GaP Decomposition Source DECO

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

Introduction

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 .

DECO 40-10-22-KS cap close-up — PBN Ga-Trapping Cap on a DECO 40-10-22-KS

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.

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 P ₂ 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.

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 Sb ₂ 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 P ₂ 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 Gasketdimensions .

Product	CF flange	Nominal capacity	Lip diameter	Cooling	Shutter*	Thermocouple	Crucible material**	UHV dimensions*** [mm]	max. electrical [W]/[A]	Power supply product code	Crucible product code
		[cm³]	[mm]			Type		[mm] / [mm]	[W] / [A]	Product code	Product code
DECO	40 -	10 -	22 -		S -	C -	P -	LxxxD34	120 / 6	PS 30-10	CRU 10-22
DECO	40 -	10 -	22 -	K	S -	C -	P -	LxxxD36	120 / 6	PS 30-10	CRU 10-22
DECO	40 -	35 -	34 -		S -	C -	P -	LxxxD35	180 / 7	PS 30-10	CRU 35-34
DECO	40 -	60 -	37 -			C -	P -	LxxxD38	520 / 13	PS 40-19	CRU 60-37
DECO	63 -	35 -	34 -	K	S -	C -	P -	LxxxD60	450 / 12	PS 40-19	CRU 35-34
DECO	63 -	125 -	54 -		S -	C -	P -	LxxxD55	800 / 14	PS 80-19	CRU 125-54
DECO	63 -	130 -	54 -		S -	C -	P -	LxxxD55	800 / 14	PS 80-19	CRU 130-54
DECO	63 -	200 -	58 -			C -	P -	LxxxD58	1280 / 16	PS 80-19	CRU 200-58
DECO	100 -	420 -	84 -			C -	P -	LxxxD84	800 / 12	PS 80-19	CRU 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.