Introduction GaN(gallium nitride) is one of the third generation of wide band gapsemiconductors in Materials Chemistry. Compared with formal semiconductormaterials, GaN (gallium nitride) has a higher frequency, higher power, andhigher density for making integrated electronics, besides, the strong radiationresistance ability for GaN (gallium nitride) could also make greatcontributions in microwave power devices filed too.Properties for GaN (gallium nitride)GaN (Galliumnitride) is very hard and stable chemical compound and it’s melting point isabout 2000K. Generally, the atomic structure for GaN (gallium nitride) isclosed-packed hexagonal structure and that results in relatively low symmetryof lattice and strong piezoelectricity and ferroelectricity. Thepiezoelectricity describes when the lattice suffers from certain directionpressure or tension, the vertical surface of the coming force will obtain equalquantity inverse charges at two sides.
The Ferroelectricity describes thespontaneous polarization when the structure of lattice don’t have a center ofsymmetry that makes the gravity center of positive and negative charges don’t coincidetogether leading to the electric moment and is not equal to 0. The two factorslead to very strong piezoelectric polarization and spontaneous polarization,totally it generates 5(MV/cm) which’s called breakdown electric energy. GaN (Gallium nitride) is regard as the thirdgeneration wide band gap semiconductor. The band gap is 3.4 eV and thermalconductivity is 1.3 W/cm*K. The band gap is presented on electronic bandstructure which’s between valence band and conduction band.
Normally valencebond refers to the band of energy occupied by the valence electrons and usuallyit’s the highest occupied band. Conduction Band is empty or may be defined asthe lowest unfilled energy band. The term ‘band gap’ refers to the energydifference between the top of the valence bond and the bottom of the conductionband.
Electrons could gain enough energy to jump to the conduction band byabsorbing either a phonon or photon. This two factors lead to the GaN (Galliumnitride) has a high working temperature and breakdown voltage and a strongability of radiation resistance. The bottom of conduction band of GaN is at ? positionwhich makes a huge energy difference with other with other valley to resist thescattering between different valleys. As a result, GaN has a very highsaturated drift velocity of electrons.
Comparing semiconductors with insulators, semiconductorshave a relatively smaller band gap and though both of them behave as insulatorat absolute zero, for semiconductors , it allow thermal excitation of electronsinto its conduction band at the temperature below its melting point. Generally,wide-band gap semiconductors materials have band gaps in the range of 2-4 eV,whereas typical semiconductors have band gaps in the range of 1-1.5 eV. Higherenergy of band gap makes it suitable for working in a high temperature.
Wideband gap semiconductors are associated with a high voltage. This is due to alarge electric filed to generate carries through impact mechanism. However,GaN also has its shortcomings. Because of it structure of energy bond, theelectron mobility is relatively low while the charge carriers have a highvaluable mass.
Preparation for GaN (gallium nitride)Thepreparation of GaN (gallium nitride) includes four main steps: metalorganicchemical vapor deposition, hydride vapor phase epitaxy, separation and secondgrowth.Inthe MOCVD step, ultra-pure gases are transferred into a reactor and finally resultin a deposition of a very thin layer of atoms onto a semiconductor wafer. Forinstance, Pin can be grown in a heated substrate by trimethylindium andphosphine.Theprecursor molecular decomposition happens in the absence of oxygen. As to theequipment, the reaction chamber is the main body that is composed by reactorwalls, liner, susceptor, gas injection units and temperature control units.Besides,two temperature should be paid attention when we heat the substrate. One isaround 823K and another isaround 1273K.
In the low temperature condition, there will be a buffer layer growingfirstly. However, in the high temperature, GaN (gallium nitrate) will growdirectly. So the temperature should becontrolled.
The hydride vaporphase epitaxy (HVPE) makes the GaN (gallium nitrate) grow continually. The hydrogenchloride is reacted at elevated temperature while the group (III) metal producinggaseous metal chlorides and then it will react with ammonia to produce group (III)metal nitride.As to the separationpart, the technique of laser lift-off is better than natural separation whichuses high power pulsed laser directly to the surface. The energy of light isbetween Esubstrate and EGaN.
Applications One of the typical application for GaN (gallium nitride) is powerdevices. Compared GaN (gallium nitride) with other materials, it has relativelysmall volume and high efficiency to transport. Nowadays, as the popularizationof 4G cell site and wireless power, the potential market could be expected too. Besides from the strong ability for GaN(gallium nitride) to transport information, the high color rendeing index andluminous efficiency of GaN (gallium nitride) also could be applied to the LED.For instance, many companies have puttheir eye on the research and exploitation on GaN (gallium nitride) materials,like Samsung, Mitsubishi etc. According to the graph mentioned on theslides, it can easily show us the promising future of GaN (gallium nitride),the statistics also show that the total value in US in 2015 has arrived at 298millionDollars, and many of the cost isconcentrated on wireless infrastructure.