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We report on quantum confined Ge1-x-ySi_ySn_xnanocrystals demonstrating both size- and composition-tunable direct visible/NIR emission (1.77 – 2.47 eV) and recombination dynamics. Temperature-dependent time-resolved photoluminescence suggests significant enhancement of oscillator strengths with Si incorporation. 

An in situ metal-organic chemical vapor phase epitaxy is used to grow a complete AlGaN/GaN metal oxide semiconductor heterojunction field effect transistor (MOSHFET) structure, gated by a gallium oxide (Ga2O3) layer; we observed reduction in the interfacial trap density compared to its version wherein the Ga2O3 was grown ex situ, after breaking the vacuum, all else being the same. A remarkable decrease in the interfacial charge density for in situ MOSHFET structures in the range of 70%–88% for 10–30 nm oxide layer thickness and improvements in other electrical parameters required for high-performing devices were observed. 

Group IV alloy nanocrystals (NCs) are a class of direct energy gap semiconductors that show high elemental abundance, low to non-toxicity, and composition-tunable absorption and emission properties. These properties have distinguished Ge1-xSnx NCs as an intriguing material for near-infrared (IR) optical studies. Achieving a material with efficient visible emission requires a modified class of Group IV alloys and the computational studies suggest that this can be achieved with Ge1-x-ySiySnx NCs. Herein, we report a colloidal strategy for the synthesis of bulk-like (10.3 ± 2.5 – 25.5 ± 5.3 nm) and quantum-confined (3.2 ± 0.6 – 4.2 ± 1.1 nm) Ge1-x-ySiySnx alloys that show strong size confinement effects and composition-tunable visible to near IR absorption and emission properties. This synthesis produces a homogeneous alloy with diamond cubic Ge structure and tunable Si (0.9 – 16.1%) and Sn (1.8 – 14.9%) compositions, exceeding the equilibrium solubility of Sn (<1%) in crystalline Si and Ge. Raman spectra of Ge1-x-ySiySnx alloys show a prominent redshift of the Ge-Ge peak and the emergence of a Ge-Si peak with increasing Si/Sn, suggesting the growth of homogeneous alloys. The smaller Ge1-x-ySiySnx NCs exhibit absorption onsets from 1.21 to 1.94 eV for x = 1.8 – 6.8% and y = 0.9 – 16.1% compositions, which are blueshifted from those reported for Ge1-x-ySiySnx bulk alloy films and Ge1-xSnx alloy NCs, indicating the influence of Si incorporation and strong size confinement effects. Solid-state photoluminescence (PL) spectra reveal core-related PL maxima from 1.77 – 1.97 eV in agreement with absorption onsets, consistent with the energy gaps calculated for ~3–4 nm alloy NCs. With facile low-temperature solution synthesis and direct control over physical properties, this methodology presents a noteworthy advancement in the synthesis of bulk-like and quantum-confined Ge1-x-ySiySnx alloys as versatile materials for future optical and electronic studies. 

Size-confined Si nanorods (NRs) have gained notable interest because of their tunable photophysical properties that make them attractive for optoelectronic, charge storage, and sensor technologies. However, established routes for fabrication of Si NRs use well-defined substrates and/or nanoscopic seeds as promoters that cannot be easily removed, hindering the investigation of their true potential and physical properties. Herein, we report a facile, one-step route for the fabrication of Si NRs via thermal disproportionation of hydrogen silsesquioxane (HSQ) in the presence of a molecular tin precursor (SnCl4) at a substantially lower temperature (450 ºC) compared to those used in the synthesis of size-confined Si nanocrystals (>1000 ºC). The use of these precursors allows the facile isolation of phase pure Si NRs via HF etching and subsequent surface passivation with 1-dodecene via hydrosilylation. The diameters (7.7–16.5 nm) of the NRs can be controlled by varying the amount of SnCl4 (0.2–3.0%) introduced during the HSQ synthesis. Physical characterization of the NRs suggests that the diamond cubic structure is not affected by the SnCl4, HF etching, and hydrosilylation. Surface analysis of NRs indicates the presence of Si0 and Sin+ species, which can be attributed to core Si and surface Si species bonded to dodecane ligands, respectively, and a systematic variation of Si0: Si-C ratio with the NR diameter. The NRs show strong size confinement effects with solid-state absorption onsets (2.51–2.80 eV) and solution-state (Tauc) indirect energy gaps (2.54–2.70 eV) that can be tuned by varying the diameters (16.5–7.7 nm), respectively. Photoluminescence (PL) and time-resolved PL (TRPL) studies reveal size-dependent emission (1.95–2.20 eV) with short, nanosecond lifetimes across the visible spectrum which trend closely to absorption trends seen in solid-state absorption data. The facile synthesis developed for size-confined Si NRs with high crystallinity and tunable optical properties will promote their application in optoelectronic, charge storage, and sensing studies. 

We demonstrated a metal-organic chemical vapor deposition (MOCVD) of smooth, thick, and monoclinic phase-pure gallium oxide (Ga2O3) on c-plane sapphire using silicon-oxygen bonding (SiOx) as a phase stabilizer. The corundum (α), monoclinic (β), and orthorhombic (ε) phases of Ga2O3 with a bandgap in the 4.4 – 5.1 eV range, are promising materials for power semiconductor devices and deep ultraviolet (UV) solar-blind photodetectors. The MOCVD systems are extensively used for homoepitaxial growth of β-Ga2O3 on (001), (100), (010), and (¯2 01) β-Ga2O3 substrates. These substrates are rare/expensive and have very low thermal conductivity; thus, are not suitable for high-power semiconductor devices. The c-plane sapphire is typically used as a substrate for high-power devices. The β-Ga2O3 grows in the (¯2 01) direction on sapphire. In this direction, the presence of high-density oxygen dangling bonds, frequent stacking faults, twinning, and other phases and planes impede the heteroepitaxy of thick β-Ga2O3. Previously phase stabilizations with SiOx have been reported for tetragonal and monoclinic hafnia. We were able to grow ~580nm thick β-Ga2O3 on sapphire by MOCVD at 750 oC through phase stabilization using silane. The samples grown with silane have a reduction in the surface roughness and resistivity from 10.7 nm to 4.4 nm and from 371.75 Ω.cm to 135.64 Ω.cm, respectively. These samples show a pure-monoclinic phase determined by x-ray diffraction (XRD); have tensile strain determined by Raman strain mapping. These results show that a thick, phase-pure -Ga2O3 can be grown on c-plane sapphire which can be suitable for creating power devices with better thermal management. 

We report the gate leakage current and threshold voltage characteristics of Al0.3Ga0.7N/GaN heterojunction field effect transistor (HFET) with metal-organic chemical vapor deposition (MOCVD) grown β-Ga2O3 as a gate dielectric for the first time. In this study, GaN channel HFET and β-Ga2O3 passivated metal-oxide-semiconductor-HFET (MOS-HFET) structures were grown in MOCVD using N2 as carrier gas on a sapphire substrate. X-ray diffraction (XRD) and atomic force microscopy (AFM) were used to characterize the structural properties and surface morphology of the heterostructure. The electrical properties were analyzed using van der Pauw, Hall, and the mercury probe capacitance-voltage (C-V) measurement systems. The 2-dimensional electron gas (2DEG) carrier density for the heterostructure was found to be in the order of ~1013 cm-2. The threshold voltage shifted more towards the negative side for the MOSHFET. The high-low (Hi-Lo) frequency-based C-V method was used to calculate the interface charge density for the oxide-AlGaN interface and was found to be in the order of ~1012 cm2eV-1. A remarkable reduction in leakage current from 2.33×10-2 A/cm2 for HFET to 1.03×10-8 A/cm2 for MOSHFET was observed demonstrating the viability of MOCVD-grown Ga2O3 as a gate dielectric. 

We report the electrical properties of Al0.3Ga0.7N/GaN heterojunction field effect transistor (HFET) structures with a Ga2O3 passivation layer grown by metal–organic chemical vapor deposition (MOCVD). In this study, three different thicknesses of β-Ga2O3 dielectric layers were grown on Al0.3Ga0.7N/GaN structures leading to metal-oxide-semiconductor-HFET or MOSHFET structures. X-ray diffraction (XRD) showed the (2¯01) orientation peaks of β-Ga2O3 in the device structure. The van der Pauw and Hall measurements yield the electron density of ~ 4 × 1018 cm−3 and mobility of ~770 cm2V−1s−1 in the 2-dimensional electron gas (2DEG) channel at room temperature. Capacitance–voltage (C-V) measurement for the on-state 2DEG density for the MOSHFET structure was found to be of the order of ~1.5 × 1013 cm−2. The thickness of the Ga2O3 layer was inversely related to the threshold voltage and the on-state capacitance. The interface charge density between the oxide and Al0.3Ga0.7N barrier layer was found to be of the order of ~1012 cm2eV−1. A significant reduction in leakage current from ~10−4 A/cm2 for HFET to ~10−6 A/cm2 for MOSHFET was observed well beyond pinch-off in the off-stage at -20 V applied gate voltage. The annealing at 900° C of the MOSHFET structures revealed that the Ga2O3 layer was thermally stable at high temperatures resulting in insignificant threshold voltage shifts for annealed samples with respect to as-deposited (unannealed) structures. Our results show that the MOCVD-gown Ga2O3 dielectric layers can be a strong candidate for stable high-power devices.