J. P. McCaffrey

Size and shape engineering of vertically stacked self-assembled quantum dots

A new procedure for the growth of stacked self-assembled quantum dot layers is described. The main effect of the procedure is to convert the quantum dot population into a population of quantum disks of approximately equal height. Proposed model of the overgrowth process for highly strained 3D islands invokes mechanisms which may lead to a variety of dot shape modification and opens up new ways of influencing the dot population by deliberate control of the overgrowth process. Demonstrated very good performance of test devices indicate that the proposed procedure may have positive impact on the further development of quantum dot lasers.

Nanocrystalline-silicon superlattice produced by controlled recrystallization

Nanocrystalline-silicon superlattices are produced by controlled recrystallization of amorphous-Si/SiO2 multilayers. The recrystallization is performed by a two-step procedure: rapid thermal annealing at 600–1000 °C, and furnace annealing at 1050 °C. Transmission electron microscopy, Raman scattering, x-ray and electron diffraction, and photoluminescence spectroscopy show an ordered structure with Si nanocrystals confined between SiO2 layers. The size of the Si nanocrystals is limited by the thickness of the a-Si layer, the shape is nearly spherical, and the orientation is random. The luminescence from the nc-Si superlattices is demonstrated and studied.

Red-Emitting Semiconductor Quantum Dot Lasers

Visible-stimulated emission in a semiconductor quantum dot (QD) laser structure has been demonstrated. Red-emitting, self-assembled QDs of highly strained InAlAs have been grown by molecular beam epitaxy on a GaAs substrate. Carriers injected electrically from the doped regions of a separate confinement heterostructure thermalized efficiently into the zero-dimensional QD states, and stimulated emission at ∼707 nanometers was observed at 77 kelvin with a threshold current of 175 milliamperes for a 60-micrometer by 400-micrometer broad area laser. An external efficiency of ∼8.5 percent at low temperature and a peak power greater than 200 milliwatts demonstrate the good size distribution and high gain in these high-quality QDs.

SiO2 film thickness metrology by x-ray photoelectron spectroscopy

Silicon dioxide films grown by industrial thermal furnace, rapid thermal, and low-pressure thermal methods were measured by x-ray photoelectron spectroscopy, transmission electron microscopy (TEM), spectroscopic ellipsometry, and capacitance–voltage analysis. Based on TEM measurements, the photoelectron effective attenuation lengths in the SiO2 and Si are found to be 2.96±0.19 and 2.11±0.13 nm, respectively. The oxide physical thicknesses (range from 1.5 to 12.5 nm) as measured by all above techniques are in good agreement. The electrical thickness is noted to be slightly thicker than the physical thickness.

Ordering and self-organization in nanocrystalline silicon

The spontaneous formation of organized nanocrystals in semiconductors has been observed during heteroepitaxial growth and chemical synthesis. The ability to fabricate size-controlled silicon nanocrystals encapsulated by insulating SiO2 would be of significant interest to the microelectronics industry. But reproducible manufacture of such crystals is hampered by the amorphous nature of SiO2 and the differing thermal expansion coefficients of the two materials. Previous attempts to fabricate Si nanocrystals failed to achieve control over their shape and crystallographic orientation, the latter property being important in systems such as Si quantum dots. Here we report the self-organization of Si nanocrystals larger than 80 Å into brick-shaped crystallites oriented along the 〈111〉 crystallographic direction. The nanocrystals are formed by the solid-phase crystallization of nanometre-thick layers of amorphous Si confined between SiO2 layers. The shape and orientation of the crystallites results in relatively narrow photoluminescence, whereas isotropic particles produce qualitatively different, broad light emission. Our results should aid the development of maskless, reproducible Si nanofabrication techniques.

InAs self‐assembled quantum dots on InP by molecular beam epitaxy

We present results of room temperature photoluminescence (PL) emission from a 0‐dimensional system in the ∼1.4 to ∼1.7 μm spectral region. Molecular beam epitaxy was used to grow InAs self‐assembled quantum dots in AlInAs on an InP substrate. Preliminary characterizations have been performed using PL and transmission electron microscopy. The low temperatures PL spectra also display excited state emission and state filling as the excitation intensity is increased.We present results of room temperature photoluminescence (PL) emission from a 0‐dimensional system in the ∼1.4 to ∼1.7 μm spectral region. Molecular beam epitaxy was used to grow InAs self‐assembled quantum dots in AlInAs on an InP substrate. Preliminary characterizations have been performed using PL and transmission electron microscopy. The low temperatures PL spectra also display excited state emission and state filling as the excitation intensity is increased.

Gadolinium silicate gate dielectric films with sub-1.5 nm equivalent oxide thickness

GdSixOy gate dielectric films were deposited on Si(001) substrates using ultra-high-vacuum electron-beam evaporation from pressed-powder targets. Transmission electron microscopy showed that the films were amorphous as deposited and remained amorphous when annealed to temperatures up to 900 °C. Capacitance–voltage measurements indicate an equivalent oxide thickness (EOT) of 13.4 A for a film with composition GdSi0.56O2.59 determined by in situ x-ray photoelectron emission spectroscopy. After forming gas annealing at 500 °C the EOT was reduced to 11.0 A, at a physical thickness of 45 A. The same film has a low leakage current of approximately 5.7×10−3 A cm−2 at +1 V, a reduction of 8.7×104 compared to current density estimates of SiO2 films with the same specific capacitance.

QUANTUM-WELL INTERMIXING FOR OPTOELECTRONIC INTEGRATION USING HIGH ENERGY ION IMPLANTATION

The technique of ion‐induced quantum‐well (QW) intermixing using broad area, high energy (2–8 MeV As4+) ion implantation has been studied in a graded‐index separate confinement heterostructure InGaAs/GaAs QW laser. This approach offers the prospect of a powerful and relatively simple fabrication technique for integrating optoelectronic devices. Parameters controlling the ion‐induced QW intermixing, such as ion doses, fluxes, and energies, post‐implantation annealing time, and temperature are investigated and optimized using optical characterization techniques such as photoluminescence, photoluminescence excitation, and absorption spectroscopy.

Characterization of Gd2 O 3 Films Deposited on Si(100) by Electron-Beam Evaporation

Gadolinium oxide films were deposited on Si(100) substrates from a rod-fed electron beam evaporator using a pressed-powder Gd 2 O 3 target. Films 25 nm thick were shown to he stoichiometric Gd 2 O 3 by Rutherford backscattering and had a dielectric constant at 100 kHz of 16.0 ± 0.3. Transmission electron microscopy and X-ray reflectivity measurements showed that films 7-13 nm thick annealed in oxygen consisted of three distinct layers, an interfacial silicon dioxide layer next to the substrate, a second amorphous oxide layer containing silicon, gadolinium, and oxygen above this, and a polycrystalline Gd 2 O 3 layer on top. Annealing in oxygen reduced the leakage currents, increased the thickness of the silicon dioxide layer, and increased the grain size of the top Gd 2 O 3 layer. The characteristics of the leakage currents through the gadolinium oxide were consistent with a Frenkel-Poole conduction mechanism with a silicon-Gd 2 O 3 band offset of 1.8 V. Interfaces with excellent electrical properties, Characteristic of good SiO 2 , were obtained after annealing in oxygen Annealing of the films in vacuum prior to oxygen annealing reduced the thickness of the interfacial silicon dioxide.

Epitaxial Y1Ba2Cu3O7 thin films on CeO2 buffer layers on sapphire substrates

Pulsed laser deposition has been used to deposit Y1Ba2Cu3O7 layer on CeO2 buffer layers on (11_02) sapphire. Both layers are epitaxial with the 〈110〉 direction of the CeO2 layer aligned with the 〈2_021〉 direction of the sapphire substrate. The c‐axis Y1Ba2Cu3O7 layer has its 〈100〉 direction alligned with the 〈110〉 direction of the CeO2. Cross‐sectional transmission electron microscopy shows the epitaxy to be coherent and the interfaces to be abrupt at an atomic level. The best films have a critical current of 9 × 106 A/cm2 at 4.2 K and lower microwave surface resistance than copper at 77 K and at a frequency of 31 GHz.