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Abstract Highly uniform InGaN-based quantum dots (QDs) grown on a nanopatterned dielectric layer defined by self-assembled diblock copolymer were performed by metal-organic chemical vapor deposition. The cylindrical-shaped nanopatterns were created on SiNx layers deposited on a GaN template, which provided the nanopatterning for the epitaxy of ultra-high density QD with uniform size and distribution. Scanning electron microscopy and atomic force microscopy measurements were conducted to investigate the QDs morphology. The InGaN/GaN QDs with density up to 8 × 1010 cm-2 are realized, which represents ultra-high dot density for highly uniform and well-controlled, nitride-based QDs, with QD diameter of approximately 22-25 nm. The photoluminescence (PL) studies indicated the importance of NH3 annealing and GaN spacer layer growth for improving the PL intensity of the SiNx-treated GaN surface, to achieve high optical-quality QDs applicable for photonics devices.

A predicted type-II staggered band alignment with an approximately 1.4 eV valence band offset at the ZnGeN2/GaN heterointerface has inspired novel band-engineered III-N/ZnGeN2 heterostructure-based device designs for applications in high performance optoelectronics. We report on the determination of the valence band offset between metalorganic chemical vapor deposition grown (ZnGe)1−xGa2xN2, for x = 0 and 0.06, and GaN using x-ray photoemission spectroscopy. The valence band of ZnGeN2 was found to lie 1.45–1.65 eV above that of GaN. This result agrees well with the value predicted by first-principles density functional theory calculations using the local density approximation for the potential profile and quasiparticle self-consistent GW calculations of the band edge states relative to the potential. For (ZnGe)0.94Ga0.12N2 the value was determined to be 1.29 eV, ∼10%–20% lower than that of ZnGeN2. The experimental determination of the large band offset between ZnGeN2 and GaN provides promising alternative solutions to address challenges faced with pure III-nitride-based structures and devices. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrated large-size free-standing single-crystal β-Ga2O3 NMs fabricated by the hydrogen implantation and lift-off process directly from MOCVD grown β-Ga2O3 epifilms on native substrates. The optimum implantation conditions were simulated with a Monte-Carlo simulation method to obtain a high hydrogen concentration with a narrow ion distribution at the desired depth. Two as grown β-Ga2O3 samples with different orientations ([100] and [001]) were used to successfully create 1.2 μm thick β-Ga2O3 NMs without any physical damage. These β-Ga2O3 NMs were then transfer-printed onto rigid and flexible substrates such as SiC and polyimide substrates. Various material characterization studies were performed to investigate their crystal quality, surface morphologies, optical properties, mechanical properties, and bandgaps before and after the lift-off and revealed that the good material quality was maintained. This result offers several benefits in that the thickness, doping, and size of β-Ga2O3 NMs can be fully controlled. Moreover, more advanced β-Ga2O3-based NM structures such as (AlxGa1−x)2O3/Ga2O3 heterostructure NMs can be directly created from their bulk epitaxy substrates; thus this study provides a viable route for the realization of high performance β-Ga2O3 NM-based electronics and optoelectronics that can be built on various substrates and platforms. [ABSTRACT FROM AUTHOR]

In this work, the interaction of n-type dopants in Si doped (AlxGa1−x)2O3 films with varying Al content over the entire composition range (x = 0%–100%) was analyzed using atom probe tomography. An almost uniform dopant distribution with dopant density in the range of 1018 cm−3 was obtained in (AlxGa1−x)2O3 layers with Al contents, x < 0.60. At x , small Si segregated zones were observed which was attributed to the increased donor-acceptor transition centers or bond length differences in Al–O, Ga–O and Si–O at high Al content. We have demonstrated that for the single phase β-(AlxGa1−x)2O3 films with Al content of x < 0.30, dopants prefer to occupy on Ga sites while Al site is preferred for high Al content (x > 0.50) (AlxGa1−x)2O3 layers. It was also observed for Al content, x = 0.30–0.50, no specific cationic site occupancy was observed, Si occupies either Al or Ga sites. This can be attributed to highly inhomogeneous layers within this Al composition range due to which dopant Si atoms are either in the Al-rich or Al-depleted regions. [ABSTRACT FROM AUTHOR]

One new secoiridoid compound swertiamarin B (1), along with a known compound lytanthosalin (2), were isolated from ethanol extract of the aerial parts of Swertia mussotii. Their structures were elucidated by the detailed analysis of comprehensive spectroscopic data. All compounds were first isolated from the Swertia genus. Their antitumor activities were evaluated for four human tumor cell lines (HCT-116, HepG2, MGC-803 and A549). Compounds 1 and 2 showed excellent cytotoxic activities toward the MGC-803 cell lines with IC50 values 3.61 and 12.04 μM, respectively. [ABSTRACT FROM AUTHOR]

The valence and conduction band offsets at (100) β-(AlxGa1−x)2O3/β-Ga2O3 heterointerfaces with the increasing Al composition are determined via x-ray photoelectron spectroscopy. The (100) β-(AlxGa1−x)2O3 thin films with the Al composition of 0.10 < x < 0.52 are grown on (100) β-Ga2O3 substrates by the metalorganic chemical vapor deposition method. By examining the onset of inelastic energy loss in core-level atomic spectra, the bandgaps of β-Ga2O3 and β-(AlxGa1−x)2O3 alloys with different Al compositions are measured from 4.83 ± 0.12 eV (x = 0) to 5.85 ± 0.08 eV (x = 0.52). The valence band offsets are determined to be −0.06 ± 0.06 eV (x = 0.10), −0.11 ± 0.06 eV (x = 0.33), and −0.19 ± 0.06 eV (x = 0.52). The conduction band offsets of 0.34 ± 0.17 eV (x = 0.10), 0.62 ± 0.17 eV (x = 0.33), and 1.21 ± 0.16 eV (x = 0.52) are determined from the extracted bandgaps of β-(AlxGa1−x)2O3 alloys. The determined band alignments at β-(AlxGa1−x)2O3/β-Ga2O3 interfaces reveal the formation of type-II (staggered gap) heterojunctions for all Al compositions investigated. The bowing parameters obtained from the quadratic fitting of both conduction band minimum and valence band maximum values are estimated to be 1.25 eV and 0.005 eV, respectively. [ABSTRACT FROM AUTHOR]

We used depth-resolved cathodoluminescence spectroscopy (DRCLS), absorption spectroscopy, and temperature-dependent Hall effect (TDH) measurements to study the effects of fluence dependent neutron irradiations on deep level defects and the associated changes of electrical properties of β-Ga2O3 grown by low pressure chemical vapor deposition and pulsed laser deposition. DRCLS enabled us to monitor systematic increases of three deep level defects after neutron irradiation which correlated with TDH measurements of significant free carrier removal and mobility decrease. The correlations between defect profiles and electrical property changes vs. irradiation dose link these dominant electrically active native point defects in Ga2O3 with their contributions to free carrier mobility, carrier density, and donor/acceptor depth profiles, further revealing their donor/acceptor electrical behavior and physical nature, consistent with the formation of compensating defects. After irradiation, temperature-dependent forming gas (FG) anneals were performed to reverse the radiation-induced damage and carrier removal. The evolution of defect concentrations with increasing neutron dose and their depth-resolved distributions with FG anneal temperature reveal an interplay between specific defects to control electronic properties. [ABSTRACT FROM AUTHOR]

β-(AlxGa1−x)2O3 thin films are grown on ( 2 ¯ 01) β-Ga2O3 substrates via metal organic chemical vapor deposition to investigate the solubility of Al in β-phase Ga2O3. The x-ray diffraction (XRD) spectra reveal crystalline quality ( 2 ¯ 01) β-(AlxGa1−x)2O3 thin films with Al compositions up to 48%. The Al compositions are further confirmed by high resolution x-ray spectroscopy measurements and energy-dispersive x-ray spectra (EDS) mapping. The bandgap energies extracted from XPS spectra range between 5.20 ± 0.06 eV and 5.72 ± 0.08 eV for x = 21%–48%. The surface morphology evaluated by both scanning electron microscopy and atomic force microscopy shows elongated features with granules along the [010] direction, which are suppressed with the increasing Al content. A systematic growth study through tuning growth parameters indicates that the chamber pressure plays an important role in both surface morphology and Al incorporation. Material characterization via high resolution scanning transmission electron microscopy and STEM-EDS reveals Al fluctuations in the sample with the 48% Al composition. Atomic resolution STEM imaging and XRD spectra for ( 2 ¯ 01) β-(AlxGa1−x)2O3/Ga2O3 superlattice structures confirm that the periodicity of the β-(AlxGa1−x)2O3/Ga2O3 sub-layers is well maintained with high-Al compositions. [ABSTRACT FROM AUTHOR]

A new record‐high room‐temperature electron Hall mobility (μRT = 194 cm2 V−1 s−1 at n ≈ 8 × 1015 cm−3) for β‐Ga2O3 is demonstrated in the unintentionally doped thin film grown on (010) semi‐insulating substrate via metal‐organic chemical vapor deposition (MOCVD). A peak electron mobility of ≈9500 cm2 V−1 s−1 is achieved at 45 K. Further investigation on the transport properties indicates the existence of sheet charges near the epilayer/substrate interface. Si is identified as the primary contributor to the background carrier in both the epilayer and the interface, originating from both surface contamination and growth environment. The pregrowth hydrofluoric acid cleaning of the substrate leads to an obvious decrease in Si impurity both at the interface and in the epilayer. In addition, the effect of the MOCVD growth condition, particularly the chamber pressure, on the Si impurity incorporation is studied. A positive correlation between the background charge concentration and the MOCVD growth pressure is confirmed. It is noteworthy that in a β‐Ga2O3 film with very low bulk charge concentration, even a reduced sheet charge density plays an important role in the charge transport properties. [ABSTRACT FROM AUTHOR]

Staggered InGaN quantum wells (QWs) are analyzed as improved gain media for laser diodes (LDs) lasing at 440 and 500 nm. The calculation of band structure is based on a 6-band k·p method taking into account the valence band mixing, strain effect, and spontaneous and piezoelectric polarizations as well as the carrier screening effect. Staggered InGaN QWs with two-layer and three-layer step-function like In-content InGaN QWs structures are investigated to enhance the optical gain as well as to reduce the threshold current density for LDs emitting at 440 and 500 nm. Our analysis shows that the optical gain is enhanced by 1.5–2.1 times by utilizing the staggered InGaN QW active region emitting at 440 nm, which leads to a reduction of the threshold current density up to 24% as compared to that of the conventional InGaN QW laser. Staggered InGaN QWs with enhanced optical gain shows significantly reduced blue-shift as carrier density increases, which enables nitride QWs with high optical gain in the green spectral regime. The use of green-emitting three-layer staggered InGaN QW is also expected to lead to reduction in the threshold carrier density by 30%. [ABSTRACT FROM AUTHOR]

Type-II InGaN–GaNAs quantum wells (QWs) with thin dilute-As (∼3%) GaNAs layer are analyzed self-consistently as improved III-nitride gain media for diode lasers. The band structure is calculated by using a six-band k·p formalism, taking into account valence band mixing, strain effect, spontaneous and piezoelectric polarizations, as well as the carrier screening effect. The type-II InGaN–GaNAs QW structure allows large electron-hole wave function overlap by confining the hole wave function in the GaNAs layer of the QW. The findings based on self-consistent analysis indicate that type-II InGaN-GaNAs QW active region results in superior performance for laser diodes, in comparison to that of conventional InGaN QW. Both the spontaneous emission radiative recombination rate and optical gain of type-II InGaN–GaNAs QW structure are significantly enhanced. Reduction in the threshold current density of InGaN–GaNAs QW lasers is also predicted. [ABSTRACT FROM AUTHOR]

(010) β-(AlxGa1−x)2O3 thin films were grown on (010) β-Ga2O3 substrates via metalorganic chemical vapor deposition with up to 40% Al incorporation by systematic tuning of the Trimethylaluminum (TMAl)/triethylgallium molar flow rate ratio and growth temperature. High crystalline quality with pure β-phase (AlxGa1−x)2O3 was achieved for films with Al composition x < 27%, while a higher Al composition induced phase segregation which was observed via X-ray diffraction spectra. Al incorporation was highly dependent on the growth temperature, chamber pressure, oxygen partial pressure, and TMAl molar flow rate. Atomic resolution scanning transmission electron microscopy (STEM) imaging demonstrated a high crystalline quality β-(Al0.15Ga0.85)2O3 film with an epitaxial interface. High resolution STEM imaging of (AlxGa1−x)2O3/Ga2O3 superlattice (SL) structures revealed superior crystalline quality for the 23% Al composition. When the Al composition reaches 40%, the SL structure maintained the β-phase, but the interfaces became rough with inhomogeneous Al distribution. N-type doping using Si in β-(AlxGa1−x)2O3 films with the Al composition up to 33.4% was demonstrated. [ABSTRACT FROM AUTHOR]