Improving the internal quantum efficiency of green InGaN ... - IOPscience

Applied Physics Express 8, 094001 (2015) http://dx.doi.org/10.7567/APEX.8.094001

Improving the internal quantum efficiency of green InGaN quantum dots through coupled InGaN/GaN quantum well and quantum dot structure Jiadong Yu, Lai Wang*, Di Yang, Zhibiao Hao, Yi Luo*, Changzheng Sun, Yanjun Han, Bing Xiong, Jian Wang, and Hongtao Li Tsinghua National Laboratory for Information Science and Technology, Department of Electronic Engineering, Tsinghua University, Beijing 100084, People’s Republic of China E-mail: [email protected]; [email protected] Received June 2, 2015; accepted July 23, 2015; published online August 17, 2015 The InGaN quantum dot (QD) is promising for use in green light-emitting diodes and laser diodes owing to its small strain and weak quantumconfined Stark effect. However, its small carrier capture cross section still sets a limit to its internal quantum efficiency (IQE). Tunneling-enhanced carrier transfer in a coupled InGaN/GaN quantum well (QW) and quantum dot structure has been studied on the basis of temperature-dependent and time-resolved photoluminescence. It is found that carriers can tunnel from a shallow QW to deep QDs at room temperature. Compared with the conventional single-QD layer, the IQE of the QDs can be enhanced more than two times to about 45%. © 2015 The Japan Society of Applied Physics

n recent years, the white light-emitting diode (WLED) has caught much of researchers’ attention owing to its applications in general lighting and liquid-crystal display backlighting.1,2) At present, a blue GaN-based LED chip along with phosphor is the most commonly used method of achieving a WLED.3–5) Although it has a high lumen efficiency and a simple structure, the short lifetime, high color temperature, and low color rendering index (CRI) of the phosphor still restrict its application.6) In addition to this, a tricolor WLED based on red, green, and blue LED chips could simultaneously have a high lumen efficiency and a high CRI. However, its performance cannot meet expectations because of the well-known “green gap”,7) which means that the internal quantum efficiency (IQE) of the green LED is much lower than those of the red and blue ones. This is attributed to the large lattice- and thermal-mismatch-induced strain as well as the strong piezoelectric polarization fields in the high-indium-composition InGaN=GaN multiquantum well (MQW),8,9) which can result in a severe quantumconfined Stark effect (QCSE).10–12) On the other hand, the InGaN=GaN QD has a small strain and a weak QCSE, and it is also compatible with current industrial manufacturing processes. Thus, it is promising for use in green LEDs and laser diodes (LDs).13–15) Nonetheless, owing to its three-dimensional islandlike structure, the carrier capture cross section of a QD layer is relatively smaller than that of a QW. Thus, part of the carriers injected into the QD active region would leak to lower the IQE. In this work, we have proposed to solve the problem of carrier leakage through the carrier tunneling mechanism,16–18) and successfully achieved a remarkable enhancement of green QDs’ IQE through the coupled InGaN= GaN QW and QD (coupled QW–QD) structure grown by metal organic chemical vapor deposition (MOCVD). We observe that carriers can tunnel from the QW to the QDs in the coupled QW–QD structure at room temperature, in which the QD layer acts as an emission center and the QW layer acts as a carrier capture reservoir to provide electrons to the QDs.19) By optimizing the width and depth of the QW and the barrier thickness between the QW and the QDs, the IQE of the QDs has been enhanced more than two times to about 45% at room temperature compared with that of the conventional single layer of QDs. In addition to this, temperature-dependent photoluminescence (TDPL) and time-re-

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(a)

(b)

Fig. 1. (a) Schematic structure of the coupled QW–QD samples’ epitaxial structure and conduction band. (b) High-resolution (HR) TEM image of sample A. The inset is Bright-field (BF) TEM image of sample A. The images were taken along the GaN ½1120.

solved photoluminescence (TRPL) measurements on the coupled QW–QDs are carried out and analyzed to further confirm that the carrier tunneling effect has occurred. Eight coupled QW–QD samples, labeled A–H, are grown on c-plane patterned sapphire substrates by MOCVD to eliminate the interference in photoluminescence as much as possible. The epitaxial structure of the coupled QW–QD samples is schematically shown in Fig. 1(a). Each sample consists of a 30-nm-thick low-temperature GaN buffer layer, a 2-µm-thick undoped GaN layer, an active region, and a 9-nm-thick GaN capping layer. The active region consists of an InGaN QW, a GaN barrier, and a layer of InGaN QDs. The QDs are grown using the “two-step” growth interruption method,20–22) which is described as follows. A nominal 1.5nm-thick In0.3Ga0.7N film is grown on the GaN barrier first, followed by a 20 s growth interruption. During the interruption, the InGaN film reforms to islandlike QDs. The growth temperature of the QW layer is about 740 °C, while that of the QDs is about 650 °C. The TMIn flow rate for QD growth is also higher than that for QW growth. Thus, the actual indium composition of the QDs is higher than that of the QW layer. Samples A–H have the same QDs but different widths and indium compositions of the QWs and different barrier thicknesses between the QW and the QDs. For comparison, another two samples, J and K, are also grown on c-plane patterned sapphire substrates by MOCVD. Sample J is a single layer of QDs without the QW, in which the QDs

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Table I. Main differences in nominal epitaxial structure parameters and IQEs of all the samples. QW Sample

Width w (nm)

In composition x (%)

Barrier width d (nm)

IQEs of QDs (%)

A (CQW-QDs)

4.5

12

4.5

45

B (CQW-QDs)

4.5

12

2.0

16

C (CQW-QDs) D (CQW-QDs)

4.5 4.5

12 12

6.5 8.5

34 29

E (CQW-QDs)

5.5

12

4.5

31

F (CQW-QDs)

3.5

12

4.5

34

G (CQW-QDs)

4.5

24

4.5

23

H (CQW-QDs)

4.5

6

4.5

26

J (QDs)

—

—

—

21

K (QW)

4.5

12

—

— Fig. 2. Integration PL spectra of nine samples at room temperature. The inset is a TDPL example of sample A.

are the same as those in samples A–H. Sample K is a single QW without the QDs, in which the QW is the same as that in sample A. The main differences in the active regions of these ten samples are listed in Table I. For structural characterization, a cross-sectional transmission electron microscopy (XTEM) observation of sample A is implemented using the in-situ FIB lift-out technique on an FEI Strata 400 Dual Beam FIB=SEM. The sample is coated with sputter Ir and E-Pt=I-Pt prior to milling, and then imaged in FEI Tecnai TF-20 FEG=TEM operated at 200 kV in the high-resolution (HR) TEM mode, as shown in Fig. 1(b). The actual height of the QDs is about 2.4 nm. The actual thicknesses of the QW, barrier, and capping layer are also very close to the nominal values. The inset is bright-field (BF) TEM image. The QW layer and especially the disconnected islandlike QDs are clearly shown. Carrier tunneling is a relatively complex quantum phenomenon, which is influenced by the coupled structure of the sample to a great extent. The conduction band of the coupled QW–QDs’ active layer is schematically shown in Fig. 1(a) regardless of the band tilt resulting from the spontaneous and piezoelectric polarization effect. As can be seen from Fig. 1(a), photon-generated carriers in the InGaN QW will tunnel into the InGaN QDs through the GaN barrier and then recombine in QDs, where the QW acts as a carrier reservoir in the coupled QW–QD structure. In order to capture and confine more carriers, the width (w) and energy depth (h) of the QW should both be as large as possible, provided that the carrier energy level is higher than that in the QDs. In particular, a wide QW, in which the QCSE is strong, will help to reduce the carrier recombination rate, so that the tunneling from the QW to the QDs can be enhanced. In addition, the thickness of the GaN barrier (d ) between the QW and the QDs is the key factor for determining whether the carrier tunneling can significantly occur. A smaller d will cause a larger trailing edge of the electron wave function into the QD layer, which will lead to a higher carrier tunneling rate.23,24) However, on the other hand, the actual parameters of the structure should also be limited by the material growth. Both w and h cannot become too large, otherwise the crystal quality of the InGaN QW will deteriorate owing to the strain caused by the lattice mismatch between InGaN and GaN. Moreover, the self-assembled growth of the QDs strongly

depends on the strain.25) A thin barrier with a small d will provide a less compressive strain to the QD layer on it; then the QD density will decrease, which has an adverse impact on the IQE of the QDs.16,26) Considering all these factors, it can be found that there exist optimal values of w, h, and d. The eight samples A–H with different w, h, and d values have been grown to study the influences of the three structural parameters on the coupling effect, whose structure parameters are shown in Table I. Here, energy depth (h) is represented by the indium composition x of the QW. In order to evaluate the coupling effect, the TDPL is measured in the temperature range from 13 to 300 K using a 405 nm laser as an excitation source with a power density of 4 W=cm2, which can avoid stimulating the carriers in GaN barriers. The PL spectra of samples A–H and J at room temperature are shown in Fig. 2. Sample J, which has only a layer of QDs in the active region, exhibits a broad peak at around 525 nm. For the coupled structure samples A–H, they also have a main peak at around 525 nm, which is considered from the QDs. The deviation of the peak wavelength may be caused by the fluctuation of the indium composition or height of the QDs owing to the different QWs and barriers beneath.27) It can be seen that the PL intensities of samples A–H have different degrees of enhancement compared with that of sample J. This is attributed to the effect of the coupled structure. In addition, for samples A–H, there is also a less obvious peak at around 470 nm, which is considered from the QW. As the temperature decreases, this peak will become more and more obvious and dominate at low temperatures. The PL spectra at different temperatures of sample A are shown in the inset of Fig. 2 to illustrate the phenomenon, while seven other coupled samples have similar results. This result is considered to be related to the carrier tunneling process, which will be discussed later. The IQE of QDs, which is calculated as the ratio of the integration PL intensity measured at 300 K to that measured at 13 K (I300K=I13K) considering that the nonradiative recombination can be ignored at 13 K,28,29) is also listed in Table I. According to the results, sample A has the highest IQE of 45% among all the samples, which is more than two times that of sample J. Samples B and G have relatively lower IQEs than the other coupled QW–QD structures. The IQE of

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(a)

(b)

(a)

(c)

(b) Fig. 3. PL spectra of three samples measured at 13 and 300 K: (a) A, (b) K, and (c) J.

sample B is low because of the small d, which may decrease the QD density owing to the relatively small strain.24,30) For sample G, x is relatively high so that the large lattice mismatch will introduce severe stress. This will reduce the crystal quality of the QW and the consequence material upon it, leading to the lower IQE. In order to prove that this enhancement effect is indeed due to carrier tunneling rather than other factors, three samples, A, J, and K, are further investigated, the active regions of which are coupled QW–QDs, a single QW, and a single layer of QDs, respectively. Figures 3(a)–3(c) show the PL spectra of the three samples measured at 13 and 300 K, respectively. In Fig. 3(a), two peaks are observed at 300 K. The one at around 470 nm is attributed to the coupled QW (labeled CQW), while the other one at around 525 nm is attributed to the coupled QDs (labeled CQDs). In Figs. 3(b) and 3(c), the peaks at around 470 and 525 nm are considered to be from the single QW (labeled UQW) and the single layer of QDs (labeled UQDs), respectively. In Fig. 3(b), two longitudinal optical (LO) phonon replicas of the QW, labeled 1-LO and 2-LO, appear at 13 K, whose peak wavelengths are 482 and 499 nm, respectively. Similarly, in Fig. 3(a), only one LO phonon replica labeled 1-LO can be observed at 13 K, which has a peak wavelength of 487 nm. The IQEs of UQW (sample K) and UQDs (sample J) are 12 and 21%, respectively. However, in sample A, the IQE of CQW decreases to 6%, while that of CQDs increases to 45%, simultaneously, which are calculated through multipeak fitting. Considering the identical growth conditions of these three samples, their different IQE characteristics would be related to the carrier tunneling effect between the CQW and the CQDs. The dependence of the normalized integration PL intensity on the temperature of the three samples is shown in Fig. 4(a) to explore the carrier transfer mechanism between the QW and the QDs. For CQW, CQDs, UQW, and UQDs, as temperature increases, the normalized integration PL intensities all decrease monotonically. However, according to Fig. 4(a), the PL intensity degradation of CQDs is faster than that of UQDs below 100 K. Meanwhile, the PL intensity degradation of CQW is lower than that of UQDs. However, the opposite

Fig. 4. (a) Dependence of normalized integration PL intensity on temperature for samples A, K, and J. The inset shows the conduction band structure of the coupled QW–QDs. (b) TRPL decay curves of three samples measured at room temperature.

PL intensity degradation phenomenon is observed when the temperature is beyond 100 K. These different PL intensity degradation phenomena between CQDs and UQDs (or CQW and UQW) with increasing temperature may be caused by different carrier tunneling mechanisms in the coupled QW–QD structure. The conduction band of sample A is briefly simulated following the design parameters using Nextnano3 software, which is shown in the inset of Fig. 4(a). It is found that the ground-state energy level of CQW (EW0) is close to the firstexcited-state energy level of the CQDs (ED1). The tunneling process phenomenon occurs here mainly because of the phonon-assisted tunneling mechanism. In the high-temperature region (T > 100 K), the energy level EW0 is slightly higher than ED1. The energy gap between EW0 and ED1 is 89 meV, which is nearly equal to a longitudinal optical phonon’s energy [as shown in the inset of Fig. 4(a)]. Thus, the probability of electrons tunneling from CQW to CQDs is relatively higher than that in the opposite direction. In addition to this, the radiative recombination lifetime in CQW is longer than that in CQDs,31) and the QCSE in CQW is more severe than that in CQDs.32,33) Therefore, the electrons at EW0 will transfer to fill in the ED1 by tunneling from CQW to CQDs and release the phonon, which will decrease (increase) the PL intensity degradation rate of CQDs (CQW). In con-

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trast, in the low-temperature region (T < 100 K), the energy level ED1 may be slightly higher than EW0 owing to the combined effect of the band filling and band shrinking,34) and thus the probability of electron tunneling from CQDs to CQW becomes higher. Besides, the radiative recombination lifetime of electrons in CQDs is also higher than that in CQW at low temperatures.31) In this case, the electrons at ED1 will transfer to fill in the EW0 by tunneling from CQDs to CQW inversely. These tunneling mechanisms are very similar to those described in previous reports.17,18) The above-mentioned result also indicates that the IQE enhancement effect of CQDs comes at the expense of the IQE degradation of CQW at room temperature, which coincides with the earlier discussion concerning Fig. 3. In addition, TRPL measurements on the three samples are performed at room temperature in order to clarify whether the tunneling happens. An Edinburgh FLS 920 fluorescence spectrometer system was used to detect the carrier lifetimes of the three samples. An ultrashort-pulsed 405 nm laser diode was used as the optical excitation source and an R928P photomultiplier tube (PMT) was used to detect the luminescence signal with an energy window of 70 meV. The testing range is 100 ns with a step size of 0.05 ns. The decay curves of PL intensity at peak wavelength are shown in Fig. 4(b). For simplicity, the carrier lifetimes are determined when their intensities reduce to 1=e of their maximum values.35) The carrier lifetimes in UQW and UQDs are estimated to be 2.68 and 1.02 ns, respectively. Those in CQW and CQDs are estimated to be 2.12 and 1.31 ns, respectively. The longer carrier lifetimes in QWs than in QDs result from the stronger QCSE in QWs.36) It is clear that the carrier lifetime of CQW is shorter than that of UQW, while that of CQDs is longer than that of UQDs. These phenomena reflect that the carriers can tunnel from QWs to QDs at room temperature, which increase the carrier lifetime in QDs and decrease the carrier lifetime in QWs. This result indicates that the coupled QW and QD structure can dramatically enhance the radiative recombination efficiency of green QDs by proper designing, which might offer a practical method in solving the “green gap” problem and developing high-brightness green lightemitting devices based on InGaN QDs.37,38) In conclusion, the tunneling-enhanced carrier transfer mechanism in the coupled InGaN=GaN QW and QD structure has been observed and studied by TDPL and TRPL. It is found that carriers can tunnel from a shallow QW to deep QDs at room temperature. Compared with the conventional single active layer of QDs, the IQE of QDs is enhanced more than two times to about 45% by optimizing the coupled QW–QD structure. Acknowledgments This work was supported by the National Basic Research Program of China (Grant Nos. 2011CB301900 and 2013CB632804), the High Technology Research and Development Program of China (Grant No. 2012AA050601), and the National Natural Science Foundation of China (Grant Nos. 61176015, 61176059, 61210014, 61321004, and 61307024).

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