Taeko Niino

Crystallization of Amorphous Silicon with Clean Surfaces

Crystallization of amorphous Si (a-Si) on SiO2 layers in ultra-high vacuum (UHV) was examined by transmission electron microscopy (TEM) and reflection high energy electron diffraction. The SiO2 growth, the a-Si deposition on the SiO2 layer, and the annealing for the crystallization were successively carried out in a UHV molecular-beam-epitaxy chamber. It has been found that the initial nucleation and the grain growth occur at the surface of the a-Si layer, in contrast with the nucleation at the a-Si/SiO2 interface ordinarily observed in the previous studies. Cross-sectional TEM observations revealed a novel mode of crystallization which resulted in the formation of mushroom-shaped Si grains at the a-Si surface. The mechanism of the crystallization was also discussed.

Selective Epitaxial Growth of Si and Si1-xGex Films by Ultrahigh-Vacuum Chemical Vapor Deposition Using Si2H6 and GeH4

Aiming at the precise profile control of Si1-xGex, selective epitaxial growth (SEG) conditions and Si1-xGex growth were investigated by ultrahigh-vacuum chemical vapor deposition (UHV-CVD) using Si2H6 and GeH4. As long as the total amount of Si2H6 did not exceed the critical amount, Si- and Si1-xGex-SEG were both achieved independent of source gas flow rate and substrate temperature. The Ge fraction x of Si1-xGex could be decided by the flow rate ratio between Si2H6 and GeH4, independent of substrate temperature. Fast gas flow switching realized the formation of a Si(120 A)/Si1-xGex(69 A) strained layer superlattice at 587°C.

SiGe Passivation for Si MBE Regrowth

One of the problems in Si molecular beam epitaxy regrowth for device application is that the cleaning conducted prior to regrowth results in unintentional etching of the Si layer grown in the preceding process. In this study, a SiGe epitaxial layer was successfully applied as a protective coating before RCA cleaning, and a one-order-lower defect density was obtained using the SiGe protective layer without any etching of the Si underlayer. The SiGe layer was completely removed by RCA solution and no Ge was detected in the regrowth layer by Auger electron spectroscopy.

(√3×√3)B structure on a (5×5)GexSi1-x/Si (111) surface

A (√3×√3)B structure was found to be formed on a (5×5) GexSi1−x/Si (111) surface on which Ga or Sn atoms did not form any superstructures. The critical B coverage at which a (7×7) pattern disappeared and only a (√3×√3) pattern was visible increased as the fraction (x) of Ge in the substrate layer increased. A Si epitaxial overlayer was grown on the (√3×√3)B/50 A Ge0.4Si0.6/Si (111) structure at a growth temperature of 300 °C. The observed (−2/3,4/3) reflection intensity in grazing x‐ray diffraction was 50 times larger than that of a Si epitaxial layer grown on a (√3×√3)B/Si (111) structure under the same condition. On a GexSi1−x substrate, the B(√3×√3) structure is well preserved at the interface probably because of relief of the interface strain that results from the small size of the boron atom.

Activation efficiency of a B√3×√3/Si(111) structure covered with molecular beam deposited amorphous Si or SiOx

The electric activation efficiencies for a‐Si/B√3×√3/Si(111) and a‐Si/B/Si(100) systems were measured before and after annealing by Hall measurement. The efficiency of the latter was lower than that of the former before annealing. But, after annealing, it rose to the former’s level, while the former’s level remained unchanged. This difference strongly suggests that almost all boron atoms are activated at the a‐Si/Si(111) interface because of the √3×√3 structure formation. The B√3×√3 structure was also preserved at the interface between the Si(111) and the SiOx layer, which had been fabricated by the codeposition of Si and O2 molecular beams at room temperature. The electric activation efficiency for B√3×√3 at the interface between SiOx and Si(111) was lower than that between a‐Si and Si(111). An a‐Si overlayer was effective to activate the boron which formed a √3×√3 structure at the interface.

Si/SiOx/Si hole-barrier fabrication for bipolar transistors using molecular beam deposition

Abstract SiOx layers were deposited on silicon substrates in a silicon MBE system by co-deposition of silicon and oxygen. The oxygen concentration increased as the deposition temperature decreased. Oxygen pressure was 5×10−5 Torr and the silicon deposition rate was 0.2 A s−1. Composition was determined by using X-ray photoelectron spectroscopy and Auger electron spectroscopy, and the electric properties of its metal-oxide-semiconductor capacitor were measured. The growth procedure used here offers high controllability in the thickness of the deposited SiOx layer and permits subsequent silicon growth in the same silicon MBE chamber. Such Si/SiOx/Si structures, which may be applied to the hole-barrier between the base and emitter layers in bipolar transistors, were also successfully grown using this method. We fabricated this type of bipolar transistor and one order higher current gain than that of usual polysilicon emitter was obtained when SiOx layer thickness was 20 A.

Interface atomic structure of Si/SiO2/Si formed by molecular beam deposition

The interface structures of SiOx/Si and Si/SiOx/Si (x≂2) formed by molecular beam deposition (MBD) were examined by high‐resolution transmission electron microscopy. The MBD SiO2/Si buffer layer interface was atomically flat in both samples. On the other hand, the Si overlayer/MBD SiO2 layer interface had a rough configuration. In the Si/SiO2/Si sample with a 7.5‐A‐thick SiO2 buried layer, the polycrystalline Si overlayer was separated from the Si buffer layer by the thin SiO2 layer. A 2.5 A reduction of the SiO2 buried layer thickness to 5.0 A led to the epitaxial growth of the Si overlayer. In this sample, the SiO2 layer formed island morphology and epitaxial information was given to the overlayer through the exposed surface of the buffer layer. The epitaxial growth mechanism of the Si overlayer was also discussed.

Photoluminescence spectra of Si 1- x Ge x /Si quantum well structures grown by three different techniques

Photoluminescence (PL) spectra of Si1-xGex/Si multiple quantum wells have been measured at 4.2 K for the samples grown by three different techniques; conventional molecular beam epitaxy (MBE), gas-source MBE, and ultra high vacuum chemical vapor deposition (UHV-CVD). Only in the case of conventional MBE, strong emission bands appear about 80 meV below the band gap of Si1-xGex. These strong emission bands disappear after the annealing at 800° C. From the dependence of the PL intensity on the excitation power, strong emission is considered to be due to some recombination center. On the other hand, in the case of gas-source MBE and UHV-CVD, the strong emission bands are undetectable, although the band-edge PL lines of Si1-xGex are clearly observed. There is no significant change in the PL spectra after the annealing. The origin of the strong emission band is considered to be defects which are characteristic of conventional MBE.

3 × 3 -B structure on a 5 × 5 GexSi1−x/Si(111) surface and its electrical conduction

Abstract The electric activation efficiencies for an a-Si/B- 3 × 3 /Si(111) system were measured by Hall measurement. The result strongly suggests that almost all boron atoms are activated at the a-Si/Si(111) interface because of 3 × 3 structure formation. A 3 × 3 -B structure was also found to be formed on a 5 × 5 Ge x Si 1 −x/Si(111) surface on which Ga or SN atoms did not form any superstructures. A Si epitaxial overlayer can be grown on the 3 × 3 -B/50 A Ge 0.4 Si 0.6 /Si(111) structure at a growth temperature of 300°C, preserving a large fraction of the 3 × 3 -B structure due to strain compensation.

Silicon etching with oxygen molecular beam assisted by predeposited germanium

Si was etched using an O2 molecular beam according to the chemical reaction 2Si+O2→2SiO↑. The minimum etching temperature was decreased by 25 °C when a Ge layer had been deposited on a clean Si surface before etching. At 800 °C, the Ge‐coated Si surface was etched while the clean Si surface was not. The O2 partial pressure during etching was 2×10−5 Torr; the etching rate was about 80 A/min at 800 °C. Auger electron spectroscopy showed that the number of Ge atoms slightly decreased during Si etching. Ge atoms on the surface are thought to weaken Si back bonds by forming a thin Ge‐Si alloy layer on the surface. Undercutting at the SiO2 mask edge was suppressed by this Ge predeposition technique at 800 °C because the sidewall without Ge was not etched at this temperature.