Surface Science 601 (2007) 4428–4432 www.elsevier.com/locate/susc
Secondary electron imaging of SiC-based structures in secondary electron microscope A.A. Suvorova a
a,*
, S. Samarin
b
Centre for Microscopy and Microanalysis, The University of Western Australia, Crawley, 6009 WA, Australia b School of Physics, The University of Western Australia, Crawley, 6009 WA, Australia Available online 22 April 2007
Abstract Secondary electron imaging plays an important role in surface science, not only in imaging of surface topography and in the ability to resolve, for example, surface defects, but also in revealing the local variations in surface barrier height or work function, e.g. p–n junctions in semiconductor devices/materials. In our study, high resolution FESEM is used to qualitatively investigate the secondary electron contrast associated with band-structure variations in the semiconductor heterostructure formed by two adjacent hexagonal (4H) and (6H) silicon carbide polytype layers. In such heteropolytipic structure, effects due to different chemical constituents, defects, incoherent interfaces, and lattice mismatching can be avoided. The study of the 4H/6H heterostructure has revealed that the 6H polytype region, with a band-gap energy of 3.05 eV, exhibits higher secondary emission than the 4H region, with a larger band-gap of 3.26 eV. Reasons for the observed contrast will be discussed, with regard to the effects of the electronic structure and the theory of secondary electron emission with its stages of generation, transport to the surface and escape. Our measurements reveal that the operating ‘‘window’’ for maximizing SE contrast is very sensitive to the operating conditions, particularly incident electron energy. The observed secondary electron contrast can be useful in revealing electronic structure variation between different components of silicon carbide semiconductor devices. 2007 Elsevier B.V. All rights reserved. Keywords: Secondary electron emission; Secondary electron microscopy; Silicon carbide; Semiconductor heterostructures
1. Introduction Secondary electron (SE) imaging plays an important role in surface science, not only in imaging of surface topography and in the ability to resolve, for example, surface defects [1,2], but also in revealing the local variations in surface barrier height or work function, e.g. p–n junctions in semiconductor devices/materials [3]. Most previous work on secondary electron contrast from semiconductor heterostructures has considered either compositional or doped superlattices, e.g. Si–GexSi1x and GaAs–AlxGa1xAs heterostructures, imaged in a field emission scanning electron microscope in high vacuum at low beam
*
Corresponding author. Tel.: +61 8 6488 8095; fax: +61 8 6488 1017. E-mail address:
[email protected] (A.A. Suvorova).
0039-6028/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.04.142
energies [4,5]. These heterostructures can have a bandgap difference of up to 0.46 eV for Si/GeSi and up to 0.74 eV for GaAs/AlGaAs. The principles underlying the secondary electron (SE) and back-scattered electron (BSE) contrast in the compositional heterostructures, such as GeSi and GaAs/AlGaAs have been discussed in several papers [4–6]. The compositional contrast observed in this type of heterostructure has been explained in terms of differences in stopping power and atomic number. The enhanced SE emission from the Ge-rich layers in GeSi heterostructures was explained qualitatively in terms of a significantly larger BSE yield from Ge relative to Si. The SE contrast from the AlGaAs/GaAs layers was interpreted following the same argument, i.e. the Z-dependence of BSE yields. The effect of band-gap variations on the emission was only considered for the case of strained GeSi layers. The effect of ionisation potential on SE emission has been
A.A. Suvorova, S. Samarin / Surface Science 601 (2007) 4428–4432
demonstrated on a heterostructure that contained strained and unstrained GeSi layers where the 0.1 eV strain-induced raising of the valence band edge results in an increased SE emission from the strained GeSi layers [5]. The doping contrast in InP/InGaAsP heterostructures has been observed by Shealy [6]. Scanning electron microscopy (SEM) has only rarely been applied to the study of silicon carbide materials. In our study, SEM is used to detect the secondary electron contrast associated with band-structure variations in the semiconductor heterostructure formed by two adjacent hexagonal (4H and 6H) silicon carbide polytype layers. This heteropolytipic structure consists of a single semiconducting material occurring with different crystal structures, i.e. depending on the stacking sequence of the atomic Si–C bilayers in the [0 0 0 1] direction [7]. In such heterostructures, effects due to different chemical constituents, defects, incoherent interfaces, and lattice mismatching can be avoided. Although some work was reported on contrast between regions of different polytypes in SiC in conventional SEM, much of this work is based on observation of electron channelling contrast [8] and theoretical consideration of wurtzite/zinc blende (3C/4H, 3C/2H) SiC heterostructures [9–11]. Spectrally resolved cathodoluminescence imaging of SiC polytypes in a SEM has been reported by Mokhov et al. [12] to be useful for polytype identification in SiC epitaxial layers. 2. Experimental procedures The samples used in the experiments were (0 0 0 1) – ntype oriented silicon carbide with a donor concentration on the order of 1 · 1017 cm3 and with regions of 4H and 6H polytypes structure that have been identified by spectral cathodoluminescence (CL) study. This combination of two polytypes represents a heterostructure with difference in energy band-gap DEg 0.2 eV (see Table 1). Investigations were performed on a Zeiss Supra 55 field emission SEM operating in high vacuum mode at 0.1– 30 kV accelerating voltages. CL measurements have been performed with an Oxford Instruments MonoCL2 system installed on a Jeol 35C SEM and with a CL spectrometer installed on a Camebax microanalyser. CL spectra have been recorded from SiC samples with a 30 keV electron beam energy, 200–380 nA beam current, 3000· display magnification, and at room temperature. Image analysis was performed using Digital Micrograph software available from Gatan Inc. To obtain contrast profiles, line scans were taken across the image and integrated over the entire image, perpendicular to the scan direction. A zero level for SE signal was determined from the
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beam-blanked image. Contrast (C) was determined according to the following definition: C¼
Ia Ib 100% Ib
ð1Þ
where Ia and Ib are the average grey level intensity from corresponding areas/regions, A and B. 3. Results A secondary electron (SE) image with a distinct contrast between adjacent SiC polytype regions marked as A and B is shown in Fig. 1. The SE image was collected on a Zeiss Supra 55 field emission SEM using an accelerating voltage of 30 kV, a working distance of 10 mm and Everhart– Thornley (ET) detector. To identify these two observed regions as corresponding to the 6H and 4H polytypes, we used CL measurements. CL spectra have been recorded from the observed regions with a 30 kV electron beam energy, at room temperature. The results of the CL are shown in Fig. 2. It was found that the CL spectra onsets for the two regions are at 3.04 eV and at 3.26 eV, respectively. These values correspond to the fundamental band-gap energy (Eg) values for the 6H and 4H polytypes, and hence identify the regions. In the 6H region, the CL emissions were approximately half the intensity of those in the 4H region. It was found that the 4H polytype region with larger band-gap energy (Eg = 3.26 eV) appears darker than the 6H region, with Eg = 3.04 eV. The contrast of the 4H/6H heterostructure as a function of energy of incident electrons, working distance and detector has been studied. The maximum spatially averaged contrast value was measured to be up to 20% for the 4H/6H heterostructure. The measurement results for different detectors and operating conditions used in this work are
Table 1 Energy gap values for major SiC polytypes SiC polytype Energy band-gap Eg (eV)
3C 2.40
6H 3.04
4H 3.26
2H 4.04
Fig. 1. SE image recorded from two regions of different SiC polytypes. The image was recorded using the ET detector, at 30 kV beam energy, at a working distance of 10 mm.
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A.A. Suvorova, S. Samarin / Surface Science 601 (2007) 4428–4432 Table 2 Measured contrast in 4H/6H heterostructure on Zeiss Supra 55 FESEM with various imaging conditions used
CL spectra 4H-SiC 6H-SiC
CL intensity (a.u.)
A
B 0 1.5
2 .0
2.5
3 .0
3.5
4 .0
Energy (eV) Fig. 2. CL spectra recorded from A and B regions of the SiC heteropolytipic structure corresponding to SE image of Fig. 1.
summarised in Table 2. Table 2 also shows imaging conditions for maximizing the heterostructure contrast. To maximize the heterostructure contrast on the Zeiss FESEM used in this work, we have found the optimum operating conditions to be an accelerating voltage of 30 kV when using an Everhart–Thornley (ET) detector and 0.5 kV when using an in-lens detector. It should be mentioned that use of an in-lens detector at low voltages substantially increases the resolution of surface structure in the SE image, however the contrast of the heterostructure is slightly reduced when compared with images recorded using an ET detector. The magnitude of the
Imaging conditions (detector, voltage and working distance)
Maximum measured contrast (%)
Everhart–Thornley detector, 30 kV, 10 mm WD In-lens detector, 0.1 kV, 3 mm WD In-lens detector, 0.5 kV, 3 mm WD In-lens detector, 1 kV, 3 mm WD In-lens detector, 3 kV, 3 mm WD In-lens detector, 5 kV, 3 mm WD In-lens detector, 10 kV, 3 mm WD
20 18 19.7 19 18 16 14
contrast we have observed using an in-lens detector at low electron beam energies (up to 5 kV) is significantly greater than that observed at higher electron beam energies (5–10 kV), as seen in Fig. 3. This observation of the increased contrast at low accelerating voltages can be explained by the fact that BSEs also contribute to the collected signal and reduce the contrast from the two polytype regions at higher electron beam energies. 4. Discussion Our study of the 4H/6H heterostructure has revealed that the 6H polytype region, with a band-gap energy of 3.05 eV, exhibits higher secondary electron emission than the 4H region, which has a larger band-gap of 3.26 eV. This result is in agreement with previous studies where the wider band-gap material in Si/GeSi and GaAs/AlGaAs heterostructures appeared darker [4,5], however these earlier
Fig. 3. Secondary electron images from the 4H/6H heterostructure recorded with different electron energies. The image was recorded using in-lens detector, at different electron beam energy (0.1–10 kV), at a working distance of 3 mm.
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results were interpreted in terms of differences in stopping power and atomic number due to compositional variations and not in terms of band-structure differences. In our experiments, there is no compositional variation and we can therefore detect only the contrast associated with band-structure variations between adjacent SiC polytype layers. It is known that the secondary electron emission in semiconductors can be divided into three stages: production of internal secondary electrons, migration of secondaries to the surface and ejection of the electrons from the surface. The SE generation depends on the ionisation potential. To interpret the observed contrast we can use Fig. 4, which is a schematic diagram of the energy level alignment between the 4H and 6H regions. It is seen that the valence band edge of the 6H region lies closer to the vacuum level than in the 4H region. Thus, the 4H polytype region has a higher ionisation energy than the 6H one. As the SE emission is higher in the 6H region, the dominant factor in the enhanced secondary emission from the 6H region could be the ionisation potential, which is higher for 4H semiconductor than for 6H one. Migration of SEs to the surface is determined by the escape depth, and the number N(Es) of emitted SEs is proportional to the escape depth K(Es) Eq. (2) [13,14]. It has been shown that a wider band-gap allows a larger escape depth for the low-energy secondary electrons, thereby resulting in the enhanced secondary emission yield [16]. This consideration of the escape depth as a factor of SE emission suggests that the escape depth for 4H region is larger than for 6H region. As the observed SE emission is higher in the 6H region, the dominant factor in the enhanced secondary emission from the 6H region could be the ionisation potential rather than the escape depth. In order for the SE to escape, the secondary electron energy Es must be greater than a barrier height Eb that is the difference between the potential in the vacuum and the bulk of semiconductor. The number N(Es) of SE emitted per
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unit energy range is a function of the barrier height as derived by Chung and Everhart [15]. dN ðEs Þ SðEs ÞKðEs Þ Eb 1 ¼ ; dEs 4 Es
Es > Eb
ð2Þ
where dNdEðEs s Þ – the number of SE emitted per unit energy range, Es – secondary electron energies measured from the bottom of the conduction band, S(Es) – secondary electron generation function, K(Es) – escape depth, Eb – barrier height. Another factor contributing to the SE signal is the emission of SEs from the surface states. Surface states change the band-structure in semiconductors, resulting in an additional modification of the surface barrier that is known as surface band bending. The band bending in n-type semiconductors creates an additional barrier for the escaping electrons. The precise location of the band levels and the surface Fermi energy dependence on doping concentration for the n-type doped 6H-SiC has been measured by XPS and was found to be varied up to 1.66 eV [17]. The maximum value of the band bending in semiconductors is known to be linearly proportional to the band-gap (Eg). The band bending found in a wide band-gap semiconductor is therefore larger than that in a narrower band-gap semiconductor [18]. Thus, for the same level of n-doping, the barrier for escaping electrons in the 4H regions will increase more than that for the 6H regions. This will further increase the contrast between the two polytype regions. Thus, by considering the effect of the ionisation potential and additional barriers on the SE emission, we can obtain a qualitative interpretation of the contrast variations observed in the 4H/6H SiC heterostructure. This approach cannot be used in other types of semiconductor heterostructures such as compositional and doped superlattices, where the additional contributions of stopping power and atomic number variations must be taken into account. 5. Conclusions
4H-SiC
6H-SiC
Evac χ Ec EF
Ec Eg=3.26 eV
Eg=3.03 eV Ev
Ev
Fig. 4. Schematic energy level diagram of the structure with two SiC polytypes (4H-SiC and 6H-SiC), with aligned Fermi levels, EF. The vacuum level of the sample is represented by Evac and electron affinity by v. The band-gap energy is denoted by Eg, the bottom level of the conduction band and the top level of the valence band are indicated by Ec and Ev, respectively.
A field emission SEM has been used to study secondary electron contrast from a heteropolytypic semiconductor structure. The heterostructure exhibits high secondary electron contrast indicating a high sensitivity of the SE contrast to variations in the energy band-gap in semiconductors. The study has revealed that the 6H-SiC polytype region, with a smaller band-gap, exhibits higher secondary emission than the 4H-SiC region, with a larger band-gap. The dominant influence on the observed contrast has been proposed to be the dependence of the SE emission on the ionisation potential. Another factor that may also be involved includes surface band bending. The observed secondary electron contrast can be useful in revealing electronic structure variation between different components of silicon carbide semiconductor devices.
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Acknowledgements The authors thank Dr. M.R. Phillips (University of Technology, Sydney) and Dr. M. Zamoryanskaya (Ioffe Institute, Saint-Petersburg) for performing the CL studies. References [1] Y. Homma, M. Tomita, T. Hayashi, Ultramicroscopy 52 (1993) 187. [2] D. Sander, W. Wulfhekel, M. Hanbucken, S. Nitsche, J.P. Palmari, F. Dulot, F.A. d’Avitaya, A. Leycuras, Appl. Phys. Lett. 81 (2002) 3570. [3] M. Buzzo, M. Ciappa, M. Stangoni, W. Fichtner, Microelectron. Reliab. 45 (2005) 1499. [4] D.D. Perovic, M.R. Castell, A. Howie, C. Lavoie, T. Tiedje, J.S.W. Cole, Ultramicroscopy 58 (1995) 104. [5] M.R. Castell, D.D. Perovic, H. Lafontaine, Ultramicroscopy 69 (1997) 279. [6] C.P. Sealy, M.R. Castell, P.R. Wilshaw, J. Electron Microsc. 49 (2) (2000) 311.
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