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.     

Carrier transport characteristics in PbSrSe thin films grown by molecular beam epitaxy

The dark-current characteristics of PbSrSe thin films grown by molecular beam epitaxy on BaF₂ substrates with Sr composition from 0.066 to 0.276 have been measured systematically under different temperatures from 77 K to 300 K. The carrier-transport characteristics have been explained on the basis of a grain-boundary barrier model. The barrier height is found to be strongly related to the Sr composition. The different conductance behavior among the PbSrSe thin films is due to the variation of the grain-boundary barrier. Both the experimental barrier height, determined from the temperature-dependent conductance, and the theoretical results, deduced from the Poisson equation, reveal that the barrier height decreases with increasing applied bias. Furthermore, the success in explaining the observed negative-capacitance phenomenon gives further evidence that the accumulation of electrons at the grain boundaries plays a key role in the carrier transport of the PbSrSe thin films. PACS 72.80.Jc; 73.61.Le; 73.50.Bk     

Observation of exciton localization in PbSrSe thin films grown by molecular beam epitaxy

Detailed temperature and excitation-intensity dependence of radiative recombination characteristics in PbSrSe thin films grown by molecular beam epitaxy has investigated. For the first time, localized excitonic structure due to the alloy disorder has been observed at low temperature, and is gradually delocalized as wave-vector-nonconserving band-to-band transitions at high temperature in PbSrSe with low Sr composition. Furthermore, we are able to observe the evolution of the exciton localization, due to the strong alloy fluctuations and lattice distortions, into either free or trapped (strongly localized) excitons at low temperature by studying a PbSrSe thin film with the Sr composition as high as 0.276. The lattice deformation has been shown to play a key role in the composition-dependent excitonic peak broadening in PbSrSe at low temperature. This gives clear evidence for the observation of excitonic effects in lead salts due to their low carrier concentration and enhanced exciton binding energy. The results are discussed in the framework of theories taking into account the carrier migration and exciton trapping due to the alloy fluctuations and lattice vibrations . PACS 78.55.Hx; 78.66.Li     

Electrochemical oxidation of La2CuO4 thin films grown by molecular beam epitaxy

Thin insulating and c-axis oriented films of La₂CuO₄ are grown using a molecular beam epitaxy technique. Subsequently, these films are oxidized electrochemically using a 1N KOH solution. This approach is used to induce superconductivity, leading to a maximum T_c0 of 31 K,, measured both resistively and inductively. The surface morphology, lattice constants and the resistivity before and after the electrochemical treatment are compared.     

Effect of double AlN buffer layer on the qualities of GaN films grown by radio-frequency molecular beam epitaxy

This paper reports that the GaN thin films with Ga-polarity and high quality were grown by radio-frequency molecular beam epitaxy on sapphire （0001） substrate with a double A1N buffer layer. The buffer layer consists of a high-temperature （HT） A1N layer and a low-temperature （LT） A1N layer grown at 800℃ and 600℃, respectively. It is demonstrated that the HT-A1N layer can result in the growth of GaN epilayer in Ga-polarity and the LT-A1N layer is helpful for the improvement of the epilayer quality. It is observed that the carrier mobility of the GaN epilayer increases from 458 to 858cm^2/V.s at room temperature when the thickness of LT-A1N layer varies from 0 to 20nm. The full width at half maximum of x-ray rocking curves also demonstrates a substantial improvement in the quality of GaN epilavers by the utilization of LT-A1N layer.     

Structural and electrical properties of single crystalline and bi-crystalline ZnO thin films grown by molecular beam epitaxy

C-oriented ZnO epitaxial thin films are grown separately on the a-plane and c-plane sapphire substrates by using a molecular-beam epitaxy technique. In contrast to single crystalline ZnO films grown on a-plane sapphire, the films grown on c-plane sapphire are found to be bi-crystalline; some domains have a 30o rotation to reduce the large mismatch between the film and the substrate. The presence of these rotation domains in the bi-crystalline ZnO thin film causes much more carrier scatterings at the boundaries, leading to much lower mobility and smaller mean free path of the mobile carriers than those of the single crystalline one. In addition, the complex impedance spectra are also studied to identify relaxation mechanisms due to the domains and/or domain boundaries in both the single crystalline and bi-crystalline ZnO thin films.     

Electrical and optical properties of ZnO films grown by molecular beam epitaxy

Zinc oxide (ZnO) films have been grown on sapphire by molecular beam epitaxy (MBE), and it is found that the grain size of the ZnO films increased with increasing the growth temperature. Photoluminescence (PL) study shows that the intensity ratio of near-band-edge emission to deep-level-related emission (NBE/DL) of the ZnO is significantly enhanced with increasing the growth temperature, and the dependence of the carrier mobility on the growth temperature shows very similar trend, which implies that there is a community factor that determines the optical and electrical properties of ZnO, and this factor is suggested to be the grain boundary. The results obtained in this paper reveal that by reducing the grain boundaries, ZnO films with high optical and electrical properties may be acquired.     

Oxidation and thermal annealing effects on native and ion-irradiated PbTe films grown by molecular beam deposition

Investigations are reported into the effect of low-pressure oxygen exposure and thermal annealing on the carrier transport properties of native and 350 eV Ar⁺ bombarded PbTe films. The electrical measurements were madein situ on MBD-grown PbTe films without breaking vacuum. On native surfaces, oxidation was initially sustained by diffusion of a donor species from the film bulk to the surface, where reaction with oxygen occured. This diffusion process was apparently inhibited on ion irradiated films and direct doping of the film surface effected a gradual reduction in the ion-induced electron accumulation resident at the film surface. The native properties and behavioural characteristics of the films could be recovered by thermal annealing of the ion-irradiated and/or oxidized films at 300–350 °C.     

Observation of regular defects formed on the surface of PbTe thin films grown by molecular beam epitaxy

Regular shape defects on the surface of PbTe thin films grown by molecular beam epitaxy (MBE) were studied by scanning electron microscope (SEM). Two types of regular shape defects were observed on Te-rich PbTe films grown at substrate temperature T ≥ 235 °C with a beam flux ratio of Te to PbTe (R_f) to be 0.5 and at 280 °C with a R_f ≥ 0.4, which include cuboids and triangular pyramids. The formation mechanism of the observed regular shape defects is interpreted as following: They are the outcome of fast growth rate along {1 0 0} crystal planes that have the lowest surface energy and the enclosure of the {1 0 0} crystal planes. The formation of the regular shape defects in the growth of PbTe needs appropriate substrate temperature and Te-rich ambience. However, when R_f is decreased low enough to make the films slightly Pb-rich, triangular pits that originate from the insufficient glide of the threading dislocations along the main 〈1 1 0〉 {1 0 0} glide system of PbTe in Cottrell atmosphere, will be the main feature on the film surface.     

分子束外延生长InGaN/AlN量子点的组分研究

报道了分子束外延生长的绿光波段InGaN/AlN量子点材料,并综合考虑InGaN量子点的应变弛豫,以及应力和量子限制斯塔克效应对量子点发光波长的影响,提出了一种结合反射式高能电子衍射原位测量与光致荧光测量确定InGaN量子点组分的方法.     

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.     

Structural, optical and electrical properties of cubic AlN films deposited by laser molecular beam epitaxy

Cubic AlN films were successfully deposited on TiN buffered Si (100) substrates by a laser molecular beam epitaxy (LMBE) technique, and their crystal structure and optical and electrical properties were studied. The results indicate that cubic AlN films show the NaCl-type structure with a (200) preferred orientation, and the lattice parameter is determined to be 0.4027 nm. The Fourier transform infrared (FTIR) pattern of the cubic AlN film displays sharp absorption peaks at 668 cm⁻¹ and 951 cm⁻¹, corresponding to the transverse and longitudinal optical vibration modes. Ellipsometric measurements evidence a refractive index of 1.66–1.71 and an extinction coefficient of about zero for the cubic AlN film in the visible range. Capacitance–voltage (C–V) traces of the metal–insulator–semiconductor (MIS) device exhibit that the cubic AlN film has a dielectric constant of 8.1, and hysteresis in the C–V traces indicates a significant number of charge traps in the film.     

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.     

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.     

HgTe/CdTe superlattices grown by molecular beam epitaxy

In this paper we present results concerning the optical absorption in HgTe-CdTe superlattices. We confirm the narrowing of the superlattice band-gap (the increase of cut-off wavelength, λ_c) compared to the band gap of the equivalent Hg_1?xCd_xTe alloy. We show also, as predicted by the theory, an increase of the cut-off wavelength of the superlattice when the HgTe layer thickness increases. At 300K, the agreement between theory and experiment is fairly good if we consider the onset of the absorption. The λ_c tail shifting towards shorter wavelengths could be explained by the interdiffusion between HgTe and CdTe layers. At 30K, no important change in the I.R. absorption is noticed for all the superlattices.We present for the first time a superlattice exhibiting an absorption in the 8–12 μm window.We have carried out Hall measurements on several superlattices and present for the first time transport properties on these alternate microstructures. The most important features concern the unexpected and not yet understood very high hole mobilities at 10K.     

Effect of thermal annealing on ZnO:Al thin films grown by spray pyrolysis

We report the effect of thermal annealing in air on the structural and optical properties of undoped and aluminium-doped (1%–4%) zinc oxide (AZO) thin films, grown by the spray pyrolysis technique on quartz substrates. Films were characterized by X-ray diffraction, low-temperature photoluminescence, electrical resistivity, and Raman spectroscopy after annealing at temperatures between 500 and 900 C. Annealing in air improves the long-range order crystalline quality of the bulk crystals, but promotes a number of point defects in the surface affecting both the resistivity and the photoluminescence.     

Correlation between structural and electrical properties of InN thin films prepared by molecular beam epitaxy

The strain-relaxation phenomena and the formation of a dislocation network in 2H-InN epilayers during molecular beam epitaxy are reported. The proposed growth model emphasizes the dominant role of the coalescence process in the formation of a dislocation network in 2H-InN. Edge type threading dislocations and dislocations of mixed character have been found to be the dominating defects in wurtzite InN layers. It is demonstrated that these dislocations are active suppliers of electrons and an exponential decay of their density with the thickness implies a corresponding decay in the carrier density. Room temperature mobility in excess of 1500 cm² V ⁻¹ s⁻¹ was obtained for 800 nm thick InN layers with dislocation densities of 3×10⁹ cm⁻².     

Thermal stability of CdZnO thin films grown by molecular-beam epitaxy

CdZnO thin films with near-band-edge (NBE) photoluminescence (PL) emission from 2.39 eV to 2.74 eV were grown by plasma-assisted molecular-beam epitaxy on c-plane sapphire substrates with 800 °C in situ annealing. CdZnO thin films evolve from pure wurtzite (wz) structure, to mixture of wz and rock-salt (rs) structures confirmed by X-ray diffraction studies. Rapid-thermo-annealing (RTA) was performed on in situ annealed CdZnO samples. Pure wz CdZnO shows insignificant NBE PL peak shift after RTA, while mixture structure CdZnO shows evident blue shifts due to phase change after annealing, indicating the rs phase CdZnO changes to wz phase CdZnO during RTA process.     

Transmission electron microscopy study of defects in Sn-doped GaAs films grown by molecular beam epitaxy

Films of GaAs, heavily doped with Sn, which have been grown by molecular-beam epitaxy are found to contain single-crystal Sn particles situated in the nearsurface region of the epilayer GaAs. The morphology and chemical composition of the particles have been examined by using cross-section transmission electron microscopy combined with energy-dispersive x-ray spectroscopy. Different growth conditions were used to study the Sn-particle formation and high-resolution transmission electron microscopy was used to investigate microstructures. The observations are discussed in terms of several models previously proposed for these phenomena.