Precise control of the hetero-epitaxy on a low-cost foreign substrate is often the key to drive the success of fabricating semiconductor devices in scale when a large low-cost native substrate is not available. Here, we successfully synthesized three different phases of Ga2O3 (α, β, and ε) films on c-plane sapphire by only tuning the flow rate of HCl along with other precursors in an MOCVD reactor. A threefold increase in the growth rate of pure β-Ga2O3 was achieved by introducing only 5 sccm of HCl flow. With continuously increased HCl flow, a mixture of βand ε-Ga2O3 was observed, until the Ga2O3 film transformed completely to a pure ε-Ga2O3 with a smooth surface and the highest growth rate (~1 µm/hour) at a flow rate of 30 sccm. At 60 sccm, we found that the film tended to have a mixture of αand ε-Ga2O3 with a dominant α-Ga2O3, while the growth rate dropped significantly (~0.4 µm/hour). The film became rough as a result of the mixture phases since the growth rate of ε-Ga2O3 is much higher than α-Ga2O3. In this HClenhanced MOCVD mode, the Cl impurity concentration was almost identical among the investigated samples. Based on our density functional theory calculation, we found that the relative energy between β-, ε-, and α-Ga2O3 became smaller thus inducing the phase change by increasing the HCl flow in the reactor. Thus, it is plausible that the HCl acted as a catalyst during 2 the phase transformation process. Furthermore, we revealed the microstructure and the epitaxial relationship between Ga2O3 with different phases and the c-plane sapphire substrates. Our HClenhanced MOCVD approach paves the way to achieving highly controllable hetero-epitaxy of Ga2O3 films with different phases for device applications.

Spontaneously-grown, self-aligned AlGaN nanowire ultraviolet light emitting diodes still suffer from low efficiency partially because of the strong surface recombination caused by surface states, i.e., oxidized surface and high density surface states. Several surface passivation methods have been introduced to reduce surface non-radiative recombination by using complex and toxic chemicals. Here, we present an effective method to suppress such undesirable surface recombination of the AlGaN nanowires via diluted potassium hydroxide (KOH) solution; a commonly used chemical process in semiconductor fabrication which is barely used as surface passivation solution in self-assembled nitride-based nanowires. The transmission electron microscopy investigation on the samples reveals almost intact nanowire structures after the passivation process. We demonstrated an approximately 49.7% enhancement in the ultraviolet light output power after 30-s KOH treatment on AlGaN nanowires grown on titanium-coated silicon substrates. We attribute such a remarkable enhancement to the removal of the surface

High-temperature operation of metal-semiconductor-metal (MSM) UV photodetectors fabricated on pulsed laser deposited β-Ga2O3 thin films has been investigated. These photodetectors were operated up to 250 °C temperature under 255 nm illumination. The photo current to dark current (PDCR) ratio of about 7100 was observed at room temperature (RT) while it had a value 2.3 at 250 °C at 10 V applied bias. A decline in photocurrent was observed from RT to 150 °C and then it increased with temperature up to 250 °C. The suppression of the blue band was also observed from 150 °C temperature which indicated that self-trapped holes in Ga2O3 became unstable. Temperature-dependent rise and decay times of carriers were analyzed to understand the photocurrent mechanism and persistence photocurrent at high temperatures. Coupled electron-phonon interaction with holes was found to influence the photoresponse in the devices. The obtained results are encouraging and significant for high-temperature applications of β-Ga2O3 MSM deep UV photodetectors.

FIG. 7. The band alignment diagram of the b-Ga 2 O 3 /AlN heterojunction, along with that of GaN.

Aluminum-gallium-nitride alloys (Al x Ga 1-x N, 0 ≤ x ≤ 1) can emit light covering the ultraviolet spectrum from 210 to 360 nm. However, these emitters have not fulfilled their full promise to replace the toxic and fragile mercury UV lamps due to their low efficiencies. This study demonstrates a promising approach to enhancing the luminescence efficiency of AlGaN multiple quantum wells (MQWs) via the introduction of a lateral-polarity structure (LPS) comprising both III and N-polar domains. The enhanced luminescence in LPS is attributed to the surface roughening, and compositional inhomogeneities in the N-polar domain. The space-resolved internal quantum efficiency (IQE) mapping shows a higher relative IQE in N-polar domains and near inversion domain boundaries, providing strong evidence of enhanced radiative recombination efficiency in the LPS. These experimental observations are in good agreement with the theoretical calculations, where both lateral and vertical band diagrams are investigated. This work suggests that the introduction of the LPS in AlGaN-based MQWs can provide unprecedented tunability in achieving higher luminescence performance in the development of solid state light sources.

In recent years, β-Ga 2 O 3 /NiO heterojunction diodes have been studied, but reports in the literature lack an investigation of an epitaxial growth process of high-quality single-crystalline β-Ga 2 O 3 /NiO thin films via electron microscopy analysis and the fabrication and characterization of an optoelectronic device based on the resulting heterojunction stack. This work investigates the thin-film growth of a heterostructure stack comprising n-type β-Ga 2 O 3 and p-type cubic NiO layers grown consecutively on c-plane sapphire using pulsed laser deposition, as well as the fabrication of solar-blind ultraviolet-C photodetectors based on the resulting p−n junction heterodiodes. Several characterization techniques were employed to investigate the heterostructure, including X-ray crystallography, ion beam analysis, and high-resolution electron microscopy imaging. X-ray diffraction analysis confirmed the single-crystalline nature of the grown monoclinic and cubic (2̅ 01) β-Ga 2 O 3 and (111) NiO films, respectively, whereas electron microscopy analysis confirmed the sharp layer transitions and high interface qualities in the NiO/β-Ga 2 O 3 /sapphire double-heterostructure stack. The photodetectors exhibited a peak spectral responsivity of 415 mA/W at 7 V reverse-bias voltage for a 260 nm incident-light wavelength and 46.5 pW/μm 2 illuminating power density. Furthermore, we also determined the band offset parameters at the thermodynamically stable heterointerface between NiO and β-Ga 2 O 3 using high-resolution X-ray photoelectron spectroscopy. The valence and conduction band offsets values were found to be 1.15 ± 0.10 and 0.19 ± 0.10 eV, respectively, with a type-I energy band alignment.

High-density dislocations in materials and poor electrical conductivity of p-type AlGaN layers constrain the performance of the ultraviolet light emitting diodes and lasers at shorter wavelengths. To address those technical challenges, we design, grow, and fabricate a novel nanowire structure adopting a graded-index separate confinement heterostructure (GRINSCH) in which the active region is sandwiched between two compositionally graded AlGaN layers, namely a GRINSCH diode. Calculated electronic band diagram and carrier concentrations show an automatic formation of a p-n junction with electron and hole concentrations of ~10 18 /cm 3 in the graded AlGaN layers without intentional doping. The transmission electron microscopy experiment confirms the composition variation in the axial direction of the graded AlGaN nanowires. Significantly lower turn-on voltage of 6.5 V (reduced by 2.5 V) and smaller series resistance of 16.7 Ω (reduced by nearly four times) are achieved in the GRINSCH diode, compared with the conventional p-in diode. Such an improvement in the electrical performance is mainly attributed to the effectiveness of polarization-induced nand p-doping in the compositionally graded AlGaN layers. In consequence, the carrier transport and injection efficiency of the GRINSCH diode are greatly enhanced, which leads to a lower turn-on voltage, smaller series resistance, higher output power, and enhanced device efficiency. The calculated carrier distributions (both

Owing to large bandgaps of BAlN and AlGaN alloys, their heterojunctions have the potential to be used in deep ultraviolet and power electronic device applications. However, the band alignment of such junctions has not been identified. In this work, we investigated the band-offset parameters of a B0.14Al0.86 N/Al0.7Ga0.3N heterojunction grown by metalorganic vapor phase epitaxy. These specific compositions were chosen to ensure a sufficiently large band offset for deep ultraviolet and power electronic applications. High resolution transmission electron microscopy confirmed the high structural quality of the heterojunction with an abrupt interface and uniform element distribution. We employed high resolution X-ray photoemission spectroscopy to measure the core level binding energies of B 1s and Ga 2p3/2 with respect to the valence band maximum of B0.14Al0.86N and Al0.7Ga0.3N layers, respectively. Then, we measured the energy separation between the B 1s and Ga 2p3/2 core levels at the interface of the heterojunction. The valence band offset was determined to be 0.40 ± 0.05 eV. As a consequence, we identified a staggered-gap (type-II) heterojunction with the conduction band offset of 1.10 ± 0.05 eV. The determination of the band alignment of the B0.14Al0.86N/Al0.7Ga0.3N heterojunction facilitates the design of optical and electronic devices based on such junctions.