- Email: [email protected]
Ge(1 0 0) interface by synchrotron radiation photoelectron spectroscopy
Applied Surface Science 256 (2010) 6057–6059
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Experimental determination of valence band offset at PbTe/Ge(1 0 0) interface by synchrotron radiation photoelectron spectroscopy C.F. Cai a , H.Z. Wu a,∗ , J.X. Si a , W.H. Zhang b , Y. Xu b , J.F. Zhu b a b
Department of Physics, State Key Laboratory for Modern Optical Instruments, Zhejiang University, Hangzhou, Zhejiang, 310027, China National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui, 230029, China
a r t i c l e
i n f o
Article history: Received 11 October 2009 Accepted 23 March 2010 Available online 30 March 2010 Keywords: SRPES PbTe/Ge heterojunction Band offsets
a b s t r a c t The band offset at the interface of PbTe/Ge (1 0 0) heterojunction was studied by the synchrotron radiation photoelectron spectroscopy. A valence band offset of EV = 0.07 ± 0.05 eV, and a conduction band offset of EC = 0.27 ± 0.05 eV are concluded. The experimental determination of the band offset for the PbTe/Ge interface should be beneficial for the heterojunction to be applied in new optoelectronic and electronic devices. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, both IV–VI compound semiconductors (such as PbTe and PbSe) and IV group element semiconductors (such as Si and Ge) have attracted much interest in the applications of solar energy conversion: PbSe and PbTe colloidal nanocrystals have demonstrated high-efficiency carrier multiplication (or multiple exciton generation), i.e. a single incident photon with energy greater than at least twice the effective band-gap of PbSe (or PbTe) can generate 2 or more electron–hole pairs, or excitons which has potential applications in the fabrication of high efficient solar cells [1,2], while Ge is widely used in solar cell fabrication as wafer substrates or active regions [3]. Electrical properties and spectral response of PbTe/Ge heterojunctions were studied and its thin film infrared detector arrays for integrated electronic structures were demonstrated [4,5]. Further, PbTe detector arrays were also fabricated on Si wafer substrates for its convenience in the integration with readout electronic circuits [6]. However, the performance of these IV–VI compound devices should be improved to compete with III–V compound devices. The understanding of electronic properties at the interface of PbTe and Ge materials are a key issue in the design and improvement of the optoelectronic devices which are made from the heterojunctions. Perfetti et al. [7] has studied the PbTe/Ge heterojunction properties by measuring the current–voltage (I–V) characteristics of the PbTe/Ge p–n diodes, and a valence band offset (VBO) of 0.26 eV and a conduction band offset (CBO) of 0.12 eV
were concluded. Cerrina et al. [8] also investigated the behavior of Ge semiconductor adsorbed on PbTe (1 0 0) substrates. However, the measured band discontinuities of the Ge/PbTe interface were much different, with a VBO of −0.35 eV (i.e. Ge valence band edge is 0.35 eV higher than PbTe valence band edge) and CBO of 0.71 eV (i.e. Ge conduction band edge is 0.71 eV higher than PbTe conduction band edge). It is seen that there is large discrepancy between the two reported experimental results. To our knowledge, there is no other experimental data on the PbTe/Ge heterojunction available. To clarify the controversial experimental data for the essential band discontinuity parameters of the PbTe/Ge interface, further investigation is needed. In this paper, we report the measurement of ultraviolet photoelectron spectroscopy (UPS) for the precise determination of the VBO at the PbTe/Ge(1 0 0) interface using synchrotron radiation as the excitation source. Photoelectron spectroscopy (PES) (including ultraviolet and X-ray photoelectron spectroscopy) has been demonstrated to be a powerful tool for direct and precise determination of EV [9–11]. With changeable excitation energy (20–200 eV) and a high-brightness incident photon flux, synchrotron radiation photoelectron spectrum (SRPES) can provide high signal to noise ratio on both the valence band maximum (VBM) and core levels (CLs), which is particularly important for the precise determination of the VBM, consequently the valence band offset of a heterojunction.
2. Experimental ∗ Corresponding author. Tel.: +86 057187953885; fax: +86 057187951328. E-mail address: [email protected] (H.Z. Wu). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.03.119
The measurements of UPS were performed at the surface physics station of National Synchrotron Radiation Laboratory (NSRL) at
6058
C.F. Cai et al. / Applied Surface Science 256 (2010) 6057–6059
Hefei, China. This experimental station is composed of a threechamber VG multi-technique ultra high vacuum (UHV) system. The analysis chamber which has a base pressure better than 2 × 10−10 mbar is equipped with an ARUPS10 hemispherical analyzer. The XPS (X-ray photoelectron spectroscopy), LEED (low energy electron diffraction), and argon-ion gun are also equipped for the surface characterizations and pre-treatment of the samples. The incident beam line covers the energy range from 20 to 200 eV and the energy resolution (E/E) is better than 1000. The details of the experimental station and the related beam line were previously described [12]. Growth of PbTe on (1 0 0)-oriented Ge substrates was carried out in our molecular beam epitaxy (MBE) system. High quality PbTe films with mirror-like surface were obtained by optimizing the growth conditions. The XRD measurement revealed that the PbTe film is in [1 0 0] orientation: only PbTe (2 0 0), (4 0 0) peaks can be detected in the XRD pattern. Three samples were prepared for the experimental measurements: (1) a thick epitaxial PbTe sample grown on a Ge (1 0 0) substrate; (2) a PbTe (2 nm)/Ge (1 0 0) heterojunction sample; (3) a Ge (1 0 0) single crystal bulk sample. Once the samples were transferred to the analysis chamber, clean PbTe (1 0 0) and Ge (1 0 0) surfaces were obtained by argon-ion (Ar+ ) sputtering (1000 eV) at a low sputtering rate, which alleviated damage to the samples. The cleanliness was checked by subsequent XPS scans for inspection of the removal of oxygen by observing the O 5s and C 1s peaks. The UPS measurements were carried out with the excitation of beam line energy of 75 eV, at which the absorption cross sections of the Pb 5d CLs and Ge 3d CLs reach maximums. Large value of the absorption cross section and intense beam line provided good sensitivity to both of VBMs and CLs measurements. The UPS spectra were measured with an incident beam angle of 45◦ to the normal of the surface and a normal emission geometry. The analyzer pass energy was set at 20 eV. All the spectra were carefully calibrated by the gold (Au) work function and the binding energy of the Au 4f7/2 CL peak to the Fermi level is 84.0 eV. The surface charge can be neglected for both PbTe films and Ge bulk crystal due to their high conductivity and good indium contacts with the equipment.
Fig. 1. The schematic diagram of energy-band for PbTe/Ge (1 0 0) heterojunction system.
Fig. 3 is the photoelectron spectrum of the PbTe/Ge(1 0 0) heterojunction sample. The thickness of PbTe film is about 2 nm, thin enough so that the photoelectron emitted from underlying Ge substrate can be detected. The spectrum for the PbTe/Ge heterojunction includes the signals from both PbTe epilayer and Ge at the interface region within the photoelectron probing depth. The Pb 5d CL peaks from the PbTe overlayer and Ge 3d CL peak from the underlying Ge can be clearly distinguished in the spectrum. The
3. Results and discussion The principle in determination of VBO (EV ) for a heterojunction by using photoelectron spectrum technique has been fully described in Ref. [9]. The VBO for the PbTe/Ge interface can be calculated from the following formula: PbTe EV = ECL + (EPb 5d
5/2
Ge where, ECL = EGe
3d
PbTe Ge − EVBM ) − (EGe
PbTe − EPb 5d
5/2
3d
Ge − EVBM )
(1)
is the binding-energy difference
between Pb 5d5/2 CL of PbTe and Ge 3d CL of Ge substrate measured PbTe PbTe ) from the thin PbTe/Ge heterojunction sample. (EPb − EVBM 5d 5/2
Ge Ge ) are the binding-energy difference between CLs and (EGe − EVBM 3d and VBMs measured from thick PbTe film and Ge bulk samples. A schematic energy-band diagram for the PbTe/Ge heterojunction is given in Fig. 1. Fig. 2 shows the photoelectron spectrum of the thick PbTe film (a) and Ge bulk (b) samples. All the CL peaks have been fitted using a Shirley background and Voigt (mixed Lorentzian–Gaussian) profile. To obtain (ECL − EVBM ), the position of EVBM is located by linear extrapolation of the valence-band leading edge. The linear method is purely experimental in nature, and will effectively remove the resolution-induced tail at the top of the valence-band (VB) [13]. The result of this procedure is shown in the inset of Fig. 2. The energy differences between Pb 5d5/2 CL peak to the PbTe VBM and Ge 3d CL peak to the Ge VBM are obtained as 18.47 ± 0.05 eV and 29.21 ± 0.05 eV, respectively.
Fig. 2. VB spectra with CLs for thick PbTe layer sample (a) and Ge bulk sample (b). All peaks have been fitted using a Shirley background and a Voigt profile. Insets show the portions of the spectra near the valence band maximum regions of the samples. The VBM is determined by two straight lines fitting the leading edge of the valence band and the background, as displayed in the inset.
C.F. Cai et al. / Applied Surface Science 256 (2010) 6057–6059
Fig. 3. The photoelectron spectra of PbTe/Ge (1 0 0) heterojunction sample. Ge 3d CL peak from the underlying Ge crystal can be clearly distinguished. Inset shows the consistency of the CL spectra of the thick PbTe film (solid line) and PbTe/Ge heterojunction sample (dash line).
energy difference between Ge 3d CL and Pb 5d5/2 CL is obtained as 10.81 ± 0.02 eV. In the experiment, we noted that the photoelectron spectrum of the PbTe/Ge heterojunction is consistent with the spectrum of the thick PbTe film sample (Fig. 2(a)), which means the 2 nm PbTe epilayer in the PbTe/Ge heterojunction sample is essentially the same in nature as the thick PbTe epilayer. The bandbending effect is an important factor that is usually difficult to be quantitatively evaluated in the measurement of ECL . In the case of PbTe/Ge heterojunction, the band-bending effect can be neglected which is explained as following. For a heterojunction, at thermal equilibrium the ratio of the potential drops at the two sides of the semiconductor interface can be expressed as: VD1 /VD2 = ε2 N2 /ε1 N1 , where N is the doping levels which are around 1017 cm−3 for both PbTe and Ge materials used in this experiment, is the static dielectric constants that are 400 for PbTe and 16 for Ge. It is evident that the band-bending is much more pronounced in Ge than in PbTe for the PbTe/Ge heterojunction. Since the photoelectron probe depth in Ge is much smaller than the width of the space charge region (∼50 nm), the potential drops at the interface of Ge side is small and neglectable. PbTe Substituting the above measured quantities: (EPb − 5d 5/2
PbTe ) = 18.47 ± 0.05 eV, EVBM Ge ECL = EGe
3d
PbTe − EPb 5d
5/2
Ge (EGe
3d
Ge ) = 29.21 ± 0.05 eV, − EVBM
= 10.81 ± 0.02 eV, to the Eq. (1), we
obtained the VBO of PbTe/Ge(1 0 0) is 0.07 ± 0.05 eV. As shown in Fig. 1 the conduction band discontinuity EC can be evaluated from the equation: EC = Eg2 − Eg1 − EV . At 300 K, the band-gap of PbTe, Eg1 , is 0.32 eV [14], and the band-gap of Ge, Eg2 , is 0.66 eV [15], therefore the EC is 0.27 ± 0.05 eV. The CBO value obtained in this work is basically in agreement with the Anderson’s electron affinity rule, which takes the conduction offset to be equal to the difference between the PbTe and Ge electron affinities, EC = q(1 − 2 ) = 0.47 ± 0.3 eV, where 1 = 4.6 ± 0.3 V for PbTe [16], and 2 = 4.13 V, for Ge. In fact, Anderson’s ideal model may be unsatisfactory in the case of PbTe/Ge heterojunction because high densities of interface states are present at the heterojunction due to large lattice mismatch between the two semiconductors. The VBO of PbTe/Ge(1 0 0) determined in this work is smaller than the previously reported value of 0.26 eV (Ref. [7]), and −0.35 eV (Ref. [8]). Perfetti et al. [7] used an indirect method in determining the valence band offset. A valence band offset of
6059
0.26 eV was attained by fitting the measured I–V characteristics of a PbTe/Ge p–n heterojunction in which more uncertainty may have been involved. In Cerrina’s experiment [8] the Ge atoms were adsorbed on PbTe substrates (actually, adsorption is quite different from epitaxial growth process) and large amount of unreacted Pb and reacted Te atoms were observed to diffuse over the Ge layer, even when the thickness of Ge overlayer exceeded 40 Å. The diffusion of the Pb and Te components from the substrate may evidently affect the band structure at the interface of the Ge/PbTe heterojunction. Contrarily, in this work PbTe with [1 0 0] orientation was epitaxially grown on Ge (1 0 0) substrates by MBE method. Rare outdiffusion of Ge atoms on PbTe overlayer occurred in the PbTe/Ge heterojunction because Ge substrate is a very stable crystal and PbTe is a single crystal. With a 2-nm PbTe overlayer coverage Ge 3d signal is significantly inhibited as seen in the Fig. 3, which indicates the PbTe/Ge interface is abrupt and the interdiffusion is neglectable. 4. Conclusion Ultraviolet photoelectron spectroscopy using synchrotron radiation as the excitation source has been utilized for the precise determination of the VBO at the PbTe/Ge(1 0 0) interfaces. XRD characterization shows the PbTe thin films grown on Ge (1 0 0) substrates by molecular beam epitaxy are in [1 0 0] orientation and fully relaxed. The UPS measurements with synchrotron radiation improve ratio of signal to noise for both VBM and CL spectra. A valence band offset of EV = 0.07 ± 0.05 eV and a conduction band offset of EC = 0.27 ± 0.05 eV are concluded. The value is in agreement with the electron affinity rule, which takes the conduction band offset to be equal to the difference between the PbTe and Ge electron affinities. The experimental determination of the band offset for the PbTe/Ge heterojunction should be beneficial for the heterojunction to be applied in new optoelectronic and electronic devices. Acknowledgment This work has been supported by the National Natural Science Foundation of China (Grant Nos. 1094174 and 60676003). References [1] R.D. Schaller, V.M. Agranovich, V.I. Klimov, Nat. Phys. 1 (2005) 189. [2] J.E. Murphy, M.C. Beard, A.G. Norman, S.P. Ahrenkiel, J.C. Johnson, P. Yu, O.I. ´ c, ´ R.J. Ellingson, A.J. Nozik, J. Am. Chem. Soc. 128 (2006) 3241. Mici [3] I. Prieto, B. Galiana, P.A. Postigo, C. Algora, L.J. Martínez, I. Rey-Stolle, Appl. Phys. Lett. 94 (2009) 191102. [4] C. Corsi, M. Miller, Thin Solid Films 20 (1974) S41. [5] C. Corsi, G. Cappuccio, A. D’amico, G. Petrocco, G. Vitali, Infrared Phys. 16 (1976) 37. [6] H. Zogg, K. Alchalabi, D. Zimin, K. Kellermann, Infrared Phys. Technol. 43 (2002) 251. [7] P. Perfetti, F. Cerrina, C. Coluzza, G. Margaritondo, J. Appl. Phys. 45 (1974) 972. [8] F. Cerrina, R.R. Daniels, Te-Xiu Zhao, V. Fano, J. Val. Sci. Technol. B 1 (1983) 570. [9] E.A. Kraut, R.W. Grant, J.R. Waldrop, S.P. Kowalczyk, Phys. Rev. Lett. 44 (1980) 1620. [10] M. Perego, G. Seguini, M. Fanciulli, J. Appl. Phys. 100 (2006) 093718. [11] J.X. Si, S.Q. Jin, H.J. Zhang, P. Zhu, D.J. Qiu, H.Z. Wu, Appl. Phys. Lett. 93 (2008) 1. [12] C.W. Zou, B. Sun, W.H. Zhang, G.D. Wang, P.S. Xu, Q.P. Wang, F.Q. Xu, H.B. Pan, Nucl. Instrum. Methods A 548 (2005) 574. [13] S.A. Chambers, T. Droubay, T.C. Kaspar, M. Gutowski, J. Vac. Sci. Technol. B 22 (2004) 2205. [14] S. Yuan, H. Krenn, G. Springholz, G. Bauer, Phys. Rev. B 47 (1993) 7213. [15] S.M. Sze, K. Kwok, Ng (Eds.), Physics of Semiconductor Devices, 3rd ed., John Wiley & Sons, 2007, p. 789. [16] W.E. Spicer, G.T. Lapeyre, Phys. Rev. 139 (1965) A565.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Experimental determination of valence band offset at PbTe/Ge(1 0 0) interface by synchrotron radiation photoelectron spectroscopy C.F. Cai a , H.Z. Wu a,∗ , J.X. Si a , W.H. Zhang b , Y. Xu b , J.F. Zhu b a b
Department of Physics, State Key Laboratory for Modern Optical Instruments, Zhejiang University, Hangzhou, Zhejiang, 310027, China National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui, 230029, China
a r t i c l e
i n f o
Article history: Received 11 October 2009 Accepted 23 March 2010 Available online 30 March 2010 Keywords: SRPES PbTe/Ge heterojunction Band offsets
a b s t r a c t The band offset at the interface of PbTe/Ge (1 0 0) heterojunction was studied by the synchrotron radiation photoelectron spectroscopy. A valence band offset of EV = 0.07 ± 0.05 eV, and a conduction band offset of EC = 0.27 ± 0.05 eV are concluded. The experimental determination of the band offset for the PbTe/Ge interface should be beneficial for the heterojunction to be applied in new optoelectronic and electronic devices. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Recently, both IV–VI compound semiconductors (such as PbTe and PbSe) and IV group element semiconductors (such as Si and Ge) have attracted much interest in the applications of solar energy conversion: PbSe and PbTe colloidal nanocrystals have demonstrated high-efficiency carrier multiplication (or multiple exciton generation), i.e. a single incident photon with energy greater than at least twice the effective band-gap of PbSe (or PbTe) can generate 2 or more electron–hole pairs, or excitons which has potential applications in the fabrication of high efficient solar cells [1,2], while Ge is widely used in solar cell fabrication as wafer substrates or active regions [3]. Electrical properties and spectral response of PbTe/Ge heterojunctions were studied and its thin film infrared detector arrays for integrated electronic structures were demonstrated [4,5]. Further, PbTe detector arrays were also fabricated on Si wafer substrates for its convenience in the integration with readout electronic circuits [6]. However, the performance of these IV–VI compound devices should be improved to compete with III–V compound devices. The understanding of electronic properties at the interface of PbTe and Ge materials are a key issue in the design and improvement of the optoelectronic devices which are made from the heterojunctions. Perfetti et al. [7] has studied the PbTe/Ge heterojunction properties by measuring the current–voltage (I–V) characteristics of the PbTe/Ge p–n diodes, and a valence band offset (VBO) of 0.26 eV and a conduction band offset (CBO) of 0.12 eV
were concluded. Cerrina et al. [8] also investigated the behavior of Ge semiconductor adsorbed on PbTe (1 0 0) substrates. However, the measured band discontinuities of the Ge/PbTe interface were much different, with a VBO of −0.35 eV (i.e. Ge valence band edge is 0.35 eV higher than PbTe valence band edge) and CBO of 0.71 eV (i.e. Ge conduction band edge is 0.71 eV higher than PbTe conduction band edge). It is seen that there is large discrepancy between the two reported experimental results. To our knowledge, there is no other experimental data on the PbTe/Ge heterojunction available. To clarify the controversial experimental data for the essential band discontinuity parameters of the PbTe/Ge interface, further investigation is needed. In this paper, we report the measurement of ultraviolet photoelectron spectroscopy (UPS) for the precise determination of the VBO at the PbTe/Ge(1 0 0) interface using synchrotron radiation as the excitation source. Photoelectron spectroscopy (PES) (including ultraviolet and X-ray photoelectron spectroscopy) has been demonstrated to be a powerful tool for direct and precise determination of EV [9–11]. With changeable excitation energy (20–200 eV) and a high-brightness incident photon flux, synchrotron radiation photoelectron spectrum (SRPES) can provide high signal to noise ratio on both the valence band maximum (VBM) and core levels (CLs), which is particularly important for the precise determination of the VBM, consequently the valence band offset of a heterojunction.
2. Experimental ∗ Corresponding author. Tel.: +86 057187953885; fax: +86 057187951328. E-mail address: [email protected] (H.Z. Wu). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.03.119
The measurements of UPS were performed at the surface physics station of National Synchrotron Radiation Laboratory (NSRL) at
6058
C.F. Cai et al. / Applied Surface Science 256 (2010) 6057–6059
Hefei, China. This experimental station is composed of a threechamber VG multi-technique ultra high vacuum (UHV) system. The analysis chamber which has a base pressure better than 2 × 10−10 mbar is equipped with an ARUPS10 hemispherical analyzer. The XPS (X-ray photoelectron spectroscopy), LEED (low energy electron diffraction), and argon-ion gun are also equipped for the surface characterizations and pre-treatment of the samples. The incident beam line covers the energy range from 20 to 200 eV and the energy resolution (E/E) is better than 1000. The details of the experimental station and the related beam line were previously described [12]. Growth of PbTe on (1 0 0)-oriented Ge substrates was carried out in our molecular beam epitaxy (MBE) system. High quality PbTe films with mirror-like surface were obtained by optimizing the growth conditions. The XRD measurement revealed that the PbTe film is in [1 0 0] orientation: only PbTe (2 0 0), (4 0 0) peaks can be detected in the XRD pattern. Three samples were prepared for the experimental measurements: (1) a thick epitaxial PbTe sample grown on a Ge (1 0 0) substrate; (2) a PbTe (2 nm)/Ge (1 0 0) heterojunction sample; (3) a Ge (1 0 0) single crystal bulk sample. Once the samples were transferred to the analysis chamber, clean PbTe (1 0 0) and Ge (1 0 0) surfaces were obtained by argon-ion (Ar+ ) sputtering (1000 eV) at a low sputtering rate, which alleviated damage to the samples. The cleanliness was checked by subsequent XPS scans for inspection of the removal of oxygen by observing the O 5s and C 1s peaks. The UPS measurements were carried out with the excitation of beam line energy of 75 eV, at which the absorption cross sections of the Pb 5d CLs and Ge 3d CLs reach maximums. Large value of the absorption cross section and intense beam line provided good sensitivity to both of VBMs and CLs measurements. The UPS spectra were measured with an incident beam angle of 45◦ to the normal of the surface and a normal emission geometry. The analyzer pass energy was set at 20 eV. All the spectra were carefully calibrated by the gold (Au) work function and the binding energy of the Au 4f7/2 CL peak to the Fermi level is 84.0 eV. The surface charge can be neglected for both PbTe films and Ge bulk crystal due to their high conductivity and good indium contacts with the equipment.
Fig. 1. The schematic diagram of energy-band for PbTe/Ge (1 0 0) heterojunction system.
Fig. 3 is the photoelectron spectrum of the PbTe/Ge(1 0 0) heterojunction sample. The thickness of PbTe film is about 2 nm, thin enough so that the photoelectron emitted from underlying Ge substrate can be detected. The spectrum for the PbTe/Ge heterojunction includes the signals from both PbTe epilayer and Ge at the interface region within the photoelectron probing depth. The Pb 5d CL peaks from the PbTe overlayer and Ge 3d CL peak from the underlying Ge can be clearly distinguished in the spectrum. The
3. Results and discussion The principle in determination of VBO (EV ) for a heterojunction by using photoelectron spectrum technique has been fully described in Ref. [9]. The VBO for the PbTe/Ge interface can be calculated from the following formula: PbTe EV = ECL + (EPb 5d
5/2
Ge where, ECL = EGe
3d
PbTe Ge − EVBM ) − (EGe
PbTe − EPb 5d
5/2
3d
Ge − EVBM )
(1)
is the binding-energy difference
between Pb 5d5/2 CL of PbTe and Ge 3d CL of Ge substrate measured PbTe PbTe ) from the thin PbTe/Ge heterojunction sample. (EPb − EVBM 5d 5/2
Ge Ge ) are the binding-energy difference between CLs and (EGe − EVBM 3d and VBMs measured from thick PbTe film and Ge bulk samples. A schematic energy-band diagram for the PbTe/Ge heterojunction is given in Fig. 1. Fig. 2 shows the photoelectron spectrum of the thick PbTe film (a) and Ge bulk (b) samples. All the CL peaks have been fitted using a Shirley background and Voigt (mixed Lorentzian–Gaussian) profile. To obtain (ECL − EVBM ), the position of EVBM is located by linear extrapolation of the valence-band leading edge. The linear method is purely experimental in nature, and will effectively remove the resolution-induced tail at the top of the valence-band (VB) [13]. The result of this procedure is shown in the inset of Fig. 2. The energy differences between Pb 5d5/2 CL peak to the PbTe VBM and Ge 3d CL peak to the Ge VBM are obtained as 18.47 ± 0.05 eV and 29.21 ± 0.05 eV, respectively.
Fig. 2. VB spectra with CLs for thick PbTe layer sample (a) and Ge bulk sample (b). All peaks have been fitted using a Shirley background and a Voigt profile. Insets show the portions of the spectra near the valence band maximum regions of the samples. The VBM is determined by two straight lines fitting the leading edge of the valence band and the background, as displayed in the inset.
C.F. Cai et al. / Applied Surface Science 256 (2010) 6057–6059
Fig. 3. The photoelectron spectra of PbTe/Ge (1 0 0) heterojunction sample. Ge 3d CL peak from the underlying Ge crystal can be clearly distinguished. Inset shows the consistency of the CL spectra of the thick PbTe film (solid line) and PbTe/Ge heterojunction sample (dash line).
energy difference between Ge 3d CL and Pb 5d5/2 CL is obtained as 10.81 ± 0.02 eV. In the experiment, we noted that the photoelectron spectrum of the PbTe/Ge heterojunction is consistent with the spectrum of the thick PbTe film sample (Fig. 2(a)), which means the 2 nm PbTe epilayer in the PbTe/Ge heterojunction sample is essentially the same in nature as the thick PbTe epilayer. The bandbending effect is an important factor that is usually difficult to be quantitatively evaluated in the measurement of ECL . In the case of PbTe/Ge heterojunction, the band-bending effect can be neglected which is explained as following. For a heterojunction, at thermal equilibrium the ratio of the potential drops at the two sides of the semiconductor interface can be expressed as: VD1 /VD2 = ε2 N2 /ε1 N1 , where N is the doping levels which are around 1017 cm−3 for both PbTe and Ge materials used in this experiment, is the static dielectric constants that are 400 for PbTe and 16 for Ge. It is evident that the band-bending is much more pronounced in Ge than in PbTe for the PbTe/Ge heterojunction. Since the photoelectron probe depth in Ge is much smaller than the width of the space charge region (∼50 nm), the potential drops at the interface of Ge side is small and neglectable. PbTe Substituting the above measured quantities: (EPb − 5d 5/2
PbTe ) = 18.47 ± 0.05 eV, EVBM Ge ECL = EGe
3d
PbTe − EPb 5d
5/2
Ge (EGe
3d
Ge ) = 29.21 ± 0.05 eV, − EVBM
= 10.81 ± 0.02 eV, to the Eq. (1), we
obtained the VBO of PbTe/Ge(1 0 0) is 0.07 ± 0.05 eV. As shown in Fig. 1 the conduction band discontinuity EC can be evaluated from the equation: EC = Eg2 − Eg1 − EV . At 300 K, the band-gap of PbTe, Eg1 , is 0.32 eV [14], and the band-gap of Ge, Eg2 , is 0.66 eV [15], therefore the EC is 0.27 ± 0.05 eV. The CBO value obtained in this work is basically in agreement with the Anderson’s electron affinity rule, which takes the conduction offset to be equal to the difference between the PbTe and Ge electron affinities, EC = q(1 − 2 ) = 0.47 ± 0.3 eV, where 1 = 4.6 ± 0.3 V for PbTe [16], and 2 = 4.13 V, for Ge. In fact, Anderson’s ideal model may be unsatisfactory in the case of PbTe/Ge heterojunction because high densities of interface states are present at the heterojunction due to large lattice mismatch between the two semiconductors. The VBO of PbTe/Ge(1 0 0) determined in this work is smaller than the previously reported value of 0.26 eV (Ref. [7]), and −0.35 eV (Ref. [8]). Perfetti et al. [7] used an indirect method in determining the valence band offset. A valence band offset of
6059
0.26 eV was attained by fitting the measured I–V characteristics of a PbTe/Ge p–n heterojunction in which more uncertainty may have been involved. In Cerrina’s experiment [8] the Ge atoms were adsorbed on PbTe substrates (actually, adsorption is quite different from epitaxial growth process) and large amount of unreacted Pb and reacted Te atoms were observed to diffuse over the Ge layer, even when the thickness of Ge overlayer exceeded 40 Å. The diffusion of the Pb and Te components from the substrate may evidently affect the band structure at the interface of the Ge/PbTe heterojunction. Contrarily, in this work PbTe with [1 0 0] orientation was epitaxially grown on Ge (1 0 0) substrates by MBE method. Rare outdiffusion of Ge atoms on PbTe overlayer occurred in the PbTe/Ge heterojunction because Ge substrate is a very stable crystal and PbTe is a single crystal. With a 2-nm PbTe overlayer coverage Ge 3d signal is significantly inhibited as seen in the Fig. 3, which indicates the PbTe/Ge interface is abrupt and the interdiffusion is neglectable. 4. Conclusion Ultraviolet photoelectron spectroscopy using synchrotron radiation as the excitation source has been utilized for the precise determination of the VBO at the PbTe/Ge(1 0 0) interfaces. XRD characterization shows the PbTe thin films grown on Ge (1 0 0) substrates by molecular beam epitaxy are in [1 0 0] orientation and fully relaxed. The UPS measurements with synchrotron radiation improve ratio of signal to noise for both VBM and CL spectra. A valence band offset of EV = 0.07 ± 0.05 eV and a conduction band offset of EC = 0.27 ± 0.05 eV are concluded. The value is in agreement with the electron affinity rule, which takes the conduction band offset to be equal to the difference between the PbTe and Ge electron affinities. The experimental determination of the band offset for the PbTe/Ge heterojunction should be beneficial for the heterojunction to be applied in new optoelectronic and electronic devices. Acknowledgment This work has been supported by the National Natural Science Foundation of China (Grant Nos. 1094174 and 60676003). References [1] R.D. Schaller, V.M. Agranovich, V.I. Klimov, Nat. Phys. 1 (2005) 189. [2] J.E. Murphy, M.C. Beard, A.G. Norman, S.P. Ahrenkiel, J.C. Johnson, P. Yu, O.I. ´ c, ´ R.J. Ellingson, A.J. Nozik, J. Am. Chem. Soc. 128 (2006) 3241. Mici [3] I. Prieto, B. Galiana, P.A. Postigo, C. Algora, L.J. Martínez, I. Rey-Stolle, Appl. Phys. Lett. 94 (2009) 191102. [4] C. Corsi, M. Miller, Thin Solid Films 20 (1974) S41. [5] C. Corsi, G. Cappuccio, A. D’amico, G. Petrocco, G. Vitali, Infrared Phys. 16 (1976) 37. [6] H. Zogg, K. Alchalabi, D. Zimin, K. Kellermann, Infrared Phys. Technol. 43 (2002) 251. [7] P. Perfetti, F. Cerrina, C. Coluzza, G. Margaritondo, J. Appl. Phys. 45 (1974) 972. [8] F. Cerrina, R.R. Daniels, Te-Xiu Zhao, V. Fano, J. Val. Sci. Technol. B 1 (1983) 570. [9] E.A. Kraut, R.W. Grant, J.R. Waldrop, S.P. Kowalczyk, Phys. Rev. Lett. 44 (1980) 1620. [10] M. Perego, G. Seguini, M. Fanciulli, J. Appl. Phys. 100 (2006) 093718. [11] J.X. Si, S.Q. Jin, H.J. Zhang, P. Zhu, D.J. Qiu, H.Z. Wu, Appl. Phys. Lett. 93 (2008) 1. [12] C.W. Zou, B. Sun, W.H. Zhang, G.D. Wang, P.S. Xu, Q.P. Wang, F.Q. Xu, H.B. Pan, Nucl. Instrum. Methods A 548 (2005) 574. [13] S.A. Chambers, T. Droubay, T.C. Kaspar, M. Gutowski, J. Vac. Sci. Technol. B 22 (2004) 2205. [14] S. Yuan, H. Krenn, G. Springholz, G. Bauer, Phys. Rev. B 47 (1993) 7213. [15] S.M. Sze, K. Kwok, Ng (Eds.), Physics of Semiconductor Devices, 3rd ed., John Wiley & Sons, 2007, p. 789. [16] W.E. Spicer, G.T. Lapeyre, Phys. Rev. 139 (1965) A565.