Etching technology requirement for III-nitride structure scaling down from micro to nano-scale
2005/10 Jung-Min Hwang
III-nitride semiconductors and their
alloys have great thermal stability and exhibit a direct band transition for optical
emission. They have great potential for application in high-power optoelectronic
devices. Moreover, GaN-based diodes have been demonstrated to be a possibility
for next-generation lighting with emission of white light.
The III-nitride has high bond strength
make them essentially chemical inert and highly resistant for etching process.
Etching damage became a serious problem for operating GaN based optical or
electronic device under high power or frequency region. The etching ability and
limitation need to be confirmed, especially for III-nitride structure scaling down from micro to nano-scale.
Title
angle IBE is a suitable technique for obtaining vertical profiles with smooth
sidewalls for mirror facets. The disordered region of the depth was reduced due to less momentum transfer
from ion to substrate with tilt ion beam incident. The
fabrication of recessed gate AlGaN/GaN HEMTs was applied the title angle IBE
technique to reduce the ion channeling effect, the transconductance was
improved from 40mS/mm to 60mS/mm compared to a device without recess and the
leakage current was reduced by nearly a factor of two.
Photo-assisted etching combines
ultraviolet laser radiation and exposure of reactive gas. The light source was
ArF excimer 193 nm. The reactive gas was Cl2. The substrate was cooling by liquid nitrogen. The
photochemical reaction was dominated in liquid nitrogen cooling to avoid the
thermal chemical reaction. It was realized the damage free etching under photon
cryogenic etching with high etching rate of 84nm/mins.
Photoelectrochemical
(PEC) etching has been demonstrated to provide a broad and smooth etched
surface with a vertical anisotropic profile. Two disadvantages of PEC etching encountered
in device fabrication are a lateral potential gradient on the GaN film and a
narrow region of operation for a smooth etched surface. Electrodeless photo-assisted wet etching was
developed to solve this problem, with an oxidizing agent peroxydisulfate (S2O8-)
replacing the Pt electrode in PEC etching. The rate of electrodeless PEC
etching might attain 40-50 nm/min for GaN, but the resulting morphology was
still not smooth, with a RMS roughness 20 nm.
Electrodeless
PEC etching of GaN with a chopped UV light source provides a smooth etched
surface of large-area and a large rate of etching with a controllable and
uniform depth of etching. The roughness of the etched GaN surface is very
important for the practical application in device fabrication. A uniform and
smooth GaN surface was obtained with a root-mean-square roughness 0.37 nm
through electrodeless PEC etching in a solution (KOH 0.01 M, K2S2O8
0.05 M) with a chopper frequency 2500 Hz. Chopped PEC was a important
development for photo-assisted wet chemical etching to be applied in the
industry with large scale, uniform, ultra smooth, mild etching method. The
p-GaN etching was also realized by ELPEC etching with a chopped UV source
(ELPEC-CS etching) using an Au mask in K2S2O8/KOH solution. The etching rate of
p-GaN was 2.8 nm/min at a
chopper frequency of 3000 Hz and a power intensity of 63 mW/cm2 in solution (0.5 M
KOH,0.05 M K2S2O8). The chopped photon was used
to suppress the recombination current at dislocation. The smooth, uniform, and
broaden etching surface in GaN by ELPEC-CS was achieved. The ELPEC-CS in p-GaN
was due to the strong hole relaxation current while the light off during the
chopped light with 2mm strip
pattern. The Au and Pt catalytic metal mask could enhance the etching rate of
p-GaN. The vertical etching profile and no lateral etching depth distribution
were achieved. There had the lateral etching effect in P/N diode or MQWs
structure for blue and green emission, the better collimated light could
reduced the lateral etching. The p-GaN, p-AlGaN, GaN, InGaN, n-GaN could be
etched by ELPEC-CS method. A new practical tool for III-Nitride based optical
or electrical device fabrication process was invented in this work.
Micron-scale light-emitting diodes have been an interest research
area of recently. The micro LED was fabricated with ICP etching methods. GaN
based micro-LEDs could offer higher light output efficiencies compared to their
broad-area conventional LED. The mechanism responsible for the improved
performance of micro-LED is still under research. The electrical properties of
micro-LEDs were measured by probe station. The ideality factor of micro-LED was increasing from 5 to 25 while the
device was scaling down from 10um to 3um. The etching damage in the etched
sidewall will enhance the non-radiated recombination. In high current injection
condition, the power saturation was not observed. This special mechanism was very
different comparing with broad-area conventional LED. The properties of the
micro-LEDs were modeling by equivalent circuit from 10um to 3um scale. The high
ideality factor indicated the etching damage was serious for micro LED device
fabricated by dry etching method.
The size of III-Nitride based structure was fabricated from 300mm to 10nm. The LED structure from 300mm to 4mm could be formed by photolithography
following by etching. By controlling photolithography in diffraction mode or
over etching the metal mask, the size could be reduced from 2mm to 0.5mm. The
mesa GaN LED or P/N diode could be fabricated. The nano structure from 100 to
30nm of GaN could be formed due to the dislocation-induced morphology during
etching in photo-assisted wet etching. The structure with 50~10nm nano-wire
could be fabricated in GaN or p-GaN during photo-assisted wet etching. The
micro-LED was successfully fabricated. Fabricating the nano-LED with small size
(100~0.1nm for nano scale) for III-nitride was the major research in the
future.
- Conference papers (會議發表):
Etching technology requirement for III-nitride structure scaling down from micro to nano-scale, Jung-Min Hwang (invited speaker), The 2nd International Symposium on Point Defect and Nonstoichiometry, October 4-6 (2005), Kaohsiung, TAIWAN (in NSYSU)
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