MENA9510 characterization course: Capacitance-voltage (CV) measurements 30.10.2017 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 Q f
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 Make good solar cells! Hall Rear contact for C-V and G-V 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, doping profiling 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 Metal E vac n-si qφ m > qχ sc E f,s E f,m
Schottky contact Metal E vac n-si E f,s E f,m
Schottky contact Metal E vac n-si φ B E f,s E f,m - - - + + + Neutral n-type
Schottky contact Metal E vac n-si φ B + E f,m Ef,s -
Schottky contact Metal E vac n-si φ B - E f,m Ef,s +
Ohmic contact Metal E vac n-si E f,s E f,m
Ohmic contact Metal E vac n-si qφ m < qχ sc E f,m E f,s
Ohmic contact Metal E vac n-si E f,m E f,s
Ohmic contact Metal E vac n-si E f,m E f,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
Deposition Metallization techniques Thermal evaporation (IFE, MiNaLab) Sputtering (IFE) E-beam (MiNalab) Metal CVD (?) Screen printing (IFE) +++
How to: ohmic contacts What is an ideal ohmic contact? Zero voltage drop Maintains equilibrium carrier concentrations for all currents But: Infinite recombination velocity I V
General How to: ohmic contacts Pick metal with the right φ M? Not always a good approach Practice deviates from theory: Interface states Interfacial oxides Mirror charges
How to: ohmic contacts General In practice: High surface doping Tunneling (Large area) φ B n++ n Note: Surface damage is not a problem, might be beneficial
Current (ma) How to: ohmic contacts Trick for p-type silcon: Al + heat treatment Al Al/Si Si (p++) Si (p) 25 20 15 10 5 0-1,5-1 -0,5 0 0,5 1 1,5-5 Voltage (V) 10 min 450 C -10-15 -20-25
General How to: schottky contact 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
Doping concentration measurements using C-V
Doping concentration measurements using C-V Capacitance of a PN junction:
C-V characterization of MOS structures
Traditional main motivation: MOSFETs Metal-Oxide-Semiconductor Field-Effect Transistor: Gate SiO 2, t ox Source Drain Substrate
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: n s n-si E c D it (E) E F E v p s
How can we reduce the SRV? n s Two main strategies for surface n-si passivation: E c D it (E) E F E v p s
How can we reduce the SRV? n s Two main strategies for surface n-si passivation: E c D it (E) Chemical passivation E F E v p s
How can we reduce the SRV? n s Two main strategies for surface n-si passivation: E c D it (E) Chemical passivation p s E F E v Field effect passivation
How can we reduce the SRV? n s n-si E c D it (E) p s E F E v Field effect passivation:
How can we reduce the SRV? Q f + + + D it (E) n s p s Field effect passivation: n-si E c - Fixed charges in passivation layer E F E v Q f
The MOS capacitance C = dq g dv g C = dq s + dq it dv ox + dφ s Q g = Q s + Q it Q s = Q n + Q p + Q b C = 1 C ox + 1 1 C p + C b + C n + C it
The MOS capacitance M O S V = 0
Negative V accumulation M O S - V ψ s < 0
Negative V accumulation M O S - V ψ s < 0
V = 0 Flat band conditions M O S V = 0 L Di ψ s = 0
Small positive V depletion M O S + V ψ s = ψ B W
Large positive V inversion (Low frequency) M O S + V ψ s > 2ψ B
Large positive V inversion (Low frequency) M O S + V ψ s > 2ψ B
Large positive V inversion (High frequency) M O S + V W ψ s > 2ψ B
The C-V curve
The flat band voltage V FB M O S
The flat band voltage V FB M O S Δφ ms
The flat band voltage V FB M O S Δφ ms Q f + + +
The flat band voltage V FB M O S Δφ ms - V FB Qf + + +
Calculation of Q f from V FB Find V FB from theoretically calculated C FB C FB = 1 1 + L Di C ox ε Si L Di = Debye length in Si, dependent on doping ++ V FB = Δφ ms qaq f C ox Q f = C ox qa Δφ ms V FB
Conductance measurements
Conductance measurements
C-V in real life
Thank you for listening!