Masashi Uematsu

Universal Theory of Si Oxidation Rate and Importance of Interfacial Si Emission

The essential role that Si atoms emitted from the interface play in determining the silicon-oxidation rate is theoretically pointed out, and a universal theory for the oxide growth rate taking account of the interfacial Si-atom emission is developed. Our theory can explain the oxide growth rate for the whole range of the oxide thickness without any empirical modifications, while the rate for an oxide thickness of less than 10 nm in dry oxidation cannot be explained with the Deal-Grove theory.

Simulation of boron, phosphorus, and arsenic diffusion in silicon based on an integrated diffusion model, and the anomalous phosphorus diffusion mechanism

Boron (B), phosphorus (P), and arsenic (As) in-diffusion profiles were simulated based on an integrated diffusion model that takes into account the vacancy mechanism, the kick-out mechanism and the Frank–Turnbull mechanism. The simulations were done using just three parameters for B and P, and four parameters for As, each of which has a clear physical meaning and a physically reasonable value, with no additional ad hoc hypothesis. These parameters correspond to the diffusion of dopant species and of point defects that contribute to dopant diffusion. For the anomalous P diffusion profile, the vacancy mechanism governs the diffusion in the plateau region, while the kick-out mechanism governs it in the deeper region, where self-interstitials dominate in the kink region and P interstitials dominate in the tail region. This changeover from the vacancy contribution to the kick-out contribution is shown to be the mechanism for the appearance of the kink-and-tail profiles of P. Moreover, the comparison among B, P...

Recombination‐enhanced impurity diffusion in Be‐doped GaAs

Recombination‐enhanced impurity diffusion (REID) in Be‐doped GaAs has been observed for the first time. Current‐induced degradation of tunnel diodes has been investigated. The Be diffusion under forward bias is enhanced by a factor of about 1015 at room temperature, and the activation energy for the diffusion is reduced from 1.8 eV for thermal diffusion to 0.6 eV for REID. The RElD of Be, in which the energy released on minority‐carrier injection at the recombination center could enhance the diffusion, is thought to be the origin of the degradation.

Self-diffusion of Si in thermally grown SiO2 under equilibrium conditions

Self-diffusion coefficients of Si in thermally grown SiO2 on a semiconductor-grade silicon wafer have been determined at temperatures between 1150 and 1300 °C under equilibrium conditions using isotope heterostructures (natSiO2/28SiO2). Si self-diffusion was induced by appropriate heat treatments, and the diffusion depth profiles of 30Si isotope from natSiO2 to 28SiO2 layers were determined by secondary ion mass spectrometry (SIMS). The diffusion coefficients found in the present study for 1150–1300 °C are more than two orders of magnitude smaller than the values measured with semiconductor-grade SiO2 in the presence of excess silicon, i.e., in nonequilibrium conditions, and agree very well with previously reported values of Si self-diffusion in fused silica under equilibrium conditions.

Charge states of vacancies in germanium investigated by simultaneous observation of germanium self-diffusion and arsenic diffusion

Diffusion of germanium (Ge) and arsenic (As) has been investigated simultaneously using As-implanted Ge isotope superlattices. No transient enhanced diffusion of As that could have arisen by the implantation damage is observed. A quadratic dependence of the Ge self-diffusion on the carrier concentration due to the Fermi level effect is observed. A precise reproduction of the Ge and As diffusion profiles by a numerical simulator lets us conclude that doubly negatively charged vacancies are the dominant point defects responsible for more than 95% of the self-diffusion in intrinsic Ge and this fraction increases even further in n-type Ge.

Effect of the Si/SiO2 interface on self-diffusion of Si in semiconductor-grade SiO2

Self-diffusion of ion-implanted 30Si in SiO2 formed directly on Si substrates by thermal oxidation was studied as a function of the temperature and SiO2 thickness (200, 300, and 650 nm). The diffusion coefficient increases by about an order of magnitude with decreasing SiO2 thickness from 650 to 200 nm when silicon–nitride capping layers are placed on top of the SiO2, i.e., the distance between the 30Si diffusers and Si/SiO2 interface has a strong influence. Because the stress on SiO2 by nitride estimated for such a change in diffusivity is unrealistically large, Si species, most likely SiO, generated at the Si/SiO2 interface and diffusing into SiO2 must be affecting the self-diffusion of Si in SiO2.

Modeling of Si self-diffusion in SiO2: Effect of the Si/SiO2 interface including time-dependent diffusivity

Self-diffusion of Si in thermally grown SiO2 is modeled taking into account the effect of SiO molecules generated at the Si/SiO2 interface and diffusing into SiO2 to enhance the self-diffusion. Based on the model, a recent self-diffusion experiment of ion-implanted 30Si in SiO2, which showed increasing self-diffusivity with decreasing distance between the 30Si diffusers and Si/SiO2 interface [Fukatsu et al., Appl. Phys. Lett. 83, 3897 (2003)], was simulated, and the simulated results fit the experimental profiles. Furthermore, the simulation predicts that the self-diffusivity would increase for a longer annealing time because more SiO molecules should be arriving from the interface. Such time-dependent diffusivity was indeed found in our follow-up experiments, and the profiles were also fitted by the simulation using a single set of parameters.

Simulation of wet oxidation of silicon based on the interfacial silicon emission model and comparison with dry oxidation

Silicon oxidation in wet ambients is simulated based on the interfacial silicon emission model and is compared with dry oxidation in terms of the silicon-atom emission. The silicon emission model enables the simulation of wet oxidation to be done using the oxidant self-diffusivity in the oxide with a single activation energy. The amount of silicon emission from the interface during wet oxidation is smaller than that during dry oxidation. The small emission rate for wet oxidation is responsible for the insignificant initial oxidation enhancement and the linear pressure dependence of the oxidation rate observed in wet oxidation. Using a unified set of parameters, the whole range of oxide thickness is fitted for both (100) and (111) substrates in a wide range of oxidation temperatures (800 °C–1200 °C) and pressures (1–20 atm).

Simulation of clustering and transient enhanced diffusion of boron in silicon

We have simulated the postimplantation clustering and transient enhanced diffusion (TED) in boron profiles with peak concentrations below the boron (B) solubility limit. First, we derive an analytical formula for B clustering in terms of the reaction between B atoms and supersaturated self-interstitials. Then, using this formula and taking into account the dissolution of B clusters to emit self-interstitials, a unified simulation is done with just three essential parameters for the B clusters and based on the self-interstitial cluster model and the B diffusion model. We have satisfactorily fitted B TED profiles not only for implanted B layers but also for initially active B layers. Moreover, a comparison with TED induced by P implantation is made in terms of the trapping and diffusivities of self-interstitials.

Si emission from the SiO2/Si interface during the growth of SiO2 in the HfO2/SiO2/Si structure

HfO2∕SiO2∕Si(001) structures were annealed in dry oxygen, and compositional depth profiles were measured by high-resolution Rutherford backscattering spectroscopy. Growth of the interfacial SiO2 layer and simultaneous surface accumulation of Si were observed. The observed result indicates that silicon species are emitted from the SiO2∕Si interface to release the stress induced by oxidation as was predicted by recent theoretical studies.