Meyer-Neldel Relationship Research Articles

Lattice site specific diffusion properties for substitutional and interstitial impurity atoms in ZnO crystals

Fundamental understanding of impurity diffusion in crystals remains a challenge due to lack of experimental capabilities for measuring the diffusion properties of atoms according to their substitutional and interstitial lattice locations. With examples of indium and silver in ZnO crystals, we demonstrate an ion beam based method to experimentally determine the energetics and entropy changes in diffusion of substitutional and interstitial impurity atoms. While the interstitial Ag diffuses much faster than the substitutional Ag, as normally expected, the interstitial In migrates slower than the substitutional In, which is attributed to a large negative entropy change (∼−10 kB), possibly caused by the large atomic size of In. The activation energy and the diffusivity pre-exponential factor for the interstitial Ag are significantly enhanced, being more than a factor of two and ∼13 orders of magnitude, respectively, relative to the case for the interstitial In. This implies two different diffusion mechanisms between these two types of interstitial atoms in ZnO crystals: the direct interstitial diffusion mechanism for the interstitial In and the kick-out diffusion mechanism for the interstitial Ag. In addition, the activation energies and the diffusivity prefactors follow the Meyer-Neldel relationship with an excitation energy of ∼92 meV.

On the influence of the cation vacancy on lithium conductivity of Li1 + xRxTi2 − x(PO4)3 Nasicon type materials

Structural features responsible for outstanding Li conductivity of Li1+xRxTi2−x(PO4)3 (LRTP) Nasicon samples (0≤x≤0.6 and R=Al, Sc, In) prepared by the ceramic route have been analyzed by XRD, ND, MAS-NMR and impedance spectroscopy. The structural analysis showed that all samples display the rhombohedral (S.G. R-3c) symmetry. The structural site occupancy has been investigated by 7Li, 27Al/45Sc and 31P MAS-NMR spectroscopy. The Fourier map differences deduced from high-resolution ND patterns of LAlTP samples revealed that Li ions occupy Li1 sites and to a lower extent Li3/Li3′ sites inside Li2 cavities. The location of Li at 3 sites minimizes electrostatic Li (Li1–Li3) repulsions, enhancing local mobility in LRTP samples. A maximum of conductivity was detected for 0.2≤x≤0.4, when a significant amount of vacant Li1 sites was created at the intersection of conduction pathways. The increment of vacant Li1 sites explains the existence of two Li motion regimes detected by 7Li NMR and impedance spectroscopy. In the low temperature regime, activation energy and migration entropy of Li have been related by the Meyer–Neldel relationship. In the high-temperature regime, further investigation is required to assess the role of vacancy in lithium conductivity.

Per-Pixel Dark Current Spectroscopy Measurement and Analysis in CMOS Image Sensors

A per-pixel dark current spectroscopy measurement and analysis technique for identifying deep-level traps in CMOS imagers is presented. The short integration time transfer gate subtraction experimental technique used to obtain accurate results is described and discussed. The activation energies obtained for molybdenum (≈0.3 eV), tungsten (≈0.37 eV), and the phosphorus-vacancy (E-center) (≈0.44 eV) trap levels in silicon match published results measured with other techniques. The Meyer-Neldel Relationship (MNR) was observed between the Arrhenius preexponential frequency factor and activation energy. The trap capture cross-sectional calculation methodology using the MNR is presented. The cross sections of molybdenum, tungsten, and the E-center were calculated as ≈1 × 10-16 cm2, ≈1.5 × 10-16 cm2, and ≈2.5 × 10-16 cm2, respectively, at 318 K. The data obtained suggest electric field enhanced emission, and Poole-Frenkel barrier force lowering of E-center defects occurs in the pinning implant regions. It is proposed that a changing Fermi level results in the correct activation energies being obtained below half the band gap and that the dark current measurement process is affected by the measurement time result of statistical mechanics. It is also tentatively suggested that, in this case, the observed MNR is a geometric relationship and not due to a physical process.

Electrical transport, Meyer–Neldel rule and oxygen sensitivity ofBi2S3 nanowires

The electrical properties of Bi2S3 nanowire bundles were investigated. The nanowires were synthesized usinga solventless reaction involving a single-source bismuth thiolate precursor andstabilizing organic ligands. For electrical testing, nanowires were dispersed insolution and drop cast onto a substrate with gold contact pads patterned byelectron beam lithography techniques (EBL). Electrical connections were madeby depositing platinum interconnect lines between the nanowires and the goldpads by focused ion beam (FIB) chemical vapour deposition. Current–voltage(I–V) curves were measured under nitrogen as a function of temperature. The data revealedactivated transport that followed a Meyer–Neldel relationship. Annealing under vacuumdecreased the nanowire resistance by nearly four orders of magnitude. The annealednanowires followed an inverse Meyer–Neldel relationship. Illumination with UV lightincreased the current, and air exposure decreased the current under constant applied bias.

Variable temperature film and contact resistance measurements on operating n-channel organic thin film transistors

We report structural and electrical properties in thin films of an n-channel organic semiconductor, N,N′-dipentyl-3,4,9,10-perylene tetracarboxylic dimide (PTCDI–C5). The structure of polycrystalline thin films of PTCDI–C5 was studied using x-ray diffraction and atomic force microscopy. Films order with single crystal-like packing, and the direction of π-π overlap is in the substrate plane. Organic thin film transistors (OTFTs) based on PTCDI–C5 were fabricated on hydrophobic and hydrophilic substrates. OTFTs showed effective mobility as high as 0.1 cm2/V s. Contact resistance of operating OTFTs was studied using resistance versus length plots and a four-probe method for three different contact metals (Au, Ag, Ca). Typical OTFTs had a specific contact resistance of 8×104 Ω cm at high gate voltage. There was no dependence of contact resistance with contact metal. Variable temperature measurements revealed that film resistance in the OTFT was activated in the temperature range 100–300 K, with typical activation energies of 60–80 meV. Contact resistance showed similar activated behavior, implying that the Schottky barrier at the contact is not the limiting resistance for the contact. Film resistance data showed a Meyer–Neldel relationship with characteristic energy EMN=20–25 meV, for various samples. The common TFT instability of threshold voltage shift (TVS) was observed in PTCDI–C5 OTFTs. A model is proposed to explain positive TVS in gate bias stress and oxygen exposure experiments. The model is based on the formation of a metastable complex between PTCDI–C5 and oxygen, which creates a deep acceptor-like trap state.

Bandtails and defects in microcrystalline silicon (μc-Si:H)

The transport and recombination mechanism of microcrystalline silicon films, μc-Si:H is determined by specific defects, Si dangling bonds, and localized bandtail states. In spite of the complex heterogeneous structure, many properties of these films can be described in terms of models that have been developed for hydrogenated amorphous silicon, a-Si:H: (1) In the high-temperature regime (T>200 K), the conductivity obeys a Meyer–Neldel relationship which suggests a statistical shift of the Fermi level due to bandtail states. (2) At low temperature (T<50 K), the photoconductivity appears to be due to hopping transport in bandtail states and the recombination occurs by distant pair recombination. The discussion addresses the question of the assignment of the defect and bandtail states to different regions in the heterogeneous structure of the films.

The Inverted Meyer-Neldel Rule in the Conductance of Nanostructured Silicon Field-Effect Devices

AbstractThin film transistors (TFTs) offer the possibility to study the electronic transport properties of an intrinsic semiconductor as a function of the Fermi level position without the introduction of dopants and/or doping related defects. Recently, we reported on the first TFTs incorporating nanostructured silicon deposited with the Hot-Wire Chemical Vapor Deposition technique. These structures offer significant advantages over conventional plasma-deposited amorphous silicon TFTs. First of all, the HW deposited nanocrystalline silicon (nc-Si:H) TFTs do not show any threshold voltage shift upon prolonged gate voltage stress. Therefore, it is now possible to study the transport characteristics at a relatively large gate voltage in a controlled fashion, unhampered by any drift of the characteristics due to the creation of metastable electronic defect states and/or charge trapping. Second, the result of the field effect is that the Fermi energy moves into the conduction band of the virtually defect-free nanocrystalline domains in the channel region of the TFT. As the effective mobility gap of the surrounding amorphous phase is higher than that of the silicon crystallites, the Fermi energy is driven deep into the band-tail distribution of the amorphous phase, a situation that could never be achieved in purely amorphous silicon TFTs nor by heavily doping an amorphous semiconductor. Thus, the nanostructured nature of the silicon thin film near the gate insulator allows to shift the Fermi level far into the tail states region of the amorphous phase. This situation reveals for the first time the inverted Meyer-Neldel relationship in an intrinsic semiconductor.

Temperature Dependence of Minority Carrier Lifetimes in a-Si:H

AbstractChanges in minority carrier lifetimes in a-Si:H, p-i-n photovoltaic cells due to light soaking have been investigated using the open circuit voltage decay (OCVD) method over the temperature range 223 K to 296 K.Using light from a Xenon flash lamp for excitation produced unexpected results: in the light soaked material, band-to-band transitions were evident at a higher temperature than for the asdeposited samples and became increasingly pronounced as the temperature was reduced. Results obtained using red light at 670 nm from a pulsed diode laser to produce relatively uniform- illumination throughout the thickness of the film, however, produced results very similar to those obtained for as-deposited films.Plots of the reciprocal of the trap activation time versus 1000/T for the results for both xenon lamp and laser excitation can be fitted by straight lines. Two distinct sets of lines with activation energies in the ranges 0.07 to 0.20 eV and 0.38 to 0.51 eV are obtained with the activation energy and the exponential prefactors exhibiting a Meyer-Neldel relationship.

Optoelectronic properties of polycrystalline silicon produced by low-temperature (600°C) solid-phase crystallization of hydrogenated amorphous silicon

Abstract We present novel data on optoelectronic properties observed in the amorphous-to-polycrystalline silicon (polysilicon) phase transition, and the effect on as-crystallized films of a simple post-hydrogenation treatment. The polysilicon thin films were produced by low-temperature (600°C) furnace crystallization of undoped hydrogenated amorphous silicon (a-Si: H). Other parameters, such as the onset of crystallization, degree of amorphization and average grain size were determined by ultraviolet reflectivity and electron microscopy. The grain size is found to increase with decreasing a-Si: H substrate temperature, and a maximum areal grain size of 0.4 μm2 is obtained. Optical absorption, d.c. conductivity and transient photoconductivity measurements are employed to examine carrier transport mechanisms. We observe a Meyer-Neldel relationship between the d.c. conductivity pre-factor [sgrave]0 and activation energy E[sgrave] . A plasma hydrogenation treatment of the as-crystallized films results in an o...

Polysilicon thin Films and Devices Produced by Low-Temperature (600°C) Furnace Crystallisation of Hydrogenated Amorphous Silicon (a-Si:H)

ABSTRACTIn this work, we report on the electronic properties of polysilicon thin films and devices realised via furnace crystallisation of undoped a-Si: H. The onset of crystallisation, degree of amorphisation and average grain size are determined by UV reflectivity and electron microscopy. Grain size is found to increase with decreasing a-Si:H substrate temperature, and a maximum areal grain size of 0.4μm2 is obtained. Optical absorption, DC conductivity and transient photoconductivity measurements are employed to examine carrier transport mechanisms. We observe a Meyer-Neldel relationship between the DC conductivity prefactor σ0 and activation energy Eσ. A plasma hydrogenation treatment of the as-crystallised films results in an order of magnitude increase in the DC conductivity and a similar increase in photoconductivity. This is consistent with a shift of the Fermi level position 0.06 eV towards the conduction band. Additionally, analysis of the transient photoconductivity infers a reduced density of states. We discuss the implications of our results for polysilicon TFT optimisation.

Developments in the understanding of the ionic conductivities of glasses

The factors which determine the magnitude of ionic conductivity are presented with particular emphasis on the role of the mobility terms. The frequency and temperature dependences of the ac conductivities of glasses are discussed and it is shown how ac and dc conductivities may be related. It is shown that ac conductivity data may be analysed to provide estimates of a characteristic frequency which may be interpreted as an effective ion hopping rate. Ion hopping rate data for glasses are reviewed and it is concluded that they are significantly enhanced by an entropic term. A rationalization of conductivity prefactors is presented which is based on a simple explanation of the magnitude of the entropy of activation. It is shown that this leads to a Meyer-Neldel relationship between prefactors and activation energy. Data of fast ion conducting glasses are presented to illustrate the various characteristics discussed.