Enhanced diode performance in cadmium telluride–silicon nanowire heterostructures
Yüklə 1,06 Mb. Pdf görüntüsü
|
|
3. Results and discussion
The XRD patterns for the CdTe thin film deposited onto the soda-lime silicate glass substrate is shown in Figure 1 (a). Three main peaks that are assigned to (111), (220) and (311) reflections for cubic CdTe lattice are evident (JCPDS 89-3053). The XRD pattern for bare Si nanowires is provided in Figure 1 (b) for comparison. The XRD patterns for the CdTe thin films deposited onto planar reference sample and nanowire arrays are provided in Figure 1 (c) and (d), respectively. No noticeable impurities or second phases were observed in the XRD spectra of the fabricated samples within the detection limits. Figure 1 (d) also includes the solid vertical reference peaks for CdTe (JCPDS 89-3053) and Si [10, 12, 28] below the XRD pattern. Extra peaks appeared around 33 , 62 and 69 in Figure 1 (c) and (d) belong to crystalline Si. From Figure 1, it is seen that the full width at half maximum (FWHM) of the (111) and (220) reflections are much smaller for the CdTe thin film deposited onto Si nanowire arrays compared to that on planar Si. Quantitatively, the FWHM values of the (111) and (220) reflections were found to be 0.244 and 0.241 degrees for the nanowire based sample and 0.273 and 0.312 in degrees for the planar reference sample, respectively. These results demonstrate higher crystallinity of the CdTe film deposited onto Si nanowires as opposed to the CdTe film deposited onto Si wafer. Rietveld refinement revealed detailed structural parameters such as average crystallite size, microstrain and dislocation density of the CdTe 9
crystallite size was calculated to be 45 nm and 25 nm for the nanowire based and the planar samples, respectively. Rietveld analysis revealed that the microstrain and dislocation density in the CdTe thin films deposited onto planar substrate are higher than the ones obtained from CdTe thin films deposited onto the Si nanowire arrays. The microstrain and dislocation density were found to be 7.7 x 10 -5 and 16 x 10 14 m -2 for the planar sample and 3.7 x 10 -5 and 4.9 x 10 14 m -2 for the nanowire based sample, respectively. The obtained lattice constant of the CdTe thin films deposited onto the nanowires and planar substrate was 0.647 nm and 0.651 nm, respectively. Lattice parameter for CdTe thin film on planar Si substrate was larger than that of the ideal bulk CdTe (a = 0.648 nm) [29]. It is known that the lattice parameter of a deposited thin film is influenced from the lattice mismatch between the film and the underlying substrate. Rietveld analysis the CdTe film deposited onto the planar − − The lattice mismatch between the CdTe thin film and Si substrate resulted in a structural disorder within the thin film and created interface states at the depletion layer of the junction. The optical bandgap (E g ) of the CdTe thin films sputtered onto glass substrates was calculated to be approximately 1.47 eV (data not shown here), which is found to be in reasonable agreement with the literature [21, 31]. 10
characterization of the structural properties of the materials, was performed in order to further investigate the crystal quality of the semiconductor CdTe films. Room-temperature Raman spectra of the CdTe thin films deposited onto the Si nanowire arrays and planar Si substrate in the spectral range between 100 - 900 cm −1 are provided in Figure 2 (a). Three pronounced vibrational features located at approximately 170, 340 and 510 cm –1 were assigned to first order longitudinal optical (LO) phonon scattering of the CdTe, its second order (2LO) and third order (3LO) overtones, respectively. Similar peaks have been previously observed and reported for the CdTe structure [32-34]. It can be noticed that the 2LO and 3 LO region for the sample deposited onto planar substrate is less prominent, suggesting formation of poorly ordered CdTe film. Particularly, distinct Raman peaks of the LO modes and their overtones with increased intensity and decreased linewidth were observed for the CdTe film deposited onto Si nanowire arrays compared to one obtained from the planar sample. This result provides strong evidence on high degree of crystallinity within the CdTe film deposited onto Si nanowires. Therefore, both the XRD and Raman analyses revealed the improved crystallinity of the CdTe thin film deposited onto Si nanowires compared to the thin film deposited onto planar Si substrate. The improved structural characteristics and high quality of the CdTe thin films on the Si nanowires indicated by the XRD and Raman data could be attributed to either the stress relief resulting from the flexibility of the nanowires due to their three-dimensional nature or to the enhanced surface area and nucleation sites on the surface of the nanowires. The small diameter of the Si nanowires leads to an elastic relaxation at the nanowire sidewall surfaces that makes possible to grow sequence of large lattice mismatch materials one on top of each other. In addition, the three-dimensional junction extending along the entire length of nanowires with larger interfacial contact area between the Si nanowire arrays and the CdTe thin films could minimize the structural disorders in the CdTe 11
these complementary materials [12]. The common features observed in the first order LO phonon band of the CdTe thin film are low frequency asymmetry and peak broadening, as shown in Figure 2 (a). In terms of the selection rules, the presence of both the LO and transverse optical (TO) phonon modes of CdTe films deposited onto Si nanowires and planar Si substrate is expected, because the (111) plane of the CdTe in cubic form permits both optical phonons. In Figure 2 (a), the first order LO phonon mode is clearly visible and is the strongest mode for the CdTe films deposited both onto nanowires and flat substrate. Besides, the TO phonon mode of the films can be identified at the low frequency side of the Raman spectra. Curve fitting procedure by superposition of the Gaussian lines allowed the determination of the positions of the different components contributing to the Raman spectrum. Figure 2 (b) shows the deconvoluted Raman spectrum from the CdTe film deposited onto Si nanowires in LO phonon band. The main peak at 171.5 cm −1 can be directly assigned to the scattering by LO phonons in the CdTe thin film [29]. The distinct lowest component at 146.5 cm −1 is ascribed to the CdTe TO phonon, because of its frequency proximity to the frequency of corresponding CdTe bulk crystal [35]. The other component peaked at 158.9 cm −1 can be attributed to the surface optical (SO) phonons, as the frequency of the SO mode is expected to be between the LO and TO phonon modes. In addition to the scattering feature at the low-frequency side of the LO peak, a noticeable high-frequency shoulder (HSF) of the LO peak is located at around 188 cm -1 , originating probably from the contribution of acoustic phonons or surface-induced vibrational modes [32]. A top-view SEM image obtained from the CdTe thin films deposited onto the planar Si substrate is provided in Figure 3 (a). Non-uniform distribution of the CdTe nanoparticles on the planar substrate is evident. The corresponding film thickness was measured to be 12
amount of the CdTe thin film was deposited onto the Si nanowires and planar Si under identical conditions. Therefore, any observed difference in the performance of the fabricated devices can be safely ascribed to the properties of the Si nanowires. The surface morphology and roughness of the deposited CdTe thin films were also examined by AFM. Figure 3 (b) shows three-dimensional AFM image for the CdTe thin film deposited onto planar Si substrate. Root mean square (RMS) roughness value of the thin film was found to be 3.36 nm, revealing the relatively smooth surface. A cross-sectional SEM image of the CdTe thin film deposited onto Si nanowires is provided in Figure 3 (c). It is quite clear from the image that the CdTe thin film deposited onto nanowire arrays forms a continuous film at top of the nanowire arrays. As clearly, the film packing on the Si nanowires increases from the bottom towards the top, resulting in formation of matchstick-like structures. The cross-sectional SEM image of the as-prepared Si nanowires (length of approximately 1 μm) is provided as an inset in Figure 3 (c), for comparison. Figure 3 (d) shows the EDX spectrum obtained from the CdTe thin film deposited onto Si nanowires, revealed the presence of the cadmium (Cd), telluride (Te) and Si. The atomic ratios of Cd, Te and Si obtained from the EDX measurement were 28.49%, 30% and 41.51%, respectively. It was found that the deposited CdTe film is slightly deficient in cadmium with a Cd:Te atomic ratio of 0.95. In order to unveil the electrical properties of the fabricated heterojunction devices, I-V measurements were conducted. A schematic configuration of a nanowire based heterojunction device with front (Au) and back (Ag) contacts is shown in Figure 4. Figure 5 (a) shows typical semilogarithmic I-V characteristics of the nanowire based and planar heterojunctions measured both in the dark and under illumination. Decent rectifying properties were obtained from both nanowire based and planar heterojunction devices. Rectifying behaviors indicated the formation of p-n junctions within the devices. 13
5 and 10 3 (I F /I R taken at 5V) were obtained for the nanowire based and planar heterojunction, respectively. It is clear that the diode performance of the heterojunction with nanowires was superior compared to the planar counterpart. This result points out the formation of a highly effective depletion region between the CdTe thin film and Si nanowires. The difference between the rectification ratios could be attributed to the enhanced surface area due to the three-dimensional nature of the Si nanowires. In addition, increased interfacial area on the nanowire surfaces can minimize the structural disorders in the thin films such as crystal defects or dislocations leading to a stress relief [10]. Under illumination, the increase in the reverse saturation current for both devices was evidenced at first sight. This is based on the generation and collection of the photogenerated minority carriers under illumination. Photocurrent (I Yüklə 1,06 Mb. Dostları ilə paylaş:
|