β-Ga2O3 Surface Research Articles

Assessment of the (010) β-Ga2O3 surface and substrate specification

Recent breakthroughs in bulk crystal growth of the thermodynamically stable beta phase of gallium oxide (β-Ga2O3) have led to the commercialization of large-area β-Ga2O3 substrates with subsequent epitaxy on (010) substrates producing high-quality films. Still, metalorganic chemical vapor deposition, molecular beam epitaxy, and processing of the (010) β-Ga2O3 surface are known to form subnanometer-scale facets along the [001] direction as well as larger ridges with features perpendicular to the [001] direction. A density function theory calculation of the (010) surface shows an ordering of the surface as a subnanometer-scale feature along the [001] direction. Additionally, the general crystal structure of β-Ga2O3 is presented, and recommendations are presented for standardizing (010) substrates to account for and control the larger-scale ridge formation.

Hetero-Integration of β-Ga2O3 and Diamond Substrates by Hydrophilic Bonding Technique

Beta-phase gallium oxide (β-Ga2O3) has a wide bandgap of ~4.9 eV and a high breakdown voltage of 8 MV/cm. Moreover, high-quality, large-size, and low-cost β-Ga2O3 substrates can be obtained by melt-growth techniques, such as Czochralski (CZ), Floating Zone (FZ), Edge-defined film-fed growth (EFG) techniques. Owing to these features, β-Ga2O3 is a possible candidate for the next-generation power electronics material. However, the primary obstacle for this application is the heat dissipation problem due to low thermal conductivity of ~10–30 W/m/k.A possible solution is to integrate β-Ga2O3 power electronics with high-thermal-conductivity materials. Our research group has developed a hydrophilic direct bonding technique of semiconductor material and diamond heat spreader[1], which has extraordinary high thermal conductivity. A diamond (111) surface treated with a H2SO4/H2O2 mixture was functionalized with OH groups, and the OH-terminated diamond substrate can form direct bonding with a OH-terminated Si substrate by a dehydration reaction. OH termination using oxygen plasma irradiation allows a strong bonding of semiconductor substrates, such as Si, SiO2, and Al2O3 (the oxide of group 13 element as Ga2O3)[2]. The purpose of this study is to form direct bonding of an oxygen-plasma-activated β-Ga2O3 substrate and the H2SO4/H2O2-treated diamond substrate by the hydrophilic direct bonding technique, as illustrated in Fig. 1.For the bonding experiment, thin β-Ga2O3 films were exfoliated along the (100) plane using β-Ga2O3 bulk crystal (from Novel Crystal Technology) with a thermal release tape. The thickness of the β-Ga2O3 film was ~10 µm. The cleaved β-Ga2O3 surface was irradiated by oxygen plasma at 200 W and 60 Pa for 30 s using our reactive ion etching (RIE) equipment. Meanwhile, diamond substrates with a 4° off-angled (111)-oriented surface (from EDP) were treated with a H2SO4/H2O2 (4:1) mixture at 75 °C for 10 min. Subsequently, they were cleaned with an NH3/H2O2/H2O (1:1:5) mixture at 75 °C for 10 min. The β-Ga2O3 and diamond surfaces were contacted with each other under atmospheric conditions. The contacted specimens were stored with desiccant for around three days and then annealed at 250 °C for 24 h for bond formation.Figure 2 shows the photograph of the β-Ga2O3 film bonded on the diamond substrate. As the β-Ga2O3 film is transparent, Newton’s rings were observed where the surfaces were not bonded. The nano-structure of the β-Ga2O3/diamond interface was investigated by a transmission electron microscope (TEM). The focused ion beam (FIB) was introduced from the β-Ga2O3 side to fabricate the ultra-thin cross-sectional specimen of the bonding interface. Figure 3 shows the TEM image around the bonding interface. The β-Ga2O3 [010] and diamond [110] directions were slightly twisted because of manual alignment. It revealed that the β-Ga2O3 and diamond surfaces were atomically bonded without interfacial voids or cracks. The β-Ga2O3 and diamond lattices were rarely damaged except for the outermost surface of the β-Ga2O3 film; The distortion is possibly due to oxygen plasma irradiation, interfacial deformation, and/or thermal stress. Generally, oxide layers having low thermal conductivity were generated at the bonding interface when semiconductor substrates formed direct bonding under atmospheric conditions. However, this study realized the direct bonding without any intermediate oxide layers because β-Ga2O3 is an oxide material and diamond never develops an oxide layer. Thus, the bonding interface is almost ideal for efficient heat dissipation from a β-Ga2O3 power device. Moreover, it can facilitate the development of future electronic devices using the β-Ga2O3/diamond interface.[1]. T. Matsumae, Y. Kurashima, H. Umezawa, and H. Takagi, Scr. Mater., 175, 24–28 (2020).[2]. T. Suni, K. Henttinen, I. Suni, and J. Mäkinen, J. Electrochem. Soc., 149, G348 (2002) http://jes.ecsdl.org/cgi/doi/10.1149/1.1477209. Figure 1

Hetero-Integration of β-Ga2O3 and Diamond Substrates by Hydrophilic Bonding Technique

A β-Ga2O3 film was directly bonded on a diamond substrate at 250ºC under atmospheric conditions. Before bonding, the β-Ga2O3 surface was activated by oxygen plasma irradiation, while the diamond surface was cleaned with H2SO4/H2O2 and NH3/H2O2 mixtures. The β-Ga2O3anddiamond surfaces adhered to one another under atmospheric conditions. By annealing the contacted specimen at 250ºC, they formed atomic bonds without voids, cracks, or severe crystallinity damages. It is believed that the direct bonding of β-Ga2O3 and diamond can facilitate the ultra-wide bandgap power electronics.

Origin of reverse leakage current path in edge-defined film-fed growth (001) β -Ga2O3 Schottky barrier diodes observed by high-sensitive emission microscopy

We utilized ultra-high sensitive emission microscopy to investigate the origin of reverse leakage current of edge-defined film-fed grown (001) β-Ga2O3 Schottky barrier diodes. In the emission patterns, we observed a partially appearing void, having a typical width and depth of 300 and 83 nm, respectively, with a base angle of 75°, below the Schottky barrier contact on the β-Ga2O3 surface. Simulations show that the electric field at the bottom of the void reached 9 × 106 V/cm at a reverse bias of −60 V and in the case of an ideally flat surface 3.1 × 106 V/cm. This indicates that the reverse leakage current originates from an electric field concentration in the partially appearing voids on the surface. On the other hand, we have confirmed that there is no clear relationship between the leakage current path and the dislocations observed by synchrotron x-ray topography.

The importance of intermediates ring-opening in preventing photocatalyst deactivation during toluene decomposition

The deactivation of photocatalysts is the main barrier that diminishes the commercial value of photocatalytic air-purification technology. It is essential to explore the intrinsic origins for the deactivation and develop effective strategy to address this key issue. This work investigates the intrinsic reasons for VOCs-decomposition induced intermediates accumulation on the surface of TiO2 (P25) based on the combined experiments and DFT calculations. The main culprit of deactivation is the accumulation of benzaldehyde intermediate on the surface of P25 because of its high reaction energy of aromatic ring opening. This will lead to the facilitated conversion of benzaldehyde to benzoic acid that will thus occupy the active sites on the surface of P25. However, the wide gap β-Ga2O3 achieved the excellent deactivation resistance during prolonged VOCs degradation, which is derived from that the fact that the adsorbed intermediates will be activated on the surface of β-Ga2O3 and strengthen the aromatic ring opening via •OH radicals, thus realizing efficient mineralization of the ring-containing intermediates into inorganic small molecules (CO2 and H2O). This work points out that the deactivation-resistant property of photocatalyst is closely related to the adsorption activation and the ring-opening characteristics of the intermediates, which could provide a new perspective in development of efficient and deactivation-resistant photocatalysts for practical applications.

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.

Defects at the surface of β-Ga2O3 produced by Ar plasma exposure

Films of β-Ga2O3 grown by halide vapor phase epitaxy on native substrates were subjected to Ar inductively coupled plasma treatment. As a result, the built-in voltage of Ni Schottky diodes deposited on the plasma treated surfaces decreased from 1 V to −0.02 V due to the buildup of deep trap concentration in the near surface region. Deep level spectra measurements indicate a strong increase in the top ∼200 nm of the plasma treated layer of the concentration of E2* (Ec − 0.8 eV) and especially E3 (Ec − 1.05 eV) deep electron traps. Capacitance-voltage profiling with monochromatic illumination also indicated a large increase in the upper ∼100 nm of the film in the concentration of deep acceptors with optical threshold for an ionization of ∼2.3 eV and 3.1 eV. Such defects at the surface led to a significant increase in reverse current, an increase in the ideality factor in forward current, and a dramatic decrease in the diffusion length of nonequilibrium charge carriers from 450 nm to 150 nm.

Effect of surface treatments on electrical properties of β-Ga2O3

The effect of various combinations of gaseous (ultraviolet/O3), liquid (HCl, buffered oxide etch, and H2O2), or plasma (CF4 and O2) treatments of the surface of β-Ga2O3 was quantified by current–voltage and capacitance–voltage measurements of rectifier structures. Plasma exposure (13.56 MHz, 24 kW/cm2) always led to significant degradation of the surface, as evidenced by large increases in rectifier reverse current and ideality factor (from 1.01 in control samples to ∼3.8 in plasma exposed samples, indicating additional defect-related carrier transport mechanisms) and lowering of the Schottky barrier height (from 1.21 eV in control samples to 0.75–0.86 eV in plasma exposed samples) and diode rectification ratio, with degraded reverse recovery characteristics. This was true of both CF4 and O2, even though it is known that fluorine incorporation in the near-surface leads to donor compensation and an increase in barrier height. Damage from the plasma exposure was not fully recovered by annealing at 500 °C. The O3 and liquid chemical cleans did lead to reduced reverse current in rectifiers, with no measurable decrease in barrier height, increase in ideality factor, or degradation of reverse recovery characteristics. Surfaces treated in this manner did not significantly change for anneals up to 500 °C; however, the Ni/Au contacts already show degradation after annealing at 350 °C.

Density Functional Theory Simulations of Water Adsorption and Activation on the (-201) β-Ga2 O3 Surface.

Density functional theory calculations are used to study the molecular and dissociative adsorption of water on the (-201) β-Ga2 O3 surface. The effect of adsorption of different water-like species on the geometry, binding energies, vibrational spectra and the electronic structure of the surface are discussed. The study shows that although the hydrogen evolution reaction requires a small amount of energy to become energetically favourable, the over potential for activating the oxygen evolution reaction is quite high. The results of our calculations provide insight as to why a high voltage is required in experiments to activate the water-splitting reaction, whereas previous studies of gallium oxide predicted very low activation energies for other energetically more favourable facets. Application of this work to studies of GaN-based chemical sensors with gallium oxide surfaces shows that it is possible to select the gate bias so that the sensors are not influenced by water-splitting reactions. It was also found that in the region where water splitting does not occur, the surface can exist in two states, that is, water or hydroxyl terminated.

MOVPE growth of violet GaN LEDs on β-Ga2O3 substrates

We report that a H2-free atmosphere is essential for the initial stage of metalorganic vapour phase epitaxy (MOVPE) growth of GaN on β-Ga2O3 to prevent the surface from damage. A simple growth method is proposed that can easily transfer established GaN growth recipes from sapphire to β-Ga2O3 with both (−201) and (100) orientations. This method features a thin AlN nucleation layer grown below 900°C in N2 atmosphere to protect the surface of β-Ga2O3 from deterioration during further growth under the H2 atmosphere. Based on this, we demonstrate working violet vertical light emitting diodes (VLEDs) on n-conductive β-Ga2O3 substrates.

Communication—Electrical Characterization of β-Ga2O3 Single Crystal Substrates

We have electrically investigated deep-level defects in unintentionally-doped β-Ga2O3 single crystal substrates fabricated by an edge-defined film-fed growth method, employing capacitance-voltage (C-V) and steady-state photo-capacitance spectroscopy (SSPC) techniques. From C-V measurements, n-type residual carriers with their concentration of ∼8.7 × 1016 cm−3 were almost uniformly distributed over the measured depth region of 188 – 336 nm from the β-Ga2O3 surface. SSPC measurements revealed four dominant deep-level defects located at ∼0.8, ∼2.04, ∼2.71, and ∼3.87 eV below the conduction band, in addition to near-band-edge emissions at ∼4.49 eV. These deep-level defects tended to be more densely present with shallowing the probing depth range.

High-quality III-nitride films on conductive, transparent (2̅01)-oriented β-Ga2O3 using a GaN buffer layer

We demonstrate the high structural and optical properties of InxGa1−xN epilayers (0 ≤ x ≤ 23) grown on conductive and transparent (01)-oriented β-Ga2O3 substrates using a low-temperature GaN buffer layer rather than AlN buffer layer, which enhances the quality and stability of the crystals compared to those grown on (100)-oriented β-Ga2O3. Raman maps show that the 2″ wafer is relaxed and uniform. Transmission electron microscopy (TEM) reveals that the dislocation density reduces considerably (~4.8 × 107 cm−2) at the grain centers. High-resolution TEM analysis demonstrates that most dislocations emerge at an angle with respect to the c-axis, whereas dislocations of the opposite phase form a loop and annihilate each other. The dislocation behavior is due to irregular (01) β-Ga2O3 surface at the interface and distorted buffer layer, followed by relaxed GaN epilayer. Photoluminescence results confirm high optical quality and time-resolved spectroscopy shows that the recombination is governed by bound excitons. We find that a low root-mean-square average (≤1.5 nm) of InxGa1−xN epilayers can be achieved with high optical quality of InxGa1−xN epilayers. We reveal that (01)-oriented β-Ga2O3 substrate has a strong potential for use in large-scale high-quality vertical light emitting device design.

Thermal stability of β-Ga2O3 in mixed flows of H2 and N2

The thermal stability of β-Ga2O3(010) substrates was investigated at atmospheric pressure between 250 and 1450 °C in a flow of either N2 or a mixture of H2 and N2 using a radio-frequency induction furnace. The β-Ga2O3 surface was found to decompose at and above 1150 °C in N2, while the decomposition of β-Ga2O3 began at only 350 °C in the presence of H2. Heating β-Ga2O3 substrates in gas flows containing different molar fractions of H2 demonstrated that the decomposition was promoted by increasing the H2 molar fractions. Thermodynamic analysis showed that the dominant reactions are in N2 and in a mixed flow of H2 and N2. The second-order reaction with respect to H2 determined for the mixed flows agrees with the experimental results for the dependence of the β-Ga2O3 decomposition rates on the H2 molar fraction.

Reconstruction of the [formula omitted]-Ga[formula omitted]O[formula omitted] (1 0 0) cleavage surface to hexagonal GaN after NH[formula omitted] nitridation

The potential use of β-Ga2O3 wafers as transparent conductive substrates for compounds with hexagonal structure, in particular the GaN-system, is shown. Nitridation of the main cleavage plane under high temperature and NH3 gas results in the substitution of O by N on the surface, and a simultaneous surface reconstruction. The 2-fold symmetry of the initial (100) plane changes to a 6-fold symmetry, keeping the [010] azimuth as a symmetry direction. The RHEED pattern suggests the formation of GaN on the β-Ga2O3 surface, with the b-axis of β-Ga2O3 parallel to the 〈112¯0〉 direction of GaN, so that β-Ga2O3 can be used as a substrate with no lattice mismatch to GaN.