1. E. Adrián Martín Tovar and R. Castro-Rodríguez
Centro de Investigación y de Estudios Avanzados Unidad Mérida
“ZnO thin films as buffer and antireflective layer in glass/ITO/ZnO/CdS structures”
E. Adrián Martín Tovar and R. Castro-Rodríguez
September 26, 2013

2. Introduction
Zinc Oxide (ZnO) is a II-VI semiconductor material with the following properties:
A 3.37 eV direct band gap
Thin film resistivity in the range of -cm
Refractive index of 2.03
High transparency in the visible region of the electromagnetic spectrum
Crystallizes in three different phases :
Hexagonal wurtzite.
Zincblende
Rocksalt

3. Thickness of 80-150 nm for wavelenghts in a range of 650- 1200 nm
Goals
To deposit ZnO thin films using PLD technique for their use as a buffer and anti-reflective layer in Glass/ITO/ZnO/CdS structures using two types of targets.
For a buffer layer high resistivities are required
-cm
As an anti-reflective layer:
Thickness of nm for wavelenghts in a range of nm

4. Experimental Procedures Target-substrate distance
Vacuum of
10-5 Torr
What’s PLD?
Laser pulse Nd:YAG
=1064 nm
~2 J/(cm2) per shot
5 Hz
Number of shots:
3000
Substrate heating
Ts = 300° C
Target-substrate distance
50 mm

5. Target (type A) ZnO sintered powder, 99.99% Area 50.3 mm2
Force = 10 Ton
T = 600° C (Heating)
Time = 2 hours

6. Target (Type B) Cured powder ZnO, 99.99%
Mixed with cyanoacrylate (C5H5NO2)
Proportion of 70:30 in weight (70% ZnO, 30% cyanoacrylate)
Dried at room temperature for three days

7. Results and discussions

8. 80-150 nm for wavelenghts in a range of 650 a1200 nm
Thickness
nm for wavelenghts in a range of 650 a1200 nm
Sample
Thickness (nm)
Deposition time
(min)
Type A
111.5 10
Type B
101.1

9. Morphological Properties
Type A
Sample
RMS (nm)
Type A
17.8
Type B
6.50
Type B

10. Structural Properties
ICCD crystallographic letter associated with ZnO.
Hexagonal wurtzite structure.
Preferential growth along the c-axis (002)
Displacements of diffraction peaks (002) to the left
FWHM (2)
Type A

11. Structural Properties
ICCD crystallographic letter associated with ZnO.
Preferential growth along (100) and (101) planes
Hexagonal wurtzite structure.
Displacements of diffraction peaks (002) to the right
FWHM (2)
Type B

12. Structurales Properties
Nano-estructured thin films
D for type A suggests a quantum confinement effect
A negative sign indicates that the unit cell of type A has a smaller volume than the standard unit cell
A positive sign indicates that the unit of type B has a greater volume than the standard unit cell
Sample
e (%)
D (nm)
Type A
-2.03
9.60
Type B
0.213
38.95
Williamson Hall equation:

13. Structurales Properties
Sample
a (Å)
c (Å)
Volume (Å3)
Volume difference (%)
Unit cell stress e (%)
Standard ZnO
3.2495
5.2069
47.615
Type A
3.1937
5.2384
46.272
-2.82
-2.03
Type B
3.2548
5.198
47.689
0.155
0.213 c b a a

14. Optical Properties Transmitance 85% Abortion edge isn’t defined UV
VIS IR

15. Band gap Values Sample Experimental Gap (eV) Grain size (nm)
Calculated gap
(eV)
Standard ZnO
3.37
Type A
4.08
9.60
4.00
Type B
3.32
38.95

16. Photocurrent Calculations
Structure
Jsc total (mA/cm2)
Loss
(mA/cm2)
Glass
23.23
2.49
Glass/ITO
21.80
3.92
Glass/ITO/CdS
18.57
7.15
Glass/ITO/ZnOtipoA/CdS
19.04
6.67
Glass/ITO/ZnOtipoB/CdS
19.09
6.63
Elimination of 7.52% of losses
Maximum value of mA/cm2

17. Resitivity Measures Measuare at RT
Sample
Resistence (M)
Resistivity (-cm)
Type A
1.17 x102
Type B
1.30 x102
Measuare at RT
For CdS/CdTe Solar cells: -cm

18. Conclusions
ZnO thin films were obtained through PLD by using two types targets.
Eg for the films A and B were 4.08 and 3.32 eV respectively
Eg sample for type A is the result of quantum confinement effects.

19. Conclusions
The samples were suitable as anti-reflective coating, the presence of ZnO resulted in an increase of about 10% of transmittance in glass/ITO/ZnO/CdS structures.
Photocurrent was obtained to be and mA/cm2 for samples A and B respectively, with a 7.52% elimination of photocurrent losses.
The type B film has the best results for an application as buffer layer and anti-reflective.

20. Thank you!