N. Ohtani

A RHEED study of the surface reconstructions of Si(001) during gas source MBE using disilane

Abstract The growth of Si(001) from a gas-source molecular beam epitaxy system (Si-GSMBE) using disilane (Si 2 H 6 ) was investigated using RHEED. Substrates were prepared by two commonly used etching techniques, one producing a surface oxide and the other a hydrogen passivated surface. Comparisons are made with regard to the growth mode and quality of the overlayer. Surface reconstructions which occurred during growth were studied as a function of both substrate temperature and surface coverage. All growth was found to be initiated by the formation of three-dimensional (3D) islands which coalesced at substrate temperatures above 600°C. Near 645°C, (2 × 2) and c(4 × 4) reconstructions occurred on the growing Si(001) surface. At higher temperatures, growth was found to proceed two-dimensionally. The Si(001) surface was found to have undergone a series of reconstructions which may be related to the number of hydrogen adatoms and Si dimers covering the surface.

RHEED investigation of Ge surface segregation during gas source MBE of Si Si1 - xGex heterostructures

Abstract Ge segregation during silicon gas source molecular beam epitaxy (Si-GSMBE) has been studied by in situ growth rate measurements using reflection-high-energy-electron diffraction (RHEED) intensity oscillations. Growth rate of Si on Si 1 − x Ge x gradually decreases to the Si homoepitaxial growth rate, which is attributed to Ge surface segregation at the growth interface. This segregation has been modelled using a mass balance equation and it has been found that the observed growth rate enhancement can be used as a direct measure of the Ge segregation. Using this novel in situ technique, concentration dependence of Ge segregation was studied, and it was found that the segregation decay curve is nonlinear, resulting in two segregation regimes dependent upon the Ge concentration, consistent with previous studies. Temperature dependence studies reveal that surface hydrogen, which is produced by the dissociation of Si 2 H 6 and GeH 4 on the surface during growth, may act as a growth controlling surfactant, and comparison with solid source growth results suggests that it significantly suppresses the Ge segregation, leading to a more precise control of the interface. Finally, the thermal stability of the segregated surfaces was examined. Growth interruption and annealing during Si overlayer growth on Si 1 − x Ge x resulted in a small increase in the surface Ge concentration, which may be ascribed to the outdiffusion effect of Ge from the near surface region.

Si(001) growth dynamics during Si GSMBE from disilane

Abstract Reflection high-energy electron diffraction (RHEED) intensity oscillations have been used to monitor the growth of Si(001) during gas source molecular beam epitaxy (Si-GSMBE) from disilane. Oscillations are easily obtained on well prepared surfaces on which a buffer layer has been grown. Oscillations of the specular beam in both the [010] and [110] azimuths have been measured as a function of temperature and disilane flow rate. Strong and damped oscillations were observed between 610 and 680°C in the two-dimensional growth regime. At higher temperatures growth by step propagation dominated, while at lower temperatures growth became three-dimensional (3D) and consequently oscillations were weak or absent. Oscillations of the fractional order and specular beams in the [110] azimuths indicate that growth occurs in a monolayer rather than a bilayer fashion, and the alternating intensity of sequential oscillations in the specularly reflected beam in the [110] azimuth may be indicative of the relative step density in the two orthogonal directions. Growth rates, as determined from the oscillations, are found to be independent of incident beam flux at substrate temperatures below 600°C, but become dependent at higher temperatures. An Arrhenius plot indicates an activation energy ( E A ) of 40.7 kcal/mol in the low temperature regime ( T C ) and an apparent E A dependence on disilane flux in the high temperature regime.

In situ observation of growth rate enhancement during gas source molecular beam epitaxy of Si1-xGex alloys on Si(100) surfaces

Using reflection‐high‐energy‐electron‐diffraction intensity oscillations the growth rate of Si1−xGex alloys at various compositions and different growth temperatures has been studied in situ. It was found that the growth rate shows a strong dependence on GeH4 flux at low temperatures (T<600 °C), while at high temperatures (T≳600 °C) the growth rate is nearly independent of the GeH4 flux but proportional to the incident Si2H6 beam flux. In addition to the enhanced growth rate, a lower activation energy is observed in the low temperature region when compared to Si homoepitaxy from Si2H6. This suggests that surface germanium atoms act as good sites for hydrogen removal which is known to inhibit Si growth from hydride sources at low temperatures. Above 600 °C, however, surface hydrogen is desorbed thermally and the addition of GeH4 has little effect on the growth rate.

Surface reconstructions of Si(001) observed using reflection‐high‐ energy‐electron diffraction during molecular‐beam epitaxial growth from disilane

The growth of Si(001) from a gas source molecular‐beam epitaxy system (Si‐GSMBE) using disilane (Si2H6) was investigated. The surface reconstructions occurring between 100–775 °C were studied as a function of both substrate temperature and surface coverage. Further, we report the first observation of (2×2) and c(4×4) reconstructions during growth at substrate temperatures near 645 °C using Si2H6. All growth was found to be initiated by the formation of 3D islands which coalesce at substrate temperatures above 600 °C, following which, growth proceeds in a two‐dimensional (2D) fashion. The Si surface was found to have undergone a series of reconstructions which were related to the number of hydrogen adatoms and Si dimers covering the surface.

Simulation studies of Ge surface segregation during gas source MBE growth of Si/Si1−xGex heterostructures

Abstract Simulation studies of Ge surface segregation have been made and compared to experimental results during gas source MBE (GSMBE) of Si/Si 1− x Ge x heterostructures. Segregation kinetics were examined through simulations using a two-site exchange model. The influence of surface hydrogen during GSMBE was investigated in terms of the energy parameters in the model; specifically, the kinetic barrier height, E 1 , and the Gibbs heat of segregation, Δ G seg . Results indicate that the addition of surface hydrogen to the system suppresses segregation due to a lowering of the apparent Gibbs heat of segregation, rather than increasing the kinetic barrier. Simulations were also performed to study the relative bulk and surface Ge concentrations, an important consideration when comparing in situ RHEED intensity oscillation data, which measures solely surface phenomena, with ex situ techniques such as SIMS which measure the final static concentration profile following growth.

Observation of reflection high energy electron diffraction intensity oscillations during Si molecular beam epitaxial growth from disilane

Reflection high energy electron diffraction (RHEED) intensity oscillations have been used to monitor the growth of Si(001) during gas source molecular beam epitaxy (Si‐GSMBE) from disilane. Oscillations are easily obtained on well‐prepared surfaces on which a buffer layer has been grown. Oscillations of the specular beam in the [010] azimuth have been measured as a function of temperature and disilane flow rate. Strong and damped oscillations were observed between 610 and 680 °C in the two‐dimensional growth regime. At higher temperatures, growth by step propagation dominated, while at lower temperatures growth became three‐dimensional (3D) and consequently oscillations were weak or absent. Growth rates, as determined from the oscillations, are found to be independent of incident beam flux at substrate temperatures below 600 °C, but become dependent at higher temperatures. An Arrhenius plot indicates an activation energy (EA) of 40.7 kcal/mol in the low‐temperature regime (T<600 °C) and an apparent EA dep...

Growth anisotropy observed on Si(001) surfaces during Si-GSMBE using disilane

Abstract Bilayer reflection high energy electron diffraction (RHEED) intensity oscillations were observed during growth on double-do-main Si(001)(2 × 1) + (1 × 2) substrates during silicon gas source molecular beam epitaxy (Si-GSMBE) using disilane. A transition from monolayer to bilayer mode oscillation behaviour was observed in the 〈110〉 azimuths during growth. Oscillations began with an asymmetric monolayer waveform which transformed into an apparent bilayer mode following several oscillation periods. Simultaneous measurement of RHEED intensity oscillations of the specular beam and (1 × 2) and (2 × 1) reconstruction related beams in the [010] azimuth showed that bilayer oscillations resulted from alternating surface reconstructions. The emergence of these bilayer oscillations during growth is discussed on the basis of the anisotropic growth kinetics on Si(001) surfaces which results in a preference of type-B domains. RHEED measurements made following growth on vicinal substrates reveal that diffraction features arising from type-B terraces show a marked increase in intensity, while type-A domain features decrease, confirming that the anisotropy results in a preferred growth on type-B terraces. Temperature studies reveal that at substrate temperatures above 600°C, this anisotropy disappears and RHEED intensity variations resume “normal” monolayer-mode growth behaviour.

RHEED studies of the growth of Si(001) by gas source MBE from disilane

The growth of Si(001) from a gas source molecular beam epitaxy system (Si-GSMBE) using disilane (Si2H6) was investigated using reflection high-energy electron diffraction (RHEED). The surface reconstructions occurring between 100 and 775°C were studied as a function of both substrate temperature and surface coverage. We report the first observation of (2x2) and c(4x4) reconstructions during growth at substrate temperatures near 645°C using Si2H6. All growth was found to be initiated by the formation of three-dimensional (3D) islands which coalesced at substrate temperatures above 600°C. The surface reconstruction was found to change from a disordered to an ordered (2x1)+(1x2) structure at 775°C via intermediate (2x2) and c(4x4) phases. Thereafter, growth was found to proceed in a 2D layer-by-layer fashion, as evidenced by the observation of RHEED intensity oscillations. This technique has been used, for the first time, to calibrate growth rates during Si-GSMBE. The intensity oscillations were measured as a function of both substrate temperature and incident beam flux. Strong and damped oscillations were observed between 610 and 680°C, in the two-dimensional growth regime. At higher temperatures, growth by step propagation dominated while at lower temperatures, growth became increasingly three-dimensional and consequently oscillations were weak or absent. Similarly, there was a minimum flux limit ( <0.16 SCCM), below which no oscillations were recorded.

Reflection‐high‐energy‐electron‐diffraction intensity oscillations of Si(111) during gas source molecular beam epitaxy

Reflection‐high‐energy‐electron‐diffraction (RHEED) intensity oscillations were used to monitor Si(111) growth during gas source molecular beam epitaxy using disilane. Intensity oscillations were measured as a function of both substrate temperature and disilane flow rate. Within the substrate temperature range of 490–560 °C where growth proceeded two dimensionally, well‐defined intensity oscillations corresponding to bilayer growth could be observed along the [211] and [110] azimuths. The oscillation frequencies were found to increase with increasing substrate temperature as well as disilane flow rate. At lower temperatures where no dissociative adsorption of disilane occurred, intensity oscillations were not observed. At higher temperatures, growth entered the step flow regime and the intensity oscillations were weak or absent.