MOCVD Growth And Performance Study Of Different Functional Layers Of GaN-based Light Emitting Diode

Light-emitting diode(LED),as a third generation lighting has many advantages such as energy saving,environment friendliness,long lifetime,high reliability,and small volume,and is used for indicators,displays,decorations,backlight sources and solid-state lighting sources.Since GaN-based semiconductor materials were prepared by Shuji Nakamura,blue LED with commercial value has been realized and tremendous progress has been made in light efficiency improving and cost saving.However,for LEDs,there are still some issues which need to be further investigated and improved,for example,the effect mechanism of growth parameters on GaN crystal quality is still unclear;the preparation of high hole concentration p-type GaN is difficult;the growth of high-quality In GaN/GaN multiple quantum wells is difficult because of the different growth regimes for InGaN and GaN and so on.For above issues,in this dissertation,different functional layers of LED epi-structure were optimized,the effects of the growth parameters and structure parameters on the structural and photoelectric properties were characterized and further themechanism was explored.The main research contents are as follows:1.The influence of the nucleation layer(NL)thickness(15,25 and 45 nm)on the GaN crystal quality grown on planner sapphire substrate was explored.High-resolution X-ray Diffraction(HRXRD)results indicate when the thickness of NL is 25 nm,the bulk GaN has the lowest(002)and(102)full width at half maximum,which are 267 and 284 arcsec,respectively.Atomic Force Microscope(AFM)results indicate: after annealing,island-like morphologies of the low-temperature GaN NL were obtained.Increasing the NL thickness is beneficial for obtaining larger island size,however,the uniformity of the island size is deteriorated.Big,uniform and low-density island is beneficial for bulk GaN crystal properties,which can be well explained by the dislocation generation and propagation process in the bulk GaN.The NL thickness as one effective solution can control the size and density of the islands and thus determine the crystal properties of bulk GaN.2.The influence of nucleation layer thickness on the GaN crystal quality grown on cone-patterned sapphire substrate(PSS)is explored.Scanning electron microscopy(SEM)and HRXRD results indicate the nucleation sites are very important for the GaN crystal quality.They have a close relationship with the nucleation layer thickness.Nucleation sites formed mainly on patterns are unfavorable for GaN crystal quality because they have different surface planes.Nucleation sites formed mainly in the trenches of PSS mounds are favorable for bulk GaN crystal quality because their surface planes are same with each other,leading to the bulk GaN with high quality and smooth surface.Nucleation layer thickness can effectively control the nucleation sites and thus determine the crystal quality of bulk GaN.3.The mechanism of an obvious increase in electron mobility and yellow luminescence(YL)band intensity in light Si-doped GaN was analyzed.It is found that the carrier mobility increases and then decreases with carrier concentration increases.The mobility behavior is attributed to the shielding of dislocation scattering by ionized impurity with the increase of Si doping concentration.At lower doping level(carrier concentration is 2.37×1017 cm-3),the shielding of dislocation scattering is dominant,which results in the increase in carrier mobility.At higher doping level(carrier concentration is 9.73×1018cm-3),the increase in ionized impurity scattering leads to the decrease in carrier mobility.Higher mobility causes longer diffusion length of nonequilibrium carrier,resulting in the participation of more dislocations in the recombination process,which induces stronger YL intensity in light Si doping GaN.4.The influence of Mg/Ga ratio on the properties of p-GaN layers grown by MOCVD was investigated.p-GaN has an optimal doping concentration to obtain high hole concentration 4.2×1017cm-3 when Mg/Ga ratio is 2%.Lower Mg/Ga ratio leads to less shallow acceptor in p-GaN.And at higher Mg/Ga ratio,MgGa-VN complexes at deep donor state form and compensate shallow acceptor.MgGa-VN complexes at deep donor state cause emission peak at 440 nm and through the increase of Mg/Ga ratio,the peak becomes dominate.Meanwhile,the excess Mg/Ga ratio causes grains at p-GaN surface and through the increase of Mg/Ga ratio,the grain size becomes bigger gradually.The results indicate that at heavy doping state,Mg impurities are not all at shallow acceptor state but parts are at deep donor state.5.The influence of well thickness on the properties of blue light InGaN/GaN multiple quantum wells(MQWs)is investigated.All samples exhibit an excitation-induced blueshift of the peak emission with the increase of excitation power density,and a larger blueshift of MQWs with wider well is measured.Under same excitation power density,all samples show redshift with the increase of well thickness and larger well thickness shows larger redshift.The shift of PL peak position with increasing excitation power density and well thickness was shown to be in agreement with the model of band filling effect in the thinner wells(1.8 and 2.7 nm)with some contribution from shielding of built-in field in the thicker wells(3.6 and 4.5 nm).The thinnest well LED sample has the highest optical output power,which is due to the weakest built-in field,however,its emitting wavelength is only 430 nm.Through increasing In flow or decreasing well temperature,the wavelength of 1.8 nm sample redshifts to 450 nm,while,its output power decreases,which is lower than that of 2.7 nm sample.Thus,2.7 nm is the proper well thickness for blue emitting LED.6.The influence of quantum well growth rate on the properties of green light InGaN/GaN multiple quantum wells(MQWs)is investigated.AFM observations show that cracks appear at the surface of MQWs and become largerand more with increasing well growth rate.Photoluminescence measurement reveals that the MQWs with slower growth rate give a stronger single peak,a narrower full width at half maximum and a shorter emission wavelength.Decreasing the growth rate leads to homogeneous indium distribution,which induces abrupt MQW interfaces and good surface morphology.