K. J. B. M. Nieuwesteeg

Tunneling effective mass in hydrogenated amorphous silicon

The tunneling effective mass of electrons in undoped a‐Si:H has been determined from measurements on Schottky diodes operating with high reverse fields. Under these conditions, the change of current with electric field is a sensitive function of effective mass. The tunneling effective mass was measured to be 0.09±0.02 me for a range of different samples giving a tunneling constant of ≊40 A.

On the current mechanism in reverse‐biased amorphous‐silicon Schottky contacts. II. Reverse‐bias current mechanism

The physical mechanisms that determine the current transport in reverse‐biased Schottky diodes on undoped ‘‘device‐grade’’ hydrogenated amorphous silicon (a‐Si:H) are elucidated. The current‐voltage (J‐V) curves for several Schottky diodes up to reverse‐biases of 40 V have been measured at temperatures between 40 and 180 °C. The reverse currents generally increase approximately exponentially with reverse bias. The decrease of the apparent barrier height as obtained from internal photoemission experiments is in good agreement with the decrease of the thermal activation energy with reverse bias. Extra information on the current transport mechanism can be obtained from the bias dependence of the prefactor in the Arrhenius plot. A theoretical model is presented which gives a semiquantitative fit to all the features observed in the experimental data. The model involves quantum‐mechanical tunnelling of a thermal distribution of carriers through an image‐force lowered triangular potential shape. At low reverse b...

Current‐stress induced asymmetry in hydrogenated amorphous silicon n‐i‐n devices

A novel manifestation of carrier induced degradation of α‐Si:H is presented. Amorphous silicon n‐i‐n devices, which have been subjected to severe constant‐current bias stress, show a strongly decreased ohmic conductivity. At the same time, the J–V characteristics in the space‐charge limited regime show pronounced asymmetry between the ‘‘forward’’ and ‘‘reverse’’ directions. Annealing the devices, without bias, restores the initial symmetrical characteristics. The decrease in the ohmic conductivity is interpreted as due to an increased density of deep (dangling bond) states in the lower part of the band gap within the i layer, resulting from the thermal equilibration with an increased density of trapped electrons in the conduction band tails. The asymmetry in the J–V characteristics is due to the formation of a spatially inhomogeneous distribution of dangling bond states in the upper half of the gap, induced by the thermal re‐equilibration to an inhomogeneous distribution of holes in the valence‐band tail ...

Tunneling through thin oxide interface layers in a-Si:H Schottky diodes

A detailed study of hydrogenated amorphous silicon (a‐Si:H) surfaces before and after thermal and plasma oxidation treatments was carried out using x‐ray photoelectron spectroscopy. The thickness of the surface oxides is correlated with the electrical properties of corresponding Mo Schottky barrier structures. Oxide layers up to 1.5 nm in thickness cause a decrease of the reverse current of nearly two orders in magnitude, while the forward current is hardly affected. For oxide thicknesses above 2.0 nm a large reduction in the forward current is observed. Surprisingly, the associated tunneling probabilities of the oxide interface layers in the a‐Si:H Schottky diodes are the same as those previously reported for c‐Si‐based tunnel diodes. Tunneling in the a‐Si:H devices cannot be simply described by the properties of a rectangular barrier, which is adopted most frequently in these studies. A potential form where the barrier height increases quadratically with thickness fits the observed tunneling characteris...

On the current mechanism in reverse-biased amorphous-silicon Schottky contacts. I. Zero bias barrier heights and current transport mechanism

A study of the zero‐bias barrier heights of hydrogenated amorphous‐silicon‐based Schottky diodes and the prevailing current transport mechanisms in these structures is made using electrical and electro‐optical techniques. Several series of devices were made using Cr, Mo, W, and Pt as Schottky metals. The current‐voltage characteristics of the devices were obtained with their temperature dependence. The barrier heights were determined independently using internal photoemission experiments at three temperatures between 270 and 380 K. In devices where the Schottky barrier is deposited on top of the semiconductor material, the saturation current density is found to be most likely determined by combined drift and diffusion of the carriers. In devices where the Schottky barrier is formed at the bottom of the diode, the transport mechanism tends towards thermionic (field) emission, but only slight effects of the prevailing transport mechanism on the electrical performance of these diodes were observed. Also, in ...

dc‐bias stress of non‐stochiometric amorphous silicon nitride thin film diodes

Photon‐emission experiments on silicon‐rich hydrogenated amorphous silicon‐nitride metal–semiconductor–metal diodes, have shown the existence of hot electrons under applied field strengths of approximately 106 V/cm. The effective temperatures and mean free path between collision for the electrons were estimated from the spectra. It is shown that, in general, asymmetrical changes in the electrical characteristics of the devices occur after prolonged dc stressing at high fields. Two drift mechanisms can be distinguished. The first is called ‘‘cathodic’’ drift and is driven by recombination between band‐tail carriers in the semiconductor. The other is called ‘‘anodic’’ drift, and results from the effects of hot electrons at the anode. The spatial and time dependence of these drift mechanism is explained using a simple model.

THE MEYER-NELDEL RULE IN HYDROGENATED AMORPHOUS SILICON NIN DEVICES

Conductivity measurements on hydrogenated amorphous silicon (a‐Si:H) nin devices with several i‐layer thicknesses are reported. The ohmic conductivity (σ) is found to follow the Meyer–Neldel rule (MNR): σ=σ00  exp(Ea/kT0) exp(−Ea/kT), where Ea is the conductivity activation energy, k is Boltzmann’s constant, and T is the temperature. It is shown that the MNR arises because of the form of the a‐Si:H gap state distribution that induces a statistical shift of the Fermi energy with the required functional form and magnitude. The characteristic temperature T0 is related to the slope G=1/kTc of the exponential conduction‐band tail state distribution according to kT0≂1.7kTc. The conductivity prefactor can be expressed as σ00=CNceμ, where Nc is the density of states (DOS) above the mobility edge, e is the electron charge, μ is the microscopic electronic mobility, and C is a constant of the order of 10−6 eV. Evaluation of the data gives σ00=10−2.9 (Ω cm)−1, and T0=570 K, i. e., Tc=340 K. This σ00 value corresponds...

On the saturation of Tb phosphors under cathode‐ray excitation. I. Excited‐state absorption in Tb‐activated phosphor powders

Optical transitions between the 5D4 excited state of the Tb3+ (4f)8 configuration and the crystal‐field split components of the (4f)7(5d) configuration are observed using high‐resolution laser excitation of Tb‐doped powder samples at room temperature. Excited‐state absorption spectra of Tb3+ in YAG, YAGaG, Y2SiO5, and LaOBr are presented. Superimposed on the broadband excitation spectrum of these transitions we find the relatively narrow 4f→4f lines. We present a theoretical model for interpreting the 4f→5d transitions, which predicts a strict proportionality between the (4f)8 5D4→(4f)7(5d) and the (4f)8 7F6→(4f)7(5d) transitions. This model is used to estimate the optical‐absorption cross section for the former transitions. Although the terminating 4f levels lie at the same energy as the 5d bands, the 4f→4f transitions can be interpreted on the basis of the Judd‐Ofelt theory. We derived a method for calibration of the Tb3+ 4f→5d fluorescence‐excitation spectra from powder samples. For YAG:Tb, the resulti...

On the saturation of Tb phosphors under cathode‐ray excitation. II. Upconversion processes in the excited‐activator bath

A physical model based on resonant electric dipole interaction between excited activators is presented that may be used for gaining insight into the second‐order energy‐loss process in cathode‐ray‐irradiated Tb phosphors. It is shown that resonant up‐conversion of the 5DJ excitons (J=3,4) to highly excited 4f and 5d states may be of equal importance. The derivation of the rate constants for this second‐order energy‐loss process is given and experimental values for the 5D4 ‐5D4 and 5D3 ‐5D4 interaction are presented for Tb‐doped YAG, YAGaG, LaOBr, and Y2SiO5. The values depend on previously determined optical cross sections for excited‐state absorption. Based on this model we explain well‐known differences in saturation behavior between these Tb‐doped phosphors under cathode‐ray excitation.

Current Induced Degradation of a-Si:H Pin and Schottky Switches

Forward-bias current stress experiments were performed on α-Si:H p-i-n and Schottky switches at several temperatures and at current densities up to 6 A/cm 2 . In Schottky diodes, current stressing results in a lowering of the forward-bias SCLC current together with an increase of its thermal activation energy. The reverse current is unaffected. The rate of degradation of the forward current increases with increasing temperature. From a comparison of the degradation behaviour of Schottkys with different barrier height we find that the rate of degradation is correlated to the minority-carrier injection ratio of the Schottky contact. The effects are interpreted as being due to metastable state creation in the bulk α-Si:H. The rectifying properties of the metal-to-semiconductor contact are relatively stable to current stress. The forward-bias I-V curves of p-i-n diodes degrade much faster than those of the Schottky switches. At the same time, the reverse-bias current increases due to the stress. The lower stability to current-stress of p-i-n diodes is ascribed to the much higher hole injection in the mesa. After a short time, the reverse-bias current becomes dominated by e-h generation from the created deep states in the i-layer and then gives a direct indication of its time dependence.