基礎工学研究科物質創成専攻吉田研究室 上岡寛明

1. Solid phase immiscibility in GaInNI-hsiu Ho and G.B. StringfellowAppl. 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 asfossil 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. 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. How do we produce the semiconductor alloy which has spatially inhomogeneous energy gap? Spinodal decomposition What is spinodal decomposition?

11. 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. Explanation of the letter It is very difficult to fabricate GaInN Although various ways had been triedsuch 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. 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 ? 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