|United States Patent
,   et al.
February 16, 1988
Silicon germanium photodetector
A photodetector, comprising a Ge.sub.x Si.sub.1-x superlattice region
between two silicon cladding layers in which the Ge.sub.x Si.sub.1-x
layers absorb light, is described.
Bean; John C. (New Providence, NJ);
Luryi; Sergey (Millington, NJ);
Pearsall; Thomas P. (Summit, NJ)
American Telephone and Telegraph Company, AT&T Bell Laboratories (Murray Hill, NJ)
November 18, 1985|
|Current U.S. Class:
||257/19; 257/21; 257/432 |
|Field of Search:
357/4 SL,30 A,30 N,16,13
References Cited [Referenced By]
U.S. Patent Documents
Capasso et al., International Electron Device Meeting Wash. D.C., (7-9 Dec.
1981), p. 284-IEDM 81.
Primary Examiner: Edlow; Martin H.
Attorney, Agent or Firm: Laumann; Richard D.
What is claimed is:
1. A photodetector comprising a first cladding layer of silicon having a
first conductivity type;
an interleaved region comprising alternating layers of Ge.sub.x Si.sub.1-x
and Ge.sub.y Si.sub.1-y, x greater than 0.0 and y and less than or equal
to 1.0, y greater than or equal to 0.0 and less than than 1.0; and
a second cladding layer having a second conductivity type.
2. A photodetector as recited in claim 1 in which said second cladding
layer comprises silicon.
3. A photodetector as recited in claim 2 in which x and y are constant.
4. A photodetector as recited in claim 1 in which said first conductivity
type is p-type.
5. A photodetector as recited in claim 4 further comprising a third layer
of silicon having intrinsic conductivity and being between said second
layer and said superlattice.
6. A photodetector as recited in claim 5 further comprising a fourth layer
of silicon having a first conductivity type and being between said third
layer and said superlattice.
7. A photodetector as recited in claim 1 further comprising a grating on
said second layer of silicon.
This invention relates generally to the field of photodetectors and
particularly to such photodetectors using a silicon germanium composition
as the absorbing medium.
BACKGROUND OF THE INVENTION
For many applications, including optical communication systems,
photodetectors are required. Silicon is a widely used material for
photodetectors but it has a bandgap of approximately 1.12 eV which
restricts its utility to those applications in which radiation having a
wavelength less than approximately 1.0 .mu.m will be detected.
Accordingly, for detection of radiation at wavelengths longer than 1.0
.mu.m, other materials must be used. Materials commonly used include Ge
and Group III-V compound semiconductors such as InGaAs.
For both p-i-n photodiodes and avalanche photodetectors, germanium is a
less than ideal semiconductor because one should use its direct bandgap,
0.8 eV, for the absorption of radiation while the relatively small
indirect bandgap, 0.66 eV, leads to large dark currents in typical device
configurations. Additionally, because the ratio of the ionization
coefficients is approximately 1.0, the rates at which the types of
carriers ionize are not significantly different. This produces an
intrinsically high noise level in an avalanche gain operating mode. As is
well known to those skilled in the art, the lowest noise avalanche
photodetectors arise when one type of carrier ionizes at a rate much
greater than the other type of carrier, i.e., the ratio of the ionization
coefficients differs significantly from 1.0. Group III-V compound
semiconductors are not ideal for avalanche photodetectors because they
also have a relatively small ratio of the ionization coefficients.
One approach to alleviating these problems in avalanche photodetectors
involves the use of separate absorption and multiplication regions. The
incident light is absorbed in a relatively small bandgap region and
avalanche multiplication occurs in a relatively large bandgap region. One
such photodetector is described in U.S. Pat. No. 4,212,019, issued on July
8, 1980 to Wataze et al. In one embodiment, his Example 3, the
multiplication region comprised a p-type silicon layer and the absorption
region comprised a p-type Ge.sub.x Si.sub.1-x layer. In another embodiment
which is depicted in his FIG. 2, the multiplication and absorption regions
are not clearly defined but rather, the composition of the Ge.sub.x
Si.sub.1-x region is gradually varied. The detailed description states
that the composition varies from pure Ge at the edge of the absorption
region to pure Si at the edge of the multiplication region.
However, a detailed consideration of this disclosure by one skilled in the
art reveals that the devices described are not suitable for use as
photodetectors at wavelengths longer than approximately 1.2 .mu.m. In
particular, they are not suitable for use as photodetectors in the 1.3 to
1.6 .mu.m wavelength range presently of interest for optical communication
systems using silica-based fibers. This range is of interest because it
includes the regions of lowest loss and minimum dispersion in the fiber.
The limited utility, with respect to wavelength, of the avalanche
photodetector arises because Ge and Si are indirect bandgap materials and
a relatively thick GeSi absorbing layer is required for high quantum
efficiency. In fact, an approximately 50 to 100 .mu.m layer will be
required for most incident light to be absorbed. However, the structure
disclosed cannot have a thick, high quality Ge.sub.x Si.sub.1-x absorbing
layer on the silicon substrate because of the large lattice mismatch
between the absorbing layer and the underlying silicon substrate. This
lattice mismatch will inevitably result in a large number of defects,
e.g., misfit dislocations, which will certainly preclude operation of the
device as an avalanche photodetector. Additionally, even if the structure
were fabricated without defects, it would not be useful for high speed
communications applications because the photogenerated carriers would have
to travel distances of the order of 50 .mu.m to reach the contacts. This
would result in a response time of the order of a nanosecond.
SUMMARY OF THE INVENTION
We have found that a photodetector comprising a first cladding layer; an
interleaved region of alternating Ge.sub.y Si.sub.1-y and Ge.sub.x
Si.sub.1-x layers, x greater than 0.0 and less than or equal to 1.0 and y
greater than or equal to 0.0 and less than 1.0; and a second cladding
layer is a useful photodetector at wavelengths longer than 1.0 .mu.m. In a
preferred embodiment, both cladding layers comprise Si. In another
preferred embodiment, the first silicon layer has p-type conductivity, and
the second silicon layer has n-type conductivity. In still another
embodiment, the device further comprises a third silicon layer having
intrinsic conductivity between the n-type layer and the superlattice. The
alloy layers in the interleaved region are normally undoped and the device
operates as an avalanche photodetector with electrons, photogenerated in a
GeSi layer, initiating the avalanche process in a silicon cladding region.
There may be a fourth p-type silicon layer between the interleaved region
and the intrinsic conductivity layer. The fourth layer is depleted during
device operation and yields the desired high-low electric field
configuration for the absorption and multiplication regions. In a
preferred embodiment, edge coupling of the light into the interleaved
region is used to obtain greater absorption than is possible with vertical
illumination. The waveguiding effect of the superlattice further increases
the absorption efficiency by increasing the optical path length. This is
accomplished, however, without a dramatic increase in the spacing between
the p- and n-type layers. A fast response time is retained. In yet another
embodiment, grating assisted coupling is used to introduce the light into
the superlattice region.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a view of one embodiment of this invention;
FIG. 2 is a depiction of an embodiment of this invention using edge
FIG. 3 is an embodiment of this invention using grating assisted coupling.
For reasons of clarity, the elements of the devices depicted are not drawn
The invention will first be described by reference to the exemplary
embodiment depicted in FIG. 1. The structure comprises a substrate 1, a
first silicon layer 3 having a first conductivity type; an interleaved
region 5 comprising a superlattice region having alternating layers of
Ge.sub.x Si.sub.1-x and Si; and a second silicon layer 7 having a second
conductivity type. In a preferred embodiment, the first conductivity type
is n-type. The superlattice region has a thickness h.sub.SL and comprises
a plurality of alternating, i.e., interleaved Ge.sub.x Si.sub.1-x and Si
layers indicated as 51, 53, 55 and 57. For reasons of clarity only 4
layers are depicted. More will typically be present. Each period, i.e.,
one Ge.sub.x Si.sub.1-x and one Si layer, has a thickness T and the
Ge.sub.x Si.sub.1-x alloy layer has a thickness h.sub.a.
Although photodetectors having only a single alloy layer are possible, they
are not as desirable as those having a superlattice region because the
effective absorption coefficient would be so low that the absorbing region
would require a horizontal dimension of several centimeters assuming edge
coupling. Of course, the resulting high capacitance is undesirable.
Vertical illumination would be unlikely to yield a useful embodiment. The
precise value of x selected for the superlattice layers is determined by
the desire to absorb light at a specific wavelength. It is generally
desirable, for a waveguide configuration, to grow the superlattice region
as thick as possible provided, of course, that stability against
dislocation formation is maintained. It should be noted that a
superlattice thicker than approximately 3000 Angstroms may be deleterious
as it may yield multimode waveguide behavior and undesirable dispersion.
Each alloy layer within the superlattice region can be grown as thick as
possible subject, of course, to the caveat that the growth should remain
It will be readily appreciated that, in general, the interleaved region
comprises alternating layers of Ge.sub.x Si.sub.1-x and Ge.sub.y
Si.sub.1-y , y greater than 0.0 and less than or equal to 1.0 and y
greater than or equal to 0.0 and less than 1.0, with x being greater than
y. If x and y are constant, the interleaved region is a superlattice. Of
course, x and y may vary within the interleaved region. Such variations
may lead to stronger waveguiding due to refractive index variations within
The structure depicted can be grown by what are now conventional and
well-known Si molecular beam epitaxy techniques. The thickness and
composition of the superlattice layers are selected, together with the
growth conditions, so that good crystal quality, that is, a small number
of misfit dislocations, is maintained during crystal growth. As the
subscript x increases, that is, as the Ge content increases, the lattice
mismatch between the alloy layer and the silicon layer becomes greater and
the maximum attainable thickness of the alloy layer becomes smaller. The
mismatch is accommodated by strain. These relationships are described in
U.S. Pat. No. 4,529,455 issued on July, 16, 1985 to John C. Bean, Leonard
C. Feldman, and Anthony T. Fiory, which is incorporated herein by
reference. The attainable superlattice thickness is determined by taking
the average value of x in the superlattice for one period, i.e., the
superlattice thickness is determined by treating it as having a misfit
equal to the misfit average of a single period. Contacts can be fabricated
by well-known techniques.
As will be readily appreciated by those skilled in the art, as the
subscript x increases, the bandgap decreases and the superlattice alloy
regions are capable of absorbing light at ever longer wavelengths.
However, the Ge.sub.x Si.sub.1-x absorbing layers will become
progressively thinner and accordingly, greater number of such layers will
be required in the superlattice region to obtain a generally complete
absorption of the incident light for vertical illumination. A possible
practical limitation to the superlattice region thickness arises because
of the presently relatively slow growth rate attainable with molecular
The incident light is absorbed in the Ge.sub.x Si.sub.1-x layers within the
superlattice regions. The device may be used as an avalanche
photodetector. The minority carrier electrons drift to the Si n-type
region and undergo avalanche multiplication. This a is desirable
configuration because electrons have a higher ionization rate than do
holes in silicon.
It is contemplated that the light will be edge coupled into the embodiment
depicted in FIG. 1, i.e., the light is coupled directly into the
superlattice region. Of course, the incident light will generally have an
intensity distribution centered on the superlattice region but also
extending into the adjacent silicon layers. Again, a thicker superlattice
region will be desirable to maximize absorption. The Ge.sub.x Si.sub.1-x
layers have a refractive index higher than that of the silicon layers, and
accordingly, the incident light is guided within the alloy layers. This
gives an absorbing distance which is effectively quite large and is
limited only by the horizontal extent of the Ge.sub.x Si.sub.1-x layers.
The device is thus useful at longer wavelengths than it is for the
vertical illumination as the superlattice region can be made thinner for
comparable absorption. However, the coupling of light from the optical
fiber into the superlattice region is quite likely to be less efficient
for the edge coupled embodiment than it is for the vertical illumination
Another embodiment of a photodetector according to this invention is
depicted in FIG. 2. Numerals identical to those used in FIG. 1 represent
identical elements. The device further comprises a third silicon layer
having p-type conductivity and a thickness .DELTA. and a fourth silicon
layer having intrinsic conductivity and a thickness d. The third layer is
adjacent the superlattice region and the fourth layer is adjacent the
third layer. The third layer is relatively thin, generally less than
10.sup.-4 cm, and is depleted during operation by the applied reverse
bias. Calculation of appropriate doping levels will be easily done by
those skilled in the art. The device should have a high-low electric field
configuration, i.e., the absorption and multiplication regions have a low
and high, respectively, electric field.
Several factors should be considered in choosing appropriate device
parameters to reduce the excess noise. For example, electrons should
initiate the avalanche process for reasons already discussed. The electric
field in the avalanche region should be near threshold and the avalanche
region should be much thicker than the inverse of the ionization
coefficient. Additionally, the electric field in the superlattice region
should not exceed the ionization threshold for germanium, otherwise impact
in the dark and thus, additional noise will result.
Some of the difficulties in the edge coupling embodiment may be avoided by
use of the embodiment depicted in FIG. 3. Numerals identical to those in
FIG. 1 represent identical elements. Shown in FIG. 3 is a grating-assisted
coupling scheme which reference numerals identical to those in FIG. 1,
represent identical elements. As can be seen, the device further comprises
a grating 17 etched on the surface of the top silicon layer. The period
and other details of the grating coupler are well known to those skilled
in the art and need not be repeated here. Fabrication techniques are also
well known to those skilled in the art. As depicted, the light can be
incident from the top surface rather than from the side of the detector.
This position facilitates coupling of the photodetector to the incident
light as positioning of the optical fiber is typically easier when
positioned with respect to the top surface than with respect to the
Still, other embodiments will be readily thought of by those skilled in the
art. For example, the cladding layer farthest from the substrate may
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