Sample preparation method for Micro-LED section surface potential test and application thereof

Abstract

The invention discloses a sample preparation method for Micro-LED cross section surface potential test and application of the sample preparation method. The Micro-LED cross section active region surface potential testing method comprises the following steps: preparing a Micro-LED cross section sample, and polishing the cross section of the sample by using an argon ion beam polishing process to form a test sample with the surface roughness of nanoscale; a Kelvin probe microscope (KPFM) is used for testing the surface potential of the test sample, and the KPFM not only can represent the surface appearance of the sample, but also can obtain the electrical property-surface potential of the surface of the sample by manufacturing a section sample and simulating the etching process, so that the difference of the surface potential of different areas can be compared more intuitively. Furthermore, the work function of the semiconductor material can be measured and calculated, and meanwhile, an energy band model can be established to analyze the transport condition of carriers on the surface.

Description

Technical Field

[0001] The present invention particularly relates to a sample preparation method for testing the surface potential of a Micro-LED cross section and an application thereof, belonging to the technical field of Micro-LED detection. Background Art

[0002] At present, the research on the optoelectronic performance of Micro-LED mainly uses macroscopic electrical methods to test external quantum efficiency, IV curve, etc. There is a lack of direct characterization of the sidewall region with high spatial resolution, and high-resolution characterization technology is now needed to study the sidewall defects and non-radiative recombination mechanisms at the sidewall. Summary of the invention

[0003] The main purpose of the present invention is to provide a sample preparation method for Micro-LED cross-sectional surface potential testing and its application, so as to overcome the deficiencies in the prior art.

[0004] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention includes:

[0005] A first aspect of an embodiment of the present invention provides a sample preparation method for testing a surface potential of a Micro-LED cross section, comprising:

[0006] Cutting the block-shaped Micro-LED epitaxial wafer according to the GaN [10-10] or [11-20] crystal zone axis to form two sample blocks, wherein the sample block includes a p-type region, an active region, and an n-type region sequentially arranged in a direction away from the C-plane, and the cross section formed by cutting includes the side walls of the p-type region, the active region, and the n-type region;

[0007] Bonding the C surfaces of the two sample blocks, with the cross sections of the two sample blocks located on the same side, and controlling the width of the bonding seam between the C surfaces of the two sample blocks to be within 1 μm, thereby obtaining a bonded sample;

[0008] The selected surface including the cross section of the bonded sample is polished until the roughness of the selected surface reaches the nanometer level, thereby obtaining a Micro-LED cross-sectional sample, wherein the interface to be measured of the Micro-LED cross-sectional sample includes the two cross sections.

[0009] A second aspect of an embodiment of the present invention provides a method for testing the surface potential of an active area of ​​a Micro-LED cross section, comprising:

[0010] Providing a Micro-LED cross-section sample obtained by the sample preparation method for testing the surface potential of the Micro-LED cross-section;

[0011] The surface potential of the interface to be measured of the Micro-LED cross-sectional sample is obtained using a Kelvin probe microscope.

[0012] A third aspect of an embodiment of the present invention provides a method for evaluating carrier transport conditions in an active region of a Micro-LED cross section, comprising:

[0013] Providing a Micro-LED cross-section sample obtained by the sample preparation method for testing the surface potential of the Micro-LED cross-section;

[0014] The surface potential of the interface to be tested of the Micro-LED cross-section sample under light irradiation and no light irradiation conditions is obtained by using the test method of the surface potential of the active area of ​​the Micro-LED cross-section;

[0015] The surface potential obtained under light irradiation conditions is subtracted from the surface potential obtained under light irradiation conditions to obtain a surface photovoltage spectrum of the interface to be measured;

[0016] The carrier transport condition is determined according to the surface photovoltage spectrum, wherein the rising edge of the surface photovoltage spectrum corresponds to the transport of electrons to the surface.

[0017] Compared with the prior art, the advantages of the present invention include:

[0018] The present invention provides a method for testing the surface potential of the active area of ​​a Micro-LED cross section, which can not only characterize the surface morphology of the sample, but also obtain the electrical performance-surface potential mapping diagram of the sample surface, which can more intuitively compare the differences in surface potentials in different areas, and then calculate the work function of the semiconductor material. At the same time, an energy band model can be established to analyze the transport of carriers at the surface.

[0019] Compared with the electrochemical method and the capacitance-voltage method, the Kelvin probe microscope test can measure the surface potential without contacting the sample surface. The test method for the surface potential of the active area of ​​a Micro-LED cross section provided by the present invention is extremely sensitive to slight changes in the surface state and has high measurement accuracy.

[0020] The present invention provides a method for testing the surface potential of the active area of ​​a Micro-LED cross section. The method can make the semiconductor sample and the needle tip sample conductive by applying metal such as indium or silver paste, instead of plating electrodes on the sample, thereby avoiding contamination of the sample during the electrode plating process. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a sample preparation flow chart for testing the surface potential of a Micro-LED cross section provided in a typical implementation case of the present invention;

[0022] FIG2 is a schematic structural diagram of a Micro-LED cross-section sample provided in a typical implementation case of the present invention;

[0023] Figure 3 It is a schematic diagram of the principle of testing a Micro-LED cross-section sample using a Kelvin probe microscope in a typical implementation case of the present invention;

[0024] Figure 4 It is a test parameter interface of a Kelvin probe microscope in a typical implementation case of the present invention. DETAILED DESCRIPTION

[0025] In view of the deficiencies in the prior art, the inventor of this case has proposed the technical solution of the present invention after long-term research and extensive practice. The technical solution, its implementation process and principle will be further explained as follows.

[0026] A first aspect of an embodiment of the present invention provides a sample preparation method for testing a surface potential of a Micro-LED cross section, comprising:

[0027] Cutting the block-shaped Micro-LED epitaxial wafer according to the GaN [10-10] or [11-20] crystal zone axis to form two sample blocks, wherein the sample block includes a p-type region, an active region, and an n-type region sequentially arranged in a direction away from the C-plane, and the cross section formed by cutting includes the side walls of the p-type region, the active region, and the n-type region;

[0028] Bonding the C surfaces of the two sample blocks, with the cross sections of the two sample blocks located on the same side, and controlling the width of the bonding seam between the C surfaces of the two sample blocks to be within 1 μm, thereby obtaining a bonded sample;

[0029] The selected surface including the cross section of the bonded sample is polished until the roughness of the selected surface reaches the nanometer level, thereby obtaining a Micro-LED cross-sectional sample, wherein the interface to be measured of the Micro-LED cross-sectional sample includes the two cross sections.

[0030] In a more specific embodiment, the sample preparation method for the Micro-LED cross-sectional surface potential test specifically includes: coating glue containing epoxy resin on the C surfaces of two sample blocks to bond the C surfaces of the two sample blocks, and applying external force along the direction of the two sample blocks docking to control the width of the bonding seam between the C surfaces of the two sample blocks to be within 1 μm.

[0031] In a more specific embodiment, the sample preparation method for the Micro-LED cross-sectional surface potential test specifically includes: first grinding the selected surface including the cross-section of the bonded sample until the selected surface is free of scratches and the active area portions that are bright on both sides of the bonded seam can be observed; and then polishing the selected surface.

[0032] Furthermore, the grinding process specifically includes: grinding the selected surface multiple times, and the roughness of the grinding tool used in the multiple grindings is gradually reduced.

[0033] Furthermore, the roughness of the grinding tool is 0.5 μm to 30 μm.

[0034] Furthermore, the polishing process specifically includes: polishing the selected surface using an argon ion beam.

[0035] Furthermore, the roughness of the selected surface after polishing satisfies: the surface average roughness Ra within the range of 2 μm×2 μm is less than 1.0 nm.

[0036] A second aspect of an embodiment of the present invention provides a method for testing the surface potential of an active area of ​​a Micro-LED cross section, comprising:

[0037] Providing a Micro-LED cross-section sample obtained by the sample preparation method for testing the surface potential of the Micro-LED cross-section;

[0038] The surface potential of the interface to be measured of the Micro-LED cross-sectional sample is obtained using a Kelvin probe microscope.

[0039] Furthermore, the method for testing the surface potential of the active area of ​​the Micro-LED cross section is characterized by specifically comprising:

[0040] Fix the side of the Micro-LED cross-section sample facing away from the interface to be tested on the metal sheet using a conductive material, and electrically connect the edge of the interface to be tested of the Micro-LED cross-section sample to the metal sheet below;

[0041] When scanning the interface to be tested of the Micro-LED cross-section sample with the probe of the Kelvin probe microscope, the set point of the Kelvin probe microscope is set to 200mV~400mV, the scanning rate is set to 0.5Hz~1Hz, the integral gain input is set to 0.5~1.0, the proportional gain input is 20%~100% larger than the integral gain input, and the lifting scanning height is set to 30nm~100nm to obtain the morphological image and potential image of the interface to be tested, wherein the potential image includes the surface potential of multiple areas of the interface to be tested.

[0042] A third aspect of an embodiment of the present invention provides a method for evaluating carrier transport conditions in an active region of a Micro-LED cross section, comprising:

[0043] Providing a Micro-LED cross-section sample obtained by the sample preparation method for testing the surface potential of the Micro-LED cross-section;

[0044] The surface potential of the interface to be tested of the Micro-LED cross-section sample under light irradiation and no light irradiation conditions is obtained by using the test method of the surface potential of the active area of ​​the Micro-LED cross-section;

[0045] The surface potential obtained under light irradiation conditions is subtracted from the surface potential obtained under light irradiation conditions to obtain a surface photovoltage spectrum of the interface to be measured;

[0046] The carrier transport condition is determined according to the surface photovoltage spectrum, wherein the rising edge of the surface photovoltage spectrum corresponds to the transport of electrons to the surface.

[0047] The technical solution, its implementation process and principle will be further explained below in conjunction with the accompanying drawings and specific implementation cases.

[0048] See also Figure 1 A sample preparation method for testing the surface potential of a Micro-LED cross section comprises the following steps:

[0049] Prepare the sample to be tested (Micro-LED epitaxial wafer);

[0050] The sample to be tested is cut according to the GaN [10-10] or [11-20] crystal zone axis to form a sample block of 1mm×3mm, and the sample block is strictly cleaned. The cleaning process and the cleaning agent used are all known in the art and are not limited here. The sample block obtained by cutting includes a p-type region (also referred to as a p-type layer, the same below), an active region (also referred to as an active layer, the same below), an n-type region (also referred to as an n-type layer, the same below), and a substrate arranged in sequence in a direction away from the C-plane. The cross section formed by cutting the sample block includes the side walls of the p-type region, the active region, and the n-type region;

[0051] Using glue containing epoxy resin to bond the C surfaces of the two sample blocks, the cross sections of the two sample blocks are located on the same side, and clamping the two bonded sample blocks with a clamp to control the width of the bonding seam between the C surfaces of the two bonded sample blocks to be within 1 μm, thereby obtaining a bonded sample;

[0052] The bonded sample was polished using sandpaper with a roughness of 30 μm, 9 μm, 6 μm, 3 μm, 1 μm, and 0.5 μm in sequence until there were no scratches on the surface of the bonded sample under an optical microscope with a magnification of 100X, and the shiny active area on both sides of the bonded seam could be observed;

[0053] The bonded sample was bonded to the baffle and polished with an argon ion beam for more than 20 hours until the roughness of the bonded seam area reached the nanometer level: the surface average roughness Ra within the range of 2μm×2μm was less than 1.0nm, and polishing was stopped to obtain a Micro-LED cross-sectional sample, such as Figure 2a , Figure 2b As shown, the interface to be tested of the Micro-LED cross-section sample includes two cross-sections.

[0054] It should be noted that cross-sectional ion beam polishing using argon ions is suitable for making areas in the millimeter range. The present invention requires the preparation of samples with a measurable range as large as possible. At the same time, the polishing damage of the argon ion beam is smaller than that of FIB. The polishing time affects the measurable range. The purpose of the present invention is to make the polishing range reach the active area on the other side of the bonding seam. According to the results obtained from the experiment of the present invention, the Micro-LED sample of the sapphire substrate needs to be polished for at least 20 hours from the substrate side to achieve the aforementioned average roughness requirement.

[0055] Specifically, when making semiconductor cross-section samples, argon ion beam polishing is a common method. Although other inert gases may also be used in some cases, argon is widely used due to its atomic properties, safety and cost. Argon is an inert gas whose atoms are relatively heavy. The mass of argon atoms is about 40u (atomic mass unit). Heavier atoms have greater momentum at the same energy and can remove materials more effectively when bombarding the sample surface. In contrast, helium (He) has an atomic mass of about 4u, which is lighter and has a relatively low sputtering efficiency at the same acceleration voltage. The mass of krypton atoms is about 84u, and the mass of xenon atoms is about 131u. From the perspective of atomic mass, they can also be used for ion beam polishing in theory, because their heavy atoms can also effectively remove materials during physical sputtering. However, their cost is relatively high, and in practical applications, except for some special research requirements (such as the preparation of certain high-precision, high-value samples, and insensitive to cost), they are generally less used. Use a relatively low energy (e.g. 500-1000 eV) argon ion beam, because the physical bombardment of the sample surface by the low energy ion beam is relatively mild, which can reduce the damage to the internal microstructure (e.g. lattice structure) of the semiconductor material during the polishing process. The beam current density range of the argon ion beam is 5 μA / cm 2 -20μA / cm 2 . Lower beam density can achieve finer polishing.

[0056] Specifically, for the Micro-LED cross-sectional samples, the present invention mainly focuses on the surface potential of the n-type region, the p-type region and the side wall of the active region. However, since the p-type region and the active region of the Micro-LED are close to the c-surface layer, the active region layer is very thin for the entire cross-sectional area. Generally, the cross-sectional sample preparation can be obtained by direct cleavage, FIB processing or argon ion polishing. However, if the previous method for making cross-sectional samples is adopted, for the Micro-LED cross-sectional sample test, the problem of needle skipping will occur near the surface layer, and the surface potential of the active region cannot be effectively measured. Specifically, when the Micro-LED cross-sectional sample is measured using a Kelvin probe force microscope (KPFM), the thickness of the active region will affect the interaction between the probe and the sample surface. When the active region is too thin, the probe may not be able to stably contact the sample surface during the scanning process, and is easily disturbed by external micro-vibrations, changes in the surface roughness of the sample, and the elasticity of the probe cantilever, resulting in needle skipping. The present invention adopts the method of gluing small-sized block samples to obtain the glued samples, and then obtains the test samples (Micro-LED cross-sectional samples) with a roughness of the nanometer size by argon ion polishing. The present invention requires to obtain a bonding gap as narrow as possible (less than 1 μm) and a surface roughness section that meets the requirements (surface average roughness Ra within the range of 2 μm×2 μm is less than 1.0 nm). The narrow bonding gap can ensure the relative stability of the sample during the test, and the sample will not move with the movement of the needle tip. The more appropriate the surface roughness, the smaller the lifting height set in the lifting mode will be, thereby ensuring sufficient signal feedback during the test.

[0057] It should be noted that the thickness of the glue will not affect the test results, because after the gluing operation, the cross-section sample will be subjected to argon ion polishing to obtain a test sample with a roughness of the nanometer size, which will ensure that the active area will not be affected by the glue, and will only appear at the position of the gluing seam (below 1μm). When focusing during the test, the active area and the gluing seam can be clearly distinguished under a light microscope.

[0058] In a typical implementation, a method for testing the surface potential of an active area of ​​a Micro-LED cross section specifically includes the following steps:

[0059] A Micro-LED cross-sectional sample obtained by the above method is provided.

[0060] The surface potential of the interface to be tested of the Micro-LED cross-section sample was measured using a Kelvin probe microscope (KPFM).

[0061] KPFM is a scanning probe microscopy technology used to measure the surface potential of samples. Its working principle is based on atomic force microscopy and combines electrostatic force detection. In KPFM, there is capacitance between the probe and the sample surface. The size of this capacitance is related to the probe-sample distance. The signal feedback mainly comes from the electrostatic force and capacitive coupling between the probe and the sample. When the probe is close to the sample surface, an electrostatic force is generated due to the interaction of the electric fields between the two. This electrostatic force will cause the cantilever to deflect, and the signal is obtained by detecting the deflection of the cantilever. At the same time, capacitive coupling also plays a key role in signal generation, because changes in capacitance will affect the size of the electrostatic force, which in turn affects the signal strength. According to the basic formula of capacitance (where C is capacitance, ∈ is dielectric constant, A is plate area, and d is plate spacing). When the sample surface roughness is small, the probe-sample spacing d is relatively stable, and the capacitance C is also relatively stable. The signal feedback strength is closely related to the capacitance. In the electrostatic force detection mode, the electrostatic force (where F is the electrostatic force, z is the coordinate direction perpendicular to the sample surface, and V is the applied voltage). The change is small, and the electrostatic force F is more stable at a given voltage V, making the signal feedback intensity more stable and stronger. For rough surfaces, d and C will change dramatically as the probe moves on the surface, causing the electrostatic force to fluctuate and the signal feedback to become weaker.

[0062] In a typical embodiment, the specific process of measuring using Kelvin probe microscopy (KPFM) includes:

[0063] 1) The samples tested using Kelvin probe microscopy (KPFM) need to be conductive, such as Figure 3 As shown, before the test, the side of the Micro-LED cross-section sample that has not been polished by the argon ion beam is fixed on the metal sheet using conductive silver paste, and the edge of the upper surface of the Micro-LED cross-section sample is connected to the metal sheet using conductive silver paste.

[0064] 2) Place the Micro-LED cross-section sample and the sample after the metal sheet is connected on the sample table, open the suction cup, and gently push with tweezers to ensure that the sample is firmly and reliably installed;

[0065] Select a conductive tip and install it on the tip holder. The Kelvin probe microscope (KPFM) is installed on the atomic force microscope (Veeco Dimension ICON). The general operation is as follows:

[0066] First, set the Surface Potential (AM-KPFM) mode, then tune the needle tip, change the default value of "target amplitude" from 900mV to 500mV, click "autotune", find the first eigenfrequency of the needle tip, click "zerophase", return the phase to zero, and finally click "exit" to exit the Tune needle interface; change the needle insertion mode from smart to standard, focus and position the sample surface according to the AFM tapping mode operating procedures, and enter the CheckParameters interface to set the initial scanning parameters. The interface is as follows: Figure 4 shown.

[0067] Before inserting the needle, start the anti-vibration table, cover the active anti-vibration table cover, turn the handle 90 degrees to the right and buckle it, turn on the anti-vibration table switch, make the green indicator light always on, and start inserting the needle for testing.

[0068] The test process is different from the existing test in that:

[0069] Under an optical microscope, the test area is located by locating the bonding seam and the surface roughness is preliminarily measured to ensure that the test requirements are met (the average surface roughness Ra within the range of 2μm×2μm is less than 1.0nm).

[0070] a) Obtain a stable topographic image of a Micro-LED cross-section sample by scanning with a Kelvin probe microscope (KPFM): Mainly by adjusting the five parameter values ​​of setpoint, scan rate, integral gain, proportional gain, and Z Range

[0071] Setpoint

[0072] Setpoint function: Setpoint is usually related to the vibration amplitude of the cantilever beam. When the cantilever beam vibrates at a certain frequency, the distance between the tip and the sample is controlled through the feedback loop to keep the vibration amplitude of the cantilever beam at the set setpoint value. The main function of this parameter is to control the average distance between the tip and the sample surface. If the setpoint value is large, the amplitude of the cantilever beam vibration is allowed to be larger, and the tip will be relatively far away from the sample surface; conversely, when the setpoint value is small, the tip will be closer to the sample surface.

[0073] The influence of setpoint on measurement: A suitable setpoint helps to obtain stable topography images and surface potential images. If the setpoint is set improperly, for example, it is set too large, it may lead to reduced resolution, because the needle tip is far away from the sample surface, and the detection sensitivity of microscopic information such as surface potential is reduced. If the setpoint is too small, the needle tip may be too close to the sample, and it is easy to collide with the sample, damaging the needle tip and the sample. At the same time, it will also cause the cantilever beam to be subjected to a large force, resulting in increased measurement noise. The setpoint setting range is between 200-400mV.

[0074] Scan Rate

[0075] Scan Rate Function: Scan rate refers to the speed at which the scanning probe moves on the sample surface. It determines the size of the area scanned per unit time. A higher scan rate means that a larger area can be scanned in a shorter time, but it also means that the acquisition time for each data point is shorter.

[0076] The impact of Scan Rate on measurement: Scan rate has a significant impact on measurement accuracy and image quality. When the scan rate is too fast, the system may not have enough time to fully collect and process the signal for each data point, resulting in decreased accuracy of the measurement data, reduced image resolution, and blur or distortion. In KPFM measurement, the measurement of surface potential requires a certain amount of time to stabilize the signal. If the scan rate is too fast, the surface potential signal may not be accurately measured. On the contrary, a lower scan rate can improve measurement accuracy and image quality, but it will increase the measurement time. Therefore, the Scan rate setting range is between 0.5-1 Hz.

[0077] Input Igain (integral gain input) and Input Pgain (proportional gain input)

[0078] The role of Input Igain and Input Pgain: Input Igain and Input Pgain play a key role in the feedback control loop of the KPFM measurement system. Proportional gain (Input Pgain) is a coefficient that performs feedback adjustment proportionally based on the current error signal (for example, the difference between the actual measured cantilever beam vibration parameter and the set value). It can quickly respond to changes in the error and make adjustments to the system in the direction of reducing the error. For example, when the cantilever beam vibration amplitude deviates from the set point, the proportional gain will quickly generate a correction signal based on the degree of deviation, so that the cantilever beam vibration returns to the set state as soon as possible.

[0079] The integral gain (Input Igain) is used for feedback regulation after the error signal is integrated. The integral gain is mainly used to eliminate the steady-state error of the system. If there is a persistent small error in the system, the proportional gain may only partially correct it, while the integral gain will accumulate and correct the error over time. For example, during a long measurement, the tip-sample spacing may have a small and persistent offset due to environmental factors. The integral gain can accumulate these small offsets and correct them so that the system finally reaches and stabilizes in the set state. A suitable Input Pgain can improve the response speed of the system. If the Input Pgain is set too high, the system may overshoot. For example, when trying to correct the error of the cantilever vibration amplitude, the correction signal is too large, which may cause the cantilever vibration amplitude to exceed the set value, and then generate a reverse error, causing the system to oscillate near the set value, affecting the stability of the measurement. If the Input Pgain is set too low, the system will respond slowly to the error, resulting in reduced measurement efficiency, and the tip-sample spacing cannot be adjusted in time when facing rapidly changing surface conditions, affecting the measurement accuracy. For Input Igain, proper setting can effectively reduce steady-state error and improve measurement accuracy. However, if Input Igain is set too high, the system may become unstable. Because the integral gain will accumulate the correction signal excessively, it may cause the system to generate self-excited oscillation, making the cantilever beam vibration unstable, and thus affecting the accurate measurement of parameters such as surface potential. The Input Igain setting range is 0.5-1.0, and usually Input Pgain is about 20%-100% larger than Input Igain.

[0080] Lift Scan Height

[0081] The role of Lift Scan Height: In KPFM measurement, there are usually two scanning modes: contact mode or tapping mode scanning to obtain topographic information, and then measuring the surface potential at the lifting scanning height position. Lift Scan Height determines how high the tip is lifted from the sample surface to perform potential scanning after completing the topographic scan. The setting of this height needs to take into account both avoiding the tip from contacting the sample surface again and affecting the accuracy of the potential measurement, and ensuring that the surface potential signal can be effectively detected. If the Lift Scan Height is set too low, the tip may still contact some tiny undulations or adsorbents on the sample surface after lifting, which will interfere with the measurement of the surface potential and produce erroneous potential signals. In addition, the tip may be damaged or the sample surface may be contaminated. On the other hand, if the Lift Scan Height is set too high, the surface potential signal will become weaker because the potential signal will attenuate to a certain extent as the distance increases. This may reduce the sensitivity and resolution of the potential measurement and reduce the quality of the measured surface potential image. The setting range of Lift Scan Height is between 30-100nm.

[0082] b) Start the lift mode:

[0083] Change scan size to 0nm, set the lift height in lift scan height, change interleave mode to lift, and set image Data Type in Channel 6 to Potential, RL Plane Fit and OL Plane Fit to None, scan line to Interleave, and linedirection to Retrace. The data obtained is the contact potential difference V between the tip and the Micro-LED cross-section sample. CPD .

[0084] V CPD =V tip -V sample

[0085] c) Set Lock-In2 Phase:

[0086] Change the scan size to 0nm and click the setphase button.

[0087] d) Optimize potential parameters

[0088] Obtaining a stable potential image: mainly through the parameter values ​​of Input Igain (integral gain input), Input Pgain (proportional gain input), lift scan height and Lock-In2 Phase. Input Igain and Input Pgain adjustment method: first try to increase the input igain until noise appears on the curve, then slightly reduce the input igain until the noise disappears. Usually Input Pgain is about 20%-100% larger than Input Igain. Lock-In2 Phase adjustment method: change the scan size to 0nm, click the set phase button in the toolbar, and the device will automatically find the phase angle so that the amplitude component is the largest in the X direction and close to 0 in the Y direction.

[0089] e) Calculate the work function of the Micro-LED cross-section sample, and establish an energy band model to analyze the carrier transport at the surface. The carrier transport is related to the built-in electric field at the surface. The corresponding energy band model can be established based on the direction of the built-in electric field.

[0090] Specifically, the method for evaluating the carrier transport condition of the active area of ​​the Micro-LED cross section specifically includes: obtaining the surface potential of the interface to be tested of the Micro-LED cross section sample under light irradiation and no light irradiation conditions respectively; subtracting the surface potential obtained under light irradiation conditions from the surface potential obtained under no light irradiation conditions to obtain the surface photovoltage spectrum of the interface to be tested; judging the carrier transport condition according to the surface photovoltage spectrum, wherein the rising edge of the surface photovoltage spectrum corresponds to the transport of electrons to the surface. Comparing the energy band models established under light conditions and no light conditions, the carrier transport condition is consistent with the transport condition judged according to the surface photovoltage spectrum.

[0091] The specific calculation model and evaluation method for measuring the work function of Micro-LED cross-section samples are as follows:

[0092] eV CPD =φ tip -φ sample (1)

[0093] Where e is the unit electron charge constant, φ tip is the work function of the needle tip, φ sample is the work function of the sample surface, V CPD is the contact potential difference between the two. In KPFM measurement, the work function of the tip can be obtained through graphite correction. The potential of the sample surface can be calculated by measuring the contact potential difference.

[0094] The work function of the surface (φ sample) can be divided into three parts: electron affinity (χ), surface energy band curvature (φ0), conduction band bottom (E c ) and the Fermi level (E F ) n ), which satisfies:

[0095] φ sample =χ+(E c -E F )+φ0=χ+E n +φ0 (2)

[0096] To ensure that the Fermi level of the surface energy band is at the same level as that of the body, the measured surface potential is compared with the GaN electron affinity. If the measured value is greater than the GaN electron affinity, the surface energy band bends upward. If the measured value is less than the GaN electron affinity, the surface energy band bends downward, thereby forming a built-in electric field at the sidewall surface. The degree of curvature of the surface energy band can be determined by E n To measure, E n The larger the absolute value of is, the greater the amount of band bending is. The amount of band bending can actually be expressed by the potential difference. The relationship between the built-in electric field and the amount of band bending (potential difference) can be obtained by integrating the Poisson equation:

[0097]

[0098] By analyzing the surface photovoltage spectrum, the method of using the surface photovoltage spectrum to analyze the damage layer of Micro-LED includes:

[0099] When the sample is irradiated with light whose photon energy is greater than or equal to the GaN band gap, the carriers absorb the photon energy and transition from the low energy level to the high energy level, generating non-equilibrium photogenerated carriers (electron-hole pairs). The photogenerated carriers are driven to the near-surface region of the semiconductor material by the concentration gradient. Driven by the built-in electric field on the surface, the electrons and holes are spatially separated, thereby forming a photogenerated voltage, called the surface photovoltage (SPV). The key change nodes of the photogenerated potential are at the corresponding wavelength of the Micro-LED emission and near the intrinsic emission wavelength of GaN, 365nm. The rising edge of the photogenerated potential corresponds to the intrinsic absorption of the bandgap width of the InGaN quantum well layer. Under the irradiation of light, the photogenerated carriers will experience dynamic carrier transport at the interface. Before the wavelength of the irradiated light reaches the wavelength of the InGaN band edge, the InGaN quantum well layer can be regarded as a transparent layer and does not absorb photon energy. When the wavelength of the irradiated light is lower than 365 nm, the InGaN quantum well layer begins to absorb photon energy, and carriers at the interface of InGaN and GaN are transported. The surface photovoltage spectrum corresponds to the transport of electrons to the surface from the rising edge. Before the wavelength of the irradiated light reaches the wavelength of 365 nm of the GaN band edge, the GaN layer can be regarded as a transparent layer and does not absorb photon energy. When the wavelength of the irradiated light is lower than 365 nm, the GaN layer begins to absorb photon energy. The result of the transport of photogenerated carriers leads to the movement of electrons to the surface, and a falling edge appears. The difference in the height of the rising edge can reflect the concentration of photogenerated holes generated in the body, and the larger concentration gradient promotes the migration ability of holes to the surface. When the wavelength of the irradiated light is lower than 365 nm, the surface photovoltage after the wavelength of 365 nm remains basically unchanged. The electrons and holes moving to the surface reach a dynamic equilibrium, and the surface photovoltage remains basically unchanged.

[0100] It should be understood that the above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A sample preparation method for testing the surface potential of a Micro-LED cross section, characterized in that: include: Cutting the block-shaped Micro-LED epitaxial wafer according to the GaN [10-10] or [11-20] crystal zone axis to form two sample blocks, wherein the sample block includes a p-type region, an active region, and an n-type region sequentially arranged in a direction away from the C-plane, and the cross section formed by cutting includes the side walls of the p-type region, the active region, and the n-type region; Bonding the C surfaces of the two sample blocks, with the cross sections of the two sample blocks located on the same side, and controlling the width of the bonding seam between the C surfaces of the two sample blocks to be within 1 μm, thereby obtaining a bonded sample; The selected surface including the cross section of the bonded sample is polished until the roughness of the selected surface reaches the nanometer level, thereby obtaining a Micro-LED cross-sectional sample, wherein the interface to be measured of the Micro-LED cross-sectional sample includes the two cross sections.

2. The sample preparation method for testing the surface potential of a Micro-LED cross section according to claim 1, characterized in that: Specifically include: Glue containing epoxy resin is applied to the C surfaces of the two sample blocks to bond the C surfaces of the two sample blocks, and external force is applied along the butting direction of the two sample blocks to control the width of the bonding seam between the C surfaces of the two sample blocks to be within 1 μm.

3. The sample preparation method for testing the surface potential of a Micro-LED cross section according to claim 1, characterized in that: Specifically include: Firstly, the selected surface including the cross section of the bonding sample is ground until the selected surface is free of scratches and the shiny active area portions on both sides of the bonding seam can be observed; then, the selected surface is polished.

4. The sample preparation method for testing the surface potential of a Micro-LED cross section according to claim 3, characterized in that: The grinding process specifically includes: grinding the selected surface multiple times, and the roughness of the grinding tool used in the multiple grindings is gradually reduced.

5. The method for preparing a sample for testing the surface potential of a Micro-LED cross section according to claim 4, characterized in that: The roughness of the grinding tool is 0.5 μm to 30 μm.

6. The sample preparation method for testing the surface potential of a Micro-LED cross section according to claim 3, characterized in that: The polishing process specifically includes: polishing the selected surface using an argon ion beam.

7. The sample preparation method for testing the surface potential of a Micro-LED cross section according to claim 1, 3 or 5, characterized in that: The roughness of the selected surface after polishing satisfies: the surface average roughness Ra within the range of 2μm×2μm is less than 1.0nm.

8. A method for testing the surface potential of an active area of ​​a Micro-LED cross section, characterized in that: include: Providing a Micro-LED cross-section sample obtained by the sample preparation method for testing the surface potential of a Micro-LED cross section according to any one of claims 1 to 7; The surface potential of the interface to be measured of the Micro-LED cross-sectional sample is obtained using a Kelvin probe microscope.

9. The method for testing the surface potential of the active area of ​​a Micro-LED cross section according to claim 8, characterized in that: Fix the side of the Micro-LED cross-section sample facing away from the interface to be tested on the metal sheet using a conductive material, and electrically connect the edge of the interface to be tested of the Micro-LED cross-section sample to the metal sheet below; When scanning the interface to be tested of the Micro-LED cross-section sample with the probe of the Kelvin probe microscope, the set point of the Kelvin probe microscope is set to 200mV~400mV, the scanning rate is set to 0.5Hz~1Hz, the integral gain input is set to 0.5~1.0, the proportional gain input is 20%~100% larger than the integral gain input, and the lifting scanning height is set to 30nm~100nm to obtain the morphological image and potential image of the interface to be tested, wherein the potential image includes the surface potential of multiple areas of the interface to be tested.

10. A method for evaluating carrier transport in an active region of a Micro-LED cross section, characterized in that: include: Providing a Micro-LED cross-section sample obtained by the sample preparation method for testing the surface potential of a Micro-LED cross section according to any one of claims 1 to 7; The surface potential of the interface to be tested of the Micro-LED cross-section sample under light irradiation and no light irradiation conditions is obtained by using the test method of the surface potential of the active area of ​​the Micro-LED cross-section according to claim 8 or 9; The surface potential obtained under light irradiation conditions is subtracted from the surface potential obtained under light irradiation conditions to obtain a surface photovoltage spectrum of the interface to be measured; The carrier transport condition is determined according to the surface photovoltage spectrum, wherein the rising edge of the surface photovoltage spectrum corresponds to the transport of electrons to the surface.

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