Epitaxial rate modulated thin layer torsion strain regulation and control structure and preparation method thereof

Abstract

The invention discloses an epitaxial rate modulated thin layer torsion strain regulation and control structure and a preparation method thereof, and the method comprises the following steps: at the initial growth stage of a strain regulation and control layer, the temperature of MOCVD is regulated and controlled to be t1, and V / III is regulated and controlled to be n1; in the later growth period of the strain regulation and control layer, the temperature of MOCVD is regulated and controlled to be t2, and V / III is regulated and controlled to be n2; wherein t1 is 5-25 DEG C higher than t2, n2 is 1.5-5 times of n1, and V / III is the ratio of the flux of the V-group source to the flux of the III-group source. The strain regulation and control structure is thinner than a traditional multi-layer gradual change buffer layer structure, lattice gradual change is achieved with higher efficiency, the dislocation density of the obtained solar cell is lower, and meanwhile the epitaxial process time is shortened.

Description

Technical Field

[0001] The invention relates to the technical field of solar cells, and in particular to an epitaxial rate modulated thin layer twisted strain control structure and a preparation method thereof. Background Art

[0002] Long-duration flights of aircraft in near-space require a continuous supply of energy, which makes solar cells the best choice for energy supply. Improving the photoelectric conversion efficiency of solar cells is an effective way to increase the power of aircraft power and increase the mass-to-power ratio under conditions such as fixed battery mass and area. The current reverse triple-junction battery has a band gap combination of 1.9 / 1.4 / 1.0eV. The infrared part below 1.0eV is not used. In order to further improve the payload capacity of near-space aircraft, it is necessary to further improve the conversion efficiency of reverse triple-junction batteries to better provide energy for aircraft.

[0003] The physical model construction and simulation research of multi-junction solar cell devices show that the cell band gap is reduced to 1.3eV (corresponding to Ga 0.96 In 0.04 As), the bottom cell bandgap is reduced to 0.9 eV (corresponding to Ga 0.64 In 0.36 As), which can increase the battery current density and help further improve the photoelectric conversion efficiency, forming a new generation of reverse triple-junction battery structure.

[0004] In order to obtain an epitaxial structure with low dislocation, it is usually necessary to adopt the component lattice gradient buffer layer CGB technology. In the new generation of reverse triple junction epitaxial structure, if the current CGB structure is used, the thickness is relatively thick, which is not conducive to thin filmization. At the same time, the relaxation degree of the epitaxial layer is also low, which is not conducive to stress release. The traditional gradient buffer layer has at least 5-6 layers and at most about 20 layers, with a total thickness of several microns. This brings about the following three problems: (1) Excessively thick epitaxial material increases the mass of the battery and reduces the mass-to-power ratio of the battery; (2) Excessively long process time leads to increased epitaxial cost; (3) Multiple interfaces serve as dislocation generation centers, resulting in a higher dislocation density in the mismatch material. In response to the above problems, the present invention proposes a strain control structure of thin layer torsion modulated by epitaxial rate. Summary of the invention

[0005] The purpose of the present invention is to provide a strain control structure of thin-layer torsion with epitaxial rate modulation to achieve high mass specific power, short process time and low dislocation density in order to address the technical defects of the prior art, such as large thickness, low mass specific power and long process time of the gradient buffer layer.

[0006] The technical solution adopted to achieve the purpose of the present invention is:

[0007] A method for preparing a strain control layer with epitaxial rate modulation and thin layer twisting:

[0008] In the initial growth stage of the strain control layer, the temperature of MOCVD is controlled to t1, and V / III is controlled to n1;

[0009] In the later stage of the growth of the strain control layer, the temperature of MOCVD is controlled to t2, and V / III is controlled to n2;

[0010] Among them, t1 is 5 to 25°C higher than t2, n2 is 1.5 to 5 times of n1, and V / III is the ratio of the flux of the V group source to the flux of the III group source.

[0011] In the above technical solution, the initial growth stage is the growth stage when the strain regulation layer has an initial thickness of 50 nm, and the late growth stage is the remaining growth stage of the strain regulation layer.

[0012] Another aspect of the present invention also includes a strain control layer of a single-layer structure obtained by the preparation method.

[0013] In the above technical solution, the thickness of the strain regulation layer is 50 to 500 nm.

[0014] In the above technical solution, the strain control layer is SiGe, AlGaInAs or AlGaInP.

[0015] Another aspect of the present invention also includes a mismatch structural material system, including a substrate, a target layer and the strain regulation layer located between the two layers, there is a torsion angle in the (001) crystal direction between the substrate and the target layer, the lattice constant of the strain regulation layer is greater than the larger value between the substrate and the target layer, and the lattice constant transition is achieved through the strain regulation layer.

[0016] In the above technical solution, the substrate and the target layer are both zinc blende structures and the lattice mismatch is within 1%.

[0017] In the above technical solution, the twist angle is greater than 0° and less than 1°, and the crystal plane cutting angle of the substrate is greater than 2°.

[0018] In the above technical solution, the substrate is the top cell and tunnel junction in the solar cell, and the target layer is the middle cell in the solar cell.

[0019] Another aspect of the present invention also includes a solar cell, including a top cell, a first tunnel junction, the strain regulation layer, a middle cell, a strain buffer layer and a bottom cell.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] 1. The purpose of the strain control structure of the present invention is to achieve a transition of lattice constants in an epitaxial mismatch structure and reduce lattice defects; the advantage of the strain control structure of the present invention is that it is thinner and more efficient than the traditional multi-layer gradient buffer layer structure to achieve lattice gradient, resulting in a lower dislocation density of the solar cell, while also shortening the epitaxial process time.

[0022] 2. The present invention introduces a metastable state during the initial growth process to achieve lattice twisting, reduce lattice mismatch, and obtain metastable materials through rapid epitaxy and high growth temperature to improve its stability, thereby avoiding stress release through dislocation generation, thereby achieving low-defect epitaxy. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The figure shows a comparison between the traditional gradient buffer layer structure and the strain regulation layer structure proposed in the present invention.

[0024] Figure 2 The figure shows a comparison between the traditional gradient buffer layer structure and the strain regulation layer structure proposed in the present invention.

[0025] Figure 3 This is the reverse triple-junction solar cell structure of Example 2. DETAILED DESCRIPTION

[0026] The present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0027] There are two ways for lattice constant transition, stress release through interface dislocation and stress release through lattice torsion. In most cases, stress release occurs through interface dislocation, such as Figure 1 (a) and Figure 2 (a) shows the gradient buffer layer. However, this will inevitably lead to an increase in the dislocation density in the epitaxial active layer, which is not conducive to improving the efficiency of the battery.

[0028] like Figure 1 (b) and Figure 2 As shown in (b) the strain control layer, the present invention introduces a metastable state in the initial growth process to achieve lattice twisting, obtains metastable materials through rapid epitaxy and high growth temperature, and improves its stability, avoiding stress release through dislocation generation, thereby achieving low-defect epitaxy

[0029] Example 1

[0030] A strain-controlled structure of epitaxial rate-modulated thin-layer twisting is prepared by the following steps:

[0031] After the growth of material A (substrate) is completed, a strain control layer such as n-AlGaInAs, n-AlGaInP or n-SiGe is grown. The dopant is Si. Taking n-AlGaInAs as an example, the growth of the n-AlGaInAs strain control layer specifically includes two stages:

[0032] In the first 50nm of the n-AlGaInAs strain control layer, the temperature of the MOCVD equipment is adjusted to 670℃ and the V / III ratio is 15; then the temperature of the MOCVD equipment is adjusted to 650℃ and the V / III ratio is 70. This achieves the release of lattice mismatch stress.

[0033] After the growth of the n-AlGaInAs strain control layer is completed, material B (target layer) is grown on the strain control layer.

[0034] Example 2

[0035] like Figure 3 As shown, a solar cell with a strain-regulated structure based on epitaxial rate-modulated thin-layer torsion is prepared by the following steps:

[0036] Using metal organic chemical vapor deposition (MOCVD) equipment, the reaction chamber pressure was set to 50 mbar. First, the core layer n-type doped GaAs was epitaxially grown on the GaAs substrate, and its dopant was Si with a doping concentration of 2×10 18 cm -3 , growth temperature is 600℃, thickness is 0.2μm;

[0037] The sacrificial layer is n-Ga 0.5 In 0.5 P, whose dopant is Si, and the doping concentration is 1×10 18 cm -3 , growth temperature is 650℃, thickness is 0.3μm;

[0038] The cap layer is n + -GaAs, whose dopant is Si, with a doping concentration of 6×10 18 cm -3 , growth temperature is 660℃, thickness is 0.5μm;

[0039] The top cell is Ga 0.5 In 0.5 P cells, including n-type doped n-Ga 0.5 In 0.5 P emitter layer and p-type doped p-Ga 0.5 In 0.5 P base layer, where n-Ga 0.5 In 0.5The dopant of the P layer is Si, and the doping concentration is 5×10 17 cm -3 , growth temperature is 650℃, thickness is 0.1μm; p-Ga 0.5 In 0.5 The dopant of the P layer is Zn, and the doping concentration is 1×10 18 cm -3 , growth temperature is 650℃, thickness is 0.5μm;

[0040] The first tunnel junction includes a heavily p-doped p ++ -Al 0.4 Ga 0.6 As layer and n-type heavily doped n ++ -GaAs layer, where p ++ -Al 0.4 Ga 0.6 The dopant of the As layer is C, and the doping concentration is 6×10 19 cm -3 , thickness is 0.03μm, growth temperature is 600℃; where n ++ The dopant of the GaAs layer is Te, with a doping concentration of 8×10 19 cm -3 , thickness is 0.02μm, growth temperature is 580℃;

[0041] Strain control layer, such as Figure 3 As shown, it is n-type doped n-(Al 0.5 Ga 0.5 ) 0.9 In 0.10 As layer, its dopant is Si, and the doping concentration is 1×10 18 cm -3 , when the thickness is 50nm before growth, the growth temperature is 670℃, the V / III ratio is 15, when the thickness is 50-150nm, the growth temperature is 650℃, the V / III ratio is 70;

[0042] Ga 1-x In x As intermediate cell, including n-doped n-(Al 0.5 Ga 0.5 ) 0.96 In 0.04 As emitter layer and p-type doped p-Ga 0.96 In 0.04 As base layer, wherein the n-(Al 0.5 Ga 0.5 ) 0.96 In 0.04 The dopant of the As emitter layer is Si, with a doping concentration of 3×10 18cm -3 , the growth temperature is 650°C, the thickness is 0.1μm; the p-Ga 0.96 In 0.04 The dopant of the As base layer is Zn, with a doping concentration of 3×10 17 cm -3 , growth temperature is 650℃, thickness is 3μm;

[0043] The second tunnel junction includes a heavily p-doped p ++ -(Al 0.6 Ga 0.4 ) 0.96 In 0.04 As layer and n-type doped n ++ -Ga 0.96 In 0.04 As layer. ++ -(Al 0.6 Ga 0.4 ) 0.96 In 0.04 The dopant of the As layer is C, and the doping concentration is 6×10 19 cm -3 , thickness is 0.03 μm, growth temperature is 550°C; wherein the n ++ -Ga 0.96 In 0.04 The dopant of the As layer is Te, and the doping concentration is 8×10 19 cm -3 , growth temperature is 550℃, thickness is 0.02μm;

[0044] The gradient buffer layer is an n-type doped layer with a gradually increasing In component x (Al 0.6 Ga 0.4 ) 1-x In x As layer, such as Figure 3 As shown. The dopant is Si, and the doping concentration is 1×10 18 cm -3 , the In composition x from the initial layer to the target gradient layer changes nonlinearly from 4% to 36% from bottom to top, while the In composition of the previous layer of the target layer is 45%, which is an overshoot layer, with a growth temperature of 670℃ and a thickness of 0.2μm;

[0045] The bottom cell is n-type doped n-Ga 0.64 In 0.36 As and p-type doped p-Ga 0.64 In 0.36 As, wherein the n-Ga 0.64 In 0.36 The dopant of the As layer is Si, and the doping concentration is 2×10 18cm -3 , with a thickness of 0.1 μm and a growth temperature of 660°C; wherein the p-Ga 0.64 In 0.36 The dopant of the As layer is Zn, and the doping concentration is 6×10 17 cm -3 , thickness is 3μm, growth temperature is 660℃;

[0046] The GaInAs cap layer is p-type doped + -Ga 0.64 In 0.36 As, the dopant is Zn, and the doping concentration is 2×10 19 cm -3 , thickness is 0.5μm, and growth temperature is 600℃.

[0047] The above epitaxial wafer is taken out from MOCVD and the subsequent device process is carried out: gold, zinc, gold and silver are deposited respectively in the metal evaporation equipment to form a seed layer; after electroplating, it is placed on a temporary substrate; the GaAs substrate is removed by corrosion; then barrier layer etching, photolithography, gold electrode evaporation, contact layer corrosion, anti-reflection film evaporation and dicing process are carried out.

[0048] The IV parameters of the prepared battery are shown in Table 1. Open circuit voltage 2.92V, short circuit current 14.8mA / cm 2 , photoelectric conversion efficiency 34%.

[0049] Table 1

[0050]

[0051] Compared with the traditional gradient buffer layer technology, the thin-layer torsional strain regulation structure involved in the present invention has a small thickness, a short growth time, and only one material and two heterogeneous interfaces (one exists between the strain regulation layer and the substrate, and the other exists between the strain regulation layer and the target layer), which is beneficial to improving the overall quality-to-power ratio of the battery, especially for thin-film batteries; it can reduce lattice mismatch dislocations; and it can be flexibly applied to multiple mismatch epitaxial structure systems, including but not limited to SiGe, InGaAs, InGaP, etc.

[0052] The above 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 principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a strain control layer of a thin layer twisted by epitaxial rate modulation, characterized in that: The following steps are involved: In the initial growth stage of the strain control layer, the temperature of MOCVD is controlled to t1, and V / III is controlled to n1; In the later stage of the growth of the strain control layer, the temperature of MOCVD is controlled to t2, and V / III is controlled to n2; Among them, t1 is 5 to 25°C higher than t2, n2 is 1.5 to 5 times of n1, and V / III is the ratio of the flux of the V group source to the flux of the III group source.

2. The preparation method according to claim 1, characterized in that The initial growth stage is the growth stage when the strain regulation layer has an initial thickness of 50 nm, and the late growth stage is the remaining growth stage of the strain regulation layer.

3. The strain control layer of the single-layer structure obtained by the preparation method according to claim 1.

4. The strain control layer according to claim 3, characterized in that: The thickness of the strain regulation layer is 50-500 nm.

5. The strain control layer according to claim 3, characterized in that: The strain control layer is SiGe, AlGaInAs or AlGaInP.

6. A mismatched structural material system, characterized in that: It comprises a substrate, a target layer and a strain control layer as described in any one of claims 1 to 5 located between the two layers, there is a twist angle in the (001) crystal direction between the substrate and the target layer, the lattice constant of the strain control layer is greater than the larger value between the substrate and the target layer, and lattice constant transition is achieved through the strain control layer.

7. The mismatched structural material system according to claim 6, characterized in that: The substrate and the target layer both have zinc blende structures and the lattice mismatch is within 1%.

8. The mismatched structural material system according to claim 6, characterized in that: The twist angle is greater than 0° and less than 1°, and the crystal plane angle of the substrate is greater than 2°.

9. The mismatched structural material system according to claim 6, characterized in that: The substrate is the top cell and tunnel junction in the solar cell, and the target layer is the middle cell in the solar cell.

10. A solar cell, characterized in that: The invention comprises a top battery, a first tunnel junction, a strain regulating layer according to any one of claims 1 to 5, a middle battery, a strain buffer layer and a bottom battery.

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