- Email: [email protected]
GaAsSb quantum dot structure
Solar Energy Materials & Solar Cells 105 (2012) 237–241
Contents lists available at SciVerse ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Improving the characteristics of intermediate-band solar cell devices using a vertically aligned InAs/GaAsSb quantum dot structure Wei-Sheng Liu a,n, Hong-Ming Wu a, Fu-Hsiang Tsao a, Tsan-Lin Hsu a, Jen-Inn Chyi b a b
Department of Photonics Engineering, Yuan Ze University, Chung-Li, Taiwan Department of Electrical Engineering, National Central University, Chung-Li, Taiwan
a r t i c l e i n f o
abstract
Article history: Received 3 April 2012 Received in revised form 8 June 2012 Accepted 12 June 2012 Available online 10 July 2012
This study demonstrates the feasibility of improving the optical properties of a vertically aligned quantum dot (QD) structure and the performance of a quantum dot intermediate band solar cell (QDIBSC) by capping a GaAsSb layer on the InAs QDs. Experimental results indicate that dot-size uniformity is significantly improved due to the strain modification in the evolution of the successive vertically aligned dot layer growth. A solar cell device with an InAs/GaAsSb columnar dot structure increases the short-circuit current density (Jsc) by 8.8%, compared to a GaAs reference cell. This dot structure also increases quantum efficiency by up to 1200 nm through the absorption of lower-energy photons. The InAs/GaAsSb QD-IBSC also improves the open-circuit voltage (Voc), indicating a reduction in misfit defect density and recombination current density. The results of this study confirm the ability of a columnar InAs/GaAsSb QD structure to enhance the device performance. & 2012 Elsevier B.V. All rights reserved.
Keywords: Antimony Columnar dots Intermediate band Quantum dot
1. Introduction Highly stacked quantum dot (QD) structures have attracted considerable interest in recent years because of their potential use as a novel structure for quantum dot intermediate-band solar cells (QD-IBSCs) [1]. An intermediate band within the GaAs energy bandgap can enhance the photocurrent through the absorption of photon energy below the GaAs bandgap [2]. Carriers in the discrete energy level of quantum dots have been demonstrated to reduce the carrier-relaxation time because of the phonon bottleneck effect, making QDs beneficial to the development of next-generation solar cells with a hot-carrier collection [3,4]. Realizing such high-performance QD photovoltaic devices requires close-coupled QD layers for the formation of an intermediate band (IB). However, the strong strain accumulation caused by a reduced spacer layer thickness between two adjacent QDs can lead to the generation of dislocations. This significantly degrades the crystalline quality of a vertically aligned QD structure [5,6]. Self-assembled InAs QDs covered by a thin GaAs1 xSbx layer have recently received considerable attention in studies on improving the performance of long-wavelength-emitting QD devices and type-II energy band alignment [7–10]. Some studies have shown that the incorporation of antimony (Sb) can increase the dot density and reduce the coalescence dots because Sb acts as a surfactant, decreasing the surface energy [11,12]. This in turn
n
Corresponding author. Tel.: þ886 3 4638800x7521. E-mail address: [email protected] (W.-S. Liu).
0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2012.06.023
increases dot quality and improve optical characteristics [13]. The resulting device performance and material characteristics demonstrate the superior crystalline quality of the InAs/GaAsSb QD structure. Therefore, this study combines the advantages of a columnar dot structure and antimony incorporation to create a vertically aligned InAs/GaAsSb QD structure that improves the material quality and characteristics of the QD-IBSC. This study demonstrates the feasibility of enhancing dot-size uniformity and thoroughly investigates the device properties of vertically aligned InAs QDs with a GaAsSb capping layer. Compared to the InAs/ GaAs reference QDSC, the proposed vertically aligned InAs/ GaAsSb QD-IBSC demonstrates improved short-circuit current density (Jsc) and open-circuit voltage (Voc) with a photoresponse spectrum extended to 1200 nm. This remarkable improvement in columnar dot quality makes the proposed InAs/GaAsSb QD structure the most promising candidate for intermediate band solar cells.
2. Experimental details Samples in this study were grown by solid-source molecular beam epitaxy system. Fig. 1 illustrates the structural design of the vertically aligned InAs/GaAs(Sb) quantum dot intermediate band solar cells. In the samples, ten layers of InAs QDs were grown by depositing 2.7 monolayers (MLs) of InAs on an n þ -GaAs substrate at 500 1C, and separated by a 10 nm-thick spacer, which is a pure GaAs layer for sample A and a combination of 5.5 nm-thick GaAs and 4.5 nm-thick GaAs1 xSbx for samples B (x ¼10%) and
238
W.-S. Liu et al. / Solar Energy Materials & Solar Cells 105 (2012) 237–241
C (x¼ 20%), respectively. When Sb composition x reaches above 14%, the GaAs1 xSbx covered InAs QDs has a Type-II band alignment [7,8,14]. Therefore, the samples B(x¼ 10%) and C(x¼ 20%) are expected to present a Type-I and Type-II band alignment, respectively. Finally, all samples were capped by a 200 nm-thick p-type GaAs layer and were processed into a solar cell device by standard photolithography and a lift-off technique with a device size of 2 2 mm2 for the QD-IBSC performances study. The 661 nm line of a laser diode was used as the excitation source for low-temperature and power-dependent photoluminescence (PL) measurements in a helium-cooled cryogenic system. A cooled InGaAs detector is utilized to measure the signal dispersed by a 0.5 m monochromator (iHR 550) via the lock-in technique. The performances of the QD-IBSC were characterized by external quantum efficiency (EQE) and current–voltage measurements under
Fig. 1. Schematic diagram of QD-IBSC with 10-stacked dot layer design of sample A: InAs/GaAs, sample B: InAs/GaAs1 xSbx (x¼ 10%) and sample C: InAs/GaAs1 xSbx (x¼ 20%).
the conditions of AM1.5 Global spectrum, 1000 W/m2 at room temperature by using the Enli QE-R3515 system.
3. Results and discussions Fig. 2(a) and (b) shows the cross-sectional transmission electron microscopy (TEM) images of vertically aligned QD structures for samples A and B, respectively. According to these figures, both of the ten-layer QD structures are defect free, as well as exhibit columnar growth of QDs along the growth direction. The vertical ordering behavior originating from the in-plane strain energy difference is attributed to the underlying InAs QDs. Such a strain difference reduces localized surface energy, leading to an energetically favored region for successive InAs adatoms of QDs to distribute selectively. Therefore, QDs in the upper layer formed above the underlying dots satisfy the lowest lattice mismatch. The diameters of successive QDs become larger with reduced dot density than the first dot layer, as indicated by the results of sample A [15,16]. However, QDs with GaAsSb capping layer in sample B find improvement in terms of the dot diameter in each layer, as indicated by the dotted arrow. Since a large lattice constant of GaAsSb capping layer plays a role of strain reduction in the InAs QD structure, the stress field of the InAs/GaAsSb columnar dot structure is modified and represents a uniform stress distribution underneath each dot layer in sample B. The improved dot volume of a vertically aligned dot structure in sample B is observed first, indicating better ground-state energy coupling to develop a miniband or intermediate band in the QD superlattice than that in sample A. Meanwhile, a larger dot height of QDs in sample B is observed in Fig. 2(b). A larger dot height of QDs in sample B can be attributed to the reduced QD decomposition or intermixing, often occurring during the GaAs capping process. The preserved dot height and identical diameter of each dot layer in sample B demonstrate the
Fig. 2. Cross-sectional TEM images of 10-stacked layers of (a)InAs/GaAs (sample A) (b) InAs/GaAsSb (sample B) quantum dots. (c) shows the standard deviation calculation of dot diameter and height from samples A and B.
W.-S. Liu et al. / Solar Energy Materials & Solar Cells 105 (2012) 237–241
importance of GaAsSb capping layer in increasing the uniformity of a columnar quantum dot structure [17–20]. Fig. 2(c) illustrates the estimated standard deviation of the dot diameter and height from the TEM results of samples A and B. Sample B reveals a smaller standard deviation of dot height and diameter than those of sample A, indicating the improvement of dot-volume uniformity with Sb incorporation in the capping layer. Additionally, the dot density of sample B is higher than that of sample A, as confirmed by the atomic force microscopy (AFM) analysis in our earlier study [18]. Fig. 3 shows the low-temperature PL measurements at 10 K for samples A, B and C. Sample B with a GaAsSb overgrown layer exhibits a reduction of the full width at half maximum (FWHM) from 30 (sample A) to 27 meV (sample B), which is likely attributed to the improvement of dot size uniformity [21]. However, the FWHM value increased from 27 meV to 110 meV when the Sb composition was increased from 10% to 20%. The induced spectral linewidth could be related to the generation of dislocation due to a large lattice mismatch between the GaAsSb overgrown layer and InAs QDs [22]. Additionally, the PL spectrum also revealed emission red-shift behavior with an increase Sb incorporation in the capping layer. The extended emission wavelength was attributed to the reduction of compressive strain or In–Ga material intermixing by the GaAsSb capping layer [23]. To investigate the band configuration of the QD heterostructure with antimony incorporation, power-dependent PL measurements were taken for samples B and C, with those results summarized in the inset, respectively. When the Sb composition in GaAsSb capping layer was increased by over 14%, energy band structure of InAs/GaAsSb vertically aligned QDs exhibits a transformation from type-I to type-II band alignment. The inset summarizes the QD ground-state energy trace of power-dependent PL (PD-PL) spectra for samples B and C. This figure revels that the spectral ground-state emissions of samples A and B remain constant when increasing the excitation power. However, the trace of sample C exhibits a linear dependent correlation with (excitation power) 1/3 until to 20 mW, which closely corresponds to the band bending behavior of a type-II band structure [24–26]. Since the photo-excited electron and hole is confined separately in InAs QDs and GaAsSb capping layer, the induced electronic field leads to bending behavior of the triangular quantum well in the type-II band alignment. Therefore, the increased PL exciting power elevates the ground-state energy, resulting in a significant spectral blue shift of sample C in the power-dependent PL experiment.
239
This study also investigated the device characteristics of the vertically aligned QD-IBSC by processing samples A, B and C along with a GaAs reference cell into 2 2 mm2 solar cell devices. Additionally, the device performances were characterized by external quantum efficiency (EQE) and current–voltage measurements at one-sun illumination. Fig. 4 shows the EQE measurement results of the QD-IBSC devices and GaAs reference cell at room temperature. The photon-absorption of GaAs band edge was found around 880 nm. Moreover, the extended photoresponse to 1000 nm of sample A was attributed to the low-energy photon absorption by the wetting layer and the InAs columnar dot structure, which correlates well with previous studies [27,28]. However, the EQE of samples B and C revealed a further extended photon-absorption wavelength range to around 1200 nm by using the GaAsSb capping layer. Since the GaAsSb capping layer preserved the QDs with reduced In–Ga intermixing and a compressive strain, absorption of photons with lower energy than sample A was observed in samples B and C. Although less antimony composition was incorporated in sample B, the improved crystalline quality over that of sample C contributed to the elevation of EQE at a long-wavelength range. Therefore, this study demonstrates the feasibility of extending EQE photoresponse wavelength towards 1200 nm. The current–voltage characteristics of the devices were measured under one sun illumination, as illustrated in Fig. 5. According to this figure, the short-circuit current density (Jsc) increased from Jsc ¼20.2 to 21.4 and 22.0 mA/cm2 of the GaAs reference cell, samples A and B, respectively; the increase of Jsc seemed to be owing to the extended external quantum efficiency of QD-IBSC at a longer wavelength range [29,30]. Additionally, a higher QD density in sample B could also contribute to the increase of photon-absorption, subsequently leading to a higher current density than its sample A counterpart. However, the decrease of open-circuit voltage (Voc) was observed in the columnar dot structure of samples A, B and C, which can be attributed to the narrower bandgap of InAs quantum dot structure than that of the GaAs reference cell [17]. Moreover, the minority carrier recombination current from the misfit defects due to the strong accumulated compressive strain appears to respond the degradation of Voc [27]. Interestingly, sample B with GaAsSb capping layer exhibits a higher Voc than the typical InAs/GaAs QD-IBSC (sample A). The voltage enhancement indicates the reduction of recombination current density and the improvement of the QD crystalline structure quality by using the GaAsSb capping layer.
GaAs_Ref Sample A Sample B Sample C
External Quantum Efficiency
1.0
Intensity (a.u.)
0.8
0.6
0.4
0.2
10
1
0.0 900
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900
Wavelength (nm)
700
800
900
1000
1100
1200
Wavelength (nm) Fig. 3. Photoluminescence spectra of samples A, B and C at low temperature of 10 K. The PL exciting power is 30 mW. The inset shows the QD ground-state energy trace of power-dependent PL measurement.
Fig. 4. External quantum efficiency (EQE) measurement of samples A, B and C along with a GaAs reference cell.
240
W.-S. Liu et al. / Solar Energy Materials & Solar Cells 105 (2012) 237–241
Current Density (mA/cm2)
References
GaAs_Ref Sample A Sample B Sample C
25
20
15
10
5
0 0.0
0.2
0.4
0.6
0.8
1.0
Voltage (V) Fig. 5. Current–voltage characteristics of samples A, B and C. Typical GaAs solar cell is also compared as a reference sample. (1000 W/m2 AM1.5 Global spectrum).
Nevertheless, Voc of sample C degrades much more than that of samples A and B. This finding implies serious generations of misfit dislocation due to large lattice mismatch, thus contributing to the increased minority-carrier recombination current density in the high Sb structural contents [27]. Although the QD-IBSCs show the same results of voltage degradation as in previous studies, the device of sample B with InAs/GaAsSb columnar QD structure still demonstrates the enhanced current density along with reduced voltage degradation over that of conventional vertically aligned In(Ga)As/GaAs QD-IBSC devices [27–31]. Results of this study thus demonstrate the superior device characteristics of columnar InAs/ GaAsSb QDSC, making it a highly promising candidate for novel intermediate band solar cell applications.
4. Conclusions In conclusion, InAs QD size uniformity and QD-IBSC performances have been improved by capping a GaAsSb overgrown layer. Optical and structural studies verify the improvements of QD crystalline quality and dot volume uniformity. To achieve the theoretically predicted high conversion efficiency of intermediate band solar cells, this study also designed and fabricated QD-IBSC with vertically aligned InAs/GaAsSb columnar dot structure. The device characteristics of InAs/GaAsSb QD-IBSC (sample B) demonstrate improvements in both short-circuit current density (Jsc) and open-circuit voltage (Voc). In addition to exhibiting an increase of Jsc by 8.8% over that a reference GaAs cell, the current measurement of sample B demonstrates an extended photoresponse up to 1200 nm due to the absorption of low-energy photon by the QDs. Meanwhile, sample B demonstrates an improvement of Voc over the typical InAs/GaAs QD-IBSC. The high quality columnar dot structure and enhanced device performance demonstrate the feasibility of achieving QD-IBSC with high conversion efficiency by using an InAs/GaAsSb columnar dot structure.
Acknowledgments The authors would like to thank the financial support of National Science Council of the Republic of China, Taiwan, under Contracts NSC 99–2221-E-155-085, and NSC 100–2221-E-155004. The Optical Sciences Center and Center for Nano Science and Technology at National Central University are appreciated for providing the research equipment.
[1] A. Luque, A. Marti, Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels, Physical Review Letters 78 (1997) 5014–5017. [2] A. Martı´, E. Antolı´n, C.R. Stanley, C.D. Farmer, N. Lo´pez, P. Dı´az, E. Ca´novas, P.G. Linares, A. Luque, Production of photocurrent due to intermediate-to conduction-band transitions: a demonstration of a key operating principle of the intermediate-band solar cell, Physical Review Letters 97 (2006) 2477011–247701-4. ¨ [3] G.J. Conibeer, D. Konig, M.A. Green, J.F. Guillemoles, Slowing of carrier cooling in hot carrier solar cells, Thin Solid Films 516 (2008) 6948–6953. [4] A.J. Nozik, Quantum dot solar cells, Physica E 14 (2002) 115–120. [5] A. Martı´, N. Lo´pez, E. Antolı´n, E. Ca´novas, A. Luque, C.R. Stanley, C.D. Farmer, P. Dı´az, Emitter degradation in quantum dot intermediate band solar cells, Applied Physics Letters 90 (2007) 233510-1. [6] T. Sugaya, Y. Kamikawa, S. Furue, T. Amano, M. Mori, S. Niki, Multi-stacked quantum dot solar cells fabricated by intermittent deposition of InGaAs, Solar Energy Materials and Solar Cells 95 (2011) 163–166. [7] H.Y. Liu, M.J. Steer, T.J. Badcock, D.J. Mowbray, M.S. Skolnick, P. Navaretti, K.M. Groom, M. Hopkinson, R.A. Hogg, Long-wavelength light emission and lasing from InAs/GaAs quantum dots covered by a GaAsSb strain-reducing layer, Applied Physics Letters 86 (2005) 143108-1–143108-3. [8] Yu-An Liao, Wei-Ting Hsu, Pei-Chin Chiu, Jen-Inn Chyi, Wen-Hao Chang, Effects of thermal annealing on the emission properties of type-II InAs/GaAsSb quantum dots, Applied Physics Letters 94 (2009) 053101-1–053101-3. [9] Wei-Sheng Liu, David M.T. Kuo, Jen-Inn Chyi, Wen-Yen Chen, HsingSzu Chang, Tzu-Min Hsu, Enhanced thermal stability and emission intensity of InAs quantum dots covered by an InGaAsSb strain-reducing layer, Applied Physics Letters 89 (2006) 243103-1–243103-3. [10] D. Guimard, M. Ishida, N. Hatori, Y. Nakata, H. Sudo, T. Yamamoto, M. Sugawara, Y. Arakawa, CW Lasing at 1.35 mm from ten InAs–SbGaAs quantum-dot layers grown by metal–organic chemical vapor deposition, Photonics Technology Letters IEEE 20 (2008) 827–829. [11] D. Guimard, M. Nishioka, S. Tsukamoto, Y. Arakawa, Effect of antimony on the density of InAs/Sb:GaAs(1 0 0) quantum dots grown by metalorganic chemical–vapor deposition, Journal of Crystal Growth 298 (2007) 548–552. [12] Cheong Hyun Roh, Young Ju Park, Kwang Moo Kim, Young Min Park, Eun Kyu Kim, Kwang Bo Shim, Defect generation in multi-stacked InAs quantum dot/GaAs structures, Journal of Crystal Growth 226 (2001) 1–7. [13] Pei-Chin Chiu, Wei-Sheng Liu, Meng-Jie Shiau, Jen-Inn Chyi, Wen-Yen Chen, Hsing-Szu Chang, Tzu-Min Hsu, Enhancing the optical properties of InAs quantum dots by an InAlAsSb overgrown layer, Applied Physics Letters 91 (2007) 153106-1–153106-3. [14] C.Y. Jin, H.Y. Liu, S.Y. Zhang, Q. Jiang, S.L. Liew, M. Hopkinson, T.J. Badcock, E. Nabavi, D.J. Mowbray, Optical transitions in type-II InAs/GaAs quantum dots covered by a GaAsSb strain-reducing layer, Applied Physics Letters 91 (2007) 021102-1–021102-3. [15] Qianghua Xie, Anupam Madhukar, Ping Chen, Nobuhiko.P. Kobayashi, Vertically self-organized InAs quantum box islands on GaAs(100), Physical Review Letters 75 (1995) 2542–2545. [16] G.S. Solomon, Electronic coupling and structural ordering of quantum dots using InAs quantum dot columns, Journal of Electronic Materials 28 (1999) 392–404. [17] Denis Guimard, Ryo Morihara, Damien Bordel, Katsuaki Tanabe, Yuki Wakayama, Masao Nishioka, Yasuhiko Arakawa, Fabrication of InAs/ GaAs quantum dot solar cells with enhanced photocurrent and without degradation of open circuit voltage, Applied Physics Letters 96 (2010) 203507-1–203507-3. [18] Wei-Sheng Liu, Hong-Ming Wu, Yu-Ann Liao, Jen-Inn Chyi, Wen-Yen Chen, Tzu-Min Hsu, High optical property vertically aligned InAs quantum dot structures with GaAsSb overgrown layers, Journal of Crystal Growth 323 (2011) 164–166. [19] J.M. Ulloa, R. Gargallo-Caballero, M. Bozkurt, M. del Moral, A. Guzma´n, P.M. Koenraad, A. Hierro, GaAsSb-capped InAs quantum dots: from enlarged quantum dot height to alloy fluctuations, Physical Review B 81 (2010) 165305-1–165305-7. [20] J.M. Ulloa, I.W.D. Drouzas, P.M. Koenraad, D.J. Mowbray, M.J. Steer, H.Y. Liu, M. Hopkinson, Suppression of InAs/GaAs quantum dot decomposition by the incorporation of a GaAsSb capping layer, Applied Physics Letters 90 (2007) 213105-1–213105-3. [21] Denis Guimard, Masao Nishioka, Shiro Tsukamoto, Yasuhiko Arakawa, High density InAs/GaAs quantum dots with enhanced photoluminescence intensity using antimony surfactant-mediated metal organic chemical vapor deposition, Applied Physics Letters 89 (2006) 183124-1–183124-3. [22] Naokatsu Kouichi Akahane, Naoki Yamamoto, Ohtani, Long-wavelength light emission from InAs quantum dots covered by GaAsSb grown on GaAs substrates, Physica E 21 (2004) 295–299. [23] J.M. Ripalda, D. Granados, Y. Gonza´lez, A.M. Sa´nchez, S.I. Molina, J.M. Garcı´a, Room temperature emission at 1.6 mm from InGaAs quantum dots capped with GaAsSb, Applied Physics Letters 87 (2005) 202108-1–202108-3. [24] Wen-Hao Chang, Yu-An Liao, Wei-Ting Hsu, Ming-Chih Lee, Pei-Chin Chiu, Jen-Inn Chyi, Carrier dynamics of type-II InAs/GaAs quantum dots covered by a thin GaAs1 xSbx layer, Applied Physics Letters 93 (2008) 0331071–033107-3.
W.-S. Liu et al. / Solar Energy Materials & Solar Cells 105 (2012) 237–241
[25] J.S. Ng, H.Y. Liu, M.J. Steer, M. Hopkinson, J.P.R. David, Photoluminescence beyond 1.5 mm from InAs quantum dots, Microelectronics Journal 37 (2006) 1468–1470. [26] T.T. Chen, C.L. Cheng, Y.F. Chen, Unusual optical properties of type-II InAs/ GaAs0.7Sb0.3 quantum dots by photoluminescence studies, Physical Review B 75 (2007) 033310-1–033310-4. [27] S.M. Hubbard, C.D. Cress, C.G. Bailey, R.P. Raffaelle, S.G. Bailey, D.M. Wilt, Effect of strain compensation on quantum dot enhanced GaAs solar cells, Applied Physics Letters 92 (2008) 123512-1–123512-3. [28] R.B. Laghumavarapu, M. El-Emawy, N. Nuntawong, A. Moscho, L.F. Lester, D.L. Huffaker, Improved device performance of InAs/GaAs quantum dot solar cells with GaP strain compensation layers, Applied Physics Letters 91 (2007) 243115-1–243115-3.
241
[29] Yoshitaka Okada, Ryuji Oshima, Ayami Takata, Characteristics of InAs/GaNAs strain-compensated quantum dot solar cell, Journal of Applied Physics 106 (2009) 024306-1–024306-3. [30] R.B. Laghumavarapu, A. Moscho, A. Khoshakhlagh, M. El-Emawy, L.F. Lester, D.L. Huffaker, GaSb/GaAs type II quantum dot solar cells for enhanced infrared spectral response, Applied Physics Letters 90 (2007) 1731251–173125-3. [31] T. Sugaya, S. Furue, H. Komaki, T. Amano, M. Mori, K. Komori, S. Niki, O. Numakami, Y. Okano, Highly stacked and well-aligned In0.4Ga0.6As quantum dot solar cells with In0.2Ga0.8As cap layer, Applied Physics Letters 97 (2010) 183104-1–183104-3.
Contents lists available at SciVerse ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Improving the characteristics of intermediate-band solar cell devices using a vertically aligned InAs/GaAsSb quantum dot structure Wei-Sheng Liu a,n, Hong-Ming Wu a, Fu-Hsiang Tsao a, Tsan-Lin Hsu a, Jen-Inn Chyi b a b
Department of Photonics Engineering, Yuan Ze University, Chung-Li, Taiwan Department of Electrical Engineering, National Central University, Chung-Li, Taiwan
a r t i c l e i n f o
abstract
Article history: Received 3 April 2012 Received in revised form 8 June 2012 Accepted 12 June 2012 Available online 10 July 2012
This study demonstrates the feasibility of improving the optical properties of a vertically aligned quantum dot (QD) structure and the performance of a quantum dot intermediate band solar cell (QDIBSC) by capping a GaAsSb layer on the InAs QDs. Experimental results indicate that dot-size uniformity is significantly improved due to the strain modification in the evolution of the successive vertically aligned dot layer growth. A solar cell device with an InAs/GaAsSb columnar dot structure increases the short-circuit current density (Jsc) by 8.8%, compared to a GaAs reference cell. This dot structure also increases quantum efficiency by up to 1200 nm through the absorption of lower-energy photons. The InAs/GaAsSb QD-IBSC also improves the open-circuit voltage (Voc), indicating a reduction in misfit defect density and recombination current density. The results of this study confirm the ability of a columnar InAs/GaAsSb QD structure to enhance the device performance. & 2012 Elsevier B.V. All rights reserved.
Keywords: Antimony Columnar dots Intermediate band Quantum dot
1. Introduction Highly stacked quantum dot (QD) structures have attracted considerable interest in recent years because of their potential use as a novel structure for quantum dot intermediate-band solar cells (QD-IBSCs) [1]. An intermediate band within the GaAs energy bandgap can enhance the photocurrent through the absorption of photon energy below the GaAs bandgap [2]. Carriers in the discrete energy level of quantum dots have been demonstrated to reduce the carrier-relaxation time because of the phonon bottleneck effect, making QDs beneficial to the development of next-generation solar cells with a hot-carrier collection [3,4]. Realizing such high-performance QD photovoltaic devices requires close-coupled QD layers for the formation of an intermediate band (IB). However, the strong strain accumulation caused by a reduced spacer layer thickness between two adjacent QDs can lead to the generation of dislocations. This significantly degrades the crystalline quality of a vertically aligned QD structure [5,6]. Self-assembled InAs QDs covered by a thin GaAs1 xSbx layer have recently received considerable attention in studies on improving the performance of long-wavelength-emitting QD devices and type-II energy band alignment [7–10]. Some studies have shown that the incorporation of antimony (Sb) can increase the dot density and reduce the coalescence dots because Sb acts as a surfactant, decreasing the surface energy [11,12]. This in turn
n
Corresponding author. Tel.: þ886 3 4638800x7521. E-mail address: [email protected] (W.-S. Liu).
0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2012.06.023
increases dot quality and improve optical characteristics [13]. The resulting device performance and material characteristics demonstrate the superior crystalline quality of the InAs/GaAsSb QD structure. Therefore, this study combines the advantages of a columnar dot structure and antimony incorporation to create a vertically aligned InAs/GaAsSb QD structure that improves the material quality and characteristics of the QD-IBSC. This study demonstrates the feasibility of enhancing dot-size uniformity and thoroughly investigates the device properties of vertically aligned InAs QDs with a GaAsSb capping layer. Compared to the InAs/ GaAs reference QDSC, the proposed vertically aligned InAs/ GaAsSb QD-IBSC demonstrates improved short-circuit current density (Jsc) and open-circuit voltage (Voc) with a photoresponse spectrum extended to 1200 nm. This remarkable improvement in columnar dot quality makes the proposed InAs/GaAsSb QD structure the most promising candidate for intermediate band solar cells.
2. Experimental details Samples in this study were grown by solid-source molecular beam epitaxy system. Fig. 1 illustrates the structural design of the vertically aligned InAs/GaAs(Sb) quantum dot intermediate band solar cells. In the samples, ten layers of InAs QDs were grown by depositing 2.7 monolayers (MLs) of InAs on an n þ -GaAs substrate at 500 1C, and separated by a 10 nm-thick spacer, which is a pure GaAs layer for sample A and a combination of 5.5 nm-thick GaAs and 4.5 nm-thick GaAs1 xSbx for samples B (x ¼10%) and
238
W.-S. Liu et al. / Solar Energy Materials & Solar Cells 105 (2012) 237–241
C (x¼ 20%), respectively. When Sb composition x reaches above 14%, the GaAs1 xSbx covered InAs QDs has a Type-II band alignment [7,8,14]. Therefore, the samples B(x¼ 10%) and C(x¼ 20%) are expected to present a Type-I and Type-II band alignment, respectively. Finally, all samples were capped by a 200 nm-thick p-type GaAs layer and were processed into a solar cell device by standard photolithography and a lift-off technique with a device size of 2 2 mm2 for the QD-IBSC performances study. The 661 nm line of a laser diode was used as the excitation source for low-temperature and power-dependent photoluminescence (PL) measurements in a helium-cooled cryogenic system. A cooled InGaAs detector is utilized to measure the signal dispersed by a 0.5 m monochromator (iHR 550) via the lock-in technique. The performances of the QD-IBSC were characterized by external quantum efficiency (EQE) and current–voltage measurements under
Fig. 1. Schematic diagram of QD-IBSC with 10-stacked dot layer design of sample A: InAs/GaAs, sample B: InAs/GaAs1 xSbx (x¼ 10%) and sample C: InAs/GaAs1 xSbx (x¼ 20%).
the conditions of AM1.5 Global spectrum, 1000 W/m2 at room temperature by using the Enli QE-R3515 system.
3. Results and discussions Fig. 2(a) and (b) shows the cross-sectional transmission electron microscopy (TEM) images of vertically aligned QD structures for samples A and B, respectively. According to these figures, both of the ten-layer QD structures are defect free, as well as exhibit columnar growth of QDs along the growth direction. The vertical ordering behavior originating from the in-plane strain energy difference is attributed to the underlying InAs QDs. Such a strain difference reduces localized surface energy, leading to an energetically favored region for successive InAs adatoms of QDs to distribute selectively. Therefore, QDs in the upper layer formed above the underlying dots satisfy the lowest lattice mismatch. The diameters of successive QDs become larger with reduced dot density than the first dot layer, as indicated by the results of sample A [15,16]. However, QDs with GaAsSb capping layer in sample B find improvement in terms of the dot diameter in each layer, as indicated by the dotted arrow. Since a large lattice constant of GaAsSb capping layer plays a role of strain reduction in the InAs QD structure, the stress field of the InAs/GaAsSb columnar dot structure is modified and represents a uniform stress distribution underneath each dot layer in sample B. The improved dot volume of a vertically aligned dot structure in sample B is observed first, indicating better ground-state energy coupling to develop a miniband or intermediate band in the QD superlattice than that in sample A. Meanwhile, a larger dot height of QDs in sample B is observed in Fig. 2(b). A larger dot height of QDs in sample B can be attributed to the reduced QD decomposition or intermixing, often occurring during the GaAs capping process. The preserved dot height and identical diameter of each dot layer in sample B demonstrate the
Fig. 2. Cross-sectional TEM images of 10-stacked layers of (a)InAs/GaAs (sample A) (b) InAs/GaAsSb (sample B) quantum dots. (c) shows the standard deviation calculation of dot diameter and height from samples A and B.
W.-S. Liu et al. / Solar Energy Materials & Solar Cells 105 (2012) 237–241
importance of GaAsSb capping layer in increasing the uniformity of a columnar quantum dot structure [17–20]. Fig. 2(c) illustrates the estimated standard deviation of the dot diameter and height from the TEM results of samples A and B. Sample B reveals a smaller standard deviation of dot height and diameter than those of sample A, indicating the improvement of dot-volume uniformity with Sb incorporation in the capping layer. Additionally, the dot density of sample B is higher than that of sample A, as confirmed by the atomic force microscopy (AFM) analysis in our earlier study [18]. Fig. 3 shows the low-temperature PL measurements at 10 K for samples A, B and C. Sample B with a GaAsSb overgrown layer exhibits a reduction of the full width at half maximum (FWHM) from 30 (sample A) to 27 meV (sample B), which is likely attributed to the improvement of dot size uniformity [21]. However, the FWHM value increased from 27 meV to 110 meV when the Sb composition was increased from 10% to 20%. The induced spectral linewidth could be related to the generation of dislocation due to a large lattice mismatch between the GaAsSb overgrown layer and InAs QDs [22]. Additionally, the PL spectrum also revealed emission red-shift behavior with an increase Sb incorporation in the capping layer. The extended emission wavelength was attributed to the reduction of compressive strain or In–Ga material intermixing by the GaAsSb capping layer [23]. To investigate the band configuration of the QD heterostructure with antimony incorporation, power-dependent PL measurements were taken for samples B and C, with those results summarized in the inset, respectively. When the Sb composition in GaAsSb capping layer was increased by over 14%, energy band structure of InAs/GaAsSb vertically aligned QDs exhibits a transformation from type-I to type-II band alignment. The inset summarizes the QD ground-state energy trace of power-dependent PL (PD-PL) spectra for samples B and C. This figure revels that the spectral ground-state emissions of samples A and B remain constant when increasing the excitation power. However, the trace of sample C exhibits a linear dependent correlation with (excitation power) 1/3 until to 20 mW, which closely corresponds to the band bending behavior of a type-II band structure [24–26]. Since the photo-excited electron and hole is confined separately in InAs QDs and GaAsSb capping layer, the induced electronic field leads to bending behavior of the triangular quantum well in the type-II band alignment. Therefore, the increased PL exciting power elevates the ground-state energy, resulting in a significant spectral blue shift of sample C in the power-dependent PL experiment.
239
This study also investigated the device characteristics of the vertically aligned QD-IBSC by processing samples A, B and C along with a GaAs reference cell into 2 2 mm2 solar cell devices. Additionally, the device performances were characterized by external quantum efficiency (EQE) and current–voltage measurements at one-sun illumination. Fig. 4 shows the EQE measurement results of the QD-IBSC devices and GaAs reference cell at room temperature. The photon-absorption of GaAs band edge was found around 880 nm. Moreover, the extended photoresponse to 1000 nm of sample A was attributed to the low-energy photon absorption by the wetting layer and the InAs columnar dot structure, which correlates well with previous studies [27,28]. However, the EQE of samples B and C revealed a further extended photon-absorption wavelength range to around 1200 nm by using the GaAsSb capping layer. Since the GaAsSb capping layer preserved the QDs with reduced In–Ga intermixing and a compressive strain, absorption of photons with lower energy than sample A was observed in samples B and C. Although less antimony composition was incorporated in sample B, the improved crystalline quality over that of sample C contributed to the elevation of EQE at a long-wavelength range. Therefore, this study demonstrates the feasibility of extending EQE photoresponse wavelength towards 1200 nm. The current–voltage characteristics of the devices were measured under one sun illumination, as illustrated in Fig. 5. According to this figure, the short-circuit current density (Jsc) increased from Jsc ¼20.2 to 21.4 and 22.0 mA/cm2 of the GaAs reference cell, samples A and B, respectively; the increase of Jsc seemed to be owing to the extended external quantum efficiency of QD-IBSC at a longer wavelength range [29,30]. Additionally, a higher QD density in sample B could also contribute to the increase of photon-absorption, subsequently leading to a higher current density than its sample A counterpart. However, the decrease of open-circuit voltage (Voc) was observed in the columnar dot structure of samples A, B and C, which can be attributed to the narrower bandgap of InAs quantum dot structure than that of the GaAs reference cell [17]. Moreover, the minority carrier recombination current from the misfit defects due to the strong accumulated compressive strain appears to respond the degradation of Voc [27]. Interestingly, sample B with GaAsSb capping layer exhibits a higher Voc than the typical InAs/GaAs QD-IBSC (sample A). The voltage enhancement indicates the reduction of recombination current density and the improvement of the QD crystalline structure quality by using the GaAsSb capping layer.
GaAs_Ref Sample A Sample B Sample C
External Quantum Efficiency
1.0
Intensity (a.u.)
0.8
0.6
0.4
0.2
10
1
0.0 900
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900
Wavelength (nm)
700
800
900
1000
1100
1200
Wavelength (nm) Fig. 3. Photoluminescence spectra of samples A, B and C at low temperature of 10 K. The PL exciting power is 30 mW. The inset shows the QD ground-state energy trace of power-dependent PL measurement.
Fig. 4. External quantum efficiency (EQE) measurement of samples A, B and C along with a GaAs reference cell.
240
W.-S. Liu et al. / Solar Energy Materials & Solar Cells 105 (2012) 237–241
Current Density (mA/cm2)
References
GaAs_Ref Sample A Sample B Sample C
25
20
15
10
5
0 0.0
0.2
0.4
0.6
0.8
1.0
Voltage (V) Fig. 5. Current–voltage characteristics of samples A, B and C. Typical GaAs solar cell is also compared as a reference sample. (1000 W/m2 AM1.5 Global spectrum).
Nevertheless, Voc of sample C degrades much more than that of samples A and B. This finding implies serious generations of misfit dislocation due to large lattice mismatch, thus contributing to the increased minority-carrier recombination current density in the high Sb structural contents [27]. Although the QD-IBSCs show the same results of voltage degradation as in previous studies, the device of sample B with InAs/GaAsSb columnar QD structure still demonstrates the enhanced current density along with reduced voltage degradation over that of conventional vertically aligned In(Ga)As/GaAs QD-IBSC devices [27–31]. Results of this study thus demonstrate the superior device characteristics of columnar InAs/ GaAsSb QDSC, making it a highly promising candidate for novel intermediate band solar cell applications.
4. Conclusions In conclusion, InAs QD size uniformity and QD-IBSC performances have been improved by capping a GaAsSb overgrown layer. Optical and structural studies verify the improvements of QD crystalline quality and dot volume uniformity. To achieve the theoretically predicted high conversion efficiency of intermediate band solar cells, this study also designed and fabricated QD-IBSC with vertically aligned InAs/GaAsSb columnar dot structure. The device characteristics of InAs/GaAsSb QD-IBSC (sample B) demonstrate improvements in both short-circuit current density (Jsc) and open-circuit voltage (Voc). In addition to exhibiting an increase of Jsc by 8.8% over that a reference GaAs cell, the current measurement of sample B demonstrates an extended photoresponse up to 1200 nm due to the absorption of low-energy photon by the QDs. Meanwhile, sample B demonstrates an improvement of Voc over the typical InAs/GaAs QD-IBSC. The high quality columnar dot structure and enhanced device performance demonstrate the feasibility of achieving QD-IBSC with high conversion efficiency by using an InAs/GaAsSb columnar dot structure.
Acknowledgments The authors would like to thank the financial support of National Science Council of the Republic of China, Taiwan, under Contracts NSC 99–2221-E-155-085, and NSC 100–2221-E-155004. The Optical Sciences Center and Center for Nano Science and Technology at National Central University are appreciated for providing the research equipment.
[1] A. Luque, A. Marti, Increasing the efficiency of ideal solar cells by photon induced transitions at intermediate levels, Physical Review Letters 78 (1997) 5014–5017. [2] A. Martı´, E. Antolı´n, C.R. Stanley, C.D. Farmer, N. Lo´pez, P. Dı´az, E. Ca´novas, P.G. Linares, A. Luque, Production of photocurrent due to intermediate-to conduction-band transitions: a demonstration of a key operating principle of the intermediate-band solar cell, Physical Review Letters 97 (2006) 2477011–247701-4. ¨ [3] G.J. Conibeer, D. Konig, M.A. Green, J.F. Guillemoles, Slowing of carrier cooling in hot carrier solar cells, Thin Solid Films 516 (2008) 6948–6953. [4] A.J. Nozik, Quantum dot solar cells, Physica E 14 (2002) 115–120. [5] A. Martı´, N. Lo´pez, E. Antolı´n, E. Ca´novas, A. Luque, C.R. Stanley, C.D. Farmer, P. Dı´az, Emitter degradation in quantum dot intermediate band solar cells, Applied Physics Letters 90 (2007) 233510-1. [6] T. Sugaya, Y. Kamikawa, S. Furue, T. Amano, M. Mori, S. Niki, Multi-stacked quantum dot solar cells fabricated by intermittent deposition of InGaAs, Solar Energy Materials and Solar Cells 95 (2011) 163–166. [7] H.Y. Liu, M.J. Steer, T.J. Badcock, D.J. Mowbray, M.S. Skolnick, P. Navaretti, K.M. Groom, M. Hopkinson, R.A. Hogg, Long-wavelength light emission and lasing from InAs/GaAs quantum dots covered by a GaAsSb strain-reducing layer, Applied Physics Letters 86 (2005) 143108-1–143108-3. [8] Yu-An Liao, Wei-Ting Hsu, Pei-Chin Chiu, Jen-Inn Chyi, Wen-Hao Chang, Effects of thermal annealing on the emission properties of type-II InAs/GaAsSb quantum dots, Applied Physics Letters 94 (2009) 053101-1–053101-3. [9] Wei-Sheng Liu, David M.T. Kuo, Jen-Inn Chyi, Wen-Yen Chen, HsingSzu Chang, Tzu-Min Hsu, Enhanced thermal stability and emission intensity of InAs quantum dots covered by an InGaAsSb strain-reducing layer, Applied Physics Letters 89 (2006) 243103-1–243103-3. [10] D. Guimard, M. Ishida, N. Hatori, Y. Nakata, H. Sudo, T. Yamamoto, M. Sugawara, Y. Arakawa, CW Lasing at 1.35 mm from ten InAs–SbGaAs quantum-dot layers grown by metal–organic chemical vapor deposition, Photonics Technology Letters IEEE 20 (2008) 827–829. [11] D. Guimard, M. Nishioka, S. Tsukamoto, Y. Arakawa, Effect of antimony on the density of InAs/Sb:GaAs(1 0 0) quantum dots grown by metalorganic chemical–vapor deposition, Journal of Crystal Growth 298 (2007) 548–552. [12] Cheong Hyun Roh, Young Ju Park, Kwang Moo Kim, Young Min Park, Eun Kyu Kim, Kwang Bo Shim, Defect generation in multi-stacked InAs quantum dot/GaAs structures, Journal of Crystal Growth 226 (2001) 1–7. [13] Pei-Chin Chiu, Wei-Sheng Liu, Meng-Jie Shiau, Jen-Inn Chyi, Wen-Yen Chen, Hsing-Szu Chang, Tzu-Min Hsu, Enhancing the optical properties of InAs quantum dots by an InAlAsSb overgrown layer, Applied Physics Letters 91 (2007) 153106-1–153106-3. [14] C.Y. Jin, H.Y. Liu, S.Y. Zhang, Q. Jiang, S.L. Liew, M. Hopkinson, T.J. Badcock, E. Nabavi, D.J. Mowbray, Optical transitions in type-II InAs/GaAs quantum dots covered by a GaAsSb strain-reducing layer, Applied Physics Letters 91 (2007) 021102-1–021102-3. [15] Qianghua Xie, Anupam Madhukar, Ping Chen, Nobuhiko.P. Kobayashi, Vertically self-organized InAs quantum box islands on GaAs(100), Physical Review Letters 75 (1995) 2542–2545. [16] G.S. Solomon, Electronic coupling and structural ordering of quantum dots using InAs quantum dot columns, Journal of Electronic Materials 28 (1999) 392–404. [17] Denis Guimard, Ryo Morihara, Damien Bordel, Katsuaki Tanabe, Yuki Wakayama, Masao Nishioka, Yasuhiko Arakawa, Fabrication of InAs/ GaAs quantum dot solar cells with enhanced photocurrent and without degradation of open circuit voltage, Applied Physics Letters 96 (2010) 203507-1–203507-3. [18] Wei-Sheng Liu, Hong-Ming Wu, Yu-Ann Liao, Jen-Inn Chyi, Wen-Yen Chen, Tzu-Min Hsu, High optical property vertically aligned InAs quantum dot structures with GaAsSb overgrown layers, Journal of Crystal Growth 323 (2011) 164–166. [19] J.M. Ulloa, R. Gargallo-Caballero, M. Bozkurt, M. del Moral, A. Guzma´n, P.M. Koenraad, A. Hierro, GaAsSb-capped InAs quantum dots: from enlarged quantum dot height to alloy fluctuations, Physical Review B 81 (2010) 165305-1–165305-7. [20] J.M. Ulloa, I.W.D. Drouzas, P.M. Koenraad, D.J. Mowbray, M.J. Steer, H.Y. Liu, M. Hopkinson, Suppression of InAs/GaAs quantum dot decomposition by the incorporation of a GaAsSb capping layer, Applied Physics Letters 90 (2007) 213105-1–213105-3. [21] Denis Guimard, Masao Nishioka, Shiro Tsukamoto, Yasuhiko Arakawa, High density InAs/GaAs quantum dots with enhanced photoluminescence intensity using antimony surfactant-mediated metal organic chemical vapor deposition, Applied Physics Letters 89 (2006) 183124-1–183124-3. [22] Naokatsu Kouichi Akahane, Naoki Yamamoto, Ohtani, Long-wavelength light emission from InAs quantum dots covered by GaAsSb grown on GaAs substrates, Physica E 21 (2004) 295–299. [23] J.M. Ripalda, D. Granados, Y. Gonza´lez, A.M. Sa´nchez, S.I. Molina, J.M. Garcı´a, Room temperature emission at 1.6 mm from InGaAs quantum dots capped with GaAsSb, Applied Physics Letters 87 (2005) 202108-1–202108-3. [24] Wen-Hao Chang, Yu-An Liao, Wei-Ting Hsu, Ming-Chih Lee, Pei-Chin Chiu, Jen-Inn Chyi, Carrier dynamics of type-II InAs/GaAs quantum dots covered by a thin GaAs1 xSbx layer, Applied Physics Letters 93 (2008) 0331071–033107-3.
W.-S. Liu et al. / Solar Energy Materials & Solar Cells 105 (2012) 237–241
[25] J.S. Ng, H.Y. Liu, M.J. Steer, M. Hopkinson, J.P.R. David, Photoluminescence beyond 1.5 mm from InAs quantum dots, Microelectronics Journal 37 (2006) 1468–1470. [26] T.T. Chen, C.L. Cheng, Y.F. Chen, Unusual optical properties of type-II InAs/ GaAs0.7Sb0.3 quantum dots by photoluminescence studies, Physical Review B 75 (2007) 033310-1–033310-4. [27] S.M. Hubbard, C.D. Cress, C.G. Bailey, R.P. Raffaelle, S.G. Bailey, D.M. Wilt, Effect of strain compensation on quantum dot enhanced GaAs solar cells, Applied Physics Letters 92 (2008) 123512-1–123512-3. [28] R.B. Laghumavarapu, M. El-Emawy, N. Nuntawong, A. Moscho, L.F. Lester, D.L. Huffaker, Improved device performance of InAs/GaAs quantum dot solar cells with GaP strain compensation layers, Applied Physics Letters 91 (2007) 243115-1–243115-3.
241
[29] Yoshitaka Okada, Ryuji Oshima, Ayami Takata, Characteristics of InAs/GaNAs strain-compensated quantum dot solar cell, Journal of Applied Physics 106 (2009) 024306-1–024306-3. [30] R.B. Laghumavarapu, A. Moscho, A. Khoshakhlagh, M. El-Emawy, L.F. Lester, D.L. Huffaker, GaSb/GaAs type II quantum dot solar cells for enhanced infrared spectral response, Applied Physics Letters 90 (2007) 1731251–173125-3. [31] T. Sugaya, S. Furue, H. Komaki, T. Amano, M. Mori, K. Komori, S. Niki, O. Numakami, Y. Okano, Highly stacked and well-aligned In0.4Ga0.6As quantum dot solar cells with In0.2Ga0.8As cap layer, Applied Physics Letters 97 (2010) 183104-1–183104-3.