Introduction to Nanotechnology

1
pair section
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9.6. APPLICATIONS 249
~ c , L ,
3 4 T T
Figure 9.1 9. Linear array of A u ligand-stabilized ~ ~ nanoparticles with interparticle resistance RT,
interparticle capacitance C, and self-capacitance Go. The single-electron current density jy
entering from the right, which tunnels from particle to particle along the line, is indicated. [From
V. Gasparian et al., in Nalwa (2000), Vol. 2, Chapter 11, p. 550.1
photodetectors. Sketches of four types of these detectors are presented in Fig. 9.20.
The conduction band is shown at or near the top of these figures, occupied and
unoccupied bound-state energy levels are shown in the wells, and the infrared
transitions are indicated by vertical arrows. Incoming infrared radiation raises
electrons to the conduction band, and the resulting electric current flow is a measure
of the incident radiation intensity. Figure 9.20a illustrates a transition from bound
state to bound state that takes place within the quantum well, and Fig. 9.20b shows a
transition from bound state to continuum. In Fig. 9.20~ the continuum begins at the
top of the well, so the transition is from bound state to quasi-bound state. Finally in
Fig. 9.20d the continuum band lies below the top of the well, so the transition is from
bound state to miniband.
The responsivity of the detector is the electric current (amperes, A) generated per
watt (W) of incoming radiation. Figure 9.2 1 shows a plot of the dark-current density
(before irradiation) versus bias voltage for a GaAs/AlGaAs bound state-continuum
photodetector, and Fig. 9.22 shows the dependence of this detector's responsivity on
Figure 9.20. Schematic conduction band (shaded) and electron transition schemes (vertical
arrows) of the following types: (a) bound state to bound state; (b) bound state to continuum;
(c) bound state to quasi-bound; (d) bound state to miniband, for quantum-well infrared
photodetectors. [Adapted from S. S. Li and M. Z. Tidrow, in Nalwa (2000), Vol. 4, Chapter 9,
p. 563.1