JP4284103B2 - Oxide semiconductor light emitting device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a semiconductor light emitting device such as a light emitting diode or a semiconductor laser, and more particularly to an oxide semiconductor light emitting device excellent in light emission characteristics, reliability, and power saving.
[0002]
[Prior art]
  As an active layer of a semiconductor light emitting device such as a semiconductor laser or a light emitting diode, a quantum well structure constituted by alternately stacking quantum well layers and potential barrier layers is often used. Since the quantum well active layer having the quantum well structure has higher quantum efficiency than the bulk active layer, it is possible to realize a semiconductor light emitting device having excellent characteristics. Such a quantum well active layer is employed in a semiconductor light emitting device using a group III nitride semiconductor that has already been put to practical use as a blue light emitting device. For example, Japanese Patent No. 2932467 discloses a quantum well active layer in which a well layer and a barrier layer are formed of InGaN mixed crystals.
[0003]
  Incidentally, zinc oxide (ZnO) is one of materials for semiconductor light emitting devices. This zinc oxide is a direct transition type semiconductor having a band gap energy of about 3.4 eV, an exciton binding energy as extremely high as 60 meV, a raw material is inexpensive, harmless to the environment and the human body, and a film forming method is simple. There is a possibility that a light-emitting device with high environmental efficiency and high efficiency and low power consumption can be realized.
[0004]
  Conventionally, as an oxide semiconductor light emitting device composed of ZnO and a mixed crystal based on ZnO, Cd_0.3Zn_0.7A well layer composed of O and Cd_0.06Zn_0.94Some include a quantum well active layer formed by repeating alternate lamination with a barrier layer made of O a plurality of times (see, for example, Patent Document 1).
[0005]
[Patent Document 1]
          International Publication No. 00/16411 Pamphlet
[0006]
[Problems to be solved by the invention]
  However, the above-described conventional oxide semiconductor light emitting device has a problem in that it has a low operating efficiency and a high oscillation threshold current as well as a light emitting efficiency as compared with a semiconductor light emitting device using a group III nitride semiconductor.
[0007]
  According to the study by the present inventors, the reason why this problem occurs is assumed as follows.
[0008]
  If both the well layer and the barrier layer constituting the quantum well active layer are composed of CdZnO mixed crystal, the potential of the barrier layer is lowered, so that electrons injected into the quantum well active layer are not uniformly confined in each well layer. . As a result, the oscillation wavelength and gain in each well layer vary.
[0009]
  Such a variation increases the potential of the barrier layer by increasing the difference in the In composition ratio between the well layer and the barrier layer in the quantum well active layer in which both the well layer and the barrier layer are composed of InGaN mixed crystals. It is solved by.
[0010]
  However, such a solution cannot be used for a quantum well active layer in which both the well layer and the barrier layer are composed of CdZnO mixed crystals. More specifically, since the CdZnO mixed crystal has a different crystal structure between CdO and ZnO, a composition range in which a CdZnO mixed crystal having a single composition having sufficient crystallinity is likely to occur when the Cd composition ratio is increased and phase separation is likely to occur. Is narrow. That is, the Cd composition ratio between the well layer and the barrier layer cannot be increased. Therefore, in the quantum well active layer in which both the well layer and the barrier layer are composed of CdZnO mixed crystals, the potential of the barrier layer cannot be increased, and characteristic nonuniformity between the quantum well layers tends to occur.
[0011]
  Accordingly, an object of the present invention is to provide an oxide semiconductor light emitting device having excellent light emitting characteristics and power saving properties.
[0012]
[Means for Solving the Problems]
  As a result of intensive studies on a technique for improving the light emission efficiency of a semiconductor light emitting device having a quantum well active layer composed of a ZnO-based semiconductor, the present inventors have optimized the structure of the quantum well active layer to achieve the above object. As a result, the present invention has been found.
[0013]
  The oxide semiconductor light emitting device of the present invention is an oxide semiconductor light emitting device composed of a ZnO-based semiconductor,
  Cd_xZn_1-xO well layer (however, 0 <x ≦ 1) and Mg_yZn_1-yAn active layer having a quantum well structure in which O barrier layers (where 0 <y ≦ 1) are alternately stacked.When,
  A clad layer including an MgZnO mixed crystal having a band gap energy larger than that of the barrier layer and sandwiching the active layer;
With
  The well layer has three or more layers, and the layer thickness of the well layer located at both ends in the stacking direction in the quantum well structure is different from the layer thickness of the well layer located at a place other than the both ends.The
  The thickness of the well layer located at both ends is thicker than the thickness of the well layer located at locations other than the both ends.It is characterized by that.
[0014]
  In this specification, a ZnO-based semiconductor includes a mixed crystal represented by ZnO and MgZnO or CdZnO based on the ZnO.
[0015]
  According to the oxide semiconductor light emitting device having the above configuration, since the well layer is configured by CdZnO having a small band gap energy and the barrier layer is configured by MgZnO having a large band gap energy, a sufficient barrier height is ensured, The carrier confinement effect of the well layer is improved. As a result, the luminous efficiency can be increased, and the operating voltage and the oscillation threshold current can be lowered. That is, an oxide semiconductor light-emitting element excellent in light emission efficiency, power saving property, and reliability can be realized.
  Moreover, the quantum levels of all the well layers can be made uniform by making the layer thickness of the well layer located at both ends different from the layer thickness of the well layer located at a place other than the both ends. That is, the quantum levels of all the well layers can be made substantially the same. Accordingly, it is possible to improve the characteristics by making the emission wavelength uniform. That is, an oxide semiconductor light-emitting element having excellent characteristics such as light emission efficiency and temperature characteristics can be realized.
  In addition, since the thickness of the well layer located at both ends is thicker than the thickness of the well layer located at locations other than the both ends, the quantum levels of all well layers are easily aligned.
[0016]
  The “active layer” is referred to as a “light emitting layer” in the case of a light emitting diode element, but is synonymous in terms of a layer that controls light emission, and is not particularly distinguished below.
  The oxide semiconductor light emitting device of the present invention is an oxide semiconductor light emitting device that can be composed of a ZnO-based semiconductor,
  Cd_xZn_1-xO well layer (however, 0 <x ≦ 1) and Mg_yZn_1-yAn active layer having a quantum well structure in which O barrier layers (where 0 <y ≦ 1) are alternately stacked.When,
  A clad layer including an MgZnO mixed crystal having a band gap energy larger than that of the barrier layer and sandwiching the active layer;
With
  The well layer has three or more layers, and the composition of the well layer located at both ends in the stacking direction in the quantum well structure is different from the composition of the well layer located at a place other than the both ends.The
  The band gap energy of the well layer located at both ends is smaller than the band gap of the well layer located at a place other than the both ends.It is characterized by that.
  In this specification, the ZnO-based semiconductor includes ZnO and a mixed crystal represented by MgZnO or CdZnO based on the ZnO.
  According to the oxide semiconductor light emitting device having the above configuration, since the well layer is configured by CdZnO having a small band gap energy and the barrier layer is configured by MgZnO having a large band gap energy, a sufficient barrier height is ensured, The carrier confinement effect of the well layer is improved. As a result, the luminous efficiency can be increased, and the operating voltage and the oscillation threshold current can be lowered. That is, an oxide semiconductor light-emitting element excellent in light emission efficiency, power saving property, and reliability can be realized.
  Further, the quantum levels of all the well layers can be made uniform by making the composition of the well layers located at both ends different from the composition of the well layers located at locations other than the both ends. That is, the quantum levels of all the well layers can be made substantially the same. Accordingly, it is possible to improve the characteristics by making the emission wavelength uniform. That is, an oxide semiconductor light-emitting element having excellent characteristics such as light emission efficiency and temperature characteristics can be realized.
  Further, since the band gap energy of the well layer located at both ends is smaller than the band gap of the well layer located at a place other than both ends, the quantum levels of all the well layers are easily aligned.
  The “active layer” is referred to as a “light emitting layer” in the case of a light emitting diode element, but is synonymous in terms of a layer that controls light emission, and is not particularly distinguished below.
  The oxide semiconductor light emitting device of the present invention is an oxide semiconductor light emitting device composed of a ZnO-based semiconductor.
  Cd_xZn_1-xO well layer (however, 0 <x ≦ 1) and Mg_yZn_1-yAn active layer having a quantum well structure in which O barrier layers (where 0 <y ≦ 1) are alternately stacked;
  A light guide layer laminated with the active layer,
  The band gap energy of the barrier layer is higher than the band gap energy of the light guide layer.
  In this specification, the ZnO-based semiconductor includes ZnO and a mixed crystal represented by MgZnO or CdZnO based on the ZnO.
  According to the oxide semiconductor light emitting device having the above configuration, since the well layer is configured by CdZnO having a small band gap energy and the barrier layer is configured by MgZnO having a large band gap energy, a sufficient barrier height is ensured, The carrier confinement effect of the well layer is improved. As a result, the luminous efficiency can be increased, and the operating voltage and the oscillation threshold current can be lowered. That is, an oxide semiconductor light-emitting element excellent in light emission efficiency, power saving property, and reliability can be realized.
  Further, since the band gap energy of the barrier layer is higher than the band gap energy of the light guide layer, a sufficient barrier height can be obtained and the carrier confinement effect of the well layer is improved. As a result, the luminous efficiency can be increased, and the operating voltage and the oscillation threshold current can be lowered. That is, an oxide semiconductor light-emitting element having excellent characteristics can be realized.
  The “active layer” is referred to as a “light emitting layer” in the case of a light emitting diode element, but is synonymous in terms of a layer that controls light emission, and is not particularly distinguished below.
[0017]
  In one embodiment, the oxide semiconductor light emitting device includes the Cd_xZn_1-xThe Cd composition ratio x of the O well layer is 0 <x ≦ 0.2.
[0018]
  According to the oxide semiconductor light emitting device of the above embodiment, the Cd composition ratio x is set within the range of 0 <x ≦ 0.2, whereby Cd_xZn_1-xThe crystallinity and composition uniformity of the O well layer can be improved.
[0019]
  In one embodiment, the oxide semiconductor light emitting device includes the Mg_yZn_1-yThe Mg composition ratio y of the O barrier layer is 0 <y ≦ 0.35.
[0020]
  According to the oxide semiconductor light emitting device of the above embodiment, the Mg composition ratio y is set within the range of 0 <y ≦ 0.35, whereby Mg_yZn_1-yThe crystallinity and composition uniformity of the O barrier layer can be improved.
[0021]
  In one embodiment, the oxide semiconductor light emitting device includes the Cd_xZn_1-xCd composition ratio x of the O well layer is 0 <x ≦ 0.08, and the above Mg_yZn_1-yThe Mg composition ratio y of the O barrier layer is 0 <y ≦ 0.33.
[0022]
  According to the oxide semiconductor light emitting device of the above embodiment, the Cd composition ratio x is in the range of 0 <x ≦ 0.08, and the Mg composition ratio y is in the range of 0 <y ≦ 0.33. By setting, Cd_xZn_1-xIn-plane lattice constant of O well layer and Mg_yZn_1-yThe in-plane lattice constant of the O barrier layer can be made substantially the same within the range of 3.250 to 3.261. As a result, the stress generated in the active layer can be extremely reduced.
[0023]
[0024]
[0025]
[0026]
[0027]
[0028]
[0029]
[0030]
[0031]
[0032]
[0033]
  In the oxide semiconductor light emitting device of one embodiment, the active layer is doped with a donor impurity.
[0034]
  According to the oxide semiconductor light emitting device of the above embodiment, since the active layer is doped with a donor impurity, the light emission efficiency is improved and the operating voltage of the device is reduced.
[0035]
  In addition, by doping the active layer with a donor impurity, energy transition through an impurity level can be generated, and light emission characteristics can be improved.
[0036]
  In one embodiment, the oxide semiconductor light emitting device according to any one of the first to fourth inventions is such that the donor impurity is doped only in the well layer.
[0037]
  According to the oxide semiconductor light emitting device of the above embodiment, since the donor impurity is doped only in the well layer, the same luminous efficiency improvement effect as that obtained when the entire active layer is doped can be obtained. The total doping amount can be reduced. As a result, the crystallinity of the active layer is improved and the reliability is increased, and the carrier absorption loss of the active layer is reduced to further improve the light emission efficiency.
[0038]
  In the oxide semiconductor light emitting device according to one embodiment, the donor impurity is doped only in the barrier layer.
[0039]
  According to the oxide semiconductor light emitting device of the above embodiment, by doping the donor impurity only in the barrier layer, an effect of improving the luminous efficiency equivalent to the case where the entire active layer is doped can be obtained, The total doping amount can be reduced. As a result, the crystallinity of the active layer is improved and the reliability is increased, and the carrier absorption loss of the active layer is reduced to further improve the light emission efficiency.
[0040]
  In the oxide semiconductor light emitting device of one embodiment, the film thickness of the well layer is in the range of 2 nm to 8 nm.
[0041]
  According to the oxide semiconductor light emitting device of the above embodiment, by setting the film thickness of the well layer in the range of 2 nm to 8 nm, the quantum efficiency and the differential gain are improved, and the light emission characteristics can be improved.
[0042]
  In one embodiment, the oxide semiconductor light-emitting device has three or more well layers, and the well layers located at positions other than both ends in the stacking direction in the quantum well structure are in the range of 3 nm to 6 nm. It is.
[0043]
  According to the oxide semiconductor light emitting device of the above embodiment, the quantum efficiency and the differential gain are further improved by setting the film thickness of the well layer located at a place other than the both ends within the range of 3 nm to 6 nm. The light emission characteristics can be further improved.
[0044]
  In one embodiment, the active layer is sandwiched between clad layers containing MgZnO mixed crystals having a band gap energy larger than that of the barrier layer.
[0045]
  According to the oxide semiconductor light emitting device of the above embodiment, since the clad layer containing the MgZnO mixed crystal having the band gap energy larger than the band gap energy of the barrier layer sandwiches the active layer, the injected carriers are the active layer. Be trapped in. As a result, since the luminous efficiency is improved, high luminous efficiency can be obtained with a low operating current.
[0046]
  An oxide semiconductor light emitting device according to an embodiment includes a carrier block layer including an MgZnO mixed crystal having a band gap energy larger than that of the cladding layer between the active layer and the cladding layer. .
[0047]
  According to the oxide semiconductor light emitting device having the above configuration, the carrier confinement effect in the well layer is further improved by forming the carrier block layer between the active layer and the cladding layer. As a result, light emission characteristics and temperature characteristics can be improved.
[0048]
DETAILED DESCRIPTION OF THE INVENTION
  Hereinafter, an oxide semiconductor light emitting device of the present invention will be described in detail with reference to embodiments shown in the drawings.
[0049]
  (Reference example)
  In this reference example, a first example in which the present invention is applied to a blue light-emitting diode element composed of a ZnO-based semiconductor will be described.
[0050]
  FIG. 1 is a schematic cross-sectional view of the blue light-emitting diode element of this reference example.
[0051]
  In the light-emitting diode element, Ga (gallium) is 3 × 10 3 on the ZnO single crystal substrate 101 whose principal surface is the zinc surface.¹⁸cm^-30.5 μm thick n-type Mg doped at a concentration of_0.2Zn_0.8O-cladding layer 102, non-doped quantum well light emitting layer 103 as an example of active layer, N (nitrogen) 1 × 10²⁰cm^-30.5 μm thick p-type Mg doped at a concentration of_0.2Zn_0.8O cladding layer 104, N is 1 × 10²⁰cm^-3A p-type ZnO contact layer 105 having a thickness of 0.5 μm doped at a concentration of 1 to 5 is stacked in this order.
[0052]
  The quantum well light-emitting layer 103 is made of Mg with a thickness of 5 nm._0.1Zn_0.9O barrier layer and 4 nm thick Cd_0.1Zn_0.9O well layers are alternately stacked. And the Mg_0.1Zn_0.9While there are 8 O barrier layers, Cd_0.1Zn_0.9There are seven O well layers.
[0053]
  On the entire main surface of the p-type ZnO contact layer 105, a translucent p-type ohmic electrode 106 in which Ni with a thickness of 15 nm is laminated is formed. Further, a bonding Au pad electrode 7 having a thickness of 100 nm is formed on the p-type ohmic electrode 106 with a smaller area than the ohmic electrode 6.
[0054]
  Under the back surface of the ZnO single crystal substrate 101, an n-type ohmic electrode 108 made of Al and having a thickness of 100 nm is formed.
[0055]
  When the light emitting diode element having the above structure was attached to a lead frame with Ag paste and molded to emit light, blue light emission with an emission peak wavelength of 430 nm was obtained.
[0056]
  FIG. 2 shows the results of examining the light emission intensity at an operating current of 20 mA by fabricating the light emitting diode element of this reference example by changing the composition ratio of the barrier layer constituting the quantum well light emitting layer 103.
[0057]
  As can be seen from FIG. 2, when the barrier layer is made of the same CdZnO mixed crystal as the well layer, the emission intensity is weak. On the other hand, if the barrier layer is composed of a MgZnO mixed crystal having a large band gap energy, the light emission intensity increases dramatically. The reason for this is that the CdZnO barrier layer has a relatively low potential barrier and cannot uniformly confine electrons in each well layer. However, as the potential barrier becomes higher, electrons are confined in the well layer. It is considered that the light emission intensity is increased because of a uniform quantum effect and a sufficient quantum effect.
[0058]
  Further, when the Mg composition ratio exceeds 0.35 in the barrier layer composed of the MgZnO mixed crystal, the light emission intensity rapidly decreases. This is thought to be due to the deterioration of crystallinity and non-uniform composition caused by the Mg composition ratio exceeding 0.35.
[0059]
  From the above, the Mg composition ratio of the barrier layer of the quantum well light-emitting layer 103 is preferably in the range of more than 0 to 0.35 because of excellent crystallinity and composition uniformity.
[0060]
  On the other hand, the Cd composition ratio of the well layer of the quantum well light-emitting layer 103 is higher in industrial utility value because visible light emission with a longer wavelength can be obtained, but as a range excellent in crystallinity and composition uniformity. Is preferably in the range of more than 0 to 0.2.
[0061]
  FIG. 3 shows Cd_xZn_1-xO and Mg_yZn_1-yFor quantum well light-emitting layers composed of O, Cd_xZn_1-xO and Mg_yZn_1-yThe relationship between the composition ratio of O and the band gap energy is examined and shown, and the relationship between the composition ratio and the in-plane lattice constant is also shown. FIG. 3 shows a comparative example by examining the above-described relationship regarding InGaN and AlGaN used as materials for the well layer and barrier layer of the group III nitride semiconductor.
[0062]
  In FIG. 3, Cd_xZn_1-xO has a Cd composition ratio x in the range of 0 to 0.08 and an in-plane lattice constant in the range of 3.249 to 3.261. Meanwhile, Mg_yZn_1-yO has an Mg composition ratio y in the range of 0 to 0.33 and an in-plane lattice constant in the range of 3.249 to 3.261. And Cd_xZn_1-xO and Mg_yZn_1-yBoth the O and the in-plane lattice constant increase as the composition ratios x and y increase.
[0063]
  Accordingly, by forming the quantum well light-emitting layer within the above composition range, that is, within the range where the Cd composition ratio x is 0 to 0.08 and within the range where the Mg composition ratio y is 0 to 0.33, the well layer And the barrier layer can have a large band gap difference, and lattice mismatch can be extremely reduced.
[0064]
  However, when the Cd composition ratio x and the Mg composition ratio y are both 0, carriers are not confined to the well layer, so x and y do not include 0, and the in-plane lattice constant is 3.250Å or more. Is preferred.
[0065]
  In the case of the group III nitride semiconductors InGaN and AlGaN shown as comparative examples, when both the In composition ratio and the Al composition ratio increase, the band gap difference increases, but the lattice mismatch becomes extremely large, and the crystal As a result, the luminous efficiency is greatly lowered due to the influence of a piezoelectric polarization electric field generated by internal stress.
[0066]
  This indicates that the oxide semiconductor light emitting device of the present invention using a ZnO-based semiconductor has a higher degree of freedom in structural design than a group III nitride semiconductor, and can produce more excellent blue to ultraviolet light emitting devices. .
[0067]
  FIG. 4 shows the relationship between the thickness of the well layer of the quantum well light-emitting layer 103 and the light emission intensity. That is, FIG. 4 shows the dependence of the emission intensity on the well layer thickness. Note that. In FIG. 4, the film thickness of the well layer of the quantum well light emitting layer 103 is described as “quantum well film thickness”.
[0068]
  As can be seen from FIG. 4, when the film thickness of the well layer is increased from 1 nm, the emission intensity is remarkably improved from the point where the film thickness of the well layer is 2 nm. As described above, the emission intensity increases as the thickness of the well layer increases. However, the emission intensity rapidly decreases when the thickness of the well layer exceeds 8 nm, and decreases more rapidly when the thickness of the well layer exceeds 10 nm. . A possible reason for this is that the binding energy of excitons is reduced by reducing the quantum effect.
[0069]
  Therefore, the thickness of the well layer of the quantum well light emitting layer 103 is preferably 2 nm or more and 8 nm or less, and more preferably 3 nm or more and 6 nm or less.
[0070]
  In order to obtain the maximum effect of uniformly confining carriers in each well layer of the quantum well light emitting layer, it is preferable to have a double heterostructure that confines carriers injected from the cladding layer into the quantum well active layer.
[0071]
  The Mg composition ratio of the MgZnO clad layer sandwiching the quantum well light-emitting layer 103 is preferably higher than the Mg composition ratio of the barrier layer of the quantum well light-emitting layer 103 and lower than 0.35 at which the crystallinity deterioration becomes remarkable.
[0072]
  P-type Mg_0.2Zn_0.8As an acceptor impurity doped into the O-clad layer 104 and the p-type ZnO contact layer 105, at least one of Li, Na, Cu, Ag, N, P, As, etc., which are Group I elements or Group V elements, is used. I can do it. N, Li and Ag are particularly preferable because they are easily activated. Further, N is N₂It is preferable to perform high-concentration doping while maintaining good crystallinity by a method in which plasma is irradiated and irradiated during crystal growth.
[0073]
  N-type Mg_0.2Zn_0.8It is preferable to use at least one of group III elements B, Al, Ga, and In, which have a high activation rate in the ZnO-based semiconductor, as a donor impurity doped in the O-clad layer 102.
[0074]
  As the substrate material, in addition to the ZnO substrate used in this reference example, a sapphire substrate, a spinel substrate, and LiGaO₂An insulating substrate such as a substrate, or a conductive substrate such as a SiC substrate or a GaN substrate can also be used.
[0075]
  When the insulating substrate is used, n-type Mg is etched by etching a part of the growth layer._0.2Zn_0.8A part of the O cladding layer 102 may be exposed, and the n-type ohmic electrode 5 may be formed on the part. Alternatively, the n-type Mg_0.2Zn_0.8More preferably, an n-type ZnO contact layer in contact with the O-clad layer 102 is provided, a part of the contact layer is exposed, and the n-type ohmic electrode 5 is formed on the part.
[0076]
  In order to obtain a growth layer with good crystallinity, n-type Mg_0.2Zn_0.8A buffer layer is preferably formed between the O-clad layer 102 or the n-type ZnO contact layer and the substrate.
[0077]
  In order to maximize the high luminous efficiency of the ZnO-based semiconductor, it is preferable to use a substrate that satisfies the following conditions (a) to (c).
[0078]
  (A) From the viewpoint of reducing defects that become non-luminescent centers in the growth layer on the substrate,
          The in-plane lattice constant of the plate is (100 ±
          3) It is within the range of%.
[0079]
  (B) The substrate has a low absorption coefficient corresponding to the emission wavelength.
[0080]
  (C) From the viewpoint of providing electrodes under the back surface of the substrate, the substrate is a conductive substrate.
[0081]
  The ZnO single crystal substrate 101 of this reference example satisfies the above conditions (a) to (c) and is most preferable. Further, it is preferable to use the zinc surface of the ZnO single crystal substrate 101 because the carrier activation rate of the p-type layer is improved and a p-type layer having a low resistance is easily obtained.
[0082]
  Further, in order to diffusely reflect the light incident on the substrate, it is preferable to form irregularities on the back surface of the substrate by a known method such as polishing or etching because the light extraction efficiency is improved.
[0083]
  As a material for the p-type ohmic electrode, one of Ni, Pt, Pd, Au, and the like can be used. Among Ni, Pt, Pd, and Au, Ni having a low resistance and good adhesion is particularly preferable as a material for the p-type ohmic electrode. The p-type ohmic electrode may be formed by alloying the plurality of metal materials. That is, the p-type ohmic electrode may be formed using a plurality of different metal materials.
[0084]
  Further, in order to obtain the highest light emission efficiency of the ZnO-based semiconductor with the maximum effect, as shown in this reference example, the light extraction efficiency can be improved by using the light-transmitting p-type ohmic electrode 106. preferable. In the p-type ohmic electrode 106, the thickness that achieves both good ohmic characteristics and high translucency is preferably in the range of 5 nm to 200 nm, and more preferably in the range of 30 nm to 100 nm.
[0085]
  Annealing treatment after the formation of the p-type electrode is preferable because adhesion is improved and contact resistance is reduced. In order to obtain an annealing effect without causing defects in the ZnO crystal, the temperature is preferably in the range of 300 ° C to 400 ° C. The atmosphere in the annealing process is O₂Or in air atmosphere, N₂On the other hand, the electrical resistance increases, which is not preferable.
[0086]
  If the Au pad electrode 7 is formed on a part of the translucent ohmic electrode 6 with a smaller area than the p-type ohmic electrode 106, the mounting process on the lead frame without impairing the effect of the translucent ohmic electrode 6 Is preferable.
[0087]
  As a material for the pad electrode, Au that is easy to bond and does not become a donor impurity even if diffused into ZnO is preferable.
[0088]
  Another metal layer may be provided between the translucent ohmic electrode 6 and the pad electrode 7 for the purpose of improving adhesion and light reflectivity.
[0089]
  As a material for the n-type ohmic electrode, one of Ti, Cr, Al and the like can be used. Among Ti, Cr, Al and the like, Al having low electric resistance and low cost, or Ti having good adhesion is preferable as a material for the n-type ohmic electrode. The n-type ohmic electrode may be formed by alloying the plurality of metal materials. That is, the p-type ohmic electrode may be formed using a plurality of different metal materials.
[0090]
  Other configurations are arbitrary and are not limited by this reference example.
[0091]
  (Embodiment 1)
  Embodiment 1 differs from the above reference example in that the thickness of the well layers at both ends in the stacking direction in the quantum well light emitting layer is different from the thickness of the remaining well layers.
[0092]
  That is, in the first embodiment, out of the seven well layers of the quantum well light emitting layer, the thickness of the two well layers located at both ends in the stacking direction is set to 5.5 nm, and the remaining five well layers A light emitting diode element is fabricated in the same manner as in the reference example except that the film thickness is 4 nm.
[0093]
  When the light emitting diode element of Embodiment 1 was separated into chips and attached to a lead frame with Ag paste and molded to emit light, blue light emission having the same emission peak wavelength of 430 nm as in the above reference example was obtained. The emission intensity was increased by 10% compared to the reference example, and the half-value width of the emission spectrum was reduced by 30% compared to the reference example, so that the monochromaticity was improved.
[0094]
  The reason is considered as follows.
[0095]
  FIG. 5A shows a schematic diagram of the conduction band diagram in the vicinity of the quantum well light emitting layer 103 of the reference example, and FIG. 5B shows the conduction band diagram in the vicinity of the quantum well light emitting layer of the first embodiment. The schematic of is shown. Note that the horizontal direction in FIGS. 5A and 5B corresponds to the stacking direction.
[0096]
  As shown in FIG. 5 (a), the quantum well light-emitting layer 103 of the above reference example has eight layers of Mg._0.1Zn_0.9O barrier layers 111, 112,..., 118 and seven layers of Cd_0.1Zn_0.9O well layers 121, 122,..., 127 are formed. The thicknesses of all the seven well layers 121, 122,..., 127 are 4 nm.
[0097]
  According to the quantum well light emitting layer 103 configured as described above, the well layers 121 and 127 at both ends in the stacking direction are strongly affected by the barrier potential of the cladding layers 102 and 104. Therefore, the quantum levels formed by the electron waves are higher in the well layers 121 and 127 at both ends in the stacking direction than the other well layers 122, 123,. That is, the quantum levels of all the well layers 121, 122,..., 127 are not aligned.
[0098]
  On the other hand, as shown in FIG. 5B, the quantum well light-emitting layer 203 of the first embodiment has eight layers of Mg._0.1Zn_0.9O barrier layers 211, 212, ..., 218 and 7 layers of Cd_0.1Zn_0.9O well layers 221, 222,..., 227. Of the seven well layers 221, 222,..., 227, the thickness of the two well layers 221 and 227 located at both ends in the stacking direction is 5.5 nm, and the remaining five well layers 222, 223, and 223 ..., 226 has a thickness of 4 nm.
[0099]
  According to the quantum well light emitting layer 203 configured as described above, the thicknesses of the two well layers 221 and 227 at both ends are thicker than the thicknesses of the other well layers 222, 223,. In the well layers 221, 227, the quantum effect is reduced and the quantum level is lowered. As a result, the quantum levels of the well layers 221 and 227 and the quantum levels of the remaining well layers 222, 223,. That is, all the quantum levels of the well layers 221, 222,. Thereby, it is considered that characteristics such as a spectrum width are improved as compared with the reference example.
[0100]
  As described above, in order to improve the characteristics by aligning the quantum levels of the plurality of well layers of the quantum well light emitting layer, the thickness of the well layers at both ends in the stacking direction of the plurality of well layers is set to be different from that of the other well layers. It is preferable to make it thicker than the film thickness. However, as described in the above reference example, the thickness of the well layer is preferably 2 nm or more and 8 nm or less. Therefore, the thickness of the well layer other than both ends in the stacking direction is preferably 3 nm or more and 6 nm or less.
[0101]
  (Embodiment 2)
  In Embodiment 2, 1 × 10 5 Ga is added to the whole quantum well light emitting layer.¹⁸cm^-3A ZnO-based light-emitting diode element was fabricated in the same manner as in the first embodiment except that doping was performed at a concentration of 1%.
[0102]
  When the light-emitting diode element of Embodiment 3 was separated into chips, attached to a lead frame with Ag paste and molded to emit light, blue light emission with an emission peak wavelength of 430 nm was obtained, and the emission intensity was the same as that of Embodiment 1 above. Compared to 20%. Moreover, since the resistance of the quantum well light emitting layer was lowered, the operating voltage was reduced by 0.1V.
[0103]
  In the light emitting diode element, since the quantum well light emitting layer is doped with Ga, this Ga forms a donor level, and light is emitted through the donor level together with exciton light emission, so that the light emission wavelength becomes longer. In addition, since the energy transition probability is improved, it is considered that the emission intensity is increased as compared with the first embodiment.
[0104]
  (Embodiment 3)
  In the third embodiment, a ZnO-based light emitting diode element was fabricated in the same manner as in the second embodiment except that only the well layer included in the quantum well light emitting layer was doped with Ga. That is, the quantum well light-emitting layer of Embodiment 3 is composed of eight non-doped Mg layers._0.1Zn_0.9O barrier layer and 1 × 10 Ga¹⁸cm^-37 layers of Cd doped at a concentration of_0.1Zn_0.9It is composed of an O well layer.
[0105]
  When the light emitting diode element of Embodiment 3 was separated into chips, attached to a lead frame with Ag paste and molded to emit light, blue light emission with an emission peak wavelength of 430 nm was obtained, and the emission intensity was the same as in Embodiment 2 above. Although the operation voltage increased by 20%, the operating voltage increased by 0.05 V compared to the second embodiment.
[0106]
  Since the light emitting diode element is doped with impurities only in the well layer, the light emission efficiency is maintained, the doping concentration of the whole quantum well light emitting layer is lowered, and the carrier absorption in the quantum well light emitting layer is reduced. It is considered that the emission intensity is increased as compared with the case where the entire layer is doped with impurities.
[0107]
  (Embodiment 4)
  In the fourth embodiment, a ZnO-based light emitting diode element was fabricated in the same manner as in the second embodiment except that only the barrier layer of the quantum well light emitting layer was doped with Ga. That is, in the quantum well light emitting layer of the fourth embodiment, Ga is 1 × 10 5.¹⁸cm^-3Layers of Mg doped at a concentration of_0.1Zn_0.9O barrier layer and 7 undoped Cd layers_0.1Zn_0.9It is composed of an O well layer.
[0108]
  When the light emitting diode element of Embodiment 4 was separated into chips, attached to a lead frame with Ag paste and molded to emit light, blue light emission with an emission peak wavelength of 425 nm was obtained, and the emission intensity was the same as in Embodiment 2 above. Compared to the second embodiment, the operating voltage increased by 15%.
[0109]
  In the above light emitting diode element, since only the barrier layer is doped with impurities, the luminous efficiency is maintained, the doping concentration of the whole quantum well light emitting layer is lowered, and the carrier absorption in the quantum well light emitting layer is reduced, so that only the well layer is provided. As a result, almost the same increase in emission intensity as in the case of doping impurities can be obtained.
[0110]
  (Embodiment 5)
  Embodiment 5 differs from the above reference example in that the composition of the well layers at both ends in the quantum well light emitting layer is different from the composition of the remaining well layers.
[0111]
  That is, in Embodiment 5, only the two well layers located at both ends in the stacking direction among the seven well layers of the quantum well light emitting layer are Cd._0.05Zn_0.95The remaining 5 well layers are composed of Cd._0.1Zn_0.9A light emitting diode element was fabricated in the same manner as in the above reference example except that it was composed of O.
[0112]
  When the light emitting diode element of Embodiment 5 was separated into chips, attached to a lead frame with Ag paste and molded to emit light, blue light emission with the same emission peak wavelength of 430 nm as in the above reference example was obtained. The intensity was increased by 10% compared to the reference example, and the half-value width of the emission spectrum was reduced by 30% compared to the reference example, so that the monochromaticity was improved.
[0113]
  The reason is considered as follows.
[0114]
  In FIG. 6, the schematic of the conduction band diagram in the quantum well light emitting layer 603 of this Embodiment 5 is shown.
[0115]
  The quantum well light-emitting layer 603 includes eight layers of Mg_0.1Zn_0.9O barrier layers 611, 612, ..., 618 and two layers of Cd_0.05Zn_0.95O well layers 621 and 627 and five layers of Cd_0.1Zn_0.9O well layers 622, 623,..., 626. In the well layers 621, 622,..., 627, the well layers 621 and 627 are located at both ends in the stacking direction.
[0116]
  The quantum well light-emitting layer 603 having the above configuration is different from the compositions of the other well layers 622, 623,..., 626 in the composition of the well layers 621, 627 at both ends in the stacking direction. The structure is such that all the quantum levels of the layers 621, 622,. As a result, it is considered that the light-emitting diode element of Embodiment 5 has improved characteristics such as a spectral width compared to the above reference example.
[0117]
  As described above, in order to improve the characteristics by aligning the quantum levels of all the well layers in the plurality of well layers, the band gap energy of the well layers at both ends in the stacking direction is set to other well layers (other than both ends in the stacking direction). It is preferable that it is smaller than the band gap energy of the well layer located at the location of Therefore, in the plurality of well layers, it is preferable to adjust the composition of each well layer so that the band gap energy of the well layers at both ends in the stacking direction is smaller than the band gap energy of the other well layers.
[0118]
  (Embodiment 6)
  In the sixth embodiment, an example in which the present invention is applied to a blue semiconductor laser element composed of a ZnO-based semiconductor will be described.
[0119]
  FIG. 7 is a schematic perspective view of the ZnO-based semiconductor laser device according to the sixth embodiment.
[0120]
  The semiconductor laser element is formed on an n-type MgO having a thickness of 0.5 μm on an n-type ZnO single crystal substrate 701 having a zinc surface as a main surface._0.2Zn_0.820-nm-thick n-type Mg as an example of an O-cladding layer 702 and a light guide layer_0.05Zn_0.95O light guide layer 703, multi-quantum well active layer 704 as an example of active layer, p-type Mg of 20 nm thickness as an example of light guide layer_0.05Zn_0.95O light guide layer 705, p-type Mg having a thickness of 1.0 μm_0.2Zn_0.8An O cladding layer 706 and a 0.5 μm thick p-type ZnO contact layer 707 are laminated in this order.
[0121]
  P-type Mg_0.2Zn_0.8The upper part of the O-cladding layer 706 is etched into a ridge stripe shape. And the p-type Mg_0.2Zn_0.8On both sides of the top of the O cladding layer 706, Mg_0.3Zn_0.7An n-type current blocking layer 708 made of O is formed. That is, the p-type Mg_0.2Zn_0.8The upper side surface of the O clad layer 706 is buried with an n-type current blocking layer 708.
[0122]
  A p-type ohmic electrode 30 is formed on the p-type ZnO contact layer 707, while an n-type ohmic electrode 709 is formed under the n-type ZnO single crystal substrate 701.
[0123]
  FIG. 8 shows a schematic diagram of a conduction band diagram near the multiple quantum well active layer.
[0124]
  The multi-quantum well active layer 704 is made of Mg with a thickness of 4 nm._0.1Zn_0.9O barrier layers 711 and 712 and Cd with a thickness of 4 nm_0.1Zn_0.9O well layers 721, 722, and 723 are alternately stacked. Mg_0.1Zn_0.9There are two O barrier layers 711 and 712, while Cd_0.1Zn_0.9There are three O well layers 721, 722, and 723. And the Mg_0.1Zn_0.9The band gap energy of the O barrier layers 711 and 712 is n-type Mg._0.05Zn_0.95O light guide layer 703 and p-type Mg_0.05Zn_0.95The band gap energy of the O light guide layer 705 is higher.
[0125]
  After producing the structure as described above, the n-type ZnO single crystal substrate 701 is cleaved to form an end face mirror, a protective film is vacuum-deposited on the end face, element isolation is performed, and the width of 300 μm of this embodiment 6 is applied. A semiconductor laser element is obtained. When a current was passed through the semiconductor laser element, blue oscillation light having a wavelength of 430 nm was obtained from the end face.
[0126]
  FIG. 9 shows the results of examining the oscillation threshold current and the characteristic temperature by fabricating the semiconductor laser device of the sixth embodiment by changing the Mg composition ratio of the barrier layer of the multiple quantum well active layer 704. In FIG. 9, the relationship between the Mg composition ratio of the barrier layer and the oscillation threshold current is indicated by a solid line, and the relationship between the Mg composition ratio of the barrier layer and the characteristic temperature is indicated by a dotted line.
[0127]
  As can be seen from FIG. 9, when the composition ratio of the barrier layer is larger than that of the light guide layer and the potential barrier with respect to the well layer is increased, the oscillation threshold current and the characteristic temperature are greatly improved. This indicates that when the band gap energy of the barrier layer is higher than the band gap energy of the optical guide layer, carriers are confined uniformly in the well layer, and various characteristics are greatly improved.
[0128]
  (Embodiment 7)
  FIG. 10 shows a schematic diagram of a conduction band diagram in the vicinity of the quantum well active layer of the seventh embodiment.
[0129]
  In the seventh embodiment, as shown in FIG. 10, n-type Mg_0.2Zn_0.8O-cladding layer 702 and n-type Mg_0.05Zn_0.95Between the O light guide layer 703 and Mg_0.3Zn_0.7An n-type carrier block layer 801 made of O and having a thickness of 5 nm is formed and p-type Mg_0.05Zn_0.95O light guide layer 705 and p-type Mg_0.2Zn_0.8Between the O clad layer 706 and Mg_0.3Zn_0.7A semiconductor laser device was fabricated in the same manner as in Embodiment 6 except that a 5 nm-thick p-type carrier block layer 802 was formed.
[0130]
  The band gap energy of the n-type carrier block layer 801 and the p-type carrier block layer 802 is n-type Mg._0.2Zn_0.8O-clad layer 702, p-type Mg_0.2Zn_0.8The band gap energy of the O cladding layer 706 is larger.
[0131]
  When a current was passed through the semiconductor laser device of the seventh embodiment, blue oscillation light having a wavelength of 430 nm was obtained from the end face as in the sixth embodiment, but the oscillation threshold current was 30% lower than that in the sixth embodiment. The characteristic temperature was improved by 20K.
[0132]
  Thus, carrier overflow from the well layer is suppressed by forming a carrier block layer having a larger band gap energy than the cladding layer between the active layer and the cladding layer. As a result, the carrier confinement ratio in the well layer can be improved, and the effects of the present invention can be further improved.
[0133]
  Since the p-type ZnO-based semiconductor has a lower carrier concentration than the n-type ZnO-based semiconductor, the hetero barrier between the active layer and the p-type cladding layer tends to be low. Therefore, the carrier block layer is preferably provided at least between the active layer and the p-type cladding layer. It is more preferable to provide a carrier block layer between the active layer and the n-type cladding layer and between the active layer and the p-type cladding layer.
[0134]
  The oxide semiconductor light emitting device of the present invention can be manufactured by a crystal growth technique such as MBE (molecular beam epitaxy), laser MBE, or MOCVD (metal organic chemical vapor deposition) using a solid or gas source. Among these crystal growth methods, the laser MBE method has a small compositional deviation between a raw material target and a thin film formed using this raw material target.₂O_FourIt is particularly preferable because generation of unintended by-products such as can be suppressed.
[0135]
  In addition, the oxide semiconductor light-emitting element of the present invention may have a single hetero structure or a double hetero structure.
[0136]
【The invention's effect】
  As is clear from the above, the oxide semiconductor light-emitting device of the present invention has the quantum well structure of the active layer as Cd._xZn_1-xO well layer (however, 0 <x ≦ 1) and Mg_yZn_1-ySince it is configured by alternately laminating with O barrier layers (however, 0 <y ≦ 1), a sufficient barrier height is secured, and the carrier confinement effect of the well layer is enhanced. As a result, the luminous efficiency can be increased, and the operating voltage and the oscillation threshold current can be lowered.
[0137]
  In the oxide semiconductor light emitting device of the present invention, in the quantum well structure including the barrier layer and the three or more well layers, the thicknesses of the well layers positioned at both ends in the stacking direction are positioned at positions other than the both ends. Since the thickness of the well layer is different, the quantum levels of all the well layers are aligned. As a result, the emission wavelength can be made uniform and the characteristics can be improved.
[0138]
  In the oxide semiconductor light emitting device of the present invention, in a quantum well structure including a barrier layer and three or more well layers, the composition of the well layer located at both ends in the stacking direction is located at a place other than the both ends. Since the composition of the well layer is different, the quantum levels of all the well layers are aligned. As a result, the emission wavelength can be made uniform and the characteristics can be improved.
[0139]
  In the oxide semiconductor light emitting device of the present invention, since the band gap energy of the barrier layer is higher than the band gap energy of the light guide layer, a sufficient barrier height is obtained and the carrier confinement effect of the well layer is improved. . As a result, the luminous efficiency can be increased, and the operating voltage and the oscillation threshold current can be lowered.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of a light-emitting diode element according to a reference example of the present invention.
FIG. 2 is a graph showing the relationship between the composition ratio of the barrier layer and the light emission intensity in the light-emitting diode device of the reference example.
FIG. 3 is a graph showing the relationship between the composition ratio of CdZnO and MgZnO, the band gap energy, and the in-plane lattice constant.
FIG. 4 is a diagram showing the relationship between the layer thickness of the well layer and the light emission intensity in the light emitting diode element of the reference example.
FIG. 5 (a) is a schematic diagram of a conduction band diagram in the vicinity of the quantum well light emitting layer in the light emitting diode device of the above reference example, and FIG. 5 (b) is a light emitting diode device according to Embodiment 1 of the present invention. It is the schematic of the conduction band diagram of the quantum well light emitting layer vicinity in FIG.
FIG. 6 is a schematic diagram of a conduction band diagram in the vicinity of a quantum well light emitting layer in a light emitting diode device according to a fifth embodiment of the present invention.
FIG. 7 is a schematic perspective view of a semiconductor laser device according to a sixth embodiment of the present invention.
FIG. 8 is a schematic diagram of a conduction band diagram near the multiple quantum well active layer in the semiconductor laser device of the sixth embodiment.
FIG. 9 is a diagram showing the relationship between the composition ratio of the barrier layer, the oscillation threshold current, and the characteristic temperature in the semiconductor laser device of the sixth embodiment.
FIG. 10 is a schematic diagram of a conduction band diagram near the quantum well active layer in the semiconductor laser device according to the seventh embodiment of the present invention.
[Explanation of symbols]
103 Quantum well light emitting layer
111, 112, ..., 118 Mg_0.1Zn_0.9O barrier layer
121, 122, ..., 127 Cd_0.1Zn_0.9O well layer
203 Quantum well light emitting layer
211, 212, ..., 218 Mg_0.1Zn_0.9O barrier layer
221, 222, ..., 227 Cd_0.1Zn_0.9O well layer
603 Quantum well light emitting layer
611,612, ..., 618 Mg_0.1Zn_0.9O barrier layer
621,627 Cd_0.05Zn_0.95O well layer
622, 623, ..., 626 Cd_0.1Zn_0.9O well layer
703 n-type Mg_0.05Zn_0.95O light guide layer
704 Multiple quantum well active layer
705 p-type Mg_0.05Zn_0.95O light guide layer
711,712 Mg_0.1Zn_0.9O barrier layer
721,722,723 Cd_0.1Zn_0.9O well layer

Claims (11)

An oxide semiconductor light emitting device composed of a ZnO-based semiconductor,
It has a quantum well structure in which Cd _x Zn _1-x O well layers (where 0 <x ≦ 1) and Mg _y Zn _1-y O barrier layers (where 0 <y ≦ 1) are alternately stacked. An active layer ,
A MgZnO mixed crystal having a band gap energy larger than the band gap energy of the barrier layer, and a clad layer sandwiching the active layer ,
The well layer is more than three layers, and the layer thickness of the well layer located at both ends in the stacking direction in the quantum well structure, unlike preparative layer thickness of the well layer located at a position other than the ends,
The oxide semiconductor light emitting element, wherein a thickness of the well layer located at both ends is greater than a thickness of the well layer located at a place other than the both ends . An oxide semiconductor light emitting device that can be composed of a ZnO-based semiconductor,
It has a quantum well structure in which Cd _x Zn _1-x O well layers (where 0 <x ≦ 1) and Mg _y Zn _1-y O barrier layers (where 0 <y ≦ 1) are alternately stacked. An active layer ,
A MgZnO mixed crystal having a band gap energy larger than the band gap energy of the barrier layer, and a clad layer sandwiching the active layer ,
The well layer is more than three layers, and the composition of the well layer located at both ends in the stacking direction in the quantum well structure, unlike the composition of the well layer located at a position other than the ends,
The band gap energy of the well layer located at the both ends is smaller than the band gap of the well layer located at a place other than the both ends . The oxide semiconductor light emitting device according to claim 1 or 2 ,
The Cd _x Zn _1-x O well layer has a Cd composition ratio x of 0 <x ≦ 0.2. The oxide semiconductor light emitting device according to claim 1 or 2 ,
The oxide semiconductor light emitting device, wherein the Mg composition ratio y of the Mg _y Zn _1-y O barrier layer is 0 <y ≦ 0.35. The oxide semiconductor light emitting device according to claim 1 or 2 ,
The Cd composition ratio x of the Cd _x Zn _1-x O well layer is 0 <x ≦ 0.08, and the Mg composition ratio y of the Mg _y Zn _1-y O barrier layer is 0 <y ≦ 0. 33. An oxide semiconductor light-emitting element, which is 33. The oxide semiconductor light emitting device according to claim 1 or 2 ,
An oxide semiconductor light-emitting element, wherein the active layer is doped with a donor impurity. The oxide semiconductor light emitting device according to claim 6 ,
The oxide semiconductor light-emitting element, wherein the donor impurity is doped only in the well layer. The oxide semiconductor light emitting device according to claim 6 ,
The oxide semiconductor light-emitting element, wherein the donor impurity is doped only in the barrier layer. The oxide semiconductor light emitting device according to claim 1 or 2 ,
The oxide semiconductor light-emitting element, wherein the well layer has a thickness in a range of 2 nm to 8 nm. The oxide semiconductor light emitting device according to claim 9 ,
Oxide semiconductor light emission characterized in that there are three or more well layers, and the film thickness of the well layers located at locations other than both ends in the stacking direction in the quantum well structure is in the range of 3 nm to 6 nm. element. The oxide semiconductor light emitting device according to claim 1 or 2 ,
An oxide semiconductor light emitting device comprising a carrier block layer containing MgZnO mixed crystal having a band gap energy larger than that of the cladding layer between the active layer and the cladding layer.

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