Hydrogen Source:

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This source generates a flux of atomic hydrogen through high tempera-ture thermal cracking of molecular hydrogen
(or deuterium) with a tungsten filament. A flux of atomic hydrogen has been shown to convert carbon and oxygen based contaminants on substrate surfaces into volatile species which evaporate readily at relatively low temperatures.
Hence an atomic hydrogen source is very useful for in-situ low temperature cleaning of substrates. It has also been suggested that growth rate in Ga-rich deposition of GaN is considerably enhanced under a flux of atomic hydrogen.
An atomic hydrogen source represents an ideal solution for in-situ low temperature surface preparation, substrate cleaning and selective MBE. All models are equipped with water cooling system to regulate the flange temperature.

Applications Atomic flow data

* GaAs, InP, CdTe substrate cleaning
* Promotes 2D epitaxial growth on GaAs by surfactant effect.
* Improves lattice matching in InAs/InP
* Si substrate preparation for GaAs growth
* Generation of atomically flat surface of SiC
* As and P etching
* Selective epitaxial growth in patterned GaAs MBE

 

Atomic flow data:

H2 Flux SCCM	P system (400 l/s)	H/(H2+H) (2200°C)	Calculated H Flux At/s
1	4 10-6 mbar	8%	8,64 10+16
0,5	2 10-6 mbar	20%	1,08 10+17
0,2	8 10-6 mbar	40%	3,60 10+16
0,05	2 10-6 mbar	60%	1,35 10+16

 

Dissociation efficiency

The curves show the dissociation efficiency of molecular hydrogen cracked on an heated tungsten surface.
The efficiency is strongly dependent of the H2 pressure, that is the flow of molecules for a given source conductance, the flows used here will give system vacuum pressures of 10-6 to 10-5 mbar with the usual pumping speed of standard MBE systems ( pumping speed of several 100 l/min). The best working conditions should take the cracking efficiency / flow rate dependance in consideration.
The following table shows some atomic H flow data at usual working conditions.

 

Specifications

Gas: H2 only (For other gases see Gas sources documentation)
Cracking efficiency: up to 40%
Operating cracking temperature: from 2000 to 2800 °K
Gas connection: VCR (0.1 to 1 cc/minute are usual flow rates)
CBr4 (option): VCR female
Thermocouples (option): Type C (W Re 5/26)
Minimum mounting flange: 35CF / 70mm / 2.75"
Power supply: DC - See attached
In-vacuum diameter: according to system
Power supply connectors System or ADDON via universal UNIPLUG

Options On request, the source can be equipped with a thermocouple which allows regulation of the temperature in the cracking zone.
The thermocouple does not give a readout of the absolute temperature of the filament because it is too high.
It can, however, be used for calibration Also as an option the source can be furnished with a second gas entry for use with gases which do not need to be cracked. In this case the W filament can be supplied with 4 Amps so as to avoid condensation in the supply tubes for this gas.
If this source is used in a system without cryopanels (LN2 or water) it is a good idea to install a water cooled panel around the cracking zone. The minimum flange size for this option is DN60CF.