MBE Components, Hydrogen Atom Beam Source HABS

HABS

Hydrogen Atom Beam Source

HABS 40
  • H2 dissociation typically 80-98%, depending on operation conditions

  • Atomic hydrogen flux density up to 1*1016/(cm2 s)

  • No high-energy particles and ions

  • Low power consumption (P < 200 W)

  • Integrated water cooling, low thermal load on other experimental equipment

  • Integrated shutter optional

  • Additional customized aperture and leak valve available

HABS 40 Hydrogen atom Beam Source on DN40CF (O.D. 2.75") flange

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The Hydrogen Atom Beam Source HABS is a thermal gas cracker that produces an absolutely ion-free hydrogen gas beam, thus avoiding ion induced damage to the substrate. In comparison to hydrogen sources based on electron bombardment heating the HABS is heated by a DC operated tungsten filament.
The source was developed and completely characterized by Dr. Karl G. Tschersich, Institute of Bio- and Nanosystems (formerly Institute of Thin Films and Interfaces) at Juelich Research Centre. Dr. Eberl MBE-Komponenten produces and markets the HABS under the license of Forschungszentrum Jülich GmbH (www.fz-juelich.de).

Usually the gas flow is regulated by a UHV leak valve or a mass flow controller which allow switching on and off the atom flux within several seconds. Short exposures can be terminated by a shutter (option S) which attenuates the hydrogen atom flux down to the detection level.
Hydrogen atoms passing the sample and hitting the chamber walls can induce reactions detrimental to the experiment. A customized aperture plate is available which limits the solid angle of hydrogen atom emission. Outside this angle no beam atoms could be detected by the QMA.

So far the HABS has been only operated with hydrogen. The source could perhaps be used as well as a radical beam source by decomposing other molecules at temperatures up to 2200 K. Please inquire for your special application. For production of atomic oxygen please refer to our source OBS.

A basic description of the cell design as well as some results from the characterization were published in the following papers:

[1] Formation of an atomic hydrogen beam by a hot capillary
     K.G. Tschersich and V. von Bonin; J. Appl. Phys. 84, 4065 (1998)
[2] Intensity of a source of atomic hydrogen based on a hot capillary
     K.G. Tschersich; J. Appl. Phys. 87, 2565 (2000)
[3] Design and characterization of a thermal hydrogen atom source
     K.G. Tschersich, J.P. Fleischhauer and H. Schuler;
     J. Appl. Phys. 104, 034908 (2008)

Further publications for HABS are listed in section References / List of Publications.

General properties of the HABS which are derived from the cell characterization in the above reference [3] are shown and discussed in the following.

Comparison standard type vs. HABS

Comparison of cracking efficiencies and principles of standard type hydrogen sources vs. HABS

flow rate of hydrogen vs. heating power

Figure 1. Hydrogen atom flux density at a distance of 60mm from the cell. The parameter is the flow rate of the hydrogen feed gas which is adjusted by a mass flow controller.

The intensity of the source can be controlled by the flow rate of hydrogen and the heating power. The heating power determines the temperature of the capillary. With respect to control of these operational parameters we suggest different procedures for high and low intensity runs.

In case of high intensity runs the gas feed is preferably maintained by means of a mass flow controller installed in the gas feed line. Mass flow controllers do not require hands-on control of the gas feed thus making unattended long-term runs feasible. They are appropriate for higher gas flow rates. When the flow rate has been preset the intensity can be adjusted by the heating power.

Figure 1 shows the on-axis (peak) H-atom flux density as a function of the heating power for different feeding gas flows. The sample is positioned 6cm in front of the capillary. The flux density can reach as much as some monolayers per second.

flow rate of hydrogen vs. gas flow rate

Figure 2. Hydrogen atom flux density at a distance of 60 mm from the cell. The parameter is the heating power which determines the capillary temperature.

Low intensities result from low gas flow rate and/or low heating power. Low flow rates can be adjusted by means of a leak valve installed in the gas feed line. In this configuration the heating power can be preset and the atom beam intensity varied by manipulating the leak valve. By measuring the pressure where the gas line is connected to the source, the flow rate can be evaluated as the product of this pressure and the flow conductance of the source. The conductance has been measured and is 6.1 cm3/s when the capillary is hot.

Figure 2 shows the on-axis hydrogen atom flux density at the same sample as above, again distance 6cm, as a function of the gas feed flow rate. (1 mbar l/s corresponds to 54.9 sccm). Flux densities as low as a tenth of a monolayer per second were measured.

Contour plot

Figure 3. Contour plot of the on-axis hydrogen atom flux density at a distance of 60mm from the cell.

The degree of dissociation depends on several factors, e.g. the temperature of the capillary. Measurements with a pyrometer showed a temperature of the capillary orifice ranging from 1570°C @ 80W heater power up to 2030°C @ 187W. (Due to its position at the bottom of the heater the TC of the HABS shows temperatures 100-200°C lower than the true capillary temperature.)

Figure 3 shows a contour plot of the parameter field. The flux density is plotted as a function of both the heating power and gas flow rate into a single diagram. The flux density increases with increasing gas flow rate as well as with increasing heater power as a result of the increased degree of dissociation.

 


Development and characterization of the HABS

Several years ago, in the former Institute of Surface Research and Vacuum Physics, Dr. Tschersich et.al. started to develop a hydrogen atom beam source intended to support thin film deposition by molecular beams. The primary goals were:

  - atom energy limited to thermal energy,
  - high beam intensity at low gas load of the vacuum chamber,
  - evaluation of the beam intensity.

To meet these requirements the group around Dr. Tschersich adopted the hot capillary design. Up to this point, the hot capillary had been heated by electron impact which involved high voltage. Although deflection plates were applied the group could not strictly avoid high-energy electrons accompanying the hydrogen atoms. Based on the new understanding of the source the group decided to switch from electron bombardment heating to the somewhat less powerful but technically much simpler resistive heating and developed the present version of the source. This design meets the first requirement due to pure thermal dissociation of the gas passing through the hot capillary. The second goal is approached by the beam formation due to molecular flow in the capillary. The molecular flow was reconsidered and an analytical expression found describing the angular distribution of the emitted hydrogen atoms, see reference [1] above. This work made it feasible to finally determine the intensity of the source from quadrupole mass analyzer measurements, reference [2].

The group built up a new QMA apparatus improving the differential pumping, the signal-to-noise ratio of the H1 and H2 signals and the accessible polar angle range. In a separate set of measurements this apparatus was calibrated with respect to its absolute sensitivity for hydrogen atoms and molecules. The performance of the source presented here was determined by the calibrated QMA apparatus. The calibration of the QMA apparatus was performed as follows. Using a special source without any obstacles (like radiation shields) ahead of the capillary orifice the unperturbed angular distribution of the QMA signal of hydrogen atoms and molecules was measured. The following two figures show some of their results.
Angular distribution of the H1 flux density Angular flux distribution of H2-molecules and H1-atoms
Figure 4. Angular distribution of the H1 flux density in a range ±30° from the cell axis, measured for different gas flow rates. Figure 5. Typical angular flux distribution of H2-molecules and H1-atoms after calibration of the system and data normalization.

The experimental data covering the polar angle range from -30 to +30 degrees were fitted by analytical functions, which were extrapolated to 90 degrees and integrated to give the integral signal representing the total flux of hydrogen atoms and molecules into the hemisphere ahead of the capillary orifice. Measuring these fluxes at different capillary temperatures, i.e. at different degrees of dissociation, but at constant mass flow rate, enables the QMA sensitivity for hydrogen atoms and molecules to be determined from the mass balance.
Parameters

J.Appl.Phys., Vol. 87, No. 5, 1 March 2000


Application

Typical applications for the HABS are low temperature surface cleaning, promotion of 2D growth of GaAs, enhancing of GaN growth rate or H surfactant growth in Si or GaAs MBE. Several publilcations to these applications are listed in section References / List of Publications.

Low Temperature Surface Cleaning of InP and GaAs
In MBE the cleaning of substrate surfaces is very important to reach high quality epitaxial films. GaAs or InP substrate wafers can be cleaned while being irradiated with atomic H. Carbon contamination is removed at temperatures as low as about 200°C and oxygen at temperatures of about 400°C.

Si Substrate Preparation / GaAs on Si
Atomic hydrogen is used for in-situ cleaning of Si substrates, leading to significant reductions in surface contamination. Atomic hydrogen irradiation has also been used during growth of GaAs on Si substrates to achieve lower defect densities.

Promotion of 2D Growth of GaAs
Improved properties of MBE grown GaAs is reported after atomic hydrogen enhanced growth, compared with standard grown GaAs.

Selective Epitaxial Growth in MBE and GS MBE
Another feature of atomic hydrogen enhanced MBE growth is selective epitaxial growth. This technique allows local selective deposition of MBE related materials onto a prepared substrate.


Gas Injection System

For optimized performance we provide a complete Gas Injection System for atomic hydrogen. The Gas Injection System is completely mounted and only an H2 gas bottle is needed to start operation. A simple turbo molecular pump can be used to evacuate the H2 gas line. The H2 gas line and the all metal leak valve can be baked up to 180°C.
  • fully UHV compatible
  • H2 gas cracking cell
  • all metal UHV leak valve
  • all metal valve for gas feed evacuation
  • H2 gas purifier
  • flexible UHV tube 1m max. length
  • bakeable system (180°C)
  • H2 bottle and stainless steel gas
    pressure reducing regulator - not included !!
Gas Injection System
Alternatively, the gas injection system can be equipped with a mass flow controller (MFC) instead of an all metal UHV leak valve. A mass flow controller in the gas line is preferred when the HABS is mainly used for high intensity long-term runs where mass flow controllers due to their automated operation require less attentiveness in comparison to the aforementioned gas feed setup.
There are many different mass flow controllers on the market. It is therefore recommended that the customer provides the MFC on his own.

  • fully UHV compatible
  • H2 gas cracking cell
  • all metal valve for shut-off during MFC maintenance
  • mass flow controller (MFC)
  • all metal valve for gas feed evacuation
  • H2 gas purifier
  • flexible UHV tube 1m max. length
  • bakeable system (depends on MFC type)
  • H2 bottle and stainless steel gas
    pressure reducing regulator - not included !!
Gas Injection System with MFC


References

Several publications about applications of our HABS are listed in section References / List of Publications.

Technical Data

Filament type tungsten filament
Gas line filament heated W capillary
Thermocouple W5%Re/W26%Re (type C)
Bakeout temperature 250°C
Operating temperature up to 2400K (about 2100 ºC)
Cooling integrated water cooling
Crucibles 2-200 cm³; PBN, PG, Al2O3 crucibles (other materials on request)
Options integrated shutter (S)
Accessories aperture plate
leak valve



Dimensions

Schematic drawing HABS Schematic drawing of the Hydrogen Atom Beam Source HABS

(drawing shows HABS 40-S with option S and accessories leak valve and aperture plate)



Specific data

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

[mm] / [mm] [W] / [A] Product code
HABS 40 - S - LxxxD36 200/ 15 PS 30-25
* rotary shutter possible on same flange
*** specify UHV length L with order


Product code:

e.g. HABS 40-S-L290D36

is a hydrogen atom beam source on DN40CF flange with integrated shutter; UHV-length 290 mm and in-vacuum diameter 36 mm.