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Description  |
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The invention relates to a method of manufacturing a semiconductor device,
the method comprising the steps of providing a monocrystalline substrate
in a deposition chamber of a molecular beam epitaxy apparatus, reducing
pressure in the deposition chamber to below atmospheric pressure, heating
the substrate and causing a plurality of successive combinations of
molecular beams to be incident on the substrate so as to form a plurality
of epitaxial layers, wherein the molecular beams each comprise a
constituent element of the respective layer material and each beam
emanates from a separate source when a shutter associated with the
respective source has been opened. Such a method may be used, for example,
for manufacturing laser diodes, field effect transistors and microwave
diodes.
In a molecular beam process, fluxes of the constituent elements of the
epitaxial layer are generated in sources (effusion cells), separate
sources being generally used for each element. The composition of the
epitaxial layer depends on the magnitudes of the fluxes of the constituent
elements of the layer, and the magnitude of each flux depends upon the
temperature of the respective source. In order to maintain each flux at a
preset value, it is necessary to maintain the temperature of each source
at a respective predetermined temperature. When a temperature equilibrium
for a source has been established, the heating provided by an associated
heating controller will exactly balance the rate at which heat is lost by
the source to the surroundings of the source. A further function of the
heating controller is to make any adjustments to the heat input of the
source as rapidly as possible consistent with maintaining stable operation
whenever the temperature of the source varies from the predetermined
temperature of that source.
Simple on-off control systems in which the heater power is either fully
on--when the source temperature is below the predetermined temperature--or
off, when the source temperature is above the predetermined temperature,
do not provide sufficiently accurate control of the source temperature.
Proportional control systems are commonly used to control the power input
to the heaters of sources used in molecular beam processes. Such control
systems overcome the problem of temperature cycling of the source by
enabling there to be continuous variation of the power input to the heater
of the source. The heater voltage is a function f of the difference
between the measured and desired temperatures of the source, where f is
defined by the equation
##EQU1##
A, B and C are programming constants, and T.sub.s and T.sub.m are the set
temperature and the measured temperature respectively. B and C are often
expressed as time constants and are sometimes referred to as integral
action and differential action time constants respectively. Such control
systems, which are known as three-term controllers or P.I.D. controllers
(Proportional Action, Integral Action and Derivative Action), provide very
accurate temperature control, but the response to changes in temperature
is slow, due to the thermal mass of the source. When growing epitaxial
structures with abrupt changes in the basic composition or in the doping,
rapidly acting mechanical shutters disposed in front of the molecular beam
sources are used to start or stop the emanation of molecular beams from
the respective sources. The act of opening or closing a shutter in front
of a source results in a significant relatively abrupt change in the rate
at which heat is lost from the front of the source and hence in a
relatively abrupt change in the temperature of that source, and for a
source operating at about 1000.degree. C., the time elapsing between the
change in position of the shutter and the restoration of the source to its
initial temperature before the shutter was moved is typically of the order
of 5 to 10 minutes, depending inter alia on the thermal mass of the
source. When a shutter is opened, the temperature of the associated source
will fall, as a result of the rate of loss of heat from that source
increasing, and when a shutter is closed, the temperature of the source
increases as a result of heat emitted from the source being reflected by
the shutter back into the source. During the investigations which led to
the present invention, it was found that when opening or closing a shutter
in front of a source, the rate at which heat is lost from that source
changes by from about 1 to 3%, depending upon the shape and reflectivity
of the shutter which in turn is influenced by material accumulated on the
shutter during the process. Most (>95%) of the
heat lost from a source is lost by radiation and so the change in the rate
of loss of heat resulting from the opening or the closing of a shutter
will result in a fall or in a rise in temperature of the order of 5K with
a source temperature of 1000K. After peaking to this value, this
temperature change will gradually be reduced to zero as the three-term
controller adjusts the rate of input of heat to the source in an
appropriate manner, but for about 5 minutes the flux emanating from the
source will be about 10% lower than the intended flux.
When growing a single layer of gallium arsenide, the effect is not too
serious, since the net result will be an initially reduced rate of
deposition with possibly a slightly higher doping level (when a dopant is
present). The situation is more serious when growing alloy films since the
alloy composition will not be constant until the source temperatures have
re-attained the predetermined values. When a superlattice structure is
grown in a molecular beam epitaxy apparatus in which the heat inputs fed
to the sources are controlled by three-term controllers, the effect of the
frequent opening and closing of one or more shutters will be to cause the
temperature(s) of the source(s) associated with the shutter(s) to
oscillate about a mean temperature which is below the desired source
temperature, resulting in the composition of the films deposited which
contain material from the source(s) whose shutter(s) is(are) being opened
and closed repeatedly varying from the desired composition.
The avoidance of changes in the temperature of a source has long been
recognised as a significant problem in the molecular beam epitaxy art, but
possibly because of the apparent contradiction between the fine degree of
accuracy of the temperature control achieved by three-term controllers and
the relatively coarse changes (for example, 10%) in the magnitude of the
flux emanated from a source after a shutter associated with that source
has been closed or opened, there has been acceptance voiced at conferences
devoted to molecular beam process technology that this problem was
insoluble.
In an article "Optical quality GaInAs grown by molecular beam epitaxy" by
G. Wicks et al., in Journal of Electronic Materials, Vol. 11, No. 2,
(1982), pages 435 to 440, it is stated that after opening the shutters on
individual Ga and In cells, the Ga and In fluxes fell over a period of
about 3 minutes to values about 7% and about 2% respectively below the
respective initial values. It was considered that the shutters acted as
heat shields. While the backs of the cells are maintained at constant
temperature by control thermocouples, opening the shutters allows the
front of the cells to cool by increased radiation, decreasing the fluxes
accordingly. This article states that alloy growth should be initiated by
rotating the substrate into the molecular beams only after the shutters
had been opened long enough for the cell temperature profile to
re-equilibrate. Such a procedure may be detrimental to the properties of
the interface due to the adsorption of impurities or the creation of
native point defects (vacancies or interstitials).
The object of the invention is to provide a method in which changes in
source temperature consequent to the opening or closure of a shutter
associated with that source are mitigated.
The method according to the present invention is characterised in that when
the shutter of one of the sources is opened the rate of input of heat to
that source is increased by a predetermined value so that the temperature
of that source does not change as a result of the opening of the shutter,
and wherein when the said shutter is closed the rate of input of heat to
that source is reduced by the said predetermined value.
The plurality of epitaxial layers may, for example, constitute a
superlattice structure. The semiconductor device may be, for example, a
double heterojunction laser diode.
In one embodiment of the invention, the heat input to a source is
controlled by a four-term controller, in which the fourth term produces
the predetermined increase in heat input to the source and is linked to
means controlling the opening and closing of the shutter associated with
that source.
In another embodiment of the invention, the predetermined increase in heat
input to each source and the operation of the respective shutters are
controlled by means of a microprocessor or a microcomputer.
It was found when using a method according to the invention, that the
source temperature both after the shutter has been opened and after the
shutter has been closed is significantly closer to the predetermined
temperature than is the case when the rate of input of heat to the source
is controlled solely with the aid of temperature-sensing means, for
example, when a three-term controller alone is used. The improvement over
the prior art systems which is achieved by the method according to the
invention is due to the use of a combination of accurate but slow-acting
control of a three-term controller or that part of a microcomputer
programme which achieves similar temperature-sensed control, and the
abruptly acting switching on or off of the predetermined increase of the
rate of input of heat.
An embodiment of the invention will be described with reference to an
Example and to FIGS. 2 to 4 of the drawings. In the drawings, FIG. 1 is a
graph on which the aluminium content a in atoms/cm.sup.3 of a Ga.sub.1-x
Al.sub.x As layer grown by a MBE process over a GaAs layer on a GaAs
substrate is plotted against the depth d (nm) from the surface of the
layer, the source temperatures in this process are controlled by a
microcomputer using a programme which does not use any shutter-linked
heating terms.
FIG. 2 is a graph similar to the FIG. 1 process, but relating to a
Ga.sub.1-x Al.sub.x As layer grown by a process in which the source
temperatures were controlled by a microcomputer using a programme
containing a shutter term for each source,
FIG. 3 is a schematic diagram of a molecular beam epitaxy growth apparatus
used in a method according to the invention, and
FIG. 4 is a schematic block diagram of a four-term power controller used to
control the power input to an effusion cell in an MBE apparatus used in a
method according to the invention.
FIG. 1 is a graph in which the aluminium concentration of a GaAs/Al.sub.x
Ga.sub.1-x As heterostructure is plotted against the depth d from the
surface of the film. The temperatures of the Ga, Al and As sources in the
MBE apparatus used to grow this structure were controlled by three-term
controllers. The growth rate of GaAs was about 1 .mu.m/hour and the growth
rate of Al.sub.x Ga.sub.1-x As was about 1.25 .mu.m/hour. Upon opening the
aluminium source shutter, the aluminium concentration increased to about
4.3.times.10.sup.21 atoms/cm.sup.3 corresponding to x.apprxeq.0.2 and then
decreased as the aluminium source temperature dropped. After about 12
minutes the aluminium concentration returned to its initial value but was
never quite constant.
FIG. 2 is a graph showing the aluminium concentration of a GaAs/Al.sub.x
Ga.sub.1-x As heterostructure grown using a four-term temperature
controller. Upon opening the aluminium source shutter, the power to the
aluminium cell was increased by about 1% which almost exactly equalled the
increase in rate at which heat was lost by radiation as a result of
removing the shutter from the front of the source. In this case the
aluminium concentration increased to about 4.4.times.10.sup.21
atoms/cm.sup.3 corresponding to x.apprxeq.0.2 and remained constant
thereafter within experimental error. Because the balance between the
rates of heat input and heat loss was constant in this case, no
temperature fluctuation of the aluminium source occurred.
For both samples the aluminium concentration was determined by Secondary
Ion Mass Spectrometry (SIMS). The absolute error in concentration is about
.+-.3%.
EXAMPLE
A molecular beam epitaxy apparatus shown in FIG. 3 comprises a vacuum
chamber 1 containing effusion cells 2, 3 and 4 which served as sources for
arsenic, gallium and aluminium respectively, which effusion cells 2, 3 and
4 are mounted within a shroud 5 cooled by liquid nitrogen. The effusion
cells 2, 3 and 4 are each provided with heating elements (not shown) and
the rates of input of heat to the heating elements of the respective
effusion cells are controlled by means of respective three-term
controllers 6, 7 and 8. Each effusion cell 2, 3 and 4 is provided with a
respective mechanical shutter 9, 10 and 11 which can be independently
operated to emanate or cut-off a molecular beam of source material from
the respective effusion cell. A rotatable substrate holder 12 bearing a
monocrystalline substrate 13 is located symmetrically with respect to a
circular aperture 14 in the shroud 5. The three-term controllers are
programmed by a microcomputer 15.
In order to determine the increase in power input to the heater of a given
effusion cell when the shutter associated is opened, the effusion cell is
allowed to stabilise with the shutter closed and the power input to the
heater is noted. The shutter is then opened, and the cell is again allowed
to stabilise. The new power input is noted, the difference between these
power inputs is the increase in power input which is supplied to the
heater while the shutter is open. This difference for the aluminium
effusion cell was found to be about 1% of the original power input.
The vacuum chamber 1 was exhausted by means of a pump system 16. The
effusion cells 2, 3 and 4 had been loaded with arsenic, gallium and
aluminium respectively, and were heated to 370.degree. C., 980.degree. C.
and 1050.degree. C. respectively, in order to produce desired fluxes, each
of the shutters 9, 10 and 11 being closed at this stage. A GaAs
monocrystalline substrate 13 was heated to 650.degree. C., and the shutter
9 was opened so that the substrate 13 was exposed to an As.sub.2 flux of
2.times.10.sup.15 mols/cm.sup.2 /sec. for 20 minutes. When the shutter 9
was opened, the microcomputer 15 increased the heat input of the effusion
cell 2 heater by a power P in order to maintain the temperature of the
cell 2 substantiallly constant. The temperature of the substrate 13 was
then reduced to 600.degree. C. and shutter 10 was opened so that the
substrate 13 was exposed to a gallium flux of 6.times.10.sup.14
mols/cm.sup.2 /sec., in order to grow a gallium arsenide layer. After 15
minutes growth, during which time 0.25 .mu.m thickness of gallium arsenide
had been grown, the substrate temperature was then raised to 700.degree.
C. Shutter 11 was then opened resulting in an aluminium flux of
1.5.times.10.sup.14 mols/cm.sup.2 /sec. of aluminium being incident on the
substrate 13. A 1 .mu.m thick layer of gallium aluminium arsenide was
grown in 48 minutes, and the shutters 10 and 11 were shut. The temperature
of the substrate 13 was reduced to below 400.degree. C. and the shutter 9
was then closed. When any of the shutters 9, 10 or 11 were opened, the
heat input to the heater of the associated effusion cell was increased so
as to maintain a
significantly constant temperature of that effusion cell. When one of these
shutters was closed, the heat input reverted to the value supplied to the
effusion cell heater before the shutter was opened.
FIG. 2 shows the variation of the aluminium concentration with thickness of
part of a GaAs/GaAlAs heterostructure device grown in the MBE apparatus
described with reference to FIG. 3. It will be seen that FIG. 2 differs
from FIG. 1 in that the aluminium content did not fall immediately after
the peak value of about 4.times.10.sup.21 atoms/cm.sup.3 (x.apprxeq.0.2)
had been reached.
Instead of using a microcomputer to control the temperatures of the
effusion cells, the power input to each effusion cell may be controlled by
a respective four-power controller. FIG. 4 is a schematic block diagram of
such a four-term controller which has been made by modifying an Oxford
Instruments three-term controller DTC-2. An input signal th from a
thermocouple (not shown) fitted to an effusion cell is converted into a
signal proportional to temperature by a signal conductioning amplifier 17.
The converted signal is compared with a reference signal generated by a
controlled source 18 in order to generate a difference signal x which is
proportional to (T.sub.s -T.sub.m), where T.sub.s is the desired preset
temperature of the effusion cell and T.sub.m is the measured temperature
of the cell. After amplification in block 19, three signals are generated
by blocks 20, 21 and 22, namely a proportional component Ax, an integral
term B xdt and a differential action term Cdx/dt. These three signals are
added togive a conventional three-term (PID) controller output z at point
23. A fourth term Dy is generated by a block 24, y having a value of zero
when the effusion cell shutter is closed and a value of 1 when this
shutter is open. This fourth term is added to the signal z at point 25 so
as to produce an output signal z.sub.1 which is defined by the
relationship.
z.sub.1 =Ax+B.intg.xdt+Cdx/dt+Dy
The signal z.sub.1 is fed to an amplifier (not shown) which supplies a
power P to the effusion cell heater, which power P is proportional to the
signal z.sub.1. The shutters are controlled electro-pneumatically, and a
simple link is provided between the shutter control and the block 24 which
generates the Dy term.
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