JP2002084045A - Nitride semiconductor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prolong the life of a nitride semiconductor element, to increase the efficiency of the element and to increase the output of the element, by modifying the substrate f or growing nitride semiconductor layers on its upper surface. SOLUTION: Nitride semiconductor layers formed into an element structure are laminated on a GaN substrate off-angled stepwise, whereby as an active layer grew on the stepped parts of steps formed on the GaN substrate is easy to turn into a quantum dot or a quantum wire structure, the efficiency of an element is enhanced. When the active layer preferably is used as an SQW or MQW having a well layer consisting of an InGaN layer, the active layer is easy to turn into a quantum dot or a wire due to the unevenness of the composition of the In the InGaN layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an LED (light emitting diode), an SLD (super luminescent diode),
Light-emitting elements such as LDs (laser diodes), light-receiving elements such as solar cells and optical sensors, or nitride semiconductors (In _X Al _Y Ga _{1 -X-YN} , 0) used for electronic devices such as transistors and power devices ≤X, 0≤Y, X + Y≤
1) Regarding elements.

[0002]

2. Description of the Related Art Nitride semiconductors have just recently been put to practical use in full-color LED displays, traffic signals and the like as materials for high-brightness blue LEDs and pure green LEDs. The LED used for these various devices has a single quantum well (SQW) InGaN between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer.
It has a double hetero structure in which an active layer is sandwiched. Wavelengths such as blue and green are determined by increasing or decreasing the In composition ratio of the InGaN active layer. The present applicant has also used this material under a pulse current under a pulse current of 410 n at room temperature.
m laser oscillation for the first time in the world {for example, Jpn.
J.Appl.Phys.35 (1996) L74, Jpn.J.Appl.Phys.35 (1996) L
217}. This laser device has a double hetero structure having a multi-quantum-well (MQW) active layer using InGaN, and has a threshold current of 610 mA, a pulse width of 2 μs and a pulse period of 2 ms. It shows a threshold current density of 8.7 kA / cm ² and oscillation of 410 nm. The present applicant has also succeeded for the first time in continuous oscillation at room temperature and has announced. {For example, Nikkei Electronics December 2, 1996
Technical Bulletin, Appl. Phys. Lett. 69 (1996) 3034, Appl.
Phys. Lett. 69 (1996) 4056-etc.]
, A continuous oscillation for 27 hours is shown at a threshold current density of 3.6 kA / cm ² , a threshold voltage of 5.5 V, and an output of 1.5 mW.

[0003] For both the LED element and the laser element, sapphire is used as a nitride semiconductor growth substrate. As is well known, sapphire has a lattice mismatch with a nitride semiconductor of 1
Since it is 3% or more, the crystal of the nitride semiconductor grown thereon has very many crystal defects. In addition to sapphire, devices using substrates such as ZnO, GaAs, and Si have also been reported. However, these substrates are not lattice-matched to nitride semiconductors, and are nitride semiconductors having better crystallinity than sapphire. Are difficult to grow, so even LEDs have not been realized.

As a technique for growing a nitride semiconductor having good crystallinity, for example, a technique for growing a nitride semiconductor on an off-angle sapphire substrate is disclosed. (See, for example, JP-A-4-299876, JP-A-4-32388)
0, JP-A-5-55631, JP-A-5-190903
These techniques attempt to obtain a nitride semiconductor with good crystallinity by using a substrate that is continuously off-angled as a growth surface, thereby reducing the interatomic distance between GaN and sapphire. However, it has not yet been put to practical use.

[0005]

In order to improve various characteristics such as the output and the life of the nitride semiconductor device, a GaN substrate lattice-matched with the nitride semiconductor is used. It is expected that good nitride semiconductors can be grown. The use of a substrate made of a material different from a nitride semiconductor, such as sapphire, ZnO, and spinel, has been attempted to improve the output and the life. Among them, sapphire has grown to practical use because it can grow a nitride semiconductor having the best crystallinity, but it has not been satisfactory as a substrate for growing a nitride semiconductor. The present invention has been made in view of such circumstances, and an object of the present invention is to improve a substrate on which a nitride semiconductor is grown by using a nitride semiconductor device having a long life, high efficiency, and high output. Is to do.

[0006]

Means for Solving the Problems In growing a nitride semiconductor on a substrate, by using a substrate that is stepped off-angle, the active layer becomes a state similar to quantum dots and quantum wires, and the device The present inventors newly found that the life and output of the device were improved, and completed the present invention.
That is, the nitride semiconductor device of the present invention is characterized in that a nitride semiconductor layer serving as an element structure is laminated on a GaN substrate that is off-angled in a step shape. However, it is desirable that the steps are formed to some extent regularly.

Preferably, the nitride semiconductor layer has an active layer having a quantum well structure including a nitride semiconductor layer containing at least indium. The nitride semiconductor containing In is preferably In _x Ga ₁ -xN (0 <x ≦ 1).
It consists of. The quantum well structure may be either SQW or MQW. In the case of a quantum structure, the thickness of the well layer is adjusted to 70 Å or less, more preferably 50 Å or less. In the case of MQW, the thickness of the barrier layer made of a nitride semiconductor having a larger band gap energy than that of the well layer is not particularly limited, but is usually formed to a thickness of 200 Å or less.

[0008] More preferably, the step of the substrate is 30 angstrom or less.
A preferred step is 25 Å or less, more preferably 20 Å or less. The lower limit is desirably 2 Å or more. If it is smaller than 2 angstroms, there is almost no difference from the conventional technology having almost no step, that is, a substrate that is off-angled at a certain angle, and therefore, there is a tendency that improvement in output cannot be expected. On the other hand, if it is larger than 30 angstroms, irregularities are generated on the surface of the nitride semiconductor layer due to steps due to the substrate during crystal growth, and the output tends to decrease.

In particular, when sapphire is used for the substrate, (0
(001) plane (hereinafter referred to as C plane) as a principal plane, and the off angle θ is within 1 degree from the C plane of the sapphire substrate. The preferable range of the off angle is adjusted to 0.8 degrees or less, more preferably 0.6 degrees or less. The lower limit is not particularly limited, but is preferably adjusted to 0.01 degrees or more. If the temperature exceeds 1 degree, the crystallinity of the nitride semiconductor deteriorates, and the output tends to decrease.

[0010]

FIG. 1 is an enlarged schematic view showing a cross section of a substrate used for a nitride semiconductor device of the present invention.
The nitride semiconductor device of the present invention is thus grown on a substrate which is stepwise off-angled (inclined). The substrate is not particularly limited as long as it is a material other than a nitride semiconductor. For example, conventionally known sapphire (C-plane, A-plane)
Plane, and R plane. ), Spinel, SiC (including 6H, 4H), GaAs, Si, ZnO and the like are used. If a GaN substrate can be manufactured industrially, the GaN substrate can also be used.

Why use a substrate that has been turned off in a step
It is not clear whether the output will improve if used, but for example
The following can be inferred. Nitrogen on a substrate with steps
When a nitride semiconductor is grown, the step is
It is inherited by semiconductors. Then grow the active layer
In this case, unevenness due to the step difference occurs in the active layer,
The active layer with the uneven area of the quantum dot, quantum wire
It will be like the following. Activity of quantum dots and quantum wires
Layers can trap carriers efficiently,
The output of the entire device is improved. Especially nitride semiconductor containing In
Body, eg In _XGa_1−XN is an In composition within the same layer.
Non-uniformity tends to occur. Therefore, nitrogen containing In
Wells of less than 70 Å in thickness
Assuming that the active layer has a door layer, the In composition X in the well layer is
A large In-rich region and an In poor with a small In composition X
Area is mixed. In rich region
And the In poor region are quantum dots in the well layer,
Forms a quantum wire. Intended on top of stepped substrate
The well layer containing In that has been grown selectively has an In
Switch region and In poorer region at the terrace
It is presumed that quantum dots and quantum wires are obtained. One
Nitride semiconductor grown on a stepped substrate
Intentionally forms quantum dots and quantum wires in the active layer
Output is improved because it can be done. Therefore, the terrace width increases
The effect of quantum dots and quantum wires is difficult to appear
In the direction. Regarding the terrace width, the height of the step
And inevitably determined by the off angle θ (eg,
(Tan θ = step height / terrace width).

The substrate off-angled in a step shape shown in FIG. 1 has a substantially horizontal terrace portion A and a step portion B. The surface unevenness of the terrace portion A is about 0. 0 on average.
It is adjusted to 5 angstroms, at most about 2 angstroms, and is formed almost regularly. On the other hand, the height of the step is adjusted to about 15 angstroms. Although the off angle θ is exaggerated, it is inclined only 0.13 ° with respect to the horizontal plane of the growth surface. The step-like portion having such an off angle is
It is desirable that the substrate is formed continuously over the entire substrate, but it may be formed partially. The off angle θ indicates an angle between a straight line connecting the bottoms of a plurality of steps and the horizontal plane of the uppermost step as shown in FIG.

[0013] A nitride semiconductor is grown on the substrate which is off-angled in such a stepwise manner. As a growth method of the nitride semiconductor, for example, a growth method such as MOVPE (metal organic chemical vapor deposition) or MBE (molecular beam chemical vapor deposition) that can control the film thickness strictly is used. These growth methods are very advantageous when an active layer having a thickness of several angstroms to several tens angstroms is grown to produce a quantum structure.

[0014]

[Embodiment 1] Off-angle angle θ = 0.13
゜, a 2 inch φ sapphire substrate 1 having a step of about 15 angstroms and a terrace width W of about 6110 angstroms and having a C plane as a main surface is prepared. On this sapphire substrate, a laser device made of a nitride semiconductor shown in FIG. 2 is manufactured by using the MOVPE method.

(N-side contact layer 2) The sapphire substrate 1 is set in a reaction vessel, and a buffer layer made of GaN is grown at 500 ° C. on the surface of the off-angle surface to a thickness of 200 Å. N made of GaN doped with Si at 1 × 10 ¹⁹ / cm ³ at 1050 ° C.
The side contact layer 2 is grown to a thickness of 5 μm. This n
Side contact layer 2 is Al mixed crystal ratio X value of 0.5 or less of Al _X
It is desirable to grow Ga ₁ -XN (0 ≦ X ≦ 0.5) with a film thickness of 1 to 10 μm. In FIG. 1, the buffer layer is not particularly shown.

(Crack prevention layer 3) Next, the temperature was raised to 800 ° C., and In _0.1 Ga _0.9 doped with Si at 5 × 10 ¹⁸ / cm ^3.
A crack prevention layer 42 of N is grown to a thickness of 500 Å. The crack prevention layer 3 is made of an n-type nitride semiconductor containing In, preferably InGaN, so that cracks can be prevented from entering the nitride semiconductor layer containing Al. The crack preventing layer is preferably grown to a thickness of 100 Å or more and 0.5 μm or less. When the thickness is less than 100 Å, it is difficult to function as a crack prevention as described above, and when the thickness is more than 0.5 μm, the crystal itself tends to turn black. Note that the crack preventing layer 3 can be omitted.

(N-side cladding layer 4) Next, at 1050 ° C.
And Si is 5 × 10¹⁸/cm^ThreeDoped n-type Al_0.2G
a _0.8A first layer of N, 20 Å;
A second layer of undoped GaN, 20
Angstroms and 100 layers alternately laminated
A superlattice structure having a thickness of 0.4 μm is formed. The n-side cladding layer 4
Acts as a carrier confinement layer and a light confinement layer,
A nitride semiconductor containing Al, preferably containing AlGaN
It is desirable to use a superlattice layer.
100 angstrom or more, 2 μm or less, more preferred
Or more than 500 Å and less than 1μm
It is desirable to make it. Cracks do not occur in the superlattice layer
A carrier confinement layer with good crystallinity can be formed.
In the nitride semiconductor layer that constitutes the superlattice layer, bandgap
Higher impurity concentration in the layer with the higher energy
Or the bandgap energy is smaller
Doping of high concentration of impurities into the layer
And the threshold value tends to decrease. In addition, the band gap
High energy nitride semiconductor layer and band gap energy
Equal impurity concentration with nitride semiconductor layer with small energy
You can also.

(N-side light guide layer 5) Subsequently, Si is added to 5 ×
An n-side optical guide layer 74 made of n-type GaN doped with 10 ¹⁸ / cm ³ is grown to a thickness of 0.1 μm. This n-side light guide layer 5 acts as a light guide layer of the active layer,
It is desirable to grow N, InGaN, usually 10
0 angstrom to 5 μm, more preferably 200
It is desirable to grow with a film thickness of angstrom to 1 μm. The n-side light guide layer 5 is usually doped with an n-type impurity such as Si or Ge to have an n-type conductivity, but may be undoped. When a superlattice is used, at least one of the first layer and the second layer may be doped with an n-type impurity or may be undoped.

(Active Layer 6) Next, at 800 ° C., a well layer made of undoped In _0.2 Ga _0.8 N, 25 Å, a barrier layer made of undoped In _0.01 Ga _0.99 N and 50 Å are alternately laminated. Multiple quantum well structure (MQW) with total thickness of 175 Å
The active layer 6 is grown. In this way, by forming the well layer of the active layer into a quantum structure of, for example, 70 Å or less, a quantum box or a quantum wire structure is formed on the step of the step substrate, and a high-power laser element is obtained. Can be

(P-side cap layer 7) Next, at 1050 ° C., the band gap energy is larger than that of the p-side light guide layer 8 and larger than that of the active layer 6, and Mg is 1 × 10 ²⁰ / g.
A p-side cap layer 7 made of p-type Al _0.3 Ga _0.7 N doped with cm ³ is grown to a thickness of 300 Å. The p-side cap layer 7 is a layer doped with a p-type impurity. However, since the film thickness is small, it may be an i-type in which an n-type impurity is doped to compensate for carriers or an undoped type, and most preferably a p-type impurity. It is a layer doped with impurities. The thickness of the p-side cap layer 7 is adjusted to 0.1 μm or less, more preferably 500 Å or less, and most preferably 300 Å or less. 0.1 μm
This is because if the layer is grown with a larger thickness, cracks are easily formed in the p-type cap layer 76, and a nitride semiconductor layer having good crystallinity is difficult to grow. A with a large Al composition ratio
When the LD element is formed as thin as lGaN, the LD element easily oscillates. For example, if the Y value is Al _Y Ga _{1 -Y} N of 0.2 or more, it is desirable to adjust the value to 500 Å or less. The lower limit of the thickness of the p-side cap layer 76 is not particularly limited, but is preferably formed to a thickness of 10 Å or more.

(P-side light guide layer 8) Next, Mg whose band gap energy is smaller than that of the p-side
A p-side optical guide layer 8 made of p-type GaN doped with × 10 ²⁰ / cm ³ is grown to a thickness of 0.1 μm. This layer
It is desirable to function as a light guide layer of the active layer and grow with GaN or InGaN, like the n-side light guide layer 5. This layer also acts as a buffer layer when growing the p-side cladding layer 9, and has a thickness of 100 Å or more.
5 μm, more preferably 200 Å to 1
By growing with a thickness of μm, it functions as a preferable light guide layer. This p-side light guide layer is usually doped with a p-type impurity such as Mg to have a p-type conductivity type, but it is not particularly necessary to dope the impurity. Note that the p-side light guide layer may be a superlattice layer. When a superlattice layer is formed, at least one of the first layer and the second layer may be doped with a p-type impurity or may be undoped.

(P-side cladding layer 9) Next, Mg was added to 1 × 1
A first layer of p-type Al _0.2 Ga _0.8 N doped with 0 ²⁰ / cm ³ , 20 Å, and 1 × 10 ¹⁹ / Mg of Mg;
A total thickness of 0.4 μm formed by alternately stacking a second layer of p-type GaN doped with cm ³ and 20 Å
The p-side cladding layer 9 composed of the superlattice layer is grown. This layer functions as a carrier confinement layer similarly to the n-side cladding layer 4, and functions as a layer for decreasing the resistivity on the p-type layer side by forming a superlattice structure. The thickness of the p-side cladding layer 9 is not particularly limited, but it is preferable that the p-side cladding layer 9 is grown at a thickness of 100 Å or more and 2 μm or less, more preferably 500 Å or more and 1 μm or less. In particular, when a nitride semiconductor layer having a superlattice structure is used as a cladding layer, providing a superlattice layer on the p-layer side is more effective in reducing the threshold current. When the p-type impurity is modulated and doped as in the case of the n-side cladding layer 4, the threshold value tends to decrease. The superlattice layer has at least p
It is preferably in the side layer, and it is preferable that the p-side layer has a superlattice layer because the threshold value is further reduced.

In the case of a nitride semiconductor device having a double hetero structure having an active layer having a quantum well layer, Al having a thickness of 0.1 μm or less having a band gap energy larger than that of the active layer is in contact with the active layer. A p-side light guide layer having a smaller gap energy than the cap layer is provided at a position farther from the active layer than the cap layer, and a more active layer than the p-side light guide layer. It is very preferable to provide a p-side cladding layer made of a superlattice layer containing a nitride semiconductor containing Al having a band gap larger than that of the p-side light guide layer at a position away from the layer. In addition, since the band gap energy of the p-side cap layer is increased, electrons injected from the n-layer are blocked by this cap layer, and the electrons do not overflow the active layer, so that the leak current of the element is reduced. .

(P-side contact layer 10) Finally, a p-side contact layer 10 of p-type GaN doped with Mg at 2 × 10 ²⁰ / cm ³ is grown to a thickness of 150 Å. The p-side contact layer has a thickness of 500 Å or less, more preferably 400 Å or less.
Adjusting the film thickness to 0 angstrom or more is advantageous in lowering the threshold voltage because the p-layer resistance is reduced.

After the reaction is completed, the wafer is annealed in a nitrogen atmosphere at 700 ° C.
Further lowering the resistance of the layer. After the annealing, the wafer is taken out of the reaction vessel, and as shown in FIG. 2, the uppermost p-side contact layer 10 and the p-side cladding layer 9 are etched by an RIE apparatus to form a ridge shape having a stripe width of 4 μm. I do.

After the formation of the ridge, as shown in FIG. 2, the p-side cladding layer 9 exposed on both sides of the ridge stripe is etched around the ridge stripe to form the n-side contact layer 2 on which the n-electrode 13 is to be formed. Expose the surface.

Next, a p-electrode 11 of Ni / Au is formed on the entire surface of the ridge. Next, as shown in FIG. 2, the p-side cladding layer 9 excluding the p-electrode 11 and the p-side contact layer 10
An insulating film 14 made of SiO2 is formed on the surface of the substrate, and a p pad electrode 12 electrically connected to the p electrode 11 via the insulating film 14 is formed. On the other hand, an n-electrode 13 made of W and Al is formed on the surface of the n-side contact layer 2 exposed earlier.

After the electrodes are formed, the back surface of the sapphire substrate of the wafer is polished to a thickness of about 50 μm, and then the wafer is cleaved on the M-plane of sapphire to produce a bar having the cleaved surface as a resonance surface. On the other hand, a bar is separated by scribe at a position parallel to the stripe-shaped electrodes to produce a laser element. FIG. 2 shows the laser element shape. When this laser element was oscillated at room temperature, sapphire C
The threshold current density and the threshold voltage were reduced by about 30%, the output was improved by about 30%, and the life was more than doubled, as compared with the laser element grown on the just-fitted substrate surface.

[Embodiment 2] Off-angle angle θ = 0.7
゜, having a step of about 10 angstroms and a terrace width A of about 820 angstroms,
A laser device was produced in the same manner as in Example 1 except that a 2-inch φ sapphire substrate having a C-plane as a main surface was used. % And decreased by about 10% in power and life.

Example 3 A GaN buffer layer was grown on the sapphire substrate used in Example 1 in the same manner as in Example 1 by 200 angstroms.
n-side contact layer 4 μm made of GaN doped with i × 1 ¹⁹ / cm ³ , active layer 20 Å of SQW structure made of undoped In _0.4 Ga _0.6 N, p-type doped with Mg 1 × 10 ²⁰ / cm ³ 0.2 μm p-side cladding layer made of Al _0.1 Ga _0.9 N, Mg is 1 × 10 ²⁰ / cm
P-side contact layer made of ^3- doped p-type GaN
1 μm is grown sequentially.

After the growth, annealing is performed in the same manner as in Example 1 to further reduce the resistance of the p-type layer, and then etching is performed from the p-side contact layer side to remove the surface of the n-side contact layer on which the n-electrode is to be formed. Expose. After forming a p-electrode on almost the entire surface of the p-side contact layer and an n-electrode on the exposed surface of the n-side contact layer, the wafer was separated into chips of 350 μm square to obtain LED elements. As compared with the conventional LED element grown on the substrate of No. 1, the Vf (forward voltage) was reduced by about 20% at 20 mA, and the output was improved by about 30%.

[0032] [Example 4] In Example 1, the n-side cladding layer 4 during the growth, and 20 Angstroms a second layer of GaN was 5 × 10 ¹⁸ / cm ³ doped with Si, an undoped Al _{0. 2} The first layer made of Ga _0.8 N is grown by 20 Å, the pair is grown 100 times, and the n-side cladding layer 4 having a superlattice structure with a total film thickness of 0.4 μm (4000 Å) is grown. When growing the p-side cladding layer 9, Mg is added to 1 × 10 ²⁰ / cm
The second layer of ^3- doped GaN is 20 Å and the first layer of undoped Al _0.2 Ga _0.8 N is
Layer is grown for 20 Å and this pair is
A laser device was obtained in the same manner as in Example 1 except that the p-side cladding layer 9 having a superlattice structure having a total film thickness of 0.4 μm (4000 angstroms) was grown. Similarly good results were obtained.

[0033]

According to the present invention, the output of the nitride semiconductor device can be improved by growing the nitride semiconductor on the GaN substrate which is stepped off-angle. This is because the crystallinity of the nitride semiconductor grown on the off-angled GaN substrate is improved, and the quantum dots of the active layer, which can be formed in the steps, that is, the uneven portions,
It is presumed that the quantum wire effect exists. Although only the GaN substrate has been described in the present invention, it goes without saying that the present invention can be applied to all substrates having a step-like off angle. Also, the present invention relates to a laser device, LE
In addition to a light emitting element such as a D element, it can be applied to light receiving elements such as a solar cell and an optical sensor, and any electronic device using a nitride semiconductor such as a transistor, and its industrial utility value is great.

[Brief description of the drawings]

FIG. 1 is an enlarged schematic cross-sectional view showing a part of a substrate used for a nitride semiconductor device of the present invention.

FIG. 2 is a perspective view showing a structure of a nitride semiconductor laser device according to one embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... N-side contact layer 3 ... Crack prevention layer 4 ... N-side cladding layer 5 ... N-side light guide layer 6 ... Active layer 7 ... P-side cap layer 8... P-side light guide layer 9... P-side cladding layer 10... P-side contact layer

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5F041 AA03 AA04 CA05 CA23 CA34 CA40 CA65 5F045 AA04 AB14 AB17 AD09 AD12 AD14 AF03 AF04 AF06 AF09 AF13 BB16 CA12 DA53 DA54 DA55 DA56 DA63 HA16 5F073 AA73 AA74 AA75 CA07 CB02 DA05

Claims (4)

[Claims] 1. A GaN substrate having a (0001) plane as a main surface, wherein at least indium is contained on a GaN substrate having an off angle θ of 0.01 ° or more and 1 ° or less from a (0001) plane. What is claimed is: 1. A nitride semiconductor device, comprising: stacking an element structure having an active layer having a quantum well structure made of a nitride semiconductor layer. 2. The nitride semiconductor device according to claim 1, wherein the GaN substrate is off-angled in a step shape. 3. The nitride semiconductor device according to claim 2, wherein a step of said step is not less than 2 angstroms and not more than 30 angstroms. 4. A quantum dot or quantum wire is formed in the well layer of the active layer.
3. The nitride semiconductor device according to item 1.