MOS Samples Research Articles

Influence of a postoxidation cooling on the interface state density of MOS structures

It has been shown by C V technique that a postoxidation cooling of Si wafers in N 2 + O 2 ambient and series of repeated irradiation-then-anneal treatments can efficiently decrease the interface state density of MOS structure introduced by 1.5 × 10 6 rad gamma irradiation. The difference between interface state distribution in Si band gap of cooled and uncooled MOS samples is demonstrated. A possible explanation of the results is proposed.

Interaction of hydrogen with defects in a-SiO 2

In this paper, the current understanding of the role of hydrogen in passivating and creating defects in irradiated MOS samples is reviewed. The first ab initio calculation of the energies of activation and reaction of the dissociation of H 2 molecules at the E′ 1 defect in α-quartz, and for a simple model for the E′ γ defect in a-SiO 2 is prestented. The effects of various improvements to standard Hartree-Fock theory, including perturbative configuration interaction and improved basis sets, were studied. It is found that the qualitative picture of H 2 dissociation is insensitive to these improvements. The estimates of the energies of reaction and activation in the adiabatic approximation are 0.25 eV and 0.78 eV, respectively. The latter energy is roughly twice the activation energy reported by Li et al. The probable sources of error are explored and it is concluded that quantum effects (zero point energy and tunneling are probably crucial for modeling this dissociation reaction. The first ab initio study of the preferred conformation of the proton in a-SiO 2 and a preliminary semiempirical calculation of the activation energy for proton hopping are also presented.

Influence of high energy electron irradiation on the interface states of the Si-SiO 2 system

Thermally stimulated charge (TSC) characteristics of MOS samples show that after high energy (11 MeV) electron irradiation four peaks are formed, which were not present in the spectrum of unirradiated samples. The height and the area of the higher temperature peaks increase with increasing irradiation dose, while the density of the shallow traps, represented by the first peak, decreases with dose increase. The energy distribution in the forbidden gap of the silicon as well as the capture cross section of the introduced interface states are calculated from TSC curves. A possible explanation of the results is proposed.

A method for the determination of the generation sources, bulk lifetime, and surface generation velocity using MOS capacitor

Various pulsed MOS capacitance techniques have been developed for the determination of the generation lifetime and the surface generation velocity in semiconductors. A method that uses a new approach to surface generation velocity evaluation is described. The pulsed MOS capacitor is routinely used to measure generation lifetime. The high-frequency capacitance transient observed after the MOS capacitor has been pulsed from accumulation into deep depletion, is usually analyzed to determine the value of bulk generation lifetime in the semiconductor concerned. The method proposed is based upon a precise MOS structure model incorporating lateral effects, the diffusion current from the bulk, and the decrease of the surface generation velocity during the measurement process. The method uses a single measurement of the nonequilibrium C-t characteristic in MOS capacitors subjected to voltage stress and is particularly useful when applied to small gate diameter MOS samples with long relaxation times.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An 8-bit 200-MHz BiCMOS comparator

The design of a fully differential BiCMOS comparator suitable for application in data conversion, instrumentation, and communication systems operating at video frequencies and above is discussed. By exploiting the advantages presented by the integration of both bipolar and CMOS devices within the same technology, the comparator dissipates less power than conventional bipolar designs without sacrificing operating speed. The comparator includes an input stage that combines MOS sampling with a bipolar regenerative amplifier. This stage dissipates no static power and, because the amplification is provided by a bipolar differential pair, no offset cancellation is needed to achieve 8-b precision. Furthermore, the need for preamplification and the attendant power-delay penalty associated with preamplifier overdrive recovery are avoided. An experimental version of the comparator, consisting of the BiCMOS regenerative input stage followed by a current-switched latch, has been integrated in a 0.8- mu m BiCMOS technology with an area of 140*75 mu m. This circuit performs comparisons to a precision of 8 b at rates up to 200 MHz. The entire circuit dissipates only 1.6 mW at the maximum clock rate while operating from a single 5-V supply. >

A transient Poole-Frenkel effect at the semiconductor surface of MAOS devices

The generation of the inversion layer in MOS and MAOS structures shows remarkable differences. Whereas for MOS samples free minority carriers build up the inversion layer, the positive layer in n-type silicon MAOS devices is mainly caused by a trap charging process. This trap charging shows peculiar properties. After a voltage step which depletes the semiconductor surface from majority carriers the gate current has a maximum at a time of about 10 −4s after the step. This current which is proved to be related to the increase of the trap charge is strongly temperature and field dependent. It is shown that the time dependent maximum in the current is caused by a transient Poole-Frenkel effect at the semiconductor surface.

Electron trapping in thin films of thermal SiO2 at temperatures between 30 and 300 K

Temperature dependence of electron trapping in thermally grown SiO2 has been studied using metal–silicon dioxide–silicon (MOS) structures. The MOS samples are charged using avalanche injection from the Si substrate at temperatures between 30 and 300 K. It is found that the rate of electron trapping increases monotonically as temperature decreases from 300 to 30 K for both oxides subjected to H2/N2 annealing and oxides subjected to no H2/N2 annealing. The majority of electrons trapped at 77 K is thermally reemitted, as the samples are warmed to room temperature. Analyses of the experimental results indicate that neutral trap centers responsible for electron trapping are characterized by the two discrete energies of 23 and 4 meV below the conduction band edge of SiO2.

Determination of interface-state parameters in a MOS capacitor by DLTS

A modification of the DLTS (Deep Level Transient Spectroscopy) technique for interface-state measurement is described in which the surface potential is used to determine the energy of the interface states contributing to the emission signal. This technique allows an accurate and unambiguous determination of interface-state energies and cross sections. Expressions are determined for interface-state emission as a function of surface potential. Measurements of interface-state density and majority-carrier cross sections as functions of energy for n- and p-type MOS samples are presented.

Asymmetry in the SiO2 tunneling barriers to electrons and holes

The potential barriers of ultrathin (%40 Å) SiO2 layers to electrons and holes tunneling between the semiconductor and the metal have been measured independently on the same MOS samples by a new experimental technique introduced in this paper. The method combines dark measurements of current and capacitance versus voltage with measurements of photocurrent suppression under optical illumination. The results show that the tunneling barriers for holes are consistently much larger than those for electrons.

Tunneling in ultrathin SiO 2 layers on silicon: Comments on dispersion relations for electrons and holes

A fundamental asymmetry between the tunneling probabilities for electrons and holes has been observed in ultrathin SiO 2 layers (20–30 Å). These probabilities have been measured on the same MOS samples using a new experimental technique combining dark characteristics with measurements of photocurrent suppression by the SiO 2 layer. Interpretation by Franz dispersion relations using conduction and valence band energies appropriate to thick SiO 2 films is not possible even assuming different effective masses in the two bands. This may be attributed to the distortion of the energy bands in the SiO 2 caused by the presence of positive oxide charge.

Optical absorption and photoconductivity in thermally grown SiO2 films

Optical transmission of unbacked thin films of thermally grown SiO2 films were measured in the vacuum uv. The absorption data indicate a band gap of 8.0±0.2 eV for SiO2, and this value is confirmed by measurements of electron and hole photoconductivity and positive charging thresholds in MOS samples. The observation of hole photoconductivity near 8 eV shows that the uppermost valence levels in SiO2 are not nearly atomic, but form a band with sufficient orbital overlap that readily transport holes at room temperature.

Low frequency conductance and capacitance measurements on MOS capacitors in weak inversion

The analysis of capacitance and conductance measurements as a function of frequency on MOS capacitors biased in depletion is well understood. However, little information is available on such measurements for capacitors biased in weak inversion. In this work it is shown that approximations to the theory of surface state response lead to a very simple technique for obtaining the density of fast surface states as a function of energy in the weak inversion region. Low frequency conductance and capacitance measurements have been made on a number of MOS samples and analysis of the data yields N ss as a function of energy in excellent agreement with results obtained by other techniques. As expected, experimental results show that N ss in weak inversion depends strongly on the annealing treatment given the MOS samples. However, annealing is found to have a much greater effect in reducing N ss in weak inversion than at midgap. It is also shown that the depletion layer generation current in the majority of samples measured is due to surface state generation rather than bulk generation.

Some observations on charge buildup and release in silicon dioxide irradiated with low energy electrons

The positive charge buildup produced in silicon dioxide by low energy electrons (0 to 30 keV) has been investigated as a function of beam energy and oxide thickness. The induced charge, as evidenced by displacement of capacitance versus voltage plots, was found to be a function of the beam energy dissipated within the oxide in the vicinity of the oxide-silicon interface. The charge induced at a particular fluence level in an oxide of given thickness increases with energy up to some level E <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</inf> beyond which the charge buildup rate falls off as the energy is increased further. Continued falloff in the buildup rate was observed in several samples irradiated at energies of 200 keV and 1 MeV. E <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</inf> has been found to correspond to the beam energy which, according to predicted range-energy data, produces maximum energy dissipation per unit path length in the oxide near the silicon interface. Constant temperature annealing of irradiated MOS samples has indicated that the annealed charge is linearly dependent on the logarithm of elapsed time over a finite time interval. This is particularly evident at room temperature where a linear dependence on In ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</tex> ) has been observed out to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> seconds. Such a time dependence of released charge can result either from thermal activation of trapped carriers from a uniform trap distribution or from thermal emission of recombination electrons over a Schottky barrier from the silicon into the oxide; however, both of these models predict the released charge to be a linear function of absolute temperature. A much stronger temperature dependence has been observed during these experiments.

RADIATION EFFECTS IN METAL-INSULATOR-SEMICONDUCTOR CAPACITORS AS DETERMINED FROM CONDUCTANCE MEASUREMENTS

Conductance-versus-voltage measurements were employed to determine the distribution of fast interface states in MOS and MNS capacitors and the changes in these distributions due to gamma irradiation with applied bias. MNS samples showed no changes due to irradiation, while MOS samples showed an increase in the conductance peak due to an increase in the density of the fast states.