JP3468082B2 - Nitride semiconductor device - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to an LED (light emitting diode), an SLD (super luminescent diode),
Nitride semiconductors (In _X Al _Y Ga _1-XY N, 0 ≦ X, which are used for light-emitting elements such as LDs (laser diodes), solar cells, light-receiving elements such as photosensors, or electronic devices such as transistors and power devices. , 0 ≦ Y, X + Y ≦ 1)
Regarding the device.

[0002]

2. Description of the Related Art We have announced that a nitride semiconductor laser device including an active layer was fabricated on a nitride semiconductor substrate to achieve the continuous oscillation of 10,000 hours or more at room temperature for the first time in the world (ICNS '97 Proceedings, October 27-31, 1997, P444-446,
And Jpn.J.Appl.Phys.Vol.36 (1997) pp.L1568-1571, Par
t2, No.12A, 1 December 1997). As a basic structure, a nitride semiconductor layer to be a laser device structure is formed on a nitride semiconductor substrate made of n-GaN selectively grown on a sapphire substrate via a partially formed SiO ₂ film. A plurality of layers are stacked. (See Jpn.J.Appl.Phys.Vol.36 for details)

[0003]

However, it is estimated that the continuous oscillation of 10,000 hours or more is 2 mW at the output. At 2 mW, the reading light source is slightly insufficient, and the writing light source requires an output 10 times or more higher than this, and further improvement in the output of the laser element and longer life are desired.

When the oscillation threshold of the laser element decreases, the amount of heat generated by the laser element decreases, so that the current value can be increased and the output can be increased. Further, the lowering of the threshold value can be applied not only to laser elements but also to other nitride semiconductor elements such as LEDs and SLDs, and it is possible to provide highly efficient and highly reliable elements. Therefore, it is an object of the present invention to reduce the oscillation threshold first in order to improve the output of the laser element and to extend the life of the laser element.

[0005]

The nitride semiconductor device according to the present invention mainly comprises three modes. The first mode is between the n-type nitride semiconductor layer and the p-type nitride semiconductor layer. In a nitride semiconductor device having an active layer containing In having a quantum well structure, the n-type nitride semiconductor layer and the p-type nitride semiconductor layer include a first nitride semiconductor layer containing Al and a first nitride layer. A superlattice layer in which a semiconductor layer and a second nitride semiconductor layer having a different composition are laminated, and between the superlattice layer and the active layer provided in the p-type nitride semiconductor layer. A layer made of a nitride semiconductor having a band gap energy larger than that of the well layer and containing Al,
The film thickness of the entire superlattice layer provided in the type nitride semiconductor layer is smaller than the film thickness of the entire superlattice layer provided in the n-type nitride semiconductor layer. The thickness of the entire n-side superlattice layer is 100 angstroms or more and 5 μm.
And the total thickness of the superlattice layer on the p-side is 5 or less.
It is 0 angstrom or more and 2 μm or less. The nitride semiconductor layer forming the superlattice layer has a single thickness of 100 angstroms or less. At least one of the n-type nitride semiconductor layer and the superlattice layer provided in the p-type nitride semiconductor layer is formed in contact with the active layer. The nitride semiconductor device is formed on the nitride semiconductor substrate by n
A nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer in this order. The superlattice layer contains impurities that determine the conductivity type, and the impurities are contained in at least one of the first nitride semiconductor layer and the second nitride semiconductor layer. The conductivity-determining impurities contained in the superlattice layer are adjusted to decrease as they approach the active layer. The Al composition of the first nitride semiconductor layer decreases as it approaches the active layer, and the impurities that determine the conductivity type contained in the superlattice layer approach the active layer. It has been adjusted so that it will decrease.

According to a second aspect, in a nitride semiconductor device having the same structure, at least one of the n-type and p-type nitride semiconductor layers has a first nitride semiconductor layer containing Al,
A superlattice layer in which a first nitride semiconductor layer and a second nitride semiconductor layer having a different composition are laminated is provided, and an impurity included in the superlattice layer that determines the conductivity type approaches the active layer. It is characterized in that it is adjusted so as to decrease.

The third mode is the most preferable state, which is a combination of the first mode and the second mode. In a light emitting device having a similar structure, the n-type and p-type are used.
Superlattice layer in which a first nitride semiconductor layer containing Al and a second nitride semiconductor layer having a composition different from that of the first nitride semiconductor layer are laminated on at least one of the type nitride semiconductor layers And the Al content of the first nitride semiconductor layer is reduced as the first nitride semiconductor layer approaches the active layer, and the impurities that determine the conductivity type contained in the superlattice layer are activated. It is characterized in that it is adjusted so as to decrease as it approaches the layer.

In all aspects of the present invention, the superlattice layer is provided on both the n-type nitride semiconductor layer and the p-type nitride semiconductor layer, and the first nitride semiconductor layer on the n-side comprises: A nitride semiconductor layer having a larger Al mixed crystal ratio than the p-side first nitride semiconductor layer is provided.

Further, all of the superlattice layers are provided in both the n-type nitride semiconductor layer and the p-type nitride semiconductor layer, and the superlattice layer is closer to the p-side than the entire film thickness on the n-side. The entire superlattice layer is thin.

All of the superlattice layers contain impurities that determine the conductivity type, and the impurities are contained in either the first nitride semiconductor layer or the second nitride semiconductor layer. It is characterized by The impurities that determine the conductivity type are, for example, Si, Se, and
A group IV element such as O, Sn, and S, and a p-type nitride semiconductor means a group II element such as Mg, Zn, Cd, Be, and Ca. (Hereinafter, the impurities contained in the n-type nitride semiconductor are referred to as donors, and the impurities contained in the p-type nitride semiconductor are referred to as acceptors.)

The layer having a thickness of 0.3 μm or less on the side of the superlattice layer closer to the active layer is characterized by being an undoped layer not doped with impurities. The thickness of the undoped region in this superlattice layer is preferably 0.2 μm.
m or less, more preferably 0.1 μm or less.
The lower limit is not particularly limited, but it is desirable that the lower limit be equal to or larger than the film thickness of the first nitride semiconductor layer or the second nitride semiconductor layer. The region composed of this undoped superlattice is n, p
It may be provided on at least one of the nitride semiconductor layers, but is preferably formed on both sides with the active layer sandwiched therebetween. In the present claims, undoped refers to a nitride semiconductor that is not intentionally doped with impurities, and for example, impurities that are diffused in from adjacent nitride semiconductors are also defined as undoped in the present invention. To do. In this case, the impurity concentration of the undoped nitride semiconductor layer often has a gradient such that it gradually decreases from the side in contact with the nitride semiconductor layer containing impurities.

Further, the superlattice layer is formed in contact with the active layer. When at least one of the superlattice layers is formed in contact with the active layer in this manner, for example, when a laser device is manufactured, the superlattice layer serves as an optical guide layer serving as a waveguide of the active layer and an optical confinement layer. It can also serve as a clad layer.

Furthermore, the first nitride semiconductor layer is made of Al _x Ga _{1 -x} N (0 <X <1), and the second nitride semiconductor layer is made of GaN. Al
A combination of GaN and GaN is very convenient in terms of production technology because only one gas flow rate needs to be adjusted when gradually changing the Al composition ratio. Also Al
Since the GaN layer having better crystallinity than GaN serves as the buffer layer, the AlGaN layer grown thereon tends to have better crystallinity as well, and the superlattice layer having good crystallinity as a whole can be formed.

[0014]

1 is a schematic cross-sectional view showing a specific structure of a nitride semiconductor device of the present invention, specifically showing a structure of a laser device, in which 8 is an active layer and 7 is an active layer. N-side clad layer made of superlattice layer, 10 made of superlattice layer p
It is a side cladding layer. In the case of a laser device, it is desirable that these superlattice layers 7 and 10 be present in the nitride semiconductor layers on both sides of n and p, but in the case of a nitride semiconductor device having a simple structure such as an LED or a light receiving device, both sides are not always required. It does not need to be present, and may be present in either one of the conductivity type nitride semiconductor layers.

The first nitride semiconductor layer forming the superlattice is a nitride semiconductor containing Al, preferably A of a ternary mixed crystal.
If l _X Ga _{1 -X} N (0 <X <1), it is easy to obtain a crystal with good crystallinity. The second nitride semiconductor layer may have any composition as long as it has a composition different from that of the first nitride semiconductor, but it is preferable that the second nitride semiconductor layer has a bandgap energy smaller than that of the first nitride semiconductor. _Y Ga
Select _1-YN (0≤Y≤1). Among them, GaN has the best crystallinity. That is, the superlattice layer is AlGa
When composed of N and GaN, GaN with good crystallinity acts like a buffer layer, and AlGaN can be grown with good crystallinity. Further, by growing and stacking a nitride semiconductor layer having a single film thickness of 100 angstroms or less, more preferably 70 angstroms or less, and most preferably 50 angstroms or less, the nitride semiconductor has an elastic critical film thickness or less, Even in a crystal such as AlGaN, which is likely to have cracks, it is possible to grow with good film quality without cracks.

In the first aspect of the present invention, the Al composition of the first nitride semiconductor layer of the n-side clad layer 7 and the p-side clad layer made of a superlattice layer is adjusted so as to become smaller as approaching the active layer. is doing. In this way, GRIN (gradient index waveguide) using both the n and p cladding layers as a superlattice
With the structure, the light emission of the active layer is guided in a region having a small Al composition, and the longitudinal mode is likely to be a single mode, which lowers the threshold value. FIG. 2 shows an energy band diagram from the n-type cladding layer 7 to the p-side cladding layer 10 in FIG.
As shown in FIG. 2, by decreasing the Al composition of the first nitride semiconductor from the n-side cladding layer 7 and the p-side cladding layer 10 to the active layer 7, the band gap energy is continuously reduced. Thus, the threshold tends to be lowered by producing the GRIN structure. The active layer has a multi-quantum well structure.

Generally, the clad layer having a double hetero structure is
Since it is necessary to make the bandgap energy larger than that of the active layer, an Al-containing nitride semiconductor such as AlGaN is used for the cladding layer of the nitride semiconductor device. In the case of AlGaN, it is theoretically known that the Al mixed crystal ratio should be increased in order to provide a difference in refractive index and a difference in bandgap energy with respect to the active layer, but Al _X Ga _{1 -X} N As the X value increases, cracks tend to form easily in the crystal.
Therefore, AlGaN with a large Al mixed crystal ratio is used for the cladding layer.
Is difficult to grow. For example, if even the superlattice, Al mixed crystal ratio X is, for example, 0.5 or more Al _X Ga
_It tends to be difficult to grow _1- XN to a film thickness required as a cladding layer for optical confinement. However, with the GRIN structure as in the present invention, the layer having a large Al mixed crystal ratio is only the outermost layer, that is, the layer farthest from the active layer, and the Al mixed crystal ratio becomes smaller as the layer approaches the active layer. Therefore, it becomes easy to form a layer having a large Al mixed crystal ratio in the outermost layer. Therefore, the refractive index difference between the clad layer and the active layer can be increased, so that the light confinement effect is increased and the threshold value is lowered. Also, the GRI in which the refractive index gradually decreases from the center (active layer) to the outside
In the N structure, light is likely to be concentrated in the center, and the threshold value is lowered.

In the second aspect of the present invention, the concentrations of donors and acceptors contained in the n-side cladding layer 7 and the p-side cladding layer made of a superlattice layer are adjusted to decrease as they approach the active layer. ing. Si, Ge, Sn, S, and O are used as the donor of the n-side cladding layer, and Si and Sn are generally used. Mg, Zn, Be, and Ca are used as the acceptor of the p-type clad layer, and Mg is generally used. Thus n,
When the p-cladding layer is used as a superlattice and the donor and acceptor concentrations contained in the superlattice are gradually reduced, light absorption in the vicinity of the active layer by the cladding layer is reduced, so that the optical loss is lowered and the threshold is lowered. Further, the crystallinity is better than that of a nitride semiconductor having a low impurity concentration and a nitride semiconductor having a high impurity concentration. Therefore, when the active layer is sandwiched between both the n and p cladding layers having a low impurity concentration and good crystallinity, the active layer having a small number of crystal defects can be grown, so that the life of the device is extended and the reliability is improved. The withstand voltage of the device also increases.

Impurities may be doped into both the first nitride semiconductor layer containing Al and the second nitride semiconductor layer.
It is preferable to dope either one.
This is called modulation doping. Doping impurities into either one of the superlattice layers improves the crystallinity of the entire superlattice layer, which is also effective in realizing a highly reliable device. Target. That is, when an impurity-doped layer is grown on a layer with good crystallinity that is not doped with impurities, the crystallinity of the impurity-doped layer is improved, and the crystallinity of the entire superlattice layer is improved. .

In the case of a donor, the impurity concentration is adjusted to 1 × 10 ^{17 to} 5 × 10 ²⁰ / cm ³ , preferably 5 × 10 ¹⁷ to 1 × 10 ²⁰ / cm ³ in the outermost layer of the n-side cladding layer. To do. In the vicinity of the active layer, for example, in the low impurity portion concentration region of the superlattice layer of 0.3 μm or less, the concentration is adjusted to 1 × 10 ¹⁹ / cm ³ or less, more preferably 5 × 10 ¹⁸ / cm ³ or less. When Si is used as the donor, the current detection limit of SIMS in the GaN matrix is about 5 × 10 ¹⁶ / cm ³ . On the other hand, in the case of the acceptor, in the case of the second mode of the p-side cladding layer, the outermost layer is 1 × 10 ^{17 to} 5 × 10 ^21.
/ Cm ³ , preferably in the range of 5 × 10 ¹⁷ to 1 × 10 ²¹ / cm ³ . In the vicinity of the active layer, for example, in the low impurity portion concentration region of the superlattice layer of 0.3 μm or less, the concentration is adjusted to 1 × 10 ¹⁹ / cm ³ or less, more preferably 5 × 10 ¹⁸ / cm ³ or less. In the case of the second aspect, the low impurity concentration layer is more important than the impurity concentration of the outermost layer, and if the impurity concentration on the side closer to the active layer is higher than 1 × 10 ¹⁹ / cm ³ , There is a tendency that absorption increases and the threshold value does not easily decrease.
Further, the crystallinity is lowered by increasing the impurity concentration, so that the life tends to be shortened. Most preferably, impurities are not intentionally doped, that is, undoped. When Mg is used as the acceptor, the current detection limit of SIMS in the GaN matrix is about 5
It is about 10 ¹⁶ / cm ³ .

The third mode shows the most preferred mode of the present invention, which is a combination of the first mode and the second mode, and the action of the superlattice layer is the same, and therefore will be omitted.

As a major feature of the device of the present invention, in all the embodiments, when the superlattice layer is provided on both the n-type nitride semiconductor layer and the p-type nitride semiconductor layer, the first n-side layer is formed.
It is preferable that the nitride semiconductor layer of 1 has a nitride semiconductor layer having a larger Al mixed crystal ratio than the p-side first nitride semiconductor layer. Preferably, the Al mixed crystal ratio of the first nitride semiconductor layer containing Al on the side farthest from the active layer is set to p
The n side is made larger than the side. This is related to light confinement. In the case of a nitride semiconductor, a transparent material, such as a nitride semiconductor substrate and an n-side contact layer, having a refractive index larger than that of the cladding layer is present outside the cladding layer on the n-side. When light leaks from the clad layer, these materials guide the light inside the clad layer and disturb the shape of the FFP of the laser light in the laser element. In addition, the lateral mode becomes multi, which causes a rise in the threshold value. Therefore, in order to prevent light from leaking to the n-side, a superlattice including the first nitride semiconductor layer having a large Al mixed crystal ratio is made to exist on the n-side. on the other hand,
On the p-side, there is almost nothing that disturbs the shape of the FFP on the outside of the p-side cladding layer, or even if there is, the film thickness is very thin, and thus it is difficult to guide the wave. Therefore, it is not necessary to provide the first nitride semiconductor layer having a large Al mixed crystal ratio on the p side unlike the n side. Further, since the nitride semiconductor layer having a large Al mixed crystal ratio has a higher resistivity than the one having a small Al mixed crystal ratio, when it is present on the p layer side, Vf tends to increase.

Further, as a next feature of the present invention, in all aspects, the superlattice layer is an n-type nitride semiconductor layer,
When provided on both the p-type nitride semiconductor layer and the p-type nitride semiconductor layer, it is desirable that the thickness of the entire superlattice layer on the p-side is smaller than the thickness of the entire superlattice layer on the n-side. This is due to the resistivity of the n and p nitride semiconductors. When a nitride semiconductor is used as a superlattice, the p-type tends to have a higher resistivity than the n-type. Moreover, the nitride semiconductor containing Al has a higher resistivity than the one not containing Al. If the p-layer side is made thicker, Vf becomes higher, and the amount of heat generated by the element tends to increase. Therefore, by making the p-layer side thinner than the n-layer side as in the present invention, it is possible to manufacture a highly reliable element in which the increase in Vf is suppressed. As a specific film thickness, the n side is 10
It is desirable that the thickness is 0 angstrom or more and 5 μm or less, and the p layer side is 50 angstrom or more and 2 μm or less.

In the device of the present invention, the n-side and p-side superlattice layers contain at least impurities that determine the conductivity type. However, if the impurities are modulated and doped as described above, the crystallinity of the superlattice layer is increased. Is improved and the threshold is lowered. Such a modulation doping method can be applied not only to the second and third aspects of the present invention but also to the first aspect. However, in the first aspect, it is not always necessary to reduce the concentration of impurities as the concentration approaches the active layer.

In all the aspects of the present invention, it is preferable that the region having a film thickness of 0.3 μm or less on the side close to the active layer of the superlattice layer is an undoped layer not doped with impurities. As mentioned above, the thickness is more preferably 0.2 μm or less, and most preferably 0.1 μm or less. If the undoped region is more than 0.3 μm, the resistivity of the undoped region increases, so that the threshold value increases and the element tends to generate heat. This tendency is particularly strong on the p-layer side, and it is desirable to make the undoped region on the p-layer side thinner than on the n-layer side. However, an acceptor such as Mg tends to diffuse more easily than a donor, and the undoped layer on the p-layer side is not completely undoped due to the diffusion of Mg from the outer Mg-doped layer. It is often in the state of.

Next, when the n-side cladding layer 7 and the p-side cladding layer 10 made of a superlattice layer are formed in contact with the active layer 8, the region of the cladding layer close to the active layer serves as an optical guide layer. As described above, the waveguide region is formed and the threshold value is lowered, but a layer made of another nitride semiconductor can be formed between the clad layer and the active layer. For example, n
It is also possible to form a layer serving as an optical guide layer of the active layer, having the same composition as the nitride semiconductor layer on the side closest to the active layer of the p-side cladding layer. Further, the light guide layer may be the first aspect, the second aspect or the third aspect of the present invention.

Further, when the active layer has a quantum well structure having at least one well layer, as shown in the band diagram of FIG. 2, the well layer is provided between the p-side superlattice layer and its active layer. When a cap layer made of a nitride semiconductor having a large bandgap energy and containing Al is formed with a film thickness of 0.1 μm or less, a light emitting element such as a laser element or an LED element has a higher output. The preferable film thickness is 800 angstroms or less, and more preferably 500 angstroms.
Be less than Angstrom. If the thickness is thicker than 0.1 μm, carriers cannot pass through the cap layer having the energy barrier due to the tunnel effect, and the improvement in output is small. The cap layer can be provided on the n side in contact with the active layer.

[0028]

【Example】

[Embodiment 1] (Third Aspect) FIG. 1 is a schematic cross-sectional view showing the shape of a laser device according to an embodiment of the present invention, showing a view when cut in a direction perpendicular to a ridge stripe. Is. The device of the present invention will be described below with reference to this drawing.

(Underlayer) Heterogeneous substrate 1 made of sapphire
On top of the G at a low temperature of around 500 ° C using the MOVPE method.
A buffer layer (not shown) made of aN is grown to a film thickness of 200 angstrom, and 90% is grown on the buffer layer.
The base layer 2 made of undoped GaN is 4 μm at 0 ° C. or higher.
Grow with a film thickness of m. This underlayer acts as an underlayer for partially forming a protective film on the surface and then performing selective growth of the nitride semiconductor substrate. Therefore, the undoped layer is most preferable in order to grow the next layer with good crystallinity. It is desirable that the thickness of the underlayer be made thicker than that of the buffer layer and adjusted to be 10 μm or less. In addition to sapphire, a substrate made of a material different from a nitride semiconductor, such as SiC, ZnO, spinel, or GaAs, which is known for growing a nitride semiconductor, can be used. Note that this underlayer has many crystal defects, for example, 10 ⁹ defects / cm ² or more, and cannot be a nitride semiconductor substrate.

(Protective film 3) After the underlayer is grown, the surface of the underlayer 2 is covered.
On the surface, form a stripe-shaped photomask, and
Depending on the location, the stripe width is 10 μm, the stripe spacing (window
Part) 2 μm SiO ₂The protective film 3 made of
To form. The shape of the protective film is striped or
It can be in any shape such as a square shape or a grid shape, but from the window
Even if the area of the protective film is increased, the crystal to be grown next
A nitride semiconductor substrate with few defects can be obtained. Material of protective film
As the material, for example, silicon oxide (SiO 2_X), Kei nitride
Elementary (Si_XN_Y), Titanium oxide (TiO_X), Zirco oxide
Ni (ZrO_X) Oxides, nitrides, etc.
In addition to multi-layer film, use metal with melting point of 1200 ℃ or more
Can be

(Nitride semiconductor substrate 4) After forming the protective film 3,
Using the MOVPE method, the nitride semiconductor substrate 4 made of undoped GaN is grown to have a film thickness of 10 μm. The grown nitride semiconductor substrate 4 has fewer crystal defects appearing on the surface than the underlayer 2, for example, 10 ⁷ / cm ² or less, and a nitride semiconductor sufficient for growing a crystal semiconductor with good crystallinity. It can be used as a semiconductor substrate.

(N-side contact layer 5) Next, on the nitride semiconductor substrate 4, G doped with Si at 1 × 10 ¹⁹ / cm ³ was formed.
The n-side contact layer 5 made of aN is grown to a film thickness of 4 μm.

(Crack prevention layer 6) Next, Si was added at 5 × 1.
A crack prevention layer 6 made of In _0.06 Ga _0.94 N doped with 0 ¹⁸ / cm ³ is grown to a film thickness of 0.15 μm. The crack prevention layer can be omitted.

(N-side clad layer 7 = superlattice layer) Next, 25 GaN layers doped with Si at 5 × 10 ¹⁸ / cm ³ were used for the first time.
Angstrom growth followed by undoped Al _0.30
A Ga _0.70 N layer is grown to a thickness of 25 Å. Then, in the second time, the amount of Si-containing gas is slightly reduced, and the GaN layer is grown to 25 angstroms.
The amount of 1-containing gas is slightly reduced, and an undoped layer of about Al _0.29 Ga _0.71 N is grown to a film thickness of 25 Å. The mixed crystal ratio of Al _0.29 Ga _0.71 N is not an accurate value. From the third time onward, the Si gas amount was made smaller than that of GaN grown before the GaN layer,
A doped GaN layer was grown, followed by the previously grown A
Undoped A with Al content much lower than that of lGaN
The lGaN layer is grown. In this way, the Si-doped GaN layer and the undoped Al _x Ga ₁ -x N layer, which have a gradually decreasing Si content as the Si content approaches the active layer, grow 1.2 μm (240 pairs) in total. After letting
The Si-containing gas was stopped, and the undoped GaN layer was 25 angstroms, more than the previously grown AlGaN.
Undoped AlGaN having a small l content is grown to 25 angstroms. Then, while changing only the composition of AlGaN, by growing so as to have undoped GaN and undoped GaN at the end with a film thickness of 0.1 μm (20 pairs), AlGaN with gradually decreasing Al content is obtained. , An n-side clad layer 7 having a superlattice structure made of GaN whose Si content is gradually reduced is grown to a film thickness of 1.3 μm.

(Active layer 8) Next, undoped In _0.01 Ga
_A barrier layer of _0.95 N is grown to a thickness of 100 Å, and a well layer of undoped In _0.2 Ga _0.8 N is grown to a thickness of 30 Å.
An active layer 8 having a multiple quantum well structure (MQW) having a total film thickness of 360 Å is grown in the order of barrier + well + barrier + well + barrier. The active layer may be undoped as in this embodiment, or may be doped with a donor and / or an acceptor. Both the well layer and the barrier layer may be doped with the donor or the acceptor, or either one of them may be doped.

(P-side cap layer 9) Next, 1 × 1 of Mg was added.
P made of p-type Al _0.3 Ga _0.7 N doped with 0 ²⁰ / cm ³
The side cap layer 9 is grown to a film thickness of 300 Å. When the p-type cap layer is formed with a film thickness of 0.1 μm or less, the output of the device tends to be improved.
The lower limit of the film thickness is not particularly limited, but it is desirable to form the film with a film thickness of 10 angstroms or more. This cap layer can also be omitted.

(P-side clad layer 10) Next, the first undoped GaN layer is grown to 25 angstroms, and then an AlGaN layer containing a trace amount of Al is grown to a film thickness of 25 angstrom by slightly flowing an Al-containing gas. Let Then, for the second time, similarly, undoped GaN is grown to 25 angstroms, and then AlGaN is grown to 25 angstroms by slightly increasing the amount of Al-containing gas. After the third time, A
An undoped AlGaN layer having a slightly higher l content is grown. In this manner, the undoped GaN layers 25 angstroms and the undoped AlGaN layers 25 angstroms in which the Al content is slightly but gradually increasing are alternately stacked to grow 500 angstroms (10 pairs). After 10 pairs of growth, a small amount of Mg-containing gas was flowed to grow a Mg-doped GaN layer having a very small amount of Mg to grow to 25 angstrom, and subsequently, the Al content was higher than that of the undoped AlGaN layer grown previously. An AlGaN layer is grown to a film thickness of 25 Å. Next, the Mg-doped GaN layer 25 angstrom in which the amount of Mg gradually increases with distance from the active layer, and the undoped Al _X Ga _1-X N in which the amount of Al also gradually increases with distance from the active layer. Alternately stack layers 25 Å and finally
8 × 10 ¹⁹ / cm ³ -doped GaN layer is grown, and then an undoped Al _0.2 Ga _0.8 N layer is grown to grow 0.75 μm (150 pairs) in total. In this way, the p-side clad layer 10 having a superlattice structure including the GaN layer having the gradually increasing Mg content and the AlGaN layer having the gradually increasing Al content with increasing distance from the active layer is formed. Grow with a film thickness of 8 μm.

As described above, the n-side cladding layer 7 made of a superlattice layer has a thickness of 1.3 μm (the maximum band gap energy is Al _0.3 Ga _0.7 N), and the p-side cladding layer 10 has a thickness of 0.8 μm.
m (maximum band gap energy is Al _0.2 Ga _0.8
By growing N) and N, it becomes easy to confine the light emission of the active layer in the waveguide region, and it is possible to realize a laser device having a lowered threshold value.

(P-side contact layer 11) Finally, a p-side contact layer 11 made of p-type GaN doped with Mg at 1 × 10 ²⁰ / cm ³ is formed on the p-side cladding layer 10 with a film thickness of 150 Å. Grow.

The wafer on which the nitride semiconductor has been grown as described above is placed in a reaction vessel in a nitrogen atmosphere at 700
Annealing is performed at 0 ° C. to further reduce the resistance of the p-type impurity-doped layer, and then the surface of the n-side contact layer 5 is exposed by RIE (reactive ion etching apparatus).

Next, as shown in FIG. 1, the p-side contact layer 11 and the p-side clad layer 10 are etched to 4 μm.
A ridge shape having a stripe width of m is used. Further, after forming an insulating film 22 made of ZrO2 on the side surface of the ridge, the p electrode 2 made of Ni and Au is formed through the insulating film.
On the other hand, the n-electrode 22 made of Ti and Al is formed in a stripe shape on the surface of the n-side contact layer 5 exposed previously.

As described above, the n electrode and the p electrode are formed.
The sapphire substrate of the formed wafer is polished to 70 μm.
Then, in the direction perpendicular to the striped electrodes,
Cleaves in a bar shape and mounts a resonator on the cleavage plane (11-00 plane).
Create. SiO on the resonator surface ₂And TiO₂A dielectric
Form a multilayer film, and finally, in the direction parallel to the p-electrode,
Cut into laser elements.

When the rear surface side of the different type substrate 1 of this laser device was set on a heat sink and each electrode was wire-bonded, and laser oscillation was attempted at room temperature, it showed laser oscillation at room temperature. Appl.Phys.Vol.36 (199
Compared with the one announced in 7), the threshold value is decreased by 50% or more, continuous oscillation for 3000 hours or more is shown at the output of 20 mW, and the shape of the laser beam is a single elliptical shape with vertical symmetry. It was in mode.

Example 2 (First Mode) In Example 1, the amount of Si doped into the GaN layer when growing the n-side cladding layer 7 was 1 × 10 ¹⁸ / cm ^3.
The superlattice layer is grown in the same manner except that the Al mixed crystal ratio is reduced as the AlGaN layer approaches the active layer. However, 0.1μ on the side close to the active layer
A region in which a GaN layer and an AlGaN layer having a film thickness of m are stacked is similarly undoped.

When growing the p-side cladding layer 10, the amount of Mg doped into GaN is kept constant at 5 × 10 ¹⁹ / cm ^3, and the Al mixed crystal ratio is increased as only the AlGaN layer is separated from the active layer. Otherwise, the superlattice layer is grown in the same manner. However, the region where the GaN layer having a film thickness of 800 Å and the AlGaN layer on the side closer to the active layer are laminated is similarly undoped.

A laser element was manufactured in the same manner as in Example 1 except for the above, and the threshold value of this laser element increased slightly as compared with Example 1, but a single mode for 2000 hours or more at an output of 20 mW. It showed continuous oscillation of.

Example 3 (Second Mode) In Example 1, when growing the n-side cladding layer 7, the Al mixed crystal ratio of the AlGaN layer was kept constant at Al _0.20 Ga _0.80 N and GaN was formed. The superlattice layer is grown in the same manner except that the amount of Si to be doped is reduced as it approaches the active layer. However, 0.1 μm on the side close to the active layer
The region in which the GaN layer and the AlGaN layer having the same thickness are stacked is similarly undoped.

[0048] Further, when growing the p-side cladding layer 10 and the Al mole fraction of the AlGaN constant at Al _{0. 15} Ga _0.85 N, the other to increase with distance the amount of Mg doped into GaN active layer Similarly grows the superlattice layer. However, the region where the GaN layer having a film thickness of 800 Å and the AlGaN layer on the side closer to the active layer are laminated is similarly undoped.

A laser element was manufactured in the same manner as in Example 1 except for the above, and the threshold value of this laser element also increased slightly as compared with Example 1, but a single mode for 1000 hours or longer at an output of 20 mW. It showed continuous oscillation of.

[Example 4] A laser device was manufactured in the same manner as in Example 1 except that the p-side cap layer 9 was not grown, and the output at the same current value was that of Example 1. In comparison with the above, the life was 2500 hours or more at an output of 20 mW, although it was slightly lowered.

[Embodiment 5] FIG. 3 is a schematic sectional view showing the structure of a laser device according to another embodiment of the present invention. The same reference numerals as in FIG. 1 indicate the same members. The following description is based on this figure.

In the same manner as in Example 1, the underlayer 2 made of undoped GaN was grown to a thickness of 4 μm on the heterogeneous substrate 1 made of sapphire by the MOVPE method, and then the striped protective film 3 was formed. To form.

(Nitride semiconductor substrate 44) After forming the protective film,
Using the MOVPE method, a nitride semiconductor layer made of Si-doped GaN is grown to a thickness of 10 μm, the upper part of the protective film 3 is covered with GaN, and then transferred to an HVPE apparatus, and Ga metal, HCl gas, NH ₃ , silane gas are transferred. Is used to grow a nitride semiconductor substrate 44 of GaN doped with Si of 5 × 10 ¹⁷ / cm ³ to a film thickness of 500 μm. After growth,
Sapphire substrate 1, protective film 3, and undoped GaN
The layer region is removed by polishing, and the nitride semiconductor substrate 44 is manufactured.

After that, a layer above the crack prevention layer 6 is laminated on the nitride semiconductor substrate 44 (the side not on the polishing side) using the MOVPE apparatus. After the growth, a high-concentration Si region doped with high-concentration Si is formed on the nitride semiconductor substrate on the polished surface to form an n-electrode forming layer 44 '.

Thereafter, a ridge is formed as shown in FIG.
A laser element is manufactured in the same manner as in Example 1 except that the n-electrode 21 is formed on almost the entire surface of the electrode formation layer 44. Figure 3
Since the laser element shown in (1) uses Si-doped GaN as a substrate, it is possible to take an electrode from the substrate side.
As for this laser element, an element having substantially the same characteristics as the laser element of Example 1 was obtained.

[0056]

As described above, in the device of the present invention, the Al composition decreases as it approaches the active layer,
Most of the light is confined in the waveguide, and the laser light becomes a single mode, and the threshold value is lowered by having the cladding layer having a low impurity concentration. Therefore, the laser device can be made to have a high output and a long life. Therefore, by using the present invention, it is very important to put the laser into practical use as a writing light source. Further, in the present specification, the laser element used under the most severe conditions has been described, but the present invention is applied not only to the laser element but also to any other electronic device using a nitride semiconductor such as an LED and a light receiving element. It is possible.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing the structure of a laser device according to an embodiment of the present invention.

FIG. 2 is a diagram showing energy bands from the n-type cladding layer 7 to the p-side cladding layer 10 in FIG.

FIG. 3 is a schematic sectional view showing the structure of a laser device according to another embodiment of the present invention.

[Explanation of symbols]

1 ... Different substrate 2 ... Underlayer 3 ... Protective film 4, 44 ... Nitride semiconductor substrate 5 ... n-side contact layer 6 ... Crack prevention layer 7 ... N-side cladding layer 8 ... Active layer 9 ... P-side cap layer 10 ... P-side cladding layer 11 ... P-side contact layer 20 ... p electrode 21 ... N-electrode 22 ... Insulating film 44 '... n-electrode forming layer

Continuation of front page (56) Reference JP-A-5-110139 (JP, A) JP-A-62-193192 (JP, A) JP-A-60-145686 (JP, A) JP-A-7-15041 (JP , A) JP-A-9-83016 (JP, A) Jpn. J. Appl. Phys. , Vol. 36 (1997), Part 2, No. 12A, pp. 1568-1571 (58) Fields investigated (Int.Cl. ⁷ , DB name) H01S 5/00-5/50 H01L 33/00

Claims (11)

(57) [Claims] 1. A nitride semiconductor device having an active layer containing In having a quantum well structure between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer, said n-type nitride semiconductor layer and p-type nitride The object semiconductor layer is provided with a superlattice layer in which a first nitride semiconductor layer containing Al and a second nitride semiconductor layer having a composition different from that of the first nitride semiconductor layer are stacked. Between the superlattice layer provided in the p-type nitride semiconductor layer and the active layer, a layer made of a nitride semiconductor having a band gap energy larger than that of the well layer and containing Al is provided.
A nitride semiconductor device, wherein the entire thickness of the superlattice layer provided in the p-type nitride semiconductor layer is smaller than the overall thickness of the superlattice layer provided in the n-type nitride semiconductor layer. 2. The thickness of the entire superlattice layer provided in the n-type nitride semiconductor layer is 100 angstroms or more and 5 μm or more.
And the thickness of the entire superlattice layer provided in the p-type nitride semiconductor layer is 50 angstroms or more and 2 μm or less.
The nitride semiconductor device according to claim 1, wherein: 3. The nitride semiconductor device according to claim 1, wherein the nitride semiconductor layer forming the superlattice layer is a nitride semiconductor layer having a single film thickness of 100 angstroms or less. 4. The superlattice layer contains an impurity that determines a conductivity type, and the impurity is contained in at least one of the first nitride semiconductor layer and the second nitride semiconductor layer. The nitride semiconductor device according to any one of claims 1 to 3, characterized in that. 5. The nitride semiconductor device according to claim 4, wherein the conductivity-determining impurities contained in the superlattice layer are adjusted so as to decrease as they approach the active layer. . 6. The first nitride semiconductor layer is configured such that the Al composition is reduced as it approaches the active layer, and an impurity that determines a conductivity type contained in the superlattice layer is activated. The nitride semiconductor device according to claim 1, wherein the nitride semiconductor device is adjusted so as to decrease as it approaches the layer. 7. The superlattice layer provided on the n-type nitride semiconductor layer and the p-type nitride semiconductor layer is formed so that at least one is in contact with an active layer.
7. The nitride semiconductor device according to any one of items 1 to 6. 8. The first nitride semiconductor layer is Al _X Ga.
8. The nitride according to any one of claims 1 to 7, wherein the nitride semiconductor layer is made of _1-X N (0 <X <1) and / or the second nitride semiconductor layer is made of GaN. Semiconductor device. 9. The nitride semiconductor device according to claim 1, wherein an n-type nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer are sequentially provided on a nitride semiconductor substrate. The nitride semiconductor device according to claim 1. 10. The nitride semiconductor substrate is a substrate obtained by growing a nitride semiconductor on a heterogeneous substrate selected from the group consisting of sapphire, SiC, ZnO, spinel and GaAs. The nitride semiconductor device described. 11. The layer having a thickness of 0.3 μm or less on the side of the superlattice layer close to the active layer is an undoped layer not doped with impurities. The nitride semiconductor device according to any one of 1.