This review seeks to extend the scope of both the experimental and theoretical work carried out since I completed my 1993 review on the electronic, optical, and to a lesser extent, the transport properties of a variety of semiconductor quantum dots (QDs). In addition to the many advances that have been made on topics such as quant um confinement effects (QCE), optical and luminescence properties, energy levels, and theoretical models that were dealt with in outline then, a number of new themes have emerged. These include detailed studies on single QDs such as InAs, InP, CuCl, etc, and this became possible due to the development of several microtechniques such as scanning near field optical microscopy, SNOM or NSOM, as well as the use of improved growth procedures such as those involving MBE and the Stranski-Krastanow (SK) growth method, or by better chemical processing. By concentrating on single dots, it has proved possible to limit the extent of the line broadening for the optical absorption and luminescence peaks due to the variation in dot sizes in the more usual types of films used. Line half widths (FWHM) in the microvolt region have now been recorded, and this has helped in the identification and resolution of excitons, biexcitons, higher excited states, and both positive and negative charged excitons, when these lie close together in energy.
Quantum dots such as CdSe and CuCl which can be considered as the model systems have been the most extensively investigated, and in the case of CdSe dots, reasonable theoretical models have been developed to predict energy levels and optical properties as a function of dot size even for the difficult case of strong confinement, when R less than or equal to a(B), the bulk exciton Bohr radius. Although problems still exist in relating predictions to all the experimental data, they have helped to identify exciton features near to and above the first main absorption peak and other optoelectronic features.
A good deal of effort has now gone into the study of the III-V systems such as InP, InAs, GaAs, and GaN QDs, as well as on porous Si (PS) and Si and Ge dots. This has been largely driven by the possibility that devices such as lasers, LEDs and devices depending on single electron transport and tunnelling might be developed, an area where there is significant technological potential. For dots such as InAs etc prepared by the SK method, where there is a mismatch in lattice parameters between the InAs and the substrate such as GaAs, the dots tend to have a roughly pyramidal shaped profile, and the dot also sits on a thin InAs wetting layer. Both 2d and 3d ordered arrays of QDs can be formed using this procedure. The photoluminescence (PL) efficiency for such systems can be unexpectedly high, and there have been attempts to explain this effect as being due to the avoidance of the so-called 'phonon bottleneck' by Auger type transitions, but this is still a controversial matter.
Other phenomena that are discussed include: (1) exciton-phonon coupling interactions, particularly as applied to QDs such as those formed from CuCl, CuBr, PbS, etc.; (2) coupled QDs for which dot-dot interactions need to be considered; (3) porous Si (PS), a system of considerable interest since the observation of strong PL emission features in the PL spectra by Canham in 1990, even though Si has an indirect gap, and on the practical side there has been much effort in the development of devices such as lasers, LEDs and other electroluminescence (EL) devices, and more recently for biological and medical applications, where PS, because of its porous structure, can be a host lattice for biochemical compounds in a manner similar to some zeolites. However the structure of PS is rat her complex, and filaments, embryonic Si dots, as well as well formed dots, oxide interfaces of uncertain composition, and compounds containing hydrogen may all be present, and this makes it difficult to make reasonable assignments to some of the optical features present in the spectra. (4) Type II QDs that concern spatially indirect systems, and this can refer to both space and wave vector k. Instead of the electron e and hole h for an e-h pair (exciton) bot h residing in dot, for most of the Type II systems the h resides in the dot while the e is in the matrix in which the dots are distributed or at the interfaces. The systems considered depend on the band offsets, and include combinations such as GaAs-AlAs and CdTe-HgTe etc. (5) Hydrogenic-type donors in semiconductor QDs. (6) Excitons, biexcitons, charged excitons (both positive and negative), or trions. (7) Quantum dot-quantum well (QD-QW) combinations, also described as thin film-QD or core-shell composites, for example CdS QDs coated with a thin layer of the smaller band gap semiconductor HgS acting as a QW followed by a further CdS coating or 'clad' or 'shell'. (8) QD-conjugated organic polymer composites, a topic developed by Alivisatos, Greenham and Bawendi and their colleagues in the mid nineties, where the polymer acts as a hole conductor in an EL or LED type of device, where the wavelength of the emitted light due to e-h recombination that occurs preferably near the interface, can be varied by altering the QD radius. The possible formation of hybrid Mott - Wannier and Frenkel excitons is also briefly considered. (9) The variation of the QD dielectric constant with QD size epsilon (2).(R), has been considered by several investigators, Their calculations suggest that the dielectric constant decreases substantially as R is reduced. This e? ect has been ignored in many contributions even though epsilon (2) enters into the equations dealing with QSE or quantum size effects, a(B), E-b, and oscillator strengths (OS), and its omission will influence the calculated estimates for these quantities. (10) Finally, single electron transport and tunnelling in single and coupled QDs, and the Coulomb blockade (CB) are considered, but only in outline since this is a large problem, but it is clearly an important topic particularly in connection with the development of computing and information processing systems.
