CN210040877U - Vertical cavity surface emitting laser with horizontal air column current injection aperture structure - Google Patents

Abstract

The utility model belongs to the technical field of the semiconductor photoelectron, a Vertical Cavity Surface Emitting Laser (VCSEL) structure of horizontal air column current injection aperture structure and preparation method thereof is related to, including the preparation method in bottom homojunction Distribution Bragg Reflector (DBR) and top homojunction DBR and current injection aperture to realize GaN base VCSEL electricity and pour into laser light source into. The utility model provides a horizontal air column current injection aperture structure contains lower current injection layer and last current injection layer, utilizes electrochemical etching technology to prepare out current injection aperture. The utility model discloses need not secondary epitaxial growth top DBR structure, only need epitaxial growth can accomplish the complete epitaxial structure of gaN VCSEL to can guarantee to obtain high-quality epitaxial material. The utility model provides a horizontal air column current injection structure can restrict the diffusion of side direction electric current effectively, reduces the threshold current density of device, solves the preparation difficult problem in gaN base VCSEL current injection aperture.

Description

Vertical cavity surface emitting laser with horizontal air column current injection aperture structure

Technical Field

The invention relates to a vertical cavity surface emitting laser with a horizontal air column current injection pore diameter structure and a preparation method thereof, belonging to the technical field of semiconductor photoelectron.

Background

In the last two decades, GaN-based semiconductor materials have made major technological breakthroughs in epitaxial growth and optoelectronic device fabrication, in which Light Emitting Diodes (LEDs) and Edge Emitting Lasers (EELs) have been industrialized, but Vertical Cavity Surface Emitting Lasers (VCSELs) with superior characteristics are still in the laboratory research phase. The VCSEL has the unique advantages of low threshold current, easiness in realizing single longitudinal mode work, high modulation frequency, small divergence angle, circular light spots, easiness in coupling with optical fibers, easiness in completing process manufacturing and detection without cleavage, easiness in realizing high-density two-dimensional arrays, photoelectric integration and the like. By virtue of the advantages, the blue-light VCSEL has wide application prospect in the fields of high-density optical storage, laser display, laser printing, laser illumination, laser televisions, underwater communication, ocean resource detection, laser biomedicine and the like.

However, since the VCSEL cavity is short (only a few microns long) and the one-way gain length is very short, it is required that the quality of the Distributed Bragg Reflector (DBR) material to be fabricated is good and the reflectivity of the DBR is very high (usually more than 99%). Research and development of GaN-based Vertical Cavity Surface Emitting Lasers (VCSELs) has been relatively slow compared to GaN-based Edge Emitting Lasers (EELs) or GaAs-based VCSELs, mainly due to the difficulty in epitaxially growing high-quality nitride DBRs and the high absorption loss of the inner cavity electrode of the ITO film resulting in high threshold current and low optical output. Research and development of GaN-based VCSELs have become the leading edge and hot spot of research in the optoelectronic field at home and abroad, and many research and development institutions at home and abroad invest a great deal of manpower and material resources to carry out basic research and application development, and industrialization is expected to be realized. China starts to research the GaN-based electro-injection blue VCSEL later and has a large difference with the international advanced level. Therefore, the research of the blue light VCSEL based on the horizontal air column current injection aperture structure has important practical significance.

In order to reduce the difficulty of epitaxially growing a nitride heterojunction double-DBR, researchers at home and abroad report solutions of a mixed-type DBR VCSEL. For example, a mixed-mode DBR VCSEL composed of a nitride heterojunction bottom DBR (epitaxial DBR) and a dielectric film top DBR (dielectric DBR) is used, the bottom nitride heterojunction DBR and a light-emitting layer are epitaxially grown on a substrate, and the top dielectric film DBR is deposited by coating. Because the dielectric film DBR is not limited by lattice matching, two dielectric materials with large refractive index difference can be freely selected, and high reflectivity and high reflection bandwidth can be obtained more easily. 1999, Science journal reports that Arakawa research group of university of Tokyo, Japan utilizes epitaxially grown AlGaN/GaN bottom nitride DBR and ZrO₂/SiO₂The mixed DBR structure VCSEL formed by the top dielectric film DBR realizes room temperature light injection pulse lasing firstly. The Hao-Chung Kuo research group of Taiwan university of transportation in 2010 prepares AlN/GaN DBR and Ta₂O₅/SiO₂The mixed DBR structure VCSEL of the dielectric film DBR realizes the room-temperature continuous electro-injection lasing, and the threshold current density is 12.4KA/cm²(ii) a The research group realizes that the threshold current density of the VCSEL is 10.6KA/cm in 2015²The output power reaches 0.9 mW. Cosendey et al, the physical research institute of condensation state of Roso-Mulberry in 2012 developed an AlInN/GaN bottom DBR and TiO that are epitaxially grown on a GaN substrate and are lattice-matched₂/SiO₂The mixed DBR structure VCSEL of the DBR at the top of the dielectric film realizes room-temperature pulse electrical injection lasing.

Meanwhile, some researchers have proposed a VCSEL solution with a double-dielectric-film DBR structure. The VCSEL with a double-Dielectric-film DBR structure (Dielectric DBR) is prepared by removing an original substrate in a film transfer mode and forming the VCSEL with a bottom Dielectric-film DBR and a top Dielectric-film DBR. Japan Song lower company and Xiamen university Zhang Bao Ping research groupRealize the ZrO₂/SiO₂VCSEL with double-dielectric-film DBR structure is subjected to room-temperature continuous electro-injection lasing, and the threshold current density of the VCSEL is reduced to 1.2 KA/cm². Nakamura research group at the university of California, Santa Barbara, USA, uses an ITO film inner cavity electrode with a thickness of less than 50nm, Ta₂O₅/SiO₂The threshold current density of the VCSEL with the double-dielectric-film DBR structure reaches 8KA/cm²(ii) a When the research group further adopts a tunnel junction to replace an ITO film inner cavity electrode with a large absorption coefficient, the threshold current density is reduced to 3.5KA/cm². Ta was reported by Japanese Sony corporation in 2016₂O₅/SiO₂And SiN/SiO₂The VCSEL with the double-dielectric-film DBR structure has the light-emitting wavelength of 453.9nm and the threshold current density of 22KA/cm²And the continuous output power at room temperature reaches 1.1mW at most. 2018 Sony corporation reported Ta dielectric films₂O₅/SiO₂The VCSEL with the double DBR structure and the single micro-curved mirror are utilized to limit the lateral optical field of the blue VCSEL, and although better beam quality is obtained, the threshold current density is increased to 141KA/cm²。

In order to solve the problem of epitaxially growing high-quality nitride heterojunction DBRs (distributed Bragg reflectors), electrochemical etching technology is utilized in recent years to dope n-type gallium nitride (n) with high doping concentration⁺GaN) sample as anode, soaking in acid or alkaline electrolyte under certain voltage⁺Electrochemical etching reaction of GaN occurs to form a nanoporous gallium nitride (NP-GaN) structure. The research on the output characteristics of the tunnel junction GaN-based Edge Emitting Laser (EEL) is reported by Suzhou nanotechnology of Chinese academy of sciences, the nano bionic research institute and Taiwan national research university in 2013, and the result shows that the structure can obtain lower threshold current density and series resistivity and improve the output power of a device. The JungHan research group at Yale university 2015 reported n-type GaN (n-GaN/n) with different doping concentrations⁺-GaN) n in DBR samples⁺The change rule of the nano-porous GaN structure with different pore sizes formed by GaN in the electrochemical etching process. The DBR with the GaN/NP-GaN structure has the advantage of larger refractive index difference (delta n is more than or equal to 0.5), so thatHigh reflectivity (R > 99%) can be achieved with fewer log DBRs, and the high reflection bandwidth is tunable in the red-blue range. The VCSEL light injection lasing light emitting of the GaN/NP-GaN DBR structure is realized for the first time by the university of south Korea in 2015. Blue-violet LED with GaN/NP-GaN DBR structure is reported by Jung Han research group of Yale university in 2017, and an InGaN/GaN microdisk laser with nano-porous gallium nitride (NP-GaN) is reported by the Jung Han research group in 2018.

The research institutes in japan and usa in 2015 reported that the I-V characteristics of tunnel junction GaN LED devices were significantly improved. Nakamura research group at university of California, Santa Barbara, USA, based on ITO membrane inner cavity electrode and Ta₂O₅/SiO₂A dual-dielectric-film DBR structure is used for researching a VCSEL (vertical cavity surface emitting laser) with an Ion implantation aperture (Ion implanted aperture) and the threshold current density is 16KA/cm²(ii) a And an Air-gap aperture (Air-gap aperture) current injection aperture structured VCSEL with a threshold current density of 22KA/cm². The study group was based on Ta in 2018₂O₅/SiO₂The double-dielectric-film DBR structure researches a VCSEL (vertical cavity surface emitting laser) contacted with an inner cavity of a tunnel junction of an ion injection current input aperture structure, realizes the continuous output of 140 mu W at room temperature, and has the threshold current density of 42.4KA/cm². The research group improves the output power of the device to 319 mu W and reduces the threshold current density to 10KA/cm by optimizing the epitaxial growth VCSEL material²。

In summary, Yale university in the United states reports the manufacturing process conditions of GaN/NP-GaN structure DBR and the change rule of the nano-porous GaN structure, and the application of the DBR structure in GaN/NP-GaN blue-violet LED and InGaN/GaN microdisk laser with nano-porous gallium nitride (NP-GaN) structure, while VCSEL research based on GaN/NP-GaN double DBR structure is not reported. The VCSEL resonant cavity adopting the ion implantation aperture and air gap aperture current injection aperture structure adopted by the university of California in the United states is of a double-dielectric-film DBR structure. Both the two current injection aperture structures adopt an ITO film inner cavity electrode, and the loss caused by absorption of the ITO film inner cavity electrode and the loss brought by an ITO/GaN interface result in higher threshold current and lower light output.

The invention provides a GaN blue light VCSEL structure with a horizontal air column current injection aperture and a preparation method thereof, which are used for realizing GaN-based electrical injection VCSELs. The method has the advantages that the complete VCSEL structure of the double DBRs only needs to be epitaxially grown once, the top DBR does not need to be manufactured secondarily, an ITO membrane electrode is not adopted, and the problem of high absorption loss of an ITO membrane inner cavity electrode can be solved. The air column current injection aperture structure can effectively limit the diffusion of lateral current, improve the uniformity of current injection into a multi-quantum well active region, and reduce the threshold current density of a device, thereby realizing the electric injection blue VCSEL. The invention solves the manufacturing problem of the VCSEL current injection aperture, breaks through the technical bottleneck of the GaN-based VCSEL by electrical injection, provides a new idea for the research and development of the GaN-based VCSEL (ultraviolet and blue-green light), promotes the industrialization progress of the GaN-based VCSEL by electrical injection, and generates great economic benefit and social benefit.

Disclosure of Invention

The invention aims to provide an epitaxial structure of an electric injection blue-green light GaN vertical cavity surface emitting laser, which realizes the vertical cavity surface emitting laser with a horizontal air column current injection aperture structure by preparing a horizontal air column current injection aperture through an electrochemical etching method.

In order to achieve the above object, the present invention provides a vertical cavity surface emitting laser structure with a horizontal air column current injection aperture structure, which sequentially comprises, from bottom to top, on a substrate layer: a sapphire substrate for epitaxially growing thereon respective layers of materials of the vertical cavity surface emitting laser; the buffer layer is made of GaN material with the thickness of 1000nm, and is manufactured on the substrate and used for preventing the transfer of defects in the substrate; the bottom DBR layer is an n-type GaN (n-GaN/n + -GaN) DBR homojunction material with different doping concentrations for epitaxial growth; a lower current injection layer formed on the bottom DBR layer; the lower barrier layer is made of GaN material with the thickness of 100nm and is manufactured on the lower current injection layer; an active region which is a multiple quantum well and is formed on the lower barrier layer; a tunnel junction, the layer fabricated on the MQW layer; an upper current injection layer formed on the tunnel junction; an upper barrier layer made of 100nm thick GaN material and formed on the active region; a top DBR layer, which is an n-type GaN (n-GaN/n + -GaN) DBR homojunction material with different doping concentrations for epitaxial growth, and is manufactured on the upper barrier layer; and an ohmic contact layer of n + -GaN material with a thickness of 300nm, the layer being formed on the top DBR layer.

The invention provides a method for manufacturing a vertical cavity surface emitting laser with a horizontal air column current injection aperture structure. The method comprises the following specific steps: first, the GaN vcsel epitaxial wafer is first photo-etched and ICP etched, and the schematic diagram is shown in fig. 2. Then, the current injection aperture of the GaN vertical cavity surface emitting laser epitaxial wafer is fabricated, and the schematic diagram is shown in fig. 3. In the experimental process, the larger the etching voltage, the faster the etching speed, and the larger the size of the porous GaN. In the process of carrying out a top DBR etching passivation process, how to select etching voltage is one of very important factors, the reaction rate of a sample when etching is started is controlled by lower etching voltage (usually, proper etching voltage is selected according to etching solution, and the etching rate is about 1 nm/s-3 nm/s), after etching reaches the designed etching depth for a period of time, the voltage is increased to be more than 1.5 times of the original etching voltage, the etching speed is sharply reduced, effective etching can not be continuously carried out as the etching reaction is stopped, even if the etching voltage is reduced to the original value, the etching reaction can not be continuously carried out, and the process is called as a DBR etching passivation process.

According to the invention, by utilizing the DBR etching passivation process, when the proper etching voltage is selected to be electrochemically etched until the effective current injection aperture is reached, the etching voltage is increased to carry out the current injection aperture etching passivation treatment process, so that the size of the current injection aperture is kept unchanged. And then, carrying out secondary photoetching and ICP etching on the GaN vertical cavity surface emitting laser epitaxial wafer, as shown in FIG. 4. And finally, manufacturing the top DBR and the bottom DBR of the epitaxial wafer of the GaN vertical cavity surface emitting laser, and selecting proper etching voltage for electrochemical etching until the top DBR and the bottom DBR are completely etched, wherein the schematic diagram is shown in FIG. 5.

Drawings

Fig. 1 is a schematic view of an epitaxial structure of an electro-injection GaN vertical cavity surface emitting laser, 1 being a substrate, 2 being a buffer layer, 3 being a bottom DBR layer, 4 being a lower current injection layer, 5 being a lower barrier layer, 6 being an active region, 7 being a tunnel junction, 8 being an upper current injection layer, 9 being an upper barrier layer, 10 being a top DBR layer, and 11 being an ohmic contact layer.

Fig. 2 is a schematic diagram of first photolithography and ICP etching of an epitaxial wafer of a GaN vertical cavity surface emitting laser, where 1 is a substrate, 2 is a buffer layer, 3 is a bottom DBR layer, 4 is a lower current injection layer, 5 is a lower barrier layer, 6 is an active region, 7 is a tunnel junction, 8 is an upper current injection layer, 9 is an upper barrier layer, 10 is a top DBR layer, 11 is an ohmic contact layer, and 20 is a first photolithography and ICP etching channel.

Fig. 3 is a schematic view of a GaN vcsel epitaxial wafer current injection aperture fabrication, where 1 is a substrate, 2 is a buffer layer, 3 is a bottom DBR layer, 4 is a lower current injection layer, 5 is a lower barrier layer, 6 is an active region, 7 is a tunnel junction, 8 is an upper current injection layer, 9 is an upper barrier layer, 10 is a top DBR layer, 11 is an ohmic contact layer, 20 is a first-lithography and ICP-etched channel, 21 is a DBR first-etching region, 30 is an ohmic contact-etching region, 31 is an upper current injection aperture region, 32 is a tunnel junction-etching region, and 33 is a lower current injection aperture region.

Fig. 4 is a schematic diagram of a second photolithography and ICP etching channel of an epitaxial wafer of a GaN vertical cavity surface emitting laser, where 1 is a substrate, 2 is a buffer layer, 3 is a bottom DBR layer, 4 is a lower current injection layer, 5 is a lower barrier layer, 6 is an active region, 7 is a tunnel junction, 8 is an upper current injection layer, 9 is an upper barrier layer, 10 is a top DBR layer, 11 is an ohmic contact layer, 20 is a first photolithography and ICP etching channel, 21 is a DBR first etching region, and 22 is a second photolithography and ICP etching channel.

Fig. 5 is a schematic diagram showing the complete etching of the top and bottom DBRs of the GaN vcsel epitaxial wafer, where 1 is a substrate, 2 is a buffer layer, 3 is a bottom DBR layer, 4 is a lower current injection layer, 5 is a lower barrier layer, 6 is an active region, 7 is a tunnel junction, 8 is an upper current injection layer, 9 is an upper barrier layer, 10 is a top DBR layer, 11 is an ohmic contact layer, 20 is a first lithography and ICP etching channel, 21 is a DBR first etching region, and 22 is a second lithography and ICP etching channel. An ohmic contact etching region 30, an upper current injection aperture region 31, a tunnel junction etching region 32, a lower current injection aperture region 33, a bottom DBR etching region 34, and a top DBR etching region 35.

Referring to fig. 1, fig. 1 shows an embodiment of the present invention: the invention provides a vertical cavity surface emitting laser epitaxial structure with a horizontal air column current injection aperture structure, which sequentially comprises the following components from bottom to top on a substrate layer: a sapphire substrate 1 for epitaxially growing thereon materials of respective layers of a vertical cavity surface emitting laser; the buffer layer 2 is made of GaN material with the thickness of 1000nm, and is manufactured on the substrate and used for preventing the transfer of defects in the substrate; a bottom DBR layer 3 for epitaxially growing n-type GaN (n-GaN/n + -GaN) DBR homojunction material with different doping concentrations, the bottom DBR layer having a total of 20 pairs and thicknesses of 40nm and 55nm, respectively, and n =1E18/cm³，n⁺GaN doping concentration n =1E19/cm³(ii) a A lower current injection layer 4 of n + -GaN material with a thickness of 50nm and a doping concentration of n =5E19/cm³(ii) a A lower barrier layer 5 made of 100nm thick GaN material and formed on the bottom DBR layer; an active region 6 which is a multiple quantum well having a light emission wavelength of 420nm to 430nm and is formed on the lower barrier layer; tunnel junction 7 of heavily doped n⁺- GaN/p⁺-GaN，n⁺The doping concentration of-GaN and p + -GaN are both 5E19/cm³15nm and 10nm in thickness, respectively, formed on the MQW layer; the upper current injection layer 8 is made of n + -GaN material with a thickness of 50nm and has a doping concentration of n =5E19/cm³The layer is made on the tunnel junction; an upper barrier layer 9 made of 100nm thick GaN material and formed on the active region; a top DBR layer 10 for epitaxially growing n-type GaN (n-GaN/n) with different doping concentrations⁺-GaN) DBR homojunction material, top DBR for a total of 15 pairs with thickness of 40nm and 55nm, respectively, n-GaN doping concentration n =1E18/cm³N + -GaN doping concentration of n =1 × E19/cm³The DBR layer is made on the upper barrier layer; the ohmic contact layer 10 is made of n + -GaN material with a thickness of 300nm and has a doping concentration of n =5E19/cm³This layer is fabricated on the top DBR layer.

Referring to fig. 2-5, fig. 2-5 show a vertical cavity surface emitting laser with a horizontal air column current injection aperture structure according to an embodiment of the present invention: the invention provides a vertical cavity surface emitting laser (DBR) with a horizontal air column current injection aperture structure and a manufacturing method of a current injection aperture. The method comprises the following specific steps: firstly, photoetching and ICP etching a GaN vertical cavity surface emitting laser epitaxial wafer for the first time. As shown in fig. 2, 1 is a substrate, 2 is a buffer layer, 3 is a bottom DBR layer, 4 is a lower current injection layer, 5 is a lower barrier layer, 6 is an active region, 7 is a tunnel junction, 8 is an upper current injection layer, 9 is an upper barrier layer, 10 is a top DBR layer, 11 is an ohmic contact layer, and 20 is a first photolithography and ICP etching channel. The etching solution is nitric acid (the mass fraction of the nitric acid is about 68 percent, and the mass fraction of the nitric acid is the same without special description below), a pulse direct-current constant-voltage power supply is adopted, the etching voltage is adjusted to be 1.5V, the pulse width of a rectangular wave voltage is adjusted to be 30s, the interval time is adjusted to be 10s, the voltage is increased to be 2.5V after etching for 4.5 hours, the reaction is finished after 5 minutes, and the current injection aperture manufacturing of the GaN vertical cavity surface emitting laser epitaxial wafer is realized. As shown in fig. 3, 20 is a first photolithography, ICP etching channel, 21 is a DBR first etching region, 30 is an ohmic contact etching region, 31 is an upper current injection aperture region, 32 is a tunnel junction etching region, and 33 is a lower current injection aperture region.

And then, carrying out second photoetching and ICP etching on the GaN vertical cavity surface emitting laser epitaxial wafer, wherein 20 is a first photoetching and ICP etching channel, 21 is a DBR first etching area, and 22 is a second photoetching and ICP etching channel, as shown in FIG. 4.

Finally, schematic diagrams of manufacturing the DBRs at the top and the bottom of the epitaxial wafer of the GaN vertical cavity surface emitting laser are shown in FIG. 5, wherein 20 is a first photoetching and ICP etching channel, 21 is a first DBR etching region, and 22 is a second photoetching and ICP etching channel. An ohmic contact etching region 30, an upper current injection aperture region 31, a tunnel junction etching region 32, a lower current injection aperture region 33, a bottom DBR etching region 34, and a top DBR etching region 35. The etching solution is nitric acid, the etching voltage is 1.5V, the reaction is finished after 4 hours of etching, and the DBRs at the top and the bottom of the epitaxial wafer of the GaN vertical cavity surface emitting laser are completely etched.

The above description is only an embodiment of the present invention, but the scope of the present invention is not limited thereto, and any modifications or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims (3)

1. A vertical cavity surface emitting laser with a horizontal air column current injection aperture structure is characterized by comprising a bottom homojunction distributed Bragg reflector, a lower current injection layer, a multi-quantum well active layer, a tunnel junction, an upper current injection layer and a top homojunction distributed Bragg reflector.

2. The VCSEL of claim 1, wherein a complete epitaxial structure of the GaN VCSEL including the bottom homojunction DBR, the lower current injection layer, the multiple quantum well active layer, the tunnel junction, the upper current injection layer, and the top homojunction DBR can be completed only by one epitaxial growth without a secondary epitaxial growth of the top DBR structure, thereby ensuring high quality epitaxial material.

3. The VCSEL of claim 1, wherein the substrate layer comprises, in order from bottom to top: a sapphire substrate (1) for epitaxially growing thereon materials of respective layers of a vertical cavity surface emitting laser; the buffer layer (2) is made of GaN materials and is manufactured on the substrate and used for preventing the defects in the substrate from being transferred; a Bragg reflector layer (3) with a homojunction at the bottom for epitaxially growing n-type n-GaN/n with different doping concentrations⁺-GaN distributed bragg reflector homojunction material; a lower current injection layer (4) formed on the bottom distributed Bragg reflector layer; a lower barrier layer (5) made of GaN material and formed on the lower current injection layer; an active region (6) which is a multiple quantum well, the layer being formed on the lower barrier layer; a tunnel junction (7) formed on the MQW layer, an upper current injection layer (8) formed on the tunnel junction, and an upper barrier layer (9) ofA GaN material fabricated on the active region; a Bragg reflector layer (10) distributed on the top for epitaxially growing n-type n-GaN/n with different doping concentrations⁺-a GaN distributed bragg reflector homojunction material, the distributed bragg reflector layer fabricated on the upper barrier layer; an ohmic contact layer (11) of n⁺-a GaN material, the layer being fabricated on the top dbr layer.

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