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FIELD OF THE INVENTION
The invention relates to the field of semiconductor processing. More
specifically, the invention relates to forming a nitrogen containing group
III-arsenide compound semiconductor using ammonia as a precursor.
BACKGROUND OF THE INVENTION
Long wavelength lasers, lasers that emit light in wavelengths between 1.3
micrometers and 1.6 micrometers, are highly desirable for
telecommunication system use because at these "telecom wavelengths", a
"wavelength window" exists where light absorption in optical fibers is
minimized. As telecommunications increasingly rely on optical signal
transmission, these long wavelength lasers have become increasingly
important.
Most optoelectronic components in the telecom wavelength range are grown on
InP substrates. An InP substrate is preferred because the substrate easily
lattice matches with high indium composition InGaAs films used in the
fabrication of devices that emit 1.3-1.6 micron wavelength light. However,
high substrate costs and low device yields makes InP-based optoelectronic
devices rather expensive. Devices based on GaAs substrates would be much
cheaper, but the difference in lattice constant normally prevents the
growth of InGaAs with high In compositions (.about.50% indium mole
fraction is preferred to achieve relevant telecom outputs) on GaAs
substrate.
The bandgap in a semiconductor laser determines the frequency of light
output by a semiconductor laser. Recently it has been demonstrated that by
incorporating small amounts (fraction of a percent to a few percent) of
nitrogen into the InGaAs film, the band gap of InGaAs alloys grown on GaAs
substrate can be reduced thereby shifting the light emitted by the
resulting devices to longer wavelengths. Indium Gallium Arsenide Nitride
alloys have been found to be excellent semiconductor materials for
fabricating the active region of VCSELS or other long-wavelength
optoelectronic devices (e.g. edge-emitting laser, photodetectors or solar
cells). Using elementary nitrogen as a group V source and a Molecular Beam
Epitaxy (MBE) process, several groups have fabricated InGaAsN based lasers
that output 1.3 micrometer wavelength light. See, M. Kondow et al., Jpn.
J. Appl. Phys., Vol. 35, 1273 (1996) and M. Kondow, S. Natatsuka, T.
Kitatani, Y. Yazawa, and M. Okai, Electron. Lett. 32, 2244 (1996).
However, MBE is a slow growth technique and therefore not well suited to
mass production of high volume optoelectronic devices such as VCSELs,
egde-emitting lasers or solar cells.
Metal Organic Chemical Vapor Deposition (MOCVD) is a suitable technique for
volume production of InGaAsN lasers. However, the high growth temperatures
and surface chemistry of MOCVD results in inefficient incorporation of
elemental nitrogen (N) in the InGaAsN material. To increase the
incorporation efficiency of Nitrogen in the InGaAsN, the Nitrogen is
typically introduced in a dimethylhydrazine (DMHy) form as described in J.
Koch, F. Hohnsdorf, W. Stolz, Journal of Electronic Materials, Vol. 29,
165 (2000) and A. Ougazzaden et al., Appl. Phys. Lett. 70, 2861 (1997)
which are hereby incorporated by reference. Large oversupplies of DMHy are
used to achieve sufficient amounts of nitrogen in the InGaAsN structure.
The use of large quantities of DMHy as a nitrogen source has two major
disadvantages. The first disadvantage is high cost. DMHy is expensive,
current costs for 100 grams of DMHy is approximately $5000. A second
disadvantage of using DMHy as a Nitrogen source is the relatively high
impurity levels that often exist in commercially available DMHy. High
impurity levels are possibly one reason why MOCVD grown InGaAsN films are
often inferior to MBE grown films.
Thus a better source of Nitrogen for forming InGaAsN structures is needed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a MOCVD system configured to use ammonia as a nitrogen source.
FIG. 2 is a graph that shows the effect of different nitrogen
concentrations on the bandgap energy and on the emission wavelength of a
laser.
FIG. 3 shows an edge-emitting laser structure with an InGaAsN active region
formed using ammonia as a nitrogen source.
FIG. 4 shows a graph that shows the effect of changing catalyst
concentrations on the nitrogen mole fraction deposited in an active layer.
FIG. 5 is a flow chart that describes the operations involved in formation
of the semiconductor laser of FIG. 3.
SUMMARY OF THE INVENTION
An improved nitrogen source to form a sample including both nitrogen and
gallium-arsenide is described. In the method, ammonia and a source of
gallium and a source for arsine is introduced into the reaction chamber.
The ammonia is decomposed releasing nitrogen atoms. A catalyst may be used
to facilitate ammonia decomposition. The released nitrogen atoms, the
gallium and the arsine together form a film including both nitrogen,
gallium and arsine.
DETAILED DESCRIPTION
An improved method of decomposing ammonia to provide a source of nitrogen
to form a sample including both nitrogen, gallium and arsenide is
described. The method is applicable to various semiconductor growth
processes, however, the most important semiconductor process for which the
described method may be used is in a MOCVD process. The details of the
MOCVD process are described in "Organomtallic Vapor-phase Epitaxy: Theory
and Practice" by G. B. Stringfellow, published by Academic Press (1989),
which is hereby incorporated by reference.
FIG. 1 shows a MOCVD system 100 that uses ammonia gas as a nitrogen source
to form an InGaAsN active layer of an optoelectronic device (e.g. VCSEL,
edge-emitting laser, or solar cell). In FIG. 1, a film 105 of InGaAsN is
grown on a GaAs substrate 104, supported by support structure 108. The
support structure 108 includes a graphite suspector 112 that rotates on a
fiber-pyrometer 116. The rotational motion promotes even growth of InGaAsN
film 105. A reaction chamber 120, typically made from quartz,
substantially encloses the film 105 and the substrate 104 enabling tight
control over environmental conditions around the film 105. One controlled
condition includes maintaining a vacuum within chamber 120 substantially
below atmospheric pressure, typically below 100 Torr. Gas flow 124
entering and exiting the reaction chamber is also tightly controlled.
In the illustrated embodiment, ammonia gas, along with other gases used to
form the film, travels through gas inlet 128 into chamber 120. The mixture
of gases depends on the structure being formed. Heating source 132, in one
example, a RF-induction heating coil, raises the temperature within
chamber 120 to a temperature that facilitates growth of the film. Typical
temperature ranges are between approximately 400 and 750 degrees
centigrade. When fabricating an indium gallium arsenide film, temperatures
around 560 degrees centigrade allow for a good growth rate with good
structural and electronic properties. Gas exhaust 136 permits venting of
un-used gases and leftover reactant products. The constant flow of gases
from gas inlet 128 to gas exhaust 136 allows tight control over the
composition of the gas mixture in chamber 120.
The concentration of gases input into chamber 120 depends on the desired
composition of the substrate being formed. When InGaAsN is formed using
ammonia gas, high concentrations of ammonia gas, typically constituting
over 50% of the gas flow into the chamber, compensates for the low overall
incorporation efficiency of Nitrogen in MOCVD growth of InGaAsN.
Typically, the ratio of [NH.sub.3 ] to [AsH.sub.3 ] during growth of
InGaAsN ranges between 5:1 and 20:1.
A second problem with using ammonia gas as a nitrogen source is the slow
decomposition rate of ammonia at low temperature. Raising the temperature
above 700 degrees centigrade may cause ammonia pyrolysis resulting in
sufficient decomposition of ammonia to provide a needed supply of free
nitrogen atoms; however, such high temperatures are undesirable for InGaAs
material growth. Instead, one embodiment of the invention uses a catalyst
to accelerate the ammonia decomposition rate. In one embodiment of the
invention, the catalyst is a chemical catalyst, typically a metal organic,
such as trimethlyaluminium (TMAl).
FIG. 3 shows a typical InGaAsN laser device structure 300. FIG. 2 is a
graph that illustrates the changes in laser emission wavelength as a
function of changing nitrogen mole fraction in the active region of the
laser device structure. In FIG. 2, the nitrogen content in the active
regions along axis 208 is plotted against the bandgap energy of the active
region represented along axis 204. The large bowing parameter of InGaAsN
alloys (bowing parameter of approximately 18-20 eV for Nitrogen
concentrations below 2%) allows significant band gap energy reductions to
be achieved by adding small amounts of nitrogen (y<2%) in
InGaAs.sub.1-y N.sub.y alloys.
The wavelength of light output by active devices fabricated from InGaAsN is
plotted on axis 212. As can be seen from the graph, increases in
wavelength output is directly related to reduced bandgap energy 204.
Reductions in bandgap energy 204 may be achieved by increasing nitrogen
content in the film. Thus, one important reason for incorporating nitrogen
in the active layer of a semiconductor laser is to reduce the bandgap
energy and thereby increase the wavelength output by the active device
formed from the GaAs substrate. Reducing the bandgap enables the
fabrication of semiconductor lasers that output light wavelengths longer
than 1.3 micrometers, preferably between 1.3 and 1.55 micrometer.
In order to facilitate decomposition of the nitrogen, a chemical catalyst,
usually including aluminum, is used. One problem with chemical catalysts
that include aluminum is that the aluminum itself forms an alloy with
GaAs. Increasing Al composition in the AlGaAs alloy increases the aluminum
gallium arsenide film bandgap. However, the decrease in band gap due to
the incorporation of nitrogen is larger than the increase in bandgap due
to the incorporation of Al in the film. Thus the overall effect is that
the increase in bandgap due to the aluminum merely lessens the overall
decrease in bandgap due to the nitrogen. To further minimize the effects
of the aluminum other catalysts that do not contain aluminum may be
substituted. One example of such a catalyst is Trimethylantimony the ratio
of ammonia to organic metal compound catalyst is greater than 100:1.
One example of a gas mixture that has been successfully used in a MOCVD
reaction chamber to form a GasAsN film with a 1.18% nitrogen composition
includes: a concentration of H.sub.2 at 6 slpm combined with AsH.sub.3 at
15 sccm (670 micromol/min), NH.sub.3 at 95 sccm (4240 micromol/min), TMGa
at 8 sccm (101 micromol/min) and TMAI (trimethylaluminum) at 5.2 sccm (4
micromol/min). In the MOCVD growth process, Hydrogen (H2) serves as the
carrier gas transporting the different metal organic (MO) compounds from
the bubbler into the chamber. AsH.sub.3 gas provides the arsine component
to the compound; NH.sub.3 gas provides the nitrogen component to the
compound; and the metal organic (MO) TMGa provides the gallium component
to the GaAs compound semiconductor film.
TMAl serves as a catalyst, enhancing the nitrogen incorporation in the
InGAsN film. The TMAl also causes incorporation of some aluminum into the
film forming an alloy with gallium, indium, nitrogen. As previously
discussed, other catalysts may be substituted for TMAl to avoid aluminum
incorporation. Typical flow ranges for H.sub.2 are in the range of 2 to 10
slpm, AH.sub.3 flow rates are in the range of 5 sccm to 200 sccm, TMGa
flow rate are in the range from 1 sccm to 100 sccm and TMAl flow rates are
in the range between 1 sccm to 100 sccm. During formation of the GaAsN
film, the temperature was maintained at 560 degrees C. while the pressure
within the chamber was maintained at 75 Torr.
Increases in the nitrogen concentration in the GaAsN films may be achieved
by increasing the catalyst concentration. For example, FIG. 4 plots the
mole fraction of nitrogen in a GaAsN films along axis 404 as a function of
the flow rate of a metal organic catalyst (TMAl) plotted along axis 408.
FIG. 4 assumes formation in a MOCVD process using a constant flow rate of
ammonia Alternatively the flow rate of ammonia can be increased to
increase the nitrogen content in the InGaAsN films.
As previously described, one problem with chemical catalysts is that the
catalyst itself may form undesirable alloys with GaAs. Using TMAl
(trimethylaluminum) as a catalyst produces undesirable aluminum gallium
arsenide compounds. Instead of chemical catalysts, one embodiment of the
invention uses a radiation source 150 that emits short wavelength light to
enhance the decomposition of the ammonia of the invention. Use of a
radiation as a catalyst can either reduce or altogether eliminate the use
of chemical catalysts and the associated alloy formation. The light
radiation is at a predetermined frequency easily absorbed by the ammonia,
typically the frequency ranges between 200-350 nanometers. Typical
radiation sources generating the desired light frequency output include
excimer laser sources or frequency tripled or frequency quadrupled solid
state laser sources such as Nd:YAG lasers.
FIG. 5 is a flow chart that describes using a MOCVD process to form the
laser structure of FIG. 3. In block 504, GaAs is formed on a GaAs
substrate 105 by combining AsH.sub.3 and TMGa in a reaction chamber. The
GaAs is positioned on a graphite suspector 112 serves as a substrate for
the formation of an InGaAsN layer. The GaAs substrate of FIG. 3 serves as
a bottom conductor or contact for the laser structure. The GaAs layer
grown on top of the GaAs substrate is typically doped n-type. One method
of achieving n-doping is uses SiH.sub.4 as a dopant source to provide
Si-dopants. The GaAs layer is typically grown at around 735 degree
Celsius.
In block 508, an aluminum containing gas such as TMAl is added to the
AsH.sub.3 and TMGa gas flows to form an AlGaAs cladding layer over GaAs
substrate 308 of FIG. 3. The AlGaAs layer grown on top of the GaAs layer
is also n-doped using SiH.sub.4 as a dopant source to provide Si dopant.
In alternative embodiments, other dopant materials such as germanium may
also be used.
The AlGaAs layers serve as cladding layer surrounding active region 312 of
FIG. 3. The active region is formed by depositing an undoped Gas waveguide
layer 316 in block 512 followed by deposition of an InGaAsN square quantum
well layer 320 in block 516. In one fabrication condition, the GaAs
waveguide layer is grown at approximately 640 degree Celsius and the
temperature is lowered to around 560 degree Celsius during growth of the
InGaAsN layer. Decomposed ammonia provides the nitrogen atoms used in
forming the InGaAsN layer. In block 520, the temperature is raised again
to approximately 640 degrees as a second undoped GaAs waveguide layer 324
is deposited over square quantum well 320. The undoped GaAs layers 316,
324 form the walls of the square quantum well and serves also as a
waveguiding layers for the separate confinement heterostucture.
A second AlGaAs cladding layer 328 is deposited in block 524. The second
AlGaAs layer grown is Mg- or C-doped using Cp.sub.2 Mg or CCl.sub.4 as a
dopant source to form a p-doped layer. In one embodiment, the AlGaAs layer
is grown at approximately 735 degree Celsius. The higher growth
temperature during the growth of the second AlGaAs layer also causes an
annealing of the InGaAsN film. In an alternate embodiment, the InGaAsN
film could be annealed by rapid thermal annealing (RTA) in a furnace under
nitrogen atmosphere and with a GaAs cap to achieve an equivalent effect.
An example annealing condition is to anneal the substrate at 750 degree
Celsius for 3 minutes.
The second AlGaAs cladding layer together with the first cladding layer
forms a waveguide structure that provides optical confinement of the
transverse optical mode. In block 528, a second GaAs contact 332 is
deposited over the cladding layer. The second GaAs contact layer may be
formed around 640 degrees Centigrade and is usually p-doped with
Mg--and/or Carbon--using Cp.sub.2 Mg and/or CCl.sub.4 as a dopant source.
Ohmic metal contacts are deposited on the n-type GaAs substrate (e.g.
AuGe) and the p-type GaAs contact layer (e.g. Ti/Pt/Au). The n- and p-type
contacts couple to a current source to supply a forward current to the
laser structure. Laser facets are cleaved along the GaAs (110) cleavage
planes to form a laser cavity and provide optical feedback.
Although the illustrated structure describes one embodiment of a
traditional edge-emitting laser structure, many variations are possible.
For example, some structures may utilize GaAs/AlAs quarter wavelength
stacks instead of AlGaAs cladding layers. The GaAs/AlAs quarter wavelength
stacks act as distributed Bragg reflectors (DBR) and form a laser cavity
perpendicular to the substrate plane. The optical feedback from the DBR
mirrors forms a vertical cavity surface emitting laser structure (VCSEL).
In addition a transparent ITO contact instead of a metal p-contact could
be used. A transparent contact allows direct current injection into the
active region while simultaneously permitting light output through the
transparent contact.
The preceding description includes many parameters, descriptions and other
details that should not be interpreted to limit the scope of the
invention. Such details are provided to facilitate understanding of the
invention and should not be construed as a necessary part of the
invention. For example, although the detailed description describes the
use fabrication of a edge-emitting structure, the concepts of the
invention may also be used to form VCSEL lasers, other passive devices
such as photodetector devices or solar cells, light detection systems, and
electronic devices, like HBT or HEMT structures. Also, a number of details
such as temperatures used during formation have been provided to
facilitate fabrication, but it should be understood that other
temperatures may be used and still within the scope of the invention.
Thus, the scope of the invention should only be limited by the claims as
set forth below.
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