A. Georgakilas

InAlN/GaN HEMTs: a first insight into technological optimization

High-electron mobility transistors (HEMTs) were fabricated from heterostructures consisting of undoped In/sub 0.2/Al/sub 0.8/N barrier and GaN channel layers grown by metal-organic vapor phase epitaxy on (0001) sapphire substrates. The polarization-induced two-dimensional electron gas (2DEG) density and mobility at the In/sub 0.2/Al/sub 0.8/N/GaN heterojunction were 2/spl times/10/sup 13/ cm/sup -2/ and 260 cm/sup 2/V/sup -1/s/sup -1/, respectively. A tradeoff was determined for the annealing temperature of Ti/Al/Ni/Au ohmic contacts in order to achieve a low contact resistance (/spl rho//sub C/=2.4/spl times/10/sup -5/ /spl Omega//spl middot/cm/sup 2/) without degradation of the channels sheet resistance. Schottky barrier heights were 0.63 and 0.84 eV for Ni- and Pt-based contacts, respectively. The obtained dc parameters of 1-/spl mu/m gate-length HEMT were 0.64 A/mm drain current at V/sub GS/=3 V and 122 mS/mm transconductance, respectively. An HEMT analytical model was used to identify the effects of various material and device parameters on the InAlN/GaN HEMT performance. It is concluded that the increase in the channel mobility is urgently needed in order to benefit from the high 2DEG density.

Heteroepitaxial growth of In-face InN on GaN (0001) by plasma-assisted molecular-beam epitaxy

The thermodynamic aspects of indium-face InN growth by radio frequency plasma-assisted molecular-beam epitaxy (rf-MBE) and the nucleation of InN on gallium-face GaN (0001) surface were investigated. The rates of InN decomposition and indium desorption from the surface were measured in situ using reflected high-energy electron diffraction and the rf-MBE “growth window” of In-face InN (0001) was identified. It is shown that sustainable growth can be achieved only when the arrival rate of active nitrogen species on the surface is higher than the arrival rate of indium atoms. The maximum substrate temperature permitting InN growth as a function of the active nitrogen flux was determined. The growth mode of InN on Ga-face GaN (0001) surface was investigated by reflected high-energy electron diffraction and atomic force microscopy. It was found to be of the Volmer–Weber-type for substrate temperatures less than 350°C and of the Stranski–Krastanov for substrate temperatures between 350 and 520°C. The number of m...

Wafer-scale integration of GaAs optoelectronic devices with standard Si integrated circuits using a low-temperature bonding procedure

A methodology for the heterogeneous integration of epitaxial GaAs wafers with fully processed standard bipolar complementary metal-oxide-semiconductor Si wafers is presented. The complete low-temperature wafer bonding process flow, including procedures for the Si wafer planarization and GaAs substrate removal, has been developed and evaluated. The implementation of an in-plane optical link, consisting of an edge-emitting laser diode, a waveguide and a photodiode, is demonstrated.

Achievements and limitations in optimized GaAs films grown on Si by molecular‐beam epitaxy

A systematic study of the growth of high‐quality films of GaAs on Si substrates has been performed for applications in devices, particularly in optoelectronic devices for cointegration in optical interconnects. The effort for optimized active layers was approached through the separate optimization of substrate preparation, growth time parameters, and postgrowth treatment. In particular, the study of growth involved the investigation of the effect of silicon substrate orientation, post‐growth treatment, as well as multilayer and, especially, silicon buffer layers. For quantification of film quality, a number of characterization methods were used both in situ: reflected high‐energy electron diffraction (RHEED); and ex situ: optical, electrical [current versus voltage (I‐V), capacitance versus voltage (C‐V), deep‐level transient spectroscopy (DLTS), Hall], transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron channeling patterns, x‐ray double‐crystal diffractometry (DDX). Schot...

Energy bandgap bowing of InAlN alloys studied by spectroscopic ellipsometry

InxAl1−xN films were heteroepitaxially grown on AlN∕Al2O3 (0001) templates by molecular beam epitaxy. The compositions studied spanned the whole ternary range. The complex dielectric function of the films was investigated by variable angle spectroscopic ellipsometry in the energy range from 0.55to6eV. The energy bandgap bowing parameter was found to strongly depend on composition, monotonically increasing with decreasing InN mole fraction. This behavior is in agreement with theoretical predictions of large charge transfer contributions to bandgap bowing.

Realistic end-to-end simulation of the optoelectronic links and comparison with the electrical interconnections for system-on-chip applications

A detailed comparison of optoelectronic versus electrical interconnections for system-on-chip applications is performed in terms of signal latency and power consumption. Realistic end-to-end models of both interconnection schemes are employed in order to evaluate critical performance parameters. A variety of electrical and optoelectronic interconnection configurations are implemented and simulated using accurate optical device and electronic circuit models integrated under an integrated circuit (IC) design computer-aided design tool. Two commercial complementary metal-oxide-semiconductor (CMOS) technologies (0.8 /spl mu/m and 0.25 /spl mu/m) are used for the estimation of the signal latency and the power consumption as a function of the interconnection length for the different link configurations. It was found that optoelectronic interconnects outperform their electrical counterparts, under certain conditions, especially for relatively long lines and multichannel data links.

Indium migration paths in V-defects of InAlN grown by metal-organic vapor phase epitaxy

InAlN thin films grown on GaN/Al2O3 (0001) templates by metal-organic vapor phase epitaxy were studied by transmission electron microscopy techniques. V-defects in the form of hexagonal inverted pyramids with {101¯1} sidewalls were observed on the films’ surfaces linked to the termination of threading dislocations. Their origin is explained by the different surface atom mobility of In and Al and the built-in strain relaxation. Indium segregation in the films is influenced by the formation of V-defects, the edges and the apexes of which function as paths of migrating indium atoms diffusing along nanopipes formed at the open-core threading dislocations.InAlN thin films grown on GaN/Al2O3 (0001) templates by metal-organic vapor phase epitaxy were studied by transmission electron microscopy techniques. V-defects in the form of hexagonal inverted pyramids with {101¯1} sidewalls were observed on the films’ surfaces linked to the termination of threading dislocations. Their origin is explained by the different surface atom mobility of In and Al and the built-in strain relaxation. Indium segregation in the films is influenced by the formation of V-defects, the edges and the apexes of which function as paths of migrating indium atoms diffusing along nanopipes formed at the open-core threading dislocations.

Biaxial strain and lattice constants of InN (0001) films grown by plasma-assisted molecular beam epitaxy

We present a systematic study, using high resolution x-ray diffraction, of the in-plane a and out-of-plane c lattice parameters of high quality InN films grown by molecular beam epitaxy on GaN∕Al2O3 (0001) substrates. It is found that their values are dependent on the nucleation and growth conditions. Films nucleated in a two- or three-dimensional growth mode exhibit biaxial compressive or tensile strain, respectively. The linear dependence of c on a is consistent with biaxial strain being present in the films. A biaxial strain relaxation coefficient of 0.43±0.04 is deduced. The values of the lattice constants for the case of strain-free InN are estimated to be in the ranges c=5.699±0.004A and a=3.535±0.005A.

Physical model of InN growth on Ga-face GaN (0001) by molecular-beam epitaxy

A consistent physical model of the growth of InN on GaN (0001) by radio-frequency plasma-assisted molecular-beam epitaxy is presented. Four distinct regimes of InN growth are observed due to the temperature dependence of indium adatoms’ mobility and of the InN decomposition rate. At substrate temperatures higher than 450°C, indium adatoms are highly mobile and a self-regulating mechanism of InN islands’ diameter takes place, so that a stoichiometric N:In atomic ratio on the top face of the islands is established. As a result, two-dimensional growth is possible only with In∕N atomic ratio on the substrate surface equal to unity. The self-regulating mechanism could be exploited to engineer self-organized nanostructures.

Mechanism of compositional modulations in epitaxial InAlN films grown by molecular beam epitaxy

A mechanism for compositional modulations in InxAl1−xN films is described which considers growth kinetics during molecular beam epitaxy. InAlN crystalline films with various indium contents, grown on GaN or AlN buffer layers to create a variation in lattice mismatch conditions, were studied by transmission electron microscopy. Films comprise of columnar domains which are observed regardless of mismatch, with increasing indium concentration toward domain edges. We propose that indium is incorporated preferentially between adjacent dynamical InAlN platelets, owing to tensile strain generated upon platelet coalescence. The resulting In-rich boundaries are potential minima for further indium adatoms, creating a permanent indium composition gradient.