Gas Source Molecular Beam Epitaxy Research Articles

Effects of plasma species on the N incorporation of GaAsSbN grown by plasma-assisted gas-source molecular-beam epitaxy

We report the effects of plasma species on the N incorporation of GaAsSbN. Optical emission spectroscopy and quadruple mass spectroscopy were used to characterize the plasma source. We found a simple correlation between the atomic N species and meta-stable molecular N2⁎ species that is independent of plasma power and N2 flow rate. In order to achieve atomic-N-rich growth conditions, we place a PBN shutter in front of our plasma source, rich in meta-stable N2* molecules, to facilitate the relaxation of N2* and turn the growth condition into an atomic-N-dominant one. When an atomic-N-rich condition is used, N incorporation rate decreases when Sb flux increases and increases as growth temperature increases. This behavior is well explained by a surface kinetics model. When a N2*-rich condition is used, the N incorporation in GaAsSbN is enhanced by increase in Sb flux and growth temperature.

Gas source MBE grown Al0.52In0.48P photovoltaic detector

Al0.52In0.48P photovoltaic detector has been fabricated using gas source molecular beam epitaxy and its characteristics have been measured in detail. The Al0.52In0.48P absorption layer shows a mismatch of +4.76×10−4 to the GaAs substrate with a full width at half maximum of 22.5arcsec. The dark current is only 0.78pA at reverse bias of 500mV and the R0A is about 1.7×108Ωcm2 at room temperature. The measured peak responsivity at zero bias is 0.287A/W at 470nm with 50% cut-on and cut-off wavelengths at 440 and 495nm, respectively.

Watt level performance of quantum cascade lasers in room temperature continuous wave operation at λ∼3.76 μm

An InP-based quantum cascade laser heterostructure emitting at 3.76 μm is grown with gas-source molecular beam epitaxy. The laser core is composed of strain balanced In0.76Ga0.24As/In0.26Al0.74As. Pulsed testing at room temperature exhibits a low threshold current density (1.5 kA/cm2) and high wall plug efficiency (10%). Room temperature continuous wave operation gives 6% wall plug efficiency with a maximum output power of 1.1 W. Continuous wave operation persists up to 95 °C.

InAs/GaAs quantum dot lasers grown by gas-source molecular-beam epitaxy

We report on the InAs/GaAs quantum dot (QD) lasers grown by gas-source molecular beam epitaxy. Ridge waveguide lasers were processed on the grown structure consisting of five-stacked InAs QD layers formed by InAs layers with slightly different thicknesses. Continuous-wave operation was achieved up to 80 °C and more than 50 mW optical power was collected from one facet at 20 °C. The characteristic temperature of the QD laser was measured as high as infinity in the temperature range of 80–185 K. In addition, the lasing lines covered the wavelength window as wide as 26 nm at 20 °C and 52 nm at 80 K, respectively. By adjusting the cavity length, the lasing wavelength can be tuned in the range of 1.05 – 1.10 μ m . Both of the broad tuning wavelength and the wide lasing spectrum indicate that the QD lasers have very broad gain profile, which is highly desirable for making widely tunable lasers.

Optical study of GaAs 1− xSb x layers grown on GaAs substrates by gas-source molecular beam epitaxy

The optical properties of GaAs 1− x Sb x (5.9% ≤ x ≤ 9.7%) layers grown on GaAs substrates by gas-source molecular beam epitaxy were characterized by photoreflectance (PR) and piezoreflectance (PzR). Identification of conduction to heavy-hole (HH) band and conduction to light-hole (LH) band transitions originated from the strained induced valence band splitting have been achieved by comparing the relative intensities of PR and PzR spectra. The results indicate increases of the valence band splitting with increasing of Sb content. The temperature dependences of near band edge transition energies were analyzed using Varshni and Bose–Einstein expressions in the temperature range from 15 to 300 K. The parameters that describe the temperature variations of the near band edge transition energies and broadening function were evaluated and discussed.

Infrared study of the absorption edge of β-InN films grown on GaN/MgO structures

Infrared optical studies were carried out in a group of cubic InN samples grown by gas source molecular beam epitaxy on MgO (001) substrates. Room temperature (RT) reflectance and low-temperature (LT) transmittance measurements were performed by using fast Fourier transform infrared spectrometry. Reflectance fittings allowed to establish that β-InN films have large free-carrier concentrations present (>1019 cm−3), a result that is corroborated by Hall effect measurements. Each sample explored exhibited a different optical absorption edge. The Varshni parameters that describe adequately the optical absorption edge responses with temperature are obtained for the set of samples studied. The observed temperatures changes, from LT to RT, are the lowest reported for III-V semiconductor binary compounds. The temperature coefficient of the conduction band depends on the strength of the electron–phonon interaction (e-ph-i), as well as on the thermal expansion. It has been predicted that cubic InN has one of the smallest e-ph-i of all III-V compounds, which is corroborated by these results. The variation in values of absorption edges is clearly consistent with the Burstein–Moss and band renormalization effects, produced by high free electron concentrations. It is shown that the conduction band in β-InN, analogous to wurtzite InN, follows a nonparabolic behavior.

InP-based InGaAs/InAlGaAs digital alloy quantum well laser structure at 2 μm

The structural and optical characteristics of InP-based compressively strained InGaAs quantum wells have been significantly improved by using gas source molecular beam epitaxy grown InAs/In0.53Ga0.47As digital alloy triangular well layers and tensile In0.53Ga0.47As/InAlGaAs digital alloy barrier layers. The x-ray diffraction and transmission electron microscope characterisations indicate that the digital alloy structures present favourable lattice quality. Photo-luminescence (PL) and electroluminescence (EL) measurements show that the use of digital alloy barriers offers better optical characteristics than that of conventional random alloy barriers. A significantly improved PL signal of around 2.1 μm at 300 K and an EL signal of around 1.95 μm at 100 K have been obtained.

GaInP–AlInP–GaAs Blue Photovoltaic Detectors With Narrow Response Wavelength Width

Ga0.51In0.49P-Al0.52In0.52In0.48P-GaAs blue photovoltaic detectors using AlInP as a light absorption active layer and GaInP as a short wavelength limiting cap layer have been fabricated by using gas source molecular beam epitaxy, and their performances have been characterized in detail. Excellent performance has been demonstrated on the detectors, resistance area product ofR 0 A = 6.9 × 108 ? · cm2, and the open-circuit voltage of V oc > 1.28 V have been measured at room temperature. The detectors show peak response at 480 nm with responsivity of 0.168 A/W at zero bias, as well as an inherent narrow wavelength width of 9.4%, which makes them a good candidate in blue light detection applications such as water related sensing and communication.

Critical thickness of β-InN/GaN/MgO structures

InN films were grown on MgO substrates with a β-GaN buffer layer using the gas source molecular beam epitaxy technique. Initially, at typical growth rates from 0.09 to 0.28 ML/sec and at 500 °C substrate temperature, the growth was performed in a layer by layer way as revealed by in situ reflection high-energy electron diffraction (RHEED). In all samples studied, a critical thickness of ∼5 ML in InN pseudomorphic layer was measured with a frame by frame analysis of RHEED patterns recorded on video. After reaching critical thickness, the InN films undergo a relaxation process, going from two-dimensional growth to three-dimensional, as evidenced by the transformation of the RHEED patterns that change from streaky to spotty. Depending on the In cell temperature, either nanocolumnar InN or flat cubic final films are grown, as can be corroborated by scanning electron microscopy. The experimental critical thickness (hc) value of 5 ML is compared to values calculated from different critical thickness models.

Thin Graphitic Structure Formation on Various Substrates by Gas-Source Molecular Beam Epitaxy Using Cracked Ethanol

We have studied the growth of graphene by gas-source molecular beam epitaxy, with a cracked-ethanol source. Three substrates – Si substrate with thin oxide layer, c-surface of sapphire, and graphene layers formed on SiC – were used to draw out clues to the optimum conditions for graphene growth. Raman spectroscopy and X-ray photoelectron spectroscopy analyses indicate that hardly any or fairly small amounts of graphene are formed on the Si and sapphire, although thin films of graphitic material can be formed. We estimated nominal growth rates on the three substrates and found that the growth rate on the graphene was much smaller than those on the other two substrates. We studied the growth process of graphene on graphene by reflection high-energy electron diffraction and found that two-dimensional graphene islands grew laterally but their lateral orientations were not aligned.

Dependence of InGaP nanowire morphology and structure on molecular beam epitaxygrowth conditions

InGaP nanowires (NWs) were grown by the Au-assisted method in a gas source molecularbeam epitaxy system. The dependence of InGaP composition, morphology and stackingfault density was studied with respect to group III and V impingement rate and size of theAu particle. Compositional analysis showed that the NWs had an In-rich core and aGa-rich shell structure. The In incorporation within the NW became limited as the Au seedparticle size diminished or the group III and V flux decreased. The NWs had wurtzite (WZ)crystal structure with zinc blende (ZB) segments (stacking faults). The density of thestacking faults decreased as the group III flux decreased and the group V flux increased.

Effect of barrier layer composition and thickness on structural and optical properties of TlInGaAsN/TlGaAs(N) triple quantum wells

Influence of the barrier layer composition and thickness on the structural and optical properties of TlInGaAsN Triple quantum wells (TQWs) was studied. Three types of TlInGaAsN TQW structures with different barriers were grown by gas-source molecular-beam epitaxy. Strong photoluminescence emission was obtained at room temperature from the TlInGaAsN TQW samples having 10 nm-thick TlGaAsN barriers, compared with the TlInGaAsN TQWs of 26 nm-thick TlGaAs barriers and those of 30 nm-thick TlGaAsN barriers. Structural investigations revealed that the TQWs with 10 nm-thick TlGaAsN barriers have good structural qualities. On the other hand, the other two samples showed the composition modulations at the interface between the lower side of the 3rd QW layer and the barrier layer. It was found that the addition of nitrogen into barrier layers and the decrease of barrier layer thickness significantly improve the crystalline quality and in turn the luminescence properties of the TlInGaAsN TQWs.

Effect of V/III flux ratio on luminescence properties and defect formation of Er-doped GaN

Erbium-doped GaN samples grown with different V/III ratios through gas source molecular beam epitaxy were prepared to investigate the influence of the V/III ratio on the defect formation and the optical activity of the Er-related luminescence center. Obvious V/III ratio dependence was observed in photoluminescence measurement. Positron annihilation spectroscopy measurements suggested that VGa-VN vacancy-complexes formed in these samples and that the VN/VGa ratio depended on the V/III ratio. The generation of Er-VN defect complexes, which act as high optical active luminescence centers, is suggested as the cause of improved optical properties of Er-doped GaN grown with a lower V/III ratio.

IMPROVING THE PERFORMANCE OF EXTENDED WAVELENGTH InGaAs PHOTODETECTORS BY USING DIGITAL GRADED HETEROINTERFACES SUPERLATTICE

The materials of extended wavelength InxGa1-xAs photodetectors(50% cut-off wavelength of 2.4μm at room temperature) with three different structures were grown by using gas source molecular beam epitaxy(GSMBE) and were processed into mesa type photodetectors.Surface morphology,x-ray diffraction rocking curve and photoluminescence measurements show that the quality of materials is obviously improved by using digital graded superlattice at the InAlAs/InGaAs heterointerfaces.The dark current at reverse bias of 10mV for the 300μm-diameter mesa type photodetectors without digital graded superlattice is 0.521μA at room temperature,however it is reduced to 0.480μA for photodetectors with digital graded superlattice.Besides,the growth of an InP buffer layer between InP substrate and InxAl1-xAs linear graded buffer layer is also beneficial to the material quality and device performance.

Device fabrication of semi-insulating surface-plasmon terahertz quantum-cascade lasers

The active region of GaAs/AlGaAs bound-to-continuum terahertz quantum-cascade laser （THz QCL） is grown by gas-source molecular beam epitaxy. The device fabrication process of semi-insulating surface-plasmon THz QCL is studied in detail. The electrical and optical characteristics of the fabricated THz QCL device are measured using a far-infrared Fourier transform infrared spectrometer with a deuterated triglycerine sulfate far-infrared detector. At 10 K，the measured lasing frequency is 32 THz and the threshold current density is 275 A/cm2.

Structural and optical analysis of GaAsP/GaP core-shell nanowires

The structural and optical properties of GaAsP/GaP core-shell nanowires grown by gas source molecular beam epitaxy were investigated by transmission electron microscopy, Raman spectroscopy, photoluminescence (PL), and magneto-PL. The effects of surface depletion and compositional variations in the ternary alloy manifested as a redshift in GaAsP PL upon surface passivation, and a decrease in redshift in PL in the presence of a magnetic field due to spatial confinement of carriers.

Ultrathin film, high specific power InP solar cells on flexible plastic substrates

We demonstrate ultrathin-film, single-crystal InP Schottky-type solar cells mounted on flexible plastic substrates. The lightly p-doped InP cell is grown epitaxially on an InP substrate via gas source molecular beam epitaxy. The InP substrate is removed via selective chemical wet-etching after the epitaxial layers are cold-welded to a 25 μm thick Kapton® sheet, followed by the deposition of an indium tin oxide top contact that forms the Schottky barrier with InP. The power conversion efficiency under 1 sun is 10.2±1.0%, and its specific power is 2.0±0.2 kW/kg. The ultrathin-film solar cells can tolerate both tensile and compressive stress by bending over a <1 cm radius without damage.

Distinction investigation of InGaAs photodetectors cutoff at 2.9 μm

Using double heterojunction structure with linearly graded In x Al 1– x As as buffer layer and In 0.9Al 0.1As as cap layer, wavelength extended In 0.9Ga 0.1As detectors with cutoff wavelength of 2.88 μm at room temperature have been grown by using gas source molecular beam epitaxy, their characteristics have been investigated in detail and compared with the detectors cutoff at 2.4 μm with similar structure as well as commercial InAs detectors. Typical resistance area product R 0 A of the detectors reaches 3.2 Ω cm 2 at 290 K. Measured peak detectivity reaches 6.6E9 cm Hz 1/2/W at room temperature.

Impedance spectroscopy characterization of GaAs nanowire bundles grown by metal-catalyzed molecular beam epitaxy

Vertically aligned GaAs nanowire (NW) bundles grown by gas-source molecular beam epitaxy on an n-doped GaAs substrate by a metal catalysis method and embedded in an insulating matrix (SU8-2) were studied by impedance spectroscopy. The DC current–voltage characteristics measured between Au dot contacts to the NW tips and the substrate exhibited Schottky behavior. A detailed analysis of the impedance data measured in reverse bias conditions is presented, which enables the elimination of the stray capacitance due to the insulating matrix, and the separation of the different contributions to the total admittance from the metal/NW Schottky interface and from the NW region beyond the barrier region. The observed NW conductances and capacitances are shown to be consistent with rough estimates based on the GaAs conductivity and permittivity data and the NW dimensions, and the NW conductance increases as a power law of the frequency. Possible charge transport mechanisms to explain this result are discussed.

High quality AlN for deep UV photodetectors

We have prepared large-area, 0.50×0.55mm2, metal-semiconductor-metal photodetectors based on AlN layers with different density of inversion domains (IDs). AlN layers were grown on (0001) sapphire substrates using gas source molecular beam epitaxy. The introduction of AlN∕GaN short period superlattices after growth of AlN nucleation layer yields significant reduction in the ID density. Photodetectors with ID density of 106cm−2 exhibit a very low dark current of 0.5fA at zero bias, which remains below 50fA up to a bias of ±30V. The peak responsivity of 0.08A∕W was obtained at a wavelength of ∼202nm.