H. H. Wieder

Effects of GaAs substrate misorientation on strain relaxation in InxGa1−xAs films and multilayers

We have investigated the effects of GaAs substrate misorientation on strain relaxation in InxGa1−xAs films and multilayers. Our calculations of shear stresses due to misfit strain, resolved on the glide plane in the glide direction, reveal that the α and β 60° slip systems are influenced in a nearly identical fashion, for all substrate misorientation directions. Thus, classical models for nucleation and glide of 60° dislocations predict that a substrate misorientation will not influence the degree of 〈110〉 asymmetry in strain relaxation in lattice-mismatched zincblende semiconductor films. Contrary to these predictions, our experimental results reveal asymmetries in strain relaxation (for partially relaxed single layers) which favor those dislocations aligned with the offcut axis. These asymmetries depend on the substrate misorientation and growth temperature, and are not easily explained by differences in the intrinsic core properties of α and β dislocations. Furthermore, in fully relaxed multilayers (gr...

Correlation of buffer strain relaxation modes with transport properties of two‐dimensional electron gases

We have investigated the effects of buffer strain relaxation on the transport properties of two‐dimensional electron gases (2DEGs). The 2DEGs consist of modulation‐doped In0.53Ga0.47As/In0.52Al0.48As heterostructures grown lattice‐mismatched to GaAs via compositionally step‐graded InxGa1−xAs buffers, with different composition gradients, or lattice‐matched to InP. We find a variation in 2DEG electronic properties which occurs simultaneously with large differences in epilayer tilt and mosaic spread in the step‐graded buffers. This indicates a correlation between the mechanism of buffer strain relaxation and the 2DEG transport properties.

Correlation of anisotropic strain relaxation with substrate misorientation direction at InGaAs/GaAs(001) interfaces

We have investigated the effects of substrate misorientation towards (111)A, (111)B, and (011) on asymmetries in the strain relaxation of InxGa1−xAs, grown on (001) GaAs substrates by molecular beam epitaxy. For epilayers grown under conditions of two‐dimensional growth, we find large anisotropies in bulk strain relaxation and epilayer rotation about an in‐plane axis (epilayer tilt) in proportion to the degree of (111)A misorientation. The residual strain asymmetry is largest for the (111)A misoriented substrate (≳50%) and smallest for the (111)B misoriented substrate (<15%). At higher growth temperatures, the bulk strain relaxation becomes isotropic while the epilayer tilt remains sensitive to the offcut direction. At all temperatures, a preference for epilayer tilt toward the [110] direction for (011) misorientations is observed.

Accumulation mode Ga0.47In0.53As insulated gate field‐effect transistors

Preliminary results obtained on enhancement‐type insulated gate field‐effect transistors are described. These are based on the surface accumulation of heteroepitaxially grown Ga0.47In0.53As layers whose residual donor impurities are compensated by deep level Fe acceptors.

Anisotropic structural, electronic, and optical properties of InGaAs grown by molecular beam epitaxy on misoriented substrates

We have investigated the structural, electronic, and optical properties of partially strain‐relaxed InxGa1−xAs layers, grown by molecular beam epitaxy on both misoriented and nominally flat (001) GaAs substrates. We find large anisotropies in bulk strain relaxation, interfacial misfit dislocation density, dark‐line defect density, and electron mobility, as well as a polarization anisotropy in cathodoluminescence for epilayers grown on misoriented substrates, in comparison with those grown on flat substrates.

Optical detection of misfit dislocation‐induced deep levels at InGaAs/GaAs heterojunctions

Using variable‐depth luminescence excitation probes, we have observed discrete emission at characteristic energies from deep electronic states associated with misfit dislocations at InGaAs/GaAs interfaces. These states are localized near the buried heterointerface within the InGaAs layer and exhibit only minor variations in energy with composition and strain. The dislocation‐induced spectral features appear only in strain‐relaxed InGaAs films and are uncorrelated with additional features due to native bulk defects.

Modulation‐doped In0.53Ga0.47As/In0.52Al0.48As heterostructures grown on GaAs substrates using step‐graded InxGa1−xAs buffers

We have grown modulation‐doped In0.53Ga0.47As/In0.52Al0.48As heterostructures on GaAs substrates using compositionally step‐graded InxGa1−xAs buffers. Triple‐axis x‐ray diffraction measurements indicate nearly complete and isotropic strain relaxation in the buffer, lattice matching of the active layers with the top of the buffer, and no significant epilayer tilt. The temperature dependence and the photoresponse of the electron mobility suggest that transport in the heterostructures is limited principally by remote ionized‐impurity scattering, with mobility values comparable to those of heterostructures grown lattice‐matched to InP.

Electron–phonon interaction studies in an In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum well structure

Joule heating measurements are carried out over a wide range of temperatures on a two-dimensional electron gas fabricated in an In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As heterostructure system that has a 25nm wide In0.53Ga0.47As quantum well region. The energy loss rate is observed to be an indicator of the electron–phonon coupling processes in these systems. The temperature dependence of the energy loss rate is studied from 30mK to 30K and points toward a possible dominant unscreened piezoelectric coupling to the acoustic modes over temperatures of 1–30K, with boundary scattering in the ohmic contacts gaining importance at very low temperatures. The temperature decay of the energy relaxation time exhibits a T−3 behavior at high temperatures that transitions to a T−1 at lower temperatures and tends to saturation at millikelvin levels.

Electric field‐induced negative photoconductivity in GaAs

High‐field negative photoconductivity observed below the fundamental band gap of GaAs is attributed to the electric field‐dependent capture cross section of an electron donor level located at 1.2 eV relative to the valence‐band edge and to a dynamic balance between emission, trapping, and recombination of optically induced charge carriers.

Electron heating measurements in an In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As heterostructure system

Electron heating experiments are carried out on a two-dimensional electron gas fabricated in an In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As heterostructure and the energy loss rates and energy relaxation times are extracted from the Shubnikov–de Haas oscillations to get a better understanding of the electron–phonon interaction in these heterostructures. The exponent of the energy loss rate dependence is used to indicate a dominant unscreened piezoelectric coupling in the temperature range of 4.2–30 K. Simulations of the quantum well band structure verify the presence of two populated subbands in this heterostructure system. The energy relaxation time is observed to be on the order of several tens of nanoseconds and its decay with temperature shows a T−3 behavior at higher temperatures and a T−1 behavior at lower temperatures.