Luminous Characteristics Of GaN-based Blue Light-emitting Diode On Si Substrate

Gallium nitride (GaN) based light-emitting diodes (LEDs) are now starting to enter general lighting applications for their advantages of high energy efficiency and long service life. Single-crystal silicon has the most mature technology, the cheapest price and the largest size among the semiconductor materials at present, thus the epitaxial growth of GaN films on Si substrate has currently become a hot researching area and gradually been improved. However, the quantum efficiency will decrease with the increasing current density at high forward injection (efficiency droop), so how to eliminate the efficiency droop and lower the cost in general lighting applications is the most difficult problem to be urgently solved at present. In this paper, GaN-based LED samples were fabricated on Si substrate by MOCVD, and five aspects of luminous characteristics were studied as following:1. The LEDs emission spectrum is affected by piezoelectric polarization. With the EL spectrum of GaN-based single quantum well (SQW) and multiple quantum wells (MQW) LED on Si substrate, it is found that the dominant wavelength (WLD) will red shift with the decreasing operating temperature under low forward current density which the piezoelectric polarization has not been screen, and this means that the effect of piezoelectric polarization on WLD is higher than increasing bandgap energy by decreasing temperature. The results also show that the intensity of piezoelectric polarization in SQW is higher than that in MQWs.2. The main factor for efficiency droop is not Auger recombination but electron leakage. The simulation results based on simple ABC model are higher than the measuring experiment results even if Auger coefficient here is several order of magnitude higher than calculated InGaN values at high forward current density, and the difference between the simulation results and the experiment results increases with increasing current density at high forward injection. But the measuring experiment results fit in well with simulation results based on simple ABC model where carrier leakage is supposed the main factor for efficiency droop.3. The electrons filling up in the wells will affect LED quantum efficiency. Judging the WLD curve and IQE curve of SQW LED as a function of forward current density in the temperature range from100to350K and the differences of those curves between SQW and MQW LEDs, it is found that the electrons will overflow from the well after filling up in higher and higher state with the increasing current density, and electron overflow is the main factor for carrier leakage.4. With the EL spectrum under different temperature the effect of carrier localized states on quantum efficiency is discussed, and the cause for severe efficiency droop at low operating temperature is analyzed. The EL spectrums at different temperature of high quality MQW LED sample show that the LEDs have worse electron spreading and less hole concentration with decreasing temperature and the electrons will overflow from the well after filled up in higher and higher state for their inhomogeneous distribution, thus efficiency droop will happen at a lower injection more severely for electron leakage under lower temperature. These results confirm that the genuine cause for electron leakage is the severe carrier localized states.5. The wells which contribute the main output power will shift in the series wells of MQW LEDs under different forward current density or varying operating temperature. With the EL spectrum under different operating temperature of series Si doped barrier LED samples, the results show that the output power is contributed by less wells with the decreasing operating temperature or the increasing forward current density, and the main light-emitting wells will be closer to p-GaN.