Silicon Sublimation Doping Source SUSI-D

• Thermal sublimation of silicon from high purity intrinsic or highly doped Si filament
• Excellent growth of thin silicon layers
• Compatible with most MBE systems
• Water-cooled electrical contacts
• Inner filament shielding with pure silicon parts
• No ceramic parts in the hot zone

Introduction

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

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.

Application

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 10 ²⁰ 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.

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.

References

Several publications base on samples grown with SUSI-D sources.  Please have a look at section References / List of Publications .

Technical Data

Dimensions

Specific data

For general information on CF mounting flanges see Flange and Gasketdimensions .

* rotary shutter possible on same flange ** maximum growth rate at 100m distance *** specify UHV length L with order

Product code: e.g. SUSI-D 63-S-L310D56 is a silicon sublimation doping source on DN63 CF-flange with shutter, in-vacuum length 310 mm and diameter 55 mm.