EP1468476A1

EP1468476A1 - Laser diode comprising a vertical resonator and a method for the production thereof

Info

Publication number: EP1468476A1
Application number: EP02706656A
Authority: EP
Inventors: Gunther Steinle; Hans-Dietrich Wolf
Original assignee: Infineon Technologies AG
Current assignee: Infineon Technologies AG
Priority date: 2002-01-25
Filing date: 2002-01-25
Publication date: 2004-10-20
Also published as: US20040042517A1; WO2003063310A1; US7075961B2

Abstract

The invention relates to a laser diode comprising a vertical resonator and to a method for the production thereof, whereby at least one active layer (2) is arranged between reflective layers. The invention is characterised in that at least one anti-oxidation layer (1) consisting of a III-V semiconductor material with a proportion of molar aluminium of less than 0.7 and/or at least one anti-oxidation layer (1) consisting of a III-V semiconductor material with an optical depth of at least two quarter waves is arranged between the reflective layers (3), thus preventing distortion caused by unintentional oxidation.

Description

description

Laser diode with vertical resonator and process for its manufacture

The invention relates to a laser diode according to the preamble of claim 1 and a method for its production according to the preamble of claim 11.

Laser diodes with a vertical resonator are known as VCSEL (vertical cavity emitting laser). In contrast to an edge-emitting laser, light is amplified transversely to a layer structure in a VCSEL, an active layer being arranged between mirror layers arranged vertically one above the other.

VCSELs are usually produced by etching a single or a multiple mesa structure with subsequent wet-thermal oxidation. For some layers, selective oxidation takes place in a targeted manner around a defined current path

(Current aperture) and to generate a certain index guidance. For example, it is known from US Pat. No. 5,262,360 to specifically oxidize Al _x Gaι- _x As layers (with x> 0.7), the aluminum content of the layers being very high.

The mirror layers (Bragg reflectors) above and below the active layer sometimes have a high aluminum content in order to achieve good reflection properties. These mirror layers with a high aluminum content are involuntarily oxidized during the production of the necessary oxide layers (eg stropore fabric). This undesirable oxidation is disadvantageous because a conversion of the semiconductor material into an oxide causes local stresses due to the volume changes in the material. To avoid these problems, the intentionally oxidized layers were previously made very thin (between 15 and 30 nm). However, the unintentionally oxidized layers cannot be made arbitrarily thin, since a certain thickness (for example λ / 4n) is required for optimal reflection properties. With an emission wavelength of λ = 850nm and a refractive index of approx. N = 3, a thickness of approx. 70nm should be available. Since VCSEL structures typically have between 60 and 70 pairs of mirrors, up to 70 layers with a high aluminum content are inadvertently oxidized, which leads to considerable tension in the outer area of the etched mesa.

The present invention has for its object to provide a laser diode with a vertical resonator, in which the tensions are reduced by unintentional oxidation.

This object is achieved by a laser diode with the features of claim 1.

Characterized in that at least one antioxidation layer made of a III-V semiconductor material with a molar aluminum content of less than 0.7 is arranged between mirror layers of a vertical resonator and / or at least one antioxidation layer made of a III-V semiconductor material with a optical thickness of at least two quarter wavelengths is arranged. This creates a layer with a reduced oxidation rate and / or ensures a controlled etching on a layer with a lower oxidation rate. Both the setting of the aluminum content and the choice of the thickness of the antioxidation layer can be used together or individually to bring about a reduced oxidation rate.

The low molar aluminum content lowers the tendency to oxidize the antioxidant layer. This minimizes local tension caused by the increase in volume in the material. The reduction in local stresses improves the reliability of the laser diode. Also will improves the defect concentration in the area of the active layer by reducing the oxide-semiconductor interfaces.

Another advantage of the antioxidation layer is the improved adhesion of dielectric layers or polymer layers and a simplified further etching after an oxidation, since little or no oxide has to be removed.

It is advantageous if an antioxidant layer consists of Al _x Gaι- _x As or In _y Al _x Gaι- _x -yAsι- _z P _z . The quintary material forms, for example, a chemical etch stop layer. The thickness of the antioxidation layer is advantageously not important in this.

It is also advantageous if at least one antioxidation layer is arranged above an active layer and / or if at least one antioxidation layer is arranged below an active layer. If the antioxidation layer is arranged above the active layer, this is not impaired in an oxidation step. If the antioxidant layer is located below the active layer one, e.g. current aperture produced by selective oxidation, which is arranged above the active layer, lie close to the active layer.

In a further advantageous embodiment of the laser diode according to the invention, an antioxidation layer and an active layer are arranged in a layer structure without the interposition of a further layer.

The antioxidation layer advantageously has an optical thickness of at least two quarter wavelengths. It is particularly advantageous if at least one antioxidation layer is designed as an etch stop and / or etch outlet layer.

If at least one antioxidant layer is at least partially absorption modulation can be minimized.

A further reduction in the local tension in the material can be achieved if at least one mirror layer, which is arranged in particular in the vicinity of an active layer, has a molar aluminum content of less than 0.9. This means that this mirror layer is less oxidized.

The object is also achieved by a method according to the preamble of claim 11.

A low-stress laser diode is obtained if at least one antioxidation layer made of a III-V semiconductor material with a molar aluminum content of less than 0.7 and / or at least one antioxidation layer made of a III-V semiconductor material with a between the mirror layers optical thickness of at least two quarter wavelengths is arranged.

If a layer of the specified optical thickness is used, it is advantageous that after the mirror layers on the antioxidation layer have been etched through, a clear change in the refractive index occurs, which can be easily recognized in the process, so that the process can be better controlled.

In an advantageous method, an antioxidation layer made of Al _x Gaι_ _x As or a chemically selective etching stop layer, in particular InyAlxGai_ _x . _y As ₁ . ₂ P ₂ installed.

The invention is explained in more detail below with reference to the figures of the drawings using several exemplary embodiments. Show it:

Figure 1 is a schematic sectional view of a laser diode with a vertical resonator according to the prior art. 2 shows a schematic sectional view of a first embodiment of the laser diode according to the invention with a vertical resonator;

3 shows a schematic sectional view of a second embodiment of the laser diode according to the invention with vertical resonator _#

1 shows a sectional view through a known laser diode with a vertical resonator. The function of such VCSEL is known in principle (e.g. Jewell et al., Vertical-Cavity Surface-Emitting Laser: Design, Growth, Fabrication, Characterization; IEEE Journal of Quantum Electronics, Vol. 27, No. 6, June 1991; pp. 1332ff ; SO Kasap, Optoelectronics and Photonics, Principles and Practices, Pretice-Hall, 2001), so that only the relationships essential to the invention are described in this description.

In the case of a VCSEL, the active layer can be arranged both in the upper, small mesa A (cf. FIG. 2) or in the lower, larger mesa B (cf. FIG. 3); for the sake of simplicity, the active layer is not shown in FIG. 1.

The layer stack of the VCSEL has a current aperture 10, which is arranged here in the upper mesa A. The current flow S with the increased current density in the area of the current aperture 10 is indicated by arrows. The current aperture 10 is formed by an intentionally oxidized layer with a high oxidation rate.

Above and below the layer with the current aperture 10, mirror layers are arranged which have a high molar aluminum content. In this process, etched through mirror layers with a high aluminum content are unintentionally oxidized on the side, the oxidized regions 11 in

Fig. 1 are shown as an example. Through this inadvertent Oxidations lead to local tension in the outer area of the etched mesa.

In the structure shown in Fig. 1, the VCSEL should be designed as a top emitter, i.e. the laser radiation leaves the layer stack at the upper edge. Alternatively, bottom emitters are also possible.

2 shows a first embodiment of the laser diode according to the invention, which is intended to avoid these tensions.

At least one antioxidation layer 1 is arranged below an active layer 2. In the second embodiment according to FIG. 3, this is exactly the opposite.

In the first exemplary embodiment shown, an intermediate layer (here mirror layer 5) is arranged between antioxidation layer 1 and active layer 2, so that the two layers do not adjoin one another directly. Alternatively, it is also possible to arrange no or more other layers (e.g. mirror layers) between the antioxidation layer 1 and the active layer 2.

The antioxidation layer 1 is formed here from AlχGa! - _x s. The molar aluminum fraction is referred to here as x. Alternatively, other III-V material systems can be used, in particular binary, ternary or quaternary (eg In-GaAlAs) or quintary (eg In _y lχGaι- _x - _y Asι- _z P _z ) material systems can be used. In these systems the molar

Aluminum content indicated in an analogous manner to the above ternary system.

The antioxidation layer 1 has a negligible oxidizability, ie the oxidized layer is only a few nanometers thick, so that the oxide which is nevertheless formed by physical processes (eg sputtering) or chemical processes is removable without affecting the rest of the structure. According to the invention, the antioxidation layer 1 has a molar aluminum content of less than 0.7. The aluminum content can be chosen to be so small that just little or no absorption occurs at the layer.

This effect was also found in the antioxidant layers 1 according to the invention with aluminum fractions of less than 0.3, in particular also with aluminum fractions in the range between 0.2 and 0.10. These examples apply to a wavelength of 850nm.

Basically, the aim is to choose a small amount of aluminum in order to obtain the best possible antioxidant effect. With Al _x Gaι_ _x As it is advantageous if the wavelength of the laser diode and the molar aluminum portion of the antioxidant layer are functionally linked by the following general relation:

0 <x <= 0.45: λ (μm)> 1.24 / (1.424 + 1.247 x)

0.45 <x <0.7: λ (μm)> 1.24 / (1.9 + 0.125 x + 0.143 x ² )

These relations can easily be converted into a functional dependency of the component x on the wavelength λ, since the wavelength is generally specified. However, for the practical application of this relation a surcharge has to be made to the aluminum part. For Al _x Gaι- _x As one then reaches the above-mentioned range of aluminum from 0.2 to 0.1.

Even if the tendency to oxidize decreases with a falling aluminum content, it is precisely the last area which is advantageous, since this represents a lower limit for a function due to the wavelength transparency.

The person skilled in the art recognizes that the molar aluminum content in each case the case is functionally linked to the wavelength of the laser diode. The functional relationships are either known or can be determined in a targeted manner.

The antioxidation layer 1 has a sufficient optical thickness, this thickness being at least two quarter-wave lengths. The thickness of the antioxidation layer is adapted to the properties of the etching process used, in particular the uniformity of the process, so that the etching can be stopped safely in the area of the antioxidation layer 1 or the etching can run out in the antioxidation layer 1. The thickness of the antioxidation layer 1 also ensures a distance between strained oxidized layers and the active layer 2.

Even if in the present case the material properties of the antioxidant layer 1 are linked to a criterion for the thickness, both criteria can also be used alone to solve the task.

The use of an antioxidation layer 1 prevents the formation of larger interface areas between the oxide material and the semiconductor material above and / or below the active layer 2. Defects of the interface areas could be induced in the active area during operation of the laser diode.

In addition to the use of an antioxidation layer 1, in the first embodiment mirror layers 5 with a reduced oxidation rate are arranged in the vicinity of the active layer 2 (Al _x Gaι_ _x As, with x <0.9) in order to reduce the local stresses. The reduced aluminum content results in less oxide volume, which leads to a reduction in tension.

To reduce the absorption losses, the antioxidation layer 1 is modulation-doped, ie the areas in which the standing wave intensity in the vertical resonator is maximum, have a lower doping. The doping in areas of minimal standing wave intensity is increased.

A p-contact 4 is arranged on the top of the small mesa A, laser light being able to emerge in an uncovered area (top emitter).

The second embodiment, which is shown in FIG. 3, differs primarily in that the antioxidation layer 1 is arranged above the active layer 2. As in the first embodiment, an intermediate layer (here mirror layer 5) is provided between the antioxidation layer 1 and the active layer 2. Alternatively, no or more intermediate layers can also be provided.

Analogous to the first embodiment, the second embodiment also has mirror layers 5 (Al _x Gaι- _x As, with x <0.9) with a reduced oxidation rate in the vicinity of the active layer 2 in order to reduce the local stresses. Analogously, the antioxidation layer can also be modulation-doped here.

Since the production of VCSEL is fundamentally known, only a few points that are important for the configuration according to the invention are to be discussed here.

In both embodiments, the oxidation is carried out immediately after the first mesa etching in order to avoid oxidation below the layers to be oxidized intentionally (e.g. current aperture).

Only one antioxidation layer 1 is shown in each of the two embodiments. In principle, several such layers can also be used in a layer structure. In principle, an antioxidation layer 1 can also be arranged above a current aperture layer 10, wherein after the etching, the exposed layers and etching flanks can be protected against oxidation by means of a suitable covering layer by subsequent process steps. A preferred material for the cover layer is, for example, CVD-SiN _x .

The embodiment of the invention is not limited to the preferred exemplary embodiments specified above. Rather, a number of variants are conceivable which make use of the laser diode according to the invention and the method for its production, even in the case of fundamentally different types.

LIST OF REFERENCE NUMBERS

1 anti-oxidation layer

2 active layer 3 mirror layer

4 p contact

5 mirror layer with reduced oxidation rate

10 current aperture layer 11 areas inadvertently oxidized

A top mesa

B lower mesa

S Current flow through current aperture

Claims

claims

1. Laser diode with a vertical resonator, in which at least one active layer is arranged between mirror layers, characterized in that between the mirror layers (3) at least one antioxidation layer (1) made of a III-V semiconductor material with a molar aluminum content of less than 0.7 and / or at least one antioxidation layer (1) made of a III-V semiconductor material with an optical thickness of at least two quarter wavelengths is arranged.

2. Laser diode according to claim 1, characterized in that an antioxidation layer (1) consists of Al _x Ga_ _x As or a chemically selective etching stop layer, in particular InyAlxGax-.x- _y ASi-zPu.

3. Laser diode according to claim 1 or 2, characterized in that at least one antioxidation layer (1) is arranged above an active layer (2).

4. Laser diode according to at least one of the preceding claims, characterized in that at least one antioxidation layer (1) is arranged below an active layer (2).

5. Laser diode according to at least one of the preceding claims, characterized in that the antioxidation layer (1) and an active layer (2) are arranged in a layer structure without the interposition of a further layer.

6. Laser diode according to at least one of the preceding claims, characterized in that at least one antioxidation layer as an etch stop and / or etch Aus- running layer is formed.

7. Laser diode according to at least one of the preceding claims, characterized in that at least one antioxidation layer (1) is at least partially modulation-doped.

8. Laser diode according to at least one of the preceding claims, characterized in that at least one mirror layer (5), in particular in the vicinity of an active layer (2), has a molar aluminum content of less than 0.9.

9. Laser diode according to at least one of the preceding claims, characterized in that between the mirror layers (3) and above at least one current aperture layer (10) at least one antioxidation layer (1) is arranged, which is designed as an etch stop and / or etch outlet layer

10. Laser diode according to at least one of the preceding claims, characterized in that an antioxidation layer 1 is arranged above a current aperture layer 10, the exposed layers being protected from oxidation by subsequent process steps after etching by means of a suitable covering layer, in particular of CVD-SiN Si become.

11. A method for producing a laser diode with a vertical resonator, which has at least one active layer between mirror layers, characterized in that between the mirror layers (3) at least one antioxidation layer (1) made of a III-V semiconductor material with a molar aluminum Share of less than 0.7 and / or at least one antioxidation layer (1) made of a III-V semiconductor material with an optical thickness of at least two quarter wavelengths is arranged.

12. The method according to claim 11, characterized in that an antioxidation layer (1) of Al _x Gaι_ _x As or a chemically selective etching stop layer, in particular In _y Al _x Gaι- _x -yAsι- _z P _{z is} installed.