The Self-Catalyzed Growth of GaAsSb Nanowires on a Si (111) Substrate Using Molecular-Beam Epitaxy

Abstract

GaAsSb semiconductor material, a ternary alloy, has long been recognized as a crucial semiconductor in the near infrared range due to its ability to finely adjust the wavelength through controlling the Sb component. In this work, we report on the pattern of orientation variation in self-catalyzed grown GaAsSb nanowires (NWs). Utilizing solid-source molecular-beam epitaxy (MBE), self-catalyzed GaAs and GaAsSb nanowires (NWs) were grown on Si (111) substrates. The influence of various Sb components on the growth direction of the nanowires in the ternary GaAsSb alloy was examined using scanning electron microscopy (SEM). The inclusion of Sb components was discovered to alter the growth direction of the nanowires, transitioning them from a vertical and inclined orientation to a configuration that encompassed vertical, inclined, and parallel orientations with respect to the Si (111) substrate. As the Sb component in GaAsSb increased, there was an increased likelihood of the nanowires growing parallel to the surface of the Si (111) substrate. A combination of X-ray diffraction (XRD) and Raman spectroscopy validated the presence of Sb components and indicated a high crystalline quality. Additionally, XRD confirmed that the Sb components aligned with the intended structure. These findings establish a solid material foundation for the development of high-performance GaAsSb-based devices.

1. Introduction

III-V semiconductor nanowires (NWs) exhibit distinctive electrical and optical characteristics that render them highly advantageous for various nanoscale optoelectronic devices [1], including transistors [2,3], light-emitting diodes (LEDs) [4,5], nanolasers [6,7], and solar cells [8,9,10]. Semiconductor silicon holds a paramount position within the industry chain as the most crucial semiconductor material. It bears the highest cost proportion among all semiconductor materials. Moreover, Si substrates possess numerous advantages, including a superior crystal quality, expansive size, and economical production costs. The growth of III-V semiconductor nanowires (NWs) on Si substrates opens up avenues for an affordable platform that enables high-performance nano-based devices [11,12]. This is primarily attributed to the elastic relaxation exhibited at the III-V/Si heterointerface [13,14]. By growing III-V ternary alloys as NWs on Si, a broad range of compositions with significant strain and adjustable properties can be achieved, which is challenging to accomplish in the planar geometry. The narrow-bandgap ternary alloy offers a remarkable bandgap tunability spanning a wide range, from approximately 870 nm (GaAs) to 1720 nm (GaSb) [15,16]. This range features a high absorption coefficient and excellent carrier mobility characteristics, and it covers the wavelengths relevant to optical communications [17,18]. To date, the self-catalyzed vapor–liquid–solid (VLS) technique has remained the primary approach for growing the majority of ternary nanowires (NWs). This technique has proven instrumental in enabling the growth of intricate NW structures, including quantum well or core-multishell NWs [16,17,18,19,20]. These advancements pave the way for the expanded utilization of such NWs in electronic and optoelectronic devices. Furthermore, the self-catalyzed method has been proven to be successful for the growth of III-V-V NWs, leading to a high yield of ternary GaAsSb and GaAsP NWs on Si substrates [21,22]. Notably, the bandgap of these NWs can be finely adjusted simply by modifying the flux of group-V elements.

The Sb-based III-V-V nanowire (NWs) system holds significant technological importance due to its unique properties, including the regulation of bandgaps and high carrier mobilities. Because of these distinctive characteristics, it is in high demand for the fabrication of optoelectronic devices that require a high-speed performance and low power consumption [23,24,25]. Following the successful preparation of defect-free zinc blende GaAsSb nanowires by Dheeraj et al. in 2008 [26], research on the growth and device applications of GaAsSb nanomaterials has continued to be investigated and documented. Multiple studies have explored the advancement of GaAsSb nanowires (NWs) using diverse growth techniques. For instance, GaAs_1−xSb_x NWs, catalyzed by Au, have been successfully cultivated on GaAs nanowire stems, employing methods such as metal-organic vapor phase epitaxy (MOVPE) and metal-organic chemical vapor deposition (MOCVD) [27,28]. Additionally, Au-catalyzed GaAsSb or GaSb NWs have also been grown on Si (111) substrates through the utilization of chemical vapor deposition (CVD) [29,30]. Nevertheless, during the nanowire growth process, the inclusion of Au introduces contamination and adversely impacts the crystal quality of the nanowires. Moreover, the unintentional introduction of Au can have severe consequences for the functionality of the device [31]. As a result, an increasing number of researchers have shifted their focus towards exploring autocatalytic growth methods for GaAsSb nanowires. One such technique gaining prominence is Molecular-Beam Epitaxy (MBE), which enables layer-by-layer growth at the atomic level under ultrahigh vacuum conditions (background vacuum environment ~10⁻¹⁰ Torr). In MBE, molecules or atomic beams undergo physical and chemical reactions on a heated substrate to facilitate the growth process. The utilization of MBE technology represents a significant breakthrough in the growth of self-catalyzed GaAsSb nanowires. The preparation method primarily involves Si substrate etching and GaAs stem formation, among others [22,23,32]. However, the direct epitaxial growth of self-catalyzed GaAsSb nanowires on Si substrates remains relatively limited. Moreover, the current research emphasis primarily revolves around the investigation of the crystal structure, electronic properties, and optical characteristics [26,33,34]. Consequently, there is an urgent need to explore and comprehend the direct growth of ternary alloy GaAsSb nanowires on Si substrates and gain insights into their growth mechanism during this stage of development.

In this study, we present the direct growth of self-catalyzed GaAs NWs and ternary GaAsSb NWs on Si (111) substrates using solid-source MBE. The growth direction of the GaAs nanowires (NWs) was observed using scanning electron microscopy (SEM) imaging, revealing both vertical and tilted orientations with respect to the Si (111) substrate. With the introduction of Sb components, the NWs demonstrated a tendency to grow along the surface of the Si (111) substrate. Moreover, as the Sb content increased, there was an increased likelihood of the GaAsSb nanowires to grow parallel to the substrate surface. This phenomenon could be attributed to the surfactant effect of the Sb, which influenced the growth direction of the GaAsSb NWs. A growth mechanism model was proposed to explain the impact of the Sb surfactant on the growth direction of the nanowires. Furthermore, both X-ray diffraction (XRD) and Raman spectra analyses confirmed the presence of Sb components in the nanowires. The XRD results also demonstrated the successful control of the Sb flux, as evidenced by the measured Sb fractional flux (FF_Sb).

2. Materials and Methods

2.1. Si (111) Substrates Treatment

To prepare the Si (111) substrates, a thorough cleaning process was conducted using ultrasonic cleaning with ethanol. Initially, the Si (111) substrate was immersed in an ethanol solution and subjected to ultrasonic treatment for 5 min to eliminate surface impurities. Subsequently, the substrate was dried using a nitrogen gun to ensure the removal of any residual water or organic residues. To further optimize the substrate’s cleanliness, several crucial degassing steps were performed. Firstly, the substrate was placed in the MBE load-lock system, creating a vacuum environment between 10⁻⁸ and 10⁻⁹ Torr, and heated to 200 °C for 2 h to remove any remaining gases. Afterward, the substrate was transferred to the MBE pre-treatment chamber, where it underwent a 2 h degassing process at a temperature of 400 °C. Finally, the substrate was transferred to the MBE growth chamber for an additional 30 min degassing step at 750 °C before the growth process commenced.

2.2. Nanowire Growth

Three sets of self-catalyzed GaAs and GaAsSb NWs were grown on Si (111) substrates using the Finland DCA P600 MBE system, denoted as samples A, B, and C, respectively.

Table 1. Growth conditions summary for the three nanowire samples.

Samples (NWs)	Annealing	Growth	GaAs	GaAsSb
	Flux (Torr)	Temperature (°C)	Flux (Torr)	Flux (Torr)
A (GaAs)	As: 1.6 × 10−6	620	Ga: 6.2 × 10−8 As: 1.6 × 10−6	/
B (GaAsSb)			/	Ga: 6.2 × 10−8 As: 1.6 × 10−6 Sb: 1.1 × 10−7
C (GaAsSb)				Ga: 6.2 × 10−8 As: 1.6 × 10−6 Sb: 4.5 × 10−7

provides a summary of the different growth conditions, including the growth temperature and flux for each sample. During the annealing process, all the samples were immersed in an As flux with a beam equivalent pressure of 1.6 × 10⁻⁶ Torr. The growth temperature for all three samples was maintained at 620 °C. Sample A (GaAs NWs) employed a Ga flux, As flux, and V/III ratio of 6.2 × 10⁻⁸ Torr, 1.6 × 10⁻⁶ Torr, and 25.8, respectively. Sample B (GaAsSb NWs) utilized a Ga flux, As flux, Sb flux, and V/III ratio of 6.2 × 10⁻⁸ Torr, 1.6 × 10⁻⁶ Torr, 1.1 × 10⁻⁷ Torr, and 27.6, respectively. Sample C (GaAsSb NWs) was grown with a Ga flux, As flux, Sb flux, and V/III ratio of 6.2 × 10⁻⁸ Torr, 1.6 × 10⁻⁶ Torr, 4.5 × 10⁻⁷ Torr, and 33.1, respectively.

2.3. Microscopy Characterizations

The morphological properties of the GaAs and GaAsSb nanowires were examined using Hitachi S-4800(Hitachi, Tokyo, Japan) field-emission scanning electron microscopy (FE-SEM). The FE-SEM was operated at an acceleration voltage of 5.0 kV and the working distance (WD) varied between 4.6 and 8.2 mm. The structural and chemical composition analysis of the three samples was conducted using Rigaku Ultima IV (Rigaku, Tokyo, Japan)X-ray diffractometer (XRD). The XRD measurements utilized Cu Ka radiation (λ = 1.54056 Å) with a scanning rate of 3°/min and a step size of 0.02°. The Raman spectra of the GaAs and GaAsSb nanowires were captured using a Horiba LabRAM HR Evolution Raman Spectrometer (Horiba, Lille, France) with backscattering geometry. The measurements were conducted at room temperature and a 532 nm wavelength laser was used as the excitation source for the Raman spectroscopy.

3. Results and Discussion

3.1. NWs Morphology and Growth Direction

In the self-catalyzed growth process of nanowires, the control of components is achieved by regulating the flux. Specifically, in the case of ternary GaAs_1−xSb_x alloy nanowires, the introduction of antimony (Sb) into the nanowires was carefully managed by adjusting the Sb fractional flux (FF_Sb) supplied to the MBE chamber. The Sb fractional flux (FF_Sb) is defined as the ratio of the Sb flux to the combined flux of the group-V precursors (As and Sb). It can be given by:

$$F{F}_{\mathit{Sb}}=\frac{F_{\mathit{Sb}}}{F_{\mathit{Sb}}+F_{\mathit{As}}}$$

(1)

where F_Sb represents the Sb flux and F_As represents the As flux. In the case of the GaAs_1−xSb_x nanowires, the As flux remained constant at a magnitude of 1.6 × 10⁻⁶ Torr, while the Sb flux exhibited variations ranging from 0 Torr to 4.5 × 10⁻⁷ Torr, resulting in corresponding fluctuations in the Sb fractional flux (FF_Sb) within the range from 0% to 21.95%. Figure 1 displays representative SEM images of the reference GaAs nanowires (sample A), as well as the GaAsSb nanowires (sample B and sample C). The morphologies of the GaAsSb ternary alloy NWs with low Sb compositions are shown in Figure 1c,d, and the morphologies of the GaAsSb alloy NWs with high Sb compositions are shown in Figure 1e,f. As shown in Figure 1a,b, it can be seen that there were two morphologies of GaAs NWs (sample A), vertical and inclined, oriented on the Si (111) substrate, respectively. However, with an increase in the Sb components, the morphologies of the NWs (sample B) were changed from two to three growth directions (shown in Figure 1c), which were perpendicular, inclined, and grew along the surface of Si substrate, respectively. As the Sb component increased, the three growth directions of the GaAsSb nanowires (sample C) persisted, but there was an increased proportion of GaAsSb nanowires lying horizontally on the Si substrate, as depicted in Figure 1e. In Figure 1c,e, the red single arrows are used to indicate the GaAsSb nanowires that grew parallel to the surface of the Si substrate. The side-view SEM images of the GaAs nanowires (Figure 1b) and GaAsSb nanowires (Figure 1d,f) yielded similar results to the 45°-tilted SEM images (Figure 1a,c,e). Moreover, as the Sb component in the GaAsSb ternary alloy increased, the proportion of vertically oriented nanowires decreased, while the quantity of nanowires growing along the Si substrate surface gradually increased. When the Sb component surpassed a certain threshold, the surfactant effect of Sb [35] became pronounced, preventing the growth of GaAsSb nanowires on the Si (111) substrate through MBE. This resulted in the formation of poorly compact GaAsSb films due to the accumulation of multiple horizontally aligned nanowires.

Figure 2 illustrates the calculation of both the nanowire density and ratio of GaAsSb ternary alloy nanowires grown parallel to the substrate surface, considering the various Sb components. As the FF_Sb increased, there was a corresponding rise in the number of nanowires growing parallel to the substrate surface. This trend became more pronounced with higher FF_Sb values, eventually reaching a point where all the nanowires exclusively grew along the surface of the substrate. This phenomenon could be attributed to the surfactant effect of the Sb, which will be further elucidated below. Notably, the variations in the Sb content (FF_Sb) did not noticeably impact the nanowire density, which remained within the range from 2.5 to 3.0 × 10⁸ cm⁻², as depicted in Figure 2. This was primarily due to the consistent treatment of the Si substrate, which promoted experiment repeatability and ensured minimal changes in the nanowire density.

3.2. NWs Growth Mechanism

To better illustrate how the Sb surfactant affected the growth direction of the GaAs_1−xSb_x NWs, we will now delve into a detailed discussion of the self-catalyzed growth mechanism of GaAsSb NWs. Figure 3 illustrates the model representing the growth mechanism of GaAs_1−xSb_x NWs. The process began by introducing a processed Si substrate into the growth chamber. Subsequently, the growth process was initiated with the introduction of Ga flux. During the pre-growth annealing step, pores were formed and Ga droplets were deposited inside these pores, serving as nucleation sites for the nanowires. Figure 3a,b illustrate the growth processes of the GaAs nanowires (NWs) and GaAsSb NWs, respectively. In Figure 3a, it can be observed that the Ga droplets deposited during the initial stage of the GaAs NWs’ nucleation remained relatively unchanged. These Ga droplets exhibited large contact angles (β) with the Si substrate, facilitating the growth of vertical or tilted NWs towards the Si substrate. Figure 3b illustrates the growth of the GaAsSb NWs. Following the deposition of the Ga droplets, As and Sb fluxes were introduced. The presence of the Sb surfactant altered the contact angle between the Ga droplets and the substrate, causing the GaAsSb NWs to preferentially grow along the surface of the substrate. As the FF_Sb increased, the influence of the Sb surfactant effect became more pronounced. Consequently, the GaAsSb NWs growing on the Si substrate were uniformly aligned parallel to the substrate surface. This explains the observation that the GaAs NWs tended to grow vertically or at an angle to the Si substrate, whereas the GaAsSb NWs appeared to grow predominantly along the surface of the substrate. This difference in growth behavior could be attributed to the increasing influence of the Sb surfactant as the FF_Sb level rose. The shape and contact angle of the catalyst Ga droplet played crucial roles in altering the free surface energy at the VLS three-phase line [36], as depicted in Figure 3c. In particular, the presence of Sb concentration led to a reduction in the surface energy at the growth front. From a thermodynamic perspective, the growth mode of the atoms in the crystal layer on the substrate was determined by the balance of the free energy of the equilibrium surface. By rearranging the Young’s equation [37], we can deduce the relationship between the contact angle and the surface energy dependency:

$${\gamma }_{\mathrm{sv}}={\gamma }_{\mathrm{sd}}+{\gamma }_{\mathrm{dv}}\mathit{cos} \beta$$

(2)

$$\beta ={\mathit{cos}}^{-1} \frac{{\gamma }_{\mathrm{sv}}-{\gamma }_{\mathrm{sd}}}{{\gamma }_{\mathrm{dv}}}$$

(3)

where β is the contact angle, and γ_sv, γ_sd, and γ_dv are the surface energies of the solid–vapor interface, solid–droplet interface, and droplet–vapor interface, respectively. The influence of the Sb surfactant caused a reduction in the surface energy at the solid–droplet interface and/or the droplet–vapor interface during the nanowire nucleation process. As a consequence, the contact angle between the droplets and substrate decreased, leading to challenges in the growth of the GaAsSb nanowires. This phenomenon also accounts for the difficulty in achieving high Sb component III-As_1−xSb_x nanowires on silicon substrates.

3.3. NWs Structure

To analyze the composition, structure, and crystallization of the GaAs_1−xSb_x alloy nanowires, XRD investigations were conducted, as depicted in Figure 4a. The XRD measurements were performed in the 2θ range from 24° to 32° with a scanning step of 0.02°. In the case of the GaAs nanowires (sample A), two peaks were observed at 27.31° and 28.48°. The higher peak originated from the diffraction of the Si (111) substrate, while the other peak corresponded to the GaAs (111) plane, in accordance with the GaAs entry (#32-0389) in the Joint Committee on Powder Diffraction Standards (JCPDS) database. These XRD investigations aimed to gain insights into the composition, structure, and crystalline nature of the ternary GaAs_1−xSb_x alloy nanowires. Figure 4a displays the XRD patterns of GaAs nanowires as a reference. In the case of the GaAsSb nanowires (sample B and sample C), aside from the Si (111) diffraction peak at 28.48°, additional diffraction peaks were observed around 27.14° and 26.80°, respectively. These peaks were shifted to smaller angles compared to the GaAs (111) diffraction peak. By referring to the GaSb entry (#07-0215) in the JCPDS standard database, the diffraction peak of GaSb (111) was identified at 25.28°. Consequently, the diffraction peaks observed at 27.14° and 26.80° were determined to correspond to GaAsSb (111) originating from the ternary alloy GaAs_1−xSb_x nanowires. The lattice constant of the GaAs_1−xSb_x nanowires was calculated using Bragg’s law:

$$2d_{hkl}\mathit{sin} \theta =n\lambda$$

(4)

where λ represents the wavelength of X-ray photons (1.54056 Å) and d_hkl denotes the interplanar spacing. The diffraction angle on the plane (hkl) is denoted by θ, while h, k, and l represent the Miller indices. The relationship between the lattice constant a and the spacing of the crystal planes can be expressed as follows:

$$d_{hkl}=\frac{a}{\sqrt{h^2+k^2+l^2}}$$

(5)

The lattice constants of the GaAs_1−xSb_x samples were determined to be approximately 5.653 Å (sample A), 5.684 Å (sample B), and 5.753 Å (sample C), respectively. These lattice constants exhibited a linear relationship with the Sb content, in accordance with Vegard’s law:

$$a_{GaA{s}_{1-x}S{b}_x}=\left(1-x\right)a_{GaAs}+x{a}_{GaSb}$$

(6)

Consequently, the Sb contents of the GaAsSb nanowires were estimated to be approximately 6.97% (sample B) and 22.46% (sample C), respectively. The relationship between the Sb component and lattice constant, as well as the fractional flux ratio of Sb (FF_Sb), were established and depicted in Figure 4b. Figure 4b illustrates the lattice constant and reveals an approximately linear relationship between the Sb content (x_Sb) and fractional flux ratio of Sb (FF_Sb). For the GaAsSb nanowires with an FF_Sb of approximately 6.43%, the resulting component error range obtained from the FF_Sb and XRD analyses (x_Sb ≈ 6.97%) was approximately 8.4%. Similarly, for the GaAsSb nanowires with an FF_Sb of around 21.95%, the component error range obtained from the FF_Sb and XRD analyses (x_Sb ≈ 22.46%) was approximately 2.3%. This comparison demonstrates that the discrepancy between the Sb components in the ternary alloy GaAsSb nanowires, as determined by the FF_Sb and XRD, was less than 10%. This indicates that the Sb contents (x_Sb) of the GaAs_1−xSb_x NWs was primarily determined by the FF_Sb during the fabrication process of the nanowires.

3.4. Raman Spectra

To gain further insights into the microscopic lattice structure associated with the integration of Sb in GaAsSb NWs, we conducted Raman spectroscopy, as depicted in Figure 5. Our previous study [16] included a Raman analysis of GaSb and GaAs NWs, revealing two distinct peaks at approximately 225.5 and 234.2 cm⁻¹ for the GaSb’s transverse optical (TO) and longitudinal optical (LO) modes, and 267.61 and 291.86 cm⁻¹ for the GaAs’s TO and LO modes, respectively. Figure 5a displays the Raman spectra of the GaAs NWs, which exhibited two mode peaks at around 267.4 and 291.3 cm⁻¹, corresponding to the GaAs TO and GaAs LO modes, respectively. Considering the vibrational aspects, the Raman spectrum of a ternary alloy comprises the phonon modes derived from the two constituent binary alloys involved in its synthesis. In the case of GaAsSb NWs, Figure 5a reveals the presence of four distinct Raman mode peaks, which could be attributed to the GaSb-like transverse optical (TO), GaSb-like longitudinal optical (LO), GaAs-like transverse optical (TO), and GaAs-like longitudinal optical (LO) modes. In comparison to the GaAs NWs, the GaAsSb NWs exhibited a noticeable red-shift in the GaAs-related transverse optical (TO) and longitudinal optical (LO) mode peaks, indicating a lower frequency. Conversely, the blue-shift observed in the peak could be attributed to the substitution of As atoms with Sb atoms. On the contrary, the primary transverse optical (TO) mode peak associated with GaSb exhibited a blue-shift when compared to the GaSb NWs [16], which could be attributed to the substitution of Sb with As. Furthermore, in comparison to the GaAsSb nanowires with an FF_Sb of 6.43% and an FF_Sb of 21.95%, a novel Raman mode peak emerged in the vicinity of 303 cm⁻¹ [38]. This newly observed peak could be attributed to the Si-2TA peak and arose due to the sparsity of the GaAsSb NWs (refer to Figure 1e).

To provide a more detailed analysis of the impact of the Sb composition on the peak positions of the Raman mode peaks related to the GaAs and GaSb, we employed multi-Lorentzian fitting on the GaAsSb NWs (sample B and sample C), as depicted in Figure 5b,c. In the case of the GaAsSb NWs with an FF_Sb of approximately 6.43%, four distinct modes, corresponding to peak positions of 226.2, 233.7, 258.6, and 275.8 cm⁻¹, were clearly observed (as shown in Figure 5b). Similarly, for the GaAsSb NWs with an FF_Sb of around 21.95%, four modes were detected with peak positions of 225.8, 233.2, 256.5, and 270.5 cm⁻¹, respectively (as shown in Figure 5c).

Table 2. Raman peak positions of the GaAs and GaAsSb NWs.

Samples	GaSb TO (cm−1)	GaSb LO (cm−1)	GaSb-Like TO (cm−1)	GaSb-Like LO (cm−1)	GaAs-Like TO (cm−1)	GaAs-Like LO (cm−1)	GaAs TO (cm−1)	GaAs LO (cm−1)	Ref.
GsAs NWs	/	/	/	/	/	/	267.61	291.86	[16]
GaSb NWs	225.5	234.2	/	/	/	/	/	/
A	/	/	/	/	/	/	267.4	291.3	Our work
B	/	/	226.2	233.7	258.6	275.8	/	/	Our work
C	/	/	225.8	233.2	256.5	270.5	/	/	Our work

presents the Raman mode peaks observed in the GaAs_1−xSb_x NWs. When comparing the GaAsSb NWs (sample B and C) to the GaAs and GaSb NWs, it was evident that the transverse optical (TO) and longitudinal optical (LO) modes of the GaSb and GaAs exhibited a red-shift. This red-shift could be attributed to an increase in the Sb components within the GaAsSb NWs (sample B and C). Consequently, the detection of these specific Raman mode peaks through Raman spectroscopy served as further evidence of the presence of Sb components and their effective and uniform integration into the NWs.

4. Conclusions

In conclusion, this study successfully grew self-catalyzed GaAs and GaAsSb nanowires with different Sb fractions on Si substrates using MBE. The Sb surfactant effect significantly influenced the NWs’ growth direction, and controlling the Sb fraction effectively controlled the Sb component in the GaAs_1−xSb_x NWs. The integration of Sb into the NWs was confirmed using XRD and Raman spectra, showing a high crystallization quality. The self-catalyzed growth of high-quality GaAsSb nanowires directly on Si substrates addresses the limitations of epitaxial growth and streamlines the material synthesis process. This breakthrough not only simplifies the fabrication steps, but also opens up exciting opportunities for preparing high-performance nano-devices and leveraging their potential applications in the near-infrared range.