The Hall mobility of holes has been measured in GaAs grown at low temperatures and in GaAs_1-xBi_x alloys for Bi concentrations x ranging from 0.94% to 5.5%. The hole mobility is found to decrease with increasing Bi content. The temperature dependence of the mobility in the 25 to 300 K range is fit with a combination of phonon scattering, ionized impurity scattering, and Bi related scattering. The hole scattering cross-section for an isolated Bi impurity is estimated to be 0.2 nm². The temperature independent mobility at the highest Bi concentration (x = 5.5%), is interpreted as being limited by scattering from Bi clusters.

Keywords: gallium arsenide; gallium compounds; Hall mobility; hole mobility; III-V semiconductors; impurities; phonons; semiconductor growth

Single crystal aluminum-gallium-oxide films have been grown by molecular beam epitaxy (MBE) in the corundum phase. Films of the (Al_1-xGa_x)₂O₃ alloys doped with neodymium have favorable properties for solid state waveguide lasers, including a high thermal conductivity sapphire substrate and a dominant emission peak in the 1090-1096 nm wavelength range. The peak position is linearly correlated to the unit cell volume, which is dependent on film composition and stress. Varying the Ga-Al alloy composition during growth will enable the fabrication of graded index layers for tunable lasing wavelengths and low scattering losses at the interfaces.

Keywords: Laser materials; Rare-earth-doped materials; Thin films

High structural quality yttrium oxide films have been grown on R-plane sapphire by molecular beam epitaxy. X-ray diffraction measurements showed clear pendellosung fringes and sharp peaks. X-ray measurements indicate that the films grow nearly perfectly up to a critical thickness with x-ray peak widths as low as 7 arc sec. This critical thickness increases with decreasing growth rate up to 7 nm at 10 nm/h. The optimal growth temperature was found to be 800 ^oC. Evidence of short range (<10 nm) surface diffusion is presented.

The photoluminescence from a Ga(AsBi) sample is investigated as a function of pump power and lattice temperature. The disorder-related features are analyzed using a Monte Carlo simulation technique. A two-scale approach is introduced to separately account for cluster localization and alloy disorder effects. The corresponding characteristic energy scales of 11 and 45 meV are deduced from the detailed comparison between experiment and simulation.

Keywords: gallium arsenide; III-V semiconductors; Monte Carlo methods; photoluminescence

Optical wave propagation in neodymium-doped yttrium oxide (Nd:Y₂O₃) films grown on R-plane sapphire substrates by molecular beam epitaxy has been studied by the prism coupler method. The measurements yield propagation loss data, the refractive index, and the dispersion relation. The refractive index of the Nd:Y₂O₃ at 632.8 nm is found to be 1.909, and the lowest propagation loss measured is 0.9 +/- 0.2 cm^-1 at 1046 nm with a polymethyl methacrylate top cladding layer on a film with 6 nm root mean square surface roughness. The loss measurements suggest that the majority loss of this planar waveguide sample is scatter from surface roughness that can be described by the model of Payne and Lacey [Opt. Quantum Electron.26, 977 (1994)].

Epitaxial films of neodymium-doped sapphire have been grown by molecular beam epitaxy on R, A and M-plane sapphire substrates. The emission spectrum features sharp lines consistent with single site doping of the Nd³⁺ ion into the host crystal. This material is believed to be a non-equilibrium phase, inaccessible by conventional high temperature growth methods. Neodymium-doped sapphire has a promising lasing line at 1096 nm with an emission cross section of 11.9×10^-19 cm², similar to the 1064 nm line of Nd:YVO₄.

Keywords: Laser materials; Rare-earth-doped materials; Thin films

Room temperature photoluminescence (PL) spectra have been measured for GaAs_1-xBi_x alloys with Bi concentrations in the 0.2%-10.6% range. The decrease in the PL peak energy with increasing Bi concentration follows the reduction in bandgap computed from density functional theory. The PL peak energy is found to increase with PL pump intensity, which we attribute to the presence of shallow localized states associated with Bi clusters near the top of the valence band. The PL intensity is found to increase with Bi concentration at low Bi concentrations, peaking at 4.5% Bi.

The optical absorption of GaAsN films grown by molecular beam epitaxy on GaAs substrates is measured using the mirage effect photothermal deflection spectroscopy (PDS). The PDS spectra were fitted with a modified Fernelius model, which takes into account multiple reflections within the GaAsN layer and GaAs substrate. This allowed the extraction of bandedge parameters for a series of GaAsN films with N content varying from 0.24% to 1.4% N. All films show a clear Urbach absorption edge with a composition-dependent bandgap consistent with literature and Urbach slope parameters roughly 3 times larger than GaAs values.

Keywords: A1. Characterization

GaAs_1-xBi_x light emitting diodes have been grown and characterized. The p-i-n structure uses a 100 nm intrinsic layer with a central 50 nm GaAs_1-xBi_x light emitting layer with 1.8 % bismuth. The diodes showed peaks in the electroluminescence (EL) emission at 987 nm from the GaAs_1-xBi_x and 870nm from the GaAs. The wavelength of the peak in the EL from the GaAs_1-xBi_x was independent of temperature in the range 100-300 K while the GaAs peak shifted with temperature as expected. Photoluminescence measurements on the same p-i-n structure show temperature dependence of the peak wavelength similar to the temperature dependence of GaAs.

Epitaxial neodymium-doped yttrium aluminum perovskite (Nd:YAlO3) has been grown by plasma-assisted molecular beam epitaxy on R-plane sapphire at . The substrates were annealed in air at to generate atomically ordered surfaces. Structural and optical characterization was performed using X-ray diffraction and photoluminescence. Emission spectra identical to that from bulk crystalline Nd:YAlO3 were obtained.

Keywords: Al. Crystal structure

We use x-ray absorption spectroscopy to investigate the local structure around Bi atoms in GaAs_1-xBi_x layers grown on GaAs as a function of Bi concentration in order to detect short-range order. We find that static disorder in the Bi next-nearest-neighbor interatomic distances dramatically increases when the Bi concentration is increased. At 1.2% Bi concentration, the Bi atoms are randomly distributed whereas at 1.9%, they tend to form next-nearest-neighbor pairs. When the Bi concentration rises to 2.4%, our results suggest that some of the Bi atoms form small Bi clusters. Such strong deviations from a random distribution are likely to play an important role in the occurrence of the giant optical bowing recently measured in this alloy.

GaAs_1-xBi_x is a mixed-anion semiconductor alloy. In the As-rich regime 0.4% x 4 % , isovalent Bi creates a series of bound states but this alloy nonetheless exhibits properties characteristic of regular semiconductors. The dual impurity-alloy character can be tuned by varying the temperature. Below 100 K, multiple Bi bound states appear at low energy in the luminescence spectrum. These states are associated with the pseudo-donor potential created by isovalent Bi impurities. Taking into consideration the concentration regime at which these bound states are observed, they likely involve excitons bound to clusters composed of a few substitutional Bi atoms, indirectly implying that the isolated Bi state is resonant with the valence band. At ambient temperature, these localized states are strongly suppressed and luminescence from the band edges is measured. The important Bi-induced atomic disorder creates a strong intraband coupling resulting in an important reduction of the band gap for a relatively small alloy concentration. These results on a pseudodonor isoelectronic alloy are reminiscent of the better known pseudoacceptor GaAsN, but offer a complementary view of this intriguing and yet little understood class of alloys.

We investigate the electronic properties of GaAs_1-xBi_x by photoluminescence at variable temperature (T = 10-430 K) and high magnetic field (B = 0-30 T). InGaAs_0.981Bi_0.019, localized state contribution to PL is dominant up to 150 K. At T = 180 K the diamagnetic shift of the free-exciton states reveals a sizable increase in the carrier effective mass with respect to GaAs. Such an increase cannot be accounted for by an enhanced localized character of the valence band states, solely. Instead, it suggests that also the Bloch states of the conduction band are heavily affected by the presence of bismuth atoms.

We describe how the Bi content of GaAs1-xBi_x epilayers grown on GaAs can be controlled by the growth conditions in molecular beam epitaxy. Nonstandard growth conditions are required because of the strong tendency for Bi to surface segregate under usual growth conditions for GaAs. A maximum Bi content of 10% is achieved at low substrate temperature and low arsenic pressure, as inferred from x-ray diffraction measurements. A model for bismuth incorporation is proposed that fits a large body of experimental data on Bi content for a wide range of growth conditions. Low growth rates are found to facilitate the growth of bismide alloys with a low density of Bi droplets.

The smoothing of weakly roughened GaAs (100) substrates is measured with elastic light scattering during homoepitaxial growth of GaAs buffer layers. The smoothing measurements are used to determine the coefficient of the linear term in the continuum growth equation for GaAs, as a function of growth rate and temperature. The temperature and growth rate dependence are in good agreement with theoretical predictions from an atomistic description of the growth process. The density of atomic steps on the surface, which is a key parameter in the continuum growth equation, is measured independently using atomic force microscopy. The linear smoothing coefficients computed from the experimental values for the step density, are found to be in good agreement with the smoothing rates measured with light scattering. These experiments provide experimental support for the continuum growth model that has been derived analytically from basic atomic level phenomena in epitaxial film growth.

In this review we describe the growth and properties of the dilute bismide semiconductor alloy GaAs_1-xBi_x and show how its properties are in certain respects complementary to the dilute nitride alloy, GaN_yAs_1-y. Like the dilute nitrides the dilute bismides show a giant band gap bowing effect in which a small concentration of the alloying element has a disproportionate effect on the band gap, however in the case of the bismide the band gap reduction is associated with an increase in the energy of the valence band maximum (VBM) rather than a reduction in the energy of the conduction band minimum (CBM). Under standard GaAs growth conditions Bi acts as a surfactant with associated improvements in surface quality. In order to incorporate Bi, growth temperatures below 400^oC are used with As₂/Ga flux ratios close to unity. The electron mobility of GaAs is only weakly affected by Bi alloying, in contrast to the dilute nitrides where the electron mobility decreases rapidly with N alloying. Bi alloying also produces a giant bowing effect in the spin orbit splitting in the valence band. Strong room temperature photoluminescence is observed. Prospects for future device applications of this new compound semiconductor alloy are discussed.

This review brings together experimental data on surface shape evolution during epitaxial growth of GaAs with kinetic Monte Carlo simulations of a solid-on-solid model and numerical solutions of a continuum growth equation derived from an adatom transport equation. Scanning probe and light scattering measurements of the surface morphology of GaAs, grown by molecular beam epitaxy, on planar as well as patterned (100) substrates are reviewed. We show that the experimental data can be described by a stable continuum growth equation that is mixed- order in the spatial derivatives, with an Edwards-Wilkinson type linear term, together with a conservative nonlinear term. The stable growth equation is derived from two coupled rate equations, one of which describes the transport of adatoms on the surface and the other describes the rate of change of surface height due to adatom incorporation into the surface at step edges. In this analysis, we assume that there is a combination of an Ehrlich-Schwoebel barrier and/or an incorporation barrier at step edges that favor a net downhill migration of adatoms across step edges, with the consequence that the growth model, like the experimental system, is stable, meaning that undulations in the surface tend to smooth out during growth. The coefficients in the growth equation depend on the growth rate and the density of steps on the surface. The continuum description of the morphological evolution is tested by comparisons to computer experiments consisting of kinetic Monte Carlo simulations of a solid-on-solid model. The methods used in this analysis of GaAs epitaxy are expected to be broadly applicable to other materials that exhibit stable epitaxial growth.

Keywords: molecular beam epitaxy; thin film; gallium arsenide;III-V semiconductors; gallium compounds;semiconductor growth; kinetic monte carlo;solid on solid;continuum growth equation;scanning probe; light scattering;adatom transport;downhill migration;stable growth

Crystalline yttrium oxide films have been grown on Si (001) and sapphire (0001) substrates by molecular beam epitaxy using molecular oxygen and thermal evaporation of yttrium. Neodymium doped films showed strong room temperature photoluminescence from the rare earth ions. The dependence of luminescence intensity on neodymium concentration was studied and a maximum was estimated to occur at 2 at.% assuming complete neodymium incorporation. The structural properties of the films were characterized using X-ray diffraction.

Keywords: Oxide thin films; Rare-earth doped oxides; Planar waveguides

An above-band-gap transition E₊ is experimentally observed in the dilute GaAs_1-xBi_x alloy. Precise measurements at very low dilutions are made of the above-band-gap transition E₊ that is observed in GaAs_1-xN_x, making it possible to compare the behavior of the different isoelectronic traps Bi and N in the common host GaAs with respect to their perturbation to the host electronic structure. We suggest that the origin of the E₊ level observed in GaAs is not the isolated isoelectronic impurity level N_x, as is presumed in the band-anticrossing model, but rather the isoelectronic-impurity-induced perturbation of the conduction band L^c₆

High resolution PL spectroscopy was performed on high quality bulk GaAs, lightly doped with the nitrogen isoelectronic impurity. The shallowest nitrogen pair bound exciton center labeled as X1 revealed in total six transitions, including transitions which had not been observed previously. The photoluminescence lines from a small ensemble of nitrogen centers showed polarization depen- dent intensity. High spectral resolution PL spectroscopy was combined with confocal spectroscopy experiments performed on a GaAs:N/AlGaAs heterostructure. The high spatial resolution achieved by this technique enables us to localize and examine individual nitrogen bound excitons. Similar spectral structure and polarization dependence was observed for individual N-pair centers in GaAs. Both techniques support the C2v symmetry of such isoelectronic impurity centers. The comparison between the PL spectra from an ensemble of nitrogen pairs and individual centers demonstrate the ability of the single impurity technique to lift the orientational degeneracy.

Keywords: thin film; gallium arsenide;III-V semiconductors; gallium compounds;nitrogen; semiconductor growth;photoluminescence;confocal microscopy

Dilute nitride semiconductors of composition GaAs_1-xN_x (0.0017<x<0.0115) are grown by plasma-assisted molecular beam epitaxy with a helical resonator plasma source for active nitrogen. The plasma source is self-starting at the operating pressure and can be operated at two different frequencies for which the emission spectrum is dominated by N₂ molecules or by N atoms. For the same power the molecular-rich mode is found to produce a higher flux of active nitrogen. After extended operation the plasma tube becomes contaminated with As which reduces the flux of active nitrogen and creates a below band gap emission band in the photoluminescence of the GaAs_1-xN_x. For the clean discharge tube no difference is observed in the photoluminescence for samples grown in the molecule-rich or atom-rich mode.

Keywords: molecular beam epitaxial growth; gallium arsenide; III-V semiconductors; gallium compounds; wide band gap semiconductors; plasma materials processing; photoluminescence; semiconductor growth; semiconductor epitaxial layers

The dilute bismide semiconductor GaAs_1-xBi_x is an interesting new semiconductor alloy with novel properties and potential device applications. Like the dilute nitride alloy GaAs_1-xN_x, the dilute bismide shows a giant bandgap bowing effect. Bi alloying primarily affects the states in the vicinity of the top of the va-lence band rather than the bottom of the conduction band as in the case of N alloying. MBE growth of dilute bismides requires low growth temperatures (320-390C) and near stochiometric group III/V ratios, due to the strong tendency of Bi to surface segregate under usual GaAs growth conditions. Optimal growth conditions are close to conditions that produce Bi and/or Ga droplets. In-situ light scattering is found to be a useful tool for defining growth conditions that do not produce droplets on the surface

We report a giant bowing of the spin-orbit splitting energy Δ_o in the dilute GaAs_1-xBi_xalloy for Bi concentrations ranging from 0% to 1.8%. This is the first observation of a large relativistic correction to the host electronic band structure induced by just a few percent of isoelectronic doping in a semiconductor material. It opens up the possibility of tailoring the spin-orbit splitting in semiconductors for spintronic applications.

We report time-resolved terahertz spectroscopy measurements of the electronic transport properties of dilute GaAs bismide and nitride alloys. The electron mobility for GaAs_1-yBi_y (y = 0.84%) extracted from Drude fits to the transient complex conductivity was  2800 cm²/Vs at a carrier density of 2.7 x 10¹⁸ cm^-3, close to the mobility of 3300 cm/²Vs measured for GaAs at a similar carrier density. The electron mobility did not decrease significantly for Bi concentrations up to 1.4% . In contrast, the GaN_xAs_1-x (x = 0.84%) and GaN_xAs_1-x-yBi_y (x = 0.85%, y = 1.4%) films exhibited non-Drude behavior with severely reduced electron mobility and suppressed conductivity at low frequencies indicative of carrier localization.

A nonlinear differential equation is derived for the surface shape evolution in epitaxial growth, from a transport equation for the adatoms. A negative Ehrlich-Schöbel barrier is assumed to be present at atomic steps favouring downhill migration of adatoms. Expressions for the coefficients in the growth equation are obtained in terms of the deposition rate, step density, step edge potential barrier and the adatom release rate from step edges. The analytical model is found to be in good agreement with a kinetic Monte Carlo simulation of a solid-on-solid model, which includes the same physical phenomena.

High resolution photoluminescence spectroscopy on heavily doped GaAs:N reveals the existence of excitons bound to a nitrogen cluster. The observed transitions are exceedingly sharp, similar to those observed for excitons bound to nitrogen pairs in high quality GaAs with the narrowest transition being only 94 ueV. Moreover, several other features can be observed originating most likely from phonon replicas of the nitrogen pair bound excitons and higher order clusters. However, the main transitions which dominate the photoluminescence spectra are thought to originate from excitons bound to a three nitrogen cluster. The sharp photoluminescence features are superimposed on a broad luminescence band indicating a strong perturbation induced by nitrogen atoms to the host GaAs lattice. The number of the allowed transitions and their polarization dependence provide important information about the arrangement of the nitrogen atoms in the cluster.

The ablation hollows or ”suncups” that form on the surface of snowfields in summer are a wonderful example of pattern formation in nature. Suncups reduce the albedo of the snow and set a characteristic length for interaction of wind with the snowpack. They also contain information about the properties of the snow and its ablation rate, that could be extracted if we had a more quantitative understanding of how suncups form. A mathematical model is proposed that explains the shape, size and dynamical behavior of suncups in terms of the interaction of solar radiation with the snowpack. Using a perturbation method, we derive a non-linear partial differential equation for the time dependent shape of the snow surface from an approximate physical model for the interaction of solar radiation with snow. The resulting equation, which is similar to the Kuramoto-Sivashinsky equation in fluid mechanics, has a characteristic length and amplitude. We find expressions for the characteristic size of suncups in terms of the spectrally averaged diffusion length of solar radiation in snow. The model correctly describes the shape of suncups, with their spatially ordered patterns of parabolic valleys and V-shaped ridges. It is also in remarkably good agreement with the observed length scales and growth rates. Depending on the relative values of the coefficients of the nonlinear terms in the differential equation, the suncup patterns can be either stationary in time, or chaotic.

The effect of dilute N alloying and Bi surfactant growth on strain relaxation in highly strained InGaAs single quantum well (QWs) was investigated by using high resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM). Dilute nitride InGaNAs QWs of varying thickness, constant lattice mismatch 1.7%, were grown by molecular beam epitaxy on oriented GaAs (001) substrates. Some samples were exposed to a flux of Bi surfactant during the growth procedure, which acts to enhance the N incorporation, increase the optical emission, and create smoother interfaces. The QWs were observed to relax through the formation of pure edge-type, misfit dislocations aligned with in-plane <100> directions. These were found to be directly associated with degradation in the optical emission, however,1% N addition, with or without Bi surfactant, did not have a detectable effect on the critical thickness nor the rate of this relaxation mechanism.

The presence of a bismuth surfactant is found to increase the nitrogen incorporation in the dilute nitride GaNxAs_1-x by as much as 60% during growth by molecular beam epitaxy. Films with nitrogen concentrations in the 0.4-0.95% range were grown using an RF plasma source for nitrogen. The Bi surface coverage is inferred from reflection high energy electron diffraction as a function of Bi flux and substrate temperature, and the nitrogen content is obtained by high-resolution x-ray diffraction. At constant substrate temperature the nitrogen content is found to increase with Bi coverage, which has the form of a Langmuir isotherm when plotted as a function of Bi flux.

We have studied GaAs_1-xBi_x (up to x ~ 3%) using Raman scattering with two different polarization configurations. Two Bi-induced phonon modes are observed at ~186 cm^-1 and ~214 cm^-1 with increasing Raman intensity as the Bi concentration increases. By comparing Raman selection rules for the observed Bi-induced phonon modes with those for the substitutional N vibrational mode (GaN mode) in GaAsN, the phonon mode at ~214 cm^-1 is identified as originating from substitutional Bi at the As site in GaAsBi.

We investigate experimentally and computationally the nonlinear replication of surface shapes during epitaxial growth. Experimental measurements of surface shapes consisting of atomic force microscope (AFM) images of epitaxially grown GaAs are compared with surface shapes computed from continuum growth equations with a conservative or a non-conservative nonlinear term. Both the non-conservative and the conservative nonlinear terms are found to be consistent with the experiments, although the conservative nonlinearity is preferred on physical grounds. Kinetic Monte Carlo (KMC) simulations cover a wider range of spatial frequencies, which enables the two different nonlinear terms to be distinguished. In this case, the observed surface shapes are consistent with the conservative nonlinearity.

We report strong band gap photoluminescence at room temperature in dilute quaternary GaN_xAs_1-x-yBi_y alloys (x<1.6%,y<2.6%) grown by molecular beam epitaxy. The band gap of the alloy can be approximated by the band gap of GaAs minus the reduction in gap associated with the effects of N and Bi alloying individually. A one-parameter method for fitting the composition dependence of the band gaps of dilute quaternary semiconductor alloys is proposed which is in excellent agreement with data for Ga_1-yIn_yN_xAs_1-x.

Incorporation of Bi in GaAs_1-xN_x epitaxial layers represents a significant interest as Bi compensates the lattice parameter reduction cased by the N incorporation while contributing to the reduction of the band gap energy. GaAs_1-x-yN_xBi_y epitaxtial layers were grown on GaAs wafers by molecular beam epitaxy. The quality of the films as well as the concentration and lattice location of Bi and N were characterized by the channeling Rutherford backscattering spectrometry and nuclear reaction analysys using 2 and 3.72 MeV He beams, respectively. The amount of nitrogen in the film was measured by means of the ¹⁴N(α, p)¹⁷O endothermic nuclear reaction and elastic recoil detection. The results indicate that high quality epitaxial layers were obtained, with y = 1.8% Bi incorporated into the layer. Angular scan along the main axes showed no strain in the film and indicated that most of the Bi atoms are located at substitutional sites. Nitrogen lattice incorporation is more difficult to establish because of the presence of Bi in t he layer, but we estimate the substitutional fraction to be 71 +/- 6%.

Kan et al. [1] recently published intriguing experimental evidence that patterned GaAs surfaces are subject to a non-monotonic amplitude decay during epitaxial growth. They compared the temporal evolution of their surface shapes with calculations based on four continuum growth models, including the KPZ equation, and found that none of them could reproduce the amplitude evolution, nor the shape of the experimental surfaces. We do not agree with their attempt to extend these conclusions to the much lower amplitude morphology ( 3^o surface slope) that we studied earlier, in which KPZ behavior was observed [2].

X-ray absorption and emission spectroscopies are used to study the effects of short-ranged ordering on the electronic states of disordered GaN. Nanocrystalline samples with crystallites as small as 3nm exhibit an electronic structure resembling a broadened version of that in crystalline GaN. The electronic structure is even more heavily broadened in amorphous GaN films containing oxygen impurities or excess gallium. The oxygen containing films show an additional peak in the density of states just above the conduction band edge, and a downward shift of the valence band edge. The signature of molecular nitrogen trapped within the films is evident in both the absorption and emission spectra.

The time evolution of surface gratings on GaAs(001) during maskless thermal Cl₂ etching is investigated using atomic force microscopy and real-time measurements of diffracted light intensity. The decay of the grating depends strongly on its orientation with respect to the crystal axes. The pattern transfer can be altered significantly by using a directional molecular beam of Cl₂ instead of exposure to a nondirectional low-pressure gas phase. In particular, if the molecular beam is incident on the sample from an off-normal direction, the grating shape develops a strong asymmetry. A numerical model consisting of two coupled partial differential equations for the surface height and the concentration of chlorine on the surface is in good quantitative agreement with the observed shape evolution. The model includes the effects of the crystal anisotropy of the etch rate, spatial inhomogeneity of the Cl₂ flux to the surface, and surface diffusion of the chlorine. Fits of the model to the surface shapes allow us to determine the diffusion length of chlorine on GaAs(001) to be on the order of 50 nm at 200 ^oC.

We have developed a continuum model based on two coupled differential equations that explains the complex surface shapes observed in epitaxial regrowth on micron scale gratings. This model describes the dependence of the surface morphology on film thickness and growth temperature in terms of a few simple atomic scale processes including adatom diffusion, step-edge attachment and detachment, and a net downhill migration of surface adatoms. The continuum model reduces to the linear part of the Kardar-Parisi-Zhang equation with a flux dependent smoothing coefficient in the long wavelength limit.

X-ray absorption and fluorescence spectra have been measured at the nitrogen K-edge of dilute GaN_xAs_1-x alloys. The x-ray spectra are in good agreement with a sp³s^* tight-binding model in which nitrogen is included in a supercell configuration and the disorder in the nitrogen distribution is neglected. A strong peak in the x-ray absorption spectrum is interpreted as a nitrogen resonant state in the conduction band rather than as an electron-hole exciton. The tight-binding calculation is also in good agreement with the observed nitrogen concentration dependence of the optical bandgap.

The evolution of one- and two-dimensional surface gratings during maskless thermal Cl₂ etching of GaAs is investigated using atomic force microscopy. It is found that the limiting factor for pattern transfer is the anisotropy of the etch rate with respect to crystal orientation. A simple numerical model based on an interpolation of measured etch rates is presented that can be used to calculate the evolution of surface patterns.

Dilute nitride GaNAs thin films and InGaNAs single quantum wells (QWs) have been grown by molecular beam epitaxy with a concurrent bismuth flux. Bi does not incorporate into the films and acts as a surfactant. Atomic force microscopy images reveal that, at sufficiently high bismuth flux, step flow growth occurs in GaN0.004As0.996 even at substrate temperatures as low as 460C. This results in an order of magnitude decrease in the surface roughness. A similar smoothing effect is obtained when growing GaAs and AlGaAs thin films with bismuth. Furthermore, Bi is found to enhance the incorporation of nitrogen into GaNAs. The peak photoluminescence intensity from an In0.26Ga0.74N0.011As0.989 QW is increased by more than a factor of two with the surfactant. We conclude that Bi reduces the incorporation of defects and/or impurities in the dilute GaNAs based alloys.

GaAs_12xBi_x epilayers with bismuth concentrations up to x53.1% were grown on GaAs by molecular beam epitaxy. The Bi content in the films was measured by Rutherford backscattering spectroscopy. X-ray diffraction shows that GaAsBi is pseudomorphically strained to GaAs but that some structural disorder is present in the thick films. The extrapolation of the lattice constant of GaAsBi to the hypothetical zincblende GaBi alloy gives 6.3360.06 Å. Room-temperature photoluminescence of the GaAsBi epilayers is obtained and a significant redshift in the emission of GaAsBi of ;84 meV per percent Bi is observed.

The band gap of GaAsBi epitaxial layers as a function of bismuth concentration up to 3.6% is determined. The optical transitions were measured by modulated electroreflectance. The energy of the band gap decreases at a linearized rate of 88 meV/% Bi, or 83 meV/% Bi for the heavy hole to conduction band transition for GaAsBi strained to GaAs. The valence-band splitting increases faster than that of GaAs under similar compressive strain whereas the temperature dependence of the observed GaAsBi transitions is similar to that of GaAs.

The surface morphology of Cl2-etched GaAs001! is measured as a function of etch time by atomic force microscopy and elastic light scattering. A flat surface is found to become rougher during the etch whereas a textured substrate becomes smoother. We have numerically simulated this behavior. It is found that the evolution of surface roughness at length scales between 50 nm and 5 mm can be described with excellent accuracy by a continuum equation for the surface height h( t), which is given by dh/dt5 n2h 2l/2(h)22K4h1 h, where his a random noise input.

We compare the strain relaxation of In0.08Ga0.92As and In0.12Ga0.88As0.99N0.01 epitaxial thin films grown on GaAs (001) by elemental-source molecular-beam epitaxy. The epilayers we studied were essentially identical in their compressive lattice mismatch (0.6260.02%), and thickness (600 nm). The strain state of the samples was determined by in situ substrate curvature monitoring, and by ex situ x-ray diffraction and plan-view transmission electron microscopy. We observe a slower rate of strain relaxation, and a 25% higher residual strain in the nitride. This is attributed to the presence of nitrogen interstitials in the InGaAsN epilayers and/or to the higher nitrogen bond strengths.

Alpha particle energy loss (AEL) spectroscopy was used to characterize a 5 mm pitch grating of silicon bars on a silicon dioxide membrane. Comparison of the data with simulated spectra shows that the angle of nonvertical grating sidewalls are readily quantified by AEL. The potential of AEL for distinguishing undercut and overcut etch profiles is assessed.

In coherent soft x-ray scattering from magnetically ordered surfaces there are contributions to the scattering from the magnetic domains, from the surface roughness, and from the diffraction associated with the pinhole aperture used as a coherence filter. In the present work, we explore the interplay between these contributions by analyzing speckle patterns in diffusely scattered x-rays from the surface of magnetic thin films. Magnetic contrast from the surface of antiferromagnetically ordered LaFeO₃ films is caused by magnetic linear dichroism in resonant x-ray scattering. The samples studies possess two types of domains with their magnetic orientations perpendicular to each other. By tuning the x-ray energy from one of the two Fe-L₃ resonant absorption peaks to the other, the relative amplitudes of the x-ray scattering from the two domains is inverted which results in speckle pattern changes. A theoretical expression is derived for the intensity correlation between the speckle patterns with the magnetic contrast inverted and not inverted. The model is found to be in good agreement with the x-ray-scattering observations and independent measurements of the surface roughness. An analytical expression for the correlation function gives an explicit relation between the change in the speckle pattern and the roughness, and magnetic and aperture scattering. Changes in the speckle pattern are shown to arise from beating of magnetic scattering with the roughness scattering and diffraction from the aperture. The largest effect is found when the surface roughness scatter is comparable in intensity to the magnetic scatter.

Using atomic force microscopy and in situ elastic light scattering we show that the surface of molecular beam epitaxy MBE! grown GaAs tends towards an equilibrium roughness independent of the initial condition, as predicted by kinetic roughening theory. Two separate continuum growth equations are consistent with the observed equilibrium roughness, namely, the Kardar-Parisi-Zhang KPZ! equation ]h/ ]t5 n2h1(l/2) 3(h)21 h, where h is the surface height and h represents nonconservative noise, and the MBE equation ]h/ ]t52 k4h2(L/2)2(h)21 hc , where hc represents conservative noise. These equations represent different physical smoothing mechanisms, so to distinguish between them we have numerically solved both equations. A novel geometric implementation of the nonlinear terms avoids instabilities associated with stiffness of the equations. We find that the time and length scale dependence of the smoothing of initially rough substrates is consistent with the KPZ equation but not the MBE equation. As the growth temperature is increased the coefficient n increases relative to l, but the KPZ description remains valid over the entire measured temperature range of 550 600 °C. Reducing the As overpressure increases the anisotropy of the surface morphology. We provide a physical interpretation of the KPZ equation in which the incorporation rate of mobile adatoms on the surface is governed by evaporation/condensation type dynamics. These results provide important insight into the MBE growth mechanism of GaAs.

An abrupt transition to a 111% faceted growth mode is observed in molecular-beam-epitaxy growth of dilute GaNxAs12x (x,0.05) films on  100! GaAs substrates. The faceted growth mode is favored by high growth temperatures, high nitrogen content, and high arsenic flux. The best electronic quality material, as measured by low-temperature photoluminescence, was obtained at high growth temperatures and high arsenic flux without exceeding the threshold for facet formation. The nitrogen content was found to be insensitive to the arsenic flux.

The mesoscale morphology of homoepitaxial GaAs surfaces is explained with an anisotropic and nonlinear Kardar-Parisi-Zhang (KPZ) model in which adatoms are incorporated into the film from a metastable surface layer. Evaporation-condensation between the film and the metastable layer is proposed as the microscopic physical origin of the KPZ description, as well as of the excess noise observed in the power spectral density. The parabolic mounds observed experimentally in films grown on rough substrates are in good agreement with the surface shape expected from the solution of the KPZ equation in the large amplitude limit.

We propose a new method for fabricating high index contrast gratings made of gallium arsenide (GaAs) and aluminum oxide (AlOx) buried in GaAs based waveguide structures. A central part of this method is a UHV-compatible thermal chlorine etch and MBE-regrowth on etched GaAs and AlxGa1-xAs. We describe results on surface roughness evolution during the etch obtained by in-situ elastic light scattering. These measurements allow us to determine the process parameters that lead to a smooth etched surface on which we demonstrate successful regrowth.

The surface morphology of homoepitaxial GaAs layers grown by molecular beam epitaxy on random and periodically textured substrates has been measured by atomic force microscopy and elastic light scattering. The random texture was obtained by thermal evaporation of the surface oxide and the periodic texture consisted of one-dimensional grating patterns fabricated by holographic lithography. The time evolution of the surface morphology was simulated numerically with a nonlinear growth equation that includes deposition noise and anisotropy in the surface diffusion. The surface of the random substrate develops shallow mounds as the large amplitude initial texture smooths out, an effect that has previously been attributed to unstable growth.

In this paper, we study the homoepitaxial growth of GaAs by molecular beam epitaxy on substrates that have different pre-growth roughness due to the method of removing the native oxide. The evolution of the surface roughness of 1µm thick GaAs films grown at 553°C was monitored in real time using ultraviolet light scattering, and compared with ex situ atomic force microscopy measurements of the power spectral density (PSD) of the surface morphology. The PSD at a spatial frequency of 2 µm-1, is approximately three orders of magnitude larger for films grown on thermally cleaned substrates than for films grown on substrates cleaned with atomic hydrogen. No mounding indicative of unstable growth was observed in the films cleaned with atomic hydrogen.

Coherent soft x-ray scattering experiments from a semiconductor sample consisting of InP islands on a smooth semiconductor substrate are described. The soft x-ray scattering was performed with 266 eV photons produced by an undulator source. Using a position sensitive detector, we are able to detect diffusely scattered x rays in the vicinity of the specular reflection, with an in-plane momentum transfer of up to 6 mm21. Using Huygens Fresnel theory and atomic force microscope images of the surface structure, we simulated the scattering assuming a finite lateral coherence length for the incident radiation. The lateral coherence length of the incident beam was found to be 20 mm from a fit to the observed diffraction pattern from a pinhole. The effect of changes in the surface morphology on the speckle pattern was simulated to explore the potential of coherent soft x-ray scattering for the study of surface structure dynamics.

We present real-time measurements of surface structure evolution during quantum dot growth in InAs/GaAs grown by molecular-beam epitaxy. The measurements were made using an ultraviolet light-scattering technique in which the 254 nm line of a mercury lamp is used as the light source. This technique provides sensitivity to roughness on lateral lengthscales as low as 154 nm for our setup. Using this technique, we can detect the onset of quantum dot formation in this system, as indicated by reflection high-energy electrondiffraction measurements. The continuous increase in the scattering signal after the dots have formed, is explained in terms of diffusion-limited growth and ripening of the large islands that coexist with the quantum dots.

The α-particle energy loss method (AEL) has been implemented in situ to monitor film thickness and composition during growth of GaAs, InP and LaF3 based materials by molecular beam epitaxy (MBE). In the AEL method, a ^228Th source is used to recoil implant a 5 mm diameter region of the surface of the wafers with the α-emitter daughter isotope ^224Ra prior to growth. The implanted nuclei decay with a half life of 3.7 days through a sequence of daughters which emit alpha particles at di!erent energies. Deposition on the surface causes the emission lines to be shifted to lower energies due to energy loss in the film. For substrates marked with a low activity (  30 kBq; similar to activity of smoke detectors) we are able to measure film thickness with +/- 6 nm uncertainty and growth rate with +/-0.01 nm/s uncertainty in real time. By measuring the relative growth rates of the di!erent materials, AEL also allows us to infer the composition of a ternary layer film as well as the sticking coefficients rates directly at different growth temperatures.

The a-particle energy loss method  AEL! has been implemented in situ to monitor film thickness during growth by molecular beam epitaxy. For InP and GaAs substrates recoil implanted with a-particle emitters, we have been able to measure thickness and composition of deposited GaAs, AlGaAs and InGaAs in real time. The AEL method yields in situ real time results comparable in accuracy to those obtained by ex situ scanning electron microscope and high-resolution x-ray diffraction measurements.

The Si 111! 737 surface is exposed at room temperature to atomic hydrogen and studied with scanning tunneling microscopy  STM! and low energy electron diffraction  LEED! in ultrahigh vacuum. For increasing exposures, the LEED pattern of the surface changes in well defined steps to a 131 pattern. The STM images of the 131 surface appear rough and disordered. The 131 pattern and rough surface are consistent with an etching of several surface layers by the atomic hydrogen. Heating the 131 surface to 560 °C and cooling to room temperature produces a surface with a )3)R30° LEED pattern. The STM images of this surface are consistent with a distribution of adatoms on an ideal Si 111! surface. Further heating results in a 737 surface.

We report elastic light scattering measurements of the surface morphology of strained InxGa1 xAs on GaAs, grown by molecular beam epitaxy at different growth temperatures and In contents. During strain relaxation through formation of interfacial misfit dislocations, the surface of the film roughens in response to inhomogeneous surface strains produced by the interfacial misfit dislocations. The time dependence of this roughening is modeled by an Edwards Wilkinson equation in which the deposition flux noise is neglected and the inhomogeneous surface stress is the only driving term.

The temperature dependences of the optical absorption edges of semi-insulating GaAs and InP have been measured from room temperature to 905 °C and 748 °C, respectively, with accuracies of 61 °C at room temperature and 65 °C at 900 °C. The temperature dependence of the optical absorption edge is adequately reproduced by an Einstein model although the Varshni model gives an improved fit to the band gap. Finally, the widths of the absorption edges are correlated with ionicity.

A combination scanning tunneling microscope  STM! and scanning electron microscope has been used to selectively pattern and image Si 111! and Si 001! surfaces in a SEM chamber. Both positive and negative STM tip polarities produce marks on the surface with linewidths as small as 20 nm. We have transferred these patterns to the silicon substrate via selective wet chemical etching. We demonstrate a well-defined voltage threshold for patterning at negative tip bias. The modifications are consistent with a model based on hydrogen depassivation and electric field mediated deposition. The physical mechanism for positive tip bias patterning is shown to be different than that for negative bias. At positive bias, the patterning is consistent with selective surface contamination.

An air-bridged, 120-nm-thick semiconductor slab with a two-dimensional  2D! square array of through holes on a 480 nm pitch  L! was fabricated using selective wet etching techniques. The second order photonic resonances of the structure were studied by comparing broadband optical scattering data with numerical solutions of Maxwell s equations. Features observed in these spectra over a 1200 cm21 range, near 9500 cm21, indicate that the 2D texture splits the energy degeneracy of slab modes with propagation constants 62p/L,0% and 0,62p/L% by as much as 14%.

The electronic structure of porous Si is investigated using soft X-ray fluorescence spectroscopy. Significant changes are observed as compared to bulk Si, which we interpret as due to altered electronic structure in the Si nanostructures. By imposing standing wave boundary conditions on the valence band wave functions, we calculate the fluorescence spectrum for thin Si sheets of different orientations. For a (100)-oriented sheet, the calculation is in good agreement with the experimental spectra, suggesting that the nanostructure in porous Si is predominantly in the form of thin Si (100)-type sheets.

A method for determining both the thickness and the average stoichiometry of thin films is presented. The method is based on implanting radioactive -sources in the substrate prior to layer growth and measuring the energy loss of the -particles as they traverse the layer. Information about the stoichiometry is obtained through the comparison of the energy loss of -particles of different initial energies. Experimental examples for the utilization of this method are presented, in which Sb was grown on Si substrates, GaAs, InAs and AlAs on GaAs and YBCO on YSZ. The experimental precision which can be expected using the method is discussed, together with specific scenarios in which it could be advantageously applied.

Elastic light scattering has been used to measure the time evolution of the power spectrum of the surface morphology of strained InGaAs layers during growth. From a combination of and Ga1 xed scattering angle measurements during growth and variable scattering angle measurements after growth, we are able to determine both the time and spatial frequency dependence of the power spectral density during relaxation of the strained lms via mis t dislocations. The data are t with an Edwards-Wilkinson model, for which the surface morphology is driven by inhomogeneous surface strains.

High resolution Fabry Perot fringe spacing measurements are used to determine the group index dispersion of TE and TM polarized modes in single quantum well InGaAs/AlGaAs/GaAs graded index separate confinement heterostructure waveguides. The TE mode data, over a ;175 nm range below the quantum well band gap, is compared with model calculations of guided mode dispersion using existing empirical formulas for the index dispersion of AlxGa12xAs and GaAs, and different phenomenological expressions for the TE index dispersion of the InGaAs quantum well. A satisfactory fit is obtained when the quantum well is modeled as a two-dimensional set of equal strength oscillators with a constant density of states, plus a single 1S excitonic level.

The equilibrium distribution of hydrogen chemisorbed to adatoms on Si 111! 737 surfaces has been determined by counting the occupied adatom sites in scanning-tunneling-microscope images of hydrogen-treated Si surfaces. From the distribution of the chemisorbed hydrogen, we are able to determine the relative binding energies of hydrogen to the four different symmetry adatoms.

A new technique is reported for obtaining a smooth growth surface during molecular beam epitaxy growth of GaAs on (001) GaAs substrates. The smoothing is achieved by exposing the surface to an indium flux during growth of the GaAs buffer layer at high substrate temperature (590^oC). In situ light scattering shows a dramatic reduction in surface roughness along the [110] direction during this process, consistent with ex situ scanning force microscopy measurements on the films, which reveal atomically flat terraces oriented along the <110> directions. The surface reconstruction shifts from (2×4) to (4×2) under the indium flux as revealed by reflection high-energy electron diffraction. The sticking coefficient of the indium as determined by secondary-ion mass spectroscopy is less than 10%. We attribute the observed effects to a transition from group V to group III termination, induced by surface segregation of the nonincorporated indium.

The relationship between the surface morphology and strain relaxation is explored in strained In_xGa_1-xAs layers grown on GaAs by molecular beam epitaxy. In situ light scattering, detected simultaneously along [110] and [1 -1 0], reveals an asymmetric surface roughening which is consistent with ex situ scanning force microscopy. Transmission electron microscopy shows that strain relaxation by misfit dislocation formation occurs before the surface roughening is detected, for In_0.18Ga_0.82As films grown at 490°C.

The temperature dependence of the optical-absorption edge (Urbach edge) of GaAs is measured in semi-insulating and n-type GaAs (n=2X 10 cmm3) over the temperature range from room temperature to 700 C. Both the optical absorption and the temperature are measured using a diffuse reflectance technique. The characteristic energy of the exponential absorption edge is found to increase linearly with temperature, from 7.5 meV at room temperature to 12.4 meV at 700 C, for semi-insulating GaAs. The temperature dependent part of the width of the Urbach edge for semi-insulating GaAs is six times smaller than predicted by the standard theory where the edge width is proportional to the phonon population.

Photoemission and x-ray absorption spectroscopy show that both the conduction and valence bands of porous silicon are shifted relative to the bands for bulk silicon, as expected in the quantum confinement model for the optical properties of porous silicon. The shift in the valence band is larger than the shift in the conduction band and proportional to it, with a proportionality constant that is consistent with effective mass theory. No oxygen is detected in the as-prepared porous silicon.

The role of the on-site Coulomb repulsion energy in the electrical excitation of rare-earth ions by minority carrier injection in semiconductors is examined. Resonant photoemission measurements of the interfacial electronic energy level alignment in NdF,/silicon heterojunctions show that the large Coulomb repulsion energy of the atomic-like 4f orbitals prevents the direct excitation of the 4f levels by either electron or hole capture.

The temperature for thermal desorption of the gallium oxide from GaAs is shown to increase linearly with oxide thickness. In addition, we show by diffuse light scattering that highly polished GaAs substrates roughen during the oxide desorption. These results are interpreted in terms of a model in which the oxide evaporates inhomogeneously.

A new optical temperature measurement technique for use in molecular beam epitaxy is demonstrated with GaAs substrates. The temperature of the semiconductor is inferred from its band gap, which is measured by the diffuse reflectivity of the substrate that is textured on the back surface. The method has a sensitivity of better than 2C and an absolute accuracy limited by the accuracy with which the band gap is known as a function of temperature. It was found to be necessary to calibrate the measurement technique with a thermocouple in contact with the sample in order to achieve satisfactory accuracy at high temperatures. Measurements of the optical absorption edge of GaAs, show that the slope of the Urbach edge is independent of temperature from room temperature to 450C.