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Realization of Highly Efficient InGaN Green LEDs with Sandwich-like Multiple Quantum Well Structure: Role of Enhanced Inter-well Carrier Transport Quanjiang Lv, Junlin Liu, Chunlan Mo, Jianli Zhang, Xiaoming Wu, Qingfeng Wu, and Fengyi Jiang ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01040 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 4, 2018
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Realization of Highly Efficient InGaN Green LEDs with Sandwich-like Multiple Quantum Well Structure: Role of Enhanced Inter-well Carrier Transport Quanjiang Lv, Junlin Liu*, Chunlan Mo, Jianli Zhang, Xiaoming Wu, Qingfeng Wu, and Fengyi Jiang National Institute of LED on Si Substrate, Nanchang University, Nanchang 330096, China
ABSTRACT The potential of multi-color semiconductor electroluminescence in solid-state lighting has been extensively pursued due to the energy-saving and smart-lighting as compared to conventional phosphor-converted white light sources. Here, we demonstrate a highly efficient 525 nm GaN-based green light-emitting diode (LED) with a sandwich-like multiple quantum well (MQW) structure grown on patterned Si (111) substrates. Performance enhancement can be achieved by adjusting the thicknesses of the three quantum barriers close to p-GaN in the interior of the sandwich MQW. Samples A, B and C with an optimized barrier thickness of 13-, 10- and 8-nm showed peak external quantum efficiency (EQE) values of 55.6%, 56.2% and 49.0%, respectively. Under normal working condition (350 mA, current density 35 A/cm2), the output power, EQE, forward voltage, and dominant wavelength of the sample representing the best performance were 306.0 mW, 37.0 %, 2.76 V, and 525 nm, respectively. This work might provide an economically feasible way to realize volume-produce of highly efficient InGaN green LEDs on silicon substrates. KEYWORDS: green light-emitting diodes, barrier thickness, high efficiency, V-pits, Si substrates III-nitride LEDs employ InGaN/GaN multi quantum wells (MQWs) as active region, covering a wide spectral region from near ultraviolet to near infrared, making them attractive in solid-state lighting (SSL).1-3 Typically, LED-based white light emission is achieved by using a blue LED to stimulate the yellow phosphor, partially converting the blue emission to the yellow/green spectral range. This conversion is related to an unavoidable energy loss known as Stokes’ loss, which is in the order of 25%.3 In fact, in pursuit of energy-saving and smart-lighting applications, it has been pointed out that the full potential of SSL can be exploited by moving towards multi-color semiconductor electroluminescence, combining the light of several colors, usually red, green, blue, and possibly yellow. This approach, more than any other, requires direct emitting LEDs with high-efficiency in the range of the visible spectrum, especially in the green/yellow range.3 Recently, Narukawa et al.4 reported a peak wall-plug efficiency exceeding 80% from the state-of-art InGaN blue LEDs, and a relatively high external quantum efficiency (EQE) about 65% for AlInGaP red LEDs was also obtained.5 However, the luminous efficiency of green LEDs are still relatively poor (~ 40%6 ), and dissatisfy the requirements of practical applications, which has been described as the “green gap” problem.2-3 Generally, there are two main reasons for the ‘‘green gap’’ problem. On the one hand, for LEDs emitting at green spectral region, InGaN/GaN MQWs with higher indium (In) composition are required,7 while the lattice mismatch strain between InN and GaN limits the incorporation of In.8 Additionally, in order to incorporate In sufficiently, it is usually accomplished by sacrificing the growth temperature of InGaN quantum wells (QWs), resulting in a significant deterioration of the material quality in the QWs due to crystal defects.9-10 On the other hand, the higher In composition in well layers induces a larger piezoelectric polarization electric field, resulting in a quantum
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confinement Stark effect (QCSE). The QCSE restricts the internal quantum efficiency (IQE) by reducing the overlap integral between the electron and hole wave functions, resulting in a lower recombination efficiency.11-12 In addition, strain-induced internal polarization field will tilt the green LED band diagrams of QWs more serious than those of blue ones, leading to a so called “efficiency droop” in InGaN-based green LEDs, which has been attributed to the inefficient hole transport and increased electron overflow out of the active region.13-14 These two problems have seriously hampered the development of SSL, especially for high-power LEDs, whose operating current is usually greater than 350mA and even up to 1A, having dimensions of approximately 1 × 1 mm2.15 Many efforts have been made to solve these two problems, such as designing novel MQW structures14-16 or optimizing growth parameters.17 Barrier thickness is one of the most important parameters of InGaN/GaN MQWs, which may affect the piezoelectric field in the well,12 the carrier transport and distribution in active layers,16 and the quality of active layers.18 Liu et al.19 reported that hole injection into the QWs away from p-GaN is significantly enhanced by reducing the thickness of the barrier layer. Nevertheless, by comparing the full width at half maximum (FWHM) of the X-ray rocking curve of samples having different QB thicknesses, Kim et al. 20 showed that the crystal quality of the extremely thin QB was lowered and the light output power (LOP) would be further reduced. In addition, it was also reported that the lower growth temperature of InGaN/GaN superlattices (SLs) embedded between the n-GaN and MQW could promote the formation of V-pits.21 Experiments and numerical simulations demonstrated that carrier injection can be enhanced through the sidewall of Vpits, resulting in non-radiative recombination centers (NRCs) being screened and improving the EQE of LEDs.22-25 Recently, Le et al.26found the average size and the density of V-pits increased with increasing QBs thickness, which causes more serious carrier leakage in both reverse and forward bias, resulting in poor luminous efficiency. Furthermore, they also observed a stronger blueshift of peak wavelength by increasing QBs thickness, which was attributed to the stronger polarization effect. In other words, in order to achieve efficient InGaN green LEDs, we should put more efforts into designing and optimizing the structure and parameters of MQWs. Here, we report high-efficiency green LEDs with a sandwich design of InGaN/GaN MQWs grown on silicon (111) substrates. Specifically, the active region was built by five pairs carrier confined QWs (CC-QWs) with a larger energy bandgap (Eg) and three pairs active-QWs (A-QWs) with a slightly smaller Eg. This active region design has a unique energy band structure, which can effectively utilize the transport characteristics of carriers at different ground state energy levels, and induce the carrier recombination mainly in the A-QWs close to p-GaN. Experimental results demonstrate that sample with thinner QBs corresponding to the A-QWs has a lower EQE under low current injection, while at high current density the efficiency droop and output power are significantly improved. The former is ascribed to the degraded crystal quality of MQWs, and the latter is attributed to the improved carrier inter-well transport. This simple and effective approach used in present research can be extended further for high efficiency ultraviolet, blue, and yellow LEDs.
EXPERIMENTAL METHODS As shown in Figure 1(a), the epitaxial structure of the green LEDs used in this work were grown on patterned silicon (111) substrates by our homemade 7×2-inch metal organic chemical vapor deposition (MOCVD) system. LED structures started from a 120 nm high temperature AlN buffer layer and a 3.3 μm n-type GaN, followed by 32 periods of InGaN/GaN (3.5nm /3.5nm) SLs for strain relief and 10 nm undoped low temperature GaN (LT-GaN). Next is the InGaN/GaN MQWs with a hybrid design. Specifically, the MQW from n-GaN to p-GaN was a combination of four pairs CCQWs (In0.24Ga0.76N) grown at 805℃ and three pairs A-QWs (In0.25Ga0.75N) grown at 800℃ plus one ACS Paragon Plus Environment
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CC-QW adjacent to p-GaN layer. Each QW has the same thickness of 3 nm and was separated by a GaN barrier layer with various thicknesses grown at 950℃. Three LED structures have the same QBs thicknesses of 13nm for all the CC-QWs, while the QBs thicknesses corresponding to the three AQWs were different, i.e., 13-, 10-, and 8-nm for samples A, B and C, respectively. Detail schematic diagrams of the fabricated LEDs are shown in the conduction band diagram in Figure 1(b) and confirmed by the secondary ion mass spectrometry measurement (see Figure S1). A 13 nm Mg-doped p-Al0.2Ga0.8N electron blocking layer (EBL) was grown followed by a 140 nm p-GaN: Mg. Finally, in the region at the top of the p-GaN was heavily doped with Mg to facilitate the formation of Ohmic contact (p++). In addition, a reference sample, sample R with regular structures was also prepared. Sample R had an epitaxial structure similar to that of the sample A except that the growth temperature of the eight QWs remained the same, and the thickness of all QBs maintained at 13 nm. For devices, the wafers were processed into vertical LED chips and packaged as LUXEON structure with silicone for LEDs encapsulation, more specific chip manufacturing process can be found in our previous work27-28 and the Supporting Information ( see Figure S2). All fabricated LEDs show a dominant wavelength about 525 nm under 35A/cm2 at room temperature. The current–voltage (I–V) measurements of the packaged LEDs were performed on a Keithley2635A semiconductor parameter analyzer. For the electroluminescence (EL) performance of devices, a calibrated integrating sphere was used for the measurement.
(a）
(b)
Figure 1. Schematic diagram of InGaN/GaN green LED on silicon substrate (a) Epitaxy layers (b) Conduction band diagrams for sample A, B and C. (flat bands without polarization effect are shown for simplicity)
RESULTS AND DISCUSSION The room-temperature EL spectra of samples A, B, C and R are first presented in Figures 2(a)(d), respectively. Only one emission peak appears in the EL spectra of the three sandwich samples, and the peak position is around 520 nm at 200mA, which is emitted from the A-QWs located close to p-GaN. Figure 2(e) shows the EL spectra of sandwich samples and the regular sample at 100 mA. The inset is the FWHM curves of samples R and A as a function of current density. According to the active region design, however, no significant bimodal emission was observed in samples A, B and C upon elevating the current level, and the sandwich samples even showed smaller FWHM values than the regular sample. This result is contrary to the conclusion that: in the case of electrical pumping, the contribution of top QW nearby the p-GaN is dominant, proposed in the previous literatures.29 We suspect that the absence of CC-QWs wavelength emission might be related to the existence of V-pits
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and the specially designed MQW structure. In this sandwich designed MQWs, since A-QWs grow at a lower temperature and have a smaller Eg. In the case of electrical injection, holes from the p-GaN layer are directly (path 1) or indirectly (path2) injected into the A-QWs in the direction of the potential energy minimum and undergo radiation recombination with the electrons, as in depicted Figure 2(f). In other words, the presence of V-pits provides a special path for readier holes to bypass the top QWs nearby the p-side,30 and the sandwich designed active region offer a subtle use of this “disadvantages”. We believe that the sandwiched A-QWs with a smaller ground state energy level, which can induce and limit carrier recombination primarily in the A-QWs. This not only brings higher efficiency with respect to carrier utilization but also achieve smaller FWHM values, which will be explained in detail later. Besides this, the results of studying the emission behavior of the top QWs in V-pit-contained InGaN/GaN MQW using 3D modeling methods are consistent with those observed in our study.25 Detailed experimental information on the carrier transport and distribution characteristics of V-pits Photon Energy (eV)
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GaN LEDs will be published elsewhere.
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(e) EL Intensity (a.u.)
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Figure 2. (a)-(d). Room-temperature EL spectra of (a) sample A, (b) sample B, (c) sample C and (d) sample R, plotted for current range from 7.5 to 200 mA. (e) EL spectra of samples A, B, C and R measured at 100 mA. The inset is the FWHM curves of samples R and A as a function of current density. Sample R has a regular structure as a reference sample with lower EL intensity and larger FWHM values. (f) Schematic diagram of V-pits generation and the model for carrier injected into the QWs near an individual V-pit. (g) Top view SEM image of V-pits after the growth of MQW. (h) Cross-section STEM image of a large V-shape pit.
Scanning electron microscopy (SEM) image and the cross-sectional scanning transmission electron microscopy (STEM) image of the LED sample are shown in Figure 2(g) and (h), respectively. The SEM image was taken after the growth of MQWs to visualize the structural property of the Vpits. The SEM and STEM images of the three LED samples are similar, so we chose the results of sample A as the representation of the V-pits. As shown in the SEM image, featuring randomly distributed V-pits with a density of approximately 6.0 × 108 cm−2, and the top diameter of V-pits is determined to be about 170–220 nm. Figure 2(h) shows the STEM image around the V-shaped pit, where the V-pit connecting with threading dislocation (TD) extend from the underlying SLs. Inside the V-pit, the V-shaped facets are clearly distinguished, the p-GaN, EBL, and InGaN/GaN MQW grown on the (1 0 1 1) plane of V-pit sidewall exhibits a much smaller thickness than those grown on (0 0 0 1) plane,31 in line with the two carrier transport paths shown in Figure 2(f). It is apparent from Figures 2(a)-(d), for all LED samples featuring various QBs thicknesses, the EL peak position with respect to the increasing injection current shifts toward shorter wavelengths (blueshift).32 These figures also demonstrated a direct link between blueshift and barriers thicknesses, ACS Paragon Plus Environment
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namely, the thicker the barrier is, the stronger the blueshift. Increased injection current from 7.5 mA to 50 mA, the blueshift is 35.0, 29.8, 25.0 and 22.4 meV for samples R, A, B and C, respectively. In general, the blueshift is strongly correlated with the QB thickness due to the QCSE caused by spontaneous and piezoelectric polarization fields in wurtzite nitride heterostructures.12 When a thicker GaN layer is grown on InGaN layer, it will results in a severer QCSE due to greater stress.18 The use of straightforward electrostatic arguments can provide a full qualitative explanation of the blueshift observed in Figures 2(a)-(d). From the conservation of the electric displacement vector across the heterostructure (∇ ∙ 𝐷 = 0), one gets the boundary condition for the electric displacement field at the QW/QB heterointerface: 𝜀𝑤𝐸𝑤 ― 𝜀𝑏𝐸𝑏 = 𝑃𝑏 ― 𝑃𝑤, where 𝐸𝑤 (𝐸𝑏) is the electric field in the well (barrier), 𝜀𝑤 (𝜀𝑏) is the well (barrier) static dielectric constant, and 𝑃𝑤 (𝑃𝑏) is the zero-field polarization of the well (barrier) material.33 For the case of periodic superlattice InGaN/GaN MQWs, the periodic boundary: 𝐿𝑊𝐸𝑊 + 𝐿𝐵𝐸𝐵 = 0, where 𝐿𝑊 and 𝐿𝐵 represent the well and barrier width, leads to33 𝐸𝑤 =
𝐿𝑏(𝑃𝑏 ― 𝑃𝑤)
(1)
𝜀𝑏𝐿𝑤 + 𝜀𝑤𝐿𝑏 and similar to the electric field of the barrier. According to the above analysis and Eq. (1), the electrostatic field in the MQWs for 8 nm thick barrier of sample C was smaller than that for 13 nm thick barrier of sample A (see Figure S5). These results indicate that QCSE can be decreased by reducing the QBs thickness, thus leading to a weaker blueshift. The LOP and EQE characteristics of samples A, B and C are plotted as a function of current density (up to J=75 A/cm2) in Figures 3(a) and (b), respectively. As depicted in Figure 3(a), LOP is significantly enhanced at high current density range as QBs thickness decreases. Specifically, at J=75 A/cm2, the LOP of samples A, B and C are 488, 523, and 542 mW, respectively. In Figure 3(b), the EQE curves of the three LED samples show a typical efficiency droop behavior, but they exhibit different droop ratios. The EQE peaks of the three LED samples are 55.6%, 56.2% and 49.0%, occurring at a current density of 1.5, 1.8 and 3.0 A/cm2, respectively. At a normal operating current density of 35 A/cm2, the EQE values of the three samples are 34.2%, 36.5% and 37.0%, respectively. The Jmax, which is the current density corresponding to the peak value of EQE, gradually increases as the barrier thickness decreases. The delay in the Jmax can be interpreted as a smaller QW field with a thinner QB, reducing the carrier leakage and increasing carrier dwell time.34 At current density of 75 A/cm2, the droop ratio for the three samples are 51.0%, 47.8% and 38.0% compared with their peak EQE, respectively. Although sample featuring the 8 nm thick QBs exhibited the minimal droop ratio and maximum LOP at high current density, but the poor peak EQE cannot be ignored. Moreover, in the region where the current density is smaller than Jmax, we can clearly see a noticeable droop in the EQE curve as the thickness of the barriers decreases, especially for the sample C, and the droop rate is quite alarming. These results showed an unanticipated phenomenon, that is, as the barrier thickness decreases, the polarization field decreases and the efficiency at low current range should be better.35 As shown in the left axis of Figure 3(b), we employ a useful parameter called S, i.e., S =∆log(L)/∆ log (I) (L and I denotes the LOP and driven current, respectively). It is said that the value of S at each bias can reflect the dominant carrier recombination process, when the Shockley Read-Hall (SRH) recombination dominates, the S value is 2.0.36 In Figure 3(b), however, the S value monotonically increases without saturation as the current decreases. At the lowest current density of 0.005A/cm2, the S values of samples A and B are all around 1.9, while the value of sample C has been significantly increased to 2.5. It is noteworthy that when the value of S exceeds 2.0, the leakage current caused by the defect-assisted tunneling is generated, and the leakage current will influence the carrier injection ACS Paragon Plus Environment
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into the active region.37 Figure 3(c) depicts the I-V characteristics of three samples. Sample C shows a large leakage current at low bias region (1.0-2.0V), and the larger tunneling current can be attributed to its higher defect states in the active region.36-38At the current density of 35 A/cm2, `the forward 2.6
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voltage of samples A, B and C are 2.73, 2.73, and 2.76 V, respectively. Figure 3. (a) Experimentally measured LOP with increasing current density for the three LED devices with various barrier thickness. (b) Plots of EQE curves and the slopes (SL-I) for the three LED samples as a function of current density. (c) Semilogarithmic I-V curves of samples A, B and C under forward bias. (d) FLM images of the InGaN/GaN green LED samples with different barrier thickness.
To analyze the relationship between tunneling current and barrier thickness in detail, we performed the fluorescence microscopy (FLM) experiment. (Nikon C-HGFI with intense 420–490 nm excitation light) The results are shown in Figure 3(d), it is found that sample A and B emit pure green light under the excitation of fluorescence, and a uniform luminous morphology of MQWs is observed, indicating a superior crystal quality. For sample C, however, featuring randomly distributed dark spots with a diameter of several micrometers are observed. These dark spots appearing in the FLM are considered as In-rich clusters associated with non-radiative areas in the QWs.39 These unexpected dark spots observed in sample C may come down to the poor crystal quality in the QWs,39which has also been confirmed by the X-ray rocking curve (see Figure S5). In the thermal treatment involving QBs growth, decorating the size and distribution of In-rich InGaN quantum dots may be crucial for ACS Paragon Plus Environment
EQE (%)
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realizing high performance LED devices, which has been proved by previous works.40 For the three LED samples, the growth temperature of the QBs is 150°C higher than that of the QWs. Sample C has the thinnest QBs and thus the shortest time of thermal treatment, resulting in poor crystal quality. Up to now, we have fully discussed the mechanism that leads to the significant decrease in the efficiency of sample C at low current. That is, the ultra-thin QBs of sample C leads to a decrease in the crystal quality and lowers the EQE at a low injection current range. In addition, according to the analysis in Figures 3(b)-(d), we can clearly see that the dominant non-radiative recombination of samples A and B would be SRH, and defect-assisted tunneling for sample C.37 Figure 4 shows the experimentally measured EL peak energy and FWHM as a function of the current density for the three LED samples. Again, the blueshift of sample A with thicker barriers is stronger than that of sample C. As analyzed in Figures 2 (a)-(c), the weakest blueshift of sample C can be explained by the weakest polarization effect due to thin QBs. It has been reported that for the recombination processes in the active region, as the injection current increases, the coulomb screening of the QCSE is accompanied by a decrease in the FWHM,41-42 while the band filling effect in QWs dominates accompanies the increase of the FWHM.17, 43As depicted in Figure 4, for all samples, we first observed that the FWHM decreased at the low injection current region (red region) and then increased again as the injection current increased (white region). Compared with samples A and B, the FWHM values of sample C are larger in the low current range, which means a poorer crystal quality in the QWs, and this result has been fully discussed in Figure 3. At the high injection current region, however, the FWHM decreases as the QBs thickness decreases. The decrease in FWHM of samples B and C indicates that the carrier density in the A-QWs is relatively low, which means that the carriers are uniformly distributed among the A-QWs and improve carrier recombination efficiency accordingly.16 Furthermore, Yoo et al.14 reported that thin barrier has a lower effective valence band height, making hole injection efficiency enhanced in the active region. This is a hint of the thinner barrier sample exhibits larger output power at high current density, especially for sample C, which has the highest EQE at high current density despite the worst crystal quality. And the answer to this
EL Peak Energy (eV)
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29 0.1
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may be related to the carrier distribution properties in the hybrid MQW. Figure 4. EL peak energy and FWHM as a function of current density for the three LED samples.
In order to explain the negative correlation between the QBs thickness and the output power at high current density in detail, the possible carrier transport mechanism in the hybrid MQWs and the valence band diagrams are shown in Figures 5(a) and (b). As we all know, LED is a bipolar device. Under the condition of electric injection, electrons and holes are injected from n-side and p-side layer, ACS Paragon Plus Environment
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respectively. To recombine effectively, these injected carriers must relax quickly to prevent the conduction and valence band states from being filled. Generally, there are two mechanisms for carrier relaxation, known as radiative recombination and non-radiative recombination as illustrated in Figure 5(a). In order to achieve higher radiation recombination efficiency, an optimally distribution of carriers in the MQW is the most desirable. Carrier escape from quantum wells had been confirmed by Schubert et al.,44 basically, there are two types of processes by which the carrier can escape from the QW, namely tunneling and hopping, as shown in Figure 5(a).45-46 Based on this, it may be deduced that carrier interaction between QWs may play an important role in this novel sandwich MQWs. This kind of interaction can modulate the distribution of carriers in the QWs because carriers are always (b) Sample A
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trying to fill the lowest energy level. Figure 5. (a) The schematic diagram of possible carrier transport processes in the sandwich-like MQWs structure. (b) Calculated valence band diagrams and hole quasi-Fermi level for LEDs with different barriers thickness under the current density of 35 A/cm2 at 300 K. (c) Experimental results of EL spectra of three LED samples at 150 K, plotted for current density of 2 A/cm2. (d) Calculated electron current density profiles near the MQWs for the LEDs with various barriers thicknesses. The electron leakage was eliminated by reducing the QBs thickness.
Note that in this carefully designed sandwich MQWs, three A-QWs adjacent to p-GaN layer have a slightly smaller Eg than that of four CC-QWs near the n-type GaN. This structural design may
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favorite for carrier’s radiation recombination in the A-QWs for two main reasons. On one hand, holes have larger effective mass and lower mobility with respect to electrons in the GaN material system. This property of holes, however, makes it difficult for holes to transport through the thicker QBs.29 In addition, as depicted in Figure 5(b), the calculated band diagrams for samples A, B and C at 35A/cm2 were investigated through the simulation. The simulation software used for calculation is ATLAS, developed by SILVACO, Inc. For a simulated LED structure, for example, the thickness of QW/QB, the In composition of QWs, the doping concentrations are as same as the experimental structure. Detailed parameters for numerical simulation can be found in Supporting Information (see S4). As depicted in Figure 5(b), as barriers thickness decreases, the overlap of the hole quasi-Fermi levels (dash line) and the valence band diagram (solid line) apex increase significantly, especially in the AQWs. Generally, this overlap can be interpreted as the presence of more holes in the QWs. Thus, we expect that the increase in barriers thickness puts the brakes on the uniformity of hole distributions within A-QWs. On the other hand, combined with the experiments, the ground state energy of CCQWs is higher than that of A-QWs due to the effect of quantum confinement in QWs. Therefore, in the case of forward bias, carriers can tunnel from the CC-QWs to the A-QWs and recombine there radiatively. However, once the carriers are captured by the A-QWs, carriers tunnel back to the CCQWs would be suppressed since a higher energy level states filling is required. The tunneling rate 𝑅𝑇 for a carrier between QWs using the zero-order Wentzel-Kramers𝐿𝐵 be―1 Brillouin (WKB) approximation as: 47 2𝑚 ∗ ∆𝑢𝑑𝑥 𝑒𝑥p ( ― ∫can 2ℏ expressed 0 × 2𝐸0 𝑚 ∗ 𝑅𝑇 = 𝐿𝑊 (2)
)
where ℏ ―1is Planck’s constant divided by 2π, 𝑚 ∗ is the effective carrier mass, ∆𝑢 is the barrier height and 𝐸0 is ground energy state of QW. The tunneling rate 𝑅𝑇 given in Eq. (2) is sensitivity depends on the barrier thickness 𝐿𝐵 and the ratio increases with decreasing barrier thickness. The above arguments qualitatively explain the results in Figure 3(a): sample with thinner barriers exhibit higher output power at high current density. This can also be understood as: for the sample with a thinner QB, the QW suffers from a smaller electric field and the carriers get less kinetic energy when they pass through a QW. For a given QW thickness, less kinetic energy means that electrons are more likely to be captured by the QW. Moreover, unlike electrons, holes have lower mobility and relatively larger effective mass, they are usually concentrated in QWs nearby the p-type layer as mentioned before. In addition, compared with samples A and B, sample C has the thinnest barriers thickness corresponding to the A-QWs (13-,10- and 8-nm for samples A, B and C, respectively). In this case, not only the carrier tunneling rate 𝑅𝑇 from CC-QW to A-QW can be enhanced, but also the barriers within A-QW’s can be regarded as transparent for carrier transport.35, 45 The actual carrier density inside the A-QWs of sample with a thinner QBs should be lower than that of sample with a thicker QBs, even though the effective volume of the active region decreases as the QBs thickness decreases. This is because of the enhanced carrier tunneling rate between two type QWs and the interaction in the A-QWs. Therefore, the dependences of the measured EQE on QB thickness can be described using 31, 48 the ABC-model, since the EQE is related to the carrier 𝐵𝑁2 density by 𝜂𝐸𝑄𝐸 = 𝜂𝐼𝑄𝐸 × 𝜂𝐿𝐸𝐸 = × 𝜂𝐿𝐸𝐸 𝐴𝑁 + 𝐵𝑁2 + 𝐶𝑁3 + 𝑓(𝑁) (3) where η𝐼𝑄𝐸, and η𝐿𝐸𝐸 represent the IQE and light extraction efficiency (constant), respectively. The recombination parameters A, B, and C are coefficients corresponding to the SRH recombination, radiative recombination, and Auger recombination, respectively. 𝑁 is the average carrier density, ACS Paragon Plus Environment
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and 𝑓(𝑁) denotes the electron leakage from the active region. The EQE is improved for lower 𝑁 at high current density. However, according to Piprek et al.,48 it is hardly to distinguish whether it is achieved by reducing Auger recombination or electron leakage. Figure 5(c) displays the semi-log scale EL spectra at 150 K. There are two peaks at about 529 nm (P1) and 417 nm (P2), which can be attributed to quantum well band to band optical transitions and Mg acceptor related transition49-50, respectively. For the latter, it has been reported that the peak energy at around 3eV in Mg doped p-GaN originates from the Mg acceptor related transitions in p-GaN, which is caused by the electron overflow from the MQWs to the p-GaN layer.49 It is generally believed that the pronounced P2 accompanies with a weak efficiency at high current.50 The intensity of P2 decreased with decreasing QBs thickness. This result implies that the ability to confine carriers is enhanced accordingly, thereby causing a weakened electron leakage from the active region. According to Eq. (3), it's easy to know that at the current density of J >Jmax, the defect-related non-radiative recombination (AN) has been already saturated (𝐵𝑁2 + 𝐶𝑁3 ≫ 𝐴𝑁). At this time, the improved EQE is mainly due to the reduction of electron leakage and the calculated results of electron current density in Figure 5(d) also backs this up. This means that in the sandwich active region the impact of electron leakage on the emission efficiency can be minimized through careful structural design. The comprehensive analysis given in the preceding discussion well explain the delayed Jmax and higher output power for the samples with thinner QBs at high current range. Although sample C with thinnest QBs exhibited the smallest efficiency droop, however, the peak EQE was significantly decreased and exhibited a poorest luminous morphology of MQWs. Alignment of efficiency droop behavior with their peak EQE, we think that the sample with 10 nm thick QBs achieved acceptable performance. Accordingly, in the sandwich-like MQW structure the optimal thickness of QBs for the A-QWs is 10 nm.
CONCLUSIONS In summary, highly efficient InGaN green LEDs employing sandwich-like MQWs were grown on patterned silicon substrates by MOCVD technique. Sandwiched MQW is composed of two types of QWs, namely CC-QW with a slightly larger bandgap and A-QW with a slightly smaller bandgap near p-GaN. The influence of the barrier thickness corresponding to the A-QWs on the optoelectronic performance of LED devices was investigated. EL measurement shows that sample with thinner QB has a weaker blueshift and higher output power at high current density. The former is attributed to the reduction of the electric field in the QWs by thinning the thickness of barriers layer, and the latter is ascribed to the enhanced carrier interaction between wells and concomitant with uniform carrier distribution in the A-QWs. Besides, a schematic diagram was employed to explain this phenomenon. Nevertheless, sample with thinnest QB has the lowest emission efficiency at low current density. From the analysis of the I-V curve and the FLM images, we can infer that this is due to a serious degradation of material quality. With a thinner QB design, the efficiency droop of the sample with 8 nm thick QBs was only 38.0%, but accompanied by the poorest peak EQE of 49.0%. When the peak EQE and efficiency droop are both taken into account, sample with 10 nm thick QBs exhibited acceptable droop behavior (47.8%) and highest peak EQE (56.2%), suggesting that LED featuring 10 nm thick QBs is promising for the future solid-state lighting with high performance.
ASSOCIATED CONTENT Supporting Information Secondary ion mass spectrometry (SIMS), LED chip fabrication process flow, X-ray rocking curve, parameters for the calculated band diagrams, calculated single conduction band diagram. ACS Paragon Plus Environment
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AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work is supported by the National Key Research and Development Program of China (Grant No. 2016YFB0400600 and 2016YFB0400601), the State Key Program of the National Natural Science of China (Grant No. 61334001), the National Natural Science Foundation of China (Grant No. 21405076, 11364034, 11674147, 61604066, 51602141 and 11604137).
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emitting diodes by temperature-dependent electroluminescence. Applied Physics Letters 2012, 100 (15), 153506.
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For Table of Contents Use Only Realization of Highly Efficient InGaN Green LEDs with Sandwich-like Multiple Quantum Well Structure: Role of Enhanced Inter-well Carrier Transport Quanjiang Lv, Junlin Liu *, Chunlan Mo, Jianli Zhang, Xiaoming Wu, Qingfeng Wu, and Fengyi Jiang *) Corresponding author, E-mail:
[email protected] Table of Contents
This Table of Contents graphic consists of three parts. The left side is the experimentally measured external quantum efficiency (EQE) curves as a function of current density for the three lightemitting diode (LED) devices with various quantum barrier thickness. The inset is an optical microscope morphology of the LED chip at a current density of 1A/cm2. The right side is schematic diagram of V-pits generation and the model for carrier injected into the quantum wells near an individual V-pit. This graphic shows that highly efficient 525 nm GaN-based green LEDs with a sandwich-like multiple quantum well (MQW) structure achieved a peak EQE of 56.2%. The presence of V-pits in the sandwich MQWs provides a special path for holes to be injected into the active region, this not only brings efficiency with respect to carrier utilization but also increases the emitting efficiency of green LED devices.
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