As4 Flux Research Articles

As-rich (2 × 2) surface reconstruction on GaAs(111)A

The atomic structures and the formation processes of the Ga- and As-rich (2×2) reconstructions on GaAs(111)A have been studied. The Ga-rich (2×2) structure is formed by heating the As-rich (2×2) phase, but the reverse change hardly occurs by cooling the Ga-rich surface under the As2 flux. Only when the Ga-rich (2×2) surface covered with amorphous As layers was thermally annealed, the As-rich (2×2) surface is formed. The As-rich (2×2) surface consists of As trimers located at a fourfold atop site of the outermost Ga layer, in which the rest-site Ga atom is replaced by the As atom.

Optimization of AlAs/AlGaAs quantum well heterostructures on on-axis and misoriented GaAs (111)B

We report systematic growth optimization of high Al-content AlGaAs, AlAs, and associated modulation-doped quantum well (QW) heterostructures on on-axis and misoriented GaAs (111)B by molecular beam epitaxy. Growth temperatures TG > 690 °C and low As4 fluxes close to group III-rich growth significantly suppress twin defects in high-Al content AlGaAs on on-axis GaAs (111)B, as quantified by atomic force and transmission electron microscopy as well as x-ray diffraction. Mirror-smooth and defect-free AlAs with pronounced step-flow morphology was further achieved by growth on 2° misoriented GaAs (111)B toward [01¯1] and [21¯1¯] orientations. Successful fabrication of modulation-doped AlAs QW structures on these misoriented substrates yielded record electron mobilities (at 1.15 K) in excess of 13 000 cm2/Vs at sheet carrier densities of 5 × 1011 cm−2.

Real-Time In Situ Monitoring of GaAs (211) Oxide Desorption and CdTe Growth by Spectroscopic Ellipsometry

We describe the growth of CdTe (211)B by molecular beam epitaxy on large-area epiready GaAs (211)B substrates. Prior to CdTe growth, GaAs substrates were thermally cleaned under an As4 flux. Oxide desorption was verified by in situ spectroscopic ellipsometry (SE) and reflection high-energy electron diffraction. The use of in situ SE played a significant role in the study of CdTe-on-GaAs growth and annealing processes. An effective medium approximation (EMA) was used to model the overlayer thickness variation of CdTe epilayers throughout growth and in situ annealing cycles. A correlation between SE-derived EMA thickness values and surface defect formation mitigation is discussed. All annealed samples (11.5 μm to 13 μm thick) exhibited excellent crystalline quality with average double crystal rocking curve full-width at half-maximum (FWHM) values of ~60 arcsec.

Erratum: In situ STM observation of site‐ controlled InAs nano‐dot growth on GaAs(001) during In and As4 irradiations

AbstractThe amounts of As4 flux, scanning time, and collapse time of InAs dot structure at 430 °C were erroneously reported and should be corrected. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Atomic-scale characterization of the N incorporation on GaAs(001)

The initial nitridation processes of GaAs(001) have been systematically studied using reflection high-energy electron diffraction, scanning tunneling microscopy, and x-ray photoelectron spectroscopy. The structure and composition of the nitrided surface strongly depends on the preparation condition. The amount of N atoms decreases with increasing substrate temperature, which is due to the desorption of N at higher temperatures. The surface nitrided without As4 flux shows a N-induced (3 × 3) reconstruction tentatively associated with a N-N dimer formation at the surface layer. N incorporation is less promoted when the initial GaAs surface is exposed to active N species under the As4 flux, because of the replacement of N atoms by the arriving As atoms. We found that N atoms are initially incorporated in the substitutional site at the central position below two As-As dimers in the β2(2 × 4) structure.

Effects of using As2 and As4 on the optical properties of InGaAs quantum rods grown by molecular beam epitaxy

We investigate the effect of the arsenic source (As2 and As4) on the optical properties of InGaAs quantum rods (QRs) grown by molecular beam epitaxy. Owing to differences in the In and Ga diffusion lengths under As2 and As4 fluxes, photoluminescence (PL) peak energies of the QR samples depend strongly on the As source when similar growth conditions are used. A marked improvement in the PL intensities from QR samples grown using As4 is achieved. However, for both As2 and As4, an increase of the As overpressure results in a PL intensity degradation, probably due to the formation of nonradiative recombination centers.

Optical characterization of AlxGa1−xAs/GaAs modulation-doped heterostructures grown under As2 and As4 fluxes

The authors have investigated the optical properties of AlxGa1−xAs/GaAs modulation-doped heterostructures grown under different experimental conditions, mainly by changing the molecular species of the As beam stemming from a cracker cell at different temperatures. By low-temperature photoluminescence (PL) spectroscopy, they observed that using As2 instead of As4, the crystal quality of the ternary alloy improves significantly since narrower lines in the near-band-gap energy of AlGaAs were obtained when the temperature of the As-cracker zone increased. The experimental position of the energy of the ternary band gap transition as a function of temperature fits the Varshni and Viña functional forms satisfactorily, while it is difficult to assess the data for those samples grown employing As4. It was found that the As2 molecular beam reduces the creation of donor acceptor levels in AlGaAs, as compared with As4. The PL spectra line shape in the GaAs band gap region shows lines associated with band-to-band transition, exciton-bound to neutral-point defects (d,X), and conduction-band to neutral carbon-acceptor (C,A0). The intensity of these transitions was found to be independent of the As molecular-beam species; nevertheless, the photoreflectance spectra of the samples show line shape changes close to the GaAs band gap energy. This region has been related to the electron mobility of two-dimensional electron-gas systems.

Fabrication of Self-Assembled InGaAs Squarelike Nanoholes on GaAs(001) by Droplet Epitaxy

The fabrication of self-assembled InGaAs squarelike nanoholes on GaAs(001) substrates grown by droplet epitaxy using molecular beam epitaxy was reported. The formation mechanism is explained by the As4 diffusion in droplets during the supply of As4 flux. The effects of substrate temperature (300–390 °C) during the InGa droplet deposition on their dimension and density were investigated. The surface morphology of InGaAs nanoholes as well as their depth profile was examined by atomic force microscopy (AFM). The square shape is oriented along [110] and [110] crystallographic directions with slightly different profiles due to anisotropy behavior. The size uniformity of the squarelike nanoholes is well controlled with less deviation at higher substrate temperatures.

Molecular beam epitaxy of GaNAs alloys with high As content for potential photoanode applications in hydrogen production

The authors have succeeded in growing GaN1−xAsx alloys over a large composition range (0<x<0.8) by plasma-assisted molecular beam epitaxy. The enhanced incorporation of As was achieved by growing the films with high As2 flux at low (as low as 100 °C) growth temperatures, which is much below the normal GaN growth temperature range. Using x-ray and transmission electron microscopy, they found that the GaNAs alloys with high As content x>0.17 are amorphous. Optical absorption measurements together with x-ray absorption and emission spectroscopy results reveal a continuous gradual decrease in band gap from ∼3.4 to <1 eV with increasing As content. The energy gap reaches its minimum of ∼0.8 eV at x∼0.8. The composition dependence of the band gap of the crystalline GaN1−xAsx alloys follows the prediction of the band anticrossing model (BAC). However, our measured band gap of amorphous GaN1−xAsx with 0.3<x<0.8 are larger than that predicted by BAC. The results seem to indicate that for this composition range the amorphous GaN1−xAsx alloys have short-range ordering that resembles random crystalline GaN1−xAsx alloys. They have demonstrated the possibility of the growth of amorphous GaN1−xAsx layers with variable As content on glass substrates.

Ordering of GaAs quantum dots by droplet epitaxy

AbstractStep‐edge‐induced ordering of GaAs QDs was achieved on GaAs(110) by droplet epitaxy in a lattice‐matched GaAs/AlGaAs system. After the growth of an AlGaAs barrier layer, step‐edges along the 〈113〉, 〈112〉, 〈111〉, and 〈221〉 directions were naturally formed. On these step‐edges, preferential nucleation of Ga droplets occurred, resulting in the formation of one‐dimensionally ordered Ga droplets. By irradiating intense As4 flux, we could form ordered GaAs QDs. Some of the arrays were longer than 1 μm and the QDs in the arrays were closely packed. After capping of the QDs followed by rapid thermal annealing, the ordered QD arrays exhibited strong photoluminescence emission. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth by Molecular Beam Epitaxy of GaNAs Alloys with High As Content for Potential Photoanode Applications in Hydrogen Production

AbstractWe have studied the low-temperature growth of GaNAs layers on sapphire substrates by plasma-assisted molecular beam epitaxy. We have succeeded in achieving GaN1-xAsx alloys over a large composition range by growing the films at temperature much below the normal GaN growth temperatures with increasing the As2 flux as well as Ga:N flux ratio. We found that the alloys with high As content x>0.1 are amorphous. Optical absorption measurements reveal a continuous gradual decrease of band gap from ˜3.4 eV to ˜1.4 eV with increasing As content. The energy gap reaches its minimum of ˜1.4 eV at the x˜0.6-0.7. For amorphous GaAsN alloys with x<0.3 the composition dependence of the band gap follows the prediction of the band anticrossing model developed for dilute alloys. This suggests that the amorphous GaN1-xAsx alloys have short-range ordering that resembles random crystalline GaN1-xAsx alloys. Such amorphous GaN1-xAsx alloys with tunable electronic structure may be useful as photoanodes in photo-electrochemical cells for hydrogen production.

The Influence of Surface Reconstruction on Arsenic Incorporation Processes During MBE of GaAs

The incorporation factors of arsenic (SAs) for As2 and As4 molecular flux during GaAs MBE were detected. It has been determined that SAs does not depend on the arsenic molecular forms in incident flux. The correlation between SAs and reconstruction state of GaAs(001) grown surface have been shown. The maximal values of SAs for GaAs(001)-(4×2) and - (2×4) surfaces are characterized by 0.42 and 0.72, respectively. The dependences of arsenic incorporation rate and SAs during GaAs MBE versus substrate temperature (Ts) and incident flux density were obtained

Room temperature midinfrared electroluminescence from InSb/InAs quantum dot light emitting diodes

Self-assembled InSb submonolayer quantum dots (QDs) in an InAs matrix have been grown by molecular beam epitaxy using Sb2 and As2 fluxes. The structures exhibit bright midinfrared photoluminescence up to room temperature. Intense room temperature electroluminescence with a peak at wavelength near 3.8 μm was observed from p-i-n light emitting diode structures containing ten InSb submonolayer QD sheets inserted within the InAs active region.

Reflectance‐anisotropy study of the dynamics of molecular beam epitaxy growth of GaAs and InGaAs on GaAs (001)

AbstractReflectance‐Anisotropy (RA) observations during the Molecular Beam Epitaxy (MBE) growth of zincblende semiconductors films were carried out using the E1 optical transition as a probe. We follow the kinetics of the deposition of GaAs and In0.3Ga0.7As on GaAs (001) at growth rates of 0.2 and 0.25 ML/s, respectively. During growth we used a constant As4 or As2 flux pressure of 5× 10–6 Torr. Clear RA‐oscillations were observed during growth with a period that nearly coincides with the growth period for a Ga‐As bilayer. RHEED was used as an auxiliary technique in order to obtain a correlation between RHEED and RA oscillations. On the basis of our results, we argue that RAS oscillations are mainly associated to periodic changes in surface atomic structure. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of As4 flux on the growth kinetics, structure, and optical properties of InAs∕GaAs quantum dots

We report the effects of variations in As4 growth flux on the evolution of molecular beam epitaxy grown InAs quantum dots (QDs) and their structures and optical properties. For InAs QDs grown under As-stable conditions, evaluated through photoluminescence and atomic force microscopy (AFM) measurements, it is evident that QD size increases with As4 pressure along with improvement in size uniformity. Furthermore, transmission electron microscopy measurements for InAs layers of critical thicknesses (∼1.7 ML) showed decreasing QD density with increasing As4 pressure accompanied by a strong reduction in photoluminescence (PL) integral intensity. These show that high As4 fluxes suppress InAs QD formation while the decreasing PL intensity seems to indicate cluster formation that features nonradiative recombination. AFM measurements show larger and denser QDs for samples grown at higher As4 pressures. These are explained on the basis of adatom condensation during surface cooling and the influence of As4 pressure on indium incorporation.

Arsenic-induced etched nanovoids on GaSb (100)

We describe in situ nanoscale etch-pit formation on GaSb (100) surfaces as a result of exposure to an As2 flux in molecular beam epitaxy. The pits form as a result of an Sb-displacement reaction that occurs between the GaSb substrate and the impinging As adatoms. The nanoscale surface features are highly crystallographic with a strong preference for {111} planes, similar to other etching techniques. Nanopit dimensions and density increase with As exposure time. For the 60 s exposure analyzed in this article, the pits vary in both size and shape with average dimensions ∼25 nm wide and 50–80 nm long and 10–70 nm deep, with density of ∼1×109/cm2. Subsequent GaAs overgrowth proceeds by a coalescence mechanism leaving interfacial nanovoids and finally highly planar bulk layers.

Self-assembled growth of ordered GaAs nanostructures

The authors have fabricated an ordered array of GaAs nanoislands using the network of misfit dislocations at the InAs∕GaAs(111)A interface. Ga nanoislands are preferably formed at compressively strained regions between buried dislocations. GaAs nanoislands fabricated by supplying As4 flux to the Ga islands also show a high degree of ordering, while simultaneous deposition of Ga and As results in random nucleation of GaAs nanoislands.

The effect of growth interruptions at the interfaces in epitaxially grown GaInAsSb/AlGaAsSb multiple-quantum-wells studied with high-resolution x-ray diffraction and photoluminescence

Molecular beam epitaxy has been used to grow GaInAsSb/AlGaAsSb multiple-quantum-well (MQW) structures. Growth has been interrupted at the interfaces between the wells and the barriers. During the growth interruptions, the interfaces have been exposed to Sbx(x=1, 2) and As2 fluxes. The structures have been studied using high-resolution x-ray diffraction (HRXRD) and photoluminescence (PL). The As content in the interface layers has been found to have a large impact on the HRXRD curves. The As content in the interface layers has been determined by simulation of HRXRD rocking curves. We also show how highly strained interfaces cause more satellite peaks to appear in HRXRD rocking curves. PL spectra show that interrupting growth at the interfaces between wells and barriers and exposing the interfaces to an Sb soak result in flatter interfaces.

Near room temperature droplet epitaxy for fabrication of InAs quantum dots

By using the droplet epitaxy method, we succeed in fabricating the InAs quantum dots (QDs) with the spatial density of 4×1010∕cm2 and an average lateral size of 20nm on GaAs (001) at the droplets deposition temperature of 50°C and subsequent annealing process under As4 flux. These QDs shows the intense photoluminescence even at room temperature indicating that high-quality InAs QDs can be fabricated by near room temperature droplets deposition and crystallization process by 400°C in situ annealing.

Surface dynamics during MBE growth of GaAs(001) monitored by in‐situ reflectance difference spectroscopy

We have used in-situ reflectance difference spectroscopy to observe changes in GaAs(001) surface reconstructions during homoepitaxy. With a constant As4 flux of 5 × 10−6 Torr and growth rates of 1.1 ML/s, the starting (2 × 4) surface exhibits a sudden decrease in the RD signal corresponding to a (3 × 1) reconstruction as simultaneously evidenced by RHEED observations. Furthermore, by varying Ga fluxes so as to control changes in surface reconstructions between c(4 × 4) and (2 × 4), we traced reproducible features that lead us to propose a possible physical origin based on strained islands, induced by surface reconstruction, in order to explain the experimental observations. This last assumption was tested by depositing some atomic layers of InGaAs on a (2 × 4) GaAs (001) where the change in strain is evident. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)