MBE Components, GaP Decomposition Source DECO

DECO

GaP Decomposition Source

DECO 40-60-37
  • High-purity phosphorus (P2) source

  • Simple and safe operation with non-inflammable GaP source material

  • Compatible with most UHV and MBE systems

  • Various crucibles from 10 to 420 cm³

  • Precise and fast flux control with high reliability

  • Optional on-flange integrated cooling shroud and shutter

DECO 63-150-60-K GaP Decomposition Source for phosphorus on DN63CF (O.D. 4.5") flange, with 150 cm³ PBN crucible and Ga-trapping PBN cap

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The Decomposition Source DECO is an ultra-pure source for P2 based on the decomposition of GaP. It is an affordable and easily operated alternative to valved phosphorus sources.

Adapted from WEZ and PEZ effusion cells, the simple construction of the DECO with its specially designed Ga-Trapping Cap Unit maintains the advantages of easy installation, full compatibility to all MBE systems, high reliability and low costs of ownership. 
Operating the DECO is similar to usual effusion cells and does not require any complicated mechanical valves or controllers. Compared to frequently used valved phosporus crackers, the deposition of white phosphorus (P4) in the growth chamber is significantly reduced due to the direct generation of pure P2 from GaP.

Two main fields of application for the DECO have emerged: the source can be built for growth applications offering elevated evaporation rates and large crucible capacities, or optimized for special doping applications (DECO-D), e.g. phosphorus doping in Si MBE, with a reduced cell shutdown time. 

The heater of the DECO consists of a Ta wire filament supported by PBN rings, similar to standard effusion cells. This design guarantees minimum outgassing at operation temperature in combination with excellent reliability, long-term stability and extended lifetime.
PBN is invariably applied As crucible material for GaP decomposition.

DECO 40-10-22-KS cap close-up

PBN Ga-Trapping Cap
on a DECO 40-10-22-KS

Ga-Trapping Cap Unit

The DECO source features a unique Ga-Trapping Cap Unit system which provides a very pure P2 beam by decomposition of GaP. The idea underlying that unit is the sublimation of phosphorus from GaP at rather moderate temperatures at which gallium has only a low vapor pressure. By special baffle plates parasitic Ga atoms are efficiently separated from the P2 beam, thus producing a very pure P2 flux.

The Ga-Trapping Cap Unit is a simple but effective design consisting of only a small number of well-designed parts which are mounted onto the PBN crucible without additional tools or holders. For source maintenance and crucible refilling the unit is likewise easily removable.

Upon special request, a complete Ga-Trapping-Cap Unit is available as a product of its own, for retrofitting standard effusion cells. 
Inquire for further information on compatibility.

Ga-Trapping Cap Unit
Principle sketch of Ga-Trapping Cap Unit

Equilibrium pressures of GaP, Ga and phosphorus
Equilibrium pressures of the components along the metal-rich boundary of the field of solidus of a Ga-P system [R.F.C. Farrow, J.Phys. D, 7, 2436 (1974)]

 

  • operating temperatures 900-1200°C for growth applications

  • precise adjustment of P2 incorporation into GaAsP. GalnAsP, etc., compound layers

  • P2/P4 ratio about 150 : 1

  • very low parasitic Ga flux (P : Ga > 105)
    (P : Ga > 103 without Ga-Trapping Cap Unit)

  • high efficiency: about 20 g P in GaP for 100µm film thickness

  • marginal white phosphorus accumulation in MBE system

  • no additional safety facilities needed in contrast to PH3, AsH3 or valved elemental phosporus crackers

  • no bakeout necessary before opening the system

  • compatible with standard III/V solid source MBE


It is also possible to use the DECO as an As2 or Sb2 source. The diagram below shows the analog curves for a Ga-As system. Please inquire for more information.
Equilibrium pressures for a Ga-As  system
Equilibrium pressures of the components along the liquidus curve for
a Ga-As system


Application

Typical applications of the DECO are the growth of phosphide compounds (GaInP, InP) in III-V MBE as well as Phosphorus (n-type) doping in Si MBE (with DECO-D, the cell that is explicitly optimized for doping applications). All WEZ standard effusion cells are in principle also available with Ga-Trapping Cap Unit. DECO sources thus combine a most efficient, well-proven heating system with the unique principle of GaP decomposition, which leads to a very stable and reproducible P2 beam formation.

For growth applications the DECO is optimized toward a high flux rates, requiring relatively high operation temperatures and a large orifice of the cap. For these sources a maximum crucible volume is typically chosen to allow extended growth campaigns.
For phosphorus evaporation in large-scale and professional MBE systems, please see the Valved GaP Compound Source VGCS that offers larger capacities and even faster flux control.
 
Single crystal GaP source material with a purity >6N is offered together with the source.


References

There are several publications based on phosphorus growth with DECO sources.


Technical Data

Filament type Ta wire heating filament
Thermocouple W5%Re/W26%Re (type C) (type K on request)
Operating temperature 900-1200°C for growth applications
Outgassing temperature 1500°C
Bakeout temperature 250°C
Cooling integrated water cooling or separate cooling shroud
Crucibles 10-420 cm³ PBN crucibles
Options integrated water cooling (K), integrated rotary shutter (S)


Dimensions

Schematic drawing DECO Schematic drawing of the GaP Compound Source

(drawing shows
DECO 40-35-34-S)



Specific data

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

[cm³] [mm] Type [mm] / [mm] [W] / [A] Product code Product code
DECO 40 - 10 - 22 - S - C - P - LxxxD34 120 / 6 PS 30-10 PBN 10-22
DECO 40 - 10 - 22 - K S - C - P - LxxxD36 120 / 6 PS 30-10 PBN 10-22
DECO 40 - 35 - 34 - S - C - P - LxxxD35 180 / 7 PS 30-10 PBN 35-34
DECO 40 - 60 - 37 - C - P - LxxxD38 520 / 13 PS 40-19 PBN 60-37
DECO 63 - 35 - 34 - K S - C - P - LxxxD60 450 / 12 PS 40-19 PBN 35-34
DECO 63 - 125 - 54 - S - C - P - LxxxD55 800 / 14 PS 80-19 PBN 125-54
DECO 63 - 130 - 54 - S - C - P - LxxxD55 800 / 14 PS 80-19 PBN 130-54
DECO 63 - 200 - 58 - C - P - LxxxD58 1280 / 16 PS 80-19 PBN 200-58
DECO 100 - 420 - 84 - C - P - LxxxD84 800 / 12 PS 80-19 PBN 420-84
* rotary shutter possible on same flange
** PBN standard
*** specify UHV length L with order

Product code:

e.g. DECO 40-10-22-KS-C-L277D36

is a GaP compound source on DN40 CF-flange for a 10cm³ crucible (lip Ø 22), integrated water cooling shroud, shutter, type C thermocouple, in-vacuum length 277mm and diameter.