Nanophotonics: Single-nanowire electrically driven lasers

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1 Nanophotonics: Single-nanowire electrically driven lasers Ivan Stepanov June 19, 2010 Single crystaline nanowires have unique optic and electronic properties and their potential use in novel photonic and electronic devices is being extensively investigated. The first demonstration of optically pumped nanowire lasers generated great excitement. Electrical pumping is however very desireable as it enables more compact systems usable in electronic devices. Massproduced telecommunication devices, information storage devices and specific tools using lasers - all rely on electrical power supply. The article [1] presents the first study of the possibility to pump single-crystal cadmium sulfide nanowires electrically and shows very promicing results. Electrically driven nanowire laser To understand how the actual object of the article, the single-nanowire electrically driven laser, is operated let us take a look at the properties of a single free standing nanowire that make it usable as a laser. The focus of the article was on cadmium sulphide (CdS) nanowires with a diameter of nm and a wurtzite structure with a [100] growth axis. A schematic drawing of the nanowire is given in the figure 1(a). The nanowire can support lasing for a number reasons: the material of the nanowire is a semiconductor and can be a gain medium, the nanowire acts as a waveguide (see figure 1(a)) and the two flat ends (assumed they are flat) act as mirrors of a Fabry-Perot cavity. CdS can act as a gain medium because it is a semiconductor with a direct band gap of 2.42 ev at 300K [2]. If the electrons are excited from the valence band into the conducting band they can recombine with the holes accompanied by spontaneous or stimulated emission. The function as a waveguide is provided by the fact that a nanowire with the refractive index n (for λ = 510nm) fulfills the requirement 1 (πd/λ)(n 2 1 n 2 0) 0,5 < 2.4, where D is the nanowire diameter, λ is the wavelength and n 0 is the refractive index of the surrounding medium. The flat ends of the nanowire 1

2 act as partially reflecting mirrors because of the difference in the refractive index and build a Fabry-Perot cavity with the length L. A Fabry-Perot cavity will support optical modes that fulfill the condition m(λ/2n 1 ) = L, where m is an integer. Figure 1: Optical properties. (a)schematic showing a nanowire as an optical waveguide with cleaved ends defining a Fabry-Perot cavity (b) SEM image of a cleaved CdS nanowire end. Scale bar 100 nm. (c) Room-temperature photoluminescence image of a CdS nanowire excited about 15 mm away from the nanowire end. Scale bar, 5 µm. Inset, an optical image of the nanowire obtained with white-light illumination. (d) Photoluminescence spectra obtained from the body of the nanowire (blue) and the end of the nanowire (green) at low pump power (10 mw). Adapted from [1] The paper presents two ways to excite CdS: optical and electrical pumping. For both pumping ways the measurements are presented and compared with each other. The photoluminescence or the electroluminescence were measured using epifluorescence microscopy. Optical pumping of nanowire lasers is proven pumping concept [3] and was used by the group to verify the assumptions mentioned above and to show that the used nanowires can act as Fabry-Perot cavities and support stimulated emission. During the optical excitation the nanowire was first pumped about 15 µm from the nanowire end with a highly focused laser beam. Significant emission was measured at the position of the excitation and at the end of the nanowire (figure 1(c)). The fact that emission is only observed at the excitation point and at the nanowire end confirms the assumption that the body of the nanowire acts as a waveguide. The following measurements were conducted under uniform pumping illumination. At low excitation power the photoluminescence spectra recorded from the body and from the end of the nanowire 2

3 are broad peaks at 512 nm and 542 nm respectively as shown in the figure 1 (d). The positions of the peaks are consistent with the room-temperature band-edge emission of CdS with red-shift observed at the end of the nanowire due to reabsorption. With increasing excitation power the emission at the end of the nanowire increases superlinearly and the broad emission peak becomes overlaid by periodic intensity variation. The spacing between the observed peaks is consistent with the assumption that the ends of the nanowire create a Fabry-Perot cavity with the length L. Measurements with nanowires of different length L also confirmed that the mode spacing is inversely proportional to L. The appearance of sharp modes in superlinear regime is also an indicator for amplified spontaneous emission. Experiment at low temperatures and at higher excitation powers also shown that a single-mode operation with the line width limited by the instrument resolution is possible. In summary the optical pumping experiments confirmed that the nanowire acts as a waveguide with its end forming a Fabry-Perot cavity and that the body of the CdS nanowire can have the function of the gain medium with the possibility of a single-mode operation under certain conditions. Significantly new results are presented on the electrically driven nanowire lasers. Similar to conventional semiconductor lasers electrical pumping is based on injection of electrons and holes into the semiconductor. To achieve this a system schematically presented in the figure 2 (a) was created. The hetero-junction needed for injection is produced by putting n-cds nanowires on the heavily p-doped Si substrate. On top of this structure nm of aluminium oxide, 40 nm titanium and 200 nm gold are placed by electron beam-lithography and electron-beam evaporation. One end of the nanowire is left uncovered to allow optical measurements, figure 2(b). If a voltage is apllied the electrons are injected over the Au/Ti-electrode and the holes are injected over the p-substrate. Because of the insulating aluminium oxide layer the current is forced through the NWsubstrate junction. Electrical measurements show a behavior typical for p-n diodes with a forward bias turn-on voltage of 2-5 V (inset of figure 2(c)). The measurements of the electroluminescence at the end of the nanowire show an initial increase of the intensity at 90 µa and a rapid non-linear increase at 200 µa. Similar to optical experiments the spectral measurements show a broad peak at low excitation currents and the appearance of sharp modes after the current has reached 200 µa. The spectrum shows a dominant peak at nm and the line width limited by the instrument resolution. The other peaks have lower intensity and are separated by mode spacings consistent with the mode spacings of a Fabry-Perot cavity of the provided length. 3

4 Figure 2: (a) Schematic showing the cross-section of the device structure. In this structure, electrons and holes can be injected into the CdS nanowire along the whole length from the top metal layer and the bottom p-si layer, respectively. (b) Top panel shows an optical image of a device described in (a). The arrow highlights the exposed CdS nanowire end. Scale bar, 5 µm. Bottom panel shows an electroluminescence image. (c) Emission intensity versus injection current. Inset shows current versus voltage for this device. (d) Electroluminescence spectra obtained from the nanowire end with injection currents of 120 ma (red) and 210 ma (green). The black arrows highlight Fabry-Perot cavity modes with an average spacing of 1.83 nm. The green spectrum is shifted upwards by 0.15 intensity units for clarity. Adapted from [1] History and prospects The published results show the worldwide first experiment in which electrical pumping of a semiconductor nanowire was sufficient enough for lasing. Moreover the electrical excitation could be provided along the whole nanowire. However prior to the publication some aspects used to create the electrically driven nanowire laser were successfully studied by other groups. Extensive studies of optical properties of nanowires made from different materials such as ZnO [4], GaN [5], ZnSe [6], InP [7] and GaAs [8] have been conducted. In optical 4

5 photoluminescence experiments similar to one described in the article many material and optical properties of semiconductor nanowires were measured such as band-edge emission, radiative efficiency, carrier and photon confinement. Demonstrations of the possibility to induce lasing in a single nanowire by optical pumping were published by a number of groups showing comparable results for different materials, for example GaN [9]. The first results by P. Yang s group at Berkeley on optically stimulated lasing in ensembles of ZnO nanowires were conducted in 2001 and were published in 2002 [10]. In their experiments nanowires were grown perpendicularly to the substrate by vapor-liquidsolid mechanism. The nanowires were then optically excited and the excitation power dependent photoluminescence spectra have been recorded. Figure 3: (A) Schematic illustration of the optical excitation and the emission of the nanowires. (B) Emission spectra of ZnO nanowires grown on α-plane sapphire substrate below and over the lasing threshold. The spectrum over the threshold is offset for easy comparison. [10] The observed dependence of the spectral behavior on the excitation power is similar to that observed in the optical experiments conducted by M. Liber s group. Above the threshold excitation power of about 40 kw/cm 2 the broad emission spectrum collapses to a narrow emission line at 380 nm. This transition is a clear indicator for lasing action. Electrical excitation followed by photon emission in nanowires was also demonstrated by M. Lieber s group. However prior to the publication of the paper review by us electrical excitation was achieved by crossing p- and n- nanowires so that the excitation region was limited to their crossing point. This excitation concept was first demonstrated by M. Lieber s group in 2001 for crossed p-n InP nanowires [11]. In a later paper [12] two InP nanowires, one of them synthesized as p-doped, the other one as n-doped, were crossed (by sequential deposition of dilute solutions with intermediate drying) on an insulating substrate and contacted by metal electrodes as shown in the inset of figure 4(b). 5

6 Figure 4: (a) Electroluminescence (EL) image of the light emitted from a forward-biased nanowire p-n junction at 2,5 V. Inset, photoluminescence (PL) image of the junction. Scale bars, 5µm. (b) EL intensity versus voltage. Inset, I-V characteristics; inset in this inset, FE-SEM image of the junction itself. Scale bar, 5 µm. The n-type and p-type nanowires forming this junction have diameters of 65 and 68 nm, respectively. [12] To confirm the doping-type of the wires two-gate measurements of the transport in the wires under different gate voltages were performed. The I-V curves of crossed nanowires (inset of figure 4(b)) show a clear rectification with the current onset at about 1.5 V showing that the electrical behavior is dominated by the p-n junction. The light emission from the spot of the p-n junction is shown in the figure 4(a) confirms the possibility of electrical excitation of semiconductor nanowires. The maximum of the measured electroluminescence is positioned at 820 nm and has a significant blue-shift from the bulk band-gap at 925 nm due to quantum confinement. However the localized charge-carrier injection and the heat generation at the junction prevent the device from being pumped above the threshold needed for lasing. In this sense our article represents a mile-stone on the way to applications based on nanowire lasers as it demonstrates the possibility of effective electrical pumping of the nanowire, a key future for the integration of the nanowires in microelectronic devices. The possible applications could be multi-colored light sources integrated into siliconbased microelectronics or lab-on-chip devices. This could have a huge impact on possibilities of laser-based applications like telecommunications, data storage and enable highly integrated chemical/biological sensors. There are however still issues that have to be addressed. The most important of them is that the laser cannot be driven substantially above threshold to achieve single-mode output at room temperature. The reason for this limitation is assumed to be the nonuniformity of the injection due to the CdS/p-Si and metal/cds junctions that still require optimization. Further only about 50 % of the nanowires are produced with flat ends and can serve as Fabry-Perot cavities. This value has to be increased in order to use nanowires reliably in applications. 6

7 Overall the article fulfills the claims of the introduction as the investigations of optical and electrical pumping clearly shows the feasibility of achieving electrically driven nanowire lasers and represent a big step towards the possible use of nanowire lasers in microelectronic devices. References [1] X. Duan, Y. Huang, R. Agarwal, C.M. Lieber, Nature, 421, p.241 (2003). [2] D. Lincot, Gary Hodes Chemical Solution Deposition of Semiconducting and Non- Metallic Films: Proceedings of the International Symposium The Electrochemical Society, [3] Huang, H. M. et al. Room-temperature ultraviolet nanowire nanolasers. Science 292, (2001). [4] P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R. He, H.-J. Choi, Adv. Funct. Mater. 12, 323 (2002) [5] H.W. Seo, S.Y. Bae, J. Park, H. Yang, K.S. Park, S. Kim, J. Chem. Phys. 116, 9492 (2002) [6] B. Xiang, H.Z. Zhang, G.H. Li, F.H. Yang, F.H. Su, R.M. Wang, J. Xu, G.W. Lu, X.C. Sun, Q. Zhao, D.P. Yu, Appl. Phys. Lett. 82, 3330 (2003) [7] M.S. Gudiksen, J. Wang, C.M. Lieber, J. Phys. Chem. B 106, 4036 (2002) [8] X. Duan, J. Wang, C.M. Lieber, Appl. Phys. Lett. 76, 1116 (2000) [9] J.C. Johnson, H.J. Choi, K.P. Knutsen, R.D. Schaller, P. Yang, R.J. Saykally, Nat. Mater. 1, 106 (2002) [10] M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292, 1897 (2001) [11] Y. Cui, C.M. Lieber, Science 851, 291 (2001) [12] Duan, X.,Huang, Y.,Wang, J., Cui, Y., Lieber, C.M. Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices. Nature 409, (2001). 7