Epitaxial Growth Mechanism Of High Indium Content InGaN Films By MOCVD And Their Photoelectric Properties

Solid-state lighting based on semiconductors can significantly reduce the energy consumption of lighting products and improve efficiency,thus lowering the emissions of greenhouse gas.However,the present commercial white light emitting diode(LED)is usually fabricated by using InGaN-based blue LED to stimulate the yellow phosphor to obtain white light,which inevitably suffers from Stokes loss caused by wavelength down-conversion.In order to achieve truly efficient solid-state lighting,it is necessary to develop efficient red,yellow,green,blue color mixed full-chip white LEDs.However,only the internal quantum efficiency(IQE)of InGaN-based blue LED has so far exceeded over 90%.With the increase of In-content,the IQE of the long wavelength InGaN-based LED drastically decreases.One of the crucial reasons is that it is difficult to grow high-quality and high In-content InGaN alloy.More indium(In)incorporation into InGaN will induce much more microstructural defects;however,the effect of these defects on the photoelectric properties of InGaN is still ambiguous.Therefore,the microstructural defects and photoelectric properties of InGaN alloys with high In-content have become a hot issue in the field of nitride light emitting devices.This dissertation mainly focuses on the epitaxial growth and related photoelectric properties of the high In-content InGaN epilayers grown on sapphire substrates.The main results are as follows:(1)The In-incorporation induced variations in crystal quality,optical properties,surface morphology,microstructural defects and strain relaxation of high In-content InGaN epilayers gorwn by MOCVD,were systematically investigated.It was found that the exciton localization effect of InGaN gradually enhanced with the increase of In incorporation.Moreover,the root mean square(RMS)roughness of InGaN epilayers increased first and then decreased,with the surface morphology evolution from the flat surface of V-shaped pits to mound-like three-dimensional(3D)surface roughness and then to progressive smoothness.Based on this,a self-regulating model of islands'proportions was proposed and such morphology evolution can be attributed to the competition between the In surfactant effect and the strain relaxation induced 3D roughness.This surfactant effect can promote the lateral migration of ad-atoms,thus contributing to the lateral growth of the islands.(2)The In-incorporation induced strain relaxation mechanism of InGaN epilayers was investigated.XRD reciprocal space mapping showed that the strain state of InGaN epilayers gradually changes from fully strained to fully relaxed with the increase of In-content.There exists In compositional stratification in InGaN when the strain is partially relaxed.In orther words,there is a low In-content interfacial region near the InGaN/GaN interface,above which there is the homogeneous InGaN epilayer.The epilayer would be fully relaxed when In-content increased to 42.2%.The cross-sectional TEM showed that with the increase of In-content,plastic relaxation via generating random stacking faults(SFs)along with the surface saw-tooth roughening becomes a more important strain relaxation mechanism.(3)The unintentional doped high-resistivity GaN films were grown by MOCVD with a high resistivity of 2.1 × 108 ?/square and background electron concentration of 1.0 × 1012 cm-3 by introducing an in-situ annealed InGaN interlayer.The compensation mechanism responsible for high resistivity was demonstrated to be the increased carbon acceptors impurities induced by increased edge-type threading dislocations in GaN.This method opens up a new way to prepare high-resistivity GaN buffer for high electron mobility transistor.(4)The microstructural defects and strain states of the InGaN active region in In0.42Ga0.58N/GaN double heterostructures(DHs)were systematically investigated.Mismatch dislocations,SFs,Moire fringes and lattice distortion can be found within the thick InGaN active region,which may adversely affect the luminescence efficiency.Special consideration was made to the void and In-precipitate defects within the high In-content InGaN/GaN DHs.Both photoluminescence and cathodoluminescence showed luminescence quenching in the In0.42Ga0.58N layer.A void model involved in surface trapping and hindered radiative recombination quantitatively reveals the luminescence quenching mechanism.Furthermore,a self-bond model for In precipitates was proposed.This model ascribed the nature of In precipitates to the complex of void and In precipitate,which traps the non-equilibrium carriers by providing similar quenching centers to voids.This model partially explained the reason behind low quantum efficiency for high In-content InGaN MQWs with only obvious In precipitates but few voids by several groups.(5)The growth of p-GaN by MOCVD on patterned sapphire substrates using In-assisted method was investigated.It was found that as the growth temperature decreased from 1005 ?to 970?,the hole concentration firstly increased then decreased,while the resistivity and mobility of p-GaN scarcely changes.AFM showed the RMS roughness of p-GaN increased with the decrease of temperature,which may be related to the reduction of the lateral migration of ad-atoms with the decrease of temperature.In addition,with the decrease of Mg flow from 360 to 200 sccm under the growth temperature of 995 ?,the resistivity of p-GaN hardly changes.However,the hole concentration firstly decreased then increased,while the hole mobility was on the contrary.After a trade-off between the crystal quality and electrical properties,the optimum condition for the growth of In-assisted p-GaN was determined to be 995'C(growth temperature)and 200 sccm(Mg flow).(6)Based on the optimized growth of In-assisted p-GaN,the Ga-polar p-down polarization-reversed LED with the electroluminescence(EL)of yellow light was successfully fabricated.Good interface quality and crystal quality were revealed from the XRD ?/20 scan.The large full width half maximum(FWHM)of PL(80 nm)indicates that an enhanced inhomogeneity of In-content in high In-content InGaN MQWs.?-? measurement showed a high turn-on voltage of 7.2 V.EL measurement revealed a small blue-shift of only 15 nm with the increase of forward voltage from 15 V to 40 V,indicating that the negative quantum-confined Stark effect caused by polarized electric field in MQWs can be weakened by such polarization-reversed structure.Finally,the 562 nm EL peak suggests that the polarization-reversed LED have the potential to enable the highly efficient long-wavelength InGaN-based LED.