We demonstrate very high luminous efficacy green light-emitting diodes employing Al0.30Ga0.70N cap layer grown on patterned sapphire substrates by metal organic chemical vapor deposition. The peak external quantum efficiency and luminous efficacies were 44.3% and 239 lm/w, respectively. At 20 mA (20 A/cm2) the light output power was 14.3 mW, the forward voltage was 3.5 V, the emission wavelength was 526.6 nm, and the external quantum efficiency was 30.2%. These results are among the highest reported luminous efficacy values for InGaN based green light-emitting diodes.
© 2016 Optical Society of America
Due to its direct and wide bandgap as well as ability to cover the whole visible light spectrum, III-nitrides semiconductors are the most promising candidates for direct emitting color mixing applications [1,2]. However, the performance of InGaN light-emitting diodes (LEDs) depends on its emission wavelength. Commercially viable InGaN-based LEDs devices are mostly limited to violet-blue wavelengths (400-460 nm) for solid state lighting (SSL). The efficiency of InGaN based light-emitting diodes (LEDs) has improved significantly in the blue spectral region since the first blue LED was demonstrated in 1993 . Since the first reports of green emitting InGaN LEDs , these devices have seen use in a variety of commercial applications. However, issues with efficiency droop and overall performance limit their utility in SSL applications.
The lack of efficient LEDs in the green spectral range, known as the “green gap”, is mainly due to the combined effects of high indium compositions, polarization fields, and possibly higher Auger recombination. A relatively high indium content in the InxGa1-xN active region (x>0.20) is required to shift the emission wavelength from the blue to the green spectral region. This requires low growth temperatures or fast growth rate to incorporate a high concentration of indium, which results in increased V-defect density and impurity incorporation [5–7]. Additionally, high indium InGaN on GaN increases film stress due to lattice mismatch leading to additional surface roughing and a breakdown of the surface morphology . Furthermore, piezoelectric-polarization fields are larger in high indium concentration InGaN quantum well (QW) due to the larger lattice mismatch GaN and InGaN, resulting in a reduced electron-hole overlap which leads to a lower spontaneous recombination rate and overall reduction in device efficiency [9–12].
Recent work from our group showed a significant improvement attributed to enhancement in internal quantum efficiency (IQE) after employing AlGaN barriers in the active region of semipolar green laser diode (LD) grown on free standing GaN semipolar substrate (20-21) . LDs with AlGaN barriers demonstrated better crystal quality, uniform emission, and higher device performance compared to devices with InGaN and GaN barriers. Researchers at Toshiba have also fabricated high efficiency green-yellow LEDs utilizing AlGaN interlayers [14,15]. They have demonstrated 532 nm green LED with light output power of 12 mW, external quantum efficiency (EQE) of 25.4% and luminous efficacy of ~80 lm/W at 20 mA (5.5 A/cm2) on 600600 μm2 die. In this paper, we demonstrate the growth and fabrication of high-luminous green LEDs on patterned sapphire substrate (PSS) using high temperature growth of the InGaN QWs immediately followed by a thin AlGaN cap layers and subsequently a higher temperature GaN barrier.
2. Experimental and device description
LED epitaxial layers were grown heteroepitaxially on PSS by atmospheric pressure metal organic chemical vaper deposition (MOCVD). Patterned sapphire substrates have been used for improving light extraction efficiency [16,17]. The LED structure consisted of 1 μm unintentionally doped (UID) GaN template layer, 2 μm Si-doped n-type GaN layer, a thirty period Si-doped superlattice (SL) with 2.5 nm In0.05Ga0.95N and 5 nm GaN layers. The InGaN/GaN SL was capped with a 10 nm undoped GaN layer prior to the growth of active region. The active region was a five MQW and was grown in two steps, where the first step consisted of growing a 3 nm In0.24Ga0.76N QW followed immediately with a 2 nm Al0.30Ga0.70N cap layer at the same temperature. In the second step, a 10 nm GaN barrier was grown at 75 °C temperature higher than the QWs and AlGaN cap layer. After the growth of the GaN barrier, the temperature was decreased for next period of QW. A 10 nm Mg-doped p-Al0.20Ga0.80N electron blocking layer (EBL) was grown on top of the last GaN barrier of the MQW followed by 100 nm Mg-doped p-GaN layer and 2 nm p++-GaN contact layer. Figure 1 shows a cross-sectional schematic of the LED structure. The thicknesses and compositions of each layer were verified via x-ray diffraction (XRD) from separate calibration samples. The structural quality of the layers was examined using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM).
For the LED fabrication, a 110 nm Sn-doped In2O3 (ITO) current-spreading layer was deposited by electron beam deposition on the contact layer due to the high resistivity of p-GaN. Rectangular mesa pattern with active area of 0.1 mm2 were formed by photolithography and inductively coupled plasma (ICP) etching with etch rate of 300 nm/min. Next, a 20/100/20/100nm Ti/Al/Ni/Au metal stack was deposited to form contacts to the exposed n-GaN layers. Finally, a 20/20/300 nm Cr/Ni/Au p-contacts was deposited on ITO. Figure 1shows a cross sectional schematic of the fabricated structure. For improving the light extraction efficiency, the wafer was diced and then packaged with a transparent vertical stand method .
3. Results and discussion
Following the MOCVD growth, the quality of the layers was inspected by HAADF-STEM. Figure 2 shows a cross section of the LED structure which exhibited excellent structural quality and thickness uniformity through the entire stack. A higher magnification scan of the active region in the inset shows an abrupt heterointerface between the AlGaN cap layer (darker contrast) and InGaN QW (lighter contrast). This result indicates that the AlGaN cap layers prevents indium desorption and QW composition fluctuations during the growth of the high temperature GaN barrier. Additionally, the AlGaN cap layers may improve the overall quality of the MQW by compensating the compressive strain in the InGaN QWs [19,20]. Further study is needed to investigate the strain compensation of very thin AlGaN layer on the InGaN QW.
Room-temperature electroluminescence (EL) measurements under direct current (DC) were performed in an integrating sphere. The dependence of light output power and forward voltage on current are shown in Fig. 3(a). At 20 mA (20 A/cm2), the output power and forward voltage were 14.2 mW and 3.5 V, respectively. Although the voltage is slightly high, it can be improved by optimizing the growth conditions and fabrication process. Figure 3(b) shows the EQE of the device at increasing DC drive current. A peak EQE of 44% at 2 mA was achieved. At 20 mA, the EQE was 30.2%.The LED exhibited a significant amount of efficiency droop, which is typical in long wavelength devices [9,21]. This magnitude of droop is comparable to results from Toshiba for green-yellow LEDs .
Figure 4(a) shows the EL peak wavelength at various current. The LED showed a blue shift in emission wavelength from 526.6 nm at 20 mA to 516 nm at 100 mA, with a total wavelength shift of 11 nm over a 80 mA range. This amount of shift is comparable to a commercial c-plane green LED which had a 12 nm total shift over same range of current density . Figure 4(b) shows the EL spectra with narrow FWHM at two different drive currents. The FWHM are 32.7 nm and 37.5 nm at 20 mA and 100 mA, respectively. The relatively small FWHM of the LED indicates QWs had minimal potential profile fluctuations.
The luminous flux and efficacy of the green LED are shown in Fig. 5. An extremely luminous efficacy of 239 lm/W was achieved at 1 mA. At 5 mA, a luminous efficacy of 175 lm/W was achieved. However, the luminous efficacy rapidly decreased with increasing current, caused by the increase in forward voltage, heating, and efficiency droop.
The AlGaN cap layer has been proposed to increase the piezoelectric-polarization induced electric field, which might increase the droop of the device . This can be explained by the simple ABC model, where A, B and C are the coefficients of Shockley-Read-Hall (SRH), radiative, and Auger recombination, respectively. The induced electric field leads to spatially separate the electron and hole wavefunctions which reduce the wavefunction overlap. The current density in the device can be expressed as J = qd (An + Bn2 + Cn3). It is clear that at given J, n will increase when the electric field in the QW increased. The Auger recombination will begin to dominate, since it depends on the cube of the carrier density, which will lead to more efficiency droop .
By employing and optimizing the AlGaN capping layers in the active region, we have demonstrated a high luminous efficacy LED on c-plane PSS with emission wavelength in the green gap spectral region. These LED showed a high luminous efficacy and EQE of 239 lm/W and 44% respectively. At 20 mA (20 A/cm2) under DC operation, the light output power and EQE were 14.2 mW and 30.2%. The emission wavelength was 526.7 nm with narrow FWHM of 32.7 nm.
KACST-KAUST-UCSB Solid State Lighting Program (SSLP) and the Solid State Lighting & Energy Electronics Center (SSLEEC); National Science Foundation (NSF) Nanotechnology Infrastructure Network (NNIN) (ECS-0335765).
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