atomic layer deposition: a process technology for functional … · 2015-01-23 · •atomic layer...
TRANSCRIPT
Atomic Layer Deposition:
a process technology for
functional ultra-thin films
Paul Chalker
VS5 and Vacuum EXPO Coventry 15 October 2014
Outline
• Atomic layer deposition processes
• Dielectric thin films in power semiconductors
• Doping in atomic layer deposition
• ZnO based transparent conducting oxides
• Some conclusions
Atomic layer deposition
Atomic layer deposition of thin films
ALD is a ‘saturative’ layer-by-layer process – highly conformal
It has to be
volatile here
By-products must be
removed here
Selection of ALD precursors
Plasma
Platen
Precursors
ALD
valves Pump
Carrier gas
It has to decompose
here when exposed to
the ‘oxidant’
6
Industry sectors:
• Buildings and Industrial
• Electronics and IT
• Renewables and Grid Storage
• and Transportation
600 V applications typically include: photovoltaic (PV)
inverters; motor drives; and power converters for
electric vehicles (EVs)
ALD dielectrics in GaN on Si power devices
ALD dielectrics in GaN on Si power devices
Silicon (111)
GaN
AlGaN S D
G - FP
ALD dielectric
2 DEG
TEM analysis of AlGaN heterostructure
TEM HREM lattice
image ([11-20])
Geometric phase
analysis of lattice strain
High-electron-mobility transistor (HEMT) structure
Two-dimensional electron gas (2DEG):
• Piezoelectric and spontaneous
• Strain dependent – [Al] content in barrier
• Can be adversely effected by charge in
dielectric
Source: L Lari, PhD thesis, University of Liverpool 2008
ALD dielectrics in GaN on Si power devices
EP/K014471/1 Silicon Compatible GaN Power Electronics
Courtesy of Edward Wasige and Iain Thayne
University of Glasgow
11
10
9
8
7
6
5
4
3
2
1
Die
lectr
ic b
reakd
ow
n s
tren
gth
(M
V/c
m)
35nm AlOx E-mode
VT ~ -1 V
Threshold voltage can be positive or negative depending on
The oxide / III-nitride interface
Robertson, Rep. Prog. Phys. 69 (2006) 327
10nm ALD AlOx TMA and water ■ or with O2 plasma ♦
ALD dielectrics in GaN on Si power devices
Challenges and opportunities for 600V technology:
• Lower on-resistances, lower conductivity losses and higher overall
efficiency
• Higher thermal conductivity allowing more efficient heat transfer
• Higher temperature operation
• Lower reverse recovery current, reducing switching losses and EMI
• Operation at >20 kHz frequencies
• Higher voltage input/output ratios enabling single stage DC-DC
conversion from 48V to 1V
Source: http://www.compoundsemiconductor.net/csc/news-details/id/19736817/name/GaN-in-power-electronics-applications.html, Sept 2013
Atomic layer deposition - doping
P1
P2
ALD P1
P2 1st monolayer
2nd monolayer
H2O
First solar is leading the way with high volume thin film PV
manufacture and breaking the $1 per watt barrier
Thin film PV (a-Si, CdTe and CIGS) will be a quarter of the
market by 2013 Commissioned: Oct 2010; Sarnia; 80 MW
ALD of Transparent Conducting Oxides
- for CdTe based photovoltaics
- Transparent electronics
Courtesy of Steve Hall and Ivona Mitrovic, EEE, University of Liverpool
Key Materials Challenges for TF-PV from
MATS-UK SRA
• Improve efficiency of energy conversion at module level.
• Reduce amount of costly semiconductor materials and
efficient materials usage.
• Use cheaper materials.
• Cheaper and lower energy processing combined with high
throughput.
• Improved durability and product life
Transparent conducting oxides: doped ZnO
[Al]
[Ga]
[Ge]
0 2 4 6 8 10 12 14 16 18 20 0
5
10
15
20
Do
pa
nt A
LD
cycle
fra
ctio
n (
%)
Dopant / (Dopant + Zn) content in film (%)
The proportion of [dopant], measured by EDX spectroscopy is proportional to
the dopant precursor ALD cycle fraction.
TEGa
TMA
DEZn
GEME
Doped ZnO – sheet resistance v’s dopant
fraction
• Minimum sheet resistance between 4 – 6% [dopant] incorporation
• Gallium doping produces lowest sheet resistances
106
105
104
103
102
Dopant ALD cycle fraction (dopant / (dopant + DEZn), %)
[Al] [Ge] [Ga]
GZO – carrier concentrations and mobility
• Carrier concentrations and
mobilities assessed by Hall
Effect
• Comparable mobilities arise
from similar microstructures
(e.g. TEM’s)
• Higher carrier concentrations
achievable with gallium
compared to germanium
dopants
0 500 1000 1500 2000 2500
Wavelength (nm)
100
80
60
40
20
0
Tra
nsm
issio
n (
%)
Ga:ZnO – optical properties
• IR ‘cut-off’ extended by
reducing carrier the
concentration
• Potential trade-off between
thermal management and
electrical conductivity
• High performance optical
properties
6.9 x 1020
8.4 x 1020
9.4 x 1020
Reflectivity
AP- MOCVD of the Cd(1-x)Zn(x)S/CdTe device
• GZO coated float glass substrate (front electrical contact)
• Cd(1-x)Zn(x)S n-type window layer (240 nm)
• CdTe p-type absorber layer (2250 nm)
• Cl treatment - in situ CdCl2 deposition & anneal
• Deionized water rinsing of excess CdCl2
• TCO contact opening
• Thermal evaporation of Au onto CdTe (back electrical contact)
Heated substrate: 200 – 450 oC
Reactor cell @ 1 atm
H2
Metal-organic precursors
Courtesy of Stuart J C Irvine, Daniel A. Lamb, Andrew J. Clayton
GZO TCO Device properties
η (%) 10.8
Jsc (mA cm-2) 23.9
Voc (V) 0.69
FF (%) 65.0
• Best GZO TCO efficiency 12 % directly comparable to commercially available
SnO2:F TCO
• Average current density/voltage of 16 devices under AM1.5 and a typical J/V curve
for a GZO TCO device
Courtesy of Stuart J C Irvine, Daniel A. Lamb, Andrew J. Clayton
Conclusions
19
• ALD can be used to deposit material with atomic scale
precision uniformly over 3D structures
• Dielectrics, transparent conductors and metallic materials
are feasible
• Application areas span IT, power devices, renewable
energy (and others e.g. optics, displays, energy storage,
catalysts etc.)
Acknowledgements
• EP/K014471/1 - Silicon Compatible GaN Power Electronics
• TSB PEARGaN - Power Electronics Applications for Reliability in GaN
• TSB PROMISE - Improved Processes and Materials for Energy Saving
Glazing
• EP/K018884/1 - ZnO MESFETs for application to Intelligent Windows