C

248 QUANTUM WELLS, WIRES, AND DOTS

248 QUANTUM WELLS, WIRES, AND DOTS

Figure 9.18. Coulomb staircase on the current i-voltage V characteristic plot from single-electron tunneling involving a 10-nm indium metal droplet. The experimental curve A was measured by a scanning tunneling microscope, and curves B and C are theoretical simulations. The peak-to-peak current is 1.8 nA. [After R. WHkins, E. Ben-Jacob, and R. C. Jaklevic, Phys. Rev. Lett. 59, 109 (1989).]

Figure 9.18. Coulomb staircase on the current i-voltage V characteristic plot from single-electron tunneling involving a 10-nm indium metal droplet. The experimental curve A was measured by a scanning tunneling microscope, and curves B and C are theoretical simulations. The peak-to-peak current is 1.8 nA. [After R. WHkins, E. Ben-Jacob, and R. C. Jaklevic, Phys. Rev. Lett. 59, 109 (1989).]

tunneling can take place between two of these Au55 ligand-stabilized clusters when they are in contact, with the shell acting as the barrier for the tunneling. Experiments were carried out with linear arrays of these Auj5 clusters of the type illustrated in Fig. 9.19. An electron entering the chain at one end was found to tunnel its way through in a soliton-like manner. Estimates of the interpartiele capacitance gave Qracro — 10-'8 F, and die estimated interpartiele resistance was Rr = 100Mil [see Gasparian et a!. (2000)]. Section 6.1.5 discusses electron tunneling along a linear chain of much larger (500-nm) gold nanoparticles connected by conjugated organic molecules.

9.6. APPLICATIONS 9.6.1. Infrared Detectors

Infrared transitions involving energy levels of quantum wells, such as the levels shown in Figs. 9.12 and 9.13, have been used for the operation of infrared

pair section

Figure 9.19. Linear array of hum ligand-siabsfeed nanopartides with irterparticte resistance R,, interparticle capacitance Cmkm, and self-capacitance CQ. 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.]

Rt. Cmicro

Figure 9.19. Linear array of hum ligand-siabsfeed nanopartides with irterparticte resistance R,, interparticle capacitance Cmkm, and self-capacitance CQ. 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.]

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. 9.20c 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 9.21 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

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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 minfoarid, for quantum-wen infrared photodetectors. [Adapted from S. S. Li and M. Z. Tidrow, in Natwa (2000), Vol. 4, Chapter 9, p. 563.]

Figure 9.21. Dark-current density versus bias voltage characteristics for a GaAs/AJGaAs quantum-well long-wavelength infrared (LWIR) photodetector measured at three indicated temperatures. A 300-K background current plot (BG, dashed curve) is also shown. [From M. Z. Tidrow, J. C. Chiang, S. S. Li, and K. Bacher, Appl. Phys. Lett. 70, 859 (1997).]

BIAS VOLTAGE (V)

Figure 9.21. Dark-current density versus bias voltage characteristics for a GaAs/AJGaAs quantum-well long-wavelength infrared (LWIR) photodetector measured at three indicated temperatures. A 300-K background current plot (BG, dashed curve) is also shown. [From M. Z. Tidrow, J. C. Chiang, S. S. Li, and K. Bacher, Appl. Phys. Lett. 70, 859 (1997).]

BIAS VOLTAGE (V)

I I I I I I I I I I I I I ï I I I I I I I I I I I I

normal incidence

45° incidence

Figure 9.22. Peak responsivity versus wavelength at 77 K for a 2-V bias at normal and 45° angles of incidence. [From M. Z. Tidrow, J. C. Chiang, S. S. Li, and K. Bacher, Appl. Phys. Lett. 70, 859 (1997).]

I I I I I I I I I I I I I ï I I I I I I I I I I I I

normal incidence

45° incidence

Figure 9.22. Peak responsivity versus wavelength at 77 K for a 2-V bias at normal and 45° angles of incidence. [From M. Z. Tidrow, J. C. Chiang, S. S. Li, and K. Bacher, Appl. Phys. Lett. 70, 859 (1997).]

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