Density Of Deep States Research Articles

Analysis of Narrow Width Effects in Polycrystalline Silicon Thin Film Transistors

In this study, narrow width effects of polycrystalline silicon thin film transistors (poly-Si TFTs) are investigated. With reducing channel width, TFT characteristics such as mobility, threshold voltage, and subthreshold swing are found to improve dramatically. To gain insight on the origin of the narrow width effects, a physically-based model is proposed to simulate the output characteristics of poly-Si TFTs. Excellent fitting with the experimental data is observed over a wide range of drain bias, gate bias, and channel width. Our model shows that both the deep state density and tail state density are reduced with reducing channel width, thus accounting for the improved TFT characteristics. In addition, subthreshold swings of poly-Si TFTs with various channel widths and lengths are compared. It is found that the subthreshold swings of poly-Si TFTs with the same channel area are identical, indicating that the grain-boundary trap density is reduced due to the reduction of channel area.

Some electronic and metastability properties of a new nanostructured material: hydrogenated polymorphous silicon

Abstract When silicon thin films are deposited by plasma enhanced chemical vapour deposition in a plasma regime close to that of the formation of powder, a new type of material, called polymorphous silicon (pm-Si), is obtained. We present here the optoelectronic and stability properties of pm-Si films deposited from a mixture of silane diluted with hydrogen at total gas pressures in the range 800–1600mTorr. A comparison with the properties of standard hydrogenated amorphous silicon (a-Si:H) is made. While some properties of both materials are similar, many others differ in a striking manner. Characterizations of as-deposited pm-Si films show that the best samples exhibit enhanced transport properties, such as the fact that the quantum efficiency–mobility–lifetime product ημτ is increased by a factor of 200–700 compared with that measured on a-Si:H under the same conditions. This correlates with a lower density of deep states. The kinetics of creation of defects, performed under 670 mWcm−2 white light illumination and at a high temperature (100°C) in order to attain a final steady state, have been studied. pm-Si samples exhibit faster kinetics of creation as well as of annealing of metastable defects than do a-Si:H samples. In their light-soaked state the best pm-Si samples exhibit ημτ products of the same order as those measured on device-grade a-Si:H in the annealed state. These enhanced transport properties, new properties and better stability are linked to the peculiar structure of pm-Si, namely ordered silicon nanoparticles embedded in an amorphous matrix.

Some electronic and metastability properties of a new nanostructured material: Hydrogenated polymorphous silicon

Abstract When silicon thin films are deposited by plasma enhanced chemical vapour deposition in a plasma regime close to that of the formation of powder, a new type of material, called polymorphous silicon (pm-Si), is obtained. We present here the optoelectronic and stability properties of pm-Si films deposited from a mixture of silane diluted with hydrogen at total gas pressures in the range 800–1600mTorr. A comparison with the properties of standard hydrogenated amorphous silicon (a-Si:H) is made. While some properties of both materials are similar, many others differ in a striking manner. Characterizations of as-deposited pm-Si films show that the best samples exhibit enhanced transport properties, such as the fact that the quantum efficiency–mobility–lifetime product ημτ is increased by a factor of 200–700 compared with that measured on a-Si:H under the same conditions. This correlates with a lower density of deep states. The kinetics of creation of defects, performed under 670 mWcm−2 white light illumination and at a high temperature (100°C) in order to attain a final steady state, have been studied. pm-Si samples exhibit faster kinetics of creation as well as of annealing of metastable defects than do a-Si:H samples. In their light-soaked state the best pm-Si samples exhibit ημτ products of the same order as those measured on device-grade a-Si:H in the annealed state. These enhanced transport properties, new properties and better stability are linked to the peculiar structure of pm-Si, namely ordered silicon nanoparticles embedded in an amorphous matrix.

Hydrogen Passivation on the Grain Boundary and Intragranular Defects in Various Polysilicon Thin-Film Transistors

We investigated the hydrogenation effects on two types of trap states in poly-Si thin-film transistors (TFT's), where the gate oxide thickness was varied from 1000 Å to 4000 Å and the active poly-Si layers were altered by different annealing methods. The decrease of threshold voltage, which is related to the density of deep states, has shown a similar tendency irrespective of device structures. However, the other device parameters, such as field-effect mobility and minimum leakage current, which are influenced by the tail states, are improved considerably in the poly-Si TFT's with a thick gate oxide or with postannealed active layers. It was verified by the comparison of trap state distributions that the tail states were decreased significantly in the above TFT's with the thick gate oxide and postannealed poly-Si layers.

Defect Distribution And Metastability in Chalcopyrite Semiconductors

AbstractDeep levels in chalcopyrite based heterojunctions are investigated by capacitance techniques. A method is presented which allows the determination of defect distributions from admittance measurements. For CuInSe2 a defect level at 280 meV above the valence band with an attempt to escape frequency of about 1011 s-1 is detected. A characteristic defect structure consisting of a shallow defect level and a broad deep structure is observed for Cu(In, Ga)Se2. The depth of the shallow defect is affected by annealing treatments which are necessary to achieve the optimum performance of the devices. CulnS2grown under an excess of CuS exhibits a high density of shallow defects whereas for Cu-poor CuInS2 defects close to midgap position with a large capture cross section are observed. These defects are metastable with respect to current injection and illumination relaxing to the equilibrium state around room temperature. The increase of the defect density after illumination results in a reduced bucking current. CV measurements can be interpreted in terms of a high density of deep states.

An analytical a-Si:H TFT DC/capacitance model using an effective temperature approach for deriving a switching time model for an inverter circuit considering deep and tail states

This paper presents an analytical a-Si:H DC/capacitance model using an effective temperature approach for deriving a switching time model for an inverter circuit considering deep and tail states. Using an effective temperature approach, the localized deep and tail states have been considered in the DC/capacitance model and the switching time model. As verified by the published data, the analytical DC/capacitance model provides an accurate prediction. Based on the analytical model, the threshold voltage of an a-Si:H TFT is proportional to the deep state density and the switching time of the TFT-inverter is dependent on the tail state density.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Carrier lifetime in amorphous semiconductors

In amorphous semiconductors, because of the band-tail and -gap states, the excess carrier lifetime becomes sensitive to many experimental details. An attempt is made to clarify the relationship between carrier lifetime, density of states, and measurement details. The results show that in a steady-state photoconductivity measurement, the loss of carriers by recombination is determined by the density of deep states and the position of the quasi-Fermi level. In a transient measurement, the limited observation time and the contact also play important roles, besides the density of states. The shallow states in the energy band control the drift mobility and thus affect the lifetime indirectly. In an extreme case, the lifetime can become longer as the defect density increases. These points are illustrated with the electron lifetime derived from photoconductivity, time of flight, and delayed-field charge collection for hydrogenated amorphous silicon and silicon-germanium alloys.

Quality factor in a-Si:H nip and pin diodes

We analyze the forward characteristics of a-Si:H nip and pin diodes. At low bias, a well-defined exponential region exists, described by a noninteger quality factor n between 1.2 and 1.7. With increasing temperature, the quality factor decreases. This behavior can be understood with a model based on electron and hole recombination in the i layer, which relates the temperature dependence of the quality factor to the distribution of localized states in the amorphous silicon. The predictions of the model are supported by numerical calculations in which the diode device equations are solved for a given distribution of localized states. The different ideality factors are due to different energy dependencies of the density of deep states in the i layer.

Extraction of Altered Localized-States in a-Si and poly-Si TFT’s Under Various Conditions

We present results regarding to extraction of altered localized-states in both amorphous silicon and polycrystalline silicon thin film transistors under various conditions, such as electrical and light stress, crystallization, and hydrogenation. Both deep and tail states have been extracted by fitting calculated current-voltage curves to measured data. The density of deep states as well as tail states in a-Si are significantly increased by stress, which may be the predominant mechanism causing the degradation in a-Si TFT performances. Also, the correlations between trap distributions in poly-Si films and their device performances have been successfully identified by analyzing the effects of crystallization and hydrogenation on the current-voltage characteristics of poly-Si TFT’s.

Post-transit-time analysis of time-of-flight photocurrents

It is shown how the density of localized gap states in amorphous semiconductors can be deduced from time-of-flight photocurrents on the basis of the trap-limited band transport model, post-transit currents giving information on deep-state densities; and how strong deep-trapping of the photo-induced excess carriers invalidates the analysis. Results from a-Si:H, p-type a-Si:H, a-Si,S:H and a-Si,C:H samples are used to illustrate these points.

Electronic stability of the reactively sputtered hydrogenated amorphous silicon thin films: The effect of hydrogen content

The total hydrogen content (CH) of reactively sputtered hydrogenated amorphous silicon (a‐Si:H) films can be independently controlled by adjusting the partial pressure of hydrogen (PH2) during deposition. We report on the electronic stability of direct‐current (dc) magnetron reactive sputtered films deposited at a constant substrate temperature of 230 °C with various PH2. The films have CH from ∼13 to 28 at. % and similar, device quality properties in the as‐deposited state. The photoconductivities of all the films are between 0.9 to 3×10−5 (Ω cm)−1 under AM‐1 (air mass‐1)illumination (100 mW/cm2). The density of deep states (DOS) measured by the constant photocurrent method is ∼6×1015 cm−3 for low CH (≤15 at. %) and ∼2×1015 cm−3 for high CH (≥17) films. Light induced degradation measurements show a systematic correlation between CH and the electronic stability; the low CH films have a slower rate of photoconductivity decrease, and slower DOS increase, than either the high CH films or one glow‐discharge d...

Computer simulation of amorphous-silicon charge-coupled devices

The electron dynamics in amorphous-silicon charge-coupled devices have been investigated numerically by taking into account tail states and deep defect states in the energy gap of the active amorphous-silicon layer. It has been revealed that the deep states far from the conduction band have a great influence on electron-transfer characteristics and that a field-free region caused by metallic electrodes prevents quick drift motion of electrons. Based on those numerical results, a novel device structure with a narrow channel has been proposed. Calculated results on device efficiency agree well with the experimental data of M. Matsumara et al. (1987), assuming a deep-state density of 2*10/sup 16/ cm/sup -4//eV.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The effect of the morphology on the transport properties of α-Si:H layers

We have studied the effect of the morphology on transient photocurrents of glow discharge α-Si:H layers. We have found a large density of deep states in our samples where monohydride bonding is dominant. In transient photocurrent measurements the range limitation has been overcome only at high electric field and light intensity. This large density of deep states seems to be independent of microstructure. The microstructure of the sample has an effect on the electron mobility. In samples showing strong columnar-like structures the mobility is in the range of 0.01 cm 2 /Vs while in those layers which seem to be nearly structureless by SEM better than 0.1 cm 2 /Vs value can be measured.

Deep states distribution in undoped amorphous silicon studied by current transient spectroscopy

Current transient spectroscopy is investigated in order to determine the density of deep states in the gap of undoped, high-resistive a-Si:H. To estimate the attempt-to-escape frequency from the states, carrier capture measurement is carried out and analyzed considering spatial variation of the carrier capture, and the density of states is deduced from the detailed analysis of J X t product.

Transient-photocurrent studies ina−Si:H

We present a detailed study of carrier transport in sputtered $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ with the use of the time-of-flight technique. Charge-collection experiments reveal that total charge collection must take place to enable the extraction of reliable dispersion parameters. Under this condition the dispersion parameters accurately follow the predictions of a multiple trapping and release transport process. By integration of the current transients we derive values of the ${\ensuremath{\mu}}_{D}{\ensuremath{\tau}}_{D}$ product for samples having a range of deep-state densities and also obtain the capture cross section of the dominant defect. Upon phosphorus doping the electron carrier lifetime increases; furthermore, changes in the dispersion parameters suggest that the conduction-band-tail---state distribution is altered with respect to the undoped material. We also present measurements of photocurrents made with the use of electrode co-planar geometry and suggest that carrier loss to deep states dominates the response at room temperature.

On the analysis of space-charge-limited current—voltage characteristics and the density of states in amorphous silicon

Abstract An expression is derived for the current-voltage characteristic under space-charge-limited conditions when the density of deep states increases exponentially with energy above the Fermi level. This shows that the slope of a logJ - log V curve increases slowly and only reaches the limiting value of (l + 1) (where l = T c/T and T c characterizes the steepness of the exponential) at voltages above those used in practical experiments. This invalidates the procedure of deriving values for T c by fitting straight lines to experimental log J - log V plots and suggests an explanation of some anomalous results published recently. Some brief comments are made on the values of T c to be expected for a-Si: H.

Direct Measurement of the Bulk Density of Gap States inn-Type Hydrogenated Amorphous Silicon

The first direct measurement is reported of the bulk density of deep states in $n$-type $a$-Si: H. The spectral distribution is considerably different from previous field-effect and $C\ensuremath{-}V$ measurements and the overall density is much lower than has previously been reported. The states seen in these samples appear to be extrinsic and suggest that the extrapolated total density of deep states in pure $a$-Si: H may be less than ${10}^{15}$ ${\mathrm{cm}}^{\ensuremath{-}3}$.