Photoinduced carrier lifetime measuring method, light incidence efficiency measuring method, photoinduced carrier lifetime measuring device, and light incidence efficiency measuring device

A photoinduced carrier lifetime measuring method capable of obtaining photoinduced carrier effective lifetime of a semiconductor substrate with high accuracy regardless of the surface state of the sample. The method includes: irradiating a microwave onto a semiconductor substrate while periodically pulse-irradiating a light onto the semiconductor substrate; detecting the microwave transmitted through the semiconductor substrate or reflected by the semiconductor substrate; and obtaining the effective lifetime of photoinduced carriers generated in the semiconductor substrate by the pulse irradiation of the light, based on an irradiation duration T1 and a non-irradiation duration T2 when performing the light pulse irradiation and an integrated value of each microwave intensity obtained by the detection.

TECHNICAL FIELD

The present invention relates to a photoinduced carrier lifetime measuring method for measuring effective lifetime of photoinduced carriers generated in a semiconductor substrate by light irradiation, and a photoinduced carrier lifetime measuring device for performing the measuring method. The present invention further relates to a light incidence efficiency measuring method for obtaining light incidence efficiency of the semiconductor substrate based on the effective lifetime, and a light incidence efficiency measuring device for performing the measuring method.

BACKGROUND ART

The photoinduced carrier lifetime is used as one of indicators for evaluating internal defects within the semiconductor substrate. The term “photoinduced carrier lifetime” means the lifetime of the photoinduced carriers (i.e., the minority carriers) generated in the semiconductor substrate by light irradiation.

A μ-PCD (microwave photoconductive decay) method (see, for example, Non-patent document 1) is known as a first example of a method and device for measuring the photoinduced carrier lifetime. In such a method, a laser is pulse-irradiated onto a semiconductor substrate for extremely short time in a state where a microwave is irradiated onto the semiconductor substrate. At this time, the reflectivity of the microwave irradiated onto the semiconductor substrate changes depending on the density of the carriers induced by the laser pulse. Thus, the effective lifetime of the photoinduced carriers of the semiconductor substrate (referred to as “effective lifetime” hereinafter) can be obtained by measuring the change of the reflectivity with time.

A QSSPC (quasi steady state photoconductivity) method (see, for example, Non-patent document 2) is known as a second example of the method and device for measuring the photoinduced carrier lifetime of a semiconductor substrate. In such a method, an inductance coil is disposed to face a semiconductor substrate, to emit RF frequency radiation. Further, a light is pulse-irradiated onto the semiconductor substrate for extremely short time. At this time, an electromagnetic wave of a RF frequency is reflected by the carriers induced by the light pulse. The photoinduced carrier effective lifetime of the semiconductor substrate can be obtained by measuring the change of the reflected wave with time as the change of the currency flowing through the coil.

Further, a microwave optical interference absorption method (see, for example, Non-patent document 3) is known as a third example of the method and device for measuring the photoinduced carrier lifetime of a semiconductor substrate. In such a method, a microwave interferometer formed of a waveguide is inserted into a semiconductor substrate, and continuous light is irradiated onto the semiconductor substrate in a state where a microwave is irradiated onto the semiconductor substrate. At this time, since the microwave is absorbed by the carriers induced by the irradiation of the continuous light, the photoinduced carrier effective lifetime can be obtained by measuring the reduction of the microwave transmittance.

PRIOR ART DOCUMENTS

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

The μ-PCD method of the first example and the QSSPC method of the second example are methods in which strong light pulse is irradiated onto the semiconductor substrate to generate photoinduced carriers (such as electrons, holes and the like) in the semiconductor substrate, and time decay-rate of the generated photoinduced carriers is measured. These methods are widely used because of their simplicity.

However, in these methods, the decay of the photoinduced carriers generated by one light pulse is measured as the change of the reflectivity of the microwave or electromagnetic wave with time, and therefore it is necessary to generate high-density photoinduced carriers in order to maintain high measurement accuracy. Thus, it is difficult to obtain the effective lifetime in the case where the density of the photoinduced carriers is low.

With the measuring system developed for performing the aforesaid methods, it is not possible to identify the incidence rate of the microwave or RF electric field incident onto the sample, and it is not possible to analyze absorption of the microwave or absorption of the RF electric field caused by the carriers generated in the sample. Therefore, it is not possible to calculate the carrier density based on the measured signal. Thus, it is not possible to obtain the effective lifetime with high accuracy.

On the other hand, the microwave optical interference absorption method as the third example is a method adapted to measure the change of the microwave transmittance caused by the photoinduced carriers generated by irradiating continuous light onto the semiconductor substrate, wherein the microwave transmittance depends on the carrier density. Thus, it is possible to detect the photoinduced carriers having a low-density of about 1×1011cm−2.

In other words, if the carrier density of the photoinduced carriers obtained based on the measured microwave transmittance is n, then the effective lifetime τeffcan be obtained by the following Equation (1) where incident light intensity is I, photon energy is hv, sample surface reflectivity is r.
τeff=nhv/(1−r)I(1)

It can be known from Equation (1) that accurate measurement of the carrier density makes it possible to accurately measure the effective lifetime. Thus, it is possible to obtain a short effective lifetime of 2 μs by using the microwave optical interference absorption method.

However, as can be known from Equation (1) that, with the aforesaid microwave optical interference absorption method, the sample surface reflectivity r of the surface of the semiconductor substrate has to be obtained in advance. Thus, in the case where the sample is a semiconductor substrate having a texture structure such as a solar cell, for example, the sample surface reflectivity r can not be determined. Further, with regard to the incident light intensity I, there is arbitrariness in signal-light intensity transfer characteristic of the detector.

Therefore, it is an object of the present invention to provide a photoinduced carrier lifetime measuring method capable of obtaining the photoinduced carrier effective lifetime of a sample semiconductor substrate with high accuracy, regardless the surface state of the sample semiconductor substrate. Further, it is another object of the present invention to provide a photoinduced carrier lifetime measuring device for achieving the aforesaid measuring method. Furthermore, it is further another object of the present invention to provide a light incidence efficiency measuring method capable of obtaining the light incidence efficiency with respect to the sample based on the effective lifetime obtained by the aforesaid measuring method, and a measuring device for performing the light incidence efficiency measuring method.

Means for Solving the Problems

To achieve the aforesaid objects, a photoinduced carrier lifetime measuring method according to an aspect of the present invention includes the following steps: irradiating a microwave onto a semiconductor substrate while periodically pulse-irradiating an light onto the semiconductor substrate; detecting the microwave transmitted through the semiconductor substrate or reflected by the semiconductor substrate; and obtaining the effective lifetime of photoinduced carriers generated in the semiconductor substrate by the pulse irradiation of the light, based on an irradiation duration T1and a non-irradiation duration T2when pulse-irradiating the light and an integrated value of each microwave intensity obtained by the detection.

Further, a light incidence efficiency measuring method according to another aspect of the present invention is characterized in that light incidence efficiency (1-r) is obtained from the following Equation (2)

obtaining light incidence efficiency (1-r) from the following Equation (2).
[Mathematical expression 1]
n=(1−r)Gτeff(2)

where r is surface reflectivity, n is carrier density of photoinduced carriers, and G is light intensity (energy of one photon).

With the photoinduced carrier lifetime measuring method and the light incidence efficiency measuring method according to the present invention, the microwave transmitted through the semiconductor substrate or reflected by the semiconductor substrate is detected in a state where the light is periodically pulse-irradiated onto the semiconductor substrate. Thus, even if the photoinduced carriers generated in the semiconductor substrate by the light irradiation is of low-density, a measurement result with high sensitivity can be obtained by obtaining an integrated value of the microwave intensity detected for each of plural periodical pulse irradiations. Therefore, it is possible to obtain a detection result with high sensitivity even in the case where the semiconductor substrate is irradiated by feeble light. Further, by changing the irradiation duration T1and the non-irradiation duration T2and periodically pulse-irradiating the light for plural times, change of the integrated value of the microwave detection intensity with respect to change of the T1, T2can be obtained, and carrier decay-rate can be known based on this change. Thus, the photoinduced carrier effective lifetime can be obtained without requiring information such as the surface reflectivity of the semiconductor substrate to be measured and the like.

Further, a photoinduced carrier lifetime measuring device according to further another aspect of the present invention includes a light source, a microwave source, a detecting section, and a calculating section. The light source is adapted to pulse-irradiate light for generating photoinduced carriers in a sample. The microwave source is adapted to generate a microwave for being irradiated onto the sample. The detecting section is adapted to detect the microwave transmitted through or reflected by the sample. The calculating section is adapted to calculate the effective lifetime of the photoinduced carriers generated in the sample by the pulse irradiation of the light based on an irradiation duration T1and a non-irradiation duration T2when periodically pulse-irradiating the light for a plurality of times and an integrated value of the intensity of the microwave detected by the detecting section.

Further, it is possible to obtain the effective lifetime based on the detection result of the microwave having high sensitivity without requiring information such as the surface reflectivity of the semiconductor substrate to be measured and the like, even in the case where the sample is irradiated by feeble light. As a result, it is possible to obtain the photoinduced carrier effective lifetime with high accuracy even in the case where the sample is a semiconductor substrate having a texture structure such as a solar cell for example.

Advantages of the Invention

Further, it is possible to obtain the effective lifetime based on the detection result of the microwave having high sensitivity without requiring information such as the surface reflectivity of the semiconductor substrate to be measured and the like, even in the case where the sample is irradiated by feeble induction light. As a result, it is possible to obtain the photoinduced carrier effective lifetime with high accuracy even in the case where the sample is a semiconductor substrate having a texture structure such as a solar cell for example.

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in the following order with reference to the attached drawings.

1. First embodiment “example of configuration of photoinduced carrier lifetime measuring device”

2. Second embodiment “modification of configuration of photoinduced carrier lifetime measuring device”

3. Third embodiment “first example of photoinduced carrier lifetime measuring method”

4. Fourth embodiment “second example of photoinduced carrier lifetime measuring method”

5. Fifth embodiment “third example of photoinduced carrier lifetime measuring method”

Incidentally, in each embodiment, like components will be denoted by like reference numerals and the explanation thereof will be omitted.

1. First Embodiment

FIG. 1shows a schematic configuration of a photoinduced carrier lifetime measuring device and a light incidence efficiency measuring device (which are to be collectively referred to as “photoinduced carrier lifetime measuring device” hereinafter). The photoinduced carrier lifetime measuring device shown inFIG. 1is adapted to be used to perform a photoinduced carrier lifetime measuring method and a light incidence efficiency measuring method described in the subsequent embodiments, and has the following configuration.

The photoinduced carrier lifetime measuring device includes a microwave source11and a light source12, wherein the microwave source11is adapted to generate a microwave to be incident onto a sample semiconductor substrate10A and a reference semiconductor substrate10B, and the light source12is adapted to emit light for generating carriers in the semiconductor substrate10A. The photoinduced carrier lifetime measuring device further includes a detecting section13A, a detecting section13B and a calculating section14, wherein the detecting section13A and the detecting section13B are adapted to detect the intensity of the microwave transmitted through the semiconductor substrates10A,10B, and the calculating section14is adapted to calculate the photoinduced carrier lifetime and the light incidence efficiency based on the microwave intensity detected by the detecting sections13A,13B.

The photoinduced carrier lifetime measuring device has a waveguide15, which constitutes a microwave interferometer, interposed between the microwave source11and the detecting sections13A,13B. The waveguide15is branched into two waveguides15A,15B in a middle portion between the microwave source11and the detecting sections13A,13B. One branched waveguide15A is provided with a gap16A into which the sample semiconductor substrate10A is inserted. The other branched waveguide15B is provided with a gap16B into which the reference semiconductor substrate10B is inserted. The semiconductor substrates10A,10B are respectively inserted into the gaps16A,16B in a direction substantially perpendicular to the extending direction of the waveguides15A,15B. The gaps16A,16B are disposed at symmetrical positions of the waveguides15A,15B.

The branched waveguides15A,15B are connected to each other at a position behind the gaps16A,16B when viewed from the microwave source11. Further, the connected waveguide15is branched again, and the two branches are respectively connected to the detecting section13A and the detecting section13B.

Thus, in the photoinduced carrier lifetime measuring device shown inFIG. 1, the microwave generated by the microwave source11passes through the waveguide15so as to be irradiated onto the semiconductor substrate10A,10B inserted into the gaps16A,16B of the waveguides15A,15B. Further, the microwave transmitted through the semiconductor substrate10A,10B passes through the waveguides15A,15B so as to be guided to the detecting sections13A,13B respectively.

In the aforesaid configuration, a light-guiding plate18for causing the light from the light source12to be incident onto the semiconductor substrate10A is inserted into the gap16A into which the sample semiconductor substrate10A is inserted. The light-guiding plate18is inserted at a position closer to the side of the detecting sections13A,13B (or the side of the microwave source11) than the position at which the semiconductor substrate10A is inserted, so that the light-guiding plate18abuts the semiconductor substrate10A inserted into the gap16A. The light-guiding plate18is connected to the light source12through an optical fiber17. Incidentally, the light-guiding plate18may also be provided at two positions where the semiconductor substrate10A is inserted, both on the side of the detecting sections13A,13B and on the side of the microwave source11.

Here, the microwave source11generates the microwave in synchronization with the light irradiation from the light source12.

The light source12is adapted to periodically pulse-irradiate light onto the sample semiconductor substrate10A so as to generate carriers in the semiconductor substrate. The light source12may be configured by a YAG (yttrium aluminum garnet) laser, for example. Note that the light source12is not limited to the YAG laser, but may also be any other light source (such as a light-emitting diode (LED), a xenon lamp, a halogen lamp and the like) as long as the light emitted by the light source has a wavelength possible to be absorbed by the semiconductor substrate, particularly has a wavelength ranging from 250 nm to 2500 nm. Further, the period of the pulse irradiation of the light of the light source12can be changed to any different period where irradiation duration T1=non-irradiation duration T2. For example, the irradiation duration T1and the non-irradiation duration T2may be arbitrarily changed in a range from 0.01 ms to 0.1 s.

The detecting section13A detects a difference IA−Bbetween a microwave transmission intensity JAof the microwave transmitted through the sample semiconductor substrate10A and a microwave transmission intensity JBof the microwave transmitted through the reference semiconductor substrate10B. Further, the detecting section13B detects a sum IA+B, of the microwave transmission intensity JAof the microwave transmitted through the sample semiconductor substrate10A and the microwave transmission intensity JBof the microwave transmitted through the reference semiconductor substrate10B. In the photoinduced carrier lifetime measuring device shown inFIG. 1, the microwave transmission intensity is detected and amplified by detecting the sum JA+Bof and the difference JA−Bbetween the microwave transmission intensity JAand the microwave transmission intensity JB, and therefore the detection sensitivity of the microwave can be improved.

Based on the microwave transmission intensity detected by the detecting section13A,13B and the period of the light emitted from the light source12, the calculating section14calculates the photoinduced carrier lifetime and light incidence efficiency of the semiconductor substrate10A inserted into the gap16A. For example, in the calculating section14, first the carrier density of the photoinduced carriers generated in the semiconductor substrate10A by the periodical light irradiation is obtained by using a method such as the method described in Non-patent document 3. Then, the photoinduced carrier effective lifetime and the effective light incidence efficiency are obtained based on the obtained carrier density. The calculation steps of the effective lifetime and the effective light incidence efficiency in the calculating section14will be described later in more detail when describing the photoinduced carrier lifetime measuring method.

Incidentally, the measuring device shown inFIG. 1may also be provided with a controller19for controlling the irradiation of the microwave irradiated from the microwave source11and the irradiation of the light irradiated from the light source12, based on the result calculated by the calculating section14.

Further, although not shown in the drawings, the measuring device shown inFIG. 1may also be provided with a position aligning section for selectively irradiating the microwave generated by the microwave source onto each of a plurality of areas of the sample semiconductor substrate10A, wherein the plurality of areas are obtained by dividing a principal surface of the sample semiconductor substrate10A. For example, a movable stage that can move the semiconductor substrate10A inserted into the gap16A in a direction perpendicular to the incident direction of the microwave may be used as the position aligning section. Further, instead of the movable stage, a microwave scanning section that can move the incident position of the microwave with respect to the semiconductor substrate10A inserted into the gap16A may be used.

Note that the measuring device shown inFIG. 1is merely an example of the photoinduced carrier lifetime and light incidence efficiency measuring device, and it is also possible to configure the photoinduced carrier lifetime and light incidence efficiency measuring device in other ways different from the aforesaid configuration.

For example, in the measuring device shown inFIG. 1, the waveguide15is branched into two waveguides, and the microwave transmittance is accurately obtained by using the sum of the microwave transmittance of the sample semiconductor substrate10A and the microwave transmittance of the reference semiconductor substrate10B, and the difference between the microwave transmittance of the sample semiconductor substrate10A and the microwave transmittance of the reference semiconductor substrate10B; however, the photoinduced carrier lifetime measuring device may also have a configuration in which the waveguide15B and the detecting section13B are eliminated.

In such a case, in photoinduced carrier lifetime measuring device includes a microwave source11that generates the microwave, a light source12that irradiates light onto a semiconductor substrate10A so as to generate carriers, a detecting section13A, a calculating section14, and a waveguide15with no branch. The waveguide15is provided with a gap into which the sample semiconductor substrate10A is inserted. A light-guiding plate18connected to the light source12by an optical fiber17is inserted into the gap. In such a measuring device, the microwave source11and the light source12have the same configurations as those in the measuring device of the first embodiment. The detecting section13A detects the microwave transmission intensity JAof the microwave transmitted through the sample semiconductor substrate10A. Based on the microwave transmission intensity JAdetected by the detecting section13A, the calculating section14calculates the carrier density, the effective lifetime and the effective light incidence efficiency of the photoinduction in the semiconductor substrate10A.

The photoinduced carrier lifetime measuring device according to the first embodiment can be used to perform the photoinduced carrier lifetime measuring method and the light incidence efficiency measuring method (which are to be described later).

2. Second Embodiment

FIG. 2shows a schematic configuration of a modification of the photoinduced carrier lifetime measuring device and the light incidence efficiency measuring device (collectively referred to as a “photoinduced carrier lifetime measuring device” hereinafter). The photoinduced carrier lifetime measuring device shown inFIG. 2is identical to the photoinduced carrier lifetime measuring device shown inFIG. 1except that, instead of the light-guiding plate18shown inFIG. 1, a diffusion reflection plate18′ is provided.

That is, in the photoinduced carrier lifetime measuring device shown inFIG. 2, a small hole is formed in the side wall of the waveguide15A. An end portion of the optical fiber17connected the light source12is attached to the hole, and the light from the light source12is guided to the waveguide15A through the optical fiber17. The diffusion reflection plate18′ is arranged inside the waveguide15A. The diffusion reflection plate18′ reflects the light from the optical fiber17to the side of the gap16A into which the semiconductor substrate10A is inserted, while diffusing the light. Further, the microwave generated by the microwave source11is transmitted through the diffusion reflection plate18′. The diffusion reflection plate18′ is formed of a fluororesin, for example.

In the waveguide15A, the diffusion reflection plate18′ and the optical fiber17connected to the diffusion reflection plate18′ are arranged at least on one of both the side of the microwave source11and the side of the detecting sections13A,13B of the gap16A into which the sample semiconductor substrate10A is inserted.

Similar to the measuring device of the first embodiment shown inFIG. 1, the measuring device of the second embodiment shown inFIG. 2may also be provided with a movable stage that that can move the semiconductor substrate10A inserted into the gap16A in a direction perpendicular to the incident direction of the microwave. Further, instead of the movable stage, a microwave scanning section that can move the incident position of the microwave with respect to the semiconductor substrate10A inserted into the gap16A may be provided.

Further, similar to the measuring device of the first embodiment shown inFIG. 1, the waveguide15B and the detecting section13B in the measuring device of the second embodiment may also be eliminated.

Similar to the measuring device of the first embodiment, the photoinduced carrier lifetime measuring device according to the second embodiment may also be used to perform the photoinduced carrier lifetime measuring method and the light incidence efficiency measuring method (which are to be described later).

Next, a first example of the photoinduced carrier lifetime measuring method will be described below. The photoinduced carrier effective lifetime measuring method and the effective light incidence efficiency measuring method will be described below by using the photoinduced carrier lifetime measuring device shown inFIG. 1orFIG. 2.

First, a data table to be used in the photoinduced carrier lifetime measuring method is previously created as follows.

First, irradiation condition of the light for generating the carriers in the sample semiconductor substrate10A is set. Here, as shown inFIG. 3Afor example, light having an intensity of I0is intermittently pulse-irradiated from the light source12with a predetermined period. In such periodical pulse irradiation of the light, the irradiation duration of the light (i.e., the pulse width) is defined as “irradiation duration T1”, and the non-irradiation duration of the light (i.e., the pulse interval) is defined as “non-irradiation duration T2”. It is preferred that both the irradiation duration T1and the non-irradiation duration T2are set in a time range that covers the value of the photoinduced carrier (minority carrier) effective lifetime of the semiconductor. Generally, the effective lifetime of a silicon film and a silicon substrate is in a range from 1 μs to 0.01 s. Thus, it is preferred that the time range of the irradiation duration T1and the non-irradiation duration T2is the range from 1 μs to 0.01 s. Here, the irradiation duration T1and the non-irradiation duration T2are set so that irradiation duration T1=non-irradiation duration T2(i.e., T1=T2).

Further, generation condition of the microwave (which is the measuring light) is set. Here, as shown inFIG. 3Bfor example, the microwave is periodically incident onto the semiconductor substrate from the microwave source11at an intensity of I0in synchronization with the pulse irradiation of the light. At this time, the irradiation duration T1of the light and the non-irradiation duration of the microwave are not superimposed on each other. In other words, the microwave is not irradiated during the irradiation duration T1of the light, and the microwave is irradiated during the non-irradiation duration T2.

In the pulse irradiation of the light set as above, the photoinduced carriers are generated in the semiconductor substrate10A by irradiating the light; and when the light irradiation is completed, carrier density n of the induced photoinduced carriers decays with the lifetime τeff. The decay can be expressed as the following Equation (3).

The n0in Equation (3) represents the carrier density of the photoinduced carriers generated in the semiconductor substrate10A in the case where the light is continuously irradiated (i.e., in the case where the light from the light source12is continuously irradiated, instead of being periodically pulse-irradiated as shown inFIG. 3A).

FIG. 3Cshows an example of the change of the carrier density with time in the case where the light is periodically pulse-irradiated onto the semiconductor substrate10A. As shown inFIG. 3C, in the semiconductor substrate10A, the carrier density increases during the irradiation duration T1of the light. Further, the carrier density decreases during the non-irradiation duration T2while the irradiation of the light is stopped. Under the generation condition of the microwave set above, the microwave is incident onto the semiconductor substrate10A in the time while the carrier density deceases.

In the incident microwave incident onto the semiconductor substrate10A, the microwave intensity of the microwave transmitted through the semiconductor substrate10A is detected, the microwave transmittance is calculated based on the detected microwave intensity, and the calculated microwave transmittance is integrated. The integrated value obtained by performing the integration is a value corresponding to the average value of the carrier density n in the non-irradiation duration T2. The average value of the carrier density n in the non-irradiation duration T2is expressed as the following Equation (4).

It can be known from Equation (4) that the integrated average value (the integrated value) of the carrier density n depends on the irradiation duration T1, the non-irradiation duration T2, the photoinduced carrier effective lifetime τeff, and the carrier density n0when continuously performing light irradiation.

Here, in the case where the irradiation duration T1is equal to the non-irradiation duration T2(i.e., T1=T2) in Equation (4), the average value of the carrier density n is expressed as the following Equation (5).

The data table is created by using Equation (5). Here, several different effective lifetimes τeffare set, and the variation of the average value <n> of the carrier density resulted from the variation of the irradiation duration T1(=non-irradiation duration T2) is calculated by using Equation (5). Further, the ratio <n>/n0of the average value <n> of the carrier density to the carrier density n0when continuously performing light irradiation is calculated for each effective lifetime τeff, and the ratio <n>/n0is plotted with respect to the irradiation duration T1. Thus, a data table shown inFIG. 4is obtained.

In other words, the data table shown inFIG. 4shows the relation between <n>/n0and the irradiation duration T1(=non-irradiation duration T2) for each photoinduced carrier effective lifetime τeff. InFIG. 4, the vertical axis represents the ratio <n>/n0of the carrier density <n> based on the microwave transmission intensity detected in the non-irradiation duration T2to the carrier density n0when continuously performing light irradiation. The horizontal axis represents the irradiation duration T1(ms) (=non-irradiation duration T2). InFIG. 4, the effective lifetimes τeffare set from 5×10−4(s) to 2.0×10−3(s) at an interval of 0.5×10−4(s). Further, the ratio <n>/n0of the carrier density <n> to the carrier density n0is calculated using the aforesaid Equation (5) for each photoinduced carrier effective lifetime τeffby changing the irradiation duration T1(=non-irradiation duration T2) from 0 ms to 3 ms, and the results are plotted into graph.

As shown inFIG. 4, the maximum of <n>/n0is 0.5 times the carrier density n0when continuously performing light irradiation. Further, <n>/n0plots regular curves with respect to both the lifetime τeffand the T1.

Next, by using the data table ofFIG. 4, the relation between <n>/n0and T1is obtained for a sample whose photoinduced carrier effective lifetime is unknown, and then the obtained the relation between <n>/n0and T1is compared with the curve of each effective lifetime τeffshown inFIG. 4to thereby obtain the effective lifetime of the sample.

To be specific, first, the sample semiconductor substrate10A is inserted into the gap16A of the waveguide15A of the photoinduced carrier lifetime measuring device shown inFIG. 1orFIG. 2. At this time, the light-guiding plate18is disposed so as to abut the irradiation surface of the light of the semiconductor substrate10A, or the semiconductor substrate10A is disposed so that the irradiation surface of the light of the semiconductor substrate10A faces the diffusion reflection plate18′. Further, the semiconductor substrates10A,10B are disposed so that they have the same direction with respect to the microwave source11.

Next, the light is periodically pulse-irradiated from the light source12onto the semiconductor substrate10A. The irradiation condition of the light is set as the aforesaid condition where, for example, the irradiation duration T1is equal to the non-irradiation duration T2as shown inFIG. 3A.

Further, the microwave is irradiated from the microwave source11onto the semiconductor substrates10A,10B in synchronization with the pulse irradiation of the light. The irradiation condition of the microwave is set as the aforesaid condition where, for example, the irradiation of the microwave has reversed phase with respect to the pulse irradiation of the light as shown inFIG. 3B; i.e., the microwave is not irradiated during the irradiation duration T1of the light, and the microwave is irradiated during the non-irradiation duration T2.

In such a state, the microwave transmitted through the semiconductor substrates10A,10B is detected by the detecting sections13A,13B. In the calculating section14, the microwave transmission intensity detected by the detecting section13A, and the microwave transmission intensity detected by the detecting section13B are summed and amplified.

Further, in the calculating section14, the microwave transmission intensity detected in each non-irradiation duration T2of the light is compared with the microwave transmission intensity detected in the preceding non-irradiation duration T2. Further, it is judged, at a point where the difference of the microwave transmission intensities compared has reached a predetermined value, that the change of the carrier density with time has become stable during the periodical pulse irradiation of the light, as shown inFIG. 3C.

After it is judged that the change of the carrier density has become stable, in the calculating section14, the microwave transmittance is calculated based on the amplified microwave intensity and further, an integrated value is obtained. The integrated value is expressed as the aforesaid Equation (5), where the average value of the carrier density in a predetermined irradiation duration T1(=non-irradiation duration T2) is <n>. Incidentally, the microwave transmittance of the reference semiconductor substrate10B, for example, may be used to calculate the microwave transmittance, and the microwave transmittance of the reference semiconductor substrate10B is obtained in advance.

Further, in the calculating section14, the relation between <n>/n0and T1is obtained based on the average value <n> of the carrier density in a predetermined irradiation duration T1(=non-irradiation duration T2), and the carrier density n0when continuously performing light irradiation calculated based on the measurement previously performed. Incidentally, the carrier density n0when continuously performing light irradiation can be determined by detecting the microwave transmission intensity using the measuring device shown inFIG. 1, and analyzing the decay-rate of the detected microwave transmission intensity.

Here, irradiation duration T1(=non-irradiation duration T2) is changed by the controller19within a ranged set when creating the data table shown inFIG. 4.

Further, <n>/n0is calculated for each changed the irradiation duration T1(=the non-irradiation duration T2), and the relation between <n>/n0and T1calculated for the unknown sample is fitted to the data of the data table shown inFIG. 4, so that the effective lifetime τeffof the unknown sample is obtained.

As described above, in the photoinduced carrier lifetime measuring method according to the third embodiment, the surface reflectivity r of the sample semiconductor substrate is not included as a parameter for obtaining the photoinduced carrier effective lifetime τeff. In a conventional microwave optical interference absorption method, the surface reflectivity r of the semiconductor substrate has to be obtained in advance. In contrast, in the photoinduced carrier lifetime measuring method according to the third embodiment, the photoinduced carrier effective lifetime τeffof the sample semiconductor substrate10A can be measured without using the surface reflectivity of the semiconductor substrate.

Further, since the light is periodically pulse-irradiated onto the sample semiconductor substrate, it is possible to obtain the measurement result with high sensitivity by obtaining an integrated value of the microwave intensity detected for each of plural periodical pulse irradiations.

Thus, it is possible to obtain the photoinduced carrier effective lifetime with high sensitivity even in the case where it is difficult to measure the surface reflectivity of the semiconductor substrate, particularly in the case where the semiconductor substrate is a solar cell structure that has a texture whose reflectivity actually can not be determined. Further, it is possible to obtain the effective lifetime based on the detection result of the microwave with high sensitivity, even in the case where the semiconductor substrate is irradiated by feeble light.

Next, a light incidence efficiency measuring method for obtaining the effective light incidence efficiency of the semiconductor substrate based on the photoinduced carrier effective lifetime obtained by the aforesaid method will be described below.

First, the carrier density n of the photoinduced carriers constantly existing in the semiconductor substrate can be expressed as the following Equation (2), which has been discussed before. In Equation (2), r is effective surface reflectivity, G is intensity (i.e., energy of one photon) of the continuously irradiated light, and τeffis the effective lifetime. Here, the effective surface reflectivity r is the actual reflectivity, which is obtained when light scattering and the like of the surface of the sample semiconductor substrate are taken into consideration, and is the reflectivity of the surface of a semiconductor substrate having a texture structure, for example.
[Mathematical Expression 5]
n=(1−r)Gτeff(2)

In the aforesaid Equation (2), the carrier density n is a value when continuously performing light irradiation (i.e., is n0), and can be determined by detecting the microwave transmission intensity using the measuring device shown inFIG. 1, and analyzing the decay-rate of the detected microwave transmission intensity. Further, the photon flux G can be determined by accurately measuring the intensity of the light irradiation. Further, the effective lifetime τeffcan be obtained by the photoinduced carrier lifetime measuring method according to the aforesaid embodiment.

Thus, it is possible to obtain effective light incidence efficiency (1-r) of the sample semiconductor substrate10A using the aforesaid Equation (2) and based on the effective lifetime τeffobtained by the aforesaid photoinduced carrier lifetime measuring method.

Further, the carrier density <n> of the photoinduced carriers in the case where periodical light is irradiated can be obtained by changing the average intensity of the photon flux G of Equation (2) into the following Equation (6) according to the irradiation duration T1and the non-irradiation duration T2.

Thus, by using the aforesaid Equation (6), it is possible to obtain the effective light incidence efficiency (1−r) of the sample semiconductor substrate10A based on the effective lifetime τeffobtained by the aforesaid photoinduced carrier lifetime measuring method.

Generally, the optical reflectivity of the semiconductor substrate and the like can be optically determined using a spectrometer. However, in the case where a transparent heteroecious thin film (such as an oxide film) is formed on the surface of the semiconductor, the reflectivity will largely change according to the light incidence angle. Thus, conventionally, in order to measure the effective optical reflectivity, it is necessary to know the distribution of the light incidence angle with respect to the sample of the light source. Further, in the case where the surface of the semiconductor substrate has convexoconcaves, there is a possibility that the light might be diffuse-reflected, and a portion of the reflected light might be incident on the semiconductor substrate again. Thus, in the case where the surface of the semiconductor substrate has convexoconcaves, it will be further difficult to spectroscopically determine the effective reflectivity.

In Example 1, an n-type silicon substrate coated with a thermally-oxidized film was used as the sample semiconductor substrate, wherein the thickness of the silicon substrate was 525 μm, and the thickness of the thermally-oxidized film was 100 nm. A light having a wavelength of 532 nm was continuously irradiated onto the silicon substrate at an intensity of 20 mW/cm2to obtain the carrier density n0of the photoinduced carriers. Further, a light having a wavelength of 532 nm was periodically pulse-irradiated onto the same silicon substrate at an intensity of 20 mW/cm2to respectively obtain the carrier densities <n> of the silicon substrate while changing the irradiation duration T1(=the non-irradiation duration T2).

Measurement of Photoinduced Carrier Lifetime

The photoinduced carrier effective lifetime of the semiconductor substrate was obtained by using the aforesaid photoinduced carrier lifetime measuring device and applying the photoinduced carrier lifetime measuring method according to the third embodiment.

In Example 1, an n-type silicon substrate coated with a thermally-oxidized film was used as the sample semiconductor substrate, wherein the thickness of the silicon substrate was 525 μm, and the thickness of the thermally-oxidized film was 100 nm. An induction light having a wavelength of 532 nm was continuously irradiated onto the silicon substrate at an intensity of 20 mW/cm2to obtain the carrier density n0of the photoinduced carriers. Further, an induction light having a wavelength of 532 nm was periodically pulse-irradiated onto the same silicon substrate at an intensity of 20 mW/cm2to respectively obtain the carrier densities <n> of the silicon substrate while changing the irradiation duration T1(=the non-irradiation duration T2).

InFIG. 5, the relation between the ratio <n>/n0of the carrier density obtained in Example 1 and the irradiation duration T1is indicated by mark ♦.FIG. 5is a graph showing the relation between <n>/n0obtained in Example 1 and T1superimposed onto the data table of the calculated values of <n>/n0and T1ofFIG. 4.

It can be known by comparing the calculated values of <n>/n0and T1shown inFIG. 4with the relation between <n>/n0and T1shown inFIG. 5that the photoinduced carrier effective lifetime τeffof the silicon substrate used as the sample in Example 1 is approximately 2.2×10−4(s).

Measurement of Photoinduced Carrier Lifetime and Measurement of Light Incidence Efficiency

The effective lifetime of the semiconductor substrate was obtained by using the aforesaid photoinduced carrier lifetime measuring device and applying the photoinduced carrier lifetime measuring method of the third embodiment, and further, the effective light incidence efficiency of the semiconductor substrate was obtained based on the obtained effective lifetime.

In Example 2, an n-type silicon substrate coated with a thermally-oxidized film was used as the sample semiconductor substrate, wherein the thickness of the silicon substrate is 525 μm, and the thickness of the thermally-oxidized film was 100 nm. The top surface of the silicon substrate was mirror-polished, and the rear surface of the silicon substrate was a rough surface not mirror-polished. The top surface and the rear surface of the silicon substrate were each coated with a thermally-oxidized film.

A light having a wavelength of 532 nm was continuously irradiated from a green surface light source onto the mirror top surface and the rough rear surface of the silicon substrate at an intensity of 1.8 mW/cm2to respectively obtain the carrier density n0of the mirror top surface and the carrier density n0of the rough rear surface. The carrier density when continuously irradiating the light onto the mirror top surface was n0=1.32×1012cm−2, and the carrier density when continuously irradiating the light on the rough rear surface was n0=1.68×1012cm−2.

Further, a light having a wavelength of 532 nm was periodically pulse-irradiated from a green surface light source onto the mirror top surface and the rough rear surface of the same silicon substrate at an intensity of 1.8 mW/cm2to respectively obtain the carrier densities <n> of the silicon substrate while changing the irradiation duration T1(=the non-irradiation duration T2).

The relation between <n>/n0of the carrier density obtained in Example 2 and irradiation duration T1was superimposed onto the data table shown inFIG. 4to compare the both so as to obtain the effective lifetime τeffof the photoinduced carriers for both the case where the light was irradiated onto the mirror top surface and the case where the light was irradiated onto the rough rear surface of the semiconductor substrate. In both the case of the mirror top surface and the case of the rough rear surface, the effective lifetime τeffwas approximately 4.0×10−4(s).

Next, the effective incidence efficiency (1−r) was calculated based on the previously calculated carrier density n0(=1.32×1012cm−2) when continuously irradiating the light onto the mirror top surface7, the photon flux G (=4.82×1015cm−2s−1) obtained by performing measurement, the previously obtained effective lifetime τeff(=approximately 4.0×10−4(s)), and the aforesaid Equation (6). As a result, the effective light incidence efficiency with respect to the mirror top surface was: (1−r)=0.68, and the effective reflectivity was r=0.32.

Further, the effective incidence efficiency (1−r) was calculated based on the previously calculated carrier density n0(=1.68×1012cm−2) when continuously irradiating the light onto the rough rear surface, the photon flux G (=4.82×1015cm−2s−1) obtained by performing measurement, the previously obtained effective lifetime τeff(=approximately 4.0×10−4(s)), and the aforesaid Equation (6). As a result, the effective light incidence efficiency with respect to the rough rear surface was: (1−r)=0.87, and the effective reflectivity was r=0.13.

Incidentally, in the aforesaid third embodiment, the value <n>/n0of the average value <n> of the carrier density to the carrier density n0when continuously performing light irradiation is used in order to normalize the average value <n> of the carrier density in the non-irradiation duration T2(=irradiation duration T1) obtained according to the integrated value of the microwave transmission intensity. However, the average value <n> of the carrier density may also be used as it is without being normalized. The average value <n> of the carrier density in the non-irradiation duration T2(=irradiation duration T1) may also be normalized as a value corresponding to the average value of the carrier density in the irradiation duration T1. In such a case, the microwave transmission intensity is also measured in the irradiation duration T1.

Next, a second example of the photoinduced carrier lifetime measuring method will be described below. The photoinduced carrier effective lifetime measuring method and the light incidence efficiency measuring method performed by using the photoinduced carrier lifetime measuring device shown inFIG. 1orFIG. 2will be described below with reference to a flowchart shown inFIG. 6.

First, the sample semiconductor substrate1OA is inserted into the gap16A of the waveguide15A of the photoinduced carrier lifetime measuring device shown inFIG. 1orFIG. 2. At this time, the light-guiding plate18is disposed so as to abut the irradiation surface of the light of the semiconductor substrate1OA, or the semiconductor substrate10A is disposed so that the irradiation surface of the light of the semiconductor substrate1OA faces the diffusion reflection plate18′. Further, the reference semiconductor substrate10B is inserted into the gap16B of the waveguide15B. It is preferred that the semiconductor substrates1OA,10B are disposed so that they have the same direction with respect to the microwave source11.

In such a state, in step S1, the light is periodically pulse-irradiated from the light source12onto the sample semiconductor substrate10A. At the same time, the microwave for measurement is irradiated from the microwave source11onto the sample semiconductor substrate10A and the reference semiconductor substrate10B.

As shown inFIG. 7A, the pulse irradiation of the light is performed at an intensity of I0and at a period of irradiation duration T1/non-irradiation duration T2. At this time, the condition of the pulse irradiation is set to a predetermined time T, wherein irradiation duration T1=non-irradiation duration T2=time T. The irradiation duration T1and the non-irradiation duration T2can be set in a range from 0.01 ms to 0.1 s as the third embodiment, and preferably can be set in a range from 0.01 ms to 10 ms.

As shown inFIG. 7B, the irradiation of the microwave is continuously performed both during the irradiation duration T1and during the non-irradiation duration T2at an intensity of J0. Here, a microwave having a frequency of 9.35 GHZ is used, for example.

Next, in step S2, the microwave irradiated onto the semiconductor substrates10A,10B and transmitted through the semiconductor substrates10A,10B in step S1is detected by the detecting sections13A,13B. Here, as described before, detected and amplified microwave transmission intensity J can be obtained by detecting the microwave by the detecting sections13A,13B.

Next, in step S3, in the calculating section14, based on the microwave transmission intensity J obtained in step S2and the set irradiation duration T1=non-irradiation duration T2=time T, a carrier density ratio P(T) of the photoinduced carriers is calculated according to the following Equation (7). The carrier density ratio P(T) calculated here is the ratio of an average carrier density Noffin the non-irradiation duration T2(T→2T) to an average carrier density Nonin the irradiation duration T1(0→T).

whereTr: microwave transmittanceTr0: microwave transmittance in dark state

In Equation (7), Tr is the microwave transmittance, and Tr0is the microwave transmittance in a dark state before the light is pulse-irradiated. The previously obtained microwave transmittance of the reference semiconductor substrate10B, for example, may be used to calculate the microwave transmittances Tr, Tr0. The microwave transmittance Tr0in the dark state is previously calculated before the measurement has been started in step S1.

Incidentally, in the calculating section14, the following judgment is performed before calculating the carrier density ratio P(T) by using the aforesaid Equation (7). In other words, two microwave transmission intensities respectively obtained in two sequential periods of the pulse irradiation of the light in step S2are compared with each other. Further, it is judged, at a point where the difference of the two microwave transmission intensities compared has reached a predetermined value, that the change of the carrier density with time has become stable during the periodical pulse irradiation of the light, as shown inFIG. 7C. Thereafter, the calculation in step S3is performed.

Next, in step S4, in the calculating section14, it is judged whether P(T)=0.859 or P(T)=0.615 has been obtained as the carrier density ratio P(T) calculated in step S3. If both results of P(T)=0.859 and P(T)=0.615 are not obtained, the process will proceed to step S5; the process proceeds to step S6only in the case where both results of P(T)=0.859 and P(T)=0.615 are obtained.

In step S5, the time T (=irradiation duration T1=non-irradiation duration T2) of the periodical pulse irradiation of the light in step S1is changed. Here, in the calculating section14, the result calculated in step S3is fed back, and the time T is changed so that the carrier density ratio P(T) becomes close to P(T)=0.859 or P(T)=0.615.

Thereafter, in step S1, the light is periodically pulse-irradiated onto the sample semiconductor substrate10A, and the measurement microwave is irradiated onto the semiconductor substrates10A,10B at a period of the time T changed in step S5. The pulse irradiation of the light and the irradiation of the microwave are performed by controlling the microwave source11and the light source12based on the time T changed in the calculating section14. The control of microwave source11and the light source12is performed by the controller19connected to the calculating section14.

Then the operation of step S1to step S5is repeatedly performed until it is judged that both results of P(T)=0.859 and P(T)=0.615 have been obtained in step S4.

On the other hand, if it is judged that both results of P(T)=0.859 and P(T)=0.615 have been obtained in step S4, the photoinduced carrier effective lifetime τeffwill be calculated in step S6by using the following Equation (8). However, T(P=0.859) is the time T at which P(T)=0.859 is reached. Further, T(P=0.615) is the time T at which P(T)=0.615 is reached. Incidentally, Equation (8) will be described later in more detail.

As described in the below Example 3, it is confirmed that the effective lifetime τeffobtained by the photoinduced carrier lifetime measuring method according to the aforesaid fourth embodiment has good consistent with photoinduced carrier lifetime when continuously performing light irradiation.

In the photoinduced carrier lifetime measuring method according to the aforesaid fourth embodiment, the photoinduced carrier effective lifetime τeffis obtained without using the surface reflectivity r of the sample semiconductor substrate as a parameter. Further, since the light is periodically pulse-irradiated onto the sample semiconductor substrate, a measurement result with high sensitivity can be obtained by obtaining an integrated value of the microwave intensity detected for each of plural periodical pulse irradiations.

Thus, similar to the method of the third embodiment, it is possible to obtain the photoinduced carrier effective lifetime with high sensitivity even in the case where it is difficult to measure the surface reflectivity of the semiconductor substrate (for example, in the case of a solar cell structure that has a texture whose reflectivity actually can not be determined).

Incidentally, in the photoinduced carrier lifetime measuring method according to the aforesaid fourth embodiment, the carrier density ratio P(T) obtained in step S2is fed back, and the time T is changed so that the carrier density ratio P(T) becomes close to P(T)=0.859 or P(T)=0.615. However, the photoinduced carrier lifetime measuring method according to the fourth embodiment is not limited to such steps, but the time T may also be sequentially changed within a predetermined ranged. In such a case, P(T)=0.859 and P(T)=0.615 are found from the carrier density ratios P(T) obtained by irradiating light at sequentially changed times T, so that T(P=0.859) and T(P=0.615) corresponding to P(T)=0.859 and P(T)=0.615 are obtained.

Equation (8) used in the fourth embodiment is derives as below.

If depth is x, time is t, and carrier volume density of the semiconductor substrate is n(x, t), then carrier density N(t) per unit area of a semiconductor substrate having thickness d can be calculated by the following Equation (9).
[Mathematical Expression 9]
N(t)=∫0dn(x,t)dx(9)

Further, the carrier density ratio P(T) of the photoinduced carrier defined as Equation (7) can be expressed as the following Equation (10) by using the carrier density N(t) per unit area in Equation (9). Incidentally, the carrier density ratio P(T) is the ratio of the average carrier density Noffin the non-irradiation duration T2(T→2T) to the average carrier density Nonin the irradiation duration T1(0→T).

whereNoffaverage carrier surface density (surface density of the entire substrate at a depth of d) in dark stateNon: average carrier surface density (surface density of the entire substrate at a depth of d) under light irradiationT: irradiation duration per pulse of light (i.e., pulse width)

Here, if the defect of the sample semiconductor substrate is bulk defect only, the surface recombination velocity on the irradiation side of the light will be Stop=0, and the surface recombination velocity on the side of the rear surface opposite to the irradiation side of the light will be Srear=0. In such a case, the carrier density Nonin the irradiation duration T1(0→T) and the average carrier density Noffin the non-irradiation duration T2(T→2T) are respectively expressed as the following Equations (11) and (12) using bulk lifetime τbof the minority carriers.

whereτb: bulk lifetime of minority carriersF: carrier production rate

The following Equation (13) can be obtained by rewriting Equation (10) based on Equations (12) and (13).

If T=τb, and T=2τbin Equation (13), then Equation (13) becomes the following Equations (14) and (15).
[Mathematical Expression 13]

If the value expressed in Equation (14) is used to express the irradiation duration T1(i.e., the pulse width) when the carrier density ratio P(T) is 0.859 as τpulse, then τpulsecan be expressed as the following Equation (16).
[Mathematical Expression 14]
τpulse=T(P=0.859)  (16)

Further, R is defined as the following Equation (17). P(0.615) is the irradiation duration T1when P(T)=0.615. T=P(0.859) is the irradiation duration T1when P(T)=0.859.

The aforesaid Equation (8) is expressed by using the τpulseand R defined as above.

The light incidence efficiency measuring method for obtaining the effective light incidence efficiency of the semiconductor substrate based on the photoinduced carrier effective lifetime obtained by the measuring method described in the fourth embodiment is performed in the same manner as described in the third embodiment.

Measurement of Photoinduced Carrier Lifetime

The photoinduced carrier effective lifetime of the semiconductor substrate was obtained by using the aforesaid photoinduced carrier lifetime measuring device and applying the photoinduced carrier lifetime measuring method according to the fourth embodiment.

In Example 3, the carrier density ratio P(T) with respect to the irradiation duration T1=non-irradiation duration T2=time T of the light was calculated using Equation (7) for each of plural silicon substrates (as semiconductor substrates) having different defect distributions. For each silicon substrate, the value of the bulk lifetime τbof the minority carriers (i.e., the photoinduced carriers), the value of the surface recombination velocity on the irradiation side of light Stop, and the value of the surface recombination velocity on the side of the rear surface opposite to the irradiation side of the light Srearwere set to respective values. These values indicate the number of the defects in each portion (i.e., the bulk, the surface onto which light is irradiated, and the surface opposite to surface onto which light is irradiated) of the semiconductor substrate.

FIG. 8shows the carrier density ratio P(T) with respect to the time T in the case where the minority carriers of the silicon substrate are holes.FIG. 9shows the carrier density ratio P(T) with respect to the time T in the case where the minority carriers of the silicon substrate are electrons.

Next, the effective lifetime τeffwas calculated for each silicon substrate having the aforesaid defect distribution by analyzing time T-carrier density ratio P(T) shown inFIGS. 8 and 9and using Equation (8) shown in the fourth embodiment.FIG. 10shows the relation between the effective lifetime τeffcalculated using Equation (8) and the photoinduced carrier lifetime when continuously performing light irradiation obtained by the following Equation (18).

[Mathematical⁢⁢expression⁢⁢16]τeffCW=τb⁢Dτb⁢(1-exp⁡(-dD⁢⁢τb))(Dτb+Srear+(Dτb-Srear)⁢exp⁡(-dD⁢⁢τb))(Dτb+Srear)⁢(Dτb+Stop)-(Dτb-Stop)⁢(Dτb-Srear)⁢exp⁡(-2⁢⁢dD⁢⁢τb)(18)τeffCW: photoinduced carrier lifetime when continuously performing light irradiationD: diffusion coefficient of minority carriersτb: bulk lifetime of minority carriersStop: surface recombination velocity on irradiation side of induction lightSrear: surface recombination velocity on side of rear surface opposite to irradiation side of induction lightd: thickness of substrate

As shown inFIG. 10, the effective lifetime calculated using Equation (8) of the fourth embodiment is in consistency with the photoinduced carrier lifetime when continuously performing light irradiation shown as Equation (18). Thus, it is confirmed that the photoinduced carrier effective lifetime τeffof the semiconductor substrate can be obtained with high accuracy.

Incidentally, in the fourth embodiment, the ratio of the average carrier density Noffin the non-irradiation duration T2(T→2T) to the average carrier density Nonin the irradiation duration T1(0→T) is used as the carrier density ratio P(T). However, the carrier density ratio P(T) is not such limited, but may also be, for example, the ratio of the average carrier density Noffin the non-irradiation duration T2(T→2T) to the carrier density n0when continuously performing light irradiation. In such a case, other suitable value is used as the T(P) that constitutes the R of Equation (8).

Next, a third example of the photoinduced carrier lifetime measuring method will be described below. Here, a method for measuring surface distribution of the photoinduced carrier effective lifetime of the semiconductor substrate will be described below by using the photoinduced carrier lifetime measuring device shown inFIG. 1orFIG. 2.

First, the sample semiconductor substrate10A on the microwave irradiation surface side is divided into a plurality of areas. Here, the sample semiconductor substrate10A on the microwave irradiation surface side is divided into a plurality of areas, where the size of each area is 1 cm×0.5 cm, for example.

Next, the photoinduced carrier lifetime measuring method according to the aforesaid third embodiment or the fourth embodiment is performed on each of the divided areas, and the photoinduced carrier effective lifetime of each area is measured. In such a case, the semiconductor substrate10A is moved in a direction perpendicular to the incident direction of the microwave by a movable stage arranged in the photoinduced carrier lifetime measuring device to thereby selectively irradiate microwave onto each area of the semiconductor substrate10A to perform measurement; or the irradiation position of the microwave is moved by a microwave scanning section arranged in the photoinduced carrier lifetime measuring device to thereby selectively irradiate microwave onto each area of the semiconductor substrate10A to perform measurement.

It is possible to obtain the surface distribution of the photoinduced carrier effective lifetime by the photoinduced carrier lifetime measuring method according to the fifth embodiment. It is possible to measure the surface distribution with high spatial resolution by using a microwave having relatively high frequency.

The light incidence efficiency measuring method for obtaining the effective light incidence efficiency of the semiconductor substrate based on the photoinduced carrier effective lifetime obtained by the measuring method described in the fifth embodiment is performed in the same manner as described in the third embodiment.

Measurement of Photoinduced Carrier Lifetime

FIG. 11shows a distribution of the effective lifetime τeffcalculated for each area of the semiconductor substrate. A microwave having a frequency of 9.35 GHZ was used to perform the measurement. Thus, it was confirmed that the photoinduced carrier lifetime could be measured with a spatial resolution of about 1 cm.

The measuring device and measuring method for detecting the microwave transmitted through the semiconductor substrate have been described in the aforesaid first embodiment to fifth embodiment; however, the present invention also includes a measuring device and measuring method for detecting the microwave reflected by the semiconductor substrate.

In such a case, as for the photoinduced carrier lifetime measuring device, in the configuration shown inFIG. 1orFIG. 2, the detecting sections13A,13B may be disposed in the position where the waveguide15, on which the microwave source11is mounted, is branched into two waveguides. Further, as for the photoinduced carrier lifetime measuring method, the microwave transmission intensity may be substituted with microwave reflection intensity.

Further, the aforesaid embodiments and examples are described base on a case where the irradiation duration T1is equal to the non-irradiation duration T2; however, in the present invention, T1and T2do not have to be equal to each other, but may also be suitably set respectively.

It is to be understood that the present invention is not limited to the embodiments described above, and various modifications and variations can be made without departing from the spirit and scope of the present invention.

EXPLANATION OF REFERENCE NUMERALS