Amorphous Silicon Schottky Diodes Research Articles

Space charge capacitance spectroscopy in amorphous silicon Schottky diodes: Theory, modeling, and experiments

Numerical modeling of capacitance spectroscopy of hydrogenated amorphous silicon Schottky diodes is carried out. We test the accuracy of the determination of the density of states at the Fermi level, g(EF), from an analytical treatment of the temperature (T) and frequency (f) dependence of the capacitance (C). Assessment of the position of the Fermi level and the attempt-to-escape frequency of states at EF is also addressed. It is shown that the precision and reliability of the determination of g(EF) is strongly dependent on the position of the Fermi level and the shape of the DOS and that the attempt-to-escape frequency is overestimated. Numerical calculations are then used to fit experimental capacitance data. Material parameters that provide the best fits are found in quite good agreement with independent modulated photocurrent and constant photocurrent measurements. Again, the attempt-to-escape frequency deduced from the simplified analytical treatment of capacitance data is found to be overestimated.

Fabrication of charge‐pump active‐matrix OLED display panel with 64 × 64 pixels

Organic light‐emitting diode (OLED) display panel using the charge‐pump (CP) pixel addressing scheme was fabricated, and the results show that it is applicable for information display. A CP‐OLED panel with 64 × 64 pixels consisting of thin‐film capacitors and amorphous silicon Schottky diodes was fabricated using conventional thin‐film processes. The pixel drive circuit passes electrical current into the OLED cell during most of the frame period as in the thin‐film transistor (TFT)‐based active‐matrix (AM) OLED displays. In this study, the panel was operated at a voltage level of below 4 V, and this operation voltage can be reduced by eliminating the overlap capacitance between the column bus line and the common electrode.

Reverse current instabilities in amorphous silicon Schottky diodes: modeling and experiments

A new model for high bias transport is reported which describes the time-dependent reverse current variations in amorphous silicon Schottky diodes. This phenomenon is of practical importance in the design and optimization of pixels for large-area optical and X-ray imaging. In the model, the main components of the reverse current, namely thermionic emission and tunneling, are both affected by the electric field at the metal/amorphous silicon interface. Time-dependent variations in this electric field arise due to the release of charges trapped in defect states in the depletion region and to charge trapping at the interface. This effect is analyzed using the approximation that the tunneling component of the current is equivalent to a lowering of the potential barrier at the interface. The calculated time-dependent reverse current is compared with the measured data.

Transient behavior of a-Si(C):H bulk barrier color detectors

The photocurrent transient of a-Si(C):H n-i-p-i-n/p-i-n-i-p color detectors under pulse illumination at different wavelengths has been investigated. In contrast to amorphous silicon Schottky diodes and p-i-n/n-i-p structures, the photocurrent decay after the end of the steady state illumination is barely influenced by the applied bias voltage. Moreover, a reversal of the photocurrent direction can be observed under certain bias when the light is being switched on. It is suggested that these properties of n-i-p-i-n/p-i-n-i-p structures are mainly attributed to the accumulation and trapping of the photogenerated carriers near the central barrier.

Time Dependence of the Reverse Current in Amorphous Silicon Schottky Diodes

ABSTRACTWe present measurement results of the time-dependent reverse current in amorphous silicon Schottky diodes for a broad range of bias voltage stress conditions. The resultant behavior can be divided into three regimes, depending on the bias. At low biases, the reverse current exhibits a power-law dependence attributable to dispersive electron transport. In the medium bias regime, the current shows a dramatic increase which may be due to enhanced thermionic emission and tunneling of electrons across the barrier, enabled by hole transport to the metal-semiconductor interface. At high biases, the effects of prolonged stress were found to be irreversible. Here, an eventual decrease in reverse current was observed, with an associated loss of rectifying characteristics.

Effect of the Depletion Region of a-Si:H Schottky Diodes on the Time-of-Flight Mobility-Lifetime Product

ABSTRACTThe effect of the depletion region in amorphous silicon Schottky diodes on the mobility-deep trapping lifetime product μτ of electrons has been investigated using transient photoconductivity techniques. We varied the laser wavelength of the exciting laser in time-of-flight (TOF) measurements. The results are correlated with measurements of the internal field by means of the charge collection efficiency, and with measurement of the deep-trapping time by the Delayed-Field TOF technique (DFTOF). We find that the electron μτ decreases from the bulk toward the top surface.

On the Role of Inversion Layer, Electronic Density of States and Interfacial Layer on the Barrier Height Formation of Amorphous-Silicon Schottky Diodes

Large variations in barrier height of a-Si:H Schottky diodes can be achieved by introducing thin inversion layer at the interface. Electronic density of states in the basic film depends upon the growth and surface preparation conditions.However, the high density of surface states of the order of 10 13 cm -2 eV -1 usually found at a-Si:H film masks the role of bulk density of states which are ∼10 16 cm -3 eV -1 . As the density of surface states increases, the barrier height increases. At very large density (∼ 5 × 10 14 cm -2 eV -1 ), Fermi level of the film pins to the neutral level and the barrier height becomes independent of the metal work function. Poisson's equation has been employed to illustrate the role of various charges contributed by the basic semiconductor, the inversion layer and the eletronic states. The concept of interfacial layer of 2 to 3 atomic dimensions with the permittivity of free space is essential to understand the mechanism of barrier height formation in a-Si:H Schottky diodes like similar devices in other semiconductors.

Transport properties and defect states of a-Si:H grown by HOMOCVD

In this paper, experimental measurements of surface and gap state densities for homogeneous chemical vapor deposited (HOMOCVD) hydrogenated amorphous silicon (a-Si:H) were obtained by C-ω-T, C-V, DLTS and ESR. These results are compared to glow discharge prepared amorphous films. The experimental data and calculated numbers are used to show better agreement with the diffusion rather than the thermionic theory for these amorphous silicon Schottky diodes.

Measurement of deep levels in hydrogenated amorphous silicon by transient voltage spectroscopy

Electronic defect levels in amorphous silicon Schottky diodes have been measured by capacitance transient spectroscopy performed in the constant capacitance mode. In hydrogenated amorphous silicon deep levels are continuously distributed in energy and of sufficient density to dominate the electrical properties. The constant capacitance mode, applied for the first time to an amorphous semiconductor, offers significant advantages over the transient capacitance mode for measuring the bulk density of gap states. For example, numerical solution of Poisson’s equation for only the steady-state charge distribution is required to analyze the transient response of a diode after a trap-filling voltage pulse. The technique has been used to record time-resolved transients, which saturate with respect to pulse width, and the analysis yields deep level distributions in the range of 1017 eV−1 cm−3 over the energy interval from 0.6 to 0.9 eV below the conduction-band mobility edge.

Post-hydrogenated chemical vapor deposited amorphous silicon Schottky diodes

Amorphous Si Schottky diodes are prepared by chemical vapor deposition and a post- hydrogenation treatment. Capacitance and photocurrent measurements on these diodes as a function of doping are discussed in terms of space-charge characteristics and carrier collection mechanisms. For doped samples a full analysis of the C(ω) and C(V) dependences enables us to obtain the space-charge width and the doping efficiency. A correlation between space-charge width and collection length of photogenerated carriers is obtained in these samples. For undoped diodes the photocurrent versus wavelength dependences are used to analyze the collection efficiency. A power efficiency of 2.5% is obtained for a fully depleted cell without antireflection coating under AMl solar simulator conditions.

Calculation of capacitance-voltage characteristics of hydrogenated amorphous silicon Schottky diodes

By adopting a new boundary condition appropriate to hydrogenated amorphous silicon diodes, and using the nonequilibrium Fermi–Dirac statistics, we predict a low-frequency capacitance-voltage relation which is quite different from that obtained by previous calculations in the literature, while in good agreement with more recent experimental results. The physical reasons for the differences are discussed.

Effects of the undoped layer on characteristics of amorphous silicon Schottky diodes

The effects of undoped layer thickness on the dark and illuminated I-V characteristics of hydrogenated amorphous silicon Schottky barrier solar cells are investigated. Schottky barrier (S.B.) metals having different work function (Cr and Pd) were deposited on the 0.22 µm - 1.45 µm thick a-Si:H films. Photovoltaic performance, J sc , V oc , FF and efficiency, are independent of thickness of the undoped layer if film thickness is larger than the depletion region width. J sc and V oc are controlled by S.B. metal and FF is independent of S.B. metal. Dark I-V characteristics depend on both S.B. metal and device thickness suggesting a barrier controlled space charge limited phenomena. Variation of turn-on (threshold) voltage with undoped layer thickness can be applied to the design of switching and memory devices.

Influence of preparation conditions on forward-bias currents of amorphous silicon Schottky diodes

We report on the open-circuit voltage Voc and the forward-current–voltage characteristic, J=Js(expqV/nKT−1), of Schottky diodes obtained by evaporation of platinum onto hydrogenated amorphous silicon (a-Si :H) fabricated by glow-discharge decomposition of silane. The diode characteristics were studied as a function of several preparation parameters, namely, silane pressure, substrate temperature, silane flow rate, annealing temperature. Rectifying behavior was obtained only for substrate temperatures between 200 and 300°C. Within this constraint, the preparation parameters have only a small effect on Voc but have large influences on n and Js. The factor n decreases towards ideality (n=1) as the silane pressure decreases, as the substrate temperature increases and with annealing of the Pt contact. The corresponding decrease of the concentration of polyhydride H sites with similar variations in the preparation parameters suggests that the recombination center responsible for nonideal value of n are associated with the polyhydride sites. We argue that the nearly constant value of Voc and therefore of the potential barrier probably originates from the crossing of the Fermi level with deep levels near the interface rather than from a previously proposed interface state. Although we were unable to use a sufficiently low temperature to choose between the thermionic emission and diffusion theories for the forward-current-voltage relationship, we point out that carrier injection in the metal base transistor structure favors a thermionic emission model. Assuming thermionic emission the value deduced for the Richardson constant is of the same order of magnitude as found for crystal Si Schottky barriers.

The interfacial layer in MIS amorphous silicon solar cells

When an insulating layer of TiOx is added beneath the Ni barrier contact of amorphous silicon Schottky diodes, increases are produced in both the open-circuit voltage and the short-circuit current. The former change is explained by an increased barrier height and diode factor, and the latter change is at least partially caused by an increase in the width of the space-charge region.