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We describe band engineering strategies to either enhance or suppress electron-initiated impact ionization relative to hole-initiated impact ionization in type II superlattice mid-wavelength infrared avalanche photodiodes. The strategy to enhance electron-initiated impact ionization involves placing a high density of states at approximately one energy gap above the bottom of the conduction band and simultaneously removing valence band states from the vicinity of one energy gap below the top of the valence band. This gives the electrons a low threshold energy and the holes a high one. The opposite strategy enhances hole-initiated impact ionization. Estimates of the electron (α) and hole (β) impact ionization coefficients predict that α/β>>1 in the first type of superlattice and α/β<<1 in the second type.
Low temperature (4.5K) photoluminescence (PL) measurements of GaAs(N):Sb on GaAs grown by solid source molecular beam epitaxy (MBE) show a Sb-related defect peak at ∼1017nm (1.22eV). The magnitude of the Sb-related impurity PL peak corresponds in intensity with the prominence of the additional two-dimensional  high-resolution x-ray diffraction (HRXRD) defect peaks. The elimination of these defects can be a measure of the improvement in crystal quality of GaAsN:Sb and a Sb flux ≥ 1.3×10−8 Torr is needed to invoke the surfactant behavior in III-V dilute nitride MBE growth for a growth rate of 1μm/hr.
Existing semiconductor electronic and photonic devices utilize the charge on electrons and holes in order to perform their specific functionality such as signal processing or light emission. The relatively new field of semiconductor spintronics seeks, in addition, to exploit the spin of charge carriers in new generations of transistors, lasers and integrated magnetic sensors. The ability to control of spin injection, transport and detection leads to the potential for new classes of ultra-low power, high speed memory, logic and photonic devices. The utility of such devices depends on the availability of materials with practical (>300K) magnetic ordering temperatures. In this paper, we summarize recent progress in dilute magnetic semiconductors such as (Ga,Mn)N, (Ga,Mn)P and (Zn,Mn)O exhibiting room temperature ferromagnetism, the origins of the magnetism and its potential applications in novel devices such as spin-polarized light emitters and spin field effect transistors.
Measurement of recombination and minority-carrier lifetimes has become a very common activity in current semiconductor technology. The two primary measurement techniques are based on photoconductive decay (PCD) and time-resolved photoluminescence (TRPL). The measurement of the “true” lifetime depends on the carriers being confined to a given spatial region of a diagnostic device. When internal electric fields exist that separate the charges, the measured value does not represent the real minority-carrier lifetime. In these cases, the measured quantity is a function of the true lifetime and the measurement technique.
The feasibility of micrometer scale dielectric periodic structures by using a single selective hydride vapour phase epitaxy (HVPE) step was assessed. HVPE is a near-equilibrium growth process which offers perfect selectivity whatever the pattern design, thus giving rise to a great flexibility. The HVPE growth is also mainly governed by the intrinsic anisotropy of the surface kinetics of the crystal. We demonstrate here that micrometer scale dielectric periodic structures, constituted of perfectly defined 1μm wide GaAs beams alternately stacked with air, can be grown by selective HVPE by controlling the hierarchy of the growth rates of the low index faces of the III-V crystal via the growth temperature and the composition of the vapour phase. Potential of the HVPE growth technic for the making of submicrometer scale structures is finally discussed.
A quantum dot (QD) medium is expected to demonstrate superior performance in various devices when compared with quantum wells (QWs). One area of interest has been the improved defect tolerance of QD media, though it was demonstrated at low temperatures so far. In this study, the defect tolerance of shape-engineered QD structures is compared with that of a QW structure at temperatures up to 300 K. To create high defect densities both QD and QW structures were irradiated with high energy (1.5 MeV) protons (with doses up to 3×1014 cm-2). Then, the relative luminescence efficiency was measured by variable temperature photoluminescence. The shape-engineered QD structure withstood two orders of magnitude higher defect density than the QWs at room temperature. This improvement is correlated with the activation energy for thermal evaporation of 390 meV, acquired through a kinetic model.
InSb1-xNx was grown on GaAs substrate by low-pressure metalorganic vapor phase epitaxy. Carrier gases were hydrogen or the mix of hydrogen and nitrogen. In both cases, X-ray analysis demonstrated that nitrogen was incorporated into InSb1-xNx up to 0.24.
GaSb-based semiconductors are of interest for mid-infrared optoelectronic and high-speed electronic devices. Accurate determination of electrical properties is essential for optimizing the performance of these devices. However, electrical characterization of these semiconductors is not straightforward since semi-insulating (SI) GaSb substrates for Hall measurements are not available. In this work, the capability of Raman spectroscopy for determination of the majority carrier concentration in n-GaInAsSb epilayers was investigated. Raman spectroscopy offers the advantage of being non-contact and spatially resolved. Furthermore, the type of substrate used for the epilayer does not affect the measurement. However, for antimonide-based materials, traditionally employed Raman laser sources and detectors are not optimized for the analysis wavelength range dictated by the narrow band gap of these materials. Therefore, a near-infrared Raman spectroscopic system, optimized for antimonide-based materials, was developed.
Ga0.85In0.15As0.13Sb0.87 epilayers were grown by organometallic vapor phase epitaxy with doping levels in the range 2 to 80 × 1017 cm-3, as measured by secondary ion mass spectrometry. For a particular nominal doping level, epilayers were grown both lattice matched to n-GaSb substrates and lattice-mismatched to SI GaAs substrates under nominally identical conditions. Single magnetic field Hall measurements were performed on the epilayers grown on SI GaAs substrates, while Raman spectroscopy was used to measure the carrier concentration of epilayers grown on GaSb and the corresponding SI GaAs substrates. Contrary to Hall measurements, Raman spectra indicated that the GaInAsSb epilayers grown on GaSb substrates have higher free carrier concentrations than the corresponding epilayers grown on SI GaAs substrates under nominally identical conditions. This is contrary to the assumption that for nominally identical growth conditions, the resulting carrier concentration is independent of substrate, and possible mechanisms will be discussed.
The carrier dynamics of a 7.5 nm GaInNAs quantum well (QW) are studied by photoluminescence (PL) at a low temperature regime of 4 K to 150 K. The PL emission efficiency of the QW is initially evaluated to examine the recombination mechanisms in the QW. A dual-activation-energy model is later found to fit the integrated PL intensity vs. temperature curve better than a single-activation-energy model. The two states that correspond to the above activation energies could have resulted in a much faster PL intensity quenching in the GaInNAs QW as compared to that of a reference GaInAs QW. One of the states is identified as a localized state that traps carriers at a low temperature range of less than ∼100 K. The other state has a larger quenching effect at temperatures higher than 100 K and this state is not studied in this paper. By fitting the original PL spectra with two Gaussian functions, the temperature dependent PL integrated intensity of both Gaussian functions was also studied to further characterize the GaInNAs QW. The analysis gives evidence of the localization behaviour in this QW.
Combination of tetrahedral and octahedral based nitrides are explored. The two cases of MnGaN and ScGaN with low Mn and Sc fractions are examined. It is found that for the MnGaN case, the Mn is incorporated under N rich conditions with little lattice change. However, for the ScGaN case, the Sc is incorporated onto the Ga sites but with a local bond angle distortion.
Highly oriented ZnO nanorods have been grown on p--Si(111) wafers using a low-pressure thermal CVD method. X-ray diffraction shows that the nanorods are grown with the c-axis normal to the substrate. An electroluminescent device with ITO/ZnS:Mn/nanorod-ZnO/p--Si structure where the ZnS:Mn and ITO layers are deposited by the electron beam deposition method on the ZnO nanorods layer operates stably in DC mode with high luminance.
Zinc oxide (ZnO) single crystals were grown by the hydrothermal method using lithium and potassium hydroxide as mineralizer and properties of the grown crystals were characterized from the viewpoints of epitaxial wafer applications. The growth sector dependence of impurity and defect concentrations were characterized by secondary ion mass spectroscopy and photoluminescence. As a result, it was clearly shown that defect and impurity distribution in the obtained crystal was anisotropic, and this anisotropy is affected by the choice of the seed crystal shape and growth direction. Annealing effect on flatness of the wafer surface was also examined, and it was found that high temperature annealing with flat single crystalline cover is appropriate for removal of scratch and formation of atomically flat surface. Moreover, we show the possible miss-evaluation of Hall coefficient of ZnO due to anisotropy in defects and impurities distributions.
Thin films of Bi, Sb, solid solutions Bi1-xSbx, as well as multilayer structures Bi-Sb-Bi-Sb--from elementary sources were produced by pulsed laser deposition for optoelectronic applications. KBr crystals were used as substrates. The solid solutions Bi1-xSbx were obtained by co-evaporation of single targets of Bi and Sb. Structural investigations show that the performance of produced films depends on both the amount of material deposited per pulse of laser energy and the ratio of this amount for bismuth and antimony. Based on this the technological regimes of growth temperature and laser intensity ranges were determined in which single-crystalline growth of films with certain x is possible. Single-crystalline films of Bi1-xSbx were obtained in the range of x (0.12—0.48), which corresponds to semiconductor state of this solution. The method of sequential deposition is used for fabrication of multilayer structures Bi/Sb with quantum-confined layers of bismuth. The growth regime with practically excluded interdiffusion of materials is found. Results of spectral investigations are shown to be in agreement with the theoretically predicted semimetal-to-semiconductor transition of bismuth as a result of quantum confinement.
Thermal processing or oxidation of 4H-SiC with n-type doping above about 3×1019 cm-3 is known to produce double Shockley stacking faults spontaneously. The resulting region of 3C stacking order acts like a quantum well in the 4H matrix and becomes negatively charged due to modulation doping. Some of these quantum well regions penetrate into the lightly-doped epilayers and intersect the wafer surface as straight lines, due to the 8° misorientation of the wafer from the c-axis. These intersections appear as bright lines in secondary electron images, which we tentatively attribute to increased secondary electron yield due to repulsion of secondary electrons from the negative charge in the quantum wells. Electrostatic force microscopy (EFM) and scanning Kelvin probe microscopy (SKPM) also produce clear images of the quantum well intersections, independent of surface topography. These images are similar to the SEM images.
This paper investigates the optical properties of bulk and epitaxial ZnO layers. High quality undoped and doped bulk ZnO crystals have been produced by melt growth techniques in addition to ZnO thin films grown by Metalorganic Chemical Vapor Deposition (MOCVD) on silicon, sapphire and the bulk ZnO substrates. This work focuses on investigating the suitability of bulk and epitaxial ZnO for waveguide applications using various spectroscopic techniques. The photoluminescence showed the dominance of strong and narrow band due to the band edge emissions for undoped ZnO. Ultraviolet-visible transmission data revealed the variation of the bandgap with different doping elements. Raman spectra showed a narrow and strong peak, corresponding to the E2 mode at 438 cm-1, characteristic of the ZnO crystallinity. A broad 2LO peak appeared near 1150 cm-1 due to the coupling between LO phonons and free carriers. A clear variation in refractive index with doping was observed by spectroscopic ellipsometery suggesting that ZnO could be used for waveguide applications.
Positron lifetime spectroscopy has been used to study the vacancy type defects in undoped gallium antimonide. Temperature dependent positron trapping into the VGa-related defect having a characteristic lifetime of 310ps was observed in the as-grown sample. The lifetime data were well described by a model involving the thermal ionization (0/-) of the VGa-related defect and its ionization energy was found to be E(0/-)=83meV. For the electron irradiated sample, the VGa-related defect with lifetime of 310ps that was found in the non-irradiated samples was also identified. Moreover, another lifetime component (280ps) was only observed in the electron irradiated sample but not in the non-irradiated sample. It was also attributed to the VGa-related defect. The two identified VGa-related defects should have different microstructures because of their difference in characteristic lifetimes. The 280ps component remains thermally stable after the 500°C annealing while the 310ps component anneals at 300°C.
We propose a compact 3C-SiC electro-optic modulator for high-speed Si-based integrated optoelectronics operating at 1.55-μm-wavelength. The device is based on an optical microresonator using sub-micron size high-index-contrast SiC waveguides on a SiC-SiO2-Si [SiC-on-Insulator (SICOI)] platform. Three different electrode configurations are analyzed. Switching times on the order of tens of ps are predicted by using low bias voltages.
Photoluminescence (PL) of annealed GaInNAs quantum well (QW) with varying temperature and laser excitation intensity is measured to understand the low temperature PL properties of annealed 6 nm GaInNAs QW. The measurements show that localization effect still exist in the QW even after annealing. This effect is characterized by an activation energy of 11 meV below the e1 state, which is obtained from fitting the integrated PL intensity vs. temperature curve with a single-activation-energy (SAE) model. This center is suggested to be related to the main localization center below the e1 state that could be resulted by N or In compositional fluctuation even after annealing.
Previously, we have made diodes[1,2] and transistors  as well as very effective realtime solid state neutron detectors  out of semiconducting boron carbide deposited on silicon or silicon carbide‥
In this work the recent fabrication of a new class of highly photosensitive boron carbide diodes is discussed. These diodes exploit the electronic behavior differences of the isomers of film precursors, the closo-dicarbododecaboranes. These differences were observed in photoemission and inverse photoemission studies where the HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap variations upon deposition varied strongly with the isomeric configuration. Based on these results, p-n junctions were formed by plasma enhanced chemical vapor of ortho and meta carborane, respectively, on both nickel and aluminum substrates. These diodes exhibit fourth-quadrant conductivity, making them exciting new photovoltaic conversion devices.
The effect of small changes in GaSb layer thickness on the photoresponse spectrum of InAs/GaSb superlattices (SLs) designed for mid-infrared detection was systematically investigated. The samples were grown by molecular beam epitaxy with precisely calibrated growth rates. The basic SL used for this study consisted of 40 periods of InAs (20.5 Å)/GaSb (X Å), where the nominal value for X was adjusted from 18 to 27 Å in four different samples. An InSb-like interface (IF) was inserted between the layers to balance the SL strain. By decreasing the GaSb width, the photoresponse cut-off wavelength (λc) was adjusted from 4.03 μm to 4.55 μm, i.e., the SL energy band gap is being decreased. This decrease in the energy separation between the first heavy hole band (HH1) and the first conduction band (C1) as the GaSb layer is narrowed is counter intuitive. However, this experimental trend can be explained by a modified envelope function approximation (EFA) calculation that includes the effect of in-plane asymmetry at InAs/GaSb interfaces. As expected, the HH band is pushed away from the top of the GaSb valence band as the GaSb layer width becomes narrower. However, at the same time the C1 band is significantly broadened by the increased wave function overlap of the electron states in the InAs layer. The trend to smaller band gap with narrower GaSb layers and other effects of the design changes on the photoresponse spectrum are discussed.