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Communication High‐Efficiency and High‐Power Multijunction InGaAs/InP Photovoltaic Laser Power Converters for 1470 nm Simon Fafard * and Denis P. Masson Broadcom (Canada), IFPD, Ottawa, ON K1A 0R6, Canada; denis.masson@broadcom.com * Correspondence: simon.fafard@broadcom.com Abstract: The high‐efficiency capabilities of multijunction laser power converters are demonstrated for high‐power applications with an optical input of around 1470 nm. The InP‐based photovoltaic power converting III‐V semiconductor devices are designed here, with 10 lattice‐matched subcells (PT10‐InGaAs/InP), using thin InGaAs absorbing layers connected by transparent tunnel junctions. The results confirm that such long‐wavelength power converter devices are capable of producing electrical output voltages greater than 4–5 V. The characteristics are compatible with common elec‐ tronics requirements, and the optical input is well suited for propagation over long distances through fiber‐based optical links. Conversion efficiencies of ~49% are measured at electrical outputs exceeding 7 W for an input wavelength of 1466 nm at 21 °C. The Power Converter Performance Chart has been updated with these PT10‐InGaAs/InP results. Keywords: optical power converters; laser power converters; power‐over‐fiber; power beaming; photovoltaic; galvanic isolation; InGaAs; InP; multijunctions semiconductor heterostructures 1. Introduction Citation: Fafard, S.; Masson, D.P. The developments in the past few decades in the field of applied semiconductor and High‐Efficiency and High‐Power optical physics served to establish a mature laser diode industry. Of particular interest, Multijunction InGaAs/InP fiber‐coupled multimode laser diodes with high output powers are commercially availa‐ Photovoltaic Laser Power ble at around 1470 nm. The low attenuation loss of optical fibers at 1470 nm is key to the Converters for 1470 nm. Photonics 2022, 9, 438. https://doi.org/10.3390/ implementation of these lasers and lends itself favorably to the propagation of optical photonics9070438 power over long distances (e.g., transmission of ~95% over 1 km). In addition, in a recent perspective paper, we reviewed the developments of optical power converters (OPCs) [1– Received: 2 June 2022 47], often also referred to as laser power converters (LPCs). The resulting Power Converter Accepted: 20 June 2022 Performance Chart [41] clearly highlights that multijunction OPCs are required for ob‐ Published: 21 June 2022 taining not only high conversion efficiencies, but also high output powers. The research Publisher’s Note: MDPI stays neu‐ related to photovoltaic devices suggests other potential future device improvements [48– tral with regard to jurisdictional 67], optical wireless power transmission applications [68–74], and system design strate‐ claims in published maps and institu‐ gies [75–78]. tional affiliations. Many OPC developments have historically been achieved at wavelengths around 808 nm. The key benefits of this wavelength option include the good availability of laser diodes and the maturity of GaAs‐based devices. More recently, ~980 nm has become an interesting wavelength option, because the laser diodes for that range can be strategic Copyright: © 2022 by the authors. Li‐ (reliability or cost) and due to the novel availability of multijunction OPCs to cover this censee MDPI, Basel, Switzerland. option [37,38,41]. Another wavelength option is in the 1310 nm region, which also has This article is an open access article interesting recent developments [30], and can benefit from the low fiber attenuation for distributed under the terms and con‐ that spectral region. Lastly, the long wavelength options of 1450–1490 nm and of the ~1550 ditions of the Creative Commons At‐ tribution (CC BY) license (https://cre‐ nm region can both benefit from the lowest fiber attenuation range and from the telecom‐ ativecommons.org/licenses/by/4.0/). munication infrastructure [10,16,35–37,39,42,44]. Much telecommunication traffic utilizes Photonics 2022, 9, 438. https://doi.org/10.3390/photonics9070438 www.mdpi.com/journal/photonics Photonics 2022, 9, 438 2 of 11 the ~1550 nm region; the ~1470 nm option can help minimize the potential for interference between optical wireless power transmission and data transmission applications. The Power Converter Performance Chart also revealed that more developments were necessary for multijunction OPCs based on the InGaAs/InP material system [42,44,46,47]. In the current work, we therefore present the results obtained with Broadcom’s initial 10 junction InGaAs/InP prototypes, herein called PT10‐InGaAs/InP, intended for operation with input from a high‐power 1470 nm laser diode. 2. Materials and Methods The InGaAs subcells lattice‐matched to InP are expected to contribute ~0.5 V of out‐ put voltage each, such as based on previous single‐junction measurements [41]. For this study, a PT10 design was selected (vertical multijunction with 10 subcells) in order to achieve an output voltage in the range of ~5 V. The schematic of the PT10‐InGaAs/InP heterostructure is depicted in Figure 1a. This is based on the previously described Vertical Epitaxial HeteroStructure Architecture (VEHSA design) [57,58], here adapted for the lat‐ tice‐matched InGaAs/InP material system. The Beer–Lambert law was used in this specific case to calculate the individual subcell’s absorber thicknesses, with each subcell absorbing ~1/10 of the incident light. A ~5 V output level is common for electronic circuitry and can also provide good output power capabilities. An output level of 3.3 V is another common option for electronics and should be achievable with a seven‐junction device. However, the PT10 design provides additional output voltage margins and was selected to ensure that sufficient output voltage can be maintained under higher operating temperatures. For a GaAs‐based system, we have demonstrated that the output voltage scales linearly up to PT30 devices with 30 subcells. Similarly, we would expect devices with more than 10 InGaAs subcells to readily be realized, although a PT10 design is expected to meet the most common output voltage requirements and was the focus of the present study. The photovoltaic vertical multijunction structure was built for operation with optical input from a powerful laser source emitting in a spectral range peaking around 1470 nm. It is designed with 10 thin (optically transparent) photovoltaic semiconductor subcells in‐ terconnected with tunnel junctions, labelled TJi in Figure 1a. Each individual subcell com‐ prises an n‐type emitter and a p‐type base. The TJs are made to be transparent to the input beam, utilizing an InGaAlAsP alloy lattice‐matched to InP with a bandgap of about 1 eV. The TJ’s current–voltage (I–V) characteristics have been verified on truncated structures (i.e., isolated TJ structures) grown under comparable conditions. For example, Figure 1b shows the I–V curve of such an isolated TJ with its negative differential resistance (NRD) region (here, 0.2 V < V < 0.6 V), confirming the tunnel current characteristics. Of particular interest is the elevated peak tunneling current capabilities (here, ~1500 A/cm ) and the related very low voltage drops under the normal multijunction OPC operations (usually with peak current densities of J < 100 A/cm ). The epitaxial layers were grown using commercial production Aixtron Metal Or‐ ganic Chemical Vapor Deposition (MOCVD) reactors. The total thickness of all the emitter and base layers from the different subcells is such that the impinging optical beam is al‐ most completely absorbed. For the InP material system, an interesting design variation consists of reducing the absorber thicknesses of the subcells and using light reflected from the back side. The InP substrate is transparent to the input light and to the InGaAs lumi‐ nescence, therefore allowing the light reflected [8] from the back side of the device to be recycled [79–82]. Nevertheless, as described previously [41,58], to realize the required photocurrent matching condition, the structure usually has increasing subcell thicknesses from the top subcell (thinnest) toward the bottom subcell (thickest). The device fabrication included standard blanket back‐metallization (SnZnIn, PdZnPdAu, or PdZnPtAu contacts) [83], front ohmic contacts (Pd/Ge/Ti/Pd/Ag/Au or In), and antireflection coatings (ARC) constructed from multiple layers of SiO2 and TiO2. The ARC was measured to reduce the reflectivity (R) of the incident beam to R < 2% for the spectral range of interest. Photonics 2022, 9, 438 3 of 11 A fiber‐coupled laser diode manufactured by BWT was used [84]. Its optical output power reaches a maximum of 14.5 W at an operating current of 6.3 A at a voltage bias of ~9.4 V. It had a numerical aperture of NA ~0.22, using a multi‐mode fiber core diameter of 400 µm and cladding of 440 µm. It was equipped with an integrated red aiming beam, which was used to align the ~1470 nm beam. For the I–V measurements, the tip of the fiber‐coupled laser was positioned to form a circular spot with a radius of about 2.6 mm from the diverging laser beam. The spot covered roughly 50% of the sample’s central area. The I–V characteristics were acquired using a Keithley 2601B source‐meter. The data were obtained at a temperature of ~21 °C and the measurements were made to avoid sig‐ nificant chip heating. Metal (a) (b) Contact layer Window InGaAs subcell 1 TJ 1 InGaAs subcell 2 TJ 2 InGaAs subcell... TJ ... InGaAs subcell 9 TJ 9 InGaAs subcell 10 ‐0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 ‐500 InP buffer ‐1000 InP Substrate ‐1500 Voltage (V) Metal Figure 1. Schematic of the PT10‐InGaAs/InP Vertical Epitaxial HeteroStructure Architecture (VEHSA design) devices prepared with 10 InGaAs subcells in (a), and I–V characteristics of the tun‐ nel junction (TJ) measured from a single TJ grown separately in a truncated structure in (b). 3. Results The key result measured with the PT10‐InGaAs/InP from this study is added as a data point (45) in Figure 2, which shows the updated power converter performance chart built from the results published in the literature [41]. Detailed results from the PT10‐In‐ GaAs/InP OPCs are then presented below. Current density (A/cm ) Photonics 2022, 9, 438 4 of 11 Figure 2. Survey of the measured performance for monolithic power converter devices at the indi‐ cated laser wavelengths. Single junction, vertical multijunction, and planar segmented (“pizza” con‐ figuration) device reports are included (module results are not included here). Updated from S. Fafard and D. P. Masson, J. Applied Physics 130, 160901 (2021). Copyright 2021 Author(s), licensed under a Creative Commons Attribution (CC BY) license [41]: (1) 1J‐GaAs (2018) Jomen et al. [1], (2) 1J‐InGaP (2021) Komuro et al. [2], (3a) Pizza‐6 (2009) Schubert et al. [3], (3b) Pizza‐4 (2009) Schubert et al. [3], (4) 6J‐GaAs (2016) Zhao et al. [4], (5) 6J‐GaAs (2017) Sun et al. [5], (6) 3J‐InGaAsP (2020) Yin et al. [6], (7) 4J (2018) Huang et al. [7], (8) Back Mirror (2021) Helmers et al. [8], (9) 1J‐GaAs (2008) Oliva et al. [9], (10) 1J‐InGaAs (2013) Mukherjee et al. [10], (11a) PT5 (2016) Fafard et al. [11], (11b) PT12/8/6 (2016) Fafard et al. [11], (12a) PT5 (2016) Fafard et al. [12], (12b) PT12/20 (2016) Fafard et al. [12], (13) 1J‐GaAs (2016) Kalyuzhnyy et al. [13], (14) 1J‐GaAs (1992) Olsen et al. [14], (15) 2J‐GaAs (2007) Krut et al. [15], (16a) 1J‐GaAs (2003) Andreev et al. [16], (16b) 1J‐GaSb (2003) Andreev et al. [16], (17a) Pizza‐4 (2003) Peña et al. [17], (17b) Pizza‐6 (2003) Peña et al. [17], (18) InGaAs (2020) Kalyuzhnyy et al. [18], (19) InGaAs‐Meta (2019) Kim et al. [19], (20) 1J‐InGaAsP (1981) Law et al. [20], (21) 1J‐GaAs (2019) Panchak et al. [21], (22) Pizza‐6 (2008) Bett et al. [22], (23) Pizza‐6 (1996) Fahrenbruch et al. [23], (24) 1J‐GaAs (1996) Fave et al. [24], (25) 1J‐Si (1992) Green et al. [25], (26) 1J‐ GaAs (2016) Höhn et al. [26], (27) 1J‐GaAs‐Rmpp = 0.38 Ohm, (2015) Shan et al. [27], (28) 1J‐GaAs (2018) Khvostikov et al. [28], (29) 1J‐GaAs (2017) Khvostikov et al. [29], (30) 1J‐InGaAsP (2020) Hel‐ mers et al. [30], (31) 1J‐GaAs (2019) Zhao et al. [31], (32) PT6 (2017) York et al. [32], (33) 6J‐GaAs (2018) Huang et al. [33], (34) 6J‐GaAs in TO (2017) Ding et al. [34], (35) 1J‐InGaAs (2014) Jarvis et al. [35], (36) 1J‐GaSb (2019) Khvostikov et al. [36], (37a) PT6‐GaAs (2021) Fafard et al. [37], (37b) PT6‐ InGaAs (2021) Fafard et al. [37], (37c) PT6‐InGaAs (2021) Fafard et al. [37], (37d) 1J‐InGaAs (2021) Fafard et al. [37], (38a) 3J‐GaAs (2021) Keller et al. [38], (38b) 5J‐InGaAs (2021) Keller et al. [38], (39a) 1J‐GaAs (1997) Wojtczuk et al. [39], (39b) 1J‐InGaAs (1997) Wojtczuk et al. [39], (40) Pizza‐4 (2010) Eggert et al. [40], (41a) 1J‐InGaP (2021) Fafard et al. [41], (41b) PT12 (2021) Fafard et al. [41], (41c) PT5 (2021) Fafard et al. [41], (41d) PT6 (2021) Fafard et al. [41], (42) 8J‐InGaAs/InP (2021) Wang et al. [42], (43) 1J‐InGaP (2022) Kurooka et al. [43], (44) 1J‐InGaAs (2022) Helmers et al. [44], (45) PT10‐ InGaAs/InP (2022) Fafard and Masson (this study). The measured I–V curves are shown in Figure 3 for various optical input powers between Pin = 2.5 W and Pin = 14.5 W. The dashed (pink) curve in Figure 3 is an ideal diode model fitted with the 5.0 W data. A good fit is obtained here, using 10 diodes all with the same ideality factor of n = 1.05 and the same photocurrent ratio corresponding to a quan‐ tum efficiency of EQE = 93%. The fit accurately reproduces the data when the overall series resistance is set at 0.00 Ohm. Furthermore, the dark I–V measurements (not shown) also support that any residual series resistance should be smaller than a fraction of 1 Ohm. It can also be deduced that the current‐matching in the individual 10 subcells is high for a Photonics 2022, 9, 438 5 of 11 spectral input at a wavelength near 1466 nm. The latter is evidenced by the high EQE value measured at ~93% and the flatness of the horizontal part of the I–V curves. Figure 3. Measured I–V properties at 21 °C for a PT10‐InGaAs/InP OPC operating with a laser peak‐ ing at 1466 nm, for input powers between 2.5 W and 14.5 W. At 14.5 W of input power, an efficiency of Eff = 48.9% is obtained with a Voc of 5.46 V. The output power (Pmpp) reaches 7.09 W with Vmpp = 4.675 V and Impp = 1.517 A. The 5.0 W curve is also fitted with a 10J ideal diode model (pink dashed line). The key characteristics of the I–V curves in Figure 3 are analyzed in more detail in Figure 4, which shows the input power dependence of key parameters. Figure 4a shows that the output power Pmpp has a measured slope efficiency of Eff ∼ 49%, with negligible deviations from linear regression for optical input powers up to 14.5 W. Here, the input power was limited by the laser’s maximum output power. Provided adequate heatsinking is available to the device; we expect that it would be possible to use higher input powers without damaging the photovoltaic laser power converter. The output voltage is shown in Figure 4b. The open‐circuit voltage (Voc) reaches a maximum value of 5.508 V, while the maximum power point voltage (Vmpp) is then 4.75 V. It corresponds to an average voltage of 0.551 V per subcell, yielding a bandgap voltage offset value of Woc = 0.187 V, where Woc = (Eg/q) − Voc with Eg being the bandgap energy (here, InGaAs lattice‐matched to InP) and q is the electronic charge. The Woc value ob‐ tained with the PT10‐InGaAs/InP is therefore very similar to that measured for the GaAs OPCs at Woc = 0.181 V [41]. The external quantum efficiency (EQE) is shown in Figure 4c. At a voltage bias of 4 V, an EQE of 92% is obtained for input powers up to ~5 W. As the input power is increased, the EQE at 4 V increases slightly, up to 93.5% at 14.5 W. The increased EQE value at higher optical intensities could be caused by a higher photon recycling, giving a better current‐ matching at higher input powers [79–82]. The output current is shown in Figure 4d. The short‐circuit current (Isc) yields a re‐ sponsivity of 1.115 A/W (taking into account the 10 junctions of the PT10). For a wave‐ length of 1466 nm, this corresponds to an EQE of 94.3%. The maximum power point cur‐ rent (Impp) is measured to have a responsivity of 1.044 A/W, corresponding to an EQE of 88.3%. Photonics 2022, 9, 438 6 of 11 oc (a) y = 0.4905x R² = 0.9999 5 mpp (b) 01 23456789 10 11 12 13 14 15 012 34 56 78 9 10 11 12 13 14 15 100% at V= 0V y = 111.54x (d) 90% R² = 0.9999 at V= 4V (c) 80% sc y = 104.4x R² = 0.9999 70% mpp 60% 50% 0 0123456 789 10 11 12 13 14 15 012 3456 789 10 11 12 13 14 15 Input Power (W) Input Power (W) Figure 4. The PT10‐InGaAs/InP’s input power dependence with an incident laser beam peaking at 1466 nm at 21 °C. The electrical output power, Pmpp, is shown in (a), the output voltage: open‐circuit voltage (Voc) and maximum power point voltage (Vmpp) are shown in (b), the external quantum ef‐ ficiency (EQE) is shown in (c), and the output current, short‐circuit current (Isc) and maximum power point current (Impp), are shown in (d). The values of the linear regressions are indicated in the plots. 4. Discussions As can be visualized from Figure 2, the PT10‐InGaAs/InP performance in Figure 3 and Figure 4 is a drastic improvement from the results previously obtained with In‐ GaAs/InP power converters. Record efficiencies are obtained for unprecedented output power capabilities at such long wavelengths. For example, the optimal load (Rmpp = Vmpp/Impp) of a PT10 is 100 times larger than that from the corresponding single junction; the impact is best observed in Figure 5. To exemplify the benefits of the multijunction devices, results were obtained with various OPC designs in different material systems. We include data from single junctions (“PT1”) to 12‐junction GaAs devices (“PT12‐ GaAs”). Here, the PT12‐GaAs has the highest output power capabilities, as can be ob‐ served from Figure 5. Our study also includes the data acquired with four OPC designs obtained using the InGaAs/InP material system, “PTN‐InGaAs/InP”, with N = 1, 2, 3, and 10, corresponding to the number of InGaAs subcells. The output voltage of the In‐ GaAs/InP multijunction devices increases by increments of Vmpp ~0.475 V per subcell (as previously shown in Figure 4b). This makes these OPC devices more suitable for opera‐ tion at higher‐input powers. The PT2‐InGaAs/InP has an output characteristic similar to the single‐junction GaAs device (“PT1‐GaAs”), and can already operate at significantly higher input power than a single junction InGaAs (“PT1‐976”) device, optimized for input at ~976 nm. The PT10‐InGaAs/InP at ~1470 nm has an output characteristic between that of a four‐junction GaAs device (“PT4‐GaAs”) and that of a six‐junction GaAs device (“PT6‐GaAs”) device, both optimized for input at ~808 nm. Figure 5 clearly demonstrates that for achieving higher OPC output powers while maintaining high efficiencies, it is necessary to increase the device output voltage; other‐ wise, the optimal load collapses to unusable small values. The latter is illustrated with the pink region of Figure 5, which corresponds to the situation when the ratio of Vmpp over Impp becomes smaller than ~1 Ohm. In these cases, it becomes very problematic to maintain high conversion efficiencies. EQE (%) P (W) mpp I (mA) sc Output Voltage (V) Photonics 2022, 9, 438 7 of 11 PT12‐GaAs PT6‐GaAs PT4‐GaAs PT10‐InGaAs/InP PT3‐InGaAs/InP PT2‐InGaAs/InP PT1‐GaAs PT1‐976 PT1‐InGaAs/InP high‐ current, low‐voltage region 0.1 0.1 1 10 Measured Output Power (W) Figure 5. The measured optimal load (Rmpp = Vmpp/Impp) as a function of the output power obtained with various OPC designs. As indicated in the caption, results are measured for the following OPC designs: single junctions from the material systems of InGaAs/InP (“PT1‐InGaAs/InP”) at ~1470 nm [37], of InGaAs/GaAs (“PT1‐976”) at ~976 nm [37], and of GaAs (“PT1‐GaAs”) at ~808 nm [37,57]; from multijunctions from the InGaAs/InP material system, “PTN‐InGaAs/InP”, with N = 2, 3, and 10, corresponding to the number of InGaAs subcells [this study]; and from multijunctions from the GaAs material system, “PTN‐GaAs”, with N = 4, 6, and 12, corresponding to the number of GaAs subcells [37,41]. The area of low voltage and high current is shown in pink and is less desirable for practical applications. 5. Conclusions In conclusion, the vertical multijunction VEHSA strategy has been proven very effec‐ tive at trading photocurrents to increase the operating voltage in long‐wavelength OPCs. Conversion efficiencies of Eff ~49% have been measured at electrical outputs exceeding 7 W for an input wavelength of 1466 nm, with the PT10‐InGaAs/InP device giving a Voc of 5.46 V. An ideality factor of n ~1.05 was found for this material system. The 4 V EQE, at high optical input powers, was measured to be ~93.5%, combined with a residual reflec‐ tivity of R < 2%. In future iterations, the EQE and IQE could therefore be further optimized by potentially up to 2% to 4%. The Woc value was measured to be ~187 mV with a lattice‐ matched InGaAs absorber. The use of an AlInGaAsP alloy for the absorber with a slightly higher bandgap (e.g., 0.8 eV) could potentially improve the Vmpp values by >50 mV per subcells. Such additional optimizations could definitely lift the conversion efficiency of long‐wavelength InP‐based power converters above 50%. Author Contributions: Conceptualization, S.F. and D.P.M.; methodology, S.F. and D.P.M.; soft‐ ware, S.F. and D.P.M.; validation, S.F. and D.P.M.; formal analysis, S.F. and D.P.M.; investigation, S.F. and D.P.M.; data curation, S.F. and D.P.M.; writing—original draft preparation S.F.; writing— review and editing S.F. and D.P.M.; visualization, S.F.; project administration, S.F. and D.P.M.; fund‐ ing acquisition, S.F. and D.P.M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. R (Ohms) mpp Photonics 2022, 9, 438 8 of 11 Informed Consent Statement: Not applicable. Data Availability Statement: The data that support the findings of this study are available from the corresponding author upon reasonable request. 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Photonics – Multidisciplinary Digital Publishing Institute
Published: Jun 21, 2022
Keywords: optical power converters; laser power converters; power-over-fiber; power beaming; photovoltaic; galvanic isolation; InGaAs; InP; multijunctions semiconductor heterostructures
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