The measurement of internal quantum efficiency of InGaN/GaN UV LEDs

For this study, we used the IQE method which S. Watanabe et al. proposed to determine the IQE of InGaN/GaN MQW UV LEDs. The internal quantum efficiency can be calculated by

where IPL and IEX are PL intensity and excitation intensity, respectively. EPL and EEX

are PL photon energy and excitation photon energy, respectively. C is a constant affected by mostly carrier injection efficiency by laser, light extraction and correction efficiency of PL, and does not depend on either excitation power density or measurement temperature. First, we measured the excitation power dependent PL intensity at low and room temperature, and then the relative PL quantum efficiency curves can be obtained by using equation 4.2.1. And the constant C would be canceled out by normalizing the curves to the peak value at the lowest temperature, because it

is independent on temperature or excitation power. From this normalization, the PL efficiency curves will not depend on carrier injection efficiency by laser, light extraction and correction efficiency of PL. Therefore, the PL efficiency would be find out from this model.

In tradition, the IQE is estimated by assuming that IQE is 100 % at low temperature regardless of excitation power density. However, IQE is strongly dependent on injected carrier density. Consequently, it is more reasonable to assume the peak of PL efficiency at lowest temperature is equal 100 %, and then the IQE curves as a function of excitation power and temperature can be understand.

Moreover, to avoid the absorption of GaN, the frequency doubled femtosecond pulse Ti: sapphire laser of 370 nm was used to excite sample, the excitation power density was changed from 0.01 to 15 mW, and calculated injection carrier density is about 2.0 × 10¹⁵ to 1.6 × 10¹⁸ cm^-3 by using the equation below:

where P is excitation power, hν is energy of incident light, φ is laser spot size, f is repetition rate of laser, dactive is the active layer thickness, αGaN is the absorption of GaN, αInGaN is absorption of InGaN, R is reflection of sample surface, and Lossobjective

is transmission loss of objective.

Fig. 4.2.1 shows the IQE of InGaN/GaN MQW UV LEDs as a function of injected carrier density at 15K and 300 K. We can observe that the IQE increases with increasing injected carrier density to reach its maximum. As injected carrier density further increases, then the IQE decreases. The tendency of two efficiency curves at 15 K and 300 K is very similar. But under low injected carrier density region, the IQE curve at 300 K increases obviously than it at 15 K. The results indicated that the IQE at 15 K saturated more easily than it at 300 K. The detailed physical mechanisms will be discussed later.

There are three possible mechanisms to explain excitation power dependence the IQE at low and room temperature:

(1) Nonradiative recombination centers

For GaN based LED, a large number of dislocation density exist in the device, and the defects would be occurred nonradiative recombination. Generally, the nonradiative centers were quenched at low temperature. In our case, as injected carriers increase, the nonradiative recombination is gradually suppressed, therefore, the radiative recombination stars to dominate the recombination process, resulting in the enhancement of IQE, which is observed in Fig 4.2.1. And IQE curve at 15 K increases not obviously than it at 300 K due to noradiative centers were quenched at low temperature. In Fig. 4.2.1, we found that the IQE of InGaN/GaN UV LED on PSS

saturated at lower injected carrier density due to the reduction of carriers captured by nonradiative recombination centers. The results demonstrated that InGaN/GaN UV LEDs on PSS has better crystal quality and smaller defect density.

(2) Coulomb screening effect

Several research groups have reported that the internal electric field existed in InGaN/GaN QW structure. This internal electric field through the QW tilts the potential band and leads to a spatial separation of electrons and holes in the QW [45], resulting a decreasing in degreed of wave function overlap which is called the QCSE.

The internal electric filed in the QW cam be screened by photogenerated carriers.

Consequently, the QCSE effect become weaker when the carrier density increased, resulting in the IQE enhanced at low injected carriers region.

(3) Band filling effect of localized states

Due to composition inhomogeneity and monolayer thickness fluctuation Of InGaN QWs self-organized In-rich region is generated in InGaN active region, resulting in potential fluctuation of energy bandgap. As injected carrier density increases further, an occupation of high energy stats of localized centers will be enhanced. And the band filling effect will make the carriers more easily escape from localized states to extendend states which decrease IQE.

The experimental results indicated that the IQE are about 61.0 %, 44.2 % at

injected carrier density is 4.7 × 10¹⁷ #/cm³(~20mA) for InGaN/GaN UV LED on PSS and conventional substrate, respectively. Table 4.3.1 shows the value of IQE and EQE, and then we calculated the extraction efficiency using the relation equation:

Extraciotn IQE

EQE η η

η

= *

We can obtained the extraction efficiency are about 70.5 %, 63.3 % for InGaN/GaN UV LED on PSS and conventional substrate, respectively. Several research groups demonstrated PSS can reduce the threading dislocations and increase the light extraction. Our results also indicated that InGaN/GaN UV LEDs on PSS increase the IQE (and light extraction efficiency.