SHORT PERIOD InAs/GaSb SUPERLATTICE INFRARED DETECTOR ON GaAs SUBSTRATES

Two type II superlattices (SLs) InAs(2ML)/GaSb(8ML) and InAs(8ML)/GaSb(8ML) were grown on GaAs substrates by molecular-beam epitaxy. High resolution X-ray diffraction showed the periods of the two SLs were 31.2 angstrom and 57.3 angstrom, respectively. Room-temperature optical transmittance spectra showed that there were clear absorption edges at 2.1 mu m and 5 mu m for the two SLs. The SWIR and MWIR photoconductor devices were fabricated by standard lithography and etched by tartaric acid solution. The spectral response and blackbody tests were carried out at low and room temperatues. The results show that the 50% cutoff wavelengths of the two photoconductors are 2.1 mu m and 5.0 mu m respectively and D-bb* is above 2 x 10(8) cmHz(1/2)/W for two kinds of photoconductors at 77K. D-bb* is above 10(8) cmHz(1/2)/W for SWIR photoconductor at room temperature.

GaAs Based InAs/GaSb Superlattice Short Wavelength Infrared Detectors Grown by Molecular Beam Epitaxy

InAs/GaSb superlattice (SL) short wavelength infrared photoconduction detectors are grown by molecular beam epitaxy on GaAs(001) semi-insulating substrates. An interfacial misfit mode AlSb quantum dot layer and a thick GaSb layer are grown as buffer layers. The detectors containing a 200-period 2ML/8ML InAs/GaSb SL active layer are fabricated with a pixel area of 800 x 800 mu m(2) without using passivation or antireflection coatings. Corresponding to the 50% cutoff wavelengths of 2.05 mu m at 77K and 2.25 mu m at 300 K, the peak detectivities of the detectors are 4 x 10(9) cm.Hz(1/2)/W at 77K and 2 x 10(8) cm.Hz(1/2)/W at 300 K, respectively.

Luminescence properties of self-assembled InAs/GaAs quantum dots covered by InAlAs and InGaAs combination strain-reducing layer

We report the photoluminescence (PL) and structural properties of self-assembled InAs/GaAs quantum dots (QDs) covered by In0.2Al0.8As and In0.2Ga0.8As combination strain-reducing layer (SRL). By introducing a thin InAlAs layer, the ground state emission wavelength redshifts, and the energy splitting between the ground and first-excited states increases to 85 meV at 10 K. The energy splitting further increases to 92 meV and the temperature dependence of full width at half maximum (FWHM) changes for QDs with different SRL after the multi-stacking. These results are attributed to the fact that the combination layer has different effects on QDs compared to the InGaAs SRL.

Temperature-induced switching-over of the luminescence transitions in GaInNAs/GaAs quantum wells

Photoluminescence (PL) spectra of the GaInNAs/GaAs single quantum well (SQW) with different N compositions are carefully studied in a range of temperatures and excitation power densities. The anomalous S-shape temperature dependence of the PL peak is analysed based on the competition and switching-over between the peaks related to N-induced localized states and the peak related to interband excitonic recombination. It is found that with increasing N composition, the localized energy increases and the turning point of the S-shape temperature dependence occurs at higher temperature, where the localized carriers in the bandtail states obtain enough thermal activation energy to be dissociated and delocalized. The rapid thermal annealing (RTA) effectively reduces the localized energy and causes a decrease of the switching-over temperature.

Luminescence emission originating from nitrogen doping of beta-Ga2O3 nanowires

Nitrogen-doped beta-Ga2O3 nanowires (GaO NWs) were prepared by annealing the as-grown nanowires in an ammonia atmosphere. The optical properties of the nitrogen-doped GaO NWs were studied by measurements of the photoluminescence and phosphorescence decay at the temperature range between 10 and 300 K. The experimental results revealed that nitrogen doping in GaO NWs induced a novel intensive red-light emission around 1.67 eV, with a characteristic decay time around 136 mus at 77 K, much shorter than that of the blue emission (a decay time of 457 mus). The time decay and temperature-dependent luminescence spectra were calculated theoretically based on a donor-acceptor pair model, which is in excellent agreement with the experimental data. This result suggests that the observed novel red-light emission originates from the recombination of an electron trapped on a donor due to oxygen vacancies and a hole trapped on an acceptor due to nitrogen doping.

Luminescence temperature and pressure studies of Zn2SiO4 phosphors doped with Mn2+ and Eu3+ ions

Zn2SiO4:Mn2+, Zn2SiO4:Eu3+ and Zn2SiO4:Mn2+ Eu3+ phosphors were prepared by a sol-gel process and their luminescence spectra were investigated. The emission bands from intra-ion transitions of Mn2+ and Eu3+ samples were studied as a function of pressure. The pressure coefficient of Mn2+ emission was found to be -25.3 +/- 0.5 and -28.5 +/- 0.9 meV/GPa for Zn2SiO4:Mn2+ and Zn2SiO4:Mn2+ Eu3+, respectively. The Eu3+ emission shows only weak pressure dependence. The pressure dependences of the Mn2+ and Eu3+ emissions in Zn2SiO4:Mn2+ Eu3+ are slightly different from that in Zn2SiO4:Mn2+ and Zn2SiO4:Eu3+ samples, which can be attributed to the co-doping of Mn2+ and Eu3+ ions. The Mn2+ emission in the two samples, however, exhibits analogous temperature dependence and similar luminescence lifetimes, indicating no energy transfer from Mn2+ to Eu3+ occurs. (c) 2005 Elsevier B.V. All rights reserved.

Thermal quenching of luminescence from buried and surface InGaAs self-assembled quantum dots with high sheet density

Variable-temperature photoluminescence (PL) spectra of Si-doped self-assembled InGaAs quantum dots (QDs) with and without GaAs cap layers were measured. Narrow and strong emission peak at 1075 nm and broad and weak peak at 1310 nm were observed for the buried and surface QDs at low temperature, respectively. As large as 210 meV redshift of the PL peak of the surface QDs with respect to that of the buried QDs is mainly due to the change of the strain around QDs before and after growth of the GaAs cap layer. Using the developed localized-state luminescence model, we quantitatively calculate the temperature dependence of PL peaks and integrated intensities of the two samples. The results reveal that there exists a large difference in microscopic mechanisms of PL thermal quenching between two samples. (c) 2005 American Institute of Physics.

Synthesis and luminescence of ZnMgS : Mn2+ nanoparticles

Efficient green emission from ZnMgS:Mn2+ nanoparticles prepared by co-doping Mg2+ and Mn2+ ions into ZnS lattices has been observed. The synthesis is carried out in aqueous solution, followed by a post-annealing process, thus showing the features of less complexity, low cost, and easy incorporation of dopants. In comparison with the emission of ZnS:Mn2+ nanoparticles, which is located generally around 590 nm, the photoluminescence of ZnMgS:Mn2+ nanoparticles is blue-shifted by 14 nm in wavelength, leading to the enhanced green emission. The X-ray diffraction, electron spin resonance, and pressure dependent photoluminescence measurements suggest that the change of the crystal field caused by Mg2+ ionic doping and the lower symmetry in the nanoparticles may account for the blue-shift of the photoluminescence. The ZnMgS:Mn2+ nanoparticles with 1% Mn2+ doping exhibit the strongest luminescence, which could potentially meet the requirements for the construction of green light emitting diodes.

Investigations on optical properties of AlGaInN epilayers grown by mocvd

Microscopic luminescence and Raman scattering study was carried on AIInGaN quaternary alloy. Based on the analyses of SEM image and cathodoluminescence spectra measured around V-defects, the correlation between V-defect formation and indium segregation was clarified. Raman scattering of thin AlInGaN epilayers was investigated by using the short wavelength excitation of 325nm laser line. The frequency shift of A(1)(LO) phonon induced by the change of Al composition in alloy was observed. The Raman scattering of LO phonons was found to be resonantly enhanced with outgoing resonance, and it is attributed to the cascade-like electron-multiphonon interaction mechanism.

Molecular beam epitaxy growth and photoluminescence study of room temperature 1.31 mu m (InyGa1-yAs/GaAs1-x Sb-x)/ GaAs bilayer quantum wells

Molecular beam epitaxy (MBE) growth of (InyGa1-yAs/GaAs1-xSbx)/GaAs bilayer quantum well (BQW) structures has been investigated. It is evidenced by photo luminescence (PL) that a strong blue shift of the PL peak energy of 47 meV with increasing PL excitation power from 0.63 to 20 mW was observed, indicating type II band alignment of the BQW. The emission wavelength at room temperature from (InyGa1-yAs/GaAs1-xSbx)/GaAs BQW is longer (above 1.2 μ m) than that from InGaAs/GaAs and GaAsSb/GaAs SQW structures (1.1 μ m range), while the emission efficiency from the BQW structures is comparable to that of the SQW. Through optimizing growth conditions, we have obtained room temperature 1.31 μ m wavelength emission from the (InyGa1-yAs/GaAs1-xSbx)/GaAs BQW. Our results have proved experimentally that the GaAs-based bilayer (InyGa1-yAs/GaAs1-xSbx)/GaAs quantum well is a useful structure for the fabrication of near-infrared wavelength optoelectronic devices. © 2005 Elsevier B.V. All rights reserved.

Luminescence properties of multi-layer InGaN quantum dots grown on C- and R-plane sapphire substrates

In this study, we report comparative luminescence properties of multi-layer InGaN quantum dots grown on C- and R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). High-density InGaN quantum dots (QDs) are formed on GaN templates by decreasing the growth temperature and increasing the adatom hopping-barrier through surface passivation. Atomic force microscopy (AFM) has been employed to estimate the size and height of these dots. Photoluminescence (PL) spectra recorded from (1120) InGaN QDs/(1102) sapphire show much stronger emission intensity compared to spectra recorded from (0001) InGaN QDs/(0001) sapphire. Due to the absence of strong spontaneous polarization and piezoelectric field, such (1150) InGaN QDs in the active layers would lead to high efficiency light emitting devices. (c) 2005 Elsevier B.V. All rights reserved.

Origin of the blueshift in the infrared absorbance of intersubband transitions in AlxGa1-x/GaN multiple quantum wells

A self-consistent calculation of the subband energy levels of n-doped quantum wells is studied. A comparison is made between theoretical results and experimental data. In order to account for the deviations between them, the ground-state electron-electron exchange interactions, the ground-state direct Coulomb interactions, the depolarization effect, and the exciton-like effect are considered in the simulations. The agreement between theory and experiment is greatly improved when all these aspects are taken into account. The ground-to-excited-state energy difference increases by 8 meV from its self-consistent value if one considers the depolarization effect and the exciton-like effect only. It appears that the electron-electron exchange interactions account for most of the observed residual blueshift for the infrared intersubband absorbance in AlxGa1-xN/GaN multiple quantum wells. It seems that electrons on the surface of the k-space Fermi gas make the main contribution to the electron-electron exchange interactions, while for electrons further inside the Fermi gas it is difficult to exchange their positions. (C) 2004 Elsevier B.V. All rights reserved.

Room-temperature spin-oriented photocurrent under near-infrared irradiation and comparison of optical means with Shubnikov de-Haas measurements in AlXGa1-XN/GaN heterostructures

The admixture of linear and circular photogalvanic effects and (CPGEs) in AlxGa1-xN/GaN heterostructures has been investigated quantitatively by near-infrared irradiation at room temperature. The spin-based photocurrent that the authors have observed solidly indicates the sizable spin-orbital interaction of the two-dimensional electron gas in the heterostructures. Further analysis shows consistency between studies by optical and magnetic (Shubnikov de-Haas) measurements on the spin-orbital coupling effects among different AlxGa1-xN/GaN heterostructures, indicating that the CPGE measurement is a good way to investigate the spin splitting and the spin polarization in semiconductors. (C) 2007 American Institute of Physics.

A photonic crystal waveguide with silicon on insulator in the near-infrared band

A two-dimensional (2D) photonic crystal waveguide in the Gamma-K direction with triangular lattice on a silicon-on insulator (SOI) substrate in the near-infrared band is fabricated by the combination of electron beam lithography and inductively coupled plasma etching. Its transmission characteristics are analysed from the stimulated band diagram by the effective index and the 2D plane wave expansion (PWE) methods. In the experiment, the transmission band edge in a longer wavelength of the photonic crystal waveguide is about 1590 nm, which is in good qualitative agreement with the simulated value. However, there is a disagreement between the experimental and the simulated results when the wavelength ranges from 1607 to 1630 nm, which can be considered as due to the unpolarized source used in the transmission measurement.

Enhanced infrared absorption of spatially ordered quantum dot arrays

We have investigated the intersubband absorption for spatially ordered and non-ordered quantum dots (QDs). It is found that the intersubband absorption of spatially ordered QDs is much stronger than that of non-ordered QDs. The enhanced absorption is attributed to the improved size uniformity concurrent with the spatial ordering for the growth condition employed. For the FTIR measurement under normal incidence geometry, using a undoped sample as reference can remove the interference effect due to multiple reflections. (c) 2006 Elsevier B.V. All rights reserved.