Transcript Slide 1

ANN NIRT GOALI
Nano-Engineering Efficient Optoelectronic Devices
NSF Grant ECS – 0609416
PIs: S. Nikishin, J. Berg, A. Bernussi, M. Holtz, H. Temkin
Nano Tech Center, Texas Tech University, Lubbock TX-79409
Motivation and Overview
Nanophotonic Texturing Experiments for Enhancing Light Extraction – GaN
Develop and apply methods of nanoengineering to improve light extraction
properties of ultraviolet light-emitting diodes (LEDs).
1. Basic process flow combines electron-beam lithography (SEM) and
plasma etching.
- Pore-to-pore pitch fixed at 1 μm.
- Pore diameter varied from 125 to 700 nm.
- Pore depth ~ 400 nm.
2. SEM-cathodoluminescence intensity found to increase systematically
with pore area fraction. Increase is ~ double starting GaN layer.
A quantitative model is derived in which the net intensity is expressed
as the sum of intensities originating from the near-surface and
pore-bottom regions. The combined terms describe well the observed
dependence (not shown).
3. SEM-cathodoluminescence peak red shifts with pore area fraction.
Shift interpreted based on relaxation of biaxial compressive stress.
Interpretation quantitatively confirmed by finite element simulation of
stress.
Starting surface (GaN)
The main efforts in year one were guided toward:
Mask removal
Active Region Nano-Engineering
Basic LED Design
p-type AlGaN
#1
#2
#3
#4
AlN/AlGaN active region
n-type AlGaN
AlGaN or AlN
template
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(a)
2D
1.0
2D  3D
2D
a
b
3D  2D
c
PL
CL
CL
4.4
Peak position (eV)
10
d
4.3
4.2
Al0.4Ga0.6N
4.1
(b)
1.0
PL
CL
CL
0.7
0
Substrate design for improved epitaxy.
(GOALI partner – TDI Inc.)
10.0
7.5
5.0
2.5
0.0
0
X-ray reciprocal space map (RSM) for
(0002) reflection of AlN template.
FWHMω-scan=100" & FWHM2Θ-ω=110".
Inset: TEM cross-section of
AlN/sapphire interface.
2.5
5.0
7.5
10.0
Plan-view 10 X 10 μm2 AFM scan
of HVPE grown 6.4 μm thick
AlN/sapphire template. The
surface RMS roughness is ~ 3
nm. Step-flow growth mode
dominates.
10
20
30
Growth time (s)
40
Ammonia flux, growth mode, and corresponding RHEED
images (insets a, b, c, and d) versus growth time of well
material. Sample #1 (21 s growth time) – onset of
3D growth mode; #2 (26 s growth time) – at least 3/5 of
well is grown in 3D mode; sample #3 (32 s growth time);
sample #4 (42 s growth time) – onset of 3D →2D transition.
CL and PL data versus growth time of well material. PL
excitation energy density is 0.72 µJ/cm2. (a) peak position,
(b) intensity, and (c) 1/e decay time. The dashed
horizontal line is the bandgap of the bulk Al0.4Ga0.6N
material. Filled circle is CL of 2D grown well material.
Intensity (a. u.)
•
Plasma etching
0.5
0.0
(c)
120
The room temperature CL intensity
Of QWs versus NH3 flux. Insets are
depiction of quantum structure
formation and TEM cross-section of
a domain.
PL
1/e (ps)
•
6
•
•
Nanoengineering of active region of UV LEDs using growth of AlGaN/AlGaN on AlGaN
and AlN template substrates (GOALI partner – TDI Inc.)
Superlattices with AlGaN quantum dots in well material
Detailed characterization of these materials using X-ray diffraction, photoluminescence,
micro-cathodoluminescence, atomic force microscopy, and transmission electron
microscopy
Fabrication of nanophotonic surfaces on Si and GaN using nanolithography and
plasma etching
Study of optical properties and light extraction efficiency of nano-structured GaN
substrates
BEP x 10 (torr)
•
E-beam lithography
100
80
60
20
25
30
35
40
Growth time (s)
Educational Impact
This project provides nine students at the BS, MS, and PhD levels with a fully integrated multidisciplinary setting for research and training. Of the students involved in this work, five are
female, and one of those was African-American. Three senior researchers are involved in research under this program.
Collaborators
Technologies and Devices International, Inc.; Army Research Laboratory; Veeco Instruments, Inc.; Soft-Impact, Ltd.; The Fox Group, Inc.
50-mm AlN free standing wafer
fabricated at TDI Inc. HVPE growth of
thick AlN layer on SiC substrates and
subsequent removal of the SiC substrate
by RIE. (Courtesy of TDI Inc.)
Select publications under this grant
“Transmission properties of nanoscale aperture arrays in metallic masks on optical fibers,” J. Appl. Phys. 101, 014303 (2007).
“Selective area growth of GaN nano islands by metal organic chemical vapor deposition: experiments and computer simulations,” Proc. Mat. Res. Soc. Symp., 955, 0955-I07 (2007).
“Enhanced luminescence from AlxGa1-xN/ AlyGa1-yN quantum wells grown by gas source molecular beam epitaxy with ammonia,” Proc. SPIE Photonics Conf., 6473, 647306 (2007).
“X-ray diffraction study of AlN/AlGaN short period superlattices”, J. Appl. Phys., JR07-3866R (2007).
“Deep UV light emitting diodes and solar blind photodetectors grown by gas source molecular beam epitaxy”, J. Mater. Sci: Mater Electron, DOI: 10.1007/s10854-007-9405-3 (2007).
“Luminescence properties of AlxGa1-xN (0.4<x<0.5)/AlyGa1-yN (0.6<y≤1) quantum structures grown by gas source molecular beam epitaxy”, physica status solidi c (2007).
“Influence of photonic nanotexture on the optical properties of GaN”, Appl. Phys. Lett. 91, 103115 (2007).