The Silicon Sublimation Doping Source SUSI-D was developed for growing thin Si layers, short period Si/Ge superlattices and Si/SiGe heterostructures. It allows for growth of thin epitaxial Si layers with a crystal quality not otherwise possible by evaporation from effusion cell crucibles. The SUSI-D is also an optimal alternative to e-beam evaporators wherever highest purity, utmost flux stability and low flux rates are required.
All main parts of the SUSI-D assembly (filament arch, base plate, shielding tube, and top plate) are made from high purity monocrystalline silicon.
Silicon sublimation from a solid filament provides a very clean and constant Si flux at a low growth rate level. Maximum growth rates of 7 Å/min and a total layer thickness of 5-10µm of pure Si with one filament are reported. The SUSI-D is therefore ideally suited for thin Si layer growth and doping in III-V MBE.
Among the various special applications of the SUSI-D are boron and phosphorus doping in Si MBE. In these cases the filament is machined from highly doped Si bulk material. See Application
Main parts of the SUSI-D assembly
The most remarkable feature of the SUSI-D is its arch-shaped, free-standing silicon filament, which is directly heated by electrical current and exclusively surrounded by silicon shielding parts of highest purity, effectively shielding the hot filament. Extreme heating of any metal and ceramic parts is further avoided by a very effective water cooling of the electrical contacts. Absolutely no insulating ceramic parts are used in the hottest zone.
As the sublimating filament is totally surrounded by Si shielding parts the SUSI-D provides an extremely clean Si flux. In a well-designed Si MBE chamber a pressure in the 10-10 Torr range is achieved while running the SUSI-D at maximum growth rate.
High purity SUSI-D silicon parts: filament arch, base plate, shielding tube, top plate
The SUSI-D is used for Si epitaxy (especially growth of high quality thin Si layers), Si doping in III-V MBE and doping in Si MBE.
Si epitaxy and growth of thin Si layers
Compared to widely used e-beam evaporators, the SUSI-D is the superior Si source wherever highest purity, stable flux and an absolutely ion-free beam is required. Due to the low growth rate the SUSI-D is an ideal source for high quality thin Si layers, whereas it is naturally less suited to growing of thick layers. The maximum total thickness of the films that can be grown with one SUSI-D 40 filament is about 5µm (10µm with SUSI-D 63) at a substrate distance of 200mm.
Si Doping in III-V MBE
For doping in III-V MBE the typically required silicon flux is considerably lower than 0.1 Å/min. The electrical current needed for providing this flux rate is between 20 to 35 A for SUSI-D 40 and 40 to 50 A for SUSI-D 63. These values may be used as a reference point to start calibration of the doping level as a function of the current. The low power operation guarantees a long lifetime without any servicing.
Doping in Si MBE
A very special application is doping in Si MBE growth by using highly boron or phosphorus doped silicon filaments. Adjusting of the doping level is performed by changing the ratio of the flux rates between undoped silicon source and the boron / phosphorus doped silicon from the SUSI-D. There is no practical limit for very low doping levels, since the ratio between the undoped silicon source and boron doped silicon from the SUSI-D can be changed very easily and quickly over a wide range by changing the SUSI-D operation parameters.
For boron doping the source material is B doped to about 1020 cm-3. The maximum boron concentration in the epitaxial film is about 2x1019 cm-3 (during silicon sublimation some boron segregates on the surface of the silicon filament). For phosphorus doping the source material is P doped to about (3-4)x1019 cm-3.
Operation and Results
The figure on the right compares the growth rates of SUSI 40 and SUSI 63, measured as a function of the electrical current through a new silicon filament.
Over the operation time the filament becomes thinner, whereby the growth rates at a given current gradually increases. The current settings should therefore be reduced from time to time to keep the flux rate constant.
The lifetime of a silicon filament strongly depends on the control of the flux rate. This is especially important when operating at higher flux rates. A runaway increase of the flux rate can result in premature burning out of the filament.
SUSI silicon growth rate as a function of the electrical current
The next figure shows the RHEED oscillations measured during Si homoepitaxy on a (001) oriented silicon substrate at 20keV electron energy and a substrate temperature of 400°C.
From the RHEED oscillation diagram a growth rate of 1.22 monolayers per minute (1.66 Å/min) can be determined. The low decay of the oscillation amplitude indicates ultra pure growth conditions.
RHEED oscillations measured during Si homoepitaxy with a SUSI-D
is a silicon sublimation doping source on DN63 CF-flange with shutter, in-vacuum length 310 mm and diameter 55 mm.