Prepartion And Physical Properties Study Of InGaN/GaN MQWs

Light-emitting diode(LED),as a new type of semiconductor solid-state light source,has been widely studied in the field of lighting owning to their superior performance.Especially the GaN-based blue LED has become the focus of international research.Although the significant breakthrough of the scientific theory and manufacturing process makes a huge improvement in the luminous efficiency and intensity of LED,there are still some issues to be solved,such as the high dislocation density,causing deterioration of crystal quality;the bowing of wafer,resulting in the wavelength nonuniformity;the polarization effect,leading to the decrease of luminous efficiency;the difficulties in the growth of high-quality In GaN/GaN multi-quantum wells(MQWs)and so on.The MQWs are the core structure of LED,which directly affect the device quality,thus it is imperative to explore the growth mechanism to improve the quality of MQWs.In this dissertation,the growth process of InGaN/GaN MQWs was optimized by metal-organic chemical vapor deposition(MOCVD),the structural and photoelectric properties were characterized,and the mechanism was deeply explored.The main research contents and conclusions are described as follows:(1)In order to improve the uniformity and consistency of the wavelength at wafer surface,the formation and evolution mechanism of bowing has been investigated.The undoped GaN(u-GaN)layer with different thickness(3.72,3.63 and 3.56 ?m),obtained by adjusting the growth time,was employed to control the bowing of wafer before the growth of MQWs,and then to control the stress during the subsequent epitaxial growth.It is found that when the thickness of u-GaN was 3.72 or 3.56 ?m,the degree of "concave" deformation was insufficient at the end of the growth of n-GaN,which cannot offset the bowing caused by the thermal stress at the following low-temperature growth process,resulting in the large bowing of wafer,which in turn affected the incorporation of In atoms.When the thickness of u-GaN layer was 3.63 ?m,the degree of "concave" deformation was sufficient,thus the curvature of the wafer was close to zero after the growth of quantum wells at low temperature,contributing to the better uniformity and consistency of wafers.Therefore,the bowing of the wafer can be controlled by adjusting the thickness of u-GaN,which can significantly improve the yield of LED chips.(2)To further improve the quality of InGaN/GaN MQWs,the effect of TMIn flow rate(30,50 and 70 sccm)on the alloy composition,crystal quality and optical properties has been investigated.As the flow rate of TMIn increased from 30 sccm to 70 sccm,the incorporation of In was increased,but the incorporation efficiency was decreased.It is also found that the dislocation density increased and the surface morphology deteriorated as the increasing of In content.Under the same excitation power density,the decreased photoluminescence intensity and the increased FWHM indicate that the crystal quality is gradually deteriorated with the increase of TMIn.Furthermore,with the excitation power density increasing,the peak wavelength shows obvious blue-shift due to the band filling effect and the carrier screening effect.On the basis of temperature-dependent PL analysis,the sample,with TMIn flow rate at 70 sccm,shows the smallest blue-shift at the range of 50–150 K,indicating the degree of carrier localization was enhanced with the increase of In content.