Method and system for characterising an optoelectronic device

Abstract

The invention relates to a method for characterising electroluminescent devices, which comprises: • illuminating the electroluminescent devices, the illumination being configured to generate charge carriers; • acquiring a series of luminescence images emitted by recombining the charge carriers in the electroluminescent devices; • determining, from the series of images and for a given electroluminescent device, a luminescence curve (20) as a function of time; • determining, from the luminescence curve (20) as a function of time, at least one parameter representative of an electroluminescence delay T, this electroluminescence delay T corresponding substantially to the start of an emission of radiation by electroluminescence of the given electroluminescent device.

Description

[0001] “Method and system for characterizing an optoelectronic device”

[0002] TECHNICAL FIELD

[0003] The present invention relates to the field of microelectronics and optoelectronics technologies. It finds a particularly advantageous application in the contactless control of optoelectronic devices, for example GaN-based micro-light-emitting diodes.

[0004] STATE OF THE ART

[0005] Typically, to form a self-emissive display screen comprising pixels emitting their own light, a plurality of optoelectronic devices of the LED (light emitting diode) or microLED or OLED (organic LED) type is required. These optoelectronic devices are first at least partly formed collectively on substrates, typically in the form of wafers, and then the devices are generally assembled individually, typically on a screen support, to manufacture the final system, typically the self-emissive display screen.

[0006] Before assembly, it is important to be able to test optoelectronic devices, collectively and / or individually, particularly to rule out malfunctioning devices. This avoids costly repairs or replacements later at the system level. However, it is counterproductive, or even harmful, to form electrical contacts on optoelectronic devices for testing before assembly. Electrical contact testing is complex to perform, particularly on microLEDs, and increases the time and cost of the manufacturing process. The presence of electrical contacts can also be problematic for subsequent assembly. Removal of the contacts may therefore be required, with the risk of damaging the optoelectronic devices.A non-destructive testing method, which can be easily implemented at different stages of the manufacturing process, is a substantial challenge for the industrial manufacturing of systems comprising a plurality of LED-type optoelectronic devices.

[0007] One solution is to develop “contactless” characterization techniques. Document US9823198B2 discloses a solution consisting of illuminating an array of LEDs and measuring a luminescence response of this array of LEDs, via a photodiode. The characteristics of the luminescence response, particularly in the transient part of the luminescence response, are interpreted to determine a junction photovoltage and an internal quantum efficiency in particular. This solution, however, does not allow individual microLEDs to be characterized. This solution also does not allow precise determination of the contributions of photoluminescence and electroluminescence to the luminescence response of the array of LEDs.

[0008] The present invention aims to at least partially overcome the drawbacks of the solutions mentioned above.

[0009] In particular, an object of the present invention is to provide a method for characterizing a set of electroluminescent devices, having improved accuracy and resolution. Another object of the present invention is to provide a system for characterizing a set of electroluminescent devices, making it possible to implement the characterization method.

[0010] Other objects, features and advantages of the present invention will become apparent upon examination of the following description and the accompanying drawings. It is understood that other advantages may be incorporated. In particular, certain features and advantages of the characterization method may apply mutatis mutandis to the characterization system, and vice versa.

[0011] SUMMARY

[0012] To achieve the above-mentioned objectives, one aspect relates to a method of characterizing a set of electroluminescent devices arranged in a matrix on a substrate, said method comprising:

[0013] - at least one illumination of the set of electroluminescent devices by a light source configured to generate charge carriers in the electroluminescent devices,

[0014] - at least one acquisition of a series of luminescence images by a sensor configured to capture radiation emitted by recombination of charge carriers generated in the electroluminescent devices,

[0015] - a determination of at least one parameter representative of an electroluminescence delay, said electroluminescence delay corresponding substantially to the start of an emission of radiation by electroluminescence of the given electroluminescent device.

[0016] Luminescence essentially integrates two contributions: a contribution due to photoluminescence and a contribution due to electroluminescence. The phenomenon of photoluminescence corresponds to a simple absorption-reemission of photons, for example directly in a quantum well. The photons coming from the illumination are absorbed by the device, which then presents an excited state. During de-excitation, the device will reemit photons, typically of lower energy. This absorption-reemission of photons occurs almost instantaneously, without delay (the reemission delay which corresponds to the radiative lifetime is typically less than a nanosecond). The phenomenon of electroluminescence corresponds to an emission of photons by recombination of charge carriers (electrons-holes). The operation of electroluminescent devices is based on this phenomenon of electroluminescence.The electroluminescent device here typically behaves like a capacitor: upon illumination, charge carriers are generated and stored in the energy bands until there are enough carriers to cross the potential barrier separating the energy bands. This accumulation of carriers under illumination corresponds to a transient regime of the electroluminescent device. The electroluminescence delay corresponds to the end of this transient carrier accumulation regime (the electroluminescence delay is significantly greater than the radiative lifetime mentioned above, typically of the order of a microsecond). When the accumulated carriers cross the potential barrier, they recombine with each other, producing photons. The precise determination of the electroluminescence delay therefore makes it possible to access the contribution of electroluminescence in the luminescence phenomenon.

[0017] In the context of the development of the present invention, it appeared that a determination of at least one parameter representative of the electroluminescence delay advantageously made it possible to reliably characterize the electroluminescent devices.

[0018] According to one possibility, said method comprises:

[0019] - at least one illumination of the set of electroluminescent devices by a light source configured to generate charge carriers in the electroluminescent devices,

[0020] - at least one acquisition of a series of luminescence images by a sensor configured to capture radiation emitted by recombination of charge carriers generated in the electroluminescent devices,

[0021] - a determination, from the series of luminescence images and for a given electroluminescent device of the set of electroluminescent devices, of a luminescence curve as a function of time,

[0022] - a determination, from the luminescence curve as a function of time, of at least one parameter representative of an electroluminescence delay, said electroluminescence delay corresponding substantially to the start of an emission of radiation by electroluminescence of the given electroluminescent device.

[0023] Advantageously, the luminescence curve is reconstructed from the data of the series of luminescence images acquired successively. Typically, a point of the luminescence curve corresponds to the same pixel or to the same group of pixels of an image of the series of images. The luminescence curve therefore advantageously comes from the same localized area on the images. A plurality of luminescence curves is thus extracted from the series of luminescence images. The characterization method is thus resolved spatially and temporally. This makes it possible to distinctly characterize several devices of the set of electroluminescent devices on the same substrate, via their luminescence curve. According to a preferred possibility, the spatial resolution of the luminescence images of the series of images makes it possible to individually distinguish each electroluminescent device on the substrate.The process thus makes it possible to obtain a luminescence curve for each electroluminescent device.

[0024] The luminescence curve integrates both contributions due to photoluminescence and electroluminescence. Accurate determination of the electroluminescence delay allows access to the electroluminescence contribution in the luminescence curve.

[0025] According to one embodiment, this determination is made by deriving the luminescence curve. The derived luminescence curve makes it possible to precisely determine the electroluminescence delay from which the electroluminescence device actually produces electroluminescence. According to one possibility, the derived luminescence curve has substantially a single peak and the electroluminescence delay corresponds to the maximum of this peak. According to another possibility, the derived luminescence curve has two peaks and the electroluminescence delay corresponds to the interval between these two peaks.

[0026] By determining the electroluminescence delay, the photoluminescence and electroluminescence contributions can be separated. The external quantum efficiency can be advantageously estimated accurately and reproducibly from the electroluminescence delay, for each electroluminescent device.

[0027] According to another embodiment, the at least one parameter representative of the electroluminescence delay corresponds to a set of statistical parameters resulting from a dimensional reduction of the luminescence curve. This set of statistical parameters can then be used to determine the external quantum efficiency of each electroluminescent device.

[0028] Another aspect relates to a system for characterizing a set of electroluminescent devices arranged in a matrix on a substrate, said system comprising:

[0029] - a light source configured to generate charge carriers in the electroluminescent devices,

[0030] - a sensor configured to capture radiation emitted by recombination of charge carriers generated in electroluminescent devices,

[0031] - a controller configured to control an illumination of the light source, and to acquire a series of luminescence images by the sensor,

[0032] - a processing module configured to:

[0033] • determine a luminescence curve as a function of time from the series of luminescence images and for a given electroluminescent device of the set of electroluminescent devices, and to • determine from the luminescence curve at least one parameter representative of an electroluminescence delay, said electroluminescence delay corresponding substantially to the start of an emission of radiation by electroluminescence of the given electroluminescent device.

[0034] Such a system advantageously makes it possible to implement the characterization method described above.

[0035] BRIEF DESCRIPTION OF THE FIGURES

[0036] The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of embodiments thereof which are illustrated by the following accompanying drawings in which:

[0037] Figure 1A illustrates an excitation signal and a first luminescence curve in response to this excitation signal, according to an embodiment of the present invention.

[0038] Figure 1B schematically illustrates a band diagram corresponding to the first luminescence curve shown in Figure 1A.

[0039] Figure 2A illustrates an excitation signal and a second luminescence curve in response to this excitation signal, according to an embodiment of the present invention.

[0040] Figure 2B schematically illustrates a band diagram corresponding to the second luminescence curve shown in Figure 2A.

[0041] Figure 3 illustrates an excitation signal, a luminescence curve in response to this excitation signal, a curve derived from the luminescence curve, and a photovoltage curve, according to an embodiment of the present invention.

[0042] Figure 4 illustrates a staircase excitation signal, a luminescence curve in response to this excitation signal, and a curve derived from the luminescence curve, according to another embodiment of the present invention.

[0043] Figure 5 illustrates a comparison of luminescence curves after correction for the photoluminescence contribution, according to one embodiment of the present invention.

[0044] Figure 6A illustrates different luminescence curves obtained experimentally, for different concentrations of magnesium, according to an embodiment of the present invention. Figure 6B illustrates different luminescence curves obtained by simulation, for different concentrations of donor traps, according to an embodiment of the present invention.

[0045] Figure 7 A schematically illustrates a characterization system, according to a first embodiment of the present invention.

[0046] Figure 7B schematically illustrates a characterization system, according to a second embodiment of the present invention.

[0047] Figure 7C schematically illustrates a characterization system, according to a third embodiment of the present invention.

[0048] Figures 8A, 8B, 8C illustrate a series of luminescence images acquired successively on a set of microLEDs, according to an embodiment of the present invention. The drawings are given as examples and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily on the scale of practical applications.

[0049] DETAILED DESCRIPTION

[0050] Before beginning a detailed review of embodiments of the invention, it is recalled that the invention according to its first aspect notably comprises the following optional characteristics which can be used in combination or alternatively:

[0051] According to one example, the method further comprises determining, from the series of luminescence images and for a given electroluminescent device of the set of electroluminescent devices, a plurality of representative luminescence values ​​as a function of time, the determination of the at least one parameter representative of an electroluminescence delay being carried out from said plurality of representative luminescence values ​​as a function of time. These representative luminescence values ​​correspond for example to photovoltage values.

[0052] According to one example, determining the at least one parameter representative of the electroluminescence delay comprises deriving the luminescence curve so as to obtain a derived luminescence curve, the at least one parameter representative of the electroluminescence delay being an interval AT between a time origin and a characteristic time corresponding to a maximum of said derived luminescence curve.

[0053] According to one example, the method further comprises, after determining the at least one parameter representative of the electroluminescence delay, a calculation of external quantum efficiency EQE according to:

[0054] Where LEL is an electroluminescence contribution to a total luminescence, and AT the interval representative of the electroluminescence delay.

[0055] According to one example, the at least one illumination is in the form of a staircase signal comprising two steps. The staircase signal makes it possible to maintain a charge in the electroluminescent device. According to one example, the at least one illumination is in the form of a pulsed signal. The pulsed signal typically comprises at least two peaks sufficiently close together to maintain a charge in the electroluminescent device. The conservation of a charge in the device makes it possible to overcome a measurement bias linked to an initiation of the charge in the device. The measurement of the electroluminescence delay is more reliable. The precision of the measurement of the electroluminescence delay is improved.

[0056] According to one example, the derived luminescence curve has at least a first peak and a second successive peak, the time origin being taken at the first peak and the characteristic time being taken at the second peak to determine the AT interval. The AT interval corresponds to the interval between the first and second peaks. The first peak of the derived luminescence curve is typically linked to the first step of the illumination staircase signal, or to a first peak of the pulsed illumination signal. The second peak of the derived luminescence curve is typically linked to the second step of the illumination staircase signal, or to a second peak of the pulsed illumination signal.

[0057] According to one example, the determination of the at least one parameter representative of the electroluminescence delay comprises a dimensionality reduction of the luminescence curve, for example by a first algorithm, configured to generate a set of statistical parameters making it possible to retain at least 90%, preferably at least 95%, and preferably at least 99% of the variance of the luminescence curve. This type of processing can advantageously be carried out by machine learning. The set of statistical parameters resulting from this processing has correlations with the electroluminescence delay, and in particular with a time derivative of the electroluminescence delay. A Pearson correlation of the order of 86% can for example be obtained between certain parameters of the set of statistical parameters and the time derivative of the electroluminescence delay.

[0058] According to one example, the method further comprises a calculation of external quantum efficiency EQE from the set of statistical parameters, by a second algorithm typically based on machine learning.

[0059] According to one example, the at least one acquisition is configured such that said images of the series of images present:

[0060] - a spatial resolution such that the electroluminescent devices are individually resolved on said luminescence images,

[0061] - a time resolution less than the duration of a transient emission phase of the electroluminescent devices.

[0062] This allows the reconstruction of a luminescence curve for each electroluminescent device in the image, with sufficient temporal resolution to account for the transient emission phase of the electroluminescent devices.

[0063] According to one example, the at least one acquisition is configured such that the series of luminescence images comprises at least five luminescence images during the transient emission phase of the electroluminescent devices.

[0064] According to one example, the at least one acquisition of the series of luminescence images is performed with an acquisition time less than or equal to 500 ns for each image of the series of luminescence images.

[0065] In one example, luminescence images have a spatial resolution less than or equal to one micrometer.

[0066] According to one example, the at least one illumination comprises a plurality of illumination pulses. According to one example, the at least one acquisition comprises a plurality of series of luminescence images, said series being synchronized with respect to the illumination pulses. This ultimately makes it possible to obtain a derived luminescence curve having several peaks corresponding to the different illumination pulses. The electroluminescence delay can be taken between two consecutive peaks of the derived luminescence curve. The measurement of the electroluminescence delay is thus carried out in a relative manner. This makes it possible to overcome possible delays linked to the measurement system, for example due to the signal transmission lines. This makes it possible to avoid possible measurement biases linked to residual charges of the devices. The measurement of the electroluminescence delay is more reliable. The precision of the measurement of the electroluminescence delay is improved.

[0067] According to one example, the at least one illumination comprises a plurality of illumination pulses. According to one example, the at least one acquisition of a series of images comprises the acquisition of a luminescence image associated with each pulse of the plurality of illumination pulses. According to one example, each acquisition is performed at an instant ti = t0 + i*ôt where t0 is the start of an illumination pulse of the plurality of illumination pulses, with 50 ns < 5t < 500 ns, for i varying from 1 to n.

[0068] According to one example, at least one, or where appropriate each acquisition is carried out after a series of two consecutive illumination pulses.

[0069] According to one example, the at least one illumination comprises a plurality of illumination pulses, and the at least one acquisition of a series of images comprises n acquisitions of a luminescence image each, each acquisition being carried out at an instant ti = tO + i*ôt where tO is the start of an illumination pulse of the plurality of illumination pulses, with 50 ns < 5t < 500 ns, for i varying from 1 to n. The illumination pulses are typically identical to each other. This makes it possible to acquire a series of images having different time intervals with respect to the instant tO corresponding to a start of an illumination pulse. This makes it possible to virtually reconstruct an acquisition frequency sufficiently high to obtain a plurality of images during the transient emission phase of the electroluminescent devices.

[0070] According to an example, each acquisition is performed after a series of two consecutive illumination pulses. The first pulse in this series allows the electroluminescent device to be charged and discharged properly. This allows any residual charges in the device to be evacuated. This allows an initial charge state to be controlled in a reproducible manner. This first pulse corresponds to a conditioning pulse, performed without acquisition. The second pulse in this series is the one from which image acquisition is performed. This avoids any measurement biases linked to residual charges in the devices. The measurement of the electroluminescence delay is more reliable. The accuracy of the measurement of the electroluminescence delay is improved.

[0071] According to one example, the illumination comprises two first pulses of the same power followed by a plurality of pulses of increasing power.

[0072] According to an example:

[0073] - the at least one illumination is in the form of a first staircase signal comprising two steps,

[0074] - the luminescence curve is presented in the form of a second staircase signal comprising two steps,

[0075] - the derived luminescence curve has two peaks corresponding to the two steps of the luminescence curve, the determination of the electroluminescence delay is done between the two peaks of the derived luminescence curve.

[0076] The electroluminescence delay measurement is also performed relatively. This eliminates possible delays related to the measurement system, for example due to signal transmission lines. This avoids possible measurement biases related to residual charges in the devices. The electroluminescence delay measurement is more reliable. The accuracy of the electroluminescence delay measurement is improved.

[0077] In one example, the portion of the luminescence curve before the electroluminescence delay is identified as a photoluminescence contribution, and the portion of the luminescence curve after the electroluminescence delay is identified as a sum of an electroluminescence contribution and the photoluminescence contribution. In one example, the method further comprises, after determining the electroluminescence delay, subtracting the photoluminescence contribution from the luminescence curve of the given electroluminescent device. This allows the electroluminescence performances of different electroluminescent devices to be compared and ranked with each other, without bias due to photoluminescence.

[0078] In one example, the external quantum efficiency is calculated as the product of the electroluminescence contribution and the electroluminescence delay.

[0079] In one example, the sensor has a pixel size smaller than a characteristic dimension of light-emitting devices. In one example, the sensor has a spatial resolution less than or equal to one micrometer.

[0080] In one example, the sensor has a time resolution of less than or equal to 500 ns.

[0081] In one example, the light source is pulsed. In one example, the controller is configured to synchronize the acquisition of series of luminescence images by the sensor with illumination pulses of the pulsed light source.

[0082] In one example, the system further includes a tracking device configured to track a drift over time in the illumination by the light source. In one example, the controller is configured to compensate for or account for said drift.

[0083] According to one example, the system further comprises a photodiode configured to acquire an average luminescence signal emitted by all of the light-emitting devices, which is not spatially resolved, said photodiode having a temporal resolution of the order of a nanosecond. The average luminescence signal acquired over all of the light-emitting devices makes it possible to increase the signal-to-noise ratio and to improve the accuracy of the measurement of the intensity, the delay and the average EQE.

[0084] According to one example, the system further comprises a module for measuring photo-induced voltage in the light-emitting devices, the set of light-emitting devices comprising first electrodes, and said module comprising second electrodes configured to form a capacitive coupling with the first electrodes, so as to measure a photo-induced voltage in the light-emitting devices. This makes it possible to have a direct measurement of the photo-induced voltage. The correlation between the electroluminescence delay and the photo-induced voltage is improved.

[0085] According to one example, the photo-induced voltage is estimated to be 1 / .

[0086] In one example, a luminescence quality criterion is assigned to the electroluminescent devices based on the electroluminescence delay. For example, for a set of electroluminescent devices, the electroluminescent devices having a electroluminescence delay greater than or equal to 70% of the maximum electroluminescence delay are considered functional. The other devices may be sorted and / or rejected. This makes it possible to obtain a subset of electroluminescent devices having good performance homogeneity around a nominal performance. The maximum electroluminescence delay is the highest electroluminescence delay that has been determined among the electroluminescent devices in the set of electroluminescent devices.

[0087] Unless inconsistent, technical features described in detail for a given embodiment may be combined with technical features described in the context of other embodiments described by way of example and not limitation, so as to form another embodiment which is not necessarily illustrated or described. Such an embodiment is obviously not excluded from the invention.

[0088] In the present invention, the method is in particular dedicated to the contactless characterization of electroluminescent devices, in particular devices of micrometric dimensions such as light-emitting diodes (LEDs). The individual LEDs or microLEDs typically have dimensions, in projection in an xy base plane, for example between 2.5 pm X 2.5 pm and 50 pm X 50 pm.

[0089] The invention can be implemented more broadly for various optoelectronic devices. The invention can, for example, be implemented in the context of laser devices.

[0090] The steps of the method as claimed are understood in the broad sense and may optionally be carried out in several sub-steps.

[0091] Several embodiments of the invention implementing successive steps of the production method are described below. Unless explicitly stated, the adjective “successive” does not necessarily imply, even if this is generally preferred, that the steps follow one another immediately, intermediate steps being able to separate them.

[0092] Furthermore, the term "step" does not necessarily mean that the actions carried out during a step are simultaneous or immediately successive. Certain actions of a first step may in particular be followed by actions linked to a different step, and other actions of the first step may be repeated subsequently. Thus, the term "step" does not necessarily mean actions that are unitary and inseparable in time and in the sequence of phases of the process.

[0093] In this patent application, the terms "light-emitting diode", "LED" or simply "diode" are used synonymously. An "LED" can also be understood to mean a "micro-LED" or even an LED associated with its own control electronics (smart LED), where applicable.

[0094] The terms "substantially", "approximately", "of the order of" mean, when they relate to a value, "within 10%" of this value.

[0095] In the present patent application, the luminescence curve may correspond to a luminous flux or power, or even an emission intensity of the device or a set of devices, as a function of time.

[0096] Figure 1A illustrates an excitation curve 10 corresponding to the illumination of a first electroluminescent device, and a first luminescence curve 20 corresponding to the response of this first device to the illumination. The curves 10, 20 are a function of time and are superimposed according to a standardized luminous flux scale, so as to compare them. The excitation curve 10 is in the form of a notch or a staircase step. The illumination reaches its maximum luminous flux without delay, it is almost instantaneous. The moment to where the illumination begins can be precisely determined without difficulty on this excitation curve 10 in notch, at 10 -7s near. The first luminescence curve 20 has a first part during which the luminous flux is almost zero, a second part where the luminous flux increases progressively, and a third part where the luminous flux stabilizes around a nominal value. The first part corresponds to an accumulation phase of the photo-generated carriers in the conduction and valence bands; the second part corresponds to a transient regime phase where the accumulated carriers begin to recombine by emitting photons; the third part corresponds to a steady-state phase where the carriers recombine by emitting photons. Such a luminescence curve 20 typically corresponds to an electroluminescence phenomenon of the first device.

[0097] Figure 1 B illustrates a band diagram schematically corresponding to the electroluminescence phenomenon described with reference to Figure 1A. In a light-emitting device comprising a PN junction between a P-type region and an N-type region, the Fermi levels EF of these regions align and a curvature of the conduction bands BC and valence bands BV is observed at the junction. A quantum well is represented here in the middle of the junction. When the conduction band is sufficiently filled, the - electrons can pass through the quantum well to recombine with the + holes of the valence band, which produces electroluminescence. Such a light-emitting device is therefore functional.

[0098] Figure 2A illustrates an excitation curve 10 corresponding to the illumination of a second electroluminescent device, and a second luminescence curve 20 corresponding to the response of this second device to the illumination. As previously, the curves 10, 20 are a function of time and are superimposed according to a standardized luminous flux scale, so as to compare them. The excitation curve 10 is in the form of a notch or a staircase step. The illumination reaches its maximum luminous flux without delay, it is almost instantaneous, at to. The second luminescence curve 20 has substantially the same shape as the excitation curve 10. The luminescence reaches its maximum luminous flux without delay, it is almost instantaneous, at to. Such a luminescence curve 20 typically corresponds to a single phenomenon of photoluminescence of the second device (no electroluminescence phenomenon).

[0099] Figure 2B illustrates a band diagram schematically corresponding to the photoluminescence phenomenon described with reference to Figure 2A. As before, the Fermi levels EF of the P-type and N-type regions align and a curvature of the conduction bands BC and valence bands BV is observed at the junction. A quantum well is represented here at the edge of the junction. In this case, the conduction band is instantly filled and the - electrons can pass through the quantum well to recombine with the + holes of the valence band without delay, which produces photoluminescence. The quantum well is poorly positioned with respect to the junction. The electroluminescence phenomenon does not take place. Such an electroluminescent device is therefore not functional.

[0100] In the context of the present invention, it has thus been observed that the time after which luminescence appears can be used to characterize functional and non-functional electroluminescent devices. One challenge is to precisely determine the electroluminescence time specific to each electroluminescent device.

[0101] Figure 3 illustrates a preferred possibility for determining the electroluminescence delay. As before, the excitation curves 10 and luminescence curves 20 are superimposed. The instant to corresponds to the start of illumination. The luminescence curve 20 is obtained by simulation on an electroluminescent device to be characterized. Curve 30 corresponds to a photo-induced voltage curve obtained by simulation on an ideal electroluminescent device, of the same type as that to be characterized. The photo-induced voltage curve 30 has three parts. The first part where the voltage is almost zero typically corresponds to the accumulation of carriers at the bottom of the conduction band: the electric field remains zero at the junction. The second part where the voltage increases progressively typically corresponds to the filling of the conduction band by the carriers up to the energy level of the quantum well: the device behaves like a capacitor.The third part where the voltage stabilizes around a nominal value corresponds to the spilling of carriers through the quantum well: the electric field no longer evolves. The transition between the second part and the third part of the photo-induced voltage curve 30 corresponds to the beginning of electroluminescence, when the electrons spilled into the quantum well recombine with the holes of the valence band. The interval between the instant to and the moment of this transition therefore corresponds to the electroluminescence delay.

[0102] In practice, it has been advantageously observed that the moment of this transition corresponds to the top of the peak of the derivative curve 21 of the luminescence curve 20. Thus, it is not necessary to carry out complex simulations to determine the electroluminescence delay. According to the invention, a derivation is carried out with respect to the time of the luminescence curve 20 to obtain the derived luminescence curve 21. According to a first possibility, the electroluminescence delay is determined from this derived luminescence curve 21, as being the interval between the instant to and the top of the peak of the derived luminescence curve 21.

[0103] Figure 4 illustrates a second possibility for determining the electroluminescence delay. In this variant, the excitation curve 10 is in the form of a staircase signal comprising two steps. In response, the luminescence curve 20 is also in the form of a staircase comprising two steps M1, M2. The steps of the signal of the excitation curve 10 are preferably configured so that the steps M1, M2 of the luminescence curve 20 are of different width and / or height. In particular, the illumination is configured so that the first step M1 is relatively smaller than the second step M2. The first step M1 typically corresponds to the initiation of a charge state in the electroluminescent device. The second step M2 typically corresponds to the electroluminescence phenomenon of the device.

[0104] In this case, the derived luminescence curve 21 has two peaks P1, P2 corresponding respectively to the two steps M1, M2 of the luminescence curve 20. The determination of the electroluminescence delay is done here between the peaks of the two peaks P1, P2 of the derived luminescence curve 21. The determination of the electroluminescence delay is done here in a relative manner. This makes it possible to overcome possible delays linked to the illumination and / or measurement system, for example linked to losses in the transmission lines. This also makes it possible to control the state of the electroluminescent device during the measurement. In particular, it is ensured that the electroluminescent device operates between two controlled charge states. The residual charges are neutralized. This makes it possible to avoid possible measurement biases. The measurement of the electroluminescence delay is more reliable and more precise.

[0105] Other variants are also possible. According to one example, the illumination is pulsed, the excitation curve 10 is in the form of square waves. The luminescence curve 20 can also be in the form of square waves. The derived luminescence curve 21 then has a plurality of peaks. The determination of the electroluminescence delay can also be done relatively in this case. The duration and / or intensity of the square waves of the excitation curve can vary.

[0106] After determining the electroluminescence delay, it is possible to subtract from the total luminescence of the device the luminescence contribution occurring before the electroluminescence delay. Such a contribution comes essentially from the photoluminescence phenomenon. It is thus possible to correct the luminescence curve so as to assess only the electroluminescence contribution of the device.

[0107] Figure 5 illustrates a comparison of luminescence curves as a function of the photoinduced voltage Vpv, after correction for the photoluminescence contribution.

[0108] As previously stated, the photo-induced voltage is related to the ability of the electroluminescent device to store a charge Q over time. The device is thus similar to a charge reservoir. The larger the reservoir, the longer the time required to fill the reservoir. The photo-induced voltage Vpv is therefore inversely proportional to the time required for filling. The value of the voltage Vbi from which electroluminescence appears (i.e. when the reservoir overflows, by analogy) is inversely proportional to the electroluminescence delay. Before the voltage value Vbi, only photoluminescence contributes to the luminescence of the device. After the voltage value Vbi, electroluminescence contributes, in addition to photoluminescence, to the luminescence of the device.

[0109] In Figure 5, curve 20a corresponds to a luminescence curve of a first device before correction of the photoluminescence contribution. Curve 20a* corresponds to the luminescence curve of the first device after correction of the photoluminescence contribution. Curve 20b corresponds to a luminescence curve of a second device before correction of the photoluminescence contribution. Curve 20b* corresponds to the luminescence curve of the second device after correction of the photoluminescence contribution. Curves 20b, 20b* are substantially superimposed because curve 20b does not show a photoluminescence contribution. Before correction, based on the total luminescence of the devices, the first device appears to perform better than the second device. When the photoluminescence contribution is subtracted, however, it appears that the second device performs better than the first device.In some cases, the photoluminescence contribution may be significant enough to bias the result of the non-contact characterization under illumination. According to a preferred possibility, the characterization method comprises a step of correcting the luminescence curves by subtracting the photoluminescence contribution. This makes it possible to evaluate the electroluminescence performance of the electroluminescent devices. The electroluminescent devices can be classified and / or sorted according to their electroluminescence performance.

[0110] After determining the electroluminescence delay, it is also possible to estimate the external quantum efficiency EQE of the device, according to the following relationship:

[0111] [Math]

[0112] Where L is the total luminescence, LEL is the electroluminescence contribution, LPL is the photoluminescence contribution, I is an electric current in the device, V is an electric voltage in the device, Vbi is the photo-induced electroluminescence voltage, and the electroluminescence delay.

[0113] By subtracting or neglecting the photoluminescence contribution, the EQE is calculated directly by the product between the electroluminescence flux and the electroluminescence delay. Since the method indicated for the determination of the electroluminescence delay is reliable and accurate, the calculation of the EQE is also reliable and accurate, especially for the purpose of ranking the relative performances of different electroluminescent devices.

[0114] The derived luminescence curve 21 can also be advantageously used to obtain further information on the device to be characterized. In the following, a simple illumination is implemented. The expected derived luminescence curve should therefore have a single peak. Figure 6A shows different derived luminescence curves 21 a, 21 b, 21 c, 21 d, 21 e, 21 f obtained experimentally for different GaN-based electroluminescent devices with varying magnesium Mg concentration. The magnesium Mg concentration here increases gradually between curve 21 a and curve 21 f. The derived luminescence curves 21 a, 21 b, 21 c, 21 d, 21 e, 21 f surprisingly have two peaks. The first peak P1 a, P1 b, P1 c... gradually decreases as the magnesium Mg concentration increases. It appears globally at the same time ti. The second peak P2a, P2b, P2c... remains substantially constant.It gradually shifts over time as the magnesium Mg concentration increases. In GaN, magnesium typically compensates for donor traps of type (Vca - O). 2- or more generally an oxygen-type impurity. By increasing the magnesium Mg concentration, the concentration of donor traps decreases.

[0115] Figure 6B shows different derived luminescence curves 21 as, 21 bs, 21 es, 21 ds, obtained by simulation for different electroluminescent devices with varying donor trap concentration. The donor trap concentration decreases progressively between the 21 as curve and the 21 ds curve. The behavior of the different peaks of the simulated curves 21 as, 21 bs, 21 es, 21 ds is similar to the behavior observed for the peaks of the experimental curves 21 a, 21 b, 21 c, 21 d, 21 e, 21 f. This tends to confirm that the first peak is in this case linked to the donor trap concentration of the device. The use of the derived luminescence curves can thus qualitatively provide information on whether the donor trap concentration is significant or not in the device. These derived luminescence curves can provide information on other characteristics of the devices, for example on residual doping of the device.

[0116] In practice, wafers (as substrates) to be inspected by luminescence comprise more or less dense sets of electroluminescent devices. It is therefore desirable that the characterization method allows the extraction of luminescence curves for each individual device. A global characterization is not suitable. A local characterization by mapping the different devices is time-consuming and may be confronted with drift problems. In the context of the present invention, a local characterization of the different devices by spatially and temporally resolved imaging has been developed. The luminescence curves are reconstructed from a series of successive images of the same area of ​​the wafer, by extracting the luminescence values ​​of the same pixel or the same group of pixels from each image.

[0117] Figures 7 A, 7B, 7C schematically illustrate different embodiments of a characterization system for implementing the characterization method by spatially and temporally resolved luminescence imaging. In particular, a high spatial resolution (submicron) camera is integrated into the system. This camera is a fast camera for acquiring images with a very short acquisition time (less than 500ns) and a precise exposure delay (<10ns offset). According to one possibility, the camera makes it possible to acquire images with a high frequency (from 2 to 30 images in one microsecond) from the start of illumination. This makes it possible to acquire, after a single illumination pulse, a series of several images during the transient emission phase of the electroluminescent devices.Alternatively, if the camera does not allow a high acquisition frequency, each image in the series of images is acquired after different identical illumination pulses, at different time intervals from the start of the illumination pulse used to trigger the acquisition. For example, a first image is acquired at t0 + times after a first illumination pulse. Then a second image is acquired at t0 + 2 times after a second illumination pulse identical to the first illumination pulse. Then a third image is acquired at t0 + 3 times after a third illumination pulse identical to the first and second illumination pulses. This sequence is continued until the acquisition of an nth image at t0 + n times after an nth illumination pulse.Intermediate illumination pulses, between the illumination pulses used to trigger image acquisitions, can be carried out in order to condition a charge / discharge state of the electroluminescent devices for example. These intermediate pulses can have different light powers from the illumination pulses used to trigger acquisitions.

[0118] The system illustrated in Figure 7A preferably comprises:

[0119] - a light source 1 configured to illuminate the electroluminescent devices of the wafer 2, by generating charge carriers in these devices. The light source 1 is typically monochromatic or quasi-monochromatic, with an emission wavelength λ centered in the near UV, for example λ = 365 nm. The light source 1 may be pulsed,

[0120] - a sensor 3 configured to acquire luminescence images of the electroluminescent devices. This sensor 3 is typically a camera having a spatial resolution less than or equal to 1 pm, with an acquisition time less than or equal to 500 ns,

[0121] - a controller 4 configured to control the illumination of the wafer 2 and the acquisition of the luminescence images.

[0122] The system comprises or is connected to a processing module configured to reconstruct the luminescence curves as a function of time from the series of luminescence images, typically from a series of pixels from the series of images. The processing module may be configured to calculate the curves derived from the luminescence curves.

[0123] The system may also comprise optical elements, such as a dichroic mirror 5, an objective 6 and one or more lenses 7. The system may also comprise a synchronization module 8 between the light source 1 and the sensor 3, so as to synchronize the illumination and the acquisition of the images.

[0124] The system illustrated in Figure 7B further comprises a tracking device or module 9 configured to track over time the characteristics of the illumination by the light source 1. This tracking module 9 may further comprise a photodiode 90 making it possible to improve the temporal detection of the start of illumination. The tracking module 9 may also comprise a light power measuring device 91 configured to track the fluctuations of the light source 1. This makes it possible to track any drift over time of the characteristics of the illumination. The controller 4 may be configured to compensate for or take this drift into account.

[0125] The system illustrated in Figure 7C further comprises an average luminescence detection of the wafer 2. This detection implements a beam splitter 92 and a photodiode 93 having a time resolution of the order of a nanosecond. The photodiode 93 is typically configured to acquire an average luminescence signal emitted by all of the electroluminescent devices, not spatially resolved. The photodiode 93 is typically connected to the light power measurement device 91.

[0126] Figures 8A, 8B, 8C illustrate a series of luminescence images acquired successively on a set of microLEDs. The image acquisition time is 200 ns. The image presented in Figure 8A corresponds to an acquisition carried out between 1 ps and 1.2 ps after the start of illumination. The image presented in Figure 8B corresponds to an acquisition carried out between 1.5 ps and 1.7 ps after the start of illumination. The image presented in Figure 8C corresponds to an acquisition carried out between 2 ps and 2.2 ps after the start of illumination. The illumination is constant for each of the three images. In the image of Figure 8A, only a few bright spots are visible. These bright spots correspond to microLEDs that emit luminescence relatively early, prematurely compared to the vast majority of “black” microLEDs that make up the image.These "white" microLEDs have little or no capacity to accumulate charges; they are typically non-functional. In the image in Figure 8B, the majority of the microLEDs begin to emit luminescence. At this stage, a slight variation in luminescence is observed across all the microLEDs. This may reflect a slight inhomogeneity in the individual performance of the microLEDs. In the image in Figure 8C, the variations in luminescence are less pronounced. The microLED array is generally homogeneous.

[0127] As illustrated through the previous examples, the characterization system and method according to the invention advantageously make it possible to evaluate without contact the performance of each microLED of a set of microLEDs, and to individually identify non-functional or defective microLEDs.

[0128] The invention is not limited to the embodiments previously described.

Claims

Claims 1. Method for characterizing a set of electroluminescent devices arranged in a matrix on a substrate (2), said method comprising: • at least one illumination of the set of electroluminescent devices by a light source (1) configured to generate charge carriers in the electroluminescent devices, • at least one acquisition of a series of luminescence images by a sensor (3) configured to capture radiation emitted by recombination of the charge carriers generated in the electroluminescent devices, • a determination of at least one parameter representative of an electroluminescence delay, said electroluminescence delay corresponding substantially to the start of an emission of radiation by electroluminescence of the given electroluminescent device.

2. Method according to the preceding claim further comprising a determination, from the series of luminescence images and for a given electroluminescent device of the set of electroluminescent devices, of a plurality of representative luminescence values ​​as a function of time, the determination of the at least one parameter representative of an electroluminescence delay being carried out from said plurality of representative luminescence values ​​as a function of time.

3. Method according to claim 1 further comprising a determination, from the series of luminescence images and for a given electroluminescent device of the set of electroluminescent devices, of luminescence values ​​as a function of time, the determination of the at least one parameter representative of an electroluminescence delay being carried out from said luminescence values ​​as a function of time.

4. Method according to claim 1 further comprising a determination, from the series of luminescence images and for a given electroluminescent device of the set of electroluminescent devices, of a luminescence curve (20) as a function of time, the determination of the at least one parameter representative of an electroluminescence delay being carried out from said luminescence curve (20) as a function of time.

5. Method according to the preceding claim in which the determination of the at least one parameter representative of the electroluminescence delay comprises: • a derivation of the luminescence curve (20) so as to obtain a derived luminescence curve (21), the at least one parameter representative of the electroluminescence delay being an interval AT between a time origin and a characteristic time corresponding to a maximum of said derived luminescence curve (21).

6. Method according to the preceding claim further comprising, after determination of the at least one parameter representative of the electroluminescence delay, a calculation of external quantum efficiency EQE according to: EQE = L EL * AT| where LEL is an electroluminescence contribution to a total luminescence, and AT the interval representative of the electroluminescence delay.

7. Method according to any one of claims 5 or 6 in which the derived luminescence curve (21) has at least a first peak and a second successive peak, the time origin being taken at the level of the first peak and the characteristic time being taken at the level of the second peak to determine the AT interval.

8. Method according to claim 7, in which the at least one illumination is in the form of a staircase signal comprising two steps or a pulsed signal.

9. Method according to any one of the preceding claims in which the determination of the at least one parameter representative of the electroluminescence delay comprises: • a dimensionality reduction of the luminescence curve (20), for example by a first algorithm, configured to generate a set of statistical parameters making it possible to conserve at least 90%, preferably at least 95%, and preferably at least 99% of the variance of the luminescence curve (20).

10. Method according to the preceding claim further comprising a calculation of external quantum efficiency EQE from the set of statistical parameters, by a second algorithm typically based on machine learning.

11. Method according to any one of the preceding claims in which the at least one acquisition is configured so that said images of the series of images present: • a spatial resolution such that the electroluminescent devices are individually resolved on said luminescence images, • a time resolution less than the duration of a transient emission phase of the electroluminescent devices.

12. Method according to the preceding claim in which the at least one acquisition is configured so that the series of luminescence images comprises at least five luminescence images during the transient emission phase of the electroluminescent devices.

13. Method according to any one of the preceding claims in which the at least one acquisition of the series of luminescence images is carried out with an acquisition time less than or equal to 500 ns for each image of the series of luminescence images.

14. Method according to any one of the preceding claims in which the luminescence images have a spatial resolution less than or equal to one micrometer.

15. Method according to any one of the preceding claims in which the at least one illumination comprises a plurality of illumination pulses, and in which the at least one acquisition of a series of images comprises the acquisition of a luminescence image associated with each pulse of the plurality of illumination pulses.

16. Method according to claim 15, in which each acquisition is carried out at an instant ti = tO + i*ôt where tO is the start of an illumination pulse of the plurality of illumination pulses, with 50 ns < ôt < 500 ns, for i varying from 1 to n.

17. Method according to one of the preceding claims in which at least one, or where appropriate each acquisition is carried out after a series of two consecutive illumination pulses.

18. System for characterizing a set of electroluminescent devices arranged in a matrix on a substrate (2), said system comprising: • a light source (1) configured to generate charge carriers in the electroluminescent devices, • a sensor (3) configured to capture radiation emitted by recombination of the charge carriers generated in the electroluminescent devices, • a controller (4) configured to control an illumination of the light source (1), and to acquire a series of luminescence images by the sensor (3), • a processing module configured to: - determining a luminescence curve (20) as a function of time from the series of luminescence images and for a given electroluminescent device of the set of electroluminescent devices, and for - determining from said luminescence curve (20) at least one parameter representative of an electroluminescence delay, said electroluminescence delay corresponding substantially to the start of an emission of radiation by electroluminescence of the given electroluminescent device.

19. System according to the preceding claim in which the sensor (3) has a pixel size smaller than a characteristic dimension of electroluminescent devices, said sensor (3) typically having a spatial resolution less than or equal to one micrometer.

20. System according to any one of claims 18 to 19 in which the sensor (3) has a time resolution less than or equal to 500 ns.

21. System according to any one of claims 18 to 20 in which the light source (1) is pulsed, and in which the controller (4) is configured to synchronize the acquisition of series of luminescence images by the sensor (3) with illumination pulses of the pulsed light source (1).

22. A system according to any one of claims 18 to 21 further comprising a tracking device (9) configured to track a drift over time of the illumination by the light source (1), and wherein the controller (4) is configured to compensate for or take into account said drift.

23. System according to any one of claims 18 to 22 further comprising a photodiode (93) configured to acquire an average luminescence signal emitted by all of the electroluminescent devices, not spatially resolved, said photodiode (93) having a temporal resolution of the order of a nanosecond.

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