Doping Carrier Concentration Research Articles

Control of Carrier Type and Density in Exfoliated Graphene by Interface Engineering

Air-stable, n-doped or p-doped graphene sheets on a chip were achieved by modifying the substrates with self-assembled layers of silane and polymer. The interfacial effects on the electronic properties of graphene were investigated using micro-Raman and Kelvin probe force microscopy (KPFM). Raman studies demonstrated that the phonon vibrations were sensitive to the doping level of graphene on the various substrates. Complementary information on the charge transfer between the graphene and substrate was extracted by measuring the surface potential of graphene flakes using KPFM, which illustrated the distribution of carriers in different graphene layers as well as the formation of dipoles at the interface. The Fermi level of single layer graphene on the modified substrates could be tuned in a range from -130 to 90 mV with respect to the Dirac point, corresponding to the doped carrier concentrations up to 10(12) cm(-2).

On the development of high-efficiency thin-film GaAs and GaInP 2 cells

This work presents an overview of recent results obtained in the field of III–V solar cell research as performed at the Radboud University Nijmegen. An n-type Al x Ga 1− x InP 2 window layer with x=1 was found to give the highest GaAs cell performance, in spite of a low doping carrier concentration as compared to GaInP 2. As a result of the optimization of the GaAs cell structure obtained by metalorganic chemical vapor deposition (MOCVD) and subsequent processing, thin-film cells transferred from their native GaAs substrate to a foreign carrier, were achieved with a conversion efficiency of 24.5%. This is a world record for single-junction thin-film cells. Difficulties in the processing of thin-film GaInP 2 cells, due to anomalous etching behavior of these layers in HCl, have been overcome by using a HBr/Br 2/H 2O mixture as an etchant. Thin-film GaInP 2 cells with efficiencies up to 11.4% were obtained.

Concentration Effects on n-GaN Schottky Diode Current-Voltage (i-v) Characteristics

We focus on the epi layer carrier concentration variation effects to improve the current – voltage (I-V) characteristics of an n-GaN schottky diode. The carrier concentration of 1×10 15cm-3, 1×1016 cm−3, 1×1017 cm−3 were employed. The simulated current was obtained by forward biasing the device of up to 2Volt at room temperature using Pt electrode. The study was conducted by using Atlas/Blaze using various models such as Consrh (Concentration Dependent Shockley Read Hall), Cvt (Lombardi Model), Fermi (Fermi Dirac), Bgn (Bandgap Narrowing), Conmob (Concentration Dependent Mobility), Auger (Auger). We found that as the concentration increases the value of forward current also increase linearly when biased at maximum of 2 volts. The reverse bias characteristics at the same concentration of the simulated diode up to 100Volt were also determined. We found that at low carrier concentration the reverse leakage current is minimum and breakdown voltage is maximum. As the carrier concentration increases there is a linear relationship between reverse leakage current and epi layer doping carrier concentration. By analyzing the forward and reverse characteristics we conclude that in forward bias for low carrier concentration the diode shows schottky rectifying behavior while for higher carrier concentration the diode shows ohmic behavior. For higher carrier concentration there is a linear relationship between carrier concentration (n) and forward current. The reverse leakage current is minimum approaching an ideal value at n≤1×1015cm-3 and breakdown voltage is maximum at these values of concentration. Increasing the concentration from n≤1×1015cm-3 the value of reverse leakage current is approaching to the maximum value as a result breakdown voltage decreases. We conclude that for n-GaN schottky diode the ideal schottky rectifying behavior of I-V characteristics is obtained at carrier concentration of n≤ 1×1015cm-3 for the simulated diodes at different carrier concentration.

In situ current-voltage characterization of swift heavy ion irradiated Au/n-GaAs Schottky diode at low temperature

A Au/n-GaAs(100) Schottky diode was irradiated at 80 r K by a 180 r MeV 107 Ag 14+ ion beam. In situ current-voltage (I--V) characterization of the diode was performed at various irradiation fluences ranging from 1 ‐ 10 10 to 1 ‐ 10 13 r ions r cm m 2 . The semiconductor was heavily doped (carrier concentration=1 ‐ 10 18 r cm m 3 ), hence thermionic field emission was assumed to be the dominant current transport mechanism in the diode. Systematic variations in various parameters of the Schottky diode like characteristic energy E 0 , ideality factor n , reverse saturation current I S , flatband barrier height K bf and reverse leakage current I R have been observed with respect to the irradiation fluence. The nuclear and electronic energy losses of the swift heavy ion affect the interface state density at the metal-semiconductor interface resulting in observed variations in Schottky diode parameters.

Effects of structural phase transitions on Superconductivity and magnetism in substituted derivatives of La 2CuO 4

Structural (neutron and x-ray diffraction) and magnetic (neutron diffraction and magnetic susceptibility) measurements of materials derived from La 2CuO 4 by isovalent and aliovalent substitution in the La 3+ site have been performed. Such substitutions permit the tilts of the CuO 6 octahedra (about the (110) and (1 10) axes of the undistorted 14/mmm structure), and the hole concentration in the CuO layers, to be independently varied. The results of these measurements, in combination with those from other research groups, demonstrate that all four tilt structures, HTT (high temperature tetragonal, 14/mmm), LTO1 (low temperature orthorhombic 1, Bmab), LTO2 (low temperature orthorhombic 2, Pccn) and LTT (low temperature tetragonal, P4 2/ncm), support bulk superconductivity, with T c ( HTT) > T c ( LTO1) > T c ( LTO2) > T c ( LTT) at the same hole concentration. Furthermore, there is a strong suppression of superconductivity when the doped carrier concentration is 1/8 hole per Cu 2+ in the LTT and LTO2 phases of these materials. The reason for this T c suppression is not established. Finally, the magnetic structures adopted by the Cu 2+ spins in the insulating phases, determined by neutron diffraction, are also influenced by the tilts of the octahedra. This may reflect a structural dependence of interlayer and intralayer magnetic anisotropies.

Electron mobility in InAs1−xSbx and the effect of alloy scattering

The significance of alloy scattering on the electron mobility of InAs1−xSbx is studied in detail. Since little experimental data are available for the InAs1−xSbx compound, calculations of the electron mobility of the binary InAs and InSb semiconductors are first evaluated from the average momentum relaxation times, 〈τm〉 of the individual mechanisms (comprising ionized and neutral impurities, acoustic phonon deformation potential and piezoelectric, optical phonon deformation potential and polar) so that they can be compared to available experimental results. Having ensured that the calculated data are in good agreement with the experimental results in these binary compounds, the electron mobility of the ternary, InAs1−xSbx as a function of x at 77 K and 300 K is calculated by introducing an alloy scattering term. At 77 K, for a total doping concentration of ≂1017 cm−3, the electron mobility of InAs1−xSbx is limited by ionized impurity scattering and alloy scattering is insignificant. However, for doping concentration of ≂1015 cm−3, alloy scattering is found to be the most significant process from about x=0.1 to 0.8. At 300 K the alloy scattering reduces the total electron mobility from between a few percent at x=0.15 to a maximum of about 11% for x=0.60. It is observed that alloy scattering is greatest at x=0.15 but it still is about 11% of the total electron mobility at this composition. The alloy mobilities at 77 K for x=0.6 and x=0.15 are 1.28×106 cm2 V−1s−1 and 7.96×105 cm2 V−1s−1, respectively. The mobilities at x=0.60 are further evaluated as a function of the doping carrier concentration at 77 and 300 K. It is found that alloy scattering becomes increasingly significant for lightly doped semiconductors at low temperatures.

Superconductivity of BaPb 1-xBi xO 3 and Ba xK 1-xBiO 3 in the coexisting state of itinerant large polarons and localized small bipolarons

From the detailed analysis of Raman spectra in BaPb1-xBixO3 (BPB) and BaxK1-xBiO3 (BKB) five electron-phonon coupled modes which originate in the breathing mode were extracted. The doped carrier concentration dependence of those modes suggests the coexistence of the itinerant large polarons and localized bipolarons. The itinerant state has a finite extent of the wave function. The extent in BKB is larger than in BPB, which advantages the high Tc. The strong interactions between these two electronic states is a possible origin of the superconductivity.

Highly conductive Ga-doped polycrystalline (Zn,Cd)S films

Highly conducting Ga-doped (Zn,Cd)S thin films with various Zn contents are used as collector/window layers in chalcopyritebased heterojunction solar cells. The films were prepared by means of thermal evaporation. In order to elucidate the effects of Ga doping on carrier transport and optical properties, Hall effect, thermoelectric power and optical transmission measurements have been carried out. Both Hall data and thermoelectric power data show a decrease of mobility and of carrier concentration with increasing Zn content. The comparison of n Hall and n thermo versus conductivity of the films indicates polar optical phonon scattering for lower concentration levels and scattering determined by charged impurities at carrier concentrations exceeding 3 X 10 19 cm 3. With increasing Ga doping, carrier concentration n Hall and mobility μ Hall show different maxima depending on Zn content. For Zn 0.2 Cd 0.8 S films, a maximum carrier concentration of about 10 20 cm -3 at 0.5 at % Ga and a maximum mobility of about 40 cm 2/ V· s at 1 at% Ga can be achieved. Above a specific Ga doping level, both carrier concentration and Hall mobility diminish. Optical data show a significant Moss-Burstein shift (up to 300 meV) for carrier concentrations exceeding 3 X 10 19 cm -3. The electrical and optical data are in agreement.

Heavy Boron Doping in Low‐Temperature Si Photoepitaxy

Heavy boron doping of up to was achieved in a photoepitaxial layer grown at 650°C. Under UV irradiation, the doped carrier concentration was independent of the diborane flow rate in the heavily doped region and doped boron atoms were completely activated up to . Without UV irradiation, no single crystals were grown and very few boron atoms were activated in the heavily doped region. We studied the crystal quality using Raman scattering spectroscopy and found that, under UV irradiation, single crystals could be grown up to . The boron atom activation ratio depended strongly on the crystal quality. UV irradiation markedly increased carrier doping efficiency due to photo‐enhancement of the diborane vapor phase reaction, which we studied using FTIR and the surface reaction of adsorbed species. The electrical properties of low‐temperature photoepitaxial layer with heavy boron doping were studied using a bipolar transistor. Excellent device characteristics were achieved. Low‐temperature photoepitaxy also produced very abrupt boron profiles.

Orientation dependence of crystal defects formation in Si molecular beam epitaxy

The origin of crystal defect in epitaxial Si film grown by the molecular beam epitaxial (MBE) method has been studied. Examination went down to the detail of crystal defect density, unintentionally doped carrier concentration, and carbon contaminations of the epitaxial film with both substrate orientations of (100) and (111). The crystal defect originated from the epitaxial interface has been affected by the excessive carbon content formed at the interface during the substrate cleaning process, because both the defect and the amount of carbon with (100) are statistically much lower than those with (111). Both unintentionally doped hole concentration which has a run number dependence and the crystal defect formed during the epitaxial growth have been much smaller with (100) than with (111). A similar trend in the run number has also been observed in terms of the residual gases of CO and CO2 in the vacuum chamber. From these facts, it has been concluded that characteristics of the MBE film grown on the (111) surface are much more sensitive to the carbon contamination than those on the (100).

Dependence of the work-function difference between the polysilicon gate and silicon substrate on the doping level in polysilicon

We correlate the work-function difference φ ps 0between the polysilicon gate and the silicon substrate in an MOS system with the doping level and carrier concentration in polysilicon. Polysilicon was doped by ion implantation with arsenic and phosphorus. The doping level was varied from 1019to 1020cm-3. Hall measurements were used to determine the carrier concentration in polysilicon at a given doping level. The Hall mobility and resistivity as a function of doping level were also obtained. The work function difference φ ps 0was determined by capacitance-voltage measurements on polysilicon-SiO 2 -Si capacitors with different oxide thicknesses. When plotted against the doping level, the work-function difference had a maximum at a dopant concentration of ≈ 5 × 1019cm-3, which corresponds to an electron concentration of 1.5 × 1019cm-3. At higher doping levels the value of φ ps 0decreases. The results can not be fully understood in terms of the Si band structure.

Silicon ribbons made by roller quenching methods

Polycrystalline silicon ribbons were prepared by Roller Quenching Methods. Ribbons were obtained flexible and of virtually continuous length, 20μ m to 200μ m in thickness, and 1 mm to 50 mm in width. Microscopic observations of etched ribbon surfaces showed that the average grain sizes were 20μ m to 30μ m. Cross-sectional views showed the ribbons to have a columnar grain structure. Unwanted impurities were undetectable by Auger analysis, except very thin oxygenated layer was present on the surface of the ribbons. By means of DC conductivity and Hall measurements of intentionally doped silicon ribbons, it was found that the active carrier concentration of the ribbons was nearly the same as the doped carrier concentration, from 1014/cm3 to 1020/cm3 , the resistivity of the ribbons were about one order of magnitude higher than that of silicon single crystals, and the Hall mobility parallel to the ribbon axis and perpendicular to the ribbon plane was 1 to 150 cm2 · V-1 · sec−1 and 50 to 700 cm2 · V−l · sec−1, respectively. A solar cell using the present ribbon was made by means of a CVD silicon deposition method. Satisfactory diode characteristics and a conversion efficiency of about 5 % were obtained.

Ultrathin doping layers as a model for 2D systems

The growth of semiconductor crystals with abrupt and ultrathin doping profiles has become feasable by the method of molecular beam epitaxy. Such doping layers introduce strong space charge potentials if incorporated in crystals of opposite bulk doping. The motion of the carriers perpendicular to the layer becomes quantized by this potential, similar to the situation in space charge layers in MOS structures. Futhermore population and energy separation of the subbands can be changed also in our system by applying external bias or by optical excitation. The electronic structure of such interface free systems is discussed with particular emphasis on the expected advantages compared to familiar MOS systems. Selfconsistent Hartree calculations of subband energies as a function of doping and free carrier concentration are presented.