CN101588019B - External cavity type multiple-active region photon crystal vertical cavity surface transmission semiconductor laser device - Google Patents

Abstract

The external-cavity multi-active-region photonic crystal vertical-cavity surface-emitting semiconductor laser belongs to the field of semiconductor optoelectronics. Ordinary vertical cavity surface emitting semiconductor lasers have problems such as small single-pass optical gain, multi-transverse mode lasing, low single-mode output power, large threshold current, and large series resistance. The present invention adopts a multiple active area structure on the active area of the device. At the same time, the defective photonic crystal structure is introduced into the upper DBR of the vertical cavity surface emitting semiconductor laser. By rationally optimizing the photonic crystal period, air aperture, etching depth, device diameter, oxidation aperture, etc., the single-mode working oxidation aperture is obtained. External cavity multi-active area photonic crystal vertical cavity surface emitting semiconductor surface emitting laser with ten microns, single-mode power of several milliwatts, series resistance of tens of ohms, and side mode suppression of more than 35 decibels.

Description

External cavity multi-active area photonic crystal vertical cavity surface emitting semiconductor laser

technical field

The invention belongs to the technical field of optoelectronics, and in particular relates to the design and manufacture of a novel vertical-cavity surface-emitting semiconductor laser. Suitable for vertical cavity surface emitting semiconductor lasers of various wavelengths (650nm, 850nm, 980nm, etc.).

Background technique

Vertical cavity surface emitting lasers (vertical cavity surface emitting lasers, VCSELS) is a semiconductor laser with excellent performance. VCSELS has the characteristics of high coupling efficiency with optical fibers, easy formation of two-dimensional arrays and integration; low threshold, single longitudinal mode, high modulation speed; reliability and high cost performance. It has good application prospects in high-density area array, optical network, data transmission, optical interconnection, optical storage, optical computing, etc., and has good application prospects in optical communication local network data communication, short-distance optical interconnection, data transmission, computer local area network and computer It is widely used in the free space optical interconnection between main boards.

The traditional vertical cavity surface emitting semiconductor laser structure is mainly obtained by epitaxy of III-V compound semiconductor materials through molecular beam epitaxy (MBE) or metal chemical vapor deposition (MOCVD). Through the semiconductor process, the traditional vertical cavity surface emission is moved to the laser device. Its basic structure is shown in Figure 1 (taking GaAs/AlGaAs with a wavelength of 850nm as an example): the upper metal electrode (P-type metal electrode) 1; the P-type ohmic contact layer 2; the upper distributed Bragg reflector (upper DBR) 3 that is periodically grown alternately ; Al _0.98 Ga _0.02 As high-aluminum oxidation confinement layer 4; single active region 5; periodically alternately grown lower distributed Bragg reflector (lower DBR) 6; substrate 7; N-type metal electrode 8; oxidation hole 9 ; The light exit hole 10 . Generally, it is a single device or an array structure. This kind of laser generally has the following disadvantages: 1. Ordinary vertical cavity surface emitting semiconductor lasers have a small one-way optical gain. In order to increase the output power, the method of increasing the light output area or increasing the current injection is generally used. Using the method of increasing the light output area will make the distribution of the carrier density in the active area worse, and the center current density will become smaller, which will increase the threshold current; when using large current injection, the carrier distribution in the active area will appear space burning. Holes affect the distribution of gain and refractive index, resulting in multi-transverse mode lasing. In addition, the thermal stability of the device deteriorates under the condition of high current injection. 2. In order to realize single-mode operation, the carrier density distribution in the central part of the active region must be relatively uniform, and it is difficult to realize single-mode operation if the light output area is too large. In order to ensure stable single-mode output of VCSEL, the oxide aperture must be below 5 μm. 3. Smaller oxidation holes will inevitably cause large series resistance, and it is difficult to control the process of making small oxidation holes. The single-mode output power is low and the threshold current is large.

Contents of the invention

The purpose of the present invention is to overcome the above shortcomings of the prior art, design and manufacture a semiconductor vertical cavity surface emitting semiconductor laser with high gain, low threshold current, small series resistance and high single-mode output power. The device parameters affecting photonic crystal vertical cavity surface emitting semiconductor laser mainly include active layer structure, photonic crystal period, air aperture, etching depth, device diameter, oxidation aperture, etc.

The external cavity type multi-active region photonic crystal vertical cavity surface emitting semiconductor laser of the present invention is characterized in that:

From bottom to top: back electrode, substrate, lower DBR, multiple active regions composed of multiple single active regions cascaded by reverse tunnel junctions; oxidation confinement layer, the center of the oxidation confinement layer is an oxidation aperture with a diameter of 10- 30μm oxidation hole; upper DBR; P-type ohmic contact layer; upper metal electrode;

Etching 1-3 microns in the DBR to produce a defect photonic crystal structure; the period of the defect photonic crystal structure is 1-7 microns, and the duty ratio is less than 0.7; the photonic crystal defect cavity 13 is surrounded by at least 3 circles with an aperture of 0.5- Surrounded by 5 micron air holes.

The present invention adopts the multiple active region 14 structure in the active region structure of the device. Using MOCVD or MBE epitaxial growth technology, epitaxial wafers of external-cavity multi-active-region photonic crystal vertical cavity surface-emitting semiconductor lasers were grown. The specific manufacturing process is as follows: the lower DBR6 is grown on the substrate 7 . Then grow multiple active regions 14 composed of multiple single active regions cascaded through the reverse tunnel junction; Al _0.98 Ga _0.02 As oxidation confinement layer 4; upper DBR3; P-type ohmic contact layer 2 multiple active region photonic crystal Epitaxial materials for vertical cavity surface emitting semiconductor lasers.

The introduction of multiple active regions 14 into the photonic crystal vertical cavity surface emitting semiconductor laser can achieve higher optical output power at a lower injection current, avoiding the problem of poor beam quality caused by a higher injection current. The problem of low gain of ordinary photonic crystal vertical cavity surface emitting semiconductor laser is solved.

Due to the introduction of the photonic crystal structure in the present invention, the mode confinement function of the oxidation hole 9 of the device has been replaced by the defect photonic crystal 12, and the main function of the oxidation hole 9 is to limit the current injection. In order to improve the single-mode output power of the device, it is necessary to increase the aperture of the oxidation hole 9, which is larger than the general VCSEL single-transverse-mode limit condition of 5 μm, regardless of the lasing mode distribution. At the same time, due to the influence of carrier diffusion, the oxidation pore size should not be too large, otherwise the uniformity of carrier injection will be reduced, the threshold current and operating current will be increased, which is not conducive to mode selection. Therefore, when making an external cavity vertical cavity surface emitting semiconductor laser, a large oxide aperture external cavity vertical cavity surface emitting semiconductor laser with an oxidation hole 9 having a diameter of 10-30 μm is produced.

The present invention introduces the defective photonic crystal structure 12 into the external cavity type vertical cavity surface emitting semiconductor laser with multiple active areas to realize the mode limitation of the vertical cavity surface emitting semiconductor laser with multiple active areas to realize single-mode output. The defect photonic crystal structure 12 is mainly in the upper DBR3. Such a structure is different from a solid photonic crystal fiber, and the working mode of the device is related to the etching depth. First of all, since the thickness of the chip used to make it is about 8 microns, it is difficult to completely engrave it from the top to the bottom when etching the smaller air hole 11. Region 14 onwards will increase non-radiative recombination. Therefore, the etching depth is usually about 1-3 microns. When the etching depth is 1.2 microns, corresponding to different periodic defect photonic crystals 12, as long as the duty ratio (the ratio of the diameter of the photonic crystal hole 11 to the photonic crystal period) is less than 0.7, the single-mode condition is met. For a device with an etching depth of 3 microns, the duty ratio of the device cannot be greater than 0.5. Therefore, the introduction of smaller perturbation by reasonable shallow etching will be more conducive to the mode selection. Of course, in the actual device fabrication, the change of the refractive index caused by the temperature drift of the device must also be considered. Generally speaking, its impact on the effective refractive index The effect is around 0.01. Therefore, the etching depth of the photonic crystal should not be too shallow. If the etching depth is too shallow, the effect of the photonic crystal will be covered by the normal working temperature drift effect. Usually the etching depth is 1-3 microns.

For the photonic crystal period (the distance between the centers of the two air holes 11), due to the size effect, when the duty ratio is less than 0.5, the single transverse mode condition in the photonic crystal waveguide structure can be satisfied. Therefore, in order to obtain a high power output and facilitate the manufacturing process, the photonic crystal period is as large as possible. However, the influence of thermal effects on the refractive index of the material must also be considered in the actual device. The central area of the device is absorbed by the material due to laser resonance, and the temperature is higher than that of the outer side. The difference in internal and external temperature leads to a difference in the refractive index of the material. It will not be affected by temperature drift, so the effective refractive index difference between the photonic crystal region and the light exit defect hole must overcome this temperature drift, and the refractive index difference of the photonic crystal becomes smaller as the period increases. In order to prevent the mode modulation effect of the photonic crystal from being annihilated by the thermal effect, the period of the photonic crystal should be as small as possible. Based on the above considerations, the present invention adopts a variety of photonic crystal periodic structures with a period of several microns.

At the same time, when the diameter of the air hole 11 is reduced, the conduction mode will shift to low frequency as a whole, and the high-order guided wave mode is restricted and cannot propagate in the waveguide of the photonic crystal defect cavity 13 . Therefore, when the duty cycle becomes smaller, the modes in the waveguide band are significantly reduced, and only the fundamental transverse mode can be transmitted in the waveguide of the photonic crystal single-defect cavity 13 . At this time, the defect cavity 13 of the photonic crystal is used as the light exit hole 10 of the VCSEL, and single-mode lasing can still be formed when the diameter is large. Since the photonic crystal realizes lateral mode selection, the oxidation aperture that determines the output power may not be subject to mode selection. limit, only the current injection needs to be adjusted individually. Because the current diffusion of the external cavity photonic crystal multi-active area vertical cavity surface emitting semiconductor laser needs to pass through the defect photonic crystal 12 and the upper DBR3, it will introduce non-radiative recombination, and the diameter of the air hole 11 is limited by the current injection. The diameter of the air hole 11 can not be too small, otherwise it will not be able to lasing. Air-hole 11 photonic crystals with apertures ranging from 0.2-3.5 microns were fabricated.

It is found through experiments that the more rows of air holes 11 in the outer layer of the photonic crystal defect cavity 13, the better the light restriction is, and as the duty cycle increases, the less light leaked out of the photonic crystal defect cavity 13, when When the duty ratio is constant, the leakage decreases as the number of rows of air holes 11 increases. At least 3 turns are needed to realize the number of turns of the single-mode confining air hole 11 .

The defect-type photonic crystal 12 is specifically manufactured by growing a layer of SiO ₂ on the surface of the device chip by using PECVD, and then throwing a layer of electron beam glue on the surface of the SiO ₂ , and using the electron beam exposure (EBL) technology to directly write the designed pattern on the electron beam. Glue on. Then by developing, the glue is applied to obtain the defect-type photonic crystal 12 pattern as shown in Figure 3, and _the unprotected SiO is etched away by inductively coupled ion etching (ICP) to remove the glue to obtain the defect shown in Figure 3 Type photonic crystal 12SiO ₂ pattern. Then use the inductively coupled ion etching technique (ICP) to etch and remove the residual SiO ₂ to manufacture the defect-type photonic crystal 12 . In addition to the above preparation methods, the defect-type photonic crystal 12 can also be prepared by using a photoresist mask by means of deep ultraviolet lithography. The specific steps are to clean the device chip with acetone ethanol deionized water in sequence, then dry it, throw a layer of photoresist on the surface of the device chip, pre-baking hard film, photolithography, development, post-baking, ICP etching, and degumming . Defect-type photonic crystals 12 are also available.

The defect-type photonic crystal 12 prepared by the above method can perform lateral mode coupling with an external cavity multi-active area vertical cavity surface emitting laser to suppress the high-order mode of the laser. The fundamental transverse mode will not be lost and exit the defect cavity 13 of the photonic crystal into the air. At the same time, the photonic crystal defect cavity 13 improves its effective refractive index relative to the surrounding area. Similar to the working principle of a solid photonic crystal fiber, it can form a waveguide structure to control the transverse mode more effectively, so that it can still be used when the light exit aperture is large. Realize the work of single transverse mode. In this way, while ensuring single-mode operation, the oxidation aperture can be relatively increased to tens of microns, so that the external-cavity photonic crystal multi-active area vertical cavity surface-emitting semiconductor laser can obtain single-mode output at high power, and the single-mode power is changed from the original 1mW The following increases to a few milliwatts. At the same time, the series resistance is also reduced. The series resistance of the general oxidation-limited vertical cavity surface emitting laser is several hundred ohms, while the resistance of the external cavity multi-active photonic crystal vertical cavity surface emitting semiconductor laser can be as small as below one hundred ohms. Therefore, the adverse influence of the thermal effect is reduced and the high-speed modulation characteristic of the device is beneficial. A higher side-mode suppression ratio was obtained, and an external-cavity multi-active-region photonic crystal vertical-cavity surface-emitting semiconductor surface-emitting laser with a size greater than several tens of decibels could be manufactured.

Compared with the prior art, the present invention has the following advantages

1. The oxidation hole 9 of the single-mode operation can be increased from the original several microns to dozens of microns, which greatly reduces the series resistance of the device, thereby improving the thermal stability of the device, and the device has a longer service life.

2. The combination of multi-active longitudinal optical coupling and single-mode transmission characteristics of photonic crystals is realized. The light-emitting area under single-mode operation is increased, and the power of single-mode is higher than that of ordinary oxidation-limited and ordinary photonic crystal vertical cavity surface emitting semiconductors. The power of the laser is large, and a low-threshold single-mode high-power output is obtained.

3. Stronger anti-interference ability, higher transmission speed, (more than 35DB side mode suppression ratio) narrower line width, stronger modulation characteristics, and the polarization direction of the laser can be controlled by using an asymmetric photonic crystal structure.

4. It can be completely transplanted to the development of other wavelength VCSELs.

Description of drawings

Below in conjunction with accompanying drawing and embodiment the present invention is described in further detail

Upper metal electrode (P-type metal electrode) 1; P-type ohmic contact layer 2; upper distributed Bragg reflector (upper DBR) 3 grown alternately in periods; Al _0.98 Ga _0.02 As oxidation confinement layer 4; single active region 5; period Alternately grown lower distributed Bragg reflectors (lower DBR) 6; substrate 7; N-type metal electrodes 8; oxidation holes 9; light exit holes 10; 11; .

Figure 1. Oxidation-confined vertical-cavity surface-emitting semiconductor laser

Figure 2. External cavity multi-active area photonic crystal vertical cavity surface emitting semiconductor laser

Figure 3. Top view of external cavity multi-active area photonic crystal vertical cavity surface emitting semiconductor laser

Specific implementation methods (taking the wavelength 850nm as an example)

1. Obtain the substrate 7 by growing it on an N ⁺ type GaAs substrate. Use the MOCVD method to sequentially grow a 0.3 micron GaAs buffer layer on the substrate and then grow N ⁺ Al _0.1 Ga _0.9 As (60nm doping concentration 3×10 ¹⁸ cm ^-3 ) and n ⁺ Al _0.9 Ga _0.1 As (68.19nm doping concentration 3×10 ¹⁷ cm ^-3 ) with 28 periods of lower DBR6, In _0.18 Al _0.12 Ga _0.7 As and Al _0.22 Ga _0.78 AS Single active region 5 heavily doped N ⁺ GaAs and P ⁺ GaAs reverse tunnel junction cascaded multiple active regions (three active regions) 14, Al _0.98 Ga _0.02 As (30nm doping concentration 1×10 ¹⁸ cm ^-3 ) oxidation confinement layer 4, Al _0.1 Ga _0.9 As (60nm doping concentration 3×10 ¹⁸ cm ^-3 ) and Al _0.9 Ga _0.1 As (68.19nm doping concentration 3×10 ¹⁸ cm ^-3 ) composition The 24-period upper DBR3, Al _0.1 Ga _0.9 As heavily doped ohmic contact layer 2 .

2. Using the traditional oxidation-limited vertical cavity surface-emitting semiconductor laser manufacturing process to produce a P electrode 1 with a mesa of 65-75 microns, a light exit hole of 10 apertures of 40-50 microns, an oxidation aperture of 10-30 microns, and 500 nanometers of TiAu The multi-active area oxidation-limited vertical cavity surface emitting semiconductor semi-finished chip (without thinning, sputtering back electrode and dissociation process)

3. Put the sample washed and dried with acetone, absolute ethanol and deionized water into chemical vapor deposition (PECVD) to deposit a dense _SiO2 oxide film with a thickness of about 300 nanometers on the surface of the sample.

4. Then put a layer of Zep520 electron beam glue on the surface of the deposited SiO ₂ oxide film, pre-baked the film, then put the sample into the electron beam exposure machine for exposure, development, and post-baking on the glue to obtain the required graphics. The period of the photonic crystal in the figure is from 1-7 microns. The duty cycle is from 0.1-0.5 and the pore size of the air holes is from 500 nanometers to 5 microns. (The graphics on the glue are shown in Figure 3)

5. Use inductively coupled ion etching (ICP) to etch off the unprotected SiO ₂ oxide film and remove the glue. Transfer the on-gel pattern to the _SiO2 oxide film. (The graph on SiO ₂ is shown in Figure 3)

6. Put the sample with SiO ₂ mask into the vacuum chamber of inductively coupled ion etching (ICP) for etching. The etching depth is 1-3 microns, and the etched sample is rinsed with SiO ₂ etching solution to remove the remaining SiO ₂ mask on the surface.

7. Thinning to about 100 microns, sputtering back electrode 8 (back electrode AuGeNiAu thickness 300nm), alloy, dissociation, pressure welding. You can get the required laser.

8. Test

By using a spectrum analyzer to test a single-defect external-cavity multi-active-region photonic crystal vertical cavity surface-emitting semiconductor laser with a period of 7 microns and a duty cycle of 0.2 and an etching depth of 1.5 microns, it is found that its spectral linewidth is 0.2 nanometers and side mode suppression than 45dB. Observation of its spot characteristics with a near-field optical microscope showed that it was single-mode. Test its single-mode power 3.5mW with a laser test system. Threshold current 4.4mA series resistor 50 ohms.

The cycle is 7 microns, the duty cycle is 0.4, the etching depth is 3 microns, the single-defect external cavity multi-active area photonic crystal vertical cavity surface emitting semiconductor laser is found to have a spectral linewidth of 0.3 nanometers, a side mode suppression ratio of 35dB, and a single-mode power 3.3mW, threshold current 3.8mA, series resistance 52 ohms.

The period is 5 microns, the duty cycle is 0.2, the etching depth is 1.5 microns, and the single-defect external cavity multi-active region photonic crystal vertical cavity surface emitting semiconductor laser is found to have a spectral linewidth of 0.3 nanometers and a side mode suppression ratio of 40DB. Single-mode power 3.2mW. Threshold current 3.8mA, series resistance 73 ohms.

The cycle is 5 microns, the duty cycle is 0.5, the etching depth is 2.0 microns, and the single defect external cavity multi-active region photonic crystal vertical cavity surface emitting semiconductor laser is found to have a spectral linewidth of 0.6 nanometers and a side mode suppression ratio of 40DB. Single-mode power 3.0mW. Threshold current 3.3mA, series resistance 45 ohms.

The period is 1 micron, the duty cycle is 0.5, the etching depth is 1 micron, and the single defect external cavity multi-active area photonic crystal vertical cavity surface emitting semiconductor laser is found to have a spectral linewidth of 0.6 nanometers and a side mode suppression ratio of 45DB. Single-mode power 0.5mW. Threshold current 0.8mA, series resistance 193 ohms.

Period is 1 micron, duty cycle is 0.7, etch depth is 1.2 microns Single-defect external cavity multi-active region photonic crystal vertical cavity surface emitting semiconductor laser is found to have a spectral linewidth of 0.5 nanometers, side mode suppression ratio of 40DB, and single-mode power 1.0mW, threshold current 0.5mA, series resistance 94 ohms.

Claims (1)

1. external cavity type multiple-active region photon crystal vertical cavity face emitting semiconductor laser is characterized in that:

Be followed successively by from bottom to up: backplate, substrate, DBR, the multiple-active-region that constitutes by a plurality of single active area of reverse tunnel knot cascade down; Oxidation limiting layer, oxidation limiting layer center are that oxide-aperture is the oxidation hole of 10-30 μ m; Last DBR; P type ohmic contact layer; Last metal electrode;

On the etching among the DBR 1-3 micron produce the deficiency photon crystal structure; The cycle of deficiency photon crystal structure is a 1-7 micron, and duty ratio is less than 0.7; Photonic crystal defect cavity (13) is that 0.5-5 micron airport surrounds by at least 3 circle apertures.

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