High-performance photodetectors based on bandgap engineered novel layer GaSe_0.5Te_0.5 nanoflakes

Abstract

Layered two-dimensional (2D) gallium monochalcogenide (GaX, X = S, Se, Te) semiconductor crystals hold great promise for potential electronics and photonics application. In this paper, we reported the optoelectronic properties of 2D bandgap engineered GaSe_0.5Te_0.5 nanoflakes. The GaSe_0.5Te_0.5 nanoflakes were synthesized by chemical vapor deposition (CVD) and characterized by XRD, SEM, TEM, XPS, Raman and PL spectra, which demonstrate the high crystal quality of as-prepared nanoflakes. The photodetector based on single GaSe_0.5Te_0.5 nanoflake shows fast response time, high reversibility and stability both in air and vacuum. The photo-responsivity is up to 22 A W⁻¹ under illumination of 532 nm light. More interesting, the GaSe_0.5Te_0.5 nanoflake photodetector demonstrate extended light response range, as compared with pure GaSe. The photo-responsivity is 13 A W⁻¹ for 650 nm red light. The present results suggest strongly that the bandgap engineered 2D GaSe_0.5Te_0.5 nanoflakes hold extensive applications in next-generation photodetection and photosensing nanodevices.

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Introduction

Single crystal 2D crystals, showing unique and fascinating properties and potential applications in next-generation electronics, have attracted great attention in recent years.^1,2 Due to the zero band gap, weak light absorbance and short exciton lifetime, nanodevices based on graphene may find limited applications in optoelectronics.³ Fortunately, the disadvantage of graphene can be overcome by 2D layered semiconductors with finite band gap and relative high carrier mobility. For example, MoS₂ and black phosphorus are excellent candidates for optoelectronics devices.^4,5 Many kinds of layered semiconductors, including layered transition metal dichalcogenides, group-III metal chalcogenides, group-IV metal chalcogenides have been peeled off from bulk materials and used to explore the corresponding optoelectronic properties.⁶ In addition to these binary layered compound, there is an increasing interesting in the ternary alloy structures, such as: MoS_2xSe_2−2x, WS₂Se_2−2x, Mo_xW_1−xSe₂, due to its continuous bandgap and emission tuning.^7–9 Extensive efforts in the research of 2D semiconductor crystals focused mainly on molybdenum and tungsten dichalcogenides.¹⁰ There are relatively little reports about the binary or ternary III–VI monochalcogenide (GaX, X = S, Se, Te) materials, whose bandgap range from near infrared to ultraviolet spectrum. These materials may find important applications.^11,12 More exploration about III–VI materials is motivated by their anisotropic crystallography characteristics and intriguing optical or optoelectronic properties.^13,14 The p-type GaTe with direct band gap of 1.7 eV, p-type GaSe with indirect bandgap of 2.0–2.1 eV and direct bandgap of 2.12–2.15 eV, n-type GaS with indirect bandgap of 2.5–2.6 eV and direct bandgap of 3.0–3.1 eV, can be used as building blocks for novel van der Waals p–n heterostructures, visible light emitter and broadband terahertz pulses generator.^15,16 GaSe_xTe_1−x and GaS_xSe_1−x alloy crystals obtained on the basis of these compounds enlarge this semiconductor family and enable the fabrication of devices with desired properties.¹⁷ Moreover, in contrast to molybdenum and tungsten dichalcogenides emitting light efficiently only with monolayer thickness, III–VI materials are bright light emitters in a wide range of thicknesses, which will relax the stringent synthesis requirements and add flexibility for fabrication excellent emitter.¹⁸

Photoconductors are sensors of light or other electromagnetic radiation and can convert optical signal into electrical signal. Photoconductors are essential element in many fields, such as optical communication, environmental monitoring, chemical/biological detection and optoelectronic memristor.^19,20 For practical applications, photodetectors with high photocurrent gain, fast response speed, high photosensitivity, good stability, high reliability, low cost and long lifetime are highly desired. Benefitting from the large surface-to-volume ratio and quantum confinement effect, low-dimensional nanostructures show huge advantages over the corresponding bulk counterpart.²¹ Up to date, various inorganic semiconductor nanostructures, such as ZnO, CdS, InAs, MoS₂, GaX (X = Te, Se, S), InSe and In₂Te₃, have been used to fabricate photodetectors.^22–24 However, the photoresponse gain of GaTe photodetector is low due to the large dark current while the GaSe nanodevices show a high resistivity and very low dark current because of the low mobility.²⁵ The compound alloy of GaTe and GaSe can improve the photoresponse gain and conduction.

Since ternary compound usually have advantages of tuned bandgap engineer and response wavelength, we fabricated here the high-performance photodetectors based on novel GaSe_0.5Te_0.5 nanoflakes, which were synthesized by a simple chemical vapor deposition method. The GaSe_0.5Te_0.5 nanoflakes photodetectors show fast response time, high stability and high responsivity both in air and vacuum. These results clearly demonstrate that the GaSe_0.5Te_0.5 nanoflakes will find extensive application in nano-optoelectronic fields.

Experimental section

The 2D GaSe_0.5Te_0.5 nanoflakes were synthesized by CVD method. Commercial GaTe and GaSe powder with 1 : 1 molar ratio were fully mixed and then used as source material, which was loaded on an alumina ceramic boat and placed at the center of tube furnace. Ar (95%)/H₂ (5%) carrier gas was introduced into the quartz tube with a constant flow rate of 100 SCCM (standard cubic centimeters per minute) to remove O₂ inside before heating. Then the furnace was rapidly heated to 950 °C and maintained at that temperature for 30 min. The synthesized products were found on the inner wall of the quartz tube in a zone locating downstream 5 cm away from the source material, where the temperature was in the range of 600–700 °C. The yield is high and the sample is easily taken off by tweezers. Fig. S1† is the photogragh of these peeled off nanoflakes.

The resulting products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), optical microscopy (OM), transmission electron microscope (TEM), energy-dispersive spectroscopy (EDS) attached on SEM and TEM, X-ray photoelectron spectroscopy (XPS). Raman spectrum and photoluminescence (PL) of single nanoflake were obtained by using a commercial Scanning Near-field Optical Microscopy (SNOM, WITec) under excitation of 532 nm laser at room temperature. The GaSe_0.5Te_0.5 nanodevices were fabricated by following lithography procedure, thermal evaporation and lift-off process. The thickness of contact electrode Au was about 120 nm. The space between the two Au electrodes was about 6 μm. The room-temperature electrical transport measurements were carried out with Keithley 2602 in air and vacuum. To measure the photoresponse properties, the whole nanodevice was exposed to 532 nm and 650 nm light perpendicularly at room temperature.

Results and discussion

Fig. 1a is SEM image of as-grown GaSe_xTe_1−x nanoflakes, which indicates explicitly the high density product with large area. The lateral dimension is up to 200 μm. No gold or others catalyst were used in the synthesis and no particle were observed at the tip of nanoflakes or nanowires. We assigned that there is self-catalyst growth process and the growth obey vapour–solid (VS) mechanism.²⁶ Fig. 1b is high magnification SEM of single nanoflake with smooth surface. Fig. 1c is corresponding EDS, which shows nearly the 2 : 1 : 1 atomic ratio of Ga : Se : Te. To examine the distribution of Ga, Te and Se element, we do element mapping of single nanoflake. Fig. 1d–f show the uniform distribution of Ga, Se and Te. Therefore, the sample is not the GaTe/GaSe heterostructures. We denoted it as GaSe_0.5Te_0.5 alloy. XRD was carried out to investigate the phase structure. The GaSe_0.5Te_0.5 nanoflakes were suspended in ethanol by sonication. Then the dispersion was dropped onto a glass slide. The GaSe_0.5Te_0.5 nanoflakes deposited evenly on the glass surface. Such dried glass slide was used to measure the XRD. The intensity of XRD pattern is weak and only a little diffraction peaks can be observed (Fig. S2†). We assigned these diffraction peaks to be (004), (002), and so on. The overwhelming (004) peaks indicate that the preferred perpendicular orientation of the GaSe_0.5Te_0.5 nanoflakes crystals is along the c-axis. These sample can be oxygenated after store for several months. In addition, we did XPS characterization to determine the component further. As see from Fig. S3(A1)–(A6),† the single nanoflake only contain Ga, Se and Te element. The ratio of Se 3d to Te 3d₅ is 4.2 : 3.21, which is a little deviation from 1 : 1. The O and Si signal are from the SiO₂/Si substrate wafer since the nanoflake is thin and the X-ray can penetrate it easily. The XPS beside the nanoflake demonstrates distinctly the strong O and Si signal and negligible Ga, Se and Te signal (Fig. S3(B1)–(B6)†).

Fig. 1 (a–c) SEM and EDS of GaSe_0.5Te_0.5 nanoflakes. (d–f) Element Ga, Se and Te mapping images of single GaSe_0.5Te_0.5 nanoflake.

TEM was used to further examine the GaSe_0.5Te_0.5 nanoflakes. Fig. 2a is low magnification TEM of GaSe_0.5Te_0.5 nanoflake, which also shows smooth surface. The EDS (inset) verifies the 2 : 1 : 1 atomic ratio of Ga : Se : Te. Fig. 2b is corresponding high-resolution TEM (HRTEM). The clear lattice stripes show high crystallinity of GaSe_0.5Te_0.5 nanoflake. The selected area electron diffraction (SAED, Fig. 2c) pattern and fast Fourier transform (FFT, Fig. 2d) also demonstrate the single crystalline quality and can be indexed to hexagonal structure. Generally, the GaSe belongs to hexagonal structure with lattice constant of a = 3.75 Å and c = 15.92 Å (JCPDS card no. 65-3507). The GaTe belong to monoclinic structure with lattice constant of a = 17.404 Å, b = 10.456 Å and c = 4.077 Å (JCPDS card no. 33-571). In GaSe_xTe_1−x, there is a phase transition from hexagonal GaSe to monoclinic GaTe structure in the composition range 0.26 ≤ x ≤ 0.60.^27,28 Thus, it is rational that the GaSe_0.5Te_0.5 is still hexagonal structure in the present case.

Fig. 2 (a and b) TEM and HRTEM of single typical GaSe_0.5Te_0.5 nanoflake. Inset of (a) is corresponding EDS. (c and d) SAED and FFT of the GaSe_0.5Te_0.5 nanoflake.

Raman spectroscopy is a powerful characterization method to understand the electronic structure and interaction of layered semiconductor materials. Fig. 3 shows the Raman spectrum of single GaSe_0.5Te_0.5 nanoflake. The Raman spectra of GaSe and GaTe were also shown in Fig. 3 for comparison. The Raman scattering peaks of GaSe crystal locate at 136.7, 215.6, 244.1 and 309.3 cm⁻¹, which are correspond to the A¹_1g, E¹_2g, E²_1g and A²_1g vibration mode, respectively.^14,29 The Raman scattering peaks of GaTe crystal locate at 117, 129, 144, 164, 178, 209 and 270 cm⁻¹, which are correspond to the A_g and B_g vibration mode, respectively.^13,24,30 The GaSe_0.5Te_0.5 shows different Raman spectrum from pure GaSe and GaTe. The spectrum profile is similar with that of GaTe. Compared with GaTe, the scattering peaks of GaSe_0.5Te_0.5 shift to larger wave number. For the ternary alloy nanostructures, it is easy to observe the Raman peaks shift with different components, such as GaS_xSe_1−x,¹⁵ WS_2(1−x)Se_2x.⁸ So, the present observed shift in the Raman peak is attributed to the alloying of GaTe crystal with Se composition. In the calculation method of GaS_0.5Se_0.5,¹⁵ the peak position of GaSSe A²_1g mode is the middle value of the GaS and GaSe A²_1g modes, which is consist with the experiment result. We also use the middle value of those GaSe and GaTe modes as the GaSe_0.5Te_0.5 peak position. These calculated middle values, (136.7 + 129)/2 = 132.9 cm⁻¹; (209 + 215.6)/2 = 212.3 cm⁻¹; are near the measurement values of 131 cm⁻¹ and 213 cm⁻¹ in the GaSe_0.5Te_0.5. So, the peaks at 131, 147, 172, 186, 213 and 278 cm⁻¹ are assigned to be A_g and B_g vibration mode of GaSe_0.5Te_0.5 nanoflakes.

Fig. 3 Raman spectrum of single GaSe_0.5Te_0.5 nanoflake. The Raman spectra of GaSe and GaTe were also shown for comparison.

Photoluminescence (PL) was used to measure further the GaSe_0.5Te_0.5 alloy. Fig. 4 is the PL of GaSe_0.5Te_0.5. The emission band from GaSe and GaTe, which may locate at 625 nm and 735 nm, are not observed. Leontie et al. reported that the PL spectra of Te doped p-GaSe single crystals contained excitonic band of GaSe, 2.0 eV band and 1.7 eV band. The latter two bands are related to the impurity levels introduced by Te atoms within the band gap of GaSe.³¹ Therefore, the only one emission band in present experiment suggests that the as-prepared sample is single phase with little impurity levels and high crystal quality. The broad PL peak centered at 688 nm represents the 1.8 eV near-bandgap emission of GaSe_0.5Te_0.5 at room temperature.³² In addition, the symmetrical emission band shape and absent emission tail indicate the GaSe_0.5Te_0.5 nanoflakes do not exhibit defect states, which will affect the band edge emission greatly.

Fig. 4 PL spectrum of GaSe_0.5Te_0.5 single nanoflake under excitation of 532 nm laser.

With the bandgap of 1.8 eV, GaSe_0.5Te_0.5 nanoflakes can act as excellent candidates for high performance photodetectors to detect the visible light. We fabricated the photodetector of single GaSe_0.5Te_0.5 nanoflake to investigate its extended photoresponse properties. The photocurrent and photoresponsivity of GaSe_0.5Te_0.5 nanoflake devices were investigated under light irradiation with different wavelength, at atmosphere and vacuum. Fig. 5a shows a schematic view of the fabricated nanoflake device, where two Au electrodes were deposited as the source and drain, respectively. Fig. 5b is current–voltage (I_ds–V_ds) curves of GaSe_0.5Te_0.5 device under dark condition as well as illumination with different wavelength in air. The upper left and bottom right inset are optical image of GaSe_0.5Te_0.5 nanoflake device and enlarged I_ds–V_ds curve at dark condition. The dark current is small, which demonstrates the poor conductance of GaSe_0.5Te_0.5. The current increase drastically when the GaSe_0.5Te_0.5 nanodevice was illuminated by green light (532 nm) and red light (650 nm). Light in the green and red wavelength region is of special interest and has a very wide range of applications. Therefore, a photodetector with good performance at such region would significantly advance the state of the art and expand the usage of imaging applications. This GaSe_0.5Te_0.5 nanoflake device can be proposed to be a simple metal–semiconductors–metal (MSM) structure consisting of two Schottky barrier contacts connected back to back.³³ One of contacts is reverse biased (cathode) while the other contact is forward (anode) under the applied voltage, creating an electric field in the GaSe_0.5Te_0.5 nanoflake. The illuminated light was absorbed strongly by GaSe_0.5Te_0.5 nanoflake, which can generate plenty of electron–hole pairs. The pairs were separated by the electric field and a pronounced photocurrent is observed. Thus, the GaSe_0.5Te_0.5 nanoflake should be a typical photon-dependent resistor in that more photocarriers would be generated by green light with higher energy than red. Generally, the red light (wavelength > 620 nm) with photon energy lower than the bandgap of GaSe cannot produce distinct photoresponse.³³ Only light with enough photon energy to excite electron from the valence band to the conduction band is able to induce a significant increase in conductance. Our present results show the extended photoresponse range of GaSe_0.5Te_0.5 alloy nanoflakes as compared to GaSe. Shen et al. reported the larger photocurrent under long (650 nm) wavelength light illumination than the short (550 nm) incident light and they ascribed this to the decreased carrier density induced by the increased surface recombination.³⁴ Our results demonstrate that the reduced surface recombination and consist with the PL spectrum.

Fig. 5 (a) Three-dimensional schematic view and the cross section view of the GaSe_0.5Te_0.5 nanoflake device. (b) I_ds–V_ds curves of GaSe_0.5Te_0.5 device under dark and illumination of 532 and 650 nm light at atmosphere. The upper left and bottom right inset are optical image of GaSe_0.5Te_0.5 device and enlarged I_ds–V_ds curve at dark condition. (c) Time-dependent photoresponse of the GaSe_0.5Te_0.5 device at V_ds = 3 V. (d) Enlarged portions of one rise and one reset process.

The response time is important factor for realizing high-performance photodetector. We investigated time-dependent photocurrent with the 532 nm and 650 nm light switched on and off at a fixed bias voltage of 3 V (Fig. 5c). The photocurrent can be reproducibly switched from the ‘ON’ state to the ‘OFF’ state by periodically turning the light on and off. The on–off ratio of photocurrent (ΔI/I_dark) is critical to determine the sensitivity of photodetector, where ΔI = I_light − I_dark and I_light, I_dark are the photocurrent and dark current, respectively. The on/off ratio was determined to be 75 for 532 nm light and 49 for 650 nm light, which indicates the high responsivity of GaTe_0.5Se_0.5 nanoflake. Wang et al. reported the high off-state current and low on/off ratio in GaTe field-effect transistor at room temperature, which originated from Ga ion vacancy.³⁵ Therefore, the present high on/off ratio manifests the high crystal quality of GaSe_0.5Te_0.5 nanoflakes. Moreover, the I_ds–T curves show almost same shape after several cycles and demonstrate high repeatability and stability of the device. The stability is important since Ga based monochalcogenide thin flakes are oxidated acceleratively under exposure to light.³⁶ Fig. 5d is the enlarged photoresponse process containing one rise and one reset, which exhibited two distinct states and changed very sharply from one state to another state. The photocurrent rise time and reset time are key parameters in determining the photodetector's sensitivity. The rise time was defined as the time needed to reach 90% of the photocurrent from dark current value after light and the reset time was defined as the time needed to reach 10% of the photocurrent after switching off the light illumination. The rising and reset time is less than 290 ms (limited by the time response rate of the measurement apparatus) in our devices, respectively, which are closely comparable to the data reported for GaTe layered material-based photodetectors.³⁴ The fast response time allow the device to act as a high quality photosensitive switch. Furthermore, the fall time is always longer than the corresponding rise time for all response spectra, which results in nonsymmetric at the rise and fall edges. These results demonstrate excellent response performance for a resistor-mode photodetector.³⁷

The photoresponsivity (R_λ) and external quantum efficiency (EQE) are both important parameters for photodetector besides the stability, reversibility and fast response speed. The R_λ and EQE can be calculated according to the following equations: R_λ = I_ph/(PS) and EQE = hcR_λ/(eλ), where I_ph is the photogenerated current, P is the incident light intensity, S is the effective illuminated area, h is Plank's constant, c is the light velocity, e is the electronic charge, and λ is the incident light wavelength. The R_λ and EQE can be estimated to be about 22 A W⁻¹, 5.1 × 10³% for 532 nm and 13 A W⁻¹, 2.5 × 10³% for 650 nm light, which may thanks to the efficient light excitation and absorption probability of GaSe_0.5Te_0.5. The R_λ and EQE are also superior than that of others III–VI monochalcogenide layered materials.^22,33,38,39 We summarized the detailed photoresponse parameters of photodetector in Table 1, which indicates the prominent performance of our devices.

Table 1 Summary of key parameters of layered III–VI monochalcogenides photoderectors

Photodetectors	Responsivity (Rλ) [A W^−1]	Quantum efficiency (EQE) [%]	Response time [ms]	Ref.
GaS	4.2	2050	30	22
GaSe	2.8	1367	20	33
GaTe	0.03	8	54	38
InSe	0.035	8.1	0.488	39
GaSe0.5Te0.5	22	5100	<290	This work

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He et al. reported that the adsorbates on GaTe surface in ambient strongly deteriorate the GaTe photodetector device performance.³⁸ After the GaTe nanodevice was pumped to around 10⁻⁵ Torr, the I–V curve shows excellent linearity and the response speed, photoresponse and mechanical stability were improved greatly.³⁸ To understand the effect of atmosphere on the device properties, we also measure the photoresponse of GaSe_0.5Te_0.5 device in vacuum, as shown in Fig. 6a. Inset of Fig. 6a is enlarged dark current. Compared with the current in air and dark condition, the I–V curve in vacuum and dark condition shows slightly more symmetrical shape, which corresponds to the improved electrode contacts and lower barrier by removing the surface adsorbates.³⁸ The current reduces a little in vacuum, although the reduce is so small. This negligible change between air and vacuum indicates the high crystal quality of GaSe_0.5Te_0.5 and the minor effect of adsorbates on the GaSe_0.5Te_0.5 nanodevice. The photocurrent in vacuum increases a little both under green and red illumination compared with that in air. This change may due to the reduced surface adsorbates, which can increase the light scattering process. Therefore, the incident light can be absorbed effectively in vacuum and then results in higher photoresponse. Fig. 6b is the time-dependent photoresponse of GaSe_0.5Te_0.5 nanoflake. Inset of Fig. 6b is the enlarged portion of one ON/OFF state, which also demonstrates the rapid response speed with a rise time and decay time of about 280 ms. The photoswitch current ratio in vacuum is almost same with that in air. So this GaSe_0.5Te_0.5 photodetector is almost not affected by surface adsorbates (vacuum) or environment effect (atmosphere ambient). These results further prove the excellent stability of the GaSe_0.5Te_0.5 device, which can be applied in common environment at room temperature.

Fig. 6 (a) I_ds–V_ds curves of GaSe_0.5Te_0.5 nanodevice under dark and illumination of 532, 650 nm light in vacuum. Inset is I_ds–V_ds curve at dark condition. (b) Time resolved photoresponse of GaSe_0.5Te_0.5 device at V_ds = 3 V. Inset is enlarged I_ds–T contained one rise and reset process.

Conclusion

In summary, the bandgap engineered GaSe_0.5Te_0.5 nanoflakes were synthesized successfully by a simple CVD method. The HRTEM and SAED demonstrate the hexagonal structure of GaSe_0.5Te_0.5 nanoflakes. Both Raman and PL spectra confirm the formation of single phase GaSe_0.5Te_0.5 alloy while not the composite of GaSe/GaTe. The current of GaSe_0.5Te_0.5 photodetectors increases drastically when exposed to red and green, showing the high responsivity, high repeatability, high stability and fast response time. Moreover, the GaSe_0.5Te_0.5 nanoflake photodetectors shows extended light response wavelength compared to GaSe. The environment play a minor contribution to the photodetector and the photoresponse mechanism is assigned to be photo-generated electron–hole pairs. These suggest strongly that the 2D bandgap engineered GaSe_0.5Te_0.5 nanoflakes are promising candidates for future optoelectronic and photosensitive photonics nanodevice with applications in common environment at room temperature.

Acknowledgements

We thank the NSF of China (term No. 51102091, 11574081, 91233203) and Research Fund for the Doctoral Program of Higher Education of China (No. 20114306120003) for financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09239j

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