Effect of Surface Morphology on Diode Performance in Vertical GaN Schottky Diodes

Abstract

The properties of thick GaN layers grown by metal organic chemical vapor deposition on 2 HVPE substrates were investigated. Although the epilayer is smooth on a microscopic scale with high quality layers as evidenced by X-ray diffraction and photoluminescence, macroscopic morphological variations are observed on the wafer surface. These variations correspond to disparities in leakage current in fabricated Schottky barrier diodes, with rougher macroscopic morphology resulting in increased leakage current. GaN-based technology has found widespread success in the electronic and optoelectronic fields, both as transistors and LEDs.1-3 These successes are mainly based on lateral device geometries or thin vertical structures based on heteroepitaxy on non-native substrates. While monolithic microwave integrated circuits (MMICs) based on high electron mobility transistor (HEMT) structures have enjoyed great success as RF power amplifiers, the lateral device geometry presents fundamental limitations, particularly for breakdown voltage, thus limiting applications for high power switching. [4][5][6][7][8] To overcome some of these limitations and reach the full potential of the material for power devices, the next-generation of GaN power switching devices needs to move to a vertical geometry, which is enabled by the commercial availability of high quality, freestanding GaN substrates produced by hydride vapor phase epitaxy (HVPE) or ammonothermal growth. 9Homoepitaxial growth of power device quality GaN films on native substrates presents unique challenges from preparing the surface for growth, both to remove impurities and reduce thermal decomposition, to reducing impurities in the film for thick drift layers. [10][11][12][13] This work will evaluate GaN films grown by MOCVD on HVPE substrates to understand the impact of surface roughness on Shcottky barrier diode performance to evaluate metrics for evaluating epitaxial layer quality and uniformity.The samples in this study were grown in an Emcore D-180 metal organic chemical vapor deposition (MOCVD) reactor on a commercially-sourced, 2 freestanding n+ GaN substrate from Kyma Technologies grown by HVPE. The MOCVD growth consisted of 5 μm of unintentionally doped GaN using standard growth conditions (3000 V/III, 200 Torr, 1030• C) and precursors (trimethylgallium and ammonia). Secondary ion mass spectroscopy (SIMS) measurements on films from this reactor under these growth conditions indicate background oxygen and carbon impurities at detection limits and a Si concentration of ∼2 × 10 16 cm −3 . As the substrate was received polished to an epi-ready surface, no additional in-situ or ex-situ cleaning procedures were undertaken. After growth, the surface morphology was characterized using multiple techniques to evaluate many length scales, including optical microscopy, atomic force microscopy (AFM), and optical profilometry. In addition, photoluminescence (PL), X-ray diffraction (XRD), and wafer bow were investigated to assess material quality.XRD maps were taken of the wafer...

Deep-UV photoemission electron microscopy for imaging nanoscale heterogeneity and defects in gallium nitride

“…Elimination of dislocations is crucial for device reliability, as dislocations in the substrate are known to propagate through the epitaxial growth, compromising the active device layers 11,12 . For example, substrates with patterned arrays of dislocation centers exhibit spatially-varying device performance and failure locations [13][14][15][16][17] . However, direct correlation between defective substrate areas, dislocations, and device performance is not straightforward: certain defects appear to be more harmful for devices than others 11,12,16,17 .…”

Hot-Wall MOCVD for High-Quality Homoepitaxy of GaN: Understanding Nucleation and Design of Growth Strategies

“…MOCVD is among the techniques of choice to fabricate large-scale GaN electronic device structures and has been demonstrated as a promising approach to obtain device-quality GaN material. ,,, Hot-wall MOCVD is a modification that offers reduced temperature gradients in both vertical and horizontal directions. It further allows for independent control of the gas-phase chemistry over the substrate and surface diffusion, which may be exploited to control defect formation and impurity incorporation in a wide growth window.…”

“…21,24 the performance of vertical homoepitaxial GaN Schottky and p−n diodes, for example, a high leakage current has been reported for rough surface areas. 25,26 The exact origin of the wavy surface has not been clarified yet. Crystal orientationdependent diffusion and migration of adatoms and high supersaturation growth conditions required for homoepitaxial GaN growth are believed to play a significant role in the surface morphology.…”

“…Although GaN homoepitaxial growth on HVPE and ammonothermal GaN substrates has been extensively studied, − critical issues such as thermal decomposition of GaN, , wavy surface and formation of hillocks and ridges, , generation of interface and V-shaped dislocations, and impurity incorporation still remain to be solved. One of the main problems in metalorganic chemical vapor deposition (MOCVD) homoepitaxial growth on (0001) GaN substrates is the wavy surface morphology and the presence of macrostep terrace structures. , Such a morphology affects negatively the performance of vertical homoepitaxial GaN Schottky and p–n diodes, for example, a high leakage current has been reported for rough surface areas. , The exact origin of the wavy surface has not been clarified yet. Crystal orientation-dependent diffusion and migration of adatoms and high supersaturation growth conditions required for homoepitaxial GaN growth are believed to play a significant role in the surface morphology. , The formation of hillocks can be eliminated using off-cut (0.25°) substrates under growth conditions, providing a large degree of adatom desorption, that is, at a high growth temperature and at a low V/III ratio .…”

Epitaxial strategies for defect reduction in GaN for vertical power devices

“…GaN substrates is the wavy surface morphology and the presence of macro-step terrace structures [29,32]. Such a morphology affects negatively the performance of vertical homoepitaxial GaN Schottky and p-n diodes grown by metalorganic chemical vapor deposition (MOCVD), e. g. a high leakage current has been reported for the rough surface areas [33,34]. The exact origin of the wavy surface has not been clarified yet.…”