Active Semiconductor Layer Research Articles

Dihexylquaterthiophene, A Two-Dimensional Liquid Crystal-like Organic Semiconductor with High Transport Properties

End-substitution of quaterthiophene with hexyl groups leads to a highly soluble conjugated oligomer, α,ω-dihexylquaterthiophene (DH4T), which has been characterized for its thermal, structural, and electrical properties. Differential scanning calorimetry indicates the existence of a 3-dimension (3D)-to-mesophase transition, occurring at 84 °C, below the melting temperature of the material (179 °C). X-ray diffraction patterns performed on crude and thermally treated films of DH4T confirm the existence of a smectic phase with two spacings that increase with temperature. This result is interpreted by a model involving alkyl chain movements, which result in spacing shrinkage, whereas the thienylene sequence remains faced at typical van der Waals distances. DH4T thus forms a 2-dimensional (2D) semiconductor with a liquid crystal-like structural organization. DH4T can be deposited as active semiconducting layer in thin-film transistors, either by vacuum evaporation or by spin coating on an octylsilane-pretreated surface. A high-field-effect carrier mobility has been obtained for both deposition techniques, μ = 3 × 10-2 and 1.2 × 10-2 cm2 V-1 s-1, respectively, together with an interesting Ion/Ioff ratio of 105. These data are discussed together with literature results on unsubstituted quaterthiophene (4T) and the parent sexithiophenes (DH6T and 6T). On one hand, results show that the semiconducting quaterthiophene core in DH4T is large enough to allow high charge transport properties, comparable to those of a sexithiophene core, whereas the core is also short enough to allow its α,ω-dialkylated derivative to be highly soluble and solution processable, contrary to the case of DH6T. Results also suggest that the larger band gap of the shorter conjugated quaterthiophenes is responsible for their lower dopant concentration, and hence of their higher dynamic ratio.

Whispering gallery mode operation in airgap verticalcavitylaser structure

Whispering gallery mode operation has been observed from a broad-area oxide-confined tunable vertical cavity laser structure which uses an airgap between the top mirror and the semiconductor active layer. Strong index guiding from both the air gap and the AlAs native oxide supports this mode of operation over a wide range of injection currents.

Scope and limits of organic–based thin–film transistors

Various organic conjugated materials have been used as active semiconducting layers in thin-film transistors. The mode of operation of these devices shows that a high carrier mobility together with a low conductivity are required. Conjugated polymers and other amorphous materials exhibit a low carrier mobility, of the order of 10 -4 −10 -5 cm 2 V -1 s -1 , and all attempts to increase it through slight doping of the organic semioconductor have failed, owing to the existence of a direct relationship between mobility and conductivity. This behaviour can be understood from the variable range hopping mechanism which describes charge transport in these materials. On the other hand, conjugated oligomers are well–defined materials, offering various physical and chemical ways of controlling the structural organization of thin films made out of them. It is thus shown that carrier mobility is directly related to the long range structural order in these films, i.e. to the decrease of grain boundaries, leading to values close to 10 -1 cm 2 V -1 s -1 , comparable to that of amorphous hydrogenated silicon. Besides which, conductivity in thin films of conjugated oligomers is mainly determined by the purity of the materials, allowing values lower than 10 -7 S cm -1 . This independent control of mobility and conductivity allows the realization of oligomer–based thin–film transistors showing characteristics close to those of classical a–Si:H–based thin–film transistors.

Tetrahedrally bonded amorphous carbon (ta-C) thinfilm transistors

The authors describe a glass compatible, carbon-based, metal-insulator- semiconductor field effect transistor (MISFET). Tetrahedral amorphous carbon (ta-C) is used, for the first time, as the active semiconductor layer in a coplanar thin film transistor (TFT) structure.

Kinetics of Interface State Generation Induced by Hot Carriers in N-Channel Polycrystalline Silicon Thin-Film Transistors

Interface state creation, induced by hot hole injection, has been frequently observed in n-channel polysilicon thin-film transistors, when subjected to prolonged bias stress with negative gate biases. In this work we propose a new model for the kinetics of interface state formation, that closely links the interface state generation to the measured gate leakage current. The interface state generation mechanism appears to be driven by the recombination of hot holes (injected from the semiconductor active layer) with electrons (injected from the gate electrode). This model is shown to fit very well the time evolution of the interface states, as determined by the sheet conductance of the damaged region close to the drain.

A Study of the Charged Excitations in Thin Films of α-Sexithiophene by Voltage-Modulation Spectroscopy and Photoimpedance Measurements

Abstract We report on the spectra of charged excitations present in solid-state thin films of α-sexithiophene. We use Voltage - Modulation Spectroscopy to charge the oligomer chains electrically by the field effect in Metal - Insulator - Semiconductor (MIS) devices in which α-sexithiophene is the active semiconductor layer. This technique allows us to probe the charged excitations in a solid state thin film device environment, whilst avoiding the steric and coulombic perturbations due to counter-ions present in chemically doped solutions. We observe the existence of two distinct charged excitations in addition to the radical cation. Photoimpedance spectra and some preliminary low temperature spectra provide further insight into these charged excitations.

Analysis of origin of nonlinear gain in 1.5 μm semiconductor active layers by highly nondegenerate four-wave mixing

The origin of the nonlinear gain effect in 1.5 μm semiconductor active layers is investigated by using highly nondegenerate four-wave mixing, where the pump-probe detuning is extended up to 2 THz. From the signal intensity measured as a function of the detuning frequency we find that both the spectral hole burning and the dynamic carrier heating contribute to the four-wave mixing. The dynamic carrier heating, however, creates the index grating rather than the gain grating, and hence, the spectral hole burning is the main origin of the nonlinear gain effect.

UHV CHEMICAL VAPOUR DEPOSITION OF UNDOPED AND IN-SITU DOPED POLYSILICON FILMS

The use of low pressure chemical vapour deposition (LPCVD) for the growth of thin films of polysilicon is widely used in integrated circuit fabrication processes. For several emerging technologies, chiefly large-area displays and multilayer memories, the crystalline quality of the as-deposited film is of paramount importance, as the polysilicon is the active semiconductor layer. It is important, in these applications, to lower the deposition temperature to avoid damage either to the substrate or circuit elements already present. The dependence of film morphology on the deposition rate, gas flow rate and reactor temperature is discussed. A reduction in pressure from typical LPCVD pressures (around 200 mTorr) causes the transition temperature to be lowered, allowing a crystalline film to be grown at relatively low temperatures. To achieve this, a special UHVCVD system has been constructed to investigate layer growth at pressures down to 0.2 mTorr. Details of work carried out on the growth of in- situ phosphorous doped polysilicon are discussed. Uniform phosphorous doped polysilicon films are grown without the need for wafer cages and/or complex gas injectors- At deposition temperatures in the region of 630°C and reactor pressures of less than 5 mTorr uniform doped polysilicon can be deposited. The resistivity and dopant concentrations can be controUed by altering the phosphine-to-silane gas flow ratio.

Method of determining C-V profiles in semiconductor substrates and active layers

A method for calculating C-V profiles by transforming the one-dimensional Poisson equation from a two-sided boundary value problem to an initial value problem is described. An analytic solution is used outside a reference plane deep in the substrate, and the rest of the solution is obtained by numerically integrating outward to the surface. This strategy speeds the calculation of quantities such as the carrier density and the apparent density that would be measured by a C-V profiler. For the same computation time, errors are smaller than those of Newton-Raphson methods because of the inherent speed and accuracy of the predictor-corrector algorithms used. By fitting Q-V pairs to a cubic spline, the C-V profiles are obtained efficiently with a moderate number of integrations. C-V profiles and threshold voltages can be calculated rapidly and accurately by this technique for a variety of semiconductor structures.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>