Nonuniform Dopant Distribution Research Articles

Nonuniform doping distribution along silicon nanowires measured by Kelvin probe force microscopy and scanning photocurrent microscopy

We use Kelvin probe force microscopy and scanning photocurrent microscopy to measure the doping distribution along single phosphorous-doped silicon nanowire grown by the vapor-liquid-solid method. A nonlinear potential drop along biased silicon nanowires is detected both by measuring the surface potential directly via Kelvin probe force microscopy and by integrating the photocurrent measured by scanning photocurrent microscopy. These variations in the potential and field are further analyzed to extract the longitudinal dopant distribution along an individual silicon nanowire. The results show a very good agreement between the two methods to quantitatively detect potential, field, and doping variations within doped silicon nanowires.

Composition, morphology, and properties of sodium-bismuth tungstate crystals

A correlation has been revealed between the formation conditions, composition, morphology, and physicochemical properties of NaBi1 − x(W1 + yO4)2 crystals (0 ≤ x ≤ 0.16, 0 ≤ y ≤ 0.11). The effect of melt deviation from stoichiometry on the decomposition rate of grown crystals under electron beam irradiation is shown by transmission electron microscopy. A nonuniform dopant distribution over the crystal boule cross-section is found by microprobe analysis. The Raman spectra of the samples depend on their crystallographic orientation.

Microstructure Study of BaTiO<sub>3</sub> Ceramics Using Convergent Beam Electron Diffraction

The nanoscale local structure of core-shell structured BaTiO3 grains with a nonuniform dopant distribution was studied by convergent beam electron diffraction. The direct measurements of the variation of lattice parameter in an individual BaTiO3 grain were achieved. It was found that the lattice parameter continuously increased from inside the grain core to the grain shell and there was no discontinuity at the core/shell interface. The increasing gradient of c-axes was slightly smaller than that of a-axes, suggesting that the perovskite unit cell in the grain shell has lower tetragonality.

A general method for the calculation of segregation profiles in floating zone grown silicon ingots with non-uniform initial distribution of the solute

In polycrystalline silicon analytics segregation effects related to radial non-uniform dopant distributions are of major importance for the interpretation of resistivity data. The determination of dopant concentrations and resistivity profiles in polycrystalline silicon inevitably requires the transformation into a single crystal. Neglecting segregation effects may therefore substantially bias the actual material properties. A tool for the computation of final concentration profiles in floating zone grown ingots with non-uniform initial distributions has been developed, which allows the re-calculation of polysilicon data from single crystal data.

Threshold Current for the Onset of Kirk Effect in Bipolar Transistors With a Fully Depleted Nonuniformly Doped Collector

We derive a generalized expression for the threshold current of the Kirk effect (base widening) in bipolar transistors that have a fully depleted collector and a nonuniform dopant distribution in the collector. This generalized expression can be helpful to the analysis of the electrical characteristics, such as the cutoff frequency as a function of bias conditions, and hence to the optimization of such devices

Degradation of 1/f noise in short channel MOSFETs due to halo angle induced VT non-uniformity and extra trap states at interface

The impact of halo implantation angle on the low-frequency noise of short channel n-MOSFET is reported. The low-frequency noise is degraded with larger tilt angle for the same implant dose and energy. The higher dose/energy of the halo implant with larger tilt angle further enhances the degradation of low-frequency noise. The larger halo angle introduces non-uniform doping distribution and creates the non-uniform threshold voltage along the channel. Additional traps can be created near the oxide/semiconductor interface due to boron pileup due to larger tilt angle. A quantitative analysis supported by experimental results confirm that the degradation of 1/f noise is due to the combined effect of non-uniformity in threshold voltage along the channel and the creation of extra trap charges near the oxide-semiconductor interface (near-interfacial charges).

Laser direct writing and doping of diamond-like carbon, polycrystalline diamond, and single crystal silicon carbide

Nanosecond pulsed Nd:yttrium–aluminum–garnet laser treatment is applied to alter the electric properties of free standing polycrystalline diamond substrates, diamond-like carbon (DLC), and 4H–SiC single crystal wafers. In the case of DLC samples, the laser irradiation, nitrogen doping, as well as the incorporation of cobalt into the DLC layer reduces its electric resistance. Laser fabrication of Schottky barrier diode at the DLC–cobalt contact is demonstrated, and its nonlinear 1/C2 vs V curve indicates a nonuniform dopant distribution. The nitrogen dopant profile in the laser-doped SiC wafer is obtained by secondary ion mass spectroscopy and the corresponding nitrogen diffusion coefficient under laser processing parameters is calculated. Laser doping enhances the nitrogen diffusivity in SiC and allows in situ fabrication of metal contacts with no additional materials. Scanning electron microscopy, wavelength dispersive spectroscopy, and x-ray photoelectron spectroscopy are used to study the surface composition and the binding states in the laser-treated samples.

The influence of charge states and elastic stresses on the diffusion of point defects in silicon

The properties of nanostructures such as ultrathin layers 50 nanometers in thickness or smaller are strongly dependent on the state of a defect subsystem in the layers. The objective of this paper is to study the influence of charge states and internal elastic stress on the transport of point defects in silicon. The equation for the diffusion of point defects taking into account the contributions of different charge states of defects to the transport process is reduced to the form convenient for numerical solution. For this purpose, the effective diffusivity of point defects was obtained as a function of the dopant concentration and empirical parameters. The effective coefficient of the absorption of point defects by immobile sinks was also obtained in a similar convenient form. Using the modified equation for the diffusion of point defects, we have simulated the transport processes of diffusing species for different charge states of defects and nonuniform dopant distributions. The cases of zero and nonzero elastic stress were investigated.

A Study of the Thermoluminescent Properties of CVD Diamond Detectors

A batch of 20 diamond detectors obtained by the chemical vapour deposition (CVD) method at the Institute for Materials Research at the Limburg University, Belgium, was investigated with respect to their thermoluminescent (TL) properties. The investigated detectors demonstrate TL sensitivity similar to that of the standard LiF : Mg, Ti (MTS) thermoluminescent detectors, lack of fading after two weeks from irradiation and apparent linearity of dose response. In spite of the persistent fluctuation of individual detector sensitivity observed in this batch, a new annealing procedure improved the stability of the TL signal. It has been concluded that 1 h annealing at 350 °C assures the highest reproducibility for this set of detectors. A 30% discrepancy of the value of the TL signal between individual detectors from the batch may be caused by non-uniform distribution of dopants in the volume of the CVD diamond. A prototype of a planar TL reader equipped with a CCD camera was employed in this investigation.

Dopant profile and gate geometric effects on polysilicon gate depletion in scaled MOS

Polysilicon depletion effects show a strong gate length dependence according to experimental p-channel MOS capacitance-voltage (C-V) data. The effect can be influenced not only by gate geometries, but also by dopant profiles in poly-gates. These effects have been modeled and verified using device simulation. Nonuniform dopant distributions in the vertical and lateral direction in the poly-gate cause additional potential drops. The potential drop in the poly-gate becomes critical as the gate geometry is scaled down due to edge and corner depletions resulting from fringing electric fields.

Comparison of the potential fluctuations in a-Si:H measured by the optical and transport methods

Hydrogenated amorphous silicon (a-Si:H) has potential fluctuations, which arise from the heterogeneities, variations in the bond angles, bond lengths, and nonuniform distribution of hydrogen and dopants, etc. We have estimated the potential fluctuations present in undoped and doped a-Si:H from the optical as well as from the transport measurements. In the optical measurements, the subgap absorption has been used to estimate the width of the potential fluctuations. In the transport method, the conductivity and the thermopower measured as a function of temperature are used to provide the width of the potential fluctuations, as shown by Overhof and Beyer. We find that in all cases, the width of the potential fluctuations measured by the optical measurements is about 20 times larger than that measured by the transport measurements on the same sample. This is explained by noting that optical absorption is a local process, which is sensitive to the short-range potential fluctuations by the order of 3 Å, whereas the transport measurements are sensitive to the long-range potential fluctuations (≈80 Å) which may have smaller width.

Scanning Electron Microscopy of Dopants in Semiconductors

Abstract It has been demonstrated that doped regions in semiconductors yield contrast in secondary electron images taken under appropriate imaging conditions. Perovic et al. attributed this "electronic" contrast to surface potentials arising from the bending of the energy bands at the surface of the semiconductor. This band-bending results from the presence of surface states which pin the Fermi level. These effects have found application to an important problem in the electronics industry -- the profiling of dopant distributions in two-dimensions. However, such dopant distributions are rarely uniform, i.e., the dopant concentration may change over many orders of magnitude within distances of only a few tens to hundreds of nanometers. Therefore, it is necessary to understand how the observed contrast from the doped regions varies for non-uniform dopant distributions to apply the technique to real electronic devices.

The Thermochemical Surface Modification of the Iron-Carbon Alloys

AbstractIron and steel samples have been treated in overheated salt -water vapor environment (oxy-doping technique). SEM and optical microscope images of modified surface demonstrated that coating with three distinct layers was deposited. XPS, XRD and SEM + EDA studies were used for the composition evaluation, and non-uniform distribution of dopants was found. Tribological properties were determined, and it was found improvement of working characteristics in 2–10 times. Thermally enhanced difflusion model describing the coating formation was discussed.

Atomic Force Microscopy and Kelvin Probe Force Microscopy Evidence of Local Structural Inhomogeneity and Nonuniform Dopant Distribution in Conducting Polybithiophene

Direct evidence of local structural inhomogeneity and nonuniform doping-level distribution in conducting polymer film has been obtained using Kelvin probe force microscopy (KFM) and atomic force microscopy (AFM). The KFM data suggests that the polymer consists of grains that constantly differ in work function and thus in the dopant concentration from the grain peripheral regions. For the as-grown polymer, most of the doping charge is located at the grain tops, whereas the electrochemically doped polymer features relatively higher doped grain periphery and less doped grain tops. The AFM study reveals two different kinds of the polymer molecular structure dependent on whether the image was taken at the top of a grain or the grain periphery. This result confirms the inherent inhomogeneity of conducting polymers demonstrated with KFM.

A new two-dimensional short channel model for the drain current-voltage characteristics of a fully depleted SOI (silicon-on-insulator) MOSFET

Abstract A new drain-current model applicable to submicrometer SOI MOSFETs is presented. The analysis is carried out for a non-uniform doping distribution in silicon film and it takes into account the field dependence of mobility of electrons in the conducting channel and the possible fringing field effects near the drain and source ends. The present two-dimensional (2-D) model considers the short-channel effect on the depletion layer charge and hence on the static characteristics of the device. The results so obtained are verified with experimental data. The analysis is extended to find an expression for transconductance in terms of terminal voltages and device parameters. The cut-off frequency of the present model lies in the microwave region; therefore, to the first-order approximation, the model could be suitable for very high frequency applications.

Analytical two-dimensional modeling for potential distribution and threshold voltage of the short-channel fully depleted SOI (silicon-on-insulator) MOSFET

A two-dimensional analytical model for fully depleted SOI MOSFETs is presented. An extensive study of potential distribution in the silicon film is carried out for non-uniform doping distribution and extended to find an expression for threshold voltage in the sub micrometer region. The results so obtained are verified with experimental data. The present model calculates a critical gate voltage (for short channel fully depleted SOI devices) beyond which gate losses its control on drain current. The advantages of SOI MOSFETs over the bulk counterparts are explained on the basis of drain induced barrier lowering [DIBL]. It is also shown that the threshold voltage for the thin film SOI MOSFET is less than that of bulk MOSFET. The short-channel effects, DIBL and threshold voltage reduction, are well predicted in the present model.

Viscosity of GeO/sub 2/-doped silica glasses

The viscosity of GeO/sub 2/-doped silica glasses is experimentally evaluated and found to be expressed as a function of the monoexponential form /spl eta//sub 0/ exp(/spl minus/A/spl Delta/e), where /spl eta//sub 0/ is the viscosity of pure silica glass, A is a positive constant, and /spl Delta/e is an equivalent relative-index difference that is introduced for a silica glass with nonuniform dopant distribution in its cross section. The constant A for GeO/sub 2/-doped silica glasses is found to be about 0.5, which is much smaller than that for fluorine-doped silica glasses and has almost no temperature dependence between 1400/spl deg/C and 2200/spl deg/C.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The effects of As overpressure and diffusion source on the diffusion of Mn in GaAs

Data are presented to show the effect of As overpressure on the diffusion of Mn in GaAs using four different Mn sources. These sources include solid Mn thin film deposited directly on the GaAs substrate and Mn vapors from pure Mn, MnAs, and Mn3As solids. In the circumstance for which a solid Mn film is used as the diffusion source, a nonuniform doping distribution and poor surface morphology is obtained due to a reaction between the Mn film and the GaAs matrix. The degraded surface consists of a layer of polycrystalline cubic alloy having a lattice constant of nearly 8.4 Å and a composition close to MnGa2 with a small amount of As. Of the remaining diffusion sources (Mn, MnAs, and Mn3As), only MnAs consistently produces a uniform doping distribution and smooth surface morphology. For diffusions at 800 °C, a uniform surface hole carrier concentration as high as 1020/cm3 can be obtained using MnAs as the source. The As overpressure is found to drastically alter the Mn diffusion profile, and Mn, like Zn, may diffuse in GaAs interstitial-substitutionally. Vapor from both the Mn and Mn3As solids degrade the GaAs surface. Mn3As, however, uncharacteristically degrades the surface more rapidly although the details of such are not well understood. With the presence of a high As overpressure, however, both surfaces of the Mn and Mn3As sources are converted to (Mn,As) compounds, the compositions being close to MnAs. High enough As overpressures are shown to completely suppress the GaAs surface degradation which is evident when Mn3As alone is used as the diffusion source.

A Mathematical Model for the Influence of Deep‐Level Electronic States on Photoelectrochemical Impedance Spectroscopy: II . Assessment of Characterization Methods Based on Mott‐Schottky Theory

The impact of the assumptions inherent in using the Mott‐Schottky theory to identify deep‐level electronic states in semiconductors was assessed by comparison to the results of a less restrictive mathematical model. The model, developed in another paper, treated the transport and recombination reactions involving electrons, holes, and electronic states located within the bandgap. The capacitive component used in standard Mott‐Schottky theory was found to be insensitive to bulk electronic states within the bandgap for concentrations significantly less than the doping level. The resistive component measured at low frequencies was much more sensitive to deep‐level states and may be used to determine their distribution. For high concentrations of deep‐level states, the model results were in agreement with the current practice of attributing changes in the slope of the Mott‐Schottky curve to partial ionization of single‐energy deep‐level states with applied potential. In the absence of frequency dispersion, these changes in slope could be attributed instead to nonuniform dopant distribution.

Dopant redistribution and crystalline damage removal during rapid thermal annealing of Be and Mg implants in Ga 0.47In 0.53As

Secondary ion mass spectrometry (SIMS) and 1.5 MeV Rutherford backscattering (RBS) have been used to correlate redistribution of Be + and Mg + implants with the reordering of damaged crystals during rapid thermal annealing (RTA) of Ga 0.47In 0.53As. Studies were made of three different implant conditions: 1 × 10 14 cm -2 Be + and 1 × 10 15 cm -2 Mg +, implanted at room temperature, and also 1 × 10 14 cm -2 Mg + implanted at 200°C. In addition, a comparison was made of two different methods of surface protection, CVD Si 3N 4 and a GaAsP “proximity cap”. It is found that redistribution of Be + differs slightly for the two types of surface protection employed, but that the use of RTA suppresses excessive indiffusion effects observed during long time furnace anneals. This allows reproducible Be + doping profiles to be achieved. 1 × 10 14 cm -2 Mg + implants performed at 200°C show considerable redistribution during encapsulation with CVD Si 3N 4 and subsequent dopant loss during RTA. Room temperature implants of 1 × 10 15 cm -2 Mg + contain an amorphous layer, the regrowth of which gives rise to non-uniform dopant distributions with depth and hinders the deposition of an effective Si 3N 4 encapsulant. It is proposed that this regrowth in GaInAs occurs in a similar manner to that observed in GaAs and InP. Proximity annealing results in retention of all the implanted Mg + dose.