We demonstrate the epitaxial growth of tetragonal platinum monoxide (PtO) on MgO, TiO2, and β-Ga2O3 single-crystalline substrates by ozone molecular-beam epitaxy. We provide synthesis routes and derive a growth diagram under which PtO films can be synthesized by physical vapor deposition. A combination of electrical transport and photoemission spectroscopy measurements, in conjunction with density functional theory calculations, reveal PtO to be a degenerately doped p-type semiconductor with a bandgap of Eg ≈ 1.6 eV. Spectroscopic ellipsometry measurements are used to extract the complex dielectric function spectra, indicating a transition from free-carrier absorption to higher photon energy transitions at E ≈ 1.6 eV. Using tetragonal PtO as an anode contact, we fabricate prototype Schottky diodes on n-type Sn-doped β-Ga2O3 substrates and extract Schottky barrier heights of ϕB > 2.2 eV.

The molecular-beam epitaxial (MBE) growth of III-O and IV-O materials (e.g., Ga2O3, In2O3, and SnO2) is known to be reaction-limited by complex 2-step kinetics and the desorption of volatile suboxides (e.g., Ga2O, In2O, SnO). We find that the different surface reactivities of suboxides and respective elements (e.g., Ga, In, Sn) with active oxygen define the film-growth-windows (FGWs) and suboxide-formation-windows (SFWs) of III-O and IV-O materials, respectively. To generalize, we provide elementary reaction pathways and respective Gibbs energies to form binary III-O, III-Se, IV-O, and IV-Se ground-states as well as their subcompounds during their MBE growth. We apply the 2-step kinetics model established for oxides to identify the subselenide-limited growth of Ga2Se3 as the specific example for III-Se materials. Our kinetic and thermodynamic conclusions suggest subcompound-limited growth may be an inherent property for the growth of III-VI and IV-VI thin films by MBE and related epitaxial growth techniques.

Gadolinium (Gd) is a promising optically active lanthanide for UV emission. In this work, the optical emission properties of Gd-implanted monoclinic gallium oxide (beta-Ga2O3) thin films are investigated. Second phase formation (gamma-Ga2O3) is observed due to implantation-induced damage of the beta-Ga2O3 lattice. Annealing the implanted films results in various beta-Ga2O3 grain orientations. The relationship between the crystalline nature and the optical properties of the beta-Ga2O3:Gd3+ films is studied. Optical activation occurs after annealing 700 degrees C, revealing a photoluminescence (PL) band at 3.92 eV. This emission is attributed to the 6P7/2 , 8S7/2 transition of Gd3+ in beta-Ga2O3. Its four constituent emission components at 3.9118 eV, 3.9153 eV, 3.9221 eV and 3.9348 eV, due to the ion's 6P7/2 Stark splitting in the beta-Ga2O3 crystal field, are investigated. The transition energies are independent of annealing temperature and film growth method, highlighting the insensitivity the 4f7orbital to minor changes in the monoclinic crystal environment.

We investigate the composition of α-phase intermediate layers at epitaxial Ga2O3/Al2O3 interfaces using high angle annular dark field scanning transmission electron microscopy. Their presence is considered a general phenomenon as they are observed independent of the growth technique [Schewski et al., Appl. Phys. Exp. 8, 011101]. Samples were grown by plasma assisted molecular beam epitaxy using different growth conditions. Almost independent of these, the quantitative evaluation of the measured intensities gave Ga concentrations of ∼25%. We show that the previously published model, based on a pure α-Ga2O3 interlayer, fails if it is adapted to the measured composition. Density functional theory (DFT) computations were used to overcome the approximations made in this model and suggest that a stabilization of the layer is possible due to the low Ga concentration (≤35%) at which the α-phase is the most stable. Our surface model computations suggest an exchange of Ga atoms at the surface with Al atoms from the underlying substrate as a possible formation mechanism.

Abstract Transparent oxide thin film transistors (TFTs) are an important ingredient of transparent electronics. Their fabrication at the back‐end‐of‐line (BEOL) opens the door to novel strategies to more closely integrate logic with memory for data‐intensive computing architectures that overcome the scaling challenges of today's integrated circuits. A recently developed variant of molecular‐beam epitaxy (MBE) called suboxide MBE (S‐MBE) is demonstrated to be capable of growing epitaxial In2O3 at BEOL temperatures with unmatched crystal quality. The fullwidth at halfmaximum of the rocking curve is 0.015° and, thus, ≈5x narrower than any reports at any temperature to date and limited by the substrate quality. The key to achieving these results is the provision of an In2O beam by S‐MBE, which enables growth in adsorption control and is kinetically favorable. To benchmark this deposition method for TFTs, rudimentary devices were fabricated.

The growth of α-Ga2O3 and α-(InxGa1−x)2O3 on m-plane α-Al2O3(101̄0) by molecular beam epitaxy (MBE) and metal-oxide-catalyzed epitaxy (MOCATAXY) is investigated. By systematically exploring the parameter space accessed by MBE and MOCATAXY, phase-pure α-Ga2O3(101̄0) and α-(InxGa1−x)2O3(101̄0) thin films are realized. The presence of In on the α-Ga2O3 growth surface remarkably expands its growth window far into the metal-rich flux regime and to higher growth temperatures. With increasing O-to-Ga flux ratio (RO), In incorporates into α-(InxGa1−x)2O3 up to x ≤ 0.08. Upon a critical thickness, β-(InxGa1−x)2O3 nucleates and, subsequently, heteroepitaxially grows on top of α-(InxGa1−x)2O3 facets. Metal-rich MOCATAXY growth conditions, where α-Ga2O3 would not conventionally stabilize, lead to single-crystalline α-Ga2O3 with negligible In incorporation and improved surface morphology. Higher TTC further results in single-crystalline α-Ga2O3 with well-defined terraces and step edges at their surfaces. For RO ≤ 0.53, In acts as a surfactant on the α-Ga2O3 growth surface by favoring step edges, while for RO ≥ 0.8, In incorporates and leads to a-plane α-(InxGa1−x)2O3 faceting and the subsequent (2̄01) β-(InxGa1−x)2O3 growth on top. Thin film analysis by scanning transmission electron microscopy reveals highly crystalline α-Ga2O3 layers and interfaces. We provide a phase diagram to guide the MBE and MOCATAXY growth of single-crystalline α-Ga2O3 on α-Al2O3(101̄0).

In this work, we investigate the growth of monoclinic β-(InxGa1-x)2O3 alloys on top of (010) β-Ga2O3 substrates via plasma-assisted molecular beam epitaxy. In particular, using different in situ (reflection high-energy electron diffraction) and ex situ (atomic force microscopy, X-ray diffraction, time-of-flight secondary ion mass spectrometry, and transmission electron microscopy) characterization techniques, we discuss (i) the growth parameters that allow for In incorporation and (ii) the obtainable structural quality of the deposited layers as a function of the alloy composition. In particular, we give experimental evidence of the possibility of coherently growing (010) β-(InxGa1-x)2O3 layers on β-Ga2O3 with good structural quality for x up to ≈ 0.1. Moreover, we show that the monoclinic structure of the underlying (010) β-Ga2O3 substrate can be preserved in the β-(InxGa1-x)2O3 layers for wider concentrations of In (x ≤ 0.19). Nonetheless, the formation of a large amount of structural defects, like unexpected (102̅) oriented twin domains and partial segregation of In is suggested for x > 0.1. Strain relaxes anisotropically, maintaining an elastically strained unit cell along the a* direction vs plastic relaxation along the c* direction. This study provides important guidelines for the low-end side tunability of the energy bandgap of β-Ga2O3-based alloys and provides an estimate of its potential in increasing the confined carrier concentration of two-dimensional electron gases in β-(InxGa1-x)2O3/(AlyGa1-y)2O3 heterostructures.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Sushma Raghuvansy, Jon P. McCandless, Marco Schowalter, Alexander Karg, Manuel Alonso-Orts, Martin S. Williams, Christian Tessarek, Stephan Figge, Kazuki Nomoto, Huili Grace Xing, Darrell G. Schlom, Andreas Rosenauer, Debdeep Jena, Martin Eickhoff, Patrick Vogt; Erratum: “Growth of β-Ga2O3 and ε/κ-Ga2O3 on AlN(0001) by molecular-beam epitaxy” [APL Mater. 11, 111113 (2023)]. APL Mater. 1 January 2024; 12 (1): 019902. https://doi.org/10.1063/5.0192370 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioAPL Materials Search Advanced Search |Citation Search

Sapphire is a technologically highly relevant material, but it poses many challenges when performing epitaxial thin-film deposition. We have identified and applied the conditions for adsorption-controlled homoepitaxial growth of c-plane sapphire. The films thus grown are atomically smooth, have a controlled termination, and are of outstanding crystallinity. Their chemical purity exceeds that of the substrates. The films exhibit exceptional optical properties, such as a single-crystal-like bandgap and a low density of F+ centers.

The growth of $\alpha$-Ga$_2$O$_3$ and $\alpha$-(In$_x$Ga$_{1-x}$)$_2$O$_3$ on $m$-plane $\alpha$-Al$_2$O$_3$(10$\bar{1}$0) by molecular beam epitaxy (MBE) and metal-oxide-catalyzed epitaxy (MOCATAXY) is investigated. By systematically exploring the parameter space accessed by MBE and MOCATAXY, phase-pure $\alpha$-Ga$_2$O$_3$(10$\bar{1}$0) and $\alpha$-(In$_x$Ga$_{1-x}$)$_2$O$_3$(10$\bar{1}$0) thin films are realized. The presence of In on the $\alpha$-Ga$_2$O$_3$ growth surface remarkably expands its growth window far into the metal-rich flux regime and to higher growth temperatures. With increasing O-to-Ga flux ratio ($R_{\text{O}}$), In incorporates into $\alpha$-(In$_x$Ga$_{1-x}$)$_2$O$_3$ up to $x \leq 0.08$. Upon a critical thickness, $\beta$-(In$_x$Ga$_{1-x}$)$_2$O$_3$ nucleates and subsequently heteroepitaxially grows on top of $\alpha$-(In$_x$Ga$_{1-x}$)$_2$O$_3$ facets. Metal-rich MOCATAXY growth conditions, where $\alpha$-Ga$_2$O$_3$ would not conventionally stabilize, lead to single-crystalline $\alpha$-Ga$_2$O$_3$ with negligible In incorporation and improved surface morphology. Higher $T_{\text{G}}$ further results in single-crystalline $\alpha$-Ga$_2$O$_3$ with well-defined terraces and step edges at their surfaces. For $R_{\text{O}} \leq 0.53$, In acts as a surfactant on the $\alpha$-Ga$_2$O$_3$ growth surface by favoring step edges, while for $R_{\text{O}} \geq 0.8$, In incorporates and leads to a-plane $\alpha$-(In$_x$Ga$_{1-x}$)$_2$O$_3$ faceting and the subsequent ($\bar{2}$01) $\beta$-(In$_x$Ga$_{1-x}$)$_2$O$_3$ growth on top. Thin film analysis by STEM reveals highly crystalline $\alpha$-Ga$_2$O$_3$ layers and interfaces. We provide a phase diagram to guide the MBE and MOCATAXY growth of single-crystalline $\alpha$-Ga$_2$O$_3$ on $\alpha$-Al$_2$O$_3$(10$\bar{1}$0).

The growth of α-Ga_2O_3 and α-(In_xGa_1-x)_2O_3 on m-plane α-Al_2O_3(101̅0) by molecular beam epitaxy (MBE) and metal-oxide-catalyzed epitaxy (MOCATAXY) is investigated. By systematically exploring the parameter space accessed by MBE and MOCATAXY, phase-pure α-Ga_2O_3(101̅0) and α-(In_xGa_1-x)_2O_3(101̅0) thin films are realized. The presence of In on the α-Ga_2O_3 growth surface remarkably expands its growth window far into the metal-rich flux regime and to higher growth temperatures. With increasing O-to-Ga flux ratio (R_O), In incorporates into α-(In_xGa_1-x)_2O_3 up to x ≤ 0.08. Upon a critical thickness, β-(In_xGa_1-x)_2O_3 nucleates and subsequently heteroepitaxially grows on top of α-(In_xGa_1-x)_2O_3 facets. Metal-rich MOCATAXY growth conditions, where α-Ga_2O_3 would not conventionally stabilize, lead to single-crystalline α-Ga_2O_3 with negligible In incorporation and improved surface morphology. Higher T_G further results in single-crystalline α-Ga_2O_3 with well-defined terraces and step edges at their surfaces. For R_O≤ 0.53, In acts as a surfactant on the α-Ga_2O_3 growth surface by favoring step edges, while for R_O≥ 0.8, In incorporates and leads to a-plane α-(In_xGa_1-x)_2O_3 faceting and the subsequent (2̅01) β-(In_xGa_1-x)_2O_3 growth on top. Thin film analysis by STEM reveals highly crystalline α-Ga_2O_3 layers and interfaces. We provide a phase diagram to guide the MBE and MOCATAXY growth of single-crystalline α-Ga_2O_3 on α-Al_2O_3(101̅0).

We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow β-Ga2O3 at a growth rate of ∼1 µm/h with control of the silicon doping concentration from 5 × 1016 to 1019 cm−3. In S-MBE, pre-oxidized gallium in the form of a molecular beam that is 99.98% Ga2O, i.e., gallium suboxide, is supplied. Directly supplying Ga2O to the growth surface bypasses the rate-limiting first step of the two-step reaction mechanism involved in the growth of β-Ga2O3 by conventional MBE. As a result, a growth rate of ∼1 µm/h is readily achieved at a relatively low growth temperature (Tsub ≈ 525 °C), resulting in films with high structural perfection and smooth surfaces (rms roughness of <2 nm on ∼1 µm thick films). Silicon-containing oxide sources (SiO and SiO2) producing an SiO suboxide molecular beam are used to dope the β-Ga2O3 layers. Temperature-dependent Hall effect measurements on a 1 µm thick film with a mobile carrier concentration of 2.7 × 1017 cm−3 reveal a room-temperature mobility of 124 cm2 V−1 s−1 that increases to 627 cm2 V−1 s−1 at 76 K; the silicon dopants are found to exhibit an activation energy of 27 meV. We also demonstrate working metal–semiconductor field-effect transistors made from these silicon-doped β-Ga2O3 films grown by S-MBE at growth rates of ∼1 µm/h.

The heteroepitaxial growth and phase formation of Ga2O3 on Al-polar AlN(0001) templates by molecular-beam epitaxy (MBE) are studied. Three different MBE approaches are employed: (i) conventional MBE, (ii) suboxide MBE (S-MBE), and (iii) metal-oxide-catalyzed epitaxy (MOCATAXY). We grow phase-pure β-Ga2O3(2̄01) and phase-pure ϵ/κ-Ga2O3(001) with smooth surfaces by S-MBE and MOCATAXY. Thin film analysis shows that the crystallographic and surface features of the β-Ga2O3(2̄01)/AlN(0001) and ϵ/κ-Ga2O3(001)/AlN(0001) epilayers are of high crystalline quality. Growth and phase diagrams are developed to synthesize Ga2O3 on AlN by MBE and MOCATAXY and to provide guidance to grow Ga2O3 on several non-oxide surfaces, e.g., AlN, GaN, and SiC, by MBE, S-MBE, and MOCATAXY.

The epitaxial growthof Ga2O3 andIn(2)O(3) on (0001)-oriented Al2O3 by plasma-assisted molecular beam epitaxy is investigated.The synthesisof both materials underlies similar growth kinetics, with the formationof volatile suboxides limiting their growth in the metal-rich regime.Quantitative analysis of beta-Ga2O3 and c-In2O3 growth kinetics has been performed on appropriatebuffer layers mimicking homoepitaxial growth conditions. In particular,we investigate the influence of the amount of supplied atomic oxygenon the growth kinetics of beta-Ga2O3. Consideringthe heteroepitaxial growth on bare (0001) Al2O3, we found that the observed nucleation window is much more limitedthan the expected growth window derived from homoepitaxial growthconditions. For Ga2O3 this was studied in detailand the upper limit of the nucleation window was determined in termsof the gallium to oxygen ratio on the growth surface and the growthtemperature. Outside the nucleation window, we report that three monolayers alpha-(Al0.75 +/- 0.04Ga0.25 -/+ 0.04)(2)O-3 formed in the initial stage of growth passivatethe surface with respect to further beta-Ga2O3 nucleation. We attribute the formation of this layer to the intermixingof a Ga-rich adlayer and Al atoms from the substrate until the solubilitylimit in the alpha-phase is reached. The observation is accompaniedby a specific RHEED pattern with satellite reflexes assigned to asurface reconstruction in the metal-rich growth regime. We report the existence of a nucleation window of beta-Ga2O3 and c-In2O3 epitaxiallygrown on (0001) Al2O3. Its size is decisivelyinfluenced by the growth temperature and the III-V surfaceratio. Outside of the nucleation window we observe solely the formationof three monolayer beta-(Al0.75Ga0.25)(2)O-3 by an intermixing process accompanied with aunique surface reconstruction.

We report the growth of α -Ga 2 O 3 on m -plane α -Al 2 O 3 by conventional plasma-assisted molecular-beam epitaxy and In-mediated metal–oxide-catalyzed epitaxy (MOCATAXY). We report a growth rate diagram for α -Ga 2 O 3 ( 10 1 ¯ 0 ), and observe (i) a growth rate increase, (ii) an expanded growth window, and (iii) reduced out-of-plane mosaic spread when MOCATAXY is employed for the growth of α -Ga 2 O 3 . Through the use of In-mediated catalysis, growth rates over 0.2 μ m h −1 and rocking curves with full width at half maxima of Δ ω ≈ 0.45° are achieved. Faceting is observed along the α -Ga 2 O 3 film surface and explored through scanning transmission electron microscopy.

The influence of In on the growth of ɛ-Ga2O3 by plasma-assisted molecular beam epitaxy is investigated. We demonstrate alloying of ɛ-Ga2O3 with In and describe its incorporation limits and catalytic effect on the growth kinetics. A special focus lies on the metal-rich growth regime, where we show that In acts as a surfactant for the ɛ-Ga2O3 growth. Both the In-incorporation and the ɛ-(In,Ga)2O3 growth rate follow a non-monotonous trend with increasing In-supply. Whereas both entities show an increase for low In-fluxes, they decrease again for very high In-fluxes, combined with the formation of an atomically smooth surface for layers with thicknesses of several hundred nanometers in this regime. Based on these results, the influence of the growth temperature and the amount of Sn, supplied to initiate the ɛ-Ga2O3 phase formation, is discussed, revealing their impact on the surfactant ability and incorporation of In. Using In as a surfactant, we demonstrate the pseudomorphic growth of ɛ-(In,Ga)2O3/ɛ-(Al,In,Ga)2O3 heterostructures with sharp interfaces and surfaces.

We observe a catalytic mechanism during the growth of III-O and IV-O materials by suboxide molecular-beam epitaxy ($S$-MBE). By supplying the molecular catalysts In$_2$O and SnO we increase the growth rates of Ga$_2$O$_3$ and In$_2$O$_3$. This catalytic action is explained by a metastable adlayer $A$, which increases the reaction probability of the reactants Ga$_2$O and In$_2$O with active atomic oxygen, leading to an increase of the growth rates of Ga$_2$O$_3$ and In$_2$O$_3$. We derive a model for the growth of binary III-O and IV-O materials by $S$-MBE and apply these findings to a generalized catalytic description for metal-oxide catalyzed epitaxy (MOCATAXY), applicable to elemental and molecular catalysts. We derive a mathematical description of $S$-MBE and MOCATAXY providing a computational framework to set growth parameters in previously inaccessible kinetic and thermodynamic growth regimes when using the aforementioned catalysis. Our results indicate MOCATAXY takes place with a suboxide catalyst rather than with an elemental catalyst. As a result of the growth regimes achieved, we demonstrate a Ga$_2$O$_3$/Al$_2$O$_3$ heterostructure with unrivaled crystalline quality, paving the way to the preparation of oxide device structures with unprecedented perfection.