Gallium Nitride (GaN)Gallium Nitride (GaN)

PHYS 571Gugun GunardiHeath Kersell

Damilola Daramola

Gallium Nitride (GaN)Gallium Nitride (GaN)IntroductionPropertiesCrystal StructureBonding TypeApplication

IntroductionIntroduction 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.

Wide band gap energy.

http://www.phy.mtu.edu/yap/images/galliumnitride.jpg

PropertiesPropertiesPROPERTY / MATERIAL

. Cubic (Beta) GaN

. Hexagonal (Alpha) GaN

. Structure Zinc Blende Wurzite Stability Meta-stable Stable

Lattice Parameter(s) at 300K 0.450 nm a0 = 0.3189 nm c0 = 0.5185 nm

Density at 300K 6.10 g.cm-3 6.095 g.cm-3

Nature of Energy Gap Eg Direct Direct

Energy Gap Eg at 293-1237 K  3.556 - 9.9x10-4T2 /

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

PropertiesProperties

Energy Gap Eg at 300 K

3.23 eV Ramirez-Flores et al 1994

.3.25 eV

Logothetidis et al 1994

3.44 eV Monemar 1974

.3.45 eV

Koide et al 1987

.3.457 eV

Ching-Hua Su et al, 2002

Energy Gap Eg at ca. 0 K3.30 eV

Ramirez-Flores et al1994Ploog et al 1995

3.50 eV Dingle et al 1971Monemar 1974

PropertiesPropertiesComparison between Common Semiconductor Material Properties

and GaNMaterialMaterial Bandgap (eV)Bandgap (eV)

Electron Electron Mobility Mobility

(cm2/Vs)(cm2/Vs)

Hole Hole Mobility Mobility

(cm2/Vs)(cm2/Vs) Critical Field Critical Field EcEc

(V/cm)(V/cm) Thermal Thermal

Conductivity Conductivity TT (W/m(W/mK)K)

Coefficient Coefficient of Thermal of Thermal Expansion Expansion (ppm/K)(ppm/K)

InSbInSb 0.17, D0.17, D 77,00077,000 850850 1,0001,000 1818 5.375.37

InAsInAs 0.354, D0.354, D 44,00044,000 500500 40,00040,000 2727 4.524.52

GaSbGaSb 0.726, D0.726, D 3,0003,000 1,0001,000 50,00050,000 3232 7.757.75

InPInP 1.344, D1.344, D 5,4005,400 200200 500,000500,000 6868 4.64.6

GaAsGaAs 1.424, D1.424, D 85008500 400400 400,000400,000 5555 5.735.73

GaNGaN 3.44, D3.44, D 900900 1010 3,000,0003,000,000 110 (200 Film)110 (200 Film) 5.4-7.25.4-7.2

GeGe 0.661, I0.661, I 3,9003,900 1,9001,900 100,000100,000 5858 5.95.9

SiSi 1.12, I1.12, I 1,4001,400 450450 300,000300,000 130130 2.62.6

GaPGaP 2.26, I2.26, I 250250 150150 1,000,0001,000,000 110110 4.654.65

SiC (3C, b)SiC (3C, b) 2.36, I2.36, I 300-900 300-900 10-3010-30 1,300,0001,300,000 700700 2.772.77

SiC (6H, a)SiC (6H, a) 2.86, I2.86, I 330 - 400 330 - 400 7575 2,400,0002,400,000 700700 5.125.12

SiC (4H, a)SiC (4H, a) 3.25, I3.25, I 700700    3,180,0003,180,000 700700 5.125.12

C C

(diamond)(diamond) 5.46-5.6, I5.46-5.6, I 2,2002,200 1,8001,800 6,000,0006,000,000 1,3001,300 0.80.8

Crystal StructureCrystal StructureGaN grown in

◦Wurtzite crystal structure ◦Zinc-blende crystal structure

The band gap, Eg, effected by crystal structure

Wurtzite Wurtzite Crystal Crystal StructureStructure•Wurtzite crystal structure is a member of the hexagonal crystal system •The structure is closely related to the structure of hexagonal diamond. • Energy gap: 3.4 eV

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

Wurtzite Wurtzite Crystal StructureCrystal StructureAn ideal angle: 1090

Nearest neighbor: 19.5 nmEnergetically favorableSeveral other compounds can take the

wurtzite structure, including Agl, ZnO, CdS, CdSe, and other semiconductors.

Zinc-blendeZinc-blendeCrystal StructureCrystal Structure

• Energy gap 3.2 eV• An ideal angle: 109.470

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

Tetrahedral bonds◦ sp3 hybridization◦ Bonding angle: 109.47°◦ Bond Length: 19.5 nm

Ga-N bonds significantly stronger than Ga-Ga interactions (based on distance)

GaN Bonding PropertiesGaN Bonding Properties

IonicityIonicity•GaN exhibits mixed ionic-covalent bonding•Ionicity of a bond is the fraction fi of ionic character compared to the fraction of fh of covalent character

•By Pauling’s definition

•Modern definition• is the ionicity phase angle

1http://www.bcpl.net/~kdrews/bonding/bonding2.html

GaN Bonding PropertiesGaN Bonding PropertiesBased on calculations using both methods, typical values are

Compound Pauling ionicity Modern ionicity2

AlN 0.430 0.449AlP 0.086 0.307AlAs 0.061 0.274GaN 0.387 0.500GaP 0.061 0.327GaAs 0.039 0.310InN 0.345 0.578InP 0.039 0.421InAs 0.022 0.357NaCl 0.668 > 0.9

C (Diamond) 0 0

2J.C. Phillips, Bonds and Bands in Semiconductors 1973

Bond Character dependent on electronegativityχN >> χP > χAs > χSb

GaN Bonding PropertiesGaN 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 compoundse.g. GaN – 3.2 eV/3.4 eV

GaP – 2.3 eV AlSb – 1.5 eV InP – 1.3 eV

ApplicationsApplicationsGallium Nitride Typical Applications: New Kind of Nanotube

Laser diodes

High-resolution Printings

Microwave radio-frequency power amplifiers

Solar Cells

New Kind of NanotubeNew Kind of Nanotube• Single Crystal Nanotubes

Fabricated

• Gallium Nitride nanotubes have diameter between 30 – 200 nm

• Potential for mimicking ion channels

GaN Laser DiodeGaN Laser DiodeNormally 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 DiodesGaN Laser DiodesApplications in:

◦‘Blu-Ray’ technology

◦Laser Printinghttp://www.aeropause.com/archives/Blu-ray-cover_plat.jpg

GaN Solar CellsGaN 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 CellsGaN Solar CellsAdvantages:

High heat capacityResistant to effects of strong radiationHigh efficiency

Difficulties:Too many crystal layers create system damaging stressToo expensive

ReferencesReferences 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