Oxygen Plasma-assisted Molecular Beam Epitaxy Research Articles

Metal–Semiconductor–Metal Photodetectors Based on β-MgGaO Thin Films

Ultrawide-bandgap (UWBG) deep-ultraviolet photodetectors have received great attention due to their versatile applications in the fields of scientific research, civilian infrastructure, military defense, etc. In this perspective, we fabricated deep-ultraviolet β-MgGaO metal–semiconductor–metal photodetectors with interdigital Pt/Au metal contacts. β-Phase MgGaO ternary alloy thin films of different Mg atomic percentages were grown using oxygen plasma-assisted molecular beam epitaxy. Ultrawide bandgaps of 5.03, 5.09, 5.15, and 5.22 eV were achieved for thin films with and without Mg2+ incorporation, and light transmittances of all samples were around 90% in the visible region. Raman spectra indicate that Mg2+ atoms have replaced the position of Ga3+ ions in both octahedral and tetrahedral chains. The responsivity of the detectors was investigated. The irradiation wavelength-, temperature-, and power-dependent I–V curves, photocurrent spectra, and dynamics of the photocurrents were measured. This work suggests that UWBG β-MgGaO semiconductors have a potential for deep-ultraviolet photodetectors and other photonic device applications.

Toward controllable Si-doping in oxide molecular beam epitaxy using a solid SiO source: Application to β -Ga2O3

The oxidation-related issues in controlling Si doping from the Si source material in oxide molecular beam epitaxy (MBE) are addressed by using its solid suboxide, SiO, as an alternative source material in a conventional effusion cell. Line-of-sight quadrupole mass spectrometry of the direct SiO-flux (ΦSiO) from the source at different temperatures (TSiO) confirmed SiO molecules to sublime with an activation energy of 3.3 eV. The TSiO-dependent ΦSiO was measured in vacuum before and after subjecting the source material to an O2-background of 10−5 mbar (typical oxide MBE regime). The absence of a significant ΦSiO difference indicates negligible source oxidation in molecular O2. Mounted in an oxygen plasma-assisted MBE, Si-doped β-Ga2O3 layers were grown using this source. The ΦSiO at the substrate was evaluated [from 2.9 × 109 cm−2 s−1 (TSiO = 700 °C) to 5.5 × 1013 cm−2 s−1 (TSiO = 1000 °C)] and Si-concentration in the β-Ga2O3 layers measured by secondary ion mass spectrometry highlighting unprecedented control of continuous Si-doping for oxide MBE, i.e., NSi from 4 × 1017 cm−3 (TSiO = 700 °C) up to 1.7 × 1020 cm−3 (TSiO = 900 °C). For a homoepitaxial β-Ga2O3 layer, a Hall charge carrier concentration of 3 × 1019 cm−3 in line with the provided ΦSiO (TSiO = 800 °C) is demonstrated. No SiO-incorporation difference was found between β-Ga2O3(010) layers homoepitaxially grown at 750 °C and β-Ga2O3(−201) heteroepitaxial layers grown at 550 °C on c-plane sapphire. However, the presence of activated oxygen (plasma) resulted in partial source oxidation and related decrease in doping concentration (particularly at TSiO < 800 °C), which has been tentatively explained with a simple model. Degassing the source at 1100 °C reverted this oxidation. Concepts to reduce source oxidation during MBE-growth are referenced.

Growth Study of New Complex Oxide PbOxSe1–x Thin Films by Oxygen Plasma-Assisted Molecular Beam Epitaxy

In this work, we introduce the growth of a new complex oxide lead oxy-chalcogenide (PbOxSe1–x) thin film using an oxygen plasma-assisted molecular beam epitaxy method. Freshly cleaved BaF2(111) wafers were used as the substrates for this growth study. Systematic characterization of X-ray diffraction peaks, Raman shifts, absorption spectra, scanning electron microscopic imaging, and Hall measurements were conducted to elucidate the structural, optical, and electric properties of the as-grown PbOxSe1–x thin films. Specifically, X-ray diffraction measurements revealed that PbOxSe1–x films maintained the same rock-salt crystal structure as the PbSe semiconductor, but a slight shift in the lattice parameter was observed. A blue shift in the optical absorption edge also suggests that the inclusion of oxygen atoms led to the formation of a ternary compound crystal structure. Furthermore, all PbOxSe1–x thin films were observed to be polycrystalline in nature and displayed preferred [100] orientated grains. Slight...

Local vibrational modes competitions in Mn-doped ZnO epitaxial films with tunable ferromagnetism

We reported spectroscopic investigations of high quality Mn-doped ZnO (ZnMnO) films grown by oxygen plasma-assisted molecular beam epitaxy. Raman scattering spectra revealed two local vibrational modes (LVMs) associated with Mn dopants at 523 and 712 cm−1. The LVMs and magnetic properties of ZnMnO films can be synchronously modulated by post annealing processing or by introducing tiny Co. The relative intensity of two LVMs clearly shows competitions arising from uncompensated acceptor and donor defects competition for ferromagnetic and nonmagnetic films. The experimental results indicated that LVM at 523 cm−1 is attributed to Mn—(Zinc-vacancy) complexes, while LVM at 712 cm−1 is attributed to Mn—(Oxygen-vacancy) complexes.

Optical identification of oxygen vacancy formation at SrTiO3–(Ba,Sr)TiO3 heterostructures

We used molecular beam epitaxy (MBE) growth to control the density of oxygen vacancy-related (VO-R) defects at (Ba,Sr)TiO3/SrTiO3 heterojunctions along with depth-resolved cathodoluminescence spectroscopy (DRCLS), surface photovoltage spectroscopy and positron annihilation spectroscopy (PAS) to identify their optical signature, energy levels and spatial distribution. We grew (Ba,Sr)TiO3 epilayers on SrTiO3 using oxygen plasma-assisted MBE under a range of oxygen pressures and temperatures designed to control oxygen extraction from the SrTiO3 substrate that feeds the (Ba,Sr)TiO3 epilayer growth and generates oxygen vacancies. DRCLS and PAS identify a 2.95 eV optical emission with VO-R defect—valence band transitions that peak below the heterointerface and their interplay with 1.8 and 2.3 eV cation vacancies versus growth conditions. These studies reveal that native defects accumulate near SrTiO3/(Ba,Sr)TiO3 interfaces and are sensitive to growth conditions.

Structural and physical properties of BiFeO3 thin films epitaxially grown on SrTiO3 (001) and polar (111) surfaces

The influence of surface polarity on the structural properties of BiFeO3 (BFO) thin films is investigated. BFO thin films are epitaxially grown on SrTiO3 (STO) (100) and polar (111) surfaces by oxygen plasma-assisted molecular beam epitaxy. It is shown that the crystal structure, surface morphology, and defect states of BFO films grown on STO substrates with nonpolar (001) or polar (111) surfaces perform very differently. BFO/STO (001) is a fully strained tetragonal phase with orientation relationship (001)[100]BFO||(001)[100]STO, while BFO/STO (111) is a rhombohedral phase. BFO/STO (111) has rougher surface morphology and less defect states, which results in reduced leakage current and lower dielectric loss. Moreover, BFO films on both STO (001) and STO (111) are direct band oxides with similar band gaps of 2.65 eV and 2.67 eV, respectively.

Surface structure of α-Cr2O3(0001) after activated oxygen exposure

The surface structure of α-Cr2O3(0001) before and after exposure to activated oxygen from an ECR plasma source was investigated by x-ray photoelectron spectroscopy (XPS) and x-ray photoelectron diffraction (XPD). Epitaxial Cr2O3(0001) thin films were deposited on Al2O3(0001) substrates by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE). When cooled or annealed in vacuum, strong evidence for a CrCrO3 termination was obtained by comparing the Cr3+ XPD azimuthal scan to single scattering simulations. However, after plasma exposure, a high binding energy feature was observed in the Cr 2p XPS spectrum that possesses an ordered structure distinct from the underlying Cr3+ of Cr2O3, which remains Cr–Cr–O3-like. Investigation of this new surface structure with simulations of various candidate structures tentatively rules out CrO2-like configurations. The high binding energy feature likely arises from a higher oxidation state of Cr. One possibility is the oxidation of the surface layer of Cr to Cr6−δ with a double chromyl structure (OCrO).

ZnO/MgO(011)的异质外延与特性

本文着重研究ZnO薄膜在MgO(011)衬底上的生长行为。利用分子束外延法(MBE)在MgO(110)衬底上生长ZnO薄膜，通过原子力显微镜(AFM)和扫描电子显微镜(SEM)表征了其表面形貌，利用X射线光电子能谱学(XPS)对薄膜的组分进行深度分层剥析，并采用透射谱(XL)研究了其光学性质。结果显示，薄膜以三维模式生长，而且薄膜表面平整度受到衬底温度的影响；而所生长的薄膜均显示了ZnO透射谱的特征峰，特征峰的位置几乎不受衬底温度的影响。不同生长时间段的薄膜结构的演变过程也通过原位的反射高能电子衍射(RHEED)和原子力显微镜进行了考察。XPS分层组分分析发现，在ZnO薄膜生长过程中，Zn离子会扩散到MgO(110)衬底之中。 In this work, the growth behaviors of ZnO thin films on MgO (011) substrates were studied. ZnO films were deposited on MgO(110) substrates using oxygen plasma-assisted molecular beam epitaxy (MBE). Their surface mor- phologies were probed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Elemental con- stituents at different depths were also analyzed by X-ray photoelectron spectroscopy (XPS) and the optical properties of the ZnO films were characterized by transmission spectra. The film growth was found to follow a three-dimensional growth mode and the surface morphologies could be monitored by substrate temperature. The characteristic band gap of ZnO was found for all the films and the value was little influenced by the substrate temperature. In addition, the evolu- tion of the structure was examined by capturing in situ reflection high-energy electron diffraction (RHEED) patterns and ex situ AFM for different growth steps. During the growth of ZnO films, the Zn ions were found to diffuse into the MgO(110) substrate.

Substrate-induced disorder in V2O3thin films grown on annealedc-plane sapphire substrates

We investigate the structural and electronic properties of V2O3 thin films deposited by oxygen plasma-assisted molecular beam epitaxy onto annealed and unannealed c-plane sapphire substrates. Annealing the substrates before growth to produce ultra-smooth surfaces improved initial epitaxy, according to in situ reflection high-energy electron diffraction. Surprisingly, films deposited on annealed substrates had a more island-like surface, broader x-ray diffraction peaks, and an increased resistivity of V2O3’s normally metallic high-temperature phase. We attribute these results to enhanced strain coupling at the interface between the substrate and film, highlighting the vulnerability of V2O3’s strongly correlated metallic phase to crystalline defects and structural disorder.

Enhanced tunnel magnetoresistance in fully epitaxial ZnO:Co-based magnetic tunnel junctions with Mg-doped ZnO barrier

The fully epitaxial ZnO-based ZnO:Co/ZnO:Mg/ZnO:Co magnetic tunnel junctions were grown on Al2O3(0001) substrate by oxygen plasma-assisted molecular beam epitaxy. The magnetoresistance behavior and spin injection through ZnO:Mg barrier were investigated. An enhanced positive tunnel magnetoresistance ratio of 85.6% is observed at 1.8 T at 5 K. The junction resistance at zero magnetic field is linear with respect to temperature power law T−4/3 between 5 K and 70 K, indicating that carriers tunnel through ZnO:Mg barrier via two localized states.

Tailoring of polar and nonpolar ZnO planes on MgO (001) substrates through molecular beam epitaxy

Polar and nonpolar ZnO thin films were deposited on MgO (001) substrates under different deposition parameters using oxygen plasma-assisted molecular beam epitaxy (MBE). The orientations of ZnO thin films were investigated by in situ reflection high-energy electron diffraction and ex situ X-ray diffraction (XRD). The film roughness measured by atomic force microscopy evolved as a function of substrate temperature and was correlated with the grain sizes determined by XRD. Synchrotron-based X-ray absorption spectroscopy (XAS) was performed to study the conduction band structures of the ZnO films. The fine structures of the XAS spectra, which were consistent with the results of density functional theory calculation, indicated that the polar and nonpolar ZnO films had different electronic structures. Our work suggests that it is possible to vary ZnO film structures from polar to nonpolar using the MBE growth technique and hence tailoring the electronic structures of the ZnO films.PACS: 81; 81.05.Dz; 81.15.Hi.

Nanoscale Thin Film Electrolytes for Clean Energy Applications

Ceria and zirconia based systems can be used as electrolytes to develop solid oxide fuel cells for clean energy production and to prevent air pollution by developing efficient, reliable oxygen sensors. In this study, we have used oxygen plasma assisted molecular beam epitaxy (OPA-MBE) to grow samaria doped ceria (SDC), to understand the role of dopant concentration and geometry of the films towards the ionic conduction in these thin films. We have also discussed the Gd doped CeO2 (GDC) and Gd stabilized ZrO2 (GSZ) multi-layer thin films to investigate the effect of interfacial phenomena on the ionic conductivity of these hetero-structures. We found the optimum concentration to be 15 mol % SmO1.5, for achieving lowest electrical resistance in SDC thin films. The electrical resistance decreases with the increase in film thickness up to 200 nm. The results demonstrate the usefulness of this study towards establishing an optimum dopant concentration and choosing an appropriate thin film thickness to ameliorate the conductance of the SDC material system. Furthermore, we have explored the conductivity of highly oriented GDC and GSZ multi-layer thin films, wherein the conductivity increased with an increase in the number of layers. The extended defects and lattice strain near themore » interfaces increase the density of oxygen vacancies, which leads to enhanced ionic conductivity in multi-layer thin films.« less

Electrical transport properties of Ti-doped Fe2O3(0001) epitaxial films

The electrical transport properties for compositionally and structurally well-defined epitaxial \ensuremath{\alpha}-(Ti${}_{x}$Fe${}_{1\ensuremath{-}x}$)${}_{2}$O${}_{3}$(0001) films have been investigated for $x$ \ensuremath{\le} 0.09. All films were grown by oxygen plasma-assisted molecular beam epitaxy using two different growth rates: 0.05--0.06 \AA{}/s and 0.22--0.24 \AA{}/s. Despite no detectable difference in cation valence and structural properties, films grown at the lower rate were highly resistive whereas those grown at the higher rate were semiconducting (\ensuremath{\rho} $=$ \ensuremath{\sim}1 \ensuremath{\Omega} \ifmmode\cdot\else\textperiodcentered\fi{} cm at 25 \ifmmode^\circ\else\textdegree\fi{}C). Hall effect measurements reveal carrier concentrations between 10${}^{19}$ and 10${}^{20}$ cm${}^{\ensuremath{-}3}$ at room temperature and mobilities in the range of 0.1 to 0.6 cm${}^{2}$/V \ifmmode\cdot\else\textperiodcentered\fi{} s for films grown at the higher rate. The conduction mechanism transitions from small-polaron hopping at higher temperatures to variable-range hopping at a transition temperature between 180 and 140 K. The absence of conductivity in the slow-grown films is attributed to donor electron compensation by cation vacancies, which may form to a greater extent at the lower rate because of higher oxygen fugacity at the growth front.

Integrated experimental and modeling study of the ionic conductivity of samaria-doped ceria thin films

Oxygen diffusion and ionic conductivity of samaria-doped ceria (SDC) thin films have been studied as a function of composition using experiment and atomistic simulation. SDC thin films were grown on Al 2O 3 (0001) substrates by oxygen plasma-assisted molecular beam epitaxy (OPA-MBE) technique. The experimental results show a peak in electrical conductivity of SDC at 15 mol% SmO 1.5. The ionic conductivity obtained from molecular dynamics simulation of the same system shows a peak at about 13 mol% SmO 1.5. The activation energy for oxygen diffusion was found to be in the range from 0.8 to 1 eV by simulations depending on the SmO 1.5 content, which compares well with the range from 0.6 to 0.9 eV given by the experimental work. The simulations also show that oxygen vacancies prefer Sm 3+ ions as first neighbors over Ce 4+ ions. The present results reveal that the optimum samaria content for ionic conductivity in single crystals of SDC is less than that in polycrystals, which can be related to the preferential segregation of dopant cations to grain boundaries in polycrystals.

Influence of growth rate on the epitaxial orientation and crystalline quality of CeO2 thin films grown on Al2O3(0001)

Growth rate-induced epitaxial orientations and crystalline quality of CeO2 thin films grown on Al2O3(0001) by oxygen plasma-assisted molecular beam epitaxy were studied using in situ and ex situ characterization techniques. CeO2 grows as three-dimensional (3D) islands and two-dimensional layers at growth rates of 1–7 Å/min and ≥9 Å/min, respectively. The formation of epitaxial CeO2(100) and CeO2(111) thin films occurs at growth rates of 1 Å/min and ≥9 Å/min, respectively. Glancing-incidence x-ray diffraction measurements have shown that the films grown at intermediate growth rates (2–7 Å/min) consist of polycrystalline CeO2 along with CeO2(100). The thin film grown at 1 Å/min exhibits six in-plane domains, characteristic of well-aligned CeO2(100) crystallites. The content of the poorly aligned CeO2(100) crystallites increases with increasing growth rate from 2 to 7 Å/min, and three out of six in-plane domains gradually decrease and eventually disappear, as confirmed by XRD pole figures. At growth rates ≥9 Å/min, CeO2(111) film with single in-plane domain was identified. The formation of CeO2(100) 3D islands at growth rates of 1–7 Å/min is a kinetically driven process unlike at growth rates ≥9 Å/min which result in an energetically and thermodynamically more stable CeO2(111) surface.

Comparison of the sputter rates of oxide films relative to the sputter rate of SiO2

There is a growing interest in knowing the sputter rates for a wide variety of oxides because of their increasing technological importance in many different applications. To support the needs of users of the Environmental Molecular Sciences Laboratory, a national scientific user facility, as well as our research programs, the authors made a series of measurements of the sputter rates from oxide films that have been grown by oxygen plasma-assisted molecular beam epitaxy, pulsed laser deposition, atomic layer deposition, electrochemical oxidation, or sputter deposition. The sputter rates for these oxide films were determined in comparison with those from thermally grown SiO2, a common reference material for sputter rate determination. The film thicknesses and densities for most of these oxide films were measured using x-ray reflectivity. These oxide films were mounted in an x-ray photoelectron or Auger electron spectrometer for sputter rate measurements using argon ion sputtering. Although the primary objective of this work was to determine relative sputter rates at a fixed angle, the measurements also examined (i) the angle dependence of the relative sputter rates, (ii) the energy dependence of the relative sputter rates, and (iii) the extent of ion beam induced reduction for some oxides. Oxide films examined include SiO2, Al2O3, CeO2, Cr2O3, Fe2O3, HfO2, In–Sn oxide, Ta2O5, TiO2 (anatase, rutile, and amorphous), and ZnO. The authors found that the sputter rates for the oxides can vary up to a factor of 2 (usually lower) from that observed for SiO2. The ratios of sputter rates relative to those of SiO2 appear to be relatively independent of ion beam energy in the range of 1–4 kV and for incident angles <50°. As expected, the extent of ion beam induced reduction of the oxides varies with the sputter angle.

The structure of the polar Sn-doped indium oxide (001) surface

Epitaxial Sn-doped In2O3 (ITO) thin films were grown using oxygen plasma-assisted molecular beam epitaxy (MBE) on (001) oriented Yttria Stabilized Zirconia. Low-energy-electron-diffraction shows that ITO(001) surface is oxygen terminated and has a c(1×1)-structure with p4g symmetry. Atomically-resolved Scanning Tunneling Microscopy suggests that surface oxygen atoms undergo dimerization; possible adsorption sites are identified. The density of surface oxygen depends on the Sn concentration and it is suggested that both, dimerization and doping stabilize the polar ITO(001) surface.

Surface Structure and Electronic Properties of In2O3(111) Single-Crystal Thin Films Grown on Y-Stabilized ZrO2(111)

We report the growth of high-quality In2O3(111) thin films on Y-ZrO2(111) by oxygen plasma-assisted molecular beam epitaxy. Ordered epitaxial growth was confirmed by X-ray diffraction and high-resolution transmission electron microscopy. Combined surface analysis techniques showed that the films exhibit atomically flat terraces supporting unreconstructed In2O3(111)-(1 × 1) surfaces. Valence band spectra excited by X-ray at hν = 1486.6 eV and hν = 6000 eV revealed a pronounced accumulation of electrons close to the surface region.

Growth and Characterization of Barium Oxide Nanoclusters on YSZ(111)

Barium oxide (BaO) was grown on a yttria-stabilized zirconia (YSZ) substrate by oxygen plasma-assisted molecular beam epitaxy. In situ reflection high-energy electron diffraction, ex situ X-ray diffraction (XRD), atomic force microscopy, and X-ray photoelectron spectroscopy (XPS) have confirmed that BaO grows as clusters on YSZ(111). During and following growth under ultrahigh vacuum conditions, we found BaO remained in single phase. When exposed to ambient conditions, the clusters transformed to BaCO3 and/or Ba(OH)2 H2O. However, in a few attempts of BaO growth, XRD results show a fairly single-phase cubic BaO with a lattice constant of 0.5418(1) nm. XPS results show that exposing BaO clusters to ambient conditions resulted in the formation of BaCO3 on the surface and partly Ba(OH)2 throughout the bulk. On the basis of the observations, it is concluded that the BaO nanoclusters grown on YSZ(111) are highly reactive in ambient conditions.

Growth-rate induced epitaxial orientation of CeO2 on Al2O3(0001)

High-quality CeO2 films were grown on Al2O3(0001) substrates using oxygen plasma-assisted molecular beam epitaxy. The epitaxial orientation of the films is found to be CeO2(100) and CeO2(111) at low (<8 Å/min) and higher growth rates (>12 Å/min), respectively. CeO2(100) film grows as three-dimensional islands, while CeO2(111) film grows as two-dimensional layers. The CeO2(100) film exhibits better epitaxial quality compared to CeO2(111) film. However, the CeO2(100) film on Al2O3(0001) shows three in-plane domains at 30° to each other. While the epitaxial quality is attributed to the close match between oxygen sublattices of CeO2(100) and Al2O3(0001), the three in-plane domains in CeO2(100) are attributed to the threefold symmetry of the substrate. The relative stability of different epitaxial orientations of CeO2 films on Al2O3(0001) obtained from molecular dynamics simulations strongly supports the experimental observations.