Band offset and an ultra-fast response UV-VIS photodetector in γ-In2Se3/p-Si heterojunction heterostructures.

High-quality γ-In2Se3 thin films and a γ-In2Se3/p-Si heterojunction were prepared using pulse laser deposition (PLD). The band offset of this heterojunction was studied by XPS and the band structure was found to be type II structure. The valence band offset (ΔE v) and the conduction band offset (ΔE c) of the heterojunction were determined to be 1.2 ± 0.1 eV and 0.27 ± 0.1 eV, respectively. The γ-In2Se3/p-Si heterojunction photodetector has high responsivity under UV to visible light illumination. The heterojunction exhibits highly stable photodetection characteristics with an ultrafast response/recovery time of 15/366 μs. The ultrafast response time was attributed to type II structure band alignment, which was good for the separation of electron-hole pairs and it can quickly reduce recombination. These excellent properties make γ-In2Se3/p-Si heterojunctions a promising candidate for photodetector applications. This journal is © The Royal Society of Chemistry.

Introduction

Two-dimensional layered semiconductors exhibit unique physical and chemical properties compared to their bulk counterparts. They have great application prospects in the field of nano-optoelectronic devices and have attracted widespread attention.[1-4] However, stable high-performance of these two-dimensional layered semiconductor materials based devices has yet to be further explored. In the past decade, layered materials such as metal dichalcogenides (MoS2,[5-7] MoSe2,[8-10] WSe2,[11-13] WS2 (ref. 14–17)), III–VI semiconductors (GaSe,[18,19] GaS,[20,21] InSe,[22,23] In2Se3 (ref. 24 and 25)), V–VI semiconductors (Bi2Te3 (ref. 26 and 27)) and elemental semiconductors (black phosphorus[28]) have found useful applications in nanoelectronics and optoelectronics. Of the various chalcogenides, In2Se3 is an interesting III–VI n-type semiconductor due to its multiphase and excellent optical properties. It is also known to have at least five crystal forms (α, β, γ, δ and κ). At different temperatures with a specific stoichiometric ratio, different phases and crystal structures can coexist in case of In2Se3.[24,29,30]

Generally, In2Se3 is a direct narrow bandgap semiconductor. Multilayered (ML) indium selenide exhibits high electron mobility, remarkable light absorption efficiency and sensitivity, and good stability which makes it one of the most promising materials for photodetection purposes. Because silicon is a general-purpose substrate for photovoltaic applications, combining In2Se3 with mature silicon technology has great potential for the preparation of photoelectric sensors with high detection performance. Current photodetection technology targets multi-spectral (wideband or dual-band) photodetectors for sensing, imaging under atmospheric conditions, object discrimination and optical communication applications.[31]

Up to now, various synthetic approaches have been used to prepare different In2Se3 heterostructures. Zhang et al. prepared the In2Se3/ZnO heterojunction and studied its offset structure.[32] Chen et al. developed an effective colloidal process involving thermal injection to synthesize uniform nanoflowers consisting of 2D γ-In2Se3 nanosheets.[33] Zheng et al. prepared a self-assembled broadband β-In2Se3/Si photodetector array for weak signal detection.[34] Yang et al. Prepared the In2Se3/Pi nanosheets and studied its photoresponsivity.[35] γ-In2Se3/Si has emerged as one of the most promising materials for visible photodetection due to its remarkable responsivity and detectivity in a wide range of wavelengths. In this work, a high-quality n type γ-In2Se3 thin film was successfully prepared by pulsed laser deposition (PLD). And we studied the valence band offset (ΔEv) and the conduction band offset (ΔEc) of the heterojunction.

The γ-In2Se3/p-Si heterojunction was prepared by depositing In2Se3 on p-Si substrate. The band offset of this heterojunction was studied by XPS. The γ-In2Se3/p-Si heterojunction exhibits significant responsivity and detectability over a wide range of wavelength. This heterojunction photodiode exhibits excellent response characteristics as an optoelectronic device.

Experimental

The In2Se3 was grown on Si substrate using the PLD method with solid targets of In2Se3 (99.99%). The deposition rate was calculated by using thickness of the grown films and deposition time and found to be 0.12 nm s−1. Background vacuum pressure was kept at 1 × 10−4 Pa. 248 nm pulsed line from a Coherent COMPexPro 102 excimer laser was used as laser source. The pulse energy and working frequency were maintained at 200 mJ and 2 Hz, respectively. The substrate temperature was kept at 600 °C during growth. At the end of growth, the samples were naturally cooled to room temperature and the sample was taken out to obtain In2Se3/p-Si heterojunction. X-ray photoelectron spectroscopy in this experiment was tested by the Kratos Axis Ultra XPS system. Al Kα (hν = 1486.6 eV) wavelength was used as an X-ray source which can accurately calibrate the work function and Fermi level.

A typical XRD spectrum shown in Fig. 1 is a well-defined set of diffraction peaks for In2Se3. The XRD standard alignment card confirms that these peaks are assigned to the hexagonal phase of In2Se3 with lattice parameters: a = 0.71, c = 1.93 nm (JCPDS card, no. 71-0250) which means that it has a gamma phase structure.[36] The Raman spectrum of gamma phase In2Se3 is shown in the inset of Fig. 1. It exhibits vibration mode at 123.8, 150.2, 173.9, and 241.0 cm−1, which are consistent with previously reported results.[30]The vibration mode at 302.0 cm−1 is consistent with reported of SiO2. The XRD pattern did not show any peak for impurity phases.

Fig. 1

The XRD patterns and the Raman spectrum of the In2Se3.

The characteristics of the band alignment of heterojunction is very important for the application of In2Se3/Si heterojunction as photodetector. There are many situations for which energy band structure diagram can be simulated by using the Anderson–Shockley model. However, in this paper, experimental methods are used to study the energy band offset of the In2Se3/Si heterojunction interface.

Fig. 2 shows Se3d core level peak from the top In2Se3 layer and Si2p core level peak originating from the underlying Si, and the difference in the core binding energy (ΔECL) for In2Se3/Si heterojunction. The valence band maximum (VBM) was determined by using linear extrapolation. The valence band offset (ΔEv) in terms of a binding energy difference ΔECL between core levels from each side of the interface is:

Fig. 2

(a) The XPS CL spectra of the Zn2p state measured from the Si sample and In2Se3/Si heterojunction, (b) the XPS CL spectra of the Se3d state measured from the In2Se3 films and In2Se3/Si heterojunction, (c) the valence-band XPS spectrum of Si and In2Se3 films, and (d) energy band diagram of type-II band alignment of the In2Se3/Si heterojunction.

The peak position of Si2p-core level in Si and In2Se3/Si found to be 98.98 ± 0.05 eV and 99.33 ± 0.05 eV, respectively (as shown in Fig. 2a). The CL of Se3d in In2Se3 was at 54.34 ± 0.05 eV, and in In2Se3/Si heterojunction at approximately 53.94 ± 0.05 eV (as shown in Fig. 2b). The valence band edge shown in Fig. 2c was used to measure the position of the VBM. The VBMs of Si and In2Se3 were determined to be 0.04 eV and 0.99 ± 0.1 eV, respectively. The Fig. 2c illustrates fitting of the low energy portion of the spectrum.

After the above data is substituted into eqn (1), the value of VBO (ΔEv) is calculated to be 1.2 eV. Since γ-In2Se3 is a direct bandgap semiconductor, the band gap of In2Se3 can be extrapolated to 2.05 eV.[32] This is consistent with the previously reported data that band gap of layered γ-In2Se3 lies at 2–2.5 eV.[37] As an indirect bandgap semiconductor, Si has a band gap of 1.12 eV at room temperature.

Therefore, the conduction band offset (CBO) value is calculated to be −0.27 eV by eqn (3). The energy band diagram of γ-In2Se3/Si heterojunction is shown in Fig. 2d. Analyzing the band structure, it can be concluded that this heterojunction has a type II aligned structure.

The typical structure of γ-In2Se3/p-Si heterojunction photodetector is shown in the inset of Fig. 3. Fig. 3 shows a typical current–voltage (I–V) characteristic of different metal semiconductor contacts. The result shows that all the three contacts: Si–In2Se3, In–Si, Au–In2Se3 are Schottky contacts, the influence of contact between Au and In2Se3 and between In and Si can be neglected; the rectification behavior of this device mainly comes from the Schottky contact of the In2Se3/Si heterojunction. And the rectification ratio is calculated 76 from the figure. Therefore, the γ-In2Se3-ML/Si heterojunction appears to be a well-defined diode with a turn-on voltage of approximately 0.7 V.

Fig. 3

A typical plots of current voltage characteristics in different metal–semiconductor contacts and In2Se3/Si heterojunction. (Inset: schematic diagram of the γ-In2Se3-ML/Si heterojunction photodetector).

Fig. 4a–d shows reverse biased portion of the I–V curve of γ-In2Se3/p-Si heterojunction in the dark, and at different light intensities using LED point light sources of 365, 420, and 500 nm for illumination (IK3301R-G). The reverse bias characteristics also confirms the rectifying behavior of the γ-In2Se3-ML/Si heterojunction. During forward bias condition, the current increases exponentially with voltage, whereas at larger reverse voltages, the increase in current tends to saturate. The reverse biased IV characteristics of γ-In2Se3/Si heterojunction in the dark and presence of light also confirms its photodiode behavior. As shown by the reverse biased I–V curves, the dark current is about −0.98 nA. Under the illumination of the LED spot light source, a notable increase in the current in the reverse biased region was observed. With three different light sources, the photoresponse was tested at different light intensities and wavelengths. Fig. 4d shows the strong photoresponse dependence on light intensity. The photocurrent increases significantly with increase in light intensity. These characteristics demonstrate high photosensitivity of the device under ultraviolet to visible light irradiation.

Fig. 4

Photoelectric characteristics of In2Se3-ML/Si heterojunction in the dark, and their photoresponse at various wavelengths and different illumination intensities: (a) 360, (b) 420, and (c) 500 nm and; (d) photocurrent as a function of light intensity under the reverse bias voltage of −5 V.

Using the power law function to fit the three curves in Fig. 4d, it was found that there is almost a linear relationship between the photocurrent and optical power density. The power law function: Iph = AP, where Iph stands for photocurrent (Iillumination − Idark); A is a constant at a specific wavelength; P is the power density of the incident light; and α is a power law index. The fitted curves show that α is 0.96, 0.86 and 0.56 for wavelengths of 365, 420 and 500 nm, respectively.

Since the heterojunction is formed by two materials with different lattice constants and lattice structures, the energy band at the interface is discontinuous and there are potential barriers and potential wells. Different lattice constants introduce defects and interface states heterojunction interface. First of all, at 500 nm, α is only 0.56. This sub-linear response is a result of complex process of electron–hole generation, trapping, and recombination in the semiconductors. At wavelengths of light of 365 and 420 nm, the alpha factor is close to the ideal state one, particularly at 365 nm, which indicates that the losses caused by recombination in the process of photoexcitation of carriers are relatively low. Both the trap states in In2Se3 and at the interface between In2Se3 and Si substrates may have recombination centers, which has also been confirmed in other photodetector materials.[37,38] It can also be seen from Fig. 4d that the photocurrent increases more slowly as the light intensity increases at a wavelength of 365 nm as compared to 420 nm incident light at the bias voltage of −5 V. In contrast, the photocurrent from 420 nm light intensity increases faster.

The responsivity (Rres) is a useful figure-of-merit for a photodetector, which can be calculated from:where Iph is the photocurrent, Iirr is the irradiance of the incident light, and A is the effective illuminated area (2 × 10−4 cm2 for this device).[39]According to photoresponse at 365 nm of 0.175 W cm−2, as shown in Fig. 4, the optical responsivity is calculated 0.54 AW−1, which is higher than other nanomaterials.[7,40]

The other important figure-of-merit is the external quantum efficiency (η), which can be calculated from:where e is the elementary charge, h is the Planck constant, and ν is the electromagnetic frequency. The external quantum efficiency is calculated 1.84, which exhibit excellent performance.

The detectivity (D) can be calculated from:where q is the fundamental unit of charge, Jd is the dark current.[39] And D is calculated 3.53 × 1012, which is higher than other nanomaterials.[7,40]

A time-resolved photo-response diagram under illumination of 365 nm light source with a reverse bias of −3 V is shown in Fig. 5a. Under the bias of −3 V, the current increases significantly from −1.5 μA (light off) to −192 μA (light on). Switch “on/off” ratio was found up to 128. The consistency of the seven switching cycles in Fig. 5a shows that the optical response has good stability and repeatability.

Fig. 5

Optoelectronic performance of In2Se3-ML/Si heterojunction: (a) the seven-cycle time-resolved photoresponse under a bias of −3 V at 365 nm light illumination, (b) a single–period plot of the time-resolved photoresponse, (c) photoresponse of the photodetector at 266 nm laser by a Q-switch Nd:YAG and (d) photoresponse of single period in (c).

Fig. 5c shows typical time response characteristics of γ-In2Se3/p-Si heterojunction photodiode. The response speed of the photodetector is usually characterized by the response time (τres)and the recovery time τrec. Whereas, response time is defined as the time required to increase 10% of its peak value to 90%, and the recovery time is the time taken to decay from 90% to 10% of its peak value. In order to extract an accurate response time, an enlarged response cycle is shown in Fig. 5d. The short response time (15 μs) and recovery time (366 μs) were observed in this work, which are much smaller than other heterojunction (such as α-In2Se3 (6 ms/12 ms), γ-In2Se3 (175 μs/226 μs), ZnO (104/-), MoS2 (31 μs/72 μs)).[26,39,41,42] The ultrafast response attributed to II structure band alignment was good at the separation of electrons–hole pairs, and can fastly reduce their recombination.

Conclusions

In this paper, high quality γ-In2Se3/p-Si heterojunction were prepared using PLD. The band alignment of In2Se3/Si heterojunction was measured by XPS. The valence band offset (ΔEv) and the conduction band offset (ΔEc) of the heterojunction were determined to be 1.2 ± 0.1 eV and 0.27 ± 0.1 eV, respectively. Analysis of the band structure indicates that the γ-In2Se3/p-Si heterojunction has a type II band alignment structure. The γ-In2Se3/p-Si heterojunction photodiodes exhibit excellent rectification characteristics in the dark and at different incident optical powers, and have a broad spectral response ranging from UV to VIS with high responsivity and stability. And the ultrafast response time (15 μs) and recovery time (366 μs) indicate that the heterojunction has a fast response and detection performance.

Conflicts of interest

There are no conflicts to declare.