JP4590574B2 - Phase lock-in type high-frequency scanning tunneling microscope - Google Patents

Description

  The present invention relates to a phase-locked-in high-frequency scanning tunneling microscope, and more particularly, a phase-locked-in high-frequency scanning tunnel capable of observing an atomic image of a conductor or semiconductor as well as an insulator or a wide bandgap semiconductor surface. It relates to a microscope.

  Since the development of the scanning tunneling microscope (STM), it has become possible to observe the atomic structure of the solid surface in real space. However, since the conventional STM using a DC bias voltage uses a tunnel current, the atomic structure of the conductor or semiconductor through which the current flows can be observed, but this is not possible with an insulator. However, a surface having a function that affects the atomic arrangement of the surface, for example, many optical elements such as optical lenses and mirrors are insulators, and many insulating materials are used for semiconductor devices. Observation of the atomic structure on the surface is strongly desired.

Currently, the practical Si surface used in the semiconductor industry is usually covered with a natural oxide film, so that STM measurement cannot be performed without solution treatment. Therefore, in order to observe the surface of an insulator such as SiO ₂ using STM, a method of applying an AC voltage between the probe and the sample or applying a sufficient DC voltage to the thin natural oxide film of Si is applied to the probe. A method is being tried.

Non-Patent Documents 1 to 3 disclose that harmonics generated by flowing a small amount of electrons between the probe and the sample and flowing tunnel electrons between the probe and the sample during a short period of a high-frequency voltage in the order of GHz. An attempt is made to perform STM operation on a sample having no electrical conductivity by using a wave as a feedback signal. However, the insulator material has only obtained a spatial resolution of up to about 1 nm. One of the causes is charging of the sample surface. This is because tunnel electrons are charged directly under the probe, thereby preventing tunneling of electrons from the probe. Second, since the AC tunneling current component having the applied frequency f ₁ is buried in the current component due to the capacitance from the probe-sample and the like that is much larger than that, it is not f ₁ . This is because harmonic components (3f ₁ etc.) are used as feedback signals.

  There are several reports of observation of the Si surface with an oxide film, which is one of the targets of the present invention, and applying a DC bias between the probe and the sample to operate as a normal STM. Among them, Non-Patent Document 4 reports an STM observation performed by applying a voltage of about 5 V to tunnel electrons to the conduction band on a Si surface having an oxide film of several nm. However, the resolution of the atomic image size has not been obtained, and the oxide film is limited to a few nm or less. This is because the natural oxide film on Si is amorphous, so there are various defect levels in the band gap, and some of the tunneled electrons fall to the defect level and the sample is charged. This is thought to be because the tunnel during STM observation is hindered.

  Thus, the resolution for observing the atomic image is not suitable in any case. In view of this, the inventors of the present invention disclosed in Patent Document 1 that the distance between the tip of the probe and the sample was very close, a tunnel current was generated by the bias voltage applied between the probe and the sample, and this tunnel current was kept constant. A scanning tunneling microscope for observing an atomic scale image of the sample surface by applying a feedback signal to the drive system under the conditions of A pulse AC bias voltage is applied, and the time constant determined by the tunnel resistance and stray capacitance between the probe and the sample is substantially matched with the time constant determined by the input impedance of the current amplifier, and only the tunnel current component is detected and rectified. We have proposed a high-frequency pulsed scanning tunneling microscope that uses a feedback signal.

  According to the method described in Patent Document 1, a HOPG atomic image can be observed relatively clearly up to a frequency of several tens of kHz. However, a clear atomic image can be observed at a frequency of 100 kHz. could not. In the method described in Patent Document 1, a high-frequency rectangular short pulse is applied as a bias voltage between the probe 101 and the sample 102 by the pulse generator 103 as shown in FIG. Because of this, overcurrent (spike current) occurs at the rise and fall of the pulse, and the tunnel current is buried in the overcurrent, so it cannot be used as a feedback signal. Therefore, an impedance matching circuit 104 composed of a capacitor C and a variable resistor R is provided, converted into a DC signal proportional to the tunnel current via the current amplifier 105 and the rectifier circuit 106, and input to the control system 107 to input the probe. Let 101 be a feedback signal for driving 101. However, when the matching circuit 104 is used, the frequency characteristics are deteriorated, and the stray capacitance changes due to the displacement of the probe. Therefore, it is impossible to completely detect only the tunnel current component even if the impedance matching circuit is provided. There was a problem, especially for a bias voltage having a high frequency, and the noise was large, and a clear atomic image could not be obtained.

In addition, the conventional STM has a structure in which observation is performed by placing the probe and sample in an ultra-high vacuum chamber with strict electromagnetic shielding in order to reduce noise as much as possible and increase detection sensitivity. It is inevitable that the apparatus is expensive and it takes a long time before the sample can be set and observed.
GPKochanski Physical Review Letters, Vol. 62 (1989) p2285 W. Seifert et al. Ultramicroscopy, Vol. 42-44 (1992) p379 B. Micheal et al. Rev. Sci. Instrum., Vol. 63, No. 9 (1992) p4080 Watanabe et al. Applied Physics Letters, Vol.72, No.16 (1998) p1987 JP 2002-365194 A

Thus, in the observation of the insulator surface (for example, SiO ₂ ) using the current STM, high spatial resolution has not been obtained. This is because tunnel electrons are charged on the surface of the sample directly under the probe, thereby preventing the tunneling of electrons from the probe. Here, the conditions for observing the insulator or semiconductor surface by STM are summarized as follows.
(1) A high frequency bias voltage is applied between the probe and the sample to prevent the sample from being charged.
(2) Only the tunnel current component is detected and used as a feedback signal that keeps the distance between the probe and the sample surface constant.
(3) In order to prevent the sample directly under the STM probe from being charged, it is necessary to reduce the amount of charge sent from the probe side to the sample side as much as possible. Therefore, the frequency of the high frequency bias voltage is made as high as possible.
(4) Tunnel electrons in the conduction band of the insulator material.

  However, when a high-frequency bias voltage is applied between the probe and the sample, the tunnel current component is buried in the current component generated by the stray capacitance from the probe to the sample and others, and electromagnetic noise It is difficult to detect the tunnel current due to the influence of the above. For example, the current value (i = 2πfCV) due to the stray capacitance in the high frequency STM is about 12 nA if the bias voltage frequency f is 100 kHz and the bias voltage V is 200 mV, even if a smaller value of 0.1 pF is selected as the stray capacitance C. Can be estimated. On the other hand, since the tunnel current value in STM observation is generally 0.1 to 1 nA, the current value derived from the stray capacitance is 1 to 2 orders of magnitude larger than that. However, although the stray capacitance varies depending on the device configuration, 0.1 pF is estimated to be sufficiently small, so that the current derived from the stray capacitance is actually much larger than the tunnel current.

  By proposing and developing a high-frequency STM, which is a new measurement method that solves these problems, it is possible to observe the surface of insulators or semiconductors with higher spatial resolution. Atomic Force Microscope (AFM) It can be expected to obtain information about electronic states that could not be performed. Of course, it is also possible to obtain information on the electronic state by observing the atomic image of the conductor, as in general STM.

Therefore, in view of the above-described situation, the present invention intends to solve the problem of a phase lock-in type high-frequency capable of observing an atomic image of an insulator such as SiO ₂ or a wide band gap semiconductor or an optical component surface. The point is to provide a scanning tunneling microscope.

  In order to solve the above-mentioned problems, the present invention provides a condition in which the distance between the tip of the probe and the sample is very close, a tunnel current is generated by a bias voltage applied between the probe and the sample, and this tunnel current is maintained constant. A scanning tunneling microscope that applies a feedback signal to the drive system and relatively scans the probe along the sample surface to observe an atomic scale image of the sample surface, and includes a rectangular wave or a gap between the probe and the sample. Input a bias power source that applies a sine wave high-frequency bias voltage, a current amplifier that amplifies a detection current that is a mixture of the tunnel current flowing between the probe and the sample and the current derived from the stray capacitance, and the current amplified by the current amplifier. The sampling signal is synchronized with the frequency of the high-frequency bias voltage and locked to the phase of the high-frequency bias voltage, and the current from the stray capacitance is removed to allow only the tunnel current. Generating a feedback signal by, to constitute a phase-lock-type high frequency scanning tunneling microscope, characterized in that a signal processing unit having a phase lock-in feature.

  Here, the high-frequency bias voltage is a rectangular wave, and an impedance matching circuit that substantially matches the time constant determined by the tunnel resistance and stray capacitance between the probe and the sample and the time constant determined by the input impedance of the current amplifier is a current amplifier. Is preferably provided on the input side.

  Further, after the high frequency bias voltage is a sine wave and the phase of the sampling signal of the signal processing means is locked to the phase of the current derived from the stray capacitance outside the tunnel region where the distance between the probe and the sample is separated, the sampling signal More preferably, the output of the electric current derived from the stray capacitance is set to 0 by shifting the phase of the current, and then the approach is started into the tunnel region where the tunnel current flows by narrowing the distance between the probe and the sample.

  In this case, after the phase of the sampling signal of the signal processing means is locked to the phase of the current derived from the stray capacitance outside the tunnel region, the output of the current derived from the stray capacitance is reduced to 0 by shifting the phase of the sampling signal by −90 °. To do.

  The frequency of the bias voltage is in the range of 100 kHz to 10 GHz, and the tunnel current used as the feedback signal is in the range of 1 pA to 10 nA.

  The probe-sample distance is kept constant, a sinusoidal high-frequency bias voltage is applied between the probe and the sample from the bias power source, and the phase of the sampling signal of the signal processing means is locked to the phase of the tunnel current. By measuring the dI / dV of the sample to obtain an IV characteristic, or by locking the phase of the sampling signal of the signal processing means to the phase of the current derived from the capacitance, the dC / dV of the sample is obtained. It is preferable to obtain CV characteristics by measurement.

  Here, it is preferable that the signal processing means is a lock-in amplifier, and the sampling signal is a reference signal of the lock-in amplifier. Further, the signal processing means is a lock-in amplifier, the sampling signal is a reference signal of the lock-in amplifier, and the current derived from the stray capacitance is obtained by automatically setting the phase of the reference signal of the lock-in amplifier outside the tunnel region. The phase is locked.

  Alternatively, the signal processing means is preferably a digital signal processor (DSP).

  According to the phase lock-in type high-frequency scanning tunneling microscope of the present invention configured as described above, a sine wave high-frequency bias voltage is applied between the probe and the sample to prevent charging of the sample and to tunnel electrons to the conduction band. And an STM image can be obtained. Here, when a sine wave or a rectangular wave having a high frequency is used as the bias voltage, the tunnel current component has the same phase as the bias voltage, but the current derived from the stray capacitance is a sine wave delayed 90 degrees from the phase of the bias voltage. By locking the phase of the sampling signal to the tunnel current component by the signal processing means having a phase lock-in function, the current derived from the stray capacitance whose phase is shifted by 90 degrees can be canceled. As a result, in the case of a sine wave, the matching circuit is not required, the performance of the current amplifier can be extracted better, and the tracking frequency up to the front of the signal processing means can be improved. Here, when the signal processing means is a lock-in amplifier and the sampling signal is a reference signal of the lock-in amplifier, a phase lock-in type high-frequency scanning tunneling microscope can be easily configured. Further, if the signal processing means is a digital signal processor (DSP), it can be configured at low cost by limiting the functions to the minimum necessary.

  In the conventional feedback control system, several tens of kHz was the limit to obtain the stability of the entire feedback loop, but by using the lock-in amplifier or DSP, the feedback signal is converted to DC by the lock-in amplifier or DSP. Therefore, the stability of the feedback loop is not related to the frequency of the bias voltage, and is stable even at a high frequency. In other words, the stability of the feedback loop greatly affects the output time constant of the lock-in amplifier or DSP, and the phase of the output current of the current amplifier immediately before the lock-in amplifier or DSP is 180 degrees from the phase of the bias voltage frequency. Even if the delay is more than that, the feedback loop is stable because the output time constant of the lock-in amplifier or DSP is stable. Therefore, pulse STM observation can be performed at a higher frequency than in the past.

  Furthermore, since the lock-in amplifier detects only signal components having frequency components near the reference signal, it is extremely resistant to frequency noise away from the reference signal, and measurement is performed with the frequency locked to a high frequency. Thus, the circuit system is extremely resistant to electromagnetic noise such as a commercial frequency of 60 Hz, whereby the S / N ratio can be improved, and feedback control can be performed even with a minute tunnel current to obtain an STM image. Similar effects occur when using a DSP.

  In addition, according to the present invention, an atomic image with a sensitivity equivalent to that of a conventional STM can be obtained even under the condition that the probe and the sample are left open to the atmosphere without using the electromagnetic shield and ultrahigh vacuum required by the conventional STM. Can be observed, and the cost of the apparatus can be greatly reduced. Similarly, the present invention is capable of observing an atomic image on the surface of an insulator or a semiconductor although it is affected by adsorbed molecules even in the atmosphere.

It is a simplified explanatory diagram between the probe and the sample showing the measurement principle according to the present invention. It is a simplified circuit diagram which shows the circuit structure of the high frequency tunnel microscope using a rectangular wave bias voltage. 3 is an oscillograph showing waveforms in various parts of the circuit of FIG. 2 when a rectangular wave bias voltage having a frequency of 4 kHz is applied. FIG. 3 is an STM image of the HOPG surface when a rectangular wave bias voltage having a frequency of 4 kHz is applied with the high-frequency tunneling microscope having the circuit configuration of FIG. 2. 3 is an oscillograph showing waveforms in various parts of the circuit of FIG. 2 when a rectangular wave bias voltage having a frequency of 100 kHz is applied. 3 is an STM image of the surface of a HOPG when a rectangular wave bias voltage having a frequency of 100 kHz is applied with the high-frequency tunneling microscope having the circuit configuration of FIG. It is a simplified circuit diagram which shows the circuit structure of the high frequency tunnel microscope using a sine wave bias voltage. It is a wave form diagram which shows the general operating principle of lock-in amplifier. It is a wave form diagram which shows the measurement principle which extracts only a tunnel current component with a lock-in amplifier from the input current which the tunnel current component (measurement signal) and the capacitive component mixed. FIG. 7 shows an adjustment procedure in the case of actually observing a sample using the high-frequency tunneling microscope of FIG. 7, wherein (a) shows a waveform in a state where the phase of the reference signal of the lock-in amplifier is locked to the phase of the capacitive component outside the tunnel region. (B) shows the waveform when the phase of the reference signal is shifted by −90 ° and the output of the capacitance component is 0, and (c) shows the waveform when only the tunnel current component is detected. FIG. 8 is an STM image of the HOPG surface when a sinusoidal bias voltage having a frequency of 100 kHz is applied with the high-frequency tunneling microscope having the circuit configuration of FIG. 7. FIG. 8 is an STM image of the HOPG surface when a sinusoidal bias voltage having a frequency of 200 kHz is applied with the high-frequency tunneling microscope having the circuit configuration of FIG. 7. FIG. 8 is an STM image obtained by observing the Si (111) surface in the direct current mode in the atmosphere with the circuit configuration shown in FIG. 7. It is the STM image which observed the Si (111) surface in air | atmosphere using the sine wave bias voltage of frequency 100kHz. It is a simplified circuit diagram which shows the circuit structure of the conventional high frequency pulse scanning tunneling microscope.

Explanation of symbols

1 Probe 2 Sample 3 Pulse generator (bias power supply)
4 Impedance matching circuit 5 Current amplifier 6 Lock-in amplifier (signal processing means)
7 Control system 8 AC power supply (bias power supply)
101 Probe 102 Sample 103 Pulse generator (bias power supply)
104 impedance matching circuit 104 matching circuit 105 current amplifier 106 rectifier circuit 107 control system

Next, the present invention will be described in more detail based on the embodiments shown in the accompanying drawings. First, the measurement principle of the high-frequency scanning tunneling microscope, which is the premise of the present invention, will be described with reference to FIG. The object of the present invention is to obtain an atomic image of the surface of an insulator or semiconductor. FIG. 1 schematically shows the exchange of electrons between the probe 1 and the sample 2. A high-frequency bias voltage whose polarity changes between positive and negative of a sine wave or a rectangular wave is applied between the probe 1 and the sample 2. In the present invention, it is particularly preferable to apply a sinusoidal high-frequency bias voltage between the probe 1 and the sample 2. Here, the sample 2 is assumed to be Si (111) having an oxide film (SiO ₂ ) formed on the surface. The frequency of the bias voltage is in the range of 100 kHz to 10 GHz, and the tunnel current used as the feedback signal is in the range of 1 pA to 10 nA.

  FIG. 1A shows a state where electrons are tunneled and flow into the conduction band of the sample 2 when a negative bias voltage is applied to the probe 1. FIG. 1B shows a state in which some of the tunneled electrons remain on the sample surface directly below the probe even when the bias voltage becomes 0, and the sample surface is charged. FIG. 1 (c) shows a state where charged electrons move to the probe 1 when the positive bias voltage is applied to the probe 1 and the charging of the surface of the sample 2 is relaxed. FIG. 1 (d) shows a state in which almost all charged electrons have disappeared and the bias voltage has returned to 0. Next, when a negative bias voltage is applied to the probe 1, the state is restored to a state in which the electron tunnel is not obstructed. It shows how they are doing.

  However, when a high frequency bias voltage is applied to the probe as shown in FIG. 1, electrons are ideally tunneled and current is detected. However, when the sample is an insulator or semiconductor, the tunnel current is very high. Since it is small, it is hidden in the current component and noise due to the stray capacitance, and thus a feedback signal cannot be obtained simply. The phenomenon that a large current flows between the probe and the sample due to the stray capacitance is not a problem in the conventional DC bias STM.

  The high-frequency scanning tunneling microscope using the rectangular wave bias voltage shown in FIG. 2 is a method in which a rectangular wave bias voltage is applied between the probe 1 and the sample 2 from a pulse generator 3 (bias power supply). A current flowing between the probe 1 and the sample 2 is detected by a current amplifier 5 through an impedance matching circuit 4 including a capacitor C and a variable resistor R. Here, since C is 100 pF and R is 0 to 2 MΩ, the time constant (CR) can be adjusted in the range of 0 to 200 μs. The output of the current amplifier 5 is input to the control system 7 that performs feedback control of the probe 1 through the lock-in amplifier 6. In the present invention, since the conventional STM is a DC bias voltage system, the use of the lock-in amplifier 6 that has not been used eliminates noise due to a stray capacitance and greatly improves the S / N. It was planned. In the present embodiment, the case where the lock-in amplifier 6 is used as the signal processing means having the phase lock-in function in the present invention will be described, but the same applies to the case where a digital signal processor (DSP) is used. In other words, the lock-in amplifier is generally configured using a DSP, but in the present invention, the current amplified by the current amplifier 5 is input and the reference signal is used as the frequency of the high-frequency bias voltage. Since the phase lock-in function is used to generate a feedback signal based only on the tunnel current by removing the current derived from the stray capacitance by locking to the phase of the high frequency bias voltage and realizing the same function in the DSP Because it can.

FIG. 3 shows signal waveforms in the apparatus having the circuit configuration shown in FIG. The rectangular wave bias voltage is 4 kHz, ± 200 mV, and its waveform is shown in the upper part of FIG. Since 4 kHz is 250 μs in terms of period, the matching circuit 4 is adjusted so that only components that change faster than this can be removed. The output waveform when the gain of the current amplifier 5 is 10 ⁷ is shown in the middle part of FIG. 3, and the output waveform of the lock-in amplifier 6 with the gain of 10 and the output time constant of 100 μs is shown in the lower part of FIG. . Here, since the lock-in amplifier 6 functions as a band-pass filter, it is sampled and output in synchronization with the frequency of the bias voltage, excluding the influence of stray capacitance appearing behind the rising and falling portions of the bias voltage. . 4A and 4B are ATM images obtained by observing the surface of HOPG as a sample. FIG. 4A is an observation region of 1 nm × 1 nm, FIG. 4B is an observation region of 2 nm × 2 nm, and FIG. X 5 nm observation region.

  FIG. 5 shows a waveform when the rectangular-wave bias voltage is 100 kHz and p-p 200 mV in the apparatus having the circuit configuration shown in FIG. Also in this case, similarly to the above, the upper part of FIG. 5 shows the bias voltage waveform, the middle part shows the output waveform of the current amplifier 5, and the lower part shows the output waveform of the lock-in amplifier 6. 6A is an STM image when the rectangular wave bias voltage is 50 kHz and p-p 200 mV, and FIG. 6B is an STM image when the rectangular wave bias voltage is 100 kHz and p-p 200 mV. Both are observation areas of 5 nm × 5 nm. Compared to FIG. 11A of the above-mentioned Patent Document 1, it can be seen that an extremely clear atomic image can be observed by the present invention, and the effect of using a lock-in amplifier has been clarified.

  Here, referring to FIG. 5, the output waveform of the current amplifier is a waveform close to a sine wave (sine wave) when the frequency becomes as high as 100 kHz in spite of the application of the rectangular wave bias voltage. It is understood that it is possible to observe the atomic image even when the bias voltage is a sine wave. On the other hand, in order to prevent charging of the sample surface, it is desirable to apply a bias voltage having a frequency as high as possible. Moreover, since the problem of spike current is solved by using a sine wave as the bias voltage, the impedance matching circuit 4 in FIG. 2 is not necessary. As a result, a high-frequency scanning tunneling microscope with a sine wave bias voltage having a simple circuit configuration as shown in FIG. 7 has been achieved.

  The phase lock-in type high-frequency scanning tunneling microscope shown in FIG. 7 applies a sinusoidal bias voltage from an AC power supply 8 (bias power supply) between the probe 1 and the sample 2, Is amplified by the current amplifier 5 and the output is input to the lock-in amplifier 6, and the lock-in amplifier 6 generates a feedback signal based only on the tunnel current, which is input to the control system 7 and searched. The needle 1 is feedback controlled. In this case, since the impedance matching circuit 4 is not provided, it is possible to prevent the frequency characteristics from being deteriorated due to the matching circuit.

  Next, based on FIG. 8 and FIG. 9, the measurement principle of using the lock-in amplifier 6 to remove the current component due to the stray capacitance and extracting only a weak tunnel current will be described. First, a general operation principle of the lock-in amplifier will be described with reference to FIG. As shown in FIG. 8A, when a measurement signal composed of a sine wave is input to a lock-in amplifier and the phase difference between the measurement signal and the reference signal of the lock-in amplifier is 0 °, the measurement signal is multiplied by the reference signal. The combined PSD output is a waveform (positive full-wave rectification) in which the half wave of the negative portion of the measurement signal is folded back positively, and the LPF output obtained by smoothing this is a positive direct current. On the other hand, as shown in FIG. 8B, when the phase difference between the measurement signal and the reference signal of the lock-in amplifier is 90 °, the PSD output has a vertically symmetric waveform and the LPF output becomes zero. Further, as shown in FIG. 8C, when the phase difference between the measurement signal and the reference signal of the lock-in amplifier is 180 °, the PSD output has a vertically inverted waveform when the phase difference is 0 °, and the LPF output is Negative direct current. In the lock-in amplifier, the measurement signal can be detected by locking the phase of the reference signal to the phase of the measurement signal. Further, in measurement signal detection, a component having a phase difference of 90 ° with respect to the measurement signal is not output.

  Next, the probe 1 and the sample 2 are replaced with a circuit in which a resistance for flowing a tunnel current and a stray capacitance are connected in parallel in an equivalent circuit. As shown in FIG. 9, the current passing between the probe and the sample flows to the lock-in amplifier while the tunnel current component (measurement signal) and the capacitance component are mixed, but the capacitance component is out of phase with the tunnel current component. It is 90 ° off. Therefore, when the phase of the reference signal is locked to the phase of the tunnel current component by the lock-in amplifier, the capacitance component shifted by 90 ° is not output, and only the tunnel current component can be detected as shown in FIG. By using this as a feedback signal, STM observation can be performed.

  Next, a procedure for actually performing STM observation using the apparatus having the circuit configuration shown in FIG. 7 will be briefly described. When AC is used as the bias voltage, if the probe is in the tunnel region, it can be considered that the resistance and capacitance are parallel between the probe and the sample, but the probe is not in the tunnel region. (See FIG. 10 (a)), the circuit between the probe and the sample is a circuit having only a capacitance. That is, when approaching, only the capacitive component flows between the probe and the sample at first, but when the probe enters the tunnel region, a current in which the capacitive component and the tunnel current component are mixed flows. First, as shown in FIG. 10A, by automatically setting the phase of the reference signal of the lock-in amplifier outside the tunnel region, the phase of the reference signal can be locked to the phase of the capacitive component. Here, the reference signal and the current of the capacitive component have the same phase. From there, the phase of the reference signal is shifted by −90 ° to set the output of the capacitive component to 0 and the approach is started (see FIG. 10B). By doing so, as shown in FIG. 10C, the output appears only when the tunnel current component flows, and only the tunnel current can be detected. In FIG. 10C, the current flowing through the tunnel gap is the sum of the tunnel current component and the capacitance component. However, as the frequency increases, the capacitance component becomes much larger than the signal level of the tunnel current component. The waveform becomes dominant. As described above, by adjusting the output of the capacitive component flowing before the approach to 0 using the lock-in amplifier, only the tunnel current component can be detected.

  11 and 12 show STM images obtained by actually observing the surface of the HOPG with the phase lock-in type high-frequency scanning tunneling microscope having the circuit configuration shown in FIG. FIG. 11 shows the result of observation of the HOPG surface using a sinusoidal bias voltage with a frequency of 100 kHz and p-p 200 mV, with a time constant of 100 μs of the lock-in amplifier, and (a) shows an observation region of 2 nm × 2 nm, (b ) Has an observation area of 5 nm × 5 nm, and an atomic image is obtained. FIG. 12 shows the result of observation of the HOPG surface using a sinusoidal bias voltage with a frequency of 200 kHz and p-p 200 mV and a time constant of 100 μs of the lock-in amplifier, and (a) shows the observation region of 2.5 nm × In 2.5 nm and (b), the observation region is 5 nm × 5 nm, and although it is unclear, an atomic image is obtained.

  In the observation using a sine wave bias voltage with a frequency of 200 kHz, the maximum use frequency of the used lock-in amplifier was 100 kHz, and therefore measurement was performed by inserting a frequency extender in front of the lock-in amplifier. It can be easily predicted that a clearer atomic image can be obtained by using wide-band current amplifiers and lock-in amplifiers.

  Finally, FIGS. 13 and 14 show the results of observation of the step-terrace structure on the Si (111) surface. First, FIG. 13 is an STM image obtained by observing the Si (111) surface in the direct current mode in the atmosphere with the circuit configuration shown in FIG. 7, where (a) is an observation region of 150 nm × 150 nm, and (b) is 500 nm. X Observation region of 500 nm. Even in the atmosphere, an STM image of a step-terrace structure on the Si (111) surface was obtained in the direct current mode.

  FIG. 14 is an STM image obtained by observing the Si (111) surface in the atmosphere using a sinusoidal bias voltage having a frequency of 100 kHz. This is the observation area. Thus, the change of the Si (111) surface was also observed in the observation in the high frequency STM. This change appears to be greater than when the bias voltage in the DC mode is negative and smaller than when it is positive. This is thought to be because the surface changes due to the influence of alternating positive and negative because measurements are performed while the polarity of the bias voltage changes alternately. However, it can be seen that a step-terrace structure is observed in the high-frequency STM image. As a result, it was confirmed that high-frequency STM observation can be performed even on a sample having a band gap.

  In this way, by selecting components with the same phase as the high-frequency bias voltage using a lock-in amplifier, the effect of stray capacitance can be minimized, and the probe-sample distance can be controlled using only the tunnel current component. It is possible to observe an insulator or a semiconductor sample by applying a sinusoidal bias voltage having a higher frequency.

  In addition, the distance between the probe and the sample is kept constant, a sine wave high-frequency bias voltage is applied between the probe and the sample from the bias power source, and the phase of the reference signal of the lock-in amplifier is locked to the phase of the tunnel current By measuring the dI / dV of the sample to obtain the IV characteristic, or by locking the phase of the reference signal of the lock-in amplifier to the phase of the current derived from the capacitance, the dC / dV of the sample is obtained. By measuring and obtaining the CV characteristics, the electrical characteristics in a minute region on the order of nm on the semiconductor surface can be examined. Here, the distance between the probe and the sample is kept constant, a sinusoidal high frequency bias voltage is applied between the probe and the sample from the bias power source, and the phase of the reference signal of the lock-in amplifier is changed to the phase of the current derived from the capacitance. By locking, the current fluctuation due to stray capacitance is eliminated, and therefore only the change in current due to the capacitive component based on the physical properties of the sample can be detected. The frequency at which the dI / dV or dC / dV of the sample is measured may be different from the frequency used for controlling the probe-sample distance.

Claims (9)

  The distance between the tip of the probe and the sample is very close, a tunnel current is generated by the bias voltage applied between the probe and the sample, and a feedback signal is applied to the drive system under the condition that this tunnel current is kept constant. A scanning tunneling microscope that scans relatively along a sample surface and observes an atomic scale image of the sample surface, and a bias power source that applies a high-frequency bias voltage of a rectangular wave or a sine wave between the probe and the sample; A current amplifier that amplifies a detection current in which a tunnel current flowing between the probe and the sample and a current derived from stray capacitance are mixed, and a current amplified by the current amplifier are input, and the sampling signal is synchronized with the frequency of the high-frequency bias voltage, and Locks to the phase of the high-frequency bias voltage to remove the stray capacitance-derived current and generate a feedback signal using only the tunnel current , Phase lock-in high-frequency scanning tunneling microscope, characterized in that a signal processing unit having a phase lock-in feature.   The high-frequency bias voltage is a rectangular wave, and an impedance matching circuit that substantially matches the time constant determined by the tunnel resistance and stray capacitance between the probe and the sample and the time constant determined by the input impedance of the current amplifier is provided on the input side of the current amplifier. The phase lock-in type high-frequency scanning tunneling microscope according to claim 1, wherein the phase-locked-in high-frequency scanning tunneling microscope is provided.   The high-frequency bias voltage is a sine wave, and after the phase of the sampling signal of the signal processing means is locked to the phase of the current derived from the stray capacitance outside the tunnel region where the distance between the probe and the sample is separated, the phase of the sampling signal The phase lock-in type high-frequency scanning according to claim 1, wherein the output of the electric current derived from the stray capacitance is set to zero by shifting and the approach is started by narrowing the distance between the probe and the sample and into the tunnel region where the tunnel current flows. Tunnel microscope.   The output of the current derived from the stray capacitance is made zero by shifting the phase of the sampling signal by -90 ° after the phase of the sampling signal of the signal processing means is locked to the phase of the current derived from the stray capacitance outside the tunnel region. 3. The phase lock-in type high-frequency scanning tunneling microscope according to 3.   5. The phase lock-in type high-frequency scanning tunneling microscope according to claim 1, wherein a frequency of the bias voltage is in a range of 100 kHz to 10 GHz, and a tunnel current used as a feedback signal is in a range of 1 pA to 10 nA.   Maintaining a constant probe-sample distance, applying a sinusoidal high-frequency bias voltage between the probe and the sample from the bias power source, and locking the phase of the sampling signal of the signal processing means to the phase of the tunnel current The dI / dV of the sample is measured to obtain the IV characteristic, or the dC / dV of the sample is measured by locking the phase of the sampling signal of the signal processing means to the phase of the current derived from the capacitance. The phase lock-in type high-frequency scanning tunneling microscope according to claim 1, wherein the CV characteristics are acquired.   The phase lock-in type high-frequency scanning tunneling microscope according to claim 1, wherein the signal processing means is a lock-in amplifier, and the sampling signal is a reference signal of the lock-in amplifier.   The signal processing means is a lock-in amplifier, the sampling signal is a reference signal for the lock-in amplifier, and the phase of the current derived from the stray capacitance is obtained by automatically setting the phase of the reference signal for the lock-in amplifier outside the tunnel region. The phase lock-in type high-frequency scanning tunneling microscope according to claim 3 or 4, wherein the phase-locked-in high-frequency scanning tunneling microscope is locked.   7. The phase lock-in type high-frequency scanning tunneling microscope according to claim 1, wherein the signal processing means is a digital signal processor (DSP).

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