High Quality Molecular Beam Epitaxy Research Articles

Soliton mode-locked fiber laser with high-quality MBE-grown Bi2Se3 film

In this work, a soliton mode-locked erbium-doped fiber laser (EDFL) with a high-quality molecular beam epitaxy (MBE)-grown topological insulator (TI) Bi2Se3 saturable absorber (SA) is reported. To fabricate the SA device, a 16-layer Bi2Se3 film was grown successfully on a 100 μm thick SiO2 substrate and sandwiched directly between two fiber ferrules. The TI-SA had a saturable absorption of 1.12% and a saturable influence of 160 MW/cm2. After inserting the TI-SA into the unidirectional ring-cavity EDFL, self-starting mode-locked soliton pulse trains were obtained at a fundamental repetition rate of 19.352 MHz. The output central wavelength, pulse energy, pulse duration, and signal to noise ratio of the radio frequency spectrum were 1530 nm,18.5 pJ, 1.08 ps, and 60 dBm, respectively. These results demonstrate that the MBE technique could provide a controllable and repeatable method for the fabrication of identical high-quality TI-SAs, which is critically important for ultra-fast pulse generation.

Quantum size effect on charges and phonons ultrafast dynamics in atomically controlled nanolayers of topological insulators Bi2Te3

Heralded as one of the key elements for next generation spintronics devices, topological insulators (TIs) are now step by step envisioned as nanodevices like charge-to-spin current conversion or as Dirac fermions based nanometer Schottky diode for example. However, reduced to few nanometers, TIs layers exhibit a profound modification of the electronic structure and the consequence of this quantum size effect on the fundamental carriers and phonons ultrafast dynamics has been poorly investigated so far. Here, thanks to a complete study of a set of high quality molecular beam epitaxy grown nanolayers, we report the existence of a critical thickness of around ~6 nm, below which a spectacular reduction of the carrier relaxation time by a factor of ten is found in comparison to bulk Bi2 Te3 In addition, we also evidence an A1g optical phonon mode softening together with the appearance of a thickness dependence of the photoinduced coherent acoustic phonons signals. This drastic evolution of the carriers and phonons dynamics might be due an important electron-phonon coupling evolution due to the quantum confinement. These properties have to be taken into account for future TIs-based spintronic devices.

Ultrafast photoresponse and enhanced photoresponsivity of Indium Nitride based broad band photodetector

InN direct band gap semiconductor is a promising material in the nitride family for high power and high frequency optoelectronic devices. However, the reports on photo-sensing ability of the material are limited with photoresponsivity < 1A/W only. Here, we report fast photoresponse from high quality molecular beam epitaxy grown InN islands delivering photoresponsivity of 13.5A/W, about 30 times higher than the recently reported InN based photodetector operating in the near infrared spectral range. The ultra-broadband response with high photoresponsivity from visible to near infrared spectral range is experimentally demonstrated. To the best of our knowledge, this study presents the working of a photodetector having fast response (38μs) and high photo detectivity (5.5 × 1010W-1Hz1/2cm) operating at room temperature. The device yields a quiet prompt saturation and decay under periodic illumination which demonstrate excellent stability and reliability of the device with switching time. The sub-linear dependent photocurrent on the bias voltage and incident power offers good tunability for multipurpose applications. This is the first report on ultra-broadband spectral range of InN based photodetectors that open up opportunities for developing the next generation high efficiency photodetectors.

Surface stability and the selection rules of substrate orientation for optimal growth of epitaxial II-VI semiconductors

The surface structures of ionic zinc-blende CdTe (001), (110), (111), and (211) surfaces are systematically studied by first-principles density functional calculations. Based on the surface structures and surface energies, we identify the detrimental twinning appearing in molecular beam epitaxy (MBE) growth of II-VI compounds as the (111) lamellar twin boundaries. To avoid the appearance of twinning in MBE growth, we propose the following selection rules for choosing optimal substrate orientations: (1) the surface should be nonpolar so that there is no large surface reconstructions that could act as a nucleation center and promote the formation of twins; (2) the surface structure should have low symmetry so that there are no multiple equivalent directions for growth. These straightforward rules, in consistent with experimental observations, provide guidelines for selecting proper substrates for high-quality MBE growth of II-VI compounds.

Magnetic anisotropy in ultrathin Fe films on GaAs, ZnSe, and Ge (001) substrates

We discuss magnetic anisotropy parameters of ferromagnetic body-centered cubic (bcc) Fe films grown by molecular beam epitaxy (MBE) on (001) substrates of face-centered cubic (fcc) GaAs, ZnSe, and Ge. High-quality MBE growth of these metal/semiconductor combinations is made possible by the fortuitous atomic relationship between the bcc Fe and the underlying fcc semiconductor surfaces, resulting in excellent lattice match. Magnetization measurements by superconducting quantum interference device (SQUID) indicate that the Fe films grown on (001) GaAs surfaces are characterized by a very strong uniaxial in-plane anisotropy; those grown on (001) Ge surfaces have a fully cubic anisotropy; and Fe films grown on ZnSe represent an intermediate case between the preceding two combinations. Ferromagnetic resonance measurements carried out on these three systems provide a strikingly clear quantitative picture of the anisotropy parameters, in excellent agreement with the SQUID results. Based on these results, we propose that the observed anisotropy of cubic Fe films grown in this way results from the surface reconstruction of the specific semiconductor substrate on which the Fe film is deposited. These results suggest that, by controlling surface reconstruction of the substrate during the MBE growth, one may be able to engineer the magnetic anisotropy in Fe, and possibly also in other MBE-grown ferromagnetic films.

Analysing Surface Structures on (Ga, Mn)As by Atomic Force Microscopy

Using atomic force microscopy, we have studied the surface structures of high quality molecular beam epitaxy grown (Ga, Mn)As compound. Several samples with different thickness and Mn concentration, as well as a few (Ga, Mn)(As, P) samples have been investigated. All these samples have shown the presence of periodic ripples aligned along the [110] direction. From a detailed Fourier analysis we have estimated the period (-50 nm) and the amplitude of these structures.

Acoustic properties of nanoscale oxide heterostructures probed by UV Raman spectroscopy

We study high quality molecular-beam epitaxy grown BaTiO3/SrTiO3 superlat-tices using ultraviolet Raman spectroscopy. In the low energy spectral region, acoustic phonon doublets are observed. These are due to the artificial superlattice periodicity and consequent folding of the acoustic phonon dispersion. From the study of samples with different BaTiO3/SrTiO3 layer thicknesses the effective sound velocities within each of the layers are obtained.

Substrate transfer process for InP-based heterostructure barrier varactor devices

We report on transferred-substrate InP-based heterostructure barrier varactor (HBV) devices. Individual HBV devices having active device diameters ranging from 10 to 40 μm have been fabricated on a SiO2 glass host substrate following a novel epitaxial layer transfer technique. This process utilizes the negative photoresist SU-8 as a photopatternable intermediate bonding layer and could be extended to other InP-based devices. The transferred HBV device characteristics have been measured and compared to devices which have been fabricated on InP. The transferred devices exhibited highly symmetrical electrical characteristics due to the preservation of the high quality molecular beam epitaxy layers during the transfer process. A leakage current of 4.5 A/cm2 at 10 V together with an associated conductance value of 40 nS/μm2 were observed. The rf measurements revealed a zero bias capacitance of 1 fF/μm2 and a capacitance ratio of 5:1.

Nanoscale magnetic domain structures in epitaxial cobalt films.

We have analyzed the magnetic domain structure in high quality molecular beam epitaxy (MBE) grown (0001)-hcp cobalt films of thicknesses varying from 10 to 500 nm for which the magnetic domain width is of the order of the film thickness. As the thickness of the film is increased, the magnetization turns from fully in plane (10 nm) to fully out of plane (50 nm) over a relatively large thickness range. Above 50 nm, the local domain imaging by magnetic force microscopy (MFM) together with overall magnetization measurements confirm the characteristic multidomain structure with perpendicular orientation predicted by Kittel. The periodicity of domains is then described by a \ensuremath{\surd}d law (where d is the film thickness) that leads to the determination of domain wall energies. The occurrence of magnetic bubbles and periodic stripe patterns is visualized by variable field MFM. \textcopyright{} 1996 The American Physical Society.

High quality Al(Ga)As/GaAs/Al(Ga)As quantum wells grown on (111)A GaAs substrates

We report the growth of high quality molecular beam epitaxy (MBE) AlGaAs, AlGaAs/GaAs/AlGaAs, and AlAs/GaAs/AlAs multiple quantum wells (MQWs) on (111)A GaAs substrates. For (111)A AlGaAs/GaAs/AlGaAs MQWs, there is no detectable photoluminescence (PL) at a growth temperature of 640 °C, and narrow PL linewidth can only be obtained at growth temperatures higher than 680 °C. A PL linewidth of 13.8 meV is measured at the growth temperature of 720 °C. To understand such growth temperature dependence of (111)A MQWs, we have investigated the material quality of (111)A AlGaAs at different growth temperatures. The strong PL integrated intensity of 640 °C grown (111)A AlGaAs indicates good material quality and a low concentration of non-radiative recombination centers. However, the broad PL linewidth of 640 °C grown (111)A AlGaAs indicates the strong compositional modulation and a rough growth front. We have used AlAs instead of AlGaAs in order to reduce the compositional modulation and smooth the interface. A PL linewidth of 13.4 meV is measured for 640 °C grown (111)A AlAs/GaAs/AlAs MQWs, which is the narrowest value for (111)A MQWs.

Chemical beam epitaxial growth of high optical quality AlGaAs — the influence of precursor purity on material properties

The influence of source precursor purity on the quality of resulting chemical beam epitaxy (CBE) grown AlGaAs epilayers has been investigated. A close correlation has been established between the presence of trace quantities of diethyl ether in a precursor and consequent oxygen contamination of AlGaAs. Careful selection and purification of the precursors to reduce ether contamination has generated significant improvements in both the optical and electrical properties of CBE-grown AlGaAs, such that it is now directly comparable with high quality molecular beam epitaxy (MBE) and metalorganic vapour phase epitaxy (MOVPE) grown material.

Chemical beam epitaxial growth of high optical quality AlGaAs - The influence of precursor purity on material properties

The influence of source precursor purity on the quality of resulting chemical beam epitaxy (CBE) grown AlGaAs epilayers has been investigated. A close correlation has been established between the presence of trace quantities of diethyl ether in a precursor and consequent oxygen contamination of AlGaAs. Careful selection and purification of the precursors to reduce ether contamination has generated significant improvements in both the optical and electrical properties of CBE-grown AlGaAs, such that it is now directly comparable with high quality molecular beam epitaxy (MBE) and metalorganic vapour phase epitaxy (MOVPE) grown material.

Molecular beam epitaxy growth of lattice-matched [formula omitted] multiple quantum well distributed feedback laser structures with gratings defined by implantation enhanced intermixing

AlGaInAs GaInAs multiple quantum wells (MQWs) are particularly suited for planar device integration using local refractive index modification by masked implantation enhanced intermixing (MIEI). Suitable molecular beam epitaxy (MBE) growth conditions are elaborated to achieve 1.3 μm AlGaInAs GaInAs MQW distributed feedback (DFB) lasers taking advantage of the capability of the MIEI process to define the gratings. High-quality MBE overgrowth on AlGaInAs necessary for device completion after H ∗ radical treatment is demonstrated.

High quality (111)B GaAs, AlGaAs, AlGaAs/GaAs modulation doped heterostructures and a GaAs/InGaAs/GaAs quantum well

We report the successful growth of high quality molecular beam epitaxy (MBE) GaAs, AlGaAs, AlGaAs/GaAs modulation doped heterostructures and a GaAs/InGaAs/GaAs quantum well on GaAs (111)B substrates. Modulation doped heterostructures show a 77 K mobility of 145 500 cm2/V s with a sheet density of 5.0×1011 cm−2. Photoluminescence of (111)B GaAs indicates a lower carbon incorporation than achieved on (100) substrates. The low growth temperature and high material quality obtainable in (111)B growth will provide advantages for laser diodes and heterostructure field effect transistors.

Molecular Beam Epitaxy: Thin Film Growth and Surface Studies

Molecular Beam Epitaxy (MBE) is a thin film deposition process in which thermal beams of atoms or molecules react on the clean surface of a single-crystalline substrate, held at high temperatures under ultrahigh vacuum conditions, to form an epitaxial film. Thus, contrary to the CVD processes described in the other articles, the MBE process is a physical method of thin film deposition.The vacuum requirements for the MBE process are typically better than 10−10torr. This makes it possible to grow epitaxial films with high purity and excellent crystal quality at relatively low substrate temperatures. Additionally, the ultrahigh vacuum environment allows the study of surface, interface, and bulk properties of the growing film in real time, by employing a variety of structural and analytical probes.Although the MBE deposition process was first proposed by Günther in 1958, its implementation had to wait for the development of the ultrahigh vacuum technology. In 1968 Davey and Pankey successfully grew epitaxial GaAs films by the MBE process. At the same time Arthur's work on the kinetics of GaAs growth laid the groundwork for the growth of high quality MBE films of GaAs and other III-V compounds by Arthur and LePore and Cho.

Time-resolved photoluminescence of the defect lines in GaAs molecular-beam epitaxy

A time-resolved photoluminescence study of the defect-induced bound excitons (DIBE) observed in the energy region 1504–1511 meV in high quality molecular-beam epitaxy grown GaAs has been performed. Our results support the model that these lines consist of two distinct recombination processes. Transient resonant excitation of these DIBE demonstrate that the lines i to s can be associated with acceptor – bound exciton transitions.

High quality molecular beam epitaxial growth on patterned GaAs substrates

In this letter we describe a procedure for high quality molecular beam epitaxy (MBE) growth over finely patterned GaAs substrates which is suitable for device fabrication requiring lateral definition of small (∼1–2 μm) dimension. This method was used for the fabrication of index guided laser arrays. Yields of individual lasers exceeded 90%, and thresholds were uniform to 10%. Temperature and flux ratio dependence of faceting during MBE growth over patterned substrates is shown for temperatures ranging from 580 to 700 °C and for As/Ga flux ratios from 1.4:1 to 4:1. The real index guided structure, which can be formed by a single MBE growth over a ridged substrate, is discussed. This technique should prove useful in the fabrication of devices which take advantage of unique features formed during regrowth by MBE.

Growth of III–V semiconductors by molecular beam epitaxy and their properties

Advances in solid state device technology in the sixties established III–V materials as a new class of semiconductors for high speed microwave and highly efficient optical devices. Molecular beam epitaxy (MBE) is an extremely versatile thin film technique which can produce single-crystal layers with atomic dimensional controls and thus permit the preparation of novel structures and devices tailored to meet specific needs. Important factors to achieve high quality MBE growth such as in situ analysis, substrate preparations, growth conditions and layer properties are discussed.