This application claims priority to and the benefit of Korean Patent Application Nos. 2005-118136, filed Dec. 6, 2005, and 2006-56212, filed Jun. 22, 2006, the disclosures of which are incorporated herein by reference in their entirety.
1. Field of the Invention
The present invention relates to a quantum dot laser diode and a method of fabricating the same, and more particularly, to a quantum dot laser diode and a method of fabricating the same which use quantum dots formed by an alternate growth method as an active layer.
2. Discussion of Related Art
Recently, there has been considerable research into a Stranski-Krastanow growth method that forms self-assembled quantum dots using a strain-relaxation process of a lattice-mismatched layer without a separate lithography process. Further, application of self-assembled quantum dots formed by the Stranski-Krastanow growth method to optical devices has been studied from various angles.
For example, application of self-assembled quantum dots in optical communication using wavelength regions of 1.3 μm and 1.55 μm is being actively researched. Here, in the 1.3 μm wavelength region, In(Ga)As quantum dots may be used. The In(Ga)As quantum dots may be easily grown by self-assembly on a GaAs substrate. In this manner, many studies on optical devices such as a laser diode using In(Ga)As quantum dots grown by self-assembly as an active layer are announced.
When In(Ga)As quantum dots are formed on a GaAs substrate to use the In(Ga)As quantum dots in a wavelength region of 1.55 μm, there is a limit to implementation of the 1.55 μm wavelength region due to effects of size of the In(Ga)As quantum dots and stress of a peripheral material. Accordingly, formation of In(Ga)As quantum dots utilized in the 1.55 μm wavelength region on an InP substrate is being actively researched.
However, an InP substrate has less lattice-mismatch with a material layer forming the quantum dots than the GaAs substrate and reacts with peripheral materials. Thus, it has difficulty in forming good quantum dots by self-assembly. Moreover, since In(Ga)As quantum dots formed on an InP substrate have an asymmetrical shape, or a very wide full-width at half-maximum (FWHM) of a photoluminescence (PL) peak and a weak intensity of the PL peak due to poor uniformity, when used for an active layer of an optical device, the efficiency of the optical device may decrease.
The present invention is directed to a quantum dot laser diode and a method of fabricating the same in which quantum dots are formed using an alternate growth method, thereby improving uniformity, increasing full-width at half-maximum (FWHM) of a PL peak, and increasing PL intensity, which in turn enhances device characteristics.
According to one aspect of the present invention, a quantum dot laser diode comprises: a first clad layer formed on an InP substrate; a first lattice-matched layer formed on the first clad layer; an active layer formed on the first lattice-matched layer, and including at least one quantum dot layer formed of an In(Ga, Al)As quantum dot or an In(Ga, Al, P)As quantum dot which is grown by an alternate growth method; a second lattice-matched layer formed on the active layer; a second clad layer formed on the second lattice-matched layer; and an ohmic contact layer formed on the second clad layer.
In the case of forming the multiple quantum dot layers, a barrier layer may be further included between the quantum dot layers. The In(Ga, Al)As quantum dot may be formed by sequentially, alternately depositing an In(Ga)As material layer and an InAl(Ga)As material layer, which are relatively more lattice-mismatched. Alternatively, the In(Ga, Al, P)As quantum dot may be formed by sequentially, alternately depositing an In(Ga)As material layer and an In(Ga, Al, As)P material layer, which are relatively more lattice-mismatched.
The In(Ga)As and InAl(Ga)As material layers, or the In(Ga)As and In(Ga, Al, As)P material layers, which may be used for the alternating deposition, may each have a thickness ranging from 1 monolayer to 10 monolayers. The In(Ga)As and InAl(Ga)As material layers, or the In(Ga)As and In(Ga, Al, As)P material layers, which are used for the alternating deposition, may have an alternating deposition period of 10 to 100.
The first lattice-matched layer, the second lattice-matched layer and the barrier layer may be formed in a hetero-junction structure (SCH structure) formed of InAl(Ga)As, In(Ga, Al, As)P or a combination thereof. In such an SCH structure, a waveguide may have a step index (SPIN) structure, and therein a quantum well may be inserted so as to symmetrically or asymmetrically surround the quantum dots (DWELLs). Alternatively, in the SCH structure, the waveguide may have a graded index (GRIN) structure, and therein a quantum well may be inserted so as to symmetrically or asymmetrically surround the quantum dots.
According to another aspect of the present invention, a method of fabricating a quantum dot laser diode comprises the steps of: forming a first clad layer on an InP substrate; forming a first lattice-matched layer on the first clad layer; forming an active layer on the first lattice-matched layer, the active layer including at least one quantum dot layer formed of an In(Ga, Al)As quantum dot and an In(Ga, Al, P)As quantum dot grown by an alternating deposition method; forming a second lattice-matched layer on the active layer; forming a second clad layer on the second lattice-matched layer; and forming an ohmic contact layer on the second clad layer.
The method may further comprise the step of forming a barrier layer between the quantum dot layers, when a plurality of quantum dot layers are stacked in the step of forming the active layer. The In(Ga, Al)As quantum dot may be formed by alternately depositing an In(Ga)As material layer and InAl(Ga)As material layer in sequence, which are relatively more lattice-mismatched. Alternatively, the In(Ga, Al, P)As quantum dot may be formed by alternately depositing an In(Ga)As material layer and In(Ga, Al, As)P material layer in sequence, which are relatively more lattice-mismatched. The alternating deposition may be performed by one of metallic organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and chemical beam epitaxy (CBE).
The present invention is related to Korean Patent Application No. 2005-85194 entitled “Method for Fabricating Quantum Dots by an Alternate Growth Process” filed by the present applicant. The present specification refers to parts of a method of fabricating quantum dots described in previously filed Korean Patent Application No. 2005-85194.
The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:
Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to attached drawings.
Referring to
Referring to
Here, the first lattice-matched layer 230 may have a quantum well inserted into the SPIN SCH structure so as to symmetrically or asymmetrically surround a following quantum dot (quantum dot in a quantum well: DWELL). Alternately, the first lattice-matched layer 230 may have a quantum well inserted into the GRIN SCH structure so as to symmetrically or asymmetrically surround a following quantum dot.
Next, an active layer 240 composed of a quantum dot layer including a plurality of quantum dots 245 is formed on the first lattice-matched layer 230 (S140). To fabricate the active layer 240, referring to
Here, the In(Ga)As material layer 241 and the InAl(Ga)As material layer 242 are deposited by one of metallic organic chemical vapor deposition (MOCVD), molecular beam epixaxy (MBE), and chemical beam epitaxy (CBE). In alternating deposition, the In(Ga)As material layer 241 and the InAl(Ga)As material layer 242 are each formed to a thickness of 1 to 10 monolayers, and are alternately deposited to 10 to 100 periods. In
Meanwhile, when it is determined that the deposition is performed as many periods as desired, the next step (S144) is processed, which is described with reference to
Referring to
Referring to
When a voltage is applied to each of the substrate 210 and the ohmic contact layer 260 of the quantum laser diode 200 fabricated by the above-described process, a hole injected through the ohmic contact layer 260 and an electron injected through the substrate 210 travel around the quantum dot 245 in the active layer 240 and are recombined. Thereby, the quantum dot laser diode 200 fabricated by the above-described process may emit a specific wavelength of laser light.
As shown in
Although exemplary embodiments of the present invention disclosed herein concern a laser diode using a quantum dot layer formed by alternately depositing an In(Ga)As material layer and an InAl(Ga)As material layer as an active layer, a laser diode using a quantum dot layer formed by alternately depositing an In(Ga)As material layer and an In(Ga, Al, As)P material layer as an active layer may be also fabricated by the above-described processes. Such a quantum dot laser diode can also provide the same effects and characteristics as the above-described exemplary embodiments. In addition, although, in the exemplary embodiments, partial stacking periods of the quantum dots are omitted for convenience of description, the stacking periods may be selected as desired.
Moreover, in the exemplary embodiments, a laser diode may be fabricated using a quantum dot layer comprising quantum dots having multiple stacking periods. However, a laser diode may be also fabricated by stacking a plurality of quantum dot layers comprising quantum dots having multiple stacking periods. When quantum dot layers are multiply stacked, barrier layers (such as a hetero-junction structure layer) are formed between the quantum dot layers.
In the above disclosure, materials enclosed in parentheses are optionally included. Thus, for example, an In(Ga)As layer may be an InAs layer or an InGaAs layer.
As described above, an ideal form of quantum dot is formed simultaneously using a self-assembly method caused by lattice-mismatch and an alternate growth method, and used as an active layer of a quantum dot laser diode. Consequently, quantum dot uniformity is good, an FWHM of a PL peak is narrow, and intensity of the PL peak is significantly increased. Thus, performance of the quantum dot laser diode is remarkably improved.
While the present 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 present invention as defined by the appended claims.
Number | Date | Country | Kind |
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10-2005-0118136 | Dec 2005 | KR | national |
10-2006-0056212 | Jun 2006 | KR | national |