Hisashi Katahama

Si doping and MBE growth of GaAs on tilted (111)A substrates

Abstract Molecular beam epitaxy of GaAs doped with Si on a vicinal surface of (111)A, (211)A and (311)A have been examined. The variations of the impurity concentrations are dependent on the growth conditions and are affected by the angle of the substrates. The impurity concentrations are compared with a model calculation which is based on microscopic surface structures and kinematical surface reactions. Growth modes are monitored by reflection high-energy electron diffraction intensity oscillations. A doping mechanism of Si atoms into GaAs films is proposed.

Incorporation Behavior of Si Atoms in the Molecular Beam Epitaxial Growth of GaAs on Misoriented (111)A Substrates

The molecular beam epitaxial growth of Si-doped GaAs on slightly (1–5°) misoriented (111)A substrates was examined. Occupation sites of Si were greatly affected by the tilted angle, the flux ratio and the substrate temperature. In the growth on an exactly (111)A-oriented substrate at low flux ratio, substantially all Si atoms acted as acceptors. With increase of the tilted angle and/or the flux ratio and with decrease of the substrate temperature, the number of donor site Si atoms increased. A doping mechanism including preferential decomposition of As4 molecules at the steps was proposed.

Effect of Heavy Boron Doping on Oxygen Precipitation in Czochralski Silicon Substrates of Epitaxial Wafers

The effect of heavy boron doping on oxygen precipitation in Czochralski silicon substrates of epitaxial wafers has been studied with transmission electron microscopy observations and a preferential etching method. Prolonged isothermal annealing between 700 and 1000°C for up to 700 h was performed on p/p+ (5–20 mΩ cm) and p/p− (10 Ω cm) wafers. It was found that, with an increase in boron concentration, the precipitate density increased, and the precipitates could nucleate at a higher temperature. The growth process of platelet precipitates was also investigated and compared with the process in polished p− wafers. It was confirmed that precipitate growth rate in p/p+ wafers was higher than that in p− wafers, and precipitate nucleation in p/p− wafers was delayed compared with p/p+ wafers. The precipitate growth in p/p+ wafers was determined to be reaction‐limited, which differed from the diffusion‐limited growth in p− wafers.

Dependence of Mechanical Strength of Czochralski Silicon Wafers on the Temperature of Oxygen Precipitation Annealing

Dependence of mechanical strength of Czochralski silicon (CZ-Si) wafers on the temperature of oxygen precipitation annealing has been studied both experimentally and theoretically. Thermal stress was applied to CZ-Si wafers after oxygen precipitation annealing at 1100°C or 1000°C after preannealing at 800°C. The warpages and the densities of slip dislocations in the wafers annealed at 1100°C are much higher than those in the wafers annealed at 1000°C, nevertheless each precipitate density is almost equal. Transmission electron microscopy observations of the 1100°C samples showed that both platelet and polyhedral precipitates were generated, but very few of these precipitates actually generated punched-out dislocations. In contrast, in the 1000°C samples, only platelet precipitates were generated, many of which generated punched-out dislocations. Further studies showed that slip dislocations formed only from platelets which did not punch out dislocations, i.e., slip dislocations formed only in the 1100°C samples. The mechanism of the generation of slip dislocation by oxide precipitates is discussed with calculated results of the system energy change due to slip dislocation generation.

Characteristics of Heavily Si-Doped GaAs grown on (111)A Oriented Substrate by Molecular Beam Epitaxy as Compared with (100) growth

Electrical and Optical characteristics of heavily Si-doped GaAs (111)A films grown by molecular beam epitaxy (MBE) were studied in comparison with those of (100) and (111)B films in relation to the occupation sites of dopant atoms determined by X-ray quasi-forbidden reflection (XFR) measurements. In the case of (100) growth, most of the doped Si atoms occupy Ga sites and the carrier saturation above [Si]~6 × 1018 cm-3 is dominantly caused by the formation of SiGa complexes, not by compensation due to SiAs acceptors. On the contrary, Si atoms in the (111)A films occupy As sites and act as acceptors up to [Si]~6 × 1019 cm-3. At higher doping, they start to occupy Ga sites resulting in the saturation of carrier concentration.

Effect of Oxide Precipitate Sizes on the Mechanical Strength of Czochralski Silicon Wafers

The effect of oxide precipitate sizes on the mechanical strength of Czochralski silicon (CZ-Si) wafers has been studied with emphasis on the mechanism of slip dislocation generation by oxide precipitates. Thermal stresses, which are larger than those set up in wafers during actual device processes, were applied to two-step annealed wafers. It was determined by X-ray topography and transmission electron microscopy observations that both platelet and polyhedral precipitates can generate slip dislocations when their size is larger than approximately 200 nm. With further experiments, it is concluded that the precipitates cannot generate slip dislocations during actual device processing when the precipitate size is smaller than 200 nm, and this conclusion is independent of the precipitate density. The stress concentration of compressive thermal stresses applied to oxide precipitates should be the cause of slip dislocation generation. Three critical stress curves of slip dislocation generation were obtained for the wafers, in which the platelet sizes are approximately 70 nm, 200 nm and from 330 nm to 490 nm.

Internal gettering for Ni contamination in Czochralski silicon wafers

Internal gettering (IG) behavior for Ni contamination in Czochralski silicon wafers was studied. The wafers were initially contaminated with Ni, and then isothermally annealed between 800 and 1000°C for up to 16 h. The density of Ni‐silicides at the polished surfaces and the density of oxide precipitates at the cleaved surfaces were obtained by the preferential etching method with Wright etchant. It was confirmed that (i) with an increase in annealing time, Ni‐silicide density decreased and became less than the detection limit, and (ii) oxide precipitates were not detected in some of the wafers, in which Ni‐silicides were not detected. The density of oxide precipitates with their size less than the detection limit was obtained after additional annealing at 1000°C for 16 h, and the size of precipitates was obtained by calculations with assuming diffusion‐limited growth. The critical size of oxide precipitates for the IG effect was defined as the size above which the Ni‐silicides were not detected. It was concluded that (i) the critical size decreased with an increase in precipitate density and (ii) the critical size became less than the detection limit of approximately 200 nm by the etching method, when the precipitate density was higher than about .

Pinning Effect on Punched-Out Dislocations in Silicon Wafers Investigated Using Indentation Method

The mechanical strength of silicon wafers was investigated using the indentation method. Sizes of rosette patterns L, generated during annealing at 900° C for 30 min, were measured for as-grown floating zone (FZ) and Czochralski (CZ) silicon wafers. It was found that (1) the rosette size L of FZ wafers was larger than that of CZ wafers and (2) L decreases in proportion to the -2/3 power of interstitial oxygen concentration ([Oi]) for CZ wafers ([ Oi]=3.7–15.5×1017 atoms/cm3). From the experimental results, it was concluded that the wafers, in which [Oi] was larger than approximately 2×1017 atoms/cm3, had the ability to pin on dislocation movements. The pinning effect on dislocations by oxide precipitates or stacking faults was also investigated using the indentation method. It was found that precipitates, of which the density was approximately 1×109 /cm3 and the average size was lower than approximately 500 nm, did not affect the rosette sizes L. On the other hand, stacking faults, of which the density was approximately 1×107 /cm3 and the average size was approximately 50 µ m, have shown the pinning effect.

Lattice relaxation of GaAs islands grown on Si(100) substrate

Initial stage of lattice relaxation of GaAs islands grown on Si(100) substrate were investigated by combination of reflection high-energy electron diffraction and molecular beam epitaxy. In addition to the lattice constants in horizontal direction (a∥) to the substrate surface, we first measured directly those in vertical one (a⊥). At the beginning of the growth, the rapid increase of a∥ and the larger Poisson’s ratios than that of bulk were observed. Atomic bond flexibility and extension induced by surface effects might cause this rapid increase of a∥ and large Poisson’s ratios.

Mechanical properties of 300 mm wafers

Abstract A simulator to calculate slip length during thermal processes on the basis of dislocation kinetics was developed. Using the simulator, slip length during the thermal process of 300 mm wafers was calculated. From the results of the calculations it was clarified that: (1) control of the ramping rate above 1000°C is important to avoid slip generation; (2) when the wafer spacing is 9.5 or 6.75 mm, slip dislocation will generate during the process at a ramping rate greater than 1.8 or 1.3°C/min, respectively; and (3) slip length decreased using a ring-like support instead of a conventional four-point support. The generation of punched-out dislocations and slip in a heavily boron-doped p+ epitaxial substrate was also examined by indentation tests and thermal stress tests. It was found that the generation of dislocations could be reduced compared with that of p/p− wafers by using p+ epitaxial substrates.