Characteristics of Vertical Ga2O3 Schottky Junctions with the Interfacial Hexagonal Boron Nitride Film.

We present the device properties of a nickel (Ni)-gallium oxide (Ga2O3) Schottky junction with an interfacial hexagonal boron nitride (hBN) layer. A vertical Schottky junction with the configuration Ni/hBN/Ga2O3/In was created using a chemical vapor-deposited hBN film on a Ga2O3 substrate. The current-voltage characteristics of the Schottky junction were investigated with and without the hBN interfacial layer. We observed that the turn-on voltage for the forward current of the Schottky junction was significantly enhanced with the hBN interfacial film. Furthermore, the Schottky junction was analyzed under the illumination of deep ultraviolet light (254 nm), obtaining a photoresponsivity of 95.11 mA/W under an applied bias voltage (-7.2 V). The hBN interfacial layer for the Ga2O3-based Schottky junction can serve as a barrier layer to control the turn-on voltage and optimize the device properties for deep-UV photosensor applications. Furthermore, the demonstrated vertical heterojunction with an hBN layer has the potential to be significant for temperature management at the junction interface to develop reliable Ga2O3-based Schottky junction devices.

Introduction

Gallium oxide (Ga2O3) with an ultrawide band gap (UWBG) has piqued the interest of researchers working on deep ultraviolet (UV) photonics and next-generation power electronic devices.[1−5] Semiconductors based on UWBG Ga2O3 materials can withstand much higher critical electric fields (6–8 MV/cm) before avalanche breakdown than silicon- (Si), silicon carbide- (SiC), and gallium nitride (GaN)-based devices.[1,6,7] Furthermore, Baliga’s figure of merit presents the advantage of Ga2O3 for power switching applications over the Si (3000×), SiC (10×), and GaN (4×).[8,9] It has been demonstrated that high-quality single-crystal Ga2O3 can be synthesized using melt growth techniques such as Czochralski, floating zone, and edge-defined film-fed growth (EFG).[10−15] In addition, halide vapor phase epitaxy (HVPE), molecular beam epitaxy (MBE), and chemical vapor deposition (CVD) methods for growing high-quality Ga2O3 have been developed.[16−25] Johnson’s figure of merit (JFOM/power–frequency product) of Ga2O3 (3× of GaN)[26,27] illustrates the potential benefit of using Ga2O3 in radio frequency (RF) devices. Chabak et al. have reported on the fabrication of the Ga2O3 MOSFET device using a thin and highly doped channel and achieving a drain-source current of 275 mA/mm at a drain to source bias of 10 V for that exhibited record-high ft/fmax = 5.1/17.1 GHz.[28] However, it is expected that Ga2O3 devices will suffer from self-heating due to the poor thermal conductivity of the material. The device model demonstrates that a homoepitaxial device suffers from an unacceptable junction temperature increase to 1500 °C at a power density of 10 W/mm, highlighting the importance of applying device-level thermal managements to individual Ga2O3 devices.[29,30] In this case, the integration of thermal conducting materials with Ga2O3-based devices may be critical for achieving dependable device performance.

The heterojunction and Schottky junction-based β-Ga2O3 devices are of great interest for the development of high-voltage devices and solar-blind UV photodetectors, considering the intrinsic n-type conductivity of β-Ga2O3 in the presence of unintentional dopants.[25−31] The conventional Ga2O3 Schottky junction devices were studied with metal electrodes such as Au, Pt, and Ni, taking into account the metals’ appropriate work function.[32,33] Again, studies on metal–insulator–semiconductor (MIS)-based SBD devices have been conducted, owing to the benefit of lowering the reverse saturation current.[34−36] The MIS-based SBD devices have been fabricated using insulating dielectric materials, such as SiO2, Si3N4, Al2O3, and so forth.[35−37,40] The thermal stability of Ga2O3 Schottky contacts is also an important aspect, for which materials with high melting temperatures and low reactivity with Ga2O3 are of significant interest.[37] Furthermore, controlling near-surface ion damage during sputtering deposition of a barrier layer/insulating dielectric[7,40] is required. In this case, the CVD-synthesized chemically inert insulating hBN layer will be significant with much higher in-plane thermal conductivity for use in Ga2O3-based Schottky junctions.[38,39]

Previously, we have demonstrated the integration of highly conducting graphene and copper iodide (γ-CuI) films with the β-Ga2O3 substrate for the fabrication of deep-UV photoresponsive devices.[41,42] The fabrication of quasi-two-dimensional metal–insulator–semiconductor field-effect transistors was demonstrated by the mechanical transfer process of hBN and nanothin flakes of β-Ga2O3.[43] In contrast to previous reports, we show how to make a vertical Ni/β-Ga2O3 Schottky junction with and without the hBN interfacial layer. The UWBG heterostructure of hBN and β-Ga2O3 can be significant due to the complementing electronic, chemical, and thermal properties of the materials. The following sections go over the specifics of hBN integration with the free-standing β-Ga2O3 substrate and the fabrication of a vertical Schottky junction. We discovered that the hBN interfacial film can be used to control the device’s turn-on voltage and current density.

Experimental Section

The hBN film was synthesized on recrystallized Cu (111) foil by the atmospheric pressure CVD (APCVD) as reported previously by the research group (Figure a).[44−46]

Figure 1

(a) Schematic diagram of the APCVD process for synthesis of the hBN film on Cu foil. (b) Transfer process of the CVD-synthesized hBN film for the fabrication of the Ni/hBN/β-Ga2O3/In heterojunction Schottky diode.

For this experiment, ammonia borane (10 mg) is used as a precursor and placed atop a magnetic boat, which was placed inside the tube about 20 cm away from the furnace to avoid precursor loss before the deposition step. The furnace was set at a high temperature of 1050 °C for the growth of the hBN film on the Cu surface. During the deposition step, the gas flow into the chamber was changed by the addition of argon gas with a flow rate of 98 sccm, and the flow rate of hydrogen gas was reduced to 2 sccm, and the mixed gas flow was 98:2 of Ar/H2. To achieve vaporization of ammonia borane, the precursor is brought close to the furnace using a magnet at 3 cm away from the furnace, as shown in the schematic diagram (Figure a). The deposition step was set for 40 min, allowing for the formation of an hBN layer on a Cu (111) substrate. Finally, the cooling step was completed by simultaneously switching the H2 gas flow, partially opening the furnace, and moving the precursor approximately 20 cm away from the furnace to avoid a secondary deposition. After the reaction chamber was cooled to room temperature, the hBN-grown Cu foil was removed and subjected to a wet transfer process described in the previous reports.[44] An hBN/β-Ga2O3 stack was obtained by transferring the CVD-synthesized hBN film on the Ga2O3 substrate. The fabricated hBN/β-Ga2O3 stack was used to fabricate an MIS-based Schottky junction device by depositing Ni and In metal electrodes, as shown in the schematic diagram (Figure b). The fabricated Schottky junction with a configuration of Ni/hBN/β-Ga2O3/In was characterized, and the effect of the interfacial hBN layer was analyzed.

The as-received β-Ga2O3 sample was characterized using X-ray diffraction (XRD), UV–vis absorption spectroscopy, and Raman spectroscopy. The XRD studies were carried out using a Rigaku Smart Lab SE with Cu Kα radiation as the X-ray source (λav = 1.5406 Å). A JEOL JSM 5600 scanning electron microscope at a voltage of 20 kV was used to examine the hBN-transferred β-Ga2O3 sample. An NRS 3300 laser Raman spectrometer was used to analyze the β-Ga2O3 sample at a laser excitation wavelength of 532.08 nm. Vertical Schottky junction devices were created by depositing Ni and In metal electrodes with the help of a metal mask. A metal mask with round-shaped holes (electrode area ∼0.003 cm2) was used to deposit the top Ni electrode, whereas the In electrode was deposited on the backside of the β-Ga2O3 substrate almost covering the entire surface. The metal electrodes were deposited using the ULVAC VPC-260F thermal evaporator under high vacuum. The current density–voltage (J–V) measurements were carried out using a two-probe system and a Keithley 2401 source meter at room temperature. The deep-UV illumination measurement was performed using a UV lamp of 254 nm wavelength and a light intensity of 614 μW/cm2.

Results and Discussion

Figure a shows the cross-sectional image of the hBN and β-Ga2O3 heterostructure with the Schottky junction. A barrier layer for the β-Ga2O3 Schottky junction can be formed by an hBN layer with a high band gap (6.0 eV) and a suitable dielectric constant. The XRD spectra of the free-standing β-Ga2O3 substrate are shown in Figure b. A strong diffraction peak at 61.01 cm–1 corresponding to the (020) reflection phase of β-Ga2O3 was observed for the sample. The XRD spectra are shown in the JCPD card number 06-0246.[41]Figure c shows a scanning electron microscopy (SEM) image for the transferred hBN layer on the β-Ga2O3 substrate.

Figure 2

(a) Cross-sectional schematic diagram of the hBN and β-Ga2O3 heterostructure. (b) XRD pattern of the free-standing β-Ga2O3 substrate. (c) SEM image of the hBN film on the β-Ga2O3 substrate. (d) Raman spectra of the β-Ga2O3 sample.

The transferred hBN film on the β-Ga2O3 substrate remains intact due to van der Waals interaction; however, SEM analysis revealed the formation of a wrinkle in the hBN thin film and surface impurities. Generally, such a type of wrinkle formation is observed for the large-area-transferred hBN film synthesized by a CVD process. The Raman spectra of the β-Ga2O3 sample are shown in Figure d, with a high-intensity Raman peak at 200 cm–1 for the Ag Raman mode, indicating the high-quality crystalline nature of the β-Ga2O3 sample used for device fabrication. The mid- and high-frequency Raman peaks are at around 354, 424, 486, 640, and 657 cm–1, corresponding to distortion of Ga2O6 octahedra and the stretching/bending of the GaO4 tetrahedra, respectively, as also discussed in previous reports.[41,42] These analyses show a successful transfer of the CVD-synthesized hBN film on the bulk β-Ga2O3 substrate. The hBN/β-Ga2O3 sample was further studied by X-ray photoelectron spectroscopy (XPS) analysis.

Figure shows the XPS analysis for the hBN/β-Ga2O3 sample, presenting the elemental composition. Figure a shows the B 1s XPS spectrum for the hBN/β-Ga2O3 heterostructure with a peak center at 192 eV. Similarly, Figure b shows the N 1s XPS spectra for the hBN film on a β-Ga2O3 substrate with a peak center at 400.1 eV. Again, Figure c shows the Ga 3d peak with a peak center at around 22.3 eV for the β-Ga2O3 sample. The XPS peak for the β-Ga2O3 was obtained considering the thin hBN film for the fabricated hBN/β-Ga2O3 heterostructure. Figure d shows the split Ga 2p peaks, where Ga 2p1/2 and Ga 2p3/2 peaks are obtained at 1119 and 1144.8 eV, respectively. The chemical structures of the hBN/β-Ga2O3 heterostructure were confirmed by XPS analysis, with the transferred CVD-synthesized hBN film remaining intact and stable on the β-Ga2O3 substrate surface. The current–voltage properties of the hBN/β-Ga2O3 heterostructure Schottky junction were studied.

Figure 3

XPS spectra for the fabricated hBN/β-Ga2O3 heterostructure sample. (a) B 1s spectra with a peak center at around 192.8 eV, (b) N 1s spectra with a peak center at 400.1 eV, (c) Ga 3d spectra with a peak center at 22.3 eV, and (d) Ga 2p3/2 and Ga 2p1/2 peaks with peak centers at 1119.4 and 1146.0 eV, respectively.

Figure 4

J–V characteristic of the Ni/β-Ga2O3/In Schottky junction: (a) J–V characteristic for the voltage range of 2 to −2 V (inset shows a schematic diagram of the fabricated device) and (b) log plot of the J–V curve. (c) J–V characteristics of the forward and reverse sweep for the voltage range of 2 to −2 V. (d) J–V curve under dark and deep-UV illumination (254 nm).

Figure a shows the J–V characteristics for the device with a voltage range of −2 to 2 V. The inset of Figure a shows the schematic diagram for the fabricated Ni/β-Ga2O3/In Schottky junction device. Figure b shows the logarithmic plot for the J–V characteristics. The J–V characteristics indicate that a suitable Schottky junction formation for the Ni/β-Ga2O3 contact has occurred. Figure c depicts the forward and reverse sweeps with a bias voltage ranging from −2 to 2 V. For the forward bias voltage, the measured J–V curve revealed no hysteresis effect. Figure d depicts the J–V characteristics with and without deep-UV light illumination (254 nm). A dark current of 0.00074 mA/cm2 was enhanced to 0.0115 mA/cm2 at a reverse bias voltage of 2 V, under the illumination of 254 nm wavelength and a power of 614 μW/cm2. The photoresponsivity of Ni/β-Ga2O3/was calculated. At a reverse bias voltage of 2 V, the current density in a Schottky junction device is 16.28 mA/W. At lower bias voltages, the photoresponsivity of the fabricated device is not considered high; however, the photoresponsivity increases with increasing reverse bias voltage.[47]

Figure shows the J–V characteristics of the fabricated device with and without UV light illumination under −8 to 8 V. The forward turn-on voltage for the Ni/β-Ga2O3/In Schottky junction was obtained to be around 3.8 V. Figure a,b shows the J–V characteristic in the dark and the corresponding logarithmic plot for bias voltages ranging from −8 to 8 V. The J–V characteristics of the Ni/β-Ga2O3 Schottky junction, Figure c, depict a schematic of the fabricated device with a device structure of Ni/β-Ga2O3In, the forward and reverse sweeps at bias voltages ranging from −8 to 8 V. For the forward bias voltage, the measured J–V curve revealed no hysteresis effect. Figure d depicts the J–V characteristics of an applied bias voltage ranging from −8 to 8 V under deep UV illumination (254 nm). Similarly, a dark current of 0.00208 mA/cm2 was enhanced to 0.0643 mA/cm2 at a reverse bias voltage of 7.2 V, under the illumination of 254 nm wavelength and a power density of 614 μW/cm2. The photoresponsivity is calculated to be 101.33 mA/W at 7.2 V reverse bias voltage for the constructed Ni/β-Ga2O3/In Schottky junction device.

Figure 5

J–V characteristic of Ni/β-Ga2O3 8 to −8 V: (a) J–V characteristics of forwarding bias voltage. (b) Log plot of the J–V curve. (c) J–V characteristics of the forward and reverse sweep of bias voltage. (d) Log plot of J–V curve dark and UV light at 254 nm.

Figure a shows a schematic diagram of the fabricated Ni/hBN/β-Ga2O3/In heterojunction Schottky junction device. The CVD-synthesized hBN film was transferred on β-Ga2O3 by wet-etching the Cu foil with a support layer of PMMA. The Ni top electrode on the hBN/β-Ga2O3 heterojunction was deposited by thermal evaporation to configure a Schottky junction device. Figure b depicts the J–V characteristics of the fabricated device with and without deep-UV light illumination for a bias voltage range of −2 to 2 V (wavelength 254 nm). In contrast to the device fabricated without the hBN layer, the forward current did not increase as the bias voltage was increased up to 2 V. The hBN film with an ultrawide band gap can act as a barrier layer at the interface of Ni and β-Ga2O3. However, at the reverse bias voltage, photoresponsivity was measured, indicating the heterojunction device’s sensitivity to deep-UV light.

Figure 6

(a) Schematic diagram of the fabricated Ni/hBN/β-Ga2O3/In Schottky junction. (b) J–V characteristics of the device for a voltage range of 2 to −2 V.

Figure a shows the J–V characteristics of the Ni/hBN/β-Ga2O3/In Schottky junction device for the voltage range of −8 to 8 V. The forward turn voltage for the Ni/hBN/β-Ga2O/In Schottky junction was obtained to be around 4.8 V, which is higher than that of the device with the hBN layer. It was also observed that the device’s forward current was significantly low up to an applied voltage of 2 V. Figure b shows the corresponding logarithmic plot for the corresponding J–V curve. Similarly, Figure c shows the J–V characteristics for the device with and without light illumination. Under the illumination of deep-UV light with a wavelength of 254 nm, a significant photoresponsivity can be observed in the reverse bias voltage. Figure d depicts the J–V curve in the reverse bias voltage range (0 to −8 V) used to measure photoresponsivity at a bias voltage. At 7.2 V bias, the photoresponsivity was 95.11 mA/W. The photoresponsivity is nearly identical to that of the device without the hBN layer; however, the forward turn-on voltage is higher, and the current density is lower. Turn-on voltage and forward current density differ significantly with and without the hBN layer on the β-Ga2O3, whereas photoresponsive reverse current density was nearly identical at an applied bias voltage of −7.2 V. Thus, a high photoresponsivity can be obtained in a Ni/hBN/β-Ga2O3/In-based heterojunction Schottky diode with high forward current turn-on voltage and low current density.

Figure 7

(a) J–V characteristics of the Ni/hBN/β-Ga2O3/In Schottky junction device for the voltage range of −8 to 8 V and (b) log plot of the J–V curve. (c) J–V characteristics with and without deep-UV illumination (254 nm) for bias voltage in the range of −8 to 8 V and (d) photoresponsivity at the reverse bias voltage (photoresponsivity of 95.11 mA/W at 7.2 V).

Figure a shows the energy band diagram for the Ni/β-Ga2O3 Schottky junction. The Ni metal contact with a higher work function (5.15 eV) forms a suitable Schottky potential with the n-type β-Ga2O3. A photoresponsivity was obtained at a reverse bias voltage for the illuminated deep-UV light considering the Schottky junction interface of Ni and Ga2O3. Similarly, Figure b shows the energy band diagram for the Ni/β-Ga2O3 Schottky junction, where the hBN layer acts as a barrier layer at the Ni and β-Ga2O3 interface.[48,49] The J–V curve shows that the diode turn-on voltage increased, but no reduction in reverse saturation current was observed. The photoresponsivity of the Ni/hBN/β-Ga2O3/Schottky junction to the illuminated deep-UV light of wavelength 254 nm was 95.11 mA/W. Thus, the Ni/hBN/β-Ga2O3/In Schottky junction demonstrated that a significant photoresponsivity can be obtained with a tunable turn-on voltage and forward current. Our study showed that the interfacial hBN layer can be used as an insulating barrier layer for the fabrication of ultrawide band gap Schottky junction devices.[49]

Figure 8

(a) Energy band diagram of the Ni/β-Ga2O3 Schottky junction vacuum level. (b) Band diagram of the Ni/hBN/β-Ga2O3/In Schottky junction vacuum level.

Conclusions

In conclusion, we have demonstrated the fabrication of a Ni/β-Ga2O3 Schottky junction device with an hBN interface layer. The hBN film was synthesized by CVD process and transferred on a free-standing β-Ga2O3 substrate by the wet chemical transfer method. XRD, Raman spectroscopy, and SEM analysis confirmed the hBN/β-Ga2O3 heterostructure of the Ga2O3 substrate and transferred hBN film. The J–V characteristics of a fabricated Schottky junction with and without the hBN interfacial layer were investigated. It was discovered that the hBN interfacial film significantly increases the turn-on voltage for the forward current of the Ni/hBN/Ga2O3/In Schottky junction. Furthermore, the created Ni/hBN/Ga2O3/In Schottky junction showed a photoresponsivity of 95.11 mA/W for the illuminated deep-UV light of wavelength 254 nm. The hBN layer acts as a suitable barrier layer as demonstrated, where the turn-on voltage, forward current density, and photoresponsivity can be optimized for the Ni/β-Ga2O3 Schottky junction.