Showing papers on "Van der Pauw method published in 1999"

Growth of Bi2Te3 and Sb2Te3 thin films by MOCVD

Abstract: Metal organic chemical vapor deposition (MOCVD) has been investigated for elaboration of Bi 2 Te 3 and Sb 2 Te 3 using TMBi (Trimethylbismuth), TESb (Triethylantimony) and DETe (Diethyltellurium) as metal–organic sources. Their thermoelectric and physical properties were studied versus growth conditions. The MOCVD elaboration of Bi 2 Te 3 and Sb 2 Te 3 was carried out in an horizontal reactor for a temperature varying from 400 to 500°C, a total hydrogen flow rate D T varying from 3 to 6 l mm −1 and ( R VI/V ) ratio ranging from 1.5 to 15. The thin films were deposited on pyrex and silicon substrates. The partial pressure of the V element varied between 0.5 10 −4 to 2 10 −4 atm to obtain high growth rate for micro-peltier applications. The cristallinity was investigated by X-ray diffraction and we observed a typical preferential c -orientation. The SEM micrographs show the layers quality and confirms the hexagonal structure. The microprobe data indicate that the stoichiometry of Bi 2 Te 3 and Sb 2 Te 3 is constant for all thickness of the epitaxial films (0.3–7 μm). The films are always n-type conduction for Bi 2 Te 3 and p-type for Sb 2 Te 3 . Seebeck coefficient and the minimum values of the resistivity were found close to −210 and +110 μV K −1 , 9 and 3.5 μΩ.m for Bi 2 Te 3 and Sb 2 Te 3 , respectively. Electrical measurements (mobility and carrier density) were performed by Van der Pauw method. For the two materials, the best values of thermoelectrical properties were obtained at a growth temperature closed to 450°C and a VI/V ratio varying from 2 to 8. The thermoelectric properties of the two materials stay constant when the growth rate is increasing to value higher than 1.5 μm h −1 . This result is very interesting for thick film applications. The previous objective of these experimental results has been to perform the thermoelectric properties of n- and p-type films by establishing first suitable deposition conditions and the elaboration of ternary alloys is now possible.

Properties of Cu(In,Ga)Se2 Thin Films Prepared by Chemical Spray Pyrolysis

Abstract: Thin polycrystalline films of Cu(In,Ga)Se2 alloy with the single-phase chalcopyrite structure were successfully grown by the chemical spray pyrolysis (CSP) method on a glass substrate at 360 and 400°C. Alloy composition in the film was well controlled by that in the spray solution. The films were characterized by X-ray diffraction, optical absorption, Raman spectroscopy, van der Pauw measurement and scanning electron microscopy. It was found that lattice constants, the optical band-gap energy and the A1 mode phonon frequency changed continuously with the alloy composition x. A grain size of about 1 µm was obtained for Cu-rich films.

Electrical and Thermoelectrical Properties of Sb2Te3 Prepared by the Metal-Organic Chemical Vapor Deposition Technique

Abstract: The V2VI3 binary compounds such as Bi2Te3, Sb2Te3 and their alloys are narrow band±gap semiconductors with a high thermoelectric ®gure of merit Z ˆ So=k, where S is the Seebeck coef®cient, o the electrical conductivity and k the thermal conductivity. These semiconductors have been extensively studied in recent years because of their promising applications especially for thermoelectrical devices [1, 2] and thermal [3] and humidity [4] sensors using the Seebeck and Peltier effects, respectively. Giani et al. [5] have grown Bi2Te3 on pyrex substrates by using the metal-organic chemical vapor deposition (MOCVD) technique in a horizontal quartz reactor. They have also studied in some detail the electrical and thermoelectrical properties of Bi2Te3. Venkatasubramanian et al. [6] have studied the MOCVD growth of Bi2Te3 and Sb2Te3 on a GaAs substrate and used their superlattice structures for thermoelectrical applications. Dauscher et al. [7] have elaborated Bi2Te3 thin ®lms by using pulsed laser deposition (PLD) and have shown that a congruent transfer of stoichiometry occurs from the target to the substrate over several cm and that a good crystallinity is achieved. Magri et al. [8] have investigated the properties of electrodeposited bismuth telluride ®lms and have shown that the ®lm composition depends on the electrolyte composition and the current density. Our studies were carried out in an attempt to make a detailed analysis of the behavior of Sb2Te3 thin ®lms regarding the effect of R ˆ VI=V ratio on electrical and thermoelectrical properties. Sb2Te3 thin ®lms were grown using the MOCVD technique in a horizontal quartz reactor. Triethylantimony (TESb) and diethyltellerium (DETe) were used as antimony and tellurium sources, respectively. To avoid the possibility of any premature decomposition, TESb and DETe sources were both maintained at 20 8C. Hydrogen was used as the carrier gas with a ow rate equal to 3 slm to obtain good results. This is due to a better cracking ef®ciency for a ow rate of 3 slm found by Giani et al. [5]. The substrate temperature was ®xed at 450 8C during the deposition process and controlled by a thermocouple in direct contact with the substrate holder. The VI=V ratio (RVI=V ˆ DETe partial pressure=TESb partial pressure) varied between 1 and 13. In addition, during the deposition of Sb2Te3, the partial pressure of the group V element (Sb) was kept constant and equal to 1 3 10 atm. A Philips X-ray diffractometer, using monochromatic CuKa radiation (e ˆ 1:54051 E A), was employed to obtain diffraction patterns from ®lms deposited on a Pyrex substrate. A wide range of e (from 58 to 308) was scanned so that all possible diffraction peaks could be detected. Surface morphology was examined by scanning electron microscopy (SEM). The composition of the deposited layers was measured using an energy dispersive X-ray (EDX) microanalyzer. To measure Seebeck coef®cients, heat was applied to the sample, which was placed between two small perfectly parallel brass cylinders. The temperature difference between these two cylinders was measured using thermocouples and a sensitive Keithley digital thermometer. The potential difference was obtained at the position of the two thermocouples using a sensitive digital multimeter. The Van Der Pauw technique was used at 300 K to evaluate the sample resistivity, its carrier's mobility and its carrier's concentration. The X-ray diffraction (XRD) pattern was compared with ASTM charts and showed that the deposited layers grew in (0 0 0 l)H and exhibited a polycrystalline phase characterized by the (1 0 1 5)H peak (Fig. 1). The same peak was observed by Mandouh [9] on vacuum-deposited Sb2Te3 thin ®lms and disappeared upon annealing at 473 K. The surface morphology and crystallinity of the deposited thin ®lms on the amorphous substrate were found to depend strongly on the VI=V ratio, and its aspect seemed quite different. The SEM micrograph shown in Fig. 2 is of one of the Sb2Te3 layers deposited at 450 8C with the VI=V ratio ˆ 7. It was observed that the shape of the crystallites was hexagonal but not symmetrical, which is in good agreement with the X-ray data and the fact that the crystallites are randomly oriented with respect to the amorphous substrate. EDX analysis showed that the binary compound was always stoichiometric. Opposite results were obtained on Bridgman Sb2Te3 [10], where progressive loss of Te occurred and where the sample composition contained excess Sb atoms,

Ionic and electronic conductivity in CaTi1−xFexO3−δ (x=0.1–0.3)

Abstract: Electrical conductivity measurements on CaTi1−xFexO3−δ (x=0.1, 0.2, 0.3) were performed on polycrystalline pressed and sintered tablets using the van der Pauw four point method in controlled atmospheres. The results were interpreted to reflect n-type, ionic and p-type conductivity at different oxygen partial pressures. An increasing iron content increases the number of oxygen vacancies and increases the ionic conductivity at high temperatures, but also increases the tendency of ordering, which suppresses the ionic conductivity at more moderate temperatures. These findings are in accordance with the phase diagram of the system CaTiO3-CaFeO2.5 based on X-ray and Mossbauer studies.

Electrical Properties of Woodceramics

Abstract: Woodceramics have recently attracted much attention as ecomaterials at low cost. Electrical properties of the woodceramics (WCM hereafter) have been characterized in the range 10–70% relative humidity and for temperatures from −20 to 100°C. The WCM bulk has been prepared by burning MDF board impregnated with phenolic resin at 650 and 750°C. Electrical resistance decreased linearly with increasing temperature, indicating the negative temperature coefficient like semiconductor. Relative humidity dependence of the resistance also indicated excellent linear characteristics between 10 and 70% RH measured here. Activation energies of 0.21, 0.15, 0.05, and 0.01 eV have been revealed from van der Pauw method. The excellent linearity for humidity and temperature is prominent advantages of WCM which may be useful as a new humidity and also temperature sensor.

Anisotropic Transport of Two-Dimensional Holes in High Landau Levels

Abstract: Magnetoresistance data taken along $[\bar{2}33]$ and $[01\bar{1}]$ directions in a GaAs/AlGaAs two-dimensional hole sample with van der Pauw geometry exhibit significant anisotropy at half-integer filling factors. The anisotropy appears to depend on both the density and symmetry of the hole charge distribution.

Magnetoconductivity of ultrathin epitaxial Ag films on Si(111) 7 × 7 at low temperatures

Abstract: Ultrathin epitaxial Ag films on Si(111) $7\ifmmode\times\else\texttimes\fi{}7$ have been shown to have at approximately 4 K a very low conductance, whereas at 100 K the metallic conductivity is evident. Therefore the magnetoconductance has been used to identify the different scattering mechanisms. The conductance and the magnetoconductance have been measured in situ with four-point probes in van der Pauw arrangement. For thicknesses from 1.8 to 20 ML different scattering mechanisms have been revealed in the temperature range from approximately 4 to 20 K and magnetic fields from $\ensuremath{-}4$ T to +4 T perpendicular to the sample surface. Whereas for films thicker than 3 ML the weak localization and antilocalization provide a complete description, the thinnest films show properties not yet described quantitatively by any theory.

Hall effect investigations of 4H-SiC epitaxial layers grown on semi-insulating and conducting substrates

Abstract: Nitrogen- and aluminum-doped 4H silicon carbide epitaxial layers were grown simultaneously on semi-insulating and conducting substrates. The layers were investigated by conventional van der Pauw Hall effect measurements and for comparison also with secondary ion mass spectrometry and capacitance voltage measurements. It was found, that the carrier concentration in the layers grown on conducting substrates were overestimated by the Hall effect measurement, which leads to an underestimation of the ionization energy of the main dopant, as compared to the layer grown on semi-insulating substrates. The difference can be explained by a two-layer Hall effect model.

Effect of threading dislocations on electron transport in In0.24Ga0.76N/GaN multiple quantum wells

Abstract: The effect of threading dislocations on electron transport in In0.24Ga0.76N/GaN multiple quantum wells has been studied by using transmission electron microscopy (TEM) and van der Pauw Hall effect measurements. From the cross-sectional TEM imaging, we observed the threading dislocations which “screw” through the multiple In0.24Ga0.76N/GaN quantum well. From the Hall effect measurement, we found that the Hall mobility decreases as the temperature decreases (μ∼T3/2) due to the threading dislocation scattering, and the Hall carrier concentration shows a transition from conduction-band transport to localized-state-hopping transport. The thermal activation energy of the residual donor level (probably Si) is about 20.2 meV.

Liquid Phase Epitaxial Growth and Properties of InSbBi Films grown from In, Bi and Sn Solutions

Abstract: InSb 1-x Bi x (0.01

Van der pauw structure to measure the resistivity of a doped area under diffusion areas and gate structures

Abstract: The present invention provides a van der Pauw semiconductor test structure for and a method of testing a resistivity of a doped area formed within a substrate of a semiconductor wafer which may be under a diffusion area or a gate structure. The test structure can include field oxide regions formed on a surface of the substrate and a base doped substrate formed within the substrate. Further, the test structure includes a first primary tub and secondary tubs that are formed within the base doped substrate, each of the secondary tubs having a first diffusion region formed adjacent to an inner isolation structure and a second diffusion region formed adjacent to an outer isolation structure. A second primary tub is located adjacent the first primary tub and a dielectric layer is formed over the substrate having contacts formed within the dielectric layer and between the isolation structures. Further still, the test structure may include a gate structure as part of the semiconductor test structure, with the gate structure having openings formed therein through which the contacts extend to the first diffusion regions and the second diffusion regions.

A thermal van der Pauw test structure

Abstract: A micromachined thermal van der Pauw test structure is reported. Similar in principle to the conventional electrical van der Pauw Greek cross test structures, it enables the in-plane thermal sheet conductivities of thin films to be determined. The analogy between the two-dimensional heat flow in thin film samples and the electrical current pattern in thin film conductors is exploited. A thermal sheet resistance of 1.87/spl times/10/sup 5/ K/W was extracted from the complete sandwich of dielectric layers of a commercial CMOS ASIC process. This is equivalent to an average in-plane thermal conductivity of the CMOS dielectric layer sandwich of /spl kappa/=1.44 Wm/sup -1/ K/sup -1/.

Nonswitching van der Pauw technique using two different modulating frequencies

Abstract: We present a nonswitching van der Pauw technique using two independent ac current sources and two lock-in amplifiers. This technique may be useful to measure the resistance of delicate samples that might be easily damaged by electric shocks induced from switching, and can be extended to measure the anisotropy of resistance.

Ionic and electronic conductivity in CaTi0.9fe0.1o3-δ

Abstract: The electrical conductivity of CaTi1−x Fe x O3-δ (x = 0.1) was measured by an alternating current van der Pauw technique versus oxygen partial pressure (10−30-1 atm) and temperature (450–1200°C). The results were interpreted to reflect n-type, ionic and p-type conductivity at respectively low, intermediate and high oxygen partial pressures. The apparent activation enthalpy for the ionic conductivity, interpreted to reflect the mobility of oxygen vacancies, was 0.87 eV. The enthalpy of intrinsic formation of electronic defects (apparent band gap E g) was 3eV. The results are compared with literature data for CaTi0.8Fe0.2O3-δ and with Fe-substituted SrTiO3 and discussed in terms of iron-oxygen vacancy association and ordering.

Photo-enhanced dissociation of hydrogen-magnesium complexes in gallium nitride

Abstract: We have studied the effect of UV light illumination during thermal annealing on the electrical properties of Mg-doped GaN films grown on (0 0 0 1) sapphire substrates by the two-flow MOCVD. We performed isochronal annealing up to 800°C for 1 h in a nitrogen atmosphere with and without UV light illumination, and measured annealing-induced changes in resistivity, hole density and mobility at 25°C using the van der Pauw method. Under no illumination, annealing around 550°C caused resistivity and mobility to decrease and simultaneously hole density to increase. This is consistent with the commonly accepted model that the hydrogen passivation of Mg is caused by the formation of electrically inactive Mg–H complexes and thermal annealing dissociates the complexes to activate Mg. The illumination of UV light with a peak wavelength around 350 nm greatly enhanced the dissociation of Mg–H complexes, reducing the temperature of resistivity reduction from 550°C to 450°C. These suggest that the dissociation of Mg–H complexes may be accelerated by the electronic excitation of the complexes and/or by the changes of their charge states. In view of application, such an effect may be useful to reduce the temperature of thermal annealing to make as-grown GaN films conductive.

Nondestructive evaluation of channel carrier concentration in modulation-doped InAlAs/InGaAs field-effect transistor structures by room-temperature photoluminescence

Abstract: The sheet carrier concentration (Ns) of channel layers in modulation-doped InAlAs/InGaAs heterostructure field-effect transistor (HFET) structures with n+InGaAs contact layers has been successfully and nondestructively determined using the room-temperature photoluminescence (PL) method. It is found that the spectral energy width between the maximum position of the main PL peak around 0.8 eV and the half-maximum position on the higher energy side has a good positive linear correlation with the Ns of the channel measured by the van der Pauw method. The scattering of the data is less than ±3×1011 cm−2. The determination of Ns is effective even if the HFET structures have not only n+InGaAs contact layers but also layers for InAlAs Schottky level-shift diodes. From a comparison with low-temperature PL spectra, the main PL peak is attributed to the e2h transition in the quantum well of the channel. It is considered that the slope of the peak stretches further to the high energy as the Fermi energy in the channe...

Reproducibility Data on SUMMiT

Abstract: SUMMiT (Sandia Ultra-planar Multi-level MEMS Technology) at the Sandia National Laboratories' MDL (Microelectronics Development Laboratory) is a standardized MEMS (Microelectromechanical Systems) technology that allows designers to fabricate concept prototypes. This technology provides four polysilicon layers plus three sacrificial oxide layers (with the third oxide layer being planarized) to enable fabrication of complex mechanical systems-on-a-chip. Quantified reproducibility of the SUMMiT process is important for process engineers as well as designers. Summary statistics for critical MEMS technology parameters such as film thickness, line width, and sheet resistance will be reported for the SUMMiT process. Additionally, data from Van der Pauw test structures will be presented. Data on film thickness, film uniformity and critical dimensions of etched line widths are collected from both process and monitor wafers during manufacturing using film thickness metrology tools and SEM tools. A standardized diagnostic module is included in each SWiT run to obtain post-processing parametric data to monitor run-to-run reproducibility such as Van der Pauw structures for measuring sheet resistance. This characterization of the SUMMiT process enables design for manufacturability in the SUMMiT technology.

Relation between the substitutional fraction and electrical activation of carbon in heavily c-ion implanted gaas

Abstract: Nuclear reaction analysis (NRA), using the 12C(d,p)13C reaction, in conjunction with Rutherford backscattering spectrometry (RBS) in the channeling geometry was used to evaluate the substitutional fraction of C in (100)-oriented semi-insulating GaAs implanted with 12C ions to a dose of 5×1016 cm−2. The substitutional fraction of the implanted 12C evaluated by NRA was 19% in the samples annealed at 600 °C, whereas the electrical activation rate of the same sample was 2.1%, as measured by the van der Pauw method. It is suggested that a possible origin of this discrepancy is the compensating centers such as As vacancy (VAs) and/or VAs-CAs complex introduced in the annealing processes. This was supported by both the surface precipitation of As observed by Raman scattering and the enhancement of the surface peak in RBS-channeling yield which was measured by using a 1.5 MeV 4He+-ion beam.

Nitrogen implantation in 4H and 6H–SiC

Abstract: We have undertaken a physico-chemical and electrical characterisation of nitrogen ion implantations in SiC epitaxies using a large variety of experimental conditions. The samples were implanted either at room temperature (RT) or at 650°C (HT). SIMS measurements were done to compare the depth-distributions with results from TRIM92 simulations. Transmission electron microscope (TEM) analysis of these samples was carried out on cross-sectional samples using Weak Beam Dark Field imaging conditions. Van der Pauw test patterns were realised to extract the thermal dependencies of resistivity and mobility. A comparison with the conductivity and mobility obtained on films doped during epitaxy and implanted films is done. It is shown that, in the lower dose range, the ionisation and activation does not depend on the implantation temperature.

Optical and electrical characterizations of Mn doped p-type β-FeSi2

Abstract: β-FeSi 2 has attracted increasing attention as a promising material for optoelectronic and thermoelectronic devices due to a high optical absorption coefficient (α) of about 10 5 cm −1 near 1.0 eV and its chemical stability at higher temperatures. For the future practical use of this material in devices, the control of each electrical conductivity type and the improvement of the material quality are highly required. Although unintentionally doped β-FeSi 2 layers formed on n-type Si(1 0 0) by the conventional electron-beam deposition (EBD) have typically shown n-type conductivity, the p-type β-FeSi 2 layers were formed by the introduction of Mn impurity using ion-implantation at room temperature (RT) and subsequent annealing procedures. In this study, we aimed to make p-type β-FeSi 2 by implantation of 55 Mn + ions into EBD-grown n-type β-FeSi 2 layers/n-Si, where 55 Mn + ions were implanted at two different temperatures ( T sub ) of RT and 250°C using an energy and a dose of 300 keV and 2.68 × 10 15 cm −2 , respectively. Their optical and electrical properties, which ought to be affected by implantation and annealing temperatures ( T a2 ), were investigated by Raman scattering, optical transmittance, reflectance and van der Pauw measurements. The results showed that the 55 Mn + doping with T sub =RT and higher thermal annealing at T a2 =900°C produced p-type layers of good quality with maximum hole mobility of 454.5 cm 2 /Vs at about 65 K.