U.S. patent application number 15/192037 was filed with the patent office on 2016-10-20 for solar cell and manufacturing method thereof.
This patent application is currently assigned to FUJITSU LIMITED. The applicant listed for this patent is FUJITSU LIMITED. Invention is credited to Kenichi KAWAGUCHI, Yoshiaki NAKATA.
Application Number | 20160308080 15/192037 |
Document ID | / |
Family ID | 50340693 |
Filed Date | 2016-10-20 |
United States Patent
Application |
20160308080 |
Kind Code |
A1 |
KAWAGUCHI; Kenichi ; et
al. |
October 20, 2016 |
SOLAR CELL AND MANUFACTURING METHOD THEREOF
Abstract
A solar cell includes a semiconductor substrate of a first
conductivity; a pillar-shaped structure constituted by a
semiconductor of the first conductivity, the pillar-shaped
structure being formed on the semiconductor substrate; a
superlattice layer including a barrier layer and a quantum
structure layer that are alternately deposited on a side wall of
the pillar-shaped structure, the quantum structure layer being
constituted by a material having a smaller energy bandgap than that
of the barrier layer, the quantum structure layer including a
wurtzite type crystal part and a zinc blende type crystal part that
are alternately arranged along an axial direction of the
pillar-shaped structure; and a semiconductor layer of a second
conductivity that is formed so as to surround the superlattice
layer, the second conductivity being an opposite conductivity to
that of the first conductivity.
Inventors: |
KAWAGUCHI; Kenichi; (Ebina,
JP) ; NAKATA; Yoshiaki; (Isehara, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FUJITSU LIMITED |
Kawasaki-shi |
|
JP |
|
|
Assignee: |
FUJITSU LIMITED
Kawasaki-shi
JP
|
Family ID: |
50340693 |
Appl. No.: |
15/192037 |
Filed: |
June 24, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14643304 |
Mar 10, 2015 |
9401444 |
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15192037 |
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PCT/JP2012/073803 |
Sep 18, 2012 |
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14643304 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/036 20130101;
H01L 31/0735 20130101; B82Y 20/00 20130101; H01L 31/184 20130101;
H01L 31/03046 20130101; H01L 31/03529 20130101; Y02P 70/50
20151101; H01L 31/1844 20130101; Y02E 10/544 20130101; H01L
31/035281 20130101; H01L 31/035236 20130101; Y02P 70/521
20151101 |
International
Class: |
H01L 31/0352 20060101
H01L031/0352; H01L 31/0304 20060101 H01L031/0304; H01L 31/0735
20060101 H01L031/0735; H01L 31/036 20060101 H01L031/036; H01L 31/18
20060101 H01L031/18 |
Claims
1-7. (canceled)
8. A manufacturing method of a solar cell, the manufacturing method
comprising: forming a pillar-shaped structure of a first
conductivity on a semiconductor substrate of the first
conductivity, the pillar-shaped structure being formed by
alternately growing a semiconductor of a wurtzite type crystal and
a semiconductor of a zinc blende type crystal such that a wurtzite
type crystal part and a zinc blende type crystal part are
alternately arranged along an axial direction; forming a
superlattice layer by alternately depositing a barrier layer and a
quantum structure layer so as to surround the pillar-shaped
structure, the quantum structure layer being constituted by a
material having a smaller energy bandgap than that of the barrier
layer, a part of the superlattice layer contacting the wurtzite
type crystal part of the pillar-shaped structure being a wurtzite
type crystal and a part of the superlattice layer contacting the
zinc blende type crystal part of the pillar-shaped structure being
a zinc blende type crystal; and depositing a semiconductor layer of
a second conductivity so as to surround the superlattice layer, the
second conductivity being an opposite conductivity to that of the
first conductivity.
9. The manufacturing method according to claim 8, wherein the
forming of the superlattice layer includes selectively forming the
superlattice layer on a side wall part of the pillar-shaped
structure.
10. The manufacturing method according to claim 8, wherein the
forming of the superlattice layer includes forming the superlattice
layer so as to cover a side wall part and a top part of the
pillar-shaped structure.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of U.S. application Ser.
No. 14/643,304, filed Mar. 10, 2015, which is a U.S. continuation
application filed under 35 USC 111(a) claiming benefit under 35 USC
120 and 365(c) of PCT Application PCT/JP2012/073803 filed on Sep.
18, 2012, the entire contents of which are incorporated herein by
reference in their entireties.
FIELD
[0002] The embodiments discussed herein are related to a solar cell
and a manufacturing method thereof.
BACKGROUND
[0003] In order to effectively utilize natural energy, there is
demand for a highly efficient solar cell. As a solar cell for
realizing high efficiency, there is proposed an intermediate band
solar cell in which quantum dots are stacked in a superlattice-like
structure. This solar cell is formed by inserting, in a bulk
light-absorbing layer of a semiconductor solar cell, a material
having a longer absorption wavelength than that of the absorbing
layer of the semiconductor solar cell, i.e., a material having low
bandgap energy, as a quantum dot superlattice. By forming such a
light-absorbing layer, it is possible to increase the light
absorption amount while maintaining the open end voltage of the
solar cell at a voltage that is determined by the bulk
light-absorbing layer, thereby attaining high efficiency.
[0004] Non-patent Document 1: A. Marti et al., "Quasi-Drift
Diffusion Model for the Quantum Dot Intermediate band solar cell",
IEEE Transactions on Electron Devices, Vol. 49, No. 9, September
2002, pp. 1632-1639
[0005] In order to realize a quantum dot intermediate-band solar
cell, the quantum dot layered structure needs to function as a
structure in which quantum dots constitute a superlattice in a
p-i-n stack direction and an intermediate-band is formed.
[0006] However, the semiconductor quantum dot layered structures
that have been used up to now, have self-forming type S-K quantum
dots based on distortion system materials such as an InAs quantum
dot/(In)GaAs barrier layer and an InAs quantum dot/In(Ga)(As)P
barrier layer. Therefore, it has not been possible to make the
barriers thin, and there has been a limit in forming a closely
stacked structure needed for functioning as a superlattice.
Furthermore, the arrangement of quantum dots is based on the
propagation of distortion, and therefore the distortion propagation
has been uneven due to inconsistencies in the sizes of the quantum
dots, thereby causing a problem in controlling the arrangements of
quantum dots.
SUMMARY
[0007] According to an aspect of the embodiments, a solar cell
includes a semiconductor substrate of a first conductivity; a
pillar-shaped structure constituted by a semiconductor of the first
conductivity, the pillar-shaped structure being formed on the
semiconductor substrate; a superlattice layer including a barrier
layer and a quantum structure layer that are alternately deposited
on a side wall of the pillar-shaped structure, the quantum
structure layer being constituted by a material having a smaller
energy bandgap than that of the barrier layer, the quantum
structure layer including a wurtzite type crystal part and a zinc
blende type crystal part that are alternately arranged along an
axial direction of the pillar-shaped structure; and a semiconductor
layer of a second conductivity that is formed so as to surround the
superlattice layer, the second conductivity being an opposite
conductivity to that of the first conductivity.
[0008] The object and advantages of the invention will be realized
and attained by means of the elements and combinations particularly
pointed out in the appended claims. It is to be understood that
both the foregoing general description and the following detailed
description are exemplary and explanatory and are not restrictive
of the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a schematic cross-sectional diagram of a structure
of a solar cell according to a first embodiment (part 1);
[0010] FIG. 2 is a schematic cross-sectional diagram of a structure
of a solar cell according to the first embodiment (part 2);
[0011] FIG. 3 is a graph indicating a relationship between a
coupling factor of quantum dots and a generation carrier density in
a quantum dot intermediate-band solar cell;
[0012] FIGS. 4A and 4B are process cross-sectional diagrams
indicating a manufacturing method of a solar cell according to the
first embodiment (part 1);
[0013] FIGS. 5A and 5B are process cross-sectional diagrams
indicating a manufacturing method of a solar cell according to the
first embodiment (part 2);
[0014] FIGS. 6A and 6B are process cross-sectional diagrams
indicating a manufacturing method of a solar cell according to the
first embodiment (part 3);
[0015] FIGS. 7A and 7B are process cross-sectional diagrams
indicating a manufacturing method of a solar cell according to the
first embodiment (part 4);
[0016] FIG. 8 is a schematic cross-sectional diagram of a structure
of a solar cell according to a second embodiment;
[0017] FIGS. 9A and 9B are process cross-sectional diagrams
indicating a manufacturing method of a solar cell according to the
second embodiment (part 1);
[0018] FIGS. 10A and 10B are process cross-sectional diagrams
indicating a manufacturing method of a solar cell according to the
second embodiment (part 2);
[0019] FIG. 11 is a schematic cross-sectional diagram of a
structure of a solar cell according to a third embodiment;
[0020] FIGS. 12A and 12B are process cross-sectional diagrams
indicating a manufacturing method of a solar cell according to the
third embodiment (part 1); and
[0021] FIGS. 13A and 13B are process cross-sectional diagrams
indicating a manufacturing method of a solar cell according to the
third embodiment (part 2).
DESCRIPTION OF EMBODIMENTS
First Embodiment
[0022] A description is given of a solar cell and a manufacturing
method thereof according to a first embodiment, with reference to
FIGS. 1 through 7B.
[0023] FIGS. 1 and 2 are schematic cross-sectional diagrams of a
structure of a solar cell according to the present embodiment. FIG.
3 is a graph indicating the relationship between the coupling
factor of quantum dots and the generation carrier density in a
quantum dot intermediate-band solar cell. FIGS. 4A through 7B are
process cross-sectional diagrams indicating the manufacturing
method of the solar cell according to the present embodiment.
[0024] First, a description is given of the structure of the solar
cell according to the present embodiment, with reference to FIGS. 1
and 2. FIG. 2 is an enlarged diagram of the part surrounded by a
dotted line in FIG. 1.
[0025] On a semiconductor substrate 10 of a first conductivity
type, pillar-shaped structures 16 are formed, which are constituted
by a semiconductor of the first conductivity type (p type or n
type). As illustrated in FIG. 2, the pillar-shaped structure 16
includes wurtzite type (hereinafter, also expressed as "WZ")
crystal parts 16WZ, and zinc blende type (hereinafter, also
expressed as "ZB") crystal parts 16ZB, which are alternately
deposited in the axial direction (vertical direction in the
diagram). Note that in the example of FIG. 1, on the WZ crystal
part 16WZ, the ZB crystal part 16ZB and the WZ crystal part 16WZ
are repeatedly deposited three times; however, the number of times
of depositing the ZB crystal part 16ZB and the WZ crystal part 16WZ
is not so limited.
[0026] On the semiconductor substrate 10, an insulating film 12 is
formed in areas other than the areas where the pillar-shaped
structures 16 are formed. On the side wall of the pillar-shaped
structure 16 protruding on the insulating film 12, a superlattice
layer 22 is formed. The superlattice layer 22 is formed by
repeatedly depositing a barrier layer 18 and a quantum structure
layer 20, so as to surround the pillar-shaped structure 16. In the
present application specification, a quantum structure layer 20
means a layer formed by a material having a lower energy bandgap
than that of the barrier layer, and a layer in which the same
quantum confining structure as that of a quantum well and a quantum
dot, etc., is formed. Note that the example of FIG. 1 indicates a
superlattice layer 22 in which three barrier layers 18 and two
quantum structure layers 20 are alternately deposited; however, the
numbers of the deposited barrier layers 18 and quantum structure
layers 20 are not so limited.
[0027] In the crystal structure of the barrier layer 18 and the
quantum structure layer 20, the crystal structure of the
pillar-shaped structure 16 that is the base is applied. That is to
say, as illustrated in FIG. 2, the barrier layers 18 and the
quantum structure layers 20 that are formed on the side walls of
the WZ crystal parts 16WZ of the pillar-shaped structure 16, are
formed by WZ crystals (WZ crystal part 18WZ, WZ crystal part 20WZ).
Furthermore, the barrier layers 18 and the quantum structure layers
20 that are formed on the side walls of the ZB crystal parts 16ZB
of the pillar-shaped structure 16, are formed by ZB crystals (ZB
crystal part 18ZB, ZB crystal part 20ZB).
[0028] On the side wall of the superlattice layer 22, a
semiconductor layer 24 of a second conductivity (n type or p type)
is formed, so as to surround the superlattice layer 22.
[0029] Similar to the superlattice layer 22, in the crystal
structure of the semiconductor layer 24, the crystal structure of
the pillar-shaped structure 16 is applied. That is to say, as
illustrated in FIG. 2, the semiconductor layer 24 formed on the
side wall of the WZ crystal part 16WZ of the pillar-shaped
structure 16 is formed by a WZ crystal (WZ crystal part 24WZ).
Furthermore, the semiconductor layer 24 formed on the side wall of
the ZB crystal part 16ZB of the pillar-shaped structure 16 is
formed by a ZB crystal (ZB crystal part 24ZB).
[0030] On the side wall of the semiconductor layer 24, a second
conductivity electrode 30 is formed, so as to surround the
semiconductor layer 24. On the back side of the semiconductor
substrate 10, a first conductivity electrode 32 is formed.
[0031] Note that FIG. 1 illustrates a state where two unit
structures of the solar cell are formed on the semiconductor
substrate 10. As for the unit structures of the solar cell, a
desired number of unit structures may be arranged, in a linear
manner or in a planar manner, according to need.
[0032] As described above, the solar cell according to the present
embodiment is a solar cell having a p-i-n junction structure, in
which a superlattice layer 22 is arranged in between the
pillar-shaped structure 16 of a first conductivity and the
semiconductor layer 24 of a second conductivity. The pillar-shaped
structure 16, the superlattice layer 22, and the semiconductor
layer 24 are formed by repeatedly depositing a WZ crystal and a ZB
crystal along the axial direction of the pillar-shaped structure
16.
[0033] The incident light enters the solar cell from above the
semiconductor substrate 10 (top side in FIG. 1). Furthermore, the
direction of applying an electric field which is applied to the
p-i-n junction via the first conductivity electrode 32 and the
second conductivity electrode 30, i.e., the movement direction of
the carriers, is the diameter direction of the cylinder forming the
p-i-n junction. That is to say, the incidence direction of the
light that is the detection target, and the direction of applying
an electric field on the p-i-n junction, are directions that
intersect each other.
[0034] The semiconductor device according to the present embodiment
is formed by a group III-V compound semiconductor. The group III-V
compound semiconductor has a unique bandgap energy according to the
material, and the bandgap energy also depends on the crystal
structure. For example, comparing a WZ crystal and a ZB crystal of
the same material, the WZ crystal has a higher bandgap energy than
the ZB crystal.
[0035] For example, focusing on the quantum structure layer 20 of
FIG. 2, the bandgap energy of the WZ crystal part 20WZ of the
quantum structure layer 20 is higher than the bandgap energy of the
ZB crystal part 20ZB of the quantum structure layer 20. Therefore,
the WZ crystal part 20WZ of the quantum structure layer 20
functions as a barrier layer with respect to the ZB crystal part
20ZB of the quantum structure layer 20. Then, by sandwiching this
quantum structure layer 20 with barrier layers 18 of a material
having a wider energy bandgap than the material of the quantum
structure layer 20, a ring-shaped quantum box surrounding the
pillar-shaped structure 16 is formed at the part of the ZB crystal
part 20ZB of the quantum structure layer 20. Thus, it becomes
possible to confine the carriers in the ZB crystal part 20ZB of the
quantum structure layer 20.
[0036] Furthermore, by forming the quantum structure layer 20 by
repeatedly depositing the barrier layer 18 and the quantum
structure layer 20 on the side wall of the pillar-shaped structure
16, a plurality of ring-shaped quantum boxes are formed in a in a
concentric fashion so as to surround the pillar-shaped structure
16. Then, in the diameter direction of these concentric circles
(horizontal direction in the diagram), a quantum structure is
formed, in which electron states are coupled like a superlattice,
and it is possible to realize the same function as that of a
quantum dot intermediate-band solar cell.
[0037] Particularly, the ZB crystal parts 18ZB, 20ZB of the barrier
layer 18 and the quantum structure layer 20 are selectively formed
on the ZB crystal part 16ZB of the pillar-shaped structure 16, and
therefore it is possible to easily align the plurality of
ring-shaped quantum boxes along the diameter direction of the
concentric circles. Accordingly, the ratio of the electronic
coupling of quantum boxes arranged along the diameter direction of
the concentric circles, is significantly increased.
[0038] Furthermore, this kind of superlattice structure is not
formed with a distortion system material, and therefore it is
possible to make the barrier layer 18 thin, and the number of
deposited barrier layers 18 and quantum structure layers 20 may be
increased. Accordingly, the efficiency of the solar cell may be
further increased.
[0039] The material system for realizing the solar cell according
to the present embodiment is not particularly limited; for example,
a material of using the superlattice structure of a InP/InGaAs
system, or a material using a superlattice system of a GaAs/InGaAs
system, may be applied.
[0040] In an example using a superlattice structure of the
InP/InGaAs system, the semiconductor substrate 10, the
pillar-shaped structure 16, the barrier layer 18, and the
semiconductor layer 24 may be formed by InP, and the quantum
structure layer 20 may be formed by InGaAs. Furthermore, in an
example using a superlattice structure of the GaAs/InGaAs system,
the semiconductor substrate 10, the pillar-shaped structure 16, the
barrier layer 18, and the semiconductor layer 24 may be formed by
GaAs, and the quantum structure layer 20 may be formed by
InGaAs.
[0041] FIG. 3 is a graph expressing changes in the generation
carrier density with respect to a coupling factor, in a quantum dot
intermediate-band solar cell having ten stacked layers of quantum
dots. This graph has been calculated by using the following formula
(1), based on detailed studies by inventors of the present
invention based on a diffusion equation described in Non-patent
Document 1. Here, D.sub.e is the diffusion coefficient of
electrons, B.sub.e is the radiation recombination coefficient,
N.sub.Dh is the state density of holes, g.sub.eh is the carrier
generation speed in the bulk absorption layer (i.e., the barrier
layer of quantum dots), and g.sub.e is the carrier generation speed
by the intermediate band of the term unique to the
intermediate-band solar cell.
D.sub.ed.sup.2.DELTA.n/dx.sup.2-B.sub.eN.sub.Dh.DELTA.n=-g.sub.e(x)-g.su-
b.eh (1)
[0042] Here, g.sub.e may be described as in the following formula
(2), as a term dependent on an effective band factor .epsilon..
Here, X is the light condensation coefficient, and 1000Sun which is
a typical light condensation solar cell is used.
g.sub.e(x)=.epsilon..intg..alpha..sub.eX sin.sup.2
.theta.2.pi./h.sup.3c.sup.2E.sup.2/(exp(E/K.sub.BT)-1)dE (2)
[0043] The effective band factor .epsilon. is obtained as follows,
where n is the number of quantum dots and .delta. is the coupling
factor of the quantum dots.
.epsilon.=n.delta..sup.n
[0044] Based on the calculation results of FIG. 3, compared to the
case where the quantum dots are coupled only by a probability of
50%, it is possible to couple the quantum dots by 100%, and the
number of generated carriers is increased by approximately two
times. This result indicates that the number of generated carriers
may also be significantly increased in a solar cell according to
the present embodiment in which the coupling ratio of quantum boxes
may be significantly increased.
[0045] Next, a description is given of the manufacturing method of
the solar cell according to the present embodiment, with reference
to FIGS. 4A through 7B.
[0046] First, the insulating film 12 is formed on the semiconductor
substrate 10 of the first conductivity by, for example, a CVD
method. As the semiconductor substrate 10 of the first
conductivity, for example, an n-InP substrate having an impurity
density of 5.times.10.sup.17 cm.sup.-3 through 1.times.10.sup.19
cm.sup.-3 may be applied. Furthermore, as the insulating film 12, a
silicon oxide film may be applied.
[0047] Next, parts of the insulating film 12, in the areas where
the pillar-shaped structures 16 are to be formed, are removed. On
the parts of the semiconductor substrate 10 that have become
exposed by removing the insulating film 12, metal particles 14 such
as Au are deposited (FIG. 4A).
[0048] For example, on the insulating film 12, a photoresist film
(not illustrated) is formed for exposing the areas where the
pillar-shaped structures 16 are to be formed, the insulating film
12 is etched by using this photoresist film as a mask, and parts of
the insulating film 12 in the areas where the pillar-shaped
structures 16 are to be formed, are removed. By removing the
photoresist film after depositing the metal particles 14, the metal
particles 14 selectively remain in the areas where the
pillar-shaped structures 16 are to be formed.
[0049] Next, for example, by a MOVPE method, by using the metal
particles 14 as a catalyst, on the semiconductor substrate 10 in
the areas that are not covered by the insulating film 12, the
pillar-shaped structures 16 are formed. The pillar-shaped structure
16 is constituted by a semiconductor of a first conductivity, in
which the WZ crystal part 16WZ and the ZB crystal part 16ZB are
alternately deposited (FIG. 4B).
[0050] The pillar-shaped structure 16 of the first conductivity may
be formed by, for example, n-InP having a diameter of 150 nm, a
length of 4 .mu.m, and an impurity density of 5.times.10.sup.17
cm.sup.-3 through 1.times.10.sup.19 cm.sup.3. The structure in
which the WZ crystal part 16WZ and the ZB crystal part 16ZB are
repeatedly deposited, may be formed by alternately switching the
doping material.
[0051] For example, when InP is grown by the MOVPE method by using
trimethylindium (TMIn) and phosphine (PH.sub.3) as the raw
materials, hydrogen sulfide (H.sub.2S) and disilane
(Si.sub.2H.sub.6), which are raw materials of an n type dopant, are
alternately supplied. When hydrogen sulfide (H.sub.2S) is used,
which is the raw material of an n type dopant, the n-InP of a WZ
crystal is grown. When disilane (Si.sub.2H.sub.6) is used, which is
the raw material of an n type dopant, the n-InP of a ZB crystal is
grown. The growth temperature is, for example, 380.degree. C.
through 400.degree. C., and the V/III ratio (flow rate ratio of
PH.sub.3 and TMI) is, for example, approximately 100 through
500.
[0052] Then length of the WZ crystal part 16WZ and the ZB crystal
part 16ZB in the axial direction of the pillar-shaped structure 16
(vertical direction in the diagram) may be controlled according to
the growth time. The axial direction size of the ZB crystal part
16ZB is preferably approximately less than or equal to 20 nm. By
setting the axial direction size of the ZB crystal part 16ZB to be
approximately less than or equal to 20 nm, it is possible to
introduce a quantum effect in the axial direction, and form a
structure having small property changes with respect to the
environmental temperature.
[0053] Next, by a generally-known etching technique, the metal
particles 14 remaining on the leading ends of the pillar-shaped
structures 16 are removed.
[0054] Next, for example, by a MOVPE method, the superlattice layer
22 is formed as a light absorption layer, so as to surround the
side walls of the pillar-shaped structure 16. The superlattice
layer 22 is formed by repeatedly depositing the barrier layer 18
and the quantum structure layer 20 by, for example, 10 cycles
through 30 cycles. Note that the diagrams illustrate the
superlattice layer 22 including three barrier layers 18 and two
quantum structure layers 20.
[0055] As the barrier layer 18, for example, InGaAs may be applied.
As the raw materials of InGaAs, for example, trimethylindium
(TMIn), trimethylgallium (TEGa), and arsine (AsH.sub.3) may be
used.
[0056] As the quantum structure layer 20, for example, InP may be
applied. As the raw materials of InP, trimethylindium (TMIn) and
phosphine (PH.sub.3) may be used.
[0057] In this case, the growth temperature is 530.degree. C.
through 580.degree. C., and as for InP, the V/III ratio (flow rate
ratio of PH.sub.3 and TMI) is 1000 through 3000, and as for InGaAs,
the V/III ratio (flow rate ratio of AsH.sub.3 and TMI+TEG) is 300
through 1000. Accordingly, it is possible to cause the barrier
layers 18 and the quantum structure layers 20 to selectively grow
on the side wall of the pillar-shaped structure 16.
[0058] The thickness of the barrier layer 18 and the quantum
structure layer 20 is not particularly limited; for example, the
thickness may be approximately 3 nm through 5 nm.
[0059] When the superlattice layer 22 is caused to grow on the side
wall of the pillar-shaped structure 16 in which the WZ crystal part
16WZ and the ZB crystal part 16ZB are alternately deposited, the
crystal structure of the pillar-shaped structure 16 is also applied
in the barrier layer 18 and the quantum structure layer 20. That is
to say, on the side wall of the WZ crystal part 16WZ of the
pillar-shaped structure 16, a barrier layer of a WZ crystal (WZ
crystal part 18WZ) and a quantum structure layer of a WZ crystal
(WZ crystal part 20WZ) are formed. Furthermore, on the side wall of
the ZB crystal part 16ZB of the pillar-shaped structure 16, a
barrier layer of a ZB crystal (ZB crystal part 18ZB) and a quantum
structure layer of a ZB crystal (ZB crystal part 20ZB) are formed
(FIG. 5A).
[0060] Next, for example, by a MOVPE method, the semiconductor
layer 24 of the second conductivity is formed on the side wall of
the pillar-shaped structure 16 on which the superlattice layer 22
is formed. As the semiconductor layer 24 of the second
conductivity, for example, a p-InP having an impurity density of
5.times.10.sup.17 cm.sup.-3 through 1.times.10.sup.19 cm.sup.-3 may
be applied. As the raw material of the p type dopant, for example,
diethyl zinc (DEZ) may be applied.
[0061] In this case, the crystal structure of the superlattice
layer 22 is applied to the semiconductor layer 24, and a
semiconductor layer of a WZ crystal (WZ crystal part 24WZ) is
formed on the side wall of the WZ crystal part 18WZ of the barrier
layer 18. Furthermore, on the side wall of the ZB crystal part 18ZB
of the barrier layer 18, a semiconductor layer of a ZB crystal (ZB
crystal part 24ZB) is formed (FIG. 5B).
[0062] Next, on the entire surface, an insulating film 26 to act as
a protection film is deposited by, for example, a CVD method (FIG.
6A). As the insulating film 26, for example, a silicon oxide film
may be applied.
[0063] Next, on the entire surface, a transparent conductive film
28 to become the second conductivity electrode 30, is deposited by,
for example, a sputtering method (FIG. 6B). As the transparent
conductive film 28, for example, a CuAlO.sub.2 film may be
applied.
[0064] Next, for example, by a CMP method, the transparent
conductive film 28 and the insulating film 26 are removed from the
top ends of the pillar-shaped structure 16, the superlattice layer
22, and the semiconductor layer 24, and the second conductivity
electrode 30 embedded between the semiconductor layers 24 is formed
(FIG. 7A).
[0065] Note that when the insulating film 26 at the tops end of the
pillar-shaped structure 16, the superlattice layer 22, and the
semiconductor layer 24, functions as an anti-reflection film, the
transparent conductive film 28 and the insulating film 26 at the
top ends of the pillar-shaped structure 16, the superlattice layer
22, and the semiconductor layer 24 need not always be removed.
Furthermore, on the second conductivity electrode 30, a grid type
metal electrode may be formed.
[0066] Next, on the back side of the semiconductor substrate 10,
the first conductivity electrode 32 is formed, and the solar cell
according to the present embodiment is completed (FIG. 7B).
[0067] As described above, according to the present embodiment, by
using the difference in the energy bandgap between the wurtzite
type crystal and the zinc blende type crystal, and a barrier layer,
a three-dimensional quantum confining structure is formed, and
therefore it is easy to closely stack and align the quantum
confining structures. Accordingly, a highly-efficient solar cell
may be realized.
Second Embodiment
[0068] A description is given of a solar cell and a manufacturing
method thereof according to a second embodiment, with reference to
FIGS. 8 through 10B. The same elements as those of the solar cell
and the manufacturing method thereof according to the first
embodiment illustrated in FIGS. 1 through 7B are denoted by the
same reference numerals, and descriptions thereof are omitted or
simplified.
[0069] FIG. 8 is a schematic cross-sectional diagram of a structure
of the solar cell according to the present embodiment. FIGS. 9A
through 10B are process cross-sectional diagrams indicating the
manufacturing method of the solar cell according to the present
embodiment.
[0070] As illustrated in FIG. 8, the solar cell according to the
present embodiment is the same as the solar cell according to the
first embodiment, except that the ZB crystal parts 16ZB, 18ZB,
20ZB, 24ZB have different sizes in the axial direction of the
pillar-shaped structure 16.
[0071] That is to say, in the solar cell according to the present
embodiment, the ZB crystal parts 20ZB of the quantum structure
layer 20 have sizes in the axial direction of the pillar-shaped
structure 16 that are not consistent as in the solar cell according
to the first embodiment; the closer the ZB crystal part 20ZB is to
the semiconductor substrate 10, the larger the size of the ZB
crystal part 20ZB. In order to form such a structure, also with
respect to the sizes in the axial direction of the pillar-shaped
structure 16 of the ZB crystal parts 16ZB, 18ZB, 24ZB of the
pillar-shaped structure 16, the barrier layer 18, and the
semiconductor layer 24, the closer the ZB crystal part 16ZB, 18ZB,
24ZB is to the semiconductor substrate 10, the larger the size of
the ZB crystal part 16ZB, 18ZB, 24ZB.
[0072] By forming the ZB crystal parts 20ZB such that the closer
the ZB crystal part 20ZB is to the semiconductor substrate 10, the
larger the size of the ZB crystal part 20ZB in the axial direction,
it is possible to absorb, in a wide wavelength area, the light of
wavelengths that are longer than the wavelength corresponding to
the bandgap energy of the barrier layer 18. Furthermore, with
respect to incident light entering from the front side, the
respective ZB crystal parts 20ZB are able to absorb the light.
[0073] Note that if the ZB crystal parts 20ZB are formed such that
the closer the ZB crystal part 20ZB is to the semiconductor
substrate 10, the smaller the size of the ZB crystal part 20ZB in
the axial direction, it becomes difficult for the ZB crystal parts
20ZB closer to the semiconductor substrate 10 to function.
Therefore, it is preferable that the closer the ZB crystal part
20ZB is to the semiconductor substrate 10, the larger the size of
the ZB crystal part 20ZB in the axial direction.
[0074] When the barrier layer energy barrier height is infinite,
the quantum dot has energy shift AE expressed by the following
formula (3) according to quantization. Here, l.sub.x, l.sub.y,
l.sub.z respectively express the sizes of the quantum dot in the x,
y, and z directions.
.DELTA.E=h.sup.2/8.pi..sup.2m(.pi./l.sub.x).sup.2+(.pi./l.sub.y).sup.2+(-
.pi./l.sub.z).sup.2 (3)
[0075] However, the actual quantum dot is surrounded by barrier
layers having a limited bandgap, and therefore the maximum shift
amount corresponds to the barrier material and the band offset
amount of the barrier material.
[0076] In the solar cell according to the present embodiment, the
energy shift amount that is changed by changing the height of the
ZB crystal part 20ZB, i.e., the l.sub.z, corresponds to the height
of the barrier formed by the WZ crystal and the ZB crystal, i.e.,
approximately 100 meV, in the case where the absorption layer
material is, for example, InGaAs. The energy difference becomes,
for example, approximately a wavelength interval of approximately
106 nm near the wavelength of 1.2 .mu.m, and approximately a
wavelength interval of approximately 142 nm near the wavelength of
1.4 .mu.m.
[0077] Next, a description is given of the manufacturing method of
the solar cell according to the present embodiment, with reference
to FIGS. 9A through 10B.
[0078] First, in the same manner as the manufacturing method of the
solar cell according to the first embodiment illustrated in FIGS.
4A and 4B, on a semiconductor substrate 10, pillar-shaped
structures 16 are formed, which are formed by repeatedly depositing
the WZ crystal part 16WZ and the ZB crystal part 16ZB. At this
time, the growth time when forming the ZB crystal parts 16ZB is
appropriately controlled to form the ZB crystal parts 16ZB such
that the closer the ZB crystal part 16ZB is to the semiconductor
substrate 10, the larger the size of the ZB crystal part 16ZB in
the axial direction (FIG. 9A).
[0079] Next, by a generally-known etching technique, the metal
particles 14 remaining on the leading ends of the pillar-shaped
structures 16 are removed.
[0080] Next, in the same manner as the manufacturing method of the
solar cell according to the first embodiment illustrated in FIG.
5A, the superlattice layer 22 is formed so as to surround the side
walls of the pillar-shaped structure 16. The superlattice layer 22
is formed by repeatedly depositing the barrier layer 18 and the
quantum structure layer 20.
[0081] When the superlattice layer 22 is caused to grow on the side
walls of the pillar-shaped structure 16 in which the WZ crystal
part 16WZ and the ZB crystal part 16ZB are alternately deposited,
the crystal structure of the pillar-shaped structure 16 is also
applied to the barrier layer 18 and the quantum structure layer 20.
That is to say, on the slide wall of the WZ crystal part 16WZ of
the pillar-shaped structure 16, a barrier layer of a WZ crystal (WZ
crystal part 18WZ) and a quantum structure layer of a WZ crystal
(WZ crystal part 20WZ) are formed. Furthermore, on the side wall of
the ZB crystal part 16ZB of the pillar-shaped structure 16, a
barrier layer of a ZB crystal (ZB crystal part 18ZB) and a quantum
structure layer of a ZB crystal (ZB crystal part 20ZB) are formed.
With respect to the sizes of the ZB crystal parts 18ZB, 20ZB, the
closer the ZB crystal part 18ZB, 20ZB is to the semiconductor
substrate 10, the larger the size of the ZB crystal part 18ZB, 20ZB
in the axial direction of the pillar-shaped structure 16 such that
the sizes of the ZB crystal parts 16ZB in the axial direction are
applied (FIG. 9B).
[0082] Next, in the same manner as the manufacturing method of the
solar cell according to the first embodiment illustrated in FIG.
5B, the semiconductor layer 24 of the second conductivity is formed
on the side wall of the pillar-shaped structure 16 on which the
superlattice layer 22 is formed.
[0083] In this case, the crystal structure of the superlattice
layer 22 is applied to the semiconductor layer 24, and a
semiconductor layer of a WZ crystal (WZ crystal part 24WZ) is
formed on the side wall of the WZ crystal part 18WZ of the barrier
layer 18. Furthermore, on the side wall of the ZB crystal part 18ZB
of the barrier layer 18, a semiconductor layer of a ZB crystal (ZB
crystal part 24ZB) is formed. With respect to the sizes of the ZB
crystal parts 24ZB, the closer the ZB crystal part 24ZB is to the
semiconductor substrate 10, the larger the size of the ZB crystal
part 24ZB in the axial direction of the pillar-shaped structure 16,
such that the sizes of the ZB crystal parts 16ZB in the axial
direction are applied (FIG. 10B).
[0084] Next, in the same manner as the manufacturing method of the
solar cell according to the first embodiment illustrated in FIGS.
6A through 7B, the insulating film 26, the second conductivity
electrode 30, and the first conductivity electrode 32 are formed,
thereby completing the solar cell according to the present
embodiment (FIG. 10B).
[0085] As described above, according to the present embodiment, by
using the difference in the energy bandgap between the wurtzite
type crystal and the zinc blende type crystal, and a barrier layer,
a three-dimensional quantum confining structure is formed, and
therefore it is easy to closely stack and align the quantum
confining structure. Accordingly, a highly-efficient solar cell may
be realized. Furthermore, by forming quantum confining structures
of different sizes, it is possible to absorb light in a wide
wavelength area, thereby further increasing the efficiency.
Third Embodiment
[0086] A description is given of a solar cell and a manufacturing
method thereof according to a third embodiment, with reference to
FIGS. 11 through 13B. The same elements as those of the solar cell
and the manufacturing method thereof according to the first and
second embodiments illustrated in FIGS. 1 through 10B are denoted
by the same reference numerals, and descriptions thereof are
omitted or simplified.
[0087] FIG. 11 is a schematic cross-sectional diagram of a
structure of the solar cell according to the present embodiment.
FIGS. 12A through 13B are process cross-sectional diagrams
indicating the manufacturing method of the solar cell according to
the present embodiment.
[0088] As illustrated in FIG. 11, the solar cell according to the
present embodiment is the same as the solar cell according to the
first embodiment, except that the superlattice layer 22 and the
semiconductor layer 24 are formed so as to cover the top part of
the pillar-shaped structure 16. By forming such a structure, it is
possible to simplify the manufacturing processes, because it is not
needed to form the insulating film 26 or to remove the second
conductivity electrode 30 at the top part of the pillar-shaped
structure 16.
[0089] Next, a description is given of the manufacturing method of
the solar cell according to the present embodiment, with reference
to FIGS. 12A through 13B.
[0090] First, in the same manner as the manufacturing method of the
solar cell according to the first embodiment illustrated in FIGS.
4A and 4B, on a semiconductor substrate 10, pillar-shaped
structures 16 are formed, which are formed by repeatedly depositing
the WZ crystal part 16WZ and the ZB crystal part 16ZB (FIG.
12A).
[0091] Next, by a generally-known etching technique, the metal
particles 14 remaining on the leading ends of the pillar-shaped
structures 16 are removed.
[0092] Next, in the same manner as the manufacturing method of the
solar cell according to the first embodiment illustrated in FIG.
5A, the superlattice layer 22 is formed so as to surround the side
walls of the pillar-shaped structure 16. The superlattice layer 22
is formed by repeatedly depositing the barrier layer 18 and the
quantum structure layer 20.
[0093] When the superlattice layer 22 is caused to grow on the side
walls of the pillar-shaped structure 16 in which the WZ crystal
part 16WZ and the ZB crystal part 16ZB are alternately deposited,
the crystal structure of the pillar-shaped structure 16 is also
applied to the barrier layer 18 and the quantum structure layer 20.
That is to say, on the slide wall of the WZ crystal part 16WZ of
the pillar-shaped structure 16, a barrier layer of a WZ crystal (WZ
crystal part 18WZ) and a quantum structure layer of a WZ crystal
(WZ crystal part 20WZ) are formed. Furthermore, on the side wall of
the ZB crystal part 16ZB of the pillar-shaped structure 16, a
barrier layer of a ZB crystal (ZB crystal part 18ZB) and a quantum
structure layer of a ZB crystal (ZB crystal part 20ZB) are formed
(FIG. 12B).
[0094] As the barrier layer 18, for example, InGaAs may be applied.
As the raw materials of InGaAs, for example, trimethylindium
(TMIn), trimethylgallium (TEGa), and arsine (AsH.sub.3) may be
used.
[0095] As the quantum structure layer 20, for example, InP may be
applied. As the raw materials of InP, trimethylindium (TMIn) and
phosphine (PH.sub.3) may be used.
[0096] In this case, the growth temperature is 450.degree. C.
through 500.degree. C., and as for InP, the V/III ratio (flow rate
ratio of PH.sub.3 and TMI) is 100 through 300, and as for InGaAs,
the V/III ratio (flow rate ratio of AsH.sub.3 and TMI+TEG) is 20
through 60. Accordingly, it is possible to cause the barrier layers
18 and the quantum structure layers 20 to grow so as to cover the
side wall and the top part of the pillar-shaped structure 16.
[0097] By applying a lower growth temperature and a higher
deposition speed than those of the first embodiment, it is possible
to cause the barrier layer 18 and the quantum structure layer 20 to
grow so as to cover the side wall and the top part of the
pillar-shaped structure 16. This is because the raw material, which
has not sufficiently reacted at the side wall of the pillar-shaped
structure 16, reacts at the top part of the pillar-shaped structure
16.
[0098] Next, in the same manner as the manufacturing method of the
solar cell according to the first embodiment illustrated in FIG.
5B, the semiconductor layer 24 of the second conductivity is formed
so as to cover the superlattice layer 22.
[0099] In this case, the crystal structure of the superlattice
layer 22 is applied to the semiconductor layer 24, and a
semiconductor layer of a WZ crystal (WZ crystal part 24WZ) is
formed on the side wall of the WZ crystal part 18WZ of the barrier
layer 18. Furthermore, on the side wall of the ZB crystal part 18ZB
of the barrier layer 18, a semiconductor layer of a ZB crystal (ZB
crystal part 24ZB) is formed (FIG. 13A).
[0100] Next, on the entire surface, the second conductivity
electrode 30 is formed by, for example, a sputtering method, by
depositing a transparent electrode material, such as a CuAlO.sub.2
film.
[0101] Next, the first conductivity electrode 32 is formed on the
back side of the semiconductor substrate 10, thereby completing the
solar cell according to the present embodiment (FIG. 13B).
[0102] As described above, according to the present embodiment, by
using the difference in the energy bandgap between the wurtzite
type crystal and the zinc blende type crystal, and a barrier layer,
a three-dimensional quantum confining structure is formed, and
therefore it is easy to closely stack and align the quantum
confining structure.
[0103] Accordingly, a highly-efficient solar cell may be realized.
Furthermore, by forming the superlattice layer and the
semiconductor layer so as to cover the pillar-shaped structure, the
manufacturing process is simplified.
Modification Embodiments
[0104] Various modifications may be made other than the above
embodiments.
[0105] For example, the above-described first through third
embodiments mainly describes a solar cell using a superlattice
structure of an InP/InGaAs system; however, the same is applicable
to a solar cell using a superlattice structure of a GaAs/InGaAs
system. In this case, on the pillar-shaped structure 16 constituted
by GaAs, a ZB crystal may be grown by setting the growth
temperature at approximately 530.degree. C. through 580.degree. C.,
and setting the V/III ratio at, for example, 2 through 8, and a WZ
crystal may be grown by setting the V/III ratio at, for example, 20
through 80.
[0106] Furthermore, in the above-described first through third
embodiments, a ring-shaped quantum box is formed by the ZB crystal
part 20ZB of the quantum structure layer 20, and a confining
structure is not formed in the circumferential direction of the
ring; however, a confining structure may also be formed in the
circumferential direction of the ring. For example, a
low-distortion material, which does not affect the depositing, may
be used to form the quantum structure layer 20 such that the
quantum structure layer 20 is formed to have a quantum dot shape,
and a confining structure is implemented in the circumferential
direction of the ring.
[0107] Furthermore, the above-described third embodiment indicates
a solar cell in which the superlattice layer 22 and the
semiconductor layer 24 are formed so as to also cover the top part
of the pillar-shaped structure 16 in the solar cell according to
the first embodiment; however, the superlattice layer 22 and the
semiconductor layer 24 may be formed so as to also cover the top
part of the pillar-shaped structure 16 in the solar cell according
to the second embodiment.
[0108] Furthermore, in the above embodiments, the cross-sectional
shape of the pillar-shaped structure 16 is not particularly
described; the shape may be symmetrical such as a circle and a
polygon, or an asymmetrical shape such as an oval.
[0109] Furthermore, in the above embodiments, the pillar-shaped
structure 16 has a fixed thickness; however, the thickness of the
pillar-shaped structure 16 need not be fixed. For example, the
pillar-shaped structure may have a frustum shape whose thickness
gradually decreases in the direction away from the semiconductor
substrate 10. Particularly, when the diameter on the side of the
semiconductor substrate 10 is large, the contact area between the
pillar-shaped structure 16 and the semiconductor substrate 10
becomes large, which is preferable in that the interface resistance
is reduced.
[0110] Furthermore, in the above embodiments, a description is
given of a solar cell in which the first conductivity is the n type
and the second conductivity is the p type; however, the same
applies to a solar cell in which the first conductivity is the p
type and the second conductivity is the n type.
[0111] Furthermore, the structure, the materials, and the
manufacturing conditions of the solar cell according to the above
amendments are merely examples, and modifications and variations
may be made according to the common general knowledge of those
skilled in the art.
[0112] According to an aspect of the embodiments, a solar cell and
a manufacturing method thereof are provided, by which it is easy to
closely stack and align the quantum confining structures, and
realize a highly-efficient solar cell.
[0113] All examples and conditional language recited herein are
intended for pedagogical purposes to aid the reader in
understanding the invention and the concepts contributed by the
inventor to furthering the art, and are to be construed as being
without limitation to such specifically recited examples and
conditions, nor does the organization of such examples in the
specification relate to a showing of the superiority and
inferiority of the invention. Although the embodiments of the
present invention have been described in detail, it should be
understood that the various changes, substitutions, and alterations
could be made hereto without departing from the spirit and scope of
the invention.
* * * * *