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We report a two-step film-growth process using suboxide molecular-beam epitaxy (S-MBE) that produces Si-doped α-Ga2O3 with record transport properties. The method involves growing a relaxed α-(AlxGa1−x)2O3 buffer layer on m-plane sapphire at a relatively high substrate temperature (Tsub), ∼750 °C, followed by an Si-doped α-Ga2O3 overlayer grown at lower Tsub, ∼500 °C. The high Tsub allows the ∼3.6% lattice-mismatched α-(AlxGa1−x)2O3 buffer with x = 0.08 ± 0.02 to remain epitaxial and phase pure during relaxation to form a pseudosubstrate for the overgrowth of α-Ga2O3. The optimal conditions for the subsequent growth of Si-doped α-Ga2O3 by S-MBE are 425 °C ≤ Tsub ≤ 500 °C and P80% O3 = 5 × 10−6 Torr. Si-doped α-Ga2O3 films grown with this method at Tsub > 550 °C are always insulating. Secondary-ion mass spectrometry confirms that both the insulating and conductive films have uniform silicon incorporation. In conductive films with 1019 ≤ NSi ≤ 1020 cm−3, the incorporated silicon is ∼100% electrically active. At NSi ≤ 1019 cm−3, the carrier concentration (n) plummets. A maximum Hall mobility (μ) = 90 cm2V·s at room-temperature is measured in a film with n = 2.9 × 1019 cm−3 and a maximum conductivity (σ) = 650 S/cm at room-temperature in a film with n = 4.8 × 1019 cm−3. A threading dislocation density of (5.6 ± 0.6) × 1010 cm−2 is revealed by scanning transmission electron microscopy, showing that there is still enormous room to improve the electrical properties of doped α-Ga2O3 thin films. 

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. 

AlScN is a new wide bandgap, high-k, ferroelectric material for radio frequency (RF), memory, and power applications. Successful integration of high-quality AlScN with GaN in epitaxial layer stacks depends strongly on the ability to control lattice parameters and surface or interface through growth. This study investigates the molecular beam epitaxy growth and transport properties of AlScN/GaN multilayer heterostructures. Single-layer Al1−xScxN/GaN heterostructures exhibited lattice-matched composition within x = 0.09–0.11 using substrate (thermocouple) growth temperatures between 330 and 630 °C. By targeting the lattice-matched Sc composition, pseudomorphic AlScN/GaN multilayer structures with ten and twenty periods were achieved, exhibiting excellent structural and interface properties as confirmed by x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). These multilayer heterostructures exhibited substantial polarization-induced net mobile charge densities of up to 8.24 × 1014/cm2 for twenty channels. The sheet density scales with the number of AlScN/GaN periods. By identifying lattice-matched growth condition and using it to generate multiple conductive channels, this work enhances our understanding of the AlScN/GaN material platform. 

Development of a high-performance, p-type oxide channel is crucial to realize all-oxide complementary metal–oxide semiconductor technology that is amenable to 3D integration. Among p-type oxides, α-SnO is one of the most promising owing to its relatively high hole mobility {as high as 21 cm2 V−1 s−1 has been reported [M. Minohara et al., J. Phys. Chem. C 124, 1755–1760 (2020)]}, back-end-of-line compatible processing temperature (≤400 °C), and good optical transparency for visible light. Unfortunately, doping control has only been demonstrated over a limited range of hole concentrations in such films. Here, we demonstrate systematic control of the hole concentration of α-SnO thin films via potassium doping. First-principles calculations identify potassium substitution on the tin site (KSn) of α-SnO to be a promising acceptor that is not (self)-compensated by native vacancies or potassium interstitials (Ki). We synthesize epitaxial K-doped α-SnO thin films with controlled doping concentration using suboxide molecular-beam epitaxy. The concentration of potassium is measured by secondary ion mass spectrometry, and its incorporation into the α-SnO structure is corroborated by x-ray diffraction. The effect of potassium doping on the optical response of α-SnO is measured by spectroscopic ellipsometry. Potassium doping provides systematic control of hole doping in α-SnO thin films over the 4.8 × 1017 to 1.5 × 1019 cm−3 range without significant degradation of hole mobility or the introduction of states that absorb visible light. Temperature-dependent Hall measurements reveal that the potassium is a shallow acceptor in α-SnO with an ionization energy in the 10–20 meV range. 

Unlike naturally occurring oxide crystals such as ruby and gemstones, there are no naturally occurring nitride crystals because the triple bond of the nitrogen molecule is one of the strongest bonds in nature. Here, we report that when the transition metal scandium is subjected to molecular nitrogen, it self-catalyzes to break the nitrogen triple bond to form highly crystalline layers of ScN, a semiconductor. This reaction proceeds even at room temperature. Self-activated ScN films have a twin cubic crystal structure, atomic layering, and electronic and optical properties comparable to plasma-based methods. We extend our research to showcase Sc’s scavenging effect and demonstrate self-activated ScN growth under various growth conditions and on technologically significant substrates, such as 6H–SiC, AlN, and GaN. Ab initio calculations elucidate an energetically efficient pathway for the self-activated growth of crystalline ScN films from molecular N2. The findings open a new pathway to ultralow-energy synthesis of crystalline nitride semiconductor layers and beyond. 

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 In₂O₃at 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. 

We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow α-(AlxGa1−x)2O3 films on (110) sapphire substrates over the 0 < x < 0.95 range of aluminum content. In S-MBE, 99.98% of the gallium-containing molecular beam arrives at the substrate in a preoxidized form as gallium suboxide (Ga2O). This bypasses the rate-limiting step of conventional MBE for the growth of gallium oxide (Ga2O3) from a gallium molecular beam and allows us to grow fully epitaxial α-(AlxGa1−x)2O3 films at growth rates exceeding 1 µm/h and relatively low substrate temperature (Tsub = 605 ± 15 °C). The ability to grow α-(AlxGa1−x)2O3 over the nominally full composition range is confirmed by Vegard’s law applied to the x-ray diffraction data and by optical bandgap measurements with ultraviolet–visible spectroscopy. We show that S-MBE allows straightforward composition control and bandgap selection for α-(AlxGa1−x)2O3 films as the aluminum incorporation x in the film is linear with the relative flux ratio of aluminum to Ga2O. The films are characterized by atomic-force microscopy, x-ray diffraction, and scanning transmission electron microscopy (STEM). These α-(AlxGa1−x)2O3 films grown by S-MBE at record growth rates exhibit a rocking curve full width at half maximum of ≊ 12 arc secs, rms roughness <1 nm, and are fully commensurate for x ≥ 0.5 for 20–50 nm thick films. STEM imaging of the x = 0.78 sample reveals high structural quality and uniform composition. Despite the high structural quality of the films, our attempts at doping with silicon result in highly insulating films. 

We report the structural and electronic properties of NbN/GaN junctions grown by plasma-assisted molecular beam epitaxy. High crystal-quality NbN films grown on GaN exhibit superconducting critical temperatures in excess of 10 K for thicknesses as low as 3 nm. We observe that the NbN lattice adopts the stacking sequence of the underlying GaN and that domain boundaries in the NbN thereby occur at the site of atomic steps in the GaN surface. The electronic properties of the NbN/GaN junction are characterized using Schottky barrier diodes. Current–voltage–temperature and capacitance–voltage measurements are used to determine the Schottky barrier height of the NbN/GaN junction, which we conclude is ∼1.3 eV.