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The light-emitting diode (LED) with an improved hole injection and straightforward process integration is proposed. p-type GaN direct hole injection plugs (DHIPs) are formed on locally etched multiple-quantum wells (MQWs) by epitaxial lateral overgrowth (ELO) method. We confirm that the optical output power is increased up to 23.2% at an operating current density of 100 A/cm2. Furthermore, in order to identify the origin of improvement in optical performance, the transient light decay time and light intensity distribution characteristics were analyzed on the DHIP LED devices. Through the calculation of the electroluminescence (EL) decay time, internal quantum efficiency (IQE) is extracted along with the recombination parameters, which reveals that the DHIPs have a significant effect on enhancement of radiative recombination and reduction of efficiency droop. Furthermore, the mapping PL reveals that the DHIP LED also has a potential to improve the light extraction efficiency by hexagonal pyramid shaped DHIPs.
© 2017 Optical Society of America
1. Introduction
The nitride-based light-emitting diodes (LEDs) have been widely used for various optical applications thanks to rapid improvements in epitaxial and novel structuring techniques. Especially, the active efforts to enhance electrical/optical conversion efficiency and product lifetime have made LED popular in large scale ranging from a little toy up to automotive lamp and city lighting where cost-effective and energy-efficient light sources are of paramount importance [1, 2].
In recent years, needs on high driving current LED are growing with increasing demands on high brightness lighting products. However, in order to operate the LED at high driving current, the chronic issue of efficiency droop should be solved. The efficiency droop is closely linked with nitride-based material properties [3–6]. The main causes underlying efficiency droop are known to include quantum confinement stark effect (QCSE) by piezoelectric field, Auger recombination, indium composition fluctuation, and different carrier distribution in quantum wells [7–11]. When the number of holes delivered from electrode to a quantum well (QW) is insufficiently small, radiative recombination occurs near the p-GaN-side QW, which leads to electron overflow from the QW and efficiency lowering. Numerous researches have been focusing their efforts to make a breakthrough out of this technical issue. For instance, in order to improve the hole injection into the multiple quantum wells (MQWs) by engineering the epitaxial structure, V-pit formation around threading dislocations (TDs) which inject holes into QWs by tunneling [12], electron blocking layer (EBL) [13–17], and graded barrier and stage relief structure [18–20] have been widely studied. By adapting these epitaxial structures, an electron overflow can be blocked and the probability of electron-hole recombination at the QWs is also increased by lowering polarization field. Furthermore, the formation of microhole and nanopillars on top of epitaxy layer is also suggested to improve the light extraction by reducing total reflection of light and strain relaxation in the MQWs [21–25]. More recently, nanopatterning and regrowth technique was proposed in one of our previous works to achieve strain relief and higher lateral hole injection at the same time [26]. However, in constructing the nanostructures by either widely used photolithography or e-beam lithography, there can be issues of non-uniform patterning over the entire wafer due to inevitable curvatures by lattice mismatch and difference in thermal expansion coefficients between the epitaxial layers and the substrate, or even among the epitaxial layers themselves. Also, it requires high cost for the fabrication process.
In this work, compared with the previous ones, we propose and demonstrate a high-efficiency LED device having hexagonal pyramid shaped p-type direct hole injection plug (DHIP) which is compatible with most of existing epitaxial techniques including EBL and strain relief layer. We confirmed that DHIP structuring not only enhances the hole injection into the MQWs but also increases the light extraction efficiency simultaneously, based on cost-effective fabrication architecture employing simple photolithography.
2. Fabrication of DHIP LED device
Figure 1 shows the fabrication process of the suggested p-type GaN LED with DHIP structures. 2-inch c-plane sapphire wafers are prepared for device fabrication and the peak wavelength is targeted at 445 nm. Here, whole epitaxial structure has u-GaN/n-GaN/n+-GaN/n-GaN (thickness = 2000 nm/1500 nm/2000 nm/6 nm), four pairs of InGaN/GaN (thickness = 3 nm/12 nm) MQW and p-GaN (thickness = 150 nm). In order to selectively fill the p-GaN on the etched MQWs, SiO2 mask (thickness = 200 nm) is deposited on the p-GaN by plasma-enhanced chemical vapor deposition (PECVD). Then, the DHIP split patterns (radius = 2 μm to 3.5 μm, total number of DHIP = 232 to 4,680 by controlling the plug pitch) are constructed by photolithography. Next, dry etching of the SiO2 layer and the MQWs are carried out using CF4 and mixture of Cl2 and BCl3 gas, respectively, to form holes where the DHIPs are regrown. Here, three QWs remain intact during the dry etch of MQWs in order to prevent loss of active region and excessive overflow of carriers leading to leakage current at high operating current. Although the measured thickness of epitaxial layers is used to adjust the etching depth of DHIP in time control based etch process, it can be automatically adjusted by detecting indium ions in the dry etch equipment. After forming the holes through the MQWs, the holes are selectively filled by p-GaN epitaxy using the same condition for plannar p-GaN (reactor pressure = 200 mbar and temperature = 98°C) [27–30]. Then, the SiO2 mask is removed by buffered HF (BHF) solution. After construction of the DHIPs, fabrication of LED devices is proceeded by the lateral LED fabrication process. The active area of a device is 440 μm × 370 μm and indium tin oxide (ITO) transparent p-electrode metal is deposited on all the active regions except the mesa-etched n-GaN regions. Therefore, the ITO layer operates as p-electrode governing the p-GaN and the DHIP together.
In order to confirm the structural formation of DHIP, the regrown structure was closely inspected by a scanning electron microscope (SEM) and a transmission electron microscope (TEM) cooperatively. Figure 2(a) shows the SEM image of MQWs partly etched in the depth direction. All the plug holes are filled by the p-GaN epitaxy and hexagonal pyramid islands are formed due to the facet directions as shown in Fig. 2(b). In the epitaxial lateral overgrowth (ELO) process, the shape of regrown pattern is changed by growth temperature and gas pressure [28]. To fill the plug patterns, the same process conditions for growing p-GaN in the early stage were adopted again. The etching is verified to reach the wanted depth as demonstrated in Figs. 2(c) and 2(d). The DHIP is regrown on the 2nd QW and completely filled with p-GaN without any void. Through this precisely depth-controlled p-GaN contact on the 2nd QW, increased amount of hole injection is made possible from the DHIP edge to the 1st and the 2nd QW sides as schematically shown in Fig. 1(g). Therefore, we expected that the light output power is associated with total DHIP circumference (DHIP number × DHIP circumference). From the measurement data, the correlations between the total DHIP circumference versus light output power can be analyzed.
Fig. 2 SEM and TEM images from the fabricated DHIP structures. (a) Holes dry-etched by ICP-RIE method for DHIP structures. (b) p-GaN islands regrown by MOCVD. (c) Cross-sectional view of a DHIP structure. (d) Enlarged image of Fig. 1(c). MQW has been partially etched above the 2nd QW successfully.
3. Results and discussion
It is expected that LED device with DHIPs would show better performance than that without DHIPs. Thus, we prepared a reference LED device (REF LED) to make a fair comparison. The REF LED went through the same fabrication process, in the same critical dimensions, with the DHIP LEDs but has no DHIP structures. Further, in order to study the effect of the total circumference of DHIPs, DHIP LED devices having different numbers of DHIPs were fabricated at the same time, where the DHIPs have the same radius of 3 μm. Thus, the final independent variable is the circumference of all the DHIPs and its effects on optical performances are investigated. Figure 3(a) shows the light output power as a function of current density. The light output power shows a steeper increase in the DHIP LED than in the REF LED. The difference in light output power becomes more noticeable in the high current region above 35 A/cm2. Compared with the DHIP LED with circumference of 104.2 mm, DHIP LED with the total circumference of 62.6 mm shows a consistently higher output power. The DHIP LED with the total circumference 62.6 mm demonstrates 23.2% increase in light output power at 100A/cm2. The efficiency droop is improved phenomenally as shown in Fig. 3(b). Nevertheless, as shown in Fig. 3(c), the reverse leakage current is increased with circumference of DHIP. It seems that the leakage path originates from the plasma damage and defects along the DHIP edge. Therefore, the total length of DHIP edge needs to be minimized to achieve higher optical output power and lower leakage current at the same time. In case of forward current-voltage (I-V) curves, the forward voltage is decreased by 0.25 V at a constant current density of 100 A/cm2 due to improvement of current conduction through the DHIPs.
Fig. 3 Comparison of optical and electrical performances among the REF LED and DHIP LED having total circumference of 62.6 mm and 104.2 mm. DHIP LEDs have same DHIP radius of 3 μm (a) Measured light output power vs. current density. (b) Wall plug efficiency vs. current density. (c) Reverse leakage current density vs. anode voltage. (d) Forward current density vs. anode voltage.
In order to confirm that the enhancement of light output power actually comes from the DHIPs, electroluminescence (EL) intensity distributions were analyzed under different current density conditions. Due to the different tendencies of light output power at low and high current density regimes, the analyses were performed at 4 A/cm2 and 75 A/cm2, separately. For fair comparison, the image capturing options were fixed and all the intensities were normalized in the same scale. As shown in Figs. 4(a) and 4(c), the EL intensity of DHIP LED device is lower than that of REF LED at low current injection. However, at high current injection, it is clearly demonstrated by Figs. 4(b) and 4(d) that the EL intensity over the entire device of DHIP LED is higher than that of REF LED and high-intensity spots are more prominent around the plugs in the DHIP LED device. Also, the current spreading is more effective in case of DHIP LED due to the distributed hole and electron conduction paths by densely and regularly arranged plugs over the entire active region.
Fig. 4 Light intensity distribution images from REF LED at current densities of (a) 4 A/cm2 and (b) 75 A/cm2 and those from DHIP LED at (c) 4 A/cm2 and (d) 75 A/cm2. (images were captured by microscope camera TUCSEN ISH300 and analyzed by EtaMax light distinct viewer).
Although it is confirmed that the EL intensity is significantly increased with the help of DHIP structures, there is a slight decrease in the optical power when the total number of DHIPs is increased. In order to perform an analysis on this unexpected result, ABC-model that can discriminate Shockley-Read-Hall (SRH) recombination (A), radiative recombination (B), and Auger recombination (C) coefficients is employed [31–34]. The EL intensity decay characteristics were analyzed to extract the recombination parameters which could be calculated by solving the current continuity equation in Eqs. (1a)-(1c). Equations (1b) and (1c) represent the current continuity equation and its solution with the initial condition, respectively. The initial carrier concentration (n0) at initial time (t0) can be calculated from Eq. (2b). Finally, by fitting the calculated EL decay characteristics on the measured curve, A, B, and C values can be extracted [35].
The current pulse is applied with amplitude of 30 A/cm2 for duration of 4 μs during a period of 6 μs to characterize EL decay. Figures 5(a)–5(c) shows the EL decaying of the REF LED and DHIP LED devices. The fitted curves using the theoretical decay characteristics show good agreements with the measurement results in all cases. As shown in the figures, DHIP LEDs show steeper decay than REF devices. By fitting the theoretical decay characteristics, recombination parameters can be determined as listed in Table 1. Among the coefficients, the SRH recombination coefficient increases as the circumference of DHIPs increases from 62.6 mm to 104.2 mm. This result implies that the regrown DHIP regions contain more point defects compared with the intact p-GaN region due to the remaining oxide atoms and plasma damages on the etched GaN surfaces. Also, unwanted Mg diffusion can take place through the side surfaces of QWs during the p-GaN regrowth process, by which non-radiative recombination can be caused. Consequently, the increased non-radiative recombination centers can lower the radiative recombination probability at low current density. On the other hand, radiative recombination has the predominance and point defects do not significantly affect the efficiency droop at high current density [36].
Fig. 5 Measured and fitted EL decay curves from (a) REF LED, (b) DHIP LED having radius of 3 μm and total circumference of 104.2 mm, and (c) DHIP LED having radius of 3 μm and total circumference of 62.6 mm.
Table 1. Calculated recombination parameters (A, SRH recombination coefficient; B, radiative recombination coefficient; C, Auger recombination coefficient).
Radiative recombination coefficient B’s of DHIP LEDs are 33% higher than that of REF LED owing to lateral hole injection. Therefore, it is clear that higher B values of DHIP LEDs contributes higher EL intensities of the devices than those of reference LED and higher A value of DHIP LED with the long circumference results in its lower EL intensity compared with the device with a short circumference. Through the extracted A, B, and C values, the internal quantum efficiency (IQE) is calculated by Eq. (2a) considering the effective volume (Vactive) of QWs formulated as Eq. (2b). The IQE also improved as shown in Fig. 6, where the same trend with the wall plug efficiency (WPE) is observed. It is found that the DHIP structure significantly improves the optical output power and increases the local defect density in the vicinity of the plugs at the same time. Although the latter effect would not be so apparent in the practical operating current region, total circumference of plugs can be optimized for better performance.
Fig. 6 Extracted IQE vs. current density curves of reference LED device and DHIP devices with different circumferences, obtained from the EL intensity decay times.
For the optimization task, we expanded the experiments to larger number of DHIP LED devices having the different circumference of DHIPs with the radius of 2 μm to 3.5 μm to set DHIP circumference as the independent variable in the new analysis. Figure 7 demonstrates that the optimal DHIP circumference is located near 70 mm according to the measurement data and the fitting curve. The injection enhancement effect by larger DHIP circumference is dominant up to the optimal point but the non-radiative recombination effect by increased point defects becomes more prominent above the point. Thus, the DHIP LED should be optimally controlled considering the total plug circumference in practice.
Fig. 7 Light output power vs. total DHIP circumference at current density of 75 A/cm2. It is confirmed that the optimal circumference length exists and is located near 70 mm.
In order to check the light extraction efficiency enhancement by the pyramids, we measured the mapping photoluminescence (mapping PL). Since the mapping PL measurement does not inject holes into MQWs directly through p-electrode, we can exclude the hole injection effect of DHIP. Figure 8 shows the mapping PL result, by which it is revealed that the PL intensity is improved due to the reduced total reflection of light. The inset of Fig. 8 represents the table of [DHIP/active area] ratio showing that the PL intensity increases with density of DHIP. In our experiment, however, the enhanced light extraction efficiency due to the pyramid structure does not seem to play an important role in improving the brightness. A crucial evidence is shown in Fig. 4(d). In this figure, it is clear that the EL intensity is improved significantly in the region around DHIP, not DHIP itself. Since the light generated within DHIP region is much weaker than the light generated outside the DHIP, the higher extraction efficiency of the pyramid structure may not contribute to the brightness significantly. Another evidence against the brightness improvement due to the pyramid structure is the fact that the brightness does not increase as a function of DHIP area. For example, the brightest LED with DHIP circumference of 62.6 mm have only 4% DHIP area out of the active area, while an LED with much lower external efficiency takes 20%. Therefore, it can be concluded that the light output power is improved mainly by hole injection effect. In order to further optimize the light extraction efficiency of pyramid structures, DHIP height should be lowered to reduce the series resistance along the thick regrown p-GaN (DHIP height = 3 μm in this experiment) and the cleaning process after the plasma hole etching prior to DHIP regrowth can be considered.
4. Conclusion
In this work, p-type GaN DHIP structure has been introduced in the GaN MQW LED for improving the hole injection efficiency, and the straightforward fabrication architecture and the characterization results have been demonstrated. It is confirmed from LI measurements that 23.2% enhancement in the light output power is achieved at 100 A/cm2 by the novel device structuring. In order to analyze the lateral hole injection efficacy in the DHIP LEDs, EL intensity distribution was high-precision mapped and revealed that the increased EL intensity attributes to the DHIPs. Also, the EL characteristics of DHIP LEDs are specifically investigated by EL decay measurements and parameter extraction for IQE evaluation. According to the calculated IQEs, density of dislocations induced by epitaxial regrowth should be minimized in designing the device structure and the process integration due to its effect of increasing non-radiative recombination under the low-current operating condition. It was found from the expanded experiments that the optimum total DHIP circumference is located around 70 mm and the hexagonal pyramid shaped DHIP has the potential to extract more lights from the MQWs.
Funding
Ministry of Science, ICT and Future Planning (MSIP) (501100003621); Global Frontier Project (CISS-2012M3A6A6054186)
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