III-N Materials

2. Mechanical and optical properties

Vicker hardness(H) and fracture toughness(Kc) of various III-N is given in table

c-BN is particularly hard material which makes it attractive for ceramic applications. AlN,GaN,InN have approximately same Hardness.

MaterialGaNAlNInNBNSiCSiH(GPa)121411.255-65339Kc(Mpa m1/2 )0.82.63.3.7Cofficient of thermal expansion(CTE)

Wz wurtzite, Sa epitaxially on sapphire substrate, c cubic, p powderCTE is a very important parameter for growth CTE and its temperature dependence have a similar impact to layer growth of heterostructures as the lattice constants.GaN(wz)GaN(sa)AlN InN BNSiCSap.Sia (10-6 K-1)3.13.82.93.61.153.24.32.6c (10-6 K-1)2.82.93.42.63.23.22.6contentsGaNAlNInNBNGaNGaN is the basic material of this class which is typically used for all devices requiring fast carrier transport, high breakdown voltage.It is used as channel material in various FETs and also as base material in HBTsMost of the ohmic contact layers in III-N devices use binary n-type or p-type GaN.

Crystal structure

Wurtzite :- At room temperature GaN, AlN, and InN are found in Wurtzite structure while BN prevails Mostly in CubicZincBlende:- The Zinc blende structure of GaN can also be found in Thin filmsRock salt phase :- rock salt phase is of no importance to electronic devices so farIn wurtzite structure the growth is typically performed along c-axis.Recently growth along m-plane has been reported as a resulting non polar material has positive influence on diode efficiency in optoelectronics.3. Dielectric constants

MaterialGaN(wz)AlN(wz)InN(wz)GaN(Zb)

BN(c)Sir9.58.515.39.57.111.9r (High freq)

5.54.778.45.354.5 In high frequency applications dielectric constant is a very crucial parameter. III-N have generally smaller dielectric constant than Si. Except InN, which has r=11.9

Basic transport propertiesElectronic transport in GaN is most understood. But a number of issues remain for further research. e.g. maximum carrier velocity in bulk material at hetero interface.The next table show the variation of mobility of GaN with temperature, doping and crystal structure. Materialn/pTND/NA

MobilityGaN(wz)n3001e17990GaN(wz)n4501e17391GaN(wz)n6001e17215GaN(wz)n3003.6e16150GaN(wz)N(2deg)30002000GaN(wz)n771e166000GaN(Zb)n3001e171100GaN(Zb)n30002100GaN(Zb)P(2deg)3001e13250From the table we can conclude that Mobility in 2 DEG is far better than bulkMobility in zb structure is little bit better than wz structure.

Factors that affect mobilityPhonon scattering by acoustic and optical phononsIonized impurity scattering ,both background scattering and surface donorsThreading dislocationsAlloy scatteringAt high fields peak velocities as high as 3e7 cm/sec are found for electron in GaNThe difference between wurtzite and zincblende structure is found to be insignificantVsat decreases with increase in donor concentration

Critical break down field

MaterialEcGaN175kv/cmAlN450 kv/cmInN65kv/cmBand structure of GaNBand structure of GaN is not fully understood ,special with respect to higher energy bands

me=rest mass of electron

Materialmn(me)mn(-K)(me)mn(-A)(me)mn(-M)(me)GaN(wz).2.36.27.33materialmp, h(me)mpl(me)mp,so(me)GaN(wz)

1.40.30.6Intervalley seperatin energirss in K-space are of practical importance for the High field transport of electron and holesThe data of wurtzite GaN are based on first principle calculationAt heterointerface between two semiconductors the energy band discontinuities and bandgap alignments are of high importance as they determine the energy barrier which carrier have to surmount.All III-N materials lead to so called type I transistion.Bandgap of GaN is about 3.43 ev at room temperatureAlNSecond to GaN, AlN is the most important material in the III-N system for electronic applicationsIt is mainly used as its ternary compound AlxGa1-xN.It is characterized to be an insulator due to high band gap and high activation energy.Usually grown as nucleation layer to start growth on SiC or sapphire substrate and as an interlayer at channel barrier interface

Mechanical and optical propertiesMass density is much smaller than GaN and InNThermal expansion and vicker hardness are similar to GaNIntrinsic thermal conductivity is better than Si,GaAs, GaN but smaller than BN,SiC,Diamond. This makes AlN a potentially attractive substrate material

Basic carrier transport propertiesThis is not of primary importance to most devices, except in very thin layers close to channel. Transport in AlN is relatively very well investigated by M.C. simulationLow field mobility =135cm2/V.sec at ND=1e17High field saturation velocity 1.4e7 cm/secPeak velocity 1.7e7 cm/sec

Band structure

Band gap of AlN is 6.2ev.Band gap of AlxGa1-xN can be modified in a broad range from value of GaN to AlN.With band gap of InN to be even smaller, wider range available in InxAl1-xN InNInN and its compound InGaN and InAlN are not so widely used in electronic devices.Indium content is low to achieve the lattice match to GaN buffer layerMOCVD growth of InN is complicated because of high growth temperature and resulting defect background concentrationThe MBE growth of InN is under development to use full range of material composition in InGaN A bulk electron mobility of 3750 cm2/V.sec at 300K is obtainedThe mobility at 150k is as high as 5100 cm2/V.secP type Inn has been recently reported which is essential for the realization of bipolar or optoelectronic p-n devices.

Mechanical and optical properties Data are given in table Data are relatively uncertain due to lack real bulk InNThe CTE and lattice constants suggests the growth on sapphire substrate (table2)Thermal conductivity and heat capacity are still primarily based on estimates and extrapolations.

BNIt can be found in several crystallographic forms but the most important insulating or semiconductor form of BN is cubic formCeramic BN is widely used for industrial tools as abrasiveThe great advantage of cubic BN is its Vicker hardness (table1) and thermal cundicitivity(750 W/m.K achieved and 1300 W/m.K can be reached theoretically)Ultravoilet LEDs can be made from c-BN despite material growth problem.which can operate as high as 5300CCubic BN is typically p-dopedHole mobility of 500 cm2/V.sec is found at carrier concentration of 5e8 /cm3 For n-type material only few experimental data existBreak down field vary from 2-6 MV/cmBand gap of cubic BN is found to be 6.4ev at RT