The Effect Of Surface Nanostructures On The Luminescence Efficiency Of GaN LED

The development of Ⅲ-Ⅴ semiconductor materials such as GaN, SiC promotes applications of the third generation semicounductor material in the field of optoelectronics and microelectronics. Because the bandgaps of GaN and its multiple solid solution can be continuously changed, the wavelength of GaN based LED covers visible light, which makes rapid development of GaN based LED for using in the field of display, lighting, backlight and biology. GaN based LED emitting light could be converted to white light through the fluorescent powder or the mixing of the three primary colours. It is expected to replace incandescent lamp and becomes a new generation lighting source.It is ambition of LED workers to package the white light LED products with high lumen efficiency. The key issue is to obtain the high internal quantum efficiency and the high light extraction efficiency. The structural quality of GaN epitaxial film has been effectively improved with the development of MOCVD technology in recent years. At present the internal quantum efficiency of GaN based LED is higher than 90%, however because the refractive index of GaN is so high that its light could not exit due to total reflection. As a result, the total efficiency of GaN based LED is not so high. For example, GaN material refractive index is 2.2-2.3, the total reflection angle is only 23°, only less than 30% light can be extracted.How to raise the luminous efficiency and extend the lifetime of LED is one of the important issues. The main method to raise LED light extraction efficiency is to increase the light escaping paths by means of changing the surface morphology of LED chip. In this dissertation, nanorod ITO thin film is fabricated by electron beam heating evaporation. The mechanism.of growth nanorod ITO film is analyzed and experimental verification with regard impact factor is carried out.In common GaN based LED, the optical output power can be promoted by using P-GaN film with rough surface, roughing ITO film and coarsening nGaN surface on flip chip. However, the strength of P-GaN film is very weak, stable technology is a difficulty and the parameter of chip is easy to fail.We focus on optical output powers of the flip-chip GaN based LEDs with different P-GaN thicknesses. The flip-chip GaN based LEDs with light exiting from N face were fabricated by coating Ag reflection glasses on the P-GaN films with different thicknesses. Experimental results show that the thinner the thickness of the P-GaN film, the higher the optical output power. It can be attributed to the optical dissipation caused by worse P-GaN film due to heaving P doping. The process of flip-chip GaN based LEDs is complicated and low yield of chip production. There is difficult to make full use the flip chip although the power chip has strong point.In this dissertation, ITO films with different morphologies are applied on the GaN LEDs by E-beam evaporation.. Compared with the light power of conventional LED, the light power of the LED with nonorod ITO film raises nearly 20%. The method of technology is easy promoted application for its reproducibility and reliability.The main content of this dissertation is as follows:1. Preparation of nanorod ITO film by electron beam heating evaporation;One-dimensional nanomaterials were often fabricated by means of VLS growth mechanism. At high temperature, the vapor composition will dissolve in alloy liquid under the right catalyst. When the concentration of crystal composition exceeds its dissolubility in the alloy, the crystal with nano size will grow on the interface of liquid-solid.Disordered nanorod ITO film is prepared by the E-beam heating evaporation in anaerobic condition. Its roughness is over several hundred nanometer. The current spreading of P-type layer is still kept although the arrangement of nanorod is disordered. The growth mechanism of ITO film is described as follows.:At first, the source of ITO is decomposed into vapor at high temperature by E-beam heating. Then, the vapor can condense into the metal droplets on the cold substrate. Around the metal droplet, ITO vapor continuously is dissolved and absorbed by the droplet. As long as the concentration of ITO exceeds its solubility in metal, ITO crystal will nucleate on the interface of liquid-solid. When the size of ITO crystal is larger than the radius of critical nucleus, ITO crystal will grow into ITO nano wisker otherwise it will be dissolved. Following the growth of ITO, the weight of wisker increases gradually. When the strength of the wisker can not support itself weight, nano wisker will bend. As a consequence, the disorder nano wisker film is finally formed.2. Influence factors on ITO film morphology by E-beam heating evaporationITO nanometer crystal wiskers were grown in the VLS mechanism during e-beam heating evaporation. Eventually, the ITO nano film was formed. We have investigated and analyzed the effect of growth parameters such as evaporation rate, oxygen flux, substrate material on the surface morphology of ITO film. The ITO growth procedure and mechanism were fully understood by the observation on the surface morphologies of ITO nanorod and nanotree films.ITO films with different surface morphologies were fabricated by the e-beam heating evaporation under different growth conditions. Their surface roughnesses range from several nanometer to hundreds nanometer. In the procedure of ITO film growth, oxygen flux plays an important role in determining the ITO growth mode. When oxygen flux increases, ITO film growth mode turn from wisker into amorphous cluster. Either ITO wisker of cluster, the faster the growth rate, the larger the ITO particle. In the growth procedure of ITO wisker, the substrate material mainly affected on size and density of alloy droplet. When the ITO wisker was grown on the Au substrate, the Au will interact with In/Sn. As a consequence, a small part of Au will dissolve into alloy droplet and the size and shape of alloy droplet will change with the growth time. Because the melting point of InSn/Au alloy is lower than that of single metal Au or InSn alloy, it causes the increase of metal droplet density and decrease of metal droplet size. Therefore, ITO can easily grow into dense nano wisker on Au substrate.3. Application of nanostructure in GaN based LED with light exiting surfaceIn common GaN based LED, the optical output power can be promoted 10-20% by coarsening ITO film. But this method need add in more techonique steps and controlling process is influenced by enviroment and materials. For the thin film of metal Ni would agglomerate through high temperature treatment, the chip power can be increased 6% by P-GaN film with rough surface which is obtained as Ni etching mask.. However, the strength of P-GaN film is very weak, the parameter of chip is easy to fail.Regular graphic stucture of the transparent conducting ITO film in GaN based LEDs is applied, the luminous efficiency of chip can be promoted 5%. But the operating voltage also raise 0.15V, it is invalid on the white efficiency.In this dissertation, we mainly focus on optical output powers of the flip-chip GaN based LEDs with different P-GaN thicknesses. The flip-chip GaN based LEDs with light exiting from N face were fabricated by coating Ag reflection glasses on the P-GaN films with different thicknesses. Experimental results show that when the thicknesses of P-GaN films are 50,100,200 and 300nm, the corresponding LED optical output powers are 170,122,78 and 60mW, respectively. Therefore, the thinner the thickness of the P-GaN film, the higher the optical output power. It can be attributed to the optical dissipation caused by worse P-GaN film due to heaving P doping.4. Fabrication of ITO films with different morphologies by electron beam heating evaporation and application in GaN based LEDUsing electron beam heating evaporation, we fabricated four kinds of ITO films, i.e. the nanorod film, nano-tree film, particle pore film and particle cluster film The roughnesses of four films are 29nm,230nm,38nm and 7nm, respectively. When GaN LEDs were fabricated using four kinds of ITO films, their optical output powers are 22.6mW,22.5mW,25.6mW and 21.2mW respectively. Compared with the optical output power of LED with nano cluster film, the optical output power of the LED with nono pore film rises 20%.In general, the optical output power of LED is proportional to the surface roughness of light exiting LED in a certain extent. However, the situation for the LED with ITO nanorod and nanotree films is not the case. The roughness of LED with nano-tree film is the highest, but its optical output power is only 22.3W.The main cause is that the roughness measurement for nanorod or nano-tree film is conducted by a small size testing prober. It is the small size testing prober that causes the measurement data scattering. For nanotree, when the prober was located on the tree, the height of film was large. Otherwise, when the prober was located under the tree, the height of film was small. As a result, the roughness of nanotree film is extraordinary larger. Actually, the apparent surface morphologies for both ananrod and nanotree are nearly same. Therefore, although the roughnesses for both LEDs with nanorod and nano-tree films are different, their optical output powers are nearly same.