Infrared absorption efficiency in AlAs/AlxGa1-xAs type-II multiple-quantum-well structure grown on (211) GaAs substrate

We have made a normal incidence high infrared absorption efficiency AlAs/Al0.55Ga0.45As multiple-quantum-well structure grown on (211) GaAs substrates by molecular beam epitaxy (MBE). A strong infrared absorption signal at 11.6 mu m due to the transition of the ground state to the first excited state, and a small signal at 6.8 mu m due to the transition from the ground state to continuum. were observed. A 45 degrees tilted incidence measurement was also performed on the same sample for the comparison with a normal incidence measurement. Both measurements provide important information about the quantum well absorption efficiency. Efficiencies which evaluate the absorption of electric components perpendicular and parallel to the well plane are eta(perpendicular to) = 25% and eta(parallel to) = 88%, respectively. The total efficiency is then deduced to be eta = 91%. It is apparent that the efficiency eta(parallel to) dominates the total quantum efficiency eta Because an electron in the (211) AlAs well has a small effective mass (m(zx)* or m(zy)*), the normal incidence absorption coefficient is expected to be higher:than that grown on (511) and (311) substrates. Thus, in the present study, we use the (211) substrate to fabricate QWIP. The experimental results indicate the potential of these novel structures for use as normal incidence infrared photodetectors.

Quantum-dot superluminescent diode: A proposal for an ultra-wide output spectrum

We propose a novel superluminescent diode (SLD) with a quantum dot (QD) active layer, which should give a wider output spectrum than a conventional quantum well SLD. The device makes use of inhomogeneous broadness of gain spectrum resulting from size inhomogeneity of self-assembled quantum dots grown by Stranski-Krastanow mode. Taking a design made out in the InxGa1-xAs/GaAs system for example, the spectrum characteristics of the device are simulated realistically, 100-200 nm full width of half maximum of output spectrum can be obtained. The dependence of the output spectrum on In composition, size distribution and injection current of the dots active region is also elaborated.

Structural and photoelectric studies on double barrier quantum well infrared detectors

GaAs/AlAs/GaAlAs double barrier quantum well (DBQW) structures are employed for making 3-5 um photovoltaic infrared (IR) detectors with a peak detectivity of 5 x 10(11) cm Hz(1/2)/W at 80 K. Double crystal X-ray diffraction is combined with synchrotron radiation X-ray analysis to determine successfully the exact thickness of GaAs, AlAs and GaAlAs sublayers. The interband photovoltaic (PV) spectra of the linear array of the detectors are measured directly by edge excitation method, providing the information about spatial separation processes of photogenerated carriers in the multiquantum wells and the distribution of built-in field in the active region. The spectral response of the IR photocurrent of the devices is also measured and compared with the temperature dependent IR absorption of the DBQW samples in order to get a better understanding of the bias-controlled optical and transport behavior of the detector photoresponse and thus to optimize the detector performance. (C) 1999 Elsevier Science Ltd. All rights reserved.

Structural and infrared absorption properties of self-organized InGaAs GaAs quantum dots multilayers

Self-organized InGaAs/GaAs quantum dots (QDs) stacked multilayers have been prepared by solid source molecular beam epitaxy. Cross-sectional transmission electron microscopy shows that the InGaAs QDs are nearly perfectly vertically aligned in the growth direction [100]. The filtering effect on the QDs distribution is found to be the dominant mechanism leading to vertical alignment and a highly uniform size distribution. Moreover, we observe a distinct infrared absorption from the sample in the range of 8.6-10.7 mu m. This indicates the potential of QDs multilayer structure for use as infrared photodetector.

Structural characterization of InGaAs/GaAs quantum dots superlattice infrared photodetector structures

InGaAs/GaAs quantum dots (QDs) superlattice grown by molecular beam epitaxy (MBE) at different substrate temperatures for fabricating 8-12 mu m infrared photodetector were characterized by transmission electron microscopy (TEM), double-crystal X-ray diffraction (DCXRD) and photoluminescence (PL). High-quality QDs superlattice can be achieved by higher growth temperature. Cross-sectional TEM shows the QDs in the successive layers are vertically aligned along growth direction. Interaction of partial vertically aligned columns leads to a perfect vertical ordering. With increasing number of bilayers, the average QDs size becomes larger in height and rapidly saturates at a certain value, while average lateral length nearly preserves initial size. This change leads to the formation of QDs homogeneous in size and of a particular shape. The observed self-organizations are attributed to the effect of strain distribution at QDs on the kinetic growth process. DCXRD measurement shows two sets of satellite peaks which corresponds to QDs superlattice and multi quantum wells formed by the wetting layers. Kinematical simulations of the wetting layers indicate that the formation of QDs is associated with a decrease of the effective indium content in the wetting layers. (C) 1999 Elsevier Science B.V. All rights reserved.

Using Fourier transform infrared grazing incidence reflectivity to study local vibrational modes in GaN

Both Fourier transform infrared (FTIR) grazing incidence reflectivity and FTIR transmission methods have been used to study GaN films grown on alpha-Al2O3 (0001) substrates by atmospheric pressure metal-organic chemical vapor deposition and low pressure metal-organic chemical vapor deposition. The results show that in the frequency range from 400 to 3500 cm(-1) the signal-to-noise ratio of the FTIR grazing incidence measurement is far higher than that of the FTIR transmission measurement. Some new vibrational structures appearing in the former measurement have been discussed. The features around 1460 and 1300 cm(-1) are tentatively assigned to scissoring and wagging local vibrational modes of CH2 in GaN, respectively. (C) 1999 American Institute of Physics. [S0021-8979(99)06509-3].

High-frequency hydrogen-related infrared modes in silicon grown in a hydrogen atmosphere

High-frequency vibrational modes have been observed at liquid-helium temperature in silicon samples grown in a H-2 or D-2 atmosphere. The highest-frequency ones are due to the overtones and combination modes of SiH fundamentals. Others are CH modes due to (C,H) complexes, but the simultaneous presence of NH modes due to (N,H) complexes cannot be excluded. The present results seem to show also the existence of centers including both SiH and CH or NH bonds. One sharp mode at 4349 cm-l is related to a weak SiH fundamental at 2210 cm(-1). The related center is ascribed to a vacancy fully decorated with hydrogen with a nearest-neighbor C atom. [S0163-1829(99)00911-X].

Neutron irradiation-infrared based measurement method for interstitial oxygen in heavily boron-doped silicon

A novel method, based on an infrared absorption and neutron irradiation technique, has been developed for the determination of interstitial oxygen in heavily boron-doped silicon. The new procedure utilizes fast neutron irradiated silicon wafer specimens. On fast neutron irradiation, the free carriers of high concentration in silicon can be trapped by the irradiated defects and the resistivity increased. The resulting calibration curve for the measurement of interstitial oxygen in boron-doped silicon has been established on the basis of the annealing behaviour of irradiated boron-doped CZ silicon.

Normally incident infrared absorption in vertically aligned InGaAs/GaAs quantum dot superlattice

30-period InGaAs/GaAs quantum dot superlattice was fabricated by MBE. Using cross sectional transmission electron microscopy, the InGaAs quantum dots were found to be perfectly vertically aligned in the growth direction (100). Under normally incident radiation, a distinct absorption in the 8.5 similar to 10.4 mu m range peaked at 9.9 mu m was observed. The normally incident infrared absorption in vertically aligned quantum dot superlattice in the 8 similar to 12 mu m range was realized for the first time. This result indicates the potential application of the quantum dot superlattice structure without grating as normally incident infrared detector focal plane arrays.

Normal-incidence intersubband (In, Ga)As/GaAs quantum dot infrared photodetectors

We report the device performance of normal-incidence (In, Ga)As/GaAs quantum dot intersubband infrared photodetectors. A primary intersubband transition peak is observed at the wavelength of 13 mu m (E-0 --> E-1) and a secondary peak at 11 mu m (E-0 --> E-2). The measured energy spacing in the conduction band of the quantum dots is in good agreement with low temperature photoluminescence measurement and calculations. A peak detectivity of 1 x 10(10) cm Hz(1/2)/W at 13 mu m was achieved at 40 K for these devices. (C) 1998 American Institute of Physics. [S0003-6951(98)01440-5].

Annealing behaviors of photoluminescence from SiOx : H

The strong photoluminescence (PL) of SiOx:H prepared by plasma enhanced chemical vapor deposition has been systematically studied in conjunction with infrared and micro-Raman spectra. We have found that each PL spectrum is comprised of two Gaussian components, a main band and a shoulder. The main band might originate from amorphous silicon clusters embedded in die SiOx network, and its redshift with annealing temperature is due to expansion of the silicon clusters. The shoulder remains at about 835 nm in spite of the annealing temperature and possibly comes from luminescent defect centers. The enhanced PL spectra after 1170 degrees C annealing are attributed to the quantum confinement effects of nanocrystalline silicon embedded in the SiO2 matrix. (C) 1998 American Institute of Physics.

Study of microstructure of high stability hydrogenated amorphous silicon films by Raman scattering and infrared absorption spectroscopy

The microstructure, hydrogen bonding configurations and hydrogen content of high quality and stable hydrogenated amorphous silicon (a-Si:H) films prepared by a simple ''uninterrupted growth/annealing" plasma enhanced chemical vapor deposition technique have been investigated by Raman scattering and infrared absorption spectroscopy. The high stability a-Si:H films contain small amounts of a microcrystalline phase and not less hydrogen (10-16 at. %), particularly, the clustered phase hydrogen, Besides, the hydrogen distribution is very inhomogeneous. Some of these results are substantially distinct from those of conventional device-quality n-Si:H film or stable cr-Si:H films prepared by the other techniques examined to date. The stability of n-Si:H films appears to have no direct correlation with the hydrogen content or the clustered phase hydrogen concentration. The ideal n-Si:H network with high stability and low defect density is perhaps not homogeneous. (C) 1998 American Institute of Physics.

Energy spectrum and persistent currents in finite-width mesoscopic ring with radial potential barrier threading a magnetic flux through its hole

The energy spectrum and the persistent currents are calculated for a finite-width mesoscopic annulus with radial potential barrier, threading a magnetic flux through the hole of the ring. Owing to the presence of tunneling barrier, the coupling effect leads to the splitting of each radial energy subband of individual concentrical rings into two one. Thus, total currents and currents carried by single high-lying eigenstate as a function of magnetic flux exhibit complicated patterns. However, periodicity and antisymmetry of current curves in the flux still preserve.

Low Temperature Deposited Nano-structured Vanadium Oxide Thin Films for Uncooled Infrared Detectors

A novel process of room temperature ion beam sputtering deposition of vanadium oxide films and low temperature post annealing for uncooled infrared detectors was proposed in this work. VOx thin films with relatively low square resistance (70 K Omega / square) and large temperature coefficient of resistance (more than 3%/K) at room temperature were fabricated using this low temperature process which was very compatible with the process of uncooled infrared detectors based on micromachined technology. Furthermore, chemical composition and film surface have been characterized using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) respectively. The results showed that the main composition of the processed thin films was V2O5 and the thin films were in the process of crystallization.

Experimental Demonstration on Structure-Parameter Dependence of Photonic Crystal Optical Spectrum

We present fabrication and experimental measurement of a series of photonic crystal waveguides and coupled structure of PC waveguide and PC micro-cavity. The complete devices consist of an injector taper down from 3 mu m into a triangular-lattice air-holes single-line-defect waveguide. We fabricated these devices on a silicon-on-insulator substrate and characterized them using tunable laser source. We've obtained high-efficiency light propagation and broad flat spectrum response of photonic-crystal waveguides. A sharp attenuation at photonic crystal waveguide mode edge was observed for most structures. The edge of guided band is shifted about 31 nm with the 10 nm increase of lattice constant. Mode resonance was observed in coupled structure. Our experimental results indicate that the optical spectra of photonic crystal are very sensitive to structure parameters.