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The following options are available:
1. 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 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.
2. A customized aperture is available which limits the solid angle of hydrogen atom emission. Outside this
angle no beam atoms could be detected by the QMA.
Although tested so far with hydrogen only, the source can be used as well as a radical beam source by decomposing other
molecules at temperatures up to 2200 K.
Development and characterization of the source
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. 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 the paper by K.G. Tschersich and V. von Bonin, "Formation of an atomic hydrogen beam by
a hot capillary", J. Appl. Phys. 84, 4065 (1998). This work made it feasible to finally determine
the intensity of the source from quadrupole mass analyzer measurements, see the paper by K.G. Tschersich,
"Intensity of a source of atomic hydrogen based on a hot capillary", J. Appl. Phys. 87, 2565 (2000).
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.Recently 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 we measured the unperturbed angular distribution of the
QMA signal of hydrogen atoms and molecules. An example is shown in the following figure.
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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.
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.
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.
[T. Sugaya, et al., Jpn. J. Appl. Phys. 30 (1991) L402]
Si Substrate Preparation / GaAs on Si
Atomic hydrogen is also 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.
[H. Shimomura, et al., Jpn. J. Appl. Phys. 31 (1992) L628]
[Y. Okada, et al., Jpn. J. Appl. Phys. 32 (1993) L1556]
[H. Shimomura, et al., Jpn. J. Appl. Phys. 32 (1993) 632]
Promotion of 2D Growth of GaAs
Improved properties of MBE grown GaAs is reported after atomic hydrogen enhanced growth, compared to standard grown GaAs.
[H. Shimomura, et al., Jpn. J. Appl. Phys. 32 (1993) L632]
[Y.J. Chun, et al., Jpn. J. Appl. Phys. 32 (1993) L1085]
Selective Epitaxial Growth in MBE and GS MBE
Another feature of atomic hydrogen enhanced MBE growth is selective epitaxial growth. This technique allows a local selective deposition of MBE related materials onto a prepared substrate.
[T. Sugaya, et al., Jpn. J. Appl. Phys. 31 (1992) L731]
[N. Kuroda, et al., Jpn. J. Appl. Phys. 32 (1993) L1627]
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HABS Data Sheet ( 325 kB pdf-file )