pubs.acs.org/NanoLett
Molecular Doping and Subsurface Dopant Reactivation in Si Nanowires ´ lvaro Miranda-Dura´n,†,‡ Xavier Cartoixa`,† Miguel Cruz Irisson,‡ and Riccardo Rurali*,†,§ A †
Departament d’Enginyeria Electro`nica, Universitat Auto`noma de Barcelona, 08193 Bellaterra (Barcelona), Spain, Instituto Polite´cnico Nacional, ESIME Culhuacan, Av. Santa Ana 1000, C.P. 04430 Me´xico D.F., Me´xico, and § Institut de Cie`ncia de Materials de Barcelona (ICMAB-CSIC), Campus de Bellaterra, 08193 Bellaterra (Barcelona), Spain ‡
ABSTRACT Impurity doping in semiconductor nanowires, while increasingly well understood, is not yet controllable at a satisfactory degree. The large surface-to-volume area of these systems, however, suggests that adsorption of the appropriate molecular complexes on the wire sidewalls could be a viable alternative to conventional impurity doping. We perform first-principles electronic structure calculations to assess the possibility of n- and p-type doping of Si nanowires by exposure to NH3 and NO2. Besides providing a full rationalization of the experimental results recently obtained in mesoporous Si, our calculations show that while NH3 is a shallow donor, NO2 yields p-doping only when passive surface segregated B atoms are present. KEYWORDS Si nanowires, molecular doping, DFT, nanoelectronics, gas sensing, electronic structure
S
through the cooperative molecular field effect.21 There, at variance with conventional chemical doping, the dipole that develops upon adsorption of a molecular monolayer is used as a control gate voltage.22-25 The possibility that an adsorbed molecule, analogous to substitutional impurities, could provide shallow electronic states that could be thermally excited has received less attention. With such an approach one would simultaneously get rid of two problems that hinder SiNW doping: (i) the competition between catalyzed and uncatalyzed doping that yields undesired axial modulation of the impurity concentration;10 (ii) the need to control the diffusion of adsorbed impurities to achieve a uniform concentration in ex situ doping,11,26 counterbalancing the tendency to surface segregation, which becomes increasingly important as the wire size decreases.27 A large body of experimental results has been obtained with mesoporous Si (meso-PSi), where electrochemical attack of a Si sample yields a disordered network of single-crystalline Si wires.28,29 Changes of the conductivity upon gas adsorption have been reported in several studies,30-39 suggesting that chemical doping of SiNWs can be accomplished through the surface adsorption of proper molecules. The mechanism has been remarkably demonstrated by Garrone et al.39 in mesoPSi, where n- and p-type doping was achieved by the exposure of the sample to NH3 and NO2, respectively. Unfortunately, in this and other related experiments,30,31,35-37 the meso-PSi sample was obtained from a highly B-doped Si substrate, making difficult to say whether the observed p-doping depended on genuine molecular doping or rather on reactivation of the passive subsurface B. In this work we study the adsorption of NH3 and NO2 on SiNWs, their interaction with subsurface B atoms, and the feasibility of molecular doping in SiNWs, as opposed to standard impurity doping. We perform first-principles electronic
ilicon nanowires (SiNWs) hold great promise to integrate conventional Si devices in future nanoelectronics applications,1 and their use as bipolar transistors,2 logic gates,3 nonvolatile memories,4 solar cells,5 biological sensors,6 and energy conversion devices7,8 has been reported. For further development of SiNW-based nanoelectronics, however, it is going to be crucial to control with as great a precision as possible the concentration and distribution of dopants, a task which is proving to be rather challenging.9 Doping of SiNWs is being carried out in two ways: (i) in situ, adding the proper gas precursor during synthesis, or (ii) ex situ, adding the doping agents at a later processing step, where surface deposition of the dopants is followed by an annealing cycle that drives impurity diffusion. In situ doping is largely used, although the competition between catalyzed and uncatalyzed incorporation of the impurities often leads to sizable disuniformities in the dopant concentration.10 Ex situ doping is receiving increasingly high attention, and successful attempts at doping various type of semiconductor nanowires are being reported.11-13 The large surface-to-bulk ratio of SiNWs suggests the possibility of doping through the external adsorption of molecules, rather than incorporating substitutional impurities. Molecular charge transfer has been extensively studied in single-walled carbon nanotubes,14-17 which, of course, offer the best conditions concerning the surface-to-volume ratio. Also, molecular doping has been obtained successfully in graphene18-20 where the vanishing band gap makes it easier to find molecular adsorbates whose HOMO (LUMO) falls in the host conduction (valence) band. Control of the electrical conductivity of Si thin films has also been explored * To whom correspondence should be addressed,
[email protected]. Received for review: 05/28/2010 Published on Web: 08/24/2010 © 2010 American Chemical Society
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FIGURE 1. Adsorption of (a) NH3 and (b) NO2 at a DB of the Si surface. NH3 adsorption yields n-type doping, pinning the Fermi level at the bottom of the condution band (panel c), while the NO2 adsorbed system stays intrinsic (panel d). The projected DOS of the side panels illustrates the contribtuon of the molecular orbitals. N and O are shown in blue and red spheres, respectively, while yellow and white spheres represent Si and H atoms.
et al.39 On the other hand, the system with the adsorbed NO2 stays intrinsic, with the bonding and antibonding states of the NO2-DB complex well inside the conduction and valence band (see Figure 1d). Hence, the origin of the p-type character in the presence of NO2 reported in the experiments must be sought in the specific interaction of the adsorbed NO2 and the subsurface B atoms. For this purpose we consider a slab with a DB-B complex, i.e., a B atom in a substitutional position right below a DB.43 This is a better model of the experimental conditions, where the Si samples were highly B doped before the electrochemical attack. It has been shown that substitutional B inhibits the etching. Therefore, highly etched systems will feature most of the B atoms immediately below the surface.49 Such DB-B complex is very stable, even toward H adsorption, andsmost importantlysis electrically passive: the B atom binds the unpaired electron of the DB and loses its acceptor character. The optimized geometries of the NH3 and NO2 molecules at a DB-B complex are shown in parts a and b of Figure 2 and do not present noticeable differences with those discussed previously (variation of the N-Si bond lengths is within 3%). On the other hand, the electronic structure of the NO2 adsorbed system undergoes a significant change and ac-
structure calculation within density-functional theory (DFT), as implemented in the Siesta package.40 We use norm-conserving pseudopotentials for the core electrons and an optimized double- ζ basis set plus polarization functions41 for the valence electrons. The exchange-correlation energy is calculated within the spin polarized generalized gradient approximation (GGA) in the parametrization of Perdew-Burke-Ernzerhof.42 We model the SiNW/meso-PSi surfaces with a Si(111) 3 × 3 surface, a system where the subsurface back-diffusion of B impurities is well documented.43-45 We use a 4 × 4 supercell of the surface unit cell in slab geometry, sampling the Brillouin zone with a grid of 4 × 4 of k-points within the Monkhorst-Pack algorithm.46 The bottom dangling bonds are passivated with hydrogen atoms. All the structures discussed have been relaxed until the all the forces on the atoms were lower than 0.04 eV/Å. At first, in order to assess their intrinsic doping tendency, we have studied the adsorption of an NH3 and a NO2 molecule at a dangling bond (DB) of a clean Si surface,47 see parts a and b of Figure 1. Both adsorption processes are favored, with the N bonding the unpaired electron of the Si DB, and chemisorption energies of 0.45 and 2.17 eV for the NH348 and the NO2, respectively. The adsorption of the NH3 yields an n-type doping (see Figure 1c), in agreement with the observations of Garrone © 2010 American Chemical Society
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FIGURE 2. Adsorption of (a) NH3 and (b) NO2 at a DB-B complex. The donor NH3 donor state of Figure 1 is passivated by the subsurface B (panel c); NO2 adsorption, on the other hand, reactivates the B acceptor state which was passive in the DB-B complex, resulting in p-type doping (panel d). The projected DOS of the side panels illustrates the contribution of the molecular orbitals and of the B impurities. Subsurface B is shown as a light green sphere.
quires a p-doping character, as reported in the experiments. Projections of the total density of states (DOS) on the different atomic species reveals a clear B character of the shallow acceptor state, as suspected.39 Therefore, the role of the adsorbed NO2 molecule is not providing itself a hole, but rather reactivating the passive subsurface B. In other words, NO2 is electronegative enough to take the electron of the DB but not enough to create a hole in the valence band. This mechanism was already described within the chemical model of Garrone et al.,50 based on a Mulliken analysis of the charge transfer. The agreement with the experimental results, however, was rather fortuitous, because Mulliken population simply quantifies the ionic character of a bond at zero temperature, but by no means the tendency to thermally populate/depopulate an electronic state. To illustrate this fact, we have performed a full structural relaxation of a B substitutional impurity in bulk Si (64-atom supercell, double- ζ polarized basis set, 2 × 2 × 2 k-points, GGA functional) obtaining a slight charge transfer from the B atom to the Si neighbors. Hence, according to a Mulliken analysis B would be a donor, while it is a known shallow acceptor (Eion ∼ 45 meV). A priori, reactivation of the subsurface B atoms can also be achieved by H adsorption at the DB. However, as we © 2010 American Chemical Society
mentioned earlier, this scenario is not realistic, because the H-DB-B adsorption process is unfavored for as much as 0.41 eV, and the H prefers to stay in the gas phase. An interesting fact that has received less attention is that also NH3 reverses its tendency to doping: while it was an active donor at a DB site, when it is adsorbed at a DB-B site the dopant state is passivated, with the N and the B forming a stable and electrically inactive complex (see Figure 3). All these adsorption reactions are moderately to strongly exothermic, meaning that whenever given the opportunity the molecules will stick to the closer adsorption site. Therefore, the doping efficiency ratio is controlled by the relative abundance of the different adsorption sites. Upon exposure to ammonia, NH3 molecules will be adsorbed at both bare DB and DB-B complexes, but only the former will yield n-type doping; the opposite happens for NO2 adsorption, where adsorption at DB-B complexes results in subsurface B reactivation, thus p-doping, while no extra carriers are generated by molecules adsorbed at bare DB. This fact explains the different doping efficiency of NH3 and NO2 observed experimentally. As discussed before, in meso-PSi the formation of the highly stable DB-B surface complex is favored.49 However, the existence of bare DB cannot be ruled out, and they are found in concentrations of up to 3.3 3592
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FIGURE 3. Isosurface plot of the wave function of the electrically inactive NB pair that forms across a Si DB.
× 1016 cm-3, even in SiNWs obtained through the much cleaner VLS bottom-up technique.51 Still, the concentration of DB-B complexes is expected to be dominant. Therefore, p-type doping is much more efficient as compared to ndoping, because most of the available adsorption sites favor B reactivation upon NO2 adsorption, while they are inert to NH3. Conversely, n-doping can be achieved only by NH3 adsorption at the comparatively less abundant bare DB. This trend is captured by the experiments which clearly show that higher NH3 pressures are required to yield measurable concentrations of carriers and that, in general, the concentration of carriers produced by NO2 adsorption is always approximately twice as large. Although the etching process in meso-PSi is very efficient in exposing most of the B at the subsurface, forming the passive DB-B complex, a small fraction of active B will still exist farther from the surface. Hence, the samples used in the experiments have a residual light p-doping.35,37,39 We model this experimental condition by adding a deeper substitutional B, which is electrically p-type active. In this situation, the first electrons provided by NH3 adsorption will compensate this residual light p-type doping, by recombining with the holes of the deeper B. Figure 3 illustrates this situation where the intrinsic character of the system has been recovered by means of the NH3 adsorption. This behavior is nicely captured by the experiments. Garrone et al.39 performed infrared spectroscopy measurements of the carrier concentration in meso-PSi exposed to increasing concentrations of NH3 and NO2. In the case of NO2 the carrier concentration starts increasing upon molecule adsorption: the holes of the reactivated subsurface B add to the holes of the deeper, previously active B. In the case of NH3 on the other hand, at first the carrier concentration is seen to decrease, it reaches a minimum (all the active B are compensated), and then starts growing similar to the case of NO2. © 2010 American Chemical Society
The results discussed until here are valid in the limit of thick wires, where the facets can be approximated with an infinite Si surface, the Si(111) 3 × 3 in our case. These wires, with diameters ranging from 50 to 200 nm, are the ones most commonly grown and relevant for the protype devices that are being fabricated. However, the extrapolation of our conclusions to wires in the nanometer range is still interesting, because our final goal is finding a molecular doping scheme that does not depend on the exact size of the nanowire. For this purpose we have considered NH3 and NO2 adsorption on a 1.5 nm SiNW grown along the 〈111〉 orientation and bounded by {111} facets (Figure 4a,b). Most of our conclusions, however, can be extended to other types of facets, as long as they are passivated. In this case, properties of the nanowiressuch as the work function or the electron affinitysthat are relevant to the alignment with the electronic states of the molecule do not depend on the specific facet considered. In order to refine this somewhat crude model, one should consider the chemistry of the DB-molecule complex, which could exhibit some dependence on the facet orientation. Considering explicitly the wire geometry allows not only describing the finite size of the facets but also accounting for the quantum confinement that broadens the energy band gap52 and deepens the impurity states.53-55 While in the limit of a large diameter treated above the NH3 donor state fell inside the conduction band, for the 1.5 nm nanowire we obtain a shallow state below the band edge (see Figure 4c), similar to the case of conventional chemical dopants for bulk Si, such as P. These results suggest that NH3 can still act as a donor agent. Unfortunately, as discussed recently by Niquet et al.,56 density-functional calculations, at least within the local and semilocal approximations to the exchangecorrelation functionals, only allow a qualititive inspection of this kind of systems and a many-body treatment is requiered for a quantitative estimation of the dopant binding energy. The case of NO2 adsorption is also qualitatively similar to what we found 3593
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FIGURE 4. Adsorption of (a) NH3 and (b) NO2 on the [110] facet of a 〈111〉 SiNW at a DB and DB-B site, respectively. Band structures showing (c) n- and (d) p-type doping.
for the adsorption on the surface and is not per se an acceptor. On the other hand, holes can be generated in the valence band by means of reactivation of subsurface B, trapped in a DB-B passive complex (Figure 4d). This fact could be important in ultrathin nanowires where surface segregation followed by DB passivation has been reported as the main deactivation mechanism for common dopants.27 The study of wires with a larger diameter is beyond the scope of this work. Yet, some qualitative observations can be made about the expected behavior of the doping efficiency. On one hand, as the diameter increases the activation energy of the donor/acceptor states will decrease due to a lower level of confinement; calculations performed in thinner nanowires showed that the broadening of the band gap makes the molecular donor level deeper, thus more difficult to activate. On the other hand, as the surface-tovolume ratio diminishes, the available surface adsorption sites will not be able to yield a satisfactory carrier concentration. While at small thicknesses these two effects can compensate, as the diameter exceeds 150-200 nm the surface density of adsorbed molecules becomes too low to guarantee sufficient carrier concentrations, at least assuming that the only available adsorption sites are the DBs that are naturally found at the wire surface. © 2010 American Chemical Society
The situation that seems relevant to the experiments is the one where the molecules are adsorbed at pre-existing surface dangling bonds. These surface defects are not found in significantly larger concentrations than the typical values considered here. However, for the sake of completeness, we have also addressed the extreme cases of half and full coverage of the wire sidewalls. We have found that increasing the coverage quickly leads to the formation of molecule-originated dispersing bands (not shown), driving the system to a semiconductormetal transition. In conclusion, we have reported electronic structure calculations of the adsorption of NH3 and NO2 in meso-PSi, explaining all the features observed in the experiments: (i) different doping efficiency of NH3 and NO2; (ii) compensation transient during NH3 adsorption that leads to an initial decrease of the carrier concentration upon adsorption. We have shown that the doping relies on two fundamentally different atomic scale mechanisms. While NH3 yields a genuine chemical doping, providing a shallow donor state close to the conduction band edge, the role of NO2 is to reactivate subsurface B atoms. These observations have important implications for the molecular doping of SiNWs and suggest that while NH3 can be used as an active n-type dopant, the use of NO2 seems specific to the particular situation of having large concentrations of inactive subsurface 3594
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B impurities. Guidelines to establish the donor or acceptor character of other candidate dopant molecules might be given in terms of electron affinities and ionization potentials. Similarly to the band alignments in metal-semiconductor junctions, we expect molecules with a low ionization potential to prefer to donate electrons, while molecules with a high electron affinity would provide holes. These considerations might be reformulated in terms of the Mulliken electronegativity, whose definition can be extended to molecules and extended systems. Acknowledgment. Financial support by the Ramo´n y Cajal program of the Ministerio de Ciencia e Innovacio´n, the scolarship-CONACYT, and funding under Contract No. TEC2006-13731-C02-01, TEC2009-06986, and FIS200912721-C04-03 are greatly acknowledged. REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26)
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