JP4756374B2 - A method for measuring electronic states in semiconductors. - Google Patents

Description

The measurement of the electronic state of impurities and defects in the semiconductor handled in the present invention provides not only evaluation of the semiconductor material itself but also important information for all electronic devices and optical devices using the semiconductor.
Currently, there are various methods for measuring the electronic states of impurities and defects in semiconductors. The measuring method according to the present invention is a simple and non-destructive method for detecting a small amount of impurities and defects that cannot be measured by the prior art with high sensitivity. The measurement method can be realized and expected to be used and developed in new industrial fields.

Impurities and defects in semiconductors, especially those having a level in the band gap, have an important effect on the electrical and optical properties of the semiconductor material, so measurements on the electronic states of impurities and defects in semiconductors Provides critical information for device design. For silicon, gallium arsenide, etc., many measurement technologies related to the electronic state (level) of impurities and defects that have a large impact on applications have been developed. The advancement of device design technology is progressing in parallel, which is linked to the basic technology of today's advanced electronic information society.
Currently, society as a whole is demanding more high-power, high-density information processing technology in a harsh environment for devices. In this case, the semiconductor material having a larger band gap than silicon or gallium arsenide is expected to have high reliability characteristics based on environment resistance, insulation resistance, and high carrier mobility. Silicon carbide, gallium nitride, zirconium oxide, etc. are actively studied as wide band gap semiconductors corresponding to them, but diamonds consisting of single elements with a wide band gap are further studied as the next generation semiconductor materials beyond that. Has been. Further, as a compound material of diamond, aluminum nitride, boron nitride, and the like are similarly studied.
From the viewpoint of materials, many of these next-generation semiconductor materials have a small electron affinity. In some surface states, it has been reported that the electron affinity is negative, and it is particularly promising for application to electron-emitting devices, such as oxides such as zirconium oxide, nitrides such as boron nitride and aluminum nitride, diamond and Searches and developments for carbon-based materials such as diamond-like carbon are underway. However, for these new materials, studies on impurity levels and defect levels in the band gap are still in the middle stage compared to silicon and the like. In particular, diamond composed of a single element has been reported to have a crystal perfection superior to that of compound materials. However, trace amounts of impurities and defects present in high-purity single crystals, especially boron contamination, have been reported. As for the evaluation technique for measuring the current, only the technique that has been developed with conventional silicon or the like is becoming difficult because it is a wide bandgap semiconductor, and a new technique is beginning to be demanded. (Non-patent documents 1 and 2)

Diamond is a semiconductor with a band gap of 5.5 eV, and boron has been reported to form an acceptor level at 0.37 eV in the upper gap from the top of the valence band as an acceptor for supplying holes. There is a need for a simple measurement technique that can detect boron with high sensitivity and nondestructiveness (Non-Patent Documents 3 to 5). On the other hand, hydrogen-terminated diamond has been reported to have a negative electron affinity surface. It has been reported that by using such a surface, unique electron emission different from that of conventional semiconductor materials is observed (see Non-Patent Documents 6 and 7). So far, there have been many reported examples of technologies that understand the characteristics of negative electron affinity and that have been applied to electron-emitting devices (Patent Documents 1 to 5). There was no.
Registration No.2798696 Japanese Patent Application No. 2003-584639 JP 2002-298777 A JP 09-161655 A JP 07-130981 A P. Hartmann et al., International Journal of Refractory Metals & Hard Materials 16 (1998) 223. R. Kravets et al., Diamond and Related Materials 13 (2004) 1785. R. Kalish et al., Applied Physics Letter 76 (2000) 757. H. Kato et al., Physica status solidi (a) 202 (2005) 2122. D. Takeuchi et al., Physica status solidi (a) 186 (2001) 269. FJ Himpsel et al., Physical Review B 20 (1979) 624. JB Cui et al, Physical Review B 60 (1999) 16135.

With conventional methods, measurement sensitivity is insufficient, spatial resolution cannot be obtained in principle, high sensitivity but quantitative evaluation cannot be performed, destructive inspection, insulation (high resistance state) ) Cannot be measured, or pre-treatment such as electrode attachment is required for measurement, and nondestructive measurement of trace amounts (10 ¹⁵ cm ^-3 or less) in high-purity high-resistance wide band gap semiconductors is easy. I could not do it.
The present invention is a method that is completely different from previous knowledge, and actively utilizes negative or very small electron affinity to efficiently detect impurities and defects in high-purity wide band gap semiconductors by photoexcitation. By doing so, non-destructive measurement of a minute amount (10 ¹⁵ cm ⁻³ or less) that could not be detected by the conventional technology can be easily performed without pretreatment such as electrode attachment. Actually, there has been no example of an evaluation technique for the electronic state of impurities and defects by positively applying electron emission from a negative electron affinity surface in this way.

The present inventors diligently investigated these problems and came up with the idea of using the principle that electrons of impurities and defects that had not been noticed by anyone until now are emitted from the negative electron affinity surface. .
Specifically, the photoemission rate measurement using the negative electron affinity state of diamond is measured in the range of 5 to 5.3 eV, which is slightly smaller than the band gap energy of diamond, at a measurement temperature above room temperature, It has been clarified that 10 ¹⁵ cm ^-3 boron atoms in a high-purity, high-resistance diamond film can be easily measured without pretreatment such as electrode attachment.

That is, the present invention provides a diamond, boron nitride, a semiconductor selected from aluminum nitride, advance the electron affinity of the negative electron affinity state allowed the the semiconductor surface, upon to the external light emission by irradiating light, the irradiation light Irradiate in a monochromatic range of energy less than the band gap energy of the semiconductor, count the number of photoelectrons with a photoelectron detector, and simultaneously measure the irradiation light intensity, and use the photoelectron emission rate spectrum for the irradiation light energy as the photoelectron emission rate This is a method for measuring an electronic state in a semiconductor, in which the presence of impurities in the diamond semiconductor is measured using a device capable of measurement.
In the present invention, room temperature or a measurement temperature higher than room temperature can be used.
Furthermore, in the present invention, as the semiconductor, one kind of thin film selected from diamond, boron nitride, and aluminum nitride having a film thickness of 200 nanometers or more can be used.

The method for measuring the electronic state of impurities and defects in a semiconductor of the present invention is a new measurement of a technical problem that has become apparent because it is a high-purity wide bandgap semiconductor only with the technology developed with conventional silicon and the like. It is clarified that it can be solved by using the principle.
In principle, since impurities and defect levels in the target band gap are directly detected, it is simple and can cover a range that cannot be detected by the conventional technique in a non-destructive manner. It is a method that consists of basic elements that can be applied to future wide band gap semiconductor evaluation technology and can be widely deployed.

The present invention is a highly sensitive, nondestructive and simple impurity / defect level measurement method using the principle that electrons occupying impurities and defect levels in a high-purity wide bandgap semiconductor are emitted from a negative electron affinity surface by photoexcitation. It is.
In addition, since the present invention can handle weak excitation light intensity and photoelectron intensity, even an insulator can be measured without any problem. Specifically, the converted current value is a maximum of 0.1 pA.
In the present invention, diamond showing a negative electron affinity can be used as a measurement target.
Furthermore, in this invention, measurement temperature can be changed arbitrarily according to a measuring object.
Furthermore, in the present invention, in order to detect boron in diamond with high sensitivity, measurement can be performed at a measurement temperature of 400 to 600K.
Furthermore, the present invention has a measurement principle that can be measured including compensated boron in diamond and is easy to measure with high sensitivity.
In the present invention, a diamond thin film having a thickness of 1 micron or more can be used.
Furthermore, in the present invention, the diamond film can be a single crystal, an epitaxial film, or a polycrystalline film having a crystal structure of (111), (100), (110) plane.

Furthermore, in the present invention, a part of the surface is diamond having a hydrogen termination structure.
In the present invention, the accuracy of quantitative measurement can be readily ensured by comparing with the measurement result of the standard sample.
Further, the present invention removes water (electrolyte) / hydrocarbon adsorbent on the diamond surface by heat treatment of diamond film in a vacuum of 10 ⁻⁸ Torr or less at a temperature of 850 to 1100 K, more preferably 900 to 1050 K. Then, a diamond surface after treatment for obtaining a clean hydrogen-terminated surface can be used.
In the present invention, either the electron counting method using an electron multiplier (channeltron or multichannel plate) or the method of directly or amplifying the current flowing through the sample stage can be used for photoelectron intensity measurement. .
In the present invention, any of a photomultiplier tube, a pyro detector, a photodiode, and a CCD can be used for light intensity measurement. Furthermore, in the present invention, by fixing the measurement optical system, even when the influence of the radiation light at the time of sample temperature control occurs, the photoelectron emission rate or the photoelectron emission can be easily obtained by referring to the light spectrum of the sample temperature control stop character. A rate spectrum can be obtained.

In the present invention, it is desirable that the sample surface has a negative electron affinity state and does not have surface contamination or surface damage that inhibits photoelectron emission. It is desirable that the excitation light can scan an energy range smaller than the band gap of the target sample. The spectroscope is preferably two or more stages in order to reduce stray light as much as possible. The slit width should be selected according to the resolution, but it is desirable to have a resolution of 50 meV or more in the ultraviolet light to be used in the wide band gap semiconductor. If the temperature is finite, the electronic states of impurities and defects have an occupation probability according to Fermi statistics, but it is desirable that the temperature can be set high in order to increase the occupation ratio and increase the detection efficiency. Alternatively, it is desirable that the change in the occupation ratio can be confirmed by changing the temperature. Photoelectron counting is preferably performed in an ultrahigh vacuum due to sensitivity issues.
The diamond used in the present invention is synthesized by a high-temperature and high-pressure method and a CVD method. In either case, the diamond surface is hydrogen-terminated by a microwave plasma CVD apparatus, and the ultrahigh height of 10 ⁻⁹ Torr or less used in the present invention is used. By removing the surface adsorbate without breaking the hydrogen termination at 1000K in a vacuum apparatus, a clean hydrogen termination surface, that is, a clean negative electron affinity surface can be formed. A xenon lamp is used as the excitation light source, infrared light is removed by a water filter, spectroscopy is performed by a double monochromator, a photomultiplier tube for measuring light intensity is measured by a magnesium fluoride beam splitter, and an ultra-high vacuum. The sample surface in the apparatus is focused and irradiated. All optical systems are applicable up to vacuum ultraviolet. The photoelectrons emitted from the sample are extracted from the signal from the electron multiplier for measurement through a capacitor, amplified by a high-speed preamplifier having a DC to 300 MHz bandwidth, introduced into a counter, and counted. The photomultiplier tube and the signal of the electron multiplier tube are finally taken into a personal computer, the photoelectron intensity for each wavelength is normalized by the light intensity by a program, and is automatically measured and processed.

  In the photoelectron emission rate measurement using the negative electron affinity used in the present invention, it is an important process for determining the sensitivity that electrons are transported to the surface through the bottom of the conduction band by photoexcitation. Therefore, when detecting the electronic state of a minute amount of impurities or defects with high sensitivity, it is desirable that the number of electron scattering mechanisms that determine the electron diffusion length is small. However, if the concentration of impurities or defects to be measured is extremely high, the electron diffusion length is shortened, while the concentration to be measured is increased. is there.

  Even if the electron affinity is not negative, if the electron affinity is small due to the law of conservation of momentum, electrons can be emitted from the inside deeper than the surface. In contrast, the method of the present invention can be applied.

As a sample, a high-temperature high-pressure synthetic diamond (001) single crystal was used. The diamond film was hydrogen-terminated by a microwave plasma CVD device and introduced into an ultra-high vacuum device. After treatment with an internal heater at 700 ° C for 1 hour, the adsorbate on the hydrogen-terminated surface and the hydrogen-terminated surface conductive layer were removed to prepare a clean hydrogen-terminated negative electron affinity surface.
This sample was transported to the photoelectron emission rate measurement position in the same ultrahigh vacuum apparatus used in the present invention. An outline of the apparatus is shown in FIG. Moreover, the schematic of the energy band structure which shows the measurement principle of this invention is shown in FIG. By utilizing the photoexcitation process I in FIG. 2, it is possible to directly measure the occupied level of impurities or defects in the band gap through the electron emission process from the negative electron affinity (NEA) surface.
FIG. 3 shows the measurement results. The excitation light energy is 5 to 5.6 eV. Each photoemission rate spectrum is shown when the measurement temperature is measured from 302K to 550K in steps of about 50K. In the figure, Eg is the light energy position corresponding to the band gap of diamond, Egx is the light energy position corresponding to the ground state of free excitons derived from the band gap structure in the diamond, and the low energy side by the lateral optical phonon from there. The dotted line indicated by a in Fig. 2 shows the main fundamental absorption edge on the low energy side of diamond at 302K, and it often appears that it moves to the low energy side to a 'at a measurement temperature of 550K. ing. This change from a to a ′ corresponds to the temperature dependence of the band gap of diamond. Then, it can be seen that the photoelectron emission rate spectrum from ionized boron in diamond can be detected in this measurement from the energy position b lower than a and a ′. It is determined to be boron from the temperature dependence of the intensity of this spectrum and the photoexcitation energy threshold value.
Although this sample was called type IIa classified as a high purity intrinsic diamond semiconductor, boron was detected by the method of the present invention. An attempt was made to detect boron contamination in this sample using the cathodoluminescence method, which is actually the prior art. The result is shown in FIG. Emission from bound excitons (BE _TO ) indicating the presence of boron in diamond was observed at a low temperature of 25K. From the emission intensity ratio with emission from free excitons (FE _TO ), the estimated amount of boron is on the order of 1016 cm −3, indicating that such a small amount of boron could be detected by the present invention. By comparing with a standard sample, it is possible to finally ensure the accuracy of quantitativeness. On the other hand, the conventional cathodoluminescence method used for confirmation has problems such as charge-up and deposition of carbon-based adsorbate during measurement by electron beam irradiation, and therefore cannot be quantitatively evaluated.

A high purity undoped CVD homoepitaxial diamond thin film (001) was used as a sample.
The precise cleaning of Example 1 was not performed, but the sample was transported to the position of the photoelectron emission rate measurement system and the measurement method of the present invention was used. Specifically, measurement was performed at an excitation light energy of 5 to 5.3 eV at a measurement temperature of 573K. FIG. 5 shows the result (white circle) together with the result of Example 1 (black square). Obviously, the electron emission rate spectrum derived from boron was confirmed. On the other hand, the strength was estimated to be about an order of magnitude lower than that in Example 1. For this sample, confirmation measurement by the cathodoluminescence method was performed in the same manner as in Example 1. However, when measured at a low temperature as in Example 1, no luminescence from bound excitons corresponding to boron was observed, and free excitation was observed. Only light from the child was emitted.
From the above, according to the present invention, it is possible to detect 10 ¹⁵ cm ^-3 units exceeding the 10 ¹⁶ cm ^-3 unit, which is considered to be the measurement limit of boron in the cathodoluminescence method, which has been considered to be the most sensitive in the past. It proved successful.

For these examples, as shown in the following comparative examples, remarkably high sensitivity, quantitativeness, and simplicity were realized compared to the conventional boron detection technology in diamond.
Comparative Example 1:
In the conventional technique “SIMS”, it is possible to quantitatively indicate the depth distribution of impurities with high sensitivity. However, since the sample is sputtered in principle, it is not a nondestructive inspection. In addition, since it is detected regardless of the electronic state, it is not known alone whether it is activated as a donor / acceptor. Since ions are used for sputtering, it is basically not applicable to insulators. Strictly speaking, it may be possible, but the sample requires various processing for antistatic purposes. Generally, the sample is not used for a device after measurement.

Comparative Example 2:
The conventional technique “Hall effect” can quantitatively measure the donor / acceptor concentration with high sensitivity. Further, a compensation ratio / compensation impurity or defect concentration can also be obtained. However, it is impossible in principle to obtain a spatial resolution, and since it is an electrical measurement, an insulator cannot be measured. Strictly speaking, there are AC type and optical Hall effect measurement that can be measured even with high resistance, but as a wide band gap semiconductor, further innovation in electrode formation technology is required. The object to be measured is free carriers, which handle comprehensive information on donors, acceptors, other impurities and defects, and assumptions are necessary to derive individual levels.

Comparative Example 3:
Even in the conventional technology “FTPS (Fourier Transform Photocurrent Spectroscopy)”, there is a document that the detection is equivalent, but there is no data corresponding to the example only in the literature by predicting the detection limit theoretically (non- Patent Document 2). In addition, it is necessary to prepare various conditions for analysis, and this method is superior in that boron in diamond as an object can be measured directly and routinely.
Table 1 summarizes the characteristics of the method for measuring an electronic state in a semiconductor containing impurities or defects according to the present invention and the conventional method.

FTPSはフーリエ変換光電流分光法、SIMSは二次イオン質量分析法、CLはカソードルミネッセンス法である。SIMSはイオンで試料をスパッタしながら測定するため破壊測定に分類される。CLは電子ビームを照射して発光を観測する非破壊測定であるが、試料を含む発光観測系の複雑さから、不純物や欠陥濃度と対応する発光強度の定量的取り扱いは原理的に不可能である。SIMSもCLも荷電ビームを入力源とするため、絶縁物に対しては原理的に取り扱い不可能である。極めて薄い金属膜や炭素膜被覆等で帯電防止を施す場合があるが、この場合定量的扱いの阻害要因を増やすのみで、定性的扱いを可能とするのみである。

FTPS is Fourier transform photocurrent spectroscopy, SIMS is secondary ion mass spectrometry, and CL is cathodoluminescence. SIMS is classified as destructive measurement because it is measured while sputtering a sample with ions. CL is a nondestructive measurement that observes luminescence by irradiating an electron beam, but due to the complexity of the luminescence observation system including the sample, quantitative handling of the luminescence intensity corresponding to the impurity and defect concentrations is impossible in principle. is there. Since SIMS and CL use a charged beam as an input source, they cannot be handled in principle for insulators. There are cases where antistatic treatment is performed with a very thin metal film or carbon film coating, but in this case only qualitative treatment is possible only by increasing the obstruction factor of quantitative treatment.

The method for measuring the electronic state of impurities and defects in a semiconductor of the present invention is a new measurement of a technical problem that has become apparent because it is a high-purity wide bandgap semiconductor only with the technology developed with conventional silicon and the like. It is clarified that it can be solved by using the principle.
In principle, since impurities and defect levels in the target band gap are directly detected, it is simple and can cover a range that cannot be detected by the conventional technique in a non-destructive manner. It is a method that consists of basic elements that can be applied to future wide band gap semiconductor evaluation technology and can be widely deployed.

1 is an example of a device configuration for realizing a method of the present invention. Feature conceptual diagram of the present invention Characteristics of the present invention Characteristics confirmation diagram using conventional example Comparison characteristics with conventional examples

Claims (3)

When a semiconductor selected from diamond, boron nitride, and aluminum nitride is used and external photoelectrons are emitted from the surface of the semiconductor whose electron affinity has previously been set to a negative electron affinity state by irradiation light, the irradiation light is irradiated with the band gap energy of the semiconductor. Using a device that can irradiate in a monochromatic range with a smaller energy range, count the number of photoelectrons with a photoelectron detector, simultaneously measure the intensity of the irradiated light, and measure the photoelectron emission rate spectrum for the irradiated light energy as the photoelectron emission rate A method for measuring an electronic state in a semiconductor, wherein the presence of impurities in the semiconductor is measured. The method for measuring an electronic state in a semiconductor according to claim 1, wherein a room temperature or a measurement temperature higher than room temperature is used. The method for measuring an electronic state in a semiconductor according to claim 1 or 2 , wherein one kind of thin film selected from diamond, boron nitride, and aluminum nitride having a film thickness of 200 nanometers or more is used as the semiconductor.

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