Christopher D. Yerino

Understanding nonpolar GaN growth through kinetic Wulff plots

In this paper we provide explanations to the complex growth phenomena of GaN heteroepitaxy on nonpolar orientations using the concept of kinetic Wulff plots (or v-plots). Quantitative mapping of kinetic Wulff plots in polar, semipolar, and nonpolar angles are achieved using a differential measurement technique from selective area growth. An accurate knowledge of the topography of kinetic Wulff plots serves as an important stepping stone toward model-based control of nonpolar GaN growth. Examples are illustrated to correlate growth dynamics based on the kinetic Wulff plots with commonly observed features, including anisotropic nucleation islands, highly striated surfaces, and pentagonal or triangular pits.

Understanding and controlling heteroepitaxy with the kinetic Wulff plot: A case study with GaN

This work represents a comprehensive attempt to correlate the heteroepitaxial dynamics in experiments with fundamental principles in crystal growth using the kinetic Wulff plot (or v-plot). Selective area growth is employed to monitor the advances of convex and concave facets toward the construction of a comprehensive v-plot as a guidepost for GaN heteroepitaxy. A procedure is developed to apply the experimentally determined kinetic Wulff plots to the interpretation and the design of evolution dynamics in nucleation and island coalescence. This procedure offers a cohesive and rational model for GaN heteroepitaxy on polar, nonpolar, and semipolar orientations and is broadly extensible to other heteroepitaxial material systems. We demonstrate furthermore that the control of morphological evolution, based on invoking a detailed knowledge of the v-plots, holds a key to the reduction of microstructural defects through effective bending of dislocations and geometrical blocking of stacking faults, paving a way t...

Morphological and microstructural evolution in the two-step growth of nonpolar a-plane GaN on r-plane sapphire

In this paper, we report a detailed study on the evolution of surface morphology and microstructure of nonpolar a-plane GaN (a-GaN) through controlled growth interruptions. Microscopy imaging shows that the two-step a-GaN growth went through a roughening-recovery process. The first-step growth (under high V/III and high pressure) produced a rough surface with tall mesas separated by voids. The second-step growth (under low V/III and low pressure) promoted the lateral growth and filled up the voids. Striations that formed during the island coalescence persisted throughout the second-step growth, but could be relieved by an additional third-step growth. The morphological evolution was explained according to the kinetic Wulff plots. The microstructure of the a-GaN films was investigated by transmission electron microscopy (TEM) and x-ray rocking curve analysis. Most of the extended defects observed in the plan-view TEM images were I1 type basal-plane stacking faults (BSFs) and their associated partial disloc...

Improving microstructural quality of semipolar (112̱2) GaN on m-plane sapphire by a two-step growth process

This letter reports a two-step growth process for improving microstructural quality of semipolar (1122) GaN on nitridized m-plane sapphire. The two-step growth of (1122) GaN, islanding growth under high pressure followed by islands coalescence under low pressure, went through a roughening-recovery process, which was found very effective in reducing the density of stacking faults and dislocations in (1122) GaN. The x-ray rocking curves of both on-axis and off-axis planes were narrowed down by more than 50%. The improvement of GaN quality was confirmed by a boost in blue and green optical output of semipolar (1122) InGaN/GaN quantum wells.

Reduction of stacking fault density in m-plane GaN grown on SiC

We report the reduction in basal-plane stacking faults (BSFs) in m-plane GaN grown on m-plane SiC. The origin of BSFs is linked to heteronucleation of m-plane GaN and the presence of N-face basal-plane sidewalls of three-dimensional islands. Graded AlGaN layers help to alleviate mismatched nucleation and the generation of BSFs. Transmission electron microscopy shows that the density of BSFs is decreased to the low 105cm−1. Anisotropy in on-axis x-ray rocking curves, a salient feature in m-plane GaN heteroepitaxial layers, is greatly reduced. A possible mechanism of BSF generation, and the demonstration of improved InGaN∕GaN quantum well emission are presented.

Using the kinetic Wulff plot to design and control nonpolar and semipolar GaN heteroepitaxy

For nonpolar and semipolar orientations of GaN heteroepitaxially grown on sapphire substrates, the development of growth procedures to improve surface morphology and microstructure has been driven in a largely empirical way. This work attempts to comprehensively link the intrinsic properties of GaN faceted growth, across orientations, in order to understand, design and control growth methods for nonpolar (1 1 2 0) GaN and semipolar (1 1 2 2) GaN on foreign substrates. This is done by constructing a comprehensive series of kinetic Wulff plots (or v-plots) by monitoring the advances of convex and concave facets in selective area growth. A methodology is developed to apply the experimentally determined v-plots to the interpretation and design of evolution dynamics in nucleation and island coalescence. This methodology offers a cohesive and rational model for GaN heteroepitaxy along polar, nonpolar and semipolar orientations, and is broadly extensible to the heteroepitaxy of other materials. We demonstrate furthermore that the control of morphological evolution, based on invoking a detailed knowledge of the v-plots, holds a key to the reduction of microstructural defects through effective bending of dislocations and blocking of stacking faults. The status and outlook of semipolar and nonpolar GaN growth on sapphire substrates will be presented.

Improved hydrogen detection sensitivity in N-polar GaN Schottky diodes

Pt/GaN Schottky diodes fabricated on m-plane (N-polar) layers grown on sapphire exhibit much larger responses to dilute concentrations (4% in N2) of hydrogen at room temperature than comparable Ga-polar devices. This is consistent with previous density functional theory indicating a very high affinity of hydrogen for the N-face surface of GaN. The rectifying current-voltage characteristics of N-face diodes make a transition to more Ohmic-like behavior after hydrogen exposure, leading to very large (∼106) maximum percentage changes in current relative to Ga-face (∼10%) or AlGaN/GaN heterostructure diodes (∼170%). The strong affinity of the N face of GaN for hydrogen also leads to a slower recovery of these diodes when hydrogen is removed from the ambient.

Effect of Controlled Growth Dynamics on the Microstructure of Nonpolar a-Plane GaN Revealed by X-ray Diffraction

This paper reports the effect of controlled growth dynamics, as monitored by in situ optical reflectance, on the microstructure of nonpolar a-plane GaN films grown on r-plane sapphire. The mosaic microstructure of a-plane GaN and its anisotropy are evaluated by X-ray rocking curve (XRC) measurements. By inserting a pronounced islanding stage followed by an enhanced lateral growth, pit-free a-plane GaN has been achieved showing an XRC linewidth of ∼0.18 and ∼0.3° for on- and off-axes planes, respectively, with only minor anisotropy. The density of basal-plane stacking faults is reduced by ∼70% as determined by a modified Williamson–Hall X-ray analysis.

Shape transformation of nanoporous GaN by annealing: From buried cavities to nanomembranes

Gallium nitride is considered a chemically inert, ceramic-like semiconductor with no effective etchants available at room temperature. In this letter, we study the shape transformation of nanoporous GaN prepared by an electrochemical process. It is found that the curvature-driven mass transport process at typical growth temperatures is effective in shaping GaN on both the nanoscale and microscale into useful configurations such as buried cavities or semiconductor-on-air structures. This process of “micromachining” GaN adds flexibilities to epitaxy and device designs. A monocrystalline GaN nanomembrane, extending millimeters in width, is reported as a proof-of-concept demonstration.

Improved photoelectrochemical water splitting efficiency of nanoporous GaN photoanode

Nanoporous (NP) GaN was used as a photoanode during photoelectrochemical water splitting and its efficiency and stability were compared with those of planar GaN. The NP GaN showed larger photocurrent density under illumination, which indicated more hydrogen generation. The enhancement was higher for a large applied voltage where the water splitting process was limited by charge transfer between semiconductor and electrolyte. Photocurrents decreased with time during the water splitting; however, the photocurrent degradation was less severe for NP GaN. It was attributed to efficient hole transport from photoanode to electrolyte in NP GaN due to its large surface area.