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Ultrafast Carrier Dynamics in Semiconductor Nanowires R. P. Prasankumar1, G. T. Wang2, T. Clement3, S. G. Choi1, S. T. Picraux1,3, and A. J. Taylor1 1
Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 2 Sandia National Laboratories, P. O. Box 5800, MS-1086, Albuquerque, New Mexico 87185 3 School of Materials, Arizona State University, Tempe, Arizona 85287-6006 Email:
[email protected]
Abstract: Time-resolved measurements of carrier dynamics in Ge and GaN nanowires reveal that carrier relaxation in these systems is governed by surface states and defects. This has significant implications for nanowire-based devices in photonics and thermoelectrics. ©2007 Optical Society of America
OCIS codes: (320.7120) Ultrafast phenomena, (320.7130) Ultrafast processes in condensed matter
Semiconductor nanowires (NW) have recently attracted much interest due to their novel electronic and optical properties along with their potential for device applications in areas including nanoscale lasers and thermoelectrics [1]. However, further development and optimization of NW-based devices will depend critically on the understanding of carrier relaxation in these unique nanostructures. For example, GaN-based photonic devices typically exhibit “yellow luminescence”, in which a broad luminescence band believed due to deep acceptor states reduces device efficiency [2]. Ultrafast optical spectroscopy can shed light on this problem by measuring carrier transfer into and out of these states, which would be important in understanding this phenomenon and optimizing device performance. To date, a substantial amount of research has been devoted to carrier relaxation in semiconductor quantum dots and wells, but relatively few time-resolved experiments have been performed on semiconductor nanowires [3-6]. In this work, we present the first optical pump-probe experiments, to the best of our knowledge, on semiconductor nanowires. Our measurements reveal that carrier relaxation is strongly affected by two dimensional confinement, with recombination and trapping primarily occurring at surface states and defects. This information is critical for the use of nanowire-based devices in applications including thermoelectrics and photonics. Ultrafast optical experiments were performed at room temperature on single crystal germanium and gallium nitride nanowires grown by chemical vapor deposition using the vapor-liquid-solid (VLS) mechanism. Gold nanoparticles were used to seed the growth of Ge NWs on Si (111) substrates (Figure 1(a)) [7]. The Ge NWs have typical diameters of 50 nm and lengths of ~1 μm. Gallium nitride NWs were grown at 800 ºC on r-plane sapphire substrates using seed Ni nanoparticles (Figure 1(b)) [2]. To avoid complications in interpreting the data due to a GaN film that grows along with the NWs, the GaN NWs were then scraped onto transparent MgO substrates, resulting in spots with low densities of randomly oriented NWs. The GaN NWs have triangular cross sections, with typical diameters of ~10 nm near the tip and ~200-400 nm near the base; they grew to lengths of ~5 μm.
(a)
(b)
(c)
Fig. 1. (a) SEM image of Ge NWs grown on a Si (111) substrate. (b) SEM image of GaN NWs grown on an r-plane oriented sapphire substrate. (c) Photoluminescence measurement on a single GaN NW grown at 800 ºC, showing the band edge luminescence (GaN BEL) and yellow luminescence band (GaN YL).
Optical pump-probe experiments were performed using a 100 kHz regeneratively amplified Ti:sapphire laser system producing 50 fs, 10 μJ pulses at 800 nm. The output beam is split into two equal parts to concurrently pump two optical parametric amplifiers (OPA). The visible OPA produces a signal beam tunable from 480-700 nm, an
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idler tunable from 930-2300 nm, and a residual pump beam at 400 nm. Similarly, the near-infrared (IR) OPA produces a signal tunable from 1.1-1.6 μm and an idler tunable from 1.6-2.4 μm, with a residual pump beam at 800 nm. This enables measurements with independently tunable pump and probe wavelengths from 400 nm to 2.4 μm.
Fig. 2. Ultrafast measurements on (a) Ge NWs with an 800 nm pump and 1200 nm probe; (b) GaN NWs with a 400 nm pump and 550 nm probe.
Measurements on Ge nanowires and an undoped reference Ge substrate were performed in reflection with an 800 nm pump (fluence ~150 μJ/cm2) and 1200 nm probe (Figure 2(a)). The probe wavelength was chosen to selectively probe carriers in the Ge NWs (band gap ~1880 nm), without probing carriers in the Si substrate (band gap~1100 nm). This allowed us to isolate dynamics due to the NW (measured signals from a Si substrate with seed Au nanoparticles were nearly two orders of magnitude smaller). It can be seen that carrier relaxation is significantly faster in the Ge NWs than in bulk Ge, likely due to carrier trapping and recombination at surface states in the NWs [5]. This has implications for thermoelectric devices based on Ge NWs, as device performance sensitively depends on the ratio of electronic to thermal transport [1]. Our experiments show that the removal of defects and surface states will be critical in optimizing performance, perhaps by incorporating core/shell NWs into these devices. We also performed 400 nm pump (fluence ~250 μJ/cm2), 550 nm probe experiments in transmission on GaN nanowires (Figure 2(b)). The 400 nm pump excites carriers at the bottom of the GaN conduction band, with the probe wavelength set to the peak of the yellow luminescence (YL) band (Figure 1(c)) to track carrier dynamics in these states. The rise time of the signal is 600 fs, which corresponds to the time for carriers to populate the deep acceptor states responsible for YL. The fast 488 fs relaxation is due to electron-phonon relaxation, with the subsequent 7.3 ps decay resulting from surface recombination and trapping. The slow 234 ps decay is due to electron-hole pair recombination. This demonstrates that the acceptor states are populated on an ultrafast time scale and recover within a few hundred ps. Recently, it has been shown that growth temperatures of 900 ºC significantly reduce YL in GaN NWs [2]; time-resolved measurements on these samples should further illuminate the origin of YL and point out methods for minimizing its effects in GaN-based optoelectronic devices. In conclusion, we have presented the first ultrafast optical pump-probe experiments on semiconductor nanowires. Our measurements demonstrate the utility of ultrafast spectroscopy in examining one-dimensional nanostructures, giving insight into carrier relaxation in these systems and suggesting methods of optimizing nanowire-based devices for applications in thermoelectrics and photonics. Future experiments will include wavelength-resolved measurements (e.g., probing across the YL band) while varying material parameters such as the nanowire diameter, length, composition, and growth temperature to further elucidate carrier dynamics in these unique nanosystems. [1] M. Law, J. Goldberger, and P. Yang, "Semiconductor nanowires and nanotubes," Annu. Rev. Mater. Res. 34, 83 (2004). [2] G. T. Wang, A. A. Talin, D. J. Werder, J. R. Creighton, E. Lai, R. J. Anderson, and I. Arslan, "Highly aligned, template-free growth and characterization of vertical GaN nanowires on sapphire by metal-organic chemical vapour deposition," Nanotechnology 17, 5773 (2006). [3] J. C. Johnson, K. P. Knutsen, H. Yan, M. Law, Y. Zhang, P. Yang, and R. J. Saykally, "Ultrafast carrier dynamics in single ZnO nanowire and nanoribbon lasers," Nano Lett. 4, 197 (2004). [4] J. K. Song, J. M. Szarko, S. R. Leone, S. Li, and Y. Zhao, "Ultrafast wavelength-dependent lasing-time dynamics in single ZnO nanotetrapod and nanowire lasers," J. Phys. Chem. B 109, 15749 (2005). [5] D. Cooke, F. A. Hegmann, Z. Wu, X. Mei, A. Shik, H. E. Ruda, J. Liu, and K. L. Kavanagh, "Transient photoconductivity of GaAs and AlGaAs nanowires," American Physical Society March Meeting (Montreal, Canada, 2004). [6] J. B. Baxter and C. A. Schmuttenmaer, "Conductivity of ZnO nanowires, nanoparticles, and thin films using time-resolved terahertz spectroscopy," J. Phys. Chem. B, published on Web 10/4/06. [7] J. W. Dailey, J. Taraci, T. Clement, D. J. Smith, J. Drucker, and S. T. Picraux, "Vapor-liquid-solid growth of germanium nanostructures on silicon," J. Appl. Phys. 96, 7556 (2004).
