MBE Films Research Articles

Superconducting Parent Compound Pr2CuO4 Achieved by Special Post-Reduction

ABSTRACTIt is commonly believed that the parent compounds of high-Tc cupratres are, universally, charge transfer insulators and triggered by Mott physics. In our experiments using metal-organic decomposition (MOD), however, accumulating evidences show that the parent compounds of “electron-doped” superconductors, RE2-xCexCuO4 [RE = rare earth ion] with x = 0, are not Mott insulators but superconductors [1-5]. They have a Tc of 30 K and crystallize in the Nd2CuO4 (T’) structure. Most likely, the sharp contradiction between our results and commonly achieved data originates from the complicated oxygen chemistry in these materials. The as-synthesized specimens contain a fair amount of impurity interstitial oxygen. Throughout the reduction process it is required to remove exclusively impurity oxygen while preserving regular oxygen site occupied in order to obtain superconductivity. With decreasing x the constraints of the reduction process are getting more tight. In this study, we systematically investigated the post-annealing process using MBE-grown T’-Pr2CuO4 films. The MBE films were reduced ex-situ in a tubular furnace following a specially designed 2-step process, as in the case of MOD films. The films were annealed at Ta = 700 - 850°C in a reducing atmosphere (PO2 = 2 x 10−5 − 2 x 10−3 atm) and finally reduced at a lower temperature Tred = 450 – 700°C under vacuum (< 10−4 Torr). The film properties systematically changed with Ta, PO2, and Tred. The optimized Tred varies from 475°C to 650°C mainly depending on Ta, since the microstructure and grain size of the films are determined by Ta. Optimal superconducting properties are Tc of 26 K, while ρ(300 K) = 250 μΩcm, and RRR ~ 10. We believe the combination of thin-film synthesis and specially designed post-reduction process enabled us to obtain nearly intact CuO2 planes. Samples prepared by above-mentioned method unveiled the intrinsic properties of the parent compounds, which are not triggered by Mott physics. This result also agrees with the recent calculation result indicating the parent compounds with T’ structure are not charge transfer insulators [6-8].

Correlation between optical properties of MBE films of AlN and morphology of their surface

AbstractIn this work we revealed correlation between presence of surface defects and peculiarities in ellipsometric spectra and hence in optical properties of AlN films grown by ammonia MBE technique. We propose to use this fact for the fast and non‐destructive checking of film quality. Principal criterions of AlN films quality was both magnitude of phase deviation in maximum and/or a value of false (effective) absorption in transparency field of AlN calculated in according with our model. Our experiments were based on using of spectroscopic ellipsometry (SE). Atomic force microscopy (AFM) was used for determination of geometric size of overlayer roughness. Quantitative parameter related with surface morphology γs was proposed and influence of surface defects upon phase of light reflected was studied to evaluate surface quality from ellipsometric spectra at once, i.e. without any model calculations. Besides, an impact of surface defects upon film extinction coefficient is demonstrated in case when roughness is not taken into account. This approach could be useful for sorting identical films produced in routine growth procedure. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

VARIATION OF IN-PLANE LATTICE CONSTANT OF Si/Ge/Si HETEROSTRUCTURES WITH Ge QUANTUM DOTS

Experimental data are presented on variations of the in-plane lattice constant of Ge and Si films in the course of the MBE film growth on the silicon (100) surface. The in-plane lattice constant of the silicon film is shown to alter as the film grows; the changes reflect the process of relaxation of elastic strains that result from the misfit of the germanium and silicon lattice constants. Due to the presence of germanium islands, a considerably thicker silicon film is required to provide the strain relaxation. The dependence of distortion penetration depth to the silicon film on the effective germanium film thickness is obtained. TEM studies indicate the vertical ordering of the germanium island layers when the thickness of the Si layer in between Ge layers is not sufficient to provide the full strain relaxation.

Doping and defect control of ferromagnetic semiconductors formed by ion implantation and pulsed-laser melting

We demonstrate a simple but effective route for the synthesis of ferromagnetic semiconductors that combines Mn ion implantation (II) and pulsed-laser melting (PLM). By this process Ga 1− x Mn x As films exhibiting a ferromagnetic Curie temperature ( T C) exceeding 130 K have been synthesized. These materials possess electrical and magnetic characteristics similar to molecular beam epitaxy-grown (MBE-grown) films that are tunable through experimental parameters such as implantation dose, laser energy fluence, and co-doping with Te donors. We have also used II–PLM to form ferromagnetic single-crystalline Ga 1− x Mn x P films. These films display magnetic properties similar to their arsenide counterparts despite remaining non-metallic and becoming strongly insulating at low temperatures. Unlike as-grown MBE films, both Ga 1− x Mn x P and Ga 1− x Mn x As formed by II–PLM do not contain Mn at interstitial sites; instead, non-substitutional Mn atoms are found to be randomly located, perhaps as clusters. As a result, post-PLM thermal annealing at temperatures higher than 300 °C leads to a decrease in T C as Mn atoms leave substitutional sites.

Investigation and characterization of Hg1−xCdxTe epilayers

The Hg1−xCdxTe epilayers of p- and n-type and different Cd concentrations x (0.22–0.24) were investigated. The experimental and theoretical studies of the optical transmission spectra at room temperature were carried out for the determination of the x value on the surface, the composition profile and the cutoff wavelength at 80K. Analysis of the optical transmission data showed that LPE-layers usually have a nonlinear composition profile in contrast to the MBE-layers with the negligibly small composition gradient. These conclusions were confirmed by independent SIMS-measurements of the LPE and MBE films. Photoresponse experiments (T = 80K) on photoresistors and photodiodes prepared from Hg1−xCdxTe epilayers confirmed that the cutoff wavelengths correspond to the compositions, determined by the optical transmission method at room temperature.

Defect formation in LT MBE InGaAs and GaAs

Results are obtained from the experimental studies (atomic force and transmission microscopy) of the influence of III/V flux ratio on the topography and internal structure of LT MBE films of InGaAs and GaAs. The layers are shown to contain both the surface growth defects — the volcano-like pits and microdrops of III group elements, and the bulk defects — dislocations, stacking faults, microtwins, and phase microheterogeneities. A high-temperature annealing results in the additional formation of thermal etching pits at the surface and the nanosized arsenic clusters in the bulk. The origin of the defects is discussed.

Determination of the Structural and Luminescence Properties of Nitrides Using Electron Backscattered Diffraction and Photo‐ and Cathodoluminescence

In this paper we describe the use of electron backscattered diffraction (EBSD) for the characterisation of nitride thin films, and report its use in the study of the spatial variation of strain across an epitaxially laterally overgrown GaN (ELOG) thin film. We also discuss the combination of luminescence and EBSD measurements to enable luminescence properties of samples to be directly correlated with their crystallographic properties. We compare photoluminescence spectra and EBSD measurements from a set of GaN thin films grown on off-axis sapphire substrates, revealing the tilt of a GaN thin film grown on a 10° off-axis sapphire substrate to be responsible for the observation of luminescence defect bands in this film. We finally report on the use of EBSD to identify zinc blende regions in a predominantly wurtzite MBE film, with cathodoluminescence used to obtain correlated luminescence spectra.

Persistent photoconductivity in ZnCdSe MBE films grown on GaAs

The effect of persistent photoconductivity (PPC) and their associated deep centers have been studied for Zn 1− x Cd x Se (0.10< x<0.26) alloys grown by molecular beam epitaxy on GaAs substrates. Low temperature photoexcitation showed a prominent PPC effect with an exponential temperature dependence of the characteristic of the carrier decay time and an energy barrier which changes with Cd concentration. Experiments of thermally stimulated currents indicated that the PPC effect may be associated to deep electron traps levels whose activation energies are a function of the Cd concentration.

Investigation on growth modes of GaN layers grown by LIRE

GaN layers are usually grown by metalorganic chemical vapour deposition (MOCVD), MBE and related processes. In this work we compare the growth of GaN layers by laser induced reactive epitaxy (LIRE) with the growth by conventional methods with special interest in growth modes of the different processes. The influence of deposition parameters as temperature and Ga/N-ratio, for LIRE represented in the energy of the laser beam, is discussed. MOCVD produces smooth films with growth steps on the surface, indication of one-dimensional growth. MBE films usually exhibit island-like growth, changing with Ga/N ratio from a meander structure over a closed surface with pits to a surface with etching-features due to desorption. The best luminescence intensities are achieved with surfaces of meander structure and with surfaces showing slight etching features. LIRE layers are found to be similar to MBE layers in surface structure and in the influence of the Ga/N-ratio.

Comparison of Growth Morphology in Ge (001) Homoepitaxy Using Pulsed Laser Deposition and MBE

ABSTRACTDifferences in the homoepitaxy of Ge(001) are explored using a dual MBE/PLD deposition system. With identical substrate preparation, temperature calibration, background pressure and analysis, the system provides a unique comparison of the processes arising only from kinetic differences in the flux and at the surface. All films show mounded growth. At substrate temperatures below 200°C, PLD films are smoother than MBE films, whereas they are similar at higher temperatures.

89 Y NMR Studies of MBE Grown DyFe 2 /YFe 2 Multilayer Films

89Y (spin I=1/2) NMR studies of DyFe2/YFe2 multilayer films at 4.2 K are reported and discussed. Results are presented for a 1000 A thick film of YFe2, and a 24,000 A thick superlattice film [300 A DyFe2/300 A YFe2]×40. For comparative purposes, measurements were also made on bulk powdered YFe2 samples. In practice, there are significant differences between the 89Y NMR in thin film and powder form. In the MBE films the NMR frequency (45.85±0.2 MHz) is shifted down in frequency by ∼0.1 MHz, with respect to bulk (powdered) YFe2 (45.94±0.09 MHz). The origin of this shift is attributed to the expanded nature of the MBE films (∼0.8%), relative to bulk YFe2. In addition, the NMR line width is broader in the MBE films by about a factor of two. This indicates either the presence of strain within the MBE films, and/or differing dipolar fields associated with the DyFe2/YFe2 interfaces. This feature is also confirmed by the spin–spin lattice relaxation time T 2, which is almost ten times longer in the superlattice film (T 2=5.1 ms), than in bulk YFe2 (T 2=0.6 ms). It is argued that this is due to the increased inhomogeneous line-width, which inhibits energy-conserving mutual spin–spin-flops.

Structure and properties of epitaxial GaAs and InGaAs films grown by low-temperature molecular-beam epitaxy

We study epitaxial GaAs and InGaAs films produced by molecular-beam epitaxy in the temperature range 150–480°C and with various arsenic partial pressures. We determine the structural and electrophysical characteristics of the film (the excess arsenic, the crystal lattice parameter, and the carrier concentration and mobility) as a function of the growth conditions. The influence of annealing on the cluster-structure formation in low-temperature MBE films, and on their properties, is also studied. In low-temperature GaAs we find a characteristic microwave absorption signal indicating the presence of a superconducting phase. We discuss the possible nature of this phase with regard to clusters of gallium or an In-Ga alloy.

Effects of H coverage on Ge segregation during Si1−xGex gas-source molecular beam epitaxy

The effects of H coverage θH on Ge segregation during Si1−xGex gas-source molecular beam epitaxy (GS-MBE) were investigated using D2 temperature programmed desorption (TPD). Si1−xGex films with x=0.01–0.30 were grown from Si2H6/Ge2H6 mixtures at Ts=450–800 °C, held at the growth temperature for 30 s, cooled to &amp;lt;200 °C, and then exposed to atomic deuterium until saturation coverage. D2 TPD spectra were fit using four peaks corresponding, in order of decreasing activation energy, to desorption from Si monodeuteride, Ge–Si mixed-dimer monodeuterides, Si dideuteride, and Ge monodeuteride. Steady-state Ge surface coverages were determined from the TPD data as a function of Ts and x. In contrast to solid-source MBE films grown in this temperature regime, Ge segregation during GS-MBE decreases with decreasing Ts due to the increasing H coverage. The results were well described by a model accounting for the Si/Ge site exchange and θH. The Ge segregation enthalpy varies from −0.28 eV at Ts⩾800 °C, where the steady-state hydrogen coverage θH approaches zero, to −0.10 eV at Ts⩽450 °C, where θH is nearly saturated.

Studies of GaN layers grown on sapphire using an RF-source

Abstract We have studied the relation between MBE growth parameters and film quality after growth of GaN on the c -plane of sapphire using a conventional Ga-source and an RF-source for the excitation of N 2 . The growth rate was varied using Ga-flux, determined by RHEED oscillations on GaAs, r GaAs = 0.1–0.8 μ m/h. The N 2 -flux was 0.5–1.5 sccm. Most layers of GaN were grown at 760°C as measured by a pyrometer, while higher temperatures were utilised to determine the dismissal of growth. The RF-power and AlN buffer layer parameters were kept constant. The GaN-thicknesses were 0.2–1 μm/h. For low growth rates of ∼ 50 nm/h, there was a preferential growth of microcrystals in the growth direction which therefore overemphasised the measured film thickness. No film could be grown above 800°C as the desorption rate was too high. For a GaN growth rate of 300 nm/h the sticking coefficient was ∼ 85%. By varying the Ga- and N 2 -fluxes it was evident that the film quality, as provided by the photoluminescence spectra, strongly depended on the growth parameters. Photoluminescence peak intensities generally improved with film thickness below 1 μm. In 1 μm thick films, we observed excitonic related peaks close to the band gap as well as peaks 50–60 meV below due to the presence of defects or impurities. We found a strong correlation between growth parameters and optimum growth and therefore the highest layer quality could be obtained only when all parameters were carefully optimised.

Surface preparation of ZnSe substrates for MBE growth of II–VI light emitters

Abstract This paper describes substrate surface preparation techniques used in the development II–VI light emitting diode and laser diode structures on high-quality, bulk ZnSe substrates supplied by Eagle-Picher Industries. The use of ZnSe substrates eliminates many of the problems associated with lattice mismatch in heteroepitaxy of II–VI light emitters on GaAs substrates. However, defects still form during nucleation of an epitaxial layer on ZnSe substrates because of surface roughness, contamination, and defects. We have employed a variety of wet chemical etches, vacuum anneals, plasma treatments, and characterization techniques such as RHEED, Auger electron spectroscopy, and SEM studies to improve the ZnSe substrate surface prior to MBE film growth. A combination of hydrogen plasma exposure and annealing was found to be the most effective way to remove contaminants from ZnSe substrates but less than optimum homoepitaxial quality showed that the surface preparation is more complex than simply cleaning the polished surface. Since polishing can leave residual damage in the form of near-surface defects, the top layer of these substrates was removed by reactive ion etching with BCl 3 . Parameters were chosen such that this etch was homogeneous and smoothed the ZnSe surface. Etch pit density measurements revealed that the polish-induced damage to ZnSe extended up to about 5 μm deep. A dramatic improvement in the characteristics of blue/green light emitting devices was observed for devices grown on ZnSe substrates from which this damaged layer had been removed. This surface preparation procedure has led to the brightest and longest lasting II–VI green LEDs made in the world today.

Optical functions of bulk and epitaxial GaSb from 0.0025 to 6 eV

The complex refraction index and dielectric functions of GaSb bulk (both p and n-type) and MBE film were accurately determined from 0.0025 to 6 eV by using reflectance and ellipsometric spectroscopies. These functions, which satisfy the Kramers-Kronig causality relations, appear independent on the doping (≤2×10 17cm −3) in the interband transition region. In particular, in comparison with previous data from literature, the E 0 fundamental gap is well evidenced for all the samples. Moreover, the optical functions around the gap are very different from those previously reported and the refractive index was modeled by a Sellmeier dispersion relation. The free-carriers instead strongly influence the far-infrared restrahlen region due to the phonon-plasmon coupling. In this range the optical functions were well fitted by Drude-Lorentz oscillators.

Optical properties of β-FeSi 2 films grown on Si substrates with different degree of structural perfection

β-FeSi 2 films, distinguished in their crystalline perfection, have been grown under different conditions of Fe and Si deposition on Si substrates. Optical transmittance, reflection, and ellipsometric investigations were carried out from the NIR to UV. Phonon spectra were studied by FTIR transmittance measurements. The spectral distribution of optical constants shows strongest differences at photon energies <3 eV for films of different structural quality. For MBE films there is considerable fine structure in the optical constants above the gap with pronounced features in between 1 and 2 eV. With higher degree of crystallinity the phonon spectrum becomes increasingly structured.

Population hole burning on the [formula omitted] zero-phonon transition in MBE films of CaF 2:Eu 2 +

Spectral holes due to the redistribution of population among the ground state hyperfine levels have been detected on the 413 nm zero-phonon transition in CaF 2:Eu 2 + MBE films grown on Si(1 1 1), in magnetic fields in excess of 3 T. These are the first long-lived (up to minutes) population holes reported for a paramagnetic ion. The hole lifetime results from phonon-induced transitions among hyperfine levels of the Eu 2 + ground electron spin states. The central hole linewidth of 40 MHz is probably determined by the EuF superhyperfine interaction which leads to spectral diffusion due to F nuclear spin flips. The hole pattern contains a narrow central hole (40 MHz) and side holes and antiholes which result from the hyperfine splittings in the ground and excited states.

ICB deposition and epitaxial growth of GaAs thin films

The hetero- and homoepitaxial deposition of GaAs films by the ionized cluster beam technique in connection with subsequent neutral beam deposition forming a double layer structure have been studied. It is found that ICB-deposited films have superior quality in comparison with conventional ion beam deposited films and comparable quality like MBE or MOCVD-deposited films. By the fabrication of device structures, it has been proved that the start of film growth from an interface formed by an ICB-deposited As layer is advantageous. Additionally, surface cleaning by low-energy ionized cluster beams has been studied. For the first time, defect-free surface cleaning by low-energy cluster ion beams has been realized.

Defect evaluation and electrical characteristics of GaSb and In0.17Ga0.83Sb films grown by molecular beam epitaxy

MBE GaSb and In0.17Ga0.83Sb films are grown on GaSb(100) and GaAs(100) substrates and the film defects are investigated by using chemical etching and optical microscopic techniques. A new wet chemical etching method that revealed film defects in thin MBE GaSb and In0.17Ga0.83Sb films is evaluated, and it is found that a hot HCl solution under illumination is most effective in revealing the defects of the thin MBE films. Dislocations and stacking faults, which are related to the substrate material selected, substrate quality, substrate surface preparation, and the insertion of a buffer layer are clearly observed. In particular, in the growth of In0.17Ga0.83Sb, the huge lattice mismatch between the In0.17Ga0.83Sb and GaAs substrate causes severe dislocations, as well as stacking faults. Meanwhile, the AISb buffer layer is able to eliminate stacking faults and reduce dislocations in In0.17Ga0.83Sb films and thereby improve the electrical characteristics of the In0.17Ga0.83Sb film.