JPH0560275B2 - - Google Patents

Description

Detailed Description of the Invention <Technical Field> The present invention relates to a semiconductor laser device, which has a structure that is particularly effective in controlling the transverse mode of laser oscillation and reducing the threshold current, and which is suitable for MBE or MO-CVD.
The present invention relates to a semiconductor laser device structure that can be manufactured using new growth techniques such as the following.

<Prior art> In recent years, there has been remarkable progress in thin film single crystal growth techniques such as molecular beam epitaxy (MBE) and metal organic vapor deposition (MO-CVD). It has become possible to obtain thin epitaxially grown layers. These advancements in manufacturing technology have also led to improvements in the conventional liquid phase epitaxial growth (LPE) method for semiconductor lasers.
This made it possible to manufacture a laser device that utilizes a new effect based on a device structure with extremely thin layers, which was difficult to manufacture. A typical example is a quantum well (QW) laser.
This QW laser is a conventional double heterojunction (DH)
Lasers take advantage of the fact that quantization levels are formed in the active layer by reducing the thickness of the active layer from several hundred Å or more to around 100 Å or less, which makes it more efficient than conventional DH lasers. It has many advantages such as lower threshold current, good temperature characteristics, and excellent transient characteristics.

(References) 1 WTT sang, Applied Physics Letters,
vol.39, No.10 pp.786 (1981).

2 NKDutta, Journal of Applied Physics,
vol.53, No.11, pp.7211 (1982).

3 H. Iwamura, T. Saku, T. Ishibashi, K.
Otsuka, Y. Horikoshi, Electronics Letters,
vol.19, No.5, pp.180 (1983). In this way, the use of thin film single crystal growth techniques such as MBE and MO-CVD has opened the way to the practical application of high-performance semiconductor lasers with new multilayer structures.

On the other hand, conventional semiconductor lasers have undergone many improvements and have been put into practical use, but one of the most important improvements is the stabilization of the transverse mode.
In early electrode stripe type semiconductor lasers, which limited only the current by forming striped electrodes, in the current region slightly above the threshold current for laser oscillation, the gain necessary for oscillation was lost only in the active region directly below the stripe. , so it oscillates in the zero-order or fundamental transverse mode. However, as the drive current increases, the carriers injected into the active layer gradually spread to both sides of the stripe region, which spreads the high gain region, leading to spread of the transverse mode and higher-order transverse mode oscillation. Such transverse mode instability and drive current dependence deteriorate the linearity of the drive current and laser output, and when modulated by pulsed current, unstable fluctuations in the laser output occur and the signal-to-noise ratio deteriorates. let In addition, since the directivity of the output light becomes unstable, it becomes difficult to efficiently and stably guide the laser output to an optical system such as an optical fiber, which causes many problems in practical use. In this regard, many structures that stabilize the transverse mode by laterally confining not only the current but also the light have been found in GaAs fabricated by LPE.
As-based and InGaAsP-based semiconductor lasers have been proposed.

FIG. 1 is a cross-sectional view of a conventional GaAAs-based transverse mode stabilized semiconductor laser manufactured by the MO-CVD method. n-Ga _0.55 on n-GaAs substrate 11
A _0.45 As clad layer (1 μm thick) 12, undoped Ga _0.85 A _0.15 As active layer (0.05 μm thick) 13, p
-Ga _0.55 A _0.45 As cladding layer (1 μm thick) 14,
The n-GaAs current blocking layer (0.8 μm thickness) 15 is MO-
It is grown continuously using the CVD method to form a multilayer crystal structure for laser operation. Next, hydrogen peroxide and ammonia water were added to selectively etch GaAs.
The current blocking layer 15 is etched away in stripes using an etching solution mixed in a ratio of H ₂ O ₂ :NH ₄ OH=5:1 to form striped grooves (width 5 μm). After that, p-Ga _0.55 A _0.45 As cladding layer (0.6 μm thick) 18, p-GaAs cap layer (0.5 μm thick)
19 (thickness) is grown, and further an n-side electrode 21 and a p-side electrode 22 are formed. Although the laser device manufactured in this way exhibits relatively stable characteristics, extra internal loss occurs because the transverse mode is stabilized by the absorption of laser light by the current blocking layer 15 outside the groove 20. However, there are inherent drawbacks such as an increase in threshold current or a decrease in differential efficiency.

<Object of the Invention> In view of the above-mentioned problems, the present invention provides a semiconductor laser element structure with a low threshold and stabilized transverse mode by utilizing the layer thickness controllability of MBE or MO-CVD. The purpose is to

<Example> FIG. 2 is a cross-sectional configuration diagram of a semiconductor laser device showing an example of the present invention. n-Ga _0.55 A _0.45 As cladding layer (1 μm thick) 1 on n-GaAs substrate 11
2. Undoped Ga _0.85 A _0.15 As active layer (0.05 μm
Thickness) 13, p-Ga _0.55 A _0.45 As clad layer (0.1 μm thickness) 14, n-GaAs/n-Ga _0.3 A _0.7
As superlattice current blocking layer (0.8μm thickness) 16 is MO−
Continuously grows using CVD method. superlattice layer 16
For example, as shown in Figure 3, n-GaAs (100 Å
30 (thick) and n-Ga _0.3 A _0.7 As (100 Å thick) 31 are laminated alternately. Each layer 30, 31
It is desirable that the thickness of the layer be approximately 500 Å or less.
Next, this superlattice current blocking layer 16 is etched away in stripes to form stripe grooves (5 μm width) 20 that will serve as current paths. Etching is carried out by photolithography. First, the superlattice current blocking layer 16 is etched to the middle using a phosphoric acid-based or sulfuric acid-based chemical etching method or an ion beam etching method that has low selectivity to the A mixed crystal ratio, and then , to selectively etch GaAs, hydrogen peroxide and ammonia water are mixed with H ₂ O ₂ :NH ₄ OH
= Etching solution and Ga _0.3 A mixed in a ratio of 5:1
_0.7 Hydrofluoric acid (HF) selectively etches As
This is done by selectively etching and removing each layer of the superlattice one after another by alternately using the following methods. After etching, p-Ga _0.55 A _0.45 As cladding layer (0.6 μm thick)
18. A p-GaAs cap layer 19 is sequentially grown, and a metal such as Au.Zn.Ni is deposited to form an n-side electrode 21.
A p-side electrode 22 is formed. In this embodiment, the refractive index of the superlattice current blocking layer 16 is GaAs and Ga _0.3 A.
Ga _0.65 A _0.35 As, which has an A mixed crystal ratio between _0.7 As and
The transverse mode is stabilized by the effective refractive index waveguide effect. Also, unlike the case of using a Ga _0.65 A _0.35 single layer, the absorption of GaAs in the superlattice increases the loss of higher-order modes, making it possible to obtain fundamental mode oscillation up to high output power, but as shown in Figure 1. Since the absorption coefficient is halved compared to the conventional semiconductor laser device structure shown, the threshold current can be suppressed low and the differential efficiency can be improved. Also,
The superlattice current blocking layer 16 does not need to be made entirely of a superlattice; as shown in FIG. 4, the side near the active layer is made a superlattice to control the refractive index and absorption coefficient. A current blocking layer may be formed by stacking semiconductors.

In the semiconductor laser device having the above structure, when carriers are injected through the p-side electrode 22 and the n-side electrode 21, the carriers do not flow in the superlattice current blocking layer 16, and the superlattice current blocking layer 16 is removed. The carrier flows concentrated only in the stripe grooves 20. Therefore, laser oscillation is started within the active layer 13 corresponding to this stripe groove 20. active layer 1
3, both junction interfaces are defined by a double heterojunction with the cladding layers 12 and 14, and light is confined by this heterojunction. Furthermore, the superlattice current blocking layer 16 has a refractive index waveguide function as described above, whereby the spot of the laser output beam is fixed within the stripe groove 20.

The above examples were fabricated by MO-CVD.
Although the GaAAs semiconductor laser was shown,
It is possible to fabricate the semiconductor laser device structure of the present invention using MBE or a portion of LPE, and the manufacturing method and semiconductor material are not limited. As a semiconductor material other than GaAAs, for example, a combination of semiconductor mixed crystals such as InGaAP and InGaAsP can be used.

It goes without saying that device characteristics can be further improved by using a quantum well structure or the like in the active region.

<Effects of the Invention> According to the present invention, by using a superlattice in the current blocking layer that controls the built-in equivalent refractive index distribution,
By changing the composition and layer thickness ratio of each layer in the superlattice, combinations of refractive index and absorption coefficient that cannot be obtained with a single layer semiconductor can be obtained, and the built-in refractive index distribution and loss distribution can be independently controlled. This makes it possible to easily optimize the structural parameters of a semiconductor laser. Furthermore, since it can be manufactured by a manufacturing method such as MBE or MO-CVD that allows good control of layer thickness and composition, a designed device can be obtained with good reproducibility.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional configuration diagram illustrating a conventional semiconductor laser. FIG. 2 is a cross-sectional configuration diagram of a semiconductor laser showing an embodiment of the present invention. FIG. 3 is a cross-sectional configuration diagram illustrating one embodiment of the configuration of the current blocking layer. FIG. 4 is a cross-sectional configuration diagram illustrating another example of the configuration of the current blocking layer. 11... n-GaAs substrate, 12... n-GaA
As clad layer, 13...GaAAs active layer, 1
4, 18...p-GaAAs cladding layer, 16...
...Current blocking layer, 19...p-GaAs cap layer.

Claims (1)

[Claims] 1 A superlattice structure in which multiple types of crystal layers with a thickness of approximately 500 Å or less are alternately laminated on a multilayer crystal structure for oscillation including a double heterojunction is different from the superlattice structure except for the striped region where current passes. A semiconductor laser element, characterized in that it is embedded in a semiconductor layer of a conductivity type.