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

• High-mobility GaAs p-type doping
• 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 12 years experience

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 a low deposition rate up to 2 Å/min. A maximum total layer thickness of 5 µm C with one filament is reported, by what layers of up to 1mm highly carbon-doped GaAs are achievable.

The SUKO-D was carefully redesigned in collaboration with Prof. W. Wegscheider, University Regensburg, Germany. Please see doping applications and references for details.

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 doping source which eliminates almost entirely these effects. The essential qualities of our SUKO-D 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-D, schematic

Main parts of the SUKO-D 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.

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-D 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 equipment page CS.

View onto the hot graphite filament of the SUKO-D

Operation and Results

Longitudinal and Hall resistance at a bath temperature <30 mK. The mobility is 1.2x10⁶cm²/Vs at a density of 2.3x10¹¹cm^-2.

The figure shows an excerpt of the results of C. Gerl et al. with the redesigned SUKO-D (Carbon doped symmetric GaAs/AlGaAs quantum wells with hole mobilities beyond 10⁶ cm²/Vs, published in Appl. Phys. Lett. 2005).

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.

During operation the SUKO-D 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-D is used in III-V MBE for p-type doping

Our SUKO-D 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 SUKO-D and SUKO, respectively, 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 operation 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 with the SUKO-D
  
  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-D.
  
  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.
  
  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 achieves 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 quality of ultrahigh mobility electron heterostructures is not affected by the employment of the new Carbon doping cell SUKO-D. The new SUKO-D uses a directly heated and specially pre-conditioned high purity pyrolytic Graphite filament which provides minimized heat load in the MBE system with long filament lifetime. It is easy to install and compatible to all commonly used MBE systems. It is particularly well suited for extremely high bulk doping levels, sharp delta doping layers, modulation doping and low resistance p-type contact formation. The SUKO-D allows 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.
  
  

References

1. Carbon doped symmetric GaAs/AlGaAs quantum wells with hole mobilities beyond 10⁶cm²/Vs
   C. Gerl, S. Schmult, H.-P. Tranitz, C.Mitzkus, W. Wegscheider Appl. Phys. Letters (2005) 86 25,2105; 86 20,2105
2. 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
3. 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.
4. 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
5. 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
6. 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
7. 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

For general information on CF mounting flanges see "Flange and Gasket Dimensions".

System manufacturer:    MBE-Komponenten, Omicron and other manufactures not separately listed
				[mm] / [mm]		[W] / [A]	[Å/min]	Product Code
		SUKO-D	40 -	LxxxD36		600 / 75	0.5	PS 15-100

 LAST UPDATE: MARCH, 2006

© 2003 Dr. Eberl MBE-Komponenten GmbH