Gallium Nitride (GaN)

PHYS 571 Gugun Gunardi Heath Kersell Damilola Daramola

Gallium Nitride (GaN)

 Introduction   Properties Crystal Structure   Bonding Type Application

Introduction

 The next important semiconductor material after silicon.  Can be operated at high temperatures.

 The key material for the next generation of high frequency and high power transistors.

http://www.phy.mtu.edu/yap/images/g alliumnitride.jpg

 Wide band gap energy.

Properties

PROPERTY / MATERIAL .

Structure Stability Lattice Parameter(s) at 300K Density at 300K Nature of Energy Gap E g Energy Gap E g at 293-1237 K

Cubic (Beta) GaN .

Zinc Blende Meta-stable 0.450 nm 6.10 g.cm

-3 Direct

Hexagonal (Alpha) GaN .

Wurzite Stable a 0 = 0.3189 nm c 0 = 0.5185 nm 6.095 g.cm

-3 Direct 3.556 - 9.9x10

-4 T 2 / (T+600) eV Ching-Hua Su et al, 2002

Properties

Energy Gap E g at 300 K 3.23 eV Ramirez-Flores et al 1994 .

3.25 eV Logothetidis et al 1994 Energy Gap E g at ca. 0 K 3.30 eV Ramirez-Flores et al1994 Ploog et al 1995 3.44 eV Monemar 1974 .

3.45 eV Koide et al 1987 .

3.457 eV Ching-Hua Su et al, 2002 3.50 eV Dingle et al 1971 Monemar 1974

Properties

Comparison between Common Semiconductor Material Properties and GaN Material InSb InAs GaSb InP GaAs GaN Ge Si GaP SiC (3C, b) SiC (6H, a) SiC (4H, a) Bandgap (eV) 0.17, D 0.354, D 0.726, D 1.344, D 1.424, D 3.44, D 0.661, I 1.12, I 2.26, I 2.36, I 2.86, I 3.25, I Electron Mobility (cm2/Vs) Hole Mobility (cm2/Vs) Critical Field (V/cm) Ec 77,000 44,000 3,000 5,400 8500 900 3,900 1,400 250 300-900 330 - 400 700 850 500 1,000 200 400 10 1,900 450 150 10-30 75 1,000 40,000 50,000 500,000 400,000 3,000,000 100,000 300,000 1,000,000 1,300,000 2,400,000 3,180,000 Thermal Conductivity (W/m  K) s T 18 27 32 68 55 110 (200 Film) 58 130 110 700 700 700 Coefficient of Thermal Expansion (ppm/K) 5.37

4.52

7.75

4.6

5.73

5.4-7.2

5.9

2.6

4.65

2.77

5.12

5.12

C (diamond) 5.46-5.6, I 2,200 1,800 6,000,000 1,300 0.8

Crystal Structure

 GaN grown in ◦ Wurtzite crystal structure ◦ Zinc-blende crystal structure  The band gap, Eg, effected by crystal structure

Wurtzite Crystal Structure

• Wurtzite crystal structure is a member of the hexagonal crystal system • The structure is closely related to the structure of hexagonal diamond.

http://en.wikipedia.org/wiki/Image: Wurtzite-unit-cell-3D-balls.png

• Energy gap: 3.4 eV

Wurtzite Crystal Structure

 An ideal angle: 109 0  Nearest neighbor: 19.5 nm  Energetically favorable  Several other compounds can take the wurtzite structure, including Agl, ZnO, CdS, CdSe, and other semiconductors.

Zinc-blende Crystal Structure

• • • Energy gap 3.2 eV An ideal angle: 109.47

0 Nearest neighbor: 19.5 nm http://en.wikipedia.org/wiki/Image:Sphalerite unit-cell-depth-fade-3D-balls.png

GaN Bonding Properties

  Tetrahedral bonds ◦ sp 3 hybridization ◦ Bonding angle: 109.47° ◦ Bond Length: 19.5 nm Ga-N bonds significantly stronger than Ga-Ga interactions (based on distance)

Ionicity

• GaN exhibits mixed ionic-covalent bonding • Ionicity of a bond is the fraction f i of ionic character compared to the fraction of f h • By Pauling’s definition of covalent character • Modern definition • is the ionicity phase angle 1 http://www.bcpl.net/~kdrews/bonding/bonding2.html

GaN Bonding Properties

Based on calculations using both methods, typical values are Compound Pauling ionicity Modern ionicity 2 AlN AlP AlAs GaN GaP GaAs InN InP InAs NaCl C (Diamond) 0.430

0.086

0.061

0.387

0.061

0.039

0.345

0.039

0.022

0.668

0 0.449

0.307

0.274

0.500

0.327

0.310

0.578

0.421

0.357

> 0.9

0 Bond Character dependent on electronegativity χ N >> χ P > χ As > χ Sb 2 J.C. Phillips,

Bonds and Bands in Semiconductors

1973

GaN Bonding Properties

• • Bonding strength determines energy gap size Large band gap evidence of strong bonding in GaN • Strongly Ionic Compounds (also insulators) LiF – 11eV; NaCl – 8.5eV; KBr – 7.5 eV • Other III-V compounds e.g. GaN – 3.2 eV/3.4 eV GaP – 2.3 eV AlSb – 1.5 eV InP – 1.3 eV

Applications

 Gallium Nitride Typical Applications:  New Kind of Nanotube  Laser diodes  High-resolution Printings  Microwave radio-frequency power amplifiers  Solar Cells

New Kind of Nanotube

• Single Crystal Nanotubes Fabricated • Gallium Nitride nanotubes have diameter between 30 – 200 nm • Potential for mimicking ion channels

GaN Laser Diode

   Normally emit ultraviolet radiation Indium doping allows variation in band gap size Band gap energies range from 0.7eV – 3.4eV

http://www.lbl.gov/Science Articles/Archive/assets/images/2002/Dec 17-2002/indium_LED.jpg

GaN Laser Diodes

 Applications in: ◦ ‘Blu-Ray’ technology ◦ Laser Printing http://www.aeropause.com/archives/Blu-ray cover_plat.jpg

GaN Solar Cells

  Indium doped (InGaN) Conversion of many wavelengths for energy  Theoretical 70% maximum conversion rate.

  Multiple layers attain higher efficiency.

Need many layers to attain 70%  Lattice matching not an issue

GaN Solar Cells

Advantages:    High heat capacity Resistant to effects of strong radiation High efficiency Difficulties:   Too many crystal layers create system damaging stress Too expensive

References

         http://www.reade.com/Products/Nitrides/Gallium-Nitride-(GaN) Powder-&-Crystals.html

http://www.semiconductors.co.uk/nitrides.htm#GaN http://www.onr.navy.mil/sci_tech/31/312/ncsr/materials/gan.asp

http://www.lbl.gov/Science-Articles/Archive/MSD-gallium-nitride nanotube.html

http://www.lbl.gov/Science-Articles/Archive/MSD-full-spectrum solar-cell.html

http://www.lbl.gov/Science-Articles/Archive/blue-light-diodes.html http://www.ioffe.ru/SVA/NSM/Semicond/GaN/bandstr.html#Basic http://nsr.mij.mrs.org/4S1/G6.3/article.pdf

http://nsr.mij.mrs.org/news/industapp97.html