Epitaxial structure of GaAs solar cell and growth method thereof

Abstract

The application provides an epitaxial structure of a GaAs solar cell and a growth method thereof, which 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, wherein the substrate serves as a bottom cell; the DBR layer is formed by alternately growing Al x Ga 1‑x As layers and In y Ga 1‑y As layers, wherein the Al x Ga 1‑x As layers are doped with C elements, and the In y Ga 1‑y As layers are doped with Zn elements; the C element has a very high doping efficiency and solubility in the Al x Ga 1‑x As layer with a high Al component, thereby overcoming the bottleneck of low Zn doping efficiency, enabling high-concentration and uniform P-type doping of both materials in the DBR layer, and greatly reducing the potential barrier at the Al x Ga 1‑x As / In y Ga 1‑y As heterojunction interface, and significantly reducing the series resistance caused by the DBR layer.

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, so as to reduce the diffusion length of the photo-generated carriers reaching the space charge region, thereby improving the collection efficiency of the photo-generated carriers. However, the reduction of the thickness of the base region 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, so as 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 a 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:

[0017] A substrate is provided, and the substrate serves as a bottom cell;

[0018] A first tunnel junction is grown on the substrate;

[0019] A DBR layer is grown on the first tunnel junction, and the DBR layer is alternately grown by Al x Ga 1-x As layers and In y Ga 1-y As layers, wherein the Al x Ga 1-x As layers are doped with C elements, and the Iny Ga 1-y As layer is doped with Zn element;

[0020] growing a middle cell on the DBR layer;

[0021] growing a second tunnel junction on the middle cell;

[0022] growing a top cell on the second tunnel junction.

[0023] After the above scheme, the beneficial effects of the present application are:

[0024] 1. In the DBR layer of the present application, Al x Ga 1-x As layer is doped with C element, In y Ga 1-y As layer is doped with Zn element, C element has very high doping efficiency and solubility in high Al component 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 of 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.

[0025] 2. In order to increase the doping concentration in the high Al component Al x Ga 1-x As layer, 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 present 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 present application maintains excellent material quality while achieving ultra-high doping concentration, thereby achieving higher open circuit voltage and conversion efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is the epitaxial structure diagram of the solar cell of the present application;

[0027] Figure 2 is the structure diagram of the DBR layer of the present application;

[0028] Figure 3 is the growth method flowchart of the present application.

[0029] REFERENCE NUMERALS:

[0030] 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-y As layer; 6, middle cell; 7, second tunnel junction; 8, top cell; 9, cap layer. DETAILED DESCRIPTION

[0031] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of the present application, and the range of the present application includes two end values.

[0032] As Figure 1 shown, the present application provides an epitaxial structure of a GaAs solar cell, which comprises, from bottom to top, 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, wherein the substrate 1 serves as a bottom cell, specifically, the substrate 1 is a P-type Ge substrate, PH3 diffusion is performed on the P-type Ge substrate to form a pn junction of the bottom cell as an emitter region, and the bottom cell is a Ge bottom cell; the middle cell 6 is an InGaAs middle cell, preferably an In 0.01 Ga 0.99 As middle cell, and the top cell 8 is a GaInP top cell, preferably a Ga 0.5 In 0.5 P top cell.

[0033] As Figure 2 shown, the DBR layer 5 is formed by alternately growing Al x Ga 1-x As layers 51 and In y Ga 1-y As layers 52, wherein 0.5≤x≤0.9 and 0.0095≤y≤0.015. In a conventional DBR growth mode, Al x Ga 1-x As is doped with Zn element, and a higher doping amount is required to obtain the doping with the element, which requires a reduced growth temperature, and Al x Ga 1-x As materials containing more than 50% Al component need a higher growth temperature to ensure the material crystal quality, which is contradictory. Therefore, the conventional DBR layer makes a trade-off between the doping amount and the material crystal quality, and generally Al x Ga1-x The doping concentration of Zn-doped Al 18 cm -3 .

[0034] The Al x Ga 1-x As layer 51 is doped with C element, and the In y Ga 1-y 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 with high Al component, which overcomes the bottleneck of low Zn doping efficiency, so that both of the two materials in the DBR layer 5 can achieve high-concentration and uniform P-type doping, which greatly reduces the potential barrier at the Al x Ga 1-x As / In y Ga 1-y As heterojunction interface, significantly reduces the series resistance caused by the DBR layer 5. Moreover, the Al x Ga 1-x As layer 51 doped with C can be grown at the conventional optimal crystal quality growth temperature, which perfectly solves the contradiction between "high doping" and "high quality".

[0035] Optionally, the reflection wavelength center value of the DBR layer 5 is 850nm-920nm, and the doping concentration of the Al x Ga 1-x As layer 51 and the In y Ga 1-y As layer 52 is 4E 18 cm -3 -8E 18 cm -3 , so that both of the two materials in the DBR layer can achieve high-concentration and uniform P-type doping, and reduce the series resistance caused by the DBR layer.

[0036] Optionally, the alternating number of the Al x Ga 1-x As layer 51 and the In y Ga 1-y As layer 52 is 5 pairs-20 pairs, and the optimal alternating number is 15 pairs.

[0037] Preferably, the optimal value of the doping concentration of the Al x Ga 1-x As layer 51 and the In y Ga 1-y As layer 52 is 6E 18 cm -3The doping concentration can improve the material interface barrier and prevent the material from diffusing to other epitaxial layers due to over-doping.

[0038] Optionally, the first tunnel junction 4 is a GaAs layer structure alternately doped with Si and C elements, and the first tunnel junction connects the bottom cell with the middle cell 6 by using a tunnel effect.

[0039] Optionally, the substrate 1 further has a nucleation layer 2 and a buffer layer 3 stacked in sequence, which are used as a window layer of the bottom cell and also as a connecting layer between the Ge substrate and subsequent epitaxial layers, the nucleation layer 2 is an N-type GaInP nucleation layer, and the buffer layer 3 is an N-type InGaAs buffer layer.

[0040] Optionally, the top cell 8 further has a cap layer 9 stacked thereon, the cap layer is a GaAs cap layer, which is used to protect the epitaxial structure and achieve high-quality ohmic contact.

[0041] The application further provides a growth method of the GaAs solar cell epitaxial structure, which is used for growing the epitaxial structure and specifically adopts an organic chemical vapor deposition (MOCVD) method to grow, and includes the following steps.

[0042] S1, providing a substrate 1, the substrate is selected as 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 to obtain a pn junction of a bottom cell as an emission area of the bottom cell, 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 subsequent epitaxial layers, the nucleation layer 2 is an N-type GaInP nucleation layer, and the buffer layer 3 is an N-type InGaAs buffer layer.

[0043] S2, growing a first tunnel junction 4 on the substrate 1, the first tunnel junction is a GaAs layer structure alternately doped with Si and C elements, and the first tunnel junction connects the bottom cell with a subsequently grown middle cell 6 by using a tunnel effect.

[0044] S3, growing a DBR layer 5 on the first tunnel junction 4, the DBR layer is composed of Al x Ga 1-x As layers 51 and In y Ga 1-yAs layer 52 is grown alternately, wherein 0.5≤x≤0.9, 0.0095≤y≤0.015, the Al x Ga 1-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 component Al x Ga 1-x As layer, overcoming the bottleneck of low Zn doping efficiency, so that both materials in DBR layer can achieve high concentration and uniform P-type doping, which greatly reduces the series resistance caused by DBR layer. x Ga 1-x As / In y Ga 1-y As heterojunction interface, significantly reducing the series resistance caused by DBR layer.

[0045] Optionally, the DBR layer has a reflection wavelength center value of 850nm-920nm, the Al x Ga 1-x As layer and In y Ga 1- y As layer have a doping concentration of 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.

[0046] Optionally, the Al x Ga 1-x As layer and In y Ga 1-y As layer have an alternating number of 5 pairs-20 pairs.

[0047] 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.

[0048] S5, growing a second tunnel junction 7 on the middle cell 6, the second tunnel junction being composed of GaInP layers and AlGaAs layers arranged alternately, wherein the GaInP layers are doped with Si element and the AlGaAs layers are doped with C element, the second tunnel junction also uses tunneling effect to connect the middle cell 6 with a top cell 8 grown subsequently.

[0049] 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.

[0050] S7, growing a cap layer 9 on the top cell 8, the cap layer being a GaAs cap layer, to protect the epitaxial structure and to achieve a high quality ohmic contact.

[0051] The following is further illustrated by specific examples and comparative examples:

[0052] Example One:

[0053] Using the growth method of the present application, in the DBR layer 5, the Al x Ga 1-x As layer 51 is an Al 0.7 Ga 0.3 As layer and is doped with C element, the In y Ga 1-y As layer 52 is an In 0.01 Ga 0.99 As layer and is doped with Zn element, the Al 0.7 Ga 0.3 As layer and the In 0.01 Ga 0.99 As layer all have a doping concentration of 6E 18 cm -3 -1, and the number of alternations is 15 pairs.

[0054] Comparative Example 1:

[0055] Using the conventional DBR growth method, the difference from Example 1 is that the Al 0.7 Ga 0.3 As layer and the In 0.01 Ga 0.99 As layer are both doped with Zn element, and the doping concentration of the Al 0.7 Ga 0.3 As layer is 2E 18 cm -3 -1, and the doping concentration of the In 0.01 Ga 0.99 As layer is 6E 18 cm -3 -1, and the number of alternations is 15 pairs.

[0056] Comparative Example 2:

[0057] Also using the conventional DBR growth method, the difference from Example 1 and Comparative Example 1 is that the Al 0.7 Ga 0.3 As layer and the In 0.01 Ga0.99 As layer is doped with Zn element, and the doping concentration is 6E 18 cm -3 , the alternating logarithm is 15 pairs, but the growth temperature is reduced by 30°C compared with the comparative example 1 and the example 1.

[0058] The solar cells prepared in the above examples and comparative examples are tested for performance, and the test results are shown in Table 1.

[0059] Table 1 - Performance test results of examples and comparative examples

[0060]

[0061] From the above table, it can be seen that the series resistance of the present example is smaller than that of the comparative example 1 and the comparative example 2, especially 0.081Ω smaller than that of the 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 finally significantly improving the final conversion efficiency of the cell, which proves the effectiveness and superiority of the technical scheme of the present application.

[0062] In addition, the open circuit voltage of the example 1 is the highest, especially 47mV higher than that of the comparative example 2, which proves that the low-temperature growth of Al x Ga 1-x As 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".

[0063] 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.

[0064] Each embodiment in the specification is described in a progressive manner, and each embodiment focuses on the differences from other embodiments, and the same and similar parts between each embodiment can be referred to each other.

[0065] The foregoing description of the disclosed embodiments enables a person skilled in the art to make or use the application. Modifications of these embodiments will occur to persons of skill in the art, and that the generic principles defined herein can be applied to other embodiments without departing from the spirit or scope of the application. Therefore, the present application is not intended to be limited to the embodiments shown herein but is to be accorded 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 The As layers are grown alternately, wherein 0.5 ≤ x ≤ 0.9, 0.0095 ≤ y ≤ 0.015, and 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, the Al x Ga 1-x As layer and In y Ga 1-y The doping concentration of the As layer is 4E18cm. -3 -8E18cm -3 .

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 The number of alternation pairs in the As layer is 5 to 20.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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 1-7, 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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