Enhanced diode performance in cadmium telluride–silicon nanowire heterostructures
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were found to be 2.06 and 15.2 nA for the nanowire based and planar diodes, respectively. I-V characteristics of the diodes are strongly affected by the parasitic resistances such as series resistance (R s ). This parameter should be taken into consideration in terms of the device performance. The value of the R s was calculated to be 1.84 kΩ for the nanowire based diode, and 58 kΩ for the planar diode. It is clear that the use of nanowires for the fabrication of heterojunction devices significantly decreased the R s . High series resistance is known to cause non-ideal diode behavior, deteriorating the device performance. An open circuit voltage of 120 mV was also obtained from the nanowire based heterojunction device under illumination. This result also confirms the formation of a much better interface between the CdTe thin film and Si nanowires as opposed to the formed within planar Si. The obtained open circuit voltage may be low for a solar cell application but the efficient CdTe thin film/Si nanowire heterojunction photovoltaic devices can be achieved by optimizing the production process of the both Si nanowires and complementary CdTe thin film. Reducing the lattice mismatch by using a buffer layer between the CdTe thin film and the nanowires could also be useful for the improvement of the device performance. Photocurrent characteristics of the photodiodes at a bias of 2 V are provided in Figure 5 (c). Both the dark and illuminated current was measured for 10 second. From Figure 5 (c), measured dark currents of the nanowire based and planar diodes were 2 and 0.4 mA, respectively. Upon illumination, current values raised to 8 and 0.8 mA, respectively. The currents of the both photodiodes decreased quickly to its original value with very good stability and reproducibility after the light was turned off. The rise and fall times were 16
Moreover, the current ON/OFF ratio was found to be 4 and 2 for the nanowire based and planar devices, respectively. The high sensitivity to the incident light of the fabricated photodiode with Si nanowire arrays suggests that it can be potentially attractive for photodetection and optical switching applications. The working mechanism of the heterojunction devices can be understood by the energy band diagram provided in Figure 5 (d). There are two energy band offsets (valence band offset ( E v ) and conduction band offset ( E c )) due to the different electron affinities and optical bandgaps of the CdTe and Si nanowires. Namely, the E g and electron affinity of the p-type CdTe are 1.47 eV (obtained from the optical measurements) and ~4.3 eV, respectively [16]. The E g and electron affinity of the Si nanowires were assumed to be 1.1 and 4.1 eV, respectively [36]. Energy band offsets were then calculated to be E c = 0.2 eV and E v = 0.57 eV using the Anderson’s rule. As a result of the differences between the work functions of the CdTe and Si, a space charge region is formed and a built-in electric field is established at the interface. Upon illumination, the photons with wavelengths shorter than 845 nm are mainly absorbed by the CdTe film, while longer wavelength photons are absorbed by the Si nanowires. Both semiconducting materials (CdTe and Si) contribute to the absorption and accordingly to the electron-hole pair generation. The photogenerated carriers that are separated through the built-in electric field can then be collected by the metallic electrodes, giving rise to the photocurrent. Among the photogenerated carriers, holes sweep to CdTe side of the junction and collected at the front contact, while the electrons travel through Si towards the back contact. The reflection of the light from the device surface is the one of the loss mechanisms that decreases the efficiency of the photodetectors [4]. Mostly, anti-reflection coatings are used to minimize such losses [18]. Si nanowire arrays are known to show superior 17
Figure 6 (a). The nanowire based device exhibited an average reflectance of less than 4%, in comparison to 35% for the planar device within the investigated wavelength range. Improved antireflective performance of the devices with Si nanowires could also contribute to the enhanced optoelectronic performance of the devices. The responsivity of a photodiode is defined as the ratio of photocurrent density to the intensity of incident light at a certain wavelength and shows the photoelectric sensitivity of the diode to incident light energy. Figure 6 (b) shows the spectral response of the fabricated heterojunction diodes measured using a calibrated light source under a reverse bias of 2 V. Two pronounced device features shows two humps located at 845 nm and 1000 nm are evident in the spectra which are associated with the absorption edges of p-type CdTe thin film and n-type Si, respectively. The planar reference device shows a photoresponse within the wavelength range of 700-1000 nm, and no response is obtained for the incoming photons with the wavelengths shorter than 700 nm. The maximum responsivity is 0.08 A W -1 at the wavelength around 1000 nm. On the other hand, a large enhancement in responsivity over the broad spectral range is observed for the nanowire based heterojunction diode. Both the range of detection and signal intensity are substantially higher as opposed to planar control sample. The detection window is extended to the ultraviolet region. The strong antireflective properties of the vertically aligned nanowire arrays in the CdTe thin film/Si nanowire heterojunction diode structure could explain the attained spectral improvement. However, it may not be solely enough to explain the observed distinct difference in the photoresponsivity of the fabricated photodiodes. In the CdTe thin film/Si nanowire heterojunction device structure, the photocurrent generation process starts with the absorption of the incoming light. Since the CdTe film is a good absorber due to its high absorption coefficient of 10 5 cm -1 [19, 20], most of the incident photons are absorbed close to the surface on the CdTe side of the 18
nature of the Si nanowires reduces the lattice mismatch-originated structural disorders (extended defects or dislocations) formed within the CdTe films and, hence decreases the number of defect states. The existence of the defect states within the bandgap of the CdTe could act as recombination centers or transport channels for the carriers created upon absorption of the light and cause some of the photogenerated carriers to be annihilated before they go through the space charge region. Based on the XRD and Raman results, a high degree of crystallinity was observed for the CdTe thin film deposited onto the Si nanowires as opposed to the film deposited onto planar Si substrate. The improved film quality together with the decreased defect density and number of defect states for the CdTe film on the Si nanowires leads to a high photocurrent for the investigated spectral interval of photocurrent spectrum (Figure 6 (b)). Additionally, the reduced effective p-n junction depth and the closer interface to the CdTe film surface together with the advantage of the three-dimensional structure of the Si nanowires make the recombination losses less pronounced. This effect is remarkable in the lower wavelength part of the photoresponsivity spectrum for the nanowire based diode. As a result of spectral photoresponsivity measurements, the maximum responsivity was detected to be 0.5 A W -1 at around 845 nm for the nanowire based device. There is almost a 7-fold higher responsivity at this wavelength. Accordingly, the CdTe thin film/Si nanowire heterojunction device shows potential promise to be used as a photodetector, which operates particularly in near-infrared wavelengths. The detectivity (D) and external quantum efficiency (EQE) for a photodiode are a figure of merits used for the characterization of the device performance. The detectivity characterizes normalized signal to noise performance of a detector and can be determined using the following equation [37], |