Intersublevel quantum-dot infrared photodetectors

This article describes the basic principles of semiconductor quantum-dot infrared photodetectors based on conduction-band intersublevel transitions. Sufficient background material is discussed to enable an appreciation of the subtle differences between quantum-well and quantum-dot devices. The article first considers infrared photon absorption and photon detection, along with some metrics for photon detectors and the detection of infrared radiation by semiconductors. It then examines the optical matrix element for interband, intersubband and intersublevel transitions before turning to experimental single-pixel quantum-dot infrared photodetectors. In particular, it explains the epitaxial synthesis of quantum dots and looks at mid-wave and long-wave quantum-dot infrared photodetectors. It also evaluates the characteristics of quantum-dot detectors and possible development of quantum-dot focal plane array imagers. The article concludes with an assessment of the challenges and prospects for high-performance detectors and arrays.

AB-INITIO STUDY OF VARIOUS PROPERTIES OF SMALL DIAMETER ARMCHAIR CARBON NANOTUBES

A comprehensive ab-initio investigation of the stability, structural, electronic, optical and Raman-active properties has been performed for the small diameter armchair carbon nanotubes. A number of new features not discussed earlier are observed in the present study. The binding energies (BEs) for the ultrathin nanotubes with respect to the graphine sheet are negative and the magnitude of the negative BE decreases with the diameter of the tube approaching zero for the graphine sheet. The separation between the two van Hove singularities (vHs) around the Fermi level increases with the diameter of the tube. The main absorption arises from the transitions between the states at nonzero values of kz lying in the range 0.38–0.50. There is a large variation in the magnitude of the optical matrix-element with the wave vector. The energy range of the strong optical absorption increases with the diameter of the tube. The presently predicted absorption and the RBM frequencies are in good agreement with the available experimental data. The variation of the radial breathing mode (RBM) frequency with diameter "d" of a tube obeys a relation which is very close to an experimentally determined relation obtained for a number of wide semiconducting nanotubes possessing a wide range of chiral angles.

Inclusion of Bandstructure and Many-Body Effects in a Quantum Well Laser Simulator

A self-consistent eight band k.p calculation, which takes into account strain and includes Hartree, exchange, and correlation terms (determined from a local density approximation) is incorporated into a QW laser simulator (MINILASE-II). The computation is performed within the envelope function approximation for a superlattice, in which all spatially varying terms of the k.p Hamiltonian, including the exchange and correlation energies are expanded in plane waves. The k.p eigenvalue equation, and Poisson's equation are solved iteratively until self-consistency is attained. Results from the k.p calculation are exported to MINILASE-II via a density of states and an energy dependent optical matrix element factor, renormalized by a Coulomb enhancement factor to account for electron-hole attraction. Results are presented for the gain spectrum and modulation response for a Ga0.8 In0.2 As/Al0.1 Ga0.9 As quantum well laser with and without the inclusion of the Coulomb enhancement factor.