Buffer Heterostructures Research Articles

Structural properties of compressive strained Ge channels fabricated on Si (111) and Si (100)

Structural properties of the strained Ge channels formed on SiGe (111) and SiGe (100) relaxed buffer layers are investigated. Compressive strained Ge(111) channels are grown on reverse graded Si0.26Ge0.74 buffer layers, and as a result, very low rms roughness of 1.7 nm and large compressive strain of −0.92% are obtained. The phosphorous doping is attempted in the SiGe buffer for the purpose of suppression of parallel conduction and very abrupt doping is realized without very little diffusion. The high-quality, doping controlled and largely strained Ge(111) channel/SiGe(111) buffer heterostructure is a promising template for high-performance Ge(111) based devices.

Investigation into the carrier distribution and energy-band profile in AlGaN/GaN heterostructures with a graded AlGaN buffer

In GaN-based high-electron mobility transistors, although excellent electron confinement has been demonstrated using a graded AlGaN buffer with linearly decreasing Al-content along [0001] direction, guidelines for graded buffer design are still lacking. To obtain overall pictures of the carrier distribution and energy-band profile in AlGaN/GaN/graded AlGaN buffer heterostructures, the influences of the related physical parameters of the buffer are studied by one-dimensional self-consistent simulation. The results show that the negative polarization charge over the buffer can induce free holes in the depths of the buffer, resulting in the coexistence of electrons and holes. By adjusting the related physical parameters of the buffer, it is even possible to form two-dimensional holes gas (2DHG) at the channel/graded buffer interface. The cause of the coexistence of electrons and holes and the formation condition of 2DHG are analyzed. In addition, in the course of the variation of the related physical parameters, the characteristics of the two-dimensional electron gas density are also exhibited. This study can provide a reference for graded AlGaN buffer design in GaN-based field-effect transistors.

Electron microscopy characterization of AlGaN/GaN heterostructures grown on Si (111) substrates

AlGaN/GaN buffer heterostructures were grown on “on axis” and 4 deg off Si (111) substrates by MOVPE. The electron microscopy study reveals the very good epitaxial growth of the layers. Almost c-plane orientated nucleation grains are achieved after full AlN layer growth. Step-graded AlGaN layers were introduced, in order to prevent the stress relaxation and to work as a dislocation filter. Thus, a crack-free smooth surface of the final GaN epitaxial layer is achieved in both cases, making the buffer structure ideal for the forthcoming growth of the heterostructure (used for HEMT device applications). Finally, the growth of the AlGaN/GaN heterostructure on top presents characteristic and periodic undulations (V-pits) on the surface, due to strain relaxation reasons. The AlN interlayer grown in between the heterostructure demonstrates an almost homogeneous thickness, probably reinforcing the 2DEG electrical characteristics.

Ultraviolet GaN photodetectors on Si via oxide buffer heterostructures with integrated short period oxide-based distributed Bragg reflectors and leakage suppressing metal-oxide-semiconductor contacts

Based on a novel double step oxide buffer heterostructure approach for GaN integration on Si, we present an optimized Metal-Semiconductor-Metal (MSM)-based Ultraviolet (UV) GaN photodetector system with integrated short-period (oxide/Si) Distributed Bragg Reflector (DBR) and leakage suppressing Metal-Oxide-Semiconductor (MOS) electrode contacts. In terms of structural properties, it is demonstrated by in-situ reflection high energy electron diffraction and transmission electron microscopy-energy dispersive x-ray studies that the DBR heterostructure layers grow with high thickness homogeneity and sharp interface structures sufficient for UV applications; only minor Si diffusion into the Y2O3 films is detected under the applied thermal growth budget. As revealed by comparative high resolution x-ray diffraction studies on GaN/oxide buffer/Si systems with and without DBR systems, the final GaN layer structure quality is not significantly influenced by the growth of the integrated DBR heterostructure. In terms of optoelectronic properties, it is demonstrated that—with respect to the basic GaN/oxide/Si system without DBR—the insertion of (a) the DBR heterostructures and (b) dark current suppressing MOS contacts enhances the photoresponsivity below the GaN band-gap related UV cut-off energy by almost up to two orders of magnitude. Given the in-situ oxide passivation capability of grown GaN surfaces and the one order of magnitude lower number of superlattice layers in case of higher refractive index contrast (oxide/Si) systems with respect to classical III-N DBR superlattices, virtual GaN substrates on Si via functional oxide buffer systems are thus a promising robust approach for future GaN-based UV detector technologies.

Lattice-engineered Si1-xGex-buffer on Si(001) for GaP integration

We report a detailed structure and defect characterization study on gallium phosphide (GaP) layers integrated on silicon (Si) (001) via silicon-germanium (SiGe) buffer layers. The presented approach uses an almost fully relaxed SiGe buffer heterostructure of only 400 nm thickness whose in-plane lattice constant is matched to GaP—not at room but at GaP deposition temperature. Single crystalline, pseudomorphic 270 nm thick GaP is successfully grown by metalorganic chemical vapour deposition on a 400 nm Si0.85Ge0.15/Si(001) heterosystem, but carries a 0.08% tensile strain after cooling down to room temperature due to the bigger thermal expansion coefficient of GaP with respect to Si. Transmission electron microscopy (TEM) studies confirm the absence of misfit dislocations in the pseudomorphic GaP film but growth defects (e.g., stacking faults, microtwins, etc.) especially at the GaP/SiGe interface region are detected. We interpret these growth defects as a residue of the initial 3D island coalescence phase of the GaP film on the SiGe buffer. TEM-energy-dispersive x-ray spectroscopy studies reveal that these defects are often correlated with stoichiometric inhomogeneities in the GaP film. Time-of-flight Secondary ion mass spectrometry detects sharp heterointerfaces between GaP and SiGe films with a minor level of Ga diffusion into the SiGe buffer.

On the band gaps and electronic structure of thin single crystalline praseodymium oxide layers on Si(111)

The influence of stoichiometry and crystal structure on the electronic properties of single crystalline cubic PrO2(111), cubic Pr2O3(111), and hexagonal Pr2O3(0001) thin film heterostructures on Si(111) was investigated by synchrotron radiation based photoemission electron spectroscopy (PES) and x-ray absorption spectroscopy (XAS). A detailed analysis of the complex satellite structures of PES Pr 3d lines of the various Pr oxide phases is given. PES was in addition applied to study the O 2p derived valence band structure and the positions of the occupied Pr 4f state density. It is found by a combined PES-XAS study that especially the band gap values strongly depend on the stoichiometry and crystal structure of the single crystalline Pr oxide layer. Furthermore, the close structure relationship between cubic Pr2O3(111) and PrO2(111) films is probably the reason for the detection of nonstoichiometric behavior, an effect which is far less pronounced in case of hexagonal Pr2O3(0001) layers. A possible origin of this effect is given by a surface modified valence change and therefore of importance to understand in future the epitaxial overgrowth of these oxide buffer heterostructures by alternative semiconductors such as germanium.

Parasitic Channels in SiGe Heterojunction p-Metal Oxide Semiconductor Field Effect Transistors

Heterojunction p-metal oxide semiconductor field effect transistors (p-MOSFETs) with -buffer heterostructure (SiGe p-MOSFETs) were fabricated and their parasitic channels induced at the (gate)/Si-cap interface were investigated. The dependence of the parasitic channel on the substrate doping concentration demonstrated that the parasitic channel caused more degradation in the transconductance of the SiGe p-MOSFETs at higher substrate doping concentrations. Si-cap thickness dependence of the transconductance was also examined. As the Si-cap thickness is decreased, the influence of the parasitic channel on the transconductance is strongly reduced. Furthermore, the reduction of the Si-cap thickness makes the parasitic channel less sensitive to the substrate doping concentration, the gate oxide thickness, and the gate voltage. At a Si-cap thickness of less than 2 nm, the SiGe p-MOSFETs can maintain their advantage over Si p-MOSFETs even in the future for smaller sizes. © 2002 The Electrochemical Society. All rights reserved.

Anisotropic magnetoresistance of (0 0 h), (0 h h) and ( h h h) La 2/3Sr 1/3MnO 3 thin films on (0 0 1) Si substrates

The epitaxial growth of magnetoresistive La 2/3Sr 1/3MnO 3 thin films with different orientations on (0 0 1) Si substrates has been successfully achieved by a suitable choice of the buffer heterostructure. The out-of-plane and in-plane microstructure has been carefully analysed and related to the magnetocrystalline anisotropy. The dependence of the magnetoresistance (MR) as a function of the angle between the current and the applied field has been explored in a wide field range. At high field the magnetisation is saturated whatever the direction of the field and we observe the intrinsic anisotropic magnetoresistance (AMR) effect having a uniaxial anisotropy while at low field the magnetocrystalline anisotropy induces higher-order angle dependence of the magnetoresistance.

Tunable epitaxial growth of magnetoresistive La2/3Sr1/3MnO3 thin films

We report on the growth of epitaxial La2/3Sr1/3MnO3 thin films on buffered Si(001) substrates. We show that a suitable choice of the buffer heterostructure allows one to obtain epitaxial (00h), (0hh), and (hhh) manganite thin films. The magnetotransport properties are investigated and we have found that the low-field magnetoresistance is directly related to the width of the normal-to-plane rocking curves, irrespective of the film orientation. The magnetic anisotropy of these films has also been determined.

Epitaxial growth of magnetoresistive (00h), (0hh), and (hhh) La2/3Sr1/3MnO3 thin films on (001)Si substrates

We show that suitable choice of the buffer heterostructure allows us to obtain epitaxial (00h), (0hh), and (hhh) La2/3Sr1/3MnO3 thin films on buffered Si(001) substrates. The magnetotransport properties are investigated. Irrespective of the film texture, it is found that the easy-magnetization direction lies within the film plane. The low-field magnetoresistance is mainly controlled by the in-plane misorientation of crystallites while differences associated with the distinct microstructure of the interfaces appear to have only a second-order effect.

Mechanisms of strain release in molecular beam epitaxy grown InGaAs/GaAs buffer heterostructures

Strain release and distribution in double InGaAs/GaAs heterostructure buffer layers were studied. A higher misfit dislocation density at the inner interface between the InGaAs layer and the substrate was found in all the samples. This corresponded to a strain release of the inner ternary layers which was much larger than that predicted by equilibrium theories. The residual parallel strain of the external layers as a function of the thickness was found to follow a curve of slope -0.502, in agreement with previous results on single InGaAs layers. These results were interpreted as evidence that the elastic energy per unit interface area remains for the strain relaxation growth. The presence of numerous dislocation loops inside the substrate was considered to be responsible for the strain relaxation occurring through dislocation multiplication due to Frank-Read sources activated during growth. A comparison with InGaAs/GaAs step graded heterostructures is also discussed. Finally, lattice plane tilts between epilayers and substrates were found and attributed to the imbalance in the linear density of misfit dislocations with opposite components of the Burgers' vector ( b eff τ) perpendicular to the interface.