JP2751306B2 - Semiconductor light emitting device - Google Patents

Description

DETAILED DESCRIPTION [Summary] For a semiconductor light emitting device such as a light emitting diode or a laser diode, for example, a light emitting device in a visible region constituted by a relatively oxidizable semiconductor material such as InGaAlP is formed by MOVPE method. An active layer made of a first semiconductor having a forbidden band width E _g1 and a forbidden band width E _g2 (where E _g2 > E _g1 ) A cladding layer made of a second semiconductor, having a forbidden band width E _g3 (where E _g1 <E _g3 ≦ E _g2 ), and having the same conductivity type as the cladding layer; A capping layer formed on the cladding layer; a capping layer formed on the cladding layer; a cap layer formed on the cladding layer; A semiconductor layer interposed between What has a shape corresponding to a desired light emitting area, the forbidden band width in the cladding layer side is E _g2, the band gap in the cap layer side is continuously changed such that E _g3 composition With a gradient,
It is constituted by including the cladding layer and a mixed crystal semiconductor layer of the same conductivity type.

[Industrial applications]

The present invention relates to a light emitting device such as a light emitting diode or a laser diode formed using a compound semiconductor.

[Conventional technology]

2. Description of the Related Art In a light emitting element such as a light emitting diode or a laser diode, a structure in which an injection carrier is caused to flow intensively in a light emitting region (active region) (carrier confinement) is required. For this purpose, a means (current constriction structure) for making it difficult for a current to flow around the active region is usually provided. The laser diode further needs a structure that can bind the generated light into a propagation path constituting a resonator, and the current confinement structure is designed to have such a light confinement function.

As one of the current constriction structures, around the active region,
There is a structure that forms a pn junction in the opposite direction to the injection carrier. This will be described with reference to FIG.

In the figure, A is an active region, and B is a current confinement region. The active region A is, for example, an n-type InP layer 2, which is epitaxially grown on the n-type InP substrate 1, an n-type or p-type InGaAsP layer.
3, formed by processing the p-type InP layer 4 and the p-type InGaAsP layer 5 into a mesa shape by etching. The InGaAsP layer 3 forms an active layer, and the InP layers 2 and 4 form a cladding layer. The p-InGaAsP layer 5 forms a cap layer for forming an ohmic contact between the electrode 6 and the p-InP layer 4. The current confinement region B, p-type Inp layer 7 ₁ and 7 ₃ and n-type InP
Layer 7 ₂ is formed, the mesa of the active area A is filled with them.

When a current flows toward the electrode 6 to the electrode 8, although the active region A forward current flows in the current confinement region B, at least npn junction by InP layer 7 _2, 7 ₁ and the InP layer 2 is formed Therefore, no current flows.

[Problems to be solved by the invention]

In the structure of FIG. 4, p-type InP layer _71, because of optical confinement, it is necessary to form deeper than InGaAsp layer 3 which is an active layer position. For this reason, the height H of the mesa in the active region A is usually about 5 μm. Therefore,
InP layer ₇₁ in the current confinement region B, 7 _2, 7 ₃ must have a thickness that is out to embed the mesa in height. Conventionally, a liquid crystal growth method (LPE) has been used as a method for obtaining an epitaxially grown layer having such a thickness.

However, since the liquid phase growth method generally has a high growth temperature, the surface of a compound semiconductor crystal containing a component having a high vapor pressure such as phosphorous (P) or a component which is easily oxidized such as aluminum (Al) becomes phosphorous. There is a disadvantage that it is susceptible to thermal denaturation such as evaporation and oxidation of aluminum. For this reason,
For example, in the visible region laser diode using InGaAlP, a modified layer is formed on the surface of the InGaAlP layer exposed on the side surface of the mesa-shaped active region. As a result, the leakage current increases and the threshold current increases. is there.

Recent advances in metal organic vapor phase epitaxy (MOVPE) technology have made it possible to grow compound semiconductors epitaxially at low temperatures and in an oxygen-free atmosphere by liquid phase epitaxy. Therefore, according to the MOPVE method,
It is expected that the problem of thermal denaturation of the liquid crystal surface as in the LPE method can be avoided.

However, the MOVPE method is a kind of vapor phase growth method, and the crystal growth mechanism is different from the liquid phase growth method. Therefore, the thickness of the current confinement region B is 5 μm like the npn junction layer.
If an attempt is made to form a semiconductor layer having a thickness of m, a specific surface of the (111) plane is formed, and it is difficult to bury the semiconductor layer flatly, and the embeddable thickness is at most about 3 μm. As a result, the current confinement structure using a pn junction as described above, for example, can not grow the InP layer 7 _1, 7 _2, 7 ₃ in a single step, every time to grow a layer of each After the etching step, the next layer must be grown, and there is a problem that many growth steps are required.

An object of the present invention is to provide a current confinement structure having no problems caused by using the pn junction as described above. Specifically, the current confinement region can be formed by a semiconductor layer having a relatively small thickness. Therefore, the current confinement region of the light emitting device in the visible region, which is formed by using a relatively oxidizable semiconductor material such as InGaAlP, can be formed by the MOPVE method, and a current confinement structure capable of controlling the transverse mode can be provided. The purpose is to provide.

[Means for solving the problem]

The object is to form an active layer made of a first semiconductor having a forbidden bandwidth E _g1 and a second semiconductor having a forbidden bandwidth E _g2 (where E _g2 > E _g1 ) and formed on the active layer. And has the forbidden band width E _g3 (where E _g1 <E _g3 ≦ E _g2 ), is of the same conductivity type as the cladding layer, and has a valence band at the junction interface with the second semiconductor. A cap layer formed on the cladding layer and a semiconductor layer interposed between the cladding layer and the cap layer, the third semiconductor layer comprising a third semiconductor in which a continuous spike-like barrier is generated; hand,
It has a shape corresponding to a desired light emitting region, and has a composition gradient that continuously changes such that the forbidden band width is E _g2 on the cladding layer side and the forbidden band width is e _g3 on the cap layer side. The semiconductor light emitting device according to the present invention is characterized by comprising the cladding layer and a mixed crystal semiconductor layer of the same conductivity type.

(Operation)

FIG. 1 is a sectional view showing the principle structure of the present invention, and FIG. 2 is an energy band diagram for explaining a current confinement operation by the structure of the present invention.

With reference to Figure 1, the heterojunction interface between the semiconductor layer 10 having a bandgap E _a semiconductor layer 11 having a band gap E _b (where E _a ≠ E _b), band gap E _a And a semiconductor layer 12 continuously changing from _Eb to _Eb . The semiconductor layer 12 is provided in a stripe shape extending in a direction perpendicular to the paper surface. Semiconductor layer 12
Is a graded layer having the same forbidden band width as the semiconductor layers 10 and 11 at the interface with the semiconductor layers 10 and 11, respectively, and the forbidden band width continuously changes between these interfaces. Such a graded layer has a mixed crystal composition of the semiconductor layer 10 and the semiconductor layer 11, for example.
It has the same composition as 10 and can be formed by giving a composition gradient on the semiconductor layer 11 side so as to have the same composition as the semiconductor layer 11.

The energy band structures at zero bias in the L ₁ -L ₂ cross section and the M ₁ -M ₂ cross section shown in FIG.
This is as shown in FIGS. FIG. 2 shows the case of a combination of p-types, and the dotted line indicates the Fermi level. L ₁ −L ₂ cross section, that is, the semiconductor layer 10
At the heterojunction interface between and, a spike-like barrier having a height ΔE _v is formed due to discontinuity of the valence band E _v . The barrier height ΔE _v has a large value as large as 0.63 eV in the case of GaAs and InGaAlP lattice-matched thereto. Such a barrier generally appears at a steep terror junction interface, and serves as a barrier to a hole current flowing through the semiconductor layers 11 to 10.

On the other hand, in the M ₁ -M ₂ cross-section, as shown in FIG. 2 (b), the semiconductor layer bandgap E _x is continuously changed in E _b from E _a between the semiconductor layer 10 and 11 Because of the interposition of 12, the spike-like barrier as described above is not formed. That is,
For a hole current flowing across the semiconductor layer 12, the barrier of ΔE _v does not exist.

Therefore, as shown in FIG. 1, the semiconductor layer 12 is provided only in the active region A, and the semiconductor layers 10 and 11 are provided in the current confinement region B.
Is formed, the current flowing through the current confinement region B is about exp (−ΔE _v / kT) times the current flowing through the active region A. In this way, the current flowing in the current confinement region B is suppressed, and the current can be concentrated and flow in the active region A.

〔Example〕

FIG. 3 is a main part view showing an embodiment of a laser diode structure to which a current confinement structure based on the above principle is applied.

On an n-type GaAs substrate 20, a cladding layer 21 of n-type In _0.5 (Ga _0.3 Al _0.7 ) _0.5 P having a thickness of 2 μm, In _{0.5 of} 0.15 μm thickness
An active layer 22 of Ga _0.5 P, p-type In _0.5 (Ga
_A cladding layer 23 made of _0.3 Al _0.7 ) _0.5 P and a graded layer 24 are sequentially epitaxially grown. The graded layer 24 is a mixed crystal of InGaAlP and GaAs, and the composition at the interface with the cladding layer 23 is In _0.5 (Ga _0.3 Al _0.7 ) _0.5 P
The composition is continuously changed so that the composition on the upper end surface is GaAs, and is a p-type conductive layer.

The graded layer 24 and the clad layer 23 are processed so as to form a stripe-shaped mesa having a height (h) extending in a direction perpendicular to the paper surface. This processing is performed by masking the active region A with an SiO ₂ film and using a normal photolithographic technique. The width (W) of the stripe-shaped mesa is 3 μm or less, and the thickness of the current confinement region B is 0.2 to 0.2 μm.
The 0.3 μm cladding layer 23 is left.

A cap layer 25 made of p-type GaAs is epitaxially grown on the stripe-shaped mesas. Then, Ti / Pt is applied to the surface of the cap layer 25 and the surface of the GaAs substrate 20, respectively.
A p-type electrode 26 made of a / Au multilayer film and an n-type electrode 27 made of an AuGe / Au multilayer film are formed.

In the laser diode having the structure shown in FIG.
According to the principle described with reference to FIGS.
, The current is difficult to flow in the current confinement region B, and the desired current confinement is achieved.
In the current confinement region B, the cladding layer 23 has a thickness of 0.2.
The cap layer 25 made of GaAs and having a small band gap is made closer to the active layer 22 made of InGaP and having a large band gap. Therefore, light generated in the active region A and diffused into the current confinement region B leaks to the cap layer 25 and is absorbed there. As a result, light contributing to laser oscillation propagates only in the active region A. That is, the laser light is confined in the active region A, and the transverse mode control is achieved.

In the case of the light emission mode, the above-described light confinement function is not required, and only the current confinement function need be provided. Therefore, it is not necessary to form a mesa structure as shown in FIG. Only the layer 24 has to be formed in a desired light emitting region shape.

According to the current confinement structure of the present invention, the active layer 22 is not processed into a mesa shape and is covered with the cladding layer 23 after its growth. There is no problem that a leakage current occurs due to thermal denaturation on the mesa side. The width (W) and height (h) of the stripe-shaped mesas in the active region A are both 3 μm or less in consideration of lateral mode control. Therefore, it is practically possible to form the GaAs cap layer 25 in which such mesas are embedded by the MOVPE method. Moreover, the cap layer 25
Is a single layer, so only one crystal growth is required after the conventional mesa processing, compared to the case where multiple crystal growth and etching steps are repeated after the mesa processing as in the case of current constriction using a pn junction. As a result, the manufacturing process can be significantly simplified.

〔The invention's effect〕

According to the present invention, a current confinement structure in a semiconductor light emitting device such as a light emitting diode or a laser diode is formed by using the MOPVD method. As a result, leakage current in the current confinement region of a semiconductor light emitting device in the visible light region using a compound semiconductor such as InGaAlP containing phosphorus (P) or Al which is easily oxidized at a high temperature is reduced, and a semiconductor having a low threshold current is used. A light emitting element can be provided. In addition, pn
Since no bonding is used, the manufacturing process is simplified, and there is an effect that the manufacturing cost and the yield of the semiconductor element can be improved.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a basic structure of the present invention, FIG. 2 is an explanatory view of a current confinement operation according to the structure of the present invention, and FIG. FIG. 4 is a cross-sectional view showing a conventional current confinement structure using a pn junction. In the figure, 1 is an InP substrate, 2, 4 and 7 ₁ and 7 ₂ and 7 ₃ InP layer, 3 and 5 InGaAsP layer, 6 and 8 electrodes, 10 and 11 and 12 the semiconductor layer, 20 a GaAs substrate , 21 and 23 are cladding layers, 22 is an active layer, 24 is a graded layer, 25 is a cap layer, 26 is a p-side electrode, and 27 is an n-type electrode.

Claims (1)

(57) [Claims] 1. An active layer comprising a first semiconductor having a forbidden band width E _g1 and a second semiconductor having a forbidden band width E _g2 (where E _g2 > E _g1 ) and formed on the active layer. And has the forbidden band width E _g3 (where E _g1 <E _g3 ≦ E _g2 ), has the same conductivity type as the cladding layer, and has a valence band at the junction interface with the second semiconductor. A capping layer formed on the cladding layer and a semiconductor layer interposed between the cladding layer and the capping layer; A composition which has a shape corresponding to a desired light emitting region, and has a forbidden band width of E _g2 on the cladding layer side and a continuously changing band _gap of E _g3 on the cap layer side. Characterized by having a gradient and a mixed crystal semiconductor layer of the same conductivity type as the cladding layer. The semiconductor light-emitting element.