K. C. Hsieh

Characterization of GaAs-based n-n and p-n interface junctions prepared by direct wafer bonding

In this study, the electrical characteristics and interface microstructures of GaAs-based n-n and p-n interface junctions prepared from direct wafer bonding have been systematically investigated through current-voltage measurements and transmission electron microscopy. It is found that a nearly continuous amorphous interface layer exists in all samples bonded at 400u2009°C. A drastic change in interface morphology caused by atomic rearrangement during high-temperature annealing at 600u2009°C leads to the formation of a locally perfect junction interface combined with an array of nanoscale, bubblelike amorphous regions. Each of them plays a different role in affecting carrier transports. The regions with local crystalline perfection can result in a considerable reduction of interface resistance for the majority carrier transport. However, the non-negligible interface resistance suggests that at interface boundary, there still exits a large number of interface states resulting from atomic imperfections, such as poi...

Low temperature wafer bonding by spin on glass

We report the development of a low temperature (∼400u200a°C) and low pressure (∼0.5u2009kg/cm2) spin-on-glass (SOG) wafer bonding technique that can bond compound semiconductors and silicon without using chemical-mechanical polishing, surface etching or other intermediate materials in the bonding process. The relation of bonding quality and applied bonding pressure was studied. Cross sectional transmission electron microscopy analysis shows that the bonding interface is smooth, uniform and did not generate dislocations. Using this SOG bonding method, simulated vertical-cavity surface-emitting laser (VCSEL) structures that consist of GaInAs and InP cavities sandwiched by Al–oxide/Si distributed Bragg reflectors (DBRs) were successfully bonded to a silicon substrate with the bonding interface located outside the bottom DBR away from the VCSEL cavity.

High indium metamorphic HEMT on a GaAs substrate

Metamorphic growth of device structures on GaAs substrates has advanced rapidly in recent years. High quality electronic and optical devices have been demonstrated. Also long-term reliability has been achieved with low noise MHEMT devices. Most of the development emphasis has been with structures conventionally grown on InP substrates. This work is motivated by the lower cost, larger diameter, and greater robustness of GaAs substrates compared to InP substrates. However an important characteristic of metamorphic growth is the degree of freedom in choosing the In/sub x/(GaAl)/sub 1-x/As composition and consequently the lattice constant between GaAs and InAs. Consequently new device structures can be achieved which are not possible by pseudomorphic growth on either GaAs or InP substrates. In this effort, solid source MBE was used to grow metamorphic HEMT structures with high indium content. For the conventional MHEMT, the indium concentration is graded to In/sub 0.52/Al/sub 0.48/As to expand the lattice constant to that of InP. Here the indium content was graded to In/sub 0.64/Al/sub 0.36/As to achieve a larger lattice constant than InP. The resulting surface roughness was examined by AFM. For a 25 /spl mu/m x 25 /spl mu/m area, the RMS roughness was 12/spl Aring/ which is very similar to the roughness present in the conventional MHEMT with less indium content.

Surface morphology control of InAs nanostructures grown on InGaAs/InP

The evolution of the surface morphology of InAs nanostructures grown on InGaAs/InP by molecular-beam epitaxy was studied through atomic force microscopy imaging. Randomly distributed quantum dots and quantum wires were reproducibly achieved by adjusting proper growth parameters such as InAs deposition thickness, growth temperature, arsenic overpressure, and InAs growth rate. It is observed that a thick InAs layer, high growth temperature, high arsenic overpressure, and high growth rate promote the formation of quantum dots. We propose that when InAs is deposited, the interaction of the total strain in the InAs layer and the surface strain distribution in the underlying matrix layer might be the determinant factor of the nanostructure morphology. Thick InAs, which increases the total strain of the InAs layer, is preferred to form quantum dots. Surface diffusion of In adatoms is another important factor affecting the surface morphology. A high growth temperature promotes homogeneous diffusion, while a high arsenic overpressure and growth rate reduces the surface diffusion of the In adatoms. These factors induce the formation of quantum dots.

Growth and characterization of metamorphic Inx(AlGa)1−xAs/InxGa1−xAs high electron mobility transistor material and devices with X=0.3–0.4

High electron mobility transistor structures containing Inx(AlGa)1−xAs and InxGa1−xAs device layers with X=0.3–0.4 were grown on metamorphic buffer layers on GaAs substrates. The structures exhibited good flatness with a root mean square roughness of 9 A. Cross-sectional transmission electron micrographs indicated that the threading dislocations from metamorphic growth were contained in the graded buffer layer. For double pulse doped structures, sheet densities up to 4×1012u200acm−2 were readily achieved. Room temperature mobilities of 8600–8800 cm2/Vu200as were obtained using In0.42Ga0.58As channel layers. Photoluminescence measurements of the InGaAs channel layer in metamorphic and pseudomorphic structures showed no significant reduction in room temperature luminescence intensity due to metamorphic growth. Transistor structures of the same device geometry were fabricated from both metamorphic and pseudomorphic GaAs high electron mobility transistor structures. At 25 GHz, the metamorphic device produced a 3 dB h...

Metalorganic chemical vapor deposition growth of high-quality InAs∕GaSb type II superlattices on (001) GaAs substrates

InAs layers and InAs∕GaSb type II superlattices (SLs) were grown on (001) GaAs substrates by metalorganic chemical vapor deposition. A thin low-temperature GaSb nucleation layer and a thicker high-temperature metamorphic GaSb buffer layer were introduced before the growth of InAs or the SLs. By optimizing the growth temperature, the interface gas switching and the growth rate, morphology, and structural properties of the grown structures are significantly improved. In some cases, nanopipes are found in these structures. Morphology studies of the InAs heteroepitaxial layers show that (1) the diameter of these nanopipes decreases with the growth temperature; (2) nanopipes are nucleated from the interface of GaSb∕InAs; and (3) the density of nanopipes depends on the passivation of GaSb surface before the growth of the InAs. In growing InAs∕GaSb SLs, we show that the growth rate of GaSb has a strong effect on the morphology and that the unintentional interfacial InSb layer formed at the InAs∕GaSb interfaces i...

Improved surface and structural properties of InAs∕GaSb superlattices on (001) GaSb substrate by introducing an InAsSb layer at interfaces

InAs∕GaSb type-II superlattices (SLs) were grown on (001) GaSb substrates by metal organic chemical vapor deposition. Besides the expected tensile stress introduced by the InAs layers in the SLs, additional tensile stress is found in the InAs∕GaSb SLs from the simulation of x-ray diffraction (XRD) curves of the SLs. High-resolution transmission electron microscopy and XRD of the SLs grown with different interface gas switching procedures suggest that the additional tensile stress is mainly located at the GaSb→InAs interface. To compensate for the tensile stress in the SL structures, we show that introducing ∼2-ML-thick InAs0.8Sb0.2 layer at the interfaces of the SL improves the morphology and the structural properties of the SLs significantly.

Growth and luminescence properties of GaP:N and GaP1−xNx

Nitrogen‐doped GaP and P‐rich GaP1−xNx alloys (x<0.03) exhibiting 77 K photoluminescence (PL) in the yellow–green to red portion of the visible spectrum have been grown on GaP substrates by gas source molecular beam epitaxy. The growth of these compounds was accomplished using gaseous NH3 and PH3 and solid Ga as the source materials. In samples with nitrogen concentration [N] < 1020 cm−3, the dominant emission occurred at 569 nm, corresponding to the isoelectronic trap NN1. The PL intensity was greatest for [N]≂ 1020 cm−3. As the nitrogen concentration was increased beyond this value, a monotonic red shift was observed in the emission wavelength, while the intensity generally decreased with increasing [N]. The red shift in the band edge is explained in terms of severe bowing in the compositional dependence of the indirect band gap for the GaP1−xNx system, as predicted by the dielectric theory of electronegativity.

Hybrid-integrated GaAs/GaAs and InP/GaAs semiconductors through wafer bonding technology: Interface adhesion and mechanical strength

In this study, the interface adhesion and mechanical strength of wafer bonded GaAs/GaAs and GaAs/InP semiconductors, each of (100) face, were characterized by combining the measurements of interface fracture energy γo and lap shear strength Es. The relations between the interface adhesion and annealing processes for four different types of bonding configurations, i.e., antiphase bonding, in-phase bonding, and twist bonding with 5° and 30° misalignments, were systematically studied. The surface free energy γα-GaAs/oxide (0.11–0.28u2009J/m2) of amorphous α-GaAs/oxide mixture was estimated based upon the reported surface free energy γc-GaAs (0.63u2009J/m2) of crystalline [100] GaAs and measured overall interface fracture energy γtotal (0.525u2009J/m2) of GaAs/GaAs bonded wafers. The micromorphologies of the bonded and debonded wafer interfaces were characterized by atomic force microscopy (AFM) and transmission electron microcopy (TEM). The interface microfailure mechanism of directly bonded GaAs wafers was proposed bas...

Improved size uniformity of InAs quantum dots grown on a lateral composition modulated InGaAs surface

The properties of InAs quantum dots (QDs) deposited on compositionally homogenous and laterally modulated surfaces is investigated by photoluminescence (PL), atomic force microscopy (AFM), and transmission electron microscopy. We use solid source molecular beam epitaxy on (100)-oriented InP substrates to fabricate the samples. It is found that QDs grown on a laterally modulated surface are more uniform in size. This is implied by a decrease of 22% in the full width at half maximum (FWHM) in the PL signal at 77 K for InAs QDs deposited on the modulated surface as opposed to the homogenous surface for equal monolayer coverage of InAs. Similarly, plan view scans taken by ex situ contact AFM also show improved size uniformity of QDs grown on the laterally composition modulated surface as evidenced by a decrease in the standard deviation of area data compiled from the images. It is shown that the improvement in the geometrical uniformity of the quantum dots as depicted by the PL FWHM and AFM data is facilitate...