1. 基礎工学研究科物質創成専攻吉田研究室 上岡寛明
Solid phase immiscibility in GaInN I-hsiu Ho and G.B. Stringfellow Appl. Phys. Lett. 69 (1996) 2701
基礎工学研究科物質創成専攻吉田研究室
上岡寛明

2. Outline Introduction My research plan for Master thesis
Quantum-dot/wire based solar cells
Spinodal decomposition
Explanation of Ho and Stringfellow’s letter
Difficulty of producing GaInN
Calculation of phase diagram
My idea for the fabrication of solar cells
Summary

3. Introduction Introduction What is the purpose of my research?
To find out new materials and fabrication method which can be used for photovoltaic solar cells by computational materials design.

4. Why solar cells ? Solar cells can produce safe and clean energy
without any limited energy resources such as fossil fuel

5. What is the ideal solar cells?
1. The ideal solar cells should be fabricated easily and at low cost.
2. The ideal solar cells should have high efficiency of energy conversion.
3. The ideal solar cells should be fabricated by environmental friendly materials.

6. To apply Quantum dot/wire to solar cells
One good way lately attracting attention to make it possible to realize high efficiency is …
To apply Quantum dot/wire to solar cells

7. Quantum dot/wire solar cells can
generate far higher efficiency
thanks to these three effects
・Discretization of energy band (Quantum dot/wire)
・Multiple-exciton formation
・Separation of electrons and holes

9. The modulation of band gap by spatially inhomogenity.

10. Spinodal decomposition
How do we produce the semiconductor alloy which has spatially inhomogeneous energy gap?
Spinodal decomposition
What is spinodal decomposition?

11. Ax-1Bx The point where dG/dx=0 is called “binodal point”
Thinking of semiconductor alloy
Ax-1Bx
The molar free energy of mixing
is approximated as
⊿G=Ωx(1-x)+kT[xln(x)+(1-x)ln(1-x)]
Ω：interaction parameter
The point where dG/dx=0 is called “binodal point”
The point where d2G/d2x=0 is called “spinodal point” T Ｘ

12. Comparing of two ways to mix up semiconductor alloys
Spinodal decomposition
The way to cool down rapidly two heated materials, (not keeping the thermal stability)
Binodal decomposition
The way to cool down slowly two heated materials, (keeping the thermal stability)

13. Why? Explanation of the letter It is very difficult to fabricate GaInN
Although various ways had been tried such as
・ Growth at 800℃ ,
・ Growth at 500 ℃,
・ Growth raising
the In pressure in the vapor,
all the ways ended in failure.
Why?
Calculation of phase diagram
Enough amount of InN didn’t mix with GaN .

14. Unknown parameter is Ω Calculate the value of Ω
The molar free energy of mixing
is approximated as
⊿G=Ωx(1-x)+kT[xln(x)+(1-x)ln(1-x)]
Ω：interaction parameter
Unknown parameter is Ω
Calculate the value of Ω

15. How ? Calculation using modified Valence-Force-Field model
(modified VFF model) on Ga1-xInxN
Before thinking of modified VFF model…

16. What is VFF model ? Need to modify… The feature of VFF model is
・To consider the interaction up to only first nearest neighbor (not enough )
・ To overestimate the strain energy ( not exact )
Need to modify…

17. Calculation of Ω by modified VFF model.
It’s allowed to consider interaction up to 2-6th nearest neighbors
average value
Ω＝5.95kcal/mol

18. The phase diagram of GaInN
Binodal region : two materials cannot mix and become separated
(phase separation)
Spinodal region : two materials mix with each other inhomogeneously
(spinodal decomposition)
This phase diagram tell us why enough amount of InN didn’t mix with GaN

19. My idea of
In the red region inside spinodal line, the form of energy gap becomes spatially inhomogeneous,
Spinodal
region
Such as

21. Summary
The obtained phase diagram proves the difficulty producing GaInN