Microstructural assessment of InN-on-GaN films grown by plasma-assisted MBE

The structural properties of InN thin films, grown by rf plasma-assisted molecular beam epitaxy on Ga-face GaN/Al₂O₃(0001) substrates, were investigated by means of conventional and high resolution electron microscopy. Our observations showed that a uniform InN film of total thickness up to 1 μm could be readily grown on GaN without any indication of columnar growth. A clear epitaxial orientation relationship of , was determined. The quality of the InN film was rather good, having threading dislocations as the dominant structural defect with a density in the range of 10⁹–10¹⁰ cm⁻². The crystal lattice parameters of wurtzite InN were estimated by electron diffraction analysis to be a=0.354 nm and c=0.569 nm, using Al₂O₃ as the reference crystal. Heteroepitaxial growth of InN on GaN was accomplished by the introduction of a network of three regularly spaced misfit dislocation arrays at the atomically flat interface plane. The experimentally measured distance of misfit dislocations was 2.72 nm. This is in good agreement with the theoretical value derived from the in-plane lattice mismatch of InN and GaN, which indicated that nearly full relaxation of the interfacial strain between the two crystal lattices was achieved.     

RHEED observation of BaTiO3 thin films grown by MBE

Ferroelectric BaTiO₃ thin films with a thickness of 10 monolayers (ML) were epitaxially grown on SrTiO₃(0 0 1) substrates by very slow deposition using molecular beam epitaxy (MBE). The investigations were carried out by two growth methods: (i) codeposition and (ii) alternate deposition of the metal elements in an oxygen atmosphere. In situ observation of reflection high-energy electron diffraction confirmed that an epitaxial cube-on-cube structure was prepared. After the deposition, X-ray diffraction measurements were carried out. The 10-ML-thick BaTiO₃ films were highly c-axis oriented single crystals with good film quality.     

Ferromagnetism of fcc iron films grown on nickel

Polycrystalline multilayers of Fe-Ni produced by thermal evaporation are studied; some results obtained by a vibrating sample magnetometer, Mössbauer spectrometer and X-ray diffractometer are reported and discussed. Fe seems to grown on Ni with fcc structure. Fe atoms at the Ni/Fe interface seem to have localizated magnetic moments depending on the crystallographic orientations of the grains of the substrate; the remaining Fe atoms between the two interfaces appear as highly susceptible metal.     

Buried waveguide double-heterostructure PbEuSe-lasers grown by MBE

Buried heterostructure (BH) lasers with a PbSe active layer and Pb_0.97Eu_0.03Se confinement layers were fabricated using molecular beam epitaxy (MBE). The waveguide was defined by photolithography and an ion milling process. By MBE overgrowth of the etched structure with a PbEuSe layer the BH-laser was formed.The lasers operated cw with low threshold currents (8 mA at 80 K) in the spectral range from 1280 cm⁻¹ (30 K) to 1800 cm⁻¹ (160K). The spectral characteristics of these lasers were superior to comparable DH stripe lasers. These lasers were the first with IV/VI-MBE overgrowth process of etched lateral structures.     

InSb/CdTe heterostructures grown by MBE

InSb/CdTe heterostructures were grown by MBE, including a 10 layer superlattice. The structures were characterized by X-ray diffraction, van der Pauw measurements, and SNMS depth profiling. The interfaces widen because of interdiffusion and through the formation of an interface compound from the reaction of Te with the InSb surface. The interface compound is identified as strained InTe(II). The interfaces are still too wide for practical devices. Thermochemical analysis indicates that the reaction can be suppressed by applying a Cd overpressure. The diffusion of In and Sb in CdTe is very fast and will necessitate a MEE growth scheme at lower temperatures.     

In-plane photovoltage measurements on MBE grown InN

Temperature dependent measurements of the in-plane photovoltage (IPV) arising from Fermi level fluctuations constitute a powerful tool for determining surface and bulk states and trap activation energies in nominally undoped semiconductors. In this work we report the first IPV measurements on high quality InN grown using the MBE technique. The temperature dependent IPV results coupled with Hall mobility and carrier density measurements provide ample evidence for the existence of a high density of states within the range 54 to 68 meV below the conduction band. Temperature dependent PL measurements indicate a band gap of 0.77 eV where a very weak temperature dependence is observed between 77 and 300 K. These results are shown to be in accord with the IPV measurements.     

Growth and structure of MBE grown TiO2 anatase films with rutile nano-crystallites

We have explored the systematics of TiO₂ polymorph nucleation during film growth by molecular beam epitaxy on perovskite substrates. The accidental lattice match between anatase (0 0 1) and LaAlO₃(0 0 1) or SrTiO₃(0 0 1) typically results in anatase nucleation at the interface. However, the growth conditions dictate whether or not rutile also nucleates, and the associated morphological and structural properties of the composite film. Four symmetry equivalent epitaxial orientations of rutile on anatase are observed when rutile nucleates as discrete particles on LaAlO₃(0 0 1). Such films constitute model systems for studying the anatase/rutile interface, which is of considerable current interest in photochemistry.     

Pressure-tuned resonance Raman scattering and photoluminescence studies on MBE grown bulk GaAs at theE 0 gap

Hydrostatic pressure has been used to tune in resonance Raman scattering (RRS) in bulk GaAs. Using a diamond anvil cell, both the photoluminescence peak (PL) and the 2 LO and LO-phonon Raman scattered intensities have been monitored, to establish RRS conditions. When theE ₀ gap of GaAs matchesħω _S orħω _L, the 2 LO and LO-phonon intensity, respectively, exhibit resonance Raman scattering maxima, at pressures determined byħω _L. With 647.1 nm radiation (ħω _L = 1.916 eV), a sharp and narrow resonance peak at 3.75 GPa is observed for the 2 LO-phonon. At this pressure the 2 LO-phonon goes through its maximum intensity, and falls right on top of the PL peak, revealing thatħω _S(2 LO) =E ₀. This is the condition for “outgoing” resonance. Experiments with other excitation energies (ħω _L) show, that the 2 LO resonance peak-pressure moves to higher pressure with increasingħω _L, and the shift follows precisely theE ₀ gap. Thus, the 2 LO RRS is an excellent probe to follow theE ₀ gap, far beyond the Γ-X cross-over point. A brief discussion of the theoretical expression for resonance Raman cross section is given, and from this the possibility of a double resonance condition for the observed 2 LO resonance is suggested. The LO-phonon resonance occurs at a pressure whenħω _L ≈E ₀, but the pressure-induced transparency of the GaAs masks the true resonance profile.     

Enhancement of ferromagnetism upon thermal annealing in plasma assisted MBE grown mixed-phase Mn-doped insulating TiO2 thin films

We have investigated the properties of Mn-doped thin films grown on SrTiO₃ (100) substrate by plasma assisted molecular beam epitaxy (PAMBE). The microstructural, electrical, optical, and magnetic properties of these thin films have been characterized systematically, with a primary focus on establishing a correlation between magnetic and electrical properties. We have shown that these thin films exhibit room temperature ferromagnetism, and appear to enhance ferromagnetism upon thermal annealing, though the films show high resistance. The possible mechanism for the phenomena is also discussed. It is suggested that the growth condition, oxygen vacancies and structural and interfacial defects, and the bound magnetic polaron (BMP) model account for the magnetism enhancement. PACS  75.50.Pp; 75.70.Ak; 71.55.Gs; 81.10.-h; 81.15.Hi; 85.75.-d     

MBE grown type-II MWIR and LWIR superlattice photodiodes

We report on the status of GaSb/InAs type-II superlattice diodes grown and fabricated at the Jet Propulsion Laboratory designed for infrared absorption 2–5 μm and 8–12 μm bands. Recent LWIR devices have produced detectivities as high as 8 × 10¹⁰ Jones with a differential resistance–area product greater than 6 Ohm cm² at 80 K with a long wavelength cutoff of approximately 12 μm. The measured internal quantum efficiency of these front-side illuminated devices is close to 30% in the 10–11 μm range. MWIR devices have produced detectivities as high as 8 × 10¹³ Jones with a differential resistance–area product greater than 3 × 10⁷ Ohm cm² at 80 K with a long wavelength cutoff of approximately 3.7 μm. The measured internal quantum efficiency of these front-side illuminated MWIR devices is close to 40% in the 2–3 μm range at low temperature and increases to over 60% near room temperature.     

Investigation of free and bound excitons in strained ZnTe films grown by MBE on GaAs(100) substrates

A study is reported of the reflectance and low-temperature photoluminescence (PL) spectra of ZnTe films grown by molecular-beam epitaxy (MBE) on GaAs substrates [(100) orientation, 3° deflection toward 〈110〉]. It is shown that the strain-induced splitting of the free-exciton energy level (ΔE _ex) does not depend on ZnTe film thickness within the 1–5.7 μm range and is due to biaxial in-plane film tension. The stresses are primarily determined by the difference between the thermal expansion coefficients of the film and the substrate. It is also shown that the residual stresses originating from incomplete relaxation of the film lattice parameter to its equilibrium value at the growth temperature likewise provide a certain contribution. The position of the spectral line of an exciton bound to a neutral acceptor (As) is well approximated in terms of the present models, taking into account the stresses calculated using the value of ΔE _ex.     

The dynamics of accumulation of electrically active radiation defects on implantation of graded-band-gap HgCdTe films grown by MBE

The dynamics of accumulation of electrically active radiation defects under ion doping of epitaxial Cd _x Hg _1−x Te films is studied for various distributions of film composition in the implantation region. The epitaxial films were irradiated by boron ions at room temperature in the continuous regime, with the dose ranging within 10¹¹−3·10¹⁵ cm⁻², energy — 20–150 keV, and ion current density — j = 0.001–0.2 μA·cm⁻². It is found that the natural logarithm of the introduction rate of electrically active radiation defects linearly depends on the epitaxial-film composition in the range of mean projected path of implanted ions. An analysis of the experimental data shows that the dynamics of accumulation of electrically active radiation defects is determined by the epitaxial-film composition in the implantation region. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 25–28, September, 2006.     

Highly-ordered SiGe-islands grown by dislocation patterning using ion-assisted MBE

Fabrication of device structures based on laterally self-ordered systems without the use of expensive and time-consuming nanolithography could have undoubted advantages. For such applications, it is proposed to use misfit dislocation networks from partially relaxed SiGe layers on (1 0 0) Si substrate as a template for the growth of highly ordered SiGe islands. Ion bombardment during molecular beam epitaxy of metastable SiGe layers leads to such a partial relaxation by misfit dislocation networks. The ions are generated by the interaction of the evaporated Si flux with the electrons in an electron beam evaporator, which causes a partial ionization of Si atoms in the molecular beam. We demonstrate by atomic force microscopy that subsequent growth of SiGe on such relaxed SiGe (25-50% Ge) layers leads to the formation of uniform three-dimensional islands highly ordered in 〈1 1 0〉 directions.     

Superconducting BSCCO thin films obtained by MBE

Summary We realized BSCCO thin films withT _c(R=0)=88 K using the Molecular Beam Epitaxy (MBE) technique. The stoichiometric control is reached by a multilayer deposition technique in which Ca−Cu−Sr−Bi are sequentially deposited using the two effusive cells and the two e-guns present in the system. EDS and X-ray analyses are used to cross-check the obtained results, after theex situ in air annealing. The recent acquisition of an atomic oxygen source will allow in the close future the completelyin situ realization of BSCCO thin films. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

Dimensional dependence of the energy transfer in MBE grown MnS layers

MBE grown MnS layers with different thicknesses have been studied by time resolved photoluminescence spectroscopy. The temporal evolution of the internal Mn²⁺ (3d⁵) luminescence is measured over 3 orders of magnitude in intensity. The decay curves obtained reveal a surprising but distinct dependence on the layer thickness with longer lifetime than thinner layers. The observed non-exponential behavior of those time transients can be well described by an energy diffusion model within the 3d states of the manganese subsystem and a subsequent dipole–dipole energy transfer to radiationless centers.     

Carrier transport mechanisms in the ZnO based heterojunctions grown by MBE

This paper presents a review of models of the current transport in different kind of heterojunctions (HJs) and their characteristics. In order to effectively deduce the dominant electron transport for the HJs based on ZnO or Zn_1?xMg_xO layers grown on Si substrate by MBE a comparison is performed – which type of the HJ exhibits better electrical properties. The current–voltage characteristics for the studied HJs were measured within 280–300 K. The transport properties of the HJs are explained in terms of Anderson model with reference to aforementioned current transport models. It is found, that the mechanisms of current transport for all of the studied HJs are similar. At a low forward voltage bias the tunneling current dominates while at medium voltage bias (0.5–1 V) multitunneling capture-emission prevails with the electron trap located at 0.1–0.25 eV below the bottom of a ZnO (Zn_1?xMg_xO) conduction band. Beyond this voltage bias space charge limited current governs the current transport.     

分子束外延生长SiGe合金中位错密度的研究

Schimmel腐蚀液腐蚀结合高倍光学显微镜观察发现，不同尺寸的掩膜窗内生长的SiGe外延层中的位错密度在整个外延层中从SiGe/Si界面到SiGe外延层表面由少到多，再由多到少明显地分成3个区，无掩膜窗限制的大面积区内的SiGe层则只呈现2个区，掩膜材料与掩膜窗尺寸不同，这3个区的位错密度也不同，掩膜形成过程中产生的应力对衬底晶格的影响，以及掩膜边界对衬底与外延层的影响是造成这种不同的根本原因。     

GaN and InN nanowires grown by MBE: A comparison

Morphological, optical and transport properties of GaN and InN nanowires grown by molecular beam epitaxy (MBE) have been studied. The differences between the two materials in respect to growth parameters and optimization procedure was stressed. The nanowires crystalline quality has been investigated by means of their optical properties. A comparison of the transport characteristics was given. For each material a band schema was shown, which takes into account transport and optical features and is based on Fermi level pinning at the surface. PACS 73.30.+y; 73.21.Hb; 73.22.-f     

Optical characteristics of MBE grown GaMnAs embedded with MnAs clusters

Photoluminescence (PL) measurements of the GaMnAs layers embedded with MnAs clusters have been performed. It was shown that the presence of MnAs clusters in the semiconducting matrix leads to appearance in the PL spectra a broad peak with local maximums at 1.36 and 1.33 eV, which are related with the defects generated in the phase separation process. The effect of the MnAs clusters on the temperature dependent band gap of GaMnAs was also observed.