Tsung-Shine Ko

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.

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...

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.

Effects of Built-In Polarization and Carrier Overflow on InGaN Quantum-Well Lasers With Electronic Blocking Layers

Effects of built-in polarization and carrier overflow on InGaN quantum-well lasers with a ternary AlGaN or a quaternary AlInGaN electronic blocking layer (EBL) have been numerically investigated by employing an advanced device-simulation program. The simulation results indicate that the characteristics of InGaN quantum-well lasers can be improved by using the quaternary AlInGaN EBL. When the aluminum and indium compositions in the AlInGaN EBL are appropriately designed, the built-in charge density at the interface between the InGaN barrier and the AlInGaN EBL can be reduced. Under this circumstance, the electron leakage current and the laser threshold current can obviously be decreased as compared with the laser structure with a conventional AlGaN EBL when the built-in polarization is taken into account in the calculation. Furthermore, the AlInGaN EBL also gives a higher refractive index than the AlGaN EBL, which is a benefit for a higher quantum-well optical confinement factor in laser operations.

Enhancement of InGaN–GaN Indium–Tin–Oxide Flip-Chip Light-Emitting Diodes With TiO

Enhancement of light extraction of GaN-based flip-chip indium-tin-oxide light-emitting diodes (FC ITO LEDs) with an omnidirectional reflector (ODR) is presented. The ODR consisting of alternating layers of TiO2 and SiO2 is designed to possess a complete photonic bandgap within the blue region of interest, and it is fabricated by E-beam deposition. At a driving current of 300mA and a chip size of 1 mmtimes1 mm, the light output power of the FC ITO LEDs with the ODR reaches 156 mW. This is an enhancement of 31% when compared with the same device with an Al mirror instead. Furthermore, by examining the radiation patterns, the FC ITO LED with the ODR shows stronger enhancement around the vertical direction. Our work offers promising potential for enhancing output powers of commercial light-emitting devices

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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.

–SiO

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.

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Extended defect reduction in GaN can be achieved via direct growth on stripe patterned (11¯02) r-plane sapphire substrates by metal organic chemical vapor deposition. The striped mesa is along [112¯0] with two etched sides in {0001} and {11¯01} faces. GaN grown on both etched facets in epitaxy exhibit different crystallographic relationships with sapphire substrate which are (11¯02)sapphire‖(112¯0)GaN and [112¯0]sapphire‖[1¯100]GaN, and (0001)sapphire‖(0001)GaN and [112¯0]sapphire‖[1¯100]GaN, respectively. The dislocation densities can be significantly reduced through epitaxial growth on the inclined lateral faces of mesas. Dislocation density in the order of ∼107cm−2 can be achieved in the tilted GaN.

Multilayer Stack Omnidirectional Reflector

The crystal quality of a-plane GaN films was improved by using epitaxial lateral overgrowth on a nanorod GaN template. The investigation of x-ray diffraction showed that the strain in a-plane GaN grown on r-plane sapphire could be mitigated. The average threading dislocation density estimated by transmission electron microscopy was reduced from 3×1010 to 3.5×108 cm−2. From the temperature-dependent photoluminescence, the quantum efficiency of the a-plane GaN was enhanced by the nanorod epitaxial lateral overgrowth (NRELOG). These results demonstrated the opportunity of achieving a-plane GaN films with low dislocation density and high crystal quality via NRELOG.

Improved hydrogen detection sensitivity in N-polar GaN Schottky diodes

The crystal quality of a-plane GaN films was improved by using epitaxial lateral overgrowth on trenched a-plane GaN buffer layers. Not only the threading dislocation density but also the difference of anisotropic in-plane strain between orthogonal crystal axes can be mitigated by using trenched epitaxial lateral overgrowth (TELOG). The low threading dislocation density investigated by the cross-sectional transmission electron microscopy was estimated to be 3×107cm−2 on the N-face GaN wing. On the other hand, the Ga-face GaN wing with a faster lateral overgrowth rate could be influenced by the thin GaN layer grown on the bottom of the trenches, resulting in higher dislocation density generated. As a result, the authors concluded that a narrower stripped GaN seeds and deeper stripped trenches etched into the surface of sapphire could derive a better quality a-plane GaN film. Finally, they demonstrated the fast coalescence process of TELOG GaN films below 10μm thick.