Wide Band Gap Semiconductors

[Summary by Dr. D.C. Agarwal and Dr. Fouran Singh]  


Wide band gap materials are the field of immense research interest not only due to their potential applications in harsh environment, but also to understand various interesting phenomenon [1-3]. Large varieties of the WBG materials have been investigated for modifications in their properties by swift heavy ion (SHI) irradiation at IUAC, such as Oxides [3-27], Nitrides [28-30], and Carbides [31].  The brief discussions are given in following sections.


1. SHI induced modification of oxide thin films and nanostructures

Among the oxide semiconductors, ZnO based thin films and nanostructures have been investigated extensively for the surface, structure, optical, luminescence, transport, and phonon structure modifications by SHI irradiations [2-16]. SHI induced surface nanostructuring studies on sputtering and evaporation grown ZnO thin film have shown that this technique can be used very effectively to modify the properties of ZnO thin film [5,9-12]. Surface roughness was found to be increased up to a critical fluence and beyond that fluence it was decreased. Density of the nanostructures has been calculated and it is noticed that the areal density of the nanostructures increases from 8 x 109 cm-2 to 3 x 1010 cm-2 upto the fluence of 1 x 1012 ions-cm-2 due to the reduction in the size of the nanostructures and after this critical fluence the density of nanostructures again decreases due to evolution of bigger structures. Studies have also shown defects induced local amorphization along with modifications in transport properties [9,15] and growth of quantum dots [16] by SHI irradiation on thin films grown by chemical route ZnO films. It is reported that the stress in the ZnO crystallites can be controlled [7] and is very useful in understanding the phonon structure in depth [4]. The studies have also shown that SHI irradiation is very efficient in the development of ZnO based materials for gas sensor [3] and spintronic [13,14] applications.  

Phase transition in TiO2 thin films from anatase to rutile in chemically grown films in chemically grown nanocrystalline films [17] is reported. While in other studies on sputtering films (grown by sputtering process), a phase transition to rutile and brookite along with ferromagnetism at room temperature by SHI irradiations are also reported [19]. In a very recent study on nanostructured TiO2, it is shown that Fermi level can be shifted by dense electronic excitations induced by SHI irradiation [20]. It is also reported that the photo-electrochemical response of TiO2 films can be improved by SHI irradiation [18]. 

SHI can induce formation of nanoparticles by the phase separation in indium sub-oxide films [21] and on the surface In2O3 thin films [22], along with s ignificant morphological changes. However, films annealed in oxygen, having In2O3 phase, do not show any phase separation under SHI. ‘In’ clusters induced by irradiation are having size in the range of 35 to 45 nm. The modifications in structural and transport properties of ITO prepared by chemical route [23] and in commercially available films for dye-sensitized solar cell (DSSC) applications [24] by SHI irradiations are also studied. The nanocrystallization of evaporated thin films of SnO2 [25] and the enhancement of the ammonia gas sensing properties nanocrystalline films [26] by SHI has also been reported. It has also been observed that the nature of sensing is changed from n-type to p-type after the irradiation. The modifications in the nanocrystalline properties of copper oxide films by SHI irradiation has also been reported [27]. 


2. SHI induced modification of Nitride thin films and nanostructures    

The pure and Co implanted MOCVD grown epilayer has been irradiated by SHI irradiations for the modifications in the structural, optical, photoluminescence and transport properties [28-30]. The PL and PL excitations studies have shown that band-to-band emission disappears by SHI irradiation and the deep level defects induced yellow emission also observed to be get modified [28]. The recovery of blue emission by thermal annealing has also been observed in this study. The studies have also shown that SHI irradiation induced compressive strain in the lattice along with broadening in the band edge absorption [29]. The damage in the epilayer has been understood by the loss of Nitrogen induced by dense electronic excitations. In other study of Co-doped GaN for diluted magnetic semiconducting applications; it is observed that RTA at 1150 °C for 20 s is the most effective annealing process in comparison to the ion irradiation process [30]. But, the cluster formation could not be observed after the treatment of RTA and SHI irradiation both.


3. SHI induced modification of SiC nanostructures          

A comparative study on buried SiC layers have been carried for the modifications in the structural properties by thermal annealing and athermal annealing by SHI irradiation [31]. This study has shown that annealing induces the formation of SiC crystallites in the near surface region of C-implanted Si, but the irradiation by 110 MeV Ni ions have not been able to induces significant re-crystallization of SiC. 



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