Inner filament shielding with pure pyrolytic graphite parts
No ceramic or metal parts in the hot zone
SUKO 40 Carbon Sublimation Source on DN40CF (O.D. 2.75") flange
The Carbon Doping Source SUKO-D is a highly optimized doping source for carbon p-type doping in III-V MBE. The SUKO-D provides a very clean and constant flux at low deposition rates of up to 2 Å/min. For growth applications a maximum total layer thickness of 5 µm C with one filament is reported, equivalent to layers of up to 1mm of highly carbon-doped GaAs.
The SUKO-D was thoroughly redesigned in collaboration with Prof. W. Wegscheider, formerly University of Regensburg. See Applications and References for details.
Sublimation of carbon from a resistance heated graphite filament requires temperatures up to 2300°C. Most other carbon doping cells are built with tantalum, molybdenum, tungsten, or ceramic parts in the hot zone. These materials will generate CO and other undesirable residual gases when used in the hottest area. The SUKO-D virtually eliminates these effects by consequently avoiding extreme heating of any metal parts and the application of pyrolytic bulk graphite material with no porosity for the complete heater assembly. The graphite filament is completely surrounded with PG parts (screws, washers, contacts, base plate and shielding tube).
Very effective internal water cooling of the electrical contacts allows for short adjusting times and rapid changes of the flux rate.
Main parts of the SUKO-D assembly
At typical operation conditions the SUKO-D filament is glowing at a temperature of about 2200°C. Especially for heat sensitive substrate materials it should be taken into account that a lot of radiation is impinging on the surface of the substrate during exposure.
The SUKO-D uses water-cooled power feedthroughs and contacts to provide ultra-pure operation conditions. An additional surrounding cooling should be provided to avoid overheating of stainless steel parts within the UHV chamber. In case the chamber has no internal (cyro-)cooling panel, separate water cooling shrouds CS are recommended for all SUKO models. Cooling shrouds can also be equipped with an integrated rotary shutter.
Close-up view of hot SUKO-D graphite filament
Operation and Results
The figure shows an excerpt of the results of C. Gerl with the redesigned SUKO-D.
Two-dimensional hole gases were prepared in a symmetric quantum well structure in the GaAs/AlGaAs heterosystem. Low temperature hole mobilities up to 1.2x106cm2/Vs at a density of 2.3x1011cm-2 were achieved on GaAs (001) substrates.
Longitudinal and Hall resistance at a bath temperature <30mK. (C. Gerl et al. "Carbon doped symmetric GaAs/AlGaAs quantum wells with hole mobilities beyond 106cm2/Vs", Appl. Phys. Lett. 2005).
During operation the SUKO-D filament becomes thinner and thereby the flux rate very slowly increases. The current should therefore be recalibrated and reduced from time to time in order to keep the flux rate constant. The lifetime of a graphite filament strongly depends on the proper control of the flux rate. This is of special importance when operating at higher currents where a runaway increase of the flux rate can result in premature burning out of the filament.
The SUKO-D is successfully used in III-V MBE for p-type doping by customers all over the world. A list of publications based on samples grown by using SUKO-D and SUKO is shown below.
In general, the electron mobility turns out to be comparable to those achieved by Be doping. Optical, REM and x-ray studies all confirmed the excellent morphology of the layers. Due to the very effective water cooling of the metal contacts and the shielding of the hot source material by pyrolytic, non-porosity graphite the vacuum conditions during operation are remarkable. In contrast to carbon gas sources no interaction with MBE equipment or memory effect is observed while operating the SUKO-D.
Carbon doping in III-V MBE with the SUKO-D
Achievable doping levels with the SUKO:
The maximum bulk p-doping level of GaAs (measured at 300K) is 6.5x10">19 cm-3, with a mobility of 29 cm²/Vs.
The maximum bulk doping level of GaAlAs (35% Al, thickness 1500 Å) (measured at 300K) is 7.5x1019 cm-3, with a mobility of 28 cm²/Vs.
The maximum delta doping level in GaAs is 2x1013 cm-2.
Minority carrier lifetime in p-doped GaAs (1.7x1019 cm-3) is 140ps.
Excellent ohmic contacts have been prepared on highly carbon doped GaAs by deposing CrAu layers without any subsequent annealing. This is particularly interesting for a lower contact resistance in laser devices.
The SUKO-D was carefully redesigned in collaboration with Prof. W. Wegscheider in the University Regensburg (Germany). In his high mobility MBE system he reproducibly achieved record hole mobilities of 1.2x106 cm2/Vs in GaAs/AlGaAs quantum wells at a carrier density of 2.3x1011cm-2 (see References below).
The new SUKO-D uses a directly heated and specially pre-conditioned high purity pyrolytic graphite filament that puts minimized heat load onto the MBE system and provides a long filament lifetime. It is particularly well suited for extremely high bulk doping levels, sharp delta doping layers, modulation doping and low resistance p-type contact formation. Various design features contribute to a very fast temperature ramping and flux switching without any memory effect in the system. The applications range from MBE growth of III/V heterostructures for basic research to electronic and optoelectronic device fabrication, as for example high power laser diode growth.
is a carbon doping source on DN40 CF-flange with in-vacuum length 300mm and diameter 36mm.