Ultrafast photoconductive structure and photoconductive antenna device
By using a combination of semiconductor substrate and superlattice photoconductive layer in terahertz photoconductive antennas, doping rare earth elements and combining epitaxial and annealing processes, the problem of low resistivity and mobility of InGaAs materials is solved, and efficient terahertz signal radiation and detection is achieved, reducing equipment cost and volume.
Patent Information
- Application Number
- CN202211591380.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2022-12-12
- Publication Date
- 2025-08-26
- Estimated Expiration
- 2042-12-12
AI Technical Summary
Among the existing terahertz photoconductive antennas, the resistivity and mobility of the InGaAs materials grown in low temperatures have low resistance, resulting in low dark resistance of the photoconductive devices, low critical breakdown field strength, limited radiation power and conversion efficiency, and large equipment costs and volume.
Using an ultrafast photoelectric structure containing a semiconductor substrate and a superlattice photoconductive layer, a carrier diffusion barrier layer and a light absorbing layer doped with rare earth elements is prepared by combining epitaxial methods and annealing process to accurately regulate the dark resistance and mobility of the material.
The ability to radiate or respond to terahertz signals at high bias voltages is achieved. The device has lower dark current, higher critical breakdown field and radiated power under excitation, and has smaller equipment costs and volume.
Smart Images
Figure CN115732582B_ABST
Abstract
Description
Technical Field
[0001] The present invention belongs to the field of terahertz wave technology and optical communication, and specifically relates to an ultrafast photoconductive structure and a preparation method thereof, and a photoconductive antenna structure. Background Art
[0002] The terahertz gap generally refers to electromagnetic waves with frequencies between 0.1 THz and 10 THz. Terahertz waves have very low energy and long penetration distances, and hold broad application prospects in communications, spectroscopy, biomedicine, astronomy, security, and atmospheric remote sensing. The key to practical applications in the terahertz band lies in achieving both terahertz radiation and detection. Researchers have conducted extensive research targeting various application scenarios. Among them, terahertz light sources and detectors based on photoconductive antennas have emerged as a key class of terahertz devices, offering advantages such as room temperature operation, fast response, wide radiation / detection range, mature manufacturing processes, and relatively low cost.
[0003] Terahertz photoconductive antennas utilize femtosecond laser pumping to the ultrafast optoelectronic material in the antenna gap. This generates photogenerated carriers within the ultrafast optoelectronic material. Under the influence of an applied voltage, these carriers form a rapidly oscillating current near the antenna surface, which radiates terahertz waves. If used without a bias and connected to an ammeter, the antenna functions as a terahertz detector. During the fabrication of terahertz photoconductive antennas, the properties of the ultrafast optoelectronic material significantly influence the performance of terahertz radiation and detection. Currently, the most commonly used ultrafast optoelectronic materials for terahertz photoconductive antennas are GaAs and InGaAs grown at low temperatures using molecular beam epitaxy. However, in practical applications, photoconductive antennas fabricated from low-temperature GaAs typically require an 800 nm titanium-sapphire femtosecond laser as the pump source, significantly increasing the cost and size of the device. Low-temperature grown InGaAs, on the other hand, can be pumped using a 1550 nm Er-doped fiber femtosecond laser, reducing device costs and enabling greater integration and miniaturization. However, the resistivity and mobility of the current low-temperature grown InGaAs material are low, resulting in insufficient dark resistance of the corresponding photoconductive devices, low critical breakdown field strength, and limited radiation power and conversion efficiency. Summary of the Invention
[0004] In order to overcome the above-mentioned deficiencies of the prior art, the present invention provides an ultrafast photoelectric structure comprising a semiconductor substrate and a photoconductive layer, a preparation method thereof, and a photoconductive antenna structure.
[0005] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:
[0006] An ultrafast photoconductive structure comprises a semiconductor substrate and a superlattice photoconductive layer, wherein the periodic structure of the superlattice photoconductive layer comprises a light absorbing layer and a carrier diffusion blocking layer, wherein the light absorbing layer is In x Ga1-x As (0≤x≤1) semiconductor layer, the carrier diffusion barrier layer is In doped with rare earth elements y Al 1-y As (0≤y≤1) or AlAs 1-z Sb z (0≤z≤1) semiconductor layer.
[0007] Furthermore, the carrier diffusion barrier layer is doped by co-deposition with a doping concentration of 1x10 15 -5x10 21 cm -3 .
[0008] Furthermore, rare earth elements include Er, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, Lu, Y, and Sc.
[0009] Furthermore, the light absorbing layer is doped with In x Ga 1-x As (0≤x≤1) semiconductor layer, the doping source is Be, C or rare earth elements, and the doping methods include δ-doping, co-doping, gradient doping and uniform doping.
[0010] Furthermore, the periodic structure further includes a first intercalation layer located between the carrier diffusion blocking layer and the light absorbing layer. Furthermore, the periodic structure further includes a second intercalation layer located on the other side of the carrier diffusion blocking layer.
[0011] Furthermore, the material of the first intercalation layer and / or the second intercalation layer is Be, C or a rare earth element.
[0012] Furthermore, the number of periods of the superlattice photoconductive layer is 50 to 500.
[0013] The present invention also provides a method for preparing an ultrafast photoconductive structure, which uses an epitaxial method to sequentially grow the above-mentioned superlattice photoconductive layers on a semiconductor substrate, and performs an annealing process on the sample.
[0014] Furthermore, the annealing process includes in-situ annealing and rapid thermal annealing. The annealing temperature is 100 to 700 ° C. Family atmosphere protection, nitrogen, hydrogen or inert gas atmosphere protection is used during rapid thermal annealing.
[0015] The present invention also provides a photoconductive antenna device, comprising an ultrafast photoconductive structure and an antenna structure, wherein the antenna structure is located on the upper surface of the superlattice photoconductive layer.
[0016] Furthermore, the shapes of the antenna structure include H-type, stripe-type, trapezoidal-type, butterfly-type, square spiral-type, logarithmic spiral-type, self-complementary spiral-type, etc., and the gap of the antenna structure is 3 μm to 30 μm.
[0017] Beneficial effects of the present invention:
[0018] The present invention utilizes a semiconductor substrate with a periodic superlattice photoconductive structure. The combination of a diffusion barrier layer and a light absorption layer satisfies the photoconductive structure's requirements for strong absorption of pump light and rapid capture of photogenerated carriers, achieving sub-picosecond carrier relaxation. Combined with the doping and intercalation structures, the material's dark resistance and mobility are precisely controlled, enabling the structure to operate at high bias voltages and rapidly radiate or respond to terahertz signals. When used in ultrafast photoconductive antennas, the ultrafast photoconductive material can be excited by femtosecond lasers of multiple frequency bands. Compared to low-temperature grown InGaAs photoconductive antennas, the device exhibits lower dark current and higher critical breakdown field, as well as higher radiation power and conversion efficiency. Furthermore, the present invention utilizes an epitaxial growth method to fabricate the ultrafast photoconductive structure, resulting in high crystal quality. The composition, period, structure, and doping method of the structure can be precisely adjusted. Combined with post-growth annealing conditions, the material's carrier lifetime, dark resistance, and mobility can be precisely modulated simultaneously. BRIEF DESCRIPTION OF THE DRAWINGS
[0019] Figure 1 Schematic diagram of the ultrafast photoconductive structure of the present invention.
[0020] Figure 2 is a schematic diagram of another ultrafast photoconductive structure in the present invention.
[0021] Figure 3 In Example 1 of the present invention, x Ga 1-x As / rare earth element doped In y Al 1-y Schematic diagram of the ultrafast photoconductive structure of the As superlattice photoconductive layer.
[0022] Figure 4 In Example 1 of the present invention, based on In x Ga 1-x As / rare earth element doped In y Al 1-y X-ray diffraction pattern of ultrafast photoconductive material of As superlattice photoconductive layer.
[0023] Figure 5 In Example 1 of the present invention, x Ga 1-x As / rare earth element doped In y Al1-y Relaxation curve of photogenerated carriers in the ultrafast photoconductive material of the As superlattice photoconductive layer at 1550nm.
[0024] Figure 6 Schematic diagram of an aerial view of the antenna structure on the surface of the ultrafast photoconductive material in Examples 1 and 2 of the present invention.
[0025] Figure 7 It is a side view of the dipole antenna structure on the surface of the ultrafast photoconductive material in Example 1 of the present invention.
[0026] Figure 8 In Example 2 of the present invention, x Ga 1-x As / rare earth element doped AlAs z Sb 1-z Schematic diagram of the ultrafast photoconductive structure.
[0027] Figure 9 It is a side view of the vertical structure antenna on the surface of the ultrafast photoconductive material in Example 2 of the present invention. DETAILED DESCRIPTION
[0028] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.
[0029] like Figure 1 As shown, the present invention provides an ultrafast photoconductive structure, comprising a semiconductor substrate 1 and a superlattice photoconductive layer 2 disposed thereon, which can be used as a carrier for generating and transmitting photogenerated carriers in a photoconductive antenna device. The periodic structure of the superlattice photoconductive layer 2 includes a light absorbing layer 21 and a carrier diffusion barrier layer 22. The light absorbing layer 21 is In x Ga 1-x As (0≤x≤1) semiconductor layer, used to absorb light with photon energy greater than the band gap of the light absorbing layer material, and then generate photogenerated carriers and serve as their main transmission medium. The carrier diffusion barrier layer 22 is In doped with rare earth elements. y Al 1-y As (0≤y≤1) or AlAs 1-z Sb z The (0≤z≤1) semiconductor layer is used to improve the material's electrical and optical properties. The introduction of rare earth elements can introduce new energy levels into the semiconductor band gap, which is wider than the pump laser energy. This enhances the material's absorption of the pump laser and facilitates the rapid capture of photogenerated carriers. The semiconductor substrate can be made of commonly used semiconductor substrate materials such as InP, GaAs, Si, and Ge-on-Si substrates.
[0030] In the present invention, the carrier diffusion barrier layer is doped by co-deposition, and the doping concentration is 1x10 15 -5x10 21 cm -3 The doped rare earth elements include Er, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, Lu, Y and Sc.
[0031] In the present invention, the light absorbing layer may be doped In x Ga 1-x As (0≤x≤1) semiconductor layer, doped with Be, C, or a rare earth element, using various doping methods including delta-doping, co-doping, gradient doping, and uniform doping. In the present invention, doping refers to the artificially controlled doping of a material, and non-doping refers to the absence of artificial doping.
[0032] In the ultrafast photoconductive structure provided by the present invention, the periodic structure of the superlattice photoconductive layer further includes a first intercalation layer 23, such as Figure 2 As shown, a first intercalation layer 23 is located between the carrier diffusion blocking layer 22 and the light absorbing layer 21. The material of the first intercalation layer is Be, C, or a rare earth element. The intercalation material modifies the periodic structure interface and dopes the light absorbing layer and carrier diffusion blocking layer, further improving the electrical properties of the photoconductive structure described herein. The periodic structure may further include a second intercalation layer 24, located on the other side of the carrier diffusion blocking layer 22. The material of the second intercalation layer 24 is Be, C, or a rare earth element, further enhancing the modifying effect of the first intercalation layer.
[0033] The superlattice structure in the ultrafast photoconductive structure of the present invention has a periodicity of 50-500, determined based on a combination of considerations including the material's absorption of pump light and dark resistance. While ensuring sufficient absorption of ultrafast laser pulses by the ultrafast photoconductive structure, epitaxial growth costs are minimized. Within a single period, the thickness of the light absorption layer 21 is 5-100 nm, and the thickness of the carrier diffusion barrier layer is 0.5-20 nm.
[0034] The present invention provides a method for preparing an ultrafast photoconductive structure, which uses an epitaxial method to sequentially grow a superlattice structure on a semiconductor substrate, and performs an annealing process on the sample in an epitaxial device. The ultrafast photoconductive structure prepared by this method has high crystal quality. At the same time, this method can accurately adjust the composition, period, structure and doping mode of the prepared structure, and combined with the annealing condition control after the material growth, the carrier lifetime, dark resistance and mobility of the material can be accurately modulated to achieve sub-picosecond rapid relaxation of the carrier, wherein the annealing temperature is 100 to 700°C. The annealing process can include in-situ annealing and rapid thermal annealing. In the in-situ annealing process, The rapid thermal annealing process is protected by a nitrogen, hydrogen or inert gas atmosphere, and a semiconductor cover is used on the surface.
[0035] The present invention also provides a photoconductive antenna device comprising an ultrafast photoconductive structure and an antenna structure, wherein the antenna structure is located on the upper surface of an ultrafast photoelectric material, i.e., the upper surface of a superlattice photoconductive layer. This photoconductive antenna structure can be used as a femtosecond laser-excited terahertz source and a terahertz detector. Standard semiconductor device fabrication processes are used to etch a mesa on the ultrafast photoelectric material and fabricate an antenna or antenna array, using electrodes made of Ti and Au or other metal materials. The antenna structure can be shaped like an H-type, strip, trapezoidal, butterfly, square spiral, logarithmic spiral, or self-complementary spiral, with a gap between the antenna structures ranging from 3 μm to 30 μm.
[0036] Example 1:
[0037] This embodiment provides a method based on In x Ga 1-x As / rare earth element doped In y Al 1-y Ultrafast photoconductive structure of As superlattice photoconductive layer, such as Figure 3 As shown, its preparation method is as follows:
[0038] Step 1: Transfer the semiconductor InP substrate to the molecular beam epitaxy equipment growth chamber, heat it to around 520℃ under As protection, and perform surface deoxidation treatment. The deoxidation treatment time is 15 minutes. Then cool it to around 490℃ and grow InP with appropriate thickness. 0.52 Al 0.48 As buffer layer;
[0039] Step 2: grow In on the buffer layer x Ga 1-x As / rare earth element doped In y Al 1-y As superlattice photoconductive layer, with a growth rate of about 1 μm / h;
[0040] Specifically, step 2 includes:
[0041] (1) Open the shutter of the Be source furnace and grow the second intercalation layer;
[0042] (2) Growth of 2 nm Er-doped In 0.52 Al 0.48 As layer;
[0043] (3) Open the shutter of the Be source furnace and grow the first intercalation layer;
[0044] (4) Growth of 10 nm In 0.53 Ga0.47 As layer;
[0045] (5) Repeat steps (1)-(4) 150 times;
[0046] Step 3: After removing the sample from the molecular beam epitaxy equipment, rapid thermal annealing is performed to complete the preparation. The annealing temperature is 600°C, and a nitrogen atmosphere is used during the annealing process.
[0047] The XRD test results and carrier lifetime test results of the ultrafast photoconductive structure of this embodiment are shown in Figures 1 and 2. Figure 4 and 5 As shown. Figure 4 It can be seen that the XRD results of the (002) surface of the structure show that the peak of the substrate coincides with the peak of the superlattice photoconductive layer and the peak shape is sharp. At the same time, the interference fringes of the buffer layer and the superlattice layer are obvious, indicating that the epitaxial layer of the structure matches the substrate and has good crystal quality. Figure 5 It can be seen that under the 1550 nm femtosecond laser degenerate pump detection test, the carrier relaxation time of this material is only 0.5 ps.
[0048] This embodiment also provides a method based on In x Ga 1-x As / rare earth element doped In y Al 1-y The photoconductive antenna of the As system ultrafast photoconductive structure has a structure such as Figure 6 and 7 As shown, including the above-mentioned In x Ga 1-x As / rare earth element doped In y Al 1-y As system ultrafast photoconductive structure and dipole antenna. The dipole antenna is located on the upper surface of the ultrafast photoconductive structure, such as Figure 7 As shown in Figure 2, a dipole antenna or dipole antenna array was fabricated on the aforementioned ultrafast optoelectronic material using standard semiconductor device fabrication techniques. The electrodes used were Ti and Au. The antenna structure was trapezoidal in shape, with a gap of 9 μm.
[0049] The photoconductive device fabricated in this embodiment can be used as a terahertz source or detector. When an ultrafast laser pulse reaches the surface of the ultrafast photoconductive layer, it generates transient photogenerated carriers within the material. When a DC bias is applied to the device's two dipole antenna electrodes, the transient photogenerated carriers within the ultrafast photoconductive layer drift under the applied voltage. The resulting transient current flows through the antenna, radiating terahertz waves into the external field, allowing the device to operate as a terahertz source. These transient photogenerated carriers can also sense changes in the electric field of the external terahertz wave and conduct it to the dipole antenna electrodes. When a highly sensitive ammeter is connected to the device's two dipole antenna electrodes, the terahertz signal can be converted into an electrical signal, enabling the device to function as a terahertz detector. The structure described in this embodiment performs better as a terahertz source than as a terahertz detector. The device has a breakdown voltage exceeding 100 kV / cm, a radiation power exceeding 400 μW, and a bandwidth exceeding 6 THz.
[0050] Example 2:
[0051] This embodiment provides a method based on In x Ga 1-x As / rare earth element doped AlAs z Sb 1-z Ultrafast photoconductive structures, such as Figure 8 As shown, its preparation method is as follows:
[0052] Step 1: Transfer the semi-insulating semiconductor InP substrate to the growth chamber of the molecular beam epitaxy equipment, heat it to around 520°C under As protection, and perform surface deoxidation treatment for 15 minutes;
[0053] Step 2: grow In on the substrate x Ga 1-x As / rare earth element doped AlAs z Sb 1-z Photoconductive layer superlattice structure, with a growth rate of about 1 μm / h;
[0054] Specifically, step 2 includes:
[0055] (1) Growth of 0.6 nm AlAs 0.46 Sb 0.54 layer, and at the same time open the shutter of the Sc source furnace;
[0056] (2) Open the shutters of the C and Sc source furnaces to grow the first intercalation layer;
[0057] (3) Growth of 30 nm In 0.53 Ga 0.47 As layer;
[0058] (4) Repeat steps (1)-(4) 100 times;
[0059] Step 3: After removing the sample from the molecular beam epitaxy equipment, rapid thermal annealing is performed to complete the preparation. The annealing temperature is 600°C, and a nitrogen atmosphere is used during the annealing process.
[0060] This embodiment also provides a method based on In x Ga 1-x As / rare earth element doped AlAs z Sb 1-z The photoconductive antenna of the system ultrafast photoconductive structure has a structure such as Figure 6 and 9 As shown, including the In x Ga 1-x As / rare earth element doped AlAs z Sb 1-z The system includes an ultrafast photoconductive structure and a dipole antenna. The dipole antenna is located on the upper surface of the ultrafast photoconductive structure. The dipole antenna or dipole antenna array is fabricated on the ultrafast photoconductive material using standard semiconductor device fabrication processes, with Ti and Au as electrode materials. The antenna structure is vertical, and the dipole antenna gap is 5 μm.
[0061] The above description is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.
Claims
1. An ultrafast photoconductive structure, characterized in that: The invention comprises a semiconductor substrate and a superlattice photoconductive layer, wherein the periodic structure of the superlattice photoconductive layer comprises a light absorbing layer and a carrier diffusion blocking layer, wherein the light absorbing layer is In x Ga 1-x As (0≤x≤1) semiconductor layer, the carrier diffusion barrier layer is In doped with rare earth elements y Al 1-y As (0≤y≤1) or AlAs 1- z Sb z (0≤z≤1) semiconductor layer, the carrier diffusion barrier layer is doped by co-deposition, and the doping concentration is 1x10 15 -5x10 21 cm -3 .
2. The ultrafast photoconductive structure according to claim 1, wherein: The rare earth elements include Er, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, Lu, Y and Sc.
3. The ultrafast photoconductive structure according to claim 1, wherein: The light absorbing layer is doped with In x Ga 1- x As (0≤x≤1) semiconductor layer, the doping source is Be, C or rare earth elements, and the doping methods include δ-doping, co-doping, gradient doping and uniform doping.
4. The ultrafast photoconductive structure according to claim 1, wherein: The periodic structure further includes a first intervening layer located between the carrier diffusion blocking layer and the light absorbing layer.
5. The ultrafast photoconductive structure according to claim 4, characterized in that: The periodic structure further includes a second intercalation layer, which is located on the other side of the carrier diffusion blocking layer.
6. The ultrafast photoconductive structure according to claim 4 or 5, characterized in that: The material of the first intercalation layer and / or the second intercalation layer is Be, C or a rare earth element.
7. The ultrafast photoconductive structure according to claim 1, wherein: The number of periods of the superlattice photoconductive layer is 50 to 500.
8. A method for preparing an ultrafast photoconductive structure, characterized in that: The superlattice photoconductive layer according to any one of claims 1 to 7 is sequentially grown on a semiconductor substrate by an epitaxial method, and an annealing process is performed on the sample.
9. The preparation method according to claim 8, characterized in that The annealing process includes in-situ annealing and rapid thermal annealing.
10. A photoconductive antenna device, characterized in that: It comprises the ultrafast photoconductive structure according to any one of claims 1 to 7, and an antenna structure, wherein the antenna structure is located on the upper surface of the superlattice photoconductive layer.
11. The photoconductive antenna device according to claim 10, wherein: The shapes of the antenna structure include H-type, strip type, trapezoidal type, butterfly type, square spiral type, logarithmic spiral type, and self-complementary spiral type. The gap of the antenna structure is 3 μm to 30 m.