Study On The Effects Of The Growth Conditions Of Preparation Layer And Quantum Well On The Properties Of GaN-based LED Grown On Si Substrates

With the rapid development of LED,it has been widely used in traffic signals,large-screen display,city night,automobile lighting and general lighting,etc.Due to superior performances,LED is gradually replacing the traditional light sources such as halogen,incandescence and fluorescence,which is becoming a new generation of lighting source.In practical use,LED still has a serious problem that its emission effiiciency appears a reduction,known as “efficiency droop”.Meanwhile,there are two practical problems: large polarization field and high dislocation density,which restrict LED's emission efficiency improvement.For high In composition LED(520~575 nm),the quantum wells suffer larger palorization field.In addition,the high In composition is normally achieved by lowering growth temperature,and the miscibility of InN in GaN is low,which facilitate more defects formation.Hence,the above problems have more seriously restriction on the improvement of high In composition GaN-based LED's efficiency.Therefore,finding approaches to reduce the large polarization field,high dislocation density and efficiency droop,promoting high In composition GaN-based LED's efficiency,is currently a hot research area.From the research into preparation layer and quantum well,in accordance with the above problems appearing in GaN-based LED,We investigated the effects of the growth parameters and structure of preparation layer and quantum well on the properties of GaN-based LED grown on Si substrates by high resolution X-ray diffraction(HRXRD),scanning electron microscope(SEM),fluorescence(FL),secondary ion mass spectroscopy(SIMS),electroluminescence(EL)and other means.The main achievements are listed as follow:1.It is creatively proposed to use SIMS and SEM to characterize the size and density of V-shaped pits in Ga N-based LED.First,the p-GaN on the surface of LED is peeled off by SIMS.Then,the LED without p-GaN is observed by SEM.This method has been used to characterize the size and density of V-shaped pits in GaN-based LED structure accurately.2.Inserting In GaN/GaN superlattice with different thicknesses under MQW,reveals the relationship between the InGaN/GaN superlattice thickness and the properties of blue LEDs.With superlattice thickness increasing,the reverse-bias leakage current is more serious.Electroluminescence(EL)measurement shows that the extent of blue-shift decreases as the superlattice thickness increasing.Nevertheless,there is no obvious discrepancy in the EL intensity between two samples with different thickness.It is observed by TEM that there is less threading dislocation in MQW area with larger superlattice thickness,which can be attributed to the more interface and larger stress.3.The effect of superlattice growth temperature on the performance of GaN-based green LED grown on Si(111)substrate has been discussed.At room temperature,the sample with higher growth temperature of superlattice has lower reverse-bias leakage current and higher emission efficiency.Temperature-dependent forward I-V curves depicts that with the increase of the growth temperature of superlattice,the forward voltage increases,especially at low temperature(<150 K).Meanwhile,the sample with higher growth temperature of superlattice shows a severer efficiency droop at cryogenic temperatures(about 100 K–150 K).Electron overflow into p-GaN is considered to be the cause of such phenomena.4.To improve the performance of LEDs in the range of green light(~520 nm),the effects of thickness ratio of In GaN to GaN in superlattice strain relief layer(SSRL)on the properties of GaN-based green LEDs grown on Si substrates has been studied.It is observed from SEM images that there are two different types of V-pits randomly distributing in all samples.The small V-pits forming at MQW cause a large decrease of the performance of device.It is found that the series resistance of the sample decreases with the increase of the thickness ratio of In GaN to GaN in SSRL.This behavior can be explained by a qualitative model of carrier vertical transport.Comparsion with cathodoluminescence(CL)images shows that the sample with larger thickness ratio of In GaN to GaN in SSRL has a higher spatial homogeneity in MQW luminescence.5.GaN-based MQW yellow LEDs with various quantum well thicknesses grown on Si(111)substrates have been investigated.It is observed from FL images that manymicro-scale indium clusters appear within MQW region when the quantum well thickness is more than 2.3 nm.And then the size and density of indium clusters increase with quantum well thickness increasing.In addition,temperature-dependent(>300 K)EL measurements shows that the sample with larger well thickness has a more serious temperature droop,especially at low current injection density.6 The influences of the number of quantum well on the GaN-based yellow-green LED grown on Si(111)has been systematically analyzed.It is showed that quantum well number has little effect on the surface morphology of samples and the uniformity of the MQW luminescence.The sample with larger number of quantum well has a lower forward voltage and more serious current leakage.Moreover,EL results shows that with quantum well number increasing,full width at half maximum(FWHM)of spectrum increases,but the increase in FWHM reduces.In addition,EQE and LOP increase firstly and then decrease as the quantum well thickness increases.Based on the test results analysis,the optimal number of quantum well for yellow-green LED in this experiment is 77.By comparing three GaN-based yellow-green LEDs with different quantum well thicknesses grown on Si(111)substrates finds that quantum well thickness has no effect on the surface morphology of samples and the uniformity of the MQW luminescence.Simultaneously,there is no obvious discrepancy in the blue-shift and FHWM among these three samples.In addition,EQE increases firstly and then decreases as the quantum well thickness increases.8.GaN-based yellow-green LEDs with various quantum barrier growth temperatures grown on Si(111)substrates has been investigated.It is found that the reverse leakage current decreases with barrier temperature increasing,which should be due to the crystal quality of InGaN/GaN MQW improvement.Moreover,the EL measurements at 300 K show that emission efficiency increases with the barrier temperature rising from 930 ? to 935 ?,but the emission efficiency reduces seriously when the barrier temperature is 940 ?.The result that the 940 ? barrier temperature leads to the lowest emission efficiency,is attributed to the poor quality of interface between InGaN well and GaN barrier.Part of the above results have been published in Journal of Applied Physics?Chinese Physics Letters and Acta Physica Sinica.Others are preparing for Journal of Applied Physics.