CPD and other imaging technics for gas sensor Mizsei, János 18-28/05/2006 Ustron Budapest University of Technology and Economics, Department of Electron Devices

Outline Introduction: potentials in general Do we really need contacts ?Ideal (static) voltmeter. Do we really need contacts ? What is it for ? Applications... …extension of the application (x-y scanning, higher resolution, (Kelvin Force microscopy) etc... Summary

Introduction: the potential working ability of a point charge in r the electric field: force on the charge it is a general „boundary condition” in the electronics electrochemical potential: advantages: it can be easily measured in a broad range, excellent for characterisation of physical systems:

VOLTMETERS ”Handy” voltmeters: 20 M  Electrometers: (?) (electron tubes, FET) Compensation: voltage measurements without current …without current. Do we really need contacts ?

The ideal VOLTMETER: R in =  How can we do that in practice? Capacitive coupling + compensation: „Vibrating reed voltmeter”

”Vibrating reed” voltmeter: static capacitive coupling: it is not applicable to transfer the information about DC voltage (except: MOS FET)static capacitive coupling: it is not applicable to transfer the information about DC voltage (except: MOS FET) solution: non-static (vibrating) capacitorsolution: non-static (vibrating) capacitor R=  Phase sensitive frquency selective current detector He

What else? Potential directly from the surface, without contact. Phase sensitive frequency selective current detector

The CPD: zero electric field between the plates ! CPD compensated: Lower work function (electron emission, positive surface charge) Higher work function (negative charge on the surface A B

Current to be detected: Capacitance: Charge:

Up to date equipment: frequency selective amplifying, phase sensitive (multiply) demodulation feedback of the DC voltage (automatic compensation) optical excitation for surface photovoltage measurements digital realisation second harmonics detection and feedback for distance control surface mapping (x-y scan).

The “ideal” energy diagram of a vibrating capacitor - semiconductor system Ideal: no surface traps around the Fermi-level, the surface index S=1 dark (equilibrium) light (non-equilibrium)

Additional light excitation: FB state Gas sensor layer

Vibrating capacitor (Kelvin) and SPV (surface photovoltage) method

V V cpd Vibration due to voltage on the tip: …stops when !!! Kelvin Force Microscopy: AFM + Kelvin

Semiconductors in gas sensitive structures

Behaviour of the semiconductor gas sensor materials Behaviour of the semiconductor gas sensor materials Experimentally observed change in the work function and change in the resistance (logarithmic scale) as function of partial pressure (root scale) 

Behaviour of the semiconductor gas sensor materials Behaviour of the semiconductor gas sensor materials Experimentally observed correlation between the work function  V K and change in the resistance (logarithmic scale, which shows  V R linearly)

Potential shift (change in the CPD) due to chemical signal: dipole adsorption on the semiconductor surface dipole adsorption on the reference electrode charged particles (ions) on the semiconductor surface change in the bulk defect (donor, acceptor) concentration due to diffusion of the adsorbed atoms change in the composition (stoichiometry) of the semiconductor materials

Semiconductor resistance/conductance response due to chemical signal: dipole adsorption on the semiconductor surface charged particles (ions) on the semiconductor surface change in the bulk defect (donor, acceptor) concentration due to diffusion of the adsorbed atoms change in the composition (stoichiometry) of the semiconductor materials dipole adsorption on the reference electrode NO YES, if the surface charge is balanced by the space charge layer in the semiconductor YES, usually at higher temperature NO

Non-ideal system Large number of surface traps: Fermi-level pinning, the surface index S<1 Charged particles (ions) on the semiconductor surface: counterpart of the charge is localized to the surface charged particles form dipole layer: CPD response: YES potential barrier, space charge, resistance response: NO (usually at lower temperature)

Activated semiconductor gas sensor surface High number of Q ss Change of the charge in the surface/interface states (Q ss ) instead of the space charge layer in the semiconductor. no conductivity response.

ALE SnO 2 layers: CPD and resistance maps 33K 30K 46K K 48K 133K K 71K K K  M 182 Chemical pictures by vibrating capacitor

Selective chemical sensing with potential mapping  360K  460K Material gradient Temperature gradient PdAgAuPtV SnO 2

Chemical pictures (surface: Pd-Ag-Au-Pt-V-Pt-SnO 2 ) PdAgAuPtV SnO 2 C 30mm 25mm 1% H 2 –in airNH 4 OH vapour (NH 3 ) CHCl 3 vapour C 2 H 5 OH vapour  460K  360K Volt Pixel

Porous silicon-p + Si as gas sensor material Extremely high amount of + charge in the porous Si Light excitation

The charge balance: from vibrating capacitor (dark- light) or from the SPV (the saturated SPV signal is proportional with the potential barrier)

Kelvin maps -= SPV map Process time/s in 1.5/3.5 HF/C 2 H 5 OH mixture with 50 mA/cm 2 current density (growth rate is ~ micron/s) P ohmcm Si inversion

Surface conditions: thick and ultrathin oxide covered Si  VV Q SS >0 Porous silicon on p + : (strong) depletion

Atomic Force and Kelvin Force Microscopy: charged surface V/ m Atomic Force: oxide step Kelvin Force: surface potential

AFM and Kelvin Force Microscopy Morphology potential distribution Semiconductor (WO 3 ) gas sensor nanograins

Summary Vibrating capacitor method included the high resolution version (Kelvin Force Microscopy)Vibrating capacitor method included the high resolution version (Kelvin Force Microscopy) Examples: analytical tool and sensor (chemical signal converter)Examples: analytical tool and sensor (chemical signal converter) Conclusion: a lot of useful application possibilitiesConclusion: a lot of useful application possibilities