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19 Oct 1992

Volume 61, Issue 16, pp. 1877-1988

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Hard x‐ray phase zone plate fabricated by lithographic techniques

B. Lai, W. B. Yun, D. Legnini, Y. Xiao, J. Chrzas, P. J. Viccaro, V. White, S. Bajikar, D. Denton, F. Cerrina, E. Di Fabrizio, M. Gentili, L. Grella, and M. Baciocchi

Appl. Phys. Lett. 61, 1877 (1992); http://dx.doi.org/10.1063/1.108400 (3 pages) | Cited 45 times

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A Fresnel phase zone plate with an unprecedented focusing efficiency of 33% was fabricated using an x‐ray lithographic technique and was tested using synchrotron x rays. Contributions by the zeroth‐order x ray to the focus were minimal. Spatial resolution in the micrometer range was achieved. The measured spot size was dominated by geometric demagnification of the source. It should be possible to obtain submicrometer resolution by aperturing the source. Experimental results of focusing efficiency measurements, intensity distribution at the focal plane, and spatial resolution tests are reported.
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42.79.Ci Filters, zone plates, and polarizers
07.85.-m X- and γ-ray instruments

Optical properties of the buried waveguide in a bistable InGaAsP semiconductor laser

I. Joindot

Appl. Phys. Lett. 61, 1880 (1992); http://dx.doi.org/10.1063/1.108401 (3 pages) | Cited 1 time

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In this letter, we first examine the power reflection coefficient at the interface between InGaAsP active and passive waveguides in a bistable laser and the optical losses of the passive waveguide. An original method is used for carrier density investigations: the spontaneous emission from the uncleaved sides of the active region is analyzed. With the experimental results and from a simple model we then calculate the carrier density difference and the reflection coefficient at the interface. After that we examine the ratio of the output power from each uncoated cleaved facet and deduce the spectrally resolved modal attenuation.
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78.66.Fd III-V semiconductors
78.66.Hf II-VI semiconductors
42.82.-m Integrated optics
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Linewidth broadening factor of a microcavity semiconductor laser

R. Jin, D. Boggavarapu, G. Khitrova, H. M. Gibbs, Y. Z. Hu, S. W. Koch, and N. Peyghambarian

Appl. Phys. Lett. 61, 1883 (1992); http://dx.doi.org/10.1063/1.108377 (3 pages) | Cited 17 times

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The emission linewidth above threshold is measured in a GaAs/AlGaAs microcavity surface‐emitting laser with a single cavity mode. The measured linewidth broadening factor is in good agreement with theoretical calculations that include the most important many‐body Coulomb effects of the electron‐hole plasma.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation

1.5 μm diode laser‐based nonlinear frequency conversions by using potassium titanyl phosphate

Weizhi Wang, Ken’ichi Nakagawa, Yasunori Toda, and Motoichi Ohtsu

Appl. Phys. Lett. 61, 1886 (1992); http://dx.doi.org/10.1063/1.108378 (3 pages) | Cited 3 times

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Both second harmonic and sum‐frequency generations by using 1.5 μm diode lasers in a type II phase matched potassium titanyl phosphate are demonstrated for the first time. A maximum second harmonic output power of 1.0 μW is obtained with the use of an external builtup cavity, and a 0.23 μW sum‐frequency output power at 0.54 μm is obtained by using 1.5 and 0.82 μm diode lasers.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.79.Nv Optical frequency converters
42.72.Bj Visible and ultraviolet sources

Reduction of the intensity noise from an erbium‐doped fiber laser to the standard quantum limit by intracavity spectral filtering

Steve Sanders, Namkyoo Park, Jay W. Dawson, and Kerry J. Vahala

Appl. Phys. Lett. 61, 1889 (1992); http://dx.doi.org/10.1063/1.108379 (3 pages) | Cited 11 times

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The high frequency intensity noise of a tandem fiber Fabry–Perot erbium‐doped fiber ring laser is reduced to the standard quantum limit, with a 0.5 dB experimental uncertainty. Noise reduction of ≳14 dB is achieved by intracavity spectral filtering of weak side modes using a narrow‐band fiber Fabry–Perot etalon.
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42.55.Wd Fiber lasers
42.60.Mi Dynamical laser instabilities; noisy laser behavior

Picosecond signal recovery in type II tunneling bi‐quantum well etalon

Atsushi Tackeuchi, Tsuguo Inata, Yoshiaki Nakata, Satoshi Nakamura, Yoshihiro Sugiyama, and Shunichi Muto

Appl. Phys. Lett. 61, 1892 (1992); http://dx.doi.org/10.1063/1.108380 (3 pages) | Cited 5 times

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We demonstrate picosecond signal recovery in all optical gate operation using a type II tunneling bi‐quantum well (TBQ) etalon. The type II TBQ consists of a series of GaAs wells, AlGaAs barriers, and AlAs layers. In this structure, photoexcited electrons in the GaAs wells escape by tunneling through the AlGaAs barriers toward X states in the AlAs layers. Therefore, the time for recovery from excitonic absorption bleaching in GaAs wells is controlled directly by the AlGaAs barrier thickness. The type II TBQ etalon with 1.7 nm barriers showed a fast signal recovery of 17 ps by carrier tunneling.
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42.65.Pc Optical bistability, multistability, and switching, including local field effects
42.79.Hp Optical processors, correlators, and modulators
78.66.Fd III-V semiconductors
78.66.Hf II-VI semiconductors
73.40.Gk Tunneling

Frequency stabilization, linewidth reduction, and fine detuning of a semiconductor laser by using velocity‐selective optical pumping of atomic resonance line

M. Kozuma, M. Kourogi, M. Ohtsu, and H. Hori

Appl. Phys. Lett. 61, 1895 (1992); http://dx.doi.org/10.1063/1.108381 (3 pages) | Cited 9 times

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A novel optical feedback technique for a semiconductor laser has been used successfully in stabilizing the laser frequency to a hyperfine transition frequency of the Rb87‐D2 line (F=2–3) and simultaneously reducing the laser field spectral linewidth 20‐times that of the free‐running laser. The locking range of this feedback was 200 MHz. The possibility of continuous detuning of the stabilized frequency as large as ±γ/2 (half of the natural linewidth of the atomic resonance) was demonstrated.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.62.Fi Laser spectroscopy

Switchable bicolor infrared detector using an electron transfer infrared modulator

V. Berger, N. Vodjdani, P. Bois, B. Vinter, and S. Delaitre

Appl. Phys. Lett. 61, 1898 (1992); http://dx.doi.org/10.1063/1.108356 (3 pages) | Cited 14 times

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In this study, we have realized a bicolor switchable detector, in which tunneling between two quantum well detectors of different widths is used to populate or deplete their ground level. The behavior of the bicolor detector is simulated using a self‐consistent model, which shows that under electric field, only the downstream well detects. The performance of the sample as a modulator and as a bicolor detector are analyzed in a Fourier transform infrared spectrometer.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
42.79.Hp Optical processors, correlators, and modulators
73.40.Gk Tunneling

Third‐order optical nonlinearities of a processable main chain polymer with symmetrically substituted tris‐azo dyes

Takashi Kurihara, Satoru Tomaru, Yuhei Mori, Makoto Hikita, and Toshikuni Kaino

Appl. Phys. Lett. 61, 1901 (1992); http://dx.doi.org/10.1063/1.108357 (3 pages) | Cited 11 times

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A new third‐order nonlinear optical (NLO) polymeric material for all‐optical waveguide devices is designed, which is a main chain polymer with symmetrically donor‐substituted NLO units as part of the polymer backbone. On the basis of the molecular design, the polymer has large third‐order NLO susceptibility, χ(3), potentially low propagation loss and high processability for channel waveguide fabrication. The magnitude of χ(3)(−3ω;ω,ω,ω) for the polymer film was 1.5×10−10 esu at the three‐photon resonant wavelength. This is the largest value for highly processable polymers.
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42.70.Jk Polymers and organics
42.65.An Optical susceptibility, hyperpolarizability
78.66.Qn Polymers; organic compounds

Observation of polarization independent electric field effect in InGaAs/InP tensile strained quantum well and its proposal for optical switch

K. G. Ravikumar, T. Aizawa, S. Suzaki, and R. Yamauchi

Appl. Phys. Lett. 61, 1904 (1992); http://dx.doi.org/10.1063/1.108358 (3 pages) | Cited 3 times

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We have experimentally observed the polarization independent electric field induced absorption coefficient variation in an InGaAs/InP tensile strained quantum well (QW) structure for the first time. We have also found that the field induced absorption coefficient/refractive index variation of strained QW for the transverse electric (TE) mode does not change appreciably from that of an unstrained one. Based on these results, we propose tensile strained QW for polarization independent optical switch/modulator.
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78.66.Fd III-V semiconductors
78.66.Hf II-VI semiconductors
78.20.Jq Electro-optical effects
42.65.Pc Optical bistability, multistability, and switching, including local field effects

Evidence for state filling effect on high speed modulation dynamics of quantum well lasers

B. Zhao, T. R. Chen, Y. Yamada, Y. H. Zhuang, N. Kuze, and A. Yariv

Appl. Phys. Lett. 61, 1907 (1992); http://dx.doi.org/10.1063/1.108359 (3 pages) | Cited 5 times

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By using different separate confinement structures for otherwise identical quantum well lasers, we find experimentally that the differential gain can vary over a factor of three, which significantly affects the high speed modulation bandwidth in such lasers. These observations are in agreement with the predictions of a theory of modulation dynamics which accounts for the carrier population in the optical confining region of the separate confinement structure (state filling effect) in quantum well lasers.
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42.55.Px Semiconductor lasers; laser diodes
42.55.Rz Doped-insulator lasers and other solid state lasers

High reflectivity broadband self‐pumped phase conjugator in near‐infrared wavelengths

Y. H. Shih, X. H. He, and H. Y. Zhang

Appl. Phys. Lett. 61, 1910 (1992); http://dx.doi.org/10.1063/1.108360 (2 pages) | Cited 1 time

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A high reflectivity broadband BaTiO3 self‐pumped phase conjugator with external ring cavity in near‐infrared wavelengths (706–850 nm) is reported. Phase conjugate reflectivity as high as 77.5% was observed at 756 nm. The losses and the reflection grating are discussed in detail.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.79.Fm Reflectors, beam splitters, and deflectors

Reactive ion etching end‐point determination by plasma impedance monitoring

V. Patel, B. Singh, and J. H. Thomas

Appl. Phys. Lett. 61, 1912 (1992); http://dx.doi.org/10.1063/1.108361 (3 pages) | Cited 8 times

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Impedance monitoring of a rf glow discharge is successfully used to determine the end point of reactive ion etching. The feasibility of using this technique is demonstrated on diode, triode, and magnetic multipole enhanced triode reactors applied to etching polycrystalline silicon on SiO2 on Si and Si3N4 on Si in an SF6 plasma, and to stripping photoresist on Al on Si in an O2 plasma. The end point of etching is determined by monitoring changes in the induced dc bias, the rf voltage, the rf current, or the phase relationship between them on either of the powered electrodes (in the cases of the triode configurations). This method is easily applicable as in in‐process end‐point detector and eliminates the need of optical access to either the plasma or the substrate.
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52.70.Ds Electric and magnetic measurements
52.80.Hc Glow; corona
81.65.-b Surface treatments
85.40.Hp Lithography, masks and pattern transfer

Photostructural defect states and residual potential in chlorine and arsenic doped amorphous selenium films

Suresh Chand, G. D. Sharma, R. C. Batheja, and Subhas Chandra

Appl. Phys. Lett. 61, 1915 (1992); http://dx.doi.org/10.1063/1.108362 (3 pages) | Cited 7 times

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The mechanism of photostructural defect states and buildup of residual potential in pure, chlorine doped, and combinationally doped, i.e., chlorine and arsenic doped amorphous selenium films (∼100 μm) has been studied using the thermally stimulated discharge current technique. Combinationally doped films show minimum buildup of residual potential even though the photostructural defect states in them lie between the pure and the chlorine doped amorphous selenium films. These results have been explained in terms of the conductivity enhancement role of chlorine and the complimentary role of arsenic in sustaining the acceptance potential in the doped films.
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61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
71.55.Ht Other nonmetals
61.72.up Other materials
73.61.Cw Elemental semiconductors
73.61.Jc Amorphous semiconductors; glasses
73.61.Le Other inorganic semiconductors

Suppressing of island formation in surfactant‐controlled solid phase epitaxy of germanium on Si(100)

H. J. Osten, E. Bugiel, and J. Klatt

Appl. Phys. Lett. 61, 1918 (1992); http://dx.doi.org/10.1063/1.108363 (3 pages) | Cited 19 times

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Smooth epitaxial 10 and 30 nm germanium layers have been grown on Si(100) by surfactant‐controlled solid phase epitaxy. The layers were characterized by reflection high energy electron diffraction, transmission electron microscopy, and x‐ray photoelectron spectroscopy. By depositing one monolayer antimony on top of the amorphous germanium layer it was possible to crystallize the germanium directly into a smooth epitaxial structure without any island formation. The obtained low‐defect layers are relaxed.
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81.15.Np Solid phase epitaxy; growth from solid phases
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Layer‐by‐layer growth of epitaxial SnO2 on sapphire by reactive sputter deposition

R. E. Cavicchi, S. Semancik, M. D. Antonik, and R. J. Lad

Appl. Phys. Lett. 61, 1921 (1992); http://dx.doi.org/10.1063/1.108364 (3 pages) | Cited 19 times

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Epitaxial films of stoichiometric tin oxide were grown on sapphire (1102) substrates by reactive sputter deposition. X‐ray diffraction showed the films to have a single (101) orientation. Lateral registry of film growth with respect to the substrate lattice was demonstrated by low energy electron diffraction. Atomic force microscopy was used to examine surface morphology and roughness. The films are extremely flat, having a rms roughness of 3 Å over a 4×4 μm2 area. Atomic steps, observed on the sapphire substrate and attributable to a 0.24° miscut, were also observed on the surface of a 400 Å film. The results indicate that the film grew via a layer‐by‐layer growth mechanism which was controlled by diffusion of the adatoms to the step edges.
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81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.15.Cd Deposition by sputtering
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Comparison between theory and experiment for InGaAs/InP self‐electro‐optic effect devices

P. J. Mares and S. L. Chuang

Appl. Phys. Lett. 61, 1924 (1992); http://dx.doi.org/10.1063/1.108365 (3 pages) | Cited 2 times

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Using the linewidth as the only fitted parameter, we present a detailed comparison between a theoretical model and a consistent set of experimental data for (i) the electronic properties, (ii) the optical properties, and (iii) the input power versus output power characteristics of In0.53Ga0.47As/InP self‐electro‐optic effect devices (SEEDs). The model includes excitonic effects, the dark current, and uses the depletion approximation to relate the applied bias to the electric field and to approximate the internal quantum efficiency. We show that the model is able to emulate and predict the behavior of quantum well modulators and SEEDs very well.
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78.66.Fd III-V semiconductors
78.66.Hf II-VI semiconductors
85.30.De Semiconductor-device characterization, design, and modeling
42.65.Pc Optical bistability, multistability, and switching, including local field effects
42.79.Hp Optical processors, correlators, and modulators

Phonon mode study of near‐lattice‐matched InxGa1−xAs using micro‐Raman spectroscopy

J. P. Estrera, P. D. Stevens, R. Glosser, W. M. Duncan, Y. C. Kao, H. Y. Liu, and E. A. Beam

Appl. Phys. Lett. 61, 1927 (1992); http://dx.doi.org/10.1063/1.108366 (3 pages) | Cited 33 times

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We identify the four allowable phonon modes in InxGa1−xAs on InP:InAs‐like transverse optical (TO) (225±2 cm−1), InAs‐like longitudinal optical (LO) (233±1 cm−1), GaAs‐like TO (255±2 cm−1), and GaAs‐like LO (269±1 cm−1), using the selectivity of first‐order Raman scattering off the (100) normal surface and the (011) cleaved plane and detailed line‐shape analysis employing a sequential simplex optimization procedure. Raman scattering off the (011) cleaved plane was achieved for the first time in thin‐film InGaAs using microprobing capabilities (∼1 μm). We also identify another phonon mode R∗ at 244 cm−1 which is attributed to an alloy disorder mode in these films. For the five identified phonon modes, a linear relationship between the Raman frequencies and composition determined from x‐ray diffraction was determined for near‐lattice‐matched conditions (0.42<1−x<0.52).
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78.30.Fs III-V and II-VI semiconductors
63.20.D- Phonon states and bands, normal modes, and phonon dispersion

Effects of annealing on the surface morphology of decapped GaAs(001)

Y.‐N. Yang, Y. S. Luo, J. H. Weaver, L. T. Florez, and C. J. Palmstrøm

Appl. Phys. Lett. 61, 1930 (1992); http://dx.doi.org/10.1063/1.108367 (3 pages) | Cited 9 times

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We have used STM and LEED to investigate As‐rich (2×4)/c(2×8) reconstructed GaAs(001) surfaces produced under different decapping conditions. The STM images show that heating to 450 °C for 10 min produces a surface that contains a high density of small islands. Heating to 450 °C for longer times allows the larger islands to grow at the expense of the small islands, but the corresponding LEED patterns do not exhibit any obvious changes. Analogous studies at 420 °C gave similar results, confirming that the observed regrowth of GaAs(001) is general to all decapping processes.
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68.35.B- Structure of clean surfaces (and surface reconstruction)
07.79.Cz Scanning tunneling microscopes
61.05.-a Techniques for structure determination
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Comparison between ruthenium‐based and other ohmic contact systems to p‐type GaAs

W. O. Barnard, G. Myburg, and F. D. Auret

Appl. Phys. Lett. 61, 1933 (1992); http://dx.doi.org/10.1063/1.108493 (3 pages) | Cited 3 times

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A conventional furnace annealed Ru/Au ohmic contact system on p‐GaAs has been investigated. Electrical and morphological characteristics of this contact system were compared with other systems such as Cr/Au, Ti/Pt, and Mn/Au. The Ru/Au contact system has been shown to have superior surface morphology and a comparable specific contact resistance value, even after annealing at 485 °C. The advantages of utilizing Ru as contact material to GaAs are that it forms high quality, thermally stable Schottky contacts to n‐GaAs and thermally stable ohmic contacts with low specific contact resistance to p‐GaAs. This dual nature of Ru contacts to GaAs makes them extremely important for future use in devices such as heterojunction bipolar transistors (HBTs) and solid state lasers.
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85.30.Pq Bipolar transistors
73.40.Ns Metal-nonmetal contacts
73.40.Cg Contact resistance, contact potential
68.35.B- Structure of clean surfaces (and surface reconstruction)

Feature size effects on selective area epitaxy of InGaAs

M. A. Cotta, L. R. Harriott, Y. L. Wang, R. A. Hamm, H. H. Wade, J. S. Weiner, D. Ritter, and H. Temkin

Appl. Phys. Lett. 61, 1936 (1992); http://dx.doi.org/10.1063/1.108368 (3 pages) | Cited 7 times

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We demonstrate the use of an ultrathin (≊5 nm) Si layer deposited on InP substrates as a mask in selective area epitaxy of InGaAs by metalorganic molecular beam epitaxy. Patterns of varying shapes and sizes, from 5 to 0.5 μm, were written on the mask by focused Ga ion beam and etched by Cl2. The growth rate of InGaAs was studied by scanning force microscopy using stripes with guard rings spaced as close as 0.5 μm from the stripes. A small increase in the growth rate was detected only when the feature size was lower than 5 μm, and the growth rate was not affected by the presence of the guard rings. This shows that precursor material is being transferred from the slow growing {111}‐planes to the (100)‐plane, and that migration of species from the Si mask to the growing areas is negligible.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
68.55.-a Thin film structure and morphology

Depth and radial profiles of defects in Czochralski‐grown silicon

S. C. Sharma, R. C. Hyer, N. Hozhabri, M. F. Pas, and S. Kim

Appl. Phys. Lett. 61, 1939 (1992); http://dx.doi.org/10.1063/1.108369 (3 pages) | Cited 3 times

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We have studied the depth and spatial profiles of vacancies in Czochralski‐grown silicon wafers by positron annihilation spectroscopy. By using a variable energy positron beam and γ‐ray spectroscopy, we have obtained depth profiles of defects in as‐grown, annealed, and 〈100〉 epitaxial Si wafers. We discuss these results in terms of vacancies and oxygen precipitates. The bulk position lifetime measurements, made as a function of axial displacement of a positron source, resolve vacancies, and divacancies in the wafer.
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61.72.Bb Theories and models of crystal defects
61.72.jd Vacancies
61.72.jj Interstitials
78.70.Bj Positron annihilation

Double modulation‐doped AlGaAs/InGaAs heterostructure with a graded composition in the quantum well

Tae‐Kyung Yoo, Pierre Mandeville, Hyunchang Park, William J. Schaff, and Lester F. Eastman

Appl. Phys. Lett. 61, 1942 (1992); http://dx.doi.org/10.1063/1.108370 (3 pages) | Cited 6 times

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A new double modulation‐doped AlGaAs/InGaAs heterostructure with a graded composition in the InGaAs quantum well (QW) has been designed and electrically characterized. An InGaAs QW with a rectangular‐like potential profile in the presence of the two dimensional electron gas is obtained by a two‐step grading of the In composition, which results in a broad and symmetric electron distribution profile even under various voltages. The Hall measurement shows a very high electron mobility of 7230 cm2/V s and an electron sheet density of 4.1×1012/cm2 at room temperature. To our knowledge, this is the highest mobility ever reported so far for the double modulation‐doped Al0.3Ga0.7As/In0.2Ga0.8As field‐effect transistor structure.
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73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.50.Dn Low-field transport and mobility; piezoresistance
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
85.30.Tv Field effect devices

Annealing effect on the carrier concentration in heavily Si‐doped n+‐InGaAs

Noriyuki Watanabe, Takumi Nittono, and Kazuo Watanabe

Appl. Phys. Lett. 61, 1945 (1992); http://dx.doi.org/10.1063/1.108371 (3 pages) | Cited 9 times

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Carrier concentration in very heavily silicon‐doped (about 2.7×1019 cm−3) n+‐In0.52Ga0.48As epilayer grown on GaAs substrate by metalorganic chemical vapor deposition is decreased by post‐growth annealing in the temperature range of 500–800 °C but keeps a value higher than 1.4×1019 cm−3. This value is fairly higher than the reported value for heavily Si‐doped GaAs after annealing. It is suggested that the decrease of carrier concentration is not caused by the formation of Si‐Si pairs or SiAs acceptors but caused by silicon atom movement from Ga‐ or In‐substitutional sites to interstitial sites at temperatures up to 600 °C and silicon atom outdiffusion at higher temperatures around 800 °C. In less doped (about 5.2×1018 and 1.6×1019 cm−3) samples, a smaller carrier or no decrease is detected after annealing.
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61.72.U- Doping and impurity implantation
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
73.61.Ey III-V semiconductors
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths

Electronic structure and optical properties of silicon crystallites: Application to porous silicon

J. P. Proot, C. Delerue, and G. Allan

Appl. Phys. Lett. 61, 1948 (1992); http://dx.doi.org/10.1063/1.108372 (3 pages) | Cited 272 times

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We have calculated the electronic structure of spherical silicon crystallites containing up to 2058 Si atoms. We predict a variation of the optical band gap with respect to the size of the crystallites in very good agreement with available experimental results. We also calculate the electron‐hole recombination time which is of the order of 10−4–10−6 s for crystallites with diameters of 2.0–3.0 nm. We conclude that small silicon crystallites can have interesting optical properties in the visible range. These results are applied to porous silicon for which we confirm that a possible origin of the luminescence is the quantum confinement.
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73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
78.55.Hx Other solid inorganic materials
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