Abstract: Cylindrical InxGa1-xAs NWs have been successfully grown at low growth temperature using MOCVD. Field Emission-Scanning Electron Microscopy (FE-SEM) characterization and Energy Dispersive X-ray (EDX) analysis have been used to investigate the morphology and chemical composition of NWs, respectively. Both characterization results consistently reinforce that the NWs growth were via direct impinging mechanism and NW have relatively uniform chemical composition.
INTRODUCTION
One-dimensional semiconductor nanowires (NWs) are expected to play to key-role not only for testing of fundamental phenomena but also for potential nanotechnology applications (Sano et al., 2007; Regolin et al., 2007; Dick et al., 2007) . Due to the direct band gap and high carrier mobility of materials, III-V compound semiconductor NWs have been receiving high attention as base components for next generation electronic and optoelectronic devices (Cornet and La Pierre, 2007). The flexibility to modify the lattice constant and energy band gap by adjusting the relative composition of the alloy leads to ternary alloy Nws have been gaining interest over elemental and binary compound NWs (Lim et al., 2008).
InxGa1-xAs NWs is one of the most amazing III-V ternary alloy NWs to be further elaborated due to some fascinating importance of this material system for application in long wavelength optical transmission and integrated photonics applications (Lim et al., 2008; Kim et al., 2006). In addition, due to low electron effective mass of the material, InxGa1-xAs is considered to be one of the fascinating materials suitable as transistors channel (Sato et al., 2008a, b). For large scale device application, the dimension of the NW (diameter and length), its crystalline structure, chemical composition as well as the surface coverage of Nws needs to be uniform and fully controlled (Joyce et al., 2006; Messing et al., 2009). To pursue these objectives, InxGa1-xAs NWs must be grown not only at suitable condition but also by appropriate growth mechanism.
Recently, semiconductor NWs with different composition have been successfully grown using seed particle via vapor-liquid-solid (VLS) (Lim et al., 2008; Wanger and Ellis, 1964; Park, 2008; Kodambaka et al., 2006; Jishiashvili et al., 2009; Schwalbach and Voorhees, 2008; Greytak et al., 2004; Hoffmann et al., 2006), vapor-solid-solid (VSS) (Adhikari et al., 2006; Lensch-Falk et al., 2007; Wang et al., 2008a; Omari et al., 2008), and solid-liquid-solid (SLS) based on self-organized growth mechanism (Kolasinski, 2006). Those mechanism depend on the growth technique, growth condition as well as the material of NWs that grown (Kolasinski, 2006). In particular, III-V NWs seed particle-assisted can be epitaxially grown by metal-organic chemical vapor deposition (MOCVD) (Joyce et al., 2007). In the growth process, a seed particle such as gold promotes anisotropic wire-like growth by either VLS or VSS mechanism (Dick, et al., 2007; Kolasinski, 2006; Joyce et al., 2006). At the growth temperature, Au nanoparticles on the semiconductor substrate surface form a liquid (in case of VLS) or solid (in case of VSS) alloy with the group III species (Joyce et al., 2006). As the group III species precipitate out at the nanoparticle-semiconductor interface, highly NW growth occurs, with Au nanoparticle atop of each NWs (Wanger and Ellis, 1964; Park, 2008; Kodambaka et al., 2006; Jishiashvili et al., 2009; Schwalbach and Voorhees, 2008; Greytak et al., 2004; Hoffmann et al., 2006; Joyce et al., 2006). Usually, the diameter of NW is mainly determined by the size of the seed particle and its length is determined by the growth time and conditions (Wanger and Ellis, 1964; Lauhon et al., 2004; Samuelson et al., 2004).
The challenges in the growth ternary compound NWs such as InxGa1-xAs NW is still difficult to produces Nws with uniform size and chemical composition (Kim et al., 2006; Joyce et al., 2007). NWs grow with tapered shape (bigger size on the bottom) have non-uniform chemical composition along the wire. In this study however, cylindrical InxGa1-xAs NWs with relatively uniform chemical composition have been successfully grown. The growth temperature was below the pseudo-binary eutectic points of Au-GaAs (630°C) (Duan and Lieber, 2000). Consequently the state of Au seed particle is solid or molten state. Its surface and interface are liquid, while the core of the seed particle is solid (Kim et al., 2006). Therefore, in this case, InxGa1-xAs NWs were grown via VSS rather than VLS mechanism.
MATERIAL AND METHODS
InxGa1-xAs NWs have been grown on undoped GaAs (111)B substrates in vertical chamber MOCVD system. The system uses low-pressure (0.1 atm) chamber with trimethylindium (TMIn), trimethylgallium (TMGa) and arsine (AsH3) as precursors and 99.9999% pure hydrogen as carrier gas. GaAs (111)B substrate was chosen due to low surface energy which is the energetically favorable growth direction (Kim et al., 2006; Cau, 2003). Prior to the NWs growth on the substrate surface, the substrate was functionalized by immersed it in 0.1% poly-L-lysine (PLL) solution for 3 min. The substrate was then cleaned with de-ionized water and subsequently blow-dried with nitrogen (N2). It was then treated with 50% gold colloid solution (30 nm diameter Au particle) for 30 sec. It was then loaded into the MOCVD chamber. The substrate was heated up to 600°C under constant partial pressure of AsH3 gas for 10 min to desorbed surface contamination and then cooled down to the desired growth temperature. Once the growth temperature was reached, TMIn, TMGa, and AsH3 was flowed into the chamber for the NWs growth. The growth time, growth temperature and V/III ratio of Nws growth were set at 30 min, 400°C and 10.0, respectively. The morphology and chemical composition of NWs were investigated using Field Emission-Scanning Electron Microscopy (FE-SEM) and Energy Dispersive X-ray (EDX) analysis, respectively.
In this growth process NWs were seeded by 30 nm diameter Au particles. Gold was chosen to initiate NWs growth due to many of the precursor materials used for growth are soluble in gold. Secondly, gold is a very soft metal and therefore is more probable to form particles with reasonably uniform shape even in the solid form. Furthermore, for many materials, growth with a solid particle would be impractically slow, but for gold however, the diffusivities of In and Ga through a solid particle are very high and therefore should give a reasonably high growth rates (Joyce et al., 2007).
| Fig. 1: | FE-SEM images of InxGa1-xAs NWs seeded by 30 nm diameter Au particles and grown for 30 min at 400°C. The images were viewed at 45o angle from the surface normal |
RESULT AND DISCUSSION
FE-SEM images in Fig. 1 shows cylindrical InxGa1-xAs NWs grow perpendicular to the substrate. It indicates at temperature of 400°C, NWs were grown by direct impinging mechanism via VSS models. In this mechanism, source atoms (precursor) directly fall onto Au seed particle (Fig. 2a) to form a true eutectic or liquid solution (partially molten state) of an alloy (stable or unstable). This is serves as a preferential site for the decomposition of the source atom via absorption (stage 1 of Fig. 2a) and diffusion mechanism (stage 2 of Fig. 2a) (Wang et al., 2008b). The amount of precursor in the vapor near the seed particle is then locally increased compared to elsewhere on the substrate (Dick, 2008) At a certain point when enough precursor material has been incorporated into the seed particle it will become supersaturated. In this case, saturation of seed particle with the growth precursor or the formation of the proper combination may lead to an induction period before the growth (Kolasinski, 2006). Then, the super saturation leads to precipitation of the semiconductor material at the particle-substrate interface referred to as nucleation (stage 3 Fig. 2a), and InxGa1-xAs NW starts to grow (Fig. 2b). Due to a continuous supply of growth precursors the growth occurs at the particle-wire interface to form InxGa1-xAs NW (Fig. 2c). The growth rate of NW depends to a large extent on the precursor concentration and growth temperature (Joyce et al., 2007).
It can be observed that InxGa1-xAs NWs grown were different in diameter and length. The distribution of diameter and surface density of NWs were likely due to the annealing effect (temperature and time) on the distribution of Au particles on the surface of substrate before the growth precursors were injected. Since they can obtain some extra energy, the adjacent particles could combine together to form the larger size through the Ostwald ripening mechanism (Cau, 2003). On the other hand, Au particle that did not combine together did not change their size.
| Fig. 2: | Schematic diagram of a direct impinging growth mechanism of InxGa1-xAs NWs. The metal droplet (see particle) is in solid or partially mlten state (liquid solution) |
Besides, the incorporation of significant amount of precursor into the seed particle is another way to change the size (volume) of seed particle. As a result, NWs grown with seed particle-assisted have different diameter. Therefore, Ostwald ripening and incorporation of growth precursor can both conspire to change the size of the Au seed particle. Due to those mechanisms some NWs were grown with diameter around six times than the diameter of Au seed particle as shown on Fig. 1b. Furthermore, NWs with smaller diameter grow shorter than the bigger ones due to different growth rate. The classical analysis of Givargizov concluded that narrower wires grow more slowly due to Gibbs-Thomson effect (Dick et al., 2006). Other studies, however, show the opposite (Givargizov, 1975), with narrow wires growth more rapidly. The difference is likely due to the different growth conditions, different growth mechanism and different material of the Nws.
EDX analysis has been used to investigate the chemical composition of individual InxGa1-xAs NWs from various detection positions; tip, middle and bottom of NW as shown on Fig. 3. Surprisingly, Au was not detected from various detection positions, even on the tip of the NW. This is most likely due to the amount of Au was very small compared to the elements of the NW. From EDX measurement the ratio of In/Ga compositional at the tip, middle and bottom of the InxGa1-xAs NW was 0.386, 0.323, and 0.319, respectively. These ratios, especially the one at the tip of the NW is close to the value of In vapor composition, xv of 0.41. The diverse In/Ga compositional ratios along the the NW were due to (In,Ga) As source atoms (precursor) fall onto the tip of the NW then move downwards to the bottom of the NW. The EDX results is the evidence that NWs grow were via direct impinging mechanism. Even though the compositional ratio from these three detection positions were not exactly the same, but all values were around of 0.3. This means that straight NWs with uniform diameter, height and a chemical composition are possible to produce.
CONCLUSION
InxGa1-xAs NWs have been grown at low growth temperature using MOCVD. FE-SEM images show cylindrical shape InxGa1-xAs NWs grow perpendicular to the substrate, indicating NWs growth via direct impinging mechanism. EDX results show (In/Ga) ratio on the tip, middle and bottom of NW was not precisely same, but all values were around of 0.3. EDX results also indicate that NWs grow via direct impinging mechanism tends to have uniform chemical composition.
| Fig. 3: | EDX measurements of InxGa1-xAs NWs from various detection positions, seeded by 30 nm diameter Au particles and grown for 30 min. At 400°C. Determined In/Ga ratios are also shown |
ACKNOWLEDGEMENTS
We would like to thank the Ministry of Higher Education, Malaysia for the financial support and Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia for laboratory facilities.