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SUKO 40, carbon sublimation source on DN40 (O.D.2.75") CF-flange

• Growth of Si-C and Si-Ge-C alloys
• Carbon Doping in III-V MBE with
  fast and precise flux control
• Ultra high purity pyrolytic graphite (PG) filament
• Water cooled electrical contacts
• Inner filament shielding with pure PG parts
• No ceramic parts in the hot zone
• Compatible with most MBE systems
• More than 10 years experience

The Carbon Sublimation Source - SUKO - was developed for carbon p-type doping in III-V MBE and for growth of Si-C and Si-Ge-C alloys in silicon MBE.

The SUKO provides a very clean and constant flux at a low deposition rate up to 2 Å/min.
A maximum total layer thickness of 5 µm C with one filament is reported.

The sublimation of carbon requires very high temperatures up to 2300°C. Therefore we designed the hot zone around the filament completely shielded with pyrolytic graphite (PG).

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.

MBE-Komponenten GmbH offers a carbon sublimation source which eliminates almost entirely these effects. The essential qualities of our SUKO are the consequent design avoiding extreme heating of any metal parts and the application of pyrolytic bulk graphite material with no porosity.

Main parts of the SUKO, schematic

Main parts of the SUKO are shown in the figures on the right.

The pyrolytic graphite filament is completely surrounded with PG parts (similar construction to our high temperature source HTS).
PG screws, PG washers, PG contacts, PG plates and a PG tube are used to protect and shield the very hot filament.

The very effective internal water cooling of the electrical contacts avoids overheating of these parts, allows rapid change of the flux rate and provides a short adjusting time.

Pyrolytic graphite (PG) parts of SUKO

During sublimation the temperature of the graphite filament is about 2200°C. Consequently, it should be taken into account that there is a lot of radiation impinging on the surface of the substrate during exposure.

The SUKO already includes water cooled power feedthroughs to provide ultra pure operation conditions. Additional separate cooling shrouds with integrated shutters are available for all types of SUKO. We recommend to always use the SUKO in combination with water or LN₂ cooling shroud in order to avoid overheating of stainless steel parts within the UHV chamber. For information about separate cooling shrouds see page cooling shrouds.

View onto the hot graphite filament of the SUKO

Operation and Results

p-type doping concentration for bulk and delta doping in GaAs
as a function of the electrical current in SUKO 40

The figure shows a typical example for the p-type doping concentration of bulk and delta doping in GaAs as a function of the electrical current for SUKO 40. The GaAs growth rate during doping is 2.3 Å/sec. The carrier concentration was measured by Hall measurements. For SUKO 63 a corresponding doping level is achieved for an electrical current which is about 25% higher than in the diagram on the left.

The values from this diagram may be used as a guide to start calibration of the flux rate as a function of the current. Due to different growth conditions and geometric arrangement of the chamber the exact values may slightly differ. Therefore we recommend to calibrate the doping levels in your system according to your requirements.

During operation the filament becomes thinner and thereby the flux rate very slowly increases. The lifetime of a graphite filament strongly depends on the control of the flux rate. This is especially important when operating at higher electrical current where a runaway increase of the flux rate can result in premature burning out of the graphite filament. The current should therefore be recalibrated and reduced from time to time in order to keep the flux rate constant.
During sublimation the temperature of the graphite filament is about 2200°C. Consequently, it should be taken into account that there is a lot of radiation impinging on the surface of the substrate during exposure.

Application

The SUKO is used in III-V MBE and Si MBE for p-type doping and growth of Si_1-xC_x or Si_1-x-yGe_xC_y alloys.
Our SUKO is a very popular cell and is used by customers all around the world. A list of publications, which are based on samples grown by using the carbon sublimation source 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 have all confirmed the excellent morphology of the layers.
The excellent vacuum conditions during growth are remarkable. This is due to the very effective water cooling of the metal contacts and the shielding of the hot source material by pyrolytic, zero porosity graphite. There is no direct contact of the hot graphite filament to metal.

• Carbon doping in III-V MBE
  Achievable doping levels with the SUKO:
  
  • The maximum bulk p-doping level of GaAs (measured at 300K) is 6.5x10¹⁹ 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.
    
    

  In contrast to carbon gas sources no interaction with MBE equipment or memory effect is observed while operating the SUKO.
  N-type HEMT structures have been grown after about 50 sample runs with C-doping. There was no degradation or memory effect detected. An n-type HEMT structure grown on the top of a GaAs buffer layer which was doped with C also showed no degradation of the electron mobility (typically 170,000 cm²/Vs at 77K).
  
  Excellent ohmic contacts have been prepared on highly carbon doped GaAs by evaporating CrAu onto it without any subsequent annealing. This is especially interesting for lower contact resistance in laser devices.
  
  

• Growing thin carbon layers in MBE
  The SUKO can be used for growing atomically thin layers of carbon, for example on Si substrates or for growing Si_1-xC_x alloy layers in Si MBE. However the maximum growth rate at a distance of 200 mm should not exceed 1Å/min.
  
  During long time operation the filament becomes thinner and thereby the flux rate gradually increases. The lifetime of a graphite filament strongly depends on the control of the flux rate. This is especially important when operating at flux rates close to the maximum rate of 5 Å/min, where a runaway increase of the flux rate can result in premature burning out of the graphite filament.
  Therefore the current should be recalibrated and eventually reduced from time to time to keep the flux rate constant.

Reference

1. 1.3µm GaInAsN Laserdiodes with improved High Temperature Performance
   M.Fischer, D. Gollub, A. Forchel Jpn.J.Appl.Phys. Vol.41 (2002) pp 1162-1163
2. Photolum. of tensile strained, exactly strain compensated, and compressively strained Si1-x-y Gex Cy Layers on Si
   Schmidt, O.G.; Eberl, K. Physical Review Letters (13 April 1998) vol.80, no.15, p.3396-9.
3. Near-Band-Edge Photoluminescence from Pseudomorphic Si 1-y C y /Si Quantum Well Structures
   K.Brunner, K. Eberl, W.Winter Physical Review Letters (1996) Vol 76, 2 pp 303
4. Heavy carbon doping of GaAs grown by solid source molecular-beam epitaxy
   C.Cianni, A. Fischer, C. Lange, K. Ploog and L.Tapfer, Appl.Phys. Lett. (1992) 61, 2 pp 183
5. Growth and strain compensation effects in the ternary Si 1-x-y Ge x C y alloy system
   Eberl, K.; Iyer, S.S.; Zollner, S.; Tsang, J.C.; Legoues, F.K. Appl. physics letters, (1992) 60, 24, pp. 3033
6. Synthesis of Si 1-y C y alloys by molecular-beam epitaxy
   Iyer, S.S.; Eberl, K.; Goorsky, M.S.; Legoues, F.K.; Tsang, J.C.; Cardone, F.; Appl.physics letters, (1992) 60, 3, pp. 356

Technical Data

Dimensions

Specific Data

 LAST UPDATE: NOVEMBER, 2004

© 2003 Dr. Eberl MBE-Komponenten GmbH