1. Design, Growth & Fabrication of InxGa1-xN
(0 ≤ x ≤ 0.25) Based Solar Cell
Dissertation
by
Rajkumar Sahu
Advisor: Mr. Sonachand Adhikari
Co-advisor: Dr. Sanjay Tiwari
CSIR-Central Electronics Engineering Research Institute, Pilani , Rajasthan
School of Studies in Electronics & Photonics , Pt. Ravishankar Shukla University, Raipur
May 22, 2014
Rajkumar Sahu
rksahu@outlook.com

2. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 2
Outline
1. Motivation & background
• Photovoltaic (PV), High-efficiency, InGaN.
2. Objectives
• Status, Research Objective, Approach
3. Theory and modeling
• Design, Silvaco-Atlas, Optimization
4. Experimental
• Growth & Fabrication
5. Conclusion

3. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 3
Motivation for PV: Population growth
•Energy Demand -

4. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 4
Motivation for PV: Energy demand
World marketed energy consumption, 1990 - 2035.
(source: Based on data from U.S. Energy Information Administration, 2011)

5. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 5
Motivation for PV: Greenhouse Gases, Acid rain

6. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 6
Motivation for PV: Global warming
(Source:-IPCC, 31st march 2014)

7. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 7
Motivation for PV: The Sun
ONE SOLUTION COMES UP EVERY MORNING!
SOLUTION

8. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 8
What is Photovoltaics (PV)?
Photovoltaics is the DIRECT method…
…of converting SUNLIGHT into ELECTRICITY…
…using a device known as SOLAR CELL.

9. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 9
Operation of a solar cell
• Working principle of Solar Cell
based on Photovoltaic effect.
• Photovoltaic effect is generation of
Electric power from light.
• Single junction solar cell is simply
PN junction under illumination of
light.
• Operating diode in fourth quadrant
generates power.

10. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 10
Advantages of PV
Green Technology
No combustion/emission, radioactivity, disposal
 High public acceptance
Infinite Resource
Fuel, semiconductor
 Flexibility and convenience
Grid connected, stand-alone, modular
Quick installation, integration
High-quality output power

11. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 11
Generations of PV
1st Generation…
Bulk Silicon, single junction
Mature technology, 93% market share
Limitation: Efficiency ~25%, Si
2nd Generation…
Thin films decrease material costs
Limitation: Low efficiencies, stability
3rd Generation…
High efficiency
 Lower cost
High output power density

12. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 12
Efficiency limit in single junction solar cell
Efficiency - 25%
Loss mechanisms in a single junction solar cell.
Transmission of low energy photons ~23%.
Thermalization of high energy photons ~29%.
Junction/Contacts ~14%.
Recombination due to material quality ~5%.
Other: Curve factor Loss, Shading, Reflection ~5%.

13. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 13
High efficiency approaches: Tandem solar cell
Solar cells with decreasing band gaps are stacked with greatest band gap
on the top.
High energy photons are absorbed by top layers decreasing
thermalization losses.
Low energy photons are transmitted to lower band gap layers.
Concept of a tandem solar cell.

14. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 14
High efficiency approaches: Quantum-well solar cell
Proposed by Keith Barnham’s group in 1990.
Multi-Quantum-Well (MQW) system is
added to the i-region of a p-i-n solar cell.
Quantum Wells (QW) can absorb photons
with energy less than that of the bulk material.
 ↑Absorption → ↑Current → ↑Efficiency
Concept of quantum well
solar cell

15. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 15
III-Nitride material system
Wide direct-band gap
Wide direct-band gap
 High absorption
Radiation hardness
 High carrier velocities
 Piezoelectric polarization

16. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 16
III-Nitride material system - Challenges
1. Substrate mismatch with GaN
Substrate Lattice
mismatch
Thermal
expansion
Sapphire 16% -34%
SiC 3% +2%
ZnO 2% -14%
Si 17% +100%
2. Material Quality
High dislocation density -1010cm2
Low lifetimes and diffusion lengths
3. P-GaN
P-type doping
Ohmic contact

17. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 17
Overview: InGaN solar cell research
Lawrence Berkeley National Lab
Proposed full spectrum InGaN solar cells.
InGaN/Si tandem (modeling).
Cornell University
Material growth.
University of Houston
Simulation, material characterization.
Novel Semiconductor Material Lab, China
Fabricated 2.7-2.8 eV InGaN p-n solar cells 0.43 VOC , FF 57%.

18. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 18
RESEARCH OBJECTIVES
Objectives & approach.

19. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 19
Research objectives
Develop an accurate modeling tool for III-nitride solar cells.
Optimize MOCVD epitaxial growth of InGaN Eg 2.51 eV
Design & fabricate InGaN solar cells Eg 2.51 eV
Understand loss mechanisms in solar cells
Material quality, fabrication issues
Develop robust & efficient fabrication scheme

20. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 20
Modeling
Silvaco-Atlas
PC1D, etc.
Fabrication
n & p contact, Current
spreading layer
Characterization
I-V, TLM
2.4 – 2.9 eV
InGaN solar cell
Growth
MOCVD
(In-situ
Characterization: GaN
Growth)
Research approach

21. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 21
THEORY
Preliminary design and modeling.

22. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 22
Modeling of solar cells: Silvaco-Atlas
Device parameter files
Structure, Region, Electrodes,
Doping, etc.
Material files
Model, Contact, Interface, Indium(%)
Optical
Refractive index, Absorpotion
Silvaco-Atlas
Solar cell simulation program
Simulate two and three-dimensional semiconductor devices.
Output
Graphical (Tony plot)
I-V, band diagram, electron & hole conc., mobility etc.

23. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 23
Modeling of solar cells: Silvaco-Atlas
Step 1: Preliminary modification Step 2: Advance modification
Material files
Models, Contact, Interface,
Indium(%)
Optical
Refractive index, Absorption
Polarization

24. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 24
Primary design: p-i-n solar cell
p-region ~ 100 nm
Maximize absorption in i-region.
Provide charge to junction.
n-region ~ 2 µm
Hole diffusion length ~ 2 µm.
Test material i-region. GaN/InGaN p-i-n solar cell.

25. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 25
Optimization : p-region
Thickness
On increasing the p-GaN
thickness, generated charge carriers
are not separated out instead they
start recombining in p-region which
results in decrease in the Jsc as
shown in fig. (a).
Doping
We also investigated the effect of
doping by taking different doping
concentration. Results shows that
Jsc first increases and then
decreases with increase in doping
concentration, as shown in fig. (a).
a
b
c
d

26. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 26
Preliminary design: i-region thickness
It can be observed that Jsc increases
with increasing i-layer thickness.
Since i-layer is low bandgap
semiconductor compare to p-GaN, it
can absorb the photons of some lesser
energy than p-GaN.
There is no significant change in the
Fill Factor(FF). However, FF starts to
decrease as we increase the thickness
because series resistance of the cell
also increases with increasing
thickness of i-layer

27. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 27
p-i-n structure with varying Indium Composition
Jsc of the double hetero-junction GaN/InGaN solar cell increases with increase in
indium composition till 20%, which contributes to increase in efficiency but beyond
this composition Jsc decreases sharply as shown in Figure.

28. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 28
Experimental
Growth and Fabrication.

29. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 29
Preliminary InGaN growth
Epitaxy: Emcore MOCVD D-125 rotating
disk reactor with short jar configuration.
 Material investigated:
InGaN: [In] 0 – 25%.
Growth variables:
Film thickness: 20 – 100 nm
Temperature: 640 – 800°C
TMIn: 30 – 250 SCCM
TMGa: 15 – 150 SCCM

30. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 30
MOCVD growth overview

31. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 31
MOCVD growth of GaN In-situ Characterization
Simulated reflectance
profile for GaN growth with extinction
coefficient of 0.001
Simulated reflectance
profile for GaN growth with extinction
coefficient of 0.153

32. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 32
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000
0
2000
4000
6000
8000
10000
12000
14000
16000
Reflection
Time (Sec)
p-i-n Solar Cell
MOCVD growth of GaN template
low temperature nucleation layer growth at 550 oC
high temperature GaN growth at 1060 oC, lateral growth, and surface roughening which induce a lightly drop
in the reflectance intensity
island coalescence which the amplitude and intensity of oscillations increases, qusi-2D GaN growth (500torr)
qusi-2D GaN growth

33. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 33
MOCVD growth of GaN template
(c) low temperature nucleation layer growth at 550 oC
(d) temperature ramp and morphology transformation
(e) high temperature GaN growth at 1060 oC, lateral growth, and surface roughening which induce
a lightly drop in the reflectance intensity
(f) island coalescence which the amplitude and intensity of oscillations increases, qusi-2D GaN
growth (500 torr)
(g) qusi-2D GaN growth

34. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 34
Baseline solar cell fabrication
p-type Contact Formation
Current Spreading Layer
n-type Contact Formation
Mesa Etching
p-GaN
i-InGaN
n-GaN
u-GaN buffer layer
Sapphire
InGaN/GaN Epi layer

35. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 35
Baseline solar cell fabrication
3535
P-type Contact Formation
Current Spreading Layer
n-type Contact Formation
Mesa Etching
p-GaN
i-InGaN
n-GaN
u-GaN buffer layer
Sapphire
InGaN/GaN Epi layer

36. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 36
Baseline solar cell fabrication
P-type Contact Formation
Current Spreading Layer
n-type Contact Formation
Mesa Etching
p-GaN
i-InGaN
n-GaN
u-GaN buffer layer
Sapphire
InGaN/GaN Epi layer
Ti/Al/Ni/Au

37. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 37
Baseline solar cell fabrication
P-type Contact Formation
Current Spreading Layer
n-type Contact Formation
Mesa Etching
p-GaN
i-InGaN
n-GaN
u-GaN buffer layer
Sapphire
InGaN/GaN Epi layer
Ni/Au
Ti/Al/Ni/Au

38. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 38
P-type Contact Formation
Current Spreading Layer
n-type Contact Formation
Mesa Etching
p-GaN
i-InGaN
n-GaN
u-GaN buffer layer
Sapphire
InGaN/GaN Epi layer
Ni/Au
Ti/Al/Ni/Au
Baseline solar cell fabrication

39. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 39
p-GaN
MQW active layer
n-GaN
u-GaN buffer layer
Sapphire
Ni/Au
Ti/Al/Ni/Au
Baseline solar cell fabrication
FINAL DEVICE
Mesa etching
n-type contact
formation
Current
spreading layer
C
O
N
T
A
C
T
I
N
G
S
C
H
E
M
S

40. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 40
n-contact resistance measurement
Fig. n-contact resistance measurement
-30 -20 -10 0 10 20 30 40 50
0
5
10
15
20
25
30
35
Mean
Linear Fit of Sheet1 Resistance
Resistance()
Gap (m)
y-Intercept = 10.46
Slope = 0.42
x-Intercept = -24.98
c
= 6.53x10-5
cm2
Contact Res. = 5.23 
Sheet Res. = 41.87 sq.

41. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 41
Conclusion
Indium Gallium Nitride is a semiconductor material with potential to be
used in photovoltaic devices.
Established InGaN as a high-efficiency photovoltaic material.
p-i-n double hetero junction structure is optimized with conventional
structure and also effect of indium variation is observed on characteristic
parameters.

42. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 42
References
Neff, H., Semchinova, O., Lima, A., Filimonov, A., Holzhueter, G.,“Photovoltaic properties and
technological aspects of In1-xGaxN/Si, Ge(0＜x＜0.6) heterojunction solar cells,” Sol. Energy
Mater. Sol. Cells 90, 982-997(2006).
Jani, O.et al., “Design and characterization of GaN/InGaN solar cells,” Appl. Phys. Lett. 91,
132117 (2007).
Shih-Wei Feng et al., “Theoretical simulations of the effects of the indium content, thickness,
and defect density of the i -layer on the performance of p - i - n InGaN single homojunction solar
cells ” Appl. Phys. Lett. 108, 093118 (2010).
Iulian Gherasoiu et al., “Photovoltaic action from InxGa1-xN p-n junctions with x > 0.2 grown on
silicon ” Phys. Status Solidi C 8, No. 7–8, 2466–2468 (2011).

43. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 43
Acknowledgment
I am thankful to the Director, CSIR-CEERI, Pilani for giving
me opportunity to work in this laboratory.
I am thankful to my supervisor Mr. Sonachand Adhikari.
I am also thankful to Dr. C. Dhanvantri (Group Leader-
ODG), Dr. S. Pal and Dr. Sumitra Singh for constant
encouragement in this work.
I also thank all ODG members for their support.
I am thankful to training in-charge Mr. Vinod K. Verma.

44. Rajkumar Sahu
rksahu@outlook.com
May 22, 2014
Slide 2