Nitride HFETs applications: Conductance DLTS The capacitance DLTS cannot be used for device trap profiling as the capacitance for the gate will be very small Conductance DLTS is similar to capacitance DLTS, but drain current, instead of capacitance, is measured while the gate is being pulsed OR gate voltage fixed and drain voltage pulsed Both E 1 and E 2 trap densities are suppressed The reduction in trap density is pointed out as a possible reason for reduction in current slump V g kept constant at -5 V but drain voltage pulsed from 0.5 V to 2.5 V Effect of SiN x passivation on conductance DLTS spectra of an AlGaN/GaN HFET Slide # 1
Three basic modes: Atomic force microscopy basics Contact mode: Here the operation is in the repulsive force region. Signal sensed is force obtained from cantilever deflection. The force is usually tens to hundreds of nn. The tip is usually a few angstroms away from the sample. Advantage: easy detection, useful for simultaneous scanning capacitance and scanning conductivity modes. Non-contact mode: Here the operation is in the positive gradient of the force-distance curve. Signal sensed is frequency (or amplitude), obtained from oscillation of the cantilever. Advantage: soft samples can be imaged, no lateral force, can be used with surface potential imaging. Intermittent contact (tapping) mode: Similar to non-contact mode. Slide # 2
Atomic force microscopy basics Amplitude IC Curve 2 (near) NC Change in amplitude Curve 1(far) ω 0 ω d Frequency Amplitude vs. frequency curve for the tip at different distances ω 0 = ( 0 k kgradient ) m Contact mode: Repulsive force monitored from cantilever deflection. Non-contact mode: Attractive force gradient is monitored from frequency shifts Slide # 3
Measurement technique AMPLITUDE DETECTOR OSCILLATOR LASER LOCK-IN CIRCUITRY CONTROLLER V ac sinωt V dc Z SCANNER SAMPLE POSITION DETECTOR CONTROLLER V dc SURFACE POTENTIAL IMAGE MORPHOLOGY IMAGE Two feedback loops, one (red) for AFM and other (blue) for Kelvin probe Slide # 4
Important result from AFM measurements RMS roughness R Surface morphology Useful for determining the growth method Gives information about surface defect structures Dislocation density rms Usually bigger pits only (pure screw and mixed screw-edge) are imaged With sharp tip smaller edge dislocations can also be imaged sometimes N i= 1 z i : the coordinate of the surface, value, N : total number of data points. = ( z z) N z i 2 : the mean Dislocations pinning 2 steps MOCVD grown GaN Dislocations pinning 2 steps (spirals) MBE grown GaN on MOCVD templates Slide # 5
Surf. Potential Scanning Kelvin probe characterization 10 nm /Div Morphology 0.1 V /Div Potential Dislocations create midgap acceptor states Dislocations are negatively charged Slide # 6
Scanning Capacitance characterization The scanning capacitance image is obtained by applying a dc bias to the sample and using another high frequency ac to measure the change in capacitance using resonant circuit detection The scanning capacitance image is proportional to the slope of the CV curve dc/dv at a particular dc bias The capacitance scan superimposed on the surface morphology scan V = V FB = -Q/C ox. Since the voltage shift is positive, so the charge at the dislocations must be negative (a) CV curve away from the dislocation, and (b) CV curve near a dislocation Slide # 7
TEM characterization TEM resolution ~1-2 Å Image obtained from transmitted beam of elastically scattered electrons Sample must be thin for proper imaging Slide # 8
TEM images of Nitride structures Dislocations Contact formation on AlGaN layer Slide # 9
TEM images Bright field: formed from direct beam Dark field: formed from the diffracted beams Diffraction Pattern (Fourier Transform of Specimen) Slide # 10