JP5125252B2 - Nitride semiconductor evaluation method and evaluation apparatus - Google Patents

Description

  The present invention relates to a nitride semiconductor evaluation method and evaluation apparatus using modulation spectroscopy, and more particularly to an evaluation method and an evaluation apparatus that can determine the internal electric field of a nitride semiconductor in a non-destructive and non-contact manner.

Nitride semiconductor, which is a generic name for gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), and mixed crystals composed of these, is physically and chemically stable and has high thermal conductivity. It is known that it has excellent heat dissipation. Therefore, a device using a multilayer film or a single layer nitride semiconductor, such as an Al _x Ga _1-x N / GaN high electron mobility transistor (HEMT) or In _x Ga _1-x N / GaN. Laser diodes (LDs) are promising as high-power elements. Many nitride semiconductor devices have been reported so far, and some have been put into practical use (for example, see Non-Patent Documents 1 and 2).

  The characteristics of semiconductor elements are strongly controlled by the structural design at the material stage. In particular, optimization of the potential structure that determines the behavior of electrons and holes that lead to characteristics is the most fundamental in structural design. In particular, since the crystal structure of a nitride semiconductor is a wurtzite type, spontaneous polarization along the c-axis direction is allowed according to the group theory. In addition, in the nitride semiconductor multilayer film, lattice mismatch distortion occurs, so that piezoelectric polarization also occurs (see, for example, Non-Patent Documents 1 and 2).

  Since these two types of electric polarization induce an electric field, the potential structure is determined. Moreover, since these polarizations are very strong in nitride semiconductors, they also dominate the electron / hole concentration. Therefore, in the structural design of a nitride semiconductor multilayer film, it is an essential task to control these electrical polarizations.

  In the development stage of semiconductor devices, structural design and creation and evaluation of devices based on it are generally performed. However, since a large number of processes are required to create a semiconductor element, it is necessary to perform evaluation in the middle of the process and feed back the result. Particularly important is material evaluation after the growth of a semiconductor multilayer film. In the nitride semiconductor multilayer film, it is necessary to measure the electric field strength or potential induced by the above-described spontaneous polarization and piezoelectric polarization.

  There are two methods for evaluating potential or electric field strength in a nitride semiconductor multilayer film. The first evaluation method is a photocurrent measurement method proposed by Yu et al. (For example, see Non-Patent Document 3). This is a method using the internal photoelectric effect in a semiconductor multilayer film. Specifically, the spectral response characteristic of the current with respect to the light irradiated from the back surface of the substrate is measured, and the potential is estimated. This method has an advantage that the height of the barrier potential with respect to the Fermi level can be directly estimated, but has a disadvantage that a destructive and erosive process of deposition of a metal electrode necessary for current measurement is accompanied. Furthermore, since the actually detected data is the barrier potential against the Fermi level in the composite system of metal / nitride semiconductor multilayer film, it may be different from the barrier potential of the nitride semiconductor multilayer film itself.

  The second evaluation method is a method using the modulation spectral characteristics of a semiconductor under an electric field (for example, see Non-Patent Documents 4-6). Modulation spectroscopy is spectroscopic measurement in which a modulation component of a dielectric function generated by applying a periodic perturbation in a sample is detected using probe light. Modulation spectroscopy is classified into various types according to the form of periodic perturbation. Among them, the light modulation reflection spectroscopy is a modulation spectroscopy used relatively often in semiconductors. Light modulation reflection spectroscopy is spectroscopic measurement in which an electric field modulation component of a dielectric function induced by excitation light irradiation with periodic intensity change is detected as a modulation reflectance using probe light. In addition, the electric field modulation by excitation light irradiation is caused by photogenerated electrons and holes.

  It is known that a light modulation reflection spectrum obtained from a sample having a certain electric field (usually 10 kV / cm or more) shows a vibration shape in the vicinity of the basic optical transition energy of the sample. This vibration shape is called Franz-Keldysh (FK) vibration. The period of the FK oscillation is determined by the electric field strength of the sample. Therefore, the internal electric field of the sample can be obtained by analyzing the period of the FK vibration obtained by the measurement. The internal electric field evaluation method using light modulation reflection spectroscopy uses light as a probe for the potential structure, and thus has a feature of non-destructive and non-contact. Therefore, processing and processing for the sample are not required, and the convenience is excellent. Taking advantage of these advantages, there have been reports of estimating the electric field strength of nitride semiconductors from FK vibrations.

  However, the internal electric field evaluation method using light modulation reflection spectroscopy has the following problems. The first problem is that light and scattered light are inevitably generated from the sample by laser light irradiation. Such light emission components and scattered light components act as stray light components for the detector for receiving the modulated reflection component of the probe light. Since the light modulation reflection signal is generally a signal smaller than the light emission component and the scattered light component, the stray light component disturbs the light modulation reflection signal.

  The second problem is related to data analysis. In the light modulation reflection measurement, since the sample is irradiated with excitation light and probe light, the electric field strength obtained from the FK vibration analysis is strictly the electric field strength under light irradiation. Under light irradiation, the electric field shielding effect accompanying the generation of carriers works, so that the electric field strength is predicted to be lower than the dark electric field strength. At the time of structural design, since light irradiation is not normally assumed, the internal electric field strength at the time of design may be different from the electric field strength obtained from the FK vibration analysis. In fact, Takeuchi et al. Have reported that in an AlGaN / GaN heterostructure, the electric field strength obtained from the FK vibration analysis is significantly different from the internal electric field strength at the time of design (see, for example, Non-Patent Document 7). For example, in the case of a GaAs system, such a difference in electric field strength can be eliminated because the electric field strength corresponding to the FK vibration analysis result can be reproduced if light irradiation is virtually performed by numerical simulation (for example, non-intensity). (See Patent Document 8, Patent Documents 1 and 2).

For a review, “Group III-Nitride Semiconductor Compounds” ed. B. Gil (Clarendon Press, Oxford, 1998) For a review, “Wide Energy Bandgap Electronic Devices” eds. F. Ren and J. C. Zolper (World Scientific Publishing, Singapore, 2003) L. S. Yu, Q. J. Xing, D. Qiao, S. S. Lau, K. S. Boutros, and J. M. Redwing, Appl. Phys. Lett. 73, 3917 (1998) For a review, M. Cardona, “Modulation Spectroscopy” (Academic Press, New York, 1969) For a review, F. H. Pollak and H. Shen, Mater. Sci. Eng. R 10, 275 (1993) For a review, H. Shen and M. Dutta, J. Appl. Phys. 78, 2151 (1995) H. Takeuchi, Y. Yamamoto, Y. Kamo, T. Oku, and M. Nakayama, Eur. Phys. J. B 52, 311 (2006) H. Takeuchi, Y. Kamo, Y. Yamamoto, T. Oku, M. Totsuka, and M. Nakayama, J. Appl. Phys. 97, 063708 (2005) “Filter circuit design for measurement”, Toshiaki Tosaka (CQ Publishing, 1998) H. Takeuchi, Y. Yamamoto, R. Hattori, T. Ishikawa, and M. Nakayama, IEICE Trans. Electron. E86-C, 2015 (2003) H. Takeuchi, Y. Yamamoto, R. Hattori, T. Ishikawa, and M. Nakayama, Jpn. J. Appl. Phys. 42, 6772 (2003) H. Takeuchi, Y. Yamamoto, R. Hattori, T. Ishikawa, and M. Nakayama, Physica E 21, 693 (2004) H. Takeuchi, Y. Yamamoto, and M. Nakayama, J. Appl. Phys. 96, 1967 (2004) Patent application 2004-316818 U.S. Patent Application 20060094133 U.S. Patent 7,038,768 Patent Application 2003-9515

  However, in the case of advanced materials typified by nitride semiconductors, there are cases where parameters used for simulation do not exist or the reported values of parameters lack reliability. Therefore, in the case of a nitride semiconductor, it is very difficult to reproduce the electric field intensity corresponding to the FK vibration analysis result using a simulator.

  This problem is solved by establishing an FK vibration measurement method using non-destructive and non-contact modulation spectroscopy other than light modulation reflection. In order to measure FK vibration, an electrical perturbation needs to be applied to the sample. However, in order to realize non-destructive / non-contact, it is not possible to directly apply voltage by providing electrodes. Therefore, it is necessary to use a non-contact-applicable perturbation that is converted into an electrical perturbation inside the sample, as in light modulation reflection.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to obtain an evaluation method and an evaluation apparatus that can determine the internal electric field of a nitride semiconductor in a non-destructive and non-contact manner. .

The nitride semiconductor evaluation method according to the present invention includes a step of irradiating a nitride semiconductor with sound waves to generate piezoelectric polarization, a step of irradiating the nitride semiconductor with probe light, and a probe reflected or transmitted by the nitride semiconductor. A step of measuring a modulation spectrum of light, a step of obtaining an internal electric field of the nitride semiconductor based on a period of the Franz-Keldish oscillation appearing in the modulation spectrum, and a positioning light beam that is collinear with respect to the propagation direction of the sound wave. A step of irradiating the nitride semiconductor; and a step of adjusting the probe light and the sound wave so as to intersect on the surface of the nitride semiconductor based on the irradiation position of the positioning light beam on the surface of the nitride semiconductor. .

A nitride semiconductor evaluation apparatus according to the present invention includes a sound wave generator for generating piezoelectric polarization by irradiating a nitride semiconductor with sound waves, a probe light source and an optical system for irradiating the nitride semiconductor with probe light, and nitriding A modulation spectrum measurement unit that measures the modulation spectrum of probe light reflected or transmitted by a semiconductor, a calculator that determines the internal electric field of the nitride semiconductor based on the period of the Franz-Keldish oscillation appearing in the modulation spectrum, and the propagation direction of the sound wave A positioning light source for irradiating a nitride semiconductor with a positioning light beam that is collinear, and an optical system . Other features of the present invention will become apparent below.

  According to the present invention, the internal electric field of a nitride semiconductor can be determined non-destructively and non-contact.

  FIG. 1 is a block diagram showing a nitride semiconductor evaluation apparatus according to an embodiment of the present invention. This nitride semiconductor evaluation apparatus employs a reflection type as a probe light detection method.

  The white light source 11 is a light source for probe light, such as a halogen lamp and a xenon lamp. The white light condenser lens 12 guides light from the white light source 11 to the spectroscope 13. The spectroscope 13 monochromaticizes the incident white light and generates probe light. The probe light condensing lens 14 condenses the probe light monochromatized by the spectroscope 13 on the surface of the sample 15 placed on the sample stage.

  The reflected light condensing lens 16 condenses the probe light reflected from the sample 15 on the photodetector 17. The photodetector 17 is a photodiode, for example, and receives the probe light reflected or transmitted by the nitride semiconductor sample 15 and converts it into an electrical signal. The sound wave generator 18 irradiates the sample 15 with a sound wave synchronized with the reference signal to generate piezoelectric polarization. The control unit 19 inputs an electrical signal from the photodetector 17 and controls the spectrometer 13 and the sound wave generator 18.

  FIG. 2 is a block diagram showing a modification of the nitride semiconductor evaluation apparatus according to Embodiment 1 of the present invention. As described above, a transmission type may be employed as a probe light detection method. The transmitted light condensing lens 20 condenses the probe light transmitted from the sample 15 on the photodetector 17. Other configurations are the same as those in FIG. This produces the same effect.

  FIG. 3 is a block diagram showing a control unit according to the embodiment of the present invention. The waveform generator 21 generates a sound wave to be applied to the sample 15 and a reference signal for lock-in detection. As the waveform, a sine wave and a square wave that are optimal for lock-in detection are used. The current-voltage converter 22 converts the photocurrent signal output from the photodetector 17 into a voltage signal. The band pass filter 23 separates the voltage signal into an alternating current component corresponding to the modulation reflectance ΔR and a direct current component corresponding to the reflectance R.

  The DC voltmeter 24 measures a DC component corresponding to the reflectance R. The lock-in amplifier 25 locks in an AC component corresponding to the modulation reflectance ΔR in synchronization with the reference signal. The modulation spectrum of the probe light can be obtained by dividing the detected AC component by the DC component. These photodetector 17, current-voltage converter 22, band pass filter 23, lock-in amplifier 25, DC voltmeter 24, and the like are modulation spectrum measuring units that measure the modulation spectrum of the probe light reflected or transmitted by the sample 15. .

  The computer 26 (computer) records the measured modulation spectrum and controls each device. Further, the computer 26 obtains the internal electric field of the nitride semiconductor based on the period of the Franz-Keldish oscillation appearing in the modulation spectrum.

FIG. 4 is a cross-sectional view showing a sample. An i-GaN buffer layer 32 and an i-Al _x Ga _1-x N layer 33 are sequentially stacked on the substrate 31. That is, a nitride semiconductor multilayer film having an Al _x Ga _1-x N / GaN heterostructure whose outermost layer is i-Al _x Ga _1-x N is formed on the substrate 31.

  Hereinafter, a sound wave modulation spectral characteristic measurement method according to an embodiment of the present invention will be described with reference to the flowchart of FIG.

  First, the sample 15 is irradiated with probe light. The white light source 11, the white light condensing lens 12, the spectroscope 13, the probe light condensing lens 14, the reflected light condensing lens 16, and the light detection so that the reflection spectrum from the sample 15 can be measured by the light detector 17. The optical system comprising the device 17 is adjusted. After adjusting the optical system, the reference signal from the waveform generator 21 is input to the sound wave generator 18 and the sound wave generator 18 is driven to irradiate the sample 15 with sound waves. At this time, the sound wave generator 18 is adjusted so that the probe light and the sound wave intersect on the surface of the sample 15 (step S1).

  Next, a reference signal is sent to the sound wave generator 18 and the lock-in amplifier 25 using the waveform generator 21. The sound wave generator 18 generates a sound wave synchronized with the reference signal, and irradiates the sample 15 to generate piezo polarization (step S2). Here, a periodic micro-stress is applied by irradiating a nitride semiconductor having a wurtzite type crystal structure with a sound wave, and stress in the c-axis direction works to generate piezo polarization. The piezoelectric polarization-induced electric field modulates the dielectric function.

  Next, the spectroscope 13 is controlled to set the wavelength of the probe light to a certain value, and the sample 15 is irradiated with the probe light. The probe light reflected or transmitted by the sample 15 is received by the photodetector 17 and converted into a photocurrent signal (electric signal) (step S3).

  Next, the photocurrent signal from the photodetector 17 is converted into a voltage signal by the current-voltage converter 22. This voltage signal includes two components, an alternating current component corresponding to the modulation reflectance ΔR and a direct current component corresponding to the reflectance R. Therefore, the band-pass filter 23 separates the voltage signal into a direct current component and an alternating current component (step S4).

  Then, the DC component is measured by the DC voltmeter 24. Further, the AC component is detected by the lock-in amplifier 25 synchronized with the reference signal. However, before the lock-in detection, it is necessary to adjust the phase of the sound wave and the modulated reflected signal in advance. Then, the modulation spectrum ΔR / R of the probe light is obtained by dividing the AC component by the DC component (step S5). In this way, the modulation spectrum of the probe light reflected or transmitted by the sample 15 is measured.

  Next, the modulation spectrum ΔR / R is recorded in the computer 26 as a function of the wavelength of the probe light or the photon energy. Then, the internal electric field of the sample 15 is obtained by the computer 26 based on the period of the Franz-Keldish vibration (FK vibration) appearing in the modulation spectrum (step S6). The FK vibration analysis method used here is the same as the FK vibration analysis method obtained by using the light modulation reflection spectroscopy (for example, Non-Patent Documents 4-6, 8, 10-13, and Patent Document 1). 4).

  As described above, by performing modulated reflection spectroscopy using sound wave irradiation, the internal electric field of the nitride semiconductor can be obtained non-destructively and non-contact.

  Here, in the measurement of the light modulation reflection signal, adjustment of the crossing of the sound wave on the sample and the probe light and the phase adjustment of the sound wave and the modulation reflection signal for lock-in detection are important. A sound wave generator suitable for these adjustments is shown in FIG.

  The sound wave generating element 41 generates a sound wave that irradiates the sample 15. The positioning light source 42 is a directional light source, for example, a laser light source, and emits a sound wave positioning beam. The optical mirrors 43 and 44 (optical system) are adjusted so that the positioning light beam is collinear with respect to the sound wave propagation direction. Thereby, the position where the sound wave is irradiated on the surface of the sample 15 can be visualized as the position of the positioning light beam.

  In addition, a drive circuit 45 that drives the sound wave generating element 41 and the positioning light source 42 in the same phase is provided. Therefore, the phase of the lock-in amplifier 25 is adjusted by using a signal obtained by receiving the positioning light beam scattered on the surface of the sample 15 by the photodetector 17 and the reference signal from the waveform generator 21. be able to.

  A phase adjustment method for performing lock-in detection will be described with reference to the flowchart of FIG.

  First, the waveform generator 21 is driven and a reference signal is input to the sound wave generator 18. Then, the sound wave generator 18 is driven to irradiate the sample 15 with a positioning light beam that is collinear with respect to the propagation direction of the sound wave (step S11). At this time, the positioning light beam and the sound wave are synchronized by the drive circuit 45.

  Next, based on the irradiation position on the surface of the sample 15 of the positioning light beam, the position of the sound wave generator 18 is adjusted so that the probe light and the sound wave intersect on the surface of the sample 15 (step S12).

  Next, the positioning light beam scattered by the sample 15 is received by the photodetector 17 and converted into an electrical signal (step S13).

  Next, the detected light beam signal for positioning is processed by the current-voltage converter 22 and the band pass filter 23 and then input to the lock-in amplifier 25 together with the reference signal from the waveform generator 21. Then, the phase of the lock-in amplifier 25 is adjusted so that the output of the lock-in amplifier 25 to which the AC component of the electrical signal of the positioning light beam is input is maximized (step S14).

  Since the modulated reflected component of the probe light by the sound wave is synchronized with the sound wave, it is in phase with the positioning light beam. Therefore, if the phase of the lock-in amplifier 25 is adjusted according to the above procedure, the modulated reflection component of the probe light can be detected in lock-in. The operation of the lock-in amplifier 25 is described in detail in Non-Patent Document 9 and the like. After the phase adjustment, the positioning light source 42 is blocked to prevent unnecessary stray light.

  Since the phase adjustment method uses signals that have passed through the photodetector 17, the current-voltage converter 22, the band-pass filter 23, and the lock-in amplifier 25, the phase shift of the signals due to the device functions of these devices is automatically performed. Can be eliminated.

It is a block diagram which shows the evaluation apparatus of the nitride semiconductor which concerns on embodiment of this invention. It is a block diagram which shows the modification of the evaluation apparatus of the nitride semiconductor which concerns on embodiment of this invention. It is a block diagram which shows the control part which concerns on embodiment of this invention. It is sectional drawing which shows a sample. It is a flowchart which shows the sound wave modulation | alteration spectral characteristic measuring method which concerns on embodiment of this invention. It is a block diagram which shows the sound wave generator which concerns on embodiment of this invention. It is a flowchart which shows the phase adjustment method for performing the lock-in detection which concerns on embodiment of this invention.

Explanation of symbols

11 White light source (light source for probe light)
12 White light condensing lens (optical system for probe light)
13 Spectrometer (Optical system for probe light)
14 Probe light condensing lens (Optical system for probe light)
15 Sample (Nitride semiconductor)
17 Photodetector (modulation spectrum measurement unit)
18 Sound wave generator 21 Waveform generator 23 Band pass filter (modulation spectrum measurement unit)
24 DC voltmeter (modulation spectrum measurement unit)
25 Lock-in amplifier (modulation spectrum measurement unit)
26 Computer (computer)
41 Sound wave generating element 42 Positioning light source 43, 44 Optical mirror (positioning optical system)
45 Drive circuit

Claims (6)

Irradiating a nitride semiconductor with sound waves to generate piezoelectric polarization;
Irradiating the nitride semiconductor with probe light; and
Measuring a modulation spectrum of the probe light reflected or transmitted by the nitride semiconductor;
Obtaining an internal electric field of the nitride semiconductor based on a period of the Franz Keldish oscillation appearing in the modulation spectrum ;
Irradiating the nitride semiconductor with a positioning light beam that is collinear with respect to the propagation direction of the acoustic wave;
Adjusting the probe light and the sound wave so as to intersect on the surface of the nitride semiconductor based on the irradiation position of the positioning light beam on the surface of the nitride semiconductor. Method for evaluating nitride semiconductor. Synchronizing the sound wave to a reference signal;
The probe light reflected or transmitted by the nitride semiconductor is received and converted into an electrical signal,
Separating the electrical signal of the probe light into a DC component and an AC component;
The AC component is detected by lock-in by a lock-in amplifier synchronized with the reference signal,
Measure the DC component with a DC voltmeter,
The nitride semiconductor evaluation method according to claim 1, wherein the modulation spectrum of the probe light is obtained by dividing the AC component by the DC component. Synchronizing the positioning light beam and the sound wave;
Receiving the positioning light beam scattered by the nitride semiconductor and converting it into an electrical signal;
3. The method according to claim 1 , further comprising a step of adjusting a phase of the lock-in amplifier so that an output of the lock-in amplifier to which an AC component of an electrical signal of the positioning light beam is input is maximized. The evaluation method of the nitride semiconductor as described. A sound wave generator for generating piezoelectric polarization by irradiating a nitride semiconductor with sound waves;
A probe light source and an optical system for irradiating the nitride semiconductor with probe light; and
A modulation spectrum measuring unit that measures a modulation spectrum of the probe light reflected or transmitted by the nitride semiconductor;
A calculator for determining an internal electric field of the nitride semiconductor based on a period of Franz-Keldish oscillation appearing in the modulation spectrum ;
An evaluation apparatus for a nitride semiconductor, comprising: a positioning light source that irradiates the nitride semiconductor with a positioning light beam that is collinear with respect to a propagation direction of the sound wave; and an optical system . A waveform generator for generating a reference signal;
The sound wave generator synchronizes the sound wave with the reference signal;
The modulation spectrum measurement unit
A photodetector that receives and converts the probe light reflected or transmitted by the nitride semiconductor into an electrical signal;
A bandpass filter that separates the electrical signal of the probe light into a DC component and an AC component;
A lock-in amplifier that locks in the AC component in synchronization with the reference signal; and
A DC voltmeter for measuring the DC component;
The nitride semiconductor evaluation apparatus according to claim 4 , wherein the modulation spectrum of the probe light is obtained by dividing the AC component by the DC component. The nitride semiconductor evaluation apparatus according to claim 4 , further comprising a drive circuit that drives the sound wave generation element of the sound wave generator and the positioning light source in the same phase.

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