Free-standing GaN from lateral overgrowth & chemical etch

Researchers from Gwangju Institute of Science and Technology and Samsung LED in Korea have developed a technique to chemically separate laterally overgrown gallium nitride (GaN) crystal from its sapphire substrate [Chu-Young Cho et al, Appl. Phys. Express, vol4, p012104, 2011]. Such free-standing GaN is an attractive substrate for growing more efficient light-emitting structures.

Commercial nitride semiconductor light-emitting diodes (LEDs) are generally produced by growing epitaxial layers on sapphire substrates. Since sapphire is electrically insulating, one has to put the device contacts on the nitride semiconductor side of the device. This creates lateral current flows that suffer from current crowding, which reduces efficiency. By removing the sapphire substrate, one is able to put the contacts on both the top and bottom of the device, enabling vertical current flow.

Laser-lift off techniques have been developed to remove the sapphire, but can cause strain-induced cracking and other damage of the device structure, impacting performance. Another approach is chemical, where defects are not expected to be generated since physical and thermal damage are avoided.

The Gwangju Institute of Science and Technology and Samsung LED combined chemical lift-off and lateral epitaxial overgrowth (LEO, also known as epitaxial lateral overgrowth/ELOG). LEO uses a mask to block threading dislocations in certain parts of the GaN template, creating regions of near-damage-free crystal from material that grows over the mask.

The technique begins with growing GaN seed layers on c-plane sapphire using metal-organic chemical vapor deposition (MOCVD). A 25nm GaN nucleation layer (grown at 550 C) was followed by 2 m-thick undoped GaN (1020 C).

The LEO process was performed using silicon dioxide coated tungsten as a mask that consisted of a series of lines 6 m wide with 6 m gaps (Figures 1a and b). The 8 m-thick LEO GaN layer was grown at 1020 C (Figures 1c and d).