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III–V dilute nitride-based multi-quantum well solar cell
ARTICLE IN PRESS
Journal of Crystal Growth 301–302 (2007) 993–996 www.elsevier.com/locate/jcrysgro
III–V dilute nitride-based multi-quantum well solar cell A. Freundlich, A. Fotkatzikis, L. Bhusal, L. Williams, A. Alemu, W. Zhu, J.A.H. Coaquira, A. Feltrin, G. Radhakrishnan Photovoltaics and Nanostructures Laboratories, Center for Advanced Materials and Physics Department, University of Houston, TX 77204-5004, USA Available online 2 February 2007
Abstract Photovoltaic properties of a preliminary set of 1–1.2 eV dilute nitride GaAsN/GaAs MQW solar cells grown by chemical beam epitaxy are investigated. The study reveals, as expected from the enhancement of effective masses, unusually high photo-conversion strength of the MQW region that exceeds by nearly two-fold those reported for conventional MQW solar cells of comparable bandgaps. Despite a current output (25 A m 2 in absence of ARC) comparable to that of conventional GaInNAs solar cells, output voltages (e.g. 0.6 V for a 1.1 eV MQW cell) appear to be significantly higher than those reported for bulk-like counterparts. Bias-dependent external quantum efficiency measurements reveal an incomplete collection of photo-generated carriers from the wells under operating conditions (forward bias), which should be mitigated by the use of thinner barrier. r 2007 Elsevier B.V. All rights reserved. PACS: 81.15.Hi; 84.60.Jt; 81.07.St; 81.05.Ea Keywords: A3. Chemical beam epitaxy; A3. Quantum wells; B2. Semiconducting III–V materials; B3. Dilute nitride; B3. Solar cells
1. Introduction The presence of small amounts of nitrogen in III–V semiconductor alloys leads to a substantial decrease of the bandgap, making them very attractive for a wide range of optoelectronic applications [1]. In particular, various schemes, involving the use of 1.0–1.25 eV GaInNAs subcells, have been devised to enhance the efficiency of existing triple and quadruple junction solar cells [2]. Nevertheless, thus far, poor minority carrier properties [3,4] and doping issues [5,6] associated with thick bulk-like dilute nitrides have hindered the success of these approaches [7–11]. Here a different strategy is evaluated, where dilute nitride multi-quantum wells (MQW) are embedded within the intrinsic region of conventional GaAs p–i–n solar cells. The insertion of quantum wells in multi-junction solar cells has been predicted to yield practical efficiencies exceeding 35% AM0 [12,13]. The use of these narrow-bandgap MQWs appears as being particularly attractive since it Corresponding author. Tel.: +713 743 3621; fax: +713 747 7724.
E-mail address: [email protected] (A. Freundlich). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.11.256
should extend the photon absorption range while alleviating minority carrier issues encountered in thick bulk-like GaInNAs. In addition the availability of larger joint density of states in the wells, as afforded by higher electron effective masses, warrants stronger absorption coefficient for these nanostructures [14]. 2. Experimental procedure 2.1. Chemical beam epitaxy of multi-quantum well devices Device structures were fabricated on n-type 2 1018 cm-3 Si doped GaAs (0 0 1) substrates by radio frequency (RF) nitrogen plasma-assisted chemical beam epitaxy in a Riber 32CTM chamber equipped with a 600 W VeecoTM nitrogen plasma source. A schematic diagram of the device structure is provided in Fig. 1. Triethyl-galium, trimethyl-indium and arsine combined with Be and Si solid-source dopants were used as growth precursors. One main advantage of CBE for PV applications is the ability to grow at higher growth rates than MBE [15]. The device structure included the deposition of highly Si doped (5 1018 cm 3) micron-thick n+-GaAs buffer and
ARTICLE IN PRESS 994
A. Freundlich et al. / Journal of Crystal Growth 301–302 (2007) 993–996
of the pseudomorphism (Fig. 1). After the growth of the MQW containing i-region, a 0.4 mm thick p-type GaAs emitter GaAs, a 20 nm Al0.85Ga0.15As window layer and a p++ GaAs contact layer were subsequently grown. The Be doping in the emitter and AlGaAs window layers was aimed at 3 1018 cm 3, whereas the contact layer was doped at about 2–5 1019 cm 3. In addition to the dilute nitride device an identical device where the i-region contained an In0.15Ga0.85As (6 nm)/ GaAs (22 nm) MQW device was grown as a reference sample. 3. Post-growth processing and characterizations Fig. 1. Schematic structure of MQW device and RHEED images recorded from the dilute nitride region during the fabrication of MQWs: (2 4) GaAsN.
0.25 mm Al0.2 Ga0.8As, acting as a back surface field (BSF), and a 1.2 mm thick n-doped GaAs base/collector layer. The Si doping of the base was set to 5 1017 cm 3. The growth temperature during the deposition of the buffer BSF and base layers was maintained to about 520 1C at a growth rate of about 2 mm/h. After the deposition of the base the growth was interrupted under arsine flux and the temperature was lowered and stabilized to about 450 1C. The growth of the device intrinsic (i) region which included a 15 period strained GaAs1 xNx (6 nm)/GaAs(22 nm) multi-quantum well (MQW) sandwiched between two 60-nm-thick not intentionally doped layers of GaAs was then resumed. Emphasis was given during growth to minimize the residual hydrogen amount in the growth chamber and its incorporation in dilute nitride layers by reducing the III/V ratio (lower H flux emanating from the arsine thermal decomposition) and by using lower H2 carrier gas flow [16,17]. During growth the hydrogen partial pressure, as measured by mass spectrometry, was maintained to about 2 10 6 Torr. In order to prevent an excessive nitridation of the surface no growth interruption was implemented during the entire growth of the i-region of the device. The RF-plasma source was ignited (with the shutter closed) during the deposition of the first intrinsic GaAs buffer and was maintained ON throughout the MQW region growth and turned OFF at the beginning of the onset of the top 60 nm undoped GaAs layer. The growth rate during the fabrication of the i-region was about 0.8 monolayer/s. The nitrogen flux and plasma conditions were adjusted following previously established conditions [18,19] to yield a nitrogen concentration of about 2% in the wells. It should be noted that even with a closed shutter a small amount of nitrogen is incorporated in GaAs layers and thus barriers and the upper region of the first 60 nm GaAs layer in the i-region have a nitrogen concentration of about 0.1–0.2%. During the growth of the MQW region a 2 4 reflection high-energy electron diffraction (RHEED) diagram was recorded attesting a highly two-dimensional growth mode and the preservation
Following the growth process samples were also subjected to rapid thermal annealing (750 1CoTo900 1C) to evaluate the effect of thermal treatments. Rapid thermal annealing (RTA) was implemented under nitrogen atmosphere and samples were capped on both sides with undoped GaAs wafers. MQW solar cell structural quality was assessed by X-ray diffraction (XRD), whereas electrochemical profilometry was used to evaluate carrier concentrations and cross-diffusions. Temperature-dependent (10–300 K) modulated photoreflectance (PR) and photoluminescence were implemented to probe confined levels and their optical properties. Dilute nitride solar cell structures along with reference bulk like p–i(GaAs)–n and conventional p–i(InGaAs/GaAs MQW)–n were mesa etched and processed using a standard methodology [15,20]. In order to facilitate comparison between different sets of devices no anti-reflect coating was applied. The application of ARC generally reduces the overall reflectivity losses by about 25–30% which results in both an increase of the short-circuit current density and a slight improvement of open-circuit voltages (an overall efficiency improvement of 25%). Device characteristics were analyzed by dark and illuminated current–voltage measurements using an Oriel AM0 solar simulator, the measurement were conducted inside an MMRTM cryostat/probe system and temperature was maintained at 298 K during illuminated data acquisition. Quantum efficiency (QE) measurements were recorded both under short-circuit conditions (V0) and under the application of an external bias at room temperature using the lock-in technique. 4. Results and discussion 4.1. MQW structural and optical properties All samples exhibited specular morphology and their morphology was preserved after RTA treatments. As grown and RTA samples exhibited superlattice like X-ray diffraction patterns and average substitutional nitrogen concentration and well barrier thicknesses were extracted from (0 0 4) diffraction rocking curves. The energy of transitions measured in near band-edge luminescence and PR spectra
ARTICLE IN PRESS A. Freundlich et al. / Journal of Crystal Growth 301–302 (2007) 993–996
of GaAsN/GaAs MQW region was modeled using a bandanticrossing formalism and structural parameters extracted from XRD analysis. The presence at room temperature of three electron- and one hole-confined levels in the multiquantum wells were identified. A typical room temperature modulated photoreflectivity spectra recorded from a GaAsN/GaAs MQW solar cell structure is reported in Fig. 2. 4.2. Device characteristics The spectral response/QE, recorded under short-circuit conditions, from all MQW GaAsN/GaAs devices was portrayed by a strong below GaAs response with peak/ shoulders approximately located at the measured fundamental and excited electron-heavy hole confined levels. All devices were characterized by cutoff wavelengths of about 1.1 eV. The relative spectral response for the first confined electron–hole and third confined electron to hole in all sample exceeded 25% and 50%, respectively. Such a relative response surpasses by over a factor of two–four those reported in the literature [21,22] for other III–V MQW solar cells and the one of the reference InGaAs/ GaAs MQW samples. This unusually high photoconversion strength of the MQW region seems to be consistent with the enhancement of effective masses. Samples subjected to RTA were characterized by a stronger photoluminescence signal of the MQW region and an overall increase of the device QE. A peculiar characteristic of the QE of as grown samples was the presence of a deep valley, indicative of strong recombination rates, in the vicinity of 1.37–1.4 eV. The magnitude of this valley was attenuated for samples that were subjected to RTA. Although the origin of this observation is not clear, the energy range corresponds roughly to the bandgap energy that one would expect from slightly nitrogen (0.2%) contaminated GaAs barriers.
Fig. 2. Room temperature spectral response of 15 period GaAs0.98N0.02(6 nm)/ GaAs(22 nm) MQW device curve with superimposed modulated photoreflectance spectra, arrows indicate calculated ground and excited electron-tohole transitions in the MQW.
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Device illuminated IV characteristics of as grown and RTA samples are shown in Fig. 3. For as-grown solar cells despite a poor short-circuit current (JSC) response (i.e. 10 times less than that of the GaAs p/n diode reference sample), the open circuit voltages (VOC0.6 eV) exceeded markedly (by 200–400 meV) that of conventional InGaAsN solar cells with similar cutoff wavelengths [2,7–10]. It is interesting to note that the open-circuit voltages obtained for these solar cells are only slightly below those reported for high end silicon solar cells [23]. A significant improvement of JSC and device fill factor was obtained for cells processed from RTA set of samples (Fig. 4), yielding an enhancement of the JSC from 15 Am 2 for the as grown solar cell to about 27 Am 2 for solar cell made with an RTA processing of 90 s at 830 1C. QE measurements as a function of external bias, bring some light to the dilemma concerning the relatively high open-circuit voltages, attesting good device minority carrier properties and the poor photo-generated carrier collection as evidenced by the mediocre JSC. The evolution of QE vs. the external bias is shown in Fig. 4. Upon application of the reverse bias although the general shape of the QE seems to be marginally modified, the QE improves significantly with the near GaAs bandedge reaching saturation at about 0.9 V. Such behavior has been previously observed for InAsP/InP MQW when the electric field across the i-region is insufficient at the operation to extract the carriers [23,25]. Another
Fig. 3. Evolution of I–V characteristic at room temperature and under 1 sun-AM0 of a 15 period multi-quantum well solar cell device as function of 830C RTA treatments, (RTA was implemented before device processing step). Cells have no anti-reflect coating.
ARTICLE IN PRESS 996
A. Freundlich et al. / Journal of Crystal Growth 301–302 (2007) 993–996
cells of comparable bandgaps. Bias-dependent external quantum efficiency measurements reveal an incomplete collection of photo-generated carriers from the wells under operating conditions (forward bias), which should be mitigated by the use of thinner barriers. Acknowledgments This work was partially supported by the NASA Grant No. NNCO4GB53N. References
Fig. 4. External quantum efficiency (spectral response) of a 15 period GaAsN/GaAs MQW solar cell as a function of applied bias (negative sign for reverse bias).
interesting feature is the increase of the relative QE of the 1.37–1.4 eV region (indicated by an arrow in Fig. 4), stressing the hindrance that is imposed to the carrier collection process by the slight nitrogen contamination of the barrier material. Since the built-in E-field across the iregion is inversely proportional to the thickness of the i-region [24], these behaviors should be mitigated by the use of thinner barriers or/and by reducing the thickness of GaAs cladding layers. 5. Conclusion Development of dilute nitride GaAsN/GaAs MQW solar cell by chemical beam epitaxy is undertaken. Device characteristics were analyzed by means of dark/illuminated (AM0) current–voltage and quantum efficiency measurements. Despite a modest current output (25 A m 2 in absence of ARC) comparable to that of conventional GaInNAs solar cells, output voltages (e.g. 0.6 V for a 1.1 eV MQWcell) appear to be significantly higher than those reported for bulk-like counterparts. The study also reveals, as theoretically expected from the enhancement of effective masses, unusually high photo-conversion strength of the MQW region that exceeds by two-fold the one obtained from conventional InAsP, InGaAs MQW solar
[1] M. Henini, Dilute Nitride Semiconductors, Elsevier, 2005, ISBN: 0080445020. [2] D.J. Friedman, S.R. Kurtz. Prog. Photovolt. 10 (5) (2002) 331. [3] D. Kwon, R.J. Kaplar, S.A. Ringel, A.A. Allerman, S.R. Kurtz, E.D. Jones, Appl. Phys Lett. 74 (19) (1999) 2830. [4] S.Y. Xie, S.F. Yoon, S. Wang, J. Appl. Phys 97 (7) (2005) 073702. [5] W. Li, M. Pessa, J. Toivonen, H. Lipsanen, Phys. Rev. B 64 (2001) 113308. [6] W. Zhu, A. Freundlich, unpublished. [7] D.J. Friedman, J.F. Geisz, S.R. Kurtz, J.M. Olson, J. Crystal Growth 195 (1–4) (1998) 409. [8] S.R. Kurtz, A.A. Allerman, E.D. Jones, J.M. Gee, J.J. Banas, B.E. Hammons, Appl. Phys. Lett. 74 (5) (1999) 729. [9] N.Y. Li, et al., in: Proceeding of 29th IEEE PVSC, vol. 986 2000. [10] B. Damilano, J. Barjon, S.W. Wan, J.Y. Duboz, M. Leroux, M. Laugt, J. Massies, IEE Proc.Optoelectron. 151 (5) (2004) 433. [11] A.J. Ptak, D.J. Friedman, S.R. Kurtz, R.C. Reedy, J. Appl. Phys. 98 (9) (2005) 094501. [12] A. Freundlich, A. Alemu, Phys. Stat. Sol. C 2 (8) (2005) 2978. [13] A. Freundlich, Multi quantum well tandem solar cells, US patents #6,372,980 (2002) and 6,147,296 (2000). [14] A. Feltrin, A. Alemu, A. Freundlich, Phys. Rev. B 73 (15) (2006) 155310. [15] A. Freundlich, F. Newman, M.F. Vilela, C. Monier, L. Aguilar, S. Street, J. Crystal Growth 209 (2–3) (2000) 481. [16] A. Fotkatzikis, M.A. Pinault, J.A.H. Coaquira, A. Freundlich, J. Vac. Sci. Technol. B 23 (3) (2005) 1333. [17] A. Fotkatzikis, M.A. Pinault, A. Freundlich, Appl. Phys. Lett. 85 (13) (2004) 2478. [18] M.A. Pinault, A. Freundlich, J.A.H. Coaquira, A. Fotkatzikis, J. Appl. Phys. 98 (2) (2005) 023522. [19] A. Fotkatzikis, A. Freundlich, J. Vac. Sci. Technol. B 24 (3) (2006) 1536. [20] A. Freundlich, F. Newman, C. Monier, S. Street, P. Dargan, M. Levy, Prog. Photovolt. 8 (3) (2000) 333. [21] C. Monier, M.F. Vilela, I. Serdiukova, L. Aguilar, F. Newman, A. Freundlich, J Crystal Growth 188 (1–4) (1998) 332. [22] A. Khan, A. Freundlich, Appl. Phys. Lett. 88 (10) (2006) 103504. [23] M.A. Green, K. Emery, D.L. King, Y. Hishikawa, W. Warta, Prog. Photovolt. 14 (5) (2006) 455. [24] I. Serdiukova, C. Monier, M.F. Vilela, A. Freundlich, Appl. Phys. Lett. 74 (19) (1999) 2812. [25] A. Alemu, J.H. Coaquira, A. Freundlich, J. Appl. Phys. 99 (8) (2006) 084506.
Journal of Crystal Growth 301–302 (2007) 993–996 www.elsevier.com/locate/jcrysgro
III–V dilute nitride-based multi-quantum well solar cell A. Freundlich, A. Fotkatzikis, L. Bhusal, L. Williams, A. Alemu, W. Zhu, J.A.H. Coaquira, A. Feltrin, G. Radhakrishnan Photovoltaics and Nanostructures Laboratories, Center for Advanced Materials and Physics Department, University of Houston, TX 77204-5004, USA Available online 2 February 2007
Abstract Photovoltaic properties of a preliminary set of 1–1.2 eV dilute nitride GaAsN/GaAs MQW solar cells grown by chemical beam epitaxy are investigated. The study reveals, as expected from the enhancement of effective masses, unusually high photo-conversion strength of the MQW region that exceeds by nearly two-fold those reported for conventional MQW solar cells of comparable bandgaps. Despite a current output (25 A m 2 in absence of ARC) comparable to that of conventional GaInNAs solar cells, output voltages (e.g. 0.6 V for a 1.1 eV MQW cell) appear to be significantly higher than those reported for bulk-like counterparts. Bias-dependent external quantum efficiency measurements reveal an incomplete collection of photo-generated carriers from the wells under operating conditions (forward bias), which should be mitigated by the use of thinner barrier. r 2007 Elsevier B.V. All rights reserved. PACS: 81.15.Hi; 84.60.Jt; 81.07.St; 81.05.Ea Keywords: A3. Chemical beam epitaxy; A3. Quantum wells; B2. Semiconducting III–V materials; B3. Dilute nitride; B3. Solar cells
1. Introduction The presence of small amounts of nitrogen in III–V semiconductor alloys leads to a substantial decrease of the bandgap, making them very attractive for a wide range of optoelectronic applications [1]. In particular, various schemes, involving the use of 1.0–1.25 eV GaInNAs subcells, have been devised to enhance the efficiency of existing triple and quadruple junction solar cells [2]. Nevertheless, thus far, poor minority carrier properties [3,4] and doping issues [5,6] associated with thick bulk-like dilute nitrides have hindered the success of these approaches [7–11]. Here a different strategy is evaluated, where dilute nitride multi-quantum wells (MQW) are embedded within the intrinsic region of conventional GaAs p–i–n solar cells. The insertion of quantum wells in multi-junction solar cells has been predicted to yield practical efficiencies exceeding 35% AM0 [12,13]. The use of these narrow-bandgap MQWs appears as being particularly attractive since it Corresponding author. Tel.: +713 743 3621; fax: +713 747 7724.
E-mail address: [email protected] (A. Freundlich). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.11.256
should extend the photon absorption range while alleviating minority carrier issues encountered in thick bulk-like GaInNAs. In addition the availability of larger joint density of states in the wells, as afforded by higher electron effective masses, warrants stronger absorption coefficient for these nanostructures [14]. 2. Experimental procedure 2.1. Chemical beam epitaxy of multi-quantum well devices Device structures were fabricated on n-type 2 1018 cm-3 Si doped GaAs (0 0 1) substrates by radio frequency (RF) nitrogen plasma-assisted chemical beam epitaxy in a Riber 32CTM chamber equipped with a 600 W VeecoTM nitrogen plasma source. A schematic diagram of the device structure is provided in Fig. 1. Triethyl-galium, trimethyl-indium and arsine combined with Be and Si solid-source dopants were used as growth precursors. One main advantage of CBE for PV applications is the ability to grow at higher growth rates than MBE [15]. The device structure included the deposition of highly Si doped (5 1018 cm 3) micron-thick n+-GaAs buffer and
ARTICLE IN PRESS 994
A. Freundlich et al. / Journal of Crystal Growth 301–302 (2007) 993–996
of the pseudomorphism (Fig. 1). After the growth of the MQW containing i-region, a 0.4 mm thick p-type GaAs emitter GaAs, a 20 nm Al0.85Ga0.15As window layer and a p++ GaAs contact layer were subsequently grown. The Be doping in the emitter and AlGaAs window layers was aimed at 3 1018 cm 3, whereas the contact layer was doped at about 2–5 1019 cm 3. In addition to the dilute nitride device an identical device where the i-region contained an In0.15Ga0.85As (6 nm)/ GaAs (22 nm) MQW device was grown as a reference sample. 3. Post-growth processing and characterizations Fig. 1. Schematic structure of MQW device and RHEED images recorded from the dilute nitride region during the fabrication of MQWs: (2 4) GaAsN.
0.25 mm Al0.2 Ga0.8As, acting as a back surface field (BSF), and a 1.2 mm thick n-doped GaAs base/collector layer. The Si doping of the base was set to 5 1017 cm 3. The growth temperature during the deposition of the buffer BSF and base layers was maintained to about 520 1C at a growth rate of about 2 mm/h. After the deposition of the base the growth was interrupted under arsine flux and the temperature was lowered and stabilized to about 450 1C. The growth of the device intrinsic (i) region which included a 15 period strained GaAs1 xNx (6 nm)/GaAs(22 nm) multi-quantum well (MQW) sandwiched between two 60-nm-thick not intentionally doped layers of GaAs was then resumed. Emphasis was given during growth to minimize the residual hydrogen amount in the growth chamber and its incorporation in dilute nitride layers by reducing the III/V ratio (lower H flux emanating from the arsine thermal decomposition) and by using lower H2 carrier gas flow [16,17]. During growth the hydrogen partial pressure, as measured by mass spectrometry, was maintained to about 2 10 6 Torr. In order to prevent an excessive nitridation of the surface no growth interruption was implemented during the entire growth of the i-region of the device. The RF-plasma source was ignited (with the shutter closed) during the deposition of the first intrinsic GaAs buffer and was maintained ON throughout the MQW region growth and turned OFF at the beginning of the onset of the top 60 nm undoped GaAs layer. The growth rate during the fabrication of the i-region was about 0.8 monolayer/s. The nitrogen flux and plasma conditions were adjusted following previously established conditions [18,19] to yield a nitrogen concentration of about 2% in the wells. It should be noted that even with a closed shutter a small amount of nitrogen is incorporated in GaAs layers and thus barriers and the upper region of the first 60 nm GaAs layer in the i-region have a nitrogen concentration of about 0.1–0.2%. During the growth of the MQW region a 2 4 reflection high-energy electron diffraction (RHEED) diagram was recorded attesting a highly two-dimensional growth mode and the preservation
Following the growth process samples were also subjected to rapid thermal annealing (750 1CoTo900 1C) to evaluate the effect of thermal treatments. Rapid thermal annealing (RTA) was implemented under nitrogen atmosphere and samples were capped on both sides with undoped GaAs wafers. MQW solar cell structural quality was assessed by X-ray diffraction (XRD), whereas electrochemical profilometry was used to evaluate carrier concentrations and cross-diffusions. Temperature-dependent (10–300 K) modulated photoreflectance (PR) and photoluminescence were implemented to probe confined levels and their optical properties. Dilute nitride solar cell structures along with reference bulk like p–i(GaAs)–n and conventional p–i(InGaAs/GaAs MQW)–n were mesa etched and processed using a standard methodology [15,20]. In order to facilitate comparison between different sets of devices no anti-reflect coating was applied. The application of ARC generally reduces the overall reflectivity losses by about 25–30% which results in both an increase of the short-circuit current density and a slight improvement of open-circuit voltages (an overall efficiency improvement of 25%). Device characteristics were analyzed by dark and illuminated current–voltage measurements using an Oriel AM0 solar simulator, the measurement were conducted inside an MMRTM cryostat/probe system and temperature was maintained at 298 K during illuminated data acquisition. Quantum efficiency (QE) measurements were recorded both under short-circuit conditions (V0) and under the application of an external bias at room temperature using the lock-in technique. 4. Results and discussion 4.1. MQW structural and optical properties All samples exhibited specular morphology and their morphology was preserved after RTA treatments. As grown and RTA samples exhibited superlattice like X-ray diffraction patterns and average substitutional nitrogen concentration and well barrier thicknesses were extracted from (0 0 4) diffraction rocking curves. The energy of transitions measured in near band-edge luminescence and PR spectra
ARTICLE IN PRESS A. Freundlich et al. / Journal of Crystal Growth 301–302 (2007) 993–996
of GaAsN/GaAs MQW region was modeled using a bandanticrossing formalism and structural parameters extracted from XRD analysis. The presence at room temperature of three electron- and one hole-confined levels in the multiquantum wells were identified. A typical room temperature modulated photoreflectivity spectra recorded from a GaAsN/GaAs MQW solar cell structure is reported in Fig. 2. 4.2. Device characteristics The spectral response/QE, recorded under short-circuit conditions, from all MQW GaAsN/GaAs devices was portrayed by a strong below GaAs response with peak/ shoulders approximately located at the measured fundamental and excited electron-heavy hole confined levels. All devices were characterized by cutoff wavelengths of about 1.1 eV. The relative spectral response for the first confined electron–hole and third confined electron to hole in all sample exceeded 25% and 50%, respectively. Such a relative response surpasses by over a factor of two–four those reported in the literature [21,22] for other III–V MQW solar cells and the one of the reference InGaAs/ GaAs MQW samples. This unusually high photoconversion strength of the MQW region seems to be consistent with the enhancement of effective masses. Samples subjected to RTA were characterized by a stronger photoluminescence signal of the MQW region and an overall increase of the device QE. A peculiar characteristic of the QE of as grown samples was the presence of a deep valley, indicative of strong recombination rates, in the vicinity of 1.37–1.4 eV. The magnitude of this valley was attenuated for samples that were subjected to RTA. Although the origin of this observation is not clear, the energy range corresponds roughly to the bandgap energy that one would expect from slightly nitrogen (0.2%) contaminated GaAs barriers.
Fig. 2. Room temperature spectral response of 15 period GaAs0.98N0.02(6 nm)/ GaAs(22 nm) MQW device curve with superimposed modulated photoreflectance spectra, arrows indicate calculated ground and excited electron-tohole transitions in the MQW.
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Device illuminated IV characteristics of as grown and RTA samples are shown in Fig. 3. For as-grown solar cells despite a poor short-circuit current (JSC) response (i.e. 10 times less than that of the GaAs p/n diode reference sample), the open circuit voltages (VOC0.6 eV) exceeded markedly (by 200–400 meV) that of conventional InGaAsN solar cells with similar cutoff wavelengths [2,7–10]. It is interesting to note that the open-circuit voltages obtained for these solar cells are only slightly below those reported for high end silicon solar cells [23]. A significant improvement of JSC and device fill factor was obtained for cells processed from RTA set of samples (Fig. 4), yielding an enhancement of the JSC from 15 Am 2 for the as grown solar cell to about 27 Am 2 for solar cell made with an RTA processing of 90 s at 830 1C. QE measurements as a function of external bias, bring some light to the dilemma concerning the relatively high open-circuit voltages, attesting good device minority carrier properties and the poor photo-generated carrier collection as evidenced by the mediocre JSC. The evolution of QE vs. the external bias is shown in Fig. 4. Upon application of the reverse bias although the general shape of the QE seems to be marginally modified, the QE improves significantly with the near GaAs bandedge reaching saturation at about 0.9 V. Such behavior has been previously observed for InAsP/InP MQW when the electric field across the i-region is insufficient at the operation to extract the carriers [23,25]. Another
Fig. 3. Evolution of I–V characteristic at room temperature and under 1 sun-AM0 of a 15 period multi-quantum well solar cell device as function of 830C RTA treatments, (RTA was implemented before device processing step). Cells have no anti-reflect coating.
ARTICLE IN PRESS 996
A. Freundlich et al. / Journal of Crystal Growth 301–302 (2007) 993–996
cells of comparable bandgaps. Bias-dependent external quantum efficiency measurements reveal an incomplete collection of photo-generated carriers from the wells under operating conditions (forward bias), which should be mitigated by the use of thinner barriers. Acknowledgments This work was partially supported by the NASA Grant No. NNCO4GB53N. References
Fig. 4. External quantum efficiency (spectral response) of a 15 period GaAsN/GaAs MQW solar cell as a function of applied bias (negative sign for reverse bias).
interesting feature is the increase of the relative QE of the 1.37–1.4 eV region (indicated by an arrow in Fig. 4), stressing the hindrance that is imposed to the carrier collection process by the slight nitrogen contamination of the barrier material. Since the built-in E-field across the iregion is inversely proportional to the thickness of the i-region [24], these behaviors should be mitigated by the use of thinner barriers or/and by reducing the thickness of GaAs cladding layers. 5. Conclusion Development of dilute nitride GaAsN/GaAs MQW solar cell by chemical beam epitaxy is undertaken. Device characteristics were analyzed by means of dark/illuminated (AM0) current–voltage and quantum efficiency measurements. Despite a modest current output (25 A m 2 in absence of ARC) comparable to that of conventional GaInNAs solar cells, output voltages (e.g. 0.6 V for a 1.1 eV MQWcell) appear to be significantly higher than those reported for bulk-like counterparts. The study also reveals, as theoretically expected from the enhancement of effective masses, unusually high photo-conversion strength of the MQW region that exceeds by two-fold the one obtained from conventional InAsP, InGaAs MQW solar
[1] M. Henini, Dilute Nitride Semiconductors, Elsevier, 2005, ISBN: 0080445020. [2] D.J. Friedman, S.R. Kurtz. Prog. Photovolt. 10 (5) (2002) 331. [3] D. Kwon, R.J. Kaplar, S.A. Ringel, A.A. Allerman, S.R. Kurtz, E.D. Jones, Appl. Phys Lett. 74 (19) (1999) 2830. [4] S.Y. Xie, S.F. Yoon, S. Wang, J. Appl. Phys 97 (7) (2005) 073702. [5] W. Li, M. Pessa, J. Toivonen, H. Lipsanen, Phys. Rev. B 64 (2001) 113308. [6] W. Zhu, A. Freundlich, unpublished. [7] D.J. Friedman, J.F. Geisz, S.R. Kurtz, J.M. Olson, J. Crystal Growth 195 (1–4) (1998) 409. [8] S.R. Kurtz, A.A. Allerman, E.D. Jones, J.M. Gee, J.J. Banas, B.E. Hammons, Appl. Phys. Lett. 74 (5) (1999) 729. [9] N.Y. Li, et al., in: Proceeding of 29th IEEE PVSC, vol. 986 2000. [10] B. Damilano, J. Barjon, S.W. Wan, J.Y. Duboz, M. Leroux, M. Laugt, J. Massies, IEE Proc.Optoelectron. 151 (5) (2004) 433. [11] A.J. Ptak, D.J. Friedman, S.R. Kurtz, R.C. Reedy, J. Appl. Phys. 98 (9) (2005) 094501. [12] A. Freundlich, A. Alemu, Phys. Stat. Sol. C 2 (8) (2005) 2978. [13] A. Freundlich, Multi quantum well tandem solar cells, US patents #6,372,980 (2002) and 6,147,296 (2000). [14] A. Feltrin, A. Alemu, A. Freundlich, Phys. Rev. B 73 (15) (2006) 155310. [15] A. Freundlich, F. Newman, M.F. Vilela, C. Monier, L. Aguilar, S. Street, J. Crystal Growth 209 (2–3) (2000) 481. [16] A. Fotkatzikis, M.A. Pinault, J.A.H. Coaquira, A. Freundlich, J. Vac. Sci. Technol. B 23 (3) (2005) 1333. [17] A. Fotkatzikis, M.A. Pinault, A. Freundlich, Appl. Phys. Lett. 85 (13) (2004) 2478. [18] M.A. Pinault, A. Freundlich, J.A.H. Coaquira, A. Fotkatzikis, J. Appl. Phys. 98 (2) (2005) 023522. [19] A. Fotkatzikis, A. Freundlich, J. Vac. Sci. Technol. B 24 (3) (2006) 1536. [20] A. Freundlich, F. Newman, C. Monier, S. Street, P. Dargan, M. Levy, Prog. Photovolt. 8 (3) (2000) 333. [21] C. Monier, M.F. Vilela, I. Serdiukova, L. Aguilar, F. Newman, A. Freundlich, J Crystal Growth 188 (1–4) (1998) 332. [22] A. Khan, A. Freundlich, Appl. Phys. Lett. 88 (10) (2006) 103504. [23] M.A. Green, K. Emery, D.L. King, Y. Hishikawa, W. Warta, Prog. Photovolt. 14 (5) (2006) 455. [24] I. Serdiukova, C. Monier, M.F. Vilela, A. Freundlich, Appl. Phys. Lett. 74 (19) (1999) 2812. [25] A. Alemu, J.H. Coaquira, A. Freundlich, J. Appl. Phys. 99 (8) (2006) 084506.