A preparation method of a thin germanium buffer layer with a low threading dislocation density based on double doping

Abstract

The present invention discloses a method for preparing a thin germanium buffer layer with a low threading dislocation density based on double doping. A silicon substrate is sent into an MBE chamber to remove the oxide layer on the surface of the silicon substrate; an N-type double-doped germanium seed layer is grown on the silicon substrate with the surface oxide layer removed; low-temperature undoped germanium epitaxial growth is carried out on the N-type double-doped germanium seed layer to form a low-temperature undoped germanium epitaxial layer; undoped germanium epitaxial growth at different temperatures is carried out on the low-temperature undoped germanium epitaxial layer to form a variable-temperature undoped germanium epitaxial layer, and after completion, cyclic annealing is carried out in the MBE chamber; high-temperature germanium capping layer growth is carried out on the annealed variable-temperature undoped germanium epitaxial layer to obtain a thin germanium buffer layer with a low threading dislocation density. This thin germanium buffer layer can be used to replace a thicker gallium arsenide buffer layer, thereby significantly reducing the total thickness of the laser without affecting the performance of the laser, thus avoiding the problem of microcracks generated during the subsequent growth of the laser and improving the production capacity of the laser.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor epitaxy, and particularly relates to a method for preparing a thin germanium buffer layer with a low threading dislocation density based on double doping. Background Art

[0002] Silicon-based optoelectronic integrated circuits use photons instead of electrons for signal transmission, and have advantages such as high computing density, high bandwidth, and low energy consumption, and can be used to meet the explosive growth demand for information throughput. The root cause restricting the further development of silicon-based optoelectronic integrated circuits is the lack of practical and reliable electrically pumped silicon-based semiconductor lasers. Heteroepitaxially growing direct bandgap III-V materials on a silicon substrate can effectively solve this problem. Compared with the mature heterogeneous integration technology, heteroepitaxially growing III-V materials on a silicon substrate can provide higher production capacity, lower cost, and are more suitable for future large-scale commercial production.

[0003] The technical difficulty of heteroepitaxy lies in the mismatch of crystal properties between III-V materials and silicon materials. Due to differences in lattice constants, material polarities, and thermal expansion coefficients, there are defects such as threading dislocations, antiphase domains, and microcracks when growing III-V materials on a silicon substrate. These defects will all form a large number of non-radiative centers, greatly reducing the performance and lifespan of the laser. During the growth process of molecular beam epitaxy (MBE), at present, the formation of antiphase domains can be suppressed by using an optimized silicon buffer layer or a bevelled silicon substrate. At present, achieving a low threading dislocation density silicon-based III-V compound material mainly relies on a superlattice dislocation filtering layer. This method requires a relatively thick III-V buffer layer to accommodate enough superlattice dislocation filtering layers so as to effectively reduce the threading dislocation density to 10 6 cm -2 . Due to the different thermal expansion coefficients of III-V materials and silicon, too thick a laser will generate a large number of microcracks, greatly reducing the production capacity of the laser. Achieving a low-thickness and low-threading dislocation density III-V buffer layer will directly determine the key to the success of high-production-capacity and low-cost silicon-based photonic integrated circuits. Summary of the Invention

[0004] The purpose of the present invention is to prepare a thin germanium buffer layer with a low threading dislocation density based on double doping. Because the lattice constant of germanium is almost the same as that of gallium arsenide, this thin germanium buffer layer can be used to replace the relatively thick gallium arsenide buffer layer, thereby greatly reducing the total thickness of the laser without affecting the performance of the laser, thus avoiding the microcrack problem generated during subsequent growth of the laser and improving the production capacity of the laser.

[0005] To solve the above technical problems, the present invention provides a method for preparing a low threading dislocation density thin germanium buffer layer based on double doping, and the method includes:

[0006] S1. Feed the silicon substrate into the MBE chamber, and use the high temperature in the MBE chamber to remove the oxide layer on the surface of the silicon substrate;

[0007] S2. Grow an N-type double-doped germanium seed layer on the silicon substrate with the surface oxide layer removed;

[0008] S3. Perform low-temperature undoped germanium epitaxial growth on the N-type double-doped germanium seed layer to form a low-temperature undoped germanium epitaxial layer;

[0009] S4. Perform undoped germanium epitaxial growth at different temperatures on the low-temperature undoped germanium epitaxial layer to form a variable-temperature undoped germanium epitaxial layer, and after completion, perform cyclic annealing in the MBE chamber;

[0010] S5. Perform high-temperature germanium cap layer growth on the annealed variable-temperature undoped germanium epitaxial layer to obtain a low threading dislocation density thin germanium buffer layer.

[0011] Preferably, the specific implementation of S1 is: Feed the silicon substrate into the MBE chamber at a temperature between 1000 - 1100 °C and hold for 20 - 40 min, thereby removing the oxide layer on the surface of the silicon substrate.

[0012] Preferably, the specific implementation of S2 is: Lower the temperature in the MBE chamber to between 200 - 250 °C and hold, and grow an N-type double-doped germanium seed layer with a thickness of 30 nm on the silicon substrate with the surface oxide layer removed, and the growth rate is 0.5 Å per second.

[0013] Preferably, the specific implementation of S3 is: Keep the temperature in the MBE chamber between 200 - 250 °C, and grow a low-temperature undoped germanium epitaxial layer with a thickness of 20 nm on the N-type double-doped germanium seed layer, and the growth rate is 0.5 Å per second.

[0014] Preferably, the specific implementation of S4 includes:

[0015] S41. Raise the temperature in the MBE chamber by 5 degrees per minute to 450 - 500 °C and hold, and while raising the temperature, grow a first variable-temperature undoped germanium epitaxial layer with a thickness of 300 nm on the low-temperature undoped germanium epitaxial layer, and the growth rate is 1 Å per second;

[0016] S42. When the temperature in the MBE chamber reaches 450 - 500 °C, continue to grow a second variable-temperature undoped germanium epitaxial layer with a thickness of 90 nm;

[0017] S43. Raise the temperature in the MBE chamber to 850 - 900 °C and hold for 5 minutes, then cool it at a rate of 50 °C per minute to 550 - 600 °C and hold for 5 minutes to complete one annealing.

[0018] S44. Repeat the operation steps of S43 five times to complete five cycles of annealing.

[0019] Preferably, the specific implementation of S5: Keep the temperature in the MBE chamber at 550 - 600 °C, and grow a high-temperature germanium capping layer with a thickness of 60 nm on the temperature-varied undoped germanium epitaxial layer after cyclic annealing at a growth rate of 1 Å per second.

[0020] Preferably, the N-type double doping in the N-type double-doped germanium seed layer in S2 is specifically doping with antimony and phosphorus while growing germanium.

[0021] Preferably, the silicon substrate in S1 is a silicon substrate with a 4° cut angle.

[0022] The above method for preparing a thin germanium buffer layer with a low threading dislocation density based on double doping grows a thin germanium buffer layer with high flatness and low threading dislocation density on a silicon substrate with surface oxides removed through temperature-varied growth and cyclic annealing. Based on this thin germanium buffer layer, the epitaxy of high-performance indium arsenide / gallium arsenide quantum dot lasers can be realized, and the thickness of the laser can be kept below the crack threshold, thereby suppressing the generation of microcracks and improving the production capacity of the laser. This thin germanium buffer layer can be used to replace a thicker gallium arsenide buffer layer, thereby significantly reducing the total thickness of the laser without affecting the performance of the laser, thus avoiding the microcrack problem generated during the subsequent growth of the laser and improving the production capacity of the laser. Description of the Drawings

[0023] Figure 1 is a flowchart of the method for preparing a thin germanium buffer layer with a low threading dislocation density based on double doping in an embodiment of the present invention;

[0024] Figure 2 is a schematic structural diagram of a silicon substrate after growing a thin germanium buffer layer in an embodiment of the present invention;

[0025] Figure 3 is a comparison of the threading dislocation density of buffer layers formed by different growth methods in an embodiment of the present invention;

[0026] Figure 4 is an elevator channel contrast imaging diagram of a thin germanium buffer layer in an embodiment of the present invention;

[0027] Figure 5 is the surface roughness of a thin germanium buffer layer based on double doping in an embodiment of the present invention.

[0028] Description of the Reference Numerals:

[0029] 1. N-type double-doped germanium seed layer; 2. Low-temperature undoped germanium epitaxial layer; 3. First variable-temperature undoped germanium epitaxial layer; 4. Second variable-temperature undoped germanium epitaxial layer; 5. High-temperature germanium capping layer; 6. Silicon substrate. Detailed implementation manners

[0030] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0031] A method for preparing a thin germanium buffer layer with low threading dislocation density based on double doping specifically includes:

[0032] S1. Feed the silicon substrate into the MBE chamber, and use the high temperature in the MBE chamber to remove the oxide layer on the surface of the silicon substrate;

[0033] S2. Grow a layer of N-type double-doped germanium seed layer on the silicon substrate with the surface oxide layer removed;

[0034] S3. Perform low-temperature undoped germanium epitaxial growth on the N-type double-doped germanium seed layer to form a low-temperature undoped germanium epitaxial layer;

[0035] S4. Perform undoped germanium epitaxial growth at different temperatures on the low-temperature undoped germanium epitaxial layer to form a variable-temperature undoped germanium epitaxial layer. After completion, perform cyclic annealing in the MBE chamber;

[0036] S5. Grow a high-temperature germanium capping layer on the annealed variable-temperature undoped germanium epitaxial layer to obtain a thin germanium buffer layer with low threading dislocation density.

[0037] Specifically, referring to Figure 1 and Figure 2 , Figure 1 is a flowchart of the method for preparing a thin germanium buffer layer with low threading dislocation density based on double doping in an embodiment of the present invention, Figure 2 is a schematic structural diagram of the silicon substrate after growing the thin germanium buffer layer in an embodiment of the present invention.

[0038] For the method for preparing a thin germanium buffer layer with low threading dislocation density based on double doping, a thin germanium buffer layer is grown on a silicon substrate by using an MBE device. First, feed the silicon substrate 6 into the MBE chamber, and use the high temperature in the MBE chamber to remove the oxide layer on the surface of the silicon substrate 6; then grow a layer of N-type double-doped germanium seed layer 1 on the silicon substrate with the surface oxide layer removed; then perform low-temperature undoped germanium epitaxial growth on the N-type double-doped germanium seed layer 1 to form a low-temperature undoped germanium epitaxial layer 2; then perform undoped germanium epitaxial growth at different temperatures on the low-temperature undoped germanium epitaxial layer 2 to form a variable-temperature undoped germanium epitaxial layer. After completion, perform cyclic annealing in the MBE chamber; then grow a high-temperature germanium capping layer 5 on the variable-temperature undoped germanium epitaxial layer after cyclic annealing to form a thin germanium buffer layer with low threading dislocation density.

[0039] In one embodiment, the specific implementation of S1 is as follows: Feed the silicon substrate into the MBE chamber at a temperature between 1000 - 1100 °C and hold for 20 - 40 min, thereby removing the oxide layer on the surface of the silicon substrate.

[0040] In one embodiment, the specific implementation of S2 is as follows: Lower the temperature in the MBE chamber to between 200 - 250 °C and hold, and grow a 30 - nm - thick N - type double - doped germanium seed layer on the silicon substrate with the oxide layer removed, with a growth rate of 0.5 Å per second.

[0041] The working principle of MBE is to evaporate the material into atoms at high temperature and then deposit them on the surface of the substrate. N - type double doping is to simultaneously open two doping sources during material growth, and the dopants will replace the positions of some atoms, providing extra electrons. Specifically, lower the temperature in the MBE chamber to between 200 - 250 °C and hold, and grow a 30 - nm - thick N - type double - doped germanium seed layer 1 on the silicon substrate 6 with the oxide layer removed at a growth rate of 0.5 Å per second. Generally, the doping sources include phosphorus (P), arsenic (As), or antimony (Sb), etc., and these materials can all play a role in promoting the reduction of threading dislocations. Preferably, the doping sources are antimony and phosphorus. Open the antimony and phosphorus doping sources while growing germanium and incorporate 3×10 18 cm -3 of antimony and 5×10 18 cm -3 of phosphorus. Since antimony is a surfactant, doping with antimony is to both reduce the dislocation density and improve the surface flatness. Since antimony atoms are larger, doping with other group - V elements will cause difficulty in doping antimony, so phosphorus with smaller atoms is specifically selected as the second dopant to be incorporated together. Secondly, during the process of N - type doping, the crystal quality first increases and then decreases as the doping concentration increases. Therefore, adding phosphorus with smaller atoms after achieving the maximum doping concentration of antimony can further improve the crystal quality.

[0042] In one embodiment, the specific implementation of S3 is as follows: Keep the temperature in the MBE chamber between 200 - 250 °C, and grow a 20 - nm - thick low - temperature undoped germanium epitaxial layer on the N - type double - doped germanium seed layer, with a growth rate of 0.5 Å per second.

[0043] Specifically, keep the temperature in the MBE chamber between 200 - 250 °C, and grow a 20 - nm - thick low - temperature undoped germanium epitaxial layer 2 on the N - type double - doped germanium seed layer 1 at a growth rate of 0.5 Å per second.

[0044] In one embodiment, the specific implementation of S4 includes:

[0045] S41. Raise the temperature in the MBE chamber by 5 degrees per minute to 450 - 500 °C and hold it. While heating up, grow a first temperature-variable undoped germanium epitaxial layer with a thickness of 300 nm on the low-temperature undoped germanium epitaxial layer at a growth rate of 1 Å per second.

[0046] S42. When the temperature in the MBE chamber reaches 450 - 500 °C, continue to grow a second temperature-variable undoped germanium epitaxial layer with a thickness of 90 nm.

[0047] S43. Raise the temperature in the MBE chamber to 850 - 900 °C and hold it for 5 minutes, then cool it down to 550 - 600 °C at a rate of 50 °C per minute and hold it for 5 minutes to complete 1 annealing.

[0048] S44. Repeat the operation step S43 for a total of 5 times to complete 5 cycles of annealing.

[0049] Specifically, first raise the temperature in the MBE chamber by 5 degrees per minute to 450 - 500 °C and hold it. Grow a first temperature-variable undoped germanium epitaxial layer 3 with a thickness of 300 nm on the low-temperature undoped germanium epitaxial layer 2 at a growth rate of 1 Å per second. When the temperature in the MBE chamber reaches 450 - 500 °C, hold it and grow a second temperature-variable undoped germanium epitaxial layer 4 with a thickness of 90 nm. Then perform 5 cycles of annealing. The operation method for each cycle of annealing is: raise the temperature in the MBE chamber to 850 - 900 °C and hold it for 5 minutes, then cool it down to 550 - 600 °C at a rate of 50 °C per minute and hold it for 5 minutes.

[0050] Generally, N-type doped germanium needs to be grown in a low-temperature environment. When growing the first temperature-variable undoped germanium epitaxial layer 3, the temperature in the MBE chamber can be directly raised to the required temperature and then grown, or it can be grown while the temperature in the MBE chamber is gradually rising. The purpose of adopting this method of growing while heating up is that it is easier to have problems with poor thermal stability after there are more types of dopants. And the method of growing while heating up can ensure that germanium is still growing during heating, playing a role in inhibiting the precipitation of dopants and ensuring the crystal quality. In addition, thermal stress will be generated during heating and cooling in the cycle annealing. The thermal stress can increase the moving speed of threading dislocations, push the threading dislocations to penetrate out from the crystal edge, or promote the meeting and mutual annihilation of two dislocations with opposite Burgers vectors.

[0051] In one embodiment, the specific implementation of S5: Keep the temperature in the MBE chamber at 550 - 600 °C, and grow a high-temperature germanium capping layer with a thickness of 60 nm on the temperature-variable undoped germanium epitaxial layer after cycle annealing at a growth rate of 1 Å per second.

[0052] Specifically, growing a germanium cap layer with a high-temperature and high-speed flow on the temperature-variable undoped germanium epitaxial layer after cyclic annealing is beneficial to reducing the surface roughness and minimizing the adverse effects of surface undulations caused by thermal annealing on subsequent growth.

[0053] In one embodiment, the silicon substrate in S1 is a 4°-cut silicon substrate.

[0054] Specifically, on a silicon substrate with a cut angle, especially on a silicon substrate with a 4° or 6° cut angle, growing a thin germanium buffer layer and then growing a III-V material thereon can suppress the problem of antiphase domains. Taking a 4°-cut silicon substrate as an example, growing a thin germanium buffer layer on a 4°-cut silicon substrate and, without affecting the performance, replacing part of the thicker gallium arsenide buffer layer with an ultra-thin germanium buffer layer and then growing a III-V material thereon can suppress antiphase domains, reduce the thickness of the III-V laser, and minimize the generation of microcracks during the epitaxial laser structure due to different coefficients of thermal expansion.

[0055] The following describes the effects of specific embodiments of the present invention in conjunction with test pictures:

[0056] See Figure 3 , Figure 3 , which shows the comparison of threading dislocation densities of buffer layers formed by different growth methods in an embodiment of the present invention. Figure 3 In, the square represents the threading dislocation density of the silicon-based gallium arsenide buffer layer, the circle represents the threading dislocation density of the silicon-based double-doped germanium buffer layer, and the triangle represents the threading dislocation density of the silicon-based antimony-doped germanium buffer layer.

[0057] The data of the threading dislocation density of the silicon-based gallium arsenide is from the buffer layer of the silicon-based III-V quantum dot laser published in "Nature Photonics" in 2016, which includes a 1-μm gallium arsenide buffer layer and 5 cycles of 10-nm / 10-nm gallium arsenide / indium gallium arsenide dislocation filtering layers, with a 300-nm gallium arsenide interlayer between each filtering layer. Only the dislocation density data of 4 dislocation filtering layers are shown here.

[0058] Figure 3 The ordinate in is plotted on a logarithmic scale, and each small grid on the ordinate represents 1. From the threading dislocation density values on the ordinate in, it can be seen that on the basis of generating buffer layers of the same thickness (500 nm), the threading dislocation density of the double-doped ultra-thin germanium buffer layer formed by the technical solution of the present invention is 6×10 Figure 3 cm 7 , and the threading dislocation density of the antimony-doped germanium buffer layer is approximately 1.5×10 -2 cm 8 (that is, 15×10 -2 ) cm 7 -2), It can be seen that the threading dislocation density of the double-doped ultrathin germanium buffer layer in the present invention is greatly reduced, about half of the threading dislocation density of the antimony-doped one with the same thickness. The threading dislocation density of the double-doped ultrathin germanium buffer layer generated by the technical solution of the present invention is similar to that of 2.2-μm silicon-based gallium arsenide (including three dislocation filtering layers). Since the lattice constants of germanium and gallium arsenide are the same, this 500-nm double-doped ultrathin germanium buffer layer can completely replace 2.2-μm gallium arsenide without affecting the performance of the laser 2. This can greatly reduce the overall thickness of the laser to near the microcrack threshold to reduce or even suppress the generation of microcracks, thereby greatly improving the production capacity of the laser.

[0059] See Figure 4 , Figure 4 is the lift channel contrast imaging diagram of the thin germanium buffer layer in an embodiment of the present invention.

[0060] In Figure 4 , the threading dislocations have been marked with circles. The threading dislocation density of this 500-nm germanium buffer layer is about 6×10 7 cm -2 , reaching the lowest value known in the industry for this thickness.

[0061] See Figure 5 , Figure 5 is the surface roughness of the thin germanium buffer layer based on double doping in an embodiment of the present invention.

[0062] In Figure 5 , the thin germanium buffer layer based on double doping has an extremely low surface roughness: the root mean square roughness shown by a 5×5 square micron atomic force microscope is 0.8 nm.

[0063] The above method for preparing a thin germanium buffer layer with a low threading dislocation density based on double doping grows a thin germanium buffer layer with high flatness and low threading dislocation density on a silicon substrate with surface oxide removed by means of temperature-variable growth and cyclic annealing. This 500-nm-thick thin germanium buffer layer can be used to replace the 2.2-μm gallium arsenide buffer layer and maintain the threading dislocation density unchanged. Based on this thin germanium buffer layer, epitaxy of a high-performance indium arsenide / gallium arsenide quantum dot laser can be realized, and the thickness of the laser can be kept below the crack threshold to suppress the generation of microcracks and improve the production capacity of the laser.

[0064] The above has introduced in detail the preparation method of the low threading dislocation density thin germanium buffer layer based on double doping. Specific examples are used in this article to elaborate on the principle and implementation manner of the present invention. The description of the above embodiments is only used to help understand the core idea of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing a low threading dislocation density thin germanium buffer layer based on double doping, characterized in that, The method includes: S1. Feed a silicon substrate into an MBE chamber, and use the high temperature in the MBE chamber to remove the oxide layer on the surface of the silicon substrate; S2. Grow an N-type double-doped germanium seed layer on the silicon substrate with the surface oxide layer removed; S3. Perform low-temperature undoped germanium epitaxial growth on the N-type double-doped germanium seed layer to form a low-temperature undoped germanium epitaxial layer; S4. Perform undoped germanium epitaxial growth at different temperatures on the low-temperature undoped germanium epitaxial layer to form a variable-temperature undoped germanium epitaxial layer, and after completion, perform cyclic annealing in the MBE chamber; S5. Perform high-temperature germanium cap layer growth on the annealed variable-temperature undoped germanium epitaxial layer to obtain a thin germanium buffer layer with a low threading dislocation density; The N-type double doping in the N-type double-doped germanium seed layer in S2 is specifically doping antimony and phosphorus while growing germanium; The specific implementation manner of S4 includes: S41. Raise the temperature in the MBE chamber by 5 degrees per minute to 450 - 500 °C and hold it. While heating up, grow a first variable-temperature undoped germanium epitaxial layer with a thickness of 300 nm on the low-temperature undoped germanium epitaxial layer, and the growth rate is 1 Å per second; S42. When the temperature in the MBE chamber reaches 450 - 500 °C, continue to grow a second variable-temperature undoped germanium epitaxial layer with a thickness of 90 nm; S43. Raise the temperature in the MBE chamber to 850 - 900 °C and hold it for 5 minutes, then cool it down to 550 - 600 °C at a rate of 50 °C per minute and hold it for 5 minutes to complete 1 annealing; S44. Repeat the operation steps of S43 for a total of 5 times to complete 5 cyclic annealings.

2. The method for preparing a dual-doped thin germanium buffer layer with low threading dislocation density according to claim 1, wherein The specific implementation manner of S1 is: Feed the silicon substrate into the MBE chamber with a temperature between 1000 - 1100 °C and hold it for 20 - 40 min, thereby removing the oxide layer on the surface of the silicon substrate.

3. The method for preparing a double-doped low threading dislocation density thin germanium buffer layer according to claim 2, wherein, The specific implementation manner of S2 is: Lower the temperature in the MBE chamber to between 200 - 250 °C and hold it. Grow an N-type double-doped germanium seed layer with a thickness of 30 nm on the silicon substrate with the surface oxide layer removed, and the growth rate is 0.5 Å per second.

4. The method for preparing a double-doped low threading dislocation density thin germanium buffer layer according to claim 3, characterized in that, The specific implementation manner of S3 is: Keep the temperature in the MBE chamber between 200 - 250 °C, and grow a low-temperature undoped germanium epitaxial layer with a thickness of 20 nm on the N-type double-doped germanium seed layer, and the growth rate is 0.5 Å per second.

5. The method for preparing a double-doped low threading dislocation density thin germanium buffer layer according to claim 4, characterized in that, The specific implementation manner of S5: Keep the temperature in the MBE chamber at 550 - 600 °C, and grow a high-temperature germanium cap layer with a thickness of 60 nm on the variable-temperature undoped germanium epitaxial layer after cyclic annealing, and the growth rate is 1 Å per second.

6. The method for preparing a double-doped low threading dislocation density thin germanium buffer layer according to claim 1, characterized in that The silicon substrate in S1 is a 4°-cut silicon substrate.

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