摘要:

A general analytical model is presented for the simulation of step‐flow epitaxial growth on two‐domain surfaces composed of alternating type‐A and B terraces. Separate terms are included for adatom attachment and crossing probabilities at ascending and descending steps on each of the two terrace types. The model is used to follow the evolution of terrace size distributions during deposition, focusing primarily on the case of single‐terrace adatom migration. Positive attachment asymmetries &Dgr;a(i.e., a larger attachment probability at ascending steps) were found, as in the case for one‐domain surfaces, to lead to a slow smoothing of size distribution fluctuations. However, even very small negative &Dgr;avalues result in a rapid increase in fluctuation amplitudes with a tendency toward step bunching and the formation of double‐height steps. The two terrace size distributions diverge essentially immediately upon initiating growth since each terrace is bordered by two terraces of the opposite type and only short‐range migration is required to stabilize the average widths of the two distributions. Fractional surface coveragesfA (B)of A (B) terraces increase at the expense of B (A) terraces when &Dgr;aB (A)≳&Dgr;aA (B). Steady‐state average terrace widths are achieved rapidly, within a few monolayers; however, size‐distribution standard deviations &sgr; evolve toward steady state slowly (for &Dgr;aA,B≳0) with &sgr;A (B)∝exp(−&Dgr;aA (B)&thgr;/&lgr;2) where &thgr; is the number of deposited monolayers and &lgr; is the fluctuation width. Allowing multiterrace migration decreases, under some growth conditions, the rate at which terrace size distributions diverge and introduces oscillations in &sgr;A,B(&thgr;). Simulation results are compared with available experimental data. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360980

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Ferromagnetic MnAs thin films grown on GaAs (001) substrates by molecular‐beam epitaxy have been studied by the methods of grazing incidence x‐ray scattering, x‐ray diffraction, and extended x‐ray‐absorption fine structure. Microstructures in two films prepared with different first‐layer growth conditions (template effects) are compared in terms of the interfacial roughness in the layer structure, lattice constants, epilayer thickness, local environment surrounding the Mn atoms, coordination number, and local disorder. Our results indicate that the template effects can cause significant differences in the local structures and crystallinity of the MnAs epitaxial layers. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360981

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We report on a structural study of Si/Ge multilayers grown by molecular‐beam epitaxy on (100)‐Si substrates. The analyses have been performed by using transmission electron microscopy, high‐resolution x‐ray diffraction, and secondary‐ion‐mass spectrometry. The investigated specimens differ in number of periods, period thickness, and in the Si/Ge periods thickness ratio. In particular, we investigate the interdiffusion of the Ge atoms in each superlattice period of the epilayer and in the epilayer as whole. The interdiffusion causes a broadening of the nominal thickness of the Ge layer producing a SixGe1−xalloy. Furthermore, the Ge content in the multilayer periods increases as a function of the growth time, i.e., the superlattice periods close to the sample surface contain more Ge atoms if compared to the periods close to the substrate/superlattice interface. We find two steps in the strain relaxation: (i) In each period the strain energy density is partially reduced by the formation of coherent islands; (ii) at a certain value of the strain energy density the shape of the islands changes and the structures relax partially or completely the accumulated strain energy by nucleation of extended defects. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360982

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Two‐dimensional, nonequilibrium molecular dynamics simulations have been applied to study the structure of thin films grown on single‐crystal Lennard‐Jones substrates. The principal microstructural features to develop within these films are single vacancies and small voids which tend to be slightly elongated and to be aligned in the growth direction. Both the void volume and the mean surface roughness of the films are found to be decreasing functions of substrate temperature and deposition kinetic energy. Voids are shown to form as a consequence of both surface roughness and shadowing effects. The attraction between deposited atoms and the sides of surface depressions lead to the formation of outgrowths from the sidewalls of the surface depression. These outgrowths shadow the open void beneath them and continue to grow across the voids by interaction with the depositing atoms until a continuous bridge is formed that closes off the void. Since this bridging mechanism leaves behind a surface depression above the closed‐off void, new voids tend to form above it. This leads to the alignment of voids along the film growth direction. The spacing of the resultant void tracks is correlated with the wavelength of the surface roughness. Increasing temperature and deposition kinetic energy enhancing surface mobility leads to an increase in the wavelength of the surface roughness and hence an increase in the spacing between void tracks. Edge dislocations tend to form within voids as a natural consequence of the void bridging process, however nondislocated voids are also observed. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360983

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The true area of contact between pharmaceutical powder particles and flat surfaces of various materials in adhesion contact has been calculated. The centrifuge technique has been used to measure the adhesion force between the particles and the surfaces after application of a defined press‐on force, which caused plastic deformation of the surface asperities. It was found that in most cases the true area of contact is less than 15% of the apparent area of contact, and for irregular particles the true area of contact is less than 10% of the apparent area of contact. Principal Component Analysis indicated that there is a strong proportionality between the work of adhesion, the indentation hardness and the surface roughness on one side, and the reduced Young’s modulus at the contact points between the surfaces and the adhesion force measured on the other side. A model equation could be proposed that allows an approximate prediction of the adhesion force from the four physical properties of the materials in contact. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360984

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The growth kinetics of ultrathin SiO2films on silicon in a nitrous oxide (N2O) ambient have been investigated as a function of oxidation temperature and time. The results show that the overall growth follows the linear‐parabolic law proposed by Deal and Grove [J. Appl. Phys.36, 3770 (1965)]. The data analysis indicates that although the oxidation proceeds by surface‐limited reaction in the initial stage, it rapidly changes into a diffusion‐controlled reaction. This behavior is evidenced from the fact that the reaction of the N2O molecule with the silicon surface produces an interfacial nitrogen‐rich layer which acts as a barrier to the oxidant passing through the SiO2/Si interface. From the Arrhenius equation for N2O oxidation, the activation energies for the linear rate constantB/Aand for the parabolic rate constantBare determined to be 1.5 and 2.3 eV, respectively. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360985

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A modified method of side data analysis of deep level transient spectroscopy (DLTS) spectra obtained by Lang’s method [J. Appl. Phys.45, 3023 (1974)] is proposed. It uses two DLTS spectra determined for the same or different ratio of the sampling times. Simple analytical formulas are given to utilize their low‐ and high‐temperature side data in the Arrhenius analysis. The proposed method eliminates the temperature dependence of the spectrum amplitude and does not require the accurate determination of the temperatures of DLTS spectra in their maxima. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360986

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The temperature dependence of the dc conductivity of electrochemically polymerized films of polypyrrole, poly(N‐methyl pyrrole) and their copolymers, poly(N‐methyl pyrrole‐pyrrole), having different percentage of BF−4ions has been investigated in the temperature range 77–350 K. The observed behavior could be explained in terms of Mott’s variable range hopping model involving a single phonon process. The estimated values of polaron radius yield the realistic values of density of states at the Fermi level which are in good agreement with the values reported earlier for other polyconjugated systems. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360987

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We report on optical and electrical properties of modulation doped InxGa1−xAs/InP single quantum wells in the composition range 0.56≤x≤0.79. Cyclotron resonance, contactless Shubnikov–de Haas and magnetophotoluminescence experiments are used to obtain two dimensional carrier densities, effective masses of electrons and holes and scattering times. We present data which give evidence for zero magnetic field spin splitting. The dispersion relation for electrons and holes is presented. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360988

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Dark current noise measurements between 101and 105Hz were carried out on a compressively strainedp‐type InGaAs/AlGaAs quantum‐well infrared photodetector as a function of temperature and bias voltage. The measured noise can be attributed to number fluctuation noise associated with the generation and recombination of holes from and to the quantum‐well bound states and the extended valence‐band states. At low bias the number fluctuation noise translates into current fluctuation noise via hole diffusion, where as at higher‐bias values the coupling is via the hole drift current component. Our measurements indicate that the field‐induced barrier lowering and the Schottky image effect strongly influence the device characteristics. In addition we observe that the thermally generated heavy holes diffuse, at low fields, on the average to the nearest neighboring quantum well where they subsequently recombine. This recombination process is triggered by hole scattering with the acceptor centers. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360989

出版商:AIP    

年代:1996

数据来源: AIP