RESEARCH REVIEWHEMTs Oxygen plasma treatment
eliminates the need for a GaN cap
If oxygen plasma treatment is used to address current collapse, thebenefits of adding a GaN cap are negligible
ONE OPTION for improving
the electrical performanceof a GaN HEMT is to add aGaN capping layer. But thisstep has almost no benefit if
devices are subjected to anoxygen plasma treatment,
according to recent workby a Japanese team fromthe University of Fukui andHokkaido University.

GaN cap with oxygen plasmatreatment produces only avery marginal improvementover just plasma treatment.
Further insight into thebenefits of plasma treatment
came from numericalmodelling of the dependenceof the NDR on the on-time ofthe device.

Simulations show that
“Using oxygen plasma
oxygen plasma treatment
treatment of the surface
completely eliminates thealone, we can avoid
two deepest trap levels,
problems associatedoccurring at 0.62 eV andwith GaN cap layers,
0.67 eV, while the GaN cap
such as obtaining good
only eliminates the deepest
ohmic contacts whentrap at 0.67 eV.
using undoped GaN; andincreased leakage, andOxygen plasma treatment is a very effective way to reduce current-collapse,
To understand the impact
the associated decrease inwhich has been evaluated by considering the ratio of dynamic on-resistance toof plasma treatment, thebreakdown voltage, when
static on-resistance.
researchers employed X-ray
using an n-type GaN cap,”
photoelectron spectroscopy
says corresponding author
featured a GaN cap.
to investigate the nature of the AlGaN
Joel Asubar from the University of Fukui.
surfaces.
Cap-free devices with and without
There are also other drawbacks of anplasma treatment produced very similar
They found that oxygen plasma treatmentn-type GaN cap, says Asubar, such asvalues for maximum drain current
creates a surface oxide layer that is about
inferior gate control of the current, which
density, threshold voltage, leakage
2 nm-thick, and probably contains astems from an increase in the gate-tocurrent and breakdown voltage. This
combination of Al2O3 and Ga2O3.
channel distance.
indicates that plasma treatment does
not have a negative impact on the DC
To highlight the futility of incorporating acharacteristics of the device.
This finding led the team to conclude
GaN cap in a GaN HEMT that has beenthat the reduction in current collapse
subjected to plasma treatment, the teamThe researchers also investigated current
produced by oxygen plasma treatment
from Japan fabricated a range of devices
collapse in a range of devices, evaluating
results from the termination of nearand carried out a series of electrical
the ratio of the dynamic on-resistancesurface gallium and aluminium atoms,
measurements on them.
to static on-resistance – the higher thisand the probable occupation of nitrogenvalue, known as the normalised dynamic
vacancies by oxygen atoms.
All devices were grown on SiC, andon-resistance (NDR), the greater thefeatured a 500 nm-thick layer of GaN,
current collapse.
Asubar and co-workers are now aiming
followed by a 25 nm-thick layer ofto realise true collapse-free operation
Al0.2Ga0.8N. After source, drain and gate
Measurements on the uncappedwith their GaN HEMTs. “We are planningstructures revealed that oxygen plasma
to combine the oxygen plasma-treatment
contacts were formed, sputtering added
treatment mitigates current collapse (seeapproach with other approaches against
a 150 nm-thick SiN passivation layer.
figure).
current collapse, such as the use of fieldplates.”
Four types of device were formed,
These experiments also show that a
distinguished by whether or not they had
GaN cap can combat current collapse,
been treated with an oxygen plasma for
J. Asubar et. al.
but this is not as effective as oxygen
60 s at a power of 100 W prior to SiN
Appl. Phys. Express 8 111001 (2015)
plasma treatment – and combining apassivation, and whether or not they

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