Wenxiang Mu

Schottky barrier diode based on β-Ga2O3 (100) single crystal substrate and its temperature-dependent electrical characteristics

The Pt/β-Ga2O3 Schottky barrier diode and its temperature-dependent current-voltage characteristics were investigated for power device application. The edge-defined film-fed growth (EFG) technique was utilized to grow the (100)-oriented β-Ga2O3 single crystal substrate that shows good crystal quality characterized by X-ray diffraction and high resolution transmission electron microscope. Ohmic and Schottky electrodes were fabricated by depositing Ti and Pt metals on the two surfaces, respectively. Through the current-voltage (I-V) measurement under different temperature and the thermionic emission modeling, the fabricated Pt/β-Ga2O3 Schottky diode was found to show good performances at room temperature, including rectification ratio of 1010, ideality factor (n) of 1.1, Schottky barrier height (ΦB) of 1.39 eV, threshold voltage (Vbi) of 1.07 V, ON-resistance (RON) of 12.5 mΩ·cm2, forward current density at 2 V (J@2V) of 56 A/cm2, and saturation current density (J0) of 2 × 10−16 A/cm2. The effective donor c...

Comparison Study of

We fabricated β-Ga₂O₃ photodetectors on bulk substrate and sapphire. Bulk Ga₂O₃ photodetector demonstrates the improved responsivity compared with the device on sapphire, due to the higher crystal quality in bulk material. Optical gain is achieved in both the devices. For the first time, we report that the Ga₂O₃ photodetector epitaxially grown on sapphire achieves a blueshift of bandgap in comparison with bulk device. Based on the measured responsivity characteristics, the direct and indirect E_G of bulk Ga₂O₃ are 4.78 and 4.59 eV, respectively. The Ga₂O₃ photodetector on sapphire exhibits a maximum cutoff wavelength at 253 nm, corresponding to the direct E_G of 4.90 eV. The increment of E_G in Ga₂O₃ on sapphire over bulk material is attributed to the residual strain in the film. The time-dependent photoresponse of the devices suggests that Ga₂O₃ on sapphire might have more oxygen vacancies than the bulk material.

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Considering their easy growth and doping in bulk crystal growth, and good crystalline quality, β-Ga2O3 single crystals, a very important wide-bandgap semiconductor, are now also considered to be a promising optical crystal candidate. In this work, a Cr4+:β-Ga2O3 single crystal has been grown successfully by the edge-defined film-fed growth method. The thermal conductivity of Cr4+:β-Ga2O3 has been measured along the a* direction at room temperature obtaining 16.2 W m−1 K−1, and it was much larger than that of the usually used Cr4+-doped crystals, such as Y3Al5O12 (YAG) or YVO4. The Raman spectrum indicated that the cutoff phonon energy of the β-Ga2O3 crystal was 767.8 cm−1. A passively Q-switched nanosecond pulsed Nd:YAG laser based on a Cr4+:β-Ga2O3 saturable absorber was experimentally demonstrated for the first time to our knowledge and its mechanism is explained by first-principles calculations. By inserting the Cr4+:β-Ga2O3 crystal into the Nd:YAG laser cavity, a Q-switched laser operation was obtained with a maximum average output power of 50 mW. The corresponding pulse repetition rate, pulse width, and pulse energy were determined to be of 421.5 kHz, 235.2 ns and 0.12 μJ, respectively.

-Ga 2 O 3 Photodetectors on Bulk Substrate and Sapphire

The difficulty in fabricating p-type Ga2O3 is a crucial issue which restricts its applications in practical devices. In the present work, we have performed first principles studies on formation energies, electronic structures and optical properties for a series of metal doped β-Ga2O3, involving a long list of main group and transition metals, even some lanthanides, to search for potential p-type dopants. Calculations have shown that Li and Be, both with small atomic radius, prefer interstitial doping rather than substitutional doping of Ga, resulting eventually in an n-type character to the doped system. In addition, an O-rich atmosphere is more favorable for p-type substitutional dopings by comparison with the Ga-rich condition. A number of metal dopants show potential in achieving p-type β-Ga2O3, for example, Na, Mg, Ca, Cu, Ag, Zn, Cd, which are all worth a further emphasis study in experiment, although a satisfying holes concentration may only be possible to achieve by simultaneously committing to both the elimination of n-type background carriers and the activation mechanism of dopants. Absorption spectra have shown that all the above-mentioned potential p-type dopants are suitable for deep UV applications. The major peaks of the absorption spectra are red-shifted in most cases, due to the introduction of new states to the forbidden gap by dopants, which have been discussed in detail by inspecting into the partial density of states. Ga2O3 doped by some transition metals show potential as magnetic devices.

An extended application of β-Ga2O3 single crystals to the laser field: Cr4+:β-Ga2O3 utilized as a new promising saturable absorber

β-Ga2O3 is an ultra-wide bandgap semiconductor with applications in power electronic devices. Revealing the transport characteristics of β-Ga2O3 devices at various temperatures is important for improving device performance and reliability. In this study, we fabricated a Pt/β-Ga2O3 Schottky barrier diode with good performance characteristics, such as a low ON-resistance, high forward current, and a large rectification ratio. Its temperature-dependent current–voltage and capacitance–voltage characteristics were measured at various temperatures. The characteristic diode parameters were derived using thermionic emission theory. The ideality factor n was found to decrease from 2.57 to 1.16 while the zero-bias barrier height Φb0 increased from 0.47 V to 1.00 V when the temperature was increased from 125 K to 350 K. This was explained by the Gaussian distribution of barrier height inhomogeneity. The mean barrier height Φ ¯ b0 = 1.27 V and zero-bias standard deviation σ0 = 0.13 V were obtained. A modified Richard...

Electronic structure and optical property of metal-doped Ga2O3: a first principles study

High-quality bulk β-Ga2O3 single crystals have been grown by optimized edge-defined film-fed growth (EFG) method. The problems of cracking and polycrystals have been effectively solved by using a high-quality 1 inch-wide seed. The crystalline quality of the as-grown crystal has been confirmed by X-ray rocking curve with a full-width at half-maximum (FWHM) of 42.1 arcsec. Moreover, an economical and efficient one-step mechanical exfoliation method has been proposed to get epi-ready β-Ga2O3 wafers directly. The root mean square (RMS) roughness of the wafers is noted to be around 0.1 nm, which means they are comparable or even better than wafers processed by chemical mechanical polishing (CMP). The mechanism of the exfoliation method is analyzed from a crystallographic view. A photodetector is fabricated on the exfoliated wafer directly and the device is proved to show good performance in solar-blind band.

Characterization of the inhomogeneous barrier distribution in a Pt/(100)β-Ga2O3 Schottky diode via its temperature-dependent electrical properties

A detailed investigation was conducted on continuous wave, wavelength tunable, and graphene passively Q-switched (PQS) lasers of Tm:LGGG crystal. In continuous wave operation, a maximum average output power of 2.26 W at 2003 nm was obtained under the absorbed power of 12.1 W, giving an optical-to-optical conversion efficiency of 18.7% and a slope efficiency of 23.6%. By using a quartz plate, the emission spectrum could be tuned from 1907.5 to 2055.9 nm, corresponding to a full-width at half-maximum (FWHM) of 83 nm. Dual-wavelength operation could be obtained by rotating the quartz plate at an appropriate angle. In graphene PQS regime, the Tm:LGGG laser produced a 1.29-μs pulse under the pulse repetition frequency of 43.9 kHz, corresponding to a pulse energy of 3.19 μJ.

One-step exfoliation of ultra-smooth β-Ga2O3 wafers from bulk crystal for photodetectors

β-Ga2O3, a semiconductor material, has attracted considerable attention given its potential applications in high-power devices, such as high-performance field-effect transistors. For decades, β-Ga2O3 has been processed through chemical mechanical polishing (CMP). Nevertheless, the understanding of the effect of OH− on β-Ga2O3 processed through CMP with an alkaline slurry remains limited. In this study, β-Ga2O3 substrates were successively subjected to mechanical polishing (MP), CMP and etching. Then, to investigate the changes that occurred on the surfaces of the samples, samples were characterised through atomic force microscopy (AFM), three-dimensional laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). LSCM and SEM results showed that β-Ga2O3 is highly vulnerable to brittle fracture during MP. AFM revealed that an ultrasmooth and nondamaged surface with a low Ra of approximately 0.18 nm could be obtained through CMP. XPS results indicated that a metamorphic layer, which mainly contains soluble gallium salt (Ga(OH)4−), formed on the β-Ga2O3 surface through a chemical reaction. A dendritic pattern appeared on the surface of β-Ga2O3 after chemical etching. This phenomenon indicated that the chemical reaction on the β-Ga2O3 surface occurred in a nonuniform and selective manner. The results of this study will aid the optimization of slurry preparation and CMP.

Diode End Pumped Continuous Wave and Graphene Q-Switched Tm:LGGG Lasers

In this letter, MOS capacitors with bilayer dielectrics consisted of large bandgap Al2O3 and high-k HfO2 in different stacking order on n-type doped (100) β-Ga2O3 are investigated through C − V and J − V measurement. The C − V measurement results reveal that incoming HfO2 makes both bilayer structures attain an increasing dielectric constant, which means a better gate control ability in transistors comparing with single Al2O3. Additionally, the interface state density extracted by high-low frequency capacitance method suggests that Al2O3/(100)β-Ga2O3 with no treatment shows a comparative Dit value (8.0 × 1012 cm-2eV-1 to 2.2 × 1011 cm-2eV-1) with HfO2/(100)β-Ga2O3 (8.4 × 1012 cm-2eV-1 to 1.0 × 1011 cm-2eV-1) in energy range of 0.2 to 0.9 eV. Furthermore, HfO2/Al2O3/Ga2O3 showing a bigger forward breakdown voltage of 11.0 V than 7.8 V of Al2O3/HfO2/Ga2O3 demonstrates that inserted larger bandgap Al2O3 insulator between Ga2O3 semiconductor and high-k HfO2 dielectric can prevent gate leakage current more effectively. Accordingly, the HfO2/Al2O3/Ga2O3 can enhance gate control ability with an acceptable gate breakdown voltage and become an alternative choice in the design of the gate structure for Ga2O3 MOSFETs.In this letter, MOS capacitors with bilayer dielectrics consisted of large bandgap Al2O3 and high-k HfO2 in different stacking order on n-type doped (100) β-Ga2O3 are investigated through C − V and J − V measurement. The C − V measurement results reveal that incoming HfO2 makes both bilayer structures attain an increasing dielectric constant, which means a better gate control ability in transistors comparing with single Al2O3. Additionally, the interface state density extracted by high-low frequency capacitance method suggests that Al2O3/(100)β-Ga2O3 with no treatment shows a comparative Dit value (8.0 × 1012 cm-2eV-1 to 2.2 × 1011 cm-2eV-1) with HfO2/(100)β-Ga2O3 (8.4 × 1012 cm-2eV-1 to 1.0 × 1011 cm-2eV-1) in energy range of 0.2 to 0.9 eV. Furthermore, HfO2/Al2O3/Ga2O3 showing a bigger forward breakdown voltage of 11.0 V than 7.8 V of Al2O3/HfO2/Ga2O3 demonstrates that inserted larger bandgap Al2O3 insulator between Ga2O3 semiconductor and high-k HfO2 dielectric can prevent gate leakage current more eff...