Electrical Characterisation of TCO thin films (method of four coefficients). Eric Don, SemiMetrics Ltd. Functional Thin Films 4 th Vacuum Symposium Thursday 17 th October 2013
Agenda TCO Basics TCO Applications TCO in-line quick comment TCO off-line characterisation Results correlation with NREL Questions
Transparent Conducting Films - Basics Currently TCO s rule Wide band gap ~ 3eV Degenerate semiconductor 1 TCO window of transmission 1 Reasonable mobility 0.8 0.8 SnO is a good example ITO electrically even better Except for the indium cost Hence the search for new Transparent conducting films 0.6 0.4 0.2 0 Reflection IR Transmission 0 11 E wp E gap Photon energy (ev) Absorption UV 0.6 0.4 0.2 0 σ = neμ Impact on the transmission window
TCO s many consumer applications. OLED FET backplane now IGZO Defrost Windows > 80% total power is for the display (I max = 1.5 A,4 x 3 ) LCD
TCO low-emissive windows Electrical Properties Percent Transmission / Reflection Percent 100 100x10 6 (µ = 10) Ideal Cold Climate Window 80 80 Cold Climate 60 40 20 AM1.5 350 K Blackbody Radiation 60 40 20 Blackbody Radiancy (W/m 3 ) 0 3 4 5 6 7 8 9 10 3 2 3 4 5 6 7 8 9 Wavelength (nm) 10 4 2 3 0 100 6 100x10 (µ = 100) Ideal Warm CIimate Window, 80 80 Warm Climate 60 40 20 AM1.5 350 K Blackbody Radiation Blackbody Radiancy (W/m 3 ) 60 40 20 0 3 4 5 6 7 8 9 1.0 2 3 4 5 6 7 8 9 Wavelength (m m) 10 2 3 0
Percent Reflection Thin Film Solar Cells 100 80 60 40 20 AM 1.5 Ideal TCO µ=100, n = 1.9 x 10 21 CTO µ = 62, n = 6.8 x 10 20 (CdTe cell) ZnO µ = 22, n = 4.7 x 10 20 (1.0 ev CIS cell) Glass TCO CdS CdTe Back Contact First Solar Inc. 1GWatt peak in one year 0 500 1000 1500 2000 2500 Wavelength (nm) Ideal Single Bandgap cell = 1.5 ev 3000 In context: 28 miles 2 windows were installed, in one year (U.S. & Europe).
In-line Electrical resistivity measurement Option eddy current µ Eddy current (non-contact) An AC current flows in a coil. The magnetic field of the coil induces circulating (eddy) currents in the sample. The eddy current measurement is actually the measurement of the electrical loss in the material, which depends on the resistivity. Four Point Probe (Contact) Measuring resistivity and sheet resistance according to SEMI MF84-0307 and SEMI MF374-0307 standards. Probe wear film damage. Wafer 7
In-line resistivity & mobility control. Optical method-ir reflection (non-contact) Hyperspectral camera Line light source Calculation processor Roll-to-roll material Computer Manufacturing equipment Additional opaque layers or other further processing
Off-line Electrical Characterization We need to go even deeper than mobility m = Velocity per unit electric field m*, effective mass Intrinsic Parameter Determined by Band/Crystal structure m qt m * t, relaxation time for carriers - Extrinsic Parameter Crystal defects Impurity scattering Grain size Grain barrier.
Relaxation Time Approximation Boltzmann s equation may be solved by approximation t a( T )E s Degenerate, parabolic band materials. t is the mean time between redistribution of carriers t can be a function of temperature and energy Then by plotting temp dependence of m ; m t T - 1 E - 1 2 Acoustic phonon scattering: Neutral impurity scattering: Ionized impurity scattering: m t T b E 0 m t T 0 E 3 2 b > 0
Method of four coefficients M.K. Zhitinskaya et al. 66 Measurement of Conductivity (s), Hall (Rh), Seebeck (α ) and Nernst (Q) coefficients gives m*, m * 3 n 2 3 q h 2 d p k B 2 T a - Q R h s t m m* q m R h s Four coefficients (α, Rh, s, Q) give m* d and s. m* d measurement independent of l (the non-parabolicity)
Transport phenomena measurement on thin-films Conductivity (van der Pauw) Hall effect V V B z Seebeck effect V T Nernst effect V B z
Schematic of 4C System Current Source Switch Matrix Voltmeter Cryostat Sample Electromagnet Computer GPIB Bus Line Temperature Controllers (2) Bipolar DC Power Supply Windows TM 4C Application Software
Four coefficients Next M.K. Zhitinskaya et al. 66 Measurement of Conductivity, Hall, Seebeck and Nernst coefficients gives s and band non-parabolicity s 3 2 Q R s a - Q Rs l Band non-parabolicity term * dm d l n m d * dn
Correlation with NREL Sample ID Temp Thickness Resistivity Hall Coeff Seebeck Coeff Nernst Coeff (K) (nm) (Ohm.cm) (m^3/c) (uv/k) (uv/k) H3522 UNN A 308 1.50E+03 2.81E-03 7.60E-08-1.12E+02-9.16E-03 H3522 NREL 295 1.50E+03 2.81E-03 7.14E-08-1.34E+02-3.70E-02 System= 22(uV/K) Sample ID Temp Thickness Doping Mobility DOS mass Tau (s) (K) (nm) (cm^-3) (cm^2/vs) (m/me) H3522 UNN A 308 1.50E+03-8.22E+19 2.71E+01 7.02E-01 1.08E-14 H3522 NREL 295 1.50E+03-8.74E+19 2.51E+01 9.76E-01 1.41E-14
Conclusion The method of four coefficients is an ideal experimental technique for transparent conductive oxides (TCO) and other low mobility samples such as semiconductor or metal thin films but equally can be used for single crystal or epitaxial thin film semiconductors. Four transport phenomena measurements: Conductivity Hall Seebeck Nernst n, µ m*, s, E Fermi
SemiMetrics 4C System is based upon work done at NREL and Colorado School of Mines (CSM) Colorado USA. We specifically acknowledge: D. L. Young, NREL, CSM V. I. Kaydanov, (CSM)