Fields modification Fields modification in high Al or In content - - PowerPoint PPT Presentation
Fields modification Fields modification in high Al or In content III- -nitrides nitrides in high Al or In content III Junyong Kang ( ) Engineering Research Center for Micro-Nano Optoelectronic Materials and Devices of State
Junyong Kang (康俊勇) Engineering Research Center for Micro-Nano Optoelectronic Materials and Devices of State Education Ministry, Fujian Provincial Key Laboratory of Semiconductors and Applications Department of Physics, Xiamen University, Xiamen 361005, P. R. China (phone) +86-592-2185962 (e-mail) jykang@xmu.edu.cn
Polarity in AlN AlN
Inhomogeneity in in InN InN
Asymmetric (GaN)m/(AlN)n GaN)m/(AlN)n superlatices superlatices
Mg-
and Si-
codoped codoped superlattices superlattices
Ultrathin compressive strained compressive strained InN/GaN InN/GaN MQWs MQWs
Blue, green, and white
Blue
Ultra Violet
Military, automobile and aircraft
√ √
Most TVs are using III nitride LEDs as backlight Lighting is expected to grow quickly in the near future
If high Al content III nitride materials and devices are developed, our manner would be profoundly changed. Only a narrow range of III nitrides has been used If high In content materials can be grown well, III nitride devices would replace almost components made
GaN AlN AlxGa1-xN x
allowing multiple junction solar cell fabrication using one material system. (Such wide band gap is not available in other established material systems)
dislocation densities
surface recombination
Fundamental and technologic problems
Strong misfit stress field Polar mixing Phase separation
Strong polarization field Optical anisotropy
Large thermal activation energy of acceptor in high Al content nitrides
(a) Cross-sectional HRTEM image
domain boundary. (b) and (c) inversion domain regions.
Al
Cross-sectional TEM images of InGaN layer a grown on a GaN surface. The inset shows In-rich dot regions.
In-rich dots If people want to control the fields well
Quantum confined Stark effect leading to carrier separation in quantum well. The effect can be deminished by fabricating QW on non-polar plane, but it is difficult to grow.
People should establish the methods
Heavy hole (HH) band 6 （x < 0.5） Favor for light extraction along c axis (Ordinary light, Ec)
2 3 4 5 0.0 1.5 3.0
2
Energy (eV) E//c Ec 20 40 60 80 100
0.0 0.1 Al composition (%) cr
Top of valence bands in Alx Ga1-x N
Crystal-field split hole (CH) band 1 （x > 0.5） Favor for light extraction vertical to c axis (Extraordinary light, E//c)
People are interested to know
Large thermal activation energy of Mg acceptor in high Al content AlGaN. Conventional SL has be proposed to modify band bending so that part of Mg levels locate above Fermi level, but the modification is insufficient.
EA
People like to develop method
Based on density function theory
charge density, potential, and energy
Band bending in the QWs
Calculated projected PDOS of different atomic layers are arranged along [0001] direction.
XMU HP Integrity Superdome 8 nodes: each with 32 CPUs Our group Lenovo R515 4 nodes: each with 4 CPUs & Dawning Tiankuo series 4 nodes: each with 4 CPUs & Lenovo DeepComp serie 12 nodes: each with 2 CPUs
, Cp2 Mg, and SiH4 Thomas Swan 32 in. CCS
Functions
structures smaller than 100nm)
6.6-1500K)
in situ nano-structural comprehensive property measurement system
Bede QC 200 XRD Accent HL5500 Hall system Varian Cary 300 UV-visible spectrophotometer Horiba Jobin Yvon UVISEL FUV Spectroscopic Ellipsometer
High melting point High pressure Polarity mixture Low crystalline quality by lower temperature epitaxy
Because of severe pre-reaction between TMA and NH3
Al-polar surface N-polar surface
Total Energy (eV) Eclean
Ew1
Ez1
Ew2
Ez2
Total Energy (eV) Eclean
Ew1
Ez1
Ew2
Ez2
z w b j z w b i z w d B
E n E n E E T k E ，其中， ) exp( Barrier heights of different paths
SEM images of SEM images of epilayers epilayers Monte Carlo simulations Monte Carlo simulations
Morphologies of Al-polar
at 1373K.
Time
Initiation from dendritic clusters → Adhesion forming continuous maze → Coalescence by fractal extension
Coverage/temperature-kinetic phase diagrams Average cluster number Average cluster size Average cluster compact degree Deposition rates:
200.6
210
310
410
Peak FW0.5M Intensity (cps) OMEGA (arcsec)
(0002) XRD peak
Atomic scale surface step AlN film grown at 1100oC (1373K) with two step technique Lower dislocation density
MOVPE depends on:
pressure over InN
temperature
Inhomogeneity Inhomogeneity
N N In In Ab Ab initio initio calculations calculations
Looking for preferable deposition sites
top top t4 t4 h3 h3
[ [-
1100] [11 [11-
20] [0001] [0001]
In In likes likes higher higher coverage coverage
4 site
site N N prefers lonely prefers lonely at t at t4
4 site
site
N will penetrate into the interstitial space between In bilayer and diffuse laterally to form tetrahedral coordination.
Diffusion path of N on In Diffusion path of N on In bilayer bilayer
t4 t4 top top h3 h3
E =
E =
E =
E =
N can only pass through the top In layer ! Nitrogen movement Nitrogen movement
trilayer
t4 t4 h3 h3 top top
E =
E =
Sample Time of TMIn(s) Time of NH3 (s) T (℃) P (Torr) A 33 33 581.5 450 B 16 33 581.5 450 C 8 33 581.5 450 D 4 33 581.5 450
ON OFF OFF ON
NH3 TMIn
Alternating supply technique Alternating supply technique to form the In to form the In bilayer bilayer
16 s 16 s 4 s 4 s
m
Local charges GaN GaN AlN AlN
The band edge anisotropy disappears when GaN well thickness becomes thinner than 6 MLs.
m /(AlN)
n superlatices
AlN
16 17 18 19 20
(deg) 1 ML 1 ML 2 ML 2 ML 3 ML 3 ML 4 ML 4 ML
AlN (0002) +1 +1 +1 +1
(0002) XRD rocking curves show that the superlattices have been fabricated. Spectroscopic Ellipsometer spectra
directions.
m /(AlN)
n superlatices
Traditional Mg-modulation- doped AlGaN/GaN SL is still difficult to get low resistivity in high Al content nitrides !!
Band alignment changes from type-I to type-II when AlGaN is doped with Mg
-doped layer: Mono- atomic layer of Mg or Si at the interfaces Potential: PAW_GGA； K-mesh: 8X8X2；
δ-doped SL1 δ-doped SL2
Remarkable band bending in both -codoped SLs Opposite trend for SL1 and SL2
Electrons transfer from Si- doped interface to Mg- doped interface
GaN AlGaN:Mg GaN Mg- Si-
4000 8000 12000 5 10 15
Intensity of
Time (s)
LT buffer Thermal cleaning 500 Torr 150 Torr 100 Torr 70 Torr AlGaN/GaN SLs
HT GaN
10450 10475 10500 10525 10550 Time (s)
On high quality thick-GaN layer -doped layers: Closing TMG & TMA and keeping on NH3 and Cp2 Mg or SiH4 Growth interruption to smooth interface
20 periods
200 250 300 350 400 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Normalized Intensity (a.u.) Wavelength (nm)
-codoped structure is superior in p-type conductivity for high Al content AlGaN
As grown wafer with peak-wavelength up to 213nm is easy to light up
blue phosphor powder green phosphor powder red phosphor powder EL spectra of as grown UV-LED wafer
Position Vf @ 1mA Vf @ 20 mA Vf @ 100mA Up 3.68 5.08 6.08 3.98 5.21 6.43 Middle 3.53 4.96 6.20 3.69 4.98 6.08 Down 3.82 5.00 6.26 3.83 5.02 6.28 Left 3.92 5.13 6.39 3.68 4.70 5.80 Right 3.87 5.09 6.37 3.90 5.11 6.35
DUV LED chips were fabricated
The relevant turn-on voltages are smaller than those of MD SL structure
Advantages of compressively strained MQW: Narrow line-width High power Low threshold current, and so on
QWs with strong piezoelectric effect.
strong stress field.
30 MLs Well width: 2 MLs
B22 W2 B22 W4 B22 W6 B22 W8 B6 W2 B14 W2 B22 W2 B30 W2
Barrier thickness: 22 MLs
Barrier
Well
Electronic structures depend on barrier and well thickness
Slightly enhanced as barrier thickness increases Markedly reduced as well width increases
Band bending
Well width > 6 MLs New VB extremum appear Ultrathin InN/GaN QW will be better
Sapphire(0001)
Sample TMG (s) TMI (s) Well Width (ML) Barrier Thickness (ML) BI 24 90 4 11 BII 48 90 4 23 BIII 96 90 4 45 WI 48 45 2 23 WII 48 90 4 23 WIII 48 180 8 23
GaN template 11×InN/GaN GaN InN 730 ℃
16000 16500 17000 17500 4000 8000 12000 16000
Intensity (a. u.) Time (s)
InGaN/GaN interface
TMIn TMGa
5000 10000 15000 4000 8000 12000 16000
MQWs
500 Torr 300 Torr
Thermal cleaning GaN template
Intensity (a. u.)
Coherent lattice and atomically sharp interfaces in HRTEM image
5 10 15 20 25 30
40 80 BII XRD data with linear fit Theoretical results Relative change /ave Ratio n/(n+m) (%) WIBIII WII WIII BI
The average strain
c c cave Coherent growth and strained control have been achieved
10
2
10
4
16.5 17.0 17.5 10
2
10
4
GaN(0002)
+1 (a)
GaN(0002) MQWs 0
th
MQWs 0
th
+1 (b)
Intensity (a.u.)
(deg)
1 2
1
1 h
e 2 h
e 3 h
e 2 h
e
Log Energy (eV) TI TII
1h-1e
2.7 3.6 4.5 0.0 0.4 Exciton
1h-1e 2h-1e
Energy (eV) 2.7 3.6 4.5
1h-1e 2 h
e
Energy (eV) Exciton
3 h
e
TI TII Agreement between the experiment results and the simulations indicates that the strain-dependent quantized levels have been realized.
2
TII TI
300 350 400 450 500 550 600 CL intensity (counts) Wavelength (nm) BI BII BIII 300 350 400 450 500 550 600 CL intensity (counts) Wavelength (nm) WI WII WIII
2.00 3.00 4.00 0.0 0.5 1.0 1.5 2.0 2.5
3.18 (a)
EL intensity (a.u.) Energy (eV)
100 mA 50 mA 1 mA
2.00 3.00 4.00 5 5 5 Energy (eV)
(b) 100 mA 50 mA 1 mA 3.14
EL spectra under different injection currents are free from
Reduction in defect recombination
Reduction in many body effect, such as Auger transition
Reduction in band-filling effect caused by phase separation Reduction in QCSE induced by the screening of the piezoelectric fields
Peak wavelength < 400nm
dynamic/kinetic properties
content nitrides by introducing GaN/AlN superlattices
Mg- and Si- codoped superlattices
compressive strained InN/GaN MQWs
National Natural Science Foundation of China “973” project of China “863” program of China Science & Technology Program of Fujian of China Science & Technology Program of Xiamen of China