MNOS Structure Research Articles

CURRENT TRANSPORT MECHENISM IN THERMAL-LYNITRIDED SiO2 THIN FILMS

A new trap-assisted two-step tunneling model is proposed to explain the conduction enhancement characteristics and conduction mechanism in heavily-nitrided oxide films. A theoretical calculation is carried out to fit the theory to the experimental results. The trap density and trap energy level are found to be in the range of 1.2×1019-7.2×1020cm-3 and 2.46-2.56 eV respectively. These results agree satisfactorily with the Auger spectroscopic data. Furthermore, this model can also be applied MNOS structure or MIS devices with other traps.

Experimental determination of the mechanism of transient single-injection current flow in dielectrics

en

Profile of trapped charge in silicon nitride films in MNOS structures

The relaxation current in the system electrolyte-dielectric-semiconductor is measured in the process of dielectric etching. A theoretic calculation of the charge profile in the MNOS structure dielectric is conducted for non-uniform distribution of trapping centres and charge injection from a field electrode. The distribution of trap density through the nitride film and their trapping cross-section are determined. The height of the triangular barrier for the electrons at the boundary nitride-aluminium is estimated. An effect enhancing the hole injection from a field electrode due to illumination is discovered. [Russian Text Ignored].

Compositional Depth Profile of a Native Oxide LPCVD MNOS Structure Using X‐Ray Photoelectron Spectroscopy and Chemical Etching

A compositional profile of a native oxide LPCVD MNOS structure has been obtained by x‐ray photoelectron spectroscopy used in conjunction with a stopped‐flow chemical etching procedure. A depth resolution of 5–10Å was achieved. Stoichiometric was found in the bulk with about 0.4 atomic percent (a/o) oxygen impurity. In contrast to profiles completed by other techniques, was found intact at the interface; complete conversion of oxide to oxynitride during LPCVD did not occur. The oxide/nitride interface is best described as sharp; the oxygen concentration drops by 80% in less than 8Å. There is also a diffuse oxynitride gradient extending into the film, but the oxygen concentration in this region is quite low. Evidence was also found for an 8Å layer containing excess bonds to silicon between the oxide and nitride layers. Possible charge trap structures that might limit device performance are discussed along with processing suggestions aimed at minimizing the formation of traps.

Effects of High Electric Field on C-V Characteristics of MNOS Structure Devices

Effects of High Electric Field on <i>C</i>-<i>V</i> Characteristics of MNOS Structure Devices

Electrical and charge storage characteristics of the tantalum oxide-silicon dioxide device

The electrical characteristics of the tantalum oxide-silicon dioxide double-dielectric structure are described. The MTOS structure (metal-tantalum oxide-silicon dioxide-silicon) is similar to the MNOS double dielectric which is used as a nonvolatile memory element except tantalum oxide (Ta 2 O 5 ) is used to replace the silicon nitride as the second dielectric. Capacitance voltage measurements show a negative Q SS with magnitudes smaller than those in compatable MNOS devices. Conduction characteristics of both anodic and thermally grown Ta 2 O 5 have been studied and both have been found to follow a Poole-Frenkel mechanism. The memory characteristics of the MTOS have been investigated and preliminary data are presented. Where MNOS data are available, the MTOS characteristics have been compared with those of the MNOS structure. It is shown that the threshold voltage of the MTOS device can be shifted using lower gate voltages than are needed for a comparable MNOS device. It thus appears that the MTOS device has some decided potential advantages over the MNOS structure as a nonvolatile memory element.

Pseudostable MNOS Structures

Thick oxide MNOS structures as studied by Frohman‐Bentchkowsky and Lenzlinger exhibit a flatband shift which saturates with time at bias. We have studied a MNOS structure (300Å of nitride and 300Å of oxide) with low conductivity nitride (10−8A cm−2 at , 373°K) which exhibits an increase in the rate of flatband voltage shift after an apparent saturation. This effect was observed on long time (2650 hr) bias‐temperature stress. Typical times to reach apparent saturation at 85°C varied from 200 to 1000 hr at 15 and 8.5V, respectively. At 165°C and 15V, the initial saturation almost merges with the subsequent rate increase of . Processing, characterization, and the bias temperature stress results for these structures are discussed. We propose that the stored charge is distributed throughout the rather than only at the interface.

Charge centroid in MNOS devices

A concise theoretical expression based on pronounced detrapping is derived for the charge-vs-centroid relationship of an MNOS structure subjected to constant current pulses which include the considerable injected charge levels encountered in practical memory device operation. Good agreement is found with available experimental charge-vs-centroid data obtained at various temperatures, thus enabling determination of Frenkel-Poole coefficient and trap depth. Values so obtained agree with those for bulk silicon nitride. An analytical expression valid for any degree of detrapping is derived for change of centroid with charge in the limit of small injected charge levels. Implications of constant voltage versus constant current charging are discussed.

Temperature behaviour of MNOS structures

A strong dependence of the flat band voltage of MNOS structure upon temperature in the range 100–400 K has been observed. This behaviour is attributed to interfacial traps with great density.

Effect of distributed charge in the nitride of an MNOS structure on the flat-band voltage

Closed-form expressions have been obtained for the flat-band voltage of an MNOS structure with charge carriers trapped in the nitride bulk according to the Arnett-Yun model. Within the applicable limit of this model, the following observations can be made from these flat-band voltage expressions: When σQ (t) ≃4 or larger, the carrier distribution can be fairly accurately approximated by a step function, so far as flat-band voltage is concerned. When σQ (t) ≃0.1 or smaller, the centroid ? of the trapped carrier distribution approaches the trapping distance x0. σ is the capture cross section and Q (t) the total number of injected carriers at time t.

Charge centroid and trapping model for MNOS structures

Charge centroid, trap density, and capture cross section for an MNOS structure are derived from measurements of the change in flat band voltage with varying pulse duration and temperature using the Arnett-Yun model. The temperature dependence of these parameters suggests that the Arnett-Yun model for the initial space-charge buildup is not entirely valid. The observations are in qualitative agreement with partially ionized traps whose occupancy by electrons decreases with increasing temperature and electric field.

Two-band conduction of amorphous silicon nitride

A method is proposed for determining the electron and hole components of stationary current flowing through an MIS structure and the hole and electron components of the stationary current in an MNOS structure have been determined. It is shown that the experimental results may be explained by a two-band Si3N4 conduction model. A two-band Si3N4 impurity conduction model with silicon microcrystals is examined, the latter serving as Poole-Frenkel centres in the nitride volume as well as trapping and recombination centres in the regions close to the electrodes. [Russian Text Ignored].

Measurements of charge propagation in Si3N4 films

A new experimental technique is described, which measures the injection and propagation of charge in the Si3N4 film of an MNOS structure caused by a pulsed electric field. Contrary to the widely held contention that the electrons injected from the Si are trapped in and very near the SiO2–Si3N4 interface, a contention resulting from the assumption that the trap density near this interface usually far exceeds the number of injected electrons, this work finds that the spatial distribution of the trapped electrons extends deep into the Si3N4 film. In fact, the centroid of the spatial distribution of the trapped electrons in a Si3N4 film ≃500 Å thick can be as much as 150 Å or more from the SiO2–Si3N4 interface, indicating that the span of the distribution extends across almost the entire thickness of the film with an average density of trapped electrons in the order of 3 × 1018/cm3. It is shown that, in general, the centroid of the trapped charge distribution in the Si3N4 film of a given device structure is a function of the applied field and its duration.

Trap-assisted charge injection in MNOS structures

Injection of charge carriers into the nitride in an MNOS structure by direct tunneling to traps in the nitride is studied in this paper. It is shown that this injection mechanism is very important for MNOS structures with thin oxides at low nitride fields (1–4 MV/cm). The dependence on field, temperature, and oxide thickness of this injection current are examined both theoretically and experimentally. It is furthermore proposed that this mechanism may be of considerable importance for charge transport in most insulators.

A uniphase charge-coupled device

A uniphase charge-coupled device (CCD) is proposed. It makes use of charge storage in an MNOS structure to define in the silicon substrate the asymmetrical potential wells required for unidirectional charge flow. A method of fabricating the device is described. The structure offers considerable advantages over previously described CCD devices.

EFFECTS OF SILICON NITRIDE GROWTH TEMPERATURE ON CHARGE STORAGE IN THE MNOS STRUCTURE

Experimental results of the effects on charge storage in the MNOS structure by varying the growth temperature of the silicon nitride are presented. The range of growth temperature examined is 650–1100°C. The initial charge stored is positive at the lower temperatures, negative at the higher temperatures. The maximum positive charge that can be stored varies continuously with growth temperature. The ability to retain the stored charge when the charging voltage is removed decreases monotonically with increasing growth temperature.