Study On Growth And Characterization Of Ⅲ-Nitride Semiconductor Epitaxial Films

In the past few decade, Ⅲ-nitride (GaN, AlN and InN) and its compounds arebeing established as materials of extreme significance for the next generationhigh-density power devices for applications in solid state lighting field, includingblue-light, green-light and visible-light light-emitting diodes (LED), ultraviolet LEDand detector in deep space exploration. After a long-period of development and research,many key obstacles in this field have been achieved progress. However, there stillremain many problems expecting to be solved. For example, although the opticalefficiency have made great progress in semiconductor lasers, ultraviolet LEDs andgreen-light LEDs, there still existed the lower internal quantum efficiency (IQE). It wasfound that IQE will drop rapidly with increasing current for blue-light LEDs possessedhigher IQE value, which is called as efficiency-droop effect. It is well known that theorder magnitude of dislocation density plays a key role in solid state lighting, andbecomes an important factor that caused the efficiency reduce. Therefore, it is importantto study deeply and character the primary problems in materials and devices ofⅢ-nitride epitaxial films for large-scale and commercialization. This thesis studied thegrowth and characterization based on AlGaN, InGaN epifilms, anharmonic effect ofGaN epifilms using dependent-temperature Raman scattering and transition mechanismfor InGaN/GaN MQW LEDs using low-frequency noise. The main research results areas follows:(1) Growth and characterization of AlGaN layer by combining low-temperatureand high-temperature AlN nucleation layer on c-plane sapphire substrateThick AlGaN layer on c-plane sapphire substrate with low-temperature AlN(LT-AlN) and high-temperature AlN (HT-AlN) nucleation layers have been grown bylow-pressure metal-organic chemical vapor deposition (LPMOCVD). Results indicatethat the insertion of LT-AlN nucleation layer between sapphire substrate andhigh-temperature AlN nucleation layer improves effectively crystal quality, reduces thesurface roughness and eliminates the threading dislocation density.(2) Growth and characterization of AlGaN layer on c-plane sapphire substrate andfree-standing GaN substrateThick AlGaN epilayers have been grown on c-plane sapphire substrate andfree-standing GaN substrate using LT-AlN nucleation layers by LPMOCVD. Resultsindicate that crystal quality can be improved greatly when grown on the free-standingGaN substrate. We calculated the threading dislocation density and found that AlGaN epifilm grown on free-standing GaN substrate is much lower in total threadingdislocation density than that grown on sapphire substrate, although surface morphologyis rougher than that of sapphire substrate.(3) Influence of growth temperature on crystal quality of InGaN epifilmsWe studied the relationships between luminescence properties, crystal quality ofInGaN epifilms and In fraction under different different growth temperatures. Resultsshow that InGaN epitaxial layers possessed better crystal quality and luminescenceproperties when indium fraction increases from4.36%to15.36%at a lower temperature(650℃), and InGaN epifilms have an improved crystal quality, a lower threadingdislocation density and smoother surface morphology.(4) Studying the wall plug efficiency for inserting an AlN nucleation layer inInGaN/GaN MQW LEDs using simulationWe have employed the software named SimuLED5.2to study the wall plugefficiency for InGaN/GaN multiple quantum well (MQW) LEDs based on Chapter IV.We compared the band diagram, emission spectra, non-radiative recombination currentand internal quantum efficiency of the convention InGaN/GaN LEDs with insertionHT-AlN nucleation layer of InGaN/GaN MQW LEDs. Conclusions show that insertingan AlN nucleation layer in InGaN/GaN MQW LEDs is beneficial to IQE, and IQE has alittle reduction with the increasing bias voltage.(5) Studying the relationships between Raman shifts and temperature range fora-plane GaN using temperature-dependent Raman scatteringRaman shifts of a-plane GaN layers grown on r-plane sapphire substrates byLPMOCVD are investigated. We studied the relationships between Raman shift andtemperature for conventional a-plane GaN epilayer with insertion AlN/AlGaNsuperlattice layers for a-plane GaN epilayer using temperature-dependent Ramanscattering. Results indicate that a critical temperature existed in the first-orderexponential decay model, which can characterize anharmonic effect of a-plane GaN indifferent temperature ranges. Further studies can calculate the thermal expansioncoefficient or Grüneisen parameters.(6) Charactering the transition mechanism for InGaN/GaN MQW LEDs usinglow-frequency noiseThe transition mechanism of InGaN/GaN MQW LEDs was investigated using thetool of low-frequency noise within the current range from0.1mA to10mA. We havestudied the relationships between power spectral density (PSD) of low-frequency noiseand frequency. Results indicated that PSD of low-frequency noise increased with thecurrent from0.1mA to10mA. At lower current range (I<1mA), it is g-r (generation-recombination) noise origin that dominated in low-frequency noise; and athigher current range (I>10mA), it is1/f noise origin that dominated in low-frequencynoise. There existed a transition mechanism in InGaN/GaN MQW LEDs between0.1mA to10mA, which indicated that the recombination mechanism of carriers fromnon-radiative recombination to stable carriers number fluctuation.