We report on a transparent gate silicon MOS photo-impedance sensor, where a gated light sensitive semiconductor layer connects fixed capacitances. The resistance of the semiconductor and the capacitance of the MOS structure change with illumination. The frequency dispersion makes the coupling of these capacitances sensitive to light intensity extending the sensor dynamic range and tuning the sensitivity of the sensor. Our modeling results demonstrate advantages of this novel sensor in terms of sensitivity and dynamic range. The design and concept of this device could be extended to many other semiconductor materials, where frequency dispersion is related either to traps, or embedded nanoparticles or carrier generation processes.
A high-quality white light source requires high luminous efficacy (lumens per input watt). Theoretically, in the "greenyellow"
spectral region (with a peak wavelength at around 555 nm), the luminous efficiency (lumens per radiant watt)
reaches a maximum based on the luminous efficiency function, V(λ), and can potentially generate high luminous
efficacy. Unfortunately, the light-emitting diode (LED) suffers from low external quantum efficiency in the "greenyellow"
region, thereby lowering the luminous efficacy value. Researchers have sought solutions such as nonpolar or
semipolar InGaN/GaN LEDs. An alternative to generating green light is to use phosphor down-conversion by exciting a
green emission phosphor with a near-UV or blue LED of higher external quantum efficiency. In this study, a
SrGa2Se4:Eu2+ phosphor with peak emission wavelength at 555 nm was initially synthesized and followed by a
systematic study of the post-synthesis annealing. The purpose of this study was to investigate how post-synthesis
annealing conditions, including annealing temperature, annealing duration, and annealing ambient atmosphere, can
affect phosphor performance. The phosphor performance was characterized in terms of external quantum efficiency and
emission properties. How the external quantum efficiency of the phosphor can be further improved is also discussed.
In this work, we report dark current reduction, which is achieved by surface treatment with Octadecylthiol (OTD), for
GaInAsSb based photodetectors. Epitaxial layers of the GaInAsSb photodetector were grown on n-type GaSb substrates
with a horizontal MOCVD reactor, and the devices were fabricated by wet chemical etching. Surface treatment was
carried out by immersing fresh-prepared detector samples in molten ODT solution maintained at 100 °C for 5 hours. The
ODT treated devices show an order of magnitude reduction in the leakage current density in comparison with the
untreated devices. The inverse of the dynamic zero bias resistance area product (1/R0A) is also lower for ODT treated
devices. XPS analyses indicate the formation of Ga-S (20.1 eV) and In-S (445.3 eV) bonds at the surface and reduction
in the formation of native oxide on ODT treated GaInAsSb surface. This means that surface treatment with ODT can
effectively passivate dangling bonds and also reduce the native oxide. These results indicate that ODT can be used for an
effective passivation technique when more sophisticated processing steps are further developed.
Surfaces of GaSb substrates currently available from various commercial vendors are nowhere close to device grade GaAs, Si or InP wafer surfaces. Hence epitaxial growth and device fabrication on as-received commercial substrates poses significant difficulties amongst antimonide based researchers. Antimonide based materials are known to have poor surface oxide quality and not so well understood chemical reactions with various chemicals used to remove the oxides prior to growth. There are no existing reports on the detailed recipe for the preparation of "atomically flat and clean" surfaces that works on wafers obtained from various commercial vendors. This paper presents a detailed recipe for obtaining atomically flat and clean GaSb surfaces, irrespective of the initial polishing source. The same recipe (with slight modification) has been found to be successful with other III-V and II-VI compounds. The novel surface preparation process developed in our laboratory includes, chemical-mechanical polishing using an agglomerate-free sub-micron alumina slurry on a soft pad such as velvet, surface cleaning using dilute ammonium or potassium hydroxide-H2O solution and surfactant or glycerol, surface degreasing using organic solvents, oxide desorption using HCl-H2O and HF-H2O mixtures, mild chemical etching using ammonium sulfide and a final rinse in high purity deionized (DI) water and methanol. Using this recipe, we have been able to achieve surfaces with atomic flatness (RMS surface roughness close to 0.5 nm over a 10 x 10 mm2) and extremely clean surfaces, irrespective of the initial contamination or the sources of the wafers. Results of wafer surfaces before and after polishing using our recipe will be presented.
III-V antimonide based devices suffer from leakage currents. Surface passivation and subsequent capping of the surfaces are absolutely essential for any practical applicability of antimonide based devices. The quest for a suitable surface passivation technology is still on. In this paper, we will present some of the promising recent developments in this area based on dry etching of GaSb based homojunction photodiodes structures followed by various passivation and capping schemes. We have developed a damage-free, universal dry etching recipe based on unique ratios of Cl2/BCl3/CH4/Ar/H2 in ECR plasma. This novel dry plasma process etches all III-V compounds at different rates with minimal damage to the side walls. In GaSb based photodiodes, an order of magnitude lower leakage current, improved ideality factor and higher responsivity has been demonstrated using this recipe compared to widely used Cl2/Ar and wet chemical etch recipes. The dynamic zero bias resistance-area product of the Cl2/BCl3/CH4/Ar/H2 etched diodes (830 Ω cm2) is higher than the Cl2/Ar (300 Ω cm2) and wet etched (330 Ω cm2) diodes. Ammonium sulfide has been known to passivate surfaces of III-V compounds. In GaSb photodiodes, the leakage current density reduces by a factor of 3 upon sulfur passivation using ammonium sulfide. However, device performance degrades over a period of time in the absence of any capping or protective layer. Silicon Nitride has been used as a cap layer by various researchers. We have found that by using silicon nitride caps, the devices exhibit higher leakage than unpassivated devices probably due to plasma damage during SiNx deposition. We have experimented with various polymers for capping material. It has been observed that ammonium sulfide passivation when combined with parylene capping layer (150 Å), devices retain their improved performance for over 4 months.
Near infrared detectors in the 1 to 2.4 μm spectral range are important for many applications such as atmospheric remote sensing, where several species have strong absorption spectra in that range. Antimonide-based III-V compound semiconductor materials are good candidates for developing detectors in that spectral range. Electrical and optical characteristics of In1-xGaxSb p-n photodetectors at different temperatures are presented. The devices were fabricated either on bulk InGaSb substrates by zinc diffusion or InGaSb epitaxial layers grown on GaSb substrates by organo-metallic vapor phase epitaxy (OMVPE). Variable area devices were fabricated. Current-voltage measurements indicated higher dark current in InGaSb devices grown on GaSb substrate, due to defects generated by the lattice-mismatch. Spectral response measurements were obtained in the 1 to 2.4 μm wavelength range at different temperatures. At room temperature, the cut-off wavelengths were observed at 2.3 and 2.1 μm for InGaSb devices grown on GaSb and for devices fabricated on bulk InGaSb substrates respectively. Reducing the operating temperature shifts the cut-off wavelength to shorter values and increases the responsivity. Noise calculations indicated a room temperature detectivities of 3.3x1010 and 5.5x1010 cmHz1/2/W at 2 μm for the GaSb and InGaSb respectively. Detectivity variation with wavelength will be presented and compared to the background limited performance.
Optical and electrical characteristics of InGaSb p-n photodetectors are presented at different temperatures. The device structures were grown on GaSb substrates using organic metal vapor phase epitaxy. Spectral calibration indicates peak responsivity around 2 µm, equivalent to 58% quantum efficiency, with 2.3-µm cutoff at room temperature. Reducing the device temperature increases the responsivity and shifts the cutoff wavelength to a shorter value. Current voltage measurements at different temperatures indicate that tunneling is the primary leakage current mechanism. Assuming Johnson limited performance, detectivity calculations resulted in 4×1010 cm Hz1/2/W indicating that InGaSb is a superior material for 2-µm detection applications.