Electrical characterization of silicon heterojunctions and silicon micro/nanowires for solar cells applications J. Alvarez, J. P. Kleider, L. Yu and P. Roca i Cabarrocas
Heterojunctions based on to a-si:h/c-si Part 1 - Band diagram under illumination : role of interfaces - Illustration of the «HIT» solar cell - Measurement and analysis tools
Band diagram under illumination : role of interfaces E C E C (n) c-si (n) a-si:h E C E F e E F h E V E V (p) a-si:h E V J. P. Kleider : "Band Lineup Theories and the Determination of Band Offsets from Electrical Measurements", in Physics and Technology of Amorphous-Crystalline Heterostructure Silicon Solar Cells, W. van Sark, L. Korte, F. Roca (eds), Springer (2011), ISBN 978-3-642-22274-0
Illustration of the «HIT» solar cell Front Metal grid TCO (p) a-si:h Undoped "i" a-si:h (n) c-si 80 nm < 20 nm a-si:h/c-si solar cells «HIT» (Heterojunctions with Intrinsic Thin layers) : > 20%, V oc > 740 mv - Best efficiencies : 23,7% 100 µm Si wafers, Sanyo, > 21% on 125 cm 2 Si substrate, INES, HETSI project, (cf EPSEC 2011, Hambourg) How to reach high efficiencies? - low volume recombination B >>ms - low interface recombination s eff <<100 cm/s undoped a-si:h (n) a-si:h Back contact contact < 20 nm Fundamental and recurrent questions : - Surface Passivation mechanism? - Role of interface defects? - Band Lineup?
Measurement and analysis tools Quasi-Steady State Photoconductance (QSSPC) Microwave PhotoConductance Decay (µw-pcd) Passivation quality Photoluminescence (PL) Modulated Photoluminescence (MPL) Capacitance spectroscopy (C(V,f,T)) Interface band lineup Conductive Probe Atomic Force Microscopy (CP- AFM) Planar Conductance (G(T)) + Modelling and simulations
CP-AFM (Conductive Probe-AFM) applied to a-si:h/c-si Heterojunctions - Motivation - CP-AFM setup - Sample preparation - Experimental results - Comparison Experimental/Simulation Part 1
Motivation (n) a-si:h/ (p) c-si and (p) a- Si:H/ (n) c-si INTERFACES have been intensively studied by capacitance, planar conductance measurements, and modelling. ALL these studies suggest a STRONG INVERSION LAYER in c-si at the interface. CP-AFM is a powerful current sensing tool capable to probe directly interfaces
CP-AFM setup - B-doped diamond tip : Radius curvature < 50 nm - Pt/Ir tip : Radius curvature < 25 nm DC bias [-10 V, 10 V], 0.01 resolution Local resistance range [10 2 10 12 ]
CP-AFM : Sample preparation - CP-AFM measurements performed on the heterojunction trench. - HF (1%) dip required to minimize surface oxide layer Cleavage Conductive AFM probe ITO a-si:h c-si ITO a-si:h i Bias V J.P. Kleider, J. Alvarez, A.V. Ankudinov, A. S. Gudovskikh, E.V. Gushina, M. Labrune, O. Maslova, W. Favre, M.E. Gueunier-Farret, P. Roca i Cabarrocas, E. I Terukov : "Characterization of silicon heterojunctions for solar cells", Nanoscale Research Letters 6:152 (2011), doi:10.1186/1556-276x-6-152
CP-AFM : Experimental results (1) Topographic image (n) a-si:h (p) c-si Electrical Image Conductive Channel : 2D-electron gas
CP-AFM : Experimental results (2) A B - A conductive channel is observed at the interface (n) a-si:h / (p) c-si which is more conductive than the bulk of c-si Strong inversion layer at the c-si surface. - 1 µm width region corresponding to the depletion region is also observed
CP-AFM : Comparison Experimental and Simulation - Resistivity profile calculations through AFORS-HET simulator confirm the quantitative CP-AFM measurements - Similar calculations on (p)a-si:h/(n)c-si reconfirms the existence of the strong inversion layer.
Silicon Nanowires Part 2 - Motivation -Vertical and horizontal silicon wire growth
Motivation L >1/α L >1/α a-si:h c-si Principle : Decoupling of light absorption and minority carrier collection Advantages : ħω Low production cost: - Lower silicon quality requirement - Low temperature deposition process (a-si:h) Tune wire dimensions 1 to promote absorption >L diff Efficiencies a-si:h/c-sinws based solar cell : Radial pn junction solar cell - 17 % conversion efficiency predicted 1-10.8 % conversion efficiency demonstrated 3 (top-down approach) a-si:h c-si ħω >L diff [1] Kelzenberg et al, 33rd IEEE PVSC (2008) [2] Kelzenberg et al, Nature Mater. 9, 239-244 (2010) [3] Y. Lu and A. Lal, Nano Lett. 10, 4651-4656 (2010) Planar pn junction solar cell
Vertical and horizontal silicon wire growth - N-type SiNWs grown by CVD, Au-catalyzed VLS method, on (100) n++-type silicon substrates. Doping of SiNWs was achieved by adding PH 3 to SiH 4. - After growth, catalysts are removed + dopant activation annealing (RTA,750 C for 5 min). - In-Plane Solid-Liquid-Solid (IPSLS) technique uses In catalyst droplets and a hydrogenated amorphous silicon (a-si:h) layer to grow horizontal SiNWs. Growth activation is done during an annealing process in the range 300-500 C. A-Si:H layer on top of ITO layer pattern 100 µm 2.1µm 440 nm In catalyst drop Region with only a-si:h covering - SiNWs embedded into SOG and planarized CMP S. Perraud et al. / Solar Energy Materials & Solar Cells 93 (2009) 1568 1571 Yu, L. and P. Roca i Cabarrocas, Physical Review B, 2010. 81(8): p. 085323 Yu, L., et al., Physical Review Letters, 2009. 102(12): p. 125501
Vertical SiNWs grown by VLS -CP-AFM analysis -Raman analysis
Electrical characterization of vertical/horizontal Si wires R AFMtip R Wcontact (a,, ) R SiNW R BackC Local resistance R Tot R AFMtip + R Wcontact (a,, ) + R SiNW + R BackC
CP-AFM Analysis (1/3) Topographic image Electrical Image 0 4.2 m 0 12 nm SiNWs brightest spots SiNWs blue spots
CP-AFM Analysis (2/3) H e i g h t p r o f i l e ( n m ) R e s i s t a n c e p r o f i l e ( ) - [PH 3 ]/[SiH 4 ] drops local resistance by several orders of magnitude confirming the phosphorus incorporation. NEXT STEP Evaluation of the Resistivity & Phosphorus concentration
CP-AFM Analysis : Local I-V measurements (3/3) [PH 3 ]/[SiH 4 ] = 0 [PH 3 ]/[SiH 4 ] = 2 10-5 [PH 3 ]/[SiH 4 ] = 2 10-3 R Tot R AFMtip + R Wcontact (a,, ) + R SiNW + R BackC SiNW 1 20-40.cm 2 0.1-1.2.cm 2 0.008-0.016.cm values versus [PH3]/[SiH4] are in good agreement with previous studies which employed 4-point probe measurements on single Si NWs 1-2. [P] = 1-2 10 14 cm -3 [P] = 0.5-7 10 16 cm -3 [P] = 2-6 10 18 cm -3 [1] Eichfeld et al. Nanotechnology 18 (2007) 315201 [2] Wang et al. Nanoletters 5 (2005) 2139
Horizontal SiNWs grown by IPSLS -CP-AFM analysis -Raman analysis
CP-AFM Analysis Si wires a-si:h In plane growth carried by a liquid catalyst drop advancing among a solid precursor. ITO pad Wire diameter and shapes are driven by : - initial hydrogen reduction of ITO - deformation of liquid catalyst drop during growth From the electrical point of view : - Local I-V measurements : n = 1.6 (V < 1 V) charge injection => Space-charge limited current n I V : n = 3 (V > 1 V) indicates trap-limited in SCLC => trap distribution Transport properties are driven by surface states : (i) higher surface/volume ratio and (ii) a-si:h (large DOS and exp. Tails) is the solid precursor used for the growth. Yu, L. and P. Roca i Cabarrocas, Physical Review B, 2010. 81(8): p. 085323 Yu, L., et al., Physical Review Letters, 2009. 102(12): p. 125501
Raman Analysis (1) Solid precursor : a-si:h which is characterized by a large DOS in the gap with exp. tails. Structural quality of Si Wires? Confocal Raman analysis at 532 nm, 100X, pinhole size : 25 µm, P< 1mW, 600 g/mm Map of integrated Raman Signal [476-550 cm -1 ]
Raman Analysis (2) Comparison SiNW Raman signal and c-si wafer (type N, 100, 10 14 cm -3 ). 3.2 cm -1 In plane SiNW 517.2 cm -1 FWHM 12 cm -1 c-si wafer 520.4 cm -1 FWHM 4 cm -1 - In plane SiNW Raman shows a peak located 517 cm -1 with a FWHM 12 cm -1 Open questions : (i) Is tensile stress the origin of the shift? (ii) the wires are not c-si but µc-si:h or nc-si:h?
Conclusion Part 1 & Part 2 CP-AFM reveals itself as a powerful current sensing technique for the analysis of heterointerfaces, electrical transport properties and doping effect on SiNWs. Demonstrated the existence of a strong inversion layer (2D-carrier gas) in the c-si layer for both types of silicon heterojunctions. Evidenced the phosphorus incorporation and estimated the phosphorus concentration in vertical SiNWs. Evidenced of SCLC and trap-limited current transport properties in horizontal SiNWs surface states are the main ingredient controlling the transport properties.
Acknowledgments - M.E. Farret-Gueunier, I. Ngo, E. Blanc, D. Diouf, R. Varache W. Favre, P. Chrétien, O. Maslova (LGEP) - E. Gushina, A.V. Ankudinov, A.S Gudovskikh, E.I. Terukov (Ioffe Institute) - C. Morin, S. Perraud (CEA-Liten) - Symposium Organizers :