Laser Molecular Beam Epitaxy Research Articles

The Effect of Mn Dopant on Structural and Optoelectronic Properties of γ-Ga2O3 thin Film Photodetectors

The metastable cubic γ-(Ga1−xMnx)2O3 thin films are successfully obtained at high growth temperature by laser molecular beam epitaxy technology. The optoelectronic properties of the solar blind Schottky-type photodetectors (PDs) based on γ-(Ga1−xMnx)2O3 thin films are reported for the first time. In this experimental system, the γ-(Ga0.96Mn0.04)2O3 PD exhibits the highest light-to-dark ratio (LDR) of 6.89 × 103, which is two orders of magnitude higher than the pure β-Ga2O3 PD prepared under the same condition. In addition, it shows a fast photoresponse decay speed of about 0.081 s. The results suggest that Mn element is expected to be one of the promising dopants to induce and stabilize the metastable γ-Ga2O3, as well as optimize the optoelectronic performance of photodetectors.

Epitaxial growth and determination of the band alignment for NixMg1-xO/MgO interface by laser molecular beam epitaxy

By laser molecular beam epitaxy, single crystalline NixMg1-xO films have been successfully synthesized on MgO(100) surface. The in situ reflection high-energy electron diffraction patterns show that the induced O2 background gas with at least 1.0 × 10−3 Pa is necessary to the epitaxial growth of NixMg1-xO films. The X-ray diffraction patterns reveal the single-phase growth along (200) direction. The energy band alignment at the interface is investigated by employing in situ X-ray photoelectron spectroscopy. The valence band offsets are determined to be from 1.47 eV to 1.50 eV with the decrease of Ni content (0.39 → 0.35). Furthermore, the work function is evaluated by using in situ ultraviolet photoemission spectroscopy, indicating the values from 4.33 eV to 4.64 eV. This study provides a promising guidance for the solar-blind device design and fabrication.

Real and Fitted Spherical Indentations

Spherical indentations that rely on original date are analyzed with the physically correct mathematical formula and its integration that take into account the radius over depth changes upon penetration. Linear plots, phase-transition onsets, energies, and pressures are algebraically obtained for germanium, zinc-oxide and gallium-nitride. There are low pressure phase-transitions that correspond to, or are not resolved by hydrostatic anvil onset pressures. This enables the attribution of polymorph structures, by comparing with known structures from pulsed laser deposition or molecular beam epitaxy and twinning. The spherical indentation is the easiest way for the synthesis and further characterization of polymorphs, now available in pure form under diamond calotte and in contact with their corresponding less dense polymorph. The unprecedented results and new possibilities require loading curves from experimental data. These are now easily distinguished from data that are “fitted” to make them concur with widely used unphysical Johnson’s formula for spheres (“P = (4/3)h3/2R1/2E∗”) not taking care of the R/h variation. Its challenge is indispensable, because its use involves “fitting equations” for making the data concur. These faked reports (no “experimental” data) provide dangerous false moduli and theories. The fitted spherical indentation reports with radii ranging from 4 to 250 μm are identified for PDMS, GaAs, Al, Si, SiC, MgO, and Steel. The detailed analysis reveals characteristic features.

Combinatorial laser molecular beam epitaxy system integrated with specialized low-temperature scanning tunneling microscopy.

We present a newly developed facility comprising a combinatorial laser molecular beam epitaxy system and an in situ scanning tunneling microscope (STM). This facility aims at accelerating the materials research in a highly efficient way by advanced high-throughput film synthesis techniques and subsequent fast characterization of surface morphology and electronic states. Compared with uniform films deposited by conventional methods, the so-called combinatorial thin films will be beneficial in determining the accurate phase diagrams of different materials due to the improved control of parameters such as chemical substitution and sample thickness resulting from a rotary-mask method. A specially designed STM working under low-temperature and ultrahigh vacuum conditions is optimized for the characterization of combinatorial thin films in an XY coarse motion range of 15 mm × 15 mm with submicrometer location precision. The overall configuration and some key aspects such as the sample holder design, scanner head, and sample/tip/target transfer mechanism are described in detail. The performance of the device is demonstrated by synthesizing high-quality superconducting FeSe thin films with gradient thickness and imaging surfaces of highly oriented pyrolytic graphite, Au (111), Bi2Sr2CaCu2O8+δ (BSCCO), and FeSe. In addition, we also have obtained clean noise spectra of tunneling junctions and the superconducting energy gap of BSCCO. The successful manufacturing of such a facility opens a new window for the next generation equipment designed for experimental materials research.

Effect of surface modification and laser repetition rate on growth, structural, electronic and optical properties of GaN nanorods on flexible Ti metal foil.

The effect of flexible Ti metal foil surface modification and laser repetition rate in laser molecular beam epitaxy growth process on the evolution of GaN nanorods and their structural, electronic and optical properties has been investigated. The GaN nanostructures were grown on bare- and pre-nitridated Ti foil substrates at 700 °C for different laser repetition rates (10–30 Hz). It is found that the low repetition rate (10 Hz) promotes sparse growth of three-dimensional inverted-cone like GaN nanostructures on pre-nitridated Ti surface whereas the entire Ti foil substrate is nearly covered with film-like GaN consisting of large-sized grains for 30 Hz growth. In case of the GaN growth at 20 Hz, uniformly-aligned, dense (∼8 × 109 cm−2) GaN nanorods are successfully grown on pre-nitridated Ti foil whereas sparse vertical GaN nanorods have been obtained on bare Ti foil under similar growth conditions for both 20 and 30 Hz. X-ray photoemission spectroscopy (XPS) has been utilized to elucidate the electronic structure of GaN nanorods grown under various experimental conditions on Ti foil. It confirms Ga–N bonding in the grown structures, and the calculated chemical composition turns out to be Ga rich for the GaN nanorods grown on pre-nitridated Ti foil. For bare Ti substrates, a preferred reaction between Ti and N is noticed as compared to Ga and N leading to sparse growth of GaN nanorods. Hence, the nitridation of Ti foil is a prerequisite to achieve the growth of dense and aligned GaN nanorod arrays. The X-ray diffraction, high resolution transmission electron microscopy and Raman studies revealed the c-axis growth of wurtzite GaN nanorods on Ti metal foil with good crystallinity and structural quality. The photoluminescence spectroscopy showed that the dense GaN nanorod possesses a near band edge emission at 3.42 eV with a full width at half maximum of 98 meV at room temperature. The density-controlled growth of GaN nanorods on a flexible substrate with high structural and optical quality holds promise for potential applications in futuristic flexible GaN based optoelectronics and sensor devices.

A comparison study of electronical and photoelectrical properties of electron gases at (1 0 0), (1 1 0) and (1 1 1) LaAlO3/SrTiO3 interfaces

(1 0 0), (1 1 0) and (1 1 1) LaAlO3/SrTiO3 (LAO/STO) interfaces show similar high mobility conduction. Here, we compare electronic transport and photoconductivity properties of electron gases at the three interfaces, considering their different polar continuity at the interface and the crystallographic symmetry. Epitaxial LAO films were grown on STO (1 0 0), (1 1 0) and (1 1 1) substrates by laser molecular beam epitaxy. All the electron gases at the interfaces exhibit metallic behaviors with close sheet carrier density (1013–1014 cm−2), the electron gas at (1 0 0) interface presents the highest mobility, which is almost one order of magnitude higher than that at (1 1 1) interface below 70 K. At lower temperatures, all the electron gases show obvious photoelectrical response to visible light illumination. Light-assisted Hall measurements indicate distinctly different mechanisms for the photoconductivity of electron gases at the three interfaces. Our results can assist the understanding of the high mobility of electron gases at oxide interfaces, which will be helpful to optimize the electronic properties for device applications.

Fluorescence of Cr3+ ions in MgAl2O4 epitaxial nanosize spinel films on SrTiO3 substrates

The fluorescence spectra of probe Cr3+ centers in MgAl2O4 spinel nanolayers of different thickness grown on SrTiO3 substrates were studied. The films were produced by laser molecular beam epitaxy. The fluorescence spectra strongly depend on nanofilm growth conditions. The Cr3+ ions 2E excited state lifetime was found to significantly differ from that in the bulk crystals. The role of Mg/Al inversion in the spinel lattice in the fluorescent properties is discussed.

Effect of substrate temperature on antiphase boundaries and spin polarization of thin Fe3O4 film on Si (100)

Polycrystalline Fe3O4 films with the thickness of 27 nm were grown at different temperature (Ts) on Si (100) substrate by Laser-Molecular Beam Epitaxy to study the influence of density of antiphase boundaries (APBs) and the spin polarization of Fe3O4. Films prepared with Ts in the range of 500 °C - 650 °C were proved to be of good crystallization, preferred orientation, and desirable values of saturation magnetization (a little smaller one for film prepared at Ts = 500 °C). The lowest density of APBs was verified by the measured bulk like electron-phonon (s-p) coupling constant for film prepared at Ts = 550 °C, along with the highest value of spin polarization and the best crystalline quality as the most obvious inflection in the curves of lnρ vs. 1000/T around Verwey transition. This optimum value of Ts can be interpreted by the APBs thermally activated migration process with an activation energy. Films prepared with Ts = 500 °C, 600 °C, and 650 °C still have a reasonable value of the s-p coupling constant compared to recently reported results, while larger magnetoresistance accompanied by the superparamagnetic behavior was observed for films prepared at lower Ts (Ts = 400 °C and 450 °C).

Magnetization reversal in NiFe2O4/SrTiO3 nanoheterostructures grown by laser molecular beam epitaxy

NiFe2O4/SrTiO3(001) nanoheterostructures have been fabricated by laser molecular beam epitaxy method. Surface morphology and crystal structure of Ni-ferrite films were analysed by atomic force microscopy (AFM), reflection of high energy electron diffraction (RHEED), X-ray diffraction (XRD). X-ray methods prove the presence of inverse spinel crystal structure of films that was confirmed by measurements of spectral dependence of optical polar Kerr (PMOKE) effect. Study of magnetization reversal for different orientations of magnetic field carried out by vibration sample magnetometry (VSM) and longitudinal magneto-optical Kerr effect (LMOKE) are presented. In-plane magnetization loops exhibit 90° period indicating presence of biaxial magnetic anisotropy. Asymmetry of LMOKE hysteresis loops is related to manifestation of quadratic in magnetization effects in reflection of light.

3D aluminum/silver hierarchical nanostructure with large areas of dense hot spots for surface-enhanced raman scattering.

Plasmonic nanomaterials possessing large-volume, high-density hot spots with high field enhancement are highly desirable for ultrasensitive surface-enhanced Raman scattering (SERS) sensing. However, many as-prepared plasmonic nanomaterials are limited in available dense hot spots and in sample size, which greatly hinder their wide applications in SERS devices. Here, we develop a two-step physical deposition protocol and successfully fabricate 3D hierarchical nanostructures with highly dense hot spots across a large scale (6×6 cm2 ). The nanopatterned aluminum film was first prepared by thermal evaporation process, which can provide 3Dquasi-periodic cloud-like nanostructure arrays suitable for noble metal deposition; then a large number of silver nanoparticles withcontrollable shape and size were decorated onto the alumina layer surfaces by laser molecular beam epitaxy, which can realize large-area accessible dense hot spots. The optimized 3D-structured SERS substrate exhibits high-quality detection performance with excellent reproducibility (13.1 and 17.1%), whose LOD of rhodamine 6G molecules was 10-9 M. Furthermore, the as-prepared 3D aluminum/silver SERS substrate was applied in detection of melamine with the concentration down to 10-7 M and direct detection of melamine in infant formula solution with the concentration as low 10mg/L. Such method to realize large-area hierarchical nanostructures can greatly simplify the fabrication procedure for 3D SERS platforms, and should be of technological significance in mass production of SERS-based sensors.

Structure and magneto-electric properties of Co-based ferromagnetic films grown on the Pb0.71Sn0.29Te crystalline topological insulator

Co, Ni, Co55Fe45 and Co40Fe40B20 layers were grown on Pb0.71Sn0.29Te (111) crystalline topological insulator films by conventional and laser molecular beam epitaxy (LMBE) methods. It was demonstrated that the Co40Fe40B20 ferromagnetic films were grown epitaxially on the crystalline topological insulator surface, with clear epitaxial relations. Obtained Co40Fe40B20 layers have a bcc crystal structure with a crystalline (111) plane parallel to the (111) plane of Pb0.71Sn0.29Te. The use of reciprocal space three-dimensional mapping of reflection high electron diffraction (RHEED) patterns made it possible to determine the epitaxial relations of the film and substrate. Using laser based angle-resolved photoemission spectroscopy (LARPES) the clean Pb0.71Sn0.29Te surface with Dirac-like dispersion of the topological surface states was demonstrated. The stable surface topological state of Pb0.71Sn0.29Te was observed at least at the monolayer thick Co coverage. It was shown that Co (or Ni) and Co55Fe45 may be used as a pair of contacts (injector and detector) with different coercivities for spin transport measurements in the “metal – topological insulator” systems. The measurements of magnetic and transport properties show distinct hysteresis – type dependence of the magnetoresistance in the range of quadratic I(U) dependence.

Cu2O-Ag nanocomposites with tunable optical property

Semiconductor-metal nanocomposites were demonstrated to be great potential applications for their unique structures. In this work, Cu2O-Ag nanocomposites with tunable optical property were prepared by laser molecular beam epitaxy with an alternative deposition way. The growth process was characterized by in situ reflection high-energy electron diffraction and in situ x-ray photoelectron spectroscopy. Due to the surface plasmon resonance, the absorption peaks of Cu2O-Ag can be easily modulated from 618 nm to 650 nm by increasing Ag laser pulses. The corresponding band gap changed from 2.01 eV to 2.74 eV. The growth mechanism was also discussed in detail. The results provide a new route to control the optical absorption of semiconductor-metal oxide nanocomposites by an easy way.

Wafer-Scale Sulfur Vacancy-Rich Monolayer MoS2 for Massive Hydrogen Production.

As one of the promising low-cost and high-efficiency catalysts for the electrochemical hydrogen evolution reaction (HER), it is well-known that there are both tiny exposed catalytic active edge sites and large-area inert basal planes in two-dimensional MoS2 structures. For enhancing its HER activity, extensive work has been done to activate the inert basal plane of MoS2. In this article, wafer-scale (2 in.) continuous monolayer MoS2 films with substantial in situ generated sulfur vacancies are fabricated by employing the laser molecular beam epitaxy process benefitting from ultrahigh vacuum growth condition and high substrate temperature. The intrinsic sulfur vacancies throughout the wafer-scale basal plane present an ideal electrocatalytic platform for massive hydrogen production. The fabricated vacancy-rich monolayer MoS2 can achieve a current density of -10 mA/cm2 at an overpotential of -256 mV. The wafer-scale fabrications of sulfur vacancy-rich monolayer MoS2 provide great leaps forward in the practical application of MoS2 for massive hydrogen production.

Evidence of a second-order Peierls-driven metal-insulator transition in crystalline NbO2

The metal-insulator transition of ${\mathrm{NbO}}_{2}$ is thought to be important for the functioning of recent niobium oxide-based memristor devices, and is often described as a Mott transition in these contexts. However, the actual transition mechanism remains unclear, as current devices actually employ electroformed ${\mathrm{NbO}}_{x}$ that may be inherently different to crystalline ${\mathrm{NbO}}_{2}$. We report on our synchrotron x-ray spectroscopy and density-functional-theory study of crystalline, epitaxial ${\mathrm{NbO}}_{2}$ thin films grown by pulsed laser deposition and molecular beam epitaxy across the metal-insulator transition at $\ensuremath{\sim}810{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. The observed spectral changes reveal a second-order Peierls transition driven by a weakening of Nb dimerization without significant electron correlations, further supported by our density-functional-theory modeling. Our findings indicate that employing crystalline ${\mathrm{NbO}}_{2}$ as an active layer in memristor devices may facilitate analog control of the resistivity, whereby Joule-heating can modulate Nb-Nb dimer distance and consequently control the opening of a pseudogap.

Energy band alignment at Cu2O/ZnO heterojunctions characterized by in situ x-ray photoelectron spectroscopy**Project supported by the National Natural Science Foundation of China (Grant No. 11404302) and the Laser Fusion Research Center Funds for Young Talents, China (Grant No. RCFPD1-2017-9).

With the increasing interest in Cu2O-based devices for photovoltaic applications, the energy band alignment at the Cu2O/ZnO heterojunction has received more and more attention. In this work, a high-quality Cu2O/ZnO heterojunction is fabricated on a c-Al2O3 substrate by laser-molecular beam epitaxy, and the energy band alignment is determined by x-ray photoelectron spectroscopy. The valence band of ZnO is found to be 1.97 eV below that of Cu2O. A type-II band alignment exists at the Cu2O/ZnO heterojunction with a resulting conduction band offset of 0.77 eV, which is especially favorable for enhancing the efficiency of Cu2O/ZnO solar cells.

Rectifying characteristics and solar-blind photoresponse in β-Ga2O3/ZnO heterojunctions**Project supported by the National Natural Science Foundation of China (Grant Nos. 51572033, 61774019, 61704153, and 11404029), the Fund from the State Key Laboratory of Information Photonics and Optical Communications (BUPT), China, and the Fundamental Research Funds for the Central Universities, China.

Heterojunctions composed of β-Ga2O3 and ZnO films are fabricated on sapphire substrates by using the laser molecular beam epitaxy method. The heterojunction possesses excellent rectifying characteristics with an asymmetry ratio over 105. Prominent solar-blind photoresponse effect is also observed in the formed heterojunction. The photodetector exhibits a self-powered behavior with a fast response speed (rise time and decay time are 0.035 s and 0.032 s respectively) at zero bias. The obtained high performance can be related to the built-in field driven photogenerated electron-hole separation.

Direct growth of self-aligned single-crystalline GaN nanorod array on flexible Ta foil for photocatalytic solar water-splitting

We report the direct growth of self-aligned single crystalline GaN nanorod array on flexible Ta metal foil using laser molecular beam epitaxy. Scanning electron microscopy reveals the vertically aligned nanorods on Ta surface having diameters in the range of 60–80 nm. The nanorods show well-defined hexagonal facets and are quite uniformly distributed across the metal foil. Transmission electron microscopy shows single crystalline nature of the individual rods having c-axis oriented growth with wurtzite structure. Room temperature photoluminescence study exhibits a sharp, intense band-to-band emission without any deep-level bands indicating the excellent optical quality of the GaN nanorod array. X-ray photoemission spectroscopy to elucidate the electronic structure of the nanorods confirms Ga–N bonding and the calculated chemical composition turns out to be slightly Ga rich. Location of valence band maxima also suggests the n-type character of GaN nanorods. The photoelectrochemical water-splitting behaviour of the self-aligned GaN nanorod arrays on Ta foil has been investigated using 1 M oxalic acid as the electrolyte with AM 1.5 G simulated solar radiation under 1 Sun (100 mW/cm2) conditions. The results demonstrate an effective way of fabricating well-aligned GaN nanorods on flexible metal foils for developing simple, relatively inexpensive, and flexible photo-electrodes for photocatalytic solar water-splitting applications.

Structural and optical properties of low temperature grown single crystalline GaN nanorods on flexible tungsten foil using laser molecular beam epitaxy

The single crystalline GaN nanorods (NRs) have been grown on flexible tungsten (W) foil using laser molecular beam epitaxy (LMBE) at low growth temperature of 600 °C. The field emission scanning electron microscopy showed the growth of GaN NRs having length, width and density of 400∼530 nm, 40–70 nm and 3.4 × 109 cm−2, respectively. The high-resolution transmission electron microscopy reveals the c-axis oriented growth of single crystalline GaN NRs on W foil. The photoluminescence spectroscopy shows that the GaN NRs possesses a strong near band edge luminescence at 3.41 eV with a line-width of ∼100 meV at room temperature which resembles the high optical quality of GaN nanorods grown on W foil. The possible mechanism of GaN nanords growth on W foil has been also discussed. The low temperature LMBE growth of single crystalline GaN NRs on flexible metal foil with high optical quality paves the way for the development of III-nitride based flexible opto-electronics devices.

Growth and physical properties of BiFeO3 thin films directly on Si substrate

Multiferroic BiFeO3 thin films have been widely studied for their intriguing fundamental physics and exotic functional properties. Integration of the BiFeO3 thin films with semiconductor Si is crucial for the practical development of novel electronic and photosensitive devices. Here, we report the fabrication and physical properties of BiFeO3 thin films grown directly on bare Si substrate by laser-molecular beam epitaxy. It was found that a predeposition process performed at room temperature and high vacuum conditions is effective and necessary for obtaining the pure-phase BiFeO3 thin films. X-ray diffraction measurements indicate that the obtained BiFeO3 thin films are in a single-phase polycrystalline state and show improved crystallinity with increasing thickness, and a transmission electronic microscopy image illustrates an interface layer around 3 nm in thickness existing between the BiFeO3 thin films and Si substrate. Electrical measurements show that the BiFeO3 thin films have good ferroelectricity and low leakage current. Moreover, optical properties investigated by spectroscopic ellipsometry demonstrate that the direct band gap of the thin films increases with decreasing thickness.

ZnO nanostructure-assisted growth of (0002)-oriented GaN thin films by laser molecular beam epitaxy

Vertically aligned zinc oxide (ZnO) nanostructures (NS) have been fabricated without any catalyst on Si (100) and sapphire (0002) substrates using high-pressure-assisted pulsed laser deposition technique. It has been observed that the substrate plays a crucial role in deciding the morphology and optical quality of the nanostructures, wherein ZnO NS on Si substrate exhibit better luminescence characteristics as compared to those grown on sapphire substrate. Gallium nitride (GaN) layers are grown using a customized laser molecular beam epitaxy system and the effect of an intermediate layer in the form of vertically aligned ZnO NS is analyzed. A comparative study is performed with GaN layers grown on ZnO NS-coated Si, bare Si, and bare sapphire substrate. The structural and room temperature photoluminescence properties of the GaN layer significantly improve when grown on ZnO-NS/Si substrates as compared to bare sapphire and Si substrate.