Gallium oxide nanowires for UV detection with enhanced growth and material properties

In the last decade, interest in the use of beta gallium oxide (β-Ga2O3) as a semiconductor for high power/high temperature devices and deep-UV sensors has grown. Ga2O3 has an enormous band gap of 4.8 eV, which makes it well suited for these applications. Compared to thin films, nanowires exhibit a higher surface-to-volume ratio, increasing their sensitivity for detection of chemical substances and light. In this work, we explore a simple and inexpensive method of growing high-density gallium oxide nanowires at high temperatures. Gallium oxide nanowire growth can be achieved by heating and oxidizing pure gallium at high temperatures (~ 1000 °C) in the presence of trace amounts of oxygen. This process can be optimized to large-scale production to grow high-quality, dense and long Ga2O3 nanowires. We show the results of morphological, structural, electrical and optical characterization of the β-Ga2O3 nanowires including the optical bandgap and photoconductance. The influence of density on these Ga2O3 nanowires and their properties will be examined in order to determine the optimum configuration for the detection of UV light.


Results and discussion
Scanning electron microscopy (SEM). To explore the growth mechanism of Ga 2 O 3 nanowires, Ga 2 O 3 was grown with and without a 5 nm Ag thin film as a catalyst at 1000 °C. Interestingly, the morphology of the nanowires grown on bare quartz was different from that of the nanowires coated with 5 nm Ag. The Ag catalyst is found to enhance nanowire growth rate as well as increase the density of nucleation sites. SEM images (Fig. 2) showed that the density and size of the nanowires were much larger on Ag-coated samples after growth. Additionally, a homogeneous coating of Ga 2 O 3 was observed under certain conditions and denser nanowires were grown at 1000 °C in the presence of 5 nm Ag compared to those grown without Ag. The Ag catalyst appears to play a role in reducing the diameters of the nanowires from 150-270 nm in the case of Ag-free growth to 120-160 nm when Ag was present. Even though denser, longer and thinner nanowires were observed by SEM imaging, no Ag NPs were seen at the tip or on the surface of Ga 2 O 3 nanowires. Consequently, other characterization techniques were performed to investigate the morphology and elemental composition of these nanowires. XRD characterization. XRD was performed with a Panalytical XPert PRO Diffractometer (Malvern Panalytical, Netherlands). Figure 3 shows the XRD patterns of the Ga 2 O 3 nanowire on quartz substrates oxidized at 1000 °C. The results are consistent with polycrystalline β-Ga 2 O 3 results in the Joint Committee on Powder Diffraction Standards (JCPDS) card # 11-370. All major peaks of the β-Ga 2 O 3 phase are seen to be present in the diffraction data, which strongly indicates the nanowires are β-Ga 2 O 3 . The apparent random orientation does not necessarily indicate the crystal growth direction is random, but more likely the nanowires are not self-aligned. Optical characterization. A UV-VIS spectrophotometer (Ocean Optics HR4000CG-UV-NIR high resolution spectrometer) was used to measure the optical properties of the nanowires, such as transmission, reflection and absorption. To determine the bandgap of these nanowires, a plot of (αhv) 2 versus photon energy was made. The bandgap was assessed by translating the transmittance curve and then correlating the bandgap ( E g ) with equation 9  www.nature.com/scientificreports/ where E g is the optical bandgap, A is a constant and hv is the incident photon energy; α is the absorption coefficient. Figure 4. shows a plot of (αhv) 2 versus photon energy. The bandgap of β-Ga 2 O 3 nanowires with and without silver was found to be 4.6 eV and 4.4 eV, respectively. Secondary Ion Mass Spectrometry (SIMS). A SIMS profile analysis (Fig. 5) was performed to provide a better understanding of the composition of nanowires surface. The beam was incident normal to the surface of Figure 4. Plot of (αhv) 2 versus photon energy. www.nature.com/scientificreports/ the sample. Due to the spot size limitation of the SIMS spectrometer, it was difficult to perform a single nanowire depth profile from tip to the base of nanowire, so the results are an aggregate of many randomly oriented and distributed nanowires. While the curve for Ga is relatively smooth, the curve of Ag has distinct peaks, demonstrating nonuniform spatial distribution of Ag within the nanowire forest. The ratio of Ag-to-Ga signal intensity versus depth has proportional increase towards the quartz surface. Figure 6 shows a STEM image of bright field (BF) and high-angle annular dark-field (HAADF) of a Ga 2 O 3 nanowire grown at 1000 °C on quartz ( Fig. 6a,b) and on quartz coated with 5 nm Ag (Fig. 6c,d). The β-Ga 2 O 3 nanowire, which was not exposed to Ag catalyst, has no particles on its surface. However, the β-Ga 2 O 3 nanowire shows several NPs with diameters between 5-10 nm that decorate the surface. Due to the transfer technique, it is not possible to determine the root or the tip of the nanowire. Han  . Bright field showed dark NPs due to diffraction contrast, since the nanoparticles are crystalline. Also, HAADF was performed to capture the z-contrast of Ag NPs. The NPs appear much brighter despite their small size, which suggests that their atomic mass and density are significantly larger than the surrounding β-Ga 2 O 3 , which points to Ag.

High Resolution Transmission Electron Microscopy (HRTEM).
To explore the presence of Ag on the surface of the grown Ga 2 O 3 nanowires, a JEOL 2100F transmission electron microscope (TEM) was used to image Ga 2 O 3 nanowires grown on a fused quartz substrate, without and with the presence of Ag as shown in Fig. 7. Ga 2 O 3 nanowires without Ag catalyst show a smooth surface in HRTEM. However, Ga 2 O 3 nanowire in the presence of Ag shows nanoparticles decorating the surface, with diameters between 2-5 nm.
The JEOL 2100F TEM was also equipped with an energy-dispersive spectroscopy (EDS) profile system, which was used to analyze the composition of Ga 2 O 3 nanowires grown on quartz (Fig. 8). It was further used to search for silver, which was detected but not seen in the SEM observation. The TEM images correspond to the elemental mapping for Ga, O and Ag. Although no Ag nanoparticles were visibly detected on the surface of the nanowire, the particles may be too small to produce a signal detectable to the EDS detector in the TEM due to the large interaction volume required by EDS for electron capture and x-ray emission. www.nature.com/scientificreports/ Selective area diffraction pattern. Selective area diffraction patterns were also taken to better understand the crystal structure of the nanowires obtained with or without Ag catalyst. The low-symmetry of the monoclinic crystal structure of β-Ga 2 O 3 makes interpretation of diffraction results difficult in general, so the analysis of the patterns was guided using the SingleCrystal TEM diffraction simulator from CrystalMaker. To identify the zone axis, the spacings between points and the angles were measured, and compared to different zone axes, as shown in Fig. 9: From the XRD results in Fig. 3, it is known in advance that the crystal structure of the nanowires is β-Ga 2 O 3 , which serves as a starting point for interpretation. It can be seen that the diffraction spots in Fig. 9 are distinct   Fig. 3, which were not due to a difference in the orientation of the nanowires, but some other factor instead. Both samples growth with Ag and without Ag showed the same orientation, indicating that the growth acceleration due to Ag catalyst does not impact the orientation of the resulting nanowires. Additionally, there   The current-voltage (I-V) characteristics were measured in dark conditions and under UV illumination for a MSM structure with and without an Ag catalyst at 10 V (Fig. 12). The MSM back-to-back Schottky diodes under UV illumination with a light intensity of 15 W/cm 2 showed results comparable to the dark current. Due to the surface plasmonic effect of Ag NPs at the surface of Ga 2 O 3 NWs, the absorption of photon is highly enhanced 14 , leading to more photoexcited carriers transport and collection at the contacts (Fig. 12c,d). Additionally, the results showed that Ga 2 O 3 on quartz with a 5 nm Ag catalyst improves the photocurrent response to the broad spectrum UV source. The results reveal that the steady photocurrent of Ag/β-Ga 2 O 3 (Fig. 12c) was three order of magnitude higher than that of the photocurrent of Ag-free/β-Ga 2 O 3 (Fig. 12a). Hence, silver as a catalyst plays a critical role in improving the electrical properties of the β-Ga 2 O 3 nanowires. The photodetection mechanism of the β-Ga 2 O 3 nanowire is credited to different aspects that mainly include the incident light absorption, carrier photogeneration and carrier transport and surface oxygen adsorption and desorption process 15 .
The metal contacts deposition method has a large impact on the dark current and photocurrent. Even though Ag/Ga 2 O 3 was sputtered with gold contacts shows high current response under illumination, it showed a large increase in the dark current as well (Fig. 12a,c). The major disadvantage of sputtering gold contacts is the induction of a damage that effects Fermi level pinning and hence the electrical measurements. This is due to the interfacial disorder of oxygen or Ag atoms vacancies 16 . Therefore, Au mesh contacts were used to avoid the consequence of sputtering technique as shown Fig. 11b. With using this technique, the results have been improved (Fig. 12b,d). The presence of Ag NPs at the surface was found to increase both photocurrent and dark current for sputtered contacts, however for mesh contacts it produced a substantial decrease in dark current. This could be due to an interaction between the Ag and deposited Au which produced a large leakage, but would not occur for mechanically applied mesh contacts.
The presence of Ag NPs at the surface plays a critical role in reducing the dark current of Ga 2 O 3 on quartz compared to the one without Ag catalyst when mesh contacts are used, as shown in the Fig. 12. The results indicated that a covering of Ag nanoparticles on the Ga 2 O 3 surface of devices can both decrease the dark current, and increase the photocurrent, both of which are beneficial. In addition, larger photocurrent can be observed for Ga 2 O 3 on quartz decorated with Ag NPs. For dark current, a Schottky barrier formed at the interface between Ag and Ga 2 O 3 due to the difference of the work function, where Ag NPs and Ga 2 O 3 have the equal Fermi energy level. However, the positively charged Ag NPs depleted the carriers near the nanowires surface. This may deplete carriers in the nanowires in the absence of light, leading to higher dark resistivity in the material and lower dark current.
The I-V measurement of the photocurrent for the photodetector was measured at different selected wavelengths such as 250, 260 and 280 nm with 1mW LED lamps to evaluate the device performance for the detection of deep UV light with high input power (Fig. 13a). interestingly, the photodetection response with Ag catalysts was improved by almost two order of magnitude. The ratio of photocurrent to dark current was measured for the Ga 2 O 3 on quartz without and with 5 nm Ag catalyst (Fig. 13b) at 10 V for Ga 2 O 3 with Ag catalyst were almost 38.3 compared to the Ag-free Ga 2 O 3 which was 4.55. Table 2 shows the performance of the developed β-Ga 2 O 3 device compared to others. www.nature.com/scientificreports/ Demonstration of UV photodetector responsivity based on a Ga 2 O 3 nanowire without and with Ag catalyst was examined (Fig. 14). The responsivity of Ga 2 O 3 on quartz with Ag catalyst was higher than the one without catalyst approximately by 1.5 order of magnitude. In addition, responsivity curve measurements were taken using a XeHg lamp coupled to a monochromator to obtain a responsivity curve, as well as the responsivity at solar wavelengths. This data is shown in Fig. 14, which shows a larger response over a broad range of wavelengths. The solar-range response (> 300 nm) is significant in both cases, however the relative responsivity in the Ag sample is about 3-5 times larger below the peak at 260 nm. To qualify a device's solar blindness, the solar-UV rejection ratio is typically used 19 , which in this case we define as the response at 320 nm, which is near the edge of the solar band, versus the response at 260 nm. For the Ag sample, the solar-UV rejection ratio is about 50, for the sample without Ag the rejection ratio is closer to 100.   [20][21][22] . Hence, highly energetic hot electrons of Ag NPs will be excited from the 4d and 5sp bands. This property could enhance the performance of Ga 2 O 3 in ultra-violet detection due to the presence of Ag NPs, which would lead to the observed shallower tail in the responsivity curve.
Transient photocurrent. The transient response of the photodetector was measured by opening and closing a shutter on a broad spectrum UV light source (Fig. 15). Ga 2 O 3 on quartz with an Ag catalyst showed a fasttransient response due to enhanced carrier transport. When the UV illumination was turned on, there was a large increase in the photocurrent. Due to the surface plasmonic effect of Ag NPs at the surface of Ga 2 O 3 NWs, the absorption of photon is highly enhanced 14 , leading to more photoexcited carriers transport and collection at the contacts. On the other hand, when the UV light is turned off, the free electrons will recombine with holes very rapidly.
In Ga 2 O 3 without an Ag catalyst, by turning the light on and off, a relatively slow response occurred in the device turning on. This slow response was attributed to the traps and surface states of oxygen generated at the surface of gallium oxide 23 . A longer tail in the photocurrent occurred when the light was turned off due to the reduction of charge carrier recombination as a result of captured hole-trap states. Hence, a longer recovery time is required because of the diffusion of oxygen molecules. The average rise time (from 10 to 90% of maximum photocurrent) and fall time (from 90 to 10% of maximum photocurrent) are 1.12 s and 2.0 s for Ag-free Ga 2 O 3 and 0.33 s and 2.2 s for Ga 2 O 3 in the presence of Ag. Hence, Ag NPs presence enhances faster rise time, but similar fall time. In Ga 2 O 3 with an Ag catalyst, silver nanoparticles enhance the adsorption of light in the sample, and thus and desorption of oxygen molecules. When Ag NPs were excited by the UV light, each nanoparticle will generate a light-induced dipole 24 . Hence, the dipole-diploe interaction of Ag nanoparticles influences each nanoparticle and those nearby it.

Silver (Ag) nanoparticles (NPs) catalyzes the growth of Ga 2 O 3 nanowires. Based on the detailed
characterization performed on the oxidation of liquid Gallium (Ga) in a quartz crucible, the presence or absence of an Ag thin film on quartz substrate could help explain the growth mechanism. The growth mechanism is summarized in Fig. 16. Ga is oxidized and forms a solid phase of gallium (III) oxide (Ga 2 O 3 ) (Eq. 1) 25,26 . Then, this Ga 2 O 3 is reduced by liquid metallic gallium and forms a gas phase of gallium suboxide (Ga 2 O) (Eq. 2) 27 .  , leading to a vapor-liquid-solid (VLS) growth mechanism. In the presence of Ag catalyst, more solid phase of Ga 2 O 3 is formed.
The growth kinetics and the mechanism of incorporation of Ag catalyst to enhance denser and longer β-Ga 2 O 3 nanowires can be explained using several factors that boost the presence of oxygen in the system. The dewetting  Photo-sensitivity of Ag NPs at the interface of Ga 2 O 3 nanowires. The experimental results of Ga 2 O 3 on quartz in the presence of Ag nanoparticles are explicitly explained using schematics of the interface of the materials in the dark and under UV illumination. To illustrate the photodetection mechanism, the energy band diagrams of a β-Ga 2 O 3 photodetector in the presence of Ag nanoparticles are plotted in Fig. 17.
In β-Ga 2 O 3 nanowires coated with Ag NPs, the work function (φ Ga2O3 ) and electron affinity (χ Ga2O3 ) of β-Ga 2 O 3 are 4.11 ± 0.05 eV and 4.00 ± 0.05 eV 34 , respectively, which is lower than the work function of Ag (φ Ag = 4.26 eV) 35 . This causes a Schottky barrier (φ B = φ Ag − χ Ga2O3 = 0.26 eV) to form. The Schottky barrier prevents the movement of electrons from Ag to Ga 2 O 3 . Consequently, a localized Schottky junction will form a charge depletion region at the interface of Ag NPs and β-Ga 2 O 3 nanowires. In dark condition, Ag NPs deplete the carriers at the interface. As the width of the depletion layer increases, the depletion region close to Ag NPs increases. Therefore, there is a large depletion width at the interface between Ag NPs and β-Ga 2 O 3 nanowires. The dark current of the photodetector in β-Ga 2 O 3 in the presence of Ag NPs thus decreases (Fig. 17a).
Under UV illumination, when the photon energy is larger than the bandgap of Ga 2 O 3 , electron-hole pairs are generated [hv → e -+ h + ]. Hence electrons move from the valence band to conduction band, leaving behind a hole.  The remaining electrons become the majority carriers that contribute to an increase in the photocurrent by generation and recombination until reaching an equilibrium level. If the incident UV light on β-Ga 2 O 3 in the presence of Ag NPs is below 320 nm, the effect of interband transitions on Ag NPs at the interface of β-Ga 2 O 3 nanowires allows the transition of highly energetic hot electrons of Ag NPs which excite from the 4d and 5sp bands [20][21][22] . These hot electrons surmount the small Schottky barrier height and form local band bending downward on Ga 2 O 3 side to enable the electron transfer from Ag NPs to the conduction band of Ga 2 O 3 nanowires. Consequently, there is a higher electron density in Ga 2 O 3 nanowires with Ag NPs compared to Ag-free Ga 2 O 3 nanowires, leading to a higher photocurrent and hence a more photo-sensitive photodetector (Fig. 17b).
In addition, the efficiency of the device is highly influenced by the presence of oxygen generated at the surface of Ga 2 O 3 nanowires, causing trap states 23 . These states at the surface of Ga 2 O 3 nanowires contains highly dense dangling bonds. Therefore, the surface of nanowires has higher sensitivity and better detection mechanism due to the large surface to volume ratio of nanowires and Ag NPs presence. It has been found that the photo-to-dark current ratio of nanowires (2.33 × 10 -7 A) was higher than the that of thin film (9.16 × 10 -8 A) 37 .

Conclusion
In brief, a β-Ga 2 O 3 nanowires for UV photodetector with and without Ag catalyst has been proposed and demonstrated. Silver plays a critical role to enhance the growth mechanism of Ga 2 O 3 nanowires due to its higher O 2 solubility and diffusivity. The growth mechanism and characterization of Ga 2 O 3 nanowires without and with Ag catalyst has been explained. It was found by SAED that the orientation of the nanowires is not affected by the use of Ag catalyst during growth. The ratio of photo-to-dark current without and with Ag catalyst of 4.55 and 38.3, respectively, was achieved, leading to more sensitive detection of UV light. The responsivity curve shows that Ga 2 O 3 nanowires in the presence of Ag has a cut-off at 320 nm due to the existence of mid-gap state. Hence, the simplicity of this fabrication method suggests a promising device for sensing UV light, particularly for mass production. Our results could offer a promising technique to grow nanowires with high sensitivity and spectral selectivity to UV light.