Method and system for testing carrier effective diffusion length

Abstract

The application discloses a kind of carrier effective diffusion length test method and test system.The test method includes: providing first epitaxial structure and second epitaxial structure;Make laser respectively irradiation produce photo-generated carrier, diffuse to multiple quantum well layer and produce recombination luminescence, form photoluminescence spectrum;Based on photoluminescence spectrum, obtain first photoluminescence intensity, second photoluminescence intensity;Based on the correlation with the thickness of corresponding test layer, calculate the carrier effective diffusion length in test layer.The test method provided by the application does not need to cut the fragment, is more convenient for practical application, and is also expected to be used for whole piece multi-point scanning type test;Test cost is low, method is simple, and operability is strong;Evaluation obtained is the carrier effective diffusion length in test layer, reflects the carrier transport capacity of epitaxial layer as a whole;Test obtained is the diffusion length in the direction perpendicular to the surface of epitaxial wafer, is favorable for directly reflecting the carrier transport characteristics of vertical device.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of semiconductor, and particularly relates to a carrier effective diffusion length testing method and a testing system. BACKGROUND

[0002] The third generation semiconductor gallium nitride (GaN) and its ternary alloy indium gallium nitride (InGaN) are currently widely used in semiconductor light-emitting devices, such as semiconductor light-emitting diodes (LEDs), semiconductor laser diodes (LDs), etc. The spectral range of the LED based on GaN and InGaN covers the visible light band, and the LD currently also realizes full coverage of the blue and green light bands. At present, the light-emitting devices based on GaN and InGaN are widely used in new generation full-color display technology, VR glasses, underwater optical communication, laser processing, smart home and even quantum communication fields.

[0003] For the effective diffusion length of carriers in GaN and InGaN epitaxial layers, the traditional test methods are mainly concentrated in the field of electrical testing. For example, the width of the surface space charge region is analyzed by Kelvin probe force microscope (KPFM), and specific references are, for example, Wang YK, Research on Micro-Region Carrier Dynamics of GaN [J]. University of Science and Technology of China, 2022, cathodoluminescence (CL), and specific references are, for example, Hafiz S, Fan Z, Monavarian M, et al. Determination of carrier diffusion length in GaN [J]. Journal of Applied Physics, 2015, 117(1): 2531, and Ino N, Yamamoto N. Low-temperature diffusion length of excitons in gallium nitride measured by cathodoluminescence technique [J]. Applied Physics Letters, 2008, 93(23): L282, etc. means of analysis, there are also surface potential combined with light analysis, time-resolved photoluminescence spectrum (TRPL), and specific references are, for example, Aleksiejunas R, Scajev P, Nargelas S, et al. Impact of Diffusivity to Carrier Recombination Rate in Nitride Semiconductors: From Bulk GaN to (In, Ga) N Quantum Wells [J]. Japanese Journal of Applied Physics, 2013, 52(8 issue 2): 08JK01, to calculate the effective diffusion length of carriers.

[0004] However, the method of analyzing surface potential by using a Kelvin probe force microscope is high in cost, and mainly reflects the carrier diffusion length close to the surface, and is not comprehensive for analyzing the average carrier effective diffusion length in a thick (In) GaN film, especially the diffusion length in the vertical epitaxial layer direction. The CL method is more complicated than the PL method for analyzing the carrier diffusion length, needs to cut the sample to a small size, and has a high requirement for the conductivity of the sample, so the CL analysis method is not suitable for application in batch production detection or product research and development. Similarly, the time-resolved photoluminescence spectrum needs a single photon counter, and even a picosecond or femtosecond pulsed laser, which is high in equipment cost and maintenance cost. SUMMARY

[0005] In view of the deficiencies of the prior art, the purpose of the present application is to provide a carrier effective diffusion length testing method and testing system.

[0006] To achieve the foregoing purposes of the application, the technical solutions adopted by the present application comprise:

[0007] In a first aspect, the present application provides a carrier effective diffusion length testing method, which comprises:

[0008] providing a first epitaxial structure and a second epitaxial structure, both of which comprise a plurality of quantum well layers and a testing layer stacked, the thickness of the testing layer in the first epitaxial structure being different from that of the testing layer in the second epitaxial structure;

[0009] making a first excitation light with a first wavelength respectively incident on the first epitaxial structure and the second epitaxial structure to generate photo-generated carriers in the two testing layers, the photo-generated carriers being capable of making the quantum well layers emit light after diffusing into the corresponding quantum well layers, thereby forming a photoluminescence spectrum, the photon energy of the first excitation light being above the band gap width of the testing layer, and the direct excitation depth of the first excitation light in any testing layer being less than the thickness of the testing layer;

[0010] obtaining a first photoluminescence intensity corresponding to the first epitaxial structure and a second photoluminescence intensity corresponding to the second epitaxial structure based on the photoluminescence spectrum;

[0011] calculating the carrier effective diffusion length in the testing layer based on the correlation between the first photoluminescence intensity, the second photoluminescence intensity and the thickness of the corresponding testing layer.

[0012] In a second aspect, the present application further provides a carrier effective diffusion length testing system for the above-mentioned testing method, which comprises a laser emission module and a photoluminescence spectrum module.

[0013] The laser emission module is used at least for emitting excitation light with a first wavelength to a test sample, the test sample comprising a first epitaxial structure and a second epitaxial structure, so that the first epitaxial structure and the second epitaxial structure generate excited light respectively;

[0014] The photoluminescence spectrum module is used at least for collecting the excited light generated by the first epitaxial structure and the second epitaxial structure, so as to obtain a first photoluminescence intensity corresponding to the first epitaxial structure and a second photoluminescence intensity corresponding to the second epitaxial structure;

[0015] In a further embodiment, the test system further comprises a sample module for fixing the first epitaxial structure and the second epitaxial structure;

[0016] Based on the above technical solution, compared with the prior art, the present application has at least the following beneficial effects:

[0017] The test method provided by the present application can be measured for the whole wafer, without cutting the wafer, and is convenient for practical application, and can also be used for whole wafer multi-point scanning test; the test can be performed by using a laser light source and simple optical instruments, the test cost is low, the method is simple, and the operability is strong; the evaluation result is the effective diffusion length of the carrier in the test layer, which reflects the carrier transport capability of the whole epitaxial layer, rather than only the part close to the surface or close to the multi-quantum well; the carrier effective diffusion length obtained by the test is the diffusion length in the direction perpendicular to the surface of the epitaxial wafer, which is beneficial to directly reflect the carrier transport characteristics of the vertical device.

[0018] The above description is only a summary of the technical solutions of the present application, in order to enable those skilled in the art to more clearly understand the technical means of the present application, and to implement the content of the description, the following is a preferred embodiment of the present application, and the detailed description is as follows. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a structural schematic diagram of the first epitaxial structure or the second epitaxial structure provided by a typical embodiment of the present application.

[0020] Figure 2 is a structural schematic diagram of the verification epitaxial structure provided by a typical embodiment of the present application.

[0021] Figure 3 is a structural schematic diagram of the test system provided by a typical embodiment of the present application.

[0022] Explanation of reference signs: 1, substrate; 2, template layer; 3, lower waveguide layer; 4, multi-quantum well layer; 5, test layer; 6, surface cover layer; 7, barrier layer. DETAILED DESCRIPTION

[0023] In the field of semiconductor, the effective diffusion length of carrier is a very important parameter for certain material layer, for example, in the structure of GaN-based LD device, in order to reduce the light loss of active region, non-doped (In) GaN waveguide layer is needed. At the same time, the carrier transport capacity in non-doped layer restricts the electrical injection efficiency of active region. Therefore, it is necessary to analyze the effective diffusion length of carrier in non-doped (In) GaN waveguide layer, and on this basis, to find suitable epitaxial layer growth process parameters and design the optimal thickness of non-doped waveguide layer in LD structure.

[0024] In view of the deficiencies of the prior art on the testing method of the effective diffusion length of carrier, the present inventors have long-term research and a large number of practices, and thus come up with the technical solution of the present application. The technical solution, the implementation process and principles thereof will be further explained as follows.

[0025] In the following description, a large number of specific details are set forth in order to facilitate a thorough understanding of the present application, however, the present application can also be implemented in other ways different from those described herein, and therefore, the protection scope of the present application is not limited by the specific embodiments disclosed below.

[0026] Moreover, the relationship terms such as "first" and "second" are only used to distinguish one from another with the same name of components or method steps, and do not necessarily require or imply any such actual relationship or sequence between the components or method steps.

[0027] In addition, before the technical solution of the present application is presented, the related technical terms and their definitions in the present application are first presented:

[0028] In x Ga 1-x N: indium gallium nitride ternary alloy, x represents the proportion of In atoms in the total III group metal atoms.

[0029] LD: laser diode, is a device that emits light based on the electromagnetic radiation emitted by the optical amplification process.

[0030] MOCVD: Metal-organic chemical vapor deposition, metal organic chemical vapor deposition.

[0031] Epitaxial layer: a semiconductor thin film with high flatness prepared on the substrate 1 by metal organic chemical vapor deposition method.

[0032] Effective diffusion length of carrier: the distance at which the carrier concentration decreases to 1 / e of the initial concentration under the condition of free diffusion of carriers.

[0033] Photoluminescence (PL): an optical phenomenon where a material emits light as a result of its excitation by photons

[0034] Multiple Quantum Wells (MQWs): in a multilayer structure formed by the growth of thin layers of two different semiconductor materials alternately, if the barrier layer is thick enough so that the coupling between the wave functions of the carriers between adjacent potential wells is small, the multilayer structure will form many separate quantum wells. Carriers are concentrated in quantum wells, and the recombination efficiency is improved.

[0035] After the relevant concepts are clear, see Figure 1 The embodiment of the present application provides a test method for the effective diffusion length of carriers, which comprises the following steps:

[0036] A first epitaxial structure and a second epitaxial structure are provided, and both the first epitaxial structure and the second epitaxial structure comprise a plurality of quantum well layers 4 and a test layer 5 which are arranged in a stack, and the thickness of the test layer 5 in the first epitaxial structure is different from the thickness of the test layer 5 in the second epitaxial structure.

[0037] The first excitation light is used to irradiate the first epitaxial structure and the second epitaxial structure respectively, so that photo-generated carriers are generated in the test layer 5, the photo-generated carriers can generate carrier recombination luminescence after diffusing into the plurality of quantum well layers 4, thereby forming a photoluminescence spectrum, and the photon energy of the first excitation light is above the band gap width of the test layer 5.

[0038] The first photoluminescence intensity corresponding to the first epitaxial structure and the second photoluminescence intensity corresponding to the second epitaxial structure are obtained based on the photoluminescence spectrum.

[0039] Based on the correlation between the first photoluminescence intensity, the second photoluminescence intensity and the thickness of the two test layers 5, the effective diffusion length of the carriers in the test layer 5 is calculated.

[0040] In some embodiments, the thickness of the test layer 5 in the first epitaxial structure and the second epitaxial structure is greater than the direct excitation depth of the first excitation light. Except for the thickness difference of the test layer, the remaining layer structures and materials in the first epitaxial structure and the second epitaxial structure are the same.

[0041] In some embodiments, the test method further comprises:

[0042] The step of verifying whether the thickness of the test layer 5 in the first epitaxial structure and the second epitaxial structure is greater than the direct excitation depth of the first excitation light.

[0043] In some embodiments, the verification specifically comprises:

[0044] A verification epitaxial structure is provided, which comprises a plurality of quantum well layers 4, a barrier layer 7 and a test layer 5 arranged in sequence, the test layer 5 in the verification epitaxial structure, the first epitaxial structure and the second epitaxial structure is of the same material, and the barrier layer 7 can block the diffusion of the photo-generated carriers into the plurality of quantum well layers 4.

[0045] The verification epitaxial structure is irradiated by the first excitation light, and a corresponding photoluminescence spectrum is obtained.

[0046] Whether the thickness of the test layer 5 is greater than the direct excitation depth of the first excitation light is determined by identifying whether the verification epitaxial structure has a characteristic luminescence peak generated by laser excitation of the plurality of quantum well layers 4 in the corresponding photoluminescence spectrum.

[0047] In some embodiments, when the characteristic luminescence peak cannot be identified, it is determined that the thickness of the test layer 5 is greater than the direct excitation depth of the first excitation light.

[0048] Based on the above technical solution, the embodiment of the present application adds a step of verifying whether the laser direct excitation depth matches the thickness of the test layer 5, which helps to improve the accuracy and repeatability of the test. In general tests, laser direct excitation does not easily penetrate the test layer 5, but in some cases, in order to make the obtained results more accurate, it is best to make the thickness difference of the measurement layer more obvious, so some measurement layers with thin thickness may be involved. In this case, the inventors found that the luminescence generated by laser direct excitation of the plurality of quantum well layers 4 significantly affects the statistical accuracy of the actual intensity of carrier recombination luminescence, therefore, in the preferred scheme of the present application, a step of using an epitaxial wafer with a specific structure to verify and avoid the above phenomenon is added.

[0049] In a specific application example, it is determined whether the direct excitation depth of the laser in the u-In x Ga1- x N test layer 5 matches the thickness of the test layer 5 (the laser direct excitation does not extend to the plurality of quantum well layers 4) based on the principle that different wavelengths of laser have different direct excitation depths in the u-In x Ga1- x N test layer 5. In the specific example of the present application, the u-In x Ga1- x N test layer 5 has an In component of not more than 4%, and the photon energy of the laser with a wavelength of 405 nm is less than the band gap of the u-In x Ga1- x N test layer 5, and does not excite interband transition. Therefore, the laser with a wavelength of 405 nm in the u-In x Gal- xThe absorption coefficient in the N testing layer 5 is small, and the laser photon with the wavelength of 325 nm can directly penetrate to the multiple quantum well layer 4 and excite the quantum well to emit light, so as to prove that the multiple quantum well layer 4 can emit light significantly under optical excitation, and the characteristic wavelength range of the direct excitation of the multiple quantum well layer 4 is obtained, which provides a basis for subsequent determination.

[0050] The laser photon with the wavelength of 325 nm has a larger energy than the u-In x Ga1- x The band gap of the N testing layer 5 is small, and the interband transition is excited. Therefore, the laser with the wavelength of 325 nm can directly penetrate to the u-In x Gal- x The absorption coefficient in the N testing layer 5 is large, and the direct excitation depth is usually not more than 100 nm. Therefore, the laser with the wavelength of 325 nm cannot directly penetrate to the multiple quantum well layer 4 to excite the light emission thereof, but excites the u-In x Ga1- x The photo-generated carriers (including two types of carriers, i.e., electrons and holes) are excited in the N testing layer 5. The excited photo-generated carriers enter the multiple quantum well layer 4 by free diffusion and recombine, so that the multiple quantum well layer 4 also emits light. At this time, since the u-Al x Ga 1-x The band gap of the N carrier blocking layer 7 is large, and a higher potential barrier is provided to block the u-In x Ga1- x The photo-generated carriers in the N testing layer 5 freely diffuse into the multiple quantum well layer 4, and the interference caused by the recombination of the carriers in the multiple quantum well layer 4 is avoided. Therefore, when the u-In x Ga1- x When the thickness of the N testing layer 5 is greater than the direct excitation depth of the laser with the wavelength of 325 nm therein, the laser with the wavelength of 325 nm is used to irradiate Figure 2 The verification epitaxial structure shown can hardly emit light, and thus it can be verified whether the thickness of the N testing layer 5 corresponding to the verification epitaxial structure is greater than the direct excitation depth.

[0051] In actual use, the thickness of the N testing layer 5 of the verification epitaxial structure is preferably less than or equal to (most preferably equal to) the minimum value of the thickness of the N testing layer 5 in the first epitaxial structure and the second epitaxial structure. In this way, the verification structure of the verification epitaxial structure can directly guide whether the first and second epitaxial structures will have direct excitation interference.

[0052] A more preferred embodiment can further provide a plurality of verification epitaxial structures with different thicknesses of the N testing layer 5, so as to statistically obtain the specific value of the actual direct excitation depth of the N testing layer 5 for a certain material and growth process, and the corresponding thicknesses of the first epitaxial structure and the second epitaxial structure are set on the basis of the value, which also belongs to the verification category.

[0053] In some embodiments, the verifying can further include:

[0054] illuminating the verifying epitaxial structure with a second excitation light having a second wavelength, obtaining a corresponding photoluminescence spectrum, thereby obtaining a wavelength range corresponding to the characteristic photoluminescence peak.

[0055] The photon energy of the second excitation light is less than the band gap width of the test layer 5.

[0056] In some embodiments, the test layer 5 of the first epitaxial structure and the second epitaxial structure both have a thickness greater than 100 nm.

[0057] In some embodiments, the material of the test layer 5 includes In x Ga 1-x N, where x ranges from 0 to 0.06.

[0058] In some embodiments, the test layer 5 is undoped.

[0059] In some embodiments, the first wavelength is 260-365 nm, preferably for example 325 nm, and the second wavelength is 400-430 nm, preferably for example 405 nm. Of course, the specific numerical range of the first wavelength and the second wavelength is affected by the material of the test layer 5, and the specific selection can be adjusted based on the above rules, and appropriate adjustments should also be within the protection scope of the present application.

[0060] In some embodiments, the first photoluminescence intensity and the second photoluminescence intensity are determined based on the logarithmic value of the integral area or the peak height of the corresponding photoluminescence peak in the photoluminescence spectrum. Using the technical solutions provided by the present application, quantitative testing can be realized more accurately by peak area calculation, and the approximate situation of the effective carrier diffusion distance can also be qualitatively reflected based on peak height calculation. Whether it is quantitative determination or qualitative reflection of the approximate situation, both utilize the inventive concept provided by the present application and are also within the protection scope of the present application.

[0061] In some embodiments, the correlation is a linear correlation.

[0062] In some embodiments, the calculation method of the effective carrier diffusion length can be, for example:

[0063] L diff = (lnI1-InI2) / (d2-d1)

[0064] wherein L diffrepresents the effective diffusion length of the carriers; I1 represents the integral area of the photoluminescence peak corresponding to the first epitaxial structure; I2 represents the integral area of the photoluminescence peak corresponding to the second epitaxial structure; d1 represents the thickness of the test layer 5 of the first epitaxial structure; d2 represents the thickness of the test layer 5 of the second epitaxial structure.

[0065] Or in other embodiments, the effective diffusion length of the carriers can be calculated by calculating the linear regression slope of the logarithmic values of the integral areas of a plurality of epitaxial structures and the thicknesses of the corresponding test layers 5, and taking the inverse of the linear regression slope as the effective diffusion length of the carriers.

[0066] In some embodiments, the first epitaxial structure and the second epitaxial structure further comprise a template layer 2, a light confinement layer, and a lower waveguide layer 3 arranged in sequence, and the multi-quantum well layer 4 is arranged on the side of the lower waveguide layer 3 away from the substrate 1.

[0067] In some embodiments, the surface of the test layer 5 is further covered with a surface cap layer 6.

[0068] As some typical application examples of the above-mentioned embodiments, the following drawings are attached Figure 1 The schematic diagram of the epitaxial wafer structure for testing the effective diffusion length of the carriers in the undoped (In)GaN waveguide layer (i.e. the first epitaxial structure and the second epitaxial structure mentioned above) is shown from bottom to top as follows: n-GaN template layer 2 grown on C-sapphire substrate 1, n-Al x Ga 1-x N light confinement layer, n-In x Ga 1-x N lower waveguide layer 3, In x Ga 1-x N / GaN multi-quantum well layer 4, undoped u-In x Ga 1-x N test layer 5, u-Al x Ga 1-x N surface cap layer 6.

[0069] The method for manufacturing the above-mentioned epitaxial structure can be as follows: the thickness of the n-GaN template layer 2 grown on C-sapphire by MOCVD is 4-5 μm, and its electron concentration is 0.5-2 x 10 19 cm -3 The thickness of the n-Al x Ga 1-x N confinement layer is 800 nm-1500 nm, the Al component is 5%-10%, and its electron concentration is 10 17 cm -3 to 10 20 cm -3 The thickness of the n-In x Ga1-x The thickness of waveguide layer 3 under N is 50 nm to 150 nm, the In composition is 2% to 6%, and its electron concentration is 10. 17 cm -3 Up to 10 20 cm -3 Between; multiple quantum well layers 4 are undoped In with 1 to 6 periods. x Ga l-x N / GaN quantum well, In x Ga 1-x The thickness of N-quantum wells ranges from 1 nm to 6 nm, with an In composition of 10% to 35%. The thickness of GaN quantum barriers ranges from 2 nm to 20 nm; undoped u-In... x Ga1- x The thickness of the N test layer 5 is 150nm–250nm, and the In composition is 0%–4%; u-Al x Ga 1-x The thickness of the N surface capping layer 6 is approximately 10 nm to 20 nm, and the Al content is approximately 10% to 30%.

[0070] Among them, u-Al x Ga 1-x The introduction of the N-surface capping layer 6 is to suppress the impact of carrier recombination on test accuracy. As for the template layer 2, the barrier layer 7, and the lower waveguide layer 3, they are naturally present as common components of normal devices, but they are not entirely indispensable. For example, omitting one or more of these layers simply to study the material and growth process of the test layer 5 will still produce the same carrier recombination luminescence phenomenon, and the corresponding data can still be obtained. However, testing with all layers present is still the most recommended approach, as this complete device structure test closely resembles the actual device structure and growth environment, and the obtained data and patterns are more consistent with real-world devices.

[0071] By adjusting the MOCVD epitaxial growth parameters such as the growth temperature and growth pressure of the test layer 5, u-In crystals with different qualities can be obtained. x Ga1- x N test layer 5, respectively, is used as the first or second epitaxial structure or even multiple epitaxial structures to perform the above measurements, thereby enabling the study of the influence of growth conditions or material composition and microstructure on the effective diffusion length, especially the diffusion length in the direction perpendicular to the thickness of the film.

[0072] Figure 2 This is a schematic diagram of the epitaxial structure for testing the laser excitation depth (i.e., the verification epitaxial structure), and... Figure 1 Similar, but in multiple quantum wells and u-In x Ga1- x Undoped u-Al was added between the N test layers 5.x Ga 1-x N carrier blocking layer 7, with a thickness of 20-30 nm and an Al component of about 10%-30%. The purpose of adding this layer is to block the diffusion of carriers into the multiple quantum well layer 4 to occur recombination luminescence, thereby affecting the test of the depth of direct laser excitation, and the layer, due to its thinness, will not have too much interference with the direct excitation of the laser, and can more accurately reflect whether the corresponding laser will penetrate the test layer 5 with a specific thickness and act on the multiple quantum well layer 4, so as to obtain the minimum limit condition of the thickness of the test layer 5 of the first and second epitaxial structures.

[0073] Based on the summary of the above examples, in some embodiments, the test method can further include:

[0074] The excitation light with the second wavelength is respectively incident on any one of the first and second epitaxial structures, and the excited light signal generated by the corresponding epitaxial structure is collected, and when the excited light signal can identify a characteristic peak and the identified characteristic peak matches the designed luminescence wavelength band of the multiple quantum well, it is determined that the multiple quantum well layer in the corresponding epitaxial structure can work normally. This step is mainly to determine whether the multiple quantum well layer can normally emit light to generate the above-mentioned signal.

[0075] Corresponding to the above-mentioned test method, the present application also provides a carrier effective diffusion length test system for executing the above-mentioned test method, which includes a laser emission module, a sample module and a photoluminescence spectrum module.

[0076] The laser emission module is used to emit first excitation light to the sample module.

[0077] The sample module is used to fix the first and second epitaxial structures.

[0078] The photoluminescence spectrum module is used to detect the photoluminescence spectrum generated by the first and second epitaxial structures after being irradiated by the laser.

[0079] In some embodiments, the laser emitted by the laser emission module is obliquely irradiated to the first and second epitaxial structures.

[0080] In some embodiments, the oblique angle of the laser incidence is 30-70°. The purpose of setting the above-mentioned angle is to reduce the reflection, and on the other hand, because the laser source and the lens group cannot be placed at the same angle.

[0081] In some embodiments, the laser emitting module is further configured to emit a second excitation light to the sample module, the second wavelength being greater than the first wavelength; and the sample module is further configured to fix a verification epitaxial structure, the verification epitaxial structure being configured to determine whether the thickness of the corresponding test layer 5 is greater than the direct excitation depth of the laser.

[0082] The present application analyzes the effective diffusion length of carriers in the vertical direction of the epitaxial layer by designing the (In)GaN epitaxial wafer structure and combining the PL measurement at room temperature.

[0083] It should be noted that the material of the substrate 1 is gallium nitride, sapphire, silicon carbide or silicon, and is not limited thereto. The material of the substrate 1 has little significant effect on the test of the present application. The epitaxial wafer structure and the test method provided by the present application can be slightly modified, such as simple exchange or deletion and insertion of epitaxial layers, but almost no structural changes affect the test performance of the epitaxial wafer in the test method. The material of the test layer 5 can include GaN, InGaN, AlGaN, AlInGaN and other single or multi-alloy nitrides, which can generate related carrier excitation and transmission functions to achieve the above test. The order of the test steps in the present patent is not affected by the test effect, such as the order of verification and test. In addition, the test scheme using cathodoluminescence CL or other wavelength laser to excite the epitaxial wafer to emit light, but the epitaxial wafer structure or test steps are basically the same as those of the present patent, also belongs to the protection scope of the present application.

[0084] The technical solutions of the present application will be further described in detail below through several embodiments in combination with the drawings. However, the selected embodiments are only used to illustrate the present application, and do not limit the scope of the present application.

[0085] Embodiment 1

[0086] This embodiment illustrates the analysis of u-In x Ga1- x The flow of testing the carrier effective diffusion length of the test layer 5 is as follows:

[0087] Select a suitable laser light source. It is recommended to select a semiconductor laser with a laser wavelength of 405 nm and a He-Cd laser with a laser wavelength of 325 nm.

[0088] Build an optical path, including a sample clamp, a lens group, a filter, and a light probe. The optical path structure and sample placement are as shown in Figure 3 The light emitted by the laser light source is obliquely incident on the sample surface, and the excited light of the sample is collected through the lens group. The collected light passes through the filter to filter out the influence of the laser light source. Finally, the spectrum of the excited light of the sample is collected by the fiber spectrometer.

[0089] Use Figure 2The laser shown in the epitaxial wafer sample test u-In x Ga1- x N test layer 5 matches the thickness of the test layer 5. First, a laser with a wavelength of 405 nm is used as a laser light source for testing, and the spectral information collected by the spectrometer is the light emitted by the multi-quantum well layer 4. The light signal should be significantly stronger than the background noise signal and match the designed light emission band of the multi-quantum well layer 4 to prove that the multi-quantum well layer 4 can work normally, and the direct excitation wavelength range of the multi-quantum well layer 4 is obtained.

[0090] Then use a laser with a wavelength of 325 nm as a laser light source for testing, and the spectral information obtained at this time should be visible corresponding to the u-In x Ga1- x N test layer 5 band edge emission (the emission peak is about 365 nm-385 nm). The light emission of the multi-quantum well layer 4 in the spectral information obtained by testing with a 325 nm laser as a laser light source should be very weak, and the light signal is almost completely submerged in the background noise signal. In this way, it can be proved that the laser with a wavelength of 325 nm in the u-In x Ga1- x N test layer 5 is less than the thickness of the test layer 5, and the two epitaxial wafers (with the same test layer 5 material) corresponding to the thickness above can be tested subsequently.

[0091] According to the structure shown in Figure 1 Two epitaxial wafer samples A and B are prepared according to the structure shown in A and B u-In x Ga1- x N test layer 5 are d1 and d2 respectively (d1 and d2 need to be at least greater than 100 nm). A laser light source with a wavelength of 325 nm is used to measure the two samples according to step 2, and the photoluminescence spectrum collected by the spectrometer is recorded (the horizontal axis is the wavelength, and the vertical axis is the light intensity). The light emission part (about 420 nm-500 nm) of the multi-quantum well layer 4 in the photoluminescence spectrum of samples A and B is integrated to obtain integral values I1 and I2. According to the formula, the u-In x Ga1- x N test layer 5 carrier effective diffusion length L diff :

[0092] L diff =(lnI1-InI2) / (d2-d1)

[0093] The carrier effective diffusion length L diff reflects the u-In x Ga1- xN test layer 5 carrier transport capacity, the larger the diffusion length is stronger, then more help to improve the carrier injection efficiency of GaN-based LD.

[0094] In a very specific test, the test layer 5 material for the specific u-In x Ga1- x N (x = 0.025), the final test of different epitaxial growth under the test layer carrier effective diffusion length of 225 nm ~ 793 nm, and the other literature disclosed in the range of 200-1000 nm diffusion length, the test results are mutually matching, to prove the test is relatively accurate; and the diffusion length is the film thickness direction of the diffusion length, the research and development design of the material layer has very important guiding significance.

[0095] Example 2

[0096] This embodiment is generally the same as example 1, the main difference is:

[0097] The number of extension epitaxial wafer samples is provided, and a plurality of different numbers of epitaxial wafers are provided, for example, 5 epitaxial wafers of different thicknesses. According to the data obtained by testing, the correlation graph is obtained by plotting lnI as the ordinate and d as the abscissa and linear regression. The reciprocal of the slope of the correlation is the effective carrier diffusion length.

[0098] This test method is more accurate than the above embodiment.

[0099] Example 3

[0100] The MOCVD epitaxial growth of u-InxGal-xN test layer 5 (x = 0.025) is adopted, and the V / III ratio in the epitaxial growth conditions is selected as a variable, and the change range is 2000 ~ 45000. The thickness of the test layer 5 is selected as 200 nm and 240 nm, and the carrier diffusion length obtained by the test method of the patent is 225 ~ 793 nm. Since the carrier diffusion length of InGaN material is less reported and the In component is lower in this embodiment, the reported results of GaN are referred to. In the literature report [1], the u-GaN diffusion length analyzed by other methods is 201 nm. In the literature report [2], the u-GaN diffusion length analyzed by other methods is 525 nm, and in the literature report [3], the n-GaN diffusion length analyzed by other methods is 920 nm.

[0101] [1]Ino N,Yamamoto N.Low temperature diffusion length of excitons ingallium nitride measured by cathodoluminescence technique[J].Applied PhysicsLetters,2008,93(23):L282.

[0102] [2]Hafiz S,Fan Z,Monavarian M,et al.Determination of carrierdiffusion length in GaN[J].Journal ofApplied Physics,2015,117(1):2531.

[0103] [3]Y.Lin,E.Flitsyian,L.Chernyak,T.Malinauskas,R.Aleksiejunas,K.Jarasiunas,W.Lim,S.J.Pearton,and K.Gartsman,Appl.Phys.Lett.95,092101(2009).

[0104] Based on the above examples, it can be clear that the embodiments provided by the present application effectively analyze the carrier transport ability of the test layer 5 in the vertical direction by designing two different GaN epitaxial wafer structures and using non-destructive photoluminescence measurement methods. It is helpful for the research, optimization and design of key epitaxial layers of GaN lasers and other devices.

[0105] The present application relies on the photoluminescence effect excited by a laser light source as the main method for analyzing the effective diffusion length of carriers, and uses the GaN epitaxial wafer structure shown in Figure 1 and Figure 2 and the calculation method introduced in the patent to analyze the effective diffusion length of carriers.

[0106] It should be understood that the above examples are only for illustrating the technical concepts and characteristics of the present application, and the purpose is to enable those skilled in the art to understand the content of the present application and implement it, and cannot limit the protection scope of the present application. Any equivalent changes or modifications made in accordance with the spirit and essence of the present application should be covered within the protection scope of the present application.

Claims

1. A method of testing the effective diffusion length of carriers, characterized in that, The application comprises the following steps: providing a first epitaxial structure and a second epitaxial structure, each of which comprises a plurality of quantum well layers and a test layer arranged in a stack, the thickness of the test layer in the first epitaxial structure being different from that of the second epitaxial structure; making a first excitation light with a first wavelength respectively incident on the first epitaxial structure and the second epitaxial structure to generate photo-generated carriers in the two test layers, the photo-generated carriers being capable of causing the quantum well layers to emit light after diffusing into the corresponding quantum well layers, thereby forming a photoluminescence spectrum, the photon energy of the first excitation light being above the band gap width of the test layer, and the direct excitation depth of the first excitation light in any test layer being less than the thickness of the test layer; obtaining a first photoluminescence intensity corresponding to the first epitaxial structure and a second photoluminescence intensity corresponding to the second epitaxial structure based on the photoluminescence spectrum; verifying whether the thickness of the test layer in the first epitaxial structure and the second epitaxial structure is greater than the direct excitation depth of the first excitation light, comprising: providing a verification epitaxial structure, the verification epitaxial structure comprising a plurality of quantum well layers, a barrier layer and a test layer arranged in a stack, the material of the test layer in the verification epitaxial structure, the first epitaxial structure and the second epitaxial structure being the same, the barrier layer being used to block the diffusion of the photo-generated carriers into the quantum well layers; irradiating the verification epitaxial structure with the first excitation light to obtain a corresponding photoluminescence spectrum; and judging whether the thickness of the test layer is greater than the direct excitation depth of the first excitation light by identifying whether the photoluminescence spectrum corresponding to the verification epitaxial structure contains a characteristic luminescence peak generated by laser excitation of the quantum well layer; irradiating the verification epitaxial structure with a second excitation light with a second wavelength to obtain a corresponding photoluminescence spectrum, thereby obtaining a wavelength range corresponding to the characteristic luminescence peak; the photon energy of the second excitation light being less than the band gap width of the test layer; calculating the effective diffusion length of the carriers in the test layer based on the correlation between the first photoluminescence intensity, the second photoluminescence intensity and the thickness of the two test layers.

2. The test method of claim 1, wherein, Except for the thickness difference of the test layer, the remaining layers and materials of the first epitaxial structure and the second epitaxial structure are the same.

3. The test method of claim 1, wherein, When the characteristic luminescence peak cannot be identified, it is determined that the thickness of the test layer is greater than the direct excitation depth of the first excitation light.

4. The test method of claim 3, wherein, Further comprising: irradiating any of the first epitaxial structure and the second epitaxial structure with the second excitation light, collecting the excited light signal generated by the corresponding epitaxial structure, and determining that the quantum well layer in the corresponding epitaxial structure can work normally when the excited light signal can identify a characteristic peak and the identified characteristic peak matches the designed luminescence wavelength band of the quantum well layer.

5. The test method of claim 4, wherein, The thickness of the test layer of the first epitaxial structure and the second epitaxial structure is greater than 100 nm; and / or, the material of the test layer comprises InxGa1-xN, wherein the value range of x is 0-0.06; and / or, the test layer is undoped; the first wavelength is 260-365 nm, and the second wavelength is 400-430 nm.

6. The test method of claim 1, wherein, The first photoluminescence intensity and the second photoluminescence intensity are determined based on integral areas or logarithmic values of peak heights of corresponding photoluminescence peaks in the photoluminescence spectrum; and the correlation is a linear correlation.

7. The test method of claim 6, wherein, The carrier effective diffusion length is calculated in the following manner: Ldiff=(lnI1-InI2) / (d2-d1), wherein Ldiff represents the carrier effective diffusion length; I1 represents an integral area of a photoluminescence peak corresponding to the first epitaxial structure; I2 represents an integral area of a photoluminescence peak corresponding to the second epitaxial structure; d1 represents a thickness of a test layer of the first epitaxial structure; and d2 represents a thickness of a test layer of the second epitaxial structure; or a linear regression slope of logarithmic values of integral areas of a plurality of epitaxial structures and corresponding thicknesses of test layers is calculated, and an inverse of the linear regression slope is taken as the carrier effective diffusion length.

8. The test method of claim 1, wherein, The first epitaxial structure and the second epitaxial structure further include a template layer, a light confinement layer and a lower waveguide layer which are sequentially stacked, and the multi-quantum well layer is arranged on a side of the lower waveguide layer away from the substrate; and a surface of the test layer is further covered with a surface cover layer.

9. A test system for carrier effective diffusion length for performing the test method of any one of claims 1 to 8, characterized by The test system includes a laser emission module and a photoluminescence spectrum detection module; the laser emission module is used at least for emitting excitation light with a first wavelength to a test sample, the test sample including a first epitaxial structure and a second epitaxial structure, so that the first epitaxial structure and the second epitaxial structure generate excited light respectively; The photoluminescence spectrum detection module is used at least for collecting the excited light generated by the first epitaxial structure and the second epitaxial structure, so as to obtain a first photoluminescence intensity corresponding to the first epitaxial structure and a second photoluminescence intensity corresponding to the second epitaxial structure; the test system further includes a sample module, the sample module being used for fixing the first epitaxial structure and the second epitaxial structure; the laser emitted by the laser emission module is obliquely incident on the first epitaxial structure and the second epitaxial structure; an oblique angle of the laser incidence is 30-70°; the laser emission module is further used for emitting second excitation light with a second wavelength to the sample module, the second wavelength being greater than the first wavelength; and the sample module is further used for fixing a verification epitaxial structure, the verification epitaxial structure being used for judging whether a thickness of a corresponding test layer is greater than a direct excitation depth of the first excitation light.

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