Influence of Carrier Distribution on The Frequency Behavior for GaN-based LEDs

WU Chun-hui, ZHU Shi-chao, FU Bing-lei, LIU Lei, ZHAO Li-xia, WANG Jun-xi, CHEN Hong-da

(1. Semiconductor Lighting Research and Development Center, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China;2. State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China;3. Science & Technology Department, CETC Electronics Equipment Group Co., Ltd., Beijing 100083, China)

Influence of Carrier Distribution on The Frequency Behavior for GaN-based LEDs

WU Chun-hui1,2, ZHU Shi-chao1*, FU Bing-lei1,3, LIU Lei1, ZHAO Li-xia1, WANG Jun-xi1, CHEN Hong-da2

(1.SemiconductorLightingResearchandDevelopmentCenter,InstituteofSemiconductors,ChineseAcademyofSciences,Beijing100083,China;2.StateKeyLaboratoryonIntegratedOptoelectronics,InstituteofSemiconductors,ChineseAcademyofSciences,Beijing100083,China;3.Science&TechnologyDepartment,CETCElectronicsEquipmentGroupCo.,Ltd.,Beijing100083,China)

The electrical and optical properties of GaN-based high power LEDs were investigated under both DC and AC bias. The results show that the carrier distribution of the active region can be modified by changing the indium concentration of the last quantum barrier. The accumulated electrons in the active region can lead to the negative capacitance effect. The improved carrier transport property for LEDs with lower quantum barrier also helps to increase the recombination rate and modulation bandwidth by 20%. This work will help to understand the influence of carrier distribution on the frequency behavior of GaN-based LEDs.

GaN-based; light emitting diodes; visible light communication; modulation bandwidth; carrier distribution

1 Introduction

High-power LEDs offer many advantages over incandescent, fluorescent, and discharge light sources, including longer lifetime, smaller size and higher energy efficiency. With the increase of the luminous efficacy, LEDs are fast replacing traditional light sources in numerous applications, such as illumination and displays. Besides, LEDs can also be modulated as sources for data transmission, which is the so-called visible light communication (VLC) technology[1-3].However, the optical modulation bandwidth of conventional commercial LEDs is still quite low[2-4],which restricts the development and application of VLC. Investigation of carrier transport characteristics and frequency behavior is important to the design of high speed LEDs for VLC.

Because of the large mobility of electrons and the existence of the electron blocking layer (EBL)[5],the carrier distribution is always asymmetry, and both electrons and holes will be mainly confined in the upmost InGaN quantum well (QW)[6-7]. Special design at the barrier close to p-type layer (namely the last barrier) is an effective method to change the carrier distribution in the active region of LEDs. The influence of the carrier distribution in the active region on the optical properties of LEDs has been studied under DC bias[8]. But how the carrier distribution will influence the modulation bandwidth under AC bias is still not clear, which is actually more important for VLC application. In this study, we designed and fabricated two kinds of LEDs with different indium concentration in the last quantum barrier. The optical properties have been investigated under both DC and AC bias. The carrier distributions have also been simulated. The results show that the carrier distribution of the active region can be effectively modified by changing the indium concentration in the last quantum barrier, and the accumulated electrons in the active region will lead to the negative capacitance effect and influence the frequency behavior. This work will benefit the design of high speed LEDs for VLC application.

2 Experiments

The LEDs were grown using metal-organic chemical vapor deposition (MOCVD), followed by a standard chip processing and encapsulation procedure. The chip active area is roughly about 1.17 mm2. The LED structure consists of a 2.0 μm thick undoped GaN layer (not shown in the figure), a 2.0 μm thick Si-doped n-GaN layer, a multiple quantum well (MQW) active region with five pairs of 6 nm GaN quantum barrier (QB) and 3 nm In0.15Ga0.85N quantum well (QW). As for the last quantum barrier, a normal GaN layer was deposited for LED-Ⅰ as usual, while for LED-Ⅱ, an In0.05Ga0.95N layer was used instead in order to change the barrier offset. Afterwards, a 3 nm p-In0.05Ga0.95N, a 40 nm thick p-Al0.15Ga0.85N EBL and a 250 nm thick main Mg-doped p-GaN layer were grown. The schematic LED structures are illustrated in Fig.1.

Fig.1 Schematic structure of two LEDs. The last quantum barrier of LED-Ⅰ and LED-Ⅱ is different, which is GaN and InGaN, respectively.

TheI-Vcurves were measured by Keithley 2400. The EL of the encapsulated chips was measured in an integrating sphere in the continuous-wave current mode. The device capacitances were measured by an Agilent 4294A impedance analyzer with the frequency range from 40 Hz to 110 MHz. During the impedance measurements, LEDs were fastened in an Agilent 16047E test fixture with a bandwidth of 110 MHz. The frequency sweep type was set as logarithmic and the voltage oscillation level was 100 mV. Optical responses from 500 kHz to 30 MHz were measured using a network analyzer (Agilent E5061B). All the measurements were carried out at room temperature.

The LED structures in the APSYS simulations were set the same as the epitaxial wafer in the experiments. As obtained from the Hall effect measurement, the hole concentration in the p-InGaN and p-GaN layer of both LEDs were set to be 4×1018cm-3and 5×1017cm-3, respectively. The energy band offset ratio between the conduction band and the valence band was assumed to be 0.7/0.3. Considering the crystal relaxation caused by the generation of misfit dislocations, 50% of the theoretical polarization charge density was assumed[9]. The Auger recombination coefficient and the SRH lifetime for electrons and holes were chosen as 5×1030cm6/s and 10 ns, respectively[10]. Most of the parameters used in this paper are the same as in Ref.[11]. Other material parameters of the semiconductors used in the simulation can be found in Ref.[12].

3 Results and Discussion

Fig.2 shows the output optical power as a function of the current (L-I) for both LEDs. It shows that the optical power of both LEDs increases with the injected current increasing. At 500 mA, the optical power of LED-Ⅰ and LED-Ⅱ is 201 and 156 mW, respectively. Compared with LED-Ⅰ, the optical power of LED-Ⅱ is decreased by 22%. The reason is mainly due to the decreased last quantum barrier in LED-Ⅱ, which led to a weaker confinement of electrons in the last several quantum wells. The inset of Fig.2 shows theI-Vcharacteristics of two LEDs. The threshold currents of both LEDs are similar, and the series resistances are roughly about 0.3 and 0.6 Ω for LED-Ⅰ and LED-Ⅱ, respectively. In addition, at reverse bias of -10 V, the current of both LEDs is roughly about 3 μA, indicating the crystal quality for the MQW active region of the two LEDs are similar as well.

Fig.2 Output optical power as a function of current for LED-Ⅰ and LED-Ⅱ. The inset shows theI-Vcurves of two LEDs.

The differential capacitances of these two different LEDs under reverse bias were compared as well. In Fig.3, the differential capacitance of LED-Ⅱ is smaller than that of LED-Ⅰ, which indicates less carriers depleted in LED-Ⅱ. The frequency dependence of the differential capacitance for the two LEDs at forward current of 10 and 90 mA were also measured. As shown in the inset of Fig.3, the capacitance is negative at low frequency or large bias current[13-16].This negative capacitance (NC) effect has been reported to be related to the carrier accumulation in the active region[16].Here, the accumulated carriers are mainly determined by the electrons in the last quantum well. The accumulated electrons under DC bias will delay or prevent the further injection of the electrons with the increase of the small signal voltage, which results in the NC effect. Since the NC effect is more obvious for LED-Ⅰ, this indicates that there are more electrons accumulated in LED-Ⅰ than that in LED-Ⅱ. With the frequency increasing, the NC effects decreased for both LEDs. This is because that, with increasing the frequency, the carrier recombination becomes more difficult to catch up with the small signal modulation voltage and herein the NC effect is only obvious at low frequency region.

Fig.3 Differential capacitances as a function of reverse voltage for two LEDs at 100 kHz. The inset shows frequency responses of the differential capacitance for these two LEDs measured at forward current of 10 and 90 mA.

In order to clarify the carrier distribution in the different LED structures, we calculated the energy band diagrams and the electron distribution using APSYS simulation. The energy band diagrams of LED-Ⅰ and Ⅱ are shown in Fig. 4(a) and (b), respectively. For both structures, the barrier heights of the EBL layer for electrons are almost the same. However, the GaN last quantum barrier of LED-Ⅱ has a lower potential than LED-Ⅰ. Thus, it can be seen from Fig.4(c) that under the same injection current, LED-Ⅱ have less confined carriers than LED-Ⅰ, which is consistent with the measured optical power. Meanwhile, less accumulated electrons will reduce the screened coulomb repulsion on the injection of electrons and accordingly a smaller negative capacitance compared with LED-Ⅰ.

Fig.4 Calculated energy band diagrams of (a) LED-Ⅰ and (b) LED-Ⅱ. Black lines are the energy bands, red lines present the Fermi level. Green dashed lines indicate the last quantum barriers and the p-InGaN layers of the two LEDs. (c) Calculated electron distribution of the two LEDs at 90 mA.

The normalized optical responses of two LEDs were also measured, as shown in the inset of Fig.5. It can be seen that, at low frequency, the optical response decreases slowly with the frequency increasing but abovef3dBthe optical response decreases rapidly. It is because that, with increasing the frequency, the carrier recombination becomes more difficult to catch up with the small signal modulation voltage. The modulation bandwidth of the two LEDs as a function of the current was also measured. The bandwidth for both devices increases with increasing the injected current.

Fig.5 Modulation bandwidth of the two LEDs as a function of bias current. The inset shows the normalized optical response of the two LEDs at bias current of 300 mA.

Based on the measuredI-VandC-Vcharacteristics in Fig.2 and Fig.3, both the RC limited bandwidths were calculated to be more than 680 MHz, which are far beyond the measured bandwidths. Therefore, the modulation bandwidths of both LEDs are mainly limited by the carrier recombination lifetime[17-18]. With the increasing of the carrier concentration， the carrier lifetime will be reduced and result in the increase of the modulation bandwidth for LEDs. In addition, as the case for LED-Ⅱ, the lower quantum barrier will improve the carrier transport probability and enhance the carrier recombination rate. Therefore, the modulation bandwidth of LED-Ⅱ increased by 20% compared with LED-Ⅰ with GaN quantum barriers. If increasing the hole injection efficiency by modulating the EBL or p-InGaN layer, it will help to improve the tradeoff between the optical power and modulation bandwidth[19], which will be further studied in future.

4 Conclusion

The influence of the carrier distribution on the frequency behavior for GaN-based high power LEDs has been investigated. The results show that the carrier distribution of the active region can be modified by changing the indium concentration in the last quantum barrier. The accumulated electrons in the active region will lead to the negative capacitance effect. Furthermore, because of the lower last quantum barrier, the improved carrier transport properties also help to increase the modulation bandwidth. Our findings will help to understand the dependence of the modulation bandwidth on the carrier distribution and design the high speed GaN-based LEDs for visible light communication.

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吴春晖(1982-)，男，河北保定人，博士研究生，2006年于清华大学获得硕士学位，主要从事可见光通信方面的研究。

E-mail: wuch@semi.ac.cn

朱石超(1990-)，男，湖北武汉人，博士研究生，2012年于华中科技大学获得学士学位,主要从事可见光通信光电器件的研究。

E-mail: sczhu@semi.ac.cn

2016-09-03；

2016-09-29

国家自然科学基金(11574306)； 中国国际科技合作计划(2015AA03A101,2014BAK02B08，2015AA033303)资助项目 Supported by National Natural Science Foundation of China(11574306); China International Science and Technology Cooperation Program(2015AA03A101,2014BAK02B08,2015AA033303)

载流子分布对GaN基LED频率特性的影响

吴春晖1,2, 朱石超1*, 付丙磊1,3， 刘 磊1， 赵丽霞1， 王军喜1， 陈宏达2

(1. 中国科学院半导体研究所 半导体照明研发中心， 北京 100083；2. 中国科学院半导体研究所 集成光电子学国家重点实验室， 北京 100083；3. 中电科电子装备集团有限公司， 北京 100070)

分别在直流偏置和交流偏置下，对大功率GaN基LED的电学和光学特性进行了研究。结果显示，通过改变靠近p型层的量子垒(也就是最后一个量子垒)中的In组分可以调控有源区中的载流子分布。有源区内积累的电子会引起负电容效应。而通过降低有源区量子垒的势垒高度，可以改善LED中载流子传输特性，并实现载流子复合速率及通信调制带宽20%的提高。这个工作将有助于理解GaN基LED中载流子分布对频率特性的影响，并为设计适用于可见光通信的大功率高速LED奠定基础。

氮化镓； 发光二极管； 可见光通信； 调制带宽； 载流子分布

1000-7032(2017)03-0347-06

TN383+.1 Document code： A

10.3788/fgxb20173803.0347

*CorrespondingAuthor,E-mail:sczhu@semi.ac.cn