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High-speed 850 nm vertical-cavity surface-emitting lasers with BCB planarization technique

Xiao-ying HE Jian DONG Shuai HU Yan HE Ben-shun LV Xin-xin LUAN Chong LI 安琪 胡 Zong-hai HU Xia GUO

何晓颖, 董建, 胡帅, 何艳, 吕本顺, 栾信信, 李冲, 胡宗海, 郭霞. 采用BCB平整技术的高速850 nm垂直面发射激光器[J]. 中国光学, 2018, 11(2): 190-197. doi: 10.3788/CO.20181102.0190
引用本文: 何晓颖, 董建, 胡帅, 何艳, 吕本顺, 栾信信, 李冲, 胡宗海, 郭霞. 采用BCB平整技术的高速850 nm垂直面发射激光器[J]. 中国光学, 2018, 11(2): 190-197. doi: 10.3788/CO.20181102.0190
HE Xiao-ying, DONG Jian, HU Shuai, HE Yan, LV Ben-shun, LUAN Xin-xin, LI Chong, 胡 安琪, HU Zong-hai, GUO Xia. High-speed 850 nm vertical-cavity surface-emitting lasers with BCB planarization technique[J]. Chinese Optics, 2018, 11(2): 190-197. doi: 10.3788/CO.20181102.0190
Citation: HE Xiao-ying, DONG Jian, HU Shuai, HE Yan, LV Ben-shun, LUAN Xin-xin, LI Chong, 胡 安琪, HU Zong-hai, GUO Xia. High-speed 850 nm vertical-cavity surface-emitting lasers with BCB planarization technique[J]. Chinese Optics, 2018, 11(2): 190-197. doi: 10.3788/CO.20181102.0190

采用BCB平整技术的高速850 nm垂直面发射激光器

doi: 10.3788/CO.20181102.0190
基金项目: 

国家自然科学基金资助项目 61335004

国家自然科学基金资助项目 61675046

国家自然科学基金资助项目 61505003

国家重大研发计划 2016YFB0400603

国家重大研发计划 2017YFB0400902

国家重大研发计划 2017YFF0104801

详细信息
    作者简介:
  • 中图分类号: TN248.4

High-speed 850 nm vertical-cavity surface-emitting lasers with BCB planarization technique

Funds: 

National Natural Science Foundation of China 61335004

National Natural Science Foundation of China 61675046

National Natural Science Foundation of China 61505003

National Key R&D Program of China 2016YFB0400603

National Key R&D Program of China 2017YFB0400902

National Key R&D Program of China 2017YFF0104801

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出版历程
  • 收稿日期:  2017-11-17
  • 修回日期:  2017-12-16
  • 刊出日期:  2018-04-01

High-speed 850 nm vertical-cavity surface-emitting lasers with BCB planarization technique

doi: 10.3788/CO.20181102.0190
    基金项目:

    国家自然科学基金资助项目 61335004

    国家自然科学基金资助项目 61675046

    国家自然科学基金资助项目 61505003

    国家重大研发计划 2016YFB0400603

    国家重大研发计划 2017YFB0400902

    国家重大研发计划 2017YFF0104801

    作者简介:

    何晓颖(1981-), 女, 湖北荆州人, 博士, 副教授, 2009年于华中科技大学获得博士学位, 主要从事半导体激光器、光纤激光器、石墨烯光电子器件等新型光电子器件的研究工作

    郭霞(1974—),女,山东青岛人,博士,教授,1996年于聊城大学获得学士学位,2003年于北京工业大学获得博士学位,主要从事半导体激光器、发光二极管等光电子器件方面的研究。E-mail:

    通讯作者: GUO Xia, E-mail: guox@bupt.edu.cn
  • 中图分类号: TN248.4

摘要: 垂直腔面发射激光器因其具有低阈值、低功耗、可实现高速调制等优势,广泛地应用于光通信和光互连等领域。寄生电容是影响激光器的调制带宽的主要因素之一。本文通过采用低k值的苯并环丁烯(BCB)平整技术有效地降低了垂直腔面发射激光器的寄生电容。详细研究了BCB平整技术的最优工艺参数,为未来高速垂直腔面发射激光器的制造技术提供参考。低k值BCB平整垂直腔面发射激光器在7 μm氧化孔径下3 dB小信号调制带宽可达15.2 GHz。

English Abstract

何晓颖, 董建, 胡帅, 何艳, 吕本顺, 栾信信, 李冲, 胡宗海, 郭霞. 采用BCB平整技术的高速850 nm垂直面发射激光器[J]. 中国光学, 2018, 11(2): 190-197. doi: 10.3788/CO.20181102.0190
引用本文: 何晓颖, 董建, 胡帅, 何艳, 吕本顺, 栾信信, 李冲, 胡宗海, 郭霞. 采用BCB平整技术的高速850 nm垂直面发射激光器[J]. 中国光学, 2018, 11(2): 190-197. doi: 10.3788/CO.20181102.0190
HE Xiao-ying, DONG Jian, HU Shuai, HE Yan, LV Ben-shun, LUAN Xin-xin, LI Chong, 胡 安琪, HU Zong-hai, GUO Xia. High-speed 850 nm vertical-cavity surface-emitting lasers with BCB planarization technique[J]. Chinese Optics, 2018, 11(2): 190-197. doi: 10.3788/CO.20181102.0190
Citation: HE Xiao-ying, DONG Jian, HU Shuai, HE Yan, LV Ben-shun, LUAN Xin-xin, LI Chong, 胡 安琪, HU Zong-hai, GUO Xia. High-speed 850 nm vertical-cavity surface-emitting lasers with BCB planarization technique[J]. Chinese Optics, 2018, 11(2): 190-197. doi: 10.3788/CO.20181102.0190
    • 850 nm vertical-cavity surface-emitting lasers(VCSELs) are standard light sources for short-reached optical interconnects and data-communication links, including supercomputer clusters and data centers, because of their high modulation speed, low power consumption, low current operation, good beam quality, and etc.[1-3]. The modulation frequency of the VCSELs is mainly limited by their intrinsic resonance frequency, damping, and extrinsic parasitic parameters. Many methods were proposed to improve the modulation frequency of VCSELs, such as applying InGaAs/AlGaAs strain quantum wells to increase the differential gain and then the modulation frequency[4], optimizing the photon lifetime through etching top distributed Bragg reflector(DBR) to reduce the damping[5-6], and growing multiple oxide layers to decrease the oxide capacitance which is the largest parasitic capacitance in the devices[7-9].

      In order to decrease the oxide capacitance, Benzocyclobutene(BCB), which is an important material for high-speed devices, is introduced into the VCSELs. Because it has a low dielectric constant of 2.65, lower to other frequently-used dielectric materials including SiO2 and polyimide with dielectric constant of 3.9 and 3.3, respectively[10]. By planarizing the low-k BCB film between the P- and the N-type ohmic contact layer, the pad capacitance can be greatly reduced due to the capacitance calculation equation[11-12].

      In this paper, high-speed oxide-confined 850 nm GaAs quantum wells(QWs) VCSELs using a low-k BCB planarization technique to reduce parasitic capacitance are reported. The small signal modulation bandwidths of the VCSEL with oxide aperture of 5 μm and 7 μm are demonstrated.

    • The epitaxial structure of the VCSEL was grown by metal-organic chemical vapor deposition on a (100)-oriented n+-GaAs substrate. A graded separate confinement heterostructure region containing an active region with 3 unstrained 7 nm thick GaAs multiple quantum wells(MQWs) separated by 8 nm thick Al0.3Ga0.7As barriers was centered in the one-λ resonant optical cavity with p-type and n-type Al0.3Ga0.7As phase compensation layers. The photoluminescence peak was located at 835 nm. 20.5 pairs of p-doped and 35 pairs of n-doped graded Al0.90Ga0.10As/Al0.12Ga0.88As DBRs were located on each side of the active region, respectively. A 30 nm thick Al0.98Ga0.02As layer, which was located near a node in the cavity standing wave profile where the optical intra-cavity scattering loss was reduced, was included in the top DBR to allow for selective oxidation during fabrication and then for transverse optical and electrical confinement. The topmost DBR layer consists of a 25 nm thick heavily p+-doped GaAs layer, located on top of the p-type DBR as a low-contact resistance layer.

      Both the top and bottom mesa were fabricated by wet etching. The oxide aperture was formed in a wet oxidation furnace at 390 ℃ with an N2 flow of 1 L/min, giving an oxidation rate of approximately 0.52 μm/min for the Al0.98Ga0.02As layers. An oxide aperture of 7 μm was selected. Ti/Au was sputtered on the top surface to form the ohmic contacts. Then, the devices were annealed in a rapid thermal annealing system at 430 ℃ in an N2 atmosphere. Coplanar ground-signal-ground(GSG) contacts were applied for direct high-frequency probing measurements, in order to avoid parasitic coupling at the probe tips and thus improve the measurement accuracy of the microwave probing test[13].

      In this paper, the BCB is used to decrease the oxide capacitance of the VCSEL. Fig. 1(a) shows the simulation results of the small signal modulation performance for the VCSELs with SiO2 and BCB passivation, which comes from the small signal modulation transfer function derived by carrier and photon kinetic equations[14]. The parasitic cutoff frequency for VCSELs with the BCB passivation can reach to ~17.8 GHz, higher than that for SiO2-passivated ones about 10.6 GHz. Fig. 1(b) demonstrated the small signal modulation response results measured by network analyzer, where the VCSELs are respectively passiated by SiO2 and BCB both with the oxide aperture of 7 μm@6 mA. The -3 dB small signal modulation bandwidth is 15.2 GHz and 9.85 GHz, respectively, which indicates the modulation bandwidth limits by the RC parasitic parameter, thus to greatly increase its cutoff frequency depending on BCB passivation.

      Figure 1.  (a) Simulation results of small signal modulation response for VCSELs with BCB and SiO2 passivation. The parasitic cutoff frequency can reach to 17.8 GHz and 10.6 GHz for BCB and SiO2-passivated VCSEL, respectively. (b)The measured small signal modulation response for VCSELs with BCB and SiO2 passivation. The -3dB bandwidth is 15.2 GHz and 9.85 GHz with the oxide aperture of 7 μm@6 mA, respectively, which indicates the parasitic capacitance limits the modulation frequency of the devices

      The schematic cross-sectional structure of high speed VCSEL devices is shown in Fig. 2(a). Fig. 2(b) presents a top-view image of the high-speed VCSEL.

      Figure 2.  (a) Schematic cross-sectional structure of high-speed VCSEL devices. (b)Top-view image of the high-speed VCSEL with coplanar GSG electrode structure

      The photosensitive BCB is coated on the VCSEL surface by spinning process. Fig. 3(a) presents the dependence of the spinning speed on the BCB thickness before and after hard baking of 1.5 min with the temperature of 70 ℃ in the oven. According to the epitaxial structure, the spinning speed is selected to be 2 000 r/min with the BCB thickness of around 6 μm. The thick film requires more exposure energy. Fig. 3(b) shows the relationship of exposure time with the diameter difference(Δd) between patterns on the mask and on the devices. Generally, the pattern size on the devices is larger than that on the mask. The diameter difference increases with the exposure time because the diffraction occurs at the edge of the mask patterns. Decreasing the exposure time will decrease the pattern size discrepancy, however, and the residues on the patterns are hard to be removed due to the thick BCB film determined by the VCSEL structure. Fig. 3(c) demonstrates the microscopic image of the exposed area. Colorful strips are caused by the refractive index difference between the residue BCB and the substrate, which will deteriorate the Ohmic contact. In our experiment, inductively coupled plasma(ICP) etching with SF6 and O2 chemistry is applied to remove these BCB residues. Fig. 3(d) demonstrates the microscopic image of the pattern after the ICP etching using SF6/O2 chemistry with ICP power of 1 000 W and RF power of 50 W.

      Figure 3.  (a) Relationship between spin speed and film thickness of BCB. (b)Difference in aperture diameter(Δd) between lithography and BCB patterns at various exposure times. (c)Top-view images of the thin BCB layer before dry etching and (d)after dry etching

    • The static light output power-current-voltage(L-I-V) characteristics were measured under the wafer probes at room temperature. The L-I-V results for BCB planarization VCSELs with the 5 μm and 7 μm oxide aperture diameter are shown in Fig. 4(a). It can be seen that the threshold currents Ith are 0.12 mA and 0.22 mA, and the threshold electrical powers are 0.65 mW and 0.84 mW at the 5 μm and 7 μm oxide aperture, respectively[15], while the threshold current density Jth is about 0.57 kA/cm2. As the current injection increases, the photon density in the active region increases and the optical power also increases. Correspondingly, the maximum output optical power increased with aperture size. The maximum output optical power reached to 0.47 mW at a 9 mA and 0.68 mW at a 12 mA respectively for the 5 μm and 7 μm oxide aperture VCSELs. That is because red shift in wavelength and decrease in efficiency would be caused by self-heating effects, and optical power therefore decreased rapidly. The differential resistance, depending on the size of the oxide aperture and epitaxial structure[16], are 212 Ω and 190 Ω at the 5 μm and 7 μm oxide aperture, which determines the thermal roll-over current of the VCSELs.

      Figure 4.  (a) Static P-I-V characteristics of BCB-planarized VCSELs with a 5 μm and 7 μm oxide aperture at room temperature. (b)Electrical-luminescence spectrum for the VCSEL at room temperature and current injection of 10.0 mA

      The small signal modulation response of the VCSELs was measured using a 40 GHz R&S ZVA 40 network analyzer and a high-frequency microwave probe(Picoprobe 40A-GSG-150-P) to contact the device under test. A multimode fiber was connected to a 25 GHz high-speed photodetector(New Focus 1414-50), and the signal after detection was connected back to the network analyzer with a high-frequency cable. Before testing the VCSELs, calibrations were made to compensate for losses caused by the cables and the probe insertion[17]. Fig. 5 show small-signal modulation response at room temperature at different bias currents for the 5 μm(a) and 7 μm(b) oxide aperture VCSELs with BCB planarization. For the 5 μm oxide aperture VCSEL, as the injection current increases from 2 mA to 5 mA, the small signal modulation bandwidth increases from 12.8 GHz to the maximum 15 GHz due to increased gain. Then the modulation bandwidth decreased to 14.3 GHz at 8 mA injection current due to heat generation inside the device. For the 7 μm oxide aperture VCSEL, as the injection current increases from 3 mA to 9 mA, the small signal modulation bandwidth increases from 14.1 GHz to the maximum 15.2 GHz, and then dropped to 13.1 GHz. Under small injection current bias at 2 mA for 5 μm oxide aperture and 3 mA for 7 μm oxide aperture, the intensity modulation can follow the current modulation up to resonance frequency. An obvious enhancement of modulation response exists at the resonance. As shown in Fig. 5, beyond the resonance, the response dropped off dramatically. While, under the large injection currents, the responses measured in Fig. 5 are flat without peak frequency of resonance. The -3 dB modulation bandwidth increases with the resonance frequency until the damping becomes more and more strong. At same time, the -3 dB modulation bandwidth decreases with the further increase of resonance frequency and the damping.

      Figure 5.  Small-signal modulation response at room temperature at different bias currents for the BCB-planarized VCSEL with (a)5 μm and (b)7 μm oxide aperture

      Fig. 6(a) plots the resonance frequency depending on the square value of injection current above the threshold current Ith for the VCSEL with oxide aperture of 5 μm and 7 μm. The data of resonance frequency is extracted from the fitting of the curves in Fig. 5 according to the transfer function[18]. As show in Fig. 6(a), resonance frequency fr increases linearly with the low injection current. The slope, which is the rate of the resonance frequency increased with bias current, could be defined as D-factor of 8.746 GHz/mA1/2 and 7.492 GHz/mA1/2, for 5 μm and 7 μm oxide aperture VCSELs, respectively. The reason is that, under the same injection current, VCSEL with small oxide aperture has a smaller active region volume than that of lager oxide aperture.

      Figure 6.  (a) Plot of the resonance frequency for the VCSELs with 5 μm and 7 μm oxide aperture versus the square root of the current injection above the threshold current at room temperature. (b)Damping rate versus resonance frequency square for the VCSELs with 5 μm and 7 μm oxide aperture at room temperature

      Fig. 6(b) plots the dependence of damping on the square of resonance frequency for the VCSELs with the oxide aperture of 5 μm and 7 μm, respectively. All the damping data are also extracted from the fitting results for the curves in Fig. 5. The damping increases in proportion to resonance frequency, which can be described as follows[19]

      (1)

      where K is the rate of the damping increased with the square of the resonance frequency, γ0 is damping factor offset. As shown in Fig. 6(b), the damping increases linearly with fr2 at the beginning and then increases dramatically. The value of K-factor, which is determined by the photon lifetime, is 0.291 and 0.305 ns for the VCSELs with the 5 μm and 7 μm oxide apertures respectively, whose slight difference comes from the measurement errors and fabrication.

    • High-speed oxide confined 850 nm GaAs QWs VCSELs were designed and fabricated using a low-k BCB planarization technique. BCB has a very low dielectric constant of 2.65 and a 6 μm thick layer was deposited using a spin-coating process. The parasitic cutoff frequency of the VCSEL using BCB planarization technique increased from 10.6 GHz to 17.8 GHz compared with the SiO2-passivated VCSEL at 7 μm oxide aperture. The small signal modulation bandwidth of the low-k BCB planarization VCSELs with 5 μm and 7 μm oxide aperture have been achieved to 15 GHz and 15.2 GHz, respectively, mainly limited by the damping and resonance frequency.

参考文献 (19)

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