Vertical-cavity surface-emitting laser and method of fabricating the same

Provided are a wavelength swept vertical-cavity surface-emitting laser and a method of fabricating the same. The laser may include a substrate, a lower reflection layer on the substrate, an active layer on the lower reflection layer, a sacrificial layer disposed on a first side of the active layer, a stopper disposed on a second side of the active layer that may be spaced apart from the sacrificial layer, and an upper reflection layer fixed on the sacrificial layer, the upper reflection layer extending over the stopper and the active layer. The stopper defines a minimum separation distance between the upper reflection layer and the active layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2012-0103345, filed on Sep. 18, 2012, in the Korean Intellectual Property Office, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Example embodiments of the inventive concept relate to a broadband wavelength swept light source, and in particular, to a vertical-cavity surface-emitting laser with a tunable cavity.

In general, an optical coherence tomography technology is achieved using a broadband wavelength swept light source, because the use of the broadband wavelength swept light source makes it possible to realize a high-resolution high-speed imaging.

A wavelength swept vertical-cavity surface-emitting laser, one of the broadband wavelength swept light sources, includes a mirror constituting a MEMS-based cavity. To achieve a stabilized operation property and a long lifetime, it is necessary to improve mechanical reliability of the mirror in the vertical-cavity surface-emitting laser. However, the conventional wavelength swept vertical-cavity surface-emitting laser suffers from low mechanical reliability of the mirror.

SUMMARY

Example embodiments of the inventive concept provide a wavelength swept vertical-cavity surface-emitting laser, which is configured to prevent a current spreading layer from being stuck on an upper reflection layer and to prevent a pull-in effect from occurring, and a method of fabricating the same.

Other example embodiments of the inventive concept provide a wavelength swept vertical-cavity surface-emitting laser with a long lifetime and a high speed operation property and a method of fabricating the same.

According to example embodiments of the inventive concepts, a vertical-cavity surface-emitting laser may include a substrate, a lower reflection layer on the substrate, an active layer on the lower reflection layer, a sacrificial layer disposed on a first side of the active layer, a stopper disposed on a second side of the active layer that may be spaced apart from the sacrificial layer, and an upper reflection layer fixed on the sacrificial layer and extending over the stopper and the active layer.

The stopper defines a minimum separation distance between the upper reflection layer and the active layer.

In example embodiments, the upper reflection layer may include a fixation part on the sacrificial layer, a spring connected to the fixation part, the spring extending from the first side to the second side, and a membrane provided on the active layer. The membrane may be connected to the spring and may be separated from the stopper.

In example embodiments, the membrane may be provided to have a hole exposing the stopper.

In example embodiments, the stopper has a diameter greater than that of the hole.

In example embodiments, the stopper may include a bottom block provided below the hole, and a capping layer covering the bottom block and on the active layer.

In example embodiments, the bottom block has substantially the same shape as that of the hole.

In example embodiments, the bottom block may include a silicon oxide layer or a silicon nitride layer.

In example embodiments, the capping layer may include a metal oxide layer.

In example embodiments, the laser may further include a current spreading layer disposed below the bottom block and the sacrificial layer to cover the active layer.

In example embodiments, the laser may further include a first electrode disposed on an edge of the lower reflection layer or substrate, a second electrode disposed on an edge of the current spreading layer, and a third electrode disposed on the fixation part of the upper reflection layer.

In example embodiments, the first electrode may be provided below the lower reflection layer or on the substrate.

In example embodiments, the capping layer may be provided to cover the first electrode, the second electrode, and the third electrode.

In example embodiments, the laser may further include an ohmic contact layer between the second electrode and the current spreading layer.

In example embodiments, the laser may further include a current confining layer between the lower reflection layer and the active layer.

In example embodiments, a top surface of the sacrificial layer may be higher than that of the stopper.

In example embodiments, the sacrificial layer may include a metal oxide layer.

In example embodiments, the spring may further include at least one of a metal layer or a metal oxide layer.

According to other example embodiments of the inventive concepts, a method of fabricating a vertical-cavity surface-emitting laser may include stacking a lower reflection layer, an active layer, a current spreading layer, a sacrificial layer, and an upper reflection layer on a substrate, removing edge portions of the upper reflection layer, the sacrificial layer, the current spreading layer, and the active layer to expose a portion of the lower reflection layer, patterning the upper reflection layer to form a fixation part, a spring, and a membrane, removing an edge portion of the sacrificial layer to expose a portion of the current spreading layer, forming a first electrode, a second electrode, and a third electrode on the lower reflection layer or the substrate, the current spreading layer, and the fixation part, respectively, forming a hole through the membrane, removing a portion of the sacrificial layer located below the hole, forming a bottom block on the current spreading layer exposed by the hole, removing other portions of the sacrificial layer located between the spring and the current spreading layer and between the membrane and the current spreading layer, and forming a capping layer on the bottom block.

In example embodiments, the bottom block may be formed by a thermal evaporation process or an e-beam deposition process.

In example embodiments, the sacrificial layer may be removed using an etching process configured to suppress the upper reflection layer from being etched.

In example embodiments, the removing of the sacrificial layer may be performed using a dry etching process, in which mixture gas of sulphur hexaflouride and silicon tetrachloride may be used as a reaction gas.

In example embodiments, the removing of the sacrificial layer may be performed using a wet etching process, in which mixture of ammonium hydroxide, hydrogen peroxide, and deionized water may be used as etching solution.

In example embodiments, the capping layer may be formed by a chemical vapor deposition or an atomic layer deposition.

DETAILED DESCRIPTION

FIG. 1is a cross-sectional view illustrating a vertical-cavity surface-emitting laser according to example embodiments of the inventive concept.FIG. 2is a top view ofFIG. 1.

Referring toFIGS. 1 and 2, a wavelength swept vertical-cavity surface-emitting laser according to example embodiments of the inventive concept may include a substrate10, a lower reflection layer20, a first electrode26, a current confining layer28, an active layer30, a current spreading layer40, an ohmic contact layer50, a second electrode52, a sacrificial layer60, an upper reflection layer70, a third electrode78, and a stopper80.

The lower reflection layer20may include a first reflection layer22and a second reflection layer24. The lower reflection layer20may be provided to have a first conductivity type. In example embodiments, the first conductivity type may be an n-type.

The lower reflection layer20may be lattice-matched to the substrate10. The first reflection layer22and the second reflection layer24may be stacked alternatingly and repeatedly. In the case where the substrate10is formed of gallium arsenide (GaAs), the first reflection layer22may include AlGaAs or GaAs. The second reflection layer24may include (Al)GaAs or AlAs. The lower reflection layer20may include AlGaAs/GaAs, AlGaAs/AlAs, or AlGaAs/AlGaAs. In the case where the substrate10is formed of indium phosphide (InP), the first reflection layer22may include InAlAs or InGaAsP. The second reflection layer24may include InAlGaAs, GaAsInP, or InP. The lower reflection layer20may include InAlAs/InAlGaAs, InGaAsP/InP, or InGaAsP/GaAsInP. In example embodiments, the lower reflection layer20may be configured to have reflectivity of about 99.9%. The first electrode26may be provided on an edge portion of the lower reflection layer20. The first electrode26may be grounded. The first electrode26may be provided around the second electrode52. In example embodiments, the first electrode26may be provided at the substrate10.

The active layer30may have a quantum well structure. The active layer30may include a semiconductor multi-layered structure that is lattice-matched to the substrate10and the lower reflection layer20. For example, the active layer30may have a multi-layered structure of GaAs/AlGaAs, InGaAs/GaAs, InGaAs/AlGaAs, AlGaAs/AlGaAs, InGaAs/InAlAs, InGaAs/InAlGaAs, InAlGaAs/InAlAs, InGaAsP/InP, InAlGaAs/InAlGaAs, or InGaAsP/InGaAsP. In example embodiments, the active layer30may be an intrinsic layer.

The current spreading layer40may be lattice-matched to the substrate10or the active layer30. The current spreading layer40may have a second conductivity type. The second conductivity type may be a p-type. The ohmic contact layer50and the stopper80may be provided on the current spreading layer40.

The ohmic contact layer50may be disposed between the second electrode52and the current spreading layer40. The second electrode52and the ohmic contact layer50may be configured to deliver an operation current to the current spreading layer40from an external source. The operation current may be an electric current that is used to generate laser light from the active layer30. The second electrode52may be disposed between the first electrode26and the upper reflection layer70. The stopper80may be configured to prevent the upper reflection layer70from being stuck to the current spreading layer40. The stopper80and the upper reflection layer70will be described in more detail below.

The sacrificial layer60may be provided on the ohmic contact layer50. The sacrificial layer60may be disposed spaced apart from the stopper80and the second electrode52. The sacrificial layer60may be configured to fix the upper reflection layer70. The sacrificial layer60may have a top surface that is higher than the stopper80. The sacrificial layer60may separate the upper reflection layer70electrically from the ohmic contact layer50.

The upper reflection layer70may extend toward a first direction, on the sacrificial layer60. The upper reflection layer70may include a fixation part72, a spring74, and a membrane76. The fixation part72may be provided on the sacrificial layer60. The third electrode78may be provided on the fixation part72. At least one solder ball34and at least one wire32may be provided on the first electrode26, the second electrode52, and the third electrode78. The third electrode78may be applied with an operation voltage. In example embodiments, the operation voltage may be used to control a distance between the membrane76and the active layer30. The spring74and the membrane76may be spaced apart from the ohmic contact layer50. The spring74may connect the fixation part72to the membrane76. The spring74may be configured to have elasticity or restoring force.

The membrane76may move vertically on the substrate10, when an electrostatic force is applied thereto. The electrostatic force may be induced by a potential difference between the membrane76and the current spreading layer40. The membrane76and the lower reflection layer20may be configured to induce optical resonance. The laser light may be generated from the active layer30between the membrane76and the lower reflection layer20. The membrane76may have reflectivity of about 99.5%. The membrane76may emit the laser light.

The membrane76may be formed to have at least one hole90. The hole90may be aligned with the stopper80. The hole90and the stopper80may have the same shape. The hole90may have a diameter smaller than that of the stopper80. The membrane76may include at least one third reflection layer77and at least one fourth reflection layer79that are stacked repeatedly. In example embodiments, the third reflection layer77and the fourth reflection layer79may be formed of the same materials as the first reflection layer22and the second reflection layer24, respectively.

The fixation part72and the spring74may have a stacking structure different from the membrane76. To control a spring constant of the spring74, a metal layer may be further provided on the spring74and a thickness of a capping layer84may be adjusted.

The stopper80may separate the membrane76from the current spreading layer40. For example, a minimum separation distance between the membrane76and the current spreading layer40may be defined by the stopper80. The stopper80may include a bottom block82and the capping layer84. The bottom block82may include a silicon oxide layer or a silicon nitride layer. The bottom block82may be provided on the current spreading layer40. The capping layer84may be provided to cover the upper reflection layer70, the bottom block82, and the current spreading layer40. The capping layer84may include metal oxide (e.g., aluminum oxide, titanium oxide, zinc oxide, tungsten oxide, or tantalum oxide).

FIG. 3is a graph showing a relationship between reflectivity and wavelength.

Referring toFIGS. 1 and 3, the membrane76may induce a resonance of single longitudinal mode laser within a high reflectivity spectrum range of the lower reflection layer20. InFIG. 3, a curve “28” shows reflectivity spectra of a laser cavity between the lower reflection layer20and the upper reflection membrane76, and a curve “32” shows resonance mode spectra of a laser light between the lower reflection layer20and the upper reflection membrane76The lower reflection layer20had high reflectivity at a wavelength range having a width of about 100 nm. For example, the laser exhibited high reflectivity at a wavelength range of about 975 nm to 1075 nm.

The membrane76may be provided in such a way that an optical path length between it and the lower reflection layer20is about three times of a center wavelength of the laser. In other words, an effective cavity length between the membrane76and the lower reflection layer20may be three times of the wavelength of the laser. In this case, a free spectral range (FSR) had a width of about 120 nm. The single longitudinal mode laser may be generated in the high reflectivity spectrum range. Accordingly, the vertical-cavity surface-emitting laser may be used for a broadband wavelength swept light source.

FIG. 4is a graph showing a relationship between a displacement of the membrane76ofFIG. 1and a wavelength of an emitted light.

Referring toFIGS. 1,3, and4, a wavelength of laser was proportional to a displacement of the membrane76. Here, an abscissa axis ofFIG. 4represents a displacement of the membrane76, which is given by a generalized value or by dividing a moving distance of the membrane76by a unit length. The membrane76may be configured in such a way that it moves within one third of a length of a movable region between the membrane76and the current spreading layer40. An ordinate axis ofFIG. 4represents the wavelength of the laser.

The membrane76may induce optical resonance within a safe displacement region62. An emitting wavelength of the laser may be within the high reflectivity spectrum range of the lower reflection layer20.

The membrane76may be mechanically unstable within a pull-in region64and be stuck to the current spreading layer40. For example, the two-thirds of the movable region of the membrane76may be the pull-in region64with mechanical instability. Due to the presence of the stopper80, it is possible to prevent the membrane76from moving into the pull-in region64. Further, the stopper80may prevent the membrane76from being stuck to the current spreading layer40.

Accordingly, the surface-emitting laser can have a long lifetime and a high speed operation property.

The surface-emitting laser may be fabricated by a method described below.

FIGS. 5 through 13are sectional views illustrating a method of fabricating a surface-emitting laser according to example embodiments of the inventive concept.

Referring toFIG. 5, the lower reflection layer20, the current confining layer28, the active layer30, the current spreading layer40, the ohmic contact layer50, the sacrificial layer60, and the upper reflection layer70may be formed on the substrate10. The lower reflection layer20, the active layer30, the current spreading layer40, the ohmic contact layer50, the sacrificial layer60, and the upper reflection layer70may be sequentially formed using a epitaxial growth technology. The epitaxial growth technology may include a metal organic chemical vapor deposition (MOCVD) or a molecular beam epitaxy process (MBE).

Referring toFIG. 6, the upper reflection layer70, the sacrificial layer60, the ohmic contact layer50, the current spreading layer40, and the lower reflection layer20may be partially removed to expose the outermost edge region of the lower reflection layer20. The removal of the upper reflection layer70, the sacrificial layer60, the ohmic contact layer50, the current spreading layer40, and the lower reflection layer20may be performed using a photolithography process and an etching process.

Referring toFIG. 7, the upper reflection layer70and the sacrificial layer60may be patterned. In example embodiments, the upper reflection layer70and the sacrificial layer60may be patterned using a photolithography process and an etching process. The patterning of the upper reflection layer70may include steps of patterning separately the fixation part72, the spring74, and the membrane76. The ohmic contact layer50may be partially exposed as the result of the patterning of the upper reflection layer70and the sacrificial layer60. Further, the current confining layer28may be formed by an oxidation process or a proton implantation process. In example embodiments, the current confining layer28may be formed of metal oxide.

Referring toFIG. 8, the first electrode26, the second electrode52, and the third electrode78may be formed on the lower reflection layer20, the ohmic contact layer50, and the upper reflection layer70, respectively. The first electrode26, the second electrode52and the third electrode78may be formed using a photolithography process, a metal deposition process and a lift-off process. The metal deposition process may include a metal thermal evaporation process or a sputtering process. In addition, the solder ball34and the wire32may be formed on the first electrode26, the second electrode52, and the third electrode78.

Referring toFIG. 9, the hole90may be formed through the upper reflection layer70. The formation of the hole90may include a photolithography process and an etching process. For example, the photolithography process may include forming a photoresist pattern92on the upper reflection layer70. The photoresist pattern92may serve as an etch mask in the etching process. The etching process may include a dry etching process or a wet etching process.

Referring toFIG. 10, the sacrificial layer60and the ohmic contact layer50exposed by the hole90may be removed. In example embodiments, a wet etching process may be used to remove the sacrificial layer60. For example, during the wet etching process, the sacrificial layer60may be isotropically removed. The photoresist pattern92may be re-used as an etch mask in the wet etching process.

Referring toFIG. 11, the bottom block82may be formed on the current spreading layer40. The bottom block82may be formed by a thermal evaporation process or an e-beam deposition process. In example embodiments, the bottom block82may be formed using a physical vapor deposition, such as a dielectric thermal evaporation and a sputtering process. Here, the bottom block82may have the same shape and diameter as those of the hole90. Thereafter, the photoresist pattern92may be removed.

Referring toFIGS. 2 and 12, the sacrificial layer60below the spring74and the membrane76may be removed. The sacrificial layer60may be removed using a dry etching process or a wet etching process. The dry etching process may be performed using mixture of sulphur hexaflouride (SF6) and silicon tetrachloride (SiCl4) as a reaction gas. The wet etching process may be performed using mixture of ammonium hydroxide/hydrogen peroxide/deionized water as etching solution. The mixture solution may be prepared to have a mixture ratio of 150:1:1. The third electrode78may be used as an etch mask layer, when the sacrificial layer60is removed. Accordingly, the spring74and the membrane76of the upper reflection layer70may be floated or separated from the current spreading layer40.

Referring toFIG. 13, the capping layer84may be formed on the substrate10. The capping layer84may be formed using an atomic layer deposition or a chemical vapor deposition. The capping layer84may be formed to cover the bottom block82and the current spreading layer40. The bottom block82and the capping layer84may constitute the stopper80. The stopper80may have a diameter greater than that of the hole90.

Further, the hole90may have a reduced size, due to the presence of the capping layer84. Accordingly, a vertical movement of the membrane76may be limited by the stopper80. The capping layer84may serve as a protection layer for the current spreading layer40. Accordingly, the use of the method makes it possible to form the stopper80, whose diameter is greater than that of the hole90.

According to example embodiments of the inventive concept, a vertical-cavity surface-emitting laser may include a substrate, a lower reflection layer, an active layer, a current spreading layer, a sacrificial layer, an upper reflection layer, and a stopper. The sacrificial layer may be locally provided on the current spreading layer. The stopper may also be locally provided on the current spreading layer. In other words, the stopper may be provided spaced apart from the sacrificial layer. The sacrificial layer may have a top surface higher than that of the stopper. The upper reflection layer may include a fixation part, a spring, and a membrane. The spring may connect the fixation part to the membrane. The fixation part may be supported by the sacrificial layer. The membrane may be vertically separated from the stopper and the current spreading layer. Due to the presence of the stopper, it is possible to prevent the membrane from being stuck to the current spreading layer. This makes it possible to improve stability in mechanical operation of the device, and to increase an operation speed of the device.

Accordingly, the surface-emitting laser according to example embodiments of the inventive concept can have a long lifetime and a high speed operation property.