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Unusually Long Free Carrier Lifetime and Metal-Insulator Band Offset in Vanadium Dioxide

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Unusually Long Free Carrier Lifetime and Metal-Insulator Band Offset in Vanadium Dioxide PHYSICAL REVIEW B 85, 085111 (2012) Unusually long free carrier lifetime and metal-insulator band offset in vanadium dioxide Chris Miller,1 Mark Triplett,1 Joel Lammatao,1 Joonki Suh,2 Deyi Fu,2,3 Junqiao Wu,2 and Dong Yu1,* 1Department of Physics, University of California, Davis, California 95616, USA 2Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA 3Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China (Received 14 December 2011; published 15 February 2012) Single crystalline vanadium dioxide (VO2) nanobeams offer an ideal material basis for exploring the widely observed insulator–metal transition in strongly correlated materials. Here, we investigate nonequilibrium carrier dynamics and electronic structure in single crystalline VO2 nanobeam devices using scanning photocurrent microscopy in the vicinity of their phase transition. We extracted a Schottky barrier height of ∼0.3 eV between the metal and the insulator phases of VO2, providing direct evidence of the nearly symmetric band gap opening upon phase transition. We also observed unusually long photocurrent decay lengths in the insulator phase, indicating unexpectedly long minority carrier lifetimes on the order of microseconds, consistent with the nature of carrier recombination between two d-subbands of VO2. DOI: 10.1103/PhysRevB.85.085111 PACS number(s): 71.30.+h, 72.20.Jv, 73.30.+y I. INTRODUCTION II. METHODS Vanadium dioxide (VO2) is one of the most studied Scanning photocurrent microscopy (SPCM) is an ideal strongly correlated materials. VO2 undergoes a first-order technique for extracting a number of physical properties ◦ 1 insulator–metal transition (IMT) at Tc = 68 C, which from quasi-one-dimensional (quasi-1D) structures, including involves a drastic change in both electric resistivity and crystal electric field distribution, local band bending, barrier heights, 7–13 structure. In its insulating (I) phase (T<Tc), VO2 has a and carrier diffusion lengths. A typical SPCM experiment monoclinic crystal structure with a band gap of ∼0.6 eV,2 involves rastering a diffraction limited laser spot over a planar which transforms into a tetragonal structure in its metallic device while recording the photocurrent as a function of the (M) phase at higher temperatures (T>Tc) [Fig. 1(a)]. The local carrier injection position. Spatially resolved photocur- 14 electronic structure at the junction of M-I domain walls in rents in VO2 NBs have been measured previously but only 2 VO2 is of great interest to understanding the mechanism at very high laser intensity (on the order of 250 kW/cm ), of IMT but has rarely been directly explored. Intriguing where the photocurrent is dominated by the thermoelectric questions remain unanswered, such as how the bands line effect. In this work, we use laser intensities 1000 times weaker up between the two phases, whether the interface is ohmic than the previous report to probe intrinsic carrier dynamics by or Schottky, and how much the bands bend at the interface. ensuring that the photoinjection does not induce unintentional The answers to these questions can shed light on the IMT phase transitions and significant temperature gradients. At this mechanism in strongly correlated materials, which is still low intensity, the local temperature rise due to laser heating is 3,4 under debate in the case of VO2 after decades of studies. estimated to be on the order of a kelvin, and we conclude as In addition, the charge carrier diffusion lengths have not yet seen below that the thermoelectric effect can be ignored. We been measured in VO2. Combined with mobilities measured found that the energy band in the n-type I phase bends upward by Hall or field effect, charge carrier lifetimes can be toward the M phase with a barrier height of ∼0.3 eV. More extracted from the diffusion lengths to elucidate the charge interestingly, we observed unexpectedly long minority carrier carrier dynamics. The difficulties of measuring charge carrier (hole) diffusion lengths in the I phase, indicating long carrier lifetimes in VO2 partly result from its inability for light lifetimes on the order of microseconds. emission, which prevents time-resolved luminescence studies, The VO2 NBs were grown by a physical vapor deposition and its unintentional phase transition during pump-probe approach in a horizontal tube furnace, as detailed in Ref. 6. measurements. Random M-I domain structures that typically Devices incorporating single VO2 NBs were fabricated by occur in VO2 thin films or bulk specimens also complicate photolithography on the as-grown substrates, where the NBs 5 spatially resolved electric measurements. Recently, single were partially embedded in the 1.1-μm-thick SiO2 layer on crystalline VO2 nanobeams (NBs) with a rectangular cross Si wafers. Cr/Au contacts were thermally evaporated, creating section have been successfully synthesized via a vapor trans- 20-μm-gap devices with NBs that typically had a rectangular 6 2 port approach. These VO2 NBs provide an excellent platform 1-μm cross section. A scanning electron microscope (SEM) for investigating the electronic structure at the M-I junction image of a typical device is shown in Fig. 2(b).TheVO2 NB (domain wall); the characteristic domain size is larger than devices showed linear current–voltage (I-V ) characteristics the lateral dimension of the NB, so a simple 1D domain with contact resistance much lower than the NB resistance, structure can be easily stabilized in the axial direction of the confirmed by four-probe measurements. The I phase VO2 NB. NBs were n-type with a relatively low electric resistivity of 1098-0121/2012/85(8)/085111(6)085111-1 ©2012 American Physical Society MILLER, TRIPLETT, LAMMATAO, SUH, FU, WU, AND YU PHYSICAL REVIEW B 85, 085111 (2012) (a) (b) (a)laser (b) Metal Insulator V 1.0 V = 2V b μ e b 6 m g Intensity = 540 W cm-2 0.5 1V VO d dπ 2 π * d 0.0 0V I // SiO I t2g E 2 F -0.5 -1V Si d d -75 -17 42 100 // // -1.0 Potential (V) -2V Photocurrent (pA) (c) (d) -1.5 O-2p 100 200 mV 400 0 2 4 6 8 10 12 L = 3.1 μm μ 50 D (c) (d) Position ( m) 200 0 V 10 0.0 0.0 S) μ Ω) -200 Ea = 80 meV -50 L = 3.5 μm Photocurrent (pA) D 1 1 Photocurrent (pA) -400 -200 mV 0 5 10 15 20 25 30 0 5 10 15 20 25 30 0.1 Injection Position (μm) Injection Position (μm) 0.1 Resistance (M Conductance ( FIG. 2. (Color online) (a) Schematic of the SPCM setup, where 310320 330 340 350 360 370 -3 5 6 7 8910x10 a diffraction limited laser spot is rastered over a VO2 NB device. Temperature (K) 1/Temperature (K-1) (b) SEM image and SPCM image of a VO2 NB at zero bias with an −2 FIG. 1. (Color online) (a) Band diagram for M and I phases of illumination intensity of 540 W cm and wavelength of 532 nm (device D1). The arrow indicates the positive current direction. VO2, showing the splitting of the d||-band and the rising of the dπ -band upon transition from the tetragonal (M) to monoclinic (I) crystal (c) Cross section of the SPCM image in (b) in the axial direction of the NB. The data are fit to the exponential form I = I0 exp[−(x − structures. (b) Kelvin probe microscopy measurement of a VO2 NB at room temperature. Shaded areas indicate electrodes. (c) Resistance x0)/LD]. The fitting parameters LD are labeled in the plot. (d) SPCM line scans under biases ranging from −200 to 200 mV with 50-mV as a function of temperature for a typical VO2 NB device (D1). Arrows indicate the temperature ramping directions. (d) Conductance increments. The dark current was subtracted. as a function of inverse temperature showing an activation energy of 80 meV. III. RESULTS AND DISCUSSIONS We first present the SPCM results of a typical single VO2 NB device (D1) at room temperature (Fig. 2), far from the ∼5 ·cm. The kelvin probe microscopy showed that the phase transition temperature (68 ◦C). The excitation laser peak electric potential was uniformly distributed along a typical NB intensity was 540 W/cm2, much lower than the phase transition at room temperature [Fig. 1(b)], indicating the NBs were free threshold; thus, the NB was completely in the I phase during of defects that might induce a local electric field. The slight scanning. A typical SPCM image at zero bias is shown in nonlinear evolution of the voltage profile near the contacts Fig. 2(b), displaying a significant photocurrent only when the is likely due to the Schottky barriers between the VO2 NB illumination is near the contacts. The Schottky junction at and the metal electrodes. The spatial resolution of our kelvin the Cr contact leads to a zero-bias photocurrent,11 and the probe microscopy is on the order of 100 nm, longer than the current direction indicates an upward band bending toward expected depletion width, and thus cannot provide a direct the electrodes. In the more than 10 devices measured to date, measure of this width. As the temperature increased and M the bending directions have been consistent and agree with the domains formed in the NBs, the device resistance decreased n-type nature of VO2. An external quantum efficiency (ratio of in discrete steps. A typical resistance–temperature curve can collected carriers to incident photons) of 0.006% is estimated be seen in Fig. 1(c). The temperature-dependent conductance from the maximum zero-bias photocurrent (∼100 pA) and of a typical NB device followed the Arrhenius behavior well the excitation intensity (540 W/cm2).
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