JPH0743294A - Photoreflectance measuring equipment - Google Patents

Abstract

PURPOSE:To allow accurate measurement of photoreflectance spectrum by eliminating the influence of scattering exciting light and photoluminescence light. CONSTITUTION:A sample 4 is irradiated with a probe light P1 modulated at a period sufficiently longer than that of an exciting light R1. A spectrometer 10 selects a specific wavelength from the reflected light H1 and a spectrometer 11 detects the reflection intensity for that wavelength. The reflected light is then sent through a high-pass filter 12a and a low-pass filter 12b to a rock-in amplifier 14. The rock-in amplifier 14 also receives a modulation period signal from a modulation chopper A2 under control of a control circuit 13 and produces a component of reflected light intensity having this modulation period.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoreflectance measuring device used for semiconductor band structure evaluation, impurity concentration evaluation, carrier trap level evaluation, substrate surface thermometer and the like.

[0002]

2. Description of the Related Art Conventionally, in measuring the photoreflectance of a semiconductor, while a sample is irradiated with excitation light such as laser light, on the other hand, it is irradiated with probe light such as white light. The measurement was performed by spectrally detecting the reflected light of light.

FIG. 13 schematically shows a conventional photoreflectance measuring device. In FIG. 13, 42 is an excitation light source,
43 is a modulation chopper, 44 is a sample, 45 is a probe light source, 46, 46a, 47, 48, 49 are lenses, 50 is a spectroscope, 51 is a detector, 52 is a lock-in amplifier, 53
Is a control circuit, r is excitation light, p is probe light, h ₁ is reflection light of probe light, h ₂ is reflection light of excitation light, f ₁ , f ₂ ,
f ₃ is a signal.

Next, the operation of the photoreflectance measuring device of FIG. 13 will be described. Probe from the probe light source 45
The black light p is applied to the sample 44 via the lenses 46 and 47. Further, the excitation light r from the excitation light source 42 is modulated by the modulation chopper 43 and then narrowed down by the lens 46a,
The sample 44 is irradiated.

On the other hand, in the reflected light h ₁ of the probe light from the sample 44, after a specific wavelength is selected by the spectroscope 50 via the lenses 48 and 49, the detector 51 detects the reflection intensity for that wavelength. Then, it is input to the lock-in amplifier 52.

The lock-in amplifier 52 has a control circuit 5
Since the synchronization signal equal to the period of the modulation signal input to the modulation chopper 43 from 3 is input, the reflected light component synchronized with the modulation period is detected.

FIG. 14 shows the reflected light intensity when the excitation light is not irradiated, and FIG. 15 shows the reflected light intensity when the excitation light is modulated by the modulation chopper and irradiated.

By irradiating the sample with excitation light from a laser or the like, thermal modulation and electric field modulation due to generation of electron-hole pairs occur, so that a photoreflectance signal is generated.

The reflected light h ₁ contains a modulation component, that is, a photoreflectance signal ΔR, which is taken out by the lock-in amplifier 52, and is reflected by the probe f ₃ detected from the signal f _3. By normalizing the photoreflectance signal ΔR with the DC component R of the signal and scanning the selected wavelength of the spectroscope 50, the photoreflectance spectrum which is a characteristic curve of wavelength and ΔR / R as shown in FIG. Can be obtained. Although the DC component R of the reflected light signal actually includes the photoreflectance signal ΔR, since ΔR is extremely smaller than R, the error can be ignored.

Since energy is a function of the wavelength of light,
From the wavelength λ ₁ at the minimum value of ΔR / R, the potential energy between certain band pairs in the band structure of the substance to be measured can be obtained.

[0011]

The conventional photoreflectance measuring device is constructed as described above, but generally ΔR is a small signal of 10 ⁻⁴ times or less of R, and the scattered light of the modulated excitation light is easily influenced.

Further, in the case of measurement at a low temperature or measurement of a superlattice structure, light emission (photoluminescence light emission) from the sample is likely to occur, and this is superposed on the photoreflectance signal ΔR, which actually causes an error in the signal amount. The signal includes S.

That is, the photoluminescence light is shown in FIG.
6 (b) has a wavelength characteristic as shown in FIG. 6 (b). When this is superimposed on the ΔR / R characteristic of FIG. 16 (a), FIG.
Since the spectrum is as shown in (1), accurate potential energy cannot be measured.

The power density of the probe light is sufficiently smaller than the power density of the excitation light, and it is not necessary to consider the influence of the scattered light and the photoluminescence light due to the probe light.

In order to remove the influence of the scattered light of the excitation light and the photoluminescence light as described above, it is necessary to separately carry out the measurement of irradiating only the excitation light and subtract the signal amount S obtained as a result. Or, the error signal had to be removed by using two detectors.

The present invention was devised to solve the above-mentioned problems, and it is easier to perform photoreflectance measurement without being affected by scattered light of excitation light and photoluminescence light, as compared with the prior art. It is an object to provide a device that can be implemented.

[0017]

The present invention is directed to a conventional photoreflectance measuring apparatus for solving the above-mentioned problems. The first invention is separate from a pumping light modulation chopper for modulating pumping light. , A modulation chopper is provided for modulating the probe light.

The second aspect of the present invention is different from the conventional photoreflectance measuring apparatus in that the modulation chopper of the excitation light is eliminated, and the portion of the sample irradiated with the probe light and the portion not irradiated with the probe light are cycled. In order to irradiate the excitation light selectively, means for varying the irradiation position of the excitation light on the sample is provided.

[0019]

According to the first aspect of the invention, the probe light modulated by the modulation chopper with a period sufficiently longer than the modulation period of the excitation light is irradiated to the sample, and the modulated excitation light is simultaneously irradiated. By detecting the periodic component corresponding to the modulation period of the probe light, the photoreflectance signal from which the error signal has been removed can be detected.

In a second aspect of the present invention, excitation light is periodically irradiated to a portion of the sample irradiated with the probe light and a portion not irradiated with the probe light, and the excitation light is irradiated from the obtained reflected light signal. By detecting the periodic component corresponding to the fluctuation period, it is possible to detect the photoreflectance signal from which the error component has been removed.

[0021]

EXAMPLES Examples of the present invention will be described below.

FIG. 1 shows a schematic structure of the photoreflectance measuring apparatus of the first invention. In FIG. 1, 1 is an excitation light source, 2 is a modulation chopper A, 3 is a modulation chopper B, 4 is a sample, 5 is a probe light source, 6, 6a, 7, 8 and 9 are lenses, 10 is a spectroscope, and 11 Is a detector, 12a is a high-pass filter, 12b is a low-pass filter, 13 is a control circuit,
14 is a lock-in amplifier, R ₁ is excitation light, P ₁ is a professional
Light, H ₁ is the reflected light of the probe light P ₁ , H ₂ is the excitation light R
₁ of the reflected _{light, F 1, F 2, F} 3 is a signal.

Next, the operation of the photoreflectance measuring device of FIG. 1 will be described. Excitation light R ₁ from the excitation light source 1 is modulated by the modulation chopper A2 which is controlled by the control circuit 13, is narrowed by the lens 6a, it is applied to the sample 4.

The probe light P from the probe light source 5 is also used.
_The sample 1 is periodically irradiated to the sample 4 via the lens 6, the modulation chopper B3 controlled by the control circuit 13, and the lens 7. The modulation cycle of the modulation chopper B3 is made sufficiently longer than the modulation cycle of the modulation chopper A2.

On the other hand, the reflected light H ₁ of the probe light is a signal as shown in FIG. 2 when the spectroscope 10 selects a specific wavelength and then the detector 11 detects the reflection intensity for that wavelength. Is obtained.

In FIG. 2, the signals obtained during the irradiation of the probe light are the reflected light signal R of the probe light and the photoreflectance signal Δ when the excitation light is not irradiated.
It is a signal in which signal errors S such as scattered light of R and excitation light and a photoluminescence signal are superposed, and only the error signal S is obtained during a period in which the probe light is not irradiated.

When this signal is passed through a high-pass filter 12a for filtering a modulation signal having a frequency higher than that of the modulation chopper A2, a signal ΔR + S in which the signal ΔR and the signal S are superposed and a periodic signal having only the signal S as shown in FIG. Is obtained.
The period of this periodic signal is equal to the period of the chopper A2.

This signal is further passed through a low-pass filter 12b which passes a frequency smaller than a frequency ν ₀ satisfying (frequency of modulation chopper A2) >> ν ₀ >> (frequency of modulation chopper B3), A signal as shown in FIG. 4 is obtained.

On the other hand, the lock-in amplifier 14 receives a periodic signal equal to the period of the modulation signal input from the control circuit 13 to the modulation chopper B3, and the signal F ₁ transmitted through the low pass filter 12b is locked in. By being input to the amplifier 14, a periodic component equal to the modulation period of the modulation chopper B3, that is, a signal of (ΔR + S) −S = ΔR is obtained.

Further, ΔR / R can be obtained by normalizing this signal by the DC component R of the reflected light signal of the probe light detected from the signal F ₃ . Although the DC component R of the reflected light signal actually includes the photoreflectance signal ΔR, since ΔR is extremely smaller than R, the error can be ignored.

Therefore, by scanning the selected wavelength of the spectroscope 10, the excitation light wavelength and ΔR /
It is possible to accurately obtain the photoreflectance spectrum which is the characteristic curve of R.

As described above, since the energy is a function of the wavelength of light, from the wavelength λ ₁ at the minimum value of ΔR / R,
It is possible to obtain the potential energy between a certain pair of bands in the band structure of the substance to be measured.

A second aspect of the invention is to irradiate a portion of the sample irradiated with the probe light and a portion not irradiated with the probe light with excitation light periodically to measure the intensity of each reflected light.
FIG. 6 shows a schematic configuration of one embodiment.

In FIG. 6, 15 is an excitation light source, 16 is a reflection plate, 17 is a rotation axis, 18 is a sample, 19 is a probe light source, 20, 20a, 20b, 21, 22, 23 are lenses, and 24 is a spectrum. , 25 is a detector, 26 is a lock-in amplifier, 27 is a control circuit, R ₂ , R ₃ and R ₄ are excitation lights, P
₂ is the probe light, H ₃ is the reflected light of the probe light P ₂ , H ₄
Is the reflected light of the excitation light R ₃ , H ₅ is the reflected light of the excitation light R ₄ , and F
₄ , F ₅ and F ₆ are signals.

Next, the operation of the photoreflectance measuring device of FIG. 6 will be described. The excitation light R ₂ changes its reflection direction due to the difference in the incident angle with respect to the reflection plate 16 which is periodically changed by the rotation axis 17, and the excitation lights R ₃ and R ₄ are the probe light P _{2 of the} sample 18. The irradiated portion and the non-irradiated portion are respectively narrowed down by lenses 20a and 20b, and then periodically irradiated onto the sample 18.

FIG. 7 shows the operation of changing the traveling direction of the excitation light R ₂ by the reflection plate 16 into the excitation lights R ₃ and R ₄ . Figure 8
(A) shows the irradiation state of the excitation light and probe light on the sample. FIG. 8B shows the irradiation state of the excitation light and the probe light on the sample as seen from the spectroscope.

In FIG. 6, the reflected light H ₃ reflected by the sample 18 is passed through the lenses 22 and 23, and the spectroscope 24
After the specific wavelength is selected in step 1, the detector 25 detects the reflection intensity for that wavelength, and a signal as shown in FIG. 10 is obtained.

In FIG. 10, when the excitation light R ₃ is applied to the portion where the probe light P ₂ is applied, the photoreflectance signal ΔR, the extra signal S such as photoluminescence, and the excitation light are applied. The sum ΔR + S + R of the reflected light signals R when not being irradiated is obtained, and when the portion where the probe light P ₂ is not irradiated is irradiated with the excitation light R ₄ , the signal S and the sum S + R of R are obtained.

On the other hand, the lock-in amplifier 26 receives a periodic signal equal to the drive signal period of the reflector 16 from the control circuit 27, and the signal F ₄ from the detector 25 is input to the lock-in amplifier 26. As a result, only the periodic component equal to the drive signal period of the reflection plate 16, that is, the signal F ₅ representing ΔR can be extracted. DC component R of the reflected light signal of the probe light obtained by detecting this signal F ₅ from F ₆
ΔR / R can be obtained by normalizing with. Although the DC component R of the reflected light signal actually includes the photoreflectance signal ΔR, since ΔR is extremely smaller than R, the error can be ignored.

Therefore, by scanning the selected wavelength of the spectroscope 24, the photoreflectance spectrum shown in FIG. 5 can be obtained.

As shown in FIG. 8C, the excitation light is moved in the orthogonal direction with respect to the spectroscope slit 24a, and the excitation lights R ₃ and R ₄ are supplied to the portion irradiated with the probe light P _2. The same photoreflectance measurement as in the above-described embodiment of FIG. 6 can be performed by irradiating the sample 18 periodically after squeezing the non-illuminated portions with the lenses 20a and 20b, respectively. In this case, the signal light H ₃ ′ from the portion irradiated with only the excitation light is input to the spectroscope 24 via the reflecting mirror a and the semi-transparent mirror b as shown in FIG. 8D.

A second method for changing the traveling direction of the excitation light is to change the excitation light source to move the irradiation position of the excitation light. A schematic configuration of one embodiment is shown in FIG.

In FIG. 11, 28 is an excitation light source, 29 is a rotation axis, 30 is a sample, 31 is a probe light source, and 32 and 32.
a, 32b, 33, 34 and 35 are lenses, 36 is a spectroscope, 37 is a detector, 38 is a lock-in amplifier, 39 is a control circuit, R ₅ and R ₆ are excitation light, P ₃ is probe light, H ₆
Is the reflected light of the probe light P ₃ , H ₇ is the reflected light of the excitation light R ₅ , H ₈ is the reflected light of the excitation light R ₆ , and F ₇ , F ₈ and F ₉ are signals.

Next, the operation of the photoreflectance measuring device of FIG. 11 will be described. The excitation light source 28 is fixed to and supported by the rotation shaft 29, which is periodically varied by the control circuit 39 to generate excitation lights R ₅ and R _{6 as} shown in FIG. 9A.
Pro - the part where the blanking light P ₃ is not irradiated with the irradiated portion, each lens 32a, after finely focused at 32b, periodically irradiating a sample 30.

Thereafter, the photoreflectance can be measured in the same manner as the embodiment shown in FIG.

As shown in FIG. 9B, the excitation light is moved in the orthogonal direction with respect to the spectroscope slit 24a, and the excitation lights R ₅ and R ₆ are supplied to the portion irradiated with the probe light P _3. The same photoreflectance measurement as in the above-described embodiment of FIG. 11 can be performed by irradiating the sample 30 periodically after narrowing the non-illuminated portions with the lenses 32a and 32b, respectively. In this case, the signal light H ₆ ′ from the portion irradiated with only the excitation light is input to the spectroscope 36 via the reflecting mirror a and the semitransparent mirror b, as shown in FIG. 9C.

A third method for changing the traveling direction of the excitation light is to move the irradiation position of the excitation light by using an acousto-optical deflector.

FIG. 12 shows a schematic structure of an acousto-optical deflector. In FIG. 12, 40 is a main body, 41 is a sound wave source, R ₇ is excitation light before entering the acousto-optical deflector,
R ₈ and R ₉ are excitation lights after being emitted from the acousto-optical deflector. A sound wave is generated by the sound wave source 41 by the control circuit, and a standing wave is made to stand in the medium in the main body 40. Since this is a periodic wave, it has the function of a diffraction grating at a certain wavelength. Therefore, by changing the frequency of the sound wave, the grating constant of the diffraction grating is changed, and the traveling direction of light can be changed. Thereby, the traveling direction of the excitation light R ₇ is changed so that the excitation light R ₈ is periodically irradiated to the portion irradiated with the probe light and the excitation light R ₉ is periodically irradiated to the portion not irradiated with the probe light. .

Also in this way, the photoreflectance measurement can be performed, and the photoreflectance measurement can be performed by using another method of changing the optical path of the excitation light.

In the above embodiment, the spectroscope is arranged between the sample and the detector. However, the spectroscope is arranged between the probe light source and the sample, and the probe light is previously dispersed to photo Reflectance measurement can also be performed.

[0051]

Since the present invention is configured as described above, the influence of scattered light of excitation light and photoluminescence light can be removed by one wavelength scan with one detection system. Therefore, an accurate photoreflectance spectrum can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an exemplary embodiment of the first invention of the present application.

FIG. 2 is a diagram showing a signal characteristic (1).

FIG. 3 is a diagram showing a signal characteristic (2).

FIG. 4 is a diagram showing a signal characteristic (3).

FIG. 5 is a diagram showing a photoreflectance spectrum.

FIG. 6 is a diagram showing a configuration of an example in the second invention of the present application.

FIG. 7 is a diagram showing how to change the traveling direction of excitation light by a reflection plate.

FIG. 8 is a diagram (1) showing optical paths of pumping light, probe light, and signal light in the second invention of the present application.

FIG. 9 is a diagram (2) showing the optical paths of the pumping light, the probe light, and the signal light in the second invention of the present application.

FIG. 10 is a diagram showing a signal characteristic (4).

FIG. 11 is a diagram showing a configuration of a modified example of the second invention of the present application.

FIG. 12 shows an acousto-optical polarization device.

FIG. 13 is a diagram showing a conventional configuration.

FIG. 14 is a diagram showing a conventional signal characteristic (1).

FIG. 15 is a diagram showing a conventional signal characteristic (2).

FIG. 16 is a diagram showing a conventional signal characteristic (3).

Claims (2)

[Claims] 1. A probe light source for irradiating the sample with probe light, an excitation light source for irradiating the sample with excitation light, an excitation light modulation chopper for modulating the excitation light, and detection of reflected light from the sample. A photoreflectance measuring device comprising a detector, wherein a modulation chopper is provided between the probe light source and the sample. 2. A photoreflectance measuring device comprising a probe light source for irradiating a sample with probe light, an excitation light source for irradiating the sample with excitation light, and a detector for detecting reflected light from the sample. 2. A photoreflectance measuring device according to claim 1, further comprising means for changing the irradiation position of the excitation light on the sample.