Control of the hydrogen bonding in reactively sputtered amorphous silicon

A reactively sputtered photoconductive amorphous silicon film having a controlled monohydride and polyhydride bond density is produced by applying a DC voltage bias to the film's substrate during deposition.

FIELD OF THE INVENTION 
The present invention relates to amorphous silicon and more particularly to 
reactively sputtered hydrogenated amorphous silicon in which the hydrogen 
is bonded selectively in monohydride or polyhydride configurations. 
BACKGROUND OF THE INVENTION 
The two principal methods of producing hydrogenated amorphous silicon are 
the glow discharge decomposition of silane and reactive sputtering in a 
mixture of argon and hydrogen. Hydrogenated amorphous silicon films 
produced by these techniques are known to have differences in optical and 
electronic properties. Those of the art attribute some these distinctions 
to the differences in the bonding configuration of hydrogen into the 
amorphous silicon network. For example, when hydrogen bonds in polyhydride 
configurations, it introduces electronic defects in the gap of the 
semiconductor and also influences the optical gap of the material by 
modifying the valence and possibly the conduction bands. In view of these 
observations, previous efforts have attempted to identify the deposition 
parameters which controls the bonding of hydrogen into the amorphous 
network. 
The silicon-hydrogen bonding characteristics in photoconductive amorphous 
silicon is conventionally explained in relation to the local environments 
of the incorporated hydrogen. Those of the art believe that the hydrogen 
can be present either in the monohydride SiH form, isolated dihydride 
SiH.sub.2 units, coupled dihydride SiH.sub.2 units, higher order 
polyhydrides such as SiH.sub.3, or combinations thereof. For purposes of 
the present application, the term polyhydride will be used to collectively 
define all silicon-hydrogen bonding forms other than the monohydride, SiH. 
Typically, infrared absorbtion or vibration spectra data is used to 
analyze the bonding characteristics of the silicon film. An analysis of 
the silicon-hydrogen vibrational spectra indicates the density of silicon 
to hydrogen bonding of the monohydride and polyhydride forms. 
Referring momentarily to the drawings, an illustration of an infrared 
absorption spectra is shown in FIG. 2. The three absorption bands are 
identified as the stretching (.about.2000cm.sup.-1), bending 
(.about.900cm.sup.-1) and wagging (.about.600cm.sup.-1) modes of SiH and 
SiHx groupings. The presence of polyhydrides in the film is evidenced by 
the bending modes, whose origin are forces opposing changes in the angles 
between Si--H bonds within SiH.sub.2 and SiH.sub.3 groups. Contrary to the 
stretching and wagging vibrations, the bending modes are absent when the 
hydrogen is bonded only in monohydride configurations. For purposes of the 
present invention, the bonding mode evidenced at 2000 cm.sup.-1 is the 
stretching vibration of monohydride groups and the bonding mode evidenced 
at 2100 cm.sup.-1 is the stretching vibration of the polyhydride groups. 
The integrated absorption under these two modes provides a measure of the 
density of monohydride and polyhydride bonds respectively. The present 
invention teaches deposition techniques for producing reactively sputtered 
photoconductive amorphous silicon films having controlled monohydride and 
polyhydride bond densities. A positive or negative DC voltage bias applied 
to the substrates during the sputter deposition increases or decreases the 
relative density of monohydride to polyhydride bonds in the resultant 
film. 
The ability to control the relative density of the monohydride and 
polyhydride bonds afforded by the present invention, further provides the 
ability to control the optical properties of the resultant film and more 
particularly the optical band gap. This ability is generally considered an 
important advantage in optimizing the photovoltaic characteristics of 
amorphous silicon hydride. 
The art has demonstrated the ability to control the relative density of 
monohydride and polyhydride bonds in amorphous silicon produced by glow 
discharge decomposition of silane. For example, Brodsky et al in a 
technical--publication entitled "Infrared & Raman spectra of the 
silicon-hydrogen bonds in amorphous silicon prepared by glow discharge and 
sputtering" teach the basic glow discharge deposition parameters which 
control the bonding of hydrogen. Films produced at low silane pressure 
(.about.0.1 monohydrides (The stretching vibration is centered around 2000 
cm.sup.-1). Films produced at low temperatures (Ts.about.25.degree. C.) 
and higher silane pressure (.about.1 Torr) contain primarily polyhydrides 
(The stretching vibration is centered around 2100 cm.sup.-1). 
Predetermined control of the bonding of hydrogen in sputtered hydrogenated 
amorphous silicon has not been previously achieved in the art. Indeed 
Brodsky et al, in Physical Review, B. 16, No. 8, 10/77 and Freeman et al, 
in Physical Review B. 18, No. 8, 10/15/78, teach that for sputter 
deposited amorphous silicon the stretching vibration is always a doublet, 
which together with the existence of the bending mode at 900 cm.sup.-1 
teach that the sputtered films always contain a mixture of monohydride and 
polyhydride bonding configurations. One technique for altering the 
densities of the monohydride and polyhydride bonds is set forth by F. R. 
Jeffrey et al in J. of Applied Physics, Vol. 50 p. 7034 (1979) wherein a 
change in the relative density of monohydride to dihydride bonding was 
noted for films sputtered at different power levels. 
SUMMARY OF THE INVENTION 
An amorphous silicon film is reactively sputtered onto a DC biased 
substrate to control the density of monohydride and polyhydride bonds 
formed in the resultant film. A positive bias applied to the substrate 
decreases the density of polyhydride bonds and increases the density of 
monohydride bonds.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention teaches a method for reactively sputtering amorphous 
silicon films having a controlled monohydride and polyhydride bonding 
density. To illustrate this invention, FIG. 1 shows a sputtering apparatus 
adapted to provide voltage biasing on a substrate to be coated. Referring 
to the drawing, a vacuum chamber 10 is capable of being evacuated by 
pumping means 12, preferably to a pressure of about 10.sup.-7 torr. 
Substrate(s) 14 is secured to platform 16 which serves simultaneously as a 
substrate support and a anode electrode. In general, substrate 14 may 
comprise any material capable of withstanding temperatures of about 
400.degree. C. For electronic applications however, substrate 14 
preferably comprises a material having a coating surface devoid of defects 
of the order of about a micron in size and free of materials which may 
migrate into the silicon to the detriment of its semiconductor properties. 
In many embodiments, substrate 14 may comprise a dielectric material such 
as glass which would electrically insulate the coating surface from the 
bias platform. For such embodiments, substrate 14 is typically coated with 
a conductive layer such as a metallic thin film, then secured to platform 
16 by conductive securing means, here shown at 18,18. The substrates used 
for infrared optical measurements are single crystal Si wafers polished on 
both sides. Chamber 10 is then closed and evacuated to a pressure 
preferably about or below 10.sup.-7 torr. 
A power supply 20, capable of supplying positive and negative DC voltage, 
is coupled to supply a DC bias voltage to platform 16 and substrate(s) 14. 
Platform 16 is electrically isolated from surrounding support structures 
which are typically held at ground potential. Power supply 20 is capable 
of supplying .+-.100 volts to the substrate, measured relative to ground 
potential. The evacuated chamber is backfilled with an inert gas such as 
argon to bring the steady state pressure of chamber 10 into a range 
suitable for sputtering, typically between about 5 m torr and about 50 m 
torr, preferably about 15 m torr. Alternatively, the argon backfilling may 
be interrupted and the chamber re-evacuated to high vacuum and 
subsequently backfilled to sputtering pressures. This process, repeatable 
to advantage, assures the removal of residuary atmosphere. 
A partial pressure of hydrogen ranging from about 1.times.10.sup.-4 torr to 
about 5.times.10.sup.-3 torr with a preferable upper range of about 
1.times.10.sup.-3 torr is fed into the vacuum chamber. As known to those 
of the art, the hydrogen constitutes the reactive sputtering gas which 
will be decomposed to its atomic state and be incorporated into the 
resultant amorphous silicon film. In many device embodiments, it is 
desirable to provide an extrinsic doping material to alter the 
semiconductor characteristics of the silicon. Partial pressures of doping 
gases such as phosphine or diborane, ranging from about 5.times.10.sup.-6 
torr to about 5.times.10.sup.-5 torr for example, may be added prior to or 
during the sputter deposition to provide a number of device 
configurations, each well known in the art. 
Throughout the deposition, the substrate(s) is maintained at a temperature 
ranging from 200.degree. C. to about 400.degree. C. Heater element 17, 
here shown as below and proximate to platform 16 and substrate(s) 14, may 
comprise any of a number of heating means capable of sustaining the 
aforedescribed requisite substrate temperature. Sputter deposition is then 
initiated by conventional capactive coupling of radio frequency, 
hereinafter RF, power to the partial pressures of gases proximate to an 
anode and cathode electrode. The cathode comprises a polysilicon target, 
5" in diameter, here shown at 22. The anode comprises the assembly of 
platform 16, substrate(s) 14 and securing means 18. As discussed earlier, 
the anode assembly is maintained at a predetermined potential relative to 
ground, typically ranging from about +100 volts to about -100 volts. The 
RF (voltage power) supplied to the cathode assembly typically ranges from 
100 watts to about 500 watts for an anode to cathode spacing of 4.5 
centimeters, resulting in a deposition rate of between 2 .ANG./second and 
4 .ANG./ second. During the sputter deposition process, a partial pressure 
of hydrogen is dissociated within the sputtering plasma. Atomic hydrogen 
is incorporated into the silicon film in varying forms. Using conventional 
sputtering techniques, wherein the substrates are maintained at ground 
potential or allowed to self-bias from the charging effect of the plasma, 
the incorporated hydrogen is bonded to the silicon in both the monohydride 
and polyhydride form as taught by the prior art. Referring now to FIG. 2, 
trace 40, the conventional deposition techniques result in a film having 
vibrational spectra for silicon-hydrogen bonding which demonstrate both 
monohydride characteristic peaks at 2000 cm.sup.-1 and polyhydride 
characteristic peaks at 2100 cm.sup.-1. This attribution is also supported 
by the existence of the bending mode at 900 cm.sup.-1. Those of the art 
believe that the presence of both bonding forms is detrimental to the 
electronic properties of the semiconductor. In contrast, the present 
invention provides control of the density of the monohydride and 
polyhydride bonds. In one embodiment, a positive DC voltage of about 
eighty volts is applied to the substrates during the deposition process. 
This DC biasing of the substrate during the reactive sputter deposition 
influences the formation of the silicon-hydrogen bonds. The silicon film 
deposited onto the positively biased substrate showed a substantial 
decrease in the density of polyhydride bonding. The bonding vibration 
spectra, shown at trace 42, evidences a substantial decrease in the 
amplitude of the polyhydride characteristic peaks at 2100 cm.sup.-1 and 
900 cm.sup.-1 as compared to the conventionally sputtered silicon film 
spectra of trace 40. The positively biased film also evidences a 
corresponding increase in the amplitude of the monohydride characteristic 
peak at 2000 cm.sup.-1. A comparison of trace 40 and trace 42 reveals that 
the aforedescribed positive bias deposition technique virtually eliminates 
the presence of polyhydride bonding in the resultant film and 
correspondingly increases the density of monohydride bonding. A further 
analysis of the hydrogen content of the film indicates the total hydrogen 
content of the film film. In a further embodiment, a negative voltage bias 
of about -100 volts is applied to the substrate(s) during the reactive 
sputtering. An analysis of the resultant film reveals a decrease in the 
density of monohydride bonds. A bonding vibration spectra, shown as trace 
44, evidences a substantial decrease in the monohydride characteristic 
peak at 2000 cm.sup.-1 and a corresponding increase for the polyhydride 
characteristic peaks at 2100 cm.sup.-1, as compared to the control sample 
having a substrate maintained at ground bias, shown as trace 40. 
Referring to FIG. 3, there is shown a graphic plot of optical band gap 
versus hydrogen content for a plurality DC biased amorphous silicon films, 
trace 50, shown in comparison to electrically grounded films, trace 52. 
With the exception of the applied bias, the preparation conditions were 
substantially identical for the two series of films. These conditions 
included a cathode to anode spacing of about 4.5 cm, a power density of 
about 1.6 watt/cm.sup.2, a substrate temperature of about 275.degree. C., 
an argon partial pressure of about 15 m Torr and a hydrogen partial 
pressure of about 1.times.10.sup.-4 Torr to about 3.times.10.sup.-3 Torr. 
The substrates of the devices resulting in trace 52 were electrically 
grounded whereas a positive DC bias of about 50 volts was applied to the 
substrates of the devices resulting in trace 50. The positive bias 
deposition, which virtually eliminates the polyhydride bonding and 
increases the monohydride bonding of silicon to hydrogen, functions to 
increase the optical band gap of the resultant film. Accordingly, an 
equivalent optical band gap material is produced containing substantially 
less hydrogen. As presently understood, the bonding of hydrogen to silicon 
compensates an otherwise (without hydrogen) dangling bond which would have 
given rise to a band gap state. It is presently believed that when the 
hydrogen bonds into the network in Sr--H monohydride forms it increases 
the optical gap substantially more strongly than when it bonds in 
SiH.sub.x polyhydride forms. Thus, by biasing the substrates positvely 
during the growth of amorphous silicon hydride films results in a small 
optical gap material with good electronic properties.