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.     

Investigation of both interface states spectrum and deep oxide states from differential isothermal transient spectroscopy

Metal/insulator/Si (MIS) structures using a high-permittivity (high-k) dielectric layer, here the perovskite compound SrTiO₃, are investigated by differential isothermal transient spectroscopy in order to study electronic states at interface and inside the oxide. The isothermal transient capacitance responses of these MIS capacitors, obtained at varying depletion bias voltages and recording times, are analysed by fast Fourier transform (FFT). Different values of the accumulation pulse duration may induce changes in the trapped charge, allowing identification of the various mechanisms for restoring thermodynamical equilibrium in the interface states. Thus, discrimination between the proper contribution of interfaces states and that of oxide deep states is evidenced. The effects of several post-annealing of the oxide layer onto the energy spectra are also described.     

Generation of surface electron states with a silicon–ultrathin-oxide interface under the field-induced damage of metal–oxide–semiconductor structures

The high-frequency capacitance–voltage characteristics of metal–oxide–semiconductor structures on n-Si substrates with an oxide thickness of 39 Å are studied upon being subjected to damage by field stress. It is shown that the action of a high, but pre-breakdown electric field on an ultrathin insulating layer brings about the formation of a large number of additional localized interface electron states with an energy level arranged at 0.14 eV below the conduction band of silicon. It is found that, as the field stress is increased, the recharging of newly formed centers provides the accumulation of excess charge up to 8 × 10¹² cm^–2 at the silicon–oxide interface. The lifetime of localized centers created under field stress is two days, after which the dependences of the charge localized at the semiconductor–insulator interface on the voltage at the gate after and before field stress are practically the same.     

Behavior of charge in a buried insulator of silicon-on-insulator structures subjected to electric fields

The behavior of charge in a buried oxide of the silicon-on-insulator structures obtained using the Dele-Cut technology was studied by keeping the structures under a voltage with an electric-field strength of 2–5.5 MV/cm. A mobile positive charge drifting under the effect of applied voltage was detected in the oxide. The expected charge accumulation in the oxide was not detected. It may be assumed that both of the observed effects are caused by the interaction of residual hydrogen, which is present in the oxide during the preparation of the structures, with traps in the thermal oxide. As a result, the traps become passivated in the buried insulator of the structure; in addition, the charge, which is mobile at room temperature, is introduced into the insulator.     

Electronic properties of InAs-based metal-insulator-semiconductor (MIS) structures

Special features of electronic processes in InAs-based MIS structures operating in the charge injection mode were investigated. These structures are used as photodetectors in the spectral range of 2.5–3.05 μm. A double-layer system consisting of an anodic oxide layer and a low-temperature silicon dioxide layer was used as an insulator. It was shown that fluorine-containing components, which were introduced into electrolyte, reduced the value of the built-in charge and the surface-state density to minimal measurable values of ≲2×10¹⁰ cm⁻² eV⁻¹. Physical and chemical characteristics of the surface states at the InAs-insulator interface and the possible causes of their absence were discussed on the basis of the phase composition data of anodic oxide obtained by X-ray photoelectron spectroscopy. An anomalous field generation was observed under the nonequilibrium depletion of the semiconductor. The processes of tunneling generation, which are important at large amplitudes of the depletion pulse, were considered. The noise behavior of MIS structures under a nonequilibrium depletion was investigated. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 9, 2001, pp. 1111–1119. Original Russian Text Copyright ? 2001 by Kuryshev, Kovchavtsev, Valisheva.     

Charge fluctuations at the bonding interface in the silicon-on-insulator structures

Starting from the data of deep-level transient spectroscopy, the charge fluctuations at the interface between the top Si layer and buried insulator in Si-on-insulator structures are evaluated. The interface was prepared by bonding Si with the thermally oxidized substrate. The magnitude of fluctuations at the interface is found to be equal or exceed (1.5–2.0)×10¹¹ cm^?2 against the charge background of ～5×10¹¹ cm^?2 at this interface. It is shown that the fluctuations are most likely associated with the negative charge at the surface states rather than with the fluctuations of the fixed positive charge within oxide.     

Neutron irradiation of insulated gate field effect transistors

Insulated gate field effect transistors and polysilicon-gated capacitors were irradiated with fast (10 keV <E < 2 MeV) neutrons. As expected, damage to the bulk silicon was detected as a degradation in the minority carrier lifetime. Optically assisted electron injection was employed for the first time to examine neutral electron trap and fixed positive charge generation in the gate insulator of the devices. While fixed positive charge densities of ≤6 x 10¹⁰ cm⁻² were detected, little or no neutral electron trap generation was observed. The small density of coulombic defects observed in the insulator could be accounted for fully by the known flux of gamma rays associated with the neutron irradiation process. This indicates that fast neutrons passing through a thin gate oxide do not produce significant amounts of damage in the oxide. Somewhat surprisingly, it was found that 1.5 keV X-rays created similar lifetime degradation effects in the bulk silicon, as did fast neutrons, even though this photon energy is not believed to be capable of producing bulk damage in the form of atom displacement in either the semiconductor or the insulator. The minority carrier lifetime of the silicon could be restored to initial values following either neutron or x-ray exposure by annealing in H₂ for 30 min at 400° C.     

Carrier trapping centers and interface states induced by r.f. sputtering of molybdenum electrodes in MOS-structure diodes

Electronic properties of interface and oxide layers of Si MOS diodes with an r.f. sputtered molybdenum metal electrodes have been studied. A remarkable density increase of both oxide charge and interface states by the sputtering has been found. IMA data show that the sputtered Mo atoms are implanted into the SiO₂ less than 100 Å from the surface. It is also found from photo I–V data that the charge centroid in the SiO₂ is shifted to a depth of 100 Å below the SiO₂ surface. The energy distribution of the carrier trapping centers having a capture cross section of the order of 10^?13 to 10^?16 cm² has been observed. The interface states density can be reduced about one order of magnitude by an annealing in N₂ 10 min/H₂ 15 min/N₂ 10 min at 450°C. The mechanism of the increase of the carrier trapping centers and the interface states is also studied. Especially it is concluded that the increase of the interface states originates from the ultraviolet-light irradiation emitted from the plasma during the r.f. sputtering.     

Properties of MIS structures based on graded-gap HgCdTe grown by molecular beam epitaxy

The effect of near-surface graded-gap layers on the electrical characteristics of MIS structures fabricated based on heteroepitaxial Hg_{1 ? x} Cd _x Te films grown by molecular beam epitaxy with a two-layer SiO₂/Si₃N₄ insulator and anodic oxide film is studied experimentally. It is shown that a larger modulation of capacitance (depth and width of the valley) is observed compared with the structures without the graded-gap layer. The field dependences of photovoltage of MIS structures with the graded-gap layers had a classical form and were characterized by a drop only in the enrichment region. For the structures without the graded-gap layer with x = 0.22, a drop in the voltage dependence of the photocurrent is observed in the region of pronounced inversion. This drop is governed by limitation of the space charge region by processes of tunneling generation via deep levels. The properties of the HgCdTe-insulator interfaces are studied.     

Enhanced Generation of Interface States in MIS IC Elements under the Action of UV and X-ray Radiations

The effect of enhanced generation of states at the Si/SiO₂ interface was first observed in polySi-gate MIS structures successively subjected to UV and X-ray radiations. It was studied on test MIS transistors of series-produced ICs by the methods of subthreshold currents, charge pumping currents, and internal photoemission. A possible reason for enhanced generation of the states is the presence of hydrogen at the interface with the polySi gate. Under UV irradiation, the hydrogen diffuses toward the substrate and causes surface state generation under subsequent irradiation by X-rays.     

Silicon-on-insulator structures with a nitrogenated buried SiO<Subscript>2</Subscript> layer: Preparation and properties

Electrical properties of silicon-on-insulator (SOI) structures with buried SiO₂ layer implanted with nitrogen ions are studied in relation to the dose and energy of N⁺ ions. It is shown that implantation of nitrogen ions with doses >3 × 10¹⁵ cm⁻² and an energy of 40 keV brings about a decrease in the fixed positive charge in the oxide and a decrease in the density of surface stares by a factor of 2. An enhancement of the effect can be attained by lowering the energy of nitrogen ions. The obtained results are accounted for by interaction of nitrogen atoms with excess silicon atoms near the Si/SiO₂ interface; by removal of Si-Si bonds, which are traps of positive charges; and by saturation of dangling bonds at the bonding interface of the SOI structure.     

Electron mobility in the inversion layers of fully depleted SOI films

The dependences of the electron mobility μ_eff in the inversion layers of fully depleted double–gate silicon-on-insulator (SOI) metal–oxide–semiconductor (MOS) transistors on the density N _e of induced charge carriers and temperature T are investigated at different states of the SOI film (inversion–accumulation) from the side of one of the gates. It is shown that at a high density of induced charge carriers of N _e > 6 × 10¹² cm^–2 the μeff(T) dependences allow the components of mobility μ_eff that are related to scattering at surface phonons and from the film/insulator surface roughness to be distinguished. The μ_eff(N _e ) dependences can be approximated by the power functions μ_eff(N _e) ∝ N _e ^?n . The exponents n in the dependences and the dominant mechanisms of scattering of electrons induced near the interface between the SOI film and buried oxide are determined for different N _e ranges and film states from the surface side.     

A study of x-ray damage effects on the short channel behavior of IGFET’s

Ionizing particles and radiation may play an important, albeit undesirable role in the processing of VLSI and ULSI circuits in that they can generate bulk charge in the gate insulator of IGFETs. In this regard, there is conflicting information in the literature on the effects of ionizing radiation on short channel phenomena in IGFETs. For example, Peckeraret al. in 1983 claimed that the effective channel length increases when positive coulombic charge is introduced during irradiation, resulting in a decrease in the short channel effect. Schrankleret al. in 1985 claimed in an experimental study, on the other hand, using 28.0 nm thick gate oxides and 0.9–10 μm channel lengths, that the effect is increased,i.e., the short channel effect begins at longer channel lengths. Wilson and Blue in 1982, in a theoretical study concluded that other than a uniform downward shift in theV _T -channel length curve due to the presence of insulator net fixed positive charge, no effect should be observed. Because of these conflicting reports, it was decided to evaluate this behavior using two different background doping levels inn-channel structures, with physical channel lengths ranging between 1.5 and 10 μm, in 0.1 and 0.5 gWcm (100) Si. To further explore the situation, gate oxide (grown at 1000° C in O₂ containing 4.5% HC1) thicknesses were varied from 17.0–35.0 nm, and the absorbed radiation dose using Al-K_α (1.5 keV) x-rays was varied between 2.4 × 10⁶ rad (SiO₂) and 2.4 × 10⁷ rad (SiO₂). For all conditions studied above, a uniform downward shift in the V_T-Channel length curve was observed, essentially corroborating the theoretical conclusions of Wilson and Blue. In addition to the above, the effects of intentionally doping the gate insulator with boron (1.2 × 10¹² B⁺ cm⁻²) implanted at 8 and 10 keV into 25.0 nm and 31.4 nm oxides, respectively, on short channel effects were evaluated for devices grown onp-type 0.5 Ω.cm substrates. Unlike the devices which did not have excess boron intentionally implanted into the gate insulator, it was found that higher concentrations of boron (2.0 × 10¹⁷ cm⁻³ in the insulator via implantation as compared to 4.2 × 10¹⁶ cm⁻³ incorporated in oxides during the oxide growth on 0.5 Ω.cm type (100) Silicon) leads to smaller short channel effects in unirradiated devices. On the other hand, these heavily doped oxides show a distinct worsening of the short channel effect after exposure to 2.4 × 10⁷ rad (SiO₂) using Al-K_α radiation. Thus, while normal devices exhibit little if any short channel improvement, or degradation following irradiation, intentionally doped insulators show an improvement in short channel characteristics prior to irradiation, and a worsening of the short channel effect following irradiation.     

Electrical stress and plasma-induced traps in SiO2

High field stress and N₂ reactive ion etching (RIE)-mode plasma-generated positive oxide charge in thin (13 nm) SiO₂–Si structures have been studied. A threshold field of about 8.5–9 MV/cm for positive charge formation is found. It is established that both high field stress and RIE-like plasma treatment create nonuniformly distributed positive charge in the depth of the oxide, in the form of bulk traps and slow states. It is found that the generation of neutral bulk traps is an attribute, only of the high field stressing. The structural nature of the process-induced traps is discussed. It is suggested that the impact ionization of oxygen vacancies accounts for the positive charge and neutral trap creation.     

Specific features of formation of radiation defects in the silicon layer in “silicon-on-insulator” structures

Specific features of formation of radiation defects in thin silicon layer of silicon-on-insulator (SOI) structures have been studied. It is shown that there are differences between variations in the structural and electrical properties of the thin silicon layer and those in bulk silicon crystals (with similar electrical characteristics) subjected to the same radiation effect. It is established that the embedded insulator in the SOI structure represents a barrier for motion of radiation-induced intrinsic interstitial silicon atoms, which brings about an increase in the dose of bombarding ions, which leads to the loss of single-crystallinity of the silicon layer in a SOI structure. It is shown that γ-ray irradiation with doses unaffecting the electrical conductivity of bulk silicon crystals appreciably affects the conductivity of the silicon layer in the SOI structures. In addition, variation in the conductivity of silicon layer is related to variation in the density of surface states at the interface between the silicon layer and the built-in insulator, rather than to generation of conventional radiation-induced structural defects in silicon.     

Effect of thermal annealing and exposure to oxygen plasma on the properties of TiO2-Si structures

The effect of thermal annealing and exposure to oxygen plasma on the phase composition, structure, and microprofile of titanium-dioxide films deposited by high-frequency magnetron sputtering on silicon substrates is studied. The influence exerted by processing modes on the capacitance-voltage and conductance-voltage characteristics of Me-TiO₂-Si-Me structures and on the density of surface states at the semiconductor-insulator interface is examined. It is shown that TiO₂ films are amorphous upon their fabrication. Upon the annealing of films at 500°C in an argon atmosphere, crystallites of anatase and rutile appear in the amorphous matrix. The treatment of a titanium-dioxide film in oxygen plasma gives rise to rutile crystallites with new crystallographic planes. As a result of annealing at 750°C, the anatase phase disappears and the film becomes polycrystalline, containing only rutile crystallites. The capacitance of Me-TiO₂-Si-Me structures in the accumulation mode reaches the maximum value upon annealing at 750°C, which is due to the transformation of titanium dioxide to the rutile phase. The specific capacitance is 5.9 × 10^?2 F/cm³. The decrease in the capacitance of the structures and in the amount of fixed charge in the insulator upon exposure to oxygen plasma is due to the diffusion of oxygen atoms across the titanium-dioxide layer to give a SiO₂ film at the TiO₂-Si interface. As a result of the annealing and treatment of a titanium-dioxide film in oxygen plasma, the energy density of surface states decreases by more than an order of magnitude as compared with the unannealed samples.     

Specific features of the field generation of minority charge carriers in Si-SiO2 structures

The specific features of generation processes occurring in Si-SiO₂ structures in the presence of a strong electric field in the oxide layer (E _ox ≈ 10 MV cm^?1) are considered. It is shown that, after a certain threshold electric-field strength is reached, an additional mechanism of minority charge-carrier generation, associated with the development of electroluminescence processes in the oxide layer, becomes operative.     

Increase in the rate and discretization of the kinetics of isothermal surface generation of minority charge carriers in metal-insulator-semiconductor structures with a planar-inhomogeneous insulator

Surface generation of minority charge carriers in silicon metal-oxide-semiconductor (MOS) structures is efficient only at the initial recombinationless stage. Quasi-equilibrium between surface generation centers and the minority-carrier band is established in a time t ～ 10^?5 s. In the absence of other carrier generation channels, an equilibrium inversion state at 300 K would need t = t_∞ > 10³ years to become established. In fact, the time t ∞ is much shorter, due to excess-carrier generation via centers located at the SiO₂/Si interface over the gate periphery. This edge-related generation can easily be simulated in an MOS structure with a single gate insulated from Si by oxide layers of various thicknesses. At gate depleting voltages V _g , the role of the periphery is played by a shallow potential well under a thicker oxide, and the current-generation kinetics becomes unconventional: two discrete steps are observed in the dependences I(t), and the duration and height of these steps depend on V _g . An analysis of the I(t) curves allows determination of the electric characteristics of the Si surface in the states of initial depletion (t = 0) and equilibrium inversion (t = t_∞), as well as the parameters of surface lag centers, including their energy and spatial distributions. The functionally specialized planar inhomogeneity of a gate insulator is a promising basis for dynamic sensors with integrating and threshold properties.     

Silicon in functional epitaxial oxides: A new group of nanostructures

The ability to integrate low-dimensional crystalline silicon into crystalline insulators with high dielectric constant (high-k) can open the way for a variety of novel applications ranging from high-k replacement in future nonvolatile memory devices to insulator/Si/insulator structures for nanoelectronic applications. We will present an approach for nanostructure fabrication by incorporation of crystalline silicon into epitaxial oxide that is based on a solid-phase epitaxy of Si. In dependence on the preparation conditions we obtained nanostructures containing an either ultra-thin single-crystalline Si quantum-well buried in single-crystalline oxide matrix with sharp interfaces or Si-nanocrystals (ncs) embedded into single-crystalline oxide layer. As an example, we demonstrate the growth of Si buried in Gd₂O₃ and the incorporation of epitaxial Si clusters into single-crystalline Gd₂O₃ on silicon as well as silicon carbide substrates using molecular beam epitaxy. The leakage current of the obtained nanostructures exhibited negative differential resistance at lower temperatures. For structures containing Si-ncs a large hysteresis in capacitance–voltage measurements due to charging and discharging of the Si-ncs was obtained.     

Oxide interface studies using second harmonic generation

We report experiments using a non-invasive second harmonic generation (SHG) technique to characterize buried Si/SiO₂ interfaces and also SIMOX thin film silicon-on-insulator (SOI) wafers. The measurements demonstrate that the SHG response can provide an indication of the quality of the buried oxide interfaces, by providing information on surface roughness, strain, defects, and metallic contamination. The potential application of SHG for comprehensive buried interface characterization and as a non-contact metrology tool for process control is described.