ROUGH DRAFT authorea.com/31911

# GaAs Nanowire Review

Abstract

Semiconductor nanowires (NW) are becoming increasingly important due to their novel electronic, photonic, thermal, electrochemical and mechanical properties and potential applications thereof in fields such as electronics and opto-electronics. (Yang 2010) (Dasgupta 2014) (Li 2006) (Yan 2009) There does not seem to be a limit to the innovative electronic designs that are being created to test these properties. Gallium Arsenide (GaAs) NWs have been explored for a myriad of possible devices including, transistors, photo-detectors, LED, solar cells, and nanolaser devices. 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.(Fang 2014) (Dick 2014) 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 GaAs NW synthesis and the most recent electronics that have been designed using GaAs NWs. The topics to be reviewed are divided into two sections, one of synthesis and one of applications.

# Synthesis

Synthesis technique advancement is key to being able to produce high quality NW for electronics applications. Synthesis of III-V NWs have undergone many changes and improvements as of late. (Fang 2014) (Dick 2014) (Dasgupta 2014) In particular, the many methods can generally be categorized as gold dependent and gold independent. But in addition to new synthesis methods, insight into the morphology of the NW has also been gained.

## Gold Catalyzed Synthesis

Many of the common NW synthesis techniques can be used for the synthesis of III-V GaAs NW, such as Vapor-Liquid-Solid (VLS), molecular beam epitaxy (MBE) and, metal organic chemical vapor deposition (MOCVD).(Meyyappan 2009) There also exist many techniques that improve on these earlier methods such as laser ablation.

Vapor-Liquid-Solid growth (VLS) is common for NW synthesis and has been used for GaAs NW growth.(Hu 1999) (Dubrovskii 2015) (Dasgupta 2014) A catalytic liquid phase droplet (in the case of GaAs NW, the catalyst could be Cu, Ag or Au) is formed on the solid surface of the substrate then, the droplet quickly absorbs the vapor containing precursors and becomes supersaturated to the point that crystal growth begins.(Meyyappan 2009) Historically it is known that the radius limit of NWs 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 NW 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 cluster sizes to prepare catalyst clusters to subsequently produce NWs via VLS.(Morales 1998) (Zhao 2007) A phase diagram must first be examined to find the conditions necessary for the catalyst to be in liquid phase and for the material to precipitate out. Then, the desired elements and catalyst are placed in a quartz furnace tube to control the temperature, pressure and residence time. Next, a laser is used to ablate the precursors and catalyst. The vapor phase species condenses into small clusters, then NW 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 “

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)