The electronic properties of inversion and accumulation layers at semiconductor-insulator interfaces and of other systems that exhibit two-dimensional or quasi-two-dimensional behavior, such as electrons in semiconductor heterojunctions and superlattices and on liquid helium, are reviewed. Energy levels, transport properties, and optical properties are considered in some detail, especially for electrons at the (100) silicon-silicon dioxide interface. Other systems are discussed more briefly.

Electronic properties of two-dimensional systems

Research in the use of organic polymers as the active semiconductors in light-emitting diodes has advanced rapidly, and prototype devices now meet realistic specifications for applications. These achievements have provided insight into many aspects of the background science, from design and synthesis of materials, through materials fabrication issues, to the semiconductor physics of these polymers.

Electroluminescence in conjugated polymers

2.2. Materials 929 2.3. Factors Influencing Charge Mobility 931 2.3.1. Molecular Packing 931 2.3.2. Disorder 932 2.3.3. Temperature 933 2.3.4. Electric Field 934 2.3.5. Impurities 934 2.3.6. Pressure 934 2.3.7. Charge-Carrier Density 934 2.3.8. Size/molecular Weight 935 3. The Charge-Transport Parameters 935 3.1. Electronic Coupling 936 3.1.1. The Energy-Splitting-in-Dimer Method 936 3.1.2. The Orthogonality Issue 937 3.1.3. Impact of the Site Energy 937 3.1.4. Electronic Coupling in Oligoacene Derivatives 938

Charge transport in organic semiconductors.

Self-localized nonlinear excitations (solitons, polarons, and bipolarons) are fundamental and inherent features of quasi-one-dimensional conducting polymers. Their signatures are evident in many aspects of the physical and chemical properties of this growing class of novel materials. As a result, these polymers represent an opportunity for exploring the novel phenomena associated with topological solitons and their linear confinement which results from weakly lifting the ground-state degeneracy. The authors review the theoretical models that have been developed to describe the physics of polyacetylene and related conducting polymers and summarize the relevant experimental results obtained for these materials. An attempt is made to assess the validity of the soliton model of polyacetylene and its generalization to related systems in which the ground-state degeneracy has been lifted.

Solitons in conducting polymers

The Al x Ga1−x As/GaAs heterostructure system is potentially useful material for high‐speed digital, high‐frequency microwave, and electro‐optic device applications Even though the basic Al x Ga1−x As/GaAs heterostructure concepts are understood at this time, some practical device parameters in this system have been hampered by a lack of definite knowledge of many material parameters Recently, Blakemore has presented numerical and graphical information about many of the physical and electronic properties of GaAs [J S Blakemore, J Appl Phys 5 3, R123 (1982)] The purpose of this review is (i) to obtain and clarify all the various material parameters of Al x Ga1−x As alloy from a systematic point of view, and (ii) to present key properties of the material parameters for a variety of research works and device applications A complete set of material parameters are considered in this review for GaAs, AlAs, and Al x Ga1−x As alloys The model used is based on an interpolation scheme and, therefore, necessitates known values of the parameters for the related binaries (GaAs and AlAs) The material parameters and properties considered in the present review can be classified into sixteen groups: (1) lattice constant and crystal density, (2) melting point, (3) thermal expansion coefficient, (4) lattice dynamic properties, (5) lattice thermal properties, (6) electronic‐band structure, (7) external perturbation effects on the band‐gap energy, (8) effective mass, (9) deformation potential, (10) static and high‐frequency dielectric constants, (11) magnetic susceptibility, (12) piezoelectric constant, (13) Frohlich coupling parameter, (14) electron transport properties, (15) optical properties, and (16) photoelastic properties Of particular interest is the deviation of material parameters from linearity with respect to the AlAs mole fraction x Some material parameters, such as lattice constant, crystal density, thermal expansion coefficient, dielectric constant, and elastic constant, obey Vegard’s rule well Other parameters, eg, electronic‐band energy, lattice vibration (phonon) energy, Debye temperature, and impurity ionization energy, exhibit quadratic dependence upon the AlAs mole fraction However, some kinds of the material parameters, eg, lattice thermal conductivity, exhibit very strong nonlinearity with respect to x, which arises from the effects of alloy disorder It is found that the present model provides generally acceptable parameters in good agreement with the existing experimental data A detailed discussion is also given of the acceptability of such interpolated parameters from an aspect of solid‐state physics Key properties of the material parameters for use in research work and a variety of Al x Ga1−x As/GaAs device applications are also discussed in detail

GaAs, AlAs, and AlxGa1−xAs: Material parameters for use in research and device applications

Conduction in pristine conjugated polymers (other than polyacetylene) is by polaron hopping between sites corresponding to conjugation lengths. The strong increase of current I with voltage V observed for both emission-limited and ohmic contacts is due in large part to mobility increase as increasing field makes it more possible to overcome internal barriers, such as energy differences between sites. For emission-limited contacts an additional source of nonlinear increase of I with increasing V is greater ability to escpe return to the injecting electrode due to the image force. For ohmic contacts additional nonlinearity comes from space charge effects. We are able to fit I vs. V for electron or hole conduction in some poly(p-phenylene vinylene), PPV, derivatives with ohmic contacts for reasonable values of the parameters involved.

Current-voltage characteristics of polymer light-emitting diodes

Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) have the potential of giving the three dimensional morphology of polymer surfaces with atomic resolution. We have imaged doped and undoped polyacetylene, poly(3-hexylthiophene), and polystyrene. Both these techniques are well suited to study the fibrillar nature and growth of these films. STM shows that the fibrils of the free side of iodine doped polystyrene are flat to within less that 10 A. The fluctuations in the surface conductivity of polyacetylene and the thermally induced motion of the polymer side groups limit the potential of these techniques to give atomic resolution.

Scanning Tunneling Microscopy and Atomic Force Microscopy of Thin Polymer Films

We show by calculations for a chain of stage-1 (16.7%) potassium-doped (CH)x that the metallic state is obtained by adding to the Su-Schrieffer-Heeger Hamiltonian the Coulomb potential of the doping ions and solitons on other chains. Good agreement with the measured Pauli susceptibility is obtained. Adding interchain coupling is found to lead to essentially the same density of states.

Insulator-Metal Transition and Conduction Processes in Trans-Polyacetylene

Experimental studies of photoinduced absorption, PA, have been carried out by several groups on composites of C 60 with conducting polymers such as poly[2-methoxy, 5-(ethyl hexyloxy) -p- phenylene vinylene], MEH-PPV, to understand the quenching of photoluminescence and increase of photoconductivity caused by the addition of small amounts of C 60 . To account for the observed PA it is first necessary to know where the C 60 molecules sit relative to the MEH-PPV chains. We found with the use of a Monte Carlo cooling algorithm that the most probable position of the C 60 is in a channel formed by the alkoxy side chains, its center approximately 10 angstrom from that of the MEH-PPV backbone. We identify the PA peak seen in MEH-PPV as the transition between the lower and upper levels of a polaron coupled to one of opposite sign on a neighboring chain. Calculations are carried out for the energy levels of neutral C 60 and C 60 - using a tight binding Hamiltonian with the parameters obtained by Harigaya. To determine the effect on the levels of the Coulomb interaction between C 60 - and a positively charged polaron, P + , on MEH-PPV we use coupled equations. Finally, we suggest that the peak of the PA in the composite is due to transitions between the two polaron levels of a free P + on MEH-PPV. Additional PA features in the composite can be identified as due to C 60 - and neutral C 60 .

Photoinduced absorption of conducting polymer-C60 composites

Theory predicts that a charge added to a conjugated polymer chain forms a polaron Recent calculations of the three-dimensional band structure of trans-polyacetylene, t-PA, and poly(phenylene vinylene), PPV, have obtained values for the interchain coupling that are large enough to destabilize the polaron Nevertheless, there is experimental evidence that polarons exist in these materials We show that the existence of chain endings and other conjugation breaks can stabilize the polaron in actual samples© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering Downloading of the abstract is permitted for personal use only

Stabilization of polarons in conjugated polymers due to finite conjugation lengths

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