JP3669848B2 - Nitride semiconductor laser device - Google Patents

Description

[0001]
[Industrial application fields]
The present invention is used for light emitting elements such as LEDs (light emitting diodes), SLD (super luminescent diodes), and LDs (laser diodes), light receiving elements such as solar cells and optical sensors, or electronic devices such as transistors and power devices. Nitride semiconductor (In _X Al _Y Ga _1-XY N, 0 ≦ X, 0 ≦ Y, X + Y ≦ 1).
[0002]
[Prior art]
Nitride semiconductors have just been put into practical use recently as full-color LED displays, traffic lights, and the like as materials for high-brightness blue LEDs and pure green LEDs. The LEDs used in these various devices have a single quantum well structure (SQW) or multiple quantum well structure (MQW) between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer. : Multi-Quantum-Well) having a double hetero structure with an active layer of InGaN sandwiched between them. The wavelengths such as blue and green are determined by increasing or decreasing the In composition ratio of the InGaN active layer. In addition, the present applicants have announced for the first time in the world a laser oscillation of 410 nm at room temperature under a pulse current using this material (for example, Jpn.J.Appl.Phys.35 (1996) L74, Jpn.J. Appl. Phys. 35 (1996) L217 etc.}. This laser device has a double hetero structure having an active layer having a multiple quantum well structure using InGaN, and has a threshold current of 610 mA and a threshold current density of 8.7 kA / cm under conditions of a pulse width of 2 μs and a pulse period of 2 ms. ² , 410 nm oscillation. The present applicant also succeeded and announced for the first time continuous oscillation at room temperature. For example, Nikkei Electronics December 2, 1996, Technical Bulletin, Appl. Phys. Lett. 69 (1996) 3034-, Appl. Phys. Lett. 69 (1996) 4056, etc. Current density 3.6kA / cm ² 27 hours continuous oscillation at a threshold voltage of 5.5 V and an output of 1.5 mW.
[0003]
In both the LED element and the laser element, sapphire is used as a growth substrate for the nitride semiconductor. As is well known, since sapphire has a lattice mismatch of 13% or more with the nitride semiconductor, the crystal of the nitride semiconductor grown thereon has a large number of crystal defects. In addition to sapphire, devices using substrates such as ZnO, GaAs, and Si have been reported, but these substrates also do not lattice match with nitride semiconductors, and nitride semiconductors with better crystallinity than sapphire Are difficult to grow, so even LEDs have not been realized.
[0004]
As a technique for growing a nitride semiconductor with good crystallinity, for example, a technique for growing a nitride semiconductor on an off-angle sapphire substrate is shown. (For example, JP-A-4-29976, JP-A-4-323880, JP-A-5-55631, JP-A-5-190903, etc.) These techniques can be achieved by using a continuously off-angle substrate as a growth surface. In order to obtain a nitride semiconductor with good crystallinity in a state in which the interatomic distance between sapphire and sapphire is made closer, it has not yet been put into practical use.
[0005]
[Problems to be solved by the invention]
In order to improve various characteristics such as output and lifetime of nitride semiconductor devices, it is predicted that a nitride semiconductor with few crystal defects and good crystallinity can be grown if a GaN substrate lattice-matched with the nitride semiconductor is used. However, since a GaN substrate does not exist industrially, improvements in output, lifetime, and the like are achieved using a substrate made of a material different from a nitride semiconductor such as sapphire, ZnO, and spinel. Among them, sapphire has been put to practical use because it can grow a nitride semiconductor having the best crystallinity, but it has not yet been satisfactory as a substrate for growing a nitride semiconductor. The present invention has been made in view of such circumstances, and its object is to improve the lifetime of a nitride semiconductor element by improving the substrate on which the nitride semiconductor is grown, high efficiency, high output, It is to improve the yield.
[0006]
[Means for Solving the Problems]
When we grow a nitride semiconductor on a substrate, we use a heterogeneous substrate that is off-angled stepwise, so that the surface of the resulting epitaxial growth layer is extremely smooth and has a good surface morphology. The inventors have newly found that the lifetime is improved and the threshold current is lowered, and the present invention has been achieved.
That is, the nitride semiconductor laser device of the present invention has an off angle of 0.3 ° to 0.5 ° A heterogeneous substrate made of a material different from a nitride semiconductor with a step (height) in the range of 1 to 10 atomic layers, and a horizontal terrace and a step on the heterogeneous substrate. At least 20 μm thick A nitride semiconductor laser device in which an element structure is stacked on a nitride semiconductor substrate comprising the first nitride semiconductor layer, wherein the heterogeneous substrate is a sapphire substrate, The terrace corresponds to a chip size having a side of about 300 to less than 1000 μm, The element structure has a waveguide, and is laminated so that a direction along the step of the nitride semiconductor substrate surface (step direction) and a resonator direction in the element structure are substantially parallel to each other. The nitride semiconductor laser device includes an active layer having a quantum well structure made of a nitride semiconductor layer containing at least indium.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail. FIG. 1 is a schematic view showing an enlarged cross section of a substrate used in the nitride semiconductor device of the present invention. The nitride semiconductor device of the present invention is thus grown on a heterogeneous substrate that is off-angled (inclined) stepwise. The heterogeneous substrate is not particularly limited as long as it is a material other than a nitride semiconductor, and conventionally known materials such as sapphire (including C-plane, A-plane, and R-plane), spinel, SiC (including 6H and 4H). ), GaAs, Si, ZnO or the like.
[0008]
Conventionally, unevenness on the surface of an off-angled dissimilar substrate, an underlayer, etc. has been inherited every time a layer is deposited, and has tended to increase or worsen. As described above, the surface of the nitride semiconductor epitaxial growth layer tends to have a hexagonal shape or a scale-like morphology only by providing a conventional off-angled heterogeneous substrate or a buffer layer on the substrate, and the unevenness is severe. Since the obtained element is formed so as to straddle it, the active layer or the like is bent, the threshold current is increased, the yield of the element is deteriorated, and the lifetime characteristic of the obtained element is also deteriorated. However, the surface morphology of the nitride semiconductor substrate formed according to the present invention has stepped steps as schematically shown in FIG. The feature is that the direction of the step is almost parallel to that of an off-angled dissimilar substrate, and the terrace is extremely flat and large in area, so that this flat terrace realizes an excellent state for depositing device structures. ing.
[0009]
Although it is not certain whether the first nitride semiconductor layer in the present invention has the surface morphology as shown in FIG. 2, if the off-angle of the heterogeneous substrate is 0.05 ° or less, the conventional hexagonal or scale-like morphology Further, by setting the thickness of the first nitride semiconductor layer grown on the different substrate to 20 μm or more, a step-like step is formed on the surface, and at least an interval (terrace width) between adjacent steps is taken out. A nitride semiconductor substrate having a terrace portion wider than the length of one side of the chip is obtained. That is, since this wide terrace is flat in all areas when viewed in chip size, a highly reliable element can be obtained by depositing each layer to form an element structure. That is, with the chip size, a nitride semiconductor substrate that is extremely flat and has a good surface morphology is realized. Here, the chip size is not particularly limited, but the chip size currently reported as a nitride semiconductor element is about 300 to less than 1000 μm on a side, and the nitride semiconductor substrate used in the present invention has this size. It has a terrace part that corresponds sufficiently, and its size is on the order of about several hundred to several thousand μm.
[0010]
In the present invention, the surface morphology of the nitride semiconductor substrate used in the device is influenced by the nitride semiconductor growth method and conditions in addition to the above. These will be described in detail below.
[0011]
In the present invention, a heterogeneous substrate off-angled in a step shape has a substantially horizontal terrace portion A and a step portion B as shown in FIG. The terrace portion A has few surface irregularities and is formed almost regularly. Although the off angle θ is exaggerated, it is inclined only 0.3 ° with respect to the horizontal plane of the growth surface. Such a step-like portion having an off angle is desirably formed continuously over the entire substrate, but may be formed partially in particular. As shown in FIG. 1, the off-angle θ refers to an angle between a straight line connecting the bottoms of a plurality of steps and a horizontal plane of the uppermost step.
[0012]
A nitride semiconductor is grown on a heterogeneous substrate off-angled in such a step shape. As a growth method of the nitride semiconductor, for example, when a growth method capable of strictly controlling the film thickness such as MOVPE (metal organic vapor phase epitaxy) or MBE (molecular beam vapor phase epitaxy) is used, several angstroms to several tens of angstroms are used. It is very advantageous when a quantum structure is produced by growing an active layer having a thickness of. In addition, a growth method generally known as a method for growing a nitride semiconductor layer can be used.
[0013]
In the present invention, the surface morphology of the first nitride semiconductor substrate grown on the off-angled heterogeneous substrate described above does not vary depending on the conditions as described above, but the off-angle and the first nitride semiconductor are not determined. It tends to depend mainly on the film thickness. In addition, stepped steps (heights) applied to different types of substrates tend to affect the surface morphology. Specifically, when the step (height) of the heterogeneous substrate is in the range of 1 to 10 atomic layers, the step (height) of the obtained nitride semiconductor surface is almost constant, and the step interval (step width) is also It tends to be constant.
[0014]
If the off-angle of the dissimilar substrate is 0.05 ° or more as described above, the surface morphology of the nitride semiconductor substrate is almost hexagonal as in the conventional case, and does not have severe unevenness, but has stepped steps. It will have. Preferably, by setting the angle to 0.3 ° or more, the hexagonal unevenness slightly observed below 0.3 °, that is, the hexagonal unevenness is not mixed with the stepped step, and the substrate surface is eliminated. A stepped step is formed in the entire area. The upper limit is not particularly limited, but the terrace width tends to be narrowed as the off-angle is increased. Therefore, if the off-angle is too large, for example, several degrees or more, it is difficult to form an element structure. More specifically, it is not limited because it differs depending on the type of substrate and the growth method, but as an example, in the case of the sapphire (0001) C plane, it is preferably 0.1 ° or more, more preferably 0.3 to 0.5 °. Within this range, the surface of the nitride semiconductor layer is more flat and the terrace width is preferably wide.
[0015]
In the present invention, the first nitride semiconductor is grown on a heterogeneous substrate at a high temperature, specifically 900 ° C. to 1100 ° C., preferably 1050 ° C. If the film thickness is at least 20 μm or more, it has a step-like surface as shown in FIG. 2 and a terrace portion is formed. More preferably, by setting the film thickness to 50 μm or more, the terrace width is wider, specifically, it is formed sufficiently wider than the length of one side of the element chip, and is stably formed on the entire surface of the substrate. The width tends to be formed almost constant. In addition, the crystallinity is good, and in addition, since the terrace width tends to increase as the thickness increases, it is preferable. Therefore, in the present invention, in order to stably manufacture the device using the nitride semiconductor substrate, it is preferable that the thickness of the first nitride semiconductor is 50 μm or more.
[0016]
As one embodiment of the present invention, the first nitride semiconductor substrate grown on the thick film is removed from the underlying layer or a part of the nitride semiconductor substrate and the heterogeneous substrate to obtain a desired thickness. In the case of taking out as a single substrate, if the film thickness of the first nitride semiconductor to be grown is at least 50 μm or more, it can be handled as a single substrate. More preferably, in order to ensure mechanical strength, the thickness is set to 100 μm or more, so that cracks and chips in the manufacturing process are almost eliminated, and handling of a single substrate becomes easy. Specifically, when the film thickness of the first nitride semiconductor is 100 μm or more, the terrace width formed on the surface corresponds to two or more in chip size, and the one reaching several thousand μm is formed. There is a tendency. Although the upper limit of the film thickness varies depending on the growth conditions, it is preferable that the thickness is 200 μm or less because warpage of the wafer (warping in a state having a different substrate) can be prevented and stable growth can be achieved. The film thickness of the obtained nitride semiconductor as a single substrate is not particularly limited, but about 100 μm is sufficient for practical use.
[0017]
In this way, by controlling the off-angle and the film thickness of the first nitride semiconductor, as well as the step (height) of the heterogeneous substrate, the nitride semiconductor is extremely flat and has a wide terrace extending to several thousand μm. A substrate can be realized and a good element can be obtained by using this.
[0018]
In the present invention, the first nitride semiconductor includes, for example, AlN, GaN, AlGaN, InGaN, etc., but preferably undoped (undoped impurity, undoped) GaN, n-type doped GaN, or Si. Doped GaN can be used. This is because undoped GaN has good crystallinity and can be stably grown into a thick film, and n-type impurity doped GaN forms electrodes so as to face each other with a nitride semiconductor substrate interposed therebetween. In some cases, this is advantageous in the production. Examples of the n-type impurity include Ge, S, Se, and the like in addition to Si.
[0019]
In the present invention, the stepped step 22 formed on the surface of the first nitride semiconductor layer is formed substantially along the step 23 of the dissimilar substrate, as shown in FIG. become. Here, the step direction is a direction along the step 22 formed on the surface of the first nitride semiconductor layer or the step 23 having a different substrate shape, and is a direction perpendicular to the paper surface in FIG.
[0020]
As described above, when an element structure is deposited on a conventional nitride semiconductor layer having irregularities on the surface, the irregularities are inherited, and when considering the element to be taken out, the active layer and the like are greatly curved in the region of the chip. For this reason, such a light-emitting element has poor reliability and a large threshold current. The present invention can solve such a conventional problem by using the above-described terrace. Here, FIG. 3 schematically shows a state in which each layer is stacked on the first nitride semiconductor layer in the present invention in a cross section perpendicular to the step direction. As can be seen in this figure, even when each layer that forms the element structure is stacked on the first nitride semiconductor in the present invention, the surface form such as the stepped step is substantially maintained, and the terrace width, step (height), etc. There are no major changes.
[0021]
Therefore, when the nitride semiconductor device obtained by the present invention is a laser device, a good laser device can be obtained by laminating the device structure so that the step direction and the resonator direction are substantially parallel. That is, in the obtained laser element, the active layer is not curved and the threshold current is also lowered. As described above, since the step on the surface of the first nitride semiconductor layer is formed in a specific direction, it is easy to make the resonator direction and the step direction substantially parallel (direction substantially perpendicular to the paper surface in FIG. 3). Can be realized. Further, although the number is smaller than the elements formed on the above-mentioned terrace, there is also an element formed across the step as seen from the hatched portion in FIG. At this time, when the step direction and the resonator direction are almost parallel to each other, the waveguide is formed so as to intersect the step when the steps are not parallel and the threshold current is greatly increased. However, the reliability of the element is also greatly reduced. However, in the case of almost parallel, there is a waveguide in parallel with the step, and the waveguide region having a width of several μm is not easily applied to the step in the element. From this, because the resonator direction and the step direction are almost parallel, even in the element formed across the step, most of them do not overlap with the waveguide region directly above the step. The device has substantially the same reliability and threshold current as those of the terrace portion. Therefore, by forming the laser element so that the direction of the resonator is substantially parallel to the step direction of the surface of the first nitride semiconductor layer, the yield can be significantly improved as compared with the case where the laser element is not.
[0022]
In addition, a buffer layer can be provided on an off-angled dissimilar substrate. At this time, as the buffer layer, for example, AlN, GaN, AlGaN, InGaN or the like is grown at a temperature of 900 ° C. or less with a film thickness of several tens of angstroms to several hundreds of angstroms. This buffer layer is formed in order to relieve the lattice constant difference between the heterogeneous substrate and the nitride semiconductor layer, but may be omitted depending on the growth method of the nitride semiconductor, the type of the substrate, and the like. However, by providing the base layer including the buffer layer between the dissimilar substrate and the first nitride semiconductor layer, the lattice constant difference between the dissimilar substrate and the nitride semiconductor layer can be relaxed, and the occurrence of crystal defects can be prevented. Therefore, the surface morphology of the first nitride semiconductor layer grown with a thick film is improved, and the reliability of the resulting device is improved.
[0023]
Further, in addition to the buffer layer described above as a base layer, a nitride semiconductor layer selectively grown as shown in Example 4 is provided, so that the morphology of the surface of the nitride semiconductor layer becomes better. This selective growth layer is formed by partially growing a protective film having a property that the nitride semiconductor does not grow or is difficult to grow on the surface of the heterogeneous substrate, buffer layer, or nitride semiconductor layer. A nitride semiconductor layer with fewer crystal defects can be obtained. In particular, the nitride semiconductor layer can be grown stably and thickly, which contributes to the formation of good surface morphology. Further, by providing an underlayer including a selective growth layer, the growth method is changed, for example, HVPE having a high growth rate in order to grow the first nitride semiconductor to a film thickness of several tens to several hundreds μm. Even in such a case, a thick nitride semiconductor can be stably manufactured with good quality.
[0024]
Here, the selective growth means that after a protective film is partially formed on a heterogeneous substrate, a buffer layer, or a nitride semiconductor layer, the nitride semiconductor is grown and then grown in a thickness direction to some extent. Lateral growth occurs and a film is formed. Here, as the protective film, specifically, silicon oxide (SiO _X ), Silicon nitride (Si _X N _Y ), Titanium oxide (TiO _X ), Zirconium oxide (ZrO) _X In addition to oxides and nitrides such as), and multilayer films thereof, metals having a melting point of 1200 ° C. or higher can be used. For the formation of the protective film, for example, a vapor deposition technique such as vapor deposition, sputtering, or CVD is used, and for the partial (selective) formation, a photolithography technique can be used. The shape of the protective film is not particularly limited. For example, the protective film can be formed in a dot, stripe, or grid pattern. As a preferable mode for adjusting the surface area of the protective film and the window portion, the protective film is formed in a stripe shape, the width of the window portion is adjusted to 3 μm or less, for example, and the lower limit value is set to 0.1 μm or more. The width of the stripe protective film is, for example, 5 to 20 μm. Within this range, a nitride semiconductor layer with few crystal defects is obtained, which is preferable. The thickness of the stripe-shaped protective film is, for example, 0.1 to 3 μm.
[0025]
【Example】
[Example 1]
Example 1 will be described below. FIG. 4 is a schematic cross-sectional view showing the structure of a laser device using a nitride semiconductor layer obtained by the growth method of the present invention as a substrate.
[0026]
2 inches φ, off-angle angle θ = 0.3 °, step step (height) about 1 atomic layer, terrace width W about 40 angstrom steps, C plane is the main surface, orientation flat surface is the A plane A sapphire substrate is prepared in which the step direction is perpendicular to the orientation flat surface, and a nitride semiconductor layer is grown by the MOVPE method. First, as shown in FIG. 2, this sapphire substrate 201 was set in a reaction vessel, and a buffer layer made of GaN was grown to a thickness of 200 angstroms on the surface of the off-angle surface at 500 ° C. as an underlayer. Next, the substrate taken out from the reaction vessel is set in the HVPE apparatus and heated to 1050 ° C., and Si is used as the first nitride semiconductor 203 at 1 × 10 6. ¹⁸ /cm ^Three Doped GaN having a thickness of 100 μm was grown. On the surface of the obtained Si-doped GaN (first nitride semiconductor), stepped steps were formed, and the step direction was substantially parallel to that of the different substrate. At this time, the orientation flat surface (A surface) of the sapphire substrate substantially corresponds to the M surface of the grown Si-doped GaN crystal, and the step direction is formed substantially perpendicular to the orientation flat surface. It is formed in a direction substantially perpendicular to the M plane. Further, the terrace portion was flat and had a good surface morphology, and most of the terrace width exceeded about 1 mm. The sapphire substrate and buffer layer of the obtained wafer were polished and removed, and the surface of the first nitride semiconductor layer 503 was exposed to make only the nitride semiconductor layer 503 having a thickness of about 80 μm.
[0027]
Next, a wafer having the first nitride semiconductor layer 503 (Si-doped GaN) as the main surface is set in a reaction vessel of the MOVPE apparatus, and the heterogeneous substrate of the first nitride semiconductor layer 503 is removed. The following layers are formed on a surface opposite to the exposed surface, that is, a surface having stepped steps. At this time, the direction of the resonator is formed in a direction perpendicular to the orientation flat surface (A surface) of the sapphire substrate and perpendicular to the M surface of GaN so that the step direction of the nitride semiconductor and the resonator direction are parallel.
[0028]
(N-side cladding layer 505)
Next, Si is 1 × 10 ¹⁹ /cm ^Three Doped n-type Al _0.2 Ga _0.8 A superlattice structure with a total film thickness of 0.4 μm is formed by alternately laminating 100 first layers made of N, 20 Å, and second layers made of undoped GaN, 20 Å.
[0029]
(N-side light guide layer 506)
Then, Si is 1 × 10 ¹⁷ /cm ^Three An n-type light guide layer 44 made of doped n-type GaN is grown to a thickness of 0.1 μm.
[0030]
(Active layer 507)
Next, Si is 1 × 10 ¹⁷ /cm ^Three Doped In _0.2 Ga _0.8 N well layer, 25 angstroms, Si 1 × 10 ¹⁷ /cm ^Three Doped In _0.01 Ga _0.99 An active layer 507 of a multiple quantum well structure (MQW) having a total film thickness of 175 Å formed by alternately stacking N barrier layers and 50 Å is grown.
[0031]
(P-side cap layer 508)
Next, Mg having a band gap energy larger than that of the p-side light guide layer 47 and larger than that of the active layer 507 is 1 × 10. ²⁰ /cm ^Three Doped p-type Al _0.3 Ga _0.7 A p-side cap layer 508 made of N is grown to a thickness of 300 Å.
[0032]
(P-side light guide layer 509)
Next, the band gap energy is smaller than the p-side cap layer 508, and Mg is 1 × 10 ¹⁸ /cm ^Three A p-side light guide layer 509 made of doped p-type GaN is grown to a thickness of 0.1 μm.
[0033]
(P-side cladding layer 510)
Next, Mg is 1 × 10 ²⁰ /cm ^Three Doped p-type Al _0.2 Ga _0.8 A first layer of N, 20 Å, and 1 × 10 Mg ²⁰ /cm ^Three A p-side cladding layer 510 made of a superlattice layer having a total thickness of 0.4 μm is formed by alternately laminating a second layer made of doped p-type GaN and 20 Å.
[0034]
(P-side contact layer 511)
Finally, Mg 2 × 10 ²⁰ /cm ^Three A p-side contact layer 511 made of doped p-type GaN is grown to a thickness of 150 Å.
[0035]
After the completion of the reaction, the wafer is annealed in a reaction vessel at 700 ° C. in a nitrogen atmosphere to further reduce the resistance of the p-type layer. After annealing, the wafer is taken out of the reaction vessel, and as shown in FIG. 4, the uppermost p-type contact layer 511 and p-type cladding layer 510 are etched by an RIE apparatus to form a ridge shape having a stripe width of 4 μm. did. At this time, the ridge is such that the resonator direction of the obtained element is substantially parallel to the step direction of the nitride semiconductor substrate. A p-electrode 514 made of Ni / Au is formed on the entire surface of the ridge surface. Next, as shown in FIG. 4, the surface of the p-side cladding layer 510 and the contact layer 511 excluding the p-electrode 514 is made of SiO. ₂ An insulating film 512 is formed, and a p-pad electrode 513 electrically connected to the p-electrode 514 through the insulating film 512 is formed.
[0036]
After forming the p-side electrode, an n-electrode 515 made of Ti / Al is formed to a thickness of 0.5 μm on the entire surface of the nitride semiconductor layer 503 where the element structure is not formed, and metallization with a heat sink is formed thereon. For this purpose, a thin film made of Au / Sn is formed.
[0037]
After that, scribing was performed from the n-electrode side 515 and cleaved at the (11-00) M plane of the first nitride semiconductor layer 503 to produce a resonance plane. SiO on both or one of the resonant surfaces ₂ And TiO ₂ A dielectric multilayer film was formed, and finally a bar was cut in a direction parallel to the p-electrode to obtain a 650 μm square laser chip. Next, the chip was placed face-up (with the substrate and the heat sink facing each other) on the heat sink, the p-pad electrode 513 was wire-bonded, and laser oscillation was attempted at room temperature. The threshold current density was 1.2 kA at room temperature. /cm ² Continuous oscillation with an oscillation wavelength of 405 nm was confirmed at a threshold voltage of 4.5 V, and the lifetime characteristics were also good.
[0038]
[ Reference example 1 ]
For comparison, a laser device was fabricated in the same manner as in Example 1 except that the resonator direction was provided so as to be perpendicular to the step direction on the surface of the first nitride semiconductor layer. The obtained laser element has a significantly lower yield than that of Example 1, and some of the obtained laser elements have a high threshold current and a reduced life characteristic, and the element characteristics vary. there were.
[0039]
[ Reference example 2 ]
A nitride semiconductor single substrate was produced in the same manner as in Example 1 except that the off-angle angle θ was 0.1 °. In the obtained nitride semiconductor single substrate, a stepped step on the surface of the first nitride semiconductor as obtained in Example 1 was observed, but a part of the surface had irregularities of hexagonal shape or scale pattern It was a morphology, and these were mixed on the substrate. In addition, a laser element manufactured using this single substrate in the same manner as in Example 1 has a higher threshold current and higher Vf than Example 1, and in particular has a hexagonal shape or a scale-patterned unevenness. Those formed were prominent in the tendency, and in addition, the reliability of the device was lowered.
[0040]
[Comparative Example 1]
Furthermore, as a comparative example Reference example 1 When the nitride semiconductor substrate was fabricated with the first nitride semiconductor layer having a thickness of 5 μm, the surface morphology was as in Example 1. Or Reference Example 1 Such stepped steps were not observed, and innumerable small irregularities were formed over the entire wafer.
[0041]
[Example 4]
As a base layer on the sapphire substrate, a nitride semiconductor layer selectively grown in addition to the buffer layer is provided. For this underlayer, after growing the buffer layer, a stripe-shaped photomask is formed on the surface of the buffer layer, and a SiO 2 having a stripe width of 10 μm and a window portion of 8 μm is formed by a CVD apparatus. ₂ A protective film is formed with a thickness of 0.1 μm. At this time, the stripe direction was a direction perpendicular to the sapphire A plane. After forming the protective film 34, the wafer is transferred to a reaction vessel, and a nitride semiconductor layer made of undoped GaN is grown to a thickness of 100 μm at 1050 ° C. using TMG and ammonia as source gases. After forming a buffer layer and a base layer composed of a selectively grown nitride semiconductor layer, a first nitride semiconductor layer 503 is formed as in Example 1, taken out as a single substrate, and an element structure is formed for nitriding A semiconductor laser device was fabricated. The obtained laser device has a yield per wafer equal to or higher than that of Example 1, and the morphology of the surface of the nitride semiconductor substrate is that of Example 1, Or Reference Example 1 The terrace width was about 900 μm or more, which was sufficient for taking out the laser element.
[0042]
[Example 5]
A 2-inch φ sapphire substrate 1 having a step of an off-angle angle θ = 0.5 ° and a step difference (height) of about 2 atomic layers and having a C plane as a main surface is prepared. A laser element made of the nitride semiconductor shown in FIG. 5 is fabricated on the sapphire substrate by using the MOVPE method.
[0043]
(N-side contact layer 603)
The sapphire substrate 601 is set in a reaction vessel, and a buffer layer made of GaN is grown on the surface of the off-angle surface at 500 ° C. to a thickness of 200 Å. Si as 1 × 10 ¹⁹ /cm ^Three An n-side contact layer 603 made of doped GaN is grown to a thickness of 50 μm. This n-side contact layer 603 has an Al mixed crystal ratio X value of 0.5 or less. _X Ga _1-X It is desirable to grow with N (0 ≦ X ≦ 0.5). Note that the buffer layer is not particularly shown in FIG. The surface of the obtained Si-doped GaN layer had an average terrace width of about 700 μm or more and Example 1, Or Reference Example 1 and Reference Example 2 The step is stepped enough to take out the element, although it is slightly narrower than that of the first embodiment. Or Reference Example 1 and Reference Example 2 It was as good as that.
[0044]
Next, the temperature is set to 800 ° C. and Si is 5 × 10 ¹⁸ /cm ^Three Doped In _0.1 Ga _0.9 A crack prevention layer 604 made of N is grown to a thickness of 500 angstroms. The crack prevention layer 604 can be prevented from being cracked in the nitride semiconductor layer containing Al by growing it with an n-type nitride semiconductor containing In, preferably InGaN. The crack prevention layer is preferably grown with a film thickness of 100 Å or more and 0.5 μm or less. If it is thinner than 100 angstroms, it is difficult to act as a crack prevention as described above, and if it is thicker than 0.5 μm, the crystal itself tends to turn black. The crack preventing layer 604 can be omitted.
[0045]
(N-side cladding layer 605)
Next, 1050 ° C. and 5 × 10 Si ¹⁸ /cm ^Three Doped n-type Al _0.2 Ga _0.8 A superlattice structure with a total film thickness of 0.4 μm is formed by alternately laminating 100 first layers made of N, 20 Å, and second layers made of undoped GaN, 20 Å. The n-side cladding layer 605 functions as a carrier confinement layer and an optical confinement layer, and is desirably a nitride semiconductor containing Al, preferably a superlattice layer containing AlGaN, and the total thickness of the superlattice layer is 100 angstroms or more. It is desirable to grow at 2 μm or less, more preferably 500 Å or more and 1 μm or less. When the superlattice layer is used, a carrier confinement layer having good crystallinity without cracks can be formed, and in the nitride semiconductor layer constituting the superlattice layer, the layer having a larger band gap energy is doped with a high concentration of impurities, or When a layer having a smaller band gap energy is doped with an impurity at a high concentration, or when modulation doping is performed, the threshold tends to decrease. Further, the impurity concentrations of the nitride semiconductor layer having a large band gap energy and the nitride semiconductor layer having a small band gap energy can be made equal.
[0046]
(N-side light guide layer 606)
Then, Si is 5 × 10 ¹⁸ /cm ^Three An n-side light guide layer 606 made of doped n-type GaN is grown to a thickness of 0.1 μm. This n-side light guide layer 606 acts as a light guide layer of the active layer, and it is desirable to grow GaN and InGaN. Usually, the n-side light guide layer 606 is grown to a thickness of 100 Å to 5 μm, more preferably 200 Å to 1 μm. desirable. The n-side light guide layer 606 is usually doped with an n-type impurity such as Si or Ge so as to have an n-type conductivity type, but can be undoped in particular. In the case of a superlattice, at least one of the first layer and the second layer may be doped with an n-type impurity or may be undoped.
[0047]
(Active layer 607)
Next, at 800 ° C., undoped In _0.2 Ga _0.8 N well layer, 25 Å, and undoped In _0.01 Ga _0.99 An active layer 607 of a multiple quantum well structure (MQW) having a total film thickness of 175 Å formed by alternately laminating N barrier layers and 50 Å is grown.
[0048]
(P-side cap layer 608)
Next, at 1050 ° C., the band gap energy is larger than that of the p-side light guide layer 609 and larger than that of the active layer 607, and Mg is 1 × 10 6. ²⁰ /cm ^Three Doped p-type Al _0.3 Ga _0.7 A p-side cap layer 608 made of N 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, the p-side cap layer 7 may be i-type in which carriers are compensated by doping an n-type impurity, or undoped, and most preferably p-type. A layer doped with impurities is used. The thickness of the p-side cap layer 608 is adjusted to 0.1 μm or less, more preferably 500 angstroms or less, and most preferably 300 angstroms or less. This is because if the film is grown to a thickness greater than 0.1 μm, cracks are likely to occur in the p-type cap layer 608, and a nitride semiconductor layer with good crystallinity is difficult to grow. When the AlGaN having a larger Al composition ratio is formed thinner, the LD element tends to oscillate. For example, Al with a Y value of 0.2 or more _Y Ga _1-Y If N, it is desirable to adjust to 500 angstroms or less. The lower limit of the thickness of the p-side cap layer 608 is not particularly limited, but it is desirable to form the p-side cap layer 608 with a thickness of 10 angstroms or more.
[0049]
(P-side light guide layer 609)
Next, the band gap energy is smaller than the p-side cap layer 608, and Mg is 1 × 10. ²⁰ /cm ^Three A p-side light guide layer 609 made of doped p-type GaN is grown to a thickness of 0.1 μm. This layer acts as a light guide layer of the active layer, and is preferably grown by GaN and InGaN, similarly to the n-side light guide layer 5. This layer also acts as a buffer layer when the p-side cladding layer 9 is grown, and acts as a preferable light guide layer by growing it at a film thickness of 100 angstroms to 5 μm, more preferably 200 angstroms to 1 μm. . This p-side light guide layer is usually doped with a p-type impurity such as Mg to have a p-type conductivity, but it is not particularly necessary to dope the impurity. The p-side light guide layer can be a superlattice layer. In the case of a superlattice layer, at least one of the first layer and the second layer may be doped with a p-type impurity or may be undoped.
[0050]
(P-side cladding layer 610)
Next, Mg is 1 × 10 ²⁰ /cm ^Three Doped p-type Al _0.2 Ga _0.8 A first layer of N, 20 Å, and 1 × 10 Mg ¹⁹ /cm ^Three A p-side cladding layer 610 made of a superlattice layer having a total thickness of 0.4 μm is formed by alternately laminating a second layer made of doped p-type GaN and 20 Å. This layer acts as a carrier confinement layer like the n-side cladding layer 605, and acts as a layer for reducing the resistivity on the p-type layer side by adopting a superlattice structure. The film thickness of the p-side cladding layer 610 is not particularly limited, but it is desirable to grow it at 100 angstroms or more and 2 μm or less, more preferably 500 angstroms 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. In addition, if the p-type impurity is modulation-doped like the n-side cladding layer 605, the threshold tends to be lowered.
[0051]
The superlattice layer is preferably at least in the p-side layer, and a superlattice layer in the p-side layer is preferable because the threshold value is further lowered.
[0052]
In the case of a nitride semiconductor device having a double hetero structure having an active layer having a quantum well layer, a nitride containing Al having a film thickness of 0.1 μm or less in contact with the active layer and having a band gap energy larger than that of the active layer A cap layer made of a semiconductor is provided, a p-side light guide layer having a bad gap energy smaller than that of the cap layer is provided at a position farther from the active layer than the cap layer, and the p-side light guide layer is separated from the active 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 larger band gap than the p-side light guide 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, so that electrons do not overflow the active layer, and the leakage current of the device is reduced. .
[0053]
(P-side contact layer 611)
Finally, Mg 2 × 10 ²⁰ /cm ^Three A p-side contact layer 611 made of doped p-type GaN is grown to a thickness of 150 Å. When the thickness of the p-side contact layer is adjusted to 500 angstroms or less, more preferably 400 angstroms or less, or 20 angstroms or more, the p-layer resistance is reduced, which is advantageous in reducing the threshold voltage.
[0054]
After completion of the reaction, the wafer is annealed in a nitrogen atmosphere at 700 ° C. in the reaction vessel to further reduce the resistance of the p layer. After annealing, the wafer is taken out of the reaction vessel, and as shown in FIG. 2, the uppermost p-side contact layer 611 and p-side cladding layer 610 are etched by an RIE apparatus to form a ridge shape having a stripe width of 4 μm. To do. At this time, the ridge is formed so that the resonator direction of the obtained element is parallel to the step direction of the surface of the first nitride semiconductor (n-side contact layer 603).
[0055]
After the ridge formation, as shown in FIG. 2, the p-side cladding layer 610 exposed on both sides of the ridge stripe is etched with the ridge stripe as the center, and the surface of the n-side contact layer 603 on which the n-electrode 615 is to be formed is etched. Expose.
[0056]
Next, a p-electrode 614 made of Ni / Au is formed on the entire surface of the ridge. Next, as shown in FIG. 2, SiO is formed on the surfaces of the p-side cladding layer 610 and the p-side contact layer 611 excluding the p-electrode 614. ₂ An insulating film 613 is formed, and a p-pad electrode 612 electrically connected to the p-electrode 614 through the insulating film 613 is formed. On the other hand, an n-electrode 615 made of W and Al is formed on the surface of the n-side contact layer 603 exposed earlier.
[0057]
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 with the M surface of sapphire to produce a bar having the cleaved surface as the resonance surface. On the other hand, a laser element is manufactured by separating the chip by scribing a bar at a position parallel to the striped electrode. The laser element shape is shown in FIG. In addition, when this laser element was laser-oscillated at room temperature, Example 1, Or Reference Example 1 , And the device yield is reduced by the narrower terrace width. But, Reference example 2 The hexagonal irregularities are not mixed as in the case where the off-angle is less than 0.3 °, and the yield is improved compared to the laser element grown on the sapphire C-plane just substrate surface. As a result, the threshold current density was lowered and the device life was greatly improved.
[0058]
[ Reference example 3 ]
A nitride semiconductor single substrate and a laser device were produced in the same manner as in Example 5 except that the off-angle angle of the sapphire substrate was θ = 0.6 °. The surface of the obtained single substrate tends to be somewhat uneven and have a narrower terrace width than the flatness of Example 1 in the terrace portion. The threshold current of the obtained laser element is also the same as that of Example 1. Compared with it, it was a little higher and the element reliability was slightly inferior.
[0059]
[ Reference example 4 ]
FIG. 6 is a schematic cross-sectional view showing the structure of one LED element using a nitride semiconductor layer obtained by the growth method of the present invention as a substrate. Below, based on FIG. Reference example 4 Will be described.
[0060]
A nitride semiconductor single substrate 703 (Si-doped GaN) was produced in the same manner as in Example 4 except that the off-angle angle of the sapphire substrate was θ = 0.7 °. The surface of this single substrate has a step, but the flatness of the terrace portion is slightly worse than in Example 4, and Reference example 3 The unevenness was conspicuous and the terrace width was narrow. This wafer is set in the reaction vessel of the MOVPE apparatus, and the silicon is removed by 1 × 10 ° C. at 1050 ° C. on the surface opposite to the surface exposed by removing the foreign substrate and the like (surface having a stepped step). ¹⁸ /cm ^Three A second buffer layer 702 made of doped GaN is grown. The second buffer layer 702 is a nitride semiconductor single crystal layer grown at a high temperature of typically 900 ° C. or higher, and is distinguished from a buffer layer grown at a low temperature for relaxing lattice mismatch with the previous substrate. .
[0061]
Furthermore, a single quantum well structure In is formed on the second buffer layer 702 with a thickness of 20 angstroms. _0.4 Ga _0.6 Active layer 707 made of N, Mg-doped Al with a film thickness of 0.3 μm _0.2 Ga _0.8 A p-side cladding layer 710 made of N and a p-side contact layer 711 made of Mg-doped GaN having a thickness of 0.5 μm are grown in this order.
[0062]
After growing the second buffer layer 702 to the p-side contact layer 711 to be an element structure, the wafer is taken out of the reaction vessel and annealed at 600 ° C. in a nitrogen atmosphere to make the p-side cladding layer 710 and the p-side contact layer 711 low resistance. To. Thereafter, etching is performed from the p-side contact layer 711 side to expose the surface of the first nitride semiconductor layer 703. Thus, by exposing the nitride semiconductor layer below the active layer by etching and providing a “cutting edge” at the time of cutting the chip, it becomes difficult to give an impact to the pn junction surface at the time of cutting. An improved and highly reliable device can be obtained.
[0063]
After etching, a light-transmitting p-electrode 714 made of Ni / Au is formed on almost the entire surface of the p-side contact layer 711 with a film thickness of 200 angstroms, and a bonding pad electrode 712 is formed on the p-electrode 714. Is formed with a film thickness of 0.5 μm.
[0064]
After forming the p-side electrode, an n-electrode 715 is formed to a thickness of 0.5 μm on the entire surface of the first nitride semiconductor layer 703 exposed by removing the sapphire substrate and the like.
[0065]
Thereafter, scribing is performed from the n-electrode side, and cleavage is performed at the M plane (101-0) of the first nitride semiconductor layer 3 and a plane perpendicular to the M plane, thereby obtaining a 300 μm square LED chip. This LED exhibits a green light emission of 520 nm at 20 mA and has a nitride obtained by growing on a sapphire substrate having an off-angle angle in the range of 0.05 to 0.3 ° or less or no off-angle. Compared to the semiconductor element structure, the threshold current was lowered, Vf was also improved, and very excellent characteristics were exhibited.
[0066]
【The invention's effect】
The nitride semiconductor device of the present invention uses a nitride semiconductor substrate having a large area and a very flat terrace portion, compared to the surface morphology of a nitride semiconductor layer grown on a conventional heterogeneous substrate. In addition, the threshold current is lowered, and the yield of the obtained laser device is improved by making the formed step and the direction of the resonator substantially parallel.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing an enlarged part of a substrate used in a nitride semiconductor device of the present invention.
FIG. 2 is a schematic cross-sectional view showing one structure of a nitride semiconductor LD device using a substrate according to the method of the present invention.
FIG. 3 is a schematic cross-sectional view showing a state in which each layer is deposited on the nitride semiconductor substrate according to the present invention.
FIG. 4 is a schematic cross-sectional view showing a nitride semiconductor laser device according to an embodiment of the present invention.
FIG. 5 is a perspective view illustrating the structure of a nitride semiconductor laser device according to an embodiment of the present invention.
FIG. 6 shows one aspect of the present invention. Reference example The schematic cross section which shows the nitride semiconductor LED element which concerns on.
[Explanation of symbols]
21 ... Terrace
22 .... Step (step)
23... Steps provided on different substrates
201, 401, 601, 701...
702... Second buffer layer
203, 503, 603 (n-side contact layer), 703... First nitride semiconductor (nitride semiconductor substrate)
604 ··· Crack prevention layer
505, 605... N-side cladding layer
506, 606... N-side light guide layer
507, 607, 707... Active layer
508, 608... Cap layer
509, 609... P-side light guide layer
510, 610, 710... P-side cladding layer
511, 611, 711... P-side contact layer
512, 612, 712,... P pad electrode
513, 613... Insulating film
514, 614, 714... P-electrode
515, 615, 715... N electrode

Claims (1)

A heterogeneous substrate made of a material different from a nitride semiconductor having an off angle in a step shape in a range of 0.3 ° to 0.5 ° and a step (height) in a range of 1 to 10 atomic layers; A nitride semiconductor laser device in which an element structure is stacked on a nitride semiconductor substrate having a horizontal terrace on the heterogeneous substrate and a first nitride semiconductor layer having a thickness of at least 20 μm having a step,
The heterogeneous substrate is a sapphire substrate;
The terrace corresponds to a chip size having a side of about 300 to less than 1000 μm,
The element structure has a waveguide, and is laminated so that the direction along the step of the nitride semiconductor substrate surface (step direction) and the resonator direction in the element structure are substantially parallel.
The element structure includes an active layer having a quantum well structure composed of a nitride semiconductor layer containing at least indium.

Images