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Composition tuning of room-temperature nanolasers
Vacuum 86 (2012) 737e741
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Composition tuning of room-temperature nanolasers C.Y. Luan a, Y.K. Liu a, b, Y. Jiang a, c, J.S. Jie a, d, I. Bello a, S.T. Lee a, J.A. Zapien a, * a
Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Ave., Kowloon Tong, Hong Kong SAR, PR China b Department of Physics, Yunnan Normal University, Kunming, PR China c School of Materials Science and Engineering, Hefei University of Technology, Anhui, PR China d School of Electronic Science & Applied Physics, Hefei University of Technology, Anhui, PR China
a b s t r a c t Keywords: Nanolasers Self-assembled nanostructures Photonics II-VI semiconductors Nanowires Waveguides
We show that careful preparation conditions enable the preparation of high quality nanoribbons capable of room-temperature lasing covering the complete spectral range from near infrared (NIR) to ultraviolet (UV). Two ternary compound II-VI semiconductors, CdS1XSeX and ZnYCd1YS, can be grown on single c-Si substrates to achieve this task. High-resolution transmission electron microscopy and x-ray diffraction showed that the ternary nanoribbons were single phase and single crystal. Room-temperature optical measurements showed that band-gap engineering could be realized via composition modulation in X and Y, with fine tuning of the lasing wavelength via composition changes (DX and DY) capable of overlapping thermally induced tuning, demonstrating the possibility of continuous tuning in the lasing wavelength throughout the complete 340e710 nm spectral range. We further demonstrate that it is possible to control the spatial composition gradient with sub-mm resolution thus opening the possibility for the fabrication of devices with full spectral optimization. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Light emitting nanostructures with finely tuned light emission in the NIR-Vis-UV spectral range are expected to provide major contributions to optic and optoelectronic applications in fields such as communication, data storage, sensing, biological, and medical instrumentation. This is because many of these applications require the ability to fabricate lasers of predetermined wavelength; a goal also exemplified by the commercial importance of green, blue and ultraviolet light emitters [1,2]. The advent of a commercial blue laser based on III-Nitride semiconductors [3] was expected to become the first of a number of finely wavelength-tunable lasers. However, a number of material problems such as phase separation have limited the expansion of III-Nitride-based lasers to the remainder of the green-blue spectral region [4]. The continuous need for higher efficiency, brighter, and low cost LEDs and lasers for the entire visible-UV range has renewed the interest in II-VI semiconductors due to their relevance as promising materials for lasing, solar cells, and other optoelectronic applications [5,6]. Indeed, band gap engineering of II-VI semiconductors has been utilized for preparing superlattices,
* Corresponding author. Tel.: þ86 852 3442 7823; fax: þ86 852 3442 0547. E-mail address: [email protected] (J.A. Zapien). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.07.004
heterostructures, and quantum wells (QW) to produce blue-green lasers [7e10]. It can be expected that the “bottom-up” approach used in nanoscale materials can assist to overcome current problems with II-VI technologies [11e14] such as (1) lack of high quality bulk single crystals of II-VI semiconductors suitable for use as substrates, and (2) defects derived from the polytypism of II-VI compounds. These limitations can be surmounted because nanotechnology enables the growth of high quality single crystal II-VI nanoribbons with fewer defects without the need of lattice matched substrates [15]. Nanowire lasing studies have been reported involving experimental [16] and theoretical [17,18] investigation of the optical cavity effects in self-assembled materials. Lasing in ZnO NWs has been attributed to excitonic emission, although it was also suggested that intense optical pumping could lead to the eventual onset of an electron-hole plasma laser [16]. Additional studies by temperature-dependent spectroscopic studies in CdS NWs provided strong evidence of the importance of exciton-based mechanism in the lasing of single CdS NWs and concluded that excitoneexciton scattering dominates from 4.2 to 75 K while exciton-LO scattering dominates at higher temperatures [19]. Some authors have also pointed out the relevance of the large oscillator strength for exciton transitions in ZnO and thus the expected increased role of lightematter interaction [20]. In fact, it was reported (Ref. [20]) that extremely strong exciton-photon coupling
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(derived from composition tuning) overlapping thermally induced tuning [21]. Similar studies have been independently reported for CdS1XSeX with lasing emission at 77 K [22], and for ZnMgO with lasing effects based on random-lasing effects [23]. More recently [24], composition tuning has been extended to quaternary ZnCdSSe compounds; however no evidence of waveguiding or lasing effects were presented. In this paper we review our work toward the development of efficient room-temperature optically pumped nanolasers covering the NIR-UV spectral region. 2. Experimental 2.1. Nanoribbon fabrication Two ternary compositions, namely CdS1XSeX [25] and ZnYCd1YS [26] were synthesized in a quartz tube mounted inside a three-zone horizontal tube furnace as previously described. Briefly, the CdS1XSeX (ZnYCd1YS) NRs were prepared from laser ablation of a CdS target prepared by pressing high-purity (>99.99 wt %) CdS powders at room temperature. The CdS target and the CdSe (ZnS) powders were placed in the middle of temperature zones I and II, respectively, while silicon substrates coated with w20 Å gold film were placed between zones II and III to achieve a temperature gradient. The tube was evacuated to a pressure of 8 106 mbar then pre-mixed argon and hydrogen (5%) was admitted into the system at a 20 sccm flow rate to achieve a pressure of w130 mbar. The temperature in the three zones of the furnace was then increased to 750, 900, and 400 C (750, 950, 500 C for ZnYCd1YS NR growth) for 30 min. A KrF laser (248 nm, pulse width 34 ns and 450 mJ per pulse at 5 pulses per second) was used to ablate the CdS target for 2.5 (2.0) hours. Gold-coated c-Si substrates were used which resulted in high quality nanoribbons. 2.2. Nanoribbon characterization Fig. 1. (a) Low and (b) higher magnification SEM images of high quality CdSSe nanoribbons. These nanoribbons show lasing emission when pumped with 266 nm pulsed (6 ns) laser with a lasing threshold of w40e60 kW/cm2. Note the lack of order in large scale (a) but the presence of some degree of local alignment (b).
in ZnO nanowires occurs at room temperature and it was suggested that the resulting polaritonic dispersion curve in the near UV could explain the surprising subwavelength guiding necessary for lasing effects to occur. Our group has demonstrated that nanostructures based on ternary II-VI composition alloys, ZnYCd1YS and CdS1XSeX, can provide efficient room-temperature lasing at any predetermined wavelength between 710 and 340 nm with tuning resolution
a
The nanoribbon morphology, composition and crystallography was characterized by scanning electron microscopy (SEM; Philips XL 30 FEG) with Energy dispersive spectroscopy (EDS), and microarea x-ray diffraction (Philips PW 1830 with Cu Ka radiation and a normal 2q scan, 40 kV, 30 mA) with a 1 mm x-ray beam, respectively. Photoluminescence (PL) measurements were conducted at room temperature using the fourth harmonic of a Nd:yttriumealuminumegarnet (Nd: YAG) laser (266 nm wavelength) with a 6 ns pulse width as the excitation source. The emitted light was detected using an ultraviolet (UV) optical fiber coupled to a 0.5 m spectrometer (Acton Research Corp., Spectra Pro 500i) with dispersion gratings of 1200 and 150 grooves mm1 for
b
Fig. 2. (a) Lasing emission of seven ZnYCd1YS and CdS1XSeX samples (including a single ZnS nanoribbon denoted by *) presenting room temperature lasing between 340 and 710 nm. (b) High resolution lasing characteristics of a single ZnS nanoribbon (after Ref. [28]) with lasing threshold w 50 kW/cm2 (inset) and resonant modes with FWHM w 0.1 nm (Q > 3000).
C.Y. Luan et al. / Vacuum 86 (2012) 737e741
Fig. 3. Lasing emission of seven CdS1XSeX samples with lasing shift Dl w 5 nm between them (80 kW/cm2 excitation) and linear regression of the experimental lasing wavelengths against the step number i (right inset).
high (w0.1 nm) and low (w0.8 nm) resolution and a Peltier cooled (20 C) intensified charge-coupled device (ICCD) camera (Roper Scientific). The measurements were performed using normal incident excitation and 10 detection were all angles measured with respect to the sample’s surface. 3. Results and discussion Careful selection of the preparation parameters enables the growth of nanomaterials with very high crystallographic quality,
739
high yields, and relatively low cost; Fig. 1a shows an example for CdSSe NWs. It is interesting to note that despite the random nature of the growth position and alignment on a large scale; in most cases it is relatively easy to identify small areas that show a relatively high level of local alignment (e.g., Fig. 1b). The composition changes in both ZnYCd1YS and CdS1XSeX nanoribbons were confirmed by energy dispersive X-ray spectrum (EDX) and X-ray diffraction (XRD). Composition changes result from the temperature gradient along the substrate, which is controlled by the temperatures of zones II and III. Such composition changes occur gradually (about 1% per mm) and are difficult to record in detail using EDX or XRD. To overcome this limitation, we interpolated the position of the maximum of the PL peak (EPL) as a function of composition X (determined from the diffraction spectra using Bragg reflection equation and Vegard’s law [27]) for selected compositions. This interpolation was used to estimate the nanoribbons composition since EPL position closely follows the variation of the energy gap, Eg, resulting from changes in the NR composition (see e.g., Refs. [25,26]). Near band gap emission is expected to occur from 710 to 510 nm for CdS1XSeX and from 510 to 340 nm for ZnYCd1YS for 0 X, Y 1. Fig. 2a presents the room-temperature PL characteristics of six ZnYCd1YS and CdS1XSeX nanoribbon ensembles with average composition X ¼ 1, 0.76, 0.52, and 0.18, and Y ¼ 0.25, and 0.72; as calculated according to Vegard’s law interpolation described above. In each case several spectra, displaced vertically for clarity, are presented at varying excitation power densities between 5 and 100 kW/cm2 with the lowest excitation power density occupying the lowest position. Clearly, smooth spontaneous luminescence bands are recorded at low excitation power densities while sharp and intense resonant modes (FWHM w 0.3e0.5 nm) are observed
Fig. 4. (a) Optical micrograph of ZnYCd1YS NRs showing a complex gradients along the x- and y- directions (axial- and transversal directions w.r.t. to the tube axis, respectively). (b) Schematic of the setup used to prepare the sample in (a) showing the close contact between the Si substrate edges and the quartz tube (see text). (c) Optical micrograph of a CdS1XSeX sample that has been prepared with minimum (below detection) composition gradient along the y- (transversal) direction. (d) Schematic of the setup used to prepare the sample in (c) where the Si substrate is supported by a fused silica thus minimized the y-direction temperature gradient.
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at higher power densities. The transition from spontaneous to amplified emission occurs in the 35e90 kW/cm2 range for different compositions and in all cases occurs simultaneously with the appearance of the resonant peaks which demonstrate that the nanoribbons are able to sustain lasing action at room temperature. We emphasize that lasing action is unlikely to occur simultaneously in all excited nanoribbons in each ensemble measurement. This is because the reduction in the effective pumping power density that results from non-optimum nanoribbon alignment with respect to the excitation laser (that is when the nanoribbon largest surface area is not perpendicular to the excitation laser beam) or by shadowing due to neighboring nanoribbons. Such geometrical consideration results in an effective selection rule by which only a limited number of nanoribbons are excited above their lasing threshold and has two main consequences: (i) it enables the clear detection of a limited number of resonant modes (as opposed to an otherwise expected large number of modes, resulting from the wide distribution of sizes, that could prevent individual modes identification); (ii) it explains the relatively large spontaneous background emission that accompanies the resonant modes since there is a large number of nanorribons being simultaneously excited below their lasing threshold. To illustrate the second point, Fig. 2 also presents the lasing characteristics of a single ZnS nanoribbon (after Ref. [28]); as expected, the corresponding emission at w340 nm is nearly free of spontaneous emission. Two additional effects complicate the interpretation of the lasing characteristics in ensemble measurements. First, differences in defect density and optical cavity quality could contribute to differences in lasing threshold for individual nanowires. Second, as mentioned above, additional selective rule is expected for randomly oriented ensembles due to non-optimum alignment with respect to the detection geometry given the high directionality of the resonant lasing emission. Clearly, the ultimate lasing characteristics of such NRs are best studied in single nanoribbon measurements. In fact, the high-resolution data of the lasing characteristics of the single ZnS nanoribbon (Fig. 2b) shows narrow resonant modes with FWHM w 0.1 nm and corresponding optical cavity quality factor Q > 3000 [28]. We have previously reported [25] that the composition-control, resulting from the temperature gradient along the Au-coated c-Si substrate, enable sub-nanometer lasing tuning capabilities. We have demonstrated this result by recording the lasing emission at seven positions along a single c-Si substrate decorated with CdS1XSeX nanoribbons such that these positions result in lasing emission changes with Dl ¼ 5 nm increments starting at 600 nm. The spectra in Fig. 3 (after Ref. [25]) were collected using a constant excitation power density (80 kW/cm2) using low (w0.8 nm) spectral resolution which prevented observation of the individual resonant modes. A linear regression (red solid line in the inset) of the experimentally determined lasing wavelength (lexp,i) yields lexp,i ¼ (660.06 0.25) nm þ (5.00 0.07) nm*i which is in excellent agreement with the intended targeted values for lasing (lT,i ¼ 660 nm þ 5 nm*i; i ¼ 0,1,..,6). We conclude that lasing tuning via compositional changes can be obtained with a relative precision better than 0.1 nm. Furthermore, changing lasing emission by Dl w 5 nm (DE w 14 meV) at w 600 nm corresponds to an estimated composition change DX w 0.025 and in fact overlaps tuning capabilities obtainable from small temperature changes DT [25]. This is because lasing in these nanostructures is attributed to exciton emission which depends with temperature T as aT. In fact a w 0.7 meV/K (for CdS a ¼ 0.714 meV/K) [29] and each targeted shift Dl ¼ 5 nm (DE ¼ 0.014 eV) could have been achieved alternatively by tuning the sample temperature by DT w 20 K. While the main composition change occurs along the tube axis; a closer inspection of the samples demonstrates a more complex
composition distribution, see e.g., Fig. 4a, which is believed to result from a secondary temperature gradient on the substrate perpendicular to the tube’s axis direction. Such gradient arises from the balance between the close contact of the Si substrate with the quartz tube and the flow of carrier gas inside the tube which results in higher substrate temperature close to the substrate edges (see schematic in Fig. 4b). As the NR composition depends critically on substrate temperature, a parabolic-like composition gradient develops (Fig. 4a). In fact, we have demonstrated that such secondary gradient can be largely avoided (Fig. 4c) by simply supporting the Si substrate on a fused silica holder (schematic in Fig. 4d). In these conditions, the fused silica holder absorbs much of the perpendicular axis gradient and a nearly-pure axial composition gradient is obtained. The suppression of the perpendicular composition gradient clearly demonstrates that it is possible to finely-tune the composition gradient of the NRs by careful experimental control and planning. 4. Conclusions Lasing has been observed in binary nanostructures such as ZnS and CdS. Furthermore, high quality nanostructure growth is also possible in ternary compounds of these materials thus opening a remarkable opportunity for sensing, lasing, light emission, and light harvesting applications. We have shown that fine lasing control via composition tuning is possible with resolution overlapping thermally induced tuning. It follows that, by combining composition and thermal-tuning, it is possible to provide any desired lasing wavelength, with high precision, in the range between 340 and 710 nm. We have demonstrated that significant control of the spatial composition gradients is possible by careful experimental design. Further developments to control the composition gradient in order to achieve compositions gradients over a spacing of w100 mm or less are expected to be possible; such control is expected to enable exciting applications in sensors, nanolasers and solar cells among others. Acknowledgments This work was fully supported by Research Grants Council of Hong Kong SAR (Grant No. CityU 103208). References [1] Iga K. Fundamentals of laser optics. New York: Plenum Press; 1994; Dragoman D, Dragoman M. Advanced optoelectronic devices. Germany: Springer-Verlag; 1999. [2] Nakamura S, Pearton S, Fasol G. The blue laser diode. 2nd ed. Berlin: SpringerVerlag; 2000. [3] Nakamura S, Senoh M, Nagahama S, Iwasa N, Yamada T, Matsushita T, et al. Appl Phys Lett 1998;72:2014. [4] Ivanov SV. Phys Stat Sol A 2002;192:157. [5] Jain M, editor. II-VI semiconductor compounds. Singapore: World Scientific; 1993; Ruda H, editor. Widegap IIeVI compounds for optoelectronic applications. London: Chapman & Hall; 1992. [6] Husain M, Beer PS, Kumar S, Sharma TP, Sebastian PJ. Solar Ener Mater Solar Cells 2003;76:399; Ando K, Ishikura H, Fukunaga Y, Kubota T, Maeta H, Abe T, et al. Phys Stat Sol B 2002;229:1065. [7] Suemune I. J Appl Phys 1990;67:2364. [8] Wang MW, McCaldin JO, Swenberg JF, McGill TC, Hauenstein RJ. Appl Phys Lett 1995;66:1974. [9] Ding J, Hagerott M, Ishihara T, Jeon H, Nurmikko AV. Phys Rev B 1993;47: 10528. [10] Cingolani R, Dio MD, Lomascolo M, Rinaldi R, Prete P, Vasanelli L. Phys Rev B 1994;49:16769. [11] Fuke S, Maezawa C, Nakamura K, Kuwahara K. J Appl Phys 1992;71:3611. [12] Oniyama H, Oniyama H, Yamaga S, Yoshikawa A. Jpn J Appl Phys 1989;28: L2137.
C.Y. Luan et al. / Vacuum 86 (2012) 737e741 [13] Dahmani R, Salamanca-Riba L, Nguyen NV, Chandler-Horowitz D, Jonker BT. J Appl Phys 1994;76:514. [14] Fujita SZ, Asano T, Maehara K, Fujita SG. Appl Surf Sci 1994;79/80:270. [15] Wang N, Tang YH, Zhang YF, Lee CS, Lee ST. Phys Rev B 1998;58:R16024e6; Peng HY, Wang N, Zhou XT, Zheng YF, Lee CS, Lee ST. Chem Phys Lett 2002; 359:241; Shi WS, Zheng YF, Wang N, Lee CS, Lee ST. Adv Mater 2001;13:591. [16] Johnson JC, Yan H, Schaller RD, Haber LH, Saykally RJ, Yang P. J Phys Chem B 2001;105:11387. [17] Maslov AV, Ning CZ. Opt Lett 2004;29:572. [18] Maslov AV, Ning CZ. IEEE J Quant Electron 2004;40:1389. [19] Agarwal R, Barrelet CJ, Lieber CM. Nano Lett 2005;5:217. [20] Van Vugt LK, Rühle S, Ravindran P, Gerritsen HC, Kuipers L, Vanmaekelbergh D. Phys Rev Lett 2006;97:147401.
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[21] Zapien JA, Liu YK, Shan YY, Tang H, Lee CS, Lee ST. Appl Phys Lett 2007;90: 213114. [22] Pan AL, Zhou WC, Leong ESP, Liu RB, Chin AH, Zou BS, Ning CZ. Nano Lett 2009;9:784. [23] Yang HY, Lau SP, Yu SF. Appl Phys Lett 2006;89:081107. [24] Pan AL, Liu RB, Sun MH, Ning CZ. ACS Nano 2010;4:671. [25] Liu YK, Zapien JA, Shan YY, Tang H, Lee CS, Lee ST. Nanotechnology 2007;18: 365606. [26] Liu YK, Zapien JA, Shan YY, Geng CY, Lee CS, Lee ST. Adv Mater 2005;17:1372. [27] Perna G, Pagliara S, Capozzi V, Ambrico M, Pallard M. Solid State Commun 2000;114:161. [28] Zapien JA, Jiang Y, Meng XM, Chen W, Au FCK, Lifshitz Y, et al. Appl Phys Lett 2004;84:1189. [29] Hvam JM. Phys Rev B 1971;4:4459.
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Composition tuning of room-temperature nanolasers C.Y. Luan a, Y.K. Liu a, b, Y. Jiang a, c, J.S. Jie a, d, I. Bello a, S.T. Lee a, J.A. Zapien a, * a
Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Ave., Kowloon Tong, Hong Kong SAR, PR China b Department of Physics, Yunnan Normal University, Kunming, PR China c School of Materials Science and Engineering, Hefei University of Technology, Anhui, PR China d School of Electronic Science & Applied Physics, Hefei University of Technology, Anhui, PR China
a b s t r a c t Keywords: Nanolasers Self-assembled nanostructures Photonics II-VI semiconductors Nanowires Waveguides
We show that careful preparation conditions enable the preparation of high quality nanoribbons capable of room-temperature lasing covering the complete spectral range from near infrared (NIR) to ultraviolet (UV). Two ternary compound II-VI semiconductors, CdS1XSeX and ZnYCd1YS, can be grown on single c-Si substrates to achieve this task. High-resolution transmission electron microscopy and x-ray diffraction showed that the ternary nanoribbons were single phase and single crystal. Room-temperature optical measurements showed that band-gap engineering could be realized via composition modulation in X and Y, with fine tuning of the lasing wavelength via composition changes (DX and DY) capable of overlapping thermally induced tuning, demonstrating the possibility of continuous tuning in the lasing wavelength throughout the complete 340e710 nm spectral range. We further demonstrate that it is possible to control the spatial composition gradient with sub-mm resolution thus opening the possibility for the fabrication of devices with full spectral optimization. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Light emitting nanostructures with finely tuned light emission in the NIR-Vis-UV spectral range are expected to provide major contributions to optic and optoelectronic applications in fields such as communication, data storage, sensing, biological, and medical instrumentation. This is because many of these applications require the ability to fabricate lasers of predetermined wavelength; a goal also exemplified by the commercial importance of green, blue and ultraviolet light emitters [1,2]. The advent of a commercial blue laser based on III-Nitride semiconductors [3] was expected to become the first of a number of finely wavelength-tunable lasers. However, a number of material problems such as phase separation have limited the expansion of III-Nitride-based lasers to the remainder of the green-blue spectral region [4]. The continuous need for higher efficiency, brighter, and low cost LEDs and lasers for the entire visible-UV range has renewed the interest in II-VI semiconductors due to their relevance as promising materials for lasing, solar cells, and other optoelectronic applications [5,6]. Indeed, band gap engineering of II-VI semiconductors has been utilized for preparing superlattices,
* Corresponding author. Tel.: þ86 852 3442 7823; fax: þ86 852 3442 0547. E-mail address: [email protected] (J.A. Zapien). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.07.004
heterostructures, and quantum wells (QW) to produce blue-green lasers [7e10]. It can be expected that the “bottom-up” approach used in nanoscale materials can assist to overcome current problems with II-VI technologies [11e14] such as (1) lack of high quality bulk single crystals of II-VI semiconductors suitable for use as substrates, and (2) defects derived from the polytypism of II-VI compounds. These limitations can be surmounted because nanotechnology enables the growth of high quality single crystal II-VI nanoribbons with fewer defects without the need of lattice matched substrates [15]. Nanowire lasing studies have been reported involving experimental [16] and theoretical [17,18] investigation of the optical cavity effects in self-assembled materials. Lasing in ZnO NWs has been attributed to excitonic emission, although it was also suggested that intense optical pumping could lead to the eventual onset of an electron-hole plasma laser [16]. Additional studies by temperature-dependent spectroscopic studies in CdS NWs provided strong evidence of the importance of exciton-based mechanism in the lasing of single CdS NWs and concluded that excitoneexciton scattering dominates from 4.2 to 75 K while exciton-LO scattering dominates at higher temperatures [19]. Some authors have also pointed out the relevance of the large oscillator strength for exciton transitions in ZnO and thus the expected increased role of lightematter interaction [20]. In fact, it was reported (Ref. [20]) that extremely strong exciton-photon coupling
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(derived from composition tuning) overlapping thermally induced tuning [21]. Similar studies have been independently reported for CdS1XSeX with lasing emission at 77 K [22], and for ZnMgO with lasing effects based on random-lasing effects [23]. More recently [24], composition tuning has been extended to quaternary ZnCdSSe compounds; however no evidence of waveguiding or lasing effects were presented. In this paper we review our work toward the development of efficient room-temperature optically pumped nanolasers covering the NIR-UV spectral region. 2. Experimental 2.1. Nanoribbon fabrication Two ternary compositions, namely CdS1XSeX [25] and ZnYCd1YS [26] were synthesized in a quartz tube mounted inside a three-zone horizontal tube furnace as previously described. Briefly, the CdS1XSeX (ZnYCd1YS) NRs were prepared from laser ablation of a CdS target prepared by pressing high-purity (>99.99 wt %) CdS powders at room temperature. The CdS target and the CdSe (ZnS) powders were placed in the middle of temperature zones I and II, respectively, while silicon substrates coated with w20 Å gold film were placed between zones II and III to achieve a temperature gradient. The tube was evacuated to a pressure of 8 106 mbar then pre-mixed argon and hydrogen (5%) was admitted into the system at a 20 sccm flow rate to achieve a pressure of w130 mbar. The temperature in the three zones of the furnace was then increased to 750, 900, and 400 C (750, 950, 500 C for ZnYCd1YS NR growth) for 30 min. A KrF laser (248 nm, pulse width 34 ns and 450 mJ per pulse at 5 pulses per second) was used to ablate the CdS target for 2.5 (2.0) hours. Gold-coated c-Si substrates were used which resulted in high quality nanoribbons. 2.2. Nanoribbon characterization Fig. 1. (a) Low and (b) higher magnification SEM images of high quality CdSSe nanoribbons. These nanoribbons show lasing emission when pumped with 266 nm pulsed (6 ns) laser with a lasing threshold of w40e60 kW/cm2. Note the lack of order in large scale (a) but the presence of some degree of local alignment (b).
in ZnO nanowires occurs at room temperature and it was suggested that the resulting polaritonic dispersion curve in the near UV could explain the surprising subwavelength guiding necessary for lasing effects to occur. Our group has demonstrated that nanostructures based on ternary II-VI composition alloys, ZnYCd1YS and CdS1XSeX, can provide efficient room-temperature lasing at any predetermined wavelength between 710 and 340 nm with tuning resolution
a
The nanoribbon morphology, composition and crystallography was characterized by scanning electron microscopy (SEM; Philips XL 30 FEG) with Energy dispersive spectroscopy (EDS), and microarea x-ray diffraction (Philips PW 1830 with Cu Ka radiation and a normal 2q scan, 40 kV, 30 mA) with a 1 mm x-ray beam, respectively. Photoluminescence (PL) measurements were conducted at room temperature using the fourth harmonic of a Nd:yttriumealuminumegarnet (Nd: YAG) laser (266 nm wavelength) with a 6 ns pulse width as the excitation source. The emitted light was detected using an ultraviolet (UV) optical fiber coupled to a 0.5 m spectrometer (Acton Research Corp., Spectra Pro 500i) with dispersion gratings of 1200 and 150 grooves mm1 for
b
Fig. 2. (a) Lasing emission of seven ZnYCd1YS and CdS1XSeX samples (including a single ZnS nanoribbon denoted by *) presenting room temperature lasing between 340 and 710 nm. (b) High resolution lasing characteristics of a single ZnS nanoribbon (after Ref. [28]) with lasing threshold w 50 kW/cm2 (inset) and resonant modes with FWHM w 0.1 nm (Q > 3000).
C.Y. Luan et al. / Vacuum 86 (2012) 737e741
Fig. 3. Lasing emission of seven CdS1XSeX samples with lasing shift Dl w 5 nm between them (80 kW/cm2 excitation) and linear regression of the experimental lasing wavelengths against the step number i (right inset).
high (w0.1 nm) and low (w0.8 nm) resolution and a Peltier cooled (20 C) intensified charge-coupled device (ICCD) camera (Roper Scientific). The measurements were performed using normal incident excitation and 10 detection were all angles measured with respect to the sample’s surface. 3. Results and discussion Careful selection of the preparation parameters enables the growth of nanomaterials with very high crystallographic quality,
739
high yields, and relatively low cost; Fig. 1a shows an example for CdSSe NWs. It is interesting to note that despite the random nature of the growth position and alignment on a large scale; in most cases it is relatively easy to identify small areas that show a relatively high level of local alignment (e.g., Fig. 1b). The composition changes in both ZnYCd1YS and CdS1XSeX nanoribbons were confirmed by energy dispersive X-ray spectrum (EDX) and X-ray diffraction (XRD). Composition changes result from the temperature gradient along the substrate, which is controlled by the temperatures of zones II and III. Such composition changes occur gradually (about 1% per mm) and are difficult to record in detail using EDX or XRD. To overcome this limitation, we interpolated the position of the maximum of the PL peak (EPL) as a function of composition X (determined from the diffraction spectra using Bragg reflection equation and Vegard’s law [27]) for selected compositions. This interpolation was used to estimate the nanoribbons composition since EPL position closely follows the variation of the energy gap, Eg, resulting from changes in the NR composition (see e.g., Refs. [25,26]). Near band gap emission is expected to occur from 710 to 510 nm for CdS1XSeX and from 510 to 340 nm for ZnYCd1YS for 0 X, Y 1. Fig. 2a presents the room-temperature PL characteristics of six ZnYCd1YS and CdS1XSeX nanoribbon ensembles with average composition X ¼ 1, 0.76, 0.52, and 0.18, and Y ¼ 0.25, and 0.72; as calculated according to Vegard’s law interpolation described above. In each case several spectra, displaced vertically for clarity, are presented at varying excitation power densities between 5 and 100 kW/cm2 with the lowest excitation power density occupying the lowest position. Clearly, smooth spontaneous luminescence bands are recorded at low excitation power densities while sharp and intense resonant modes (FWHM w 0.3e0.5 nm) are observed
Fig. 4. (a) Optical micrograph of ZnYCd1YS NRs showing a complex gradients along the x- and y- directions (axial- and transversal directions w.r.t. to the tube axis, respectively). (b) Schematic of the setup used to prepare the sample in (a) showing the close contact between the Si substrate edges and the quartz tube (see text). (c) Optical micrograph of a CdS1XSeX sample that has been prepared with minimum (below detection) composition gradient along the y- (transversal) direction. (d) Schematic of the setup used to prepare the sample in (c) where the Si substrate is supported by a fused silica thus minimized the y-direction temperature gradient.
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at higher power densities. The transition from spontaneous to amplified emission occurs in the 35e90 kW/cm2 range for different compositions and in all cases occurs simultaneously with the appearance of the resonant peaks which demonstrate that the nanoribbons are able to sustain lasing action at room temperature. We emphasize that lasing action is unlikely to occur simultaneously in all excited nanoribbons in each ensemble measurement. This is because the reduction in the effective pumping power density that results from non-optimum nanoribbon alignment with respect to the excitation laser (that is when the nanoribbon largest surface area is not perpendicular to the excitation laser beam) or by shadowing due to neighboring nanoribbons. Such geometrical consideration results in an effective selection rule by which only a limited number of nanoribbons are excited above their lasing threshold and has two main consequences: (i) it enables the clear detection of a limited number of resonant modes (as opposed to an otherwise expected large number of modes, resulting from the wide distribution of sizes, that could prevent individual modes identification); (ii) it explains the relatively large spontaneous background emission that accompanies the resonant modes since there is a large number of nanorribons being simultaneously excited below their lasing threshold. To illustrate the second point, Fig. 2 also presents the lasing characteristics of a single ZnS nanoribbon (after Ref. [28]); as expected, the corresponding emission at w340 nm is nearly free of spontaneous emission. Two additional effects complicate the interpretation of the lasing characteristics in ensemble measurements. First, differences in defect density and optical cavity quality could contribute to differences in lasing threshold for individual nanowires. Second, as mentioned above, additional selective rule is expected for randomly oriented ensembles due to non-optimum alignment with respect to the detection geometry given the high directionality of the resonant lasing emission. Clearly, the ultimate lasing characteristics of such NRs are best studied in single nanoribbon measurements. In fact, the high-resolution data of the lasing characteristics of the single ZnS nanoribbon (Fig. 2b) shows narrow resonant modes with FWHM w 0.1 nm and corresponding optical cavity quality factor Q > 3000 [28]. We have previously reported [25] that the composition-control, resulting from the temperature gradient along the Au-coated c-Si substrate, enable sub-nanometer lasing tuning capabilities. We have demonstrated this result by recording the lasing emission at seven positions along a single c-Si substrate decorated with CdS1XSeX nanoribbons such that these positions result in lasing emission changes with Dl ¼ 5 nm increments starting at 600 nm. The spectra in Fig. 3 (after Ref. [25]) were collected using a constant excitation power density (80 kW/cm2) using low (w0.8 nm) spectral resolution which prevented observation of the individual resonant modes. A linear regression (red solid line in the inset) of the experimentally determined lasing wavelength (lexp,i) yields lexp,i ¼ (660.06 0.25) nm þ (5.00 0.07) nm*i which is in excellent agreement with the intended targeted values for lasing (lT,i ¼ 660 nm þ 5 nm*i; i ¼ 0,1,..,6). We conclude that lasing tuning via compositional changes can be obtained with a relative precision better than 0.1 nm. Furthermore, changing lasing emission by Dl w 5 nm (DE w 14 meV) at w 600 nm corresponds to an estimated composition change DX w 0.025 and in fact overlaps tuning capabilities obtainable from small temperature changes DT [25]. This is because lasing in these nanostructures is attributed to exciton emission which depends with temperature T as aT. In fact a w 0.7 meV/K (for CdS a ¼ 0.714 meV/K) [29] and each targeted shift Dl ¼ 5 nm (DE ¼ 0.014 eV) could have been achieved alternatively by tuning the sample temperature by DT w 20 K. While the main composition change occurs along the tube axis; a closer inspection of the samples demonstrates a more complex
composition distribution, see e.g., Fig. 4a, which is believed to result from a secondary temperature gradient on the substrate perpendicular to the tube’s axis direction. Such gradient arises from the balance between the close contact of the Si substrate with the quartz tube and the flow of carrier gas inside the tube which results in higher substrate temperature close to the substrate edges (see schematic in Fig. 4b). As the NR composition depends critically on substrate temperature, a parabolic-like composition gradient develops (Fig. 4a). In fact, we have demonstrated that such secondary gradient can be largely avoided (Fig. 4c) by simply supporting the Si substrate on a fused silica holder (schematic in Fig. 4d). In these conditions, the fused silica holder absorbs much of the perpendicular axis gradient and a nearly-pure axial composition gradient is obtained. The suppression of the perpendicular composition gradient clearly demonstrates that it is possible to finely-tune the composition gradient of the NRs by careful experimental control and planning. 4. Conclusions Lasing has been observed in binary nanostructures such as ZnS and CdS. Furthermore, high quality nanostructure growth is also possible in ternary compounds of these materials thus opening a remarkable opportunity for sensing, lasing, light emission, and light harvesting applications. We have shown that fine lasing control via composition tuning is possible with resolution overlapping thermally induced tuning. It follows that, by combining composition and thermal-tuning, it is possible to provide any desired lasing wavelength, with high precision, in the range between 340 and 710 nm. We have demonstrated that significant control of the spatial composition gradients is possible by careful experimental design. Further developments to control the composition gradient in order to achieve compositions gradients over a spacing of w100 mm or less are expected to be possible; such control is expected to enable exciting applications in sensors, nanolasers and solar cells among others. Acknowledgments This work was fully supported by Research Grants Council of Hong Kong SAR (Grant No. CityU 103208). References [1] Iga K. Fundamentals of laser optics. New York: Plenum Press; 1994; Dragoman D, Dragoman M. Advanced optoelectronic devices. Germany: Springer-Verlag; 1999. [2] Nakamura S, Pearton S, Fasol G. The blue laser diode. 2nd ed. Berlin: SpringerVerlag; 2000. [3] Nakamura S, Senoh M, Nagahama S, Iwasa N, Yamada T, Matsushita T, et al. Appl Phys Lett 1998;72:2014. [4] Ivanov SV. Phys Stat Sol A 2002;192:157. [5] Jain M, editor. II-VI semiconductor compounds. Singapore: World Scientific; 1993; Ruda H, editor. Widegap IIeVI compounds for optoelectronic applications. London: Chapman & Hall; 1992. [6] Husain M, Beer PS, Kumar S, Sharma TP, Sebastian PJ. Solar Ener Mater Solar Cells 2003;76:399; Ando K, Ishikura H, Fukunaga Y, Kubota T, Maeta H, Abe T, et al. Phys Stat Sol B 2002;229:1065. [7] Suemune I. J Appl Phys 1990;67:2364. [8] Wang MW, McCaldin JO, Swenberg JF, McGill TC, Hauenstein RJ. Appl Phys Lett 1995;66:1974. [9] Ding J, Hagerott M, Ishihara T, Jeon H, Nurmikko AV. Phys Rev B 1993;47: 10528. [10] Cingolani R, Dio MD, Lomascolo M, Rinaldi R, Prete P, Vasanelli L. Phys Rev B 1994;49:16769. [11] Fuke S, Maezawa C, Nakamura K, Kuwahara K. J Appl Phys 1992;71:3611. [12] Oniyama H, Oniyama H, Yamaga S, Yoshikawa A. Jpn J Appl Phys 1989;28: L2137.
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