Optical properties and structural characteristics of ZnO thin films grown on a-plane sapphire substrates by plasma-assisted molecular beam epitaxy

We investigated structural and optical properties of ZnO thin films grown on (112?0) a-plane sapphire substrates using plasma-assisted molecular beam epitaxy. Negligible biaxial stress in ZnO thin films is due to the use of (112?0) a-plane sapphire substrates and slow substrate cooling. The 14 K photoluminescence spectrum shows a blueshift of energy positions compared with ZnO single crystal. A donor with binding energy of 43 meV and an acceptor with binding energy of ～170 meV are identified by well-resolved photoluminescence spectra. A characteristic emission band at 3.320 eV (so-called A-line) is studied. Based on analysis from photoluminescence spectra, the origin of the A-line, it seems, is more likely an (e, A°) transition, in which defect behaves as an acceptor. The room-temperature photoluminescence is dominated by the FX at 3.307 eV, which is an indication of strongly reduced defect density in ZnO thin films.     

Growth of InN thin films on ZnO substrate by plasma-assisted molecular beam epitaxy

We have studied the microstructure property of InN epitaxial films grown on ZnO substrate by plasma-assisted molecular beam epitaxy. We found that the In₂O₃ compound was produced on ZnO substrate and many pits were formed on the InN films when InN was directly grown on ZnO substrate with the N/In flux ratio less than 40. We demonstrated that the quality of InN film was significantly improved when the In₂O₃ layer was used as a buffer to prevent the reaction between In and the ZnO substrate.     

Layer by layer growth of ZnO on (0001) sapphire substrates by radical-source molecular beam epitaxy

Layer by layer growth of ZnO epilayers on (0001) Al₂O₃ substrates is achieved by radical-source molecular beam epitaxy. A thin MgO buffer, followed by a low-temperature ZnO buffer was used in order to accommodate the lattice mismatch between ZnO and sapphire. Reflection high-energy electron diffraction intensity was employed for the optimization of the ZnO growth. The surface morphology of the samples was studied with atomic force microscopy. Investigation of the nature of the influence of the MgO buffer layer on the formation of ZnO on sapphire substrate was carried out using Transmission Electron Microscopy. For the first time it was shown that a thin spinel (magnesium aluminum oxide) layer is formed on the interface of the sapphire substrate and MgO buffer layer leading to the crystalline quality improvement of the ZnO main layer. X-Ray diffractometry measurements of the obtained ZnO layers show excellent quality of the single crystalline ZnO heteroepitaxially grown on sapphire. The crystalline quality of the ZnO layers is even better than that of our previously reported layers grown employing hydrogen peroxide as an oxidant. The full width at half maximum of the XRD (0002) rocking curve is as low as 25 arc s. The influence of growth parameters (Zn/O flux ratio, temperature, etc.) on the structural properties as well as on the surface morphology of the zinc oxide layers on sapphire is investigated and discussed.     

Growth of high-quality perovskite (110)-SrIrO_3 thin films using reactive molecular beam epitaxy

Recently, 5d transition metal iridates have been reported as promising materials for the manufacture of exotic quantum states. Apart from the semimetallic ground states that have been observed, perovskite SrIrO_3 is also predicted to have a lattice-symmetrically protected topological state in the(110) plane due to its strong spin–orbit coupling and electron correlation. Compared with non-polar(001)-SrIrO_3, the especial polarity of(110)-SrIrO_3 undoubtedly adds the difficulty of fabrication and largely impedes the research on its surface states. Here, we have successfully synthesized high-quality(110)-SrIrO_3 thin films on(110)-SrTiO_3 substrates by reactive molecular beam epitaxy for the first time. Both reflection high-energy electron diffraction patterns and x-ray diffraction measurements suggest the expected orientation and outstanding crystallinity. A(1 × 2) surface reconstruction driven from the surface instability, the same as that reported in(110)-SrTiO_3, is observed. The electric transport measurements uncover that(110)-SrIrO_3 exhibits a more prominent semimetallic property in comparison to(001)-SrIrO_3.     

Characterization of nitrogen-doped ZnO thin films grown by plasma-assisted pulsed laser deposition on sapphire substrates

The crystalline, optical and electrical properties of N-doped ZnO thin films were measured using X-ray diffraction, photoluminescence and Hall effect apparatus, respectively. The samples were grown using pulsed laser deposition on sapphire substrates coated priorly with ZnO buffer layers. For the purpose of acceptor doping, an electron cyclotron resonance (ECR) plasma source operated as a low-energy ion source was used for nitrogen incorporation in the samples. The X-ray diffraction analyses indicated some deterioration of the ZnO thin film with nitrogen incorporation. Temperature-dependent Van der Pauw measurements showed consistent p-type behavior over the measured temperature range of 200–450 K, with typical room temperature hole concentrations and mobilities of 5×10¹⁵ cm⁻³ and 7 cm²/V s, respectively. Low temperature photoluminescence spectra consisted of a broad emission band centered around 3.2 eV. This emission is characterized by the absence of the green deep-defect band and the presence of a band around 3.32 eV.     

Graphene films grown on sapphire substrates via solid source molecular beam epitaxy

A method for growing graphene on a sapphire substrate by depositing an SiC buffer layer and then annealing at high temperature in solid source molecular beam epitaxy (SSMBE) equipment was presented. The structural and electronic properties of the samples were characterized by reflection high energy diffraction (RHEED), X-ray diffraction Φ scans, Raman spectroscopy, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The results of the RHEED and Φ scan, as well as the Raman spectra, showed that an epitaxial hexagonal α-SiC layer was grown on the sapphire substrate. The results of the Raman and NEXAFS spectra revealed that the graphene films with the AB Bernal stacking structure were formed on the sapphire substrate after annealing. The layer number of the graphene was between four and five, and the thickness of the unreacted SiC layer was about 1--1.5 nm.     

在预刻蚀的衬底上通过分子束外延直接生长出拓扑绝缘体薄膜的微器件

在利用光刻将拓扑绝缘体外延薄膜加工成微米尺寸结构的过程中,所用的各种化学物质会导致薄膜质量的下降.在实验中,通过在钛酸锶衬底上预先光刻出Hall bar形状的凸平台并以此为模板进行拓扑绝缘体(Bi x Sb1-x)2Te3薄膜的分子束外延生长,直接获得了薄膜的Hall bar微器件,从而避免了光刻过程对材料质量的影响.原子力显微镜和输运测量结果均显示该微器件保持了(Bi x Sb1-x)2Te3外延薄膜原有的性质.这种新的微器件制备方法有助于在拓扑绝缘体中实现各种新奇的量子效应,并可推广于其他外延生长的低维系统.     

Growth and annealing of zinc-blende CdSe thin films on GaAs (0 0 1) by molecular beam epitaxy

CdSe thin films have been grown on GaAs (0 0 1) substrates by molecular beam epitaxy (MBE). The effects of substrate temperature and annealing treatment on the structural properties of CdSe layers were investigated. The growth rate slightly decreases due to the accelerated desorption of Cd from CdSe surface with an increase in the temperature. The sample grown at 260 °C shows a polycrystalline structure with rough surface. As the temperature increases over 300 °C, crystalline CdSe (0 0 1) epilayers with zinc-blende structure are achieved and the structural quality is improved remarkably. The epilayer grown at 340 °C displays the narrowest full-width at half-maximum (FWHM) from (0 0 4) reflection in double-crystal X-ray rocking curve (DCXRC) and the smallest root-mean-square (RMS) roughness of 0.816 nm. Additionally, samples fabricated at 320 °C were annealed in air for 30 min to study the films’ thermal stability. X-ray diffraction (XRD) results indicate that the zinc-blende structure remains unchanged when the annealing temperature is elevated to 460 °C, meaning a good thermal stability of the cubic CdSe epilayers.     

Optical spectroscopy of a-plane-oriented ZnO epilayers grown by plasma-assisted molecular beam epitaxy

a-plane ZnO layers were successfully grown, by plasma-assisted molecular beam epitaxy, on r-plane (011–2) sapphire substrates. Several features attributed to the A, B and C free excitonic transitions are identified through temperature-dependent photoluminescence (PL) and reflectivity measurements. The temperature dependence of the peak energy positions of these transitions was studied from 8 K to 300 K. So, the PL peak energy of the A free exciton was plotted and fitted with a Varshni empirical equation. In the σ polarization (E⊥c), for which A and B are allowed, the reflectivity spectrum measured at 8 K was fitted by using a program based on the theory of the spatial resonance dispersion Hopfield model. Our results indicate that the A and B free excitonic features are at a higher energy than those in ZnO on c-oriented sapphire and show a good PL even at 250 K. These results also show that the new and intense emission peak observed in the region of the first phonon replica (3.33–3.28 eV) and identified as a stacking fault in the epilayer disappears at high temperature above 150 K.     

Semipolar (11\bar 2\bar 2) ZnO thin films grown on LaAlO3‐buffered LSAT (112) single crystals by pulsed laser deposition

Semipolar (11\bar 2 \bar 2) ZnO was successfully grown on (112) LaAlO₃/(LaAlO₃)_0.29(Sr₂AlTaO₆)_0.35 substrate by pulsed laser deposition. The epitaxial relationship is [11\bar 23]_{\rm ZnO} // [11\bar 1]_{\rm LAO/LSAT} with the polar axis of [000\bar 1]_{\rm ZnO} pointing to the surface. For ZnO films with thickness of 1.6 μm, the threading dislocation density is ～1 × 10⁹ cm^–2, and the density of basal stacking faults is below 1 × 10⁴ cm^–1. The (11\bar 2 \bar 2) ZnO exhibits strong D⁰X emissions with a FWHM of 9 meV and very few green–yellow emissions in the low‐temperature (10 K) and room‐temperature photoluminescence spectra, respectively.

     

Growth kinetics of graphene and graphite films on the \operatorname{Re} (10\bar 10) surface

Growth kinetics of graphene and graphite layers on the surface of carbon-saturated rhenium has been investigated. The experimental data agree well with the results of the mathematical analysis of diffusion processes in the metal bulk with the participation of carbon atoms and the subsequent precipitation of carbon on the surface. It has been established that graphene layers of rigorously specified thickness in the range of 1–50 layers can be grown on metal in the temperature range of 1300–1500 K.     

Growth of nanostructured ZnO thin films on sapphire

Growth of ZnO nanostructures on c-plane sapphire has been investigated using three different methods. Pulsed laser deposition (PLD) at low incident pulse energies can yield nanorods, the majority of which are aligned at an angle of ∼50° to the substrate plane. Selected area electron diffraction reveals that the nanorods display two distinct epitaxial relationships with the sapphire substrate. Those inclined to the surface normal exhibit the relationships (112̄4)_ZnO//(0001)_sap; [101̄0]_ZnO//[112̄0]_sap and (0001)_ZnO//(101̄4)_sap; [101̄0]_ZnO//[112̄0]_sap. Members of the second family are aligned along the surface normal, with (0001)_ZnO//(0001)_sap and [101̄0]_ZnO//[112̄0]_sap; the relative yield of this latter class increases at higher incident pulse energies. Hydrothermal synthesis and chemical vapour deposition on sapphire substrates that have been pre-coated (by PLD) with a thin ZnO layer result, respectively, in well-aligned ZnO microrod and nanorod arrays, both of which satisfy the relationships (0001)_ZnO//(0001)_sap; [101̄0]_ZnO//[112̄0]_sap. In contrast, employing these latter methods with a bare sapphire substrate results in, respectively, poorly aligned structures and localized islands of growth. PACS  81.07.-b; 81.10.-h; 81.15.Aa; 81.15-z; 68.65.-k     

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.     

氮掺杂(1120) ZnO 薄膜磁性质研究

采用基于密度泛函理论(DFT)和局域密度近似(LDA)的第一性原理分析了氮掺杂(1120) ZnO 薄膜的磁性质.首先,研究了一个N原子掺杂ZnO薄膜的磁性质,结果表明N 2p,O 2p和Zn 3d 发生自发自旋极化.其次,研究了二个N原子掺杂ZnO薄膜的磁性质,9个不同几何结构的计算结果表明N原子之间具有FM耦合稳定性,而且具体分析了N掺杂ZnO铁磁稳定性的产生原因.最后,讨论了氮 关键词： 第一性原理 半导体 铁磁性     

Growth and magnetic properties of ultra-thin Co(0001) films on patterned {\rm Mo(110)/Al_2O_3(11\overline{2}0)} substrates

Single crystalline patterned Mo(110) films have been prepared on ${\rm Al_2O_3(11\overline{2}0)}$ using standard lithographical techniques, that can be used as a substrate for 3d magnetic metals epitaxy. The preparation procedure, and the structural and morphological characteristics are presented. We show that the patterned Mo(110) films may be used in a similar way than single crystal Mo(110) substrates. The structural and magnetic properties of a 3 nm-thick cobalt film deposited on the patterned Mo(110) film have been studied. The epitaxial Co layer reveals a uniaxial in-plane magnetic anisotropy as measured by Kerr magnetometry.     

High quality PdTe_2 thin films grown by molecular beam epitaxy

PdTe_2,a member of layered transition metal dichalcogenides(TMDs),has aroused significant research interest due to the coexistence of superconductivity and type-II Dirac fermions.It provides a promising platform to explore the interplay between superconducting quasiparticles and Dirac fermions.Moreover,PdTe_2 has also been used as a substrate for monolayer antimonene growth.Here in this paper,we report the epitaxial growth of high quality PdTe_2 films on bilayer graphene/SiC(0001)by molecular beam epitaxy(MBE).Atomically thin films are characterized by scanning tunneling microscopy(STM),X-ray photoemission spectroscopy(XPS),low-energy electron diffraction(LEED),and Raman spectroscopy.The band structure of 6-layer PdTe_2 film is measured by angle-resolved photoemission spectroscopy(ARPES).Moreover,our air exposure experiments show excellent chemical stability of epitaxial PdTe_2 film.High-quality PdTe_2 films provide opportunities to build antimonene/PdTe_2 heterostructure in ultrahigh vacuum for future applications in electronic and optoelectronic nanodevices.     

Structural and optical properties of non-polar ZnO/Zn0.81Mg0.19O multiple quantum wells grown on r -plane sapphire substrates by plasma-assisted molecular beam epitaxy

This work investigates the structural and optical properties of non-polar ZnO/Zn_0.81Mg_0.19O multiple quantum wells (MQWs), which have been prepared on $r$ -plane sapphire substrates by plasma-assisted molecular beam epitaxy (MBE). The MQWs are ( $11\bar{2}0$ ) oriented ( $a$ -plane) as identified by the X-ray diffraction pattern. Structural properties are anisotropic and surfaces of MQWs show stripes running along the ZnO $c$ -axis direction. Sharp interfaces between the well layers and barrier layers can be clearly resolved by the secondary ion mass spectroscopy (SIMS) analysis. The room-temperature photoluminescence (PL) resulting from the well regions exhibits a significant blueshift with respect to ZnO single layer. Exciton emission in the ZnO QW is resolved into two components in the temperature dependence of the PL spectra. Two types of excitons are responsible for this feature. The excitons trapped by the potential minima dominate at low temperature, and the excitons localized in the “free exciton states” dominate at relatively high temperature. An activation energy of 7.3 meV for quenching of the exciton emission is in good agreement with the transition of the two types of excitons.     

GaAs(001)衬底上分子束外延生长InNSb单晶薄膜

利用射频氮等离子辅助分子束外延(RF-MBE)技术在GaAs(001)衬底上生长稀氮 InNSb半导体薄膜,并通过原子力显微镜(AFM)、扫描电子显微镜(SEM)、X射线衍射仪(XRD)和拉曼散射光谱等测量手段对样品的微结构和N组分等进行了表征.结果显示样品有较好的晶体质量,N组分可高达0.84%(XRD的结果).本文还对样品的输运性质进行了表征,结果显示样品在室温下具有较低的载流子浓度和较高的迁移率.另外,初步研究表明在InSb中掺入N可导致其室温磁阻明显下降. 关键词： 分子束外延 稀氮半导体 X射线衍射 拉曼光谱     

X-ray photoelectron spectroscopy study on the CrN surface grown on sapphire substrate to control the polarity of ZnO by plasma-assisted molecular beam epitaxy

We have studied on the polarity selection procedure of ZnO grown on CrN buffer by using X-ray photoelectron spectroscopy (XPS). XPS studies have been performed on the O- and Zn-treated CrN/Al₂O₃surfaces and revealed that Cr₂O₃ and Zn-chromate-like structures are formed on O- and Zn-treated CrN surfaces, respectively. The polarity selection procedure is explained in terms of the variation of bonding coordination by the formation of ZnO on the topmost O- and Zn-atoms of each surface.