Spin injection into semiconductors using dilute magnetic semiconductors

So far, attempts at realizing spin-polarized current injection into a semiconductor using metallic ferromagnetic contacts have yielded unsatisfying results. In this paper, we present a simple model of diffusive transport, which shows that the principle reason for these negative results is a conductivity mismatch between the ferromagnetic contacts and the semiconductor. Moreover, we demonstrate that this problem can be addressed by using dilute magnetic semiconductor (DMS) contacts instead of metallic contacts. We present experimental results of optical measurements on a GaAs/AlGaAs diode fitted with a DMS spin injector contact. These measurements show a spin polarization of around 90% in the semiconductor. Furthermore, we discuss a novel magnetoresistance effect based on the suppression of one of the spin channels in the semiconductor which should allow the detection of a spin-polarized current by magnetoresistance measurements.     

Magnetic semiconductors

As far as the electrical conductivity is concerned, solids are usually classified as metals, semiconductors, or insulators. In metals the concentration of the charge carriers responsible for the electrical conductivity is large, whereas in semiconductors and insulators the carrier concentration is much smaller. The distinction between semiconductors and insulators is based on a difference in the nature of the conductivity. For semiconductors the charge carriers (electrons or holes) occupy the states of energy bands; these states are not Iocalized on particular atoms, but spread throughout the entire crystal. In such a situation the mobility of the carriers can be quite high and would, in fact, be infinite in a rigid periodic lattice; in this model the thermal motion of the atoms induces a scattering of the carriers and thus limits the conductivity to finite values. The classical examples of semiconductors are the elements Si and Ge and compounds such as GaAs, InSb, CdTe, ZnS, etc.     

Amorphous semiconductors

There has been a recent surge of interest in the subject of amorphous semiconductors, a field which only a few years ago was generally considered to be about as scientific as witchcraft or alchemy. The major reason for the change in attitude is not difficult to pinpoint: the turning point was the publication by Ovshinsky¹ detailing the various types of switching phenomena that characterize a large class of amorphous solids, and the subsequent publicity describing many potential applications of these phenomena.^2,3 But, in addition, the field presents several surprising features that stimulate pure academic interest. Firstly, a serious investigation of the properties of amorphous semiconductors requires not only familiarity with one particular area of science or engineering, but rather a detailed knowledge of results from the combined fields of physics, chemistry, metallurgy, and electrical engineering. Secondly, although the study of amorphous semiconductors, especially when compared with that of crystalline materials, is certainly in its infancy, and much fundamental experimental and theoretical work remains to be done, still much sophisticated thought has been devoted to the subject over the past 10 years and great strides have been made in understanding the basic principles. Finally, even a superficial investigation of amorphous semiconductors brings home rather strikingly the fact that solid state physics, even up to the present time, is being taught incorrectly!     

Doped amorphous semiconductors

The article reviews the advances that have been made during the past two years in the new field of substitutionally doped a-semiconductors, particularly a-Si. After introducing some of the basic concepts used in the treatment of a-materials, the preparation and doping of a-semiconductors from the gas phase and the role of hydrogen are discussed in some detail. Recent results on doping by ion implantation have also been included. The following sections are concerned with the effect of n and p-type impurities on the electronic transport properties of a-Si and a-Ge. The discussion is based on results from conductivity, drift mobility, Hall effect and thermoelectric power measurements and leads to the main conclusion that the donors introduce a new hopping path through the system which begins to dominate over tail-state hopping when their density exceeds about 10¹⁸ cm^-3. Recent photo-conductivity and luminescence results on doped a-Si are then discussed; they give information on recombination and also demonstrate the pronounced effect of doping on these properties. In the final section amorphous p-n junctions are considered as well as their possible applications in photovoltaic solar energy conversion.     

Isotopically controlled semiconductors

A review of recent research involving isotopically controlled semiconductors is presented. Studies with isotopically enriched semiconductor structures experienced a dramatic expansion at the end of the cold war when significant quantities of enriched isotopes of elements forming semiconductors became available for worldwide collaborations. Isotopes of an element differ in nuclear mass, may have different nuclear spins and undergo different nuclear reactions. Among the latter, the capture of thermal neutrons, which can lead to neutron transmutation doping, can be considered the most important one for semiconductors. Experimental and theoretical research exploiting the differences in all the properties has been conducted and will be illustrated with selected examples. Manuel Cardona, the longtime editor-in-chief of solid state communications has been and continues to be one of the major contributors to this field of solid state physics and it is a great pleasure to dedicate this review to him.     

Spintronic with semiconductors

Soon afterwards the discovery of the giant magnetoresistance in metallic multilayers, researchers have attempted to integrate spintronic properties with semiconductor materials. They came up against several difficulties related to the structural and electronic properties of the ferromagnetic metal-semiconductor interface. We will report on the recent progress made in this field of spintronic with semiconductors. First of all we will explain the interfacial resistance conditions required to inject and detect efficient spin current in a semiconductor and in a second part we will show that efficient spin injection experiments have been now achieved thanks to the addition of a tunnel resistance at the interface. We will then report on the magnetoresistance experiment performed with diluted magnetic semiconductors as ferromagnetic material. This type of material can constitute an alternative road to achieving electrical control spintronic devices. Finally, we will finish by reporting on research for a highly spin-polarized source to inject spin-polarized current in a semiconductor. It will be mainly focused on tunnel magnetoresistance junctions with semiconductor barriers and hot electron transistor. To cite this article: J.-M. George et al., C. R. Physique 6 (2005).     

Heavy fermion semiconductors

The heavy fermion semiconductors, or Kondo insulators, are very narrow gap semiconductors in which the properties show unusual temperature dependencies. We shall review their properties and show how they can be interpreted in terms of an electronic band structure, with a temperature dependent hybridization gap together with temperature dependent quasi-particle lifetimes. The properties of these semiconductors are very sensitive to impurities, which can enhance the incipient antiferromagnetic correlations and precipitate a magnetic instability.     

Formation of Graphene on Polycrystalline Nickel

Technical Physics - The results of studying the formation of graphene layers during thermal pyrolysis of methane on the surface of polycrystalline nickel are presented. The studies have been...     

二维半导体中的能谷电子学

文章介绍了能谷电子学背后的基本物理原理,并回顾了此方向在材料实现上的进展。在理论背景部分简单回顾了基本模型和有关贝里曲率导致量子输运和光选择的重要概念,在材料实现部分除了总结在真实材料中重要的实验和理论的发现,也讨论了在这些材料中的自旋轨道耦合和近邻诱导的塞曼效应,最后展望了能谷电子学的发展前景。     

Photoconductivity of amorphous semiconductors

An analysis of photoconductivity is presented for a material with a carrier mobility which decreases as a power law with time. We calculate the steady state and transient response, and show that the model applies to hydrogenated amorphous silicon and other amorphous semiconductors. In a-Si:H the effective mobility is obtained from 10^?6 to 1 sec. The recombination is of bimolecular type and the carrier density is almost independent of excitation intensity.     

Polaritons in anisotropic semiconductors

We show how to compute the optical properties (reflection and absorption) of anisotropic semiconductors in the exciton energy region, taking into account polariton and electron-hole coherence effects. The method is applied to a GaAs/Ga_1–x Al _x As superlattice, and the modifications in the optical properties with respect to GaAs are related to the anisotropy.     

Diamagnetism of narrow-gap semiconductors

Computations are performed of the diamagnetic susceptibility of a degenerate electron gas in narrow-gap semiconductors. The structure of the diamagnetic contribution is analyzed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 71–74, February, 1987.     

Cathodic Arc Evaporation of Polycrystalline Silicon

Russian Physics Journal - The paper presents the experimental results of silicon cathodic arc evaporation in a continuous mode with the superimposed external arched magnetic field. Volt-ampere...     

High-Resolution Single-Grain Diffraction of Polycrystalline Materials

Polycrystalline bulk materials are ubiquitous in everyday life, including biological, geological, and engineered structural and functional materials. Their fundamental units are individual grains, which are characterized by their microstructure; i.e., the arrangement of lattice defects. The microstructure usually influences the materials properties critically.     

Impurity diffusion into semiconductors

A method is described by which it is possible to obtain thin diffusion layers with various impurity distributions within the layer. Calculations and graphs are given which may be used to determine the junction thickness and impurity concentration at any point of the diffusion layer when the diffusion conditions are known.     

Dispersion-induced nonlinearities in semiconductors

A dispersive and saturable medium is shown, under very general conditions, to possess ultrafast dynamic behaviour due to non-adiabatic polarisation dynamics. Simple analytical expressions relating the effect to the refractive index dispersion of a semiconductor are derived and the magnitude of the equivalent Kerr coefficient is shown to be in qualitative agreement with measurements on active semiconductor waveguides.     

Superconductivity in covalent semiconductors

This review covers recent advances in superconductivity of diamond, Si, SiC, group III–V and II–IV semiconductors, metal-intercalated graphite and fullerites. The results are critically analyzed and prospects are given for future research directions. In particular, it is argued that the highest transition temperatures of ∼9 K in diamond and 11.5 K in CaC₆ can further be enhanced and that no reliable evidence exists yet for superconductivity in III-V semiconductors.