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n-GaAs(0 0 1) structure
Applied Surface Science 244 (2005) 293–296 www.elsevier.com/locate/apsusc
Incorporation of SiO2 for the band alignment control of Gd2O3/n-GaAs(0 0 1) structure Jun-Kyu Yang, Hyung-Ho Park* Department of Ceramic Engineering, Yonsei University, 134 Sinchon-dong, Seodaemun-ku, Seoul 120-749, Republic of Korea Received 31 May 2004; accepted 22 September 2004 Available online 7 January 2005
Abstract Band alignment of Gd2O3 gate oxide films on n-GaAs(0 0 1) was controlled by the incorporation of SiO2. The photoelectron binding energy shifts in Gd2SiO5 film could be interpreted with relative electronegativity of second nearest neighbor element. The surface and interface morphology of Gd2SiO5/n-GaAs structure was smooth due to the absence of crystalline phase. Energy band gaps were estimated as 5.8 and 6.6 eV for Gd2O3 and Gd2SiO5, respectively, by combining photoemission with absorption spectra. A decrease of leakage current density and a saturated accumulation capacitance indicate an enhanced band offset and small roughness in Gd2SiO5/n-GaAs system. # 2004 Published by Elsevier B.V. PACS: 73.40.Q (metal–insulator–semiconductor structures) Keywords: GaAs; MOS; Gd2O3; Silicate; Band offset
1. Introduction A gate dielectric on GaAs has been investigated for a long time due to a large input voltage excursion and simple circuit design [1]. Nevertheless, most of III–V devices adopt a metal gate because of serious interfacial reaction between oxide and GaAs. Surface passivation and appropriate insulator material should be employed to overcome the aforementioned problems. Among the various gate dielectrics such * Corresponding author. Tel.: +82 2 2123 2853; fax: +82 2 365 5882. E-mail address: [email protected] (H.-H. Park). 0169-4332/$ – see front matter # 2004 Published by Elsevier B.V. doi:10.1016/j.apsusc.2004.09.164
as SiO2, Al2O3, Ga2O3(Gd2O3), etc [2–5], Gd2O3 film has been one of very attractive materials because it shows an epitaxial growth on GaAs(0 0 1). However, impediments in preparing clean surface and local structural/electrical deformation in polycrystalline phase drive us to search a new alternative amorphous film. Al2O3 and Ga2O3 have been recently focused as amorphous oxide–GaAs system, since their films showed high breakdown field and atomically abrupt interface with GaAs [4,6]. In addition, considering a close relation between band parameters and electrical properties, the band offset control must be one of the most significant requirements for the design of metal-oxide–semiconductor field effect transistor
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J.-K. Yang, H.-H. Park / Applied Surface Science 244 (2005) 293–296
(MOSFET). So then, an amorphous oxide film with large band offsets can be an optimized host for the gate of GaAs MOS devices. In this work, a rare-earth silicate amorphous film was employed as a gate dielectric on n-GaAs. The band alignment of Gd2O3 with regard to n-GaAs was effectively controlled by incorporating SiO2. We have correlated the change of band structure and the interfacial properties with electrical characteristics such as dielectric capacitance and leakage current.
2. Experimental procedure n-GaAs(0 0 1) wafer (Si-doping of 8 1016 cm 3) was used in this experiment. Au/Ge/Ni/Au was served as a low resistance ohmic contact by subsequent evaporation and anneal. After sulfur passivation of GaAs surface, Gd2O3 and Gd2SiO5 (hereafter denoted as GSO) films were deposited by e-beam evaporation of Gd2O3 and mixed oxides of (Gd2O3)0.8(SiO2)0.2, respectively, under 2 10 7 Torr at 300 8C of substrate-anneal. The experiments using synchrotron radiation source was performed at the Pohang Light Source. For the characterization of valence band and O 1s absorption spectra, photoemission spectroscopy (PES) and X-ray absorption spectroscopy (XAS) were employed at the 4B1 beam line. Structural analyses using X-ray diffraction (XRD) and reflectivity (XRR) were performed at the 3C2 beam line. For an electrical measurement of MOS structure, capacitance–voltage (C–V) and leakage current density were obtained from HP4284A precision LCR meter and HP4145B semiconductor parameter analyzer, respectively.
3. Results and discussion Fig. 1 represents photoemission spectra of Gd 4d and Si 2p. The peak position of Gd 4d corresponds to 3+ ionic state and showed a peak shift to high binding energy with GSO film. Si 2p peak was observed at 102.3 eV, lower than the binding energy of Si 2p in SiO2. The binding energy shifts were caused by the relative electro-positive/negative nature of second nearest-neighbor [7]. A relatively high covalence of Si induces Gd of Gd–O(–Si) bond in GSO more ionic
Fig. 1. Photoemission spectra (hn = 450 eV) of (a) Gd 4d and (b) Si 2p in Gd2O3 and GSO films.
than Gd of Gd–O(–Gd) in Gd2O3, whereas Si of Si– O(–Gd) in GSO shows more covalent than SiO2, i.e., a chemical shift to low binding energy. The dielectric thickness and interface roughness were estimated from the reflectivity measurement as shown in Fig. 2. The oxide thickness was determined as 18 nm from the width of oscillation period. A gradual decay of specula reflectivity at low angle indicated that crystalline Gd2O3 film has rougher interface than that of GSO film. The interface roughness was estimated as 0.57 and 0.32 nm for Gd2O3 and GSO, respectively. Yu et al. reported that any crystalline oxides could destroy the interface and cause a Fermi-level pinning [6]. As indicated in the inset of Fig. 2, root mean square (RMS) roughness of GSO film surface was calculated as 0.60 nm, corresponding to the result of XRR. From the X-ray diffraction pattern in Fig. 3, (4 4 0) and (4 3 1) diffractions were observed with Gd2O3 film of bixbyte structure (C-type rare-earth). While, no diffraction peak was observed with GSO film.
J.-K. Yang, H.-H. Park / Applied Surface Science 244 (2005) 293–296
Fig. 2. X-ray reflectivity of Gd2O3 and GSO films on nGaAs(0 0 1). The inset shows respective surface morphologies observed by AFM (t: film thickness, s1: interface roughness, s2: surface roughness, and s2*: root mean square roughness).
Fig. 4 shows the change of energy band gap in Gd2O3 with the incorporation of SiO2. The results of PES were combined with those of XAS to express the unoccupied and occupied electronic states. Fermi energy in XAS could be determined from the binding energy of O 1s core level. The values of valence band maximum were obtained as 3.5 and 3.7 eV for
Fig. 3. X-ray diffraction patterns of (a) Gd2O3 and (b) GSO films on n-GaAs(0 0 1). u–2u scans were performed using the incident photon energy of 8.05 eV (l = 0.154 nm).
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Fig. 4. Valence band spectra and O 1s X-ray absorption spectra were combined with an energy scale relative to Fermi energy. The band gap was defined as the energy difference from valence-band maximum to the absorption edge obtained from the first derivative of XAS. The EF in XAS was determined from O 1s binding energy.
Gd2O3 and GSO, respectively. The conduction-band minimum could be defined as a maximum position of the first derivative [8]. The band gap was calculated as 5.8 eV for Gd2O3 and 6.6 eV for GSO films, which was 2 eV smaller than that of SiO2. Assuming the band gap of GaAs as 1.4 eV and no Fermi level-pinning, the increase of band offset values such as (DEV and DEC) at the GSO/n-GaAs interface could be simply interpreted. For example, 2.1 and 2.3 eV were estimated for the DEV and DEC with Gd2O3/n-GaAs while 2.3 and 2.9 eV with GSO/n-GaAs, respectively. Both the band alignment and surface or interface structure were closely correlated with the electrical properties. As shown in Fig. 5, a stretched C–V curve with deep depletion was attained in Gd2O3 film. Incomplete horizontal curve in the accumulation region indicated that the large roughness at the interface and small band offset induced a high leakage current. The curve of GSO film showed lower interface state density compared with that of Gd2O3 film. The large band offset and atomic level smoothness at the GSO/n-GaAs interface provided high carrier mobility without localized breakdown field. In the inset of Fig. 5, a re-plot of the J–V curves, a larger negative slope of GSO film indicated the higher tunneling barrier height, DEC than that of Gd2O3 [9].
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J.-K. Yang, H.-H. Park / Applied Surface Science 244 (2005) 293–296
GaAs interface. The J–V results confirmed an enhanced conduction band offset in the GSO/n-GaAs structure, which encouraged the design of GaAs MOSFET to be improved.
Acknowledgements This work was supported by Korea Research Foundation Grant (KRF-2003-041-D00375). The experiments at PLS were supported in part by MOST and POSTECH.
Fig. 5. High frequency (1 MHz) C–V characteristics of MOS structure for Gd2O3 and GSO. The inset represents Fowler–Nordheim tunneling mechanism, extracted from the relationship in log (J/E2) vs. 1/E in J–V curves.
4. Conclusions The incorporation of SiO2 into Gd2O3 prohibited the crystallization and induced amorphous GSO with smoother interface and surface than Gd2O3. It was revealed that the band gap of Gd2O3 could be controlled with the addition of SiO2 from the observations of valence band and O 1s absorption spectra. As a result, the C–V curve with GSO showed more saturated behavior in the accumulation region than Gd2O3, indicating an amelioration of the oxide/
References [1] C.W. Wilmsen, Physics and Chemistry of III–V Compound Semiconductor Interfaces, Plenum Press, New York, 1985, p. 165. [2] W.T. Tsang, M. Olmstead, R.P.H. Chang, Appl. Phys. Lett. 34 (1979) 408. [3] A. Callegari, P.D. Hoh, D.A. Buchanan, D. Lacey, Appl. Phys. Lett. 54 (1989) 332. [4] D. Ye, G.D. Wilk, B. Yang, J. Kwo, S.N.G. Chu, S. Nakahara, H.J.L. Gossmann, J.P. Mannaerts, M. Hong, K.K. Ng, J. Bude, Appl. Phys. Lett. 83 (2003) 180. [5] M. Hong, J. Kwo, A.R. Kortan, J.P. Mannaerts, A.M. Sergent, Science 283 (1999) 1897. [6] Z. Yu, C.D. Overgaard, R. Droopad, M. Passlack, J.K. Abrokwah, Appl. Phys. Lett. 82 (2003) 2978. [7] T.L. Barr, J. Vac. Sci. Technol. A 9 (1991) 1793. [8] H. Purdum, P.A. Montano, G.K. Shenoy, T. Morrison, Phys. Rev. B 25 (1982) 4412. [9] T.S. Lay, M. Hong, J. Kwo, J.P. Mannaerts, W.H. Hung, D.J. Huang, Solid-State Electron. 45 (2001) 1679.
Incorporation of SiO2 for the band alignment control of Gd2O3/n-GaAs(0 0 1) structure Jun-Kyu Yang, Hyung-Ho Park* Department of Ceramic Engineering, Yonsei University, 134 Sinchon-dong, Seodaemun-ku, Seoul 120-749, Republic of Korea Received 31 May 2004; accepted 22 September 2004 Available online 7 January 2005
Abstract Band alignment of Gd2O3 gate oxide films on n-GaAs(0 0 1) was controlled by the incorporation of SiO2. The photoelectron binding energy shifts in Gd2SiO5 film could be interpreted with relative electronegativity of second nearest neighbor element. The surface and interface morphology of Gd2SiO5/n-GaAs structure was smooth due to the absence of crystalline phase. Energy band gaps were estimated as 5.8 and 6.6 eV for Gd2O3 and Gd2SiO5, respectively, by combining photoemission with absorption spectra. A decrease of leakage current density and a saturated accumulation capacitance indicate an enhanced band offset and small roughness in Gd2SiO5/n-GaAs system. # 2004 Published by Elsevier B.V. PACS: 73.40.Q (metal–insulator–semiconductor structures) Keywords: GaAs; MOS; Gd2O3; Silicate; Band offset
1. Introduction A gate dielectric on GaAs has been investigated for a long time due to a large input voltage excursion and simple circuit design [1]. Nevertheless, most of III–V devices adopt a metal gate because of serious interfacial reaction between oxide and GaAs. Surface passivation and appropriate insulator material should be employed to overcome the aforementioned problems. Among the various gate dielectrics such * Corresponding author. Tel.: +82 2 2123 2853; fax: +82 2 365 5882. E-mail address: [email protected] (H.-H. Park). 0169-4332/$ – see front matter # 2004 Published by Elsevier B.V. doi:10.1016/j.apsusc.2004.09.164
as SiO2, Al2O3, Ga2O3(Gd2O3), etc [2–5], Gd2O3 film has been one of very attractive materials because it shows an epitaxial growth on GaAs(0 0 1). However, impediments in preparing clean surface and local structural/electrical deformation in polycrystalline phase drive us to search a new alternative amorphous film. Al2O3 and Ga2O3 have been recently focused as amorphous oxide–GaAs system, since their films showed high breakdown field and atomically abrupt interface with GaAs [4,6]. In addition, considering a close relation between band parameters and electrical properties, the band offset control must be one of the most significant requirements for the design of metal-oxide–semiconductor field effect transistor
294
J.-K. Yang, H.-H. Park / Applied Surface Science 244 (2005) 293–296
(MOSFET). So then, an amorphous oxide film with large band offsets can be an optimized host for the gate of GaAs MOS devices. In this work, a rare-earth silicate amorphous film was employed as a gate dielectric on n-GaAs. The band alignment of Gd2O3 with regard to n-GaAs was effectively controlled by incorporating SiO2. We have correlated the change of band structure and the interfacial properties with electrical characteristics such as dielectric capacitance and leakage current.
2. Experimental procedure n-GaAs(0 0 1) wafer (Si-doping of 8 1016 cm 3) was used in this experiment. Au/Ge/Ni/Au was served as a low resistance ohmic contact by subsequent evaporation and anneal. After sulfur passivation of GaAs surface, Gd2O3 and Gd2SiO5 (hereafter denoted as GSO) films were deposited by e-beam evaporation of Gd2O3 and mixed oxides of (Gd2O3)0.8(SiO2)0.2, respectively, under 2 10 7 Torr at 300 8C of substrate-anneal. The experiments using synchrotron radiation source was performed at the Pohang Light Source. For the characterization of valence band and O 1s absorption spectra, photoemission spectroscopy (PES) and X-ray absorption spectroscopy (XAS) were employed at the 4B1 beam line. Structural analyses using X-ray diffraction (XRD) and reflectivity (XRR) were performed at the 3C2 beam line. For an electrical measurement of MOS structure, capacitance–voltage (C–V) and leakage current density were obtained from HP4284A precision LCR meter and HP4145B semiconductor parameter analyzer, respectively.
3. Results and discussion Fig. 1 represents photoemission spectra of Gd 4d and Si 2p. The peak position of Gd 4d corresponds to 3+ ionic state and showed a peak shift to high binding energy with GSO film. Si 2p peak was observed at 102.3 eV, lower than the binding energy of Si 2p in SiO2. The binding energy shifts were caused by the relative electro-positive/negative nature of second nearest-neighbor [7]. A relatively high covalence of Si induces Gd of Gd–O(–Si) bond in GSO more ionic
Fig. 1. Photoemission spectra (hn = 450 eV) of (a) Gd 4d and (b) Si 2p in Gd2O3 and GSO films.
than Gd of Gd–O(–Gd) in Gd2O3, whereas Si of Si– O(–Gd) in GSO shows more covalent than SiO2, i.e., a chemical shift to low binding energy. The dielectric thickness and interface roughness were estimated from the reflectivity measurement as shown in Fig. 2. The oxide thickness was determined as 18 nm from the width of oscillation period. A gradual decay of specula reflectivity at low angle indicated that crystalline Gd2O3 film has rougher interface than that of GSO film. The interface roughness was estimated as 0.57 and 0.32 nm for Gd2O3 and GSO, respectively. Yu et al. reported that any crystalline oxides could destroy the interface and cause a Fermi-level pinning [6]. As indicated in the inset of Fig. 2, root mean square (RMS) roughness of GSO film surface was calculated as 0.60 nm, corresponding to the result of XRR. From the X-ray diffraction pattern in Fig. 3, (4 4 0) and (4 3 1) diffractions were observed with Gd2O3 film of bixbyte structure (C-type rare-earth). While, no diffraction peak was observed with GSO film.
J.-K. Yang, H.-H. Park / Applied Surface Science 244 (2005) 293–296
Fig. 2. X-ray reflectivity of Gd2O3 and GSO films on nGaAs(0 0 1). The inset shows respective surface morphologies observed by AFM (t: film thickness, s1: interface roughness, s2: surface roughness, and s2*: root mean square roughness).
Fig. 4 shows the change of energy band gap in Gd2O3 with the incorporation of SiO2. The results of PES were combined with those of XAS to express the unoccupied and occupied electronic states. Fermi energy in XAS could be determined from the binding energy of O 1s core level. The values of valence band maximum were obtained as 3.5 and 3.7 eV for
Fig. 3. X-ray diffraction patterns of (a) Gd2O3 and (b) GSO films on n-GaAs(0 0 1). u–2u scans were performed using the incident photon energy of 8.05 eV (l = 0.154 nm).
295
Fig. 4. Valence band spectra and O 1s X-ray absorption spectra were combined with an energy scale relative to Fermi energy. The band gap was defined as the energy difference from valence-band maximum to the absorption edge obtained from the first derivative of XAS. The EF in XAS was determined from O 1s binding energy.
Gd2O3 and GSO, respectively. The conduction-band minimum could be defined as a maximum position of the first derivative [8]. The band gap was calculated as 5.8 eV for Gd2O3 and 6.6 eV for GSO films, which was 2 eV smaller than that of SiO2. Assuming the band gap of GaAs as 1.4 eV and no Fermi level-pinning, the increase of band offset values such as (DEV and DEC) at the GSO/n-GaAs interface could be simply interpreted. For example, 2.1 and 2.3 eV were estimated for the DEV and DEC with Gd2O3/n-GaAs while 2.3 and 2.9 eV with GSO/n-GaAs, respectively. Both the band alignment and surface or interface structure were closely correlated with the electrical properties. As shown in Fig. 5, a stretched C–V curve with deep depletion was attained in Gd2O3 film. Incomplete horizontal curve in the accumulation region indicated that the large roughness at the interface and small band offset induced a high leakage current. The curve of GSO film showed lower interface state density compared with that of Gd2O3 film. The large band offset and atomic level smoothness at the GSO/n-GaAs interface provided high carrier mobility without localized breakdown field. In the inset of Fig. 5, a re-plot of the J–V curves, a larger negative slope of GSO film indicated the higher tunneling barrier height, DEC than that of Gd2O3 [9].
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J.-K. Yang, H.-H. Park / Applied Surface Science 244 (2005) 293–296
GaAs interface. The J–V results confirmed an enhanced conduction band offset in the GSO/n-GaAs structure, which encouraged the design of GaAs MOSFET to be improved.
Acknowledgements This work was supported by Korea Research Foundation Grant (KRF-2003-041-D00375). The experiments at PLS were supported in part by MOST and POSTECH.
Fig. 5. High frequency (1 MHz) C–V characteristics of MOS structure for Gd2O3 and GSO. The inset represents Fowler–Nordheim tunneling mechanism, extracted from the relationship in log (J/E2) vs. 1/E in J–V curves.
4. Conclusions The incorporation of SiO2 into Gd2O3 prohibited the crystallization and induced amorphous GSO with smoother interface and surface than Gd2O3. It was revealed that the band gap of Gd2O3 could be controlled with the addition of SiO2 from the observations of valence band and O 1s absorption spectra. As a result, the C–V curve with GSO showed more saturated behavior in the accumulation region than Gd2O3, indicating an amelioration of the oxide/
References [1] C.W. Wilmsen, Physics and Chemistry of III–V Compound Semiconductor Interfaces, Plenum Press, New York, 1985, p. 165. [2] W.T. Tsang, M. Olmstead, R.P.H. Chang, Appl. Phys. Lett. 34 (1979) 408. [3] A. Callegari, P.D. Hoh, D.A. Buchanan, D. Lacey, Appl. Phys. Lett. 54 (1989) 332. [4] D. Ye, G.D. Wilk, B. Yang, J. Kwo, S.N.G. Chu, S. Nakahara, H.J.L. Gossmann, J.P. Mannaerts, M. Hong, K.K. Ng, J. Bude, Appl. Phys. Lett. 83 (2003) 180. [5] M. Hong, J. Kwo, A.R. Kortan, J.P. Mannaerts, A.M. Sergent, Science 283 (1999) 1897. [6] Z. Yu, C.D. Overgaard, R. Droopad, M. Passlack, J.K. Abrokwah, Appl. Phys. Lett. 82 (2003) 2978. [7] T.L. Barr, J. Vac. Sci. Technol. A 9 (1991) 1793. [8] H. Purdum, P.A. Montano, G.K. Shenoy, T. Morrison, Phys. Rev. B 25 (1982) 4412. [9] T.S. Lay, M. Hong, J. Kwo, J.P. Mannaerts, W.H. Hung, D.J. Huang, Solid-State Electron. 45 (2001) 1679.