Enhanced diode performance in cadmium telluride–silicon nanowire heterostructures
Yüklə 1,06 Mb. Pdf görüntüsü
|
|
1. Introduction
Today, silicon (Si) is the best known and widely used semiconductor material in semiconductor industry due to its abundance, tractable electrical and optical properties, and well established processing techniques. Si based solar cells and photodetectors are crucial commercial products of the semiconductor industry. An explosion of research interest on crystalline Si has seen in the last decade since the production of optoelectronic devices is mainly based on Si and Si-related materials. Nowadays, there is an extremely effort to 3
properties. The one-dimensional Si nanostructures such as nanowires reveal noticeable electronic performance in microelectronic devices because of their attractive physical properties which are differ from those of typical bulk counterparts [1-3]. Thereby, Si nanowires have caused researchers and companies to consider its utilization in optoelectronic applications. The Si nanowires could represent the next step in the development of optoelectronics devices since the Si nanowire based devices have considerable advantages when compared to typical Si optoelectronic devices. For instance, the utilization of the nanowire junctions instead of typical planar junctions in Si based thin film devices causes an increase in the amount of absorbed light by the device surface and a decrease the high recombination rate in the junction. In addition, increased surface area of the nanowires can lead to the enhanced optoelectronic properties in the devices [4, 5]. There are numerous successful techniques to fabricate the Si nanowires including vapour liquid solid [6], metal- assisted etching (MAE) [5, 7], molecular beam epitaxy [8] and electron beam evaporation [9]. Among these methods, the MAE is a simple and low-cost process using fewer amounts of materials for the Si nanowires synthesis with ability to control the length of the nanowires. The Si nanowires fabricated through the MAE method have same doping type and high quality crystal structure of the used Si wafer. Furthermore, the MAE can successfully be applied for the fabrication of Si nanowire arrays over large areas under ambient conditions [10]. The nanowires complemented with organic or inorganic materials are of interest for a variety of optoelectronic device applications due to its geometrical advantages relating to the junction area. Different semiconductor materials can be combined with the nanowire arrays to form heterostructures which gives them functionalities like p-n heterojunctions. More recently, the combinations of the Si nanowires with several complementary semiconducting 4
cadmium telluride (CdTe) with Si may be a promising route toward effectively utilization of the solar light for optoelectronic applications. Particularly, combining of the Si nanowires with CdTe thin film is very promising for realization of cost-effective highly efficient optoelectronic devices. Despite a great number of individual studies on the CdTe and Si, very little effort focusing on combined system of these two materials has been done in the published literature. There are few reported studies containing CdTe thin film/Si wafer heterojunctions [13-15]. An interesting combination of the Si nanowires with Sb-doped CdTe nanoribbon was also presented in another work [16]. However, to the best of our knowledge, no comparable study is presented on the fabrication and characterization of the CdTe thin film/Si nanowire heterojunctions in the literature. In our device structure, one-dimensional semiconducting Si nanowires are coated with a thin layer of CdTe film in order to construct nano-sized heterojunctions. Three-dimensional structure of nanowires leads to formation of p-n junctions extending along the entire length of the nanowires with larger interfacial contact area between the Si nanowire arrays and the CdTe thin film. Moreover, the vertical arrangement of the ordered Si nanowires with respect to the substrate traps and scatters incoming light efficiently at short wavelengths in particular and provides omnidirectional light absorption within the nanowire arrays [17-19]. Depending on the size, geometry, orientation, relative position and period of the Si nanowire arrays more light can be absorbed compared to thick solid crystalline Si [20]. Our device configuration offers not only excellent antireflection and superior absorption properties in the visible range of the optical spectra, but also it is the optimal design for efficient charge collection. Si nanowire arrays can decouple the direction of photon absorption and that of charge transportation by incorporating radial p- n junctions. The three dimensionality of the junctions orthogonalizes photon absorption and carrier transport. All these advantages of such structure could be beneficial for the realization 5
regard, CdTe can be considered one of the most promising semiconductor materials for the heterojunction device applications. The advantages of the CdTe compared to the other semiconductors for optoelectronic device applications include its direct optical bandgap (1.4 – 1.5 eV) that matches with the most generous part of the solar spectrum, high mobility of carriers and high absorption coefficient (about 10 5 cm -1 ) [21, 22] for the visible light. CdTe is a unique semiconductor among the other II –VI group compounds because it exhibits both n- type and p-type conductivity. This allows the fabrication of diodes, field effect transistors and homo/heterojunction solar cells using the CdTe [23]. Aforementioned features make CdTe a potential semiconducting material for the fabrication of Si nanowire based heterojunction devices. Different physical and chemical techniques have been utilized for the deposition of CdTe thin films such as chemical vapor deposition [24], molecular beam epitaxy [25], vacuum deposition [26] and sputtering [27]. Among these, RF sputtering is a commonly used and an easy method. RF sputtering provides the deposition of high quality and reproducible thin films and optimum coating uniformity. This paper presents the fabrication and characterization of CdTe thin film/Si nanowire heterojunction photodiodes. The RF magnetron sputtering method was used for the deposition of a thin CdTe layer on the Si nanowire arrays. The fabricated nanowire based devices consisted of vertically oriented n-type Si nanowires complemented with the p-type CdTe film. The planar heterojunction device was also fabricated as a reference sample. Following the fabrications, structural and optoelectronic properties of the diodes have been systematically investigated. Improved structural, electrical and photodiode performances for the nanowire based devices have been obtained compared to the planar reference sample. The ideality factor and rectification ratio was found to be 1.9 and 10 5 , respectively. Moreover, low reverse leakage current and low series resistance, crucial parameters to produce an ideal 6
room temperature. Yüklə 1,06 Mb. Dostları ilə paylaş:
|