Distribution Of Band Tail States Research Articles

Trap-state suppression and band-like transport in bilayer-type organic semiconductor ultrathin single crystals

Fundamental study of the carrier transport in both the channel layer and the electrode-channel contact of organic semiconductor crystals is indispensable for achieving high-performance organic field-effect transistors. In this paper, we report the temperature-dependent carrier transport of high-mobility organic semiconductors called $2\text{--}\text{decyl}\text{--}7\text{-phenyl-}[1]\text{benzothieno}[3,2\text{--}b][1]\mathrm{benzothiophene}$ (Ph-BTBT-C10) and $3\text{-decyl-}9\text{-phenyl-}[1]\text{benzothieno}[3,2\text{--}b]\text{naphtho}[2,3\text{--}b]\text{thiophene}$ (Ph-BTNT-C10). The use of single-crystal films with controlled bilayer-number thickness enabled the simultaneous study of intralayer and interlayer transport at cryogenic temperatures. Four-probe measurement of two-bilayer- and three-bilayer-thick films of Ph-BTBT-C10 suggests that the access resistance is dominated by tunneling transport across the insulating alkyl-chain layers. Single-crystal thin-film transistors of these materials showed band-like carrier transport down to 80 K and the carrier mobility of Ph-BTBT-C10 reached $34\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$. Detailed analysis of the low-temperature characteristics revealed small activation energy of approximately 5 meV and a sharp distribution of band tail states. These findings suggest that high crystallinity owing to the bilayer-type crystal structure effectively suppresses the localization of gate-induced carriers.

Noise and detectivity limits in organic shortwave infrared photodiodes with low disorder

To achieve high detectivity in infrared detectors, it is critical to reduce the device noise. However, for non-crystalline semiconductors, an essential framework is missing to understand and predict the effects of disorder on the dark current. This report presents experimental and modeling studies on the noise current in exemplar organic bulk heterojunction photodiodes, with 10 donor–acceptor combinations spanning wavelength between 800 and 1600 nm. A significant reduction of the noise and higher detectivity were found in devices using non-fullerene acceptors (NFAs) in comparison to those using fullerene derivatives. The low noise in NFA blends was attributed to a sharp drop off in the distribution of bandtail states, as revealed by variable-temperature density-of-states measurements. Taking disorder into account, we developed a general physical model to explain the dependence of thermal noise on the effective bandgap and bandtail spread. The model provides theoretical targets for the maximum detectivity that can be obtained at different detection wavelengths in inherently disordered infrared photodiodes.

Recombination dynamics of a localized exciton bound at basal stacking faults within the m-plane ZnO film

We investigated the carrier dynamics near basal stacking faults (BSFs) in m-plane ZnO epitaxial film. The behaviors of the type-II quantum wells related to the BSFs are verified through time-resolved and time-integrated photoluminescence. The decay time of the emission of BSFs is observed to have a higher power law value and longer decay time than the emission of the donor-bound excitons. The spectral-dependent decay times reveal a phenomenon of carriers migrating among band tail states, which are related to the spatial distribution of the type-II quantum wells formed by the BSFs. A high density of excited carriers leads to a band bending effect, which in turn causes a blue-shift of the emission peak of BSFs with a broadened distribution of band tail states.

Application of Exponential Tail-State Distribution Model to the Above-Threshold Characteristics of Zn-Based Oxide Thin-Film Transistors

Our experimental results for amorphous InGaZnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> and polycrystalline ZnO TFTs show a gate-voltage-dependent transconductance that is somewhat different from that for crystalline MOSFETs. We show that a model of the exponential distribution of band tail states can be used to describe the above-threshold behavior of Zn-based oxide TFTs.

Above-threshold drain current model including band tail states in nanocrystalline silicon thin-film transistors for circuit implementation

A simple analytical expression for the above threshold voltage drain current is derived in nanocrystalline silicon thin-film transistors (TFTs), based on an exponential energy distribution of band tail states. When the characteristic temperature distribution of the band tails is equal to 1.5 times the lattice temperature, the derived expression leads to the basic “quadratic” metal-oxide-semiconductor current expression. By including the impact ionization effect and using the same trap distribution parameters, the model describes adequately the output characteristics of TFTs with different channel dimensions, making the proposed model suitable for the design of circuits with nc-Si TFTs.

Analytical current-voltage model for nanocrystalline silicon thin-film transistors

An analytical model for the drain current above threshold voltage, based on an exponential energy distribution of band tail states, has been applied to bottom-gated nanocrystalline silicon (nc-Si) thin-film transistors (TFTs). Analysis of the model shows that the slope of the exponential band tails determines the behavior of the device current-voltage characteristics. Comparison with experimental data shows that few fundamental model parameters, related to the material quality and different physical effects, can be used to describe consistently both output and transfer characteristics of nc-Si TFTs over a wide range of channel lengths.

Optical properties and transport in PLD-GaN

We present structural, optical and transport data on GaN samples grown by hybrid, two-step low temperature pulsed laser deposition. The band gap of samples with good crystallinity has been deduced from optical spectra. Large below gap band tails were observed. In samples with the lowest crystalline quality the PL spectra are quite dependent on spot laser incidence. The most intense PL lines can be attributed to excitons bounded to stacking faults. When the crystalline quality of the samples is increased the ubiquitous yellow emission band can be detected following a quenching process described by a similar activation energy to that one found in MOCVD grown samples. The samples with the highest quality present, besides the yellow band, show a large near band edge emission which peaked at 3.47 eV and could be observed up to room temperature. The large width of the NBE is attributed to effect of a wide distribution of band tail states on the excitons. Photoconductivity data supports this interpretation.

Effect of Ga content on defect states in CuIn1−xGaxSe2 photovoltaic devices

Defects in the band gap of CuIn1−xGaxSe2 have been characterized using transient photocapacitance spectroscopy. The measured spectra clearly show response from a band of defects centered around 0.8 eV from the valence band edge as well as an exponential distribution of band tail states. Despite Ga contents ranging from Ga/(In+Ga)=0.0 to 0.8, the defect bandwidth and its position relative to the valence band remain constant. This defect band may act as an important recombination center, contributing to the decrease in device efficiency with increasing Ga content.

Gap states and stability of rapidly deposited hydrogenated amorphous silicon films

The sub-band gap absorption spectrum α( hv) in a-Si:H films prepared at high deposition rates was measured by the constant photo-current method (CPM) for photon energy ranging from 0.8 to 1.7 eV in a thermally-annealed state and light-soaked state. The Simmons–Taylor theory and occupation statistics of correlated defects are used to model the distribution of band tail and gap states. It is found that the density of gap states increases after light soaking, however, there is no evident change found in the density and distribution of band tail states. Measurements of the light-induced changes find that the photoconductivity decreases by less than one order of magnitude after long time of light illumination for the high rate deposited a-Si:H. This demonstrates that the high rate deposited samples have relatively high stability compared with conventionally deposited a-Si:H.

Light-induced changes in the photoconductivity of rapidly-deposited hydrogenated amorphous silicon films

The temperature dependence of steady-state photoconductivity in hydrogenated amorphous silicon films prepared at high deposition rates have been measured in thermally-annealed states (state A) and light-soaked states (state B). The light-induced changes (the Staeble-Wronski effect) in the photoconductivity were studied using a computer simulation calculation process. The Simmons-Taylor theory and occupation statistics of correlated defects were used to describe the exponential distribution band tail states and the dangling bond states. Tunneling recombination of excess carriers was taken into account at low temperature. It was found that the density of dangling bond states increases after light soaking, but there is no change in the density and distribution of band tail states. A discussion of the experimental and simulation results is given.

Absorption in amorphous silicon doping-modulated multilayers

Photothermal deflection spectroscopy (PDS) measurements on doping-modulated multilayers of a-Si : H show that the measured width of the absorption due to band tails increases as the relative intrinsic layer thickness decreases. This increased width is due to a wider distribution of band tail states in the n- and p-doped layers. A large absorption (∼ 100 cm −1) well below the gap is observed in all samples.