Patent Application
20110012146
The present invention relates to a semiconductor light-emitting device and a method for manufacturing the same.
Semiconductor light-emitting devices such as light-emitting diodes (LED) and the like have, for example, a structure in which a first compound semiconductor layer 11 with an n-type conductivity type, an active layer 12, and a second compound semiconductor layer 13 with a p-type conductivity type are laminated in order on a substrate 10. In addition, a first electrode (n-side electrode) 15 is provided on the substrate or an exposed portion 11A of the first compound semiconductor layer 11, and a second electrode (p-side electrode) 114 is provided on the top of the second compound semiconductor layer 13. Such semiconductor light-emitting devices can be classified into the two types including a type of semiconductor light-emitting device in which light from the active layer 12 is emitted through the second compound semiconductor layer 13 and a type (referred to as a “bottom emission type” for convenience) of semiconductor light-emitting device in which light from the active layer 12 is emitted through the first compound semiconductor layer 11 as disclosed in, for example, WO2003/007390.
Usually, a conventional bottom emission-type semiconductor light-emitting device frequently uses, as the second electrode 114, a reflecting electrode which reflects visible light from the active layer 12 in order to maintain a high efficiency of emission as shown in
However, in order to cover the second electrode 114 with the coating layer 100, various processes, for example, formation of the coating layer 100 based on a physical vapor deposition method (PVD method) and patterning of the coating layer 100 by lithography technique and etching technique are required. Thus, the manufacturing cost of a semiconductor light-emitting device is inevitably increased.
Accordingly, an object of the present invention is to provide a semiconductor light-emitting device including a second electrode which exhibits a stable behavior in a process for manufacturing the semiconductor light-emitting device or during an operation of the semiconductor light-emitting device and provide a method for manufacturing the semiconductor light-emitting device.
In order to achieve the object, a semiconductor light-emitting device according to the present invention includes:
(A) a first compound semiconductor layer with an n-type conductivity type;
(B) an active layer formed on the first compound semiconductor layer and composed of a compound semiconductor;
(C) a second compound semiconductor layer with a p-type conductivity type formed on the active layer;
(D) a first electrode electrically connected to the first compound semiconductor layer; and
(E) a second electrode formed on the second compound semiconductor layer;
the second electrode being composed of a titanium oxide, having an electron concentration of 4×1021/cm3 or more, and reflecting light emitted from the active layer.
The semiconductor light-emitting device according to the present invention can take a form in which the semiconductor layer is doped with niobium (Nb), tantalum (Ta), or vanadium (V). In other words, the semiconductor light-emitting device can take a form in which a doping impurity of the second electrode is at least one atom selected from the group consisting of niobium (Nb), tantalum (Ta), and vanadium (V).
In addition, the semiconductor light-emitting device according to the present invention including the above-described preferred form can be configured so that compound semiconductors constituting the first compound semiconductor layer, the active layer, and the second compound semiconductor layer are AlXGaYIn1-X-YN (0≦X≦1, 0≦Y≦1, 0≦X+Y≦1).
Further, the semiconductor light-emitting device according to the present invention including the above-described preferred form and configuration is preferably configured so that the crystal structure of the titanium oxide is a rutile structure.
Further, the semiconductor light-emitting device according to the present invention including the above-described preferred form and configuration is preferably configured so that the top surface of the second compound semiconductor layer on which the second electrode is formed has a (0001) plane (also referred to as a “C plane”). In this way, the top surface of the second compound semiconductor layer has a C plane so that high lattice matching with the second electrode can be achieved depending on the compound semiconductor constituting the compound semiconductor layer.
In order to achieve the object, a method for manufacturing a semiconductor light-emitting device according to the present invention includes at least the steps of:
(a) forming a light-emitting portion by laminating in order a first compound semiconductor layer with an n-type conductivity type, an active layer, and a second compound semiconductor layer with a p-type conductivity type on a substrate; and
(b) then forming a second electrode on the second compound semiconductor layer;
wherein compound semiconductors constituting the first compound semiconductor layer, the active layer, and the second compound semiconductor layer are AlXGaYIn1-X-YN (0≦X≦1, 0≦Y≦1, 0≦X+Y≦1); and
in the step (b), the second electrode composed of a titanium oxide having a rutile crystal structure is epitaxially grown on the top surface of the second compound semiconductor layer having a (0001) plane in a state of being doped with an impurity so that the electron concentration is 4×1021/cm3 or more.
The method for manufacturing a semiconductor light-emitting device according to the present invention can take a form in which an impurity is niobium (Nb), tantalum (Ta), or vanadium (V). In other words, the method can take a form in which a doping impurity of the second electrode is at least one atom selected from the group consisting of niobium (Nb), tantalum (Ta), and vanadium (V). In addition, according to circumstances, after the second electrode is formed, an impurity may be implanted in the second electrode by, for example, an ion implantation method from the viewpoint of addition of an impurity.
In the method for manufacturing a semiconductor light-emitting device according to the present invention including the above-described preferred form, the second electrode is preferably formed on the basis of a pulse laser deposition method (PLD method) which is a physical vapor deposition method, but the method is not limited to this and the second electrode can also be formed on the basis of another method, such as a molecular beam epitaxy method (MBE method) or a sputtering method.
In addition, in the semiconductor light-emitting device or the method for manufacturing the same according to the present invention including the above-described preferred form and configuration, a material constituting the second electrode preferably has a property, i.e., a metal-like property, that electric conductivity increases with increases in temperature. In addition, in order to determine whether or not the property is like metals, besides measurement of electric conductivity, a state density of a substance can be measured on the basis of an X-ray photoelectron spectroscopy method (XPS method) to determine whether or not a Fermi edge is present.
Further, the semiconductor light-emitting device or the method for manufacturing the same according to the present invention including the above-described preferred form and configuration preferably has a form in which in the second electrode, a plasma frequency (ω) represented by expression (1) below is 425 nm or less.
ω={(ne·e2)/(ε0·me)}1/2 (1)
wherein
In addition, the semiconductor light-emitting device or the method for manufacturing the same according to the present invention including the above-described preferred form and configuration is preferably configured so that light emitted from the active layer is emitted to the outside through the first compound semiconductor layer. Namely, the semiconductor light-emitting derive is preferably a bottom emission type.
In the semiconductor light-emitting device according to the present invention including the above-described preferred form and configuration or the method for manufacturing the semiconductor light-emitting device according to the present invention including the above-described preferred form and configuration (may be generically named “the present invention” hereinafter), as described above, as the compound semiconductor, not only a GaN-based compound semiconductor (including an AlGaN mixed crystal or AlGaInN mixed crystal, and a GaInN mixed crystal) but also an InN-based compound semiconductor and an AlN-based compound semiconductor can be used. In addition, as the method for forming (method for depositing) layers of these semiconductors, a metal-organic chemical vapor deposition method (MOCVD method), a MBE method, and a hydride vapor deposition method in which a halogen contributes to transport or reaction can be used. Examples of an n-type impurity added to compound semiconductor layers include silicon (Si), selenium (Se), germanium (Ge), tin (Sn), carbon (C), and titanium (Ti). Examples of a p-type impurity include zinc (Zn), magnesium (Mg), beryllium (Be), cadmium (Cd), calcium (Ca), barium (Ba), and oxygen (O).
Substrates which can be used as the substrate in the present invention include a sapphire substrate, a GaAs substrate, a GaN substrate, a SiC substrate, an alumina substrate, a ZnS substrate, a ZnO substrate, an AlN substrate, a LiMgO substrate, a LiGaO2 substrate, a MgAl2O4 substrate, an InP substrate, a Si substrate, a Ga substrate, and these substrates each including an underlying layer and a buffer layer formed on a surface (main surface). In the present invention, the semiconductor light-emitting device is first provided on the substrate, but a form in which the semiconductor light-emitting device is formed on the substrate or a form in which the substrate is removed can be used as a final form of the semiconductor light-emitting device.
In the semiconductor light-emitting device according to the present invention including the above-described preferred form and configuration or the method for manufacturing the semiconductor light-emitting device according to the present invention including the above-described preferred form and configuration (may be generically named “the semiconductor light-emitting device, etc. of the present invention” hereinafter), as a material constituting the first electrode, the first electrode can be composed of titanium (Ti) or a titanium alloy such as TiW or TiMo (for example, a TiW layer, a Ti layer/Ni layer/Au layer, or the like), or aluminum (Al), an aluminum alloy, AuGe, AuGe/Ni/Au, or the like. In addition, when the electrode has a multilayer structure, a material shown in front of “/” is disposed on the substrate side. For the first electrode and the second electrode, a contact portion (pad portion) including a multilayer metal layer having a laminated configuration, such as [adhesive layer (a Ti layer, a Cr layer, or the like)/[barrier metal layer (a Pt layer, a Ti layer, a TiW layer, a Mo layer, or the like)]/[metal layer having good compatibility in mounting (for example, an Au layer)], for example, a Ti layer/Pt layer/Au layer or the like, may be provided according to demand. The first electrode and the contact portion (pad portion) can be formed by any of various PVD methods such as a vacuum evaporation method and a sputtering method, various chemical vapor deposition methods (CVD methods), and a plating method.
Specifically, for example, a light-emitting diode (LED), an edge emission-type semiconductor laser, a surface emitting laser device (vertical resonator laser, VCSEL) can be configured using the semiconductor light-emitting device, etc. of the present invention.
In the semiconductor light-emitting device, etc. of the present invention, the second electrode reflects light emitted from the active layer, but the expression “the second electrode reflects light” represents that optical reflectance at a wavelength for 380 nm to 800 nm is 40% or more. The optical reflectance of the second electrode can be determined by comparing relative reflected light intensity with a dielectric multilayer film as a base theoretically having an optical reflectance of 100%. The electron concentration of the second electrode can be determined on the basis of hole measurement (Van der Pauw method).
In the present invention, since the second electrode is composed of a titanium oxide having a high electron density, not only high conductivity can be achieved, but also high optical reflectance can be achieved. As a result, the emission efficiency of the semiconductor light-emitting device can be significantly improved. In addition, since the second electrode is composed of a titanium oxide, a problem of deterioration due to oxidation does not occur, and electric migration does not occur because the titanium oxide is a very stable substance. Therefore, the second electrode need not be covered with a coating layer, and thus an attempt can be made to simplify the process for manufacturing the semiconductor light-emitting device and decrease the manufacturing cost of the semiconductor light-emitting device.
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Although the present invention is described below on the basis of an embodiment with reference to the drawings, a second electrode of the present invention is taken into consideration prior to the description.
It is known that high conductivity equal to that of ITO can be obtained by doping a titanium oxide with pentavalent niobium (Nb) or tantalum (Ta), i.e., forming Ti1-ZAZO2 (A is a pentavalent impurity) (refer to, for example, T. Hitotsugi, A. Ueda, Y. Furubayashi, Y. Hirose, S. Nomura, T. Shimada, T. Hasegawa, Jpn. J. Appl. Phys. 46, L86 (2007), N. Yamada, T. Hitotsugi, N. L. Kuong, Y. Furubayashi, Y. Hirose, T. Shimada, T. Hasegawa, Jpn. J. Appl. Phys. 46, 5275 (2007), T. Hitotsugi, Y. Furubayashi, A. Ueda, K. Itabashi, K. Inaba, Y. Hirose, G. Kinoda, Y. Yamamoto, T. Shimada, T. Hasegawa, Jpn. J. Appl. Phys. 44, L1063 (2005), Y. Furubayashi, T. Hitotsugi, Y. Yamamoto, K. Inaba, G. Kinoda, T. Shimada, T. Hasegawa, Appl. Phys. Lett. 86, 252101 (2005), and T. Hitotsugi, A. Ueda, S. Nakano, N. Yamada, Y. Furubayashi, Y. Hirose, T. Shimada, T. Hasegawa, Appl. Phys. Lett. 90, 212106 (2007)). Here, assuming free electrons without itinerancy on the basis of the Drude's electron theory, plasma frequency (ω) referred to as “plasmon” can be determined according to expression (1) (refer to, for example, C. Kittel “Introduction to Solid State Physics, 7th edition, p. 304 (1998)).
ω={(ne·e2)/(ε0·me)}1/2 (1)
wherein
The plasmon is quantized electron oscillation and light with energy lower than this is theoretically totally reflected. Therefore, light in the visible region can be totally reflected by increasing the electron concentration in a titanium oxide to a plasma frequency or more. It is said that in a titanium oxide, the activation rate of an impurity such as Nb or Ta is about 80%. Therefore, the electron concentration (carrier concentration) becomes 4×1021/cm3 by doping with 5×1021/cm3 of impurity, and light up to about 423 nm can be totally reflected (refer to
In addition, a titanium oxide is a very stable substance, and thus the second electrode composed of the titanium oxide has very strong process resistance, heat resistance, and electric resistance. Further, it is known that a rutile-type titanium oxide (titanium dioxide, titanium(IV) oxide) is stable in air, and a titanium oxide (may be referred to as “rutile-type TiO2”) having a rutile structure undergoes heteroepitaxial growth on a C-plane of GaN (refer to, for example, T. Hitotsugi, Y. Hirose, J. Kasai, Y. Furubayashi, M. Ohtani, K. Nakajima, T. Chilyow, T. Shimada, T. Hasegawa, Jpn. J. Appl. Phys. 44, L1503 (2005). Therefore, heteroepitaxial growth of the second electrode on the C-plane of the second compound semiconductor layer can provide an electrically good interface at an interface between the compound semiconductor layer and the second electrode without causing a level due to a defect or the like.
Embodiment 1 relates to a semiconductor light-emitting device of the present invention and a method for manufacturing the same.
A semiconductor light-emitting device of Embodiment 1 includes a light-emitting diode (LED), and as shown in
(A) a first compound semiconductor layer 11 having an n-type conductivity type;
(B) an active layer 12 formed on the first compound semiconductor layer 11 and composed of a compound semiconductor;
(C) a second compound semiconductor layer 13 formed on the active layer 12 and having a p-type conductivity type;
(D) a first electrode 15 electrically connected to the first compound semiconductor layer 11; and
(E) a second electrode 14 formed on the second compound semiconductor layer 13.
In addition, the second electrode 14 is composed of a titanium oxide, specifically a titanium oxide having a rutile crystal structure, has an electron concentration of 4×1021/cm3 or more, and reflects light emitted from the active layer 12. That is, the semiconductor light-emitting device of Embodiment 1 is a bottom emission type in which light emitted from the active layer 12 is emitted to the outside through the first compound semiconductor layer 11. Herein, in the second electrode 14, a titanium oxide (rutile-type TiO2) is doped with niobium (Nb) or tantalum (Ta). The value of electron concentration determined in the second electrode 14 based on hole measurement (Van der Pauw method) is 4×1021/cm3. Therefore, in the second electrode 14, the plasma frequency (ω) represented by the above expression (1) is 425 nm or less. The material (rutile-type TiO2 doped with Nb or Ta) constituting the second electrode 14 has a property, i.e., a metal-like property, that the electric conductivity increases with increases in temperature.
Herein, a substrate 10 includes a substrate 10A composed of, for example, sapphire, and an underlying layer 10B formed on the substrate 10A and composed of GaN. In addition, the compound semiconductors constituting the first compound semiconductor layer 11, the active layer 12, and the second compound semiconductor layer 13 are composed of AlXGaYIn1-X-YN (0≦X≦1, 0≦Y≦1, 0≦X+Y≦1), more specifically GaN-based compound semiconductors. That is, the first compound semiconductor layer 11 is composed of Si-doped GaN (GaN:Si), the active layer 12 is composed of an InGaN layer (well layer) and a GaN layer (barrier layer) and has a multiquantum well structure. In addition, the second compound semiconductor layer 13 is composed of Mg-doped GaN (GaN:Mg). In addition, a light-emitting portion is configured by a laminated structure in which the first compound semiconductor layer 11, the active layer 12, and the second compound semiconductor layer 13 are laminated. Further, the top surface of the second compound semiconductor layer 13 on which the second electrode 14 is formed has a (0001) plane which is a C-plane.
Further, the first electrode 15 is provided on a portion 11A of the first compound semiconductor layer 11 which is exposed by partially removing (etching) the second compound semiconductor layer 13 and the active layer 12. In addition, when a current is passed from the second electrode 14 to the first compound semiconductor layer 11 and the first electrode 15 through the active layer 12 which is positioned immediately below the remaining second compound semiconductor layer 13, in the active layer 12, the quantum well structure of the active layer 12 is excited by current injection, creating a light emission state over the entire surface. In addition,
Also, in the light-emitting diode of Embodiment 1, a first contact portion (first pad portion) 18A is formed on the first electrode 15 to extend from a first opening 17A provided in an insulating layer 16 to the top of the insulating layer 16, and a second contact portion (second pad portion) 18B is formed on the second electrode 14 to extend from a second opening 17B provided in the insulating layer 16 to the top of the insulating layer 16. A material constituting the insulating layer 16 can be exemplified by a SiOX-based material, a SiNX-based material, a SiOXNY-based material, Ta2O5, ZrO2, AlN, and Al2O3.
The method for manufacturing the semiconductor light-emitting device of Embodiment 1 is described below with reference to schematic partial sectional views of
First, the substrate 10A composed of sapphire is transferred to a MOCVD apparatus and cleaned at a substrate temperature of 1050° C. for 10 minutes in a carrier gas containing hydrogen, and then the substrate temperature is decreased to 500° C. Then, the underlying layer 10B composed of GaN is crystal-grown on a surface of the substrate 10A by supplying a trimethylgallium (TMG) gas as a gallium raw material while supplying an ammonia gas as a nitrogen raw material on the basis of a MOCVD method, and then supply of the TMG gas is stopped.
Next, the first compound semiconductor layer 11 having an n-type conductivity type, the active layer 12, and the second compound semiconductor layer 13 having a p-type conductivity type are laminated in order on the substrate 10 to form the light-emitting portion.
Specifically, on the basis of the MOCVD method, the substrate temperature is increased to 1020° C., and then supply of a monosilane (SiH4) gas as a silicon raw material is started at normal pressure to crystal-grow the first compound semiconductor layer 11 on the underlying layer 10B, the first compound semiconductor layer 11 being composed of Si-doped GaN (GaN:Si) and having an n-type conductivity type and a thickness of 3 μm. In addition, the doping concentration is about 5×1018/cm3.
Then, supply of the TMG gas and the SiH4 gas is stopped, and the substrate temperature is decreased to 750° C. Then, triethyl gallium (TEG) gas and trimethyl indium (TMI) gas are used and these gases are supplied by valve switching to crystal-grow the active layer 12 composed of InGaN and GaN and having a multiquantum well structure.
For example, in a light-emitting diode with an emission wavelength of 400 nm, a multiquantum well structure (for example, including 2 well layers) composed of InGaN having an In ratio of about 9% and GaN (thicknesses of 2.5 nm and 7.5 nm, respectively) may be formed. In addition, in a blue light-emitting diode with an emission wavelength of 460 nm±10 nm, a multiquantum well structure (for example, including 15 well layers) composed of InGaN having an In ratio of about 15% and GaN (thicknesses of 2.5 nm and 7.5 nm, respectively) may be formed. Further, in a green light-emitting diode with an emission wavelength of 520 nm±10 nm, a multiquantum well structure (for example, including 9 well layers) composed of InGaN having an In ratio of about 23% and GaN (thicknesses of 2.5 nm and 15 nm, respectively) may be formed.
After the formation of the active layer 12 is completed, supply of the TEG gas and the TMI gas is stopped, the carrier gas is changed from nitrogen to hydrogen, the substrate temperature is increased to 850° C., and supply of a TMG gas and biscyclopentadienylmagnesium (Cp2MG) gas is started to crystal-grow the second compound semiconductor layer 13 on the active layer 12, the second compound semiconductor layer 13 being composed of Mg-doped GaN (GaN:Mg) and having a thickness of 100 nm. In addition, the doping concentration is about 5×1019/cm3. Then, supply of the TMG gas and the Cp2MG gas is stopped, and the substrate temperature is decreased to room temperature, completing crystal growth.
After the crystal growth is completed as described above, the p-type impurity (p-type dopant) is activated by annealing at about 800° C. for 10 minutes in a nitrogen gas atmosphere.
Next, as the second electrode (p-side electrode) 14, Ti1-ZAZO2 (A is a pentavalent impurity, specifically Nb or Ta) is deposited on the second compound semiconductor layer on the basis of a PLD method. Specifically, the second electrode 14 is deposited, for example, under the conditions shown in Table 1 below. Next, TiOX (X>2) constituting the peroxidized second electrode 14 can be converted to TiO2 by, for example, reduction annealing under the conditions of 400° C. for 5 minutes in a hydrogen atmosphere. In this case, the impurity concentration is such a value that the carrier concentration is 4×1021/cm3 or more so that the light emitted from the active layer 12 is reflected by the second electrode 14. For example, as described above, in case of Nb or Ta, the dose may be about 5×1021/cm3 or more because the electric activation rate in TiO2 is about 80%. In addition, in general, assuming that the electric activation rate of impurity in TiO2 is α, the dose of the impurity may be 4×1021×(1/α)/cm3 or more. Also, rutile-type TiO2 is known to grow heteroepitaxially on a C-plane of GaN, and a good interface can be easily obtained.
Then, the first compound semiconductor layer 11 is partially exposed. Specifically, a portion 11A of the first compound semiconductor layer 11 is exposed by partially removing the second electrode 14, the second compound semiconductor layer 13, and the active layer 12 on the basis of a lithography technique and a dry etching technique (see
Then, at least an exposed portion of the first compound semiconductor layer 11, an exposed portion of the active layer 12, an exposed portion of the second compound semiconductor layer 13, and a portion of the second electrode 14 are covered with the insulating layer 16 (refer to
In Embodiment 1, the second electrode 14 is composed of a titanium oxide doped with an impurity and having a high electron density, and thus not only high conductivity can be achieved, but also high optical reflectance can be achieved. As a result, the emission efficiency of the semiconductor light-emitting device can be significantly improved. In addition, since the second electrode 14 is composed of a titanium oxide, deterioration due to oxidation does not occur, and electrical migration does not occur. Therefore, unlike in a conventional technique, the second electrode 14 need not be covered with a coating layer, and thus an attempt can be made to simplify the process for manufacturing the semiconductor light-emitting device and decrease the manufacturing cost of the semiconductor light-emitting device. In addition, since a titanium oxide doped with an impurity has a high work function, a low Schottky barrier is produced between the second electrode 14 and the second compound semiconductor layer 13 having a p-type conductivity type, and thus a good ohmic contact can be achieved as an electric connection between the second electrode 14 and the second compound semiconductor layer 13.
Although the present invention described above on the basis of the preferred embodiment, the present invention is not limited to this embodiment. The configurations and structures of the semiconductor light-emitting device described in the embodiment, the materials constituting the semiconductor light-emitting device, and the manufacturing conditions and various numerical values of the semiconductor light-emitting device are exemplary and can be appropriately changed. For example, in the semiconductor light-emitting device described in Embodiment 1, the form of being formed on the substrate is described as the final form of the semiconductor light-emitting device, but alternatively, a structure can be formed, in which the first electrode 15 is formed on the first compound semiconductor layer 11 exposed by polishing or etching the substrate. In addition, by using a substrate with conductivity, the first compound semiconductor layer, etc. may be formed on a main surface (front surface) of the substrate, and the first electrode 15 may be formed on the back surface of the substrate.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-074875 | Mar 2008 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/054849 | 3/13/2009 | WO | 00 | 9/16/2010 |
