Volume 50 Issue 10
Oct.  2021
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Jia Tiantian, Dong Hailiang, Jia Zhigang, Zhang Aiqin, Liang Jian, Xu Bingshe. Influence of indium composition of n waveguide layer on photoelectric performance of GaN-based green laser diode[J]. Infrared and Laser Engineering, 2021, 50(10): 20200489. doi: 10.3788/IRLA20200489
Citation: Jia Tiantian, Dong Hailiang, Jia Zhigang, Zhang Aiqin, Liang Jian, Xu Bingshe. Influence of indium composition of n waveguide layer on photoelectric performance of GaN-based green laser diode[J]. Infrared and Laser Engineering, 2021, 50(10): 20200489. doi: 10.3788/IRLA20200489

Influence of indium composition of n waveguide layer on photoelectric performance of GaN-based green laser diode

doi: 10.3788/IRLA20200489
Funds:  Supported by the National Natural Science Foundation of China (61904120, 21972103, 61604104,51672185); the National Key R&D Program of China (2016YFB0401803); the Basic Research Projects of Shanxi Province (201801D221124, 201801D121101, 201901D111111, 201901D211090, 201601D202029), the Key Shanxi Provincial R&D Program (201803D31042); the Shanxi Provincial Key Innovative Research Team in Science and Technology (201605D131045-10)
  • Received Date: 2020-11-18
  • Rev Recd Date: 2021-01-19
  • Publish Date: 2021-10-20
  • The theoretical simulation of the extension structure of high-power GaN-based laser diodes is of great significance to improve the photoelectric performance of GaN-based laser diodes. A green laser diode extension structure with an n-side dual-wave conductor structure was designed. The effect of indium parts in the n-InxGa1−xN waveguide layer on its photoelectronic performance in laser extension structure was discussed. And the mechanism of the n-InxGa1−xN waveguide layer on the photoelectronic performance of laser diode was clarified. The results showed that when the indium part of the n-side InxGa1−xN waveguide layer was 0.07, the photon loss was minimal, and the threshold current was the lowest. When the indium part of the n-side waveguide layer was high or low, photon loss and operating voltage were increased, and meanwhile, the output power of the laser diode was reduced. Therefore, by regulating indium parts in the n-InxGa1−xN waveguide layer and controlling the optical field distribution of the outer layer, the photon loss was reduced by 0.2 cm−1, and the threshold current was reduced by 193.49 mA to 115.98 mA, and the operating voltage was reduced, which increased the output power and electro-optical conversion efficiency of the laser diode, increased the laser output power to 234.95 mW at 6 kA/cm2. The n-side dual-waveguide structure design provides theoretical guidance and data support for the preparation of high-power green laser diodes.
  • [1] Liu Y, Liu Y, Xiao H D, et al. 638 nm narrow linewidth diode laser with a grating external cavity [J]. Chinese Optics, 2020, 13(6): 1249-1256. (in Chinese) doi:  10.37188/CO.2020-0249
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Influence of indium composition of n waveguide layer on photoelectric performance of GaN-based green laser diode

doi: 10.3788/IRLA20200489
  • 1. Key Laboratory of Interface Science and Engineering in Advanced Materials Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China
  • 2. College of Textile Engineering, Taiyuan University of Technology, Taiyuan 030024, China
  • 3. College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China
  • 4. Institute of Atomic and Molecular Science, Shaanxi University of Science & Technology, Xi’an 710021, China
Fund Project:  Supported by the National Natural Science Foundation of China (61904120, 21972103, 61604104,51672185); the National Key R&D Program of China (2016YFB0401803); the Basic Research Projects of Shanxi Province (201801D221124, 201801D121101, 201901D111111, 201901D211090, 201601D202029), the Key Shanxi Provincial R&D Program (201803D31042); the Shanxi Provincial Key Innovative Research Team in Science and Technology (201605D131045-10)

Abstract: The theoretical simulation of the extension structure of high-power GaN-based laser diodes is of great significance to improve the photoelectric performance of GaN-based laser diodes. A green laser diode extension structure with an n-side dual-wave conductor structure was designed. The effect of indium parts in the n-InxGa1−xN waveguide layer on its photoelectronic performance in laser extension structure was discussed. And the mechanism of the n-InxGa1−xN waveguide layer on the photoelectronic performance of laser diode was clarified. The results showed that when the indium part of the n-side InxGa1−xN waveguide layer was 0.07, the photon loss was minimal, and the threshold current was the lowest. When the indium part of the n-side waveguide layer was high or low, photon loss and operating voltage were increased, and meanwhile, the output power of the laser diode was reduced. Therefore, by regulating indium parts in the n-InxGa1−xN waveguide layer and controlling the optical field distribution of the outer layer, the photon loss was reduced by 0.2 cm−1, and the threshold current was reduced by 193.49 mA to 115.98 mA, and the operating voltage was reduced, which increased the output power and electro-optical conversion efficiency of the laser diode, increased the laser output power to 234.95 mW at 6 kA/cm2. The n-side dual-waveguide structure design provides theoretical guidance and data support for the preparation of high-power green laser diodes.

  • 近年来,GaN基激光二极管(Laser Diodes,LD)在高密度存储、显示、投影方面具有广泛的应用[1-7],并且InGaN材料的发光光谱从红外到紫外覆盖了整个可见光光谱[8]。相对于技术较成熟的蓝光LD和红光LD,目前市场上比较稳定的绿光LD普遍是倍频的固体LD,其通过倍频晶体转换红外波长激光,获得绿光激光[9]。国内外在InGaN蓝光LD研究方面取得了重大进展[10],不仅蓝光LD的输出功率得到明显提高,而且光学性质得到改善。随着InGaN蓝光LD波长向绿光长波方向扩展,InGaN波导层和AlGaN限制层材料之间的折射率差值减小会导致绿光光子反射率降低,导致光场向有源区p侧偏移,限制层对光场的限制作用减小,降低光学限制因子,进而产生光子泄漏。因此,光场偏移导致了光子损耗增加,不仅增加了阈值电流,而且降低了LD的输出功率[10-11]。然而,为了获得高性能更长波长的激光二极管,一般采用增加n-AlGaN包层的厚度的方法来减少光子泄漏,但是这对减少光子泄漏效果差。该方法主要用于提高激光光束的质量,但厚的包层会导致LD工作电压升高。同时,金属有机化学气相沉积方法生长厚的AlGaN层生长会导致器件的稳定性降低[12]。采用提高n-InGaN波导层铟组分或增加n-InGaN波导层厚度的方法来减少光子泄漏,不仅增加了生长高质量的激光器的难度,而且会增大LD的串联电阻,从而导致工作电压升高[13];采用对称单波导层结构的LD存在光子在p型层的吸收比较大和光子泄漏大的问题。采用非对称n侧双波导InGaN蓝光LD结构,提高InGaN波导层厚度使光学模在GaN衬底方向的光子泄漏完全被抑制[14-15]

    针对绿光InGaN LD存在严重的光子泄漏和低输出功率的问题,采用k·p理论模型设计了非对称双波导结构,通过调控n侧波导层的铟组分增加波导层和限制层的折射率差,从而调控外延层中光场的分布,将光场控制在量子阱区和波导区,增加光学限制因子,不仅降低光子损耗,而且还降低LD的工作电压和阈值电流;通过调控n-InxGa1−xN波导层铟组分来调节波导层的带隙,不仅降低空穴注入的势垒,而且增加空穴注入效率,降低非辐射复合效率,提高内量子效率,从而提高LD的输出功率和电光转换效率。

  • 采用SiLENSe (Simulator of Light Emitters based on Nitride Semiconductors)仿真软件进行理论仿真实验。InGaN基绿光LD的外延结构如图1所示,其外延参数如表1所示。文中将优化下n侧InxGa1−xN层波导铟组分,取值0.04、0.05、0.06、0.07和0.075,其他结构参数都相同。通过改变n-InxGa1−xN波导层铟组分实现调控LD的光场分布。对LD器件结构进行模拟,设置偏压3~12 V,腔长800 μm,宽度10 μm,在设计材料时外延层的电子和空穴的迁移率分别为200 cm2V−1s−1和20 cm2V−1s−1,带偏移比ΔEcEv=0.7/0.3,俄歇系数(CPCn) InN为2.5×10−30 cm6s−1,GaN为0 cm6s−1,电子和空穴的非辐射寿命为0 s。激光器器件的光电性能都是在工作温度为300 K时计算获得,整个外延结构中每层的缺陷密度为1×106 cm−2

    Figure 1.  Epitaxial structure of the InGaN-based green laser diode

    NameThicknessConcentration
    /cm−3
    Mobility
    /cm2V−1s−1
    p-Contact layer 100 nm p-GaN 1×1020 200/20
    p-Cladding layer (CL) 500 nm p-Al0.12In0.01Ga0.87N 1×1020 200/20
    p-
    Waveguide layer (WG)
    70 nm p-In0.04Ga0.96N 2×1019 200/20

    Electron blocking layer (EBL)
    14nm p-Al0.18In0.01Ga0.81N 5×1018 200/20

    Quantum well (QW)/
    Quantum barrier
    (QB) (×2)
    3.5 nm In0.29Ga0.71N/11 nm Al0.05In0.01Ga0.94N 0/6×1018 3000/30/
    200/20
    QW 3.5 nm In0.29Ga0.71N 0 3000/30
    n-WG 47.5 nm InxGa1-xN 5×1018 200/20
    n-WG 100 nm n-GaN 5×1018 200/20
    n-CL 550 nm n-Al0..09In0.01Ga0.9N 6×1018 200/20
    n-Contact layer 100 nm n-GaN 6×1018 200/20

    Table 1.  Structural parameters of InGaN-based green laser diode

  • 由于电子对光子的吸收较小,而空穴对光子的吸收较大,相对于p侧的波导层和限制层,光子在传输过程中在n侧波导层和限制层区的损耗较小,p限制层中的空穴浓度较高,光损耗也较大。为了减少p侧空穴对光子的吸收。因此,调控光场向n侧分布有助于降低光子损耗。由于低铟组分的n-InxGa1−xN波导层和限制层之间的折射率差减小,限制光场的能力减弱,导致光子泄漏。因此,n-InxGa1−xN波导层中铟组分是影响折射率差值的关键因素之一,是调控光场分布和实现高功率输出的关键参数[12]

    对于525 nm 激光器而言,InN和GaN的折射率分别设置为2.88[12]和2.42[13],波长在525 nm时x为0.04~0.08的InxGa1−xN折射率曲线如图2所示。

    Figure 2.  Refractive index of InxGa1-xN with x composition 0.04-0.08 at the wavelength of 525 nm

    为了研究波导层铟组分变化对光场分布以及折射率差的影响,将讨论n-InxGa1−xN波导层不同铟组分的光场分布,如图3所示,TE模限制在波导层区和有源区,这主要是由于波导层与限制层之间折射率的差值较大,限制光的能力较大。由图3(a)可以看出:铟组分从0.04~0.075,TE1模的强度先增大后减小,这表明限制层对光场的限制先增加后减小。同时光场向n侧波导移动,使光场偏离p侧波导区,减少光场在p限制层区的泄漏。当铟组分增加到0.075时,波导层和限制层间的折射率差从0.06增加到0.1,限制层对光场的限制作用逐渐增强。但铟组分为0.075时,光子泄漏增加,光场强度降低,如图3(b)所示。这主要是由于铟组分增加,InxGa1−xN 折射率增大,限制层对光子限制减弱,从而导致向衬底泄漏会增强[14]Гv从n-InxGa1−xN波导层铟组分为0.04时的1.90%增加到铟组分为0.07时的2.75%,而后降低到0.075时的1.54%,进而验证了随着n-InGaN波导层铟组分的增加,波导层和限制层之间的折射率对比度增加,减少了光子向n限制层方向泄漏[11, 13]。但当铟组分过高时,限制层对光子限制减弱 [14]。因此,当铟组分为0.07时,光场主要分布在波导层区,减少了p型限制层对光的吸收,从而降低了光子损失。光子损耗机理将在下面部分进行详细讨论。

    Figure 3.  Refractive index profiles and intensity distributions of TE-modes versus the different n-InxGa1−xN waveguide indium content ((b) is the magnification of (a) from 550 nm to 1050 nm)

    LD的阈值电流密度受内部光子损耗影响较大,内部光子损耗越大,LD的阈值电流越高,斜率效率越低。因此,光子损耗的研究对于提升GaN基LD的性能具有重要意义。光场分布影响着光子损耗,这主要是由于外延层中电子和空穴对光子的吸收系数不同导致[16]。光子损耗越大,光子泄漏越多,光场强度越弱。内部光子损耗(αTotal)则是由量子阱内(αQW)和阱外(αout)的损耗组成,从图4中可以看出,αoutαTotal最小值的铟组分是0.07,而αQW最小值为0.04。当铟组分从0.04增加到0.07时,在注入电流为0.5 A时(电流密度为6 kA/cm2),αTotalαout值减少了0.2 cm−1,从0.07增加到0.075时,αTotalαout值增加了0.05 cm−1,如图4(a)(c)所示。这主要是由于铟组分增加使n侧InGaN波导层和AlGaN限制层材料之间的折射率差值增大,减少了光子损耗,从而降低了αout[10, 14],但过高的铟组分由于多模激射会增加光子损耗。组分从0.04增加到0.07时,αQW值增加了0.04 cm−1,如图4(b)所示,这是由于铟组分的增加,量子阱和n侧波导层之间的折射率差就会增加,使光场远离p型区域,尤其是具有较大吸收系数的电子阻挡层,从而导致大的αQW[17]。但组分从0.04增加到0.07时,αQW值降低了0.02 cm−1,这也是多模激射引起的。因此,波导层铟组分为0.07时光子泄漏最少,光损耗最小。

    Figure 4.  Optical loss outside multiple quantum wells ( αout) (a), optical loss in multiple quantum wells (αQW) (b) and total optical loss (αTotal) (c) of laser diode versus the injected current for different n-InxGa1−xN waveguide indium content (The arrow indicates 0.5 A or 6 kA/cm2)

  • 内量子效率(Internal Quantum Efficiency,IQE)、输出功率和电光转换效率是LD器件重要的电学参数,漏电流对IQE有着重要的影响。在注入电流为6 kA/cm2时,铟组分为0.04器件的漏电电流密度最大,注入效率最小,而0.07的器件漏电电流密度比较小,从图中也可以看出铟组分的变化对泄漏电流密度对影响不大,同时0.07的注入效率最大,如图5(a)(b)所示。注入效率越大,漏电电流流密度越小,器件的IQE越大[18]。从图5(c)中可以看出,铟组分是0.07时IQE最大。这主要是由于当空穴的注入增加,电子的泄漏减少,空穴和电子在有源区的辐射复合就会增强,进而提高IQE。注入电流为6 kA/cm2时IQE最大值为80%,因此,n-InxGa1−xN波导层最佳组分为0.07时获得最高的内量子效率。

    Figure 5.  Leakage current density (a), injection efficiency (b) and IQE (c) of laser diode versus the injected current for different n-InxGa1−xN waveguide indium content

    从能带结构的角度对漏电流、注入效率与n-InxGa1−xN波导层铟组分之间的相互影响关系进行解释。图6为LD在电流密度为6 kA/cm2时不同铟组分的能带结构图。从铟组分为0.04、0.05、0.06、0.07和0.075的能带图中可以观察到,价带中的三角形势垒平整。为了揭示能带对电子泄漏的影响,电子费米能级与n-InxGa1−xN WG或AlGaN EBL的导带(Conduction bands)之间的电位差(Potential Difference,PD)表示为PDn-InxGa1-xN WG-cond 或PDEBL-cond。电子泄漏的势垒高度为PDn-InxGa1-xN WG-cond和PDEBL-cond之间的差值。从图6可以看出,铟组分为0.04、0.05、0.06、0.07和0.075电子泄漏的势垒高度分别为2.14、2.15、2.15、2.15、2.15 eV。阻挡电子泄漏的势垒高度从2.14 eV先增加到2.15 eV。因此,提高铟组分对增强势垒高度来减少电子泄漏并没有太大的影响,但是过高铟组分会降低电子泄漏势垒高度。此外,铟组分对空穴注入的影响可以用来解释价带对空穴传输的影响。在图6中,PDn-InxGa1-xN WG -Vale为n-InxGa1−xN WG层价带费米能级与AlGaN EBL的价带(Valence bands)之间的电位差;PDEBL-Vale为在AlGaN-EBL空穴费米能级与AlGaN势垒层(Barrier)价带之间的电位差。势垒的高度为PDn-InxGa1-xN WG -Vale和PDEBL-Vale之和。铟组分为0.04、0.05、0.06、0.07和0.075时势垒高度的值分别为2.80、2.83、2.81、2.86、2.81 eV。InxGa1−xN带隙公式:Eg(x)=1.43x2−4.08x+3.42[11]。随着铟组分的增加,InxGa1−xN带隙减小,从而减小空穴注入的势垒。从上述对能带的分析可以得出结论,铟组分为0.07时的LD不仅增加了电子泄漏的势垒高度,而且也降低了空穴注入的势垒高度[19-20],从而LD在铟组分为0.07时IQE获得最高值为80%。

    Figure 6.  Band energy versus of different n-InxGa1−xN waveguide indium contents at injection current 0.5 A (Injection density is 6 kA/cm2

    LD的IQE同时受非辐射电流密度影响较大。n-InxGa1−xN波导层铟组分的增加可以减少电子和空穴在除量子阱区外的其他区复合,提高辐射复合率,从而提高内量子效率[21]。当载流子浓度在有源区较高时,非辐射复合主要依赖于俄歇复合,如图7所示。因为非辐射复合率被定义为Rnr=Cnp2,其中C为俄歇复合系数;np分别为有源区区电子和空穴的浓度。在室温下的III-V族半导体中,系数C实际上与载流子浓度无关。因此,非辐射复合过程的速率主要取决于载流子浓度,而有源区在铟组分为0.04时载流子浓度最大,故非辐射复合电流密度在0.04时最大,如图7(d)所示。此外,当注入电流超过阈值电流时,俄歇复合电流密度保持不变,同时非辐射复合电流密度也不变。当铟组分为0.07时,非辐射复合电流密度降到最小。因此,器件在铟组分为0.07时获得了高的IQE。

    Figure 7.  Curves of nonradiative current density (a), SRH current density (b), Auger current density (c) and active region carrier concentration (d) versus injected current at different indium components of n-InxGa1−xN waveguide layers

    图8给出了LD的阈值电流和电压与n-InxGa1−xN波导层铟组分的关系。从图8(a)中可以看出,铟组分为从0.04增加到0.075,阈值电流从193.19 mA先降低到115.98 mA后升高到132.33 mA。LD阈值电流的大小主要取决于光学损耗和IQE [22],当铟组分提高时,光子损耗先降低后升高和IQE的提高会导致阈值电流的先升高后降低。从图8(b)中可以看出,波导层的铟组分对LD的工作电压并没有明显的变化,曲线呈增大的趋势。这主要是由于LD的电压与外延层的串联电阻有关,等效电阻越大,工作电压就会随之增大[23]。铟组分为0.04时器件的工作电压最大,组分0.07时器件的电压最小。这主要是由于铟组分增加使得电子泄漏先减少后增加,空穴注入先增加后减小,从而使得电压先减小后增加。因此,铟组分为0.07时,器件的阈值电流和电压最低。

    Figure 8.  Threshold current (a) and voltage (b) of laser diode with n-InxGa1−xN waveguide indium contents versus injected current

    LD的光输出功率和电光转换效率是LD最关键的电学参数。由图9(a)中可以看出,随着InGaN波导层铟组分从0.04增加至0.075,LD的光最大输出功率从152.9 mW增加到234.95 mW后降低至164.42 mW。这是由于随着n-InxGa1−xN波导层铟组分的增加,LD的光子损耗先逐渐减小后增大,同时载流子注入效率先逐渐增加后减小所致。随着铟组分增加,可以将光场限制在n侧波导层中,光子损耗先减少后增大,同时波导层折射率差增加[14],光限制因子先增加后减小,使得阈值电流先减小后增大[16],同时使得输出功率增大[24]。铟组分为0.07时器件的输出功率最高。

    Figure 9.  Simulated power (a) and conversion efficiency (b) of laser diode with n-InxGa1−xN waveguide indium contents versus injected current

    LD的光电转换效率取决于LD的功率P、电流I和电压V。在注入电流为6 kA/cm2时,0.07时器件的电光转换效率接近7.8%,如图9(b)所示。主要原因是随着n-InxGa1−xN波导层铟组分增加,输出功率先增大,且工作电压降低,从而电光转换效率增加;但铟组分继续增加时,输出功率降低,工作电压提高,电光转换效率降低。并且LD的电光转换效率取决于LD的阈值电流[18, 25],阈值电流越大,光电转换效率越低。因此,当n-波导层铟组分为0.07时,可以改善器件的光子泄漏、输出功率以及电光转换效率等特性。

  • 分析了光场中各种横模分布、光子损耗以及电学性能,对InGaN半导体绿光LD的n-InxGa1−xN侧波导层不同铟组分进行了仿真计算。研究结果表明,n波导层铟组分增加,使限制层和波导层间的折射率差增加,调控光场在外延层分布,使光学限制因子从1.89%增加到2.75%,光子泄漏减少,光子损耗减少。同时随着n波导铟组分的增加,载流子注入效率增加,电子泄漏减少,非辐射复合电子密度降低,从而内量子效率提高,且使得激光二极管的工作电压随n波导铟组分的增加而减小,阈值电流从195.51 mA降低到112.76 mA。当注入电流为0.5 A时,获得80%的内量子效率、7.2%的最佳电光转换效率和284.28 mW的输出功率。通过调控InGaN波导层铟组分为制备高输出功率和光学特性的边发射激光二极管提供了理论实验数据。

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