Z. Hussain (zhussain@dircon.co.uk) is with Department of Electrical and Electronic Engineering, Imperial College of Science, Technology and Medicine, Exhibition Road, London SW7 2BT United Kingdom.
Vacuum temperature-dependent ellipsometric studies on
WO3 thin films are reported at a single wavelength,
λ = 0.633 µm, and across a temperature range of
100 < T ≤ 453 K. All the measurements were
made in an optical cryostat fixed in the sample compartment of the
ellipsometer. Experimental results involving reduction and
oxidation of WO3 are discussed in terms of electrochromic
characteristics and structural changes, which can be helpful for many
and various technological applications. Temperature-dependent
drifts in the real part of the refractive index n and
extinction coefficient k have been explained by use of a
variety of chemical relations and have also been utilized to evaluate
their temperature coefficients. Moreover, polaronic excitations
between localized states around the Fermi level are put forward to
explain the ellipsometric results at or above room temperature, and
both polaronic and bipolaronic transitions are proposed for
interpreting low-temperature ellipsometric measurements.
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p, ap,
as are analyzed into four zones in relevance to the
polarizer, P (deg) and analyzer, A (deg), azimuths.
Conversion formulas for ellipsometric angles
ψ° and Δ°.
Table 4
Vacuum Ellipsometric Measurements on a 7059 Glass Slide
(Owing to Strain Effects because of Optical Windows) Kept Under
Vacuum and in Open Aira
Pressure
Optical Windows
Δ (deg)
Ψ (deg)
n
k
2 × 10-5 Torr
With
1.77
5.51
1.5296
0.00041
In air (atm)
With
1.32
5.55
1.5276
0.00031
In air (atm)
Without
1.58
5.53
1.5284
0.00036
At room temperature in both cases.
Table 5
Vacuum Ellipsometric Data on a 7059 Glass Slide (with
Back Surface Roughened by Silicon Carbide)
Instrument Type
Δ (deg)
Ψ (deg)
ne
Abbe Readings (deg)
na
Manual
0.06
5.34
1.5296
—
—
ellipsometer
Automatic
1.96
5.04
1.5264
—
—
ellipsometer
Abbe
—
—
—
24.743
1.5283
Note: Data (on the same glass slide) from other optical
instruments set at a wavelength of 632.8 nm (and all maintained at
room temperature) are also included.
Table 6
Manual and Autoellipsometric (Comparative) Data on
WO3 Thin Films Deposited on 7059 Glass Slides
Type
Thickness (µm)
Δ (deg)
Ψ (deg)
n
k
Manual
0.280
156.49
11.79
2.0410
0.00059
Auto
0.286
130.08
10.16
2.0220
0.00058
Manual
0.370
-83.22
4.28
1.9905
0.01514
Auto
0.364
82.68
6.84
1.9810
0.01612
Note: ϕ ∼ 60.
Table 7
Vacuum Ellipsometric Temperature-Dependent Data on
WO3 Thin Film Deposited on a 7059 Glass Slide
Temperature (K)
Δ (deg)
Ψ (deg)
n
k
First heating cycle
295.0
-81.17
4.79
2.06123
0.00869
333.0
-81.92
4.83
2.06182
0.00860
373.0
-82.93
4.80
2.06221
0.00889
413.0
-81.77
4.45
2.05986
0.00987
453.0
-78.30
4.46
2.05810
0.00918
First cooling cycle
373.0
-80.17
4.30
2.05830
0.01006
297.0
-82.29
4.45
2.06010
0.00996
Second heating cycle
297.0
-82.29
4.45
2.06010
0.00996
453.0
-78.93
4.69
2.05950
0.00860
Second cooling cycle
295.0
-81.20
4.57
2.06020
0.00928
Note: Thickness, 0.522 µm; ϕ =
60.32°.
Table 8
Vacuum-Annealed Ellipsometric Data on WO3 Thin
Film Upheld at Different Temperatures
Temperature (K)
Time (h)
Δ (deg)
Ψ (deg)
n
k
First heating cycle
295.0
—
2.56
5.48
2.0266
0.00020
373.0
—
-7.46
5.16
2.0323
0.00098
373.0
14.0
-57.07
4.31
2.0576
0.00655
First cooling cycle
333.0
—
-58.26
4.22
2.0578
0.00703
303.0
48.0
-58.71
4.15
2.0579
0.00733
Second heating cycle
297.0
—
-21.52
3.44
2.0386
0.00713
413.0
—
-68.54
4.41
2.0639
0.00772
413.0
10.0
-93.04
4.50
2.0769
0.01201
Second cooling cycle
373.0
—
-94.60
4.51
2.0776
0.01231
333.0
—
-96.32
4.53
2.0786
0.01270
295.0
12.0
-97.32
4.49
2.0789
0.01309
Third heating cycle
295.0
—
-96.41
4.49
2.0784
0.01289
Note: Thickness, 0.518 µm; ϕ =
60.33°.
Table 9
Vacuum-Low-Temperature Ellipsometric Measurements on
WO3 Thin Film with Liquid Nitrogen Used as a Coolant and
the Temperature Directed by a 407 Controller
Temperature (K)
Time (h)
Δ (deg)
Ψ (deg)
n
k
First heating cycle
295.0
—
93.18
8.78
2.0107
0.00771
373.0
—
100.24
8.88
1.9931
0.00820
First cooling cycle
333.0
—
98.70
9.09
1.9971
0.00996
153.0
3.0
29.50
3.90
2.1123
0.00860
105.0
1.0
21.83
5.41
2.1146
0.00980
Second heating cycle
295.0
—
88.93
9.00
2.0200
0.00967
Second cooling cycle
243.0
—
-37.48
7.74
2.1282
0.00615
101.0
—
-45.45
7.29
2.1346
0.00390
Third heating cycle
183.0
—
-48.22
5.10
2.1314
0.00430
298.0
—
-27.72
2.44
2.1106
0.01485
373.0
—
-94.75
4.45
2.0773
0.01260
373.0
6.0
-91.23
3.81
2.0714
0.01377
Third cooling cycle
297.0
4.0
-94.94
3.85
2.0732
0.01446
Fourth heating cycle
413.0
—
-87.93
4.09
2.0718
0.01221
413.0
4.0
-83.51
4.05
2.0695
0.01143
453.0
—
-84.23
4.32
2.0714
0.01065
453.0
2.0
-51.92
3.33
2.0519
0.00948
453.0
4.0
-51.30
3.21
2.0514
0.00967
Fourth cooling cycle
295.0
8.0
-59.46
3.76
2.0568
0.00869
Note: Thickness = 0.325 µm, ϕ
= 60.34°.
Table 10
Ellipsometric Data Collected on WO3 Thin Film
Owing to Hydrogen and Oxygen Plasma Treatments while Film Upheld at
T = 373 K
Type of Plasma
Time (min)
Δ (deg)
Ψ (deg)
n
k
No plasma
—
156.61
11.73
2.0397
0.0006
Hydrogen
5.0
117.58
8.14
1.8446
0.1982
3.0
112.71
9.08
1.8208
0.2348
3.0
104.13
9.35
1.7499
0.2559
35.0
89.07
12.74
1.5747
0.3599
15.0
89.73
12.54
1.5849
0.3545
45.0
89.87
12.32
1.5908
0.3480
40.0
90.05
11.97
1.5996
0.3375
Oxygen
90.0
97.77
9.74
1.6965
0.2722
150.0
95.27
9.18
1.6826
0.2568
270.0
97.70
8.72
1.7033
0.2428
Note: Thickness, 0.27 µm; ϕ =
60.32°.
Table 11
Ellipsometric Data Collected on WO3 Thin Film
Owing to Hydrogen and Oxygen Plasma Treatments while Film Upheld
at T = 453 K
Type of Plasma
Time (h)
Δ (deg)
Ψ (deg)
n
k
No plasma
—
158.52
11.86
2.0324
0.00108
Hydrogen
0.92
96.83
10.61
1.6862
0.31270
0.83
94.68
10.55
1.6710
0.31050
0.83
93.11
10.56
1.6594
0.30990
Oxygen
0.75
97.44
10.02
1.6989
0.29654
0.50
97.96
9.98
1.7032
0.29546
14.00
99.50
8.38
1.7302
0.25313
36.00
103.73
6.65
1.7618
0.20938
8.00
106.39
6.00
1.7740
0.19322
1.00
101.01
7.90
1.7278
0.21780
2.00
102.06
7.63
1.7342
0.20967
8.00
105.24
7.07
1.7505
0.19175
10.00
106.47
6.77
1.7549
0.18272
Note: Thickness, 0.245 µm; ϕ =
60.33°.
Table 12
Vacuum Ellipsometric Data on Evaporated WO3
Thin Films Prepared at Different Substrate (7059 Glass)
Temperatures
Substrate Temperature (K)
Thickness (µm)
Δ (deg)
Ψ (deg)
n
k
295.0
0.280
156.49
11.79
2.0410
0.0005
353.0
0.520
-83.35
4.80
2.0624
0.0089
373.0
0.348
-100.10
5.92
2.1039
0.0133
Note: ϕ = 60.32°, rate of evaporation,
950 Å/min.
Table 13
Vacuum Ellipsometric Data on WO3 Thin Films
when the Rate of Evaporation is Varied while the Substrate (7059
glass) Is at Room Temperature
Thickness (µm)
Rate of Evaporation (Å/min)
Pressure ×10-6 (Torr)
Δ (deg)
Ψ (deg)
n
k
3220.0
900.0
1.8
88.05
7.56
2.0445
0.00147
3700.0
1200.0
2.2
-82.57
6.04
1.9996
0.00096
Note: ϕ = 60.33°.
Tables (13)
Table 1
Vacuum Ellipsometric Alignment Data with Reference to the
Stainless-Steel Mirror
P (deg)
Q (deg)
Amin (deg)
0.00
0.00
90.19
0.00
90.00
90.00
0.00
180.00
90.23
0.00
270.00
90.02
90.00
0.00
359.94
90.00
90.00
0.00
90.00
180.00
359.95
90.00
270.00
0.01
180.00
0.00
90.18
180.00
90.00
89.99
180.00
180.00
90.22
180.00
270.00
90.03
270.00
0.00
359.94
270.00
90.00
0.01
270.00
180.00
359.95
270.00
270.00
0.01
Total mean error = +0.06
Note: ϕ = 60.34°, λ = 632.8 nm.
Table 2
Vacuum Ellipsometric Alignment Data when Platinum Thick
Film on a Microscopic Glass Slide is Used
Polarizer Settings P (deg)
Quarter-Wave Settings Q (deg)
Average of Two Measurements of Minimum Analyzer Settings Amin (deg)
p, ap,
as are analyzed into four zones in relevance to the
polarizer, P (deg) and analyzer, A (deg), azimuths.
Conversion formulas for ellipsometric angles
ψ° and Δ°.
Table 4
Vacuum Ellipsometric Measurements on a 7059 Glass Slide
(Owing to Strain Effects because of Optical Windows) Kept Under
Vacuum and in Open Aira
Pressure
Optical Windows
Δ (deg)
Ψ (deg)
n
k
2 × 10-5 Torr
With
1.77
5.51
1.5296
0.00041
In air (atm)
With
1.32
5.55
1.5276
0.00031
In air (atm)
Without
1.58
5.53
1.5284
0.00036
At room temperature in both cases.
Table 5
Vacuum Ellipsometric Data on a 7059 Glass Slide (with
Back Surface Roughened by Silicon Carbide)
Instrument Type
Δ (deg)
Ψ (deg)
ne
Abbe Readings (deg)
na
Manual
0.06
5.34
1.5296
—
—
ellipsometer
Automatic
1.96
5.04
1.5264
—
—
ellipsometer
Abbe
—
—
—
24.743
1.5283
Note: Data (on the same glass slide) from other optical
instruments set at a wavelength of 632.8 nm (and all maintained at
room temperature) are also included.
Table 6
Manual and Autoellipsometric (Comparative) Data on
WO3 Thin Films Deposited on 7059 Glass Slides
Type
Thickness (µm)
Δ (deg)
Ψ (deg)
n
k
Manual
0.280
156.49
11.79
2.0410
0.00059
Auto
0.286
130.08
10.16
2.0220
0.00058
Manual
0.370
-83.22
4.28
1.9905
0.01514
Auto
0.364
82.68
6.84
1.9810
0.01612
Note: ϕ ∼ 60.
Table 7
Vacuum Ellipsometric Temperature-Dependent Data on
WO3 Thin Film Deposited on a 7059 Glass Slide
Temperature (K)
Δ (deg)
Ψ (deg)
n
k
First heating cycle
295.0
-81.17
4.79
2.06123
0.00869
333.0
-81.92
4.83
2.06182
0.00860
373.0
-82.93
4.80
2.06221
0.00889
413.0
-81.77
4.45
2.05986
0.00987
453.0
-78.30
4.46
2.05810
0.00918
First cooling cycle
373.0
-80.17
4.30
2.05830
0.01006
297.0
-82.29
4.45
2.06010
0.00996
Second heating cycle
297.0
-82.29
4.45
2.06010
0.00996
453.0
-78.93
4.69
2.05950
0.00860
Second cooling cycle
295.0
-81.20
4.57
2.06020
0.00928
Note: Thickness, 0.522 µm; ϕ =
60.32°.
Table 8
Vacuum-Annealed Ellipsometric Data on WO3 Thin
Film Upheld at Different Temperatures
Temperature (K)
Time (h)
Δ (deg)
Ψ (deg)
n
k
First heating cycle
295.0
—
2.56
5.48
2.0266
0.00020
373.0
—
-7.46
5.16
2.0323
0.00098
373.0
14.0
-57.07
4.31
2.0576
0.00655
First cooling cycle
333.0
—
-58.26
4.22
2.0578
0.00703
303.0
48.0
-58.71
4.15
2.0579
0.00733
Second heating cycle
297.0
—
-21.52
3.44
2.0386
0.00713
413.0
—
-68.54
4.41
2.0639
0.00772
413.0
10.0
-93.04
4.50
2.0769
0.01201
Second cooling cycle
373.0
—
-94.60
4.51
2.0776
0.01231
333.0
—
-96.32
4.53
2.0786
0.01270
295.0
12.0
-97.32
4.49
2.0789
0.01309
Third heating cycle
295.0
—
-96.41
4.49
2.0784
0.01289
Note: Thickness, 0.518 µm; ϕ =
60.33°.
Table 9
Vacuum-Low-Temperature Ellipsometric Measurements on
WO3 Thin Film with Liquid Nitrogen Used as a Coolant and
the Temperature Directed by a 407 Controller
Temperature (K)
Time (h)
Δ (deg)
Ψ (deg)
n
k
First heating cycle
295.0
—
93.18
8.78
2.0107
0.00771
373.0
—
100.24
8.88
1.9931
0.00820
First cooling cycle
333.0
—
98.70
9.09
1.9971
0.00996
153.0
3.0
29.50
3.90
2.1123
0.00860
105.0
1.0
21.83
5.41
2.1146
0.00980
Second heating cycle
295.0
—
88.93
9.00
2.0200
0.00967
Second cooling cycle
243.0
—
-37.48
7.74
2.1282
0.00615
101.0
—
-45.45
7.29
2.1346
0.00390
Third heating cycle
183.0
—
-48.22
5.10
2.1314
0.00430
298.0
—
-27.72
2.44
2.1106
0.01485
373.0
—
-94.75
4.45
2.0773
0.01260
373.0
6.0
-91.23
3.81
2.0714
0.01377
Third cooling cycle
297.0
4.0
-94.94
3.85
2.0732
0.01446
Fourth heating cycle
413.0
—
-87.93
4.09
2.0718
0.01221
413.0
4.0
-83.51
4.05
2.0695
0.01143
453.0
—
-84.23
4.32
2.0714
0.01065
453.0
2.0
-51.92
3.33
2.0519
0.00948
453.0
4.0
-51.30
3.21
2.0514
0.00967
Fourth cooling cycle
295.0
8.0
-59.46
3.76
2.0568
0.00869
Note: Thickness = 0.325 µm, ϕ
= 60.34°.
Table 10
Ellipsometric Data Collected on WO3 Thin Film
Owing to Hydrogen and Oxygen Plasma Treatments while Film Upheld at
T = 373 K
Type of Plasma
Time (min)
Δ (deg)
Ψ (deg)
n
k
No plasma
—
156.61
11.73
2.0397
0.0006
Hydrogen
5.0
117.58
8.14
1.8446
0.1982
3.0
112.71
9.08
1.8208
0.2348
3.0
104.13
9.35
1.7499
0.2559
35.0
89.07
12.74
1.5747
0.3599
15.0
89.73
12.54
1.5849
0.3545
45.0
89.87
12.32
1.5908
0.3480
40.0
90.05
11.97
1.5996
0.3375
Oxygen
90.0
97.77
9.74
1.6965
0.2722
150.0
95.27
9.18
1.6826
0.2568
270.0
97.70
8.72
1.7033
0.2428
Note: Thickness, 0.27 µm; ϕ =
60.32°.
Table 11
Ellipsometric Data Collected on WO3 Thin Film
Owing to Hydrogen and Oxygen Plasma Treatments while Film Upheld
at T = 453 K
Type of Plasma
Time (h)
Δ (deg)
Ψ (deg)
n
k
No plasma
—
158.52
11.86
2.0324
0.00108
Hydrogen
0.92
96.83
10.61
1.6862
0.31270
0.83
94.68
10.55
1.6710
0.31050
0.83
93.11
10.56
1.6594
0.30990
Oxygen
0.75
97.44
10.02
1.6989
0.29654
0.50
97.96
9.98
1.7032
0.29546
14.00
99.50
8.38
1.7302
0.25313
36.00
103.73
6.65
1.7618
0.20938
8.00
106.39
6.00
1.7740
0.19322
1.00
101.01
7.90
1.7278
0.21780
2.00
102.06
7.63
1.7342
0.20967
8.00
105.24
7.07
1.7505
0.19175
10.00
106.47
6.77
1.7549
0.18272
Note: Thickness, 0.245 µm; ϕ =
60.33°.
Table 12
Vacuum Ellipsometric Data on Evaporated WO3
Thin Films Prepared at Different Substrate (7059 Glass)
Temperatures
Substrate Temperature (K)
Thickness (µm)
Δ (deg)
Ψ (deg)
n
k
295.0
0.280
156.49
11.79
2.0410
0.0005
353.0
0.520
-83.35
4.80
2.0624
0.0089
373.0
0.348
-100.10
5.92
2.1039
0.0133
Note: ϕ = 60.32°, rate of evaporation,
950 Å/min.
Table 13
Vacuum Ellipsometric Data on WO3 Thin Films
when the Rate of Evaporation is Varied while the Substrate (7059
glass) Is at Room Temperature