Inorganic perovskite photodetector with chromium interdigitated electrodes is investigated. The active material is a solution processed CsPbI3 synthesized by one step deposition. The α-phase of this material is unstable at room temperature and it converts to the δ-phase at temperatures below 320°C. The stabilization of the α-phase was conducted in nitrogen environment by adding 5% weight ratio of polyethylene oxide to the equimolar mixture of PbI2 and CsI in dimethyl sulfoxide. The absorbance spectrum of the CsPbI3 with polyethylene oxide showed the emergence of a second peak at ∼ 690 nm related to the α-phase in addition to the peak at 420 nm of the δ phase. The peak in the visible range was also observed in the μ-photoluminescence spectrum at 690 nm. The interdigital structure was prepared by photolithography method to deposit 80 nm of chromium interdigitated electrodes on glass substrate with 10 μm channel width. The detector's on/off ratio and the detectivity were calculated from the current voltage curve under 100 mW/cm2 to be 9.64×103 and 9.2×109 Jones respectively.

Charge transport between nickel oxide nanoparticles and CdSe/ZnS alloyed core/shell nanocrystals is investigated. The crystal structure and composition of the nickel oxide nanoparticles are evaluated using X-ray diffraction, Raman and X-ray photoelectron spectroscopies. The nanoparticles are near-stoichiometric with very low defect densities. The optical properties of the materials are studied by measuring the absorbance and time resolved photoluminescence spectra. The band gap of the nickel oxide nanoparticles is around 4.42 eV. The CdSe/ZnS nanocrystals exhibit shorter average lifetimes when mixed with nickel oxide nanoparticle powder. The lifetime quenching can be attributed to the efficient charge transport from the CdSe/ZnS nanocrystals to nickel oxide nanoparticles due to the relative valence band alignment.

Anatase and rutile titanium dioxide thin films grown by a low temperature process are investigated for their use as a single layer antireflection coating for GaAs solar cells. The thin films are obtained by spin coating a layer from the TiO2 sol-gel and subsequently annealing at 150 °C. The sol-gel is synthesized by the hydrolysis of titanium isopropoxide in the presence of an acid or a base. By controlling the pH of the sol-gel during growth, pure anatase and rutile phases are obtained. A pH of around 3.0 yields anatase phase while a pH of 9.0 yields pure rutile phase TiO2. The two different phases of TiO2 are characterized by measuring the Raman scattering spectra. The optical constants, thickness and reflectance of the thin films on GaAs are obtained using a spectroscopic ellipsometer. The sol-gel is spin coated on GaAs based solar cells and annealed at 150 °C to form the anti-reflective layer. The performance of the solar cells is evaluated before and after coating with the TiO2 films. The anatase TiO2 anti-reflective films performed better than the rutile with a maximum power conversion efficiency enhancement of 50%. Quantum efficiency enhancement of 58% and 25% are obtained with anatase and rutile phase films respectively. The performance enhancement of the solar cells using these thin films can be attributed to the destructive interference of light associated with a single layer coating on the solar cell surface.

All inorganic quantum dot light emitting devices with solution processedtransport layers are investigated. The device consists of an anode, a holetransport layer, a quantum dot emissive layer, an electron transport layer and acathode. Indium tin oxide coated glass slides are used as substrates with theindium tin oxide acting as the transparent anode electrode. The transport layersare both inorganic, which are relatively insensitive to moisture and otherenvironmental factors as compared to their organic counterparts. Nickel oxideacts as the hole transport layer, while zinc oxide nanocrystals act as theelectron transport layer. The nickel oxide hole transport layer is formed byannealing a spin coated layer of nickel hydroxide sol-gel. On top of the holetransport layer, CdSe/ZnS quantum dots synthesized by hot injection method isspin coated. Finally, zinc oxide nanocrystals, dispersed in methanol, are spincoated over the quantum dot emissive layer as the electron transport layer. Thematerial characterization of different layers is performed by using absorbance,Raman scattering, XRD, and photoluminescence measurements. The completed deviceperformance is evaluated by measuring the IV characteristics,electroluminescence and quantum efficiency measurements. The device turn on isaround 4V with a maximum current density of ∼200 mA/cm2 at9 V.

A study on light absorption enhancement of an InAs quantum dots embedded into InxGa1-xAs quantum well with GaAs as a barrier solar cells was carried out. Solar cell devices were fabricated from different structures, which were grown by using molecular beam epitaxy, with the In mole fraction (x) varied between 0 – 25 %. Poly-L-Lysine ligands and ZnO sol-gel was used to modify the surface of the solar cells and act as anti-reflection coatings. The anti-reflection characteristic of the ligands and the sol-gel were investigated by measuring the solar cell characteristics before and after the solar cells surface modifications. The current-voltage characteristics were measured of the fabricated solar cells before and after Poly-L-Lysine and ZnO coatings. A significant enhancement on the order of 40 % of the solar cells performance was observed. This type of enhancement was observed in the power conversion efficiency, spectral response measurements, and external quantum efficiency.

Highly pure iron pyrites (FeS2) cubic phased nanocrystals of diameter ∼ 20 nm were synthesized by colloidal method using only amines acting both as a coordinating and surfactant ligands. The details of synthetic condition at temperature 175 °C, 215 °C, 230 °C, 245 °C were compared and elucidated. The best synthetic conditions using an octylamine as a ligand at 230 °C for 2h have been optimized in an inert atmosphere. The XRD measurement shows diffraction peaks of pure cubic iron pyrite crystal structure without any detectable presence of marcasite, pyrrotite, greigyte and other impurity structures. The UV-Vis spectra depict clear absorption onset at 1200 nm in best sample with estimated band gap of ∼1.03 eV. These high pure and nanostructures based iron pyrite processed from solution route may offer excellent manufacturing scalability at very low cost since it can be used as inks for large scale fabrication. The morphological and optical characterizations are carried out by using XRD, UV-Vis, and SEM techniques.

Solar cells based on InAs quantum dots embedded in InxGa1-xAs quantum wells grown on n-type GaAs substrate were fabricated and tested. Solar cells with In mole fraction (x) in the range of 0-40% were investigated. The performance of the solar cells was evaluated using current-voltage characteristics, spectral response, and quantum efficiency measurements. The spectral response and quantum efficiency spectra possess several peaks along the lower energy side of the spectra, which are attributed to the interband transitions in the structure. These peaks are red shifted as x is increased above 0 %. The device power conversion efficiency was extracted from the current-voltage characteristics using an AM 1.5 solar simulator. The short circuit current density increased as the x is increased above 0 %. But the overall power conversion efficiency decreased due to decrease in the open circuit voltage. The decrease in open circuit voltage is due strain induced dislocations caused by lattice mismatch.

To develop alternative and low cost photovoltaic technologies we have synthesized CuInS2 nanocrystals with tunable optical properties and characterization was carried out thoroughly with TEM, SEM, EDAX and XRD. Furthermore large self-organized arrays of TiO2 nanotubes were fabricated on Ti foil followed by simple electrochemical anodization technique and characterized their structure by SEM and then for the first time coupled both the nanocrystals and nanotubes to form a p-n junction type photovoltaic device. The current-voltage (I-V) characteristics of photovoltaic cells were measured to test the proof of concept. Some preliminary experiments showed that device generates some current upon illumination. However, in our case we fabricated a device without sandwiching any buffer or barrier layers in between nanocrystals and nanotube arrays. We have been optimizing our solar cells efficiency by improving quality of nanotubes and nanocrystals. Some of the interesting finding are presented and discussed.

Rapid thermal annealing (RTA), with fast ramp up and down rates, was performed on several Cu(In,Ga)Se2 (CIGS) films and solar cells under various peak annealing temperatures and holding times. The XRD, SEM, Hall- effect, photo J-V, and quantum efficiency (Q-E) measurements were made on CIGS films and cells before and after RTA treatments to study the effects of RTA on the CIGS film properties and cell performance. The results show that RTA treatments under optimal annealing condition can provide significant improvements in the electrical properties (resistivity, carrier concentration, and mobility) of CIGS films and cell performance while preserving the film composition and microstructure morphology.

The optical absorption spectra of intersubband transitions in In0.3Ga0.7As/GaAs multiple quantum dots (MQDs) grown by molecular beam epitaxy were investigated. By varying the number of In0.3Ga0.7As monolayers deposited, a series of samples with varying dot sizes ranging from 10 – 50 monolayers were obtained. The quantum dots grown with size less than 15 monolayers or more than 50 monolayers did not yield any observable measurements of intersubband transition. This suggests that there exist a critical upper and lower limit of In0.3Ga0.7As quantum dots for infrared detectors. A wavelength range of 8.60 – 13.70 μm is achieved for structures grown with the above monolayers range. The theoretical line-shape of the intersubband transition absorption was compared to the experimental measurements. From the lineshape, it was deduced that bound-to-continuum transtition is present in thick quantum dots and bound-to-bound transition is present in thinly grown quantum dots.

The emission from bias voltage driven In0.06Al0.08Ga0.86As/Al0.3Ga0.7As edge-emitting diode lasers with cavity lengths of 0.6 and 0.9 mm was measured at 290 K using a high resolution Fourier-transform infrared spectrometer. The light-current (L-I) characteristic curves showed that a threshold current of ∼ 0.5 and 0.8 A was needed to reach the stimulated emission in laser diodes with cavity lengths of 0.6 and 0.9 mm, respectively. The laser diode longitudinal modes were observed in the stimulated emission spectrum for each of the devices. These spectra enabled us to calculate the refractive index, total loss, and gain of the active region. On the other hand, the cavity length was determined by utilizing the interferogram spectrum, longitudinal modes spacing, and direct measurements of the laser dimensions. A good agreement between the cavity length values obtained from the three methods was achieved.

We present an approach for spectroscopic strain analysis in semiconductor quantum-well devices. This approach is applicable to all types of semiconductor materials, and to spectroscopic techniques which employ the electronic band-structure of the material, such as photoluminescence, photoreflection, photocurrent, and transmittance. The approach is based on two components, namely the theoretical calculation of the strain-sensitivity of the spectral positions of the relevant quantum-confined optical transitions within a particular quantum-well, and the spatially resolved measurement of a substantial part of the optical transition sequence within the quantum-well. The primary experimental technique applied in our approach is photocurrent spectroscopy. InAlGaAs/GaAlAs/GaAs, high-power lasers serve as the model species.

Intersubband transitions (ISTs) in GaN/AlxGa1-xN multiple quantum wells (MQWs) were investigated using an optical absorption technique. Several samples were grown by either Molecular Beam Epitaxy (MBE) or Metal-Organic Chemical Vapor Deposition (MOCVD) and were investigated using both normal incident and waveguide configurations. The waveguides were fabricated by dicing each sample into 2 mm wide by 5 mm long pieces with two facets polished at 45 degrees with respect to the surface such that light propagates across the sample's width. Preliminary results indicate that ISTs are observable in Si-doped and undoped GaN/AlxGa1-xN MQWs. The source of these charge carriers in the undoped samples are explained as being due to the spontaneous polarization effect which exists at the GaN/AlxGa1-xN interfaces where the GaN surface has Ga-polarity. Scanning Electron Microscopy indicates that a sample containing what appeared to be a large number of cracks and or hexagonal voids lacked the presence of ISTs.

Photoluminescence (PL) spectra of interband transitions in GaInNAs/GaAs single quantum wells grown by metalorganic chemical vapor deposition technique on semi-insulating GaAs substrates were measured at 77 K for several samples grown with different In composition and dimethylhydrazine (DMH)/III ratios. The results show that the PL intensity increases as the In mole fraction is increased from 0 to 25%, but the PL intensity is degraded for samples with an In mole fraction of 30% or higher. The peak position energies of the PL spectra were investigated as a function of the DMH/III ratio. Thermal annealing effect induced a blue-shift in the PL spectra peak position energy in samples grown with high DMH/III ratio.

Radiation hardness on intersubband transition in proton-irradiated InGaAs/InAlAs multiple quantum wells is investigated using Fourier transform infrared absorption (FTIR) technique and compared to the results observed for the GaAs/AlGaAs multiple quantum wells. It is observed that the intersubband transition remains almost unchanged in samples irradiated with 1 MeV protons and with doses as high as 1×1014 cm-2. This dose on the other hand, was found to completely deplete the intersubband transition in GaAs/AlGaAs multiple quantum wells samples. The intersubband transition in InGaAs/InAlAs multiple quantum wells was almost washed out in samples irradiated with doses as high as 3×1015 cm-2. The partial thermal annealing recovery of the depleted intersubband transition in the proton irradiated InGaAs/InAlAs multiple quantum wells will be reported.

Optical absorption measurements were used to investigate deep defects in proton irradiated doped and undoped AlGaN thin films grown on sapphire substrates. Several samples were proton irradiated with energies ranging between 10 keV and 1 MeV. In certain samples, multiple-energy ion implantation was found necessary to produce a defect, which is responsible for the absorption band observed at 4.61 eV with a shoulder at around 4.10 eV in Al0.6Ga0.4N. Furnace thermal annealing of the irradiated samples show that this absorption band starts to anneal out at temperature as low as 200 oC. A combined isochronal and isothermal annealing in the temperature range of 200- 350°C shows that the activation energy (enthalpy associated with the migration process) of this defect is approximately 0.41 eV. This leads us to conclude that this absorption band is due to a N-vacancy related defect. It is observed that the peak position energy of the absorption band due to this defect is shifted depending on the Al mole fraction in good agreement with the theoretical predictions.

Thermal annealing effects on carbon-hydrogen (C-H) complexes defects in AlGaN grown on sapphire by metalorganic chemical vapor deposition (MOCVD) technique have been investigated using Fourier transform infrared spectroscopy (FTIR). The CH complexes in AlGaN, formed either during growth or by proton irradiation, exhibit five local vibrational modes (LVMs) due to the symmetric and asymmetric vibrational stretching modes of C-H in CHn (n=1–;3) defect complexes. It was found that the annealing temperature (Ta) of 500°C is sufficient enough to dissociate most of the C-H complexes in AlGaN samples. A turning point annealing temperature is found around 300°C for un-irradiated Mg-doped sample, below which the total integrated area of the C-H LVMs continued to increase with increasing annealing temperature and reach the maximum value around 300°C. At Ta > 300°C, the total integrated area of the C-H LVMs starts to decrease and the C-H complexes seem to be completely depleted at Ta > 600°C. The depleted C-H LVMs were observed to partially recover after thermal annealing at Ta > 500°C and waiting for aging periods of several days. This recovery behavior is explained in terms of the hydrogen being remained inside the crystal after the dissociation of C-H complexes, subsequent diffusion and recombining again with carbon atom to reform C-H complexes.

Intersubband transitions in In0.52Ga0.48As/In0.52Al0.48As multiple quantum wells grown on lattice matched InP substrates were investigated using Fourier transform infrared (FTIR) absorption and photoluminescence (FTPL) techniques. The well width was tailored to produce excited states resonant in the conduction band, at the edge of the conduction band, and confined in the quantum wells. Interband transitions were also probed using FTPL and optical absorption techniques. The FTPL spectra show that three interband transitions exist in the quantum well structures with well width larger than 30 Å. The intersubband transitions in this class of quantum wells seem to withstand proton irradiation with doses higher than those used to deplete the intersubband transitions in the GaAs/AlGaAs multiple quantum wells.

Local vibrational modes (LVMs) of carbon-hydrogen (C-H) complexes in proton implanted AlGaN grown on sapphire by metalorganic chemical vapor deposition (MOCVD) were investigated using Fourier transform infrared (FTIR) spectroscopy. The LVMs exhibit five distinctive peaks in the spectral region of 2846∼2963 cm−1, which are due to the symmetric and asymmetric stretching modes of C-Hn (n=1–3). The LVMs intensities in doped AlGaN are increased as irradiation dose is increased in the entire irradiation dose range used without reaching the saturation stage. On the other hand, undoped samples show that LVMs intensity increase then either decreased or saturated as the irradiation dose is increased above 5×1016 cm−2. Proton irradiation causes a drastic increase in the CH3 LVM while electron irradiation causes the opposite effect suggesting strongly that the observed LVMs are truly due to CH complexes.

Localized vibrational modes of carbon-hydrogen complexes in metalorganic chemical vapor deposition grown GaN on sapphire were studied using a Fourier-transform infrared spectroscopy technique. Three distinctive localized vibrational modes were observed around 2850, 2922, and 2959 cm−1 for undoped, Si- and Mg-doped samples. These peaks are related to CH, CH2, and CH3 defect complexes, respectively. It is also observed that the frequencies and intensities of the localized vibrational modes are sample dependent.