Surface emitting laser, method for manufacturing surface emitting laser, and image forming apparatus

A surface emitting laser includes a lower multilayer mirror and an upper multilayer mirror which are provided on a substrate. A first oxidizable layer is partially oxidized to form a first current confinement layer including a first conductive region and a first insulating region. A second oxidizable layer is partially oxidized to form a second current confinement layer including a second conductive region and a second insulating region, a boundary between the first conductive region and the first insulating region being disposed inside the second current confinement layer in an in-plane direction of the substrate. The first oxidizable layer and the second oxidizable layer or layers adjacent to the respective oxidizable layers are adjusted so that when both layers are oxidized under the same oxidizing conditions, the oxidation rate of the first oxidizable layer is lower than that of the second oxidizable layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface emitting laser, a method for manufacturing a surface emitting laser, and an image forming apparatus.

2. Description of the Related Art

A vertical cavity surface emitting laser (VCSEL) which is one of surface emitting lasers is capable of extracting light in a direction vertical to a surface of a substrate. Therefore, a two-dimensional array can be easily formed.

High density and high speed can be realized by parallel processing with a plurality of beams emitted from the two-dimensional array, and various industrial applications such as optical communication and the like are expected. For example, when a surface emitting laser array is used as an exposure light source of an electrophotographic printer, an image forming process with a plurality of beams can be improved in density and speed.

Electrophotographic applications of the type described above require the formation of stable and micro laser spots on a photosensitive drum. Accordingly, stable operations in a single transverse mode and a single longitudinal mode are required laser characteristics of a VCSEL.

In surface emitting lasers, in order to improve performance, there has been developed a method for injecting a current only in a necessary region by forming a current confinement structure using a selective oxidation technique.

This method for forming a current confinement structure includes providing an AlGaAs layer (for example, Al0.98Ga0.02As) having a high Al composition ratio in a multilayer mirror and selectively oxidizing the layer in a high-temperature steam atmosphere to form the current confinement structure. Since an oxidized region is converted from a conductive region to an insulating region, a current can be injected into a desired portion of an active layer region.

In order to achieve high output in a selective oxidation-type VCSEL, it is necessary to increase the diameter of an aperture serving as a conductive region of a current confinement structure. However, in a distribution of current carriers, the carriers are concentrated in the edge portion of the aperture, which is a boundary between a conductive region and an insulating region. Therefore, when the diameter of the aperture is increased, high-order transverse mode oscillation with light intensity highly distributed to the edge portion is easily generated.

In an attempt to solve the problem of carrier concentration at the edge portion of the aperture, a method of using two current confinement structures is disclosed by H. J. Unold et al., in “Large-Area Single-Mode Selectively Oxidized VCSELs: Approaches and Experimental”, Proceedings of SPIE Photon, West, Vol. 3946 (2000), pp. 207-218, (hereafter “the Unold document”). To illustrate the method of using two current confinement structures, FIG. 10(b) of the Unold document is shown inFIG. 9herein.

In the method using two current confinement structures, as illustrated inFIG. 9, a current confinement structure930having a smaller aperture diameter than a current confinement structure920disposed near an active layer910is disposed further away from the active layer910than the current confinement structure920. As a result, charge carriers are concentrated in a central portion of the aperture in the current confinement structure920provided nearer to the active layer910. Since the current confinement structure920provided nearer to the active layer910controls the mode of resonant light, the efficiency of coupling of the carriers and fundamental mode light is enhanced by injecting the carriers in the central portion of the aperture. Therefore, the use of two current confinement structures can suppress high-order mode oscillation and form high-output surface emitting lasers as compared with use of one current confinement structure.

In order to achieve a single transverse mode, effective coupling of carriers and the fundamental mode light is required. Therefore, in the technique of providing two current confinement structures as described in the Unold document, it is necessary that the aperture diameter of the current confinement structure away from the active layer is smaller than that of the current confinement structure near the active layer.

For example, when the aperture diameter of the current confinement structure920disposed near the active layer910is 6 to 7 μm, the aperture diameter of the current confinement structure930disposed away from the active layer910is about a half, i.e., about 3 to 4 μm.

In addition, U.S. Pat. No. 5,493,577 (hereafter “the '577 patent) describes that two oxidized regions constituting two current confinement layers may be the same or different (see, e.g., column 15). The '577 patent also describes that different oxidized regions can be formed by controlling an Al composition; and that different oxidized regions can be formed by using a stepped mesa.

As described above, the Unold document discloses that two current confinement structures are provided, and the aperture diameter of the current confinement structure away from the active layer is smaller than that of the current confinement structure near the active layer.

The '577 patent describes that different oxidized regions can be formed by controlling an Al composition ratio of an oxidizable layer which becomes the current confinement structure. Since the oxidation rate increases as the Al composition ratio of a semiconductor increases, when semiconductor layers having different Al composition ratios are oxidized for the same time, a semiconductor layer having a higher Al composition ratio has a smaller aperture diameter than a semiconductor layer having a lower Al composition ratio.

However, the inventors of the present invention confirmed that from the viewpoint of reliability of a device, a problem is present in a surface emitting laser having a plurality of current confinement structures formed by controlling the Al composition ratio of an upper oxidizable layer to be higher than that of a lower oxidizable layer.

SUMMARY OF THE INVENTION

The present invention provides a surface emitting laser having secured device reliability and including a plurality of current confinement structures, a method for manufacturing the surface emitting laser, and an image forming apparatus including a surface emitting laser array in which a plurality of the surface emitting lasers are arranged.

A surface emitting laser according to an embodiment of the present invention includes a lower multilayer mirror, an active layer, and an upper multilayer mirror which are provided in that order on a substrate; a first current confinement layer provided in the upper multilayer mirror or between the upper multilayer mirror and the active layer and including a first insulating region and a first conductive region which are formed by partially oxidizing a first oxidizable layer; and a second current confinement layer provided at a smaller distance from the active layer than that between the active layer and the first current confinement layer and including a second insulating region and a second conductive region which are formed by partially oxidizing a second oxidizable layer. In the surface emitting laser, a boundary between the first conductive region and the first insulating region is disposed in the second conductive region in an in-plane direction of the substrate, and any one of the following conditions (1) to (3) is satisfied.

(1) The first oxidizable layer and the second oxidizable layer contain Al, and the first oxidizable layer has a lower Al composition ratio than that of the second oxidizable layer.

(2) The first oxidizable layer is thinner than the second oxidizable layer.

(3) One of the layers adjacent to the first oxidizable layer has a higher Al composition than that of one of the layers adjacent to the second oxidizable layer.

A method for manufacturing a surface emitting laser according to an embodiment of the present invention is a method for manufacturing a surface emitting laser including a laminate of a lower multilayer mirror, an active layer, and an upper multilayer mirror which are provided in that order on a substrate. The method includes a step of forming a second oxidizable layer in the laminate, a step of forming a first oxidizable layer above the second oxidizable layer in the laminate, a step of partially oxidizing the first oxidizable layer to form a first current confinement layer including a first conductive region and a first insulating region, and a step of partially oxidizing the second oxidizable layer to form a second current confinement layer including a second conductive region and a second insulating region after the step of forming the first current confinement layer, a boundary between the first conductive region and the first insulating region being disposed inside the second current confinement layer in an in-plane direction of the substrate. The first oxidizable layer and the second oxidizable layer or a layer adjacent to the first oxidizable layer and a layer adjacent to the second oxidizable layer are adjusted so that when both layers are oxidized under the same conditions, the oxidation rate of the first oxidizable layer is lower than that of the second oxidizable layer.

According to the present invention, it is possible to provide a surface emitting laser having secured device reliability and including a plurality of current confinement structures, a method for manufacturing the surface emitting laser, and an image forming apparatus including a surface emitting laser array in which a plurality of the surface emitting lasers are arranged.

DESCRIPTION OF THE EMBODIMENTS

As described above, it is necessary that the Al composition ratio of an oxidizable layer for forming a structure having a small aperture diameter is higher than that of an oxidizable layer for forming a structure having a large aperture diameter.

However, in a method of controlling a carrier distribution by a plurality of oxide current confinement structures, a current density increases as the aperture diameter decreases. In particular, the current density increases in a boundary (oxidation front) between a conductive region and an insulating region.

On the other hand, at the oxidation front of a current confinement structure, volumetric contraction is produced by oxidizing an oxidizable layer, and residual stress is concentrated accompanying this volumetric contraction. The stress increases as the Al composition in the oxidizable layer increases.

That is, a high current density and high stress are applied to a current confinement structure disposed away from an active layer, thereby causing deterioration during turning on electricity. There is thus the problem of decreasing reliability of a device.

In addition, in order to form a small aperture diameter, besides control of the Al composition ratio, it is considered to increase the oxidation rate by increasing the thickness of the oxidizable layer. However, when the thickness of the oxidizable layer is increased, residual stress is concentrated due to volumetric contraction after oxidation.

On the other hand, in order to form a small aperture diameter, it is considered to increase the oxidation rate of the oxidizable layer by decreasing the Al composition ratio of a layer adjacent to the oxidizable layer. However, when the Al composition ratio of a layer adjacent to the oxidizable layer is decreased, residual stress in the oxidizable layer after oxidation cannot be relieved, and thus the stress remains.

As described above, the inventors found that when the oxidation rate of the oxidizable layer is increased for decreasing the aperture diameter of a current confinement structure disposed away from the active layer, the reliability of a device is degraded.

A surface emitting laser, a method for manufacturing the surface emitting laser, and the like which are intended for resolving the above-mentioned problems are described in detail below.

First Embodiment

(Configuration of Surface Emitting Laser)

FIG. 1Ais a drawing schematically illustrating a sectional view (along line IA-IA ofFIG. 1B) of a surface emitting laser100according to an embodiment of the present invention.FIG. 1Bis a drawing schematically illustrating a top view of a surface emitting laser100according to an embodiment of the present invention.

InFIG. 1A, a lower multilayer mirror120, a lower spacer layer130, an active layer140, an upper spacer layer150, and an upper multilayer mirror160are formed in that order on a substrate110, forming a laser resonator in a direction vertical to a surface of the substrate110. When carriers are injected into the active layer140from an upper electrode170provided on the upper multilayer mirror160and a lower electrode180provided below the substrate110, the active layer140emits light, resulting in oscillation of the surface emitting laser100.

A first current confinement layer210is provided in the upper multilayer mirror160. The first current confinement layer210has a first insulating region214and a first conductive region212. The first insulating region214and the first conductive region212are formed by partially oxidizing a first oxidizable layer250. The first current confinement layer210has the function of injecting current in a central portion of the active layer140.

A second current confinement layer220is provided at a position nearer to the active layer140than the first current confinement layer210. For example, the second current confinement layer220is provided between the first current confinement layer210and the active layer140. The second current confinement layer220has a second insulating region224and a second conductive region222that are formed by partially oxidizing a second oxidizable layer260. The second confinement layer220has the function to control a mode of resonant light.

Although, inFIG. 1A, the first current confinement layer210and the second current confinement layer220are provided above the active layer140, the second current confinement layer220may be provided below the active layer140. In addition, inFIG. 1A, the first current confinement layer210is provided in a layer of the upper multilayer mirror160, but may not be necessarily provided in the upper multilayer mirror160. Moreover, it should be noted that the denomination of “first” and “second” layers is used in this specification for purposes of convenience only. As long as at least two confinement layers exist, it does not matter the order in which the layers are denominated.

A trench structure240is provided to extend from the upper surface of the upper multilayer mirror160to at least the upper surface of the second current confinement layer220, passing through the first current confinement layer210. Therefore, a semiconductor layer which faces the trench structure240is oxidized from the side wall to form the first insulating region214and the second insulating region224. The length (or depth) of trench structure240extends from the upper surface of the upper multilayer mirror160and may be stopped at a center of the second current confinement layer220or may pass through the second current confinement layer220.

When the surface emitting laser100is viewed from above, as shown inFIG. 1B, the first conductive region212has a smaller size than the second conductive region222, and the boundary between the first conductive region212and the first insulating region214is found inside the second conductive region222.

This state may be expressed as “the boundary between the first conductive region212and the first insulating region214is present inside the second conductive region222in an in-plane direction of the substrate”.

The presence of the first conductive region212increases coupling of a fundamental mode light intensity distribution and a current distribution as shown inFIG. 8.FIG. 8is a plot of the light intensity distribution of the fundamental mode of a surface emitting laser, and the current density distributions near an active layer. The horizontal axis ofFIG. 8represents the distance from the center of the optical axis in the in-plane direction of a substrate. Therefore, the surface emitting laser100can be operated in a single transverse mode within a wide range of driving currents.

In addition, the magnitude of residual stress produced in the boundary between the first conductive region212and the first insulating region214in a step of forming both regions is controlled to be smaller than that produced in the boundary between the second conductive region222and the second insulating region224in a step of forming both regions. As a result, the reliability of the device is improved as compared with the case in which such control is not performed.

In order to control the magnitude of residual stress as described above, the first oxidizable layer250and the second oxidizable layer260are controlled so that under the same oxidizing conditions, the oxidation rate of the first oxidizable layer250is lower than that of the second oxidizable layer260. Alternatively, a layer adjacent to the first oxidizable layer250and a layer adjacent to the second oxidizable layer260are controlled so that under the same conditions, the oxidation rate of the first oxidizable layer250is lower than that of the second oxidizable layer260.

The oxidation rate of an oxidizable layer decreases as the Al composition ratio of the oxidizable layer decreases, the thickness of the oxidizable layer decreases, or the Al composition ratio of the periphery of the oxidizable layer increases. On the other hand, the residual stress in a boundary between an oxidized region and an unoxidized region after oxidation of the oxidizable layer decreases as the Al composition ratio of the oxidizable layer decreases, the thickness of the oxidizable layer decreases, or the Al composition ratio of the periphery of the oxidizable layer increases. In addition, the residual stress in the boundary between an oxidized region and an unoxidized region after oxidation of the oxidizable layer decreases as the oxidation rate of the oxidizable layer decreases.

(Method for Manufacturing Surface Emitting Laser)

In a layer structure in which under the same conditions, the oxidation rate of the first oxidizable layer is lower than that of the second oxidizable layer, when the two oxidizable layers are simultaneously oxidized, the second conductive region222becomes smaller than the first conductive region212. In this case, effective single transverse mode characteristics cannot be achieved.

Therefore, this embodiment uses the manufacturing method including forming the first current confinement structure by oxidizing the first oxidizable layer, and then forming the second current confinement structure by oxidizing the second oxidizable layer. Consequently, the sizes of the first conductive region212and the second conductive region222can be determined independently of the oxidization rates of the oxidizable layers.

Next, the specified configuration of the surface emitting laser including a substrate and a laminate stacked on the substrate according to the embodiment and the manufacturing method therefor are described.

The substrate110is, for example, an n-type doped GaAs substrate. The laminate including a lower multilayer mirror120, an active layer140, and an upper multilayer mirror160provided on (stacked) the substrate110can be formed in stages using known fabrication methods. The n-type lower multilayer mirror120is formed by alternately laminating Al0.9Ga0.1As with an optical thickness of λ/4 and Al0.5Ga0.5As with an optical thickness of λ/4 on the substrate110. For example, the number of pairs is 70. In addition, λ is the resonant wavelength of a resonator and is, for example, 680 nm in terms of vacuum wavelength.

For example, the AlGaInP-based active layer140, specifically, the active layer140having a GaInP/AlGaInP multiquantum well structure having an emission peak at λ=680 nm, is formed by crystal growth on the lower multilayer mirror120. In addition, the lower spacer layer130and the upper spacer layer150are formed, for phase adjustment of the resonator, below and above the active layer140, respectively. The total optical thickness of the lower spacer layer130, the active layer140, and the upper spacer layer150is an integral multiple of λ/2, for example, λ.

The p-type upper multilayer mirror160is grown on the active layer140. The upper multilayer mirror160is composed of, for example, repetitions of 40 pairs of Al0.9Ga0.1As with an optical thickness of λ/4 and Al0.5Ga0.5As with an optical thickness of λ/4. A portion of the upper multilayer mirror160is replaced with an AlGaAs layer having a higher Al composition ratio than those of AlGaAs constituting the pairs of the upper multilayer mirror160, providing an oxidizable layer. The oxidizable layer is partially oxidized to form a current confinement layer including an oxidized insulating region and a unoxidized conductive region. For example, when oxidizable layers are formed at two positions in the upper multilayer mirror160, the second oxidizable layer260and the first oxidizable layer250are disposed at the positions of the 1st pair and the 6th pair, respectively, in the upper multilayer mirror160from the active layer140. The present invention is not limited to this form, but may have another form in which, for example, the first oxidizable layer250is disposed in the upper multilayer mirror160, and the second oxidizable layer260is disposed in the lower multilayer mirror120.

As the oxidizable layers, for example, an AlGaAs layer can be used. For example, AlxGa1-xAs (0.95≦x≦1) is easily oxidized to an insulator containing an Al oxide by, for example, heating to 300° C. or more and exposure to steam.

In addition, a semiconductor contact layer is formed as a top layer of the upper multilayer mirror160in order to make contact with the upper electrode170. The semiconductor contact layer is, for example, a GaAs layer of 20 nm.

Crystal growth of the above-described laminate is performed by, for example, a MOCVD (Metal-Organic Chemical Vapor Deposition) method.

(1) Al Composition Ratio of an Oxidizable Layer

When the Al composition ratio of the first oxidizable layer250is decreased to be lower than that of the second oxidizable layer260, residual stress can be decreased. For example, Al0.98Ga0.02As can be used for the first oxidizable layer250, and AlAs can be used for the second oxidizable layer260.

When these layers are oxidized, specifically, the rates of volume changes of AlAs and Al0.98Ga0.02As are −11% and −2%, respectively. With a lower Al composition ratio, the volume reduction due to oxidation is small, thereby decreasing the residual stress applied to the boundary between the oxidized region and the unoxidized region.

With respect to the oxidation rate, when a layer of Al0.5Ga0.5As is adjacent to AlAs and Al0.98Ga0.02As each having a thickness of 30 nm, for example, the oxidation rate ratio is about 2 times in oxidation at 440° C. and about 30 times in oxidation at 370° C.

According to the manufacturing method described below, the first oxidizable layer can be little oxidized by oxidation of the second oxidizable layer. Therefore, from this viewpoint, the difference in oxidation rate between the first oxidizable layer and the second oxidizable layer is opposite to that of related art. Therefore, as described above, the Al composition ratio of the first oxidizable layer250is lower than that of the second oxidizable layer260. The difference between the Al composition ratios is preferably 1% or more and more preferably 2% or more. When the Al composition ratios of the first oxidizable layer250and the second oxidizable layer260are excessively decreased, the AlGaAs layer of the multilayer reflecting mirror is also oxidized, thereby decreasing the mechanical strength of the device. Therefore, the Al composition ratio of the second oxidizable layer260is preferably 98% or more, more preferably 99% or more, and most preferably 100%.

(2) Thickness of an Oxidizable Layer

As the thickness of an oxidizable layer is decreased, the amount of volume reduction by oxidation can be decreased, thereby decreasing the residual stress. Therefore, the first oxidizable layer is thinner than the second oxidizable layer. For example, the thickness of the first oxidizable layer250is 20 nm, and the thickness of the second oxidizable layer is 30 nm.

As described above, for the convenience of the manufacturing method according to the present invention, the oxidation rate of the first oxidizable layer250is different from that of the second oxidizable layer260.

The difference in oxidation rate due to the thickness of a layer becomes remarkable as the thickness decreases. Therefore, the first oxidizable layer may be somewhat thin. The thickness of the first oxidizable layer is preferably 30 nm or less and more preferably 20 nm or less.

In addition, the dependency of the oxidation rate on the thickness increases as the oxidation temperature decreases. For example, oxidation of AlAs with a thickness of 60 nm and AlAs with a thickness of 30 nm at 440° C. causes substantially no difference in oxidation rate. However, oxidation of AlAs with a thickness of 60 nm and AlAs with a thickness of 30 nm at 370° C. causes a difference of about 2 times in oxidation rate. Therefore, the second oxidizable layer is oxidized at as low temperature as possible, and thus the Al composition ratio of the second oxidizable layer is set to be higher than that of the first oxidizable layer. Therefore, a more desirable design is achieved by combining (1) and (2).

(3) Al Composition Ratio of an Adjacent Layer

When a layer adjacent to an oxidizable layer has such an Al composition ratio (e.g., 0.9 or less) that it is not oxidized, the residual stress applied to a boundary of an oxidized region of the oxidizable layer decreases as the Al composition ratio of the adjacent layer increases. This is because the adjacent layer having a higher Al composition ratio can relieve the stress on the oxidized oxidizable layer due to a difference in surface tension as compared with the adjacent layer having a lower Al composition ratio. Therefore, the layer adjacent to the first oxidizable layer250desirably has a higher Al composition ratio than that of the layer adjacent to the second oxidizable layer260. In addition, according to the manufacturing method described below, the first oxidizable layer250may be little oxidized by oxidation of the second oxidizable layer260. Therefore, also from this viewpoint, the layer adjacent to the first oxidizable layer250desirably has a higher Al composition ratio than that of the layer adjacent to the second oxidizable layer260. When the Al compositions of the adjacent layers are adjusted as described above, the oxidation rate of the first oxidizable layer250becomes lower than that of the second oxidizable layer260.

As the Al composition ratio of an oxidizable layer increases, the difference between the oxidation rates of the oxidizable layers due to the difference in Al composition ratio between the layers adjacent to the respective oxidizable layers becomes remarkable. Also, as the oxidation temperature decreases, the difference becomes remarkable.

When a layer adjacent to an oxidizable layer has such a high Al composition ratio that it is not oxidized, the adjacent layer has the effect of decreasing electric resistance (hetero barrier) at the interface between an unoxidized region of the oxidizable layer and the adjacent layer. Therefore, a layer having a high Al composition ratio can be formed as the layer adjacent to the first oxidizable layer250in which the first current confinement structure with the highest current density is formed. For example, when the first oxidizable layer250is composed of Al0.98Ga0.02As, a graded layer in which the composition is graded from Al0.9Ga0.1As to Al0.5Ga0.5As from the oxidizable layer side can be used as the layer adjacent to the first oxidizable layer250. The graded layer can be used from the viewpoint of decreasing the electric resistance at the interface of the oxidizable layer.

Then, a semiconductor process shown inFIGS. 2A to 2F,3G to3L, and4is performed for a wafer subjected to the above-described growth.

The process described below is a process of performing oxidation of the first oxidizable layer250and oxidation of the second oxidizable layer260with different timings. Specifically, the oxidation of the second oxidizable layer260is performed after the oxidation of the first oxidizable layer250.

First, as shown inFIG. 2A, a first dielectric layer300is deposited as a protective layer on the upper multilayer mirror160. The first dielectric layer300is composed of, for example, SiO2having a thickness of 1 μm and can be deposited by, for example, a plasma CVD method.

Then, photoresist310is applied to the first dielectric layer300and then subjected to patterning and development so that an aperture pattern is formed at the position corresponding to the trench structure240.

The trench structure240is has an inner diameter of, for example, 27 μm. The outer diameter of the trench structure240is, for example, 33 μm. The trench structure240is not shown inFIG. 2A.

Next, as shown inFIG. 2B, the first dielectric layer300is etched using the patterned resist310as a mask. The etching may be, for example, wet etching with BHF (buffered hydrofluoric acid) or dry etching with a plasma of CHF3gas.

Then, as shown inFIG. 2C, semiconductor layers are dry-etched using the resist310and the first dielectric layer300as a mask to form the trench structure240. In this case, the etching is performed so that the trench structure240reaches at least the top surface of the first oxidizable layer250, but not reach the top surface of the second oxidizable layer260. The dry etching is performed with, for example, a plasma of SiCl4gas and Ar gas.

Next, as shown inFIG. 2D, the resist310remaining on the first dielectric layer300is removed by, for example, asking with an oxygen plasma.

Next, as shown inFIG. 2E, the first oxidizable layer250is oxidized from the side wall exposed in the trench structure240to form the first insulating region214. The oxidation is performed by, for example, heating the substrate to 450° C. and exposing it to stream. An oxidized portion of the oxidizable layer becomes an insulator composed of polycrystal or amorphous Al oxide as a main component. In addition, an unoxidized region, i.e., the first conductive region212, remains in the central portion of the first oxidizable layer250. The first conductive region212has, for example, a circular shape having a diameter of 4 μm.

Next, as shown inFIG. 2F, semiconductor layers are etched from the bottom of the trench structure240using the first dielectric layer300as a mask to expose the second oxidizable layer260. The etching is dry etching with, for example, a plasma of SiCl4gas and Ar gas. The etching may be wet etching.

For example, in order to improve heat radiation, an AlAs layer may be used as a low-refractive-index layer in the lower multilayer mirror120. In this case, etching of the trench structure240is stopped so as not to expose AlAs of the lower multilayer mirror120.

Next, as shown inFIG. 3G, the second oxidizable layer260is oxidized from the side wall exposed in the trench structure240to form the second insulating region224. The oxidation is performed by, for example, heating the substrate to 400° C. and exposing it to stream.

Since the surface of the first oxidizable layer250has already been oxidized, in many cases, oxidation of the first oxidizable layer250little proceeds even when the substrate is heated and exposed to stream in this step.

However, in order to minimize the oxidation of the first oxidizable layer250in this step, the heating temperature of the substrate in the oxidation of the second oxidizable layer260is as lower as possible than that in the oxidation of at least the first oxidizable layer250. Therefore, the Al composition ratio of the second oxidizable layer260is set to be higher than that of the first oxidizable layer250or the thickness of the second oxidizable layer260is set to be larger than that of the first oxidizable layer250. Of course, both the Al composition ratio and the thickness may be controlled. In addition, when the Al composition ratio of one of the semiconductor layers adjacent to the first oxidizable layer250is set to be higher than that of one of the semiconductor layers adjacent to the second oxidizable layer260, oxidation of the first oxidizable layer250can be further suppressed.

Further, the unoxidized region, i.e., the second conductive region222, is left in the central portion of the second oxidizable layer260. The second conductive region222has, for example, a circular shape having a diameter of 6 μm, a square shape having a side of 6 μm, or a shape intermediate between the circular and square shapes. In this case, the distance between the boundary (oxidation front) between the second conductive region222and insulating region and the inner periphery of the trench structure240is 10.5 μm. That is, the oxidation distance is 10.5 μm.

Next, as shown inFIG. 3H, the remaining first dielectric layer300has been removed using, for example, buffered hydrofluoric acid.

Next, as shown inFIG. 3I, an insulating film190is deposited over the whole device. The insulating film190includes, for example, a dielectric layer with an optical thickness of λ/2 and composed of, for example, silicon oxide, and can be deposited by, for example, plasma CVD.

Next, as shown inFIG. 3J, a photoresist330is applied and subjected to patterning exposure and development for partially removing the insulting film190in order to make contact between the semiconductor contact layer and the upper electrode170which is formed latter.

Next, as shown inFIG. 3K, the insulating film190is partially removed using the resist330as a mask. Then, the resist330is removed.

Next, as shown inFIG. 3L, a photoresist340for liftoff is applied and subjected to patterning for forming the upper electrode170by a liftoff method, forming a resist pattern for liftoff.

Next, as shown inFIG. 4, the upper electrode170is formed by, for example, electron beam evaporation or the like and the liftoff method. The upper electrode170is composed of, for example, Ti/Au.

The upper electrode170has, for example, a ring shape having an opening at the center thereof. The opening is larger than the first conductive region212so as not to interfere with leaser emitted light therethrough. The size of the opening in the upper electrode170may be larger or smaller than the second conductive region222. When the size of the opening is smaller than the second conductive region222, the upper electrode170overlaps a portion of the periphery of an emission region. In a higher-order mode than the fundamental mode, the overlap between the light intensity distribution and the upper electrode170is increased. Therefore, when the upper electrode170causes a light loss such as a scattering effect or the like in each of the modes, the upper electrode170has the effect of further suppressing a higher-order mode.

Finally, the lower electrode180is formed on the back of the substrate by, for example, resistance heating evaporation. The lower electrode180is composed of, for example, AuGe/Au.

In the above-describe process, oxidation of the first oxidizable layer250and oxidation of the second oxidizable layer260are performed with different timings.

The second oxidizable layer260is oxidized under conditions (second oxidation conditions) different from conditions (first oxidation conditions) for oxidizing the first oxidizable layer250. Specifically, the second oxidizable layer260is oxidized under such conditions that the oxidation rate is lower than that of oxidation of the first oxidizable layer250. As a result, oxidation of the first oxidizable layer250can be prevented from proceeding in the step of oxidizing the second oxidizable layer260.

As these conditions, for example, the substrate temperature of the second oxidation conditions can be made lower than the substrate temperature of the first oxidation conditions. In the second oxidation conditions, the oxidizer concentration in the atmosphere can be made lower than that in the first oxidation conditions. As the oxidizer, for example, H2O can be used. In addition, in the second oxidation conditions, the oxidation inhibitor concentration in the atmosphere can be made higher than that in the first oxidation conditions. As the oxidation inhibitor, for example, O2can be used. The reaction rate in the step of oxidizing AlGaAs can be decreased by increasing the amount of O2.

Second Embodiment

In the first embodiment, the second oxidizable layer is not exposed in the trench structure when the first oxidizable layer is oxidized. The second oxidizable layer is exposed by further etching the bottom of the trench structure after the first oxidizable layer is oxidized.

However, in the second embodiment, as shown inFIG. 5, another trench structure245can be newly provided from the bottom of a trench structure240.

The trench structure240first formed is referred to as the “first trench structure” and the trench structure245newly formed from the bottom of the trench structure240is referred to as the “second trench structure”.

In this embodiment, a protective layer can be formed on the sidewall of the exposed first oxidizable layer in the first embodiment. Therefore, the second embodiment has the specific effect of further suppressing oxidation of the first oxidizable layer250during oxidation of the second oxidizable layer260.

FIGS. 6A to 6Dshow exemplary steps of a manufacturing method of the second embodiment. The manufacturing method of the second embodiment is the same as the manufacturing method described in the first embodiment up to the step shown inFIG. 2E, and thus description up to the step shown inFIG. 2Eis omitted.

As shown inFIG. 6A, a protective layer320is deposited on the bottom and side of the first trench structure240and on the remaining dielectric layer300. The protective layer320is composed of, for example, silicon oxide and deposited by a plasma CVD method. The protective layer320protects the sidewall of the first oxidizable layer when the second oxidizable layer is oxidized.

Next, a resist is applied and subjected to patterning for forming the second trench structure245. Specifically, a pattern having an opening at the bottom of the first trench structure240is formed. Then, as shown inFIG. 6B, a portion of the protective layer320at the bottom of the first trench structure is removed by etching using the resist pattern as a mask.

Next, as shown inFIG. 6C, the second trench structure245is formed by etching the semiconductor layers using the resist and the protective layer320as a mask. The second trench structure245is formed by etching up to at least the top of the second oxidizable layer260. The second trench structure245may be stopped at an intermediate position of the second oxidizable layer260or may pass through the second oxidizable layer260.

The etching is, for example, dry etching with SiCl4+Ar plasma. Wet etching may be performed.

Next, when the resist remains, the resist is removed, and then, as shown inFIG. 6D, the second oxidizable layer260is oxidized from the sidewall exposed in the second trench structure245to form the second insulating region224. The oxidation is performed by, for example, heating the substrate to 400° C. and exposing it to stream.

Then, the protective film320is removed using, for example, buffered hydrofluoric acid or the like. Subsequent steps are the same as the manufacturing process of the first embodiment, specifically the step shown inFIG. 3Iand the subsequent steps, and are thus not described.

Third Embodiment

An image forming apparatus using a surface emitting laser array in which a plurality of surface emitting lasers as described in the first or second embodiment are arranged is described.

FIGS. 7A and 7Bare structural drawings showing an electrophotographic recording-type image forming apparatus provided with a surface emitting laser array according to the present invention.FIG. 7Ais a plan view of the image forming apparatus, andFIG. 7Bis a side view of the same apparatus. A surface emitting laser array514serves as a recording light source and is configured to be turned on or off according to an image signal from a laser driver (not shown).

A modulated laser beam is applied to a polygon mirror510from the surface emitting laser array514through a collimator lens520. The polygon mirror510is rotated in an arrow direction by a motor512so that the laser beam output from the surface emitting laser array514is reflected as a deflected beam while the emission angle is continuously changed by the reflection surfaces with rotation of the polygon mirror510. The reflected light is subjected to correction of distortion and the like with a f-θ lens522, applied to a photosensitive drum500through a reflecting mirror516, and scan on the photosensitive drum (photosensitive member)500in a main scanning direction. At this time, an image corresponding to a plurality lines of the surface emitting laser array514is formed in the main scanning direction of the photosensitive drum500by reflection of the beam through one surface of the polygon mirror510. In this embodiment, the surface emitting laser array514of 4×8 is used so that an image of 32 lines is formed.

The photosensitive drum500is previously electrically charged by a charger502and successively exposed to light by scanning laser beams to form an electrostatic latent image.

The photosensitive drum500is rotated in an arrow direction (circular arrow inFIG. 7B) so that the formed electrostatic latent image is developed by a development device504, and the developed visible image is transferred to transfer paper (not shown) by a transfer charger506.

The transfer paper to which the visible image is transferred is moved to a fixing device508and then discharged to the outside of the apparatus after fixing.

Although this embodiment uses the surface emitting laser array of 4×8, the surface emitting laser array is not limited to this, and a surface emitting laser array of m×n (m, n: a natural number (excluding 0)) may be used.

This application claims the benefit of Japanese Patent Application No. 2009-175391, filed Jul. 28, 2009, which is hereby incorporated by reference herein in its entirety.