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Efficiency enhancement of green light emitting diodes by improving the uniformity of embedded quantum dots in multiple quantum wells through working pressure control
Journal of Alloys and Compounds 669 (2016) 156e160
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Efficiency enhancement of green light emitting diodes by improving the uniformity of embedded quantum dots in multiple quantum wells through working pressure control Sheng-Chieh Tsai a, b, Hsin-Chiao Fang b, Yen-Lin Lai b, Cheng-Hsueh Lu b, Chuan-Pu Liu a, * a b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan Research Center, Genesis Photonics Incorporation, Tainan 74144, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 December 2015 Received in revised form 26 January 2016 Accepted 29 January 2016 Available online 4 February 2016
InGaN/GaN multiple-quantum-wells (MQWs) in green light emitting diodes (G-LEDs) containing embedded quantum dots (QDs) inside or extruded 3D-like QDs with various size distributions produced via spinodal decomposition are grown by metal-organic chemical vapor deposition. The average size of QDs changes from 3.05 nm to 2.40 nm as the working pressure decreases from 500 torr to 300 torr. The growth mechanism of QDs is discussed. The photoluminescence and electroluminescence results show that smaller and more uniform embedded QDs can improve recombination efficiency, and thus achieve higher peak intensity with smaller peak broadening. More importantly, this work demonstrates that the embedded QDs undergo higher strain relaxation, with a smaller piezoelectric field and better droop performance. Accordingly, the performance of external quantum efficiency is enhanced, leading to a 20% increase in light output power in lamp-form package LEDs. © 2016 Published by Elsevier B.V.
Keywords: Nitride materials Green light-emitting diodes Quantum dots Droop
1. Introduction Light-emitting diodes (LEDs) have attracted much attention in solid-state lighting, backlighting, and other applications of optoelectronic devices, due to the urgent need to consume less energy [1e4]. In addition, the tunable bandgap energy that can be obtained by controlling the indium fraction in the active InGaN layers means that LEDs can emit light from ultraviolet to infrared. Based on these revolutionary lighting technologies, the Noble Prize in Physics was awarded for the work on InGaN-based blue LEDs in 2014. LEDs technologies have progressed very rapidly in recent years with the advent of advanced equipment design, as well as improved epitaxial methods and chip process abilities. Nevertheless, green LEDs and laser diodes (LDs) based on InGaN/GaN multiple-quantum-wells (MQWs) are of the lowest quantum efficiency among all such devices, and thus a number of challenges remain to be overcome with regard to this technology. More In content in the InGaN active layers is essential for green LEDs and LDs, resulting in a larger lattice mismatch at the interface between InGaN active layers and GaN barriers compared to their blue
* Corresponding author. E-mail address: [email protected] (C.-P. Liu). http://dx.doi.org/10.1016/j.jallcom.2016.01.237 0925-8388/© 2016 Published by Elsevier B.V.
counterparts. This also cause larger piezoelectric polarization field across the quantum wells along the c-axis growth direction. As a result, a strong quantum confined Stark effect (QCSE) arises especially in green LEDs. The low quantum efficiency of such devices can be significantly improved by incorporating quasi-quantum dots (QDs) as In-rich regions in the active layers. The advantages of using QDs include (1) localizing excitons from transporting to surrounding dislocations and defects, (2) acting as radiative recombination centers, (3) releasing misfit strain and thus inducing a smaller piezoelectric field, and (4) increasing the spatial overlap between electron and hole wave functions by decreasing the piezoelectric field. The capturing both electrons and holes into confined QDs in the active layers contributes to a small leakage current as well as better injection efficiency [5e12]. The presence of QDs can thus enhance the recombination efficiency and increase the total light emission intensity in highly defective GaN-based LEDs. However, In-rich QDs can exist in different morphologies resulting from different growth mechanisms, and it remains unclear as to the optimum morphology of QDs for the best emission efficiency. The formation mechanisms of QDs have been widely investigated. Yao et al. obtained QDs with different width and height by varying the interruption time, and showed better output power
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with smaller QDs [13]. Lu et al. studied the dependence of QD size and density on the V/III ratio when growing InGaN active layers and concluded that light output power is higher with larger QD density [14]. Wang et al. observed that QDs could be destroyed if the annealing temperature for p-GaN activation is too high [15]. Lai et al. found that the temperature used when growing barriers could affect the parameters of pre-existing QDs in MQWs and thus device performance [16]. However, among the growth parameters, the working pressure when growing active layers is usually crucial for determining the growth mode, but details of this have not yet been reported yet. This work demonstrates an effective method to improve recombination efficiency by simply varying the working pressure in growing active layers. The mechanism for enhancing recombination efficiency is discussed. The enhancement in the external quantum efficiency (EQE) and light output power of lamp-formed packaged LEDs is demonstrated. 2. Experimental procedure Two samples of InGaN/GaN MQWs were grown on c-oriented pattern sapphire substrates in a low-pressure metal-organic chemical vapor deposition (MOCVD) system (Thomas Swan, UK). With H2 or N2 as a carrier gas, trimethylgallium (TMGa), trimethylindium (TMIn), ammonia (NH3), silane (SiH4), and biscyclopentadienyl-magnesium (Cp2Mg) were used as the precursors for Ga, In, N, Si, and Mg, respectively. Each sample consisted of a low temperature GaN buffer layer followed by a 3-mm-thick undoped GaN and 2-mm-thick n-type GaN layer doped with Si, then nine pairs of InGaN/GaN heterostructures. Finally, a 0.1-mm-thick ptype GaN layer doped with Mg was grown to complete a typical LED structure to emit green light with the dominant wavelength at about 521 nm, as determined by lamp-type packaged measurements at room temperature. All the growth conditions including growth temperature, V/III ratio and carrier gas for each of the layers were identical between these two samples, except the working pressure in the InGaN active layers, which was set at 300 torr and 500 torr, denoted as S-300 and S-500, respectively. Room-temperature Photoluminescence (PL) measurements were carried out using a 55-mW continuous-wave HeeCd laser at 325 nm. The microstructure of the samples was characterized by high-resolution transmission electron microscopy (HRTEM; JEOL 2100F) operated at an acceleration voltage of 200 kV. For the characterization of opto-electrical performance, each sample was made into the package in lamp-type form. The process began by depositing an indium tin oxide (ITO) layer on the p-type GaN layer as a transparent contacting layer, followed by defining and exposing n-type regions with a square mask (12 13 mil2) by photolithography and inductively coupled plasma dry etching (SAMCO ICP-RIE101iPH) until etching into the n-type GaN layer to a depth of 1.5 mm. Subsequently, trilayers of Cr/Pt/Au (50/20/200 nm) were deposited as p-/n-electrodes and bonding pads to obtain the LED chips. Finally, the LED chips were mounted onto lamp lead frames, followed by wire-bonding and encapsulation for the measurement of the opto-electric properties. The light output powerecurrent curves (LeI curves) of the lamp-formed LEDs were measured using a calibrated integrating sphere detector with a Keithley 2400 semiconductor parameter analyzer at room temperature. 3. Results and discussion Fig. 1 shows the PL spectra of the two samples. The PL peak intensity of S-300 is higher than that of S-500, while the peak wavelength at 520.7 nm from S-300 is slightly shorter than that at
Fig. 1. Photoluminescence spectra of S-300 and S-500, where the InGaN active layers of sample S-300 and sample S-500 were grown under the working pressure of 300 torr and 500 torr, respectively.
521.7 nm from S-500. To explore the mechanism responsible for this difference, the average indium contents in the InGaN active layers were measured accurately from the ue2q spectra of x-ray diffraction analysis (not shown), and the results showed that they are identical, at 20 ± 2%, for the two samples. This implies that the difference in the PL spectra position is not caused by the variation in the indium content. Besides, Fig. 1 also shows the full width at half maximum (FWHM) of the peak is 43 nm for S-300, narrower than 62 nm for S-500. Therefore, the PL peak from S-300 is more intense, narrower and located at a shorter wavelength than that from S-500. Fig. 2 shows cross-sectional TEM dark-field images taken under a two-beam condition using g ¼ 0 0 0 2 close to the ½1120 zoneaxis for the two samples. The images were always taken from fresh areas within a reasonably short time to avoid beam-damage or beam-induced effects [17,18], and thus ensure the existence of dots in the samples. Fig. 2(a) shows the QDs are tiny and uniformly imbedded inside the quantum wells of S-300, whereas Fig. 2(c) reveals the InGaN MQWs in S-500 are completely composed of much coarser and more distinct QDs. Some of the quantum dots are pointed out by the red arrows (in the web version). Fig. 2(a) and (c) shows the absence of a wetting layer underneath the QDs in both samples, which suggests that the QDs do not result from the StranskieKrastanov (SK) growth mode [14,19e21]. The formation of QDs in this case is via spinodal decomposition, according to the phase diagram calculated based on thermodynamic theory [22], where the derived average indium content of 20 ± 2% for both samples is located at the immiscibility gap. In order to reach lower Gibbs energy, phase separation into the In-rich region QDs and Inpoor matrix takes place once heat is supplied either during the growth of the InGaN active layers or post annealing in the subsequent layer growth. Under these circumstances, the distribution of QDs is kinetically limited by diffusion length, which in this case is affected by working pressure given that all the other conditions are identical. With the higher working pressure used for growing S500, more organic molecules were cracked and deposited on the surface leading to the shorter mean free path for adatoms than seen in S-300. In other words, with sufficient energy, the diffusion length of the adatoms is longer at a lower working pressure, in favor of 2D growth. In contrast, a shorter diffusion length promotes the segregation of In adatoms for forming In-rich regions in favor of 3D growth. Consequently, this causes distinct QWs features between both samples in Fig. 2, where the low working pressure (300 torr)
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Fig. 2. Two-beam dark field (g ¼ 0 0 0 2) TEM images showing QD contrast in the InGaN active layers of (a) S-300 and (c) S-500 with the corresponding statistics of QD size distribution in (b) and (d), respectively.
favors 2D growth for S-300, and the high working pressure (500 torr) promotes a 3D-like growth mode for S-500. The statistical QD distribution is also estimated in Fig. 2, where the average size of the embedded QDs in S-300 is 2.40 ± 0.19 nm (Fig. 2(b)), and that of the distinct QDs in S-500 is 3.05 ± 0.23 nm (Fig. 2(d)). The larger average QD size in S-500 causes the lower PL peak emission intensity at a larger wavelength attributed to the lower recombination efficiency and the wider QD size distribution contributes to the broader emission peak [11,16,23,24]. Fig. 3 shows the EL spectra obtained with different currents from the lamp-type package of both samples. The measurements
were carried out for more than 20 LEDs lamps to ensure that the spectra shown here demonstrate the typical behavior for each sample. In general, S-500 exhibits lower emission intensity with higher peak broadening for every peak under different currents than S-300, which is consistent with the PL results. Furthermore, as the current is applied from 1 mA to 60 mA, the emission peak undergoes blue shift to be 16.3 nm and 21.5 nm for S-300 and S500, respectively, which is ascribed to the different degree of strain relaxation involved. By comparing Fig. 2(a) with Fig. 2(c), the more uniform and smaller embedded QDs in S-300 help relieve the strain more effectively from lattice mismatch, while larger QDs in S-500
Fig. 3. Electroluminescence spectra with different current from the lamp-type packages of (a) S-300 and (b) S-500.
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might possess higher residual strain. Consequently, smaller strain leads to a smaller piezoelectric field by the smaller QDs in S-300, which then causes a smaller screen compensation effect from the injected carriers under the released QCSE [25]. Therefore, S-300 possesses a smaller blue shift in the emission peak under different currents. Fig. 4 shows the FWHM values of the EL spectra for S-300 and S500 measured with different currents at room temperature. With the applied current from 0 mA to 60 mA, the FWHM value in S-300 is from 28.7 nm to 34.2 nm and from 34.9 nm to 38.2 nm for S-500. The higher FWHM value in S-500 is obtained. The wider range of quantum dots size distribution in S-500 leads to diverse confinement [26], which results in wider wavelength distribution and larger FWHM value. The FWHM value in S-300 of PL spectrum at the room temperature shown in Fig. 1 is 43 nm and that is 62 nm in S-500. The results of larger FWHM value in EL and PL spectra are also attributed to wider quantum dots size distribution. Fig. 5 shows the EQE versus current density from S-300 and S500 in lamp-type package. The EQE can be given by the following formula [26]:
EQE ¼ ðPopt=EphotÞ=ðI=qÞ
159
Fig. 5. External quantum efficiency as a function of current density for lamp-type packages of S-300 and S-500.
(1)
where Popt is the light output power, Ephot is the photon energy, I is the injection current, and q is the charge. The EQE is extracted from the EL measurements, which were obtained by using a pulsed current source in order to eliminate the heating effect. The peak values of S-300 and S-500 at low current density are 41.2% and 39.5%, respectively. For the normal high power operation condition at 350 mA, corresponding to the current density of 26.8 A/cm2, the EQE are 21.6% and 17.1% for S-300 and S-500, respectively. It is believed that this large increase can be attributed to the relaxation of layer strain in the InGaN/GaN MQWs. The more uniform and smaller embedded QDs in S-300 help relieve the strain. The decrease in the piezoelectric field leads to an increase in spatial overlapping between the electron and hole wave functions until the full screening of the QCSE, resulting in an increased probability of radiative recombination. The S-300 thus has a better EQE performance throughout the whole current range measured [27e29]. The internal quantum efficiency (IQE) can be deduced from EQE and light extraction efficiency (LEE), which can be given by the following expression [26]:
Fig. 4. The FWHM value with different current from the lamp-type packages of S-300 and S-500.
EQE ¼ IQE LEE
(2)
The EQE of S-300 (smaller QDs) is shown to be greater than that of S-500 (larger QDs) for all current densities. It is believed that rough surface in LEDs structure would help to improve the LEE [30,31]. By examining Fig. 2(a) and (c), S-500 exhibits rougher interfaces than S-300 due to larger and irregular quantum dots, leading to higher LEE. Therefore, S-500 with higher LEE but lower EQE apparently should result in lower IQE compared to S-300. Fig. 6 shows the typical light output characteristics (LeI curves) of packaged G-LEDs lamps fabricated from the two samples. As the current changes from 1 mA to 60 mA, the output power increases from 2.2 mW to 23.6 mW in S-300, which is higher than that from 0.6 mW to 17.1 mW in S-500 due to enhanced carrier recombination in the smaller QDs of S-300. Specifically, the LEDs lamp fabricated from S-300 has a light output power of 10.7 mW at 20 mA, which is about 20% higher than that of 8.8 mW from S-500. It is well known that the light output power of LEDs devices is governed by carrier recombination in InGaN active layers. The poor
Fig. 6. Light output characteristics (LeI curves) of lamp-type packages of S-300 and S500.
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light output power from S-500 is thus due to its larger and more irregular QDs.
[5] [6] [7] [8]
4. Conclusions [9]
In summary, this work demonstrates a method to control QD morphology and size in the form of embedded QDs in 2D quantum wells or 3D island-like QDs as the active layers by simply varying the working pressure. This study also shows that the uniform QD size distribution in the 2D case enhances carrier recombination efficiency, and thus produces a higher intensity of the emission peak with smaller peak broadening in the PL and EL spectra. Moreover, the smaller QDs embedded in the InGaN layers promote strain relaxation leading to a smaller piezoelectric field for stronger electronehole wave function overlap and better droop performance. Finally, the light output power from the uniform and smaller QDs size embedded in the InGaN layers is efficiently enhanced by increasing the carrier recombination efficiency by approximately 20% compared with conventional devices grown on a compositionally inhomogeneous layer. This work thus provides an economic and promising method for the fabrication of highefficiency InGaN-based green LEDs. Acknowledgments This work was financially supported by the Ministry of Science and Technology of Taiwan (grant no. 101-2221-E-006-131-MY3) and Genesis Photonics Inc.. References [1] S. Nakamura, J. Vac. Sci. Technol. A 13 (1995) 705e710. [2] P. Waltereit, O. Brandt, A. Trampert, H.T. Grahn, J. Menniger, M. Ramsteiner, M. Reiche, K.H. Ploog, Nature 406 (2000) 865e868. [3] P. Tao, H. Liang, X. Xia, Y. Liu, J. Jiang, H. Huang, Q. Feng, R. Shen, Y. Luo, G. Du, Superlatt. Microstruct. 85 (2015) 482e487. [4] S. Min-Ho, H. Hyun-Guk, K. Hyo-Jun, K. Young-Joo, Appl. Phys. Express 7 (2014) 052101.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Efficiency enhancement of green light emitting diodes by improving the uniformity of embedded quantum dots in multiple quantum wells through working pressure control Sheng-Chieh Tsai a, b, Hsin-Chiao Fang b, Yen-Lin Lai b, Cheng-Hsueh Lu b, Chuan-Pu Liu a, * a b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan Research Center, Genesis Photonics Incorporation, Tainan 74144, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 December 2015 Received in revised form 26 January 2016 Accepted 29 January 2016 Available online 4 February 2016
InGaN/GaN multiple-quantum-wells (MQWs) in green light emitting diodes (G-LEDs) containing embedded quantum dots (QDs) inside or extruded 3D-like QDs with various size distributions produced via spinodal decomposition are grown by metal-organic chemical vapor deposition. The average size of QDs changes from 3.05 nm to 2.40 nm as the working pressure decreases from 500 torr to 300 torr. The growth mechanism of QDs is discussed. The photoluminescence and electroluminescence results show that smaller and more uniform embedded QDs can improve recombination efficiency, and thus achieve higher peak intensity with smaller peak broadening. More importantly, this work demonstrates that the embedded QDs undergo higher strain relaxation, with a smaller piezoelectric field and better droop performance. Accordingly, the performance of external quantum efficiency is enhanced, leading to a 20% increase in light output power in lamp-form package LEDs. © 2016 Published by Elsevier B.V.
Keywords: Nitride materials Green light-emitting diodes Quantum dots Droop
1. Introduction Light-emitting diodes (LEDs) have attracted much attention in solid-state lighting, backlighting, and other applications of optoelectronic devices, due to the urgent need to consume less energy [1e4]. In addition, the tunable bandgap energy that can be obtained by controlling the indium fraction in the active InGaN layers means that LEDs can emit light from ultraviolet to infrared. Based on these revolutionary lighting technologies, the Noble Prize in Physics was awarded for the work on InGaN-based blue LEDs in 2014. LEDs technologies have progressed very rapidly in recent years with the advent of advanced equipment design, as well as improved epitaxial methods and chip process abilities. Nevertheless, green LEDs and laser diodes (LDs) based on InGaN/GaN multiple-quantum-wells (MQWs) are of the lowest quantum efficiency among all such devices, and thus a number of challenges remain to be overcome with regard to this technology. More In content in the InGaN active layers is essential for green LEDs and LDs, resulting in a larger lattice mismatch at the interface between InGaN active layers and GaN barriers compared to their blue
* Corresponding author. E-mail address: [email protected] (C.-P. Liu). http://dx.doi.org/10.1016/j.jallcom.2016.01.237 0925-8388/© 2016 Published by Elsevier B.V.
counterparts. This also cause larger piezoelectric polarization field across the quantum wells along the c-axis growth direction. As a result, a strong quantum confined Stark effect (QCSE) arises especially in green LEDs. The low quantum efficiency of such devices can be significantly improved by incorporating quasi-quantum dots (QDs) as In-rich regions in the active layers. The advantages of using QDs include (1) localizing excitons from transporting to surrounding dislocations and defects, (2) acting as radiative recombination centers, (3) releasing misfit strain and thus inducing a smaller piezoelectric field, and (4) increasing the spatial overlap between electron and hole wave functions by decreasing the piezoelectric field. The capturing both electrons and holes into confined QDs in the active layers contributes to a small leakage current as well as better injection efficiency [5e12]. The presence of QDs can thus enhance the recombination efficiency and increase the total light emission intensity in highly defective GaN-based LEDs. However, In-rich QDs can exist in different morphologies resulting from different growth mechanisms, and it remains unclear as to the optimum morphology of QDs for the best emission efficiency. The formation mechanisms of QDs have been widely investigated. Yao et al. obtained QDs with different width and height by varying the interruption time, and showed better output power
S.-C. Tsai et al. / Journal of Alloys and Compounds 669 (2016) 156e160
157
with smaller QDs [13]. Lu et al. studied the dependence of QD size and density on the V/III ratio when growing InGaN active layers and concluded that light output power is higher with larger QD density [14]. Wang et al. observed that QDs could be destroyed if the annealing temperature for p-GaN activation is too high [15]. Lai et al. found that the temperature used when growing barriers could affect the parameters of pre-existing QDs in MQWs and thus device performance [16]. However, among the growth parameters, the working pressure when growing active layers is usually crucial for determining the growth mode, but details of this have not yet been reported yet. This work demonstrates an effective method to improve recombination efficiency by simply varying the working pressure in growing active layers. The mechanism for enhancing recombination efficiency is discussed. The enhancement in the external quantum efficiency (EQE) and light output power of lamp-formed packaged LEDs is demonstrated. 2. Experimental procedure Two samples of InGaN/GaN MQWs were grown on c-oriented pattern sapphire substrates in a low-pressure metal-organic chemical vapor deposition (MOCVD) system (Thomas Swan, UK). With H2 or N2 as a carrier gas, trimethylgallium (TMGa), trimethylindium (TMIn), ammonia (NH3), silane (SiH4), and biscyclopentadienyl-magnesium (Cp2Mg) were used as the precursors for Ga, In, N, Si, and Mg, respectively. Each sample consisted of a low temperature GaN buffer layer followed by a 3-mm-thick undoped GaN and 2-mm-thick n-type GaN layer doped with Si, then nine pairs of InGaN/GaN heterostructures. Finally, a 0.1-mm-thick ptype GaN layer doped with Mg was grown to complete a typical LED structure to emit green light with the dominant wavelength at about 521 nm, as determined by lamp-type packaged measurements at room temperature. All the growth conditions including growth temperature, V/III ratio and carrier gas for each of the layers were identical between these two samples, except the working pressure in the InGaN active layers, which was set at 300 torr and 500 torr, denoted as S-300 and S-500, respectively. Room-temperature Photoluminescence (PL) measurements were carried out using a 55-mW continuous-wave HeeCd laser at 325 nm. The microstructure of the samples was characterized by high-resolution transmission electron microscopy (HRTEM; JEOL 2100F) operated at an acceleration voltage of 200 kV. For the characterization of opto-electrical performance, each sample was made into the package in lamp-type form. The process began by depositing an indium tin oxide (ITO) layer on the p-type GaN layer as a transparent contacting layer, followed by defining and exposing n-type regions with a square mask (12 13 mil2) by photolithography and inductively coupled plasma dry etching (SAMCO ICP-RIE101iPH) until etching into the n-type GaN layer to a depth of 1.5 mm. Subsequently, trilayers of Cr/Pt/Au (50/20/200 nm) were deposited as p-/n-electrodes and bonding pads to obtain the LED chips. Finally, the LED chips were mounted onto lamp lead frames, followed by wire-bonding and encapsulation for the measurement of the opto-electric properties. The light output powerecurrent curves (LeI curves) of the lamp-formed LEDs were measured using a calibrated integrating sphere detector with a Keithley 2400 semiconductor parameter analyzer at room temperature. 3. Results and discussion Fig. 1 shows the PL spectra of the two samples. The PL peak intensity of S-300 is higher than that of S-500, while the peak wavelength at 520.7 nm from S-300 is slightly shorter than that at
Fig. 1. Photoluminescence spectra of S-300 and S-500, where the InGaN active layers of sample S-300 and sample S-500 were grown under the working pressure of 300 torr and 500 torr, respectively.
521.7 nm from S-500. To explore the mechanism responsible for this difference, the average indium contents in the InGaN active layers were measured accurately from the ue2q spectra of x-ray diffraction analysis (not shown), and the results showed that they are identical, at 20 ± 2%, for the two samples. This implies that the difference in the PL spectra position is not caused by the variation in the indium content. Besides, Fig. 1 also shows the full width at half maximum (FWHM) of the peak is 43 nm for S-300, narrower than 62 nm for S-500. Therefore, the PL peak from S-300 is more intense, narrower and located at a shorter wavelength than that from S-500. Fig. 2 shows cross-sectional TEM dark-field images taken under a two-beam condition using g ¼ 0 0 0 2 close to the ½1120 zoneaxis for the two samples. The images were always taken from fresh areas within a reasonably short time to avoid beam-damage or beam-induced effects [17,18], and thus ensure the existence of dots in the samples. Fig. 2(a) shows the QDs are tiny and uniformly imbedded inside the quantum wells of S-300, whereas Fig. 2(c) reveals the InGaN MQWs in S-500 are completely composed of much coarser and more distinct QDs. Some of the quantum dots are pointed out by the red arrows (in the web version). Fig. 2(a) and (c) shows the absence of a wetting layer underneath the QDs in both samples, which suggests that the QDs do not result from the StranskieKrastanov (SK) growth mode [14,19e21]. The formation of QDs in this case is via spinodal decomposition, according to the phase diagram calculated based on thermodynamic theory [22], where the derived average indium content of 20 ± 2% for both samples is located at the immiscibility gap. In order to reach lower Gibbs energy, phase separation into the In-rich region QDs and Inpoor matrix takes place once heat is supplied either during the growth of the InGaN active layers or post annealing in the subsequent layer growth. Under these circumstances, the distribution of QDs is kinetically limited by diffusion length, which in this case is affected by working pressure given that all the other conditions are identical. With the higher working pressure used for growing S500, more organic molecules were cracked and deposited on the surface leading to the shorter mean free path for adatoms than seen in S-300. In other words, with sufficient energy, the diffusion length of the adatoms is longer at a lower working pressure, in favor of 2D growth. In contrast, a shorter diffusion length promotes the segregation of In adatoms for forming In-rich regions in favor of 3D growth. Consequently, this causes distinct QWs features between both samples in Fig. 2, where the low working pressure (300 torr)
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Fig. 2. Two-beam dark field (g ¼ 0 0 0 2) TEM images showing QD contrast in the InGaN active layers of (a) S-300 and (c) S-500 with the corresponding statistics of QD size distribution in (b) and (d), respectively.
favors 2D growth for S-300, and the high working pressure (500 torr) promotes a 3D-like growth mode for S-500. The statistical QD distribution is also estimated in Fig. 2, where the average size of the embedded QDs in S-300 is 2.40 ± 0.19 nm (Fig. 2(b)), and that of the distinct QDs in S-500 is 3.05 ± 0.23 nm (Fig. 2(d)). The larger average QD size in S-500 causes the lower PL peak emission intensity at a larger wavelength attributed to the lower recombination efficiency and the wider QD size distribution contributes to the broader emission peak [11,16,23,24]. Fig. 3 shows the EL spectra obtained with different currents from the lamp-type package of both samples. The measurements
were carried out for more than 20 LEDs lamps to ensure that the spectra shown here demonstrate the typical behavior for each sample. In general, S-500 exhibits lower emission intensity with higher peak broadening for every peak under different currents than S-300, which is consistent with the PL results. Furthermore, as the current is applied from 1 mA to 60 mA, the emission peak undergoes blue shift to be 16.3 nm and 21.5 nm for S-300 and S500, respectively, which is ascribed to the different degree of strain relaxation involved. By comparing Fig. 2(a) with Fig. 2(c), the more uniform and smaller embedded QDs in S-300 help relieve the strain more effectively from lattice mismatch, while larger QDs in S-500
Fig. 3. Electroluminescence spectra with different current from the lamp-type packages of (a) S-300 and (b) S-500.
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might possess higher residual strain. Consequently, smaller strain leads to a smaller piezoelectric field by the smaller QDs in S-300, which then causes a smaller screen compensation effect from the injected carriers under the released QCSE [25]. Therefore, S-300 possesses a smaller blue shift in the emission peak under different currents. Fig. 4 shows the FWHM values of the EL spectra for S-300 and S500 measured with different currents at room temperature. With the applied current from 0 mA to 60 mA, the FWHM value in S-300 is from 28.7 nm to 34.2 nm and from 34.9 nm to 38.2 nm for S-500. The higher FWHM value in S-500 is obtained. The wider range of quantum dots size distribution in S-500 leads to diverse confinement [26], which results in wider wavelength distribution and larger FWHM value. The FWHM value in S-300 of PL spectrum at the room temperature shown in Fig. 1 is 43 nm and that is 62 nm in S-500. The results of larger FWHM value in EL and PL spectra are also attributed to wider quantum dots size distribution. Fig. 5 shows the EQE versus current density from S-300 and S500 in lamp-type package. The EQE can be given by the following formula [26]:
EQE ¼ ðPopt=EphotÞ=ðI=qÞ
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Fig. 5. External quantum efficiency as a function of current density for lamp-type packages of S-300 and S-500.
(1)
where Popt is the light output power, Ephot is the photon energy, I is the injection current, and q is the charge. The EQE is extracted from the EL measurements, which were obtained by using a pulsed current source in order to eliminate the heating effect. The peak values of S-300 and S-500 at low current density are 41.2% and 39.5%, respectively. For the normal high power operation condition at 350 mA, corresponding to the current density of 26.8 A/cm2, the EQE are 21.6% and 17.1% for S-300 and S-500, respectively. It is believed that this large increase can be attributed to the relaxation of layer strain in the InGaN/GaN MQWs. The more uniform and smaller embedded QDs in S-300 help relieve the strain. The decrease in the piezoelectric field leads to an increase in spatial overlapping between the electron and hole wave functions until the full screening of the QCSE, resulting in an increased probability of radiative recombination. The S-300 thus has a better EQE performance throughout the whole current range measured [27e29]. The internal quantum efficiency (IQE) can be deduced from EQE and light extraction efficiency (LEE), which can be given by the following expression [26]:
Fig. 4. The FWHM value with different current from the lamp-type packages of S-300 and S-500.
EQE ¼ IQE LEE
(2)
The EQE of S-300 (smaller QDs) is shown to be greater than that of S-500 (larger QDs) for all current densities. It is believed that rough surface in LEDs structure would help to improve the LEE [30,31]. By examining Fig. 2(a) and (c), S-500 exhibits rougher interfaces than S-300 due to larger and irregular quantum dots, leading to higher LEE. Therefore, S-500 with higher LEE but lower EQE apparently should result in lower IQE compared to S-300. Fig. 6 shows the typical light output characteristics (LeI curves) of packaged G-LEDs lamps fabricated from the two samples. As the current changes from 1 mA to 60 mA, the output power increases from 2.2 mW to 23.6 mW in S-300, which is higher than that from 0.6 mW to 17.1 mW in S-500 due to enhanced carrier recombination in the smaller QDs of S-300. Specifically, the LEDs lamp fabricated from S-300 has a light output power of 10.7 mW at 20 mA, which is about 20% higher than that of 8.8 mW from S-500. It is well known that the light output power of LEDs devices is governed by carrier recombination in InGaN active layers. The poor
Fig. 6. Light output characteristics (LeI curves) of lamp-type packages of S-300 and S500.
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light output power from S-500 is thus due to its larger and more irregular QDs.
[5] [6] [7] [8]
4. Conclusions [9]
In summary, this work demonstrates a method to control QD morphology and size in the form of embedded QDs in 2D quantum wells or 3D island-like QDs as the active layers by simply varying the working pressure. This study also shows that the uniform QD size distribution in the 2D case enhances carrier recombination efficiency, and thus produces a higher intensity of the emission peak with smaller peak broadening in the PL and EL spectra. Moreover, the smaller QDs embedded in the InGaN layers promote strain relaxation leading to a smaller piezoelectric field for stronger electronehole wave function overlap and better droop performance. Finally, the light output power from the uniform and smaller QDs size embedded in the InGaN layers is efficiently enhanced by increasing the carrier recombination efficiency by approximately 20% compared with conventional devices grown on a compositionally inhomogeneous layer. This work thus provides an economic and promising method for the fabrication of highefficiency InGaN-based green LEDs. Acknowledgments This work was financially supported by the Ministry of Science and Technology of Taiwan (grant no. 101-2221-E-006-131-MY3) and Genesis Photonics Inc.. References [1] S. Nakamura, J. Vac. Sci. Technol. A 13 (1995) 705e710. [2] P. Waltereit, O. Brandt, A. Trampert, H.T. Grahn, J. Menniger, M. Ramsteiner, M. Reiche, K.H. Ploog, Nature 406 (2000) 865e868. [3] P. Tao, H. Liang, X. Xia, Y. Liu, J. Jiang, H. Huang, Q. Feng, R. Shen, Y. Luo, G. Du, Superlatt. Microstruct. 85 (2015) 482e487. [4] S. Min-Ho, H. Hyun-Guk, K. Hyo-Jun, K. Young-Joo, Appl. Phys. Express 7 (2014) 052101.
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