Study On Fabrication And Performance Of GaN-based LED On Carbon Nanotube Patterned Sapphire Substrates

Gallium nitride(GaN)materials has aroused considerable interest in the last few decades.Nowadays,GaN-based LEDs have been widely applied in lighting,display and other areas of dairy life.The substrate of GaN based LED is an important step in the fabrication of LED.Most of the GaN heteroepitaxy is growed directly on sapphire substrate.However,there are plenty of non-radiative recombination center formed by defects and dislocations in the conventional GaN epitaxy for the lattice mismatch between the GaN and sapphire substrate,which leads to the decrease of radiative efficiency.The patterned sapphire substrate(PSS)is fabricated by dry etching and wet etching.The crystalline quality of GaN grown on PSS is increased for the lateral epitaxial growth.However,GaN-based LEDs and related patterned substrates still suffered following problems.(1)Poor thermal conductivity of the conventional PSS leads to poor heat dissipation of the LED;(2)The fabrication of conventional patterned substrates is difficult and expensive for the stable properties of sapphire.To solve the problems above,the research on impact of carbon nanotubes(CNTs)pattern layers on GaN-based LED is investigated.The CNTs patterned sapphire substrate(CNTPS)is fabricated by laying the CNTs on sapphire substrate,which leads to the increasement of lattice quality,relaxation of strain and decrease of dislocation of GaN grown on CNTPS for the CNTs nano-pattern.What’s more,the CNT helps the thermal conductive in the LEDs.Lateral LEDs and vertical LEDs are fabricated with GaN epitaxy with different layers of CNTs,analysed with results of I-V test,Electroluminescent,Photoluminescence,X-Ray Diffraction,Raman Spectra and ABC model.The conclusions are as follows:1.The CNTs can improve the crystalline quality of GaN epitaxy,decrease the density of defect and dislocation.Among the LEDs with different layers of CNTs,the GaN epitaxy with 2 layers CNTs exhibits the best crystalline quality,and the crystalline quality and strain decrease along with the increase of the CNTs layers.2.The threshold energy of the laser lift off is decrease from 26.0mJ to 20.7mJ by the CNTs.With the increasement of the CNTs layers,the threshold of laser lift off decreases and reaches to saturation when the layers come to 3.3.The CNTs can improve the luminous efficiency of lateral LED,and the lateral LED with 2 layers of CNTs exhibits the highest luminous efficiency,which is demonstrated by the results of TDPL and the fitting of ABC model.Compared with the LEDs without CNTs,the external quantum efficiency of lateral LED with 2 layers of CNTs is improved by 406%,for its higher crystalline quality and light extraction efficiency.4.Vertical LEDs are fabricated with CNTs patterned GaN epitaxy and it’s demonstrated that CNTs can improve the luminous efficiency of vertical LED,in which the 3-CNTs devices exhibit the highest luminous efficiency.The IQE of VLED with 3 layers of CNTs is the highest for the strain relaxation and lower polarization field;After laser lift off,the light extraction efficiency of 3-CNTs devices improved by 58%,so its external quantum efficiency is higher than that of two-layer carbon nanotube devices.5.Comparation between lateral LEDs and vertical LEDs are investigated.After laser lift off,the strain in vertical LEDs are relaxed and the QCSE is weakened.The IQE of vertical LEDs with 2,3,and 0 layers of CNTs are 4.82%,6.23%,16.65%lower than lateral LEDs,which attributes to the damage caused during the laser lift off.However,The 5 CNTs vertical LED has 9.91%internal quantum efficiency lower than lateral LED.With poor crystalline quality,the vertical LED with 5CNTs was thermal annealed by the high energy laser,improving the crystalline quality.What’s more,the change of light extraction efficiency for laser lift off leads to the change of external quantum efficiency.The external quantum efficiency of vertical LEDs with 3 CNTs and 5 CNTs improve while the 2CNTs device decreases.