Monometallic waveguide GaN-based terahertz quantum cascade laser and preparation method thereof

Abstract

The invention discloses a monometallic waveguide GaN-based terahertz quantum cascade laser, which is characterized in that the bottommost layer of the structure is a substrate, an n-type highly-doped n + + layer, a QCL active region and an n-type doped layer are sequentially arranged on the substrate, a cathode electrode layer is arranged on the n-type highly-doped n + + layer, an anode electrode layer is arranged on the n-type doped layer, and an anode electrode layer is arranged on the anode electrode layer. Wherein the substrate is a GaN substrate, an AlN substrate or a SiC substrate. The invention also discloses a preparation method. On the basis of systematic research on refractive index related parameters of GaN under THz frequency, absorption of single-metal waveguide structure parameters on QCL light field limiting factors and free carriers is analyzed, and QCL with high light field limiting factors and low loss of different substrates is designed. Besides, according to the designed active region structure, the gain can reach 88 / cm at 10K and can reach 34 / cm at 280K, the corresponding wavelength is 6.6 THz, and the frequency which cannot be achieved by a traditional GaAs system is achieved.

Description

Technical Field

[0001] The invention relates to a single metal waveguide GaN-based terahertz quantum cascade laser and a preparation method thereof, belonging to the technical field of semiconductor devices. Background Art

[0002] Since the invention of the world's first QCL in 1994, this research field has made great progress. QCL has covered 2.6-25μm in the mid-infrared band, and can achieve continuous lasing at room temperature in the 4-20μm band, with an operating temperature of more than 400K. Since the invention of GaAs / AlGaAs-based THz-QCL in 2002, it has covered the 0.8-5.4THz frequency band; the maximum peak power in pulse mode is 2.4W (10K), and the maximum operating temperature is 250K (2006 to 170K; 2012 to 200K; 2020 to 250K); the maximum continuous lasing power is 100mW (10K), and the maximum operating temperature is 132K. However, due to the inherent characteristics of GaAs semiconductor materials, the operating temperature of 250K basically reaches the theoretical limit.

[0003] However, GaAs / AlGaAs terahertz quantum cascade lasers are severely limited in the frequency gap of solid-state coherent light sources (5.5 12THz) for two main reasons: (1) the polariton bandgap of GaAs material is about 7-9THz; (2) in this band, the non-radiative transition caused by longitudinal optical (LO) phonon scattering is much faster than the radiative transition. The potential of III-nitride GaN series wide bandgap semiconductor materials in the field of QCL has attracted attention. Compared with traditional THz-QCL, the advantages and potential of GaN THz-QCL mainly lie in its large longitudinal optical (LO) phonon energy (91meV; 22THz), which is 2.5 times that of GaAs LO phonon energy (36meV; 8.8THz). This brings two obvious advantages: First, it can greatly suppress the scattering of laser upper energy level phonons caused by thermal excitation. This non-radiative recombination is the main physical reason why GaAs-based THz-QCLs are difficult to operate at room temperature. The greater the LO phonon energy, the more it can suppress this non-radiative recombination, maintaining room temperature or higher temperature population inversion to achieve THz lasing. Second, the phonon absorption band is far away from the THz frequency band, which can broaden the operating frequency of THz-QCL to the blank frequency band of 5.4-12THz. In comparison, due to strong phonon absorption, GaAs-based THz-QCLs cannot reach this high frequency band.

[0004] Due to lattice mismatch and thermal mismatch, the current growth technology of III nitride is not very mature, and the active area design is also limited by the growth technology. In addition to the QCL active area structure and epitaxial technology, high light field confinement factor and low loss waveguide structure are also crucial. For traditional terahertz quantum lasers, there are mainly two types of waveguides, bimetallic waves (DMW) and single metal semi-insulating surface plasma waveguides (SI-SPWs). The light field confinement factor of the bimetallic waveguide is close to 1, but its process cannot be mass-produced at present. The single metal waveguide process is fully compatible with the current microelectronics process, but the light field will leak to the substrate, so the choice of substrate and the design of the waveguide structure have a huge impact on its light field characteristics.

[0005] Although the process of single metal semi-insulating surface plasmon waveguide (SI-SPWs) QCL is simple, it has higher requirements for structural design. Different from the dual metal waveguide QCL, the light field confinement factor and waveguide loss of the single metal waveguide QCL will vary greatly with the waveguide structure. Therefore, in order to obtain a larger light field confinement factor and a smaller internal loss to improve the performance of the laser, the waveguide structure needs to be designed very carefully. However, the current research on the optical properties of GaN-related materials in the THz band is not mature enough. The plasma frequency of GaN material is just at the THz frequency of QCL operation, and is related to the doping concentration, such as Figure 1 and Figure 2 It can be seen that the real and imaginary parts of the refractive index at different THz frequencies are very different due to different doping concentrations. Therefore, it is very complicated to analyze the optical field confinement factor and internal loss at THz frequencies. Different substrates and different operating frequencies will have very different designs. Summary of the invention

[0006] The object of the present invention is to provide a single metal waveguide GaN-based terahertz quantum cascade laser.

[0007] The purpose of the present invention is achieved through the following technical solutions:

[0008] A single metal waveguide GaN-based terahertz quantum cascade laser, characterized in that the bottom layer of its structure is a substrate, on which an n-type highly doped n++ layer, a QCL active area, and an n-type doped layer are arranged in sequence, a cathode electrode layer is arranged on the n-type highly doped n++ layer, and an anode electrode layer is arranged on the n-type doped layer, wherein the substrate is a GaN substrate, an AlN substrate or a SiC substrate.

[0009] Preferably, the QCL active region layer includes a plurality of periodic structures, each periodic structure is composed of three AlGaN barrier layers with an Al component of 10-18% and three well layers in sequence, and the thickness of the structure sequence is: The thickness of the barrier layer is indicated by bold underline, and n-type doping is performed in the widest potential well layer, with a sheet doping density of 2-8×10 10 cm -2 .

[0010] Preferably, the QCL active region layer includes 50-250 periodic structures.

[0011] Preferably, when the substrate is a GaN substrate and the laser frequency is less than 4 THz, the thickness of the n++ layer is 360-440 nm;

[0012] When the substrate is a GaN substrate and the laser frequency is 4-8THz, the thickness of the n++ layer is 300-400nm; when the substrate is a GaN substrate and the laser frequency is greater than 8THz, the thickness of the n++ layer is 100-330nm; when the substrate is a SiC substrate or an AlN substrate, the thickness of the n++ layer is 100-300nm;

[0013] The doping concentration of the n++ layer is 1×10 19 -5×10 19 / cm 3 .

[0014] Preferably, the thickness of the n-type doping layer is 20-250 nm, and the doping concentration is 1×10 18 -5×10 18 / cm 3 .

[0015] Preferably, the anode electrode layer adopts a Ti / Al / Ni / Au structure, and the thicknesses thereof are 25-35 / 440-460 / 45-55 / 250-4500 nm respectively.

[0016] Preferably, the cathode electrode layer adopts a Ti / Al / Ni / Au structure, and the thicknesses thereof are 25-35 / 440-460 / 45-55 / 100-200 nm, respectively.

[0017] The present invention also discloses a method for preparing the above-mentioned single metal waveguide GaN-based terahertz quantum cascade laser, the steps of which include:

[0018] (1) Clean the substrate;

[0019] (2) epitaxially grow an n-type highly doped n++ layer on the substrate surface, followed by the QCL active region, and finally the n-type doped layer;

[0020] (3) First photolithography, processing the anode electrode on the surface of the n-type doped layer by photolithography, electron beam evaporation and lift-off;

[0021] (4) ICP etching: Using the anode electrode as a mask, a platform deep into the n-type highly doped n++ layer is etched on the device surface;

[0022] (5) Second photolithography, through photolithography, electron beam evaporation and stripping, the cathode electrode is processed on the n++ layer;

[0023] (6) Cleavage and packaging to obtain a single metal waveguide GaN-based terahertz quantum cascade laser.

[0024] Preferably, the substrate is thinned to 100-150 μm before cleavage.

[0025] Preferably, the cleavage cavity is 600-1200 μm long.

[0026] The present invention designs a GaN-based terahertz quantum cascade laser with a single metal waveguide structure, which has a simple structure and is easy to process, and a light field confinement factor of up to 20%-50%. Based on a systematic study of the refractive index-related parameters of GaN at THz frequencies, the present invention analyzes the effects of the single metal waveguide structure parameters on the QCL light field confinement factor and free carrier absorption, designs a QCL with high light field confinement factor and low loss on different substrates, and uses an n++ layer to effectively improve the light field confinement factor. In addition, the active region structure designed by the present invention has a gain of up to 88 / cm at 10K and up to 34 / cm at 280K, and the corresponding wavelength is 6.6THz, achieving a frequency that the traditional GaAs system cannot achieve. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the real part of the refractive index of GaN with different doping concentrations in the THz band.

[0028] Figure 2 is the imaginary part of the refractive index of GaN with different doping concentrations in the THz band.

[0029] Figure 3 A schematic cross-sectional view of a laser device.

[0030] Figure 4 Schematic diagram of the structure of one period of active area QCL.

[0031] Figure 5 The waveguide loss of the QCL device in the THz band based on GaN substrate with different n++ layer thicknesses, where the doping concentration of the active region is 5×10 15 cm -3 .

[0032] Figure 6 is the light field confinement factor of the QCL device in the THz band based on GaN substrate with different n++ layer thicknesses, where the doping concentration of the active region is 5×10 15 cm-3 .

[0033] Figure 7 is the waveguide loss of GaN substrate QCL devices in the THz band with different carrier concentrations, where the doping concentration of the active region is 5×10 15 cm -3 .

[0034] Figure 8 The waveguide loss of QCL devices in the THz band based on SiC substrate with different n++ layer thicknesses, where the doping concentration of the active region is 5×10 15 cm -3 .

[0035] Fig. 9 is the light field confinement factor of the QCL device in the THz band based on SiC substrate with different n++ layer thicknesses, where the doping concentration of the active region is 5×10 15 cm -3 .

[0036] Fig.10 The waveguide loss of QCL devices in the THz band based on AlN substrate with different n++ layer thicknesses, where the doping concentration of the active region is 5×10 15 cm -3 .

[0037] Fig.11 is the light field confinement factor of the QCL device with different n++ layer thickness based on AlN substrate in the THz band, where the doping concentration of the active region is 5×10 15 cm -3 .

[0038] Fig.12 Figure 2 shows the relationship between peak gain and applied bias voltage per cycle at different temperatures.

[0039] Fig.13 Light field distribution of single metal waveguide GaN-based terahertz quantum cascade laser.

[0040] Fig.14 Figure 3 shows the light field distribution when using n++ layer laser and when not using n++ layer laser. DETAILED DESCRIPTION

[0041] The following is a clear and complete description of the technical solutions in the embodiments of the present invention in combination with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians without creative work are within the scope of protection of the present invention.

[0042] Example 1

[0043] like Figure 3 As shown, the GaN-based terahertz quantum cascade laser is a multi-layer structure, the bottom layer is a substrate 360, on which are arranged in sequence an n-type highly doped n++ layer 340, a QCL active region 330, and an n-type doped layer 320, a cathode electrode 350 is arranged on the n-type highly doped n++ layer 340, and an anode electrode 310 is arranged on the n-type doped layer 320. The structure of the present invention is simple and only requires two times of photolithography to process the structure, but its light field confinement factor is still as high as 20%-50%, such as Fig.13 shown.

[0044] The n-type doping layer 320, as an n-type contact layer, forms a good ohmic contact with the anode electrode 310. The thickness of the layer is 20-250 nm, and the doping concentration is 1×10 18 -5×10 18 / cm 3 .

[0045] The n-type highly doped n++ layer 340, in addition to being an n-type contact layer and forming a good ohmic contact with the cathode electrode 350, the design of this layer also affects the light field confinement factor and loss of the QCL. The doping concentration is 1×10 19 -5×10 19 / cm 3 , a higher light field confinement factor and relatively low loss can be obtained.

[0046] The substrate is a non-doped GaN or carbon-doped GaN substrate, or a SiC substrate or an AlN substrate. Since the dielectric constant of the n++ layer is much lower than that of the metal layer, the light field can penetrate the substrate (about tens of microns), so the optimization of the substrate structure and the optimization of the n++ layer structure with high n-type doping are crucial to improving the light field confinement factor of the QCL and reducing the waveguide loss.

[0047] At present, high-quality substrates for GaN-based optoelectronic devices include GaN single crystal substrates, GaN-sapphire composite substrates, AlN substrates, and SiC substrates. Among them, the dielectric constant of sapphire substrates in the THz band is higher than that of GaN, so GaN-sapphire composite substrates are not suitable for QCL waveguide structures.

[0048] For GaN substrates, the effect of n++ layer thickness on waveguide loss and optical field confinement factor can be found in Figure 5 and Figure 6 .like Figure 3As shown in the figure, with the increase of the thickness of the n++ layer, the waveguide loss increases rapidly and the increase is getting faster and faster. The main reason for this phenomenon is that the increase of the thickness of the n++ layer leads to the enhancement of free carrier absorption; in addition, due to the high doping concentration of the n++ layer, the plasma frequency increases. Therefore, with the increase of frequency, the waveguide loss also increases rapidly. The effect of the thickness of the n++ layer on the light field confinement factor can be divided into several bands. When the frequency is less than 4THz, the thickness of 360-440nm is the most suitable, at which time the waveguide loss and confinement factor are both optimal; the higher light field confinement factor in the 4-8THz band always corresponds to a higher loss. Considering that the increase of the waveguide loss is large and has exceeded the gain, the thickness of 300-400nm is the better choice; when the frequency exceeds 8THz, 100-330nm is the best choice, at which time the confinement factor is the largest and the loss is the smallest. Figure 7 is the waveguide loss under different carrier concentrations of GaN substrate. It can be seen that when the carrier concentration exceeds 2×10 15 cm -3 The back waveguide loss increases rapidly, so the GaN substrate should be a non-doped GaN or carbon-doped GaN substrate.

[0049] For SiC substrates, the effect of n++ layer thickness on waveguide loss and optical field confinement factor can be found in Figure 8 and Fig. 9 .Depend on Figure 8 It can be seen that the waveguide loss increases with the increase of the thickness of the n++ layer. Fig. 9 It can be seen that when the frequency is higher than 7THz and lower than 4THz, the thinner n++ layer thickness has a better light field confinement factor, and the n++ layer thickness between 4-7THz has little effect on the light field confinement factor. Considering that the n++ layer also needs to be used as an ohmic contact layer, in order to provide sufficient redundancy for device processing and preparation, the n++ layer cannot be too thin, so the thickness of the n++ layer in the present invention is 100-300nm.

[0050] For AlN substrate, the effect of n++ layer thickness on waveguide loss and light field confinement factor is shown in Fig.10 and Fig.11 The variation trend of its waveguide loss and light field confinement factor with the increase of n++ layer thickness is similar to that of SiC substrate. Therefore, for the same reason, the thickness of the n++ layer based on AlN substrate in the present invention is 100-300nm.

[0051] In addition, the dielectric constant of the substrates gradually decreases from AlN, SiC to GaN, so the light field confinement factor of the QCL using the three substrates will also gradually decrease. The AlN substrate has the largest light field confinement factor, but the AlN substrate is currently expensive and small in size, and the output cannot meet industrial demand, so it is mainly used for laboratory exploration. SiC substrates are currently low in cost and large in size, and have a high light field confinement factor, so they have the potential for industrial production. GaN substrates currently have the highest epitaxial quality, and the size of the substrate is gradually increasing and the price is gradually decreasing. Although the light field confinement factor is small, it is still the best choice for experimental development and industrial production.

[0052] The structure of the QCL active region 330 is a terahertz QCL structure with AlGaN as the barrier and Al content of 10-18% and GaN as the potential well, with triple-well phonon (RP). A typical cycle is as follows Figure 4 As shown, there are 6 layers from AlGaN barrier layers 410, 430, 450 to GaN well layers 420, 440, 460, and the thickness of the structure sequence (in Units): The thickness of the barrier layer is indicated by bold underline. The widest potential well is n-type doped, and the sheet doping density is 2-8×10 10 cm -2 .Depend on Fig.12 It can be seen that at 10K, the peak gain is 88 / cm, the photon energy is 27meV, and the corresponding wavelength is 6.6THz, which is not currently achievable by GaAs / AlGaAs QCL devices. When the temperature is as high as 280K, the peak gain is 34 / cm, which is still higher than the waveguide loss, indicating that the active region structure is expected to achieve THz lasing at room temperature. The total thickness of the QCL active region 330 is 40nm-5000um, corresponding to 20-250 cycles.

[0053] The anode electrode adopts a Ti / Al / Ni / Au structure, and its thickness is 25-35 / 440-460 / 45-55 / 250-4500nm respectively. The anode electrode 310 has three functions in this structure: (1) forming an ohmic contact with n-type doped GaN, serving as the anode of the device; (2) using the metal layer as the top cladding, the dielectric constant of the metal is very small compared to the semiconductor material, which can play a good role in limiting the light field and improving the light field limitation factor; (3) serving as a self-aligned mask for etching and preparing the ridge waveguide during device processing. The present invention has additional requirements for the thickness of the metal anode (cladding): as the cladding of the waveguide structure, in order to prevent the leakage of the light field, the thickness of the last Au layer of the device anode is required to be at least more than 250nm; as a self-aligned mask during etching, a part of the film layer will be sacrificed during etching, and the ridge waveguide of the QCL needs to be etched to a depth of several hundred to several microns (depending on the structural design and power requirements), so in order to ensure the thickness of the cladding, the Au thickness is 250-4500nm (depending on the etching depth). The cathode electrode adopts Ti / Al / Ni / Au structure, and its thickness is 25-35 / 440-460 / 45-55 / 100-200nm respectively. The cathode electrode 350 in this structure mainly acts as a cathode to energize the device, and has no effect on its waveguide characteristics, so the thickness of the Au electrode does not need to be too thick.

[0054] Example 2

[0055] This GaN-based terahertz quantum cascade laser is epitaxially grown by molecular beam epitaxy (MBE). Because the entire growth process is carried out in an ultra-high vacuum environment, the influence of impurities is avoided, so high-purity films can be prepared. The growth rate is low and controllable, which is convenient for precise control of the thickness, composition and doping amount of the film at the atomic scale, and the sample growth process can be monitored in situ in real time using reflection high-energy electron diffraction (RHEED), so QCL is particularly suitable for MBE growth.

[0056] A heavily doped n++plasmonics confinement layer is grown on the surface of the cleaned substrate. This layer can improve the optical field confinement factor of the active region and can also serve as a contact layer for the cathode.

[0057] Then, 20-250 cycles of QCL structures are grown. A typical structure of one cycle is as follows: Figure 4 As shown, considering the thickness and composition fluctuations, the actual designed structure thickness (in Units): The thickness of the barrier layer is indicated by bold underline, the AlGaN component is 10%-18%, and the well layer is GaN. N-type doping is performed at the widest potential well, and the sheet doping density is 2-10×10 10 cm -2 Finally, a 20-250nm n-type doped n+GaN contact layer is grown epitaxially.

[0058] After the epitaxial growth is completed, the device structure is prepared through two photolithography processes.

[0059] First photolithography: Anode electrodes are processed on the n-type doped layer by photolithography, electron beam evaporation and lift-off. The anode width is 80-400μm, such as Figure 3 As shown in 310.

[0060] ICP etching: Using the anode as a mask, a ridge waveguide is etched on the device through ICP etching. The etching depth must at least reach the n++plasmonics confinement layer, and at most 20nm of the n++plasmonics confinement layer must be retained as an ohmic contact layer.

[0061] Second photolithography: The cathode area is processed by photolithography, electron beam evaporation and lift-off. The cathode width is 80-400μm and the cathode edge is 30-50μm away from the table. Figure 3 As shown in Figure 350.

[0062] The substrate is thinned to 100-150 μm, cleaved, and packaged to obtain the device, and the cleavage cavity length is 600-1200 μm.

[0063] Comparative Example 1

[0064] A GaN-based terahertz quantum cascade laser, the structure of which includes a substrate of 350 μm, an n-type doping layer of 100 nm, and a doping concentration of 5×10 18 / cm 3 , QCL active area 2μm, n-type doping layer 20nm, doping concentration 5×10 18 / cm 3 A cathode electrode layer is provided on one n-type doped layer, and an anode electrode layer is provided on another n-type doped layer. In addition to using a laser with a thickness of 400nm and a doping concentration of 5×10 19 / cm 3 Except for the n++ layer, the rest of the laser is the same as the comparison device. Compared with the laser using the n++ layer, the light field distribution of the laser is as follows Fig.14 As shown, from Fig.14 It can be seen that the light field of the n++ device at 4 THz is mostly confined to the active region, with a light field confinement factor as high as 40.27%, while the light field of the comparison device mostly leaks into the substrate, with a light field confinement factor of only 2.40%.

[0065] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A single metal waveguide GaN-based terahertz quantum cascade laser, characterized in that The bottom layer of the structure is a substrate, on which are arranged in sequence a highly n-type doped n++ layer, a QCL active area, and an n-type doped layer. A cathode electrode layer is arranged on the highly n-type doped n++ layer, and an anode electrode layer is arranged on the n-type doped layer. The substrate is a GaN substrate, an AlN substrate, or a SiC substrate.

2. The single metal waveguide GaN-based terahertz quantum cascade laser according to claim 1, characterized in that: The QCL active region layer includes a plurality of periodic structures, each of which is composed of three AlGaN barrier layers with an Al content of 10-18% and three GaN potential well layers in sequence, and the thickness of the structure sequence is: The thickness of the barrier layer is indicated by bold underline, and n-type doping is performed in the widest potential well layer, with a sheet doping density of 2-8×10 10 cm -2 .

3. The single metal waveguide GaN-based terahertz quantum cascade laser according to claim 2, characterized in that: The QCL active region layer includes 50-250 periodic structures.

4. The single metal waveguide GaN-based terahertz quantum cascade laser according to any one of claims 1 to 3, characterized in that: When the substrate is a GaN substrate and the laser frequency is less than 4 THz, the thickness of the n++ layer is 360-440 nm; when the substrate is a GaN substrate and the laser frequency is 4-8 THz, the thickness of the n++ layer is 300-400 nm; When the substrate is a GaN substrate and the laser frequency is greater than 8 THz, the thickness of the n++ layer is 100-330 nm; when the substrate is a SiC substrate or an AlN substrate, the thickness of the n++ layer is 100-300 nm; The doping concentration of the n++ layer is 1×10 19 -5×10 19 / cm 3 .

5. The single metal waveguide GaN-based terahertz quantum cascade laser according to claim 4, characterized in that: The thickness of the n-type doping layer is 20-250nm, and the doping concentration is 1×10 18 -5×10 18 / cm 3 .

6. The single metal waveguide GaN-based terahertz quantum cascade laser according to claim 4, characterized in that: The anode electrode layer adopts a Ti / Al / Ni / Au structure, and its thickness is 25-35 / 440-460 / 45-55 / 250-4500nm respectively.

7. The single metal waveguide GaN-based terahertz quantum cascade laser according to claim 4, characterized in that: The cathode electrode layer adopts a Ti / Al / Ni / Au structure, and its thickness is 25-35 / 440-460 / 45-55 / 100-200nm respectively.

8. The method for preparing a single metal waveguide GaN-based terahertz quantum cascade laser according to any one of claims 1 to 7, characterized in that The steps include: (1) Clean the substrate; (2) epitaxially grow an n-type highly doped n++ layer on the substrate surface, followed by the QCL active region, and finally the n-type doped layer; (3) The first photolithography process is to process the anode electrode on the surface of the n-type doped layer by photolithography, electron beam evaporation and lift-off; (4) ICP etching: Using the anode electrode as a mask, a platform deep into the n-type highly doped n++ layer is etched on the device surface; (5) Second photolithography, through photolithography, electron beam evaporation and stripping, the cathode electrode is processed on the n++ layer; (6) Cleavage and packaging to obtain a single metal waveguide GaN-based terahertz quantum cascade laser.

9. The preparation method according to claim 8, characterized in that Before cleavage, the substrate is thinned to 100-150μm.

10. The preparation method according to claim 8, characterized in that The cleavage cavity length is 600-1200μm.

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