Fotodetectores infravermelhos de alta eficiência baseados em poços quânticos crescidos por epitaxia de feixes moleculares

Photodetectors based on quantum wells (QWIPs – Quantum-Well Infrared Photodetectors) have numerous applications in the fields of medicine, engineering, defense, meteorology and environmental monitoring. The absorption spectrum of QWIPs has high wavelength selectivity, and this type of photodetector is the current choice for the fabrication of high-resolution cameras operating in the infrared. Currently, Brazil faces restrictions to import such a technology, imposed by the developed countries, due to its possibility of being used in military applications. In this thesis, we propose the development and optimization of photodetectors based on intraband transitions in quantum wells grown by molecular beam epitaxy (MBE) on GaAs substrates. The epitaxial growth of the quantum wells was investigated, and the samples were analyzed by photoluminescence (PL) to verify the quality and reproducibility of the heterostructures. The calculations of the energy levels and wavefunctions of the quantum wells were done by numerical implementation of the transfer matrix method [21] in the Mathematica software. This method was also applied to the self-consistent calculations involving the doping of the structures. From the values of the confinement energies, the wave functions could be obtained as well, and some physical quantities such as the theoretical absorption coefficient, the dark current and noise could be estimated. A model was also developed for the calculation of the energy levels of a hydrogenoid impurity inside a GaAs quantum well with AlGaAs barriers. It is believed that this system could have better noise characteristics when compared to a common QWIP in which the impurities are completely ionized [25] [26] [27]. The sample processing for the manufacture of the photodetectors was developed, optimized, and involved conventional photolithography techniques to define the physical size of the devices (followed by wet etching) as well as metallic film deposition to obtain ohmic contacts . Several characterization techniques were developed and installed to determine the wavelength of operation, the responsivity, the intrinsic noise and the dark current of the QWIPs manufactured in our laboratory. When this PhD project started, none of the characterization techniques was available in the lab. A complete set of experimental data was achieved by measuring the absorption coefficient and the spectral response by Fourier transform infrared spectroscopy (FTIR), the photocurrent using a blackbody, the dark current using I-V curves, and the noise with a spectrum analyzer. The measurements were performed as a function of the bias voltage at different temperatures. Several QWIPs samples were grown for absorption in the atmospheric windows from 3μm to 5μm and from 8μm to 12μm. The absorption curve of each sample was measured, and full characterization was performed on two QWIPs, in the region of 8μm to 12μm. The best results were obtained in a QWIP with peak absorption at 9. 3μm, which showed a detectivity of 5×10 10 cm.Hz 1/2 /W at 10K and 4×10 9 cm.Hz 1/2 /W at 70K.