Method for improving modulation rate of semiconductor laser

Abstract

The invention discloses a method for improving the modulation rate of a semiconductor laser. Relates to the technical field of semiconductor lasers, in particular to a method for improving the modulation rate of a semiconductor laser. According to the invention, based on a basic electrical principle in a semiconductor laser device, parameters such as inductance, resistance and resistance of the semiconductor laser device are adjusted by adjusting parameters such as an electrode metal lead, ohmic contact performance and a metal coverage area in the device, so that the modulation rate of the semiconductor laser device is improved from multiple aspects. The invention discloses a method for improving the modulation rate of a semiconductor laser. Method 1, method 2 and method 3; the method 1 comprises the following steps of: selecting a short and thick electrode lead in the packaging process of the semiconductor laser; method 2: through a rapid annealing process, the ohmic contact performance of the semiconductor laser is improved, and the contact resistance is reduced; and 3, reducing the stray capacitance effect by reducing the metal coverage area of the surface of the active region.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of semiconductor laser, in particular to a method for improving the modulation rate of semiconductor laser. BACKGROUND

[0002] Semiconductor laser has the advantages of small volume, low power consumption, direct modulation and easy integration, and is an important core component of optical fiber communication system. It has developed rapidly in bandwidth, power consumption and without refrigeration, and has become one of the most core light source devices in optical communication system. As a key device of optical transmitting end, the modulation rate of high-speed semiconductor laser directly determines the transmission rate and bandwidth performance of communication link.

[0003] Semiconductor laser shoulders the heavy responsibility of converting electrical signal into optical signal in optical module. Commonly used laser diodes are divided into surface emission and edge emission according to the type of light emission. Among them, surface emission type laser is mainly vertical cavity surface emitting laser (VCSEL); there are many types of edge emission type laser, and the most widely used is Fabry-Perot laser (FP) laser, distributed feedback laser (DFB) and electro-absorption modulated laser (EML). Among them, DFB laser is widely used in long-distance optical fiber communication due to its narrow linewidth, high side mode suppression ratio and good modulation characteristics; VCSEL becomes the mainstream light source of short-distance data center interconnection due to its low power consumption, circular output spot and easy array packaging advantage.

[0004] However, as the communication rate continues to increase to 100 Gb / s or even higher, the traditional semiconductor laser faces multiple technical bottlenecks in high-frequency modulation performance, and improving the modulation rate of semiconductor laser is an effective means to directly improve the information transmission capacity per unit time.

[0005] At present, the main method to improve the modulation rate of semiconductor laser is to improve the relaxation oscillation frequency According to the relationship of relaxation oscillation frequency , represents the injected current, threshold current, represents the photon lifetime, it can be seen that the most direct method to improve the relaxation oscillation frequency is to reduce the threshold current, improve the output power and shorten the photon lifetime.

[0006] ​However, reducing the threshold current usually means reducing the minimum current required to start the laser, but this can cause the noise of the laser to increase, especially when modulated at high frequencies, the noise can be amplified significantly, affecting the stability and output quality of the laser. The increase in noise can cause distortion of the laser output signal, especially in applications that require high fidelity (such as high-speed data transmission or precision measurement) can cause problems; Increasing the output power helps to enhance the output intensity of the laser, thereby improving the modulation response, but it also increases the thermal load. Excessive output power is prone to heat accumulation, which can cause thermal drift, affecting the frequency stability and optical performance of the laser. Thermal effects can also cause degradation of the performance of the laser material, thereby reducing overall efficiency and lifespan; Shortening the photon lifetime can accelerate the response speed of the laser, but the reduction of photon lifetime usually leads to the reduction of the interaction efficiency between photons and carriers, resulting in the decrease of the light efficiency of the laser. In order to reduce the photon lifetime, it may be necessary to sacrifice a certain light output power or increase the driving current, which may make the laser lose balance between stability and efficiency.

[0007] Although the above methods can further improve the modulation rate of the laser by increasing the relaxation oscillation frequency in theory, they must also balance the side effects of increased noise, thermal management problems, and decreased light efficiency. Therefore, in practical applications, the existing relaxation oscillation frequency method affects the long-term stability and high efficiency of the semiconductor laser. SUMMARY In view of the problem that the modulation rate of the semiconductor laser is limited in optical fiber communication, especially in the process of continuously improving the communication rate, the present application provides a method for improving the modulation rate of the semiconductor laser. The present application starts from the basic electrical principle of the semiconductor laser device, adjusts the parameters such as electrode metal lead, ohmic contact performance, and metal coverage area in the device, and then adjusts the inductance, resistance, and resistance of the semiconductor laser device, to improve the modulation rate of the semiconductor laser from multiple aspects.

[0008] The method comprises the following steps: The method for improving the modulation rate of the semiconductor laser comprises method 1, method 2, and method 3; Method 1 specifically sets the length of the laser electrode metal lead to be in the range of 0.5mm~2mm; The cross-sectional diameter of the laser electrode metal lead is set to be in the range of 50μm~100μm; One end of the laser electrode metal lead is connected with the upper electrode metal contact layer of the laser, and the other end is connected with the upper surface metal contact layer of the heat sink electrode pin; Method 2 specifically optimizes the ohmic contact performance through a rapid annealing process; Method 3 specifically involves creating a window in the region where the electrode metal contact layer of the laser overlaps with the ridge waveguide using photolithography. Methods 1, 2, and 3 can be combined in any way to improve the modulation rate of semiconductor lasers.

[0009] Furthermore, in Method 3, the window area is equal to the area of ​​the ridge waveguide. .

[0010] Furthermore, the material of the electrode metal leads is .

[0011] Furthermore, the upper surface metal contact layer material of the heat sink electrode pin is... alloy.

[0012] Furthermore, the rapid annealing process includes: after the P-side electrode growth is completed, the laser chip is placed in a rapid annealing furnace for nitrogen atmosphere treatment. Anneal for 2 minutes, and set the heating time of the laser chip to 5 to 10 seconds; the heating range is from room temperature to 420°C. After rapid annealing, the cooling time of the laser chip is set to no more than 30 seconds; the cooling range is from 420℃ to room temperature.

[0013] Furthermore, the rapid annealing process also includes: after the N-side electrode growth is completed, the laser chip is placed in a rapid annealing furnace for nitrogen atmosphere treatment. Rapid annealing for 2 minutes; setting the laser chip's heating time to 5-10 seconds; heating range: from room temperature to 420℃. After rapid annealing, the cooling time of the laser chip is set to no more than 30 seconds; the cooling range is from 420℃ to room temperature.

[0014] Furthermore, in method 3, the region in the electrode metal contact layer of the laser that is not windowed is called the non-windowed region; the non-windowed region is formed by depositing a 200nm thick layer. A thin film is formed to create a dielectric isolation layer.

[0015] The beneficial effects of the method described in this invention are as follows: (1) The method described in this invention shortens the transmission path, improves the carrier flow capacity, reduces the line inductance, and increases the current change rate by selecting “short and thick” electrode metal leads during the packaging process, thereby improving the modulation rate (response speed) of the semiconductor laser.

[0016] (2) The method of the present invention improves ohmic contact performance and reduces contact resistance (contact resistance from) through a rapid annealing process. Reduce to On the other hand, by reducing the metal coverage area of the active region surface, the parasitic capacitance is reduced, that is, the capacitance value of the metal semiconductor interface is reduced. The method of the present application reduces the RC time constant through the above two aspects of adjustment, and finally improves the modulation rate (response speed) of the semiconductor laser through the reduction of the RC time constant. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 The electrode metal lead of the present application is shown in the schematic diagram. DETAILED DESCRIPTION

[0018] The technical solutions of the present application will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0019] Embodiment 1, The present embodiment provides a method for improving the modulation rate of a semiconductor laser.

[0020] The present embodiment takes a 1550nm edge-emitting distributed feedback semiconductor laser (DFB) as an example for further detailed description. The present application is not limited to this, but also applicable to vertical cavity surface emitting laser (VCSEL) structure.

[0021] When a semiconductor is modulated at high frequency (communication rate is constantly improved), the parasitic inductance of the packaging lead (electrode metal lead) in the (high-speed) semiconductor laser will cause voltage reflection and phase delay of high-frequency signals in the transmission process, thereby limiting the modulation response speed of the laser. In the traditional semiconductor laser, the chip and the packaging pin are usually connected by a long and thin metal lead. This structure will introduce a large parasitic inductance when working at high frequency, which affects the modulation rate. The schematic diagram of the electrode metal lead in the electron microscope is shown in Figure 1 .

[0022] The present application proposes to select a short and thick electrode metal lead during the packaging process of the semiconductor laser, that is, method 1, which is specifically: The length of the laser electrode metal lead is set to be in the range of 0.5mm~2mm; The cross-sectional diameter of the laser electrode metal lead is set to be in the range of 50μm~100μm; In this embodiment, a wire with a length of 1mm and a cross-sectional diameter of 75μm is selected as the laser electrode metal lead, and the laser electrode metal leads are arranged in parallel. ​

[0023] The one end of the laser electrode metal lead is connected with the upper electrode metal contact layer of the laser, and the other end is connected with the upper surface metal contact layer of the heat sink electrode pin. Alloy.

[0024] The (circuit) inductance is closely related to the length and cross-sectional area of the current path. When the current passes through the electrode metal lead, a magnetic field is generated around it. The longer the current path, the stronger the magnetic field generated, and the greater the inductance. By selecting a shorter electrode lead, the length of the current path can be reduced, thereby reducing the circuit inductance. In addition, by increasing the cross-sectional diameter of the electrode metal lead (i.e., selecting a thicker lead), the resistance when the current flows can be reduced, and the impedance when the current passes through can be reduced, thereby further reducing the circuit inductance. Therefore, the short and thick electrode metal lead selected by the present application can accelerate the response speed of the current, and thus improve the modulation rate and dynamic response performance of the laser.

[0025] The quality of the ohmic contact between the laser electrode and the semiconductor layer directly affects the resistance characteristics and modulation response of the device. The contact resistance of the traditional semiconductor laser ohmic contact structure is relatively high, and excessive voltage drop and RC delay are easily generated during modulation.

[0026] The present application proposes to improve the ohmic contact performance of the semiconductor laser and reduce the contact resistance by means of rapid annealing process, i.e. method 2, specifically: after the growth of the P-face electrode and the N-face electrode is completed, the chip of the laser is sent into a rapid annealing furnace, and a nitrogen atmosphere annealing for 2 minutes, and cooling to room temperature within 30 seconds.

[0027] In this embodiment, the P-face electrode is grown by magnetron sputtering Ti / Pt / Au, the thickness of Ti / Pt / Au deposited by the magnetron sputtering equipment is 50nm / 50nm / 300nm, after the growth of the P-face electrode is completed, the chip of the laser is sent into a rapid annealing furnace, and a nitrogen atmosphere annealing for 2 minutes, and setting the temperature rising process to rise from room temperature to 420℃ within 5-10 seconds, after the rapid annealing is completed, cooling from 420℃ to room temperature within 30 seconds.

[0028] In this embodiment, the N-face electrode is also grown by magnetron sputtering: the thickness of Ni / AuGe / Au deposited by the magnetron sputtering equipment is 5nm / 50nm / 300nm, after the growth of the N-face electrode is completed, the chip of the laser is sent into a rapid annealing furnace, and a nitrogen atmosphere annealing for 2 minutes, and setting the temperature rising process to rise from room temperature to 420℃ within 5-10 seconds, after the rapid annealing is completed, cooling from 420℃ to room temperature within 30 seconds.

[0029] Finally, a 200 nm thick Au film is deposited. After the rapid annealing process of Method 2, the contact resistance of the ohmic contact is reduced from the original to , reducing the RC delay effect and improving the response capability of the laser when modulated at high speed.

[0030] A parasitic capacitance is formed between the upper electrode metal layer and the underlying semiconductor layer of a semiconductor laser, which severely limits the signal bandwidth when the device is operated at high speed. The present invention proposes to reduce the parasitic capacitance effect by reducing the metal coverage area of the active region surface, i.e. Method 3, which is specifically: through a photolithography process, a window is opened in the laser upper electrode metal contact layer, in the region overlapping the ridge waveguide, the window area is .

[0031] The area of the laser upper electrode metal contact layer that is not subjected to windowing is the non-windowed area; the non-windowed area forms a dielectric isolation layer by plating a 200 nm thick film.

[0032] In this embodiment, the ridge waveguide area is 5000 (the ridge waveguide length is: 500 , the ridge waveguide width is: 10 ), the window area is of the ridge waveguide area, specifically 2000 (the window length is: 400 , the window width is: 5 ).

[0033] After the above-mentioned Method 3 processing, the effective coupling area between the laser upper electrode metal contact layer and the semiconductor laser is reduced, thereby reducing the capacitance value between the laser upper electrode metal contact layer and the laser upper electrode metal contact layer.

[0034] The size of the capacitance is closely related to the contact area and distance between the two conductors. When the effective coupling area between the upper electrode metal contact layer and the semiconductor laser is reduced, the area of the capacitor plate becomes smaller, thereby reducing the ability to store electric charge, and thus the capacitance value is also reduced. The formation of the capacitance depends on the electric field distribution between the metal electrode and the semiconductor material, and the reduction of the coupling area makes the range of the electric field smaller, reducing the capacitance value. This change helps to improve the response speed of the circuit, because the reduced capacitance reduces the time constant of charging and discharging, making the laser able to respond to external modulation signals more quickly.

[0035] Any combination of the method 1, the method 2 and the method 3 can be used to improve the modulation rate of the semiconductor laser, and the combination includes single combination, pairwise combination, and combination of the method 1, the method 2 and the method 3.

[0036] In the embodiment, the combination of the method 1, the method 2 and the method 3 is used, and the combination of the method 2 and the method 3 can further reduce the RC time constant and improve the modulation rate of the semiconductor laser.

[0037] The RC time constant determines the response speed of the circuit to the current change in the semiconductor laser. The smaller the RC time constant is, the smaller the delay of the current and voltage change is, and the faster the laser can respond to the external modulation signal, thereby improving the modulation frequency and the overall dynamic response capability.

[0038] The contact resistance affects the flow efficiency of the current, and reducing the contact resistance (the method 2) can reduce the energy loss in the current conduction process, so that the current can change more easily and rapidly. The capacitance determines the charging and discharging capability of the electric charge, and reducing the capacitance (the method 3) means that the charging and discharging process of the electric charge is faster. Therefore, reducing the contact resistance and the capacitance can accelerate the response speed of the current, thereby shortening the RC time constant and improving the modulation performance and the response capability of the laser.

[0039] In the embodiment, other preparation processes (not described in the embodiment) of the 1550nm edge-emitting distributed feedback semiconductor laser (DFB) are the same as the conventional preparation processes, and will not be described herein again, and the person skilled in the art can set them according to the actual needs.

Claims

1. A method for improving the modulation rate of a semiconductor laser, characterized in that, The methods for increasing the modulation rate of a semiconductor laser include: Method 1, Method 2, and Method 3; Method 1 specifically involves setting the length of the laser electrode metal leads to a range of 0.5mm to 2mm. The cross-sectional diameter of the laser electrode metal leads is set to be in the range of 50μm~100μm; One end of the laser electrode metal lead is connected to the upper electrode metal contact layer of the laser, and the other end is connected to the upper surface metal contact layer of the heat sink electrode pin. Method 2 specifically involves optimizing ohmic contact performance through a rapid annealing process; Method 3 specifically involves creating a window in the region where the electrode metal contact layer of the laser overlaps with the ridge waveguide using photolithography. Methods 1, 2, and 3 can be combined in any way to improve the modulation rate of semiconductor lasers.

2. The method for improving the modulation rate of a semiconductor laser according to claim 1, characterized in that, In Method 3, the window area is equal to the area of ​​the ridge waveguide. .

3. The method for improving the modulation rate of a semiconductor laser according to claim 2, characterized in that, The material of the electrode metal leads is .

4. The method for improving the modulation rate of a semiconductor laser according to claim 3, characterized in that, The upper surface metal contact layer material of the heat sink electrode pin is alloy.

5. The method for improving the modulation rate of a semiconductor laser according to claim 4, characterized in that, The rapid annealing process includes: after the P-side electrode growth is completed, the laser chip is placed in a rapid annealing furnace for nitrogen atmosphere treatment. Anneal for 2 minutes, and set the heating time of the laser chip to 5 to 10 seconds; the heating range is from room temperature to 420°C. After rapid annealing, the cooling time of the laser chip is set to no more than 30 seconds; the cooling range is from 420℃ to room temperature.

6. The method for improving the modulation rate of a semiconductor laser according to claim 5, characterized in that, The rapid annealing process also includes: after the N-side electrode growth is completed, the laser chip is placed in a rapid annealing furnace for nitrogen atmosphere treatment. Rapid annealing for 2 minutes; setting the laser chip's heating time to 5-10 seconds; heating range: from room temperature to 420℃. After rapid annealing, the cooling time of the laser chip is set to no more than 30 seconds; the cooling range is from 420℃ to room temperature.

7. A method for improving the modulation rate of a semiconductor laser according to claim 6, characterized in that, In Method 3, the region in the electrode metal contact layer of the laser that is not windowed is called the non-windowed region; the non-windowed region is formed by depositing a 200nm thick layer. A thin film is formed to create a dielectric isolation layer.

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