Showing papers on "Van der Pauw method published in 2012"

Measurement of the electrical resistivity and Hall coefficient at high temperatures

Abstract: The implementation of the van der Pauw (VDP) technique for combined high temperature measurement of the electrical resistivity and Hall coefficient is described. The VDP method is convenient for use since it accepts sample geometries compatible with other measurements. The technique is simple to use and can be used with samples showing a broad range of shapes and physical properties, from near insulators to metals. Three instruments utilizing the VDP method for measurement of heavily doped semiconductors, such as thermoelectrics, are discussed.

AlGaN/GaN/AlGaN Double Heterostructures Grown on 200 mm Silicon (111) Substrates with High Electron Mobility

Abstract: In this work, we demonstrate, for the first time, Al0.35GaN/GaN/Al0.25GaN double heterostructure field effect transistors on 200 mm Si(111) substrates. Thick crack-free Al0.25GaN buffer layers are achieved by optimizing Al0.75GaN/Al0.5GaN intermediate layers and AlN nucleation layers. The highest buffer breakdown voltage reaches 1380 V on a sample with a total buffer thickness of 4.6 µm. According to Van der Pauw Hall measurements, the electron mobility is 1766 cm2 V-1 s-1 and the electron density is 1.16×1013 cm-2, which results in a very low sheet resistance of 306±8 Ω/square.

Mobility and carrier density in p-type GaAs nanowires measured by transmission Raman spectroscopy.

Abstract: The unambiguous measurement of carrier concentration and mobility in semiconductor nanowires remains a challenging task. This is a consequence of their one-dimensional nature and the incompatibility with Hall or van der Pauw measurements. We propose a method that allows the direct determination of mobility and carrier concentration in nanowires in a contact-less manner. We demonstrate how forward Raman scattering enables the measurement of phonon–plasmon interactions. By applying this method to p-type GaAs nanowires, we were able to directly obtain values of the carrier concentration between 3.0 × 1017 and 7.4 × 1018 cm−3 and a mobility of 31 cm2 (V s)−1 at room temperature. This study opens the path towards the study of plasmon–phonon interactions in semiconductor nanowires.

The relationship between the doping levels and some physical properties of SnO2:F thin films spray-deposited on optical glass

Abstract: The relationship between the fluorine doping level and the electrical, structural and optical properties of the SnO2:F films are investigated using the Hall effect measurement set-up in van der Pauw configuration, the XRD patterns, UV–vis spectrophotometry and atomic force microscopy (AFM). The X-ray diffraction patterns taken at room temperature show that the films are polycrystalline. The preferred directions of crystal growth in the diffractogram of SnO2:F (FTO) films correspond to the reflections from the (1 1 0), (2 0 0), (2 1 1) and (3 0 1) planes. Thin film thickness and the grain size vary from 280 to 1545 nm and from 17.45 to 33.22 nm, respectively. AFM study reveals the surface of FTO to be made of nanocrystalline particles. The electrical study reveals that the films are degenerate and exhibit n-type electrical conductivity. The FTO films have a minimum resistivity of 5.29 × 10 − 4 Ω·cm, carrier density of 0.09 × 1020 cm − 3 and mobility of 377.02 cm2/V·s. The sprayed FTO film has the minimum sheet resistance of 5.69 Ω/cm2 and the highest figure-of-merit of 204 × 10 − 4 Ω − 1 at 700 nm. The resistivity attained for the doped film in this study is lower than the values reported for 20 wt.% fluorine-doped tin oxide films prepared from the aqueous solution of SnCl2·2H2O precursor. The highest visible transmittance (700 nm) of the deposited films is 91.8% for 25 wt.% fluorine-doped tin oxide films. The obtained results reveal that the structures and properties of the films are greatly affected by doping levels. These films are useful as conducting layers in electrochromic and photovoltaic devices.

Characterization of 4H-SiC Epitaxial Layers and High-Resistivity Bulk Crystals for Radiation Detectors

Abstract: Defect and electrical characterization of bulk semi-insulating (SI) 4H-SiC crystals and SI and n-type 4H-SiC epitaxial layers grown by chemical vapor deposition (CVD) on highly doped (0001) 4H-SiC substrates is reported. Optical microscopy, electron beam induced current (EBIC) imaging, current-voltage ( $I$ – $V$ ) measurements, thermally stimulated current (TSC) spectroscopy (94 K–620 K), Hall effect, and van der Pauw measurements have been conducted for characterization and defect correlation studies. Both epitaxial layers exhibited relatively shallow levels related to Al, B, $L$ - and D-centers. Deep level centers in the n-type epitaxial layer peaked at ${\sim} 400$ K ( $E_{a} \sim 1.1$ eV), and ${\sim} 470$ K were correlated with $IL_{2}$ defect and 1.1 eV center in high-purity bulk SI 4H-SiC. The SI epitaxial layer exhibited peak at ${\sim} 290$ K ( $E_{a} = 0.82\hbox{--}0.87$ eV) that was attributed to $IL_{1}$ and HK2 centers, and at ${\sim} 525$ K that was related to intrinsic defects and their complexes with energy levels close to the middle of the band-gap. Results of EBIC and optical microscopy showed segregation of threading dislocations around comet tail defects in the n-type epitaxial layer. The $I$ – $V$ characteristics of the devices on SI epitaxial layer exhibited steps corresponding to the ultimate trap filling of deep centers. The high-temperature resistivity measurements of bulk SI 4H-SiC sample revealed resistivity hysteresis that was attributed to the filling of the deep-level electron trap centers. The responsivity of the n-type epitaxial 4H-SiC detector in the soft X-ray energy range is reported for the first time.

Growth Studies on Quaternary AlInGaN Layers for HEMT Application

Abstract: Quaternary barrier layers for GaN-based high-electron-mobility transistors (HEMT) have recently been a focus of interest because of the possible lattice-matched growth to GaN. This results in a reduction of strain-related defects, while having the option of adjusting the bandgap separately. A further benefit of the quaternary approach is the possibility to achieve high polarization and high carrier mobility simultaneously. This may improve the performance of such devices beyond what is possible with ternary barrier layers. In this work, we report on growth and characterization of AlxInyGa1−x−yN barrier layers within the range of 16% to 56% Al, 2% to 45% In, and 20% to 82% Ga deposited on conventional GaN buffer layers on sapphire. We present an effective way to change the composition of quaternary layers and discuss the influence of tensile and compressive strain on structural and electrical properties. From high-resolution x-ray diffraction (HRXRD), Rutherford backscattering spectroscopy (RBS), and wavelength-dispersive x-ray spectroscopy (WDX), we determined the compositions and strain states of the AlInGaN layers. The bandgaps (Eg) were obtained by spectroscopic ellipsometry (SE). Hall and van der Pauw measurements on thin heterostructure layers yielded high mobilities in excess of 1550 cm2/V s and 5350 cm2/V s at room temperature and 77 K, respectively.

Low Temperature Hall Effect Investigation of Conducting Polymer-Carbon Nanotubes Composite Network

Abstract: Polypyrrole (PPy) and polypyrrole-carboxylic functionalized multi wall carbon nanotube composites (PPy/f-MWCNT) were synthesized by in situ chemical oxidative polymerization of pyrrole on the carbon nanotubes (CNTs). The structure of the resulting complex nanotubes was characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The effects of f-MWCNT concentration on the electrical properties of the resulting composites were studied at temperatures between 100 K and 300 K. The Hall mobility and Hall coefficient of PPy and PPy/f-MWCNT composite samples with different concentrations of f-MWCNT were measured using the van der Pauw technique. The mobility decreased slightly with increasing temperature, while the conductivity was dominated by the gradually increasing carrier density.

Effect of N doping on hole density of Cu2O:N films prepared by the reactive magnetron sputtering method

Abstract: N-doped Cu 2 O thin films have been deposited on glass substrate by reactive magnetron sputtering method under various N 2 /O 2 flow ratios from 0 to 1.0. The structural and electronic properties of Cu 2 O:N films were investigated by X-ray diffraction (XRD), four-point probe and Hall effect measurements. XRD pattern showed that crystalline structures of all the samples retained single phase of Cu 2 O with the increase of N 2 /O 2 flow ratio from 0 to 1.0. However, the crystalline quality of Cu 2 O:N films reduced with the increase of the N 2 /O 2 flow ratio. The phenomenon of peak shift of Cu 2 O(1 1 1) implied that N atoms have been doped into Cu 2 O film. The square resistance of Cu 2 O:N films linearly decreased from 28.1 to 1.5 (10 4 Ω / ☐) with the increase of N 2 /O 2 flow ratio from 0.2 to 0.6 initially, and then it changed slowly with the increase of N 2 /O 2 flow ratio from 0.8 to 1.0. Hole density of Cu 2 O:N films with various N 2 /O 2 flow ratios from 0 to 0.6 was measured using the Van der Pauw method. All the samples are p -type, and the hole density of Cu 2 O:N films was enhanced from 1.2 × 10 16 cm −3 to 3.1 × 10 19 cm −3 with the increase of N 2 /O 2 flow ratio from 0 to 0.6. The experimental results demonstrated that N doping was an effective method to enhance hole density of p -type Cu 2 O film.

Structural, electrical and optical studies of gold nanostructures formed by Ar plasma-assisted sputtering

Abstract: Ultrathin gold layers with different thickness of (10–100 nm) on the glass substrate were obtained by Ar plasma-assisted sputtering. The effects of annealing on gold structures sputtered onto glass substrate were studied using AFM, SEM, UV–Vis methods and electrical measurements. Concentration of free charge carriers were determined from the measured resistance and the Hall constant measured by the Van der Pauw method. We have shown that post-deposition thermal treatment leads in significant change in surface morphology of the sputtered Au structures. Our results suggest that the annealing affects electrical properties of the Au coverage namely electrical sheet resistance, free carriers volume concentration, the saturation of which is in comparison with as-sputtered samples shifted towards thicker structures. While semi-conductive character of as-sputtered samples diminishes close to the Au structure thickness of ca 20 nm, in the case of the annealed structures zero-level saturation is achieved for the thickness of 60 nm.

Characterization of a-FeSi2/c-Si heterojunctions for photovoltaic applications

Abstract: Amorphous iron disilicide (a-FeSi2) shows potential as a photovoltaic material due to its bandgap of ?0.9 eV and high absorption coefficient. We present a detailed characterization of a-FeSi2, with particular emphasis on the electrical properties of a-FeSi2/c-Si heterostructures, under both dark and illuminated conditions. The samples were prepared on quartz and silicon substrates using RF co-sputtering of an iron/silicon target. Optical transmission spectroscopy was used to confirm the bandgap of the samples. Van der Pauw measurements and current?voltage analysis techniques were used to determine the carrier type and conduction mechanisms of the samples. The results show that a-FeSi2?forms a rectifying p?n heterojunction on p-type crystalline silicon. The silicide is characterized by very high carrier concentrations, resulting in the depletion region being almost entirely formed within the silicon substrate. Initial J?V results suggest carrier recombination within the silicide to be the dominant contribution to the conduction across the junction, with photovoltaic effects having been observed under AM1.5 conditions.

Taming transport in InN

Abstract: Taming transport in InN Joel W. Ager III and Nate R. Miller Materials Sciences Division, Lawrence Berkeley National Lab, Berkeley, CA 94720 USA Emcore, Albuquerque, NM 87123 USA Keywords Indium nitride, thermopower, doping, conductivity The large electron affinity of InN, close to 6 eV and the largest of any III-V semiconductor, creates a strong driving force for native donor formation, both in the bulk and at surfaces and interfaces. Moreover, all InN surfaces, regardless of crystal orientation or doping, have been observed to have a surface accumulation layer of electrons, which interferes with standard electrical measurements. For these reasons, until recently, it was uncertain whether or not compensation by donor defects would prevent “real” p-type activity (i.e. existence of sufficiently shallow acceptors and mobile holes). A coordinated experimental approach using a combination of electrical (Hall effect) and electrothermal (Seebeck coefficient) measurements will be described that allows definitive evaluation of carrier transport in InN. In Mg-doped InN films, the sensitivity of thermopower to bulk hole conduction, combined with modeling of the parallel conducting layers (surface/bulk/interface), enables quantitative measurement of the free hole concentration and mobility. In undoped (n-type) material, combined Hall and thermopower measurements, along with a considering of the scattering mechanisms, leads to a quantitative understanding of the crucial role of charged line defects in limiting electron transport. 1 Introduction Depending on the band offset values used, the electron affinity of InN is in the range of 5.5-5.8 eV [1-3]. This is the largest electron affinity of all group III-V semiconductors and has a number of important consequences for the study of electron and hole transport in this material. Based on considerations of both the charge neutrality level (also known as the branch point energy) [4,5] and of the amphoteric native defect (Fermi level stabilization) model [6,7], InN would be expected to have its surface Fermi level pinned high above the conduction band minimum. Indeed, surface pinning corresponding to a Fermi level position in the range of 0.7-0.9 eV above the CBM has been observed experimentally by a number of techniques including electron energy loss spectroscopy, x- ray photoelectron spectroscopy, angle-resolved photoemission, and electrolyte contacted capacitance voltage (CV) measurements [8-12]. The large electron affinity of InN also affects bulk conduction properties. Native point defects have a strong driving force to be donors. Energetic particle irradiation experiments have shown that their average energy is 0.9 eV above the CBM, similar to the position of the surface pinning energy [13,14]. Both hydrogen and also certain types of dislocations are also predicted to be donors in InN [15,16]. As a result, undoped InN thin films grown to date by molecular beam epitaxy have been degenerate n-type, with the lowest reported electron concentration in the low 10 17 cm -3 range [17,18]. The presence of electron-rich surface layers and the strong propensity for the formation of native donors created challenges for producing p-type InN. While CV techniques were able to show that Mg forms acceptors, as in GaN [11], direct measurement of the transport properties had been prevented by the n-type surface inversion layer. Recently, thermopower [19-22] and Hall effect/conductivity studies [23-24] have shown that Mg doping produces mobile holes in InN, but there are only a few reports concerning the hole concentration and mobility. In this work, we are “taming” the transport of InN in two ways. (1) We use the ability of an electrolyte to form an insulating surface layer to deplete InN’s surface accumulation/inversion layers to reveal the bulk transport properties of InN epitaxial films [25]. (2) We take advantage of the sensitivity of thermopower measurements to bulk hole conduction to obtain the free hole concentration and mobility in p-type InN:Mg films grown by MBE. The additional use of thermopower yields a more precise assessment of hole conduction properties than with Hall/conductivity analysis alone. 2 Experimental Methods for the MBE growth of the films used in this study are described in the literature by our collaborators [17,26,27]. Hall effect measurements were measured using a 3000 Gauss magnet with contacts placed in the van der Pauw configuration. Thermopower measurements were performed using a system Corresponding author: JWAger@lbl.gov

Improved electron mobility in InSb epilayers and quantum wells on off-axis Ge (001) substrates

Abstract: Two types of InSb structures, epilayers and quantum wells (QWs), have been grown on on-axis and 6°-off-axis Ge (001) substrates and examined by reflection high-energy electron diffraction, transmission electron microscopy, X-ray diffraction, atomic force microscopy, and the van der Pauw and Hall effect techniques. Anti-phase domain defects, which prevail in these InSb structures when grown on on-axis Ge (001) substrates, are significantly decreased by the use of 6° off-axis Ge (001) substrates. Such off-axis substrates also lead to reductions in the densities of micro-twins and threading dislocations. Room-temperature electron mobilities in 4.0-μm-thick InSb epilayers and 25-nm-thick InSb QWs grown on 6°-off-axis Ge (001) substrates are 59 000 and 14 000 cm2/(V s), respectively, which are ∼1.5 times higher than their counterparts grown on on-axis Ge (001) substrates. These improved mobilities are the highest among the reported values for each type of structure grown on Ge (001) substrates.

Application of the electrostatic Thompson–Lampard theorem to resistivity measurements

Abstract: A method for measurement of resistivity of flat samples and thin layers complementary to the well-known van der Pauw technique has been proposed. The method is based on the application of the Thompson–Lampard theorem of electrostatics used in metrology for the realization of calculable capacitor, according to which a large variety of electrode systems can be designed. A prototypic electrode arrangement is shown on which the practical performance of the method was tested.

Dimers as Fast Diffusing Species for the Aggregation of Oxygen in Boron-Doped Czochralski Silicon: Formation of New Thermal Donors

Abstract: The thermal behaviors of oxygen-related complexes in boron doped Czochralski Silicon (Cz-Si) wafers at 450°C and 800°C were investigated using Fourier transform infrared spectroscopy (FTIR) and Hall mobility measurements. Activation of thermal donors (TDs) at 450°C leads to a decrease of both mobility and majority carrier concentration using the four point probes configuration of Van Der Pauw. It was found that annealing at 450°C would possibly affect the electronic properties of the Si wafers via the formation of interstitial dioxygen defects (IO2i), which exhibit an IR absorption band positioned at 545 cm–1. A strengthening of the IR bands peaking at around 1595 cm–1, 1667 cm–1, 1720 cm–1 and 1765 cm–1 occurs at 450°C, while they disappear at 800°C. At high temperatures, the precipitation of interstitial oxygen becomes predominant over all other oxygen-related reactions. The dynamic of oxygen-thermal donor generation-annihilation in Cz-Si involving the formation of small oxygen clusters is discussed.

Carbon‐doped p‐type (0001) plane AlGaN (Al = 6–55%) with high hole density

Abstract: In this paper, promising experimental results for the p-type electrical properties of carbon-doped (C-doped) AlGaN are discussed. P-type conductivity was experimentally achieved in C-doped (0001) plane AlGaN layers with from a small amount to 55% solid Al composition, but not in (0001) plane GaN. The maximum free hole density (determined by van der Pauw geometry-Hall effect measurement) achieved for an AlGaN layer with 10% solid Al composition was p = 3.2 × 1018 cm−3. The maximum net ionized acceptor densities (NIAD), , which were determined by capacitance–voltage measurement, for AlGaN with 6, 10, 27, and 55% solid Al compositions, were all in the range of (6–7) × 1018 cm−3. Moreover, the electrical activity of the carbon acceptors was estimated to be 55–71% from the NIAD and secondary-ion microprobe mass spectrometry analysis data on the carbon concentration. A p–n junction was also fabricated using the C-doped p-type AlGaN.

Low-cost Cu 2 ZnSnS 4 thin films for large-area high-efficiency heterojunction solar cells

Abstract: Cu 2 ZnSnS 4 (CZTS) thin films have been deposited on soda-lime glass (SLG) and Mo-coated SLG substrates using a low-cost spray pyrolysis technique followed by sulfurization under H 2 S flow at 540°C. Aqueous solution containing CuCl, ZnCl 2 , SnCl 4 and thiourea was used as precursor. Spray deposition was carried out at three different substrate temperatures of 280°C, 360°C and 440°C. Deposition conditions were optimized to obtain the best film properties. Structural, morphological and compositional analysis of the as-deposited and sulfurized CZTS films were carried out by x-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive x-ray spectroscopy (EDX) and x-ray photoelectron spectroscopy (XPS). Optical and electrical properties were measured by UV-Vis spectroscopy, van der Pauw and Hall effect measurements. XRD spectra confirmed the formation of kesterite CZTS films. Grown CZTS films showed an absorption coefficient >104 cm−1 and the bandgaps were found to lie between 1.42–1.72 eV at room temperature depending on deposition conditions and post deposition sulfurization treatments. All films were found to be of p-type conductivity with an average carrier concentration in the order of 1018–1020 cm−3. Optimum quality films were obtained for films fabricated at 360°C and no secondary phases were observed. n-CdS window layer was deposited on p-CZTS films prepared at 360°C substrate temperature to fabricate p-CZTS/n-CdS heterojunction solar cells. The heterojunction exhibited an open-circuit voltage (V OC ) of 290 mV and a short-circuit current density (J SC ) of 3.1 mA/cm2 under AM 1.5 illumination. Details of CZTS thin film fabrication, processing, and characterization results are presented.

The effect of growth temperature and spraying rate on the properties of ZnO:In films

Abstract: ZnO:In thin films were deposited by chemical spray pyrolysis from zinc acetate solutions containing 3 at% of InCl3 The films were grown in the temperature region of 400-500 °C using solution spraying rates 06-71 ml/min ZnO:In films were characterized by XRD, Hall and van der Pauw methods The dependence of the film resistivity vs growth temperature is parabolic ZnO:In films with resistivity minimum of 3x10-3 Ωcm can be deposited at 450 °C The solution spraying rate has strong effect on the film resistivity, deposition rates below 3 ml/min result in high resistivities (© 2012 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim)

Contactless Magnetoresistance in Large Area Epitaxial Graphene Grown on SiC Substrates

Abstract: The present contactless measurements of magnetoresistance of large area graphene films grown by chemical vapor deposition method on semi-insulating SiC substrates. For this purpose we propose microwave technique using single post dielectric resonator operating at frequency about 13.5 GHz. Comparison of microwave measurements with classic van der Pauw method shows good agreement and proves usefulness of contactless technique in magnetoresistance studies. Experiments have been performed at 4.2 K in magnetic fields up to 7 T. Significant differences in graphene magnetoresistance depending on the orientation and crystal structure of SiC substrates have been observed.

Anisotropy and inhomogeneity measurement of the transport properties of spark plasma sintered thermoelectric materials

Abstract: We report on the development and capabilities of two new measurement systems developed at Fraunhofer-IPM. The first measurement system is based on an extension of the Van der Pauw method and is suitable for cube-shaped samples. A mapping of the electrical conductivity tensor of a Skutterudite-SPS samples produced at the Instituto de Microelectronica de Madrid is presented. The second measurement system is a ZTmeter also developed at the Fraunhofer-IPM. It enables the simultaneous measurement of the electrical conductivity, Seebeck coefficient and thermal conductivity up to 900 K of cubes at least 5x5x5 mm 3 in size. The capacity of this measurement system for measuring the anisotropy of the transport properties of a (Bi,Sb)2Te3 SPS sample produced by KTH is demonstrated by simply rotating the samples.

Ion Implant Technology for Intermediate Band Solar Cells

Abstract: This chapter describes the creation of an Intermediate Band (IB) on single crystal silicon substrates by means of high-dose Ti implantation and subsequent Pulsed Laser Melting (PLM). The Ti concentration over the Mott limit is confirmed by Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) measurements and the recovery of the crystallinity after annealing by means of Glancing Incidence X Ray Diffraction (GIXRD) and Transmission Electron Microscopy (TEM). Rutherford Backscattering Spectroscopy (RBS) measurements show that most of the atoms are located interstitially. Analysis of the sheet resistance and mobility measured using the van der Pauw geometry shows a temperature-dependent decoupling between the implanted layer and the substrate. This decoupling and the high laminated conductivity of the implanted layer could not be explained except if we assume that an IB has been formed in the semiconductor. A specific model for the bilayer electrical behaviour has been developed. The fitting of this model and also the simulation of the sheet resistance with the ATLAS code allow to determine that the IB energetic position is located around 0.36–0.38 eV below the conduction band. Carriers at the IB have a density very similar to the Ti concentration and behave as holes with mobilities as low as 0.4 cm2 Vs− 1.