WO2008102967A1 - Semiconductor laser diode with quantum wells structure - Google Patents

Semiconductor laser diode with quantum wells structure Download PDF

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Publication number
WO2008102967A1
WO2008102967A1 PCT/KR2008/000932 KR2008000932W WO2008102967A1 WO 2008102967 A1 WO2008102967 A1 WO 2008102967A1 KR 2008000932 W KR2008000932 W KR 2008000932W WO 2008102967 A1 WO2008102967 A1 WO 2008102967A1
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layer
sch
type
layers
laser diode
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PCT/KR2008/000932
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French (fr)
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Sang Wan Ryu
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Industry Foundation Of Chonnam National University
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/30—Structure or shape of the active region; Materials used for the active region
    • H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
    • H01S5/2004—Confining in the direction perpendicular to the layer structure
    • H01S5/2009—Confining in the direction perpendicular to the layer structure by using electron barrier layers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/30—Structure or shape of the active region; Materials used for the active region
    • H01S5/305—Structure or shape of the active region; Materials used for the active region characterised by the doping materials used in the laser structure
    • H01S5/3077—Structure or shape of the active region; Materials used for the active region characterised by the doping materials used in the laser structure plane dependent doping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/30—Structure or shape of the active region; Materials used for the active region
    • H01S5/32—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures
    • H01S5/3211—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures characterised by special cladding layers, e.g. details on band-discontinuities
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/30—Structure or shape of the active region; Materials used for the active region
    • H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
    • H01S5/343—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser
    • H01S5/34306—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser emitting light at a wavelength longer than 1000nm, e.g. InP based 1300 and 1500nm lasers

Definitions

  • the present invention relates to a semiconductor laser diode, and more particularly, to a semiconductor laser diode having a quantum well structure and an improved temperature characteristic.
  • InGaAsP/InP-based material is generally used. Crystal growth and regrowth are easy with this material, and related processing techniques have been developed. Thus, the material is used for fabricating optical devices for communication.
  • FIG. 1 illustrates an energy -band structure of a semiconductor laser diode having a quantum well structure according to conventional art.
  • undoped first and second Separate Confinement Het- erostructure (SCH) layers 2 and 4 are interposed between a first cladding layer 1 of n- type InP and a second cladding layer 5 of p-type InP, and an active layer 3 consisting of a plurality of quantum well layers 3a and barrier layers 3b is formed in the middle of the SCH structure.
  • SCH Separate Confinement Het- erostructure
  • the first and second SCH layers 2 and 4 have smaller band gaps than the first and second cladding layers 1 and 5, and the quantum well layers 3a have a smaller band gap than the first and second SCH layers 2 and 4. Electrons and holes inserted through the first and second cladding layers 1 and 5 are captured in a quantum well and can provide optical gain.
  • the quantum well is formed by inserting, between different semiconductor layers, a thin semiconductor layer having a smaller energy band gap than the different semiconductor layers.
  • electrons or holes are confined by an energy barrier and thus move in a two-dimensional plane.
  • the quantum well layers 3a denote thin layers having a small energy band gap
  • the barrier layers 3b denote layers having a larger energy band gap than the quantum well layers 3a and surrounding the quantum well.
  • electrons and holes in the quantum well layers 3a are confined to quantum wells by the barrier layers 3b.
  • the quantum well structure is prepared, electrons and holes in the active layer 3 are recombined in the quantum wells and emit light.
  • optical characteristics of the active layer 3 are determined by the quantum wells.
  • InGaAsP/InP-based material has a small conduction band offset, and thus electrons injected into the quantum wells easily escape to the barrier layers 3b or the SCH structure. Since the amount of escaping electrons remarkably increases according to temperature, characteristics of the laser deteriorate at high temperature.
  • InGaAlAs-based material has a relatively large conduction band offset, and thus characteristics slightly deteriorate according to an increase in temperature.
  • the present invention is directed to providing a semiconductor laser diode having a quantum well structure in which n-type and p-type modulation-doping are respectively performed on outer regions of Separate Confinement Heterostructure (SCH) layers to change energy band shapes of the SCH layers, and thus a temperature characteristic is improved.
  • SCH Separate Confinement Heterostructure
  • the present invention is also directed to providing a semiconductor laser diode having a quantum well structure in which n-type modulation-doping is performed on an SCH layer adjacent to a p-type cladding layer to a specific thickness, and thus an energy band shape of the SCH layer is changed to improve a temperature characteristic.
  • One aspect of the present invention provides a semiconductor laser diode having a quantum well structure, comprising: a lower cladding layer formed on a substrate; a first Separate Confinement Heterostructure (SCH) layer formed on the lower cladding layer and having an n-type doped region in an upper part of the first SCH layer and a p-type doped region in a lower part of the first SCH layer; an active layer formed on the first SCH layer and having a multi-quantum- well structure in which a plurality of quantum well layers and barrier layers are alternately formed; a second SCH layer formed on the active layer and having an n-type doped region in a lower part of the second SCH layer and a p-type doped region in an upper part of the second SCH layer; an upper cladding layer formed on the second SCH layer; and a contact layer formed on the upper cladding layer, wherein the n-type doped regions are formed in regions of the first and second SCH layers adjacent to the active layer to face
  • the first and second SCH layers may be Graded Index (GRIN) SCH layers in which a plurality of different layers are arranged in order of band gap.
  • GRIN Graded Index
  • Another aspect of the present invention provides a semiconductor laser diode having a quantum well structure, comprising: n-type and p-type cladding layers; an active layer of a multi-quantum- well structure having a plurality of quantum well layers and barrier layers alternately formed between the n-type and p-type cladding layers; a first Separate Confinement Heterostructure (SCH) layer formed between the n-type cladding layer and the active layer; and a second SCH layer formed between the p-type cladding layer and the active layer, wherein n-type and p-type doped regions are respectively formed to specific thicknesses in upper and lower parts of the second SCH layer, the n-type doped region is formed in a region of the second SCH layer adjacent to the active layer to face the active layer, and the p-type doped region is formed in a region of the second SCH layer adjacent to the p-type cladding layer.
  • SCH Separate Confinement Heterostructure
  • Still another aspect of the present invention provides a semiconductor laser diode, comprising: n-type and p-type cladding layers; an active layer of a multi-quantum-well structure having a plurality of quantum well layers and barrier layers alternately formed between the n-type and p-type cladding layers; a first Separate Confinement Heterostructure (SCH) layer formed between the n-type cladding layer and the active layer; and a second SCH layer formed between the p-type cladding layer and the active layer, wherein an n-type doped region is formed to a specific thickness in a region of the second SCH layer adjacent to the active layer.
  • SCH Separate Confinement Heterostructure
  • a p-type doped region may be formed in a region of the first SCH layer adjacent to the n-type cladding layer, or an n-type doped region may be formed in a region of the second SCH layer adjacent to the active layer.
  • n-type and p-type modulation-doping are respectively performed on outer regions of Separate Confinement Heterostructure (SCH) layers to change energy band shapes of the SCH layers, and thus a temperature characteristic is improved.
  • SCH Separate Confinement Heterostructure
  • FIG. 1 illustrates an energy -band structure of a semiconductor laser diode having a quantum well structure according to conventional art.
  • FIG. 2 is a cross-sectional view of a semiconductor laser diode having a quantum well structure according to an exemplary embodiment of the present invention.
  • FIG. 3 illustrates an energy-band structure of a semiconductor laser diode having a quantum well structure according to an exemplary embodiment of the present invention.
  • FIG. 4 illustrates an energy -band structure of a semiconductor laser diode having a quantum well structure according to another exemplary embodiment of the present invention.
  • FIG. 5 illustrates an energy-band structure of a semiconductor laser diode having a quantum well structure according to still another exemplary embodiment of the present invention.
  • FIG. 6 is a reference diagram showing detailed materials of a band diagram according to an experimental embodiment of the present invention.
  • FIG. 7 illustrates graphs showing high-temperature operating characteristics of
  • LDs Fabry-Perot Laser Diodes
  • FIG. 8 illustrates graphs showing a characteristic of a single-mode Distributed
  • FIG. 2 is a cross-sectional view of a semiconductor laser diode having a quantum well structure according to an exemplary embodiment of the present invention.
  • FIG. 3 illustrates an energy-band structure of a semiconductor laser diode having a quantum well structure according to an exemplary embodiment of the present invention
  • FIG. 4 illustrates an energy-band structure of a semiconductor laser diode having a quantum well structure according to another exemplary embodiment of the present invention.
  • the semiconductor laser diode having a quantum well structure includes a substrate 100, and a buffer layer 200, a lower cladding layer 300, a first Separate Confinement Heterostructure (SCH) layer 400, an active layer 500, a second SCH layer 600, an upper cladding layer 700 and a contact layer 800 formed on the substrate 100 in sequence.
  • SCH Separate Confinement Heterostructure
  • the substrate 100 may be a semiconductor substrate such as an n-type InP substrate.
  • the buffer layer 200 is formed on the substrate 100 to compensate a surface unevenness of the substrate 100 and stack the upper layers as evenly as possible.
  • the buffer layer 200 may be formed of, for example, n-type InP or
  • the buffer 200 has a thickness of about 1 D and is doped with, for example, silicon (Si) as n-type impurities.
  • the lower cladding layer 300 is formed on the buffer layer 200 and may be formed of, for example, n-type InP or InGaAsP.
  • the lower cladding layer 300 has a thickness of about 1 D and is doped with, for example, silicon (Si) as n-type impurities.
  • the first SCH layer 400 which is an optical guide layer guiding oscillation of a laser beam, is formed on the lower cladding layer 300, and may be formed of, for example, undoped InGaAsP.
  • n-type and p-type doped regions 400a and 400b having specific thicknesses are formed in upper and lower parts of the first SCH layer 400, respectively.
  • the first and second SCH layers 400 and 600 may consist of a single layer.
  • a modulation- doped structure of the present invention may be applied to the first and second SCH layers 400 and 600, and thus the first and second SCH layers 400 and 600 may be Graded Index (GRIN) SCH layers in which a plurality of different layers 400- 1 to 400-3 and 600-1 to 600-3 are arranged in order of band gap.
  • GRIN Graded Index
  • the active layer 500 is formed on the first SCH layer 400 and has a multi- quantum- well structure in which a plurality of quantum well layers 500a and barrier layers 500b are alternately formed.
  • the second SCH layer 600 which is an optical guide layer guiding oscillation of a laser beam, is formed on active layer 500, and may be formed of, for example, undoped InGaAsP.
  • n-type and p-type doped regions 600a and 600b having specific thicknesses may be formed in lower and upper parts of the second SCH layer 600 to be symmetrical to the first SCH layer 400 with respect to the active layer 500, re- spectively.
  • n-type and p-type doped regions 600a and 600b of the second SCH layer 600 have the same constitution and effect as the above described n-type and p-type doped regions 400a and 400b of the first SCH layer, and thus a description thereof will not be repeated.
  • the upper cladding layer 700 is formed on the second SCH layer 600 and may be formed of, for example, p-type InP or InGaAsP.
  • the upper cladding layer 700 has a thickness of about 1 D and is doped with, for example, zinc (Zn) as p-type impurities.
  • the contact layer 800 is formed on a part of the upper cladding layer 700 and may be formed of, for example, p-type InGaAs.
  • the contact layer 800 has a thickness of about 100 D and is doped with, for example, Zn as p-type impurities.
  • a pair of n-type and p-type modulation-doped regions are formed in the first and second SCH layers 400 and 600 according to an exemplary embodiment of the present invention.
  • the present invention not being limited thereto, the pair of n-type and p-type modulation-doped regions may be formed in only one of the SCH layers.
  • n-type and p-type modulation doping is performed on the upper and lower parts of the first SCH layer 400 to generate ions in the modulation- doped regions.
  • an electric field is generated in the region.
  • Such an electric field causes an energy band to bend and finally forms an energy band structure as shown in FIG. 3 or 4.
  • an energy barrier to electrons is relatively increased in such a structure.
  • overflow of electrons is suppressed, and a temperature characteristic of the semiconductor laser diode can be improved.
  • all impurities are depleted from the n-type and p-type doped regions
  • two-dimensional doping concentrations of the n-type and p-type doped regions 400a and 400b must be adjusted to be identical.
  • Equation 1 A relationship between an electric field E generated by such impurity ions and a doping concentration is given as a one-dimensional Poisson s equation shown in Equation 1 below.
  • Equation 1 When Equation 1 is used, it is possible to calculate the strength of an electric field (
  • Equation 2 Equation 2
  • ⁇ 5 nd denotes a two-dimensional charge density. Assuming that a distance between an n-type doped region and a p-type doped region is L, an increase ⁇ V in potential barrier due to modulation doping is calculated using Equation 3 below.
  • 1 8 n-type and p-type regions are doped in a concentration of 1x10 and have a thickness of 10 D.
  • an electric field generated by modulation doping has a strength of 1.4x10 V/cm.
  • a distance between the n-type and p- type regions is 50 D
  • a height of a potential barrier generated from the above mentioned factors is 0.7 V.
  • an energy barrier to electrons increases by 0.7 eV due to modulation doping.
  • the value is remarkably larger than 0.1 to 0.2 eV that is a general conduction band energy barrier of an InP-based laser for communication. Consequently, the proposed structure is expected to remarkably suppress overflow of electrons.
  • the buffer layer 200 is grown on the substrate 100 using a Metal Organic
  • MOCVD Chemical Vapor Deposition
  • the SCH layer 400, the active layer 500 having a multi-quantum- well structure, the second SCH layer 600, the upper cladding layer 700 and the contact layer 800 are stacked in sequence.
  • modulation doping is performed so that the n-type and p-type doped regions
  • 400a and 400b are formed to specific thicknesses in upper and lower parts of the first SCH layer 400, which is the feature of the present invention.
  • modulation doping is performed so that the n-type and p-type doped regions 600a and 600b having specific thicknesses are formed in lower and upper parts of the second SCH layer 600 to be symmetrical to the first SCH layer 400 with respect to the active layer 500, respectively.
  • FIG. 5 illustrates an energy -band structure of a semiconductor laser diode having a quantum well structure according to still another exemplary embodiment of the present invention.
  • the semiconductor laser diode having a quantum well structure includes a substrate 100, and a buffer layer 200, a lower cladding layer 300, a first SCH layer 400, an active layer 500, a second SCH layer 600, an upper cladding layer 700 and a contact layer 800 formed on the substrate 100 in sequence.
  • the active layer 500 has a multi-quantum- well structure in which a plurality of quantum well layers and barrier layers are alternately formed between the lower cladding layer (n-type cladding layer) 300 and the upper cladding layer (p-type cladding layer) 700.
  • an n-type doped region 400a is formed to a specific thickness in a region adjacent to the active layer 500 in the second SCH layer 600.
  • the effect of the present invention can be partially obtained even when the n-type doped region 400a alone is formed.
  • a p-type doped layer 600b may be formed in the second SCH layer 600 adjacent to the upper cladding layer (p-type cladding layer) 700.
  • the upper cladding layer (p-type cladding layer) 700 is p-type doped, and thus the p-type doped region 600b may not be formed in the second SCH layer 600.
  • n-type and p-type doped regions 440a and 440b are preferably formed in upper and lower parts of the first SCH layer 400.
  • FIG. 6 is a reference diagram showing detailed materials of a band diagram according to this experimental embodiment of the present invention.
  • Inventors of the present invention designed an epitaxial structure having a modulation-doped region to be employed in an active layer of a 1.3 D multi- quantum- well structure.
  • an SCH region of the active layer a pair of n- and p-type doped regions having a thickness of 5 D were formed.
  • a doping concentration of the n- and p-type doped regions was changed from 1x10 cm (sample A) to 1x10 cm (sample B).
  • Table 1 The detailed structure is shown in Table 1 below.
  • 1.0Q, 1.1Q, etc. denote materials that are obtained by lattice-matching InGaAsP, which is a compound of the four elements, with InP, and respective elemental compositions thereof have band gaps corresponding to wavelengths of 1.0 D, 1.1 D, and so on.
  • a substrate having a thickness of 350 Dand formed of InP was used.
  • FIG. 7 illustrates graphs showing high-temperature operating characteristics of Fabry- Perot laser diodes fabricated using an epitaxial structure according to this experimental embodiment of the present invention.
  • the reference sample and samples A and B have similar temperature characteristics.
  • the modulation-doped sample has an excellent temperature characteristic at a temperature of 50 0 C or above, and samples A and B have better temperature characteristics at a high temperature. Meanwhile, it can be seen that sample A has a better laser characteristic than sample B at a high temperature.
  • the maximum operating temperatures of the reference sample, sample A and sample B were measured to be 70 0 C, 85 0 C and 80 0 C, respectively. Consequently, it was possible to increase an operating temperature by about 15 0 C using a modulation-doped epitaxial structure.
  • FIG. 8 illustrates graphs showing a characteristic of a single-mode DFB laser fabricated using an epitaxial structure according to an experimental embodiment of the present invention.
  • FIG. 7 a single-mode DFB laser was fabricated, and its temperature characteristic was compared with that of a reference sample.
  • the maximum operating temperatures of DFB lasers according to the reference sample and sample A were 60 0 C and 80 0 C, respectively. Thus, it was confirmed that sample A had an excellent effect. In the result, it could be seen that the maximum operating temperature of the single-mode DFB laser was increased by 20 0 C using the modulation-doped epitaxial structure.
  • the inventors of the present invention attached a Thermoelectric Cooler (TEC) for changing temperature, a Subminiature version A (SMA) connector, an Aluminum Nitride (AIN) submount and a microstrip line to a metal block for high-speed measurement, bonded them with wires, and then measured a small- signal modulation characteristic while changing a temperature from 25 0 C to 75 0 C.
  • an operating current was set to be the sum of a threshold current and 10 D.
  • the structure according to this experimental embodiment had a good modulation characteristic at all operating temperatures.
  • a DFB laser having a modulation-doped structure a 3-D small-signal modulation frequency was measured to be 5.5 Dat normal temperature. It was expected that a small-signal modulation frequency of 6 to 7 D required for 10-Gbps operation could be obtained when an operating current increased.

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Abstract

Provided is a semiconductor laser diode having a quantum well structure. The semiconductor laser diode includes: a substrate; a buffer layer formed on the substrate; a lower cladding layer formed on the buffer layer; a first Separate Confinement Heterostructure (SCH) layer formed on the lower cladding layer and having an n-type doped region in an upper part thereof and a p-type doped region in a lower part thereof; an active layer formed on the first SCH layer and having a multi-quantum- well structure in which a plurality of quantum well layers and barrier layers are alternately formed; a second SCH layer formed on the active layer and having an n-type doped region in a lower part thereof and a p-type doped region in an upper part thereof; an upper cladding layer formed on the second SCH layer; and a contact layer formed on the upper cladding layer. According to the semiconductor laser diode, energy band shapes of the first and second SCH layers are changed, and thus temperature characteristics can be improved.

Description

Description
SEMICONDUCTOR LASER DIODE WITH QUANTUM WELLS
STRUCTURE
Technical Field
[1] The present invention relates to a semiconductor laser diode, and more particularly, to a semiconductor laser diode having a quantum well structure and an improved temperature characteristic. Background Art
[2] Lately, with the development of optical subscriber networks, a demand for a low- priced communication light sources is growing. To manufacture a low-priced light source for communication, it is necessary to fabricate a laser that does not need a thermoelectric cooling device even at high temperature due to an excellent temperature characteristic. In general, characteristics of semiconductor Laser Diodes (LDs) tend to deteriorate as their operating temperatures increase.
[3] To fabricate a semiconductor laser diode having a wavelength band of about 1.3 to
1.6 D used in optical communication, InGaAsP/InP-based material is generally used. Crystal growth and regrowth are easy with this material, and related processing techniques have been developed. Thus, the material is used for fabricating optical devices for communication.
[4] FIG. 1 illustrates an energy -band structure of a semiconductor laser diode having a quantum well structure according to conventional art.
[5] Referring to FIG. 1, undoped first and second Separate Confinement Het- erostructure (SCH) layers 2 and 4 are interposed between a first cladding layer 1 of n- type InP and a second cladding layer 5 of p-type InP, and an active layer 3 consisting of a plurality of quantum well layers 3a and barrier layers 3b is formed in the middle of the SCH structure.
[6] Here, the first and second SCH layers 2 and 4 have smaller band gaps than the first and second cladding layers 1 and 5, and the quantum well layers 3a have a smaller band gap than the first and second SCH layers 2 and 4. Electrons and holes inserted through the first and second cladding layers 1 and 5 are captured in a quantum well and can provide optical gain.
[7] The quantum well is formed by inserting, between different semiconductor layers, a thin semiconductor layer having a smaller energy band gap than the different semiconductor layers. In the quantum well, electrons or holes are confined by an energy barrier and thus move in a two-dimensional plane.
[8] Here, the quantum well layers 3a denote thin layers having a small energy band gap, and the barrier layers 3b denote layers having a larger energy band gap than the quantum well layers 3a and surrounding the quantum well. As a result, electrons and holes in the quantum well layers 3a are confined to quantum wells by the barrier layers 3b. When the quantum well structure is prepared, electrons and holes in the active layer 3 are recombined in the quantum wells and emit light. Thus, optical characteristics of the active layer 3 are determined by the quantum wells.
[9] However, InGaAsP/InP-based material has a small conduction band offset, and thus electrons injected into the quantum wells easily escape to the barrier layers 3b or the SCH structure. Since the amount of escaping electrons remarkably increases according to temperature, characteristics of the laser deteriorate at high temperature.
[10] To solve this problem, new InGaAlAs/InP-based material is being researched.
InGaAlAs-based material has a relatively large conduction band offset, and thus characteristics slightly deteriorate according to an increase in temperature.
[11] However, crystal growth is difficult with this material. In particular, oxidation occurring upon regrowth makes it difficult to obtain a laser diode having good characteristics.
Disclosure of Invention Technical Problem
[12] The present invention is directed to providing a semiconductor laser diode having a quantum well structure in which n-type and p-type modulation-doping are respectively performed on outer regions of Separate Confinement Heterostructure (SCH) layers to change energy band shapes of the SCH layers, and thus a temperature characteristic is improved.
[13] The present invention is also directed to providing a semiconductor laser diode having a quantum well structure in which n-type modulation-doping is performed on an SCH layer adjacent to a p-type cladding layer to a specific thickness, and thus an energy band shape of the SCH layer is changed to improve a temperature characteristic. Technical Solution
[14] One aspect of the present invention provides a semiconductor laser diode having a quantum well structure, comprising: a lower cladding layer formed on a substrate; a first Separate Confinement Heterostructure (SCH) layer formed on the lower cladding layer and having an n-type doped region in an upper part of the first SCH layer and a p-type doped region in a lower part of the first SCH layer; an active layer formed on the first SCH layer and having a multi-quantum- well structure in which a plurality of quantum well layers and barrier layers are alternately formed; a second SCH layer formed on the active layer and having an n-type doped region in a lower part of the second SCH layer and a p-type doped region in an upper part of the second SCH layer; an upper cladding layer formed on the second SCH layer; and a contact layer formed on the upper cladding layer, wherein the n-type doped regions are formed in regions of the first and second SCH layers adjacent to the active layer to face the active layer, and the p-type doped regions are respectively formed in regions of the first and second SCH layers adjacent to the lower and upper cladding layers.
[15] The first and second SCH layers may be Graded Index (GRIN) SCH layers in which a plurality of different layers are arranged in order of band gap.
[16] Another aspect of the present invention provides a semiconductor laser diode having a quantum well structure, comprising: n-type and p-type cladding layers; an active layer of a multi-quantum- well structure having a plurality of quantum well layers and barrier layers alternately formed between the n-type and p-type cladding layers; a first Separate Confinement Heterostructure (SCH) layer formed between the n-type cladding layer and the active layer; and a second SCH layer formed between the p-type cladding layer and the active layer, wherein n-type and p-type doped regions are respectively formed to specific thicknesses in upper and lower parts of the second SCH layer, the n-type doped region is formed in a region of the second SCH layer adjacent to the active layer to face the active layer, and the p-type doped region is formed in a region of the second SCH layer adjacent to the p-type cladding layer.
[17] Since the second SCH layer is in contact with the p-type cladding layer, and electrons overflow to the p-type region in the second SCH layer, an effect of the present invention can be obtained by inserting a modulation-doped layer in the p-type region alone.
[18] Still another aspect of the present invention provides a semiconductor laser diode, comprising: n-type and p-type cladding layers; an active layer of a multi-quantum-well structure having a plurality of quantum well layers and barrier layers alternately formed between the n-type and p-type cladding layers; a first Separate Confinement Heterostructure (SCH) layer formed between the n-type cladding layer and the active layer; and a second SCH layer formed between the p-type cladding layer and the active layer, wherein an n-type doped region is formed to a specific thickness in a region of the second SCH layer adjacent to the active layer.
[19] A p-type doped region may be formed in a region of the first SCH layer adjacent to the n-type cladding layer, or an n-type doped region may be formed in a region of the second SCH layer adjacent to the active layer.
Advantageous Effects
[20] According to the inventive semiconductor laser diode having a multi-quantum- well structure, n-type and p-type modulation-doping are respectively performed on outer regions of Separate Confinement Heterostructure (SCH) layers to change energy band shapes of the SCH layers, and thus a temperature characteristic is improved. Brief Description of the Drawings
[21] FIG. 1 illustrates an energy -band structure of a semiconductor laser diode having a quantum well structure according to conventional art.
[22] FIG. 2 is a cross-sectional view of a semiconductor laser diode having a quantum well structure according to an exemplary embodiment of the present invention.
[23] FIG. 3 illustrates an energy-band structure of a semiconductor laser diode having a quantum well structure according to an exemplary embodiment of the present invention.
[24] FIG. 4 illustrates an energy -band structure of a semiconductor laser diode having a quantum well structure according to another exemplary embodiment of the present invention.
[25] FIG. 5 illustrates an energy-band structure of a semiconductor laser diode having a quantum well structure according to still another exemplary embodiment of the present invention.
[26] FIG. 6 is a reference diagram showing detailed materials of a band diagram according to an experimental embodiment of the present invention.
[27] FIG. 7 illustrates graphs showing high-temperature operating characteristics of
Fabry-Perot Laser Diodes (LDs) fabricated using an epitaxial structure according to an experimental embodiment of the present invention.
[28] FIG. 8 illustrates graphs showing a characteristic of a single-mode Distributed
Feedback (DFB) laser fabricated using an epitaxial structure according to an experimental embodiment of the present invention. Mode for the Invention
[29] Hereinafter, exemplary embodiments of the present invention will be described in detail. However, the present invention is not limited to the exemplary embodiments disclosed below, but can be implemented in various types. Therefore, the present exemplary embodiments are provided for complete disclosure of the present invention and to fully inform the scope of the present invention to those ordinarily skilled in the art.
[30] FIG. 2 is a cross-sectional view of a semiconductor laser diode having a quantum well structure according to an exemplary embodiment of the present invention. FIG. 3 illustrates an energy-band structure of a semiconductor laser diode having a quantum well structure according to an exemplary embodiment of the present invention, and FIG. 4 illustrates an energy-band structure of a semiconductor laser diode having a quantum well structure according to another exemplary embodiment of the present invention.
[31] Referring to FIGS. 2 to 4, the semiconductor laser diode having a quantum well structure according to an exemplary embodiment of the present invention includes a substrate 100, and a buffer layer 200, a lower cladding layer 300, a first Separate Confinement Heterostructure (SCH) layer 400, an active layer 500, a second SCH layer 600, an upper cladding layer 700 and a contact layer 800 formed on the substrate 100 in sequence.
[32] Here, the substrate 100 may be a semiconductor substrate such as an n-type InP substrate.
[33] The buffer layer 200 is formed on the substrate 100 to compensate a surface unevenness of the substrate 100 and stack the upper layers as evenly as possible.
[34] In addition, the buffer layer 200 may be formed of, for example, n-type InP or
InGaAsP. The buffer 200 has a thickness of about 1 D and is doped with, for example, silicon (Si) as n-type impurities.
[35] The lower cladding layer 300 is formed on the buffer layer 200 and may be formed of, for example, n-type InP or InGaAsP.
[36] In addition, the lower cladding layer 300 has a thickness of about 1 D and is doped with, for example, silicon (Si) as n-type impurities.
[37] The first SCH layer 400, which is an optical guide layer guiding oscillation of a laser beam, is formed on the lower cladding layer 300, and may be formed of, for example, undoped InGaAsP.
[38] In addition, n-type and p-type doped regions 400a and 400b having specific thicknesses are formed in upper and lower parts of the first SCH layer 400, respectively. As illustrated in FIG. 3, the first and second SCH layers 400 and 600 may consist of a single layer. On the other hand, as illustrated in FIG. 4, a modulation- doped structure of the present invention may be applied to the first and second SCH layers 400 and 600, and thus the first and second SCH layers 400 and 600 may be Graded Index (GRIN) SCH layers in which a plurality of different layers 400- 1 to 400-3 and 600-1 to 600-3 are arranged in order of band gap.
[39] The active layer 500 is formed on the first SCH layer 400 and has a multi- quantum- well structure in which a plurality of quantum well layers 500a and barrier layers 500b are alternately formed.
[40] The second SCH layer 600, which is an optical guide layer guiding oscillation of a laser beam, is formed on active layer 500, and may be formed of, for example, undoped InGaAsP.
[41] In addition, n-type and p-type doped regions 600a and 600b having specific thicknesses may be formed in lower and upper parts of the second SCH layer 600 to be symmetrical to the first SCH layer 400 with respect to the active layer 500, re- spectively.
[42] The n-type and p-type doped regions 600a and 600b of the second SCH layer 600 have the same constitution and effect as the above described n-type and p-type doped regions 400a and 400b of the first SCH layer, and thus a description thereof will not be repeated.
[43] The upper cladding layer 700 is formed on the second SCH layer 600 and may be formed of, for example, p-type InP or InGaAsP.
[44] In addition, the upper cladding layer 700 has a thickness of about 1 D and is doped with, for example, zinc (Zn) as p-type impurities.
[45] The contact layer 800 is formed on a part of the upper cladding layer 700 and may be formed of, for example, p-type InGaAs.
[46] In addition, the contact layer 800 has a thickness of about 100 D and is doped with, for example, Zn as p-type impurities.
[47] A pair of n-type and p-type modulation-doped regions are formed in the first and second SCH layers 400 and 600 according to an exemplary embodiment of the present invention. However, the present invention not being limited thereto, the pair of n-type and p-type modulation-doped regions may be formed in only one of the SCH layers.
[48] As described above, n-type and p-type modulation doping is performed on the upper and lower parts of the first SCH layer 400 to generate ions in the modulation- doped regions. In the result, an electric field is generated in the region. Such an electric field causes an energy band to bend and finally forms an energy band structure as shown in FIG. 3 or 4.
[49] In particular, an energy barrier to electrons is relatively increased in such a structure. Thus, overflow of electrons is suppressed, and a temperature characteristic of the semiconductor laser diode can be improved.
[50] Preferably, all impurities are depleted from the n-type and p-type doped regions
400a and 400b of the first SCH layer 400. To this end, two-dimensional doping concentrations of the n-type and p-type doped regions 400a and 400b must be adjusted to be identical.
[51] In this case, electrons and holes are confined to a quantum well, recombined together and then disappeared. Thus, only impurity ions exist as charges in the first SCH layer 400.
[52] A relationship between an electric field E generated by such impurity ions and a doping concentration is given as a one-dimensional Poisson s equation shown in Equation 1 below.
[53] Equation 1
[54] dE/dx=en/s
[55] Here, e denotes an amount of electric charges, n denotes a doping concentration, ε denotes a permittivity of a material, and x denotes a distance in a growth direction. [56] When Equation 1 is used, it is possible to calculate the strength of an electric field (
AE
) generated while passing through a modulation-doped region using Equation 2 below. [57] Equation 2
[58]
Figure imgf000008_0001
[59] Here,
<5 = nd denotes a two-dimensional charge density. Assuming that a distance between an n-type doped region and a p-type doped region is L, an increase ΔV in potential barrier due to modulation doping is calculated using Equation 3 below.
[60] Equation 3
[61]
AV=ΔE - L
[62] A change in potential barrier according to a doping concentration and a thickness will be described using the relation equation with reference to an example. Given that
1 8 n-type and p-type regions are doped in a concentration of 1x10 and have a thickness of 10 D.
[63] According to Equation 2 above, an electric field generated by modulation doping has a strength of 1.4x10 V/cm. Assuming that a distance between the n-type and p- type regions is 50 D, a height of a potential barrier generated from the above mentioned factors is 0.7 V. When the height of the potential barrier is applied to an electron energy diagram, an energy barrier to electrons increases by 0.7 eV due to modulation doping. The value is remarkably larger than 0.1 to 0.2 eV that is a general conduction band energy barrier of an InP-based laser for communication. Consequently, the proposed structure is expected to remarkably suppress overflow of electrons.
[64] In an actual application, it is expected that a smaller electric field than the value calculated using the above equations will be formed due to a screening effect by carriers. In a quantum well structure, however, most carriers are confined to a quantum well, and a carrier concentration of an SCH layer is not high. Thus, the proposed structure is expected to operate without a problem.
[65] A method of fabricating a semiconductor laser diode having a quantum well structure according to an exemplary embodiment of the present invention will be described in detail with reference to FIGS. 2 to 4.
[66] First, the buffer layer 200 is grown on the substrate 100 using a Metal Organic
Chemical Vapor Deposition (MOCVD) technique.
[67] Subsequently, using the MOCVD technique, the lower cladding layer 300, the first
SCH layer 400, the active layer 500 having a multi-quantum- well structure, the second SCH layer 600, the upper cladding layer 700 and the contact layer 800 are stacked in sequence.
[68] Here, modulation doping is performed so that the n-type and p-type doped regions
400a and 400b are formed to specific thicknesses in upper and lower parts of the first SCH layer 400, which is the feature of the present invention. In addition, modulation doping is performed so that the n-type and p-type doped regions 600a and 600b having specific thicknesses are formed in lower and upper parts of the second SCH layer 600 to be symmetrical to the first SCH layer 400 with respect to the active layer 500, respectively.
[69] A process after this is the same as that of a general semiconductor laser diode, and thus a detailed description thereof will be omitted.
[70] FIG. 5 illustrates an energy -band structure of a semiconductor laser diode having a quantum well structure according to still another exemplary embodiment of the present invention.
[71] Referring to FIGS. 2 and 5, the semiconductor laser diode having a quantum well structure according to still another exemplary embodiment of the present invention includes a substrate 100, and a buffer layer 200, a lower cladding layer 300, a first SCH layer 400, an active layer 500, a second SCH layer 600, an upper cladding layer 700 and a contact layer 800 formed on the substrate 100 in sequence.
[72] The active layer 500 has a multi-quantum- well structure in which a plurality of quantum well layers and barrier layers are alternately formed between the lower cladding layer (n-type cladding layer) 300 and the upper cladding layer (p-type cladding layer) 700. According to this exemplary embodiment, an n-type doped region 400a is formed to a specific thickness in a region adjacent to the active layer 500 in the second SCH layer 600.
[73] The effect of the present invention can be partially obtained even when the n-type doped region 400a alone is formed. On the other hand, in the second SCH layer 600 adjacent to the upper cladding layer (p-type cladding layer) 700, a p-type doped layer 600b may be formed. However, the upper cladding layer (p-type cladding layer) 700 is p-type doped, and thus the p-type doped region 600b may not be formed in the second SCH layer 600.
[74] Meanwhile, n-type and p-type doped regions 440a and 440b are preferably formed in upper and lower parts of the first SCH layer 400.
[75] Experimental embodiment
[76] According to this experimental embodiment, a pair of n-type and p-type modulation doped regions are formed in upper and lower parts of an active layer to generate an internal electric field. By the internal electric field, a band diagram is bent toward an active layer, and thus a band structure is changed. Consequently, leakage current of electrons is reduced at a high temperature, improving a high-temperature characteristic. FIG. 6 is a reference diagram showing detailed materials of a band diagram according to this experimental embodiment of the present invention.
[77] Inventors of the present invention designed an epitaxial structure having a modulation-doped region to be employed in an active layer of a 1.3 D multi- quantum- well structure. In an SCH region of the active layer, a pair of n- and p-type doped regions having a thickness of 5 D were formed. Here, a doping concentration of the n- and p-type doped regions was changed from 1x10 cm (sample A) to 1x10 cm (sample B). The detailed structure is shown in Table 1 below. Using an epitaxial structure having a modulation-doped region, a buried heterostructure laser structure having a current-binding structure was fabricated. In Table 1, 1.0Q, 1.1Q, etc., denote materials that are obtained by lattice-matching InGaAsP, which is a compound of the four elements, with InP, and respective elemental compositions thereof have band gaps corresponding to wavelengths of 1.0 D, 1.1 D, and so on. A substrate having a thickness of 350 Dand formed of InP was used.
[78] Table 1
Figure imgf000011_0001
[79] Meanwhile, a Fabry-Perot laser diode was fabricated using the buried het- erostructure laser structure. According to samples A and B and a reference sample not having the proposed modulation-doped structure, Fabry-Perot laser diodes were fabricated, and then their high-temperature operating characteristics were compared. FIG. 7 illustrates graphs showing high-temperature operating characteristics of Fabry- Perot laser diodes fabricated using an epitaxial structure according to this experimental embodiment of the present invention.
[80] Referring to FIG. 7, the reference sample and samples A and B have similar temperature characteristics. However, the modulation-doped sample has an excellent temperature characteristic at a temperature of 50 0C or above, and samples A and B have better temperature characteristics at a high temperature. Meanwhile, it can be seen that sample A has a better laser characteristic than sample B at a high temperature. The maximum operating temperatures of the reference sample, sample A and sample B were measured to be 70 0C, 85 0C and 80 0C, respectively. Consequently, it was possible to increase an operating temperature by about 15 0C using a modulation-doped epitaxial structure.
[81] Next, a single-mode Distributed Feedback (DFB) laser was fabricated using the buried heterostructure laser structure. FIG. 8 illustrates graphs showing a characteristic of a single-mode DFB laser fabricated using an epitaxial structure according to an experimental embodiment of the present invention. [82] According to sample A that is proven to have a better temperature characteristic in
FIG. 7, a single-mode DFB laser was fabricated, and its temperature characteristic was compared with that of a reference sample. The maximum operating temperatures of DFB lasers according to the reference sample and sample A were 60 0C and 80 0C, respectively. Thus, it was confirmed that sample A had an excellent effect. In the result, it could be seen that the maximum operating temperature of the single-mode DFB laser was increased by 20 0C using the modulation-doped epitaxial structure.
[83] Meanwhile, to measure a high-speed modulation characteristic of the single-mode
DFB laser according to change in temperature, the inventors of the present invention attached a Thermoelectric Cooler (TEC) for changing temperature, a Subminiature version A (SMA) connector, an Aluminum Nitride (AIN) submount and a microstrip line to a metal block for high-speed measurement, bonded them with wires, and then measured a small- signal modulation characteristic while changing a temperature from 25 0C to 75 0C. Here, an operating current was set to be the sum of a threshold current and 10 D. In the results, the structure according to this experimental embodiment had a good modulation characteristic at all operating temperatures. As for a DFB laser having a modulation-doped structure, a 3-D small-signal modulation frequency was measured to be 5.5 Dat normal temperature. It was expected that a small-signal modulation frequency of 6 to 7 D required for 10-Gbps operation could be obtained when an operating current increased.
[84] While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

Claims
[1] A semiconductor laser diode having a quantum well structure, comprising: a lower cladding layer formed on a substrate; a first Separate Confinement Heterostructure (SCH) layer formed on the lower cladding layer and having an n-type doped region in an upper part of the first SCH layer and a p-type doped region in a lower part of the first SCH layer; an active layer formed on the first SCH layer and having a multi-quantum- well structure in which a plurality of quantum well layers and barrier layers are alternately formed; a second SCH layer formed on the active layer and having an n-type doped region in a lower part of the second SCH layer and a p-type doped region in an upper part of the second SCH layer; an upper cladding layer formed on the second SCH layer; and a contact layer formed on the upper cladding layer, wherein the n-type doped regions are formed in regions of the first and second SCH layers adjacent to the active layer to face the active layer, and the p-type doped regions are respectively formed in regions of the first and second SCH layers adjacent to the lower and upper cladding layers.
[2] The semiconductor laser diode of claim 1, wherein the first and second SCH layers are Graded Index (GRIN) SCH layers in which a plurality of different layers are arranged in order of band gap.
[3] The semiconductor laser diode of claim 1 or 2, wherein two-dimensional doping concentrations of the n-type and p-type doped regions in the first and second SCH layers are adjusted to be identical so that impurities are depleted from the n- type and p-type doped regions of the first and second SCH layers.
[4] A semiconductor laser diode having a quantum well structure, comprising: n-type and p-type cladding layers; an active layer of a multi-quantum- well structure having a plurality of quantum well layers and barrier layers alternately formed between the n-type and p-type cladding layers; a first Separate Confinement Heterostructure (SCH) layer formed between the n- type cladding layer and the active layer; and a second SCH layer formed between the p-type cladding layer and the active layer, wherein n-type and p-type doped regions are respectively formed to specific thicknesses in upper and lower parts of the second SCH layer, the n-type doped region is formed in a region of the second SCH layer adjacent to the active layer to face the active layer, and the p-type doped region is formed in a region of the second SCH layer adjacent to the p-type cladding layer.
[5] The semiconductor laser diode of claim 4, wherein n-type and p-type doped regions are formed to specific thicknesses in lower and upper parts of the first
SCH layer.
[6] The semiconductor laser diode of claim 4, wherein the first and second SCH layers are Graded Index (GRIN) SCH layers in which a plurality of different layers are arranged in order of band gap.
[7] The semiconductor laser diode of claim 4, wherein two-dimensional doping concentrations of the n-type and p-type doped regions in the first and second SCH layers are adjusted to be identical so that impurities are depleted from the n-type and p-type doped regions of the first and second SCH layers. [8] A semiconductor laser diode having a quantum well structure, comprising: n-type and p-type cladding layers; an active layer of a multi-quantum- well structure having a plurality of quantum well layers and barrier layers alternately formed between the n-type and p-type cladding layers; a first Separate Confinement Heterostructure (SCH) layer formed between the n- type cladding layer and the active layer; and a second SCH layer formed between the p-type cladding layer and the active layer, wherein an n-type doped region is formed to a specific thickness in a region of the second SCH layer adjacent to the active layer. [9] The semiconductor laser diode of claim 8, wherein a p-type doped region is further formed in a region of the second SCH layer adjacent to the n-type cladding layer. [10] The semiconductor laser diode of claim 8, wherein an n-type doped region is further formed in a region of the first SCH layer adjacent to the active layer.
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