Combination Of Molecular Beam Epitaxy Research Articles

Highly ordered catalyst-free and mask-free GaN nanorods onr-plane sapphire

Self-organized and highly ordered GaN nanorods were grown without catalyst onr-plane sapphire using a combination of molecular beam epitaxy and metal–organicvapor-phase epitaxy. AlN nucleation centers for the nanorods were prepared bynitridation of the sapphire in a metal–organic vapor-phase epitaxy reactor, while thenanorods were grown by molecular beam epitaxy. A coalesced two-dimensionalGaN layer was observed between the nanorods. The nanorods are inclined by62° towards the -directions of the a-plane GaN layer. The high degree of ordering and the structural perfection were confirmedby micro-photoluminescence measurements.

Dielectric functions and critical points of PbTiO3, PbZrO3, and PbZr0.57Ti0.43O3 grown on SrTiO3 substrate

Single crystalline PbTiO3, PbZrO3, and PbZr0.57Ti0.43O3 thin films on SrTiO3 (001) substrates were grown by a combination of molecular beam epitaxy and rf sputtering methods. The authors measured the dielectric functions of the thin films using spectroscopic ellipsometry and determined the interband critical point energies using standard critical point model. They compared the critical point energies to the band structure calculations in the literature. The data suggest that anticrossing behavior occurs between Ea and Eb near Zr=0.17. This phenomenon is attributed to a coupling between X1c and X3c bands caused by intrinsic alloy disorder.

Optical modes in pyramidal GaAs microcavities

Pyramidal GaAs structures on top of GaAs∕AlAs distributed Bragg reflectors are investigated as candidates for true three-dimensional cavities with potentially low mode volume and high quality-factor. Different types of single and coupled resonators with base lengths of a few microns are realized using a combination of molecular-beam epitaxy, electron-beam lithography, and wet chemical etching. Embedded InGaAs quantum dots are utilized as light sources to verify the resonator modes. Furthermore, a spatially localized emission through the pyramid facets indicates the future possibility of coupling cavity modes to optical fibers. This could be interesting within the context of single photon emitters.

Growth temperature dependence of MBE-grown ZnSe Nanowires

The growth orientation and crystalline perfection of ZnSe nanowires (NWs) grown by the combination of molecular beam epitaxy (MBE) and vapor–liquid–solid (VLS) reaction was found dependent on the growth temperature. Transmission electron microscopy (TEM) observations reveal that the preferred growth orientation of NWs depends on both the size of NWs and the growth temperature. A growth temperature dependence of the thickness of the terminal growth zone under the metal/semiconductor interface is introduced to modify our previous proposed phenomenological model to explain these observations. Tapering growth (the size of the NWs becomes narrower towards the top) was observed for the samples grown at 400 °C, which can be attributed to the diffusion-induced VLS mode that can be activated at relatively low growth temperature but inhibited at higher growth temperature. A cubic to hexagonal phase transformation was observed on the tapered NWs and its formation mechanism was proposed.

Nucleation, coarsening and kinetic scaling of two-dimensional islands on GaSb(0 0 1)

Using a combination of molecular beam epitaxy and in situ surface X-ray diffraction, we investigate the nucleation and coarsening of monolayer high islands on GaSb(0 0 1) during deposition in real time. We find an activation energy for island nucleation of 1.55 ± 0.16 eV, indicating a stable nucleus size larger than two atoms. For intermediate temperatures where GaSb homoepitaxy is stable, the lateral coarsening of the islands after deposition is described by Ostwald ripening. The average island sizes during coarsening are isotropic, although with different size distributions in different directions. The size distributions do not change during coarsening, implying kinetic scaling.

Band structure investigation of chalcopyrite CuInSe2(001) by angle-resolved photoelectron spectroscopy

AbstractClean and ordered chalcopyrite CuInSe2 surfaces are a precondition for the study of the electronic structure by angle-resolved photoelectron spectroscopy. The preparation of welldefined CuInSe2(001) surfaces by the combination of molecular beam epitaxy and a selenium capping and decapping process is described. The surface structure of CuInSe2 epilayers with different bulk composition is compared and analysed by low-energy electron diffraction.Employing near-stoichiometric surfaces, the valence electronic structure of CuInSe2 was investigated by angle-resolved photoelectron spectroscopy at the synchrotron source BESSY 2. This is the first study of the valence band structure of a copper chalcopyrite semiconductor material by photoelectron spectroscopy. The valence band dispersion along τT, i.e. the [001] direction, was investigated by a variation of the excitation energy from 10 to 35 eV under normal emission, and the band dispersion along τT, i.e. the [110] direction, was analysed by angular scans with hv = 13 eV.The valence bands derived from antibonding and bonding Se4p-Cu3d hybrid orbitals, nonbonding Cu3d states and In-Se hybrid states are clearly indentified. The strongest dispersion is found for the topmost valence band with a bandwidth of ∼0.7 eV from τ to T. From τ to N, the observed dispersion was 0.5 eV. The experimental valence bands are discussed in relation to calculated band structures in the literature.

Magnetotransport and magnetic properties of InAs/Mn digital alloys

We have investigated the magnetic and magneto-transport properties of a systematic sequence of five InAs/Mn digital alloys grown by a combination of molecular beam epitaxy and atomic layer epitaxy. The samples consist of 30 periods of Mn fractional monolayers (ML) (0.17– 0.5 ML ) separated by 14 ML thick InAs spacer layers in a superlattice configuration. Four samples show n-type electrical conduction while the fifth (0.25 ML Mn ) is p-type. Squid magnetization measurements performed on these samples show remnant magnetization above room temperature, which is apparently related to a second phase.

Formation of lateral quantum dot molecules around self-assembled nanoholes

We fabricate groups of closely spaced self-assembled InAs quantum dots (QDs)—termed lateral QD molecules—on GaAs (001) by a combination of molecular-beam epitaxy and AsBr3 in situ etching. An initial array of homogeneously sized nanoholes is created by locally strain-enhanced etching of a GaAs cap layer above InAs QDs. Deposition of InAs onto the nanoholes causes a preferential formation of the InAs QD molecules around the holes. The number of QDs per QD molecule ranges from 2 to 6, depending on the InAs growth conditions. By decreasing the substrate temperature, the number of QDs per QD molecule increases, but the statistical distribution is wider due to a reduced In atom diffusion length. Our photoluminescence investigation documents the nanohole and QD molecule formation step by step and confirms the high crystal quality of these structures. An analysis of the nanohole geometry as a function of annealing time and InAs filling allows us to propose a model for the QD molecule formation process.

Fabrication of high quality two-dimensional electron gases by overgrowth of focused ion beam implantation doped Al xGa 1− xAs

We use a combination of molecular beam epitaxy (MBE) and focused ion beam (FIB) implantation to fabricate selectively doped inverted GaAs/Al x Ga 1− x As heterostructures containing two-dimensional electron gases (2DEGs). The FIB implantation doping is carried out after growth of the Al x Ga 1− x As barrier and enables the lateral patterning of the doped area. During the growth interruption we use an amorphous arsenic layer to protect the surface against contamination. We find an in situ MBE annealing step of 730°C for 20 s prior to overgrowth as a means of significantly improving the electron mobility. Electron mobilities up to 9.5×10 5 cm 2/ V s at 4.2 K after illumination have been obtained. For ex situ annealed samples the peak mobility was only 1.2×10 5 cm 2/ V s , suggesting a higher quality of the regrown layer for the in situ annealed sample.

InSb 1− xN x growth and devices

Indium antimonide (InSb) has the smallest energy gap of any of the binary III–V materials, leading to a cut-off wavelength of 7 μm at 300 K. The addition of small proportions of nitrogen to InSb offers the prospect of extending the response wavelength into the 8–12 μm range, which is important for thermal imaging in that atmospheric transmission window and because it encompasses the absorption lines of several environmentally important gases and can therefore be used for monitoring the gases. We report on the growth, by a combination of molecular beam epitaxy and a nitrogen plasma source, of InSb 1− x N x with up to 10% nitrogen. Structural characterisation techniques of TEM, AFM and SIMS have enabled some optimisation of material quality to be demonstrated by biasing the sample during growth. Measurements on light emitting diodes comprising a superlattice of InSb 0.945N 0.055/InSb show an emission wavelength of 10.5 μm, which is confirmed by free electron laser assessment. Comparison with first principles band-structure calculations indicate that approximately 10% of the nitrogen is active. Hall effect measurements of 1 μm thick bulk layers indicate an increasing n-type behaviour, the degeneracy effects of which mean, however, that this is only a lower limit.

Novel ferromagnetism in digital GaAs/Mn and GaSb/Mn alloys

Ferromagnetic III-Mn-V semiconductors are under intensive investigation for spintronic applications. In the present work digital alloys of GaAs/Mn and GaSb/Mn were fabricated by a combination of molecular beam epitaxy and atomic layer epitaxy. The Mn fraction in the layers was varied at constant III–V spacer thickness, and the III–V spacer-layer thickness was varied at constant Mn fraction (0.5 monolayer). Transmission electron micrographs showed good crystal quality with no evidence of three-dimensional (3D) Mn-V precipitates. The GaAs/Mn samples have Curie temperatures in the vicinity of 40 K and exhibit an anomalous Hall effect (AHE) similar to that seen in GaMnAs random alloys. These samples all show thermally activated resistance at zero field, characteristic of hopping conduction with evidence of Coulomb-gap-like behavior. The GaSb/Mn samples exhibit ferromagnetism (with temperature dependent hysteresis loops) and a strong AHE up to 400 K . At low temperatures the remanent magnetization initially drops rapidly with increasing temperature, indicative of Curie temperatures between 10 and 50 K depending on the Mn concentration. However, a substantial remanent magnetization persists to high temperature, suggesting a second phase with a Curie temperature above 400 K . These samples all showed essentially metallic behavior and weak negative magnetoresistance at low temperatures. These results are discussed in the context of a model of quasi-2D MnSb islands (for which there is direct evidence) embedded in a matrix of GaMnSb in the Mn-containing layers.

Difference frequency generation of 8-µm radiation in orientation-patterned GaAs

First-order quasi-phase-matched difference frequency generation of narrowband tunable mid-infrared light is demonstrated in orientation-patterned GaAs. The all-epitaxial orientation-patterned crystal is fabricated by a combination of molecular beam epitaxy and hydride vapor phase epitaxy. Lasers at 1.3 and 1.55 microm were mixed to give an idler output at 8 microm, with power and wavelength tuning consistent with theoretical estimates, indicating excellent material uniformity over the 19-mm-long and 500-microm-thick device.

Measurement of the nonlinear coefficient of orientation-patterned GaAs and demonstration of highly efficient second-harmonic generation

Quasi-phase-matched (QPM) GaAs structures, 0.5 mm thick, 10 mm long, and with 61-mum grating periods, were grown by a combination of molecular-beam epitaxy and hydride vapor phase epitaxy. These were characterized by use of mid-IR second-harmonic generation (SHG) with a ZnGeP(2) (ZGP) optical parametric oscillator as a pump source. The SHG efficiencies of QPM GaAs and QPM LiNbO(3) were directly compared, and a ratio of nonlinear coefficients d(14)(GaAs)/d(33) (LiNbO(3))=5.01+/-0.3 was found at 4.1-mum fundamental wavelength. For input pulse energies as low as 50muJ and approximately 60-ns pulse duration, an internal SHG conversion efficiency of 33% was measured in QPM GaAs.

Epitaxial growth of Pb(Zr0.2Ti0.8)O3 on Si and its nanoscale piezoelectric properties

We have demonstrated a route to epitaxial Pb(Zr0.2Ti0.8)O3 on (001) Si that exhibits a uniform piezoelectric response down to nanoscale levels through the utilization of an insulating, single-crystalline SrTiO3 transition layer. These structures, which were grown by a combination of molecular-beam epitaxy and off-axis magnetron sputtering, have a surface roughness of <5 Å, with piezoelectric microscopy measurements revealing a piezoelectric coefficient of ∼50 pm/V that is switchable down to sub-100-nm dimensions.

Selective area deposited blue GaN–InGaN multiple-quantum well light emitting diodes over silicon substrates

We report on fabrication and characterization of blue GaN–InGaN multi-quantum well (MQW) light-emitting diodes (LEDs) over (111) silicon substrates. Device epilayers were fabricated using unique combination of molecular beam epitaxy and low-pressure metalorganic chemical vapor deposition growth procedure in selective areas defined by openings in a SiO2 mask over the substrates. This selective area deposition procedure in principle can produce multicolor devices using a very simple fabrication procedure. The LEDs had a peak emission wavelength of 465 nm with a full width at half maximum of 40 nm. We also present the spectral emission data with the diodes operating up to 250 °C. The peak emission wavelengths are measured as a function of both dc and pulse bias current and plate temperature to estimate the thermal impedance.

Fabrication of thin-film cleaved cavities using a bonding and cleaving fixture

A novel method for the fabrication of cleaved-cavities has been developed that uses a copper plate assembly to support semiconductor layers after substrate removal. PbSe layers were grown through a combination of molecular beam epitaxy and liquid phase epitaxy on Si [100] substrates using CaF/sub 2/ and BaF/sub 2/ buffer layers. After growth the sample was bonded to the edges of a copper plate assembly epilayer down and the BaF/sub 2/ buffer layer was etched away allowing for growth substrate removal. This technique allows fabrication of cleaved Fabry-Perot resonant cavities by separating the copper plates after the substrate is removed.

IV–VI Semiconductor growth on silicon substrates and new mid-infrared laser fabrication methods

This paper reviews results from research conducted at the University of Oklahoma on the development of new IV–VI semiconductor (lead salt) epitaxial growth and laser fabrication procedures that can ultimately lead to dramatic increases in mid-IR laser operating temperatures. Work has focused on growth of IV–VI semiconductor laser structures on silicon substrates using buffer layers that contain BaF 2. Recent experiments show that it is possible to obtain high crystalline quality IV–VI semiconductor layer structures on (111)-oriented silicon substrates using molecular beam epitaxy (MBE) or on (100)-oriented silicon using a combination of MBE and liquid phase epitaxy (LPE). Experimental data for IV–VI semiconductor layer structures grown on silicon substrates including crystalline quality information as determined by high resolution X-ray diffraction (HRXRD) measurements and absorption edge information as determined by Fourier transform infrared (FTIR) transmission measurements are presented. Results show that these materials can be used to fabricate lasers that cover the 3 μm (3333 cm −1) to 16 μm (625 cm −1) spectral range. Removal of IV–VI semiconductor laser structures from the silicon growth substrate by dissolving BaF 2 buffer layers with water is also demonstrated. This allows epitaxially-grown laser structures to be sandwiched between two heat sinks with a minimum of thermally resistive IV–VI semiconductor material. Theoretical modeling predicts that IV–VI lasers fabricated this way will have maximum continuous wave (cw) operating temperatures at least 60° higher than those of IV–VI lasers fabricated on PbSe or PbTe substrates.

Atomically flat interface in SiGe/Si heterostructures formed by solid phase epitaxy: Significant increase in two-dimensional electron mobility

A new SiGe/Si heterostructure is fabricated through a combination of molecular beam epitaxy (MBE) and solid phase epitaxy (SPE). The hetro-interface obtained by SPE was atomically flat, as confirmed by transmission electron microscope (TEM) observation. This interface enables an ultrahigh electron mobility of the two-dimensional electron gas (2DEG).

Temperature dependence of the magnetization reversal in Co(fcc)–BN–Co(poly hcp) structures

The magnetic properties of multilayer structures with two magnetic layers of the same metal (Co) but with different crystallographic structures separated by an insulating BN layer have been studied. These structures were prepared on Si (001) substrates by a combination of molecular beam epitaxy (metallic layers) and electron cyclotron resonance-assisted sputtering (BN layer). An fcc Co single-crystal layer (60 Å) was first stabilized by growing it on a copper fcc buffer layer and subsequently a polycrystalline Co layer (70 Å) with hcp structure was grown on top of the insulating BN layer. A CoO antiferromagnetic layer, formed adjacent to this hcp Co layer, significantly influenced the magnetic behavior of the polycrystalline hcp Co layer. The magnetic hysteresis loops for these structures were measured at temperatures ranging from 5 to 350 K with the magnetic field applied along the easy (110) in-plane axis of the fcc Co. A very sharp flipping of the magnetization was found for the fcc Co layer with a nearly temperature-independent coercive field that increased from 14 mT below 100 K to 16 mT at 300 K. In contrast, the magnetization reversal in the hcp Co layer was smoother and its coercivity varied significantly with temperature depending on the strength of the exchange coupling with the adjacent CoO layer. At 5 K the coercivity was greater than 0.2 T and decreased with increasing temperature, becoming essentially zero above room temperature. When cooling in a magnetic field, an exchange offset was observed below 150 K that increased to about 0.1 T at 5 K.

Characteristics of Si3N4/GaAs metal-lnsulator-semiconductor interfaces with coherent Si/Al0.3Ga0.7As interlayers

Si3N4/GaAs metal-insulator-semiconductor (MIS) interfaces with Si(10A)/ Al0.3Ga0.7As (20A) interface control layers have been characterized using capacitance-voltage (C-V) and conductance methods. The structure was in situ grown by a combination of molecular beam epitaxy and chemical vapor deposition. A density of interface states in the 1.1 × 1011 eV-1 cm-2 range near the GaAs midgap as determined by the conductance loss has been attained with an ex situ solid phase annealing of 600°C in N2 ambient. A dip quasi-static C-V demonstrating the inversion of the minority-carrier verifies the decent interface quality of GaAs MIS interface. The hysteresis and frequency dispersion of the MIS capacitors were lower than 100 mV, some of them as low as 50 mV under a field swing of about ±2 MV/cm. The increase of the conductance loss at higher frequencies was observed when employing the surface potential toward conduction band edge, suggesting the dominance of faster traps. Self-aligned gate depletion mode GaAs metal-insulator-semiconductor field-effect transistors with Si/Al0.3Ga0.7As interlayers having 3 μm gate lengths exhibited a transconductance of about 114 mS/mm. The present article reports the first application of pseudomorphic Si/ Al0.3Ga0.7As interlayers to ideal GaAs MIS devices and demonstrates a favorable interface stability.