Abstract

In this paper, the internal structure of novel multiphase gallium nitride nanowires in which multiple zinc-blende and wurtzite crystalline domains grow simultaneously along the entire length of the nanowire is investigated. Orientation relationships within the multiphase nanowires are identified using high-resolution transmission electron microscopy of nanowire cross-sections fabricated with a focused ion beam system. A coherent interface between the zinc-blende and wurtzite phases is identified. A mechanism for catalyst-free vapor–solid multiphase nanowire nucleation and growth is proposed.

1. Introduction

Semiconducting nanowires represent a new class of device building blocks with properties enhanced by their small size and large aspect ratios. Nanowires can be made from a variety of different semiconducting materials and therefore a wide assortment of band gaps, hole and electron mobilities, and mechanical properties are available for device engineering.

Gallium nitride nanowires in particular have received much attention due to their unique material properties. Gallium nitride (GaN) is a direct, wide band gap material [1] . It is capable of forming in the zinc-blende or wurtzite crystalline phases, further extending its versatility for device design. GaN nanowire optical and electronic devices, including lasers [2, 3] , LEDs [4] , UV detectors [5] , and field effect transistors [6] , and mechanical systems using a GaN nanowire as the primary device element [7] have been fabricated.

Nanowires must be of very high material purity, crystalline quality, and have tailored and reproducible crystallographic orientations to be viable in high performance devices. Plain-view high-resolution transmission electron microscopy (HRTEM) imaging with selective area electron diffraction (SAED) is a standard method for determining the crystal quality of as-grown semiconducting nanowires. It is, however, difficult to assess the internal nanowire structure using this method. The surface energy versus the volume total free energy contributions to the total nanowire free energy are differently proportioned than in bulk and can energetically enable complex internal structural arrangements [8, 9] . Such arrangements can lead to enhanced device properties, or induce defects which may significantly degrade device performance. Therefore, investigation of the internal structures of as-grown nanowires is important for both fundamental understanding and for successful nanodevice engineering [10] [11] [12] [13] .

The multiphase nanowire structures reported here incorporate highly crystalline zinc-blende and wurtzite phases simultaneously that extend in the longitudinal direction along the entire length of the nanowire [14] . The nanowires were synthesized in a quartz tube furnace at 850 • C in a direct reaction of gallium vapor and flowing ammonia (NH 3 ) [15] [16] [17] . Plain-view HRTEM (JEOL 2200FS, 200 kV) investigations of over 30 nanowires showed that the zinc-blende/wurtzite multiphase nanowires were consistently obtained using this growth method [18] . However, the internal orientation relationships that enable the multiphase structure could not be accurately assessed using plain-view HRTEM. Nanowire cross-sections, fabricated using a focused ion beam (FIB) system, proved important to identify details of the multiphase nanowire structure as they revealed internal crystallographic orientations that would be extremely difficult to reconstruct from plain-view methods. In particular, all crosssections of multiphase GaN nanowires examined to date have contained one or more coherent interfaces between zinc-blende {111} planes and wurtzite (0001) planes at domain interfaces. These coherent interfaces are considered to be important to the multiphase nanowire structure stability and growth.

2.1. Nanowire Growth

GaN nanowires were grown in a tube furnace by reaction of gallium with ammonia as previously described in [15] [16] [17] . Gallium (99.999% purity/metals basis) was placed in a small BN boat (1 cm × 3 cm). The boat was placed into a 150 mm long 20 mm inside diameter quartz tube, which in turn was placed in the center of a 25 mm diameter, 1 m long quartz tube. The tubes and the BN boat were centered in a tube furnace. The 1 m process tube was sealed at both ends to form a vacuum system and evacuated by a two stage mechanical pump operating with Fromblin oil. The system was pumped down to a base pressure of ∼20 mTorr. An electronic mass flow controller then controlled the flow of electronic grade ammonia through the growth system. The flow rate was set between 80 and 90 sccm. A small valve at the end of the flow tube regulated the pressure in the system as measured by a capacitance manometer at the end of the process tube furthest from the pump. The furnace was rapidly ramped up to the 850 • C growth temperature. After 2 h, the furnace was turned off and cooled to ∼500 • C, at which point the ammonia flow was turned off and nitrogen substituted. When the system had cooled to room temperature, the inner quartz tube was removed and the GaN material removed. The width of the nanowires were between 60 and 150 nm and up to 500 µm in length.

2.2. Fib Process

Cross-section TEM samples of the nanowires were fabricated using a FIB dual beam system (FEI Quanta 200 3D). The FIB was used to extract and mill nanowire samples until they were transparent to a TEM electron beam. For cross-section preparation, the nanowires and growth matrix were placed in ethanol and sonicated to free the nanowires from the matrix. The suspensions were then deposited on silicon wafers that had a layer of native oxide. A 5 nm titanium layer and 20 nm gold layer were thermally evaporated over the nanowires and substrate. These layers provided additional protection from ion beam damage in the initial phases of sample preparation. A thick layer of platinum, ∼4 µm, was deposited using reactive gas/ion interaction so that the nanowires would not be damaged during the ion milling process. The Pt/Au/Ti/nanowire/silicon layers were ion milled and extracted from the substrate using a micromanipulator and placed and secured on TEM grids using reactive gas/ion Pt deposition. The samples were milled further using the FIB so that they were thin enough to be transparent to the TEM electron beam.

3. Results And Discussion

Figure 1(a) shows a typical plain-view HRTEM image of a multiphase nanowire. A very sharp interface with no observable defects between the zinc-blende and wurtzite phases is observed, indicated by the solid line. The nanowire growth is in the [011] direction for the zinc-blende phase and in the [2110] direction for the wurtzite phase, as determined by selected area electron diffraction (SAED) patterns, not shown. The sharp interface is identified as (111) zinc-blende/(0001) wurtzite, is a totally coherent interface, and extends the entire length of the nanowire. Figure 1 (b) shows a typical TEM image of a multiphase nanowire and where a cross-section slice might be taken. Contrast variation in this image resulted not only from variations in thickness but also different crystalline phases. The white box indicates where the HRTEM image shown in figure 1(a) was taken. Figure 2 shows details of the internal zinc-blende/wurtzite structure of a multiphase nanowire. The structure consisted of multiple zinc-blende and wurtzite domains in several orientations within the triangular nanowire. An HRTEM image of the multiphase GaN nanowire cross-section is shown in figure 2(a). The contrast variations seen in this image result from the multi-domain internal structure of the nanowire. The triangular cross-section is typical of the zinc-blende/wurtzite multiphase nanowire growth and consistent with previous AFM and SEM investigations. Platinum, gold and titanium metal layers deposited to protect the nanowire from ion beam damage during ion beam milling are labeled in the image. divided into five distinct crystallographic domains, indicated by the dashed lines, shown in figure 2(c). Fast Fourier transforms (FFTs) taken from HRTEM images of each domain show their corresponding crystallographic orientations. FFTs from domains 1-4 were indexed to the wurtzite structure along the [2110] zone axis, and the FFT from domain 5 was indexed to the zinc-blende structure along the [011] zone axis. These zone axes were consistent with the corresponding nanowire growth directions found in plain-view TEM studies, as shown in figure 1(a) , and previously reported by our group [14] . Arrows in these FFTs are given to show the [0001] direction of the wurtzite domains and the [111] direction of the zincblende domain. The solid line indicates a long, ∼80 nm, totally coherent (111)/(0001) interface between zinc-blende domain 5 and wurtzite domain 4, that extended from the center of the nanowire to the outside edge.