Hot Photoluminescence in γ-In2Se3Nanorods
© to the authors 2008
Received: 10 July 2008
Accepted: 11 September 2008
Published: 30 September 2008
The energy relaxation of electrons in γ-In2Se3nanorods was investigated by the excitation-dependent photoluminescence (PL). From the high-energy tail of PL, we determine the electron temperature (T e) of the hot electrons. TheT evariation can be explained by a model in which the longitudinal optical (LO)-phonon emission is the dominant energy relaxation process. The high-quality γ-In2Se3nanorods may be a promising material for the photovoltaic devices.
KeywordsInSe nanorods Hot photoluminescence Energy relaxation
The III–VI semiconductors have been the subject of many investigations due to their peculiar electrical and optical properties, and their potential applications in electronic and optoelectronic devices [1–4]. Among these semiconductors, γ-In2Se3 has attracted attention because it is suitable for use in photovoltaic applications . In the recent years, many researchers have been interested in the synthesis of the nanoscale materials due to their unique properties and novel applications in optoelectronic and electronic devices [6–8]. Although some progress has been achieved regarding the growth and characterization of γ-In2Se3 epilayers, the γ-In2Se3 nanostructures have not been grown and investigated yet. The γ-In2Se3 nanostructures may show potential applications in optoelectronic device such as lasers, light emitting diodes (LEDs), and solar cells, due to their high surface-to-volume ratio.
When excess energy is supplied to a carrier by optical excitation or an applied electric field, the energetic carrier becomes hot. The hot carriers then relax toward less energetic state by two competing processes, namely scatterings with other carriers and emission of phonons . The understanding of this energy relaxation process constitutes a direct probe of a very fundamental interaction in condensed matter physics, namely, the electron–phonon and electron–electron interactions. Also, the subject is of obvious technological significance since many devices work mostly in high-field conditions. High electric fields may lead to carrier heating and, consequently, transport effects related to the hot carrier distribution function. A knowledge of hot carrier relaxation mechanisms is thus essential not only for understanding the fundamental process in semiconductor materials but also for evaluating optical device performance.
In this study, the single phase γ-In2Se3nanorods on silicon (111) substrates were grown by metal-organic chemical vapor deposition (MOCVD). The excitation power dependence of photoluminescence (PL) in γ-In2Se3nanorods was studied. The high-energy tails of the low-temperature PL were characterized by effective electron temperatures which increase with increasing excitation intensity. It is found the main path of energy relaxation of the hot electrons in the γ-In2Se3nanorods is the LO-phonon emission.
The γ-In2Se3nanorods were grown on Si (111) substrates by using an MOCVD system at atmospheric pressure with a vertical reactor. The liquid MO, a TMIn compound, and gaseous H2Se were employed as the reactant source materials for In and Se, respectively. The gaseous N2was used as the carrier gas in the process. The substrates used in this experiment were cut from a 6-inchp-type vicinal (111)-oriented Si wafer. Before the growth, Si substrates were baked at 1100 °C for 10 min in gaseous HCl and H2to remove the native oxide. After the thermal etching process, the reactor was cooled down to 425 °C and the γ-In2Se3started to grow. The gaseous flow rate of TMIn was kept at 3 μmol/min and that of H2Se was controlled at 40 μmol/min. The gaseous H2Se was mixed with 85% hydrogen and 15% H2Se. The γ-In2Se3nanorods were grown at 425 °C during a total growth time of 50 min. The structure of the γ-In2Se3nanorods was examined by the X-ray diffraction (XRD) in a θ–2θ geometry. The XRD measurements were performed by using the CuKα-radiation (λ = 1.541 Å) to test the phases of samples. PL was made using the Ar-ion laser operating at a wavelength of 514.5 nm. The room-temperature PL measurements were performed using a confocal microscopy. The collected luminescence was dispersed by a 0.75 m spectrometer and detected with a photo-multiplier tube (PMT).
Results and Discussion
where τph is the effective phonon lifetime, E LO is the LO-phonon energy, , and K 0 is the modified Bessel function of the order of zero. In the steady state, the power input per electron P e is equal to the power loss to the lattice through phonon scattering. Taking values of 19 meV, 1.12×1016 cm−3, 2.12, 2.41 eV, 4.8 × 10−6 cm for E LO n W h ν0 d, respectively, the solid line in Fig. 4 displays the fitted T e with the power loss per electron. Good agreement between experiments and calculations indicates that the model based on the carrier scattering by LO-phonon is able to explain the measuredT e variation with excitation power. It demonstrates again that the LO-phonon emission is the dominant energy loss mechanism in the energy relaxation processes of hot electrons in γ-In2Se3 nanorods.
In summary, the γ-In2Se3nanorods were successfully grown on Si (111) substrates by using MOCVD. A clear room-temperature PL with the peak position of 1.95 eV was observed, corresponding to the near band edge emission. The high-energy tail of PL can be characterized by an effectiveT ewhich increases with increasing excitation intensity. The relationship between theT eand the electron energy loss rate can be explained by a model based on the carrier scattering by the LO-phonons.
This project was supported by the National Science Council under the Grant numbers NSC 93-2112-M-033-010 and 93-2120-M-033-001, and the Center-of-Excellence Program on Membrane Technology, the Ministry of Education, Taiwan.
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