GaAs Nanowire Synthesis Review


Semiconductor nanowires (NW) are becoming increasingly important due to their interesting properties and potential applications thereof in fields such as electronics and opto-electronics. In order for applications in these fields to be successful there must be synthetic control of NW quality; including, synthetic control of phase puritiy, chemical composition, surface:volume ratio, NW length, NW diameter, and NW shape. All these NW attributes consequently control the NW properties that are of interest, and even small variations or inconsistencies can have large effects on NW performance. This article reviews the methods of Gallium Arsenide (GaAs) nanowire synthesis and new insight into the shape, structure, mechanism of formation, and controllable growth parameters for tunable NW structure. The topics to be reviewed are divided into three sections, the first encompassing gold dependent synthesis methods, the second, gold independent synthesis, and the third section reviews crystal structure and ways to control NW morphology.

Gold Catalyzed Synthesis

Many of the common NW synthesis techniques can be used for the III-V NW synthesis of GaAs, such as Vapor-Liquid-Solid (VLS), molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD). There also exist many techniques that improve on these earlier methods such as laser ablation.

Vapor-Liquid-Solid growth (VLS) is common for nanowire synthesis and has been used for GaAs nanowire growth.(Hu 1999) A catalytic liquid phase droplet is formed (in the case of GaAs NW, the catalyst could be Cu, Ag or Au) on the solid surface of the substrate, the droplet quickly absorbs the vapor containing precursors then becomes supersaturated to the point that crystal growth begins.(Meyyappan 2009) Historically it is known that the radius limit of nanowires grown this way depends on the formation and stability of the liquid catalyst cluster beads, for which the minimum radius (\(r_{min}\)) of the wire is given by equation \ref{eq:vls_limit} and is limited by equilibrium conditions (\(T\) is growth temperature, \(V_l\) is the liquid molar volume, \(\sigma\) is the degree of vapor pressure saturation and \(\sigma_{LV}\) is the liquid-vapor interfacial energy) .(Meyyappan 2009) \[\label {eq:vls_limit} r_{min}=\frac{2\sigma_{LV}V_{L}}{RT \sigma}\] Using typical values of GaAs nanowire growth for equation \ref{eq:vls_limit} a limit of \(2 \mu m\) is found. (Hu 1999) Laser ablation can overcome the limitation of equilibrium clusters sizes to prepare catalyst clusters to subsequently produe nanowires via VLS.(Morales 1998) The mechanism works as follows...First a phase diagram must be examined to find the conditions necessary for the catalyst to be in liquid phase and for the material to precipitate out. The desired elements and catalyst are placed in a quartz furnace tube to control the temperature, pressure and residence time. Then a laser is used to ablate the precursors and catalyst. The vapor phase species condenses into small clusters, then nanowire growth begins once the liquid becomes supersaturated and continues as long as the nanoclusters remains in the liquid phase. Growth ends when the NW are cooled.(Morales 1998)

Schematic of laser ablation setup. 1 = laser source, 2=focusing lens, 3=quartz tube with sample, 4=furnace, 5=cooled substrate collector, 6=gas flow. Figure 1 from “ A Laser Ablation Method for the Synthesis of Crystalline Semiconductor Nanowires”, reference (Morales 1998).

Simplifying the ternary phase diagram of a GaAsM (M:Au,Ag,Cu) system to pseudo binary phase diagrams allows for an analysis of necessary growth conditions for NW production.(Duan 2000)(Panish 1967) conditions for growth using laser ablation were found to be \(800-1030^{\circ}C\) \(100-500 Torr\), \(Ar:H_{2} (95:5)\) with laser specifications of (\(\lambda\) = 1064nm; 10Hz pulse; 2.5 W power). GaAs have been synthesized with radius as small as \(1.5 nm\) using this laser ablation method.(Duan 2000)

MOCVD is a technique that is very similar to the VLS described above. Au nanoparticles are deposited onto a semiconductor substrate surface to alloy and initiate growth of vapor phase group (III) and group (V) precursors, such as \(Ga(CH_{3})_{3}\) and \(AsH_{3}\). These precursors are used to decompose to the elements necessary to make the GaAs NW.(Joyce 2011) (Stringfellow 1999) After annealing the substrate to rid it of impurities it is cooled to between 350 and \(550^{\circ}C\). The group V flow rate is adjusted for growth and the group III precursors are fed to the reaction chamber to initiate growth. Growth times are generally between 30 seconds and 120 minutes, chosen according to the growth rate and the desired nanowire length. Upon completion of growth, the samples are cooled under group V overpressure.(Joyce 2011)