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Three II-VI wide bandgap compound semiconductors have been investigated for surface passivation of various photovoltaic devices. First part of this work focuses on the surface passivation of HgCdTe IR detectors using CdTe. A new metalorganic chemical vapor deposition (MOCVD) process has been developed that involves depositing CdTe films at much lower temperature (< 175°C) than the conventional processes used till now. Deposition rate as high as 420nm/h was obtained using this novel experimental setup. Favorable conformal coverage on high aspect ratio HgCdTe devices along with a significant minority carrier lifetime improvement was obtained. Another II-VI semiconductor, namely, CdS was investigated as a surface passivant for HgCdTe IR detectors. It was deposited by MOCVD as well as atomic layer deposition (ALD) and was studied for optimal conformal coverage on high aspect ratio structures. Surface passivation of p-type Si wafer has also been demonstrated using p-ZnTe grown by MOCVD, for possible application in solar cells. Preliminary work showed a remarkable improvement in the minority carrier lifetime of Si light absorbing layer after passivation with a thin layer of ZnTe.
Heteroepitaxial growth of high-quality II-VI-alloy materials on Si substrates is a well-established commercial growth process for infrared (IR) detector devices. However, it has only recently been recognized that these same processes may have important applications for production of high-efficiency photovoltaic devices. This submission reviews the process developments that have enabled effective heteroepitaxy of II-VI alloy materials on lattice-mismatched Si for IR detectors as a foundation to describe recent efforts to apply these insights to the fabrication of multijunction Si/CdZnTe devices with ultimate conversion efficiencies >40%. Reviewed photovoltaic studies include multijunction Si/CdZnTe devices with conversion efficiency of ∼17%, analysis of structural and optoelectrical quality of undoped CdTe epilayer films on Si, and the effect that a Te-rich growth environment has on the structural and optoelectronic quality of both undoped and As-doped heteroepitaxial CdTe.
We present a new concept applicable to the epitaxial growth of dislocation-free semiconductor structures on a mismatched substrate with a thickness far exceeding the conventional critical thickness for plastic strain relaxation. This innovative concept is based on the out-of-equilibrium growth of compositionally graded alloys on deeply patterned substrates. We obtain space-filling arrays of individual crystals several micrometers wide in which the mechanism of strain relaxation is fundamentally changed from plastic to elastic. The complete absence of dislocations at and near the heterointerface may pave the way to realize CMOS integrated SiGe X-ray detectors.
The control of ferromagnetic properties by external stimuli is of great interest in the electronics community. One method of producing such a control is through proximity of a ferromagnetic film with a material that has a semiconductor-to-metal transition (SMT). In order for these magnetic heterostructures to be beneficial, they must consist of high-quality, crystalline films. Epitaxial films increase the reproducibility of both devices and properties. We have investigated the trend in magnetic coercivity in epitaxial nickel films on VO2. We show that not only does the interaction between the Ni and VO2 change the normal coercivity trend found in Ni M-H curves with no proximity to VO2, but that the crystalline growth mode of the Ni film also impacts the magnetic coercivity as a function of temperature.
In this study, the initial AlN layer and the vertical-direction breakdown voltage (VDBV) of AlGaN/GaN high-electron-mobility transistors (HEMTs) were characterized. Prior to the formation of the interface between the AlN layer and the Si substrate, only trimethylaluminum (TMA) was introduced without ammonia to control the crystal quality of initial AlN layer (TMA preflow). HEMT structures were simultaneously grown on identical AlN layers on Si substrates (AlN/Si templates) grown using different TMA preflow temperatures. The density of screw- or mixed-type dislocations in the initial AlN layer decreased as the TMA preflow temperature increased. Further, the VDBV of the HEMT structure increased as the TMA preflow temperature increased. It is supposed that the screw- or mixed-type dislocations are the possible source of the vertical leakage current in the HEMT structures. The improvement in the crystal quality of the initial AlN layer affects the increase in the VDBV of the AlGaN/GaN HEMTs on Si substrates.
Contact metallization is an essential obstacle for utilizing low temperature achievable polycrystalline ZnO in any discrete devices and integrated circuits. To develop ZnO based semiconductor devices with advanced feature of flexibility, transparency and compatibility with low temperature processing, rectifying junctions must be fully developed. In this work, nanoscale polycrystalline ZnO thin films are fabricated with via low temperature (<200 °C) by atomic layer deposition (ALD). A vertical structure of bottom Schottky metallized diode is developed with copper (Cu) sputtered in room temperature. A control of Cu surface oxidation is realized with an in-situ remote plasma treatment. The results indicate that preparation of the copper surface substantially affects the electrical behavior of the diode. Thermal reliability of Cu metallized Schottky diode is subsequently carried out by annealing up to a maximum temperature of 300 °C before it breaks. This work considers the current transport mechanism evolved deviating current vs voltage (I-V) characteristics from conventional thermionic emission theory.
In this paper, the electrical properties of bottom-gate (BG) polycrystalline silicon (poly-Si) thin-film transistors (TFTs) by NiSi2 seed-induced lateral crystallization (SILC) and its applications are presented. Sequential lateral solidification (SLS), which is one of crystallization methods, is known to have poor electrical properties of TFTs with BG structures due to problems induced by laser. Therefore, the laser method cannot be used to well-developed production line of amorphous-Si (a-Si) TFT, resulting in large initial investment cost to change fabrication procedures. On the other hand, the BG poly-Si TFT by SILC (SILC-BGPS TFT) has basically compatible process flows with that of the a-Si TFT. The SILC-BGPS TFT exhibited threshold voltage of -3.9 V, steep subthreshold slope of 130 mV/dec, a high field-effect mobility of 129 cm2/Vs , and Ion/Ioff ratio of ∼106.
Spectrum tuning in phosphor converted white light emitting diodes (pc-WLED) is done by mixing powders of phosphor compounds (with different emission wavelengths) in different weight ratios. In this paper, a new methodology for designing unique full spectrum phosphor mixtures (with fixed weight ratios of different emission phosphor) has been presented that could provide a wide range of pc-WLED spectrum. This is done by optimizing the excitation and emission spectra of the phosphor compounds used in the mixture. A unique phosphor mixture comprising of Eu2+ and Ce3+-Na1+ activated compounds of SrGa2S4, CaGa2S4, SrS, CaS and CaF2 was used to produce full spectrum warm, neutral and cool white LEDs with color temperatures between 2500 K and 7500 K and with color rendering index exceeding 95.
Amorphous oxide semiconductors (AOS) are important candidates for next generation display transistors, but instability under illumination with month-long transients is a significant drawback and may limit broader use. Several models have been developed to fit transient photoconductivity observed in AOS and relate it to a spectrum of weighted time constants, equivalent to either a density of states distribution of deep traps within the activated energy model, or to a time-dependent relaxation time constant in other models. In this work, we classify fits of the time constant spectrum to the transient data as either “descriptive” if they make no presumption about the spectral shape, or “predictive” if they assume a spectral shape a priori characterized by a few simple parameters. By fitting both descriptive and predictive models to simulated transients, it is observed that the best fit converges for the descriptive model if the measurement duration exceeds the mode (“peak”) value of the time constant distribution. The predictive models can converge orders of magnitude faster, but rely on a proper identification of the correct lineshape a priori. Therefore, it is recommended that first an unbiased descriptive model of sufficient measurement duration be performed. Then the known lineshape can be applied as a predictive model for future measurements, reducing subsequent measurement durations by orders of magnitude.