Cross-Section High Resolution Transmission Electron Microscopy Investigation of Internal Structures of Gallium Nitride Nanowires


Gallium nitride nanowires and rods were analyzed with plain-view and cross-section high resolution transmission electron microscopy. Crosssection studies revealed details of the internal structures including totally coherent internal interfaces and nanopipes. The internal structures have implications for electronic and opto-electronic device performance.

I. Introduction

Semiconducting nanowires represent a new class of device building blocks with properties enhanced by their small size and large aspect ratios. Gallium nitride (GaN) nanowires in particular have received much attention due to their unique material properties. GaN nanowire-based blue/UV lasers are an especially desirable application under investigation by several groups [1] [2] [3] [4] . To date, optical pumping of GaN nanowire lasers has been successfully demonstrated; however electronic pumping for a compact all solid-state device has not yet been clearly realized. Electronic pumping requires well defined energy states in a highly crystalline material as an essential characteristic for efficient lasing action. Nanowires are known for their high crystallinity. However, very recent results [5] [6] [7] [8] [9] [10] [11] indicate that nanowires may possess an internal structure, which is not readily apparent even with investigation by plan-view high resolution transmission electron microscopy (HRTEM). In the present investigations, GaN nanowires and rods were analyzed with plan-view and cross-section HRTEM. Cross-sections for HRTEM were fabricated using a focused ion beam (FIB, FEI Quanta 200 3D) system. Details of the cross-section process may be found in Refs. [9] [10] . The cross-section studies revealed details of the internal structures of GaN nitride nanowires and rods that would have been difficult to reconstruct from planview HRETM alone.

I . G A N Na No Wi Re Cro S S -S Ecti O N I Nv Es Ti G A Ti O Ns

The GaN nanowires and rods used in this study were synthesized by way of a catalyst-free direct reaction of gallium vapor and ammonia [12] . Nanowires and rods with two orientations were obtained using this growth method, approximately dependent on the furnace growth temperature. At growth temperatures between 850°C and 950°C, unique multiphase zinc-blende/wurtzite GaN nanowires with triangular cross-sections, ranging between 60-150 nm in width with growth orientations along the <011> direction for zinc-blende and the <2-1-10> direction for wurtzite were commonly obtained [10, 11] . At growth temperatures above 1000°C, single-phase wurtzite GaN nanowires and rods with hexagonal cross sections, ranging between 200-5000 nm in width with growth orientation along the [0001] direction were commonly obtained [9] .

GaN nanowires grown at furnace temperatures of 850-950°C were multiphase with multiple highly crystalline zinc-blende and wurtzite domains extending along the entire length of the nanowire. Extensive plan-view TEM of over 50 nanowires to date has been used to check that the multiphase character extends over the full length of the nanowire

Fig. 1. HRTEM of cross-section of GaN nanowire grown at 850°C. The totally coherent wurtzite/zinc-blende interface is indicated by the straight line. (adapted from Fig. 2, Ref. [10]).

The HRTEM analysis of a cross-section taken from a nanowire grown at 850°C is shown in Fig. 1 . Three large 30 nm wurtzite domains and one large 40 nm zincblende domain are evident. A long 80 nm totally coherent interface between wurtzite region 4 and zincblende region 5 is observed in the image and verified by the Fast Fourier Transforms (FFTs) of regions 4 and 5, shown right. Also, triangular region 2, discussed in detail in Ref. [10] , is a small wurtzite region with smaller zinc-blende ‘spacers’ at each apex that enable it to fit smoothly with regions 1, 3, and 4. Three short 10 nm coherent interfaces (not shown) were observed between the zinc-blende and wurtzite components of region 2. The HRTEM analysis of a cross-section taken from a nanowire grown at 950°C is shown in Fig. 2 . Three large 30 nm wurtzite domains and one smaller 20 nm zincblende domain are evident. Wurtzite domains 2 and 3 are separated into a,b regions by the development of a stacking fault in each, indicated as a dotted line. Region 2a shares a totally coherent (0001)/(111) interface with zinc-blende region 1, indicated by the horizontal arros in the FFTs, right. Furthermore, regions 3b and 3c are almost coherent with each other as well as with zinc-blende region 1 between them, as shown by the FFTs, right. A 7-10 atomic layer stacking fault region is observed between regions 3b and 1 and a 10-12 atomic layer stacking fault region is observed between regions 3c and 1. The character of the stacking fault regions is mixed wurtzite/zinc-blende, rather than random disorder. All cross sections of multiphase GaN nanowires grown at furnace temperatures of 850-950°C examined to date involved one or more coherent interfaces between zincblende {111} planes and wurtzite (0001) planes at domain interfaces. Coherent domain interfaces are considered to be important to the multiphase nanowire structure stability. Nanowire nucleation and growth is discussed below. It is noted here that a zinc-blende nucleation site for step-ledge growth has not yet been identified. The results shown above suggest the possibility that the formation of the zincblende regions may be driven by lowering of internal stresses.