GaAs Quantum Well Lasers Grown by Molecular

Jun 5, 2017 - As a promising new class of near-infrared light emitters, GaAsBi laser diodes (LDs) are considered to have a high energy efficiency and ...
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1.142 μm GaAsBi/GaAs Quantum Well Lasers Grown by Molecular Beam Epitaxy Xiaoyan Wu,†,‡,∥ Wenwu Pan,†,‡ Zhenpu Zhang,†,§ Yaoyao Li,*,† Chunfang Cao,† Juanjuan Liu,†,‡ Liyao Zhang,† Yuxin Song,† Haiyan Ou,∥ and Shumin Wang*,†,§,⊥ †

State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, CAS, Shanghai 200050, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § ShanghaiTech University, Shanghai 201210, China ∥ Department of Photonics Engineering, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark ⊥ Department of Microtechnology and Nanoscience, Chalmers University of Technology, 41296 Gothenburg, Sweden ABSTRACT: As a promising new class of near-infrared light emitters, GaAsBi laser diodes (LDs) are considered to have a high energy efficiency and an insensitive temperature dependence of the band gap. In this paper, we realize the longest ever reported lasing wavelength up to 1.142 μm at room temperature in GaAsBi0.058/GaAs quantum well LDs grown by molecular beam epitaxy. The output power is up to 127 mW at 300 K under pulsed mode. We also demonstrate continuous wave mode operation up to 273 K for the first time. The temperature coefficient of the GaAsBi/GaAs LD is 0.26 nm/K in the temperature range of 77−350 K, lower than that of both InGaAsP/InP and InGaAs/GaAs LDs. The characteristic temperature is extracted to be 139 K in the temperature range of 77−225 K and decreases to 79 K at 225−350 K. KEYWORDS: GaAsBi, molecular beam epitaxy, laser diodes, quantum well, uncooled laser when the ΔSO is larger than Eg.8 In GaAsBi alloys, the ΔSO > Eg band structure is present when the Bi composition is bigger than 10%, where the alloy band gap is close to 1.55 μm.8 This is significant for the development of highly efficient and uncooled GaAs-based lasers in optical communication systems.11,20−22 It is well known that the crystal quality of III−V semiconductors is highly affected by the growth temperature. Due to the metastable nature of GaAsBi, a low growth temperature is required compared to that for epitaxial growth of typical (Al)GaAs alloys.23−26 This leads to an increase in defect density and optical quality degradation. Thus, the main challenge to achieve a GaAsBi LD in the range of 1.3−1.6 μm is the relatively high Bi composition (>10%) with good material quality as required for laser structures. To push the GaAsBi LD wavelength to the telecom wavelength range of 1.3−1.6 μm, a concerted effort to develop high-quality GaAsBi LDs has been made. The first electrically pumped GaAsBi laser was demonstrated by Ludewig et al. in 2013 grown by metal−organic vapor phase epitaxy (MOVPE) (containing 2.2% Bi), lasing at room temperature (RT) (∼947 nm).27 Then, the first molecular beam epitaxy (MBE)-grown electrically pumped GaAsBi laser was demonstrated by Fuyuki et al. in 2014 with Bi up to 4% (∼1.045 μm at RT).10 The

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urrently InP-based laser diodes (LDs) are widely used in wavelength division multiplexing (WDM) optical communication systems, despite the limitations of the low characteristic temperature (T0 = 60 K) and the wavelength fluctuation depending on the ambient temperature. Thus, thermoelectric coolers are required in practical use, which leads to increasing cost. Therefore, uncooled LDs that can perform very well in energy saving are highly appealing for cost-effective communication systems. Among different approaches including dilute nitride quantum wells (QWs),1,2 InAs quantum dots (QDs),3 and dilute bismide QWs, lasers based on GaAsBi are attracting increasing interest due to the suppressed Auger recombination,4,5 intervalence band absorption (IVBA),6−9 and temperature-insensitive band gap.10−14 These significant properties make GaAsBi LDs a promising candidate for energy-efficient near-infrared devices in datacom/telecom systems.15−17 The GaAs-based dilute nitride QW LDs tend to suffer from large defect-related recombination in addition to Auger recombination.18 The InAs QD LDs have similarly shown to suffer from Auger recombination and have no significant improvement of temperature stability compared to conventional QW LDs unless p-doping is employed.19 For dilute bismides, as reported before, the incorporation of Bi can strongly reduce the band gap (Eg) and increase the spin−orbit splitting energy (ΔSO).7,8 In practical terms, the Auger recombination and IVBA can be significantly suppressed © XXXX American Chemical Society

Received: March 10, 2017 Published: June 5, 2017 A

DOI: 10.1021/acsphotonics.7b00240 ACS Photonics XXXX, XXX, XXX−XXX

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threshold current was about 8−15 kA/cm2. Up to now, the longest wavelength of GaAsBi LD reported was 1.06 μm, demonstrated by Butkute et al.28 using hybrid MOVPE/MBE and containing ∼6% Bi with three QWs, while the threshold current was relatively high, more than 30 kA/cm2. In this paper, we report the laser oscillation from electrically pumped GaAsBi LD grown by MBE with ∼5.8% Bi. At RT, the lasing wavelength is up to 1.142 μm and the output power is up to 127 mW. We have achieved electrically pumped pulsed operation up to 350 K and continuous wave (CW) mode operation up to 273 K for the first time. In addition, we investigate the temperature characteristics of GaAsBi LD and reveal the temperature dependence of the lasing wavelength is lower than that of InGaAsP/InP and InGaAs/GaAs LDs.

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Figure 2. Lasing spectra of GaAs/(Al)GaAs single QW LD with 6 × 1000 μm2 (black line) and GaAsBi/GaAs single QW LD with 6 × 1500 μm2 (red line) at room temperature under pulsed excitation.

RESULTS AND DISCUSSION Figure 1 shows the bright-field transmission electron microscope (TEM) image of the GaAsBi LD structure with 5.8% Bi

relatively high Jth value of GaAsBi LDs is considered to be related to the following effects: (1) low electron confinement in the GaAs conduction band, leading to electrons escaping from the GaAsBi QW into GaAs barriers, and (2) a nonradiative recombination process. Low-temperature growth of the GaAsBi QW introduces some point defects, leading to many injected carriers being wasted through nonradiative recombination. As shown in Figure 2, the lasing spectrum of GaAsBi LD is broader than that of the GaAs LD. There are two factors that contribute to the line width broadening of the GaAsBi LD spectrum. (1) The one to two monolayer transition region related to interface roughness usually introduces line width broadening of the gain spectrum. For a 15 nm thick GaAsBi QW, this is estimated to be 10 meV or less. (2) GaAsBi bulks and QWs often reveal a broad PL spectrum (fwhm >100 meV) due to nonuniform Bi incorporation and/or Bi-induced clusters, which is the main reason for the gain broadening. This implies the carriers are likely spatially localized similar to the case of quantum dots, showing strong but broad photoluminescence. The temperature characteristics of GaAsBi LDs are investigated under both pulse and CW excitation mode. Figure 3 shows the P−I curve of the GaAsBi LD at 273 K under the CW mode. The inset shows a lasing spectrum under the same conditions. The GaAsBi LD lasing spectrum is relatively broad, containing multiple Fabry−Perot modes. This indicates a significantly broadened gain spectrum in GaAsBi. As shown in Figure 3, the lasing wavelength reaches 1.135 μm (Jth = 5.2 kA/

Figure 1. TEM cross-sectional image of the GaAs1−xBix (x = 5.8%) QW LD structure.

calibrated by X-ray diffraction (XRD). The dark area represents the GaAsBi layer, whereas the surrounding bright area represents the GaAs layer. As is shown in Figure 1, the thickness of the GaAsBi layer is about 15 nm and the surrounding GaAs layers are about 38−42 nm. We have also done TEM-EDX mapping, and the Bi composition is found to be about 5.8% in the GaAsBi layer, in agreement with the XRD estimation (not shown here). Besides, Figure 1 shows a smooth and well-defined interface with a transition region of only 1−2 monolayers (MLs). In the GaAsBi QW, the Bi distribution appears to be very homogeneous without any noticeable fluctuations in Bi concentration in the plane. Figure 2 shows the lasing spectra of GaAs (as a reference) and GaAsBi (5.8%) LDs at an injection current (I) of ∼1.2 threshold current (Ith) at RT measured under pulsed excitation. The lasing wavelength of a GaAs LD with 6 × 1000 μm2 stripes and GaAaBi (5.8%) with 6 × 1500 μm2 stripes is 0.851 and 1.142 μm, respectively. The GaAs and GaAsBi (5.8%) LDs’ threshold current density (Jth) is 3.65 and 3.89 kA/cm2, respectively. Despite the Jth value being relatively high compared to that of standard (InGa)As LDs, the similar Jth values of the low-temperature-grown GaAsBi LD and the hightemperature -grown GaAs LD indicate the high quality of the GaAsBi layer. Considering the low-temperature growth of the GaAsBi layer, the Bi surfactant effect plays an important role in achieving high-quality QW layer growth. As reported before, there is a significant difference in the performance of devices with and without Al in the barrier layer.29 The lowest Jth of the GaAsBi LD (2.2%) of ∼1 kA/cm2 was measured in the laser with 12% Al in the barrier layer, while the GaAsBi LD (2.2%) with a GaAs barrier showed a high Jth of 7.5 kA/cm2. The

Figure 3. P−I curve of GaAsBi LDs at 273 K under the CW mode. The inset shows the lasing spectrum. B

DOI: 10.1021/acsphotonics.7b00240 ACS Photonics XXXX, XXX, XXX−XXX

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cm2), while under a pulsed mode, the GaAsBi (5.8%) LD operates well up to 350 K with the spectrum shown in the inset of Figure 4. These relatively high operating temperatures are encouraging for the application of GaAsBi LDs in a WDM system.

Ith(T ) = I0 exp(T /T0)

(1)

where I0 is the extrapolated Ith at T = 0 K. Using eq 1, T0 is calculated to be 139 K at 77−225 and 79 K at 225−350 K. This is higher than the reported T0 = 60 K from typical 1.3 μm InGaAsP FP-LDs. The high T0 value is considered to be associated with a strong carrier confinement of the GaAsBi/ GaAs/AlGaAs system than that of InGaAsP/InP. At a low temperature (T < 225 K), the nonradiative recombination processes are expected to be minimized and the carrier can be effectively confined in the QW layer, while as the temperature increases (T > 225 K), the carriers more easily escape from the GaAsBi QW to the GaAs barrier layer and the nonradiative recombination processes increase. This leads to a relatively high Jth and a low T0. To reduce the carrier leakage, Al can be introduced to the GaAs barrier layer. However, increasing the Al content has the negative effect of reducing both the lasing wavelength and the refractive index contrast between the active region and the AlGaAs waveguide layers. The later will reduce optical confinement, leading to a decrease in modal gain. Therefore, to optimize the device performance, one should take into account two opposite effects related to adding Al in the waveguide and barrier layers.

Figure 4. Dependence of the GaAsBi lasing wavelength on temperature. The inset shows the lasing spectrum at 350 K under pulsed mode.

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CONCLUSION In this work, we have grown GaAsBi/GaAs single QW LD structures by MBE and fabricated ridge waveguide LDs. Lasing oscillation up to 1.142 μm at RT is achieved with a threshold current density about 3.89 kA/cm2, which is the longest reported lasing wavelength from GaAsBi LDs to date. The output power is measured up to 127 mW at 300 K under pulsed mode. The temperature dependence of GaAsBi LDs is investigated under both pulse and CW excitation mode. We have achieved electrically pumped pulsed operation up to 350 K and CW mode operation up to 273 K for the first time. The temperature coefficient of the GaAsBi LD is 0.26 nm/K in the temperature range of 77−350 K, which is lower than that of both InGaAsP/InP and InGaAs/GaAs LDs. The characteristic temperature is extracted to be 139 K in the temperature range of 77−225 K and decreases to 79 K at 225−350 K. The demonstrated MBE-grown GaAsBi LDs with a relatively high characteristic temperature and a low temperature coefficient are attractive for highly efficient and cooler-free GaAs-based longwavelength lasers.

To investigate the temperature dependence of the lasing peak wavelength, a series of lasing spectra at different temperatures are measured in the range of 77−350 K under pulsed operation. Figure 4 shows the wavelength shift of the GaAsBi LDs with temperature. The peak lasing wavelength is extracted using a Gaussian curve fitted from the lasing spectrum. The temperature coefficient of the GaAsBi single QW LD is 0.26 nm/K in the temperature range of 77−350 K, which is lower than that of 1.3 μm InGaAsP/InP (0.4 nm/K)30 and 1.0 μm InGaAs/GaAs LDs (0.345 nm/K).31 As reported before, the temperature sensitivity of the GaAsBi band gap is lower than that of GaAs.11 Thus, the temperature coefficient reduction in GaAsBi LDs is attributed to the temperature coefficient reduction of its band gap. Figure 5a shows P−I curves for GaAsBi LDs at different temperatures from 77 to 350 K. The extracted Ith values at different temperatures are shown in Figure 5b. The characteristic temperature, T0, is obtained by fitting the experimental temperature dependence of Ith to

Figure 5. (a) P−I curves at different temperatures from 77 to 350 K. (b) Temperature dependence of the threshold current from 77 to 350 K. C

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METHODS A GaAsBi/GaAs single QW LD structure was grown by solid source MBE on n-doped GaAs(001) substrates. The GaAs1−xBix (x = 0 or 5.8%) QW layer was about 15 nm thick surrounded by 40 nm GaAs spacer layers plus 160 nm AlGaAs layers with a graded Al content varying from 0.25 to 0.45. Both the n-type and p-type AlGaAs waveguide layers were 1500 nm and doped at a doping level of 1 × 1017 cm−3 by silicon (Si) and 1 × 1018 cm−3 by beryllium (Be), respectively. n-AlGaAs:Si (1 × 1018 cm−3) and p-AlGaAs:Be (5 × 1018 cm−3) were 160 nm outside the AlGaAs waveguide layers. A 200 nm thick p+-GaAs:Be layer at a doping level of 3 × 1019 cm−3 was grown on top of the structure in order to improve the metal− semiconductor contact. For the reference sample (GaAs LD structure), all the layers were grown at 590 °C. For the GaAs1−xBix (x = 5.8%) sample, the GaAsBi layer was grown at 375 °C, and other layers were grown at 590 °C. The LDs were fabricated using a standard photolithography process. The stripe width was 6, 8, 10, and 12 μm, respectively, with Ti (20 nm)/Pt (20 nm)/Au (400 nm) metal deposited on top of p+-GaAs. The substrate was thinned to ∼180 μm. Ge (13 nm)/Au (33 nm)/Ni (30 nm)/Au (200 nm) ohmic contacts were deposited on the back side of the substrate and alloyed at 370 °C for 40 s to form ohmic contacts. The as-cleaved GaAs1−xBix (x = 0 or 5.8%) LDs were investigated using both pulsed bias (200 ns/50 kHz) and CW mode. Temperature characteristics of GaAsBi LDs were also investigated under both pulse and CW excitation mode, respectively. Emission spectra were measured using a Nicolet Magna 860 Fourier transform infrared spectrometer-based PL system, in which an InGaAs detector and a CaF2 splitter were used. The output power was measured by an InGaAs photodiode optical power meter. The samples were mounted into a closed-cycle refrigerator for the low-temperature lasing spectra and P−I measurements.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiaoyan Wu: 0000-0003-2598-8660 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge the financial support of the Key Program of Natural Science Foundation of China (Grant No. 61334004), the National Basic Research Program of China (Grant No. 2014CB643902), the Natural Science Foundation of China (Grant Nos. 61404152 and 11274329), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA5-1), the Foundation of National Laboratory for Infrared Physics, the Key Research Program of the Chinese Academy of Sciences (Grant No. KGZD-EW-804), and the Creative Research Group Project of Natural Science Foundation of China (Grant No. 61321492). The Swedish Research Council (VR) is also acknowledged for financial support. D

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luminescence Study of Exciton Recombination in Bulk GaAsBi. Nanoscale Res. Lett. 2014, 9, 19. (21) Fuyuki, T.; Yoshioka, R.; Yoshida, K.; Yoshimoto, M. LongWavelength Emission in Photo-Pumped GaAs1-xBix Laser with Low Temperature Dependence of Lasing Wavelength. Appl. Phys. Lett. 2013, 103, 202105. (22) Mohmad, A. R.; Bastiman, F.; Hunter, C. J.; Ng, J. S.; Sweeney, S. J.; David, J. P. R. The Effect of Bi Composition to the Optical Quality of GaAs1-xBix. Appl. Phys. Lett. 2011, 99, 042107. (23) Yoshimoto, M.; Murata, S.; Chayahara, A.; Horino, Y.; Saraie, J.; Oe, K. Metastable GaAsBi Alloy Grown by Molecular Beam Epitaxy. Jpn. J. Appl. Phys. 2003, 42, L1235−L1237. (24) Wang, P.; Pan, W.; Wu, X.; Liu, J.; Cao, C.; Wang, S.; Gong, Q. Influence of GaAsBi Matrix on Optical and Structural Properties of InAs Quantum Dots. Nanoscale Res. Lett. 2016, 11, 280. (25) Wang, P.; Pan, W.; Wu, X.; Cao, C.; Wang, S.; Gong, Q. Heteroepitaxy Growth of GaAsBi on Ge (100) Substrate by Gas Source Molecular Beam Epitaxy. Appl. Phys. Express 2016, 9, 045502. (26) Vardar, G.; Paleg, S. W.; Warren, M. V.; Kang, M.; Jeon, S.; Goldman, R. S. Mechanisms of Droplet Formation and Bi Incorporation during Molecular Beam Epitaxy of GaAsBi. Appl. Phys. Lett. 2013, 102, 042106. (27) Ludewig, P.; Knaub, N.; Hossain, N.; Reinhard, S.; Nattermann, L.; Marko, I. P.; Jin, S. R.; Hild, K.; Chatterjee, S.; Stolz, W. Electrical Injection Ga (AsBi)/(AlGa) As Single Quantum Well Laser. Appl. Phys. Lett. 2013, 102, 242115. (28) Butkutė, R.; Geižutis, A.; Pačebutas, V.; Č echavičius, B.; Bukauskas, V.; Kundrotas, R.; Ludewig, P.; Volz, K.; Krotkus, A. MultiQuantum Well Ga (AsBi)/GaAs Laser Diodes with More than 6% of Bismuth. Electron. Lett. 2014, 50, 1155−1157. (29) Marko, I. P.; Ludewig, P.; Bushell, Z. L.; Jin, S. R.; Hild, K.; Batool, Z.; Reinhard, S.; Nattermann, L.; Stolz, W.; Volz, K. Physical Properties and Optimization of GaBiAs/(Al) GaAs Based nearInfrared Laser Diodes Grown by MOVPE with up to 4.4% Bi. J. Phys. D: Appl. Phys. 2014, 47, 345103. (30) Higashi, T.; Yamamoto, T.; Ogita, S.; Kobayashi, M. Experimental Analysis of Temperature Dependence of Oscillation Wavelength in Quantum-Well FP Semiconductor Lasers. IEEE J. Quantum Electron. 1998, 34, 1680−1689. (31) Rakić, A. D.; Majewski, M. L. Modeling the Optical Dielectric Function of GaAs and AlAs: Extension of Adachi’s Model. J. Appl. Phys. 1996, 80, 5909−5914.

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DOI: 10.1021/acsphotonics.7b00240 ACS Photonics XXXX, XXX, XXX−XXX