Results and Discussion…

The injected carrier density is determined primarily by the power of pumping laser (P), the energy of injected photon (hv), the spot size of pumping laser (φ), the thickness of GaN and active region (dGaN ,d_active), the repetition rate of pumping laser (f), the absorption efficiency of GaN and InGaN (αGaN, αInGaN), and the reflectance of pumping laser (R), as expressed by the following equation. Experimentally, we choose φ = 50 μm, dGaN = 200 nm, dactive =270 nm,

αInGaN =10⁵ cm^-1, and R=0.17 to calculate the injected carrier density in our samples. Here we

ignore the absorption of GaN since the energy of pumping photons is less than its energy bandgap,i.e. αGaN =0. For the measurement of temperature dependent time-resolved PL (TRPL), the frequency doubled femto-second-pulse Ti:sapphire laser was operated on 390 nm with 2mW.

The repetition rate of the laser is 76 MHz whose time interval is 13 ns. The luminescence decay was measured with time correlated single photon counting (TCSPC) system in conjunction with a 0.55-meter monochromator. Time-resolution for the detection is about 4 ps.

4-3 Results and Discussion

We first performed the power-dependent PL measurement at 15 and 300 K and plot the efficiency curves of both PSS and reference samples in figure 4.5 after normalizing to its peak at low temperature. For the LED grown on planar sapphire substrate, one could clearly see that the IQE increases with injected carrier density to reach its maximum and decreases as the

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injected carrier density further increases. The tendency of two efficiency curves at 15 K and 300 K is quite similar. But under low injection carrier density range, the IQE at 300 K increases more pronounced than it at 15 K. Moreover, the corresponding density to the peak efficiency (~62%) in 300 K is at injected carrier density of about 10¹⁷ cm^-3, which is lager than it at 15 K, about 10¹⁶ cm^-3. A much larger variation of the IQE was observed over the injection carrier range at 300 K than 15 K.

For the LED grown on PSS, a similar dependence of the IQE on the injected carrier density was observed. However, in term of the peak efficiency (~70%) in 300 K at injected carrier density of 1x 10¹⁷ cm^-3, the IQE of LED grown on the PSS was enhanced by ~13%. It means that under the same injected power of pumping laser, there is about 13% enhancement for the converted photon carriers within the active region, as compared to the conventional LED structure grown on planar sapphire substrate. We believe the higher IQE for the LED grown on PSS is due to the better crystalline quality, attributed to the interaction between stack faults and threading dislocations, as studied in our previous work [9]. We now discuss the mechanism responsible for the dependence of IQE on the injected carrier density for both LEDs grown on planar sapphire substrate and PSS.

In general, the collected PL intensity, L , is proportional to the injected carrier density, I , with a power index P which could be expressed as [11, 12]

L α I P (4-4)

where parameter P physically reflects the various recombination processes. If P equals to 1, it indicates the radiative recombination dominates. On the other hand, if P>1, the Shockley-Read-Hall (SRH) recombination occurs, relating to the presence of nonradiative centers that provide a shunt path to the current. Figure 4.6 summarized the relationship between injected carrier density and the PL intensity for both LED grown on planar sapphire substrate (top one) and PSS (bottom one).

At 15 K for both samples, the intensity is linearly varied with excitation power density (P = 1), which indicates that the radiative recombination dominates the recombination process at all injected carrier density range and the nonradietive centers are quenched at low temperature. However, under low excitation power density at 300 K, the superlinear dependence of L on I are observed for both samples, showing that the defect related nonradiative recombination dominates in this low carrier injection range. But as injected carriers continuously increased, the linear dependence of the PL intensity to the injected carrier density is exhibited. It shall be noted here for both samples in 300 K, the value of P decreases to 1 gradually with the increasing of injected carrier density, instead of jump from

1.49 to 1 for PSS sample (or 2 to 1 for planar sample). It means the nonradiative centers are saturated and leads to the gradual suppression of the nonradiative recombination with the injected carrier density; therefore, the radiative recombination starts to dominate the recombination process, resulting in the pronounced increasing of the IQE, as shown in figure 4.5, for the region of injected carrier density less than 10¹⁷ cm^-3. In addition, since the LED grown on the planar sapphire substrate has higher threading dislocations than that grown on the PSS, the value of P in the superlinear zone is greater for the LED grown on planar sapphire substrate (P=2) than for the LED grown on PSS (P=1.49). Here we mainly focused on analyzing the transition process from the region dominated by nonradiative at low injection region to radiative recombination at high injection region. It should be noted that at an extremely high injection region, which is over 10¹⁸ cm^-3, another mechanism will be dominated to lower the efficiency which is called efficiency droop. This mechanism strongly deteriorates the device performance especially for the high current operating devices. More discussion at this efficiency droop phenomenon could be found in the next chapter.

To further study the mechanisms responsible for the variation of the IQE in figure 4.5, more optical properties were investigated as below. We first investigate these optical characteristics of both samples at low temperature. Figure 4.7 and 4.8 shows the emission peak energy and the FWHM of spectra as a function of the injected carrier density at 15 K for both LEDs grown on the planar sapphire substrate (figure 4.7) and the PSS (figure 4.8).

Accompanied with the emission energy and FWHM, the corresponding carrier lifetime for both samples were also measured and provided in both figures for supporting the analysis of carrier dynamics.

In figure 4.7(a), for the LED grown on planar sapphire substrate in 15 K, several unique optical properties were observed. First, the emission peak energy gradually increases with the injected carrier density. Second, the FWHM of spectra shrinks when the injection carrier density ranging from 5 x 10¹³ cm^-3 to 1 x 10¹⁶ cm^-3, and an opposite trend was observed as the injection carriers further increased. In general, there are two possible mechanisms for the blue-shift of emission energy with increasing injected carrier density. The first is coulomb screening of the quantum-confined Stark effect(QCSE). For the InGaN/GaN MQWs material, because the internal field direction is parallel to MQW growth direction, the device endures a strong QCSE. It caused a band tilting and a separation of wavefunction between electrons and holes, which resulting in a wavelength redshift and recombination efficiency reduction. On the other hand, the increasing of injected carrier density weakens the QCSE; that is what we called coulomb screening effect. Of course, an increasing of transition energy and efficiency

should be observed if this effect dominates. Besides, as the screening effect dominates the emission process, it accompanied a reduction in FWHM. The second is band filling effect of localized states. That is an effect for carriers filled at a higher energy level while the injected carrier density increased continuously.

Due to indium composition inhomogeneity and monolayer thickness fluctuation of the InGaN MQWs, self-organized In-rich region is generated in InGaN active region, resulting in potential fluctuation of the energy bandgap [13-15]. Further increasing of injected carrier density, the filling effect of high energetic localized centers starts interfering and becomes dominated, that also induces a blue-shift of emission energy. However, unlike the effect of QCSE, this effect accompanies the broadening of FWHM. Clearly, we can conclude that in the region of injected carrier density from ~5 x 10¹³ cm^-3 to ~1 x 10¹⁶ cm^-3, the gradual increase of emission energy and shrink of FWHM for the LED grown on planar sapphire substrate is mainly due to the coulomb screening of the QCSE, and that increases the overlap probability of wavefunction of electron and hole. Therefore, in the same region of injected carrier density (~5 x 10¹³ cm^-3 to ~1 x 10¹⁶ cm^-3), its IQE also gradually increases, as shown in figure 4.5 (open square). As the injected carrier density is further increased (> 1 x 10¹⁶ cm^-3), band filling of localized states starts interfering and becomes dominated; the effect prompts the injected carriers to escape more easily from localized states. As a result, the IQE was further deteriorated with the injected carrier density larger than 1 x 10¹⁶ cm^-3.

In figure 4.7(b), we observed the gradually reduced carrier lifetime with the increasing of the injected carrier density. The decreasing of carrier lifetime with increasing injected carrier density could be attributed to the coulomb screening of internal electric in InGaN MQWs [16].

As injected carrier density in the QW increases, more excited carriers can screen the built-in electric field in QW, and that animates the recombination of electron-hole pair. As a result, the carrier recombination rate was accelerated, leading to a decrement of carrier lifetime. For higher injection carrier density (> 1 x 10¹⁶ cm^-3), we observe that the carrier lifetime keep decreasing, but a saturated tendency as injected carrier density higher than 2 x 10¹⁷ cm^-3. That is mainly due to the fact that the carrier at higher state by band filling effect would have shorter lifetime. And as long as these higher energy states were fully occupied, the corresponding carrier lifetime of emitted photon shall keep constant, even with further injection of carrier density. Here it is clear that our measurement of carrier lifetime well agrees with the shift of main emission peak.

As for the LED grown on the PSS in 15 K (figure 4.7(a)), basically we can adopt the same carrier dynamics to clarify the dependence of emission peak energy and the FWHM on

the injected carrier density. However, in the bottom of figure 4.8(b), we observe that under exactly the same injected carrier density, the carrier lifetime of the LED grown on the PSS is around 10ns longer than that of the LED grown on the planar sapphire substrate. We believe it is due to the fewer threading dislocations in the LED grown on the PSS. In general, the nonradiative recombination rate is much quicker than the radiative recombination rate, and more nonradiative centers were expected in the LED grown on the planar sapphire substrate.

Therefore, in term of the carrier lifetime of emitted photon, the LED grown on the PSS shall have longer carrier lifetime than that grown on the planar sapphire substrate. It is also consistent with the evaluation of the IQE in figure 4.5, as one can observe the higher efficiency value on the LED grown on the PSS than that grown on the planar sapphire substrate in 15 K.

To further examine the mechanism responsible for the tendency of the IQE in 300 K, we perform the similar analysis for both LED devices. For the LED grown on the planar sapphire substrate as shown in figure 4.9, we could divide three parts of this curve for discussion. At first glance, the tendency in region of the injected carrier density from ~5 x 10¹⁵ cm^-3 to ~7 x 10¹⁷ cm^-3 in figure 4.9, that is very similar to it at 15 K. Moreover, as compared to the top of Fig. 3 (solid square, 300 K), the parameter P in this region equals to 1, indicating the radiative recombination dominates. Therefore, the physical mechanisms of this region can be classified to two stages. First is preliminary dominated coulomb screening of the QCSE and then the subsequent interfering of band filling effect of localized states.

However, for the initial case of the region with the injected carrier density from 1 x 10¹⁴ cm^-3 to 1 x 10¹⁵ cm^-3, different phenomenon was observed. First, in contrast to the results measured at 15 K as shown in figure 4.7, the emission energy shows the red-shift and the FWHM broadens with increasing injected carrier density. Second, the parameter P in this region equals to 2, indicating the nonradiative recombination dominates the carrier recombination process. Third, as compared to figure 4.7, it can be found the emission energy at 300 K under low injected carrier density is higher than it at 15 K, which shows that the carriers recombined at higher energy states at 300K under low injected carrier density condition. Finally, in figure 4.9(b), the carrier lifetime increases to the maximum of ~45 ns with the injected carrier density from 1 x 10¹⁴ cm^-3 to 1 x 10¹⁵ cm^-3, and remains unchanged from 1 x 10¹⁵ cm^-3 to 5 x 10¹⁵ cm^-3.As further increase the injected carrier density (>5 x 10¹⁵ cm^-3), the corresponding carrier lifetime droops gradually.

To summarize the different phenomenon mentioned above, at lowest injected carrier density, due to nonradiative process dominates the carrier recombination process (P=2), the

carrier lifetime is shortened, and that prompts excited carriers to recombine at higher energy extended states before reaching into lower energy localized states [17]. Thus as compared to the recombination in low energy localized state in 15 K, the transition of these higher energy extended states would emit higher photon energy. Since there may exist many of higher energy extended states, this kind of recombination would accompany by the broadening of FWHM. Moreover, with the increasing of the injected carrier density from 1 x 10¹⁴ cm^-3 to 1 x 10¹⁵ cm^-3, the nonradiative recombination was gradually bleached out and, on the contrary, the radiative process comes to be dominating. Hence an increment of lifetime was observed in this region (figure 4.9(b)). Once the carrier lifetime increases, the excited carriers can transfer from higher extended states to lower localized states, and accompanied with the red-shift of emission energy. As a result, in the region of injected carrier density from 1 x 10¹⁴ cm^-3 to 1 x 10¹⁵ cm^-3 in 300 K, we can expect the higher emission energy than that in 15 K, the red-shift of emission energy, the broadening of the FWHM, and the increasing of the carrier lifetime.

For the region of the injected carrier density from 1 x 10¹⁵ cm^-3 to 5 x 10¹⁵ cm^-3, it could be seemed as a quasi-equilibrium state of the injected carriers, thus an unchanged of carrier lifetime was observed. After that (> 5 x 10¹⁵ cm^-3), similar phenomenon as we discussed in figure 4.7 was observed. Therefore, we could conclude that for the LED grown on the planar sapphire substrate, the main mechanism for causing the difference between the IQE curves in 15 K and 300 K is the thermal activated nonradiative centers especially at the stage of low injected carrier density. The majority of injected carrier was exhausted by the nonradiative centers and fails to effectively screen the QCSE resulting in low IQE at 300 K.

In figure 4.10(a), we show the emission energy and the FWHM as function of injected power density at 300 K for the LED grown on the PSS. The corresponding carrier lifetime for the sample is also shown in figure 4.10(b). As compared to the sample LED measured in 15 K (Fig. 4b), for the injected carrier density from 1 x 10¹⁴ cm^-3 to 1 x 10¹⁵ cm^-3, we can observe the higher emission energy, the red-shift of emission energy, and the increasing of the carrier lifetime, and that is similar to the previous analysis for the LED grown on the planar sapphire substrate. However, we do not observe the broadening of the FWHM in this region. We believe it is due to the fewer threading dislocations existing in the LED grown the PSS, and thus reduces the transition from higher energy extended states. For the inject carrier density from 1 x 10¹⁵ cm^-3 to 5 x 10¹⁵ cm^-3, we also observe the unchanged of the carrier lifetime;

however, its maximum carrier lifetime of ~60 ns is longer than that of the LED grown on the planar sapphire substrate( ~45 ns). It means fewer nonradiative centers would interfere and affect the transition of injected carriers, and that is well consistent with the experimental

observation of FWHM. Again, for the injected carrier density larger than 5 x 10¹⁵ cm^-3, the radiative recombination becomes dominated (P=1), and that would lead to the preliminary coulomb screening of the QCSE and the subsequent band filling effect of localized states as we mentioned before.

Finally, we would like comment briefly on the origin of higher IQE for the LED grown on the PSS. All the information provided above is the summary and comparison of both samples at 15 and 300 K. Here, we collected the temperature dependent PL intensity and plot the normalized intensity with inversed temperature which is understood as Arrhenius plot in figure 4.11. Then, this temperature dependent curve could be fitted by the following equation to get the activation energy:

0/[1 exp( / ) exp( / )]

T a b

I =I +A −E kT +B −E kT (4-5)

where IT, I0 are the integrated PL intensity for T and 0 K, A and B are constants, k is the Boltzmann constant, T is the temperature, Ea is the activation energy for PL quenching, and Eb is generally associated to the free exciton binding energy [18]. The fit activation energy for the LED grown on the planar sapphire substrate and the PSS are 32.4 meV and 61.7 meV, respectively. In general, the activation energy can be explained by the ability to confine the carriers within the potential minima. One could understand it by the schematic of activation energy in figure 4.12. In InGaN material system, because the In inhomogeneous, there are a large In fluctuation and a various localization states. The average of this localization could be quantized as an effective localized state. On the other hand, because of a large density of dislocation which results in a nonradiative recombination, we could also summarize this state as a effective defect state. The difference between these two states could be viewed as the activation energy. Besides, one could also realize it as energy to enable a confined electron trapped in the localized state escaping from it. Thus the higher value of the activation energy indicates the stronger confinement of injected carriers and that certainly promises the higher IQE.

In addition to the fitted activation energy from the temperature dependent PL experiments, similar evidence could be found in the comparison of emission wavelength. We compared the emission wavelength and energy of samples grown on PSS and flat sapphire substrate as shown in figure 4.13(a) and (b). As the injected carrier density increased, the emission wavelength continuously increased in both samples. Nevertheless, if we focus on comparing the wavelength difference between carrier density of 1x10¹⁸ cm^-3 and 10¹⁷ cm^-3, PSS has a larger wavelength difference, 5.75 nm, than that of flat sample, 5 nm, at low

temperature. Similarly, the difference is 3.54 nm for PSS sample and 5.9 nm for flat sample at room temperature. As we understood, the high injection period is dominated by band filling effect; the larger wavelength shift implies the samples could endure a larger carrier density but still effectively recombination. These observations provide evidence that the PSS do benefit for a better crystal quality and improve the IQE performance.

Reference:

[1] Y. J. Lee, H. C. Kuo, S. C. Wang, T. C. Hsu, M. H. Hsieh, M. J. Jou, and B. J. Lee, IEEE Photonic Technology Letter 17, 2289 (2005).

[2] T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. DenBaars, and S. Nakamura, Applied Physics Letter 84, 855 (2004).

[3] S. Fan, P.R. Villeneuve, J.D. Joannopoulos, and E.F. Schubert, Physics Review Letter 78, 3294 (1997).

[4] T. N. Oder, K. H. Kim, J. Y. Lin, and H. X. Jiang, Applied Physics Letter 84, 466 (2004).

[5] C. C. Kao, H. C. Kuo, H. W. Huang, J. T. Chu, Y. C. Peng, Y. L. Hsieh, C. Y. Luo, S. C.

Wang, C.C. Yu and C. F. Lin, IEEE Photonics Technology Letters 17, 19 (2005).

[6] Y. J. Lee, J.M.Hwang, T. C. Hsu, M. H. Hsieh, M. J. Jou, B. J. Lee, T.C. Lu, H.C. Kuo, and S.C. Wang, EEE Photonic Technology Letter 18, 724 (2006).

[7] S. Watanabe, N. Yamada, M. Nagashima, Y. Ueki, C. Sasaki, Y. Yamada, T. Taguchi, K.

Tadatomo, H. Okagawa, and H. Kudo, Applied Physics Letter 83, 4906 (2003).

[8] C. Netzel, T. Wernicke, U. Zeimer, F. Brunnera, M. Weyers, M. Kneissl, Journal of Crystal Growth 310, 8 (2008)

[9] Y. J. Lee, T.C. Lu, H.C. Kuo, S.C. Wang, K. W. Ng, K. M. Lau, Z. P. Yang, Allan S.P.

Chang, and S. Y. Lin, IEEE Journal of Lightwave Technology 26, 1455 (2008).

[10] Y. J. Lee, J.M. Hwang, T. C. Hsu, M. H. Hsieh, M. J. Jou, B. J. Lee, T.C. Lu, H.C. Kuo, and S.C. Wang, IEEE Photonic Technology Letter 18, 1152 (2006) .

[11] I. Ma’rtil, E. Redondo, and A. Ojeda, Journal of Applied Physics 81, 2442 (1997).

[12] X. A. Cao, E. B. Stokes, P. M. Sandvik,, S. F. LeBoeuf, J. Kretchmer, and D. Walker, IEEE Electronic Device Letter 23, 535 (2002).

[13] S. Chichibu, T. Sota, K. Wada, and S. Nakamura, Journal of Vacuum Science and

[13] S. Chichibu, T. Sota, K. Wada, and S. Nakamura, Journal of Vacuum Science and