Abstract
The semiconductor gallium nitride is the material at the centre of energy-efficient solid-state lighting and is becoming increasingly important in high-power and high-frequency electronics. Reducing the dislocation density of gallium nitride planar layers is important for improving the performance and reliability of devices, such as light-emitting diodes and high-electron-mobility transistors. The patterning of selective growth masks is one technique for forcing a three-dimensional growth mode in order to control the propagation of threading defects to the active device layers. The morphology of the three-dimensional growth front is determined by the relative growth rates of the different facets that are formed, and for GaN is typically limited by the slow-growing {1−101} facets. We demonstrate how the introduction of nanodash growth windows can be oriented in an array to preserve fast-growing {11−22} facets at the early stage of growth to accelerate coalescence of three-dimensional structures into a continuous GaN layer. Cathodoluminescence and Electron Channelling Contrast Imaging methods, both used to measure the threading dislocation density, reveal that the dislocations are organised and form a distinctive pattern according to the underlying mask. By optimising the arrangement of nanodashes and the nanodash density, the threading dislocation density of GaN on sapphire epilayers can be reduced significantly from 109cm−2 to 3.0×107cm−2. Raman spectroscopy, used to monitor the strain in the overgrown GaN epilayers, shows that the position of the GaN E2H phonon mode peak was reduced as the dash density increases for a sample grown via pendeo-epitaxy whilst no obvious change was recorded for a sample grown via more conventional epitaxial lateral overgrowth. These results show how growth mask design can be used to circumvent limitations imposed by the growth dynamics. Moreover, they have revealed a greater understanding of the influence of the growth process on the dislocation density which will lead to higher performing electronic and optoelectronic devices as a result of the lower dislocation densities achieved.
Highlights
The electronic and optoelectronic properties of gallium nitride and its alloys are widely exploited for the creation of electronic and optoelectronic devices
Metal-organic vapour phase epitaxy (MOVPE), hydride phase epitaxy (HVPE) and liquid phase epitaxy (LPE), notably on the control of growth through the intrinsic growth parameters, and through external influences. The latter is exemplified by the use of selective masks on the growth substrate to influence the morphology of the subsequent growth, resulting in epitaxial lateral overgrowth [1]
The linear dependency of TDD as a function of dash density suggests that the TDD is related to the dislocation regeneration upon GaN coalescence events: the design reduces the density of coalescence events as the number of dashes decrease, which leads to a reduction of TDD
Summary
The electronic and optoelectronic properties of gallium nitride and its alloys are widely exploited for the creation of electronic and optoelectronic devices. In the past few years there has been substantial interest in the use of nanorods to create planar GaN epilayers with reduced dislocation density through dislocation bending and annihilation [1,2,3,4] and reduced strain [5,6,7,8] in order to mitigate the use of thermaland lattice-mismatched substrates Such improvements will lead to longer device lifetimes, and superior performance [9]. We introduce a pattern designed to reduce the influence of the slow-growing {1 À1 0 1} nanopyramid facets to the final overgrown layer This is achieved by elongating the circular nanorod geometry into an elliptical or rectangular ‘nanodash’ shape, where the major axis is aligned carefully to the underlying crystal orientation. The impact of the patterning on the strain of the GaN overgrown layer is investigated by Raman spectroscopy
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
