Dr. Eberl MBE-Komponenten GmbH - hydrogen atom beam source HABS

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SUSI I SUKO I DECO I DCS I HABS I HCS I GRZ I TGI I ISES I VACS I OREZ

	HABS 40 on DN40 (O.D. 2.75") CF-flange		Thermal dissociation of hydrogen in a hot tungsten capillary heated by DC current (I < 15 A, U < 15 V) Thermal energy, no high-energy particles Efficient thermal radiation shielding, low power consumption (N < 200 W) Encapsulating water cooling, low thermal load on other experimental equipment Customized aperture (optional) Sensitive temperature monitoring (optional) Efficient shutter (optional)


	The Hydrogen Atom Beam Source - HABS - is a thermal gas cracker which produces an absolutely ion-free hydrogen gas beam to avoid ion induced damage of the substrate. In comparison to the HCS the HABS provides lower gas throughput and is heated by a DC operated tungsten filament. The source was developed by Dr. Karl Georg Tschersich, Institut für Schichten und Grenzflächen, Forschungszentrum Jülich GmbH. (see http://www.fz-juelich.de/isg/hydrogen_atom_beam_source) MBE-Komponenten manufactures the HABS under the licence of the Forschungszentrum Jülich GmbH. The hydrogen atom beam source is fully characterized. 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.		Main parts of the SUKO, schematic

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 hydrogen atom flux density at a sample positioned 6 cm in front of the capillary. The flux density can reach as much as some monolayers per second.

Figure 1. Hydrogen atom flux density at a sample positioned 6 cm in front of the capillary. The parameter is the flow rate of the hydrogen feed gas, which is adjusted by a mass flow controller.

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 a sample positioned, as before, 6 cm in front of the capillary. Flux densities as low as a tenth of a monolayer per second were measured.

Figure 2. Hydrogen atom flux density at a sample positioned 6 cm in front of the capillary. The parameter is the heating power determining the capillary temperature. The flow rate of the hydrogen feed gas is varied by means of a leak valve.

The in-vacuum length of the atomic hydrogen beam source can be fabricated to the customer’s specification. The following options are available: The source can be equipped with a thermocouple attached to the radiation shielding. The temperature indicated is not that of the capillary, but it is strongly related to it and is therefore useful to independently monitor the temperature level of the source, especially during long-term operation. When the heating power is increased from standby to operation, the atom flux reaches 90 % of the stationary flux in less than 1 minute, which is fast enough to switch long-term runs on and off. Short exposures can be terminated by a shutter which attenuates the hydrogen atom flux down to the detection level of our quadrupole mass analyzer (QMA). Hydrogen atoms passing the sample and hitting the chamber walls can induce reactions detrimental to the experiment. 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 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.

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.

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]

Technical Data

Filament type	tungsten filament
Gas line	filament heated W capillary
Thermocouple	W5%Re/W26%Re (type C)
Bakeout temperature	300°C
Operating temperature	up to 2400K (about 2100 ºC)
Cooling	integrated water cooling

	Dimensions

Specific Data

Product	Mounting Flange	in-vacuum D / Length		Power	Filament Current	Filament Voltage
	[mm / inch]	[mm]	[mm]	[W]	[A]	[V]
HABS 40	DN40 CF (O.D.2.75")	33	220-400	< 200	< 15	< 15



LAST UPDATE: NOVEMBER, 2004		© 2003 Dr. Eberl MBE-Komponenten GmbH