JPH11298090A - Nitride semiconductor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To lower the oscillation threshold by comprising an active layer having a multiple quantum well structure composed of laminated In-containing nitride semiconductor well layers and barrier layers between and p-side nitride semiconductor layers, where the well layers the active layer are not doped but barrier layers are doped with an n-type impurity. SOLUTION: An active layer 25 has undoped In0.2 Ga0.8 N well layers grown to 40 Å thickness and Si-doped by 5×10<18> /cm<3> In0.01 Ga0.95 N barrier layers grown up to 100 Å thick at the same temperature. They are laminated in the order of the well, barrier, well, and barrier layers as a multiple quantum well structure of the active layer grown. The number of the well layers is closely related to the threshold current density which lowers most, when the number of well layers is 2, and hence the oscillation threshold can be made to lower.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a light emitting diode element, a light-emitting element such as a laser diode element, a solar cell, a light receiving element such as an optical sensor or transistor, a power device such as a nitride semiconductor (an In _X used for electronic devices, A
1 _Y Ga _{1 -XYN} , 0 ≦ X, 0 ≦ Y, X + Y ≦ 1).

[0002]

2. Description of the Related Art A nitride semiconductor is a high-brightness pure green light emitting LE.
D and blue LEDs have already been put to practical use in various light sources such as full-color LED displays, traffic signal lights, and image scanner light sources. These LED elements basically have a buffer layer made of GaN on a sapphire substrate,
N-side contact layer made of Si-doped GaN, InGaN having a single quantum well structure or an active layer having a multiple quantum well structure having InGaN, p-side cladding layer made of Mg-doped AlGaN, and p-layer made of Mg-doped GaN
Side contact layer is sequentially laminated,
At 20 mA, 5 mW with a 450 nm emission wavelength blue LED, 9.1% external quantum efficiency, 520 nm green LE
D shows an excellent characteristic of 3 mW and an external quantum efficiency of 6.3%.

[0003] In addition, we have produced a nitride semiconductor laser device including an active layer on a nitride semiconductor substrate, and announced that the world's first continuous oscillation at room temperature of 10,000 hours or more has been achieved (ICNS). '97 Proceedings, October 27-31, 1997, P444-4
46, and Jpn.J.Appl.Phys.Vol.36 (1997) pp.L1568-157
1, Part 2, No. 12A, 1 December 1997). The basic structure is such that a partially formed SiO 2 is formed on a sapphire substrate.
_A plurality of nitride semiconductor layers forming a laser element structure are stacked on a nitride semiconductor substrate made of GaN selectively grown via _two films. (For details, see Jpn.J.Appl.Phys.Vol.36
reference)

[0004]

However, the continuous oscillation of 10,000 hours or more was estimated at 2 mW in output. At 2 mW, the light source for reading is slightly insufficient, and the light source for writing needs an output ten times or more than this. Therefore, further improvement in the output of the laser element and a longer life are desired.

When the oscillation threshold value 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 fact that the threshold value is reduced means that the present invention can be applied not only to a laser element but also to other nitride semiconductor elements such as an LED and an SLD, and a highly efficient and highly reliable element can be provided. Accordingly, it is an object of the present invention to improve the output of a laser device and to extend the life of the device by lowering the oscillation threshold value.

[0006]

Means for Solving the Problems The nitride semiconductor device of the present invention mainly comprises two types of embodiments.
A well layer made of a nitride semiconductor containing In and a barrier layer made of a nitride semiconductor having a different composition from the well layer are stacked between the nitride semiconductor layer on the p-side and the nitride semiconductor layer on the p-side. Wherein the well layer of the active layer is undoped, and the barrier layer is doped with an n-type impurity.

According to a second aspect of the present invention, a well layer made of a nitride semiconductor containing In, a composition of a well layer and a well layer formed between an n-side nitride semiconductor layer and a p-side nitride semiconductor layer. An active layer having a multiple quantum well structure in which barrier layers made of different nitride semiconductors are stacked;
The side close to the nitride semiconductor ends with a barrier layer, at least the last well layer is undoped, and the last barrier layer is doped with an n-type impurity. In the present invention, the n-side nitride semiconductor refers to a semiconductor layer side including at least an n-type nitride semiconductor layer in one nitride semiconductor layer with an active layer interposed therebetween, and the p-side nitride semiconductor refers to an active layer. Indicates the nitride semiconductor on the opposite side to the nitride semiconductor on the n side. In the first and second aspects of the present invention, the threshold of the multiple quantum well structure tends to be lower when starting from the well layer.

In the first and second aspects of the present invention,
Undoped refers to a nitride semiconductor layer that is not intentionally doped with impurities. For example, it is possible to mix impurities contained in a raw material, contamination of impurities due to contamination in a reaction apparatus, and intentionally doping impurities. In the present invention, a layer in which an impurity is mixed due to unintended impurity diffusion from the layer of is also defined as undoped (substantially undoped).

In the first and second aspects of the present invention,
The n-type nitride semiconductor layer has a first semiconductor made of GaN or a nitride semiconductor containing In on a side close to the active layer.
And a second nitride semiconductor layer comprising a superlattice including a nitride semiconductor layer containing Al on a side farther from the active layer than the first nitride semiconductor layer. Features. The first nitride semiconductor layer may or may not be in contact with the active layer, but is preferably formed in contact with the active layer.

Further, the p-side nitride semiconductor layer includes:
A nitride semiconductor containing Al or a third nitride semiconductor layer made of GaN is provided on a side closer to the active layer, and Al is provided on a side farther from the active layer than the third nitride semiconductor layer is.
And a fourth nitride semiconductor layer including a superlattice including a nitride semiconductor layer including: Similarly, the third nitride semiconductor layer may or may not be in contact with the active layer, but is preferably formed in contact with the active layer.

Further, in the first and second aspects of the present invention,
At least one of the n-side and the p-side nitride semiconductor layers has a fifth nitride semiconductor layer made of a superlattice containing a nitride semiconductor containing Al, and the fifth nitride semiconductor layer Is characterized in that the Al content is reduced as it approaches the active layer. This sets the superlattice to G
RIN structure (gradient index waveguide) is meant.

The fifth nitride semiconductor layer is doped with an impurity which determines the conductivity type of the layer, and the impurity is adjusted so as to decrease as the impurity approaches the active layer. I do.

[0013] The barrier layer contains a p-type impurity by impurity diffusion. Impurity diffusion refers to, for example, a state in which a p-type impurity is diffused into a barrier layer during or after growth of a layer containing a p-type impurity which is adjacent to, close to (not necessarily in contact with), or after growth. . For example, when a p-type impurity is contained by impurity diffusion, the layer often has a concentration gradient of the p-type impurity. As impurities mixed by diffusion, Mg, B
It is often an impurity that has been confirmed to be p-type when doped into a nitride semiconductor such as e or Ca. It is often smaller than the n-type impurity.

Further, the present invention is characterized in that the total number of well layers in the multiple quantum well structure is two. When the number of wells is 2, the threshold value is most likely to decrease. That is, from the n side, a barrier + well + barrier + well + barrier, well + barrier + well + barrier, well +
It is most preferable to have a multiple quantum well structure in which a barrier and a well (the structure ending with the well is applied only to the first embodiment) are stacked.

[0015]

FIG. 1 is a schematic sectional view showing the structure of one laser device according to the present invention. The basic structure is such that an n-side contact layer 21 and a crack prevention layer are formed on a GaN substrate 20. (Can be omitted) 22, n-side cladding layer 23,
an n-side optical guide layer 24, an active layer 25 having a multiple quantum well structure,
It has a structure in which a p-side cap layer 26, a p-side guide layer 27, a p-side cladding layer 28, and a p-side contact layer 29 are sequentially laminated.
1 shows an index-guided laser device having a striped waveguide region etched up to 1.

In the active layer 25 having a multiple quantum well structure, the well layer is made of a nitride semiconductor containing at least In, preferably In _x Ga ₁ -xN (0 <x <1). on the other hand,
As the barrier layer, a nitride semiconductor having a band gap energy larger than that of the well layer is usually selected, and preferably, In _Y Ga
_{1-Y N (0 ≦ Y} <1, X> Y) or _{Al Z Ga 1-Z N (} 0
<Z <0.5). However, the well layer and the barrier layer are made of InAl.
N can also be set. The thickness of the well layer is adjusted to 100 Å or less, more preferably 70 Å or less, and most preferably 50 Å or less. If the thickness is more than 100 Å, the output tends to decrease. Although the thickness of the barrier layer is not particularly limited, it is usually formed to be the same as or thicker than the well layer, and 200 Å or less, preferably 15 Å or less.
0 Å or less, and most preferably 100 Å or less. Further, the thickness of the barrier layer may be smaller than that of the well layer.

In the first embodiment of the present invention, the well layer of the active layer 25 is undoped (in a state where no impurity is intentionally doped), and only the n-type impurity is doped into the barrier layer.
When the barrier layer is doped with an n-type impurity, the carrier concentration of the well layer increases, so that the threshold value decreases. Conversely, if the barrier layer is intentionally doped with a p-type impurity, it tends to be less likely to decrease. Therefore, it is desirable to dope only the n-type impurity into the barrier layer. Furthermore, when the barrier layer is doped with an n-type impurity, the carrier concentration of the well layer increases,
It is presumed that the spatial separation of electrons and holes due to the quantum Stark effect of the piezo effect due to distortion is screened, and the threshold value is lowered. Conversely, if the well layer is doped with impurities, the crystallinity deteriorates and carrier scattering increases,
The threshold value tends to be higher. In the first aspect of the present invention, in the multiple quantum well structure, the side close to the p-side nitride semiconductor layer may end with the barrier layer or the well layer, but as in the second aspect, Desirably ends in layers.

On the other hand, in the second embodiment, the active layer 25 ends on the side closer to the p-side nitride semiconductor with a barrier layer, and at least the last well layer is undoped, and the last barrier layer is doped with an n-type impurity. Have been. That is, the layers are stacked from the n-side nitride semiconductor so as to be a well + barrier + well + barrier or a barrier + well + barrier + well + barrier, the last well layer is undoped, and the last barrier layer is formed. Doping with an n-type impurity. The other well layers and barrier layers may or may not be doped with an impurity, and the impurity doping is not particularly limited. Preferably, as in the first aspect,
It is desirable that the barrier layer is doped with an n-type impurity and the well layer is undoped. In the second embodiment, the threshold of the multiple quantum well structure tends to be lower when the barrier layer is terminated than when the barrier layer is terminated, and the last well layer is undoped, and the barrier layer is doped with an n-type impurity. Then, the threshold value further decreases. Although the reason is not clear, in the case of a nitride semiconductor, the effective mass of holes is large, the holes injected into the active layer are localized on the p-layer side, and it is considered that light is emitted only from the well layer on the p-layer side. Can be Therefore, the well layer approaching the n-layer side does not contribute much to light emission, and the well layer approaching the p-layer side contributes more to light emission. For this reason, it is presumed that the efficiency is most improved when the well layer closest to the p-layer side is undoped and the barrier layer is doped with an n-type impurity.

The n-type impurity doped into the barrier layer is Si,
Group IV such as Ge, Sn, S, O, Ti, Zr, or VI
Group elements can be used, preferably Si, Ge, S
n is used. The doping amount of the n-type impurity is 5 × 10 ¹⁶ / cm ³
５5 × 10 ²¹ / cm ³ , preferably 1 × 10 ¹⁷ / cm ³ -1 ×
10 ²⁰ / cm ³ , more preferably 1 × 10 ¹⁸ / cm ³ to 1 ×
Adjust to the range of 10 ¹⁹ / cm ³ . If it is less than 5 × 10 ¹⁶ / cm ³ , the threshold value does not decrease so much, while 5 × 10 ²¹ / cm 3
^{If the} number is more than ³ , the crystallinity of the barrier layer is deteriorated, and the life is shortened.

In the first and second aspects of the present invention,
A nitride semiconductor containing In or a first nitride semiconductor layer made of GaN is provided on a side closer to the active layer 25, and Al is contained on a side farther from the active layer than the first nitride semiconductor layer is. It has a second nitride semiconductor layer made of a superlattice including the nitride semiconductor layer. In the case of FIG. 1, the n-side light guide layer 24 corresponds to a first nitride semiconductor layer. By providing this layer, for example, in a laser element, it acts as a layer forming a waveguide region, and in other elements, it acts as a buffer layer before growing an active layer, and grows an active layer with good crystallinity. Make it easier.

Further, in the case of FIG. 1, the n-side cladding layer 23 made of a superlattice corresponds to the second nitride semiconductor layer. This layer acts as a cladding layer for confining carriers and confining light. The superlattice is a multilayer film in which nitride semiconductor thin films having different compositions are stacked. When forming a superlattice, one of the nitride semiconductors is a nitride semiconductor containing Al, preferably a ternary mixed crystal of Al _x Ga ₁ -xN (0
When <X <1), a material having good crystallinity is easily obtained.
The other may be of any composition as long as it has a different composition from the nitride semiconductor containing Al, but is preferably a nitride semiconductor having a band gap energy smaller than that of the nitride semiconductor, such as In _Y Ga ₁ -YN (0 ≤Y≤1). Among them, GaN is most excellent in crystallinity.
That is, when the superlattice layer is composed of AlGaN and GaN,
GaN with good crystallinity acts like a buffer layer,
AlGaN can be grown with good crystallinity. The single thickness of the superlattice layer is 100 Å or less, more preferably 7 Å.
0 angstrom or less, most preferably 50 angstrom or less. By growing the thin film in this way, the nitride semiconductor has an elastic critical thickness or less.
Even a crystal such as AlGaN that easily cracks can be grown with good film quality without cracking. In addition, InAlN, InGaAlN, or the like can be formed in the superlattice layer.

In the first and second aspects of the present invention,
A nitride semiconductor containing Al or a third nitride semiconductor layer made of GaN is provided on the side closer to the active layer 25, and further contains Al on a side farther from the active layer than the third nitride semiconductor layer. It has a fourth nitride semiconductor layer composed of a superlattice including the nitride semiconductor layer. In the case of FIG. 1, the p-side cap layer 26 or the p-side light guide layer 27 corresponds to the third nitride semiconductor layer, and the p-side cladding layer 28 made of a superlattice
Corresponds to the fourth nitride semiconductor. p-side cap layer 26
Is a nitride semiconductor having a band gap energy larger than that of the barrier layer of the active layer, and has a thickness of 0.1 μm.
By making it thinner, the output tends to improve. The p-side light guide layer 27 has a band gap energy smaller than that of the p-side cap layer, and thus acts as a layer forming a waveguide region in a laser device, for example. It acts as a buffer layer before growing the layer 28, and facilitates the growth of a cladding layer having good crystallinity.

Further, in the case of FIG. 1, the p-side cladding layer 28 made of a superlattice corresponds to the fourth nitride semiconductor layer. The function and the preferred configuration of this layer are exactly the same as those of the n-side cladding layer, and therefore the description is omitted.

Further, in the present invention, the n-side cladding layer 23,
The p-side cladding layer 29 may have a super lattice GRIN structure. Assuming a GRIN structure, the light emission of the active layer is Al
Since the light is guided in a region having a small composition, the longitudinal mode tends to become a single mode, and the threshold value is reduced. In general, a clad layer having a double hetero structure needs to have a band gap energy larger than that of an active layer.
Is used. Further GRIN
In the structure, only the outermost layer, that is, the layer farthest from the active layer, needs to have a large Al mixed crystal ratio, and the Al mixed crystal ratio decreases as approaching the active layer. A layer having a large crystal ratio is easily formed. Therefore, the refractive index difference between the cladding layer and the active layer can be increased, so that the light confinement effect increases and the threshold value decreases.
In a GRIN structure in which the refractive index gradually decreases from the center (active layer) toward the outside, light tends to converge at the center, so that the threshold value decreases. When the n-side cladding layer has a GRIN structure, the n-side light guide layer 24 and the p-side light guide layer 28 may be omitted since the active layer side of the clad layer forms the waveguide region.

Further, when the n-side cladding layer 23 and the p-side cladding layer 28 have a superlattice GRIN structure, at least one of the n-type impurity concentration and the p-type impurity concentration contained in the cladding layer approaches the active layer. Therefore, it is desirable to make adjustments so as to reduce the number. The impurity may be doped into both the first nitride semiconductor layer containing Al and the other nitride semiconductor layer, but it is preferable to dope either one. This is called modulation doping, and by doping one of the superlattice layers with an impurity, the crystallinity of the entire superlattice layer is improved, which is also an effect for realizing a highly reliable device. It is a target. In other words, when a layer doped with impurities is grown on a layer with good crystallinity without impurities, the crystallinity of the layer doped with impurities is improved, and the crystallinity of the entire superlattice layer is improved. . As the n-type impurities, Si, G
e and Sn are preferably used. Mg as a p-type impurity
Be and Ca are preferably used. As described above, when the clad 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 clad layer is reduced, so that light loss is reduced and the threshold is lowered. Further, the crystallinity is better than a nitride semiconductor having a low impurity concentration and a nitride semiconductor having a high impurity concentration. Therefore, n having a low impurity concentration and good crystallinity,
When the active layer is sandwiched between the p-type cladding layers, an active layer having few crystal defects can be grown, so that the life of the element is prolonged, the reliability is improved, and the breakdown voltage of the element is increased.

In the case of the GRIN structure, the impurity concentration
In the case of an n-type impurity, the outermost layer of the n-side cladding layer is 1 × 10
¹⁷~ 5 × 10²⁰/cm^Three, Preferably 5 × 10¹⁷~ 1 × 1
0²⁰/cm^ThreeAdjust to the range. Also near the active layer, for example
In the low impurity portion concentration region of the superlattice layer of 0.3 μm or less, 1 ×
10¹⁹/cm^ThreeHereinafter, more preferably, 5 × 10¹⁸/cm^ThreeLess than
Adjust down. On the other hand, in the case of a p-type impurity,
1 × 10 at the outermost layer ¹⁷~ 5 × 10^{twenty one}/cm^Three, Preferably
Is 5 × 10¹⁷~ 1 × 10^{twenty one}/cm^ThreeAdjust to the range. Ma
In the vicinity of the active layer, for example, in the low impurity concentration region of the superlattice layer.
For 3 μm or less, 1 × 10¹⁹/cm^ThreeBelow, even more preferred
Ku 5 × 10¹⁸/cm^ThreeAdjust to the following. Outermost layer impurities
Rather than the concentration, the low impurity concentration layer is more important,
The impurity concentration on the side close to the active layer is 1 × 10¹⁹/cm^ThreeYo
If the amount is too high, the light absorption will increase and the threshold will not easily decrease.
Tend to be. Also, due to the increased impurity concentration
The life tends to be shortened due to a decrease in crystallinity. most
Preferably, no impurities are intentionally doped, that is,
And dope.

[0027]

[Embodiment 1] (First and Second Embodiments) Hereinafter, the present invention will be described in detail with reference to FIG. 1. However, Embodiment 1 shows both the first and second embodiments. .

(GaN substrate 20) A protective film made of striped SiO ₂ is formed on GaN grown on a heterogeneous substrate made of 2 inch φ sapphire. An undoped GaN layer is grown to a thickness of 10 μm by MOVPE, and then HV
Similarly, an undoped GaN layer is grown to a thickness of 300 μm by the PE method. After the growth, the sapphire substrate, the protective film, and the GaN layer grown by MOVPE are polished and removed.
A GaN substrate 20 of 10 ⁵ / cm ² or less is obtained. In addition to polishing, as a means for separating sapphire and the GaN substrate, a high-power excimer laser is irradiated from the back side of the polished sapphire substrate at a spot size of about 100 μm and scanned to remove GaN at the GaN interface. Although there is a method of separation by alteration, this method may be used.

(N-side contact layer 21) Next, on the GaN substrate (surface opposite to the polished side), Si is applied at 1 × 10 ¹⁹ / cm 2.
³ doped 4μ the n-side contact layer 5 made of GaN
It is grown to a thickness of m. The n-side contact layers 21 to p
All layers up to the side contact layer 29 are grown by MOVPE.

(Crack prevention layer 22) Next, 5 ×
A crack preventing layer 22 made of In _0.06 Ga _0.94 N doped with 10 ¹⁸ / cm ³ is grown to a thickness of 0.15 μm.
The crack prevention layer can be omitted.

(N-side cladding layer 23 = superlattice layer) Subsequently, a layer made of undoped Al _0.16 Ga _0.84 N is grown to a thickness of 25 Å, then TMA is stopped, silane gas is flown, and Si A layer made of × 10 ¹⁸ / cm ³ doped n-type GaN is grown to a thickness of 25 Å. These layers are alternately stacked to form a superlattice layer, and an n-side cladding layer 23 made of a superlattice having a total film thickness of 1.2 μm is grown. As described above, the superlattice is composed of a nitride semiconductor containing Al, and the other is composed of a nitride semiconductor having a smaller bandgap energy, and one of the layers is doped with impurities. It is desirable to carry out.

(N-side light guide layer 24) Subsequently, an n-side light guide layer 24 made of undoped GaN is grown to a thickness of 0.1 μm. The n-side light guide layer 24 has Si, G
An n-type impurity such as e or Sn may be doped.

(Active Layer 25) Next, undoped In_0.2
Ga_0.8The N well layer is a 40 Å film
Thick, followed by 5 × 10 5 Si at the same temperature.¹⁸/
cm^ThreeDoped In_{0.0 1}Ga_0.951 N barrier layer
It is grown to a thickness of 00 Å. in this way
Wells + barriers + wells + barriers are stacked in this order, with a total thickness of 280 mm.
Layer with multiple quantum well structure (MQW)
Grow. The last barrier layer of this active layer is
Mg diffused from the contacting p-side cap layer
0¹⁸/cm ^ThreeWas included.

(P-side cap layer 26) Next, Mg was added to 1 ×
A p-side cap layer 26 of 10 ²⁰ / cm ³ doped p-type Al _0.3 Ga _0.7 N is grown to a thickness of 150 Å. When the p-type cap layer is formed with a thickness of 0.1 μm or less, the output of the device tends to be improved. Although the lower limit of the film thickness is not particularly limited, it is desirable to form the film with a film thickness of 10 Å or more. This cap layer can also be omitted.

(P-side light guide layer 27) Next, Mg was added to 5 × 1
P-side optical guide layer 2 made of 0 ¹⁶ / cm ³ doped GaN
7 is grown to a thickness of 0.1 μm.

(P-side cladding layer 28) Subsequently, a layer made of undoped Al _0.16 Ga _0.84 N is grown to a thickness of 25 Å, and a layer made of GaN doped with Mg at 1 × 10 ¹⁹ / cm ³ is formed. Grown to a thickness of 25 angstroms, stacked alternately, and a total thickness of 0.7 μm
A p-side cladding layer 28 of m superlattice layers is grown.

(P-side contact layer 29) Finally, a p-side contact layer 29 made of p-type GaN doped with 1 × 10 ²⁰ / cm ^{3 of} Mg is grown to a thickness of 150 Å.

The wafer grown in this manner is
The mask was taken out of the VPE reaction vessel, and a mask having a stripe width of 1.5 μm made of SiO ₂ was formed on the surface of the uppermost p-side contact layer using a CVD apparatus. As shown in
The etching is performed until the surface of the side contact layer 21 is exposed. Then, T is applied to the exposed surface of the n-side contact layer 21.
An n-electrode 32 made of i / Al is formed.

After the formation of the n-electrode, a SiO ₂ mask is formed on the surface of the n-electrode while the SiO ₂ mask is kept on the surface of the p-side contact layer 29, and a striped strip is etched and exposed as shown in FIG. An insulating film 30 made of ZrO2 is formed on a side surface of the nitride semiconductor layer.

After the formation of the insulating film 30, S is formed by a lift-off method.
After removing the iO ₂ mask, p
On the surface of the side contact layer 29, a p-electrode 31 made of Ni / Au is formed.

After forming the electrodes, the GaN substrate was placed on the M-plane (11-0).
When a laser device was manufactured using the cleavage plane opposing the cleavage at 0) as a resonance plane, continuous oscillation was exhibited at a threshold current density of 1.2 kA / cm ² , and continuous oscillation was observed at 20 mW output for 2000 hours or more.

FIG. 2 shows the relationship between the threshold current density of the laser element and the number of well layers in the active layer 25 of the laser element. In FIG. 2, the number 1 of the well layers indicates a single quantum well, and the subsequent shows a multiple quantum well structure in which the well layers are sandwiched between barrier layers. As shown in this figure, the number of well layers is closely related to the threshold current density, and it can be seen that the threshold current density decreases most when the number of well layers is two.

Embodiment 2 (Second Aspect) In the first embodiment, when growing the active layer 25,
A barrier layer made of × 10 ¹⁸ / cm ³ doped In _0.01 Ga _0.95 N is grown to a thickness of 100 Å, followed by 1 × 10 ¹⁸ Si doped In _0.2 at the same temperature.
A Ga _0.8 N well layer is grown to a thickness of 40 Å. And Si-doped barrier + Si-doped well + Si
After stacking the doped barriers, the last well layer is undoped, followed by the growth of the last Si doped barrier layer, resulting in a multiple quantum well structure (MQW) with a total thickness of 380 Å.
And Other than that, when a laser device was manufactured in the same manner as in Example 1, the threshold current density was slightly increased as compared with Example 1, but at the same 20 mW output, the laser current was increased to 2000.
It showed a lifetime of more than hours.

Embodiment 3 FIG. 3 is a schematic sectional view showing the structure of a laser device according to Embodiment 3 of the present invention. Embodiment 3 will be described below with reference to FIG.

(GaN substrate 20 ′) On GaN grown on a heterogeneous substrate made of 2 inch φ sapphire,
A protective film made of striped SiO ₂ is formed, and an undoped GaN layer is grown on the striped protective film by MOVPE to a thickness of 10 μm.
Ga doped with 1.times.10@18 / cm @ 3 Si by VPE
An N layer is grown to a thickness of 300 μm. After the growth, the sapphire substrate, the protective film, and the undoped GaN layer are polished and removed to obtain a GaN substrate doped with Si.

(Crack Prevention Layer 22) Next, in the same manner as in Example 1, 5 × 10 ¹⁸ / cm ^{3 of In} doped with Si was used.
_The crack preventing layer 22 made of _0.06 Ga _0.94 N is 0.2 μm thick.
It is grown to a thickness of m.

(N-side cladding layer 23 ′ = superlattice GRIN
Structure) Next, a first GaN layer doped with 5 × 10 ¹⁸ / cm ³ of Si is grown at 25 Å, and an undoped Al _0.30 Ga _0.70 N layer is grown at a thickness of 25 Å. Then, a second time, the GaN layer is grown at 25 Å with a slightly reduced amount of Si-containing gas, followed by a slightly reduced amount of Al-containing gas to reduce the undoped Al _0.29 Ga _0.71 N layer to approximately 25 Å. It grows with the film thickness. The mixed crystal ratio of Al _0.29 Ga _0.71 N is not an accurate value. From the third time on, the amount of Si gas is further reduced than that of GaN grown before the GaN layer, and the Si-doped GaN layer is grown, and the Al content is further lower than that of AlGaN grown subsequently. Grow an undoped AlGaN layer. Thus, S
As the i content approaches the active layer, the Si-doped GaN layer gradually decreases and the undoped Al _x G
After growing 1.2 μm (240 pairs) together with the a _1-x N layer, the Si-containing gas is stopped, and the undoped GaN layer is 25 Å, and the undoped GaN layer has a lower Al content than the previously grown AlGaN. AlGaN 2
Grow 5 angstroms. Then, while changing only the composition of AlGaN, AlGaN is grown to a thickness of 0.1 μm (20 pairs) so as to become undoped GaN and undoped GaN, so that the Al content gradually decreases. Then, an n-side cladding layer 23 'having a superlattice structure composed of GaN having a gradually decreasing Si content is grown to a thickness of 1.3 [mu] m. In addition,
Although the amount of the n-type impurity was adjusted for GaN, it can also be adjusted for the AlGaN layer. Although the Al composition ratio of AlGaN was finely divided and adjusted so as to gradually decrease,
For example, it can be roughly reduced in steps in about three steps, which is within the scope of the present invention.

(Active Layer 25)
A well layer of In _0.2 Ga _0.8 N of 0 Å and Si of 100 Å at 5 × 10 ¹⁸ / cm ³
An active layer having a multiple quantum well structure (MQW) having a total thickness of 280 Å is grown by stacking a barrier layer made of doped In _0.01 Ga _0.95 N in the order of well + barrier + well + barrier. Further, an n-side light guide layer may be formed between the active layer 25 and the n-side cladding layer 23 'in the same manner as in the first embodiment.

(P-side cladding layer 28 ′) Next, an undoped GaN layer is grown for the first time at 25 Å, and then an Al-containing gas is flowed slightly to form an AlGaN layer containing a trace amount of Al with a thickness of 25 Å. Let it grow. Then, the second time, 25 g of undoped GaN
Angstrom growth followed by a slight increase in the amount of Al-containing gas to grow 25 Å of AlGaN. After the third time, an undoped AlGaN layer having a slightly higher Al content than the previously grown AlGaN is grown. In this way, the undoped GaN layer 25 Å and the undoped AlGaN layer 25 Å having a small but gradually increasing Al content are alternately laminated and grown to 500 Å (10 pairs). After 10 pairs of growth, the Mg-containing gas is slightly flown to grow the Mg-doped GaN layer doped with a very small amount of Mg to 25 Å, and the Al content is higher than that of the previously grown undoped AlGaN layer. An AlGaN layer is grown to a thickness of 25 Å. Next, the Mg-doped GaN layer 25 Å in which the amount of Mg gradually increases as the distance from the active layer increases, and the undoped Al _X Ga _{1 -X} N also increases in the amount of Al as the distance increases from the active layer. Alternately with 25 Å layers and finally Mg
Is grown to a GaN layer doped with 8 × 10 ¹⁹ / cm ³ , and then an undoped Al _0.2 Ga _0.8 N layer is grown, for a total of 0.75 μm (150 pairs). In this manner, the p-side cladding layer 10 having a superlattice structure composed of a GaN layer having a gradually increasing Mg content and an AlGaN layer having a gradually increasing Al content as the distance from the active layer increases is reduced to 0.1. It is grown to a thickness of 8 μm. Similarly,
Although the p-type impurity amount is adjusted for GaN, it can also be adjusted for the AlGaN layer. In addition, the Al composition ratio of AlGaN was adjusted so as to be finely divided and gradually increased.
For example, it can be roughly increased in steps in about three steps, which is within the scope of the present invention. Also, the active layer 2
The p-side cap layer 26 and the p-side light guide layer 27 may be formed between the layer 5 and the p-side cladding layer 28 'in the same manner as in the first embodiment.

(P-side contact layer 29)
Similarly, a p-side contact layer 29 made of p-type GaN doped with 1 × 10 ²⁰ / cm ^{3 of} Mg is grown to a thickness of 150 Å.

The wafer grown in this manner is
Take out from the VPE reaction vessel, and
A mask made of O ₂ and having a stripe width of 1.5 μm is formed, and the GaN substrate 2 is formed by RIE as shown in FIG.
Etching is performed until the surface of 0 'is exposed. And
While keeping the SiO ₂ mask on the surface of the p-side contact layer 29, the same SiO ₂ mask is formed on the surface of the n-electrode,
As shown in FIG. 1, the insulating film 3 made of ZrO ₂ is formed on the side surface of the striped nitride semiconductor layer exposed by etching.
0 is formed.

After the formation of the insulating film 30, as shown in FIG.
An n-electrode 32 made of Ti / Al is formed on the back surface of the N substrate 20 ', and the GaN substrate is cleaved on the M surface (11-00 = a surface corresponding to the side surface of a hexagonal prism), and the opposing cleavage surface is used as a resonance surface. When a laser device was manufactured, a laser device having substantially the same characteristics as in Example 1 was obtained. Needless to say, the laser device of FIG. 1 can have a structure in which an n-electrode is provided on the back surface side of the GaN substrate as in FIG.

[Embodiment 4] In the third embodiment, when growing the n-side cladding layer 23 ', Si
Is doped at 1 × 10 ¹⁸ / cm ³ . Also, when growing the p-side cladding layer 28 ', Mg was added to all the GaN layers having a superlattice structure by 1 ×.
A laser device was obtained in the same manner except that doping was performed at 10 ¹⁹ / cm ³ , and although the threshold current density was slightly increased as compared with the third embodiment, the device also exhibited a life of 2,000 hours or more at a power of 20 mW.

[0054]

As described above, in the first and second embodiments of the present invention, the well layer of the active layer is undoped, and only the barrier layer is doped with an n-type impurity, or at least the p-side. By undoping the last well layer and doping the barrier layer with an n-type impurity, a laser device having a low threshold current density and a long life can be manufactured. The active layer having such a multiple quantum well structure can be applied not only to a laser element but also to a light emitting element such as an LED and a super luminescent diode, a light receiving element such as a solar cell and an optical sensor. The value is very large.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing the relationship between the number of well layers in an active layer and the threshold current density of a laser device.

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

[Explanation of symbols]

 20, 20 ′ GaN substrate 21 n-side contact layer 22 crack preventing layer 23, 23 ′ n-side cladding layer 24 n-side light guide layer 25 active layer 26 p-side cap layer 27 p-side light guide layer 28, 28 'p-side clad layer 29 p-side contact layer 30 insulating film 31 p-electrode 32 ..N-electrode

Claims (8)

[Claims] 1. A well layer made of a nitride semiconductor containing In and a barrier made of a nitride semiconductor having a different composition from the well layer between an n-side nitride semiconductor layer and a p-side nitride semiconductor layer. A multi-quantum well structure active layer formed by stacking layers, wherein the well layer of the active layer is undoped, and the barrier layer is doped with an n-type impurity. . 2. A well layer made of a nitride semiconductor containing In and a barrier made of a nitride semiconductor having a different composition from the well layer between an n-side nitride semiconductor layer and a p-side nitride semiconductor layer. And an active layer having a multi-quantum well structure in which layers are stacked. In the multi-quantum well structure, the side close to the p-side nitride semiconductor ends with a barrier layer, and at least the last well layer is undoped. A nitride semiconductor device, wherein the last barrier layer is doped with an n-type impurity. 3. The n-side nitride semiconductor layer includes a nitride semiconductor containing In or Ga on a side close to the active layer.
A second nitride semiconductor comprising a superlattice including a first nitride semiconductor layer made of N and further including a nitride semiconductor layer containing Al on a side farther from the active layer than the first nitride semiconductor layer The nitride semiconductor device according to claim 1, further comprising a layer. 4. The p-side nitride semiconductor layer includes a nitride semiconductor containing Al or Ga on a side close to an active layer.
A fourth nitride semiconductor having a third nitride semiconductor layer made of N and further comprising a superlattice including a nitride semiconductor layer containing Al on a side farther from the active layer than the third nitride semiconductor layer; The nitride semiconductor device according to claim 1, further comprising a layer. 5. A semiconductor device according to claim 1, wherein at least one of the n-side and p-side nitride semiconductor layers has a fifth nitride semiconductor layer made of a superlattice containing a nitride semiconductor containing Al. The nitride semiconductor device according to claim 1 or 2, wherein the Al content of the nitride semiconductor layer of (5) decreases as approaching the active layer. 6. The fifth nitride semiconductor layer is doped with an impurity that determines the conductivity type of the layer, and the fifth nitride semiconductor layer is adjusted so that the impurity decreases as approaching the active layer. The nitride semiconductor device according to claim 5, wherein 7. The nitride semiconductor device according to claim 1, wherein the barrier layer contains a p-type impurity by impurity diffusion. 8. The multi-quantum well structure in which the total number of well layers is 2
The nitride semiconductor device according to any one of claims 1 to 7, wherein