Patent Application
20160005919
The present invention relates to a novel deep ultraviolet light emitting device that employs a nitride semiconductor and whose emission wavelength is in the range of 200 to 300 nm.
Under present circumstances, gas discharge lamps using heavy hydrogen, mercury, or the like are used as deep ultraviolet light sources whose emission wavelength is 300 nm or less. There are inconveniences of shortlivedness, large size and so on in these gas discharge lamps. In addition, mercury is a substance on which more and more conventions are regulating activities. Thus, the realization of deep ultraviolet light emitting devices employing semiconductors that are possible to overcome these inconvenience and that are easy to be treated is awaited.
However, there are problems with light emitting devices employing semiconductors that light output is lower compared to that from gas discharge lamps such as heavy hydrogen-vapor lamps and mercury-vapor lamps; and the light emission efficiency is also low.
It is a cause of insufficient light output from semiconductor light emitting devices that in nitride semiconductor light emitting devices, the effective mass of electrons is smaller compared to holes, and the career density is high; thus, electrons cross over active layers (region), to overflow p-type layers, which results in their low light emission efficiency. Such an overflow of electrons to p-type layers results in a further low light emission efficiency under the condition of high injection currents, and at the same time, heating values are increased. As a result, light output has stopped increasing, and it becomes difficult to obtain light output corresponding to the amount of injected carriers.
The problem of an overflow of electrons to p-type layers in nitride semiconductor light emitting devices is not only with deep ultraviolet light emitting devices whose emission wavelength is 300 nm or less (for example, see Non Patent Literature 1). For example, Patent Literature 1 describes the art of, in a semiconductor light emitting device whose emission wavelength is over 300 nm, forming an electron blocking layer having a band gap larger than that of an active layer, between the active layer and a p-type layer, and whereby preventing the outflow of electrons from the active layer region toward the p-type layer, and improving the light emission efficiency. Non Patent Literature 2 describes that an electron blocking layer as described above is tried to be applied to a deep ultraviolet light emitting device (see Non-Patent Literature 2).
However, according to the inventor's study, it has been found out that the light emission efficiency is not sufficiently improved only by the arrangement of an electron blocking layer in a nitride semiconductor light emitting device whose emission wavelength is 300 nm or less. The inventor assumes the reason as follows: that is, it is required that a band gap of a p-type layer in a deep ultraviolet light emitting device whose emission wavelength is 300 nm or less is larger than that in a near ultraviolet light emitting device or a visible light emitting device; as a result, the activation rate is more decreased and the effective mass becomes large concerning holes in the p-type layer in the deep ultraviolet light emitting device; thus it is considered that the overflow of electrons is easier to occur.
Therefore, an object of the present invention is to solve the problem as above with nitride semiconductor light emitting devices whose emission wavelength is from 200 to 300 nm, and to provide nitride semiconductor deep ultraviolet light emitting devices of the high light emission efficiency.
The inventor has intensively done studies to solve the above problem. Specifically, the inventor has studied the relationship among band gaps of layers in detail. Thus, the inventor has found that it is possible to effectively improve the light emission efficiency of a nitride semiconductor deep ultraviolet light emitting device with the arrangement of an electron blocking layer having a band gap larger than those of layers composing an active layer and a p-type layer, and at least one first p-type layer having a band gap larger than that of a layer which has the smallest band gap in an n-type layer (hereinafter this layer may referred to as “first n-type layer”), and has completed the present invention.
The first aspect of the present invention is:
[1] a nitride semiconductor light emitting device having emission wavelength of 200 to 300 nm, including
an n-type layer consisting of a single layer or a plurality of layers having different band gaps,
a p-type layer consisting of a single layer or a plurality of layers having different band gaps,
an active layer arranged between the n-type layer and the p-type layer, and
an electron blocking layer having a band gap larger than any band gap of layers composing the active layer and the p-type layer,
wherein the p-type layer includes a first p-type layer having a band gap larger than a band gap of a first n-type layer which has a smallest band gap in the n-type layer, and
the electron blocking layer is arranged between the active layer and the first p-type layer
[2] In the first aspect of the present invention, the p-type layer may consist of the plurality of layers having different band gaps.
[3] In the first aspect of the present invention, it is preferable that the active layer includes a well layer and a barrier layer; the p-type layer includes a p-type cladding layer and a p-type contacting layer; the nitride semiconductor light emitting device includes a stacked structure in which the n-type layer, the active layer, the electron blocking layer, the p-type cladding layer, and the p-type contacting layer are stacked in the order mentioned; the barrier layer is represented by a composition formula AlaGa1-aN (0.34≦a≦0.89); the p-type cladding layer is represented by a composition formula AlbGa1-bN (0.44<b<1.00); and a difference (b-a) between the Al composition of the p-type cladding layer and the Al composition of the barrier layer is greater than 0.10 and no more than 0.45.
It is noted that the above p-type cladding layer is preferably the first p-type layer.
[4] In the first aspect of the present invention of the embodiment as the above [3], it is preferable that the well layer is represented by a composition formula AlcGa1-cN (0.33≦e≦0.87); and a difference (a-e) between the Al composition of the barrier layer and the Al composition of the well layer is no less than 0.02
[5] In the first aspect of the present invention of the embodiments as the above [3] to [4], the well layer preferably has a thickness of 4 to 20 nm.
[6] In the first aspect of the present invention of the embodiments as the above [3] to [5], it is preferable that the electron blocking layer is p-type or i-type; the electron blocking layer is represented by a composition formula AlcGa1-cN (0.45≦c≦1.00); the p-type cladding layer is represented by a composition formula AlbGa1-bN (0.44<b<1.00); the Al composition (c) of the electron blocking layer is greater than the Al composition (b) of the p-type cladding layer; a difference (c-a) between the Al composition of the electron blocking layer and the Al composition of the barrier layer is 0.11 to 0.98; and a difference (b-a) between the Al composition of the p-type cladding layer and the Al composition of the barrier layer is greater than 0.10 and no more than 0.45
[7] The first aspect of the present invention of the embodiments as the above [3] to [6] preferably includes a plurality of the barrier layers; and the plurality of barrier layers includes a first barrier layer contacting the n-type layer, and a second barrier layer contacting the electron blocking layer.
[8] The second aspect of the present invention is a nitride semiconductor wafer including stacked structures of the nitride semiconductor light emitting device according to the first aspect of the present invention.
According to the present invention, electrons in nitride semiconductor deep ultraviolet light emitting devices whose emission wavelength is 300 nm or less can be prevented from overflowing, and thus it is possible to improve the light emission efficiency of the nitride semiconductor deep ultraviolet light emitting devices.
First, the basic summary of a nitride semiconductor light emitting device will be described.
In the present invention, a nitride semiconductor light emitting device having the emission wavelength from 200 to 300 nm (hereinafter may be simply shortened to “deep ultraviolet light emitting device”) can be manufactured by, for example, metalorganic chemical vapor deposition (MOCVD). Specifically, the nitride semiconductor light emitting device can be manufactured with an apparatus on the market, by supplying a group III raw material gas, for example, an organo-metallic gas such as trimethylaluminium, and a nitrogen source gas, for example, a raw material gas such as an ammonia gas to the top of a single-crystalline substrate described later or on the top of a substrate of a stack. Conditions of known methods can be employed for those for manufacturing the nitride semiconductor light emitting device by MOCVD. The nitride semiconductor light emitting device of the present invention can be manufactured by a method other than MOCVD as well.
In the present invention, the nitride semiconductor light emitting device is not specifically limited as long as its emission wavelength is from 200 to 300 nm. Specifically, the composition of each layer may be determined from compositions including nitrogen and at least one selected from boron, aluminum, indium and gallium and represented by a general formula: BXAlYInZGa1-x-y-zN (0≦x≦1, 0<y≦1, 0≦z<1 and 0<x+y+z≦1), to compose the nitride semiconductor light emitting device whose emission wavelength is from 200 to 300 nm. More specifically, for example, if an active layer is composed by the composition represented by AlaGa1-aN, the composition of 0.2≦a≦1 is necessary.
Generally, it is likely that the more the proportion of B and/or Al increases, the larger band gaps are, and the more the proportion of In and/or Ga increases, the smaller band gaps are. Thus, the band gap of each layer can be controlled by the proportion of these constituent elements. The proportion of the constituent elements can be obtained, by measuring the manufactured nitride semiconductor light emitting device with an SIMS (Secondary Ion-microprobe Mass Spectrometer), by TEM-EDX (Transmission Electron Microscope-Energy Dispersive X-ray spectrometry), by three dimensional atom prove (3DAP) or the like. The proportion of the constituent elements of each layer can be converted into the band gap. The band gap of each layer can be directly obtained by analyzing the nitride semiconductor light emitting device using cathodeluminescence spectroscopy (CL) or photo luminescence (PL) as well. In a case where the constituent elements are Al, Ga and N, the Al-composition can be specified using a conversion formula by the value of the band gap.
In Examples and Comparative Examples of the present invention, the proportion of constituent elements of each layer was measured by X-ray diffraction (XRD), and band gaps were obtained by PL. When the technical scope of the invention disclosed in this application is judged, values measured by XRD shall be employed for the composition of each layer, and values determined by PL shall be employed for the band gap of each layer unless there are specific circumstances.
The nitride semiconductor light emitting device according to one aspect of the present invention will be described using drawings in detail.
As depicted in
The nitride semiconductor light emitting device 100 further includes an electrode for n-type 60 that is arranged on an exposed surface of the n-type layer 20 that is exposed by etching removal of the second p-type layer 52, the first p-type layer 51, the electron blocking layer 40, the active layer 30 and a part of the n-type layer 20, and an electrode for p-type 70 that is arranged on the second p-type layer 52. The electrode for n-type 60 and the electrode for p-type 70 can be formed by a known method. Each layer will be described in detail hereinafter.
(Substrate 10)
A known substrate manufactured by a known method can be employed as the substrate 10 without any specific limitation. Concrete examples of substrates that can be employed as the substrate 10 include an AlN substrate, a GaN substrate, a sapphire substrate, an SiC substrate and an Si substrate. Among them, an AlN substrate where a c-plane is a growth surface, or a sapphire substrate where a c-plane is a growth surface is preferable. The thickness of the substrate 10 is not specifically limited. However, the range of 0.1 mm to 2 mm is preferable.
(n-type Layer 20)
The n-type layer 20 is a layer where an n-type dopant is doped. In the deep ultraviolet light emitting device 100 of
When the n-type layer 20 is a single layer, the band gap of the n-type layer 20 is not specifically limited as long as its band gap is smaller than that of the first p-type layer, which is described below. Though, it is preferable that the value of the band gap of the n-type layer 20 is in the range of 4.15 eV to 6.27 eV in order to improve the productivity and to enhance the use of the nitride semiconductor light emitting device whose emission wavelength is from 200 to 300 nm. It is more preferable that its range is from 4.20 eV to 6.25 eV. It is specifically preferable that its range is from 4.50 eV to 5.50 eV.
Examples of preferable composition of the n-type layer 20 include the composition where Al composition (d) is 0.34 to 1.00 when the n-type layer 20 is represented by the composition formula, AldGa1-dN. When the n-type layer 20 is represented by the composition formula, AldGa1-dN, the Al composition (d) is more preferably from 0.34 to 0.90; further preferably from 0.34 to 0.80; and most preferably from 0.45 to 0.70. It is also preferable that the n-type layer 20 is composed by a single crystal.
The film thickness of the n-type layer 20 is not specifically limited. It can be in the range of 1 nm to 50 μm.
While the n-type layer 20 included in the nitride semiconductor light emitting device 100 of
(Active Layer 30)
The active layer 30 has a quantum well structure including at least one well layer and at least one barrier layer (hereinafter may simply referred to as “quantum well structure”). In the energy band diagram of
Band gaps of layers composing the active layer are not specifically limited as long as those are smaller than that of the electron blocking layer. When the active layer has the quantum well structure including at least one well layer and at least one barrier layer, generally, the barrier layer has a band gap larger than the well layer in the active layer. Thus, in a case where the band gap of a barrier layer that has the largest band gap in the active layer is smaller than that of the electron blocking layer, the band gaps of layers comprising the active layer are not limited. The band gaps of the well layers can be properly determined depending on other layers, and the range of 4.13 eV to 6.00 eV is preferable, the range of 4.18 eV to 5.98 eV is more preferable, and the range of 4.20 eV to 5.00 eV is further preferable. The band gaps of the barrier layers are not specifically limited as well, and the range of 4.15 eV to 6.02 eV is preferable, the range of 4.20 eV to 6.00 eV is more preferable, and the range of 4.30 eV to 5.50 eV is specifically preferable.
It is preferable that the thickness of each well layer and barrier layer is from 1 to 50 nm.
(Barrier Layers 30b, 31b, 32b, 33b and 34b)
Each barrier layer may be composed by a single crystal having the composition represented by the composition formula, AlaGa1-aN (0.34≦a≦1.00), and preferably, is composed by a single crystal having the composition represented by the composition formula, AlaGa1-aN (0.34≦a≦0.89). As described later, in a case where the first p-type layer (p-type cladding layer) 51 is formed by a single crystal represented by the composition formula AlbGa1-bN (0.44<b<1.00), it is preferable that difference between the Al composition of the first p-type layer (p-type cladding layer) 51 and that of each barrier layer (b-a) is greater than 0.10 and no more than 0.45. In this case, in view of improving the productivity and also further improving the light emission efficiency, it is more preferable that the Al composition of each barrier layer (a) is 0.34≦a≦0.80 and the difference in Al composition (b-a) is no less than 0.12 and no more than 0.45; and it is specifically preferable that the Al composition of each barrier layer (a) is 0.40≦a≦0.70 and the difference in Al composition (b-a) is no less than 0.12 and no more than 0.45.
In a case where a plurality of the barrier layers are included in the active layer like the barrier layers 30b, 31b, 32b, 33b and 34b in
(Well Layers 30a, 31a, 32a and 33a)
It is preferable that each well layer is composed by a single crystal having the composition represented by the composition formula, AleGa1-eN (0.33≦e≦0.87). Each well layer is composed so as to have a band gap smaller than the barrier layers. Thus, in a case where each of the well layers and the barrier layers is composed by a single crystal of AlGaN, each well layer is composed by a single crystal of AlGaN having the Al composition lower than that of each barrier layer.
In a case where each well layer is composed by a single crystal represented by the composition formula, AleGa1-eN and each barrier layer is composed by a single crystal represented by the composition formula, AlaGa1-aN, it is preferable that difference between the Al composition of each barrier layer (a) and that of each well layer (e) (a-e) is no less than 0.02. The upper limit of the difference (a-e) is not specifically determined. However, the difference (a-e) is preferably no more than 0.87.
An absolute value of the Al composition of each well layer (e) when each well layer is composed by a single crystal represented by the composition formula, AleGa1-eN may be determined depending on band gaps of other layers. This absolute value has only to satisfy the composition formula AleGa1-eN (0.33≦e>1.00), preferably satisfies the composition formula AleGa1-eN (0.33≦e≦0.87), more preferably satisfies the composition formula, AlcGa1-cN (0.33≦e≦0.78), and specifically preferably satisfies the composition formula, AlcGa1-cN (0.33≦e≦0.68).
The thickness of each well layer is preferably from 4 nm to 20 nm. When the difference between the Al composition of the p-type cladding layer and that of each barrier layer (a-e) is within the above preferable range, the light emission efficiency is more improved by the thickness of each well layer being from 4 nm to 20 nm. The efficiency of hole injection is improved by a comparatively thick well layer, that is, no less than 4 nm, which makes it possible to reduce the overflow of carriers in a high current injection region. On the other hand, while internal fields in the active layer functions as spatially decoupling wave functions of electrons and holes that are confined in the well layer from each other, to make the efficiency of the recombination low, the internal fields can be screened enough by injection careers because the thickness of each well layer is no more than 20 nm. Thus, it is possible to suppress spatial decoupling between the wave function of electrons and that of holes, to improve the probability of the recombination. In more detail, not a rectangular potential but a triangular potential is formed on a heterointerface, where the composition is different, of the nitride semiconductor, specifically the nitride semiconductor light emitting device represented by AlGaN because of the effect of spontaneous polarization. Therefore, in a case where a quantum well layer is composed, injected electrons and holes are unevenly distributed to opposite interfaces to each other because of the quantum confined stark effect (hereinafter simply referred to as “QCSE”) due to the internal fields, and thus are spatially decoupled. As a result, the probability of the recombination of electrons and holes are decreased, and therefore, the internal quantum efficiency is decreased. On the contrary, since the internal fields can be screened enough by injection careers because the thickness of each well layer is no more than 20 nm, QCSE can be suppressed. In view of the same, the thickness of each well layer is preferably from 4 nm to 18 nm, and more preferably, from 4 nm to 15 nm.
In a case where a plurality of the well layers are included in the active layer like the well layers 30a, 31a, 32a and 33a in
(Structure of Active Layer 30)
As depicted in
Either a p-type or an n-type dopant may be doped into the barrier layers 30b to 34b. In a case where a p-type dopant is doped into the barrier layers 30b to 34b, the effect of suppressing the overflow of carriers and the effect of decreasing QCSE can be improved. In a case where an n-type dopant is doped into the barrier layers 30b to 34b, the effect of decreasing QCSE can be improved.
(Electron Blocking Layer 40)
The electron blocking layer 40 is a layer for suppressing leakage of part of electrons that are injected from the n-type layer to the active layer by applying an electric field, into the p-type layer side. Therefore, it is necessary for the electron blocking layer 40 to have a band gap larger than those of any layers that compose the active layer 30 and the p-type layer 50, which is described later. It is also necessary for the electron blocking layer 40 to be formed between the active layer 30 and the first p-type layer (p-type cladding layer) 51, which is described later.
The band gap of the electron blocking layer 40 is larger than those of any layers constituting the active layer 30, and also larger than those of any layers constituting the p-type layer 50.
The band gap of the electron blocking layer 40 is not specifically limited as long as being larger than those of any layers constituting the active layer 30 and the p-type layer 50. The band gap of the electron blocking layer 40 is preferably larger than that of a barrier layer having the largest band gap in the active layer 30 by no less than 0.03 eV, more preferably by no less than 0.05 eV, and specifically preferably by no less than 0.20 eV. The upper limit of the difference between the band gap of the electron blocking layer 40 and the largest band gap of the active layer 30 is not specifically determined. In view of the productivity, no less than 2.15 eV is preferable. It is preferable that the band gap of the electron blocking layer 40 is larger than that of a layer having the largest band gap in layers constituting the p-type layer 50 (first p-type layer (p-type cladding)51) by no less than 0.02 eV, more preferably no less than 0.04 eV, and specifically preferably no less than 0.10 eV. The upper limit of the difference between the band gap of the electron blocking layer 40 and the largest band gap in the layers constituting the p-type layer 50 is not specifically determined. In view of the productivity, the difference is preferably no more than 2.14 eV, more preferably no more than 2.09 eV, and specifically preferably no more than 1.20 eV.
An absolute value of the band gap of the electron blocking layer 40 is not specifically limited. However, the absolute value is preferably no less than 4.18 eV and no more than 6.30 eV, more preferably no less than 4.25 eV and no more than 6.30 eV, and specifically preferably no less than 4.70 eV and no more than 6.30 eV.
It is preferable that the electron blocking layer 40 is composed by a single crystal of AlGaN, In a case where each of the active layer 30, the p-type layer 50 and the electron blocking layer 40 are composed by a single crystal of AlGaN, it is preferable that the electron blocking layer 40 is composed by a single crystal of AlGaN whose Al composition proportion is higher than any other layers constituting the active layer 30 and the p-type layer 50. In a case where the n-type layer 20 is composed by a single crystal of AlGaN, it is preferable that the electron blocking layer 40 is composed by a single crystal of AlGaN whose Al composition is higher than that of the n-type layer 20, while the electron blocking layer 40 may be composed by a single crystal of AlGaN whose Al composition is lower than that of the n-type layer 20. That is, it is preferable that the electron blocking layer 40 is composed by a single crystal of AlGaN whose Al composition is higher than any other layers.
When the electron blocking layer 40 is represented by the composition formula, AlcGa1-cN, the Al composition of the electron blocking layer 40 (c) is preferably 0.45≦c≦1.00, and specifically preferably, 0.53≦c≦1.00. In view of further improving the effect of the present invention, it is preferable that difference between the Al composition of the electron blocking layer 40 (c) and that of each barrier layer (a) (c-a) is from 0.11 to 0.98, more preferably from 0.13 to 0.80, and further preferably from 0.13 to 0.60.
As described below, in a case where the first p-type layer (p-type cladding layer) 51 is composed by a single crystal represented by the composition formula, AlbGa1-bN (0.44<b<1.00), it is preferable that the Al composition of the electron blocking layer 40 (c) is higher than that of the first p-type layer (p-type cladding layer) 51 (b). Specifically, it is preferable that difference between the electron blocking layer 40 and the first p-type layer (p-type cladding layer) 51 in Al composition (c-b) is greater than 0.00 and no more than 0.88, more preferably, greater than 0.00 and no more than 0.80, and further preferably no less than 0.01 and no more than 0.70.
A p-type dopant may be doped into the electron blocking layer 40, or the electron blocking layer 40 may be an undoped layer. In a case where a p-type dopant, for example, Mg is doped into the electron blocking layer 40, the impurity concentration is preferably from 1×1016 to 1×1021 (cm−3). Furthermore, both an area where a p-type dopant is doped and an undoped area may exist in the electron blocking layer 40. If the electron blocking layer has both the doped area and the undoped area, the impurity concentration of whole of the electron blocking layer 40 is preferably from 1×1016 to 1×1021 (cm−3).
The thickness of the electron blocking layer 40 is not specifically limited. It is preferably from 1 nm to 50 μm.
(p-type Layer 50)
The nitride semiconductor light emitting device 100 has, in the p-type layer 50, the first p-type layer (p-type cladding layer) 51 whose band gap is larger than that of the first n-type layer (20), which has the smallest band gap in the n-type layer 20, together with the electron blocking layer 40.
In the nitride semiconductor light emitting device 100 of
(First p-type Layer (p-type Cladding Layer) 51)
Arranged in the nitride semiconductor light emitting device 100 are the first p-type layer (p-type cladding layer) 51 having a band gap larger than that of the first n-type layer (n-type layer 20 in
Difference between the n-type layer 20 (first n-type layer) and the first p-type layer (p-type cladding layer) 51 in band gap is not specifically limited. The first p-type layer (p-type cladding layer) has preferably a band gap larger than that of the first n-type layer 20 by no less than 0.01 eV, and more preferably, by no less than 0.10 eV. The upper limit of the difference between the n-type layer 20 (first n-type layer) and the first p-type layer (p-type cladding layer) 51 in band gap is not specifically limited. In view of the productivity, this difference is preferably no more than 1.50 eV, more preferably, no more than 1.00 eV, and specifically preferably, no more than 0.50 eV.
An absolute value of the band gap of the first p-type layer 51 is not specifically limited as long as being larger than that of the first n-type layer 20. However, this absolute value is preferably no less than 4.16 eV and no more than 6.28, more preferably no less than 4.21 eV and no more than 6.26 eV, and specifically preferably no less than 4.60 eV and no more than 5.60 eV.
It is preferable that the first p-type layer (p-type cladding layer) 51 is composed by a single crystal represented by the composition formula, AlbGa1-bN (0.44<b<1.00). Its Al composition (b) is preferably no less than 0.52 and no more than 0.99. It is preferable that the difference between the Al composition of the first p-type layer (p-type cladding layer) (b) and that of each barrier layer (b-a) is greater than 0.10 and no more than 0.45, and more preferably, no less than 0.12 and no more than 0.45 as described above.
The thickness of the first p-type layer (p-type cladding layer) 51 is not specifically limited. It is preferably from 1 nm to 1 μm.
(Second p-type Layer (p-type Contacting Layer) 52)
In the present invention, the p-type layer 50 may be a single layer (in this case, the p-type layer 50 is the first p-type layer (p-type cladding layer)). However, composition of the second p-type layer (p-type contacting layer) 52 makes it easy to realize ohmic contact with the electrode for p-type 70, and to reduce the contact resistance against the electrode for p-type 70.
The second p-type layer (p-type contacting layer) 52 has a band gap smaller than that of the first p-type layer (p-type cladding layer) 51. Specifically, it is preferable that the band gap of the second p-type layer (p-type contacting layer) 52 takes a smaller value than that of the first p-type layer (p-type cladding layer) 51; and its absolute value is preferably no less than 0.70 eV and no more than 6.00 eV, and more preferably the absolute value is no less than 3.00 eV and no more than 4.50 eV. Typical examples include an embodiment of the second p-type layer (p-type contacting layer) 52 composed by GaN (band gap: 3.4 eV).
The second p-type layer (p-type contacting layer) 52 is preferably composed by a single crystal of AlGaN. In a case where each first p-type layer (p-type cladding layer) 51 and second p-type layer (p-type contacting layer) 52 is composed by a single crystal of AlGaN, the second p-type layer (p-type contacting layer) 52 is preferably lower than the first p-type layer (p-type cladding layer) 51 in Al composition. In a case where the second p-type layer (p-type contacting layer) 52 is composed by a single crystal represented by the composition formula, AlfGa1-fN, its Al composition (f) may be from 0.00 to 1.00, preferably from 0.00 to 0.70, and more preferably from 0.00 to 0.40. When the second p-type layer (p-type contacting layer) 52 is composed by GaN as the above typical example, f=0.00. The second p-type layer (p-type contacting layer) 52 may include In as long as any effect of the present invention is not blocked.
The thickness of the second p-type layer (p-type cladding layer) 52 is preferably from 1 nm to 250 nm.
While mainly given as an example in the above description concerning the present invention is the nitride semiconductor light emitting device 100 wherein the active layer 30 has a quantum well structure and four well layers are included therein, the present invention is not limited to this embodiment. When the active layer has a quantum well structure in the nitride semiconductor light emitting device of the present invention, the number of the well layers may be either one or plural. While the upper limit of the number of the well layers is not specifically limited, preferably no more than 10 in view of the productivity of the nitride semiconductor light emitting device. The nitride semiconductor light emitting device can take such an embodiment that the active layer therein does not have a quantum well structure but has a bulk structure (double heterostructure). In a case where the active layer 30 has a bulk structure, the thickness of the active layer 30 is preferably from 20 to 100 nm.
While mainly given as an example in the above description concerning the present invention is the nitride semiconductor light emitting device 100 wherein the active layer 30 has the quantum well structure including the well layers 30a to 33a and the barrier layers 30b to 34b; a layer contacting the n-type layer 20 is the barrier layer 30b; and a layer contacting the electron blocking layer 40 is the barrier layer 34b, the present invention is not limited to this embodiment. The nitride semiconductor light emitting device can take such an embodiment as to have a first well layer that contacts the n-type layer and a second well layer that contacts the electron blocking layer.
The nitride semiconductor light emitting device can take such an embodiment that a layer contacting the n-type layer is a barrier layer and a layer contacting the electron blocking layer is a well layer.
The nitride semiconductor light emitting device can take such an embodiment that a layer contacting the n-type layer is a well layer and a layer contacting the electron blocking layer is a barrier layer.
While mainly given as an example in the above description concerning the nitride semiconductor light emitting device of the present invention is the nitride semiconductor fight emitting device 100 wherein the active layer 30 and the electron blocking layer 40 are in contact with each other directly, the present invention is not limited to this embodiment. The nitride semiconductor light emitting device can take such an embodiment that a third p-type layer is arranged between the active layer and the electron blocking layer.
Arrangement of the third p-type layer 53 makes it possible to suppress impurities (dopants) from diffusing from the other p-type layers (first p-type layer (p-type cladding layer) 51 and second p-type layer (p-type contacting layer) 52) to the active layer 30, specifically to the well layer 33a, which is the nearest well layer to the p-type layer. Thus, the quality of the active layer 30 can be improved.
The third p-type layer 53 may be either a layer into which a p-type dopant is doped when composed as well as the other p-type layers, or such a layer that: after an undoped layer is once composed, this undoped layer gets the p-type conductivity by diffusion of dopants of other p-type layers. This third p-type layer 53 is composed over the active layer 30, and the electron blocking layer 40 is composed thereover.
It is preferable that the band gap of the third p-type layer 53 is the same as that of the active layer 30, specifically those of the barrier layers 30b to 34d. The thickness of the third p-type layer 53 is preferably no less than 1 nm and no more than 50 nm.
While mainly given as an example in the above description concerning the present invention is the nitride semiconductor light emitting devices 100 and 200 wherein the n-type layer is a single layer, that is, the n-type layer 20 is the first n-type layer having the smallest band gap in the n-type layer, the present invention is not limited to this embodiment. The nitride semiconductor light emitting device can take an embodiment of having the n-type layer that consists of plural layers. The nitride semiconductor light emitting device of the present invention according to such other embodiments will be described below.
The n-type underlayer 20A is a layer for easing lattice mismatch, interface roughening, and the like between the substrate 10 and growth layers (in
The n-type cladding layer 20B is a layer that plays the same role as the n-type layer in the embodiment where the n-type layer is a single layer (for example, the n-type layers 20 in the nitride semiconductor light emitting devices 100 and 200 as above), and is a layer for supplying electrons to the active layer 30 along with the n-type underlayer 20A.
These n-type underlayer 20A and n-type cladding layer 20B preferably include Si as a dopant so that the impurity concentration is from 1×1016 to 1×1021 (cm−3), in order to be made to be n-type layers. These impurities (dopant) may be distributed among the n-type underlayer 20A and the n-type cladding layer 20B either homogeneously or unhomogeneously. Part of the side of the n-type underlayer 20A, which contacts the substrate 10, may be undoped.
It is not specifically limited but, for example, in a case where the substrate 10 is a sapphire substrate or on an AlN substrate, as depicted in
It is also not specifically limited but, for example, in a case where the band gap of the n-type underlayer 20A is larger than that of the n-type cladding layer 20B (see FIG. 9(A)), difference between the n-type underlayer 20A and the n-type cladding layer 20B in band gap is preferably no less than 0.025 eV and no more than 2.00 eV. On the other hand, in a case where the band gap of the n-type underlayer 20A is smaller than that of the n-type cladding layer 20B (see FIG. 9(B)), difference between the n-type underlayer 20A and the n-type cladding layer 20B in band gap is preferably no less than 0.025 eV and no more than 2.00 eV. An absolute value of the band gap of the n-type underlayer 20A is preferably no less than 3.4 eV and no more than 6.30 eV. An absolute value of the band gap of the n-type cladding layer 20B is preferably no less than 4.15 eV and no more than 6.27 eV. The preferable ranges of the absolute values of the band gaps of these n-type underlayer 20A and n-type cladding layer 20B are same as is in the case where other n-type layers are further composed.
The thickness of the n-type underlayer 20A is preferably no less than 1 nm and no more than 50 μm. The thickness of the n-type cladding layer 20B is preferably no less than 1 nm and no more than 50 μm.
While in the above description, given as an example is the nitride semiconductor light emitting device 300 that has the embodiment of having the first p-type layer (p-type cladding layer) 51 and the second p-type layer (p-type contacting layer) 52, the present invention is not limited to this embodiment. The nitride semiconductor light emitting device can take such an embodiment that the third p-type layer is further arranged between the active layer and the electron blocking layer like the nitride semiconductor light emitting device 200, which is given as an example already.
The n-type hole blocking layer 20C is a layer for suppressing a part of holes that are injected from the p-type layer into the active layer due to application of an electric field from leaking into the n-type layer side. The n-type cladding layer 20B is a layer that plays the function same as the n-type layer in the embodiment of the n-type layer of a single layer (for example, the n-type layers 20 in the above nitride semiconductor light emitting devices 100 and 200), and is a layer for supplying electrons to the active layer 30. It is preferable that these n-type cladding layer 20B and n-type hole blocking layer 20C include, for example, Si as a dopant so that the impurity concentration is from 1×1016 to 1×1021 (cm−3), in order to be made to be n-type layers. These impurities may be distributed among the n-type cladding layer 20B and the n-type hole blocking layer 20C either homogeneously or unhomogeneously.
It is not specifically limited but as depicted in
The thickness of the n-type cladding layer 20B is preferably no less than 1 nm and no more than 50 μm. The thickness of the n-type hole blocking layer 20C is preferably no less than 1 nm and no more than 1 μm.
While in the above description, given as an example is the nitride semiconductor light emitting device 400 that has the embodiment of including the first p-type layer (p-type cladding layer) 51 and the second p-type layer (p-type contacting layer) 52, the present invention is not limited to this embodiment. The nitride semiconductor light emitting device can take such an embodiment of further arranging the third p-type layer between the active layer and the electron blocking layer.
As to a semiconductor light emitting device necessary for current injection in the horizontal direction (in the direction of stacked planes in the stacked structure inside the device), generally, the distance required for carriers in the horizontal direction of the light emitting device is long enough compared to that in the depth direction of the light emitting device (in the direction of the normal line of the stacked planes in the stacked structure inside the device). Thus, drive voltage of the light emitting device increases as the resistance proportional to the distance required for careers in the horizontal direction increases. Therefore, the structure of using a two dimensional electron gas is generally utilized in order to improve the carrier conductivity in the horizontal direction of the device. Such a structure is called a current spreading layer. In the n-type current spreading layer 20D, Fermi level is at an upper side than the bottom end of the conduction band due to formation of a triangular potential. The n-type cladding layer 20B is a layer that plays a role of supplying electrons to the active layer. It is preferable that these n-type cladding layer 20B and n-type current spreading layer 20D include, for example, Si as a dopant so that the impurity concentration is from 1×1016 to 1×1021 (cm−3), in order to be made to be n-type layers. These impurities (dopant) may be distributed among the n-type cladding layer 20B and the n-type current spreading layer 20D either homogeneously or unhomogeneously.
The n-type current spreading layer 20D may have a larger band gap than the n-type cladding layer 20B as long as a two dimensional electron gas can be generated due to the formation of a triangular potential. FIG. 13(3) is a view to explain an energy band diagram in a case where the band gap of the n-type current spreading layer 20D is larger than that of the n-type cladding layer 20B. In this case, a layer of the smallest energy gap in the n-type layer (first n-type layer) is the n-type cladding layer 20B.
It is not specifically limited but difference between the n-type cladding layer 20B and the n-type current spreading layer 200 in band gap is preferably no less than 0.03 eV and no more than 2.00 eV. In a case where the absolute value of the band gap of the n-type current spreading layer 20D is smaller than that of the n-type cladding layer 20B (see FIG. 13(A)), the absolute value of the band gap of the n-type current spreading layer 20D is preferably no less than 4.15 eV and no more than 6.27 eV, and the band gap of the n-type cladding layer 20B is preferably no less than 4.18 eV and no more than 6.30 eV. In a case where the absolute value of the band gap of the n-type current spreading layer 20D is larger than that of the n-type cladding layer 20B (see FIG. 13(B)), the absolute value of the band gap of the n-type current spreading layer 20D is preferably no less than 4.18 eV and no more than 6.30 eV, and the band gap of the n-type cladding layer 20B is preferably no less than 4.15 eV and no more than 6.27 eV.
The thickness of the n-type cladding layer 20B is preferably no less than 1 nm and no more than 50 μm. The thickness of the n-type current spreading layer 200 is preferably no less than 1 nm and no more than 1 μm.
Given as an example in the above description mainly is the nitride semiconductor light emitting device 500 that has the embodiment where the n-type current spreading layer 20D is arranged independently from the n-type cladding layer 20B, and the active layer 30 is stacked while contacting the n-type current spreading layer 20D. However, the present invention is not limited to this embodiment. The nitride semiconductor light emitting device can take such an embodiment that the n-type current spreading layer is composed inside the n-type cladding layer, and thus, the active layer is not in contact with the n-type current spreading layer directly.
While in the above description, mainly given as an example is the nitride semiconductor light emitting device 500 that has the embodiment of including the first p-type layer (p-type cladding layer) 51 and the second p-type layer (p-type contacting layer) 52 as the p-type layer, the present invention is not limited to this embodiment. The nitride semiconductor light emitting device can take such an embodiment that the third p-type layer is further arranged between the active layer and the electron blocking layer.
(Other Combination in n-type Layer)
The nitride semiconductor light emitting devices 300, 400 and 500 having the embodiment of including the n-type layer consisting of combination of two layers, are given as examples in the above description concerning the nitride semiconductor light emitting device of the present invention, which has the embodiment of including the n-type layer consisting of a plurality of layers. However, the present invention is not limited to the embodiment. The nitride semiconductor light emitting device can take an embodiment of including the n-type layer consisting of other combination of a plurality of layers. For example, a plurality of layers composing the n-type layer can be at least two layers selected from the n-type underlayer, the n-type cladding layer, the n-type hole blocking layer and the n-type current spreading layer. The thickness of each layer is as described above. The order of stacking the given plurality of layers composing the n-type layer on the substrate as an example is preferably the following (layers enclosed by parentheses mean that such layers are not always necessary. The stacked order of the n-type hole blocking layer and the n-type current spreading layer is not specifically limited):
substrate/(n-type underlayer)/n-type cladding layer/(n-type hole blocking layer, n-type current spreading layer)
In a case where the plurality of layers composing the n-type layer is such a combination, a layer having the smallest band gap among the n-type underlayer, n-type cladding layer, n-type hole blocking layer and n-type current spreading layer corresponds to the first n-type layer.
Mainly given as an example in the above description concerning the present invention are the nitride semiconductor light emitting devices 100, 100′, 100″, 100′″, 200, 300, 400 and 500, which have the embodiment of having the p-type layer consisting of a plurality of layers of different band gaps. However, the present invention is not limited to the embodiment. The nitride semiconductor light emitting device can take an embodiment of the p-type layer of a single layer.
The second aspect of the present invention is a nitride semiconductor wafer that has the stacked structures described above concerning the nitride semiconductor light emitting device of the present invention. In the nitride semiconductor wafer of the present invention, generally, the stacked structures of the nitride semiconductor light emitting device of the present invention described above is formed. A plurality of the nitride semiconductor light emitting devices of the present invention can be obtained by cutting each device out of the nitride semiconductor wafer.
The present invention will be described in detail with Examples and Comparison Examples hereinafter. The present invention is not limited to the following examples.
In the following Examples and Comparative Examples, the proportion of constituent elements of each layer was measured by X-ray diffraction (XRD), and band gaps were obtained by photo luminescence (PL). X'Pert PRO manufactured by PANalytical B. V. was used for the XRD measurement, and HR800 UV manufactured by HORIBA, Ltd. was used for the measurement by PL. SMS-500 manufactured by SphereOptics GmbH was used for the measurement of the emission wavelength, and the wavelength of the strongest emission intensity was recorded as the emission wavelength. The external quantum efficiency was measured with the same apparatus as used for the measurement of the emission wavelength.
The nitride semiconductor light emitting device having a stacked structure depicted in
First, an Al0.75Ga0.25N layer where Si was doped (first n-type layer; band gap: 5.23 eV, Si concentration: 1×1019 cm−3, and thickness: 1.0 μm) was formed on a c-plane of the AlN substrate 10, which was 7 by 7 mm square and 500 μm in thickness, by MOCVD as the n-type layer (20).
The active layer (30) having a quantum well structure that included four quantum wells (see
An AlN layer where Mg was doped (band gap: 6.00 eV, Mg concentration: 5×1019 cm−3, and thickness: 30 nm) was formed over the active layer (30) (that is, over the barrier layer that was the outermost layer of the active layer) as the electron blocking layer (40).
An Al0.8Ga0.2N layer where Mg was doped (band gap: 5.38 eV; Mg concentration: 5×1019 cm−3, and thickness: 50 nm) was formed over the electron blocking layer (40) as the first p-type layer (p-type cladding layer) (51). A GaN layer where Mg was doped (band gap: 3.40 eV, Mg concentration: 2×1019 cm−3, and thickness 100 nm) was formed over the first p-type layer (p-type cladding layer) (51) as the second p-type layer (p-type contacting layer) (52).
Next, heat treatment was carried out in a nitrogen atmosphere for 20 minutes at 900° C. After that, a predetermined resist pattern was formed on the surface of the second p-type layer (p-type cladding layer) (52) by photolithography, and etching was carried out on a window that is a part where the resist patter was not formed by reactive ion etching until the surface of the n-type layer (20) was exposed. Then, a Ti(20 nm)/Al(200 nm)/Au(5 nm) electrode (anode) was formed on the surface of the n-type layer (20) by evaporation, and heat treatment was carried out in a nitrogen atmosphere for 1 minute at 810° C. Next, an Ni(20 nm)/Au(50 nm) electrode (cathode) was formed on the surface of the second p-type layer (p-type contacting layer) (52) by evaporation, and after that, heat treatment was carried out in an oxygen atmosphere for 3 minutes at 550° C., to manufacture the nitride semiconductor light emitting device.
The obtained nitride semiconductor light emitting device had the emission wavelength of 267 nm when the current injection was 10 mA, and its external quantum efficiency was 2.2%.
The nitride semiconductor light emitting device was manufactured with the same operation as Example 1 except that in Example 1, the first p-type layer (p-type cladding layer) (51) was changed to have the composition Al0.75Ga0.25N and the band gap of 5.23 eV (Mg concentration: 5×1019 cm−3).
The obtained nitride semiconductor device had the emission wavelength of 267 nm when the current injection was 10 mA, and its external quantum efficiency was 1.7%.
The nitride semiconductor light emitting device was manufactured with the same operation as Example 1 except that in Example 1, the first p-type layer (p-type cladding layer) 51 was changed to have the composition Al0.7Ga0.3N and the band gap of 5.09 eV (Mg concentration: 5×1019 cm−3).
The obtained nitride semiconductor light emitting device had the emission wavelength of 267 nm when the current injection was 10 mA, and its external quantum efficiency was 1.3%.
A wafer including a plurality of the nitride semiconductor light emitting devices of stacked structures depicted in
First, an Al0.75Ga0.25N layer of 1.0 μm in thickness where Si was doped (band gap: 5.23 eV, Si concentration: 1×1019 cm−3) was formed on a c-plane of the AlN substrate (10), which was 7 by 7 mm square and 500 μm in thickness, by MOCVD as the n-type layer (20).
The active layer (30) having a quantum well structure that included three quantum wells was formed over the n-type layer (20) by composing four barrier layers where Si was doped (composition: Al0.75Ga0.25N, band gap: 5.23 eV, Si concentration: 1×1018 cm−3, and thickness: 7 nm) and three well layers (composition: Al0.5Ga. 5N, band gap: 4.55 eV, undoped, and thickness: 2 nm) so that the barrier layers and the well layers were stacked by turns. One of the barrier layers was formed so as to contact the n-type layer (20) and another barrier layer was formed as the outermost layer.
An AlN layer where Mg was doped (band gap: 6.00 eV, Mg concentration: 5×1019 cm−3, and thickness: 15 nm) was formed over the active layer (30) (that is, over the barrier layer that was the outermost layer of the active layer) as the electron blocking layer (40).
An Al0.80Ga0.20N layer where Mg was doped (band gap: 5.38 eV; Mg concentration: 5×1019 cm−3, and thickness: 50 nm) was formed over the electron blocking layer (40) as the first p-type layer (p-type cladding layer) (51). A GaN layer where Mg was doped (band gap: 3.40 eV, Mg concentration: 2×1019 cm−3, and thickness 100 nm) was formed over the first p-type layer (p-type cladding layer) (51) as the second p-type layer (p-type contacting layer) (52).
Next, heat treatment was carried out in a nitrogen atmosphere for 20 minutes at 900° C.. After that, a predetermined resist pattern was formed on the surface of the second p-type layer (p-type contacting layer) (52) by photolithography, and etching was carried out on a window that is a part where the resist patter was not formed by reactive ion etching until the surface of the n-type layer (20) was exposed. Then, a Ti(20 nm)/Al(200 nm)/Au(5 nm) electrode (anode) was formed on the surface of the n-type layer (20) by evaporation, and heat treatment was carried out in a nitrogen atmosphere for 1 minute at 810° C. Next, an Ni(20 nm)/Au(50 nm) electrode (cathode) was formed on the surface of the second p-type layer (p-type contacting layer) (52) by evaporation, and after that, heat treatment was carried out in an oxygen atmosphere for 3 minutes at 550° C., to manufacture the nitride semiconductor wafer having the above stacked structure. The nitride semiconductor light emitting device was manufactured by cutting the obtained nitride semiconductor wafer into pieces 700 by 700 μm square.
The obtained nitride semiconductor device had the emission wavelength of 272 nm when the current injection was 100 mA, and its external quantum efficiency was 2.0%.
The nitride semiconductor wafer and nitride semiconductor light emitting device were manufactured with the same operation as Example 2 except that in Example 2, each barrier layer was changed to have the composition Al0.65Ga0.35N (band gap: 4.95 eV, Si concentration: 1×1018 cm−3). The obtained nitride semiconductor light emitting device had the emission wavelength of 267 inn when the current injection was 100 mA, and its external quantum efficiency was 2.3%.
The nitride semiconductor wafer and nitride semiconductor light emitting device were manufactured with the same operation as Example 3 except that in Example 3, the thickness of each well layer was changed from 2 nm to 4 nm. The obtained nitride semiconductor light emitting device had the emission wavelength of 270 nm when the current injection was 100 mA, and its external quantum efficiency was 2.7%.
The nitride semiconductor wafer and nitride semiconductor light emitting device were manufactured with the same operation as Example 3 except that in Example 2, each barrier layer was changed to have the composition Al0.60Ga0.40N (band gap: 4.81 eV, Si concentration: 1×1018 cm−3) and the thickness of each well layer was changed from 2 nm to 6 nm. The obtained nitride semiconductor light emitting device had the emission wavelength of 263 nm when the current injection was 100 mA, and its external quantum efficiency was 32%.
<Evaluation Result>
The compositions and evaluation results of Examples 1 to 5 and Comparative Examples 1 to 2 are represented in Table 1.
Ga
N
Ga
N
Ga
N
Ga
N
Ga
N
Ga
N
indicates data missing or illegible when filed
The nitride semiconductor devices of Examples 1 to 5 presented good light emission efficiency compared with the nitride semiconductor light emitting device of Comparative Examples 1 to 2, where the p-type layer of a larger band gap than the smallest band gap in the n-type layer was not included in the side of the electron blocking layer opposite to the active layer. Every nitride semiconductor devices of Examples 1 to 5 had a stacked structure where the n-type layer, the active layer having the well layer and the barrier layer, the electron blocking layer, the first p-type layer (p-type cladding layer) and the second p-type layer (p-type contacting layer) were stacked in this order, the barrier layer was represented by the composition formula AlaGa1-aN (0.34≦a≦0.89), and the first p-type layer (p-type cladding layer) was represented by the composition formula AlbGa1-bN (0.44<b<1.00). Among them, the nitride semiconductor light emitting devices of Examples 3 to 5 where the difference between the Al composition of the first p-type layer (p-type cladding layer) and that of each barrier layer (b-a) was over 0.10 and no more than 0.45 presented the light emission efficiency superior to the nitride semiconductor light emitting devices of Examples 1 to 2, where the difference was not as the above. Moreover, the nitride semiconductor light emitting devices of Examples 4 to 5, where the thickness of each well layer was in the range of 4 to 20 nm presented an specifically superior light emission efficiency.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-020723 | Feb 2013 | JP | national |
| 2013-105182 | May 2013 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2014/052474 | 2/2/2014 | WO | 00 |
