Terahertz Nanoprobing of Semiconductor Surface Dynamics - Nano

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Terahertz nano probing of semiconductor surface dynamics Geunchang Choi, Young-Mi Bahk, Taehee Kang, Yoojin Lee, Byung Hee Son, Yeong Hwan Ahn, Minah Seo, and Dai-Sik Kim Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03289 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 25, 2017

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Terahertz nano probing of semiconductor surface dynamics Geunchang Choi,†,‡ Young-Mi Bahk,§ Taehee Kang,† Yoojin Lee,† Byung Hee Son,║ Yeong Hwan Ahn,║ Minah Seo*,‡ and Dai-Sik Kim*,†

†

Department of Physics and Astronomy and Center for Atom Scale Electromagnetism, Seoul

National University, Seoul 151-747, Korea ‡

Sensor System Research Center, Korea Institute of Science and Technology, Seoul 136-791,

Korea §

Department of Physics, Incheon National University, Incheon 22012, Korea

║

Department of Physics and Department of Energy Systems Research, Ajou University,

Suwon 443-749, Korea

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ACS Paragon Plus Environment

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Abstract Most semiconductors have surface dynamics radically different from its bulk counterpart due to surface defect, doping level, and symmetry breaking. Due to the technical challenge of direct observation of the surface carrier dynamics, however, experimental studies have been allowed in severely shrunk structures including nanowires, thin films, or quantum wells where the surface-to-volume ratio is very high. Here, we develop a new type of terahertz (THz) nano probing system to investigate the surface dynamics of bulk semiconductors, using metallic nano gap accompanying strong THz field confinement. We observed that carrier lifetimes of InP and GaAs dramatically decrease close to the limit of THz time resolution (~1 ps) as the gap size decreases down to nano scale, and that they return to their original values once the nano gap patterns are removed. Our THz nano probing system will open up pathways towards direct, and nondestructive measurements of surface dynamics of bulk semiconductors.

Keywords: Nano probing, Optical pump- Terahertz probe spectroscopy, Semiconductor surface, Carrier dynamics

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Surface carrier dynamics in semiconductor materials are related to the band structure of the materials,1 ion-doping,2 and surface states,3-7 which can be drastically different from its bulk counterpart. Understandably, such carrier dynamics and surface properties are crucial for the performance of semiconductor-based optoelectronic8,

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and photovoltaic devices.10

Despite their importance, direct measurements of the surface properties have been hindered by technical difficulties. Those have been limitedly provided in nano-sized structures such as nanowires or thin films which have very large surface to volume ratio.11-13 Even in nano scale samples, most carrier dynamics still contain both surface and bulk dynamics; more importantly, it is essential to probe surface dynamics of bulk semiconductors without resorting to, say, nanowires of the same material. Optical pump-optical probe technique based on femtosecond laser has been conducted in various semiconductor materials for measurements of different carrier relaxation processes such as ultrafast non-equilibrium dynamics,14, 15 carrier-carrier interaction,16 and carrier-phonon interaction.17 As a probe beam, Terahertz (THz) waves with low photon energy have many advantages such as being nondestructive, and being far below the band gap of the targeted semiconductor materials.18, 19 Here, we measured the surface carrier dynamics of semi-insulating (SI)-InP and SIGaAs in picosecond time scale using the optical pump-THz probe (OPTP) method. By fabricating nano-antenna onto the targeted materials (SI-InP and SI-GaAs), the carrier dynamics at extreme surface can be efficiently examined. The nano-antenna arrays are composed of slot antennas with a few hundred micrometers in length but with a nanoscale width, accompanying strong field confinement (Fig. 1(a)).20 It enables us to sensitively capture the surface-only carrier dynamics of bulk semiconductor in OPTP experiment. The faster carrier dynamics near the surface becomes invisible again once the nano-patterns are etched out and the bulk dynamics are restored (Fig. 1(b)). From the measured subliminal surface carrier dynamics, we characterize surface recombination velocities and diffusion 3


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coefficients of the semiconductors by analytical carrier density fitting. For the OPTP experiment, a femtosecond Ti:sapphire laser (80 MHz repetition rate, 800 nm center wavelength and 130 fs pulse duration) is divided into three paths for optical pump, THz generation, and THz detection via electro-optical sampling at low-temperature grown GaAs photoconductive antenna.21, 22 The optical pump is focused on a 1 mm by 1 mm aluminum aperture located at the focal spot of THz waves with a power of 70 mW. The optical pump impinges on a sample at an angle of 45 degrees while the THz beam is normally incident. The nano-patterns were fabricated by electron beam lithography, and we used focused ion beam or atomic layer lithography for sub-100 nm gap samples (Supporting Information). To avoid direct metal contacts on target semiconductor materials, which might affect surface states of semiconductors, we added aluminum oxide layers between the semiconductors and nano-patterned gold films (Supporting Information). The polarization of the incident THz wave is perpendicular to long axis of the antennas. We monitored photoinduced change in THz transmission, ∆Τ/Τ, at the peak of THz signal in time domain by changing relative delay of the optical pump beam. Because THz waves are sensitive to conductivities of the sample materials, the optical pump responses of semiconductors can be described with interband conductivities which depends on photo-excited carrier densities. We note that ∆T/T linearly increases with the optical pump power, implying that the measured THz transmission changes are originated from the excited carrier densities in semiconductors (Supporting Information).1 To take into account confined depth of the THz probe beam and effective penetration depth of the optical pump beam which play crucial roles in the measurements of surface carrier dynamics, we investigate near-field characteristics of electromagnetic waves for two different wavelengths (pump and probe). Figure 2(a) presents electric intensity distribution 4


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(|Ex|2) of THz wave and optical power (P) obtained from analytical calculation based on modal expansion and COMSOL simulation, respectively. From the calculations, we plot normalized |Ex|2 of THz probe beam (violet) and P of optical pump beam (red) along the zaxis for 500 nm (top) and 50 nm (bottom) gap patterned on InP (left) and GaAs (right), respectively (Fig. 2(b)). The different depth of pump or probe beam obviously indicates stronger field confinement for longer wavelength of electromagnetic waves (λpump ~ 800 nm and λprobe ~ 600 µm). The probe depth of THz light, though its wavelength is 800 times larger than that of the pump beam is shallower. THz nano probing beam therefore achieves the smaller spot size required by an ideal probe, because the penetration into the sample can be fully described by the electrostatic leakage field of a parallel capacitor at length scale in orders of magnitude smaller than its wavelength.20 We define the effective probe depth of the confined THz waves, dprobe, by full width at half maximum (FWHM) of the THz electric intensity, while the effective pump depth of optical beam, dpump, is defined by an effective penetration depth at which the optical power drops to 1/e from the surface value. Figure 2(c) renders dprobe and dpump as a function of the gap size, showing that the confined probe depths of THz waves where the smallest gap is around 10 nm. With the increasing gap size, the confined depth of the THz probe beam linearly increases while the effective penetration depth of the pump beam increases and then saturates close to the penetration depth determined by the extinction coefficient of the materials at the wavelength of 800 nm (dInP ~ 300 nm and

dGaAs ~ 750 nm). By considering optical field enhancement (~ 1) at the nano gap,23, 24 the calculated photoexcited carrier is below 5 x 1016 cm-3 for both InP and GaAs samples, which is low enough for linear THz response (Supporting Information),25 and the optical pump is far away from the resonance of our antenna, thereby having no influence on spontaneous emission rate.26 Furthermore, we experimentally confirmed that the carrier dynamics of GaAs 5


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and InP are constant for increased pump power (Supporting Information). Controlling the depth of pump or probe beam by the gap size enables to observe extreme surface carrier dynamics of bulk semiconductors. Figure 3(a) and (b) show normalized transmission changes of THz probe beam as a function of the OPTP time delay for bare and nano-patterned InP and GaAs at various gap sizes. Since the energy of surface states due to dangling bonds in InP is close to the conduction band while that of surface states in GaAs is closer to the mid gap, the Fermi-level at the surface of GaAs is pinned near the mid gap leading to band bending upward. This Fermi-level pinning position which is related to the dangling bonds energy results in the surface recombination velocity of GaAs being faster than that of InP even though they have similar band structures and optical properties.27 In bare InP and bare GaAs, the carrier lifetimes (τ) are relatively slow, up to several hundreds of picosecond. In stark contrast, with decreasing the gap size down to nano scale, τ dramatically decreases to picosecond scale (τInP ~ 2.1 ps for 50 nm gap and τGaAs ~ 2 ps for 200 nm gap). Carrier lifetime for 35 nm-gappatterned GaAs is saturated at 2 ps close to the limit of THz time resolution as shown in the inset of the figure. The faster carrier lifetimes in nano-patterned samples are originated from the extreme surface, observed by our THz nano probing system. Thus far, to measure the surface carrier dynamics effectively, it has been required to severely change surface-tovolume ratio by newly fabricating nanowires or nanomaterials11-13, 28 otherwise to use shorter wavelength as a probe beam for much smaller penetration depth.29 In THz nano probing system, we can choose a broad range of wavelengths with changing the antenna length, without affecting the shortened carrier lifetime (Supporting Information). Furthermore, the system enables permanent reuse of target materials, as the bulk lifetimes of semiconductors (InP and GaAs) are completely restored after etching the nano-patterns by hydrogen fluoride (HF) solution (Fig. 1(b)). However, there are some limitations for probing surface carrier 6


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dynamics that the signal-to-noise ratio decreases due to the small confined probe volume and the target material has to be detectable in transmission geometry. To quantitatively investigate the surface carrier dynamics near the gap, we extracted the surface recombination velocity, S, and diffusion coefficient, D, from the carrier density distribution. An analytical expression for the normalized carrier density N is as follows30 N (t , z ) =

 z 2   1 z   w α Dt − exp −   2 4 Dt 2 Dt    

 S + αD  −  S − αD

 z w α Dt + 2 Dt 

  2S t z  + w S +  S − α D D 2 Dt  

    ,

where t is the delay time, z is a distance from surface, α is the absorption coefficient at the excitation wavelength, and w(ζ) = exp(ζ2)[1-erf(ζ)]. Especially, for applying nano antenna samples, α is replaced with the effective absorption coefficient αeff (= 1/dpump) where dpump is shown in Fig. 2(c). The values of αeff for each nano antenna sample are presented in Supporting Information. Since ∆Τ/Τ is proportional to the change of carrier density as a function of time, the normalized carrier densities are integrated from the surface to dprobe at each time delay (Supporting Information). Here, S and D can be treated as variables to get the best fitting from our model where diffusion to the in-plane direction is neglected owing to the much tighter confinement along the z-direction compared with that along the x-direction (Supporting Information). The obtained S and D values thus are S = 1.5ⅹ105 cm/s, D = 0.3 cm2/s for InP; S = 1.1ⅹ106 cm/s, D = 45 cm2/s for GaAs, which fit the experimental data very well (lines in Fig. 3). As expected, the S value is much larger than that reported for an intrinsic InP,32 but is similar to that reported for another semi-insulating InP (Fe-doped) result,

S = 1ⅹ105 cm/s.33 For GaAs, the S value matches well with that in a previous work.34 We note that, for these S and D values, carrier dynamics within the first few picoseconds is largely due to the surface recombination, while later dynamics is governed by diffusion and bulk recombination properties.31 Therefore, a few picoseconds lifetime in the small nano gaps 7


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(in Fig. 3) represents surface recombination sped up by an effective thickness smaller than the gap width. In Fig. 4, we plot the inverse of the carrier lifetime, 1/τ, for nano-patterned InP (red) and GaAs (blue), as a function of the inverse of the effective probe depth, 1/dprobe, controlled by the gap size. The carrier lifetimes are extracted by fitting the calculated lines (circular dot) in Fig. 3 with a single exponential function. As we decrease the gap size below 200 nm (50 nm), the carrier lifetime of GaAs (InP) dramatically decreases even below 1 ps, caused by surface state and surface defect driving the faster recombination process at extreme the surface of semiconductors. Our approach provides the most direct approach to the underlying surface dynamics of bulk semiconductors without inferring to nanostructures such as quantum wires or quantum wells. Moreover, with the help of nano-patterning, the lifetimes of bulk semiconductor can be engineered with huge dynamic range, opening up new ultrafast optical applications. In conclusion, we demonstrated ultrafast surface dynamics measurements of bulk semiconductors (SI-InP and SI-GaAs) using THz nano probing method. Taking advantage of the confined THz near-field, we can effectively measure the subliminal surface carrier dynamics for InP and GaAs, by confining both pump and probe beams spatially. The sample retains its original properties, as the complete restoration of bulk dynamics after removal of the nano-patterns is demonstrated. Through both experimental results and calculations, the surface recombination velocities and diffusion coefficients of the semiconductor materials were extracted. With ever decreasing the feature sizes we envision ultrafast switching applications using surface dynamics only, bypassing the much slower bulk dynamics.

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Associated content

Supporting information Supporting information is available free of charge Sample fabrication. Effect of aluminum oxide layer on carrier dynamics. Dependence of optical pump power and antenna resonant frequency on carrier dynamics. Calculation of integrated carrier densities. Effective absorption coefficient and diffusion effect in nano antenna (PDF)

Author Information

Corresponding author [email protected] and [email protected]

Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIP: NRF-2015R1A3A2031768) (MOE: BK21 Plus Program21A20131111123) (MSIP: Global Frontier Program-2016M3A6B3936653) (MSIP: Mid-

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carrier Researcher Program-2016R1A2B2010858) and KIST intramural grants with No. 2E27270 and 2V05550.

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Figure 1. (a) A schematic representation of photo induced carriers around nano patterns on a semiconductor. As closer to the surface, the recombination time of carriers (electrons in blue and holes in red) is getting faster as described. The faster carrier dynamics near the extreme surface, then, can be captured by a tightly localized THz probe. (b) Carrier dynamics of bare and nano-patterned InP (top) and GaAs (bottom) measured from optical pump THz-probe spectroscopy. The gap sizes is 150 nm and 500 nm for InP and GaAs, respectively. Once the patterns are etched, the carrier recombination processes are completely recovered to the characteristics of bare samples.

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Figure 2. (a) Electric field intensity |Ex2| distribution of THz probe beam (left) and Poynting vector |S| distribution of optical probe beam (right) near 50 nm-sized nano gap patterned on InP, obtained by analytical calculation based on modal expansion and COMSOL simulation, respectively (b) Normalized |Ex|2 of THz probe (violet) and |S| of optical pump (red) along the z-axis for 500 (top) and 50 (bottom) nm-gap-patterned on InP (left) and GaAs (right), respectively. (c) Effective pump depth (dpump) of optical pump beam for InP (red dot) and GaAs (blue dot) and effective probe depth (dprobe) of THz probe beam (black dot) as a function of the gap size.

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Figure 3. Normalized THz transmission changes for different metallic gap samples on (a) InP and (b) GaAs as a function of pump-probe delay time. The decaying time constants τ of InP are 73 ps, 28 ps, 5 ps, and 2.1 ps, for the gap sizes of 1 µm, 500 nm, 150 nm, and 50 nm, respectively, meanwhile τ of GaAs are 86 ps, 35 ps, 22 ps, 18 ps and 7.5 ps for bare GaAs and the gap sizes of 3 µm, 2.5 µm, 1.5 µm and 500 nm, respectively. Further narrow gap (200 nm and 35 nm) of GaAs, the carrier lifetimes are saturated due to the THz time resolution limit. Each fitting lines with experimental data are calculated by normalized carrier densities as a function of time. From the fitting, we extract the parameters S = 1.5ⅹ105 cm/s, D = 0.3 cm2/s for InP and S = 1.1ⅹ106 cm/s, D = 45 cm2/s for GaAs. 16


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Figure 4. Inverse of carrier lifetime (1/τ) of nano-patterned InP (red) and GaAs (blue) as a function of the inverse of the effective probe depth (1/dprobe) controlled by the gap size. Carrier lifetimes are extracted from single exponential fits of experimental data (triangular dot) and the fitted lines (dotted line) in Fig.3. The solid lines are guide to the eye, which start to saturate around a recombination rate of 1/(2ps) due to the limited time resolution of the setup.

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Terahertz Nanoprobing of Semiconductor Surface Dynamics - Nano

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