Integration of laser with photodiode for feedback control

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FIELD OF THE INVENTION

This application relates to the integration of surface light emitting devices with photodetectors and, in particular, to monolithic integration of superluminescent light emitting diodes (SLEDs) or vertical cavity surface emitting lasers (VCSELs) with photodetectors for feedback control of the radiation intensity of the SLEDs or VCSELs.

BACKGROUND OF THE INVENTION

Semiconductor lasers are widely used in applications such as optical communications and compact disk players, etc. In those applications, the optical intensity of the lasers is usually monitored for any intensity variation due to changing of the environment (e.g. temperature), heat generated by the laser circuitry, long-term drift of the laser properties, or drift of the properties of the circuitry that drives the laser. Generally, for conventional edge-emitting lasers, the intensity of the laser is monitored by a separate photodetector installed in the vicinity of the laser. The photodetector receives a small portion of the total optical radiation emitted from the laser and generates a feedback signal that corresponds to the intensity of the emitted radiation. This feedback signal is then provided to a feedback circuit that sends a control signal to the circuit that provides current to the laser. Accordingly, any drift of the laser intensity is detected and compensated by adjusting the current applied to the laser. In digital applications wherein the laser is modulated by a series of current pulses, the photodetector is also used to verify the output optical pulses against the input of the electrical pulses.

A new class of semiconductor lasers, vertical cavity surface emitting lasers (VCSELs), has been developed. Unlike conventional edge emitting lasers that emit light in a direction parallel to the semiconductor substrates where the lasers are formed, a VCSEL has an optical cavity perpendicular to the substrate and emits optical radiation in a direction perpendicular to the substrate. Because of this structure, a VCSEL is more readily integrated with a photodiode than a conventional edge emitting laser.

In an article "Monolithic Integration of Photodetector with Vertical Cavity Surface Emitting Laser, " Electronic Letters, Vol. 27, No. 18, pp.1630-1632 (1991), Hasnain et al. disclose an integrated device comprising a VCSEL and a PIN photodiode. In this device, the VCSEL is epitaxially formed on a semiconductor substrate and the PIN photodiode is epitaxially formed on the VCSEL. When operating, the VCSEL emits optical radiation which traverses the PIN photodiode. Accordingly, the photodiode absorbs a small portion of the optical radiation and generates a photocurrent which corresponds to the intensity of the radiation. However, in this device, the PIN photodiode undesirable reflects and absorbs a portion of the light emitted from the VCSEL, which compromises the optical efficiency of the VCSEL.

It is therefore an object of the present invention to develop an integrated optoelectronic device comprising a VCSEL and a photodiode without compromising the optical efficiency of the VCSEL.

Certain semiconductor diodes known as superluminescent light emitting diodes (SLEDs) also emit radiation in a direction perpendicular to the plane of the p-n junction. These SLEDs have a construction that is similar to that of a VCSEL except that one of the mirrors either is absent or has reduced reflectance such that the threshold conditions for lasing cannot be achieved.

It is yet another object of the invention to monolithically integrate SLEDs with photodiodes in various structures.

SUMMARY OF THE INVENTION

In one embodiment of the invention, an integrated optoelectronic device comprises a vertical cavity surface emitting laser (VCSEL) and a photodiode. The VCSEL comprises first and second mirrors forming therebetween an optical cavity perpendicular to a substrate where the VCSEL is formed, and an active region disposed between the mirrors. The photodiode is formed within the second mirror. Advantageously, the photodiode comprises a plurality of semiconductor layers which also form a portion of the second mirror. Therefore, the photodiode is integrated with the VCSEL without introducing any undesirable optical perturbation from the photodiode. The photodiode comprises materials having an energy bandgap equal to or smaller than the energy bandgap corresponding to the optical radiation, so that the photodiode intercepts and absorbs at least a portion of the optical radiation and generates a corresponding electrical signal.

Preferably, the second mirror is an epitaxially formed semiconductor distributed Bragg reflector (DBR) comprising a plurality of alternating semiconductor layers having high and low indices of refraction, with each layer having a thickness of .lambda./4n where .lambda. is the wavelength of the optical radiation emitted from the laser and n is the index of refraction of the layer. The photodiode is a PIN photodiode that comprises an undoped region interposed between a p-type region and an n-type region. Accordingly, each of the regions that form the PIN photodiode includes at least one of the layers that forms the second mirror.

In another embodiment of the invention, an integrated optoelectronic device comprises a substrate, a surface light emitting device formed on the substrate, the device emitting optical radiation having a wavelength, .lambda., and a photodiode non-epitaxially formed on the surface emitting device. The surface light emitting device may be a SLED or a VCSEL. In the case of a VCSEL, the VCSEL comprises first and second mirrors which form therebetween a vertical optical cavity, and an active region disposed between the mirrors. The photodiode is non-epitaxially formed over the second mirror of the VCSEL. The second mirror of the laser can be a semiconductor DBR or a dielectric DBR. Preferably, the photodiode is a polysilicon PN photodiode comprising a p-type polysilicon layer formed over an n-type polysilicon layer. The photodiode receives optical radiation emitted from the VCSEL and generates a signal which corresponds to the intensity of the radiation. In the case of a SLED, the structure of the device is the same except that one of the mirrors either is absent or has reduced reflectance.

In another embodiment, an integrated optoelectronic device comprises a VCSEL formed on a first surface of a substrate and a photodiode non-epitaxially formed on a second surface of the substrate. The photodiode is substantially vertically aligned to the VCSEL for receiving optical radiation emitted from the laser.

In another embodiment, an integrated optoelectronic device comprises a semiconductor substrate, a surface light emitting device having a light-generating region formed on the substrate, at least the light-generating region forming a mesa having a substantially vertical outer surface, and a photodiode formed over the vertical outer surface of the mesa. The surface light emitting device emits a first optical radiation substantially orthogonal to the substrate, and a second optical radiation substantially parallel to the substrate. The photodiode receives the second optical radiation and generates a corresponding electrical signal.

The surface light emitting device may be a SLED or a VCSEL. When the surface light emitting device is a VCSEL, it is formed on a substrate and a photodiode non-epitaxially formed on the side of the laser. Specifically, the VCSEL includes first and second mirrors that form a vertical optical cavity, and an active region disposed between the mirrors. A mesa that includes at least the second mirror and the active region is formed. The mesa has a sidewall which has a vertical outer surface. Accordingly, the photodiode is formed on at least a portion of the outer surface of the mesa. The photodiode receives lateral optical radiation from the active region of the VCSEL and generates a corresponding feedback signal. In the case of a SLED, the structure of the device is the same except that one of the mirrors either is absent or has reduce reflectance.

In another embodiment, an integrated optoelectronic device comprises a VCSEL epitaxially formed on a first surface of a substrate and an implanted photodiode formed within the substrate and at a second surface of the substrate. The implanted photodiode is vertically aligned with the VCSEL and receives optical radiation emitted from the laser. Preferably, the implanted photodiode is a PN photodiode comprising an implanted p-type region formed in an n-type substrate.

In another embodiment, an integrated optoelectronic device comprises a lateral injection VCSEL and a lateral PIN photodiode. The lateral injection VCSEL comprises first and second mirrors and an active region disposed between the mirrors. Additionally, the laser further contains a first implantation region having a first conductivity type and a second implantation having a second conductivity type. Both the first and second implanted regions are formed at the periphery of the active region for applying a current to the active region. The device further includes a third implantation region having a first conductivity type formed at the periphery of the active region; and the first, second, and third implantation regions do not overlap each other. Accordingly, the third implantation region, active region, and second implantation region together form a PIN photodiode, whereby a voltage between the second and third implantation regions indicates the intensity of the optical radiation emitted from the laser.

Alternatively, in addition to the first, second, and third implantation regions, a fourth implantation region having a second conductivity type is formed at the periphery of the active region. Accordingly, the first and second implantation regions are utilized to inject current into the active region, whereas the third and fourth implantation regions and the active regions form the PIN photodiode.

Finally, in another embodiment, an integrated optoelectronic device comprises a substrate having a first conductivity type, a first mirror having a first conductivity type formed on the substrate, a first undoped spacer formed on the first mirror, an active region formed on the first spacer, a second undoped spacer formed on the active region, and a second mirror having a second conductivity type formed on the second spacer. The device further includes an implanted isolation region formed within at least the second mirror, the spacers and the active region. The implanted isolation region extends vertically from the second mirror to the first mirror, thereby dividing the second mirror, active region, and spacers into two portions, a first portion which is a VCSEL, and a second portion which is a PIN photodiode. The PIN photodiode receives a laterally emitted optical radiation from the active region of the laser and generates a corresponding signal. The isolation region separates the laser and the PIN photodiode so that a voltage applied to the photodiode does not affect the laser.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, objects, and advantages of the present invention will become more apparent from the detailed description in conjunction with the appended drawings in which:

FIG. 1a depicts a cross-sectional view of a VCSEL integrated with a PIN photodiode epitaxially formed in a mirror of the VCSEL;

FIG. 1b illustrates, in more detail, a portion of the PIN photodiode of FIG. 1a;

FIG. 1c depicts an alternative embodiment of the VCSEL of FIG. 1a.

FIG. 2a shows an equivalent circuit diagram of the integrated optoelectronic device of FIG. 1a;

FIG. 2b illustrates an exemplary circuit configuration utilizing the device shown in FIG. 1a;

FIG. 2c is a circuit diagram of feedback control of the laser intensity using the device shown in FIG. 1a;

FIG. 3a illustrates a cross-section of a VCSEL integrated with an epitaxially formed PN photodiode;

FIG. 3b is a more detailed illustration of a portion of the PN photodiode shown in FIG. 3a;

FIG. 4 depicts a cross-section of a VCSEL integrated with a non-epitaxially formed PN photodiode;

FIG. 5 illustrates a cross-section of a VCSEL integrated with a non-epitaxially formed PN photodiode formed on a back surface of a substrate;

FIG. 6 depicts a cross-section of a VCSEL integrated with a PN photodiode non-epitaxially formed on the side of a mesa of the VCSEL;

FIGS. 7a and 7b illustrate cross-sectional and top views of an alternative embodiment of the device shown in FIG. 6;

FIG. 8 illustratively shows a cross-section of a VCSEL integrally formed with an implanted PN photodiode;

FIGS. 9a-9c depict a cross-sectional view, a top view, and an equivalent circuit diagram, respectively, of a lateral injection VCSEL integrated with a lateral PIN photodiode;

FIG. 9d depicts a top view of an alternative embodiment of the device shown in FIG. 9a;

FIG. 9e depicts a top view of another alternative embodiment of the device shown in FIG. 9a;

FIG. 9f is a schematic circuit diagram for operating the device shown in FIG. 9e;

FIG. 9g shows an equivalent circuit diagram of another possible circuit configuration using the device shown in FIG. 9e;

FIG. 10a illustrates a top view of a VCSEL integrated with a PIN photodiode formed alongside of the VCSEL;

FIG. 10b illustrates a cross-section of the VCSEL shown in FIG. 10a;

FIG. 10c depicts a cross-sections of the VCSEL and the PIN photodiode shown in FIG. 10a; and

FIG. 10d is an equivalent circuit diagram of the device shown in FIG. 10a.

DETAILED DESCRIPTION

In accordance with the invention, an optoelectronic device is disclosed comprising a vertical cavity surface emitting laser (VCSEL) or a superluminescent light emitting diode (SLED) and a photodiode which is integrally formed with the VCSEL or SLED. When in operation, the VCSEL or SLED emits optical radiation, and the photodiode intercepts the radiation and generates a feedback signal that corresponds to the intensity of the radiation.

In one embodiment, an integrated optoelectronic device comprises a VCSEL formed on a substrate and a photodiode formed in a mirror of the laser. The mirror is a semiconductor DBR comprising epitaxially formed layers having alternating high and low indices of refraction. Advantageously, the photodiode is at least a portion of the mirror and is an integral part of the VCSEL. As a result, the device of the present invention offers improved optical efficiency as compared with prior art devices.

The photodiode is a PIN photodiode or a PN photodiode. The photodiode comprises semiconductor materials having a smaller bandgap than the effective bandgap of an active region of the VCSEL for absorbing the emitted light.

FIG. 1a illustrates a cross-sectional view of a preferred embodiment of the device. It comprises an n-type first mirror 15, a first spacer 20, an active region 25 comprising AlGaAs/GaAs quantum wells, a second spacer 30, and a second mirror 35 formed on one side of an n-type GaAs substrate 10. Alternatively, the active region comprises InGaAs/GaAs quantum wells.

Second mirror 35 has two portions, a mesa portion 34, and a contact portion 33. Preferably, contact portion 33 is doped to have a p-type conductivity to provide an ohmic contact with contact 32. Contact 5 for the substrate is formed on the other side of the substrate.

Illustratively, lower mirror 15 is an n-doped semiconductor DBR comprising 20-30 periods of AlAs/AlGaAs layers. Each of these AlAs and AlGaAs layers has a thickness of .lambda./(4n) where .lambda. is referred hereinafter as the wavelength of the emitted optical radiation in free space and n is the index of refraction of the layer.

Second mirror 35 is also a semiconductor DBR comprising alternating semiconductor layers of high and low indices of refraction. The layers that form the second mirror are doped so that they form the PIN photodiode.

Specifically, as shown in FIG. 1b, second mirror comprises three portions, a lower p-type portion 40, an undoped layer 45, and an upper n-type portion 50. Lower portion 40 comprises p-doped alternating layers of AlAs and AlGaAs, each layer having a thickness of .lambda./(4n). Layer 45 is an undoped GaAs layer having a thickness of .lambda./(4n). Portion 50 comprises n-doped alternating layers of AlAs and AlGaAs, each layer having a thickness of .lambda./(4n). Thus, portions 40, 45, and 50 form a PIN diode. GaAs layer 45 in the PIN photodiode has a smaller bandgap than the effective bandgap of the active region for absorbing a portion of radiation emitted from the laser. Alternatively, if the active region comprises InGaAs/GaAs quantum wells, layer 45 is an InGaAs layer having an effective bandgap no greater than that of the active region for absorbing the optical radiation. Illustratively, optical radiation 60 is emitted through the second mirror and the PIN photodiode absorbs a small portion of the radiation.

In FIG. 1b, the undoped portion of the PIN photodiode is a single layer having a thickness of .lambda./(4n). Alternatively, this undoped portion can consist of more than one layer, each layer having a thickness of .lambda./(4n).

FIG. 2a depicts an equivalent circuit diagram of the device shown in FIG. 1a. In FIG. 2a, a VCSEL 70 is serially connected with PIN photodiode 80. The VCSEL and photodiode anodes are common.

FIG. 2b depicts one of the possible configurations for utilizing the PIN photodiode to detect the radiation emitted from the VCSEL. In this configuration, contact 32 is grounded. A positive voltage V.sub.L is applied between contacts 32 and 5 to generate current I.sub.L which causes VCSEL 70 to radiate. PIN diode 80 is reversed biased by applying a positive voltage V.sub.P between contacts 55 and 32. The light emitted from the VCSEL generates a photocurrent I.sub.P which corresponds to the intensity of the emitted radiation. This photocurrent can be utilized as a signal for feedback control of the VCSEL intensity or as an indicator of the VCSEL intensity.

FIG. 2c illustrates a schematic diagram of a circuit for using the PIN photodiode for feedback control of the VCSEL intensity. As shown in this figure, photocurrent I.sub.P is sent to a feedback circuit 85 which generates a signal 90 that corresponds to photocurrent I.sub.P. Signal 90 is then sent to a laser controller 95. The laser controller then adjusts a laser driving current. I.sub.n in response to feedback signal 90 so as to maintain a constant magnitude of VCSEL intensity.

Instead of photocurrent, a photovoltage generated from the photodiode can also be utilized as the feedback signal that relates to the intensity of the radiation. Additionally, instead of grounding contact 32, contacts 5 and/or 55 may be grounded.

The integrated optoelectronic device as shown in FIG. 1a is fabricated similarly to conventional VCSELs. All semiconductor layers, including layers for the first and second mirrors, the first and second spacers, and the active region, are epitaxially formed on a semiconductor substrate. Typically, Molecular Beam Epitaxy (MBE) or Metal Organic Chemical Vapor Deposition (MOCVD) technology is employed to epitaxially form these layers. Subsequently, the mesa portion of the second mirror is defined by etching the portion of the layers in region that surrounds the mesa. Contact 5 for contacting n-doped substrate 10 and contact 32 for contacting p-doped mirror portion 33 are also formed; and a ring-shaped contact 55 is formed on top of second mirror 35 to contact upper portion 50 of the PIN photodiode. Note that contacts 5 and 55 are contacts for n-doped semiconductor material, whereas contact 32 is a contact for p-type semiconductor material.

Various other device structures or materials that are typically utilized in conventional VCSELs can also be incorporated in this device. For example, portions 50 and 40 of the second mirror can be formed by depositing alternating layers of Al.sub.(1-x) Ga.sub.x As and Al.sub.(1-y) Ga.sub.y As where x and y are different values; or an annular p-type implanted region can be formed in the second spacer under contact 32 for reducing the contact resistance.

Alternatively, as illustrated in FIG. 1c, instead of removing portions of the layers of the second mirror to form the mesa structure, the planar structure formed by deposition of the layers of the VCSEL can be left in place and an annular implantation region 36 can be formed in the planar structure. Implantation region 36 should have a p-type conductivity type and extend vertically downward at least to the layer in p-type portion 40 of mirror 35. In this structure, contact 32 is formed on the surface of the implantation region and therefore is co-planar with contact 55. Preferably, the implantation region should extend to second spacer 30 to reduce serial resistance of the laser.

In another embodiment of the invention, a PN photodiode is formed in a mirror of a VCSEL. The cross-section of a preferred embodiment of this integrated optoelectronic device is illustrated in FIG. 3a. Since this device is very similar to the above described device wherein the detector is a PIN photodiode, like elements are designated with the same number in FIGS. 3a and 3b for convenience.

As shown in FIG. 3a, second mirror 35 comprises two portions, a p-type portion 40 and an n-type portion 50 that is formed on the p-type portion. In order to absorb a portion of the emitted radiation, at least one of the layers at the interface of portions 40 and 50 should be a semiconductor layer having a bandgap equal or smaller than the effective bandgap of the active region. As illustrated in FIG. 3b, the two layers adjacent to the interface between p-type portion 40 and n-type portion 50 are a p-type AlAs layer and an n-type GaAs layer. These two layers form a PN junction.

Illustratively, portion 40 comprises alternating p-type layers of AlAs and AlGaAs, and portion 50 comprises an n-type GaAs layer formed on portion 40 and alternating n-type layers of AlAs and AlGaAs formed on the GaAs layer.

The rest of the embodiment of FIG. 3a is the same as the embodiment shown in FIG. 1a and is not described here.

In another embodiment of the invention, an integrated optoelectronic device comprises a surface light emitting device such as a SLED or a VCSEL and a photodiode non-epitaxially formed on the surface light emitting device. This non-epitaxially formed photodiode intercepts optical radiation emitted from the device and produces a feedback signal that corresponds to the intensity of the radiation.

FIG. 4 illustrates a preferred embodiment wherein the surface light emitting device is a VCSEL. The device comprises an n-type GaAs substrate 100, a first mirror 105 formed on the substrate, a first spacer 110, an active region 115 formed on the first spacer, a second spacer 125 formed on the active region, a first portion of a second mirror 130, a second portion of the second mirror 140, a polysilicon PN photodiode 158, and a protection layer 165 formed on the PN photodiode. This device further includes an implanted annular current confinement region 132, an annular p-contact 135, contacts 145 and 160 for contacting the photodiode, and contact 95 for contacting the n-doped substrate.

Illustratively, first mirror 105 is a semiconductor DBR comprising 20-30 periods of AlAs/AlGaAs layers. Each of the AlAs or AlGaAs layers has a thickness of .lambda./(4n). In addition, the first mirror is doped to have n-type conductivity.

Active region 115 comprises at least one GaAs quantum well. Preferably, the two spacers 110, 125 surrounding the active region are un-doped or low doped.

The first portion of the second mirror 130 is a p-type semiconductor DBR. Since the first portion is mainly used to provide ohmic contact with p-type contact 135, it typically contains only a few layers. Preferably, the first portion comprises approximately one to three periods of AlAs/AlGaAs layers, each AlAs or AlGaAs layer being .lambda./(4n) thick. In addition, this first portion is doped to have p-type conductivity to assure ohmic contact with contact 135. Advantageously, since this first portion only contains a few layers, it is doped with p-dopant to a very high doping density (e.g., 10.sup.18 -10.sup.20 /cm.sup.3) without significantly compromising the optical characteristics of the VCSEL.

The second portion of the second mirror is a dielectric DBR comprising alternating layers of SiO.sub.2 80 /and Si.sub.3 N.sub.4, each of the layers having a thickness of .lambda./(4n). Thus, the first and second portions of the second mirror form the second mirror of the VCSEL. Accordingly, the first and second mirrors form therebetween a vertical optical cavity.

Current confinement region 132 is an annular region peripherally formed around the active region, the second spacer, the first portion of the second mirror, and a part of the first spacer. It is formed by implanting conductivity reducing ions such as Argon (Ar) or Helium (He). Consequently, this region exhibits high resistivity compared with its neighboring regions.

Contact 135 is used to contact p-doped portion 130. Illustratively, it comprises gold-beryllium (AuBe) or gold-zinc (AuZn) alloys. The substrate contact 95 is formed to make ohmic contact to the n-type GaAs substrate and illustratively comprises gold-germanium alloy (AuGe).

Polysilicon photodiode 158 is formed on the second mirror. It comprises a p-type polysilicon layer 150, and a n-type polysilicon layer 155 formed on the p-type layer. Two peripheral contacts are formed to contact the two polysilicon layers. Illustratively, contact 160 is peripherally formed on the n-doped layer for contacting the n-type polysilicon layer 155, and contact 145 is peripherally formed under the p-doped layer for contacting the p-doped layer 150. Preferably, both contacts 145 and 160 are titanium silicide. Protection layer 165 is a SiO.sub.2 layer formed on the photodiode.

The fabrication of this device begins with sequentially growing the layers that form the first mirror, the first spacer, the active region, the second spacer, and the first portion of the second mirror. Next, the current confinement region is formed by performing an annular implantation of Argon or Helium. The implanted region is damaged by the implantation so it has a substantially higher resistivity than the non-implanted regions. Therefore, current applied between contacts 135 and 95 is substantially confined to the active region. After the implantation process, contact 135 is formed by depositing a layer of gold-beryllium alloy and removing portions of the layer to define the shape of contact 135.

Subsequently, the second portion of the second mirror is formed by depositing alternating layers of Si.sub.3 N.sub.4 and SiO.sub.2. The deposition of Si.sub.3 N.sub.4 and SiO.sub.2 layer is well-known and is accomplished through techniques such as sputtering or chemical vapor deposition (CVD).

Following the formation of the second mirror, a layer of titanium (Ti) is uniformly deposited onto the second mirror and etched to form contact 145. Next, the polysilicon PN diode is formed by first depositing p-doped polysilicon layer 150 and then n-doped polysilicon layer 155. Layers 150 and 155 form a PN junction.

Subsequently, contact 160 is formed by depositing another layer of titanium (Ti) and etching. Then, protective layer 165 is formed by depositing a layer of SiO.sub.2. Etching is subsequently performed to expose portions of contacts 160 and 145 so that they can be contacted by bonding wire 170 and 148.

The structure is subsequently annealed so that contacts 145 and 160 titanium react with the polysilicon layers to form titanium-silicide. These titanium silicide layers form low-resistance ohmic contacts with layers 150 and 155, respectively. Typically, the annealing temperature is between 400.degree. to 500.degree. C. and the annealing time is about 30 minutes. The thickness of Ti layers 145 and 160 should be judiciously chosen so that they only form silicide within the layer they intend to contact. The formation of a silicide contact to a polysilicon layer is a well-developed technique in silicon semiconductor technology and therefore is not discussed in detail here.

Finally, contact 95 on n-type GaAs is formed by depositing a layer of Ni and AuGe on substrate 100 and annealing.

In this device, when an electrical current is applied between contacts 135 and 95, optical radiation 175 is emitted from the laser. If the applied current exceeds the threshold current of the laser, coherent optical radiation 175 is generated. Optical radiation 175 traverses polysilicon PN photodiode 158 which is reverse biased. Since the bandgap of polysilicon is smaller than the effective bandgap of the active region, a portion of the emitted radiation is absorbed by the polysilicon PN photodiode. Consequently, the PN photodiode produces a photocurrent that corresponds to the intensity of emitted radiation. Such current can be utilized as a feedback signal for controlling the intensity of radiation 175.

An alternative way to contact polysilicon layer 150 is to form contact 145 peripherally on layer 150, instead of forming it under layer 150. The process to fabricate such structure includes forming polysilicon layers 150 and 155 onto the second mirror prior to forming contact 145. Subsequently, layer 155 is peripherally etched to expose a portion of layer 150. Contact 145 is then formed on this exposed area of layer 150.

In another embodiment, as shown in FIG. 5, an integrated optoelectronic device comprises a VCSEL formed on a first surface of a substrate and a PN photodiode 188 non-epitaxially formed on a second surface of the substrate. Additionally, the PN photodiode is vertically aligned with the VCSEL. Thus, the PN photodiode intercepts optical radiation 203 emitted from the VCSEL and produces a current or voltage which corresponds to the intensity of radiation 230. Such current or voltage can be used as a feedback signal for controlling the intensity of radiation 230.

Illustratively, the VCSEL comprises a first mirror 205 formed on a first surface 201 of substrate 200, a first spacer 210 formed on the first mirror, an active region 215 formed on the first spacer, a second spacer 220 formed on the active region, and a second mirror 225 formed on the second spacer. An isolation layer 195 is formed on second surface 199 of the substrate. The PN photodiode comprises a p-type polysilicon layer 190 non-epitaxially formed on the isolation layer, and a n-type polysilicon layer 185 non-epitaxially formed on the p-type polysilicon layer. Preferably, a protective SiO.sub.2 layer 180 is formed on the n-type polysilicon layer.

In the VCSEL, first mirror 205 and second mirror 225 form therebetween a vertical optical cavity. First mirror 205 is partially transmissive to allow radiation 203 to reach the photodiode. Preferably, to reduce absorption due to the substrate, radiation 203 has a wavelength corresponding to an energy bandgap that is not greater than that of the substrate. This can be achieved by forming the active region so that it has an effective energy bandgap that is not greater than that of the substrate. Alternatively, the substrate can be thinned to minimize absorption effects. Preferably, radiation 203 is only a portion of the total optical radiation emitted from the laser; for example, it is approximately 10% of the total radiation, whereas radiation 230 is about 90% of the total radiation.

First mirror 205 is a semiconductor DBR comprising a plurality of alternating layers of AlGaAs and AlAs, each of AlGaAs or AlAs having a thickness of .lambda./(4n). The number of periods of AlGaAs/AlAs layers and the lasing wavelength are judiciously chosen so that about 10% of total radiation transmits through the mirror 205.

Preferably, active region 215 comprises one or more GaAs quantum wells and has an effective bandgap smaller than that of GaAs substrate 200. As result, radiation 203 goes through the substrate and reaches the photodiode without suffering significant attenuation in the substrate.

In the case when the active region comprises one or more GaAs/AlGaAs quantum wells, GaAs substrate 200 may be required to have a thickness of less than 100 .mu.m. This reduces the absorption of radiation 203 in the substrate, so radiation 203 has the desired intensity when it reaches the photodiode.

The active region is laterally defined by an annular implanted current confinement region 217 which has substantially higher resistivity than the active region. The current confinement region includes peripheral portions of spacers 210 and 220.

Second mirror 225 comprises two portions, a first portion 221 and a second portion 222. First portion 221 is a semiconductor DBR comprising one or more periods of AlGaAs/AlAs layers, each of the AlGaAs or AlAs layer having a thickness of .lambda./4n. Additionally, the first portion of the second mirror is doped with p-type dopants to provide an ohmic contact with contact 223.

Second portion of the second mirror 222 is a dielectric DBR, or a semiconductor DBR. Preferably, portion 222 is a dielectric Bragg reflector comprising a plurality of alternating layers of Si.sub.3 N.sub.4 and SiO.sub.2, each of the Si.sub.3 N.sub.4 or SiO.sub.2 layer having a thickness of .lambda./4n.

As illustrated, contacts 198, 189, and 182 are formed to provide electrical contact to the substrate and the photodiode, respectively.

To fabricate this device, epitaxial layers forming the first mirror, the spacers, the active region and the first portion of the second mirror are first grown on the first Surface of the GaAs substrate. Subsequently, current confinement region 217 is formed by an annular implantation of Ar ions. Next, the second portion of the second mirror is constructed by depositing a series of alternating layers of SiO.sub.2 and Si.sub.3 N.sub.4 onto the first portion of the second mirror. The deposition is conventionally accomplished by sputtering or other techniques.

Next, isolation layer 195 is formed by depositing a layer of SiO.sub.2 onto the back surface of the substrate. The polysilicon PN photodiode is then constructed by depositing first a p-type polysilicon layer and then a n-type polysilicon layer. The polysilicon layers are formed by conventional Low Pressure Chemical Vapor Deposition (LPCVD) or evaporation. Subsequently, a SiO.sub.2 protection layer is formed on the exposed polysilicon layer.

Next, mesa etching is performed to define the second portion of the second mirror and to expose an annular area of the first portion of the second mirror for formation of contact 223. Similarly, etching is also performed to expose peripheral areas of the substrate, the p-type polysilicon layer, and the n-type polysilicon layer. Finally, contacts for the laser and the polysilicon PN photodiode are formed on those exposed areas.

In the preferred embodiment, the polysilicon photodiode is vertically aligned to the active region to receive maximum optical radiation emitted toward the photodiode.

In another embodiment, an integrated optoelectronic device comprises a surface light emitting device having a mesa, and a photodiode formed on the sidewall of the mesa. The surface light emitting device may be a SLED or a VCSEL. The photodiode intercepts optical radiation laterally emitted from the surface light emitting device through the sidewall and provides a feedback signal that corresponds to the intensity of total radiation.