furrey undergraduate research symposium v3 public

Al-Induced Crystallization: Cheaper, More Efficient Solar CellsPATRICK FURREY, ELISABETH MCCLURE, SETH HUBBARD

NANOPOWER RESEARCH LABS, ROCHESTER NY

What is a single junction solar cell?

Undergraduate Research Symposium 2015August 7, 2015 Rochester, NY

¹ http://pveducation.org/pvcdrom/solar-cell-operation/solar-cell-structure ² William Shockley and Hans J. Queisser, "Detailed Balance Limit of Efficiency of p-n Junction Solar Cells", Journal of Applied Physics, Volume 32 (March 1961), pp. 510-519; doi:10.1063/1.1736034

2

• Generation of light-generated carriers

• Collection light-generated carriers to generate current

• Generation of large voltage across solar cell

• Dissipation of power in load and parasitic resistances

• Shockley-Queisser limit maximizes single p-n junction with 1.34eV bandgap at AM1.5 to be 33.7% ²

¹

http://pveducation.org/pvcdrom/solar-cell-operation/solar-cell-structure

http://pveducation.org/pvcdrom/solar-cell-operation/solar-cell-structure

http://jap.aip.org/resource/1/japiau/v32/i3/p510_s1

https://en.wikipedia.org/wiki/Digital_object_identifier

https://dx.doi.org/10.1063/1.1736034

What is a multi junction solar cell?• III-V solar cells are direct bandgap

semiconductors, as opposed to Si and is are great absorbers• Multijunction cells can account for

otherwise lost photons• Grown by MOCVD, GaInP/GaInAs/Ge

cells can reach 31.3% efficiency at AM1.5 and 41.6% under concentration


3

History of AIC and its relation to our Goal


• AIC uses inexpensive techniques, such as sputtering or thermal evaporation, to work towards these kinds of efficiencies; however, much work is still needed to make it commercially viable.• Polycrystalline materials have grain

boundaries which act as defects, whereas small grains can lead to increased nonradiative recombination that impedes current generation.

4

Highly (111)-oriented Ge thin films on insulators formed by Al-induced crystallizationToko, K. and Kurosawa, M. and Saitoh, N. and Yoshizawa, N. and Usami, N. and Miyao, M. and Suemasu, T., Applied Physics Letters, 101, 072106 (2012), DOI:http://dx.doi.org/10.1063/1.4744962

Our goal is to pick up where Toko, et al, left off in exploring different variables for increasing grains size and preferential orientation.

Our Process and Tools

• Started with 1 1 1 Si wafers• RCA cleaned them to proceed to grow a 1-micron oxide layer• Sputtered/thermally evaporated 50nm of Al• Broke vacuum for 30 mins/remained under vacuum

• Sputtered/thermally evaporated 50nm of Ge• Annealed for 10/20 hour


5

CVC 601 Sputter System Kurt J. Lesker PVD 75c Lindberg Blue Tube Furnace

Si

SiO2AlGe

Al2O3

Al2O3Ge

Variable Time


6

Constants:• 260 C anneal• No Al2O3• Sputtered

Pre anneal shows no Ge peak meaning the Ge is amorphous

Both anneals showed Ge peaks, meaning crystals formed

20h Ge peak had a higher intensity than the 10h, showing a greater number of crystals

Variable Time- SEM


7

Constants:• 260 C anneal• No Al2O3• Sputtered

Pre anneal shows no Ge peak meaning the Ge is amorphous

Both anneals showed Ge peaks, meaning crystals formed

20h Ge peak had a higher intensity than the 10h, showing a greater number of crystals

20 hour anneal

10 hour anneal

Scherrer EquationA formula that relates X-ray diffraction readings to grain sizes.

K is a dimensionless shape factor that has a typical value of 0.9, varying with crystalline structure. is the X-ray wavelength.The Scherrer Equation is applicable up to 0.1 to 0.2um.

A K of 0.9 and the of Cu, 1.541838 Å, was used.

8

Sample FWHM () Bragg Angle ()

Mean Size (or less)

10h, no Al2O3 0.230 27.2º 6 Å20h, no Al2O3 0.226 27.1º 6 Å

Patterson, A. (1939). "The Scherrer Formula for X-Ray Particle Size Determination". Phys. Rev. 56 (10): 978–982. Bibcode:1939PhRv...56..978P.doi:10.1103/PhysRev.56.978.


With or Without Al2O3


9

Constants:• 10h period• 260 C anneal• Sputtered

Since Al2O3 acts as a barrier, layer transfer cannot occur at such a low temperature and the Ge remains amorphous

As the Al tries to diffuse, it is impeded by the Al2O3 and forms dendritic grains

With or Without Al2O3 – Nomarski

Microscopy


10

Constants:• 10h period• 260 C anneal• Sputtered

Since Al2O3 acts as a barrier, layer transfer cannot occur at such a low temperature and the Ge remains amorphous

The Al peak shift is either an Al alloy created or Al2O3

Without Al2O3

With Al2O3

Different Deposition Methods


11

Constants:• 20h period• 260 C anneal• With Al2O3

Neither method shows Ge crystals formed

Sputtering shows larger peaks meaning bigger crystals or more uniform crystal orientation

Different Deposition

Methods - SEM


12

Constants:• 20h period• 260 C anneal

Neither method shows Ge crystals formed

Sputtering shows larger peaks meaning bigger crystals or more uniform crystal orientation

Thermal Evaporation with Al2O3

Sputter without Al2O3

Variable Temperature


13

Constants:• 20h period• With Al2O3• Thermally

Evaporated

The Al2O3 barrier is overcome and layer transfer can occur at 325 C. This results in 111 Ge grains.

Variable Temperature –

Nomarski Microscopy


14

Constants:• 20h period• With Al2O3• Thermally

Evaporated

The Al2O3 barrier is overcome and layer transfer can occur at 325 C. This results in 111 Ge grains.

325 C Anneal

260 C Anneal

Conclusions and Further Work• THE THERMAL EVAPORATED SAMPLE AT 325 C SHOWED

THE BEST IMAGES THROUGH MICROSCOPY.• THE 20H ANNEAL WITH NO VACUUM BREAK SHOWED THE

STRONGEST XRD PEAKS FOR 1 1 1 GERMANIUM.• HYPOTHESIS IS THAT THE ALUMINUM GROWS

DENDRITICALLY AND THE GERMANIUM GROWS IN A MORE ROUNDED SHAPE.

• Changing the thicknesses of Al and Ge• More higher temperature anneals, >260 C, needed to

find the “sweet spot”• Explore 20h anneal sputtered with Al2O3 layer.


15

Acknowledgements• GEORGE NELSON• MICHAEL SLOCUM, NATHAN COX, JASON STAUB• ZAC BITTNER• THE OFFICE OF NAVAL RESEARCH

Questions?

N00173-14-1-G011


16

furrey undergraduate research symposium v3 public

Documents