been deposited by the pulsed laser deposition technique at the temperatures of 200 C and 400 C, respectively. Structural, electrical and optical properties of ZnO thin films grown on sapphire (Al₂O₃) substrate with 1, 5, and 9 nm thick ZnO buffer layers were investigated. A minute shift of the (101) peak was observed which indicates that the lattice parameter was changed by varying the thickness of the buffer layer. High resolution transmission electron microscopy (TEM) was used to investigate the thickness of the ZnO buffer layer and the interface involving a thin ZnO buffer between the film and substrate. Selected area electron diffraction (SAED) patterns show high quality hexagonal ZnO thin film with 30 in-plane rotation with respect to the sapphire substrate. The use of the buffer can reduce the lattice mismatch between the ZnO thin film and sapphire substrate; therefore, the lattice constant of ZnO thin film grown on sapphire substrate became similar to that of bulk ZnO with increasing thickness of the buffer layer.     

Characterizations of n-type ferromagnetic GaMnN thin film grown on GaN/Al2O3 (0 0 0 1) by metal-organic chemical vapor deposition

A high-quality ferromagnetic GaMnN (Mn=2.8 at%) film was deposited onto a GaN buffer/Al₂O₃(0 0 0 1) at 885 °C using the metal-organic chemical vapor deposition (MOCVD) process. The GaMnN film shows a highly c-axis-oriented hexagonal wurtzite structure, implying that Mn doping into GaN does not influence the crystallinity of the film. No Mn-related secondary phases were found in the GaMnN film by means of a high-flux X-ray diffraction analysis. The composition profiles of Ga, Mn, and N maintain nearly constant levels in depth profiles of the GaMnN film. The binding energy peak of the Mn 2p_3/2 orbital was observed at 642.3 eV corresponding to the Mn (III) oxidation state of MnN. The presence of metallic Mn clusters (binding energy: 640.9 eV) in the GaMnN film was excluded. A broad yellow emission around 2.2 eV as well as a relatively weak near-band-edge emission at 3.39 eV was observed in a Mn-doped GaN film, while the undoped GaN film only shows a near-band-edge emission at 3.37 eV. The Mn-doped GaN film showed n-type semiconducting characteristics; the electron carrier concentration was 1.2×10²¹/cm³ and the resistivity was 3.9×10⁻³ Ω cm. Ferromagnetic hysteresis loops were observed at 300 K with a magnetic field parallel and perpendicular to the ab plane. The zero-field-cooled and field-cooled curves at temperatures ranging from 10 to 350 K strongly indicate that the GaMnN film is ferromagnetic at least up to 350 K. A coercive field of 250 Oe and effective magnetic moment of 0.0003 μ_B/Mn were obtained. The n-type semiconducting behavior plays a role in inducing ferromagnetism in the GaMnN film, and the observed ferromagnetism is appropriately explained by a double exchange mechanism.     

Characterization of single-crystalline In2O3 films deposited on Y-stabilized ZrO2 (1 0 0) substrates by MOCVD

Epitaxial In₂O₃ films have been deposited on Y-stabilized ZrO₂ (YSZ) (1 0 0) substrates by metalorganic chemical vapor deposition (MOCVD). The films were deposited at different substrate temperatures (450-750 °C). The film deposited at 650 °C has the best crystalline quality, and observation of the interface area shows a clear cube-on-cube epitaxial relationship of In₂O₃(1 0 0)||YSZ(1 0 0) with In₂O₃[0 0 1]||YSZ[0 0 1]. The Hall mobility of the single-crystalline In₂O₃ film deposited at 650 °C is as high as 66.5 cm² V⁻¹ s⁻¹ with carrier concentration of 1.5 × 10¹⁹ cm⁻³ and resistivity of 6.3 × 10⁻³ Ω cm. The absolute average transmittance of the obtained films in the visible range exceeds 95%.     

Fabrication of ZnO/Al2O3 core-shell nanostructures and crystalline Al2O3 nanotube

We have demonstrated the crystalline ZnO-Al₂O₃ core-shell nanowire structure by atomic layer deposition (ALD) at a temperature 100 °C. The core-shell structure could have potential applications in the fabrication of ZnO field effect transistor. After dissolving the ZnO core, shape defined, rigid and robust crystalline Al₂O₃ shelled nanostructures have been fabricated. Nanowire ZnO nanostructures have been replicated by alumina shell. This is one of the most effective techniques for producing core-shell or shell/hollowed nanostructures of any desired objects. The Al₂O₃ shelled nanostructures could have potential applications as space confined nanoreactors, drug delivery, nanofluidic channels and optical transmitting.     

The effect of hydrogen irradiation and annealing on the low-temperature growth of homoepitaxial ZnO layers grown on (0 0 0 1) ZnO substrates by plasma-assisted molecular beam epitaxy

Low-temperature growth of high quality homoepitaxial ZnO is realized by using hydrogen irradiation and annealing processes (modified method). By modified method, two-dimensional growth and atomically smooth surface with steps (terrace length ∼75 nm) are achieved at 400 °C. Furthermore, FWHMs of high resolution X-ray rocking curves for (0 0 0 2) and (10-11) reflections are evaluated as narrow as 21 arcsec, which indicates that high crystallinity of the ZnO grown at 400 °C by modified method is almost similar to that grown at 600 °C by conventional method. Photoluminescence results show the considerable improvement of optical properties, such as an emersion of free exciton (FX) and a decrease of donor bound exciton (D°X) linewidth, by using modified method, even at growth temperature as low as 400 °C.     

Superconducting molybdenum nitride epitaxial thin films deposited on MgO and α-Al2O3 substrates by molecular beam epitaxy

Molybdenum nitride Mo₂N_x films were grown on MgO(0 0 1) and on α-Al₂O₃(0 0 1) substrates by molecular beam epitaxy under nitrogen radical irradiation. X-ray photoelectron spectroscopy revealed that the composition of the film varied in the range of Mo₂N_1.4-Mo₂N_2.8 depending on the growth temperature. The deposition at 973 K gave well-crystallized films on both substrates. The high-resolution reciprocal space mapping by X-ray diffraction showed that the nitrogen-rich γ-Mo₂N crystalline phase (the composition: Mo₂N_1.4) was epitaxially grown on MgO at 923 K with a slight tetragonal distortion (a = 0.421 and c = 0.418 nm) to fit the MgO lattice (a = 0.421 nm). On α-Al₂O₃(0 0 1), nitrogen-rich γ-Mo₂N (Mo₂N_1.8) was grown at 973 K with (1 1 1) planes parallel to the substrate surface. X-ray diffraction analysis with a multi-axes diffractometer revealed that the γ-Mo₂N on α-Al₂O₃(0 0 1) had a slight rhombohedral distortion (a = 0.4173(2) and α = 90.46(3)°). Superconductivity was observed below 2.8-3 K for the films grown at 973 K on MgO and on α-Al₂O₃(0 0 1).     

Structural properties of Al2O3 dielectrics grown on TiN metal substrates by atomic layer deposition

We investigated on the structural properties of Al₂O₃ dielectrics grown on TiN metal substrates using an atomic layer deposition technique with tri-methyl-aluminum and either O₃ or H₂O as the precursor and oxidant, respectively. The structural and morphological features of these films were examined by atomic force microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy measurements. We find that Al₂O₃ dielectric films with the O₃ oxidant exhibit a rough morphology, a thick TiO₂ film, and a small amount of contaminants such as carbon and hydrogen. The reason for the rapid diffusion of oxygen atoms into the TiN lattice leads to the formation of TiO₂ layer on the TiN substrate. This is due to the higher oxidation potential of the O₃ compared to the H₂O.     

Effects of oxygen flow rate on the properties of HfO2 layers grown by metalorganic molecular beam epitaxy

The investigations on the properties of HfO₂ dielectric layers grown by metalorganic molecular beam epitaxy were performed. Hafnium-tetra-tert-butoxide, Hf(C₄H₉O)₄ was used as a Hf precursor and pure oxygen was introduced to form an oxide layer. The grown film was characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), and capacitance–voltage (C–V) and current–voltage (I–V) analyses. As an experimental variable, the O₂ flow rate was changed from 2 to 8 sccm while the other experimental conditions were fixed. The XPS spectra of Hf 4f and O 1s shifted to the higher binding energy due to the charge transfer effect and the density of trapped charges in the interfacial layer was increased as the oxygen flow rate increased. The observed microstructure indicated the HfO₂ layer was polycrystalline, and the monoclinic phases are the dominant crystal structure. From the C–V analyses, k = 14–16 and EOT = 44–52 were obtained, and the current densities of (3.2–3.3) × 10⁻³ A/cm² were measured at −1.5 V gate voltage from the I–V analyses.     

Electrical transport properties of Fe3−xCrxO4 ferrite films on MgO (0 0 1) grown by molecular beam epitaxy

In this report, we fabricated a series of Fe_3−xCr_xO₄(0≦x≦2) films by plasma-oxygen-assisted molecular beam epitaxy (MBE) and did structural and electrical characterizations of these films. These films show textured single phase quality and the lattice parameters are consistent with those of the bulk at low Cr composition (x<0.9). However, the lattice parameters show severe deviation from the bulk value in the intermediate region of 0.9≦x≦1.5 and no diffraction can be resolved at x∼2. These discrepancies may be attributed to the cation distributions and the instability of spinal structure as Cr concentration becomes dominant. The resistivity presents a typical Arrhenius temperature dependence with ρ=ρ₀ exp (E_p/k_BT) indicating that the transport is due to a hopping mechanism. The prefactor ρ₀ increases in Fe_3−xCr_xO₄, at smaller x but tends to level out for x>1, suggesting that Cr³⁺ ions may start to replace Fe³⁺ ions at the A site in the high x region. The activation energy of electrical hopping gradually increases at low Cr concentration but abruptly rises to ∼110 meV at x>0.9, suggesting a crossover from electron-hopping mediated transport to a thermally activated band gap excitation.     

Characteristics of sandwich-structured Al2O3/HfO2/Al2O3 gate dielectric films on ultra-thin silicon-on-insulator substrates

Sandwich-structure Al₂O₃/HfO₂/Al₂O₃ gate dielectric films were grown on ultra-thin silicon-on-insulator (SOI) substrates by vacuum electron beam evaporation (EB-PVD) method. AFM and TEM observations showed that the films remained amorphous even after post-annealing treatment at 950 °C with smooth surface and clean silicon interface. EDX- and XPS-analysis results revealed no silicate or silicide at the silicon interface. The equivalent oxide thickness was 3 nm and the dielectric constant was around 7.2, as determined by electrical measurements. A fixed charge density of 3 × 10¹⁰ cm⁻² and a leakage current of 5 × 10⁻⁷A/cm² at 2 V gate bias were achieved for Au/gate stack /Si/SiO₂/Si/Au MIS capacitors. Post-annealing treatment was found to effectively reduce trap density, but increase in annealing temperature did not made any significant difference in the electrical performance.     

Ge(001)衬底上分子束外延生长高质量的Ge1-xSnx合金

Ge_1-xSn_x是一种新型IV族合金材料, 在光子学和微电子学器件研制中具有重要应用前景. 本文使用低温分子束外延(MBE)法, 在Ge(001)衬底上生长高质量的Ge_1-xSn_x合金, 组分x分别为1.5%, 2.4%, 2.8%, 5.3%和14%, 采用高分辨X射线衍射(HR-XRD)、卢瑟福背散射谱(RBS) 和透射电子显微镜(TEM)等方法表征Ge_1-xSn_x合金的材料质量. 对于低Sn组分(x≤ 5.3%)的样品, Ge_1-xSn_x合金的晶体质量非常好, RBS的沟道/随机产额比(χ_min)只有5.0%, HR-XRD曲线中Ge_1-xSn_x衍射峰的半高全宽(FWHM)仅100' 左右. 对于x=14%的样品, Ge_1-xSn_x合金的晶体质量相对差一些, FWHM=264.6'. 关键词： 锗锡合金 锗 分子束外延