DE102008013068B4 - Method and device for the spatially resolved determination of carrier lifetime in semiconductor structures - Google Patents

Method and device for the spatially resolved determination of carrier lifetime in semiconductor structures

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DE102008013068B4
DE102008013068B4 DE200810013068 DE102008013068A DE102008013068B4 DE 102008013068 B4 DE102008013068 B4 DE 102008013068B4 DE 200810013068 DE200810013068 DE 200810013068 DE 102008013068 A DE102008013068 A DE 102008013068A DE 102008013068 B4 DE102008013068 B4 DE 102008013068B4
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excess charge
semiconductor structure
time
s2
charge carriers
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Klaus Ramspeck
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Institut fuer Solarenergieforschung GmbH
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using infra-red, visible or ultra-violet light
    • G01N21/62Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/64Fluorescence; Phosphorescence
    • G01N21/6489Photoluminescence of semiconductors
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using infra-red, visible or ultra-violet light
    • G01N21/17Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/1717Systems in which incident light is modified in accordance with the properties of the material investigated with a modulation of one or more physical properties of the sample during the optical investigation, e.g. electro-reflectance
    • G01N2021/1719Carrier modulation in semiconductors
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01NINVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00Investigating or analysing materials by the use of optical means, i.e. using infra-red, visible or ultra-violet light
    • G01N21/17Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infra-red light
    • G01N21/3563Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infra-red light for analysing solids; Preparation of samples therefor
    • G01N2021/3568Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infra-red light for analysing solids; Preparation of samples therefor applied to semiconductors, e.g. Silicon
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01RMEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/26Testing of individual semiconductor devices
    • G01R31/2648Characterising semiconductor materials
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01RMEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/26Testing of individual semiconductor devices
    • G01R31/265Contactless testing
    • G01R31/2656Contactless testing using non-ionising electromagnetic radiation, e.g. optical radiation

Abstract

A method for spatially resolved determination of a local carrier lifetime in a semiconductor structure (1), the method comprising:
Generating excess charge carriers in the semiconductor structure (1) within a predefinable excitation time interval (31);
Determining, simultaneously for each of a plurality of sub-surfaces of the semiconductor structure, in each case of location-dependent first measured values S1 by integrating measurement signals dependent on a concentration of the excess charge carriers within a first time subinterval (25) in which the concentration of the excess charge carriers has not reached a quasi-stationary state , and spatially dependent second measurements S2 by integrating excess charge carrier dependent sensing signals within a second time subinterval (27) in which the concentration of excess carriers has reached a quasi-steady state, the first time subinterval (25) preceding the second time subinterval (27) ends;
Determining a value of the local carrier lifetime for each of the plurality of sub-surfaces of the semiconductor structure by relating the respective first measured value S1 with the respective second measured value S2.

Description

  • FIELD OF THE INVENTION
  • The present invention relates to a method and a device for the spatially resolved determination of carrier lifetimes in semiconductor structures.
  • BACKGROUND OF THE INVENTION
  • One of the decisive parameters for determining the quality of semiconductor materials is the charge carrier lifetime, ie the average time that lingers an electron-hole pair generated by an excitation on average in the excited state before it returns to its initial state by recombination. Methods for determining this material parameter are therefore of crucial importance in the monitoring and development of processes involving such materials.
  • Particularly in photovoltaics, where solar cells are produced as large-area components made of inexpensive semiconductor materials as possible, one often has to deal with considerable spatial inhomogeneities of the charge carrier lifetime over the surface. Therefore, imaging methods which make it possible to determine the distribution of the material quality, as expressed inter alia in the local charge carrier lifetime, spatially resolved, are of great importance here.
  • In principle, a distinction is made between static processes which reduce the service life under static conditions, ie. H. determine quasi-steady state conditions, from the amplitude of a lifetime-related signal, and dynamic processes that determine the lifetime from the dynamic history of a lifetime associated signal as the conditions change (eg, injection level).
  • The existing camera-based methods for measuring the carrier lifetime, namely ILM (Illuminated Lifetime Mapping), see, for example, US Pat. B. Patent DE 199 15 051 C2 , and PLI (Photoluminescence Imaging) described, for. In "Photoluminescence imaging of silicon wafers", Appl. Phys. Lett. 89, 044107 (2006) or in "Quantitative Lifetime Measurements with Photoluminescence Imaging". Proc. of the 22nd EPVSEC, Milan, Italy (2007), p. 354, use measurements of the charge carrier concentration under static conditions.
  • The ILM is based on the emission / absorption of electromagnetic radiation by free charge carriers in the material. By pulsed excitation of additional charge carriers by means of a modulatable light source, the infrared emission / absorption of the free charge carriers in the material is increased, which can be detected spatially resolved with the aid of a suitable infrared camera. The determination of the density of the additionally excited by the light free charge carriers in the material in the region of a specific partial surface of a semiconductor substrate, ie the determination of the local excess charge carrier density .DELTA.n (x, y), is done using a calibration of the camera signal with wafers of known majority carrier density. An important prerequisite is that the calibration wafers and the samples to be measured have identical optical properties. Furthermore, a homogeneous and known illumination intensity and a homogeneous temperature of both the sample to be measured and the background must be ensured and the front reflection of the sample to be measured at the excitation wavelength must be known. The temperatures must also be the same when calibrating as during the measurement. If these prerequisites are met, the measurement of the additional emission / absorption of the excess charge carriers excited in the samples to be examined, which is usually carried out using a lock-in method, can be directly deduced to Δn (x, y). Together with the known generation rate G, the carrier lifetime results in: τ (x, y) = Δn (x, y) / G (1)
  • To determine the charge carrier lifetime on optically inhomogeneous samples, a method has been proposed which converts the measured ILM signal with the aid of an emission image to an optic corresponding to the calibration wafers. This method assumes a linear relationship between the absolute emission of a wafer and the additional emission by the additionally excited charge carriers. This linear relationship is not exactly fulfilled when taking into account reabsorption processes in the material and therefore represents only an approximation.
  • In PLI photoluminescence measurements, the intensity of radiative recombination is measured using a Si-CCD camera or an InGaAs camera. This intensity depends on the carrier concentration in the material according to: S PL = C Opt * B * n * A (2)
  • Here S PL denotes the signal strength, B is the coefficient of radiative recombination and n and p the electron and hole concentration, respectively. In addition, influences of the optics of the sample and the probability of detection are included in the intensity determined with the aid of a camera (C opt ). Therefore, a calibration is necessary here in order to convert the measured signal into absolute lifetimes. Furthermore, for the correct determination of the excess charge carrier concentration, the reabsorption of the luminescence photons in the material of the sample to be examined must be taken into account, which presupposes the knowledge of the depth distribution of the excess charge carrier density. The generation rate G must also be known, which in turn requires a homogeneous and known illumination intensity as well as front reflection of the sample to be examined. In addition to calibration by comparison measurements with other methods for determining the charge carrier lifetime, a self-consistent method was proposed here, which uses the hysteresis of the signal with respect to the excitation in the case of triangular excitation. However, also in this method, the dynamic method of hysteresis is used only to determine a calibration factor with which the luminescence signal measured under static conditions is converted into absolute lifetimes. Therefore, this method does not use the advantages of dynamic methods over static methods.
  • In dynamic methods, the lifetime can be determined directly from the time course of a signal, which is proportional to the concentration of the charge carriers in the semiconductor structure to be examined. Therefore, in dynamic methods, the particular lifetime does not depend on pre-factors that can affect the amplitude of the signal, such as, for example. Example, the detection probability of a detector or the emissivity of the sample. Such dynamic methods are therefore in principle more robust and can be used more easily for different sample structures.
  • Dynamic methods for the spatially resolved measurement of the charge carrier lifetime have been used to measure the sample by scanning it point by point and only then to assemble it into an image. This leads to long measuring times, since 389,376 individual measurements must be carried out for samples of a size of, for example, 156 × 156 mm 2 at a resolution of 250 μm per dot. With a carrier lifetime of, for example, 1 ms, each measurement requires at least a time of approximately 10 ms. This gives a measurement duration of at least 65 minutes for the entire sample, which can no longer be reduced by technical improvements. However, real-world measurement systems often require multiple measurements for each point to achieve a sufficiently strong signal, meaning typical measurement times of often several hours.
  • The most widely used dynamic scanning method is the MWPCD (microwave detected photo-conductance decay). B. in the German patent DE 691 08 563 T2 is described. Here, charge carriers in the semiconductor material to be examined are locally excited by laser pulses. The decrease in the charge carrier concentration after switching off the light is determined by measuring the microwave reflection. The time constant determined when fitting an exponential function to the asymptotic refraction of the microwave reflection in this case directly gives the charge carrier lifetime.
  • In the US 5,153,503 For example, a method and apparatus for time-dependent measurement of carrier lifetime of a 4th group semiconductor is described.
  • In the DE 100 56 770 A1 For example, a method and apparatus for measuring properties of a sample in multiple points of the sample is described.
  • Existing static methods for camera-based carrier lifetime measurement require elaborate calibrations and have the fundamental problem that the lifetime values determined depend on the optical properties of the sample. The only proposed dynamic measuring method with triangular excitation still requires knowledge of the doping concentration as well as the generation rate - and thus the frontal reflection of the samples to be measured. By contrast, dynamic methods are performed by scanning and therefore require considerable measurement periods.
  • SUMMARY OF THE INVENTION
  • There may therefore be a need for a method for the spatially resolved determination of the charge carrier lifetime and for a corresponding device, in which, in particular, the above-mentioned problems of the prior art are at least partially overcome. In particular, a method may be required which allows the determination of the carrier lifetime independently of the knowledge of the front reflection, generation rate, sample optics or doping concentration and which thus allows a determination of the lifetimes on any samples without additional measurements or calibrations. Furthermore, a method can be sought in which the use of a camera for the measurement also allows low measurement times for large samples with good spatial resolution.
  • This need can be met by the subject-matter of the independent claims. Advantageous embodiments of the present invention are described in the dependent claims.
  • According to a first aspect of the present invention, a method is proposed for the spatially resolved determination of a local charge carrier lifetime in a semiconductor structure, the method comprising the following steps: generating excess charge carriers in the semiconductor structure within a predefinable excitation time interval; Determining, simultaneously for each of a plurality of sub-surfaces of the semiconductor structure, respectively of location-dependent first measurement values S1 by integrating measurement signals dependent on a concentration of the excess charge carriers within a first time subinterval in which the concentration of the excess charge carriers has not reached a quasi-stationary state; location-dependent second measurement values S2 by integrating measurement signals dependent on a concentration of the excess charge carriers within a second time subinterval in which the concentration of the excess charge carriers has reached a quasi-stationary state, wherein the first time subinterval ends before the second time subinterval; Determining a value of the local carrier lifetime for each of the plurality of sub-surfaces of the semiconductor structure by relating the respective first measured value S1 with the respective second measured value S2.
  • In other words, the inventive method according to the first aspect can be considered to be based on the following idea:
    In a semiconductor structure, excess charge carriers are generated by, for example, illuminating the semiconductor structure with light. The excitation time interval, that is to say the illumination duration in this case, can be preset. It may be advantageous if the excitation intensity, that is to say the light intensity in the example, rises as quickly as possible from an initial value to a maximum value, for example within less than 5 μs. In other words, the time profile of the excitation intensity may have a rectangular profile with a steep edge at the beginning and / or end of the excitation time interval.
  • In order to be able to determine the charge carrier lifetime with spatial resolution, in each case a first measured value S1 and a second measured value S2 are determined for each of a plurality of partial surfaces of the semiconductor structure by integrating a measuring signal dependent on a concentration of the local excess charge carrier. Such measurement signals dependent on the excess charge carrier concentration can be, for example, the infrared emission or infrared absorption of the semiconductor structure, the emission / absorption being higher the higher the local excess charge carrier concentration. Alternatively, the local photoluminescence can be measured as a measurement signal due to radiative recombination of excess charge carrier pairs.
  • It is essential for the method according to the invention that the first time subinterval, within which the first measured values S1 are measured, ends before the second time subinterval. In principle, the first and the second time subintervals may start at the same time but end at different times. However, it is preferred that the first time sub-interval ends before the second time sub-interval begins. Both time sub-intervals may preferably end within the excitation time interval or, alternatively, end after the excitation time interval. It may be preferred that the first time subinterval lies in the region of the beginning of the excitation time interval, that is to say within a time period in which the excess charge carrier concentration still increases and has not yet reached a quasi-stationary state. The first time subinterval may begin shortly before, exactly at or shortly after the start of the excitation time interval, for example less than 10 ms before or after. The duration of the first time subinterval should be chosen such that even at the end of the first time subinterval, the excess charge carrier concentration has not yet reached a quasi-stationary state. The second time subinterval may then be chosen to be, at least in large part, within a time period in which the excess charge carrier concentration has reached a quasi-steady state.
  • The measured values S1, S2 determined within the two time subintervals for each of the sub-surfaces are then used to determine the local charge carrier lifetime prevailing there for each of the sub-surfaces. For this purpose, the two location-dependent measured values S1, S2 are related to one another. In this context, an "in-relational setting" can be understood as meaning that a value for the local charge carrier lifetime can only be determined taking into account both measured values S1 and S2, whereas the knowledge of only one measured value does not suffice to determine the charge carrier lifetime. Thus, the "putting in-relation" is different from a pure mean value formation, in which even the knowledge of a single measured value can be used to determine - albeit less accurately - a parameter value. The "putting into relation" can be, for example, the formation of a quotient S1 / S2 of the two measured values or a subtraction S2-S1. It is explicitly pointed out that at least two measured values S1, S2 are determined for the subsequent in-relation setting, but also any desired more measured values S1, S2, S3,... Are determined within different time subintervals and then put into relation can. For example, an additional third measured value can be determined that takes account of a background or noise-related contribution.
  • By determining two measured values S1, S2 depending on the local, instantaneous charge carrier concentration, in each case according to the invention, for each partial area of a semiconductor structure, the first measured value S1 preferably being determined during a non-quasi-stationary state of the excess charge carrier excitation and the second measured value S2 being determined during one quasi-stationary state of the excess charge carrier excitation is determined, and the two measured values S1, S2 are related to each other, external effects such as the optical reflection properties of the surface of the semiconductor structure or the spatial homogeneity of the excitation used to generate the excess charge carriers can be eliminated. The measurement of the local charge carrier lifetime is largely independent of such external influences and can therefore be carried out without previous or parallel calibration. Furthermore, since the measurement can take place simultaneously for a plurality of sub-surfaces of the semiconductor structure, it is possible to perform a very fast spatially resolved determination of the local carrier lifetime for an entire substrate surface.
  • Hereinafter, further features, advantages and embodiments of the method according to the invention will be described according to the above first aspect.
  • The semiconductor structure whose local charge carrier lifetime is to be determined in a spatially resolved manner can be, for example, a wafer made of silicon or another semiconductor material. The semiconductor structure may also be a semiconductor thin film or an already partially processed semiconductor device such as a partially processed solar cell. The semiconductor structure can have a homogeneous surface, as can be found, for example, in monocrystalline semiconductor structures. However, the method is also particularly suitable for semiconductor structures with inhomogeneous surfaces such as multicrystalline semiconductor structures or semiconductor structures with a surface texturing. Particularly in the case of semiconductor structures with inhomogeneous surfaces, the advantage of the method comes into play, that the local charge carrier lifetime can be determined independently of the local reflection properties of the semiconductor structure.
  • Within the semiconductor structure, excess charge carriers can be generated in different ways during a predefinable excitation time interval. For example, the semiconductor structure can be illuminated over a surface area by a light source during the excitation time interval, so that electron-hole pairs within the semiconductor structure are produced by the absorption of the incident light. The concentration of the excess charge carriers thus generated depends on the intensity of the light absorbed and absorbed in the semiconductor structure. In this case, the intensity of the incident light can rise rapidly from zero to a predetermined maximum. The fastest possible increase, that is, the steepest possible slope of the excitation intensity, is advantageous for the process. Alternatively, the excitation intensity can also be raised from a previously set basic level, for example a permanently prevailing backlight (bias light) during the excitation time interval to a correspondingly higher stimulation level and then drop back to the base level. As a result, for example, the charge carrier lifetime can also be determined as a function of different original excess charge carrier concentrations.
  • The measured values S1 and S2, which are determined by integrating a measured quantity which depends on the excess charge carrier concentration, can be determined simultaneously for a plurality of partial surfaces of the semiconductor structure in accordance with the invention. In contrast to conventional dynamic methods, in which a local carrier lifetime is determined only for a single partial surface of the semiconductor structure and the entire surface of the semiconductor structure has to be scanned step by step, which leads to very long measurement times, the method according to the invention allows the location-dependent local carrier lifetime be determined simultaneously for large areas or the entirety of the surface of the semiconductor structure. This allows very short measuring times in the range of a few seconds. For the simultaneous determination of the plurality of measured values S1 or S2, for example, a camera with a planar detector may be used, each one of the camera-like pixels of the camera respectively measuring one of the plurality of partial surfaces with respect to electromagnetic radiation emitted there, for example infrared light ,
  • The first and second measured values S1, S2 measured in this way can then be used separately for each of the plurality of sub-surfaces of the semiconductor structure for determining a value of the local charge carrier lifetime.
  • According to one embodiment of the method according to the invention, the local Charge carrier lifetime determined from the quotient S1 / S2 of a respective first measured value S1 and a respective second measured value S2. In this particular form of "in-relation setting", the quotient formation leads to pre-factors, as in the determination of the individual measured values S1 and S2, for example by external factors such as locally prevailing reflection, inhomogeneities with respect to the locally generated excess charge carrier concentration, inhomogeneities of the local Temperature of the semiconductor structure, etc. occur, away. Due to the fact that the first time subinterval ends before the second time subinterval, the carrier lifetime can be determined from the quotient S1 / S2.
  • According to another embodiment of the present invention, the first and second time subintervals both end within the excitation time interval. Alternatively, the first and second time subintervals both end after the excitation time interval. Using the example of an excess charge carrier generation by illumination of the semiconductor structure with light and detection of a local excess charge carrier concentration dependent emission / absorption of infrared light or a local luminescence by charge carrier recombination using a camera, this means that the camera takes two images, the two periods, within which the camera integrates the incident light signal, both ends during the illumination time interval (first alternative) or after switching off the light source, ie after the illumination time interval end (second alternative). Due to the fact that the first time subinterval ends before the second time subinterval, the first and second measurement signals S1, S2 can be determined at different time periods within the excitation time interval, for example at the beginning of the excitation time interval, if no quasi-stationary state has yet settled with respect to the excess charge carrier concentration, and towards the end of the excitation time interval when the excess charge carrier concentration has assumed a quasi-steady state value. The same applies to a recording of the measured values S1, S2 after switching off the excitation, when the excess charge carrier concentration generated during the excitation time interval decreases gradually from a quasi-stationary value to zero or a base value by recombination of the excess charge carriers.
  • According to a further embodiment of the present invention, the first time interval ends at the latest 10 ms, preferably at the latest 5 ms, and more preferably at the latest 1 ms after the beginning or, alternatively, after the end of the excitation time interval. In other words, the first measured values S1 should be determined as soon as possible after switching on or off the excitation source, for example the light source for generating the excess charge carriers. Within such a time subinterval, the excess charge carrier concentration continuously increases due to the generation of additional charge carriers associated with the excitation or continuously decreases when the excitation source is switched from an original quasi-stationary output value, but has not yet assumed a quasi-stationary target value. The first time subinterval may start simultaneously with the excitation time interval or shortly before or after it, for example 1 ms before or after the start of the excitation time interval.
  • According to a further embodiment, the first and second measured values S1, S2 are respectively determined by integrating measurement signals dependent on a concentration of the excess charge carriers over an integration period of less than 10 ms, preferably less than 5 ms and more preferably less than 1 ms. The integration duration may be selected such that the excess charge carrier concentration has not reached a quasi-stationary state during the first time subinterval. Since this quasi-stationary state is reached the faster, the shorter the charge carrier lifetime, the shortest possible integration time is sought, in order to be able to measure semiconductor structures with very short carrier lifetimes, for example in the range of a few microseconds. However, it must be taken into account that with decreasing integration time, the measurement signal is weaker and thus a signal-to-noise ratio is worse. Therefore, integration times of less than 100 μs should be avoided as a rule. In particular, it may be advantageous to adapt the integration duration to the expected service life of a specific semiconductor structure.
  • According to a further embodiment of the present invention, an excitation intensity with which the excess charge carriers are generated in the semiconductor structure increases to a maximum within less than 50 μs, preferably less than 20 μs and more preferably less than 5 μs. In other words, an excitation intensity with which excess charge carriers are generated in the semiconductor structure should change as quickly as possible from an initial value to a final value. The time course with which the excitation intensity changes should therefore have the steepest possible starting edge or trailing edge. Preferably, the time profile of the excitation intensity has a rectangular profile. The faster the excitation intensity rises from an initial level, that is, for example, off-centered excitation or excitation with a baseline level (eg bias light) to a maximum level, the better Ignore influences due to the timing of the excitation intensity in the subsequent determination of the local charge carrier lifetime.
  • According to a further embodiment of the present invention, the excess charge carriers are generated by means of a light source illuminating the sub-surfaces of the semiconductor structure. It can be used any light source, such as incandescent or discharge lamps, with light sources that can quickly reach a maximum light emission and allow a homogeneous illumination, such as LED arrays or lasers may be preferred. The emitted light intensity can be selected as high as possible in order to obtain strong measurement signals for determining the first and second measured values S1, S2. However, it may also be advantageous to select the emitted light intensity in accordance with a light intensity used in the later use of the semiconductor structure. For example, when the semiconductor structure is a solar cell, a light intensity corresponding to normal solar radiation may be desirable to determine the carrier lifetime at such a light intensity.
  • According to a further embodiment of the present invention, the location-dependent first and second measured values S1, S2 can be determined with the aid of a camera. The camera can have a planar detector with pixels arranged like a matrix. Each detector pixel can detect the electromagnetic radiation emitted by an associated sub-surface of the semiconductor structure and integrate it over a corresponding time subinterval. The detector should be sufficiently sensitive to still provide a sufficiently strong measurement signal at short integration intervals. Suitable detectors are, for example, GaAs detectors (gallium arsenide detectors), InSb detectors (indium antimony detectors), QWIP detectors (Quantum Well Infrared Photon), MCT detectors (Mercurium Cadmium Telluride), S1-CCD detectors. Detectors (silicon charge coupled device) or InGaAs detectors (indium gallium arsenide).
  • According to another embodiment of the present invention, the location-dependent first and second measured values S1, S2 are determined by determining the local absorption or emission of infrared radiation. The intensity of the emitted or absorbed infrared radiation depends directly on the concentration of the free excess charge carriers and their temperature. By, for example, producing a temperature difference between the semiconductor structure to be measured and the background detected by the detector, a measurement signal dependent on the local concentration of the excess charge carrier can thus be determined by detecting the absorption or emission of infrared radiation with the aid of a corresponding infrared camera. For example, a heated semiconductor structure in front of a cold background, or conversely, a cold semiconductor structure in front of a heated background may be used for measurement purposes.
  • According to a further embodiment of the present invention, a plurality of first measured values S1 and a plurality of second measured values S2 are determined for each partial surface by means of a lock-in method within cyclically successive excitation time intervals. The first and second measured values S1, S2 can then be correlated with a sine function and a cosine function in order to be able to derive a measure of the carrier lifetime from the quotients of the measured values correlated with these two functions, for example. In this way, reliable statements about the local charge carrier lifetime can be determined in a metrologically simple manner by, for example, cyclically switching on and off an excitation source for generating excess charge carriers and locking the first and second measured values S1, S2 by a lock-in. Procedures are measured in time dependence on the cyclic switching on and off and then put in relation to each other in a simple manner.
  • According to a further aspect of the present invention, an apparatus for the spatially resolved determination of a local carrier lifetime in a semiconductor structure is proposed, wherein the device is designed to carry out the method described above. In particular, the device can have an excess charge carrier generation device which is designed to generate excess charge carriers in the semiconductor structure within a predefinable excitation time interval. Furthermore, it can have a measured value determination device that is designed to simultaneously for each of a plurality of sub-surfaces of the semiconductor structure each location-dependent first measured values S1 by integrating of a concentration of the excess charge carrier-dependent measurement signals within a first time subinterval, in which the concentration of the excess charge carriers is not quasi -stationary state, and location-dependent second measurement values S2 by integrating measurement signals dependent on a concentration of the excess charge carriers within a second time subinterval in which the concentration of the excess charge carrier has reached a quasi-stationary state, the first time subinterval ending before the second time subinterval, to determine. The device may further include a Have evaluation, which is designed to determine a value of the local charge carrier lifetime for each of the plurality of sub-surfaces of the semiconductor structure by In-relation set each of the first measured value S1 with the second measured value S2 for the respective sub-surface of the semiconductor structure.
  • It is noted that embodiments, features and advantages of the invention have been described mainly with respect to the method according to the invention for the spatially resolved determination of a local charge carrier lifetime in a semiconductor structure. However, one skilled in the art will recognize from the foregoing and also from the following description that, unless stated otherwise, the embodiments and features of the invention can also be analogously applied to the device according to the invention for the spatially resolved determination of a local charge carrier lifetime in a semiconductor structure. In particular, the features of the various embodiments can also be combined with one another in any desired manner.
  • BRIEF DESCRIPTION OF THE DRAWINGS
  • Other features and advantages of the present invention will become apparent to those skilled in the art from the following description of exemplary embodiments, which should not be construed as limiting the invention, and with reference to the accompanying drawings.
  • 1 shows a device for the spatially resolved determination of a local carrier lifetime in a semiconductor substrate according to an embodiment of the present invention.
  • 2 shows by way of example the time profile of an excitation for generating excess charge carriers in a semiconductor structure during a predeterminable excitation interval and the corresponding time subintervals during which the measured values S1, S2 are determined according to an embodiment of the present invention.
  • All figures are only schematic and not to scale.
  • DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
  • Embodiments of methods according to the invention for measuring the effective charge carrier lifetime in a semiconductor structure will first be described.
  • The basis is a camera system with short exposure times of less than about 5 ms for the detection of a signal which is determined by the excess charge carrier concentration in the semiconductor material to be examined. For the measuring method according to the invention, a preferably rectangular excitation of charge carriers in the semiconductor material to be examined is used with steep edges as far as possible over time. This excitation can be done for example by a modulated light source of suitable wavelength. The light source may be, for example, a laser or a light-emitting diode array.
  • The excitation of the charge carriers in the semiconductor material to be examined takes place with a generation rate G. Assuming a temporally constant lifetime τ increases with increasing excess carrier concentration Δn and the recombination rate proportional to Δn: R = Δn / τ and Δn (t) = G · t - ∫R (t) dt (3)
  • Thus follows for the carrier concentration during an on / off time interval of the period T, the following dependence:
    during the excitation time interval, that is during the on-time interval: Δn (t) = Gτ (1-exp (-t / τ)) tτ ≤ T / 2 (4)
  • Outside the excitation interval, that is during the off-time interval: Δn (t) = Gτ (1 - exp (- T / 2 / τ)) · exp (- t - T / 2 / τ) ≈ Gτ · exp (- t - T / 2 / τ) with T / 2>> τ for t> T / 2 (5)
  • The approximation T / 2 >> τ corresponds to the condition that the system in each case during each period or during each excitation time interval and between two excitation time intervals reached a quasi-steady state (steady state state). The charge carrier concentration determining the detected measurement signal is thus shifted in time relative to the excitation.
  • If one measures mm with sufficiently short integration time, typically less than 5 ms, a quantity proportional to the charge carrier concentration, such as the luminescence radiation or the infrared emission or infrared absorption of the free charge carriers directly after switching on the excitation, then the measured signal strength depends on the Life of the charge carriers in the semiconductor structure.
  • If one divides a first measured value S1 corresponding to this signal by a measured value S2, which is determined on the basis of a signal which is measured a certain time later in the same excitation time interval, then the information about the carrier lifetime is Ratio, that is in the quotient, containing the two measured values. In this case, optical effects as well as inhomogeneities of the measurement setup are contained in identical form in both measured values and are eliminated by the use of the quotient.
  • Since the signals which can be measured in a short integration time of a few milliseconds are typically very small, the measuring method according to the invention in a preferred embodiment involves the averaging of several measured values S1, S2 or else the use of a lock-in algorithm for improving the signal -Rausch ratio.
  • It may be important that the determination of the first measured value occurs shortly before or shortly after the start of the excitation time interval, whereby the end of the time subinterval during which the first measured value is integrated must lie after the beginning of the excitation time interval. For the special case that the measured values S1, S2 are recorded by image recording with the aid of a camera and the excess charge carriers in the semiconductor structure are produced by illumination with a light source, this means that the image acquisition of the first image occurs shortly before or after the light source is switched on , wherein the time of switching on the light source must be before the end of the image integration time of the camera.
  • A simple relationship between charge carrier lifetime and the ratio of the measured values S1 in the image which is recorded directly after the light source is switched on and the measured values S2 in the image, which takes place some time later, after quasi-stationary conditions have been established have to be recorded. This allows a charge carrier lifetime value to be calculated for each pixel, which can be determined independently of the optics of the sample as well as of other factors influencing the absolute measurement signal.
  • According to a further embodiment of the method according to the invention, a lock-in method can be used to measure the very small signals in the determination of the first and second measured values S1, S2 with a high signal-to-noise ratio and to suppress the influences of test setup and sample optics , The lock-in measurement system is designed for 4-point correlation to achieve noise suppression by averaging over many periods and by correlating with periodic functions. For example, by correlation with a sine and a cosine function, two signal components, namely the sine-correlated signal S sin and the cosine-correlated signal S cos can be determined as follows:
    Figure 00220001
  • Where: T = 2π / ω; m can be a number greater than or equal to 4, which allows a 4-point correlation, for example, 4, 8, 10, 12, 16 .... The constant c here contains the influences of the measuring system and the sample optics and is thus location-dependent. So are the two parts of the signal, as well as the signal amplitude
    Figure 00220002
    Of optics and measuring system dependent and thus not clearly associated with the carrier lifetime. The so-called phase of the signal is not influenced by influences of the sample optics and the measuring system. The phase of such a lock-in measurement can be defined as:
    Figure 00230001
  • The above-described interferences such as sample optics, etc. are eliminated in the division of the two parts of the signal, since they are identical in both contain. From the equations given above, an associated phase can now be calculated for each lifetime. This allows the self-consistent determination of carrier lifetime on optically inhomogeneous samples.
  • An essential component of the method according to the invention according to one embodiment is the image acquisition directly or very shortly after switching on and directly or very shortly after switching off an excitation for generating excess charge carriers. As a result, a particularly high sensitivity of the determined phase over the life is achieved. Depending on the carrier lifetime to be determined and the integration time of the camera used, the image acquisition directly after switching on or off or even taking a picture at a later date may lead to optimal measurement results. It is provided in the embodiment of the method and the device according to the invention, the use of a measuring system with the possibility of a time shift between image acquisition and excitation. This time shift can be between -10 ms and +10 ms.
  • Furthermore, in one embodiment of the method according to the invention, the use of an additional illumination as background light (bias light) for setting an injection level in the semiconductor structure to be examined is provided if this appears expedient for the measurement.
  • By measuring the time shift between the excitation and the determined measuring signal for the first and second measured values S1, S2 in accordance with the embodiment, optical effects are eliminated from camera-based measurements. Furthermore, no spatially perfect homogeneous excitation is required for the proposed method, and in the case of the ILM, no homogeneous temperature control of the semiconductor structure and the background. For the phase-sensitive ILM, only a sufficiently large temperature difference between sample and background is needed to generate a sufficiently strong ILM signal.
  • An essential advantage of the method according to an embodiment of the invention over conventional methods for spatially resolved measurement of the effective charge carrier lifetimes is that no calibration of the measurement signal for determining the carrier lifetime is needed.
  • 1 schematically shows a device for the spatially resolved determination of a local carrier lifetime according to an embodiment of the present invention.
  • A not yet finished solar cell processed silicon wafer 1 For example, having a surface passivating layer of silicon nitride may be on a gold or silver mirror, for example 3 be brought to a temperature of for example 70 ° C. The background shown by the mirror 5 may be at a temperature of 10 ° C, for example.
  • Excess charge carriers may be present in the silicon wafer 1 For example, be excited by light of a wavelength of 450 nm, for example. This light can be from a light emitting diode array 7 generated, which is modulated, for example, with a frequency of 40 Hz. The signal of infrared emission due to the generated excess charge carriers may be detected, for example, by an infrared camera 9 be detected in the wavelength range of 4-5 microns. This camera can take pictures at a refresh rate of, for example, 160 Hz and an image integration time of, for example, 1,100 μs.
  • For example, the images can be taken directly after the illumination by the light-emitting diode array 7 , that is to say at the beginning of the excitation time, after half of the excitation time, directly after the end of the excitation and after half of the time without stimulation. Using a lock-in process, the images can be correlated in real time with a sine function and a cosine function. This can be done by the camera 9 recorded image data via a line 11 to a computer system 13 be transmitted. The computer system 13 is again on the one hand via a control line 15 with the camera 9 connected to trigger them, and on the other hand is via a line 17 with a control unit 19 connected to the light emitting diode array 7 controls.
  • 2 shows an example of the time course of a perfectly rectangular excitation signal 21 , are generated with the excess charge carriers in a semiconductor structure, the waveform 23 a carrier lifetime of 1000 μs and the time subintervals 25 . 27 . 33 . 35 Image acquisition with an image integration time of 1,100 μs.
  • Like from the 2 is taken during an excitation time interval 31 initially at the beginning within a time subinterval 25 a first measured value S1 is determined. This is the first time subinterval 25 in a period of time within which the excess charge carrier concentration 23 still continuously increased, that is not yet reached a quasi-stationary state. A second measured value S2 is during a second time subinterval 27 determines that within the excitation time interval 31 later, that is, at a time when the excess charge carrier concentration is substantially no longer increasing and is in a quasi-steady state.
  • In order to increase the overall measuring accuracy, further measured values can be used at later time subintervals 33 . 35 immediately after switching off the excitation and at a later time, when a quasi-stationary resting state has set to be recorded. The excitation time intervals 31 and the following idle time intervals 37 can be cyclically repeated periodically, so as to enable the determination of several measured values S1, S2 and, as a result, the averaging or the correlation with a sine or cosine function.
  • For this configuration, analytically, a relationship between the carrier lifetime and the determined time shift between the excitation and the time-dependent measured values S1, S2 or the determined phase of the lock-in signal can be calculated, as has been partially expressed in the equations described above , From this, a table with lifetime-phase pairs can be created. For each recorded pixel, an assignment of the measured phase to a lifetime value can now be performed.
  • Finally, it should be noted that the terms "comprise", "comprise", etc., do not exclude the presence of other elements. The term "a" does not exclude the presence of a plurality of objects. The reference numerals in the claims are merely for ease of reading and are not intended to limit the scope of the claims in any way.

Claims (15)

  1. Method for the spatially resolved determination of a local charge carrier lifetime in a semiconductor structure ( 1 ), the method comprising: generating excess charge carriers in the semiconductor structure ( 1 ) within a predefinable excitation time interval ( 31 ); Determining, simultaneously for each of a plurality of sub-surfaces of the semiconductor structure, in each case of location-dependent first measured values S1 by integrating measurement signals that depend on a concentration of the excess charge carriers within a first time subinterval ( 25 ), in which the concentration of the excess charge carriers has not reached a quasi-stationary state, and of spatially dependent second measured values S2 by integrating measurement signals that depend on a concentration of the excess charge carriers within a second time subinterval ( 27 ), in which the concentration of the excess charge carriers has reached a quasi-stationary state, wherein the first time subinterval ( 25 ) before the second time subinterval ( 27 ) ends; Determining a value of the local carrier lifetime for each of the plurality of sub-surfaces of the semiconductor structure by relating the respective first measured value S1 with the respective second measured value S2.
  2. The method of claim 1, wherein the local carrier lifetime is determined from the quotient S1 / S2 of a respective first measured value S1 and a respective second measured value S2.
  3. The method of claim 1 or 2, wherein the first and second time sub-intervals ( 25 . 27 ) both within the excitation time interval ( 31 ) or the first and second time subintervals ( 25 . 27 ) both after the excitation time interval ( 31 ) end up.
  4. Method according to one of claims 1 to 3, wherein the first time subinterval ( 25 ) at the latest 10 ms after the beginning or after the end of the excitation time interval ( 31 ) ends.
  5. Method according to one of claims 1 to 4, wherein the first and the second measured values S1, S2 are respectively determined by integrating on a measurement of the concentration of the excess charge carrier measuring signals over an integration period of less than 10 ms.
  6. Method according to one of claims 1 to 5, wherein an excitation intensity with which the excess charge carriers in the semiconductor structure ( 1 ) is increased to a maximum within less than 50 μs.
  7. Method according to one of claims 1 to 6, wherein the excess charge carriers by means of one of the partial surfaces of the semiconductor structure ( 1 ) illuminating light source ( 7 ) be generated.
  8. Method according to one of claims 1 to 7, wherein the location-dependent first and second measured values S1, S2 by means of a camera ( 9 ).
  9. Method according to one of claims 1 to 8, wherein the location-dependent first and second measured values S1, S2 is determined by determining the local absorption or emission of infrared radiation.
  10. Method according to one of claims 1 to 9, wherein for each sub-surface a plurality of first measured values S1 and a plurality of second measured values S2 by means of a lock-in method within cyclically successive excitation time intervals ( 31 ) and correlated with a sine function and a cosine function.
  11. Apparatus for the spatially resolved determination of a local charge carrier lifetime in a semiconductor structure, the apparatus comprising: an excess charge carrier generation device ( 7 ), which is designed to remove excess charge carriers in the semiconductor structure ( 1 ) within a predefinable excitation time interval ( 31 ) to create; Measured value determination device ( 9 ), which is designed to be used simultaneously for each of a plurality of sub-surfaces of the semiconductor structure ( 1 ), in each case location-dependent first measured values S1 by integrating measurement signals which depend on a concentration of the excess charge carriers within a first time subinterval ( 25 ), in which the concentration of the excess charge carriers has not reached a quasi-stationary state, and location-dependent second measured values S2 by integrating measurement signals that depend on a concentration of the excess charge carriers within a second time subinterval ( 27 ), in which the concentration of the excess charge carrier has reached a quasi-stationary state, wherein the first Time subinterval ( 25 ) before the second time subinterval ( 27 ) ends to determine; an evaluation device ( 13 ), which is adapted to calculate a value of the local charge carrier lifetime for each of the plurality of sub-surfaces of the semiconductor structure ( 1 ) by setting each of the first measured value S1 with the second measured value S2.
  12. Apparatus according to claim 11, wherein the excess charge carrier generating means comprises a light source ( 7 ) having.
  13. Apparatus according to claim 11 or 12, wherein the measured value determination device is a camera ( 9 ) having.
  14. Apparatus according to claim 13, wherein the camera ( 9 ) has an integration duration of less than 10 ms.
  15. Device according to one of claims 11 to 14, further comprising a lock-in device for temporally correlating the excitation time interval ( 31 ) and the first and second time subintervals ( 25 . 27 ).
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