5466
OPTICS LETTERS / Vol. 38, No. 24 / December 15, 2013
Low-temperature-grown InGaAs terahertz photomixer embedded in InP thermal spreading layer regrown by metalorganic chemical vapor deposition Kiwon Moon,1 Dong Woo Park,3 Il-Min Lee,1 Namje Kim,1 Hyunsung Ko,1 Sang-Pil Han,1 Donghun Lee,2 Jeong-Woo Park,1 Sam Kyu Noh,3 and Kyung Hyun Park1,* 1 2
THz Photonics Creative Research Center, ETRI, Daejeon 305-700, South Korea
Photonic/Wireless Convergence Components Department, ETRI, Daejeon 305-700, South Korea 3
Nano Materials Evaluation Center, KRISS, Daejeon 305-340, South Korea *Corresponding author: [email protected]
Received September 26, 2013; revised November 19, 2013; accepted November 19, 2013; posted November 19, 2013 (Doc. ID 198155); published December 13, 2013 A novel buried photomixer for integrated photonic terahertz devices is proposed. The active region of the mesastructure InGaAs photomixer is buried in an InP layer grown by metalorganic chemical vapor deposition (MOCVD) to improve heat dissipation, which is an important problem for terahertz photomixers. The proposed photomixer shows good thermal properties compared to a conventional planar-type photomixer. The MOCVD regrowth process indicates the possibility for THz photomixers to be integrated monolithically with conventional photonic devices. © 2013 Optical Society of America OCIS codes: (300.6495) Spectroscopy, terahertz; (130.3130) Integrated optics materials; (120.6810) Thermal effects. http://dx.doi.org/10.1364/OL.38.005466
Terahertz (THz) waves are electromagnetic waves with wavelengths between the mid-infrared (MIR) and millimeter wave bands. They are broadly applicable to areas such as spectroscopy, imaging, the food industry, nondestructive inspection, communications, medicine, and biology [1]. Devices such as gyrotrons [2], backwardwave oscillators [3], and free-electron lasers [4] offered high-power THz radiation, but a cost-effective, portable, and broadband tunable continuous wave (CW) THz source can be achieved with photonics-based photomixing techniques [5–8]. Photomixing relies on the heterodyne downconversion of two laser beams generated by individual laser diodes [5] or a dual-mode laser diode [7,8]. The photomixer requires a photoconductive material with a high dark resistivity and a short carrier lifetime of less than several picoseconds. Ideal photoconductive properties can be realized in III–V compound semiconductors with the introduction of engineered defects such as deep levels by using ion irradiation [9], erbium nanoparticles [10,11], and arsenic precipitates. Of these, As precipitates in InGaAs [12–14], GaAs [12,15], and GaBiAs [16] grown by molecular beam epitaxy (MBE) at low temperatures, have been well characterized. To realize a high-performance THz photomixer, the growth conditions and subsequent TA temperatures of the photoconductive material should be optimized. The initial excess-As concentration is mainly determined by the MBE growth temperature, while the size and density of the precipitated metallic defects [12], i.e., the carrier lifetime [15] and resistivity [13,14] are affected by the TA temperature. It is common to deploy rapid thermal annealing (RTA) with a capping GaAs wafer or to perform TA in an MBE chamber under As ambient after the growth [14]. The photoconductive materials also have been adopted for the broadband pulse THz timedomain spectroscopy to generate THz pulses [17–19]. Multilayered structure [18], mesa-type photomixers 0146-9592/13/245466-04$15.00/0
[19], and plasmonic nanostructures [20] have been introduced to enhance the THz radiation efficiency. Heat dissipation is another important issue with the photomixer [21]. Improvements in dissipating heat from the laser-irradiated photomixer active region [22,23] and improved packaging techniques [24] have recently led to better efficiency. Here we propose a buried-mesa-type THz photomixer to enhance the heat dissipation and to show the possibility of monolithic integration with MOCVD-grown photonic devices, such as laser diodes and Shottky diodes to realize a one-chip THz emitter and detector. The active region of the proposed photomixer is embedded in a high-thermal-conductivity InP layer grown by metalorganic chemical vapor deposition (MOCVD) under optimized growth conditions. The buried-mesa-type photomixer showed 3.8 times improvement in the maximum power and operates under higher bias compared to a reference photomixer without a buried mesa structure. By thermo-graphic camera, we confirmed the improvements were due to the enhanced thermal dissipation. The annealing behavior of low-temperature grown (LTG) InGaAs layers places restrictions on subsequent processing temperatures, and, for MOCVD, the growth temperature should be decided according to the carrier lifetime and resistivity. To determine the best TA conditions, we began by preparing a 1.2 μm thick In0.53 Ga0.47 As layer grown by MBE on a (100)-oriented Fe-doped semi-insulating InP wafer at a growth temperature, beam equivalent ratio, and growth rate of 160°C, 12, and 1 μm∕h, respectively. The layer was doped with beryllium at a doping concentration of 2 × 1018 cm−3 to obtain a short carrier lifetime. After the growth, the LTG-In0.53 Ga0.47 As wafer was divided into several pieces that were subject to RTA with a GaAs capping wafer at a temperature of 380°C, 440°C, 500°C, 560°C, or 620°C for 10 min. After annealing, x-ray diffraction (XRD) rocking © 2013 Optical Society of America
December 15, 2013 / Vol. 38, No. 24 / OPTICS LETTERS
curves, carrier concentrations, Hall mobilities, and carrier lifetimes of each sample were measured. The carrier concentration of the as-grown, nonannealed n-type LTG-InGaAs sample was 8.67 × 1016 cm−3 . As the annealing temperature increased from 380°C to 620°C (Fig. 1), the carrier concentration decreased monotonically to 3.25 × 1015 cm−3 , indicating carrier depletion due to the formation of metallic clusters. The mobility, however, increased as the annealing temperature increased, reaching a maximum value of 1510 cm2 ∕V · s at 620°C. The increased mobility can be attributed to TA improving the crystal quality. The sheet resistance of the as-grown LTG-In0.53 Ga0.47 As sample was 644 Ω∕sq, whereas that of the samples annealed at 620°C was 10.59 kΩ∕sq. We estimated the carrier lifetimes of the as-grown and annealed samples from the transient transmittance [two examples are shown in Fig. 2(a)]. The carrier lifetime of the as-grown LTG-In0.53 Ga0.47 As sample layer was determined to be 0.74 ps from the fast decay lifetime of a bi-exponential fitting. For the annealed samples, an increase in the annealing temperature resulted in an increase in the carrier lifetime to a maximum of 2.07 ps [Fig. 2(b)]. The XRD rocking curves [Fig. 2(b)] showed that the excess arsenic concentration of the as-grown LTG-InGaAs is about 1% [13]. Results show that the mobility saturates above 560°C, but the carrier lifetime increases as the TA temperature increases. In the MOCVD, the growth process replaces the annealing process; no explicit annealing process is required. The growth time is also important, but the precipitation process was reported to be saturate in less than several minutes [14]. To confirm this, we performed TA in the MOCVD reactor at 560°C under AsH3 flow, obtained similar characteristics. Thus we adopted an MOCVD growth temperature of 560°C. To fabricate the buried-mesa-type InGaAs photomixer, we employed a selective InP regrowth process using MOCVD. The device structure and photographs of the fabricated photomixer are shown in Fig. 3. The mesa structure was defined on a PECVD-deposited SiO2 mask layer by optical lithography and wet etching with HBr solution. The size of the mesa structure was varied between 10 μm × 10 μm and 20 μm × 20 μm. The etch depth
Fig. 1. (a) Carrier concentrations (blue) and Hall mobilities (black) of the as-grown and RTA-annealed LTG-In0.53 Ga0.47 As samples. The carrier is n-type.
5467
Fig. 2. (a) The transient transmittance of two LTGIn0.53 Ga0.47 As samples. The annealed sample data are offset for clarity. Inset shows the TEM photograph of arsenic clusters after annealing at 560°C. (b) Carrier lifetime (black) and XRD mismatch (blue) as a function of annealing temperature.
was around 1.2 μm, which corresponds to the thickness of the LTG-InGaAs layer. After defining the mesa, an undoped InP layer was selectively grown around the mesa by MOCVD using the SiO2 layer as a mask. The growth temperature, V/III ratio, and growth rate were 560°C, 170, and 5.6 Å∕ sec, respectively. To prevent surface degradation, the PH3 flow was maintained at 350 sccm while the sample was heated in the MOCVD reactor. In general, growth rates vary for different crystallographic directions in selective growth, and this may result in local protrusions around the mesa structure. To prevent this, the isotropic etching property of the HBr solution was exploited to produce a SiO2 mask that overhung the mesa. The relatively planar surface profile is obtained after regrowth and assists in the subsequent metallization process. Figure 3(a) shows a scanning electron microscope (SEM) image of a 20 μm × 20 μm mesa structure, and the mask overhang can be seen in the inset. To obtain an optimum sidewall profile, the thickness of the InP layer was set to be 0.6 μm. A Normaski image of the photomixer structure after the InP regrowth is shown in Fig. 3(b). The final structure [Fig. 3(c)] was designed with an interdigitated finger structure and a planar log-spiral antenna. The finger had a width of 0.8 μm with a gap of 2.4 μm. The gap between the spirals was set to 10 μm, and the antenna structure was designed to have broadband characteristics using a commercial simulator.
5468
OPTICS LETTERS / Vol. 38, No. 24 / December 15, 2013
Fig. 3. (a) SEM image of the 20 μm × 20 μm mesa structure, the inset shows the SiO2 mask overhang. (b) Photograph taken in Normaski mode of the LTG-InGaAs mesa after InP regrowth. (c) Photograph of the fabricated buried-mesa-type InGaAs photomixer with a mesa size of 15 μm × 15 μm. (d) Schematic of the buried-mesa-type photomixer.
For comparison, we also fabricated a reference photomixer without performing the mesa etching and regrowth process. Therefore, the reference photomixer is identical to the buried-mesa type, except for the regrown u-InP layer region in Fig. 3(d), and is replaced by the initially grown LTG-InGaAs. The reference photomixer was subject to TA in the MOCVD reactor under AsH3 flow for a fair comparison. To compare the performances of the fabricated photomixers, we prepared a fiber-based broadband-pulse THz system, as shown in Fig. 4(a). A 1.55 μm center wavelength femtosecond laser was used to generate and detect the THz pulses. A dispersion-compensating fiber and single-mode fibers were used for dispersion management. The width and average power of the femtosecond laser pulses were 120 fs and 7 mW, respectively. The fast Fourier transform (FFT) amplitude of the THz pulse from the buried-type photomixer and the reference sample are shown in Fig. 4(b). As expected, the performance of the buried photomixer was better than the reference photomixer, especially at high frequencies. After changing the femtosecond laser with a 30 mW dual-mode laser, which generates 1.5 μm center wavelength optical beating signal at THz frequencies [7,8], we measured CW THz emission efficiency at 700 GHz as a function of the photocurrent [Fig. 4(c)]. At the same bias voltage, the photocurrent of the buried mesa type was smaller than the reference photomixer. However, within the measured current range, the THz power was improved by more than two times. In addition, the buried mesa type photomixer sustained 18% higher electric bias than the reference photomixer. As a result, the maximum power was enhanced by 3.8 times, which can be attributed to enhanced thermal dissipation through the InP layer. The improved thermal dissipation was confirmed from thermal images of the photomixers (Fig. 5) obtained by using a MIR camera (FLIR Systems, Thermo Vision A40M), which show that the surface temperature of the buried-mesa-type photomixer was approximately 5°C lower than that of the reference photomixer. The
Fig. 4. (a) THz-TDS measurement setup. DCF, dispersioncompensating fiber; SMF, single-mode fiber. (b) FFT amplitude spectra of the buried-mesa-type and reference photomixers. (c) CW THz power as a function of the photocurrent.
photomixers were subject to the same conditions of an optical CW pumping power of 19 dBm and a bias of 3 V. In conclusion, we have developed a buried-mesa-type THz photomixer by replacing the conventional TA process with an MOCVD regrowth process. The improved heat dissipation leads to improved performance of the buried-mesa-type photomixer compared to the conventional planar-type photomixer. Our work is a first step in realizing single-chip functional THz devices by monolithic integration with conventional electronic and photonic devices such as photodiodes lasers [7,8], and Shottky diodes. This work was partly supported by the IT R&D program of MOTIE/KEIT [10045238, Development of the portable scanner for THz imaging and spectroscopy], Joint Research Projects of ISTK, the Public Welfare & Safety Research Program through the National Research
December 15, 2013 / Vol. 38, No. 24 / OPTICS LETTERS
Fig. 5. Thermal images of the buried-mesa-type photomixer (a) before and (b) after the optical input is applied. (c) and (d) Corresponding thermal images of the reference photomixer. Note that the log-periodic antenna has been integrated because of experimental restrictions.
Foundation of Korea (NRF) Technology (NRF-20100020822), and Nano·Material Technology Development Program through the NRF of Korea (NRF2012M3A7B4035095). References 1. B. Ferguson and X.-C. Zhang, Nat. Mater. 1, 26 (2002). 2. M. Y. Glyavin, N. S. Ginzburg, A. L. Goldenberg, G. G. Denisov, A. G. Luchinin, V. N. Manuilov, V. E. Zapevalov, and I. V. Zotova, Terahertz Sci. Technol. 5, 67 (2012). 3. A. Dobriu, M. Yamashita, Y. N. Ohshima, Y. Morita, C. Otani, and K. Kawase, Appl. Opt. 43, 5638 (2004). 4. B. A. Knyazev, G. N. Kulipanov, and N. A. Vinokurov, Meas. Sci. Technol. 21, 054017 (2010). 5. E. R. Brown, K. A. McIntosh, K. B. Nichols, and C. L. Dennis, Appl. Phys. Lett. 66, 285 (1995). 6. C. Baker, I. S. Gregory, M. J. Evans, W. R. Tribe, E. H. Linfield, and M. Missous, Opt. Express 13, 9639 (2005). 7. N. Kim, J. Shin, E. Sim, C. W. Lee, D.-S. Yee, M. Y. Jeon, Y. Jang, and K. H. Park, Opt. Express 17, 13851 (2009). 8. N. Kim, H.-C. Ryu, D. Lee, S.-P. Han, H. Ko, K. Moon, J.-W. Park, M. Y. Jeon, and K. H. Park, Laser Phys. Lett. 10, 085805 (2013).
5469
9. J. Mangeney, A. Merigault, N. Zerounian, P. Crozat, K. Blary, and J. F. Lampin, Appl. Phys. Lett. 91, 241102 (2007). 10. J. E. Barnason, T. L. J. Chan, A. W. M. Lee, E. R. Brown, D. C. Driscoll, M. Hanson, A. C. Gossard, and R. E. Muller, Appl. Phys. Lett. 85, 3983 (2004). 11. A. Schwagmann, Z.-Y. Zaho, F. Ospald, H. Lu, D. C. Driscoll, M. P. Hanson, A. C. Gossard, and J. H. Smet, Appl. Phys. Lett. 96, 141108 (2010). 12. M. R. Melloch, J. M. Woodall, E. S. Harmon, N. Otsuka, F. H. Pollak, D. D. Nolte, R. M. Feenstra, and M. A. Lutz, Annu. Rev. Mater. Sci. 25, 547 (1995). 13. R. A. Metzger, A. S. Brown, L. G. McCray, and J. A. Henige, J. Vac. Sci. Technol. B 11, 798 (1993). 14. C. Baker, I. S. Gregory, W. R. Tribe, I. V. Bradley, M. J. Evans, E. H. Linfeld, and M. Missous, Appl. Phys. Lett. 85, 4965 (2004). 15. E. S. Harmon, M. R. Melloch, J. M. Woodall, D. D. Nolte, N. Otsuka, and C. L. Chang, Appl. Phys. Lett. 63, 2248 (1993). 16. P. Pačebutas, A. Bičiūnas, S. Balakauskas, A. Krotkus, G. Andriukaitis, D. Lorenc, A. Pugžlys, and A. Baltuška, Appl. Phys. Lett. 97, 031111 (2010). 17. A. Takazato, M. Kamakura, T. Matsui, J. Kitagawa, and Y. Kadoya, Appl. Phys. Lett. 91, 011102 (2007). 18. R. Takahashi, Y. Kawamura, and H. Iwamura, Appl. Phys. Lett. 68, 153 (1996). 19. H. Roehle, R. J. B. Dietz, H. J. Hensel, J. Böttcher, H. Künzel, D. Stanze, M. Schell, and B. Satorius, Opt. Express 18, 2296 (2010). 20. B. Heshmat, H. Pahlevaninezhad, Y. Pang, M. MasnadiShirazi, R. B. Lewis, T. Tiedje, R. Gordon, and T. E. Darcie, Nano Lett. 12, 6255 (2012). 21. S. Verghese, K. A. Mclntosh, and E. R. Brown, Appl. Phys. Lett. 71, 2743 (1997). 22. S. A. Daffaie, O. Yilmazoglu, F. Küppers, and H. Hartnagel, in 37th International Conference on Infrared, Milimeter and Terahertz Waves (IRMMW-THz), Wollongong, Australia, September 23–28, 2012, Mon-C-1–2. 23. E. Peyatavit, S. Lepilliet, F. Hindle, C. Coinon, T. Akalin, G. Ducournau, G. Mouret, and J.-F. Lampin, in 37th International Conference on Infrared, Milimeter and Terahertz Waves (IRMMW-THz), Wollongong, Australia, September 23–28, 2012, Thu-C-1–2. 24. S.-P. Han, N. Kim, H. Ko, H.-C. Ryu, J.-W. Park, Y. J. Yoon, J.-H. Shin, D. H. Lee, S.-H. Moon, S.-W. Choi, H. S. Chun, and K. H. Park, Opt. Express 20, 18432 (2012).
OPTICS LETTERS / Vol. 38, No. 24 / December 15, 2013
Low-temperature-grown InGaAs terahertz photomixer embedded in InP thermal spreading layer regrown by metalorganic chemical vapor deposition Kiwon Moon,1 Dong Woo Park,3 Il-Min Lee,1 Namje Kim,1 Hyunsung Ko,1 Sang-Pil Han,1 Donghun Lee,2 Jeong-Woo Park,1 Sam Kyu Noh,3 and Kyung Hyun Park1,* 1 2
THz Photonics Creative Research Center, ETRI, Daejeon 305-700, South Korea
Photonic/Wireless Convergence Components Department, ETRI, Daejeon 305-700, South Korea 3
Nano Materials Evaluation Center, KRISS, Daejeon 305-340, South Korea *Corresponding author: [email protected]
Received September 26, 2013; revised November 19, 2013; accepted November 19, 2013; posted November 19, 2013 (Doc. ID 198155); published December 13, 2013 A novel buried photomixer for integrated photonic terahertz devices is proposed. The active region of the mesastructure InGaAs photomixer is buried in an InP layer grown by metalorganic chemical vapor deposition (MOCVD) to improve heat dissipation, which is an important problem for terahertz photomixers. The proposed photomixer shows good thermal properties compared to a conventional planar-type photomixer. The MOCVD regrowth process indicates the possibility for THz photomixers to be integrated monolithically with conventional photonic devices. © 2013 Optical Society of America OCIS codes: (300.6495) Spectroscopy, terahertz; (130.3130) Integrated optics materials; (120.6810) Thermal effects. http://dx.doi.org/10.1364/OL.38.005466
Terahertz (THz) waves are electromagnetic waves with wavelengths between the mid-infrared (MIR) and millimeter wave bands. They are broadly applicable to areas such as spectroscopy, imaging, the food industry, nondestructive inspection, communications, medicine, and biology [1]. Devices such as gyrotrons [2], backwardwave oscillators [3], and free-electron lasers [4] offered high-power THz radiation, but a cost-effective, portable, and broadband tunable continuous wave (CW) THz source can be achieved with photonics-based photomixing techniques [5–8]. Photomixing relies on the heterodyne downconversion of two laser beams generated by individual laser diodes [5] or a dual-mode laser diode [7,8]. The photomixer requires a photoconductive material with a high dark resistivity and a short carrier lifetime of less than several picoseconds. Ideal photoconductive properties can be realized in III–V compound semiconductors with the introduction of engineered defects such as deep levels by using ion irradiation [9], erbium nanoparticles [10,11], and arsenic precipitates. Of these, As precipitates in InGaAs [12–14], GaAs [12,15], and GaBiAs [16] grown by molecular beam epitaxy (MBE) at low temperatures, have been well characterized. To realize a high-performance THz photomixer, the growth conditions and subsequent TA temperatures of the photoconductive material should be optimized. The initial excess-As concentration is mainly determined by the MBE growth temperature, while the size and density of the precipitated metallic defects [12], i.e., the carrier lifetime [15] and resistivity [13,14] are affected by the TA temperature. It is common to deploy rapid thermal annealing (RTA) with a capping GaAs wafer or to perform TA in an MBE chamber under As ambient after the growth [14]. The photoconductive materials also have been adopted for the broadband pulse THz timedomain spectroscopy to generate THz pulses [17–19]. Multilayered structure [18], mesa-type photomixers 0146-9592/13/245466-04$15.00/0
[19], and plasmonic nanostructures [20] have been introduced to enhance the THz radiation efficiency. Heat dissipation is another important issue with the photomixer [21]. Improvements in dissipating heat from the laser-irradiated photomixer active region [22,23] and improved packaging techniques [24] have recently led to better efficiency. Here we propose a buried-mesa-type THz photomixer to enhance the heat dissipation and to show the possibility of monolithic integration with MOCVD-grown photonic devices, such as laser diodes and Shottky diodes to realize a one-chip THz emitter and detector. The active region of the proposed photomixer is embedded in a high-thermal-conductivity InP layer grown by metalorganic chemical vapor deposition (MOCVD) under optimized growth conditions. The buried-mesa-type photomixer showed 3.8 times improvement in the maximum power and operates under higher bias compared to a reference photomixer without a buried mesa structure. By thermo-graphic camera, we confirmed the improvements were due to the enhanced thermal dissipation. The annealing behavior of low-temperature grown (LTG) InGaAs layers places restrictions on subsequent processing temperatures, and, for MOCVD, the growth temperature should be decided according to the carrier lifetime and resistivity. To determine the best TA conditions, we began by preparing a 1.2 μm thick In0.53 Ga0.47 As layer grown by MBE on a (100)-oriented Fe-doped semi-insulating InP wafer at a growth temperature, beam equivalent ratio, and growth rate of 160°C, 12, and 1 μm∕h, respectively. The layer was doped with beryllium at a doping concentration of 2 × 1018 cm−3 to obtain a short carrier lifetime. After the growth, the LTG-In0.53 Ga0.47 As wafer was divided into several pieces that were subject to RTA with a GaAs capping wafer at a temperature of 380°C, 440°C, 500°C, 560°C, or 620°C for 10 min. After annealing, x-ray diffraction (XRD) rocking © 2013 Optical Society of America
December 15, 2013 / Vol. 38, No. 24 / OPTICS LETTERS
curves, carrier concentrations, Hall mobilities, and carrier lifetimes of each sample were measured. The carrier concentration of the as-grown, nonannealed n-type LTG-InGaAs sample was 8.67 × 1016 cm−3 . As the annealing temperature increased from 380°C to 620°C (Fig. 1), the carrier concentration decreased monotonically to 3.25 × 1015 cm−3 , indicating carrier depletion due to the formation of metallic clusters. The mobility, however, increased as the annealing temperature increased, reaching a maximum value of 1510 cm2 ∕V · s at 620°C. The increased mobility can be attributed to TA improving the crystal quality. The sheet resistance of the as-grown LTG-In0.53 Ga0.47 As sample was 644 Ω∕sq, whereas that of the samples annealed at 620°C was 10.59 kΩ∕sq. We estimated the carrier lifetimes of the as-grown and annealed samples from the transient transmittance [two examples are shown in Fig. 2(a)]. The carrier lifetime of the as-grown LTG-In0.53 Ga0.47 As sample layer was determined to be 0.74 ps from the fast decay lifetime of a bi-exponential fitting. For the annealed samples, an increase in the annealing temperature resulted in an increase in the carrier lifetime to a maximum of 2.07 ps [Fig. 2(b)]. The XRD rocking curves [Fig. 2(b)] showed that the excess arsenic concentration of the as-grown LTG-InGaAs is about 1% [13]. Results show that the mobility saturates above 560°C, but the carrier lifetime increases as the TA temperature increases. In the MOCVD, the growth process replaces the annealing process; no explicit annealing process is required. The growth time is also important, but the precipitation process was reported to be saturate in less than several minutes [14]. To confirm this, we performed TA in the MOCVD reactor at 560°C under AsH3 flow, obtained similar characteristics. Thus we adopted an MOCVD growth temperature of 560°C. To fabricate the buried-mesa-type InGaAs photomixer, we employed a selective InP regrowth process using MOCVD. The device structure and photographs of the fabricated photomixer are shown in Fig. 3. The mesa structure was defined on a PECVD-deposited SiO2 mask layer by optical lithography and wet etching with HBr solution. The size of the mesa structure was varied between 10 μm × 10 μm and 20 μm × 20 μm. The etch depth
Fig. 1. (a) Carrier concentrations (blue) and Hall mobilities (black) of the as-grown and RTA-annealed LTG-In0.53 Ga0.47 As samples. The carrier is n-type.
5467
Fig. 2. (a) The transient transmittance of two LTGIn0.53 Ga0.47 As samples. The annealed sample data are offset for clarity. Inset shows the TEM photograph of arsenic clusters after annealing at 560°C. (b) Carrier lifetime (black) and XRD mismatch (blue) as a function of annealing temperature.
was around 1.2 μm, which corresponds to the thickness of the LTG-InGaAs layer. After defining the mesa, an undoped InP layer was selectively grown around the mesa by MOCVD using the SiO2 layer as a mask. The growth temperature, V/III ratio, and growth rate were 560°C, 170, and 5.6 Å∕ sec, respectively. To prevent surface degradation, the PH3 flow was maintained at 350 sccm while the sample was heated in the MOCVD reactor. In general, growth rates vary for different crystallographic directions in selective growth, and this may result in local protrusions around the mesa structure. To prevent this, the isotropic etching property of the HBr solution was exploited to produce a SiO2 mask that overhung the mesa. The relatively planar surface profile is obtained after regrowth and assists in the subsequent metallization process. Figure 3(a) shows a scanning electron microscope (SEM) image of a 20 μm × 20 μm mesa structure, and the mask overhang can be seen in the inset. To obtain an optimum sidewall profile, the thickness of the InP layer was set to be 0.6 μm. A Normaski image of the photomixer structure after the InP regrowth is shown in Fig. 3(b). The final structure [Fig. 3(c)] was designed with an interdigitated finger structure and a planar log-spiral antenna. The finger had a width of 0.8 μm with a gap of 2.4 μm. The gap between the spirals was set to 10 μm, and the antenna structure was designed to have broadband characteristics using a commercial simulator.
5468
OPTICS LETTERS / Vol. 38, No. 24 / December 15, 2013
Fig. 3. (a) SEM image of the 20 μm × 20 μm mesa structure, the inset shows the SiO2 mask overhang. (b) Photograph taken in Normaski mode of the LTG-InGaAs mesa after InP regrowth. (c) Photograph of the fabricated buried-mesa-type InGaAs photomixer with a mesa size of 15 μm × 15 μm. (d) Schematic of the buried-mesa-type photomixer.
For comparison, we also fabricated a reference photomixer without performing the mesa etching and regrowth process. Therefore, the reference photomixer is identical to the buried-mesa type, except for the regrown u-InP layer region in Fig. 3(d), and is replaced by the initially grown LTG-InGaAs. The reference photomixer was subject to TA in the MOCVD reactor under AsH3 flow for a fair comparison. To compare the performances of the fabricated photomixers, we prepared a fiber-based broadband-pulse THz system, as shown in Fig. 4(a). A 1.55 μm center wavelength femtosecond laser was used to generate and detect the THz pulses. A dispersion-compensating fiber and single-mode fibers were used for dispersion management. The width and average power of the femtosecond laser pulses were 120 fs and 7 mW, respectively. The fast Fourier transform (FFT) amplitude of the THz pulse from the buried-type photomixer and the reference sample are shown in Fig. 4(b). As expected, the performance of the buried photomixer was better than the reference photomixer, especially at high frequencies. After changing the femtosecond laser with a 30 mW dual-mode laser, which generates 1.5 μm center wavelength optical beating signal at THz frequencies [7,8], we measured CW THz emission efficiency at 700 GHz as a function of the photocurrent [Fig. 4(c)]. At the same bias voltage, the photocurrent of the buried mesa type was smaller than the reference photomixer. However, within the measured current range, the THz power was improved by more than two times. In addition, the buried mesa type photomixer sustained 18% higher electric bias than the reference photomixer. As a result, the maximum power was enhanced by 3.8 times, which can be attributed to enhanced thermal dissipation through the InP layer. The improved thermal dissipation was confirmed from thermal images of the photomixers (Fig. 5) obtained by using a MIR camera (FLIR Systems, Thermo Vision A40M), which show that the surface temperature of the buried-mesa-type photomixer was approximately 5°C lower than that of the reference photomixer. The
Fig. 4. (a) THz-TDS measurement setup. DCF, dispersioncompensating fiber; SMF, single-mode fiber. (b) FFT amplitude spectra of the buried-mesa-type and reference photomixers. (c) CW THz power as a function of the photocurrent.
photomixers were subject to the same conditions of an optical CW pumping power of 19 dBm and a bias of 3 V. In conclusion, we have developed a buried-mesa-type THz photomixer by replacing the conventional TA process with an MOCVD regrowth process. The improved heat dissipation leads to improved performance of the buried-mesa-type photomixer compared to the conventional planar-type photomixer. Our work is a first step in realizing single-chip functional THz devices by monolithic integration with conventional electronic and photonic devices such as photodiodes lasers [7,8], and Shottky diodes. This work was partly supported by the IT R&D program of MOTIE/KEIT [10045238, Development of the portable scanner for THz imaging and spectroscopy], Joint Research Projects of ISTK, the Public Welfare & Safety Research Program through the National Research
December 15, 2013 / Vol. 38, No. 24 / OPTICS LETTERS
Fig. 5. Thermal images of the buried-mesa-type photomixer (a) before and (b) after the optical input is applied. (c) and (d) Corresponding thermal images of the reference photomixer. Note that the log-periodic antenna has been integrated because of experimental restrictions.
Foundation of Korea (NRF) Technology (NRF-20100020822), and Nano·Material Technology Development Program through the NRF of Korea (NRF2012M3A7B4035095). References 1. B. Ferguson and X.-C. Zhang, Nat. Mater. 1, 26 (2002). 2. M. Y. Glyavin, N. S. Ginzburg, A. L. Goldenberg, G. G. Denisov, A. G. Luchinin, V. N. Manuilov, V. E. Zapevalov, and I. V. Zotova, Terahertz Sci. Technol. 5, 67 (2012). 3. A. Dobriu, M. Yamashita, Y. N. Ohshima, Y. Morita, C. Otani, and K. Kawase, Appl. Opt. 43, 5638 (2004). 4. B. A. Knyazev, G. N. Kulipanov, and N. A. Vinokurov, Meas. Sci. Technol. 21, 054017 (2010). 5. E. R. Brown, K. A. McIntosh, K. B. Nichols, and C. L. Dennis, Appl. Phys. Lett. 66, 285 (1995). 6. C. Baker, I. S. Gregory, M. J. Evans, W. R. Tribe, E. H. Linfield, and M. Missous, Opt. Express 13, 9639 (2005). 7. N. Kim, J. Shin, E. Sim, C. W. Lee, D.-S. Yee, M. Y. Jeon, Y. Jang, and K. H. Park, Opt. Express 17, 13851 (2009). 8. N. Kim, H.-C. Ryu, D. Lee, S.-P. Han, H. Ko, K. Moon, J.-W. Park, M. Y. Jeon, and K. H. Park, Laser Phys. Lett. 10, 085805 (2013).
5469
9. J. Mangeney, A. Merigault, N. Zerounian, P. Crozat, K. Blary, and J. F. Lampin, Appl. Phys. Lett. 91, 241102 (2007). 10. J. E. Barnason, T. L. J. Chan, A. W. M. Lee, E. R. Brown, D. C. Driscoll, M. Hanson, A. C. Gossard, and R. E. Muller, Appl. Phys. Lett. 85, 3983 (2004). 11. A. Schwagmann, Z.-Y. Zaho, F. Ospald, H. Lu, D. C. Driscoll, M. P. Hanson, A. C. Gossard, and J. H. Smet, Appl. Phys. Lett. 96, 141108 (2010). 12. M. R. Melloch, J. M. Woodall, E. S. Harmon, N. Otsuka, F. H. Pollak, D. D. Nolte, R. M. Feenstra, and M. A. Lutz, Annu. Rev. Mater. Sci. 25, 547 (1995). 13. R. A. Metzger, A. S. Brown, L. G. McCray, and J. A. Henige, J. Vac. Sci. Technol. B 11, 798 (1993). 14. C. Baker, I. S. Gregory, W. R. Tribe, I. V. Bradley, M. J. Evans, E. H. Linfeld, and M. Missous, Appl. Phys. Lett. 85, 4965 (2004). 15. E. S. Harmon, M. R. Melloch, J. M. Woodall, D. D. Nolte, N. Otsuka, and C. L. Chang, Appl. Phys. Lett. 63, 2248 (1993). 16. P. Pačebutas, A. Bičiūnas, S. Balakauskas, A. Krotkus, G. Andriukaitis, D. Lorenc, A. Pugžlys, and A. Baltuška, Appl. Phys. Lett. 97, 031111 (2010). 17. A. Takazato, M. Kamakura, T. Matsui, J. Kitagawa, and Y. Kadoya, Appl. Phys. Lett. 91, 011102 (2007). 18. R. Takahashi, Y. Kawamura, and H. Iwamura, Appl. Phys. Lett. 68, 153 (1996). 19. H. Roehle, R. J. B. Dietz, H. J. Hensel, J. Böttcher, H. Künzel, D. Stanze, M. Schell, and B. Satorius, Opt. Express 18, 2296 (2010). 20. B. Heshmat, H. Pahlevaninezhad, Y. Pang, M. MasnadiShirazi, R. B. Lewis, T. Tiedje, R. Gordon, and T. E. Darcie, Nano Lett. 12, 6255 (2012). 21. S. Verghese, K. A. Mclntosh, and E. R. Brown, Appl. Phys. Lett. 71, 2743 (1997). 22. S. A. Daffaie, O. Yilmazoglu, F. Küppers, and H. Hartnagel, in 37th International Conference on Infrared, Milimeter and Terahertz Waves (IRMMW-THz), Wollongong, Australia, September 23–28, 2012, Mon-C-1–2. 23. E. Peyatavit, S. Lepilliet, F. Hindle, C. Coinon, T. Akalin, G. Ducournau, G. Mouret, and J.-F. Lampin, in 37th International Conference on Infrared, Milimeter and Terahertz Waves (IRMMW-THz), Wollongong, Australia, September 23–28, 2012, Thu-C-1–2. 24. S.-P. Han, N. Kim, H. Ko, H.-C. Ryu, J.-W. Park, Y. J. Yoon, J.-H. Shin, D. H. Lee, S.-H. Moon, S.-W. Choi, H. S. Chun, and K. H. Park, Opt. Express 20, 18432 (2012).
