Larry Grazulis

Optimum growth window for InAs/GaInSb superlattice materials tailored for very long wavelength infrared detection

The authors report growth studies to develop an InAs/GaInSb superlattice (SL) material for very long wavelength infrared detection. They select a SL structure of 47.0 A InAs/21.5 A Ga0.75In0.25Sb that is designed for the greatest possible detectivity, and tune growth conditions to achieve the best quality ternary material. Since the material quality of grown layers is particularly sensitive to extrinsic defects such as nonradiative recombination centers generated during the growth process, the authors investigate the effect of the growth temperature (Tg) on the spectral photoresponse (PR) and carrier recombination lifetime using photoconductivity and time-resolved differential reflectivity measurements. Results indicate that a molecular beam epitaxy growth process the authors developed produces a consistent energy gap around 50 meV, determined from the PR spectra, but the intensity of the spectra is sensitive to Tg. For SLs grown at Tg between 390 and 470 °C, the PR signal intensity gradually increases as...

Combined in situ and ex situ analysis of hydrogen radical and thermal removal of native oxides from (001) GaAs

We are currently involved in the study of regrowth of InAs on nanopatterned GaAs surfaces. The nanopatterning is accomplished through the movement of the sample while in contact with a diamond tip maintained at a constant load. Native oxides present on these surfaces introduce an obstacle to the subsequent regrowth. Therefore, the removal of this oxide is a prerequisite step for the study of the subsequent regrowth on these patterned surfaces. In this study we used in situ spectroscopic ellipsometry (SE) and reflection high energy electron diffraction (RHEED) as well as ex situ atomic force microscopy to follow the hydrogen cleaning and thermal removal of the native oxides from the GaAs surface. SE and RHEED were used to follow the oxide desorption process in situ and were used to determine when the surface was clean. Post AFM analysis indicated that the thermally desorbed oxide surface contained pits which were approximately 100 A deep and covered 15% of the surface. Hydrogen radical cleaning was studied...

Review Article: Rare-earth monosulfides as durable and efficient cold cathodesa)

In their rocksalt structure, rare-earth monosulfides offer a more stable alternative to alkali metals to attain low or negative electron affinity when deposited on various III-V and II-VI semiconductor surfaces. In this article, the authors first describe the successful deposition of lanthanum monosulfide via pulsed laser deposition on Si and MgO substrates. These thin films have been characterized by x-ray diffraction, atomic force microscopy, high resolution transmission electron microscopy, ellipsometry, Raman spectroscopy, ultraviolet photoelectron spectroscopy, and Kelvin probe measurements. For both LaS/Si and LaS/MgO thin films, the effective work function of the submicron thick thin films was determined to be about 1 eV from field emission measurements using the scanning anode field emission microscopy technique. The physical reasons for these highly desirable low work function properties were explained using a patchwork field emission model of the emitting surface. In this model, nanocrystals of ...

Characterization and field emission properties of lanthanum monosulfide nanoprotrusion arrays obtained by pulsed laser deposition on self-assembled nanoporous alumina templates

Three distinct types of nanostructures—nanodomes, nanodots, and nanowires—have been simultaneously self-assembled by pulsed laser deposition of lanthanum monosulfide on anodic alumina films containing hexagonal arrays of pores about 50nm wide and 500nm deep. The nanostructures have been characterized by x-ray diffraction, atomic force microscopy (AFM), and field emission scanning electron microscopy (FE-SEM). Nanodomes preferentially grow on the boundary separating regions (grains) of the alumina template that have near perfect pore ordering, and their density is ∼109∕cm2. The diameter of a nanodome at the base is about 100nm and their aspect ratio (height/diameter at the base) is between 1 and 3. Additionally, nanodots nucleate on top of the alumina walls that separate adjacent pores. They have a diameter of ∼50nm, a density equal to the pore density (1010∕cm2), and an aspect ratio less than 1. Finally, cross sectional FE-SEM images of the templates indicate that LaS nanowires grow inside the pores with ...

Strain relaxation in the growth of planar InAs

In this work, we look at the strain relaxation of InAs growth on GaAs under metal rich conditions. InAs layers, grown under these conditions, have been found to maintain a planar morphology for thicknesses well beyond a Matthews-Blakeslee critical thickness of ∼2 ML. We employed reflection high-energy electron diffraction, in situ spectroscopic ellipsometry and atomic force microscopy to follow the InAs growth under In stabilized conditions. The critical thickness was found to increase with a reduction in growth temperature. A region of growth was seen in which three-dimensional diffraction was not visible in the reflection high energy electron diffraction pattern before annealing. For thicknesses less than the critical thickness observed by atomic force microscope, the planar morphology has been found to be stable with annealing. These results indicate that strain relaxation via the formation of Lomer-type dislocations is essential to stabilizing planar morphology during growth.

Growth and spectroscopic ellipsometry evaluation of composite layers of ErAs and InAs nanoparticles

Metal nanoparticles coupled to semiconductor quantum dots have been studied recently due to the enhancement in absorption, emission, and nonlinearities expected from these hybrid structures. These properties stem from the ability of the metal to focus light as well as shift the phase, which occurs at the metal–dielectric interface. To date, most quantum dots metal nanoparticle couples are formed by the attachment of a ligand to both particles. The extension of this idea to bulk semiconductor films is being attempted by the formation of a composite structure of ErAs, which forms semimetallic nanoparticles (SMNP) in GaAs, and InAs self-assembled quantum dots (SAQD). In this work, the authors analyze structures composed of periods of InAs SAQDs and ErAs SMNPs and analyze these with spectroscopic ellipsometry in the spectral region 0.7–4.0 eV with 0.02 eV steps. Initially, individual structures composed of InAs SAQD stacks or ErAs SMNP stacks, both capped with layers of GaAs, are analyzed. The authors have al...

Field emission properties of metallic nanostructures self-assembled on nanoporous alumina and silicon templates

Two types of nanoscale field emitter arrays have been self-assembled using either flexible alumina templates containing hexagonal close packed pores that are 50nm wide and 500nm long or nanoporous Si templates. The first type is an array of gold “nanopinetrees” obtained by e-beam evaporation of a few nanometers of gold on bare alumina templates. The second type is a nickel “nanoblade” array formed by e-beam evaporation of a few nanometers of nickel on nanoporous Si templates. The field emission characteristics of both mesoscopic structures were measured using a scanning field emission microscope, and results were analyzed in terms of the cathode material emission. This synthetic approach for producing nanoscale field emitters could lead to a versatile and inexpensive technology for synthesizing flexible arrays of nanoscale cold cathode emitters.

Field emission from lanthanum monosulfide thin films grown on the (100) magnesium oxide substrates

Lanthanum monosulfide (LaS) films were grown by pulsed laser deposition on the (100) magnesium oxide (MgO) substrates at an elevated substrate temperature and in a background gas of H2S. The thin films have been characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), and high resolution transmission electron microscopy. The film surface is composed of grainlike features with an average size of approximately 34nm. The root-mean-square variation of the film surface roughness measured over a 2×2μm2 area by AFM was found to be approximately 1.5nm. XRD data indicate that the average size of the nanocrystalline grains in the film is about 26nm, which is about twice the size of the grains found in LaS thin films deposited at room temperature on Si. The field emission (FE) properties of the films have been characterized by scanning anode field emission microscopy and are interpreted in terms of a recently developed patchwork FE model. The FE data indicate that there is roughly a seven times incre...

Planar InAs growth on GaAs(001) and subsequent quantum dot formation by a surface induced morphological instability

InAs growth on GaAs has been studied extensively and most recently for the self-assembly of quantum dots on the surface. In this work, we have studied the growth conditions to form planar InAs, at an In beam-equivalent pressure of 3.1×10−8 Torr, under the metal rich (4×2) reconstruction and the subsequent formation of quantum dots (QDs) condensed from this layer by a rapid change in the As4 pressure. Atomic force microscopy (AFM) studied the surface morphology of InAs resulting from a nominal 12 ML growth with various As4 growth pressures while maintaining a metal rich condition. It was found that the surface roughness was dependent on the As4 pressure. At the higher As4 pressures studied, the growth was extremely rough and much smoother at the lower As4 pressures. For a beam-equivalent pressure of 1×10−7 Torr for As4 and 3.1×10−8 Torr for In, growth was seen to remain planar for a deposition thickness up to 3 ML. AFM analysis showed that a fingered morphology was formed with a roughness of ∼1 ML. When th...

Kinetically controlled dewetting of thin GaAs cap from an ErAs/GaAs nanoparticle composite layer

ErAs metal nanoparticles (NPs) embedded in GaAs have multiple applications in plasmonic, terahertz, and tunneling devices. Growing a high quality thin GaAs layer over the ErAs NP layer is vital to these applications. In this work, the authors study the surface stability of a thin GaAs cap (1–5 nm) annealed in a temperature range of 450–620 °C. The thin GaAs cap covered a single layer of ErAs NPs [0.5–1.33 monolayer (ML)] grown using molecular beam epitaxy on GaAs(001) substrates at 450–500 °C. For 1.33 ML ErAs coverage, although a 1 nm GaAs cap exhibited a root-mean-square surface roughness close to 0.3 nm, the authors expected that 1 nm GaAs was not thick enough to overgrow the NPs in a height of 3–4 nm; thus, a large number of pinholes should be left on the surface. By increasing the GaAs cap thickness to 3 nm, the authors were able to achieve atomically smooth surfaces with few remaining pinholes. At a lower coverage of ErAs, 0.5 ML, the authors were able to achieve atomically flat pinhole-free GaAs caps with a thickness of 3–5 nm. However, the key finding from this study is that distinct changes in the surface morphology occurred upon annealing depending on the film thickness and NP density. In the case of 1 nm GaAs caps, clumps were formed when annealed, whereas in the case of GaAs caps of 3–5 nm in thickness, the GaAs film uncharacteristically dewetted at the ErAs NP/GaAs composite interface. Thermodynamically, this dewetting is driven by the high interfacial energy resulting from the difference in the crystal structure between GaAs and ErAs (zinc-blende and rock salt); however, surface mobility plays an important kinetic role in this process. It has been demonstrated that the dewetting can be prevented by combining a higher As overpressure, a low growth/annealing temperature, lower surface coverage of ErAs NP, and thicker GaAs caps.ErAs metal nanoparticles (NPs) embedded in GaAs have multiple applications in plasmonic, terahertz, and tunneling devices. Growing a high quality thin GaAs layer over the ErAs NP layer is vital to these applications. In this work, the authors study the surface stability of a thin GaAs cap (1–5 nm) annealed in a temperature range of 450–620 °C. The thin GaAs cap covered a single layer of ErAs NPs [0.5–1.33 monolayer (ML)] grown using molecular beam epitaxy on GaAs(001) substrates at 450–500 °C. For 1.33 ML ErAs coverage, although a 1 nm GaAs cap exhibited a root-mean-square surface roughness close to 0.3 nm, the authors expected that 1 nm GaAs was not thick enough to overgrow the NPs in a height of 3–4 nm; thus, a large number of pinholes should be left on the surface. By increasing the GaAs cap thickness to 3 nm, the authors were able to achieve atomically smooth surfaces with few remaining pinholes. At a lower coverage of ErAs, 0.5 ML, the authors were able to achieve atomically flat pinhole-free GaAs ca...