Stephen D. Hersee

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%.

Unusually strong space-charge-limited current in thin wires.

The current-voltage characteristics of thin wires are often observed to be nonlinear, and this behavior has been ascribed to Schottky barriers at the contacts. We present electronic transport measurements on GaN nanorods and demonstrate that the nonlinear behavior originates instead from space-charge-limited current. A theory of space-charge-limited current in thin wires corroborates the experiments and shows that poor screening in high-aspect ratio materials leads to a dramatic enhancement of space-charge limited current, resulting in new scaling in terms of the aspect ratio.

Selective growth of Ge on Si(100) through vias of SiO2 nanotemplate using solid source molecular beam epitaxy

We demonstrate that Ge can be selectively grown on Si(100) through openings in a SiO2 nanotemplate by solid source molecular beam epitaxy. The selectivity relies on the thermal instability of GeO and SiO near 650 °C. Ge islands grow in the template windows and coalesce on top of the template, forming an epitaxial lateral overgrowth (ELO) layer. Cross-sectional transmission electron microscopy images show that the Ge seeds and the ELO layer are free of threading dislocations. Only stacking faults are generated but terminate within 70 nm of the Ge–Si interface, while twins along {111} planes are observed in the ELO layer. The threading-dislocation-free Ge seeds and ELO layer are attributed to epitaxial necking as well as Ge–Si intermixing at the interface.

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.

Heteroepitaxy of high-quality Ge on Si by nanoscale Ge seeds grown through a thin layer of SiO2

We demonstrate that high-quality Ge can be grown on Si covered with a thin layer of chemical SiO2. When the oxidized Si substrate is exposed to Ge molecular beam, 7-nm-wide seed pads form in the oxide layer and “touchdown” on the underlying Si. Upon continued exposure, Ge selectively grows on the seed pads rather than on SiO2, and the seeds coalesce to form an epitaxial lateral overgrowth (ELO) layer. The Ge ELO layer is characterized by transmission electron microscopy and etch-pit density (EPD). The Ge ELO layer is free of dislocation network, but stacking faults exist near the Ge-SiO2 interface. A fraction of these stacking faults propagate to the surface, resulting in EPD less than 2×106cm−2. The high quality Ge ELO layer is attributed to a high density of nanoscale Ge seed pads interspaced by 2–12-nm-wide SiO2 patches.

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.

Morphological evolution and strain relaxation of Ge islands grown on chemically oxidized Si(100) by molecular-beam epitaxy

We have previously demonstrated that high-quality Ge can be grown on Si by the touchdown process, where chemically oxidized Si is exposed to a Ge molecular beam. The causes of strain relaxation in the Ge epilayer were also proposed and discussed. Herein, we present a detailed analysis on the morphological evolution and strain relaxation of nanoscale Ge islands on SiO2-covered Si in order to identify the mechanisms by which the high-quality epilayer forms. During the touchdown, the Ge seeds are anchored to the underlying Si. This immobility of Ge islands gives rise to a unique bimodal size distribution during coarsening. Three events are observed during coalescence: (1) merging of two small (<10nm) islands largely driven by surface diffusion, (2) merging of a small island and a big island (∼50nm), and (3) merging of two big islands. The coalescence of two small islands is characterized by the formation of twins or stacking faults at the two merging fronts. In contrast, no stacking fault or grain boundary r...

Fabrication of GaN nanowire arrays by confined epitaxy

The authors report the fabrication of GaN nanowire arrays inside a thick SiNx, selective growth mask that was patterned by interferometric lithography and dry etching. The GaN nanowires are molded by the apertures in the selective growth mask and the growth is epitaxial with respect to the underlying GaN layer. The precise location and diameter of each nanowire in the array are controlled by the growth mask patterning, and the resulting array has a long-range order that is compatible with photonic crystal applications. This process uses conventional metal organic precursors and does not require any additional metal catalysts.

Surface-emitting laser-based smart pixels for two-dimensional optical logic and reconfigurable optical interconnections

A monolithic smart pixel technology based on the integration of two-dimensional arrays of cascadable optical switches, optical logic gates, and optical switching nodes consisting of vertical-cavity surface-emitting lasers and heterojunction phototransistors or photothyristors is described. Different combinations of these components perform optical switching, logic, routing, memory, and regeneration. Latching, nonlatching, and bistable switches with high optical gain and contrast are demonstrated, along with all the simple single-stage Boolean optical logic functions. A 2*2 optical bypass-exchange node and a reconfigurable, multistage, two-dimensional optical switching network architecture are also described. >

GaN linear electro‐optic effect

Measurements of the linear (Pockels) electro‐optical coefficient of wurtzite GaN are reported. The values for the electro‐optic coefficients r33 and r31 are 1.91±0.35 and 0.57±0.11 pm/V at 633 nm, respectively, in agreement with extrapolations from measured second‐harmonic generation coefficients (χ33(2)=−20±6 pm/V and χ31(2)=10±3 pm/V) suggesting that the dominant contributions are electronic in origin. Measurements were performed using a Mach–Zehnder interferometer with LiNbO3 as a reference material. Piezoelectric effects were also observed.