Semiconductor superlattice micro-/nanowires could greatly increase the versatility and power of modulating electronic or excitonic, and photonic transport, and related optical properties. In this paper we report the synthesis of alloyed semiconductor superlattice microwires (SMs) of CdS1-xSex/Sn: CdS1-xSex based on the mciro-environmental controlled co-evaporation technique. The alloyed SMs can produce color-tunable multimode emission with wavelength from 513 nm to 596 nm by controlling the composition x from 0 to 0.4. In addition, the alloyed segments in the superlattices form many optical microcavities in queue which can lead to the coupled optical cavities which confine both excitons and photons, producing multiple cavity emission modes. This structure may be used in color-tunable nonlinear optical devices, and study light-matter interaction.
© 2011 OSA
Superlattice nanowires (SNs) with modulated microstructures or compositions along the axial direction have long been studied for their fascinating electronic and optical properties due to the electronic confinement [1–4]. Among the fancy nanostructures, we can use one type of SNs with one semiconductor material work as nanosized segments incorporated into a semiconductor backbone, which should be of particular interests if the segments has a low band gap but bigger refractive index than that of the backbone, so the segment can confine photons within backbone while backbone confine electrons or excitons within segments. Therein this structure can form the coupled optical cavity and exciton lattice , with realization of confining and/or transporting photon/exciton for wide use in studying light-matter interaction, coupled optical cavties, slowing light engineering, weak nonlinear optical devices, solar cell and so on. The past years have witnessed various semiconductor SNs which have successfully been fabricated by well-developed methods [6–9]. In those methods, vapor-liquid-solid growth mechanism takes effect in SNs growth via altering the reaction atmosphere periodically step by step, their interfaces between heteromaterials is usually not very good. Our lab recently developed a simple co-evaporation technique with micro-environmental control to grow superlattice microwires (SMs) [10,11], which provide a facile route for growing such one-dimensional (1D) periodic wire structures in one step. So far, the semiconductor segments in SNs and SMs have been obtained only for fixed compositions. Yet, for practical applications, it is very important to fabricate the SNs or SMs with composition-tunable materials or multicomponents, leading to varied optical properties at a spatial modulation. Recently, Zhao and associates provided an effective approach based on employing graded growth-temperature for achieving InGaN superlattice structure with different In-content [10,11], which demonstrated wavelength-tunable emission [12,13]. Advances in ternary 1D semiconductor nanostructure also have shown that their band gaps and therein optical emission and lasing lines can be tuned by changing their constituent stoichiometry [14,15], whose structure extension to 1D superlattice structure will be good to improve their properties and applications.
In this paper we report the synthesis of alloyed semiconductor SMs of CdS1-xSex/Sn: CdS1-xSex based on the mciro-environmental controlled co-evaporation technique. Our result demonstrate that the alloyed SMs can produce color-tunable emission with wavelength from 513 nm to 596 nm by controlling the composition x from 0 to 0.4. Furthermore, such SMs can modulate the photons and/or excitons transport to produce multiple selected emission modes.
In a typical synthesis of the alloyed SMs of CdS1-xSex/Sn: CdS1-xSex, we used a horizontal tube reactor with engineered temperature gradient along the position of loaded Si wafer substrates. The mixed semiconductors powders of CdS (0.1g, Alfa Aesar, 99.995% purity), CdSe (0.05g, Alfa Aesar, 99.995% purity) and SnO2 (0.02g, 99.5% purity) was selected as source materials, since CdSe and CdS can form alloyed CdS1-xSex . Several pieces of Si (100) wafers were placed downstream of the carrier gas flow (Ar + H2 (10%)) and separately about 7-12 cm from the source powders. Typically, the furnace was heated to about 1000 °C at a rate of about 100 °C /min, kept at this temperature for 30 min, and then cooled to room temperature. During the growth process, the 20 sccm mixture of carrier gas was injected into the tube. After the reaction, bright yellow to brown-red products were deposited on the surface of silicon wafers and on the inner wall of the quartz tube at the deposition temperature of 650-800 °C downstream.
The morphology and the composition of the obtained samples are controlled and strongly affected by the weight ratio of the source powder, the growth time, the source temperature, and the substrate temperature (or the substrate distance from the source). When the weight ratio of SnO2/ (CdS + CdSe) is higher than 2/5, the uniform Sn-core/CdSSe-shell wires in short growth time (<40 min) and the partially hollow CdSSe wires in long growth time (>60 min) can be obtained from the silicon substrate separately about 10 cm from the source powders (at a temperature of ~700°C). However, the morphologies of the obtained sample are thick tapered belts at temperature higher than 800°C and are thin wire (<0.5 μm) at temperature lower than 600°C, whatever the molar ratio of SnO2/ (CdS + CdSe) in the source powder. Further investigations indicate the superlattice wires only can be found in the proper deposition temperature or position, where reduced Sn can embed periodically in the domain CdSSe wire. This was similar to the results reported in . So, under the growth condition given in experimental section, the alloyed SMs of CdS1-xSex/Sn: CdS1-xSex in the composition range from x = 0 to x = 0.4, instead of from x = 0 to x = 1, can be achieved on the surface of the Si substrate.
The structure of the product was characterized by X-ray powder diffraction (XRD, Siemens D-5000). The morphology and composition of the product were investigated by a field-emission scanning electron microscopy (SEM, JSM-6700F) on an instrument equipped for energy-dispersive X-ray spectrometry (EDS). Room-temperature photoluminescence (PL), optical waveguide and mapping images were taken on a confocal Scanning Near-field Optical microscope (SNOM, alpha 300 series, Witec, Germany) using an Ar-ion laser with a wavelength of 488 nm as an excitation light source..
3. Results and discussion
Figures 1(a) and 1(b) show the SEM image of the as-grown sample obtained at temperature ~650°C and ~800°C, respectively, in which their morphology showed little variation in the temperature range. The wires have diameters of 400 nm to 4 μm and lengths up to several hundreds of micrometers. Inset in 1(a) and 1(b) are the magnified morphology images of a typical sample, ending with a hexagonal solid wire and a microsphere at two sides. The main composition of the microsphere is Sn which indicated a likely Sn-catalyst vapor-liquid-solid process for the formation of these wires. The typical bright-field optical images of an individual wire at different growth temperature ~650°C and ~800°C are shown in Figs. 1(c) and 1(d), respectively. The long microwire with dark-bright periodic structure is clearly seen, which represent alternate indices of refraction, that is, SMs. The compositional modulation within SMs was examined by EDS with line-scanning technique, where the EDS intensities are plotted along the longitudinal axis of wire. From the EDS intensity profiles in Fig. 1(e) (for low temperature sample), it was clearly confirmed that Cd, S and Sn are effectively modulated with precise periods. Combined with the X-ray diffraction spectra in Fig. 2 , it can be concluded that the bright section in Fig. 1(c) is CdS and the dark section is the composite CdS doped with Sn (VI). Figure 1(f) is the EDS intensity profiles taken on the individual SM in Fig. 1(d) (high temperature sample). After careful observation, we can also distinguish a little discrepant intensity profiles of Cd, S, Se and Sn, which indicated the alloyed SM of CdS1-xSex/Sn: CdS1-xSex. The composition boundary ambiguity for the relatively high temperature sample is because the Sn of low melting point may be prone to diffuse into the master CdS1-xSex at that growth temperature.
Figures 2(a)-2(d) shows the normalized X-ray diffraction patterns of several representative CdS1-xSex/Sn: CdS1-xSex SMs samples. Curves (a) is for the sample obtained at ~650°C, in which the main diffraction peaks are match well with a wurtzite (hexagonal) structure of bulk CdS (JCPDS No. 2-549). Besides the two weak Sn (JCPDS No. 65-296) diffraction peaks (Sn sphere at the head of wire), no characteristic peaks from other impurities, such as CdO, SnO and SnO2, were detected. These results confirm the product is composed of CdS and Sn, which support CdS1-xSex/Sn: CdS1-xSex SMs structure with x = 0. Curves (b-d) are the samples in queue deposited on the substrate at gradually rising temperature, in which the samples show with rising Se concentrations. It is clearly seen that the main crystallographic phase of all the samples is in good agreement with that of the typical hexagonal wurtzite CdS1-xSex crystals. For comparison, enlarged spectra of the diffraction peak in the ranges of 23-30 degree are showed in Figs. 2(a1)-2(d1), in which the diffraction peaks shift gradually toward low angles, indicating that the lattice constants of the alloyed wire increase when Se concentration increases. The labeled x of the alloyed composition were be determined from Vegard’s law using the lattice parameters deduced from the XRD data .
Figure 3 shows the dark-field photoluminescence (PL) images of an individual CdS1-xSex/Sn: CdS1-xSex SM which was obtained from a confocal optical system using Ar-ion laser line 488 nm as excitation at room temperature. Images (a-d) are corresponding to the SMs samples which were characterized by XRD in Figs. 2(a)-2(d). The inserted top left image is the bright-field image. The PL photograph in (a-d) exhibit the color of periodic light change from green (consistent to x = 0) to red (consistent to x = 0.4) by naked eyes, whose Micro-PL spectra show peaks shift from 513 nm to 596 nm (Fig. 3(e)). As reported in , the band gap of ternary CdS1-xSex films becomes smaller with increasing x and the position of the strongest PL peak of our samples are in good agreement with the results of those corresponding CdS1-xSex films (0≤ x ≤0.4), so the spectral shift of the main band emission for the alloyed wires at varied deposition temperatures should come from the variety of their band-edge, which is due to the different ratio of Se or S composition.
Figure 4 shows the typical micro-PL spectrum of a single binary CdS wire (a) and an alloyed CdS0.78Se0.22 superlattice microwire (b) at the same high excitation power at room temperature. It can be seen that the emission intensity for alloyed microwire is about ten times higher than that of binary CdS wire, indicating their more potential applications in the tunable nano/micro optoelectronic devices in the future. Besides, more accompanying shoulder peaks can be observed on the low energy side of the main band. The periodical cavity structures for this 1D SMs induce these accompanying peaks, which will be discussed in next section.
Further optical characterizations were conducted on the SM sample to study their spectra. Figure 5(a) is the bright-field optical image of an individual CdS0.85Se0.15/Sn: CdS0.85Se0.15 SM. In the corresponding far-field PL emission image in Fig. 5(b), the bright white-yellow spot (the arrowhead denoted) is in situ PL under laser excitation; the emitted light can be effectively transport along the long axis and periodically emitted at the adjacent “joint”, where the Sn:CdS0.85Se0.15 segments stay. In this wire, the alloyed CdS0.85Se0.15 has a direct band gap of about 2.25 eV and refractive index of about 2.42. The SnS2 components within Sn: CdS0.85Se0.15 has a refractive index of about 3. Hence the neighboring alloyed segments in the superlattices can form many optical microcavities in queue which confine coupled photons, segments of Sn: CdS0.85Se0.15 works as the reflection interfaces partially and some light may leak out at these joints. The periodic bright emissions arise exactly from the interference of coherently oscillated light within the coupled microcativities, produces the multipeaks at the low energy side of the main band. The light with energy above the main band cannot transport long within the wire for the strong absorption at above bandgap, so cannot produce waveguide modes. Figure 5(c) is the corresponding Micro-PL spectrum, which exhibits a strong CdS0.85Se0.15 band edge emission at ~551 nm and several accompanying shoulder peaks at ~564 nm, 575 nm, and 588 nm respectively. These peaks are not from the typical Fabry-Pe’rot (F-P) modes at ends, but coupled cavity modes, since the calculated mode spacing for a F-P cavity suggests 4.6 μm, but in fact the wire is about hundreds micrometers while the unit cavity size is about 3.3 μm. A far-field PL mapping technique in confocal optical microscope was used to manifest the origin of these peaks. The mapping image of the wavelength range 551-553 nm (peak 1) are shown in Fig. 5(d), which exhibits a series of bright segments separated by periodic dark joint. The bright regions represent the intense waveguding photons, which results from the emitted photon and/or exciton transporting within the cavity area of this periodic SM. Further investigations by mapping the other peaks (2-4) show different bright segments existing along the long axis of SM, which indicate the waveguiding photons within the wire, while the dark segments show the photon forbidden area at which the photon is blocked or reflected (Figs. 5(d1)-5(d4)). These results indicated the combination of the photonic modulation and possible exciton coherent emission in different optical microcavities. Thus, we can conclude that these emission profiles were produced by the combined contributions of coupled optical microcavity modes  and local excitons within this 1D photonic crystal wire.
In summary, we have reported the fabrication of alloyed semiconductor SMs of CdS1-xSex/Sn: CdS1-xSex in the composition range from x = 0 to x = 0.4 based on a simple co-evaporation method. We demonstrated that the SMs can produce color-tunable periodic light with continuously wavelength tunable emission between 513 nm and 596 nm. Such unique SMs can modulate the photons and/or excitons to produce multiple emission modes, which could provide a new materials platform for wide applications in producing tunable low-threshold lasing, color-by-design light emission diodes, multicolor detectors, and study light-matter interaction.
This work was supported by the NSFC (Nos. 51002009, 90606001, 20873039, 90923014 and 10974050), China Postdoctoral Science Foundation founded project (No. 20100470211), and BIT111-201101.
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