Yukio Takano

The effect of temperature oscillations at the growth interface on crystal perfection

Abstract Dislocation-free silicon crystals are grown with the Czochralski technique under various growth conditions, including changed heater positions, pulling rates, and crystal and crucible rotation rates. The correlation between the microdefect density in crystals and temperature oscillations near the growth interface is investigated. It is found that the microdefect density decreases with higher heater position, increased pullong rate and decreased crystal rotation rate. Theseresults are in accord with the behavior of the temperature oscillation amplitude. Thus, the density of microdefects decreases as the oscillation amplitude is reduced. The relation between microdefect density and temperature oscillation amplitude is well explaines by the crystal remelt model. The microdefect density in a crystal varies for annealing at more than 1000δC immediately after growth. Thus, the reduction of microdefect density by annealing depends on the defect distribution in the as-grown crystals.

Characterization of tungsten-related deep levels in bulk silicon crystal

Tungsten‐related deep levels in a silicon crystal are investigated using microwave and conventional deep level transient spectroscopy. It is found that tungsten atoms have two diffusion constants. Most tungsten atoms stay near the surface of Si crystal, but the remainder go deeper than 1 μm from the surface. It is also found that tungsten atoms make both a hole trap of 0.41 eV and an electron trap of 0.22 eV. Device characteristics are found to be seriously degraded due to deep levels made of deeply diffused tungsten atoms.

Measurements of Compositional Change in Semi-Insulating GaAs Single Crystals by Precise Lattice Parameter Measurements

Precise lattice parameter measurements have been made for semi-insulating GaAs, and it is found that the lattice parameters vary greatly (order of 10-4 A) among GaAs wafers and in a wafer. This variation of lattice parameter is closely related to the threshold voltage of the FETs, which are made by direct ion implantation on the crystal. The change of the lattice constant is probably caused by the interstitial As atoms, and a slight change of crystal growth condition may cause its variation in the wafer. Variation of the As concentration affects the threshold voltage of MESFETs when they are made by direct ion implantation.

Dependence of Deep Level Concentration on Nonstoichiometry in MOCVD GaAs

Investigations have been made of deep levels in undoped GaAs crystals grown by metal organic chemical vapor deposition (MOCVD) on liquid encapsulated Czochralski (LEC) GaAs wafers. The dependence of electron trap level EL2 concentration and a lattice constant on nonstoichiometric composition of epitaxial GaAs crystals has been clarified by changing the mole flux ratio of arsine (AsH3) to trimethyl gallium (TMG), [AsH3]/[TMG], from ten to eighty. The dependence of EL2 concentration and a lattice constant on [AsH3]/[TMG] strongly suggests that an interstitial arsenic atom or group of interstitial arsenic atoms leads to development of EL2.

A New Method of X-Ray Diffraction Topography Using Monochromatic Divergent Beams Made by a Curved Crystal

A new method of X-ray diffraction topography is proposed, in which monochromatic divergent beams made by a curved crystal are used as the incident beam and diffracted successively from different positions of the specimen crystal by oscillating it. The method is simple in mechanism and easy in experimental procedure. As a remarkable merit, a topograph of fairly high resolution and contrast is obtained from the whole region of the specimen even if it is heavily distorted. Some reflection as well as transmission topographs from a LiF crystal and a Si wafer locally coated with oxide film are shown and compared with those taken by the Lang method. A modified use is also described in which the specimen and photographic plate are rotated step-wise and by which information on the relative misorientation of various parts of the specimen is given.

Nondestruetive Characterization of Deep Levels in Semi-Insulating GaAs Wafers Using Microwave Impedance Measurement

A new technique called microwave DLTS is developed and proven to be a powerful method for the characterization of deep levels in semi-insulating GaAs wafers. Because this new technique is essentially nondestructive and contactless, it is quite suitable for the direct characterization of GaAs wafers for IC fabrication processes. Using microwave DLTS, density distributions of EL2 in undoped semi-insulating GaAs wafers were measured nondestructively.

Dislocation Generation due to Stress Induced by Oxidation in Si Grooves

Stress induced by thermally grown SiO2 in grooves formed on a Si surfaceis experimentally and theoretically discussed. It is shown that dislocations are generated from the fine groove when the Si crystal is annealed at 1000°C in an O2 gas. However, dislocations are hardly generated when the Si crystal is annealed at 1100°C. Growing SiO2 film on the bottom of a groove exerts compressive stress, because Si volume expands about 2.3 times when it changes into SiO2 and because the grown SiO2 cannot horizontally expand. The stress is calculated by using Maxwell viscoelastic theory to explain the experimental results.

A Modification for X-Ray Diffraction Topography of Oscillating Method Using Monochromatic Divergent Beams

A new technique for the oscillation method using monochromatic divergent beams (the O.M.D. method) is proposed for obtaining an undistorted image of X-ray diffraction topograph. In taking topographs over a wide region of the specimen, the specimen and the photographic plate are oscillated around the same rotating axis in the usual O.M.D. method, while they are rotated around two parallel rotating axes in the present method. The rotation speeds of the specimen and the photographic plate are the same in both methods. An application of the new technique to a single crystal of NaCl is presented.

New characterization method of deep levels in semi‐insulating GaAs wafers using microwave impedance measurement

A new contactless characterization technique based on microwave impedance measurement was developed and applied to the nondestructive measurement of deep level concentration in semi‐insulating GaAs crystals. Using this new technique, the nonuniform distribution of EL2 was measured in GaAs wafers used for IC fabrication. Taking advantage of the nondestructive and contactless feature of this new technique, both the EL2 concentration and the threshold voltage (Vth) of metal semiconductor field effect transistors were measured in the same sample. By analyzing the relation of these two quantities, it is found that the activation efficiency of implanted Si atoms is lower in the EL2 richer region, and vice versa. A possible mechanism to explain the experimental data is discussed in relation with crystal nonstoichiometry.

Development of EXAFS spectrometer and structural characterization of amorphous silicon

The design features and performance of an EXAFS (Extended X-ray Absorption Fine Structure) spectrometer are presented. This device allows soft X-ray absorption experiments for structural studies of thin films composed of light elements such as silicon. The spectrometer is equipped with a totally reflecting mirror to reduce the intensity of higher harmonics to about 1% of that of the fundamental. The application of the spectrometer to amorphous silicon has shown that an increment in the deposition rate gives rise to a larger magnitude of the first peak in the radial distribution, indicating the formation of a more ordered phase during deposition.