Direct Imaging of p–n Junction in Core–Shell GaN Wires

Abstract

While core−shell wire-based devices offer a promising path toward improved optoelectronic applications, their development is hampered by the present uncertainty about essential semiconductor properties along the threedimensional (3D) buried p−n junction. Thanks to a crosssectional approach, scanning electron beam probing techniques were employed here to obtain a nanoscale spatially resolved analysis of GaN core−shell wire p−n junctions grown by catalyst-free metal−organic vapor phase epitaxy on GaN and Si substrates. Both electron beam induced current (EBIC) and secondary electron voltage constrast (VC) were demonstrated to delineate the radial and axial junction existing in the 3D structure. The Mg dopant activation process in p-GaN shell was dynamically controlled by the ebeam exposure conditions and visualized thanks to EBIC mapping. EBIC measurements were shown to yield local minority carrier/exciton diffusion lengths on the p-side (∼57 nm) and the n-side (∼15 nm) as well as depletion width in the range 40−50 nm. Under reverse bias conditions, VC imaging provided electrostatic potential maps in the vicinity of the 3D junction from which acceptor N a and donor N d doping levels were locally determined to be N a = 3 × 10 18 cm −3 and N d = 3.5 × 10 18 cm −3 in both the axial and the radial junction. Results from EBIC and VC are in good agreement. This nanoscale approach provides essential guidance to the further development of core−shell wire devices.

Core–shell InGaN/GaN nanowire light emitting diodes analyzed by electron beam induced current microscopy and cathodoluminescence mapping

“…On the contrary, EBIC offers excellent spatial resolution and allows to address individual nanostructures composing the macroscopic device. [8][9][10][11][12][13][14] In particular, EBIC microscopy is a perfectly dedicated tool to investigate the structural and the electrical properties of the three-dimensional nanostructured LEDs. In the past years, several EBIC investigations of single nanowires and nanowire devices built of different materials have been reported, [15][16][17][18][19][20] sometimes coupled with cathodoluminescence (CL) mapping.…”

“…19 Recently, Tchoulfian et al have proposed a cross-PAPER Nanoscale 2 | Nanoscale, 2015, 00, 1-- 3 This journal is © The Royal Society of Chemistry 2015 sectional approach based on cleaving core/shell nanowires along their axis, which has been applied to GaN p-n junction core-shell nanowires. 13 However no cross-sectional studies of full nanowire LEDs has ever been reported. In addition, no correlation between induced current and optical mapping has ever been performed on core/shell nanowires.…”

Location and Visualization of Working p-n and/or n-p Junctions by XPS

“…Recently, scanning transmission electron microscopy (STEM) using differential phase-contrast has also been proven to reveal the built-in electric field at a p-n junction7. Recent SEM characterizations of GaN nanorods have extended conventional mapping by adding in-situ electrical biasing thus enabling the direct imaging of the p-n junction under operational conditions and yielding critical information on its depletion width89. Other techniques like cathode-luminescence in a scanning transmission microscope (CL-STEM)10 and conductive atomic force microscopy (AFM) have also been utilized for the characterization of the p-n junction in GaN nanorods11 and porous GaN based LEDs12.…”

Spectroscopic XPEEM of highly conductive SI-doped GaN wires

“…We first note that the nitride bonding states can be viewed as Si atoms in Ga substitutional sites (Si Ga ), bonded to nitrogen, as expected in intentional n-doping with fully ionized, electrically-active dopants. The elemental silicon is representative of Si(-Si) 3 bonding or bonding of Si atoms with elements having similar electronegativity: the first situation would correspond to clustering of Si atoms already reported in GaN at very high doping levels [25]. In the second case Si atoms in nitrogen sites would be bonded directly to Ga of close (1.5) electronegativity to that of Si: this is again consistent with the N-deficient character of the m-facet surface as revealed by SAM and would tend to electrically compensate the expected n-type doping induced by Si Ga .…”

“…Beyond the issues involved at the processing level, the physical characterization is often challenging because its deals with individual objects. It is also of utmost importance, within the perspective of a comprehensive understanding of the specific doping phenomena in wires, to develop reliable methods enabling to study individual wire structures [3]. Electrical (four-probe) and measurements on single wires are time consuming and with a limited spatial resolution.…”