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Abstract
In this report, we demonstrate high spectral responsivity (SR) solar blind deep ultraviolet (UV) β-Ga2O3 metal-semiconductor-metal (MSM) photodetectors grown by the mist chemical-vapor deposition (Mist-CVD) method. The β-Ga2O3 thin film was grown on c-plane sapphire substrates, and the fabricated MSM PDs with Al contacts in an interdigitated geometry were found to exhibit peak SR>150A/W for the incident light wavelength of 254 nm at a bias of 20 V. The devices exhibited very low dark current, about 14 pA at 20 V, and showed sharp transients with a photo-to-dark current ratio>105. The corresponding external quantum efficiency is over 7 × 104%. The excellent deep UV β-Ga2O3 photodetectors will enable significant advancements for the next-generation photodetection applications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Solar blind ultraviolet (UV) photodetectors have a vast and ever growing number of military and civil surveillance applications such as missile tracking, secure communication, fire detection, ozone holes monitoring, chemical/biological analysis, and corona detection, etc [1,2]. Principally, photodetectors work in three fundamental modes: photoconductive detectors, photodiode p-n junctions, and Schottky barrier detectors [3]. Large numbers of materials have been applied to the UV photodetectors. For example, the wide-band gap semiconductors, such as ZnMgO [4], GaN [5], and AlGaN [6], have been developed for UV photodetectors. However, the utilization of UV photodetectors based on these materials are limited by their rigorous growth conditions, low-crystalline quality, low responsivity (<1A/W) and low external quantum efficiency (EQE) (10%) [6]. Therefore, it is very important to find alternative materials which are very sensitive to UV light.
β-Ga2O3 is a wide band gap oxide semiconductor material, in the forbidden bandwidth between 4.5~4.9 eV, and has advantages of high electron mobility, low dielectric constant, stable physical chemistry and high mechanical strength [7,8]. β-Ga2O3 also offers economic advantages as conventional crystal growth techniques such as edge-defined film-fed growth (EFG), float-zone and Czochralski methods can be employed towards enabling scalable and large-area single crystal wafers. Thus, it is very attractive to apply β-Ga2O3 in solar blind UV photodetectors.
Beside the bulk material growth, thin film growth of β-Ga2O3 on foreign substrates has also been widely reported using different growth techniques including molecular beam epitaxy (MBE), metal organic chemical vapour deposition (MOCVD), mist CVD and radio frequency magnetron sputtering for different applications. Based on these film growth methods, deep UV β-Ga2O3 photodetectors with Schottky and MSM architectures on foreign substrates have been studied in the recent years. However, the growth of β-Ga2O3 thin films normally requires either a high substrate temperature or ultra-high vacuum, which increases fabrication cost. MOCVD epitaxial growth technique is mature and has been around since the early days of GaAs research, but the growth rates are as slow as MBE and the MOCVD equipment is also expensive. Sputter method can only obtain the polycrystalline films. Compared to above methods, nonvacuum solution-processed mist CVD offers a highly scalable, low-cost route for the mass production of electronic devices at significantly lower temperatures [9–13]. In this work, we demonstrate solar blind deep UV β-Ga2O3 metal-semiconductor-metal (MSM) photodetectors grown by the mist CVD method. These photodetectors exhibits a high spectral responsively (SR) and external quantum efficiency (EQE).
2. Method
2.1 Material growth
Gallium acetylacetonate, 99.99%, was acquired from Alfa. Hydrochloric acid, 30%, was purchased from Wako Pure Chemical Industries. C-plane sapphires were purchased from Optical & Fine Materials Supermarket. A 0.05-M solution was prepared from 100 ml distilled H2O, 1ml HCl and 1.85g Ga(Acac)3. The solution was atomized via ultrasonication at a frequency of 1.7 MHz. The ultrasonically generated mist particles were transferred via an air carrier gas, to a c-plane sapphire substrate heated to the temperatures of 400°C, 470°C, 550°C, or 600°C, on which the thin film was grown for 30mins. The film used in this work is 500 nm in thickness.
2.2 Device fabrication
MSM photodetectors were fabricated on the Mist-CVD grown Ga2O3 film. After following the standard lithography procedures, the Al (100nm) metal was deposited using sputter for contacts. The metal electrodes had an interdigitated geometry with 18 fingers: 80μm long, 3μm wide, and 3μm finger spacing with an active area of 5010μm2. Figure 1 shows the structure of the device.
2.3 Device characterization
The film properties of Ga2O3 films were investigated using a range of complementary techniques: X-ray diffraction (XRD) patterns were obtained from an X-ray diffractormeter (D8 Advance, Bruker, Germany); the surface morphology was obtained by an atomic force microscopy (AFM) (Agilent 5500); and the optical properties were obtained by a dual-beam 950 UV-Vis spectrometer. The I-V characteristics were measured by using a low pressure mercury lamp and a Keysight B1500 semiconductor parameter analyser. All the measurements were performed under ambient atmosphere at room temperature.
3. Results and discussion
Figure 2 shows the Ga2O3 film surface topographies measured by AFM using an Agilent 5500 system in tapping mode for the films growth at 400°C(a), 470°C(b), 550°C(c) and 600°C(d). The films have the Root-Mean-Square (RMS) surface roughness values of 12.45nm, 18.8nm, 4.14nm and 34.7nm, respectively. It is shown that the film grown at 550°C has a much smooth surface. To further investigate the crystalline structure of the grown Ga2O3 films, we performed XRD measurement on these films with a 500nm thickness. XRD data for the as-deposited Ga2O3 samples are shown in Fig. 2(e) over the diffracted angle range of 2θ = 15~65°. The XRD spectra have mainly four peaks at 19°, 38°, 40°, and 59°. The intensity of α-Ga2O3(0006)was appeared at 400°C and decreased as the growth temperature was rising, suggesting a slight inclusion of α-phase owing to their lower growth temperatures. This conclusion is in accordance with [14]. Finally, α-Ga2O3 (0006) diffraction peak was completely disappeared at 550°C. This implies that the phase of α-Ga2O3, which is more stable at low temperatures [15–17]. Finally, approximate single β-phase films were obtained at 550°C and 600°C.
Fig. 2 (a) AFM surface images of Ga2O3 films grown at different temperatures. (b)Those of XRD data for the Ga2O3 samples grown on Sapphire .
Optical absorption measurements (200–500nm) were performed on Ga2O3 films deposited on sapphire substrates using a dual-beam 950 UV-Vis spectrometer. As shown in Fig. 3, optical transmission across the visible part of the spectrum is very high with a mean value of 80%. The transmittance spectrum did not show marked change from that of the as-grown Ga2O3 for the growth temperature up to 550°C and the optical band gap energy remained at 4.7~4.8 eV, which is corresponding to β-Ga2O3. However, the spectrum drastically changed for the growth temperature higher than 600°C. The reduction of transmittance may be caused by rough surface.
To check the solar blind UV responsivity of the Ga2O3 photodetectors, we measured their current voltage (I-V) in the dark and under irradiations of 254nm and 365nm, respectively. The UV lights with power densities of 200μW/cm2 are vertically irradiated on the sample. And the effective irradiated area was ~5010μm2. Figure 4 shows the photo and dark current-voltage (I-V) characteristics of the device (linear scale (a) and log scale (b)) at room temperature. The photocurrent was measured at the illumination with light wavelength of 254 nm. High photo-current (1.5μA at 20 V), very low dark current (14pA at 20 V) were measured and the photo-to-dark current ratio > 105 were obtained. In summary, the photo detector current increases by more than five orders of magnitude with 254 nm (200μW/cm−2) UV light illumination. A low value of dark current is important for the practical photodetectors. The low dark current indicates a high sensitivity and low noise for device [19]. In contrast, the photocurrent is only 230pA under 365nm irradiation. The increase of the current under 365nm irradiation may be caused by the defects of Ga2O3 film. The photocurrent across the film increased under UV illumination compared to the dark current, which can be attributed to the fact that the light illumination can excite electron–hole pairs in the Ga2O3 film and resulted in an increase of the conductance.
Fig. 4 (a) The current voltage (I-V) characteristics of the Ga2O3 films in dark, 254nm and 365nm illumination,(b) and its log scale.(c) Time-dependent photoresponse of the detector obtain at 550°C under 254 nm illumination at 20 V (log scale), (d) and those of 400°C.
Spectral responsivity (R) and external quantum efficiency (EQE) are two critical parameters to evaluate the sensitivity for photodetectors. R is defined as the photocurrent generated per unit power of incident light on the effective area of a photodevice and EQE is related to the number of electron–hole pairs excited by a photodetector per adsorbed photon and per unit time. EQE expressed in the following equation:
where h is Planck’s constant, c is the velocity of light, and q is the electronic charge. From the UV photodetectors point of view, high internal quantum efficiency does not always warrant high external efficiency. In particular, it is well known that a large amount of light is usually trapped inside the luminescent layer that results in a low external efficiency. Hence, it is important to obtain high efficiency for application in short wavelength light sources and optoelectronic devices and is very important to improve the responsivity of Ga2O3 based UV photodetectors. For our device grown at 550°C, the photoresponsivity at a 20 V applied bias was 150 A/W and the external quantum efficiency reached 7.39 × 104% under 254 nm light illumination. The small dark current, excellent photoresponsivity, and high external quantum efficiency of the Ga2O3 based photodetectors validate its applicability.Figure 4(c) shows a representative time-dependent photoresponse of Ga2O3 photodetectors. At the bias of 20 V, the photocurrent increases instantaneously to approximately 1.5μA when illumination is on, and reduces to 30pA when illumination is off. For a more detailed comparison of the response time of the Ga2O3 film, the quantitative analysis of the process of the current rise and decay process involves the fitting of the photoresponse curve with a bi-exponential relaxation equation. The rise time tr and the decay time td were estimated to be 1.8s and 0.3s, respectively. The response (rising) edges and the recovery (fall) edges usually consist of two components, with a fast response component and a slow-response component [18, 19]. Generally, the fast-response component can be attributed to the rapid change of the carrier concentration as soon as the light is turned on or off, while the slow-response component is caused by the carrier trapping/releasing owing to the existence of oxygen vacancies defects in Ga2O3 thin films. As shown in Fig. 4(c), the ON/OFF switching behavior can be well retained even after scores of measurement cycles, which indicates the device has excellent stability and reversibility. Compared to the device grown at 550°C, the device grown at 400°C only exhibits a very low on/off ratio 102 as shown in Fig. 4(d) and it has a low photodetector performance.
As seen in Fig. 5(a), we compared photocurrents of devices with different growth temperatures under 254 nm light illumination. The device grown at 400°C shows photocurrent achieved to be 3.2μA. However, it has a high dark current 41nA at a bias voltage of 20V. The photo-to-dark current ratio is only 100, which is greatly lower than the device grown at 550°C (photo-to-dark current ratio is 105). In fact, all other devices show an inferior performance compared with the device grown at 550°C. When the growth temperature increases to 600°C, photocurrent decreases to 35nA under 254 nm light illumination at a bias voltage of 20V. Finally, we obtain the optimal device which is grown at 550°C. It can be seen from Fig. 5(b) that the current increases with the increases of bias voltage under different illumination conditions. The measured I–V characteristics curve shows that the photocurrent of the thin film is sensitive to 254 nm UV light. It is also shown that the light intensity can affect the device performance and this may be the nonideal Ohmic contact between the Al electrode and the Ga2O3 film. The device performance could be enhanced by improving the electrode contact, which will be our future work. In order to clarify the influence of the different growth temperature on the device performance, we add the support data to have a contrast.
Fig. 5 (a)Photocurrent at diffenent growth temperatures(400°C,470°C,550°C,600°C) under 254nm illumination. (b) I–V characteristics of the 550°C device under irradiation with 254 nm light at different light intensities.
4. Conclusion
In this paper, we have grown β-Ga2O3 thin film using the Mist-CVD method with different temperatures at atmospheric pressure. All the Ga2O3 films exhibit high uniformity and high optical transparency 80% due to the wide optical band gap (4.8eV). In conclusion, we demonstrate a large responsivity of 150 A/W at 20 V for deep UV detectors based on Mist-CVD grown Ga2O3 with low dark current <14pA (at 20 V), high photo to dark of ratio is 105 and high EQE of 73999.69%, fast response speed (about 1.3s) and excellent stability. All these outstanding characteristics indicate that the Mist-CVD grown Ga2O3 has great potential applications on solar blind UV photodetectors.
Disclosures
The authors declare no conflict of interest.
Funding
National 111 Center (Grant No. B12026); Fundamental Research Funds for the Central Universities (JBX171103); National Natural Science Foundation of China (61604119); China Postdoctoral Science Foundation (Grant No. 2016M602771).
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