In-situ monitoring of interface formation using the phase shift of reflection high-energy electron diffraction intensity oscillations

A novel phase shift of Reflection High-Energy Electron Diffraction (RHEED) intensity oscillations during heterointerface formation allows direct monitoring of segragation at GaAs/Al _x Ga_1–x As interfaces. The effect should be applicable to other materials systems as it is due to the reconfiguration of the surface structure at the heterointerface.     

In situ characterization of MBE grown GaAs and Al x Ga1−x As films using RHEED,SIMS, and AES techniques

A combination of the surface diagnostic techniques Auger electron spectroscopy (AES), reflection high energy electron diffraction (RHEED), and secondary ion mass spectroscopy (SIMS) was used in order to get more detailed information on basic processes which lead to the formation of high quality monocrystalline GaAs and Al _x Ga_1−x As films by molecular beam epitaxy (MBE) under ultra-high vacuum conditions. The formation and changes of reconstructed surface structures on (100) GaAs as a function of growth parameters were observedduring growth by RHEED. AES was used to determine the relative ratio of Ga/As on the surface for different reconstructed structures, to investigate the impurity contamination on substrate surfaces and grown films, and to study the surface segregation of Sn in MBE GaAs during doping. Finally, intentional and unintentional impurities incorporated during the growth of GaAs and Al _x Ga_1−x As by MBE were detected by the SIMS technique immediately after growth within the reaction chamber.     

Well defined superlattice structures made by phase-locked epitary using RHEED intensity oscillations

Long continuing intensity oscillations of the RHEED pattern in the [100] azimuth on a (001) oriented substrate were observed during MBE growth of GaAs and Al_xGa_1−xAs. Using these oscillations, growth rates of GaAs and Al_xGa_1−xAs, and the Al mole fraction x of the Al_xGa_1−xAs were accurately monitored during the growth. The phase of the RHEED oscillations was analyzed by computer and molecular beam shutters were operated at a particular phase of the oscillations. This computer controlled phase-locked epitaxy (PLE) was used to grow precisely defined (GaAs)₂(AlAs)₂ bi-layer superlattices. Raman scattering spectra of the bi-layer superlattice showed split lines characteristic of superlattices. From TEM observation of a GaAs-AlAs multi-layered structure, it was verified that one cycle of oscillations corresponds to one monolayer growth of GaAs and AlAs. This PLE has a great advantage over the conventional MBE growth method for the precise control of very thin films and superlattice structures because it is invulnerable to fluctuations of molecular beam flux intensity.     

Photoluminescence of Al x Ga1−x As/GaAs quantum well heterostructures grown by molecular beam epitaxy

Nominally undoped Al_xGa_1–xAs grown by molecular beam epitaxy from As₄ species at elevated substrate temperatures of 670°C exhibits well-resolved excitonic fine structure in the low-temperature photoluminescence spectra, if the effective As-to-(Al+Ga) flux ratio on the growth surface is kept within a rather narrow range of clearly As-stabilized conditions. In contrast to previous results on Al_xGa_1–xAs of composition 0.15not to shift in energy by changing the excitation intensity. This implies a simple freeelectron carbon-acceptor recombination mechanism for the line without any participation of a donor. In Al_xGa_1–xAs of composition close to the direct-to-indirect cross-over point, two distinct LO-phonons separated by 34 and 48 meV from the (D ⁰,C ⁰) peak position at x=0.43 were observed which were before only detectable by Raman scattering experiments. The intensity of the carbon-impurity related luminescence lines in bulk-type Al_xGa_1–xAs and GaAs layers was found to be strongly reduced, as compared to the excitonic recombination lines, if the respective active layer was covered by a very thin confinement layer of either GaAs on top of Al_xGa_1–xAs or vice versa grown in the same growth cycle.     

Growth of wide band gap wurtzite ZnMgO layers on (0001) Al2O3 by radical-source molecular beam epitaxy

Wurtzite structure ZnMgO layers have been grown using radical-source molecular beam epitaxy on high-quality ZnO buffer layers grown on (0001) sapphire substrates. The thickness of the ZnO buffer layers is 300 nm, with full width at half maxim of the HR-XRD (0002) rocking curves as low as 25 arcsec. In-situ Reflection High-Energy Electron Diffraction (RHEED) was employed for the optimization of the ZnMgO growth. RHEED and X-Ray Diffractometry measurements did not reveal any phase change from the wurzite structure to the rocksalt structure. The C-lattice parameter of Zn_1−xMg_xO films decreased from 5.209 to 5.176 Å with increasing x to 0.2. The surface morphology of the samples was studied with atomic force microscopy. The root mean square roughness values of 200 nm thick ZnMgO (x=0.2) was less than 1 nm. The main photoluminescence peak of Zn_1−xMg_xO shifted to as high as 3.77 eV owing to the increasing Mg composition of up to x=0.2.     

Photoluminescence of strained-layer superlattices

The strained-layer superlattices (SLS's) of In_xGa_1?xAsGaAs and the single hetero-structure of In_xGa_1?xAs on GaAs were grown by MBE method. The samples obtained have a perfect surface morphology. The alloy composition of In_xGa_1?xAs layer and the growth rate were determined with high accuracy by in situ observation of the intensity oscillation of RHEED pattern. Photoluminescence peak energies of SLS's are in agreement with the calculated value by the Kronig-Penny analysis.     

Extremely narrow luminescence linewidth in GaAs single quantum wells by insertion of thin AlAs smoothing layers

We propose a new method to considerably reduce the overall growth interruption for high-quality GaAs single quantum wells during molecular beam epitaxy. The insertion of ultrathin AlAs smoothing layers at the constituent GaAs/Al _x Ga_1–x As heterointerfaces and growth interruptions of not more than 15 s yields an improvement of the luminescence linewidth (FWHM) to 0.56 meV for a 13 nm wide GaAs well and to a value as low as 0.195 meV for a 27 nm wide GaAs well. In addition, no Stokes shift between absorption and emission and no line splitting due to monolayer fluctuations in the well width is observed.     

Optical anisotropy of GaNAs grown on GaAs(0 0 1) substrate

In this paper GaN_xAs_1−x surfaces during growth are observed using reflectance difference or reflectance anisotropy spectroscopy (RDS or RAS). The epi-layer was grown by solid-source molecular beam epitaxy (MBE) system with a RF nitrogen prasma source. RD spectra showed broader structure and reduced amplitude compared to those of GaAs surfaces; GaAs(2 × 4)-like features were still observed with weak and blue-shifted peaks. In the low growth temperature region, an extra structure was also observed around 3.02 eV. We proposed that GaN_xAs_1−x surface can be classified into three types of the surface.     

Hydrogen sulphide doping of GaAs and AlxGa1−xAs grown by molecular beam epitaxy (MBE)

H₂S gas has been used during molecular beam epitaxy (MBE) growth of GaAs and Al _x Ga_1–x As as sulphur vector forn-type doping. Doping efficiencies are less than 10^–3 at usual growth temperatures, and are limited by an incorporation competitive surface process, probably 2Ga+H₂SGa₂S+H₂. In Al_xGa_1–x As forx0.2 the doping efficiency is further reduced by carrier freeze-out at deep levels. Measured thermal activation energies depend on growth conditions and remain relatively low even up to the direct-indirect bandgap crossover for substrate temperatures in the 585–645 C range.     

Ge quantum dots on a large band gap semiconductor: the first growth stages on 4H–SiC(0 0 0 1)

The Ge growth on SiC(0 0 0 1) follows a Stranski–Krastanov mode for Si-rich (3×3) and reconstructed surfaces. For Ge deposit in particular temperature conditions, a new (4×4) superstructure takes place and the reflection high energy electron diffraction (RHEED) specular spot intensity presents one oscillation proving a wetting layer formation. An island nucleation is then ascertained by the oscillation vanishing and by the appearance of a k-modulated RHEED pattern. On the other hand, on a C-rich surface, a direct Ge island nucleation is observed from the first growth stage. Indeed, for 1 ML Ge, the RHEED diagram consists in spots and rings, and the atomic force microscopy analysis indicates a high density (8×10¹⁰ cm⁻²) of small islands (30 nm, h3 nm). The RHEED spot analysis shows a preferential epitaxial relationship with the substrate Ge(1 1 1)//SiC(0 0 0 1). The Ge–C bonding being energetically unfavourable, Ge tends to form islands immediately rather than wetting the graphite-terminated surface. The Ge growth mode on C-rich surface is thus of Volmer–Weber type.     

Selectively dopedn-AlxGa1−xAs/GaAs heterostructures with high-mobility two-dimensional electron gas for field effect transistors

In selectively dopedn-Al_xGa_1–xAs/GaAs heterostructures with high-mobility two-dimensional electron gas (2 DEG) at the heterointerface a second conductive channel exists, if the Al_xGa_1–xAs layer is not totally depleted from free carries. The occurrence of parallel conductance has a deleterious effect on the performance of high-electron mobility transistors (HEMTs) fabricated from this material. Although in principle computable, parallel conductance depends on a large number of design parameters to be chosen for the heterostructure, which are additionally affected by the presence of deep electron traps inn-Al_xGa_1–xAs of composition 0.25n-Al_xGa_1–xAs/GaAs heterostructures is shown.     

Photo production of surface recombination centers in gallium arsenide

The near band edge photoluminescence (–870 nm) at 300 K of semi-insulating and p-type GaAs, a potentially useful tool for GaAs wafer mapping, decreases with time under illumination from the 514 nm line of an Ar⁺ laser. The photoluminescence bleaching has been studied by optical and photo Hall-effect techniques. It recovers only partially on a time scale of days and does not show a distinct intensity threshold behavior. From lifetime measurements and experiments on samples covered with epitaxial layers of GaAs _x P_1–x or Si₃N₄ it is concluded that creation of surface recombination centers causes the PL bleaching.     

Materials and device properties of pseudomorphic In x Ga1−x As/Al0.3Ga0.7As/GaAs high electron mobility transistors (0<x<0.5)

Modulation doped Al_0.3Ga_0.7As/In _x Ga_1–x As/GaAs high electron mobility transistor structures for device application have been grown using molecular beam epitaxy. Initially the critical layer thickness for InAs mole fractions up to 0.5 was investigated. For InAs mole fractions up to 0.35 good agreement with theoretical considerations was observed. For higher InAs mole fractions disagreement occurred due to a strong decrease of the critical layer thickness. The carrier concentration for Al_0.3Ga_0.7As/In _x Ga_1–x As/GaAs high electron mobility transistor structures with a constant In _x Ga_1–x As quantum well width was investigated as a function of InAs mole fraction. If the In _x Ga_1–x As quantum well width is grown at the critical layer thickness the maximum carrier concentration is obtained for an InAs mole fraction of 0.37. A considerable higher carrier concentration in comparison to single-sided -doped structures was obtained for the structures with -doping on both sides of the In _x Ga_1–x As quantum well. Al_0.3Ga_0.7As/In _x Ga_1–x As/GaAs high electron mobility transistor structures with InAs mole fractions in the range 0–0.35 were fabricated for device application. For the presented field effect transistors best device performance was obtained for InAs mole fractions in the range 0.25–0.3. For the field effect transistors with an InAs mole fraction of 0.25 and a gate length of 0.15 m a f _T of 115 GHz was measured.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

A Ground Miniband Width of a Superlattice with Rectangular Quantum Wells versus Constructive Parameters

A convenient formula for the ground miniband width as a function of constructive parameters of the superlattice is derived from the solution of the Kronig–Penney equation using the perturbation theory. The conditions of applicability of this formula are verified for the GaAs/Al_xGa_1–x As superlattices used as infrared photodetectors.     

Photoluminescence of AlxGa1−xAs/GaAs quantum well heterostructures grown by molecular beam epitaxy

Low-temperature photoluminescence measurements on nominally undoped Al_xGa_1–xAs/GaAs quantum well heterostructures (QWHs) grown by molecular beam epitaxy (MBE) exemplified the exclusivelyintrinsic free-exciton nature of the luminescence under moderate excitation conditions. Neither any spectroscopic evidence for alloy clustering in the Al_xGa_1–xAs barriers nor any extrinsic luminescence due to recombination with residual acceptors has been detected in single and double QWHs when grown at 670 °C under optimized MBE growth conditions. Carrier confinement in Al_xGa_1–xAs/GaAs QWHs starts at a well width ofL _z30 nm when x0.25. The minor average well thickness fluctuation ofL _z=4×10^–2nm as determined from the excitonic halfwidth allowed the realization of well widths as low asL _z=1 nm and thus a shift of the free-exciton line as high as 2.01 eV which is close to the conduction band edge of the employed Al_0.43Ga_0.57As confinement layer. The measurements further revealed a strongly enhanced luminescence efficiency of the quantum wells as compared to bulk material which is caused by the modified exciton transition probabilities due to carrier localization.     

Morphological stability in epitaxy of semiconductors — application to optoelectronic monolithically integrated structures

A simple selection rule for epitaxial growth techniques, which is based on morphological stability of the substrate surface is proposed. According to this rule a certain growth technique should be used for preparing a specific device structure of a three-dimensional monolithically integrated optical or electronic circuit. The formulae for morphological stability functions for LPE, MO, VPE, and MBE growth techniques are given. Calculations performed for the GaAs/Al _x Ga_1–x As material system by using the linear morphological stability theory of Mullins and Sekerka suggest that from the point of view of morphological stability the most suitable growth technique for fabrication of three-dimensional monolithically integrated optical and electronic device structures is the MBE technique.On leave from the Institute of Physics, Polish Academy of Sciences, PL-02-668 Warsaw, Poland     

Effect of structural defects on InSb/AlxIn1−x Sb quantum wells grown on GaAs (0 0 1) substrates

We investigate oriented abrupt steps (OASs), a type of surface defect in InSb/Al_xIn_1−xSb quantum-well (QW) samples grown on GaAs (0 0 1) substrates. Previous atomic force microscopy studies have reported that the OASs are oriented along the [1 1 0] and directions and have an inclination angle of 5°–15° with respect to the sample surface. Our plan-view and cross-sectional transmission electron microscopy analyses reveal that the OASs are the terminal edges of threading micro-twins at the sample surface. Hall effect measurements indicate that the density of OASs correlates with the electron mobility in the InSb QWs.     

Polaritons in anisotropic semiconductors

We show how to compute the optical properties (reflection and absorption) of anisotropic semiconductors in the exciton energy region, taking into account polariton and electron-hole coherence effects. The method is applied to a GaAs/Ga_1–x Al _x As superlattice, and the modifications in the optical properties with respect to GaAs are related to the anisotropy.     

Barrier-disorder induced exciton relaxation via LO-phonons in GaAs/Al x Ga1−x As multiple quantum wells

Hot exciton relaxation is observed in GaAs/Al _x Ga_1–x As multiple quantum wells. The photolumnescence excitation spectra of the localized exciton emission at low temperatures and excitation densities are composed of narrow equidistant peaks exactly separated by the GaAs LO-phonon energy (36 meV). The relaxation mechanism via LO-phonons is found to be important for localized excitons in multiple quantum wells with GaAs layer thicknesses of about 50 Å, where pronounced alloy fluctuations in the barriers provide a strong additional lateral potential which suppresses the dissociation of hot excitons.     

Giant Hall effect in (GaAs)Fe granular films

A series of (GaAs)_1 − xFe_x (x: volume fraction) films with Fe granules embedded in GaAs matrix were prepared by magnetron sputtering. Hall Effect of the films was characterized. The largest saturated Hall resistivity of was observed in (GaAs)₃₀Fe₇₀ film at room temperature, which is over 2 orders larger than that of pure Fe, about 1 order larger than that of (NiFe)–(Al₂O₃) and (NiCo)–(SiO₂) granular films prepared under the same preparation conditions, and 150% larger than that of Ge₃₀Fe₇₀.