 |
Description  |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a compound semiconductor device composed
of nitride compound semiconductors formed on a substrate, and more
particularly to a light emitting device composed of compound
semiconductors.
2. Description of the Related Art
Recently, a nitride compound semiconductor, expressed by the general
formula of BAlGaInN, has been known as a material for blue or purple light
emitting diodes. The well-known diode using such a material has an MIS
(Metal-Insulator-Semiconductor) structure. However, the diode of such an
MIS type has disadvantages that its operating voltage is high, its
luminous strength is weak and further its element life is short. As one of
means to overcome these disadvantages, there has already been known to
shift to a pn-junction type.
The above materials are grown on a sapphire substrate principally through a
metalorganic chemical vapor deposition method (hereinafter referred to as
an MOCVD method) or molecular beam epitaxy method (an MBE method). Even if
these methods are not used, the nitride compound semiconductor has a
variety of problems.
The first problem concerning the nitride compound semiconductor takes place
during its crystal growth. Generally, in the crystal growth process
utilizing a non-equilibrium state, such as by an MOCVD or MBE method, the
growth pursues the following process, i.e., firstly crystal nucleuses for
growing are formed; secondly the entire nucleuses grow (the
threedimensional growth); then the nucleuses associate together to become
a flat film; and finally the film grows in a growth-axis direction (the
two-dimensional growth). As the result of crystal growth experiments using
a sapphire substrate by the inventors, a problem has been found that, in
case of n-type crystal growth by adding Si, a shift period of time from
the three-dimensional growth to the two-dimensional growth is longer than
that of a so-called undoped film or a p-type film intentionally doped with
Mg, and it is especially difficult to obtain a flat film where the film
has a thickness of 1 .mu.m or less.
The second problem concerning the nitride compound semiconductor is of
crystal quality. Generally, it is well known that, in the nitride compound
semiconductor, nitrogen vacancies tend to be produced inside the crystal,
because of very high vapor pressure of N.sub.2. Each nitrogen vacancy
forms a deep donor level of energy. Therefore, particularly as to a
light-emitting element, light generated at a light-emitting layer is
absorbed due to transition of low energy concerned with the deep donor
level, and light take-out efficiency is reduced. As the means to fill such
nitrogen vacancies, it is known to use an element of group V, such as P or
As, except N, as described in Jap. Pat. Appln. KOKAI Publication No.
49-29770. However, GaP (gallium phosphide) or GaAs (gallium arsenide) is
extremely lower in energy gap than GaN. Therefore, when such impurities,
for instance P or As, are added to GaN, energy gaps of these mixed crystal
semiconductor GaPN or GaAsN become extremely decreased, and there is a
problem that a light emission of short wave length, utilizing a large
energy gap belonging to characteristics of the nitride compound
semiconductor, cannot be obtained.
The third problem concerning the nitride compound semiconductor is also of
crystal quality. When a nitride compound semiconductor film is formed on a
semiconductor substrate, such as Si or GaAs, used generally for formation
of semiconductor elements, the nitride compound semiconductor film formed
on the substrate takes over the crystal system, a cubic system, of the
substrate, and thereby the epitaxial layer becomes a film of a cubic
system and not of a hexagonal system. Therefore, conventionally, a
sapphire substrate is used, so as to form a nitride compound semiconductor
film having a hexagonal system. However, where the sapphire substrate
which has no conductivity is used, it is necessary to etch some layers
from the surface so as to attain an electrical contact with a layer which
is not exposed to the surface, in the case of a semiconductor element,
especially a light-emitting element, having a layered structure of nitride
compound semiconductor films. As a result of experiments conducted by the
inventors, a problem has been found that a p-type nitride compound having
added Mg is very difficult to be eliminated by etching treatments.
The fourth problem concerning the nitride compound semiconductor is of
impurity diffusion during element formation following the crystal growth.
Besides the nitride compound semiconductor belonging to the subject
according to the present invention, during manufacturing process of
semiconductor elements, heat treatments following crystal growth are
generally performed in order to obtain various states. During this
treatment, impurities which are expected to be kept at desired positions
are diffused, and there becomes a problem that it results in harmful
effects on several characteristics of elements. From experimental results
by the inventors, it has been found that Mg diffusion is most remarkable
among the impurities generally utilized.
As mentioned above, the nitride compound semiconductor film has, from a
view point of characteristics of crystal itself or crystal growth, a
variety of problems, such as difficulty in formation of a flat thin film,
presence of nitrogen vacancies, difficulty in crystal etching and impurity
diffusion during heat treatment. These problems have become severe in case
of forming a semiconductor element, especially a light-emitting element,
of nitride compound semiconductor films, particularly using sapphire as a
substrate.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above problems,
and it is an object of the present invention to solve these problems by
means of impurity addition during formation of the nitride compound
semiconductor layers.
According to a first aspect of the present invention, there is provided a
compound semiconductor device comprising:
a crystal substrate; and
a first semiconductor film composed of n-type or i-type (intrinsic or
neutral conductivity type) nitride compound, supported by the substrate
through vapor phase growth, wherein the first semiconductor film contains
1.times.10.sup.15 cm.sup.-3 to 1.times.10.sup.17 cm.sup.-3 of magnesium or
1.times.10.sup.15 cm.sup.-3 to 1.times.10.sup.17 cm.sup.-3 of zinc so as
to accelerate a shift from its three-dimensional growth to its
two-dimensional growth.
According to a second aspect of the present invention, there is provided a
compound semiconductor device comprising:
a crystal substrate; and
a first semiconductor film composed of nitride compound, supported by the
substrate through vapor phase growth,
wherein the first semiconductor film contains 1.times.10.sup.16 cm.sup.-3
to 5.times.10.sup.17 cm.sup.-3 of carbon, 1.times.10.sup.18 cm.sup.-3 to
1.times.10.sup.20 cm.sup.-3 of oxygen, 1.times.10.sup.16 cm.sup.-3 to
1.times.10.sup.18 cm.sup.-3 of selenium, or 1.times.10.sup.16 cm.sup.-3 to
1.times.10.sup.18 cm.sup.-3 of sulfur so as to fill nitrogen vacancies
thereof.
According to a third aspect of the present invention, there is provided a
compound semiconductor device comprising:
a crystal substrate; and
a second semiconductor film composed of p- or i-type nitride compound,
formed to be supported by the substrate through vapor phase growth and
containing magnesium, wherein the second semiconductor film contains
1.times.10.sup.16 cm.sup.-3 to 8.times.10.sup.17 cm.sup.-3 of silicon so
as to facilitate etching thereof.
According to a fourth aspect of the present invention, there is provided a
compound semiconductor device comprising:
a crystal substrate; and
first and second semiconductor films supported by the substrate and
mutually laminated, the first semiconductor film being composed of n- or
i-type nitride compound, formed through vapor phase growth, the second
semiconductor film being composed of p- or i-type nitride compound
containing magnesium,
wherein the first semiconductor film contains 3.times.10.sup.18 cm.sup.-3
to 1.times.10.sup.20 cm.sup.-3 of hydrogen so as to prevent magnesium from
diffusing thereinto from the second semiconductor film.
Problems to be solved are dependent upon elements. Therefore, functions of
the elements to the problems will be described respectively. These
elements are divided into four groups, i.e., magnesium (Mg) and zinc (Zn);
carbon (C), oxygen (O), selenium (Se) and sulfur (S); silicon (Si); and
hydrogen (H).
Initially, the group of Mg and Zn will be described. These elements are
related to the first problem, mentioned above, of crystal flatness during
its growth. From experimental results by the inventors, as already
mentioned above, a crystal film of n-type nitride compound semiconductor
having Si added thereto, has been found to be slow in its shift from the
three-dimensional growth to the two-dimensional growth. On the contrary,
when Mg is also added, its shift from the three-dimensional growth to the
two-dimensional growth occurs at an earlier time, i.e., during thin film
state, in comparison to when no Mg is added. That is to say, a flat thin
film may be formed by adding Mg. However, since Mg is essentially an
acceptor impurity, its addition in a large amount will avoid formation of
an n-type crystal. Therefore, from experimental results by the inventors,
it has been found that an Mg concentration range, which is effective in
formation of the n-type crystal without disturbing the formation by Mg
addition, is from 1.times.10.sup.15 cm.sup.-3 to 1.times.10.sup.17
cm.sup.-3. A similar effect with Zn also is obtained, and the effective
concentration of Zn has been found to fall in a range of from
1.times.10.sup.15 cm.sup.-3 to 1.times.10.sup.17 cm.sup.-3.
Next, the group of C, O, Se and S will be described. These elements are
related to the second problem of nitrogen vacancies in crystal. Since
atoms of these elements tend to enter the nitrogen site of the nitride
compound semiconductor, the nitrogen vacancies peculiar to this compound
semiconductor may be filled. Therefore, a deep donor level caused by the
nitrogen vacancies may be eliminated. Also, since the above four kinds of
impurity elements are impurities which form shallow donors or acceptors by
themselves, formation of deep impurity level does not occur by their
additions, therefore, loss of light take-out efficiency also does not
occur. On the other hand, excess addition of these impurities is not
desirable, since reduction in crystalline property causes decrease in
light-emitting efficiency. And, as the result of searching for a proper
range of impurity concentration, it has been found that C falls in a range
of from 1.times.10.sup.16 cm.sup.-3 to 5.times.10.sup.17 cm.sup.-3, O
falls in a range of from 1.times.10.sup.18 cm.sup.-3 to 1.times.10.sup.20
cm.sup.-3, Se falls in a range of from 1.times.10.sup.16 cm.sup.-3 to
1.times.10.sup.18 cm.sup.-3 and S falls in a range of from
1.times.10.sup.16 cm.sup.-3 to 1.times.10.sup.18 cm.sup.-3.
Thirdly, Si will be described. This element is related to the third
problem, mentioned above, of difficulty of crystal etching. From
experimental results by the inventors, it has been found that a general
crystal layer of p-type nitride compound semiconductor having Mg added
thereto is very difficult to eliminate by etching. However, it has been
found that an additional small amount of Si make the etching easy.
Further, since Si is essentially a donor impurity of the n-type, its
excess addition avoids formation of the p-type crystal. Therefore, from
experimental results by the inventors, it has been found that an Si
concentration range, which is effective in formation of the n-type crystal
without disturbing the formation by Si addition, is from 1.times.10.sup.16
cm.sup.-3 to 8.times.10.sup.17 cm.sup.-3.
Finally, H will be described. A pn-junction structure formed on a substrate
and composed of an n-type GaN layer and a p-type GaN layer having Mg added
thereto will be considered. When the n-type layer contains no H and
further the structure is heat-treated, Mg diffuses from the p-type layer
to the n-type layer. Such heat diffusion is remarkably found by Mg among
the impurities generally used. This heat diffusion causes the carrier
concentration in essentially n-type crystal to be decreased significantly,
thereby bringing about a phenomena in which the n-type layer does not
serve as n-type. However, when a proper amount of H is present within the
n-type layer, Mg diffusion is restricted, and diffusion from the p-type
layer to the n-type layer hardly occurs. Thus, it is considered that H has
effects to restrict impurity diffusion. From experimental results by the
inventors, it has been found that an H concentration capable of
restricting diffusion falls in a range of from 1.times.10.sup.18 cm.sup.-3
to 1.times.10.sup.20 cm.sup.-3.
Additional objects and advantages of the invention will be set forth in the
description which follows, and in part will be obvious from the
description, or may be learned by practice of the invention. The objects
and advantages of the invention may be realized and obtained by means of
the instrumentalities and combinations particularly pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part
of the specification, illustrate presently preferred embodiments of the
invention and, together with the general description given above and the
detailed description of the preferred embodiments given below, serve to
explain the principles of the invention.
FIG. 1 is a sectional view showing a light-emitting element according to
the first embodiment of the present invention;
FIG. 2 is a sectional view showing a light-emitting element according to
the second embodiment of the present invention;
FIG. 3 is a sectional view showing a light-emitting element according to
the third embodiment of the present invention;
FIG. 4 is a sectional view showing a light-emitting element according to
the fourth embodiment of the present invention;
FIG. 5 is a sectional view showing a light-emitting element according to
the fifth embodiment of the present invention;
FIG. 6 is a sectional view showing a semiconductor laser element according
to the sixth embodiment of the present invention;
FIG. 7 is a sectional view showing a light-emitting element according to
the seventh embodiment of the present invention; and
FIG. 8 is a timing chart showing an operation when the temperature is
decreased in a process of manufacturing the light-emitting element
according to the first embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention will be described with reference to
the drawings.
›First Embodiment!
FIG. 1 is a sectional view showing a light-emitting diode 10 according to
the first embodiment of the present invention.
The light emitting diode 10 has a sapphire substrate 11, on the main face
11a of which a buffer layer 12 of 50 nm thickness is formed in order to
relax lattice mismatching. Further, on the buffer layer 12 are laminated,
in sequence, an n-GaN layer 13 of 4 .mu.m thickness, an n-InGaN
light-emitting layer 14 of 50 .mu.m thickness, a p-AlGaN layer 15 of 150
nm thickness as a clad layer and a p-GaN layer 16 of 300 nm thickness. The
n-GaN layer 13 serves also as a clad layer.
Within each layer from the n-GaN layer 13 to the p-GaN layer 16,
2.times.10.sup.17 cm.sup.-3 of carbon is present. Also, as will be
described in the second embodiment, a small amount of Si is present in the
p-AlGaN layer 15 and the p-GaN layer 16 for making etching easier.
After crystal growth, the p-GaN layer 16 to the n-InGaN layer 14 are etched
until the n-GaN layer 13 is exposed, and then almost the entire face is
covered with a SiO.sub.2 film 17 of 400 nm thickness. At the required
portion on the SiO.sub.2 film 17 are formed some holes, and an Au-Ni film
18 to the p-layer 16 and an Al film 19 to the n-layer 13 are formed to
dispose ohmic electrodes.
Hereinafter, an example of manufacturing processes of the light emitting
diode 10 will be described in sequence.
The light emitting diode 10 was prepared by vapor phase growth through a
metalorganic chemical vapor deposition method (an MOCVD method). Ammonia
(NH.sub.3), silane (SiH.sub.4) and carbon tetrachloride (CCl.sub.4) as raw
material gases as well as H.sub.2 and N.sub.2 as carrier gases were used.
And, trimethylgallium ((CH.sub.3).sub.3 Ga) (hereinafter referred to as
TMG), trimethylaluminum ((CH.sub.3).sub.3 Al) (hereinafter referred to as
TMA), trimethylindium ((CH.sub.3).sub.3 In) (hereinafter referred to as
TMI), dimethylzinc ((CH.sub.3).sub.2 Zn) (hereinafter referred to as DMZ)
and biscyclopentadienyl magnesium ((C.sub.5 H.sub.5).sub.2 Mg)
(hereinafter referred to as CP.sub.2 Mg) were used as organometal raw
materials.
Initially, the single crystal sapphire substrate 11, the main face 11a of
which is of c-face cleaned by organic-solvent cleaning, acid cleaning and
heat treatment, was mounted on a heatable susceptor placed in the reaction
room of an MOCVD unit. Then, the main face 11a of the sapphire substrate
11 was vapor-phase etched for about 10 minutes under the normal pressure
at 1050.degree. C. during a H2 flow rate of 10 L/min.
Next, the temperature of the sapphire substrate 11 was lowered to
510.degree. C., and then the buffer layer 12 was formed by flowing H.sub.2
at a flow rate of 15 L/min, N.sub.2 at 5 L/min, NH.sub.3 at 10 L/min and
TMG at 25 cc/min, respectively, for 6 minutes.
The temperature of the sapphire substrate 11 was then increased to
1020.degree. C. and maintained at this temperature, and then the n-GaN
layer 13 was formed by flowing H.sub.2 at a flow rate of 15 L/min, N.sub.2
at 5 L/min, NH.sub.3 at 10 L/min, TMG at 25 cc/min, SiH.sub.4 of 100 ppm
at 1 cc/min and CCl.sub.4 at 5 cc/min, respectively, for 60 minutes.
The temperature of the sapphire substrate 11 was then lowered to
800.degree. C., and then the n-InGaN layer 14 was formed by flowing
H.sub.2 at a flow rate of 5 L/min, N.sub.2 at 15 L/min, NH.sub.3 at 10
L/min, TMG at 3 cc/min, TMI at 100 cc/min, SiH.sub.4 at 1 cc/min, DMZ at
10 cc/min and CCl.sub.4 at 5 cc/min, respectively, for 15 minutes.
The temperature of the sapphire substrate 11 was then increased until
1020.degree. C. and maintained at this temperature, the p-AlGaN layer 15
was formed by flowing H.sub.2 at a flow rate of 15 L/min, N.sub.2 at 5
L/min, NH.sub.3 at 10 L/min, TMG at 50 cc/min, TMA at 25 cc/min, CP.sub.2
Mg at 30 cc/min, CCl.sub.4 at 25 cc/min and SiH.sub.4 at 0.1 cc/min,
respectively, for 5 minutes.
Further, while the temperature of the sapphire substrate 11 was maintained
at 1020.degree. C., the p-GaN layer 16 was formed by flowing H.sub.2 at a
flow rate of 15 L/min, N.sub.2 at 5 L/min, NH.sub.2 at 10 L/min, TMG at 25
cc/min, CP.sub.2 Mg at 30 cc/min, CCl.sub.4 at 5 cc/min and SiH.sub.4 at
0.1 cc/min, respectively, for 5 minutes. By using CCl.sub.4 as a growth
gas in this way, each growth layer should contain C.
Thereafter, TMG, CP.sub.2 Mg and CCl.sub.4 flows were stopped, and during
flowing H.sub.2, N.sub.2 and NH.sub.3 at a flow rate of 15, 5 and 10
L/min, respectively, the sapphire substrate 11 was lowered in temperature
to 700.degree. C. Further, the flow of H.sub.2 and NH.sub.3 was stopped,
and, during flowing of N.sub.2 at a flow rate of 5 L/min, the sapphire
substrate 11 was left on a susceptor to lower its temperature to room
temperature (see FIG. 8).
In the method described above, the temperature at which H.sub.2 and
NH.sub.3 are stopped while only N.sub.2 is allowed to flow should be
300.degree. C. or more, and preferably 500.degree. C. or more. This is
because, under a hydrogen atmosphere at a high temperature, impurities
present near the surface of a growing crystal film may be made inactive by
active hydrogen. Under such a hydrogen atmosphere at a high temperature,
the ratio of activated impurities is as low as about 1%. In this case,
non-activated impurities form lattice defects to function as non-radiative
recombination centers, thereby greatly lowing the efficiency of the
element. In contrast, it has been found that, where switching of the gases
is carried out at a high temperature, at least 7% of, and generally 10% or
more of the added impurities are activated.
The substrate is desirably cooled at a rate of 50.degree. C./min or less.
Where the substrate is cooled faster than this rate, cracks may occur in
the surface of crystal due to thermal stress of the crystal, especially
when using a mixed crystal having added Al, such as AlGaN.
As a gas used during the lowering of the temperature, N.sub.2, which is one
of the components of the matrix crystal, is preferably used, but another
inactive gas, such as He or Ar can be used.
By performing the above-described steps, it becomes unnecessary to carry
out, e.g., a thermal annealing step conventionally adopted for improving
the activated ratio of impurities, so that the process is simplified and
the period of time needed for the process is shortened. Further, it is
possible to attain an activated ratio higher than that obtained by a post
step, such as a thermal annealing step.
The sapphire substrate 11, on which the nitride compound semiconductor
layers grew, was then removed from the MOCVD unit, and then, was etched by
an alkaline aqueous solution until the n-GaN layer was exposed, while a
resist or the like was used as a mask, in order to make a mesa structure.
Then, on the surface was formed a SiO.sub.2 film 17 of about 400 nm
thickness in a CVD unit. With this film, leak current at the portion
adjacent to the pn-junction interface exposed at the mesa structure side
was lowered, and thereby degradation of the element was lowered.
In the SiO.sub.2 film 17 two holes of about 100 .mu.m square and 100 .mu.m
diameter, respectively, were then formed so that the p-GaN layer 16 and
n-GaN layer 13 were exposed respectively, by using a hydrofluoric acid
solution. Through these holes, an Au-Ni film 18 of about 1 .mu.m thickness
to the p-GaN layer 16 and an Al film 19 of about 600 nm thickness to the
n-GaN layer 13 were formed respectively, in order to make ohmic
electrodes. By means of the above process the light-emitting diode was
prepared.
Impurity concentrations within each layer of the light emitting diode 10
are as follows. Only Mg concentration within the p-GaN layer 16 is
1.times.10.sup.20 cm.sup.-3, and concentrations of Si, Zn and Mg within
the n-GaN layer 13, n-InGaN layer 14 and p-AlGaN layer 15 were
2.times.10.sup.19 cm.sup.-3, respectively. Also, a C concentration within
the layers from the n-GaN layer 13 to the p-GaN layer 16 was
2.times.10.sup.17 cm.sup.-3, respectively.
The light-emitting diode 10 formed in this manner was die-cut into a size
of about 350 .mu.m square, was mounted on a stem and then was molded to
form a lamp. By this diode, a light output of about 2-3 mW was obtained
with respect to a forward current of 20 mA, and its life of about 20,000
hours was established. These values are improved by about 2 to 3 times in
comparison with characteristics of a light-emitting diode which contains
substantially no C or a C concentration of identification limit or less
but has a similar structure.
At a carbon concentration which falls in a range of from 1.times.10.sup.16
cm.sup.-3 to 5.times.10.sup.17 cm.sup.-3, similar effects as mentioned
above were obtained. At values lower than this range, the take-out
efficiency was decreased because of formation of a deep level of nitrogen
vacancies. That is to say, effects by C addition were not found. On the
other hand, at values higher than this range, carbon was precipitated
within the crystal, which became a non-radiative recombination center or
crystal defect, and effects were hardly noticed. In order to obtain
significant effects, a C concentration is preferred to fall in the range
of from 5.times.10.sup.16 cm.sup.-3 to 5.times.10.sup.17 cm.sup.-3, and
further at a C concentration of from 1.times.10.sup.17 cm.sup.-3 to
3.times.10.sup.17 cm.sup.-3, the take-out efficiency gave a maximum value.
Hitherto, a InGaN light-emitting layer has been described. Similar effects
also were obtained with a GaN or InGaAlN light-emitting layer.
›Second Embodiment!
FIG. 2 is a sectional view showing a light-emitting diode 20 according to
the second embodiment of the present invention.
The light-emitting diode 20 has a sapphire substrate 21, whereon a buffer
layer 22 of 50 nm thickness is formed, in order to relax lattice
mismatching. And on the buffer layer 22 are laminated, in sequence, a
n-GaN layer 23 of 3 .mu.m thickness, a n-InGaN light-emitting layer 24 of
100 nm thickness, a p-AlGaN layer 25 of 300 nm thickness as a clad layer
and a p-GaN layer 26 of 500 nm thickness.
Impurity concentrations within each layer are as follows. Within the p-GaN
layer 26, the Mg concentration is 1.times.10.sup.20 cm.sup.-3 and the Si
concentration is 1.times.10.sup.17 cm.sup.-3. Within the p-AlGaN layer 25,
the Mg concentration is 2.times.10.sup.19 cm.sup.-3 and the Si
concentration is 1.times.10.sup.17 cm.sup.-3. Within the n-InGaN layer 24,
the Si concentration is 2.times.10.sup.19 cm.sup.-3 and the Zn
concentration is 1.times.10.sup.18 cm.sup.-3. Within the n-GaN layer 23,
the Si concentration is 2.times.10.sup.19 cm.sup.-3.
Hereinafter, an example of manufacturing processes of the light emitting
diode 20 will be described in sequence.
Initially, the single crystal sapphire substrate 21, the main face of which
is of a-face, which is of the (11-20) plane, cleaned by organic-solvent
cleaning, acid cleaning and heat treatment, was mounted on a heatable
susceptor placed in the reaction section an MOCVD unit. Then, the main
face of the sapphire substrate 21 was vapor-phase etched for about 10
minutes under normal pressure at 1050.degree. C. during a H.sub.2 flow
rate of 10 L/min.
Next, the temperature of the sapphire substrate 21 was lowered to
510.degree. C., and then the buffer layer 22 was formed by flowing H.sub.2
at a flow rate of 15 L/min, N.sub.2 at 10 L/min, NH.sub.3 at 5 L/min and
TMG at 25 cc/min, respectively.
The temperature of the sapphire substrate 21 was then increased to
1020.degree. C. and maintained at this temperature, and then the n-GaN
layer 23 was formed by flowing H.sub.2 at a flow rate of 15 L/min, N.sub.2
at 10 L/min, NH.sub.3 at 5 L/min, TMG at 25 cc/min and SiH.sub.4 at 5
cc/min, respectively, for 60 minutes.
The temperature of sapphire substrate 21 was then decreased to 800.degree.
C., and then an n-InGaN layer 24 was formed by flowing H.sub.2 at a flow
rate of 10 L/min, N.sub.2 at 15 L/min, NH.sub.3 at 5 L/min,
triethylgallium ((C.sub.2 H.sub.5).sub.3 Ga) (hereinafter referred to TEG)
at 3 cc/min, TMI at 30 cc/min, DMZ at 1 cc/min and SiH.sub.4 at 1 cc/min,
respectively, for 30 minutes. Such Zn addition during layer formation
serves not only to give radiative centers, but to aid growth of a flat
thin film.
Next, after the temperature of the sapphire substrate 21 was increased to
1020.degree. C. and maintained at this temperature, the p-AlGaN layer 25
was formed by flowing H.sub.2 at a flow rate of 15 L/min, N.sub.2 at 10
L/min, NH.sub.3 at 5 L/min, TMG at 50 cc/min, TMA at 25 cc/min, CP.sub.2
Mg at 50 cc/min and SiH.sub.4 at 1 cc/min, respectively, for 10 minutes.
Further, while the temperature of the sapphire substrate 21 was maintained
at 1020.degree. C., the p-GaN layer 26 was formed by flowing H.sub.2 at a
flow rate of 15 L/min, N.sub.2 at 10 L/min, NH.sub.3 at 5 L/min, TMG at 25
cc/min, CP.sub.2 Mg at 100 cc/min and SiH.sub.4 at 1 cc/min, respectively,
for 10 minutes.
Hereinafter, the TMG, CP.sub.2 Mg and SiH.sub.4 flow was stopped, and
during flowing H.sub.2, N.sub.2 and NH.sub.3 at a flow rate of 15, 10 and
5 L/min, respectively, the temperature of the sapphire substrate 21 was
lowered until 800.degree. C. Further, during flowing N.sub.2 at a flow
rate of 10 L/min, the sapphire substrate 21 was left on the susceptor to
lower it to room temperature.
The sapphire substrate 21, where a nitride compound semiconductor layer
grew, was then removed from the MOCVD unit, and then, was etched by an
alkaline aqueous solution until the n-GaN layer was exposed, while a
resist or the like was used as a mask, in order to make a mesa structure.
Then, on the surface a SiO.sub.2 film 27 was formed of about 200 nm
thickness in a CVD unit. With the film, the leak current was lowered at
the portion adjacent to the pn-junction interface exposed at the mesa
structure side, and degradation of the element was also lowered.
Further, on the SiO.sub.2 film 27 two holes of about 100 .mu.m square and
120 .mu.m diameter, respectively, were formed by using a hydrofluoric acid
solution so that the p-GaN layer 26 and n-GaN layer 23 are exposed
respectively. Through these holes, an Au-Ni film 28 of about 2 .mu.m
thickness to the p-GaN layer 26 and an Al-Ti film 29 of about .mu.m
thickness to the n-GaN layer 23 were formed, in order to make ohmic
electrodes.
The light emitting diode 20 formed in this manner was die-cut into a size
of about 350 .mu.m square, was mounted on a stem and then was molded to
form a lamp. The light-emitting diode 20 showed similar performances as
that of the first embodiment, with respect to its light-emitting strength
as well as reliability.
A Si concentration in each of the p-GaN layer 26 and p-AlGaN layer 25,
which is a characteristic point of the diode 20, was 2.times.10.sup.17
cm.sup.-3 as mentioned above. At this concentration, the p-GaN layer 26
and p-AlGaN layer 25 could be removed by etching, and elements were
formed. An Si concentration capable of etching was required of
1.times.10.sup.16 cm.sup.-3 or more. On the other hand, since Si is
essentially a donor impurity, its excess addition creates a problem that
the GaN layer 26 and AlGaN layer 25, which are to be originally the
p-type, results in a shift to the n-type. Therefore, it has been found
that, although the upper limit of Si concentration is dependent upon an
acceptor concentration of the layer, the shift to the n-type rarely
occurred at a Si concentration of 8.times.10.sup.17 cm.sup.-3 or less,
since a carrier concentration of the p-type, required to form elements, is
1.times.10.sup.18 cm.sup.-3. Further, the Si concentration was preferred
to be 5.times.10.sup.17 cm.sup.-3 or less, and, most preferably, to be
from 5.times.10.sup.16 cm.sup.-3 to 5.times.10.sup.17 cm.sup.-3 in terms
of element formation.
›Third Embodiment!
FIG. 3 is a sectional view showing a light emitting diode 30 according to
the third embodiment of the present invention. The light emitting diode 30
has a AlN substrate 31, whereon are laminated, in sequence, a n-AlGaN
layer 33 of 4 .mu.m thickness, a n-GaN light-emitting layer 34 of 100 nm
thickness, a p-AlGaN layer 35 of 300 nm thickness as a clad layer and a
p-GaN layer 36 of 500 nm thickness. And, within each layer from the
n-AlGaN layer 33 to the p-GaN layer 36, 1.times.10.sup.18 cm.sup.-3 of
oxygen is present.
Hereinafter, an example of manufacturing processes of the light emitting
diode 30 will be described in sequence.
Initially, the AlN substrate 31 which was cleaned by organic-solvent
cleaning, acid cleaning and heat treatment was mounted on a heatable
susceptor placed in the reaction section of an MOCVD unit. Then, the main
face of the AlN substrate 21 was vapor-phase etched for about 10 minutes
under normal pressure at 1050.degree. C. during a H.sub.2 flow rate of 10
L/min.
Next, the temperature of the AlN substrate 31 was lowered to 1000.degree.
C. and maintained at this temperature, and then the n-AlGaN layer 33 was
formed by flowing H.sub.2 at a flow rate of 15 L/min, N.sub.2 at 10 L/min,
NH.sub.3 at 5 L/min, TMA at 25 cc/min, TMG at 50 cc/min, SiH.sub.4 at 1
cc/min and oxygen (O.sub.2) diluted to 0.1% at 20 cc/min, respectively,
for 60 minutes.
While the AlN substrate 31 was kept at a temperature of 1000.degree. C.,
the n-GaN layer 34 was formed by flowing H.sub.2 at a flow rate of 10
L/min, N.sub.2 at 15 L/min, NH.sub.3 at 5 L/min, TEG at 3 cc/min, DMZ at
10 cc/min, SiH.sub.4 at 1 cc/min and diluted oxygen at 300 cc/min,
respectively, for 4 minutes.
After the AlN substrate 31 was maintained at a temperature of 1000.degree.
C., the p-AlGaN layer 35 was then formed by flowing H.sub.2 at a flow rate
of 15 L/min, N.sub.2 at 10 L/min, NH.sub.3 at 5 L/min, TMG at 50 cc/min,
TMA at 25 cc/min, CP.sub.2 Mg at 100 cc/min and diluted O.sub.2 at 200
cc/min, respectively, for 5 minutes. Further, while the AlN substrate 31
was maintained at a temperature of 1000.degree. C., a p-GaN layer 36 was
formed by flowing H.sub.2 at a flow rate of 15 L/min, N.sub.2 at 10 L/min,
NH.sub.3 at 5 L/min, TMG at 50 cc/min, CP.sub.2 Mg at 30 cc/min and
diluted O.sub.2 at 200 cc/min, respectively, for about 20 minutes.
Then, the temperature of the AlN substrate 31 was lowered to 900.degree. C.
and was left on a susceptor during flowing N.sub.2 and NH.sub.3 each at a
flow rate of 10 L/min.
The AlN substrate 31, on which the nitride compound semiconductor layers
grew, was removed from the MOCVD unit, and then was etched by an alkaline
aqueous solution until the n-AlGaN layer 33 was exposed, while a resist or
the like was used as a mask, in order to make a mesa structure. Then, on
the surface a SiO.sub.2 film 37 was formed of about 300 nm thickness in a
CVD unit. With the film, the leak current at the portion adjacent to the
pn-junction interface exposed at the mesa structure side was lowered, and
degradation of the element also was lowered.
In the SiO.sub.2 film 37 two holes of about 100 .mu.m square and 100 .mu.m
diameter, respectively, were then formed by using a hydrofluoric acid
solution so that the p-GaN layer 36 and n-AlGaN layer 33 were exposed
respectively. Through these holes, an Au-Ni film 38 of about 2 .mu.m
thickness to the p-GaN layer 36 and an Al film 39 of about 1 .mu.m
thickness to the n-AlGaN layer 33 were formed, in order to make ohmic
electrodes.
The light-emitting diode 30 formed in this manner was die-cut into a size
of about 400 .mu.m square, was mounted on a stem and then was molded to
form a lamp. The light emitting diode 30 showed similar performances as
that of the first embodiment with respect to its light-emitting strength
as well as reliability.
Impurity concentrations within each layer of the light emitting diode 30
are as follows. Only Mg concentration within the p-GaN layer 36 was
1.times.10.sup.20 cm.sup.-3, and concentrations of Si, Zn and Mg within
the n-AlGaN layer 33 to the p-AlGaN layer 35 were 2.times.10.sup.19
cm.sup.-3 respectively. Also, with reference to the oxygen concentration,
which is a characteristic point of this embodiment, the effect of filling
nitrogen vacancies was obtained, in a range of from 1.times.10.sup.18
cm.sup.-3 to 1.times.10.sup.20 cm.sup.-3, including 1.times.10.sup.18
cm.sup.-3 mentioned above. At values lower than this range, the effect on
light-emitting strength was not found. At values higher than this range, a
problem of safety of the unit may be present, because of the possibility
of explosion. Above all, characteristics as a light-emitting element was
highly improved at an oxygen concentration of from 1.times.10.sup.18
cm.sup.-3 to 1.times.10.sup.19 cm.sup.-3.
›Fourth Embodiment!
FIG. 4 is a sectional view showing a light emitting diode 40 according to
the fourth embodiment of the present invention. The light emitting diode
40 has a sapphire substrate 41, whereon a buffer layer 42 of 50 nm
thickness is formed in order to relax lattice mismatching. And, on the
buffer layer 42 are laminated, in sequence, a n-GaN layer 43 of 4 .mu.m
thickness, a n-InGaN light-emitting layer 44 of 50 nm thickness, a p-AlGaN
layer 45 of 150 nm thickness as a clad layer and a p-GaN layer 46 of 300
nm thickness. Within each of the n-GaN layer 43 and n-InGaN lightemitting
layer 44, 5.times.10.sup.18 cm.sup.-3 of hydrogen is present.
Further, after crystal growth, layers from the n-InGaN layer 44 to the
p-GaN layer 46 are etched until the n-GaN layer 43 is exposed, and then
almost the entire surface is covered with an SiO.sub.2 film 47 of 400 nm
thickness. Holes are formed at the required portions of the SiO.sub.2 film
47, and an Au-Ni film 48 to the p-layer 46 and an Al film 49 to the
n-layer 43 are formed respectively, in order to dispose ohmic electrodes.
Hereinafter, an example of manufacturing | | |
