Patent Description:
High power semiconductor laser diodes have become important components in the technology of optical communication, particularly because such laser diodes can be used for fiber pumping (amplification of optical signals) and other high power applications. In most cases features such as long lifetime, reliable and stable output, high output power, high electro-optic efficiency, and high beam quality are generally desirable. One key for the long-term reliability of modern high-power laser diodes depends on the stability of the laser facets cleaved to form the opposing mirrors of the laser cavity.

The physical degradation of laser facets is a complex reaction that can be driven by light, current, and heat, resulting in power degradation and, in severe cases, to catastrophic optical damage (COD) of the mirror surfaces themselves. A process developed by IBM and referred to as "E2 passivation" has been used to address these concerns and minimize the possibility of COD. As described in IBM's <CIT>, the E2 process involves the deposition of a layer of silicon (or perhaps germanium or antimony) as a coating over the bare facet (mirror) surfaces. The presence of the coating functions as a passivation layer, preventing the diffusion of impurities capable of reacting with the mirror facet interface. <CIT> and <CIT> both disclose light emitting elements.

In particular, <CIT> discloses a nitride semiconductor laser device in which an aluminum oxynitride film having a thickness of <NUM> is formed as a first coat film on the face at the light emitting side, and an aluminum oxide film having a thickness of <NUM> is formed as a second coat film on the aluminum oxynitride film, with reflectivity of <NUM>%. On the other hand, an aluminum oxynitride film having a thickness of <NUM> is formed on the facet at the light reflecting side. An aluminum oxide film having a thickness of <NUM> is formed on the aluminum oxynitride film. A high reflection film with reflectivity of <NUM>% or higher is formed on the aluminum oxide film by stacking four pairs of a <NUM>-thick silicon oxide film and a <NUM>-thick titanium oxide film (stacked starting from the silicon oxide film) and thereafter forming a silicon oxide film having a thickness of <NUM> on the outermost surface. Each of the aforementioned films may be formed for example by ECR (Electron Cyclotron Resonance) sputtering, or may be formed by any other sputtering, EB (Electron Beam) evaporation, CVD (Chemical Vapor Deposition), or the like. Furthermore, a heating process may be performed after the above-noted films are formed on the facet at the light emitting side and the facet at the light reflecting side. Thus, removal of moisture contained in the above-noted film and improvement in film quality by the heating process can be expected. The heating process may be performed by heating with a heater, ultraviolet laser radiation, or the like.

Today's laser diodes are operated at relatively high powers and these prior art passivation layers, as deposited, have been found to break down and allow for damage of the mirror surfaces to occur. Therefore, in order to obtain stable mirrors for infrared high power laser diodes, it has now become a standard practice to "condition" the passivation layer. As performed today, conditioning is an extremely time-consuming process that requires operating the laser diode at a reduced current level for a prolonged period of time (e.g., tens to hundreds of hours) so as to form a crystalline structure inside the as-deposited amorphous passivation layer, forming a stable interface between the passivation layer and the mirror facet. Besides the time period required for this conditioning process, it is necessarily performed on a device-by-device basis, further extending the time and expense of the fabrication process.

The need to reduce the time required for laser facet conditioning is addressed by the present invention, which relates to laser devices and, more particularly, edge-emitting laser diodes having mirror facet passivation coatings that are conditioned using an "ex-situ" irradiation process in place of the conventional reduced current operation approach. In a first aspect of the present invention, a method of passivating facets of an edge-emitting laser diode is provided as recited bin accordance with claim <NUM>. Further aspects and preferred embodiments are set out in claim <NUM> et seq.

In accordance with the present invention, an external energy source is utilized to irradiate the material used as the facet passivation layer. The passivation layer should preferably be insulating (or low conducting). In particular, it may be formed using materials such as silicon, germanium or antimony. The irradiation process itself takes only seconds or a few minutes, compared to the extended hours of time required for the prior art "burn-in" conditioning process.

In accordance with the present invention, the external energy source comprises a laser source. The energy source may be operated in either CW or pulsed fashion, where the passivation layer is irradiated with an irradiation dose sufficient to condition the complete thickness of the layer of passivation material. This ex-situ conditioning treatment is applied to facets of the laser diode and is preferably performed while the devices are in bar form (i.e., before dicing). However, it is to be understood that the inventive ex-situ conditioning process may also be applied to individual devices after dicing, performing ex-situ conditioning of either individual unmounted dies or mounted dies (e.g., devices mounted on cards, carriers, or submounts).

An ex-situ method of passivating facets of an edge-emitting laser diode in accordance with one or more embodiments of the present invention includes the following steps: a) depositing, in a reaction chamber, one or more layers of passivation materials on bare facet surfaces of the edge-emitting laser diode to form a facet coating of a predetermined thickness; b) removing the laser diode from the reaction chamber; and, c) irradiating the facet coating with a beam from an external laser energy source for a period of time sufficient to condition the facet coating by forming a crystalline structure through the predetermined thickness; wherein the one or more passivation layers consist of materials selected from the group consisting of: silicon, germanium, antimony, as well as oxides of silicon germanium, antimony, and nitrides of germanium, antimony. In another method, the outer coating layers may be deposited over the passivation layers prior to performing the irradiation step (thus fully conditioning and providing stabilization of the combination of the passivation layer and coating layer).

Other and further embodiments and aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.

As will be described in detail below, the present invention is directed to the utilization of an ex-situ process to fully condition the passivation layer used as a coating on exposed facets of a laser diode. Ex-situ is used here to emphasize the difference between conditioning as formed in accordance with the principles of the present invention (i.e., conditioning provided by using an external energy source) and the prior art "in-situ" conditioning achieved by operation of the laser diode device itself (typically at a reduced current level for an extended period of time). For the purposes of the present invention, the phrase "fully condition" primarily means to condition the material comprising the passivation layer (e.g., silicon, germanium, antimony) through the complete thickness of the layer. To "fully condition" can also be described for the purposes of the present invention as providing an ex-situ stabilization of the complete facet overlay, including both the passivation layer and a standard coating layer overlying the passivation layer (as well as all interfaces therebetween, such as the passivation film-chip interface).

As will be discussed below, the ex-situ approach of the present invention allows for the conditioning to be performed on a bar of laser diodes (prior to dicing into individual devices) thus significantly improving the efficiency of the process over the prior art product-by-product approach. Additionally, the ex-situ process of the present invention takes only a matter of seconds or minutes to perform, depending on the size/area to be treated, not the hours required by the conventional device-operated conditioning approach.

Turning now to the figures, an exemplary laser diode is schematically illustrated in the plan view of <FIG>. The laser is formed in a semiconductor optoelectronic chip (or "bar") <NUM> having a front facet <NUM> and an opposing rear facet <NUM>. Bar <NUM> includes a vertical structure (not shown in detail) that is typically composed of layers of AlGaAs, GaAs, and related III-V semiconductor materials epitaxially deposited on a GaAs substrate. However, it is to be understood that other material combinations are possible within the scope of the present invention.

In the commercial production of these devices, a large number of such bars are simultaneously formed on a single GaAs wafer, with the wafer then cleaved along natural cleavage planes to form a large number of separate bars <NUM> having the front and rear facets <NUM>, <NUM>, as well as the perpendicularly-arranged sides <NUM>, <NUM>, as shown in <FIG>. The semiconductor processing performed on the wafer also forms a waveguide structure <NUM> extending between and perpendicular to facets <NUM>, <NUM>. While in most cases waveguide structure <NUM> is a ridge waveguide, other configurations are possible (e.g., a buried heterostructure waveguide, which may be preferred for high power applications). For many high power applications, waveguide structure <NUM> may have a width substantially larger than the lasing wavelength so as to form a broad-area laser.

As part of the fabrication process, cleaved facets <NUM>, <NUM> are subjected to the conventional E2 passivation process. That is, bars <NUM> are loaded into a reaction chamber and passivation material(s) are deposited to a predetermined thickness to provide a coating over mirror surfaces of facets <NUM> and <NUM>. The passivation materials need to be insulating (or low conducting), comprising silicon, germanium, or antimony, and may also comprise any oxide of these materials as well as nitrides of germanium or antimony. The as-deposited materials are shown as passivation layers <NUM>, <NUM> in <FIG>. It is at this point in the process that the ex-situ conditioning process of the present invention may be used.

In accordance with one or more embodiments of the present invention, conditioning of passivation layers <NUM>, <NUM> is provided by an external system <NUM>, as shown in <FIG>. External system <NUM> includes an energy source <NUM> for generating a beam <NUM>, which is typically in the visible range (e.g., <NUM>) but may also comprise a UV or IR beam. Energy source <NUM> may emit in either CW or pulsed mode. In the specific embodiment illustrated in <FIG>, beam <NUM> from energy source <NUM> subsequently passes through a focusing lens <NUM> and scans along a portion <NUM> of passivation layer <NUM> which overlies an active region of the laser diode bar <NUM>. Laser diode bar <NUM> may be mounted on a conventional sub-mount fixture <NUM> and moved with respect to the radiation from energy source <NUM> so that the focused beam is scanned across the lateral extent of passivation layer <NUM>. Energy source <NUM> comprises a radiation source capable of emitting radiation at an energy sufficient to create the desired homogeneous conditioning of the passivation material. In accordance with the present invention, energy source <NUM> comprises a laser source that creates a beam with an energy sufficient to condition passivation layer <NUM> through its complete thickness.

A spectrometer <NUM>, also shown in <FIG>, may be utilized to monitor the conditioning process. For example, scattered/redirected radiation from passivation layer <NUM> can be analyzed within spectrometer <NUM> using conventional means to determine the point in time when full conditioning has been achieved. Once the monitoring signal has leveled off, the external energy system may be de-activated.

It is to be understood that the same ex-situ radiation process can be used to fully condition passivation layer <NUM> along the opposite endface of the laser diode. Indeed, it is possible to configure a system where both facets are simultaneously conditioned. It has been found that the conditioning provided by this ex-situ irradiation process results in a homogeneous conditioning of the passivation materials through the complete thickness of the passivation layer. This is a clear advantage over the prior art process of activating the devices and performing the conditioning at a reduced power level, which has been found to result at times in a partial, inhomogeneous conditioning of the passivation materials.

As mentioned above, it is also possible to perform the conditioning process of the present invention after both the passivation layers and reflective coating layers have been applied over the laser facets. <FIG> illustrates an exemplary laser diode, similar in structure to the configuration shown in <FIG>, but in this case further processed to deposit a first coating layer <NUM> over passivation layer <NUM> and a second coating layer <NUM> over passivation layer <NUM>. In most cases, silicon nitride is used as coating layers <NUM>, <NUM>. Other suitable coating materials include, but are not limited to, silicon, germanium, gallium arsenide, silicon oxide, aluminum oxide, titanium oxide, aluminum nitride, and tantalum oxide.

Similar to the embodiment of <FIG>, energy source <NUM> is used to irradiate both coating layer <NUM> and underlying passivation layer <NUM> so as to fully condition and stabilize the laser diode structure. Under irradiation, the structures of both the coating and passivation layers change in a manner that stabilizes the device and results in creating the required high COD levels. In an example not forming part of the present invention, when silicon nitride is used as the coating material, the silicon nitride remains amorphous during irradiation (as opposed to crystallizing), but the atomic configurations in the nitride material does change. At the same time, this irradiation crystallizes the passivation layer and forms an interface between the passivation layer and the chip.

Thus, in accordance with this <FIG> embodiment of the present invention, the phrase "fully condition" means to structurally change the coating layer, crystallize the passivation layer, and create an interface between the laser chip and the passivation layer. The ex-situ conditioning process of the present invention can therefore be thought of as "stabilizing" the laser diode itself by virtue of the changes made to these layers.

The COD current of devices formed in accordance with the present invention has been compared against devices using the conventional burn-in process. It is recalled that "COD current" is defined as the current at which the laser facet experiences catastrophic optical damage. <FIG> illustrates the results of this comparison. In particular, <FIG> contains a set of plots I showing the COD power as a function of current for devices that have been subjected only to the conventional E2 process (without any post-process conditioning). Plots II are associated with devices created using the same prior art E2 process, followed by the conventional "in-situ" conditioning process of operating the devices at low current/power levels. Clearly, the performance of these conditioned devices exceeds those in the first group, with much higher COD levels. Plots III are associated with devices formed in accordance with the present invention; that is, using an ex-situ conditioning process to provide full conditioning of the passivation layers. In particular, the results shown in <FIG> were obtained from devices formed in accordance with the embodiment discussed above in association with <FIG>, where the ex-situ conditioning process was formed to stabilize both the coating and passivation layers.

It is observed that the devices formed in accordance with the present invention exhibit a somewhat higher level of COD than those of the prior art. While this is clearly one goal of the present invention, the fact that full conditioning can be performed on the complete laser bar (instead of at the individual device level) is also significant and a great improvement over the prior art. Moreover, the inventive ex-situ condition process is orders of magnitude more efficient than the standard burn-in process, able to fully condition/stabilize the structure in a matter of seconds or minutes, in comparison to the tens to hundreds of hours required for low-current level burn-in.

Summarizing, the process of the present invention has been found to homogeneously and fully condition the standard E2 passivation layer (as well as the overlying coating layer when present), eliminating the vertical and lateral conditioning inhomogeneity as found in the prior art. The inventive process is found to maximize the current level at which mirror damage occurs (i.e., the COD current/optical power) without burn-in. This eliminates the prior art's need to perform chip training by chip operation. The distribution of COD current within a production lot has also been found to be reduced.

Moreover, as mentioned above, it is possible to perform ex-situ full conditioning of laser facets at the bar level (i.e., before chip separation). This allows for the full conditioning of a large number of bars in a short period of time, as preferred for mass production situations. Indeed, the inventive approach also eliminates the need for a customer to perform any conditioning steps on the devices, as was the case in certain situations in the past.

Claim 1:
A method of passivating facets of an edge-emitting laser diode comprising:
a) depositing, in a reaction chamber, one or more passivation layers (<NUM>, <NUM>) on bare facet surfaces (<NUM>, <NUM>) of the edge-emitting laser diode to form a facet coating of a predetermined thickness;
b) removing the edge-emitting laser diode from the reaction chamber; and
c) irradiating the facet coating with an external laser energy source (<NUM>) for a period of time sufficient to condition the facet coating by forming a crystalline structure through the predetermined thickness;
wherein the one or more passivation layers (<NUM>, <NUM>) consist of materials selected from the group consisting of: silicon, germanium, antimony, as well as oxides of silicon, germanium, antimony, and nitrides of germanium, antimony.