Structural Changes in a Single GaN Nanowire under Applied Voltage Bias

Abstract

GaN nanowires (NWs) are promising building blocks for future optoelectronic devices and nanoelectronics. They exhibit stronger piezoelectric properties than bulk GaN. This phenomena may be crucial for applications of NWs and makes their study highly important. We report on an investigation of the structure evolution of a single GaN NW under an applied voltage bias along polar [0001] crystallographic direction until its mechanical break. The structural changes were investigated using coherent X-ray Bragg diffraction. The three-dimensional (3D) intensity distributions of the NWs without metal contacts, with contacts, and under applied voltage bias in opposite polar directions were analyzed. Coherent X-ray Bragg diffraction revealed the presence of significant bending of the NWs already after metal contacts deposition, which was increased at applied voltage bias. Employing analytical simulations based on elasticity theory and a finite element method (FEM) approach, we developed a 3D model of the NW bending under applied voltage. From this model and our experimental data, we determined the piezoelectric constant of the GaN NW to be about 7.7 pm/V in [0001] crystallographic direction. The ultimate tensile strength of the GaN NW was obtained to be about 1.22 GPa. Our work demonstrates the power of in operando X-ray structural studies of single NWs for their effective design and implementation with desired functional properties.

Combining Nanofocused X-Rays with Electrical Measurements at the NanoMAX Beamline

“…The strong tilts are due to bending of the nanowire, which is induced by the metal contacts on both ends of the nanowire [15,17]. The shape of the nanowire can be reconstructed using a line integral [52], although such an analysis is outside of the scope of the present article.…”

“…Conversely, an electrical bias can be applied on a nanodevice, with X-rays as a probe. In particular, X-ray diffraction (XRD) can be used to quantify electric-field induced changes in single nanodevices [14][15][16][17]. XRD is in principle able to characterize any changes that couple to the crystal lattice, such a piezoelectricity and heating.…”

“…Unlike scanning XRD, which has been frequently used to study nanowires [15,17,19,[29][30][31][32][33][34][35], there have only been a few examples of nanowire devices studied using XBIC [8,9,11,36]. XBIC has been employed for studying solar cells since the early 2000s [37,38], when it was mostly used to examine grain boundaries and precipitated metal in polycrystalline silicon for solar cells at micrometer resolution [39][40][41].…”

Direct Growth of Single Crystalline GaN Nanowires on Indium Tin Oxide-Coated Silica

“…The exact mechanism of the lowering of the turn-on voltage still requires further investigation. Previous reports have shown that applying a high voltage to the material might have induced current-carrying paths through electrical breakdown [21, 22], or modify the structure of the GaN nanowire itself [23] leading to improvement in turn-on voltage.…”

Influence of Contacts and Applied Voltage on a Structure of a Single GaN Nanowire

“…In our previous study [22], we analyzed the NWs of the 350-400 nm diameter with one type of contacts. We have demonstrated that deposition of the Au contacts, as well as the applied voltage bias deform single GaN nanowires leading to their bending.…”