C. Priester

Surface roughness, strain, and alloy segregation in lattice-matched heteroepitaxy

Spinodal decomposition of ternary semiconductor alloys during lattice-matched heteroepitaxy is considered here. It has been previously demonstrated that a perfectly flat surface (with no step) would forbid alloy demixing. The case of a rough surface is the purpose of this article. How the possibility of a better strain relaxation introduced by the surface roughness can favor alloy demixing is analyzed first. The present results are exemplified by the AlInAs lattice matched to the InP case. Second, a step-by-step model is proposed to simulate the growth process on a rough surface. This model leads to a description of the strain and alloy demixing during this growth. This study clearly shows how and why the atoms corresponding to binary materials with lower surface tension naturally tend to segregate towards bumped areas.

Accuracy of zero‐charge and zero‐dipole approximations for the determination of valence‐band offsets at semiconductor heterojunctions

A tight‐binding treatment of (100) heterojunctions with common anions is performed. A new zero‐dipole approximation is presented together with a first‐order correction that explicitly depends upon the static macroscopic dielectric constant. The difference between the zero‐charge and zero‐dipole approximations is discussed. These two approximations are then compared to a complete self‐consistent calculation and shown to provide band offsets within 0.1 eV of the exact value. The corrected zero‐dipole treatment gives a much better accuracy of order of 0.02 eV. Agreement with experiment is very good for the three lattice matched systems: AlAs–GaAs, CdTe–HgTe, and AlSb–GaSb.

Electronic structure of sodium atoms adsorbed on the GaAs(110) surface

The electronic structure of Na atoms adsorbed on the GaAs(110) surface is calculated in the tight‐binding approximation and the zero charge limit. For a monolayer and half a monolayer coverage, two different adsorption sites are considered. The surface atom and sodium core level shifts are calculated and compared to the experiment. The adatom binding energy is found to be maximum for adsorption on the surface Gallium dangling bonds. Finally the density of states and the interface state dispersion energy are compared with photoemission results.

Strained layer epitaxy: How do capping layers and oppositely strained intermediate layers enhance the critical thickness?

Within a valence force field framework, we calculate the critical thickness of a film that is lattice mismatched to the substrate on which it is epitaxially deposited. A capping layer that is lattice matched to the substrate is known to enhance the critical thickness. We calculate the efficiency of the capping layer as a function of its thickness. We also demonstrate that this efficiency can be improved by using a capping layer that is not lattice matched to the substrate. When the capping layer is undesirable, another way of enhancing the critical thickness is to use an oppositely strained intermediate (OSI) layer between the substrate and the film. To maximize efficiency, the OSI layer lattice constant should be larger (respectively, smaller) than that of the substrate if the film is in tension (respectively, compression). We demonstrate that a single OSI layer has no significant effect on the critical thickness. A microscopic description of the strain on all of system is provided, and the mechanism of strain compensation is explained.

Calculation of band offsets in strained II–VI heterojunctions

Abstract The self-consistent tight-binding description of unstrained and strained heterojunctions has recently provided theoretical values for the offset of the average valence band Δ E av v which agree well with experimental data for III–V and II–VI semiconductor compounds. We present here calculations of Δ E av v in other strained heterojunctions with common anion or cation atoms such as ZnS, ZnSe, ZnTe, HgTe and CdTe. The charge transfer in the interface neighbourhood is taken into account by a self-consistent description using realistic values of the dielectric constant e ( q ). First, we have obtained the hydrostatic deformation potentials. Then the Δ E av v are calculated for different situations which can occur for strained heterojunctions according to the strain is confined (i) to one side of the heterojunction as, for instance, CdTe strained to ZnTe or (ii) to the other side: ZnTe strained to CdTe. Results are discussed and compared with those obtained for similar compounds and also to experimental values.

Heterojunction band offset: A local feature

After an extensive compilation of calculated band offsets obtained from a self‐consistent tight‐binding description, it is shown how these results can be recovered by the use of a local description based on the molecular model of the semiconductor band structures. This local description allows us to explain the mechanism which relates the band offset modification to the chemical structure of the interface.

Lateral Organization of Quantum Dots on a Patterned Substrate

The work reported here focuses on the role of nanopatterning in strained heteroepitaxy. This study makes use of an atomistic description. Two types of prepatterning are considered: (i) A perfectly periodic strain field induced by a buried array of twist interface dislocations in a twist-bonded bilayer substrate. Network periodicity is controlled by the misorientation angle between the substrate and the surface bonded layer. For a thin enough surface bonded layer (a few tens of nm) the strain field variations appear to be strong enough to laterally organize quantum dots when a strained layer is grown (Ge deposited on a Si/Si twist bonded sample). The mechanism is similar to what happens in vertical alignment in multi-quantum dots layers. (ii) Nanomesas: a quite regular array of nanomesas can be obtained by using stress-selective etching of the surface of twist bonded samples. A preliminary study of strained growth on nanomesas compared to strained growth on ideally flat substrates is also reported. It is shown how and why the elastic relaxation at the edges of the mesas can delay or even inhibit the 2D-3D transition. However, related to the design parameters of these nanomesas, one still gets 3D quantum dots (whose shapes are quite different from the usual self-assembled quantum dots shapes) which are very well laterally organized and calibrated.

Composition modulation and local structure in strained diluted GaInNAs nitride alloy thin layers on GaAs substrates

The theoretical study reported here is devoted to diluted GaInNAs nitride alloys, and focuses on correlation between local chemical atomic neighborhood and atomic distances. The model used is Valence Force Field approximation, and we model a periodic Ga 1−x In x N y As 1−y alloy film deposited on a GaAs substrate. The surface is dimerised (for simplicity we consider 2×1 anion rich surfaces). First starting from a random film, first and second nearest neighbor distances are calculated, and the corresponding histogram drawn. Then a pseudo-annealing process is simulated by allowing the N atoms to choose their optimal location. This pseudo-annealing strongly enhances the number of In-N bonds, in agreement to experimental studies. From this statistical study, fine structure of each peak of the histograms is shown to be not directly related to a given surrounding chemical distribution up to 3rd nearest neighbors. The positions (in distance) of the peaks appear not to be modified by alloy composition nor alloy segregation, which only alter relative intensities and peak shapes. Last, we consider stripes of In-rich and In-poor zones: calculated energy variations show a strong tendency for N atoms to completely desert In-poor zones.

Plastic Relaxation Mechanics in Systems with a Twist-Bonded Layer

Abstract : With a view to investigating how a thin film twist-bonded to a host substrate can have compliant behavior from a plasticity point of view the onset and spread of edge dislocations throughout a mesa are studied. The discussion focuses on the energy relaxed by such dislocations in a mesa made from two coherently bonded lattice-mismatched layers twist-bonded onto a host substrate and patterned down to the film/host substrate interface. Our theoretical results show that the confinement of threading dislocations into a thin twist-bonded film is energetically favorable allowing the overgrowth of a mismatched layer exempt of any threading dislocation at least as far as mesas are concerned.