U.S. patent application number 09/442590 was filed with the patent office on 2002-01-24 for semiconductor devices with selectively doped iii-v nitride layers.
Invention is credited to GOETZ, WERNER, KERN, SCOTT R..
Application Number | 20020008245 09/442590 |
Document ID | / |
Family ID | 23757381 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020008245 |
Kind Code |
A1 |
GOETZ, WERNER ; et
al. |
January 24, 2002 |
SEMICONDUCTOR DEVICES WITH SELECTIVELY DOPED III-V NITRIDE
LAYERS
Abstract
A semiconductor device is provided having n-type device layers
of III-V nitride having donor dopants such as germanium (Ge),
silicon (Si), tin (Sn), and/or oxygen (O) and/or p-type device
layers of III-V nitride having acceptor dopants such as magnesium
(Mg), beryllium (Be), zinc (Zn), and/or cadmium (Cd), either
simultaneously or in a doping superlattice, to engineer strain,
improve conductivity, and provide longer wavelength light
emission.
Inventors: |
GOETZ, WERNER; (PALO ALTO,
CA) ; KERN, SCOTT R.; (SAN JOSE, CA) |
Correspondence
Address: |
SKJERVEN MORRILL MACPHERSON LLP
25 METRO DRIVE
SUITE 700
SAN JOSE
CA
95110
US
|
Family ID: |
23757381 |
Appl. No.: |
09/442590 |
Filed: |
November 17, 1999 |
Current U.S.
Class: |
257/87 |
Current CPC
Class: |
H01L 33/04 20130101;
H01S 5/305 20130101; H01S 5/32341 20130101; H01L 33/12 20130101;
H01L 33/325 20130101 |
Class at
Publication: |
257/87 |
International
Class: |
H01L 027/15; H01L
031/12; H01L 033/00 |
Claims
The invention claimed is:
1. A device comprising: a substrate; an III-V nitride layer
positioned over said substrate; an active layer positioned over
said III-V nitride layer; a second III-V nitride layer positioned
over said active layer; and said III-V nitride layer doped with an
element dopant from a group consisting of germanium, tin, oxygen,
magnesium, beryllium, zinc, cadmium, and a combination thereof
whereby stress is controlled and embrittlement of said III-V
nitride layer is minimized.
2. A device as claimed in claim 1 wherein: said III-V nitride layer
contains a second dopant selected from a group consisting of
silicon, germanium, tin, oxygen, magnesium, beryllium, zinc,
cadmium, and a combination thereof.
3. The device as claimed in claim 1 wherein: said III-V nitride
layer contains a second dopant selected from a group consisting of
silicon, germanium, tin, oxygen, magnesium, beryllium, zinc,
cadmium, and a combination thereof, said second dopant changes
concentration towards said active layer.
4. The device as claimed in claim 1 wherein: said III-V nitride
layer contains a second dopant selected from a group consisting of
silicon, germanium, tin, oxygen, magnesium, beryllium, zinc,
cadmium, and a combination thereof, said second dopant increases
concentration towards said active layer; and said dopant increases
concentration away from said active layer.
5. The device as claimed in claim 1 wherein: a portion of said
III-V nitride layer is doped with a dopant from a group consisting
of silicon, germanium, tin, oxygen, and a combination thereof; and
a second portion of said III-V nitride layer is doped with a dopant
different from the dopant used in said first portion and selected
from a group consisting of silicon, germanium, tin, oxygen,
magnesium, beryllium, zinc, cadmium, and a combination thereof.
6. The device as claimed in claim 1 wherein: a plurality of
portions of said III-V nitride layer are doped with a dopant from a
group consisting of silicon, germanium, tin, oxygen, magnesium,
beryllium, zinc, cadmium, and a combination thereof; a plurality of
second portions of said III-V nitride layer are doped with a dopant
different from the dopant used in said plurality of portions and
selected from a group consisting of silicon, germanium, tin,
oxygen, magnesium, beryllium, zinc, cadmium, and a combination
thereof; and said plurality of portions and said plurality of
second portions are alternated whereby a superlattice structure is
formed.
7. The device as claimed in claim 1 including: an undoped layer in
said III-V nitride layer.
8. The device as claimed in claim 1 wherein: said III-V nitride
layer includes a heavily doped portion using a different dopant
than said dopant to increase the lattice parameter adjacent to said
active layer.
9. The device as claimed in claim 1 wherein: said III-V nitride
layer is heavily doped to render it highly conductive to be a
contact layer for the device.
10. The device as claimed in claim 1 wherein: said III-V nitride
layer contains a plurality of dopants to selectively vary the
composition and properties of said III-V nitride layer from said
structure to said active layer.
11. The device as claimed in claim 1 wherein: said III-V nitride
layer and the second III-V nitride layer range from 10 .ANG. to 10
.mu.m in thickness.
12. A light-emitting device comprising: a substrate containing an
element from a group consisting of aluminum, carbon, gallium,
indium, nitrogen, oxygen, silicon, and a combination thereof; a
III-V nitride layer positioned over said substrate and containing
an element selected from a group consisting of aluminum, gallium,
indium, nitrogen, and a combination thereof; an active layer
positioned over said III-V nitride layer and containing an element
selected from a group consisting of aluminum, gallium, indium,
nitrogen, and a combination thereof; a second III-V nitride layer
positioned over said active layer and containing an element
selected from a group consisting of aluminum, gallium, indium,
nitrogen, and a combination thereof; and said III-V nitride layer
doped with an element dopant from a group consisting of germanium,
tin, oxygen, magnesium, beryllium, zinc, cadmium, and a combination
thereof whereby compression or tension cracking and embrittlement
of said III-V nitride layer is minimized.
13. The light-emitting device as claimed in claim 12 wherein: said
III-V nitride layer contains a second dopant selected from a group
consisting of silicon, germanium, tin, oxygen, magnesium,
beryllium, zinc, cadmium, and a combination thereof.
14. The light-emitting device as claimed in claim 12 wherein: said
III-V nitride layer contains a second dopant selected from a group
consisting of silicon, germanium, tin, oxygen, magnesium,
beryllium, zinc, cadmium, and a combination thereof, said second
dopant changes concentration towards said active layer whereby the
lattice constant of said III-V nitride layer changes towards the
lattice constant of said active layer.
15. The light-emitting device as claimed in claim 12 wherein: said
III-V nitride layer contains a second dopant selected from a group
consisting of silicon, germanium, tin, oxygen, magnesium,
beryllium, zinc, cadmium, and a combination thereof, said second
dopant increases concentration towards said active layer; and said
dopant increases concentration away from said active layer whereby
high conductivity and long wavelength emissions from the active
layer are achieved.
16. The light-emitting device as claimed in claim 12 wherein: a
first portion of said III-V nitride layer is doped with a dopant
from a group consisting of silicon, germanium, tin, oxygen,
magnesium, beryllium, zinc, cadmium, and a combination thereof; and
a second portion of said III-V nitride layer is thicker than said
first portion and is doped with a dopant different from the dopant
used in said first portion and selected from a group consisting of
silicon, germanium, tin, oxygen, magnesium, beryllium, zinc,
cadmium, and a combination thereof.
17. The light-emitting device as claimed in claim 12 wherein: a
plurality of first portions of said III-V nitride layer are doped
with a dopant from a group consisting of silicon, germanium, tin,
oxygen, magnesium, beryllium, zinc, cadmium, and a combination
thereof, a plurality of second portions of said III-V nitride layer
are doped with a dopant different from the dopant used in said
plurality of first portions and selected from a group consisting of
silicon, germanium, tin, oxygen, magnesium, beryllium, zinc,
cadmium, and a combination thereof; and each of said plurality of
first portions and each of said plurality of second portions are
alternated whereby the conductivity and the stress in the device
are controlled.
18. The light-emitting device as claimed in claim 12 including: a
plurality of undoped layers in said III-V nitride layer and in said
second III-V nitride layer.
19. The light-emitting device as claimed in claim 12 wherein: said
III-V nitride layer includes a heavily doped portion using a
different dopant than said dopant to increase the lattice parameter
adjacent to said active layer.
20. The light-emitting device as claimed in claim 12 wherein: said
III-V nitride layer has at least a portion heavily doped to render
it a contact layer.
21. The light-emitting device as claimed in claim 12 wherein: said
III-V nitride layer contains a plurality of dopants to selectively
vary the composition and properties of said III-V nitride layer
from said substrate to said active layer.
22. The light-emitting device as claimed in claim 12 wherein: said
III-V nitride layer and the second III-V nitride layer range from 1
.ANG. to 10 .mu.m in thickness.
Description
TECHNICAL FIELD
[0001] The present invention relates generally to semiconductor
devices and more particularly to doping III-V nitride
light-emitting devices.
BACKGROUND ART
[0002] Silicon (Si) is the donor of choice for doping n-type III-V
nitrides due to its favorable properties. In particular, during
metal-organic chemical vapor deposition (MOCVD), Si atoms can be
delivered to the growing crystal by flowing silane (SiH.sub.4),
which is available as a high purity grade gas. In addition, Si
incorporates efficiently onto the gallium (Ga) sites in the gallium
nitride (GaN) lattice where it acts as a donor. Further, Si in GaN
(SiGa) is a shallow donor with an activation energy for ionization
of .about.20 meV.
[0003] However, with Si doping the achievable n-type conductivity
of an III-V nitride layer is limited due to the fact that the
incorporation of Si leads to the formation of cracks for
heteroepitaxially-grown III-V nitride materials (particularly on
sapphire substrates). For a given material thickness, the material
cracks when the Si doping level exceeds a certain critical
concentration. Likewise, for a given doping concentration, the
material starts to crack when the material thickness exceeds a
certain critical thickness.
[0004] Both a high doping concentration and a large material
thickness are desirable to reduce the electrical resistivity of a
semiconductor material. For example, for an .about.3.5 .mu.m thick
GaN material, as typically employed in a light-emitting diode (LED)
structure, the doping concentration is limited to .about.5e18
cm.sup.-3. As a consequence of the aforegoing, the series
resistance of an aluminum indium gallium nitride (AlInGaN) LED is
dominated by the resistance of the Si-doped GaN layer. This is the
case for growth on non-conductive substrates such as sapphire where
the current passes laterally through the Si-doped GaN layer as well
as growth on conductive substrates such as silicon carbide (SiC)
and hydride vapor phase epitaxy (HVPE) grown GaN where the current
passes vertically through the thick Si-doped GaN layer. Higher
doping concentrations and/or thicker n-type GaN materials (for
growth on non-conductive substrates) would be advantageous for the
fabrication of III-V nitride based LEDs with low series
resistance.
[0005] Further, in addition to Si, germanium (Ge) and tin (Sn) have
been studied as potential donor impurities for III-V nitride
materials. However, there are reports on Ge doping experiments
where it was concluded that doping with Ge is problematic. In the
S. Nakamura, T. Mukai, and M. Senoh, Si- and Ge-Doped GaN Materials
Grown with GaN Buffer Layers, Jpn. J. Appl. Phys. 31, 2883, 1992,
it is reported that the doping efficiency of Ge is about one order
of magnitude lower than for Si. Furthermore, they concluded that
the maximum carrier concentration for Ge-doped GaN is limited to
.about.1.times.10.sup.19 cm.sup.-3 because at this doping level the
surface of the Ge-doped GaN materials becomes rough and shows pits.
X. Zhang, P. Kung, A. Saxler, D. Walker, T. C. Wang, and M.
Razeghi, Growth of Al.sub.xGa.sub.1-xN:Ge on sapphire and Si
substrates, Appl. Phys. Lett. 67, 1745 (1995), concluded the
Ge-doped aluminum gallium nitride (AlGaN) materials have low
electron mobilities and that Ge doping is not useful for growing
low resistivity materials.
[0006] For a long time, a solution has been sought to the problem
of material cracking which occurs with Si doping levels exceeding
certain concentrations at certain critical thicknesses. Further, Si
doping is known to cause the III-V nitride materials to embrittle,
which further enhances the tendency of the material to crack, and a
solution to this problem has long been sought. It has also been
shown that there is a large piezoelectric effect due to the lattice
mismatch between GaN and its alloys. For example, an indium gallium
nitride (InGaN) layer grown between two GaN layers will have a high
piezoelectric sheet charge associated with each interface.
DISCLOSURE OF THE INVENTION
[0007] The present invention provides a semiconductor device having
n-type device layers of III-V nitride having donor dopants such as
germanium (Ge), silicon (Si), tin (Sn), and/or oxygen (O) and/or
p-type device layers of III-V nitride having acceptor dopants such
as magnesium (Mg), beryllium (Be), zinc (Zn), and/or cadmium (Cd),
either simultaneously or in a doping superlattice, to engineer
strain, improve conductivity, and provide longer wavelength light
emission.
[0008] The present invention further provides a semiconductor
device using Ge either singularly or in combination, as a
co-dopant, with Si and Sn as donor dopants either simultaneously or
in a doping superlattice to engineer strain. Unlike Si, the Ge
doping concentration can range from .about.10.sup.19 cm.sup.-3 to
.about.10.sup.20 cm.sup.-3 at layer thicknesses of 3 .mu.m and
higher without causing cracking problems.
[0009] The present invention further provides donor impurities
which do not cause embrittlement of III-V nitride materials.
[0010] The present invention further provides multi-donor impurity
doping for III-V nitride materials to control doping and strain
engineering separately.
[0011] The present invention further provides highly conductive,
n-type, Ge-doped, gallium nitride (GaN) materials for utilization
in contact layers of III-V nitride devices.
[0012] The present invention further provides a light-emitting
device with donor impurities which promote growth of high indium
nitride (InN) containing indium gallium nitride (InGaN) light
emission layers for light emission at long wavelengths
(.lambda..gtoreq.500 nm). This allows the InGaN active region to
contain a higher InN composition with higher quality and thus a
higher efficiency, longer wavelength light emission or the growth
of an AlGaN layer on top of GaN without cracking.
[0013] The present invention further provides a light-emitting
device co-doped using a combination of Si, Ge, Sn, oxygen (O),
magnesium (Mg), beryllium (Be), zinc (Zn), or cadmium (Cd) to
improve the conductivity of III-V nitride materials which stabilize
the structural integrity of heteroepitaxially-grown III-V nitride
materials on lattice mismatched substrates.
[0014] The present invention further provides a light-emitting
device using different donor dopants for conductive and contact
layers.
[0015] The present invention further provides a light-emitting
device where a bottom layer is doped with Ge and a layer on top
doped with a different species (e.g. Si, Sn, or a combination of
Si, Ge, and Sn). This permits adjustment of the in-plane lattice
constant of GaN closer to the in-plane lattice constant of a
ternary compound (e.g., InGaN or aluminum gallium nitride (AlGaN)).
This allows the InGaN active region to contain a higher InN
composition with higher quality and thus a higher efficiency,
longer wavelength light emission or the growth of an AlGaN layer on
top of GaN without cracking.
[0016] The present invention further provides a method of
controlling strain and, thus, the effects of piezoelectricity in
III-V nitride layers. Strain engineering plays a major role in
controlling piezoelectric interface charges.
[0017] The above and additional advantages of the present invention
will become apparent to those skilled in the art from a reading of
the following detailed description when taken in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a light-emitting device incorporating the doped
III-V nitride layer of the present invention;
[0019] FIG. 2 is a light-emitting device having the doped
superlattice of the present invention; and
[0020] FIG. 3 is a light-emitting device incorporating the strain
engineered doping of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0021] Referring now to FIG. 1, therein is shown an electronic
device such as a light-emitting device 10 which could be a
light-emitting diode (LED) or laser diode (LD). The light-emitting
device 10 includes an optional substrate 11 of sapphire, silicon
carbide (SiC), silicon (Si), gallium arsenide (GaAs), or gallium
nitride (GaN). It should be understood that the substrate 11 could
be discarded in the formation of the light-emitting device 10 after
deposition of the various layers which will hereinafter be
described.
[0022] Due to difficulties in nucleation of the single crystalline
III-V nitride layers on foreign substrates, a low temperature
buffer layer 12 is often disposed on the substrate 11. The buffer
layer 12 is of a material such as GaN or aluminum nitride (AlN)
deposited on sapphire at low temperatures around 500.degree. C.
[0023] A highly conductive, n-type, light-emitting, III-V nitride
layer 13 is deposited on the buffer layer 12. The nitride layer 13
is made of a doped GaN, an indium gallium nitride (InGaN), an
aluminum gallium nitride (AlGaN), an aluminum indium nitride
(AlInN), or an aluminum gallium indium nitride (AlGaInN). These
materials enable low driving voltages for the light-emitting device
10 due to reduced resistance in the n-layer, excellent electron
injection due to high electron concentration near the p-n junction,
and the formation of electrodes to the layers with the ohmic
electrical characteristics. In the preferred embodiment, the dopant
is germanium (Ge) instead of silicon (Si) or combinations of Si,
Ge, tin (Sn), and oxygen (O).
[0024] An active layer 14 is deposited on the nitride layer 13. The
active layer 14 can have a single-quantum well (SQW),
multiple-quantum well (MQW), or double-hetero (DH) structure.
Generally, this layer is GaN, AlGaN, AlInN, InGaN, or AlInGaN.
[0025] A highly conductive p-type, III-V nitride layer 15 is
deposited on the active layer 14. The p-type nitride layer 15 is
similar to the n-type nitride layer 13 except with a p-type dopant
being used.
[0026] Final device layers 16, such as cladding and/or contact
layers, may be deposited on top of the p-type nitride layer 15.
[0027] The various layers may be grown using techniques such as
metal organic chemical vapor deposition (MOCVD), molecular beam
epitaxy (MBE), gas source MBE (GSPMBE), or hydride vapor phase
epitaxy (HVPE). Also, the composition and/or doping of the various
layers may change abruptly from one layer to another, may be
smoothly graded over a finite thickness, may be graded over the
entire thickness of a layer, or may be combined with undoped
layers.
[0028] Referring now to FIG. 2, therein is shown an electronic
device, such as a light-emitting device 20. The light-emitting
device 20 includes an optional substrate 21 of sapphire, silicon
carbide (SiC), silicon (Si), gallium arsenide (GaAs), or gallium
nitride (GaN). Again, due to difficulties in nucleation of the
single crystalline III-V nitride layers on foreign substrates, a
low temperature buffer layer 22 is often disposed on the substrate
21. The buffer layer 22 is of a material such as GaN or AlN
deposited on sapphire at low temperatures.
[0029] A highly conductive, n-type III-V nitride layer is deposited
on the buffer layer 22. This nitride layer can be GaN with layers
doped with different donors, GaN and InGaN layers doped with
different donors, InGaN, AlGaN, and AlGaInN doped with different
donors, or these layers with undoped layers in between. This
layered structure may be termed a "superlattice", although the
layers are thicker than those in a conventional superlattice
structures since these layers should range from 10 .ANG.
(angstroms) to 10 .mu.m (microns) in thickness. It has been
determined that this greater thickness provides greater strain
control.
[0030] The doped layers are designated as nitride layers 23 through
29, which have combinations of Si, Ge, Sn, and O as dopants. The
combination of dopants is alternated such that the odd numbered
doped nitride layers, designated as nitride layers 23, 25, 27, and
29, use one or more dopant(s) and the other nitride layers, the
even numbered doped nitride layers, designated as nitride layers
24, 26, and 28, use another dopant or combination of dopants to
achieve a desired state of strain. For example, the nitride layers
23, 25, 27, and 29 are Ge doped and the nitride layers 24, 26, and
28 are Si doped.
[0031] An active layer 30 is deposited on the nitride layer 29. The
active layer 30 can have a SQW, MQW, or DH structure. Generally,
this layer is an InN containing InGaN or AlInGaN. With a higher InN
composition in the InGaN active region, a longer wavelength light
emission can be obtained.
[0032] A highly conductive p-type, III-V nitride layer 31 is
deposited on the active layer 30. The p-type nitride layer 31 is
the same as the n-type nitride layer 23 except with a p-type dopant
being used.
[0033] Final device layers 32, such as cladding and/or contact
layers, may be deposited on top of the p-type nitride layer 31. The
final device layers 32 are the other layers required by the
light-emitting device 20.
[0034] Referring now to FIG. 3, therein is shown another electronic
device, such as a light-emitting device 50. The light-emitting
device 50 includes a substrate 51 of sapphire, SiC or GaN. Due to
difficulties in nucleation of the single crystalline III-V nitride
layers on foreign substrates, a low temperature buffer layer 52 is
often deposited on the substrate 51. The buffer layer 52 is of a
material such as GaN or AlN.
[0035] A highly conductive, n-type III-V nitride layer is deposited
on the buffer layer 52. This nitride layer is made of doped GaN,
InGaN, AlGaN, AlInN, or AlGaInN. Here, one dopant species is used
in a nitride layer designated as nitride layer 53 and a second in a
nitride layer designated as nitride layer 54. In the preferred
embodiment, the dopants are combinations of Si, Ge, Sn, and O.
Where the nitride layer 53 is doped with Si and the nitride layer
54 with Ge, the nitride layer 54 can be a contact layer.
[0036] An active layer 55 is deposited on the nitride layers 53 and
54. The active layer 55 can have a SQW or MQW structure. Generally,
this layer is an InN containing InGaN or AlInGaN.
[0037] A highly conductive p-type, III-V nitride layer 56 is
deposited on the active layer 55. The p-type nitride layer 56 is
similar to the n-type nitride layers 53 and 54 except with a p-type
dopant being used.
[0038] Final device layers 57, such as cladding and/or contact
layers, may be deposited on top of the p-type nitride layer 56. The
final device layers 57 are the other layers required by the
light-emitting device 50.
[0039] In the past, Si has been the donor of choice for doping
n-type, III-V nitride layers due to its favorable properties.
However, with Si doping the achievable n-type conductivity of an
III-V nitride layer is limited due to the fact that the
incorporation of Si leads to the formation of cracks in
heteroepitaxially-grown GaN due to differences in lattice constants
and in coefficients of thermal expansion with the substrate. It is
possible that the cracking problem is a consequence of the small
ionic radius of Si donors as compared to Ga host atoms. Si has an
ionic radius of 0.41 .ANG. while Ga has an ionic radius of 0.62
.ANG.. For example, it has been determined that for growth on
c-plane sapphire, Si doping leads to more compressive strain in the
c-axis direction for high Si doping concentrations. As a
consequence, the basal plane of GaN is put into more tensile
strain. Two potential donor impurities, Ge and Sn for III-V nitride
materials possess larger ionic radii than Si and are much closer to
the ionic radius of Ga. Ge has an ionic radius of 0.53 .ANG. and Sn
has an ionic radius of 0.71 .ANG..
[0040] Further, like Si, both Ge and Sn doping sources are readily
available as gases, germanium hydride (GeH.sub.4) and tin hydride
(SnH.sub.4), for use with conventional MOCVD processes. And, the
donor ionization energies of Ge in GaN (Ge.sub.Ga) and Sn in GaN
(Sn.sub.Ga) are expected to be similar to that of silicon in GaN
(Si.sub.Ga). This makes these ions ideal dopants.
[0041] With reference to the structure shown in FIG. 1, the nitride
layer 13 can be doped with Si, Ge, Sn, or O alone or together in
combination. In contrast to Si-doped GaN, heavily Ge-doped GaN will
not crack when grown thicker than .about.1 .mu.m. Further, the
nitride layer 13 with Ge doping levels in the range from
.about.10.sup.19 cm.sup.-3 to .about.10.sup.20 cm.sup.-3 typically
form ohmic contacts with various metals, and thus make good contact
layers. Ge doping at such concentration has been deemed to be
unobtainable in the literature, as indicated by Nakamura et al,
supra.
[0042] Also with reference to FIG. 1, the nitride layer 13 may be
co-doped using a combination of the following donors: Si, Ge, Sn,
and O. The different donor species are introduced simultaneously to
stabilize the structural integrity of heteroepitaxially-grown III-V
nitride on lattice mismatched substrates. For example, tensile
strain can be reduced by using combinations of Si and Ge, Si and
Sn, and Ge and Sn. In addition, the use of O is highly desirable as
it will occupy the N-lattice site. Hence, there is no site
competition with Si, Ge, or Sn which occupies the Ga lattice site,
and higher doping levels may be achieved. With higher doping
levels, it is possible to achieve much higher conductivity. Using
co-dopants of Si/Ge, Si/Sn, and Ge/Sn, it is possible to stabilize
the lattice and avoid cracking. The dopants and their percentages
are chosen differently for the growth of GaN, InGaN, and AlGaN to
adjust the strain state that is desirable for overgrowth of GaN,
AlGaN, AlInN, InGaN, or AlInGaN.
[0043] With reference to FIG. 2, the light-emitting device 20 is a
solution to the problem reported by Nakamura, et al., supra, that
the doping efficiency for Ge is an order of magnitude lower than
for Si. To circumvent this problem, Si layers could be sandwiched
between Ge-doped layers so that the total thickness of the Si-doped
layers does not exceed the critical thickness for cracking at the
given doping level. Thus, the Si-doped layers 24, 26, and 28 would
be relatively thin. The Ge-doped layers 23, 25, 27, and 29 can be
doped to the same concentration, but can be made thick enough to
provide the desired high conductivity.
[0044] Referring back to FIG. 1, the different dopants in the
nitride layer 13 can be at a single dopant concentration or one
which gradually changes from the buffer layer 12 to the nitride
layer 13. It is also practical for the nitride layer 13 to start
with one dopant at the buffer layer 12 and gradually decrease the
concentration of the one dopant and gradually increase the
concentration of the second dopant.
[0045] For example, a gradual adjustment of the strain for
subsequent overgrowth of an InGaN or AlGaN active layer 14 will be
possible by choosing a specific combination of dopants and grading
their relative concentration. The in-plane lattice parameters for
InGaN and AlGaN are larger and smaller, respectively, than the
lattice parameters for GaN.
[0046] As a consequence, co-doping of a GaN nitride layer 13 with
two different donor species and increasing the concentration of the
donor species that increases the in-plane lattice constant towards
the interface with an InGaN active layer 14 will adjust the lattice
for overgrowth of InGaN. For example, co-doping with Si and Ge, and
increasing Si concentration towards the InGaN interface.
[0047] On the other hand, choosing an alternative pair of donor
dopants for a GaN nitride layer 13 and increasing the concentration
of the donor that decreases the in-plane lattice constant towards
the interface with an AlGaN active layer 14 will be advantageous
for overgrowth of thick AlGaN layers as required for growth of
mirror stacks in surface emitting lasers, for example, co-doping
with Si and Ge, and increasing Ge concentration towards the AlGaN
interface.
[0048] With reference now to FIG. 3, similar results to those just
described above can be achieved by the introduction of a separately
doped layer. For example, the nitride layer 53 of GaN could be
doped with Ge. This would allow for high doping and the layer could
also be thick. On top of the Ge-doped GaN layer, a heavily doped
nitride layer 54 is grown using a different donor species such as
Si which increases the lattice parameter in the c-plane and
therefore will allow InGaN with high InN compositions to grow. With
a higher InN composition in the InGaN active region, a longer
wavelength light emission can be obtained. Also, higher tensile
strain in the basal plane will reduce the piezoelectric sheet
charge at the InGaN interface.
[0049] Alternately, the nitride layer 53 would contain GaN:Si and
the nitride layer 54 of Ge-doped GaN would be grown on top with a
Ge doping concentration of .about.10.sup.20 cm.sup.-3. This high
Ge-doped nitride layer 54 would be a contact layer which is thick
enough so that it can be easily reached by etching even if the etch
depth varies. The active layer 55 is then grown on top of this
contact layer.
[0050] While the invention has been described in conjunction with a
specific best mode, it is to be understood that many alternatives,
modifications, and variations will be apparent to those skilled in
the art in light of the aforegoing description. For example, the
structure is further applicable to highly doped p-layers in
semiconductor devices where the dopants would be Mg, Be, Zn, or Cd.
Accordingly, it is intended to embrace all such alternatives,
modifications, and variations which fall within the spirit and
scope of the included claims. All matters set forth herein or shown
in the accompanying drawings are to be interpreted in an
illustrative and non-limiting sense.
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