Device structure for semiconductor lasers

Abstract

A vertical-cavity surface-emitting laser structure is provided. The device comprises a structure which consists of: a substrate; a multi-layered structure stacked over the substrate, which consists of a bottom distributed Bragg reflector, a bottom cladding or spacer layer, a light-emitting active layer, a top cladding or spacer layer, a top distributed Bragg reflector (DBR). An annular disordered region (or disordered absorber) with an aperture is formed in a part of top or bottom DBR for transverse modes control; an active region aligned with the aperture of the annular disordered region is formed on the light-emitting active layer; and a p-electrode and an n-electrode are formed on a p-type and an n-type layers respectively. The device structure is to provide a vertical-cavity surface-emitting laser that can operate in a stable single-mode with a sufficient output power and high yield production.

Description

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to semiconductor lasers, and particularly relates to semiconductor vertical-cavity surface-emitting lasers that utilize disordered region structure to achieve single mode and high power operation.

[0004] 2. Description of the Relate Art

[0005] In recent years, vertical-cavity surface-emitting lasers (VCSELs)[Koyama et al “Room-temperature continuous wave lasing characteristics of a GaAs vertical-cavity surface-emitting laser,” Appl. Phys. Lett. vol. 55, 221-222, 1989] have become important light sources for the various optical communication and storage systems due to their unique features, such as the low threshold current, single longitudinal mode operation, and low divergent beam. Particularly, vertical-cavity surface-emitting lasers with stable single-mode operation that operates in both a single longitudinal mode and a single transverse mode are highly desirable for high speed long haul communication to minimize dispersion effects, for wavelength-division-multiplex (WDM) system to avoid interchannel crosstalk, and for optical storage and printing systems to obtain a single circular pattern. Here we define the stable single-mode operation as a lasing with a single-mode that can maintain over the entire drive current range above the threshold current. The vertical-cavity surface-emitting lasers typically operate in a single longitudinal mode due to their built-in distributed Bragg reflectors (DBRs) and the wide mode spacing (30-40 nm). However, the single transverse mode is more difficult to achieve because it requires either a good current confinement scheme in transverse direction to form a single transverse mode active region with diameter usually less than ˜5 μm, or an optical structure in the cavity for single transverse mode selection. In the prior art, although various vertical-cavity surface-emitting laser (VCSEL) structures have been fabricated, only few devices exhibited stable single-mode operation. The etched mesa vertical-cavity surface-emitting laser (VCSEL) [Jewell et al “Low threshold electrically pumped vertical-cavity surface-emitting microlaser,” Electron. Lett., vol. 25, pp. 1123-1124, 1989] using a mesa structure to confine the injection current and optical field usually operates in multimode due to the strong index-guided structure. The ion-implanted vertical-cavity surface-emitting laser (VCSEL) [Geel et al “Low threshold planarized vertical-cavity surface-emitting lasers,” IEEE Photon. Technol. Lett., vol. 2, pp. 234-236, 1990] with active regions defined by ion implantation can maintain single-mode operation only at lower current levels, and exhibits multimode at higher current levels. Although the vertical-cavity surface-emitting laser (VCSEL) with a passive antiguide region [Wu et al “High-yield processing and single-mode operation of passive antiguide region vertical-cavity lasers,” IEEE J. Select. Topics Quantum Electron. Vol. 3, pp-429-434, 1997] can operate in a stable single-mode, the device fabrication which requires a crystal regrowth is more complicated. The oxide-confined vertical-cavity surface-emitting laser (VCSEL) [Grabherr et al “Efficient single-mode oxide-confined GaAs VCSEL's emitting in the 850 nm wavelength regime,” IEEE Photon. Technol. Lett. vol. 9, pp. 1304-1306, 1997] requires an oxidation process to convert an AlAs layer to an AlOx layer forming an active region less than 3 μm diameter to have stable single-mode operation. This laser structure demands a very critical control on both the epilayer growth to vertically position the AlOx layer close to a node of the optical standing-wave and the oxidation process to laterally oxidize the AlAs layer to a desired length within ˜1 μm accuracy. The vertical-cavity surface-emitting laser with an etched surface on the top side [Unold et al “Increased-area oxidized single-fundamental mode VCSEL with self-aligned shallow etched surface relief,” Electron. Lett., vol. 35, pp. 1340-1341, 1999] to suppress the higher-order modes can perform single-mode operation only up to five times threshold current, it also requires a critical control on the etched depth within a range of 50 nm (0.05 μm). The same inventor previously demonstrated that a vertical-cavity surface-emitting laser with a top selectively disordered mirror formed by zinc (Zn) diffusion through the entire (100%) top distributed Bragg reflector (DBR) [Dziura, T. G., Yang, Y. J., et al “Single mode surface emitting laser using partial mirror disordering,” Electron. Lett., vol. 29, pp. 1236-1237, 1993] can maintain stable single-mode operation, but the device suffers from a large optical loss that includes mirror transmission and absorption loss due to a reduced reflectance and a high doping concentration respectively in the deep Zn diffused disordered DBR region (>3 μm,—>100% DBR), resulting in a higher threshold current and a very low output power (<0.25 mW) which is insufficient for practical use. All of the abovementioned vertical-cavity surface-emitting laser structures and associated fabrication methods, though widely used, have exhibited either an unsatisfactory performance with an unstable single-mode operation or a high threshold current and an insufficient output power, or a great difficulty in fabrication, particularly for the large scale production. Consequently, there is a demand for a device that can operate in a stable single-mode with a sufficient output power (>1 mW) for most applications, and can be readily fabricated by the conventional semiconductor technology and also compatible with the mass production.

SUMMARY OF THE INVENTION

[0006] The purpose of the present invention is to provide a vertical-cavity surface-emitting laser that can operate in a stable single-mode including both a single longitudinal mode and a single transverse mode with a sufficient output power useful for most applications. The device also can be readily fabricated by the conventional semiconductor technology and compatible with the mass production.

[0007] The vertical-cavity surface-emitting laser comprises a structure, which consists of:

[0008] a substrate;

[0009] a multi-layered structure stacked over the substrate, which consists of a bottom distributed Bragg reflector (DBR), a bottom cladding or spacer layer, a light-emitting active layer, a top cladding or spacer layer, a top distributed Bragg reflector (DBR).

[0010] an annular disordered region (or disordered absorber) with an aperture where the DBR is intact (or non-disordered) is formed in a part of the top or bottom DBR for transverse modes control;

[0011] an active region with its center aligned with the aperture of the annular disordered region (or disordered absorber) is formed on the light-emitting active layer; and

[0012] a p-electrode and an n-electrode are formed on a p-type and an n-type layers respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a cross-sectional view, schematically showing a vertical-cavity surface-emitting laser according to an embodiment of the invention, with an annular disordered region (or disordered absorber) formed in the distributed Bragg reflector and an active region formed by ion implantation.

[0014] FIG. 2 is a graph of the light output power versus current characteristics of the device of the present invention.

[0015] FIG. 3 is a graph of the emission spectra at different current levels of the device of the present invention.

[0016] FIG. 4 is a cross-sectional view, schematically showing a vertical-cavity surface-emitting laser according to an embodiment of the invention, with an annular disordered region (or disordered absorber) formed in the distributed Bragg reflector and an active region formed by oxidation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] In the present invention, the terms “vertical-cavity surface emitting laser”, “distributed Bragg reflector”, and “single transverse mode” are used in an ordinary sense in the field of semiconductors.

[0018] In the present invention, the term “stable single-mode operation” means that the laser devices operate in single-mode over the entire drive current range above the threshold current.

[0019] In the present invention, the term “disordered region” (or disordered absorber) means that a multilayer including DBR of the corresponding region is either totally or partially mixed together in terms of composition and layer structure, which is usually induced by diffusion or thermal process. The disordered region will cause a large optical loss that includes mirror transmission and absorption loss due to a reduced reflectance and a high doping concentration respectively in a heavily doped multilayer region.

[0020] FIG. 1 schematically shows the cross-section of a vertical-cavity surface emitting laser (VCSEL) (10). It comprises of a substrate (11) and a multi-layered structure stacked over the substrate, which consists of a bottom distributed Bragg reflector (12), a bottom cladding or spacer layer (13), a light-emitting active layer (14) which can be of a single layer, or a quantum-well structure, a top cladding or spacer layer (15), a top distributed Bragg reflector (16).

[0021] Both the top and bottom distributed Bragg reflector (12, 16) are typically made of many pairs of alternate layers (17), such as GaAs/AlAs, InxGal−xAsyPl−y/InP, each alternate layer (17) has a thickness of a quarter of the corresponding lasing wavelength, and the pair number of alternate layers (17) is designed to be large enough to give a very high reflectivity (>99%) with respect to the lasing wavelength, usually it is preferably between 20 and 40. The pair of alternate layer (17) usually consists of an interface of grading composition.

[0022] In a part of the laser multi-layers a torus region is selectively heavily doped (>5×1018/cm3) with an aperture (18) that remains intact to form an annular disordered region (or disordered absorber) (19) to suppress the higher-order modes by using a conventional process, such as diffusion, implantation, or regrowth. The annular disordered region (or disordered absorber) (19) can be doped with a p-type dopant such as zinc (Zn), magnesium (Mg), beryllium (Be), strontium (Sr), or barium (Ba), or an n-type dopant such as silicon (Si), germanium (Ge), selenium (Se), sulfur (S), or tellurium (Te). The purpose of forming an annular disordered region (or disordered absorber) (19) with an aperture (18), which allows only the light of fundamental mode transmit through, is to suppress the lasing of higher-order modes other than the fundamental mode. To achieve this end, the aperture (18) is preferably to have a diameter or a longest diagonal between 1 and 8 μm, more preferably between 4 and 6 μm. The thickness of the annular disordered region (or disordered absorber) (19) will determine the optical loss of the light passing through and the degree of the higher-order mode suppression. Since the part (<10%) of the fundamental mode light may also couple to the annular disordered region (or disordered absorber) (19) resulting in an optical loss, the thickness of the annular disordered region (or disordered absorber) (19) needs to be optimized.

[0023] Therefore, to have an annular disordered region (or disordered absorber) (19) thick enough to suppress the higher-order mode but not to cause a significant optical loss for the fundamental mode, the annular disordered region (or disordered absorber) (19) thickness is preferably between 3 and 95%, more preferably between 10 and 50%, most preferably between 15 and 40%, of the thickness of the distributed Bragg reflector where the annular disordered region (or disordered absorber) (19) is usually located.

[0024] To reduce the optical loss of the fundamental mode, the annular disordered region (or disordered absorber) (19) is preferably formed at the far end of the distributed Bragg reflector away from the light-emitting active layer (14).

[0025] Around the light-emitting active layer (14), a current confinement structure (20) is formed to confine the injection current in an active region (21) with its center aligned with the aperture (18) of the annular disordered region (or disordered absorber) (19), by using a conventional semiconductor process, such as implantation, diffusion, oxidation, or mesa etching. The diameter of the active region (21) is preferably between 1 and 50 μm, more preferably between 5 and 15 μm.

[0026] To form a p-n junction of the device, the semiconductor layers above and below the light-emitting active layer (14) are formed to be p- and n-typed, or n- and p-typed respectively.

[0027] A p-electrode (22) and an n-electrode (23) are formed on a p-type layer and an n-type layer of the laser structure respectively. An opening with its center aligned with the active region (21) and the aperture (18) of the annular disordered region (or disordered absorber) (19) is formed on either the p- or n-electrode to allow the light emitted out.

[0028] In the present invention, the vertical-cavity surface-emitting laser (10) can be made of materials of semiconductor and dielectric systems, such as AlxGal−xAs, AlxGayInl−x−yAs, InxGal−xAsyPl−y, AlxGayInl−x−yP/AlxGal−xAs, InxGayNl−x−yAs, GaxAlyInl−x−yN, GaAsxSbl−x, ZnxCdl−xSySel−y, SiO2/Si3N4, SiO2/TiO2, or Si/SiO2. The lasing wavelength of the vertical-cavity surface-emitting laser (10) is mainly determined by the material and structure used.

[0029] The present invention will be described below by way of the following examples.

EXAMPLE 1

[0030] The wafer used for the fabrication of a single-mode 850 nm vertical-cavity surface-emitting laser (VCSEL) consisted of a typical VCSEL epilayer structure, a three GaAs/AlGaAs multiquantum wells (MQW) with the top and bottom cladding layers sandwiched by a 30-pair n-type and a 20-pair p-type Al0.12Ga0.88As/Al0.9Ga0.1As layers with interfaces of grading composition. The completed device shown in FIG. 1 was fabricated as follows: First a Si3N4 mask with 5 μm diameter circles was formed on the sample, then the masked sample with a Zn2As3 source was sealed in a vacuumed quartz ampoule and put into a 650° C. furnace for 8 min short time Zn diffusion. It was intended to have a Zn diffused region with a thickness less than 0.5 μm, corresponding to 15% of the top p-type distributed Bragg reflector multilayer, outside the Si3N4 masked area to form an annular disordered region (or disordered absorber). Following the Zn diffusion the sample was selectively implanted with proton of an energy of 300 keV and a dosage of 1×1014, using a 6 μm thick photoresist layer with a 15 μm diameter as an implanted mask. After the photoresist layer was stripped off, a Cr/Au film with a 15×15 μm2 emitting window was deposited on the top and a Ge/Au film was deposited on the backside of the sample to form a p- and an n-type electrodes respectively. FIG. 2 shows the typical light output power and voltage versus current characteristics of a fabricated device. The performance with a low threshold current of 3.0 mA and a maximum output power of >3.0 mW was obtained, which was substantially better than that of the vertical-cavity surface-emitting laser with a thick zinc diffusion region (>3.0 μm, 100% of the top distributed Bragg reflector), in which typically the threshold current was >8 mA and the output power was only 0.25 mW. FIG. 3 shows the corresponding emission spectra of the device at different current levels, which indicates that the device operates in a stable single-mode with a higher-order mode suppression ratio better than 40 dB up to the maximum drive current where the light output saturates. The number of laser devices fabricated from the wafer was about 12,000. The yield of better than 95% stable single-mode laser devices was obtained.

[0031] Thus, it was confirmed that, according to the present invention, a vertical-cavity surface-emitting laser with a stable single-mode operation and a sufficient output power (>1 mW) is provided.

EXAMPLE 2

[0032] The same wafer as in Example 1 was used to fabricate a single-mode 850 nm vertical-cavity surface-emitting laser. The completed device shown in FIG. 4 was fabricated as follows: First a Si3N4 mask with 5 μm diameter circles was formed on the sample, then a Zn(3000 A)/Au(1000 A) film was deposited on the masked sample, which was put into a 650° C. furnace for 10 min open-tube Zn diffusion. It was intended to have a Zn diffused region with a thickness <0.8 μm, corresponding to 25% of the top p-type distributed Bragg reflector, outside the Si3N4 masked area to form an annular disordered region (or disordered absorber). After the Zn diffusion the sample was etched to form mesa/moat structure. The moats were etched with ˜1 μm deeper than the light-emitting active layer to expose the AlGaAs layer and isolate the devices electrically. Then the sample was put in a 415° C. furnace with 90° C. H2O vapor flowing to oxidize the exposed AlGaAs to form a current confined active region. Following the oxidation a 2000 Å SiO2 film was deposited over the entire sample and a 30×30 μm2 square window was opened on the top of mesa. At final step a Cr/Au film with a 15×15 μm2 window was deposited on the top and a Ge/Au film was deposited on the backside of the sample to form a p- and an n-type contacts respectively. The performances of the vertical-cavity surface-emitting laser fabricated were substantially the same as those of the devices of Example 1. The yield was also the same as in Example 1.

EXAMPLE 3

[0033] The same wafer as in Example 1 was used to fabricate a single-mode 850 nm vertical-cavity surface-emitting laser. The completed device essentially same as that shown in FIG. 1 was fabricated as follows: First a 3000 A thick Au film with 5 μm diameter circles was formed photolithographically on the sample, then the masked sample was selectively implanted with Zn of an energy of 2.5 MeV and a dosage of 1×1015/cm2. After the implantation the sample was annealed at 900° C. for 30 sec to activate the implanted Zn forming an annular disordered region (or disordered absorber). Following the Zn implantation and annealing the sample was selectively implanted with proton of an energy of 300 keV and a dosage of 3×104, using a 6 μm thick photoresist layer with a 15 μm diameter as an implanted mask. After the photoresist layer was stripped off, a Cr/Au film with a 15×15 μm2 emitting window was deposited on the top and a Ge/Au film was deposited on the backside of the sample to form a p- and an n-type electrode respectively. The performances of the vertical-cavity surface-emitting laser fabricated were substantially the same as those of the devices of Example 1. The yield was also the same as in Example 1.

EXAMPLE 4

[0034] The wafer for the fabrication of a single-mode 1.3 μm vertical-surface emitting-laser (VCSEL) consisted of a typical VCSEL epilayer structure, a three InGaAsP/InP multiquantum wells (MQW) with the top and bottom cladding layers sandwiched by a 40-pair n-type and a 35-pair p-type InGaAsP/InP layers with interfaces of grading composition. The completed device was fabricated as follows: First a Si3N4 mask with 5 μm diameter circles was formed on the sample, then the masked sample with a Zn2As3 source was sealed in a vacuumed quartz ampoule and put into a 650° C. furnace for 15 min Zn diffusion. It was intended to have a Zn diffused region with a thickness less than 1.5 μm, corresponding to 20% of the top p-type distributed Bragg reflector multilayer, outside the Si3N4 masked area to form an annular disordered region (or disordered absorber). Following the Zn diffusion the sample was selectively implanted with proton of energy 650 KeV and a dosage of 1×1014, using a 10 μm thick photoresist layer with a 15 μm diameter as an implanted mask. After the photoresist layer was stripped off, a Ti/Pt/Au film with a 15×15 μm2 emitting window was deposited on the top and a Ni/AuGe/Ni/Au film was deposited on the backside of the sample to form a p- and an n-type electrode respectively.

Claims

1. A vertical-cavity surface-emitting laser comprises: a substrate; a multi-layered structure stacked over the substrate, which consists of a bottom distributed Bragg reflector (DBR), a bottom cladding or spacer layer, a light-emitting active layer, a top cladding or spacer layer, a top distributed Bragg reflector (DBR); an annular disordered region (or disordered absorber) with an aperture where the DBR is intact (or non-disordered) formed in a part of said top or bottom DBR for transverse modes control, said annular disordered region (or disordered absorber) having a partial thickness of said top or bottom DBR, wherein the thickness of said annular disordered region (or disordered absorber) is controlled to be less than the total thickness of located said top or bottom DBR to minimize the optical loss for the lasing modes, so as to achieve a higher output power. an active region with its center aligned with said aperture of said annular disordered region (or disordered absorber) formed on said light-emitting active layer; and a p-electrode and an n-electrode formed on a p-type and an n-type layer respectively.

2. The device according to claim 1, wherein said annular disordered region (or disordered absorber) is formed of a heavily doped region with a dopant concentration larger than 5×1018/cm3, which has to be large enough to cause

3. The device according to claim 1, wherein said aperture of said annular disordered region (or disordered absorber) has a diameter or a longest diagonal of 1 to 8 μm, or an area of 1 to 60 μm2 to suppress the higher-order transverse modes and enhance the fundamental transverse mode (TEMOO).

4. The device according to claim 2, wherein said dopant is formed of zinc (Zn), magnesium (Mg), beryllium (Be), strontium (Sr), barium (Ba), cadmium (Cd), silicon (Si), germanium (Ge), tin (Sb), selenium (Se), sulfur (S), or tellurium (Te).

5. The device according to claim 1, wherein said annular disordered region (or disordered absorber) has a thickness of 3% to 90% of that of located said top or bottom DBR.