David Zubia

Nanoheteroepitaxy: The application of nanostructuring and substrate compliance to the heteroepitaxy of mismatched semiconductor materials

This article describes an approach to the heteroepitaxy of lattice mismatched semiconductors, that we call nanoheteroepitaxy. The theory developed here shows that the 3D stress relief mechanisms that are active when an epilayer is nucleated as an array of nanoscale islands on a compliant patterned substrate, will significantly reduce the strain energy in the epilayer and extend the critical thickness dramatically. Calculations show that with the scale of patterning that is achievable with advanced lithography (10–100 nm) we can eliminate mismatch dislocations from heterojunctions that are mismatched by as much as 4.2%.

Nanoheteroepitaxial growth of GaN on Si by organometallic vapor phase epitaxy

Nanoheteroepitaxy has recently been proposed as a technique for significantly extending the thickness of pseudomorphic growth in mismatched heterostructures. This letter reports the experimental application of nanoheteroepitaxy for the growth of GaN on patterned 〈111〉 oriented silicon-on-insulator substrates by organometallic vapor phase epitaxy. Transmission electron microscopy reveals that the defect concentration decays rapidly away from the heterointerface as predicted by nanoheteroepitaxy theory. The melting point of the nanoscale islands is found to be significantly reduced, enhancing substrate compliance and further reducing the strain energy in the GaN epitaxial layer.

Nanoheteroepitaxy for the integration of highly mismatched semiconductor materials

We describe an ongoing study of nanoheteroepitaxy (NHE), the use of nanoscale growth-initiation areas for the integration of highly mismatched semiconductor materials. The concept and theory of NHE is briefly described and is followed by a discussion of the design and fabrication by interferometric lithography of practical sample structures that satisfy the requirements of NHE. Results of NHE growth of GaAs-on-Si and GaN-on-Si are described, following the NHE process from nucleation through to coalescence. Micro-Raman measurements indicate that the strain in partially coalesced NHE GaN-on-Si films is <0.1 GPa.

Nanoheteroepitaxy: Nanofabrication route to improved epitaxial growth

Nanoheteroepitaxy is a fundamentally new epitaxial approach that utilizes three-dimensional stress relief mechanisms available to nanoscale heterostructures to eliminate defects provided the island diameter is below a critical value 2lc. Analysis shows that 2lc∼(15–30)× the critical thickness hc. In the case of GaAs on Si (∼4% misfit), 2lc∼40 nm. In material systems such as GaN on Si (∼20% misfit), where the misfit is much larger and interfacial defects are unavoidable, the nanoheteroepitaxial structure is shown to reduce the formation and propagation of threading defects. Nanostructured substrate parameters that impact growth are discussed and interferometric lithography is introduced as a method for fabrication of large-area substrates for nanoheteroepitaxy. Si nanoisland diameters as small as 20 nm are demonstrated. Scanning and transmission electron microscopy data of GaN grown on Si (via organometallic vapor phase epitaxy) shows reduced threading defects in nanostructured samples compared to growth o...

Strain partitioning in coherent compliant heterostructures

A self-consistent set of equations describing strain partitioning in planar bilayers is developed using a typical definition of strain, the assumption of a coherent interface and a mechanical equilibrium criterion. This approach eliminates the need for the concepts of lattice mismatch and compatibility of deformation, leading to a general solution for the strains and in-plane lattice parameter in bilayer structures. Using the strain equations, the strain energies in the system are calculated as a function of the epilayer to substrate thickness ratio. It was found that for a given substrate thickness, the epilayer strain energy contains a maximum at a layer thickness ratio of ∼1. The peak epilayer strain energy is only ∼25% of the maximum possible in the system. A criterion based on energy considerations is proposed for determining whether to use the epilayer or substrate dislocation formation energy when calculating the epilayer critical thickness. This criterion is applied to the GexSi1−x/Si(100) materia...

MOCVD growth of InNxAs1-x on GaAs using dimethylhydrazine

InNAs/GaAs multiple-quantum-well samples were grown by MOCVD on (100) n + -GaAs substrates at 500 °C and 60 Torr using uncracked dimethylhydrazine (DMHy). Quantum well layers were grown using trimethylindium, tertiarybutylarsine, and 95-97.5% of DMHy in the vapor phase, while GaAs buffer, barrier, and cap layers were grown using trimethylgallium and arsine. The crystalline quality and solid phase composition were evaluated using high-resolution X-ray diffraction analysis. The nitrogen content in InNAs wells was determined to be 18%. Surface morphology was investigated by atomic force microscopy (AFM) and field emission microscopy (FEM). Photoluminescence measurements confirm that the bandgap energy of InNAs is significantly lower than that of InAs. The peak emission wavelength of ∼6.5 μm at 10 K is the longest reported so far for dilute nitride semiconductors.

Defect formation dynamics during CdTe overlayer growth

The presence of atomic-scale defects at multilayer interfaces significantly degrades performance in CdTe-based photovoltaic technologies. The ability to accurately predict and understand defect formation mechanisms during overlayer growth is, therefore, a rational approach for improving the efficiencies of CdTe materials. In this work, we utilize a recently developed CdTe bond-order potential (BOP) to enable accurate molecular dynamics (MD) simulations for predicting defect formation during multilayer growth. A detailed comparison of our MD simulations to high-resolution transmission electron microscopy experiments verifies the accuracy and predictive power of our approach. Our simulations further indicate that island growth can reduce the lattice mismatch induced defects. These results highlight the use of predictive MD simulations to gain new insight into defect reduction in CdTe overlayers, which directly addresses efforts to improve these materials.

High-fidelity simulations of CdTe vapor deposition from a bond-order potential-based molecular dynamics method

CdTe has been a special semiconductor for constructing the lowest-cost solar cells and the CdTe-based Cd1-xZnxTe alloy has been the leading semiconductor for radiation detection applications. The performance currently achieved for the materials, however, is still far below the theoretical expectations. This is because the property-limiting nanoscale defects that are easily formed during the growth of CdTe crystals are difficult to explore in experiments. Here we demonstrate the capability of a bond order potential-based molecular dynamics method for predicting the crystalline growth of CdTe films during vapor deposition simulations. Such a method may begin to enable defects generated during vapor deposition of CdTe crystals to be accurately explored.

Characterisation of nitride thin films by electron backscattered diffraction

Abstract In this paper, we describe the technique of electron backscattered diffraction (EBSD) and illustrate its use in the characterisation of nitride thin films by describing our results from a silicon-doped 3 μm thick GaN epilayer grown on a sapphire substrate misoriented by 10° towards the m-plane (10-10). We show that the EBSD technique may be used to reveal the relative orientation of an epilayer with respect to its substrate (a 90° rotation between the GaN epilayer and sapphire substrate is observed) and to determine its tilt (the GaN epilayer was found to be tilted by 12±3° towards [10-10] GaN ).

Resistive Switching of SnO2 Thin Films on Glass Substrates

Resistive switching of SnO2 thin films deposited by RF magnetron reactive sputtering at room temperature was investigated. Ag/SnO2/Ti structures were fabricated on glass substrates for current-voltage characteristics evaluation. Repeatable unipolar switching was observed using a compliance current of 10 mA and limiting the reset voltage between 0.8 and 1.2 V. Different top contact area were fabricated indicating a filamentary forming mechanism. Furthermore, a retention memory analysis was performed indicating an acceptable device behavior through time. An Ohmic conduction process was found in LRS and HRS. However for HRS, Ohmic conduction was observed only at voltages lower than 0.3 V. At higher voltages, conduction is not explained well by Ohmic, Poole-Frankel, Schottky emission, or space-charge-limited conduction. This indicates that a material structural change occurs at voltages above 0.3 V which is the onset to switching from HRS to LRS.