KR20090026299A - Nitride semiconductor light emitting device - Google Patents

Abstract

At a completely different point of time from the prior art, a nitride semiconductor light emitting device having improved carrier injection efficiency from a p-type nitride semiconductor layer to an active layer by simple means is provided. On the sapphire substrate 1, a buffer layer 2, an undoped GaN layer 3, an n-type GaN contact layer 4, an InGaN / GaN superlattice layer 5, an active layer 6, an undoped InGaN layer 7 ), the p-type GaN-based contact layer 8 is laminated, and the p-electrode 9 is mesa-etched on the p-type GaN-based contact layer 8 to expose the n-type GaN contact layer 4. The n electrode 10 is formed. An intermediate semiconductor layer formed between the well layer closest to the p side of the active layer having a quantum well structure and the p-type GaN-based contact layer 8 includes an undoped InGaN layer 7 and the total film thickness of the intermediate semiconductor layer. By making 20 nm or less, the carrier injection efficiency to the active layer 6 can be improved.

Description

Nitride Semiconductor Light Emitting Device {NITRIDE SEMICONDUCTOR LIGHT EMITTING ELEMENT}

The present invention relates to a nitride semiconductor light emitting device having a quantum well structure and having an active layer composed of nitride including a well layer.

Background Art In recent years, short wavelength semiconductor lasers have been developed with a focus on application to high density optical disc recording and the like. As a short wavelength semiconductor laser, hexagonal compound semiconductors (hereinafter, simply referred to as nitride semiconductors) containing nitrogen such as GaN, AlGaN, InGaN, InGaAlN, GaPN and the like are used. LEDs using nitride semiconductors have also been developed.

Although the light emitting element of the MIS structure was used for the said nitride semiconductor light emitting element, since the i-type GaN type semiconductor of high resistance is laminated | stacked, it was a problem that light emission output was very low generally. In order to solve this problem, electron irradiation or annealing is performed on the i-type GaN-based semiconductor layer.

In addition, even in the case of a nitride semiconductor light-emitting device in which a p-type GaN-based semiconductor layer is formed, efforts have been made to increase the light emission output. For example, as shown in Patent Literature 1, ohmic contact between the p-electrode and the p-type GaN contact layer is prevented. It is proposed to improve the luminous efficiency by obtaining a thin film thickness of the p-type GaN contact layer or lowering the forward voltage Vf.

Further, in Patent Document 1, in order to obtain the p-type characteristics of the p-type AlGaN cladding layer, by using Mg as the p-type dopant or by defining the film thickness and Al composition of the p-type AlGaN cladding layer, the crystallinity is improved. It is also proposed to improve luminous efficiency.

Patent Document 1: Japanese Patent Publication No. 2778405

(Tasks to be solved by the invention)

However, as in the prior art, the luminous efficiency is improved by improving the ohmic contact between the p-electrode and the p-type GaN contact layer, the film thickness of the p-type GaN contact layer, the p-type dopant, and the crystallinity of the p-type AlGaN cladding layer. Even if it improved, there was a limit to the improvement effect, and when it wanted to further improve luminous efficiency, there was no effective means.

The present invention was devised to solve the above-mentioned problems, and from a completely different viewpoint from the prior art, the carrier injection efficiency from the p-type nitride semiconductor layer to the active layer was improved by simple means, and the luminous efficiency was improved. It is an object to provide a nitride semiconductor light emitting device.

(Means to solve the task)

The nitride semiconductor light emitting device of the present invention is a nitride semiconductor light emitting device having a structure in which a well layer has an active layer having a quantum well structure composed of a nitride including In between a p-type nitride semiconductor layer and an n-type nitride semiconductor layer. And an undoped InGaN layer included in the intermediate semiconductor layer formed between the well layer disposed closest to the p side of the active layer and the p-type nitride semiconductor layer, wherein the total film thickness of the intermediate semiconductor layer is 20 nm or less. Make a point.

MEANS TO SOLVE THE PROBLEM The present inventors discovered that there exist a completely different means from the said prior art as a means of improving the hole injection efficiency from a p-type semiconductor layer to an active layer. That is, a part of the intermediate semiconductor layer formed between the well layer disposed at the position closest to the p side of the active layer and the p-type nitride semiconductor layer is composed of an undoped InGaN layer, and the total film thickness of the intermediate semiconductor layer is 20 nm or less. As a result, it was found that the hole injection efficiency from the p-type nitride semiconductor layer to the active layer changes rapidly.

In addition, when the intermediate semiconductor layer is composed of an active layer barrier layer and an undoped InGaN layer, the undoped InGaN layer may also be an In composition gradient layer whose In composition decreases toward the p-type nitride semiconductor layer. .

In addition, when Mg-doped p-type Al _x GaN (0.02≤x≤O.15) is formed as part of the p-type nitride semiconductor layer, the hole carrier concentration is in the range of 2x10 ¹⁷ cm ^-3 or more. It also makes a point.

In addition, the nitride semiconductor light-emitting device of the present invention, in addition to the above-mentioned point, when the In composition ratio of the well layer which is the active layer becomes 10% or more and the emission wavelength is long, from the end of film formation of the final well layer in the growth direction of the active layer. It is a summary that the sum total of the film-forming time whose growth temperature exceeds 950 degreeC is 30 minutes or less until the completion | finish of film-forming of the p-type contact layer formed in contact with a p electrode which is a part of p-type nitride semiconductor layer. InGaN is particularly thermally unstable, and if the above conditions are exceeded, there is a risk of decomposition. In the worst case, In is separated and the wafer becomes black.

(Effects of the Invention)

In the nitride semiconductor light emitting device of the present invention, a part of the intermediate semiconductor layer formed between the well layer closest to the p side of the active layer having a quantum well structure and the p-type nitride semiconductor layer is composed of an undoped InGaN layer. Since the total film thickness of is formed to be 20 nm or less, the injection efficiency of the holes into the active layer can be extremely increased, and the light emission efficiency is improved.

In the case where the intermediate semiconductor layer is composed of an active layer barrier layer and an undoped InGaN layer, the undoped InGaN layer is formed into an In composition gradient layer whose In composition decreases toward the p-type nitride semiconductor layer. This becomes easy to be injected and the luminous efficiency is improved.

Further, since p-type Al _x GaN (0.02≤x≤0.15) is laminated on the intermediate semiconductor layer, and the hole carrier concentration due to the p-type impurity is formed to be 2 x 10 ¹⁷ cm ^-3 or more, the hole injection amount into the active layer Can be taken sufficiently, and the luminous efficiency can be improved.

In addition, during the film formation time when the growth temperature exceeds 950 ° C from the end of film formation of the final well layer in the growth direction of the active layer to the end of film formation of the p-type contact layer formed in contact with the p-electrode which is a part of the p-type nitride semiconductor layer. Since the total is less than 30 minutes, especially in a nitride semiconductor light emitting device having a long emission wavelength, that is, an element having an In composition ratio of 10% or more in the well layer of the active layer, deterioration of the active layer can be prevented and high The luminous intensity can be maintained.

1 is a view showing a cross-sectional structure of a first nitride semiconductor light emitting device of the present invention.

2 is a diagram showing a layer structure near the active layer.

3 is a view showing a layer structure different from that of FIG. 2 near the active layer.

4 is a view showing a cross-sectional structure of a second nitride semiconductor light emitting device of the present invention.

Fig. 5 is a graph showing the relationship between the total film thickness of the intermediate semiconductor layer formed between the final well layer of the active layer and the p-type nitride semiconductor layer and the luminance of the nitride semiconductor light emitting device.

FIG. 6 is a diagram showing an emission spectrum when the film thickness of an undoped InGaN layer is 350 kPa. FIG.

FIG. 7 is a diagram showing an emission spectrum when the film thickness of an undoped InGaN layer is 120 kPa. FIG.

8 is a diagram showing a relationship between In composition of an undoped InGaN layer and luminance of a nitride semiconductor light emitting device.

9 is a diagram illustrating a state of band gap energy in the vicinity of an active layer.

It is a figure which shows the relationship between the relative ratio of In flow volume for every growth temperature, and the In composition ratio of an InGaN layer.

11 is a graph showing the relationship between the growth temperature of the InGaN layer and the In composition ratio.

12 is a conceptual diagram for calculating the EL integral relative strength.

Fig. 13 is a diagram showing a state in which the EL integral relative strength changes depending on the type of intermediate semiconductor layer formed between the last well layer of the active layer and the p-type nitride semiconductor layer.

Fig. 14 is a diagram showing a state in which the EL integral relative intensity changes depending on the kind of intermediate semiconductor layer formed between the last well layer of the active layer and the p-type nitride semiconductor layer.

FIG. 15 is a graph showing the relationship between the Al composition ratio of AlGaN and the luminance of the nitride semiconductor light emitting element. FIG.

Fig. 16 is a graph showing the relationship between AlGaN growth temperature and emission spectrum.

It is a figure which shows the state in which the value which integrated PL intensity changed with temperature.

FIG. 18 is a diagram illustrating a relationship between a growth temperature and an internal quantum efficiency of a p-type nitride semiconductor layer. FIG.

19 is a diagram showing a relationship between a growth time and internal quantum efficiency for each growth temperature of a p-type nitride semiconductor layer.

(Explanation of the sign)

1: sapphire substrate 2: buffer layer

3: undoped GaN layer 4: n-type GaN contact layer

5 InGaN / GaN superlattice layer 6 Active layer

6a: barrier layer 6b: barrier layer

6c: well layer 7: undoped InGaN layer

8: p-type GaN-based contact layer 9: p-electrode

10: n electrode 11: p-type AlGaN cladding layer

1 shows a cross-sectional view of an example of a first nitride semiconductor light emitting device of the present invention. On the sapphire substrate 1, a buffer layer 2, an undoped GaN layer 3, an n-type GaN contact layer 4, an InGaN / GaN superlattice layer 5, an active layer 6, an undoped InGaN layer 7 ), the p-type GaN-based contact layer 8 is laminated, and a part of the region is mesa-etched from the p-type GaN-based contact layer 8, so that the n-type GaN contact layer 4 is exposed. The n electrode 10 is formed. The p-electrode 9 is formed on the p-type GaN-based contact layer 8. Here, the p-type GaN-based layer is composed of GaN or a compound containing GaN doped with a p-type impurity, and the undoped InGaN layer is composed of an InGaN layer which is not intentionally doped with impurities.

As described above, the n-type GaN contact layer 4 and the InGaN / GaN superlattice layer 5 are formed as the n-type nitride semiconductor layer, and the p-type GaN-based contact layer 8 is formed as the p-type nitride semiconductor layer. The nitride semiconductor light emitting device of the present invention has a double heterostructure in which an active layer is maintained between these n-type nitride semiconductor layers and p-type nitride semiconductor layers.

The buffer layer 2 is made of GaN, AlN, Al _{x 1} GaN (0 < _x1 ≦ 0.1), or the like, and is formed to a film thickness of 50 to 300 GPa, preferably 100 to 200 GPa. The undoped GaN layer 3 stacked on the buffer layer 2 has a film thickness of 1 to 3 µm, and the n-type GaN contact layer 4 formed on the undoped GaN layer 3 has a Si doping concentration of 1 to 5 x. It consists of 10 ¹⁸ cm <^-3> , film thickness 1-5 micrometers. Further, the InGaN / GaN superlattice layer 5 relieves the stress of InGaN and GaN having a large lattice constant difference and makes it easier to grow InGaN of the active layer 6, for example, the Si doping concentration is 1 to 5 x 10 ^18. A structure in which In _x GaN (0.03? _X ? 0.1) having a thickness of 10 m ³ and a thickness of 10 m and a GaN having a thickness of 20 m 2 are alternately laminated for about 10 cycles is used.

The active layer 6 is an active layer having a quantum well structure, and has a structure in which a well layer (well layer) is sandwiched between barrier layers (barrier layers) having a larger band gap than the well layer. . This quantum well structure may be multiplexed instead of one, and in this case, a multi quantum well structure is used. In addition, the active layer 6 is composed of InGaN of a three-way mixed crystal system. The undoped InGaN layer 7 is formed in contact with the active layer 6 and includes a role of a cap layer that suppresses thermal decomposition of In of the active layer 6.

2 illustrates the structure of the active layer 6 in detail. The barrier layer 6b is disposed on the side where the active layer 6 is in contact with the InGaN / GaN superlattice layer 5, and the well layer 6c is stacked thereon, and the barrier layer 6b and the well layer 6c are disposed thereon. After several alternating cycles of this cycle, 6a is formed as the last barrier layer, and an undoped InGaN layer 7 is laminated on the barrier layer 6a. A p-type GaN-based contact layer 8 is formed on the undoped InGaN layer 7.

Here, the barrier layer 6b has a non-doping or Si doping concentration of 5 × 10 ¹⁶ cm ^{-3 to} 5 × 10 ¹⁸ cm ^-3, and has an In _z1 GaN (0) of 100 to 350 Pa, preferably 150 to 300 Pa. ≤ z1 <1). On the other hand, the well layer 6c may be composed of, for example, non-doped In _y1 GaN (0 <y1 <1, y1> z1) having a film thickness of 30 GPa. However, when doping impurities, the Si doping concentration is 5 ×. It is preferable to set it as 10 ¹⁸ cm <^-3> or less. Moreover, it is comprised so that a well layer may be 3-8 layers, Preferably it is 5-7 layers. In the active layer 6, the emission wavelength can be changed from purple to red by changing y1 in the range of 0 <y1 <1.

As shown in Fig. 2, after forming the last well layer 6c in the growth direction, the barrier layer 6a is formed. The barrier layer 6b and the Si doping concentration so far may be the same, but the film thickness of the barrier layer 6a is made thinner than the barrier layer 6b and is formed to about 20 to 30 kPa. The barrier layer 6a may be formed of GaN (z1 = 0). However, in order to improve the luminous efficiency, InGaN (z1 ≠ 0) is preferable. In this case, the composition of In is 0.5 to 1% is good. 3, the undoped InGaN layer 7 may be formed in contact with the last well layer 6c in the growth direction. In this case, the undoped InGaN layer 7 serves as an electron barrier layer that prevents electrons from flowing into the p side from the active layer 6, and when the temperature becomes high, In of the well layer 6c is easily sublimed and broken, thereby preventing it. It also serves as a cap layer.

In either case of FIGS. 2 and 3, the film thickness of the undoped InGaN layer 7 is, for example, about 10 to 200 kPa. Thus, in the case of FIG. 2, the total film thickness of the barrier layer 6a and the undoped InGaN layer 7 is formed so that it may be 20 nm (200 micrometers) or less. On the other hand, in the case of Fig. 3, the film thickness of the undoped InGaN layer 7 formed in contact with the last well layer of the active layer 6 is formed to be 20 nm or less.

3, when the undoped InGaN layer 7 is formed in contact with the last well layer of the active layer 6, in order to block electrons by making band gap energy larger than the well layer 6c, It is preferable to have a band gap energy equal to or higher than the barrier layer 6b, and accordingly, it is preferable to set it as the undoped InGaN layer 7 of In composition of composition ratio z1 or less. Undoped InGaN layer 7, the p-type GaN-based con tact layer 8 is formed on, the p-type InGaN or p-type GaN is used, Mg doping concentration ^{3 × 10 19 ㎝ -3 ~ 3} × 10 20 ㎝ - ³ , and the film thickness is grown to be about 200 to 3000 kPa (most preferably 700 kPa to 1000 kPa).

Fig. 5 shows the total film thickness (horizontal axis) of the last well layer in the growth direction of the active layer, that is, the intermediate semiconductor layer formed between the well layer disposed at the position closest to the p side of the active layer and the p-type nitride semiconductor layer, and the luminance of the light emitting element. The relationship between the (vertical axis) is shown. The light emission intensity (luminance) was measured by changing the film thickness of the intermediate semiconductor layer. The vertical axis is shown on the basis of the luminance at 250 Hz. Here, the total film thickness of the intermediate semiconductor layer corresponds to the total film thickness of the barrier layer 6a and the undoped InGaN layer 7 as long as it is the configuration of FIG. 2, and on the other hand, in the case of FIG. It corresponds to the film thickness of the InGaN layer 7 itself. The horizontal axis represents the total film thickness of the intermediate semiconductor layer, and the vertical axis represents the luminance (arbitrary unit). When the total film thickness is 200 kPa (20 nm) or less, it can be seen that the luminance is rapidly improved.

This can be considered as follows. FIG. 6 shows the emission spectrum in the case of the film thickness of the undoped InGaN layer 7 in the light emitting device having the structure of FIG. 3 in the configuration of FIG. The vertical axis is shown relative to the light emission intensity of the standard LED. In FIG. 6, not only the original emission spectrum of the active layer 6 but also the spectrum of the undoped InGaN layer 7 are mixed, and the recombination of holes and electrons is performed not only in the active layer 6 but also in the undoped InGaN layer 7. Since the holes do not sufficiently move from the p-type GaN-based contact layer 8 to the active layer 6, the luminous efficiency of the active layer 6 is lowered.

On the other hand, Fig. 7 shows the emission spectrum when the film thickness of the undoped InGaN layer 7 is 120 Hz, but only the original emission spectrum of the active layer 6 is shown, and the undoped InGaN layer 7 as shown in Fig. 5 is shown. The spectrum of does not appear. This is because the thinner the thickness of the undoped InGaN layer 7 improves the injection efficiency of holes from the p-type GaN-based contact layer 8 into the active layer 6. Therefore, the thinner the film thickness of the undoped InGaN layer 7 becomes, the larger the light emission intensity of the light emitting element is. And as an optimal value of this film thickness, it turns out that it is 200 micrometers (20 nm) or less from FIG.

Next, the structure of the 2nd nitride semiconductor light emitting element of this invention is shown in FIG. The same code | symbol as FIG. 1 shows the structure similar to FIG. The difference between the second nitride semiconductor light emitting device and the first nitride semiconductor light emitting device is that the p-type AlGaN cladding layer 11 is inserted between the undoped InGaN layer 7 and the p-type GaN-based contact layer 8. The p-type AlGaN cladding layer 11 has a role of an electron block layer and is for further increasing the injection efficiency of holes, and uses p-type Al _x GaN (0.02 ≦ _x ≦ 0.15) or the like. As described later, the carrier concentration of p-type Al _x GaN by the doping of the impurity Mg is preferably in the range of 2 × 10 ¹⁷ cm ⁻³ or more, for example, a film thickness of 150 to 300 kPa (most preferably 200 kPa). Al is composed of _0.07 GaN. In addition, the intermediate semiconductor layer can have the structure of FIG. 2 or FIG. 3.

In the second nitride semiconductor light emitting device (constitution of FIG. 4), the luminance was measured by changing the film thickness of the undoped InGaN layer 7, and even in this case, the graph shape of FIG. 5 was obtained. Therefore, also in the structure of FIG. 4, when the film thickness of the undoped InGaN layer 7 becomes 200 kPa or less, brightness will improve rapidly.

8 shows the relationship between the In composition ratio of the undoped InGaN layer 7 and the luminance of the nitride semiconductor light emitting element. The horizontal axis represents the In composition ratio of the undoped InGaN layer 7, and the vertical axis represents the luminance (arbitrary unit). The vertical axis represents relative to the luminance when the In composition ratio is 0.5%. This luminance measurement was performed with the light emitting element which has the layer structure in which the undoped InGaN layer 7 was formed in contact with the last well layer of the active layer 6 like FIG. As can be seen from the figure, up to about 2.5% of the In composition ratio can be used as the luminescence brightness, but after that the luminescence brightness is considerably close to zero and becomes unusable. This is because in InGaN, since the residual electron concentration is large, and the In composition ratio is increased, the residual electron concentration also increases, and it is understood that it is preferable to reduce the In composition ratio in order to increase the carrier (hole) injection amount. In addition, it is shown that the In composition is about 0.5%-1% that can maintain the state with the highest brightness | luminance.

Next, the case where the undoped InGaN layer 7 is an In composition gradient layer will be described below. In order for the undoped InGaN layer 7 to be an In composition gradient layer, as shown in FIG. 2, the intermediate semiconductor layer must be composed of the barrier layer 6a and the undoped InGaN layer 7. 9 shows a band gap energy diagram in the conduction band before and after the active layer 6. A quantum well structure is formed by the barrier layer 6b and the well layer 6c, and the barrier layer 6a is formed in contact with the last well layer 6c at the p side, and is undoped in contact with the barrier layer 6a. Although the InGaN layer 7 is formed, the In composition ratio of the undoped InGaN layer 7 is continuously from the connection point with the barrier layer 6a toward the direction of the p-type nitride semiconductor layer (the right direction toward the drawing). Configured to be reduced. The smaller In composition ratio toward the p-type nitride semiconductor layer means that the band gap energy of the undoped InGaN layer 7 increases toward the p-type nitride semiconductor layer.

When the In composition is inclined, as shown in Fig. 9, since the band structure in the conduction band responsible for hole conduction lowers the potential toward the well layer, the holes are easily introduced, which is preferable. In addition, if the growth temperature is high, the residual electron concentration decreases. Therefore, the composition gradient of In is preferably produced by increasing the growth temperature.

Next, the manufacturing method of the said 1st and 2nd nitride semiconductor light emitting element is demonstrated. On the sapphire substrate 1, the PLD method (laser polishing method) is used to form the buffer layer 2 made of single crystals such as GaN, AlN, and Al _{x 1} GaN (0 < _{x 1} ? 0.1).

First, the sapphire substrate 1 is placed in a load lock chamber, heated at a temperature of about 400 ° C. for 5 to 10 minutes, and the remaining moisture is blown off. Thereafter, the sapphire substrate 1 is conveyed in a vacuum chamber having a pressure in the chamber of 1 × 10 ⁻⁶ Torr or less, disposed to face the target, and the sapphire substrate 1 is mounted on a heating source, thereby providing a substrate. The temperature of the target is maintained at 600 ° C. to 1000 ° C., for example, by irradiating the target with a KrF excimer laser light having an oscillation wavelength of 248 nm from the quartz window of the vacuum chamber, thereby subliming (polishing) the material of the target. The sublimed atoms adhere to the surface of the sapphire substrate 1, and the single crystal buffer layer 2 grows. The buffer layer 2 is formed, for example, 100 mV to 200 mV.

As the target, for example, a sintered GaN target is used. Of course, a sintered compact target of AlN, AlGaN or InGaN can be used. However, when using a sintered compact target, in the InGaN sintered compact target, since it is a substance which In hardly enters, it is hard to determine a composition. Therefore, the sintered compact target of GaN, AlN, or AlGaN is preferable.

Next, as described above, the sapphire substrate 1 on which the buffer layer 2 is formed is placed in the load lock chamber of the MOCVD apparatus, heated at a temperature of about 400 ° C. for 5 to 10 minutes, and the remaining moisture or the like is blown away. The substrate is returned to the reaction chamber of the apparatus. 1100 ℃ from the MOCVD apparatus, a thermal cleaning is carried out for 30 minutes in atmosphere of NH _3.

Subsequently, the substrate temperature is raised to 1065 ° C., for example, an undoped GaN layer 3 is stacked, for example, 1 μm, and Si-doped n-type GaN is grown on the undoped GaN layer 3 by 2.5 μm. The substrate temperature is lowered to 760 ° C, and the InGaN / GaN superlattice layer 5 is formed at, for example, 300 kPa. The substrate temperature is lowered to 750 ° C. and the active layer 6 is formed, for example, 3/17 nm.

Here, after forming the last barrier layer 6a in the structure of FIG. 2, after growing the last well layer 6c in the structure of FIG. 3, the undoped InGaN layer 7 remains at a substrate temperature of 750 ° C. )). As described above, the film thickness is adjusted so that the total film thickness of the intermediate semiconductor layer is 20 nm or less. When the undoped InGaN layer 7 is not an In composition gradient layer, the In composition of InGaN is 2.5% or less, as can be seen from FIG. 8, but 0.5% to 1% is most suitable.

Next, in the case of the structure of FIG. 1, as a p-type GaN type contact layer 8, a growth temperature is raised to 1000-1030 degreeC (for example, 1010 degreeC), and a p-type GaN layer is grown, for example, 700 microseconds. In addition, as will be described later, a p-type InGaN layer doped with Mg may also be used, and in this case, for example, 700 mW is grown.

After removing the native oxide film on the surface of the p-type GaN-based contact layer 8 with hydrochloric acid, a multilayer metal film such as Ti / Au is formed by vapor deposition or sputtering as the p-electrode 9. Next, a mesa pattern is formed, and the GaN-based semiconductor laminate is etched until the n-type GaN contact layer 4 is exposed. At this time, it is preferable to form a pattern such that pillars are formed in the periphery of the mesa at the same time and to make the surface of the n-type GaN contact layer 4 become rough surface, so that light extraction becomes large. However, in the case of not being a rough surface, the etching depth until the n-type GaN contact layer 4 is exposed is sufficient, but in the case of being a rough surface, the etching depth is 1 µm or more deeper than the exposed surface of the n-type GaN contact layer 4. If it is performed, light extraction will become large and it is preferable.

After completion of the mesa etching, Al is formed as the n-electrode 10 on the n-type GaN contact layer 4, and annealing treatment for taking an ohmic at 500 ° C to 700 ° C is performed to complete the configuration of FIG. 1.

By the way, without forming the p-electrode 9 on the p-type GaN-based contact layer 8, and stacking a ZnO electrode on the p-type GaN-based contact layer 8, the p-electrode 9 may be formed. good. In this case, a Ga-doped ZnO electrode is formed on the p-type GaN-based contact layer 8 by, for example, Molecular beam epitaxy (MBE) or Pulsed Laser Deposition (PLD). At this time, if the specific resistance of ZnO is high, current enlargement is not obtained. Therefore, the specific resistance must be at least 1 × 10 ⁻³ Ωcm or less, and preferably 1-5 × 10 ⁻⁴ Ωcm. Thereafter, it is preferable to form irregularities on the ZnO surface as on the n-type GaN contact layer surface described above.

Since the ZnO electrode is of a predetermined size, after etching to the p-type GaN-based contact layer 8 using wet etching of hydrochloric acid or dry etching such as RIE, the entire ZnO is etched into SiN, SiON, SiO ₂ , Al. Cover with insulator such as ₂ O ₃ , ZrO ₂ .

Thereafter, as described above, mesa etching is performed to form the n electrode 10 on the n-type GaN contact layer 4, and then partially forms a contact hole on the ZnO electrode to form a contact hole, and ZnO through the contact hole. Ti / Au or the like is formed as a p-electrode so as to be in contact with the electrode. At this time, Ti / Au is added also on Al as an n electrode, and it is set as the metal for wire bonding. Thereafter, the whole may be covered with an insulator such as SiN, SiON, SiO ₂ , Al ₂ O ₃ , ZrO ₂ , a hole in the metal part is formed, the sapphire substrate 1 is made thin, and then chipped. good.

4, the p-type AlGaN cladding layer 11 is formed before the p-type GaN-based contact layer 8, that is, after the formation of the active layer 6, for example, 200 mV. AlGaN growth may be performed at a temperature of about 950 ° C, but when it is desired to improve crystallinity, it is preferable to set it to about 1000 ° C or more. Formation of each subsequent layer is as described above.

Next, the manufacturing method of an undoped InGaN layer when the undoped InGaN layer 7 is comprised from the In composition gradient layer which has In composition gradient as shown in FIG. 9 is demonstrated. Usually, the undoped when growing an InGaN layer, trimethyl indium (TMI), triethyl gallium (TEG), nitrogen (N _2), NH _3, the H ₂ supply, but control the composition ratio of In in the growth chamber If desired, it is common to increase or decrease the flow rate (feed ratio) of trimethylindium under a constant temperature.

However, as shown in FIG. 10, when it is going to cover the range of the composition ratio of In widely, only the control ratio of a trimethyl indium supply cannot control broadly the composition ratio of In. Fig. 10 shows the relationship between the trimethylindium flow rate relative ratio and the In composition ratio when InGaN is produced. The trimethyl indium flow rate relative ratio is a ratio of the respective TMI flow rates when a certain flow rate is arbitrarily determined and the flow rate is 1, and a graph is drawn for each specific growth temperature.

For example, when the TMI flow rate relative ratio becomes about 0.2 or less, it can be seen that the In composition suddenly changes toward 0, and it becomes difficult to control the In composition ratio in this range. Therefore, it is intended to simply form an In composition gradient layer having a wide range of composition ratios by using a region in which the composition ratio of In hardly changes even if the supply ratio of trimethyl indium is increased or decreased.

As can be seen from FIG. 10, when the growth temperature is made constant, the In composition ratio remains saturated even if the supply ratio of trimethyl indium is increased or decreased in the vicinity of the point S (about 1.3) of the TMI flow rate. It is.

Thus, for example, the point S is taken as the value of the relative TMI flow rate in the region where the composition ratio of In hardly changes, the TMI flow rate relative ratio is fixed at the point S, and In of the curve for each growth temperature corresponding to the point S is obtained. If the composition ratio is P1, P2, P3, P4, it is found that when the growth temperature is changed from 770 ° C to 840 ° C, at least the In composition ratio changes from P1 to P4, that is, from about 18.5% to about 8%. Can be.

Thus, when the TMI flow rate relative ratio is fixed at S point, FIG. 11 plots the In composition ratio at the time of changing a growth temperature from 770 degreeC to 840 degreeC, and further high temperature is shown in the graph. 11, the horizontal axis represents the growth temperature of undoped InGaN, and the vertical axis represents the In composition ratio of undoped InGaN.

In this manner, when the growth temperature is raised without changing the relative flow rate of the TMI, the width of the upper and lower portions of the In composition ratio can be widened, and the In composition gradient layer can be easily produced.

In the configuration of FIG. 1, and also having the structure of FIG. 2, after the active layer 6 is grown at a growth temperature of 750 ° C., for example, as the p-type GaN-based contact layer 8, a p-type InGaN layer doped with Mg In the case of forming a film, the growth temperature may be increased to about 850 ° C., so that an undoped InGaN layer having an In composition gradient can be automatically formed in the process of raising the growth temperature. Specifically, when the growth temperature is sequentially raised to about 850 ° C., an undoped InGaN layer having a composition gradient curve as shown in FIG. 10 is formed. In the configuration of FIG. 4, when the p-type AlGaN cladding layer 11 is grown at a temperature of about 950 ° C., a gradient curve from the composition gradient curve shown in FIG. 11 to a growth temperature of about 950 ° C. can be obtained.

As described above, the range of the composition gradient curve shown in FIG. 10 is determined by the start point and the end point of the growth temperature, but the change in the In composition ratio of the In composition gradient layer is, for example, from 18% to 3%. In the case where it is desired to form continuously, the growth temperature may be changed to T1 to T2, and when the change in the In composition ratio is to be continuously formed to 3% to 0.5%, the growth temperature may be changed to T2 to T3. As described above, if the growth temperature is high, the residual electron concentration is decreased. Therefore, it is preferable to produce the In composition gradient layer by increasing the growth temperature, and it is also preferable to make the starting point of the growth temperature high.

By the way, in the structure of FIG. 1 or FIG. 4, the luminous efficiency at the time of using three types of structures as an intermediate semiconductor layer was compared. The curves of X1 to X3 all use the structure of FIG. 3, and the curve of X1 is a semiconductor layer in contact with the last well layer of the active layer 6, and the low temperature undoped grown at a low temperature of 750 to 800 ° C. instead of the undoped InGaN layer. In the case of using a GaN layer (film thickness of 400 kPa), the curve of X2 is a curve of X3 in the case of using an undoped InGaN layer (film thickness of 200 kPa) having no In composition gradient as a semiconductor layer in contact with the last well layer of the active layer 6. Shows a case where an undoped InGaN layer (film thickness 200 m) having an In composition gradient is used as a semiconductor layer in contact with the last well layer of the active layer 6. This is calculated by obtaining the relative intensity of electroluminescence (EL) integration. Although FIG. 12 is an example of photoluminescence (PL), since it is exactly the same as EL, it demonstrates as this. First, the temperature is changed to measure the emission spectrum (PL intensity distribution), and the integrated value of the PL intensity distribution for each temperature is obtained.

For example, the PL intensity integrated value in the case of absolute temperature 12K (K represents Kelvin, the same applies below) corresponds to the area of the curve of 12K in the figure. Next, when the predetermined absolute temperature is expressed by RT, the PL intensity integral value at that RT corresponds to the area of the RT curve in the figure. RT is changed from 12K to about 290K, and the PL intensity integral value for each temperature is obtained and shown in the graph. An example of this graph is shown in Fig. 17. Usually, since the luminous efficiency deteriorates when the temperature rises, the PL intensity integrated value becomes small. As shown in FIG. 17, the average of the PL intensity integrated value of the state with the best luminous efficiency is represented by I (12K), and this I (12K) becomes a reference | standard.

Next, when the PL intensity integral value for the temperature parameter RT is called I (RT), the PL integral relative intensity is expressed by I (RT) / I (12K). Fig. 13 shows I (RT) / I (12K), where the vertical axis is the EL integral relative intensity (PL integral relative intensity), and the horizontal axis is the inverse of the absolute temperature and is an Arennius plot. T of (1000 / T) indicated in the description of the abscissa is absolute temperature and the unit is K (Kelvin). The above measurement and calculation were performed, and the graph of X1-X3 was obtained.

In FIG. 13, the one closer to zero in the horizontal axis corresponds to the direction in which the temperature rises. Therefore, even if it is close to 0 of the horizontal axis, the one where the value of the EL integral relative intensity is closer to 1 becomes better in light emission efficiency. The good luminous efficiency means that the hole injection efficiency from the p-type nitride semiconductor layer of the p-type GaN-based contact layer or the p-type AlGaN cladding layer is good, and the semiconductor is in contact with the last well layer of the active layer 6 due to the structure of the nitride semiconductor element. Since only the layers compare different things, it can be seen from which semiconductor layer the most implantation efficiency is improved.

As can be seen from Fig. 13, since the curve of X3 shows the state of the highest luminous efficiency over the temperature of 12K to 290K, the semiconductor layer having the In composition gradient as the semiconductor layer in contact with the last well layer of the active layer 6 is shown. In the case where the dope InGaN layer is used, the injection efficiency of most holes is improved.

On the other hand, curve Y2 of FIG. 14 has an intermediate semiconductor layer formed between the well layer disposed at the position closest to the p side of the active layer 6 and the p-type nitride semiconductor layer as the structure of FIG. 2, and the intermediate semiconductor layer is undoped. A barrier layer 6a formed of In _z1 GaN (0 ≦ _z1 <1) and an undoped InGaN layer having an In composition inclination were made to have a total film thickness of 100 GPa and thinner to a total film thickness of 20 nm or less. The EL integrated relative intensity in the case is shown. Moreover, curve Y1 shows EL integral relative intensity curve like curve X3 shown in FIG.

In the same manner as in Fig. 13, a graph is drawn with the EL axis relative intensity and the horizontal axis as 1000 / T. Comparing the curves Y1 and Y2, it can be seen that the closer to the room temperature, the more the injection efficiency of the hole increases on the Y2 side. This is because the film thickness of the undoped InGaN layer 7 is reduced. The total film thickness of the intermediate semiconductor layer is 200 Pa or less, and the total injection film thickness is made thinner, so that the hole injection efficiency is increased and light emission is achieved. It shows that the efficiency is improved.

Next, as shown in Fig. 4, Fig. 15 shows the relationship between the Al composition, the hole carrier concentration, and the light emission intensity of the nitride semiconductor light emitting element in the configuration of Fig. 4 when the p-type AlGaN cladding layer 11 is formed. A graph of the case where the Al composition ratio of p-type AlGaN is shown on the horizontal axis, the emission intensity is shown on the vertical axis, and the hole carrier concentration is changed. When the carrier concentration is less than 2 × 10 ¹⁷ cm ^-3 , such as the curve of carrier concentration 8 × 10 ¹⁶ cm ^-3 or the curve of 5 × 10 ¹⁶ cm ^-3 , the slope of the curve becomes extremely large and the Al composition ratio becomes small. Accordingly, the luminous intensity drops extremely.

In general, when the Al composition of the p-type AlGaN is increased, the barrier height can be easily secured, but the band gap is increased, the impurity activation rate is reduced, and the carrier concentration is low even at the same impurity concentration. Since the enhancement of the carrier concentration determines a certain barrier height for the electrons, the range to be used appropriately is determined. The use range is Al _x GaN (0.02? _X ? 0.15). Within this range, it is understood that the carrier concentration should be at least 2 × 10 ¹⁷ cm ⁻³ or more if the luminescent intensity does not decrease to an extreme and finds a practically enduring state.

By the way, the growth of the p-type AlGaN cladding layer can be formed even at a substrate temperature of 950 ° C. However, in the case of p-type AlGaN, the crystallinity is improved to prevent the occurrence of carrier compensation effect and increase of the residual electron concentration, thereby increasing the hole concentration. In order to keep (carrier concentration) high, as mentioned above, the growth temperature of 1000 degreeC or more is preferable.

Fig. 16 shows a state in which crystallinity changes with growth temperature. The vertical axis represents photoluminescence intensity (arbitrary unit), and the horizontal axis represents emission wavelength. The vertical axis represents the measured photoluminescence intensity (PL intensity) and is relatively shown based on the peak intensity (peak intensity of GaN) of the PK portion shown in the drawing. This is a configuration in which undoped GaN is laminated on a sapphire substrate, and an AlGaN monolayer 2000 Å is laminated on this undoped GaN. The excitation light source was measured at an excitation intensity of 2.5 mW and a measurement temperature of 12 K using a He-Cd laser. . Where K is Kelvin representing the absolute temperature.

Although p-type AlGaN may be grown at a substrate temperature of 950 ° C, as shown in FIG. 16, when grown at a substrate temperature of 950 ° C, a phenomenon called deep level emission occurs. This indicates that the carrier compensation effect in AlGaN or the new level, that is, crystal defects, occurs in the band gap, resulting in a decrease in the hole concentration. On the other hand, when growing at a substrate temperature of 1010 ° C. to improve crystallinity, deep state light emission does not occur, so that the hole concentration is maintained as it is, and deterioration of the hole injection efficiency can be prevented. Therefore, in order to further improve the crystallinity of p-type AlGaN, it turns out that the growth temperature of 1000 degreeC or more is preferable.

As described in FIG. 16, in order to make the crystallinity of p-type AlGaN very good, the growth temperature of 1000 ° C. or higher is better. In order to produce, the growth temperature is preferably at least 950 ° C or higher. However, when Al _X Ga _Y N (where X + Y = 1, 0 ≦ X <1, 0 <Y ≦ 1) used for a p-type current injection layer is grown at a high temperature of 950 ° C. or higher, a good p-type Crystals exhibiting conductivity are obtained, but when fabricated at a temperature lower than 950 ° C., the imperfections of the crystals become very large, and the hole concentration does not improve due to the carrier compensation effect or the increase in the residual electron concentration, and the crystal exhibits good p-type conductivity. This is not obtained.

By the way, especially in the visible light LED which light-emits at the peak wavelength of 410 nm or more using nitride which is especially important industrially, although the In composition of the InGaN well layer 6c of the active layer 6 may be 10% or more, The higher the temperature, the easier Sub is sublimed and broken, leaving the light emission efficiency extremely low. Therefore, when the p-type Al _X Ga _Y N is grown at a high temperature exceeding 950 ° C, the crystallinity of the p-type Al _X Ga _Y N layer is improved, but the In component in the active layer having a high In composition ratio is already formed. Decomposition | disassembly and the luminous efficiency fell significantly, there existed a problem.

This state is shown in FIG. As the nitride semiconductor light emitting device, the In composition ratio range of the active layer 6 was changed as follows using the configuration of FIG. 1 or FIG. 4 described above. As an example of the configuration of the active layer 6 when the In composition of the InGaN well layer 6c has a peak wavelength of 10% or more, that is, 410 nm or more, the barrier layer 6b has a Si doping concentration of 5 × 10 ¹⁶ cm ^{−. It was} 3-5 * 10 <^18> cm <^-3> and consisted of Inz2 GaN (0 <= _{z2 <} = 0.03) of 100-350 Pa of film thickness, Preferably it is 150-300 Pa. On the other hand, the well layer 6c is composed of, for example, non-doped In _y2 GaN (0.15≤y2≤0.18) having a film thickness of 30 kPa. In addition, when doping an impurity in the well layer 6c, it is preferable to make Si doping concentration into 5 * 10 <^18> cm <^-3> or less. Moreover, it is comprised so that a well layer may be 3-8 layers, Preferably it is 5-7 layers.

Fig. 17 shows how the light emitting efficiency of the nitride semiconductor light emitting device changes with the growth temperature of the p-type GaN-based contact layer or the p-type AlGaN cladding layer. For example, in the configuration shown in FIG. 1, the p-type GaN-based contact layer is a p-type GaN contact layer, the growth temperature is kept constant, and the growth time of the p-type GaN contact layer is 27 minutes to form a light emitting device. The internal quantum efficiency was measured, and the growth temperature of the p-type GaN contact layer was changed to measure the internal quantum efficiency for each growth temperature. The growth temperature was set at 880 ° C in the first measurement, 950 ° C in the second measurement, and 1010 ° C in the third measurement and 1060 ° C in the fourth measurement. In Fig. 17, the horizontal axis represents the growth temperature of the p-type GaN contact layer, and the vertical axis represents the internal quantum efficiency (%) of the light emitting device.

By the way, the internal quantum efficiency is obtained as follows. As shown in FIG. 12, PL (photoluminescence) integral intensity value (area of the curve of 12K of a figure) in the case of absolute temperature 12K (K represents Kelvin) is represented by J (12K). Next, the PL intensity distribution curve when the absolute temperature is 290 K is integrated, the PL integrated intensity value (area of the curve of RT = 290 K in FIG. 12) is obtained, and the PL integrated intensity value is set to I (290 K). In this way, the PL integral intensity value at some point between 12K and 290K is obtained, and as shown in FIG. 13, the graph is plotted by plotting. The abscissa in FIG. 17 is the inverse of the absolute temperature and is an Arrhenius plot.

The average of the PL intensity integrated values in the state with the best luminous efficiency is represented by I (12K), and this I (12K) is used as a reference. It is represented by internal quantum efficiency η = I (290 K) / I (12 K). Accordingly, the higher the internal quantum efficiency is, the better the light emission efficiency is and the higher the light emission intensity is.

As can be seen from FIG. 18 shown on the basis of the internal quantum efficiency obtained as described above, the emission efficiency deteriorates rapidly after exceeding 1010 ° C. Thus, as a growth temperature which does not deteriorate the InGaN well layer 6c of the active layer 6, while maintaining the crystallinity of a p-type GaN layer and a p-type AlGaN layer favorable, it is from 950 degreeC to 1010 degreeC from FIG. It is preferable to set it as between.

In Fig. 18, the growth time is fixed at 27 minutes, and since the relationship between the growth temperature and the growth time is not known, the following items were also measured. For example, in the configuration of FIG. 1, the nitride semiconductor light emitting device in which the p-type GaN-based contact layer 8 is a p-type GaN contact layer and the In composition of the well layer 6c is 10% or more as described above. The relationship between the growth time and the internal quantum efficiency from the end of film formation of the well layer closest to the p side among the well layers of the active layer 6 to the end of film formation of the p-type GaN contact layer was measured. The results are shown in FIG. 19, the horizontal axis representing the growth time described above, the vertical axis representing the internal quantum efficiency, and the growth temperature from the completion of the deposition of the well layer closest to the p side to the completion of the deposition of the p-type GaN contact layer once. The first time was changed to 900 ° C, the second time to 950 ° C, and the third time to 1010 ° C, and measured for each growth temperature.

Here, the growth time from the completion of the formation of the well layer closest to the p side to the completion of the deposition of the p-type GaN contact layer means that the barrier layer 6a, the undoped InGaN layer 7, and the p have the structure of FIG. 2. On the other hand, when it has the structure of FIG. 3, it becomes the sum total of each growth time of the undoped InGaN layer 7 and p-type GaN contact layer.

Of the three measurement points shown in FIG. 19, the intermediate measurement point represents 27 minutes of growth time. As shown in the figure, when the growth temperature is 900 ° C., even though the growth time is long, the influence on the luminescence intensity is small. Can be. This is because if the InGaN well layer 6c of the active layer 6 is heated to a high temperature for a long time, it is deteriorated by In sublimation or the like. That is, when growing a semiconductor layer at the growth temperature of 950 degreeC or more from completion | finish of film formation of the well layer closest to p side of an active layer, it can be understood that 30 minutes is a limit as a total of growth time.

In addition, in the case of the nitride semiconductor light emitting device of the configuration shown in FIG. 4, the p-type AlGaN cladding layer is increased in addition to the configuration of FIG. 1, so that the growth temperature of the p-type AlGaN cladding layer is increased to 950 ° C. or more. The total of time should be within 30 minutes.

Although the In composition of the InGaN well layer 6c of the active layer 6 has a peak wavelength of 10% or more, that is, 410 nm or more, the method of manufacturing the nitride semiconductor light emitting device of FIGS. 1 and 4 is basically the method described above. Is the same as Therefore, when the structure of FIG. 1 has the structure of FIG. 2 or FIG. 3, the target layer whose growth temperature is 950 degreeC or more becomes only the p-type GaN type contact layer 8, and a p-type GaN type contact layer It corresponds to making growth time of (8) into 30 minutes or less. On the other hand, in the structure of FIG. 4, when having the structure of FIG. The total of the growth times of these two layers may be 30 minutes or less.

However, the undoped InGaN layer 7 may be heat treated at a high temperature of 950 ° C. or higher without causing the growth temperature to be about 750 ° C., and the surface unevenness may be reliably eliminated, thereby reducing the carrier compensation center reliably. In order to make the total of time to become temperature 950 degreeC or more within 30 minutes, the film thickness of each layer needs to be adjusted.

By the way, in the manufacturing method mentioned above, in the case of the structure of FIG. 1, using a p-type GaN layer as the p-type GaN type contact layer 8, a growth temperature is raised to 1000-1030 degreeC (for example, 1010 degreeC), For example, although 700 Å growth was performed, especially in the case of a green LED or the like having a high In composition, the InGaN well layer 6c is thermally decomposed also in this case. Suppresses to ~ 900 degreeC. Instead of setting the growth temperature to 800 to 900 ° C, a p-type InGaN layer doped with Mg that can give a high concentration of hole carrier concentration at this growth temperature is used as the p-type GaN-based contact layer 8. The In composition ratio of the p-type InGaN layer is determined by the growth temperature, but is about 0.5% to 3% is sufficient. In this way, by making the total of the growth times at the growth temperature of 950 ° C or more very small, it is possible to cope with a green LED or the like having a particularly high In composition.

In the configuration of FIG. 1, when the p-type InGaN contact layer is used, the film formation of the well layer closest to the p-type nitride semiconductor layer among the well layers of the active layer 6 until the end of the deposition of the p-type GaN-based contact layer 8 is formed. The cumulative growth time of which the growth temperature exceeds 950 ° C. can be zero, which is an effective means in the case of a green LED having a particularly high In composition.

On the other hand, although the p-type AlGaN cladding layer 11 is formed at 200 kPa, for example, the AlGaN growth is performed at a temperature of about 950 ° C., preferably about 1000 ° C. or more, at this time, The contact layer 8 is adjusted so that the growth time of 950 degreeC or more may be 30 minutes or less by raising a ratio or taking a thin film thickness. As much as possible 15 minutes or less is preferable.

Claims (11)

In a nitride semiconductor light emitting device having a structure in which a well layer has an active layer having a quantum well structure composed of a nitride including In between a p-type nitride semiconductor layer and an n-type nitride semiconductor layer, An intermediate semiconductor layer formed between the well layer disposed closest to the p side of the active layer and the p-type nitride semiconductor layer includes an undoped InGaN layer, and the film thickness of the intermediate semiconductor layer is 20 nm or less. Nitride semiconductor light emitting device. The method of claim 1, The intermediate semiconductor layer is composed of only an undoped InGaN layer. The method of claim 2, A nitride semiconductor light emitting device according to claim 1, wherein the In composition ratio of the undoped InGaN layer is 2.5% or less. The method of claim 1, The intermediate semiconductor layer is formed of a barrier layer of the active layer and an undoped InGaN layer. The method of claim 4, wherein The undoped InGaN layer is a nitride semiconductor light emitting device, wherein the In composition is an In composition gradient layer that decreases toward the p-type nitride semiconductor layer. The method of claim 5, wherein The In inclination of the In composition gradient layer is formed by a temperature rising process until reaching a growth temperature for forming the p-type nitride semiconductor layer. The method according to any one of claims 1 to 6, A p-type contact layer is formed as part of the p-type nitride semiconductor layer to contact the p-electrode, and the p-type contact layer is formed of a nitride semiconductor light emitting device comprising Mg-doped InGaN or Mg-doped GaN. . The method of claim 7, wherein Mg-doped p-type Al _x GaN (0.02≤x≤0.15) is formed between the undoped InGaN layer and the p-type contact layer as part of the p-type nitride semiconductor layer. The method of claim 8, The hole carrier concentration of the p-type Al _x GaN (0.02≤x≤0.15) is in the range of 2 × 10 ¹⁷ cm ⁻³ or more. The method according to claim 8 or 9, The p-type Al _x GaN (0.02 _{≤ x ≤} 0.15) is grown at a temperature of 1000 ℃ or more nitride semiconductor light emitting device. The method according to any one of claims 1 to 10, The In composition ratio of the well layer is 10% or more, and the growth temperature becomes 950 ° C or more during the end of the well layer deposition at the position closest to the p side of the active layer until the end of the deposition of the p-type nitride semiconductor layer. A nitride semiconductor light emitting element, wherein the sum of time is within 30 minutes.

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