Adam M. Crook

Low resistance, nonalloyed Ohmic contacts to InGaAs

We report extremely low specific contact resistivity (ρc) nonalloyed Ohmic contacts to n-type In0.53Ga0.47As, lattice matched to InP. Contacts were formed by oxidizing the semiconductor surface through exposure to ultraviolet-generated ozone, subsequently immersing the wafer in ammonium hydroxide (NH4OH, 14.8 normality), and finally depositing either Ti∕Pd∕Au contact metal by electron-beam evaporation or TiW contact metal by vacuum sputtering. Ti∕Pd∕Au contacts exhibited ρc of (0.73±0.44)Ωμm2—i.e., (7.3±4.4)×10−9Ωcm2—while TiW contacts exhibited ρc of (0.84±0.48)Ωμm2. The TiW contacts are thermally stable, showing no observable degradation in resistivity after a 500°C annealing of 1min duration.

Enhanced conductivity of tunnel junctions employing semimetallic nanoparticles through variation in growth temperature and deposition

We report ErAs nanoparticle-enhanced tunnel junctions grown on GaAs with low specific resistances (<2×10−4 Ω cm−2), approximately tenfold lower than previous reports. A reduction in specific resistance was achieved by modifying the ErAs nanoparticle morphology through the molecular beam epitaxial growth conditions, particularly lower growth temperatures. A further investigation of the variation in tunnel junction resistance with the amount of ErAs deposited and growth temperature shows that nanoparticle surface coverage may not be the only factor determining tunnel junction resistance.

Growth and characterization of LuAs films and nanostructures

We report the growth and characterization of nearly lattice-matched LuAs/GaAs heterostructures. Electrical conductivity, optical transmission, and reflectivity measurements of epitaxial LuAs films indicate that LuAs is semimetallic, with a room-temperature resistivity of 90 μΩ cm. Cross-sectional transmission electron microscopy confirms that LuAs nucleates as self-assembled nanoparticles, which can be overgrown with high-quality GaAs. The growth and material properties are very similar to those of the more established ErAs/GaAs system; however, we observe important differences in the magnitude and wavelength of the peak optical transparency, making LuAs superior for certain device applications, particularly for thick epitaxially embedded Ohmic contacts that are transparent in the near-IR telecommunications window around 1.3 μm.

Frequency Limits of InP-based Integrated Circuits

We examine the limits in scaling of InP-based bipolar and field effect transistors for increased device bandwidth. With InP-based HBTs, emitter and base contact resistivities and IC thermal resistance are the major limits to increased device bandwidth; devices with 1-1.5 THz simultaneous ftau and fmax are feasible. Major challenges faced in developing either InGaAs HEMTs having THz cutoff frequencies or InGaAs-channel MOSFETs having drive current consistent with the 22 nm ITRS objectives include the low two-dimensional effective density of states and the high bound state energies in narrow quantum wells.

Structural and optical studies of nitrogen incorporation into GaSb-based GaInSb quantum wells

We investigate the incorporation of nitrogen into (Ga,In)Sb grown on GaSb and report room temperature photoluminescence from GaInSb(N) quantum wells. X-ray diffraction and channeling nuclear reaction analysis, together with Rutherford backscattering, were employed to identify the optimal molecular beam epitaxial growth conditions that minimized the incorporation of non-substitutional nitrogen into GaNSb. Consistent with this hypothesis, GaInSb(N) quantum wells grown under the conditions that minimized non-substitutional nitrogen exhibited room temperature photoluminescence, indicative of significantly improved radiative efficiency. Further development of this material system could enable type-I laser diodes emitting throughout the (3-5 μm) wavelength range.

Suppression of planar defects in the molecular beam epitaxy of GaAs/ErAs/GaAs heterostructures

We present a growth method that overcomes the mismatch in rotational symmetry of ErAs and conventional III-V semiconductors, allowing for epitaxially integrated semimetal/semiconductor heterostructures. Transmission electron microscopy and reflection high-energy electron diffraction reveal defect-free overgrowth of ErAs layers, consisting of >2× the total amount of ErAs that can be embedded with conventional layer-by-layer growth methods. We utilize epitaxial ErAs nanoparticles, overgrown with GaAs, as a seed to grow full films of ErAs. Growth proceeds by diffusion of erbium atoms through the GaAs spacer, which remains registered to the underlying substrate, preventing planar defect formation during subsequent GaAs growth. This growth method is promising for metal/semiconductor heterostructures that serve as embedded Ohmic contacts to epitaxial layers and epitaxially integrated active plasmonic devices.

Ultra-Low Resistance Ohmic Contacts to InGaAs/InP

An extremely low in situ metal contacts can be formed to (In,Ga)As on InP. it is also possible to form ultra low resistance ex situ contacts by improved surface treatment. However, unlike in situ contacts, ex situ contacts are very sensitive to surface preparation. Similar contact resistivities to InAs on GaAs were reported by Nittono et al. This is the first time such low metal-semiconductor contacts have been demonstrated in In0.53Ga0.47As system that does not deteriorate at least upto 500 degC. These thermally stable, extremely low resistances, ohmic contacts are an enabling technology for THz bandwidth InGaAs/InP HBTs, mm wave InGaAs HEMT technologies, and the evolving III-V MOSFET technologies.

On the Feasibility of few-THz Bipolar Transistors

We review the limits faced in seating of InP-based bipolar transistors for increased device bandwidth. Emitter and base contact resistivities and IC thermal resistance are the major limits to increased device bandwidth. Devices with 1-1.5 THz simultaneous ftau, and fmax are feasible; these will enable 750 GHz monolithic amplifiers and medium-scale digital ICs at ~400-500 GHz clock rate.

Surface segregation effects of erbium in GaAs growth and their implications for optical devices containing ErAs nanostructures

We report on the integration of semimetallic ErAs nanoparticles with high optical quality GaAs-based semiconductors, grown by molecular beam epitaxy. Secondary ion mass spectrometry and photoluminescence measurements provide evidence of surface segregation and incorporation of erbium into layers grown with the erbium cell hot, despite the closed erbium source shutter. We establish the existence of a critical areal density of the surface erbium layer, below which the formation of ErAs precipitates is suppressed. Based upon these findings, we demonstrate a method for overgrowing ErAs nanoparticles with III-V layers of high optical quality, using subsurface ErAs nanoparticles as a sink to deplete the surface erbium concentration. This approach provides a path toward realizing optical devices based on plasmonic effects in an epitaxially-compatible semimetal/semiconductor system.

Highly Strained Mid-Infrared Type-I Diode Lasers on GaSb

We describe how growth at low temperatures can enable increased active layer strain in GaSb-based type-I quantum-well diode lasers, with emphasis on extending the emission wavelength. Critical thickness and roughening limitations typically restrict the number of quantum wells that can be grown at a given wavelength, limiting device performance through gain saturation and related parasitic processes. Using growth at a reduced substrate temperature of 350 °C, compressive strains of up to 2.8% have been incorporated into GaInAsSb quantum wells with GaSb barriers; these structures exhibited peak room-temperature photoluminescence out to 3.96 μm. Using this growth method, low-threshold ridge waveguide lasers operating at 20°C and emitting at 3.4 μm in pulsed mode were demonstrated using 2.45% compressively strained GaInAsSb/GaSb quantum wells. These devices exhibited a characteristic temperature of threshold current of 50 K, one of the highest values reported for type-I quantum-well laser diodes operating in this wavelength range. This temperature stability is attributable to the increased valence band offset afforded by the high strain values, due to the simultaneously high quantum well indium and antimony mole fractions. Exploratory experiments using bismuth both as a surfactant during quantum well growth, as well as in dilute amounts incorporated into the crystal were also studied. Both methods appear to be promising avenues to surmount current strain-related limitations to laser performance and emission wavelength.