Molecular Beam Epitaxial Growth Of Si Research Articles

Towards controlled molecular beam epitaxial growth of artificially stacked Si: Study of boron adsorption and surface segregation on Si(1 1 1)

Creating rotation twins periodically in a defined distance within Si layers could lead to the formation of miscellaneous Si crystal structures. This could be realized by several growth and annealing cycles on heavily B-covered Si(111) exhibiting (√3×√3)R30° surface superstructure. However, surface defects due to imperfections of the B-induced surface structure give rise to an inhomogeneous Si nucleation, which limits the structure size. Therefore, surface structure formation induced by both adsorption and surface segregation of B on Si(111) and its influence on the Si molecular beam epitaxial growth mode has been investigated using ultraviolet photoelectron spectroscopy and accompanying reflection high-energy electron diffraction. Based on these studies, conditions have been established to prevent surface defects. Furthermore, annealing samples with 0.6 monolayers (ML) B buried below several ML Si at 1080K results in a renewal of the B-induced Si surface structure without any defects. This indicates a dominance of B surface segregation over bulk diffusion, which becomes significant only above 1100K.

Auger spectroscopy thermodesorption of Sb on Si 1− xGe x layers grown on Si( [formula omitted]) substrates

Desorption of Sb from Si 1− x Ge x layers ( x=0, 0.05, 0.1, 0.2, 1) grown by molecular beam epitaxy (MBE) on Si(1 0 0) substrates is investigated using Auger electron spectroscopy thermodesorption (TD-AES). Sb desorption process on Si 1− x Ge x is well described by a first-order reaction. No extra TD-AES peaks are observed on Si 1− x Ge x compared to Si. For 1 ML of Sb coverage the TD-AES peak shifts to lower temperature when Ge bulk concentration increases. The Sb monolayer desorbs at 801, 752, 740, 715, and 706 °C for x=0, 0.05, 0.1, 0.2 and 1, respectively. We explain the non-linear decrease of the Sb desorption energy when x increases by the strong Ge surface segregation during the MBE growth of Si 1− x Ge x layers, resulting in an almost pure Ge surface even for low x.

Monitoring of Si Molecular-Beam Epitaxial Growth by an Ellipsometric Method

Ellipsometric measurements were applied to investigate Si homoepitaxial growth by molecular-beam epitaxy on a Si(111) substrate, and the changes in film thickness and optical parameters were monitored. The reflection high-energy electron diffraction (RHEED) intensity oscillation monitoring method was also employed for comparison. In the epitaxial growth mode, Ψ and Δ values showed only a small spiral variation and the analyzed optical constants were nearly the same as those of single-crystal Si. The film thickness obtained by the ellipsometric method exhibited good agreement with that obtained by RHEED oscillation as well as by a quartz crystal monitor. Thus, it was shown that the film thickness monitoring by ellipsometry is useful even for the Si/Si homoepitaxial growth with a step flow mechanism, where the RHEED oscillation monitoring method is not available.

Formation of twinned two-bilayer-high islands during initial stages of Si growth on Si(111) [formula omitted]-B

We investigated the initial stages of Si molecular-beam epitaxial growth on a Si(111) 3 × 3 -B surface using scanning tunneling microscopy. In the initial stages, Si islands one or two bilayers (BL) high are formed. The number of Si atoms contained in these islands depends on the surface B concentration. The larger the surface B concentration, the more Si atoms there are in the 2BL-high islands. Furthermore, comparison between the positions of adatoms in the 3 × 3 reconstruction on the Si islands and the substrate shows that the 2BL-high islands are twinned with the substrate, but that the BL-high islands are not twinned.

A Low Temperature Silicon‐on‐Insulator Fabrication Process Using Si MBE on Double‐Layer Porous Silicon

A series of low temperature processes incorporated in the silicon on insulator (SOI) formation using Si molecular beam epitaxial growth on porous silicon dramatically reduces the impurity ramp‐up from the porous silicon substrate and lowers the doping level in the epitaxial Si layer. A double‐layer porous silicon system with a spongy microstructure layer on top of a dendritic microstructure layer was fabricated on a moderately doped p‐type Si wafer using a two‐step anodization process, and was used as the substrate for Si epitaxy. An extended low temperature annealing in high vacuum was found to be helpful in stabilizing the porous silicon and making it less susceptible to microstructure change during the subsequent heat‐treatment. Prior to epitaxy, a Ge‐beam treatment was used to lower the temperature necessary for the Si surface cleaning. A low temperature preoxidation process stabilized the porous silicon during the subsequent high temperature oxidation. The choice of a moderately doped substrate and all these low temperature processes helped to achieve a Si epitaxial layer doping of 6 to , which is very close to meeting the requirement for direct device fabrication in the SOI without any additional doping compensation.

RHEED-AES observation of Sb surface segregation during Sb-mediated Si MBE on Si(0 0 1)

The Sb surface segregation during Si molecular beam epitaxy (MBE) on a Sb-saturated Si(0 0 1) surface has been investigated by the simultaneous usage of reflection high-energy electron diffraction and Auger electron spectroscopy (RHEED-AES). From a quantitative analysis of Sb MNN Auger electron spectra as excited by a grazing-incidence electron beam for RHEED, the Sb bulk incorporation rate, k, and the Sb surface coverage, Θ Sb, during the initial Si MBE growth were evaluated as a function of the substrate temperature T s. The temperature dependence of k, and Θ Sb suggests that the Sb surface segregation kinetics are divided into three temperature regions: (I) T s<∼500°C, (II) ∼500°C< T s<∼700°C and (III) ∼700°C< T s. k decreases with increase in T s in region I whereas k increases with T s in region II. k also shows long-time-scale decrease with growth time for the regions I and II. These findings are explained in terms of Sb diffusion through MBE Si layers and the crystallinity of the MBE Si layers.

Monte-Carlo simulation of Ge on Si(111) MBE growth: analysis of percolative structure

Reflection high energy electron diffraction (RHEED) intensity oscillations were observed during molecular beam epitaxy (MBE) growth of Ge on Si(111) surface. At 250 °C the oscillations continue up to 6 monolayers. The intensity of the reflected beam is calculated by solving the one-dimensional Schrödinger equation. Monte-Carlo simulation was used for the growth simulation in order to investigate fundamental behaviours of reflectivity change during the Stranski-Krastanov growth of Ge on the Si(111) surface at 250 °C.

Temperature dependence of boron surface segregation in Si molecular beam epitaxial growth on the Si(111) √3 × √3- B surface

Si epitaxial growth on a 1 monolayer (ML) boron-adsorbed Si(111) surface was studied. Reflection high-energy electron diffraction (RHEED) intensity oscillations revealed that when the substrate temperature was below 600°C, layer-by-layer growth with 4 ML high two-dimensional (2D) islands occuurred as long as the surface segregated boron was more than 1 3 ML. On the other hand, layer-by-layer growth with normal 2 ML high 2D islands occurred at 600°C even though the surface segregated boron was more than 1 3 ML. This result also indicates that the surface segregated boron suppresses the Si surface migration.

Migration of Ga atoms during Si molecular beam epitaxial growth on a Ga-adsorbed Si(111) surface

Ga atom migration during Si molecular beam epitaxial growth on a Ga-adsorbed Si(111)√3 × √3 surface is investigated by microprobe reflection high-energy electron diffraction. Ga atom migration in the depth direction (surface segregation) and termination of Si dangling bonds by Ga atoms cause a reduction of the production rate of two-dimensional nuclei and an enhancement of surface diffusion of Si atoms during Si-MBE. This improves the crystallinity of Si-grown films. Ga atom migration in the lateral direction is also investigated using a partially Ga-adsorbed surface. The Ga-adsorbed areas are preserved during Si-MBE at low substrate temperature ( $ ̃ 300°C), and during annealing of the sample at high substrate temperature ( $ ̃ 600°C). The adsorbed areas clearly change during Si-MBE at high substrate temperature ( $ ̃ 600°C). This indicates that Ga atoms migrate in the lateral direction on the surface when interchange between Si atoms and Ga atoms occurs in the surface segregation process of Ga atoms during Si-MBE.

Microprobe RHEED and SREM studies of Si MBE on Ga-adsorbed Si(111) surface

Molecular beam epitaxial (MBE) growth of silicon on the gallium-adsorbed Si(111)√3 × √3 surface structure is investigated by microprobe reflection high-energy electron diffraction (RHEED) and scanning reflection electron microscopy (SREM). Improvement in crystallinity is observed for a Si film grown on a Ga-adsorbed surface compared with that grown on a clean Si(111)7 × 7 surface structure. Substrate temperature dependence of the denuded zone width of two-dimensional (2D) Si nuclei is measured for various growth rates. The measurements give the activation energy of surface diffusion and the critical 2D nucleus size. Due to gallium absorption on the surface, the activation energy becomes lower and the size becomes larger than on the clean Si surface. This improves the crystallinity of the grown film. Si MBE growth on partially adsorbed surfaces is also reported. It is found that a film with good crystallinity grows selectively on the Ga-adsorbed area with a stripe shape.

Molecular beam epitaxial growth of Si on Ga-activated Si(111) surface

Molecular beam epitaxial (MBE) growth of silicon on gallium-activated Si(111) surface is investigated by microprobe reflection high-energy electron diffraction. Improvement in crystallinity is observed for a film grown on Ga adsorbed surface compared with that grown on a clean Si surface. Substrate temperature dependence of denuded zone width of two-dimensional (2D) nuclei is measured for various growth rates. The measurements give the activation energy of surface diffusion and the critical 2D nucleus size. Due to gallium adsorption on the surface, the activation energy becomes lower and the size becomes larger than those on the clean Si surface. This improves the crystallinity of the grown film. MBE growth of Si on partially activated surface is also reported. It is found that a film with good crystallinity grows selectively on Ga-adsorbed area.

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&amp;lt;600 °C) and an apparent EA dependence on disilane flux in the high-temperature regime.

Use of Low Temperature Si MBE Growth Techniques for High Performance SiGe/Si Electronics

ABSTRACTThrough the use of low temperature Si molecular beam epitaxy (MBE), we have fabricated high performance Si1−x Gex/Si heterojunction bipolar transistors (HBTs) and bipolar inversion-channel field effect transistors (BICFETs). Our growth method employs a high temperature Si-assisted desorption followed by MBE growth at a temperature only slightly in excess of the critical temperature for two-dimensional layer-by-layer growth. [1] At this temperature, segregation of Sb has previously been shown to be kinetically limited. [2] In addition, significantly more strain can be frozen into such an epitaxial layer as compared with those grown at higher temperatures.[3] Secondary ion mass spectroscopy (SIMS) data verify the abruptness of the Sb doping profiles in our device structures. High resolution x-ray diffraction (HRXRD) data are consistent with planar, coherently strained Si1−x Gex layers in our HBTs. A gain of 2690 (3210) is observed in our BICFETs at 300 K (7 K).

Self-limitation in the surface segregation of Ge atoms during Si molecular beam epitaxial growth

Self-limitation in the surface segregation of Ge atoms in the Si epitaxial overlayers arising from the surface bond geometry on Si(100) during molecular beam epitaxy has been investigated theoretically. It was found that Ge surface segregation is strongly limiting when the Ge concentration exceeds 0.01 monolayer. As a result of this self-limitation, segregation profiles of Ge in Si overlayers are found to decay nonexponentially in the growth direction with a kink in the profile around 3×1020 cm−3, which is in close agreement with the experimental observation. The kinetic barrier of the Ge surface segregation is estimated to be 1.63±0.1 eV.

B doping effect on reflection high-energy electron diffraction intensity oscillation during gas source silicon molecular beam epitaxial growth

B doping effect on reflection high-energy electron diffraction (RHEED) intensity oscillation was studied during gas source Si molecular beam epitaxial growth. During high doping of B above 1020 cm−3, no RHEED oscillation was observed on Si (100) surfaces. This is caused by surface B atoms which disturb surface migration of disilane (Si2H6) molecules. On the other hand, RHEED oscillation was observed on Si (111) √3×√3 B surfaces. At √3×√3 B surfaces, B exists in a subsurface substitutional site, directly underneath a Si adatom. This is the reason why surface migration was not disturbed by surface B atoms on Si (111) √3×√3 B surfaces.

Influence of the Si evaporation source on the incorporation of In during Si molecular-beam epitaxy growth: A comparative study of magnetically and electrostatically-focused electron-gun evaporators

The incorporation behavior of thermally evaporated In during the growth of Si(100) by molecular-beam epitaxy has been observed to depend critically on whether a magnetically focused electron gun (MFG) or electrostatically focused electron gun (EFG) evaporation source was used for Si evaporation. When a MFG source was used, the incorporation probability σ was found to be very low during growth on grounded substrates, but could be increased considerably by the application of a negative substrate bias between 100 and 2000 V. This increase was due to Si+ ion bombardment of the In surface overlayer resulting in secondary implantation during growth. For deposition on electrically grounded substrates, σ was found to be much higher during growth with the EFG source than with the MFG source. This was primarily due to bombardment of the In overlayer by energetic neutral Si atoms emitted from the EFG source.

OBSERVATION OF INTENSITY OSCILLATIONS OF RHEED DURING SILICON MOLECULAR BEAM EPITAXIAL GROWTH

The growth behavior during silicon molecular beam epitaxy on Si(lll) is observed by measuring the intensity oscillation of reflection high energy electron diffraction. The oscillation behaves as what is expected with bilayer model. The characteristics of damping and recovery of intensity oscillation during Si molecular beam epitaxy growth on Si(lll) are different from those on Si(100), but are very similar to those during GaAs molecular beam epitaxy growth.

Boron doping of Si molecular-beam epitaxy layers: A new high-temperature effusion cell

The high-temperature effusion cell (HTEC) presented in this paper is designed for materials requiring evaporation temperatures above 1000 °C. It employs a novel concept of charge heating, indirectly, through a graphite crucible, by electron bombardment combined with thermal radiation. The cell is particularly well suited to molecular-beam epitaxy (MBE) systems and we have used it as a source of B in the MBE growth and doping of Si, obtaining the resulting B concentration NB between 1015/cm3 and 5×1019/cm3. Some characteristics of Si:B layers thus fabricated are presented and it is shown that NB is: (i) an exponential function of Tcell ; (ii) inversely proportional to the growth rate; and (iii) does not depend on the substrate temperature. We conclude from (ii) and (iii) that B is totally incorporated in the crystal when coevaporated with Si. Because of the total incorporation of B and the absence of its surface segregation very abrupt doping profiles can be obtained. NB gradients as steep as one decade per 100 Å have been obtained. A comparison of the secondary ion mass spectroscopy (SIMS) data and electrical measurements results indicates that B impurities are all electrically active. The values of resistivity ρ, and the Hall mobility μ, at room temperature are both bulklike and both ρ(NB) and μ(NB) follow the standard bulk Si dependences. The MBE layers grown without intentional doping are n-type, and have an electron concentration of 5×1013/cm3 and μ=1450 cm2/V s at room temperature, like pure n-type Si.

マイクロプローブ反射電子回折法による表面研究

Si (111) surface topography changed during Si molecular beam epitaxial growth were observed by reflection electron microscope images using microprobe reflection high-energy electron diffraction (RHEED) . When RHEED intensity oscillations were observed at low substrate tempe-rature (350°C), it was found that the shape of atomic steps on the substrate was preserved during the growth and the surface topographies changed repeatedly with the period of the oscillations. When almost no oscillations were observed at higher substrate temperature (500°C), the shape of the atomic steps changed during the growth. These observations provide direct evidence that RHEED intensity oscillations occur as the result of layer-by-layer two-dimensional nucleation growth.

Observation of Si(111) surface topography changes during Si molecular beam epitaxial growth using microprobe reflection high-energy electron diffraction

Si(111) surface topography changes during Si molecular beam epitaxial growth were observed by reflection electron microscope images using microprobe reflection high-energy electron diffraction (RHEED). When RHEED intensity oscillations were observed at low substrate temperature (350 °C), it was found that the shape of atomic steps on the substrate was preserved during the growth and the surface topographies changed repeatedly with the period of the oscillations. When almost no oscillations were observed at higher substrate temperature (500 °C), the shape of the atomic steps changed during the growth. These observations provide direct evidence that RHEED intensity oscillations occur as the result of layer-by-layer two-dimensional nucleation growth.