Quantum Dot

Figure 9A Steps in the formation of a quantum wire or quantum dot by electron-beam lithography: (a) initial quantum well on a substrate, and covered by a resist; (b) radiation with sample shielded by template; (c) configuration after dissolving irradiated portion of resist by developer; (d) disposition after addition of etching mask; (e) arrangement after removal of remainder of resist; (f) configuration after etching away the unwanted quantum-well material; (g) final nanostructure on substrate after removal of etching mask. [See also T. P. Sidiki and C. M. S. Torres, in Nalwa (2000), Vol. 3, Chapter 5, p. 250 ]

The first step of the lithographic procedure is to place a radiation-sensitive resist on the surface of the sample substrate, as shown in Fig. 9.4a. The sample is then irradiated by an electron beam in the region where the nanostructure will be located, as shown in Fig. 9.4b. This can be done by using either a radiation mask that contains the nanostructure pattern, as shown, or a scanning electron beam that strikes the surface only in the desired region. The radiation chemically modifies the exposed area of the resist so that it becomes soluble in a developer. The third step in the process (Fig. 9.4c) is the application of the developer to remove the irradiated portions of the resist. The fourth step (Fig. 9.4d) is the insertion of an etching mask into the hole in the resist, and the fifth step (Fig. 9.4e) consists in lifting off the remaining parts of the resist. In the sixth step (Fig. 9.4f) the areas of the quantum weli not covered by fee etching mask are chemically etched away to produce the quantum structure shown in Fig. 9.4f covered by die etching mask. Finally the etching mask is removed, if necessary, to provide the desired quantum structure (Fig. 9.4g), which might be the quantum wire or quantum dot shown in Fig. 9.3b.

In Chapter 1 we mentioned the most common process called electron-beam lithogmpky, which mate use of an electron beam for the radiation. Other types of lithography employ neutral atom beams (e.g., Li, Na, K, Rb, Cs), charged ion beams (e.g., Ga+), or electromagnetic radiation such as visible light, ultraviolet light, or X rays. When laser beams are utilized, frequency doublets and quadruple┬╗ can bring the wavelength into a range (e.g., >l~150nm) that is convenient for quantum-dot fabrication. Photochemical etching can be applied to a surface activated by laser light.

The lithographic technique can be used to make more complex quantum structures than the quantum wire and quantum dot shown in Fig. 9.3b. For example one might start with a multiple quantum-well structure of die type illustrated in Fig. 9.5, place the resist on top of it, and make use of a mask film or template with six circles cut out of it, as portrayed at the top of Fig. 9.5. Following the lithographic procedure outlined in Fig. 9.4, one can produce die 24-quantum-dot array consisting of six columns, each containing four stacked quantum dots, that is sketched in

Figure 9.5. Four-cycle muttiple-quantum-weH arrangement mounted on a substrate and covered by a resist. A radiation shielding template for lithography is shown at the top.

Quantum Dots

Quantum Dots

Figure 9.6. Quantum-dot array formed by lithography from the initial configuration of Fig. 9.5. This 24-fold quarrtum-dot array consists of six columns of four stacked quantum dots.

Fig 9.6. As an example of the advantages of fabricating quantum dot arrays, it has been found experimentally that die arrays produce a greatly enhanced photo-iuminescent output of light Figure 9.7 shows a photoluminescence (PL) spectrum from a quantum-dot array that is much more than 100 times stronger than the spectrum obtained from the initial multiple quantum wells. The principles behind the photoluminescence technique are described in Section 8.3.1. The main peak of the spectrum of Fig. 9.7 was attributed to a localized exciton (LE), as explained in Section 9.4. This magnitude of enhancement shown in the figure has been obtained from initial samples containing, typically, a 15-period superlattice (SL) of alternating a c

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