A. Sacedón

Design of InGaAs linear graded buffer structures

The relaxation of compositionally graded InGaAs buffers, with and without uniform cap layers, has been studied. Simple InGaAs linear‐graded layers on GaAs substrates never reach complete relaxation. The residual strain in these structures produces a dislocation‐free strained top region while the rest of the buffer is nearly completely relaxed through misfit dislocations, as observed by transmission electron microscopy (TEM). This strained top region is analyzed and its thickness compared with theoretical calculations. The effects of different cap layers on the relaxation behavior of the graded buffer has been studied by double crystal x‐ray diffraction, TEM, and low temperature photoluminescence, and results compared with predictions of the models. The optical quality of the cap layer improves when its composition is close to the value that matches the lattice parameter of the strained surface of the grade. The design of linear graded buffers having a strain‐free cap layer with high crystalline quality is...

Strain relief in linearly graded composition buffer layers: A design scheme to grow dislocation‐free (<105 cm−2) and unstrained epilayers

The strain relaxation in linearly graded composition InGaAs layers grown on (001) GaAs substrates by molecular beam epitaxy is studied by transmission electron microscopy (TEM) and double crystal x‐ray diffraction (DCXRD). The dislocation distribution in these layers does not coincide with the predicted equilibrium dislocation distribution [J. Tersoff, Appl. Phys. Lett. 62, 693 (1993)]. The dislocation density in the dislocation‐rich layer thickness is slightly smaller than the equilibrium density. The thickness of the dislocation‐rich region is different in the [110] and [110] directions. A good correspondence exists between the TEM and DCXRD strain measurements. The dislocation distribution observed by TEM has made it possible to design a scheme to grow dislocation‐free and unstrained top layers on linearly graded composition buffer layers.

DEPENDENCE ON THE IN CONCENTRATION OF THE PIEZOELECTRIC FIELD IN (111)B INGAAS/GAAS STRAINED HETEROSTRUCTURES

A series of (111)B InxGa1−xAs/GaAs multiple quantum well p‐i‐n structures have been investigated via low temperature photocurrent and photoluminescence spectroscopies. The indium mole fraction was in the range of 0.07–0.23. Evolution of the experimental blueshifts of the transition energies, with the external bias, agreed very well with the theoretical calculations. This allowed us to obtain precise information about the piezoelectric constant, e14, for the various In compositions. For the range of x investigated, we have found e14(x) to be linear with x but significantly lower than predicted by a simple linear interpolation of the accepted values for GaAs and InAs.

Dislocation behavior in InGaAs step‐ and alternating step‐graded structures: Design rules for buffer fabrication

A comparison between compositionally stepped and alternating step‐graded structures used in the production of a relaxation buffer layer is carried out by means of transmission electron microscopy. The latter shows higher efficiency in relieving the strain. A simple balance force model permits us to understand the reason for a higher generation of threading dislocations observed in the alternating step‐graded structures. The presented results can be applied as new design rules for buffer fabrication that contrast in some key points with previous published rules as, for example, the ‘‘zero‐net‐strain’’ precept [D. Dunstan, P. Kidd, P. F. Fewster, N. L. Andrew, L. Gonzalez, Y. Gonzalez, A. Sacedon, and F. Gonzalez‐Sanz, Appl. Phys. Lett. 65, 845 (1994)].

Relaxation mechanism of InGaAs single and graded layers grown on (111)B GaAs

Abstract In this work we study the influence of the composition (strain) and the layer thickness of InGaAs grown on (111)B GaAs substrates on the surface morphology and defect distribution (strain relief). Samples of uniform (x=0.13 or 0.22) and graded (xinitial=0.10 for each sample; xfinal=0.17 and 0.25) composition have been studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). The relaxation mechanism for the two series of samples is the same. SEM observations reveal a surface roughness with a triangular structure, where the `cross-hatch pattern distribution is more isotropic and homogeneous as the thickness and In composition increase. Misfit dislocations and deformation twins (band twins), anisotropically and inhomogeneously arranged, are observed in all the studied samples. Deformation twins are formed by the nucleation of partial dislocations from the film surface. A mechanism of the observed band twins formation is proposed.

Step-graded buffer layer study of the strain relaxation by transmission electron microscopy

Abstract The lattice relaxation behavior in an In x Ga 1−x As/GaAs linearly step-graded structure is studied by transmission electron microscopy (TEM). From the misfit dislocation densities measured by TEM at each interface the relaxation parameters such as strain and percentage relaxation are deduced for each layer. The obtained results are compared with the predictions of the Dunstan et al. model which describe the dislocation behavior during relaxation in such structures. A different relaxation behavior than that described by Dunstan et al. is observed. This is attributed to the fact that the individual layer thickness is lower than the critical layer thickness of Dunstan et al. Work-hardening processes are found to induce a linear increase in the residual strain with increasing layer thickness.

Dislocation Distribution in Graded Composition IngaAs Layers

The defect distribution of a graded composition InGaAs layer grown on GaAs by MBE has been characterized by TEM (XTEM, PVTEM, HREM). The observed configuration does not correspond completely with that theoretically predicted. Dislocation misfit segments are in a quantity much bigger than in constant composition layers. Dislocation density is quite uniform up to a certain layer thickness t 1 . Few dislocations are observed between this t 1 thickness and a larger thickness t 2 . Dislocation density is below the detection limit of XTEM for thicknesses bigger than t 2 . Some dislocations are observed to penetrate in the GaAs substrate. Several mechanisms (reactions between 600 dislocations, Hagen-Strunk and modified Frank-Read processes) are proposed to explain the interactions of dislocations in the epilayer and their penetration in the substrate.

Sequential tunneling in [100]‐ and [111]‐oriented InGaAs/GaAs multi‐quantum wells by photocapacitance

The features present in the capacitance– and conductance–voltage characteristics of [100]‐ and [111]‐oriented InGaAs/GaAs multi‐quantum well (MQW) p‐i‐n structures under illumination are described. Sequential tunneling of electrons and holes is proposed to explain the different peaks observed, as they occur at voltages corresponding to resonant alignment of energy levels of adjacent wells. Two kinds of piezoelectric [111] devices are analyzed, with positive or negative average electric fields in the MQW region. The voltage corresponding to zero average electric field in the MQW is detected by a feature in the dark device capacitance. For the sample with negative average electric field, the capacitance characteristics clearly reflect the presence and evolution with voltage of a long range screening of the piezoelectric fields by the photogenerated carriers.

Structural study of AlGaAs/InGaAs superlattices grown by MBE on (111)B GaAs substrates

Abstract AlGaAs/InGaAs strained layers superlattices (SLS) have been grown by molecular beam epitaxy on (111)B and (100) GaAs substrates at different temperatures (520, 540 and 560 °C). The heterostructures are studied by transmission electron microscopy, scanning electron microscopy and low-temperature photoluminescence. The controversy concerning the growth temperature for In-based/Al-based III-V alloys on (111)B GaAs is discussed. In this context, the (111)B and their corresponding (100) structures are compared. The best growth temperature is 560 °C, as a smooth surface and the lowest defect (dislocations, planar defects and stacking faults tetrahedra) density is obtained at this temperature.

Transmission electron microscopy study of InxGa1-xAs/GaAs multilayer buffer structures used as dislocation filters

To investigate the efficiency of threading dislocation (TD) filtering in multilayer (ML) buffers, InxGa1-xAs/GaAs structures are studied by transmission electron microscopy. The TDs are generated at the first In0.3Ga0.7As thick layer. Different MLs harder and softer on average than this TD generator thick layer are used as filters. The efficiencies of the ML filters are compared with that of a bulk layer with identical average In content. As the individual ML thickness is below the Matthews-Blakeslee critical thickness, no TD bending should occur. However, in contrast with the softer buffer behavior, MLS buffer harder than the TD generator layer are shown to filter. This difference in the TD behavior between the two kinds of structure is attributed to yield strength effects. From a kinetic model, an expression for the TD density vs. the height is deduced. On application of this model to the measured TD densities, filtering parameters are deduced.