Dopant Source DECO-D

10²⁰ /cm³ n-type doping in Si/SiGe MBE
P-doping through GaP decomposition
High P incorporation rate
Minimum memory effects
Easy to install and to operate
Sharp doping profiles
Precise and fast flux control

Introduction

The Phosphorus Doping Source DECO-D is an ultra pure source for P ₂ , based on the decomposition of GaP. At typical operation temperatures of about 600°C - 700°C GaP is essentially decomposed to Ga and P₂ . Additional cracking is not necessary and the accumulation of P₄ (White Phosphorus) is reduced to a minimum.

Principle sketch of Ga-Trapping Cap Unit

The figure shows an I-U characteristic with a record high peak-to-valley current ratio of a Si/SiGe Esaki diode. The data are from R. Duschl et al., Appl. Phys. Lett. 76 (2000) 879, measured at the Max-Planck-Institute for Solid State Research in Stuttgart.

On the last figure a SIMS profile of a sharp p-n junction in a Si/SiGe/Si Esaki diode decice is presented. The structure was grown at the Max-Planck-Institute for Solid State Research in Stuttgart, in the group of Dr. O. Schmidt. It shows a record high peak-to-valley current ratio.

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 \[R.F.C. Farrow, J.Phys. D, 7, 2436 (1974)\]

SIMS measurement of phosphorus delta-doping in Si-MBE with a DECO-D 40-10-22-KS; (Data courtesy of C.Tolksdorf, Physics Institute ET9, Uni-BW Munich)

The simple construction of the DECO-D, based on our WEZ and PEZ cells, and the specially designed Ga-Trapping-Cap System have the advantages of easy mounting, full compatibility to all MBE systems, high reliability and low cost. The operation of the DECO-D is comparable to usual effusion cells and does not require complicated mechanical valves and controllers. MBE-Komponenten GmbH has developed special sources based on the Ga-Trapping-Cap System. We build the DECO-D optimized for special doping applications, e.g. as phosphorus doping cell for Si MBE with a reduced cell shutdown time.

The heater of the DECO-D is designed very similarly to the WEZ-heater which consists of a Ta wire filament supported by PBN rings. Crucible material is PBN. With this design minimum outgassing at operation temperature is achieved, in combination with excellent reliability, long-term stability and extended lifetime.

Excellent operation temperature stability of ±0.1°K is achieved by an improved design of the thermocouple assembly. This assembly touches the crucible near the base. The direct touch technique and the use of double pairs of TC-alloy wires ensure a reproducible and stable measurement of the real crucible temperature.

Ga-Trapping Cap Unit

The DECO source features a unique Ga-Trapping Cap Unit system that 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 negligible 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.

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

Application

Typical applications of our DECO-D phosporus doping source are high level and sharp delta-doping with Phosphorus in Si/SiGe MBE. Small crucible charges, large double layer shutters, integrated water cooling and reduced shielding allow a fast shutdown of the cell after the doping. High 10²⁰/cm ³doping and sharp delta-doping have been reported in literature applying the DECO-D from MBE-Komponenten GmbH. Due to its small dimensions and easy operation the DECO-D is ideally suited for all types of MBE systems.

Single crystal GaP material with a purity >6N is recommended as source material.

The DECO-D has been successfully operated in many MBE laboratories worldwide. The following three figures demonstrate the unique performance of devices fabricated by means of the DECO-D.

The figure below shows a sharp P-doping profile in a Si layer with two P-delta-layers. The experiment was performed at the Universät der Bundeswehr in Munich, in the group of Prof. Eisele.

References

There are several publications based on P-doping in Si MBE with DECO-D sources.

Technical Data

Filament type	Ta wire heating filament, optimized for doping application
Thermocouple	W5%Re/W26%Re (type C); type K on request
Operating temperature	600-700°C for doping applications
Outgassing temperature	1500°C
Bakeout temperature	250°C
Cooling	integrated water cooling or separate cooling shroud
Crucibles	10-35 cm³ PBN crucibles
Options	integrated water cooling shroud ( K ), integrated 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-D	40 -	10 -	22 -		S -	C -	P -	LxxxD34	120 / 6	PS 30-10	CRU 10-22
DECO-D	40 -	10 -	22 -	K	S -	C -	P -	LxxxD36	120 / 6	PS 30-10	CRU 10-22
DECO-D	40 -	35 -	34 -		S -	C -	P -	LxxxD35	180 / 7	PS 30-10	CRU 35-34
DECO-D	40 -	60 -	37 -			C -	P -	LxxxD38	240 / 8	PS 30-10	CRU 60-37
DECO-D	63 -	35 -	34 -	K	S -	C -	P -	LxxxD60	180 / 7	PS 30-10	CRU 35-34

* rotary shutter possible on same flange ** PBN standard *** specify UHV length L with order

Product code: e.g. DECO-D 40-10-22-KS-C-L277D36 is a phosphorus doping 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.