Efficiency and droop improvement in green InGaN/GaN light-emitting diodes on GaN
nanorods template with SiO2 nanomasks
Da-Wei Lin, Chia-Yu Lee, Che-Yu Liu, Hau-Vei Han, Yu-Pin Lan, Chien-Chung Lin, Gou-Chung Chi, and Hao-Chung Kuo
Citation: Applied Physics Letters 101, 233104 (2012); doi: 10.1063/1.4768950
View online: http://dx.doi.org/10.1063/1.4768950
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/101/23?ver=pdfcov Published by the AIP Publishing
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Da-Wei Lin, Chia-Yu Lee, Che-Yu Liu, Hau-Vei Han, Yu-Pin Lan, Chien-Chung Lin, Gou-Chung Chi,1and Hao-Chung Kuo1,b)
1
Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
2
Institute of Photonic System, College of Photonics, National Chiao-Tung University, No. 301, Gaofa 3rd Road, Guiren Township, Tainan County 71150, Taiwan
(Received 7 October 2012; accepted 12 November 2012; published online 3 December 2012) This study presents the green InGaN/GaN multiple quantum wells light-emitting diodes (LEDs) grown on a GaN nanorods template with SiO2 nanomasks by metal–organic chemical vapor
deposition. By nanoscale epitaxial lateral overgrowth, microscale air voids were formed between nanorods and the threading dislocations were efficiently suppressed. The electroluminescence measurement reveals that the LEDs on nanorods template with SiO2 nanomasks suffer less
quantum-confined Stark effect and exhibit higher light output power and lower efficiency droop at a high injection current as compared with conventional LEDs. VC 2012 American Institute of
Physics. [http://dx.doi.org/10.1063/1.4768950]
In recent decades, group-III nitride material has been regarded as one of the most promising materials for develop-ing full-color light-emittdevelop-ing diodes (LEDs) because it has a wide range of direct bandgaps (0.7–6.2 eV). The high effi-ciency blue InGaN-based LEDs have been combined with red and yellow phosphors to provide next generation white-light sources. However, the internal quantum efficiency (IQE) of InGaN-based LED becomes low as the emission energy decrease to green or red emission light. The ineffi-ciency of green and red InGaN-based LEDs is caused by the severe quantum-confined Stark effect (QCSE) induced by high indium composition in quantum wells and strong internal piezoelectric field. An alternative quaternary material InGaAlP has been used to develop high quantum efficiency red LEDs for years,1while the green LEDs are still left without a suitable so-lution. The lag of the green LED efficiency limits the develop-ment of LED in both solid-state lighting (SSL) and display applications. As a result, solving this so-called “efficiency green gap” and developing the equally efficient red, green, blue (RGB) LEDs are the most significant issues.
The external quantum efficiency (EQE) of an LED can be expressed as the product of current injection efficiency, internal quantum efficiency, and light extraction efficiency (LEE). Our previous studies have demonstrated that by using nanoscale patterned sapphire substrate (NPSS) technique, the IQE and LEE of a blue LED can be greatly improved.2,3 In addition, we also employed the technologies of nanopyra-mid structure4and m-plane bulk substrate5to fabricate semi-polar and nonsemi-polar blue-green LEDs. Owing to their high carrier density dependent wavelength stability and low effi-ciency droop, they show a promising potential in developing high indium content LEDs. In order to develop high effi-ciency green LEDs, Liet al. also reported an improvement in both IQE and LEE for a 525 nm green LED on the
nano-patterned sapphire substrate.6In this work, a nanoscale pat-terned substrate, comprised high aspect ratio GaN nanorods (NRs) and SiO2nanomasks on the top of NRs, was applied
to develop high performance green LEDs. The growth mech-anism, the enhancement of LEE, and the properties of band diagram were also discussed in detail.
The GaN nanorods template was prepared in the follow-ing procedures:7(1) deposition of a 2 lm thick undoped GaN on c-plane sapphire by metal–organic chemical vapor depo-sition (MOCVD); (2) depodepo-sition of a 200 nm SiO2layer by
plasma-enhanced chemical vapor deposition (PECVD); (3) evaporation of a 10 nm thick Ni layer followed by rapid ther-mal annealing (RTA) with a flowing nitrogen gas at 850C for 1 min to form self-assembled Ni clusters with approxi-mately 200 nm in diameter; (4) dry etching with the Ni clus-ters served as etch masks for forming NRs by reactive ion etching (RIE) and inductive coupled plasma (ICP); (5) re-moval the residual Ni masks by dipping the sample into ni-tric acid solution (HNO3) at 100C for 5 min. Figure 1(a)
shows the cross-sectional scanning electron microscopy (SEM) image of the GaN NRs with SiO2 nanomasks. The
height of the GaN NRs is about 2 lm while the diameters of them are in a range of 200–300 nm. The green InGaN/GaN multiple quantum wells (MQWs) LED structure was grown on this GaN NRs template by a low pressure MOCVD (Veeco D75) system. For comparison, a sample with the same green LED structure was also grown on c-plane sap-phire. The epitaxial structure consists of a 3 lm n-GaN, six periods of 3 nm InGaN QWs and 12 nm GaN barriers, a 20 nm p-Al0.1Ga0.9N electron blocking layer (EBL), and a 0.2 lm
p-GaN. Typical n-(Si) and p-(Mg) type dopants were used. Subsequently, the LED chips were fabricated by regular chip process with 300 300 lm2 diode mesas, indium-tin-oxide (ITO) current spreading layer, and Ni/Au contact pads.
To investigate the mechanism of nanoscale epitaxial lateral overgrowth (NELOG) in detail, the transmission elec-tron microscopy (TEM) was employed. Figure 1(b) shows
a)Electronic mail: chienchunglin@faculty.nctu.edu.tw. b)
Electronic mail: hckuo@faculty.nctu.edu.tw.
the cross-sectional TEM image of GaN epilayer overgrown on the GaN NRs template with SiO2nanomasks. Owing to
each NR has a SiO2nanomask on the top of it, the GaN
epi-layer can only grow on the sidewall of NRs. Our previous study have showed that this growth process is a combination of GaN epilayer grown on the M-plane (10-10) sidewall of NR and inclined R-plane facets (1-102) which are close to the top of the NRs.8By laterally growing GaN epilayer on these two kinds of facets, the spacing between NRs become closer and finally coalesces. However, because of the rapid lateral growth and the high aspect ratio of NRs template, microscale air voids were formed between NRs, as shown in Figure1(b). In addition, one can observe that there are many threading dislocations (TDs) below the SiO2 nanomasks
layer, while the TDs above this layer become much less than below. There are two reasons for the reduction of TDs in GaN epilayer above the SiO2nanomasks layer. First, the
for-mation of air voids can act as microscale mask to prevent the TDs from going through to the upper epilayer. In addition, the SiO2 nanomasks have the similar function as the air
voids, which also block parts of TDs. Second, due to the strong lateral growth by using NELOG, the TDs tend to fol-low the growth direction. This characteristic gives the TDs a chance to bend to the air voids and the SiO2nanomasks. The
cross-sectional TEM image indicates that many TDs are ended with these two types of masks, which results in a high crystalline quality GaN epilayer.
One of the problems leading to the inefficiency of green LED is the QCSE induced by high indium composition in quantum wells and strong internal piezoelectric field. To keep the wavelength of green LED, it is hard to decrease the indium composition for improving the QCSE. Under this cir-cumstance, it is much important to decrease the residual
strain in the epitaxial layer. The Raman scattering spectrum was performed to measure the strain of the GaN epilayer on both GaN NRs template and sapphire substrate. The E2
(high) peaks of the Raman scattering spectrum of the GaN epilayers on NRs template and sapphire substrate are at 568.5 and 570.5 cm1, respectively (not shown here). From the E2(high) peaks, the in-plane compressive strain can be
estimated to be 0.88 GPa and 1.77 GPa by using the follow-ing equation:9
Dx¼ xE2 x0¼ Cr; (1)
where Dx is the Raman shift peak difference between the strained GaN epitaxial layer xE2and the unstrained GaN
epi-taxial layer x0(566.5 cm1), and C is the biaxial strain
coef-ficient, which is 2.25 cm1/GPa. The Raman scattering spectrum indicates that the GaN epitaxial layer on GaN NRs template exhibited lower compressive strain than GaN epi-layer on sapphire. With the strain relaxation characteristic of GaN epilayer grown on GaN NRs template, one can expect that the QCSE in the green MQWs grown on this template can be suppressed, which causes an increase in wave func-tion overlap between holes and electrons and consequently enhance the IQE of the MQWs.
In order to clarify the influence of QCSE on the LED devices, the electroluminescence (EL) system was employed. Figures 2(a) and 2(b) show the EL spectra under different injection current for the LED devices with the dimensions of 300 300 lm2on GaN NRs template and sapphire substrate,
respectively. The insets of them show the corresponding EL emission peak wavelength as a function of injection current. The emission peak wavelength of NR-LED is slightly red-shifted from C-LED since the effect of strain relaxation may
FIG. 1. Cross-sectional (a) SEM image of GaN NRs template and (b) TEM image of the GaN epilayer overgrown on GaN NRs template.
FIG. 2. The EL emission peak wavelength as a function of injection current for (a) NR-LED and (b) C-LED. (c) Forward voltage and light output power as a function of injection current of NR-LED and C-LED.
slightly increase the indium incorporation rate.10,11 In addi-tion, as the injected current increases, the emission peak wavelength of NR-LED exhibits a blue-shift by 5.25 nm, which is less than 7.03 nm for C-LED. This result reveals that the QCSE is weaker for NR-LED because parts of resid-ual strain had been released through the NELOG. Figure2(c)
shows the power-current-voltage (L-I-V) curves of NR-LED and C-LED. The forward voltages (Vf) at an injected current
of 20 mA for NR-LED and C-LED were 3.35 and 3.38 V, respectively, which reveal the electrical characteristics of these two devices are similar. On the other hand, the light output powers of NR-LED were 33.3% and 65.5% higher than that of C-LED at 20 mA and 100 mA current injection.
The enhancement of light output power for NR-LED can be attributed to several reasons. First, the TDs were greatly reduced by the microscale air voids and the SiO2nanomasks,
which can effectively suppress the formation of nonradiative centers. Second, the weaker QCSE for the MQWs on NRs template enhances the recombination of electron-hole pairs. Third, the LEE for a blue LED with the embedded air voids can be greatly enhanced.7To quantify the light extraction effi-ciency for NR-LED and C-LED, a 3D finite difference time domain (FDTD) simulation was applied by usingFULLWAVETM
program.12 The simulated model of NR-LED includes the 5 5 random NRs with the height of 2 lm and 7.56 lm2
for each unit cell. In addition, the microscale air voids and SiO2
nanomasks were extracted from the SEM image in Figure
1(b). A detector was placed on the p-side of the simulated LED structure to collect the light output intensity emitted
implies that the NR-LED with better light extraction effi-ciency, which results in higher light intensity. The overall enhancement of LEE acquired from the steady-state light in-tensity for NR-LED was 1.37 times higher than that of C-LED. However, the enhancement of LEE might be a little overestimated due to the imperfection of air void shapes and sizes and the random distribution of them in the real GaN epi-layer. This is why the light output power of NR-LED was only 1.33 times higher than that of C-LED at 20 mA.
Moreover, the normalized efficiency of NR-LED and C-LED as a function of injection current is shown in Figure
4(a). The efficiency droop, which is defined by gpeak-g100 mA/
gpeak, for NR-LED (25.9%) is smaller than C-LED (56.1%).
To explore the origin of efficiency droop for NR-LED and C-LED, the APSYS simulation software was used to analyze
the band structure of the green MQWs for both cases.13The simulated parameters, including a Shockley–Read–Hall recombination lifetime and an Auger recombination coeffi-cient were set as 5 ns and 2 1030cm6/s, respectively. The simulated results of efficiency droop for both NR-LED and C-LED, which are also shown in Figure4(a), agreed with the experimental data. Moreover, the corresponding energy band diagram of NR-LED and C-LED at a forward current density 100 mA is shown in Figure 4(b). It is reasonable that the band diagram of C-LED is severely bent due to the high pie-zoelectric field. The severe upward conduction band edge from n-GaN side to MQWs and the triangular MQWs greatly suppressed the radiative recombination of electron-hole pairs. In comparison, the band diagram of NR-LED is more uniform due to less QCSE. As a result, the NR-LED exhibits lower efficiency droop than the conventional one.
In conclusion, we have demonstrated the green InGaN/ GaN MQWs LEDs grown on a GaN NRs template with SiO2
nanomasks. The improvement of EQE and efficiency droop can be attributed to high IQE and LEE for green LEDs on NRs template. The threading dislocations and the residual strain are effectively suppressed by using NELOG. The em-bedded microscale air voids and SiO2 nanomasks not only
improve the crystalline quality but also greatly enhance the LEE of the green LEDs. The corresponding simulated results are agreed well with experimental results.
This work was supported by the National Science Coun-cil NSC 100-3113-E-009-001-CC2 in Taiwan, ROC.
FIG. 3. 3D-FDTD calculated electric-field distribution of NR-LED and C-LED.
FIG. 4. (a) Experimental and simulated EQE as a function of injection current for NR-LED and C-LED. (b) The calcu-lated band diagram of NR-LED and C-LED under a 100 mA forward bias operation.
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