Undoped Region Research Articles

Electrochemical growth of poly(3-dodecylthiophene) and its interpretation as a fractal

Poly(3-dodecylthiophene) with a fractal pattern has been prepared by an electrochemical method on the surface of a solution. The fractal dimension has been calculated by four methods. At an applied voltage of about 10 V, the fractal dimension indicates a maximum value of 1.79. The growth rate in the radial direction has a maximum value at about 7.5 V and the conductivity in the iodine doped state is a maximum for the sample prepared at a voltage of about 15 V in accordance with the voltage dependence of fractal dimension. Although the conducting polymer is usually doped by the counter-ion, BF4-, in polymerisation an undoped region has been observed in the branches of the conducting polymer at high voltage. The time dependence of the fractal dimension has been calculated and the rise time has been found to depend on the applied voltage. By short circuiting the electrode after the polymerisation, the undoped region in the branch has proceeded concentrically from the tip of the growth to the needle electrode.

Polysilicon resistor trimming by laser link making

A technique for laser trimming of polysilicon resistors has been developed. In this scheme an undoped polysilicon film is patterned lithographically, and then photoresist patterns are used to prevent doping of certain narrow areas of each resistor during the impurity implant; thus, the protected areas remain insulating after wafer processing. Subsequent laser scanning of the undoped regions can produce precise reductions in resistance by lateral diffusion of dopant impurities. Trimming can be accomplished by changing either the laser power or the position. Q-switched Nd:YAG (1.06- mu m wavelength) and second-harmonic (0.53- mu m wavelength) radiations have produced successful results. The process was applied to passivated resistors with negligible (perhaps zero) post-trim drift, and the temperature coefficients of resistivity (TCRs) of trimmed resistors can be matched to the TCRs of untrimmed resistors in the same film.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Extrinsic doping at low concentrations for CDxHg1- xTe layers grown by MOVPE

The study of arsenic doping has resulted in the capability to control the acceptor level between 1×10 16 cm -3 and 2×10 17 cm -3 in Cd x Hg 1- x Te grown by MOVPE. An understanding of the dopant incorporation mechanism and diffusion rate has enabled the growth parameters to be adjusted to ensure high (≈100%) electrical activity and homogeneous distribution of the dopant atoms. The acceptor ionization energy obtained from such layers was consistent with extrinsically doped material. Minority carrier lifetime data are also presented. Doped/undoped heterostructures have been produced which have demonstrated p-n junctions following Hg annealing. The high- x regions can behave as barriers to Hg in-diffusion if they cap the undoped region.

Realization of a staircase photodiode: Towards a solid-state photomultiplier

Abstract A few years ago a low noise avalanche photodiode with a near unity excess noise factor was proposed. This detector achieves a high ratio of ionization coefficient κ via a suitable staircase band diagram. This APD can in principle exhibit photomultiplier-like noise behavior. In this paper we report the first realization of a staircase APD, grown in AlGaAs/GaAs by MBE, consisting of a ten-stage undoped staircase high-field region between p+ and n+ GaAs layers. Typical dark currents were 200 pA at the breakdown onset. The electron initiated avalanche gain was greater than 100 at the same voltage. Ionization ratios greater than 10 were estimated. Pulse responses with a full width at half maximum (FWHM) of 200 ps were demonstrated.

Structural investigation of a nonconjugated conducting polymer by solid state 13C nuclear magnetic resonance spectroscopy

The charge-transfer mechanism in 1,4 cis-polyisoprene is studied using solid state 13C NMR spectroscopy as a function of doping level of iodine. The CP/MAS NMR data show that the concentration of double bonds decreases as a consequence of doping. The overall results are consistent with previous observations and indicate that cation radicals –C+‖–C⋅‖– are formed upon doping and charge transfer. The double bonds are reformed as the doped samples are compensated with ammonia indicating reversibility of the doping. The measurements also indicate that the chain mobility is restricted in the doped regions compared to the undoped regions.

Delta -doped FET with sidewall source and drain by atomic layer epitaxy

Summary form only given. The DC and RF performance of planar doped FET devices degrades severely due to parasitic source and drain excess resistances. The authors report a novel approach to reducing these parasitic resistances based on sidewall growth of source and drain using atomic layer epitaxy (ALE). The delta -doped structures were first etched to remove the undoped regions under the source and drain areas, followed by ALE sidewall regrowth at 450 degrees C. The regrown ALE GaAs film is 500 AA thick and has ten planes of Se atoms with equivalent carrier concentrations of 2*10/sup 19//cm/sup 3/. ALE allows the uniform regrowth of the sidewalls, since it proceeds at a monolayer/cycle irrespective of the orientation of the etched structure. The grown layer thus makes a direct contact to the channel of the delta -doped FET and reduces both R/sub s/ and R/sub D/. The performances of delta -doped FETs with the same ALE contacting layers were compared for both etched and nonetched structures. Even with nonalloyed sidewall source and drain contacts, the excess source and drain resistances were reduced by 30-40%. >

Room-temperature optical absorption and confinement effect in Ga0.47In0.53As/Al0.48In0.52As MQW

Room-temperature optical absorption in MBE-grown multiple-quantum well (MQW) samples of GaInAs/AlInAs with 100 AA wells and 100 AA barrier has been investigated. The specimens consisted of 30 periods of a nominally undoped MQW region sandwiched between 100 AA layers of undoped AlInAs with a 50 AA GaAs cap. The optical absorption data, in conjunction with calculated energy levels based on nonparabolic effective-mass theory, yielded information about the quantum confinement effect in the structure.

Collision broadening of optical gain in semiconductor lasers

The energy-dependent intraband relaxation with polar-optical-phonon scattering is incorporated in the calculation of the linear gain and the refractive index change of the GaAs injection laser with an undoped active region. Comparison with the conventional model, which assumes a constant intraband relaxation, shows that it is difficult to fit the overall gain spectra exactly by single, energy-independent intraband-relaxation time. Moreover, the calculated gain spectra wth phonon broadening show strong temperature dependence due to the change of the relaxation time and the Fermi functions.

Investigation of (AlGa)As/GaAs quantum wells by optical and phototransport measurements

AbstractAn optical and phototransport characterization of (AlGa)As/GaAs multi‐quantum‐well (MQW) structures grown by molecular beam epitaxy (MBE) is reported. Photovoltage, photoluminescence, and optical transmission spectra are studied on p‐i‐n photodiodes in which the intrinsic (nominally undoped) region contains the wells. The thickness of GaAs and (AlGa)As layers is in the range 8 to 11 and 12 to 17 nm, respectively. The Al x‐molar fraction is in the interval 0.25 to 0.30. In the photovoltage spectra transitions between the fundamental and also the first excited state of electron and hole subbands in the quantum wells are observed. Agreement is found between these spectra and the results of luminescence and transmission measurements. Theoretical calculation fits the temperature dependence of the peaks in the spectra; it is possible to estimate the well width with an accuracy of about 10%.

Impurity-induced layer disordering of In0.53Ga0.47As/In0.52Al0.48As heterostructures

Impurity-induced layer disordering of In0.53 Ga0.47 As/In0.52 Al0.48 As heterostructures grown by molecular beam epitaxy has been observed by Auger electron spectroscopy depth profiling. We find that Si+ ion implantation to concentrations greater than 2×1019 atoms cm−3 enhances the intermixing of Ga and Al in these heterostructures at an annealing temperature of 1075 K. However, the relatively high temperature which is required to activate the interdiffusion of Ga and Al in the region of high Si concentration is sufficient to induce In diffusion in regions of lower Si concentration. Zinc diffusion is found to completely intermix the Ga and Al in the heterolayers at temperatures as low as 825 K, which is below the temperature at which significant In diffusion occurs in undoped regions.

Negative differential resistance in AlAs/NiAl/AlAs metal base quantum wells: toward a resonant tunneling transistor

The authors report the observation of a negative differential resistance region (NDRR) in the axial current-voltage characteristics of a nominally 13-monolayer epitaxial NiAl film sandwiched between two 12-monolayer AlAs layers. The active area is separated by two 100-AA undoped GaAs regions from the heavily doped n-type GaAs electrodes on both sides. The NDRR is observable at room temperature in many of the devices but varies in strength across the wafer. The highest documented peak-to-valley ratio at room temperature is 2.0, but the majority of the devices exhibit lower ratios, typically around 1.2. The onset of the NDRR occurs around 800 meV. This value, however, contains the voltage drop at the top contact which is believed to be substantial. The origin of this effect is believed to be resonant tunneling through quantized electronic levels is the thin metal layer. This appears to be the first direct observation of size quantization in semiconductor-metal heterostructures.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Numerical study of an AlGaAs/GaAs heterostructure bipolar transistor: Emitter design and compositional grading

The effect of various forms of compositional grading of Al in the emitter of a HBT is studied via numerical simulation. The governing drift and diffusion equations and Poisson's equation are solved in two dimensions. For the nominal doping levels and layer widths considered, linear grading over a 300 Å region provides the best overall performance in terms of collector current and high frequency characteristics. When linear grading is introduced over a 300 Å-wide undoped region (spacer layer) the turn-on voltage is found to decrease; the f T is also higher than for other forms of grading at I c below 3000 A/cm 2. When a parabolic grading profile is implemented in the emitter, the resulting performance is not significantly different from linearly graded devices to warrant complicated process steps.

Energy levels and optical absorption associated with Ga0.47In0.53As/Al0.48In0.52As MQW

Multiple-quantum-well (MQW) samples of GaInAs/AlInAs, with 100 AA, barriers and 50 AA wells, were grown by MBE. The samples contained 10, 20 or 50 periods and were either nominally undoped throughout or consisted of nominally undoped MQW region sandwiched between thick doped layers. Measurements of optical absorption yielded information about the sub-band structure of the superlattice and gave a measure of the absorption coefficients. Non-parabolic effective-mass theory was used to compare calculated energy levels with observations and satisfactory agreement was obtained.

Fundamental studies and device application of ?-doping in GaAs Layers and in AlxGa1?xAs/GaAs heterostructures

In addition to the realization of atomically abrupt interfaces in III–V semiconductors by molecular beam epitaxy, the confinement of donor and acceptor impurities to an atomic plane normal to the crystal growth direction, calledδ-doping, is important for the fabrication of artifically layered semiconductor structures. The implementation ofδ-function-like doping profiles by using Si donors and Be acceptors generates V-shaped potential wells in GaAs and AlxGa1−xAs with a quasi-two-dimensional (2D) electron (or hole) gas. In this review we define three areas of fundamental and device aspects associated withδ-doping. (i) The prototype structure ofδ-doping formed by a single atomic plane of Si donors in GaAs allows to study the 2D electron gas by magnetotransport and tunneling experiments, to study the metal-insulator transition, and to study central-cell and multivalley effects. In addition, non-alloyed ohmic contacts to GaAs and GaAs field-effect transistors (δ-FETs) with a buried 2D channel of high carrier density can be fabricated fromδ-doped material. (ii) GaAs sawtooth doping superlattices, consisting of a periodic sequence of alternating n- and p-typeδ-doping layers equally spaced by undoped regions, emit light of high intensity at wavelengths of 0.9 <λ <1.2 [μm], which is attractive for application in photonic devices. The observed carrier transport normal to the layers due to tunneling indicates the feasibility of this superlattice as effective-mass filter. (iii) The confinement of donors (or acceptors) to an atomic (001) plane in selectively doped AlxGa1−xAs/GaAs heterostructures leads to very high mobilities, to high 2D carrier densities, and to a reduction of the undesired persistent photo-conductivity. Theseδ-doped heterostructures are thus important for application in transistors with improved current driving capabilities.

Transient analysis of a triangular-barrier bulk unipolar diode

The transient analysis of a triangular-barrier bulk unipolar diode (TB) is based on the conduction currents associated with both the majority and minority carriers together with a displacement current associated with the space change regions. During the turn-on and turn-off transients the conduction current, which is determined by thermionic emission-diffusion theory, is shown to have negligible effect compared with the displacement current. The transients and steadystate currents of a typical GaAs TB are compared to those from a device simulator which includes thermionic emission over the barrier, and are shown to be in excellent agreement. 10% time constants are determined by an analytic expression for both turn-on and turn-off and are shown to be equivalent and dependent on the applied DC bias and independent of the position of the p+plane in the undoped region.

Detectors for monolithic optoelectronics

Abstract In this paper, we present data on the electrical characteristics and the optical response of photodetectors integrated on GaAs substrates with FET devices. We compare the differences between devices fabricated on globally implanted areas versus the undoped semi-insulating regions of the same wafer.

Single heterostructures for optical transport experiments

We introduce a new GaAs/AlxGa1−xAs single heterostructure, which allows local optical injection of a one-component confined plasma. The heterojunction is placed in the undoped region of a p-i-n structure, which provides a high built-in electric field for the separation of injected electrons and holes, as well as for the two-dimensional confinement of electrons. A time-of-flight measurement of the majority-electron drift velocity at room temperature is demonstrated.

The optical properties of AsF 5-doped Trans-polyacetylene

Measurements of the anisotropic optical properties on AsF 5-doped highly-oriented non-fibrous crystalline trans-(CH) x are presented. The reflectance parallel to the chain direction shows a Drude-like behaviour of strongly damped carriers and no indication of any interband contributions from undoped regions. Perpendicular to the chain direction, the reflectance is very low and constant throughout the region of the plasma edge for light polarized parallel to the chains. A Kramers-Kronig analysis of the parallel reflectance data yields the complex refractive index and allows the real and imaginary parts of the dielectric function and the optical conductivity to be deduced.

Ga0.47In0.53As/InP superlattice avalanche photodiode grown by metalorganic chemical vapor deposition

We report the operation of the first Ga0.47In0.53As/InP superlattice avalanche photodiode grown by metalorganic chemical vapor deposition. The undoped region of the p-i-n structure consists of 100 periods of alternating InP and Ga0.47In0.53As layers 100 A&amp;ring; thick with no intentional doping. dc multiplication higher than 20 and high-speed response with full width at half-maximum of 200 ps have been measured.

Tunneling-assisted impact ionization for a superlattice

Tunneling-assisted impact ionization across the conduction-band-edge discontinuity of quantum-well heterostructures is investigated and applied to a new superlattice structure. We consider multiquantum-well structures where the quantum-well regions are heavily doped and the undoped barrier regions are essentially insulating. Incident hot electrons due to the applied electric field perpendicular to the heterointerface interact with the two-dimensional electrons confined to the quantum wells through Coulomb force. The resultant electrons can either have enough energy to get out of the wells or tunnel through the triangular barriers. A new analytical approximation for the impact ionization rate is given which compares favorably with numerical results. The tunneling-assisted impact ionization rates and the ionization coefficients are calculated. It is shown that the tunneling effect reduces the ionization threshold and enhances the ionization rate significantly.