MENA9510 characterization course: C-V measurements 27.10.2014
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MENA9510 characterization course: C-V measurements 27.10.2014 Halvard Haug Outline • • • • • • Overview of interesting sample structures Ohmic and schottky contacts Why C-V for solar cells? The MOS capacitor The C-V curve Calculation of Qf Suggested reading • Schroder, D. K. (2006). Semiconductor material and device characterization, Chapter 6. (I have a PDF!) • Principles of Electronic Devices: – http://ecee.colorado.edu/~bart/book/book/toc6.htm Interesting structures: • Ohmic contacts – Necessary for reliable electrical measurements • I-V • Resisitivity • Hall • Rear contact for C-V and G-V – Make good solar cells! • Schottky contacts – Characterization of bulk material • Bulk defect levels (DLTS) • Doping profiling (CV) – (Induced-junction solar cells) Interesting structures: • PN-junctions – Devices (solar cells, duh..) – Rectifying junctions for characterization • DLTS ++ • MIS capacitors – Important component in many devices • Field effect transistors, • Solar cell coating layers – Characterization of insulator, interface and underlying semiconductor Metal-semiconductor contacts Schottky contact Evac Metal qφm > qχsc Ef,s Ef,m n-Si Schottky contact Evac Metal Ef,s Ef,m n-Si Schottky contact Evac Metal Ef,s φB Ef,m --- +++ Neutral n-type n-Si Schottky contact Evac Metal n-Si φB + Ef,m Ef,s - Schottky contact Evac Metal n-Si φB - Ef,m Ef,s + Ohmic contact Evac Metal Ef,s Ef,m n-Si Ohmic contact Evac Metal qφm < qχsc Ef,m Ef,s n-Si Ohmic contact Evac Metal Ef,m Ef,s n-Si Ohmic contact Evac Metal n-Si Ef,m Ef,s Accumulation of electrons near the surface No schottky barrier +++ --- Neutral n-type Metal/silicon barrier heights http://www.cleanroom.byu.edu/ohmic-schottky.phtml Metallization techniques • Deposition – Thermal evaporation – Sputtering – E-beam – Metal CVD – Screen printing – +++ (IFE, MiNaLab) (IFE) (MiNalab) (?) (IFE) How to: ohmic contacts • What is an ideal ohmic contact? – Zero voltage drop – Maintains equilibrium carrier concentrations for all currents – But: Infinite recombination I velocity V How to: ohmic contacts • General – Pick metal with the “right” φM? • Not always a good approach • Practice deviates from theory: – Interface states – Interfacial oxides – Mirror charges • “All semiconductor/metal junctions are rectifying” How to: ohmic contacts • General – In practice: • High surface doping • Tunneling • (Large area) – Note: Surface damage is not a problem, might be beneficial φB n++ n How to: ohmic contacts • Trick for p-type silcon: – Al + heat treatment Current (mA) 25 Al Al/Si 20 15 Si (p++) 10 Si (p) 5 0 – 10 min 450 °C – Tested with evaporated and sputtered contacts – Also works on 50 nm a-Si -1,5 -1 -0,5 0 -5 -10 -15 -20 -25 0,5 1 Voltage (V) 1,5 How to: schottky contact • General – Larger barrier height is better – Important to have an abrupt interface with few surface defects • Remove oxide with HF • Deposit metal carefully under high vacuum and low T • Evaporation probably better than sputtering due to surface damage C-V characterization of MOS structures Traditional main motivation: MOSFETs Metal-Oxide-Semiconductor Field-Effect Transistor: SiO2, tox Gate Source Substrate Drain Our main motivation(?): Solar cell surface layers n++ p Why surface passivation? Wafer quality Cell quality How can we reduce the SRV? SRV depends upon: ns n-Si Ec EF Dit(E) Ev ps How can we reduce the SRV? ns Two main strategies for surface n-Si passivation: Ec Dit(E) ps EF Ev How can we reduce the SRV? ns Two main strategies for surface n-Si passivation: Ec Dit(E) • Chemical passivation ps EF Ev How can we reduce the SRV? ns Two main strategies for surface n-Si passivation: Ec Dit(E) • Chemical passivation ps EF Ev • Field effect passivation How can we reduce the SRV? ns n-Si Ec Dit(E) ps EF Ev • Field effect passivation: How can we reduce the SRV? + Qf + + ns • Field effect passivation: n-Si - Dit(E) ps Ec Fixed charges in passivation layer EF Ev Qf The MOS capacitance πππ πΆ= πππ ππ = − ππ + πππ‘ πππ + ππππ‘ πΆ=− ππππ₯ + πππ ππ = ππ + ππ + ππ πΆ= 1 1 1 + πΆππ₯ πΆπ + πΆπ + πΆπ + πΆππ‘ The MOS capacitance M V=0 O S Negative V ο accumulation M O -V ππ < 0 S Negative V ο accumulation M O -V ππ < 0 S V = 0 ο Flat band conditions M O S V=0 πΏπ·π ππ = 0 Small positive V ο depletion M O S +V π ππ = ππ΅ Large positive V ο inversion (Low frequency) M O S +V ππ > 2ππ΅ Large positive V ο inversion (Low frequency) M O S +V ππ > 2ππ΅ Large positive V ο inversion (High frequency) M O S +V π ππ > 2ππ΅ The C-V curve The flat band voltage VFB M O S The flat band voltage VFB M Δπππ O S The flat band voltage VFB M O S Δπππ ππ + + + The flat band voltage VFB M O S Δπππ ππ - VFB + + + Calculation of Qf from VFB Find VFB from theoretically calculated CFB πΆπΉπ΅ = ππΉπ΅ 1 πΏπ·π 1 + πΆππ₯ πππ ππ΄ππ = Δπππ − πΆππ₯ πΆππ₯ ππ = Δπππ − ππΉπ΅ ππ΄ πΏπ·π = Debye length in Si, dependent on doping ++ Conductance measurements Conductance measurements C-V in real life Thank you for listening!
