Irradiated SiO2 Research Articles

Structural disorder in neutron irradiated quartz and MgO

Abstract We measure diffuse X-ray scattering close to Bragg reflections in SiO2 and MgO single crystals after irradiation with fast neutrons. In SiO2 smaller and many times fewer defect clusters are created. In neutron irradiated SiO2 there exist additional planes of diffuse intensity. By inelastic neutron scattering no energy broadening (Δν = 0.045 THz) was observed. The intensity distribution in reciprocal space can be explained by chains in x-direction of statically tilted SiO4 tetrahedra.

Nucleation of antimony films on an irradiated SiO surface

The nucleation of antimony on SiO was investigated by means of transmission electron microscopy and Auger spectroscopy. It was shown that the Auger analysis permits the estimation of the activation energy of nucleation; this was in good agreement with the theory of nucleation. New experimental data for the deposition of antimony on SiO under low energy electron irradiation is presented. The electron irradiation was found to raise the critical temperature of condensation and it changes markedly the morphology of the deposit. Possible applications are discussed.

Electron-Hole Recombination in Irradiated SiO2 from a Microdosimetry Viewpoint

The fields of radiation chemistry and microdosimetry have produced models for the calculation of the extent of reactions involving radiation-produced ions. This paper presents a model based on ideas developed by Magee and coworkers which are in turn based on the old columnar recombination model of Jaffe. The key contribution of this model is a method for the determination of the spatial distribution of reactants into several differently shaped deposition regions. The implications of this multiple shape recombination (MSR) model for the treatment of electron-hole recombination in MOS oxides will be discussed. In addition, radiation damage in SiO2 appears to offer a better means for testing such a model than any which has been previously available. That is, the study of electronic devices is benefiting from ideas which originated in the fields of microdosimetry and radiation chemistry, and is in turn providing unique tests of such ideas.

Electron trapping in electron-beam irradiated SiO2

In addition to the well-known positive space charge, electron irradiation of MOS capacitors with 25-keV electrons is shown to introduce additional uncharged electron traps into the oxide layer. These traps persist after most of the positively charged defects have been removed by the usual low-temperature (? °C) anneals. Their presence after this anneal is determined by injecting hot electrons into the oxide where they are captured by existing defects. The effective trap densities increase with increasing electron fluence and are reduced by forming-gas anneals at temperatures in excess of 500 °C. Observed electron-capture cross sections are between 10−15 and 10−18 cm2. The residual radiation damage in oxides exposed to 10−4 C cm−2 of 25-keV electrons and subsequently annealed at 400 °C results in an additional neutral density of 5×1011 traps cm−2 with cross sections distributed over the above range. Electron-trapping cross sections and effective trap densities associated with this damage are found to be identical at 77 and 295 K. The traps are possibly associated with dipolar defects formed when valence electrons localize around an ion after the bonds are broken.

Role Transport and Charge Relaxation in Irradiated SiO2 MOS Capacitors

The transient response of SiO2 gate-insulator MOS capacitors to pulsed electron beam irradiation was studied. The radiation-induced flatband voltage shift (Δ VFB) in SiO2 MOS capacitors was measured with a fast C-V technique from 70 μsec to 1000 sec after a 60 krad radiation pulse for temperatures from 80 to 293 K. In complementary experiments, the post-irradiation charge displacement in the MOS capacitors was measured with an integrating picoammeter. By correlating the relaxation of the flatband voltage with integrated current measurements, it was established that the measured response is dominated by hole transport and trapping in the SiO2 film. The temporal and temperature dependences of the hole transport are well described by a stochastic hopping model based on a continuous time random walk (CTRW). The essential feature of the CTRW is that the transport occurs via a carrier hopping (phonon assisted) process between localized sites randomly distributed in the amorphous SiO2. Since the carriers do not require excitation to the band edges in order to be mobilized, the activation energy for conduction (tunneling) is independent of the optical excitation energy.

Electron−hole pair creation energy in SiO2

The average energy W required to create an electron−hole pair in SiO2 has been determined to be approximately 18 eV by considering the energy loss of fast electrons in solids. This energy loss occurs primarily by plasmon production and subsequent decay of the plasmons into electron−hole pairs. It is also demonstrated that recent data on electron−irradiated SiO2 films can be explained remarkably well by a columnar recombination model. The extrapolation to infinite electric field of the columnar model fit to the data yields a value for W which is in excellent agreement with the value obtained from the energy loss calculation.

Sodium mobility in irradiated SiO2

The distribution of sodium in the oxide films of MOS capacitors was determined by etch-off procedures, using sodium-22 as a tracer. The effects on sodium distribution of ionizing radiation, of in-diffused aluminum, and of bias-temperature stressing were then determined and compared with C-V measurements of the total oxide charge in the same samples. Aluminum films sintered for 30 minutes at 500°C provided high aluminum concentrations (∼10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) in the oxide near the surface, while sintering for 60 minutes caused aluminum to diffuse about three times deeper. In both cases low concentrations (∼10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) were found next to the silicon. Aluminum-rich portions of the oxide were found to trap sodium, and thus the oxides with the 30-minute aluminum sinter had very little sodium concentrated near the silicon substrate (0.2 to 0.6 &time; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ); the interfacial sodium accounted for only 2% of the oxide charge determined by C-V measurements. Oxides without aluminum or with 60-minute aluminum sinter had nearly 10-fold higher interfacial sodium concentrations (1 to 6 &time; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and this interfacial sodium accounted for 10% of the oxide charge from C-V measurements. Ionizing radiation (10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> rads in one hour from cobalt-60 at 25°C with a positive bias of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> volts/cm) increased the total oxide charge by 14 to 17 &time; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> electronic charges/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , but caused very small increases, in interfacial sodium (0.2 to 0.9 &time; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). However, the combination of ionizing radiation followed by positive bias stress (10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> volts/cm) at 300°C for 10 minutes caused an appreciable movement of sodium to the interface in some of the oxides, especially those with no aluminum or with aluminum sintered for 60 minutes. Positive bias at 300°C for 10 minutes in aluminum-doped oxides caused large increases in negative voltage shifts (corresponding to an increase of 10 to 20 &time; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> positive charges per cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) but in the same tests no more than 3 &time; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> sodium ions per cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> moved into the oxide next to the silicon substrate. These results suggest that the drift of positively charged aluminum interstitials may be the cause of much of the voltage shift in 300°C tests.

Effects of stoichiometry on the radiation response of SiO2

Defect center production in irradiated silica is shown to exceed that found in stoichiometric SiO 2 . The defect centers observed are the E′ type which are associated with oxygen vacancies. It is suggested that a large concentration of E′ centers is expected near the oxygen deficient Si-SiO 2 interface of irradiated MOS devices and that the presence of these defects is one source of positive space charge buildup within the first hundred Angstroms of the oxide. In addition, the 4500A emission band observed in irradiated SiO 2 has been shown to be associated with the decay of E′ centers.

Parallel between Surface States at the Si–SiO2 Interface and the B2 Center in Irradiated SiO2

The effect of annealing in nitrogen and hydrogen atmospheres on surface-state densities Nss at the Si–SiO2 interface has been measured using the linear voltage-ramp method which gives Nss vs silicon surface potential. A mercury probe was used in order to eliminate the effect of aluminum on surface-state reduction. The dependence of annealing of surface states on atmosphere and temperature closely parallels that of the B2 center in sputtered SiO2, an absorption center with maximum absorption at 5.1 eV which is observed in irradiated SiO2. In N2, temperatures between 700°C and 800°C are required to anneal completely the B2 center; Nss reaches its lowest values after annealing in N2 at 700°C. In H2, the B2 center anneals completely and Nss is reduced to its lowest values after heating at 400°C. Annealing at 600°C or 700°C in H2 increases Nss and introduces a surface-state level ∼0.1 V above the midgap energy Ei.