JPH1051070A - Semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a low contact resistance, by providing a p-type contact layer formed out of a p-type InN semiconductor or a p-type In-Ga-N-based semiconductor on the p-type clad layer of the laser structure including an active layer, an n-type clad layer and a p-type clad layer using GaN-based semiconductors. SOLUTION: In a semiconductor laser, a contact layer has the composition of x>0.2 in p-type Inx Ga1-x N. Further, in the laser, an Si-doped n-type GaN buffer layer 2, an Si-doped n-type Al0.15 Ga0.85 N clad layer 3, an Si-doped n-type GaN optical confinement layer 4, an undoped InGaN multiple quantum well(MQW) active layer 5, an Mg-doped p-type GaN optical confinement layer 6, an Mg-doped p-type Al0.15 Ga0.85 N clad layer 7, an Mg-doped p-type contact layer 8, a p-type InGaN inclined-composition layer 9 with a gradually increased In composition and an Mg-doped p-type InN contact layer 10 are grown in succession on a substrate 1. The p-type InGaN inclined composition layer 9 with a gradally increased In composition reduces a potential barrier present between the Mg-doped p-type contact layer 8 and the Mg-doped p-type InN contact layer 10 to make a low contact resistance obtainable.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser using a GaN-based semiconductor suitable for oscillating short-wavelength light.

[0002] In this specification, a GaN-based semiconductor is
A semiconductor containing GaN as a component.

[0003]

BACKGROUND ART GaN-based semiconductor, particularly a GaN-based nitride semiconductor _{(GaN, In x Ga 1-} x N, Al y Ga 1-y N
), Which has a relatively wide bandgap and is particularly attracting attention as a constituent material of a light-emitting element having an emission wavelength in the blue to ultraviolet region. For example, if a blue-emitting or ultraviolet-emitting semiconductor laser can be used in a digital information recording device such as a magneto-optical disk in which a red-emitting semiconductor laser is currently used, the recording density can be greatly improved.

FIG. 2 schematically shows the layer structure of a GaN-based laser first realized by a group of Nichia Corporation in 1996 (S. Nakamura et al, Jpn. J. Appl. Phy.
s. 35, L74 (1996)). On a sapphire substrate 51, an n-type GaN buffer layer 51, an n-type AlGaN cladding layer 5
2. After stacking the n-type GaN optical confinement layer 53, the InG
An aN multiple quantum well (MQW) active layer 54 is formed. M
The QW active layer 54 is made of In _0.2 Ga _{0.8 with} a thickness of 2.5 nm.
It consists of 26 periods of an N well layer and a barrier layer of In _0.5 Ga _0.5 N having a thickness of 5 nm.

On the MQW active layer 54, a p-type GaN light confinement layer 55, a p-type AlGaN cladding layer 56, a p-type G
By stacking the aN contact layers 57, a pn junction diode is formed. In this pn junction structure, the n-type layer is doped with Si, and the p-type layer is doped with Mg. p-type G
A part of the n-type GaN buffer layer 51 is removed by etching from the surface of the aN contact layer 57 to form a stripe-shaped mesa structure.

An n-type electrode 60 is provided on the surface of the n-type buffer layer 51.
Is formed, and the p-type electrode 6 is formed on the surface of the p-type contact layer 57.
1 is formed. The n-type electrode 60 is formed of a Ti / Al stack, and the p-type electrode 61 is formed of Ni / Au. It is reported that oscillation was obtained at a wavelength of 417 nm by this configuration.

[0007]

It is not easy to make good ohmic contact to GaN having a wide band gap. In particular, the resistance of the p-type electrode to the p-type region tends to increase. Usually, as a dopant of p-type GaN,
Mg is used.

However, the carrier concentration of the Mg-doped p-type layer is generally about 1 × 10 ¹⁸ cm ⁻³ , and is exceptionally high at about 5 × 10 ¹⁸ cm ⁻³ . The work function of GaN is far from the vacuum level, and has a characteristic that it is easy to make a low-resistance contact in the n-type region but hard to make a low-resistance contact in the p-type region. Under such circumstances, the contact resistance of the p-type electrode is about 10 ⁻³ Ωcm, and the resistance is not sufficiently low as a laser device.

When the contact resistance is large, a large voltage is required for driving the laser. The voltage drop in the contact resistance portion generates wasteful power consumption, and the wasteful power consumption also generates heat, thereby deteriorating laser characteristics.

An object of the present invention is to provide a GaN-based semiconductor laser having a low contact resistance of a p-type electrode.

[0011]

According to one aspect of the present invention, there is provided a laser structure using a GaN-based semiconductor containing at least GaN as a component, including an active layer, an n-type cladding layer, and a p-type cladding layer. Formed on the mold cladding layer,
And a p-type contact layer formed in the mold InN or p-type _{In x Ga 1-x N (} x> 0.2).

InN has a narrow band gap (about 1.9 eV) as compared with GaN, and is suitable for forming a low resistivity region. InGaN mixed crystals with a high In composition also
Has properties close to If the In composition is larger than 0.2, an effect similar to InN can be expected. Such In
When a p-type electrode is brought into contact with an InGaN layer having a high N or In composition, a low contact resistance can be obtained.

[0013]

FIG. 1 shows a G according to an embodiment of the present invention.
1 schematically shows a configuration of an aN-based semiconductor laser. Crystal structure 6
Each layer necessary for forming a semiconductor laser is grown on a (0001) plane substrate of SiC having H by metal organic chemical vapor deposition (MOVPE). First, a Si-doped n-type GaN buffer layer 2 having a thickness of about 2 μm and an impurity concentration of 5 × 10 ¹⁸ cm ⁻³ is grown on the surface of the substrate 1 and then has a thickness of about 0.4 μm.
μm, an Si-doped n-type Al _0.15 Ga _0.85 N clad layer 3 having an impurity concentration of about 5 × 10 ¹⁷ cm ⁻³ , a thickness of about 0.1 μm,
Si-doped n-type GaN with an impurity concentration of about 5 × 10 ¹⁷ cm ^-3
The light confinement layer 4 is grown. The cladding layer 3 has a lower refractive index than the light confinement layer 4 and has a light confinement effect on light in the light confinement layer 4.

As shown in FIG. 1B, the light confinement layer 4
Undoped InGaN multiple quantum wells (M
QW) The active layer 5 is formed. The MQW active layer 5 includes 30 In _0.3 Ga _0.7 N well layers 5 a each having a thickness of 3 nm.
Between them, a GaN barrier layer 5b having a thickness of 3 nm is interposed therebetween.

On the MQW active layer 5, symmetrically with the n-side region
The p-side region is formed. That is, on the MQW active layer 5
Has a thickness of about 0.1 μm and an impurity concentration of about 5 × 10¹⁷cm^-3
Mg-doped p-type GaN optical confinement layer 6 having a thickness of about 0.4
μm, impurity concentration about 5 × 10 ¹⁷cm^-3Mg-doped p-type
Al_0.15Ga_0.85The N cladding layer 7 is grown. Cladding
On the layer 7, a thickness of about 0.3 μm and an impurity concentration of about 2
× 10¹⁸cm^-3, Mg-doped p-type GaN contact layer
Grow 8.

On this p-type contact layer 8, a graded p-type InGaN composition layer 9 whose In composition gradually increases is formed. The composition gradient layer has, for example, a thickness of about 0.1 μm and a p-type impurity concentration of 2 × 10 ¹⁸ cm ⁻³ or more. The In composition is about 0 on the contact layer 8 side and about 1 on the upper surface.

The In composition of the InN composition gradient layer 9 is as follows:
It is desirable that the density gradually increases from the GaN contact layer 8 side toward the InN contact layer 10 side. The change of the In composition may be changed continuously or stepwise.

Further, when a superlattice is formed by laminating an InN layer and a GaN layer, characteristics similar to InGaN mixed crystal can be realized. By gradually changing the ratio of the thicknesses of the adjacent InN layer and GaN layer in the superlattice, it is possible to obtain the same characteristics as those obtained by changing the mixed crystal composition. The composition gradient layer 9 can also be formed by such a superlattice structure.

On the InGaN composition gradient layer 9, a Mg-doped p-type InN contact layer 10 having a thickness of about 0.5 μm and an impurity concentration of about 1 × 10 ¹⁹ cm ^-3 is formed. InN is
Since the band gap is relatively narrow, the acceptor can be doped to a high concentration. InGaN composition gradient layer 9
Reduces the potential barrier between the p-type contact layer 8 and the p-type contact layer 10 and enables low contact resistance.

InN can grow a single crystal on GaN, but has lattice mismatch. Although lattice mismatch is reduced by inserting the InGaN composition gradient layer 9, it is difficult to avoid generating lattice defects above the vicinity of the interface of the GaN contact layer 8 by forming the InN contact layer 10. However, even if such a lattice defect occurs, the lattice defect is located at a position distant from the MQW active layer 5 and has little adverse effect on the laser element characteristics.

After crystal growth of the stack, etching is performed from the surface of the InN contact layer 10 to the middle of the GaN buffer layer 2 while leaving a striped region. The direction of the stripe is the [1-100] direction. This is to make it possible to form a resonator using the (1-100) plane of the 6H-SiC substrate and each epitaxial growth layer as a cleavage plane. This etching can be performed by, for example, dry etching using chlorine.

On the exposed surface of the n-type GaN buffer layer 2, an n-type electrode 11 of Ti / Pt / Au laminated in stripes
Is formed, and a p-type electrode 12 is formed on the surface of the InN contact layer 10 by Ni / Au lamination.

After that, the epitaxial growth layer and the substrate are cleaved along the (1-100) plane to form a laser resonator long in the [1-100] direction. In FIG. 1A, the direction perpendicular to the paper is the resonance direction of the laser resonator. Thus, a GaN-based semiconductor laser is produced.

The substrate may be made of another material other than SiC. For example, the sapphire substrate described with reference to FIG. 2 may be used. The active layer is not limited to the configuration described above. For example, an MQW active layer formed of a well layer and a barrier layer having the mixed crystal composition described with reference to FIG. 2 may be used. Also,
An active layer other than the MQW active layer may be used. In addition, the design can be changed by changing the mixed crystal composition or the like.

The present invention has been described in connection with the preferred embodiments.
The present invention is not limited to these. For example, it will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

[0026]

As described above, according to the present invention,
A GaN-based semiconductor laser having a low contact resistance of a p-type electrode can be provided.

[Brief description of the drawings]

FIG. 1 is a sectional view schematically showing a configuration of a GaN-based semiconductor laser according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view schematically showing a configuration of a GaN-based semiconductor laser according to a conventional technique.

[Explanation of symbols]

 Reference Signs List 1 SiC substrate 2 n-type GaN buffer layer 3 n-type AlGaN cladding layer 4 n-type GaN light confinement layer 5 InGaN MQW active layer 6 p-type GaN light confinement layer 7 p-type AlGaN cladding layer 8 p-type GaN contact layer 9 p-type AlGaN composition Inclined layer 10 p-type InN contact layer 11 n-type electrode 12 p-type electrode

Claims (4)

[Claims] 1. Ga containing at least GaN as a component
A laser structure including an active layer, an n-type cladding layer, and a p-type cladding layer using an N-based semiconductor; and a p-type InN or p-type layer formed on the p-type cladding layer.
A semiconductor laser having a p-type contact layer formed of In _x Ga _1-x N (x> 0.2). 2. The method according to claim 1, further comprising a composition gradient layer disposed between the p-type cladding layer and the p-type contact layer and composed of an InGaN layer whose substantial In composition changes gradually. Semiconductor laser. 3. The semiconductor laser according to claim 2, wherein the composition of In in said composition gradient layer changes monotonically and continuously or monotonically and stepwise. 4. The composition-graded layer is an InN / GaN superlattice layer, wherein the thickness ratio between the adjacent InN layer and the GaN layer gradually changes.
A semiconductor laser as described in the above.