CHAPTER 6
Low dimensional structures

6.1 Small really is beautiful

As any semiconductor scientist can tell you, low-dimensional structures (LDS) is a drug. It came to the notice of the scientific community in the early 1970s and, since then, the number of addicts has grown in a quite alarming fashion. The great developments of the 1960s were concerned with epitaxy, which led to the first well-controlled heterostructures and, in 1970, the ultimate success of the DH laser. The following decade 1970–80 saw the extension of these skills to encompass the growth of a very special type of heterostructure, characterized by its hitherto undreamed-of smallness. The double heterostructure (DH) laser was based on a structure with an active GaAs layer as thin as 0.1μ m (100 nm)—during the next decade this was to be dwarfed (on an inverse scale!) by structures with dimensions in the range 1–10 nm. Indeed, it was not long before dimensions were being quoted in ‘monolayers’, rather than nanometres, a monolayer of GaAs being a single molecular layer, one plane of Ga atoms plus one of As atoms, having an overall thickness (in the (0 0 1) direction) of 2.83 Å (0.283 nm). But what was the point? In a word, the point was that such miniscule amounts of material demonstrate physical properties, which are distinctly different from those of bulk material and these new features can be put to a surprisingly wide range of uses, both scientific and technological. It is a moot point whether the introduction of these so-called ‘Low Dimensional Structures’ has had greater impact in the sphere of new semiconductor physics or of new semiconductor devices but it is not a question which will long detain us—we simply welcome both aspects, while recognizing once again the vitality of the interaction between them.

In attempting to understand these new structures, the two-questions confront us: why is size relevant? and what order of size is important? There are, in fact, two complementary ways in which we can approach these questions, both depending on the quantum mechanical description of electron behaviour. The first of these concerns the property of electron ‘tunnelling’ through a potential barrier and the second depends on the relationship between the size of the space allowed to an electron and its energy. Tunnelling came first so we will deal with it first. In classical mechanics, applied to macroscopic particles, the presence of a potential

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The Story of Semiconductors
Table of contents

Table of contents

  • Title Page iii
  • Preface v
  • Contents xi
  • Chapter 1 - Perspectives 1
  • Chapter 2 - The Cat's Whiskers 19
  • Chapter 3 - Minority Rule 47
  • Chapter 4 - Silicon, Silicon, and Yet More Silicon 93
  • Chapter 5 - The Compound Challenge 149
  • Chapter 6 - Low Dimensional Structures 213
  • Chapter 7 - Let There Be Light 277
  • Chapter 8 - Communicating with Light 331
  • Chapter 9 - Semiconductors in the Infrared 385
  • Chapter 10 - Polycrystalline and Amorphous Semiconductors 447
  • Index 503
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