F. Shimura

Infrared reflection spectroscopy of the SiO2‐silicon interface

An infrared reflection technique, devised to study the structure of very thin films on substrates of high refractive index, yields an optical spectrum amplification of three orders of magnitude. With the aid of an infrared polarizer, an unanticipated peak at 1240 cm−1 in the internal reflection spectrum of thin (5–100 A) thermal SiO2 films on silicon has been identified as a longitudinal optical phonon peak. The unambiguous identification of this peak supports a similar interpretation of the 1230‐cm−1 peak in oxygen‐containing silicon first proposed by Hu in 1980.

Infrared spectroscopy of thin silicon dioxide on silicon

An infrared technique has been devised to study the structure of very thin films on substrates of high refractive index. Optical spectrum amplification of three orders of magnitude is theoretically available. A series of refractive index enhanced multiple internal reflection spectra reveals a clear thickness‐dependent structural transformation in thermal SiO2. The spectra suggest a shift in ring statistics, from smaller to larger, with increasing distance from the oxide‐silicon interface.

Noncontact minority-carrier lifetime measurement at elevated temperatures for metal-doped Czochralski silicon crystals

Minority‐carrier recombination lifetimes have been extensively measured with a noncontact laser/microwave method for metal‐doped p‐type Czochralski silicon crystals in the temperature range from room temperature to 250 °C. The contrasting behavior of the lifetime as a function of temperature is shown for Fe‐ and Cr‐doped silicon crystals. Iron doping greatly shortens the lifetime in the entire temperature range. Although Cr doping also greatly shortens at room temperature, the degradation effect completely disappears at elevated temperatures. Doping the Na, Ni, Cu, and W shows little effect on lifetime; Na doping results in a rather higher lifetime compared with that of undoped silicon. Moreover, lifetime measurement as a function of holding time at an elevated temperature clearly distinguishes uncontaminated silicon from metal‐doped silicon.

Bulk and surface components of recombination lifetime based on a two‐laser microwave reflection technique

An algorithm for separating the bulk and surface components of recombination lifetime, tailored for contactless measurement techniques with laser excitation, is presented in the paper. In order to analyze the carrier decays and subtract the surface recombination term, two lasers operating at 910 and 830 nm are applied. A separation of carrier decay resulting from the different contribution of surface and bulk components due to difference in the light absorption is observed for such a case. This separation is a function of surface recombination velocity S. An experimental verification of the analysis is presented using microwave absorption/reflection measurements.

Multiple internal reflection infrared spectroscopy of silicon surface structure and oxidation process at room temperature

It is demonstrated that the multiple internal reflection infrared (IR) spectroscopy using a germanium prism is a very powerful nondestructive diagnostic technique for the study of silicon wafer surfaces in a wide range of IR irradiation region. The technique limits neither the shape of samples nor the IR range due to the absorption by silicon itself. With this technique, it is demonstrated that; (i) dangling bonds of a silicon surface treated with HF solution and de‐ionized (DI) water are terminated mostly with H atoms, (ii) native oxide growth is enhanced by DI water rinsing, and the interstitial oxygen concentration in the silicon surface region increases during native oxide growth process, and (iii) DI water rinsing after HF etching replaces Si—F bonds with Si—H and Si—OH bonds on a silicon surface.

Separation of the bulk and surface components of recombination lifetime obtained with a single laser/microwave photoconductance technique

An algorithm for separating the bulk and surface components of recombination lifetime obtained via a contactless single laser excitation/microwave reflection decay measurement is presented. The surface recombination component of lifetime is determined by extrapolating the tail portion of the carrier decay curve to the carrier axis. Although the slope of this curve depends on both surface and bulk properties, it is shown that the y intercept depends only on the surface component of lifetime. A wide range of surface lifetimes, corresponding to surface recombination velocities from 102 to 105 cm/s, and bulk lifetimes from a few microseconds to several hundred microseconds can be measured. An experimental verification of the analysis is presented using microwave absorption/reflection measurements on silicon wafers representing a wide variety of bulk and surface lifetime components.

Outdiffusion of oxygen and carbon in Czochralski silicon

The outdiffusion behavior of oxygen and carbon in heat‐treated Czochralski (CZ) silicon has been investigated by secondary ion mass spectroscopy. The results show that oxygen diffusion is greatly retarded by oxygen precipitation and strongly support a vacancy‐dominant diffusion mechanism for oxygen in silicon. In carbon‐doped CZ silicon, the diffusion of both oxygen and carbon is greatly enhanced at 750 °C, but is significantly retarded at 1000 °C. In conjunction with the infrared absorption data, the enhanced diffusion has been attributed to the formation of fast‐diffusing O‐C complexes, while the retarded diffusion of carbon has been tentatively attributed to the formation of slow‐diffusing complexes, such as Si‐O‐C.

Hydrogen enhanced out‐diffusion of oxygen in Czochralski silicon

Out‐diffusion of oxygen in Czochralski silicon wafers annealed at 1000 and 1200 °C under a hydrogen ambient is studied with secondary ion mass spectroscopy (SIMS). The oxygen diffusivity, 1.41×102 exp(−3.1 eV/kT) cm2 s−1, obtained from fitting the oxygen SIMS profile is significantly larger than normally expected. This hydrogen enhancement effect is found at temperatures much higher than those reported (<500 °C) in literature, and is attributed to the direct interaction between in‐diffused hydrogen and interstitial oxygen atoms. Estimation of the oxygen diffusivity made from hydrogen solubility and diffusivity data is in reasonable agreement with the experimental result. It is suggested that the intrinsic/internal gettering may benefit from the enhancement effect as well as a very low surface oxygen concentration, which is also observed in this work, due to hydrogen treatment.

Temperature dependence of minority‐carrier lifetime in iron‐diffused p‐type silicon wafers

Minority‐carrier recombination lifetime was measured with a noncontact laser/microwave method for nondiffused and iron‐diffused p‐type silicon wafers in the temperature range from 28 °C to 230 °C. The lifetime increased monotonically with temperature in nondiffused silicon, while the lifetime in iron‐diffused silicon showed a broad peak around 110 °C and a depression around 170 °C. The temperature dependence of the lifetime in iron‐diffused silicon was analyzed based on Shockley–Read–Hall statistics. The origin of the lifetime temperature dependence was attributed to the dissociation of iron‐boron pairs. Our experimental data supported that an electron trap for an iron‐boron pair at Ec−0.29 eV was more effective as a recombination center than a hole trap at Ev + 0.1 eV. It was also shown that the effect of iron in concentrations as low as 1×1011 cm−3 on the lifetime can be detected with the noncontact laser/microwave method.

Highly resolved separation of carrier‐ and thermal‐wave contributions to photothermal signals from Cr‐doped silicon using rate‐window infrared radiometry

It is shown that the new photothermal technique of lock‐in rate‐window infrared radiometry is capable of completely separating out photoexcited free‐carrier‐wave and thermal‐wave contributions to the photothermal signal from an n‐type, Cr‐doped Si wafer with a simple experimental procedure, and with superior temporal resolution in the determination of the electronic lifetime and thermal transport time constant.