Effect of Surface Chemistry on Quantum ... - ACS Publications

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Effect of Surface Chemistry on Quantum Confinement and Photoluminescence of Ammonia-Passivated Silicon Nanocrystals Navneethakrishnan Salivati,† Nimrod Shuall,‡ Joseph M. McCrate,† and John G. Ekerdt*,† †

Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712, and ‡D.C. Sirica Ltd., Nesher, 36680 Israel

ABSTRACT Silicon (Si) nanocrystals (NCs) less than 5 nm in diameter are grown on SiO2 surfaces using hot wire chemical vapor deposition, and the dangling bonds and the reconstructed bonds at the NC surface are passivated and transformed with D and NDx by using deuterated ammonia (ND3), which is predissociated over a hot filament. At low hot wire ND3 doses, photoluminescence (PL) emission is observed from a defect state at 1000 nm corresponding to reconstructed surface bonds capped by predominantly monodeuteride and Si-ND2 species. As the hot wire ND3 dose is increased, di- and trideuteride species form, and intense PL is observed around 800 nm, which does not shift with NC size and is associated with defect levels resulting from NDx insertion into the strained Si-Si bonds forming Si2dND. A clean bandgap can be realized with fully relaxed and fully terminated NC surfaces consisting of di- and trideuterides and SiND2. SECTION Nanoparticles and Nanostructures

ilicon (Si) nanocrystals (NCs) less than 5 nm in diameter exhibit size-dependent light emission and such materials can potentially be used to fabricate low-cost, light-emitting devices (LEDs).1-5 However, the ratio of surface atoms to the total number of atoms in nanostructured materials is very large, and surface effects play an important role in determining the optoelectronic properties.6-9 Dangling bonds and surface reconstructions create defect states in the bandgap, which can result in nonradiative recombination of excitons.10-13 Defect states can also be introduced by surface passivants such as oxygen, sulfur, or nitrogen when present in the bridged or double-bond configurations.14-20 A complete understanding of the influence of surface chemistry in determining the optoelectronic properties is still not available. In this paper, we describe in detail the effect of the various NDx and deuteride species on the photoluminescence (PL) emitted from Si NCs subjected to hot wire doses of ND3. This work is the first to show experimentally that an appropriate choice of deuteride and NDx surface passivation allows one to obtain PL that is characteristic of either quantum confinement or a defect state in the band gap. In a previous paper, we examined the chemistry of Si NC surfaces exposed to deuterated ammonia (ND3) using two different dosing techniques - dissociative (thermal) adsorption and hot wire ND3 predissociation.21 The surface chemistry of Si NCs was studied using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). The dissociative (thermal) adsorption of ND3 on Si NCs resulted in the formation of Si-ND2 species. D2 desorption was observed only from the monodeuteride species at 780 K. No PL was

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observed when the NCs were subjected to dissociative (thermal) adsorption of ND3 because, as reported herein, thermal adsorption did not generate enough ND2 and D to saturate the dangling bonds and relax the Si-Si bonds. Umemoto et al. have shown that ammonia decomposition on a hot tungsten filament results in the formation of predominantly H atoms and NH2 radicals, with NH radicals formed as a minor product.22 In separate experiments, a hot tungsten filament was used to predissociate ND3 before adsorption on the NC surface.21 XP spectra revealed that Si-ND2 species are present initially, but as the ND3 dose is increased, Si2dND species dominate. D2 desorption was observed from the mono-, di-, and trideuteride species when ND3 was predissociated. PL was observed from the Si NCs only when the hot filament was used to predissociate ND3, and this was attributed to the higher concentration of deuterides formed on the NC surface as compared to dissociative (thermal) adsorption. We use the same nomenclature for the deuteride states on the surface of the NC as described earlier.21 The mono- and dideuteride are denoted by β1 and β2, which desorb around 780 and 680 K, respectively; the trideuteride denoted by β3 appears as a broad low temperature feature before the dideuteride feature. In the N 1s XP spectra, the Si-ND2 species appear at 398.5 eV and the Si2dND species appear Received Date: May 6, 2010 Accepted Date: June 8, 2010 Published on Web Date: June 11, 2010

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DOI: 10.1021/jz100581c |J. Phys. Chem. Lett. 2010, 1, 1957–1961

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Figure 1. PL from 4 nm Si NCs subjected to various hot wire ND3 doses. Inset: N 1s XP spectra from 4 nm Si NCs subjected to the various hot wire ND3 doses.

Figure 2. D2þ TPD signals from 4 nm Si NCs subjected to various hot wire ND3 doses. Inset: variation in the normalized area of the D2þ TPD signals with increasing hot wire ND3 dose.

at 398 eV.21 ND3 exposures are reported in Langmuirs (1 L = 10-6 Torr s). Average particle sizes are specific to the growth chamber, filament configuration, and dosing conditions. Scanning electron microscopy (SEM) images of the NCs grown using hot wire chemical vapor deposition (HWCVD) can be found in ref 6. The particle number density is estimated to be in the range of 8 1011 cm-2. Si NCs grown using 15 min of HWCVD (∼4 nm diameter) were subjected to various hot wire doses of ND3 at 375 K and were moved to the transfer chamber where PL was measured (Figure 1) at 310 K. No PL was observed when the NCs were subjected to dissociative (thermal) adsorption of ND3 at 375 K.21 Hot wire predissociation of ND3 was necessary to observe PL. At the lowest hot wire dose of 1500 L, a weak PL was observed at 1000 nm (1.24 eV). At a dose of 3000 L, a small feature at 800 nm (1.55 eV) appears along with the peak at 1000 nm. As the ND3 dose is increased to 12 000 L, PL intensity at 800 nm increases. The PL peak at 1000 nm is not observed at doses greater than 3000 L. As the ND3 dose is increased beyond 12000 L, PL intensity decreases. XP spectra from the above samples are shown in the inset of Figure 1. At low hot wire doses, the NC surface is covered mainly by Si-ND2 species. As the hot wire dose is increased, the N 1s intensity increases, and the NC surface is covered by mainly Si2dND species. Density functional theory calculations by Zaitseva et al. indicate that the N atoms prefer to be on the surface than in the core of Si NC and that the surface is likely to be covered by several NH2 and NH groups.23 D2þ TPD spectra from the above samples are shown in Figure 2. The TPD spectrum from a 1500 L ND3 dose indicates the presence of mostly monodeuteride species on the NC surface. As the hot wire dose is increased to 3000 L, D2 desorption from di- and trideuteride species is observed. Trideuteride species can also abstract a neighboring D atom and form SiD4(g), which results in etching of the NC surface. Although trideuterides are present on the NC surface, the presence of N atoms has been shown to suppress the etching reaction.21 Thus, most of the trideuteride species decompose to di- and monodeuteride species during TPD. An intense β3 feature is observed at an ND3 dose of 21 000 L, and this can be attributed to suppression of the etching reaction, which results in enhanced trideuteride decomposition. As the hot wire ND3 dose is increased, D2þ intensity in TPD also increases. The hot


Figure 3. PL from 3 nm Si NCs subjected to various hot wire ND3 doses.

wire technique results in a highly deuterated surface when compared to the case of dissociative ND3 adsorption. The variation in the normalized area of the D2þ TPD signals with increasing hot wire ND3 dose is displayed in the inset of Figure 2. The normalization is done with respect to the maximum dose of 21 000 L. The ordinate at 0 L corresponds to the normalized area under the D2þ TPD signal for the case of dissociative/thermal adsorption. As the hot wire ND3 dose is increased, the area under the D2þ TPD signal increases rapidly; however, beyond 12000 L, the D2þ TPD signal begins to saturate. Si NCs grown using 11 min of HWCVD (∼ 3 nm diameter) were subjected to various doses of ND3, and the PL-XPSTPD experimental run was performed as indicated earlier. PL is shown in Figure 3. Reducing the HWCVD growth time reduces the particle size, and PL peak position should shift to a lower wavelength in accordance with quantum confinement. Such a quantum shift has been observed in our earlier work with deuterium-passivated particles.6 However, with the ND3-passivated NCs, no quantum shift is discernible. Similar to the results with the 4 nm Si NCs, the PL signal is observed at 1000 nm for a 1500 L hot wire ND3 dose. As the hot wire dose is increased, intense PL is observed at 800 nm, and the PL peak at 1000 nm vanishes. At extremely high doses above 12 000 L, PL intensity is observed to decrease. XP spectra

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and D2þ TPD spectra (not shown) are similar to those presented at Figures 1 and 2 For both the 4 and 3 nm Si NCs, a weak PL signal is observed at 1000 nm for a 1500 L hot wire ND3 dose. XPS indicates that predominantly Si-ND2 species are adsorbed on the NC surface at these low ND3 doses. The dissociation of NH3 (ND3) over a hot filament results in the formation of predominantly H (D) atoms and NH2 (ND2) radicals.22 These radicals initially passivate the dangling bonds on the NC surface at low doses, leaving the reconstructed surface dimers and steps intact. In TPD spectra, D2 desorption is observed only from the monodeuteride species at 780 K, which indicates the presence of reconstructed dimers and steps on the NC surface. Theoretical calculations indicate that the reconstructed surface bonds are highly distorted, and such reconstructions introduce defect states in the band gap, which can trap excitons.12,13 The PL intensity at 1000 nm can be attributed to recombination of excitons from such defect states in the bandgap. Thus, an NC surface in which the reconstructed surface bonds are capped by monodeuteride species alone is insufficient for a clean (defect state free) bandgap. As the ND3 dose is increased, D radicals from the hot filament adsorb on the NC surface in substantial amounts and relieve the strained reconstructed bonds, forming di- and trideuteride species. The D2þ intensity in TPD also increases. The PL peak at 1000 nm, which is due to the presence of monodeuteride, vanishes, and intense PL centered at 800 nm is observed that increases in intensity as the concentration of di- and trideuterides is increased. An NC surface devoid of surface reconstructions is expected to have a clean bandgap.12 However, as discussed below, the Si2dND species that also form due to insertion of NDx radicals into the strained Si-Si reconstructed bonds introduce a new recombination defect. Beyond 12 000 L, PL intensity decreases, and the D2þ TPD area remains relatively constant, while N 1s XP spectra indicate that the concentration of Si2dND species on the NC surface increases. Strong trideuteride features are observed in the D2þ TPD at large ND3 doses in Figure 2. The XPS and TPD data (Figures1 and 2) indicate that N atoms are being inserted into the Si-Si bonds without an appreciable increase in deuterides. A similar result was observed in our earlier work with Si NCs passivated using atomic deuterium generated from D2 on the hot tungsten filament;6 at extremely high atomic deuterium doses, the surface saturated with trideuteride species and the D2þ area did not increase any further. Once the NC surface is covered with substantial amounts of trideuteride species, atomic D can attack the Si backbonds, resulting in the formation of an amorphous layer on the NC surface, and this amorphous layer was shown to quench the PL signal.6 For the present study we conclude that the NC surface is covered with substantial trideuteride species, and, with increasingly high doses, the NC surface becomes amorphized as D radicals begin to attack the Si backbonds, and this reduces the PL intensity. Unlike our previous report on Si NCs passivated by cracking D2,6 no quantum shift is observed when the NC size is reduced. In Figure 4a, PL intensity from 3 nm Si NCs initially subjected to 6000 L dissociative (thermal) exposures of ND3


Figure 4. (a) PL intensity from Si NCs grown using different HWCVD times and subjected to dissociative (thermal) doses of ND3 followed by hot wire D2 dose. PL from an 11 min HWCVD growth subjected to a 12 000 L hot wire dose of ND3 is plotted for comparison. (b) PL intensity from Si NCs grown using 11 min of HWCVD, subjected to sequential hot wire exposures of ND3 followed by hot wire exposure of D2.

followed by hot wire exposures of D2 is shown. PL from 3 nm Si NCs subjected to a 12 000 L hot wire ND3 dose is also plotted for comparison. No PL is observed after the dissociative (thermal) ND3 exposure as indicated by the gray line. After exposure to hot wire D2, the PL for 3 nm NCs is clearly blueshifted as compared to the case when samples are exposed to 12 000 L of hot wire ND3. When the HWCVD growth time is reduced to 10 min (the size for a 10 min growth cannot be measured in SEM), PL shifts to a lower wavelength as compared to an 11 min HWCVD growth (3 nm size). There was no difference observed in the PL peak positions of samples subjected to dissociative ND3 adsorption and followed by hot wire D2 exposures when compared to samples exposed to simply hot wire D2 (not shown). Clearly, subjecting the samples to dissociative (thermal) exposures of ND3, which leads to Si-ND2, prior to hot wire D2 doses does not appear to affect quantum confinement. In order to analyze this further, 3 nm Si NCs (grown using 11 min of HWCVD) were initially subjected to hot wire doses of ND3 followed by hot wire doses of D2. The results are displayed in Figure 4b. Si NCs are exposed to 1500 and 3000 L hot wire ND3 doses followed by a 1900 L dose of hot wire D2. The PL emission at 1000 nm vanishes, and strong PL is observed at 800 nm. No quantum confinement is observed, although the initial hot wire ND3 dose is quite low. This clearly

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suggests that the PL at 800 nm is due to recombination of excitons from a defect state caused by the hot wire cracking of ND3. It is likely that nitrogen-containing radicals (ND2 and ND) generated at the hot filament insert into the strained Si-Si reconstructed bonds forming Si2dND species where the N atom is in the bridged (Si-N-Si) configuration. In the case of dissociative adsorption, no radicals are generated, and the ND3 gas dissociates into ND2 and D due to the presence of dangling bonds on the NC surface, and this adsorbed amine ligand does not insert into Si-Si bonds. Theoretical calculations by Dal Negro et al. indicate that the bandgap of Si NCs is only slightly reduced by the presence of amine (-NH2) groups on the NC surface.24,25 However, the presence of bridging N atoms (Si-N-Si) results in a substantial reduction in the bandgap of Si NCs. Our experiments show that the quantum confinement effect is observed when the NC surface is covered by Si-ND2 groups, but not when the NC surface is covered by Si2ND groups, which is a validation of the theoretical predictions of Dal Negro et al. Several authors have tried to develop LEDs based on Si quantum dots embedded in silicon nitride; however, such devices have extremely low efficiencies.1-3 Our experiments show that, by tuning the surface passivation, it is possible to obtain PL emission from either defect states or quantum confined states, and this insight is central for the development of efficient NC-based devices. In conclusion, PL emission from ND3-passivated Si NCs is highly sensitive to surface chemistry. The key to a clean bandgap is a fully relaxed NC, whose dangling bonds are completely passivated. At low hot wire ND3 doses, PL emission is observed at 1000 nm (1.24 eV) due to the presence of reconstructed surface bonds capped with monodeuteride and Si-ND2 species. At higher hot wire doses, the NC surface is covered by Si2dND species and PL wavelength peaks at 800 nm (1.55 eV), which is attributed to emission from a defect state. Si NCs subjected to dissociative (thermal) adsorption of ND3 followed by atomic D adsorption, exhibit sizedependent PL, and the presence of Si-ND2 species on the NC surface does not affect quantum confinement effects. Thus, the manner of filling the various NDx and deuteride species plays an important role in determining the optoelectronic properties of Si NCs. While not specifically addressed in this paper, we further note that the stability of the passivated surface that produces the clean bandgap is likely limited to temperatures close to the threshold for trideuteride desorption (∼500 K Figure 2).

current of 4 A was used that led to an estimated filament temperature of 1775 K.6,21 Disilane (Voltaix; 4% in He) partial pressure during HWCVD was 1.6 10-7 Torr and particle size was controlled by varying the HWCVD time. After growth, the samples were cooled to 375 K, the hot filament was turned on, and the samples were subjected to various doses of ND3 (Voltaix; 1% in He) or deuterium (Voltaix; 99.99%) under a partial pressure of 5 10-6 Torr. The samples were then transferred in situ to the PL chamber where PL was measured at a substrate temperature of 310 K. The sample was next transferred to the analytical chamber, where XP spectra and TPD spectra were collected, respectively. A fresh sample was grown and used for each PL-TPD run. A 405 nm (3.06 eV) continuous wave diode laser attached to a quartz view port to enable in situ PL measurements (output power of 20 mW, spot size 1 mm2) was used for excitation, and a QE65000 Ocean optics spectrometer with a wavelength range of 300-1100 nm was used to record the PL spectrum at the sample center.

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AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. Address: The University of Texas at Austin, Department of Chemical Engineering C0400, Austin, TX 78712. Fax: 512.471.7060; phone: 512.471.4689; e-mail: [email protected].

ACKNOWLEDGMENT The authors thank the Welch Foundation (Grant No. F-1502) and Sirica Corporation for funding.

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The experimental apparatus consists of a load lock, growth chamber, a PL chamber, and an analytical chamber, all connected to each other via an intermediate transfer chamber under a base pressure of 5 10-9 Torr. More details of the system are available elsewhere26,27 and the experimental procedure has been described earlier.6,21 Si(100) wafers with 10 nm of thermal oxide were cut into squares of 1.6 cm 1.6 cm and inserted into the growth chamber and heated to a growth temperature of 875 K. A hot tungsten filament is used in HWCVD; a constant filament


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