Formation of ZnTe:Cu/Ti Cont

High Temperature for CdS/CdTe 

T. A. Gessert, S. Asher, S. Johnston

M.R. Young, and T. Moriarty

National Center for Photovoltaic

National Renewable Energy Labora

Acknowledgements

Alan Fahrenbruch (ALF)

Research supported by United States Departme

Contract No. DE-AC36-99GO10337

NREL/PR-520-39845Presented at the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion (WMay 7-12, 2006 in Waikoloa, Hawaii.
Tim Gessert, 4WCPEC, 5/11/06

Disclaimer and Government License

This work has been authored by Midwest Research Institute (MRI) under Contract No. DE-AC36Department of Energy (the “DOE”). The United States Government (the “Government”) retains athe work for publication, acknowledges that the Government retains a non-exclusive, paid-up, irrepublish or reproduce the published form of this work, or allow others to do so, for Government puNeither MRI, the DOE, the Government, nor any other agency thereof, nor any of their employeeor implied, or assumes any liability or responsibility for the accuracy, completeness, or usefulnesproduct, or process disclosed, or represents that its use would not infringe any privately owned ri

specific commercial product, process, or service by trade name, trademark, manufacturer, or othimply its endorsement, recommendation, or favoring by the Government or any agency thereof. authors and/or presenters expressed herein do not necessarily state or reflect those of MRI, the agency thereof. 
Tim Gessert, 4WCPEC, 5/11/06

ZnTe:Cu/Ti Contact Proces

(All-Dry, High-Temperature [300

ZnTe:C

High Temp.

Vacuum

CdTe/CdS +

Vapor CdCl2

 Heating

CdTe CdTe CdTe 

ZnTe:Cu 

Glass Glass Glass G

Ion-Beam  Milling

 r.f. Sputter  Deposition  (ZnTe:Cu)

d.De 
Tim Gessert, 4WCPEC, 5/11/06

CdS CdTe

CdS CdTe

Ti

Ti

ZnTe:Cu

Constant Temperature ~300°C

Vary ZnTe:Cu Thickness

Constant T

Vary ZnT

Cu 
Tim Gessert, 4WCPEC, 5/11/06

10171018101910201021

Cu Concentration (cm-3

)

0 1 2 3 4 5

Depth (μm)

ZnTe:Cu CdTe CdS

 240C 300C 320C 340C 360C

SIMS Analysis of Temperature D

Some Insig
Tim Gessert, 4WCPEC, 5/11/06

Some Insig

302010

0

-10

Current Density (mA cm

2

)

-0.2 0.0 0.

V

   

Cu Diffusion Less Than Optimum Cu Diffusion 

302010

0

-10

Current Density (mA cm

2

)

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (Volts)

 ~240°C (24V) ~280°C (28V) ~320°C ( 32V, Optimum)

Insight from Temperature Depende

(ZnTe:Cu Contact Temperature)
Tim Gessert, 4WCPEC, 5/11/06

Some Insig

Dark IV Analysis of Temperature Dep

(Low Jo and Increase in Rs at Higher Contact 

10

-6

10

-5

10

-4

10

-3

10

-2

10

-1

10

0

10

1

Current Density (mA/cm2)

0.0 0.2 0.4 0.6 0.8 

Voltage (Volts)

 UC697A_ UC694A_ UC696A_ UC695A_ UC698A_

?
Tim Gessert, 4WCPEC, 5/11/06

Light and Voltage-Dependen

100

80604020

0

Quantum Efficiency (%)

400 500 600 700 800 900

Wavelength (nm)

 24 Volt Heater

 Insufficient Cu

 32 Volt Heater

 Optimum Cu

 36 Volt Heater

 Excessive Cu

1-Sun Bias+0.4 Volts

QE Comparison Suggests

Narrow Junction 

for Excessive Cu Devices

Red-Light B

Photoco

For Exces

120100

80604020

0

Apparent Quantum Efficienty (%)

400 500 
Tim Gessert, 4WCPEC, 5/11/06

ZnTe:Cu/Metal Contact D

Some Resulting Question

•Is Cu diffusion from the contact always “good” in CdTe an•Is there a maximum Cu concentrations that can be tolerate•If Cu in the CdS can be minimized, does the Voc increase?

•Can the Cu diffusion from contact be controlled advantag

10171018101910201021

Cu Concentration (cm-3

)

0 1 2 3 4 

Depth (μm)

ZnTe:Cu CdTe 

 240C 300C 320C 340C 360C
Tim Gessert, 4WCPEC, 5/11/06

ZnTe:Cu/Metal Contact D

ZnTe:Cu Contact Parameters - Th

•Ion Beam Mill Depth 1000 •Cu Concentration in ZnTe 9 atom•ZnTe:Cu Thickness (0.5 μ

•Vary: 0.04-

•ZnTe:Cu Deposition Temperature (300°

•Increase: 360°C 

•Outer Metallization Ti, 0.

•Voc ~800 mV

•Jsc ~19 mA/cm2

•FF ~71%•Efficiency ~11%

Device Performance (This Study, VTD Production 
Tim Gessert, 4WCPEC, 5/11/06

Co

10

17

10181019102010

21

Cu Concentration (cm-3) 

-1 0 1 2 3 4

Depth (microns)

ZnTe:Cu CdTe CdS

 1.0 μm, 360C 0.5 μm, 360C 0.2 μm, 360C Previous, 1.0 μm, 360 Previous, 1.0 μm, 320 This Study

Maximum CuMidway BetwePast Studies

10171018101910201021

Cu Concentration (cm-3

)

0 1 2 3 4 5

Depth (μm)

ZnTe:Cu CdTe CdS

 240C 300C 320C 340C 360C

Previous StudieToo Much Cu???Just Right Cu?Too Little Cu?
Tim Gessert, 4WCPEC, 5/11/06

(100 kHz, 10 mV, -8.0 to +0.6 Sweep)

Capacita

Comparison of Previous and Present C-V 

10 1310 1410 1510 1610 1710 18

N

a-Nd (cm -3

)

0 1 

De

 ZnTe:Cu ThickneContact Temperature 3 100 KHz, 10 mV Sweep -8.0 V - +0.6 

 1 μm0.5 μm 0.2 μm 0.1 μm

0 V Bias10,000 Å 

101310141015101610171018

N

A-ND (cm-3

)

1 2 3 4

Depletion Width (μm)

 ZnTe:Cu Depositon Temperature

 240C 300C 320C 340C 360C

340C 320C

240C

360C

300C

Previous Data - Contact Temperature New Study - 
Tim Gessert, 4WCPEC, 5/11/06

High 

Can

Insuffi

Unl

V

But s

0.2 μm 

- 

10

-7

10

-6

10

-5

10

-4

10

-3

10

-2

10

-1

10

0

10

1

Current Density (mA cm-2)

0.0 0.2 0.4 0.6 0.8 1.0

Voltage (Volts)

 ZnTe:Cu Thickness 

Devices Contacted at 360°C 

 1.0 μm 0.5 μm 0.2 μm 0.1 μm 0.04 μm

40302010

0

-10-20

Current Density (mA cm

-2)

0.0 0.2 0.4 0.6 0.8 1.0

Voltage (Volts)

ZnTe:Cu Thickness

Devices Contacted at 360°C

 1 μm 0.5 μm 0.2 μm 0.1 μm 0.04 μm
Tim Gessert, 4WCPEC, 5/11/06

SCAPS-1D Simulation Model De

(Vary Shallow CdS ND and CdTe NA Only - No Deep De

+ 4.5 μm

0.3 μm

CdS

(Vary ND)

CdTe

(Vary NA)

=

CdS/CdTe

Simulated

Junction
Tim Gessert, 4WCPEC, 5/11/06

Comparison of C-V Measuremen

and SCAPS-1D C-V Simulatio

10 1310 1410 1510 1610 1710 18

N

a-Nd (cm -3

)

0 1 2 3 4

Depletion Width (um)

 ZnTe:Cu ThicknessContact Temperature 360°C 100 KHz, 10 mV Sweep -8.0 V - +0.6 Volts

 1 μm0.5 μm 0.2 μm 0.1 μm

0 V Bias10,000 Å 

0 V Bias 5,000 Å

0 V Bias 2,000 Å

0 V Bias 1,000 Å

10

13

10

14

10

15

10

16

10

17

10

18

N

A (cm -3

)

0 1 

1e141e11

1e141e13

S

100 kH

 NA 

 CC46_2e1 CC46_4e1 CC46_8e1
Tim Gessert, 4WCPEC, 5/11/06

Comparison of LIV/DIV Measure

and SCAPS-1D Simulation

40302010

0

-10-20

Current Density (mA cm -2)

0.0 0.2 

 SCAPS 

CdTe N

 LIV_2e13 LIV_1e14 LIV_1e14 LIV_1e14 DIV46_1 DIV46_1 DIV46_1

40302010

0

-10-20

Current Density (mA cm

-2)

0.0 0.2 0.4 0.6 0.8 1.0

Voltage (Volts)

ZnTe:Cu Thickness

Devices Contacted at 360°C

 1 μm 0.5 μm 0.2 μm 0.1 μm 0.04 μm
Tim Gessert, 4WCPEC, 5/11/06

Probable Cause of Apparent Series Res

For CdS/CdTe Devices with High Cu Concen

CompensatingCu Acceptors

Insufficient-Optimum Cu

Excessive Cu
Tim Gessert, 4WCPEC, 5/11/06

Conclusions

•Cu diffusion from a ZnTe:Cu contact causes goo

•The Good (Cu in CdS < low 1018 cm-3)

•Increase in CdTe NA-ND that leads to Voc and FF improve

•The Bad (Cu in CdS > low 1018 cm-3)

•Possibly decreased of shunt resistance (?)•Depletion width in CdTe can become too narrow for collection at JMPP•Donor reduction in CdS (Significant FF loss in LIV)•Excessive Cu diffusion into CdS readily observed by 

•Next Question - Can we get the Good without th