JP2008300418A - Nitride semiconductor laser element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-reliability nitride semiconductor laser element by controlling an impurity density in a p-type layer of the nitride semiconductor laser element, and reducing the drive voltage of the semiconductor element. <P>SOLUTION: A nitride semiconductor laser element comprises a first layer, comprising an n-type nitride semiconductor, a second layer comprising a p-type nitride semiconductor, and a light-emitting layer between the first layer and the second layer. The light-emitting layer has a quantum well structure, comprising a well layer constituted of AlGaInN and a barrier layer constituted of AlGaN, and the p-type impurity density of the second layer is changed in the second layer. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a nitride semiconductor laser device, and more particularly, to a nitride semiconductor laser device that operates stably at high output for a long period of time with reduced driving voltage by controlling the impurity concentration profile of a p-type layer. It is.

  In recent years, an optical disk system using a nitride-based semiconductor laser element and intended for large-capacity recording has entered a practical stage. These new discs require highly reliable high-power blue-emitting semiconductor laser devices in order to enable higher density (corresponding to double-layer discs) and high-speed writing. Conventionally, a blue light emitting semiconductor laser element having a light emitting layer of InGaN and having a multiple quantum well structure and a p-type AlGaN cladding layer is known (see Non-Patent Document 1).

  This blue light-emitting semiconductor laser device has an n-type GaN layer, an n-type AlGaN clad layer responsible for optical confinement, an n-type GaN light guide layer that distributes light in the vicinity of the light-emitting layer, and different In compositions on the substrate. A light-emitting layer having an InGaN multiple quantum well structure, a p-type AlGaN carrier block layer that improves carrier confinement in the light-emitting layer, a p-type GaN light guide layer that distributes light in the vicinity of the light-emitting layer, and p-type AlGaN responsible for light confinement It has a structure in which a cladding layer and a p-type GaN contact layer are laminated by epitaxial growth.

  Further, it is usually formed by dry etching such as RIE, has a ridge portion for confining light in the lateral direction, and an insulating film is formed for current confinement. Further, an n-type electrode and a p-type electrode for injecting current are vacuum deposited. The element having such a structure is cleaved with a thickness of several hundred μm, and the cleaved surface becomes an optical resonator. On the cleaved surface, an AR (Anti-Reflection) coat film and an HR (Hi-Reflection) coat film made of a dielectric multilayer film for improving the reflectance of the front and back end faces are formed by vacuum deposition to form a chip. Usually, the chip is stacked on a submount having high thermal conductivity for heat dissipation, and further sealed in a stem to complete a laser device.

However, since the driving voltage of the conventional blue light emitting semiconductor laser device is higher than that of the semiconductor laser device in the red or infrared region, the input power during driving of the device is high, the thermal damage is large, the high output operation and There was a problem with reliability.
JPN. J. et al. APPl. Phys. 2000. Vol. 39. PP. L647-L650

In the nitride semiconductor laser element, the main cause of the high drive voltage is that a p-type nitride semiconductor having low resistance and good crystal quality cannot be obtained. Usually, Mg, Zn, and Be are suitable as the p-type impurity of the nitride semiconductor, and Mg is most often used. However, Mg taken into the nitride semiconductor has its bonds inactivated by hydrogen, and has a high resistance and does not exhibit p-type conductivity as it grows. For this reason, usually, the Mg-doped film is subjected to heat treatment or electron beam irradiation treatment to break the bond between Mg and hydrogen to activate Mg to obtain a p-type film. Even when the p-type treatment is performed, the impurity activation rate is on the order of several percent, and only a significantly lower p-type carrier concentration can be obtained as compared with the concentration of Mg atoms taken into the crystal. Therefore, the required Mg reaches a concentration close to the composition of the host crystal, 10 ²⁰ to 10 ²¹ cm ⁻³ .

In order to obtain a low resistance p-type film, it is necessary to increase the Mg atom concentration in the crystal or to greatly increase the activation rate. However, the activation rate is usually on the order of several percent, and further improvement is extremely difficult, and even if the crystal growth conditions or heat treatment conditions are devised, it has been realized that the activation rate is on the order of 10%. Absent. Further, when the Mg concentration is increased, 10 ²² cm ⁻³ or more is required, so that the crystallinity of the host crystal is deteriorated, which is caused by light absorption or an increase in defects, leading to deterioration of device characteristics. The problem with the above Mg doping is the same even when Zn or Be is used, and is an essential problem in the p-type nitride semiconductor film. Further, in the case of AlGaN containing Al as a composition, the band gap is wide, so that the p-type impurity order is deepened, which is more disadvantageous than GaN in terms of p-type conversion.

  For the above reasons, in the conventional nitride semiconductor laser element, the resistance in the p-type layer and the contact resistance are high, and the drive voltage of the element is high. Therefore, the input power to the element during driving is also high, and the heat loss in the p-type layer having high resistance is large, and the conventional element disclosed in Non-Patent Document 1 is particularly reliable during high output driving. There's a problem. The problem to be solved by the present invention is to provide a highly reliable nitride semiconductor laser device by controlling the impurity concentration in the p-type layer of the nitride semiconductor laser device to reduce the driving voltage of the semiconductor device. .

  The nitride semiconductor laser device of the present invention includes a first layer made of an n-type nitride semiconductor, a second layer made of a p-type nitride semiconductor, and a light emitting layer between the first layer and the second layer. The light emitting layer has a quantum well structure composed of a well layer made of AlGaInN and a barrier layer made of AlGaN, and the second layer has a p-type impurity concentration that changes in the second layer. It is characterized by. The p-type impurity is preferably Mg, Zn, Be, or a mixture thereof. The second layer is composed of a plurality of layers, and is a layer located on the light emitting layer side of the second layer, and the p-type impurity concentration in the boundary region of at least one pair of adjacent layers is the second level. A mode in which the concentration is lower than the p-type impurity concentration in other regions of the layer is preferable. Further, it is preferable that the second layer is composed of a plurality of layers, and the Al composition ratio of the second layer located on the light emitting layer side is higher than the Al composition ratio in other regions of the second layer. .

  A driving voltage of the semiconductor element can be reduced, and a highly reliable nitride semiconductor laser element can be provided.

  The present invention relates to a nitride semiconductor laser device having an n-type first layer, a p-type second layer, and a light emitting layer between the first layer and the second layer, the light emitting layer comprising: , Having a quantum well structure composed of a well layer made of AlGaInN and a barrier layer made of AlGaN, and the second layer is characterized in that the p-type impurity concentration is changed in the second layer. To do. By controlling the profile of the p-type impurity concentration in the second layer made of the p-type nitride semiconductor, the electrical resistance in the p-type layer can be lowered and the driving voltage of the semiconductor laser element can be lowered.

  Specifically, the second layer is composed of a plurality of layers, and is a layer located on the light emitting layer side of the second layer, and the p-type impurity concentration in the boundary region between at least one pair of adjacent layers is set. By controlling the concentration lower than the p-type impurity concentration in the other region of the second layer, band discontinuity at the heterojunction of the adjacent layer can be reduced, the electrical resistance can be lowered, and the drive voltage of the element is lowered. be able to. For this reason, the input electric power to the element at the time of driving is also reduced, the heat loss in the p-type layer having a low resistance is small, and compared with the conventional element disclosed in Non-Patent Document 1, especially at the time of high output driving Reliability can be improved.

  From this point of view, the p-type impurity concentration in the boundary region between at least one pair of adjacent layers is a layer located on the light-emitting layer side of the second layer, and the p-type impurity in the other region of the second layer. The aspect which reduces to 10% or less of a density | concentration is preferable, and the aspect which reduces to 8% or less is more preferable. In addition, the layer for reducing the p-type impurity concentration in the boundary region between adjacent layers is located on the light emitting layer side of the second layer in that the light absorption from the active layer can be reduced as much as possible and the device characteristics can be further improved. The aspect which makes it a layer to perform is preferable.

  Further, from the viewpoint of preventing overflow of carriers injected into the active layer, the second layer is composed of a plurality of layers, and the Al composition ratio of the layer located on the light emitting layer side of the second layer is the second layer. An aspect higher than the Al composition ratio in the other region of this layer is preferable.

Embodiment 1
The structure of the nitride semiconductor laser device manufactured in this embodiment is shown in FIG. FIG. 1A is a cross-sectional view, and FIG. 1B is a perspective view. The nitride semiconductor laser element 116 includes an n-electrode 113, a p-electrode 112, and a ridge portion 110 that is an optical waveguide, and an AR coating film 114 made of Al ₂ O ₃ and a SiO ₂ / TiO ₂ film on the cleavage plane. The HR coat film 115 is formed of the nine-layer film. On the GaN substrate 101 having a thickness of 100 μm, a first layer made of an n-type nitride semiconductor, a light emitting layer, and a second layer made of a p-type nitride semiconductor are formed. That is, an n-type GaN layer 102 having a layer thickness of 0.2 μm, an n-type Al _0.05 Ga _0.95 N cladding 103 having a layer thickness of 2.5 μm, and an n-type GaN guide layer 104 having a layer thickness of 0.1 μm are formed on the substrate 101. Then, a multiple quantum well light emitting layer 105 composed of three well layers made of AlGaInN having a thickness of 4 nm and four barrier layers made of AlGaN having a thickness of 8 nm is formed.

Next, a p-type Al _0.3 Ga _0.7 N carrier blocking layer 106 having a layer thickness of 20 nm, a p-type GaN guide layer 107 having a layer thickness of 0.08 μm, and a p-type Al _0.062 Ga _0.938 N cladding layer 108 having a layer thickness of 0.5 μm. Then, a p-type GaN contact layer 109 having a layer thickness of 0.1 μm is sequentially stacked. The multi-quantum well light emitting layer 105 is usually formed in the order of barrier layer / well layer / barrier layer / well layer / barrier layer / well layer / barrier layer, but well layer / barrier layer / well layer / barrier layer. ../ A structure that starts with a well layer and ends with a well layer, such as a well layer, may be used.

As the n-type impurity, Si is usually used, and the impurity concentration is on the order of 10 ¹⁸ cm ⁻³ . It is known that the impurity of the n-type nitride semiconductor is almost 100% activated at room temperature with the crystal grown, and the n-type carrier concentration is substantially equal to the impurity concentration. In addition to Si, C, Ge, or O can be used as the n-type impurity. As the p-type impurity, Mg is suitable, Zn or Be can be used, and a mixture thereof is also preferably used. Mg is usually supplied during crystal growth as Cp ₂ Mg (Bis-Cyclopentadienyl Mg) or EtCp ₂ Mg (Ethyl-Bis-Cyclopentadienyl Mg).

Since impurities in the p-type nitride semiconductor are inactivated by H bonding when the crystal is grown, heat treatment or electron beam treatment is performed after the crystal growth in order to make it p-type. In general, activation is performed by heat treatment, and the temperature is about 800 ° C. to 900 ° C. and is maintained for a maximum of about 30 minutes. The atmosphere at that time is N ₂ or a mixed gas of N ₂ and O ₂ , and the O ₂ concentration when using the mixed gas is on the order of several percent at the maximum. The p-type guide layer 107 and the p-type cladding 108 are processed by dry etching such as RIE, and are patterned by the insulating film 111 for current confinement.

The nitride semiconductor laminate is sequentially formed on the GaN substrate 101 by the MOCVD apparatus from the n-type GaN layer 102 to the p-type GaN contact layer 109 in the order shown in FIG. Thereafter, the ridge 110 is patterned by dry etching such as RIE or ICP, and an insulating film 111 mainly made of SiO ₂ , ZrO ₂ or the like is formed for current confinement. Thereafter, the p-electrode 112 is formed, and then the surface of the GaN substrate 101 on which the laminated film is not formed is ground and polished until the film thickness is about 100 μm. After the damaged layer formed on the back surface of the GaN substrate 101 in the polishing step is removed by vapor phase etching such as RIE, an n electrode 113 made of Ti / Al is formed by EB vapor deposition. Thereafter, it is divided into bars, and an AR coating film 114 is formed on the light exit surface.

In this embodiment, when doping the Mg into the p-type layer, Cp ₂ Mg is used as a raw material, but EtCp ₂ Mg can be used in the same manner. Cp ₂ Mg, which is solid at room temperature, needs to be maintained at about 50 ° C. or higher in order to sublimate into the carrier gas during MOCVD growth. Cp ₂ Mg and H ₂ introduced carrier within the filled cylinders, have been Cp ₂ Mg are mixed in a carrier by sublimation is transported on the substrate 101 by crystal growth, and H at a temperature of above about 1000 ° C. It decomposes into bonded Mg and is taken into the crystal as p-type impurities together with GaN or AlGaN that grows.

The concentration of Mg taken into GaN or AlGaN can be controlled by the temperature of the cylinder filled with the raw material and the flow rate of the carrier gas introduced into the cylinder. It can also be controlled by the growth rate of the host crystal. Usually, the higher the cylinder temperature and the higher the carrier flow, the higher the concentration of Mg. Furthermore, the Mg concentration can also be controlled by changing the flow rate of NH ₃ which is a group V raw material when growing the base crystal.

In the present embodiment, the boundary region between the adjacent carrier block layer 106 and the p-type guide layer 107 and the boundary region between the p-type guide layer 107 and the p-type cladding layer 108 are located on the light emitting layer side of the second layer. In FIG. 2, the Mg concentration in each boundary region was adjusted to be lower than the Mg concentration in other regions of the second layer by changing the NH ₃ flow rate, and the p-type impurity concentration was changed in the second layer. . When the carrier block layer 106 is grown to 18 nm, the NH ₃ flow rate is increased by a factor of three to grow the remaining 2 nm, and then the p-type guide layer 107 is maintained while the NH ₃ flow rate is increased. Grows to a thickness of 0.008 μm. Subsequently, the NH ₃ flow rate is reduced to a normal state to grow 0.064 μm. Thereafter, the NH ₃ flow rate is again increased three times to grow the remaining 0.008 μm of the p-type guide layer 107. Subsequently, the p-type cladding layer 108 is grown by 0.05 μm with the NH ₃ flow rate increased three times.

Thereafter, the normal NH ₃ flow rate is restored and the p-type cladding layer 108 and the p-type contact layer 109 are grown. Through the above growth of the p-type layer, the supply amount of Cp ₂ Mg was kept constant. FIG. 2 shows the change over time in the ammonia flow rate during crystal growth in the present embodiment. The nitride semiconductor laser device sample thus grown was heat-treated at 900 ° C. for 10 minutes in an N ₂ atmosphere to activate Mg. Separately from experiments, it was found that both p-type GaN and p-type AlGaN exhibit p-type conductivity by performing a heat treatment within a range of 700 ° C. to 950 ° C. within 30 minutes. The atmosphere at that time was N ₂ mixed with O ₂ with an upper limit of 5%.

After the heat treatment, a part of the nitride semiconductor laser device was cut out and the Mg concentration was measured by SIMS (secondary ion mass spectrometer). As a result, a profile of Mg concentration in the p-type layer as shown in FIG. 3 was obtained. . For comparison, FIG. 3 shows the Mg concentration of a conventional product grown by a normal method. The Mg concentration of each of the carrier block layer 106, the p-type guide layer 107 and the p-type cladding layer 108 grown at a normal NH ₃ flow rate was constant at about 10 ²⁰ cm ⁻³ . On the other hand, in the boundary region between the carrier block layer 106 and the p-type guide layer 107 and the boundary region between the p-type guide layer 107 and the p-type cladding layer 108, as a result of triple the NH ₃ flow rate, the Mg concentration is It decreased to about 10 ¹⁹ cm ⁻³ . Further, as shown in FIG. 3, the change in Mg concentration was not abrupt but characteristically changed relatively slowly. This is a characteristic when the doping concentration is controlled by changing the flow rate of NH ₃ . On the other hand, for example, when the Mg concentration is controlled by changing the amount of Cp ₂ Mg, the change in the Mg concentration is steep.

The sample grown by the above method was divided into chips through a normal process and mounted on a stem to evaluate the characteristics. As a result, in the device of the present invention, the driving voltage at an optical output of 200 mW was 4.5V, which was lower than the 5.5V of the device having the conventional structure. In addition, a sample in which the Mg concentration profile was similarly controlled by changing the flow rate of Cp ₂ Mg was mounted in the same manner, and the characteristics were confirmed. As a result, the driving voltage at an optical output of 200 W was 4.9 V. Therefore, the method of controlling the Mg concentration profile by changing the flow rate of NH ₃ showed a remarkable voltage reduction effect than the method of controlling the Mg concentration profile by changing the flow rate of Cp ₂ Mg. In the method of changing the flow rate of NH ₃ , the change in Mg concentration in the boundary region is gradual, so that the degree of relaxation of the band discontinuity in the hetero boundary region of GaN-AlGaN or AlGaN-GaN is large. It is done.

  Next, the life test of the device of the present invention was conducted under the condition of 80 ° C. with an optical output of 200 mW. As a result, it was found to have a lifetime of 5000 hours or more. The element having a conventional structure has a high driving voltage and a large input power, and therefore has a life of 3000 hours or less due to heat. Therefore, it was possible to extend the life.

Embodiment 2
In the present embodiment, a semiconductor laser device was manufactured by controlling the Zn concentration by changing the NH ₃ flow rate in the same manner as in the first embodiment except that Zn was used as the p-type impurity instead of Mg. The obtained laser element was mounted in the same manner and the characteristics were confirmed. As a result, the driving voltage at an optical output of 200 W was 4.8V. Therefore, although the driving voltage could be reduced as compared with the element having the conventional structure, the driving voltage was higher than that of the element controlling the Mg concentration profile in the first embodiment. This is because, since the Bohr radius of Zn and Mg is Mg> Zn, Zn has higher binding energy of valence electrons than Mg, and is in the forbidden band when taking the upper end of the valence band in the energy band as a reference. This is probably because the energy ranking is Zn> Mg. Further, as in the first embodiment, a life test was performed under the conditions of an optical output of 200 mW and 80 ° C. As a result, a life of 4000 hours was confirmed. The reason why the reliability is lower than that of Mg is considered to be that Zn is diffused more than Mg in the host crystal.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  It is suitable for an optical disk system for large-capacity recording, and can provide a highly reliable high-power blue light emitting semiconductor laser element capable of high density and high speed writing.

2A and 2B are diagrams showing the structure of the nitride semiconductor laser device according to the first embodiment, where FIG. 3A is a cross-sectional view, and FIG. FIG. 3 is a diagram showing a change in the ammonia flow rate during crystal growth in the first embodiment. FIG. 5 is a diagram showing a profile of Mg concentration in the p-type layer of the nitride semiconductor laser element in the first embodiment.

Explanation of symbols

  101 substrate, 102 n-type GaN layer, 103 n-type AlGaN cladding layer, 104 n-type GaN guide layer, 105 multiple quantum well light emitting layer, 106 p-type AlGaN carrier block layer, 107 p-type GaN guide layer, 108 p-type AlGaN cladding Layer, 109 p-type GaN contact layer, 110 ridge portion, 111 insulating film, 112 p electrode, 113 n electrode, 114 AR coat film, 115 HR coat film, 116 nitride semiconductor laser element.

Claims (4)

A nitride semiconductor laser device having a first layer made of an n-type nitride semiconductor, a second layer made of a p-type nitride semiconductor, and a light emitting layer between the first layer and the second layer. ,
The light emitting layer has a quantum well structure including a well layer made of AlGaInN and a barrier layer made of AlGaN,
The nitride semiconductor laser device according to claim 1, wherein the second layer has a p-type impurity concentration changed in the second layer.   2. The nitride semiconductor laser device according to claim 1, wherein the p-type impurity is Mg, Zn, Be, or a mixture thereof.   The second layer is composed of a plurality of layers, and is a layer located on the light emitting layer side of the second layer, and the p-type impurity concentration in the boundary region of at least one pair of adjacent layers is The nitride semiconductor laser element according to claim 1, wherein the nitride semiconductor laser element is lower than a p-type impurity concentration in another region of the layer.   The second layer is composed of a plurality of layers, and the Al composition ratio of the second layer located on the light emitting layer side is higher than the Al composition ratio in other regions of the second layer. The nitride semiconductor laser device according to claim 1.

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