C. Kadow

SELF-ASSEMBLED ERAS ISLANDS IN GAAS: GROWTH AND SUBPICOSECOND CARRIER DYNAMICS

We report the growth of self-assembled ErAs islands embedded in GaAs by molecular beam epitaxy. The nucleation of ErAs on GaAs occurs in an island growth mode leading to spontaneous formation of nanometer-sized islands. Several layers of ErAs islands separated by GaAs can be stacked on top of each other to form a superlattice. X-ray diffraction shows superlattice fringes from such samples. Pump–probe measurements indicate carrier capture times as short as 120 fs. These capture times are strongly correlated with the period of the superlattice.

An ultra-low power InAs/AlSb HEMT Ka-band low-noise amplifier

The first antimonide-based compound semiconductor (ABCS) MMIC, a Ka-Band low-noise amplifier using 0.25-/spl mu/m gate length InAs/AlSb metamorphic HEMTs, has been fabricated and characterized on a 75 /spl mu/m GaAs substrate. The compact 1.1 mm/sup 2/ three-stage Ka-band LNA demonstrated an average of 2.1 dB noise-figure between 34-36 GHz with an associated gain of 22 dB. The measured dc power dissipation of the ABCS LNA was an ultra-low 1.5 mW per stage, or 4.5 mW total. This is less than one-tenth the dc power dissipation of a typical equivalent InGaAs/AlGaAs/GaAs HEMT LNA. Operation with degraded gain and noise figure at 1.1 mW total dc power dissipation is also verified. These results demonstrate the outstanding potential of ABCS HEMT technology for mobile and space-based millimeter-wave applications.

Electronic structure and conduction in a metal–semiconductor digital composite: ErAs:InGaAs

We have grown epitaxial superlattice structures of layers of semimetallic ErAs particles embedded in an InGaAs matrix on (001) Fe-doped InP substrates. Temperature-dependent Hall measurements, x-ray diffraction, and transmission electron microscopy were performed on the materials. The carrier mobility and the temperature dependence of the charge density imply conduction in the InGaAs matrix. We calculate an offset between the conduction-band minimum of the InGaAs matrix and the Fermi level of the ErAs particles that is strongly dependent on the amount of ErAs deposited. As the size of the ErAs particles increases, the Fermi level decreases from ∼0.01 eV above the InGaAs conduction-band edge to ∼0.2 eV below the InGaAs conduction-band edge and the electrical conduction properties change from metallic to semiconducting.

Self-assembled ErAs islands in GaAs for optical-heterodyne THz generation

We report photomixer devices fabricated on a material consisting of self-assembled ErAs islands in GaAs, which is grown by molecular beam epitaxy. The devices perform comparably and provide an alternative to those made from low-temperature-grown GaAs. The photomixers frequency response demonstrates that the material is a photoconductor with subpicosecond response time, in agreement with time-resolved differential reflectance measurements. The material also provides the other needed properties such as high photocarrier mobility and high breakdown field, which exceeds 2×10^5 V/cm. The maximum output power before device failure at frequencies of 1 THz was of order 0.1 µW. This material has the potential to allow engineering of key photomixer properties such as the response time and dark resistance.

Transistor and circuit design for 100-200 GHz ICs

Compared to SiGe, InP HBTs offer superior electron transport properties but inferior scaling and parasitic reduction. Figures of merit for mixed-signal ICs are developed and HBT scaling laws introduced. Device and circuit results are summarized, including a simultaneous 450 GHz f/sub /spl tau// and 490 GHz f/sub max/ DHBT, 172-GHz amplifiers with 8.3-dBm output power and 4.5-dB associated power gain, and 150-GHz static frequency dividers (a digital circuit figure-of-merit for a device technology). To compete with advanced 100-nm SiGe processes, InP HBTs must be similarly scaled and high process yields are imperative. Described are several process modules in development: these include an emitter-base dielectric sidewall spacer for increased yield, a collector pedestal implant for reduced extrinsic C/sub cb/, and emitter junction regrowth for reduced base and emitter resistances.

An ultra-low power InAs/AlSb HEMT W-band low-noise amplifier

An antimonide-based compound semiconductor (ABCS) microstrip MMIC, a W-Band low-noise amplifier using 0.2-μm gate length InAs/AlSb metamorphic HEMTs, has been fabricated and characterized on a 50 μm GaAs substrate. The compact 1.2 mm 2 five-stage W-band LNA demonstrated a 3.9 dB noise-figure at 94 GHz with an associated gain of 20.5 dB. The measured dc power dissipation of the ABCS LNA was an ultra-low 1.2mW per stage, or 6.0 mW total which is less than one-tenth the dc power dissipation of a typical equivalent InGaAs/AlGaAs/GaAs HEMT LNA. Operation with degraded gain and noise figure at 3.5 mW total de power dissipation is also verified. These results demonstrate the outstanding potential of ABCS HEMT technology for mobile and space-based millimeter-wave applications.

InAs/AlSb HFETs with f τ and f max above 150 GHz for low-power MMICs

Very low-power InAs/AlSb HFETs with excellent RF performance are reported. These metamorphic HFETs on GaAs substrates combine high microwave g/sub m/ of at least 1.1 S/mm with low parasitic resistances to offer simultaneous measured f/sub /spl tau// and f/sub max/ values of 160 GHz for both figures of merit. This performance is obtained at a drain bias voltage of only 0.35 V for an HFET with a 0.25-/spl mu/m gate length. The high current gain (f/sub /spl tau//) is attributable to the improved charge control due to scaling of the barrier thickness to 180 /spl Aring/. The maximum power gain (f/sub max/) depends on both g/sub m/ and the HFET output conductance, which is fundamentally limited by the low breakdown voltage gap of the InAs channel (E/sub g/ = 0.36 eV).

Imaging of acoustic charge transport in semiconductor heterostructures by surface acoustic waves

We demonstrate room-temperature acoustic charge transport of electrons and holes in an InGaAs/GaAs heterostructure. The carriers are optically generated by interband absorption and then separated, stored, and transported in the piezoelectric potential superlattice of a surface acoustic wave. The charge distribution is detected with a spatial resolution of a few acoustic wavelengths by a second orthogonal probe beam, genererated by so-called tapered transducers. The image information is given as a phase shift signal in frequency space and allows for the direct comparison of the number of generated and transported carriers. Regions of mere carrier drag and full carrier capture and transport are observed simultaneously.

Imaging interedge-state scattering centers in the quantum Hall regime

After years of study and two Nobel prizes, the quantum Hall effect continues to provide important challenges to both experimentalists and theorists. Many of the most interesting questions concern the nonuniform spatial structures that can occur within a two-dimensional electron gas ~2DEG! in high magnetic fields. These structures arise from competition between the effects of Landau level ~LL! quantization, Coulomb interactions, and external potentials and include striped phases 1 and insulating phases in the bulk 2 as well as conducting states localized at the edges of the sample ~edge states!. 3,4 Scanned probe techniques offer a new approach to investigate these structures directly. They have recently been used to probe the Hall voltage profile and the properties of the insulating state within a quantum Hall plateau. 5‐9 Here we use a scanned probe to investigate the microscopic effects of the spatial structure in a 2DEG on electron transport by examining the nature of the scattering between edge states in a quantum Hall conductor. Transport in the integer quantum Hall regime is now well understood in terms of transport through both quasi-1D edge

n/sup +/-InAs-InAlAs recess gate technology for InAs-channel millimeter-wave HFETs

We report a submicrometer, self-aligned recess gate technology for millimeter-wave InAs-channel heterostructure field effect transistors. The recess gate structure is obtained in an n/sup +/-InAs-InAlAs double cap layer structure with a citric-acid-based etchant. From molecular-beam epitaxy-grown material functional devices with 1000-, 500-, and 200-nm gate length were fabricated. From all three device geometries we obtain drive currents of at least 500 mA/mm, gate leakage currents below 2 mA/mm, and RF-transconductance of 1 S/mm. For the 200-nm gate length device f/sub /spl tau// and f/sub max/ are 162 and 137 GHz, respectively. For the 500-nm gate length device f/sub /spl tau// and f/sub max/ are 89 and 140 GHz, respectively. We observe scaling limitations at 200-nm gate length, in particular a negative threshold voltage shift from -550 to -810 mV, increased kink-effect, and a high gate-to-drain capacitance of 0.5 pF/mm. The present limitations to device scaling are discussed.