Epitaxial structure of GaAs solar cell and growth method thereof

Abstract

The invention provides an epitaxial structure of a GaAs solar cell and a growth method thereof, the epitaxial structure comprises a substrate, a first tunnel junction, a DBR layer, a middle cell, a second tunnel junction and a top cell which are sequentially stacked from bottom to top, and the substrate is used as a bottom cell; the DBR layer is formed by alternately growing an Al < x > Ga < 1-x > As layer and an In < y > Ga < 1-y > As layer, the Al < x > Ga < 1-x > As layer is doped with a C element, the In < y > Ga < 1-y > As layer is doped with a Zn element, and the C element has very high doping efficiency and solubility in the Al < x > Ga < 1-x > As layer with a high Al component, so that the bottleneck of low doping efficiency of Zn is overcome, two materials in the DBR layer can both realize high-concentration and uniform P-type doping, and the doping efficiency is improved. The barrier at the Al < x > Ga < 1-x > As / In < y > Ga < 1-y > As heterogeneous interface is greatly reduced, and the series resistance caused by the DBR layer is remarkably reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of semiconductor devices, and particularly relates to an epitaxial structure of a GaAs solar cell and a growth method thereof. BACKGROUND

[0002] The space GaInP / InGaAs / Ge three-junction solar cell has the characteristics of high photoelectric conversion efficiency, strong anti-radiation capability, and good stability in on-orbit operation environment, and is widely used in the field of aerospace. 0.5 In 0.5 P / In 0.01 Ga 0.99 As / Ge three-junction solar cell is widely used in the field of aerospace. The spacecraft is subjected to the radiation of high-energy particles in space during on-orbit service. Research shows that the damage caused by high-energy particles entering the solar cell to the In 0.01 Ga 0.99 As material of the middle cell is serious, and the thicker the In 0.01 Ga 0.99 As thickness is, the more serious the damage of high-energy particles is.

[0003] In view of the irradiation attenuation problem of the space Ga 0.5 In 0.5 P / In 0.01 Ga 0.99 As / Ge three-junction solar cell, the most mainstream solution at present is to reduce the thickness of the base region of the middle cell, reduce the diffusion length of the photo-generated carriers reaching the space charge region, and thereby improve the collection efficiency of the photo-generated carriers. However, the reduction of the base region thickness will lead to the decline of the absorption capacity of low-energy photons, resulting in current loss. In order to solve this problem, the common practice in the industry is to add a set of Bragg reflector (DBR) between the middle cell and the bottom cell, to reflect the photons with energy higher than the band gap width of the middle cell back to the middle cell for reabsorption, thereby increasing the photo-generated current.

[0004] However, the introduction of DBR brings new technical problems. The traditional DBR layer is grown by alternating AlGaAs and InGaAs materials, and usually uses single Zn as a P-type dopant. Due to the significant difference in the incorporation efficiency of Zn in different materials, especially in high Al component AlGaAs material, the doping efficiency of Zn in AlGaAs material is much lower than that in InGaAs material, which leads to the formation of a higher potential barrier at the interface of the two materials in the DBR, significantly increasing the series resistance of the solar cell. The increase of the series resistance directly leads to the decrease of the fill factor (FF) of the cell, which seriously restricts the further improvement of the final conversion efficiency (EFF) of the cell. In addition, if the epitaxial growth temperature is lowered in order to obtain a higher Zn doping concentration in high Al component AlGaAs, the crystal quality of the material will be sacrificed, leading to the attenuation of the open circuit voltage, which also has a negative impact on the performance of the solar cell.

[0005] Therefore, there is an urgent need for a new DBR doping method that can effectively reduce the series resistance of the DBR layer while ensuring the crystal quality of the material, so as to fully exert the optical advantages of the DBR structure and realize a leap-forward improvement in the conversion efficiency of the space multi-junction solar cell. SUMMARY

[0006] The purpose of the present application is to provide an epitaxial structure of a GaAs solar cell and a growth method thereof, which can effectively reduce the series resistance of the DBR layer while ensuring the crystal quality of the material.

[0007] To achieve the above purpose, the solution of the present application is to provide an epitaxial structure of a GaAs solar cell, which comprises a substrate, a first tunnel junction, a DBR layer, a middle cell, a second tunnel junction and a top cell stacked in order from bottom to top, the substrate serving as a bottom cell; the DBR layer is grown by alternating Al x Ga 1-x As layer and In y Ga 1-y As layer, wherein the Al x Ga 1- x The Al y Ga 1-y As layer is doped with C element, and the In

[0008] Optionally, in the Al x Ga 1-x As layer and In y Ga 1-y As layer, 0.5≤x≤0.9, 0.0095≤y≤0.015.

[0009] Optionally, the Alx Ga 1-x As layer and In y Ga 1-y The doping concentration of the As layer is 4E 18 cm -3 -8E 18 cm -3 .

[0010] Optionally, the Al x Ga 1-x As layer and In y Ga 1-y The alternating pairs of the As layer are 5 pairs-20 pairs.

[0011] Optionally, the reflection wavelength center value of the DBR layer is 850nm-920nm.

[0012] Optionally, the substrate is a P-type Ge substrate, the P-type Ge substrate is diffused with PH3 to form the pn junction of the bottom cell, and the bottom cell is a Ge bottom cell; the middle cell is an InGaAs middle cell, and the top cell is a GaInP top cell.

[0013] Optionally, the first tunnel junction is a GaAs layer structure alternately doped with Si elements and C elements, and the second tunnel junction is composed of GaInP layers and AlGaAs layers alternately stacked, wherein the GaInP layers are doped with Si elements, and the AlGaAs layers are doped with C elements.

[0014] Optionally, the substrate further has a nucleation layer and a buffer layer stacked in sequence thereon, the nucleation layer is an N-type GaInP nucleation layer, and the buffer layer is an N-type InGaAs buffer layer.

[0015] Optionally, the top cell further has a cap layer stacked thereon, and the cap layer is a GaAs cap layer.

[0016] The application further provides a growth method of a GaAs solar cell epitaxial structure, which is used for growing the epitaxial structure, and comprises the following steps: providing a substrate, which serves as a bottom cell; growing a first tunnel junction on the substrate; growing a DBR layer on the first tunnel junction, wherein the DBR layer is composed of Al x Ga 1-x As layers and In y Ga 1-y As layers alternately grown, wherein the Al x Ga 1-x As layers are doped with C elements, and the In y Ga 1-yThe As layer is doped with Zn element; Growth of a middle cell on the DBR layer; Growth of a second tunnel junction on the middle cell; Growth of a top cell on the second tunnel junction.

[0017] After the above scheme, the application has the following advantages: 1. In the DBR layer of the application, Al x Ga 1-x The As layer is doped with C element, In y Ga 1-y The As layer is doped with Zn element, C element has very high doping efficiency and solubility in the Al x Ga 1-x As layer, which overcomes the bottleneck of low Zn doping efficiency, so that both materials in the DBR layer can achieve high concentration and uniform P-type doping, which greatly reduces the series resistance of the DBR layer. x Ga 1-x As / In y Ga 1-y The barrier at the heterojunction interface is significantly reduced.

[0018] 2. In order to increase the doping concentration in the Al x Ga 1-x As layer with high Al component, the traditional single Zn doping scheme has to reduce the growth temperature, which seriously sacrifices the crystal quality of the material, resulting in a significant decrease in open circuit voltage of the cell. The Al x Ga 1-x As layer of the application can be doped with C at the conventional best crystal quality growth temperature, perfectly solving the contradiction between "high doping" and "high quality". The application obtains ultra-high doping concentration while maintaining excellent material quality, thereby obtaining higher open circuit voltage and conversion efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is the epitaxial structure diagram of the solar cell of the application; Figure 2 is the structure diagram of the DBR layer of the application; Figure 3 is the growth method flowchart of the application.

[0020] Label explanation: 1. Substrate; 2. Nucleation layer; 3. Buffer layer; 4. First tunnel junction; 5. DBR layer; 51. Al x Ga 1-x As layer; 52. In y Ga 1-yAs layer; 6. Middle cell; 7. Second tunnel junction; 8. Top cell; 9. Cap layer. Detailed Implementation

[0021] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application. The scope value described in this invention includes two endpoint values.

[0022] like Figure 1 As shown, this application provides an epitaxial structure for a GaAs solar cell, comprising a substrate 1, a first tunnel junction 4, a DBR layer 5, a middle cell 6, a second tunnel junction 7, and a top cell 8, stacked sequentially from bottom to top. The substrate 1 serves as the bottom cell; specifically, the substrate 1 is a P-type Ge substrate. PH3 diffusion is performed on the P-type Ge substrate to form the pn junction of the bottom cell, which serves as the emitter region of the bottom cell, thus making the bottom cell a Ge bottom cell. The middle cell 6 is an InGaAs middle cell, preferably InGaAs. 0.01 Ga 0.99 In the As-type battery, the top battery 8 is a GaInP top battery, preferably Ga... 0.5 In 0.5 P-top battery.

[0023] like Figure 2 As shown, the DBR layer 5 is composed of Al x Ga 1-x As layer 51 and In y Ga 1-y The As layers are grown alternately in layers 52, where 0.5 ≤ x ≤ 0.9 and 0.0095 ≤ y ≤ 0.015. In traditional DBR growth methods, Al... x Ga 1-x As is doped with Zn. To obtain a high doping level using Zn, the growth temperature needs to be lowered. However, Al containing more than 50% Al content... x Ga 1-x To ensure the quality of the crystal structure, asphalt (Al) materials require high growth temperatures, creating a trade-off between the two. Therefore, traditional DBR layers strike a balance between doping level and crystal quality. x Ga 1-x The doping concentration of Zn in the As layer is in the range of 2E. 18 cm -3 about.

[0024] And the Al of this application x Ga1-x As layer 51 is doped with C element, the In y Ga 1-y As layer 52 is doped with Zn element. C element has very high doping efficiency and solubility in high Al x Ga 1-x As layer, overcoming the bottleneck of low Zn doping efficiency, so that both materials in DBR layer 5 can achieve high concentration and uniform P-type doping, which greatly reduces the series resistance caused by DBR layer 5. x Ga 1-x As / In y Ga 1-y As heterojunction interface, significantly reducing the series resistance caused by DBR layer 5. Moreover, Al x Ga 1-x As layer 51 doped with C can be grown at a conventional optimal crystal quality growth temperature, perfectly solving the contradiction between "high doping" and "high quality".

[0025] Optionally, the reflection wavelength center value of the DBR layer 5 is 850nm-920nm, the doping concentration of the Al x Ga 1-x As layer 51 and In y Ga 1-y As layer 52 is 4E 18 cm -3 -8E 18 cm -3 , so that both materials in DBR layer can achieve high concentration and uniform P-type doping, reducing the series resistance caused by DBR layer.

[0026] Optionally, the number of pairs of the Al x Ga 1-x As layer 51 and In y Ga 1-y As layer 52 is 5 pairs-20 pairs, and the optimal number of pairs is 15 pairs.

[0027] Preferably, the doping concentration of the Al x Ga 1-x As layer 51 and In y Ga 1-y As layer 52 is 6E 18 cm -3 , which can not only improve the material interface barrier, but also prevent the material from being doped too high and diffusing to other epitaxial layers, affecting the growth of other epitaxial layers.

[0028] Optionally, the first tunnel junction 4 is a GaAs layer structure alternately doped with Si element and C element, and the first tunnel junction connects the bottom cell with the middle cell 6 by using tunnel effect. The second tunnel junction 7 is composed of GaInP layers and AlGaAs layers alternately stacked, wherein the GaInP layers are doped with Si element and the AlGaAs layers are doped with C element, and the second tunnel junction 7 also connects the middle cell 6 with the top cell 8 by using tunnel effect.

[0029] Optionally, the substrate 1 further has a nucleation layer 2 and a buffer layer 3 stacked in sequence thereon, which are used as a window layer of the bottom cell and also as a connecting layer between the Ge substrate and the subsequent epitaxial layer. The nucleation layer 2 is an N-type GaInP nucleation layer, and the buffer layer 3 is an N-type InGaAs buffer layer.

[0030] Optionally, the top cell 8 further has a cap layer 9 stacked thereon, and the cap layer is a GaAs cap layer, which is used to protect the epitaxial structure and realize high-quality ohmic contact.

[0031] The application further provides a growth method of the GaAs solar cell epitaxial structure, which is used for growing the above epitaxial structure and is specifically grown by using an organic chemical vapor deposition (MOCVD) method, and comprises the following steps: S1, providing a substrate 1, wherein the substrate is selected to be a P-type Ge substrate, N-type PH3 diffusion is performed on the P-type Ge substrate, the Ge of the topmost layer is diffused into N-type, a pn junction of a bottom cell is obtained, and the topmost layer is used as an emission region of the bottom cell, i.e., the substrate is used as the bottom cell, and the bottom cell is a Ge cell; then a nucleation layer 2 and a buffer layer 3 matched with the Ge lattice are grown on the substrate 1, which are used as a window layer of the bottom cell and also as a connecting layer between the Ge substrate and the subsequent epitaxial layer. The nucleation layer 2 is an N-type GaInP nucleation layer, and the buffer layer 3 is an N-type InGaAs buffer layer.

[0032] S2, growing a first tunnel junction 4 on the substrate 1, wherein the first tunnel junction is a GaAs layer structure alternately doped with Si element and C element, and the first tunnel junction connects the bottom cell with a subsequently grown middle cell 6 by using tunnel effect.

[0033] S3, growing a DBR layer 5 on the first tunnel junction 4, wherein the DBR layer is composed of Al x Ga 1-x As layers 51 and In y Ga 1-y As layers 52 alternately grown, wherein 0.5≤x≤0.9 and 0.0095≤y≤0.015, the Al x Ga 1-x As layers 51 are doped with C element, and the In y Ga 1-yThe As layer 52 is doped with Zn element. The C element has high doping efficiency and solubility in the Al x Ga 1-x As layer, overcoming the bottleneck of low Zn doping efficiency, so that both materials in the DBR layer can achieve high-concentration and uniform P-type doping, which greatly reduces the series resistance caused by the DBR layer. x Ga 1-x As / In y Ga 1-y As heterojunction interface, significantly reducing the series resistance caused by the DBR layer.

[0034] Optionally, the DBR layer has a reflection wavelength center value of 850nm-920nm, and the Al x Ga 1-x As layer and In y Ga 1- y The doping concentration of the As layer is 4E 18 cm -3 -8E 18 cm -3 , so that both materials in the DBR layer can achieve high-concentration and uniform P-type doping, reducing the series resistance caused by the DBR layer.

[0035] Optionally, the Al x Ga 1-x As layer and In y Ga 1-y As layer are alternately arranged for 5 pairs-20 pairs.

[0036] S4, growing a middle cell 6 on the DBR layer 5, the middle cell being an InGaAs middle cell, preferably an In 0.01 Ga 0.99 As middle cell.

[0037] S5, growing a second tunnel junction 7 on the middle cell 6, the second tunnel junction being composed of alternately arranged GaInP layers and AlGaAs layers, wherein the GaInP layers are doped with Si element and the AlGaAs layers are doped with C element, and the second tunnel junction also uses tunneling effect to connect the middle cell 6 with a subsequently grown top cell 8.

[0038] S6, growing a top cell 8 on the second tunnel junction 7, the top cell being a GaInP top cell, preferably a Ga 0.5 In 0.5 P top cell.

[0039] S7, growing a cap layer 9 on the top cell 8, the cap layer being a GaAs cap layer, used to protect the epitaxial structure and achieve high-quality ohmic contact.

[0040] The following specific examples and comparative examples further illustrate this point: Example 1: Using the growth method described in this application, in DBR layer 5, Al x Ga 1-x As layer 51 is Al 0.7 Ga 0.3 As layer, and doped with C element, In y Ga 1-y As layer 52 is In 0.01 Ga 0.99 As layer, and doped with Zn element, Al 0.7 Ga 0.3 As layer and In 0.01 Ga 0.99 The doping concentration of the As layer is 6E. 18 cm -3 The number of alternating pairs is 15.

[0041] Comparative Example 1: The traditional DBR growth method is used, which differs from Example 1 in that: Al 0.7 Ga 0.3 As layer and In 0.01 Ga 0.99 All As layers are doped with Zn, and Al 0.7 Ga 0.3 The doping concentration of the As layer is 2E. 18 cm -3 In 0.01 Ga 0.99 The doping concentration of the As layer is 6E. 18 cm -3 The number of alternating pairs is 15.

[0042] Comparative Example 2: It also adopts the traditional DBR growth method, but differs from Example 1 and Comparative Example 1 in that: Al 0.7 Ga 0.3 As layer and In 0.01 Ga 0.99 The As layers are all doped with Zn, and the doping concentration is 6E. 18 cm -3 The number of alternating logs was 15, but the growth temperature was reduced by 30°C compared to Comparative Example 1 and Example 1.

[0043] The solar cells fabricated in the above embodiments and comparative examples were subjected to performance tests, and the test results are shown in Table 1.

[0044] Table 1 - Performance test results of the examples and comparative examples

[0045] From the above table, it can be seen that the series resistance of the tandem cell of the present embodiment is smaller than that of Comparative Example 1 and Comparative Example 2, and in particular, 0.081 Ω smaller than that of Comparative Example 1, which leads to the highest fill factor and the highest conversion efficiency. Furthermore, by the method of co-doping C and Zn, the present application effectively reduces the series resistance of the DBR layer, thereby improving the fill factor and ultimately significantly improving the final conversion efficiency of the cell, which proves the effectiveness and superiority of the technical solution of the present application.

[0046] In addition, the open-circuit voltage of Example 1 is the highest, in particular, 47 mV higher than that of Comparative Example 2, which proves that the Al x Ga 1-x Low-temperature growth of GaAs material will affect the crystal quality of the material, and the doping method of the present application does not require low-temperature growth, which solves the contradiction between "high doping" and "high quality".

[0047] It is worth noting that the thickness of the substrate 1, the nucleation layer 2, the buffer layer 3, the first tunnel junction 4, the DBR layer 5, the middle cell 6, the second tunnel junction 7, the top cell 8 and the cap layer 9 shown in the drawings of the present application are only examples, and do not represent the true thickness. Moreover, the true ratio between the substrate 1, the nucleation layer 2, the buffer layer 3, the first tunnel junction 4, the DBR layer 5, the middle cell 6, the second tunnel junction 7, the top cell 8 and the cap layer 9 is not as shown in the drawings, but only for reference.

[0048] The various embodiments in the specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0049] The above description of the disclosed embodiments enables a person skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An epitaxial structure for a GaAs solar cell, characterized in that: The battery comprises, from bottom to top, a substrate, a first tunneling junction, a DBR layer, a middle cell, a second tunneling junction, and a top cell, wherein the substrate serves as the bottom cell; the DBR layer is made of Al. x Ga 1-x As layer and In y Ga 1-y As layers are grown alternately, wherein the Al x Ga 1-x The As layer is doped with C element, and the In y Ga 1-y The As layer is doped with Zn.

2. The epitaxial structure of a GaAs solar cell as described in claim 1, characterized in that: The Al x Ga 1-x As layer and In y Ga 1-y In layer A, 0.5≤x≤0.9, 0.0095≤y≤0.

015.

3. The epitaxial structure of a GaAs solar cell as described in claim 1, characterized in that: The Al x Ga 1-x As layer and In y Ga 1-y The doping concentration of the As layer is 4E. 18 cm -3 -8E 18 cm -3 .

4. The epitaxial structure of a GaAs solar cell as described in claim 1, characterized in that: The Al x Ga 1-x As layer and In y Ga 1-y The alternation pairs in the As layer are 5 to 20 pairs.

5. The epitaxial structure of a GaAs solar cell as described in claim 1, characterized in that: The center value of the reflection wavelength of the DBR layer is 850nm-920nm.

6. The epitaxial structure of a GaAs solar cell as described in claim 1, characterized in that: The substrate is a P-type Ge substrate, and PH3 diffusion is performed on the P-type Ge substrate to form a pn junction of the bottom cell, thus the bottom cell is a Ge bottom cell; the middle cell is an InGaAs middle cell, and the top cell is a GaInP top cell.

7. The epitaxial structure of a GaAs solar cell as described in claim 1, characterized in that: The first tunneling junction is a GaAs layer structure with alternating Si and C elements, and the second tunneling junction is composed of alternating GaInP and AlGaAs layers, wherein the GaInP layer is doped with Si element and the AlGaAs layer is doped with C element.

8. The epitaxial structure of a GaAs solar cell as described in claim 1, characterized in that: A nucleation layer and a buffer layer are also sequentially stacked on the substrate. The nucleation layer is an N-type GaInP nucleation layer, and the buffer layer is an N-type InGaAs buffer layer.

9. The epitaxial structure of a GaAs solar cell as described in claim 1, characterized in that: The top battery is also stacked with a capping layer, which is a GaAs capping layer.

10. A method for growing an epitaxial structure of a GaAs solar cell, used to grow an epitaxial structure of a GaAs solar cell as described in any one of claims 1-9, characterized in that, include: A substrate is provided, the substrate serving as a bottom cell; The first tunnel junction is grown on the substrate; A DBR layer is grown on the first tunnel junction, the DBR layer being composed of Al x Ga 1-x As layer and In y Ga 1-y As layers are grown alternately, wherein the Al x Ga 1-x The As layer is doped with C element, and the In y Ga 1-y The As layer is doped with Zn. Cells are grown on the DBR layer; A second tunnel junction is grown on the intermediate cell; A top cell is grown on the second tunnel junction.

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