Modulated Molecular Beam Epitaxy Research Articles

Molecular beam epitaxial growth of AlSb/InAsSb heterostructures

Abstract AlSb/InAsSb heterostructures have been successfully grown on GaAs substrate by modulated molecular-beam epitaxy (MMBE). New shutter sequence has been presented and room temperature mobility of 16,170 cm 2 /V s has been achieved with our non-intentionally doped structures. With a view for optimization, we analyze variation of electron mobility induced by growth temperature and InAsSb thickness. By increasing growth temperature and thickness of InAsSb, improvement of electron mobility has been observed. With our optimized AlSb/InAsSb heterostructures, accurate control of composition in InAsSb alloy and reduced interface mixing have been confirmed by X-Ray diffraction and Raman spectroscopy measurements.

Structural properties of InAs/InAs1–xSbx type-II superlattices grown by molecular beam epitaxy

Strain-balanced InAs/InAs1−xSbx type-II superlattices (SLs) have been proposed for possible long-wavelength infrared applications. This paper reports a detailed structural characterization study of InAs/InAs1−xSbx SLs with varied Sb composition grown on GaSb (001) substrates by modulated and conventional molecular beam epitaxy (MBE). X-ray diffraction was used to determine the SL periods and the average composition of the InAs1−xSbx alloy layers. Cross-section transmission electron micrographs revealed the separate In(As)Sb/InAs(Sb) ordered-alloy layers within individual InAs1−xSbx layers for SLs grown by modulated MBE. For the SLs grown by conventional MBE, examination by high-resolution electron microscopy revealed that interfaces for InAs1−xSbx deposited on InAs were more abrupt, relative to InAs deposited on InAs1−xSbx: this feature was attributed to Sb surfactant segregation occurring during the SL growth. Overall, these results establish that strain-balanced SL structures with excellent crystallinity can be achieved with proper design (well thickness versus Sb composition) and suitably optimized growth conditions.

2 μm GaInAsSb/AlGaAsSb midinfrared laser grown digitally on GaSb by modulated-molecular beam epitaxy

Stimulated emission at 1.994 μm was demonstrated from an optically pumped, double quantum well, semiconductor laser that was digitally grown by modulated-molecular beam epitaxy. This “digital growth” consists of short period superlattices of the ternary GaInAs/GaInSb and GaAsSb/GaSb/AlGaSb/GaSb alloys grown by molecular beam epitaxy with the intent of approximating the band gaps of quaternary GaInAsSb and AlGaAsSb alloys in the active region and barriers of the laser, respectively. For a 50 μs pulse and a 200 Hz repetition rate, the threshold current density was 104 W/cm2 at 82 K. The characteristic temperature (T0) was 104 K, the maximum operating temperature was 320 K and the peak output power was 1.895 W/facet at 82 K with pumping power of 7.83 W.

Microstructural properties of superlattices and InAsxSb1 − x ordered alloys grown by modulated molecular beam epitaxy

Modulated molecular beam epitaxy (MMBE) has been demonstrated to be useful in controlling group-V alloys such as InAs x Sb 1-x and AlAs x Sb 1-x . Further studies of the MMBE grown InAs x Sb 1-x ordered alloys and InAs/InAs x Sb 1-x superlattices by using cross-sectional scanning tunneling microscopy show well defined interfaces between InAs x Sb 1-x ordered alloys and InAs. Clear composition modulation is observed in the InAs x Sb 1-x ordered alloy layers. Transmission electron diffraction and microscopy, X-ray diffraction, and low temperature photoluminescence experiments show no obvious sign of CuPt orderings. These results suggest that MMBE provides a possible means to bypass the ordering problem of InAs x Sb 1-x random alloys.

Atomic-scale structure of InAs/InAs1−xSbx superlattices grown by modulated molecular beam epitaxy

We present cross-sectional scanning tunneling microscopy (STM) studies of the atomic-scale structural and electronic properties of InAs/InAs1−xSbx superlattices grown by modulated molecular-beam epitaxy. In this technique the Group V composition is controlled by rapid modulation of the Group V beams rather than adjustment of the Group V flux ratios. A superlattice sample was grown at 475 °C consisting of 52 Å InAs1−xSbx alternating with 172 Å InAs for 30 periods on a GaSb (001) substrate. The InAs1−xSbx alloy layers consisted nominally of 7.8 Å InAs alternating with 5.2 Å InSb for four periods. X-ray diffraction was used to determine an average composition of InAs0.76Sb0.24 for the alloy layers. Constant-current STM images of the superlattice exhibit clear, electronically induced contrast between the InAs layers and InAs0.76Sb0.24 layers, and also reveal ordering within the InAs0.76Sb0.24 layers. Interfaces between the InAs layers and the InAs0.76Sb0.24 layers appear sharp, though there is evidence of more atomic intermixing at the InAs-on-InAs0.76Sb 0.24 interface. Significant variation in composition within individual InAs0.76Sb 0.24 alloy layers and apparent Sb incorporation from the InAs0.76Sb0.24 layers into the surrounding InAs layers are also observed.

Improved crystalline quality of AlAsxSb1−x grown on InAs by modulated molecular-beam epitaxy

This letter reports a dramatic improvement in crystalline quality of AlAs0.16Sb0.84 ordered alloys compared with random ones grown by modulated molecular-beam epitaxy (MMBE) on InAs substrates. The average As/Sb ratio in these ordered alloys is accurately controlled by the modulation of As2 and Sb2 beams. The surface morphology of the epilayers is mirrorlike to the naked eye and appears texture- and cluster-free under an electron scanning microscope. X-ray diffraction measurements reveal that the lattice-matched conditions for AlAsxSb1−x epilayers to InAs can be easily achieved by simply changing the As-shutter dutycycle. Distinct satellite peaks corresponding to the ordered structure with a period of about 11 Å are also clearly observed. Strong photoluminescence (PL) is observed from GaAs0.08Sb0.92/AlAs0.16Sb0.84 single quantum well structures up to room temperature under a very low photoexcitation (5 W/cm2). At 5 K the PL linewidth is 4.5 meV.

Structural characterizations of initial nucleation of gaas on si films grown by modulated molecular beam epitaxy

Structural characterizations by plan-view and high resolution cross-sectional transmission electron microscopy (TEM) on 150A thick GaAs on Si films grown by a modulated molecular beam (MMBE) technique and conventional molecular beam epitaxy (MBE) at 300° C are reported. From the coverage of Moire interference fringes on plan-view electron micrographs, it is found that the nucleation of GaAs deposited by MMBE is more two-dimensional than conventional MBE. High resolution cross-sectional TEM micrographs show that the interfacial defects consist almost solely of partial dislocations for both samples. A model is constructed to explain this phenomenon by considering the magnitude of strain relieved by each type of misfit dislocation relative to its energy of formation. The effect of thermal annealing on the interfacial defect structures of these samples is also reported.

Nucleation and Defect Structures of GaAs Films Grown on Reactive Ion Etched Si Substrates

ABSTRACTThe nucleation and interfacial defect structure of thin GaAs films grown on reactive ion etched Si substrates by normal molecular beam epitaxy (MBE) and modulated molecular beam epitaxy (MMBE) at 300°C were studied by plan view and high resolution cross-sectional electron microscopy (TEM). Plan view TEM micrographs show a pronounced three-dimensional (3-d) island type nucleation for the MBE grown sample. A high density of microtwins is also found in these nucleated islands from high resolution cross-sectional TEM micrographs. The 3-d nucleation and the interfacial twinning is suppressed however in the MMBE grown samples. The FWHM of the (400) Bragg peak for 3 μm thick GaAs on Si films shows a reduction of 60 arcseconds when the initial buffer layer is grown by MMBE as compared to normal MBE.

Photodetectors fabricated on heteroepitaxial GaAs/Si structures grown by molecular beam epitaxy

Photodetectors were fabricated on GaAs/Si epitaxial structures grown on high resistivity Si substrates by modulated molecular beam epitaxy. Our efforts were focused on enhancing the substrate surface quality and optimizing the deposition conditions during the early stages of growth of the GaAs on the Si substrates. The photodetector structures investigated consisted of a 300Å thick GaAs/AlAs nucleation layer, a 5-period GaAs (100Å)/AlAs(100Å) accomodation layer grown at 300°C, two GaAs/InGaAs strained layer superlatices separated by a GaAs spacer layer, and a 2μm undoped GaAs active photodetector layer, a 0.2μm n-Gaas and a n +-GaAs contact layer. Various photodetector configurations fabricated in these GaAs/Si material structures were investigated. The photodetector response measurements were made using a 840nm wavelength pulsed laser with a pulse width of 5ns and rise (t r) and fall (t f) times of 200ps. Typical rise and fall times of the photodetectors were in the 1–2 ns and 3–6 ns ranges, respectively. The responsivity and quantum efficiency values of these photodetectors were in the ranges of 0.5–1.0 A/W and 0.3–1.5, respectively.

Improvement in the crystalline quality of heteroepitaxial GaAs on Si films grown by modulated molecular beam epitaxy

GaAs films were grown on Si substrates by a two-step method in which a thin buffer layer was first deposited using a modulated molecular beam epitaxy (MBE) technique followed by a thick layer grown by conventional molecular beam epitaxy. The film quality was evaluated using 77 K photoluminescence (PL) and double-crystal x-ray rocking curves. It was found that GaAs films grown in this way have a superior crystalline quality compared to the films prepared by normal two-step MBE. To investigate the nucleation of this buffer layer, thin GaAs layers (150 Å) were grown on Si substrates and examined by plan-view and cross-sectional transmission electron microscope. A thin, two-dimensional nucleation pattern was found in this sample, in clear contrast to the three-dimensional nucleation islands found in samples grown by conventional MBE. This showed conclusively that modulated beam molecular beam epitaxy resulted in a more uniform and two-dimensional nucleation during the initial stage of the growth as speculated earlier and is believed to be the reason for the improvement of the crystalline quality of the overgrown layer.

Initial Nucleation Studies of Heteroepitaxial GaAs films on Si Substrates by Modulated Molecular Beam Epitaxy

AbstractNominal 100 Å and 150 Å thick GaAs layers were deposited on Si substrates by a modulated molecular beam technique and normal molecular beam epitaxy (MBE) at 300 °C and 375 °C respectively for plan view and cross-sectional transmission electron microscopy (TEM) examinations. From coverage of Moire fringes, it is found that the nucleated GaAs films grown by the modulated molecular beam technique were thinner, streaker and more two-dimensional than the MBE grown films. The same modulated molecular beam technique was also used for the deposition of the initial buffer layer of 3µm thick GaAs on Si films. Results from the 77k photoluminescence(PL) and double crystal X ray diffraction measurements showed that these films have superior optical and structural quality compared to similar films grown by normal two-step MBE. The improvement is attributed to a more two-dimensional nucleation of GaAs films associated with the modulated molecular beam growth technique.