Density Of Deep States Research Articles

A proposal and simulation analysis for a novel architecture of gate-all-around polycrystalline silicon nanowire field effect transistor

A proposal for a novel gate-all-around (GAA) polycrystalline silicon nanowire (poly-SiNW) field effect transistor (FET) is presented and discussed in this paper. The device architecture is based on the realization of poly-SiNW in a V-shaped cavity obtained by tetra methyl ammonium hydroxide (TMAH) etch of monocrystalline silicon (100). The device’s behavior is simulated using Silvaco commercial software, including the density of states (DOS) model described by the double exponential distribution of acceptor trap density within the gap. The electric field, potential, and free electron concentration are analyzed in different nanowire regions to investigate the device's performance. The results show good performance despite the high density of deep states in poly-SiNW. This can be explained by the strong electric field caused by the corner effect in the nanowire, which favors the ionization of the acceptor traps and increases the free electron concentration.

Light soaking effect on defect states distribution of Hydrogenated amorphous silicon investigated By means of constant photocurrent technique

In the present paper we have investigated, by using the constant photocurrent method in dc-mode (dc-CPM), the effect of the light soaking (LS) on the deep defect density (Nd) and the slope of the Urbach tail (E0) of a slightly phosphorus-doped hydrogenated amorphous silicon (a-Si:H) film prepared by Plasma-Enhanced Chemical Vapour Deposition (PECVD). By applying the derivative method, we have converted the measured data into a density of states (DOS) distribution in the lower part of the energy gap. The evolution of the sub-band-gap absorption coefficient and the CPMdetermined density of gap-states distribution within the gap versus the illumination time leads to: (i) an increase in the deep defect absorption without any significant changes in the Urbach tail (exponential part), (ii) a presence of more charged than neutral defects as predicted by the defect pool model, and (iii) a saturation point of the degradation of both optical absorption coefficient and density of deep states of slightly P-doped sample measured by dc-CPM. The constant photocurrent technique in dc-mode as a spectroscopy method for the defect distribution determination is, therefore, most reliable to study the light soaking effect on the stability of hydrogenated amorphous silicon layers used in solar cells manufacturing.

Correlating the Density of Trap-States and the Field-Effect Performance in Metal-Oxide Thin-Film Transistors With High-$\kappa$ Gate Dielectrics via a Trap-Limited Conduction Method

We performed a quantitative study using a trap-limited conduction method to correlate the density of localized states with the performance of Sm2O3- and HfO2-gated metal-oxide thin-film transistors (TFTs) fabricated at room temperature. We found that TFT performance with a Sm2O3 gate dielectric exceeds that with a HfO2 gate dielectric. Sm2O3-gated TFTs with an In-Ga-Zn-O (In-Zn-O) channel show a saturation mobility of 57.1 (114.9) cm $^2$ V $^{-1}$ s $^{-1}$ , effective threshold voltage of 0.9 (1.6) V, and subthreshold swing of 0.116 (0.256) Vdec $^{-1}$ . Based on a trap-limited conduction model, the improved performance correlates with the lower density of localized states in the tail and deep regions. A lower density of tail states leads to higher mobility, and the lower density of deep states tends to reduce both the threshold voltage and the subthreshold swing. Using semi-empirical quantum chemistry simulations, we show that the decrease in the number of localized states is associated with the inherent characteristics of Sm cations, due to their lower electropositivity and larger ionic radius compared to Hf, which leads to less ionic distortion in the channel.

Parameter Extraction and Compact Modeling of 1/f Noise for Amorphous ESL IGZO TFTs

In this paper, the flicker noise properties of bottom-gate ESL structured amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) from two different technologies have been studied and modeled. Model development is carried out by adapting the Unified Model parameter Extraction Method (UMEM), developed for parameter extraction and compact modeling of TFT devices, and the unified 1/f noise model. Furthermore, comparative study of device figures of merits is performed to point out the device performance enhancements that the different fabrication technologies have brought about. It is found out that both devices have low deep state density, whereas the second device demonstrated higher performance in terms of carrier mobility and subthreshold slope. Both the DC and noise models are validated by matching them against experimental data obtained in different regimes of device operation.

Electrical Performance and Bias-Stress Stability of Amorphous InGaZnO Thin-Film Transistors with Buried-Channel Layers

To improve the electrical performance and bias-stress stability of amorphous InGaZnO thin-film transistors (a-IGZO TFTs), we fabricated and characterized buried-channel devices with multiple-stacked channel layers, i.e., a nitrogen-doped a-IGZO film (front-channel layer), a conventional a-IGZO film (buried-channel layer), and a nitrogen-doped a-IGZO film (back-channel layer). The larger field-effect mobility (5.8 cm2V−1s−1), the smaller subthreshold swing value (0.8 V/dec, and the better stability (smaller threshold voltage shifts during bias-stress and light illumination tests) were obtained for the buried-channel device relative to the conventional a-IGZO TFT. The specially designed channel-layer structure resulted in multiple conduction channels and hence large field-effect mobility. The in situ nitrogen-doping caused reductions in both the front-channel interface trap density and the density of deep states in the bulk channel layers, leading to a small subthreshold swing value. The better stability properties may be related to both the reduced trap states by nitrogen-doping and the passivation effect of the nitrogen-doped a-IGZO films at the device back channels.

Effect of Deep Centers on Charge-Carrier Confinement in InGaN/GaN Quantum Wells and on LED Efficiency

The deep-center-assisted tunneling of carriers in p–n structures of light-emitting diodes (LEDs) with InGaN/GaN quantum wells (QWs) makes smaller the effective height of the injection barrier, but leads to a dependence of the radiation efficiency on the density and energy spectrum of defects in GaN. In the case of hopping conduction across the space charge region, the forward voltage mainly drops near the QW boundary, where the density of deep states at the quasi Fermi-level is the lowest. As a result, band bending at the boundary decreases, and, with increasing current, the direction of the electric field also changes, which leads to a weaker confinement of holes, to their non-radiative recombination in the n barrier, and to an efficiency droop. The low efficiency of green GaN LEDs is associated with the dominance of deep centers and insufficient density of shallow centers in the energy spectrum of defects in barrier layers near the boundaries with the QW. The proposed model is confirmed by the stepwise experimental dependences of the current, capacitance and efficiency of green and blue LEDs in the case of forward bias, which reflect the contribution of color centers responsible for the defect photoluminescence bands in GaN.

Влияние глубоких центров на конфайнмент носителей в квантовых ямах InGaN/GaN и эффективность светодиодов

AbstractThe deep-center-assisted tunneling of carriers in p–n structures of light-emitting diodes (LEDs) with InGaN/GaN quantum wells (QWs) makes smaller the effective height of the injection barrier, but leads to a dependence of the radiation efficiency on the density and energy spectrum of defects in GaN. In the case of hopping conduction across the space charge region, the forward voltage mainly drops near the QW boundary, where the density of deep states at the quasi Fermi-level is the lowest. As a result, band bending at the boundary decreases, and, with increasing current, the direction of the electric field also changes, which leads to a weaker confinement of holes, to their non-radiative recombination in the n barrier, and to an efficiency droop. The low efficiency of green GaN LEDs is associated with the dominance of deep centers and insufficient density of shallow centers in the energy spectrum of defects in barrier layers near the boundaries with the QW. The proposed model is confirmed by the stepwise experimental dependences of the current, capacitance and efficiency of green and blue LEDs in the case of forward bias, which reflect the contribution of color centers responsible for the defect photoluminescence bands in GaN.

Determination of bulk and interface density of states in metal oxide semiconductor thin-film transistors by using capacitance–voltage characteristics

A physical-based straightforward extraction technique for interface and bulk density of states in metal oxide semiconductor thin film transistors (TFTs) is proposed by using the capacitance–voltage ( C – V ) characteristics. The interface trap density distribution with energy has been extracted from the analysis of capacitance–voltage characteristics. Using the obtained interface state distribution, the bulk trap density has been determined. With this method, for the interface trap density, it is found that deep state density nearing the mid-gap is approximately constant and tail states density increases exponentially with energy; for the bulk trap density, it is a superposition of exponential deep states and exponential tail states. The validity of the extraction is verified by comparisons with the measured current–voltage ( I – V ) characteristics and the simulation results by the technology computer-aided design (TCAD) model. This extraction method uses non-numerical iteration which is simple, fast and accurate. Therefore, it is very useful for TFT device characterization.

Deep-level transient spectroscopy on an amorphous InGaZnO4 Schottky diode

The first direct measurement is reported of the bulk density of deep states in amorphous IGZO (indium-gallium-zinc oxide) semiconductor by means of deep-level transient spectroscopy (DLTS). The device under test is a Schottky diode of amorphous IGZO semiconductor on a palladium (Pd) Schottky-barrier electrode and with a molybdenum (Mo) Ohmic contact at the top. The DLTS technique allows to independently measure the energy and spatial distribution of subgap states in the IGZO thin film. The subgap trap concentration has a double exponential distribution as a function energy, with a value of ∼10 19 cm-3 eV-1 at the conduction band edge and a value of ∼1017 cm-3 eV-1 at an energy of 0.55 eV below the conduction band. Such spectral distribution, however, is not uniform through the semiconductor film. The spatial distribution of subgap states correlates well with the background doping density distribution in the semiconductor, which increases towards the Ohmic Mo contact, suggesting that these two properties share the same physical origin. cop. 2014 AIP Publishing LLC.

Quantitative and Depth-Resolved Investigation of Deep-Level Defects in InGaN/GaN Heterostructures

Deep-level defects in In0.17Ga0.83N/In0.02Ga0.98N/p-GaN:Mg heterostructures were studied using deep-level optical spectroscopy (DLOS). Depth-resolved DLOS was achieved by exploiting the polarization-induced electric fields to discriminate among defects located in the In0.17Ga0.83N and the In0.02Ga0.98N regions. Growth conditions for the InxGa1� xN layers were nominally the same as those in InGaN/GaN multi-quantum-well (MQW) structures, so the defect states reported here are expected to be active in MQW regions. Thus, this work provides important insight into defects that are likely to influence MQW radiative efficiency. In0.17Ga0.83N-related bandgap states were observed at Ev + 1.60 eV and Ev + 2.59 eV, where Ev is the valence-band maximum, compared with levels at Ev + 1.85 eV, Ev + 2.51 eV, and Ev + 3.30 eV in the In0.02Ga0.98N region. A lighted capacitance–voltage technique was used to determine the areal density of deep states. The possible origins of the associated defects are considered along with their potential roles in light-emitting diodes.

Relationship of deep defects to oxygen and hydrogen content in nanocrystalline silicon photovoltaic materials

We report measurements of the structural and compositional properties of a range of hydrogenated nanocrystalline films. We employed Raman spectroscopy for crystallinity and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) for impurity characterizations. The crystalline volume fractions and impurity levels are correlated with the deep state densities determined by drive level capacitance profiling. Those defects were found to have a thermal emission energy of 0.65±.05 eV. We found that the overall crystallinity correlated reasonably well with the density of such defect states and also found a strong correlation between the defect density and the levels of oxygen impurities. Possible origins of these defects are discussed.

Compact model for forward subthreshold characteristics in polymer semiconductor transistors

We present a compact model for polymer thin film transistors (TFTs) operating in forward subthreshold region. Due to the threshold voltage shift in these devices, the bias point of a device operating for a sufficiently long time moves toward the subthreshold region. Therefore, modeling subthreshold operation in polymer semiconductor based TFTs is important. We particularly address two areas where polymer TFTs are different from other disordered materials such as amorphous silicon. Firstly, the shape of the density of deep states cannot be assumed to be purely exponential. Such an assumption does not provide fine modeling of the subthreshold slope. Secondly, for subthreshold operation particularly under high drain-source bias, we need to include Poole–Frenkel type transport mechanisms such as variable range hopping and space charge limited current. This paper includes these features and develops a compact closed form model.

Grain-size-related transient terahertz mobility of femtosecond-laser-annealed polycrystalline silicon

We investigated carrier-relaxation dynamics of femtosecond laser annealed (FLA) polycrystalline silicon (poly-Si). The correlation between morphology and electrical properties of poly-Si after femtosecond laser annealing is elucidated by optical-pump–terahertz-probe and terahertz time-domain spectroscopies. The transient conductivities of FLA-processed poly-Si with large (∼500 nm) and small (∼50 nm) grain sizes were both well fitted by the Drude model in the terahertz regime from 0.4 to 2 THz. The transient mobilities of these materials were determined to be 175±19.4 and 94.5±20.2 cm2/V s, respectively. After annealing, reduction of deep-state density rather than tail-state density in large-grain poly-Si is responsible for its higher mobility.

Stability of continuous-wave laser-crystallized epilike silicon transistors

Stability of high-hole-mobility thin-film transistors (TFTs) on single-grainlike silicon channels formed by continuous-wave laser crystallization during hot-carrier stressing (HCS) was studied. As channel layers become thicker, laser-mediated channel crystallinity increases, increasing channel roughness. On such epilike polycrystalline silicon substrates, the poorer interface quality for thicker channels, even those with lower tail-state densities of grain traps, is responsible for the extensive charge trapping and creation of deep-state densities in the fabricated TFTs due to HCS. Hence, on a thin channel with a thickness of 50nm and ultrasmooth surfaces, HCS hardly degrades the electrical parameters of the devices.

Trap-state density in continuous-wave laser-crystallized single-grainlike silicon transistors

This investigation characterizes electrical characteristics of continuous-wave green laser-annealed single-grainlike silicon thin-film transistors in relation to trap-state densities. As laser power increases, highly crystalline channels form, reducing tail-state densities to as low as 3×1019eV−1cm−3. This occurrence is responsible for high field-effect electron mobility of 284cm2∕Vs. In contrast, increasing laser power initially reduces the deep-state density and then increases it to 3×1016eV−1cm−3. This reversal in deep-state density and thus in the subthreshold slope as well as a saturating reduction in threshold voltage are associated with the formation of extra interface defects caused by laser-crystallization-enhanced surface roughness.

Properties of dislocation networks formed by Si wafer direct bonding

Reproducible formation of well-controlled dislocation structures is a prerequisite to use dislocations as an active part of devices. Regular dislocation networks have been formed at the interface by Si wafer direct bonding. The barriers of interface were generally smaller than 100 meV. The temperature dependence of the electron-beam-induced current (EBIC) contrast of the interface indicates a deep state density of a few 10E5 per cm along the dislocation lines in the network. It is also found that the dislocation networks in Si can act as effective channel for carrier transport. Photoluminescence (PL) reveals that the D line spectrum related to the dislocation networks can be tailored by the bonding misorientation. So, the D1 line can be made the dominating feature in the PL spectrum. It is suggested that regular dislocation networks represent an interesting new nanosystem for future applications, such as accommodation biomolecules onto silicon, dislocation-based LED or buried nanowires.

Effect of localized traps on the anomalous behavior of the transconductance in nanocrystalline TFTs

It has been reported that nanocrystalline and microcrystalline devices show an anomalous behavior in the transconductance where several rates of increase of the transconductance with applied gate voltage, not present in amorphous TFTs are observed. In this paper we show that the anomalous effect of the transconductance is observed for an acceptor tail states activation energy similar to the normal values for hydrogenated silicon amorphous devices, (a-Si:H), provided that some conditions are met regarding the density of trapped charge in tail and deep states and the density of free charge in the material, which does not necessarily suggest a behavior in between amorphous and polycrystalline. The effect appears if the density of deep tail states, is smaller (higher) than the typical values in a-Si:H. The localized state distribution present in a nanocrystalline TFT prepared by hot wire deposition technique is estimated by comparison of experimental and simulated transconductance curves. In our case a lower density of deep states is obtained, which corresponds with their better light and bias stability.

Density of localized electronic states in a-Se from electron time-of-flight photocurrent measurements

Electron time-of-flight transient photocurrents have been investigated in stabilized a-Se as a function of electric field, annealing, aging (relaxation), and alloying with As and doping with Cl. The distribution of localized states (DOS) in stabilized a-Se has been investigated by comparing the measured and calculated transient photocurrents. The samples were prepared by conventional vacuum deposition techniques. The theoretical analysis of multiple-trapping transport has been done by the discretization of a continuous DOS and the use of Laplace transform formalism. The resulting DOS has distinct features: A first peak at ∼0.30eV below Ec with an amplitude ∼1017eV−1cm−3, a second small peak (or shoulder) at 0.45–0.50 eV below Ec with an amplitude 1014–1015eV−1cm−3, and deep states with an integral concentration 1011–1014cm−3 lying below 0.65 eV, whose exact distribution could not be resolved over the time scale of present experiments. The influence of doping, aging, annealing, and substrate temperature on the DOS distribution has been investigated. The doping with Cl does not affect the amplitudes of the first and second peaks while the concentration of deep states increases dramatically. The alloying with As reduces the density of deep states and seems to increase the amplitude of first and second peaks. The aging substantially reduces the deep states density and the amplitude of the second peak while the amplitude of the first one remains practically unchanged. The results have been interpreted primarily in terms of thermodynamic and intrinsic structural defects in the chalcogenide glass structure.

Electrical Measurement of Recombination Lifetime in Blue Light Emitting Diodes

ABSTRACTA recombination lifetime of approximately 25 ns was extracted from measured reverse recovery storage times in AlGaN/GaN/AlGaN double heterojunction blue light emitting diodes. This experimentally determined lifetime is expected to arise from a combination of radiative and non-radiative processes occurring in the diodes. The non-radiative processes are likely to be due the presence of a high concentration deep-states as identified from the current-voltage and capacitance-voltage measurements. Current-voltage characteristics of these diodes were highly non-ideal as indicated by high values of the ideality factor ranging from 3.0 – 7.0. Logarithmic plots of the forward characteristics indicated a space-charge-limited-current (SCLC) conduction in presence of a high density of “deep-level states” in the active region of the diodes. An analysis of these characteristics yielded an approximate density of these deep-level states as 2 × 1017/cm3. The density of deep-states extracted from capacitance-voltage measurements were in good agreement with that obtained from current-voltage measurements.

New procedure for the extraction of a-Si:H TFTs model parameters in the subthreshold region

In a previous paper we presented a new procedure to extract basic AIM-Spice model parameters of amorphous thin film transistor model in the above-threshold region, based on the integration of the experimental data current. In this paper we present a new procedure to determine subthreshold TFT parameters highly dependent on the fabrication technology as the density of deep states and characteristic temperature of deep states. These parameters are extracted in a simple and direct way from the experimental measurements, with no need of assigning predetermined values to any other model parameter. Other parameters required for device modeling in the subthreshold regime are also determined. The validity of the procedure is tested for a-Si:H TFTs with channel length down to 4 μm. We also show the good coincidence between calculated, using parameter values obtained with our new extraction procedure and experimental curves, in all working regions of the devices. Finally we discuss effects in the behavior of I DS vs. V DS curve related to the series resistance at the drain and source, which become evident during the device modeling.