Interface states and deep-level centers in silicon-on-insulator structures

Deep-level centers in a split-off silicon layer and trap levels were studied by deep-level transient spectroscopy both at the Si/SiO₂ interface obtained by direct bonding and also at the Si(substrate)/〈thermal SiO₂〉 interface in the silicon-on-insulator structures formed by bonding the silicon wafers and delaminating one of the wafers using hydrogen implantation. It is shown that the Si/〈thermal SiO₂〉 interface in a silicon-on-insulator structure has a continuous spectrum of trap states, which is close to that for classical metal-insulator-semiconductor structures. The distribution of states in the upper half of the band gap for the bonded Si/SiO₂ interface is characterized by a relatively narrow band of states within the range from E _c −0.17 eV to E _c −0.36 eV. Furthermore, two centers with levels at E _c −0.39 eV and E _c −0.58 eV are observed in the split-off silicon layer; these centers are concentrated in a surface layer with the thickness of up to 0.21 μm and are supposedly related to residual postimplantation defects. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 8, 2001, pp. 948–953. Original Russian Text Copyright ? 2001 by Antonova, Stano, Nikolaev, Naumova, Popov, Skuratov.     

Post-irradiation formation of Si-SiO2 interface states in a hydrogen atmosphere at room temperature

Si MOSFETs were irradiated with x-rays and then exposed to various partial pressures of H₂ at either room temperature or 125 °C. The number of interface traps and the net positive oxide trapped charged were measured during the hydrogen exposure using spectroscopic charge pumping techniques. During the hydrogen exposure the gate electrode was held at a positive bias to maintain a field of 0.65 MV/cm across the gate oxide. It was found that during the room temperature hydrogen exposure the number of interface traps increased by a factor of about two. The change in the oxide trapped charge during hydrogen exposure indicated that the decrease in the number of positively charged oxide traps was approximately the same as the increase in the number of interface traps. The time evolution and bias dependence of these changes are explained by a model that we previously proposed. In this model positively charged radiation induced defects in the oxide crack the H₂ to form H⁺. Under positive gate bias the H⁺ then drifts to the Si-SiO₂ interface where it forms an interface state, while at the same time removing positive charge from the oxide.     

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.     

Special features of photoluminescence in silicon-on-insulator structures implanted with hydrogen ions

The special features of photoluminescence spectra of silicon-on-insulator structures implanted with hydrogen ions are studied. An increase in the photoluminescence intensity with increasing hydrostatic pressure P during annealing and the formation of narrow periodic photoluminescence peaks in the spectral range from ～500 to 700 nm are revealed for the structures annealed at P > 6 kbar. It is shown that the fine structure of the photoluminescence spectra correlates with the slowing-down of hydrogen effusion from the implanted samples and with the suppression of the formation of hydrogen microbubbles in the surface layer. These processes promote the formation of an optical resonator, with the mirrors formed by the “silicon-on-insulator-air” and “silicon-on-insulator-SiO₂” interfaces and with the optically active layer formed by hydrogen ion implantation and subsequent annealing.     

Optically active interface states in MOS structures

The results of studies of the interface states in clean n-type MOS capacitors by the low-temperature photocapacitance technique are reported. Energy distribution of the interface states density and photoionization cross-section are determined from the analysis of the photocapacitance kinetic. It is shown that the energy distribution of the interface state density can be described by the Gaussian distribution with maximum at about 0.7 eV below the conduction band edge and standard deviation of about 0.1 eV. The photoionization cross-section of these states can be described by the Lucovsky relationship. The maximum of the photoionization cross-section vs photon energy was equal to about 5 × 10^?20/(E_c ? E_ss) cm² for a given energy of the interface states E_ss.From the comparison of the photocapacitance and quasi-static C-V measurements the existence of two types of interface states is suggested: “optically active states” with density decreasing towards the band edges, and “optically inactive states” with density increasing towards the band edges. No photocapacitance associated with “optically inactive states” was observed.     

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.     

Charge trapping and interface states in hydrogen annealed HfO2–Si structures

Metal–oxide–semiconductor (MOS) capacitors based on HfO₂ gate stacks with Al and TiN gates are compared to study the effect of the gate electrode material to the properties of insulator–semiconductor interface. The structures under study were shown to contain interface trap densities of around 2 × 10¹¹ cm⁻² eV⁻¹ for Al gate and up to 5.5 × 10¹² cm⁻² eV⁻¹ for TiN gate. The peak in the surface state distribution was found at 0.19 eV above the valence band edge for Al electrode. The respective capture cross-section is 6 × 10⁻¹⁷ cm² at 200 K.The charge injection experiments have revealed the presence of hole traps inside the dielectric layer. The Al-gate structure contains traps with effective capture cross-section of 1 × 10⁻²⁰ cm², and there are two types of traps in the TiN-gate structure with cross-sections of 3.5 × 10⁻¹⁹ and 1 × 10⁻²⁰ cm². Trap concentration in the structure with Al electrode was considerably lower than in the structure with TiN electrode.     

The effect of post-oxygen-implant annealing temperature on the channel mobilities of CMOS devices in oxygen-implanted silicon-on-insulator structures

The effects of post-oxygen-implant annealing temperature on the characteristics of MOSFET's in oxygen-implanted silicon-on-insulator (SOI) substrates are studied. The results show significant improvements in the electron and hole mobilities near the silicon/buried-oxide interface and in the electron mobility of the front-gate n-channel transistors in SOI substrates with higher post-oxygen-implant annealing temperature. The improvements in the transistor characteristics hence are attributed to the annihilation of oxygen precipitates and the reduction of defect density in the residual silicon film. By comparing the ring oscillators fabricated in SOI substrates annealed at 1150°C and 1250°C after oxygen implantation, a speed improvement of 15 percent is observed in substrates annealed at higher temperature.     

Defect structure and generation of interface states in MOS structures

The correlation between the defect structure of the SiO₂ and the generation of interface traps upon avalanche injection of electrons and holes in MOS capacitor was investigated. Using samples with widely varying densities of intrinsic and H₂O-related trapping centers, and with different oxide thicknesses and gate electrodes it was established from experiments around room temperature that a one to one correlation between the densities of captured carriers and generated interface states exists as long as only a single type of carrier is involved. If simultaneously the second type of carrier is injected the picture becomes more complex. We confirm reports by other authors that at reduced temperatures (typically 77 K) two steps may be distinguished in the generation process, one of which is thermally activated. In this case the “yield” of interface states drops to values distinctly below one, even after warm up. In many cases the energy distribution of the states shows characteristic features: at E = E_v + 0.45 eV for electron injection in samples with H₂O related traps, at E = E_v + 0.75 eV for hole injection. Samples exhibiting these features always show the occurrence of slow states.     

Crystallization induced by thermal annealing with millisecond pulses in silicon-on-insulator films implanted with high doses of hydrogen ions

The crystallization of silicon-on-insulator films, implanted with high doses of hydrogen ions, upon annealing with millisecond pulses is studied. Immediately after hydrogen-ion implantation, the formation of a three-phase structure composed of silicon nanocrystals, amorphous silicon, and hydrogen bubbles is detected. It is shown that the nanocrystalline structure of the films is retained upon pulsed annealing at temperatures of up to ～1000°C. As the temperature of the millisecond annealing is increased, the nanocrystal dimensions increase from 2 to 5 nm and the fraction of the nanocrystalline phase increases to ～70%. From an analysis of the activation energy of crystal phase growth, it is inferred that the process of the crystallization of silicon films with a high (～50 at %) hydrogen content is limited by atomic-hydrogen diffusion.     

Properties of silicon-on-insulator structures and devices

The physical grounds for making SOI structures by the DeleCut (ion irradiated deleted oxide cut) method are considered. This method is a modification of the commonly known Smart Cut? technique and aims at eliminating the disadvantages of the basic method [1]. The proposed method makes it possible to considerably lower the annealing temperature and the content of radiation defects in SOI structures. It allows the thickness of a split-off Si layer and a transition layer between the SOI layer and a buried oxide to be reduced. The method also reduces the nonuniformity in the thickness of the SOI layer and the insulator to several nanometers. By using DeleCut, new SOI structures were formed on wafers with diameters as large as 150 mm; the structures included dislocation-free SOI layers of 0.003–1.7 μm in thickness and a buried thermal SiO₂ oxide (0.05–0.5 μm). These structures have good electrical characteristics, which is supported by fabricating the submicrometer (0.2–0.5 μm) SOI-based CMOS transistors and test integrated circuits. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 9, 2001, pp. 1075–1083. Original Russian Text Copyright ? 2001 by Popov, Antonova, Frantsuzov, Safronov, Feofanov, Naumova, Kilanov.     

Effect of aluminum linewidth on the annealing of fast states in MOS structures

Empirical data are presented showing that the effect of aluminum in the annealing of fast states in MOS structures is dependent on the linewidth of the aluminum. The density of fast states under 0.2-mil-wide aluminum is found to be essentially the same as that in regions not covered by aluminum.     

Anodization of nanoscale Si layers in silicon-on-insulator structures

Anodization of silicon-on-insulator (SOI) layers is studied as a function of the SOI layer thickness in the range from 5 to 500 nm. It is found that thinning of the silicon film to less than 100 nm is accompanied by a sharp decrease in the anodization rate. For SOI films thinner than or on the order of magnitude of ∼10 nm, the limiting thickness of the oxidized silicon layer is 0.4 nm. It is shown that the main cause of a decrease in the anode current efficiency during oxidation of nanoscale Si layers is an increase in the resistance of the silicon active layer, which limits the hole current in the film plane and, hence, the number of silicon cations coming to the SiO₂/electrolyte interface for their subsequent oxidation.     

Electrical conductivity of silicon-on-insulator structures prepared by bonding silicon wafers to a substrate using hydrogen implantation

Silicon-on-insulator structures were prepared by exfoliating a thin layer from a silicon wafer owing to hydrogen implantation, transferring this layer to another substrate, and bonding to it. The influence of hydrogen and the doping level in the original wafers on the free-carrier concentration and the conductivity type in the split-off silicon layer was investigated. A high boron concentration in the original material, together with a high concentration of residual hydrogen in the silicon layer exfoliated from the wafer, was shown to result in n-type conduction, which is retained up to annealing temperatures of 1100°C in these structures. A decrease in the residual hydrogen concentration owing to additional annealing creates the conditions under which the conductivity of resulting structures corresponds to the conductivity type of the original material.     

氢气退火在UV-LIGA制作折叠波导慢波结构中的影响

详细研究了利用紫外-光刻、电铸(UV-LIGA)技术制作全铜结构折叠波导各个阶段中氢气退火工艺对实验的影响。氢气退火工艺主要用于两个阶段：第一阶段为对铜基板的烧氢处理(前期),第二阶段为实现金属结构后对基板和铸层整体的烧氢处理(后期)。实验发现,前期氢气退火除清洁基板、降低内应力外,还能发生晶界迁移,使晶粒在高温下生长趋于稳定,利于生长与之结合更紧密的电铸层。但该处理需提前至基板抛光之前,否则会导致平整度变差。后期氢气退火除检测全铜结构能否经受高温焊接外,还有助于进一步去除光刻胶,并促进基板和铸层在高温下生长为结合紧密的共同体。     

On occupation functions of donor- and acceptor-like interface states in metal-insulator-semiconductor tunnel structures

This paper presents a theoretical study on the occupation functions of interface states in MIS tunnel structures. Based on Shockley-Read-Hall (SRH) statistics and considering carriers tunneling between the metal and interface states, occupation functions of both donor- and acceptor-like interface states at arbitrary energy level within the semiconductor band gap are derived and analyzed. It reveals that, for the same energy level, the occupation functions of donor-like and acceptor-like interface states are remarkably different in magnitude. The deviation in occupation functions of these two types of interface states is a nonlinear function of the ratio of capture cross-section of the charge states to that of the neutral states (C_C/C_N) and the semiconductor surface conditions. If the insulating layer (SiO₂) is relatively thick (> 50 Å) or thin (< 10 Å), interface states are in equilibrium with the semiconductor or the metal, respectively. Under the circumstances, the occupation functions of the two types of interface states are no longer distinguishable. Alternatively, they can be approximated by the well-known Fermi-Dirac distribution function.

In this paper, quantitative influences of key parameters of MIS tunnel structures such as insulating layer thickness, electron and hole density at the semiconductor surface, capture cross-sections for charged and neutral states, etc., on interface states occupation function are discussed. For Gaussian-distributed donor- and acceptor-like interface states, the quantitative roles of interface states in charge storage and current-assisting effects are also demonstrated.     

Hot-carrier-induced degradation of the back interface inshort-channel silicon-on-insulator MOSFETS

The tolerance of silicon-on-insulator MOSFETs to hot-carrier injection into the buried oxide is investigated. It is shown that stressing of the back channel results in reversible electron trapping and formation of localized defects at the buried interface. This damage is responsible for the transconductance overshoot, large threshold voltage shift, and attenuated kink effect. It is also noticed that even in moderately thin films the back oxide damage does not affect the front-channel operation and, conversely, stressing the front channel does not generate defects at the buried interface. These findings indicate that the hot-carrier degradation of the buried oxide might be chosen as a sensitive criterion for optimizing SIMOX (separation by implantation of oxygen) structures     

Growth of silicon nanoclusters in thermal silicon dioxide under annealing in an atmosphere of nitrogen

It is found for the first time that silicon nanoclusters are formed in the surface layer of thermal silicon dioxide under high-temperature annealing (T = 1150°C) in dried nitrogen. Analysis of the cathodoluminescence spectra shows that an imperfect surface layer appears upon such annealing of silicon dioxide, with silicon nanoclusters formed in this layer upon prolonged annealing. Transmission electron microscopy demonstrated that the silicon clusters are 3–5.5 nm in size and lie at a depth of about 10 nm from the surface. Silicon from the thermal film of silicon dioxide serves as the material from which the silicon nanoclusters are formed. This method of silicon-nanocluster formation is suggested for the first time.     

Estimation of long-term transmission loss increase in silica-basedoptical fibers under hydrogen atmosphere

The irreversible loss increase in silica-based optical fibers due to hydrogen is discussed on the basis of results of various high-temperature tests. The results show that germanium-doped-core fibers have different behavior with respect to irreversible loss increase, and that a pure-silica-core fiber fabricated under optimum conditions is very stable against irreversible loss increase. The estimation of long-term transmission loss stability is also discussed, and high-temperature testing is certified to be effective for estimating the long-term loss stability under low temperature