Laser Molecular Beam Epitaxy Research Articles

Influence of growth temperature on laser molecular beam epitaxy and properties of GaN layers grown on c-plane sapphire

We have investigated the influence of growth temperature on the in-plane strain, structural, optical and mechanical properties of heteroepitaxially grown GaN layers on sapphire (0001) substrate by laser molecular beam epitaxy (LMBE) technique in the temperature range 500–700 °C. The GaN epitaxial layers are found to have a large in-plane compressive stress of about 1 GPa for low growth temperatures but the strain drastically reduced in the layer grown at 700 °C. The nature of the in-plane strain has been analyzed using high resolution x-ray diffraction, atomic force microscopy (AFM), Raman spectroscopy and photoluminescence (PL) measurements. From AFM, a change in GaN growth mode from grain to island is observed at the high growth temperature above 600 °C. A blue shift of 20–30 meV in near band edge PL emission line has been noticed for the GaN layers containing the large in-plane strain. These observations indicate that the in-plane strain in the GaN layers is dominated by a biaxial strain. Using nanoindentation, it is found that the indentation hardness and Young's modulus of the GaN layers increases with increasing growth temperature. The results disclose the critical role of growth mode in determining the in-plane strain and mechanical properties of the GaN layers grown by LMBE technique.

Growth and Characterization of Sn Doped β-Ga2O3 Thin Films and Enhanced Performance in a Solar-Blind Photodetector

Ga2−x Sn x O3 thin films were deposited on c-plane Al2O3 (0001) substrates with different Sn content by laser molecular beam epitaxy technology (L-MBE). The Sn content x was varied from 0 to 1.0. ( $$\bar{2}01$$ ) oriented β-phase Ga2−x Sn x O3 thin films were obtained at the substrate temperature of 850°C in the vacuum pressure of 5 × 10−5 Pa. The crystal lattice expanded and the energy band-gap decreased with the increase of Sn content for Sn4+ ions incorporated into the Ga site. The n-type conductivity was generated effectively through doping Sn4+ ions in the Ga2O3 lattice in the oxygen-poor conditions. The solar-blind (SB) photodetectors (PDs) based on Ga2−x Sn x O3 (x = 0, 0.2) thin films were fabricated. The current intensity and responsivity almost increased by one order of magnitude and the relaxation time constants became shorter for x = 0.2. Our work suggests that the performance of PD can be improved by doping Sn4+ ions in Ga2O3 thin films.

Improvement for the performance of solar-blind photodetector based on β-Ga2O3 thin films by doping Zn

Highly oriented () Ga2−xZnxO3 thin films with different doping concentrations were grown on (0 0 0 1) sapphire substrates by laser molecular beam epitaxy technology. The expansion of lattice and the shrinkage of band gap with increasing doping level confirms the chemical substitution of Zn2+ ions into the Ga2O3 crystal lattice. The emission intensity of blue-violet emission bands enhanced with the increase of (ZnGa)′ under 254 nm ultraviolet excitation, and the maximum was obtained at x = 0.8. A metal–semiconductor–metal structured solar-blind photodetector based on Ga2−xZnxO3 (x = 0, 0.8) was made, the increasing responsivity and diminishing relaxation time constants for β-Ga2−xZnxO3 (x = 0.8) photodetector were observed with 254 nm ultraviolet illumination.

Modulation of ultrafast laser-induced magnetization precession in BiFeO3-coated La0.67Sr0.33MnO3 thin films

The ultrafast laser-excited magnetization dynamics of ferromagnetic (FM) La0.67Sr0.33MnO3 (LSMO) thin films with BiFeO3 (BFO) coating layers grown by laser molecular beam epitaxy are investigated using the optical pump-probe technique. Uniform magnetization precessions are observed in the films under an applied external magnetic field by measuring the time-resolved magneto-optical Kerr effect. The magnetization precession frequencies of the LSMO thin films with the BFO coating layers are lower than those of uncoated LSMO films, which is attributed to the suppression of the anisotropy field induced by the exchange interaction at the interface between the antiferromagnetic order of BFO and the FM order of LSMO.

The important role of Ga vacancies in the ferromagnetic GaN thin films

Room-temperature ferromagnetism was observed in the unintentionally doped GaN films, which were fabricated using laser molecular beam epitaxy followed by annealing process at different temperatures from 800 to 1000 °C for 25 min in flowing nitrogen gas. The annealing temperature has a prominent effect on the crystalline quality, the surface morphology, the concentration of Ga vacancies and the magnetic properties of the GaN films. It is found that high temperature annealing process is an effective means of introducing Ga vacancies, which play an important role in the origin of room-temperature ferromagnetism in the GaN samples. The Ga vacancy-related ferromagnetism may be an assistant mechanism in the Mn-doped GaN films, but it is dominant in the unintentionally doped GaN films, which deserves further study to develop novel diluted magnetic semiconductors without dopants.

Impurity Compensation Effect Induced by Tin Valence Change in α-Ga1.4Sn0.6O3 Thin Films.

Corundum-structured α-phase Ga1.4Sn0.6O3 thin films have been deposited on m-plane Al2O3(300) substrates using laser molecular beam epitaxy technology. With increasing of the oxygen partial pressure, the crystal lattice of Ga1.4Sn0.6O3 films expands due to tin ions valence changes from Sn4+ to Sn2+. The resistivity of the film deposited under 3 × 10-5 Pa is 3.54 × 104 Ω·cm, which decreases by about 2 orders of magnitude than that fabricated under 3 × 10-1 Pa. The mixture valence of Sn2+ and Sn4+ ions leads to the impurity altitude compensation effect. The deep ultraviolet photodetector based on α-phase Ga1.4Sn0.6O3 thin films was fabricated. With the oxygen partial pressure reducing gradually, the dark current and the photocurrent increase, and the relaxation time constants diminish, respectively.

Unsaturated magnetoconductance of epitaxial La0.7Sr0.3MnO3 thin films in pulsed magnetic fields up to 60 T

We report on the temperature and field dependence of resistance of La0.7Sr0.3MnO3 thin films over a wide temperature range and in pulsed magnetic fields up to 60 T. The epitaxial La0.7Sr0.3MnO3 thin films were deposited by laser molecular beam epitaxy. High magnetic field magnetoresistance curves were fitted by the Brillouin function, which indicated the existence of magnetically polarized regions and the underlying hopping mechanism. The unsaturated magnetoconductance was the most striking finding observed in pulsed magnetic fields up to 60 T. These observations can deepen the fundamental understanding of the colossal magnetoresistance in manganites with strong correlation of transport properties and magnetic ordering.

Growth of Y3Fe5O12/GaN layers by laser molecular-beam epitaxy and characterization of their structural and magnetic properties

Laser molecular-beam epitaxy has been employed to obtain layers of yttrium-iron garnet (YIG) Y3Fe5O12 on gallium nitride substrates. It was found that there exists a polycrystalline YIG phase without admixtures of other structural phases. A magnetic anisotropy of films of the “easy-magnetic plane” type was found. The gyromagnetic ratio and the demagnetizing field 4πMS were calculated.

Deep ultraviolet photodetectors based on p-Si/i-SiC/n-Ga2O3 heterojunction by inserting thin SiC barrier layer

Deep ultraviolet photodetectors based on p-Si/n-Ga2O3 and p-Si/i-SiC/n-Ga2O3 heterojunctions were fabricated by laser molecular beam epitaxial (L-MBE), respectively. In compare with p-Si/n-Ga2O3 heterostructure-based photodetector, the dark current of p-Si/i-SiC/n-Ga2O3-based photodetector decreased by three orders of magnitude, and the rectifying behavior was tuned from reverse to forward. In order to improve the quality of the photodetector, we reduced the oxygen vacancies of p-Si/i-SiC/n-Ga2O3 heterostructures by changing the oxygen pressure during annealing. As a result, the rectification ratio (I F/I R) of the fabricated photodetectors was 36 at 4.5 V and the photosensitivity was 5.4 × 105% under the 254 nm light illumination at −4.5 V. The energy band structure of p-Si/n-Ga2O3 and p-Si/i-SiC/n-Ga2O3 heterostructures was schematic drawn to explain the physic mechanism of enhancement of the performance of p-Si/i-SiC/n-Ga2O3 heterostructure-based deep UV photodetector by introduction of SiC layer.

Characterization of hexagonal ɛ-Ga1.8Sn0.2O3 thin films for solar-blind ultraviolet applications

Ga1.8Sn0.2O3 thin films were deposited on c-plane Al2O3 (0001) substrates by laser molecular beam epitaxy technology. Well crystallized (002) oriented ɛ-phase Ga1.8Sn0.2O3 thin films were obtained at the substrate temperature above 750 °C and the oxygen partial pressure more than 5 × 10−3 Pa. The band-gap slightly shrinks with Sn4+ ions incorporated into Ga3+ sites, showing an excellent solar-blind ultraviolet (UV) characteristic. The conductivity of hexagonal ɛ-Ga1.8Sn0.2O3 films is very low in the dark, and permitting the design and fabrication of solar-blind photodetector. The photodetector exhibits obvious photo-response under 254 nm UV light irradiation, and it increases in photocurrent with both the rise of applied bias and optical input power. The results suggest that ɛ-Ga1.8Sn0.2O3 thin film is a promising candidate for using in solar-blind photodetectors.

Fast-response solar-blind ultraviolet photodetector with a graphene/β-Ga2O3/graphene hybrid structure

A high performance vertical solar-blind ultraviolet photodetector based on β-Ga2O3 thin films sandwiched between two graphene sheets has been fabricated by laser molecular beam epitaxy. The photodetector exhibits obvious rectifying characteristics and excellent solar-blind UV photoresponse. The fast rising and decay time of our detector are 0.96 s and 0.81 s under 254 nm illumination. The ratio of I254/Idark is up to 82.88 and the responsivity of graphene/β-Ga2O3/graphene photodetector increases to 9.66 A/W at 10 V bias. Our results suggest that this performance is attributed to the existence of Schottky barriers between β-Ga2O3 and two graphene sheets. The combination of Ga2O3 and graphene might open up new possibilities for future UV integrated optoelectronic devices.

Reduced growth temperature of Bi6FeCoTi3O18 thin films by conductive bottom layers

The Aurivillius layered oxide homologous series attract wide interests due to their room temperature multiferroic properties. Unfortunately, the synthesis of such layered oxide epitaxial thin films has been a major challenge owing to the occurrence of growth defects and narrow growth temperature window. To obtain high quality epitaxial Bi6FeCoTi3O18 (BFCTO) thin films, the effects of insulating and conductive bottom layers were studied by laser molecular beam epitaxy. We found that the optimal deposition temperature for growth on conductive bottom layers is more than 90°C lower than that on insulating bottom layers, which indicates the interface between BFCTO and conductive bottom layers has smaller interfacial energy than the interface between BFCTO and insulating bottom layers. The magnetic and ferroelectric properties of the optimized BFCTO thin films on insulating substrate and conductive bottom layers were studied. This study is important to control the growth of complex layered oxide thin films and exploit the applications for future room temperature multiferroic devices.

Dielectric Materials: Enhancement of Dielectric Properties in Epitaxial Bismuth Ferrite–Bismuth Samarium Ferrite Superlattices (Adv. Electron. Mater. 8/2016)

The front cover illustrates the enhanced charge storage capacity of bismuth ferrite (blue)- bismuth samarium ferrite (green) ferroelectric superlattices, as reported by R. Maran et al. in article number 1600170. The precise composition control enabled by combinatorial laser molecular-beam epitaxy is exploited to design superlattice structures with enhanced ferroelectric coupling between individual layers and these can be used to to tailor desired dielectric properties in thin film systems. The authors acknowledge Jiaqi Dai for the cover artwork.

Laser MBE-grown yttrium iron garnet films on GaN: characterization of the crystal structure and magnetic properties

Yttrium iron garnet (YIG) films were grown on GaN substrates using the laser molecular beam epitaxy method. X-ray diffraction data showed polycrystalline YIG layers without additional structural modifications. The magnetic properties of the YIG films were studied at room temperature with the aid of a vibration sample magnetometer, the magneto-optical Kerr effect and ferromagnetic resonance methods. ‘Easy-plane’-type magnetic anisotropy was found in the films. The gyromagnetic ratio and 4 πMS value were calculated.

Evidence of weak localization in quantum interference effects observed in epitaxial La0.7Sr0.3MnO3 ultrathin films.

Quantum interference effects (QIEs) dominate the appearance of low-temperature resistivity minimum in colossal magnetoresistance manganites. The T1/2 dependent resistivity under high magnetic field has been evidenced as electron-electron (e-e) interaction. However, the evidence of the other source of QIEs, weak localization (WL), still remains insufficient in manganites. Here we report on the direct experimental evidence of WL in QIEs observed in the single-crystal La0.7Sr0.3MnO3 (LSMO) ultrathin films deposited by laser molecular beam epitaxy. The sharp cusps around zero magnetic field in magnetoresistance measurements is unambiguously observed, which corresponds to the WL effect. This convincingly leads to the solid conclusion that the resistivity minima at low temperatures in single-crystal manganites are attributed to both the e-e interaction and the WL effect. Moreover, the temperature-dependent phase-coherence length corroborates the WL effect of LSMO ultrathin films is within a two-dimensional localization theory.

Low-relaxation spin waves in laser-molecular-beam epitaxy grown nanosized yttrium iron garnet films

Synthesis of nanosized yttrium iron garnet (Y3Fe5O12, YIG) films followed by the study of ferromagnetic resonance (FMR) and spin wave propagation in these films is reported. The YIG films were grown on gadolinium gallium garnet substrates by laser molecular beam epitaxy. It has been shown that spin waves propagating in YIG deposited at 700 °C have low damping. At the frequency of 3.29 GHz, the spin-wave damping parameter is less than 3.6 × 10−5. Magnetic inhomogeneities of the YIG films give the main contribution to the FMR linewidth. The contribution of the relaxation processes to the FMR linewidth is as low as 1.2%.

Epitaxial growth and magnetic properties of ultraviolet transparent Ga2O3/(Ga1-xFex)2O3 multilayer thin films.

Multilayer thin films based on the ferromagnetic and ultraviolet transparent semiconductors may be interesting because their magnetic/electronic/photonic properties can be manipulated by the high energy photons. Herein, the Ga2O3/(Ga1−xFex)2O3 multilayer epitaxial thin films were obtained by alternating depositing of wide band gap Ga2O3 layer and Fe ultrathin layer due to inter diffusion between two layers at high temperature using the laser molecular beam epitaxy technique. The multilayer films exhibits a preferred growth orientation of crystal plane, and the crystal lattice expands as Fe replaces Ga site. Fe ions with a mixed valence of Fe2+ and Fe3+ are stratified distributed in the film and exhibit obvious agglomerated areas. The multilayer films only show a sharp absorption edge at about 250 nm, indicating a high transparency for ultraviolet light. What’s more, the Ga2O3/(Ga1−xFex)2O3 multilayer epitaxial thin films also exhibits room temperature ferromagnetism deriving from the Fe doping Ga2O3.

Growth and characterization of α-phase Ga2−xSnxO3 thin films for solar-blind ultraviolet applications

Ga2−xSnxO3 thin films on m-plane (300) sapphire substrates were deposited by laser molecular beam epitaxy technology. Corundum-structured α-phase Ga2−xSnxO3 thin films with different Sn content were obtained at 850 °C in vacuum pressure of 5 × 10−5 Pa. The band-gap energy of the α-phase Ga2−xSnxO3 thin films, determined from the absorption spectrum, decreases linearly with increasing Sn content. A metal–semiconductor–metal structured solar-blind photodetector based α-phase Ga2−xSnxO3 thin films was fabricated, and excellent solar-blind ultraviolet characteristics were demonstrated. The ratio of I254/Idark was up to 1.40 × 102 and the responsivity increased to 9.55 × 10−2 A W−1. The results suggest that α-phase Ga2−xSnxO3 thin films are promising candidates for use in solar-blind photodetectors.

The Integration of Functional Oxide Thin Film with GaAs By Laser Molecular Beam Epitaxy

Integration of crystalline oxide thin films with semiconductors has attracted considerable attention in recent years. Compared to Si, GaAs have a higher saturated electron velocity and electron mobility, transistors based on GaAs can function at a much higher frequencies. In this talk, SrTiO3 films were grown on GaAs (100) substrates as an intermediate buffer layer for the epitaxial growth of oxide thin films including ferroelectric BaTiO3 and semiconductor ZnO by laser molecular beam epitaxy (LMBE). The properties of the multilayers in terms of growth modes, strain and interface structures were characterized by the insitu reflective high energy electron diffraction (RHEED) pattern. The crystalline quality and surface morphology of the oxide/GaAs heterostructure were investigated respectively by a combination of Xray diffraction (XRD) and atomic force microscopy (AFM). These heterostructure exhibit good functional properties such as ferrelectrisity and resistive switching. Furthermore, the fundamental interface properties of these heterostructure with an emphasis on the strain effect were discussed in relation with the electrical properties. The results provide an understanding of the integration of oxide/GaAs heterostructure with full control over the interface structure at the atomic scale.

Influence of laser repetition rate on the structural and optical properties of GaN layers grown on sapphire (0001) by laser molecular beam epitaxy

High-quality GaN layers were grown on sapphire (0001) substrates using laser molecular beam epitaxy (LMBE) by laser ablating a solid GaN target at different laser repetition rates (10–40 Hz) under a constant supply of r.f. nitrogen plasma. The effect of laser repetition rate on the structural and optical properties of GaN layers was systematically studied using high-resolution X-ray diffraction (HRXRD), field emission scanning electron microscopy, atomic force microscopy, Raman spectroscopy and photoluminescence (PL) spectroscopy. High-resolution X-ray rocking curve measurements revealed highly c-axis oriented GaN layers on sapphire grown at 30 Hz with a calculated screw dislocation density of ~1.42 × 107 cm−2, whereas the GaN layers grown at 10 or 40 Hz consisted the screw dislocation density in the range of 108–109 cm−2. Surface morphological analysis revealed a change in grain size as well as surface roughness as a function of laser repetition rate and is explained on the basis of growth kinetics. Vibrational Raman spectroscopy revealed that the GaN layer grown at 10 Hz shows an in-plane compressive stress of ~1 GPa, while the film grown at 30 Hz exhibits a minimum stress of ~0.3 GPa. The PL measurements show a highly luminescent band-to-band emission of GaN at 3.44 eV for the 10 Hz grown highly strained GaN layer and at 3.41 eV for the less strained film grown at 30 Hz along with a broad defect band emission centered around 2.28 eV. It is found that the GaN layers grown at 30 Hz have excellent structural and optical properties. We expect that the less strained thin and highly oriented GaN film grown by LMBE can further be utilized for developing prodigious low-temperature-grown nitride-based multilayer structures and devices.