Herb Goronkin

TAMM STATES AND DONORS AT INAS/ALSB INTERFACES

Localized interface states, viz., generalized Tamm states, can be induced by an interfacial InSb bond in the InAs/AlSb heterojunction system, as confirmed by electronic structure calculations. The calculated energies of the interfacial Tamm states, however, are too low to account for the observed carrier concentrations in InAs quantum wells. Native defects capable of accounting for the observed carrier concentrations are identified, and their electronic structures calculated: AlSb in an AlSb layer is responsible for the semi‐insulating character of thin InAs quantum wells and the n‐type character of wide wells, and AsAl at an AlAs‐like interface is responsible for the high values of electron concentration in the wells. The decrease of electron concentrations with temperature can be attributed to partial freezing of electrons into the shallow levels associated with ionized donors.

Remote n‐type modulation doping of InAs quantum wells by ‘‘deep acceptors’’ in AlSb

Due to the fact that impurities normally change their doping characters when they undergo shallow to deep transitions or deep‐to‐false‐valence transitions, a single defect, such as a cation on an Sb site, can explain all of the following facts for nonintentionally doped AlxGa1−xSb alloys and InAs/AlxGa1−xSb superlattices and quantum‐well structures: (i) Bulk GaSb is p type; (ii) bulk AlSb is semi‐insulating; (iii) InAs/AlSb superlattices with InAs quantum wells thicker than a critical thickness dc(x=1.0) are n type, where the InAs shallow–deep critical thickness function dc(x) is around ≂100–≂150 A for 0.5<x≤1.0 for InAs/AlxGa1−xSb superlattices; (iv) InAs/AlSb superlattices with InAs quantum wells thinner than dc(x=1.0) are semi‐insulating. In addition, the theory predicts that Al0.5Ga0.5Sb and AlSb will be semi‐insulating when nonintentionally doped, but can be converted to p type by the application of hydrostatic pressure P: P≳90 kbar and P≳150 kbar, respectively. These changes of doping character, whi...

Spatially Selective Formation of InAs Self-organized Quantum Dots on Patterned GaAs (100) Substrates

Self-organized InAs quantum dots are selectively formed on patterned GaAs (100) substrate by chemical beam epitaxy. Dot formation on top of sub-µm sized mesa stripes is a function of the stripe top width, the stripe orientation, as well as the growth conditions. The dot density is higher for stripes aligned in [001] direction, and lower for stripes aligned in the [011] direction, respectively, when compared to that obtained on a non-patterned substrate under the same growth conditions. We attribute these effects to dissimilar surface migration behavior of In adatoms on different side facets of the mesa stripes. The spatial distribution of the self-organized quantum dots (SOQDs) on top of [001] stripes becomes more uniform as the top facet width decreases, and self-alignment of the dots into well-defined rows is achieved by manipulating the growth conditions and mesa size.

A MMIC lumped element directional coupler with arbitrary characteristic impedance and its application

In this paper the design technique for MMIC lumped element directional couplers, based on arbitrary termination impedance, is described. Arbitrary impedance terminations allow matching the linear and nonlinear elements to the coupler without the requirement of transformer networks. The techniques capability is demonstrated through the design of a single balanced mixer prototype at 1.8GHz for which measured and simulated data are provided.

Functional Nanoscale Devices

The recent emergence of fabrication tools and techniques capable of constructing structures with dimensions ranging from 0.1 to 50 nm (see Fig. 5.1) has opened up numerous possibilities for investigating new devices in a size domain heretofore inaccessible to experimental researchers. The WTEC nanotechnology panel reviewed research in the United States, Japan, Taiwan, and Europe to find that there is considerable nanoscience and technology activity in university, industrial, and government laboratories around the world. The insight gained from this survey suggests areas of strength and areas of possible improvement in the field.

Explanation of the origin of electrons in the unintentionally doped InAs/AlSb system

Theoretical studies are summarized which show that (i) anion‐site antisite defects AlSb in AlSb barriers and (ii) interfacial native defects such as AsAl at AlAs‐like interfaces are the origin of electrons in nominally undoped InAs quantum wells of AlSb/InAs heterojunctions. Tamm interface states, while present at the InSb‐like interfaces, lie at too low energy to account for the observed carrier densities.

Quantum MMIC (QMMIC) VCO's for Wireless Applications

The monolithic integration of heterostructure tuneling diodes with other semiconductor devices, such as HFETs, creates novel, quantum functional devices and circuits. The enhanced functionality of these devices enables design of both digital and analog circuits with reduced complexity, smaller size and better performance. Several types of QMMIC VCOs operating in L-band frequency range have been designed and characterized. VCOs achieved output power of 8-10 dBm at L-band frequency range. All VCOs exhibited very low phase noise (in the range of ¿107 to ¿115 dBc/Hz) at 1.0 MHz away from the carrier frequency.

Resonant tunneling devices and circuits

We review results of logic and memory devices and circuits based on the negative differential resistance associated with resonant tunneling and interband tunneling effects. We have fabricated resonant interband tunneling field effect transistors on both InAs/GaSb/AlSb and InGaAs/InAlAs/InP material systems. A new exclusive-NOR device has also been demonstrated. Preliminary results of a FULL ADDER are shown. Static random access memory based on the bistability of two serially connected diodes is also achieved. We show simulations and compare our devices with other approaches and discuss important issues related to applications of resonant tunneling devices and circuits.

Enhancement of Mobility in Pseudomorphic FET's with Up and Down Monolayers

We have used molecular beam epitaxy (MBE) to grow single molecular layers (monolayers) in quantum wells in order to improve electron transport. The quantum well energy levels, wave functions, and the carrier concentrations are obtained by self-consistently solving the Schrodinger and Poisson equations. Photoluminescence (PL) and Hall measurements confirmed the energy level shifts and the mobility enhancement due to the monolayer insertions.

CMOS and beyond

Abstract Lithography has played a key role in the scaling of CMOS-based integrated circuits. To fabricate sub-70 nm features, new techniques based on electron projection and extreme ultraviolet radiation are being developed. These and other lithographic solutions are discussed. For the ultimate in scaling, an alternate approach would be to start with objects that are inherently nano-scale in size, and use chemical techniques to have these objects self-assemble into units that provide electronic functionality. Several classes of molecules, including carbon nanotubes, deoxyribonucleic acid (DNA) and custom-synthesized organic molecules, are potentially suitable for this approach. Some recent advances in their research will be reviewed.