Measurement of Depositing and Bombarding Species Involved in the Plasma
Production of Amorphous Silicon and Silicon/Germanium Solar Cells

Quarterly Technical Report

Dec. 1, 2003 -- Feb. 29, 2004

DOE Interagency Agreement De-AC36-02G010244

Principal investigator: Alan Gallagher

Contributors: Peter Horvath, Damir Kujundcik

DOE Program Officer: Ms. Lizana Pierce

NREL Technical monitor: Dr. Bolko Von Roedern

The technical approach to this project is to utilize mass spectrometry
to measure radical and ion species that arrive at the substrate of a
plasma-enhanced-chemical-vapor-deposition (PECVD) reactor. Threshold
ionization mass spectrometry (TIMS) is utilized for the radical
detection. A small-scale reactor mimics those used to produce
hydrogenated amorphous (a-Si:H) and microcrystalline (μc-Si) silicon and
silicon/germanium (a-Si:Ge:H) solar cells. Radio frequency (RF) and high
frequency (HF) discharges will normally be studied, and the reactor
utilizes a similar electrode gap (2-3 cm), substrate temperature (20-250
°C), gas pressures (0.1-5 Torr), gas mixtures and discharge power
density to that used in an industrial reactor.

As noted in previous reports, the vacuum, gas handling and
silane-discharge apparatus is constructed and operating properly. This
includes the pyrolyser that breaks up the toxic or flammable effluents
(silane and/or germane) into hydrogen gas and solid. The quadrupole mass
spectrometer (QMS) is also working well, using our home designed and
built control electronics. This includes the ability to operate the QMS
at different RF frequencies to allow accurate H atom and Si~x~H~n~
radical detection. The discharge is working as designed, with the
discharge RF voltage under computer control. The differential pumping
through three chambers, and finally into the mass spectrometer chamber,
is working as designed.

A major effort during the last quarter has been testing and improvement
of the threshold- ionization electron gun and ion-collection optics.
This has been done first by utilizing argon gas, since it does not coat
the chamber with a-Si:H or pyrolyse on the hot cathode of the electron
gun. Several difficulties were encountered in trying to obtain the
designed electron beam behavior. In particular, the beam was severely
deflected from the design path. The electron beam is magnetically
confined, but the confining field is only \~ 20 Gauss to avoid a
fringing field in the discharge region that could influence the
discharge. Under these "weak confinement" conditions other sources of
magnetic field, although relatively minor, can distort the electron
path. A common solution with electron beams is to utilize molybdenum
electron-gun elements, but many of internal components other than the
electron gun elements are also nearby. The three-chamber differential
pumping arrangement required for the experiment requires several large
metal vacuum components near the electron gun, and it was most
convenient to make these of stainless steel since this is most easily
machined and welded. The apparatus parts near the electron gun were made
of type 304 stainless steel, which is normally non magnetic. However,
machining strains the stainless steel surfaces, and can thereby produce
magnetization. Having discovered problems with the electron beam, we
evaluated the magnetic properties of these components, and decreased
their magnetism with a combination of annealing and "degausing", where
large alternating fields are used to randomize the dipoles within each
part. Another change was to increase the confining field, to about 40
Gauss, thereby decreasing the spiraling radius of the electrons.
Finally, the cathode-anode region was more fully shielded from nearby
grounded surfaces, to minimize off-axis electric fields that increase
spiraling. The combination of these mitigation efforts has produced the
designed electron beam behavior; in particular 1-5 μA passing about 7 mm
below the orifice to the discharge chamber.

Another accomplishment during this quarter is programming of a computer
to control the experiment and take data. This involves controlling the
discharge power and timing, and recording several types of data. The
measurements include the discharge pressure, RF voltage, stable-gas
constituents, ionizer electron beam current and energy, the a-Si:H film
growth rate, the mass selected for the mass spectrometer, and the ion
signal from the mass spectrometer.

A major change in staffing of the project occurred in December. Dr.
Karoly Rozsa returned to his laboratory in Budapest, Hungary, after
about 5 years of off-and--on work on the NREL and now DOE contracts. He
was typically here for about half of the year, returning to guide work
at his laboratory in between work in Boulder. We will miss his guidance
and council, gained through several decades of research into the physics
and applications of gas discharges. The current program is now staffed
by two Ph.D. candidate students, and of course the PI. These students
are Peter Horvath, a Physics Department student of Budapest University,
and Damir Kujundzic, a EE student at the University of Colorado.