12 November 2020
MOCVD tunnel junctions for GaN LEDs
University of California Santa Barbara (UCSB) in the USA claims the highest performance so far for tunnel-junction micron-sized blue light-emitting diodes (TJ μLEDs) grown by metal-organic chemical vapor deposition (MOCVD) [Panpan Li et al 2020 Semicond. Sci. Technol., vol35, p125023, 2020].
In particular, very low forward voltages (Vf) were achieved by enhancing the tunneling probability and the activation efficiency of the p-GaN layers by inserting an indium gallium nitride (InGaN) layer into the tunnel junction and by using a selective-area growth (SAG) process, respectively. The performance of the TJs is comparable to that from molecular beam epitaxy (MBE) growth processes, it is claimed.
The InGaN insertion layer uses a difference in charge polarization relative to GaN to introduce band-banding effects that reduce the tunneling distance, hence increasing the tunneling probability.
The researchers see their work as overcoming size limitations on TJ μLEDs, enabling low Vf even for larger μLEDs and for regular-size LEDs. MOCVD is a much preferred process over MBE in large-scale, high-output manufacturing contexts. “This work solves the key challenges of MOCVD-grown TJs,” the team adds.
Tunnel junctions are seen as an alternative to transparent conductive oxides (TCOs) like indium tin oxide (ITO) for improving current spreading and thus emission uniformity and efficiency in GaN-based LEDs. Unfortunately, up to now the increased Vf penalty has wiped out any potential benefit in terms of reduced light absorption.
Further, TJs could provide interconnections between stacked ‘cascaded LEDs’ of devices with different color emissions, possibly enabling a reduced complexity assembly process for blue/green/red LED displays. Other potential benefits could be seen for producing edge-emitting laser and vertical-cavity surface-emitting laser (VCSEL) diodes.
The researchers used commercial blue LED material on patterned sapphire substrate (PSS) as a template for the overgrowth of TJ structures. Solvents including aqua regia were used to clean the surface before the overgrowth of III-N materials by MOCVD. The layer structure (Figure 1) consisted of 3nm silicon-doped n-In0.15Ga0.85N, 20nm n+-GaN, 350nm n-GaN, and 15nm n-GaN cap. A sample without the InGaN interlayer was also produced for comparison. A further variation was a structure produced by SAG.