Identification of Trapping Effects and Ion Neutralization at the Insulator/Semiconductor Interface of MIS Structures from Dynamic Current–Voltage Characteristics of Ion Depolarization

The manifestations of ion traps, ion neutralization, and minority carrier generation at the insulator/semiconductor interface (hereafter, interface for brevity) in MIS structures are judged from isothermal dependences of ion depolarization current J and high-frequency capacitance C _s of the depletion layer in the semiconductor on gate potential V _g and the rate of potential change _v = dV _g/dt = const. In the general case, even for a single type of mobile ions in the insulator, the dynamic current–voltage characteristics (CVCs) may exhibit three current peaks. The transfer of some nonlocalized (free) ions at the interface through the insulator, depletion of ion traps, and decomposition of neutral ion–electron associates are responsible for the peaks. The sequence and number (down to one) of the peaks depend on the activation energies of the associated processes, value of _v, and energy of activation of minority carrier generation. Depending on these parameters, the peaks may appear, disappear, or merge into a broad peak, which may erroneously be identified as a result of the depletion of ion traps that have an energy spectrum. In other words, the CVC with a single peak does not necessarily mean that there exist several types of mobile ions. From the J, C _s = f(T, V _g, n ₀, _v) families, one can discriminate between purely ionic and electronic phenomena and identify free, neutralized, and/or trapped ions present at the interface (here, T is the temperature and n ₀ is the initial total surface concentration of particles (ions) and neutral associates at the interface).     

Manifestations of the deneutralization of mobile charges in SiO2 in the spectroscopy of the silicon-oxide interface

The total number of mobile ions in the oxide film in a Si-based MOS structure is determined by the conventional methods of recording capacitance-voltage and dynamic current-voltage characteristics. The fraction of ions in the neutral state at the Si-SiO₂ interface is determined. Spectroscopy of the interface reveals a peak of the effective density of interface states. It is shown that the number of states in this peak corresponds to the number of neutralized particles. The mechanism for neutralization of the mobile charge of ions is discussed. Fiz. Tekh. Poluprovodn. 32, 1445–1449 (December 1998)     

Ion transfer processes in an insulating layer containing traps

Abstrect A theory of ion transfer processes in an insulating layer containing a uniform distribution of traps throughout its volume is formulated. It is shown that free ions localized near the surface are held in a potential well created by repulsion from trapped carriers. Accordingly, the activation energy of the free-ion current is higher than the mobility activation energy and decreases as the voltage is increased, while detrapping takes place with a time delay. The temperature dependence of the depolarization current has two or three peaks, whose positions and profiles change with the voltage. A distinctive feature of the transfer of ions through an insulator containing traps is the “memory” of the electric field driving the carriers toward the surface prior to the start of the transfer process. Fiz. Tekh. Poluprovodn. 31, 43–48 (January 1997)     

Model of the behavior of MOS structures under ionizing irradiation

A quantitative model describing the behavior of MOS structures under ionizing irradiation is developed. The model is based on the capture of holes by hydrogen-containing traps. Some traps are charged and thus form a positive space charge in the insulator. The other traps decay to release positive hydrogen ions. These ions migrate in the insulator electric field to the insulator-semiconductor interface, where they depassivate surface states. The charging of surface states both under irradiation and during measurement of the threshold voltage is taken into account.     

Deep trapping of implanted Na+ and Li+ ions near the Si/SiO2 interface in metal-oxide-silicon structures

Na⁺ and Li⁺ ions have been implanted in the oxide layer of MOS structures with doses ranging from 3 × 10¹¹ to 3 × 10¹³ ions/cm². Part of the implanted ions can be retraced as mobile ions: this fraction decreased with increasing dose. The trapping of the mobile ions near the Si/SiO₂ interface has been investigated by means of the thermally stimulated ionic current (TSIC) technique. The average energy depth of the ionic traps appeared to increase with increasing dose. Moreover, we found that Li⁺ ions are trapped deeper than Na⁺ ions under equivalent experimental conditions. The influence of the applied electric field on the detrapping has been studied. In the case of 3 × 10¹³ Na⁺ implantation, the barrier lowering corresponds with the Poole-Frenkel theory. We have also paid attention to the effects of bias-temperature stress treatments on the trapping kinetics. We observed a decrease of the mobile ion current after long BTS treatments.     

Stabilization of charge at the interface between the buried insulator and silicon in silicon-on-insulator structures

The effect of additional implantation of hydrogen ions into the region of the interface between the split-off silicon layer and the buried insulator in silicon-on-insulator structures and subsequent high-temperature annealing on the parameters of the structures and their radiation resistance is studied. This modification of silicon-on-insulator structures gives rise to the following effects. The mobile charge present in the oxide of the initial structures becomes immobilized, which stabilizes the characteristics of silicon-on-insulator structures and simultaneously increases the fixed charge near the boundary with the split-off silicon layer. Furthermore, additional traps are introduced into the oxide; these are predominantly electron traps that accumulate negative charge during irradiation. As a result, the charge in the oxide of silicon-on-insulator structures is decreased somewhat at the initial stage of irradiation but then remains nearly unchanged up to doses of 10⁷ rad. Conventional accumulation of positive charge occurs at the second boundary of the structure and is typical also of initial (unmodified) silicon-on-insulator structures.     

The charge accumulation in an insulator and the states at interfaces of silicon-on-insulator structures as a result of irradiation with electrons and gamma-ray photons

The accumulation of charge in an insulator and the states at interfaces in silicon-on-insulator structures irradiated with 2.5-MeV electrons and 662-keV gamma-ray photons were studied. It was found that an additional positive charge appears in the buried insulator of the structures as a result of irradiation. The concentration of hole traps generated by radiation in the oxide is higher at the boundary with substrate than at the bonding interface between a split-off silicon layer and oxide. It is shown that the presence of even a weak built-in field in the structures (F?5×10³ V/cm) gives rise to efficient separation of charge carriers. There is no generation of additional states at the Si/SiO₂ interfaces in the silicon-on-insulator structures for both irradiation types, although this generation is observed in the initial thermal oxide.     

Manifestation of percolation conductivity of short-channel field-effect transistors in the spectrum of shallow interface states

The effective density of shallow interface states N _ss is investigated in the temperature range T=77–300 K using the field-effect method in short-channel (0.5–5 μm) Si-MNOS and GaAs-based FET’s with high (greater than 10¹² cm⁻²) concentrations of built-in charge in the subgate insulator. A peculiarity of the density of electronic states N _ss was found having the form of a peak, which manifests itself more distinctly at lower temperatures, higher concentrations of built-in charge, and shorter gates. The peak was observed at the same values of the channel conductance G∼q ²/h, regardless of variations in the above-enumerated parameters, the thickness of the sub-gate insulator, and the channel-length-to-width ratio. This means that the energy depth of the peak (∼40–120 meV) varies in proportion to T, which contradicts the current understanding of the interface states caused by both the fluctuation potential (FP) and surface defects or traps. The results are interpreted within the framework of percolation theory applied to the conductivity of strongly disordered systems. The N _ss peculiarity is associated with a transition from the conductivity of a two-dimensional effective solid, which occurs when the fluctuation potential is strongly screened by surface electrons, to conductivity via a quasi-one-dimensional potential trough organized by local regions with reduced surface potential under conditions of a strong fluctuation potential. Fiz. Tekh. Poluprovodn. 31, 1460–1467 (December 1997)     

Influence of a flatband-voltage variation along the channel on the drain characteristics of a TFT

The theoretical model for a thin film transistor (TFT) is extended to include the effect of a flatband-voltage variation along the channel as caused by the presence of mobile ions in the insulator and of slow surface traps in the semiconductor-insulator interface. The parameters determining the flatband-voltage in a common experimental situation are discussed. Finally, some approximations are given, resulting in a very simple model for the influence of mobile ions on the drain characteristics.     

Resonance transfer of charge carriers in Si/CaF<Subscript>2</Subscript> periodic nanostructures via trap states in insulator layers

A model of the resonance-tunneling transport of charge carriers via discrete-level traps in insulator layers in Si/CaF₂ periodic low-dimensional structures is proposed. Upon application of the external bias voltage to the structure, the resonance-tunneling transport occurs in the cases when the energy of the charge carriers in Si wells coincides with the energy of the trap level in CaF₂ layers. It is shown that filling of the traps and violation of the conditions for the resonance-tunneling transport of charge carriers via the trap level, which take place when the energy of the carriers in the wells exceeds the energy of the trap state in the insulator, result in a drop in the current through the structure; thus, a region of negative differential resistance is formed in the current-voltage characteristics of Si/CaF₂ periodic structures. Simulation of this effect shows that devices based on these structures may operate in a wide range of temperatures from 77 to 300 K. Another advantage is their compatibility with silicon integrated-circuit technology.     

Free ion transport in the insulator layer and electron-ion exchange at an insulator-semiconductor phase boundary produced as a result of thermally stimulated ionic depolarization of silicon MOS structures

The nature of thermally stimulated depolarization currents in silicon MOS structures is investigated. The analysis is based on the experimentally established fact that the activation energy of the depolarization current in the initial section of growth is independent of the magnitude of the depolarizing voltage and on the previously discovered phenomenon of the formation of neutral associates ion + electron at the insulator-semiconductor interface. Transport of free ions in the insulator layer (the transit time is of a thermal activation character) and electron-ion exchange processes at the Si-SiO₂ phase boundary, which include tunneling ionization (decay) of neutral associates, are studied. The ion transfer in SiO₂ layers, found from the thermally stimulated depolarization curves using the developed theory, agrees well with data from independent experiments. Fiz. Tekh. Poluprovodn. 33, 962–968 (August 1999)     

Electrical properties of FeIn<Subscript>2</Subscript>Se<Subscript>4</Subscript> single crystals

The temperature dependence of electrical conductivity and current-voltage characteristics of FeIn₂Se₄ single crystals were studied. It is shown that the current in the nonlinear range of the current-voltage characteristic is caused by the field effect. The activation energy of charge carriers, the concentration of traps, and the shape of a potential well in the region of a trap are determined.     

The effect of ionization and displacement damage on minority carrier lifetime

Based on 1 MeV electrons and 40 MeV Si ion irradiations, the contribution of ionization and displacement damage to the decrease in the minority carrier lifetime of gate controlled lateral PNP (GLPNP) transistors is investigated by gate sweeping (GS) technique. Molecular hydrogen is employed to increase the ionization radiation sensitivity and help to understand the relationship between the minority carrier lifetime and ionization damage. Experimental results show that 1 MeV electrons mainly induce ionization damage to GLPNP transistors, 40 MeV Si ions primarily produce displacement defects in silicon bulk. For 40 MeV Si ions, with increasing the irradiation dose, the densities of interface trap and oxide charge are almost no change, the minority carrier lifetime obviously decreases. The decrease of the minority carrier lifetime is due to bulk traps induce by 40 MeV Si ions. For 1 MeV electrons, with increasing the irradiation dose, the densities of interface trap and oxide charge for the GLPNP with and without soaked in H₂ increase, and the minority carrier lifetime decreases. Compared with the GLPNP transistors without soaking in H₂, the density of the interface traps the irradiated GLPNP transistors by 1 MeV electrons and soaked in H₂ are larger and the minority carrier lifetime is lower. Therefore, both ionization and displacement damage can induce the decreases in the minority carrier lifetime including bulk minority carrier lifetime and surface minority carrier lifetime.     

On the origin of the thermal-field asymmetry in ionic polarization/depolarization of oxide in Si-MOS structures

Based on the concepts of the significant role of the surface neutralization of positive ions at the boundaries of the oxide layer in the processes of ion transport in an insulating gap of MOS structures, the origin of a well-pronounced asymmetry in the temperature and temporal characteristics of the volume-charge ionic polarization/depolarization of an insulator are analyzed. The neutralization of ions occurs owing to the tunneling capture of electrons from semiconducting and metallic contacts. Experimental data obtained in a wide range of variations in the gate potential V _g and related to the thermally stimulated and isothermal polarization of oxide in Si-MOS structures consistently support the asymmetry model that accounts for a higher degree of neutralization of ions and a higher coupling of ions to electrons at the metal surface (gate) than at the semiconductor surface. The transients exhibit three stages during polarization. The first of these is related to the transport of unneutralized (free) ions; in the initial stages of thermally stimulated and isothermal polarization for V _g=const, the ions move in the oxide ballistically. In the second stage, a transition from the mode of free-ion drift to the modes of hyperbolic and (or) exponential kinetics of relaxation is observed; in the latter case, the current becomes virtually independent of the field, temperature, and the rates of the field or temperature scans and becomes a single-valued function of actual time. In this case, the law of relaxation is defined by the rate of tunneling ionization of neutral associations of ion + electron and (or) by their diffusion and thermal decomposition in the bulk of the insulator.     

Distribution of mobile ions in thin insulator films at the insulator-semiconductor interface

The influence of a semiconductor on the mobile ion distribution in insulator thin films at the insulator-semiconductor interface was considered. The degree of ion localization at the interface under the field effect in the film was calculated. The threshold of the ion delocalization with a decrease in the voltage applied to the structure was determined. The relation between the delocalization thresholds and ion current peaks in dynamic current-voltage characteristics of the system is discussed.     

Profiling of traps in SiO2/Al2O3 gate stack by the charge pumping technique

In this paper, we present our results on the distribution and generation of traps in a SiO₂/Al₂O₃ transistor. The investigation has been carried out by using charge pumping measurements, both variable voltage and frequency techniques, and constant voltage stress.By increasing the amplitude of the gate pulse we observe an increase of the charge recombined per cycle closely related to the contribution of shallow traps near the SiO₂/Al₂O₃ interface. By reducing the pulse frequency we measure an increase in the charge pumping current due to traps located deeper in the Al₂O₃. By combining charge pumping and constant voltage stress measurements, we found that the traps are mostly generated near the Si/SiO₂ interface.     

Anomalous drain voltage dependence in bias temperature instability measurements on high-κ field effect transistors

We find that changes in threshold voltage induced by negative bias temperature stressing of p-channel field effect transistors with HfSiON gate dielectrics are modulated by the drain voltage, in measurements wherein the drain current is measured during stressing. This effect is not observed in SiO₂ gate devices. Short channel effects are excluded as explanations, leading us to conclude that positive charge in the dielectric stack is laterally mobile and is conducted out of the insulator via the drain. Further, a simple qualitative model of charging kinetics allows us to extract the density of interface states as a function of time, and shows that these defects build in time, reaching numbers on the order of 10¹¹ cm⁻² after hundreds of seconds.     

Electrical characterisation of Si3N4/SiO2 double layers on p-type 6H–SiC

MNOS, MNS and MOS devices have been fabricated on p-type 6H–SiC substrates without epitaxial layers. They have been characterised using high frequency CV, GV, and IV measurements. The high frequency CV characteristics of p-type 6H–SiC MNOS structures indicate a very similar interface quality to p-type 6H–SiC MOS devices. A lower effective fixed insulator charge Q_I is found in MNOS devices with a higher oxide thickness x_ox. An x_ox of 10 nm is effective in avoiding charge instability. The effective fixed insulator charge Q_I can be modified in the 10 nm oxide SiC MNOS devices by injecting carriers into the nitride. Similar leakage current characteristics compared to p-type 6H–SiC MNS structures have been found for p-type 6H–SiC MNOS devices, but the SiO₂/Si₃N₄ insulator current is lower, particularly for positive electric fields. At the oxide breakdown limit (−10 MV/cm), Poole–Frenkel conduction is observed in the nitride for negative electric fields due to direct tunnelling of holes into the nitride.     

Theory of dynamic charge current and capacitance characteristics in MIS systems containing distributed surface traps

Theoretical studies have been made on the dynamic characteristics of the metal-insulator-semiconductor (MIS) capacitor containing distributed surface traps. It has been shown that when the surface traps are in dynamic equilibrium with the voltage ramp, the device exhibits steady-state charge, current and capacitance characteristics. When the surface traps are out of equilibrium with the voltage ramp, then the emission of trapped charge is a function of time only and not of voltage. Under such conditions, the characteristics are considered to be non-steady-state in nature.In the steady state, the emission of electrons from a continuum of surface traps accounts for the reduction in the slopes of the C–V curves from the ideal ones. Kinks are manifested when the traps just empty the last of their electrons.In the non-steady state, electron emission can be described by the non-steady-state (time-dependent) occupancy function derived herein, which is shown to be similar in shape to the Fermi function. This means that electron emission takes place from a narrow band of energy positioned near the uppermost-filled traps. Hysteresis effects are manifested in the C–V characteristics due to the non-steady-state emission of trapped charge. At the transition from the steady state to the non-steady state and vice versa, kinks are exhibited in the charge and capacitance characteristics, while step changes in current components are also predicted.The physical processes involved have been stressed and closed-form expressions have been obtained for the charge, current and capacitance in terms of the trapping parameters, sweep-rate and temperature.