Effect of annealing on the electric and magnetic properties of GaMnAs and Be-codoped GaMnAs

GaMnAs and Be-codoped GaMnAs films grown via molecular beam epitaxy (MBE) were heat treated and the stability of Mn in the matrix was investigated. MnAs had a stable phase at the low growth temperature, but MnGa was stable at the annealing temperature. Be-codoping did not prevent the precipitation processes, but Be itself was stable during the annealing process to maintain the GaAs matrix at the high conductivity.     

Magnetic behavior and role of the antiphase boundaries in Fe3O4 epitaxial films sputtered on MgO (001)

Magnetite Fe₃O₄ films were grown on single crystal MgO (001) substrates using facing target sputtering technique. Conversion Electron M?ssbauer Spectroscopy and magneto optical polar Kerr spectra have confirmed the stoichiometric repartition of Fe cations corresponding to the inverse spinel structure and the electronic structure characteristic of bulk Fe₃O₄. Hysteresis loops carried out at room temperature show that, in a 1 T applied magnetic field, only 60% of the saturation magnetization is detected. This behavior is discussed in correlation to the antiphase boundaries (APBs) observed by electron microscopy. Magnetic force microscopy studies show that magnetic domains are larger than the mean distance between APBs. Received 2 July 2001     

Quantum rings formed in InAs QDs annealing process

InAs quantum dots (QDs) were grown by molecular beam epitaxy in the Stranski-Krastanow growth mode. The samples were placed between two undoped GaAs slices and annealed in nitrogen ambient at different temperature. Effect of annealing temperature on the evolution of QDs morphology is investigated by the AFM. This behavior can be attributed to the mechanisms of QDs ripening, intermixing and segregation in the annealing process. A number of QDs have evoluted into the uniform distribution quantum rings (QRs) when the sample was annealed at the temperature of 800 °C. The results indicated that high density and uniform QRs can be obtained by the post-growth technique.     

Asymmetrical twin boundaries and highly dense antiphase domains in BaNb0.3Ti0.7O3 thin films

Asymmetrical twin boundaries and highly dense antiphase domains were identified by means of transmission electron microscopy in a perovskite-based BaNb_0.3Ti_0.7O₃ thin film grown by laser molecular beam epitaxy on a SrTiO₃ (001) substrate. The microstructural characteristics of the BaNb_0.3Ti_0.7O₃ film were clarified in terms of lamellar {111} twins and antiphase domains, the domain boundaries of which are 1/2?110? stacking faults. It is proposed that the intersections of (111) twinning with the antiphase domain boundaries result in the asymmetrical twin boundaries.     

Photoluminescence characteristics of GaAsSbN/GaAs epilayers lattice-matched to GaAs substrates

The photoluminescence (PL) characteristics of GaAsSbN/GaAs epilayers grown by molecular beam epitaxy (MBE) are carefully investigated. The results show that antimony (Sb) incorporation into GaNAs material has less influence on the N-induced localization states. For the same N concentration, GaAsSbN material can reach an emission wavelength near 1.3 μm more easily than GaInNAs material. The rapid thermal annealing (RTA) experiment shows that the annealing induced rearrangement of atoms and related blueshift in GaAsSbN epilayers are smaller than those in GaNAs and GaInNAs epilayers. The GaAsSbN material can keep a longer emission wavelength near 1.3 μm-emission even after the annealing treatment. Raman spectroscopy analysis gives further insight into the structure stability of GaAsSbN material after annealing.     

Antiphase disorder in GaAs/Ge heterostructures for solar cells

Antiphase disorder in metal organic vapour phase epitaxy grown GaAs/(100)Ge heterostructures has been studied both in as-grown materials and in GaAs solar cells by chemical etching, transmission electron microscopy, and cathodoluminescence. All the samples are single domains at the surface due to the self-annihilation of antiphase domains whose size decreases as the misorientation angle increases. Completely antiphase domain-free epitaxy has been achieved for substrate miscuts greater than 3 degrees off towards [111]. A reversal in sublattice location has been found in the GaAs layers varying the misorientation angle and the growth temperature. A model to explain this result has been proposed based on the role of surface steps in the nucleation process. Strong interaction between antiphase boundaries and misfit dislocations has been found in all the heterostructures. In solar cells antiphase domains have been observed in high densities in the initial layer of GaAs deposited on Ge. The successful realisation of high efficiency solar cells is due to the overgrowth of these domains by single phase material over most of the wafer area.     

InGaP/GaAs heterojunction bipolar transistor grown by solid-source molecular beam epitaxy with a GaP decomposition source

Lattice-matched InGaP epilayers on GaAs (001) and InGaP/GaAs heterojunction bipolar transistors (HBTs) were successfully grown by solid-source molecular beam epitaxy (SSMBE) with a GaP decomposition source. A 3 μm thick InGaP epilayer shows that low temperature photoluminescence (PL) peak energy is as large as 1.998 eV, full width at half maximum (FWHM) is 5.26 meV, which is the smallest ever reported, and X-ray diffraction (XRD) rocking curve linewidth is as narrow as that of GaAs substrate. The electron mobilities at room temperature of nominally undoped InGaP layers obtained by Hall measurements are comparable to similar InGaP epilayer grown by solid-source molecular beam epitaxy (SSMBE) with other sources or other growth techniques. The large area InGaP/GaAs HBTs show very good Dc characteristics.     

Temperature dependence of excitonic emission in cubic CdSe thin film

A detailed photoluminescence investigation of the thermal redshift and broadening of the excitonic line of cubic CdSe film grown by molecular beam epitaxy is presented. Free excitonic emission from the cubic CdSe film was observed at low temperature. Temperature-dependent measurement was performed to obtain material parameters related to exciton-phonon interaction by fitting the experimental data to the phenomenological model. The relative contribution of both acoustic and optical phonon to the band gap shrinkage and exciton linewidth broadening are discussed. Exciton binding energy of 16±1.5 meV was determined from the Arrhenius analysis.     

Observation of antiphase domains in Cd<Subscript>x</Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te films on silicon by the phase contrast method in atomic force microscopy

It has been shown that phase contrast in atomic force microscopy (AFM) can be used to obtain adequate information on the density and distribution of antiphase domains on the surface of CdHgTe films grown by molecular beam epitaxy on a Si(301) substrate. By comparing the AFM phase images of the film surface with TEM images of structural defects in the near-surface region, the relation between microstructure and micromorphology of the films is revealed.     

Transmission electron microscopy characterization of HfO<Subscript>2</Subscript>/GaAs(001) heterostructures grown by molecular beam epitaxy

Transmission electron microscopy structural characterization of HfO₂/GaAs(001) heterostructures grown by molecular beam epitaxy with a film thickness of ∼ 5 nm was conducted. The study indicates that the room-temperature as-grown films are amorphous and the films crystallize into the monoclinic phase upon in situ post annealing at 540 °C in the growth chamber. Both types of films show an atomically sharp interface with GaAs(001) substrates. The crystalline monoclinic HfO₂ films exhibit c-oriented epitaxy on the substrate and consist of 90° domains. The formation of 90° domains in the heterostructures, the details of the domain-wall configurations, and the possible impact of the walls and the frequently observed anti-phase boundaries in the films on electrical properties of the heterostructures are discussed. PACS 68.37.Lp; 68.37.Og; 68.35.bg     

Molecular beam epitaxy growth of GaAs on an offcut Ge substrate

Molecular beam epitaxy growth of GaAs on an offcut Ge (100) substrate has been systemically investigated. A high quality GaAs/Ge interface and GaAs film on Ge have been achieved. High temperature annealing before GaAs deposition is found to be indispensable to avoid anti-phase domains. The quality of the GaAs film is found to strongly depend on the GaAs/Ge interface and the beginning of GaAs deposition. The reason why both high temperature annealing and GaAs growth temperature can affect epitaxial GaAs film quality is discussed. High quality In_0.17Ga_0.83As/GaAs strained quantum wells have also been achieved on a Ge substrate. Samples show flat surface morphology and narrow photoluminescence line width compared with the same structure sample grown on a GaAs substrate. These results indicate a large application potential for III--V compound semiconductor optoelectronic devices on Ge substrates.     

{1 1 1} Quantum wells of dilute nitrides grown on GaAs by molecular beam epitaxy

Dilute nitrides are promising alloys in view of extending potential micro- and opto-electronics applications of GaAs technology. Orientation effects on nitrogen incorporation in GaAs have been scarcely addressed. Here, GaAsN on (1 0 0) and on As(B)- and Ga(A)-rich (1 1 1) substrates was grown by molecular beam epitaxy at different substrate temperatures. Nitrogen content measured by secondary ion mass spectrometry as a function of the growth temperature highlights the influence of orientation on nitrogen incorporation. Furthermore, thermal annealing is shown to improve the optical quality of GaAsN quantum wells whatever their substrate orientations.     

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     

Effect of thermal annealing on GaN pn-junction diode with Pt/Ag as ohmic contact

In this paper, the effect of thermal annealing on gallium nitride (GaN) pn-junction photodiode grown on Si(1?1?1) by RF-plasma assisted molecular beam epitaxy is described. Platinum (Pt) and silver (Ag) were used as ohmic contact for GaN pn-junction photodetector. The structural evolution and temperature dependence of the current of Pt/Ag contacts on GaN pn-junction at various annealing were investigated by scanning electron microscopy, atomic force microscopy, high resolution X-ray diffraction, and current–voltage (I–V) measurements, respectively. The temperature dependence of the current may be attributed to changes of the surface morphology and surface roughness of Pt/Ag contacts on the sample. The lower surface roughness was achieved at thermal annealing temperature of 700?°C.     

Monte Carlo simulation of antiphase boundaries and growth of antiphase domains in Al5Ti3 phase in Al-rich γ-TiAl intermetallics

Predominantly two kinds of antiphase boundaries (APBs) form in Al₅Ti₃, which is an Al-rich ordered derivative of the γ-TiAl (L1₀) phase. This phase can be viewed as a periodic arrangement of lean rhombs and squares on the Ti-rich (002) planes of the tetragonal L1₀. Energies of the two types of APBs were varied in a Monte Carlo simulation by suitably changing the pair interaction parameters. APBs of both types form boundaries of Al₅Ti₃ antiphase domains (APDs), which coarsen with time. An important observation in this regard is that mostly facetted APBs form at lower ageing temperatures, whereas curved APBs appear to form at relatively higher ageing temperatures. The findings of this work suggest that there exists a critical temperature, akin to the roughening transition temperature for crystals, that marks the transition from facetted to curved APBs.     

Surface chemistry and growth mechanisms studies of homo epitaxial (1 0 0) GaAs by laser molecular beam epitaxy

In this paper, GaAs thin film has been deposited on thermally desorbed (1 0 0) GaAs substrate using laser molecular beam epitaxy. Scanning electron microscopy, in situ reflection high energy electron diffraction and in situ X-ray photoelectron spectroscopy are applied for evaluation of the surface morphology and chemistry during growth process. The results show that a high density of pits is formed on the surface of GaAs substrate after thermal treatment and the epitaxial thin film heals itself by a step flow growth, resulting in a smoother surface morphology. Moreover, it is found that the incorporation of As species into GaAs epilayer is more efficient in laser molecular beam epitaxy than conventional molecular beam epitaxy. We suggest the growth process is impacted by surface chemistry and morphology of GaAs substrate after thermal treatment and the growth mechanisms are discussed in details.     

Study of the structural perfection and distribution/redistribution of silicon in epitaxial GaAs films grown by molecular beam epitaxy on (100), (111)A, and (111)B substrates

Silicon distribution before and after thermal annealing in thin doped GaAs layers grown by molecular beam epitaxy on (100)-, (111)A-, (111)B-oriented substrates is studied by X-ray diffraction and SIMS. The surface morphology of the epitaxial films inside and outside an ion etch crater that arises during SIMS measurements is studied by atomic force microscopy. Distinctions in the surface relief inside the crater for different orientations have been revealed. Observed differences in the doping profiles are explained by features of the surface relief developing in the course of ion etching in SIMS measurements and by enhanced Si diffusion via growth defects.     

(In, Mn)As nanowires with ultrahigh Mn concentration: Growth, morphology and magnetic anisotropy

(In,Mn)As nanowires with ultrahigh Mn concentration have been successfully grown on GaAs(001) substrates by molecular beam epitaxy. The morphology dependences on Mn concentration and growth temperature are investigated. High Mn concentration and high growth temperature are both necessary for the growth of nanowires. All the (In,Mn)As nanowires are self-aligned along [−110]GaAs, and therefore have the shape magnetic anisotropy with the easy axis along the alignment orientation of the nanowires.     

MBE growth of SiGe with high Ge content for optical applications

The molecular beam epitaxy is a powerful technology for integrating optoelectronic devices in standard Si microelectronics. The MBE growth of high speed germanium detectors is discussed. The necessary lattice accommodation between Si and Ge is realized by an ultra thin virtual substrate. Contact layers with very high doping concentration and very sharp transitions are grown with special doping strategies. As special growth method the differential epitaxy allows the growth of epitaxial layers in oxide windows.     

Properties of Strain Compensated Symmetrical Triangular Quantum Wells Composed of InGaAs/InAs Chirped Superlattice Grown Using Gas Source Molecular Beam Epitaxy

We investigate the properties of symmetrical triangular quantum wells composed of InGaAs/InAs chirped superlattice, which is grown by gas source molecular beam epitaxy via digital alloy method. In the quantum well structure tensile AlInGaAs are used as barriers to partially compensate for the significant compressive strain in the wells, the strain compensation effects are confirmed by x-ray measurement. The photoluminescence spectra of the sample are dominated by the excitonic recombination peak in the whole temperature range. The thermal quenching, peak energy shift and line-width broadening of the PL spectra are analysed in detail, the mechanisms are discussed.