High power diamond traveling wave amplifier

An FET device especially useful in common gate amplifier circuits used as amplifiers of microwave and millimeter wave signals. The device has a diamond film layer constituting the device's channel. Device geometry is selected so that, in a common gate amplifier circuit, device input and output are impedance matched to avoid phase cancellation between input and output. In one embodiment a boron nitride layer is disposed heteroepitaxially with the diamond channel and separating the channel from the gate. In another embodiment plural such devices are yoked together integrally source to drain in such a manner that charge carriers entering the second and subsequent stages do so at maximum velocity without the need to accelerate from zero or low velocity. The resulting device has a higher power handling capacity, upper frequency range, and dynamic range.

Reference is made to my co-pending application "Yoked, Orthogonally 
Distributed Traveling Wave Amplifier", filed Aug. 27, 1987, Ser. No. 
089,892, the disclosure of which is incorporated herein by reference. 
Reference is also made to co-pending applications, "Yoked Orthogonally 
Distributed Equal Reactance Non-Coplanar Traveling Wave Amplifier" of 
Yoder and Morgan, filed Aug. 27, 1987, Ser. No. 095,487; and the 
application of Robert Markunas et al., filed Sept. 24, 1987, Ser. No. 
100,477Reference is also made to my co-pending application Ser. No. 
091,133, filed Aug. 31, 1987. 
BACKGROUND OF THE INVENTION 
The invention pertains to unipolar amplifier devices especially useful as 
amplifiers of signals in the microwave to millimeter wave range. 
Field Effect Transistors (FET's) are commonly used as amplifiers of high 
frequency signals, most commonly in common source amplifier circuits. The 
common source configuration, however, has the inherent drawback that its 
input and output portions are theoretically impedance mismatched. For this 
reason, phase cancellation between the input and output occurs unless the 
length of the device in the direction of wave propagation (orthogonal to 
current carrier transit) is made a small fraction of a wavelength. This 
limitation on size inherently limits the power handling capacity of any 
such amplifier. The common gate amplifier configuration, however, can 
theoretically have impedance matched input and output portions, and is 
thus an excellent candidate for use as an amplifier of high frequency 
signals if provided with FET's that are internally impedence matched. 
Another limitation on amplifer power capacity is the inherent breakdown 
voltage of semiconductors, made worse by the peculiar property of some 
semiconductors to form weak cross bonds at the semiconductor's edge or 
interface with other non-lattice matched materials. These weakly held 
interface or surface electrons are much more easily raised from the 
valence to the conduction band, and in FET's cause breakdown at a much 
lower voltage than the inherent breakdown voltage of the bulk 
semiconductor of which the FET's channels are made. Another limitation on 
power capacity is the low thermal conductivity of many semiconductor 
materials. 
A limitation on the dynamic range of devices of this kind is the inherent 
breakdown voltage between the FET gate and the FET channel; for isolated 
gate FET's, the voltage is the breakdown potential of the isolation 
material. 
High frequency semiconductor amplifiers commonly use compounds of elements 
of column III and column V of the periodic table, such as gallium arsenide 
and indium phosphide, because of their extremely high maximum steady state 
drift velocities. Unfortunately, however, this inherent advantage is 
offset somewhat because the relationship between electric field potential 
and steady state drift velocity in both gallium arsenide and indium 
phosphide becomes negative shortly after the velocity peak. Thus 
semiconductor devices employing either gallium arsenide or indium 
phosphide require extremely highly doped regions along the path of carrier 
movement to insure that these carriers entering subsequent gain stages 
experience no electric field potential sufficiently higher than that 
corresponding to the maximum drift velocity. This requires slowing, then 
accelerating, carriers four times per rf cycle, partially offseting the 
value of the extremely high maximum drift velocity in these compounds, 
wasting considerable energy, and creating much excess heat. Moreover, 
because of the negative differential drift velocity versus electric field 
slope of these compounds, charge carriers at higher potential may, in 
fact, be moving slower than carriers at a lower potential, resulting in 
carrier bunching and the formation of localized dipole domains within the 
FET channel. To prevent such dipole domains from degrading device 
operation, the highly doped regions must be doped sufficiently to rapidly 
collect these domains and quench them. 
As with all semiconductor devices, increasing tensile strength can simplify 
fabrication and make for a much more rugged and marketable product. 
Additionally, eliminating device parts, and thus the fabrication steps 
necessary to manufacture these parts, makes such devices simplier and 
easier to manufacture, and more reliable in the field because the 
probability of a fatal fabrication error increases with the number of 
steps necessary to fabricate a device. 
Objects of the Invention 
Accordingly, an object of the invention is to provide an FET device 
especially useful in common gate amplifier circuits used to amplify 
millimeter wave and microwave signals. 
Another object of the invention is to provide such a device having a higher 
frequency range by forming the device's channel of semiconductor material 
having a relatively high maximum steady state drift velocity. 
Another object of the invention is to extend yet further the device's upper 
frequency range, and the device's dynamic range, by forming the channel of 
a semiconductor material having no appreciable negative slope in its 
characteristic curve of electric field intensity versus steady state drift 
velocity. 
Another object of the invention is to extend the device's power handling 
capacity and dynamic range by forming the device's channel of a material 
having a high breakdown potential. 
Another object of the invention is to further extend the device's power 
handling capacity by forming a gate isolation layer heteroepitaxially with 
the channel to insure that the channel's breakdown potential occurs by 
bulk rather by skin (surface), breakdown. 
Another object of the invention is to further increase the device's power 
handling capacity, and reliability of operation, by making the channel of 
a material having a relatively small dielectric constant and high thermal 
conductivity. 
Another object of the invention is to further extend the device's dynamic 
range by forming the gate isolation layer of a material having a high band 
gap so as to increase the potential at which unwanted gate current occurs. 
Another object of the invention is to make the device more rugged by 
increasing the tensile strength of one or more of the device's 
semiconductor layers. 
Another object of the invention is to provide an integrated configuration 
wherein charge carriers enter the second and subsequent gain stages at 
high velocity and without the requirement to accelerate from zero velocity 
as in other semiconductors. 
Another object of the invention is to make the fabrication of the device's 
gate simpler and more reliable by forming the isolation layer of a 
material highly resistant to etching, so as to act as a practical etch 
stop. 
In accordance with these and other objects made apparent hereinafter, the 
invention is an FET device especially useful in a common gate 
configuration as an amplifier of millimeter wave and microwave signals. 
The device has a channel made of a thin epitaxial film layer which, 
because of its high thermal conductivity, high breakdown voltage, and 
higher tensile strength, provides clearly superior performance. Most 
importantly, however, semiconductor diamond has a high maximum drift 
velocity but unlike such semiconductors as gallium arsenide and indium 
phosphide, the maximum drift velocity in diamond semiconductor remains 
virtually constant and near its peak value for all measured electric field 
intensities. Thus unlike FET devices using gallium arsenide or indium 
phosphide, diamond channel integrally yoked FETs need not have highly 
doped regions and their attendant disadvantages. In a preferred 
embodiment, the FET device is made in a single monolith and of a geometry 
making the device's input portion (source and gate) and output portion 
(drain and gate) mirror images of one another. With this geometry and the 
exceptional uniformity of material between the input and output portions 
obtainable by forming the device in a single monolith, the input and 
output impedance of the device can be made virtually identical. This 
insures that the phase velocity of signal propagation in the input and 
output portions are also virtually identical, eliminating phase 
cancellation between stages. This enables one to build the device long in 
the direction of wave propagation making for a much larger device with the 
attendant increase in power capacity. In one embodiment, the diamond 
channel and the device gate are separated by a layer of boron nitride 
heteroepitaxial with the diamond channel. The high band gap of boron 
nitride increases the breakdown potential between gate and channel at 
which unwanted gate current occurs, thus increasing the device's dynamic 
range. The diamond channel and the boron nitride layer, being 
heteroepitaxial and lattice-matched, have no uncompleted or weakly 
completed covalent bonds or trapped charges at the diamond channel's edge, 
insuring that channel breakdown occurs in bulk rather than by skin 
(surface) breakdown. In another embodiment, plural amplifier devices are 
yoked together integrally in a single monolith, source to drain, adjacent 
sources and drains of adjacent stages being made unitary, thus eliminating 
device parts and shortening the device in the direction of carrier flow. 
This results in yet further incremental increases in the device's 
frequency response (primarily by injecting charge carriers at high 
velocity into second and subsequent stages), as well as simplifying the 
device's fabrication by eliminating parts. 
The invention is more fully understood from the following detailed 
description of the preferred embodiments, it being understood, however, 
that the invention is capable of extended application beyond the precise 
details of the preferred embodiments. Changes and modification can be made 
that do not affect the spirit of the invention nor exceed its scope, as 
expressed in the appended claims. Accordingly, the invention is described 
with particular reference to the accompanying drawings, wherein:

Detailed Description of the Invention As is seen in FIG. 1, material such 
as gallium arsenide and indium phosphide are much better suited to high 
frequency applications than is silicon because the maximum drift 
velocities of charge carriers in gallium arsenide and indium phosphide are 
much higher than silicon. Unfortunately, unlike silicon, gallium arsenide 
and indium phosphide's maximum velocities do not remain steady over a wide 
span of electric field intensity, but rather decline sharply after 
reaching peak values, at about 3.5.times.(10.sup.3) kV/cm for gallium 
arsenide and about 10.sup.4 kV/cm. for indium phosphide. Diamond, however, 
combines the best advantages of both silicon and the column III-V 
compounds, having a very high maximum drift velocity which remains 
relatively constant for all measured values of electric field intensity, 
with no negative fall-off with increasing field intensity. 
With special reference to FIG. 2, a two-stage device according to the 
invention is shown. (Although two stages are illustrated, it is plain that 
the advantages of the invention are achieved by a one stage device or a 
device of three or more stages.) Device 1 has a unitary layer 2 of doped 
semiconductor diamond epitaxially grown on underlying diamond layer 6. In 
the preferred embodiments, the diamond is n-doped, although diamond can be 
advantageously n or p-doped. Overlaying diamond layer 2, are metallic 
microstrip terminations 3 and 5. Microstrip 3 (along with region 30 of 
layer 2) corresponds to the source of the first stage of device 1, and 5 
(along with region 50) to the drain of device l's second stage. On the 
opposite side of layer 2 from microstrips 3, 5 in insulating diamond 
substrate layer 6. are cut (etched) gate trenches 7, 8 which, when filled 
with metal, form the gates of the two FET stages. Between gate 9 and gate 
10, is an internally formed (deposited nichrome) resistor 13, which serves 
to set the correct bias point for gate 10 of the second FET stage. 
Underlaying the metal of gates 9, 10 is dielectric 11 (preferentially 
deposited boron nitride) which, in turn, is underlain by deposited metal 
substrate 12, completing the device, 
In operation, an input signal enters device 1 at the point on microstrip 3 
denominated "IN", and propagates along microstrip 3 parallel to the "x" 
direction illustrated in FIG. 2. The electric field, being transverse to 
the direction of wave propagation, extends along the "y" and "z" 
directions causing modulation of charge carrier flow through channels 2. 
Gates 9 and 10 divide the monolith into the two separate FET gain stages, 
creating a field node at region 4 equidistant between first stage source 
3, 30 and second stage drain 5, 50. Although materially identical to, and 
unitary with, the remainder of layer 2, region 4 acts as a virtual drain 
for the first FET stage and virtual source for the second. Each FET stage 
of device 1 can be viewed as a pair of transmission lines, the source and 
gate being the input line of the stage, the drain and gate being the 
output line. Because of the mirror image symmetry between input and 
output, and the high uniformity of material between corresponding points 
of the input and output, each stage's input and output are impedence 
matched. (Although the difference in resistivity between virtual 
source-drain 4, and heavily doped source 30 and drain 50 will offset this 
match somewhat, the loss of performance will not be great.) This virtually 
eliminates phase cancellation between each stage's input and output, and 
between stages. Because virtual source-drain 4 is internal to device 1, it 
needs no metalization like 3 and 5, simplifying fabrication of device 1. 
It is to be appreciated that charge carriers enter the second gain stage at 
maximum velocity. Unlike GaAs or InP materials, the electric field need 
not be reduced between gain stages. The average velocity within the second 
and subsequent gain stages is thus nearly twice the value if accelerated 
from zero velocity. 
With particular reference to FIG. 3, a device 1' is shown. Device 1' is 
like device 1, but with an additional layer 14 of boron nitride disposed 
between diamond layer 2 and semi-insulating diamond layer 6. Crystalline 
boron nitride layer 14 is heteroepitaxially grown on diamond layer 6 and 
diamond layer 2 is heteroepitaxially grown on boron nitride layer 14. The 
lattice-matched diamond to boron nitride interface precludes trapped 
charges, which in turn precludes low voltage breakdown in channel 
interface regions 2. The high bandgap of layer 14 allows much higher input 
signals to exist between the sources and gates, and the drains and gates, 
across layer 14 before unwanted gate current arises, thus expanding the 
device's dynamic range. Layer 14 also enables gate trenches 7 and 8 to be 
formed more easily and reliably by merely etching in diamond layer 6 until 
the differentially high etchability of boron nitride halts etching at the 
boron nitride surface. 
Diamond film technology is relatively new. So as to permit a better 
understanding of the invention, the following description is provided of 
an apparatus (chemical reactor) and method for producing the kind of 
diamond films that are discussed above. This method and apparatus are not 
part of the invention, and are the subject of a separate patent 
application by Robert Markunas et al. of Research Triangle Institute, 
Serial No. 100,477, filed Sept. 24, 1987. 
The reactor design and operating criteria discussed hereinafter are based 
on the principle of a remote region of activation of a gas or mixture of 
gases. The activated gas (gases) then plays several roles leading to the 
deposition of the semiconducting film. Because of the central importance 
of "remote region of activation" to the present invention, this 
terminology is first defined referring to FIG. 4. 
FIG. 4 shows schematically a section of a flow tube 112. A feed gas stream 
(single gas, vapor, or mixture) enters at input inlet 110. In the region 
of activation 114, the feed gas has its chemical reactivity increased. 
Chemical reactivity of the feed gas can be increased in many ways. For 
example, one or more components of the feed gas may be ionized; one or 
more components of the feed gas may be dissociated into more reactive 
species, such as converting water vapor into hydrogen and oxygen; or the 
internal energy of the feed gas may be increased without ionization. This 
can be accomplished by many methods. Some of these methods can be internal 
to the flow tube. A sample of these internal methods might include 
heaters, or catalytic surfaces, and electron or ion bombardment sources. 
Some methods could be external to the flow tube. A sample of these 
external methods might include broad range optical sources (with an 
appropriately transparent tube), microwave or radio frequency power 
sources, or simple heaters. Whatever the feed gas(es), the combined means 
for activation, or the reactive species formed, in the activation region 
114, energy is coupled into one or more gases, and that energy can 
contribute to subsequent chemical reactions. 
In experimental studies performed to date, an external radio frequency coil 
114a, shown in FIG. 5, concentric with the flow tube was used to activate 
the gas stream. 
Referring to FIG. 4, the concept of a "remote" region of activation in the 
present remote plasma enhanced chemical vapor deposition (RPECVD) 
technique will be described. By remote region of activation is meant two 
things: (1) the substrate is not located in a remote region of activation; 
(2) in any remote region of activation, only gas(es) from the inlet of 
that region of activation is(are) present, other gas(es) that may be 
present in other regions of the apparatus can not reach a remote region of 
activation by diffusion or other processes that would allow such gas(es) 
to enter through the exit of a region of activation. To ensure this 
requires both a suitable reactor design and a proper selection of 
operating parameters. In the flow system, shown in FIGS. 4 and 5, the 
design of the physical separation of the various regions of the reactor, 
coupled with the flow velocity of the gas stream (which of course depends 
on the selection of process parameters) in those regions, determines 
whether back-diffusion of gases into a region of activation can occur. 
As shown in FIG. 4, the system has a feed gas inlet 110 through which a 
feed gas is entered into a plasma tube 112. In an activation region 114, 
the chemical reactivity of the feed gas is increased to produce reactive 
species of the feed gas which pass downstream of the exit plane 114b in 
the downward direction shown in FIG. 4. Between the exit plane 114b and 
the carrier gas inlet 118, the feed gas reactive species are filtered such 
that only the desired specie reaches the gas inlet 118 where it mixes and 
interacts with a carrier gas introduced via the inlet 118a in a mixing and 
interaction region 120. 
In a working embodiment used to date, a radio frequency coil 114a 
concentric with the flow tube 112 has been used to create a "plasma" (glow 
discharge) of the feed gas in the activation region 114. Working examples 
have used either a pure noble gas plasma feed, as discussed hereinafter, 
or noble gas mixtures with hydrogen. The plasma environment in the 
activation region 114 contains many species, even with a simple feed gas 
like helium. In fact, the feed gas reactive species produced in the 
activation region 114 include ions, electrons, and a host of excited 
species all with different composite lifetimes which are influenced by 
various factors. The flow through the activation region 114 carries the 
species downstream towards the carrier gas inlet 118 and a substrate 122 
mounted in a deposition region 124 downstream of the inlet 118. The 
distance that the various species can travel before they are annihilated 
will depend on their composite lifetimes and the flow velocity. The flow 
velocity of the feed and carrier gases are controlled so as to control the 
relative abundance of selected or the reacted species at a given distance 
downstream of the region of activation, such as at the mixing and 
interaction region 120 and at the carrier gas inlet 118. Thus, by 
controlling the gas flow rates, and by requiring the reactive species of 
the feed gas to pass from the exit plane to the mixing and interaction 
region 120, a spatial filtering region 116 is provided downstream of the 
exit plane 114b in which undesired reactor species are annihilated and 
only the selected of the reactive species are passed downstream towards 
the mixing and interaction region 120. 
Spatial filtering as above described involves two aspects. First, some 
physical distance between the activation region which excites the feed gas 
and the region where the carrier gas is introduced must exist. And second, 
the lifetimes of the desired reactive species must be substantially longer 
than the lifetimes of those species not desired. Once these criteria are 
established, spatial filtering occurs because the pumping velocity of the 
reactive species determines how far downstream from the activation region 
various species will travel before they decay or are annihilated. For 
example, in a He discharge, electron impact excites He atoms into a host 
of excited electronic states. These states include 2.sup.3 P, 2.sup.1 P, 
3.sup.3 S, 3.sup.1 S, 3.sup.1 P, 3.sup.3 P, 3.sup.1 D, and 3.sup.3 D. All 
these states all have energies greater than the metastable 2.sup.3 S 
state. However, these states quickly decay to the ground state or one of 
the lower metastable states (e.g. 2.sup.1 S or 2.sup.3 S) exponentially 
with a characteristic decay time. This decay time is less than 10.sup.-7 
s. As the species are pumped from the discharge regions, the metastables 
and ground state He atoms are dominant Of the two metastable states, the 
2.sup.1 S state has the shorter decay time or effective lifetime 
2.times.10.sup.-8 sec vs 6.times.10.sup.-3 sec for the 2.sup.3 S state. 
Thus, the host of highly excited He states in the plasma region have been 
spatially filtered to produce a desired flux of metastable 2.sup.3 S He 
atoms at the entrance to the gas mixing region. The unwanted excited 
species are completely attenuated exponentially along the length of the 
spatial filter compared to a factor of 3-150 attenuation (for plug 
velocities of 10-50 m/sec and spatial filter length of 0.3m) for the 
desirable metastable specie. For this particular spatial filter design and 
system operating parameters, all activated gas feed species having 
effective lifetimes less than 4.times.10.sup.-3 sec will be completely 
annihilated in the spatial filter. 
The spatial filtering region 116 also acts as a backstreaming isolation 
region which in conjunction with the selected gas flow rates prevents 
injected carrier gas from the inlet 118 from back diffusing to the exit 
plane of 114b of the activation region 114. 
The flux of activated noble gas species (and by activated it is 
specifically meant in the sense of internal energy and not kinetic energy) 
partially dissociates and activates (in the gas phase) the carrier gas. 
The flux of the activated noble gas species completely reacts and 
crystallographically orders the activated carrier species onto the 
substrate 122 and results in the growth of an epitaxial or heteroepitaxial 
semiconductor on the substrate. The flux of activated spatially filtered 
noble gas species enhances surface reactivity and reactant surface 
mobility in the growth of a single crystal epitaxial layer. The technique 
of the invention can be used in a low pressure process where the mean free 
path between the exit plane 114b of the activation region 114 and the 
substrate 122 is such that no gas phase collisions occur. 
Reference is made to the schematic of the remote plasma enhanced chemical 
vapor deposition reactor, shown in FIG. 4. This representation of a RPECVD 
reactor primarily consists of a plasma tube 112, the region of activation 
114, including an activation source such as an rf coil 114a, a gas 
dispersal ring 118, and the substrate susceptor 128. Additional components 
include an electron gun 130, phosphorous screen 132, and a manipulator arm 
134 used together to perform reflection high energy electron diffraction 
(RHEED) characterizations of the substrate 122 and the epitaxial 
semiconductor film deposited thereon. The plasma tube 112 used consists of 
a 7.6 cm inside diameter pyrex tube. The plasma is driven by a 13.56-MHz 
rf generator with matching network. The substrates 122 are clamped to a 
graphite susceptor 128 heated internally by a tungsten halogen lamp (not 
shown). Substrate temperatures are calibrated using thermocouples (not 
shown) attached to the surface of a silicon substrate. Gasses are 
introduced through two separate gas feeds, the plasma feed gas inlet 112 
and the carrier gas feed 118a to the gas dispersal ring 118, which serves 
as the carrier gas feed inlet. The plug velocity of He or other noble gas 
through the 7.6 cm plasma tube 112 is high enough to prevent 
back-diffusion of CH.sub.4 carrier gases. The plug velocity used is 200 
m/s for diamond deposition. Also shown is an outlet 136 for high vacuum 
pumping via a turbomolecular pump (not shown), an outlet 138 for pumping 
the process gasses using a roots blower (not shown) together with a direct 
drive mechanical pump (not shown). Typical pressures are less than 
5.times.10.sup.-10 Torr minimum base pressure when the process gasses are 
not flowing and 1-300 mTorr during epitaxial growth of a semiconductor 
layer The vacuum intake to the roots blower is ballasted with a constant 
gas load to prevent antibackstreaming of oil vapors. 
Epitaxial growth of diamond may be accomplished by flowing a Noble gas (He, 
Ar, or Xe) through the plasma tube, and methane, CH.sub.4, through the gas 
dispersal ring. One important factor that distinguishes growth of diamond 
is the poly-phasic nature of the deposited material. Depending upon the 
growth conditions, the deposited layers may be diamond, graphite, 
amorphous or glassy carbon, or mixtures of these materials. When a 
hydrocarbon such as methane is excited in a plasma, radicals of the form 
CH.sub.x are formed. As in the silane example, these radicals interact in 
the gas phase to form carbon-carbon bonds. The added complication in the 
carbon case results from the ability of carbon to form not one, but three 
hydridizations. Thus we get carbon-carbon bonding of the ethane form 
(sp.sup.3 hybridization), of the ethylene form (sp.sup.2 hybridization), 
and of the acetylene form (sp.sup.1 hybridization). The parallel between 
these gas phase precursors and their solid phase analogues is striking. 
Diamond (sp.sup.3 hybridization) has ethane type bonding, graphite 
(sp.sup.2 hybridization) has ethylene type bonding, and carbynes (sp 
hybridization) are chainlike compounds with acetylene type bonding. To 
grow semiconducting diamond it is necessary to preclude the incorporation 
of wrong bonds of graphite-like or carbyne-like hybridization. The flux of 
gaseous precursors with incorrect hybridization onto the film surface is 
inevitable if the undesirable methane radicals (i.e., the ethylene and 
carbyne) are formed. Consequently, the design of the growth reactor and 
the choice of the growth parameters must be properly chosen to form 
precursors which upon condensation on the substrate promote sp.sup.3 
hybridization and diamond growth. The growth of diamond proceeds in the 
following manner. Typically, 500 sccm of He flows through the plasma 23 
tube 112 with a 30 sccm dilute mixture of He, H.sub.2, and CH.sub.4 
(4:10:1 by volume) flowing from the gas dispersal ring 118. A rf discharge 
of 80 W is sustained in the activation region 114 during deposition at a 
typical pressure of 10 mTorr. The substrate temperature is varied from 
650.degree.-850.degree. C. The quartz plasma tube size is 1.5 in. o.d. 
insuring a high plug velocity necessary for transporting metastables and 
radicals to the substrate. Using these growth parameters, diamond films 
have been grown at the rate of approximately 2000 Angstroms per hour. 
The proper choice of noble gas and methane diluent is critical for 
promoting diamond growth. Because the energy of the He metastable is so 
high (-20 eV), the cross-section for collisional dissociation of the 
CH.sub.4 molecule is low. Thus, the depositional precursor species created 
by the He are CH.sub.4 +, CH.sub.3 +, or CH.sub.3, all of which are highly 
saturated CH.sub.x radicals. The hydrogen serves two roles. First as a 
source of atomic hydrogen to the nucleating film, it more preferentially 
etches the graphitic bonds than the diamond bonds Second, it moderates the 
gas phase chemistry promoting higher saturation of the CH.sub.x radicals. 
One of the foremost problems in low temperature chemical vapor deposition 
is the removal of hydrogen from the nucleating film. For CH.sub.4 the 
spontaneous desorption of hydrogen occurs around 1000.degree. C. Growth of 
diamond then requires some other process besides thermal desorption to rid 
the deposited layers of hydrogen. One approach is to supply the surface 
which some other source of energy, photons, electrons, ions, etc. 
One way to do this is to supply a flux of metastables to the deposition 
surface. The same metastables which dissociate bonds in the gas phase can 
liberate hydrogen bonded on the nucleating solid. This is accomplished by 
keeping the carrier gas concentrations low to prevent total quenching of 
the metastables in the reaction zone. However, because quenching of the 
metastables is necessary to form precursor species which can deposit at 
lower temperatures, there is a compromise made between the deposition rate 
determined by how many metastables are quenched and the dehydrogenation 
rate determined by how many metastables survive the reaction zone and are 
incident on the substrate. 
One approach uses a sequentially pulsed growth technique where one deposits 
for some determinant period of time with the carrier gas flowing, removes 
the carrier gas, and dehydrogenates for some determinant period of time. 
The grown period is sustained long enough to deposit a monolayer of 
material. The dehydrogenation period is sustained long enough to rid the 
deposited monolayer of hydrogen Because the carrier gasses are not 
quenching the metastable flux, the metastable flux to the surface would be 
maximum and the hydrogenation time minimized. One might also expect that 
the metastable flux to the surface to impart energy to the adsorbed atoms 
and increase their mobility on the growth surface. In general, the higher 
the surface atom mobility is, the better the crystal will grow. The 
following are key operating parameters of the pulsed growth sequence 
technique : 
Deposition Sequence 
Ar 200 sccm plasma tube 
Ar 50 sccm ring feed 
SiH.sub.4 10 sccm ring (2% SiH.sub.4 in He) 
Pressure 0.200 Torr 
rf power 40W 
substrate temperature 200.degree. C. 
deposition time 1 min 
Dehydrogenation Sequence 
Ar 200 sccm plasma tube 
Ar 50 sccm ring feed 
SiH.sub.4 0 sccm ring 
Pressure 0.200 Torr 
rf power 40W 
substrate temperature 200.degree. C. 
dehydrogenation time 30 sec 
The overall sequence includes repeated alternate performances of the above 
noted deposition and dehydrogenation sequences. 
In any type of epitaxial process the order and cleanliness of the starting 
surfaces are of paramount importance. This is especially true in any low 
temperature epitaxial process where adsorbed atoms may not have enough 
mobility unless the energy is provided by some other source other than 
thermal. In the RPECVD apparatus and process, two techniques that use the 
flux of excited or dissociated species from the plasma region to clean 
substrates of residual contaminants have been developed. The first 
technique involves dissociation of molecular hydrogen in the plasma region 
and transport of atomic hydrogen to the substrate surface. There the 
hydrogen reacts with residual carbon and oxygen atoms forming volatile 
compounds which leave the surface. Typical operating conditions for this 
process are 80-100 sccm H.sub.2 plasma, 4-5 mTorr, 35 Watts, 300.degree. 
C. substrate temperature, and 20 s time duration. Because atomic hydrogen 
may react with the glass walls of the plasma tube, this process has been 
refined. Now, metastable species of Ar, generated in the plasma region 
114, interact with hydrogen flowing from the ring feed 118 to form atomic 
hydrogen. The plug velocity of the Ar is kept high to prevent hydrogen 
from back-diffusing into the plasma region. Typical operating conditions 
for the refined cleaning process are 200 sccm Ar plasma, 50 sccm H.sub.2 
ring, 100 mTorr, 50 Watts, 300.degree. C. substrate temperature, and 30 s 
time duration. Here as before, the atomic hydrogen reacts with the 
residual contaminants on the substrate to form volatile compounds and 
leave the surface. Without these effective hydrogen cleaning procedures, 
none of the epitaxial work would be possible. As with the epitaxial 
growth, it is the flux of particular selected species from the excitation 
region to the substrate that is the key to these processes. 
Following use of the cleaning procedure and before limitation of the 
epitaxial growth process it is very important to prevent re-contamination 
of the substrate surface by undersirable gas species which constitute the 
background or "base" pressure of the growth apparatus. These contaminants 
can impinge the substrate from a variety of sources including the several 
surfaces present in the growth apparatus. Therefore, it is very important 
to minimize the concentration of these contaminant species in the 
apparatus through maintaining a very low base pressure, less than 
5.times.10.sup.10 Torr. Base pressures larger than this value, through 
contamination and disruption of the crystalline order of the cleaned 
substrate surface, will significantly degrade the quality of the epitaxial 
grown layer For example, at a base pressure of 5.times.10.sup.-10 Torr, 10 
percent of the substrate surface, will be covered with new contaminants 
after only 4 minutes. Yet, a 10 percent surface contamination will create 
an unacceptably large crystalline point defect density between 10.sup.20 
and 10.sup.21 in the epitaxial layer. Consequently, the requirement of 
base pressure less than 5.times.10.sup.-10 Torr is a minimum requirement. 
The invention has been shown in what is considered to be the most practical 
and preferred embodiments. It is recognized, however, that obvious 
modifications may occur to those with skill in this art. Accordingly, the 
scope of the invention is to be discerned solely by reference to the 
appended claims.