Semiconductor Epitaxial Layers Research Articles

Characterization of GaAs-based micro-origami mirrors by optical actuation

We have performed optical actuation to characterize GaAs-based micro-mirrors fabricated by the micro-origami technique. Micro-origami makes use of the energy stored as strain between lattice-mismatched semiconductor epitaxial layers to create self-positioning, three-dimensional structures with simple and robust hinges. Mirrors studied in this work were GaAs plates grown on In 0.22Ga 0.78As/GaAs strained layers. Optical actuation was achieved by uniformly shining the 488-nm line of a high-power Ar + laser on the mirrors. The laser light was either continuous wave (CW) or chopped with an acousto-optical modulator. Under a CW illumination of about 450 mW/mm 2, an angular deflection of the mirror of 0.5° was obtained. When actuated with modulated light, the first resonant mode of the mirror was experimentally detected at 25 kHz. Finite element analysis showed that the experimental results correspond to a longitudinal-bending resonant mode of the mirror.

Electromechanical coupling in free-standing AlGaN/GaN planar structures

The strain and electric fields present in free-standing AlGaN/GaN slabs are examined theoretically within the framework of fully coupled continuum elastic and dielectric models. Simultaneous solutions for the electric field and strain components are obtained by minimizing the electric enthalpy. We apply constraints appropriate to pseudomorphic semiconductor epitaxial layers and obtain closed-form analytic expressions that take into account the wurtzite crystal anisotropy. It is shown that in the absence of free charges, the calculated strain and electric fields are substantially different from those obtained using the standard model without electromechanical coupling. It is also shown, however, that when a two-dimensional electron gas is present at the AlGaN/GaN interface, a condition that is the basis for heterojunction field-effect transistors, the electromechanical coupling is screened and the decoupled model is once again a good approximation. Specific cases of these calculations corresponding to transistor and superlattice structures are discussed.

Crack initiation and termination in III-V epitaxial layers

Cracks in III-V epitaxial layers of cubic and hexagonal semiconductors have been studied to elucidate mechanisms of nucleation and termination. Pre-existing intersecting cracks and oval defects are very effective terminators. Cracks can be initiated at scribe marks and deep pits with clearly defined facets. However, for the majority of cracks propagated during the growth phase an additional nucleation feature which permits a crack to grow outwards in opposite directions is required but has not yet been observed.

Overview of antimonide based III-V semiconductor epitaxial layers and their applications at the center for quantum devices

The properties of Sb-based III-V semiconductor compounds for optoelectronic applications in the mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) range were reviewed. The growths of the Sb-based binary, ternary and quaternary were studied by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). The structural, optical and electrical characterizations were carried out. Focal plane array, photoconductors and photodiodes were fabricated for the MWIR and LWIR range. Doublehetero structure (DH), multi-quantum well (MQW) and strained superlattice (SSL) lasers in the 3–5 μ m range were fabricated. InAs-GaSb type-II superlattices were designed, grown and fabricated into photodetectors for the MWIR and LWIR range.

Development of a novel epitaxial-layer segmentation method for optoelectronic device fabrication

Describes the development of a new process for segmenting epitaxial-layer material and fabricating cleaved-cavity lasers. The technique, which involves growth substrate removal and use of a novel epitaxial-layer cleaving tool, is applicable to a variety of different materials and devices. IV-VI semiconductor epitaxial layers grown by molecular beam epitaxy on silicon [111] substrates with a water-soluble barium fluoride buffer layer were used. The layers were metallurgically bonded to the rectangular tips (500×1040 μm) of copper bars held together by a small vise apparatus, the substrate was removed by dissolving the barium fluoride buffer layer in water, and the copper bars were separated by releasing the vise assembly. Current versus voltage characterization of separated p-n junction IV-VI epilayer structures on the tips of the copper bars showed nonlinear behavior with a forward bias current onset voltage /spl sim/250 mV, a value consistent with the room temperature bandgaps of the IV-VI alloys used. These results demonstrate the utility of this technique for obtaining epitaxial-layer structures with typical cleaved-cavity in-plane laser dimensions.

Geometrical Contribution to Yield Strength in Small Volumes

AbstractCritical thickness theory was developed to account for the ability of thin epitaxial metal and semiconductor layers to support high misfit or coherency strain. The thermodynamic equilibrium theory is correct, and is well approximated by a theory based on geometrical arguments. The latter theory is readily extended to arbitrary misfit strain profiles including linearly graded layers, for which a surface layer free of misfit dislocations. This result applies as well to linear strain gradients introduced by deformation, in which case the misfit dislocations are known as geometrically necessary dislocations. We show that a consequence of this surface layer is that the apparent yield strength of the material increases in small structures such as thin wires in torsion. Under large plastic deformation, even if the material is perfectly plastic, critical thickness theory also predicts an apparent work-hardening. The predictions of critical thickness theory are in excellent agreement with experimental data in the literature.

High-resolution optical detection of electron spin resonance in epitaxial semiconductor layers by coherent Raman spectroscopy

We describe the application to semiconductors of a microwave-frequency optical heterodyne technique that enables electron-spin-resonance spectra to be detected through coherent Raman scattering. The technique is sufficiently sensitive to detect spectra from epitaxial layers and has the added advantage of optical selectivity. We demonstrate its effectiveness with studies of a ZnSe epitaxial layer in which there is a variation in strain. The spin-resonance linewidths are sufficiently narrow for the gyromagnetic ratio to be determined with a precision of 1 part in ${10}^{4}$ and, as the laser is tuned to resonance with differently strained parts of the material, the g value changes at a rate of approximately $0.4 {\mathrm{eV}}^{\ensuremath{-}1}.$ We have carried out the experiment in both transmission and reflection geometries and the technique promises to be of wide applicability.

Experimental confirmation by galvanomagnetic methods of a complex transport model in In0.53Ga0.47As layers deposited by MBE on SI-InP

At low temperatures In0.53Ga0.47As samples show an increase of carrier concentration, which can be explained in terms of a two carriers transport model. This type of problem exists since the beginning of the semiconductor era, dating back to monocrystalline germanium.We propose that in all the investigated layers, there are X atoms or charged dislocations in the region of the first monolayers, which are built in during epitaxial growth. The layers were intentionally undoped. They form an impurity band in which low mobility carriers dominate over the localised electron scattering due to the s–d exchange interaction. These carriers do not freeze out at liquid helium temperature and give rise to two transport media for electrons; a conduction band at higher temperatures and an impurity band at lower temperatures. The electron which fall down onto the previously ionised X atoms, then move by thermally activated hopping. We show that the two carriers model for In0.53Ga0.47As epitaxial layers are confirmed by the carrier concentration–temperature, carrier concentration–magnetic field, resistivity–magnetic field behaviour, and also by YKA theory also. The differences between the two transport models are so distinctive that observed phenomena may exist. This paper presents experimental results, which constitute comprehensive evidence for the complicated structure of the semiconductor epitaxial layers on the sample of n-type In0.53Ga0.47As/InP layer with n=2.2×1015/cm3.

Characterization of the electrical properties and thickness of thin epitaxial semiconductor layers by THz reflection spectroscopy

We have measured the dielectric properties and thickness of thin semiconductor epitaxy layers by the reflection of THz radiation from the surface of a two-layered semiconductor wafer. When reflecting from two interfaces the electromagnetic pulse has a destructive interference at a specific wavelength dependent on the thickness of the outer layer and its dielectric function. Near that frequency the reflection coefficient has a significant drop. By extending the incident pulse spectrum to include this interference frequency, a measurement of the thickness can be obtained together with a direct measurement of the carrier number density. By this technique epitaxy layers of thickness down to 15 μm are characterized.

Nucleation of III nitride semiconductors in heteroepitaxy

The nucleation of III nitride semiconductors in heteroepitaxy is theoretically investigated using GaN nucleation on the AlN surface as an example. It is inferred that the mechanism of this process is determined by the temperature at the initial stage of the layer formation (T). At low temperatures (T 650°C, there arise only GaN nuclei. It is revealed that the GaN nucleation is governed by the generalized diffusion coefficient of GaN, which is a combination of the diffusion coefficients for gallium and nitrogen atoms. It is shown that the generalized diffusion coefficient of GaN on the crystal surface increases by seven orders of magnitude as the growth temperature increases from 600 to 800°C. This is accompanied by a change in the growth mechanism of the III nitride semiconductor epitaxial layers.

Phonon-induced photoconductive response in doped semiconductors

We present an unusual photoconductive response in doped GaAs and InP semiconductor epitaxial layers and demonstrate that such a response is due to the longitudinal optic (LO) phonon-induced Fano resonance instead of the traditional kinetic mechanism. Theoretical calculations based upon the present mechanism are in excellent agreement with the experimental results and the developed formalism should be universally applicable to the related photoconductive processes of other doped semiconductors.

TEM Study of strain states in III-V semiconductor epitaxial layers.

ABSTRACTThe strain states induced by a lattice mismatch in epitaxial systems have been studied by Transmission Electron Microscopy (TEM) using the moiré fringe technique on plane view samples. For the GaSb/(001)GaAs system, moiré patterns suggest that the GaSb layer is free of stress and homogeneously relaxed by a perfect square array of Lomer dislocations. A 10 nm thick layer of GaInAs (20% In concentration) grown on (001)GaAs does not give any moiré fringes for all low-index Bragg reflections: this result indicates that the effective misfit strain does not correspond to the theoretical one described by the elastic theory. Segregation effects are expected to play an important role in the relaxation of the misfit strain.

TEM analysis of epitaxial semiconductor layers with high stacking fault densities considering artifacts induced by the cross-section geometry

Epitaxial semiconductor layers, in particular II–VI compound semiconductors with the component Se deposited on III–V semiconductor substrates like GaAs or InAs, can contain high densities of stacking faults. Cd x Mg 1− x Se layers with Cd-concentrations x of 93% and 57% on InAs(0 0 1) substrates were investigated as typical representatives of this class of heterostructures. The defect structure of the layers is dominated by a high density of stacking fault (SF) pairs bound by Shockley partial dislocations with Burgers vectors b= 1 6 〈1 1 2〉 and a stair-rod dislocation with b= 1 6 〈1 1 0〉 at the intersection line of the pairs. Plan-view transmission electron microscopy (TEM) is generally applied to obtain information about the type, density and arrangement of the stacking faults in thin epilayers up to moderate SF densities. Cross-section TEM is more frequently carried out for thick layers and at high SF densities. It will be demonstrated by a detailed analysis of cross-section images that a careful interpretation of the observed SF morphologies is necessary due to artifacts induced by the cross-section geometry.

Effect of rare earth addition on liquid phase epitaxial InP-based semiconductor layers

The influence of rare earth (RE) element (Ho, Er or Nd) addition during liquid phase epitaxial growth (LPE) on the electro-optical properties of InP and GaInAsP semiconductor layers has been studied. The effect of Nd admixture is reported for the first time. Series of InP-based layer samples were prepared by LPE from the melt containing 0–0.35 wt.% of RE admixture. Temperature-dependent Hall effect and capacitance–voltage curves show a quite dramatic impact of Er or Nd on donor and free-carrier concentrations: they were decreased by as much as three orders of magnitude in some cases. Low-temperature photoluminescence (PL) spectra have been measured for various levels of excitation power. The major manifestation of the RE admixture was the pronounced narrowing of PL curves and the corresponding appearance of the fine features in the excitonic band, characteristic of pure material, low in defects. The effects are attributed to RE atoms acting as very efficient gettering agents.

Growth of III-N materials and devices by metalorganic chemical vapor deposition

The characteristics of III-V nitride semiconductor epitaxial layers grown by metalorganic chemical vapor deposition are of interest for the realization of many technologically important devices. This paper will review heteroepitaxial growth on (0001) sapphire substrates as well as the selective-area and subsequent lateral epitaxial overgrowth on masked substrate surfaces.

Planar quantum transistor based on 2D–2D tunneling in double quantum well heterostructures

We report on our work on the double electron layer tunneling transistor (DELTT), based on the gate control of two-dimensional–two-dimensional (2D–2D) tunneling in a double quantum well heterostructure. While previous quantum transistors have typically required tiny laterally defined features, by contrast the DELTT is entirely planar and can be reliably fabricated in large numbers. We use a novel epoxy-bond-and-stop-etch flip-chip process, whereby submicron gating on opposite sides of semiconductor epitaxial layers as thin as 0.24 μm can be achieved. Because both electron layers in the DELTT are 2D, the resonant tunneling features are unusually sharp, and can be easily modulated with one or more surface gates. We demonstrate DELTTs with peak-to-valley ratios in the source-drain I–V curve of order 20:1 below 1 K. Both the height and position of the resonant current peak can be controlled by gate voltage over a wide range. DELTTs with larger subband energy offsets (∼21 meV) exhibit characteristics that are nearly as good at 77 K, in good agreement with our theoretical calculations. Using these devices, we also demonstrate bistable memories operating at 77 K. Finally, we briefly discuss the prospects for room temperature operation, increases in gain and high speed.

Impurity Diffusion in SiGe Alloys: Strain and Composition Effects

ABSTRACTThe advent of epitaxial-growth techniques dedicated to the growth of epitaxial layers of group-IV semiconductors and their alloys has opened up new possibilities for novel types of diffusion experiments and thus for critical tests of existing diffusion theories. It is now possible to grow test structures for diffusion consisting of well-defined narrow distributions of tracer impurities in epitaxial layers of biaxial-strained or relaxed alloys of different compositions. This enables studies of alloy effects and/or strain effects on diffusion with varying strain energy for a well-defined type of strain (tensile or compressive). A review is presented on recently published results on impurity diffusion in strained and relaxed, epitaxial SiGe alloy layers. As almost all these studies have focused on boron and antimony diffusion, these two impurities will have the leading roles in this review.

Impurity Diffusion in SiGe Alloys: Strain and Composition Effects

ABSTRACTThe advent of epitaxial-growth techniques dedicated to the growth of epitaxial layers of group-IV semiconductors and their alloys has opened up new possibilities for novel types of diffusion experiments and thus for critical tests of existing diffusion theories. It is now possible to grow test structures for diffusion consisting of well-defined narrow distributions of tracer impurities in epitaxial layers of biaxial-strained or relaxed alloys of different compositions. This enables studies of alloy effects and/or strain effects on diffusion with varying strain energy for a well-defined type of strain (tensile or compressive). A review is presented on recently published results on impurity diffusion in strained and relaxed, epitaxial SiGe alloy layers. As almost all these studies have focused on boron and antimony diffusion, these two impurities will have the leading roles in this review.

Measuring and calculating the Hall mobility of semiconductor epitaxial layers by a contactless method

A new probe was made from microwave dielectric waveguides. Using this probe, we can measure the magnetoresistance of Al x Ga 1− x As, Hg 1− x Cd x Te and other epitaxial layers easily. It is a contactless and non-destructive method. During the test, we do not need to specially treat the surface and the shape of the samples. The measuring area is about 4 × 4 mm 2. This article also presents a method of calculating the mobility of these samples from the magnetoresistance data for different semiconductor materials.

Integrated Optoelectronics Using Thin Film Epitaxial Liftoff Materials and Devices

The separation of single crystal thin film epitaxial compound semiconductor layers from a lattice matched growth substrate through selective etching, with subsequent bonding of the epitaxial thin film devices onto host substrates, is an emerging tool for multi-material, hybrid integration. Progress to date in this area, presented herein, includes advanced thin film devices, thin film material separation and device integration processing techniques, and thin film material and device integration with host substrates which include silicon circuitry, polymers, glass, and lithium niobate.