GaAs multilayer solar cells

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ScienceDirect Solar Energy 132 (2016) 587–590 www.elsevier.com/locate/solener

Brief Note

Defect distribution in InGaAsN/GaAs multilayer solar cells A. Kosa a,⇑, L. Stuchlikova a, L. Harmatha a, M. Mikolasek a, J. Kovac a, B. Sciana b, W. Dawidowski b, D. Radziewicz b, M. Tlaczala b a

Institute of Electronics and Photonics, Faculty of Electrical Engineering and Information Technology, Slovak University of Technology, Bratislava 81219, Slovakia b Division of Microelectronics and Nanotechnology, Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology, Wroclaw 50372, Poland Received 7 December 2015; received in revised form 3 March 2016; accepted 17 March 2016 Available online 11 April 2016 Communicated by: Associate Editor Bibek Bandyopadhyay

Abstract Deep Level Transient Fourier Spectroscopy (DLTFS) experiments were realized to study emission and capture processes in InGaAsN multilayer solar cells grown on GaAs substrates by Atmospheric Pressure Metal Organic Vapor Phase Epitaxy (APMOVPE). As a referent structure for comparison purposes a basic GaAs p–n sample grown in the same system was also utilized. All the structures exhibited variety of deep energy levels with high concentrations. In addition to the most commonly described arsenic antisite defect, with activation energies 0.73–0.78 eV, possible traces of oxygen–arsenic vacancies with 0.52 eV and nitrogen interstitial complexes were evaluated. Most dominant electron trap at about 0.53 eV below the conduction band EC was observed at different measurement conditions. Based on various references, this electron trap can be associated with a split interstitial defect containing two nitrogen atoms on the same As lattice site. Calculated energies and possible origins of these results were confirmed by Arrhenius curve comparison. Ó 2016 Elsevier Ltd. All rights reserved.

Keywords: Deep Level Transient Fourier Spectroscopy; InGaAsN/GaAs; Tandem solar cell; Semiconductor defect

Solar energy is one of the many energy forms harnessed by humanity in order to produce electricity in an environmental friendly and efficient way. Today it is reliable, long-lasting and pollution free alternative of electricity generation, which has rapidly grown with high importance in the last decade (Lee et al., 2015). In order to maintain reliability and low cost not only the market leading silicon solar cells, but also novel materials such as organic or hybrid organic–inorganic materials are extensively studied (Bella, 2015a, 2015b). III–V compound based multi-junction solar cells have the potential for achieving high conversion efficiencies and are promising for space ⇑ Corresponding author.

E-mail address: [email protected] (A. Kosa). http://dx.doi.org/10.1016/j.solener.2016.03.057 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.

and terrestrial applications (Yamaguchi et al., 2005, 2008). Dilute-nitride InGaAsN based solar cells lattice matched to GaAs were demonstrated as working prototypes since 1998 (Friedman et al., 1998). Sub cells with adequate performances for 3 junction cells were reported in 2007 (Jackrel et al., 2006). World record efficiency 43.5% has been achieved by multijunction solar cells employing GaInNAsSb junctions (Sabnis et al., 2012; Kim et al., 2014). New approaches for InGaAsN based structures are continuously studied to realize 50% efficiency by 4- or 5-junction solar cells lattice matched to GaAs and Ge substrates (Yamaguchi et al., 2012a). Application of this promising material however requires the understanding of electrically active defect generation and effects on device performance. Investigation must be

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correlated also with the growth process and conditions in order to develop the fabrication. The aim of the presented study is to investigate the emission and capture processes on solar cell structures based on GaAs and InGaAsN and to evaluate the defect distribution according to correlating experimental results. The investigated multilayer (tandem) InGaAsN solar cell samples were grown by APMOVPE (Sciana et al., 2012). These structures consisted of two p–i–n junctions sandwiched between a n type Si-doped GaAs substrate with doping concentration n = 1–2  1018 cm 3 and a 50 nm thick Zn-doped cap layer. Light falling on the p–i– n structures, optimized for different wavelengths of the solar spectrum, is absorbed by the intrinsic i layer where electron–hole pairs are generated. The top GaAs p–i–n with thicknesses 200/800/200 nm was connected with the bottom InyGa1 yAs1 xNx p–i–n (y = 5.8%, x = 0.5%) 50/250/200 nm via a tunnel junction. The inter layers connecting the p–i–n junctions included a n+ doped GaAs 100 nm thick layer, with concentration of n = 1– 2  1018 cm 3, moreover a 15 nm thick n++ InyGa1 yAs y = 4.7% n = 1–2  1019 cm 3, and a p++ InyGa1 y As y = 4.7% n = 2  1019 cm 3 layer (Fig. 1). These are allowing the recombination of holes coming from the top p–i–n with electrons coming from the bottom. As dopant sources silane (20 ppm of SiH4 in H2) and diethylzinc (DEZn – Zn (C2H5)2) were used for n-type and p-type dopants respectively. High purity hydrogen was employed as a carrier gas. Trimethylgallium (TMGa), trimethylindium (TMIn), tertiarybutylhydrazine (TBHy) and arsine (AsH3: 10% mixture in H2) were applied as growth precursors (Dawidowski

et al., 2014). Main difference between samples Tandem A and Tandem B (Fig. 1) is the value of hydrogen flow rate through a gallium source during the deposition of InGaAsN layers. In the case of B the H2 flow rate through TMGa precursor was 10 ml/min while for A it was 7 ml/ min. When comparing two structures it is necessary to point out the equality of layer widths and dissimilarities in doping concentrations. In overall view sample A had higher doping concentrations, thus in means of trap concentrations should have a more significant reflection on DLTFS responses. Metallization was prepared for DLTFS experiments by Au contacts on the top side of the structures. DLTFS investigations were realized by the measurement system BioRad DL 8000 in temperature range from 80 K to 550 K. Lower temperature states were established by liquid nitrogen cryostat system. Wide range of measurement conditions were tested, but only the most reliable data are shown. Comparison of measured results included one referent p–n structure (p–n A), and two multilayer InGaAsN solar cell samples (Tandem A, Tandem B). DLTFS investigations at various measurement conditions and comparison of calculated Arrhenius curves revealed 6

Fig. 2. Compared DLTFS spectra of investigated basic GaAs p–n and InGaAsN multilayer solar cell samples.

Fig. 1. Epitaxial structures of the investigated multilayer InGaAsN/GaAs tandem solar cell samples highlighting differing parameters.

Fig. 3. Compared DLTFS spectra of InGaAsN multilayer solar cell samples.

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Fig. 4. Calculated Arrhenius curves of investigated basic GaAs p–n and InGaAsN multilayer solar cell samples. Comparison of referent (lines) and calculated (symbols) data.

Table 1 Calculated parameters of evaluated deep energy levels with possible referent data.

T1 T2 T3 T4 T5 T6

DET (eV)

rT (cm2)

0.48 0.52 0.47 0.70 0.36 0.52

3.16  10 2.03  10 2.44  10 1.13  10 8.32  10 4.39  10

14 14 16 14 17 17

deep energy levels. Since in the case of p–i–n structures depletion region is located at the p/i and i/n interfaces the carrier assignment is not really obvious. However assuming the convention, all the positive peaks were defined as majority responses. Different approaches and evaluation procedures were taken into account to state the proper type of defects. However interaction between majority and minority responses in samples with an intrinsic layer made difficult to state the defect parameters (Tompuu et al., 2010; Bouabdallah et al., 2009). Four GaAs impurities were stated as majority traps by the evaluation of the referent sample p–n A (Fig. 2). A broad peak measured from 240 K to 400 K (Fig. 2, black rectangles) was with high probability a result of a complex response. Multi level separation method showed traces of an arsenic vacancy T1 (M4 0.499 eV Eisen et al., 1992), oxygen placed in an arsenic vacancy T2, (EL3 0.594 eV Masse et al., 1989) and probable presence of nickel T3 (EC2 0.480 eV Partin, 1979). Most commonly described arsenic antisite defect T4 (EL2 0.780 eV Hardalov et al., 1992, EX2 0.730 eV Masse et al., 1989) was also observed at about 440 K. All these levels were confirmed also by corresponding reference curves (Fig. 3) (see Fig. 4). Tandem solar cell samples A and B also exhibited traces of these defects, where similarities of Arrhenius curve temperature positions were evident (Fig. 3. hexagons and triangles). In each case evaluated points were in good agreement with the reference data. Results mostly correlated with T1 and T3 but in the case of T5 at 300 K and T6 at around 400 K lower Arrhenius curve slopes were observed. Trap T1 was only observed in the referent sample and in Tandem A. This indicated the attribution of In/N to the evaluated defect complexes. Activation energies T5 0.36 eV and

NT (cm-3)

Origin

1.31  1013 1.93  1013 1.04  1013 5.63  1012 1.11  1015 5.01  1015

VAs M4 0.49 eV (Eisen et al., 1992) OAs EL3 0.59 eV (Masse et al., 1989) Ni EC2 0.48 eV (Partin, 1979) AsGa EL2 0.78 eV (Hardalov et al., 1992) (N-As)As 0.33 eV (Yamaguchi et al., 2012b) (N-N)As 0.57 eV (Krispin et al., 2003)

T6 0.52 eV were stated and were associated with nitrogen induced defect complexes, thus nitrogen split interstititals on a As site (N-As)As 0.33 eV (Yamaguchi et al., 2012b) and (N-N)As 0.57 eV (Krispin et al., 2003) (Table 1.). Moreover T6 exhibited the strongest signal in sample Tandem A and was evaluated as a complex defect state. Several points of the Arrhenius curve correlated with T4, thus the presence of T4 in this sample was also assumed. Most common defect response in all samples was evaluated as presence of nickel, thus level T3. It is probable, that Ni was unintentionally introduced during the growth or metallization process, however the source is unknown. In overall view, similar correlating DLTFS curve characters were measured at around 300 K in all samples, where with high probability GaAs defects were also involved. Most of these responses are related to an oxygen contaminated growth process. Sample Tandem A exhibited more significant trap concentrations, influencing also typical cell characteristics obtained by I–V measurements. Conversion efficiency g = 1.21 % was stated in comparison with g = 3.27 % of Tandem B. It is important to notice, that low efficiencies were obtained due to low transmission properties of the top contacts, since the structures were optimalized for DLTFS experiments. According to the above stated results we can assume that sample Tandem B is more suitable for efficient solar cell structures.

Acknowledgments This work has been supported by the Scientific Grant Agency of the Ministry of Education of the Slovak Republic (Projects VEGA 1/0377/13, VEGA 1/0651/16 and

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VEGA 1/0739/16). This work was supported by Wroclaw University of Technology statutory grants. References Bella, F., 2015a. Electrochim. Acta 175, 151–161. Bella, F., 2015b. Chem. Soc. Rev. 44 (11), 3431–3473. Bouabdallah, B., Bourezig, Y., Brahimi, R., 2009. J. Mater. Process. Technol. 209, 1495. Dawidowski, W. et al., 2014. J. Electron. Telecommun. 60, 151. Eisen, J. et al., 1992. J. Appl. Phys. 72, 5593. Friedman, D.J. et al., 1998. J. Cryst. Growth 195, 409–415. Hardalov, Ch. et al., 1992. J. Appl. Phys. 71, 2270. Jackrel, D. et al., 2006. Photovolt. Energy Convers. 1, 783. Kim, Y. et al., 2014. Sol. Energy 102, 126–130.

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