We review the physical properties of diluted magnetic semiconductors (DMS) of the type AII1−xMnxBVI (e.g., Cd1−xMnxSe, Hg1−xMnxTe). Crystallographic properties are discussed first, with emphasis on the common structural features which these materials have as a result of tetrahedral bonding. We then describe the band structure of the AII1−xMnxBVI alloys in the absence of an external magnetic field, stressing the close relationship of the sp electron bands in these materials to the band structure of the nonmagnetic AIIBVI ‘‘parent’’ semiconductors. In addition, the characteristics of the narrow (nearly localized) band arising from the half‐filled Mn 3d5 shells are described, along with their profound effect on the optical properties of DMS. We then describe our present understanding of the magnetic properties of the AII1−xMnxBVI alloys. In particular, we discuss the mechanism of the Mn++‐Mn++ exchange, which underlies the magnetism of these materials; we present an analytic formulation for the magnetic susc...

Diluted magnetic semiconductors

A II-VI compound semiconductor laser diode (10) is formed from overlaying layers of material including an n-type single crystal semiconductor substrate (12), adjacent n-type and p-type guiding lasers (14) and (16) of II-VI semiconductor forming a pn junction, a quantum well active layer (18) of II-VI semiconductor between the guiding layers (14) and (16), first electrode (32) opposite the substrate (12) from the n-type guiding layer (14), and a second electrode (30) opposite the p-type guiding layer (16) from the quantum well layer (18) Electrode layer (30) is characterized by a Fermi energy A p-type ohmic contact layer (26) is doped, with shallow acceptors having a shallow acceptor energy, to a net acceptor concentration of at least 1 x 1017 cm-3, and includes sufficient deep energy states between the shallow acceptor energy and the electrode layer Fermi energy to enable cascade tunneling by charge carriers

Blue-green laser diode

Transport of electrons in a single molecule junction is the simplest problem in the general subject area of molecular electronics. In the past few years, this area has been extended to probe beyond the simple tunnelling associated with large energy gaps between electrode Fermi level and molecular levels, to deal with smaller gaps, with near-resonance tunnelling and, particularly, with effects due to interaction of electronic and vibrational degrees of freedom. This overview is devoted to the theoretical and computational approaches that have been taken to understanding transport in molecular junctions when these vibronic interactions are involved. After a short experimental overview, and discussion of different test beds and measurements, we define a particular microscopic model Hamiltonian. That overall Hamiltonian can be used to discuss all of the phenomena dealt with subsequently. These include transition from coherent to incoherent transport as electron/vibration interaction increases in strength, inelastic electron tunnelling spectroscopy and its interpretation and measurement, affects of interelectronic repulsion treated at the Hubbard level, noise in molecular transport junctions, non-linear conductance phenomena, heating and heat conduction in molecular transport junctions and current-induced chemical reactions. In each of these areas, we use the same simple model Hamiltonian to analyse energetics and dynamics. While this overview does not attempt survey the literature exhaustively, it does provide appropriate references to the current literature (both experimental and theoretical). We also attempt to point out directions in which further research is required to answer cardinal questions concerning the behaviour and understanding of vibrational effects in molecular transport junctions. (Some figures in this article are in colour only in the electronic version)

/pdf/molecular-transport-junctions-vibrational-effects-4dr06nme1x.pdf

Molecular transport junctions: vibrational effects

Structural, FTIR and photoluminescence studies of Cu doped ZnO nanopowders by co-precipitation method

This paper combines background information about the IBM Journal of Research and Development with guidelines for the preparation of Journal manuscripts. The purpose is to acquaint authors with the Journal as a primary, professional publication and to present suggestions to ease the work of author and editor in preparing clear, concise, and useful manuscripts.

IBM journal of research and development: information for authors

The piezomodulated and the photomodulated reflectivity spectra of Mn-based diluted magnetic semiconductors (${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te,${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$ Te,${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se,${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Se) reveal sharp signatures associated with the excitonic transitions near the band gap. The dependence of the excitonic energy on the manganese concentration (x), on temperature (T), and, in the wurtzite structure, on the polarization with respect to the optic axis, are determined from the modulation experiments. The excitonic energy in the ``hypothetical'' tetrahedrally coordinated MnTe and MnSe, viewed as the end members of the alloy systems, are deduced from the x dependence of the excitonic transition. A signature associated with ${\mathrm{Mn}}^{2+}$ is observed in piezomodulated ``pseudoreflection,'' when the energy gap is sufficiently large; its energy is insensitive to x. The absence of the Zeeman effect for the ${\mathrm{Mn}}^{2+}$ signature, in contrast to the large Zeeman effect of the excitonic feature, demonstrates that the former is an optical transition associated with the 3${d}^{5}$ shell of ${\mathrm{Mn}}^{2+}$.

Energy gap, excitonic, and "internal" Mn2+ optical transition in Mn-based II-VI diluted magnetic semiconductors.

In this paper, we present a numerical method for evaluating the full Wigner function throughout a device by solving a steady-state quantum kinetic equation in two dimensions, in the linear-response regime. This method has two advantages over conventional treatments of mesoscopic devices. First, dissipative processes can be included within the device, thus allowing a smooth transition from the quantum to the semiclassical regime. Second, the contacts are treated in the same manner as in semiclassical device analysis. A short phase-breaking time can be used in the contact regions so that oscillations in the electron density due to interference effects die out quickly; this is particularly useful when obtaining self-consistent solutions with the Poisson equation. Any quantity of interest, such as electron density or current density per unit energy, can be computed throughout the entire device. We will first show that under low-bias, low-temperature conditions, the diagonal elements of the Wigner function can be used to define a local electrochemical potential (\ensuremath{\mu}) that lends insight into the internal transport physics. We show that separate electrochemical potentials ${\mathrm{\ensuremath{\mu}}}_{\mathit{L}}$ and ${\mathrm{\ensuremath{\mu}}}_{\mathit{R}}$ for left- and right-moving electrons show unphysical behavior when defined in a local sense. But sensible results are obtained when these potentials are defined in an average sense over regions the size of a de Broglie wavelength. We then examine the difficulties associated with measuring \ensuremath{\mu}, with numerical examples. Next, we use the local electrochemical potential profile to clarify the nature of the spreading resistance associated with the narrowing of a current lead. Finally, we show that the electrostatic potential (\ensuremath{\varphi}) can be viewed as a convolution of \ensuremath{\mu} with a screening function and present example computations of \ensuremath{\varphi}.

Voltage drop in mesoscopic systems : a numerical study using a quantum kinetic equation

Premiere observation de l'effet d'un champ magnetique externe sur la transition optique de Mn 2+ a 2,2 eV, et description de ses implications dans les differents modeles proposes dans la litterature

Origin of the Mn2+ optical transition in Mn-based II-VI diluted magnetic semiconductors.

We report the epitaxial growth of CdSe, Zn1−x
 Cd
 x
 Se (0 ≤x
 ≤ 1) and Cd1−x
 Mn
 x
 Se (0 ≤x
 ≤ 0.8) on (100) GaAs. X-ray diffraction (XRD), electron diffraction and transmission electron microscopy (TEM) indicate that all the epilayers have the cubic (zinc-blende) structure of the GaAs substrate. The energy gaps of these materials were measured using reflectivity measurements. We also report the growth of ZnSe/Zn1−x
 Cd
 x
 Se superlattices. TEM and XRD measurements show that high quality modulated structures with sharp interfaces are possible.

Molecular beam epitaxy of CdSe and the derivative alloys Zn 1- x Cd x Se and Cd 1- x Mn x Se

Optical transitions associated with the electronic levels of a single quantum well in GaAs/AlxGa1−xAs heterostructures are observed with exceptional clarity in the piezomodulated reflectivity spectrum measured down to liquid helium temperature. The splitting of such transitions in multiple quantum well structures due to the coupling of the electronic levels in the adjacent wells is illustrated with results on a double quantum well.

Y. R. Lee

Papers

Energy gap, excitonic, and "internal" Mn2+ optical transition in Mn-based II-VI diluted magnetic semiconductors.

Voltage drop in mesoscopic systems : a numerical study using a quantum kinetic equation

Origin of the Mn2+ optical transition in Mn-based II-VI diluted magnetic semiconductors.

Molecular beam epitaxy of CdSe and the derivative alloys Zn 1- x Cd x Se and Cd 1- x Mn x Se

Piezomodulated electronic spectra of semiconductor heterostructures: GaAs/AlxGa1−xAs quantum well structures