We performed interfacial electric and electronic studies of both in-situ and ex-situ atomic-layer deposited (ALD) Al₂O₃ films on InGaAs. Self-aligned inversion-channel metal-oxide-semiconductor field-effect-transistors (MOSFETs) with a 1 μm gate length (L_g) from the in-situ sample have extrinsic drain currents (I_d) of 1.8 mA/μm, transconductances (G_m) of 0.98 mS/μm, and an effective mobility (μ_eff) of 1250 cm²/V s. MOSFETs that employ ex-situ ALD-Al₂O₃ have an I_d of 0.56 mA/μm, G_m of 0.28 mS/μm, and μ_eff of 410 cm²/V s. Synchrotron radiation photoemission reveals no AsO_x residue at the Al₂O₃/InGaAs interface using the in-situ approach, whereas some AsO_x residue is detected using the ex-situ method.

Single-crystal atomic-layer-deposited (ALD) Y\(_{\mathrm{2}}\)O\(_{\mathrm{3}}\) films 2 nm thick were epitaxially grown on molecular beam epitaxy (MBE) GaAs(001)-4 \(\times\) 6 and GaAs(111)A-2 \(\times\) 2 reconstructed surfaces. The in-plane epitaxy between the ALD-oxide films and GaAs was observed using \textit{in-situ} reflection high-energy electron diffraction in our uniquely designed MBE/ALD multi-chamber system. More detailed studies on the crystallography of the hetero-structures were carried out using high-resolution synchrotron radiation X-ray diffraction. When deposited on GaAs(001), the Y\(_{\mathrm{2}}\)O\(_{\mathrm{3}}\) films are of a cubic phase and have (110) as the film normal, with the orientation relationship being determined: Y\(_{\mathrm{2}}\)O\(_{\mathrm{3}}\)(\(110\))[\(001\)][\(\overline{1}10\)]//GaAs(\(001\))[\(110\)][\(1\overline{1}0\)]. On GaAs(\(111\))A, the Y\(_{\mathrm{2}}\)O\(_{\mathrm{3}}\) films are also of a cubic phase with (\(111\)) as the film normal, having the orientation relationship of Y\(_{\mathrm{2}}\)O\(_{\mathrm{3}}\)(\(111\))[\(2\overline{1}\overline{1}\)] [\(01\overline{1}\)]//GaAs (\(111\)) [\(\overline{2}11\)][\(0\overline{1}1\)]. The relevant orientation for the present/future integrated circuit platform is (\(001\)). The ALD-Y\(_{\mathrm{2}}\)O\(_{\mathrm{3}}\)/GaAs(\(001\))-4 \(\times\) 6 has shown excellent electrical properties. These include small frequency dispersion in the capacitance-voltage CV curves at accumulation of ~7% and ~14% for the respective p- and n-type samples with the measured frequencies of 1 MHz to 100 Hz. The interfacial trap density (Dit) is low of ~10\(^{12}\) cm\(^{−2}\)eV\(^{−1}\) as extracted from measured quasi-static CVs. The frequency dispersion at accumulation and the D\(_{it}\) are the lowest ever achieved among all the ALD-oxides on GaAs(\(001\)).

A low interfacial trap density (D _it) of 2.2 × 10¹¹ eV⁻¹ cm⁻² has been achieved with an atomic layer deposited (ALD) single crystal Y₂O₃ epitaxially on n-GaAs(001), along with a small frequency dispersion of 10.3% (2.6%/decade) at the accumulation region in the capacitance–voltage (C–V) curves. The D _it and frequency dispersion in the C–V curves in this work are the lowest among all of the reported ALD-oxides on n-type GaAs(001). The D _it was measured using the conductance–voltage (G–V) and quasi-static C–V (QSCV) methods. Moreover, the heterostructure was thermally stable with rapid annealing at 900 °C under various durations in He and N₂, which has not been achieved in the heterostructures of ALD-Al₂O₃ or HfO₂ on GaAs.

Achieving high material perfection in aluminum (Al) films and their associated Al/AlOx heterostructures is essential for enhancing the coherence time in superconducting quantum circuits. We grew Al films with thicknesses ranging from 3 to 30 nanometers (nm) epitaxially on sapphire substrates using molecular beam epitaxy (MBE). An integral aspect of our work involved electron-beam (e-beam) evaporation to directly deposit aluminum oxide (Al2O3) films on the freshly grown ultrathin epitaxial Al films in an ultra-high-vacuum (UHV) environment. This in situ oxide deposition is critical for preventing the oxidation of parts of the Al films, avoiding the formation of undesired native oxides, and thereby preserving the nm-thick Al films in their pristine conditions. The thicknesses of our Al films in the study were accurately determined; for example, coherence lengths of 3.0 and 20.2 nm were measured in the nominal 3.0 and 20 nm thick Al films, respectively. These Al films were epitaxially grown on sapphire substrates, showing an orientational relationship, denoted as Al(111)⟨21¯1¯⟩∥sapphire(0001)[21¯1¯0]. The Al/sapphire interface was atomically ordered without any interfacial layers, as confirmed by in situ reflection high-energy electron diffraction (RHEED) and cross-sectional scanning transmission electron microscopy (STEM). All sample surfaces exhibited smoothness with a roughness in the range of 0.1–0.2 nm. The Al films are superconducting with critical temperatures ranging from 1.23 to around 2 K, depending on the film thickness.