U.S. patent application number 10/763269 was filed with the patent office on 2005-01-13 for avalanche photodiode.
This patent application is currently assigned to Hitachi, Ltd.. Invention is credited to Fujisaki, Sumiko, Matsuoka, Yasunobu, Tanaka, Shigehisa.
Application Number | 20050006678 10/763269 |
Document ID | / |
Family ID | 33562494 |
Filed Date | 2005-01-13 |
United States Patent
Application |
20050006678 |
Kind Code |
A1 |
Tanaka, Shigehisa ; et
al. |
January 13, 2005 |
Avalanche photodiode
Abstract
An avalanche photodiode includes at least one crystal layer
having a larger band-gap than that of an absorption layer formed by
a composition or material different from that of the absorption
layer formed on a junction interface between a compound
semiconductor absorbing an optical signal and an Si multiplication
layer, and the crystal layer may be intentionally doped with n or p
type impurities to cancel electrical influences of the impurities
containing oxides present on the junction interface of compound
semiconductor and surface of Si.
Inventors: |
Tanaka, Shigehisa;
(Kunitachi, JP) ; Fujisaki, Sumiko; (Hachioji,
JP) ; Matsuoka, Yasunobu; (Hachioji, JP) |
Correspondence
Address: |
Stanley P. Fisher
Reed Smith LLP
Suite 1400
3110 Fairview Park Drive
Falls Church
VA
22042-4503
US
|
Assignee: |
Hitachi, Ltd.
OpNext Japan, Inc.
|
Family ID: |
33562494 |
Appl. No.: |
10/763269 |
Filed: |
January 26, 2004 |
Current U.S.
Class: |
257/292 ;
257/E31.064 |
Current CPC
Class: |
H01L 31/1075
20130101 |
Class at
Publication: |
257/292 |
International
Class: |
H01L 031/062 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 9, 2003 |
JP |
2003-194005 |
Claims
What is claimed is:
1. An avalanche photodiode comprising: an absorption layer
absorbing light to create carriers; and a multiplication layer
multiplying the created carriers, wherein the multiplication layer
is formed of Si and the absorption layer is formed of a compound
semiconductor, and wherein a semiconductor interface layer having a
wider band-gap than that of the absorption layer is formed between
the multiplication layer and the absorption layer.
2. An avalanche photodiode according to claim 1, wherein the
absorption layer is formed of an InGaAs mixed crystal or InGaAlAs
mixed crystal or InGaAsP mixed crystal, and the semiconductor
interface layer is formed of the InGaAlAs mixed crystal or InGaAsP
mixed crystal.
3. An avalanche photodiode according to claim 1, wherein the
absorption layer is formed of an InGaAs mixed crystal or InGaAlAs
mixed crystal or InGaAsP mixed crystal, and the semiconductor
interface layer is formed of InP or GaAs.
4. An avalanche photodiode according to claim 1, wherein the
absorption layer is formed of a semiconductor containing Sb.
5. An avalanche photodiode according to claim 1, wherein a junction
between the multiplication layer and the semiconductor interface
layer is formed by a fusion.
6. An optical module mounting an avalanche photodiode, said
avalanche photodiode comprises: an absorption layer absorbing light
to create carriers; and a multiplication layer multiplying the
created carriers, wherein the multiplication layer is formed of Si
and the absorption layer is formed of a compound semiconductor, and
wherein a semiconductor interface layer having a wider band-gap
than that of the absorption layer is formed between the
multiplication layer and the absorption layer.
7. An optical receiver mounting an avalanche photodiode, said
avalanche photodiode comprises: an absorption layer absorbing light
to create carriers; and a multiplication layer multiplying the
created carriers, wherein the multiplication layer is formed of Si
and the absorption layer is formed of a compound semiconductor, and
wherein a semiconductor interface layer having a wider band-gap
than that of the absorption layer is formed between the
multiplication layer and the absorption layer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an avalanche photodiode and
more particularly to a fast, highly sensitive, wideband avalanche
photodiode with a large gain for use in optical communication.
[0003] 2. Description of the Related Art
[0004] The avalanche photodiode is a light receiving device with a
built-in function of amplifying an optical signal and, because of
its high sensitivity and fast operation, has found a wide range of
applications as an optical communication light receiving device.
The amplification function of the avalanche photodiode is realized
by taking advantage of an avalanche breakdown phenomenon that
occurs in semiconductors. A principle by which an amplification
occurs during the avalanche breakdown is briefly explained as
follows.
[0005] Electrons or holes moving in a semiconductor are scattered
by a crystal lattice when they strike it. Applying a large electric
field to the semiconductor accelerates these carriers, resulting in
an increase in their moving speed. As the moving speed of the
carriers in the semiconductor increases and their kinetic energy is
higher than a bandgap of the semiconductor, a probability of
breaking bonds of lattice increases when they hit the crystal
lattice, newly creating free-moving electron-hole pairs. An atom
with its bonds broken loses electric charges and looks as if it is
ionized. This phenomenon is therefore called an impact electrolytic
dissociation or impact ionization, and a measure of how many
electron-hole pairs are generated by the impact ionization after an
electron or hole has traveled a unit distance is also called an
ionization rate. A ratio of an ionization rate based on electrons
to an ionization rate based on holes is further called an
ionization rate ratio.
[0006] Newly created carriers (electrons or holes) produced by the
impact ionization are also accelerated by the electric field and
acquire a kinetic energy, with subsequent impact ionizations
further creating new carriers. As the impact ionization
repetitively occurs, the number of carriers increases rapidly,
creating a large current. This is the phenomenon called an
avalanche breakdown. In a semiconductor that is applied an electric
field of a magnitude just below the avalanche breakdown, an
injection of carriers, even in a small number, can produce a large
number of new carriers through the impact ionizations, resulting in
a sudden increase in current. That is, a large current can be
obtained even with an injection of a small number of carriers. This
is a principle by which amplification is accomplished during the
avalanche breakdown. The avalanche photodiode uses photo-induced
carriers produced by an optical absorption for the carrier
injection that triggers this phenomenon.
[0007] As well known, an important factor in terms of a high-speed
response of the avalanche photodiode is the ionization rate ratio.
The more the ionization rate ratio is away from unity, the better
the performance of the avalanche photodiode becomes. Conversely, as
the ionization rate ratio approaches unity, the amplification rate
at high speed deteriorates, making it impossible to produce an
avalanche photodiode with a good performance. Since infrared light
is used in a high-speed optical communication, the fabrication of
the light receiving device has so far used compound semiconductors,
such as InP and InGaAs. However, the ionization rate ratio of InP,
a typical compound semiconductor used in optical communication, is
0.5, relatively close to unity. Even with InAlAs the ionization
rate ratio is 4 or 5 at most. Thus, an applicable frequency is
about 10 GHz at most. As a result, a satisfactory performance
cannot be obtained for high-speed devices of 40 GHz or higher.
[0008] On the other hand, Si has a very large ionization rate ratio
ranging from 10 to more than 100 and thus can make a fast, highly
sensitive avalanche photodiode. However, since Si cannot absorb
light in an infrared frequency range used for optical
communication, Si has not been able to be used for optical
communication.
[0009] To overcome this drawback of the Si avalanche photodiode, an
attempt has been made to combine Si with a compound semiconductor
that has a sensitivity in the infrared range. For example,
epitaxially growing a compound semiconductor on Si has been
explored for a couple of decades now. However, no crystal with a
satisfactory quality has been realized for practical use.
[0010] An example method for alleviating this quality problem of
such a compound semiconductor on Si is disclosed in U.S. Pat. No.
6,384,462B1, which is briefly explained with reference to FIG. 2.
In this patent, an avalanche photodiode is formed by directly
fusing a Si multiplication layer 23 onto an InGaAs layer 22
epitaxially grown on a compound semiconductor substrate 21, as
shown in FIG. 2. Further, by using ion implantation and diffusion
techniques, a contact layer 24 and a guard ring 25 are formed. The
use of the fusing technique keeps the crystalline structure of both
the compound semiconductor and Si intact, so a high quality light
receiving device can be obtained.
[0011] In the structure described above, however, the Si
multiplication layer is directly fused at elevated temperatures to
the InGaAs layer of a low carrier concentration that absorbs
optical signal. Normally, on an interface of a junction between the
InGaAs layer and the Si multiplication layer, there are many
impurities including oxides. These impurities infiltrate into the
InGaAs layer near the junction during the fusing process. As a
result, the carrier concentration in the InGaAs layer near the
junction increases, resulting in a high electric field being
applied. The InGaAs layer has a narrow bandgap, so that when it is
applied a high electric field, a dark current increases, degrading
the sensitivity down to a level not suitable for practical use. In
fact, in a device which has a Si multiplication layer directly
fused to an InGaAs layer, the dark current exceeds a microampere,
making the sensitivity of the device three or more orders of
magnitude worse than those of conventional avalanche photodiodes in
practical use. Further, a high electric field gives rise to a
problem of causing an avalanche breakdown even in the InGaAs layer
and thus degrading a high-speed response.
[0012] An object of the present invention is to provide an
avalanche photodiode having a low dark current, a high sensitivity
and a high speed and made of a combination of a compound
semiconductor and Si, and to provide a method of manufacturing the
same.
SUMMARY OF THE INVENTION
[0013] The avalanche photodiode of this invention has a structure
in which, in an interface between a compound semiconductor that
absorbs an optical signal (referred to as an absorption layer) and
a Si multiplication layer, at least one crystal layer formed of a
composition or material different from that of the absorption layer
and having a larger bandgap than that of the absorption layer
(referred to as an interface layer) is formed.
[0014] Other objects, features and advantages of the invention will
become apparent from the following description of the embodiments
of the invention taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a cross-sectional view of a semiconductor light
receiving device as a first embodiment of the present
invention.
[0016] FIG. 2 is a cross-sectional view of a semiconductor light
receiving device as a conventional example.
[0017] FIG. 3 is a cross-sectional view of a semiconductor light
receiving device as a second embodiment of the present
invention.
[0018] FIG. 4 is a cross-sectional view of a semiconductor light
receiving device as a third embodiment of the present
invention.
[0019] FIGS. 5A to 5I are explanatory diagrams showing a process of
fabricating a semiconductor light receiving device of the third
embodiment of the present invention.
[0020] FIG. 6 is a graph showing a wavelength dependency of an
absorption coefficient of InGaAs.
[0021] FIG. 7 is a cross-sectional view of a semiconductor light
receiving device as a fourth embodiment of the present
invention.
[0022] FIG. 8 is a cross-sectional view of a semiconductor light
receiving device as a fifth embodiment of the present
invention.
[0023] FIG. 9 is a cross-sectional view of a semiconductor light
receiving device as a sixth embodiment of the present
invention.
[0024] FIG. 10 is a cross-sectional view of a semiconductor light
receiving device as a seventh embodiment of the present
invention.
[0025] FIG. 11 is a cross-sectional view of a semiconductor light
receiving device as a eighth embodiment of the present
invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Embodiment 1
[0026] FIG. 1 illustrates an example structure of the avalanche
photodiode of this invention. Denoted 11 is a Si substrate (n-type,
2xE18 cm.sup.-3), 12 a Si multiplication layer (n-type, 1E15
cm.sup.-3, 0.2 .mu.m), 13 an InAlAs interface layer (p-type, 1E18
cm.sup.-3, 0.05 .mu.m), 14 an InGaAs absorption layer (p-type, 2E15
cm.sup.-3, 1.2 .mu.m), 15 an InAlAs capping layer (p-type, 2E18
cm.sup.-3, 1 .mu.m), and 16 an InGaAs contact layer (p-type, 5E19
cm.sup.-3, 0.1 .mu.m). Reference number 17 represents a SiN film
protecting the surface of the device. Reference number 18 is a
metal electrode. The structure of the device shown is of a surface
illuminated type, and an optical signal enters from a surface of
the Si substrate 11 or the contact layer 16. The light receiving
surface may be provided with a non-reflective coat film or an
appropriate window structure or lens to enhance the optical signal
receiving efficiency.
Embodiment 2
[0027] FIG. 3 illustrates another embodiment of this invention. In
this embodiment, the device has a planar structure for improved
reliability. Denoted 31 is a guard ring and a p-type impurity is
doped through ion implantation or diffusion.
Embodiment 3
[0028] FIG. 4 illustrates still another embodiment of this
invention. In this embodiment the basic structure of the element is
similar to that of FIG. 1 except that a guard ring is provided for
improved reliability. Designated 41 is an InGaAlAs interface layer
(p-type, 1E18 cm.sup.-3, 0.05 .mu.m). The composition of InGaAlAs
is adjusted so that the interface layer has a bandgap wavelength of
1.1 .mu.m (equivalent to 1.13 eV) to prevent an optical signal of a
1.3-.mu.m band from being absorbed. Denoted 42 is a guard ring
formed of high-resistance InP. The guard ring may be p- or n-type
InP if the carrier concentration is low.
[0029] A process of manufacturing this structure will be explained
by referring to FIGS. 5A to 5I. First, a compound semiconductor and
Si to be joined together are prepared separately. As shown in FIG.
5A, a highly resistive Si multiplication layer 52 with a low
carrier concentration is epitaxially grown on an n-type Si
substrate 51 through an appropriate method. Alternatively, an
n-type impurity may be diffused into a highly resistive Si
substrate to form the same structure. It is also possible to
diffuse a p-type impurity into an n-type Si substrate to increase
the resistance of the surface and thereby form the same structure.
Next, as shown in FIG. 5B, this structure is formed into a
trapezoidal shape (mesa) as by photolithography and dry or wet
etching. The dimensions of the mesa structure need to be set to
produce a proper capacity for high frequency use. In this
embodiment, the mesa structure is shaped like a truncated cone
which at its top measures about 25 .mu.m in diameter for use in a
10 GHz range. Then, a dielectric film 53 of SiN or SiO.sub.2 is
formed over the surface by a proper chemical vapor deposition
method to protect the surface. In the case of SiO.sub.2, the
dielectric film may be formed by a thermal oxidation method. Next,
as shown in FIG. 5C, only the top portion of the dielectric film is
removed by photolithography and dry or wet etching to expose the
surface 54 of Si. Now, the preparation of Si is complete.
[0030] The compound semiconductor is prepared as follows. First, as
shown in FIG. 5D, a p-type InGaAs contact layer 56 (with a carrier
concentration of 5E19 cm.sup.-3 and a thickness of 0.1 .mu.m), a
p-type InGaAlAs capping layer 57 (2E18 cm.sup.-3, 1 .mu.m), a
p-type InGaAs absorption layer 58 (1E15 cm.sup.-3, 1 .mu.m) and a
p-type InGaAlAs interface layer 59 (1E18 cm.sup.-3, 0.05 .mu.m) are
epitaxially grown in that order over the InP substrate 55 by a
molecular beam epitaxy. These layers are adjusted in their
composition so as to have a lattice match with the InP substrate,
and are also doped with Be, a p-type impurity, to control their
carrier concentrations.
[0031] The composition of InGaAlAs used in the cap and interface
layers is adjusted so that its bandgap will be 1.1 .mu.m. This
adjustment is made to ensure that the device does not absorb light
of a 1.3-.mu.m band, which represents an optical signal. FIG. 6
shows a relation between an optical absorption coefficient and a
wavelength of light for InGaAs. It can be seen from this graph that
InGaAs absorbs almost no light when the optical wavelength is about
0.1 .mu.m longer than its bandgap wavelength. Thus, if the bandgap
wavelength is set shorter than 1.2 .mu.m, InGaAlAs used in the cap
and interface layers no longer absorbs a 1.3-.mu.m band optical
signal, thus avoiding an unwanted loss of the optical signal. That
is, the composition of InGaAlAs used in the cap and interface
layers need only have a bandgap wavelength shorter than 1.2 .mu.m,
and its bandgap wavelength is not limited to 1.1 .mu.m.
[0032] However, InGaAlAs used in the cap and interface layers also
has a limit value on a shorter wavelength side of the bandgap
wavelength, which is restricted by a difference in bandgap between
it and the InGaAs absorption layer. That is, when the difference in
bandgap between InGaAlAs used in the cap and interface layers and
the InGaAs absorption layer becomes too large, the electrons and
holes cannot ride over the energy difference at the interface and
build up there, resulting in a loss of a high-speed response, a
so-called pileup phenomenon. Thus, the bandgap of InGaAlAs used in
the cap and interface layers must not be set excessively large.
Normally, to obtain a 10-GHz response speed, the energy difference
in a conduction or valence band between the bandgap of InGaAlAs
used in the cap and interface layers and the bandgap of the InGaAs
absorption layer needs to be set to about 0.5 eV. Based on this,
the limit value on the shorter wave-length side of the bandgap
wavelength of InGaAlAs used in the cap and interface layers is
calculated to be approximately 700 nm.
[0033] These layers may be grown by a metalorganic vapor phase
epitaxy or a proper chemical vapor deposition. The p-type dopant
may be Zn. Next, this structure is processed by photolithography
and dry or wet etching into a trapezoidal shape (mesa), as shown in
FIG. 5E. A top of the truncated cone structure thus formed has a
diameter of about 25 .mu.m, as in FIG. 5B. Now, the preparation of
the compound semiconductor is complete.
[0034] Next, Si 510 of FIG. 5C and the compound semiconductor 511
of FIG. 5E, prepared as described above, are joined as follows. As
shown in FIG. 5F, Si of FIG. 5C and the compound semiconductor of
FIG. 5E are arranged so that their top portions oppose each other
and, in this condition, they are placed in a radio frequency plasma
system. A small amount of argon gas is introduced into a chamber of
the system to clean the surfaces of the structures to be joined.
Immediately after cleaning, the top portions are brought into
contact to join Si of FIG. 5C and the compound semiconductor of
FIG. 5E. This joining may be done by heating though it can be
performed at an ordinary temperature. Then, the joined structure is
immersed in a weak hydrochloric acid-based etching liquid to
selectively remove unwanted InP substrate. Then, as shown in FIG.
5G, the combined structure is subjected to the photolithography and
dry or wet etching to process only the compound semiconductor into
a trapezoidal shape again. After this, as shown in FIG. 5H, a
dielectric mask 512 is formed by photolithography and dry or wet
etching. This is followed by a highly resistive InP layer 513 being
grown by a metalorganic vapor phase epitaxy or a proper chemical
vapor deposition. Then, as shown in FIG. 5I, the dielectric mask is
removed, after which a SiN film 514 for the protection of the
entire device is formed by the plasma chemical vapor deposition and
a hole for electrode connection is formed in the SiN film by
photolithography. Then, a metal electrode 515 is formed by vapor
deposition, photolithography and liftoff process. In the last step,
a non-reflective coat 516 is formed over the Si substrate surface
which constitutes a light incident surface. Now, the light
receiving device is complete.
[0035] When a reverse bias was applied to the device fabricated in
this manner, a breakdown voltage Vb was 35 V and a dark current at
32 V, about 90% of the breakdown voltage, was as low as 50 nA. As
for the high frequency characteristic, a multiplication factor of
10-GHz optical signal was 25 at maximum and uniform within a light
receiving range. Further, in a reverse bias conduction test at an
elevated temperature (200.degree. C., 100 mA constant), a voltage
variation after 1000 hours was less than 1 V, and a breakdown
voltage and a dark current at room temperature showed no change
from those before the test.
Embodiment 4
[0036] FIG. 7 shows yet another embodiment of this invention. The
device of this embodiment has a structure similar in cross section
to that of Embodiment 1 of FIG. 1, except that it is shaped like a
waveguide. In FIG. 7, parts identical with those of FIG. 1 are
given like reference numbers. While in Embodiment 1 an optical
signal strikes the substrate at right angles or at angles close to
90 degrees to it, this embodiment has the optical signal enter the
substrate parallel or nearly parallel to it. This device has a high
speed and sensitivity of 40 GHz or higher and is suited for surface
mounting.
Embodiment 5
[0037] FIG. 8 shows a further embodiment of this invention. The
device of this embodiment has a surface illuminated type structure
similar to that of Embodiment 1, except that a compound
semiconductor substrate is used as a base on which a Si
multiplication layer is formed. Denoted 81 is an InP substrate
(n-type, 2xE18 cm.sup.-3), 82 an InGaAs optical absorption layer
(n-type, 2E15 cm.sup.-3, 1.2 .mu.m), 83 an InGaAsP interface layer
(n-type, 1E18 cm.sup.-3, 0.05 .mu.m), 84 a Si multiplication layer
(p-type, 1E15 cm.sup.-3, 0.2 .mu.m), and 85 a Si contact layer
(p-type, 2E18 cm.sup.-3, 0.1 .mu.n). The composition of InGaAsP is
adjusted for the same reason as Embodiment 3 so that it has a
bandgap wavelength of 1.1 .mu.m to prevent an optical signal of a
1.3 .mu.m band from being absorbed.
Embodiment 6
[0038] FIG. 9 shows a further embodiment of this invention. Instead
of a simple Si substrate, the device of this embodiment uses a
substrate formed with a Si or SiGe integrated circuit and has an
avalanche photodiode similar to Embodiment 4 formed on that
substrate. Denoted 91 is a preamplifier made of a Si or SiGe
integrated circuit on a Si substrate, and 92 an avalanche
photodiode of FIG. 7. It is noted that a single substrate is
commonly used as the Si substrate 11 and the integrated circuit Si
substrate 91.
Embodiment 7
[0039] FIG. 10 shows a further embodiment of this invention. This
embodiment represents an example optical module having the
avalanche photodiode 101 of FIG. 4, a preamplifier integrated
circuit device 102 and an optical fiber 103 all accommodated in a
single case 104.
Embodiment 8
[0040] FIG. 11 shows a further embodiment of this invention. This
embodiment represents an example optical receiver having the
optical module 110 of FIG. 10 mounted on a package 111
incorporating an analog-digital converter and a decoder.
[0041] With the embodiments of this invention, even if an electric
field strength at an interface between Si and a compound
semiconductor fused together becomes abnormally high due to an
effect of impurities present at the interface, a large bandgap of
the compound semiconductor material at the interface can minimize
an increase in a dark current. By deliberately doping impurities in
the interface layer to nullify electric influences of the interface
impurities, it is possible to suppress electric field anomalies at
the interface. As a result, a highly sensitive, fast avalanche
photodiode for optical communications with a much lower dark
current can be realized.
[0042] It should be further understood by those skilled in the art
that although the foregoing description has been made on
embodiments of the invention, the invention is not limited thereto
and various changes and modifications may be made without departing
from the spirit of the invention and the scope of the appended
claims.
* * * * *