Microcantilevers III Cantilever based sensors: 1 The cantilever based sensors can be classified into three groups (i)General detection of any short range forces: Scanning probe microscopy (ii)Detection of mass attachment: based on changes in frequency response (iii)Detection of molecular adsorption: based on changes in surface stress of a functionalized cantilever. This can also be detected based on resonance amplitude change. (iv)Detection of radiation (IR or nuclear): based on deflection of a bimaterial cantilever, or from deflection changes (v)Detection of charges or electric or magnetic fields: Based on resonance frequency or amplitude changes Note that although cantilevers are highly sensitive to stress changes, but resonance frequency based changes are the most sensitive due to quality factor enhancements.

Goutam Koley Kelvin probe technique Kelvin probe technique measures surface charge, surface potential, and surface work function. The advantages are quantitative nature, and ease of operation.  Si Eg,Si p + Silicon Ec Ev,EF, Si  semi EF,se mi qsqs qVcon = qVdc =  Si + Eg,Si - (  semi + q  s) Probe TipSemiconductor Sample Evac EC qVcon G. Koley and M. G. Spencer, J. Appl. Phys. 90, 337 (2001)

Goutam Koley Mathematical Model for SKPM G. Koley and M. G. Spencer, J. Appl. Phys. 90, 337 (2001)

Goutam Koley SKPM Measurement System LOCK-IN CIRCUITRY CONTROLLER SURFACE POTENTIAL IMAGE MORPHOLOGY IMAGE CONTROLLER AMPLITUDE DETECTOR POSITION DETECTOR LASER SCANNER OSCILLATOR Vacsin  t Vdc SAMPLE Z G. Koley and M. G. Spencer J. Appl. Phys. 90, 337 (2001)

Goutam Koley Electronic characterization of dislocations MorphologyPotential 0.1 V /Div 10 nm /Div Surf. Potential G. Koley and M. G. Spencer, Appl. Phys. Lett. 78, 2873 (2001)

Goutam Koley Surface potential patterning using mask UV light 20  m circle quartz mask HFET Sample (35% Al in barrier, 44 nm AlGaN layer)

Goutam Koley Spatial resolution of charge storage UV exposure through a mask of 1, 2, 5, 10 and 20  m squares Spatial resolution on the order of ~1-2  m 3.5  m G. Koley et al. JAP (2004)

Goutam Koley Measurements in GaN based transistors AFM scanning probe Biasing Probes

Goutam Koley Surface morphology and potential profiles in dc biased transistors Gate SourceDrain MorphologySurface Potential Vd = 2V, Vg = -1.5 V

Goutam Koley The Cantilever effect Smaller R results in larger measurement accuracy Goutam KoleyG. Koley et al. Appl. Phys. Lett. 79, 545 (2001)

Goutam Koley Measurement of transients +ve dc bias Probe tip -ve dc bias or square pulse Measurement setup schematic AA 20  resistor AFM scanning probe Biasing Probes Source Gate Drai n Gate Drai n Source Goutam KoleyG. Koley et al. IEEE Trans. Electron Dev. 50, 886 (2003)

Goutam Koley Drain current variations due to stress Type II: continuous stress at Vd = 20V, Vg = -12 V for 2 mins 100  m HFET Vd = 1 V, Vg = 0 V during measurement. Type I: pulsed stress at Vd = 5V, Vg = pulsed from 0 to -10 V Goutam KoleyG. Koley et al. IEEE Trans. Electron Dev. 50, 886 (2003)

Goutam Koley Correlation between drain current and surface potential Device stressed at Vd = 20 V, Vg = -12 V for 2 mins Surface potential Drain current 100  m HFET Vd = 1 V, Vg = 0 V during measurement. Goutam KoleyG. Koley et al. IEEE Trans. Electron Dev. 50, 886 (2003)

Goutam Koley Stressed at V g = -12V, V d = 20 V for 2 mins Maximum variation observed ~0.3  m from the gate edge Charges take a long time to reach equilibrium value Potential variation with distance and time 150  m HFET Goutam KoleyG. Koley et al. IEEE Trans. Electron Dev. 50, 886 (2003)

Goutam Koley Surface conductivity measurements (a) Morphology, (b) conductivity, and (c) overlap of the surface morphology and conductivity images

Goutam Koley Scanned gate microscopy Scanned gate microscopy is useful to determine the variation of conductivity along a thin channel, and where direct measurement of conductivity is difficult (a) Experimental set up for SGM, (c) the SGM image of a single-walled CNT bundle for V tip = 1 V; Black corresponds to very high resistance Refer to handouts for scanning thermal microscopy

Goutam Koley Scanning capacitance microscopy Scanning capacitance technique actually measures the dC/dV signal which is inversely proportional to doping. The advantages of this technique include a large measurement range (10 15 – cm -3 ), and resolution of <10 nm For capacitance measurement a low frequency ac voltage is applied to the sample. The ac voltage periodically changes the tip-sample capacitance. The sensor produces a high frequency signal to measure very small capacitance changes.

Goutam Koley Application of capacitance microscopy Cross-sectional measurement in a MOSFET under actual operation

Goutam Koley Applications to GaN samples The dC/dV decreases around the dislocations indicating the reduction in the background carrier concentration Morphology image Capacitance image C-V curve