1. Introduction

GaN-based light-emitting diodes (LEDs) are attracting considerable attention and are widely applied in display technology, back lighting and general illumination [1]. Although very high luminous efficacy has been achieved for InGaN blue LEDs [1], this efficacy has shown to drop rapidly especially as the emission wavelength goes into green (λ = 510~570 nm) spectral region, or the called “green gap” [2]. The quantum-confined Stark effect (QCSE) induced by internal piezoelectric fields with a magnitude of ~MV/cm associated with the in-plane strain in InGaN/GaN multiple quantum wells (MQWs), decreases the overlap of electron and hole wave-functions and the internal quantum efficiency (IQE), which is more severe for the InGaN green LEDs [3]. Another cause is the presence of non-radiative defects in the high In composition InGaN material [4,5].

Several specific structure designs have been proposed to enhance the IQE for the green LEDs [2,6–17], such as employing semipolar InGaN QWs [2,6], InGaN nanowire structure [7–9], InGaN quantum dots [10], staggered InGaN QWs [11], graded MQWs [12], InGaN-delta InN QWs [13] and InGaN shallow QWs structure [14]. The green LEDs grown on semipolar bulk GaN substrate is promising due to the alleviation of the electrostatic field and QCSE but the reported external quantum efficiency (EQE) is still less than 20%, possible related to Shockley-Read-Hall (SRH) non-radiative recombination [2]. The nanowire based InGaN green LEDs, which related to a complicated material growth and device fabrication, also show a low EQE [10–12]. A-J. Tzou et al. reported a high efficiency green LEDs using quaternary InAlGaN/GaN superlattice (SLs) electron blocking layer [17]. Nevertheless, the material growth of high crystal quality quaternary InAlGaN/GaN SLs is still difficult. Therefore, the realization of highly efficient InGaN-based green LEDs with high EQE and WPE remains challenging and highly demanding.

In this study, we demonstrate very high performance of InGaN green LEDs grown on low cost patterned sapphire substrate (PSS) using metal organic chemical vapor deposition (MOCVD). Very high luminous efficient 527 nm InGaN green LEDs were achieved. Materials characterizations by high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) were carried out and the optical and electrical properties of the fabricated devices were discussed.

2. Experiments

The InGaN-based green LEDs were grown on low cost c-plane (0001) PSS (around 10$/2 inch) by MOCVD. Trimethylgallium (TMGa), triethylgallium (TEGa), trimethylindium (TMIn), ammonia (NH₃), silane (SiH₄) and bicyclopentadienyl (Cp₂Mg) sources were used as precursors and dopants. The schematic epitaxial structure is consisted of a 30 nm low-temperature GaN nucleation layer, a 4.0 μm undoped GaN, a 3.0 μm Si-doped nGaN layer, 12 pairs In_0.08Ga_0.92N/GaN (2 nm/7 nm) SLs, 16 pairs In_0.25Ga_0.75N QWs/GaN quantum barriers (QBs) (3 nm/12 nm), a 3 nm thin undoped AlGaN cap layer (AlGaN1) on top of last QB, a 40 nm Mg-doped pGaN (pGaN1), a 16 nm p-type AlGaN electron blocking layer (AlGaN2), a 200 nm pGaN (pGaN2), a 15 nm Mg heavily doped p + GaN and a 2 nm p-InGaN layer [18]. The TMIn and TEGa flow switch diagram and temperature profile during MQWs growth are shown in the following Fig. 1, where the growth strategy can be well understood. Firstly, a very thin ~1 nm GaN layer was deposited with a V/III ratio of 21500 and a 3 nm InGaN QW (In/Ga ratio: 0.66) was grown at the same temperature of 755°C. After the growth of each QW layers, TEGa was introduced into the reactor for 20 more seconds and it kept opening during the temperature ramping up by 50 °C, to form a cap layer. Finally, a GaN barrier was grown at a temperature of 935°C with a V/III ratio of 10820. A very low growth rate of 0.17 Å/s was used for the growth of the QWs. The 12 nm thick GaN barrier could help suppress the strain relaxation within the QW [19]. A 3 nm thin undoped AlGaN cap layer on top of last QB, which likely compensates the compressive strain induced by the QWs. The pGaN1 was grown at a relatively lower temperature than that of QBs to protect the InGaN QWs from indium desorption during the growth of the pAlGaN electron blocking layer and p-type layers [20]. LEDs devices with a size of 20 × 40 mil² (0.5 mm²) were fabricated using a conventional mesa structure. The nGaN layer was initially exposed by inductively coupled plasma (ICP) etching. Indium Tin Oxide (ITO) and Cr/Al/Cr/Pt/Au metals were deposited on top of pGaN and nGaN as ohmic contact layer, respectively. SiO₂ layer was deposited on the side walls and Au metals were evaporated as p/n contact pads. A patterned design of metal electrodes with multiple-branches was employed to avoid the current crowding effect [21]. Finally, the LEDs were diced, packaged on silver header, encapsulated with silicone, mounted in the stage and measured in a calibrated integrating sphere (INSTRUMENT SYSTEMS, MODEL: ISP500-220). There is a temperature control to keep a constant temperature of 25°C for the stage to make sure the accurate measurement of the output power in the integrating sphere. The fabricated green LEDs chip at low injection of 3 mA and the packaged devices are shown in Figs. 2(a) and 6(b), respectively. Very uniform green light emission can be observed due to the multiple fingers design of p/n electrodes, indicating a good current spreading effect.

 

Fig. 1 TMIn and TEGa flow switch diagram and temperature profile during MQWs growth.

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Fig. 2 (a) Green emission of the fabricated green LEDs at low injection current of 3 mA and (b) The packaged green LEDs.

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3. Results and discussion

High crystalline quality of GaN material was grown on PSS and X-ray diffraction (XRD) rocking curves show that the FWHM along [0002] and [10–12] was 240 and 225 arcsec, respectively, as shown in Fig. 3(a). XRD reciprocal space mapping (RSM) measured along [10–15] is plotted in Fig. 3(b). The reciprocal lattice point (RLP) of the GaN layer and the InGaN satellite peaks stand in a straight line, indicating the InGaN layers grown on the GaN template were fully strained. Narrow InGaN satellite peaks by RSM from −5 to + 2 orders can be clearly observed, suggesting high crystalline quality and good periodicity of the InGaN/GaN MQWs.

 

Fig. 3 (a) XRD rocking curve along [0002] and [10–12]; (b) RSM measured along [10–15].

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HRTEM was performed to investigate the detail of p-type layers and active region, as presented in Figs. 4(a) and 4(b). The alternately stacked bright and dark layers correspond to the (Al)GaN layers and InGaN QWs with abrupt interface and the thicknesses of the InGaN QWs and GaN QBs are measured to be around 3.1 nm and 12 nm, respectively. A linear scanning energy-dispersive X-ray spectroscopy analysis (EDXA) on the Al and In composition distribution is carried out across the p-type layers/AlGaN layers/the first two InGaN QWs and the composition of the In and Al is labeled in Fig. 4(c). The atomic percentage for Al and In compositions are not normalized here. Two Al peaks related to two AlGaN layers can be obviously observed from the Al/In atom profile, which shows highly consistence with the originally designed structure. The high crystal quality, good periodic and sharp interface of the InGaN/GaN MQWs are confirmed by the narrow satellite peaks from −5 up to + 2 orders by the (002) ω-2theta scan in Fig. 4(d), which agrees well with the RSM results and TEM result in Fig. 4(b). This growth strategy of the InGaN/GaN MQWs provides a high crystal quality of InGaN/GaN green MQWs as well as a high IQE, which is very important for the realization of high efficiency green LEDs as described later.

 

Fig. 4 (a) HRTEM image of the p-type layers/AlGaN layers/the first two InGaN QW; (b) HRTEM results of InGaN/GaN MQWs; (c) X-ray spectroscopy analysis (EDXA) on the Al and In composition distribution profile across the area labeled in (a); (d) (002) XRD ω-2theta scan.

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Al-containing layer on top of InGaN QWs has been proposed to balance the strain within the InGaN MQWs in previous study [22,23]. To investigate the effect of the single 3 nm thick AlGaN1 layer on top of the last GaN barrier in compensating the strain, APSYS simulation program is carried out on our green LEDs and the conventional LEDs as a reference [17,20]. Firstly, a comparison of the calculated electrostatic fields at 20 A/cm² is shown in the Fig. 5(a). It can be seen that the electrostatic field in the last QW of our green LEDs was decreased from −1.55 MV/cm to −1.21 MV/cm, suggesting the strain compensation effect by the single 3 nm AlGaN1 on top of last GaN QB [21]. Moreover, from the band energy in Figs. 5(b) and 5(c), the effective potential height for hole in valence band (Δφ_h) can be reduced from 691 meV to 378meV, while the effective potential height for electron in conductive band (Δφ_e) is increased from 261 meV to 371 meV, which are helpful for the hole injection and electron confinement [17,20].

 

Fig. 5 (a) A comparison of the calculated electrostatic fields at 20 A/cm². Band diagrams of (b) conventionial green LEDs and (c) our green LEDs.

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The electroluminescence (EL) spectrum of the InGaN green LEDs at 20 A/cm² is described in Fig. 6(a). The peak emission wavelength and the FWHM is 527 nm and 31.7 nm, respectively. The dependence of peak emission wavelength and FWHM on the injection current is demonstrated in Fig. 6(b). The emission wavelength shows a large blue-shift from 541 nm to 523 nm with a current increasing from 2 mA to 200 mA, which is caused by the large piezoelectric polarization electric field within the MQWs along c-plane orientation. The FWHM is narrow at a low injection current and then increases linearly with the current, which can be well explained by the combined screening of the piezoelectric electric field and carriers filling of localized states [24,25].

 

Fig. 6 (a) EL spectrum of the InGaN green LEDs at 20 A/cm² and (b) Emission wavelength and FWHM versus current.

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The current-power and current-voltage (L-I-V) characteristics are presented in Fig. 7. At 100 mA, the output power is 92 mW and the forward voltage is extremely low as 2.86 V. Such low forward voltage is ascribed to the good current spreading effect by the multiple-fingers electrodes of the fabricated chips.

 

Fig. 7 Current-power and current-voltage (L-I-V) characteristics.

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Finally, the dependence of EQE, WPE and electrical efficiency (EE) on the driven current is shown in Fig. 8(a). A peak EQE of 53.3% and a peak WPE of 54.1% are achieved at round 5 mA, which is the firstly reported InGaN-based green LEDs with both EQE and WPE exceeding 50% in the literatures. Such high efficiency is mainly caused by the high material quality of the InGaN MQWs in our growth strategy and the improvement of hole injection and electron confinement by the special p-type layers design. The high EQE of our green LEDs means a high IQE of the InGaN/GaN active region, which agrees well with the previous results of the high crystalline quality by the XRD ω-2theta scan and RSM. It is more attractive of our high efficiency green LEDs since only traditional InGaN/GaN MQW system grown on c-plane substrate is employed. Further characterization of the materials quality of the InGaN MQWs would be carried out later. Both EQE and WPE exhibit significant droop with increasing current, which is typically observed in long wavelength InGaN LEDs on c-plane substrate. This magnitude of droop is comparable to previous results for green LEDs [16], which is primarily caused by the fundamental non-radiative Auger recombination [26]. At a conventional working current density (J) of 20 A/cm², the green LEDs exhibit a very high EQE of 38.4% and a WPE of 32.1%. At very current density, the EE is more than unit and similar observation has been reported in literatures [27,28]. Meanwhile, the luminous flux and efficacy of the green LEDs are plotted in Fig. 8(b). An extremely high peak luminous efficacy of 329 lm/W is achieved at 5 mA. At a J of 20 A/cm², the green LEDs show a luminous flux of 45 lm and a high luminous efficacy of 156 lm/W, following by a color temperature of 7886 K. To our best knowledge, these values are among the best reported luminous efficacy for InGaN-based green LEDs. Especially, it is worth to point out that our growth strategy provides many benefits, such as low cost, easier control and high efficiency, without involving any complicated process, as compared to the nanowire based green LEDs and semipolar green LEDs on sapphire template et, al.

 

Fig. 8 (a) Dependence of EQE and WPE on driven current; (b) Luminous flux and efficacy of the InGaN green LEDs at various currents.

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A very high efficiency can be realized by operating the devices at a low current density for LEDs grown on c-plane. A very high EQE and WPE more than 50% and a luminous efficacy of 259 lm/W can be achieved at a low J of 4 A/cm² with an output power of 24 mW. Our green LEDs show a much higher EQE and WPE at low current density as compared to the one reported in the literatures, which is a significant improvement for the efficiency of green LEDs. In general, it is very promising to produce these InGaN green LEDs on low cost substrates with very high efficiency by operating the LEDs at low current density.

4. Summary

In summary, we demonstrate an easier way to achieve very high efficient InGaN green LEDs with a peak EQE of 53.3%, a peak WPE of 54.1% and a peak luminous efficacy of 329 lm/W. Operating the very low cost InGaN green LEDs on PSS at a low J of 4 A/cm² leads to an ultra-high efficiency with both EQE and WPE more than 50%. This demonstration presents significant progress in exploring the efficiency capability of InGaN-based green LEDs on c-plane.