Sample preparation method for micro-led cross-section surface potential test and application thereof

Abstract

The application discloses a kind of Micro-LED section surface electric potential test sample preparation method and application thereof.The test method of Micro-LED section active area surface electric potential includes: preparing Micro-LED section sample, argon ion beam polishing process is applied to the sample section Polishing, form the test sample with nanometer size order surface roughness;Surface electric potential of the test sample is tested using Kelvin probe force microscope (KPFM), by making section sample, simulation etching process, KPFM can not only characterize the surface topography of sample, but also obtain the electrical properties of sample surface-surface electric potential, can be more intuitive to compare the difference of surface electric potential in different regions.Furthermore, the work function of semiconductor material can be calculated, and the energy band model can be established to analyze the transport of surface carriers.

Description

TECHNICAL FIELD

[0001] The application relates to a Micro-LED cross-section surface potential test sample preparation method and application thereof, and belongs to the technical field of Micro-LED detection. BACKGROUND

[0002] At present, the photoelectric performance of Micro-LED is mainly tested by macroscopic electrical methods such as external quantum efficiency and I-V curve. There is a lack of high spatial resolution direct characterization of the side wall region, and high-resolution characterization technology is needed to study the side wall defects and non-radiative recombination mechanism at the side wall. SUMMARY

[0003] The main purpose of the application is to provide a Micro-LED cross-section surface potential test sample preparation method and application thereof, so as to overcome the shortcomings of the prior art.

[0004] To achieve the above-mentioned purposes, the technical scheme adopted by the application comprises:

[0005] The first aspect of the embodiment of the application provides a Micro-LED cross-section surface potential test sample preparation method, which comprises the following steps:

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

[0007] Bonding the C faces of the two sample blocks, the cross-sections of the two sample blocks being located on the same side, and controlling the width of the bonding seam between the C faces of the two sample blocks to be within 1 mu m, so as to obtain a bonded sample;

[0008] Polishing a selected surface comprising the cross-section of the bonded sample until the roughness of the selected surface reaches the nanometer level, so as to obtain a Micro-LED cross-section sample, and the interface to be tested of the Micro-LED cross-section sample comprising two cross-sections.

[0009] The second aspect of the embodiment of the application provides a Micro-LED cross-section active region surface potential test method, which comprises the following steps:

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

[0011] Using a Kelvin probe microscope to obtain the surface potential of the interface to be tested of the Micro-LED cross-section sample.

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

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

[0014] obtaining surface potentials of the interface to be tested of the Micro-LED cross-section sample under light irradiation and no light irradiation respectively by using the surface potential testing method of the Micro-LED cross-section active region;

[0015] subtracting the surface potential obtained under no light irradiation from the surface potential obtained under light irradiation to obtain a surface photovoltage spectrum of the interface to be tested;

[0016] judging the carrier transport 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 present application has the following advantages:

[0018] The surface potential testing method of the Micro-LED cross-section active region provided by the present application can not only characterize the surface morphology of the sample, but also obtain the electrical property-surface potential mapping of the sample surface, so that the differences in surface potential of different regions can be compared more intuitively, and the work function of the semiconductor material can be calculated, and the 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 microscopy testing can measure the surface potential without contacting the sample surface, and the surface potential testing method of the Micro-LED cross-section active region provided by the present application is extremely sensitive to small changes in surface state and has high measurement accuracy.

[0020] The surface potential testing method of the Micro-LED cross-section active region provided by the present application can make the semiconductor sample conductive with the needle sample by using a metal such as point indium or silver paste, instead of plating an electrode on the sample, thereby avoiding the pollution of the sample during the plating process. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is a sample preparation flowchart for surface potential testing of a Micro-LED cross-section provided in a typical embodiment of the present application;

[0022] Fig. 2 is a structure schematic diagram of a Micro-LED cross-section sample provided in a typical embodiment of the present application;

[0023] Figure 3 is a schematic diagram of the principle of testing a Micro-LED cross-section sample by a Kelvin probe microscope in a typical embodiment of the present application;

[0024] Figure 4 is a test parameter interface of a Kelvin probe microscope in a typical embodiment of the present application. DETAILED DESCRIPTION

[0025] In view of the deficiencies in the prior art, the present inventors have long studied and practiced to come up with the technical solution of the present application. The technical solution, its implementation process and principles will be further explained as follows.

[0026] A first aspect of the embodiments of the present application provides a sample preparation method for Micro-LED cross-section surface potential testing, which comprises:

[0027] Cutting a bulk Micro-LED epitaxial wafer according to GaN [10-10] or [11-20] crystal band axis to form two sample blocks, the sample blocks comprising a p-type region, an active region and an n-type region arranged in sequence in a direction away from the C face, and the cross-section formed by the cutting comprising side walls of the p-type region, the active region and the n-type region;

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

[0029] Polishing a selected surface comprising the cross-section of the bonded sample until the roughness of the selected surface reaches the nanometer level, thereby obtaining a Micro-LED cross-section sample, the interface to be tested of the Micro-LED cross-section sample comprising the two cross-sections.

[0030] In a more specific embodiment, the sample preparation method for Micro-LED cross-section surface potential testing specifically comprises: coating glue containing epoxy resin on the C faces of the two sample blocks to bond the C faces of the two sample blocks, and applying external force in the direction of the butt joint of the two sample blocks to control the width of the bonding seam between the C faces of the two sample blocks to be within 1 μm.

[0031] In a more specific embodiment, the sample preparation method for Micro-LED cross-section surface potential testing specifically comprises: first polishing a selected surface comprising the cross-section of the bonded sample until the selected surface is free of scratches and the active region parts on both sides of the bonding seam can be observed to be bright; and then polishing the selected surface.

[0032] Further, the polishing process specifically includes: polishing the selected surface for multiple times, roughness of polishing tools used in the multiple times of polishing is gradually reduced.

[0033] Further, the roughness of the polishing tool is 0.5 μm-30 μm.

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

[0035] Further, the roughness of the selected surface after the polishing process satisfies: surface average roughness Ra in a range of 2 μm*2 μm is less than 1.0 nm.

[0036] The second aspect of the embodiment of the application provides a method for testing surface potential of a Micro-LED cross-section active region, which comprises the following steps:

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

[0038] obtaining surface potential of an interface to be tested of the Micro-LED cross-section sample by using a Kelvin probe microscope.

[0039] Further, the method for testing surface potential of the Micro-LED cross-section active region specifically comprises the following steps:

[0040] fixing one side of the Micro-LED cross-section sample, which is opposite to the interface to be tested, on a metal sheet by using a conductive material, and electrically connecting an edge of the interface to be tested of the Micro-LED cross-section sample with the metal sheet below;

[0041] scanning the interface to be tested of the Micro-LED cross-section sample by using a probe of the Kelvin probe microscope, setting a set point of the Kelvin probe microscope to 200 mV-400 mV, setting a scanning rate to 0.5 Hz-1 Hz, setting an integral gain input to 0.5-1.0, setting a proportional gain input to be 20%-100% larger than the integral gain input, and setting a lifting scanning height to 30 nm-100 nm, so as to obtain a topographic image and a potential image of the interface to be tested, the potential image comprising surface potential of multiple regions of the interface to be tested.

[0042] The third aspect of the embodiment of the application provides a method for evaluating carrier transport of a Micro-LED cross-section active region, which comprises the following steps:

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

[0044] The surface potential of the active region of the Micro-LED cross section was obtained under both light irradiation and no light irradiation conditions using the aforementioned test method.

[0045] The surface photovoltage spectrum of the interface under test is obtained by subtracting the surface potential obtained under no-light conditions from the surface potential obtained under light irradiation conditions.

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

[0047] The following will provide a further explanation of the technical solution, its implementation process, and its principles, in conjunction with the accompanying drawings and specific implementation examples.

[0048] Please see Figure 1 A sample preparation method for measuring the surface potential of a Micro-LED cross-section includes the following steps:

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

[0050] The sample to be tested was cut into 1mm × 3mm sample blocks along the GaN [10-10] or [11-20] zone axis. The sample blocks were then thoroughly cleaned. The cleaning process and cleaning agents used were known in the art and are not limited here. The cut sample block included a p-type region (also called a p-type layer, hereinafter the same), an active region (also called an active layer, hereinafter the same), an n-type region (also called an n-type layer, hereinafter the same), and a substrate, arranged sequentially along the direction away from the C-plane. The cross-section formed by cutting the sample block included the sidewalls of the p-type region, the active region, and the n-type region.

[0051] Two sample blocks are bonded together using an epoxy resin-containing adhesive, with the cross-sections of the two sample blocks on the same side. The two bonded sample blocks are clamped together with a clamp to control the width of the bonding seam between the C-surfaces of the two bonded sample blocks to within 1 μm, thereby obtaining bonded samples.

[0052] The bonded samples were polished with sandpaper with roughness of 30μm, 9μm, 6μm, 3μm, 1μm and 0.5μm in sequence until the surface of the bonded samples was free of scratches when observed under an optical microscope at a magnification of 100X, and the bright active areas on both sides of the bonded seam could be observed.

[0053] The sample was adhered to a baffle and polished with an argon ion beam for more than 20 hours until the roughness of the adhesion seam area reached the nanometer level: the average surface roughness Ra in a 2μm × 2μm range was less than 1.0 nm. Polishing was then stopped, thus obtaining a Micro-LED cross-section sample. Figure 2a ,Figure 2b As shown, the interface to be measured of the Micro-LED cross-section sample includes two cross-sections.

[0054] It should be noted that cross-section ion beam polishing using argon ions is suitable for making regions in the millimeter range, and the present application needs to prepare samples with a measurable range as large as possible, and at the same time, the polishing damage of argon ion beam is smaller than FIB. The polishing time affects the measurable range, and the purpose of the present application is to make the polishing range reach the active region on the other side of the seam. According to the results obtained by the experiments of the present application, it is necessary to polish the Micro-LED sample of the sapphire substrate for at least 20 hours from the side of the substrate to polish, so as to meet the requirements of the average roughness mentioned above.

[0055] Specifically, when making semiconductor cross-section samples, argon ion beam polishing is a commonly used method, although other noble gases can also be used in some cases, but argon is widely used due to its atomic characteristics, safety and cost, etc. Argon is an inert gas, and its atom is relatively heavy. The atomic mass of argon is about 40 u (atomic mass unit). The heavier atom has a larger momentum at the same energy, and can more effectively remove materials when bombarding the sample surface. In contrast, the atomic mass of helium (He) is about 4 u, which is relatively light, and its sputtering efficiency is relatively low at the same acceleration voltage. The atomic mass of krypton is about 84 u, and the atomic mass of xenon is about 131 u. 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 in the physical sputtering process. However, their cost is relatively high, and in practical applications, except for some special requirements of research (such as the preparation of some high-precision, high-value samples, and not sensitive to cost), they are generally less used. A lower energy (such as 500-1000 eV) argon ion beam is used because the physical bombardment of the low-energy ion beam on the sample surface is relatively mild, which can reduce the damage to the internal microstructure (such as the crystal lattice structure) of the semiconductor material during polishing. The beam current density of the argon ion beam is in the range of 5 μA / cm 2 -20 μA / cm 2 . A lower beam current density can achieve more delicate polishing.

[0056] Specifically, for the Micro-LED cross-section sample, the present application mainly focuses on the surface potential at the sidewall of the n-type region, the p-type region and the active region. However, due to the p-type region and the active region of the Micro-LED being close to the c-plane surface layer, the active region layer is very thin for the entire cross-section area, and the cross-section sample can be obtained by direct cleavage, FIB processing or argon ion polishing. However, if the previous cross-section sample preparation method is adopted, the Micro-LED cross-section sample test will have a needle jumping problem at the position close to the surface layer, and the active region surface potential cannot be effectively measured. Specifically, when the Kelvin probe force microscope (KPFM) is used to measure the Micro-LED cross-section sample, 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 scanning, and is easily disturbed by external slight vibration, sample surface roughness change and probe cantilever elasticity, etc., resulting in the needle jumping phenomenon. The present application adopts the method of sticking small size block-shaped samples to obtain the stuck sample, and then obtains the test sample (Micro-LED cross-section sample) with nanometer size roughness through argon ion polishing. The present application requires a narrow stuck seam (below 1 μm) and a required surface roughness (the surface average roughness Ra in the range of 2 μm x 2 μm is less than 1.0 nm) cross-section. The narrow stuck seam 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 appropriate surface roughness will make the lift height set in the lift mode smaller, 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 the cross-section sample will be subjected to argon ion polishing after the sticking operation, and the test sample with nanometer size roughness will be obtained, which will ensure that the active region will not be affected by the glue, and only the stuck seam position (below 1 μm) will appear. The position of the active region and the stuck seam can be clearly distinguished under the light microscope when focusing during the test.

[0058] In a more typical embodiment, a test method for the surface potential of the active region of the Micro-LED cross-section is provided, which specifically comprises the following steps:

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

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

[0061] KPFM is a scanning probe microscopy technique for measuring the potential of a sample surface. Its working principle is based on atomic force microscopy, combined with electrostatic force detection. In KPFM, there is a capacitance between the probe and the sample surface, and the size of the capacitance is related to the probe-sample spacing. The signal feedback is mainly derived from the electrostatic force and the capacitive coupling between the probe and the sample. When the probe approaches the sample surface, due to the interaction of the electric field between the two, an electrostatic force is generated, which causes the cantilever to deflect. By detecting the deflection of the cantilever, a signal is obtained. At the same time, capacitive coupling also plays a key role in signal generation, because the change of capacitance will affect the size of electrostatic force, and then affect the signal strength. According to the basic formula of capacitance (where C is the capacitance, ∈ is the dielectric constant, A is the area of the plate, and d is the spacing between the plates). When the roughness of the sample surface 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). Due to the small roughness, the capacitance C is stable, and the electrostatic force F is more stable under a given voltage V, so that the signal feedback strength is more stable and stronger. For rough surfaces, d and C will change dramatically with the movement of the probe on the surface, resulting in fluctuations in the electrostatic force and weaker signal feedback.

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

[0063] 1) The sample to be tested by Kelvin probe force microscopy (KPFM) needs to be conductive, as shown in Figure 3 Before testing, the side of the Micro-LED cross-section sample that has not been polished by an argon ion beam is fixed on a 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 metal sheet after connecting the sample on the sample stage, turn on the suction cup, and gently push it with tweezers to ensure that the sample is securely and reliably installed;

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

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

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

[0068] During the test, the difference from the existing test is:

[0069] Under the light microscope, the test area is located by the adhesive joint, the surface roughness is preliminarily measured, and the test requirements are ensured (the average surface roughness Ra in the range of 2μm×2μm is less than 1.0nm).

[0070] a) Obtain the stable topographic image of the Micro-LED cross-section sample by Kelvin probe force microscopy (KPFM) scanning: mainly by adjusting the setpoint, scan rate, integral gain, proportional gain and Z Range

[0071] Setpoint

[0072] Setpoint: 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 a feedback loop, so that the vibration amplitude of the cantilever beam remains 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 allows a larger range, and the tip will be relatively far from the sample surface; on the contrary, when the setpoint value is small, the tip will be closer to the sample surface.

[0073] Setpoint's Effect on Measurement: Proper setpoint helps to obtain stable topography images and surface potential images. If setpoint is set improperly, for example, set too large, it may lead to reduced resolution because the tip is far away from the sample surface, and the sensitivity of detecting microscopic information such as surface potential is reduced. If setpoint is too small, the tip may be too close to the sample, which is easy to collide with the sample, damaging the tip and sample, and at the same time, it will also make the cantilever receive a larger force, leading to increased measurement noise. Setpoint is set in the range of 200-400 mV.

[0074] Scan Rate

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

[0076] Effect 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 blurred or distorted images. In KPFM measurement, the measurement of surface potential requires a certain amount of time to stabilize the signal, and 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 is set in the range of 0.5-1 Hz.

[0077] Input Igain and Input Pgain

[0078] Effect of Input Igain and Input Pgain: In the feedback control loop of the KPFM measurement system, Input Igain and Input Pgain play a key role. The proportional gain (Input Pgain) is a coefficient that adjusts the feedback in proportion to the current error signal (for example, the difference between the actual measured cantilever vibration parameter and the set value). It can quickly respond to changes in error and make adjustments to the system to reduce error. For example, when the cantilever vibration amplitude deviates from the setpoint, the proportional gain will quickly produce a correction signal according to the degree of deviation, allowing the cantilever vibration to return to the set state as soon as possible.

[0079] The integral gain (Input Igain) is used to feedback adjust the error signal after integral operation. The integral gain is mainly used to eliminate the steady-state error of the system. If there is a small error that persists in the system, the proportional gain can only partially correct it, while the integral gain will accumulate the error over time for correction. For example, during a long measurement process, due to environmental factors, etc., there may be a small persistent shift in the tip-sample distance. The integral gain can accumulate these small shifts for correction, so that the system can eventually reach and stabilize at the set state. The appropriate Input Pgain can improve the response speed of the system. If the Input Pgain is set too high, the system may have an overshoot phenomenon. For example, when trying to correct the error of the cantilever beam vibration amplitude, due to the large correction signal, the cantilever beam vibration amplitude may exceed the set value, then generate an opposite error, cause the system to oscillate around the set value, and affect the stability of the measurement. If the Input Pgain is set too low, the system's response to error will be very slow, resulting in reduced measurement efficiency, and when facing rapidly changing surface conditions, the tip-sample distance cannot be adjusted in time, affecting the measurement accuracy. For the Input Igain, the appropriate setting can effectively reduce the steady-state error and improve the measurement accuracy. However, if the Input Igain is set too high, the system may become unstable. Because the integral gain will excessively accumulate the correction signal, it may cause the system to generate self-excited oscillation, making the cantilever beam vibration unstable, and thus affecting the accurate measurement of surface potential and other parameters. The Input Igain is usually set in the range of 0.5-1.0, and the Input Pgain is usually about 20%-100% larger than the Input Igain.

[0080] Lift Scan Height (Lift Scan Height)

[0081] The role of Lift Scan Height: In KPFM measurement, there are usually two scanning modes: contact mode or tapping mode to obtain topography information, and then the surface potential measurement is carried out at the position of lifting the scanning height. Lift Scan Height determines how high the tip is lifted from the sample surface after completing the topography scan to perform potential scanning. The setting of this height needs to consider both avoiding the tip from contacting the sample surface again to affect the accuracy of potential measurement and ensuring that the surface potential signal can be effectively detected. If Lift Scan Height is set too low, the tip may still contact some small undulations or adsorbed substances on the sample surface after lifting, which will interfere with the measurement of surface potential and produce false potential signals. Moreover, it may damage the tip or contaminate the sample surface. On the other hand, if Lift Scan Height is set too high, it will result in a weak surface potential signal because the potential signal will attenuate to some extent with the increase of distance. This may reduce the sensitivity and resolution of potential measurement, and the quality of the measured surface potential image will decrease. The setting range of Lift Scan Height is between 30-100 nm.

[0082] b) Start the lift mode:

[0083] Change scan size to 0 nm, 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, line direction to Retrace. The data obtained is the contact potential difference V between the tip and the cross-section sample of the Micro-LED CPD .

[0084] V CPD = V tip -V sample

[0085] c) Set Lock-In2 Phase:

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

[0087] d) Optimize potential parameters

[0088] Obtaining 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 input igain as much as possible, until noise appears on the curve, then slightly reduce input igain until the noise disappears, usually Input Pgain is about 20%-100% larger than Input Igain, Lock-In2 Phase adjustment method: change scan size to 0nm, click set phase button in the toolbar, the device will automatically find the phase angle, so that the amplitude component is maximum in X and close to 0 in Y direction.

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

[0090] Specifically, the evaluation method of the carrier transport in the active region of the Micro-LED cross-section includes: obtaining the surface potential of the Micro-LED cross-section sample under light irradiation and no light irradiation conditions; subtracting the surface potential obtained under no light irradiation conditions from the surface potential obtained under light irradiation to obtain the surface photovoltage spectrum of the interface to be measured; determining the carrier transport 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 and no light conditions, the transport of carriers is consistent with the transport determined according to the surface photovoltage spectrum.

[0091] The specific calculation model and evaluation method for calculating the work function of the Micro-LED cross-section sample 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, φ sample is the work function of the sample surface, and V CPD is the contact potential difference between them. In KPFM measurement, the work function of the needle can be obtained by graphite correction. The contact potential difference obtained by measurement can be used to calculate the potential of the sample surface.

[0094] The work function of the surface (φ sample) can be divided into three parts: electron affinity (χ), surface band bending amount (φ0), and the difference between the conduction band bottom (E c ) and the Fermi level in the body (E F ) (E 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 band is at the same level as that in the body, the measured surface potential is compared with the electron affinity of GaN. If the measured value is greater than the electron affinity of GaN, the surface band is bent upward, and if the measured value is less than the electron affinity of GaN, the surface band is bent downward, thereby forming a built-in electric field at the sidewall surface. The degree of bending of the surface band can be measured by E n , and the greater the absolute value of E n , the greater the band bending amount. The band bending amount can actually be represented by the potential difference, and the relationship between the built-in electric field and the band bending amount (potential difference) can be obtained by integrating the Poisson equation:

[0097]

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

[0099] When the sample is irradiated with light having a photon energy greater than or equal to the band gap of GaN, the carriers absorb the photon energy and transition from a low energy level to a high energy level, generating non-equilibrium photo-generated carriers (electron-hole pairs). The photo-generated carriers are driven by the concentration gradient to the near-surface region of the semiconductor material, and under the driving of the surface built-in electric field, the electrons and holes are spatially separated, thereby forming a photo-generated voltage, referred to as surface photovoltage (SPV). The key change nodes of the photo-generated potential are at the wavelengths corresponding to the Micro-LED light emission and the intrinsic emission wavelength of GaN of 365 nm, respectively, and the rising edge of the photo-generated potential corresponds to the intrinsic absorption of the band gap width of the InGaN quantum well layer. Under the irradiation of light, the dynamic transport phenomenon of carriers occurs at the interface. Before the wavelength of the irradiation 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, and when the wavelength of the irradiation light is lower than the wavelength of the InGaN band edge, the InGaN quantum well layer starts to absorb photon energy, and the carriers at the interface between InGaN and GaN are transported, and the surface photovoltage spectrum has a rising edge corresponding to the transport of electrons to the surface. Before the wavelength of the irradiation light reaches the wavelength of the GaN band edge of 365 nm, the GaN layer can be regarded as a transparent layer and does not absorb photon energy, and when the wavelength of the irradiation light is lower than 365 nm, the GaN layer starts to absorb photon energy, and the transport of photo-generated carriers results in 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 photo-generated holes in the bulk, and a larger concentration gradient promotes the migration ability of holes to the surface. When the wavelength of the irradiation light is lower than 365 nm, the surface photovoltage after 365 nm remains basically unchanged. The electrons moving to the surface and the holes reach a dynamic equilibrium, and the surface photovoltage remains basically unchanged.

[0100] It should be understood that the above embodiments are only to illustrate 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 to 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 for preparing a sample for Micro-LED cross-section surface potential testing, characterized in that, The method comprises the following steps: cutting a bulk Micro-LED epitaxial wafer along a GaN [10-10] or [11-20] crystal band axis to form two sample blocks, the sample blocks comprising a p-type region, an active region, and an n-type region arranged in sequence in a direction away from a C face, and a cross section formed by the cutting comprising side walls of the p-type region, the active region, and the n-type region; bonding the C faces of the two sample blocks, the cross sections of the two sample blocks being located on the same side, and controlling a width of a bonding seam between the C faces of the two sample blocks to be within 1 μm, thereby obtaining a bonded sample; polishing a selected surface comprising a cross section of the bonded sample until a roughness of the selected surface reaches a nanometer level, thereby obtaining a Micro-LED cross section sample, and an interface to be measured of the Micro-LED cross section sample comprising the two cross sections.

2. The sample preparation method for cross-section surface potential test of a micro-LED according to claim 1, wherein, The method specifically comprises: applying glue containing epoxy resin to the C faces of the two sample blocks to bond the C faces of the two sample blocks, and applying an external force in a direction in which the two sample blocks are butted to control the width of the bonding seam between the C faces of the two sample blocks to be within 1 μm.

3. The sample preparation method for cross-section surface potential test of a micro-LED according to claim 1, wherein, The method specifically comprises: first polishing a selected surface comprising a cross section of the bonded sample until the selected surface is free of scratches and a bright active region part on both sides of the bonding seam can be observed, and then polishing the selected surface.

4. The sample preparation method for cross-section surface potential test of a micro-LED according to claim 3, wherein, The polishing specifically comprises: polishing the selected surface for multiple times, and roughnesses of polishing tools used in the multiple times of polishing are gradually reduced.

5. The sample preparation method for cross-section surface potential test of micro-LEDs according to claim 4, wherein: The roughness of the polishing tool is 0.5 μm to 30 μm.

6. The sample preparation method for cross-section surface potential test of a micro-LED according to claim 3, wherein, The polishing specifically comprises: polishing the selected surface by using an argon ion beam.

7. The sample preparation method for micro-LED cross-section surface potential testing according to claim 1 or 3 or 5, wherein, After the polishing, a roughness of the selected surface satisfies: a surface average roughness Ra within a 2 μm×2 μm range is less than 1.0 nm. 8.A method for testing surface potential of a cross-section active region of a Micro-LED, characterized in that, The method comprises the following steps: providing a Micro-LED cross section sample obtained by a sample preparation method of Micro-LED cross section surface potential testing according to any one of claims 1 to 7; obtaining a surface potential of an interface to be measured of the Micro-LED cross section sample by using a Kelvin probe force microscope.

9. The method for testing a surface potential of a Micro-LED cross section active region according to claim 8, wherein: a back of the Micro-LED cross section sample opposite to the interface to be measured is fixed on a metal sheet by using a conductive material, and an edge of the interface to be measured of the Micro-LED cross section sample is electrically connected to the metal sheet below; a probe of the Kelvin probe force microscope is used to scan the interface to be measured of the Micro-LED cross section sample, a set point of the Kelvin probe force microscope is set to 200 mV to 400 mV, a scanning rate is set to 0.5 Hz to 1 Hz, an integral gain input is set to 0.5 to 1.0, a proportional gain input is set to be 20% to 100% larger than the integral gain input, and a lifting scanning height is set to 30 nm to 100 nm, so as to obtain a topography image and a potential image of the interface to be measured, and the potential image comprises surface potentials of multiple regions of the interface to be measured. 10.A method for evaluating carrier transport in a cross-section active region of a Micro-LED, the method comprising: providing a Micro-LED structure; and performing a simulation on the Micro-LED structure to obtain a carrier transport in a cross-section active region of the Micro-LED. The method comprises the following steps: The Micro-LED cross-section sample obtained by the sample preparation method of the Micro-LED cross-section surface potential test in any one of claims 1-7; The surface potential of the interface to be tested of the Micro-LED cross-section sample obtained by the Micro-LED cross-section active region surface potential test method in claim 8 or 9 under light irradiation and no light irradiation conditions; Subtracting the surface potential obtained under the no light irradiation condition from the surface potential obtained under the light irradiation condition 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.

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