JP2005251922A - Semiconductor light emitting device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a semiconductor light emitting device using a gallium nitride series semiconductor, wherein the semiconductor light emitting device such as a light emitting diode or the like having a high luminous efficiency even by using a silicon substrate is obtained. <P>SOLUTION: The semiconductor light emitting device has such a structure that the surface of an active layer 6 composed of the gallium nitride series semiconductor has the uneven structure of a pyramid shape, and the recess of this uneven structure is filled with a first clad layer 7 composed of a p-type gallium nitride series semiconductor. Preferably, the dislocation defect density of a second clad layer 5 composed of an n-type gallium nitride series semiconductor forming a part of an underlaying layer of the active layer 6 is 10<SP>9</SP>to 10<SP>11</SP>/cm<SP>2</SP>, and preferably, the gap of a dislocation defect in the second clad layer 5 is 120 to 170 nm. Further, preferably, the entire thickness of the underlaying layer is 3 μm or less. The active layer 6 is formed by an organic metal vapor deposition, and its film forming pressure conditions are set to atmospheric pressure or rather lower pressure than the atmospheric pressure. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a semiconductor light emitting device such as a light emitting diode using a gallium nitride based semiconductor such as gallium indium nitride (InGaN).

Gallium nitride-based semiconductors typified by gallium indium nitride are used in semiconductor light-emitting elements such as light-emitting diodes having a wide light-emitting region ranging from the ultraviolet region to the visible region.
As a semiconductor light emitting element using such a gallium nitride based semiconductor, it has recently been studied to use a silicon substrate as the substrate. Compared to a sapphire substrate that has been widely used as a conventional substrate, this is because it is conductive, so that the electrode can be easily formed, the substrate itself is inexpensive, and the substrate is relatively soft and easily cut with a dicing saw. For reasons such as being able to.

However, even in the semiconductor light emitting device using such a silicon substrate, the following problems to be solved remain.
Even in a light emitting device using a silicon substrate, a buffer layer made of AlN, GaN or the like is first formed on the silicon substrate, and an n-type GaN layer serving as a cladding layer is stacked on the buffer layer. Residual stress caused by the difference in thermal expansion coefficient between GaN and silicon acts as a tensile force on the GaN layer near room temperature. As a result, when the GaN layer is grown to a thickness of 1 μm or more, this GaN layer breaks. Arise.

On the other hand, in order to reduce dislocation defects caused by the difference in lattice constant between GaN and silicon and form a GaN layer with good crystallinity, it is necessary to take measures such as depositing mask material to terminate dislocation defects. Inevitably, a thick GaN layer is required.
However, in a GaN layer having a thickness of 1 μm or more, cracks occur as described above, so that a thick GaN layer cannot be grown, and the dislocation defect density cannot be sufficiently reduced.
For this reason, there are many dislocation defects in the GaN layer serving as the cladding layer, and dislocation defects also occur in the active layer having a quantum well structure made of InGaN or the like on the GaN layer. Stays.
JP 2002-368269 A JP 2000-277441 A

  Accordingly, an object of the present invention is to obtain a semiconductor light emitting device such as a light emitting diode having high light emission efficiency even when a silicon substrate is used in a semiconductor light emitting device using a gallium nitride based semiconductor.

To solve this problem,
In the invention according to claim 1, the surface of the active layer made of a gallium nitride semiconductor has a pyramidal uneven structure, and the recess of the uneven structure is filled with a first cladding layer made of a p-type gallium nitride semiconductor. The semiconductor light emitting device having the above structure.
According to a second aspect of the present invention, the dislocation defect density of the second cladding layer made of an n-type gallium nitride semiconductor forming a part of the underlayer of the active layer is 10 ⁹ to 10 ¹¹ / cm ² . It is a semiconductor light emitting device.

The invention according to claim 3 is the semiconductor light emitting element according to claim 2, wherein the distance between dislocation defects in the second cladding layer is 120 to 170 nm.
The invention according to claim 4 is the semiconductor light-emitting element according to claim 2 or 3, wherein the total thickness of the underlayer is 3 μm or less.
The invention according to claim 5 is the method according to any one of claims 1 to 4, wherein the active layer is formed by metal organic vapor phase epitaxy, and the film forming pressure condition is atmospheric pressure or a pressure slightly lower than atmospheric pressure. It is a semiconductor light emitting device.

According to the present invention, most of the current applied to the light emitting element flows to the apex of the convex portion of the pyramid structure of the active layer, and flows so as to avoid dislocation defects existing in the active layer. For this reason, almost no current flows through the dislocation defect serving as a non-luminescent center, and the luminous efficiency is increased.
Moreover, since the multiple quantum well structure of the active layer is formed on a slope inclined from the c-axis of the active layer, the polarization of the active layer due to stress is smaller than that on the c-plane. For this reason, the spatial distance between electrons and holes due to the quantum Stark effect is reduced, and the recombination efficiency is increased. This also improves the light emission efficiency.
Therefore, a semiconductor light emitting device with high light emission efficiency can be obtained even though the active layer is formed on the second cladding layer in which many dislocation defects exist.

  Furthermore, since the dislocation defect density allowed in the second cladding layer is large, it is not necessary to use a thick film to reduce the dislocation defect density as the second cladding layer, and a thin second cladding layer may be used. The cost for forming the two cladding layers can be reduced. Moreover, since the dislocation defect density allowed in the second cladding layer is large, a silicon substrate can be used, and a sapphire substrate can also be used.

The present invention will be described in detail below.
FIG. 1 shows a light emitting diode (LED) as an example of the semiconductor light emitting device of the present invention, and reference numeral 1 in the drawing denotes a silicon substrate.
The silicon substrate 1 is made of an n-type silicon crystal and has a (111) plane as a surface.
The silicon substrate 1 is preliminarily heat-treated in ammonia gas, and the contact resistance at the interface with the shielding layer 2 made of AlN formed on the substrate 1 is obtained by the heat treatment in the ammonia atmosphere. The effect of decreasing is obtained.

On this silicon substrate 1, a shielding layer 2 made of AlN and having a thickness of 2 to 3 nm is provided. The shielding layer 2 is for preventing the gallium of the conductive layer 3 made of AlGaN formed on the shielding layer 2 from reacting with the silicon of the silicon substrate 1.
On the shielding layer 2, a conductive layer 3 made of n-type AlGaN and having a thickness of about 20 nm is provided. Furthermore, a multilayer 4 is formed on the conductive layer 3 by alternately stacking 20 to 30 GaN layers having a thickness of about 20 nm and AlN layers having a thickness of about 50 nm. The multi-layer 4 is for alleviating thermal stress generated between the silicon substrate 1 and the second clad layer 5 made of n-type GaN stacked on the multi-layer 4.

On the multiple layer 4, a second cladding layer 5 made of n-type GaN having a thickness of about 200 nm is provided. The second cladding layer 5 forms a double hetero structure with the active layer 6 sandwiched together with the first cladding layer 7 described later. The thickness of the second cladding layer 5 does not need to be increased, and 200 nm is sufficient.
An active layer (light emitting layer) 6 made of InGaN having a thickness of 100 to 200 nm is formed on the second cladding layer 5. The active layer 6 has a multiple quantum well structure (MQW) in which a well layer having a thickness of about 3 nm and a barrier layer having a thickness of about 5 nm are alternately stacked for 10 to 20 periods.

Further, a first cladding layer 7 made of p-type AlGaN having a thickness of about 20 nm is stacked on the active layer 6. The first cladding layer 7 forms a double heterojunction structure in which the active layer 6 is sandwiched with the second cladding layer 5.
An electrode layer 8 made of p-type GaN having a thickness of about 200 nm is provided on the first cladding layer 7, and a transparent electrode 9 made of Ni film / Au film is formed on the electrode layer 8. A p-type ohmic electrode 10 made of a Ni film / Au film is formed on the transparent electrode 9.

Further, an n-type ohmic electrode 11 made of AuSb film / Au film is formed on the back surface of the silicon substrate 1, and the light emitting diode of this example is configured.
In this light emitting diode, light is emitted from the active layer 6 by applying a direct current between the p-type ohmic electrode 10 and the n-type ohmic electrode 11 and functions as a light emitting element.
Each layer on the silicon substrate 1 of the light emitting diode is formed by a well-known metal organic chemical vapor deposition method (MOCVD method).

FIG. 2 schematically shows an enlarged main part in the laminated structure shown in FIG.
In FIG. 2, in the second clad layer 5 made of n-type GaN, a number of dislocation defects A extending in the thickness direction of the layer are spaced at intervals of 100 to 150 nm and have a density of 10 ⁹ to 10 ¹¹ / cm ² . Exists. These dislocation defects A are propagated and formed in each layer so as to extend from the shielding layer 2 due to a lattice constant misfit between the silicon of the silicon substrate 1 and the semiconductor forming each layer on the substrate 1.

  In addition, the active layer 6 has a pyramid structure in which a large number of pyramids having a quadrangular pyramid are formed adjacent to the first cladding layer 7. This pyramid structure gradually decreases as the c-planes of the multi-quantum well layer 61 and the barrier layer 62 forming the active layer 6 go upward, and an inclined surface inclined with respect to the C-axis is formed. In addition, a multiple quantum structure including a well layer 61 and a barrier layer 62 is formed. In this pyramid structure, as shown in the figure, the dislocation interval of the second cladding layer 5 is substantially equal to the base of the base, and one pyramid is formed between adjacent dislocation defects.

  Further, the dislocation defects in the second cladding layer 5 are partially terminated near the bottom of the pyramid structure, and the dislocation defects reaching the first cladding layer 7 through the active layer 6 are considerably reduced. doing.

  The present invention is characterized by such a pyramid structure of the active layer 6 and exhibits the above-described excellent effects. That is, the p-type AlGaN forming the first cladding layer 7 has a larger band gap and higher resistivity than the electrode layer 8 made of p-type GaN, so that most of the current injected into the diode is the active layer 6. The current that flows toward the apex of the pyramid structure and flows into dislocation defects is greatly reduced.

In addition, since most of the multiple quantum well structure of the active layer 6 is formed on an inclined surface inclined from the c-axis, the polarization of the active layer 6 due to stress is compared to the multiple quantum well structure on the c-plane. small. For this reason, the spatial distance between electrons and holes due to the quantum Stark effect is reduced, and the recombination probability is increased. As a result, the adoption of such a pyramid structure increases the recombination probability in the multiquantum well structure formed on the inclined surface at the same time as the majority of the current flows avoiding dislocation defects that act as non-luminescent centers. Can do.
Therefore, even if the active layer 6 is formed on the second cladding layer 5 where many dislocation defects remain, high light emission efficiency can be realized.

Next, requirements necessary for the expression of the pyramid structure in the active layer 6 will be described.
First, it is necessary that the active layer 6 has an appropriate strain, in other words, an appropriate stress is applied. In general, when a large stress is applied during crystal growth, the crystal grows in the vertical direction rather than in the horizontal direction because the strain energy becomes smaller and becomes dominant. In such a case, if there are dislocation defects with a constant density in the second cladding layer 5, the lateral continuity is divided and a pyramid structure as shown in FIG. 2 is formed.

  In order to apply an appropriate strain to the active layer 6, it is necessary that an appropriate strain exists in the second cladding layer 5. To make an appropriate strain exist in the second cladding layer 5, the second cladding layer 5 Therefore, it is necessary to appropriately adjust the stress at room temperature due to the difference in thermal expansion coefficient between the silicon substrate 1 and the silicon substrate 1, and the multilayer 4 is provided for this purpose.

Secondly, the dislocation defect density in the second cladding layer 5 needs to be in an appropriate range. In the previous example, it is about 10 ¹⁰ / cm ² according to transmission electron microscope observation. A preferable range of dislocation defect density is 10 ⁹ to 10 ¹¹ / cm ² , but this range is not necessarily required.
In order to form such a dislocation defect density, the configuration of each layer of the shielding layer 2, the conductive layer 3, and the multi-layer 4, that is, the composition ratio, thickness, film forming conditions, etc. of the elements constituting each layer is controlled. It becomes possible.

If the dislocation defect density in the second cladding layer 5 is small and the dislocation defect interval is wide, it is necessary to increase the thickness of the active layer 6 in order to obtain a pyramid structure. In this case, the active layer 6 If it is thin, a flat portion appears at the top of the pyramid structure, and current flow is not concentrated on the top.
Further, when the dislocation defect density in the second cladding layer 5 is large and the dislocation defect interval is narrow, the pyramid structure having a uniform shape can be obtained by reducing the thickness of the active layer 6. However, even in this case, it is necessary to appropriately adjust the amounts of In in the well layer 61 and the barrier layer 62 in order to generate sufficient strain in the active layer 6. Since the In amount and thickness of the well layer 61 change the emission wavelength, the strain amount is adjusted by adjusting the lattice constant difference between the entire active layer 6 and the second cladding layer 5 by the In amount and thickness of the barrier layer 62. An appropriate value is desirable.

  Next, the amount of strain due to the difference in lattice constant between the second cladding layer 5 and the well layer 61 of the multiple quantum well structure forming the active layer 6 is related. This amount of strain is a function of the thickness of the well layer 61, the thickness of the entire active layer 6, and the growth conditions. In the previous example, a 15-cycle multiple quantum well structure comprising a 3 nm well layer 61 and a 5 nm barrier layer 62 is formed, and the film formation temperature is 780 ° C. However, since the thickness of the well layer 61 and the barrier layer 62 affects the emission wavelength, it is necessary to determine the thickness in consideration of these.

  Moreover, the pressure at the time of forming the active layer 6 is also involved in the expression of the pyramid structure. Normally, each layer on the silicon substrate 1 is formed by metal organic vapor phase epitaxy, and the film formation pressure is reduced under a reduced pressure of 760 to 76 Torr. In this example, only the active layer 6 is formed at atmospheric pressure or a pressure near atmospheric pressure, so that a pyramid structure with a flat inclined surface can be formed. However, since the influence of the pressure at the time of film formation depends on the metal organic chemical vapor deposition apparatus (MOCVD apparatus) to be used, it cannot be uniquely determined only by the pressure condition at the time of film formation of the active layer 6.

  Furthermore, the thickness of the underlayer composed of the second cladding layer 5, the multilayer 4, the conductive layer 3, and the shielding layer 2 is also involved. If the thickness of the underlayer exceeds 3 μm, the stress applied to the active layer 6 is relieved, and therefore it is not preferable. Usually, the thickness is preferably about 1 μm.

  As described above, the requirements for causing the active layer 6 to exhibit the pyramid structure are diverse, and can be realized by optimizing these requirements.

In the present invention, examples of the GaN-based semiconductor forming the active layer 6 include AlInGaN in addition to the above-mentioned InGaN.
Specifically, Al _0.01 In to _0.05 Ga to _0.94 N having a thickness of about 2 nm is used as a well layer having a multiple quantum well structure forming the active layer 6, and a thickness of about 15 nm is used as a barrier layer. such as _Al 0.2 _{in ~0.01 Ga ~0.79} N is those laminated to about 5 layers alternately respectively used.

  In this case, it is necessary to provide an intermediate layer such as n-type AlGaN having a thickness of about 50 nm between the active layer 6 and the second cladding layer 7 made of n-type GaN. In this case, the second cladding layer 7 made of n-type GaN may be omitted, and the intermediate layer may be used as the second cladding layer.

When a material containing Al is used as the GaN-based semiconductor forming the active layer 6, the active layer 6 tends to grow flat and it is difficult to form a pyramid structure. The pyramid structure can be realized by selecting.
On the other hand, if the GaN-based semiconductor forming the active layer 6 does not contain In, it is difficult to form a pyramid structure. From this point, an InGaN-based semiconductor is preferable as the GaN-based semiconductor forming the active layer 6.

Hereinafter, although a specific example is shown, this invention is not limited to this specific example.
(Example 1)
After carrying the n-type silicon (111) substrate into the MOCVD apparatus, heat treatment is performed at 1130 ° C. for 10 minutes in a hydrogen atmosphere to remove the oxide film on the silicon substrate surface, and heat treatment is performed at 1130 ° C. for 60 seconds in an ammonia gas atmosphere. went. Next, an AlN layer (thickness 3 nm) serving as a shielding layer at 1180 ° C., an n-type Al _0.27 Ga _0.73 N layer (thickness 20 nm) serving as a conductive layer, and GaN (20 nm) / AlN (including 20 layers) 5 nm), an n-type GaN layer (thickness 200 nm) to be the second cladding layer, an InGaN active layer, a p-type Al _0.15 Ga _0.85 N layer to be the first cladding layer, and an electrode layer A p-type GaN layer (200 nm) was sequentially formed.

The active layer made of InGaN has a 15-cycle multiple quantum well structure composed of a 3 nm In _0.18 Ga _0.82 N well layer and a 5 nm In _0.01 Ga _0.99 N barrier layer. The temperature was 780 ° C., and the growth pressure was atmospheric pressure. Hydrogen and nitrogen were used as the carrier gas. The dislocation defect density of the second cladding layer made of the n-type GaN layer was 10 ¹⁰ / cm ² by observation with a transmission electron microscope. Further, cyclopentadienyl magnesium was used for the p-type dopant, and silane was used for the n-type dopant.

  Further, a Ni film / Au film serving as a transparent electrode and a Ni film / Au film (12/100 nm) serving as a p-type ohmic electrode are vapor-deposited in vacuum on the p-type GaN layer of the electrode layer, and at 610 ° C. for 3 minutes. Then, heat treatment was performed in nitrogen to form a p-type ohmic electrode. Then, an AuSb film / Au film (18/100 nm) is vacuum-deposited on the back surface of the n-type silicon substrate, and heat-treated in a nitrogen atmosphere at 380 ° C. for 1 minute to form an n-type ohmic electrode. It was created.

  The current-voltage characteristics of the obtained light emitting diode are shown in FIG. 3, and the light output characteristics are shown in FIG. When the injection current was 20 mA, the operating voltage was 4.1 V, the optical output was 18 μW, and the peak emission wavelength was 478 nm. The light output is a value measured by a photodetector placed at a position 10 mm away from the p-type ohmic electrode. The light output of this light emitting diode is equivalent to that of a GaN-based light emitting diode having a similar structure using a conventional sapphire substrate.

When this light emitting diode was subjected to a life test at 27 ° C. and 80 ° C. at a constant injection current (20 mA), the decrease rate of the light output after the elapse of 1000 hours at 27 ° C. and 80 ° C. was −2% respectively. And -6%.
Further, a transmission electron micrograph in the vicinity of the active layer of the light emitting diode is shown in FIG. From this photograph, the pyramid structure in the active layer can be clearly confirmed.

(Example 2)
A light emitting diode was obtained in the same manner as in Example 1 except that the growth temperature of the active layer was 740 ° C. and the growth pressure was atmospheric pressure.
The emission wavelength of the obtained light-emitting diode was a green light-emitting diode having a wavelength of 505 nm, and the current-voltage characteristics, light output characteristics, and reduction rate of light output in the life test were the same as those of the light-emitting diode of Example 1.

(Example 3)
In Example 1, the thickness of the n-type GaN layer of the second cladding layer was 150 nm, and an intermediate layer made of n-type Al _0.07 Ga _0.93 N having a thickness of 50 nm was provided on the second cladding layer. . Also, the active layer is Al _0.01 In to _0.05 Ga to _0.94 N with a well layer of 2 nm in thickness, and the barrier layer is Al _0.2 In to _0.01 Ga to _{0 with} a thickness of 15 nm. _.79 N in which about 5 layers were alternately laminated was used. The deposition temperature of the active layer was 820 ° C., and the pressure was atmospheric pressure.
The obtained light emitting diode emitted light at a wavelength of 375 nm, and a pyramid structure was confirmed in the active layer by observation with a transmission electron microscope.

  The semiconductor light emitting device of the present invention is useful as, for example, a blue light emitting diode.

It is a schematic sectional drawing which shows an example of the structure of the semiconductor light-emitting device of this invention. It is the schematic sectional drawing which expanded and showed typically the principal part in FIG. It is a graph which shows the current-voltage characteristic of the light emitting diode obtained by the specific example. It is a graph which shows the light output characteristic of the light emitting diode obtained by the specific example. It is a transmission electron micrograph of the principal part of the light emitting diode obtained by the specific example.

Explanation of symbols

5 ... 2nd cladding layer, 6 ... Active layer, 7 ... 1st cladding layer

Claims (5)

  A semiconductor light emitting device having a structure in which a surface of an active layer made of a gallium nitride semiconductor has a pyramidal uneven structure, and a recess of the uneven structure is filled with a first cladding layer made of a p-type gallium nitride semiconductor. ^2. The semiconductor light emitting element according to claim 1, wherein the dislocation defect density of the second cladding layer made of an n-type gallium nitride semiconductor forming a part of the base layer of the active layer is 10 ⁹ to 10 ¹¹ / cm ² .   The semiconductor light emitting device according to claim 2, wherein a distance between dislocation defects in the second cladding layer is 120 to 170 nm.   4. The semiconductor light emitting device according to claim 2, wherein the total thickness of the underlayer is 3 [mu] m or less. 5. The semiconductor light emitting device according to claim 1, wherein the active layer is formed by metal organic vapor phase epitaxy, and a film forming pressure condition is atmospheric pressure or a pressure slightly lower than atmospheric pressure.

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