Strained-layer Heterostructures Research Articles

Introduction of misfit dislocations into strained-layer GaAs/InxGa1–xAs/GaAs heterostructures by mechanical bending

The stability of strained-layer heterostructure lasers can be assessed by their response to stimuli for the introduction of dislocations. Three-point bending at elevated temperatures has been applied to GaAs/InxGa1−xAs/GaAs heterostructures to apply such a thermomechanical stimulus. In each case, the middle-layer thickness was below the critical thickness predicted by the Matthews–Blakeslee model, so that the pre-test structures were fully strained with no observed misfit dislocations. The tensile stress of 46.4 MPa produced during the tests resulted in the formation of 60° misfit dislocations whose configurations changed according to the alignment of the bending axis. For bending in the [110] orientation, the misfit dislocations formed parallel to each other and to the bending axis. For [100] bending, they formed an orthogonal pattern with each dislocation at 45° to the bending axis. In each case, these misfit dislocations caused relaxation of the strained-layer structures, even though the unloaded structures had been considered thermodynamically stable and the test temperatures were lower than those used during the original fabrication of the structures. These findings challenge existing assumptions of strained-layer stability and have implications for the design of lasers intended to be “buried and forgotten” in optical telecommunications.

Optimization of NBED simulations for disc-detection measurements

Nano-beam electron diffraction (NBED) is a method which can be applied to measure lattice strain and polarisation fields in strained layer heterostructures and transistors. To investigate precision, accuracy and spatial resolution of such measurements in dependence of properties of the specimen as well as electron optical parameters, simulations of NBED patterns are required which allow to predict the result of common disc-detection algorithms. In this paper we demonstrate by focusing on the detection of the central disc in crystalline silicon that such simulations require to take several experimental characteristics into account in order to obtain results which are comparable to those from experimental NBED patterns. These experimental characteristics are the background intensity, the presence of Poisson noise caused by electron statistics and blurring caused by inelastic scattering and by the transfer quality of the microscope camera. By means of these optimized simulations, different effects of specimen properties on disc detection – such as strain, surface morphology and compositional changes on the nanometer scale – are investigated and discussed in the context of misinterpretation in experimental NBED evaluations. It is shown that changes in surface morphology and chemical composition lead to measured shifts of the central disc in the NBED pattern of tens to hundreds of µrad. These shifts are of the same order of magnitude or even larger than shifts that could be caused by an electric polarisation field in the range of MV/cm.

Engineering quantum spin Hall insulators by strained-layer heterostructures

Quantum spin Hall insulators (QSHIs), also known as two-dimensional topological insulators, have emerged as an unconventional class of quantum states with insulating bulk and conducting edges originating from nontrivial inverted band structures and have been proposed as a platform for exploring spintronics applications and exotic quasiparticles related to the spin-helical edge modes. Despite theoretical proposals for various materials, however, experimental demonstrations of QSHIs have so far been limited to two systems—HgTe/CdTe and InAs/GaSb—both of which are lattice-matched semiconductor heterostructures. Here, we report transport measurements in yet another realization of a band-inverted heterostructure as a QSHI candidate—InAs/InxGa1−xSb with lattice mismatch. We show that the compressive strain in the InxGa1−xSb layer enhances the band overlap and energy gap. Consequently, high bulk resistivity, two orders of magnitude higher than for InAs/GaSb, is obtained deep in the band-inverted regime. The strain also enhances bulk Rashba spin-orbit splitting, leading to an unusual situation where the Fermi level crosses only one spin branch for electronlike and holelike bands over a wide density range. These properties make this system a promising platform for robust QSHIs with unique spin properties and demonstrate the strain to be an important ingredient for tuning spin-orbit interaction.

The calculation of semipolar orientations for wurtzitic semiconductor heterostructures: application to nitrides and oxides

We investigate how the template crystal orientation indices (hk. ℓ) can influence the intensity of density of elastic energy and polarization fields in wurtzite nitrides and oxides semiconductor strained layer heterostructures. We propose analytical relations between the angle Θ, defined as the direction of the sixfold axis of unstrained material and the direction normal to the growth plane, and (i) the value of the total polarization and (ii) the density of elastic energy stored in the strained layer. We find that quasi-cancellation of quantum confined stark effect (QCSE) can be generally obtained by carefully selecting the (hk. ℓ) set. This situation does not lead to minimal strain density of elastic energy, but the increase of this parameter may be moderate compared to the minimum value. In the case of materials submitted to biaxial tension, we observe that the total density of elastic energy stored shows no minimum except when Θ = 0.

Misfit Dislocations in SiGe/Si Heterostructures: Nucleation - Propagation - Multiplication

We present experimental data on the effect of low-temperature buffer layers on the dislocation structure formation in SiGe/Si strained-layer heterostructures under thermal annealing. Specific subjects include mechanisms of misfit dislocation nucleation, propagation and multiplication as well as the role of intrinsic point defects in these processes. Samples with lowtemperature Si (400°C) and SiGe (250°C) buffer layers were grown by MBE. In general, the processes of MD generation occur similarly in the heterostructures studied independently of the alloy composition (Ge content: 0.15, 0.30) and kind of buffer layer. Intrinsic point defects related to the low-temperature epitaxial growth influence mainly the rate of misfit dislocation nucleation.

Thermally induced strain relaxation in SiGe/Si heterostructures with low‐temperature buffer layers

Processes of misfit dislocation (MD) nucleation and multiplication in SiGe/Si strained-layer heterostructures under thermal annealing were studied. Specific subjects include the kinetics of dislocation network formation in heterostructures with low-temperature (LT) buffer layers and mechanisms of dislocation nucleation. Samples with LT-Si (400 oC) and LT-SiGe (250 oC) buffer layers were grown by MBE. In general, the processes of MD generation occur similarly in the heterostructures studied independently of the alloy composition (Ge content: 0.15, 0.30) and kind of buffer layer. Intrinsic point defects related to the LT epitaxial growth influence mainly the rate of MD nucleation. We suggest a new mechanism of MD generation which includes a nucleation of incipient dislocation loops at heterogeneous sources within SiGe epitaxial layer and formation of spiral sources at threading V-shaped dislocation half-loops. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Piezoelectric field effects on electron density in a δ-doped AlGaAs/InyGa1-yAs/GaAs pseudomorphic HEMT

We have calculated the low-temperature electron density in a δ-doped AlGaAs/InGaAs/GaAs heterostructure in the presence of a piezoelectric field. Growth of a strained InGaAs layer on (N11) GaAs substrates causes a piezoelectric field to be built into the quantum well of a pseudomorphic high electron mobility transistor (HEMT). The presence of this field modifies the electronic properties of strained-layer heterostructures. A self-consistent analysis is made of these δ-doping systems to solve simultaneously the Schrodinger and Poisson equations taking into account exchange-correlation and strain effects. Thus, we have found the confining potential, the electron density, the subband energies, the eigenenvelope wavefunctions, and the Fermi-energy level in the quantum well. We have studied the effects of the InGaAs channel width and the indium composition on the electron density. Our results show a larger increase of the electron density when the calculated value is for a (111) GaAs substrate rather than for equivalent (001) and (311) substrates. We have also investigated the effects of the environmental Al concentration seen by the Si δ-doped on the electron density.

Type-I Ge∕Ge1−x−ySixSny strained-layer heterostructures with a direct Ge bandgap

The electronic properties of Ge∕Ge1−x−ySixSny strained-layer heterostructures are predicted theoretically. It is found that a lattice-matched system with fully strained Ge layers and relaxed Ge1−x−ySixSny alloys can have a direct fundamental bandgap with spatial localization in the Ge layers (type I). The Si and Sn concentrations for which such a direct bandgap obtains are close to those that have already been experimentally demonstrated [M. Bauer, C. Ritter, P. A. Crozier, J. Ren, J. Menendez, G. Wolf, and J. Kouvetakis, Appl. Phys. Lett. 83, 2163 (2003)]. The required level of tensile strain in the Ge layers is compatible with Si–Ge technology. The predicted direct bandgap values are as high as 0.6eV.

Growth of strained Si and strained Ge heterostructures on relaxed Si1−xGex by ultrahigh vacuum chemical vapor deposition

Numerous applications require the growth of planar strained-layer heterostructures on relaxed Si1−xGex. After briefly reviewing these applications as well as the challenges in growing such heterostructures, we provide experimental examples illustrating the influence of lattice mismatch, growth temperature, and film composition on the morphology of thin strained layers in the Ge–Si alloy system. Procedures for growing strained Si and strained Ge single and double heterostructures via ultrahigh vacuum chemical vapor deposition are described in detail. We demonstrate planar growth of strained Ge layers with lattice mismatches as high as 2%, planar Si layers on any Si1−xGex lattice constant, and double heterostructures that are comprised of a strained Ge layer capped with strained Si. Notably, the techniques described here have already been applied to the fabrication of extremely high mobility p- and n-channel metal–oxide–semiconductor field-effect transistors and germanium-on-insulator substrates.

Formation of misfit dislocations in strained-layer GaAs/InxGa1−xAs/GaAs heterostructures during postfabrication thermal processing

It is demonstrated that relaxation of GaAs/InxGa1−xAs/GaAs strained-layer heterostructures can be brought about by postfabrication thermal processing. Misfit dislocations are introduced into the structure during thermal processing, even though the thickness of the strained layer is well below the critical value predicted by the Matthews–Blakeslee model. The misfit dislocations are observed to be of both 60° mixed type and 90° pure edge type. As no relaxation occurs at the lower temperatures encountered during fabrication by molecular-beam epitaxy, it can be inferred that the critical condition for the formation of misfit dislocations is not only a function of strained-layer thickness and composition, but also of temperature. This observation cannot be accounted for by differential thermal expansion or diffusion across the strained-layer interfaces, but the temperature-dependent Peierls force may offer an explanation. The high temperature required to produce relaxation of these structures suggests that they are sufficiently thermally stable for most practical applications.

Quantum-ballistic transport in an etch-defined Si/SiGe quantum point contact

Ballistic constrictions are fabricated on a high-mobility Si/SiGe strained-layer heterostructure which exhibit conductance quantization in units of 4e2/h at T=4.2 K. Under finite drain voltage a half-plateau develops at 2e2/h and a series of oscillations appear which enable us to extract the energy separation ΔEN+1,N between successive one-dimensional subbands. The result is ΔE2,1=2.0 meV and ΔE3,2=1.4 meV.

Spectroscopic ellipsometry of Si xGe 1− x/Si: a tool for composition and profile analysis in strained heterostructures used in the microelectronics industry

Si x Ge 1− x /Si strained-layer heterostructures play a primary role in today’s developments in Si-based fast electronics. Either real-time epitaxial growth control of this alloy or ex situ on-line characterization of the processed wafers is required. Indeed, spectroscopic ellipsometry (SE) is a fast and non-destructive technique compared to secondary ion mass spectroscopy (SIMS) and/or X-ray diffraction XRD X-ray analysis. The heterostructure-base-transistor (HBT) ellipsometry studies, with either graded or abrupt profiles, have been reported. The analysis presented herein was based on an optical database of indices currently provided in the literature for relaxed materials. For strained pseudomorphic structures, using a calibration procedure to access the actual x alloy composition, the in-depth variation x could be determined even with the presence of a silicon capping layer. Achievement was completed for a dummy-load of test wafers, as well as for patterned processed wafers, where analysis required minimum areas of 30×30 μm.

Investigation of threading dislocation blocking in strained-layer InGaAs/GaAs heterostructures using scanning cathodoluminescence microscopy

Threading dislocation glide relieves strain in strained-layer heterostructures by increasing the total length of interface misfit dislocations. The blocking theory proposed by Freund [J. Appl. Phys. 68, 2073 (1990)] predicts the thickness above which gliding threading dislocations are able to overcome the resistance force produced by existing orthogonal misfit dislocations. A set of wedge-shaped samples of InxGa1−xAs/GaAs (x=0.04) strained-layer heterostructures was grown using molecular-beam epitaxy in order to test the theory of dislocation blocking over a range of thicknesses within one sample. Scanning cathodoluminescence microscopy techniques were used to image the misfit dislocations. The cathodoluminescence results confirm the model proposed by Freund.

High-frequency noise performance of SiGe p-channel MODFETs

For the first time, an investigation into the microwave noise performance of SiGe-based MODFETs is presented. The devices were 0.1 µm gate-length p-channel MODFETs fabricated on a high-mobility SiGe strained-layer heterostructure grown by UHV-CVD and had a unity gain cutoff frequency fT of 42 GHz and maximum frequency of oscillation fmax of 69 GHz at a drain-to-source bias voltage Vds of –0.6 V. A minimum noise figure Fmin of 1.1 dB and an associated gain Ga of 18 dB were obtained at 3 GHz, while values of Fmin = 1.7 dB and Ga = 12 dB were obtained at 10 GHz. These values are comparable to those of SiGe production HBTs, and demonstrate the suitability of SiGe MODFETs for RF and microwave communications applications.

Formation of misfit dislocations during growth of InxGa1-xAs/GaAs strained-layer heterostructures

Transmission electron microscopy of GaAs/InxGa1-xAs/GaAs double heterostructures has enabled the onset and subsequent development of misfit dislocations to be followed for increasing strained-layer thicknesses, from sub- to supercritical. It has been observed that misfit segments are introduced into threading dislocations when the strained-layer thickness is close to, but below, the critical thickness predicted by the Matthews-Blakeslee (M-B) model. Analysis shows that threading dislocations may be able to glide to form interfacial misfit dislocation segments even though the critical thickness predicted by the M-B model has not been reached. It has also been observed that the total dislocation density rises slowly as the layer thickness increases above its critical value, until a sudden increase occurs. It is suggested that the sudden increase in dislocation density is associated with a different mechanism of misfit dislocation formation, which dominates the global relaxation of the structure.

Defects formed during 1 MeV Si ion-irradiation of GeSi/Si strained-layer heterostructures at elevated temperatures

The strain relief observed in GeSi/Si strained-layer heterostructures irradiated with MeV ions at elevated temperature (253°C) is shown to be consistent with the accumulation of excess vacancies in the alloy layer. Partial recovery of the as-grown strain during subsequent annealing to temperatures <750°C coincides with the appearance of macroscopic voids. Since the tensile strain per vacancy is reduced by clustering this is believed to account for the observed strain recovery. At higher temperatures, >750°C, accelerated strain relaxation is shown to coincide with the growth of large intrinsic dislocation loops from the alloy–substrate interface. After annealing at 900°C, unirradiated alloys contain a low density of orthogonal misfit dislocations. Irradiated samples contain intrinsic irregular loop-like defects extending between the alloy–substrate interface and the surface.

High-fT n-MODFETs fabricated on Si/SiGe heterostructures grown by UHV-CVD

The authors present high-frequency results of n-channel MODFETs fabricated on high-mobility Si/SiGe strained layer heterostructures grown by ultrahigh vacuum chemical vapour deposition (UHV-CVD). Devices with gate length Lg = 0.2 µm and drain-source separation Lds = 0.9 µm displayed unity current gain cutoff frequencies as high as fT = 45 GHz (47 GHz) at Vds = +0.6 V (+1.5 V). Similar devices with Lg = 0.2 µm and Lds = 0.5 µm produced values of fT = 61 GHz (62 GHz) at Vds = +0.6 V (+1.0 V). The value fT = 62 GHz is the highest unity current gain cutoff frequency reported to date for an n-channel strained Si MODFET.

High-performance 0.1 [micro sign]m gate-length Ge/Sio0.4Ge0.6 p-channel MODFETs

Results are reported of high-transconductance p-channel MODFETs fabricated on Ge/Sio0.4Ge0.6 strained-layer heterostructures grown by UHV-CVD. Devices with 0.1 mm gate length were fabricated on compressively-strained pure-Ge channels with a Hall mobility of 1750 cm2/Vs (30900 cm2/Vs) at room temperature (T = 77 K). These devices displayed room-temperature peak extrinsic transconductances as high as 317 mS/mm, at Vds = –0.6 V, while the output conductance under the same bias conditions was only 18 mS/mm, corresponding to a maximum voltage gain of 18. At T=77 K, peak extrinsic transconductances as high as 622 mS/mm were obtained at bias voltages as low as Vds = –0.2 V. To our knowledge, the 77 K transconductance is the highest ever reported for a p-type field effect transistor.

Investigation of oval defects in InGaAs/GaAs strained-layer heterostructures using cathodoluminescence and wavelength dispersive spectroscopy

Oval defects in In0.04Ga0.96As/GaAs strained-layer heterostructures have been investigated using cathodoluminescence (CL) and wavelength dispersive x-ray spectroscopy (WDS). WDS studies showed that the particulates seen at the center of oval defects are indium rich and gallium depleted. A luminescent halo was seen around the indium rich particulates in the CL mode. When the halo was studied further, it was shown that the peak obtained from CL spectroscopy due to the luminescent halo shifts to lower wavelengths as the beam is moved from the center of the oval defect to the edge of the halo region, indicating a decreasing gradient in indium concentration.

In Situ Studies of the Interaction of Dislocations with Point Defects during Annealing of Ion Implanted Si/SiGe/Si (001) Heterostructures.

: Strained layer heterostructures provide ideal systems with which to study the dynamics of dislocation motion via in situ transmission electron microscopy, as the geometry, strain state, and kinetics can be characterized and directly controlled. We discuss how these structures are used to study dislocation-point defect interactions, emphasizing the experimental requirements necessary for quantification of dislocation motion. Following ion implantation, different concentrations and types of point defects are introduced within the SiGe epilayer depending on the implantation species, energy, and current density. By annealing samples in situ in the transmission electron microscope (TEM) following implantation, we can directly observe dislocation motion and quantify the effect of dislocation-point defect interactions on dislocation velocities. We find that dislocation motion is impeded if the implantation dose peak lies within the epilayer, as dislocations pin at point defect atmospheres. Shallow BF2 implantation into the sample capping layer results in more complicated behavior. For low current density implants, dislocation velocities may be dramatically increased; at higher current densities the magnitude of this increase is significantly smaller. Implantation of different ions separately implicates fluorine as the species responsible for the observed increases in dislocation velocities, presumably due to an electrical effect on the rate of dislocation kink nucleation.