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Si(100) heterostructures formed by ECR Ar plasma CVD without substrate heating
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Electronic properties of Si/Si-Ge Alloy/Si(100) heterostructures formed by ECR Ar plasma CVD without substrate heating ⁎
Naofumi Ueno, Masao Sakuraba , Yoshihiro Osakabe, Hisanao Akima, Shigeo Sato Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 9808577, Japan
A R T I C L E I N F O
A BS T RAC T
Keywords: Plasma chemical vapor deposition Heteroepitaxial growth Silicon Silicon-germanium alloy Valence band pn junction diode
By using our low-energy Ar plasma enhanced chemical vapor deposition (CVD) at a substrate temperature below 100 °C during plasma exposure without substrate heating, modulation of valence band structures and infrared photoluminescence can be observed by change of strain in a Si/strained Si0.4Ge0.6/Si(100) heterostructure. For the strained Si0.5Ge0.5 film, Hall mobility at room temperature was confirmed to be as high as 660 cm2 V−1 s−1 with a carrier concentration of 1.3×1018 cm−3 for n-type carrier, although the carrier origin was unclear. Moreover, good rectifying characteristics were obtained for a p+Si/nSi0.5Ge0.5 heterojunction diode. This indicates that the strained Si-Ge alloy and Si films and their heterostructures epitaxially grown by our lowenergy Ar plasma enhanced CVD without substrate heating can be applicable effectively for various semiconductor devices utilizing high carrier mobility, built-in potential by doping and band engineering.
1. Introduction In the field of next-generation Si large-scale integrated circuits (LSIs) development, lowering power consumption is increasingly required and will be achieved by highly-integrated transistors, e.g. metal-oxide-semiconductor field-effect transistor (MOSFET) and heterojunction bipolar transistor (HBT), as well as efficient circuit configuration by novel functional devices; e.g. Esaki-tunnel diode and transistor with lower subthreshold swing [1–7] and resonant-tunneling device with intrinsic negative differential conductance [8–10]. Therefore, towards human-friendly society based on world-wide information and communication technology with less energy, efforts to develop new types of quantum-effect emphasized device as described above and to integrate them onto conventional Si LSIs are expected to be valuable. For realization of Si-based quantum-effect emphasized devices, additionally to control of doping, electronic band modulation using strained Si-Ge-C alloy/Si heterostructures is a key technology [11–13]. Low-temperature process is necessary for fabricating high quality heterostructures because suppressed intermixing at heterointerface is indispensable [14–17]. In such a trend, electron-cyclotron-resonance (ECR) Ar plasma chemical-vapor deposition (CVD) process at a substrate temperature below 100 °C during plasma exposure without substrate heating is one of the low-temperature deposition processes and epitaxial growth of strained/unstrained films of Si, strained Si-Ge
⁎
alloy and Ge without substrate heating has been achieved in our group [18–22]. In this paper, for acquisition to control quantum-effect phenomena by heterostructures in the Si-based semiconductor devices, we report characterization results of electronic band structures and electronic properties of Si/strained Si1−xGex (x=0.5–0.6)/Si(100) heterostructures grown by the ECR Ar plasma CVD process. Here, by utilizing perfect Si crystal and lattice matching to it, high-quality single-crystalline heterostructure can be integrated on Si-LSIs and it will provide quantum-effect phenomena (e.g. interband and/or intraband tunneling) in addition to a use of excellent physical and electrical properties of Si crystal. Typically, Ge fraction of around 0.5–0.6 with thickness up to around 10 nm is expected to be practically applicable from a view point of trade-off relation between band gap narrowing and critical thickness. 2. Experimental methods By using an ECR Ar plasma CVD (Fig. 1) without substrate heating [18], Si/strained Si1−xGex (x=0.5–0.6)/Si(100) heterostructures were deposited on Si(100) by reaction of GeH4 and SiH4 under low-energy Ar plasma irradiation as follows:. Typically, substrates used were partially SiO2 covered p-type or ntype Si(100) wafers and they were treated in a few % dilute-HF (DHF) solution to remove the native oxide and rinsed with deionized water just before loading into the reactor chamber. By the DHF treatment, it
Corresponding author. E-mail address: [email protected] (M. Sakuraba).
http://dx.doi.org/10.1016/j.mssp.2016.09.035 Received 18 July 2016; Received in revised form 21 September 2016; Accepted 27 September 2016 Available online xxxx 1369-8001/ © 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: Ueno, N., Materials Science in Semiconductor Processing (2016), http://dx.doi.org/10.1016/j.mssp.2016.09.035
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subtraction” was performed as follows: When a photoelectron count rate for a certain binding energy in a valence band and a peak area of photoelectron spectrum for a core level of Ge 3d (around 29−30 eV) are defined as IVB(d) and AGe(d) for Si0.4Ge0.6/Si(100) with a d nm-thick Si cap, the photoelectron spectrum only for a valence band of the Si cap is simply calculated by using an equation of IVB(d)−{AGe(d) ⁄ AGe(0)}×IVB(0) for each binding energy under consideration of photoelectron decay characteristics in the Si cap. In order to determine lattice constants of Si0.4Ge0.6 and lattice matching with Si(100) or degree of strain relaxation, rocking curve of X-ray diffraction (XRD) for (400) from the Si/strained Si0.4Ge0.6/ Si(100) heterostructures was measured by X’Pert PRO MRD diffractometer (PANalytical Japan, Spectris Co., Ltd.). Furthermore, XRD reciprocal space map (XRD-RSM) for (422) [24] was measured by using X’Pert3 MRD diffractometer with 3D detection system (PIXcel3D) (PANalytical Japan, Spectris Co., Ltd.) with high-incidence asymmetric-reflection technique for improvement of effective resolution (X-ray wavelength of 0.154 nm (Cu Kα1 line)). In order to evaluate residual H atoms, from Si cap/Si-Ge alloy/Si(100) heterostructures, Fourier-transform infrared (FTIR) absorbance spectra were taken by an attenuated-total reflection (ATR) method with a single-reflection type prism of Ge crystal. Electronic band structures of Si/strained Si0.4Ge0.6/p-type Si(100) heterostructure was evaluated by means of XPS (for valence band [25]) and photoluminescence (PL; Horiba Jobin Yvon PL-F: a PbS detector (thermoelectrically-cooled at −15 °C) with a chopper and a lock-in amplifier). Here, PL at 8 K was excited by Ar+ ion laser light (515 nm) irradiation with a power of about 100 mW. Especially for a 11 nm-thick strained Si0.5Ge0.5 film with no intentional doping, resistivity measurement by a four-point probe (4PP) and Hall-effect measurement at room temperature were performed to determine carrier type (n or p), carrier concentration and carrier mobility. Especially for these electrical measurements, a p-type Si-on-insulator (SOI) substrate (Shin-Etsu Handotai Co., Ltd.) was used to reduce parallel conduction in the substrate material. Here, the SOI substrate has a 88 nm-thick p-type Si(100) with resistivity of 9– 18 Ω cm (sheet resistivity 1–2 MΩ/square, carrier concentration < 1015 cm−3) as a top Si layer and a 145 nm-thick SiO2 as a buried oxide layer on a thick p-type Si(100) substrate. In the 4PP resistivity measurement, a 4PP probe with pin-to-pin distance of 1 mm and pin-top radius of 150 µm (Kyowa Riken; K-89PS150 and K-504RB) was used and resistivity was calculated with a correction factor of 4.53 [26]. In the Hall-effect measurement, Hall voltage was measured by van der Pauw method [27–29] for 12 mm×12 mm square-shape sample with another type of hand-made 4PP probe (Au-plated BeCu pins with inner spring pressure of 108 g and pin-to-pin distance of 10 mm and magnetic flux of 0.25 T with double Neodymium magnets). For both of measurements, current and voltage were measured by a 2channel digital multimeter (ADC Corp. 7352 A). Finally, Hall mobility was calculated using the carrier concentration and the resistivity obtained by the 4PP-resistivity measurements. A p+Si/nSi0.5Ge0.5 heterojunction diode was fabricated by a simplified process, i.e. Al wet etching followed by B-doped Si wet etching with single resist pattern (6 mm diameter) as shown later in Fig. 9. As a substrate, n-type Si(100) wafer (1–3 Ω cm, carrier concentration around 1015 cm−3) was used. After DHF treatment for native oxide removal on the substrate and transferring through cleanroom air, an intentionally-undoped Si0.5Ge0.5 film (11 nm-thick, the Hall-effect measurement shows “n-type” as clarified later) was deposited. And then, after transferring the substrate in cleanroom air to another ECR CVD chamber, a B-doped Si film (11 nm-thick, hole concentration 7×1019 cm−3) was deposited in the same manner as the Si0.5Ge0.5 deposition. Here, partial pressures for SiH4, B2H6 and H2 were 1.0×10−4 Pa, 1.0×10−5 Pa and 1.0×10−3 Pa, respectively, microwave power 200 W and deposition rate 1.1 nm/min. Then, after DHF treatment for native oxide removal and transferring the substrate in
Ultraclean Gas Supply System Ar Microwave (2.45 GHz) Quartz Window Magnetic Coil
Substrate
SiH4 GeH4 (Ar or He Diluted) B2H6 (H2 Diluted)
ECR Plasma Chamber N2
Reactor Chamber Turbo Molecular Pump
Dry Pump
Susceptor
N2 Purged Transfer Chamber
Fig. 1. Schematic of ECR Ar plasma CVD system without substrate heating.
is well known that a Si atom on top of Si(100) surface is terminated by a few hydrogen atoms and the surface is effectively protected from native oxide formation at room temperature [23]. To minimize air contamination into the reactor chamber and to enhance desorption of water molecules physically-adsorbed on the substrate, wafer loading were performed through a N2 purged transfer chamber combined with a gate valve. After wafer loading and evacuating, Ar gas was continuously supplied into a plasma generation chamber at a partial pressure of 2.1 Pa and then reactant gases of GeH4 and SiH4 (diluted by He or Ar) were directly introduced into the reactor chamber. Subsequently, microwave (2.45 GHz, 200 W) was supplied through a quartz window on top of the plasma generation chamber and electroncyclotron resonance was induced at a specific magnetic field of 0.0875 T. Especially to avoid plasma damage and intermixing at heterointerfaces, a low-energy ECR Ar plasma condition with an Ar pressure as high as a few Pa is important to obtain peak ion energy as low as a few eV. Substrate temperature was suppressed below 100 °C during plasma exposure even for a few hundred seconds in the present condition up to 200 W [18]. Here, it should be noted that in-situ surface cleaning was not performed intentionally before deposition. For the Si-Ge alloy with Ge fraction (x) of 0.5 and 0.6, GeH4 partial pressure was 0.8×10−4 Pa and 1.0×10−4 Pa and deposition rate was 0.73 nm/min and 0.80 nm/min, respectively. SiH4 partial pressure was fixed to be 1.0×10−4 Pa. For the Si cap, SiH4 partial pressure was 1.0×10−4 Pa and deposition rate was 0.33 nm/min. Here, all the Si-Ge alloy was intentionally undoped. Crystallinity of the Si-Ge alloy or Si films was ex-situ evaluated by reflection high-energy electron diffraction (RHEED) patterns obtained in a high-vacuum electron beam irradiation system with an electron acceleration voltage of 16 kV (electron wavelength 9.7 pm) after sample transfer within 5 min in cleanroom air. Deposited thickness was measured by atomic force microscope after lift-off technique for thin film deposition on the partially SiO2-covered Si(100). Ge fraction of the deposited Si-Ge alloy film was determined by photoelectron intensities from core levels of Si 2p and Ge 3d measured from X-ray photoelectron spectroscopy (XPS) (Kratos Analytical: AXIS Nova) under consideration of sensitivity difference with a monochromatized X-ray of Al Kα line at 1487 eV and a take-off angle for photoelectron of 90° (perpendicular to the surface) without any surface treatment (such as ion beam etching). Absolute values of binding energy were corrected typically by the peak position for Au 4f (83.98 eV) and Cu 2p (932.66 eV). In order to extract a photoelectron spectrum only for a valence band of a Si cap near the heterointerface, “Si0.4Ge0.6 signal 2
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low-energy ECR Ar plasma CVD without substrate heating. This point of view is quite important for device application based on a use of strained-layer epitaxy on a perfect crystalline Si wafer. On the other hand, in the case of thicker strained Si0.4Ge0.6 films than 24 nm, broadening of diffraction peak, increase in an in-plane lattice constant and decrease in a vertical lattice constant are observed, while the lineshape diffraction peak becomes weak. These results indicates that strain relaxation proceeds in the deposited strained Si0.4Ge0.6 films and its critical thickness is around 20 nm for strained Si0.4Ge0.6 on Si(100) which is not so different from the reported value for high-temperature epitaxy [11,30–32].. Strained Si0.4Ge0.6 thickness dependence of FTIR-ATR spectra of Si/strained Si0.4Ge0.6/Si(100) heterostructures is shown in Fig. 3. The reported wavenumbers for FTIR absorbance by Si-hydride [33] and Ge-hydride [34] are useful to analyze behavior of hydrogen atoms. Especially in the range of 1980–2060 cm−1, absorbance tends to increase with the Si0.4Ge0.6 thickness and decrease by the heat treatment at 400 °C. It has been known that hydrogen atoms bonded to a Ge atom can be rapidly desorbed at 400 °C [35]. Additionally, from the fact that reduction of absorbance by the heat treatment is small in the range of 2100–2140 cm−1 and that hydrogen atoms bonded to a Si atom as dihydride (Si-H2) and trihydride (Si-H3) can be rapidly desorbed at 400 °C [36], hydrogen atoms might tend to remain stably as Si monohydride (Si-H). Moreover, STM images have clarified that Si dihydride can be a dominant structure on the DHF treated Si(100) [37]
cleanroom air, Al (150 nm) was deposited by RF sputtering without substrate heating. Here, it should be noted again that in-situ surface cleaning was not performed intentionally before all the film deposition. Current-voltage characteristics of the p+Si/nSi0.5Ge0.5 heterojunction diode were measured using the 4PP resistivity measurement system described above in order to neglect a large voltage drop at a highresistivity contact between a pin metal and nSi0.5Ge0.5 or nSi due to the use of simplified fabrication process. 3. Results and discussion Rocking curves for (400) and XRD-RSMs for (422) of Si/strained Si0.4Ge0.6/Si(100) heterostructures are shown in Fig. 2. In the case of a strained Si0.4Ge0.6 film as thin as 12 nm, a clear diffraction peak in Fig. 2(a) and a narrow-width line-shape diffraction peak along a vertical axis in Fig. 2(b) can be clearly observed at Qx=5.208 nm−1 and Qy=7.00−7.15 nm−1. Because a value of √(22+22)/Qx=0.543 nm corresponds to an in-plane (horizontal) lattice constant, it is obvious that an in-plane (horizontal) lattice constant is well aligned with that of unstrained Si(100). Additionally, the above line-shape diffraction peak is located near the calculated vertical lattice constant for strained Si0.4Ge0.6 on Si(100) rather than that for relaxed Si0.4Ge0.6. These results indicate that a compressively-strained Si0.4Ge0.6 is epitaxially grown under lattice matching with Si(100). Moreover, this implies that dislocation generation is well suppressed in epitaxial growth by our
Fig. 2. (a) Rocking curves of X-ray diffraction for (400) and (b)–(d) XRD-RSMs for (422) measured from the Si/strained Si0.4Ge0.6/Si(100) heterostructures with strained Si0.4Ge0.6 thickness of 12 nm, 24 nm and 36 nm. Si cap thickness of 10 nm.
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also for strained (crystalline) Si0.4Ge0.6 and relaxed (crystallinitydegraded) Si0.4Ge0.6, crystallinity degradation is expected as an origin of the change in spectra. Unfortunately, despite of apparent structural changes, the strain relaxation in strained Si0.4Ge0.6 seems to induce small influence on the band discontinuity amount, and there could be difficulty (e.g. band bending effect) in detecting an expected band discontinuity roughly around 0.4 eV in our XPS measurements... PL spectra of Si/strained Si0.4Ge0.6/p-type Si(100) heterostructures with a thicker strained Si0.4Ge0.6 thickness are shown in Fig. 6. Especially in the case of strained Si0.4Ge0.6 thickness at 36 nm, a broad PL peak can be clearly observed and the peak disappeared after heat treatment at 400 °C. Because the reduction of FTIR absorption related to Ge-H bonds has been confirmed after the heat treatment as described above, it is considered that aggregates of hydrogen-passivated defects might act as a radiative recombination center and the peak wavelength might shift dependently on the microstructure (i.e. grain size and crystal/amorphous fraction) similarly to a case of hydrogenated-amorphous or microcrystalline Ge [41] and that radiative recombination was suppressed in the epitaxial region of unrelaxed strained Si0.4Ge0.6 for some reasons (e.g. n-type doping against p-type substrate and/or high carrier mobility as indicated later in Figs. 8 and 9). Moreover, PL spectra for Si/strained Si0.4Ge0.6 (3 nm)/buffer Si/ptype Si(100) are shown in Fig. 7. By increasing Si cap thickness, it is found that a PL peak from Si substrate (around 1140 nm [42]) tends to be decreased, and additional components at 1500–1800 nm can be observed. Combining the results in Figs. 6 and 7, it can be considered that PL is weak in the strained Si0.4Ge0.6 without strain relaxation. From the fact that the strained Si0.5Ge0.5 deposited by our ECR-plasma CVD apparatus was confirmed to be n-type as shown later, there is a possibility that the weakness of PL is originated from carrier separation in the depletion region of p/n junction between n-type Si0.5Ge0.5 and ptype Si(100). Such luminescence at a longer wavelength seems reasonable as an expected radiative recombination of generated electron-hole
Fig. 3. Strained Si0.4Ge0.6 thickness dependence of FTIR-ATR spectra of Si/strained Si0.4Ge0.6/Si(100) heterostructures. Si0.4Ge0.6 thickness was (a) 12 nm, (b) 24 nm and (c) (d) 36 nm. Si cap thickness was 7.2 nm. Sample (d) was measured after heat treatment at 400 °C in N2 atmosphere for 30 min. Measurements were done at room temperature.
and areal density of hydrogen atoms can be determined roughly about 1.4×1015 cm−2. Therefore, the spectrum of the DHF-treated Si(100) can be used as a reference to evaluate hydrogen concentration in the films. From comparison of peak area with that of the DHF-treated Si(100), average hydrogen atom concentration is estimated to be as low as below 1 at%. However, differential peak area between (b) and (c) corresponds to around 2 at% and is apparently higher than that between (a) and (b) ( < 1 at%). Therefore, it is clear that hydrogen incorporation is enhanced by strain relaxation, and this implies a possibility that hydrogen atoms tend to be incorporated at crystal defects generated by strain relaxation.. For different strained Si0.4Ge0.6 thickness, XPS spectra for valence bands are shown in Figs. 4 and 5. Valence-band structures have been reported by calculation and seem to be similar to measured XPS spectra [38]. Especially to evaluate the valence-band structure of strained Si0.4Ge0.6 by XPS (Fig. 4), most of the Si cap was removed by repetition of sub-nanometer thick wet Si-oxide formation (in a H2SO4 and H2O2 solution mixture) and the Si-oxide removal (in a dilute-HF solution). For larger strained Si0.4Ge0.6 thickness, it is clear that the valence-band structure of strained Si0.4Ge0.6 is complicatedly changed with strain relaxation in strained Si0.4Ge0.6. Similarly, valenceband structure of the Si cap near the heterointerface (Fig. 5) is also changed and this is considered to be due to crystallinity degradation induced by the strain relaxation in strained Si0.4Ge0.6 in relation to difference of valence band structures between crystalline Si and hydrogenated-amorphous Si [39,40], in which binding energies for typical peaks (2–2.5, 3, 7.5, 9.0 and 11 eV) differ dependently on crystallinity. From such a view point, between the Si0.4Ge0.6 spectrum (Fig. 4(c)) and that of the Si cap (Fig. 5(c)), there is similarity typically in the peak position of around 3 eV and 9 eV. Therefore, at present,
Fig. 4. XPS spectra for valence band of strained Si0.4Ge0.6 (after thinning Si cap) in the Si/strained Si0.4Ge0.6/Si(100) heterostructures. Si0.4Ge0.6 thickness was (a) 12 nm, (b) 24 nm and (c) 36 nm. Initial Si cap thickness was 7.2 nm and thinned down to 0–0.4 nm by wet etching.
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Fig. 7. Si cap thickness dependence of PL spectra of Si/strained Si0.4Ge0.6/buffer Si/ Si(100) heterostructures. Thickness for Si buffer and strained Si0.4Ge0.6 was 10 nm and 3 nm, respectively. Measurements were done at 8 K.
Fig. 5. XPS spectra for valence band of Si cap (after thinning Si cap and “Si0.4Ge0.6 signal subtraction” noted in “Section 2”) in the Si/strained Si0.4Ge0.6/Si(100) heterostructures. Si0.4Ge0.6 thickness was (a) 12 nm, (b) 24 nm and (c) 36 nm. Initial Si cap thickness was 7.2 nm and thinned down to 1.5–3 nm by wet etching.
Fig. 8. Electrical characteristics by 4PP-resistivity measurement and Hall-effect measurement for a 11 nm-thick strained Si0.5Ge0.5 on Si(100). Measurements were done at room temperature. For calculation of carrier concentration n, a factor of 3π/8 was used because n was apparently much lower than a degenerated level around 1019 cm−3.
band bending effect near the surface as well as quantum-confinement effect... From electrical characteristics obtained by the 4PP-resistivity measurement and Hall-effect measurement (Fig. 8), it is clarified that the 11 nm-thick intentionally-undoped strained Si0.5Ge0.5 film on Si(100) is n-type with carrier concentration of 1.3×1018 cm−3 and carrier mobility as high as 660 cm2 V−1 s−1. The mobility value seems to be at a reasonable and comparable level with the drift mobility of unstrained Si and Ge (300–400 cm2 V−1 s−1 and 2000 cm2 V−1 s−1, respectively, for 1018 cm−3 at room temperature [43]) even with consideration of effects by strain and alloy scattering (typical mobility of 400 cm2 V−1 s−1 for undoped Si0.5Ge0.5) [44]. Although origin of slightly high carrier concentration of 1.3×1018 cm−3 is not yet clear, a few candidates which will act as a donor can be imagined, e.g.
Fig. 6. Strained Si0.4Ge0.6 thickness dependence of PL spectra of Si/strained Si0.4Ge0.6/ Si(100) heterostructures. Strained Si0.4Ge0.6 thickness was (a) 12 nm, (b) 24 nm and (c) (d) 36 nm. Si cap thickness was 7.2 nm. Sample (d) was measured after heat treatment at 400 °C in N2 atmosphere for 30 min. Measurements were done at 8 K.
pairs at strained Si0.4Ge0.6 with narrower bandgap than unstrained Si. However, the wavelength shift is still difficult to be imagined only due to increase of Si cap thickness. And, towards electronic and photonic device application, further investigations are needed to clarify detailed origin of the PL and relationship with electronic band structures or 5
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on Si(100) can be applicable effectively to high-mobility channel of field-effect transistor and Esaki tunnel junction with high carrier concentration above 1019 cm−3.. Current-voltage characteristics of a p+Si/nSi0.5Ge0.5 heterojunction diode fabricated on nSi(100) were shown in Fig. 9. In the reverse bias condition (voltage < 0), a leakage current is around 10−6−10−5 A cm−2
crystallographic defects, residual N2 gas in the reactor and/or oxygen [45] possibly come from a quartz cover inside of the plasma generation chamber. These are issues for improving our ECR plasma CVD process towards establishment as a universal tool of epitaxy for semiconductor device fabrication. On the other hand, because the carrier mobility is still high fortunately, it can be concluded that our strained Si-Ge alloy
Fig. 9. (a) Fabrication process, (b) 4PP electrical measurement setup and (c) current-voltage characteristics of a p+Si/nSi0.5Ge0.5 heterojunction diode fabricated on n Si(100) (red filled square). Measurements were done at room temperature. Characteristics for a diode without Si0.5Ge0.5 (blue open square) is also shown as a reference. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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effectively for various semiconductor devices utilizing high carrier mobility, built-in potential by doping and band engineering.
and it seems sufficiently low if it is used in small-dimension devices such as MOSFET or HBT, while much lower leakage current around 10−10−10−8 A cm−2 has been successfully observed by using relatively high-temperature and careful junction fabrication process [46–48]. Especially in a lower voltage region (0 to −0.6 V), the current is much larger than that for Si p+/n junction diode. This is considered to be due to narrower bandgap of nSi0.5Ge0.5. Moreover, in a higher voltage region above −1V, the current is almost the same level as that for Si p+/ n junction diode. This is considered to be due to much higher carrier concentration in nSi0.5Ge0.5 than that in the nSi substrate, which results in a thinner depletion width in nSi(100). In addition to generation current through crystal defects, there is a possibility that carrier generation through surface states on the sidewall [49] is enhanced, because no passivation film is formed after etching of the B-doped Si into a mesa shape. By addition of sidewall passivation as well as improvement of epitaxial quality, suppression of the recombination and generation currents might be expected. Therefore, development of insulator deposition method without substrate heating is another concern for us. In the forward bias condition (voltage > 0), for both the cases with and without nSi0.5Ge0.5, it is clear that current tends to increase exponentially with voltage and the slope is almost ideal (60 mV/decade shown by dotted lines). Looking at a detail for the case with nSi0.5Ge0.5, especially in the smaller current level around 10−6 A cm−2, measured current seems to deviate slightly from the ideal case (dotted line), and this indicates that recombination current becomes dominant relatively by reducing carrier injection across the junction [50]. However, the recombination current around 10−6 A cm−2 is negligible small due to large current enhancement by introducing nSi0.5Ge0.5 having narrower bandgap, which is considered to be the same effect as higher injection efficiency at emitter/base junction of HBT [51]. The current enhancement can be also expressed by a lower voltage shift of 0.19 V from the reference without nSi0.5Ge0.5. Because a built-in potential between the junction is considered to become about 0.2 eV larger due to larger carrier concentration in nSi0.5Ge0.5 than nSi(100), an effective bandgap narrowing can be estimated to be about 0.4 eV as a sum of these values. This value is almost the same as the calculated value and is much larger than a bandgap difference between relaxed Si0.5Ge0.5 and relaxed Si (around 0.2 eV [52]). This implies that keeping strain in the Si-Ge alloy on Si is quite important to introduce a larger electronic effect by bandgap narrowing and/or band-edge discontinuity. In this way, it is concluded that crystal defect generation can be negligibly small due to superior current enhancement which is effectively caused by high-quality heterojunction formation of highly-strained Si0.5Ge0.5 and unstrained Si. From the fact that in-situ surface cleaning was not performed intentionally before all the film deposition, we suggest that our Ar plasma enhanced CVD without substrate heating has a potential to make a part of semiconductor device fabrication more simplified and lower cost by eliminating high-temperature processing apparatus, waiting time for heat-up and cool-down, and cross contamination problem which is caused by thermal diffusion..
Acknowledgments This study was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan, the Cooperative Research Project Program of the Research Institute of Electrical Communication, Tohoku University and the Japan Society for the Promotion of Science (JSPS) Core-toCore Program, A. Advanced Research Networks “International Collaborative Research Center on Atomically Controlled Processing for Ultralarge Scale Integration”. We are grateful to Mr. Koichi Seo and Dr. Shuji Kusano (PANalytical Japan, Spectris Co., Ltd.) for their ultrafast XRD-RSM measurements using X’Pert3 MRD diffractometer with 3D detection system (PIXcel3D) and useful advice on our rocking-curve measurements using conventional X’Pert PRO MRD diffractometer. References [1] S.-Y. Chung, R. Yu, N. Jin, S.-Y. Park, P.R. Berger, P.E. Thompson, IEEE Electron Dev. Lett. 27 (2006) 364. [2] A. Ramesh, P.R. Berger, R. Loo, Appl. Phys. Lett. 100 (2012) 092104. [3] M. Oehme, Thin Solid Films 520 (2012) 3341. [4] A.M. Ionescu, H. Riel, Nature 479 (2011) 329. [5] Y. Morita, T. Mori, S. Migita, W. Mizubayashi, A. Tanabe, K. Fukuda, T. Matsukawa, K. Endo, S. O’uchi, Y.X. Liu, M. Masahara, H. Ota, IEEE Electron Dev. Lett. 35 (2014) 792. [6] Y. Morita, K. Fukuda, T. Mori, W. Mizubayashi, S. Migita, K. Endo, S. O’uchi, Y. Liu, M. Masahara, T. Matsukawa, H. Ota, Jpn. J. Appl. Phys. 55 (2016) 04EB06. [7] M. Kim, Y.K. Wakabayashi, M. Yokoyama, R. Nakane, M. Takenaka, S. Takagi, IEEE Trans. Electron Dev. 62 (2015) 9. [8] R. Ito, M. Sakuraba, J. Murota, Semicond. Sci. Technol. 22 (2007) S38. [9] T. Seo, K. Takahashi, M. Sakuraba, J. Murota, Solid-State Electron 53 (2009) 912. [10] K. Takahashi, M. Sakuraba, J. Murota, Solid-State Electron 60 (2011) 112. [11] R. People, IEEE J. Quantum Electron (1986) 1696 (QE-22). [12] A.R. Powell, K. Eberl, B.A. Ek, S.S. Iyer, J. Cryst. Growth 127 (1993) 425. [13] H.J. Osten, J. Appl. Phys. 84 (1998) 2716. [14] K. Goto, J. Murota, T. Maeda, R. Schütz, K. Aizawa, R. Kircher, K. Yokoo, S. Ono, Jpn. J. Appl. Phys. 32 (1993) 438. [15] P.M. Garone, V. Venkataraman, J.C. Sturm, IEEE Electron Dev. Lett. (1992) 56 (EDL-13 ). [16] J. Murota, M. Sakuraba, B. Tillack, Jpn. J. Appl. Phys. 45 (2006) 6767. [17] S. Takehiro, M. Sakuraba, T. Tsuchiya, J. Murota, Thin Solid Films 517 (2008) 346. [18] M. Sakuraba, D. Muto, M. Mori, K. Sugawara, J. Murota, Thin Solid Films 517 (2008) 10. [19] M. Sakuraba, K. Sugawara, J. Murota, Key Eng. Mater. 470 (2011) 98. [20] Y. Abe, M. Sakuraba, J. Murota, Thin Solid Films 557 (2014) 10. [21] Y. Abe, S. Kubota, M. Sakuraba, J. Murota, S. Sato, ECS Trans. 58 (9) (2013) 223. [22] (a) N. Ueno, M. Sakuraba, J. Murota, S. Sato, Thin Solid Films 557 (2014) 31; (b) N. Ueno, M. Sakuraba, S. Sato, ECS Trans. 64 (6) (2014) 99. [23] M. Sakuraba, J. Murota, S. Ono, J. Appl. Phys. 75 (1994) 3701. [24] E. Koppensteiner, G. Bauer, H. Kibbel, E. Kasper, J. Appl. Phys. 76 (1994) 3489. [25] A. Ohta, K. Makihara, S. Miyazaki, M. Sakuraba, J. Murota, IEICE Trans. Electron (5) (2013) 680 (E96-C). [26] F.M. Smits, Bell Sys. Tech. J. 37 (1958) 711. [27] E.H. Hall, Am.J. Math, 2 (1879) 287. [28] (a) L.J. van der Pauw, Philips Res. Rep. 13 (1958) 1; (b) L.J. van der Pauw, Philips Tech. Rev. 20 (8) (1958) 220. [29] S.M. Sze K.K. Ng 3rd ed. Physics of Semiconductor Devices 2007 John Wiley & Sons, Inc. Hoboken, New Jersey 33 35. [30] J.C. Bean, L.C. Feldman, A.T. Firoy, S. Nakahara, I.K. Robinson, J. Vac. Sci. Technol. A 2 (1987) 436. [31] R. People, J.C. Bean, Appl. Phys. Lett. 47 (1985) 322 (Erratum; 49 (1986) 229). [32] E. Kasper, S. Heim, Appl. Surf. Sci. 224 (2004) 3. [33] Y.J. Chabal, G.S. Higashi, K. Raghavachari, V.A. Burrows, J. Vac. Sci. Technol. A 7 (1989) 2104. [34] J.E. Crowell, G.Q. Lu, J. Electron Spectrosc. Relat. Phenom. 54/55 (1990) 1045. [35] L. Papagno, X.Y. Shen, J. Anderson, G. Schirripa Spagnolo, G.J. Lapeyre, Phys. Rev. B 34 (1986) 7188. [36] P. Gupta, V.L. Colvin, S.M. George, Phys. Rev. B 37 (1988) 8234. [37] Y. Morita, H. Tokumoto, Appl. Phys. Lett. 67 (1995) 2654. [38] J.R. Chelikowsky, M.L. Cohen, Phys. Rev. 14 (1967) 556. [39] W.B. Jackson, S.M. Kelso, C.C. Tsai, J.W. Allen, S.-J. Oh, Phys. Rev. B 31 (1985) 5187. [40] T.M. Hayes, J.W. Allen, J.L. Beeby, S.-J. Oh, Proceedings of the 17th International Conference on the Physics of Semiconductors, 1985, p.791. [41] (a) S. Ishii, M. Kurihara, T. Aoki, K. Shimakawa, J. Singh, J. Non-Cryst. Solids 266 (2000) 721; (b) S. Kobayashi, T. Shimizu, T. Aoki, T. Asakawa, J. Mat. Sci.: Mater. Electron. 14
4. Conclusions By using our low-energy Ar plasma enhanced CVD at a substrate temperature below 100 °C during plasma exposure without substrate heating, modulation of valence band structures and infrared photoluminescence can be observed by change of strain in a Si/strained Si0.4Ge0.6/Si(100) heterostructure. For the strained Si0.5Ge0.5 film, Hall mobility at room temperature was confirmed to be as high as 660 cm2 V−1 s−1 with a carrier concentration of 1.3×1018 cm−3 for ntype carrier, although the carrier origin was unclear. Moreover, good rectifying characteristics were obtained for p+Si/nSi0.5Ge0.5 heterojunction diode. This indicates that the strained Si-Ge alloy and Si films and their heterostructures epitaxially grown by our low-energy Ar plasma enhanced CVD without substrate heating can be applicable 7
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Electronic properties of Si/Si-Ge Alloy/Si(100) heterostructures formed by ECR Ar plasma CVD without substrate heating ⁎
Naofumi Ueno, Masao Sakuraba , Yoshihiro Osakabe, Hisanao Akima, Shigeo Sato Laboratory for Nanoelectronics and Spintronics, Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 9808577, Japan
A R T I C L E I N F O
A BS T RAC T
Keywords: Plasma chemical vapor deposition Heteroepitaxial growth Silicon Silicon-germanium alloy Valence band pn junction diode
By using our low-energy Ar plasma enhanced chemical vapor deposition (CVD) at a substrate temperature below 100 °C during plasma exposure without substrate heating, modulation of valence band structures and infrared photoluminescence can be observed by change of strain in a Si/strained Si0.4Ge0.6/Si(100) heterostructure. For the strained Si0.5Ge0.5 film, Hall mobility at room temperature was confirmed to be as high as 660 cm2 V−1 s−1 with a carrier concentration of 1.3×1018 cm−3 for n-type carrier, although the carrier origin was unclear. Moreover, good rectifying characteristics were obtained for a p+Si/nSi0.5Ge0.5 heterojunction diode. This indicates that the strained Si-Ge alloy and Si films and their heterostructures epitaxially grown by our lowenergy Ar plasma enhanced CVD without substrate heating can be applicable effectively for various semiconductor devices utilizing high carrier mobility, built-in potential by doping and band engineering.
1. Introduction In the field of next-generation Si large-scale integrated circuits (LSIs) development, lowering power consumption is increasingly required and will be achieved by highly-integrated transistors, e.g. metal-oxide-semiconductor field-effect transistor (MOSFET) and heterojunction bipolar transistor (HBT), as well as efficient circuit configuration by novel functional devices; e.g. Esaki-tunnel diode and transistor with lower subthreshold swing [1–7] and resonant-tunneling device with intrinsic negative differential conductance [8–10]. Therefore, towards human-friendly society based on world-wide information and communication technology with less energy, efforts to develop new types of quantum-effect emphasized device as described above and to integrate them onto conventional Si LSIs are expected to be valuable. For realization of Si-based quantum-effect emphasized devices, additionally to control of doping, electronic band modulation using strained Si-Ge-C alloy/Si heterostructures is a key technology [11–13]. Low-temperature process is necessary for fabricating high quality heterostructures because suppressed intermixing at heterointerface is indispensable [14–17]. In such a trend, electron-cyclotron-resonance (ECR) Ar plasma chemical-vapor deposition (CVD) process at a substrate temperature below 100 °C during plasma exposure without substrate heating is one of the low-temperature deposition processes and epitaxial growth of strained/unstrained films of Si, strained Si-Ge
⁎
alloy and Ge without substrate heating has been achieved in our group [18–22]. In this paper, for acquisition to control quantum-effect phenomena by heterostructures in the Si-based semiconductor devices, we report characterization results of electronic band structures and electronic properties of Si/strained Si1−xGex (x=0.5–0.6)/Si(100) heterostructures grown by the ECR Ar plasma CVD process. Here, by utilizing perfect Si crystal and lattice matching to it, high-quality single-crystalline heterostructure can be integrated on Si-LSIs and it will provide quantum-effect phenomena (e.g. interband and/or intraband tunneling) in addition to a use of excellent physical and electrical properties of Si crystal. Typically, Ge fraction of around 0.5–0.6 with thickness up to around 10 nm is expected to be practically applicable from a view point of trade-off relation between band gap narrowing and critical thickness. 2. Experimental methods By using an ECR Ar plasma CVD (Fig. 1) without substrate heating [18], Si/strained Si1−xGex (x=0.5–0.6)/Si(100) heterostructures were deposited on Si(100) by reaction of GeH4 and SiH4 under low-energy Ar plasma irradiation as follows:. Typically, substrates used were partially SiO2 covered p-type or ntype Si(100) wafers and they were treated in a few % dilute-HF (DHF) solution to remove the native oxide and rinsed with deionized water just before loading into the reactor chamber. By the DHF treatment, it
Corresponding author. E-mail address: [email protected] (M. Sakuraba).
http://dx.doi.org/10.1016/j.mssp.2016.09.035 Received 18 July 2016; Received in revised form 21 September 2016; Accepted 27 September 2016 Available online xxxx 1369-8001/ © 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: Ueno, N., Materials Science in Semiconductor Processing (2016), http://dx.doi.org/10.1016/j.mssp.2016.09.035
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subtraction” was performed as follows: When a photoelectron count rate for a certain binding energy in a valence band and a peak area of photoelectron spectrum for a core level of Ge 3d (around 29−30 eV) are defined as IVB(d) and AGe(d) for Si0.4Ge0.6/Si(100) with a d nm-thick Si cap, the photoelectron spectrum only for a valence band of the Si cap is simply calculated by using an equation of IVB(d)−{AGe(d) ⁄ AGe(0)}×IVB(0) for each binding energy under consideration of photoelectron decay characteristics in the Si cap. In order to determine lattice constants of Si0.4Ge0.6 and lattice matching with Si(100) or degree of strain relaxation, rocking curve of X-ray diffraction (XRD) for (400) from the Si/strained Si0.4Ge0.6/ Si(100) heterostructures was measured by X’Pert PRO MRD diffractometer (PANalytical Japan, Spectris Co., Ltd.). Furthermore, XRD reciprocal space map (XRD-RSM) for (422) [24] was measured by using X’Pert3 MRD diffractometer with 3D detection system (PIXcel3D) (PANalytical Japan, Spectris Co., Ltd.) with high-incidence asymmetric-reflection technique for improvement of effective resolution (X-ray wavelength of 0.154 nm (Cu Kα1 line)). In order to evaluate residual H atoms, from Si cap/Si-Ge alloy/Si(100) heterostructures, Fourier-transform infrared (FTIR) absorbance spectra were taken by an attenuated-total reflection (ATR) method with a single-reflection type prism of Ge crystal. Electronic band structures of Si/strained Si0.4Ge0.6/p-type Si(100) heterostructure was evaluated by means of XPS (for valence band [25]) and photoluminescence (PL; Horiba Jobin Yvon PL-F: a PbS detector (thermoelectrically-cooled at −15 °C) with a chopper and a lock-in amplifier). Here, PL at 8 K was excited by Ar+ ion laser light (515 nm) irradiation with a power of about 100 mW. Especially for a 11 nm-thick strained Si0.5Ge0.5 film with no intentional doping, resistivity measurement by a four-point probe (4PP) and Hall-effect measurement at room temperature were performed to determine carrier type (n or p), carrier concentration and carrier mobility. Especially for these electrical measurements, a p-type Si-on-insulator (SOI) substrate (Shin-Etsu Handotai Co., Ltd.) was used to reduce parallel conduction in the substrate material. Here, the SOI substrate has a 88 nm-thick p-type Si(100) with resistivity of 9– 18 Ω cm (sheet resistivity 1–2 MΩ/square, carrier concentration < 1015 cm−3) as a top Si layer and a 145 nm-thick SiO2 as a buried oxide layer on a thick p-type Si(100) substrate. In the 4PP resistivity measurement, a 4PP probe with pin-to-pin distance of 1 mm and pin-top radius of 150 µm (Kyowa Riken; K-89PS150 and K-504RB) was used and resistivity was calculated with a correction factor of 4.53 [26]. In the Hall-effect measurement, Hall voltage was measured by van der Pauw method [27–29] for 12 mm×12 mm square-shape sample with another type of hand-made 4PP probe (Au-plated BeCu pins with inner spring pressure of 108 g and pin-to-pin distance of 10 mm and magnetic flux of 0.25 T with double Neodymium magnets). For both of measurements, current and voltage were measured by a 2channel digital multimeter (ADC Corp. 7352 A). Finally, Hall mobility was calculated using the carrier concentration and the resistivity obtained by the 4PP-resistivity measurements. A p+Si/nSi0.5Ge0.5 heterojunction diode was fabricated by a simplified process, i.e. Al wet etching followed by B-doped Si wet etching with single resist pattern (6 mm diameter) as shown later in Fig. 9. As a substrate, n-type Si(100) wafer (1–3 Ω cm, carrier concentration around 1015 cm−3) was used. After DHF treatment for native oxide removal on the substrate and transferring through cleanroom air, an intentionally-undoped Si0.5Ge0.5 film (11 nm-thick, the Hall-effect measurement shows “n-type” as clarified later) was deposited. And then, after transferring the substrate in cleanroom air to another ECR CVD chamber, a B-doped Si film (11 nm-thick, hole concentration 7×1019 cm−3) was deposited in the same manner as the Si0.5Ge0.5 deposition. Here, partial pressures for SiH4, B2H6 and H2 were 1.0×10−4 Pa, 1.0×10−5 Pa and 1.0×10−3 Pa, respectively, microwave power 200 W and deposition rate 1.1 nm/min. Then, after DHF treatment for native oxide removal and transferring the substrate in
Ultraclean Gas Supply System Ar Microwave (2.45 GHz) Quartz Window Magnetic Coil
Substrate
SiH4 GeH4 (Ar or He Diluted) B2H6 (H2 Diluted)
ECR Plasma Chamber N2
Reactor Chamber Turbo Molecular Pump
Dry Pump
Susceptor
N2 Purged Transfer Chamber
Fig. 1. Schematic of ECR Ar plasma CVD system without substrate heating.
is well known that a Si atom on top of Si(100) surface is terminated by a few hydrogen atoms and the surface is effectively protected from native oxide formation at room temperature [23]. To minimize air contamination into the reactor chamber and to enhance desorption of water molecules physically-adsorbed on the substrate, wafer loading were performed through a N2 purged transfer chamber combined with a gate valve. After wafer loading and evacuating, Ar gas was continuously supplied into a plasma generation chamber at a partial pressure of 2.1 Pa and then reactant gases of GeH4 and SiH4 (diluted by He or Ar) were directly introduced into the reactor chamber. Subsequently, microwave (2.45 GHz, 200 W) was supplied through a quartz window on top of the plasma generation chamber and electroncyclotron resonance was induced at a specific magnetic field of 0.0875 T. Especially to avoid plasma damage and intermixing at heterointerfaces, a low-energy ECR Ar plasma condition with an Ar pressure as high as a few Pa is important to obtain peak ion energy as low as a few eV. Substrate temperature was suppressed below 100 °C during plasma exposure even for a few hundred seconds in the present condition up to 200 W [18]. Here, it should be noted that in-situ surface cleaning was not performed intentionally before deposition. For the Si-Ge alloy with Ge fraction (x) of 0.5 and 0.6, GeH4 partial pressure was 0.8×10−4 Pa and 1.0×10−4 Pa and deposition rate was 0.73 nm/min and 0.80 nm/min, respectively. SiH4 partial pressure was fixed to be 1.0×10−4 Pa. For the Si cap, SiH4 partial pressure was 1.0×10−4 Pa and deposition rate was 0.33 nm/min. Here, all the Si-Ge alloy was intentionally undoped. Crystallinity of the Si-Ge alloy or Si films was ex-situ evaluated by reflection high-energy electron diffraction (RHEED) patterns obtained in a high-vacuum electron beam irradiation system with an electron acceleration voltage of 16 kV (electron wavelength 9.7 pm) after sample transfer within 5 min in cleanroom air. Deposited thickness was measured by atomic force microscope after lift-off technique for thin film deposition on the partially SiO2-covered Si(100). Ge fraction of the deposited Si-Ge alloy film was determined by photoelectron intensities from core levels of Si 2p and Ge 3d measured from X-ray photoelectron spectroscopy (XPS) (Kratos Analytical: AXIS Nova) under consideration of sensitivity difference with a monochromatized X-ray of Al Kα line at 1487 eV and a take-off angle for photoelectron of 90° (perpendicular to the surface) without any surface treatment (such as ion beam etching). Absolute values of binding energy were corrected typically by the peak position for Au 4f (83.98 eV) and Cu 2p (932.66 eV). In order to extract a photoelectron spectrum only for a valence band of a Si cap near the heterointerface, “Si0.4Ge0.6 signal 2
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low-energy ECR Ar plasma CVD without substrate heating. This point of view is quite important for device application based on a use of strained-layer epitaxy on a perfect crystalline Si wafer. On the other hand, in the case of thicker strained Si0.4Ge0.6 films than 24 nm, broadening of diffraction peak, increase in an in-plane lattice constant and decrease in a vertical lattice constant are observed, while the lineshape diffraction peak becomes weak. These results indicates that strain relaxation proceeds in the deposited strained Si0.4Ge0.6 films and its critical thickness is around 20 nm for strained Si0.4Ge0.6 on Si(100) which is not so different from the reported value for high-temperature epitaxy [11,30–32].. Strained Si0.4Ge0.6 thickness dependence of FTIR-ATR spectra of Si/strained Si0.4Ge0.6/Si(100) heterostructures is shown in Fig. 3. The reported wavenumbers for FTIR absorbance by Si-hydride [33] and Ge-hydride [34] are useful to analyze behavior of hydrogen atoms. Especially in the range of 1980–2060 cm−1, absorbance tends to increase with the Si0.4Ge0.6 thickness and decrease by the heat treatment at 400 °C. It has been known that hydrogen atoms bonded to a Ge atom can be rapidly desorbed at 400 °C [35]. Additionally, from the fact that reduction of absorbance by the heat treatment is small in the range of 2100–2140 cm−1 and that hydrogen atoms bonded to a Si atom as dihydride (Si-H2) and trihydride (Si-H3) can be rapidly desorbed at 400 °C [36], hydrogen atoms might tend to remain stably as Si monohydride (Si-H). Moreover, STM images have clarified that Si dihydride can be a dominant structure on the DHF treated Si(100) [37]
cleanroom air, Al (150 nm) was deposited by RF sputtering without substrate heating. Here, it should be noted again that in-situ surface cleaning was not performed intentionally before all the film deposition. Current-voltage characteristics of the p+Si/nSi0.5Ge0.5 heterojunction diode were measured using the 4PP resistivity measurement system described above in order to neglect a large voltage drop at a highresistivity contact between a pin metal and nSi0.5Ge0.5 or nSi due to the use of simplified fabrication process. 3. Results and discussion Rocking curves for (400) and XRD-RSMs for (422) of Si/strained Si0.4Ge0.6/Si(100) heterostructures are shown in Fig. 2. In the case of a strained Si0.4Ge0.6 film as thin as 12 nm, a clear diffraction peak in Fig. 2(a) and a narrow-width line-shape diffraction peak along a vertical axis in Fig. 2(b) can be clearly observed at Qx=5.208 nm−1 and Qy=7.00−7.15 nm−1. Because a value of √(22+22)/Qx=0.543 nm corresponds to an in-plane (horizontal) lattice constant, it is obvious that an in-plane (horizontal) lattice constant is well aligned with that of unstrained Si(100). Additionally, the above line-shape diffraction peak is located near the calculated vertical lattice constant for strained Si0.4Ge0.6 on Si(100) rather than that for relaxed Si0.4Ge0.6. These results indicate that a compressively-strained Si0.4Ge0.6 is epitaxially grown under lattice matching with Si(100). Moreover, this implies that dislocation generation is well suppressed in epitaxial growth by our
Fig. 2. (a) Rocking curves of X-ray diffraction for (400) and (b)–(d) XRD-RSMs for (422) measured from the Si/strained Si0.4Ge0.6/Si(100) heterostructures with strained Si0.4Ge0.6 thickness of 12 nm, 24 nm and 36 nm. Si cap thickness of 10 nm.
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also for strained (crystalline) Si0.4Ge0.6 and relaxed (crystallinitydegraded) Si0.4Ge0.6, crystallinity degradation is expected as an origin of the change in spectra. Unfortunately, despite of apparent structural changes, the strain relaxation in strained Si0.4Ge0.6 seems to induce small influence on the band discontinuity amount, and there could be difficulty (e.g. band bending effect) in detecting an expected band discontinuity roughly around 0.4 eV in our XPS measurements... PL spectra of Si/strained Si0.4Ge0.6/p-type Si(100) heterostructures with a thicker strained Si0.4Ge0.6 thickness are shown in Fig. 6. Especially in the case of strained Si0.4Ge0.6 thickness at 36 nm, a broad PL peak can be clearly observed and the peak disappeared after heat treatment at 400 °C. Because the reduction of FTIR absorption related to Ge-H bonds has been confirmed after the heat treatment as described above, it is considered that aggregates of hydrogen-passivated defects might act as a radiative recombination center and the peak wavelength might shift dependently on the microstructure (i.e. grain size and crystal/amorphous fraction) similarly to a case of hydrogenated-amorphous or microcrystalline Ge [41] and that radiative recombination was suppressed in the epitaxial region of unrelaxed strained Si0.4Ge0.6 for some reasons (e.g. n-type doping against p-type substrate and/or high carrier mobility as indicated later in Figs. 8 and 9). Moreover, PL spectra for Si/strained Si0.4Ge0.6 (3 nm)/buffer Si/ptype Si(100) are shown in Fig. 7. By increasing Si cap thickness, it is found that a PL peak from Si substrate (around 1140 nm [42]) tends to be decreased, and additional components at 1500–1800 nm can be observed. Combining the results in Figs. 6 and 7, it can be considered that PL is weak in the strained Si0.4Ge0.6 without strain relaxation. From the fact that the strained Si0.5Ge0.5 deposited by our ECR-plasma CVD apparatus was confirmed to be n-type as shown later, there is a possibility that the weakness of PL is originated from carrier separation in the depletion region of p/n junction between n-type Si0.5Ge0.5 and ptype Si(100). Such luminescence at a longer wavelength seems reasonable as an expected radiative recombination of generated electron-hole
Fig. 3. Strained Si0.4Ge0.6 thickness dependence of FTIR-ATR spectra of Si/strained Si0.4Ge0.6/Si(100) heterostructures. Si0.4Ge0.6 thickness was (a) 12 nm, (b) 24 nm and (c) (d) 36 nm. Si cap thickness was 7.2 nm. Sample (d) was measured after heat treatment at 400 °C in N2 atmosphere for 30 min. Measurements were done at room temperature.
and areal density of hydrogen atoms can be determined roughly about 1.4×1015 cm−2. Therefore, the spectrum of the DHF-treated Si(100) can be used as a reference to evaluate hydrogen concentration in the films. From comparison of peak area with that of the DHF-treated Si(100), average hydrogen atom concentration is estimated to be as low as below 1 at%. However, differential peak area between (b) and (c) corresponds to around 2 at% and is apparently higher than that between (a) and (b) ( < 1 at%). Therefore, it is clear that hydrogen incorporation is enhanced by strain relaxation, and this implies a possibility that hydrogen atoms tend to be incorporated at crystal defects generated by strain relaxation.. For different strained Si0.4Ge0.6 thickness, XPS spectra for valence bands are shown in Figs. 4 and 5. Valence-band structures have been reported by calculation and seem to be similar to measured XPS spectra [38]. Especially to evaluate the valence-band structure of strained Si0.4Ge0.6 by XPS (Fig. 4), most of the Si cap was removed by repetition of sub-nanometer thick wet Si-oxide formation (in a H2SO4 and H2O2 solution mixture) and the Si-oxide removal (in a dilute-HF solution). For larger strained Si0.4Ge0.6 thickness, it is clear that the valence-band structure of strained Si0.4Ge0.6 is complicatedly changed with strain relaxation in strained Si0.4Ge0.6. Similarly, valenceband structure of the Si cap near the heterointerface (Fig. 5) is also changed and this is considered to be due to crystallinity degradation induced by the strain relaxation in strained Si0.4Ge0.6 in relation to difference of valence band structures between crystalline Si and hydrogenated-amorphous Si [39,40], in which binding energies for typical peaks (2–2.5, 3, 7.5, 9.0 and 11 eV) differ dependently on crystallinity. From such a view point, between the Si0.4Ge0.6 spectrum (Fig. 4(c)) and that of the Si cap (Fig. 5(c)), there is similarity typically in the peak position of around 3 eV and 9 eV. Therefore, at present,
Fig. 4. XPS spectra for valence band of strained Si0.4Ge0.6 (after thinning Si cap) in the Si/strained Si0.4Ge0.6/Si(100) heterostructures. Si0.4Ge0.6 thickness was (a) 12 nm, (b) 24 nm and (c) 36 nm. Initial Si cap thickness was 7.2 nm and thinned down to 0–0.4 nm by wet etching.
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Fig. 7. Si cap thickness dependence of PL spectra of Si/strained Si0.4Ge0.6/buffer Si/ Si(100) heterostructures. Thickness for Si buffer and strained Si0.4Ge0.6 was 10 nm and 3 nm, respectively. Measurements were done at 8 K.
Fig. 5. XPS spectra for valence band of Si cap (after thinning Si cap and “Si0.4Ge0.6 signal subtraction” noted in “Section 2”) in the Si/strained Si0.4Ge0.6/Si(100) heterostructures. Si0.4Ge0.6 thickness was (a) 12 nm, (b) 24 nm and (c) 36 nm. Initial Si cap thickness was 7.2 nm and thinned down to 1.5–3 nm by wet etching.
Fig. 8. Electrical characteristics by 4PP-resistivity measurement and Hall-effect measurement for a 11 nm-thick strained Si0.5Ge0.5 on Si(100). Measurements were done at room temperature. For calculation of carrier concentration n, a factor of 3π/8 was used because n was apparently much lower than a degenerated level around 1019 cm−3.
band bending effect near the surface as well as quantum-confinement effect... From electrical characteristics obtained by the 4PP-resistivity measurement and Hall-effect measurement (Fig. 8), it is clarified that the 11 nm-thick intentionally-undoped strained Si0.5Ge0.5 film on Si(100) is n-type with carrier concentration of 1.3×1018 cm−3 and carrier mobility as high as 660 cm2 V−1 s−1. The mobility value seems to be at a reasonable and comparable level with the drift mobility of unstrained Si and Ge (300–400 cm2 V−1 s−1 and 2000 cm2 V−1 s−1, respectively, for 1018 cm−3 at room temperature [43]) even with consideration of effects by strain and alloy scattering (typical mobility of 400 cm2 V−1 s−1 for undoped Si0.5Ge0.5) [44]. Although origin of slightly high carrier concentration of 1.3×1018 cm−3 is not yet clear, a few candidates which will act as a donor can be imagined, e.g.
Fig. 6. Strained Si0.4Ge0.6 thickness dependence of PL spectra of Si/strained Si0.4Ge0.6/ Si(100) heterostructures. Strained Si0.4Ge0.6 thickness was (a) 12 nm, (b) 24 nm and (c) (d) 36 nm. Si cap thickness was 7.2 nm. Sample (d) was measured after heat treatment at 400 °C in N2 atmosphere for 30 min. Measurements were done at 8 K.
pairs at strained Si0.4Ge0.6 with narrower bandgap than unstrained Si. However, the wavelength shift is still difficult to be imagined only due to increase of Si cap thickness. And, towards electronic and photonic device application, further investigations are needed to clarify detailed origin of the PL and relationship with electronic band structures or 5
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on Si(100) can be applicable effectively to high-mobility channel of field-effect transistor and Esaki tunnel junction with high carrier concentration above 1019 cm−3.. Current-voltage characteristics of a p+Si/nSi0.5Ge0.5 heterojunction diode fabricated on nSi(100) were shown in Fig. 9. In the reverse bias condition (voltage < 0), a leakage current is around 10−6−10−5 A cm−2
crystallographic defects, residual N2 gas in the reactor and/or oxygen [45] possibly come from a quartz cover inside of the plasma generation chamber. These are issues for improving our ECR plasma CVD process towards establishment as a universal tool of epitaxy for semiconductor device fabrication. On the other hand, because the carrier mobility is still high fortunately, it can be concluded that our strained Si-Ge alloy
Fig. 9. (a) Fabrication process, (b) 4PP electrical measurement setup and (c) current-voltage characteristics of a p+Si/nSi0.5Ge0.5 heterojunction diode fabricated on n Si(100) (red filled square). Measurements were done at room temperature. Characteristics for a diode without Si0.5Ge0.5 (blue open square) is also shown as a reference. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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effectively for various semiconductor devices utilizing high carrier mobility, built-in potential by doping and band engineering.
and it seems sufficiently low if it is used in small-dimension devices such as MOSFET or HBT, while much lower leakage current around 10−10−10−8 A cm−2 has been successfully observed by using relatively high-temperature and careful junction fabrication process [46–48]. Especially in a lower voltage region (0 to −0.6 V), the current is much larger than that for Si p+/n junction diode. This is considered to be due to narrower bandgap of nSi0.5Ge0.5. Moreover, in a higher voltage region above −1V, the current is almost the same level as that for Si p+/ n junction diode. This is considered to be due to much higher carrier concentration in nSi0.5Ge0.5 than that in the nSi substrate, which results in a thinner depletion width in nSi(100). In addition to generation current through crystal defects, there is a possibility that carrier generation through surface states on the sidewall [49] is enhanced, because no passivation film is formed after etching of the B-doped Si into a mesa shape. By addition of sidewall passivation as well as improvement of epitaxial quality, suppression of the recombination and generation currents might be expected. Therefore, development of insulator deposition method without substrate heating is another concern for us. In the forward bias condition (voltage > 0), for both the cases with and without nSi0.5Ge0.5, it is clear that current tends to increase exponentially with voltage and the slope is almost ideal (60 mV/decade shown by dotted lines). Looking at a detail for the case with nSi0.5Ge0.5, especially in the smaller current level around 10−6 A cm−2, measured current seems to deviate slightly from the ideal case (dotted line), and this indicates that recombination current becomes dominant relatively by reducing carrier injection across the junction [50]. However, the recombination current around 10−6 A cm−2 is negligible small due to large current enhancement by introducing nSi0.5Ge0.5 having narrower bandgap, which is considered to be the same effect as higher injection efficiency at emitter/base junction of HBT [51]. The current enhancement can be also expressed by a lower voltage shift of 0.19 V from the reference without nSi0.5Ge0.5. Because a built-in potential between the junction is considered to become about 0.2 eV larger due to larger carrier concentration in nSi0.5Ge0.5 than nSi(100), an effective bandgap narrowing can be estimated to be about 0.4 eV as a sum of these values. This value is almost the same as the calculated value and is much larger than a bandgap difference between relaxed Si0.5Ge0.5 and relaxed Si (around 0.2 eV [52]). This implies that keeping strain in the Si-Ge alloy on Si is quite important to introduce a larger electronic effect by bandgap narrowing and/or band-edge discontinuity. In this way, it is concluded that crystal defect generation can be negligibly small due to superior current enhancement which is effectively caused by high-quality heterojunction formation of highly-strained Si0.5Ge0.5 and unstrained Si. From the fact that in-situ surface cleaning was not performed intentionally before all the film deposition, we suggest that our Ar plasma enhanced CVD without substrate heating has a potential to make a part of semiconductor device fabrication more simplified and lower cost by eliminating high-temperature processing apparatus, waiting time for heat-up and cool-down, and cross contamination problem which is caused by thermal diffusion..
Acknowledgments This study was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan, the Cooperative Research Project Program of the Research Institute of Electrical Communication, Tohoku University and the Japan Society for the Promotion of Science (JSPS) Core-toCore Program, A. Advanced Research Networks “International Collaborative Research Center on Atomically Controlled Processing for Ultralarge Scale Integration”. We are grateful to Mr. Koichi Seo and Dr. Shuji Kusano (PANalytical Japan, Spectris Co., Ltd.) for their ultrafast XRD-RSM measurements using X’Pert3 MRD diffractometer with 3D detection system (PIXcel3D) and useful advice on our rocking-curve measurements using conventional X’Pert PRO MRD diffractometer. References [1] S.-Y. Chung, R. Yu, N. Jin, S.-Y. Park, P.R. Berger, P.E. Thompson, IEEE Electron Dev. Lett. 27 (2006) 364. [2] A. Ramesh, P.R. Berger, R. Loo, Appl. Phys. Lett. 100 (2012) 092104. [3] M. Oehme, Thin Solid Films 520 (2012) 3341. [4] A.M. Ionescu, H. Riel, Nature 479 (2011) 329. [5] Y. Morita, T. Mori, S. Migita, W. Mizubayashi, A. Tanabe, K. Fukuda, T. Matsukawa, K. Endo, S. O’uchi, Y.X. Liu, M. Masahara, H. Ota, IEEE Electron Dev. Lett. 35 (2014) 792. [6] Y. Morita, K. Fukuda, T. Mori, W. Mizubayashi, S. Migita, K. Endo, S. O’uchi, Y. Liu, M. Masahara, T. Matsukawa, H. Ota, Jpn. J. Appl. Phys. 55 (2016) 04EB06. [7] M. Kim, Y.K. Wakabayashi, M. Yokoyama, R. Nakane, M. Takenaka, S. Takagi, IEEE Trans. Electron Dev. 62 (2015) 9. [8] R. Ito, M. Sakuraba, J. Murota, Semicond. Sci. Technol. 22 (2007) S38. [9] T. Seo, K. Takahashi, M. Sakuraba, J. Murota, Solid-State Electron 53 (2009) 912. [10] K. Takahashi, M. Sakuraba, J. Murota, Solid-State Electron 60 (2011) 112. [11] R. People, IEEE J. Quantum Electron (1986) 1696 (QE-22). [12] A.R. Powell, K. Eberl, B.A. Ek, S.S. Iyer, J. Cryst. Growth 127 (1993) 425. [13] H.J. Osten, J. Appl. Phys. 84 (1998) 2716. [14] K. Goto, J. Murota, T. Maeda, R. Schütz, K. Aizawa, R. Kircher, K. Yokoo, S. Ono, Jpn. J. Appl. Phys. 32 (1993) 438. [15] P.M. Garone, V. Venkataraman, J.C. Sturm, IEEE Electron Dev. Lett. (1992) 56 (EDL-13 ). [16] J. Murota, M. Sakuraba, B. Tillack, Jpn. J. Appl. Phys. 45 (2006) 6767. [17] S. Takehiro, M. Sakuraba, T. Tsuchiya, J. Murota, Thin Solid Films 517 (2008) 346. [18] M. Sakuraba, D. Muto, M. Mori, K. Sugawara, J. Murota, Thin Solid Films 517 (2008) 10. [19] M. Sakuraba, K. Sugawara, J. Murota, Key Eng. Mater. 470 (2011) 98. [20] Y. Abe, M. Sakuraba, J. Murota, Thin Solid Films 557 (2014) 10. [21] Y. Abe, S. Kubota, M. Sakuraba, J. Murota, S. Sato, ECS Trans. 58 (9) (2013) 223. [22] (a) N. Ueno, M. Sakuraba, J. Murota, S. Sato, Thin Solid Films 557 (2014) 31; (b) N. Ueno, M. Sakuraba, S. Sato, ECS Trans. 64 (6) (2014) 99. [23] M. Sakuraba, J. Murota, S. Ono, J. Appl. Phys. 75 (1994) 3701. [24] E. Koppensteiner, G. Bauer, H. Kibbel, E. Kasper, J. Appl. Phys. 76 (1994) 3489. [25] A. Ohta, K. Makihara, S. Miyazaki, M. Sakuraba, J. Murota, IEICE Trans. Electron (5) (2013) 680 (E96-C). [26] F.M. Smits, Bell Sys. Tech. J. 37 (1958) 711. [27] E.H. Hall, Am.J. Math, 2 (1879) 287. [28] (a) L.J. van der Pauw, Philips Res. Rep. 13 (1958) 1; (b) L.J. van der Pauw, Philips Tech. Rev. 20 (8) (1958) 220. [29] S.M. Sze K.K. Ng 3rd ed. Physics of Semiconductor Devices 2007 John Wiley & Sons, Inc. Hoboken, New Jersey 33 35. [30] J.C. Bean, L.C. Feldman, A.T. Firoy, S. Nakahara, I.K. Robinson, J. Vac. Sci. Technol. A 2 (1987) 436. [31] R. People, J.C. Bean, Appl. Phys. Lett. 47 (1985) 322 (Erratum; 49 (1986) 229). [32] E. Kasper, S. Heim, Appl. Surf. Sci. 224 (2004) 3. [33] Y.J. Chabal, G.S. Higashi, K. Raghavachari, V.A. Burrows, J. Vac. Sci. Technol. A 7 (1989) 2104. [34] J.E. Crowell, G.Q. Lu, J. Electron Spectrosc. Relat. Phenom. 54/55 (1990) 1045. [35] L. Papagno, X.Y. Shen, J. Anderson, G. Schirripa Spagnolo, G.J. Lapeyre, Phys. Rev. B 34 (1986) 7188. [36] P. Gupta, V.L. Colvin, S.M. George, Phys. Rev. B 37 (1988) 8234. [37] Y. Morita, H. Tokumoto, Appl. Phys. Lett. 67 (1995) 2654. [38] J.R. Chelikowsky, M.L. Cohen, Phys. Rev. 14 (1967) 556. [39] W.B. Jackson, S.M. Kelso, C.C. Tsai, J.W. Allen, S.-J. Oh, Phys. Rev. B 31 (1985) 5187. [40] T.M. Hayes, J.W. Allen, J.L. Beeby, S.-J. Oh, Proceedings of the 17th International Conference on the Physics of Semiconductors, 1985, p.791. [41] (a) S. Ishii, M. Kurihara, T. Aoki, K. Shimakawa, J. Singh, J. Non-Cryst. Solids 266 (2000) 721; (b) S. Kobayashi, T. Shimizu, T. Aoki, T. Asakawa, J. Mat. Sci.: Mater. Electron. 14
4. Conclusions By using our low-energy Ar plasma enhanced CVD at a substrate temperature below 100 °C during plasma exposure without substrate heating, modulation of valence band structures and infrared photoluminescence can be observed by change of strain in a Si/strained Si0.4Ge0.6/Si(100) heterostructure. For the strained Si0.5Ge0.5 film, Hall mobility at room temperature was confirmed to be as high as 660 cm2 V−1 s−1 with a carrier concentration of 1.3×1018 cm−3 for ntype carrier, although the carrier origin was unclear. Moreover, good rectifying characteristics were obtained for p+Si/nSi0.5Ge0.5 heterojunction diode. This indicates that the strained Si-Ge alloy and Si films and their heterostructures epitaxially grown by our low-energy Ar plasma enhanced CVD without substrate heating can be applicable 7
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