Molecular Beam Epitaxy Samples Research Articles

Comparative study of LaNiO3/LaAlO3 heterostructures grown by pulsed laser deposition and oxide molecular beam epitaxy

Variations in growth conditions associated with different deposition techniques can greatly affect the phase stability and defect structure of complex oxide heterostructures. We synthesized superlattices of the paramagnetic metal LaNiO3 and the large band gap insulator LaAlO3 by atomic layer-by-layer molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) and compared their crystallinity and microstructure as revealed by high-resolution transmission electron microscopy images and resistivity. The MBE samples show a higher density of stacking faults but smoother interfaces and generally higher electrical conductivity. Our study identifies the opportunities and challenges of MBE and PLD growth and serves as a general guide for the choice of the deposition technique for perovskite oxides.

Analysis of the relation between spectral response and absorptivity of GaAs photocathode

In order to study the relation between spectral response and absorptivity of GaAs photocathode, two kinds of GaAs photocathodes are prepared by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD), respectively. The samples grown by the MBE include varying doping GaAs photocathodes with different values of emission layer thickness from A to E. The thickness of GaAs emission layer is 1.6 μm or 2 μm. The Al component is 0.5 or 0.63. The samples grown by the MOCVD include varying doping or various component GaAs photocathodes with different values of emission layer thickness and different window layer components from F to J. The thickness values of GaAs emission layer are 1.4 μm, 1.6 μm or 1.8 μm, respectively. The Al component is 0.7 or varies from 0.9 to 0. The doping concentration of the GaAs emission layer is divided into 8 sections between 1×1018 cm-3 and 1×1019 cm-3. The experimental spectral response curves for all samples are obtained by the optical spectrum analyzer. And the experimental reflectivity and transmittivity curves are measured by the ultraviolet visible near infrared spectrohootometer. Based on the law of energy conservation, the absorptivity curves are obtained according to the experimental reflectivity and transmittivity. In the same coordinate system, both the curves are obtained by unitary processing according to the max. A similar surface barrier can be given by dividing the normalized absorptivity by the normalized spectral response, and those are termed the similar I barrier and the similar Ⅱ barrier, respectively. The results indicate that for both the GaAs photocathodes, the experimental spectral response curves both tend to move to the infrared band compared with the experimental absorptivity curves. The average energy differences between absorptivity and spectral response are calculated to be 0.3101 eV for the MBE sample, and 0.3025 eV for the MOCVD sample, respectively. The red-shifts of the photocathodes grown by MBE are a bit bigger than those of the photocathodes grown by MOCVD. In the shortwave region, the absorptivity is very large, but the spectral response cuts off nearby 500 nm. In the visible wavelength region, the peak position of the spectral response curve shifts toward the infrared band for several hundred meV in comparison with the absorptivity curve. In the near infrared region, a red shift of several meV appears at the cut-off position of the spectral response curve in comparison with the absorptivity curve. The results have the guiding significance for improving the photoemission performance of wide-spectrum GaAs photocathode by optimizing the optical performance.

Cyclotron resonance in ferromagnetic InMnAs and InMnSb

We present experimental and theoretical studies of the magneto-optical properties of $p$-type In${}_{1\ensuremath{-}x}$Mn${}_{x}$As and In${}_{1\ensuremath{-}x}$Mn${}_{x}$Sb ferromagnetic semiconductor films in ultrahigh magnetic fields oriented along [001]. Samples were fabricated by molecular beam epitaxy (MBE) and metal-organic vapor phase epitaxy (MOVPE). To model the results, we used an 8-band Pidgeon-Brown model generalized to include the wave vector dependence of the elec-tronic states along ${k}_{z}$ as well as the s-d and p-d exchange interactions with the localized Mn $d$ electrons. The Curie temperature is taken as an input parameter and the average Mn spin is treated in mean-field theory. We compared Landau level and band structure calculations with observed cyclotron resonance (CR) measurements. While differences between the CR measurements are seen for MBE and MOVPE samples, our calculations indicate that they arise from differences in the carrier densities. In addition, the difference in the carrier densities suggests significantly larger average spin for the MOVPE structures; this fact could be responsible for higher Curie temperatures in this material system.

Contactless electroreflectance studies of Fermi level position on c-plane GaN surface grown by molecular beam epitaxy and metalorganic vapor phase epitaxy

Contactless electroreflectance (CER) has been applied to study the Fermi-level position on c-plane GaN surface in Van Hoof structures grown by molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE). A clear CER resonance followed by strong Franz-Keldysh oscillation (FKO) of various periods was clearly observed for the series of samples of different thicknesses (30, 50, and 70 nm) of undoped GaN layer. The built-in electric field in this layer has been determined from the period of GaN-related FKO. A good agreement between the calculated and measured electric fields has been found for the Fermi-level located ∼0.4 and ∼0.3 eV below the conduction band for the MBE and MOVPE samples, respectively.

Patterned gold-assisted growth of GaP nanowires on Si

The patterning of metal seed particles for the vapor–liquid–solid (VLS) growth of semiconductor nanowires is examined. We report the challenges of obtaining GaP nanowires grown by molecular beam epitaxy (MBE) from a patterned array of Au seed particles created by electron beam lithography. Transmission electron microscopy studies revealed the presence of a thin silicon oxide layer over the patterned Au seed particles. Au acted as a catalyst for silicon oxide growth at even moderate temperatures. In particular, the annealing of Au-patterned Si (1 1 1) substrates at low temperatures required for indium mounting (200–300 °C) was responsible for the formation of an oxide layer that was detrimental to Au-assisted growth. Removal of the oxide layer by a buffered HF etch prior to MBE sample loading enabled patterned VLS growth.

Defect-induced magnetism in undoped wide band gap oxides: Zinc vacancies in ZnO as an example

To shed light on the mechanism responsible for the weak ferromagnetism in undoped wide band gap oxides, we carry out a comparative study on ZnO thin films prepared using both sol-gel and molecular beam epitaxy (MBE) methods. Compared with the MBE samples, the sol-gel derived samples show much stronger room temperature ferromagnetism with a magnetic signal persisting up to ∼740 K, and this ferromagnetic order coexists with a high density of defects in the form of zinc vacancies. The donor-acceptor pairs associated with the zinc vacancies also cause a characteristic orange-red photoluminescence in the sol-gel films. Furthermore, the strong correlation between the ferromagnetism and the zinc vacancies is confirmed by our first-principles density functional theory calculations, and electronic band alteration as a result of defect engineering is proposed to play the critical role in stabilizing the long-range ferromagnetism.

Comparison of Fe and Si doping of GaN: An EXAFS and Raman study

Abstract The effect of Fe and Si doping in GaN grown epitaxially on Al 2 O 3 is studied using Extended X-ray Absorption Fine Structure (EXAFS) and Raman spectroscopies. The EXAFS analysis shows that in GaN samples grown by Molecular Beam Epitaxy (MBE) at 750 °C the Ga–Ga distance is about 0.19% longer than the corresponding distance in samples grown by Metalorganic Vapour Phase Epitaxy (MOVPE) at 1050 °C. As this distance is affected strongly by the value of the a lattice constant, its smaller value complies with the higher compressive biaxial thermal strain, present in samples grown at a higher temperature. This finding is in line with the smaller blue shift observed for the E 2 Raman peak in the MBE samples. Si doping, contrary to Fe doping, reduces the mean square relative displacement of the Ga atoms in the second nearest neighboring shell of Ga. Most probably a stress relaxation mechanism, e.g. formation of dislocations, affects the c / a ratio which in turn modifies the difference of the Ga–Ga distances in- and out- of the c -plane. Contrary to Fe doping, the incorporation of Si atoms in the Ga sublattice induces compressive hydrostatic strain that blue-shifts the E 2 Raman peak. Finally, the carrier concentration determined by proper fitting of the A 1 (LO) Raman peak, that takes into account the phonon–plasmon coupling, shows that continuous Fe doping compensates slightly the n-type conductivity of GaN.

Study on NiO/Fe interface with X-ray photoelectron spectroscopy

Different monolayers (ML) of Fe atoms were deposited on NiO (001) substrates or NiO underlayers using molecular beam epitaxy (MBE), pulse laser deposition (PLD), and magnetron sputtering (MS). The magnetic properties and microstructure of the films were studied. The apparent magnetic dead layer (MDL) is found to exist at the NiO/Fe interfaces of the MBE sample (about 2 ML MDL), the PLD sample (about 3 ML MDL), and the MS sample (about 4 ML MDL). X-ray photoelectron spectroscopy indicates the presence of ionic Fe (Fe2+ or Fe3+) and metallic Ni at the NiO/Fe interfaces, which may be due to the chemical reactions between Fe and NiO layers. This also leads to the formation of MDL. The thickness of the MDL and the reaction products are related with the deposition energy of the atoms on the substrates. The interfacial reactions are effectively suppressed by inserting a thin Pt layer at the NiO/Fe interface.

Localization of the dopant in Ge:Mn diluted magnetic semiconductors by x-ray absorptionat the Mn K edge

A unitary picture of the structural properties ofMnxGe1 − x diluted alloys fabricated by either ion implantation or molecular beam epitaxy(MBE), at various growth temperatures (from 80 to about 623 K) and few per centconcentrations, is proposed. Analysis is based on synchrotron radiation x-ray absorptionspectroscopy at the Mn K edge. When the growth temperature exceeds 330 K, the MBEsamples show a high number of precipitated ferromagnetic nanoparticles, mainlyMn5Ge3, nucleated from the previous occupation of interstitial tetrahedral sites. Efficient substitutionis observed in the case of MBE samples made by alternate layers of GeMn alloys grown atT ≤ 433 K and undoped Ge thick layers. Similar good dilution properties are obtained byimplanting Mn ions at low temperatures (80 K). Possible precursors to preferentialmechanisms in the alloy formation are discussed on the basis of the present comparativestudy.

Comparison of deep level incorporation in ammonia and rf‐plasma assisted molecular beam epitaxy n‐GaN films

AbstractThe use of rf‐plasma and ammonia nitrogen sources for growth of GaN films by molecular beam epitaxy (MBE) are compared in terms of defect incorporation using deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS). To better improve the understanding of ammonia‐based MBE growth of GaN and potential defect sources as opposed to the more studied plasma source‐based MBE‐grown material several V/III ratios were also investigated, which were generated via systematic adjustment of the ammonia flow rates during growth. The DLOS spectra, comparing deep traps within the n‐GaN grown using N‐plasma and ammonia sources, reveal the presence of the same deep levels due to background carbon and gallium vacancies, with energy levels at EC‐3.28, EC‐2.62, and EC‐1.28. The DLTS results of the N‐plasma and ammonia‐based MBE samples show two similarly dominant electron traps at EC‐0.60, and EC‐0.24 in each sample. Measurements made as a function of V/III flux ratio for ammonia‐based MBE growth indicate a large dependence of the EC‐0.24 eV trap concentration on growth flux ratio, which is significant for guiding continued optimization of this promising MBE growth method for GaN devices. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface and structural disorder in MBE and sputtering deposited Cu thin films revealed by X-ray measurements

Copper thin films have been deposited on Si substrates by molecular beam epitaxy (MBE) at different deposition rates varying from 1 up to 22 Å/s. X-ray reflectivity and θ–2 θ measurements have shown that the surface roughness correlation length, the structural disorder and the grain dimensions are strongly affected by the deposition rate. Comparing these results with those obtained for sputtered deposited thin films with a low deposition rate (2.5 Å/s), a clear similarity between the MBE samples deposited with the highest deposition rate and the sputtering Cu films is observed. This result has been interpreted considering the different energies of the particles that approach the substrate in the two deposition techniques.

The optical excitation mechanism in ZnS: Sm 3+ grown by molecular-beam epitaxy

We fabricated a thin film of Sm 3+ doped ZnS by molecular-beam epitaxy (MBE) and investigated its photoluminescence properties. The excitation spectrum of the Sm 3+ luminescence at 10 K is dominated by broad bands peaked at the free exciton energies, in contrast to many earlier reports on rare-earth-doped semiconductors. This result shows that the free exciton contributes significantly to the excitation of Sm 3+ in our MBE sample, probably because the concentration of the trap centers which capture free excitons but do not transfer energy to Sm 3+ is sufficiently low in our sample. The thermal quenching characteristic of the Sm 3+ luminescence under interband excitation of the ZnS host is fitted well to a formula expressed by taking into account two deactivation channels. The origins of these deactivation channels are argued on the basis of previously proposed excitation mechanisms of the rare-earth ions in semiconductors.

Bonding at copper–alumina interfaces established by different surface treatments: a critical review

The present study summarizes the effects of processing conditions and substrate cleaning procedures on the structure, chemistry and bonding of Cu/(0001)Al2O3 interfaces as determined by advanced transmission electron microscopy techniques. The Cu/(0001)Al2O3 samples were prepared by molecular beam epitaxy (MBE) and solid-state diffusion bonding. Investigations of the MBE samples showed that the Al2O3 cleaning procedure alters the interfacial bonding. Metallic bonds occurred for an Ar+ ion sputtering and subsequent ultra-high vacuum (UHV) annealing treatment. Strongly ionic–covalent bonds were found for a wet chemical cleaning process followed by UHV annealing. The interfacial electronic structure did not reveal any significant changes compared to the bulk electronic structure for samples where the substrate surface was annealed in an oxygen-containing atmosphere after Ar+ ion pre-sputtering and UHV annealing. The results obtained at the solid-state diffusion-bonded Cu/Al2O3 samples indicated that the processing parameters such as temperature and load do not change the bonding behavior. Post-annealing of the solid-state diffusion-bonded Cu/Al2O3 samples in a well-defined oxygen partial pressure led to the formation of CuAlO2 at the interface between Cu and Al2O3, which improved the adhesion.

Ion-cut of Si facilitated by interfacial defects of Si substrate/epitaxial layer grown by molecular-beam epitaxy

We have shown that the Si/Si interface produced by molecular-beam epitaxy (MBE) growth of Si on a Si substrate can significantly enhance the efficiency of ion cutting. MBE growth is performed at 650°C. Samples are then implanted at room temperature by 62keV H− to a dose of 7×1016ions/cm2. The implantation energy locates H-peak in the vicinity of the Si/Si interface, which is 600nm below the Si surface. Scanning electron microscopy shows that, after post-implantation annealing at 300°C for 50min, the H implanted MBE Si has bubbles formed with an average diameters of 33μm, which is around one order of magnitude larger than that observed in the control bulk silicon sample. It is also observed that the area covered with blisters is a factor of 2 larger for the MBE samples, a trend that is systematically observed for anneals carried out in the range of 300–550°C.

Current transport property of n-GaN∕n-6H–SiC heterojunction: Influence of interface states

Heterostructures of n-GaN∕n-6H–SiC grown by hydride vapor phase epitaxy (HVPE) and molecular-beam epitaxy (MBE) are characterized with the current–voltage (I–V), capacitance–voltage (C–V), and deep level transient spectroscopy (DLTS) techniques. Using different contact configurations, the I–V results reveal a rectifying barrier in the n-GaN∕n-6H–SiC heterostructures. When GaN is negatively biased, the current is exponentially proportional to the applied voltage with the built-in barrier being 0.4–1.1eV for the HVPE samples and 0.5eV for the MBE sample. DLTS measurements reveal intense band-like deep level states in the interfacial region of the heterostructure, and the Fermi-level pinning by these deep level defects is invoked to account for the interfacial rectifying barrier of the heterostructures.

Interface transparency and proximity effect in Nb/Cu triple layers realized by sputteringand molecular beam epitaxy

We have investigated, in the framework of the proximity effect theory, the interfacetransparency between Nb and Cu in the case of high quality Nb/Cu trilayers fabricated by molecularbeam epitaxy (MBE) and sputtering deposition techniques. The obtained values do not seem to be strongly influenced by the fabrication methods but moreby the intrinsic properties of the two metals; a slightly higher value for has even been deduced for the MBE prepared samples. The proximity effect in thesesamples has also been studied in the presence of an external magnetic field. In the parallelconfiguration a significant shift towards lower values of the 2D–3D crossover temperaturehas been observed for MBE samples, in good agreement with very recent theoreticalpredictions. In the perpendicular case a positive curvature of the temperaturedependence of the upper critical field has been detected, which was less pronounced forsputtered samples. Both the effects have been observed only for trilayers with low Nbthickness (<600 Å) which confirms the crucial influence of the interface transparency on the values of theupper critical field in such samples.

Comparison of linear and nonlinear optical spectra of various ZnO epitaxial layers and of bulk material obtained by different experimental techniques

AbstractWe investigate ZnO epitaxial layers grown by MBE (Molecular Beam Epitaxy) and MOVPE (Metal Organic Vapor Phase Epitaxy) techniques. The samples show similar optical behavior in temperature dependent photoluminescence measurements, reflection and photoluminescence excitation spectroscopy in the low density regime. High excitation measurements show different behavior. While the MBE sample leads to stimulated emission from the exciton‐exciton‐scattering, an electron hole plasma is formed in the MOVPE sample which leads to stimulated emission at higher excitation intensities. The gain value measured by the variable stripe length method is much higher for the MBE grown sample. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Screw dislocations in GaN grown by different methods.

A study of screw dislocations in hydride-vapor-phase-epitaxy (HVPE) template and molecular-beam-epitaxy (MBE) overlayers was performed using transmission electron microscopy (TEM) in plan view and in cross section. It was observed that screw dislocations in the HVPE layers were decorated by small voids arranged along the screw axis. However, no voids were observed along screw dislocations in MBE overlayers. This was true both for MBE samples grown under Ga-lean and Ga-rich conditions. Dislocation core structures have been studied in these samples in the plan-view configuration. These experiments were supported by image simulation using the most recent models. A direct reconstruction of the phase and amplitude of the scattered electron wave from a focal series of high-resolution images was applied. It was shown that the core structures of screw dislocations in the studied materials were filled. The filed dislocation cores in an MBE samples were stoichiometric. However, in HVPE materials, single atomic columns show substantial differences in intensities and might indicate the possibility of higher Ga concentration in the core than in the matrix. A much lower intensity of the atomic column at the tip of the void was observed. This might suggest presence of lighter elements, such as oxygen, responsible for their formation.

Extended X-ray Absorption Fine Structure Studies of InGaN Epilayers

ABSTRACTThe local structure around In atoms in InGaN epilayers grown by Molecular Beam Epitaxy (MBE) and by Metal-Organic Chemical Vapour Deposition (MOCVD) was studied by means of Extended X-ray Absorption Fine Structure (EXAFS). The averaged In fraction of MOCVD grown samples ranged from 10% to 40% as estimated by Electron Probe Microanalysis (EPMA). The In fraction of MBE grown samples spanned the range from 13% to 96%. The In–N bond length was found to vary slightly with composition, both for MBE and MOCVD grown samples. Moreover, for the same In content, the In-N bond lengths in MOCVD samples were longer than those in MBE grown samples. In contrast, the In-In radial separations in MOCVD and MBE samples were found to be indistinguishable for the same In molar fraction. The In-Ga bond length was observed to deviate fromaveragecation-cation distance predicted by Vegard's law for MBE grown samples which indicates alloy compositional fluctuations.

The composition dependence of the optical properties of InN-rich InGaN grown by MBE

ABSTRACTStudy of the relationship between the composition and optical energies of InxGa1-xN has generated much interest and intrigue over the last decade and beyond. In this paper we describe data from InxGa1-xN epilayers covering the full range of composition (0 &lt; × &lt; 1), grown by both Metal-Organic Vapour Phase Epitaxy (MOVPE) and Molecular Beam Epitaxy (MBE). In particular we concentrate on a set of state-of-the-art InN rich MBE layers (0.6 &lt; × &lt; 1.0). Wavelength dispersive X-ray microanalysis is employed for accurate measurement of the InN fraction and of the group III : group V ratio. The InN rich layers are shown to be highly stoichiometric. The composition results are correlated with luminescence spectra, which show peaks covering the range 1.3 to 0.7 eV. Inclusion of our data from sets of MOVPE and MBE epilayers with InN fractions up to 0.4, measured using identical techniques, allows the composition dependence of the luminescence peak energy to be plotted across the entire composition range. A quadratic fit gives good agreement with both the low-InN MOVPE and high-InN MBE samples but not for the intermediate region. Possible reasons for this are discussed.