Device for the contactless determination of a surface photovoltage and a photoemission current of a test object and method for the contactless determination of the surface photovoltage or the photoemission current of a test object using the device

Abstract

The invention relates to a device (100) for the contactless determination of a surface photovoltage (USPV) and a photoemission current (i) of a test object (2), wherein the device (100) comprises: - an electrically conductive surface (1) on which the object under investigation (2) can be positioned, - a voltage source (5) designed to generate an electrical potential in the electrically conductive surface (1), - an electrode (3) arranged opposite the electrically conductive surface (1), such that when the object under investigation (2) is positioned on the electrically conductive surface (1), it is positioned between the electrically conductive surface (1) and the electrode (3), - a transimpedance amplifier (TIA), wherein the input (21) of the transimpedance amplifier (TIA) is connected to the electrode (3) and the output (22) of the transimpedance amplifier (TIA) is connected to a surface photovoltage signal output (9), at which an output voltage (UA) output by means of the transimpedance amplifier (TIA) can be measured, which is indicative of the surface photovoltage (USPV) of the object under investigation (2), as well as - a frequency response correction device (10) for at least partial compensation of a frequency response fall-off of the output voltage (UA) of the transimpedance amplifier (TIA), which remains indicative for the photoemission current (i), wherein an input (31) of the frequency response correction device (10) is connected to the output (22) of the transimpedance amplifier (TIA), and an output (32) of the frequency response correction device (10) is connected to or forms a photoemission current output (11) at which the at least partially frequency response-compensated output voltage (UA) can be measured. Furthermore, the invention relates to a method for non-contact measurement of the surface photovoltage (USPV) and a method for non-contact measurement of the photoemission current (i) of an object under investigation (2) using the device (100).

Description

[0001] The invention relates to a device for the contactless determination of a surface photovoltage and a photoemission current of an object under investigation, a method for the contactless determination of the surface photovoltage of an object under investigation, and a method for the contactless determination of the photoemission current using the device.

[0002] In materials science, particularly in semiconductor technology and the study of thin films, the precise determination of time-varying surface photostresses and photoemission currents is of great importance. These quantities provide essential information about the physical properties of material surfaces, which is indispensable for the optimization and development of new materials.

[0003] However, current measurement methods require separate instruments to measure these different parameters. This means that the samples under investigation must be transferred from one measuring device to another. This process is not only time-consuming but also consumes valuable laboratory space and results in measurements being performed under non-identical conditions, which impairs the comparability of the results. Furthermore, the need to use different measuring instruments increases the risk of measurement errors and complicates the calibration and integration of the obtained data.

[0004] German patent application DE 10 2019 117 989 B3 discloses a device for non-contact voltage measurement. This device is based on the concept of a field mill and is designed to detect AC and DC components of a surface voltage in a single measurement by generating a summed signal. For this purpose, an equalizer is connected to a first output of the device and / or to an amplifier output. The equalizer is configured to modify the amplifier voltage, at least temporarily, as if the amplifier of the device had a lower cutoff frequency of less than 10 Hz. Furthermore, the device is connected via the first output and a second output for the regulator voltage to a summing amplifier. This summing amplifier is configured to add the modified amplifier voltage from the equalizer and the regulator voltage to produce a single output voltage and output it.

[0005] US Patent 5,393,980 A relates to a device in which impurities on surfaces are determined using optically stimulated electron emission (OSEE), based on the external photoelectric effect. The device disclosed herein employs, among other things, an electromechanical Kelvin probe with which the contact potential of different metals on a surface can be determined. The contact potential influences the results of the OSEE measurements, which are then corrected. In the Kelvin probe, a conductive cantilever and the sample are piezoelectrically vibrated, thereby inducing a current which is regulated to zero by an external voltage. This voltage corresponds to the contact potential.

[0006] A method with an associated apparatus for determining the minority charge carrier surface recombination lifetime constant (ts) of a semiconductor material sample is disclosed in EP 0 656 643 A1. A flat semiconductor sample is clamped in a holder in which a counter electrode is arranged. The apparatus also includes a capacitive measuring probe with a light source for exciting photocurrents in the surface, the measuring probe being based on a reference electrode. The measuring probe is pressed onto the sample to detect the measurement signal – induced photocurrents in the surface.

[0007] The present invention is based on the objective of providing a device that enables both the contactless determination of the surface photovoltage and the photoemission current of a test object. Furthermore, it is an objective of the present invention to provide a method for the contactless determination of the surface photovoltage as well as a method for the contactless determination of the photoemission current of a test object.

[0008] These problems are solved by a device having the features of claim 1, a method having the features of claim 8, and a method having the features of claim 10.

[0009] Advantageous embodiments of these aspects of the invention are specified in the corresponding dependent claims and are described below.

[0010] A first aspect of the invention relates to a device for the contactless determination of a surface photovoltage and a photoemission current of a test object. The device comprises: - an electrically conductive surface on which the object under investigation can be positioned, - a voltage source designed to generate an electrical potential in the electrically conductive surface, - an electrode positioned opposite the electrically conductive surface, such that when the object under investigation is positioned on the electrically conductive surface, it is located between the electrically conductive surface and the electrode, - a transimpedance amplifier, wherein the input of the transimpedance amplifier is connected to the electrode and the output of the transimpedance amplifier is connected to a surface photovoltage signal output, at which an output voltage provided by means of the transimpedance amplifier can be measured, which is indicative of the surface photovoltage of the object under investigation, as well as - a frequency response correction device for at least partial compensation of a frequency response drop in the output voltage of the transimpedance amplifier, which remains indicative for the photoemission current, wherein an input of the frequency response correction device is connected to the output of the transimpedance amplifier, and an output of the frequency response correction device is connected to or forms a photoemission current output at which the at least partially frequency response-compensated output voltage can be measured.

[0011] The output voltage UA of the transimpedance amplifier is indicative of the surface photovoltage of the object under investigation by means of the relationship UA = USPV*(-CL / C(8)), where CL denotes the capacitance of the medium between the conductive surface or the object under investigation and the electrode, for example air, and C(8) is the capacitance of a negative feedback capacitor of the transimpedance amplifier.

[0012] On the other hand, the output voltage UA of the transimpedance amplifier is also indicative for the photoemission current i by means of the relationship UA -i*R(7), where R(7) denotes a resistance value of a negative feedback resistor of the transimpedance amplifier.

[0013] The measurement bandwidth for determining the surface photovoltage depends solely on the operational amplifier of the transimpedance amplifier and can extend well into the MHz range. The characteristic frequency f1 of the transimpedance amplifier is f1 = 1 / (2*π*R(7)*C(8)). For example, f1 is only f1 = 0.16 Hz for R(7) = 1 TΩ and C(8) = 1 pF, and here it acts as a lower cutoff frequency. Therefore, surface photovoltage signals are measurable in the range above 0.16 Hz. To determine the photoemission current, the object under investigation is negatively biased using the voltage source, typically at voltages in the range of -50 V and -300 V. The electrical conditions differ from those used to determine the surface photovoltage: the measured quantity is now the continuous electron flow from the object under investigation to the electrode, in the form of the photoemission current. The same characteristic frequency resulting from R(7) and C(8) of, for example,In this case, 0.16 Hz is the upper measurable cutoff frequency. Accordingly, photoemission current signals are measurable in the frequency range from 0 Hz to 0.16 Hz.

[0014] In particular, the frequency response correction device according to the invention makes it possible to at least partially, preferably completely, compensate for the frequency response falloff of the output voltage indicative of the photoemission current, so that both the surface photovoltage and the photoemission current of a test object can be determined using the device. This compensation is achieved by frequency-dependent adjustment of the gain of the transimpedance amplifier's output voltage indicative of the photoemission current by means of the frequency response correction device. The output voltage of the transimpedance amplifier provided by the frequency response correction device at the photoemission current output is frequency-response compensated according to the gain.

[0015] For example, the object under test can be positioned on the electrically conductive surface by placing it on the surface. In this example, positioning is gravity-driven, with the electrode positioned above the conductive surface, against the direction of gravity. Alternatively or additionally, the object under test can also be positioned on the conductive surface using fasteners. This allows, for example, positioning the sample on the conductive surface against the direction of gravity, with the electrode positioned below the conductive surface.

[0016] The electrode is advantageously spaced away from the electrically conductive surface, so that the object under investigation can be positioned between the electrode and the electrically conductive surface.

[0017] Positioning the object under test on the electrically conductive surface ensures that the voltage source can impart a defined electrical potential to the object relative to ground. Preferably, the electrically conductive surface is a metallic surface.

[0018] According to one embodiment of the device according to the invention, the characteristic first frequency f1 of the transimpedance amplifier, resulting from a feedback resistor and a feedback capacitor of the transimpedance amplifier, is at most 1 Hz, preferably at most 0.1 Hz or exactly 0.1 Hz, and the frequency response correction device has a frequency-independent gain below the first frequency f1. The gain or frequency response of the transimpedance amplifier when determining the photoemission current is naturally linear below the first frequency f1. Therefore, the frequency response correction device advantageously has no gain or a linear characteristic in this range.

[0019] According to one embodiment of the device according to the invention, the frequency response correction device has a gain that increases above the first frequency f1 up to a second frequency f2, in particular a gain that increases by 20 dB per decade. The gain advantageously compensates at least partially, and preferably completely, for the frequency response falloff of the output voltage of the transimpedance amplifier that is indicative of the photoemission current.

[0020] In a further embodiment of the device according to the invention, the amplification above the second frequency f2 transitions back into a frequency-independent curve, wherein the second frequency f2 is at least 10 times, preferably 100 times, greater than the first frequency f1. Accordingly, the amplification can be applied over, for example, two decades without any adverse effects on the signal-to-noise ratio; that is, the dimensioning is based on f2 = 100*f1, and for the frequency-dependent amplification: V2 = 100*V1. In the example considered, the determination of photoemission current signals now has a linear frequency response from 0 Hz to 16 Hz, so that it is also possible to work with modulated light intensity and lock-in amplifiers.

[0021] Advantageously, the first frequency f1 of the transimpedance amplifier is at most 1 Hz, preferably at most 0.1 Hz or exactly 0.1 Hz, and the frequency response correction device has a frequency-independent gain below the first frequency f1, and an increasing gain above the first frequency f1, in particular a gain increasing by 20 dB per decade, which transitions back into a frequency-independent response above a second frequency f2, wherein the second frequency f2 is at least a factor of 10, preferably a factor of 100, greater than the first frequency f1.

[0022] According to a further embodiment of the device according to the invention, the frequency response correction device has a third frequency f3, exceeding f2, in particular wherein the ratio f3 / f2 lies between 1 and 5, above which the gain decreases by at least 20 dB per decade, preferably by 60 dB to 100 dB per decade. In this region, which typically no longer contains meaningful measurement signals, broadband noise can thus be suppressed. For example, 5th-degree Bessel low-pass filters are used for this purpose.

[0023] According to a further embodiment of the device according to the invention, the electrode can be arranged or is arranged at a distance of less than 10 mm from the electrically conductive surface, so that test objects with a thickness of several mm can be arranged between the electrode and the electrically conductive surface, and the electrode is spaced between 0.05 mm and 1 mm from a side of the test object facing the electrode. In the context of the present invention, a "thickness of several mm" refers to thicknesses of test objects in the range of between 0 mm and 10 mm, and in particular in the range of 0.01 mm to 10 mm.

[0024] According to a further embodiment of the device according to the invention, the electrode has at least one opening, in particular a plurality of openings, through which electromagnetic radiation can be radiated onto the object under investigation, in particular wherein the electrode is designed as a plate with holes for the entry of the electromagnetic radiation.

[0025] In a further embodiment of the device according to the invention, the electrode and / or the electrically conductive surface is coupled to an actuator, so that the electrode and the electrically conductive surface, and in particular when the object under investigation is positioned on the electrically conductive surface and arranged between the electrically conductive surface and the electrode, the electrode and the object under investigation, are movable relative to each other.

[0026] In particular, the relative motion occurs with at least a spatial component, and especially exclusively along a distance direction between the electrode and the conductive surface. The distance direction can, for example, be defined by an axis whose orientation is determined by the shortest distance vector between the electrode and the conductive surface.

[0027] For example, the device may have a single actuator coupled either to the electrode or to the conductive surface, enabling it to generate relative motion between the electrode and the conductive surface, or between the electrode and the object under investigation on the conductive surface, by moving either the electrode or the conductive surface. Alternatively, a single actuator may be coupled to both the electrode and the conductive surface, enabling it to generate relative motion between the electrode and the conductive surface, or between the electrode and the object under investigation on the conductive surface, by moving both. The device may also comprise multiple actuators, particularly two, so that the electrode and the conductive surface are each moved by separate actuators.

[0028] The relative movement between the electrode and the conductive surface, or

[0029] The object under investigation advantageously allows the determination of a DC voltage component of the surface photovoltage of the object under investigation.

[0030] According to a further embodiment of the device according to the invention, it comprises an electromagnetic radiation source which is configured to generate the electromagnetic radiation. The electromagnetic radiation can then be used to induce the surface photovoltage or the photoemission current in the object under investigation.

[0031] A second aspect of the invention relates to a method for the contactless determination of the surface photovoltage of a test object using the device according to the first aspect of the invention. According to this method, the test object is positioned on the electrically conductive surface and arranged between the electrically conductive surface and the electrode, and irradiated with electromagnetic radiation, whereby the resulting output voltage of the transimpedance amplifier is measured at the surface photovoltage signal output, which is indicative of the surface photovoltage of the test object.

[0032] According to one embodiment of the method as described in the second aspect of the invention, the electrode and the object under investigation are moved relative to each other, and a DC component of the surface photovoltage of the object under investigation is determined by compensating for the resulting alternating current in the electrode by varying a voltage supplied by the voltage source. In particular, the movement is periodic and occurs along a specific distance direction between the electrode and the object under investigation. Specifically, the relative movement between the electrode and the object under investigation can occur only once, or at least once, and in only one direction, for example, in the sense of a single approach between the electrode and the object under investigation, while the voltage supplied by the voltage source is varied such that the AC component of the output voltage of the transimpedance amplifier is minimized.It is also possible to use an aperiodic relative motion, during which the DC component of the surface photovoltage of the object under investigation is determined.

[0033] Alternatively, the relative movement between the electrode and the object under test can also be periodic. In this case, the frequency of the voltage supplied by the voltage source is adjusted to the frequency of the periodic change in distance between the object under test and the electrode. For this purpose, the voltage source can be integrated into a control loop that adjusts the voltage so that the AC component of the output voltage of the transimpedance amplifier is minimized.

[0034] A third aspect of the invention relates to a method for the contactless determination of the photoemission current of a test object using the device according to the first aspect of the invention. According to this method, the test object is positioned on the electrically conductive surface and arranged between the electrically conductive surface and the electrode, wherein an electrical potential is generated in the electrically conductive surface and the test object by means of the voltage source, and wherein the frequency response falloff of the output voltage of the transimpedance amplifier, which is still indicative for the photoemission current of the test object, is at least partially compensated by means of the frequency response correction device and measured at the photoemission current output.

[0035] In the following, exemplary embodiments as well as further features and advantages of the invention will be explained with reference to the figures. The figures show: Fig. 1 an embodiment of a device according to the first aspect of the invention; Fig. 2 measured spectra for modulated photoemission current ( Fig. 2a) and modulated surface photovoltage signals ( Fig. 2b) an exemplary test object 2 in the form of a p-type doped Si crystal with a hydrogen-terminated surface; and Fig. 3. For comparison, spectra for DC photoemission ( Fig. 3a) and DC surface photovoltage signals ( Fig. 3b) of the H-terminated p-type Si crystal in a vacuum.

[0036] Fig. Figure 1 shows an embodiment of a device 100 for the contactless determination of a surface photovoltage USPV and a photoemission current i of a test object 2 according to the first aspect of the invention.

[0037] The device 100 comprises an electrically conductive surface 1 on which a test object 2 is positioned. The test object 2 can be, for example, a metallic or semiconducting object. Furthermore, the device 100 comprises a voltage source 5 configured to generate an electrical potential in the electrically conductive surface 1. Through the electrical contact between the electrically conductive surface 1 and the test object 2 resting on it, the test object 2 can be supplied with a defined potential relative to ground by the voltage source 5.

[0038] Furthermore, the device 100 has an electrode 3 arranged opposite the electrically conductive surface 1, so that the object under investigation 2 is as described in Fig. 1 is evidently arranged between the electrically conductive surface 1 and the electrode 3. In this embodiment, the electrode 3 has a plurality of openings through which electromagnetic radiation 4 from a radiation source (not shown) can be directed onto the object under investigation 2 to induce a surface photovoltage USPV or a photoemission current i, both of which can be determined using the device 100. The conductive surface 1, the object under investigation 2, and the electrode 3 are arranged in Fig. 1 can be seen in an area of ​​the device 100 designated as signal source SQ.

[0039] Furthermore, the device 100 comprises a transimpedance amplifier TIA with an operational amplifier 6, a feedback resistor 7, and a feedback capacitor 8. An input 21 of the transimpedance amplifier TIA is electrically connected to the electrode 3, and an output 22 of the transimpedance amplifier TIA is electrically connected to a surface photovoltage signal output 9, at which an output voltage UA provided by the transimpedance amplifier TIA can be measured. This output voltage UA is indicative of the surface photovoltage USPV of the object under investigation 2 via the relationship UA = USPV*(-CL / C(8)), so that the surface photovoltage USPV can be determined by measuring the output voltage UA of the transimpedance amplifier TIA.Here, C(8) denotes the capacitance of the negative feedback capacitor 8 of the transimpedance amplifier TIA, and CL denotes the capacitance of the medium between the object under investigation 2 and the electrode 3, for which the capacitance of air is assumed in the present embodiment. Therefore, the surface photovoltage USPV of the object under investigation 2 can be determined using the device 100.

[0040] To determine the photoemission current i, the device 100 also includes a frequency response correction device 10 for at least partially compensating for a frequency response drop in the output voltage UA of the transimpedance amplifier TIA, which remains indicative for the photoemission current i. The output voltage UA is indicative for the photoemission current i via the relationship UA = -i*R(7), where R(7) denotes the resistance value of the negative feedback resistor 7 of the transimpedance amplifier TIA. An input 31 of the frequency response correction device 10 is connected to the output 22 of the transimpedance amplifier TIA, and an output 32 of the frequency response correction device 10 is connected to a photoemission current output 11, at which the at least partially frequency-response-compensated output voltage UA can be measured. Therefore, the photoemission current i of the object under investigation 2 can also be determined using the device 100.

[0041] A characteristic first frequency f1 of the transimpedance amplifier TIA, resulting from the negative feedback resistor 7 and the negative feedback capacitor 8, is at most 1 Hz, preferably 0.1 Hz. As in the frequency response correction device 10 in Fig. As graphically indicated in Figure 1, below the first frequency f1, the gain is frequency-independent. Above the first frequency f1, the gain increases, in particular by 20 dB per decade, up to a second frequency f2, above which the gain again transitions to a frequency-independent curve. The second frequency f2 is at least 10 times, preferably 100 times, greater than the first frequency f1.

[0042] Furthermore, in Fig. As can be seen from Figure 1, the frequency response correction device 10 here has a third frequency f3, exceeding f2, above which the gain decreases by at least 20 dB per decade, preferably by 60 dB to 100 dB per decade. The ratio f3 / f2 is preferably between 1 and 5.

[0043] The in Fig. The device 100 shown enables a method for the contactless determination of the surface photovoltage USPV of an object of investigation 2 according to the second aspect of the invention as well as a method for the contactless determination of the photoemission current i of an object of investigation 2 according to the third aspect of the invention.

[0044] For the method of contactless determination of the surface photovoltage of a test object 2 according to the second aspect of the invention, the test object 2 is positioned on the electrically conductive surface 1 and arranged between the electrically conductive surface 1 and the electrode 3. Subsequently, the test object 2 is irradiated with the electromagnetic radiation 4, and the resulting surface photovoltage of the test object 2 is measured at the surface photovoltage signal output 9.

[0045] Optionally, the device 100 comprises at least one actuator coupled to the electrode 3 and / or the conductive surface 1, such that the electrode 3 and the test object 2 positioned on the conductive surface 1 can be moved periodically relative to each other. Thus, with regard to the method for the contactless determination of the surface photovoltage of a test object 2 according to the second aspect of the invention, a DC voltage component of the surface photovoltage USPV of the test object 2 can be determined by compensating for a resulting alternating current in the electrode 3 by means of the voltage source 5 during a relative movement between the electrode and the test object 2 caused by the at least one actuator.

[0046] When determining surface photovoltages USPV with modulated or pulsed electromagnetic radiation 4, the capacitance between the electrode 3 and the object under investigation 2, as well as the capacitance of the feedback capacitor 8 of the transimpedance amplifier TIA, determine the gain. In the present embodiment, the capacitance between the electrode 3 and the object under investigation 2 is assumed to be that of air CL. For abrupt or not very low-frequency changes in the surface photovoltage USPV of the object under investigation 2, whose frequency components lie above the characteristic frequency defined by R(7) and C(8), the output voltage UA of the transimpedance amplifier TIA is given by the following formula, as described above: UA = USPV*(-CL / C(8)), where C(8) denotes the capacitance of the feedback capacitor 8. The measurement bandwidth depends only on the operational amplifier 6 of the transimpedance amplifier TIA and can be well in the MHz range.The characteristic frequency f1 of the transimpedance amplifier TIA is f1 = 1 / (2*π*R(7)*C(8)), where R(7) denotes the resistance value of the feedback resistor 7 of the transimpedance amplifier TIA. For example, f1 is only f1 = 0.16 Hz for R(7) = 1 TΩ and C(8) = 1 pF, and here it acts as a lower cutoff frequency. Therefore, surface photovoltage signals are measurable in the range above 0.16 Hz. This relatively low lower cutoff frequency is advantageous for measuring even very slow surface photovoltage transients with low distortion. The surface photovoltage signals are output at the output of the transimpedance amplifier TIA in the form of the output voltage UA.Furthermore, the device 100 can also be used to measure very slowly changing surface photovoltage signals by switching to the DC regime, in which the electrode 3 is set into periodic motion and the resulting alternating current is adjusted to zero by means of a voltage provided by the voltage source 5. The change in voltage under the influence of light corresponds to the DC voltage component of the surface photovoltage USPV.

[0047] For the method of contactless determination of the photoemission current i of a test object 2 according to the third aspect of the invention, the test object 2 is also positioned on the electrically conductive surface 1 and arranged between the electrically conductive surface 1 and the electrode 3. Subsequently, an electrical potential is generated in the electrically conductive surface 1, and thus in the test object 2, by means of the voltage source 5, and the frequency response falloff of the output voltage UA, which in this case is indicative for the photoemission current i, is at least partially compensated by means of the frequency response correction device 10 in the transimpedance amplifier TIA. The resulting photoemission current i is finally determined via the at least partially frequency-response-compensated output voltage UA at the photoemission current output 11.

[0048] Accordingly, when determining the photoemission current, the object under investigation 2 is negatively biased by the voltage source 5, typically at voltages in the range of -50 V and -300 V. The electrical conditions differ from those used when determining the surface photovoltage: the measured quantity is now the continuous electron flow from the object under investigation 2 to the electrode 3, in the form of the photoemission current i. The output voltage UA of the transimpedance amplifier TIA is then given by: UA = -i*R(7). The same characteristic frequency, e.g., 0.16 Hz, resulting from R(7) and C(8), is now the upper cutoff frequency. Accordingly, photoemission current signals in the frequency range from 0 to 0.16 Hz are measurable. In practical operation, this inertia can prove to be very disruptive, especially if the intensity of the electromagnetic radiation 4 is to be periodically modulated.Advantageously, the frequency response equalization 10 is therefore dimensioned such that f1 equals the aforementioned frequency, in this example 0.16 Hz. The frequency response drop of 20 dB per decade occurring in the transimpedance amplifier TIA is precisely compensated by the 20 dB per decade increase in the frequency response equalization 10. Without any negative impact on the signal-to-noise ratio, this increase can occur over approximately two decades; that is, the dimensioning is based on f2 = 100*f1, and the frequency-dependent gain is given by V2 = 100*V1. In the example under consideration, the measurement of photoemission current signals now has a linear frequency response from 0 to 16 Hz, so that it is also possible to work with modulated light intensity and lock-in amplifiers.

[0049] For signal display, for example on an oscilloscope, or for reading with an A / D converter, the broadband noise above f2 can be suppressed. In this range, no meaningful signal components typically occur. This can be achieved using a low-pass filter with a third frequency, f3. To effectively suppress mains hum components of 50 Hz—for example, in the form of capacitive interference on the device under investigation 2—and 100 Hz—here, for example, due to modulated ambient light—the gain drops sharply above f3, for example, by more than 20 dB per decade, preferably by 60 dB to 100 dB per decade. Suitable devices for this purpose include, for example, 5th-degree Bessel low-pass filters. The photoemission current signals are output at output 32 of the frequency response correction device 10 in the form of the at least partially, and in particular fully, frequency-response-compensated output voltage UA.

[0050] Separately outputting the signal indicative of the surface photovoltage USPV, namely the output voltage UA of the transimpedance amplifier TIA before the frequency response correction device 10, and the signal indicative of the photoemission current i, in the form of the at least partially frequency-response-compensated output voltage UA after the frequency response correction device 10, allows for convenient operation. Furthermore, switching between the surface photovoltage and photoemission current measurement regimes at the transimpedance amplifier TIA is unnecessary, and the advantageously large time constant R(7)*C(8), and the resulting very low characteristic frequency, can be maintained during the determination of the surface photovoltage USPV and the photoemission current i. This yields surface photovoltage and photoemission current measurement signals with low noise and good frequency responses.

[0051] Fig. Figure 2 shows measured spectra for modulated photoemission current ( Fig. 2a) and modulated surface photovoltage signals ( Fig. 2b) of an exemplary test object 2 in the form of a p-type doped Si crystal with a hydrogen-terminated surface. Such a test object 2 is characterized by a particularly low surface density of states and leads to a particularly pronounced influence of photoemission current and surface photovoltage signals in spectra of modulated measurements. In this sense, the test object 2 used has model character for the separation of AC photoemission current and AC surface photovoltage signals. The spectra each show the output voltage UA of the transimpedance amplifier TIA, which is used to determine the surface photovoltage USPV directly at the output 22 of the transimpedance amplifier TIA.can be tapped at the surface photovoltage signal output 9 and is measurable at the photoemission current output 11 at the output 32 of the frequency response correction device 10 for the determination of the photoemission current i as an at least partially frequency response compensated output voltage UA (see . Fig. 1).

[0052] In Fig. 2. The in-phase and 90° phase-shifted signal components correspond to a signal response that is fast or slow compared to the modulation period. Furthermore, it should be noted that if the signals rise and fall when the light is switched on and off without a change in sign, the in-phase and 90° phase-shifted signal components have opposite signs. In contrast, if a change in sign occurs during the signal decay, the in-phase and 90° phase-shifted signal components have the same sign.

[0053] Above 5.1 eV, the signs of the in-phase and 90° phase-shifted photoemission current signal components are opposite, with the in-phase signals being positive. The region with opposite signs for the in-phase and 90° phase-shifted photoemission current signal components corresponds to the modulated photoemission current signals. In contrast, the in-phase and 90° phase-shifted signal components in the spectral range between approximately 1.1 and 5.1 eV show the same signs. Under illumination, these signal components represent, practically speaking, the flow of electrons predominantly towards the surface, without any electrons actually leaving the sample. This directed electron flow (charge carrier separation) is caused by the space charge boundary layer inside the semiconductor under illumination.When the illumination is switched off, electrons effectively flow back from a region near the surface towards the bulk of the semiconductor, thus changing the direction of the current. Consequently, signals in regions of the modulated photoemission current spectra with the same sign for the in-phase and 90° phase-shifted signal components can be clearly attributed to charge separation and not to photoelectron emission.

[0054] The signs of the in-phase and 90° phase-shifted SPV signal components are opposite between approximately 1.1 eV and 5.1 eV, with the in-phase signals being negative. The region with opposite signs for the in-phase and 90° phase-shifted SPV signal components corresponds to the modulated SPV signals, where light-excited electrons are separated towards the surface, and the charge separation decays again after the light is switched off. In contrast, the in-phase and 90° phase-shifted signal components show the same sign in the spectral range above 5.1 eV. The positive in-phase signals indicate that some of the electrons excited at high photon energies are continuously escaping from the sample, thus creating an increased electron concentration in front of the semiconductor surface.The emitted electrons diffuse into regions of the vacuum chamber, so that when the light is switched off, an excess of positive charges remains near the sample surface. This causes a reversal of the potential near the surface, resulting in a reduction of the negative surface photovoltage. Consequently, the signals in regions of the modulated SPV spectra with the same sign for the in-phase and 90° phase-shifted signal components can be unambiguously attributed to photoelectron emission and not to charge separation.

[0055] Fig. Figure 3 shows spectra for DC photoemission ( for comparison). Fig. 3a) and DC surface photovoltage signals ( Fig. 3b) of the H-terminated p-type Si crystal in a vacuum. The object of investigation 2 therefore corresponds to the one from Fig.2. The positive, fixed photoemission current was measured at a voltage of -100 V applied to the back of the test object 2 by means of the voltage source 5. The DC surface photovoltage signals were determined by a measurement during relative movement between electrode 3 and test object 2. The photoemission current signals begin at a photon energy of approximately 5.1 eV and drop drastically above 6.5 eV due to oxygen absorption in the air-operated monochromator used. The DC surface photovoltage signals begin at around 1.1 eV and are negative. Negative surface photovoltage signals are typical for p-type doped semiconductors with a depletion surface layer. No DC surface photovoltage signals appeared in the DC photoemission current measurements, and conversely, no DC photoemission current signals appeared in the DC surface photovoltage measurements. Reference symbol list 1 Conductive surface 2. Object of investigation 3 electrode 4 Electromagnetic radiation 5 Voltage source 6 operational amplifiers 7 Feedback resistor 8 Feedback capacitor 9 Surface photovoltage signal output 10 Frequency response correction device 11 Photoemission current output 21 Input of the transimpedance amplifier 22 Output of the transimpedance amplifier 31 Input of the frequency response correction device 32 Output of the frequency response correction device 100 Device l Photoemission flux SQ signal source TIA Transimpedance Amplifier USPV surface photovoltage

Claims

[1] Device (100) for non-contact determination of a surface photovoltage and a photoemission current (i) of an object under investigation (2), wherein the device (100) comprises: - an electrically conductive surface (1) on which the object under investigation (2) can be positioned, - a voltage source (5) designed to generate an electrical potential in the electrically conductive surface (1), - an electrode (3) arranged opposite the electrically conductive surface (1), such that when the object under investigation (2) is positioned on the electrically conductive surface (1), it is positioned between the electrically conductive surface (1) and the electrode (3), - a transimpedance amplifier (TIA), wherein the input (21) of the transimpedance amplifier (TIA) is connected to the electrode (3) and the output (22) of the transimpedance amplifier (TIA) is connected to a surface photovoltage signal output (9), at which an output voltage (UA) output by means of the transimpedance amplifier (TIA) can be measured, which is indicative of the surface photovoltage of the object under investigation (2), as well as - a frequency response correction device (10) for at least partial compensation of a frequency response fall-off of the output voltage (UA) of the transimpedance amplifier (TIA), which is also indicative for the photoemission current (i), wherein an input (31) of the frequency response correction device (10) is connected to the output (22) of the transimpedance amplifier (TIA), and an output (32) of the frequency response correction device (10) is connected to or forms a photoemission current output (11) at which the at least partially frequency response-compensated output voltage (UA) can be measured. [2] The device (100) according to claim 1, wherein a characteristic first frequency f1 of the transimpedance amplifier (TIA) resulting from a feedback resistor (7) and a feedback capacitor (8) of the transimpedance amplifier (TIA) is at most 1 Hz and the frequency response correction device (10) has a frequency-independent gain below the first frequency f1 and an increasing gain above the first frequency f1, which transitions back into a frequency-independent response above a second frequency f2, wherein the second frequency f2 is at least 10 times greater than the first frequency f1. [3] The device (100) according to claim 2, wherein the frequency response correction device (10) has a third frequency f3 exceeding f2, above which the gain decreases by at least 20 dB per decade. [4] The device (100) according to one of the preceding claims, wherein the electrode (3) can be arranged or is arranged at a distance of less than 10 mm from the electrically conductive surface (1), so that test objects (2) with a thickness of several mm can be arranged between the electrode (3) and the electrically conductive surface (1) and the electrode (3) is spaced between 0.05 mm and 1 mm from a side of the test object (2) facing the electrode (3). [5] The device (100) according to one of the preceding claims, wherein the electrode (3) has a plurality of openings through which electromagnetic radiation (4) can be radiated onto the object under investigation (2). [6] The device (100) according to one of the preceding claims, wherein the electrode (3) and / or the electrically conductive surface (1) is / are coupled to an actuator such that the electrode (3) and the electrically conductive surface (1) are movable relative to each other. [7] The device (100) according to one of the preceding claims, further comprising an electromagnetic radiation source which is configured to generate an electromagnetic radiation (4). [8] Method for non-contact determination of a surface photovoltage of an object under investigation (2) using the device (100) according to one of claims 1 to 7, wherein the object under investigation (2) is positioned on the electrically conductive surface (1) and is arranged between the electrically conductive surface (1) and the electrode (3) and is irradiated with electromagnetic radiation (4), and wherein the resulting output voltage (UA) of the transimpedance amplifier (TIA) is measured at the surface photovoltage signal output (9), which is indicative of the surface photovoltage of the object under investigation (2). [9] The method according to claim 8, wherein the electrode (3) and the object under investigation (2) are moved relative to each other and a DC voltage component of the surface photovoltage of the object under investigation (2) is determined by compensating an alternating current resulting therefrom in the electrode (3) by varying a voltage provided by the voltage source (5). [10] Method for the contactless determination of a photoemission current (i) of an object under investigation (2) using the device (100) according to one of claims 1 to 7, wherein the object under investigation (2) is positioned on the electrically conductive surface (1) and arranged between the electrically conductive surface (1) and the electrode (3), wherein the electrical potential is generated in the electrically conductive surface (1) and the object under investigation (2) by means of the voltage source (5), and wherein the frequency response fall-off of the output voltage (UA) of the transimpedance amplifier (TIA), which is also indicative for the photoemission current (i) of the object under investigation (2), is at least partially compensated by means of the frequency response correction device (10) and is measured at the photoemission current output (11).

Images