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