| dc.description.abstract | In this work, alloyed II-VI group CdZnTe, HgCdTe colloidal quantum dots (QDs) have been synthesised and their optical, material, electrical properties are studied for optoelectronic applications. The bandgap of the alloyed QDs could be tuned by varying the size as well as composition. The bandgap of CdZnTe QDs correspond to the visible range 420-600 nm, whereas that of HgCdTe QDs correspond to mid-infrared range 2200-5000 nm.
Colloidal CdTe and Cdx Zn1-x Te QDs by synthesized by reflux method with N-acetyl l cysteine as capping agent in aqueous medium. UV-Vis absorption spectra show the excitonic peaks of the QDs, which are blue-shifted and narrower for Cdx Zn1-x Te QDs relative to CdTe QDs. XRD pattern confirms the crystal lattice and the TEM images show the uniformly distributed spherical nanocrystals. Zn incorporation in Cdx Zn1-x Te QDs is verified by XPS and ICP-MS is used to determine the atomic composition of zinc and cadmium in the quantum dots. Quantum yield (QY) of the QDs are determined in reference to standard Rhodamine B in water and Cdx Zn1-x Te QDs are found to be superior to CdTe QDs in QY as well as long-term colloidal, PL stability.
To understand the alloying effects on photophysical properties of Cdx Zn1-x Te QDs, temperature dependent photoluminescence (PL) measurements are performed. PL spectra is recorded as a function of temperature in the range of 80-300 K and the PL peak energy, linewidth, integrated intensity are found to be strongly sensitive to the measurement temperature. Varshni, O'Donnell & Chen relations introduced for bulk semiconductor bandgap analysis, are also found to fit well with the QDs structures. It implies that processes such as thermal dilatation of the lattice are behind the temperature dependent peak-shift, similar to bulk materials. The physical parameters of average phonon energy, Huang-Rhys parameter, exciton-phonon coupling co-efficient are determined by fitting the experimental PL peak energy and FWHM data with theoretical relations. Also, the effect of quantum confinement on these parameters are also discussed. The inhomogeneities in the ensemble of QDs due to size distribution, composition, crystal shape etc. are found to be lower in Cdx Zn1-x Te QDs with respect to CdTe QDs. Also, the surface trap states in QDs are also significantly reduced upon alloying as indicated by the suppression of the broad tail emission in PL spectra. The activation energy for the non-radiative recombination path is found to be higher for alloyed QDs than the parent CdTe QDs due to the reduction in defect states. High quantum yield, narrow emission, biocompatibility of Cdx Zn1-x Te QDs QDs make it a potential candidate for applications related to optoelectronic devices like LEDs, solar cells, and also in photocatalysis, biomedicine. Thus, colloidal Cdx Zn1-x Te semiconductor QDs help us to understand the effects of alloying in optical and vibrational properties of the material, along with furnishing a promising material for future devices with better performance.
In the case of HgCdTe QDs, the work focusses on the photodetection performance of the material rather than photoemission properties. Bulk HgCdTe crystals are essentially employed for IR imaging in the entire infrared region with excellent performance. In this infrared region, only HgTe QDs are explored to cover the SWIR, MWIR, LWIR range and HgCdTe QDs have been explored only upto near-IR region due to synthesis limitations.
Ternary alloyed HgCdTe QDs are synthesised with good stability, exhibiting excitonic absorption edges in the mid-ir region. The tetrapod shape of the crystals and stoichiometric excess of cations are analogous to the case of parent HgTe QDs. Ohmic behaviour is obtained with gold and photoconductor devices are fabricated with simple drop casted films on interdigitated gold electrodes. After EDT ligand exchange, the resistance of the devices drop to the order of kOhms and demonstrate promising photo-response under excitation of visible (532 nm), IR (810 nm, 1550 nm) wavelengths. The responsivity value is typically around 1 mA W-1 at 1 V bias and within 1-2 V, an exponential increase in value is observed. A consistent linearity in photo-response is also observed over irradiance range of 0.025 - 2.5 W/cm2. A simple polymer encapsulation of the photoconductor is employed for preventing oxidation. Apart from the photoconductor device, a metal-semiconductor rectifying contact is also formed with aluminium. The photodiode structure is characterised by temperature dependent dark I-V, C-V curves, photocurrent, responsivity measurements. I-V curves reveal the trap-controlled space charge transport mechanism in the device, while the C-V curves reveal the role of interfacial states in the device functioning. The rectifying contact in the device is also utilized for characterising the defect levels in the device by thermal admittance spectroscopy technique. A bulk trap and interface traps are identified which also matches with the activation energy obtained from I-T Arrhenius plot. These electrical characterizations are important for understanding the physical processes which will further pave the way for improvement in performance.
These insights on electrical properties of HgCdTe QDs and further efforts on optimizing film thickness, stoichiometry, device geometry, improving surface passivation, monodispersivity will pave the way for their possible applications for developing next generation of NIR and MWIR optoelectronic devices. | en_US |
