GaN based LED Fabrication Process:
Paper: fabrication of GaN based LEDs
The fabrication process mentioned in this paper produces blue light emitting diodes based on GaN material. To obtain white LED lamps, coating procedures are used on blue LED chips. In this fabrication process, thin layers of GaN are developed on (0001)-orientation sapphire substrates. This process is done by metal organic chemical vapour deposition (MOCVD). A highlight of MOCVD innovation is that it can make a thin continuous film in nanometer units. This is a truly important feature for developing the GaN layers on a sapphire substrate. After MOCVD, using several microfabrication methods, the blue GaN based LED chips are formed.
Using MOCVD in the EMCORE D125 vertical geometry, rotating disk reactor, all the required samples were grown. The EMCORE D125 MOCVD machine has in-situ optical reflectance monitoring as an optimization device in the development of GaN based structures. Fig. 1 shows the image and the schematic diagram of the EMCORE D125 MOCVD growth chamber.
During the growth:
EMCORE D125 MOCVD chamber pressure: 100-300 Torr.
Al, Ga, In and N sources: Trimethylaluminum (TMAI), Trimethylgallium (TMGa), Trimethylindium (TMIn) and ammonia (NH3) respectively.
p -type doping source: Biscyclopentadienyl magnesium (CP2Mg)
n-type doping source: Disilane (Si2H6)
Prior to growth processes on substrates:
31813507667625Substrate temperature was increased to 727 ?C to develop a 250nm Mg doped p-type GaN layer
00Substrate temperature was increased to 727 ?C to develop a 250nm Mg doped p-type GaN layer
32289756505575After the growth of the active region
00After the growth of the active region
30384755000625Temperature was increased to 1020 ?C to develop a 1µm GaN layer and then a 1µm Si-doped, n-type GaN layer
00Temperature was increased to 1020 ?C to develop a 1µm GaN layer and then a 1µm Si-doped, n-type GaN layer
right3905250After cooling down from thermal cleaning
0After cooling down from thermal cleaning
59626493105150To remove native oxide from the substrate surface
00To remove native oxide from the substrate surface
60960080645Annealing of grown LED structure at 730 ?C for 2 min in N2 ambient to set off the Mg doped p-type GaN layers
00Annealing of grown LED structure at 730 ?C for 2 min in N2 ambient to set off the Mg doped p-type GaN layers
Fig.2 Schematic drawing of the sample structure.
Fabrication of GaN MQW LED chips:
Contact metallisation processes on the surface of as-grown samples
3048000628650For metallization, E-beam evaporator of Auto 360 is used.
00For metallization, E-beam evaporator of Auto 360 is used.
Fig.3 sample structure of the fabricated blue LED chips
Fig.3 shows the sample structure of the fabricated blue LED chips. These blue LED chips were fabricated on sapphire substrate quarters having diameter of 2- inch. Then, the surface of blue LED chip was coated with a solution of yellow phosphor and epoxy resin to make white light emitting diode. Fig. 4 shows final blue LED chips with area of 315 × 315 µm2.
Fig. 4 Image of several blue LED chips
This characteristic was obtained with an I-V measurement system at room temperature. Fig. 5 shows characteristic for InGaN/GaN same size blue LED chips as a function of forward driving current. Results shows turn on voltage of 4V and contact resistance of 125 ?.
Fig.5 I-V characteristic of the fabricated blue LED chips.
This measurement was taken at room temperature for InGaN MQW LED at the injection current of 40mA. Fig.6 (a) shows the Electroluminescence (EL) spectra having strong emission peak at 466.5 nm wavelength. To measure effective operation of the fabricated chips, this measurement was performed at the different values of forward injection currents as shown in Fig.6(b). Result shows that, photodetector becomes saturated at 50mA and above value of forward injection currents on LED chips.
Luminous Intensity measurement:
This measurement was taken by an LED chip tester. The result shows the luminous intensity of fabricated blue LED chips was 80 millicandela (mcd) approximately with current of 20mA. The wavelength at this measurement was about 450nm. After coating process, power measurement of the fabricated white LED lamps is about 40 lmW-1 at 20 mA current.
Fig.6 Electroluminescence measurement of the fabricated LED chips.