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Abstract
A specially designed InGaN/GaN superlattice (SL) interlayer was inserted between n-GaN and a multiple quantum well to enhance the performance of yellow light-emitting diodes (LEDs) grown on Si (111). The number of SL periods was determined to be the key to enhancing the external quantum efficiency and reducing forward voltage. Our results show that more SLs could suppress nonradiative recombination by eliminating micron-scale indium-rich clusters and could promote hole injection with increased V-pit size. However, too many SLs reduce the effective luminescence area and lead to many voids formed in the p-type layer. We demonstrate that 32 is the optimum number of SLs for yellow InGaN/GaN LEDs, obtaining a high light output power of 63 mW with a dominant wavelength of 568 nm, and a low forward voltage of 2.38 V at 200 mA (20 A/cm2).
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Currently, phosphor-conversion white-light emitters based on InGaN blue light emitting diodes (LEDs) are widely used for solid-state lighting because their efficiencies are much higher than those of incandescent and fluorescent lamps [1]. However, there is still room for improvement in their power efficiency and spectrum quality. Therefore, color-mixing white LEDs have been proposed, which have great potential for higher efficiency, quality, and intelligence than current phosphor-conversion white-light emitters. It is well known that yellow is one of the basic colors. However, yellow LEDs still suffer from limited quantum efficiency and high forward voltage. Generally, the inefficiency of yellow InGaN-based LEDs can be attributed to their high indium content. To incorporate more indium, a lower temperature is required in the growth process of InGaN, leading to more defects [2,3]. Moreover, more indium would enhance indium phase separation [4] owing to the finite solubility of InN in GaN. Additionally, the electron–hole wave function overlap would be reduced owing to the enhanced quantum confined Stark effect (QCSE) in high-indium-content multiple quantum wells (MQWs). All these factors impair the performance of yellow InGaN LEDs. At present, only c-plane GaN is used in the industrial production of LEDs; hence, c-plane substrates have attracted significant attention. Some approaches based on c-plane substrates have been suggested to improve the efficiency of InGaN LEDs in the green or yellow range, including structure modifications [5–7] and optimization of the growth conditions of MQWs [8–11]. A high-output-power LED of 11.0 mW and 559 nm at 20 mA has been reported, but with a high forward voltage of 5.71 V [10], whereas a 560-nm LED of normal forward voltage (3.39 V) has been achieved at the same current, but with relatively lower output power (2.14 mW) [7]. Additionally, R. Hashimoto et al. achieved a 570-nm yellow LED of 8.4 mW at an injection current of 20 mA, but did not report its forward voltage [11].
Our group has devoted many efforts to growing yellow InGaN-based LEDs on silicon (111) substrates, and primary production has been gained. In this work, we employed an InGaN/GaN superlattice (SL) interlayer. To develop high-efficiency InGaN-based LEDs, the SL structure is often used between n-GaN and MQWs as a strain-relief layer [12–17] or a current spreading layer [18,19]. Some other mechanisms of SLs have also been reported, such as the reduction of dislocations in MQWs by bending threading dislocation lines [20] and determining the diameter of V-pits, which could lead to a higher potential barrier around dislocations and to a higher hole injection efficiency [21,22]. Besides the SLs located between the MQWs and n-GaN, SLs were also positioned between MQWs and the electron barrier layer (EBL) to improve hole injection [23,24]. Overall, the SL structures play an important role in the performances of GaN-based optoelectronic devices. However, most previous related works were based on blue LEDs grown on sapphire substrates, and recent researches were conducted green LEDs, whereas studies on yellow LEDs and other substrates are relatively sparse. Moreover, with the increase of In content, the strain state of GaN would be changed and the influence of the SL interlayer may become more complicated considering the relatively inferior crystalline quality and more severe In segregation [25]. In this work, we focused on the influences of the number of SL periods on the performance of yellow InGaN/GaN LEDs grown on silicon substrates. A specially designed InGaN/GaN SL interlayer with a wide well and a thin barrier was inserted between n-GaN and the MQWs. We observed that the increase in the number of SL periods could help to eliminate the micrometer-scale large In-rich clusters, promote hole injection with increased V-pit size, and lead to higher efficiency and lower voltage.
2. Experiments
The samples studied in this work were grown on patterned Si (111) substrates by metal–organic chemical vapor deposition (MOCVD). As shown in Fig. 1, epitaxy started from an 110-nm high-temperature AlN buffer layer. Then, three-dimensional GaN islands were grown on the AlN buffer layer for dislocation reduction and strain relaxation, followed by a GaN recovery layer to coalesce the separated islands. Subsequently, a 2.4-μm n-type GaN layer with Si doping of 5 × 1018 cm−3 was grown. After that, a 10-nm low-temperature GaN (LT-GaN) layer was deposited, followed by a SLs interlayer with multi-periods of 5 nm In0.07Ga0.93N/1 nm GaN grown with no doping. V-pits can easily initialize at the sites of dislocations within the SLs. Then, another 10-nm layer of LT-GaN with Si doping of 2 × 1018 cm−3 was grown as an electron injection layer. The active region consisted of 5 periods of 3 nm In0.3Ga0.7N/13 nm GaN QWs. The p-type layers consisted of heavily Mg-doped (2 × 1020 cm−3) p-GaN, a p-AlGaN EBL with Mg doping of 9 × 1019 cm−3, a lightly Mg-doped (2 × 1019 cm−3) p-GaN layer, a V-pit recovery layer, and a heavily doped (1.5 × 1020 cm−3) p-GaN contact layer. To investigate the effect of the number of SLs on the performance of the InGaN/GaN yellow LEDs, three samples were prepared with a varied number of SLs periods, with period numbers 16, 24, and 32 for Sample A, Sample B, and Sample C, respectively. It is worth noting that the SLs used in this work are specifically designed for a wide well (5 nm InGaN) and a thin barrier (1 nm GaN), in contrast to conventional SLs [13,14,16]. We believe that these SLs will help to incorporate more indium in the InGaN layer owing to their higher average In content. The as-grown epitaxial wafers were fabricated into vertical thin-film LEDs with the top surface (n-side) roughened and the bottom surface (p-side) coated with Ag reflector, in size of 1 × 1 mm2. Details on the applied processes have been reported in [26].
3. Results and discussions
The indium composition of the MQWs was investigated using photoluminescence (PL) spectra obtained with a 410-nm laser as the excitation source at room temperature, as well as high-resolution X-ray diffraction (HRXRD). As shown in Fig. 2, there is a clear decrease in the radiated photon energy from sample A to sample C. Generally, the radiation energy of the MQWs depends on the well thickness and the indium content of the InGaN wells [27]. Therefore, we performed an Omega-2theta scan of (002) using HRXRD to determine the structure of the MQWs. As shown in Fig. 3, the distances between the satellite peaks of the MQWs of the three samples are almost the same. Therefore, we can deduce that there is no difference in the well thickness among samples. Hence, the increase in the PL wavelength should be caused by the increase in the indium content of the InGaN wells. Additionally, the improved indium utilization achieved by the reduction of In-rich clusters (as shown in Fig. 4) should also contribute to the increase of the wavelength. This result implies that increasing the number of InGaN/GaN SL periods is beneficial for the indium incorporation of the yellow MQWs, owing to the strain relief effect of the SL interlayer [17,28].
Fig. 2 PL Spectra of the three samples, obtained by excitation with 410-nm laser light at room temperature.
Fig. 3 Omega-2theta scan spectra of the yellow InGaN LED structures grown on silicon substrates, obtained with HRXRD (PANalytical X’Pert).
Fig. 4 FL images of sample A (a), with 16 SL periods, sample B (b), with 24 SL periods, and sample C (c), with 32 SL periods. The images were obtained through excitation with 420–490-nm intense fluorescent light.
It must be noted that QWs-0 is not observed in Fig. 3. Using the simulation software -Jordan Valley RADs V5.1, we determined that QWs-0 is very close to SLs-0 and covered by SLs-0 owing to its lower intensity. Moreover, as shown in Fig. 2, the positions of SLs-0 are shifted away from the GaN peak as the number of SL periods increases. The position of SLs-0 depends on the average In content of the SLs interlayer and its strain state. The latter should be excluded because the SLs are commonly used as a strain relief layer [16,17] and the reduction in strain would cause the peak to move to the opposite direction (towards the GaN peak) [28]. Hence, the shift of SLs-0 indicates an increase in the average In content of the SLs interlayer, which is consistent with the increase in the In content of the QWs.
From Fig. 2, it can be also observed that sample C shows a larger full width at half-maximum (FWHM) compared to the other samples, which may be caused by the enhanced localization effect of the MQWs. Q. Mu et al. reported that the localization effect would be stronger owing to slight indium composition fluctuations or to a small phase separation in the MQWs caused by the strain release effect of the SLs [12]. This is very consistent with the fluorescence spectra (FL) obtained in this work, as shown in Fig. 4. We determined that the phase separation was suppressed effectively by the increase in the number of SL periods, which is illustrated by the reduction of dark spots (In-rich clusters). The stronger localization effect means the presence of more localized states, which could lead to a larger FWHM.
FL is very useful for observing the luminous morphology of MQWs; the FL spectra were acquired using Nikon C-HGFI with intense 420–490-nm excitation light. We observed many dark reddish-brown spots with a diameter of several micrometers in Sample A, whereas the number and diameter of these spots in Sample B decrease significantly. In sample C, these spots have almost disappeared. These dark reddish-brown spots are considered to be In-rich clusters, which are common in high-indium-content InGaN alloys [29–31]. The micron-scale In-rich clusters (M-ICs) decrease with increasing number of SL periods, which implies an improvement in the quality of the MQWs. The elimination of such M-ICs could be related to the strain relief of the MQWs. On the one hand, the SLs are well known to act as a strain relief layer [17,28]. On the other hand, the HRXRD results show that the peaks of the SLs shift towards smaller degrees, which means a greater lattice distance on the a-axis. As a result, the compressive stress in the InGaN wells is reduced. Meanwhile, the greater lattice distance would help indium incorporation owing to the large diameter of the indium atom. Hence, more SL periods could help stress relief and indium incorporation of the MQWs, thereby substantially eliminating M-ICs and achieving a more uniform indium distribution. Additionally, because the MQWs of the samples were grown under the same indium flow and temperature, the decrease of M-ICs means more effective indium incorporation or longer wavelengths, which is consistent with the PL wavelength.
Electroluminescence (EL) tests were performed on the LED chips under the pulse mode at room temperature using a Keithley Instruments 2635A source meter and an Instrument Systems CAS 140CT spectrometer. As shown in Fig. 5, the LEDs with more SL periods have higher external quantum efficiency (EQE) and lower forward voltage than the other LEDs. At approximately 2 A/cm2, the EQE values reach their maxima. By increasing the number of SL periods from 16 to 32, EQEmax increases by about 28%; this increase of the EQE remains almost stable as the current density increases. For the common operating current density of 20 A/cm2, the EQE of sample C (0.77) increases by approximately 30% compared with sample A (0.58), and the forward voltage decreases from 2.56 V to 2.38 V. Yellow LEDs with an EQE of 14.3%, wavelength of 568 nm, and a forward voltage of 2.38 V at 200 mA (20 A/cm2) were achieved. It is valuable to get such low forward voltage which is still a tough question for high-efficiency InGaN-based LEDs in the long wavelength range [7,10]. The factors and mechanism that contribute to these improvements were studied in detail, as described in the following.
Fig. 5 Comparison of EQE and forward voltage among samples with different numbers of SL periods. All the samples have the same dominant wavelength of 568 nm at 200 mA.
As mentioned earlier, the FL results show that more SL periods could help to eliminate M-ICs in yellow MQWs. Nanoscale In-rich clusters (N-ICs) and quantum dots have been well known to be localized radiative centers, which are responsible for the high efficiency of InGaN-based emitting devices [32–35]. However, once N-ICs grow into M-ICs, the effective carrier localization will rapidly decrease and many nonradiative centers will be produced [36]. This is because the formation of such large M-ICs could generate many additional energy steps within the merged regions, giving rise to a number of nonradiative centers (e.g., dislocations) [36], which is illustrated in Fig. 6. Therefore, the opportunities for carriers to be trapped by nonradiative centers will increase greatly, resulting in lower efficiency. Hence, the increase of the quantum efficiency of LEDs with more SLs contributes to the reduction of non-radiative centers and to effective carrier localization during the elimination of M-ICs. Q. Mu et al. also reported a stronger localization effect for the sample with less strain and phase separation in InGaN wells [12].
Fig. 6 Diagrams explaining the opposite effects of (a) N-ICs and (b) M-ICs on luminescence. Within the M-ICs, many nonradiative centers (e.g., dislocations) are generated in the merged regions, as shown by the short red lines, leading to a reduced radiation recombination rate.
However, the elimination of M-ICs does not explain why the LED with more SLs also has a lower forward voltage, in addition to a higher EQE. Further investigations were performed using scanning electron microcopy (SEM) with a HITACHI SU8010 SEM system to observe the surface morphology of the MQWs. As shown in Fig. 7, there are a number of inverted hexagonal pits on the surface of the MQWs, which are called V-pits, and their size increases noticeably with the number of SL periods. Our previous studies and Y. Li et al. have proven that there are two ways to achieve hole injection into MQWs: via the flat c-plane or via the side-walls of V-pits, as shown by Fig. 7(d). Hole injection becomes easier with larger V-pits and the hole distribution becomes more uniform among QWs of different depth [37–41]. Hence, the increased V-pit size helps to achieve a higher EL efficiency, as well as a lower forward voltage. Furthermore, it has also been reported that the potential barrier around dislocations formed by the sidewalls of V-pits could effectively suppress non-radiative recombination [21,42,43].
Fig. 7 (a), (b), and (c)SEM images of the V-pits for the three samples, obtained with a HITACHI SU8010 SEM system; (d) diagram showing the two ways to inject holes into the MQWs: via the flat area or via the side-walls of V-pits.
Generally, within the range of 32 periods, more SLs could upgrade the quality of yellow InGaN/GaN MQWs by eliminating M-ICs and promote hole injection with increasing V-pit size, which may be the reason why yellow LEDs with more SLs yield higher efficiency and lower voltage.
However, as the area of the c-plane of a chip is fixed, improving the efficiency of the MQWs by modulating the SL layer would involve a trade-off between the area of the V-pits and the c-plane MQWs, where light emission mainly occurs, i.e., the number of SL periods has an optimal value. Besides, if the V-pits are too large, it is difficult to obtain good crystal quality for the subsequently grown p-type GaN. As shown in Fig. 8(a)–(c), in the range up to 32 SLs, all the yellow InGaN-based LED films exhibit good single-crystal surfaces; the step-flow growth mode can be seen in Fig. 8(e)–(f). However, the LEDs employing 40 SLs, shown in Fig. 8(d), present a rough surface with many voids. This is because when the V-pits are over-enlarged, crystal grains that fill the V-pits are more difficult to coalesce, resulting in poor surfaces, severe leakage, and lower efficiency. Therefore, InGaN/GaN with 32 SL periods may be optimum for the yellow LEDs studied in this work. Figure 9 shows an image of a LED chip emitting at an injection current of 30 mA, together with the EL spectra and the output power, as a function of the injection current. A high light output power of 63 mW is achieved with a dominant wavelength of 568 nm, and a forward voltage of 2.38 V at 200 mA. A blue shift of the peak emission wavelength is observed with the increase of the injection current, which is approximately 13 nm in the range 30–750 mA. The FWHM of the EL spectra for an injection current of 200 mA is approximately 41 nm.
Fig. 8 Atomic force microscopy (AFM) images of the surface of the p-type GaN layers of LEDs with varied SL periods. The numbers of SL periods are 16 ((a) and (e)), 24 ((b) and (f)), 32 ((c) and (g)), and 40 ((d) and (h)).
Fig. 9 (a) Optical photograph of the emitting LED. (b) EL spectra and (c) light output power as a function of the injection direct current of sample C with 32 SLs.
4. Conclusion
To summary, we studied yellow InGaN/GaN LEDs with a specially designed SL interlayer. We observed that the In content of yellow InGaN/GaN MQWs increases with the number of SL periods. Moreover, more SL periods help to yield higher efficiency and lower voltage, which can be attributed to the following two factors. One, the M-ICs, which include many non-radiative centers, can be eliminated substantially with more SLs. Second, the size of V-pits could be adjusted by the number of SL periods. More SLs lead to larger V-pits, which improve hole injection effectively. Hence, the SLs have a significant influence on the performance of LEDs. However, the increase of the number of SL periods increases the area of V-pits and thus reduces the effective luminescence area owing to the limited area of the chip. Moreover, too many SLs would lead to a poor p-type layer with many voids. In this work, 32 SLs was demonstrated to be the optimum for yellow InGaN/GaN LEDs grown on silicon substrates, achieving an improvement of approximately 30% in the EQE, as well as reduced voltage, compared with LEDs the 16 SLs. High-power InGaN/GaN LEDs with a wavelength of 568 nm were achieved with a light output power of 63 mW and a forward voltage of 2.38 V at 200 mA (20 A/cm2).
Funding
National Science and Technology Major Project of China (Grants No. 2016YFB0400600, 2016YFB0400601); National Natural Science Foundation of China (Grants No. 61334001, 61604066, 21405076, 11604137, 11674147, 51602141); and the National High Technology Research and Development Program of China (Grant No. 2015AA03A102).
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