Bias Voltage Controlled System

The first STM experiments were performed under UHV conditions, and so the bias potential was simply applied as a difference across the tip and sample. However, introducing an electrolyte above the sample brought with it some particular problems. It is no longer sufficient simply to apply a bias voltage equal to the potential difference between tip and sample as this means that the potentials of the tip and sample are undefined with respect to any fixed reference, a wholly undesirable situation. Consequently, modern electrochemical STM systems operate under bipotentiostatic control with the tip and sample controlled and monitored independently with respect to the reference electrode. The bias potential is then still given by (Fs — FT), but VT and Fs are now potentials with respect to the reference electrode. 

When the E-T detector is used for compositional contrast, the Faraday cage should be negatively charged (-50 V) to exclude SEs with energies lower than the bias voltage of the cage. Some SEM systems are equipped with a dedicated detector for BSE collection, which is placed above the specimen. To obtain compositional contrast, we can switch detection from the SE mode to the BSE mode by simply pushing a button on the SEM control panel. 

Here u(f) is the inhibitor and a x, t) is the activator variable. In the semiconductor context u t) denotes the voltage drop across the device and a(x, t) is the electron density in the quantum well. The nonlinear, nonmonotonic function /(a, u) describes the balance of the incoming and outgoing current densities of the quantum well, and D(a) is an effective, electron density dependent transverse diffusion coefficient. The local current density in the device is j a, u) = (/(a, u) + 2a), and J = j jdx is associated with the global current. Eq. (5.22) represents Kirchhoff s law of the circuit (5.3) in which the device is operated. The external bias voltage Uq, the dimensionless load resistance r R, and the time-scale ratio e = RhC/ra (where C is the capacitance of the circuit and Ta is the tunneling time) act as control parameters. The one-dimensional spatial coordinate x corresponds to the direction transverse to the current flow. We consider a system of... 

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