Z. A. Weinberg

On tunneling in metal‐oxide‐silicon structures

The analysis of Fowler‐Nordheim tunneling data in metal‐oxide‐silicon structures is reviewed. It is concluded that a parabolic dispersion relation for SiO2 and an electron effective mass of mox = 0.5m provide the best description of the experimental results, this conclusion is consistent with recent band structure calculations for SiO2. Also included is a brief discussion of the transverse momentum conservation issue for tunneling from silicon of 〈100〉, 〈110〉, and 〈111〉 orientation into SiO2.

Location of positive charges in SiO2 films on Si generated by vuv photons, x rays, and high‐field stressing

The location of positive trapped charge in the dry thermally grown films of SiO2 on Si in MOS structures has been investigated by combining the internal photoemission‐voltage dependence from both interfaces with the capacitance‐voltage technique. Trapped holes have been produced in the SiO2 by vacuum ultraviolet (vuv) photons, x rays, and high‐field stressing. After irradiation under positive gate bias, trapped holes have been found to reside near the Si‐SiO2 interface with an upper limit of about 50 A determined for their centroid from this interface. After irradiation under negative bias, a similar situation was found to occur near the Al‐SiO2 interface; and in addition some positive charge was found approximately at the Si‐SiO2 interface. After high‐field stressing under negative bias, positive charge was found approximately at the Si‐SiO2 interface. The charge locating technique is described in detail as well as the implications of the results to radiation damage and insulator breakdown.

The effect of gate metal and SiO2 thickness on the generation of donor states at the Si‐SiO2 interface

Two experimental observations are reported concerning the degradation of the Si–SiO2 interface during electron injection in metal‐oxide‐semiconductor structures. First, the generation of the interfacial positive charge during avalanche injection can be strongly inhibited by employing magnesium, instead of aluminum, as gate metal, or enhanced by employing gold. This correlates with the different work functions of the metals. Second, during negative bias high‐field injection in Al‐gate capacitors with thin oxides (≲100 A), a threshold in gate voltage, of 7–8 V, is found for the generation of the positive charge. Both observations are consistent with a model which assumes that holes generated in the anode by hot electrons, via emission of surface plasmons, are injected into the SiO2 and are subsequently trapped at the Si–SiO2 interface. Other possible mechanisms are also discussed.

SiO2‐induced substrate current and its relation to positive charge in field‐effect transistors

Experimental data are presented for the substrate current (holes), which accompanies electron injection into the oxide of n‐channel field‐effect transistor structures, in the tunneling regime. Dependencies of the effect on oxide thickness and on the metal gate material were investigated. An inverse relation was found between the initial rise time of oxide current transients and both the electron and hole currents. It is shown that these initial current increases are related to positive charge, therefore a correlation exists between the positive charge and electron or hole currents. The strength of impact ionization in SiO2 is discussed on the basis of band‐structure arguments and it is concluded that there are difficulties in explaining the substrate current by impact ionization. A technique for fast measurements of capacitance‐voltage shifts at the end of an applied high field pulse is described.

The relation between positive charge and breakdown in metal‐oxide‐silicon structures

The relation between positive‐charge accumulation at the Si‐SiO2 interface and the occurrence of high‐field breakdown in metal‐oxide‐silicon structures has been investigated. Oxides having different hole‐trapping properties were prepared with the addition of short rapid thermal anneals in O2. Experiments testing hole trapping, high‐field stressing, the initial current transients at constant gate voltage, and breakdown statistics were performed on these oxides to examine the correlation between positive charge and breakdown. The conclusion is that positive‐charge generation is only one of the processes occurring during high‐field stress but is not the main cause for breakdown. Large current increases were observed for oxides that have large hole‐trapping efficiencies, but the current increase is followed by fast current decay. The mechanism causing the current decay was investigated and was found to be an intrinsic mechanism which is related to the neutralization of the positive charge. These processes alw...

Investigation of the SiO2‐induced substrate current in silicon field‐effect transistors

An experimental investigation is presented for the substrate current (holes) appearing in n‐channel field‐effect transistors having SiO2 as their gate insulator. In these experiments, the gate is biased by a high and positive voltage, causing an electron current to be injected from the device channel into the oxide. This current is accompanied by the substrate current whose origin is not clear. The experiments were performed by application of short pulses (400 μsec) to the gate. It is shown that the substrate current is too large to be explained by simple electron tunneling from the silicon valence band into the oxide. Temperature‐dependence measurements, down to 20 K, show that some of the data are not consistent with models for hole transport from the oxide into the silicon valence band. It is argued that the substrate current may be related to the energy loss experienced by hot electrons as they traverse the oxide. It is further argued that the same mechanism responsible for the substrate current may produce positive charge at the injecting electrode and thus lead to breakdown in thin oxide devices.An experimental investigation is presented for the substrate current (holes) appearing in n‐channel field‐effect transistors having SiO2 as their gate insulator. In these experiments, the gate is biased by a high and positive voltage, causing an electron current to be injected from the device channel into the oxide. This current is accompanied by the substrate current whose origin is not clear. The experiments were performed by application of short pulses (400 μsec) to the gate. It is shown that the substrate current is too large to be explained by simple electron tunneling from the silicon valence band into the oxide. Temperature‐dependence measurements, down to 20 K, show that some of the data are not consistent with models for hole transport from the oxide into the silicon valence band. It is argued that the substrate current may be related to the energy loss experienced by hot electrons as they traverse the oxide. It is further argued that the same mechanism responsible for the substrate current may p...

Hole conduction and valence‐band structure of Si3N4 films on Si

Transport measurements were performed on thin films of Si3N4 deposited on Si using carrier injection from low‐energy corona ions and a shallow junction detector. Large hole conduction is found for both corona polarities. Examination of the electronic structure of Si3N4 by x‐ray photoemission spectroscopy (XPS or ESCA) reveals one broad structure 10 eV wide (FWHM) at the top of the valence bands which results from the bonding of the Si 3s, Si 3p, and N 2p orbitals. This finding is consistent with the hole conduction we observe. The XPS results are compared with those from amorphous SiO2. The tops of the valence band of Si3N4 and SiO2 are found to lie 1.5±0.2 eV and 4.5±0.2 eV, respectively, below the Fermi level of a thin overlayer of gold.

Reduction of electron and hole trapping in SiO2 by rapid thermal annealing

Reduction of electron or hole trapping in SiO2 was achieved by short‐time lamp annealing. The trapping characterization was done by the avalanche injection technique on metal‐oxide‐silicon capacitor structures with aluminum gates and SiO2 thickness of 50 nm. Electron trapping on water related centers is reduced by 10‐s anneals in Ar or N2 ambients at 600–800 °C. Hole trapping is reduced by short anneals in an O2 ambient at 1000 °C with an optimal time of 100 s. The O2 short anneal is much more effective if the oxide had a long post‐oxidation anneal in N2 at 1000 °C which produces an interfacial nitrogen‐rich layer at the Si‐SiO2 interface.

Exciton or hydrogen diffusion in SiO2

The appearance of positive interfacial charge at the Si‐SiO2 interface in metal (Al) ‐SiO2‐silicon structures illuminated by vacuum ultraviolet (≳ 9 eV) light (Al‐negative) or subjected to electron‐avalanche injection (Al‐positive) has been found to depend on water exposure of the SiO2 layer. This finding raises the possibility that these effects can be explained by the diffusion of water‐related species (especially H), rather than by the diffusion of excitons, as has been previously proposed. Although we cannot unambiguously decide whether the source of the vacuum ultraviolet effect is diffusion of excitons or water‐related species, it appears more likely that the water enhances the detection efficiency of the diffusing species as they reach the Si‐SiO2 interface.

Hole conduction in Si3N4 films on Si

Dominant hole conduction is observed in thin Si3N4 films deposited on silicon with holes being injected from either aluminum or gold electrodes. Hole transport is confirmed by employing a shallow junction diode detector. Such a diode can serve also as excellent means for separating conduction currents from transient displacement currents under pulsed conditions.