Applied Science and Convergence Technology 2022; 31(6): 141-144
Published online November 30, 2022
Copyright © The Korean Vacuum Society.
aDepartment of Electronic and Information Materials Engineering, Division of Advanced Materials Engineering, and Research Center of Advanced Materials Development, Jeonbuk National University, Jeonju 54896, Republic of Korea
bDepartment of Physics, Kangwon National University, Chuncheon 24341, Republic of Korea
We report the structural and optical properties of InN nanowires (NWs) formed on p-type Si(111) substrates using a plasma-assisted molecular-beam epitaxy. The InN NWs were formed on a Si(111) substrate using a new growth method, the indium (In) pre-deposition (InPD) method, in which In droplets are initially formed by supplying only In flux to the substrate, where they work as nucleation sites for the formation of subsequent InN NWs. Field-emission scanning-electron microscopy images show that the InN NWs have symmetric shapes along the vertical direction. In addition, most of the InN NWs were unidirectionally grown in the direction perpendicular to the substrate. Strong and narrow peaks corresponding to InN(0002) can be clearly observed in the double crystal X-ray diffraction rocking curves of the NW samples. In the transmission electron microscopy images of the InN NWs, stacking faults, typically observed in Si-based III-nitride semiconductors, are rarely observed. A strong free-exciton peak was observed from the InN NWs at the wavelength of 1297 nm with a narrow linewidth at room temperature. Structural and optical characterizations of the NW samples indicate that highly crystalline InN NWs were formed on Si(111) using the new InPD growth method.
Keywords: Plasma-assisted molecular-beam epitaxy, InN nanowires, Si, In pre-deposition method, High crystallinity
Recently, III-nitride semiconductors are being actively studied for optoelectronic applications including solar cells, photodetectors, lightemitting diodes, and optical sensors operating in the wavelength window from ultraviolet to near infra-red (NIR). The wide operation wavelength is typically obtained by manipulating the composition of group III elements in (Ga,In,Al)N . Among the III-nitride materials, InN has the highest electron mobility and saturation velocity due to its smallest effective electron mass. Because the InN epitaxial layer shows a band gap from 0.6 to 1.1 eV [2–4], it is suitable for optoelectronic devices operating in the NIR wavelength window, especially those operating at 1.3 and 1.55 µm [5,6]. According to previous reports, InN is typically formed on Si or sapphire [7,8]. However, the formation of highly crystalline InN layers is very difficult due to significant differences in material properties, including lattice parameters and coefficient of thermal expansion between InN and substrate. In addition, dissociation of In atoms may be significantly higher than those of other group-III atoms, Ga and Al, during epitaxial growth. More seriously, there is significant In segregation even at low growth temperature [7–9]. These intrinsic growth-limiting factors make it is difficult to obtain highly crystalline InN films on Si or sapphire. To overcome this intrinsic problem of the epitaxial layer, InN nanowires (NWs) are attracting much attention because their small footprints can relax the strain originating from lattice mismatch through the NW sidewall . Also, for nanophotonic and nanoelectronic device applications, onedimensional structures such as NWs and nanorods are promising due to their strong anisotropic optical absorption, directionality of light emission, and confined charge-carrier transport [10,11]. For the formation of InN NWs on a Si substrate, there has been wide adoption of the vapor-liquid-solid method, in which metal catalysts such as Au and Ni are used as nucleation sites for NWs. However, the metal catalysts show high probability of remaining inside or on the top surfaces of the InN NWs, resulting in the introduction of deep trap levels for carriers and consequent degradation of optical and electrical signals [12,13]. To overcome the problems caused by the use of metal catalysts, the Volmer-Weber mode has been used to promote self-catalyst and catalyst-free formation of InN NWs on Si. However, the formation of highly crystalline InN NWs with high structural symmetry remains difficult. Moreover, the growth direction of the NWs is random with respect to the substrate, causing difficulty in the device fabrication process . According to previous reports, Si-based InN NWs had asymmetric, conical or tapered structures . For example, Stoica et al. reported that InN NWs formed on Si were tapered and nonuniform along the vertical direction . The asymmetric shapes of the NWs caused significant Fermi-level fluctuations along the NW axial direction, resulting in degradation of one-dimensional properties and of overall device performance [17–20]. However, the previously reported InN NWs still had asymmetric and non-uniform shapes with random growth direction [21,22]. Considering these results, it is essential to obtain a comprehensive understanding of the overall growth mechanism so that highly crystalline InN NWs having symmetric and uniform shapes can be formed.
In this paper, we report a newly proposed growth method, the In pre-deposition (InPD) method, to improve the structural properties, including the shape uniformity, of InN NWs. In droplets were initially formed using the InPD method on a Si(111) substrate and act as nucleation centers for subsequent growth of InN NWs. Morphology of InN NWs was manipulated by controlling growth temperature and V/III flux ratio, which is defined as the ratio of the relative amount of nitrogen (N) flux to In flux. Field-emission scanning electron microscopy (FE-SEM), double-crystal X-ray diffraction (DCXRD), and aberration-corrected (Cs) transmission electron microscopy (TEM) measurements were used to analyze the structural properties of the InN NWs. Photoluminescence (PL) spectroscopy was used to analyze the optical properties of the InN NWs.
A schematic diagram of the growth of InN NWs using InPD method is shown in Fig. 1. InN NWs were formed on a p-type Si(111) substrate using a plasma-assisted molecular-beam epitaxy system. In the first growth process of InN NWs, because there is a naturally formed native oxide layer on the surface of the Si(111) substrate, annealing was performed at 900 °C for 1 h inside a chamber to remove it. Because the bonding energy between In and N atoms is stronger than that between In and Si atoms, a nitridation process, in which N-plasma flux was provided only to the Si(111) substrate at a growth temperature of 800 °C, was performed [17,23]. After the nitridation process, SiN
Figures 2(a)-(d) shows plan-view (top) and cross-sectional (bottom) FE-SEM images of the NW1, NW2, NW3, and NW4 samples, respectively. In the plan-view FE-SEM images, the surfaces of the InN NWs show hexagonal shapes, indicating that InN NWs have wurtzite (WZ) crystal structures. The average lengths of the NW1, NW2, NW3, and NW4 samples were found to be 308, 407, 529, and 565 nm, respectively. Average length of the InN NWs increases with increasing growth temperature, largely due to the increase in the effective diffusion length of In adatoms. This result can be explained by the following equation .
The V/III ratio was reduced from 180 to 111 at a fixed growth temperature of 500 °C, to evaluate its effect on the shape and uniformity of InN NWs. Figures 3(a)-(c) shows plan-view (top) and cross-sectional (bottom) FE-SEM images of the NW2, NW5, and NW6 samples, respectively. The average lengths of the NW2, NW5, and NW6 samples were 407, 466, and 490 nm, respectively. As the V/III ratio decreased, the average length of the InN NWs increased and the degree of change to tapered shape intensified because the N-blocking effect on the migration of In adatoms from the substrate to the top surface of NWs through their sidewall decreased .
Figure 4(a) shows DCXRD patterns of InN NW samples according to growth temperature. DCXRD peaks corresponding to the (0002) crystal plane of InN were observed at 31.30, 31.26, 31.23, and 31.22 ° for the NW1, NW2, NW3, and NW4 samples, respectively. These results are consistent with InN of WZ crystal structure. The peak position corresponding to the (0002) crystal plane of InN increased slightly with decreasing average length of NWs. This peak shift was due to the decrease in tensile strain resulting from the difference in material properties between InN and Si(111) . The full width at half maximum (FWHM) values of the peaks for the (0002) crystal plane of InN were measured as 0.18, 0.16, 0.17, and 0.19 ° for the NW1, NW2, NW3, and NW4 samples, respectively. All FWHM values of InN NWs were significantly narrower than those (~ 0.46 °) in previously reported results [27,28]. This result indicates that InN NWs formed on Si(111) have high crystallinity. Figure 4(b) shows DCXRD patterns of the InN NW samples grown at three different V/III ratios. DCXRD peaks corresponding to the (0002) crystal plane of InN were observed at 31.26, 31.24, and 31.24 ° for the NW2, NW5, and NW6 samples, respectively. With decreasing V/III ratio, the peak position decreased slightly; this is also attributed to the increase in the average length of the NWs. FWHM values of the peaks of the (0002) crystal plane of InN were measured as 0.16, 0.19, and 0.18 ° for the NW2, NW5, and NW6 samples, respectively. In the DCXRD patterns of the NW samples, the NW2 sample had the narrowest FWHM value, indicating that the NW2 sample had the best crystal quality.
A Cs-TEM image of a single InN NW of the NW2 sample is shown in Fig. 5. High-resolution TEM (HRTEM) images and selective-area electron diffraction (SAED) patterns of the InN NW, measured from three different positions, are shown on the right-hand side of the TEM image. Typically, Si-based III-V compound-semiconductor NWs have significant quantities of stacking faults [14,29]. However, stacking faults were rarely observed in the HRTEM images of the InN NWs in this work. From this investigation, we can conclude that InN NWs with high crystallinity were grown on Si(111) substrates via the InPD method. SAED patterns obtained for the three different regions show the growth directions of  and [01-10] originated from InN NW with WZ crystal structure . This result is consistent with the DCXRD rocking curves shown in Fig. 4.
Figure 6 shows the PL spectra of the NW2 sample measured at 10 K and room temperature (RT). The excitation power was set to 3 mW/cm2. A free-exciton (FX) peak was found at the wavelength of 1288 nm with a linewidth of 21 nm at 10 K. Observation of FX peaks from Si-based III-nitride NWs at RT is quite difficult because there are typically many stacking faults and defects inside NWs [14,31]. In other words, a considerable number of carriers generated by light absorption can become trapped in the stacking faults inside NWs. As a result, radiative recombination becomes insufficient and non-radiative recombination increases [32,33]. However, in this paper, a strong FX peak was found at the wavelength of 1,297 nm with a narrow linewidth of 25 nm at RT. The PL peak of the NW2 sample at RT was slightly red-shifted from that at 10 K, because atomic vibration due to heat increased with increasing temperature . The valley observed around the wavelength of 1,330 nm is associated with water absorption . Compared with the previously reported emission wavelength of InN (~ 1.63 µm), the emission wavelength of the InN NWs was significantly blue-shifted. This result can be explained as a combined effect of the quantum confinement effect of the InN NWs and the Moss- Burstein effect [36,37].
In conclusion,highly crystalline InN NWs with symmetric shapes were successfully formed on a Si(111) substrate by adapting the InPD method and varying the growth parameters, growth temperature, and V/III ratio. In FE-SEM images of the InN NWs, symmetric shapes along the vertical growth direction were observed. Strong and narrow DCXRD rocking curves corresponding to InN(0002) were observed. In the HRTEM images, stacking faults and defects were rarely observed. The SAED patterns of the InN NWs show the growth directions of  and [01-10]. A strong FX peak was observed from InN NWs with narrow FWHM values. These results indicate that highly crystalline InN NWs having WZ crystal structure formed on Si(111).
This research was supported by Jeonbuk National University.
The authors declare no conflicts of interest.