High Efficiency InGaN Green LEDs with Additional Optimized p-AlGaN Interlayer

YU Hao, ZHENG Chang-da, DING Jie, MO Chun-lan, PAN Shuan, LIU Jun-lin, JIANG Feng-yi

(National Institute of LED on Silicon Substrate, Nanchang University, Nanchang 330096, China)

Abstract: Significantly improved external quantum efficiency was achieved by growing an additional optimized 25 nm low-doped p-AlGaN interlayer(IL) after the conventional p-AlGaN electron blocking layer for InGaN/GaN green LEDs with V-pits on Si(111) substrate. At 35 A/cm2 current density, external quantum efficiency(EQE) and output power reach up to 43.6% and 362.3 mW with the dominant wavelength of 520 nm. This is a new record for green InGaN-based LEDs. The underlying physical mechanism is attributed to the enhanced holes injection efficiency via V-shaped pits assisted by the optimized p-AlGaN interlayer. This paper provides an effective approach to improve efficiency especially suitable for those InGaN/GaN LED with V-shape pits.

Key words: green LEDs; p-AlGaN interlayer (IL); external quantum efficiency; V-shaped pits; holes injection efficiency

1 Introduction

Due to the widely tunable wavelength from ultraviolet to blue/green/yellow, GaN-based light-emitting diodes(LEDs) are used in various applications, especially in solid-state lighting[1-2]. The higher EQE of GaN-based green LEDs is achieved at low current density, while efficiency droop occurs at high current density[3]. Many possible factors contribute to the efficiency droop, like electron leakage from active layer into p-type layer[4], Auger recombination[5], carrier delocalization[6]. Moreover, the poor holes injection efficiency and nonuniform holes distribution are the main factors which influence efficiency droop[7]. To promote the hole transportation, many investigations have been carried out. A electron blocking layer with graded aluminum composition can not only prevent electron leakage, but also enhance hole injection[8]. A new AlGaN/GaN/AlGaN-type EBL can manipulate hole transport mechanism by inserting a thin GaN into EBL, enhancing the hole injection efficiency[9]. By increasing the Mg-doping level in EBL, the hole concentration can increase and the potential barrier of hole could reduce, leading to enhancement of hole-tunneling efficiency[10]. Moreover, high density of threading dislocations will appear due to the large lattice mismatch and the different thermal expansion coefficient between GaN epilayer and substrate, resulting in the formation of V-pits[11]. And it is found that the V-pits sidewall can improve hole injection efficiency due to the lower polarization charge densities, so the radiative recombination rate will increase[12]. The unintentionally doped EBL can increase the proportion of hole injection rateviaV-pits sidewall, bringing about the appearance of sidewall quantum well emission peak at cryogenic temperature[13]. There have been a variety of improved hole transportation, but it is still far from perfect, deserving further research.

In this work, an additional low-doped p-AlGaN interlayer (IL) after the conventional p-AlGaN electron blocking layers was adopted to optimize the holes injection route and improve the recombination efficiency. The effect of the p-AlGaN IL thickness on the property of InGaN/GaN green LEDs was studied and high efficient green LED was obtained with optimized 25 nm p-AlGaN IL.

2 Experiments

InGaN/GaN green LED structure was grown on 1.2 mm×1.2 mm patterned 2-inch silicon (111) substrates by a “31×2” Thomas Swan metal organic chemical vapor deposition (MOCVD) system. Fig.1(a) shows the epitaxial structure which is consist of 120 nm AlN layer, 2.8 μm n-GaN layer, thirty-two periods of InGaN/GaN(5 nm/2 nm) superlattices layers. Active region consists of ten pairs of green InGaN/GaN MQWs. All the InGaN QWs width is fixed to 3 nm. The barriers are divided to two types with 13 nm GaN barrier and 10 nm GaN barrier respectively, so as to enhance holes injection into the QWs by tunneling process[14]. Lvetal. have reported a highly efficient green InGaN green LED by employing a combination of 13 nm QB and 10 nm QB[15]. Afterwards, the p-type layers consist of 1.5×1019cm-3Mg-doped p-Al0.2Ga0.8N electron blocking layer (EBL) with a fixed 20 nm thickness and then a lowered 7.5×1018cm-3Mg-doped p-Al0.2Ga0.8N interlayer(IL) with different thickness, heavily Mg-doped p-GaN, p-AlGaN/InGaN superlattices layers, lightly Mg-doped p-GaN layer, and heavily Mg-doped p-GaN contact layer. The epitaxial structures have no different except the thickness of lowered 7.5×1018cm-3Mg-doped p-AlGaN IL. Four samples with 0, 6, 25, 38 nm p-AlGaN IL were prepared and labeled as sample A, B, C and D, respectively. All samples contain V-pits in MQWs regions, and have the same V-pits density and top diameter of V-pits. The density is about 8.2×108cm-2and the top diameter is about 170 nm. Fig.1(b) shows the typical V-pits top view SEM image after MQWs growth of these samples. All wafers were bonded on a new Si template and fabricated into 1.1 mm×1.1 mm vertical structure LED chips with roughened n-side up and p-side Ag mirror. The detailed chip fabrication process has been reported[16]. The EL test is characterized by the system consisting of IP250 integrating sphere and the CAS140CT spectrometer made by Instrument System. The temperature-controlling unit made by MMR Technologies, Inc. Chips with 520 nm dominate wavelength under the forward current density 35 A/cm2were evaluated in this work.

Fig.1 (a) Epitaxial structure schematic of the samples with different p-AlGaN interlayer thickness. (b) Typical top view SEM image of V-pits after MQWs growth.

3 Results and Discussion

Fig.2(a)and 2(b) show the curves of EQE and output power as function of operating current density at room temperature, respectively. At 35 A/cm2current density, EQE is 38.2%, 41.8%, 43.6% and 42.5%, the output power is 318.0, 347.7, 362.3, 353.2 mW for sample A, B, C and D respectively. Significantly, EQE and the output power have been greatly improved by employing 25 nm p-AlGaN IL. Especially the very high peak EQE of sample C (25 nm) has been up to 55.4% at 3 A/cm2. From Fig.2(a), the current density can be divided into two regions. The region Ⅰ corresponds to low current density between 0.001 A/cm2and 0.75 A/cm2; the region Ⅱ corresponds to the high current density from 0.75 A/cm2to 75 A/cm2. At the region Ⅰ, the EQE of sample B is higher than sample A, then the EQE of sample C remains same with sample B, but the EQE value decreases severely for sample D with thickest 38 nm p-AlGaN IL. At the region Ⅱ, EQE successively increases from sample A o sample C, but the EQE value turns to decline for sample D with 38 nm p-AlGaN IL. Fig.2(c) indicates that the WPE shows the same trend as EQE, and has also been greatly improved up to 51.3% peak efficiency by employing 25 nm p-AlGaN IL. Fig.2(d) shows the current-voltage (I-V) curves of the samples at room temperature. When the voltage exceeds the threshold voltage, it can be seen that the voltage increases gradually with the rise of p-AlGaN IL thickness. The reason is that the series resistance enlarges gradually with increasing p-AlGaN IL thickness.

Fig.2 (a) EQEversuscurrent density of all samples at room temperature. (b) Output powerversuscurrent density of all samples at room temperature. (c) WPE curves current density of all samples at room temperature. (d)I-Vcurves of all samples at room temperature.

As mentioned above in Fig.1, there is an additional p-AlGaN interlayer(IL) in the epitaxial structure. We consider that different EQE behaviors at low and high current density are correlated with the different p-AlGaN IL thickness, based on LEDs structure with V-pits. Fig.3(a) shows the cross-sectional TEM image located around the large V-pits. This figure shows five periods of MQWs with 13 nm QB and five periods of MQWs with 10 nm QB from n-GaN to p-GaN. Fig.3(b) shows the schematic diagram of V-pits structure, there is a p-AlGaN IL after the EBL. The thickness and indium concentration of V-pits sidewall MQWs are usually much lower compared to that ofc-plane[17]. The p-AlGaN IL thickness of V-pits sidewall is also much smaller than that ofc-plane. Thec-plane p-AlGaN IL thickness is defined asTC, while the sidewall p-AlGaN IL thickness is defined asTV. Correspondingly, thec-plane p-AlGaN IL resistance is defined asRC, while the sidewall p-AlGaN IL resistance is defined asRV. As the p-AlGaN IL thickness increases, Δ(TC-TV) rises gradually. And the Mg-doping concentration of p-AlGaN IL is much lower than that of traditional p-AlGaN. As a result, a large voltage difference betweenc-plane and sidewall of p-AlGaN IL is induced because Δ(RC-RV) enlarges with p-AlGaN IL thickness. The large voltage difference would regulate the proportion of holes injectionviac-plane and V-pits. Path 1 corresponds to the holes injection channel fromc-plane into MQWs, while path 2 corresponds to holes injection channel from V-pits into MQWs. It has been reported that more proportion of holes would be injected from V-pits into the deeper wells close to n-GaN[12-18].

Fig.3 (a) Cross-sectional TEM image of large V-pits. (b) Schematic diagram of V-pits and the difference paths of hole injection into MQWs.

The reported bandgap of V-pits sidewall MQWs is several hundreds of meV higher than that ofc-plane[11]. The higher energy barrier prevents holes from flowing into MQWsviaV-pits. At low current density(region Ⅰ), holes tend to inject mainly through the path 1, we consider that there are two competing mechanisms. One is the defects screening effect promoted by V-pits with increasing the p-AlGaN thickness[19]. The other is the holes injection efficiency reduced with the p-AlGaN IL thickness. It has been reported that the Mg activation energy in p-AlGaN becomes higher and holes concentration would be lower as increasing the AlGaN thickness[20]. The defect screening effect promoted by V-pits is dominant when the p-AlGaN IL thickness increases from 0 nm to 6 nm, the radiative recombination efficiency gets improved. The decline of holes injection efficiency will counteract the enhanced defects screening effect by V-pits with the thickness up to 25 nm, so the EQE remains the same. While the decline of holes injection efficiency becomes dominant when the thickness further increases to 38 nm, so the radiative recombination efficiency gets worse.

Wuetal. reported that the increase of voltage difference can offset the higher energy barrier of sidewall MQWs and drive more proportion of holes flowing through V-pits sidewall[13]. At high current density (region Ⅱ), holes have enough energy to reach MQWs over the height of sidewall barrier. Δ(RC-RV) enlarges by increasing p-AlGaN IL thickness, which leads to the rise of proportion of holes injectionviaV-pits sidewall. So, EQE of sample C gets improved greatly when the thickness increases to 25 nm. But the series resistance of sample D is much larger than others as showed in Fig.2(c), due to the too thick p-AlGaN IL.RVwill also increase with the p-AlGaN IL thickness. Although the proportion of holes injection into QWs risesviaV-pits, the total holes concentration would reduce. So, EQE begins to decrease when the thickness further increases to 38 nm. At the same time, electron leakage is further suppressed with the rise of p-AlGaN IL thickness, the efficiency droop will decrease. So EQE of sample D is higher than that of sample A.

EL properties were also tested at cryogenic temperature. It is well known that the Shockley-Read-Hall (SRH) recombination will be suppressed with the temperature decline[21]. Due to the high ionization energy of Mg acceptor, holes concentration reduces with the decline of temperature[20]. The normalized EL spectra of samples A, B, C and D with the current density at cryogenic temperature of 100 K are shown in Fig.4(a), (b), (c) and (d) respectively. The right EL peak P1 corresponds to the emission from five periods of QWs with 10 nm QB, the left EL peak P2 corresponds to that from five periods of QWs with 13 nm QB. At low current density, only P1 appears for all samples. When the current density rises to 7.5 A/cm2, compared to samples A and B, P2 begins to appear for samples C and D. The intensity of P2 becomes higher in samples C and D from 7.5 A/cm2to 35 A/cm2. Compared to sample C, P2 intensity of sample D is more obvious.

Fig.4 Normalized EL spectra of all samples as a function of injection current density at 100 K. (a) Sample A. (b) Sample B. (c) Sample C. (d) Sample D.

Two types of green quantum wells with a 13 nm barrier and 10 nm barrier were designed for all samples, as described in Fig.3(a). The strain in the QWs with 13 nm barrier is larger than that in the QWs with 10 nm barrier, resulting in the decline of indium incorporation[22].And the underlaying InGaN wells have the strain relief effect on the upper InGaN wells with the increase of QW numbers, resulting in the enhancement of indium incorporation for the upper InGaN wells[23-24]. So, the five periods of green QWs with 10 nm barrier have higher indium composition than five periods of green QWs with 13 nm barrier, the emission wavelength of five periods of green QWs with 10 nm barrier is longer than that of the latter. For sample C and D, P2 peak emitted from five-period green QWs with 13 nm barrier can be found in spectra, which can’t be found for samples A and B. This means that more proportion of holes are injected into MQWsviaV-pits assisted by the p-AlGaN interlayer, and can be injected to deeper QWs close to n-GaNviapath 2.

In the samples A and B, Δ(RC-RV) is not the predominant factor, the p-AlGaN IL has no significant effect on enhancing holes injectionviaV-pits. So, most proportion of holes are injected into MQWs by path 1, can mainly flow into the five periods of green QWs with 10 nm barrier. So there is only the right EL peak P1 whether at low or high current density. For sample C and D, more proportion of holes will reach MQWs from path 1 at low current density due to higher energy barrier of sidewall MQWs, so there is also only one the right EL peak P1. When the current density enlarges, the increased Δ(RC-RV) by p-AlGaN IL has effects on enhancing holes injectionviaV-pits. Although the total concentration of holes will reduce with thicker p-AlGaN IL, the proportion of holes injected into MQWs from path 2 increase at high current density, and can be injected into the five periods of green QWs with 13 nm barrier. So, the left EL peak P2 arises for samples C and D, and the P2 intensity of sample D is more obvious than that of sample C. The p-AlGaN IL thickness variation can optimize the holes injection route and enhance holes injection into deeper QWsviaV-pits, so two types of emission peaks appear.

4 Conclusion

In summary, an additional optimized 25 nm p-AlGaN interlayer was designed to significantly improve the external quantum efficiency for InGaN/GaN green LEDs with V-pits on Si (111) substrate. At 35 A/cm2current density, the external quantum efficiency (EQE) increases from 38.2% to 43.6% and the output power increases from 318.0 mW to 362.3 mW. This is a new record for green InGaN-based LED. The physical mechanisms are analyzed and attributed to the enhanced holes injection efficiencyviaV-shaped pits assisted by the p-AlGaN interlayer. At the same time, more holes would be injected to the deeper quantum wells, which leads to the short wavelength emission occurring in EL spectrum. These results are meaningful for regulating holes injection especially for those InGaN/GaN LED with V-shape pits, and developing high efficiency InGaN/GaN LED.