With the rapidly increasing demand for solar-blind ultraviolet (UV) photodetectors (PDs) on various military and civilian applications, including flame sensors, missile interception, biological analysis, and UV radiation monitoring below the ozone hole, Ga2O3 with an ultrawide bandage of 4.9 eV is considered as a promising candidate for solar-blind detection , , , . Due to the unique structural and optical properties of nanowires, Ga2O3 nanowires can enhance the performance of solar-blind UV PDs , , , . The vertical nanowire array structures display more superior optical absorption ability, higher carrier generation, and higher recovery efficiency, resulting from high surface-to-volume ratio, surface carrier recombination, and effective optical coupling between the nanowire arrays and the incident light compared to thin films or flat disordered nanowires . At present, some methods [e.g. vapor-liquid-solid mechanism, pulsed laser deposition (PLD), thermal evaporation, molecular beam epitaxy (MBE), and metal-organic chemical vapor deposition] have been used to grow Ga2O3 nanowires , , , . Guo et al.  grew vermicular Ga2O3 nanowire thin film by PLD and fabricated the metal-semiconductor-metal UV PDs. The rise and decay times of PDs under 254 nm illumination were estimated to be 4.3 and 8.4 s, respectively. Wu et al.  reported the growth of β-Ga2O3 nanowires using a vapor-phase transport method on SiO2/Si template. The fabricated β-Ga2O3 nanowire solar-blind PDs exhibited a responsivity of 3.43×10−3 A/W and response time in the order of seconds. However, the structures of nanowires in current reports on PDs based on Ga2O3 nanowires were mostly unordered or inclined, which cannot make full use of the effect of nanowires to improve the performance of solar-blind UV PDs. The epitaxial growth of vertical Ga2O3 nanowire arrays is very difficult to achieve and the construction of PDs based on vertical Ga2O3 nanowire arrays via a simple and feasible method is still a great challenge.
As it is well known, GaN or GaAs can be oxidized into Ga2O3 at high temperature in O2, O3, N2O, or H2O vapor ambient , , , . Huang et al.  investigated the realization of β-Ga2O3 thin film by simple furnace oxidation of GaN thin film and the fabrication of a solar-blind β-Ga2O3 PD with an extremely large deep UV-to-visible rejection ratio. In addition to the epitaxy technologies, thermal oxidation is also a feasible method to realize Ga2O3 nanowires.
In this work, vertical Ga2O3 nanowire arrays were prepared by thermally oxidizing GaN nanowires grown by MBE on n-doped Si substrate and a monolayer graphene film was transferred to Ga2O3 nanowires to form the graphene/Ga2O3 heterojunction and transparent electrodes. Based on the graphene/vertical Ga2O3 nanowire array heterojunction, solar-blind UV PDs with high performance were proposed and demonstrated. Meanwhile, the results showed that the graphene/vertical Ga2O3 nanowire array heterojunction structure realized by thermal oxidation might provide a new development direction of UV PDs based on Ga2O3.
Vertical Ga2O3 nanowire arrays were obtained from GaN nanowires by thermal oxidization. First, vertical GaN nanowire arrays were grown along the (002) orientation on n-type Si substrate (resistivity 2–4 Ω cm) by MBE. Then, GaN nanowires were oxidized in a quartz tube purged with O2 gas at 1000°C for 10 min after removing the native oxide layer of GaN nanowires by diluted hydrochloric acid-water solution. The rear SiO2 layer on Si substrate formed during thermal oxidation was removed and Ti/Al (50/100 nm) was deposited by electron beam evaporation as the rear electrodes followed by rapid thermal annealing at 400°C for 30 min to form the ohmic contact. Then, a 200-nm-thick SiO2 dielectric layer was deposited by inductively coupled plasma chemical vapor deposition at 75°C and selectively etched by reactive ion etching to expose Ga2O3 nanowires of square regions. After deposition, a monolayer graphene film was transferred to Ga2O3 nanowires to form the graphene/vertical Ga2O3 nanowire array heterojunction and transparent electrodes by wet transfer . Finally, the proposed device was finished after the deposition of the front electrode (Ti/Au 50/200 nm). The specific fabrication process is shown in Figure 1.
The morphology and properties of the graphene were characterized by optical microscopy and micro-Raman spectroscopy (LABRAM HR, Horiba, Tokyo, Japan) with an Ar+ laser (excitation wavelength 532 nm). The morphology of the nanowires was observed by field-emission scanning electron microscopy (FESEM; Hitachi S-4800, Hitachi, Tokyo, Japan) and high-resolution transmission electron microscopy (HR-TEM; Tecnai G2 F20 S-TWIN, Thermo Fisher Scientific, Hillsboro, OR, USA). The elemental analysis of Ga2O3 nanowires on Si substrate was measured by energy-dispersive spectrometer (EDS; Quanta 400 FEG, Thermo Fisher Scientific, Hillsboro, OR, USA). The crystal structure was determined by X-ray diffraction (XRD; Bruker D8 Advance, Bruker, Karlsruhe, Germany). The current-voltage (I-V) and transient responses characteristics of PDs were measured by Agilent B1505A, Agilent, Santa Clara, CA, USA. The spectral response characteristics were measured with the photoelectric measurement system consisting mainly of mercury lamp, optical chopper, phase-locked, and power supply.
3 Results and discussion
Figure 2A depicts the Raman spectrum of the monolayer graphene film and the inset is the optical microscopy image of the graphene/SiO2/Ga2O3 nanowire structure. A strong G peak (1594 cm−1) and 2D peak (2694 cm−1) with the Raman intensity ratio of I2D/IG=2.3 are detected. The defect related D peak is barely visible, confirming the high crystal quality of the monolayer graphene film.
FESEM images of vertical GaN nanowire arrays on Si substrate before and after thermal oxidization are shown in Figure 2B and C, respectively. After thermal oxidization, the diameter of the oxidized nanowires increases by approximately 10 nm, but height has nearly no change. It should be noted that the oxidized nanowires are slightly curved, which could be attributed to GaN decomposition at high temperatures. The HR-TEM image of the partial enlargement of oxidized nanowires (Figure 2D) demonstrates that the oxidized nanowires are polycrystalline with good crystallinity and large grains. The spacing between two parallel fringes is approximately 0.368 nm corresponding to the interplanar distance of (201) lattice planes of monoclinic β-Ga2O3. To further determine whether the oxidized nanowires are Ga2O3 or not, the EDS spectrum (Figure 2E) is extracted from the oxidized sample. The Si signal from the substrate at 1.7 keV has been removed to make other signals more visible. Only Ga, O, and C signals are observed in the spectrum, where the small amount of C signal should be related to contamination. It implies that GaN nanowires have been totally oxidized into Ga2O3 nanowires. In the meantime, XRD patterns (Figure 2F) of vertical GaN nanowire arrays on Si (111) substrate before and after thermal oxidation also attest to this result. The GaN (002) peak disappears and a new Ga2O3 peak appears at 49.84° after thermal oxidation. This new diffraction peak is assigned as (402) planes of β-Ga2O3 with monoclinic structure (JCPDS Card No. 43-1012), which could be the preferred orientation in the growth process of Ga2O3 nanowires. As mentioned above, because grown Ga2O3 nanowires are polycrystalline with some defects and grain boundaries, the intensity of the Ga2O3 (402) peak is weak. In addition, Figure 2G shows the reflection spectra of vertical GaN nanowire arrays on Si substrate before and after thermal oxidization, in which the light reflectivity of Ga2O3 and GaN nanowires is observed to decrease when the wavelength is reduced to 281 and 366 nm, corresponding to their cutoff absorption edges, respectively , . The decrease in reflectivity of Ga2O3 nanowires at 281 nm is attributed to Ga3+ vacancies in the conduction band . To the best of our knowledge, it is the first time that vertical Ga2O3 nanowire arrays have been realized by thermal oxidation. However, lattice mismatch and built-in stress cause the appearance of defects and grain boundaries in Ga2O3 nanowires during thermal oxidation. The defects and grain boundaries in Ga2O3 nanowires can adversely affect the performance of PDs. When UV illumination is irradiated on Ga2O3 nanowires, the defects and grain boundaries can trap photogenerated carriers, which can reduce the photocurrent and responsivity of the device. In addition, the capture and release of photogenerated carriers caused by the defects and grain boundaries in Ga2O3 nanowires can reduce the response speed and increase the response time of PDs. The improvement of crystal quality of Ga2O3 nanowire is a matter of future investigations to enhance the performance of PDs.
To check the solar-blind UV photoresponse of PDs, I-V characteristics should be measured. As monolayer graphene allows 90 T% (transmission rate) of the incident light in the range of 200 to 1000 nm, UV light could easily penetrate and irradiate on the heterojunction, enhancing the light harvesting . Figure 3 exhibits the I-V curves of PDs under dark. It is obvious that nonlinear I-V characteristics can be used as a reflection of the rectifying characteristics ascribed to the graphene/vertical Ga2O3 nanowire array heterojunction. The rectifying ratio (I1 V/I−1 V) is over 102 with no illumination and the dark current is 1.14 μA at −1 V. Moreover, further analysis has been performed on PDs by fitting the equation, which is described as (1)(2)
where I0 is the saturation current, A* is the modified Richardson constant for semiconductor, S is the contact area, n is the diode ideality factor, ϕB is the barrier height, q is the fundamental electric charge, k is the Boltzmann constant, and T is the absolute temperature. The A* value used in this analysis is estimated to be 41 A cm−2·K−2 using the electron effective mass of 0.342 m0 at room temperature. Under forward bias, the above equation can be rearranged to extract the n and ϕB as shown in Figure 3 (inset). Accordingly, n and ϕB are estimated at values of 1.69 and 0.67 eV, respectively. These values are comparable to previously reported results for graphene/Ga2O3 heterojunction .
To quantitatively assess the device performance of PDs, the responsivity defined as the photocurrent generated per unit power of the incident light on the effective area of the PD is calculated . From the spectral response of PDs, (Figure 4), the PD shows a peak responsivity of 0.185 and 0.029 A/W at 258 nm wavelength at the bias of −5 and −3 V, respectively. The ratio of the photo-dark current is 57.2 and the rejection ratio (R258 nm/R365 nm) of the device is about 3×104 at −5 V, which illustrates the strong selectivity of PD to the solar-blind UV. Figure 5A and B displays the transient response of PDs based on the graphene/vertical Ga2O3 nanowire array heterojunction to 254 nm illumination at −5 V, from which both the rising time (τon, defined as the time during which the current rises to the waveform’s maximum height) and the decay time (τoff, defined as the time during which the current decreases to the base line) are extracted. As shown in Figure 5B, the device exhibits fast response times (9 and 8 ms for τon and τoff, respectively) superior to some reported values , , [25, 26, 27, 28]. The improvement of the response time may be attributed to two factors : (i) the property of Ga2O3 nanowires and (ii) the heterojunction structure between graphene and Ga2O3 nanowires. Compared to Ga2O3 thin films, vertical Ga2O3 nanowire arrays have higher surface-to-volume ratio and surface carrier recombination, which leads to much higher carrier generation and recovery efficiency. Besides, the heterojunction structure between graphene and Ga2O3 nanowires also contributes to the fast response time of PDs. Figure 6 illustrates an energy band diagram of the graphene/Ga2O3 nanowire array heterojunction. When UV illumination is irradiated on PDs, Ga2O3 nanowires absorb the photons and the electrons in the valence band are inspired to the conduction band, leading to the generation of electron-hole pairs. Then, photogenerated electron-hole pairs in the depletion region caused by the upwardly bended energy band are separated quickly. After turning off the UV illumination, the electron-hole pairs recombine very rapidly under the influence of the heterojunction structure.
In conclusion, a solar-blind UV PD based on the graphene/vertical Ga2O3 nanowire array heterojunction was demonstrated and vertical Ga2O3 nanowire arrays were realized by thermally oxidizing GaN nanowires grown by MBE on n-doped Si substrate for the first time. The fabricated device exhibited the large rejection ratio (R258 nm/R365 nm) of 3×104 and responsivity of 0.185 A/W at the bias of −5 V. Furthermore, thanks to the nanowire structure and heterojunction between graphene and Ga2O3 nanowires, the improved rise and decay times were 9 and 8 ms under 254 nm illumination, respectively. These results indicate that PDs based on the graphene/vertical Ga2O3 nanowire array heterojunction realized by thermal oxidation might open up new possibilities for future optoelectronic systems.
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