Gallium arsenide (GaAs) is an important III–V compound semiconductor and provides advantages in high-speed electronic and optoelectronic devices thanks to the direct bandgap and outstanding electron transport properties.^[1–3] In the past two decades, GaAs nanowires (NWs) have become a hot topic in nanoscience and nanotechnology with the progress of synthesis methods.^[4–6] Due to the small diameter, large surface to volume ratio, highly anisotropic shape, and especially the capability of coupling with integrated silicon circuits, GaAs NWs show unique properties and have novel applications in optoelectronics and nanoscale lasering devices.^[7–11] The mechanical and electronic properties are critical for the performance of GaAs NWs devices and have attracted intensive scientific interest. Remarkable elasticity and stiffness have been found in GaAs NWs with elastic strain limit round 11%.^[12–14] The electron mobilities of GaAs NWs vary in the range from to depending on the surface quality and the ballistic transport effect is found to be strongly affected by the stacking faults and impurities of NWs.^[15,16] By now, the modification methods for mechanical and electronic properties of GaAs NWs are mainly focused on the improvement on NWs quality which is limited by the state of the art in synthesis methods.

As a fundamental thermodynamic variable, pressure can directly and efficiently reduce interatomic distances and change electronic orbitals and bonding patterns. High-pressure techniques have been widely used to search for new structures and modify the physical properties.^[17–20] Under pressure up to 12 GPa, the zinc-blende (ZB) phase GaAs begins to transform into an orthorhombic (OR) phase.^[21] The OR phase of GaAs was demonstrated to have the Cmcm symmetry and predicted as a metallic phase because the energy bands cross the Fermi level during the phase transition.^[22,23] In 2016, Wu et al. observed the semiconductor–metal phase transition in GaAs at 12 GPa by in situ alternating-current (AC) impedance spectroscopy and temperature dependent electrical resistivity measurements.^[24] For the GaAs NWs, high pressure studies have been focused on NWs with diameter from 80 nm to 150 nm, and the irreversible phase transition to OR structure was observed around 19 GPa in both ZB and wurtzite (WZ) NWs.^[25,26] It is noticeable that the mechanical properties of GaAs NWs show significant size-dependent effects, especially with diameters down to a few tens of nanometers.^[12–14] So it is expected that we will observe unique high-pressure behaviors in GaAs NWs with smaller diameters. The small size and high density surface state of GaAs NWs increase the technical difficulties in electrical resistivity measurements under high pressure, and there is still no report about the metallicity of GaAs NWs under high pressure.

In this work, we investigate the high-pressure behaviors of GaAs NWs with diameter around 40 nm by using synchrotron x-ray diffraction (XRD) and in-situ infrared reflectance (IRR) spectroscopy methods up to 26.2 GPa at room temperature. The reversible phase transition from ZB structure to OR structure was observed in GaAs NWs at around 20 GPa. In the same pressure range, a dramatic increase of infrared reflectance indicated the metallization of GaAs NWs.

The GaAs NWs under investigation were grown by a solid source molecular-beam epitaxy (MBE) system (VG 80) in a self-catalyzed growth manner. Commercial p-type Si (111) wafers were used as the substrates. Before being loaded into the MBE chamber, the Si substrates were pretreated by chemical etching. At first, we removed the native oxidized layer completely using a HF solution (5%). Then, the substrate was coated with a new oxidized layer by dipping the Si substrate in a solution of H₂SO₄ (98%) and H₂O₂ (30%) (volume ratio = 4:1).^[27] The growth of NWs was commenced by opening the gallium source shutter for 8 s, and then opening the arsenic source shutter at a temperature of 590 °C. The V/III beam equivalent pressure ratio was 7.3 and the growth time was 50 min.

The morphology and crystal structure of GaAs NWs were characterized by scanning electron microscopy (SEM, Nova NanoSEM 650) and synchrotron XRD. The XRD data were collected with a wavelength of 0.6199 Å at the BL15U1 beam line of Shanghai Synchrotron Radiation Facility (SSRF) at room temperature. The average diameter of the GaAs NWs is about 40 nm and the average length is according to the SEM image shown in Fig. 1(a). There is a typical diffraction pattern of ZB structure as shown in Fig. 1(b).

Fig. 1.

	Figure Option ViewDownloadNew Window
	Fig. 1. (a) SEM image of GaAs NWs. (b) XRD patterns of ZB GaAs NWs at ambient pressure.

High pressure experiments were carried out at room temperature with a diamond anvil cell (DAC). The pressure was measured by the fluorescence shift of ruby. The IRR spectroscopy measurements were performed with a Bruker Vertex 80 V FT-IR spectrometer equipped with a nitrogen-cooled mercury cadmium telluride (MCT) detector. The GaAs NWs were directly attached to a culet surface of the diamond anvil without pressure transmitting medium. The diameter of the diamond culet was and the sample chamber of drilled steel gasket was about in diameter and in thickness. The reflection spectra were measured at the interface between the sample and diamond anvil. Spectra taken at the internal diamond–air interface of the empty cell were used as the reference for normalizing the sample spectra. We applied additional normalization procedures to consider variations in source intensity. All of the reflectivity spectra that are reported in this paper refer to the absolute reflectivity at the diamond–sample interface. The high-pressure synchrotron XRD experiments were performed at beam line 4w2, Beijing Synchrotron Radiation Facility (BSRF, λ =0.6199 Å). The collected two-dimensional diffraction patterns were analyzed by integrating the two-dimensional image with the FIT2D software. For the XRD experiments, the diamond culet diameter was and the sample chambers of drilled steel gasket were about in diameter and in thickness. Methanol–ethanol (4:1) was used as the pressure transmitting medium.

The XRD patterns of GaAs NWs under high pressure up to 26.2 GPa are presented in Fig. 2(a). In the low pressure range (2.0–19.0 GPa), all the diffraction peaks of ZB phase become broadening and shift to the higher angles with increasing pressure. Up to 20.0 GPa, several diffraction peaks appear at around 14.37°, 14.74°, 15.39°, 20.56°, and 21.19°, which can be indexed to the OR phase.^[26] While at the same pressure, the intensity of the diffraction peaks of ZB phase decreases significantly. These results indicate that the phase transition from ZB to OR occurs at 20.0 GPa, which is higher than the transition pressure of bulk GaAs.^[21] With the pressure increasing a little bit to 20.9 GPa, all the diffraction peaks of ZB phase disappear, which suggests that the GaAs NWs completely transform into the OR phase. The whole phase transition process accomplishes in a shorter pressure range (20.0–20.9 GPa) comparing with that of bulk GaAs.^[21] Then, the OR phase remains stable up to 26.2 GPa, as all the diffraction peaks of OR phase can be observed without any new diffraction peaks.

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. (a) Synchrotron XRD patterns of GaAs NWs upon compression from 2.0 GPa to 26.2 GPa. Asterisks in the pattern measured at 20.0 GPa represent the diffraction peaks belonging to OR phase, ticks of OR phase are assigned at 23 GPa (Ref. [26]). (b) Synchrotron XRD patterns of GaAs NWs upon decompression from 26.2 GPa to 0.7 GPa.

The evolution of the XRD patterns of GaAs NWs in decompression process is shown in Fig. 2(b). All the diffraction peaks of OR phase remain down to 7.7 GPa and the (311) diffraction peak of ZB phase appears indicating the phase transition from OR to ZB. With further decreasing pressure to 1.7 GPa, all the diffraction peaks of OR phase disappear and only the diffraction peaks of ZB phase can be observed. These results suggest that GaAs NWs transform to the initial ZB phase in the depressurizing process. Comparing with the compression process, the OR–ZB transition in GaAs NWs shows ∼13 GPa hysteresis in transition pressure. For the pressure-induced reversible phase transition with evident volume collapse, the converse transition can be observed at lower pressure. The ZB–OR transition in GaAs is a first-order phase transition with large volume collapse and the similar hysteretic OR–ZB transition process upon decompression has already been observed in bulk GaAs.^[22,23]

While high pressure studies have focused on GaAs NWs with larger diameters (80–150 nm), the phase transition from ZB to OR was suggested irreversible. We believe that the reversibility of phase transition is mainly affected by the mechanical properties of the NWs with different diameters, for the size effect on the mechanical properties of GaAs NWs has been demonstrated in previous works.^[12–14] The special high pressure behaviors of GaAs NWs in this work could be attributed to the super elasticity and reversible plasticity observed in GaAs NWs with diameters in the range of 50–60 nm.^[28]

The cell volumes were determined by Rietveld refinement of XRD patterns using GSAS software package and the cell volumes at various pressures are shown in Table 1. Figure 3 shows the pressure–volume diagram of GaAs NWs. The cell volume of GaAs NWs at ambition pressure is 182(3.6) ų. At 20.0 GPa, the corresponding volume change is ∼16.5%. The experimental data of GaAs MWs were fitted to the third order Birch–Murnaghan equation of state (EOS)

Fig. 3.

	Figure Option ViewDownloadNew Window
	Fig. 3. Pressure–volume diagram of GaAs NWs. The black-solid circles represent the experimental data. The various color solid lines are the fitting results generated via the third-order Birch–Murnaghan equation of state with different fixed , and the red-dashed line is the fitting results without fixed .

Table 1.

Table 1.

Table 1. The cell volumes of ZB GaAs NWs at various pressures. . P/GPa V/Å3 0 182(3.6) 2.2 175(4.4) 5.8 168 (3.0) 9.7 164(4.5) 13.9 160(4.3) 15.4 159(2.9) 18.0 156(5.9) 19.0 154(2.3) 20.0 151(5.7)

Table 1. The cell volumes of ZB GaAs NWs at various pressures. .

Table 2.

Table 2.

Table 2. The bulk modulus B0 of ZB GaAs with fixed or unfixed . . 3.6 (fixed) 4.0 (fixed) 4.4 (fixed) 4.8 (fixed) 3.1 (not fixed) B0/GPa 89.9(6.7) 86.3(6.8) 82.9(6.9) 79.6(7.1) 94.4(20.2)

Table 2. The bulk modulus B0 of ZB GaAs with fixed or unfixed . .

To investigate the electronic properties of GaAs NWs under high pressure, in-situ IRR spectroscopy measurements were performed. Figure 4(a) shows the reflectivity spectra with pressure up to 25.6 GPa and the data between 1550 cm⁻¹ and 2700 cm⁻¹ were cut out from the spectra due to the absorption of diamond. In the pressure range of 1.9–16.7 GPa, there is almost no change of reflectivity. At 18.4 GPa, the reflectivity increases slightly. With the pressure further increasing to 19.3 GPa, a dramatic increase of reflectivity can be observed. Above 20.7 GPa, the reflectivity increases gradually. As shown in Fig. 4(b), the pressure dependence of reflectivity at low frequencies gives the electronic evolution under pressure, and higher reflectivity indicates higher carrier density in the measured samples.^[29] Below 18.4 GPa, the reflectivity shows an independent behavior with pressure, suggesting no evident increase of carrier density. The sharp increase of reflectivity can be observed at around 19.3 GPa, indicating the pressure-induced metallization of GaAs NWs. The metallicity of GaAs NWs can be further improved continually with pressure up to 25.6 GPa. The similar transition processes confirm that metallization originates from the phase transition (ZB to OR), which is consistent with the previous reports about bulk GaAs.^[22–24]

Fig. 4.

	Figure Option ViewDownloadNew Window
	Fig. 4. (a) IRR spectra of GaAs NWs up to 25.6 GPa. (b) Pressure–reflectivity diagram of two different wavenumbers (1500 cm−1 and 3000 cm−1) upon compression.

The reflectivity spectra of GaAs NWs in decompression process are shown in Fig. 5(a). The reflectivity changes gradually in the pressure range of 17.3–25.2 GPa. Below 14.8 GPa, the reflectivity decreases significantly until 8.9 GPa. With further decreasing pressure, the reflectivity changes slightly and recovers back to the initial value. More details can be found in the pressure dependence of reflectivity at low frequencies as shown in Fig. 5(b). The reflectivity spectra of the compression and decompression processes demonstrate pressure-induced reversible metallization in GaAs NWs in agreement with our XRD results.

Fig. 5.

	Figure Option ViewDownloadNew Window
	Fig. 5. (a) IRR spectra of GaAs NWs upon decompression at various pressures. (b) Pressure–reflectivity diagram of two different wavenumbers (1500 cm−1 and 3000 cm−1) upon decompression.

Due to the high surface to volume ratio and nanosize effects, the GaAs NWs have high surface energy which can affect the optical and mechanical properties, and even the stabilities under high pressure.^[18] Enhanced transition pressure with decreasing particle size has been demonstrated in several kinds of nanomaterials mainly due to the surface energy differences.^[30–33] It is reasonable that the hysteretic transition pressure of metallization (phase transition from ZB to OR) can be attributed to their high surface energy in GaAs NWs with smaller diameters.

In summary, the structural phase transition and electronic properties of GaAs NWs were investigated under high pressure by synchrotron XRD and in-situ IRR spectroscopy methods. A pressure-induced reversible phase transition from ZB to OR was observed at around 20.0 GPa. In the same pressure range, pressure-induced reversible metallization of GaAs NWs was confirmed by IRR spectra. The reversibility of phase transition depends on the mechanical properties of GaAs NWs. Compared to bulk materials, GaAs NWs show larger bulk modulus and enhanced transition pressure due to the size effects and high surface energy.