ix
TABLE OF CONTENTS
CHAPTER
1
2
TITLE
PAGE
DECLARATION
ii
ACKNOWLEDGEMENT
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF TABLES
x
LIST OF FIGURES
xi
LIST OF SYMBOLS
xv
INTRODUCTION
1.1
Amorphous Semiconductor
1
1.2
Potential Applications
2
1.3
Research Objectives
5
1.4
Research Scope
5
1.5
Layout of Thesis
5
LITERATURE REVIEW
2.1
2.2
Hydrogenated Amorphous Silicon (a-Si:H)
7
2.1
8
Hydrogenation of a-Si
Conventional Thin Film Deposition Techniques
13
x
2.2.1 Chemical Vapor Deposition
13
2.2.2 DC Glow Discharge
14
2.2.3 RF Glow Discharge
18
2.3
Plasma Enhanced Chemical Vapor Deposition (PECVD)
19
2.4
Plasma Fundamentals
19
2.4.1
19
2.5
3
Non-equilibrium Glow Discharges
2.4.2 Potentials in rf Glow Discharges
21
Qualitative Model for PECVD Reactions
22
METHODOLOGY
3.1
3.2
Deposition of Amorphous Silicon Films
27
3.1.1 Introduction
27
3.1.2
27
Plasma Enhanced Chemical Vapor Deposition
3.1.3 PECVD Setup
28
3.1.4
PECVD System Deposition Process
33
3.1.5
Preparation of a-Si:H Samples
34
3.1.5.1 Substrate Preparation
34
3.1.5.2
34
Deposition Parameters
Analytical Tools
36
3.2.1
36
Spectroscopy
3.2.2 Absorption
3.2.3
37
3.2.2.1 Optical Absorption
37
3.2.2.2
42
UV-3101-PC Spectrophotometer
Infrared (IR) Absorption Spectroscopy
45
3.2.3.1 Introduction
45
3.2.3.2
45
Mechanism of IR Absorption
3.2.3.3 FTIR – Fourier Transform Infrared
Spectroscopy
3.2.4
3.2.5
46
Photoluminescence
49
3.2.4.1
Basic Theory of Photoluminescence
49
3.2.4.2
Luminescence Spectrometer LS 55
51
Atomic Force Microscopy (AFM)
54
xi
3.2.5.1
Contact Mode
3.2.5.2 Non-contact Mode
57
3.2.5.3
Tapping Mode
57
3.2.5.4
Image Display
57
3.2.5.5 Surface Roughness
3.3
4
5
56
Film Thickness Measurements
58
58
RESULTS AND DISCUSSIONS
4.1
Surface Morphology and Deposition Rate
61
4.2
X-Ray Diffraction (XRD) Analysis
70
4.3
Infrared (IR) Transmission Spectrum
72
4.3.1 Dependence on rf Power
74
4.3.2
79
Dependence on Substrate Temperature
4.4
Optical Energy Gap
84
4.5
Photoluminescence
89
4.5.1
89
Dependence on Substrate Temperature
CONCLUSION
5.1
Summary
96
5.2
Recommendation
98
REFERENCES
99
PRESENTATIONS
105
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LIST OF TABLE
TABLE NO.
3.1
TITLE
PAGE
The preparation conditions for a-Si:H thin films under
different RF power and substrate temperature. Note that
4.1
other parameters are kept constant.
35
The observed mode frequency and their assignments.
72
xiii
LIST OF FIGURES
FIGURE NO.
1.1
TITLE
PAGE
Schematic representation of a typical thin film a-Si:H
solar cell on glass. The incoming photons with an energy
larger than the band gap are absorbed in the intrinsic
a-Si:H film creating holes-electron pairs.
3
1.2
Schematic of a general active matrix array.
4
2.1
A model of hydrogenated amorphous silicon. The small
ball represent hydrogen atoms while the large ball
represents silicon atoms.
2.2
Model for the adsorption, diffusion and recombination
of SiH3 on the growing surface.
2.3
9
12
A common implementation of CVD
(AX, X, BX – gases, AB – solid material).
14
2.4
Current-voltage characteristics in a DC glow discharge.
15
2.5
Luminous zones and dark spaces in a DC glow
discharges.
2.6
The potential distribution in a DC glow discharge.
(Vp – plasma potential, Vc – cathode potential).
2.7
17
Schematic of fundamental transport and reaction
processes underlying CVD.
2.8
16
23
Equivalent circuit representation of the sequence of
steps in thermally driven CVD. The Ri (with i = 1-7)
represent the seven steps describe in the text.
2.9
Equivalent circuit representation showing the thermal
CVD path, and the parallel plasma-enhanced path
24
xiv
represented by the Ri*s.
2.10
25
Activation energy diagram for a thermally driven
(solid line) and plasma enhanced (dashed line) reaction.
The activation energy for the plasma enhanced reaction,
∆E*, is typically less than that for the thermally driven
reaction, ∆E.
26
3.1
NPN2 gas.
29
3.2
PN2 gas rack.
29
3.3
Silane (SiH4) gas cabinet.
30
3.4
Scrubber.
30
3.5
Schematic of the rf PECVD reactor system.
31
3.6
The rf PECVD reactor system.
32
3.7
A schematic of the optical absorption curves of
amorphous materials.
3.8
39
Electron transition for semiconductors having a
direct gap and indirect gap
40
3.9
Shimadzu UV-3101-PC Spectrophotometer.
44
3.10
Examples of infrared active and inactive absorption
bands in CO2.
3.11
46
Fourier transform infrared spectrometer (Perkin Elmer
Spectrum GX).
48
3.12
A schematic of the PL process.
50
3.13
Photoluminescence spectrometer LS 55.
52
3.14
Luminescence spectrometer schematic diagram.
54
3.15
Beam deflection system, using a laser and
photodetector to measure the beam position.
55
3.16
Force between tip and sample surface
56
3.17
Colour mapping scheme for height
58
3.18
Dektak3 surface profiler.
59
4.1
Representation of surface morphology of a-Si:H films.
63
4.2
The 10 µm × 10 µm AFM images with surface height
given by the grey scales extending from 0 (black) to
400 nm (white): (a) sample A deposited at 27°C;
(b) sample B deposited at 100°C; (c) sample C
xv
deposited at 200°C; (d) sample D deposited at 300°C.
4.3
Deposition rate of a-Si:H films deposited at different
temperature as a function of rf power.
4.4
69
X-ray diffraction pattern of a-Si:H thin film
deposited at room temperature and rf power of 100 W.
4.7
68
Model of surface reaction where Si-Si is formed
releasing H2.
4.6
67
Deposition rate of a-Si:H films deposited at different
rf power as a function of substrate temperature.
4.5
64
70
X-ray diffraction pattern of a-Si:H thin film deposited
at 200˚C and rf power of 50 W.
71
4.8
Local Si-H vibrations for SiH, SiH2 and SiH3 groups.
72
4.9
IR transmission of a-Si:H thin films deposited at
different rf power. Emphasize on the 2000 cm-1 regime.
4.10
IR transmission of a-Si:H thin films deposited at
different rf power. Emphasize on the 600 cm-1 regime.
4.11
77
IR transmission of a-Si:H thin film prepared under
different rf power by Lucovsky et al. (1979)
4.13
76
IR transmission of a-Si:H thin films deposited at
different rf power. Emphasize on the 800 cm-1 regime.
4.12
75
78
IR transmission of a-Si:H thin films deposited at
different substrate temperatures. Emphasize on the
2000 cm-1 regime. RF power is at 100 W.
4.14
81
IR transmission of a-Si:H thin films deposited at
different substrate temperatures. Emphasize on the
800 cm-1 regime. RF power is at 100 W.
4.15
82
IR transmission of a-Si:H thin films deposited at
different substrate temperatures. Emphasize on the
600 cm-1 regime. RF power is at 100 W.
4.16
The (αhν)1/2 vs hν graphs for a-Si:H deposited at
different rf power.
4.17
84
The (αhν)1/2 vs hν graphs for a-Si:H deposited at
different substrate temperature.
4.18
83
Energy gap of a-Si:H films prepared under different
85
xvi
substrates temperatures as a function of rf power.
4.19
86
Luminescence spectrum of amorphous silicon at
various substrate temperatures with rf power kept
constant at 50W.
4.20
90
Luminescence spectrum of amorphous silicon at
various substrate temperatures with rf power kept
constant at 100W.
4.21
91
Luminescence spectrum of amorphous silicon at
various substrate temperatures with rf power kept
constant at 150W.
4.22
91
Luminescence spectrum of amorphous silicon at
various substrate temperatures with rf power kept
constant at 200W.
4.23
92
Luminescence spectrum of amorphous silicon at
various substrate temperatures with rf power kept
constant at 250W.
4.24
92
Flow diagram representing the interrelation of the
excitation and recombination processes appropriate
to amorphous silicon.
4.25
94
Schematic plot of the relaxation and recombination
probabilities versus energy from midgap.
95
xvii
LIST OF SYMBOL
VB
-
Breakdown voltage
VG
-
Glow discharge voltage
Vp
-
Plasma potential
Vc
-
Cathode potential
Vf
-
Floating potential
A
-
Gas species
A*
-
Reactive species
e-
-
Electrons
e
-
Unit electron charge (1.60 × 10-19 C)
k1
-
Reaction rate coefficient
Te
-
Electron temperature
Ts
-
Substrate temperature
mi
-
Ion mass
me
-
Electron mass
k
-
Boltzmann constant (1.38 × 10-23 J/K)
∆E*
-
Activation energy for plasma enhanced reaction
∆E
-
Thermally driven activation energy
Eg
-
Optical gap
Eo
-
Urbach energy
α
-
Optical absorption
h
-
Planck constant (6.625 × 10-34 J-s)
ν
-
Frequency
B
-
Edge width parameter
A
-
Absorbance
T
-
Transmittance
Io
-
Incident light intensity
xviii
I
-
Intensity of light transmitted through sample
d
-
Film thickness
t
-
Deposition time
c
-
Speed of light (2.998 × 108 m/s)
λ
-
Wavelength of light
Ds
-
Surface diffusion
τs
-
Staying time
dr
-
Deposition rate
CH
-
Bonded H content
XC
-
Volume fraction of microcrystalline