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15 Jun 1993

Volume 73, Issue 12, pp. R1-8664

Page 1 of 4 Pages Next Page | Jump to Page

Frequency control of semiconductor lasers

M. Ohtsu, K. Nakagawa, M. Kourogi, and W. Wang

J. Appl. Phys. 73, R1 (1993); http://dx.doi.org/10.1063/1.353354 (17 pages) | Cited 3 times

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This article reviews our recent works on frequency control of semiconductor lasers. The magnitudes of quantum noise limited frequency modulation (FM) noise, realized by the negative electrical feedback, are given for four methods of using an external Fabry–Perot cavity as a frequency demodulator. It is shown that the theoretical expression for the quantum noise‐limited FM noise of the feedback laser contains a factor of 1/8 as compared with that of the free running laser, which is due to the different ways of injecting the vacuum fluctuations to the laser cavity and to the external Fabry–Perot cavity for negative electrical feedback. The FM sideband technique is shown to be an effective method to reject the contribution of laser power fluctuations to the FM noise detection for the negative electrical feedback system. As a candidate for a high reflectivity and frequency selective external reflector for the optical feedback, characteristics of the semiconductor laser as a phase conjugate mirror, i.e., the characteristics of the nearly degenerate four‐wave mixing and the nondegenerate four‐wave mixing in a semiconductor laser, are shown. Optical feedback by using a velocity selective optical pumping and polarization spectroscopy of an atomic vapor is proposed as an effective method to realize simultaneously the center frequency stabilization and linewidth reduction of the field spectrum of the laser, and also the fine detuning of the stabilized center frequency.
For the heterodyne frequency locking between two lasers, a spectroscopic method of using a Doppler‐free spectrum of the three‐level atomic vapor, obtained by using the phenomenon of coherent population trapping, is shown. In order to realize a highly efficient nonlinear optical frequency conversion for wideband frequency sweep of semiconductor lasers, a method of adding the output powers of several lasers, i.e., the coherent addition, is presented. After emphasizing that the wideband frequency sweep (covering from the near‐infrared to the visible region) can be realized by using the techniques of nonlinear optical frequency conversions and the optical phase locking, relevant experimental results of nonlinear optical frequency conversions are presented which are the second harmonics generation, sum and difference frequency conversions. A highly accurate optical frequency measurement system is proposed using an optical frequency comb generator with a modulation sidebands up to several THz. Performances of the optical frequency comb generator used for this system are presented. As a candidate for an ultrafast and wavelength insensitive photodetector for optical frequency counting, nonbolometric optical response characteristics of a high transition temperature (Tc) oxide superconducting film are demonstrated.  
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42.55.Px Semiconductor lasers; laser diodes
42.60.Fc Modulation, tuning, and mode locking

Depth profiling of the Ge concentration in SiGe alloys using in situ ellipsometry during reactive‐ion etching

G. M. W. Kroesen, G. S. Oehrlein, E. de Frésart, and M. Haverlag

J. Appl. Phys. 73, 8017 (1993); http://dx.doi.org/10.1063/1.353916 (10 pages) | Cited 10 times

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The complex refractive index of SiGe alloys at 632.8 nm has been measured as a function of the Ge concentration using in situ ellipsometry while the material is slowly removed from a silicon substrate using reactive‐ion etching (RIE). Homogeneous, strained epitaxial SiGe films on silicon substrates were used. The Ge concentration was obtained by Rutherford backscattering. If an unknown SiGe structure is etched with RIE, in situ ellipsometry yields combinations of the ellipsometric angles Ψ and Δ with time. Starting at the Si substrate, these points are, on a point‐to‐point basis, converted into combinations of complex refractive index and depth in a numerical procedure. For this inversion of the ellipsometry equations, the known relation between the real and the imaginary part of the refractive index of SiGe is used. Finally the refractive indices are converted into Ge concentrations. Thus the depth profile of the Ge concentration in an unknown epitaxial SiGe structure can be inferred from an in situ ellipsometric measurement during RIE of the unknown structure. The obtained resolutions in depth and Ge concentration are 0.3 nm and 0.3%, respectively.
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07.60.Fs Polarimeters and ellipsometers
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
68.55.Nq Composition and phase identification
81.65.-b Surface treatments

Role of cladding layer thicknesses on strained‐layer InGaAs/GaAs single and multiple quantum well lasers

D. C. Liu, C. P. Lee, C. M. Tsai, T. F. Lei, J. S. Tsang, W. H. Chiang, and Y. K. Tu

J. Appl. Phys. 73, 8027 (1993); http://dx.doi.org/10.1063/1.353917 (8 pages) | Cited 10 times

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The influences of cladding layer thicknesses on the performance of strained‐layer InGaAs/GaAs graded‐index separated confinement heterostructure quantum well lasers have been studied. The waveguiding property of the laser structure was analyzed using the transfer matrix method. In this work, experimental results and the calculated results showed that threshold current densities and external quantum efficiencies both were crucially dependent on the thicknesses of cladding layer for both single and multiple quantum well lasers. The minimum cladding layer thicknesses needed to maintain low threshold current densities and low internal total loss for both single and multiple quantum well devices were determined experimentally and theoretically.
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42.55.Px Semiconductor lasers; laser diodes
78.66.Fd III-V semiconductors

Line shape analysis of higher order nonlinear processes in time‐resolved degenerate four‐wave mixing in GaAs

Song Wu

J. Appl. Phys. 73, 8035 (1993); http://dx.doi.org/10.1063/1.353918 (6 pages) | Cited 2 times

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We theoretically calculate the line shapes of the third order, the cascaded third order, and the fifth order nonlinear processes in a femtosecond time‐resolved four‐wave mixing experiment. On the basis of the calculations, experimentally we distinguish the wavelength dependent contributions to the direct fifth order process and the cascaded third order process in bulk GaAs. Our results show that the cascaded process dominates at wavelengths near band gap resonance; however the direct process dominates away from resonance. The nonlinear susceptibilities χ(3) and χ(5) are also obtained.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
78.20.Bh Theory, models, and numerical simulation

Nonlinear optical properties of CdSe quantum dots

S. H. Park, Michael P. Casey, and Joel Falk

J. Appl. Phys. 73, 8041 (1993); http://dx.doi.org/10.1063/1.353919 (5 pages) | Cited 6 times

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The nonlinear optical properties of CdSe quantum dots are investigated using self‐saturation and degenerate four‐wave mixing techniques. The saturation of the room temperature absorption coefficient, for a wavelength in the vicinity of the first exciton peak, is measured. The measured data can be predicted from a description of the quantum dot as a two level, homogeneously broadened, saturating system. The variation of the four‐wave mixing reflectivity with intensity and with wavelength is reported. All four‐wave mixing data are also consistent with the homogeneously‐broadened two‐level model.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
78.66.Hf II-VI semiconductors

Upconversion dynamics of Er3+:YAlO3

D. J. Simkin, J. A. Koningstein, P. Myslinski, S. A. Boothroyd, and J. Chrostowski

J. Appl. Phys. 73, 8046 (1993); http://dx.doi.org/10.1063/1.354092 (4 pages) | Cited 10 times

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The dynamics of the green (2H11/12, 4S3/2) and red (4F9/2) upconversion emission excited by 15 ns pulses from an Er:fibre laser at 1500 nm are presented and analyzed. The upconversion process is found to fit a kinetic model, which includes both cross relaxation and energy transfer.  
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42.70.Hj Laser materials
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
78.55.Hx Other solid inorganic materials

Studies of photorefractive diffraction dynamic in paraelectric KTa1−xNbxO3 crystals

Peixian Ye, Changxi Yang, Yingwu Lian, Dadi Wang, Qincai Guan, and Jiyang Wang

J. Appl. Phys. 73, 8050 (1993); http://dx.doi.org/10.1063/1.353920 (9 pages)

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The dynamics of photorefractive diffraction in paraelectric KTa1−xNbxO3 crystals during writing, reading, and erasing have been studied systematically. Experimental observations related not only to the diffraction from the first‐order photorefractive grating but also to that from the second‐order photorefractive grating, and where a writing beam with spots smaller than, as well as larger than, the entrance surface of the crystal was used. All the complicated behaviors observed have been explained qualitatively. An electron‐hole transport model and a nonequilibrium screening model have been applied, and the existence of an internal dc electric field induced during writing simultaneously by an optical field and an externally applied field has been suggested and demonstrated. The significant influence of this induced field is indicated.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
77.84.Fa KDP- and TGS-type crystals

Short pulse electron beam excitation of the high‐pressure atomic Ne laser

Jong W. Shon, Robert L. Rhoades, Joseph T. Verdeyen, and Mark J. Kushner

J. Appl. Phys. 73, 8059 (1993); http://dx.doi.org/10.1063/1.353921 (7 pages) | Cited 7 times

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The high‐pressure atomic Ne laser operates on four visible transitions between the 3p and 3s manifolds. Oscillation at 585 nm (3p′[1/2]0→3s′[1/2]10) at efficiencies of ≳1% have been demonstrated by others. The upper laser level is believed to be populated by dissociative recombination of Ne2+, while state‐selective Penning reactions relax the lower laser levels. To investigate these pumping mechanisms, experimental and modeling studies have been performed on a short pulse e‐beam excited Ne laser using He/Ne/Ar mixtures. We found that the electron temperature in the afterglow following the e‐beam pulse largely determines the time at which oscillation starts. The electron temperature during the afterglow is partly controlled by a slow relaxation of excited states in Ar. Laser oscillation does not occur until these manifolds are depleted and the electron temperature decreases, thereby increasing the rate of dissociative recombination.
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42.55.Lt Gas lasers including excimer and metal-vapor lasers
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
52.80.Sm Magnetoactive discharges (e.g., Penning discharges)

Fabrication and optical properties of lead‐germanate glasses and a new class of optical fibers doped with Tm3+

J. Wang, J. R. Lincoln, W. S. Brocklesby, R. S. Deol, C. J. Mackechnie, A. Pearson, A. C. Tropper, D. C. Hanna, and D. N. Payne

J. Appl. Phys. 73, 8066 (1993); http://dx.doi.org/10.1063/1.353922 (10 pages) | Cited 45 times

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In this article we present a study of a new class of optical fibers based on lead germanate glass. The maximum vibrational frequency of this glass is intermediate between silica and zirconium barium lanthanum aluminum fluoride glass, causing a beneficial change in nonradiative decay and therefore quantum efficiency for particular laser transitions. Fabrication of high‐strength, low‐loss fibers of this glass has been acheived by modification of the composition to produce optimal physical properties for fiber drawing, while retaining the useful vibrational properties of the original PbGeO2 glass. Measurements of both the thermal and optical properties are described. The fibers produced are ideal for many applications in fiber devices.
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42.70.Ce Glasses, quartz
42.81.Bm Fabrication, cladding, and splicing
78.55.Hx Other solid inorganic materials

Optical Kerr‐effect measurement for a series of alcohols

Neil J. Harrison and Barry R. Jennings

J. Appl. Phys. 73, 8076 (1993); http://dx.doi.org/10.1063/1.353923 (5 pages)

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Nanosecond optical Kerr‐effect (OKE) measurements are reported using a modified apparatus, designed to enable rapid and precise data recording in pure liquids. Careful design of the apparatus enables measurements to be made at several inducing wavelengths without substantial apparatus modifications. The first measurement of the optical Kerr effect for benzene at an inducing wavelength of 532 nm is presented together with novel OKE data for the hitherto unstudied homologous alcohol series from methanol to 1‐dodecanol. Analysis of the results indicates for this series the existence of a linear relationship between the carbon chain length and the optically induced Kerr constant somewhat similar to the behavior previously observed in the n‐alkanes.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
78.20.Jq Electro-optical effects

Spatial coherence measurements and x‐ray holographic imaging using a laser‐generated plasma x‐ray source in the water window spectral region

I. C. E. Turcu, I. N. Ross, M. S. Schulz, H. Daido, G. J. Tallents, J. Krishnan, L. Dwivedi, and A. Hening

J. Appl. Phys. 73, 8081 (1993); http://dx.doi.org/10.1063/1.353924 (7 pages) | Cited 6 times

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The properties of a coherent x‐ray point source in the water window spectral region generated using a small commercially available KrF laser system focused onto a Mylar (essentially carbon) target have been measured. By operating the source in a low‐pressure (approximately 20 Torr) nitrogen environment, the degree of monochromaticity was improved due to the nitrogen acting as an x‐ray filter and relatively enhancing the radiation at a wavelength of 3.37 nm (C vi 1s‐2p). X‐ray pinhole camera images show a minimum source size of 12 μm. A Young’s double slit coherence measurement gave fringe visibilities of approximately 62% for a slit separation of 10.5 μm at a distance of 31.7 cm from the source. To demonstrate the viability of the laser plasma as a source for coherent imaging applications a Gabor (in‐line) hologram of two carbon fibers, of different sizes, was produced. The exposure time and the repetition rate was 2 min and 10 Hz, respectively.
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42.40.Kw Holographic interferometry; other holographic techniques
07.85.-m X- and γ-ray instruments
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)

Transient heating and melting transformations in argon‐ion laser irradiation of polysilicon films

Xianfan Xu, Scott L. Taylor, Hee K. Park, and Costas P. Grigoropoulos

J. Appl. Phys. 73, 8088 (1993); http://dx.doi.org/10.1063/1.353925 (9 pages) | Cited 4 times

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Undoped, thin silicon films have been deposited at different temperatures on fused quartz substrates by low‐pressure chemical vapor deposition. The heating of these films by continuous wave, argon‐ion laser beam irradiation has been studied. In situ, normal incidence reflectivity measurements have been obtained at specified locations in the semiconductor films. Melting and recrystallization phenomena have been probed by transient measurements. The static film reflectivity at elevated temperatures, up to about 1400 K, has also been measured. The temperature field has been analyzed numerically, using a modified enthalpy model. Thin‐film optics were used to calculate the argon‐ion laser light absorption in the polysilicon layer and the transient reflectivity response to the probing laser light. The predicted and experimentally measured reflectivity histories have been compared. The initial stages of the phase change process have been captured by high‐speed photography.
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68.55.-a Thin film structure and morphology
44.10.+i Heat conduction
64.70.K- Solid-solid transitions

Analysis of a diode with a ferroelectric cathode

L. Schachter, J. D. Ivers, J. A. Nation, and G. S. Kerslick

J. Appl. Phys. 73, 8097 (1993); http://dx.doi.org/10.1063/1.353926 (14 pages) | Cited 39 times

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It has been shown experimentally that electron current densities of more than 30 A/cm2 can be achieved from a cathode made of ferroelectric ceramic, when applying a field of order 0.1 MV/m. This current exceeds the Child–Langmuir current by two orders of magnitude. The current in the diode varies linearly with the applied voltage, provided that the latter is positive. In this theoretical study we show that the ferroelectric material plays a crucial role in the emission process. When a voltage is applied to the ferroelectric, the internal polarization field varies and the amount of screening charge required decreases. As a result, the electrons distribution near the cathode changes, forming a cloud which fills part of the diode gap. If now a positive voltage is applied to the anode, electrons are readily transferred through the diode gap. The qualitative and quantitative results of the theory are in good accordance with the experiment.
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52.80.Vp Discharge in vacuum
79.70.+q Field emission, ionization, evaporation, and desorption
41.75.Fr Electron and positron beams

Plasma‐sheath approximate solutions for planar and cylindrical anodes and probes

O. Biblarz and G. S. Brown

J. Appl. Phys. 73, 8111 (1993); http://dx.doi.org/10.1063/1.353927 (11 pages)

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Anode sheaths are explored in detail for collisional, identical‐specie‐temperature plasmas where sheath dimensions are in the micron range. The selected approach involves postulation of a specific electric field distribution with three shape factors. Previous research using this approach is verified and expanded upon using greater parameter ranges. z, a dimensionless quantity specifying plasma composition and condition, groups diverse plasmas into families exhibiting similar sheath characteristics. η, a nondimensional ratio of electrical energy to thermal energy in the sheath, allows temperature effects to be studied. This investigation focuses on plasma families that span a z range of 1.1729 to 2.1493 at η values defined by discrete plasma temperatures of 6000, 3000, and 300 K. Results indicate that at the lower temperatures, charge production in the outer sheath is somewhat generic to the electric field distribution, and that the sheaths themselves are nearly unaffected by substantial changes in temperature (i.e., η). Conversely, sheath density and extent are shown to vary significantly for differing z values. Equations governing cylindrical anodes generate sheaths that are virtually identical to corresponding planar cases. It is shown that only those anodes whose radii are comparable to the plasma’s characteristic radius (γ) must be treated with the cylindrical formulation; low‐ and moderate‐pressure plasmas would require micron‐diameter anodes to be thus affected. Finally, an analytical approach shown yields solutions that confirm the numerical results, and offers an algebraic approximation for high‐η plasmas.  
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52.40.Hf Plasma-material interactions; boundary layer effects
52.75.Di Ion and plasma propulsion
52.70.Ds Electric and magnetic measurements

Hydrogen‐atom spectroscopy of the ionizing plasma containing molecular hydrogen: Line intensities and ionization rate

Keiji Sawada, Kouji Eriguchi, and Takashi Fujimoto

J. Appl. Phys. 73, 8122 (1993); http://dx.doi.org/10.1063/1.353930 (4 pages) | Cited 45 times

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Our previous calculation of the excited atom population [T. Fujimoto, K. Sawada, and K. Takahata, J. Appl. Phys. 66, 2315 (1989)] is revised on the basis of new assessment of the cross sections for excitation from atomic hydrogen and dissociative excitation from molecular hydrogen. The effective ionization rate of molecular hydrogen is also calculated by the method of the collisional‐radiative model. Its ratio to the Balmer α photon emission rate is higher than that for the atomic hydrogen case by more than one order of magnitude.
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52.25.-b Plasma properties
52.70.Kz Optical (ultraviolet, visible, infrared) measurements

Electron and metastable densities in parallel‐plate radio‐frequency discharges

K. E. Greenberg and G. A. Hebner

J. Appl. Phys. 73, 8126 (1993); http://dx.doi.org/10.1063/1.353451 (8 pages) | Cited 42 times

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Electron densities were measured in continuous and pulse‐modulated, 13.56‐MHz, helium and argon discharges. These measurements were made in a symmetrically driven Gaseous Electronics Conference Reference Cell. Pulse modulation of the applied potential allowed observation of the time required for the electron density to achieve steady state. In general, helium discharges reached steady state in approximately 1.5 ms, taking three to ten times longer than argon discharges under similar operating conditions. As much as a threefold increase of the electron density was observed in the afterglow of a pulse‐modulated helium discharge, indicative of large metastable densities. Absorption spectroscopy was used to measure the helium singlet and triplet metastable densities throughout the volume of the discharge. For a 1.0‐Torr discharge, helium triplet metastable densities at the center of the discharge were as large as 2.5×1011 cm−3 while the peak singlet density was 3.0×1010 cm−3. The steady‐state electron density varied from 3.8×1010 cm−3 at 50 V to 2.3×1011 cm−3 at 200 V for the 1.0‐Torr helium discharge.
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52.80.Pi High-frequency and RF discharges
52.70.Kz Optical (ultraviolet, visible, infrared) measurements

K‐shell x‐ray yield scaling for aluminum x‐pinch plasmas

D. H. Kalantar, D. A. Hammer, K. C. Mittal, N. Qi, F. C. Young, S. J. Stephanakis, P. G. Burkhalter, G. Mehlman, and D. A. Newman

J. Appl. Phys. 73, 8134 (1993); http://dx.doi.org/10.1063/1.353452 (5 pages) | Cited 12 times

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We report results from experiments performed to measure and characterize the intense K‐shell radiation from aluminum x‐pinch plasmas at peak driving currents ranging from 280 kA to 1.0 MA. Single pulse aluminum K‐shell (predominantly line radiation at 1.6–2 keV) x‐ray yields ranged from 7.6 J at 290 kA to 240 J at 1.0 MA. In the range from 280 to 470 kA, the yield scales with current to the power of 3.6, whereas nonoptimized K‐shell yields at 800 kA and 1.0 MA indicate a power of about 3 or higher.
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52.80.Qj Explosions; exploding wires
52.70.La X-ray and γ-ray measurements
52.55.Ez Theta pinch

A fundamental mode, high‐power, large‐orbit gyrotron using a rectangular interaction region

D. J. Radack, K. Ramaswamy, W. W. Destler, and J. Rodgers

J. Appl. Phys. 73, 8139 (1993); http://dx.doi.org/10.1063/1.353453 (7 pages) | Cited 4 times

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This paper reports on experimental and theoretical studies of a fundamental mode, large‐orbit gyrotron oscillator designed to produce high‐power radiation in the rectangular TE10 mode at about 650 MHz. A basic linear theory for design and operation is presented and experiments confirm its applicability. Results of initial experiments have demonstrated 40±4.2 MW of microwave radiation in pulses equivalent in duration to the electron beam, corresponding to an efficiency of about 16%. In some cases, much greater power levels of 100±20 MW were observed at the expected frequency but in pulses of very short duration. In both cases, the observed frequency and mode pattern were consistent with operation in the fundamental mode. Experiments also demonstrated that the observed radiation was linearly polarized, as expected.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.59.Px Hard X-ray sources
52.27.Ny Relativistic plasmas

Surface cleaning of C‐doped p+ GaAs with hydrogen electron cyclotron resonance plasma

Noriyuki Watanabe, Takumi Nittono, Hiroshi Ito, Naoto Kondo, and Yasushi Nanishi

J. Appl. Phys. 73, 8146 (1993); http://dx.doi.org/10.1063/1.353454 (5 pages) | Cited 10 times

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We report surface cleaning of C‐doped p+‐GaAs epilayers with hydrogen electron cyclotron resonance plasma. Native oxides on the surface of the p+‐GaAs layer can be removed at a very low substrate temperature of 150 °C. In addition, carrier concentrations decrease after cleaning at about 300 °C, which is attributed to the hydrogenation of carbon acceptors caused by hydrogen plasma exposure. A cleaning temperature of about 400 °C, a cleaning time of about 5 min, and microwave power of about 30 W appear to be optimum for the regrowth process.
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81.65.-b Surface treatments
52.40.Hf Plasma-material interactions; boundary layer effects
73.61.Ey III-V semiconductors

Temporal evolution of the electron energy distribution function in oxygen and chlorine gases under dc and ac fields

Ping Jiang and Demetre J. Economou

J. Appl. Phys. 73, 8151 (1993); http://dx.doi.org/10.1063/1.353455 (10 pages) | Cited 12 times

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An analysis of the temporal evolution of the electron energy distribution function (EEDF) and the electron swarm parameters in oxygen and chlorine gases is presented. The spatially homogeneous time‐dependent Boltzmann equation is solved for dc and radio‐frequency ac electric fields by a finite‐element method. A comparison is made of the swarm parameters obtained for the following three cases: (a) under the actual ac field; (b) assuming that the EEDF follows faithfully the applied ac field [quasi‐steady‐state (QSS) approximation]; and (c) using an ‘‘effective’’ dc field (effective dc approximation). It is shown that the effective dc approximation is not applicable to either oxygen or chlorine for frequencies <10 MHz; however, the QSS approximation is justified for chlorine discharges at <13.56 MHz. This has important implications for reducing the computation time in modeling the bulk plasma of glow discharge reactors. It is also shown that atomic chlorine resulting from molecular dissociation has a significant effect on the swarm parameters, especially for large degrees of gas dissociation.  
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52.80.Pi High-frequency and RF discharges
51.50.+v Electrical properties (ionization, breakdown, electron and ion mobility, etc.)

Lattice position of Si in GaAs determined by x‐ray standing wave measurements

A. Shih, P. L. Cowan, S. Southworth, L. Fotiadis, C. Hor, B. Karlin, F. Moore, E. Dobisz, and H. Dietrich

J. Appl. Phys. 73, 8161 (1993); http://dx.doi.org/10.1063/1.353430 (8 pages) | Cited 1 time

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The x‐ray standing wave (XSW) technique was applied to determine the lattice location of Si impurity atoms in GaAs(100) crystals. The synchrotron radiation of X24A at the national synchrotron light source was utilized to set up backreflection XSW, an experimental geometry which drastically relaxes the otherwise stringent requirement on the lattice perfection. Specifically, the lattice sites were determined with respect to the [311] reflection planes which differentiate a Ga site from an As site. With the aid of an appropriate choice of the x‐ray fluorescence filter, we were able to study GaAs(100) samples with very low levels of Si impurities. On a sample doped with 4×1018 cm−3 Si during the molecular‐beam epitaxy growth, we found that the Si atoms predominantly occupied the Ga sites. On both an ion‐implanted sample after annealing and a sample with Si impurities introduced by thermal diffusion, about 30% of the Si atoms occupied the Ga sites, and the rest occupied random sites. The As site occupation was less than 6%. Suggestions are made for further experiments with improved sensitivity.
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61.72.Dd Experimental determination of defects by diffraction and scattering
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
07.85.-m X- and γ-ray instruments

X‐ray scattering studies of FeSi2 films epitaxially grown on Si(111)

J. M. Gay, P. Stocker, and F. Réthoré

J. Appl. Phys. 73, 8169 (1993); http://dx.doi.org/10.1063/1.353431 (10 pages) | Cited 10 times

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A laboratory x‐ray diffractometer for surface and thin‐film studies is presented. Ex situ structural characterization of FeSi2 films epitaxially grown on Si(111) is reported. Both specular and nonspecular reflectivities are measured on β‐FeSi2 films grown by solid‐phase epitaxy and reactive deposition epitaxy techniques. A detailed comparison is performed of the electron density profile of the films normal to the surface, as well as of their surface roughness. In‐plane diffraction is also measured at grazing incidence. For the β‐FeSi2 sample investigated, the (110) epitaxy on Si(111) is clearly shown. For a film grown by molecular‐beam‐epitaxy codeposition at 550 °C, the existence of a new metastable phase which is in registry with silicon along the Si〈1 10〉 direction and slightly out of registry, (3.0±1.0)% compressed along the Si〈112〉 direction, is reported.
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61.05.cf X-ray scattering (including small-angle scattering)
61.05.cj X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc.
68.55.-a Thin film structure and morphology

Optical doping of soda‐lime‐silicate glass with erbium by ion implantation

E. Snoeks, G. N. van den Hoven, and A. Polman

J. Appl. Phys. 73, 8179 (1993); http://dx.doi.org/10.1063/1.353432 (5 pages) | Cited 30 times

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Soda‐lime‐silicate glass has been implanted with 500 keV Er ions at fluences between 8.6×1014 and 1.8×1016/cm2 with the aim to optically dope the material in the near surface region. The ion range was 100 nm, and Er concentrations in the range 0.09—1.9 at. % were obtained. The characteristic photoluminescence (PL) of Er3+ around 1.54 μm is observed at room temperature in as‐implanted glass. The PL intensity increases by an order of magnitude after annealing above 500 °C, as a result of annihilation of implantation‐induced defects. Annealing causes an increase in PL lifetime. As a function of Er fluence, the PL intensity first increases, but levels off above ∼6×1015 Er/cm2 (0.6 at. % Er peak concentration). The PL lifetime decreases from 13 to 1.5 ms for increasing Er concentration. The decrease in PL efficiency with concentration is attributed to concentration quenching caused by Er‐Er interactions. The optimal combination of PL intensity and lifetime is reached at ≊0.4 at. % peak concentration, for which the lifetime is 6 ms. For high Er concentrations and high pump intensities (∼3 kW/cm2) an additional, intensity dependent quenching mechanism (possibly cooperative upconversion) is observed.
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61.72.uj III-V and II-VI semiconductors
78.55.Hx Other solid inorganic materials
42.70.Hj Laser materials

Radiation damage in SiO2/Si induced by low‐energy electrons via plasmon excitation

Takashi Yunogami and Tatsumi Mizutani

J. Appl. Phys. 73, 8184 (1993); http://dx.doi.org/10.1063/1.353433 (5 pages) | Cited 8 times

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We have found definite evidence which proves that radiation damage in SiO2/Si induced by low‐energy electrons is caused by plasmon excitation in SiO2. The SiO2/Si sample was irradiated by accelerated thermoelectrons, and the flat‐band voltage shift, ΔVFB, of the sample was measured by the C‐V method. The effective charge generation yield, Rf, in SiO2/Si was evaluated from the ΔVFB and the electron dose. The effective positive charges were measured in p‐type SiO2/Si, and the effective negative charges were measured in n‐type SiO2/Si. This is because interface states behave like positive charges in p‐type SiO2/Si and negative charges in n‐type SiO2/Si. The Rf in p‐type SiO2/Si oscillated as a function of the incident electron energy of 5–150 eV with several clear peaks. These peaks correspond to the quantum energy of a bulk plasmon or a surface plasmon. Both plasmons decay into electron‐hole pairs. The holes that escape from recombination with the electrons are trapped at the SiO2/Si interface and cause effective positive charges. The Rf was maximum when the SiO2 thickness was 20 nm, independent of the incident electron energy. This is because the plasmon region extends to about 20 nm in SiO2, and the hole‐trapping efficiency is expected to be maximum at a SiO2 thickness of 20 nm.
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61.80.Fe Electron and positron radiation effects
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)

Scanning probe microscopy studies of electromigration in electroplated Au wires

M. Paniccia, P. Flinn, and R. Reifenberger

J. Appl. Phys. 73, 8189 (1993); http://dx.doi.org/10.1063/1.353434 (9 pages) | Cited 18 times

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Scanning probe studies of electromigration in 4‐μm‐wide by 1‐μm‐thick electroplated Au wires reveal changes in surface morphology. Features that develop on a submicrometer length scale were observed using current densities up to 7.5×106 A/cm2 and temperatures up to 280 °C for time intervals ranging from a few minutes up to a few days. Scanning probe images of the same area taken before, during, and after current stressing reveal substantial changes in surface morphology. Both hillock growth and void formation have been observed in situ. Changes in surface morphology caused by electromigration can be quantitatively described from the scanning probe images and new techniques for studying mass flow due to electromigration are now possible.
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68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.37.Ps Atomic force microscopy (AFM)
68.37.Rt Magnetic force microscopy (MFM)
68.37.Uv Near-field scanning microscopy and spectroscopy
66.30.Qa Electromigration
73.50.-h Electronic transport phenomena in thin films
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