ITMI940860A1 - METHOD FOR THE OPERATING COOLING OF HIGH-EMISSION SEMICONDUCTOR LASER TERMINAL MIRRORS AND RELATED DEVICE - Google Patents

Description

Introduction :

The present invention refers to a process for the treatment of semiconductor laser facets, adapted to prevent their degradation during the operation in the high power regime, and to the relative device.

In recent years, laser devices with high light emitting power have gained important fields of application: they are currently used, for example, in optical recording systems, in laser printers, in optical communication systems.

One of the factors that most limits the emission power of semiconductor laser devices with an emission wavelength less than about one micrometer (excluding with this the emitters based on Indium phosphide, which seem immune to these problems) is the so-called catastrophic degradation of mirrors.

This phenomenon appears as a localized fusion of the laser mirrors, practically nullifying the power emitted by the device. all the characteristics of a self-catalyzing process that is consumed on a time scale of microseconds.

If the intimate mechanism is still partially obscure, the associated phenomenology has been variously investigated over the last few years, for example in the article by A. Moser entitled "Thermodynamics of facet damage in cleaved AlGaAs lasers" published in Applied Physics Letters you . 59, year 1991, from page 522, an empirical formalism is proposed for the prediction of the average life of devices based on AlGaAs alloys, whose main failure mechanism is the catastrophic degradation of the mirrors.

The mode of degradation is shared by devices based on alloys that do not contain Aluminum, as highlighted in the article by D. Sala et al, entitled "Analysis of catastrophic optical damage for AlGaAs / InGaAs and InGaP / InGaAs strained quantum well lasers" presented at the ESREF 93 conference, held in Bordeaux on 4-7 October 1993.

This phenomenon is predominant in the regime of high emission power, while at low power the average life of these devices can exceed 10A5 hours, as well shown in the article by M. Okayasu et al, entitled "Estimation of the reliability of 0.98 um InGaAs / GaAs strained quantum well lasers "appeared in the Journal of Applied Physics vol. 72, year 1992, from page 2119.

Degradation mechanism:

Broad consensus is found in the specialized literature regarding the chain of events that leads to the catastrophic degradation of the mirrors. The phenomenon appears as a localized fusion of the laser facets, resulting in the self-absorption of the radiation emitted by the device itself and promoted by a non-radiative recombination rate.

The high speed of surface recombination at the interface of the semiconductor with the air leads to an increase in the temperature in the mirror region with respect to the internal areas of the device. This localized heating causes a local reduction of the energy gap in the mirror region with respect to to the internal areas of the device; therefore the absorption coefficient can increase considerably, resulting in the production of more heat. It is clear how this positive feedback mechanism can rapidly lead to localized melting of the facets.

Prior art:

On the basis of this interpretative model, the specialized literature reports various possible solutions to the problem.

For example, in the article by Y. Shima et al, entitled "Effects of facet coatings on the degradation characteristics of GaAs / GaAlAs lasers", published in Applied Physics Letters vol.31, year 1977, from page 625, the effect is shown protective dielectric coatings on the reliability of the devices. However, this method is not sufficient to completely eliminate the phenomenon of catastrophic degradation of the mirrors, as well shown in the aforementioned article by A. Moser.

Another proposed solution is presented in the article by T. Shibutani et al, entitled "A novel high power laser structure with current-blocked region near cavity facets" published in IEEE Journal of Quantum Electronics vol. QE-23, year 1987, from page 760, where the use of regions suitable for preventing the flow of current in the vicinity of the laser facets is proposed and at the same time the size of the asset in the same area is reduced in order to suppress the 'local temperature increase, responsible for the catastrophic degradation of mirrors. the manufacturing process of these devices is particularly complicated, requiring two epitaxial growths on non-planar substrates, and can only be performed with epitaxial deposition techniques with great anisotropy in the speed of growth (such as liquid phase deposition) and furthermore, sufficient data to support the affirmation of the effectiveness of the method are not shown in the cited article.

Purpose of the invention:

The object of the invention is to propose a method of operative cooling of the semiconductor laser facets which allows to eliminate the catastrophic degradation of the laser mirrors, thus allowing the use at high emission powers of these devices.

The purpose is achieved by applying the method which is described in detail in the following paragraph, and which can be generalized to laser devices formed by materials other than those indicated as an example in the process flow under examination.

Detailed description:

The operational cooling process is applied on laser structures, the manufacturing process of which is briefly described below.

A multilayer structure composed of a substrate, for example of Gallium arsenide (GaAs), on which a sequence of epitaxial layers is deposited with techniques such as Metal-Organic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE).

This structure includes:

i) n-type buffer of GaAs

ii) lower cladding of n-type AlGaAs

iii) optical guide composed of AlGaAs with aluminum content lower than that of

layers ii) and vi)

iv) active layer of InGaAs

v) optical guide composed of AlGaAs with aluminum content lower than that of

layers ii) and vi)

vi) superior cladding of p-type AlGaAs

vii) p-type GaAs layer for the electrical contact of the structure

The processing of the epitaxial slices thus obtained involves a sequence of technological steps suitable for forming the guiding structure, for carrying out the electrical contact, to arrive at the flaking of the bars from which to obtain the devices proper.

In order to realize the waveguide, for example of the Ridge V / aveguide (RW) type, non-selective attachments can be used, which allow to realize the guide in a single attachment process.

Two possible known solutions are:

a) a solution used belongs to the category of H2S04: H202: H20 mixtures widely described and characterized in the literature.

b) a further possibility to realize this type of structures involves a plasma Reactive Ion Etching (RIE) process. In this case the planar structure, suitably masked with Ni in order to obtain the confined guide, is placed on the cathode inside a vacuum bell.

Process gases, such as a combination of CH4 / H2 / Ar, are introduced in succession and a 13.56 MHz discharge is triggered.

The surface of the semiconductor is therefore in contact with reactive ions that attack it both physically (by bombardment) and chemically, while the depth of the attack is continuously controlled internally by means of a laser interferometer.

Finally, laser bars are separated from the epitaxial slice, obtained by mechanical engraving and flaking to form the mirrors of the device.

The operating cooling process described below is applied to the laser bars thus formed.

The bars, whose flaked faces have been exposed to the air from the moment of separation and therefore show a considerable degree of oxidation, are introduced into a parallel plate rf (13.56 MHz) reactor Plasma Enhanced Chemical Vapor Deposition (PECVD) with a basic vacuum P base <10A (-5) bar and operating at a variable pressure between 0.1-0.5 bar.

The first phase of the process consists in the removal of the oxides of the elements of group V, present on the laser mirror, by creating a reducing environment of hydrogen plasma.

Oxygen bound, for example, to arsenic or phosphorus, in the presence of a strongly reducing environment, reacts with the elements of group III forming more stable oxides such as GaO (x), A10 (x) and InO (x). volatile hydrides are formed of the elements of group V, such as AsH3 and PH3, which are removed from the surface.

The removal of contaminants and oxides, at this stage of the process, is already sufficient to reduce the degradation of the laser devices to which this first part of the treatment object of the present invention is applied.

During the process of removing oxides, contaminants and elementary species of group V, a diffusion of elementary hydrogen occurs inside the semiconductor.

It is known, see for example the article by A.Y. Pakhomov et al, entitled "Hydrogen passivation: relevance to semi-insulating III-V materials" presented at the "6th Conference on Semi-insulating materials, Toronto, Canada, 1990", which hydrogen inactivates the deep electron levels in III-V materials.

The passivation by hydrogen is maintained even at high temperature, up to about 600 C, thus stably reducing the non-radiative recombination processes associated with these electronic levels, thus obtaining a three-dimensional diffuse layer with low heat generation capacity.

Surprisingly, this passivation method has never been applied to semiconductor laser devices, and this knowledge has remained confined to basic solid-state physics studies.

In a second phase an ammonia plasma is produced in the PECVD reactor, in order to remove the remaining group III oxides, bringing the laser facet back to compositional stoichiometry conditions and free from contaminants.

These processes take place under temperature conditions T = 250-350 (300) C, at a pressure P = 0.1-0.5 (0.3S) bar, at a power rf = 5-20 (7) W for a time t = 10- 1000 (500) s, where preferred conditions are indicated in parentheses.

The complete removal of native oxides and elementary As or P clusters leads to a reduction of the antisite defects associated with As or P with a drastic decrease in the surface recombination speed and the local heating connected to it is therefore considerably reduced.

On the surface of the semiconductor, brought back to almost ideal conditions, a thin layer (d = l-100 nm) of metallic aluminum is deposited using the PECVD technique, using organometallic sources such as trimethyl aluminum or triethyl aluminum.

Deposition preferably takes place with partial pressure of the aluminum precursor P (Al) = 10Λ (-5) -10A (-6) (4 * 10Λ (-6)) bar, at a temperature T = 100 ~ 200 (180) C for a time t = 10-1000 (400) s, where the preferred conditions are indicated in brackets.

The process of interdiffusion of the aluminum deposited on the surface is then carried out by means of an "in-situ" annealing in an H2 / N2 environment at a temperature T = 350-450 (400) C and for a time t = 10 -1000 (400) s, where preferred conditions are shown in parentheses.

The metallurgical layer that is formed on the surface creates a potential barrier to the diffusion of minority carriers, thus preventing surface non-radiative recombination of the electron-hole pairs.

In this way, an effective operational cooling of the laser mirror is obtained with a consequent decrease in the degradation associated with it.

The mere formation of metallurgical alloy in the area adjacent to the end mirrors leads to a marked improvement in the reliability characteristics of the devices treated according to this part of the process object of the invention.

The surface thus prepared maintains a certain degree of reactivity towards the external environment. A protective dielectric layer is then deposited, such as SiN (x), in order to isolate the surface of the laser mirror from the external environment.

This deposition process takes place in immediate sequence to the previously described steps, so as not to interrupt the continuity of the process and to maintain a strongly reducing environment.

The encapsulating layer is deposited under conditions of minimum stress with respect to the supporting semiconductor, by means of a gradual adjustment of its nitrogen content, and by varying the hydrogen content present in the film.

The final phase of the process involves a thermal stabilization cycle of the passivated structure which is carried out in an H2 / N2 environment at a temperature T = 300-450 (400) C for a time t = 5-60 (30) s, where the preferred conditions are indicated in parentheses.

This process allows a further settling of the structure, further reducing its stress. This process sequence takes place under time and temperature conditions compatible with the process on laser bars, i.e. on structured semiconductor material, i.e. in the presence of metallization layers and dielectric coatings.

The process described above can alternatively be carried out in an Electron Cyclotron Resonance (ECR) type reactor operating under more severe basic vacuum conditions,

Base P <10 ^ (- ll) bar, with reduction of the damage associated with the physical component of the process, that is the ionic bombardment of the surface.

It can also be applied to semiconductor laser structures formed by different materials such as, for example, alloys of the inGaAsP type both for the direct replacement of alloys containing Aluminum and for the realization of laser structures emitting in the visible spectral region through the use of structures based on alloys, for example, of the InGaAlP type.

The encapsulating layer can also be formed by AlN (x), deposited under conditions of minimum stress, by means of a gradual adjustment of its nitrogen content, and by varying the hydrogen content present in the film.

The deposition of dielectric layers with high / low reflection optical functions is carried out on the laser bars obtained at the end of the process.

These depositions are indispensable both to further protect the laser mirrors from oxidation and to make the best use of the power emitted by the device which, intrinsically symmetrical, would radiate it equally from the two end facets.

For the antireflection layers, single λ / 4 layers of alumina are generally deposited, where an appropriate number (2-3) of alumina and silicon bi-layers are used for the high reflection.

Experimental Evidence:

The presence of oxides or contaminants on the end mirrors of the laser results, as widely described above, in an increase in the speed of surface recombination.

The photoluminescence (PL) diagnostic technique is capable of detecting the state of the surface, as the intensity of the PL peaks is inversely proportional to the speed of surface recombination.

This technique also allows to measure the temporal evolution of the PL signal, or to follow any surface degradation phenomena. Figure 1 shows the trend of the PL signal in the case of untreated samples according to the process object of this invention.

It can be noted that the PL signal coming from treated samples not only does not show degradation, but shows a progressive stabilization of the surface with a consequent increase in the PL signal.

Claims (9)

Claims: 1) Method for the operative cooling of the terminal mirrors of heterojunction semiconductor lasers, characterized by the fact of depositing on the surface of said terminal mirrors a material which, following an interdiffusion process, can locally form a metallurgical layer with a forbidden band of higher energy than the constituents of said laser. 2) Method according to claim 1), characterized in that said material consists of Aluminum. 3) Method according to claim 2), characterized in that a layer of Aluminum with a thickness included is deposited between 1 and 100 nm. 4) Method according to claim 1), characterized in that said interdiffusion process occurs by means of thermal treatment at a temperature between 350 and 450 C, and for a time between 10 and 1000 s. 5) Method according to claim 1), characterized in that it further comprises the preventive step of treating said laser mirrors in a strongly reducing environment in order to eliminate native oxides and surface contaminants. 6) Method according to claim 5), characterized in that said reducing environment comprises nascent hydrogen. 7) Method according to claim 6), characterized in that said reducing environment is generated by hydrogen plasma. 8) Method according to claim 6), characterized in that said reducing environment is suitable for the diffusion of hydrogen in the zone adjacent to said mirrors. 9) Method according to claim 5), characterized in that it further comprises the step of treating the laser mirrors in ammonia plasma, 10) Method according to claim 6) or 7), characterized by using a reactor of the PECVD type. 11) Method according to claim 10), characterized in that said reactor operates in the temperature range between 250 and 350 C, in the pressure range between 0.1 and 0.5 bar, in the rf power range between 5 and 20 W and for a duration between 10 and 1000 s. 12) Method according to claim 1), characterized in that it further comprises the step of physically coating said mirrors. 13) Method according to claim 12), characterized in that said coating is constituted by a layer of SiN (x). 14) Method according to claim 12), characterized by the fact that said coating is constituted by a layer of AlN (x), 15) Method according to claim 13) or 14), characterized in that said coating is deposited under conditions of minimum stress of the materials, by creating a compositional nitrogen gradient and by varying the hydrogen content of said layer . 16) Heterojunction semiconductor laser device characterized in that it has a metallurgical layer of AlGaAs in the area adjacent to the end mirrors. 17) Method according to claim 16), characterized in that it further comprises a layer comprising hydrogen. 18) Method according to claim 16), characterized in that it does not present native oxides and contaminants on the surface of the end mirrors. 19) Method according to claim 16), characterized by the fact of presenting a protective coating deposited on the end mirrors,