Broadband high-reflective distributed Bragg reflectors based on amorphous silicon films for semiconductor laser facet coatings Xiang-Yu Guan,1 Jung Woo Leem,1 Soo Hyun Lee,1 Ho-Jin Jang,2 Jeong-Ho Kim,3 Swook Hann,3 and Jae Su Yu1,* 1
Department of Electronics and Radio Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin 446-701, South Korea 2
Optowell Co., Ltd., 824 Palbok-dong 2ga, Deokjin-gu, Jeonju 561-844, South Korea
3
Korea Photonics Technology Institute, Laser-IT Research Center, 9 Cheomdan Bencheo-ro 108, Buk-gu, Gwangju 500-779, South Korea *Corresponding author: [email protected] Received 5 November 2014; revised 12 December 2014; accepted 26 December 2014; posted 6 January 2015 (Doc. ID 226218); published 4 February 2015
We fabricated amorphous silicon (a-Si)-based distributed Bragg reflectors (DBRs) consisting of alternating dense/porous films (i.e., pair) for a center wavelength (λc ) of 0.96 μm by oblique angle deposition (OAD) technique using an electron-beam evaporation system. The dense (high refractive index, i.e., high-n) and porous (low-n) a-Si films were deposited at two incident vapor flux angles of 0° and 80° in the OAD, respectively. Their optical reflectance characteristics were investigated in the wavelength range of 0.6–1.5 μm, including theoretical comparison using a rigorous coupled-wave analysis method. Above three pairs, the reflectivity (R) of a-Si DBRs was almost saturated at wavelengths around 0.96 μm, exhibiting R values of >97%. For the a-Si DBR with only three pairs, a broad normalized stop bandwidth (Δλ∕λc ) of ∼22.5% was obtained at wavelengths of ∼0.87–1.085 μm, keeping high R values of >95%. To simply demonstrate the feasibility of device applications, the a-Si DBR with three pairs was coated as a high-reflection layer at the rear facet of GaAs/InGaAs quantum-well laser diodes (LDs) operating at λ 0.96 μm. For the LDs coated with three-pair a-Si DBR, external differential quantum efficiency (ηd ) was nearly doubled compared to the uncoated LDs, indicating the ηd value of ∼50.6% (i.e., ηd ∼ 25.5% for the uncoated LDs). © 2015 Optical Society of America OCIS codes: (140.5960) Semiconductor lasers; (230.1480) Bragg reflectors; (310.1860) Deposition and fabrication; (310.6860) Thin films, optical properties. http://dx.doi.org/10.1364/AO.54.001027
1. Introduction
Semiconductor laser diodes (LDs) have been widely used in a variety of fields, such as optical fiber communications, information storages, displays, medical instruments, and measurement systems [1–3]. Generally, LD has a Fabry–Perot optical cavity that can 1559-128X/15/051027-05$15.00/0 © 2015 Optical Society of America
effectively give positive feedback by a pair of facet reflectors. Thus, a high-reflection (HR) coating at the rear-side facet is crucial because it reduces optical mirror losses in lasers, leading to improved device performance. Distributed Bragg reflectors (DBRs) have been generally utilized in semiconductor lasers as a HR facet coating layer [4–6]. For DBRs, the reflectivity and stop bandwidth are directly affected by the refractive index (n) difference between the high-n and low-n materials. As a result, 10 February 2015 / Vol. 54, No. 5 / APPLIED OPTICS
1027
the larger refractive index contrast provides higher reflectivity and wider stop bandwidth [7]. However, conventional DBRs consisting of two different alternating dielectric materials (TiO2 ∕SiO2 [8], Ta2 O5 ∕ SiO2 [9], ZrO2 ∕SiO2 [10], SiNx ∕SiOx [11], etc.) have fundamental limitations, such as low refractive index contrast, material selection, thermal expansion mismatch, and poor heat dissipation capability due to a low thermal conductivity [12,13]. Thus, the DBRs consisting of the same material, fabricated in an effective way including cost-effective and simple processes, are rather desirable for practical applications. Over the past years, some research works have been reported on the same material-based DBRs with only a few pairs for Si [14–17], Ge [18], TiO2 [19], ITO [20], and so forth. The low-n film with a high porosity can be formed by a relatively low-cost and simple electron beam (e-beam) evaporation using only one source material via the oblique angle deposition (OAD) method. The OAD technique can control the air volume fraction within the film because of its enhanced self-shadowing effect and limited mobility of incoming atoms [21,22]. In the case of amorphous silicon (a-Si), it has a low absorption and a high refractive index (i.e., na-Si > 2.3) in the nearinfrared (NIR) wavelength region of 0.8–1.1 μm, which allows for a larger refractive index difference between dense and porous films by the OAD as well as a relatively better thermal conductivity. There is very little or no report on the use of the a-Si DBRs as a HR facet coating layer in semiconductor LDs via the OAD method. Therefore, it is very meaningful to investigate the device performance of semiconductor LDs incorporated with the a-Si DBR for HR facet coating. In this work, we fabricated the a-Si DBRs consisting of dense/porous film pair structures for different pairs of one to four by the OAD technique via e-beam evaporation. The effect of a-Si DBR-based HR facet coating layer on the device characteristics of GaAs/InGaAs quantum-well (QW) LDs was studied. For optical reflectance properties, a theoretical analysis was also carried out using a rigorous coupled-wave analysis (RCWA) method. 2. Experimental and Simulation Modeling Details
Figure 1 shows the schematic illustration for the fabrication procedure of a-Si DBRs consisting of dense (high-n)/porous (low-n) film pair structures on GaAs substrates by the OAD method via an e-beam evaporation. The cross-sectional and top-view scanning electron microscope (SEM) images of a-Si DBRs with one pair are also shown in Fig. 1. To fabricate the a-Si DBRs, Si granules with 99.99% purity were used as a target source. The chamber was evacuated to a base pressure of 95%. This is due to the large refractive index contrast between the a-Si films deposited at θα 0° and 80° by the OAD, as can be seen in Fig. 2(a). In RCWA simulations, similarly, as the number of pairs in a-Si DBRs is increased from one pair to six pairs, the reflectivity is also increased and the HR band is narrowed. Above three pairs, the R value of a-Si DBRs was almost saturated at wavelengths around 0.96 μm, exhibiting R values of >97%. The highreflective properties of a-Si DBRs can be also confirmed in E-field intensity distribution calculations. In Fig. 3(c), for the a-Si DBR with three pairs, the E-field intensity in the GaAs substrate is lower than that of a-Si DBR with one pair due to its higher reflectivity at λ 0.96 μm, as shown in Fig. 3(b). In order to investigate the effect of the a-Si DBRs on the performance of actual devices, the a-Si DBRs with three pairs were coated at the rear facet of GaAs/InGaAs QW LDs for a HR coating. Figure 4(a) shows the schematic of the GaAs/InGaAs QW LD 10 February 2015 / Vol. 54, No. 5 / APPLIED OPTICS
1029
(a)
100
Reflectance (%)
80 70
n-AlGaAs waveguide
Light
60
n-GaAs clad GaAs substrate
50 a-Si DBR
40
n-contact
(b)
30 20
Measurements 0.7
0.8
0.9
1.0
1.1
400 Active region
Low-n a-Si film
High-n a-Si film
350
3.0 1.2
1.3
1.4
1.5
Wavelength (μm) 6
a-Si DBR 5 R (%) 100
99% 4
75
97%
2.5 2.0
Light emission λ= 0.96 μm
HR coated rear-facet
300 250
GaAs/InGaAs QW LD
a-Si DBR (3 pairs)
200
1.5
150
1.0
100
0.5 0.0 0
Uncoated HR coated
200
400
600
800
50 0 1000
50
Injection Current (mA)
25
Fig. 4. (a) Schematic of the GaAs/InGaAs QW LD structure and (b) CW L-I-V of the uncoated and HR-coated LDs at room temperature. The schematic of the HR-coated GaAs/InGaAs QW LD is also shown.
3 0
2
Voltage (V)
0 0.6
Number of Pairs
4.0 3.5
10
(b)
Passivation (SiO2)
p-contact p-GaAs clad AlGaAs p-waveguide GaAs/InGaAs QW active region
1 pair 2 pairs 3 pairs 4 pairs
a-Si DBR
90
Optical Output Power (a.u.)
(a)
Calculations 1
0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
Wavelength (μm)
(c) Z (μm)
2
Ey-field Intensity Max
Air Air
1
Si DBR 1 pair
Si DBR 3 pairs
0 GaAs substrate
-1 -2
-1
0
X (μm)
1
GaAs substrate 2 -2
-1
0
1
Min
2
X (μm)
Fig. 3. (a) Measured reflectance spectra of the a-Si DBRs with different pairs of one to four, (b) contour plot of variations of calculated reflectance spectra of the a-Si DBR as functions of number of pairs in a-Si DBRs and wavelength, and (c) calculated E-field intensity distributions of the a-Si DBRs with one pair and three pairs at λ 0.96 μm.
structure. The LD consists of the GaAs/InGaAs QW active region between AlGaAs waveguides. The p- and n-GaAs layers were used as clad layers, respectively. The p-GaAs clad layer was etched, and then the silicon dioxide (SiO2 ) layer was covered for passivation by using a plasma-enhanced chemical vapor deposition system. Finally, p- and n-contact metal layers were coated. Figure 4(b) shows the CW L-I-V of uncoated and HR-coated LDs at room temperature. The schematic of the HR-coated GaAs/InGaAs QW LD is also shown in the inset of Fig. 4(b). As shown in Fig. 4(b), the device performance was considerably improved by incorporating the a-Si DBR into the rear facet of the LD. Both the uncoated and HR-coated LDs showed the almost 1030
APPLIED OPTICS / Vol. 54, No. 5 / 10 February 2015
similar I-V characteristics (i.e., no obvious electrical degradation). From the I-L curves, on the other hand, the threshold current (I th ) decreased and output power (Po ) increased compared to the uncoated LD. For the HR-coated LD, a lower I th of 168 mA and a higher Po of 271.8 mW at an injection current of 1000 mA were obtained, exhibiting a slope efficiency (dP∕dI) of 0.3267 mW/mA (i.e., I th 225 mA, Po 127.7 mW, and dP∕dI 0.1648 mW∕mA for the uncoated LD). This is attributed to the reduced mirror loss by the HR a-Si DBR coated at the rear facet of LDs. The mirror loss (αm ) is given by the equation [26] αm
1 1 ln ; 2L R1 R2
(1)
where L is the cavity length, and R1 and R2 are the reflectivities of both the facets of LDs. It is assumed that the cleaved facet has the mirror reflectivity of ∼33.1%, which is a calculated value for the active material at λ 0.96 μm in these experiments. For L 2 mm, the αm of uncoated LD is estimated to be ∼5.53 cm−1 , while it decreased to 2.85 cm−1 for the HR-coated LD, indicating the reduction percentage of ∼48.5%. Thus, the external differential quantum efficiency (ηd 2e∕hν · dP∕dI, where hν is the photon energy and e is the electron charge), which is defined by the number of photons emitted by each net electron above threshold, is increased from ∼25.5% to ∼50.6%, showing the increment of ∼1.98 times. From these results, the a-Si DBR with only
few pairs can be used as the HR facet coating of optoelectronic devices without any degradation of electrical properties. 4. Conclusion
The device performance enhancement of GaAs/ InGaAs QW LDs (λ 0.96 μm) coated with threepair a-Si DBRs at the rear facet was demonstrated. At λc 0.96 μm, the a-Si DBRs consisting of dense/ porous film pair structures were fabricated for high-n at θα 0° and for low-n at θα 80° by the OAD method. As the number of pairs in a-Si DBRs was increased, the maximum R was increased while the HR bandwidth was narrowed. For the a-Si DBR with only three pairs, a broad Δλ∕λc of ∼22.5% was achieved in the wavelength range of 0.87–1.085 μm, keeping high R values of >95%. The introduction of a-Si DBR with three pairs as a HR facet coating of LDs led to the increased Po and reduced I th due to the decrease of mirror losses, showing the ηd enhancement of ∼1.98 times. These results suggest that the broadband high-reflective DBRs with only few pairs consisting of dense/porous a-Si films made of the same material are very promising in semiconductor lasers as well as other optoelectronic applications. This work was financially supported by the grant from the Industrial Source Technology Development Program (10044650) of the Ministry of Knowledge Economy (MKE) of Korea. References 1. H. D. I. Abarbanel, M. B. Kennel, L. Illing, S. Tang, H. F. Chen, and J. M. Liu, “Synchronization and communication using semiconductor lasers with optoelectronic feedback,” IEEE J. Quantum Electron. 37, 1301–1311 (2001). 2. S. Shinada, F. Koyama, N. Nishiyama, M. Arai, and K. Iga, “Analysis and fabrication of microaperture GaAs-GaAlAs surface-emitting laser for near-field optical data storage,” IEEE J. Sel. Top. Quantum Electron. 7, 365–370 (2001). 3. H. Yokoyama, A. Sato, H.-C. Guo, K. Sato, M. Mure, and H. Tsubokawa, “Nonlinear-microscopy optical-pulse sources based on mode-locked semiconductor lasers,” Opt. Express 16, 17752–17758 (2008). 4. R. P. Schneider and J. A. Lott, “InAlP/InAlGaP distributed Bragg reflectors for visible vertical cavity surface-emitting lasers,” Appl. Phys. Lett. 62, 2748–2750 (1993). 5. D. Hofstetter, H. P. Zappe, J. E. Epler, and J. Söchtig, “Singlegrowth-step GaAs/AlGaAs distributed Bragg reflector lasers with holographically-defined recessed gratings,” Electron. Lett. 30, 1858–1859 (1994). 6. W. W. Chow, K. D. Choquette, M. H. Crawford, K. L. Lear, and G. R. Hadley, “Design, fabrication, and performance of infrared and visible vertical-cavity surface-emitting lasers,” IEEE J. Quantum Electron. 33, 1810–1824 (1997). 7. O. Blum, I. J. Fritz, L. R. Dawson, A. J. Howard, T. J. Headley, J. F. Klem, and T. J. Drummond, “Highly reflective, long wavelength AlAsSb/GaAsSb distributed Bragg reflector grown by molecular beam epitaxy on InP substrates,” Appl. Phys. Lett. 66, 329–331 (1995). 8. I. W. Feng, S. Jin, J. Li, J. Lin, and H. Jiang, “SiO2/TiO2 distributed Bragg reflector near 1.5 μm fabricated by e-beam evaporation,” J. Vac. Sci. Technol. A 31, 061514 (2013).
9. C. C. Kao, Y. C. Peng, H. H. Yao, J. Y. Tsai, Y. H. Chang, J. T. Chu, H. W. Huang, T. T. Kao, T. C. Lu, H. C. Kuo, S. C. Wang, and C. F. Lin, “Fabrication and performance of blue GaNbased vertical-cavity surface emitting laser employing AlN/ GaN and Ta2O5/SiO2 distributed Bragg reflector,” Appl. Phys. Lett. 87, 081105 (2005). 10. S. F. Chichibu, T. Ohmori, N. Shibata, and T. Koyama, “Dielectric SiO2/ZrO2 distributed Bragg reflectors for ZnO microcavities prepared by the reactive helicon-wave-excitedplasma sputtering method,” Appl. Phys. Lett. 88, 161914 (2006). 11. Y. H. Lin, C. L. Wu, Y. H. Pai, and G. R. Lin, “A 533-nm selfluminescent Si-rich SiNx/SiOx distributed Bragg reflector,” Opt. Express 19, 6563–6570 (2011). 12. J. Boucart, C. Starck, F. Gaborit, A. Plais, N. Bouché, E. D erouin, J. C. Remy, J. Bonnet-Gamard, L. Goldstein, C. Fortin, D. Carpentier, P. Salet, F. Brillouet, and J. Jacquet, “Metamorphic DBR and tunnel-junction injection: a CW RT monolithic long-wavelength VCSEL,” IEEE J. Sel. Top. Quantum Electron. 5, 520–529 (1999). 13. D. J. Ripin, J. T. Gopinath, H. M. Shen, A. A. Erchak, G. S. Petrich, L. A. Kolodziejski, F. X. Kärtner, and E. P. Ippen, “Oxidized GaAs/AlAs mirror with a quantum-well saturable absorber for ultrashort-pulse Cr4+:YAG laser,” Opt. Commun. 214, 285–289 (2002). 14. S. J. Jang, Y. M. Song, C. I. Yeo, C. Y. Park, and Y. T. Lee, “Highly tolerant a-Si distributed Bragg reflector fabricated by oblique angle deposition,” Opt. Mater. Express 1, 451–457 (2011). 15. C. Mazzoleni and L. Pavesi, “Application to optical components of dielectric porous silicon multilayers,” Appl. Phys. Lett. 67, 2983–2985 (1995). 16. F. Duerinckx, I. Kuzma-Filipeck, K. Van Nieuwenhuysen, G. Beaucarne, and J. Poortmans, “Reorganized porous silicon Bragg reflectors for thin-film silicon solar cells,” IEEE Electron Device Lett. 27, 837–839 (2006). 17. W. H. Zheng, P. Reece, B. Q. Sun, and M. Gal, “Broadband laser mirrors made from porous silicon,” Appl. Phys. Lett. 84, 3519–3521 (2004). 18. J. W. Leem and J. S. Yu, “Broadband and wide-angle distributed Bragg reflectors based on amorphous germanium films by glancing angle deposition,” Opt. Express 20, 20576–20581 (2012). 19. J. W. Leem, X. Y. Guan, and J. S. Yu, “Tunable distributed Bragg reflectors with wide-angle and broadband highreflectivity using nanoporous/dense titanium dioxide film stacks for visible wavelength applications,” Opt. Express 22, 18519–18526 (2014). 20. M. F. Schubert, J. Q. Xi, J. K. Kim, and E. F. Schubert, “Distributed Bragg reflector consisting of high- and low-refractiveindex thin film layers made of the same material,” Appl. Phys. Lett. 90, 141115 (2007). 21. T. Karabacak, G.-C. Wang, and T.-M. Lu, “Quasi-periodic nanostructures grown by oblique angle deposition,” J. Appl. Phys. 94, 7723–7728 (2003). 22. J. W. Leem and J. S. Yu, “Glancing angle deposited ITO films for efficiency enhancement of a-Si:H/μc-Si:H tandem thin film solar cells,” Opt. Express 19, A258–A269 (2011). 23. Rosft Design Group, DiffractMOD, www.rsoftdesign.com (retrieved May 2014). 24. SOPRA, http://www.sopra‑sa.com (accessed 1 May 2014). 25. J. W. Leem and J. S. Yu, “Physical properties of electrically conductive Sb-doped SnO2 transparent electrodes by thermal annealing dependent structural changes for photovoltaic applications,” Mater. Sci. Eng. B 176, 1207–1212 (2011). 26. Y. H. Ko, S. W. Park, and J. S. Yu, “Cavity length and temperature dependent characteristics of compressively strained InGaAsP MQW BH lasers with a two-step compositional SCH structure,” Semicond. Sci. Technol. 23, 095011 (2008).
10 February 2015 / Vol. 54, No. 5 / APPLIED OPTICS
1031
Department of Electronics and Radio Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin 446-701, South Korea 2
Optowell Co., Ltd., 824 Palbok-dong 2ga, Deokjin-gu, Jeonju 561-844, South Korea
3
Korea Photonics Technology Institute, Laser-IT Research Center, 9 Cheomdan Bencheo-ro 108, Buk-gu, Gwangju 500-779, South Korea *Corresponding author: [email protected] Received 5 November 2014; revised 12 December 2014; accepted 26 December 2014; posted 6 January 2015 (Doc. ID 226218); published 4 February 2015
We fabricated amorphous silicon (a-Si)-based distributed Bragg reflectors (DBRs) consisting of alternating dense/porous films (i.e., pair) for a center wavelength (λc ) of 0.96 μm by oblique angle deposition (OAD) technique using an electron-beam evaporation system. The dense (high refractive index, i.e., high-n) and porous (low-n) a-Si films were deposited at two incident vapor flux angles of 0° and 80° in the OAD, respectively. Their optical reflectance characteristics were investigated in the wavelength range of 0.6–1.5 μm, including theoretical comparison using a rigorous coupled-wave analysis method. Above three pairs, the reflectivity (R) of a-Si DBRs was almost saturated at wavelengths around 0.96 μm, exhibiting R values of >97%. For the a-Si DBR with only three pairs, a broad normalized stop bandwidth (Δλ∕λc ) of ∼22.5% was obtained at wavelengths of ∼0.87–1.085 μm, keeping high R values of >95%. To simply demonstrate the feasibility of device applications, the a-Si DBR with three pairs was coated as a high-reflection layer at the rear facet of GaAs/InGaAs quantum-well laser diodes (LDs) operating at λ 0.96 μm. For the LDs coated with three-pair a-Si DBR, external differential quantum efficiency (ηd ) was nearly doubled compared to the uncoated LDs, indicating the ηd value of ∼50.6% (i.e., ηd ∼ 25.5% for the uncoated LDs). © 2015 Optical Society of America OCIS codes: (140.5960) Semiconductor lasers; (230.1480) Bragg reflectors; (310.1860) Deposition and fabrication; (310.6860) Thin films, optical properties. http://dx.doi.org/10.1364/AO.54.001027
1. Introduction
Semiconductor laser diodes (LDs) have been widely used in a variety of fields, such as optical fiber communications, information storages, displays, medical instruments, and measurement systems [1–3]. Generally, LD has a Fabry–Perot optical cavity that can 1559-128X/15/051027-05$15.00/0 © 2015 Optical Society of America
effectively give positive feedback by a pair of facet reflectors. Thus, a high-reflection (HR) coating at the rear-side facet is crucial because it reduces optical mirror losses in lasers, leading to improved device performance. Distributed Bragg reflectors (DBRs) have been generally utilized in semiconductor lasers as a HR facet coating layer [4–6]. For DBRs, the reflectivity and stop bandwidth are directly affected by the refractive index (n) difference between the high-n and low-n materials. As a result, 10 February 2015 / Vol. 54, No. 5 / APPLIED OPTICS
1027
the larger refractive index contrast provides higher reflectivity and wider stop bandwidth [7]. However, conventional DBRs consisting of two different alternating dielectric materials (TiO2 ∕SiO2 [8], Ta2 O5 ∕ SiO2 [9], ZrO2 ∕SiO2 [10], SiNx ∕SiOx [11], etc.) have fundamental limitations, such as low refractive index contrast, material selection, thermal expansion mismatch, and poor heat dissipation capability due to a low thermal conductivity [12,13]. Thus, the DBRs consisting of the same material, fabricated in an effective way including cost-effective and simple processes, are rather desirable for practical applications. Over the past years, some research works have been reported on the same material-based DBRs with only a few pairs for Si [14–17], Ge [18], TiO2 [19], ITO [20], and so forth. The low-n film with a high porosity can be formed by a relatively low-cost and simple electron beam (e-beam) evaporation using only one source material via the oblique angle deposition (OAD) method. The OAD technique can control the air volume fraction within the film because of its enhanced self-shadowing effect and limited mobility of incoming atoms [21,22]. In the case of amorphous silicon (a-Si), it has a low absorption and a high refractive index (i.e., na-Si > 2.3) in the nearinfrared (NIR) wavelength region of 0.8–1.1 μm, which allows for a larger refractive index difference between dense and porous films by the OAD as well as a relatively better thermal conductivity. There is very little or no report on the use of the a-Si DBRs as a HR facet coating layer in semiconductor LDs via the OAD method. Therefore, it is very meaningful to investigate the device performance of semiconductor LDs incorporated with the a-Si DBR for HR facet coating. In this work, we fabricated the a-Si DBRs consisting of dense/porous film pair structures for different pairs of one to four by the OAD technique via e-beam evaporation. The effect of a-Si DBR-based HR facet coating layer on the device characteristics of GaAs/InGaAs quantum-well (QW) LDs was studied. For optical reflectance properties, a theoretical analysis was also carried out using a rigorous coupled-wave analysis (RCWA) method. 2. Experimental and Simulation Modeling Details
Figure 1 shows the schematic illustration for the fabrication procedure of a-Si DBRs consisting of dense (high-n)/porous (low-n) film pair structures on GaAs substrates by the OAD method via an e-beam evaporation. The cross-sectional and top-view scanning electron microscope (SEM) images of a-Si DBRs with one pair are also shown in Fig. 1. To fabricate the a-Si DBRs, Si granules with 99.99% purity were used as a target source. The chamber was evacuated to a base pressure of 95%. This is due to the large refractive index contrast between the a-Si films deposited at θα 0° and 80° by the OAD, as can be seen in Fig. 2(a). In RCWA simulations, similarly, as the number of pairs in a-Si DBRs is increased from one pair to six pairs, the reflectivity is also increased and the HR band is narrowed. Above three pairs, the R value of a-Si DBRs was almost saturated at wavelengths around 0.96 μm, exhibiting R values of >97%. The highreflective properties of a-Si DBRs can be also confirmed in E-field intensity distribution calculations. In Fig. 3(c), for the a-Si DBR with three pairs, the E-field intensity in the GaAs substrate is lower than that of a-Si DBR with one pair due to its higher reflectivity at λ 0.96 μm, as shown in Fig. 3(b). In order to investigate the effect of the a-Si DBRs on the performance of actual devices, the a-Si DBRs with three pairs were coated at the rear facet of GaAs/InGaAs QW LDs for a HR coating. Figure 4(a) shows the schematic of the GaAs/InGaAs QW LD 10 February 2015 / Vol. 54, No. 5 / APPLIED OPTICS
1029
(a)
100
Reflectance (%)
80 70
n-AlGaAs waveguide
Light
60
n-GaAs clad GaAs substrate
50 a-Si DBR
40
n-contact
(b)
30 20
Measurements 0.7
0.8
0.9
1.0
1.1
400 Active region
Low-n a-Si film
High-n a-Si film
350
3.0 1.2
1.3
1.4
1.5
Wavelength (μm) 6
a-Si DBR 5 R (%) 100
99% 4
75
97%
2.5 2.0
Light emission λ= 0.96 μm
HR coated rear-facet
300 250
GaAs/InGaAs QW LD
a-Si DBR (3 pairs)
200
1.5
150
1.0
100
0.5 0.0 0
Uncoated HR coated
200
400
600
800
50 0 1000
50
Injection Current (mA)
25
Fig. 4. (a) Schematic of the GaAs/InGaAs QW LD structure and (b) CW L-I-V of the uncoated and HR-coated LDs at room temperature. The schematic of the HR-coated GaAs/InGaAs QW LD is also shown.
3 0
2
Voltage (V)
0 0.6
Number of Pairs
4.0 3.5
10
(b)
Passivation (SiO2)
p-contact p-GaAs clad AlGaAs p-waveguide GaAs/InGaAs QW active region
1 pair 2 pairs 3 pairs 4 pairs
a-Si DBR
90
Optical Output Power (a.u.)
(a)
Calculations 1
0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
Wavelength (μm)
(c) Z (μm)
2
Ey-field Intensity Max
Air Air
1
Si DBR 1 pair
Si DBR 3 pairs
0 GaAs substrate
-1 -2
-1
0
X (μm)
1
GaAs substrate 2 -2
-1
0
1
Min
2
X (μm)
Fig. 3. (a) Measured reflectance spectra of the a-Si DBRs with different pairs of one to four, (b) contour plot of variations of calculated reflectance spectra of the a-Si DBR as functions of number of pairs in a-Si DBRs and wavelength, and (c) calculated E-field intensity distributions of the a-Si DBRs with one pair and three pairs at λ 0.96 μm.
structure. The LD consists of the GaAs/InGaAs QW active region between AlGaAs waveguides. The p- and n-GaAs layers were used as clad layers, respectively. The p-GaAs clad layer was etched, and then the silicon dioxide (SiO2 ) layer was covered for passivation by using a plasma-enhanced chemical vapor deposition system. Finally, p- and n-contact metal layers were coated. Figure 4(b) shows the CW L-I-V of uncoated and HR-coated LDs at room temperature. The schematic of the HR-coated GaAs/InGaAs QW LD is also shown in the inset of Fig. 4(b). As shown in Fig. 4(b), the device performance was considerably improved by incorporating the a-Si DBR into the rear facet of the LD. Both the uncoated and HR-coated LDs showed the almost 1030
APPLIED OPTICS / Vol. 54, No. 5 / 10 February 2015
similar I-V characteristics (i.e., no obvious electrical degradation). From the I-L curves, on the other hand, the threshold current (I th ) decreased and output power (Po ) increased compared to the uncoated LD. For the HR-coated LD, a lower I th of 168 mA and a higher Po of 271.8 mW at an injection current of 1000 mA were obtained, exhibiting a slope efficiency (dP∕dI) of 0.3267 mW/mA (i.e., I th 225 mA, Po 127.7 mW, and dP∕dI 0.1648 mW∕mA for the uncoated LD). This is attributed to the reduced mirror loss by the HR a-Si DBR coated at the rear facet of LDs. The mirror loss (αm ) is given by the equation [26] αm
1 1 ln ; 2L R1 R2
(1)
where L is the cavity length, and R1 and R2 are the reflectivities of both the facets of LDs. It is assumed that the cleaved facet has the mirror reflectivity of ∼33.1%, which is a calculated value for the active material at λ 0.96 μm in these experiments. For L 2 mm, the αm of uncoated LD is estimated to be ∼5.53 cm−1 , while it decreased to 2.85 cm−1 for the HR-coated LD, indicating the reduction percentage of ∼48.5%. Thus, the external differential quantum efficiency (ηd 2e∕hν · dP∕dI, where hν is the photon energy and e is the electron charge), which is defined by the number of photons emitted by each net electron above threshold, is increased from ∼25.5% to ∼50.6%, showing the increment of ∼1.98 times. From these results, the a-Si DBR with only
few pairs can be used as the HR facet coating of optoelectronic devices without any degradation of electrical properties. 4. Conclusion
The device performance enhancement of GaAs/ InGaAs QW LDs (λ 0.96 μm) coated with threepair a-Si DBRs at the rear facet was demonstrated. At λc 0.96 μm, the a-Si DBRs consisting of dense/ porous film pair structures were fabricated for high-n at θα 0° and for low-n at θα 80° by the OAD method. As the number of pairs in a-Si DBRs was increased, the maximum R was increased while the HR bandwidth was narrowed. For the a-Si DBR with only three pairs, a broad Δλ∕λc of ∼22.5% was achieved in the wavelength range of 0.87–1.085 μm, keeping high R values of >95%. The introduction of a-Si DBR with three pairs as a HR facet coating of LDs led to the increased Po and reduced I th due to the decrease of mirror losses, showing the ηd enhancement of ∼1.98 times. These results suggest that the broadband high-reflective DBRs with only few pairs consisting of dense/porous a-Si films made of the same material are very promising in semiconductor lasers as well as other optoelectronic applications. This work was financially supported by the grant from the Industrial Source Technology Development Program (10044650) of the Ministry of Knowledge Economy (MKE) of Korea. References 1. H. D. I. Abarbanel, M. B. Kennel, L. Illing, S. Tang, H. F. Chen, and J. M. Liu, “Synchronization and communication using semiconductor lasers with optoelectronic feedback,” IEEE J. Quantum Electron. 37, 1301–1311 (2001). 2. S. Shinada, F. Koyama, N. Nishiyama, M. Arai, and K. Iga, “Analysis and fabrication of microaperture GaAs-GaAlAs surface-emitting laser for near-field optical data storage,” IEEE J. Sel. Top. Quantum Electron. 7, 365–370 (2001). 3. H. Yokoyama, A. Sato, H.-C. Guo, K. Sato, M. Mure, and H. Tsubokawa, “Nonlinear-microscopy optical-pulse sources based on mode-locked semiconductor lasers,” Opt. Express 16, 17752–17758 (2008). 4. R. P. Schneider and J. A. Lott, “InAlP/InAlGaP distributed Bragg reflectors for visible vertical cavity surface-emitting lasers,” Appl. Phys. Lett. 62, 2748–2750 (1993). 5. D. Hofstetter, H. P. Zappe, J. E. Epler, and J. Söchtig, “Singlegrowth-step GaAs/AlGaAs distributed Bragg reflector lasers with holographically-defined recessed gratings,” Electron. Lett. 30, 1858–1859 (1994). 6. W. W. Chow, K. D. Choquette, M. H. Crawford, K. L. Lear, and G. R. Hadley, “Design, fabrication, and performance of infrared and visible vertical-cavity surface-emitting lasers,” IEEE J. Quantum Electron. 33, 1810–1824 (1997). 7. O. Blum, I. J. Fritz, L. R. Dawson, A. J. Howard, T. J. Headley, J. F. Klem, and T. J. Drummond, “Highly reflective, long wavelength AlAsSb/GaAsSb distributed Bragg reflector grown by molecular beam epitaxy on InP substrates,” Appl. Phys. Lett. 66, 329–331 (1995). 8. I. W. Feng, S. Jin, J. Li, J. Lin, and H. Jiang, “SiO2/TiO2 distributed Bragg reflector near 1.5 μm fabricated by e-beam evaporation,” J. Vac. Sci. Technol. A 31, 061514 (2013).
9. C. C. Kao, Y. C. Peng, H. H. Yao, J. Y. Tsai, Y. H. Chang, J. T. Chu, H. W. Huang, T. T. Kao, T. C. Lu, H. C. Kuo, S. C. Wang, and C. F. Lin, “Fabrication and performance of blue GaNbased vertical-cavity surface emitting laser employing AlN/ GaN and Ta2O5/SiO2 distributed Bragg reflector,” Appl. Phys. Lett. 87, 081105 (2005). 10. S. F. Chichibu, T. Ohmori, N. Shibata, and T. Koyama, “Dielectric SiO2/ZrO2 distributed Bragg reflectors for ZnO microcavities prepared by the reactive helicon-wave-excitedplasma sputtering method,” Appl. Phys. Lett. 88, 161914 (2006). 11. Y. H. Lin, C. L. Wu, Y. H. Pai, and G. R. Lin, “A 533-nm selfluminescent Si-rich SiNx/SiOx distributed Bragg reflector,” Opt. Express 19, 6563–6570 (2011). 12. J. Boucart, C. Starck, F. Gaborit, A. Plais, N. Bouché, E. D erouin, J. C. Remy, J. Bonnet-Gamard, L. Goldstein, C. Fortin, D. Carpentier, P. Salet, F. Brillouet, and J. Jacquet, “Metamorphic DBR and tunnel-junction injection: a CW RT monolithic long-wavelength VCSEL,” IEEE J. Sel. Top. Quantum Electron. 5, 520–529 (1999). 13. D. J. Ripin, J. T. Gopinath, H. M. Shen, A. A. Erchak, G. S. Petrich, L. A. Kolodziejski, F. X. Kärtner, and E. P. Ippen, “Oxidized GaAs/AlAs mirror with a quantum-well saturable absorber for ultrashort-pulse Cr4+:YAG laser,” Opt. Commun. 214, 285–289 (2002). 14. S. J. Jang, Y. M. Song, C. I. Yeo, C. Y. Park, and Y. T. Lee, “Highly tolerant a-Si distributed Bragg reflector fabricated by oblique angle deposition,” Opt. Mater. Express 1, 451–457 (2011). 15. C. Mazzoleni and L. Pavesi, “Application to optical components of dielectric porous silicon multilayers,” Appl. Phys. Lett. 67, 2983–2985 (1995). 16. F. Duerinckx, I. Kuzma-Filipeck, K. Van Nieuwenhuysen, G. Beaucarne, and J. Poortmans, “Reorganized porous silicon Bragg reflectors for thin-film silicon solar cells,” IEEE Electron Device Lett. 27, 837–839 (2006). 17. W. H. Zheng, P. Reece, B. Q. Sun, and M. Gal, “Broadband laser mirrors made from porous silicon,” Appl. Phys. Lett. 84, 3519–3521 (2004). 18. J. W. Leem and J. S. Yu, “Broadband and wide-angle distributed Bragg reflectors based on amorphous germanium films by glancing angle deposition,” Opt. Express 20, 20576–20581 (2012). 19. J. W. Leem, X. Y. Guan, and J. S. Yu, “Tunable distributed Bragg reflectors with wide-angle and broadband highreflectivity using nanoporous/dense titanium dioxide film stacks for visible wavelength applications,” Opt. Express 22, 18519–18526 (2014). 20. M. F. Schubert, J. Q. Xi, J. K. Kim, and E. F. Schubert, “Distributed Bragg reflector consisting of high- and low-refractiveindex thin film layers made of the same material,” Appl. Phys. Lett. 90, 141115 (2007). 21. T. Karabacak, G.-C. Wang, and T.-M. Lu, “Quasi-periodic nanostructures grown by oblique angle deposition,” J. Appl. Phys. 94, 7723–7728 (2003). 22. J. W. Leem and J. S. Yu, “Glancing angle deposited ITO films for efficiency enhancement of a-Si:H/μc-Si:H tandem thin film solar cells,” Opt. Express 19, A258–A269 (2011). 23. Rosft Design Group, DiffractMOD, www.rsoftdesign.com (retrieved May 2014). 24. SOPRA, http://www.sopra‑sa.com (accessed 1 May 2014). 25. J. W. Leem and J. S. Yu, “Physical properties of electrically conductive Sb-doped SnO2 transparent electrodes by thermal annealing dependent structural changes for photovoltaic applications,” Mater. Sci. Eng. B 176, 1207–1212 (2011). 26. Y. H. Ko, S. W. Park, and J. S. Yu, “Cavity length and temperature dependent characteristics of compressively strained InGaAsP MQW BH lasers with a two-step compositional SCH structure,” Semicond. Sci. Technol. 23, 095011 (2008).
10 February 2015 / Vol. 54, No. 5 / APPLIED OPTICS
1031
