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Functional Inorganic Materials and Devices

Investigation of the mechanism for Ohmic contact formation in Ti/Al/Ni/Au contacts to #-Ga2O3 nanobelt field-effect transistors Jin-Xin Chen, Xiao-Xi Li, Hongping Ma, Wei Huang, ZhiGang Ji, Changtai Xia, Hong-Liang Lu, and David Wei Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09166 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

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ACS Applied Materials & Interfaces

Investigation of the mechanism for Ohmic contact formation in Ti/Al/Ni/Au contacts to β-Ga2O3 nanobelt field-effect transistors

Jin-Xin Chen ‡ ,1, Xiao-Xi Li ‡ ,1, Hong-Ping Ma1, Wei Huang1, Zhi-Gang Ji2, Changtai Xia3, Hong-Liang Lu1*, David Wei Zhang1 1State

Key Laboratory of ASIC and System, Shanghai Institute of Intelligent Electronics

& Systems, School of Microelectronics, Fudan University, Shanghai 200433, China 2Department

of Electronics and Electrical Engineering, Liverpool John Moores

University, Liverpool L3 3AF, UK 3Key

Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and

Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China

E-mail: [email protected] ‡, These authors contributed equally

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ABSTRACT The issue of contacts between electrode and channel layer is crucial for wide-bandgap semiconductors, especially the β-Ga2O3 due to its ultra-large bandgap (4.6-4.9 eV). It affects the device performance greatly and thus needs special attention. In this work, the high-performance β-Ga2O3 nano-belt field-effect transistors with Ohmic contact between multilayer metal stack Ti/Al/Ni/Au (30/120/50/50 nm) and unintentionallydoped β-Ga2O3 channel substrate have been fabricated. The formation mechanism of Ohmic contacts to β-Ga2O3 under different annealing temperatures in N2 ambient is systematically investigated by X-ray photoelectron spectroscopy. It is revealed that the oxygen vacancies at the interface of β-Ga2O3/intermetallic compounds formed during rapid thermal annealing is believed to induce the good Ohmic contacts with low resistance. The contact resistance (𝑅𝑐) between electrodes and unintentionally-doped βGa2O3 reduces to ~ 9.3 Ω・mm after annealing. This work points to the importance of contact engineering for future improved β-Ga2O3 device performance, and lays a solid foundation for wider application of β-Ga2O3 in electronics and optoelectronics. KEYWORDS: wide-bandgap semiconductors, gallium oxide, field-effect transistors, Ohmic contact, multilayer metal stack, oxygen vacancies

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INTRODUCTION Recently, β-Ga2O3 has gained great attention due to its extraordinary properties, such as ultra-large bandgap Eg (4.6~4.9 eV),1-5 high theoretical breakdown field Eb (~8 MV/cm),6 excellent thermal and chemical stability.7,8 Combined with ~300 cm2V-1s-1 carrier mobility, β-Ga2O3 displays much larger Baliga’s figure-of-merit (BFOM, ~3444) than SiC and GaN.9,10 It is thus an attractive channel material for the next-generation power devices including field effect transistors (FETs) and Schottky barrier diodes (SBDs). A variety of simple and low-cost β-Ga2O3 bulk single-crystal growth methods are already commercially available including the vertical Bridgman technique, the edge-defined film-fed growth (EFG), optical floating zone (OFZ), and Czochralski method.11-15 Similar to many other conventional two-dimensional (2D) materials (e.g., graphene, black phosphorus and transition metal dichalcogenides), (001) and (100) oriented β-Ga2O3 substrate can be acquired by mechanical cleavage technique even though it’s not a van der Waals 2D material, due to the large lattice constant. In comparison with radio-frequency magnetron sputtering, metal organic vapor phase epitaxy (MOVPE), and other Ga2O3 film growth techniques, mechanical exfoliation technique can effectively eliminate the potential fault caused by strain in subsequent device fabrication. Attempts have been made to fabricate field effect transistors (FETs) using exfoliated β-Ga2O3 micro-flakes,16-19 however, the formation of Ohmic contacts between the electrode and channel layer is arduous because of its ultra-large bandgap. It is known that the quality of the contact affects the device performance significantly including the on-state current, on/off current ratio (Ion/Ioff), field-effect mobility (𝜇𝐹𝐸), 3



subthreshold slope (SS) and I-V line-shapes.19,20 Therefore, the exploration of βGa2O3/metal contacts is urgently required to achieve excellent device performance. Several attempts have been made in the past few years. Sasaki et al.21 controlled the electron concentration in β-Ga2O3 by using Si-ion (Si+) implantation doping method to achieve low-resistance Ohmic contact to β-Ga2O3. The lowest contact resistivity in their work was approximately 1.4×10-2 Ω · mm, along with high effective donor concentrations (Nd) in the order of 1019 cm-3. Yao et al.22 investigated nine different metals (i.e., Zr, Sn, Ag, Zn, Sc, Mo, W, In, and Ti) as potential Ohmic electrodes to the Sn-doped β-Ga2O3 with high electron carrier concentration of 5×1018 cm-3. Carey IV et al.23,24 utilized AZO and ITO as a low resistance intermediary layer between Si-doped Ga2O3 substrate and Ti/Au metallization and formed Ohmic contacts with low contact resistance through annealing. However, all the above-mentioned attempts require the high doping density of the channel material. Recent studies have revealed that the high channel doping may reduce the gate controllability and lead to larger off-state current at relatively high channel thickness and high operating temperatures.7,25,26 Moreover, the deep understanding on the reactions at the β-Ga2O3/metal interface was rare due to the initial stages of β-Ga2O3 based device development. Li et al.19 prepared high-quality Ohmic contacts in β-Ga2O3 FETs and analyzed the interface reactions between β-Ga2O3 and titanium layer during 300 oC furnace annealing (FA) process via X-ray photoelectron spectroscopy (XPS). Unfortunately, as compared with conventional GaN and SiC, a systematic study of Ohmic contacts using multilayer metal stacks on β-Ga2O3 single-crystal substrate is currently lacking. There are still 4


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many questions regarding electrical contacts (e.g., interfacial analysis of the Ga2O3/metals, long-term stability of contacts, and others) to explore to guarantee highperformance β-Ga2O3 based devices and establish the potential of β-Ga2O3 for power electronics. In addition, various deep ultraviolet photodetectors based on β-Ga2O3 fieldeffect transistors have been experimentally demonstrated to date. 3,18 These transistors with excellent Ohmic contact can easily obtain ultrahigh photo-to-dark current ratio, and therefore greatly improve the application of photodetection. In this paper, we explored the feasible solution to achieve high quality Ohmic contact with channel using unintentionally-doped β-Ga2O3 which thus maintains the devices’ high performance. The multilayer metal stacks Ti/Al/Ni/Au (30/120/50/50 nm) were specifically used in which nickel and gold were used respectively as the barrier layer and the capping layer due to their high melting points. With the optimized processing conditions, the high-performance β-Ga2O3 nanobelt field-effect transistors with Ti/Al/Ni/Au (30/120/50/50 nm) Ohmic electrodes were experimentally demonstrated. In addition, we further explored the underlying mechanism of the Ohmic contact formation to β-Ga2O3 under different annealing temperatures in N2 ambient using XPS analysis. Therefore, this work provides a pathway for fabricating highperformance β-Ga2O3 nanobelt field-effect transistors in future.

EXPERIMENTAL SECTION Preparation of the β-Ga2O3 Nano-belt FETs. In this study, unintentionally ndoped β-Ga2O3 bulk single crystal with free electron concentration (Nd) of ~1017 cm-3 5



was produced by the edge-defined film-fed growth (EFG) method and used as starting sample. The β-Ga2O3 nanobelts were mechanically exfoliated from bulk single crystals using the Scotch tape method, and dry-transferred onto the SiO2/p+ +-Si substrate with SiO2 thickness of 110 nm. Two electrode regions (i.e., source and drain) were defined by the electron-beam lithography (EBL), followed by Ti/Al/Ni/Au (30/120/50/50 nm) metallization and lift-off processes. The growth temperature and base pressure for the preparation of metallic contact on the Ga2O3 substrates using evaporation method were 50 oC and 5 × 10-4 Pa, respectively. Prior to the electrodes pattern, the SiO2/p+ +-Si substrates along with transferred β-Ga2O3 nanobelts were cleaned using the standard solvent degreasing procedure and rinsed with DI water for 30 s, followed by a nitrogen blow-dry. Moreover, the AS-One rapid thermal processing (RTP) system was employed to anneal the contact stacks in nitrogen atmosphere for 70 s under different annealing temperatures. The ramp-up rate was approximately 25 oCs-1, and the rampdown duration was set as 1 s. Sample Characterization. X-ray diffractometer (XRD; Bruker, D8) was employed to analyze the crystalline structure of β-Ga2O3 bulk. The surface morphology and thickness of the transferred β-Ga2O3 (100) nano-flakes were characterized by optical microscope (Leica, DM2700M), scanning electron microscope (SEM; Hitachi, SU1510), and tapping-mode atomic force microscope (AFM; Bruker, Icon), respectively. Micro-Raman spectra were recorded using Horiba Jobin Yvon LabRAM ARAMIS system equipped with an excitation wavelength of 532 nm to analyze the structural properties of the transferred β-Ga2O3 thin flakes. The current-voltage (I-V) 6


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electrical characteristics of β-Ga2O3 devices were measured with the Agilent B 1500A semiconductor device analyzer using Tungsten probe tips in air at room temperature. XPS measurements were performed via a SPECS XPS system equipped with a monochromatic Al Kα source (hv=1486.6 eV) as the excitation of photoelectrons. The analysis region was defined as a round spot with a diameter of 1 mm. Charge correction was performed with the C 1s peak at 284.8 eV. Narrow scans with a step of 0.05 eV were performed for 15-20 times for binding energy of specific elements. The spectral deconvolution was carried out by Shirley background subtraction utilizing a Voigt function.

RESULTS AND DISCUSSION The XRD spectrum of the unintentional doped β-Ga2O3 is presented in Fig.1. (a). The clear high-intensity peaks of narrow width can be observed, suggesting the nature of single crystallinity of the bulk material. The bandgap of the single crystal β-Ga2O3 can be obtained by the linear fit on the loss spectra curve from XPS measurement near the approximate location of onset of inelastic losses.4 As shown in Fig.1. (b), the bandgap was estimated to be 4.87 eV. Furthermore, the micro-Raman spectroscopy was carried out to evaluate the quality of the exfoliated β-Ga2O3 nanobelt, as given in Fig.1. (c&d). There was no significant difference in the position of phonon peaks in the exfoliated β-Ga2O3 nanobelt compared with the initial bulk one. It suggests that the prepared nanobelt has low defect density. The change of the base lines in Raman spectra is ascribed to the 110-nm SiO2/p+ +-Si substrate beneath the β-Ga2O3 nano-flakes. 7



Fig.1. Characteristics of the unintentional doped β-Ga2O3 single crystals. (a) XRD pattern for β-Ga2O3 bulk single crystal. (b) The reflection electron energy loss spectrum (i.e., O1s core level spectrum) from the unintentional doped β-Ga2O3 crystal via XPS measurements. Micro-Raman spectra of (c) the β-Ga2O3 bulk sample and (d) the mechanically exfoliated β-Ga2O3 nanobelt on 110 nm SiO2/p+ +-Si substrate.

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Fig.2. (a) Typical cross-sectional height profile of the exfoliated β-Ga2O3 nano-flake on SiO2/p+ +-Si substrate (the white dashed line in the inset determined by AFM). (b) Corresponding AFM image of the surface of β-Ga2O3 nano-flake after mechanical exfoliation. (c) Schematic diagram of the back-gate β-Ga2O3 channel FET with Ti/Al/Ni/Au (30/120/50/50 nm) contact stacks. (d) Optical image of the fabricated βGa2O3 channel FET on SiO2 (110 nm-thick)/p+ +-Si substrate after annealing. The inset displays the SEM image of the corresponding β-Ga2O3 channel area.

Fig.2. (a) shows a typical AFM image of the β-Ga2O3 nanobelt on the SiO2/p+ +-Si substrate. The thickness of the nanobelt is estimated to be ~153 nm. The surface morphology of the corresponding β-Ga2O3 nanobelt is given in Fig.2. (b). A root mean square (RMS) roughness is detected to be ~0.19 nm in a 500×500 nm square. This value 9



is comparable to the works reported by Liu et al. and Zhou et al.16,18 Moreover, all the exfoliated β-Ga2O3 samples exhibit smooth surfaces without spots or cracks. The obtained nanobelt area is generally above 20 μm2, which is perfect for the quasi-twodimensional transistor fabrication. Fig.2. (c) shows the schematic structure of β-Ga2O3based back-gate FET with Ti/Al/Ni/Au (30/120/50/50 nm) as the metal electrodes. Similar to the conventional 300-nm SiO2/Si substrate, the optical identification of exfoliated β-Ga2O3 nanobelts on the 110-nm SiO2/Si substrate is also facile, as can be seen in Fig.2. (d). The inset in Fig.2. (d) shows the SEM image of the corresponding βGa2O3 channel area. The spacing between two contact electrodes (i.e., channel length 𝐿𝑔) and the channel width (𝐿𝑤) are ~3.59 and ~2.62 μm, respectively. The electrical performance of the back-gate β-Ga2O3 FET under two different temperatures during rapid thermal annealing (RTP) was systematically investigated. After annealing at a temperature of 400 oC in N2 for 70 s, the β-Ga2O3 FET still maintains high contact resistance, as shown in Fig.3. (a). The entire Id-Vd curve at Vgs=0 V shown in the inset, exhibits strong Schottky behavior. The source-drain on-resistance (𝑅𝑡𝑜𝑡) is up to 4.0×104 Ω・mm. The corresponding output curves under at high Vds regime are also presented in Fig.3. (c). Strong current blocking effect can be clearly observed at small Vds values. Fig.3. (e) presents the corresponding Id-Vbg transfer curves with different Vds after annealing at 400 oC. The on/off current ratio is only of the order of 102. Meanwhile, small on-state current (1.08×10-3 mA/mm), unstable threshold voltage and abnormal line-shapes can also be observed. In contrast, the device performance improved dramatically after annealing at a higher temperature of 470 oC 10


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in N2 atmosphere for 70 s. Fig.3. (b) shows the linear Id-Vds curves at low Vds regime, indicating good Ohmic contacts at drain/source without significant charge injection barriers at room temperature. For comparison, the inset of Fig.3. (b) gives the Id-Vds curve at Vgs = 0 V from the same device before annealing. Evidently, the typical asymmetric Schottky behavior can be seen from the back-gate β-Ga2O3 FET without RTP annealing. Fig.3. (d) shows the Ids-Vds output characteristics of the corresponding β-Ga2O3 FET under high Vds after 470 oC annealing. The Id can be effectively tuned by the back-gate bias (Vbg) with good saturation and sharp pinch-off characteristics, demonstrating that the channel material is n-type. This property is ascribed to the presence of oxygen vacancies and unintentional impurities in the intrinsic β-Ga2O3 substrate.8 The corresponding Id-Vbg transfer characteristics of the β-Ga2O3 FET in both linear- and log-scale are shown in Fig.3. (f). This β-Ga2O3 back-gated FET with Ti/Al/Ni/Au Ohmic electrodes exhibit excellent electronic performance after annealing at 470 oC, including low cutoff current (~1 pA), large on/off current ratio (Ion/Ioff >106) and steep sub-threshold swing (SS, ~250 mV/dec). The field-effect mobility (𝜇𝐹𝐸) extracted from the transfer curve in linear scale was based on the following equation, 𝐿𝑔 𝑑𝐼𝑑

1

(1)

𝜇𝐹𝐸 = 𝐿𝑤𝑑𝑉𝑏𝑔𝐶𝑜𝑥𝑉𝑑𝑠

where 𝐶𝑜𝑥, 𝐿𝑤, and 𝐿𝑔 are the oxide capacitance, channel width, and channel length in the β-Ga2O3 FET, respectively. 𝐼𝑑, 𝑉𝑏𝑔, and 𝑉𝑑𝑠 are the drain current, back-gate voltage, and drain voltage in the FET, respectively. The maximum 𝜇𝐹𝐸 reaches 43.15 cm2/(V・s), which is comparable to recently reported back-gate devices with heavily doped Ga2O3 channels (Nd, in the order of 1018 cm-3).16,18,19,27 These electrical results

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indicate that a proper annealing temperature during RTP treatment is of critical importance to improve the performance of the β-Ga2O3 nano-belt FETs.

Fig.3. I-V electrical characteristics of the back-gated β-Ga2O3 FET with Ti/Al/Ni/Au (30/120/50/50 nm) as the contacts under different annealing temperatures. Id-Vds family curves from the β-Ga2O3 FET with ~153-nm-thick channel after annealing at 400 oC in N2 for 70 s under (a) low Vds and (c) high Vds values. The inset in (a) shows the entire Id-Vd curve at Vg of 0 V to reveal strong Schottky-like behavior in the contact. Id-Vds output curves from the β-Ga2O3 FET after annealing at 470 oC in N2 for 70 s under (b)

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low Vds and (d) high Vds. The inset image in (b) gives the Id-Vds curve at Vgs of 0 V from the same device before annealing. Id-Vbg transfer curves of the corresponding βGa2O3 device in linear and logarithmic scale at various Vds values after annealing in N2 ambient at (e) 400 oC and (f) 470 oC for 70 s.

To determine the contact resistance (𝑅𝑐) of the β-Ga2O3 nano-belt FETs after 470 oC

annealing, transmission line measurements (TLM) were performed using the

following equation, 𝑅𝑡𝑜𝑡 = 2𝑅𝑐 + 𝜌𝑐ℎ𝑎𝑛𝑛𝑒𝑙 ∙

𝐿𝑔

(2)

𝐴

where 𝑅𝑡𝑜𝑡 and 𝑅𝑐 are the total resistance and contact resistance in the β-Ga2O3 FET, respectively. 𝜌𝑐ℎ𝑎𝑛𝑛𝑒𝑙, 𝐿𝑔 , and 𝑉𝑑𝑠 are the channel resistivity, channel length, and cross-sectional area in the FET, respectively. Two-probe measurements for the β-Ga2O3 FETs with different Lg/A ratios are shown in Fig.4. (a). Fig.4. (b) correspondingly plots the total resistance 𝑅𝑡𝑜𝑡 as a function of the ratio Lg/A. The 𝑅𝑐 extracted from half of the y-intercept of the fitted total resistance curve is ~9.3 Ω・mm, which is much lower than the value (i.e., 606 Ω・mm) reported by Yang et al.17 for the unintentionally ndoped β-Ga2O3 nanobelt. In fact, the 𝑅𝑐 can be further reduced by using heavily doped β-Ga2O3 (Nd, above 1018 cm-3) channel materials.16 As a result, the use of Ti/Al/Ni/Au contact stack is a promising approach for reducing contact resistance in the β-Ga2O3 based devices.

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Fig.4. Contact resistance (Rc) measurements of the β-Ga2O3 FET with Ti/Al/Ni/Au metal electrodes after annealing at 470 oC in N2 for ~70 s. (a) I-V characteristics of the β-Ga2O3 FETs with different Lg/A ratios measured by two-probe technique. (b) Total resistance (Rtot) plotted as a function of the Lg/A ratio in β-Ga2O3 FETs.

In order to understand the underlying mechanism of the Ohmic contacts formation at the channel/electrode interface, high-resolution XPS was used to systematically investigate the evolution of interfacial chemical states of β-Ga2O3/metals during the RTP annealing process. Four types of samples were used in our XPS experiments: (1) pure intrinsic β-Ga2O3 single-crystal substrate after RTP annealing at 470 oC for 70 s in N2 ambient, (2) Ti/Al-coated (2/2 nm) β-Ga2O3 bulk crystal after annealing at 400 oC

for 70 s in N2 ambient, (3) Ti/Al-coated (2/2 nm) β-Ga2O3 bulk crystal after

annealing at 470 oC for 70 s in N2, and (4) Ti-coated (~2.5 nm) β-Ga2O3 bulk crystal annealed at 470 oC for 70 s in N2. Prior to the metal deposition, the β-Ga2O3 crystals were cleaned via the standard solvent degreasing procedure.

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Fig.5. XPS measurements from the intrinsic n-type β-Ga2O3 bulk samples. (a) Ga 2p3/2 and (b) Ga 3d core level spectra from the β-Ga2O3 bulk single crystal after annealing at 470 oC in N2 ambient for ~70 s. (c) Ga 2p3/2 and (d) Ga 3d core level spectra from the Ti/Al-coated (2/2 nm) β-Ga2O3 bulk after annealing at 400 oC in N2 ambient for 70 s.

Generally, for the pure β-Ga2O3 single-crystal substrate without annealing, the Ga 2p3/2 spectrum is often fitted by a single contribution attributed to Ga-O bonding,5,28-30 and the Ga 3d can be fitted by two separate peaks, attributed to Ga 3d3/2 and Ga 3d5/2.19,31 However, after annealing at 470 oC for 70 s in N2 ambient, additional small-intensity peaks at lower binding energies (i.e., 1116.02±0.2 eV and 18.55±0.2 eV) cannot be neglected from XPS measurements, as shown in Fig.5. (a)-(b), which is due to the formation of a small number of oxygen vacancies at the β-Ga2O3 surface in an oxygenpoor atmosphere.32,33 The results agree well with the work Li et al.19 recently reported about β-Ga2O3 tube furnace annealing in Ar ambient. In Fig.5. (c)-(d), similar new 15



subpeaks (i.e., 1116.22±0.2 eV and 19.78±0.2 eV) can be seen in the Ti/Al-coated (2/2 nm) β-Ga2O3 substrate after annealing at relatively low temperature of 400 oC in N2. Note that the intensities of new subpeaks associated with oxygen vacancies both improved slightly, as compared to annealed pure β-Ga2O3 substrate. Fig.6. (c)-(d) respectively shows the Ga 2p3/2 and Ga 3d core-level spectra for the Ti/Al-coated (2/2 nm) β-Ga2O3 substrate under higher annealing temperature of 470 oC for 70 s in N2. Surprisingly, the total peak shapes both changed greatly, which is similar to the peak shapes of annealed Ga2O3/Al2O3 and Ga2O3/TiO2 samples Serykh et al.34 previously demonstrated. In Fig.6. (c), it can be clearly seen that two strong subpeaks exist at lower binding energies compared with the neighboring peak (i.e., Ga 2p3/2 for Ga2O3). One sub-peak locates at 1116.72±0.2 eV (i.e., the main peak), corresponding to the reduced Ga δ + state. The other is at 1114.30±0.2 eV, which is due to the presence of minor amount of zero-valence gallium. In Fig.6. (d), an intense low-binding-energy subpeak can also be observed at 18.76 ± 0.2 eV near the main peak. All of them are in the intermediate region between the β-Ga2O3 and pure gallium peaks,31, 35-37 indicating the presence of large number of oxygen vacancies at the β-Ga2O3/metal interface after 470 oC

RTP annealing. The comparative study of annealing at 400 and 470 oC via XPS is

in good agreement with the electrical data of β-Ga2O3 FETs in Fig.3.

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Fig.6. XPS measurements from the intrinsic n-type β-Ga2O3 bulk. (a) Ga 2p3/2 and (b) Ga 3d core level spectra from the Ti-coated (~2.5 nm) β-Ga2O3 bulk single crystal after annealing at 470 oC in N2 atmosphere for 70 s. (c) Ga 2p3/2 and (d) Ga 3d core level spectra for the Ti/Al-coated (2/2 nm) β-Ga2O3 bulk sample after annealing at 470 oC in N2 for 70 s. (e) Schematic diagram for the role of Ti in the generation of oxygen vacancies. (f) Schematic for the formation process of oxygen vacancies at the interface of β-Ga2O3/metal. 17



To further analyze the mechanism of Ohmic metal stacks on β-Ga2O3, XPS measurements from the Ti-coated (~2.5 nm) β-Ga2O3 bulk crystal annealed at 470 oC for 70 s in N2 are also presented in Fig.6. (a)-(b). Interestingly, the Ga 2p3/2 spectrum for the Ti-coated (~2.5 nm) β-Ga2O3 was fitted by a single peak at 1118.39±0.2 eV, assigned to Ga-O bonding. The oxygen vacancy associated subpeak cannot be detected with XPS. From the Ga 3d spectrum (Fig.6. b), three subpeaks are observed at 20.68± 0.2 eV, 20.23±0.2 eV and 19.19±0.2 eV, corresponding to Ga 3d3/2, Ga 3d5/2, and the oxygen vacancy associated. However, similar to annealed pure β-Ga2O3 substrate, the intensity of the subpeak associated with oxygen vacancy is very small, illustrating the negligible number of oxygen vacancies at the interface. These results demonstrate that single Ti layer cannot generate large number of oxygen vacancies at the β-Ga2O3/metal interface in short time during the same annealing process, as shown in Fig.6. (e). An additional Al layer is essential. During the short-time/high-temperature annealing process in poor oxygen ambient, The Al atoms can diffuse through the Ti layer rapidly, leading to the formation of Ti-Al inter-metallic phase with lower work function. It acted as chemically active contacts to reduce β-Ga2O3 and generated large numbers of oxygen vacancies at the interface of β-Ga2O3/intermetallic compounds, as shown in Fig.6. (f).38 These locally distributed oxygen vacancies served as donors in the intrinsic n-type βGa2O3, which improved electron transport across the hetero-interface and facilitated Ohmic contact formation.

CONCLUSIONS In summary, the high-performance β-Ga2O3 nano-belt FETs were fabricated. Wherein, the use of Ti/Al/Ni/Au (30/120/50/50 nm) metal stacks on β-Ga2O3 substrate is shown to produce thermally stable Ohmic contacts with low contact resistance after 18


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annealing around 470 oC in N2 ambient for 70 s. The underlying mechanism of Ohmic contact formation to β-Ga2O3 under different annealing temperatures was analyzed by XPS. It is revealed that the involvement of Al can assist generating large number of oxygen vacancies in short time under 470 oC annealing temperature, and thereby improve Ohmic contacts on β-Ga2O3 FETs. The obtained results provide useful information on the understanding and design of high-quality β-Ga2O3-based electronic devices.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Hong-Liang Lu: 0000-0003-2398-720X Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work is supported by the National Key R&D Program of China (No.2016YFE0110700), National Natural Science Foundation of China (No. U1632121, 11804055, 51861135105 and 61874034), and Natural Science Foundation of Shanghai (No. 18ZR1405000).

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Figure for Abstract 600x399mm (300 x 300 DPI)


Ni

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