Study On Epitaxy Growth Of GaN Based LED And P-type GaN Activation

The representative of III-nitride semiconductor materials, GaN, has become theworld hot research for its superior optical and electrical properties in recent years.And its applications are increasingly widespread, showing a tantalizing prospect andbroad market value in the field of solid-state lighting. The n-type GaN growth, designof quantum well structure and p-type GaN carrier concentration level have muchimportant impact on GaN-based high-brightness LEDs. The p-type GaN epitaxialgrowth typically exhibits a high impedance characteristic and low hole concentration,which is bad to form an ohmic contact between the metal and the p-type GaN, andlimiting the application of GaN. The growth parameters of n-type GaN such as NH3flow rate, carrier gas etc. can affect its growth rate, while its growth rate has aninfluence of intentional and unintentional doping. The LED with a rectangularquantum well structure will appears quantum efficiency decline when the forwardcurrent density from small increases to a certain value,which "efficiency droop"phenomenon. How to reduce droop effect on the LEDs need certain requirements ofquantum well structure. In this theis, the p-type GaN activation by Ni/Ag assistannealing, the n-type GaN growth rate and intentional and unintentional dopinginfluenced by growth parameters, the optical and electrical properties of GaN basedgreen LED with a trapezoid quantum well were studied. The main results of ourresearch are as following:1.The hydrogen concentration of the p-type GaN film annealed with Ni/Ag assistwas analyzed by SIMS depth profiling. The results showed that cover with Ni/Agannealing can significantly promote the hydrogen desorption in p-type GaN, andactivated Mg acceptor. The (Mg-H) complexes decomposed during the annealingprocess and H atoms moved from p-type GaN to the metal layer. The Ni/Ag layer wasalso oxided and generated Ag2O and NiO. Ga atoms and N atoms were diffused outfrom p-type GaN living Ga vacancies and N vacancies, while NiO can be a barrier toN atoms diffuse, thus make Ga vacancies more than N vacancies in p-type GaN. The(Mg-H) complex decomposed and more Ga vacancies can lead a improvement of holeconcentrations, and activated Mg acceptors successfully. 2.For the lack of method to measurement the hole concentration of p-type GaNin whole structure LED, we try to use dots transmission line model method, throughthe quantitative comparison Ag (100nm)/Ni (1.5nm)/p-GaN contact characteristics toexamine the carrier concentration in p-type GaN. It was found that it cannot activatedthe Mg acceptor when the annealing temperature is too low, while it will make thecontact characteristics deteriorate when the annealing temperature is too high.Combined I-V curve and the curve H depth profiling analysis, although Hconcentration is reduced when the annealing temperature increases, the holeconcentration did not show changes in synchronization. We consider that certainamount of nitrogen vacancies were produced at high-temperature annealing, and thenitrogen vacancies will compensate the already activated acceptors.The optimalconditions of p-type GaN activation by Ni/Ag assist annealing is Ni5nm,450℃.3.The n-type GaN growth with high V/III ratio by MOCVD has a lower growthrate at bigger NH3flow whenever use H2or N2as carrier gas. The reason is that theparasitic reactions existing in the MOCVD system which consumed part of Ga source,mead Ga source for GaN growth become little. Use H2as carrier gas has a fastergrowth rate than N2when the NH3flow is same, maybe for MO source move faster inH2gas.4. We compared the effects of the GaN growth rate on Si and C concentration.The lower growth rate is better to Si merge into GaN lattice and lead higherconcentration. The C concentration is lower in bigger NH3flow and N2carrier gas forlittle N vacancies in GaN.5.LED with a trapezoid quantum well has better properties than with a rectanglequantum well. The LED with a trapezoid quantum well worked at high currentdensity has a bigger output power,weaker EQE efficiency droop and smaller EL blueshift at room temperature. We think the adjust of quantum well energy band by thetrapezoid quantum well, reduce of internal electric field in the quantum wells andmore overlap of electron and hole wave functions were contribute to better propertiesof the trapezoid quantum well LED.