JP4936653B2 - Sapphire substrate and light emitting device using the same - Google Patents

Description

  The present invention relates to a sapphire substrate for growing a nitride-based semiconductor and a light-emitting device using the sapphire substrate.

Aluminum nitride (hereinafter referred to as AlN), gallium nitride (hereinafter referred to as GaN), indium nitride (hereinafter referred to as InN), or a mixed crystal of them, aluminum gallium indium nitride (hereinafter referred to as Al _x Ga _{1−). Since} nitride-based semiconductors such as _xy In _y N (referred to as 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1) can be used for light emitting / receiving elements and electron transit elements, A wide range of research has been conducted on crystal growth and application to semiconductor devices.

  Since nitride-based semiconductors cannot grow large bulk single crystals, they are generally heteroepitaxially grown using a heterogeneous substrate such as sapphire as a substrate for semiconductor growth.

  Epitaxial growth methods include metalorganic vapor phase epitaxy (MOVPE), molecular beam epitaxy (MBE), and halide vapor phase epitaxy (HVPE). The most common method for practical use is MOVPE. .

  Further, in the light emitting device using the semiconductor element as described above, a structure in which a nitride-based semiconductor layer is stacked is epitaxially grown on the entire surface of the sapphire substrate, processed into a desired device shape, and then an electrode is formed. .

  When a sapphire substrate having a (0001) plane or a (11-20) plane as a main surface is used, a nitride-based semiconductor grown on the sapphire substrate has [0001] orientation, so the (0001) plane is the main surface. However, in the case of nitride-based semiconductors, when a [0001] -oriented light-emitting element structure is stacked, the band structure of the active layer is distorted by the influence of the piezoelectric field, which is peculiar to this system material, and the luminous efficiency is reduced. There was a problem of letting it go. Due to the band structure distortion of the active layer, the characteristics of the nitride-based semiconductor, which is a direct transition type semiconductor, cannot be fully utilized. As a result, the energy loss is mainly consumed as thermal energy in the light emitting element, so that it is difficult to improve the luminance of the manufactured light emitting element, thus preventing the realization of a further high luminance light emitting element.

  Therefore, as a solution, it has been desired to grow a light emitting element structure in which crystals are oriented in a direction not affected by a piezoelectric field.

In general, when a nitride-based semiconductor is grown on a (01-12) plane sapphire substrate, it is known to have [11-20] orientation, which has already been described in, for example, Patent Document 1. In addition, a prototype example of a light emitting device has already been reported. Non-Patent Document 1 discloses a GaN / GaInN multiple quantum well in which a (11-20) plane GaN layer is formed on a (01-12) plane sapphire substrate. Prototype examples of structured light-emitting devices have already been reported. According to the above document, as shown in FIG. 6, an n-type GaN layer (30 μm) 611 and an n-type Al _0.1 Ga _{0 are formed} on a sapphire substrate 11 having a (01-12) plane as a main surface by the MOVPE method. _.9 N clad layer (100 nm) 612, GaN / In _0.15 Ga _0.85 N multiple quantum well structure active layer 613, p-type Al _0.1 Ga _0.9 N clad layer (50 nm) 614, p-type GaN A light emitting element structure 61 formed by sequentially laminating layers (200 nm) 615 is formed. The light emitting device 6 is formed by performing mesa processing by RIE and forming the p-side electrode 62 and the n-side electrode 63.

Here, the light emitting element structure 61 is [11-20] oriented. This document is useful in that the piezo electric field of the active layer 613 has been studied, but an n-type GaN cladding layer having a thickness of 30 μm, which is too thick for the light emitting element structure 61, is used. The growth time is not only too long, but the warpage of the epitaxial substrate after growth is a big problem, which is a technique unsuitable for practical use. This is because when the grown nitride semiconductor has a small film thickness, the surface morphology has an uneven shape, and thus the film thickness is increased to flatten the surface.
JP 2002-374003 A APPLIED PHYSICS LETTERS: VOL. 84, NUMBER 18, P.M. 3663 (2004)

  As described above, the surface flatness of the light-emitting element structure is a very important factor when manufacturing a light-emitting element, but the surface is flattened in a light-emitting element structure with a thickness of about several μm suitable for practical use. It was difficult to do.

In order to solve the above-described problems, the present application relates to a sapphire substrate and Al _x Ga _1-xy In _y N (0 ≦ x, y, x + y ≦ 1) grown on the main surface of the sapphire substrate. And a light emitting device structure having a thickness of 0.5 μm or more and 8 μm or less , the light emitting device comprising: a nitride-based semiconductor represented by : wherein the main surface of the sapphire substrate is (01-12) It is inclined at an off angle α from the surface to the (0001) plane direction, satisfies −0.75 ° ≦ α ≦ −0.25 °, and from the (01-12) plane of the main surface of the sapphire substrate to (0001) )surface
Provided is a light emitting device characterized in that an off-angle β representing an inclination angle in a direction perpendicular to the direction is 0 ° ≦ | β | ≦ 0.04 ° .

The surface roughness of the light emitting element structure is preferably 8 nm or less.

The present application is also a sapphire substrate for crystal growth of a nitride-based semiconductor represented by Al _x Ga _1-xy In _y N (0 ≦ x, y, x + y ≦ 1), wherein the nitride The main surface on which the semiconductor crystal is grown is inclined at an off-angle α from the (01-12) plane to the (0001) plane, and satisfies −0.75 ° ≦ α ≦ −0.25 °, and the sapphire The off-angle β representing the inclination angle of the main surface of the substrate from the (01-12) plane to a direction perpendicular to the (0001) plane direction is 0 ° ≦ | β | ≦ 0.04 °. A sapphire substrate is also provided.

  The surface can be flattened without being affected by variations in the structure of the light emitting element, and a highly efficient light emitting element can be manufactured stably. Further, the growth time of the light emitting element structure can be shortened to reduce the product cost. Further, even if the light emitting element structure is formed on a large-diameter substrate, no large warp is generated, which is suitable for mass production of light emitting devices.

  Embodiments of the present invention will be described below.

  FIG. 1 is a cross-sectional view of the sapphire substrate 11 of the present invention divided by a (11-20) plane that is an A plane. Therefore, [0001] (c in the figure) which is the C axis is inclined with respect to the main surface 12 of the sapphire substrate 11. The sapphire substrate 11 of the present invention is a surface that is inclined (off) in the (0001) plane direction from the (01-12) plane in which the main surface 12 is the R plane.

  Here, in the present invention, the definition of the off angle α when the (01-12) plane is turned off in the (0001) plane direction will be described. α is expressed as an angle formed between the main surface 12 of the sapphire substrate 11 and the (01-12) plane of sapphire. Since [0001] of hexagonal sapphire is uniquely determined, the case where the (01-12) plane is turned off in the direction approaching (0001) as shown in FIG. A case where the power is turned off in a direction away from (0001) like a ′ is defined as a minus sign.

  In the present invention, the off-angle α satisfies −0.75 ° ≦ α ≦ −0.25 °. Here, the reason why α is in the above range is that the surface flatness of the crystal grown on the main surface 12 of the sapphire substrate 11 deteriorates when α is in the other range.

  Next, another embodiment of the present invention will be described.

  FIG. 2 is also a cross-sectional view when the sapphire substrate 11 of the present invention is divided by a plane perpendicular to the (11-20) plane. In this case, [0001] of the sapphire substrate cannot be described on the paper surface of FIG. In this cross-sectional view, an off-angle β can be defined when the (01-12) plane is turned off in a direction perpendicular to the off-angle α. β is expressed as an angle formed by the main surface 12 of the sapphire substrate 11 and the (01-12) plane (b in the figure) of sapphire, as in α, but β is turned off by 90 ° from α. It is necessary to pay attention to. Note that there is no difference in the range defined in the present invention regardless of the sign of β, so there is no need to define the sign. Therefore, in the present invention, it is defined by | β | that does not depend on the sign.

  In the sapphire substrate 11 of the present invention, the off angle α is within the above-described range, and the off angle β satisfies 0 ° ≦ | β | ≦ 0.05 °. Here, β is in the above range because, when | β | is larger than 0.05 °, the surface flatness of the crystal grown on the main surface 12 of the sapphire substrate 11 deteriorates.

  FIG. 3 is a three-dimensional illustration of the crystal orientation and off-angle direction of the sapphire substrate 11 of the present invention. In FIG. 3, r represents [01-12], and the off angles α and β of the (01-12) plane are represented as [01-12] off angles in order to simplify the drawing.

FIG. 4 is a sectional view showing an example of a light emitting device using the sapphire substrate 11 of the present invention. On the main surface 12 of the sapphire substrate 11, an AlN layer 41, an undoped GaN layer 42, an n-type GaN cladding layer 43, a GaN / GaInN multiple quantum well structure active layer 44, a p-type Al _0.2 Ga _0.8 N layer 45, p A type Al _0.07 Ga _0.93 N clad layer 46 and a p ⁺ -type GaN layer 47 are sequentially laminated. As described above, by using the sapphire substrate 11 in which the off-angle of the main surface 12 is strictly defined, the [11-20] -oriented light emitting device structure 4 on the main surface 12 has a conventional 0.5 μm or more and 8 μm. Flattening is possible with the following film thickness. That is, by setting the off-angle α within a small range and also setting the off-angle β within a predetermined range, the crystal orientation of the sapphire substrate 11 is strictly defined, and the light-emitting element in the epitaxial growth process of the nitride-based semiconductor The surface can be flattened without adjusting the structure.

Here, the flatness of the epitaxially grown nitride-based semiconductor layer may be evaluated by an arithmetic average surface roughness measured by an atomic force microscope (AFM), and expressed by a root mean square surface roughness R _rms . In this case, it needs to be 10 nm or less. A light-emitting device using a light-emitting element structure that is not planarized is problematic because the luminance does not increase.

The light-emitting element structure 4 of the present invention generally has a light-emitting element structure composed of a nitride-based semiconductor represented by Al _x Ga _{1 -xy} In _y N (0 ≦ x, y, x + y ≦ 1). It only has to include at least an n-type cladding layer, an active layer, and a p-type cladding layer. By using the sapphire substrate, a planarization effect is obtained, and thus the light emitting element structure itself is not specified.

  As a first embodiment, an embodiment of the present invention will be described with reference to FIG.

First, the light-emitting element structure 4 was epitaxially grown. A MOVPE method was used as the growth method, and a (01-12) sapphire substrate was used as the semiconductor growth substrate 11. In this case, the after angle α when the (01-12) plane is turned off in the (0001) plane direction is set to −0.5 °. On this sapphire substrate, an AlN layer (200 nm) 41, a GaN layer (2 μm) 42, an n-type GaN cladding layer (500 nm) 43, a GaN / GaInN multiple quantum well structure active layer 44 (30 nm), a p-type Al _0.2 Ga _0.8 An N layer 45 (2 nm), a p-type Al _0.07 Ga _0.93 N clad layer 46 (3 nm), and a p ⁺ -type GaN layer 47 (3 nm) were sequentially laminated to constitute the light emitting element structure 4. The n-type layer was doped with silicon, and the p-type layer was doped with magnesium. The film thickness of all the light emitting element structures was 3 μm or less, and the layer structure was suitable for mass production.

  The growth temperature of each layer may be set so as not to impair the characteristics of the epitaxial substrate such as the composition, crystallinity, surface roughness, and electrical characteristics.

  X-ray diffraction measurement confirmed that the light-emitting element structure 4 was [11-20] oriented.

  Next, the manufacturing method of the light-emitting device 6 is demonstrated using FIG.

  First, after forming a mask by photolithography, dry etching using a chlorine-based gas was performed. The etching depth was controlled so that the etching was completed in the middle of the n-type GaN cladding layer 43.

  Next, Ni / Au is deposited to a thickness of 20/80 nm by electron beam evaporation, respectively, p-side ohmic electrode 51, Ti / Al / Ti is deposited to a thickness of 30/100/20 nm, respectively, and an n-side ohmic electrode is deposited. 52 was formed.

  In this way, the light emitting device 5 was obtained. This light-emitting device 5 is a blue light-emitting device having a peak wavelength of 450 nm, and the luminance is improved as compared with the case where a light-emitting element structure similar to the above is [0001] oriented.

  Hereinafter, Examples 1 to 6 and Comparative Examples 1 and 2 were prepared and evaluated under the conditions shown in Table 1 in order to compare the same evaluations as described above while changing the conditions.

  First, as in Comparative Examples 1 and 2, when the off-angle β was fixed at 0 ° and the off-angle α was changed, α was −0.2 ° and −0.8 ° (01-12). In the case of using the sapphire substrate 11 whose surface is the main surface, the surface roughness of the light emitting element structure 4 is increased.

  On the other hand, from Examples 1 to 3, the surface roughness can be reduced when the off-angle α is in the range of −0.75 ° ≦ α ≦ −0.25 °.

Next, as in Examples 4 to 6, the off angle α was fixed at −0.5 ° and the off angle β was changed. When | β | was less than 0.5 °, the surface roughness was I was able to keep it small.

It is sectional drawing explaining the crystal orientation of the sapphire substrate of this invention. It is sectional drawing explaining the crystal orientation of the sapphire substrate of this invention. It is a perspective view explaining the crystal orientation of the sapphire substrate of the present invention. It is sectional drawing which shows the light-emitting device of this invention. It is sectional drawing which shows the light-emitting device of this invention. It is sectional drawing which shows the conventional light-emitting device.

Explanation of symbols

11: Sapphire substrate 12: Main surface a: (01-12) plane (when α> 0 °)
a ′: (01-12) plane (when α <0 °)
c: (0001) axis b: (01-12) plane 4: light emitting device structure 41: AlN layer 42: undoped GaN layer 43: n-type GaN cladding layer 44: GaN / GaInN multiple quantum well structure active layer 45: p-type Al _0.2 Ga _0.8 N layer 46: p-type Al _0.07 Ga _0.93 N cladding layer 47: p ⁺ -type GaN layer 5: light emitting device 51: p-side electrode 52: n-side electrode

Claims (3)

A sapphire substrate and a nitride-based semiconductor expressed by Al _x Ga _1-xy In _y N (0 ≦ x, y, x + y ≦ 1), which is crystal-grown on the main surface of the sapphire substrate, A light emitting device having a film thickness of 0.5 μm or more and 8 μm or less ,
The main surface of the sapphire substrate is inclined at an off-angle α from the (01-12) plane to the (0001) plane direction, and satisfies −0.75 ° ≦ α ≦ −0.25 °,
The off-angle β representing the inclination angle of the main surface of the sapphire substrate from the (01-12) plane to a direction perpendicular to the (0001) plane direction is 0 ° ≦ | β | ≦ 0.04 °. A light emitting device characterized by the above.   2. The light emitting device according to claim 1, wherein the surface roughness of the light emitting element structure is 8 nm or less. A sapphire substrate for crystal growth of a nitride-based semiconductor represented by Al _x Ga _1-xy In _y N (0 ≦ x, y, x + y ≦ 1),
The main surface on which the nitride-based semiconductor is grown is inclined from the (01-12) plane to the (0001) plane at an off-angle α, and satisfies −0.75 ° ≦ α ≦ −0.25 °. ,
The off-angle β representing the inclination angle of the main surface of the sapphire substrate from the (01-12) plane to a direction perpendicular to the (0001) plane direction is 0 ° ≦ | β | ≦ 0.04 °. A sapphire substrate.

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