Introduction to Nanotechnology

9.7. SUPERCONDUCTIVITY 253
horizontal stripe on the figure labeled QD) is a 30-nm-thick GaAs region, and
centered in this region are 12 monolayers of In,,,Ga,,,As quantum dots with a
density of 1.5 x 10'o/cmZ. The inset at the bottom of the figure was drawn to
represent the details of the waveguide region. The length L, and the width W varied
somewhat from sample to sample, with L, = ranging from 1 to 5 mm, and W varying
between 4 and 60 pm. The facets or faces of the laser were coated with ZnSe/MgF,
high-reflectivity (> 95%) coatings that reflected the light back and forth inside to
augment the stimulated emission. The laser light exited through the lateral edge of
the laser.
The laser output power for continuous-wave (CW) operation at room temperature
is plotted in Fig. 9.25 versus the current for the laser dimensions L, = 1.02 mm and
W = 9 pm. The near-infrared output signal at the wavelength of 1.32 pm for a
current setting just above the 4.1-mA threshold value, labeled point a, is shown in
the inset of the figure. The threshold current density increases sharply with the
temperature above 200K, and this is illustrated in Fig. 9.26 for pulsed operation.
9.7. SUPERCONDUCTIVITY
Superconductors exhibit some properties that are analogous to those of quantum
dots, quantum wires, and quantum wells. This is the case, in part, because their
characteristic length scales ;1 and 5 are in the nanorange, as the representative data in
Table 9.6 indicate. The table also gives the transition temperature T, below which
each material superconducts, that is, has zero electrical resistance. The majority of
the values of i, and 5 listed in the table are 200nm or less, and several are below
6 nm. The penetration depth 2 is a measure of the distance that an externally applied
1
QD Edge Emitter, HR / HR
9 pm x 1020 wm
160 T=295K,CW
0
1.26 1.30 1.34 1.38
Wavelength (pm)
1 I I I I
0 10 20 30 40
Current (mA)
Figure 9.25. Dependence of the near-infrared light output power on the current for a contin-
uous-wave, room-temperature, edge-emitting quantum dot laser of the type illustrated in
Fig. 9.24. [From Park et al. (1999).]