Dual-band photodetector with a hybrid Au-nanoparticles/β-Ga₂O₃ structure

Abstract

Hybrid Au-nanoparticles (NPs)/β-Ga₂O₃ thin films were fabricated by laser molecular beam epitaxy of high quality β-Ga₂O₃ thin films on c-plane sapphire substrates followed by spin coating and post annealing of Au NPs. Photodetectors (PDs) made of hybrid Au-NPs/β-Ga₂O₃ thin films and bare β-Ga₂O₃ thin films are comparatively studied. It is found that Au-NPs can obviously improve the performances of PDs. Lower dark current, higher photoresponse and faster switching time under 254 nm light illumination are obtained by introducing Au-NPs. The localized surface plasmonic resonance from Au-NPs could be attributed to the enhanced performance. Meanwhile, a photoelectric response to 532 nm light illumination is observed in an Au NPs/β-Ga₂O₃ based PD. It is suggested that our hybrid Au-NPs/β-Ga₂O₃ PD could serve as a dual-band detector in one device.

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1. Introduction

In recent years, β-Ga₂O₃, with a wide direct band gap (∼4.9 eV) and high thermal stability, has been considered as an attractive and promising material for solar-blind deep UV detectors,^1–3 gas sensor materials,⁴ magnetic semiconductors⁵ and power devices.⁶ Among these applications, UV detectors have a significantly huge market in civil and military areas. So far, a lot of encouraging advances on β-Ga₂O₃-based detectors have been reported in terms of their low dark current, high responsivity and high sensitivity in a variety of detector structures such as films,^3,7,8 single crystals⁹ and nanostructures.^10–13 In spite of these advantages, the performance of β-Ga₂O₃-based UV detectors is still lower than expected. Surface plasmons (SPs)^13,14 can decrease dark current and improve photocurrent. Fortunately, SPs can form a dual-band (visible and UV) response with Ga₂O₃ films by introduction of Au-NPs due to the absorption wavelength of Au NPs being around 532 nm. The advantage of dual-band PDs is that a PD can integrate two PDs, which significantly reduces size and cost.

SPs, excited by the interaction between light and electron plasma waves at the metal surface, have been widely used to improve the efficiency of photovoltaics,¹⁵ photocatalysis,¹⁶ PDs and solar cells.^17–20 Because of the localized field amplification that occurs, in metal NPs on the surface of the semiconductor, excitation of surface plasmons can enhance scattering and optical absorption of incident photons within the semiconductor region near each NP. Among the metals, the SPs of Au-NPs has been actively used to enhance PDs performance owing to chemical stability,^20–24 and which has been continued interest in the plasmonic properties and sensing capabilities over the past 30 years.^25–27 However, the dual-band response of PD has been far less experimental work. Besides, Au NPs unpin Fermi level at the metal–semiconductor interface suppress photogenerated carriers are captured from Ga₂O₃ thin film.^28–30 Hence, it is worthwhile to explore the effect of Au NPs on the performance of PD. In this study, Au NPs/β-Ga₂O₃ hybrid PD that can be operated at 254 nm and 532 nm is fabricated. Meanwhile, the physical, electrical, and optical properties of the Au NPs/β-Ga₂O₃ hybrid PD are discussed.

2. Experimental details

The bare β-Ga₂O₃ epilayer films were grown on (0001) sapphire substrates by laser molecular beam epitaxy (L-MBE). A high-purity (99.995%) Ga₂O₃ ceramic was used as the target of L-MBE. Before depositing, the chamber was pumped to a vacuum degree of around 1 × 10⁻⁶ Pa. The laser ablation was carried out at a laser fluence of 5 J cm⁻² with a repetition rate of 1 Hz using a KrF excimer laser with a wavelength of 248 nm. The distance between target and substrate was 5 cm, and the substrate was rotated during deposition to improve the film uniformity. The sample was deposited upon 1 × 10⁻³ Pa of oxygen pressure for 10 000 pulse at 750 °C. The thicknesses of the films were estimated by scanning electron microscope (SEM) to be ∼400 nm. After the growth, Au colloids with about 30 nm size were deposited by spin coating method onto the Ga₂O₃ thin film, which was rotated at 450 rpm for 6 s following 4000 rpm 1 min by SIYOUYEN KW-4A/5 spin coater. After depositing by spin coating, the film was subsequently inserted into a tube furnace and annealed in atmosphere at 300 °C for 10 min in a furnace to evaporate the solvent and remove organic residuals. The surface morphologies were characterized by a Hitachi S-4800 SEM. The crystal structures were measured by a Bruker D8 Advance X-ray diffractometer (XRD). To construct a metal–semiconductor–metal photodetector, radio frequency magnetron sputtering technique was used to deposit four pairs of Au/Ti interdigital electrode on the β-Ga₂O₃ thin films using a shadow mask. The current–voltage (I–V) and time-dependent photoresponse of the bare β-Ga₂O₃ PD and Au NPs/β-Ga₂O₃ PD were measured by Keithely 2450. A 6 W lamp was employed to provide the 254 nm and 365 nm UV for the photocurrent measurement.

3. Results and discussion

Fig. 1(a–c) show the SEM images of the bare β-Ga₂O₃ thin film, 40% Au NPs/β-Ga₂O₃ thin film and 100% Au NPs/β-Ga₂O₃ thin film (40% Au NPs means Au colloids diluted to 40%, and 100% Au NPs means Au colloids without diluting). The bare β-Ga₂O₃ thin film shows a dense and smooth surface morphology. The size of the uniform dispersal Au NPs is 30–40 nm. And Au NPs increase on the β-Ga₂O₃ thin film with increasing Au colloids concentration. The crystal structure of the Ga₂O₃ thin films were investigated by XRD patterns. Fig. 1(d) shows the XRD patterns of the bare Ga₂O₃ thin film, 40% and 100% Au NPs/β-Ga₂O₃ thin films. The sharp peaks of the three samples correspond to (2̄01), (4̄02) and (6̄03), which can be indexed as β-Ga₂O₃ (JCPDS file 43-1012). Except the diffraction peak of the substrate, no other peaks are found, indicating a single (2̄01) plane orientation growth. Moreover, the intensity of the β-Ga₂O₃ peaks is nearly the same, indicating that the Au NPs on the surface of β-Ga₂O₃ thin film have no effect on the crystal quality of the films. Fig. 1(e) shows the UV-vis absorbance spectra of bare β-Ga₂O₃ thin film, 40% and 100% Au NPs/β-Ga₂O₃ thin films on sapphire substrates. It is observed that a significant absorption of the bare β-Ga₂O₃ thin film only exhibit a significant absorption edge at wavelengths about 250 nm. After introducing Au-NPs, the samples consist of two peaks obtained around 250 nm and 560 nm. Due to the characteristic surface plasmons band of Au NPs the second peak is observed at λ = 560 nm. We can also observe that the absorption peaks intensity increases with the increasing of Au NPs.

Fig. 1 (a–c) are the SEM images of bare β-Ga₂O₃ thin film, 40% and 100% Au NPs/β-Ga₂O₃ thin film; (d) the XRD patterns of the pristine Ga₂O₃ film and other films with Au NPs for 30–40 nm; (e) UV/vis absorbance spectra of bare β-Ga₂O₃ thin film, 40% and 100% Au NPs/β-Ga₂O₃ thin film.

The schematic diagram of the fabricated prototype PD device is shown in Fig. 2(a). The I–V characteristic curves of the bare β-Ga₂O₃, 40% and 100% Au NPs/β-Ga₂O₃ PDs under different illumination are shown in Fig. 2(b), (c) and (d), respectively. The linear I–V characteristics indicate that ohmic contact has been obtained. Among three PDs, the dark current of 100% Au NPs PD with 18.2 nA is smallest than that of 40% Au NPs/β-Ga₂O₃ PD (21.0 nA) and bare β-Ga₂O₃ PD (31.6 nA) at 1 V. And the photocurrent with 259.5 nA under 254 nm light illumination is largest than that of the 40% Au NPs/β-Ga₂O₃ PD (178.1 nA) PDs and bare β-Ga₂O₃ PD (126.0 nA) at 1 V. Meanwhile, we investigate the effect of 532 nm. It is obvious that the current of bare β-Ga₂O₃ PD increases as the applied bias increases both 254 nm and under 254 nm + 532 nm light illumination conditions, because absorption edge of β-Ga₂O₃ thin film is about 254 nm. The I–V curve with 37.2 nA measured under 532 nm illumination shows little increase, which may arise from the defects in β-Ga₂O₃ thin film. However, the I–V curves of 40% and 100% Au NPs/β-Ga₂O₃ PDs (73.2 nA and 49.3 nA) measured show obvious increase with increasing Au NPs under 532 nm light illumination, because absorption edge of Au NPs is located at around 532 nm. Furthermore, the I–V curves of 100% and 40% Au NPs/β-Ga₂O₃ PDs (317.5 nA and 212.9 nA) have significantly increased under 254 nm + 532 nm light illumination compared with 254 nm light illumination, due to the existence of SPs by Au NPs on the surface of β-Ga₂O₃ thin film.

Fig. 2 (a) The schematic illustration of the Au NPs/β-Ga₂O₃ PDs; (b), (c) and (d) are the I–V cures of bare β-Ga₂O₃ PD, 40% and 100% Au/β-Ga₂O₃ PDs upon different light illumination, respectively.

To investigate the photo dual-band modulation, the time dependence of bare β-Ga₂O₃, 40% and 100% Au/β-Ga₂O₃ PDs in dark and under 254 nm, 532 nm and 254 + 532 nm light illumination (120 μW cm⁻²) at 1 V, as shown in Fig. 3(a–c). It can be clearly seen that the photocurrent of bare β-Ga₂O₃ PD responds sensitively only to 254 nm light illumination. Nevertheless, the photocurrents of 40% and 100% Au NPs/β-Ga₂O₃ PDs respond more and more sensitively to 254 nm and 532 nm light illumination with increasing Au NPs, respectively. In addition, they are enhanced under 254 nm + 532 nm light illumination. In order to further investigate the performance of these PDs, the transient response with a 1 V applied bias and at 254 nm light illumination measured as the UV excitation is switched on and off, as shown in Fig. 3(d). Three PDs exhibit excellent on–off switching performance. The currents of three PDs mostly exhibit two distinct states with the light irradiation on and off, a low-current state in dark and a high-current state under 254 nm light illumination. The current increases very sharply from one state to another state, indicating an obvious response speed of the two PDs. The results of the time-resolved photocurrent reveal that the response speeds of the PDs increase with increasing Au NPs. For further detailed comparison of the response time of three PDs, the quantitative analysis of the current rise and decay process involve the fitting of the photoresponse curve with a bi-exponential relaxation equation of the following type:³¹

where I₀ is the steady state photocurrent, A and B are scaling constant, t is the time when the light was turned on or off, τ₁ and τ₂ are two relaxation time constants. As shown in Fig. 3(e) (top), (middle) and (bottom), the photoresponse processes are well fitted. τ_r and τ_d are the time constants for the rise edge and decay edges, respectively. Here for our bare β-Ga₂O₃ PD [Fig. 3(e) (top)], the estimated time constants for the rise process are: τ_r1 = 1.3 s, τ_r2 = 29.7 s. In contrast, that of 40% and 100% Au NPs/β-Ga₂O₃ PDs are: τ_r1 = 1.2 s, τ_r2 = 16.5 s, and τ_r1 = 0.8 s, τ_r2 = 9.0 s. The decay time constant τ_d1/τ_d2 are estimated to be 1.2 s/19.0 s, 1.0 s/17.0 s and 0.7 s/15.0 s for bare β-Ga₂O₃ PD, 40% and 100% Au NPs/β-Ga₂O₃ PD, respectively. τ₁ (in both rise and decay processes) should be related to the carrier generation and recombination processes in the bare β-Ga₂O₃ thin film, τ₂ (in both the rise and decay processes) should be mainly related to hole trapping and release processes in the depletion layer on the surface of the bare β-Ga₂O₃ film.

Fig. 3 (a–c) are temporal responses of the three PDs measured by shutting on/off the 254 nm, 532 nm and 254 + 532 nm lights illumination (120 μW cm⁻²) under a 1 V applied bias. (d) and (e) are time-resolved photoresponse of the three PDs, respectively. The 254 nm light illumination (120 μW cm⁻²) were turned on/off manually to generate pulsed light.

Fig. 4(a) show the values of dark current, photocurrent and photoconductive gain obtained for three PDs at 1 V. The dark current for a bare β-Ga₂O₃ based UV PD is about 31.6 nA (Table 1). The dark current for the 40% and 100% Au NPs/β-Ga₂O₃ PDs are about 21.0 and 18.2 nA. It is found that the value of dark currents of PDs obviously decreases with the increasing of Au NPs [Fig. 4(a)], due to the formation of a Schottky junction at the interface of the Au NPs with the semiconducting β-Ga₂O₃ thin film. Values of the rise time and fall time obtained for three PDs are plotted in Fig. 4(b). Both the rise time and fall time for PDs are reduced with increasing Au NPs.

Fig. 4 (a) Variations of dark current, photocurrent and photoresponse (K) of the fabricated UV PDs. (b) Variation of rise time and fall time of the fabricated UV PDs.

Table 1 UV photoresponse parameters obtained for all prepared PDs: (a) dark current (I_off), (b) photocurrent (254 nm + 532 nm) (I_on), (c) photoresponse (K), (d) rise time (T_r) and (e) fall time (T_f)

Samples prepared	Dark current (nA)	Photocurrent (nA)	Photoconductor gain Ion/Ioff	Rise time Tr (s)	Fall time Tf (s)
Bare Ga2O3	31.6	126.6	4.0	1.3	1.2
40% Au/Ga2O3	21.0	212.9	10.1	1.2	1.0
100% Au/Ga2O3	18.2	317.5	17.4	0.8	0.7

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The photoconductive gains for PDs increase with increasing Au NPs (Table 1). The photoconductive gain (K) of a UV PD is determined as K = I_on/I_off, where I_on is the photocurrent is measured under UV radiation and I_off is the dark current measured in dark. The enhanced photoconductive gain obtained for the Au NPs/β-Ga₂O₃ PDs are attributed to two main factors. One is related to the obvious reduction in the value of low dark current due to formation of the Schottky barrier at the interface of β-Ga₂O₃ thin film and Au NPs. The other factor is SPs caused by Au NPs on the surface of β-Ga₂O₃ thin film, as discussed earlier.

Fig. 5(a–c) show the photocurrent response of the bare β-Ga₂O₃, 40% and 100% Au NPs/β-Ga₂O₃ PDs at the 1 V under 254 nm light illumination. The sequence of the test is with power densities range from 100 μW cm⁻² → 50 μW cm⁻² → 10 μW cm⁻² → 50 μW cm⁻² → 100 μW cm⁻². The consequence demonstrates that the photocurrent increases steadily as the irradiance intensity increases, reaching 80 nA, 83 nA and 93 nA at 10 μW cm⁻², respectively. And it can be seen that the photocurrent response detectors are extremely consistent and repeatable.

Fig. 5 (a–c) are the current versus time under various 254 nm light illumination intensities of the bare β-Ga₂O₃, 40% and 100% Au NPs/β-Ga₂O₃ PDs at 1 V.

In order to illustrate the aforementioned mechanism, the energy band diagrams of β-Ga₂O₃ PDs without and with Au are plotted as shown in Fig. 6. Oxygen molecules are absorbed on the surface n-type β-Ga₂O₃ and decrease the carrier density in the β-Ga₂O₃ film by trapping free electrons [O₂(g) + e⁻ → O₂⁻(ad)] from the n-type semiconductor.²⁸ It created a depletion layer with low conductivity near the surface. When covered with Au NPs, which means a localized Schottky junction that will creates a charge depletion region, takes shape at the Au NPs–β-Ga₂O₃ interfaces. The negatively charged Au NPs deplete the carriers near the surface of β-Ga₂O₃ films. As the width of the depletion layer increases, the depletion region close to the Au NPs increase accordingly. And the width of the depletion layer is relevant to the barrier height. In addition, it is the reflection of the unpinned Fermi level at the metal–semiconductor interface.^29,30 All of these cause the decreased dark current in Au NPs/β-Ga₂O₃ PD. And dark current decreased with increasing Au NPs on the β-Ga₂O₃ interfaces.

Fig. 6 Schematic illustrating the charge transfer process between β-Ga₂O₃ PDs without and with Au NPs. Upward band bending creates a Schottky barrier, which allows hot electrons with energy >Φ_b to inject into the conduction band of the Ga₂O₃ epilayer.

When upon 254 nm light illumination, electron–hole pairs are generated [hν → e⁻ + h⁺]. The photogenerated carriers immediately affect the photocurrent due to the sudden increase in the carrier density in the β-Ga₂O₃ film. Then, holes flow to the surface and recombine with the O₂-trapped electrons and releasing O₂⁻ from the surface [O₂⁻(ad) + h⁺ → O₂(g)]. The remaining unpaired electrons become the major carriers that contribute to the photocurrent until they are trapped again. When covered with Au NPs, the SPs of the Au NPs have enhanced the scattering of the 254 nm light, which made more photons scatter on the β-Ga₂O₃ film. Then more photogenerated electron–hole pairs are created. The impact of SPs increases with the increasing of Au NPs. As a result, Au NPs/Ga₂O₃ PD exhibits significantly high value of photocurrent.

4. Conclusion

In this paper, we demonstrated that Au NPs on the surface of β-Ga₂O₃ thin film can improve the performance of PD. An obvious enhancement of the response sensitivity and good reproducibility for the Au NPs-decorated PD can be observed, and the performance of PD is enhanced with the increasing of the quantity of Au NPs. Meanwhile, dual-band (visible and UV) response is obtained with Ga₂O₃ PD by introducing Au-NPs and the absorption wavelength of Au NPs is around 532 nm. All these enhanced performances are attributed to the coupling between the excitons of β-Ga₂O₃ thin film and SPs of Au NPs. So, it is a viable approach to improve the performance of photodetectors.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), People's Republic of China, the National Natural Science Foundation of China (Grant No. 51572033, 61274017, 51572241, and 11404029), the Fundamental Research Funds for the Central Universities (Grant No. 2014RC0906).

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