Radiative and nonradiative relaxation phenomena in hydrogen- and oxygen-terminated porous silicon
© Arad-Vosk and Sa'ar; licensee Springer. 2014
Received: 2 September 2013
Accepted: 7 January 2014
Published: 28 January 2014
Using time-resolved photoluminescence spectroscopy over a wide range of temperatures, we were able to probe both radiative and nonradiative relaxation processes in luminescent porous silicon. By comparing the photoluminescence decay times from freshly prepared and oxidized porous silicon, we show that radiative processes should be linked with quantum confinement in small Si nanocrystallites and are not affected by oxidation. In contrast, nonradiative relaxation processes are associated with the state of oxidation where slower relaxation times characterize hydrogen-terminated porous silicon. These results are in a good agreement with the extended vibron model for small Si nanocrystallites.
78.55.Mb; 78.67.Rb; 78.47.jd
KeywordsPorous silicon Photoluminescence Quantum confinement Surface chemistry Nonradiative processes The vibron model
The efficient room-temperature visible photoluminescence (PL) from porous silicon (PSi) has attracted much attention in recent years, mainly due to open questions and controversies concerning the mechanism responsible for the PL emission [1–7]. In addition, numerous PSi-based devices having potential applications in diverse fields such as photonics, optoelectronics, and photovoltaics, were proposed and investigated [8–15]. In particular, PSi has been considered as an attractive candidate for sensing applications [16–21] where its large surface area can be exploited for enhancing the sensitivity to surface interactions. In such a sensor, the PL emitted from PSi can be used as a transducer that converts the chemical interaction into a measurable optical signal. For example, PL quenching due to surface interactions with various chemical species has been utilized for developing various biophotonic sensors [16, 22, 23].
Originally, the efficient PL from PSi was attributed to quantum confinement (QC) of charged carriers in Si nanocrystallites located in the PSi matrix . Experimental evidences supporting this model include a shift of the energy bandgap with size [1–3, 25, 26], resonant PL at low temperatures [27–29], and PL decay time spectroscopy [1, 2, 27]. However, the QC model cannot account for other experimental observations, mainly the dependence of the PL on surface treatments [30–34]. Several reports proposed a more complex picture of QC combined with localization of charged carriers at the surface of the nanocrystals [35–38], particularly the work of Wolkin et al.  who demonstrated a strong dependence of the PL on surface chemistry. This group has shown that while in fresh PSi the PL peak energy depends on the size of the nanocrystals (i.e., follows the QC model), the QC model cannot account for the limited PL shift observed for oxidized PSi. By introducing surface traps into the model, the behavior of the PL peak energy for oxidized PSi could be explained . Other reports have shown that both QC and surface chemistry shape the PL characteristics [37, 38]. The extended vibron (EV) model provides a simple explanation to the mutual role of surface chemistry and QC [39–41]. According to this model, QC affects radiative processes that are less sensitive to the state of the surface, while nonradiative relaxation processes are mostly influenced by the surface chemistry. However, both QC and surface chemistry contribute to the efficient PL from PSi.
In this work, we investigate the role of surface chemistry, particularly the relationship between the state of oxidation and the PL characteristics of luminescent PSi samples. We examine the contribution of radiative and nonradiative decay processes to the overall PL lifetime and the sensitivity of these processes to surface treatments. Furthermore, we examine the EV model by comparing radiative and nonradiative decay times of freshly prepared hydrogen-terminated PSi (H–PSi), with those of oxidized PSi (O–PSi). This allows to experimentally test the hypothesis that radiative processes are not sensitive to surface treatments while nonradiative processes are. Utilizing temperature-dependent, time-resolved PL (TR-PL) spectroscopy , we extend our previous work on silicon nanocrystals embedded in SiO2 matrices and silicon nanowires [37, 41, 43, 44] to PSi, as this system allows a modification of the surface chemistry by simple means and tracing quite accurately the state of the surface.
PSi samples were prepared by electrochemical etching of p-type (10 to 30 Ω⋅cm) silicon wafers under standard dark anodization conditions [25, 26]. A 1:1 mixed solution of aqueous hydrofluoric (HF) acid (49%) and ethanol was used as the electrolyte at a current density of 70 mA cm-2 for 200 s to yield a PSi layer of approximately 9.5 μm (measured by scanning electron microscope) with average pore size of a few nanometers . The freshly prepared PSi is terminated by Si-hydrogen bonds that are known to be quite unstable under ambient conditions. These bonds are subsequently replaced by the more stable Si-oxygen bonds upon exposure to air. Hence, in order to investigate the optical properties of H–PSi, we introduced the freshly prepared samples into a vacuum optical cryostat and kept them under vacuum conditions for the entire experiment. Oxygen-terminated O–PSi was obtained after taking the same PSi sample out of the vacuum cryostat and letting it age under ambient conditions for 6 days. The state of the PSi surface (having either Si-O or Si-H bonds) was monitored by Fourier transform infrared (FTIR) spectroscopy. To eliminate interference phenomena, thinner PSi samples were prepared for these measurements (10 s of anodization under the same conditions, resulting in approximately 450 nm thick PSi film). Bruker's Vertex-V70 vacuum FTIR spectrometer (Bruker Optik GmbH, Ettlingen, Germany), equipped with a mercury-cadmium-telluride (MCT) photovoltaic detector, has been exploited for these experiments. Measurements were performed in the grazing angle reflection mode, at an incidence angle of 65° and under p-polarization (to enhance the sensitivity to surface bonds ). For continuous wave (cw) PL and TR-PL measurements, the samples were excited by Ar+ ion laser operating at 488 nm while the PL signal was dispersed by a 1/4-m monochromator and detected by a photomultiplier tube. For time-resolved measurements, the laser beam was modulated by an acousto-optical modulator driven by a fast pulse generator, while the PL signal has been analyzed by a gated photon counting system. During PL measurements, the samples were kept under vacuum, in a continuous-flow liquid helium optical cryostat that allows temperature control from approximately 6 K up to room temperature.
where τR-1 is the radiative transition rate (given by Equation 2), τNR-1 is the nonradiative relaxation rate, and τ-1 is the total decay rate. The integrated PL (i.e., the area below the PL spectrum shown at the inset to Figure 1) is proportional to the quantum yield that is given by the ratio of the radiative to the total decay rate, . The variation of the integrated PL with temperature is shown in Figure 3b on a semi-logarithmic scale, similar to that of Figure 3a for the PL lifetime. Notice that while the PL lifetime varies by approximately two orders of magnitude over the 30 to 300 K temperature range, the integrated PL varies by less than 3. Hence, one concludes that at this temperature range, τR < < τNR, leading to, τ ≈ τR (Equation 3), and η ≈ constant (as in reference ). Thus, at temperatures above 30 to 40 K the measured lifetime is dominated by radiative transitions. In addition, the strong dependence of the upper singlet lifetime on photon energy (a decrease from 6 to 7 μs at 1.6 eV down to 200 to 300 ns at 2.3 eV; see Figure 4a), suggests again that this lifetime should be associated with radiative transitions (where τU ~ τRU < < τNRU). In this case, the fast radiative lifetime is due to the influence of confinement on the spontaneous emission rates in small Si nanocrystals [39, 40]. On the other hand, the lower triplet lifetime that is dominant at low temperatures is approximately constant (varies by less than factor of 2 over the same range of energies) and roughly independent of the photon energy that probes a given size of nanocrystals. This suggests that the low-temperature lifetime should be associated with a nonradiative relaxation time (of the whole system) that dominates over the (forbidden) radiative triplet lifetime, in agreement with [37, 39, 40].
Turning back to our main findings, we conclude that oxidation (in ambient conditions) has a minor impact on the size of the nanocrystals (giving rise to about 3% blue shift of the PL spectrum) and no noticeable effect on the radiative lifetime and the excitonic energy splitting (via their dependence on photon energy). On the other hand, nonradiative relaxation times, which are associated with the state of the surface, are expected to be sensitive to oxidation and to a modification of surface bonds as experimentally observed (see Figure 4c). This result can be explained by the EV model [39, 40], which assigns the slow nonradiative relaxation times to resonant coupling between surface vibrations and quantized electronic sublevels in the conductance/valence bands of the nanocrystals. The stronger is the coupling between these electronic states and surface vibrations, the slower are the nonradiative lifetimes [39, 40]. Hence, according to this model, the longer lifetime measured for O-PSi (compared to H-PSi) should be assigned with the larger electronegativity of oxygen (relative to hydrogen) that gives rise to larger dipole strength of the Si-O-Si vibration .
Finally, let us point out that the conclusion τR < < τNR (for both types of PSi; see Figure 4) implies that the PL quantum yield should approximately be constant. This conclusion provides a simple explanation to the slight variation of the PL intensity under oxidation, as oxidation modifies nonradiative relaxation times associated with the PSi surface. However, this has a minor impact on the PL quantum yield as the PL efficiency is practically independent of the nonradiative relaxation times at high temperatures [39, 56, 57] and is mostly influenced by the nanocrystals size [59, 60].
In conclusion, using temperature-dependent, time-resolved PL spectroscopy for probing both radiative and nonradiative relaxation processes in freshly prepared and oxidized PSi, we were able to show that radiative processes should be associated with quantum confinement in the core of the Si nanocrystallites and therefore, are not affected by oxidation. On the other hand, nonradiative relaxation processes are affected by oxidation and by the state of the nanocrystallites surface. These results are consistent with the extended vibron model that assigns radiative relaxation to QC, while nonradiative processes are assigned to surface chemistry.
Hydrogen-terminated porous silicon
Oxidized porous silicon
- TR-PL spectroscopy:
Time-resolved photoluminescence spectroscopy.
This work has been partially supported by the Israel Science Foundation (ISF), grant no. 425/09. NAV acknowledges the support of Dr. Ilana Levitan fellowship for women in physics.
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