Diamond and semiconductor devices

Abstract

To improve the performance of a semiconductor device including a FET using diamond.SOLUTION: The present disclosure allows a diamond substrate 201 to function as a nitrogen-containing and hydrogen-terminated diamond layer without providing a non-doped layer. In other words, in the present disclosure, a diamond layer which is conductive, substantially includes nitrogen, and is hydrogen-terminated forms a diamond substrate 201.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to diamond and semiconductor devices. [Background technology]

[0002] Japanese Patent Application Laid-Open No. 2017-092398 (Patent Document 1) describes a technology relating to a diamond electronic device having a diamond layer into which nitrogen is introduced. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2017-092398 Summary of the Invention [Problem to be solved by the invention]

[0004] For example, field-effect transistors (hereinafter sometimes referred to as FETs) that use wide-bandgap semiconductor materials, which have a bandgap larger than that of silicon, have superior characteristics compared to FETs that use silicon. For this reason, FETs that use wide-bandgap semiconductor materials are expected to be the next generation of FETs.

[0005] In this regard, diamond is a wide-bandgap semiconductor material with a bandgap of 5.5 eV, and has excellent properties such as high breakdown voltage, high thermal conductivity, and high mobility. For these reasons, diamond is expected to be the next-generation wide-bandgap semiconductor material following silicon carbide (SiC) and gallium nitride (GaN).

[0006] However, there are hurdles that must be overcome to improve the performance of diamond-based FETs, and further improvements are needed to make them practical. [Means for solving the problem]

[0007] In one embodiment, the diamond is substantially nitrogen-containing and hydrogen-terminated diamond, and is electrically conductive.

[0008] In one embodiment, the semiconductor device includes a field effect transistor having a p-type diamond layer, a source electrode electrically connected to the p-type diamond layer, a drain electrode electrically connected to the p-type diamond layer, and a gate electrode disposed between the source electrode and the drain electrode.

[0009] Here, the p-type diamond layer is a diamond layer that substantially contains nitrogen, is hydrogen-terminated, and has electrical conductivity. [Effects of the Invention]

[0010] According to one embodiment, diamond having novel properties can be obtained, and the performance of semiconductor devices including field effect transistors using this diamond can be improved. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram illustrating a surface conduction FET in the related art. [Figure 2] FIG. 1 is a cross-sectional view showing the configuration of a surface conduction FET in an embodiment. [Figure 3] FIG. 1 is a cross-sectional view showing the configuration of a surface conduction FET in an embodiment. [Figure 4] FIG. 1 is a diagram illustrating a planar measurement position in SIMS measurement. [Figure 5] 1 is a table showing the relationship between nitrogen concentration and electrical conductivity. [Figure 6] 1 is a graph showing the relationship between nitrogen concentration and threshold voltage. [Figure 7] 10 is a table showing the relationship between applied device examples, threshold voltages, and nitrogen concentrations. [Figure 8]10 is a graph showing drain current-drain voltage characteristics when the nitrogen concentration is 2.5×10 17 / cm 3 . [Figure 9] FIG. 10 is a diagram showing the configuration of a surface conduction FET according to Modification 1. [Figure 10] FIG. 10 is a diagram showing the configuration of a surface conduction type FET according to Modification 2. DETAILED DESCRIPTION OF THE INVENTION

[0012] In all the drawings for explaining the embodiments, the same components are generally designated by the same reference numerals, and repeated explanations thereof will be omitted. In addition, hatching may be used even in plan views to make the drawings easier to understand.

[0013] <The usefulness of diamond as a semiconductor material> For example, future mobile communications, satellite communications, and ultra-small radars will require FETs capable of high-power and high-frequency transmission. Semiconductor materials such as silicon (Si) and gallium arsenide (GaAs) reach their power density limits at frequencies above several GHz. For this reason, wide-bandgap semiconductor materials such as silicon carbide (SiC), gallium nitride (GaN), and diamond are being considered for use in FETs. Diamond, in particular, has the highest thermal conductivity of any material (four times that of SiC and 16 times that of GaN) and the highest dielectric breakdown field strength of any semiconductor material (three times that of SiC and 10 times that of GaN). Furthermore, the hole mobility and hole saturation velocity in diamond are comparable to those of silicon.

[0014] Therefore, due to the high thermal conductivity of diamond, its heat dissipation characteristics are excellent, enabling suppression of heat generation in semiconductor devices and also allowing for high-temperature operation of semiconductor devices. In addition, due to the high dielectric breakdown field strength of diamond, semiconductor devices are less likely to be damaged even when a high voltage is applied, making it suitable for semiconductor devices in high-power applications. Furthermore, the high carrier mobility of diamond suggests that diamond has great potential as a semiconductor device for high-frequency applications.

[0015] From the above, it can be seen that diamond as a semiconductor material is regarded as promising in realizing next-generation semiconductor devices including FETs capable of high-output and high-frequency transmission.

[0016] <Difficulty in fabricating n-type diamond> As described above, diamond is a wide-bandgap semiconductor material, and it is considered that n-type diamond can be fabricated by introducing n-type impurities called donors into diamond. Specifically, it is considered that n-type diamond can be realized by supplying electrons from the donor levels of the donors to the conduction band of diamond.

[0017] Here, nitrogen can be cited as a donor, but in diamond, the donor levels of nitrogen do not exist near the conduction band and exist at "deep levels" far from the conduction band. Specifically, the donor levels of nitrogen exist at an energy position 1.7 eV lower than the conduction band of diamond. This means that in diamond, the activation energy for exciting electrons from the donor levels of nitrogen to the conduction band of diamond becomes large.

[0018] As a result, even when nitrogen, which is a donor, is introduced into diamond, it is difficult to increase the electrons supplied to the conduction band, making it difficult for diamond with nitrogen introduced to function as n-type diamond.

[0019] On the other hand, it is thought that p-type diamond can be created by introducing p-type impurities called acceptors into diamond. Specifically, it is thought that p-type diamond can be realized by exciting electrons from the valence band of diamond to the acceptor level of the acceptor, thereby generating holes in the valence band of diamond.

[0020] Here, boron can be mentioned as an acceptor, and the acceptor level of this boron exists near the valence band of diamond. Specifically, the acceptor boron forms an acceptor level at an energy position 0.37 eV higher than the valence band of diamond. In other words, in diamond, the acceptor level of boron does not constitute a "deep level" like the donor level of nitrogen. For this reason, the activation energy for exciting electrons from the valence band of diamond to the acceptor level of boron is not very large. As a result, it is easier to produce p-type diamond from diamond than to produce n-type diamond.

[0021] Therefore, FETs manufactured using diamond generally do not use n-channel FETs, which require n-type diamond, but rather p-channel FETs, which use p-type diamond, which is easier to fabricate than n-type diamond. In other words, FETs manufactured using diamond are realized as FETs that use holes as carriers, rather than FETs that use electrons as carriers. In particular, FETs manufactured using diamond include p-channel FETs called "surface conduction FETs." This "surface conduction FET" will be explained below.

[0022] <Surface conduction FET> When diamond is hydrogen-terminated, there is a phenomenon in which holes (two-dimensional hole gas) are induced on the surface of the hydrogen-terminated diamond. An FET that utilizes this phenomenon is called a "surface-conduction FET." In other words, a "surface-conduction FET" is an FET that uses the two-dimensional hole gas induced on the surface of the hydrogen-terminated diamond as the FET channel, and performs switching operations by controlling the conduction / non-conduction of the channel by changing the gate voltage applied to the gate electrode. This "surface-conduction FET" has the advantage of being able to operate at high temperatures with high voltage resistance. This is because hydrogen-terminating diamond forms "CH" bonds, which are more stable than the "CC" bonds in diamond.

[0023] The mechanism by which holes are induced on the surface of hydrogen-terminated diamond has not been fully elucidated, but two theories are considered to be the most likely.

[0024] One theory is the "transfer doping model," which states that differences in chemical potential caused by adsorbates or changes in the pH of the surface cause electrons in the diamond's valence band to move to levels determined by the chemical potential of the surface, generating holes near the surface where these electrons are lost.

[0025] On the other hand, the other theory is the "negative ion model." This theory is as follows: In the "CH" bond created by hydrogen-terminating diamond, hydrogen becomes positively charged and carbon becomes negatively charged due to the difference in electronegativity. As a result, negative ions in the atmosphere are adsorbed onto the positively charged hydrogen, and these adsorbed negative ions attract holes to the surface of the diamond.

[0026] Although the mechanism is not fully understood, holes are actually induced on the surface of hydrogen-terminated diamond. This phenomenon has been utilized to create a superior FET called a "surface conduction FET," which is capable of high-temperature operation with high breakdown voltage.

[0027] Below, we will explain the related technology of "surface conduction FET" and then explain the room for improvement that exists in the related technology.

[0028] <Description of Related Art> The term "related art" as used herein refers to art that is not publicly known, but has problems that the inventors have discovered, and is a prerequisite for the present invention.

[0029] FIG. 1 is a diagram illustrating a surface conduction FET 100 according to the related art.

[0030] In FIG. 1, a surface conduction FET 100 has a diamond substrate 101 into which nitrogen is introduced, and an undoped layer 102 is provided on the diamond substrate 101.

[0031] Here, the non-doped layer 102 referred to in this specification is a diamond layer in which the nitrogen impurity concentration is 10, which is the lower limit of detection of a measuring device using SIMS (Secondary Ion Mass Spectrometry). 16 / cm 3 Diamond layer having a thickness of less than 100 nm.

[0032] The surface of the non-doped layer 102 is hydrogen-terminated. As a result, a hole accumulation layer 103 containing a two-dimensional hole gas is formed on the surface of the non-doped layer 102.

[0033] Next, a contact layer 104 and a contact layer 105 are provided on the non-doped layer 102. Each of the contact layers 104 and 105 is a p-type layer in which, for example, boron, which is a p-type impurity (acceptor), is introduced at a high concentration. +The contact layers 104 and 105 are each made of a diamond layer. Specifically, a high concentration of boron is introduced into each of the contact layers 104 and 105 so as to form ohmic contact with the non-doped layer 102. A source electrode 106 is provided on the contact layer 104. Meanwhile, a drain electrode 107 is provided on the contact layer 105.

[0034] Subsequently, a gate insulating film 108 is provided on the non-doped layer 102 between the contact layer 104 and the contact layer 105, and a gate electrode 109 is provided on this gate insulating film 108. In this manner, the surface conduction FET 100 is configured.

[0035] Next, the operation of the surface conduction FET 100 will be described.

[0036] The surface-conduction FET 100 is a p-channel FET, and includes a hole accumulation layer 103 having a two-dimensional hole gas formed near the surface of an undoped layer 102. With 0 V applied to the gate electrode 109, for example, a positive potential is applied to the source electrode 106, while a reference potential of 0 V is applied to the drain electrode 107. As a result, a hole current flows through the source electrode 106, the contact layer 104, the hole accumulation layer (channel) 103 formed on the surface of the undoped layer 102, the contact layer 105, and the drain electrode 107. In other words, the surface-conduction FET 100 is a normally-on FET.

[0037] Next, when a positive potential equal to or greater than the threshold voltage is applied to the gate electrode 109, a repulsive force acts from the gate electrode 109 at the positive potential on the two-dimensional hole gas formed directly below the gate electrode 109. As a result, the two-dimensional hole gas formed directly below the gate electrode 109 is moved away from the surface of the non-doped layer 102. As a result, the channel consisting of the two-dimensional hole gas directly below the gate electrode 109 disappears. In other words, when a positive potential equal to or greater than the threshold voltage is applied to the gate electrode 109, a depletion layer is formed directly below the gate electrode 109, and this depletion layer blocks the channel consisting of the two-dimensional hole gas. As a result, the hole current flowing through the hole accumulation layer 103, which serves as the channel, is blocked, and the surface-conduction FET 100 is turned off.

[0038] As described above, the switching operation (ON / OFF operation) of the surface-conduction FET 100 can be controlled by adjusting the gate voltage applied to the gate electrode 109. Therefore, in order to stabilize the switching operation of the surface-conduction FET 100, it is important to stabilize the threshold voltage of the gate voltage applied to the gate electrode 109.

[0039] <Consideration of improvements> In the surface-conduction FET 100 of the related art described above, even FETs of the same design have large variations in threshold voltage. As a result, the surface-conduction FET 100 of the related art cannot stably control switching operations, and the on-state current value varies depending on the FET. This is a major obstacle to practical application of the surface-conduction FET 100. In other words, in the related art, the variation in threshold voltage, one of the most important characteristics of an FET, has become apparent as an area for improvement.

[0040] In particular, the threshold voltage of the surface-conduction FET 100 depends heavily on the carrier density of the two-dimensional hole gas, which in turn is significantly affected by the "CH" bond density. The "CH" bond density varies significantly depending on the substrate and layer conditions, such as surface roughness. Consequently, even surface-conduction FETs 100 with the same design can have significantly different threshold voltages. In other words, in the related art, the threshold voltage depends on the "CH" bond density of the hydrogen termination, a difficult-to-control characteristic, making it difficult to stably control the threshold voltage. Thus, the surface-conduction FET 100 in the related art still has room for improvement in terms of suppressing variations in threshold voltage.

[0041] Furthermore, there is room for improvement in the related art, and this point will be explained.

[0042] In related art, an undoped layer 102 is used as a layer for hydrogen-terminating diamond. The present inventors have newly discovered that the undoped layer 102 contains not only a current that can be controlled by the gate electrode 109, but also a current that is difficult to control by the gate electrode 109, which is caused by a two-dimensional hole gas formed on the surface. Specifically, the inventors' investigations have revealed that, as shown in FIG. 1 , a "deep current" that is difficult to control by the gate electrode 109 exists in a deep region away from the surface of the undoped layer 102. The present inventors have newly discovered that the gain of the surface-conduction FET 100 is limited due to this "deep current."

[0043] This point will be explained below.

[0044] The gain of a surface conduction FET is generally expressed by the following formula (1):

[0045] Av≒gm×Ro (Formula 1) Av: Gain (output gain) gm: transconductance (mutual conductance) Ro: Output impedance The output impedance can be calculated from the reciprocal of the slope of the Vd (drain voltage)-Id (drain current) curve in the saturation region.

[0046] Based on the above-mentioned (Equation 1), it is desirable to increase the transconductance (gm) and output impedance (Ro) in order to increase the gain (Av). In this regard, the transconductance increases in inverse proportion to the gate length of the gate electrode 109. That is, the transconductance can be increased by shortening the gate length of the gate electrode 109. However, shortening the gate length of the gate electrode 109 tends to result in a decrease in output impedance due to the so-called short channel effect. In other words, there is a trade-off between the transconductance and the output impedance. As a result, it is difficult to significantly increase the gain, which is the output gain.

[0047] In this regard, the present inventors have newly discovered that the output impedance is reduced due to the aforementioned "deep current." Therefore, it is believed that the output impedance can be increased if the "deep current" can be suppressed. This means that while shortening the gate length of the gate electrode 109 increases the transconductance, the output impedance decreases due to the short-channel effect. However, if the "deep current," which is one factor that reduces the output impedance, can be suppressed, it is possible to increase the transconductance while maintaining a high output impedance. In this case, the gain of the surface-conduction FET can be significantly improved, which is believed to be of great technological significance for the development of surface-conduction FETs.

[0048] Therefore, there is room for improvement in the surface conduction FET 100 of the related art from the viewpoint of suppressing the "deep current" in order to improve the gain.

[0049] For the above reasons, there is room for improvement in the related art from the viewpoint of suppressing variations in threshold voltage and suppressing "deep current." Therefore, in this embodiment, a device is implemented to overcome the room for improvement that exists in the related art. The technical concept of this embodiment, which implements this device, will be described below.

[0050] <Basic Concept of the Embodiment> The basic idea of ​​this embodiment is to provide, in place of the non-doped layer 102 in a surface conduction FET, a layer having a property capable of controlling the threshold voltage and capable of generating a hole accumulation layer having a two-dimensional hole gas.

[0051] According to this basic concept, the threshold voltage is controllable, and by adjusting this characteristic, the threshold voltage can be stably controlled.

[0052] Specifically, the above-mentioned basic idea can be realized by providing a diamond layer that substantially contains nitrogen and is made of hydrogen-terminated diamond (hereinafter, may be referred to as a nitrogen-containing-hydrogen-terminated diamond layer) instead of the non-doped layer 102. This is because, since the threshold voltage of a surface conduction FET generally depends on the concentration of conductive impurities introduced into the diamond layer, the threshold voltage can be stably controlled by adjusting the concentration of this conductive impurity.

[0053] For example, when nitrogen is used as the conductive impurity, the threshold voltage of the surface conduction FET depends on the concentration of nitrogen introduced into the diamond layer, and therefore, by adjusting the concentration of this nitrogen, the threshold voltage can be stably controlled. In other words, the "characteristic that can control the threshold voltage" of the basic concept is, for example, the concentration of nitrogen introduced into the diamond layer. In this way, the basic concept can be realized by providing a nitrogen-containing hydrogen-terminated diamond layer in place of the non-doped layer 102 in the surface conduction FET.

[0054] Here, "substantially containing nitrogen" means that the nitrogen impurity concentration in the diamond layer is 10 16 / cm 3 This means that the concentration is equal to or higher than the concentration of nitrogen contained in the non-doped layer 102. The reason for the phrase "substantially containing nitrogen" is to clarify that it is different from the non-doped layer 102. In other words, the non-doped layer 102 has a nitrogen impurity concentration in the diamond layer that is equal to or higher than the detection limit of 10 16 / cm 3 Hereinafter, the term "substantially containing nitrogen" is used to clarify that the nitrogen-containing hydrogen-terminated diamond layer is a different layer from the undoped layer 102.

[0055] For example, in related technologies, a non-doped layer 102 is used in which a hole accumulation layer having a two-dimensional hole gas is formed on the surface by hydrogen termination, but this non-doped layer 102 does not have the property of easily controlling the threshold voltage. This is because, for example, nitrogen exists in the non-doped layer 102 only at a concentration below the detection limit, making it difficult to control the threshold voltage with the nitrogen concentration. For this reason, in related technologies, the threshold voltage of a surface-conduction FET depends on the "C-H" bond density of the hydrogen termination, which is difficult to control, making it difficult to stably control the threshold voltage.

[0056] In contrast, according to the basic concept of this embodiment, a nitrogen-containing hydrogen-terminated diamond layer is used instead of the non-doped layer 102. In this case, the threshold voltage of the surface conduction FET depends not only on the difficult-to-control characteristic of the "CH" bond density of the hydrogen termination, but also on the concentration of nitrogen introduced into the nitrogen-containing hydrogen-terminated diamond layer, so the threshold voltage can be stably controlled by adjusting the concentration of this nitrogen.

[0057] Therefore, the basic idea of ​​providing a nitrogen-containing hydrogen-terminated diamond layer instead of the non-doped layer 102 has great technical significance in that it has a characteristic (nitrogen concentration) that can control the threshold voltage, and by adjusting this characteristic, the threshold voltage can be stably and easily controlled.

[0058] Here, in the conventional understanding, the diamond that contains nitrogen and is terminated with hydrogen is insulating, but the present inventor has found that under certain conditions, the nitrogen-containing hydrogen-terminated diamond layer has electrical conductivity as a new finding, and has arrived at a basic idea based on this new finding.That is, because the nitrogen-containing hydrogen-terminated diamond layer has electrical conductivity, it can form the channel of surface conduction FET, and as a result, it can make surface conduction FET operate, and only with this new finding can the basic idea be realized.

[0059] In this specification, conductivity means that the resistance value can be measured. For example, when the sheet resistance value is measured and is on the order of MΩ / □ or higher, it often exceeds the upper limit of measurement and is determined to not have conductivity. On the other hand, when the sheet resistance value is measured and is on the order of kΩ / □, it is measurable and is determined to have conductivity.

[0060] The following describes the novel findings that led to the above-mentioned basic idea.

[0061] <<New findings discovered by the inventors>> For example, nitrogen is a donor for diamond, which is a wide bandgap semiconductor material, but since the donor level of nitrogen is a "deep level," diamond containing nitrogen does not have many electrons in the conduction band and is essentially an insulator.

[0062] Furthermore, conventionally, nitrogen-containing and hydrogen-terminated diamond is also considered an insulator. This is because, for example, commercially available diamond substrates currently contain 10 18 / cm 3This is because a higher concentration of nitrogen is introduced than in the case of the conventional structure, and it is thought that the holes that make up the two-dimensional hole gas generated by the hydrogen termination recombine with electrons excited to the conduction band due to the high concentration of nitrogen, resulting in the disappearance of the two-dimensional hole gas.

[0063] In other words, even if the donor level of nitrogen is a "deep level," electrons are slightly excited from the donor level to the conduction band of diamond, so that some electrons exist in the conduction band. As a result, electrons and holes existing in this conduction band recombine, and it is conventionally recognized that diamond containing nitrogen and being hydrogen-terminated is also an insulator.

[0064] Because of this conventional understanding, related technologies use an undoped layer 102 instead of nitrogen-containing, hydrogen-terminated diamond. In other words, the hydrogen-terminated undoped layer 102 generates a two-dimensional hole gas on the surface, but because the undoped layer 102 contains almost no nitrogen, the holes that make up the two-dimensional hole gas do not recombine with electrons in the conduction band due to nitrogen and are not completely annihilated. Therefore, the undoped layer 102 allows the two-dimensional hole gas to be used in the channel of a surface-conduction FET.

[0065] From the above, it can be seen that, based on conventional understanding, it is necessary to provide an undoped layer 102 on a commercially available diamond substrate. However, in this case, the additional step of forming the undoped layer 102 not only increases the number of manufacturing steps but also increases the variability in device performance, such as threshold voltage. For this reason, efforts have been made to thin the undoped layer 102. Finally, the present inventors have confirmed that surface-conduction FETs can operate on commercially available diamond substrates without providing an undoped layer 102. This is due to the fact that the performance of commercially available diamond substrates has improved, resulting in a decrease in the nitrogen concentration contained in the diamond substrate. In other words, the present inventors have newly discovered through the above-mentioned process that, if the nitrogen concentration contained in the diamond substrate is reduced to a certain extent, conductivity due to the two-dimensional hole gas generated by hydrogen termination can be maintained even if the nitrogen concentration does not decrease to the level required to form the undoped layer 102.

[0066] This novel finding can be understood by the following qualitative mechanism. That is, a decrease in the nitrogen concentration of the diamond substrate means that fewer electrons are excited into the conduction band of diamond, which means that the two-dimensional hole gas generated by hydrogen termination does not completely disappear through recombination. As a result, two-dimensional hole gas remains on the surface of the hydrogen-terminated diamond substrate, and it can be considered that this two-dimensional hole gas ensures conductivity.

[0067] Based on the novel finding that, under certain conditions, the nitrogen-containing hydrogen-terminated diamond layer is conductive, it has been found that it is possible to operate a surface conduction FET even if a nitrogen-containing hydrogen-terminated diamond layer is used instead of the non-doped layer 102, and furthermore, a basic concept has been arrived at based on the novel finding described above.

[0068] The above-mentioned new findings suggest that it may be possible to easily fabricate surface-conduction FETs from commercially available diamond substrates in the future. From this perspective, the new findings of the present inventors have important technical significance for the practical application of surface-conduction FETs.

[0069] The following describes embodiments that embody the basic concept described above.

[0070] <Realization mode> <<Configuration of surface conduction FET>> FIG. 2 is a cross-sectional view showing the configuration of a surface conduction type FET 200 in an embodiment.

[0071] In FIG. 2, a surface-conduction FET 200 has a diamond substrate 201 into which nitrogen has been introduced. The surface of the diamond substrate 201 is hydrogen-terminated. That is, the diamond substrate 201 in this embodiment functions as a nitrogen-containing, hydrogen-terminated diamond layer. In other words, the diamond substrate 201 is a p-type diamond layer, which is a diamond layer that substantially contains nitrogen and is hydrogen-terminated, and is composed of an electrically conductive diamond layer. As a result of the hydrogen-terminated surface of the diamond substrate 201, a hole accumulation layer 103 made of a two-dimensional hole gas is formed on the surface of the diamond substrate 201.

[0072] Next, a contact layer 104 and a contact layer 105 are provided on the diamond substrate 201 having the hole accumulation layer 103. Each of these contact layers 104 and 105 is made of, for example, a p-type impurity (acceptor) containing boron at a high concentration. + Specifically, a high concentration of boron is introduced into each of the contact layers 104 and 105 so as to form ohmic contact with the diamond substrate 201.

[0073] Specifically, the concentration of boron is, for example, 5×10 19 / cm 3 That's it, 1 x 10 22 / cm 3 The thickness of each of the contact layer 104 and the contact layer 105 is, for example, about 20 nm or more and 300 nm or less.

[0074] A source electrode 106 is provided on the contact layer 104. Meanwhile, a drain electrode 107 is provided on the contact layer 105. Subsequently, a gate insulating film 108 is provided on the diamond substrate 201 between the contact layer 104 and the contact layer 105, and a gate electrode 109 is provided on this gate insulating film 108.

[0075] The electrode materials of the source electrode 106, the drain electrode 107, and the gate electrode 109 are, for example, gold (Au), ruthenium (Ru), aluminum (Al), titanium (Ti), molybdenum (Mo), copper (Cu), chromium (Cr), lead (Pb), zinc (Zn), platinum (Pt), or a combination thereof (e.g., Ti / Mo / Au). The thickness of each of the source electrode 106, the drain electrode 107, and the gate electrode 109 is, for example, about 10 nm or more and 100 nm or less.

[0076] On the other hand, the gate insulating film 108 is made of, for example, aluminum oxide (Al2O3), calcium fluoride (CaF2), magnesium fluoride (MgF2), silicon oxide (SiO2), silicon nitride (Si3N4), etc. The thickness of the gate insulating film 108 is, for example, about 5 nm or more and 100 nm or less.

[0077] 2, the gate electrode 109 does not overlap with the contact layer 104, and the gate electrode 109 does not overlap with the contact layer 105. Furthermore, in a plan view, the gate electrode 109 does not overlap with the contact layer 104, and the gate electrode 109 does not overlap with the contact layer 105.

[0078] In this manner, the surface conduction FET 200 is configured.

[0079] <<Operation of Surface Conduction FET>> Next, the operation of the surface conduction FET 200 will be described.

[0080] The surface-conduction FET 200 is a p-channel FET, and has a hole accumulation layer 103 containing a two-dimensional hole gas formed near the surface of a diamond substrate 201. Here, with 0 V applied to the gate electrode 109, for example, a positive potential is applied to the source electrode 106, while a reference potential of 0 V is applied to the drain electrode 107. As a result, a hole current flows through the source electrode 106 → contact layer 104 → hole accumulation layer (channel) 103 formed on the surface of the diamond substrate 201 → contact layer 105 → drain electrode 107. In other words, the surface-conduction FET 200 is a normally-on FET.

[0081] Next, when a positive potential equal to or greater than the threshold voltage is applied to the gate electrode 109, a repulsive force acts from the gate electrode 109 at the positive potential on the two-dimensional hole gas formed directly below the gate electrode 109. As a result, the two-dimensional hole gas formed directly below the gate electrode 109 is repelled away from the vicinity of the surface of the diamond substrate 201. As a result, the channel consisting of the two-dimensional hole gas directly below the gate electrode 109 disappears.

[0082] 3, when a positive potential equal to or greater than the threshold voltage is applied to the gate electrode 109, a depletion layer (dot region) is formed directly below the gate electrode 109, and this depletion layer blocks the channel made of two-dimensional hole gas. As a result, the hole current flowing through the hole accumulation layer 103, which is the channel, is blocked, and the surface-conduction FET 100 is turned off.

[0083] As described above, by adjusting the gate voltage applied to the gate electrode 109, the switching operation (ON / OFF operation) of the surface conduction type FET 200 can be controlled.

[0084] <<Features in Realization Mode>> Next, the features of the embodiment will be described.

[0085] The first feature is that the diamond substrate 201 functions as a nitrogen-containing, hydrogen-terminated diamond layer without providing the non-doped layer 102. In other words, the first feature is that the diamond substrate 201 is made of a diamond layer that substantially contains nitrogen, is hydrogen-terminated, and has electrical conductivity. This makes it possible to realize a surface-conduction FET without using the non-doped layer 102.

[0086] Here, in order to make the nitrogen-containing hydrogen-terminated diamond layer conductive, the nitrogen concentration must be 1×10 18 / cm 3 The nitrogen concentration must be less than 1 × 10 18 / cm 3 This is because it has been found that only those having a thickness less than 1000 nm are conductive.

[0087] In addition to SIMS measurement, other techniques for measuring nitrogen concentration include FTIR measurement (Fourier transform infrared spectroscopy), Hall effect measurement, and CV (capacitance-voltage) measurement. Below, we will explain the results of SIMS measurement.

[0088] For example, when ions are incident on a sample surface, various particles, such as electrons, neutral particles, and ions, are emitted from the sample surface. SIMS is a technique for qualitative and quantitative analysis of the components contained in a sample by detecting ions among these particles and measuring the detected amount at each mass. Specifically, when primary ions such as oxygen or cesium are irradiated onto the sample surface, atoms near the sample surface are agitated, and some of the agitated atoms are ejected into the vacuum. SIMS is a technique for analyzing the components contained in a sample by mass spectrometry of the ions (secondary ions) among the ejected particles.

[0089] In this embodiment, the nitrogen concentration contained in the diamond substrate is measured by SIMS under the following measurement conditions.

[0090] 1. Venue: Materials Science and Technology Foundation (MST) 2. Measurement conditions (1) Measuring device: CAMECA IM-7f (2) Primary ion species: Cs + (3) Primary accelerating voltage: 15.0 kV (4) Detection area: 30 (μmφ) 3. Measurement points (1) Plane position: Near the center of the diamond substrate (see Figure 4) (2) Depth: The range in which a stable value can be measured without taking into account noise near the surface. In Figure 4, the planar shape of the diamond substrate is quadrangular, but this is just one example, and the planar shape of the diamond substrate as a sample is not limited to this and may be a polygonal shape such as a pentagon or hexagon, a circular shape including a perfect circle or ellipse, or any other shape including an irregular shape, and in this case the planar measurement position is a position near the center of each planar shape.

[0091] FIG. 5 is a table showing the relationship between nitrogen concentration and electrical conductivity.

[0092] In Figure 5, the nitrogen concentration is 3.5 x 10 19 / cm 3 " or "1.3 x 10 18 / cm 3 ", the sheet resistance is MΩ / □ or more, exceeding the upper limit of measurement, and it is found that the film does not have conductivity. On the other hand, when the nitrogen concentration is "2.5 × 10 17 / cm 3 ", the sheet resistance is within the measurable range of 25 kΩ / □, indicating that the film is conductive.

[0093] Therefore, based on the measurement results shown in Figure 5, the nitrogen concentration is 1 × 10 18 / cm3 If the thickness is less than 100 nm, the nitrogen-containing hydrogen-terminated diamond layer will have electrical conductivity, and it will be understood that a surface conduction FET can be realized without using the non-doped layer 102.

[0094] Furthermore, according to the first feature, since a nitrogen-containing hydrogen-terminated diamond layer is used, the threshold voltage of the surface-conduction FET depends on the concentration of nitrogen introduced into the nitrogen-containing hydrogen-terminated diamond layer. Therefore, according to the surface-conduction FET in the embodied mode, the threshold voltage can be stably controlled by adjusting the concentration of this nitrogen.

[0095] FIG. 6 is a graph showing the relationship between nitrogen concentration and threshold voltage. In FIG. 6, the horizontal axis represents the nitrogen concentration (1 / cm 3 ) and the vertical axis is the threshold voltage (V).

[0096] Here, the nitrogen concentration of the electrically conductive nitrogen-containing hydrogen-terminated diamond layer, which is the first characteristic, is 1×10 18 / cm 3 The range of substantially containing nitrogen is less than the lower limit of measurement, 1 × 10 16 / cm 3 Therefore, the nitrogen concentration of the conductive nitrogen-containing hydrogen-terminated diamond layer in the first feature point is 1×10 16 / cm 3 More than 1×10 18 / cm 3 On the other hand, the nitrogen concentration of the non-doped layer 102 is less than the lower limit of measurement, 1×10 16 / cm 3 6, the nitrogen concentration of the non-doped layer 102 is 1×10 16 / cm 3 The nitrogen concentration of the electrically conductive nitrogen-containing hydrogen-terminated diamond layer is 1 × 10 or less. 16 / cm 3 More than 1×10 18 / cm 3It can be seen that in the range of less than 10 ...

[0097] According to a first feature of the embodiment, the threshold voltage of the surface-conduction FET can be stably controlled by adjusting the nitrogen concentration. Therefore, according to the embodiment, by utilizing the technique of controlling the threshold voltage by adjusting the nitrogen concentration, it is possible to fabricate surface-conduction FETs with threshold voltages suited to various applications.

[0098] This point will be explained below.

[0099] FIG. 7 is a table showing the relationship between the threshold voltage and the nitrogen concentration for example application devices.

[0100] In FIG. 7, for example, when realizing an FET for an environmentally resistant analog circuit, the threshold voltage must be 4.5 V or less. Therefore, the nitrogen concentration in this case is set to 2×10 16 / cm 3 That's 1 x 10 18 / cm 3 It is desirable to keep it below this.

[0101] Furthermore, if you want to keep the threshold voltage low in an FET for environmentally resistant analog circuits, the threshold voltage must be 4.0 V or less. In this case, the nitrogen concentration is set to 6.5 × 10 16 / cm 3 That's 1 x 10 18 / cm 3 It is desirable to keep it below this.

[0102] For example, to realize a FET for a high frequency circuit, the threshold voltage must be 3.2 V or less. In this case, the nitrogen concentration must be 2 × 10 17 / cm 3 That's 1 x 10 18 / cm 3 It is desirable to keep it below this.

[0103] For example, when realizing an FET for an analog circuit amplifier, the threshold voltage must be 3.0 V or less, so the nitrogen concentration in this case is set to 2.5 × 10 17 / cm 3 That's 1 x 10 18 / cm 3 It is desirable to keep it below this.

[0104] Furthermore, for example, when realizing a FET for a power device, the threshold voltage must be 2.5 V or less, so the nitrogen concentration in this case must be 4 × 10 17 / cm 3 That's 1 x 10 18 / cm 3 It is desirable to keep it below this.

[0105] As described above, according to the embodiment, by adjusting the nitrogen concentration, it is possible to fabricate surface-conduction FETs having threshold voltages suited to various applications.

[0106] According to an embodiment, the threshold voltage of a surface-conduction FET can be controlled by adjusting the nitrogen concentration. In this regard, methods for controlling the threshold voltage include, in addition to adjusting the nitrogen concentration, (1) a method for controlling the threshold voltage by changing the carrier concentration based on changes in the in-film charge or interfacial charge resulting from the type or formation conditions of the gate insulating film or the structure (film thickness, etc.) of the gate insulating film, and (2) a method for controlling the threshold voltage by changing the work function by changing the material of the gate electrode, and the threshold voltage may be controlled by combining these methods.

[0107] Next, according to the first characteristic point described above, the following advantages can be obtained.

[0108] For example, in a surface conduction FET, a "deep current" occurs, and this "deep current" reduces the output impedance. In this regard, in the embodiment, the diamond substrate 201 is formed from a conductive diamond layer that substantially contains nitrogen and is hydrogen-terminated, without providing the non-doped layer 102.

[0109] This means that, according to the first feature, nitrogen is also introduced into the region where the "deep current" flows. Therefore, holes that constitute the "deep current" are more likely to recombine with electrons excited from the nitrogen donor level to the conduction band of diamond. As a result, according to the embodiment, the "deep current" can be suppressed.

[0110] Therefore, for example, shortening the gate length of the gate electrode 109 increases the transconductance while decreasing the output impedance due to the short channel effect, but the first feature can suppress the "deep current," which is one factor that decreases the output impedance, and as a result, it is possible to increase the transconductance while maintaining a large output impedance. Therefore, the first feature in this embodiment can significantly improve the gain of the surface conduction FET.

[0111] Specifically, Figure 8 shows the results for a nitrogen concentration of 2.5 × 10 17 / cm 3 1 is a graph showing the drain current-drain voltage characteristics for the case where the gate voltage is increased by ΔVg=+0.5V in increments of ΔVg.

[0112] For example, from FIG. 8, when the gate voltage Vg is -2V, (1) the output impedance when nitrogen is added is 11 kΩ, while (2) the output impedance when nitrogen is not added is 8 kΩ. Therefore, from this result, it can be seen that adding nitrogen can increase the output impedance. In other words, the first feature can suppress the "deep current", thereby increasing the output impedance.

[0113] In order to improve the gain of a surface-conduction FET, it is desirable to increase the transconductance. In this regard, shortening the gate length of the gate electrode 109 is an effective way to increase the transconductance, but another method for increasing the transconductance is to reduce the parasitic resistance by reducing the distance between the gate electrode 109 and the source electrode 106. These methods may be combined to increase the transconductance.

[0114] Furthermore, according to the first characteristic point described above, the following advantages can be obtained.

[0115] For example, a surface conduction FET according to the related art can be obtained by forming an undoped layer 102 on a diamond substrate 101 and hydrogen-terminating the surface of this undoped layer 102. In this case, it is necessary to form the undoped layer 102 anew on a commercially available diamond substrate 101, which complicates the manufacturing process and also causes the undoped layer 102 to increase the variability in device characteristics, so improvements are being considered.

[0116] In this regard, in a first feature of the embodied embodiment, the diamond substrate 201 functions as a nitrogen-containing, hydrogen-terminated diamond layer without providing the non-doped layer 102. That is, in the embodied embodiment, the diamond substrate 201 is constituted of a diamond layer that substantially contains nitrogen, is hydrogen-terminated, and has electrical conductivity.

[0117] As a result, according to the embodiment, it is not necessary to form the non-doped layer 102, which complicates the manufacturing process and is a factor in increasing the variation in device characteristics. The nitrogen concentration of commercially available diamond substrates is, for example, 1×10 16 / cm 3 More than 1×10 18 / cm 3 When the surface conduction FET is less than 1000 nm, hydrogen-terminating the commercially available diamond substrate can easily make the diamond substrate function as a conductive nitrogen-containing hydrogen-terminated diamond layer. This means that surface conduction FETs can be easily manufactured from commercially available diamond substrates, and this has important technical significance for the practical application (mass production) of surface conduction FETs.

[0118] 2, the gate electrode 109 does not overlap with the contact layer 104, and the gate electrode 109 does not overlap with the contact layer 105. As a result, the surface-conduction FET 200 having the second feature can improve high-frequency characteristics, and can provide a promising FET for realizing next-generation semiconductor devices including FETs capable of high-frequency transmission.

[0119] This point will be explained below.

[0120] For example, there is the normally-off FET. This normally-off FET is a type of FET that is off under normal conditions (when a gate voltage smaller than the threshold voltage is applied to the gate electrode), but turns on when a gate voltage equal to or greater than the threshold voltage is applied to the gate electrode, forming a channel called an inversion layer directly below the gate electrode. In this normally-off FET, to form the inversion layer, it is necessary to apply the channel modulation effect from the gate electrode to the entire channel. For this reason, in a normally-off FET, the gate electrode and contact layer must overlap.

[0121] However, overlapping the gate electrode and contact layer generates parasitic capacitance between the gate electrode and source and between the gate electrode and drain. This parasitic capacitance adversely affects the high-frequency characteristics of the FET. Therefore, a normally-off FET poses a hurdle that must be overcome to realize an FET capable of high-frequency transmission.

[0122] In contrast, the surface-conduction FET in the present embodiment is a normally-on FET. This normally-on FET is an FET in which, under normal conditions (when a gate voltage smaller than the threshold voltage is applied to the gate electrode), the FET is on and current flows, but when a gate voltage equal to or greater than the threshold voltage is applied to the gate electrode, a depletion layer extends from directly below the gate electrode to cut off the channel, turning the FET off.

[0123] In this normally-on FET, there is no need to extend the depletion layer over the entire channel; instead, it is sufficient to block the portion of the channel directly below the gate electrode with the depletion layer, so there is no need to overlap the gate electrode with the contact layer. This reduces the parasitic capacitance caused by overlapping the gate electrode with the contact layer, resulting in improved frequency characteristics. Therefore, it can be seen that normally-on FETs are more promising than normally-off FETs for realizing FETs capable of high-frequency transmission.

[0124] <Variation 1> FIG. 9 is a diagram showing the configuration of a surface conduction FET 200A according to the first modification.

[0125] 9, in a surface conduction FET 200A, a nitrogen-containing hydrogen-terminated diamond layer 202 is provided on a diamond substrate 201. As a result of the hydrogen termination of the surface of the nitrogen-containing hydrogen-terminated diamond layer 202, a hole accumulation layer 103 containing two-dimensional hole gas is formed on the surface. In this way, instead of making the diamond substrate 201 itself function as the nitrogen-containing hydrogen-terminated diamond layer, a configuration may be adopted in which the conductive nitrogen-containing hydrogen-terminated diamond layer 202 is provided on the diamond substrate 201.

[0126] In particular, the configuration of the present modification 1 is, for example, a commercially available diamond substrate 201 having a nitrogen concentration of 1×10 16 / cm 3 More than 1×10 18 / cm 3 In this case, it is effective when the surface roughness is outside the range of 1×10 16 / cm 3 More than 1×10 18 / cm 3 By providing the nitrogen-containing hydrogen-terminated diamond layer 202 having a nitrogen concentration falling within the range of less than 0.1%, a surface conduction FET that embodies the basic concept can be realized.

[0127] <Variation 2> FIG. 10 is a diagram showing the configuration of a surface-conduction FET 200B according to the second modification.

[0128] 10 , in Modification 2, the gate electrode 109 is in direct contact with the hole accumulation layer 103. In other words, no gate insulating film is interposed between the gate electrode 109 and the hole accumulation layer 103. In this way, the gate electrode 109 may be in direct contact with the hole accumulation layer 103. In this case, for example, the gate electrode 109 and the hole accumulation layer 103 are in Schottky contact. When a gate voltage equal to or higher than the threshold voltage is applied to the gate electrode 109, a depletion layer extends from directly below the gate electrode 109, thereby blocking the channel.

[0129] The invention made by the inventor has been specifically described above based on the embodiments thereof, but it goes without saying that the present invention is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit of the invention. [Explanation of symbols]

[0130] 100 Surface conduction FET 200 Surface conduction FET 200A surface conduction FET 200B surface conduction FET 101 Diamond substrate 102 Non-doped layer 103 Hole accumulation layer 104 Contact layer 105 Contact layer 106 Source electrode 107 Drain electrode 108 Gate insulating film 109 Gate electrode 201 Diamond substrate 202 Nitrogen-containing hydrogen-terminated diamond layer

Claims

1. A diamond that is substantially nitrogen-containing and hydrogen-terminated, The diamond has electrical conductivity, Diamond, wherein the concentration of nitrogen is less than 1×10 18 / cm 3 .

2. 2. The diamond according to claim 1, Substantially containing nitrogen is 1×10 １６ / cm ３ This means that the nitrogen contained in the above-mentioned compound is 0.1 or more.

3. 2. The diamond according to claim 1, The diamond is a p-type diamond.

4. 2. The diamond according to claim 1, The concentration of the nitrogen is 2×10 １６ / cm ３ That's 1 x 10 １８ / cm ３ is less than.

5. 2. The diamond according to claim 1, The concentration of the nitrogen is 6.5×10 １６ / cm ３ That's 1 x 10 １８ / cm ３ is less than.

6. 2. The diamond according to claim 1, The concentration of the nitrogen is 2×10 １７ / cm ３ That's 1 x 10 １８ / cm ３ is less than.

7. 2. The diamond according to claim 1, The concentration of the nitrogen is 2.5×10 １７ / cm ３ That's 1 x 10 １８ / cm ３ is less than.

8. 2. The diamond according to claim 1, The concentration of the nitrogen is 4×10 １７ / cm ３ That's 1 x 10 １８ / cm ３ is less than.

9. A semiconductor device including a field effect transistor, The field effect transistor is A diamond layer; a source electrode electrically connected to the diamond layer; a drain electrode electrically connected to the diamond layer; a gate electrode provided between the source electrode and the drain electrode; and the diamond layer is a diamond layer that substantially contains nitrogen and is hydrogen-terminated, and has electrical conductivity; The semiconductor device, wherein the concentration of the nitrogen is less than 1×10 18 / cm 3 .

10. 10. The semiconductor device according to claim 9, A gate insulating film is interposed between the diamond layer and the gate electrode.

11. 10. The semiconductor device according to claim 9, The field effect transistor is a surface conduction field effect transistor.

12. 10. The semiconductor device according to claim 9, The field effect transistor is a normally-on type transistor.

13. 10. The semiconductor device according to claim 9, The field effect transistor is a first contact layer provided between the diamond layer and the source electrode; a second contact layer provided between the diamond layer and the drain electrode; It has.

14. 14. The semiconductor device according to claim 13, the gate electrode does not overlap the first contact layer in a plan view; In a plan view, the gate electrode does not overlap the second contact layer.

15. A semiconductor device including a field effect transistor, The field effect transistor is A diamond layer; a source electrode electrically connected to the diamond layer; a drain electrode electrically connected to the diamond layer; a gate electrode provided between the source electrode and the drain electrode; and A semiconductor device, wherein the diamond layer is made of the diamond according to any one of claims 1 to 8.

16. A material containing nitrogen at 1×10 16 / cm 3 or more, It has a p-type conductivity, Diamond has electrical conductivity and exhibits a sheet resistance value of less than 1 MΩ / □.

17. The diamond according to claim 16, The semiconductor does not contain elements that become p-type impurities at a concentration of 1×10 16 / cm 3 or more.

18. The diamond according to claim 16, The diamond has at least a partially hydrogen-terminated region, The diamond contains less than 1×10 18 / cm 3 of nitrogen.

19. A semiconductor device including a field effect transistor, comprising: The field effect transistor is a diamond layer comprising the diamond of claim 16; a source electrode electrically connected to the diamond layer; a drain electrode electrically connected to the diamond layer; a gate electrode provided between the source electrode and the drain electrode; A semiconductor device comprising at least:

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