One-dimensional semiconductor nanowires have attracted considerable attention as unique building blocks for various applications in integrated electronic and optoelectronic devices, such as nanoscale lasers,^[1–3] field effect transistors,^[4–6] solar cells,^[7] and photodetectors (PDs).^[8,9] Nanowire-based PDs, as potential key functional units in on-chip information communication and processing, have attracted considerable interest.^[10–12] Particularly, PDs constructed by III–V semiconductor nanowires are widely used in near-infrared and infrared detection due to their narrow bandgap.^[13,14] Compared to the binary III–V semiconductor nanowires, the alloyed III–V semiconductor wires usually have wider and adjustable spectral response ranges due to their tunable bandgap nature, which means that the III–V alloy nanowires are ideal systems that are promising candidates for constructing wide-spectral-response-range and high-performance PDs in the infrared region.^[15–21] Among the many III–V alloys, Ga_1−xIn_xAs_ySb_1−y is an important quaternary alloy with tunable bandgap from 1.42 eV (x = 0, y = 1) to 0.1 eV (x = 1, y = 0), this widely tunable bandgap range covers three important spectral regions 1–3 μm, 3–5 μm, and 8–14 μm.^[22] So far, GaInAsSb layers based PDs have been successfully constructed,^[23,24] however, the photoelectric measurement based on quaternary GaInAsSb alloy nanowires is rarely reported in the literature.

Quaternary Ga_1−xIn_xAs_ySb_1−y alloys have usually been grown by liquid phase epitaxy (LPE),^[25] organometallic vapor phase epitaxy (MOVPE),^[26] molecular beam epitaxy (MBE),^[27] and metal-organic chemical-vapor-deposition (MOCVD).^[28] For the preparation of GaInAsSb quaternary alloy nanowires, there is a challenge to obtain the GaInAsSb nanowires with compositions in the miscibility gap through LPE or other thermodynamic equilibrium methods. Although, the non-equilibrium growth techniques, such as MBE and MOCVD, can effectively overcome this difficulty, the above growth strategies usually have a high cost and the preparation process is complicated. To the best of our knowledge, there is no report on the GaInAsSb alloy nanowires synthesized via a chemical vapor deposition (CVD) method, which is a cheaper method and easy to operate.

In this work, high-quality GaInAsSb quaternary alloyed nanowires were synthesized via a simple CVD method. Both Raman spectrum and photoluminescence studies show bandgap engineered optical properties consistent with the composition of the alloy nanowires. Single-nanowire-PDs with high responsivity and external quantum efficiency in the near-infrared region were constructed. Our results of these novel PDs will advance the development of photonics and optoelectronics as well as the integrated devices.

The samples were grown in a horizontal furnace, where a moving source method was applied for the composition modulation.^[29] Typically, an alumina boat with GaSb powder (Aladdin, 99.99%) was firstly pulled into the heating zone of the furnace, and another boat with InAs powder (Aladdin, 99.99%) was placed far away from the heating zone. A quartz rod driven by a step motor through magnetic force was used to push the boats into or out of the heating zone during the growth. Silicon wafers coated with 5-nm-thick gold film were placed in the deposition area to collect the samples. Before heating, mixed gas (Ar with 5% H₂) was flowed into the system at a rate of 45 sccm and the pressure was maintained at 4 Torr. Then the furnace was heated to 750 °C at a rate of 30 °C·min⁻¹. After 60 min of growth at 750 °C, the InAs powder was shifted to the center of the heating zone slowly. After keeping on the growth for another 60 min, the furnace was naturally cooled to room temperature.

The morphology and structure of the as-prepared sample were characterized by a field emission scanning electron microscope (FE-SEM, Zeiss sigma-HD), an x-ray diffraction spectrometer (XRD, Rigaku D/Max 2500), and a transmissionelectron microscope (TEM, Tecai F20) combined with energy-dispersive x-ray spectroscopy (EDX). The Raman scattering spectra were collected on a laser confocal Raman spectrometer (LABRAM-010, France), while PL measurements were conducted via a home-built infrared micro-PL setup with a femtosecond laser excitation source (Spectra Physics Tsunami, 800 nm, 80 fs pulse duration, 80 MHz repetition rate).

PDs were constructed with the as-prepared single Ga_0.75In_0.25As_0.49Sb_0.51 nanowires. The nanowires were first dispersed on a p-type Si substrate with a 300 nm thickness SiO₂ layer. Then AMMA and PMMA were spin-coated on the surface of the mask. The electron-beam lithography system (RAITH: 150-TWO) was employed to design the electrode patterns. Two Cr/Au (10/60 nm) electrodes that acted as the source and drain were fabricated by a thermally deposited process. The single nanowire connected with the Cr/Au Schottky contact electrodes constitutes a typical M-S-M PD. The as-fabricated PDs were investigated by a semiconductor parameter analyzer (Keithley 4200) under dark conditions and the illumination of a monochromatic light beam, all the measurements were carried out at room temperature.

The SEM image of the as-grown sample (Fig. 1(a)) shows that the synthesized nanowires have a length of up to 10 μm and a diameter ranging between 150 nm and 300 nm. The inset of Fig. 1(a) shows a high-magnification SEM image of a typical nanowire. Figure 1(b) exhibits the XRD pattern of the as-deposited quaternary alloy nanowires. For comparison, we list the standard XRD patterns of zinc-blende (ZB) GaAs (JCPDS card No. 89-2770; a = 5.65330 Å) and ZB InSb (JCPDS card No. 70-2515; a = 6.47937 Å) in the top and bottom of the graph, respectively. The sharp diffraction peaks indicate that the as-grown nanowires are GaInAsSb alloy with pure ZB crystallographic phase and high phase purity.

Fig. 1.

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	Fig. 1. (color online) (a) SEM image of the as-grown nanowires. Inset: high-resolution SEM image of a single nanowire with catalyst. (b) The XRD pattern of the as-deposited alloy nanowires.

Figure 2(a) is the TEM image of a representative nanowire. There is a typical spherical catalytic particle at the tip of the nanowire, which demonstrates that the growth of these nanowires follows the typical metal-catalyzed vapour–liquid–solid (VLS) mechanism.^[30] Figure 2(b) shows the high-resolution TEM (HRTEM) image taken from the nanowire (the red rectangle in Fig. 2(a)). The d-spacing of (220) planes was measured to be 0.2124 nm, falling in between the values of GaAs and InSb (Cf. d_GaAs = 1.9988 Å and d_InSb = 2.2907 Å). The insert of Fig. 2(b) plots the corresponding reciprocal lattice space image extracted by fast-Fourier transform (FFT) along the zone axes. According to the FFT patterns, the crystal structure of the nanowire can be confirmed to be the ZB structure, and their growth direction is [110]. Meanwhile, the clear and orderly atomic arrangement further reveals that the nanowire has a high-quality single-crystalline structure without significant stacking faults or twin-plane polytypic defects. Figure 2(c) plots the EDS spectrum collected from the wire (the red dot in the inset of the figure), which illustrates that the nanowire consists of elements Ga, In, As, and Sb with the ratio of 0.75:0.25:0.49:0.51, revealing that the composition of the achieved wires is Ga_0.75In_0.25As_0.49Sb_0.51. Figure 2(d) displays the two-dimentional (2D) elemental mapping of this nanowire, indicating that Ga, In, As, and Sb are homogeneously distributed across the whole nanowire.

Fig. 2.

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	Fig. 2. (color online) (a) TEM image of a representative single GaInAsSb nanowire. (b) HRTEM image taken from the red rectangle of (a), the insert is the diffraction pattern converted by fast-Fourier transform where the zone axis can be identified. (c) Corresponding EDX profiles measured at the red dot of the nanowire. (d) The 2D elemental mapping of the four detected elements Ga, In, As, and Sb.

In order to further characterize the microstructural properties of the as-grown Ga_0.75In_0.25As_0.49Sb_0.51 nanowires, we have performed the Raman and PL spectra measurements. As displayed in Fig. 3(a), the Raman spectrum excited with a continuous 632.8 nm laser was performed on a single nanowire. This spectrum shows a multi-mode phonon vibration behavior of GaInAsSb quaternary mixed crystal lattice. Distinct peaks in the broad spectrum ranging from 100 cm⁻¹ to 300 cm⁻¹ are observed. The four phonon modes at 148 cm⁻¹, 236 cm⁻¹, 260 cm⁻¹, and 277 cm⁻¹ are related to the GaSb-like longitudinal optical phonon (LO) modes,^[31] the GaSb-like transverse optical phonon (TO) modes, the GaAs-like TO modes, and the GaAs-like LO modes, respectively.^[32] This confirms that the achieved nanowires have good compositional homogeneity and high crystal quality. The PL spectrum of the nanowires at 77 K is shown in Fig. 3(b), which exhibits a single emission band with peak wavelength at 1950 nm, corresponding to the bandgap of ∼ 0.636 eV. The bandgap can be calculated to be 0.615 eV through the equation^[33]

Fig. 3.

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	Fig. 3. (color online) (a) The Raman spectrum of the Ga0.75In0.25As0.49Sb0.51 nanowire. (b) The PL spectrum of Ga0.75In0.25As0.49Sb0.51 nanowires at 77 K.

In order to investigate the photoresponse properties of the as-grown nanowires, PDs were constructed with the as-prepared single Ga_0.75In_0.25As_0.49Sb_0.51 nanowires. The single nanowire connected with the Cr/Au Schottky contact electrodes constitutes a typical M-S-M PD. Figure 4(a) provides the schematic image of the nanowire PDs. Figure 4(b) shows the I–V curves of a representative nanowire PD (see the inset) with a wire diameter of ∼ 200 nm and a channel length of ∼ 5.5 μm, measured in dark conditions and under the illumination of a monochromatic light beam with a wavelength of 980 nm at different light intensities. It can be seen that the electrical conductance of the device significantly increases when the light intensity rises from 10 mW/cm² to 260 mW/cm². Figure 4(c) plots the light intensity-dependent photocurrent with the working voltage at 1.0 V, which demonstrates an approximate linear dependency. The largest photo-excited current can reach 0.32 μA when the light intensity is raised to 260 mW/cm² (the dark current is about 150 nA obtained in Fig. 4(b)), showing that the photon-generated carriers are enhanced with the increased light intensity. The light intensity-dependent photoconductive gain demonstrates that the nanowire is a typical photon-dependent resistor, which is beneficial for applications in light power detection.

Fig. 4.

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Fig. 4. (color online) (a) Schematic image of Ga0.75In0.25As0.49Sb0.51 nanowire PDs. (b) I–V curves of the device under 980 nm illumination at different light intensities. The inset shows the SEM image of a fabricated PD, scale bar: 1 μm. (c) Light intensity-dependence of the photocurrent at VDS = 1.0 V. (d) Spectral responsivity R and EQE as functions of light intensity at VDS = 1.0 V, respectively.

The spectral responsivity (R) and external quantum efficiency (EQE) are two critical parameters of a PD, which can be expressed as^[34,35]

In summary, the Ga_0.75In_0.25As_0.49Sb_0.51 quaternary alloy nanowires have been successfully synthesized through a simple CVD method. Along the length of the nanowires, the composition distribution of the nanowires is uniform, and the nanowires are high quality zinc-blende single-crystal structured. These nanowires have strong light emission at 1950 nm, which is in accordance with their bandedge emission. The constructed PDs based on the alloy nanowires show a strong light response in the near-infrared region with high R (158 A/W) and EQE (2.0 × 10⁴%). The synthesized quaternary alloy nanowires in this work will be a good candidate for construction of functional electronic and photoelectric devices.