Charge generation in thin SiO2 polysilicon-gate MOS capacitors

Charge generation in the bulk of thin oxides (10–33 nm) during high-field (Fowler-Nordheim) stress is investigated on polysilicon-gate MOS capacitors using constant current injection and high frequency C-V measurements. Our data show that initially positive charges are generated in the bulk of the oxide, followed by negative ones. The mechanisms that govern the creation of these different charges are investigated. A qualitative physical model is presented. We propose that positive charges are created by electron detrapping. Negative charges are due to electron trapping at preexistent and possibly, newly created, trap sites in the region of the oxide where the injected electrons have low kinetic energy. This model helps us understand the wear-out and breakdown of thin oxides.     

Minority carrier recombination in MOS capacitors switched from inversion to accumulation

By pulsing an MOS capacitor between inversion and accumulation, the minority carrier lifetime can be measured. The method is very neat in principle but one may “a priori” expect experimental results to be affected by errors due to several collateral phenomena, such as high-injection, recombination at the ohmic contact, lateral diffusion and surface-states trapping. In this paper the simple theory is presented and all these effects are thoroughly examined theoretically as well as experimentally. It is shown that the method proposed is reliable, provided some care is used in the interpretation of the results, which are not negligibly influenced by the charging and discharging of surface states.     

Investigation of electron-hole generation in MOS capacitors on 4H SiC

This paper investigates electron-hole generation in n-type MOS capacitors on 4H-SiC with the gate oxide directly grown in either 100% NO or 10% N/sub 2/O. High-temperature capacitance-transient measurements were used to determine and compare the contributions of carrier generation in the bulk and at the SiO/sub 2/-SiC interface. The effective generation rate in the bulk is similar in the MOS capacitors with either type of gate oxide, whereas the effective surface-generation rate is much lower in the case of oxides grown in 100% NO. Moreover, the effective surface-generation rate in these oxides is reduced to the level that is comparable to the effective bulk-generation rate. This result demonstrates the high quality of MOS capacitors with the gate oxide directly grown in 100% NO.     

A comparative analysis of substrate current generation mechanisms in tunneling MOS capacitors

This paper presents a critical analysis of the origin of majority and minority carrier substrate currents in tunneling MOS capacitors. For this purpose, a novel, physically-based model, which is comprehensive in terms of impact ionization and hot carrier photon emission and re-absorption in the substrate, is presented. The model provides a better quantitative understanding of the relative importance of different physical mechanisms on the origin of substrate currents in tunneling MOS capacitors featuring different oxide thickness. The results indicate that for thick oxides, the majority carrier substrate current is dominated by anode, hole injection, while the minority carrier current is consistent with a photon emission-absorption mechanism, at least in the range of oxide voltage and oxide thickness covered by the considered experiments. These two currents appear to be strictly correlated because of the relatively flat ratio between impact ionization and photon emission scattering rates and because of the weak dependence of hole transmission probability on oxide thickness and gate bias. Simulations also suggest that, for thinner oxides and smaller oxide voltage drop, the photon emission mechanism might become dominant in the generation of substrate holes.     

Random errors in MOS capacitors

The effects of random edge variations and deviations of oxide thickness and permittivity are examined. Making only a few basic assumptions, it is shown that edge effects introduce a relative capacitance error /spl Delta/C/C/spl alpha/C/SUP -3/4/, while the oxide variations cause /spl Delta/C/C/spl alpha/C/SUP -1/2/. Error bounds are derived for C in terms of the variances of the linear dimensions and oxide permittivity. For a capacitor C realized as a parallel connection of n unit capacitors of values C/n, the relative error caused by edge effects is n/SUP 1/4/ times larger than for a single capacitor of value C. The relative error due to oxide variations remains the same for the two realizations. All theoretical results agree with physical consideration, as well as the Monte Carlo simulations performed.     

Determination of generation lifetime from the small-signal transient behavior of MOS capacitors

The theory and an experimental proof of a method for determining generation-lifetime in silicon with impurity concentrations up to 10¹⁸ cm^?3 are described. Generation-lifetime is obtained by analyzing the transient behavior of the total charge stored in a MOS-capacitor after a very small depleting voltage step. The analytical expression allowing this analysis is deduced from the Shockley-Read-Hall-expression of the carrier generation rate by considering small deviations from thermal equilibrium. The small signal expression thus obtained differs in a constant factor only from the well known large signal expression. A comparison of this small-signal method with well known large signal methods practiced on samples with doping levels of 10¹⁵ cm^?3 shows that the results are in very good agreement.     

Monitoring trapped charge generation for gate oxide under stress[MOS capacitors]

A measurement method to extract the respective quantities and centroids of positive and negative trapped charges, i.e., Q_p and Q_n, generated by the negative current stress for gate oxides is proposed and demonstrated. The method is based on neutralization of and by a low positive current stress to differentiate the effects of Q_p and Q_n. From the extracted quantities and centroids of Q_p and Q_n of negatively stressed oxides, it was found that Q_p and Q_n are generated near the oxide/substrate interface and Q_p is initially much larger than Q_n. After the continuous stressing, Q_p saturates and moves closer to the interface, but Q_n keeps increasing and moves away from the interface, especially for those oxides after the post-poly anneal (PPA) treatment. Q_p is very unstable and easily neutralized, either by a small stress of opposite polarity or the same polarity. For the latter, Q_p is mainly dependent on the level of the final stressing field     

Unpinned GaAs MOS capacitors and transistors

Metal-oxide-semiconductor (MOS) capacitors and field-effect transistors (MOSFETs) in the GaAs semiconductor system using an unpinned interface are described. The structures utilize plasma-enhanced chemical-vapor deposition (PECVD) for the silicon-dioxide insulator on GaAs that has been terminated with a few monolayers of silicon during growth by molecular beam epitaxy. Interface densities in the structures have been reduced to ~10¹² cm^-2·eV^-1 . High-frequency characteristics indicate strong inversion of both p-type and n-type GaAs. The excellent insulating quality of the oxide has allowed demonstration of quasi-static characteristics. MOSFETs operating in depletion mode with a transconductance of 60 mS/mm at 8.0-μm gate lengths have been fabricated     

Degradation of electron irradiated MOS capacitors

We have investigated the degradation of MOS structure due to high energy electron irradiation as a function of radiation dose and gate bias applied during the irradiation. Devices have been characterized by current–voltage measurements, in order to study charge accumulation also at the gate interface. Three types of oxide charge have been observed: the unstable positive charge, due to trapped holes induced by the electron irradiation; the negative charge in the oxide bulk, deriving from capture of electrons injected during electrical measurements in radiation generated traps; and border traps, at both oxide interfaces.     

Frequency dependence of of MOS capacitors

The frequency dependence of ΔV/Δ(C⁻²) of an MOS capacitor, which plays an important role in determining the semiconductor doping profile, is studied theoretically and experimentally. Useful expressions relating the measurable quantities to the doping profile are derived systematically. It is shown how interface states and majority carriers influence the frequency dependence of ΔV/Δ(C⁻²) and give rise to errors in profile determinations. The techniques of measuring the various types of the frequency dependence of ΔV/Δ(C⁻²) are also described.     

Mechanism of stress-induced leakage current in MOS capacitors

Stress-induced leakage current (SILC) is examined both below and above the voltage at which the preexisting Fowler-Nordheim tunneling current dominates. Based on these results, it is argued that SILC is the result of inelastic rather than elastic trap-assisted tunneling. This clarification explains the well-known thickness dependence of the SILC at low fields that has identified it as a scaling limitation for nonvolatile memory tunnel oxide. It also explains a newly observed different thickness dependence at high fields and facilitates modeling of the electric field/voltage and trap density dependencies of the SILC     

Postmetallisation annealing of aluminium-silicon gate MOS capacitors

Postmetallisation annealing (PMA) is widely used to reduce midgap interface trap densities in aluminium gate MOS structures. It is shown that the presence of 1% silicon in the aluminium inhibits PMA and that increased PMA times result in an increase rather than a decrease in midgap interface trap densities for AlSi.     

Effect of ionizing radiation on MOS capacitors

A numerical model of metal-oxide-semiconductor (MOS) capacitor has been developed to investigate the effect of ionizing radiation on the characteristics of the device during exposure and also in the post-irradiated condition. The model takes into account the effect of radiation-induced changes in silicon-dioxide as well as in silicon substrate of MOS structure. It is found that the total high frequency capacitance of the device during exposure to radiation is different from its value in the post-irradiated condition. The results of the study are expected to be useful in predicting the behavior of MOS based devices operating in radiation environment.     

Radiation damage of thermally oxidized MOS capacitors

The influence of preparation parameters and the effect of X-rays (150 keV, 10⁴rad (Si)) on oxide charge Qoxand interface state density Nssin thermally oxidized MOS varactors under different biasing conditions during irradiation has been investigated. The interface state density was determined by the ac conductance method before and after irradiation. The oxide charge has been evaluated with regard to the charge Qssof the interface states. Qsshas beeu discussed with the aid of simple models concerning the energetic distribution and recharge character of the interface states. The results indicate a similar dependence between flatband voltage, interface state density, and normalized oxide charge density as a function of gate bias during irradiation. Furthermore, the so-called "oxidation triangle" of oxide charge before irradiation exists for interface states as well. Calculations on the basis of the Schottky barrier model of the irradiated MOS structure show that the radiation-induced charge exists at both interfaces in the oxide layer. Radiation tolerance of the MOS capacitors as a function of technological parameters is discussed.     

Breakdown properties of thin oxides in irradiated MOS capacitors

The influence of ionizing radiation on breakdown characteristics of MOS structures with thin gate oxides is investigated. It is found that, as the result of irradiation, a significant reduction in the number of medium-voltage defect-related breakdown events takes place. This improvement effect suggests that there is an interaction between defects responsible for early breakdowns and defects generated by radiation, which results in the hardening of oxide weak spots. The observed shortening of the time-to-breakdown for irradiated structures is explained in terms of radiation-induced positive charge and electron trap generation which, during subsequent TDDB stress, leads to enhancement of the internal oxide field. As the result of irradiation, a significant reduction of the field acceleration factor of oxide breakdown (determined for high, near critical electric fields) is also observed.     

Thickness measurement of thin oxide layers in MOS capacitors

The validity of the SiO/sub 2/ layer thickness determined from capacitance-voltage measurements in MOS devices is demonstrated by comparison with results obtained by X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and spectroscopic ellipsometry published elsewhere in the literature.<>     

On the mechanism for interface trap generation in MOS transistorsdue to channel hot carrier stressing

The classical concept and theory suggest that the degradation of MOS transistors is caused by interface trap generation resulting from “hot carrier injection.” We report three new experiments that use the deuterium isotope effect to probe the mechanism for interface trap generation in n-MOS transistors in the presence of hot hole and electron injection. These experiments show clearly that hot carrier injection into the gate oxide exhibits essentially no isotope effect, whereas channel hot electrons at the interface exhibit a large isotope effect. This leads to the conclusion that channel hot electrons, not carriers injected into the gate oxide, are primarily responsible for interface trap generation for standard hot carrier stressing     

Effects of high field generation of charge trapping in the oxideand interface traps on MOS capacitors and MOSFETs

Oxide charge trapping and interface state generation phenomena under the various high-field stress conditions have been investigated using capacitors fabricated on both p-and n-type substrates, and p- and n-channel MOSFETs. It was found that prediction based on MOSFET devices yielded shorter lifetimes than predictions based on capacitors     

Transient response of MOS capacitors under localized photoexcitation

Lateral surface transport of charges under the gate electrode of MOS capacitors from localized photogeneration of electrons and holes are studied by observing the transient photocurrent through the MOS capacitor. The photocurrent is highly non-linear under a step function illumination. It shows a large initial spike (phase I), whose length is independent of device area, followed by a slow decay (phase II), whose length increases for high light intensity and for large device area. Detailed time dependence data indicate that phage I comes from the flattening of the surface energy band in the illuminated area, which causes an initial surge of charges in the lateral direction driven by a constant voltage source, and phase II from the slow and areally uniform charging up of the entire interface under the gate. Theoretical calculations of the photocurrent waveforms were carried out using a simple transmission line model for phase I and one-lump model for phase II based on physical considerations. Excellent agreements between the theories and the observed photocurrent waveforms are obtained.     

Positive flatband voltage shift in MOS capacitors on n-type GaN

GaN MOS capacitors were fabricated using silicon dioxide deposited by low-pressure chemical vapor deposition oxide at 900°C. The MOS capacitor flatband voltage shift versus temperature was used to determine a pyroelectric charge coefficient of 3.7 × 10⁹ q/cm²-K, corresponding to a pyroelectric voltage coefficient of 7.0 × 10⁴ V/m-K