Letter pubs.acs.org/NanoLett
Electronic Level Scheme in Boron- and Phosphorus-Doped Silicon Nanowires Keisuke Sato,† Antonio Castaldini,† Naoki Fukata,‡ and Anna Cavallini*,† †
Department of Physics, University of Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
‡
S Supporting Information *
ABSTRACT: We report the first observation of the electronic level scheme in boron (B)- and phosphorus (P)-doped nanowires (NWs). The NWs’ morphology dramatically depends on the doping impurity while a few deep electronic levels appear in both kinds of nanowires, independently of the doping type. We demonstrate that the doping impurities induce the same shallow levels as in bulk silicon. The presence of two donor levels in the lower half-bandgap is also revealed. In both kinds of NWs, B- and P-doped, the donor level (0/ +) at Ev + 0.36 eV of the gold−hydrogen complex is observed. This means that the gold diffusion from the NW tip introduces an electronically active level, which might negatively affects the electrical characteristics of the NWs. In P-doped NWs, we observed a further donor level at 0.26 eV above the valence band due to the phosphorus-vacancy pairs, the E-center, well-known in bulk silicon. These findings seriously question both diffusion modeling of impurities in NWs and the technological aspects arising from this. KEYWORDS: Silicon nanowires, shallow levels, deep levels, gold incorporation, E-center
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ow-dimensional structures, such as quantum dots1 and nanowires (NWs)2,3 made from a variety of materials, are recognized as the essential building blocks for emerging technologies in nanotechnology. In particular, NWs represent a great promise toward novel nanoscale electronic device applications, such as solar cells and nanowire field-effect transistors (NWFETs).4−6 Among several different semiconductors, silicon is the most important since the Si-based devices dominate the microelectronic field. In this frame, SiNWs are the building blocks bridging to nanoscale the Si-based microelectronics. Si-NWs have generated much attention as a high-performance platform for solar cells,7,8 NWFETs with back-gate9,10 and surround-gate (gate-all-around)11 structures, due to their relatively low reflectivity,8,12 high carrier mobility,10,11 and high drive current.9−11 The precise control of the morphology, including diameter, length and growth direction, of the composition and of the impurity doping of Si-NWs is, however, the necessary condition for their application to viable technologies. The metal nanoparticle-mediated vapor−liquid−solid (VLS) technique7,9,12,13 is a particularly powerful tool, as it allows a precise morphological control. Additionally, such growth technique has also allowed the direct incorporation and compositional control of p- and n-type dopant atoms, namely, boron (B) and phosphorus (P), into the crystalline Si core of NWs, thus permitting the formation of p/n junctions.7,9,14 Using Au as catalyst enables synthesizing at low temperature due to the low Au−Si eutectic temperature (360 °C).15 © XXXX American Chemical Society
However, this gives rise to Au-rich precipitates on the B-doped regions of NWs16 and Au incorporation, either onto the Si-NW sidewall surface or into the NW volume, due to surface diffusion of Au from the NWs tip during the growth.17 Au incorporation, specifically, could be detrimental to the electronic device performance since Au might result in deep electronic levels as occurs in bulk Si.15,18 There is so far no experimental evidence of Au-related electronic levels in SiNWs, but a recent paper by Motayed and co-workers,19 who found an electronic level at 0.39 eV possibly associated to Au. Further levels might exist in the bandgap of Si-NWs, first of all those possibly related to the dopants, the knowledge of which could have important consequences on the dopant diffusion modeling and in turn on the device realization. In this respect, the literature reports a large body of work on the impurity doping of Si-NWs, mainly based on electrical transport measurements.20−22 Micro-Raman scattering and electron spin resonance (ESR) spectroscopy have also been used to determine the impurity behavior and their segregation kinetics in NWs.23 However, ESR, a well-recognized technique used for identifying sites and types of defect structures in nanostructured materials,14 does not allow to determine the bandgap energy levels. At present, notwithstanding the attractive properties and the long-time Received: February 27, 2012 Revised: April 27, 2012
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studies on the Si-NWs, the investigation on their electrical activity at the atomic scale, namely their level scheme, is still at its infancy. However, the study of the electronic levels in SiNWs is a prerequisite to improve the fundamental knowledge of the NWs and to optimize their application in nanoscale Sibased devices. In this paper, we report the first experimental evidence of the presence of bandgap levels in Si-NWs grown by the VLS technique. Deep level transient spectroscopy (DLTS)24,25 and photoinduced current transient spectroscopy (PICTS)25,26 measurements have been used to detect any shallow and deep levels in the bandgap of B-doped and P-doped Si-NWs and possibly understand their relationship to dopant species and to Au-diffusion. Scanning electron microscopy (SEM) observations have provided morphology features that are critical to the interpretation of DLTS and PICTS results. B-doped Si-NWs contain one shallow trap with parameters peculiar of the B-acceptor level in bulk Si, and two deep traps, one of them related to the donor level of the gold−hydrogen (Au−H) complex.27−29 P-doped NWs show evidence of four bandgap levels: one shallow level, the fingerprint of which corresponds to the P-donor level in bulk Si, and three deep levels. The thermodynamic parameters of two of these levels match with the donor states of the most studied defects in bulk Si, the E-center and the Au−H complex. Finally, we show that the deepest levels in B- and P-doped NWs, respectively, correspond to the E(0/+) donor level AuH1 of the Au−H complex. B-doped and P-doped Si-NWs have been synthesized at 600 °C in flowing 19 sccm of silane (100% SiH4) as silicon reactant, diborane (1% B2H6 in H2) as p-type dopant precursor and phosphine (1% PH3 in H2) as n-type dopant precursor, respectively, and 30 sccm of nitrogen (N2) as carrier gas via the VLS mechanism in a chemical vapor deposition (CVD) system. A reaction time of 30 min has been used in both cases. The substrates are thermally oxidized n-type Cz-Si wafers, with a resistivity equal to 1−4 ohm·cm, (carrier concentration of 3 × 1015 to 1 × 1016 cm−3). The thermal oxide thickness is 200 nm. Nanocolloidal Au particles of 3 nm in diameter have been used as metal catalyst. During the NW growth, the flow rate of both B2H6 and PH3 gases has been set at 0.2 sccm. The total B concentration, NA, in B-doped Si-NWs is in the range 1018− 1019 cm−3, as estimated by the intensity of the Fano broadening and the local vibrational peaks of B.30 In the DLTS calculations reported here below, we have adopted the value NA = 1018 cm−3 to obtain the order of magnitude of the level concentrations, being aware that they could be slightly underestimated. The total P concentration, ND, in P-doped Si-NWs results in the order of 1018 cm−3 by capacitance−voltage (C−V) measurements at room temperature. Table 1 summarizes the growth parameters. The morphology of B-doped and P-doped Si-NWs has been investigated by SEM evidencing significant differences in their aspect ratio. The planar view SEM micrographs of typical mats of B- and P-doped Si-NWs are shown in Figure 1a and 2a, respectively. B- and P-doped Si-NWs show cylindrical structures with an almost uniform diameter along their growth axes. Their features notably differ, namely in orientation and diameter distribution, significantly depending on the dopant type. The diameter distributions of B- and P-doped Si-NWs in the SEM images, based on a statistical analysis of more than 200 NWs for each set of samples, are also reported in Figures 1b and 2b, respectively. The fitting lines have been obtained by
Table 1. Nanowire Growth Parameters growth method substrate resistivity of Si substrate [Ωcm] dopant dopant concentration [cm−3] SiH4 flow [sccm] B2H6 flow [sccm] PH3 flow [sccm] growth temperature [°C] growth time [min]
chemical vapor depositon n-type Si (111) with SiO2 layer 1−4 B P 1018−1019 1018 19 19 0.2 0.2 600 600 30 30
Figure 1. (a) SEM planar view (beam voltage Eb = 10 kV) of B-doped Si-NWs. (b) Histogram representing the diameter distribution of the left-side Si-NW mat. The fitting line is obtained by the Extreme Value Distribution Theory.33
Figure 2. (a) SEM planar view (beam voltage Eb = 10 kV) of P-doped Si-NWs. (b) Histogram of the diameter distribution of the left-side SiNW mat. The fitting line is obtained by the Extreme Value Distribution Theory.33
the “Extreme Value Distribution” method.31−33 The diameters of B-doped NWs exhibit a broad, Gaussian-like, distribution, ranging from 100 to ∼530 nm. Their mean value is 380 nm with a standard error of ±50 nm. Differently, the diameters of the P-doped NWs exhibit a strongly asymmetric distribution with a sharp peak followed by a long tail, where the diameters range from about 15 to 160 nm. Their mean diameter is 26 nm with a standard error of ±9 nm. These diameter values are large enough to exclude quantum confinement effects. The differences in the diameter distribution and in the average diameter between B- and P-doped Si-NWs is most likely due to effects of surface doping during the NW growth.16,34,35 Additionally, the mean length of both B-doped and P-doped Si-NWs results in a mat thickness of about 20 μm. To detect the bandgap electronic levels, DLTS and PICTS measurements have been carried out in the temperature range 40−300 K. It is worth reminding that DLTS detects majority carrier traps,36 as exploring the relevant half bandgap, while B
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PICTS detects both minority and majority carrier traps,26 by scanning the whole bandgap. DLTS and PICTS measurements have been carried out on B- and P-doped mats, respectively, according to their peculiar electrical activity to optimize the output signal and, in turn, the significance of the achieved emission spectra. As a matter of fact, PICTS would be the most suitable choice to detect all levels in the bandgap, even if the distinction between majority and minority carrier traps would be lost. However, it could not be applied to B-doped mats since the quantum efficiency37 of B-doped NWs is orders of magnitude lower than that of P-doped ones.38 DLTS has been carried out with Schottky Au-contacts on the mat and ohmic, gallium-painted, contacts on the back-surface or, alternatively, on the top surface after removing the NWs, obtaining the same results. PICTS has been performed with both ohmic contacts on the mat, using an excitation light wavelength equal to 890 nm. Figure 3 shows the schematics of the DLTS and PICTS measurements.
Figure 4. (a) Typical DLTS spectrum of B-doped NWs, obtained with an emission rate ep = 465 s−1 and bias applied Va = −0.30 V. The labels identify the three hole traps emitting at the peaked positions. (b) Arrhenius plot reporting the thermal emission rates (T2-corrected) of the hole traps detected in B-doped NW mats. The level activation energy is next to the relevant line.
H1 has a value significantly lower than NA (Table 2). Thus, B is only partially electrically activated and the H1 density is likely to be the threshold of B dissolution in substitutional positions. The remaining B would dissolve as interstitial B, segregate at the surface, form electrically inactive complexes as boron− hydrogen (B−H) or is passivated by hydrogen.41−43 Level H2 has not been identified. Level H3 at 0.36 eV from the valence band exhibits the thermodynamic properties peculiar of the donor level belonging to the electrically active Au−H complex observed in bulk Si, that is, its fingerprint is in good agreement with literature results.28,44 They demonstrate that the transition metals Au, Ag, Pt, and Pd form substitutional defects in bulk Si and that they possess donor or (0/+) levels around Ev + 0.35 eV.27−29,44,45 Recently, by fitting low-frequency noise spectroscopy results on Si-NWs grown at 850 °C, Motayed et al. observed a single deep level at 0.39 eV from the band edge that could be tentatively associated with the Au donor level.19 In ptype Si-NWs grown at 600 °C, our DLTS results doubtlessly demonstrate that Au incorporation actually occurs under the form of Au−H complexes. Therefore, this process occurs even at temperatures lower than 850 °C, being probably related to the surface diffusion of Au from the tip of NWs during growth17 that gives rise to an electrically active hole trap. Our present findings demonstrate that in Si-NWs (i) also deep levels, besides impurity-related shallow levels, recover bulk characteristics in convenient growth conditions, and (ii) Au diffusion introduces the active donor level AuH (0/+), which should be controlled to avoid catalyst contamination that drastically changes the NW electrical characteristics. The
Figure 3. Schematics of DLTS and PICTS. DLTS has been carried out with the Schottky Au-contact on the mat and the ohmic contact on the back-surface (sketch a) or with ohmic contact on the top surface after partially removing the NWs (sketch b). PICTS has been performed with both ohmic contacts on the mat (sketch c), using an excitation light wavelength equal to 890 nm.
Figure 4a shows a typical DLTS spectrum of B-doped Si-NW mats. It exhibits three peaks, corresponding to the emission by hole traps, the Arrhenius curves of which are plotted in Figure 4b. The fingerprints of these traps,24 that is, their apparent activation energy, ΔHp, and capture cross section, σp, as determined from the Arrhenius plots of T2/ep versus 1/T, are reported in Table 2, which specifies the thermodynamic parameters of all the detected bandgap levels in both B- and P-doped NWs. Even though the quantitative estimates of the level concentrations in the mats are questionable, there are some features that need to be pointed out. The H1 activation energy and apparent capture cross section (see Table 2) corresponds to those of the shallow level due to B dopant in substitutional position in bulk Si.39,40 This is the first experimental proof supporting the theoretical result of “ab initio” total energy calculations41 that in not-confined NWs, that is, in NWs with diameter larger than 3 nm, the impurity shallow levels recover bulk characteristics. The concentration of C
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Table 2. Thermodynamic Parameters of the Electronic Levels in B-Doped and P-Doped Si-Nanowires B-Doped Si-NWs
level
activation energy ΔHp [eV]
H1 H2
0.055 0.10
H3
0.36
capture cross section σp [cm2] −17
1.1 × 10 3 × 10−19 3 × 10−18
P-Doped Si-NWs level density NT [cm−3] NA = 1018
level
5 × 10 8 × 1016 16
5 × 10
E1 E2 E2B E3
15
accuracy of these experimental results does not allow to quantitatively relate the concentrations of the doping density, NA, the density of the trap H1 due to shallow B acceptor level and the density of the H3 level due to deep AuH donor27 with the possible presence of B−H pairs acting as sources of the atomic hydrogen needed to form the Au−H complexes.27 However, this hypothesis should be accounted for and this subject should in future deserve great attention in connection with B−H dissociation and formation of Au−H complexes also in NWs, as observed in bulk Si in the past. It is worth noting that the concentration of H3 (in the order of 5 × 1015 cm−3), accounting for the total concentration of the Au-related level in the NWs as a whole (core and sidewalls), is 2 orders of magnitude higher than the equilibrium solubility of Au in bulk Si (3 × 1013 cm−3 at 600 °C).46 This result is in good agreement with Putnam et al.,47 who demonstrated that Au incorporation into the Si-NWs during VLS growth using Au catalyst might arise at concentrations much higher than the equilibrium solubility level of bulk Au in Si and suggested that this might be due also to the changes of the surface chemistry. As we will show below, an excess solubility of Au as Au−H complexes is shown only in B-doped NWs. It could be therefore suggested that the presence of B−H complexes or passivated B could play a role on the enhancement of the Au solubility. Figure 5a shows a typical PICTS spectrum of P-doped NW mats, which exhibits four peaks, the Arrhenius curves of which are plotted in Figure 5b. Table 2 reports their concentration, apparent activation energy, ΔEn,p, and capture cross section, σn,p. These peaks could be due to either majority or minority carrier traps since by shining light both types of carriers are injected. Fortunately, their emission characteristics allow to identify most of them by comparison with the DLTS data on ptype NWs and with literature data referring to bulk Si. Similarly to what is described above on B-level, the thermodynamic properties of level E1, its activation energy, and capture cross section coincide with those of the P-level in bulk Si. This result confirms that in NWs P is doped in substitutional sites and electrically activated23 and that P recovers bulk characteristics in NWs with large enough diameters.41 Similarly to what described about B impurities in p-type NWs, the concentration of E1 is significantly lower than the doping density, meaning that only part of P is activated and that in these NWs its dissolution threshold is around 7 × 1015 cm−3. The high peak E2 could not be identified. Level E2B has the emission characteristics of the donor level E(0/+) at an energy of ∼0.27 eV from the valence band edge Ev peculiar of the phosphorus-vacancy (PV) pairs of the well-known E-center on which wide literature referring to bulk Si exists.29,48−50 Its fingerprint is reported in Figure 5b. As recently demonstrated by Nylandsted Larsen et al.50 about bulk Si defects, the Ecenter, consisting of a vacancy trapped next to a substitutional
activation energy ΔEn,p [eV] 0.06 0.25 0.26 0.38
capture cross section σn,p [cm2] 4.3 4.6 8.7 1
× × × ×
−15
10 10−11 10−16 10−15
level density NT [cm−3] ND = 1018 7 × 1015 2 × 1016 5 × 1014 5 ×1014
Figure 5. (a) Typical PICTS spectrum of P-doped NWs, obtained with an emission rate ep = 512 s−1 and a pulse width of 10 ms. The labels identify the four traps emitting at the peaked positions. (b) Arrhenius plot reporting the thermal emission rates (T2-corrected) of the traps detected in P-doped NW mats. The level activation energy is next to the relevant line. For comparison purpose, we have also reported the Arrhenius plot of the level H3 relevant to B-doped mats. Also results from literature are plotted: the magenta dashed line is the fingerprint of the donor level of the E-center50 at Ev + 0.27 eV and capture cross section σp = 5 × 10−16 cm2, the green dashed line is the fingerprint of the donor level of the Au−H complex44 at Ev + 0.36 eV and capture cross section σp = 3.3 × 10−15 cm2.
group-V atom (P, As, Sb), has also a donor level besides the acceptor level predicted for the vacancy-assisted group-V impurity diffusion. This makes the E-center and its donor level of paramount importance since the diffusion of the technologically important group-V dopants (P, As, Sb) takes place via this defect51 and electrical deactivation of donor atoms could result from the formation of this defect. Even if the Ecenter is one of the most studied defects in bulk Si for ages,52 its donor level at Ev + 0.27 eV has lain hidden for so long time. In fact, it is not detectable by electron paramagnetic resonance (EPR) because it is not paramagnetic nor by DLTS D
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investigations of the lower half-band in p-type Si, where PV pairs are not present. Using PICTS, and hence investigating the whole bandgap width, has allowed us to reveal it. For sake of comparison, Figure 5b reports also the emission characteristics of this level (magenta dashed line) reported in ref 50. The level E3 has the same features as level H3 detected in B-doped NWs, that is, the thermodynamic characteristics of the donor (0/+) level at about Ev + 0.36 eV of the Au−H complex, and hence E3 can be identified with this defect. In Figure 5b, the Arrhenius plot of H3 is also reported for comparison as well as the fingerprint of the donor level cited in ref 44 (green dashed line). The significantly higher concentration of this level in Bdoped NWs than in P-doped NWs could be attributed not only to the presence of B−H pairs acting as hydrogen source but to the different surface diffusion of Au14 in P-doped Si-NWs. While these findings cannot yet be fully explained, it is possible that they are also related to the difference in the wire diameters of B- and P-doped NWs, and hence in the core-sidewall Au concentration.47 In conclusion, we report for the first time the electronic level scheme of Si-NWs, with the identification of the origin of most levels. We experimentally observed that the impurity levels recover bulk characteristics in NWs and that B-doping and Pdoping give rise to electrically active defects with shallow character. The existence of these shallow defects is in agreement with “ab initio” calculations on NWs in absence of quantum confinement effects. We also demonstrated the existence of two deep donor levels previously observed in bulk Si, that is, the donor level of the PV pairs, the E-center in P-doped mats, at Ev + 0.26 eV and the donor level of the Au−H complex at Ev + 0.36 eV. This proves that in NWs defect deep levels, not only the shallow ones, also recover their features peculiar of bulk Si. The different density of the level H3/E3 related to the Au−H complex in B- and P-doped NWs indicates that the dynamic behavior and segregation mechanisms significantly depend on the doping impurity, as already observed by ESR and micro-Raman scattering measurements.14 Finally, the evidence of the presence of electrically active PV pairs is expected to affect both diffusion modeling and device design while that of Au-related electrically active defects might refocus attention on the contamination by metal catalysts.
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ASSOCIATED CONTENT
S Supporting Information *
Additional information and figure. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is partly supported by a Funding Program for Next Generation World-Leading Researchers (NEXT Program) in Japan.
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