Single Crystalline β-Ga2O3 Research Articles

High crystalline quality homoepitaxial Si-doped β-Ga2O3(010) layers with reduced structural anisotropy grown by hot-wall MOCVD

A new growth approach, based on the hot-wall metalorganic chemical vapor deposition concept, is developed for high-quality homoepitaxial growth of Si-doped single-crystalline β-Ga2O3 layers on (010)-oriented native substrates. Substrate annealing in argon atmosphere for 1 min at temperatures below 600 °C is proposed for the formation of epi-ready surfaces as a cost-effective alternative to the traditionally employed annealing process in oxygen-containing atmosphere with a time duration of 1 h at about 1000 °C. It is shown that the on-axis rocking curve widths exhibit anisotropic dependence on the azimuth angle with minima for in-plane direction parallel to the [001] and maximum for the [100] for both substrate and layer. The homoepitaxial layers are demonstrated to have excellent structural properties with a β-Ga2O3(020) rocking curve full-widths at half-maximum as low as 11 arc sec, which is lower than the corresponding one for the substrates (19 arc sec), even for highly Si-doped (low 1019 cm−3 range) layers. Furthermore, the structural anisotropy in the layer is substantially reduced with respect to the substrate. Very smooth surface morphology of the epilayers with a root mean square roughness value of 0.6 nm over a 5 × 5 μm2 area is achieved along with a high electron mobility of 69 cm2 V−1 s−1 at a free carrier concentration n=1.9×1019 cm−3. These values compare well with state-of-the-art parameters reported in the literature for β-Ga2O3(010) homoepitaxial layers with respective Si doping levels. Thermal conductivity of 17.4 Wm−1K−1 is determined along the [010] direction for the homoepitaxial layers at 300 K, which approaches the respective value of bulk crystal (20.6 Wm−1K−1). This result is explained by a weak boundary effect and a low dislocation density in the homoepitaxial layers.

Enhancing the quality of homoepitaxial (−201) β-Ga2O3 thin film by MOCVD with in situ pulsed indium

This article innovatively uses pulsed metal-organic chemical vapor deposition technology to optimize the quality of β-Ga2O3 thin films on (−201) β-Ga2O3 homo-substrate using indium pulse-assisted technology. The results demonstrate that the pulsed indium-assisted method, when compared with the traditional indium-assisted method, effectively suppresses the desorption of Ga2O, enhances the flatness of the β-Ga2O3 film, and reduces the surface roughness from 34.8 to 0.98 nm. The optimized single crystalline β-Ga2O3 film was grown with pulsed-indium, and the full width at half maximum of x-ray diffraction rocking curve was 30.42 arc sec, smaller than that of the continuous indium β-Ga2O3 (56.1 arc sec). In combination with the x-ray photoelectron spectroscopy O1s split-peak fitting analysis, the relative content of oxygen vacancies in the film was significantly reduced by pulsed indium-assisted method. The Hall mobility of films assisted by pulsed-indium is approximately 14 times higher than that of films assisted by traditional indium. The pulsed indium technology provides an idea for homoepitaxial growth of high-quality β-Ga2O3 films.

Dissimilar thermal transport properties in κ-Ga2O3 and β-Ga2O3 revealed by homogeneous nonequilibrium molecular dynamics simulations using machine-learned potentials

The lattice thermal conductivity (LTC) of Ga2O3 is an important property due to the challenge in the thermal management of high-power devices. In this work, we develop machine-learned neuroevolution potentials (NEPs) for single-crystalline β−Ga2O3 and κ−Ga2O3 and demonstrate their accuracy in modeling thermal transport properties. Combining NEP-driven homogeneous non-equilibrium molecular dynamics simulations with tensor analysis, we determine the spatial distributions of LTCs for two Ga2O3 crystals, showing dissimilar thermal behaviors. Specifically, β−Ga2O3 shows isotropic thermal transport properties, with the LTCs along [100], [010], and [001] directions being predicted to be 10.3±0.2, 19.9±0.2, and 12.6±0.2 W/(m K), respectively, consistent with previous experimental measurements. For κ−Ga2O3, our predictions suggest nearly isotropic thermal transport properties, with the LTCs along [100], [010], and [001] being estimated to be 4.5±0.1, 3.9±0.1, and 4.0±0.1 W/(m K). The reduced LTC of κ−Ga2O3 vs β−Ga2O3 stems from its restricted low-frequency phonons up to 5 THz. Furthermore, we find that the β phase exhibits a typical temperature dependence slightly stronger than ∼T−1, whereas the κ phase shows a weaker temperature dependence, ranging from ∼T−0.5 to ∼T−0.7.

Wafer-scale single-crystalline β-Ga2O3 thin film on SiC substrate by ion-cutting technique with hydrophilic wafer bonding at elevated temperatures

Heterogeneous integration of β-Ga2O3 on a highly thermal conductive SiC substrate is an efficient solution to solve its bottleneck of thermal dissipation for high-power electronics. In this work, a 2-inch high-quality \(\left( {\bar 201} \right)\) β-Ga2O3 single-crystalline film was transferred to the 4H-SiC substrate via the ion-cutting technique with hydrophilic bonding at elevated temperatures. The evolution process of the surface blistering on the hydrogen-implanted β-Ga2O3 together with the internal pressure in blisters were investigated systematically to understand the physical mechanisms of the ion-cutting of β-Ga2O3 thin film. As suggested by the finite element simulation, the hydrophilic bonding was carried out at an elevated bonding temperature of 96°C to prevent the debonding of β-Ga2O3/4H-SiC during the ion-cutting process via reducing the thermal stress. The as-transferred β-Ga2O3 thin film exhibited a narrow full width at half maximum of the X-ray diffraction of 79.2 arcsec, and an extremely smooth surface with a root-mean-square roughness of 0.1 nm was achieved after chemical mechanical polishing. It is expected that the β-Ga2O3/4H-SiC heterogeneous integration material obtained by the ion-cutting technique with hydrophilic bonding at elevated temperatures will serve as a practical platform for high-performance β-Ga2O3 power devices.

(Digital Presentation) Study on the Electrical Characteristics and Band Diagrams of Ga2O3 MOS Gate Stacks

β-Ga2O3 is expected as one of the candidate materials for high-efficient and cost-effective power devices for high-voltage applications [1]. For the fabrication of MOSFETs the understanding of band diagrams of MOS structure is inevitably important. However, the previous reports on band alignments of β-Ga2O3 and various dielectrics including Al2O3 and SiO2 are controversial with significant inconsistencies in their values [2]. Since the ideal band diagram referring to the vacuum level is not clarified we cannot tell the Fermi level of β-Ga2O3 and the flat-band voltage (Vfb) shift of MOS capacitors. In this study we fabricated MOS structures and its band diagram was investigated from photoelectron spectroscopy.We employed SiO2 as the gate dielectric. Single crystalline β-Ga2O3 (001) wafers with ~2×1016 cm-3 n-type doped epitaxial layers were cleaned in diluted HF solution. Then SiO2 films were deposited by electron-beam evaporation of Si in O2 ambient of ~1×10-2 Pa, followed by post-deposition annealing (PDA) in 1 atm-O2 at various temperatures. Au was evaporated as gate electrode to form MOS capacitors. Ultraviolet photoelectron spectroscopy (UPS) was conducted to determine the energy level of the valence band edge on the surface of β-Ga2O3 epitaxial layer referring to the vacuum level. The valence band spectrum of ~4nm-thick SiO2/β-Ga2O3 was also investigated by x-ray photoelectron spectroscopy to determine the valence band offset at the interface.From C-V characteristics of MOS capacitors with different PDA conditions, the higher PDA temperature was found to be beneficial to achieve nearly-ideal C-V characteristics. A negligibly-small hysteresis was obtained for the one fabricated with O2-PDA at 1000℃ [3]. The energy profiles of interface state density (Dit) were determined by conductance method for the samples of different PDA conditions. The values of Dit decreased for higher PDA temperature down to ~1010 cm-3 at the energy level of EC−0.2 eV [3]. These values are significantly lower than typically reported ones for ALD-Al2O3 MOS structures on β-Ga2O3 [4]. The additional annealing at low temperature at 400℃ works for the further reduction of hysteresis to less than 1 mV/nm.Next the band diagrams of the MOS structures were investigated. From the UPS spectra we could deduce the energy level of valence band edge referring to the vacuum level, from the difference between minimum (cut-off) and maximum edges of the obtained kinetic energy spectra of photoelectrons. We also evaluated the valence band offset from the deconvolution of the valence band XPS of thin SiO2/β-Ga2O3 stack. From the energy edge difference between the deconvoluted spectra corresponding to SiO2 and Ga2O3, respectively, the valence band offset ~1.1 eV was deduced, which is not conflicting with the estimated valence band edge energy of Ga2O3 and the well-reported valence band edge energy of SiO2. The conduction band edge energy was also estimated assuming the bandgap of β-Ga2O3 (4.7eV) [1]. Considering the Fermi-level of β-Ga2O3 with n-type doping of ~2×1016 cm-3 was approximated to be ~0.1 eV from the conduction band edge, the Fermi-level on the surface of β-Ga2O3 epitaxial layer after 1000℃ O2 annealing was estimated to be 3.6-3.7eV below the vacuum level [5]. Note that this value would be significantly smaller than the one simply estimated from the previously reported electron affinity of Ga2O3 [6].Finally the Vfb shifts were investigated for the MOS capacitors fabricated with O2 annealing at 1000℃ with various thicknesses, referring to the ideal flat-band voltage determined with the Fermi-level of β-Ga2O3 as discussed above and the work function of Au electrode which was also determined with UPS measurement. As a result, we found the most of the Vfb values were around the ideal flat-band voltage irrespective of the film thickness from 20 to 60 nm, which indicates a small density of fixed charges <2×1011 cm-2 [5].In conclusion, we found that SiO2/β-Ga2O3 (001) MOS capacitors showed nearly-ideal electrical characteristics with small fixed charge density by applying O2-PDA at 1000℃. The band diagram for the MOS stacks referring to the vacuum level was clarified by photoelectron spectroscopy, which enables us to understand the β-Ga2O3 MOS interface properties correctly.

Catalyst-free growth of β-Ga2O3 microstructures by thermal oxidation

In this study, single crystalline β-Ga2O3 microstructures (wires, belts, and sheets) were simultaneously synthesized by catalyst-free thermal oxidation. Monoclinic phase of crystalline Ga2O3 microstructures with different morphologies (wires, belts, sheets) formed. The largest size of sheet-like structure was 450 ​μm in length and 206 ​μm in width. The PL spectrum of these microstructures exhibited the emission at a blue light band centered at 456 ​nm. The growth mechanism of β-Ga2O3 microstructures with different morphologies was discussed. These β-Ga2O3 microstructures might be meaningful in application of opto-electrical functional devices.

Efficient thermal dissipation in wafer-scale heterogeneous integration of single-crystalline β-Ga2O3 thin film on SiC

The semiconductor, β-Ga2O3 is attractive for applications in high power electronic devices with low conduction loss due to its ultra-wide bandgap (∼4.9 eV) and large Baliga's figure of merit. However, the thermal conductivity of β-Ga2O3 is much lower than that of other wide/ultra-wide bandgap semiconductors, such as SiC and GaN, which results in the deterioration of β-Ga2O3-based device performance and reliability due to self-heating. To overcome this problem, a scalable thermal management strategy was proposed by heterogeneously integrating wafer-scale single-crystalline β-Ga2O3 thin films on a highly thermally conductive SiC substrate. Characterization of the transferred β-Ga2O3 thin film indicated a uniform thickness to within ±2.01%, a smooth surface with a roughness of 0.2 nm, and good crystalline quality with an X-ray rocking curves (XRC) full width at half maximum of 80 arcsec. Transient thermoreflectance measurements were employed to investigate the thermal properties. The thermal performance of the fabricated β-Ga2O3/SiC heterostructure was effectively improved in comparison with that of the β-Ga2O3 bulk wafer, and the effective thermal boundary resistance could be further reduced to 7.5 m2K/GW by a post-annealing process. Schottky barrier diodes (SBDs) were fabricated on both a β-Ga2O3/SiC heterostructured material and a β-Ga2O3 bulk wafer. Infrared thermal imaging revealed the temperature increase of the SBDs on β-Ga2O3/SiC to be one quarter that on the β-Ga2O3 bulk wafer with the same applied power, which suggests that the combination of the β-Ga2O3 thin film and SiC substrate with high thermal conductivity promotes heat dissipation in β-Ga2O3-based devices.

Effect of O2 plasma exposure time during atomic layer deposition of amorphous gallium oxide

Amorphous gallium oxide thin films were grown by plasma-enhanced atomic layer deposition on (100) silicon substrates from trimethylgallium Ga(CH3)3 precursor and oxygen plasma. At 200 °C, the growth per cycle is in the range of 0.65–0.70 Å for O2 plasma exposure times ranging from 3 up to 30 s during each cycle. The effect of O2 plasma exposure times on the interfacial SiOx regrowth and the electrical properties was investigated. In situ spectroscopic ellipsometry shows that the SiOx regrowth occurs during the first three cycles and is limited to 0.27 nm for plasma times as long as 30 s. Increasing the O2 plasma exposure during each ALD cycle leads to a drastic decrease in the leakage current density (more than 5 orders of magnitude for 30 nm films), which is linked to the suppression of oxygen vacancy states as evidenced by spectroscopic ellipsometry. Interestingly, an increase in the dielectric constant with increasing O2 plasma exposure time is observed, reaching a value of εr∼14.2, larger than that of single crystalline β-Ga2O3. This study highlights the crucial role of oxygen plasma exposure time in the control and tuning of the electrical properties of amorphous gallium oxide films.

Single-Crystalline All-Oxide α-γ-β Heterostructures for Deep-Ultraviolet Photodetection.

Recent advancements in gallium oxide (Ga2O3)-based heterostructures have allowed optoelectronic devices to be used extensively in the fields of power electronics and deep-ultraviolet photodetection. While most previous research has involved realizing single-crystalline Ga2O3 layers on native substrates for high conductivity and visible-light transparency, presented and investigated herein is a single-crystalline β-Ga2O3 layer grown on an α-Al2O3 substrate through an interfacial γ-In2O3 layer. The single-crystalline transparent conductive oxide layer made of wafer-scalable γ-In2O3 provides high carrier transport, visible-light transparency, and antioxidation properties that are critical for realizing vertically oriented heterostructures for transparent oxide photonic platforms. Physical characterization based on X-ray diffraction and high-resolution transmission electron microscopy imaging confirms the single-crystalline nature of the grown films and the crystallographic orientation relationships among the monoclinic β-Ga2O3, cubic γ-In2O3, and trigonal α-Al2O3, while the elemental composition and sharp interfaces across the heterostructure are confirmed by Rutherford backscattering spectrometry. Furthermore, the energy-band offsets are determined by X-ray photoelectron spectroscopy at the β-Ga2O3/γ-In2O3 interface, elucidating a type-II heterojunction with conduction- and valence-band offsets of 0.16 and 1.38 eV, respectively. Based on the single-crystalline β-Ga2O3/γ-In2O3/α-Al2O3 all-oxide heterostructure, a vertically oriented DUV photodetector is fabricated that exhibits a high photoresponsivity of 94.3 A/W, an external quantum efficiency of 4.6 × 104%, and a specific detectivity of 3.09 × 1012 Jones at 250 nm. The present demonstration lays a strong foundation for and paves the way to future all-oxide-based transparent photonic platforms.

VLS growth of pure and Au decorated β-Ga2O3 nanowires for room temperature CO gas sensor and resistive memory applications

High-density single crystalline β-Ga2O3 nanowires on silicon (1 0 0) substrates were grown by vapour-liquid-solid growth method. We have characterized the pure β-Ga2O3 nanowires along with the Au-decorated β-Ga2O3 nanowires. The CO gas sensors at room temperature (RT) have been studied for pure and Au decorated nanowires with multiple-networked array and single nanowire devices. The diameter of the 1D nanostructure ranged from 127 ± 5 nm. The synthesized nanowires were studied using Field Emission Scanning Electron Microscope (FESEM), Transmission Electron Microscope (TEM), Energy Dispersive X-ray Spectroscopy (EDS), Gracing Incidence X-ray Diffraction (GI-XRD), Photoluminescence (PL), Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS). Using the Focussed Ion Beam (FIB) technique, single nanowire gas sensor devices were fabricated. Single nanowire RT CO gas sensors using the proposed Au decorated β-Ga2O3 nanowire achieved remarkable sensitivity for 100 ppm CO gas at room temperature. Besides, we have compared the RT CO gas sensing properties of multiple-networked Au decorated β-Ga2O3 nanowires with single Au-decorated β-Ga2O3 nanowire and single pure β-Ga2O3 nanowire. In addition, bipolar resistive switching property is inspected for the Au/pure β-Ga2O3 nanowires/p-Si and Au/Au decorated β-Ga2O3 nanowires/p-Si structures.

On the origin of red luminescence from iron-doped β-Ga2O3 bulk crystals

Currently, Fe doping in the ∼1018 cm−3 range is the most widely available method for producing semi-insulating single crystalline β-Ga2O3 substrates. Red luminescence features have been reported from multiple types of Ga2O3 samples, including Fe-doped β-Ga2O3, and attributed to Fe or NO. Herein, however, we demonstrate that the high-intensity red luminescence from Fe-doped β-Ga2O3 commercial substrates consisting of two sharp peaks at 689 nm and 697 nm superimposed on a broader peak centered at 710 nm originates from Cr impurities present at a concentration near 2 ppm. The red emission exhibiting a twofold symmetry, peaks in intensity for excitation near the absorption edge, seems to compete with the Ga2O3 emission at a higher excitation energy and appears to be intensified in the presence of Fe. Based on the polarized absorption, luminescence observations, and the Tanabe–Sugano diagram analysis, we propose a resonant energy transfer of photogenerated carriers in the β-Ga2O3 matrix to octahedrally coordinated Cr3+ to give red luminescence, possibly also sensitized by Fe3+.

Single crystalline β-Ga2O3 homoepitaxial films grown by MOCVD

β gallium oxide (β-Ga2O3) homoepitaxy films were grown on (2‾01) β-Ga2O3 substrates by metal organic chemical vapor deposition (MOCVD). The effect of growth temperature on the crystalline quality was measured and systematically analyzed. The results showed the growth temperature played an important role in crystalline quality of β-Ga2O3. The optimized single crystalline β-Ga2O3 film was grown at 750 °C, and the full width at half maximum (FWHM) of X-ray diffraction (XRD) rocking curve was 21.6 arcsec, smaller than that of the β-Ga2O3 substrate (26.3 arcsec). The film exhibited a greatly smooth surface and the root-mean-square (RMS) roughness was 0.68 nm. And the arrangement of atoms inside the film was in good accordance with lattice constants of β-Ga2O3. In addition, the effect of the band structure on the luminescent properties of β-Ga2O3 was discussed.

Photo-Electrochemical Etching of Single Crystalline β-Ga2O3 Thin Film

β-Ga2O3 is a next-generation semiconductor material having ultra-wide bandgap energy of 4.8 eV and excellent electrical properties. So it is expected to be suitable for various applications such as power electronics, solar-blind UV photodetectors and gas sensors. However, there are some issues to overcome in order for β-Ga2O3 to be commercialized. Among them, crystal defects are important factor that significantly affects device performance. Defects are formed intrinsically during the growth process, or caused by high-energy plasma ion during dry etching. It has been reported that the electrical characteristics are deteriorated in the device such as MOSFET and MISFET fabricated by inductively coupled plasma etching. Unlike dry etching, which can cause damage by plasma ions, wet etching is a damage-free process that uses a chemical reaction between the target material and etchant. In addition, wet etching may be used for removing defects by using the etching characteristic, which the chemically unstable defect areas are preferentially etched. However, β-Ga2O3 hardly reacts with etchant at room temperature, and has been reported to have etch rate of several tens of nm/min when acid etchant are used at higher than 100 °C.In our study, we have tried to increase the etch rate of β-Ga2O3 and remove defects effectively by Photo-electrochemical (PEC) etching. PEC etching is a method that promotes the reaction by irradiated light having energy above bandgap and applied external bias. Single crystalline β-Ga2O3 thin films are cleaved in the [100] direction with mechanical force from single crystalline β-Ga2O3 substrate. Ti/Au was deposited to form Ohmic contact with β-Ga2O3 thin films. With the PEC etching method, we could etch at a lower temperature than conventional wet etching. In addition, it was confirmed that the etch rate can be controlled according to the external bias condition. The details of our experiment conditions and results will be presented at the meeting.

Synthesis and optoelectronic properties of ultrathin Ga2O3 nanowires

Synthesis using a gallium sulfide precursor and Au or Ag catalysts produces ultrathin single-crystalline β-Ga2O3 nanowires at low substrate temperatures.

Improvement of dry etch-induced surface roughness of single crystalline β-Ga2O3 using post-wet chemical treatments

Post-wet chemical treatments using sulfuric acid and hydrogen peroxide mixture (SPM) or tetramethyl ammonium hydroxide (TMAH) solutions were performed to reduce dry etch-induced surface damage of β-Ga2O3 caused by the inductively coupled plasma-reactive ion etching (ICP-RIE) process using the Cl2/BCl3 gas mixture. The addition of BCl3 to the Cl2/BCl3 gas mixture resulted in a significant enhancement of the etch rate of β-Ga2O3. However, it increased the surface roughness with the formation of the nonvolatile etch by-product B2O3 as a result of the BCl3/β-Ga2O3 reaction. The post-SPM treatment led to the reduction of dry etch-induced surface damage of β-Ga2O3 to a certain extent with the removal of B2O3. On the other hand, the post-TMAH treatment significantly improved the surface morphology of the dry etched β-Ga2O3 surface. During the post-TMAH treatment, the chemical reaction between the damaged β-Ga2O3 layer caused by ICP-RIE process and OH− in the TMAH solution led to the formation of the Ga oxide, followed by the simultaneous dissolution of the Ga oxide and the B2O3. Thus, the post-TMAH treatment effectively recovered the surface damage of β-Ga2O3 induced by the ICP-RIE process using the Cl2/BCl3 gas mixture.

Catalyst-modified vapor-liquid-solid (VLS) growth of single crystalline β-Gallium Oxide (Ga2O3) thin film on Si-substrate

In the current work, a single crystalline β-Gallium Oxide (Ga2O3) thin film (~15 nm) is grown on Si-substrate by employing catalyst-modified vapor-liquid-solid (VLS) method. The FESEM and AFM images indicate the formation of a continuous film with small surface roughness. XRD measurement confirms the formation of a highly crystalline <1¯11>-plane of β-phase of Ga2O3. The chemical states and optical properties of such films are analyzed by X-ray photoelectron spectroscopy (XPS) and Spectroscopic ellipsometry (SE). The energy bandgap, refractive index and extinction coefficient of the grown β-Ga2O3 film are obtained to be 4.78 eV, 1.84 and 0.10, respectively. The vacancies/defect states are studied from Photoluminescence (PL) spectra where the major PL-peaks corroborate with the transitional energy values obtained from SE measurement. Therefore, the study suggests that the VLS method, amongst the state-of-the-art growth techniques, is capable of growing high-quality single crystalline β-Ga2O3 thin film in a relatively simple and cost-effective approach.

Deep-Ultraviolet Photodetection Using Single-Crystalline β-Ga2O3/NiO Heterojunctions.

In recent years, β-Ga2O3/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-Ga2O3/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-Ga2O3 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 (2̅01) β-Ga2O3 and (111) NiO films, respectively, whereas electron microscopy analysis confirmed the sharp layer transitions and high interface qualities in the NiO/β-Ga2O3/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/μm2 illuminating power density. Furthermore, we also determined the band offset parameters at the thermodynamically stable heterointerface between NiO and β-Ga2O3 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.

Nanowire-Seeded Growth of Single-Crystalline (010) β-Ga2 O3 Nanosheets with High Field-Effect Electron Mobility and On/Off Current Ratio.

2D β-Ga2 O3 nanosheets, as fundamental materials, have great potential in next generations of ultraviolet transparent electrodes, high-temperature gas sensors, solar-blind photodetectors, and power devices, while their synthesis and growth with high crystalline quality and well-controlled orientation have not been reported yet. The present study demonstrates how to grow single-crystalline ultrathin quasi-hexagonal β-Ga2 O3 nanosheets with nanowire seeds and proposes a hierarchy-oriented growth mechanism. The hierarchy-oriented growth is initiated by epitaxial growth of a single-crystalline β-Ga2 O3 nanowire on a GaN nanocrystal and followed by homoepitaxial growth of quasi-hexagonal (010) β-Ga2 O3 nanosheets. The undoped 2D (010) β-Ga2 O3 nanosheet field effect transistor has a field-effect electron mobility of 38 cm2 V-1 s-1 and an on/off current ratio of 107 with an average subthreshold swing of 150 mV dec-1 . The from-nanowires-to-nanosheets technique paves a novel way to fabricate nanosheets, which has great impact on the field of nanomaterial synthesis and growth and the area of nanoelectronics as well.

Growth of Ga2O3 Nanowires via Cu-As-Ga Ternary Phase Diagram

Currently, it is challenging to develop new catalysts for semiconductor nanowires (NWs) growth in a complementary-metal-oxide-semiconductor (CMOS) compatible manner via a vapor-liquid-solid (VLS) mechanism. In this study, chemically synthesized Cu2O nano cubes are adopted as the catalyst for single crystalline β-Ga2O3 NWs growth in chemical vapor deposition. The growth temperature is optimized to be 750 to 800 °C. The NW diameter is controlled by tuning the sizes of Cu2O cubes in the 20 to 100 nm range with a bandgap of ~4.85 eV as measured by ultraviolet-visible absorption spectroscopy. More importantly, the catalyst tip is found to be Cu5As2, which is distinguished from those Au-catalyzed Au-Ga alloys. After a comprehensive phase diagram investigation, the β-Ga2O3 NWs are proposed to be grown by the ternary phase of Cu-As-Ga diffusing Ga into the growth frontier of the NW, where Ga react with residual oxygen to form the NWs. Afterward, Ga diminishes after growth since Ga would be the smallest component in the ternary alloy. All these results show the importance of the catalyst choice for CMOS compatible NW growth and also the potency of the ternary phase catalyst growth mode in other semiconductor NWs synthesis.

Catalyst-free growth of single crystalline β-Ga2O3 microbelts on patterned sapphire substrates

The high density and large quantity β-Ga2O3 microbelts were synthesized by changing the growth temperature on patterned sapphire substrates (PSS) by chemical vapor deposition equipment. The width of the microbelts was about 1–1.5 μm, and the length was about 15 μm. The results of different growth temperature influenced for surface morphology and crystal structure of β-Ga2O3 indicated that the optimal growth temperature of β-Ga2O3 microbelts was at 900 °C. Furthermore, the growth mechanism of microbelts was also studied. It was found that the hemispherical PSS played an important role for β-Ga2O3 microbelts formation. The optical absorption spectrum indicated that optical band gap energy of microbelts was about 4.78 eV.