research  review Supressing leakage in


nitride diodes Leakage currents in Schottky barrier diodes plummet by seven orders of magnitude when the barrier composition is shifted from Al0.21


Ga0.79 N to Al0.11 Ga0.89 N.


ENGINEERS from Central Research Lab, Hitachi, have shown that reverse leakage currents in AlGaN/GaN Schottky barrier diodes can plummet through reductions in sheet carrier density.


High leakage currents are an Achilles heel for today’s GaN diodes, which hold much commercial promise thanks to their combination of high temperature, high frequency and high power.


It is widely believed that the dominant cause of leakage is structural defects, especially dislocations, which can result from strain in the AlGaN layer.


However, initial studies by the Hitachi researchers indicated that sheet carrier density also influences leakage current.


This finding motivated these engineers to carry out further work involving three AlGaN/GaN heterostructures that shared a 3.6 µm, highly-resistive GaN buffer layer and a 5 nm GaN cap.


In these samples, barrier compositions were Al0.11 Al0.21


Ga0.89 Ga0.79 N, Al0.16 Ga0.84 N and N, and corresponding room-


temperature two-dimensional electron gas densities determined by Hall measurements were 3.6 x 1012 6.6x1012


cm-2 respectively.


“We prepared those three samples with our own MOCVD [tool] under precisely controlled conditions,” says lead author Akihisa Terano. “We think that the material quality of our three samples is almost the same.”


All three epiwafers were processed into planar Schottky barrier diodes with contacts formed by electron beam evaporation (see Figure 1).


Measurements on the devices revealed that a three-fold fall in the density of the two-dimensional electron gas results in a reduction in leakage current by seven


and 1.0 x 1013 cm-2


cm-2 ,


, orders of magnitude (see Figure 2).


Unfortunately, slashing leakage current with this approach has its downsides.


“A reduction in the sheet density is thought to lead to some increase in the resistance of the drift layer, resulting in some decrease of the forward current,” explains Terano.


He and his co-workers are now planning to tackle this issue head-on, developing devices that combine a sufficiently high forward current with a low leakage.


A. Terano et al. Electron. Lett. 48 274 (2012)


Modelling of growth dynamics is a challenging computational task as it requires simulation of realistic QD sizes with atomistic resolution.


By performing a systematic set of multi- million atom atomistic simulations, the researchers in Ireland found a correlation between their calculations and the experimentally measured data.


The results quantitatively show the influence of indium-gallium intermixing and indium segregation effects on the polarisation properties of the QDs.


M. Usmanet et al. Nanotechnology 23 165202 (2012)


April / May 2012 www.compoundsemiconductor.net 51


Figure 2.Massive changes in leakage current result from relatively small changes in the density of the two- dimensional electron gas


Multi-million-atom QD metrology


Researchers have used a simulation of InAs quantum dots to accurately reproduce experimentally measured optical spectra


GROWTH DYNAMICS in real quantum dot devices has been modelled by an international team from Tyndall National Institute, Ireland, the National Nanotechnology Laboratory-CNR, Italy, and Purdue University, Indiana. The researchers used multi-million atom simulations that are claimed to deliver unprecedented precision.


Figure 1.Schottky diodes built by Hitachi feature 1000 µm by 200 µm cathodes and anodes,separated by 200 µm


The semiconductor device simulation tool, called NEMO 3D, was developed at NASA and Purdue. This tool can model structures containing up to 50 million atoms through parallelised computation on high performance supercomputing clusters.


By accurately mimicking indium-gallium intermixing and indium-segregation effects with an innovative two-layer composition model for quantum dots (QDs), the researchers can reproduce polarisation- dependent optical emission spectra of an MBE-grown InAs QD.


Tyndall has proposed an innovative two- composition model for the InAs QDs, comprised of an indium-rich central core surrounded by an indium-poor region close to the edges of the QD. This model allowed the researchers to reproduce the experimentally measured optical spectra.


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