1 Introduction

During past decades, the semiconductor nanowire (NWs) has been interest due to their potential in low dimension such as nano science, nano material and nanotechnology [1–4]. Due to their low dimension properties, that they are controllable, tunable conductivity, variable and diameter dependent band gap, flexible surface chemistry, large surface e area, enhanced exciton binding energy, and increased surface scattering for electrons and phonons, etc. [1]. Especially, the Ga2O3/GaAs NW growth have attracted substantial attention for various applications on nanoelectronics, optoelectronics, and nanosensors, etc. [2–5], as well as the Ga2O3/GaAs NWs growth will expect many exciting opportunities on nanoscale science and technology [6–9].

Up to now, we have had many number of methods for purpose of synthesis the semiconductor Ga2O3 NWs such as vapor liquid solid—VLS, chemical vapor deposition—CVD, laser ablation catalytic growth—LCG, pulsed laser deposition—PLD, solution liquid solid—SLS, oxide assisted growth—OAG, and molecular beam epitaxy—MBE [1, 6–8, 10], and among them, the VLS method has been a closely and usefully method. Because research the VLS method began as early as in 1964 by Wagner et al. [10] for silicon whisker growth, and then this method has being applied for NWs growth on GaAs substrate. In this case, the VLS method strongly depends on many technological factors: the growing temperature, catalyst metal, vapor pressure and gas volume, the bulk and surface diffusion of the catalyst metal [7, 9, 11]. Secondly, the sizes of catalyst metal particles also strongly effect the seed nuclear formation on the solid surface, the features of the phase diagram of the catalyst metal–material system to NWs growth process [1, 7, 9, 12].

Although, there have had studies on the successful synthesis of Ga2O3/GaAs NW by using different catalyst metals and structures as: nano thin-films, and nanoparticle of Au, Ag, Pt, Fe, Cr, Ni materials. But most of them have deposited on the surface of wafer by physical deposition methods, for example thermal evaporation, direct current—DC/or radio frequency—RF sputtering, thermal effusion, etc. [9]. On the other hand, several kinds of deposition methods have existed high cost and difficultly controlled. In order to solve these drawbacks, another choice of the deposition catalyst metal is of importance to deposit materials on the GaAs surface [1, 9].

The sol–gel method has widely used to synthesize nanoscale materials, which can produce solid nanoparticle materials from small molecules (precursors). This mechanism uses either a chemical solution (sol short for solution) or colloidal particles (sol for nano particle) to form an integrated network in a continuous liquid (gel) [13]. In this case, the monomers (precursors) for synthesizing the Ag colloid consisted AgNO3 salt and HF acid solvent. And then, these nanoparticles were dispersed in a solid and deposited on the substrate by spaying, dipping or doctor blade method.

According to our knowledge on both experimental and theory model, there has not been a combination between sol–gel and VLS method to grow Ga2O3/GaAs NW, yet. Therefore, in this paper, continuing several our before works [6, 7, 11], we report the producing Ag nanoparticle (AgNP) on GaAs wafer by using sol–gel method, which has investigated to effects of the AgNP concentration, growth temperature on formation Ga2O3/GaAs NW by VLS method. The morphology results and structural property of Ga2O3 NW are also discussed.

2 Experimental procedures

The (100) GaAs semiconductor undoped single crystalline wafer with resistivity of 1.3–2.7 × 108 Ωcm was used as the substrate, which provided from AXT, Inc. The chemical HF 40 % solution, and 2.5 % (w/v) AgNO3 powder reagents provided from Sigma Aldrich, Alfa Aesar, respectively. Before experiments, the GaAs wafers were cut of 0.5 × 0.5 cm2 pieces. The Ag deposition on the GaAs surface was done by immersing the wafers in the aqueous solution of 0.4 mM AgNO3 and 0.14 M HF at 25 °C for 5, 20, and 35 s, respectively. According to previously reported of authors [14–17], we decided to use above concentration reagents and deposition time for AgNP high uniformity. Neither too sparse nor too dense, this condition chose to obtain the AgNP size diameter of range from 50 to 70 nm and ~1 × 1010 cm−2 of particle density, respectively. After silver coated, the samples were rinsed with deionized water and dried at room temperature by pure nitrogen gas. For steps of the NW growth process, with which are experimentally carried out and repeated many times on the different investigation temperatures range from 650 to 830 °C at the low vapor-pressure inside of ~10−1 Torr (listed Table 1). The growth temperature profiles in the closed quartz tube were automatically measured and controlled by a Keithley 2000 multimeter (see Fig. 2b). And then, the morphology of NWs were investigated by field emission scanning electron microscope (FE-SEM) (Hitachi, Japan) equipment, energy-dispersive X-ray (EDX) techniques on JED-2300 AnalysStation equipment.

Table 1 Technological conditions for the experiments of Ga2O3/GaAs NW growth by VLS method
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3 Experimental results and discussions

The Fig. 1a–c shown the AgNP densities and sizes on (100) surface GaAs substrate after AgNP deposited by the HF/AgNO3 solution. As same concentration of HF/AgNO3 solution and the different deposition times, we can see the density and size of AgNPs as a function of deposition time. The Fig. 1c shown the ~2 × 1010 of AgNP density, which is uniformly distributed in whole GaAs wafer surface. The diameter of the congregated particles are clearly observed ~70 nm of the short deposition time (Fig. 1a) and ~50 nm of long deposition time (Fig. 1c). Also, SEM cross-sectional image analysis of AG153 sample (Fig. 1d) shown that the average height and width of particles are ~70, ~50 nm in range, respectively. These results can use to estimate the diameters of Ga2O3 NWs, which will be grown by below VLS mechanism. From the SEM results of AgNPs-coated, we shall select AgNP concentration as in Fig. 1c to next experiments. According to technology conditions [6, 7, 11], especially growth temperature was exactly controlled by a Keithley 2000 multimeter with two-steps as Fig. 2b. For step one at 400 °C of T2 for 10 min, the AgNP will divide into smaller nanosize and adhere to the GaAs surface. These affected factors have still not understood yet to control in the first stage of VLS method, such as the contact angle, alloy temperature, and metal–crystalline. They can also use to determine the surface energy of a NW growth process [9, 18, 19].

Fig. 1
figure 1

Top view SEM images of AgNPs coated on the GaAs suface as a function of deposition time, a 5 s; b 20 s; c 35 s; and d SEM cross-sectional of AgNPs deposited on GaAs wafer for 35 s, as AG151, AG152, AG153 samples, respectively

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Fig. 2
figure 2

a The growth furnace scheme with 10−1 torr low vacuum quartz tube, and b the NWs growth by two steps temperature profile modes, where the growth temperatures T3 are near the eutectic temperature Te

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For step two at (from 650 to 830 °C range) T3 of different growth temperatures of this process (Fig. 2b), the gallium and oxygen atoms partly will be entered into the AgNP by surface absorb mechanism. Therefore, the NWs are immediately grown. So far, we also regard to investigate our experimental series due to the effect of NW morphologies at growth (T3) temperature, and AgNP sizes. At the T3 temperature, the NWs are continuously synthesized, in which will survey most of growth conditions depend on NW morphologies. The Fig. 3 shows the NW morphologies as a function of AgNP density that the NW successfully grown at 830 °C of substrate temperature and ~10−1 Torr of vacuum–pressure inside quarts tube. In this case, we will investigate the effect of NW growth process depend on silver NPs density. At the high AgNP density (Fig. 3a, b), the NW has been random grown with very long of few hundred micrometers of length, and plentifully of area surface, as well as they also have smaller in diameter. In contrast, they have larger diameter and shorter length, in which we can see existent of Ag droplets on the top of the NW (Fig. 3c) that this phenomenon is in good agreement with the previously conventional explanation on VLS method [6, 7, 9, 10]. Also, they looked like tangly whisker structure and sidewall smooth shape. The different morphologies of NWs are often appeared during the NW growth that due to effects of monocentric and polycentric nucleation and its combination with periodic stable and unstable growth. As a NW begins to grow, its height increases by an amount and the radius of the contact area decreases by an amount, then the NW growth continues, also the base of the NWs increases from the zero value follow Young’s model. The detail explains of this mechanism could be found elsewhere [1, 2, 7, 9–11]. As far, these NW morphologies (Fig. 3a, b) will better candidate to nano electric circuit, nanosensor, and applications in the future.

Fig. 3
figure 3

SEM image results of Ga2O3 NW morphologies which depend of different AgNP densities on GaAs surface at same growth temperature and pressure a 20 s, b 35 s, c 5 s of AgNP deposition times, as AG158, AG159, and AG1510 samples, respectively

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The Fig. 4 shows the investigation results of NW which depend on different substrate temperatures at the similar AgNP density. As you see the Fig. 4a–c, the NWs have successfully synthesized as a function of substrate temperature, that mean the NWs can easily grow at the higher substrate temperature (~Te), as well as longer in length. This believed due to low impurity incorporation at high growth temperature [9]. To more detailed explain, we need to understand eutectic point (Te) of Ag–GaAs alloy [20] and the important role of AgNP catalytic during growth process. At the nearly Te (Fig. 4d), and phase edge region of catalytic liquid alloy, in which can rapidly absorb a vapor to become stability levels of thermodynamic equilibrium state, and then a new crystal growth can subsequently appear as nucleated seed at the liquid–solid interface. However, if growth temperature is too high (above Te) causing a significant amount of gallium and oxygen desorption then reduce the NW quality. On the other hand, the high substrate will reduce impurity of alloy and just only alloy phase desired to form NW nuclear seeds [9], as well as the high growth temperature (>850 °C) is unacceptable for the real usage. But the advantages of high growth temperature is offset (~Te) by the degrading effect of vacancies from contamination.

Fig. 4
figure 4

SEM image results of Ga2O3 NW morphologies which depend of different growth temperature with similar AgNPs density on the GaAs surface a 650 °C, b 700 °C, c 780 °C d 830 °C, as AG154, AG155, AG156, and AG157 samples, respectively

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The Fig. 5 shows EDX measurement result of Ga2O3 NW on AG159 sample which grown at 830 °C of substrate temperature for 30 min. Among NW length, the structure composition of NW has similar chemical element. From the EDX results, the wt% composition magnitudes of NW configuration are Oxygen of 47.99 wt%, Gallium of 37.60 wt% and small Carbon amount impurity of 7.10 wt%. Also, we obtained the Arsenic of 7.31 wt%. Evidence of the residual As molecular small amount was absorbed on the NW surface by physical absorption during cooling down process of the substrate. Nevertheless, this As amount will not effect on NW structure and they also will disappear at As vapor temperature/or high temperature. To deeply understand these mechanisms, we can consider the surface physical absorption/or desorption mechanism.

Fig. 5
figure 5

EDX results of Ga2O3 NWs grown at 830 °C for 30 min with line scan on AG159 sample

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4 Conclusions

In summary, we have successfully synthesized Ga2O3/GaAs NWs using VLS method based on Ag catalytic metal NPs deposited on the GaAs substrate by chemical deposition of HF/AgNO3 solution and investigated the effects of Ag catalyst as well as the NW growth properties. During investigation process, we finally found that: (a) the NW can random grow at anywhere of GaAs wafer surface area and the NW morphology strongly depend on the AgNP density, (b) the NW growth temperature is found around the Te: 700–830 °C, (c) the two-step temperature modes for dividing Ag catalyst metal particles into smaller size and grow NW at low vacuum–pressure (~10−1 Torr), at which can synthesis longer of length and control diameter of NWs depend of AgNP size. These results indicated also that the VLS mechanism is still dominated with Ag droplet on the top of NW. Therefore, in future, these Ga2O3 NWs are good candidate for the micro/or nano device applications.