A method for measuring heterojunction interface recombination rate by electron beam induced current

Abstract

The application is a method for measuring heterojunction interface recombination rate by electron beam induced current, which considers both depletion zone bulk recombination and front interface recombination, and considers both diffusion movement and drift movement caused by non-uniform built-in electric field in the depletion zone, and obtains a new carrier collection probability function. The application analyzes the characteristics of heterojunction interface recombination by electron beam induced current, which is the most important recombination mechanism affecting short-wave response of heterojunction device. EBIC / I b The application calculates the recombination rate S p of heterojunction interface, thereby obtaining a set of parameters capable of reliably evaluating the quality of heterojunction interface.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor heterojunction photoelectric property measurement and analysis, and relates to a method for characterizing the recombination rate at the heterojunction interface. Background Technology

[0002] Heterojunction-based optoelectronic devices, such as high-speed response photodiodes and high-efficiency photovoltaic cells, are gradually replacing homojunction-based optoelectronic devices. Interfacial recombination in these devices has a significant impact on their performance. Accurate characterization of interfacial recombination during the development of such devices helps increase the reliability of the research process and promotes researchers' understanding of the physical mechanisms of heterojunction photoelectric conversion. Photoresponse spectroscopy or transient fluorescence spectroscopy are commonly used methods to characterize the degree of carrier recombination at different microscopic locations within the device. Based on the assumption that light absorption attenuation is negatively correlated with wavelength, short-wavelength values ​​are used to handle carrier recombination at the surface, while long-wavelength values ​​are used to handle carrier recombination in the bulk. However, this characterization method is affected by the optical properties of the material, making it difficult to eliminate the interference of the non-uniform light absorption coefficient unique to heterojunctions. Electron beam induced current (EBIC) is a commonly used tool for characterizing semiconductor micro-defects (CN112313769A). The induced current efficiency η... EBIC Theoretically, this corresponds to the carrier collection probability f of a heterojunction. This characterization tool can analyze carrier recombination at different locations in a material without being affected by its light absorption properties. In the field of photovoltaic cells, electron beam induced current is often used to study the microscopic distribution of carrier recombination in thin-film heterojunction cells (Progress in Photovoltaics: Research and Applications, (2023) 31: 678-689.), but most theoretical methods used to analyze electron beam induced current signals are based on overly simplistic or unrealistic assumptions. For example, an early group of German researchers (Solar Energy Materials and Solar Cells, (2011) 95: 887-893.) neglected carrier diffusion and bulk recombination in the depletion region, deriving a collection probability f = (1 + S / μE). -1Several years later, scientists in Germany and the United States (Journal of Applied Physics, (2013) 114:134504; Journal of Applied Physics, (2016) 120:7.) considered interface recombination at interface 2, which is far from the light-receiving surface, but neglected interface recombination at heterojunction interface 1, the region with the most significant photoelectric conversion. These shortcomings resulted in a large deviation between the device defect characteristics characterized by the electron beam induced current and the actual spectral response characteristics of the device. Therefore, this invention, through theoretical analysis, derives a carrier collection probability function that can characterize heterojunction interface recombination and bulk recombination. Using this function in conjunction with the electron beam induced current to characterize the defect characteristics of the device interface and bulk, the recombination rate at the heterojunction interface can be calculated, thus providing a reliable basis for quality control in the manufacture of heterojunction devices. Summary of the Invention

[0003] To overcome the shortcomings of existing methods in characterizing the defect features of heterojunction devices by analyzing electron beam induced current, and considering that photoelectric conversion mainly occurs near the heterojunction interface close to the light-receiving surface, this invention re-analyzes the carrier collection equation in the depletion region of the heterojunction, and takes into account both bulk recombination in the depletion region and recombination at the front interface, proposes a method for analyzing the electron beam induced current characterization signal in heterojunction devices.

[0004] The technical solution adopted by this invention to solve its technical problem is:

[0005] A method for measuring the heterojunction interface recombination rate using electron beam induced current simultaneously considers carrier diffusion and drift motion caused by non-uniform built-in electric fields in the depletion region, and also considers heterojunction interface recombination and depletion region bulk recombination, thus obtaining a carrier collection probability such as:

[0006]

[0007] Integral S p It is the recombination rate at the heterojunction interface, D n and D p λ is the bulk diffusion coefficient of charge carriers, a0 is the characteristic length related to the doping concentration of the absorber layer, and a is the width of the depletion region. p =Eμ p τ p ,λ n =Eμ n τ n These are the drift lengths related to the lifetimes of holes and electrons, respectively.

[0008] Furthermore, considering that interfacial recombination plays a much larger role than bulk recombination near the heterojunction interface, equation (1) is simplified to:

[0009]

[0010] It is known that for the known semiconductor materials that make up a heterojunction, the carrier collection probability near the heterojunction interface is only equal to that of S. p / D p Related; diffusion coefficient D as a bulk property p The electron diffusion coefficient D is directly measured through the electrical transport of a single-layer film; and for known materials, the electron diffusion coefficient D is... n and hole diffusion coefficient D p The ratio N is converted using effective mass; therefore, as long as the carrier collection probability curve within the heterojunction is obtained, the interfacial recombination rate S of the heterojunction can be calculated. p The calculation is performed; on the other hand, electron beam induced current technology is used, through the ratio of current f(x) = I. EBIC / I b Calculate the carrier collection probability S p :

[0011]

[0012] This invention provides a method for characterizing the photoelectric conversion efficiency of passive optoelectronic heterojunction devices. This method is independent of the light absorption properties of the heterojunction and has high reliability. The method includes a function that calculates the interfacial recombination rate using the carrier collection probability. This function considers both interfacial recombination near the light-receiving surface and bulk recombination in the depletion region. The function is a function of doping concentration, film thickness, interfacial recombination rate, and bulk diffusion coefficient. The ratio of the interfacial recombination rate to the mobility is a unified whole, revealing that interfacial recombination and bulk diffusion compete to influence the carrier collection probability. The magnitude of the carrier collection probability at the heterojunction interface is independent of the carrier lifetime. This invention, combined with electron beam induced current technology, accurately characterizes the heterojunction interfacial recombination rate, providing research parameters for heterojunction fabrication processes.

[0013] The beneficial effects of this invention are mainly reflected in the following aspects: the data processing process used in the method is simple and practical, and can be used to analyze the interface quality of heterojunction semiconductors, evaluate the heterojunction preparation process, and predict the corresponding spectral response curve of heterojunction. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of carrier collection theory.

[0015] Figure 2 It is the theoretical probability curve of carrier collection in the depletion region of a heterojunction.

[0016] Figure 3 The EBIC curves and fittings for the MZO / CdTe heterojunction are shown.

[0017] Figure 4 This is a graph showing the calculated recombination rate at the MZO / CdTe heterojunction interface.

[0018] Figure 5 The EBIC curves and fittings for CdS / CIGS heterojunctions are shown.

[0019] Figure 6 This is a graph showing the calculated recombination rate at the CdS / CIGS heterojunction interface. Detailed Implementation

[0020] The present invention will now be further described with reference to the accompanying drawings.

[0021] Reference Figures 1-6 A method for measuring the heterojunction interface recombination rate using electron beam induced current simultaneously considers carrier diffusion and drift motion caused by non-uniform built-in electric fields in the depletion region, and also considers heterojunction interface recombination and depletion region bulk recombination, thus obtaining the following carrier collection probability:

[0022]

[0023] Integral S p It is the recombination rate at the heterojunction interface, D n and D p λ is the bulk diffusion coefficient of charge carriers, a0 is the characteristic length related to the doping concentration of the absorber layer, and a is the width of the depletion region. p =Eμ p τ p ,λ n =Eμ n τ n These are the drift lengths related to the lifetimes of holes and electrons, respectively.

[0024] Furthermore, considering that interfacial recombination plays a much larger role than bulk recombination near the heterojunction interface, equation (1) is simplified to:

[0025]

[0026] It is known that for the known semiconductor materials that make up a heterojunction, the carrier collection probability near the heterojunction interface is only equal to that of S. p / D p Related; diffusion coefficient D as a bulk property p The electron diffusion coefficient D is directly measured through the electrical transport of a single-layer film; and for known materials, the electron diffusion coefficient D is... n and hole diffusion coefficient D p The ratio N is converted using effective mass; therefore, as long as the carrier collection probability curve within the heterojunction is obtained, the interfacial recombination rate S of the heterojunction can be calculated. pThe calculation is performed; on the other hand, electron beam induced current technology is used, through the ratio of current f(x) = I. EBIC / I b Calculate the carrier collection probability S p :

[0027]

[0028] Therefore, combining electron beam induced current technology can accurately characterize the heterojunction interface recombination rate and provide research parameters for heterojunction fabrication processes.

[0029] Figure 1 This is a theoretical schematic diagram of the collection probability function (1). An electron beam injects electrons at point x1 within the depletion region. Under the drift effect of the built-in electric field and the diffusion effect of the carrier concentration within the depletion region, an induced current is generated. The induced current consists of both electron and hole flows within the semiconductor. 'a' is the width of the depletion region, which is the thickness of the heterojunction film.

[0030] Heterojunction interface composite S p This will lead to a decrease in the probability of collection. For example... Figure 2 The figure shows the simplified collection probability function (2) as a function of the heterojunction interface recombination rate S. p The change in depth is the distance from the heterojunction interface to the point of investigation within the depletion region. After selecting an appropriate diffusion coefficient D, it can be seen that surface recombination has a significant impact on the collection probability within a 60 nm range near the interface. Therefore, to reliably calculate the surface recombination rate at the heterojunction interface using this method, a detailed EBIC scan near the heterojunction interface is required.

[0031] Example 1: Selecting Zn 1-x Mg x The object is the O / CdTe heterojunction. 1-x Mg x O / CdTe heterojunctions are a core material commonly used in photovoltaic thin-film batteries in recent years. Zn with a fixed composition x is prepared on glass using magnetron sputtering. 1-x Mg x O, then use near-space sublimation in Zn 1-x Mg x A CdTe layer was prepared on O. Zn 1-x Mg x The different crystal structures and lattice constants of O and CdTe result in a heterojunction interface filled with defects. These defects affect the collection of photogenerated carriers.

[0032] First, a Hall electrode was fabricated on the CdTe surface. After eliminating side effects such as potential inequality and thermoelectric potential difference, the mobility μ of majority carrier holes was obtained by Hall measurement. p= 8 cm / Vs. Meanwhile, for cadmium telluride, the electron mobility is 8 times the hole mobility, therefore μ n = 64 cm / Vs. The diffusion coefficients D can be obtained as follows: p =0.2cm 2 / s, D n =1.6cm 2 / s. For thin-film batteries, the depletion region occupies the entire film layer, so the width of the depletion region 'a' is approximately equal to the film thickness of 2.88 μm.

[0033] Next, a cutting tool is used to cut the cross-section of the heterojunction, exposing it on the surface. The cut sample is then loaded into an electron beam system. EBIC signal testing is performed on the sample. Figure 3 As shown, the test results indicate that the heterojunction has good quality. The collection probability is around 90% in most areas of the depletion region. (The last part, "D," appears to be an unrelated fragment and is omitted from the translation.) p =0.2cm 2 / s, D n =1.6cm 2 / s, a=2.88μm are substituted into function (2), and the interface recombination rate parameter S is continuously modified. p To fit EBIC efficiency data in the 0-40nm range, such as Figure 3 The surface recombination rate corresponding to the curve with the smallest fitting deviation shown is S. p ≈2.9×10 9 cm / s.

[0034] Another method is to substitute the values ​​of f(x) at different x points and the integral values ​​of F1(x) and F3(x) into formula (3) to calculate S. p .like Figure 4 As shown, S p ≈(2.3~4.5)×10 9 cm / s, take the average value S p ≈3.1×10 9 cm / s. S obtained by both methods p The results are fairly consistent.

[0035] Example 2: A CdS / CdTe heterojunction was selected as the object. The CdS / CdTe heterojunction is another type of heterojunction different from Zn. 1- x Mg xThe O / CdTe thin-film battery structure is the mainstream commercial thin-film battery structure. CdS is prepared on glass using a water bath method, and then a CdTe layer is prepared on the CdS using near-space sublimation. The similar chemical structures and lattice constants of CdS and CdTe result in better interface quality in this heterojunction. However, the alloying of CdS and CdTe leads to a structure riddled with bulk defect recombination. These defects affect the collection of photogenerated carriers deep within the depletion region.

[0036] As in Example 1, the mobility of CdTe in this heterojunction was measured. μ p = 9.6 cm / Vs. Therefore, μ n = 76.8 cm / Vs. The diffusion coefficients D can be obtained as follows: p =0.24cm 2 / s, D n =1.92cm 2 / s. The width of the depletion region is approximately equal to the film thickness of 3.4 μm.

[0037] Similar to Example 1, the EBIC signal of the sample was tested. Figure 5 As shown, the test results indicate that this heterojunction has better interface quality. The collection probability is around 90% in the region near the interface. However, the collection probability begins to decrease significantly deeper into the depletion region, which is due to larger bulk recombination. [The text then abruptly shifts to a seemingly unrelated topic: "D..."] p =0.2cm 2 / s, D n =1.6cm 2 / s, a=2.88μm are substituted into function (2), and the interface recombination rate parameter S is continuously modified. p To fit EBIC efficiency data in the 0-35nm range, such as Figure 5 The surface recombination rate corresponding to the curve with the smallest fitting deviation shown is S. p ≈5.5×10 8 cm / s.

[0038] Another method is to substitute the values ​​of f(x) at different x points and the integral values ​​of F1(x) and F3(x) into formula (3) to calculate S. p .like Figure 6 As shown, S p ≈(4.0~5.9)×10 8 cm / s, take the average value S p ≈5.3×10 9 cm / s. S obtained by both methods p The results are fairly consistent.

[0039] The embodiments described in this specification are merely examples of implementations of the inventive concept and are for illustrative purposes only. The scope of protection of this invention should not be considered limited to the specific forms described in these embodiments; rather, it extends to equivalent technical means conceived by those skilled in the art based on the inventive concept.

Claims

1. A method for measuring the recombination rate at a heterojunction interface using electron beam induced current, characterized in that, By simultaneously considering carrier diffusion and drift caused by the non-uniform built-in electric field in the depletion region, as well as heterojunction interface recombination and depletion region bulk recombination, the following carrier collection probability was obtained: (1)； Integral , S p It is the recombination rate at the heterojunction interface. D n and D p It is the bulk diffusion coefficient of charge carriers. a 0 is a characteristic length related to the doping concentration of the absorption layer. a It is the width of the exhaustion region. These are the drift lengths related to the lifetimes of holes and electrons, respectively. μ p The mobility of the charge carrier holes, μ n Electron mobility; Considering that interfacial recombination plays a much larger role than bulk recombination near the heterojunction interface, equation (1) is simplified to: (2)； It is known that for the known semiconductor materials that make up a heterojunction, the carrier collection probability near the heterojunction interface is only equal to... S p / D p Correlation; diffusion coefficient as a bulk property D p The electron diffusion coefficient can be directly measured through the electrical transport of a single-layer film; and for known materials, the electron diffusion coefficient is... D n and hole diffusion coefficient D p ratio N Convert using effective mass; Obtain the carrier collection probability curve within the heterojunction and calculate the interfacial recombination rate of the heterojunction. S p On the other hand, electron beam induced current technology is used, through the ratio of current... f ( x )= I EBIC / I b Calculate the carrier collection probability f ( x ): (3)。

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