Feedback from ISSLED 2017
Margaret Hopkins from University of Bath gave a poster presentation entitled “Indium-free Transparent Ohmic Contacts to N-polar n-typeGaN” describing results from the INREP project.
Low resistance transparent contacts to N-face n-type GaN have the potential to greatly simplify the processing of high-brightness flip-chip or vertical light-emitting diodes (V-LEDs). To date work on forming transparent contacts to the top N-face n-GaN layer in V-LEDs has focused on using indium tin oxide (ITO). However, a technology based on earth-abundant elements is preferred for cost, supply and environmental reasons. Such Indium-free wide band gap transparent materials typically form Schottky diodes on n-GaN2 and methods for overcoming this limitation are required. The INREP partners report a study of the electrical properties of transparent Aluminium or Boron-doped Zinc oxide (AZO or BZO) contacts to moderately doped (n - 3 x 1018 cm-3) N-polar n-GaN for vertical LEDs. These contacts are non-ohmic, displaying leaky Schottky barrier behaviour.
However, exposure of the n-GaN surface to a hydrogen or oxygen plasma prior to deposition of the transparent conducting oxide gives ohmic contacts with contact resistances below 2 x 10-5 Ωcm2, which is close to state of the art. The mechanisms by which these contacts are formed has been investigated through the temperature dependence of the contact resistances and XPS measurements of the interfaces. The thermal stability of these contacts has also been investigated up to 250°C. This technique opens the door to creating In-free transparent conducting electrodes for efficient large-area V-LEDs.
The ISSLED is a biannual international meeting for scientists and engineers worldwide, both in academia and in industry, to meet together to share and exchange the latest discoveries and progress on light emitting semiconductors,
with a focus on III-nitride devices. It started in 1996/1998 in Chiba/ Japan and
went through Berlin/ Germany, Cordoba/ Spain, Gyeongju/ Korea, Montpellier/ France, Phoenix/ Arizona, Beijing/ China, Berlin/ Germany, and most recently Kaohsiung/ Taiwan in 2014.