Method of forming silicon structures with selectable optical characteristics

Silicon and metal are coevaporated onto a silicon substrate in a molecular beam epitaxy system with a larger than stoichiometric amount of silicon so as to epitaxially grow particles of metal silicide embedded in a matrix of single crystal epitaxially grown silicon. The particles interact with incident photons by resonant optical absorption at the surface plasmon resonance frequency. Controlling the substrate temperature and deposition rate and time allows the aspect ratio of the particles to be tailored to desired wavelength photons and polarizations. The plasmon energy may decay as excited charge carriers or phonons, either of which can be monitored to indicate the amount of incident radiation at the selected frequency and polarization

Atomic Step Organization in Homoepitaxial Growth on GaAs(111)B Substrates

When homoepitaxial growth is performed on exactly oriented (singular) (111) GaAs substrates, while maintaining the √19 x √19 surface reconstruction, the originally flat surface spontaneously evolves vicinal (111) facets that are tilted approximately 2.5° toward the \u3c 211 \u3e azimuthal directions. These facets form pyramid-like structures where the distance between adjacent peaks can be varied from as little as 1 μm to tens of μm. When these surfaces are observed with atomic force microscopy (AFM), we find that they are extremely smooth with the observed tilt resulting from atomic steps which are spaced at approximately 7.5 nm. We have also studied growth on vicinal GaAs(111) substrates. Our results are interpreted as indicating that the 2.5° vicinal (111) surface has a minimum free energy for the √19 x √19 reconstruction (i.e., that 10 nm spacing of \u3c 011 \u3e steps is thermodynamically preferred). Exactly oriented (111) facets are only observed when their facet width is less than a couple of micrometers implying a minimum nucleation size. This is a surprising result since conventional wisdom argues the surfaces with low Miller indexes are preferred. A possible explanation is an anisotropy in the surface in the two degenerate phases of √19 x √19 reconstruction which are rotated ±23° from the unreconstructed surface

GaN-Ready Aluminum Nitride Substrates for Cost-Effective, Very Low Dislocation Density III-Nitride LED's

The objective of this project was to develop and then demonstrate the efficacy of a costeffective approach for a low defect density substrate on which AlInGaN LEDs can be fabricated. The efficacy of this “GaN-ready” substrate would then be tested by growing high efficiency, long lifetime InxGa1-xN blue LEDs. The approach used to meet the project objectives was to start with low dislocation density AlN single-crystal substrates and grow graded AlxGa1-xN layers on top. Pseudomorphic AlxGa1-xN epitaxial layers grown on bulk AlN substrates were used to fabricate light emitting diodes and demonstrate better device performance as a result of the low defect density in these layers when benched marked against state-of-the-art LEDs fabricated on sapphire substrates. The pseudomorphic LEDs showed excellent output powers compared to similar wavelength devices grown on sapphire substrates, with lifetimes exceeding 10,000 hours (which was the longest time that could reliably be estimated). In addition, high internal quantum efficiencies were demonstrated at high driving current densities even though the external quantum efficiencies were low due to poor photon extraction. Unfortunately, these pseudomorphic LEDs require high Al content so they emit in the ultraviolet. Sapphire based LEDs typically have threading dislocation densities (TDD) > 108 cm-2 while the pseudomorphic LEDs have TDD ≤ 105 cm-2. The resulting TDD, when grading the AlxGa1-xN layer all the way to pure GaN to produce a “GaN-ready” substrate, has varied between the mid 108 down to the 106 cm-2. These inconsistencies are not well understood. Finally, an approach to improve the LED structures on AlN substrates for light extraction efficiency was developed by thinning and roughening the substrate

The 2020 UV emitter roadmap

Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments

Phase transitions in disordered systems: I. exciton/electron-hole liquid/plasma system in germanium and II. amorphous ferromagnets and spin glasses

In Part I of this thesis, we present experimental evidence demonstrating that a metal-insulator (M-I) phase transition can occur separately from the liquid-gas (L-G) transition in the exciton/electron-hole (e-h) gas/e-h liquid system in stressed Ge. Using a strain wel1 to confine the photoexcited carriers, we analyzed the spectral content and spatial distribution of e-h recombination luminescence. we observe a line of first-order transitions between the exciton gas am p-h gas occurring UP to 7 K which we associate with the M-I transition. '!his is well above the temperature of the L-G critical point which we found to be 4.5 + 0.5 K with a critical density of 16 3 + 1 x 10em Spectroscopic evidence is also presented for a triple point which we estimate to be about 4 K. At this temperature, we were able to fit our measured luminescence spectra at particular photoexcitation powers only by including a theoretical line shape for the e-h gas with a density of 16 -3 2.0 + .5 x 10 em A simple expansion of the free energy of the e-h system is presented from which we calculate a critical temperature for the M-I transition of 5.4 K which, possible because of quantum effects, is substantially below our measured value. Lifetime measurements of the different phases of the e-h system are also made. In particular, we have measured an extremely long lifetime of 1.5 ms for strain-confined excitons iv below 3.2 K. This compares favorably with our predicted radiative lifetime of 2.0 rns in stressed (',e. M:x:Ne 3.2 K, a rapid decrease j.n the exciton lifetime is observed with increasing temperature, concurrent with an exponential decrease in the observed luminescence intensitv. Three models for thermally-activated loss of strain-confined excitons are considered as possible explanations. In Part II of this thesis. we obtain the thermodynamic properties of a system of Ising spins interacting with various random {X)tentials in the Bethe-Peierls-Wiss (BPW) approximation. When the effective number of neighbors z approaches infinity, we show that all the magnetic properties arising from the BPW approximation, the mean-random-field (MRF) and the Sherrington-Kirkpatrick (SK) replica treatment are identical. Also. the microscopic internal energy in the BPW method can bP. integrated approxiately to obtain a microscopic free energy which is identical to that derivprJ hv diagrammatic expansions of the disordered Hamiltonian. Usi.ng this free energy and our calculated distribution of internal fieMs, we show that the BPW met.hcx:1 reproduces all the results of Sf( includinq a negative entropy of -k/(2n) at T =O. We also show by analytical means that a square hole or gap arises in the low-temperature distribution of the single-particle excitation fields h at h = 0 in the limit of infinite z. In an externally applied o 0 field, the hole remains centered about the zero value of the total (internal plus external) field. The reasons for the unphysica1 low-temperature results occurring in both the SF and BPW treatments are clarified in our discussion of this gap. The phase diagram as a function of z is calculated within the MRF approximation. We find that for z > 8 the phase diaqram is already very close to that of the infinite z case Finally, we compare magnetization versus temperature curves which have been calculated within Handrich's approach with those calculated within the BPW approach in the limit of infinite z. BPW approximations are very similar for small amounts of disorder but Handrich's method breaks down as the account of disorder approaches the spin-glass boundary.U of I OnlyThesi

GaN-Ready Aluminum Nitride Substrates for Cost-Effective, Very Low Dislocation Density III-Nitride LED's

The objective of this project was to develop and then demonstrate the efficacy of a costeffective approach for a low defect density substrate on which AlInGaN LEDs can be fabricated. The efficacy of this “GaN-ready” substrate would then be tested by growing high efficiency, long lifetime InxGa1-xN blue LEDs. The approach used to meet the project objectives was to start with low dislocation density AlN single-crystal substrates and grow graded AlxGa1-xN layers on top. Pseudomorphic AlxGa1-xN epitaxial layers grown on bulk AlN substrates were used to fabricate light emitting diodes and demonstrate better device performance as a result of the low defect density in these layers when benched marked against state-of-the-art LEDs fabricated on sapphire substrates. The pseudomorphic LEDs showed excellent output powers compared to similar wavelength devices grown on sapphire substrates, with lifetimes exceeding 10,000 hours (which was the longest time that could reliably be estimated). In addition, high internal quantum efficiencies were demonstrated at high driving current densities even though the external quantum efficiencies were low due to poor photon extraction. Unfortunately, these pseudomorphic LEDs require high Al content so they emit in the ultraviolet. Sapphire based LEDs typically have threading dislocation densities (TDD) > 108 cm-2 while the pseudomorphic LEDs have TDD ≤ 105 cm-2. The resulting TDD, when grading the AlxGa1-xN layer all the way to pure GaN to produce a “GaN-ready” substrate, has varied between the mid 108 down to the 106 cm-2. These inconsistencies are not well understood. Finally, an approach to improve the LED structures on AlN substrates for light extraction efficiency was developed by thinning and roughening the substrate