Plasma/radiation assisted molecular beam epitaxy method

A molecular beam epitaxy (MBE) growth method and apparatus is disclosed which achieves a significantly improved sticking coefficient for materials like Hg upon a substrate, and thus a higher efficiency. A highly ionized, low pressure plasma is formed consisting of a mixture of ions of one substance of a compound to be epitaxially grown, neutral particles of the substance and electrons, and also preferably both ionization and excitation radiation. The plasma is directed onto a substrate together with a flux of the other substance in the compound; the flux can be in the form of either a vapor, or a second plasma. Radiation assisted epitaxial growth for Hg compounds in which ionization and excitation radiation are formed from Hg vapor and used to assist epitaxial growth with neutral Hg particles is also described. The plasma is formed in a special discharge chamber having a hollow cathode with an emissive-mix-free cathode insert. The source is preferably a refractory metal such as rolled tantalum foil, which is substantially emissive-material-free and does not contaminate the plasma. Good results are obtained by allowing the plasma to simply diffuse out through an exit port in the discharge chamber, without special extraction assemblies required by prior ion thrusters. Hg sticking coefficients have been improved by a factor of 40 or more.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to molecular beam epitaxial methods and apparatus 
for epitaxial growth of a compound upon a substrate. 
2. Description of the Related Art 
The present invention is concerned in general with the growth of structures 
by molecular beam epitaxy (MBE), and in particular with the use of MBE for 
the epitaxial growth of mercury telluride (HgTe), cadmium telluride (CdTe) 
and mercury cadmium telluride (HgCdTe) single crystal alloys, and 
HgTe/HgCdTe superlattices. HgCdTe is difficult to prepare for use in 
detection devices by either bulk or epitaxial techniques. The most 
commonly used epitaxial growth process for these materials is currently 
liquid phase epitaxy. Although high performance infrared detectors have 
been realized with growth by liquid phase epitaxy, the technique cannot 
produce abrupt heterojunctions and superlattices required for advanced 
opto-electronic devices. A review of various growth techniques is provided 
in J.P. Faurie et al., "Latest Developments in the Growth of Hg.sub.l-x 
Cd.sub.x Te and CdTe-HgTe Superlattices by Molecular Beam Epitaxy", J. 
Vac. Sci. Technol. A, Vol. 1, No. 3, Jul.-Sep. 1983, pages 1593-97. 
The MBE technique, on the other hand, is suitable for the growth of high 
quality epilayers, abrupt heterojunctions and alternate microstructures 
such as superlattices. This technique is described in J.P. Faurie et al., 
"Molecular Beam Epitaxy of II-VI Compounds: Hg.sub.l-x Cd.sub.x Te", J. 
Cryst. Growth, Vol. 54, No. 3, pages 582-85, 1981. However, the growth of 
HgCdTe by MBE is hard to control because of the excessive mercury 
re-evaporation from the surface during the deposition process. 
MBE is a vacuum deposition process. The current implementation of the 
process uses several effusion cells, each cell comprising an electrically 
heated crucible containing one of the substances of the compound to be 
grown. Upon heating, the cells produce atomic or molecular beam fluxes of 
mercury, cadmium and tellurium. The fluxes are directed onto the surface 
of the substrate, where they react with each other and produce an 
epitaxial layer. 
The growth rate of an MBE process is critically dependent upon the 
"sticking coefficient" of the materials being grown, i.e., the probability 
that a particle of the flux will adhere to the surface of the substrate. 
In the case of HgCdTe and HgTe/HgCdTe superlattice growth, the Hg sticking 
coefficient is very low. For a substrate temperature range of 
170.degree.-200.degree. C., the Hg sticking coefficient has been found to 
vary between about 10.sup.-4 and 10.sup.-3. With conventional MBE growth, 
therefore, large Hg fluxes must be used. For example, as described in J.P. 
Faurie et al., "Molecular Beam Epitaxy of Alloys and Superlattices 
Involving Mercury", J. Vac. Sci. Technol., A3(1), 1985, pages 55-59, kg of 
mercury is required to grow a 75 micron thick layer of Hg.sub.l-x Cd.sub.x 
Te. This is an undesirably high rate of mercury consumption, and also 
requires a relatively high substrate temperature. Furthermore, it is 
difficult to control the electrical properties and to attain abrupt 
junctions for heterostructures. 
SUMMARY OF THE INVENTION 
In view of the above problems with the related art, the purpose of the 
present invention is to provide a new MBE method and apparatus that 
substantially increases the mercury sticking coefficient when used with a 
mercury compound, lowers both the amount of mercury consumption and the 
substrate temperature, provides a greater degree of control over both the 
growth and substrate conditions, and avoids unintended contamination of 
the growth material. 
The invention accomplishes these purposes by providing one of the 
substances to be grown in the form of a highly ionized, low pressure 
plasma which, in the preferred embodiment, includes both ions and neutral 
particles of the substance, electrons, and also ionization and excitation 
radiation fields of the substance. Radiation can also be used by itself to 
assist epitaxial growth, but not as effectively as the full plasma. The 
plasma is formed by bombarding a neutral gas of the substance with 
electrons, the electron energy (discharge voltage), emission current and 
substance flow rate being controllable to control the relative proportions 
of ions and neutral particles, and of ionization as opposed to excitation 
radiation. 
A chamber is described for forming the highly ionized, low pressure plasma 
which has an opening on one side for the simple outward diffusion of the 
plasma, without any special extraction facilities. Means are provided to 
introduce a gas of a desired substance such as mercury into the chamber, 
and for providing electrons to ionize and excite at least some of the gas. 
An anode is also provided within the chamber and maintained at a more 
positive voltage than the cathode to accelerate electrons and thereby 
produce ionization. Excitation radiation results from the spontaneous 
decay of excited gas particles, while ionization radiation results from 
ions and electrons recombining on interior surfaces. The substrate is held 
in the path of the plasma emitted from the chamber. The substrate is 
positioned with respect to the chamber so that at least about 20% of the 
radiation exiting the chamber reaches the substrate along with the plasma. 
A tube with radiation reflective walls may be used to confine the 
radiation between the chamber and substrate, with a magnetic coil 
preventing the plasma from re-combining at the tube walls. Means are 
provided for forming a flux from the other substance of the desired 
compound, and directing that flux onto the substrate to react with the 
plasma. The electron discharge is a DC mechanism, and the pressure is kept 
relatively low within the approximate range of 10.sup.-5 -10.sup.-4 Torr. 
The described technique has been found to produce a marked increase in the 
mercury sticking coefficient, with an accompanying significant reduction 
in mercury consumption. Also, contamination of the plasma is avoided by 
employing a hollow cathode as the electron source with an emissive 
mix-free insert such as a rolled tantalum foil. 
Further features and advantages of the invention will be apparent to those 
skilled in the art from the following detailed description of preferred 
embodiments, taken together with the accompanying drawings, in which:

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 illustrates the principal components of the apparatus employed in 
the present invention to produce a more efficient MBE. In accordance with 
the invention, one of the substances in a compound to be epitaxially grown 
is provided in the form of a highly ionized, low pressure plasma flux that 
includes neutral particles, ions, electrons, and both ionization and 
excitation radiation. The other substance of the compound is provided as a 
flux in either a vapor or plasma format. While FIG. 1 illustrates the 
preferred embodiment in which a plasma-delivered Hg is epitaxially grown 
together with evaporated Cd, Te or CdTe on a substrate, the invention is 
generally applicable to numerous compounds formed from Group II-VI and 
III-V combinations of the periodic table, and also to Group IV doping of 
compounds. However, because of the very substantial improvement offered to 
the sticking coefficient of Hg, the remainder of this specification will 
discuss this application of the invention. 
The apparatus is enclosed within a vacuum housing 2, which is maintained at 
a pressure on the order of 10.sup.-7 Torr by vacuum pump 4 and LN.sub.2 
cryowall 5. Hg from a source 6 is introduced into a cylindrical plasma 
discharge chamber 8 within the housing through flow lines 10 and 12. Hg 
flowing through line 10 is vaporized and enters the chamber through a 
hollow cathode 14, which is energized by a current-regulating power supply 
16 to inject electrons into the chamber at a specified discharge voltage 
(V.sub.D) and current (J.sub.E). The electrons ionize at least some of the 
Hg vapor within the chamber, forming a weakly ionized, low pressure plasma 
consisting of a mixture of neutral Hg particles, ions, electrons, 
excitation radiation and ionization radiation. The exact mechanism by 
which the plasma is formed is explained in further detail below. 
The plasma diffuses out of chamber 8 through an exit port 18 onto a 
crystalline substrate 20, which is supported on an electrically isolated, 
thermally controlled substrate holder 22. A crucible 24 is heated by a 
current source 26 to produce a flux of either molecular CdTe, Cd, or Te. 
The flux is directed onto the substrate 20, where it reacts with the Hg 
plasma to produce an epitaxial growth of the desired compound. The 
particular Hg plasma source shown was capable of supplying a flux on the 
order of 10.sup.16 ions/sec/cm.sup.2 (several mA/cm.sup.2) of low energy 
(on the order of 30 eV) Hg.sup.+ ions to the substrate, while with a 
substrate temperature fixed at 170.degree. C. a neutral Te.sub.2 flux from 
crucible 24 was adjusted to produce an indicated partial pressure at the 
substrate location of 1.5.times.10.sup.-6 Torr (about 2.times.10.sup.14 
atoms/cm.sup.2 /sec). A significant increase in the Hg sticking 
coefficient compared to prior systems was noted, thus permitting a 
corresponding reduction in the Hg flux required for MBE growth. 
The growth enhancement at the substrate 20 has been found to vary in 
positive proportion to the amount of radiation reaching the substrate 
along with the plasma. The radiation leaving the chamber 8 through exit 
port 18, however, progressively diverges to encompass a greater area as it 
travels away from the chamber, the radiation density generally following 
an inverse square law relationship with distance from the exit port. The 
substrate 20 should accordingly be positioned close enough to the exit 
port so that enough radiation reaches it to produce a significant 
enhancement in growth. In general, a radiation density at the substrate 
equal to at least about 20% of the radiation density at the exit port 18 
is considered necessary to produce a significant growth enhancement, 
although a radiation density of 40% or more is preferable. In one 
experiment the diameter of the exit port was 3", and the substrate was 
located between 1" and 2" away, with good growth results. The substrate 
was about 1/4" square (the accompanying figures are not to scale), so that 
the gradient in radiation intensity from one side of the substrate to the 
other due to the substrate being angled to the axis of the 
plasma/radiation path was not significant. The plasma also disperses with 
distance from the exit port 18, but at a lesser rate than the radiation. 
In the system described thus far, the Hg plasma supplied to the substrate 
can be varied by changing V.sub.D and/or J.sub.E and/or the mercury flow 
rate. However, in some applications a very rapid change in the plasma 
reaching the substrate is desired, such as in the formation of 
superlattices with very abrupt boundaries between Hg and Te layers. In 
this case, a more rapid response in the plasma reaching the substrate 
might be achieved by applying a voltage signal from a variable voltage 
source 28 to the substrate 20, via substrate holder 22. The density of 
ions or electrons in the plasma reaching the substrate might then be 
controlled by biasing the substrate voltage either positive or negative. 
Although not yet demonstrated, it is believed that a very rapid response 
might be achieved with this technique. 
Further details of plasma chamber 8 are furnished in FIG. 2. A cylindrical 
anode 28 is positioned within the chamber adjacent to the cylindrical 
chamber wall between cathode 14 and exit port 18. The anode 28 serves to 
collect discharge current and plays a role in the production of neutral 
particles and ionization radiation, as discussed below. It is preferably 
water-cooled and coated with Cd, and is maintained at a positive voltage 
relative to the cathode. A divergent magnetic field which efficiently 
confines the low pressure, highly ionized plasma is produced by alnico 
permanent magnet bars 29 arranged along the perimeter of the discharge 
chamber. 
The exit port 18 is preferably formed by a metal plate 30, such as 
stainless steel, which is coated with Cd and has a central opening 
approximately 7.5 cm in diameter. Unlike related ion thrusters which 
employ ion extraction mechanisms, the plasma inside the chamber simply 
diffuses out of exit port 18; a typical prior ion thruster is described in 
Harold R. Kaufman, "Technology of Electron-Bombardment Ion Thrusters", 
Advances in Electronics and Electron Physics, Vol. 36 (L. Marton, ed.) 
Academic Press, New York, 1974, pp. 265-373. Despite this, the large 
majority of the output from the plasma chamber may be ions, as opposed to 
neutral particles. This is because the ions are driven out by internal 
electric fields within the chamber, whereas neutral atoms are moved out by 
random thermal motion; since a relatively low temperature source is used, 
the density of neutral particles in the chamber output can be strictly 
limited. The interior of the discharge chamber, including the anode 28, is 
preferably coated with Cd to minimize sputter-induced impurities evolving 
from the plasma source. 
The cathode 14 has a unique structure which prevents it from adding 
impurities to the Hg introduced into the chamber. Liquid Hg is fed through 
supply line 10, which terminates at a porous tungsten plug 32 in the 
cathode tube. Plug 32 is heated to a temperature of about 300.degree. C. 
by a surrounding heating coil 34, which is powered by a variable current 
source 36. The liquid Hg vaporizes and migrates through the pores of plug 
32 to enter the cathode chamber 38. As the Hg vapor continues downstream, 
it passes through the interior of an insert sleeve 40, which is heated by 
an electric coil heater 42 to emit electrons into the Hg vapor passing 
through the insert. The design of the insert 40 is unique, and offers a 
distinct advantage over prior ion thrusters. Inserts employed in the past 
have typically been chosen to have a low work function, so that electrons 
can be easily emitted. A tungsten insert impregnated with BaO, having a 
work function of about 2-2.5 eV, has commonly been used. However, BaO is 
an emissive material which gives off oxide and Ba, Ca, and Al emissions 
that can contaminate the substrate. 
In accordance with the present invention, an emissive-mix-free insert is 
used in the hollow cathode 14. The new insert is formed from a refractory 
metal, such as tantalum or tungsten, in the form of a foil which is rolled 
up in a hollow spiral. Tantalum is preferred because it is less brittle 
than tungsten. The work function of tantalum is about 4.4 eV. Once the 
plasma is lit the cathode heater can be turned off, and the plasma 
self-heats the cathode insert. Ions from the plasma bombard the cathode 
with an energy sufficient to heat the insert to thermionic emission 
temperatures. 
A small loop anode 44 called a keeper is positioned within the discharge 
chamber immediately in front of hollow cathode 14. This device draws 
sufficient electron current to initiate and maintain the cathode discharge 
in the presence of low emission conditions and plasma fluctuations. The 
cathode is isolated from the main discharge chamber via a baffle system 
46, consisting of a cylindrical baffle 48 surrounding the cathode exit, 
and a circular baffle plate 50 in-line with the cathode exit. The baffle 
system, typically 5-8 cm in diameter, tends to isolate the plasma in the 
cathode from that in the main discharge chamber. The magnetic field from 
the permanent magnet bars 29 efficiently confines the plasma. 
Hg is also introduced directly into the discharge chamber through a main 
plenum 52. This is fed with liquid Hg from line 12, and includes a porous 
tungsten plug 54 which is heated by a surrounding electrical coil 56 
supplied by a variable current source 58. As with the initial portion of 
the hollow cathode, a tungsten plug 54 in the main plenum is heated to 
about 300.degree. C. to vaporize the liquid Hg, which then migrates 
through the pores of the plug directly into the discharge chamber. The 
flow rate of mercury vapor produced by the main plenum plug 54 and cathode 
plug 32 can be adjusted by varying the currents applied to their 
respective heating coils. 
Further details of the hollow cathode are provided in FIG. 3. The 
emissive-mix-free tantalum insert 40 is formed from a foil about 0.013-mm 
thick, rolled in six or seven turns with an inside diameter of about 
3.8-mm. The insert is about 25-mm long. The tube forming the cathode 
chamber 38 is about 6.4-mm in diameter, is preferably formed from Mo or 
Ta, and is surrounded by about thirteen turns of the heating coil 42 in 
the vicinity of the tantalum insert. An orifice plate 60 is positioned at 
the end of the cathode tube, and has a flared central opening with a 
minimum diameter of 0.76 mm to emit the Hg vapor and electrons into the 
discharge chamber. Orifice plate 60 is preferably formed from thoriated 
tungsten (W with about 2% Th). Hg is introduced into the discharge chamber 
through the hollow cathode 14 and main plenum 52 at a pressure preferably 
in the range of about 10.sup.-5 to 10.sup.-4 Torr. 
The manner in which a plasma consisting of neutral Hg particles, Hg ions, 
electrons, ionization radiation and excitation radiation is produced by 
the electrons supplied by cathode 14 is illustrated in FIG. 4. Electrons 
62 are emitted from cathode 14 at a given discharge voltage V.sub.D and 
current J.sub.E. The electrons bombard neutral mercury atoms within the 
chamber. This bombardment will result in a combination of excited Hg* 
atoms, Hg.sup.+ ions and electron pairs, and neutral Hg atoms; the 
proportion of each is determined by the electron energy and plasma 
confinement conditions. There will also be a small number of Hg.sup.++ 
ions produced, primarily by electron bombardment of Hg.sup.+, but for 
simplicity this reaction is not considered in this description. The 
excited Hg* atoms spontaneously decay to neutral Hg atoms, giving off 
excitation radiation h.upsilon.*. The Hg.sup.+ ion and electron pairs 
either diffuse out of the discharge chamber, or to the anode 28, cathode 
14, or cathode surfaces 8 where the ions and electrons recombine to form 
Hg atoms, giving off ionization energy h.upsilon..sup.+ in the process. 
The various proportions of Hg, Hg.sup.+, electrons, h.upsilon.* and 
h.upsilon..sup.+ will all diffuse out of the discharge chamber and be 
applied to the substrate to participate in the epitaxial growth. 
It has been found that the relative proportion of ions to neutrals and 
radiation can be varied by varying V.sub.D, J.sub.E and/or the mercury 
flow rate. V.sub.D also controls the ion energy, and an ability to control 
this parameter is important in limiting the ion energy so that lattice 
damage is prevented during ion deposition. An ionization level of 
approximately 10% is preferably established. 
The manner in which V.sub.D can be controlled is known from the prior art 
of ion thrusters. To increase V.sub.D for a constant cathode current 
J.sub.E, the Hg flow rate through the cathode is first reduced by reducing 
the power from current source 36 (referring to FIG. 2). For a fixed 
cathode current, this will cause the discharge voltage V.sub.D to 
increase. Since reducing the cathode flow rate reduces the amount of 
propellant introduced into the discharge chamber, the ion current exiting 
the source 36 will be reduced. To maintain this ion current constant, the 
Hg flow through the main plenum 52 is simultaneously increased. This will 
have a tendency to reduce the discharge voltage as well, but is easily 
compensated for by additionally reducing the cathode flow rate slightly. 
An increase in the discharge voltage in the manner described will always 
increase the ionization efficiency of the ionized substance (i.e., 
increase the ratio of ions to neutrals). The Hg flow through the cathode 
14 and main plenum 48 can be varied in exactly the opposite manner to 
reduce V.sub.D and shift the plasma away from ions and more towards 
neutral particles. 
If it is desired to keep the ion energy fixed while increasing the 
ions/neutrals ratio, J.sub.E is increased instead of V.sub.D. To increase 
the proportion of ions, J.sub.E is increased by adjusting power supply 16, 
leading to an increase in V.sub.D. The flow of Hg through cathode 14 is 
then increased to cancel the rise in V.sub.D. At the same time, the Hg 
flow rate through the main plenum 28 is reduced to maintain the absolute 
number of ions within the discharge chamber constant, but with an 
increased ion ratio because of the increase in J.sub.E. 
A large variation in the proportion of ions is possible with the present 
invention. An ion output of at least 90% has been demonstrated, without 
any form of ion extraction assembly. 
Marked improvements have been noted in the Hg sticking coefficient. While 
the greatest improvement is achieved with a plasma consisting of neutral 
particles, ions, electrons, and both ionization and excitation radiation 
reaching the substrate, greater sticking coefficients have also been 
achieved with neutrals and both ionization and excitation radiation alone 
reaching the substrate. In one demonstration, a fine wire mesh was placed 
over the exit aperture 18 of the plasma chamber to prevent the Hg plasma 
from exiting. Instead, only Hg atoms and Hg-excitation and ionization 
radiation were allowed to impinge upon the substrate. In this experiment 
an ionization gauge was rotated into the location of the substrate to 
measure the Hg partial pressure and to verify that little, if any, plasma 
was reaching the substrate. The discharge chamber was operated to produce 
an Hg.sup.o flux of 5.times.10.sup.15 atoms/cm.sup.2 -sec 
(.perspectiveto.0.8 mA/cm.sup.2) to the substrate. As will be shown in the 
discussion below, this should have resulted in an Hg-deficient HgTe layer. 
The neutral Te.sub.2 flux was adjusted to produce a partial pressure of 
1.5.times.10.sup.-6 Torr (1.9.times.10.sup.14 atom/cm.sup.2 -sec) at the 
location of the substrate. In the presence of the Hg-excitation and 
ionization radiation emitted from the plasma source, an enhancement of 
about 10% in the Hg sticking coefficient was noted. 
In another demonstration, a single, crystal CdTe (111) substrate was 
confined within an LN.sub.2 -pumped vacuum system with a baseline vacuum 
pressure of 10.sup.-7 Torr, at a temperature of 170.degree. C. Controlled 
fluxes of Te.sub.2 and Hg atoms were produced by electrically heated 
crucibles, with the Te.sub.2 and Hg fluxes directed onto the substrate. An 
exposed ionization gauge was rotated in the location of the substrate to 
measure the partial pressures of the fluxes. The Te.sub.2 flux was 
maintained at a pressure of 1.5.times.10.sup.-6 Torr (1.9.times.10.sup.14 
atoms/cm.sup.2 -sec). The Hg flux was then varied over three orders of 
magnitude, ranging from 3.2.times.10.sup.14 atoms/cm.sup.2 -sec to 
4.times.10.sup.16 atoms/cm.sup.2 -sec. 
The variation of the atomic percentage concentration of Hg and Te in the 
grown HgTe films with the amount of incident Hg flux is charted in FIG. 5. 
The results indicate that for an incident Hg flux of less than 
1.8.times.10.sup.16 atoms/cm.sup.2 -sec, the grown HgTe films were Te-rich 
(Hg-deficient). For higher Hg fluxes, the films were Hg-rich 
(Te-deficient). 
To assess the effect of a radiation assisted molecular beam epitaxy 
technique, the Hgo flux was next set to a value of 1.8.times.10.sup.15 
atoms/cm.sup.2 -sec. Based upon the results illustrated in FIG. 5, this 
Hg.sup.o flux would be expected to result in a Hg-deficient film. For this 
experiment, a Hg lamp which provided a radiation source with predominantly 
Hg-excitation radiation was mounted approximately 5 cm from the CdTe 
substrate. The lamp radiated at 255 nm, which is the first excitation 
potential of Hg. The power density of the lamp at a distance of 1.9 cm was 
4.5 mW/cm.sup.2. 
The variation in the atomic percentage of Hg and Te in the HgTe films 
formed with and without the Hg-radiation lamp is shown in FIG. 6. In the 
presence of Hg excitation radiation, the Hg atomic percentage increased 
from a Hg-deficient level (approximately 35%) to Hg-rich (approximately 
59%). In a further experiment the substrate temperature was increased from 
170.degree. C. to 180.degree. C., and the incident Hg.sup.o flux was 
increased to 6.times.10.sup.15 atoms/cm.sup.2 -sec. Under these 
conditions, the Hg atomic percentage again increased substantially, from 
5% in the absence of the radiation field to 13% in the presence of the 
field. 
The results shown in FIGS. 5 and 6 suggest that for HgTe films grown by 
conventional MBE techniques, the "sticking coefficient" of Hgo atoms can 
be approximately doubled by performing the growth in the presence of Hg 
excitation radiation. It is anticipated that further enhancements can be 
achieved by optimum selections for the intensity of the light source, 
substrate temperature, and incident Hg and Te.sub.2 flux conditions. 
When a full plasma consisting of Hg.sup.+, Hg, electrons, ionization 
radiation and excitation radiation was applied from the discharge chamber 
described above, the Hg sticking coefficient was increased by a multiple 
of at least 40, with fluxes of about 90% Hg.sup.+ and about 10% Hg 
reaching the substrate. The wavelength of the excitation radiation was 
calculated as 259 nm, while that of the ionization radiation was 
calculated as 119 nm. 
Referring now to FIG. 7, another embodiment of the invention is shown in 
which a flux of CdTe or Te plasma is provided from a second discharge 
chamber 64, rather than from a heated crucible. Discharge chamber 64 for 
CdTe, Cd or Te is similar to Hg discharge chamber 8, with a CdTe or Te 
source 66 supplying a cathode 68 and a main plenum 70; a current regulated 
power supply 72 is provided for cathode 68. A CdTe, Cd or Te plasma is 
emitted through exit port 74 onto the substrate 20, where it reacts with 
the Hg plasma to produce an epitaxial growth. 
FIG. 8 illustrates another embodiment in which a radiation confining tube 
76 is employed in conjunction with the positioning of substrate 20 so that 
sufficient radiation reaches the substrate to significantly enhance the 
growth there. Tube 76 is formed with inner radiation reflective walls such 
as aluminum or molybdenum to effectively confine the radiation. It is 
preferably equal in diameter to the exit port 18, and extends from the 
exit port to a location close to substrate 20. A magnetic coil 78 is wound 
around the tube to guide the plasma along the interior of the tube, 
thereby preventing it from re-combining at the tube walls. 
Various embodiments of a highly efficient MBE system have thus been shown 
and described. Since numerous variations and alternate embodiments will 
occur to those skilled in the art, it is intended that the invention be 
limited only in terms of the appended claims.