- Email: [email protected]
Electrical properties of an epitaxial Si film prepared by RF magnetron plasma at low temperature
Thin Solid Films 475 (2005) 348 – 353 www.elsevier.com/locate/tsf
Electrical properties of an epitaxial Si film prepared by RF magnetron plasma at low temperature Toshifumi Yujia, Youl-Moon Sungb,* b
a Department of Nuclear, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan Department of Electrical Electronic Engineering, University of Miyazaki, 1-1 Gakuenkibanadai Nishi, Miyazaki 889-2192, Japan
Available online 26 August 2004
Abstract This article reports the formation of epitaxial Si film that were formed by directly depositing a Sb-doped n-type epilayer on a p-type substrate by using a DC bais RF magnetron sputter system at a low temperature of 400 8C and a conventional vacuum of 5107 Torr. In addition, the plasma parameters were quantitatively investigated to examine the deposition condition. The electron density (n e) of about 1017 m3 was obtained at the plasma region under the conditions where gas pressure was 3 mTorr, the power of the RF source was 350 W and the electron temperature (Te) and ion saturation current (I 0) were in the range of 3–4 eV and 1–1.5 mA/cm2, respectively. The p–n junction diode fabricated by the Si epitaxy shows, under optimum conditions, a reverse current density (RCD) as low as 9.5106 mA/cm2 at a reverse bias voltage of 5 V and an ideality factor of 1.05. The reverse current density has a good correlation with the crystallinity of the deposited films, which, in turn, depends on deposition gas pressures and substrate biases. D 2004 Elsevier B.V. All rights reserved. Keywords: Si epitaxy; RF magnetron sputtering; Electron temperature; Electron density
1. Introduction Epitaxial Si film formation at low temperature is essential for the fabrication of high-performance Si devices. A low growth temperature (400–700 8C) prevents problems such as autodoping from the substrate and impurity redistribution. To achieve such low temperature film growth, an ultrahigh vacuum environment usually is required, such as ultrahigh vacuum chemical vapor deposition (UHV-CVD) [1], UHV electron-cyclotron-resonance (ECR) CVD [2], molecular-beam epitaxy [3], UHV RF plasma deposition [4] and neutral loop discharge (NLD) plasma [5]. For an industrial process application, however, a conventional vacuum level achievable using a usual vacuum system is much preferred. Unfortunately, epitaxial Si films formed at a low temperature and a conventional vacuum around 107 Torr are still not good enough for the device application. Recently, we have performed a low-temperature process * Corresponding author. Tel.: +81 985 58 7350; fax: +81 985 58 7350. E-mail address: [email protected] (Y.-M. Sung). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.07.037
technique for an epitaxial Si growth by using a sputter method [6]. The RF magnetron plasma has many advantageous features, such as high-density and low-temperature plasma production at low gas pressures of 103 Torr. The addition of magnetron capability to the sputtering system confines the secondary electrons emitted from the target. This trapping of the high-energy electrons reduces the electron bombardment of the film surface and allows better control of the substrate temperature. Using this system, crystallographically perfect single-crystal Si has been produced at a low temperature of 400 8C and a conventional vacuum of 5107 Torr. The next step in determining the suitability of these epilayers for practical Si devices is a thorough electrical characterization because the defects, impurity and interface quality, which are often responsible for the degradation of device characteristics, cannot be showing clearly in the crystallinity evaluation. The characteristics of p–n junctions, which serve as the foundation of semiconductor devices, would show us important information for device fabrication. The reverse bias leakage current is one of the fundamental parameters that determine the
T. Yuji, Y.-M. Sung / Thin Solid Films 475 (2005) 348–353
349
quality of p–n junctions. For epitaxial films fabricated using the above method, the calculated depletion layer widths at a reverse bias voltage of 5 V for the n region and p substrate are 7 2 and 2.3 Am, respectively, indicating that the epilayer–substrate interface is within the depletion layer. Therefore, the p–n junction behavior gives us a clear characterization of the integrity of the epilayer near the interface, as well as the epilayer–substrate interface. The purpose of this article is to report on the study of the currentvoltage (I-V) characteristics of p–n junctions formed by directly depositing an Sb-doped n-type epilayer on a p-type substrate using an RF magnetron sputtering system under the low temperature of 400 8C and conventional vacuum level of 5107 Torr, which shows remarkably good performance in terms of the reverse current density (RCD) and an ideality factor. Fig. 1. Schematic diagram of an RF magnetron sputter system.
2. Experiment The RF magnetron sputter system can be briefly described as follows. The chamber was of stainless steel of internal diameter 300 mm. The RF power ( P rf) of 350 W was supplied to a heavily Sb-doped n-type Si target having a resistivity of 0.01 V cm, which was used for growth of epitaxial layers with high doping concentrations. The target diameter was 100 mm, and the distance between the target and substrate was 150 mm. The DC power sources were applied to the substrate to control the substrate potential independently during the deposition. A temperature controller and heater controlled the substrate temperature. Cooling water is circulated through the target and chamber to prevent overheating during deposition. High ion current densities of more than 1.5 mA/cm2 were obtained in the plasma region at a low Ar gas pressure of 3 mTorr, when the RF power was 350 W. A uniformity of ion current density of F8% was obtained at substrate position over the area having a diameter of 60 mm around the plasma axis. The plasma parameters were quantitatively investigated to examine the process condition. The ion current density I 0 and electron temperature Te and density n e were measured with a double probe method as shown in Fig. 1. The measurement results of the current-voltage characteristics of the double probe and ion energy analyzer (IEA) were recorded with an X–Y plotter and an oscilloscope. A Z-shaped double probe, which minimizes measurement errors from the magnetic field effect, was used to measure the Te, n e and I 0 distributions around the substrate and target regions. The probe tips made of tungsten had a diameter of 0.4 mm and a length of 4 mm and were separated by a piece of ceramic material for electrical insulation. The measurement points were 20 (z=20 mm) and 120 mm (z=120 mm) from the substrate plane (z=0 mm). The Z-axis is the central axis of the chamber. The p–n junctions were fabricated using the following procedures. Bdoped p-type (100) Si wafers having a resistivity of 9–12 V cm were used as substrates. A 300-nm-thick SiO2 film was
formed on the substrate by dry oxidation, and windows having diameters in the ranges of 1.0–2.0 mm were patterned by lithography; then the oxide film in the window areas was etched by buffer HF solution. A standard RCA chemical cleaning then was performed to remove the remaining native oxide in the windows; after that, the sample was dipped in 0.5% HF solution for 30 s to terminate the Si surface by H, which would keep a stable Si surface to prevent the oxide growth. Finally, high-purity nitrogen gas blowing was used to prepare a water-free surface. Following this procedure, the sample was immediately loaded into the vacuum chamber, which was subsequently pumped down to the base pressure of less than 5107 Torr. The sample was kept at a temperature of 400 8C. Hence, an n-type Si film of about 400 nm thick was deposited from the Sb-doped Si target on the substrate by using the RF magnetron sputtering. The deposition time was 15–20 min. The deposited Si film was a single crystal at the window area where the bare Si surface was exposed, while the film on the oxide was amorphous Si. An electron backscattering diffraction measurement has confirmed these results. Amorphous Si film was then patterned to isolate each junction. Al metallization was carried out on both the epitaxial layer and the backside of the substrate to form electrodes. It should be noted that the processing temperature never exceeded 400 8C both during and after the epitaxial growth.
3. Results and discussion Fig. 2(a–c) shows the radial distribution of the measured I 0, n e and Te of the RF magnetron plasma system. The I 0, n e and Te at the substrate region (z=20 mm) were in the range of 0.55–0.7 mA/cm2, 4.61016–6.51016 m3 and 2.5– 3 eV, respectively. The I 0, n e and Te near the target (z=120 mm) were in the range of 0.5–1.7 mA/cm2, 6.51016– 8.81016 m3 and 3.1–4.3 eV, respectively.
350
T. Yuji, Y.-M. Sung / Thin Solid Films 475 (2005) 348–353
Fig. 2. (a) Distribution of the measured I 0 of RF magnetron sputter system. (b) Distribution of the measured Te of the RF magnetron sputter system. (c) Distribution of the measured n e of the RF magnetron sputter system.
The dependence of plasma parameters on the gas pressure and RF power was examined. The I 0, n e and Te near the substrate region (r=0 mm, z=20 mm) were plotted against gas pressure, as shown in Fig. 3(a). The value of n e was increased with the working gas pressure, whereas Te was decreased. Fig. 3(b) shows the I 0, n e and Te near the substrate region (r=0 mm, z=20 mm) as a parameter of RF power. It could be seen from Fig. 3(b) that the measured values were increased with the increase of RF power. The XPS wide spectrum of the Si thin film prepared by RF magnetron sputter system is shown in Fig. 4. The Si (2 s) and Si (2 p) photoelectron peaks were evidently detected at the binding energy of about 151 and 99.7 eV, respectively. An additional peak of C (1 s) on the top of the surface was detected at the binding energy of about 290 eV, and it was a peculiar property of XPS analysis. Fig. 5(a) shows the electron beam scattered diffraction (EBSD) pattern of the sample obtained at a pressure of 2.2 mTorr. Clear Kikuchi lines are evident in the EBSD, implying that the deposited Si film is crystalline. Miller indices of the EBSD pattern are shown in Fig. 5(b). This mapping leads to the conclusion that the crystal orientation of the film is completely coincident with the Si substrate. This means that the deposited Si film grew epitaxially on the Si substrate [7]. In addition, these investigations were carried out at 400 detected points in an area of 11 mm2, and all the data showed epitaxial growth of the deposited Si
film. The same results were obtained from the several areas of the sample, leading to the conclusion that the epitaxial Si layer formed uniformly on the Si substrate. The epitaxial film quality increased with increasing gas pressure, and the best film was obtained at the gas pressure of 3 mTorr. According to Hall measurement results, the carrier concentration and resistivity of the epilayer were about 51018 cm3 and 0.01 V cm, respectively, which are almost the same as those of the sputtering target. Details of the Hall measurement results will be presented elsewhere. In this article, we focus our attention on the I-V characteristics of p–n junctions to evaluate the integrity of the epitaxial layer, especially at the epilayer–substrate interface. The reverse IV characteristics of p–n junctions formed at different substrate bias (V s) and gas pressure ( P g) of 2.7 mTorr are shown in Fig. 6. It is to be noted that the substrate potential without bias voltage V s was the floating potential, denoted by V f, with respect to the ground potential. V f was measured for each discharge condition. The diode diameter was 1.0 mm. It can be seen that the reverse current is very sensitive to V s, the bias voltage during deposition, indicating that the ion bombardment energy is a very important factor for the deposition of a high-quality film. When V s is equal to V f, the leakage current is very large. This suggests that some defects might have formed in the film and interface because of the excess ion bombardment energy. Because V f, for a deposition gas pressure of 2.7 mTorr, is about 5 V, the 10-V
T. Yuji, Y.-M. Sung / Thin Solid Films 475 (2005) 348–353
351
Fig. 5. (a) EBSD pattern of the sample obtained at a pressure of 2.2 mTorr. (b) Miller indices of the EBSD pattern.
Fig. 3. (a) Measured I 0, n e and Te near the substrate region plotted against gas pressure. (b) Measured I 0, n e and Te near the substrate region plotted against RF power.
bias applications to the substrate means that the ion bombardment energy is decreased by 5 eV. When 10-V bias was used, the leakage current decreased by about two orders of magnitude. When V s was increased further by an additional 10 V, the leakage current increased again, which
suggests that the impinging ion energy is not sufficient for the Si surface activation and/or surface mobility [8]. To clarify the combined effects of P g and V s on the quality of deposited films, reverse current densities (RCD) of p–n junctions fabricated at different P g and V s values were investigated. Fig. 7 shows the RCD of p–n junctions formed at different conditions. The RCD at a low P g of 2.2 mTorr shows rather large values, although the substrates were biased in the range of 10–40 V. This means that both high P g and appropriate V s are important parameters for the epitaxial growth. The high P g not only increases the ion
Fig. 4. XPS wide spectrum of the Si thin film prepared by RF magnetron sputter system.
352
T. Yuji, Y.-M. Sung / Thin Solid Films 475 (2005) 348–353
Fig. 6. Reverse current-voltage characteristics of p–n junction diodes with diameter of 1.0 mm. Samples were prepared at P g of 2.7 mTorr with different V s. Plasma conditions for deposition: P rf=350 W, Ts=400 8C.
density of Ar plasmas but also decreases the ion energy. Therefore, low ion energy prevents the damage by ion bombardment and high ion flux guarantees enough energy for Si surface activation or surface mobility [8]. Thus, an RCD of about 1105 mA/cm2 could be obtained only at a P g of 3 mTorr and a V s of 10 V. The reverse and forward I-V characteristics for the best sample fabricated at a high P g of 3 mTorr and V s of 10 V are shown in Fig. 8. The ideality factor n is defined in the forward bias regime by the following empirical relationship [9]: I ¼ I0 expðeV =nkT Þ
ð1Þ
where I 0 is the proportionality constant, e is the electronic charge, k is the Boltzmann constant and T is the temperature. The ideality factor ranges from 1.0 when diffusion currents dominate the diode characteristics (ideal diode behavior) to 2.0 when space-charge layer recombination currents dominate the diode characteristics (indicating poor quality material). The ideality factor n=1.05 was obtained
Fig. 7. Reverse current density of p–n junction diodes prepared at different P g and V s. Plasma conditions for deposition were P rf=350 W and Ts=400 8C.
Fig. 8. Current-voltage characteristics of p–n junction diodes with diameter of 1.0 mm fabricated by depositing an n-type epilayer directly on a p-type substrate. Plasma conditions for deposition were P g=3 mTorr, V s=10V, P rf=350 W and Ts=400 8C.
from the 0.1- and 0.4-V data. This value of n indicates that the Si thin film was high quality. Also, an RCD of 9.5106 mA/cm2 at 5 V was achieved. It is worthwhile to note that diodes having such low RCD were produced using the original wafer surface as an interface of the p–n junction at a conventional vacuum of 5107 Torr and a low temperature of 400 8C. The corresponding ideality factor of 1.05 is very close to the ideal value. It is excellent by any standards for thin films formed under such conditions. Hence, these films show a high potential for use in semiconductor device fabrication.
4. Summary and conclusion The I-V characteristics of epitaxial-type Si p–n junction diodes fabricated at a low temperature of 400 8C and a conventional vacuum of 5107 Torr by using an RF magnetron sputter system were reported. It was shown that the p–n junction diodes using the original wafer surfaces as their junction interfaces exhibited a quite low reverse current density of 9.5109 A/cm2 and an ideality factor of 1.05, which is very close to the ideal value. It should be pointed out that the p–n junctions having these excellent behaviors were performed at the conventional vacuum level and without any further thermal treatment after epitaxial deposition. In addition, a good correlation was also demonstrated between the reverse current density of p–n junction diodes and the degree of crystallinity for the deposited epitaxial films. This work was focused on the low temperature epitaxy process using RF magnetron sputtering. Although we first tried to make silicon p–n junction device, the detail application field was not yet decided. Further study should be necessary to make this clear. Furthermore, this work was mainly performed from the viewpoint of the electrical properties of silicon p–n junction diode and
T. Yuji, Y.-M. Sung / Thin Solid Films 475 (2005) 348–353
optimum plasma circumstances and not on the detail crystal properties. It is very difficult to obtain perfect epitaxial result with low-temperature process technique. However, we believed that the sample obtained at the gas pressure of 3 mTorr was the best and formed uniformly on all the substrate area. Further research on the defect density will be also conducted.
Acknowledgements The author wishes to thank professors C. Honda and M. Otsubo of University of Miyazaki and professor T. Watanabe of Tokyo Institute of Technology for their valuable discussions.
353
References [1] B.S. Meyerson, F.K. LeGoues, T.N. Nguyen, D.L. Harame, Appl. Phys. Lett. 50 (1987) 113. [2] I. Nagai, T. Takahagi, A. Ishitani, H. Kutoda, M. Yoshikawa, J. Appl. Phys. 64 (1988) 5183. [3] A. Ishizaka, Y. Shiraki, J. Electrochem. Soc. 133 (1986) 604. [4] T. Ohmi, T. Ichikawa, H. Iwabuchi, T. Shibata, J. Appl. Phys. 66 (1989) 4756. [5] Y.M. Sung, K. Uchino, K. Muraoka, T. Sakoda, J. Vac. Sci. Technol. A18 (2000) 2149. [6] J. Wang, H. Nakashima, J. Gao, K. Furukawa, K. Muraoka, Y.M. Sung, J. Vac. Sci. Technol. B19 (2001) 333. [7] J. Gao, H. Nakashima, J. Wang, K. Furukawa, K. Muraoka, Jpn. J. Appl. Phys. 39 (2000) 834. [8] W.R. Burger, R. Rei, J. Appl. Phys. 63 (1988) 368. [9] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981, p. 92.
Electrical properties of an epitaxial Si film prepared by RF magnetron plasma at low temperature Toshifumi Yujia, Youl-Moon Sungb,* b
a Department of Nuclear, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan Department of Electrical Electronic Engineering, University of Miyazaki, 1-1 Gakuenkibanadai Nishi, Miyazaki 889-2192, Japan
Available online 26 August 2004
Abstract This article reports the formation of epitaxial Si film that were formed by directly depositing a Sb-doped n-type epilayer on a p-type substrate by using a DC bais RF magnetron sputter system at a low temperature of 400 8C and a conventional vacuum of 5107 Torr. In addition, the plasma parameters were quantitatively investigated to examine the deposition condition. The electron density (n e) of about 1017 m3 was obtained at the plasma region under the conditions where gas pressure was 3 mTorr, the power of the RF source was 350 W and the electron temperature (Te) and ion saturation current (I 0) were in the range of 3–4 eV and 1–1.5 mA/cm2, respectively. The p–n junction diode fabricated by the Si epitaxy shows, under optimum conditions, a reverse current density (RCD) as low as 9.5106 mA/cm2 at a reverse bias voltage of 5 V and an ideality factor of 1.05. The reverse current density has a good correlation with the crystallinity of the deposited films, which, in turn, depends on deposition gas pressures and substrate biases. D 2004 Elsevier B.V. All rights reserved. Keywords: Si epitaxy; RF magnetron sputtering; Electron temperature; Electron density
1. Introduction Epitaxial Si film formation at low temperature is essential for the fabrication of high-performance Si devices. A low growth temperature (400–700 8C) prevents problems such as autodoping from the substrate and impurity redistribution. To achieve such low temperature film growth, an ultrahigh vacuum environment usually is required, such as ultrahigh vacuum chemical vapor deposition (UHV-CVD) [1], UHV electron-cyclotron-resonance (ECR) CVD [2], molecular-beam epitaxy [3], UHV RF plasma deposition [4] and neutral loop discharge (NLD) plasma [5]. For an industrial process application, however, a conventional vacuum level achievable using a usual vacuum system is much preferred. Unfortunately, epitaxial Si films formed at a low temperature and a conventional vacuum around 107 Torr are still not good enough for the device application. Recently, we have performed a low-temperature process * Corresponding author. Tel.: +81 985 58 7350; fax: +81 985 58 7350. E-mail address: [email protected] (Y.-M. Sung). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.07.037
technique for an epitaxial Si growth by using a sputter method [6]. The RF magnetron plasma has many advantageous features, such as high-density and low-temperature plasma production at low gas pressures of 103 Torr. The addition of magnetron capability to the sputtering system confines the secondary electrons emitted from the target. This trapping of the high-energy electrons reduces the electron bombardment of the film surface and allows better control of the substrate temperature. Using this system, crystallographically perfect single-crystal Si has been produced at a low temperature of 400 8C and a conventional vacuum of 5107 Torr. The next step in determining the suitability of these epilayers for practical Si devices is a thorough electrical characterization because the defects, impurity and interface quality, which are often responsible for the degradation of device characteristics, cannot be showing clearly in the crystallinity evaluation. The characteristics of p–n junctions, which serve as the foundation of semiconductor devices, would show us important information for device fabrication. The reverse bias leakage current is one of the fundamental parameters that determine the
T. Yuji, Y.-M. Sung / Thin Solid Films 475 (2005) 348–353
349
quality of p–n junctions. For epitaxial films fabricated using the above method, the calculated depletion layer widths at a reverse bias voltage of 5 V for the n region and p substrate are 7 2 and 2.3 Am, respectively, indicating that the epilayer–substrate interface is within the depletion layer. Therefore, the p–n junction behavior gives us a clear characterization of the integrity of the epilayer near the interface, as well as the epilayer–substrate interface. The purpose of this article is to report on the study of the currentvoltage (I-V) characteristics of p–n junctions formed by directly depositing an Sb-doped n-type epilayer on a p-type substrate using an RF magnetron sputtering system under the low temperature of 400 8C and conventional vacuum level of 5107 Torr, which shows remarkably good performance in terms of the reverse current density (RCD) and an ideality factor. Fig. 1. Schematic diagram of an RF magnetron sputter system.
2. Experiment The RF magnetron sputter system can be briefly described as follows. The chamber was of stainless steel of internal diameter 300 mm. The RF power ( P rf) of 350 W was supplied to a heavily Sb-doped n-type Si target having a resistivity of 0.01 V cm, which was used for growth of epitaxial layers with high doping concentrations. The target diameter was 100 mm, and the distance between the target and substrate was 150 mm. The DC power sources were applied to the substrate to control the substrate potential independently during the deposition. A temperature controller and heater controlled the substrate temperature. Cooling water is circulated through the target and chamber to prevent overheating during deposition. High ion current densities of more than 1.5 mA/cm2 were obtained in the plasma region at a low Ar gas pressure of 3 mTorr, when the RF power was 350 W. A uniformity of ion current density of F8% was obtained at substrate position over the area having a diameter of 60 mm around the plasma axis. The plasma parameters were quantitatively investigated to examine the process condition. The ion current density I 0 and electron temperature Te and density n e were measured with a double probe method as shown in Fig. 1. The measurement results of the current-voltage characteristics of the double probe and ion energy analyzer (IEA) were recorded with an X–Y plotter and an oscilloscope. A Z-shaped double probe, which minimizes measurement errors from the magnetic field effect, was used to measure the Te, n e and I 0 distributions around the substrate and target regions. The probe tips made of tungsten had a diameter of 0.4 mm and a length of 4 mm and were separated by a piece of ceramic material for electrical insulation. The measurement points were 20 (z=20 mm) and 120 mm (z=120 mm) from the substrate plane (z=0 mm). The Z-axis is the central axis of the chamber. The p–n junctions were fabricated using the following procedures. Bdoped p-type (100) Si wafers having a resistivity of 9–12 V cm were used as substrates. A 300-nm-thick SiO2 film was
formed on the substrate by dry oxidation, and windows having diameters in the ranges of 1.0–2.0 mm were patterned by lithography; then the oxide film in the window areas was etched by buffer HF solution. A standard RCA chemical cleaning then was performed to remove the remaining native oxide in the windows; after that, the sample was dipped in 0.5% HF solution for 30 s to terminate the Si surface by H, which would keep a stable Si surface to prevent the oxide growth. Finally, high-purity nitrogen gas blowing was used to prepare a water-free surface. Following this procedure, the sample was immediately loaded into the vacuum chamber, which was subsequently pumped down to the base pressure of less than 5107 Torr. The sample was kept at a temperature of 400 8C. Hence, an n-type Si film of about 400 nm thick was deposited from the Sb-doped Si target on the substrate by using the RF magnetron sputtering. The deposition time was 15–20 min. The deposited Si film was a single crystal at the window area where the bare Si surface was exposed, while the film on the oxide was amorphous Si. An electron backscattering diffraction measurement has confirmed these results. Amorphous Si film was then patterned to isolate each junction. Al metallization was carried out on both the epitaxial layer and the backside of the substrate to form electrodes. It should be noted that the processing temperature never exceeded 400 8C both during and after the epitaxial growth.
3. Results and discussion Fig. 2(a–c) shows the radial distribution of the measured I 0, n e and Te of the RF magnetron plasma system. The I 0, n e and Te at the substrate region (z=20 mm) were in the range of 0.55–0.7 mA/cm2, 4.61016–6.51016 m3 and 2.5– 3 eV, respectively. The I 0, n e and Te near the target (z=120 mm) were in the range of 0.5–1.7 mA/cm2, 6.51016– 8.81016 m3 and 3.1–4.3 eV, respectively.
350
T. Yuji, Y.-M. Sung / Thin Solid Films 475 (2005) 348–353
Fig. 2. (a) Distribution of the measured I 0 of RF magnetron sputter system. (b) Distribution of the measured Te of the RF magnetron sputter system. (c) Distribution of the measured n e of the RF magnetron sputter system.
The dependence of plasma parameters on the gas pressure and RF power was examined. The I 0, n e and Te near the substrate region (r=0 mm, z=20 mm) were plotted against gas pressure, as shown in Fig. 3(a). The value of n e was increased with the working gas pressure, whereas Te was decreased. Fig. 3(b) shows the I 0, n e and Te near the substrate region (r=0 mm, z=20 mm) as a parameter of RF power. It could be seen from Fig. 3(b) that the measured values were increased with the increase of RF power. The XPS wide spectrum of the Si thin film prepared by RF magnetron sputter system is shown in Fig. 4. The Si (2 s) and Si (2 p) photoelectron peaks were evidently detected at the binding energy of about 151 and 99.7 eV, respectively. An additional peak of C (1 s) on the top of the surface was detected at the binding energy of about 290 eV, and it was a peculiar property of XPS analysis. Fig. 5(a) shows the electron beam scattered diffraction (EBSD) pattern of the sample obtained at a pressure of 2.2 mTorr. Clear Kikuchi lines are evident in the EBSD, implying that the deposited Si film is crystalline. Miller indices of the EBSD pattern are shown in Fig. 5(b). This mapping leads to the conclusion that the crystal orientation of the film is completely coincident with the Si substrate. This means that the deposited Si film grew epitaxially on the Si substrate [7]. In addition, these investigations were carried out at 400 detected points in an area of 11 mm2, and all the data showed epitaxial growth of the deposited Si
film. The same results were obtained from the several areas of the sample, leading to the conclusion that the epitaxial Si layer formed uniformly on the Si substrate. The epitaxial film quality increased with increasing gas pressure, and the best film was obtained at the gas pressure of 3 mTorr. According to Hall measurement results, the carrier concentration and resistivity of the epilayer were about 51018 cm3 and 0.01 V cm, respectively, which are almost the same as those of the sputtering target. Details of the Hall measurement results will be presented elsewhere. In this article, we focus our attention on the I-V characteristics of p–n junctions to evaluate the integrity of the epitaxial layer, especially at the epilayer–substrate interface. The reverse IV characteristics of p–n junctions formed at different substrate bias (V s) and gas pressure ( P g) of 2.7 mTorr are shown in Fig. 6. It is to be noted that the substrate potential without bias voltage V s was the floating potential, denoted by V f, with respect to the ground potential. V f was measured for each discharge condition. The diode diameter was 1.0 mm. It can be seen that the reverse current is very sensitive to V s, the bias voltage during deposition, indicating that the ion bombardment energy is a very important factor for the deposition of a high-quality film. When V s is equal to V f, the leakage current is very large. This suggests that some defects might have formed in the film and interface because of the excess ion bombardment energy. Because V f, for a deposition gas pressure of 2.7 mTorr, is about 5 V, the 10-V
T. Yuji, Y.-M. Sung / Thin Solid Films 475 (2005) 348–353
351
Fig. 5. (a) EBSD pattern of the sample obtained at a pressure of 2.2 mTorr. (b) Miller indices of the EBSD pattern.
Fig. 3. (a) Measured I 0, n e and Te near the substrate region plotted against gas pressure. (b) Measured I 0, n e and Te near the substrate region plotted against RF power.
bias applications to the substrate means that the ion bombardment energy is decreased by 5 eV. When 10-V bias was used, the leakage current decreased by about two orders of magnitude. When V s was increased further by an additional 10 V, the leakage current increased again, which
suggests that the impinging ion energy is not sufficient for the Si surface activation and/or surface mobility [8]. To clarify the combined effects of P g and V s on the quality of deposited films, reverse current densities (RCD) of p–n junctions fabricated at different P g and V s values were investigated. Fig. 7 shows the RCD of p–n junctions formed at different conditions. The RCD at a low P g of 2.2 mTorr shows rather large values, although the substrates were biased in the range of 10–40 V. This means that both high P g and appropriate V s are important parameters for the epitaxial growth. The high P g not only increases the ion
Fig. 4. XPS wide spectrum of the Si thin film prepared by RF magnetron sputter system.
352
T. Yuji, Y.-M. Sung / Thin Solid Films 475 (2005) 348–353
Fig. 6. Reverse current-voltage characteristics of p–n junction diodes with diameter of 1.0 mm. Samples were prepared at P g of 2.7 mTorr with different V s. Plasma conditions for deposition: P rf=350 W, Ts=400 8C.
density of Ar plasmas but also decreases the ion energy. Therefore, low ion energy prevents the damage by ion bombardment and high ion flux guarantees enough energy for Si surface activation or surface mobility [8]. Thus, an RCD of about 1105 mA/cm2 could be obtained only at a P g of 3 mTorr and a V s of 10 V. The reverse and forward I-V characteristics for the best sample fabricated at a high P g of 3 mTorr and V s of 10 V are shown in Fig. 8. The ideality factor n is defined in the forward bias regime by the following empirical relationship [9]: I ¼ I0 expðeV =nkT Þ
ð1Þ
where I 0 is the proportionality constant, e is the electronic charge, k is the Boltzmann constant and T is the temperature. The ideality factor ranges from 1.0 when diffusion currents dominate the diode characteristics (ideal diode behavior) to 2.0 when space-charge layer recombination currents dominate the diode characteristics (indicating poor quality material). The ideality factor n=1.05 was obtained
Fig. 7. Reverse current density of p–n junction diodes prepared at different P g and V s. Plasma conditions for deposition were P rf=350 W and Ts=400 8C.
Fig. 8. Current-voltage characteristics of p–n junction diodes with diameter of 1.0 mm fabricated by depositing an n-type epilayer directly on a p-type substrate. Plasma conditions for deposition were P g=3 mTorr, V s=10V, P rf=350 W and Ts=400 8C.
from the 0.1- and 0.4-V data. This value of n indicates that the Si thin film was high quality. Also, an RCD of 9.5106 mA/cm2 at 5 V was achieved. It is worthwhile to note that diodes having such low RCD were produced using the original wafer surface as an interface of the p–n junction at a conventional vacuum of 5107 Torr and a low temperature of 400 8C. The corresponding ideality factor of 1.05 is very close to the ideal value. It is excellent by any standards for thin films formed under such conditions. Hence, these films show a high potential for use in semiconductor device fabrication.
4. Summary and conclusion The I-V characteristics of epitaxial-type Si p–n junction diodes fabricated at a low temperature of 400 8C and a conventional vacuum of 5107 Torr by using an RF magnetron sputter system were reported. It was shown that the p–n junction diodes using the original wafer surfaces as their junction interfaces exhibited a quite low reverse current density of 9.5109 A/cm2 and an ideality factor of 1.05, which is very close to the ideal value. It should be pointed out that the p–n junctions having these excellent behaviors were performed at the conventional vacuum level and without any further thermal treatment after epitaxial deposition. In addition, a good correlation was also demonstrated between the reverse current density of p–n junction diodes and the degree of crystallinity for the deposited epitaxial films. This work was focused on the low temperature epitaxy process using RF magnetron sputtering. Although we first tried to make silicon p–n junction device, the detail application field was not yet decided. Further study should be necessary to make this clear. Furthermore, this work was mainly performed from the viewpoint of the electrical properties of silicon p–n junction diode and
T. Yuji, Y.-M. Sung / Thin Solid Films 475 (2005) 348–353
optimum plasma circumstances and not on the detail crystal properties. It is very difficult to obtain perfect epitaxial result with low-temperature process technique. However, we believed that the sample obtained at the gas pressure of 3 mTorr was the best and formed uniformly on all the substrate area. Further research on the defect density will be also conducted.
Acknowledgements The author wishes to thank professors C. Honda and M. Otsubo of University of Miyazaki and professor T. Watanabe of Tokyo Institute of Technology for their valuable discussions.
353
References [1] B.S. Meyerson, F.K. LeGoues, T.N. Nguyen, D.L. Harame, Appl. Phys. Lett. 50 (1987) 113. [2] I. Nagai, T. Takahagi, A. Ishitani, H. Kutoda, M. Yoshikawa, J. Appl. Phys. 64 (1988) 5183. [3] A. Ishizaka, Y. Shiraki, J. Electrochem. Soc. 133 (1986) 604. [4] T. Ohmi, T. Ichikawa, H. Iwabuchi, T. Shibata, J. Appl. Phys. 66 (1989) 4756. [5] Y.M. Sung, K. Uchino, K. Muraoka, T. Sakoda, J. Vac. Sci. Technol. A18 (2000) 2149. [6] J. Wang, H. Nakashima, J. Gao, K. Furukawa, K. Muraoka, Y.M. Sung, J. Vac. Sci. Technol. B19 (2001) 333. [7] J. Gao, H. Nakashima, J. Wang, K. Furukawa, K. Muraoka, Jpn. J. Appl. Phys. 39 (2000) 834. [8] W.R. Burger, R. Rei, J. Appl. Phys. 63 (1988) 368. [9] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981, p. 92.