Space Charge Limited Current Measurements Research Articles

Charge transport properties of a metal-free phthalocyanine discotic liquid crystal

Discotic liquid crystals can self-align to form one-dimensional semiconducting wires, many tens of microns long. In this letter, we describe the preparation of semiconducting films where the stacking direction of the disc-like molecules is perpendicular to the substrate surface. We present measurements of the charge carrier mobility, applying temperature-dependent time-of-flight transient photoconductivity, space-charge limited current measurements, and field-effect mobility measurements. We provide experimental verification of the highly anisotropic nature of semiconducting films of discotic liquid crystals, with charge carrier mobilities of up to 2.8x10$^{-3}$cm$^2$/Vs. These properties make discotics an interesting choice for applications such as organic photovoltaics.

Dielectric and transport properties of magnetron sputtered titanium dioxide thin films

Titanium dioxide (TiO 2) thin films were prepared by DC magnetron sputtering onto well-cleaned p-type silicon substrates. The thickness, composition and surface morphology of the films were analyzed using alpha step profilometer, Auger electron spectroscopy (AES) and atomic force microscope (AFM) respectively. The X-ray diffraction (XRD) studies reveal the amorphous nature of the deposited film. Thin film capacitors of the type Al/TiO 2/Si/Al have been fabricated. Dielectric and AC conduction studies were performed at various frequencies (10 kHz to 10 MHz) and temperatures (300–390 K). Dielectric constant value of TiO 2 film of thickness 140 nm was evaluated at room temperature and at a frequency of 1 MHz as 5.5. The mechanisms responsible for the AC and DC conduction in these films have been identified. For the first time, the trap density, mobility values of TiO 2 thin films are evaluated from the space charge limited current (SCLC) measurements as 1.637×10 17 cm −3, 2×10 −11 cm 2 V −1 s −1, respectively. The AC and DC activation energies have been calculated as 0.097 and 0.136 eV, respectively.

Space-charge-limited currents in GaN Schottky diodes

Unusual dark current–voltage ( I– V) characteristics were observed in GaN Schottky diodes. I– V characteristics of the GaN Schottky diodes were measured down to the magnitude of 10 −14 A. Although these Schottky diodes were clearly rectifying, their I– V characteristics were non-ideal which can be judged from the non-linearity in the semi-logarithmic plots. Careful analysis of the forward bias I– V characteristics on log–log scale indicates space-charge-limited current (SCLC) conduction dominates the current transport in these GaN Schottky diodes. The concentration of the deep trapping centers was estimated to be higher than 10 15 cm −3. In the deep level transient spectra (DLTS) measurements for the GaN Schottky diodes, deep defect levels around 0.20 eV below the bottom of the conduction band were identified, which may act as the trapping centers. The concentration of the deep centers obtained from the DLTS data is about 5 × 10 15 cm −3. SCLC measurements may be used to probe the properties of deep levels in wide bandgap GaN–AlGaN compound semiconductors, as is the case with insulators in the presence of trapping centers.

Current–voltage characteristics of a tetracene crystal:Space charge or injection limited conductivity?

We show that current–voltage characteristics, measured on a tetracene crystal sandwiched between two gold electrodes, are in quantitative agreement with the concept of dopant-assisted charge injection across a metal–organic interface. This notion explains the wild range of carrier mobilities recently found by space-charge limited current measurements in apparently identical tetracene crystals [de Boer et al., J. Appl. Phys. 95, 1196 (2004)] and the difference between these data and the mobility measured in the time-of-flight experiments.

Hall effect, space-charge limited current and photoconductivity measurements on TlGaSe2 layered crystals

TlGaSe2 layered crystals are studied through dark electrical conductivity, Hall mobility, space-charge limited current and illumination- and temperature-dependent photoconductivity in the temperature ranges 120–350 K, 220–350 K, 260–350 K and 120–350 K, respectively. The Hall effect measurements revealed the extrinsic p-type conduction. The Hall mobility increase with decreasing temperature is limited by the thermal lattice scattering. The space-charge limited current and dark conductivity measurements predicted the existence of a single discrete trapping level located at 330 meV with a trap concentration of (1.4–2.2) × 1013 cm−3. The dark electrical conductivity and photoconductivity measurements reflect the existence of three other energy levels located at 95, 46 and 26 meV at high, moderate and low temperatures, respectively. The photocurrent is observed to increase with increasing temperature up to a maximum temperature of 320 K. The illumination dependence of photoconductivity is found to exhibit sublinear, linear and supralinear recombinations at high, moderate and low temperatures, respectively. The change in recombination mechanism is attributed to the exchange in the behaviour of sensitizing and recombination centres.

Evaluation of the gap state distribution in a-Si:H by SCLC measurements

The states in the mobility gap of amorphous hydrogenated silicon play an important role in determining its optical and electrical properties, and it is essential to know their density and distribution in the gap. Space Charge Limited Current (SCLC) is a classic and a well-known method for determining this density of states [1, 2, 3, 4]. In SCLC measurements, the superlinear I–V dark characteristics due to injected charges from the ohmic contacts are measured. When the injection is large enough to displace significantly the quasi Fermi level from its equilibrium position, the current start to rise super-linearly. The magnitude of the current and the shape of the curve depend on the density of states just above the equilibrium Fermi level, so this portion of the curve can be used to determine this quantity.

Band-Like Charge Transport in C60 Single Crystals

Charge transport properties of vapor grown C60 single crystals are investigated in the temperature range from 5–300 K by field-effect transistor and space charge limited current measurements in vacuum. Room temperature mobilities of 2.1 and 1.8 cm2/Vs are observed for holes and electrons, respectively. Moreover, the mobilities increase with decreasing temperature following a power law behavior (∝T—1.6). Values as high as 60 (electrons) and 95 cm2/Vs (holes) are achieved at low temperatures. The discontinuity around 255 K is ascribed to the first-order phase transition of C60. The pronounced electric field dependence can be explained by acoustic phonon scattering. Hence, we conclude, that band-like charge transport in delocalized states rather than hopping motion can be assumed as intrinsic charge transport mechanism in undoped C60.

Influence of disorder on the electron transport properties in fluorinated copper-phthalocyanine thin films

Electron transport in polycrystalline thin films of fluorinated copper phthalocyanine is investigated by space charge limited current, photoconductivity, and field-effect transistor measurements in the temperature range from 200 to 300 K. In this region the conductivity follows the Meyer–Neldel rule. The Meyer–Neldel energy EMN is connected to the characteristic energy of the exponential tail state distribution of localized states determined by voltage-dependent space charge limited current spectroscopy. Moreover, a correlation of the conductivity prefactor and EMN is observed revealing a maximum energy barrier of 0.52 eV for the relaxation of trapped electrons.

Density of States in a-Si:H from SCLC and Its Application in Modeling a Vertical TFT

ABSTRACTSteady-state space-charge limited current (SCLC) measurements are used to investigate the density of states (DOS) in the mobility gap of hydrogenated amorphous silicon (a-Si:H). The density of states is calculated by different methods based on both continuous DOS and discrete traps assumptions. The density of states found by the SCLC measurements is used to set the trap densities and trap energy levels to model a vertical a-Si:H thin-film transistor (TFT) using the Medici device simulation package. The effect of different sets of traps in the bulk of a-Si:H and variation of the physical dimensions of the device on the characteristics of the vertical TFT is studied. The simulation on the space-charge limited current is performed to verify the validity and accuracy of the SCLC method.

Electrical properties of n-GaSe single crystals doped with chlorine

Hall-effect and space-charge-limited-current (SCLC) measurements were performed on Cl-doped GaSe single crystals grown by the Bridgmann–Stockbarger method. The temperature dependence of the free electron density shows the characteristics of a partially compensated n-type semiconductor. The electrical properties are dominated by a deep donor level at about 0.57 eV below the conduction band. An electron trapping level between 0.56 and 0.62 eV below the conduction band has been observed by SCLC measurements. The trapping level concentration depends on the amount of dopant. Finally, the conduction band density-of-states effective mass was estimated to be 1.1 m0.

Determination of the density of states at the Fermi level of hydrogenated amorphous silicon in thin-film transistor structure by space charge limited current measurement

We studied the space charge limited current effect in hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs). We demonstrate that the drain current is space charge limited when the source-drain voltage is large and the gate voltage is small. Using this space charge limited current we determined the density of states in the gap of a-Si:H in a-Si:H TFT.

Refinement on the theories of measurement for trap density from space-charge-limited current

The theory of space-charge-limited current (SCLC) measurement for determining an arbitrary energy distribution of traps of insulators and semiconductors is put forward in this article. As a result, no restriction is given to the trap densities of the sample tested, which is caused by neglecting free carriers in the previous theories of SCLC measurement. It is shown by numerical computations that the relative errors near the band edges that usually differ by as much as several orders of magnitude is greatly improved. The refinement is also expected to be useful in the high-temperature region.

Assessment of density of states extraction from the space charge limited current measurement: A numerical simulation

The den Boer analysis of single carrier space charge limited currents is commonly used to determine the density of states function within the gap of amorphous silicon hydride and other newly emerging materials. By using a device simulator, we assess the validity and accuracy of this analysis. In particular, den Boer’s assumptions of 0 K occupation statistics, a spatially uniform distribution of injected charge carriers, spatially uniform material, no generation/recombination, and no contact effects are investigated. We find that in the absence of contact effects and with spatially uniform states the den Boer analysis is accurate to within a factor of 2. However, current limiting due to contact effects at higher current values and diffusion dominated current at lower applied biases should not be mistaken for features of the density of states. In addition, if the gap states are not spatially uniform, gross overestimates (by as much as an order of magnitude for the device studied here) of the density of states can result. In general, only a limited range of the gap of the semiconductor can be mapped with this technique for a single device length, although multiple device lengths can be used to increase this range.

Space-charge-limited currents in amorphous gallium sulfide thin films

The density N(E) of midgap states in vacuum-evaporated and thermally annealed amorphous GaS films has been deduced from space-charge-limited-current (SCLC) measurements. Specimens with an In/a-GaS/stainless-steel sandwich structure have been used to obtain current-voltage characteristics at room temperature. By using an analytical method the density N(E) over an energy range extending about 0.18 eV above the Fermi level at thermal equilibrium has been calculated. The conductivity data for variable-range hopping [log(σ)∝T−1/4] have also been studied, but the calculated values of N(E) are approximately one order of magnitude higher than those obtained by SCLC measurements.

Acceptor states distributed in energy in Cd-doped Cu 2O

The effect of Cd-doping and annealing at different temperatures on the trap density in Cu 2O is considered. From space charge limited current (SCLC) measurements and Hall effect measurements it is demonstrated that there is a trap-band distribution in the area of the Fermi level, which always lies within the trap-band. For resistivities of the material higher than about 200 Ωcm the maximum of concentration of traps is located just above the Fermi level. For resistivities lower than about 200 Ωcm this maximum lies below the Fermi level. By fitting the experimental data to theory we calculated the parameters of the trap-band. Annealing of the samples in air at 500°C decreases the resistivity of the material by two orders of magnitude and at the same time the concentration of traps decreases. The trap-band width becomes narrower in these samples.

Trap centers in cuprous oxide

The effect of annealing temperature on the trap density in Cu 2O is investigated. From space charge limited current (SCLC), Hall effect and DLTS measurements it is demonstrated that there is a trap-band distribution with the Fermi level lying within the trap-band. The maximum of this trap-band distribution is just above the Fermi level. A theoretical model is introduced to explain this trap-band distribution. Annealing of the samples in air at 400–500°C for 5 h gives the lowest total trap concentration and the lowest resistivity of the material. The above annealing decreases the total number of traps by a factor of 3 compared to the as grown samples. The resistivity decreases also by at least one order of magnitude.

Improved Stability of RF Glow Discharge Deposited a-Si:H Achieved by Hydrogen Dilution of Silane.

The magnitude of metastable light-Induced degradation of intrinsic hydrogenated amorphous silicon films deposited by rf glow discharge from different selene-hydrogen mixtures wan studied. By stone of spacecharge-limited current measurements the Fermi-lava1 density of states was determined for as-grown and light-soaked n + -t-n + devices. Films deposited using a mixture of 55 vol.% silane and 45 vol.% hydrogen and a relatively high rf power level of 62 mW/cm 2 to obtain a high growth rate exhibited an improved stability with regard to light-Induced degradation of the film, compared with films grown under the conditions of using pure etlane and an rf power Just sufficient to seintaln the plasma, conditions known to ylald good quality films. The results are explained in terms of an improvement in the structural oraer of the film.

Density of states near mid gap in hydrogenated amorphous silicon

The density of states distributions near mid gap in a series of hydrogenated amorphous silicon films have been determined from space charge limited current measurements. The measurements were made on Au/a-Si:H Schottky diode structures prepared by reactive rf sputter deposition. Samples with hydrogen concentrations near 16% as determined from infrared absorption had densities of states of 3 × 10 14 states/cm 3 eV. The experimental results indicate that high quality a-Si:H films with low densities of states can be obtained under certain deposition conditions and that the density of states at mid gap is hydrogen concentration dependent with a minimum near 16%. For a given hydrogen concentration, films thicker than 2 μm yielded the lowest density of states consistent with a model in which diffusion currents can be neglected and where surface and interface layers have a higher defect density than the bulk of the film.

The effect of injection electrodes on the determination of the density of states in a-Si:H by the space-charge-limited-current technique

Abstract The electric field at the interface between the n+-layer and the central i-region has been used to describe the injection properties of the electrode, i.e. the injecting junction region, of n+-i-n+ devices of hydrogenated amorphous silicon. The effect of this field on the determination of the density of states (DOS) in amorphous silicon from space-charge-limited-current (SCLC) measurements has been investigated. It is shown that the electrode has little effect on the deduced DOS values for reasonable interface field and it is concluded that SCLC measurements on a-Si:H are representative of a bulk property of the material.

Electrical properties and degradation behavior of hydrogenated amorphous Si alloys for solar cells

The electrical properties and the degradation behavior of hydrogenated amorphous silicon alloys (a-Si1−x A x : H, with A=C, Ge, B, P) in designs of pin, pip, nin, and MOS structures are investigated by measuring the dark and light I(V) characteristics and the spectral response as well as the space-charge-limited current (SCLC), the time of flight (TOF) of carriers and the field effect (FE). These investigations give an overview of our recent work combined with new results emphasizing the physics of the a-Si:H pin solar cells. We discuss the stabilizing influence on the degradation behavior achieved by profiling the i layers of the pin solar cells with P and B. Two kinds of pin solar cells, namely glass/SnO2/p(C)in/metal and glass/metal/pin/ITO, are investigated and an explanation of their different spectral response behavior is given. SCLC measurements lead to the conclusion that trapping is also involved in the degradation mechanism, as is recombination. TOF experiments on a-Si1−x Ge x : H pin diodes indicate that the incorporation of Ge widens the tail-state distribution below the conduction band. FE measurements showed densities of gap states of about 5×l016cm−3eV−1.