A vertical-cavity surface-emitting laser and its fabrication method

Abstract

The invention discloses a vertical cavity surface emitting laser and a preparation method thereof, and relates to the field of semiconductor lasers, in particular to a vertical cavity surface emitting laser, which solves the problems of high current threshold and low slope efficiency of the existing laser. The laser comprises, from top to bottom, a P electrode, a P-type Bragg reflector group, a first oxide layer, a first active region, a first hole blocking layer, a first quantum well intercalation tunnel junction, a second oxide layer, a second active region, a second hole blocking layer, an N-type Bragg reflector group, a substrate, and an N electrode. By arranging a quantum well intercalation tunnel junction in the laser and introducing an AlGaAs material containing a high Al component to form a multi-layer hole blocking layer, the series resistance of the device is reduced, the carrier tunneling probability and the slope efficiency are increased, and the light absorption of the tunnel junction is reduced.

Description

Technical Field

[0001] The invention relates to the field of semiconductor lasers, in particular to a vertical cavity surface emitting laser. Background Art

[0002] Vertical cavity surface emitting laser (VCSEL) has unique advantages such as small size, small divergence angle, high beam quality, low cost, and easy two-dimensional integration. In recent years, it has aroused widespread research interest in the field of semiconductor lasers and has also expanded rapidly in the application-end market, including: 3D facial recognition, laser medical beauty, gas detection, smart home, lidar and other applications.

[0003] The current mainstream vertical cavity surface emitting laser (VCSEL) chip mainly adopts a single active area structure, and the single hole light output power is generally only 5-10mW. In order to make the VCSEL chip achieve a higher light output power, it is usually considered to increase the device aperture. This approach increases the difficulty of optical design and reduces the light output power density, so it is difficult to meet the needs of high integration and high power density. Connecting multiple active areas through highly doped tunnel junctions is currently an effective way to improve power density, quantum efficiency and slope efficiency. By optimizing and controlling the tunnel junction doping, the tunnel junction resistance can be effectively reduced. However, high doping also inevitably increases the free carrier absorption loss, which is not conducive to low threshold lasing. At the same time, due to the carrier leakage effect in the active area, electrons and holes will break away from the constraints of the quantum well barrier and directly enter the hole and electron transport layer, greatly reducing the internal quantum efficiency of the laser.

[0004] In view of the above problems, the present invention designs a high-performance triple-junction vertical cavity surface emitting laser based on a quantum well intercalation tunnel junction and a hole barrier layer. Summary of the invention

[0005] The purpose of the present invention is to solve the problems of high current threshold and low slope efficiency of existing vertical cavity surface emitting lasers. The present invention provides a high-performance multi-junction vertical cavity surface emitting laser based on quantum well intercalation tunnel junction and hole blocking layer and a preparation method thereof. At the same time, the hole blocking layer is introduced to reduce the inter-valence band absorption at the tunnel junction while ensuring a lower laser threshold and series resistance, thereby realizing a 905nm high-performance multi-junction vertical cavity surface emitting laser.

[0006] In order to achieve the above-mentioned purpose, the present invention specifically adopts the following technical solutions:

[0007] A vertical cavity surface emitting laser comprises, from top to bottom, a P-electrode, a P-type Bragg reflector group, a first oxide layer, a first active region, a first hole blocking layer, a first quantum well intercalation tunnel junction, a second oxide layer, a second active region, a second hole blocking layer, an N-type Bragg reflector group, a substrate, and an N-electrode.

[0008] Preferably, the laser comprises, from top to bottom, a P-electrode, a P-type Bragg reflector group, a first oxide layer, a first active region, a first hole blocking layer, a first quantum well intercalation tunnel junction, a second oxide layer, a second active region, a second hole blocking layer, a second quantum well intercalation tunnel junction, a third oxide layer, a third active region, a third hole blocking layer, an N-type Bragg reflector group, a substrate, and an N-electrode.

[0009] Optionally, the quantum well intercalation tunnel junction includes from top to bottom: p-type heavily doped Al x Ga 1-x As, n-type heavily doped GaAs quantum well, n-type lightly doped Al x Ga 1-x As,Al x Ga 1-x The As component variation range is: x=0-0.1.

[0010] Optional, p-type heavily doped with Al x Ga 1-x The concentration range of As is 3×10 19 cm -3 —1×10 20 cm -3 , thickness is 5-10nm; n-type lightly doped Al x Ga 1-x The concentration range of As is 5×10 17 cm -3 —2×10 18 cm -3 .

[0011] Optionally, the first hole blocking layer, the second hole blocking layer and the third hole blocking layer are n-type lightly doped Al x Ga 1-x As hole blocking layer, Al x Ga 1-x The As component variation range is: x=0.45-0.9.

[0012] Optional, Al x Ga 1-x As doping concentration range is 5×10 17 cm -3 —2×10 18 cm -3 , thickness is 10-30nm.

[0013] The present invention also provides a method for preparing a vertical cavity surface emitting laser, comprising the following steps:

[0014] S1, epitaxially growing an N-type Bragg reflector group, a third hole blocking layer, a third active region, a third oxide layer, a second quantum well intercalation tunnel junction, a second hole blocking layer, a second active region, a second oxide layer, a first quantum well intercalation tunnel junction, a first hole blocking layer, a first active region, a first oxide layer, and a P-type Bragg reflector group in sequence on a substrate;

[0015] S2. A P electrode is fabricated on a side of the substrate close to the P-type Bragg reflector group, and an N electrode is fabricated on a side of the substrate close to the N-type Bragg reflector group, to complete fabrication of the laser.

[0016] Compared with the prior art, the advantages of the present invention are:

[0017] 1. A vertical cavity surface emitting laser and a preparation method thereof involved in the present invention can effectively reduce the tunneling distance and increase the tunneling probability by designing a quantum well intercalation tunnel junction and embedding narrow-bandgap GaAs into wide-bandgap AlGaAs. In addition, for the GaAs material system, the absorption loss of free electrons is 2-3 times smaller than that of free holes, and placing the GaAs quantum well on the n-side is conducive to reducing the absorption loss.

[0018] 2. The vertical cavity surface emitting laser and its preparation method involved in the present invention introduce AlGaAs material containing a high Al component to form a hole blocking layer, which effectively limits the escape of carriers, inhibits hole leakage current, reduces the device series resistance, increases the probability of carrier tunneling, and reduces the light absorption of the tunnel junction.

[0019] 3. The vertical cavity surface emitting laser and its preparation method involved in the present invention, the arrangement of multiple hole blocking layers can effectively suppress the hole leakage current, greatly improve the quantum efficiency in the device, and significantly increase the oblique line efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A schematic diagram of the structure of a vertical cavity surface emitting laser.

[0021] Figure 2 A schematic diagram of the structure of a vertical cavity surface emitting laser.

[0022] Figure 3 Schematic diagram of optical and electrical performance testing in Experiment 1.

[0023] Figure annotations: P-DBRs-P type Bragg reflector group, oxide layer 1-first oxide layer, active area 1-first active area, hole blocking layer 1-first hole blocking layer, tunnel junction 1-first quantum well intercalation tunnel junction, oxide layer 2-second oxide layer, active area 2-second active area, hole blocking layer 2-second hole blocking layer, tunnel junction 2-second quantum well intercalation tunnel junction, oxide layer 3-third oxide layer, active area 3-third active area, hole blocking layer 3-third hole blocking layer, N-DBRs-N type Bragg reflector group.

[0024] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0025] Therefore, the following detailed description of the embodiments of the present invention is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. DETAILED DESCRIPTION

[0026] See also Figure 1 As shown, a vertical cavity surface emitting laser comprises, from top to bottom, a P-electrode, a P-type Bragg reflector group, a first oxide layer, a first active region, a first hole blocking layer, a first quantum well intercalation tunnel junction, a second oxide layer, a second active region, a second hole blocking layer, an N-type Bragg reflector group, a substrate, and an N-electrode.

[0027] Preferably, a laser with higher power comprises, from top to bottom, a P-electrode, a P-type Bragg reflector group, a first oxide layer, a first active region, a first hole blocking layer, a first quantum well intercalation tunnel junction, a second oxide layer, a second active region, a second hole blocking layer, a second quantum well intercalation tunnel junction, a third oxide layer, a third active region, a third hole blocking layer, an N-type Bragg reflector group, a substrate, and an N-electrode.

[0028] It is understandable that, depending on the power in actual applications, different numbers of oxide layers, active regions, hole blocking layers, quantum well intercalation tunnel junctions, etc. can be selected for superposition, and different numbers of layers can be changed according to actual needs.

[0029] It can be understood that the vertical cavity surface emitting laser involved in the present invention forms a standing wave in the active area, so that the photon energy is amplified and finally forms lasing, thereby realizing the light emission of the laser. By setting a quantum well intercalation tunnel junction and introducing AlGaAs material containing a high Al component to form a hole blocking layer, while ensuring a lower laser threshold and series resistance, the inter-valence band absorption at the tunnel junction is reduced, and a high-performance vertical cavity surface emitting laser of 905nm is realized, the device series resistance is reduced, the probability of carrier tunneling is increased, and the light absorption of the tunnel junction is reduced, solving the problems of high threshold and low slope efficiency in the prior art.

[0030] Furthermore, in a vertical cavity surface emitting laser involved in the present invention, the arrangement of multiple hole blocking layers effectively suppresses hole leakage current, greatly improves the quantum efficiency in the device, and significantly increases the oblique line efficiency.

[0031] Furthermore, in a vertical cavity surface emitting laser involved in the present invention, an oxide layer is provided on one side of the p-region of each active region. In addition to limiting the aperture of the light-emitting region, the oxide layer can also effectively suppress the leakage of electrons, further improving the quantum efficiency of the device.

[0032] In the present invention, a GaAs substrate is preferred.

[0033] The quantum well intercalation tunnel junction of the present invention comprises from top to bottom: p-type heavily doped Al x Ga 1-x As, n-type heavily doped GaAs quantum well, n-type lightly doped Al x Ga 1-x As,Al x Ga 1-x The As component variation range is: x=0-0.1.

[0034] The p-type heavily doped Al x Ga 1-x The concentration range of As is 3×10 19 cm -3 —1×10 20 cm -3 , thickness is 5-10nm; n-type lightly doped Al x Ga 1-x The concentration range of As is 5×10 17 cm -3 —2×10 18 cm -3 .

[0035] Specifically, p-type heavily doped Al x Ga 1-x The concentration range of As is 8×10 19 cm -38, thickness 8nm; n-type lightly doped Al x Ga 1-x The concentration range of As is 1×10 18 cm -3 .

[0036] In the present invention, it is preferred that p-type heavily doped Al x Ga 1-x The concentration range of As is 1×10 20 cm -3 , preferably with a thickness of 10 nm; preferably with n-type lightly doped Al x Ga 1-x The concentration range of As is 2×10 18 cm -3 .

[0037] In the present invention, the two sides of the GaAs quantum well are respectively p-type heavily doped and n-type lightly doped AlxGa 1-x As, it can be understood that such a setting embeds narrow-bandgap GaAs into wide-bandgap AlGaAs, which can effectively reduce the tunneling distance and increase the tunneling probability.

[0038] It can be understood that in a tunnel junction, the tunneling probability of electrons is mainly determined by the effective mass, the bandgap width, and the tunnel length. Selecting materials with small effective mass and bandgap width to prepare tunnel junctions is conducive to obtaining a higher tunneling probability, thereby obtaining a high-quality tunnel junction with low reverse bias and low series resistance. However, reducing the bandgap width of the tunnel junction material will also introduce greater absorption losses (such as free carrier absorption and intrinsic absorption), increasing the operating threshold of the laser. Traditional near-infrared multi-junction VCSELs (such as 905, 940, and 980nm) use GaAs homojunctions.

[0039] Specifically, for the traditional 905nm multi-junction VCSEL, the fluorescence peak of the active region material at room temperature is around 890nm, and the corresponding photon energy is 1.393eV, while the bandgap width of the intrinsic GaAs material at room temperature is 1.424eV, and the difference between the two is only 31meV. Considering the bandgap shrinkage caused by the heavily doped band tail effect and carrier screening, as well as the red shift of the intrinsic absorption edge caused by the Franz-Keldysh effect, the GaAs homojunction may have intrinsic absorption of photons in this band, thereby increasing the absorption loss of the laser.

[0040] It can be further understood that the AlGaAs / GaAs / AlGaAs quantum well intercalation tunnel junction of the present invention reduces the threshold current of the laser without increasing the series resistance.

[0041] Specifically, p-type heavily doped Al x Ga 1-xThe concentration of As is approximately lower than that of n-type lightly doped Al x Ga 1-x The concentration of As is about one order of magnitude higher. This setting makes the depletion layer of the PN junction mainly fall on the n-region side, and the tunneling probability of electrons is greatly affected by the n-region material. Placing the GaAs quantum well on the n-region side improves the tunneling performance and reduces free carrier absorption, avoiding increasing the absorption loss of the laser.

[0042] The first hole blocking layer, the second hole blocking layer and the third hole blocking layer of the present invention are n-type lightly doped Al x Ga 1-x As hole blocking layer, Al x Ga 1-x The As component range is: x = 0.45-0.9. x Ga 1-x As doping concentration range is 5×10 17 cm -3 —2×10 18 cm -3 , thickness is 10-30nm.

[0043] Specifically, in this embodiment, the first hole blocking layer, the second hole blocking layer and the third hole blocking layer are n-type lightly doped Al 0.5 Ga 0.5 As hole blocking layer. Al 0.5 Ga 0.5 As doping concentration range is 6×10 17 cm -3 , thickness is 20nm.

[0044] It can be understood that, under normal circumstances, the carrier concentration in the active region of a semiconductor laser is as high as 10 18 —10 19 cm -3 The barrier layers on both sides of the quantum well cannot effectively limit the escape of carriers. Some electrons and holes injected into the quantum well region cannot participate in stimulated radiation in a short time, and migrate to the hole transport layer and the electron transport layer respectively, thereby generating leakage current and releasing energy in a non-radiative recombination manner, affecting the laser's diagonal efficiency.

[0045] Therefore, in the traditional multi-junction VCSEL, the material composition of the active region n-region side is Al x Ga 1-x As (x≤0.4), note that since x≤0.4, Al is low and this layer of material cannot effectively suppress the hole leakage current in the active area.

[0046] Each junction area of ​​a traditional multi-junction VCSEL is designed with a high-Al oxide layer, which can effectively limit the electron leakage current. However, the gap between the tunnel junction and the quantum well region is usually low-Al Al. x Ga 1-x As (x < 0.4), such an amount of Al cannot effectively prevent the escape of holes. x Ga 1-x As (x>0.4) hole blocking layer can better suppress hole current.

[0047] The present invention also provides a method for preparing a vertical cavity surface emitting laser, comprising the following steps:

[0048] S1, epitaxially growing an N-type Bragg reflector group, a third hole blocking layer, a third active region, a third oxide layer, a second quantum well intercalation tunnel junction, a second hole blocking layer, a second active region, a second oxide layer, a first quantum well intercalation tunnel junction, a first hole blocking layer, a first active region, a first oxide layer, and a P-type Bragg reflector group in sequence on a substrate;

[0049] S2, making a P electrode on the side of the substrate close to the P-type Bragg reflector group, and making an N electrode on the side of the substrate close to the N-type Bragg reflector group, to complete the production of the laser.

[0050] In the specific epitaxial process, there is a buffer layer between the N-type Bragg reflector group, the third hole blocking layer, the third active area, the third oxide layer and other layers involved. The parameters are adjusted according to the actual preparation requirements to perform epitaxial growth on each buffer layer to meet the actual needs.

[0051] The preparation method of the present invention can use metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) to perform epitaxy on each layer.

[0052] Example 1

[0053] A vertical cavity surface emitting laser comprises, from top to bottom, a P-electrode, a P-type Bragg reflector group, a first oxide layer, a first active region, a first hole blocking layer, a first quantum well intercalation tunnel junction, a second oxide layer, a second active region, a second hole blocking layer, a second quantum well intercalation tunnel junction, a third oxide layer, a third active region, a third hole blocking layer, an N-type Bragg reflector group, a substrate, and an N-electrode.

[0054] In this embodiment, the quantum well intercalation tunnel junction includes from top to bottom: p-type heavily doped Al 0.05 Ga 0.95 As, n-type heavily doped GaAs quantum well, n-type lightly doped Al 0.05 Ga 0.95As. p-type heavily doped Al 0.05 Ga 0.95 The concentration range of As is 3×10 19 cm -3 , thickness is 8nm; n-type lightly doped Al 0.05 Ga 0.95 The concentration range of As is 1×10 18 cm -3 .

[0055] In this embodiment, the first hole blocking layer, the second hole blocking layer and the third hole blocking layer are n-type lightly doped Al 0.6 Ga 0.4 As hole blocking layer. Al 0.6 Ga 0.4 The As doping concentration range is 1×10 18 cm -3 , thickness is 20nm.

[0056] Example 2

[0057] A vertical cavity surface emitting laser comprises, from top to bottom, a P-electrode, a P-type Bragg reflector group, a first oxide layer, a first active region, a first hole blocking layer, a first quantum well intercalation tunnel junction, a second oxide layer, a second active region, a second hole blocking layer, a second quantum well intercalation tunnel junction, a third oxide layer, a third active region, a third hole blocking layer, an N-type Bragg reflector group, a substrate, and an N-electrode.

[0058] In this embodiment, the quantum well intercalation tunnel junction includes from top to bottom: p-type heavily doped Al 0.06 Ga 0.94 As, n-type heavily doped GaAs quantum well, n-type lightly doped Al 0.06 Ga 0.94 As. Among them, p-type heavily doped Al 0.06 Ga 0.94 The concentration range of As is 8×10 19 cm -3 , thickness is 6nm; n-type lightly doped Al 0.06 Ga 0.94 The concentration range of As is 9×10 17 cm -3 .

[0059] In this embodiment, the first hole blocking layer, the second hole blocking layer and the third hole blocking layer are n-type lightly doped Al 0.8 Ga 0.2 As hole blocking layer. Among them, Al 0.8 Ga 0.2 As doping concentration range is 8×10 17 cm-3 , thickness is 28nm.

[0060] In order to verify the optical and electrical performances of the emitting laser involved in the present invention, the following test examples were carried out.

[0061] Test case to check the optical and electrical performance of the transmitting laser

[0062] 1.1 Experimental design

[0063] Two treatment groups were set up in the experiment. The experimental group was the vertical cavity surface emitting laser involved in Example 1, and the comparative group was the vertical cavity surface emitting laser using Al 0.05 Ga 0.95 The optical and electrical performances of the As homogeneous tunnel junction vertical cavity surface emitting laser were tested under the same experimental conditions.

[0064] Test results see Figure 2 The dotted line represents the 905nm triple junction VCSEL with quantum well intercalation tunnel junction and hole blocking layer (experimental group), and the solid line represents the Al 0.05 Ga 0.95 905nm triple-junction VCSEL with As homogeneous tunnel junction (control group).

[0065] After testing, the current-power curve representing the optical performance and the current-voltage curve representing the electrical performance were obtained.

[0066] 1.2 Results Analysis

[0067] See also Figure 2 As can be seen from the current-power curve, in terms of optical performance, the lasing threshold of the experimental group is 1.2mA. Compared with the lasing threshold of the control group of 1mA, the experimental group has a higher lasing threshold, that is, a larger turn-on current. It can be seen that the experimental group introduced a GaAs intercalation quantum well tunnel junction, resulting in greater absorption losses. Furthermore, although the experimental group increased the lasing threshold, due to the presence of the hole blocking layer, the internal quantum efficiency of the device is further improved, the diagonal efficiency is higher, and there is a greater output power under high current injection conditions.

[0068] Referring to the current-voltage curve, in terms of electrical performance, the curve of the experimental group is steeper, that is, the differential resistance of the experimental group is smaller than that of the control group. This shows that the GaAs intercalation tunnel junction of the experimental group does have a shorter tunneling distance, which increases the tunneling probability of electrons.

[0069] Combining the two sets of curves, although the experimental group has a larger lasing threshold, it performs better in differential resistance and slope efficiency and has a higher photoelectric conversion efficiency.

[0070] In summary, the vertical cavity surface emitting laser involved in the present invention can effectively reduce the tunneling distance and increase the tunneling probability by designing a quantum well intercalation tunnel junction and embedding narrow-bandgap GaAs into wide-bandgap AlGaAs. In addition, for the GaAs material system, the absorption loss of free electrons is 2-3 times smaller than that of free holes, and placing the GaAs quantum well on the n-side is conducive to reducing the absorption loss. The introduction of AlGaAs material containing a high Al component to form a hole blocking layer can effectively limit the escape of carriers, suppress hole current, reduce the series resistance of the device, increase the probability of carrier tunneling, and reduce the light absorption of the tunnel junction.

[0071] The above embodiment is only one implementation of the present invention, and its description is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be based on the attached claims.

Claims

1. A vertical cavity surface emitting laser, characterized in that: The laser is composed of a P electrode, a P-type Bragg reflector group, a first oxide layer, a first active region, a first hole blocking layer, a first quantum well intercalation tunnel junction, a second oxide layer, a second active region, a second hole blocking layer, an N-type Bragg reflector group, a substrate, and an N-electrode from top to bottom. The quantum well intercalation tunnel junction includes: a p-type heavily doped Al x Ga 1-x As, n-type heavily doped GaAs quantum well, n-type lightly doped Al x Ga 1-x As,Al x Ga 1-x As composition range: x = 0-0.1, the hole blocking layer is n-type lightly doped Al x Ga 1-x As hole blocking layer, Al x Ga 1-x The As component variation range is: x=0.45-0.

9.

2. A vertical cavity surface emitting laser according to claim 1, characterized in that: The laser comprises, from top to bottom, a P-electrode, a P-type Bragg reflector group, a first oxide layer, a first active region, a first hole blocking layer, a first quantum well intercalation tunnel junction, a second oxide layer, a second active region, a second hole blocking layer, a second quantum well intercalation tunnel junction, a third oxide layer, a third active region, a third hole blocking layer, an N-type Bragg reflector group, a substrate, and an N-electrode.

3. A vertical cavity surface emitting laser according to claim 1, characterized in that: p-type heavily doped Al x Ga 1-x The concentration range of As is 3×10 19 cm -3 —1×10 20 cm -3 , thickness is 5-10nm; n-type lightly doped Al x Ga 1-x The concentration range of As is 5×10 17 cm -3 —2×10 18 cm -3 .

4. A vertical cavity surface emitting laser according to claim 1, characterized in that: Al x Ga 1-x As doping concentration range is 5×10 17 cm -3 —2×10 18 cm -3 , thickness is 10-30nm.

5. The method for preparing a vertical cavity surface emitting laser according to any one of claims 1 to 4, characterized in that: The following steps are involved: S1. On the substrate, sequentially epitaxially grow an N-type Bragg reflector group, a third hole blocking layer, a third active region, a third oxide layer, a second quantum well intercalation tunnel junction, a second hole blocking layer, a second active region, a second oxide layer, a first quantum well intercalation tunnel junction, a first hole blocking layer, a first active region, a first oxide layer, and a P-type Bragg reflector group; S2. Make a P electrode on the side of the substrate close to the P-type Bragg reflector group, and make an N electrode on the side of the substrate close to the N-type Bragg reflector group, to complete the production of the laser.

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