M. Yakimov

Metal-oxide-semiconductor capacitors on GaAs with high-k gate oxide and amorphous silicon interface passivation layer

We demonstrate the electrical properties of metal-oxide-semiconductor capacitors on molecular beam epitaxial GaAs in situ passivated with ultrathin amorphous Si (a-Si) layer and with ex situ deposited HfO2 gate oxide and TaN metal gate. Minimum thickness of the Si interface passivation layer of 1.5 nm is needed to prevent the Fermi level pinning and provide good capacitance-voltage characteristics with equivalent oxide thickness of 2.1 nm and leakage current of ⩽1.0mA∕cm2. Transmission electron microscopy analysis showed that the Si layer was oxidized up to 1.4 nm during ex situ processing while the interface between the GaAs and a-Si remained atomically sharp without any sign of interfacial reaction.

Room-temperature defect tolerance of band-engineered InAs quantum dot heterostructures

Using photoluminescence (PL) at 77–420K and high-energy proton implantation (1.5MeV, dose up to 3×1014cm−2) we have studied the thermal quenching of PL and defect tolerance of self-assembled shape-engineered InAs quantum dots (QDs) embedded into GaAs quantum wells (QWs). At room temperature, QDs appeared to withstand two orders of magnitude higher proton doses than QWs without PL degradation. A simple dynamic model was used to account for both dose and temperature dependence of PL efficiency. At low temperatures, the defect-related quenching is mainly controlled by a reduction in the density of defect-free QDs. At and above room temperature, both thermal and defect-related quenching of PL are due to the escape of carriers from dots to wells that act as barriers with low damage constants. A relatively large barrier for escape (450meV) as well as low nonradiative recombination rate in QDs is shown to account for unsurpassed room-temperature defect tolerance and high PL efficiency at room and elevated temper...

Strained Quantum Wells for P-channel InGaAs CMOS

We present experimental results on the effect of strain on hole transport in InGaAs quantum well (QW) structures. Indium content was varied from lattice matched to high compressive stress in InGaAs/InP QW and the transport properties were analyzed at various temperatures (T = 77-300 K) using Hall measurements. The effect of QW thickness (4-20 nm) on hole transport is also presented. The current best results include room temperature mobility and sheet resistance of 390 cm 2 /V-s and 8500 Ω/sq., respectively. It was observed that the mobility had a T -1.8 dependence indicating similar scattering mechanism in almost all of the samples with prominent mechanism being due to interface and barrier scattering. Further optimization of p-channel for InGaAs CMOS needs to be performed using the above results as guidelines.

Nano-Engineering Approaches to Self-Assembled InAs Quantum Dot Laser Medium

A number of nano-engineering methods are proposed and tested to improve optical properties of a laser gain medium using the self-assembled InAs quantum dot (QD) ensemble. The laser characteristics of concern include higher gain, larger modulation bandwidth, higher efficiency at elevated temperatures, higher thermal stability, and enhanced reliability. The focus of this paper is on the management of QD properties through design and molecular beam epitaxial growth and modification of QD heterostructures. This includes digital alloys as high-quality wide-bandgap barrier; under- and overlayers with various compositions to control the dynamics of QD formation and evolution on the surface; shape engineering of QDs to improve electron-hole overlap and reduce inhomogeneous broadening; band engineering of QD heterostructures to enhance the carrier localization by reduction of thermal escape from dots; as well as tunnel injection from quantum wells (QWs) to accelerate carrier transfer to the lasing state. Beneficial properties of the developed QD media are demonstrated at room temperature in laser diodes with unsurpassed thermal stability with a characteristic temperature of 380 K, high waveguide modal gain >50 cm−1, unsurpassed defect tolerance over two orders of magnitude higher than that of QWs typically used in lasers, and efficient emission from a two-dimensional (2-D) photonic crystal nanocavity.

Concept of feedback-free high-frequency loss modulation in detuned duo-cavity vertical cavity surface-emitting laser

The authors propose and demonstrate a novel concept for ultrahigh-speed loss modulation of a vertical cavity surface-emitting laser (VCSEL). A duo-cavity device architecture is used to optically decouple the ac feedback component of a loss-modulation section from the VCSEL active region. The elimination of feedback allows modulation of the VCSEL output far beyond the optoelectronic relaxation frequency by eliminating both resonance and intrinsic high-frequency response roll-off. Precise detuning of the resonances of both coupled cavities is used to achieve ac feedback elimination by control of thickness during molecular beam epitaxy growth. Variation in the applied bias at the multiple-quantum-well modulator section allows adjustment of detuning to change coupling between the two sections, resulting in resonance features in the modulation response. In the ideal case, the resulting resonance-free high-frequency modulation response is limited only by parasitics of the modulator section. A flat (±3 dB) modul...

Oxidation lift-off method for layer transfer of GaAs∕AlAs-based structures

A method for layer transfer via attachment and release of GaAs-based components onto silicon platform in a single-step process (oxidation lift-off method) is proposed. The method involves moderate temperature (∼400°C) alloy bonding of GaAs devices with simultaneous removal of the GaAs substrate by lateral oxidation of sacrificial AlAs layer. Selectivity with respect to Al content is high enough to release the vertical cavity laser structures containing layers of AlxGa1−xAs with x=0.9. This characteristic of the oxidation process allows for the release of components and form oxide apertures during a single step. The technology can be employed for heterogeneous integration of various compound semiconductor devices with Si or other substrates.

High frequency Q-switched operation of a VCSEL with intracavity electroabsorption modulator

We report on active Q-switching of AlGaAs/GaAs VCSELs with In0.16Ga0.84As MQW active region and absorber. Pulse widths of 40 ps at repetition rates of 4 GHz were measured at 5 dBm incident RF power.

Structural and Optical Effects of Capping Layer Material and Growth Rate on the Properties of Self-Assembled InAs Quantum Dot Structures

In order to develop nanoengineering methods to control electronic spectrum of self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy, we have utilized atomic force microscopy (AFM), photoluminescence (PL) and TEM methods to investigate the effects of capping layer growth on the physical/chemical properties as well as the optical/electronic performance of QD device structures. Capping layer material choice (or its absence all together) has been found to directly influence QD dimensions (size, height), and subsequently, to affect QD emission wavelength. We report results of QD lateral size and height as well as densities of InAs QDs capped with 2ML (monolayers) of AlAs or GaAs grown at various rates. Our AFM results are complemented by PL measurements, where the optical properties of capped versus non-capped QDs have been explored and direct correspondence between structural differences induced by capping and the electronic/optical properties of QDs is demonstrated. Analysis of the data shows that the results can be explained by two competing surface processes. The first of which is the redistribution of indium between QDs on top of the 2D wetting layer, resulting in the increase of QD size with time. The second effect is the diffusion of indium out of the QDs and onto the top of the capping layer. TEM with multislice image simulation has supported our AFM and PL observations with the demonstration of “indium driven” alloy intermixing in the overlayer as well as significant alloying in the InAs wetting layer.

Optical Decoupling in a Loss-Modulated Dual-Cavity VCSEL

We have demonstrated the principle of optical decoupling in a loss-modulated VCSEL by use of a dual-cavity geometry. Adjusting detuning between cavities controls the decoupling amount. Flat (+/-3db) response up to 20 GHz is demonstrated.

Quantification of interface trap density above threshold voltage by gated hall method in InGaAs buried quantum well MOSFET

Low density of states (DOS) and typically high interface and border trap densities (D<;sub>it<;/sub>) in high mobility group III-V semiconductors provide difficulties in quantification of D<;sub>it<;/sub> near the conduction band edge. The trap response above the threshold voltage can be very fast, and conventional D<;sub>it<;/sub> extraction methods, based on capacitance/conductance response (CV methods) of MOS capacitors at frequencies <;1MHz, cannot distinguish conducting and trapped carriers. In addition, the CV methods have to deal with high dispersion in the accumulation region that makes it a difficult task to measure the true oxide capacitance C<;sub>ox<;/sub> value. Another implication of these properties of III-V interfaces is an ambiguity of determination of electron density in the MOSFET channel. Traditional evaluation of carrier density by integration of the C-V curve, gives significantly overestimated results even if corrected by D<;sub>it<;/sub>. It happens because the CV methods can distinguish free and trap carriers exclusively by their response kinetics, and therefore all trapped electrons responding faster than ~1μs are treated as free electrons. In this work, we are using a gated Hall method [1,2] which allows for direct measurement of free carrier density, and therefore, when combined with CV measurements or electrostatic modeling allows for accurate quantification of Dit spectrum. In addition, the former approach does not need knowledge of Cox.