- Email: [email protected]
Sulfur passivation of Ge (0 0 1) surfaces and its effects on Schottky barrier contact
ARTICLE IN PRESS
Materials Science in Semiconductor Processing 9 (2006) 706–710
Sulfur passivation of Ge (0 0 1) surfaces and its effects on Schottky barrier contact Tatsuro Maedaa,, Shinichi Takagia,b, Tsuyoshi Ohnishic, Mikk Lippmaac a
MIRAI Project, ASRC-AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan b The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan c Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan Available online 18 September 2006
Abstract The structure of a passivating sulfide layer on Ge (0 0 1) substrates treated in an aqueous sulfide solution is investigated using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and coaxial impact-collision ion scattering spectroscopy (CAICISS). It is found from XPS analysis that the contribution from twofold co-ordinated Ge atoms is dominant in the S-treated Ge surface. The incident and azimuth angle dependencies of the Ge signal intensity are compared between the S-passivated and clean Ge surfaces in CAICISS measurements. The azimuth angle dependence exhibits clear 4-fold symmetry in both the S-passivated and the clean surfaces. From the incident angle dependence along [1 0 0] direction, almost the same shadowing and focusing effects are observed in both the surfaces. These results suggest that Ge surface is passivated by S atoms occupying bridge positions between adjacent surface Ge atoms. In order to examine the effect of S-passivation of Ge surfaces on the interface electrical properties, we perform I– V measurements of Au/S-passivated Ge Schottky diodes. It is found that the S-passivation is effective for obtaining uniform diode characteristics. r 2006 Elsevier Ltd. All rights reserved. Keywords: Ge surface; Sulfur passivation; Schottky barrier
1. Introduction Recently, Germanium (Ge) is increasingly being studied for metal oxide semiconductor field-effect transistor (MOSFET) applications because of its high intrinsic carrier mobilities [1]. Contrary to Si, the thermodynamic instability of GeOx may allow to realize a high-k gate stack on Ge without a lowerk interfacial layer, and thereby to break through the Corresponding author. Tel.: +81 29 861 5122; fax: +81 29 861 2642. E-mail address: [email protected] (T. Maeda).
1369-8001/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2006.08.025
EOT scaling barrier [2,3]. On the other hand, the lower-k Ge-oxynitride interfacial layer has conventionally provided the best improvements in the electrical properties among a variety of high-k/Ge gate stacks [4,5], which is similar to Si gate stack trend. To overcome this dilemma, an atomic passivation technology of Ge surfaces to provide a minimum interfacial layer and good interface properties should be investigated for future scalable gate technologies. An effective passivation treatment is supposed to be chemically stable, protect the substrate from the unnecessary oxidation, remove surface-induced
ARTICLE IN PRESS T. Maeda et al. / Materials Science in Semiconductor Processing 9 (2006) 706–710
Dangling bond
(a)
707
Sulfur atoms
(b)
Fig. 1. Side view into the [0 1 1] direction of (a) clean (1 1) and (b) the S-passivated Ge (0 0 1) surfaces.
2. Experimental For simple processing, the wet chemical sulfidation of Ge (0 0 1) was carried out. The sulfidation of III–V semiconductor surfaces by this method was found to remove surface states [8] and to retard deleterious oxidation reaction [9]. It has also been reported that aqueous sulfur treatment of Ge substrates can produce the S-passivated Ge (1 1)
(a) HF-treated O
C dN (E)/dE
carrier recombination states, and exhibit a low density of interface charges. Since the (0 0 1) face of a diamond-structure semiconductor has two dangling bonds per surface atom (Fig. 1(a)), a monolayer (ML) of a Group VI element, which forms two covalent bonds, can satisfy all the dangling bonds of the substrate by occupying a bridge site in a (1 1) geometry (Fig. 1(b)). Sulfur and higher chalcogens are promising candidates as adsorbates to obtain an ideal (1 1) termination of the bivalent (0 0 1) surfaces of silicon and Ge, while oxygen is known to penetrate and form SiO2 and GeO2. Whereas the adsorption system S/Si(0 0 1) does not realize an ideal surface termination [6], the system S/Ge (0 0 1) does. Accordingly, it has been demonstrated that 1 ML of elemental S deposited on Ge (0 0 1) in ultrahigh vacuum (UHV) leads to this predicted arrangement [7]. Hence, this atomic passivation technology of Ge surfaces has a possibility to control metal or insulator/Ge interface properties as well as to fabricate epitaxial heterostructures directly on Ge substrates. In this study, we report our investigation of the surface structures of Ge (0 0 1) substrates treated in an aqueous (NH4)2S solution by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and coaxial impact-collision ion scattering spectroscopy (CAICISS). In order to examine the effect of the S-passivation treatment of Ge surfaces on the interface electrical properties, we studied I– V measurements of Au/S-passivated Ge Schottky diodes.
Ge
(b) S-treated S
100
200 300 400 Kinetic Energy (eV)
500
Fig. 2. Auger spectra of (a) HF-treated and (b) S-treated Ge substrates.
surface [10]. In this study, Ge substrates were dipped first in HF to remove the native oxide [11] and then treated for 20 min in a hot aqueous (NH4)2S solution. After this S-treatment, the samples were rinsed in running water and dried before being introduced into the vacuum chambers. 3. Results and discussion Fig. 2 shows the Auger spectra obtained from (a) HF-treated and (b) S-treated Ge substrates. As a result of cyclic HF dip and ultra pure water (UPW) rinse by the method presented in Ref. [11], oxygen and carbon transitions at kinetic energies of 276 and 514 eV are observed slightly (Fig. 2(a)). This means that GeO2 can be etched by HF-treatment, but oxygen and carbon contaminations still remain in our experiments. These signals may originate from a combination of adventitious carbon, hydrocarbon and water absorbed onto the surface during its exposure to ambient condition after the wet treatments. The S-treatment followed the HF-treatment. A strong sulfur transition at a kinetic energy of 148 eV appears in Fig. 2(b), indicating the existence
ARTICLE IN PRESS T. Maeda et al. / Materials Science in Semiconductor Processing 9 (2006) 706–710
708
of sulfur on the S-treated Ge surfaces. A negligibly small signal from oxygen means higher stability of the S-treated surfaces against oxidation than that of HF-treated surfaces because of the same interval between wet treatments and the insertion to the vacuum chamber. A typical surface Ge 2p3/2 core level spectrum obtained after the S-treatment is shown in Fig. 3. The inset shows the S 2p peak of the same sample. The Ge 2p3/2 region of the XPS spectrum is commonly used to study the removal of the oxide layer and other surface chemical states because of its sensitivity to the different oxidation states of Ge [11]. The Ge 2p band is more surface sensitive than the Ge 3d band because of the smaller mean free path associated with the Ge 2p core level photoelectrons in the 200–300 eV kinetic energy range. In Fig. 3, it is clearly observed that Ge 2p3/2 peak from the S-treated surface is composed of main bulk component and other chemical shifted components. Because of the existence of a strong S 2p signal and a negligibly small O 1s signal, at first stage for the deconvolution of Ge 2p3/2 peak, we put four possible chemical shifted components whose binding energy shift induced by sulfur is DE ¼ 0.33 eV per S-Ge bond [7]. Here, we assume that the value of the binding energy shift in Ge 3d band is the same in Ge 2p band. From the numerical deconvolution of Ge 2p3/2 peak using a liner background subtraction, we found two sulfur-induced chemical shifts with 0.66 (S1) and 1.28 (S2) eV and non-negligible contribution (S3) at higher binding energy as shown in Fig. 3. The peak with the lowest binding energy is
Normalized Intensity (arb. units)
S 2p
Ge 2p I 164 160 Binding Energy(eV)
I S1 I S3 1222
I S2
bulk
1220 1218 1216 Binding Energy (eV)
1214
Fig. 3. Ge 2p3/2 peak with peak fitting after S-treatment. The inset shows the S 2p peak of the same sample.
the Ge bulk peak of 1217.4 eV. One major chemical shifted peak (S1) is consistent with doubly sulfur coordinated Ge with a chemical shift of 0.66 eV. This state corresponds to adsorbed sulfur atoms occupying the bridged site between neighboring Ge atoms [7]. The other small peak (S2) is quadruply sulfur co-ordinated Ge with a chemical shift of 1.28 eV, which is possibly attributed to the formation of GeS2 [7]. Last chemical shifted component (S3) with relatively full width at half-maximum (FWHM) can be interpreted as Ge atoms with oxygen, since this binding energy shift is larger than that of sulfurinduced one. For example, the value of the chemical shift of GeO2 is reported as 2.8 eV [12]. Oxide could remain after the wet treatment as shown in the previous AES observation. Analyses of the XPS spectra reveal that some Ge oxidation states are presented for the aqueously sulfided Ge (0 0 1) surface, indicating an ideal surface termination has not been formed completely [13]. However, the predominant chemical state is the doubly sulfur coordinated Ge which has a potential to eliminate surface dangling bonds on Ge (0 0 1) surface. For further understanding of the Ge surface structures with the S-treatment, it is important to observe the positions of sulfur atoms on the surface in respect to the host Ge atoms. RHEED patterns of the S-treated exhibit a clear 1 1 structure, indicating the smooth and well-ordered surfaces. The RHEED patterns, however, seem not to reflect the structure of exact top surface, but to give information averaged over a few atomic layers from the top. Therefore, we used CAICISS to clarify the structure of the S-treated surface. CAICISS is a specialized technique in low-energy ion scattering spectroscopy (ISS) to monitor the intensity of the quasi-single scattered ions. The scattered ions are detected at an angle close to 1801 with respect to the incident ions and the energy of the scattered particles is analyzed by a time-of-flight (TOF) technique [14]. In this configuration, the TOF spectrum is composed of peaks corresponding to head-on collisions between incident He ions and target atoms on the topmost surface. Hence, the surface atomic species and the arrangement can be determined from the incident and azimuth angular dependences of the TOF spectrum, by taking the shadowing and focusing effects into account. Fig. 4 shows normalized Ge signal intensity dependencies on the incident angle along [1 0 0] from Ge surfaces of (a) clean, (b) S-treated and (c) native oxide. We prepared a clean Ge surface as a
ARTICLE IN PRESS T. Maeda et al. / Materials Science in Semiconductor Processing 9 (2006) 706–710
(c) native oxide Normalized Ge Signal Intensity (arb.units)
Normalized Ge Signal Intensity (arb.units)
(c) native oxide
(b) S:Ge
(a) clean Ge
0
15
30
45 60 75 Incident Angle (deg.)
709
90
(a) cleanGe
-45 [110]
105
Fig. 4. Normalized Ge signal intensity dependencies on the incident angle along [1 0 0] from Ge surfaces of (a) clean, (b) S-treated and (c) native oxide.
(b) S:Ge
45 [110]
90 [010]
135 [110]
Azimuth Angle (deg.) Fig. 5. Normalized Ge signal intensities as a function of the azimuth angle from Ge surfaces of (a) clean, (b) S-treated and (c) native oxide. The incident angle was fixed at 451.
Diode Current (A)
10-4
reference with the perfect atom-arrangement by native oxide removal through thermal desorption in UHV [15]. It is observed in Fig. 4(c) that the nonordered structure of native oxides with the thickness of approximately 1 nm indicates no significant variation of signal intensity. On the other hand, the variations in the signal intensities of both the clean and the S-treated Ge surfaces have clear minima at incident angles of 901, 721, and 451 and clear maxima at around 1101, 811, 601, and 331, as shown in Fig. 4(a) and (b). This dependence of the signal intensity on the direction of the incident beam can be explained by shadowing and focusing effects due to the atom arrangement in diamond structures. When the ion beam is incident on a Ge crystal with an angle of 451 in the (1 0 0) plane, for example, Ge atoms below the top-most layer are shadowed by the top-most atoms due to an atom-arrangement along the [0 1 1] direction. This leads to a minimum in the signal intensity at the incident angles of 451 in the range of incident angle between 601 and 331. Since no considerable difference is observed in the signal intensity variations between the clean and the S-treated surfaces, topmost surface sulfur atoms can achieve the same shadowing and focusing effects. Fig. 5 shows the azimuthal scan of Ge for incident angle of 451. The azimuth angle dependencies exhibit clear 4-fold symmetry in both the S-treated and the clean Ge surfaces. Compared with both azimuth angle dependences, there is no significant difference. These observations imply that sulfur atoms occupy of same atom positions of Ge in the diamond structure.
0 [100]
Reverse bias
10-5
Forward bias
(a) S-treated
10-6
10-7
(b) HF-treated
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
Diode Voltage (V) Fig. 6. Typical diode current–voltage (I–V) characteristics of (a) the S-treated and (b) the HF-treated Schottky diodes.
Although various surface analytical techniques have been employed to study the S-passivation of Ge substrates, there is no report of electrical characteristics using this system. In order to examine the effects of the S-passivation treatment of Ge surfaces on electrical properties, we fabricated simple Au/S-treated Ge and Au/HF-treated Ge Schottky diodes. Fig. 6 shows typical diode current–voltage (I–V) characteristics of (a) the S-treated and (b) the HF-treated Schottky diodes. The S-treated samples exhibit higher diode current in both forward and reverse bias than that of the HF-treated one, indicating the reduction of Schottky barrier height for electron. This observation is confirmed by the Weibull plot of both forward and reverse diode currents at the diode voltage
ARTICLE IN PRESS T. Maeda et al. / Materials Science in Semiconductor Processing 9 (2006) 706–710
710
Vd=-0.1V
Vd=0.1V
Weibull Ln (-Ln (1-P))
S-treated
HF-treated
treatment of Ge substrates. We have confirmed that this treatment produces the S-passivated Ge surface. From CAICISS analyses, we observed no significant difference in the Ge signal intensity dependencies on azimuth and incident angles. These results suggest that S atoms are occupying bridge positions between adjacent surface Ge atoms. It is found that the S-passivation yields the lower Schottky barrier height for electrons and the improvement of the uniformity in the diode characteristics. Acknowledgments
10-6
10-5
Diode Current (A) Fig. 7. Weibull plot of both forward and reverse diode currents at Vg ¼ 0.1 and 0.1 V, respectively, for (a) the S-treated and (b) the HF-treated Schottky diodes.
(Vd) ¼ 0.1 and 0.1 V, respectively, as shown in Fig. 7. It is also found that the fluctuation of diode currents is suppressed in the S-treated samples, suggesting high uniformity of diode characteristics between the samples. These quantitative differences between the S-treated and the HF-treated Ge Schottky diodes suggest the realization of more idealistic Ge/metal Schottky interfaces by the Streatment. One possible explanation of lower Schottky barrier height for the S-passivated samples is the presence of interface traps which are passivated by S atoms. Electrically, surface states originated from dangling bonds often pin the surface Fermi level, causing surface band bending. In case of n-type Ge substrates, an inversion layer formation at the interface is implied in the NiGe/Ge diode structure [16]. If S atoms mitigate the dangling bonds effectively and reduce interface charges, Schottky barrier height for electron should be lowered. Less dangling bonds and negligible unstable Ge oxides due to the S-passivation also can stabilize the dominant current path and inhibit the variation of diode current. We can expect further improvement of the interface electronic properties of Ge by optimizing the S-treatment process. 4. Conclusion As an atomic passivation technology of Ge surfaces, we have studied the aqueous sulfur
The authors are grateful to Drs. M. Hirose and T. Kanayama for their continuous supports. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO).
References [1] Shang H, Okorn-Schmidt H, Chan KK, Copel M, Ott JA, Kozlowski PM, et al. IEDM Tech Dig 2002;441. [2] Chui CO, Ramanathan S, Triplett BB, McIntyre PC, Saraswat KC. IEEE Electron Dev Lett 2002;23:473. [3] Kim H, Chui CO, Saraswat KC, McIntyre PC. Appl Phys Lett 2003;83:2647. [4] Bai WP, Lu N, Liu J, Ramirez A, Kwong DL, Wristers D, et al. VLSI Tech Dig 2003;121. [5] Chui CO, Kim H, McIntyre PC, Saraswat KC. IEEE Electron Dev Lett 2004;25:274. [6] Weser T, Bogen A, Konrad B, Schnell RD, Schug CA, Steinmann W. In: Engstro¨m O, editor. Proceedings of the 18th international conferences on the physics of semiconductors. Singapore: World Scientific; 1987. [7] Weser T, Bogen A, Konrad B, Schnell RD, Schug CA, Steinmann W. Phys Rev B 1987;35:8184. [8] Iyer R, Chang RR, Lile DL. Appl Phys Lett 1988;53:134. [9] Lu ZH, Graham MJ, Feng XH, Yang BX. Appl Phys Lett 1992;60:2773. [10] Anderson GW, Hanf MC, Norton PR, Lu ZH, Graham MJ. Appl Phys Lett 1995;66:1123. [11] Deegan T, Hughes G. Appl Surf Sci 1998;123:66. [12] Bodlaki D, Yamamoto H, Waldeck DH, Borguet E. Surf Sci 2003;543:63. [13] Lyman PF, Sakata O, Marasco DL, Lee TL, Breneman KD, Keane DT, et al. Surf Sci 2000;462:594. [14] Katayama M, Nomura E, Kanekama N, Soejima H, Aono M. Nucl Instrum Methods B 1988;33:857. [15] Maeda T, Yasuda T, Nishizawa M, Miyata N, Morita Y, Takagi S. Appl Phys Lett 2004;85:3181. [16] Chi DZ, Lee RTP, Chua SJ, Lee SJ, Ashok S, Kwong D-L. J Appl Phys 2005;97:113706.
Materials Science in Semiconductor Processing 9 (2006) 706–710
Sulfur passivation of Ge (0 0 1) surfaces and its effects on Schottky barrier contact Tatsuro Maedaa,, Shinichi Takagia,b, Tsuyoshi Ohnishic, Mikk Lippmaac a
MIRAI Project, ASRC-AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan b The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan c Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan Available online 18 September 2006
Abstract The structure of a passivating sulfide layer on Ge (0 0 1) substrates treated in an aqueous sulfide solution is investigated using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and coaxial impact-collision ion scattering spectroscopy (CAICISS). It is found from XPS analysis that the contribution from twofold co-ordinated Ge atoms is dominant in the S-treated Ge surface. The incident and azimuth angle dependencies of the Ge signal intensity are compared between the S-passivated and clean Ge surfaces in CAICISS measurements. The azimuth angle dependence exhibits clear 4-fold symmetry in both the S-passivated and the clean surfaces. From the incident angle dependence along [1 0 0] direction, almost the same shadowing and focusing effects are observed in both the surfaces. These results suggest that Ge surface is passivated by S atoms occupying bridge positions between adjacent surface Ge atoms. In order to examine the effect of S-passivation of Ge surfaces on the interface electrical properties, we perform I– V measurements of Au/S-passivated Ge Schottky diodes. It is found that the S-passivation is effective for obtaining uniform diode characteristics. r 2006 Elsevier Ltd. All rights reserved. Keywords: Ge surface; Sulfur passivation; Schottky barrier
1. Introduction Recently, Germanium (Ge) is increasingly being studied for metal oxide semiconductor field-effect transistor (MOSFET) applications because of its high intrinsic carrier mobilities [1]. Contrary to Si, the thermodynamic instability of GeOx may allow to realize a high-k gate stack on Ge without a lowerk interfacial layer, and thereby to break through the Corresponding author. Tel.: +81 29 861 5122; fax: +81 29 861 2642. E-mail address: [email protected] (T. Maeda).
1369-8001/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2006.08.025
EOT scaling barrier [2,3]. On the other hand, the lower-k Ge-oxynitride interfacial layer has conventionally provided the best improvements in the electrical properties among a variety of high-k/Ge gate stacks [4,5], which is similar to Si gate stack trend. To overcome this dilemma, an atomic passivation technology of Ge surfaces to provide a minimum interfacial layer and good interface properties should be investigated for future scalable gate technologies. An effective passivation treatment is supposed to be chemically stable, protect the substrate from the unnecessary oxidation, remove surface-induced
ARTICLE IN PRESS T. Maeda et al. / Materials Science in Semiconductor Processing 9 (2006) 706–710
Dangling bond
(a)
707
Sulfur atoms
(b)
Fig. 1. Side view into the [0 1 1] direction of (a) clean (1 1) and (b) the S-passivated Ge (0 0 1) surfaces.
2. Experimental For simple processing, the wet chemical sulfidation of Ge (0 0 1) was carried out. The sulfidation of III–V semiconductor surfaces by this method was found to remove surface states [8] and to retard deleterious oxidation reaction [9]. It has also been reported that aqueous sulfur treatment of Ge substrates can produce the S-passivated Ge (1 1)
(a) HF-treated O
C dN (E)/dE
carrier recombination states, and exhibit a low density of interface charges. Since the (0 0 1) face of a diamond-structure semiconductor has two dangling bonds per surface atom (Fig. 1(a)), a monolayer (ML) of a Group VI element, which forms two covalent bonds, can satisfy all the dangling bonds of the substrate by occupying a bridge site in a (1 1) geometry (Fig. 1(b)). Sulfur and higher chalcogens are promising candidates as adsorbates to obtain an ideal (1 1) termination of the bivalent (0 0 1) surfaces of silicon and Ge, while oxygen is known to penetrate and form SiO2 and GeO2. Whereas the adsorption system S/Si(0 0 1) does not realize an ideal surface termination [6], the system S/Ge (0 0 1) does. Accordingly, it has been demonstrated that 1 ML of elemental S deposited on Ge (0 0 1) in ultrahigh vacuum (UHV) leads to this predicted arrangement [7]. Hence, this atomic passivation technology of Ge surfaces has a possibility to control metal or insulator/Ge interface properties as well as to fabricate epitaxial heterostructures directly on Ge substrates. In this study, we report our investigation of the surface structures of Ge (0 0 1) substrates treated in an aqueous (NH4)2S solution by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and coaxial impact-collision ion scattering spectroscopy (CAICISS). In order to examine the effect of the S-passivation treatment of Ge surfaces on the interface electrical properties, we studied I– V measurements of Au/S-passivated Ge Schottky diodes.
Ge
(b) S-treated S
100
200 300 400 Kinetic Energy (eV)
500
Fig. 2. Auger spectra of (a) HF-treated and (b) S-treated Ge substrates.
surface [10]. In this study, Ge substrates were dipped first in HF to remove the native oxide [11] and then treated for 20 min in a hot aqueous (NH4)2S solution. After this S-treatment, the samples were rinsed in running water and dried before being introduced into the vacuum chambers. 3. Results and discussion Fig. 2 shows the Auger spectra obtained from (a) HF-treated and (b) S-treated Ge substrates. As a result of cyclic HF dip and ultra pure water (UPW) rinse by the method presented in Ref. [11], oxygen and carbon transitions at kinetic energies of 276 and 514 eV are observed slightly (Fig. 2(a)). This means that GeO2 can be etched by HF-treatment, but oxygen and carbon contaminations still remain in our experiments. These signals may originate from a combination of adventitious carbon, hydrocarbon and water absorbed onto the surface during its exposure to ambient condition after the wet treatments. The S-treatment followed the HF-treatment. A strong sulfur transition at a kinetic energy of 148 eV appears in Fig. 2(b), indicating the existence
ARTICLE IN PRESS T. Maeda et al. / Materials Science in Semiconductor Processing 9 (2006) 706–710
708
of sulfur on the S-treated Ge surfaces. A negligibly small signal from oxygen means higher stability of the S-treated surfaces against oxidation than that of HF-treated surfaces because of the same interval between wet treatments and the insertion to the vacuum chamber. A typical surface Ge 2p3/2 core level spectrum obtained after the S-treatment is shown in Fig. 3. The inset shows the S 2p peak of the same sample. The Ge 2p3/2 region of the XPS spectrum is commonly used to study the removal of the oxide layer and other surface chemical states because of its sensitivity to the different oxidation states of Ge [11]. The Ge 2p band is more surface sensitive than the Ge 3d band because of the smaller mean free path associated with the Ge 2p core level photoelectrons in the 200–300 eV kinetic energy range. In Fig. 3, it is clearly observed that Ge 2p3/2 peak from the S-treated surface is composed of main bulk component and other chemical shifted components. Because of the existence of a strong S 2p signal and a negligibly small O 1s signal, at first stage for the deconvolution of Ge 2p3/2 peak, we put four possible chemical shifted components whose binding energy shift induced by sulfur is DE ¼ 0.33 eV per S-Ge bond [7]. Here, we assume that the value of the binding energy shift in Ge 3d band is the same in Ge 2p band. From the numerical deconvolution of Ge 2p3/2 peak using a liner background subtraction, we found two sulfur-induced chemical shifts with 0.66 (S1) and 1.28 (S2) eV and non-negligible contribution (S3) at higher binding energy as shown in Fig. 3. The peak with the lowest binding energy is
Normalized Intensity (arb. units)
S 2p
Ge 2p I 164 160 Binding Energy(eV)
I S1 I S3 1222
I S2
bulk
1220 1218 1216 Binding Energy (eV)
1214
Fig. 3. Ge 2p3/2 peak with peak fitting after S-treatment. The inset shows the S 2p peak of the same sample.
the Ge bulk peak of 1217.4 eV. One major chemical shifted peak (S1) is consistent with doubly sulfur coordinated Ge with a chemical shift of 0.66 eV. This state corresponds to adsorbed sulfur atoms occupying the bridged site between neighboring Ge atoms [7]. The other small peak (S2) is quadruply sulfur co-ordinated Ge with a chemical shift of 1.28 eV, which is possibly attributed to the formation of GeS2 [7]. Last chemical shifted component (S3) with relatively full width at half-maximum (FWHM) can be interpreted as Ge atoms with oxygen, since this binding energy shift is larger than that of sulfurinduced one. For example, the value of the chemical shift of GeO2 is reported as 2.8 eV [12]. Oxide could remain after the wet treatment as shown in the previous AES observation. Analyses of the XPS spectra reveal that some Ge oxidation states are presented for the aqueously sulfided Ge (0 0 1) surface, indicating an ideal surface termination has not been formed completely [13]. However, the predominant chemical state is the doubly sulfur coordinated Ge which has a potential to eliminate surface dangling bonds on Ge (0 0 1) surface. For further understanding of the Ge surface structures with the S-treatment, it is important to observe the positions of sulfur atoms on the surface in respect to the host Ge atoms. RHEED patterns of the S-treated exhibit a clear 1 1 structure, indicating the smooth and well-ordered surfaces. The RHEED patterns, however, seem not to reflect the structure of exact top surface, but to give information averaged over a few atomic layers from the top. Therefore, we used CAICISS to clarify the structure of the S-treated surface. CAICISS is a specialized technique in low-energy ion scattering spectroscopy (ISS) to monitor the intensity of the quasi-single scattered ions. The scattered ions are detected at an angle close to 1801 with respect to the incident ions and the energy of the scattered particles is analyzed by a time-of-flight (TOF) technique [14]. In this configuration, the TOF spectrum is composed of peaks corresponding to head-on collisions between incident He ions and target atoms on the topmost surface. Hence, the surface atomic species and the arrangement can be determined from the incident and azimuth angular dependences of the TOF spectrum, by taking the shadowing and focusing effects into account. Fig. 4 shows normalized Ge signal intensity dependencies on the incident angle along [1 0 0] from Ge surfaces of (a) clean, (b) S-treated and (c) native oxide. We prepared a clean Ge surface as a
ARTICLE IN PRESS T. Maeda et al. / Materials Science in Semiconductor Processing 9 (2006) 706–710
(c) native oxide Normalized Ge Signal Intensity (arb.units)
Normalized Ge Signal Intensity (arb.units)
(c) native oxide
(b) S:Ge
(a) clean Ge
0
15
30
45 60 75 Incident Angle (deg.)
709
90
(a) cleanGe
-45 [110]
105
Fig. 4. Normalized Ge signal intensity dependencies on the incident angle along [1 0 0] from Ge surfaces of (a) clean, (b) S-treated and (c) native oxide.
(b) S:Ge
45 [110]
90 [010]
135 [110]
Azimuth Angle (deg.) Fig. 5. Normalized Ge signal intensities as a function of the azimuth angle from Ge surfaces of (a) clean, (b) S-treated and (c) native oxide. The incident angle was fixed at 451.
Diode Current (A)
10-4
reference with the perfect atom-arrangement by native oxide removal through thermal desorption in UHV [15]. It is observed in Fig. 4(c) that the nonordered structure of native oxides with the thickness of approximately 1 nm indicates no significant variation of signal intensity. On the other hand, the variations in the signal intensities of both the clean and the S-treated Ge surfaces have clear minima at incident angles of 901, 721, and 451 and clear maxima at around 1101, 811, 601, and 331, as shown in Fig. 4(a) and (b). This dependence of the signal intensity on the direction of the incident beam can be explained by shadowing and focusing effects due to the atom arrangement in diamond structures. When the ion beam is incident on a Ge crystal with an angle of 451 in the (1 0 0) plane, for example, Ge atoms below the top-most layer are shadowed by the top-most atoms due to an atom-arrangement along the [0 1 1] direction. This leads to a minimum in the signal intensity at the incident angles of 451 in the range of incident angle between 601 and 331. Since no considerable difference is observed in the signal intensity variations between the clean and the S-treated surfaces, topmost surface sulfur atoms can achieve the same shadowing and focusing effects. Fig. 5 shows the azimuthal scan of Ge for incident angle of 451. The azimuth angle dependencies exhibit clear 4-fold symmetry in both the S-treated and the clean Ge surfaces. Compared with both azimuth angle dependences, there is no significant difference. These observations imply that sulfur atoms occupy of same atom positions of Ge in the diamond structure.
0 [100]
Reverse bias
10-5
Forward bias
(a) S-treated
10-6
10-7
(b) HF-treated
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
Diode Voltage (V) Fig. 6. Typical diode current–voltage (I–V) characteristics of (a) the S-treated and (b) the HF-treated Schottky diodes.
Although various surface analytical techniques have been employed to study the S-passivation of Ge substrates, there is no report of electrical characteristics using this system. In order to examine the effects of the S-passivation treatment of Ge surfaces on electrical properties, we fabricated simple Au/S-treated Ge and Au/HF-treated Ge Schottky diodes. Fig. 6 shows typical diode current–voltage (I–V) characteristics of (a) the S-treated and (b) the HF-treated Schottky diodes. The S-treated samples exhibit higher diode current in both forward and reverse bias than that of the HF-treated one, indicating the reduction of Schottky barrier height for electron. This observation is confirmed by the Weibull plot of both forward and reverse diode currents at the diode voltage
ARTICLE IN PRESS T. Maeda et al. / Materials Science in Semiconductor Processing 9 (2006) 706–710
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Vd=-0.1V
Vd=0.1V
Weibull Ln (-Ln (1-P))
S-treated
HF-treated
treatment of Ge substrates. We have confirmed that this treatment produces the S-passivated Ge surface. From CAICISS analyses, we observed no significant difference in the Ge signal intensity dependencies on azimuth and incident angles. These results suggest that S atoms are occupying bridge positions between adjacent surface Ge atoms. It is found that the S-passivation yields the lower Schottky barrier height for electrons and the improvement of the uniformity in the diode characteristics. Acknowledgments
10-6
10-5
Diode Current (A) Fig. 7. Weibull plot of both forward and reverse diode currents at Vg ¼ 0.1 and 0.1 V, respectively, for (a) the S-treated and (b) the HF-treated Schottky diodes.
(Vd) ¼ 0.1 and 0.1 V, respectively, as shown in Fig. 7. It is also found that the fluctuation of diode currents is suppressed in the S-treated samples, suggesting high uniformity of diode characteristics between the samples. These quantitative differences between the S-treated and the HF-treated Ge Schottky diodes suggest the realization of more idealistic Ge/metal Schottky interfaces by the Streatment. One possible explanation of lower Schottky barrier height for the S-passivated samples is the presence of interface traps which are passivated by S atoms. Electrically, surface states originated from dangling bonds often pin the surface Fermi level, causing surface band bending. In case of n-type Ge substrates, an inversion layer formation at the interface is implied in the NiGe/Ge diode structure [16]. If S atoms mitigate the dangling bonds effectively and reduce interface charges, Schottky barrier height for electron should be lowered. Less dangling bonds and negligible unstable Ge oxides due to the S-passivation also can stabilize the dominant current path and inhibit the variation of diode current. We can expect further improvement of the interface electronic properties of Ge by optimizing the S-treatment process. 4. Conclusion As an atomic passivation technology of Ge surfaces, we have studied the aqueous sulfur
The authors are grateful to Drs. M. Hirose and T. Kanayama for their continuous supports. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO).
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