| Summary: | 博士 === 國立成功大學 === 電機工程學系碩博士班 === 90 === In this dissertation, a high quality ZnSe buffer layer was grown onto the etched GaAs substrate by molecular beam epitaxy (MBE) while the etched substrate surface having a (4×1) reconstructed pattern during deoxidization. It was found that the orders of periodic fringes of the DCXRD spectrum of a ZnSe buffer layer are over six and that peak splitting between GaAs and ZnSe is 800 arc second which agrees well with the fully strained case. Furthermore, in the critical thickness (hc) determining, an hc value deduced from the x-ray data and from the PL data is about 1700 ? and 800 ?, respectively. It was concluded that the photoluminescence spectra are more sensitive to determine the critical thickness.
The high quality ZnSeTe/ZnSe multi quantum wells (MQWs) were also grown and investigated. The ZnSe0.99Te0.01 emission line due to Ten-cluster bound exciton is located at 2.67eV, and the full widths at half maximum (FWHM) are 184 meV and 171 meV for 5 QW and 3QW, respectively. Meanwhile, the high quality ZnSSeTe epilayer grown on ZnSe/GaAs layers was achieved by the incorporation of sulfur element into the epitaxial films comparing with the lattice-mismatched ZnSeTe films. It was found that the PL spectra of ZnSSeTe layer are basically not sensitive on the sulfur concentration but sensitive on Te content. It was also found that the ZnSSeTe epitaxial layers not only have excellent high crystal quality but also keep the emission properties of Ten-cluster bound state. Moreover, one-dimensional configuration coordinate diagram of the ZnSSeTe epitaxial layer at 15K have been drawn and compared the lattice of a ZnSSeTe/GaAs system with a ZnSeTe/GaAs system. It has been concluded that the ZnSSeTe epitaxial layer was suitable for thick film application.
For n-type doping, it was found that the net donor concentrations are logarithmic linearly dependent on the temperature from 120℃ to 160℃. When the doping concentration increases from 3×1018 cm-3 to 1.2×1019 cm-3, the related deep level emission at 2.1 eV increases about two orders. In addition, in such high doping concentration of 1.2×1019 cm-3, the peak of I2 line shift from 2.8 to 2.82 eV and get more broaden. It has been concluded that this emission associated with the continuous states built by heavy doping between the donor and conduction levels. On the other hand, the net acceptor concentration gradually saturated with increasing power from 110 W to 140 W. When the RF power was raised to about 140 W, the doping level, eventually, has been saturated at 8×1017 cm-3 and is independent on RF power. For contact study, thermal reliability of Au/AuBe and Au/AuBe/Cr were investigated. It was found that Zn out-diffusion toward sample surface is the main reason for the increase of ρc for Au/AuBe contact. In other words, Au/AuBe/Cr contact is more thermally reliable. Such a property makes Au/AuBe/Cr attractive in device application.
In Te application, high quality quaternary ZnSSeTe epitaxial layers were successfully grown by MBE. A ZnSSeTe MSM photodetector was fabricated for the first time. It was found that we could achieve a photo current to dark current contrast higher than five orders of magnitude by applying a 10V reverse bias. It was also found that the maximum photo responsivity is about 0.4 A/W under a 10V reverse bias. In addition, ITO layers were deposited onto n-ZnMgSSe films by DC magnetron sputtering and ITO ZnMgSSe MSM photodetectors were fabricated for the first time. It was found that we could achieve a photocurrent-to-dark current contrast higher than four orders of magnitude by applying a 10 V reverse bias. Moreover, the maximum photoresponsivity at 400 nm is 0.27 A/W under a 5 V reverse bias. Such a value corresponds to an external quantum efficiency of 41.5%.
Finally, the orange light emitting diodes (LEDs) have been successfully fabricated by using the ZnCdSeTe/ZnSSe MQWs. It was found that a 15% Te can result in a 78 nm EL red-shift at RT. Furthermore, It was found that we could modulate the color of LEDs by varying the well numbers (i.e. ZnCdSe, or ZnCdSeTe), and the dominated emission may be a nearest neighbor-well layer beside the p-ZnSSe layer. Therefore, the x-y values of chromaticity diagram of mixed color LEDs are located in the region strongly depending on what a nearest neighbor-well layer is. In addition, the probability of a ZnCdSeTe well-state emission may increase while the well thickness is thin enough.
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