Solid phase immiscibility in GaInNI-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

• IntroductionMy research plan for Master thesisQuantum-dot/wire based solar cellsSpinodal 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

Outline

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.

Introduction

Why solar cells ?

Solar cells can produce safe and clean energy

without any limited energy resources such as fossil fuel

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.

One good way lately attracting attention to make it possible to realize high efficiency is …

To apply Quantum dot/wire to solar cells

・ Discretization of energy band (Quantum dot/wire) ・ Multiple-exciton formation

・ Separation of electrons and holes

Quantum dot/wire solar cells can generate far higher efficiency thanks to these three effects

The modulation of band gap by spatially inhomogenity.

How do we produce the semiconductor alloy which has spatially inhomogeneous energy gap?

Spinodal decomposition

What is spinodal decomposition?

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”

Thinking of semiconductor alloy Ax-1Bx

Ｘ

T

Comparing of two ways to mix up semiconductor alloys

Spinodal decomposition

Binodal decomposition

The way to cool down rapidly two heated materials, (not keeping the thermal stability)

The way to cool down slowly two heated materials, (keeping the thermal stability)

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?

Enough amount of InN didn’t mix with GaN .

Calculation of phase diagram

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

How ?

Calculation using modified Valence-Force-Field model (modified VFF model) on Ga1-xInxN

Before thinking of modified 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 )

What is VFF model ?

Need to modify…

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

The phase diagram of GaInNBinodal 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

Such as

My idea of

In the red region inside spinodal line, the form of energy gap becomes spatially inhomogeneous,

Spinodal region

Summary

•The obtained phase diagram proves the difficulty producing GaInN