JP4904653B2 - Nitride semiconductor light emitting device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a GaN-based semiconductor light-emitting element such as a semiconductor laser having an oscillation wavelength from the visible region to the ultraviolet region, which is expected to be applied to high-density optical information processing and display fields.
[0002]
[Prior art]
Nitride semiconductors containing nitrogen (N) as a group V element have an emission band in a wide wavelength range from the visible region to the ultraviolet region due to the size of the band gap, and are particularly promising as materials for short-wavelength light emitting devices. Yes. In particular, gallium nitride-based compound semiconductors (GaN-based semiconductors: AlxGayInzN (0≤x, y, z≤1, x + y + z = 1)) are actively studied, and blue light-emitting diodes (LEDs) and green LEDs are It has been put into practical use. In addition, in order to increase the capacity of optical disk devices, semiconductor lasers having an oscillation wavelength in the 400 nm band have been eagerly desired, and semiconductor lasers made of GaN-based semiconductors have attracted attention and are now reaching a practical level.
[0003]
FIG. 6 is a structural sectional view of a GaN-based semiconductor laser in which laser oscillation is achieved. A GaN buffer layer 602 and u-GaN 603 are grown on the sapphire substrate 601 by metal organic vapor phase epitaxy (MOVPE). Next, the substrate is taken out of the crystal growth furnace, and the SiO ₂ mask 604 is patterned on the stripes using a photolithography technique. Next, the substrate is again placed in a crystal growth furnace, and ELO-GaN 605 is regrown. At this time, GaN grows laterally on the SiO ₂ mask 604. In this region, the effect of mismatching between the lattice constant and the thermal expansion coefficient between GaN and sapphire is small, so a high-quality GaN crystal with a low dislocation density can be obtained. Next, crystal growth is continued, and an n-GaN layer 606, an n-Al0.07Ga0.93N cladding layer 607, an n-GaN first guide layer 608, Ga1-xInxN / Ga1-yInyN (0 <y <x <1 ), A p-GaN optical guide layer 610, a p-Al0.07Ga0.93N cladding layer 611, and a p-GaN contact layer 612 are grown. A ridge stripe having a width of about 3 to 10 microns is formed on the p-GaN contact layer 612, and both sides thereof are buried with an insulating film 614. Thereafter, a p-electrode 613 made of, for example, Ni / Au is formed on the ridge stripe and the insulating film 614, and an n-electrode 615 made of, for example, Ti / Al is formed on the surface etched partially until the n-GaN layer 606 is exposed. In this device, when the n electrode 615 is grounded and a voltage is applied to the p electrode 613, holes are injected from the p electrode 613 side and electrons from the n electrode 615 side toward the multiple quantum well active layer 609, and the multiple quantum wells are injected. An optical gain is generated in the well active layer 609, and laser oscillation in an oscillation wavelength band of 400 nm occurs. The oscillation wavelength varies depending on the composition and film thickness of the Ga1-xInxN / Ga1-yInyN thin film, which is the material of the multiple quantum well active layer 609. Currently, continuous oscillation above room temperature has been realized.
[0004]
The surface of the laser resonator is formed by polishing and cleaving the sapphire substrate surface on which the nitride semiconductor layer is not formed, as disclosed in, for example, Japanese Patent Application Laid-Open No. 8-213694. The substrate width of the laser chip is 400 μm, and the resonator length is 500 μm. The ridge stripe on the p-GaN contact layer 612 is preferably in the range of 1 to 50 μm from the viewpoint of optical confinement. Regarding the dimensions of the nitride-based semiconductor laser chip, other than that, Applied Physics Letters, Vol. 69, p. In 1477, 1996, it was reported that the resonator length was 600 μm, the ridge stripe on the p-GaN contact layer was 10 μm, and the width of the ridge stripe for forming the n electrode on the n-GaN contact layer was 150 μm. .
[0005]
[Problems to be solved by the invention]
However, in the nitride semiconductor laser chip on the sapphire substrate, the thermal stress (in this case) is applied to the nitride semiconductor due to the difference in thermal expansion coefficient between the sapphire substrate and the nitride semiconductor grown on the sapphire substrate. There is a problem that the chip itself is warped due to the occurrence of compressive stress. In particular, if the substrate warpage is large in the direction of the resonator, the resonator surfaces are not parallel to each other, and the loss of light is so large that optical feedback cannot be performed sufficiently, leading to an increase in the laser threshold. As a result of extensive studies by the inventors using simulation calculations, the warpage in this direction is such that the width of the ridge produced for forming the n-electrode is larger than the cavity length, as in the dimensions of a conventional laser chip. It was found that this was caused by the anisotropy that the thermal stress in the cavity length direction was stronger due to the short length.
[0006]
Furthermore, as a result of extensive investigations by the inventors, the laser chip size and the thermal stress anisotropy (relationship between the stress values) in the direction perpendicular to the resonance direction of the laser are the same. It has been found that this is not always sufficient for threshold current reduction. See, for example, Journal of Applied Physics, Volume 80, p. As shown in 6868, 1996, hexagonal nitride-based semiconductors can reduce the valence band density of states by applying anisotropic stress that breaks symmetry in the C plane. It has been reported that the threshold current required for laser oscillation is reduced, and that the threshold current is more effectively reduced if anisotropic stress is applied in a more advantageous direction of coupling with light. As a result of calculations by the inventors, in the conventional laser chip, the ridge stripe on the p-contact and the ridge stripe for forming the n-electrode are produced, so that the thermal stress is released in the direction perpendicular to the stripe in the C plane. However, it has been found that the effect cannot be sufficiently obtained with the size of the conventional laser chip.
[0007]
The present invention has been made in view of the above problems, and provides a high-performance semiconductor laser by controlling the anisotropic stress peculiar to hexagonal nitride semiconductors fabricated on different substrates. is there.
[0008]
[Means for Solving the Problems]
The nitride-based semiconductor device of the present invention is characterized in that the width of the ridge stripe including the active layer has a length equal to or longer than the laser resonator length.
[0009]
The nitride-based semiconductor device of the present invention is characterized in that the width of the step stripe closest to the substrate side of the nitride-based semiconductor layer fabricated on the substrate is different from the laser resonator length.
[0010]
In addition, the nitride semiconductor device of the present invention is characterized by having a groove structure in a ridge stripe including an active layer. In particular, the groove is formed from the surface side of the ridge stripe including the active layer to a position deeper than the active layer.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The nitride semiconductor growth method of the present invention is not limited to metal organic vapor phase epitaxy (MOVPE), but nitriding such as hydride vapor phase epitaxy (H-VPE) and molecular beam epitaxy (MBE). The present invention can be applied to all methods proposed so far for growing a physical semiconductor layer. Further, in the MOVPE growth method, the growth pressure may be any of reduced pressure, atmospheric pressure, and pressurized pressure (760 Torr or higher), or may be switched to an optimum pressure in each layer. The carrier gas for supplying the raw material to the substrate is supplied as a gas containing at least an inert gas such as nitrogen or hydrogen.
[0012]
(Embodiment 1)
FIG. 1 is a sectional view of an element of a GaN-based semiconductor laser showing a first embodiment.
[0013]
An n-GaN layer 102 (film thickness 4 μm), an n-Al _0.07 Ga _0.93 N cladding layer 103 (film thickness 0.7 μm), an n-GaN light guide layer 104 (film thickness 0.12 μm), GaInN (film thickness) 3 nm) / GaN (9 nm thickness) multiple quantum well (MQW) active layer 105, p-Al _0.14 Ga _0.86 N cap layer 106 (thickness 20 nm), p-GaN light guide layer 107 (thickness 0.12 μm), p -Al _0.14 Ga _0.86 N / GaN superlattice clad layer 108 (film thickness 0.5 μm) and p-GaN contact layer 109 (film thickness 0.04 μm) are sequentially deposited. Next, the p-GaN contact layer 109 and the p-Al _0.14 Ga _0.86 N / GaN superlattice cladding layer 108 are processed into a ridge stripe (p stripe 113), and both sides of the ridge are covered with the SiO ₂ insulating film 111. A current injection region is formed. The stripe width of the p stripe 113 is about 2 to 5 microns. A p-electrode 110 is provided on the surface of the p-GaN contact layer 109 in the opening of the insulating film 111 and part of the insulating film 111. An n-electrode 112 is formed on the surface etched until a part of the n-GaN layer 102 is exposed (formation of the p / n stripe 114). Here, the width of the p / n stripe 114 is 400 μm.
[0014]
For the substrate 101, SiC, Si, GaAs or the like is used in addition to the sapphire C surface. Moreover, the board | substrate which consists of a GaN-type crystal produced on the said various board | substrates using the selective growth method may be sufficient.
[0015]
Next, after producing the laminated structure, a laser resonator end face is formed by cleavage to produce a laser bar. Finally, scribing is performed to separate the laser chip. The resonator length is 350 μm and the chip width is 600 μm. Although the resonator length of the present invention is shorter than that of the conventional structure, as a manufacturing method, a method of scribing after polishing the substrate, laser beam irradiation (ablation), or the like can be sufficiently processed to this extent. is there.
[0016]
Next, the effectiveness of setting the relationship between the p / n stripe width and the resonator length as in the present invention will be described.
[0017]
FIG. 4 shows the relationship between laser chip dimensions and warpage due to thermal stress. If the p / n stripe width L1 is shorter than the resonator length L2, the device warps more greatly in the resonator length direction due to thermal stress, and the opposite relationship (L ₁ > L ₂ ) It turns out that it warps more greatly in the vertical direction. As a result of the simulation analysis performed by the inventors using the finite element method, it has been found that this is caused by the release of the thermal stress generated between the substrate and the GaN crystal by the p / n stripe. That is, if the substrates are made of different materials, which are different from the nitride-based semiconductor, the thermal expansion coefficients are different from each other, and thermal stress is generated due to the difference in the thermal expansion coefficients.
[0018]
FIG. 5 shows the relationship of thermal stress found from the simulation. FIG. 5A shows the stress acting on the laser structure when a GaN-based laser structure 502 is formed on the substrate 501. In this case (for example, a sapphire substrate), the thermal expansion coefficient of the substrate is larger than that of the GaN-based laser structure, so that compressive stress acts on the laser structure when cooled from the process temperature (about 1000 ° C. to room temperature). (B) creates a p stripe 503 and a p / n stripe 504, so that the compressive stress is released at the ends of the p stripe 503. If the width of the p / n stripe 504 is shorter than the resonator length , The compressive stress in the direction perpendicular to the stripe becomes smaller in the plane C. According to the calculation, the difference in compressive thermal stress between the stripe direction and the perpendicular direction is several tens of MPa, especially p. It has been found that the stress anisotropy of the active layer existing directly under the p stripe has a great influence due to the magnitude relationship between the length of the / n stripe 504 and the resonator length.
[0019]
By making the length of the p / n stripe longer than the resonator length as in the present invention, the effect of releasing compressive thermal stress in the resonator length direction is released by the p / n stripe. Since it becomes more prominent than the effect, compressive stress is applied in the direction perpendicular to the resonator length direction, the warpage in the resonator direction is reduced, the parallelism of the opposing resonator surface is increased, and the resonance loss is reduced, The laser oscillation threshold current is reduced.
[0020]
It should be noted that the resonator length and the laser chip dimensions are examples, and it goes without saying that the effects of the present invention can be obtained as long as the above size relationship is satisfied.
[0021]
Further, the same effect can be obtained with another similar structure as shown in FIG.
[0022]
That is, as shown in FIG. 2A or 2B, a stripe-shaped step 215 or step 217 is provided in a part of the n-GaN layer. The thickness of each step is a = 1 μm, b = 1.5 μm (therefore, the remaining thickness from the bottom of the step of n-GaN 102 is 1.5 μm), c = 1 μm, and d = 1.5 μm. In FIG. 2A, the width of each stripe is 200 μm for the p / n stripe 114 and 450 μm for the step width 216. The laser cavity length is 350 μm. In FIG. 2B, the width of the p / n stripe 114 is 200 μm, the step width 218 is 400 μm, and e = 50 μm.
[0023]
This step is characterized by processing a part of the n-GaN layer and does not engrave the substrate surface. The step width 216 or 217 is made longer than the resonator length of 350 μm. As a result, in the active layer, the thermal stress in the direction perpendicular to the resonator direction in the C plane becomes larger, and the warpage in the resonator direction is reduced. As a result, the lasing threshold current was reduced.
[0024]
In the calculation of thermal stress, when a plurality of step stripes are formed on a nitride semiconductor fabricated on a substrate, the thermal stress is anisotropic due to the size relationship between the width of the step stripe closest to the substrate and the resonator length. It became clear that there was a gender relationship. Accordingly, it is effective to reduce the laser oscillation threshold current by making the width of the step stripe closest to the substrate longer than the resonator length.
[0025]
FIG. 7A shows the relationship between the p / n stripe width of the warp in the resonator direction or the step stripe width at the bottom (closest to the substrate). The definition of warp is the difference between the height of the center of the resonator and the height of both ends when the laser chip is placed horizontally. As shown in the figure, when the p / n stripe width or the step stripe width at the bottom (closest to the substrate) exceeds the resonator length, it is rapidly reduced. This is considered to be due to the fact that the anisotropy of thermal stress is switched by the magnitude relationship between the stripe and the resonator length.
[0026]
(Embodiment 2)
Next, a nitride-based semiconductor device structure in which the threshold current of the laser is reduced by the effect of anisotropic stress by effectively controlling the thermal stress will be described.
[0027]
FIG. 3 shows a cross-sectional view of the element. In each layer, the same reference numerals as those in FIG. 1 are the same layers. As shown in the figure, V grooves are formed on both sides of the p stripe 113. Although it is more desirable for the trench to reach the n-type layer through both sides of the active layer 105, it does not necessarily need to reach the active layer 105. The shape of the groove is not limited to the V-groove shape, and the bottom may be flat. It is important to carve both sides of the p stripe 113. In this embodiment, the groove is 2 μm away from the end of the p stripe 113. The width of the groove is about 10 μm. The depth of the groove reaches the n-AlGaN cladding layer 103. The film thickness of each layer is the same as that in the first embodiment. The groove can be selectively formed using a photolithographic technique and a dry etching process or laser irradiation. Further, the position of the groove may be anywhere on both sides of the p stripe 113, but it is desirable that the groove be as close to the end of the p stripe 113 as possible. Further, it is desirable that the groove be on both sides of the p stripe 113, but it may be either one.
[0028]
FIG. 7B shows the dependence of thermal stress on the groove depth in the vicinity of the active layer. If there is no groove, a large compressive thermal stress is applied. However, as the groove depth is increased, the compressive stress is relaxed, and the groove is dug beyond the thickness f corresponding to the active layer surface from the surface shown in FIG. Rather, it becomes tensile stress. That is, the thermal stress undergoes a dramatic change as the groove depth is carved deeper than the active layer.
[0029]
FIG. 5C shows the distribution of thermal stress when the V-groove is provided. While compressive stress is applied to the bottom of the V-groove (on the side close to the substrate), tensile stress acts rather in the vicinity of the upper light emitting region 503 (active layer) due to the compressive stress in the lower portion of the groove. As a result, compressive stress is applied in the direction of the resonator in the light emitting region, and tensile stress is applied in the direction perpendicular to the resonator in the C plane. That is, the thermal stress anisotropy can be greatly obtained as compared with the laser chip having the conventional structure. As a result of the application of this anisotropic stress, band splitting was effectively realized in the band structure of the valence band. As a result of actually fabricating a device using this, the threshold current of laser oscillation was reduced to about ½ reflecting the reduction in the state density of the valence band.
[0030]
This structure also has the effect of reducing the capacitance of the p / n junction, and is effective for high-speed modulation and noise reduction.
[0031]
【Effect of the invention】
As described above, the nitride semiconductor device of the present invention has the effect of releasing compressive thermal stress in the resonator length direction by making the length of the p / n stripe longer than the resonator length. However, since the effect is more pronounced than the effect of being released by the p / n stripe, the compressive stress is applied in the direction perpendicular to the resonator length direction, and the warpage in the resonator direction is reduced. The parallelism is increased, the resonance loss is reduced, and the laser oscillation threshold current is reduced.
[0032]
In addition, the nitride-based semiconductor device of the present invention uses a V-groove structure in the vicinity of the active layer to significantly increase the anisotropy of thermal stress, thereby reducing the valence band state density and causing the laser oscillation threshold current. Reduced to about ½.
[Brief description of the drawings]
FIG. 1 is a structural cross-sectional view of a GaN-based semiconductor laser showing a first embodiment of the invention. FIG. 2 is a structural cross-sectional view of a GaN-based semiconductor laser showing a second embodiment of the invention. FIG. 4 is a structural cross-sectional view of a GaN-based semiconductor laser showing a third embodiment of the present invention. FIG. 4 is a diagram showing the effect of the present invention on the relationship between warpage and a laser chip. FIG. 6 is a sectional view of an element of a conventional GaN-based quantum well semiconductor laser. FIG. 7 is a diagram illustrating the effect of the present invention.
101 Substrate 102 n-GaN layer 103 n-AlGaN cladding layer 104 n-GaN light guide layer 105 active layer 106 p-AlGaN cap layer 107 p-GaN light guide layer 108 p-AlGaN / GaN superlattice cladding layer 109 p-GaN Contact layer 110 p electrode 111 SiO2
112 n electrode 113 p stripe 114 p / n stripe 215 step 216 step width 217 step 218 step width 315 V groove 501 substrate 502 laser structure 503 p stripe 504 p / n stripe 505 V groove 601 substrate 602 buffer layer 603 u− GaN layer 604 SiO ₂ mask 605 ELO-GaN
606 n-GaN layer 607 n-AlGaN cladding layer 608 n-GaN light guide layer 609 active layer 610 p-GaN light guide layer 611 p-AlGaN cladding layer 612 p-GaN contact layer 613 p electrode 614 SiO 2
615 n electrode

Claims (3)

A nitride-based semiconductor light-emitting device characterized in that the width of the ridge stripe including the active layer has a length equal to or longer than the laser resonator length. A nitride-based semiconductor light-emitting device characterized in that the width of a step stripe closest to the substrate side of a nitride-based semiconductor layer fabricated on a substrate has a length greater than or equal to the laser resonator length. The nitride-based semiconductor light-emitting device according to claim 1, wherein the nitride-based semiconductor layer formed on the substrate has a plurality of step stripes having different widths.

Images