JP2007157765A - Gallium nitride semiconductor light emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gallium nitride semiconductor light emitting element which can prevent the depletion of carrier due to spontaneous polarization or piezo polarization occurring on a boundary between an AlGaN semiconductor layer and a GaN semiconductor layer, and can stabilize driving voltage. <P>SOLUTION: A gallium nitride semiconductor crystal 2 including a light emission area is formed on a sapphire substrate 1. The gallium nitride semiconductor crystal 2 is covered with film so that its growth surface may be become an N (nitrogen) polarity surface. In addition, a low-temperature buffer layer 4 is laminated on a bulk substrate 3, and the gallium nitride semiconductor crystal 2 including the light emission area is formed on the low-temperature buffer layer 4. The gallium nitride semiconductor crystal 2 is covered with film so that its growth surface may be become an N (nitrogen) polarity surface. The direction of an electric field generating on a boundary GaN/AlGaN on p side is reversed to bring in positive holes to the side of the light emission area. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a gallium nitride semiconductor light emitting device using GaN.

  As a semiconductor light emitting element such as a semiconductor laser element or a light emitting diode emitting blue or violet light, there is a gallium nitride semiconductor light emitting element. When manufacturing a GaN-based semiconductor element, it is difficult to manufacture a substrate made of GaN. Therefore, a GaN-based semiconductor layer is epitaxially grown on a substrate made of sapphire, SiC, Si, or the like.

  For example, using MOCVD (metal organic chemical vapor deposition) on the (0001) surface of a sapphire substrate, an undoped GaN buffer layer, n-GaN contact layer, n-AlGaN cladding layer, n-GaN light guide layer, InGaN A multiple quantum well (MQW) active layer and the like are sequentially formed, and a p-GaN light guide layer, a p-AlGaN cladding layer, a p-GaN contact layer, and the like are sequentially formed on the active layer.

  From the p-GaN contact layer to a partial region of the n-GaN contact layer is removed by etching, the n-GaN contact layer is exposed, an n-electrode is formed on the exposed upper surface of the n-GaN contact layer, and p-GaN A p-electrode is formed on the upper surface of the contact layer.

  FIG. 11 shows a unit cell diagram showing the plane orientation of a sapphire single crystal, and the crystal structure of sapphire can be approximated by a hexagonal system as shown in the figure. When a GaN-based semiconductor layer is stacked on a sapphire substrate, the C-plane (0001) of the sapphire substrate is used, and the GaN-based semiconductor stacked on the (0001) -oriented sapphire substrate is a (0001) -oriented wurtzite type. The cation element of Ga has a crystal polarity (growth in the C-axis direction) in the growth surface direction. That is, the sapphire substrate is laminated with the C axis [0001] in the vertical direction.

FIG. 12 shows the band gap energy in the valence band from the n-AlGaN cladding layer 21 to the p-AlGaN cladding layer 25 in the gallium nitride semiconductor light emitting device in which a GaN-based semiconductor layer is stacked on the sapphire substrate described above.
JP 2004-140339 A

  As in the above prior art, the GaN-based semiconductor layer laminated on the C-plane (0001) of the sapphire substrate has a Ga polar plane in the growth surface direction, and thus has grown when the growth direction has polarity. Piezoelectric polarization due to spontaneous polarization and interfacial stress due to the wurtzite structure in which there is no symmetry in the C-axis direction at the GaN / AlGaN heterobond interface of the GaN-based semiconductor layer and the front and back surfaces of the epitaxial film grown on the C plane are generated. Occurs, a polarization charge is generated, and an electric field is generated at the heterojunction interface. This electric field has a small problem because electrons are drawn into the active layer on the n side.

  However, on the p side, an electric field E is generated from the p-GaN light guide layer 24 toward the p-AlGaN cladding layer 25 as shown in FIG. 12, and the generated electric field E causes the p-AlGaN cladding layer 25 to p. -Holes flowing into the GaN light guide layer 24 are electrically repelled and cannot flow into the MQW active layer 23, carrier depletion occurs, and the drive voltage rises. The increase in the driving voltage has been a problem because it also shortens the lifetime of the gallium nitride semiconductor light emitting device.

  Further, in Patent Document 1, as a device having a nitride-based heterostructure, there is a description that a growth surface of a part of a semiconductor layer is made into nitrogen polarity or Ga polarity, but p-side GaN This is not a problem of carrier depletion due to spontaneous polarization or piezo polarization at the / AlGaN interface or the interface between the GaN-based semiconductor layer and the AlGaN semiconductor layer, and the above problem cannot be solved.

  The present invention was devised to solve the above-described problems, and prevents driving depletion due to spontaneous polarization or piezo-polarization generated at the interface between the AlGaN semiconductor layer and the GaN-based semiconductor layer, thereby reducing the driving voltage. An object of the present invention is to provide a gallium nitride semiconductor light emitting device that can be stabilized.

  In order to achieve the above object, an invention according to claim 1 is provided with at least an n-type semiconductor layer, a light emitting region, and a p-type semiconductor layer on a substrate in order, and an AlGaN semiconductor layer formed on the p-type semiconductor layer side, A gallium nitride semiconductor light emitting device having an interface with a GaN semiconductor layer located on the n side of the AlGaN semiconductor layer, wherein the growth surface is formed in the nitrogen polarity direction of GaN from the n-type semiconductor layer to the AlGaN semiconductor layer A gallium nitride semiconductor light-emitting device characterized by being manufactured.

  According to a second aspect of the present invention, at least an n-type semiconductor layer, a light emitting region, and a p-type semiconductor layer are provided on a substrate in this order, and an AlGaN semiconductor layer formed on the p-type semiconductor layer side and the AlGaN semiconductor layer. A gallium nitride semiconductor light emitting device having an interface with an InGaN semiconductor layer located on the n side, wherein the growth surface is formed in the nitrogen polarity direction of GaN from the n-type semiconductor layer to the AlGaN semiconductor layer. This is a gallium nitride semiconductor light emitting device.

According to a third aspect of the present invention, at least an n-type semiconductor layer, a light emitting region, and a p-type semiconductor layer are sequentially provided on a substrate, and the Al _x GaN semiconductor layer formed on the p-type semiconductor layer side and the p-type Al _A gallium nitride semiconductor light emitting device having an interface with an Al _Y GaN semiconductor layer (X> Y) located on the n side of the _X GaN semiconductor layer, wherein the growth from the n-type semiconductor layer to the Al _X GaN semiconductor layer A gallium nitride semiconductor light emitting device characterized in that the surface is formed in the nitrogen polarity direction of GaN.

  According to the present invention, the growth direction (direction perpendicular to the substrate) of the gallium nitride semiconductor stacked on the substrate is formed in the N (nitrogen) polarity direction of GaN, that is, the [000-1] axis direction. Therefore, the electric field generated at the interface between the p-side AlGaN semiconductor layer and the GaN-based semiconductor layer can be reversed by spontaneous polarization or piezo-polarization, so that carrier depletion can be prevented and the driving voltage can be stabilized.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a schematic configuration of a gallium nitride semiconductor light emitting device of the present invention. In the gallium nitride semiconductor light emitting device of FIG. 1A, a gallium nitride semiconductor crystal 2 including a light emitting region is formed on a sapphire substrate 1. The gallium nitride semiconductor crystal 2 is formed so that its growth surface is an N (nitrogen) polar surface. Further, as shown in FIG. 1B, a gallium nitride semiconductor crystal in which a low-temperature buffer layer 4 is stacked on a bulk substrate 3 such as sapphire, SiC, Si, or GaAs and includes a light-emitting region on the low-temperature buffer layer 4. 2 is formed. The gallium nitride semiconductor crystal 2 is formed so that its growth surface is an N (nitrogen) polar surface.

  FIG. 2 shows the positional relationship between the plane orientation of the gallium nitride semiconductor crystal 2 and the plane orientation of the sapphire substrate 1 or the bulk substrate 3. White circles indicate the positions of N (nitrogen) atoms, and black circles indicate the positions of Ga (gallium) atoms. FIG. 3 schematically shows the crystal structure of the gallium nitride semiconductor crystal 2 epitaxially grown on the C-plane of the sapphire substrate 1 or the bulk substrate 3.

  As described above, the sapphire substrate 1 or the bulk substrate 3 as a growth substrate uses the (0001) plane, but the growth direction of the gallium nitride semiconductor crystal 2 is the N-polar direction, that is, [[ 000-1] is formed in the axial direction. In this way, by reversing the polarity of the growth direction of the gallium nitride semiconductor crystal 2 from Ga polarity to N polarity, the direction of the electric field generated at the p-side GaN / AlGaN interface is reversed to emit holes. It tries to pull in to the area side.

  FIG. 4 shows an example of an LED in a gallium nitride semiconductor light emitting device in which a gallium nitride semiconductor crystal 2 is grown on a sapphire substrate 1 so that the growth surface has N polarity. The substrate 11 is a sapphire substrate, and the buffer layer 12, the n-type contact layer 13, the n-type superlattice layer 15, the MQW active layer 16, the p-type electron block layer 17, and the p-type contact layer 18 are formed on the substrate 11. . Further, a positive electrode (p electrode) 20 is formed on the p-type contact layer 18, and a negative electrode (n electrode) 14 is formed on the n-type contact layer 13. The buffer layer 12, the n-type contact layer 13, the n-type superlattice layer 15, the MQW active layer 16, the p-type electron block layer 17 and the p-type contact layer 18 all have a crystal structure with an N-polar growth surface.

  Here, the buffer layer 12 is undoped GaN, the n-type contact layer 13 is n-GaN, and the n-type superlattice layer 15 is a superlattice structure in which n-GaN thin films and n-InGaN thin films are alternately stacked for 5 to 10 periods, The MQW active layer 16 is a multi-quantum well structure of a well layer made of InGaN and a barrier layer made of GaN or InGaN, the p-type electron block layer 17 is made of p-AlGaN, and the p-type contact layer 18 is made of p-GaN. The band gap energy in the valence band is shown in FIG.

  As shown in FIG. 12, when a gallium nitride semiconductor crystal is formed with a conventional Ga polar plane as a growth surface, the GaN semiconductor layer is replaced with an AlGaN layer at the interface between the p-GaN light guide layer and the p-AlGaN cladding layer. The electric field E is generated toward the semiconductor layer. However, when the N polar face is used as the growth surface, the electric field E is generated from the p-AlGaN electron block layer 17 toward the GaN barrier layer as shown in FIG. . Thus, since the direction of the electric field E is reversed, holes injected from the positive electrode 19 side are easily drawn into the MQW active layer 16 side which is a light emitting region, and carrier depletion can be prevented.

  In order to grow the crystal as described above, in the configuration of FIG. 1A, the gallium nitride semiconductor crystal 2 is formed on the sapphire substrate 1 by using the MBE method (molecular beam epitaxy method). The MBE method is a method in which each raw material is heated and its molecular beam reaches the substrate to perform crystal growth, so that an intermediate reaction of the raw material atoms can be avoided and the temperature of the raw material atoms can be controlled independently. The molecular beam intensity can be controlled by the cell temperature.

  As shown in FIG. 6, the MBE apparatus arranges a constituent element in a cell as a solid evaporation source, and heats the element to about 500 ° C. to 800 ° C. to generate vapor of the constituent element. The film is supplied to the substrate as a beam) to form a film, and the molecules do not collide with each other until they reach the substrate. In this example, Al, Ga, N, and In, which are constituent elements of the gallium nitride semiconductor crystal 2, were arranged in the cell. The material supply can be turned ON / OFF by the cell shutter control, and the inside of the apparatus is evacuated during film formation. Further, since growth is performed in a non-thermal equilibrium state, low temperature growth is possible.

6, for example, on the R surface of the sapphire substrate 11, the buffer layer 12 made of undoped GaN is about 1 to 3 μm, the Si-doped GaN contact layer 13 is about 1 to 5 μm, and the Si-doped InGaN. / GaN superlattice layer 15, MQW active layer 16, Mg-doped AlGaN electron blocking layer 17, and Mg-doped GaN contact layer 18 are sequentially stacked in a thickness of about 0.2 to 1 μm. The MQW active layer 16 is formed by alternately laminating a well layer made of In _0.18 Ga _0.83 N of 1 to 3 nm and a barrier layer made of 10 to 20 nm of In _Z GaN (0 ≦ Z ≦ 0.05). Thus, a multilayer structure having 3 to 10 cycles was formed. In order to obtain a good quality crystal in the N polarity direction, the In composition ratio of the InGaN well layer should be 50% or less, preferably 35% or less. In this embodiment, it was 18% as described above.

  After forming the p-GaN contact layer 18, the p-GaN contact layer 18, the p-AlGaN electron block layer 17, the MQW active layer 16, the n-InGaN / GaN superlattice layer 15, and a part of the n-GaN contact layer 13 Are removed by mesa etching by reactive ion etching or the like. Thereafter, the negative electrode 14 is formed on the etched surface of the n-GaN contact layer 13 by vapor deposition, and the positive electrode 19 is formed on the p-GaN contact layer 18 by vapor deposition.

Next, the manufacturing method of the structure shown in FIG.1 (b) is demonstrated. The bulk substrate 3 is a sapphire substrate, and the low-temperature buffer layer 4 is laminated thereon. In order to laminate the low-temperature buffer layer 4, it is grown at a low temperature (600 ° C. or lower) using a laser ablation method. The low temperature buffer layer 4 formed in this way has a growth surface of N polarity. In the laser ablation method, as shown in FIG. 7, the bulk substrate 3 and the target 33 are arranged to face each other in a vacuum chamber (not shown) having a chamber internal pressure of 1 × 10 ⁻⁶ Torr or less, and the bulk substrate 3 is heated. The target is placed on the source 31 and the substrate temperature is maintained at 600 ° C. or lower, and the target 33 is irradiated with a laser beam 34 made of, for example, a KrF excimer laser having an oscillation wavelength of 248 nm from the quartz window of the vacuum chamber. The material 33 is sublimated (ablated) to form the bloom 8, and the sublimated atoms adhere to the surface of the bulk substrate 3, so that the low-temperature buffer layer 4 serving as a buffer layer of the GaN single crystal can be grown.

As the target 33, for example, a sintered body such as GaN, AlN, AlGaN-based compound, InGaN-based compound, or the like can be used. In general, Al _x Ga _y In _1-xy N (0 ≦ x ≦ 1) , 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1), and can be determined according to the composition of the nitride semiconductor crystal layer grown on the low-temperature buffer layer 4. In this case, since In is a substance that is difficult to enter, it should be noted that the composition tends to fluctuate. When a single crystal thin film of a GaN layer is formed, the buffer layer of GaN single crystal can be grown also by sublimation in a gas atmosphere of ammonia or nitrogen plasma.

  First, a sapphire substrate as the bulk substrate 3 is placed in a load lock chamber, and first heated at a temperature of about 400 ° C. for about 5 to 10 minutes to remove excess moisture and the like. And the board | substrate 3 is conveyed in a chamber and a substrate temperature is set to 600 degrees C or less. By irradiating the target 33 with the KrF excimer laser beam 34, for example, GaN of the target 33 is sublimated and can be formed on the surface of the sapphire substrate to form the low-temperature buffer layer 4. If the growth time is lengthened, the low temperature buffer layer 4 can be grown by a desired thickness.

  Since the growth surface of the low-temperature buffer layer 4 is an N-polar surface, if the gallium nitride semiconductor crystal 2 is subsequently formed by MOCVD (metal organic chemical vapor deposition) or the like, these crystals also have an N-growth surface. It is formed as a polar surface. The structure of the gallium nitride semiconductor crystal 2 can be the same as that described in FIG. The substrate 11 in FIG. 4 corresponds to the bulk substrate 3, and the low-temperature buffer layer 4 corresponds to the buffer layer 12.

By the way, the generation of the electric field E due to piezo polarization caused by spontaneous polarization and interface stress is observed not only at the AlGaN / GaN interface but also at the interface between AlGaN and other GaN-based semiconductor layers. In particular, a similar electric field is generated at the AlGaN / InGaN interface and the Al _X GaN / Al _Y GaN (X> Y) interface.

For example, FIG. 8 shows an example of an LD in a gallium nitride semiconductor light emitting device in which a gallium nitride semiconductor crystal 2 is grown on a sapphire substrate 1 so that the growth surface has N polarity. As the manufacturing method, the above-described MBE method (molecular beam epitaxy method) or laser ablation method is used. On the R surface of the sapphire substrate 41, a buffer layer 42 made of undoped GaN is about 1 to 3 μm, an n-type cladding layer 44 made of a Si-doped AlGaN / GaN superlattice layer is about 1 to 5 μm, and Si-doped InGaN / GaN. An n-type superlattice layer 45 made of, an MQW active layer 46, a p-type electron blocking layer 47 made of Mg-doped AlGaN, and a p-type cladding layer 48 made of Mg-doped AlGaN / GaN superlattice layer are about 0.2 to 1 μm. Laminate sequentially. The MQW active layer 46 is formed by alternately stacking a well layer made of In _0.17 Ga _0.83 N having a thickness of 1 to 3 nm and a barrier layer made of In _X GaN (0 ≦ X ≦ 0.05) having a thickness of 10 to 20 nm. Thus, a multilayer structure having 3 to 10 cycles was formed.

  After the p-type cladding layer 48 is patterned by etching to form a ridge portion, the insulating layer 49 covers the side surface of the ridge portion to the flat portion of the p-type cladding layer 48, and an Mg layer is formed on the ridge portion of the p-type cladding layer 48. A p-type contact layer 50 made of doped GaN is stacked.

  After the p-type contact layer 50 is formed, the p-type cladding layer 48, the p-type electron blocking layer 47, the MQW active layer 46, the n-type superlattice layer 45, the n-type cladding layer 44, and a part of the buffer layer 42 are made reactive. It is removed by mesa etching by ion etching or the like. Thereafter, an n-electrode 43 is formed on the etched surface of the buffer layer 42 by vapor deposition, and a p-electrode 51 is formed on the p-type contact layer 40 by vapor deposition. The buffer layer 42, the n-type cladding layer 44, the n-type superlattice layer 45, the MQW active layer 46, the p-type electron blocking layer 47, the p-type cladding layer 48, and the p-type contact layer 50 are all crystals having an N-polar growth surface. It becomes a structure.

As described above, if the barrier layer of the MQW active layer 46 is In _Z GaN (0 ≦ Z ≦ 0.05) and Z ≠ 0, the p-type electron blocking layer 47 made of AlGaN and the barrier layer are An AlGaN / InGaN interface is formed. Further, an Al _X GaN / Al _Y GaN (X> Y) interface is formed between the p-type electron blocking layer 47 and the p-type cladding layer 48 made of an AlGaN / GaN superlattice layer.

  FIG. 9 shows the state of the AlGaN / InGaN interface together with the band gap energy in the valence band. When the epitaxial growth direction of the gallium nitride semiconductor crystal on the substrate is a Ga polar plane, FIG. As shown in FIG. 2, an electric field E is generated from the InGaN semiconductor layer toward the AlGaN semiconductor layer at the AlGaN / InGaN interface, and holes from the p-electrode side are less likely to flow into the light emitting region due to electrical repulsion. Become.

Further, FIG. _10, although the state of the interface of the _{Al X GaN / Al Y GaN (} X> Y) illustrates with the band gap energy in the valence band, as shown in FIG. 10 (a), Al _X At the GaN / Al _Y GaN (X> Y) interface, an electric field E is generated from the Al _Y GaN semiconductor layer toward the Al _X GaN semiconductor layer, and holes from the p-electrode side are electrically repelled. It becomes difficult to flow into the light emitting area.

Therefore, if the growth direction of the semiconductor layer of the gallium nitride semiconductor crystal on the substrate is an N polarity plane as shown in FIGS. 2 and 3 by the above-described method, as shown in FIG. At the interface, the electric field E in FIG. 9A is reversed and generated from the AlGaN semiconductor layer toward the InGaN semiconductor layer, and holes from the p-electrode side can be drawn into the light emitting region. Further, as shown in FIG. 10 _(b), even in _{Al X GaN / Al Y GaN (} X> Y) interface, _Al Y GaN semiconductor layer from _Al X GaN semiconductor layer electric field E is inverted shown in FIG. 10 (a) It is generated toward (X> Y), and holes from the p-electrode side can be drawn into the light emitting region. In this way, carrier depletion can be prevented and the drive voltage can be stabilized.

It is a figure which shows schematic structure of the gallium nitride semiconductor light-emitting device of this invention. It is a figure which shows the positional relationship of the surface orientation of a gallium nitride semiconductor crystal, and the surface orientation of a growth substrate. It is a figure which shows typically the crystal structure of the gallium nitride semiconductor crystal epitaxially grown on the C surface of the growth substrate. It is sectional drawing which shows an example of the LED structure in a gallium nitride semiconductor light-emitting device. It is a figure which shows the band gap energy in the valence band of LED of FIG. It is a figure which shows schematic structure of a MBE apparatus. It is a figure which shows schematic structure of a laser ablation apparatus. It is sectional drawing which shows an example of LD structure in a gallium nitride semiconductor light-emitting device. It is a figure which shows the state of the electric field of an AlGaN / InGaN interface. _{Al X GaN / Al Y GaN (} X> Y) is a diagram showing a state of an electric field of the interface. It is a unit cell figure which shows the hexagonal system surface orientation. It is a figure which shows the band gap energy in the valence band of the conventional gallium nitride semiconductor light-emitting device.

Explanation of symbols

1 Sapphire substrate 2 Gallium nitride semiconductor crystal 3 Bulk substrate 4 Low temperature buffer layer

Claims (3)

A substrate comprising at least an n-type semiconductor layer, a light emitting region, and a p-type semiconductor layer in order, and comprising an AlGaN semiconductor layer formed on the p-type semiconductor layer side and a GaN semiconductor layer located on the n-side from the AlGaN semiconductor layer A gallium nitride semiconductor light emitting device having an interface,
A gallium nitride semiconductor light-emitting device characterized in that the growth surface is formed in the nitrogen polarity direction of GaN from the n-type semiconductor layer to the AlGaN semiconductor layer. An AlGaN semiconductor layer formed on the p-type semiconductor layer side and an InGaN semiconductor layer located on the n-side of the AlGaN semiconductor layer, comprising at least an n-type semiconductor layer, a light emitting region, and a p-type semiconductor layer on the substrate A gallium nitride semiconductor light emitting device having an interface,
A gallium nitride semiconductor light-emitting device characterized in that the growth surface is formed in the nitrogen polarity direction of GaN from the n-type semiconductor layer to the AlGaN semiconductor layer. At least the n-type semiconductor layer on a substrate, the light emitting region comprises a p-type semiconductor layer in this order, located in the n side of the p-type semiconductor layer Al X _GaN semiconductor layer formed on the side and the p-type Al X _GaN semiconductor layer A gallium nitride semiconductor light emitting device having an interface with an Al _Y GaN semiconductor layer (X> Y),
A gallium nitride semiconductor light-emitting device characterized in that a growth surface is formed in the nitrogen polarity direction of GaN from the n-type semiconductor layer to the Al _x GaN semiconductor layer.

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