Challenges in the implementation of Nanoelectronics

We investigate photo-luminescence (PL) and cathodo-luminescence (CL) spectra of InAsP/InP quantum well (QW) structures under exposure to low energy ions, and more generally to the particles generated in a plasma reactor typically used for etching such materials. The samples contain QWs of constant thickness (7 to 8 nm) and graded As/P concentration such that each QW produces, at low temperature, a PL line at a well-defined and resolved energy. Adjusting the depth of the QWs below the sample surface, the PL/CL of these QWs is thus the signature of the depth-dependent interaction with the ions and particles from the plasma. We have exposed our QW structures to different kinds of plasmas, containing variable mixtures of halogen-based molecules / hydrocarbons / nitrogen and Ar. Previous studies [1], in particular based on a similar approach of using the QW as an internal probe of the ion – material interaction, have shown that atomic ions with energies in the range of 100 to a few 100’s eV penetrate III-V materials (like GaAs or InP) through a channeling process [2], even if their direction of impingement is not favorable for channeling. The angular dispersion is in general sufficient to produce channeling. The ions channel a certain distance into the crystal, and then stop due to de-channeling mechanisms. As a consequence, a quenching of the PL for the QWs in the region where the de-channeling occurred takes place. We extended the analysis looking not only at the intensity of the PL lines, but also at their spectral characteristics upon exposure to the ion fluxes. Fig. 1 illustrates a selection of the effects that we were able to identify and explain [3]. On fig. 1-a, a sample was exposed to the particles flux from a plasma discharge in SiCl4/H2/Ar. Under the conditions of this plasma discharge, etching actually occurs at a rate ≈ 250-300 nm/min., and we had to be careful not to etch the whole QW structure immediately. Fig. 1-a shows that: - the signal for the two QWs at the lower wavelength seems to disappear. These QWs are the ones closest to the sample surface - the emission bands from the other QWs exhibit energy shifts Transmission electron microscopy and secondary ion mass spectrometry (SIMS) show that actually the two QWs closest to the top surface have not been etched. SIMS shows the penetration of Cl-species below the surface, which probably play a role in the quenching of the PL for these QWs The sample on fig. 1-a was undoped. On fig. 1-b, the same experiment is conducted for a sample with the same QW sequence as that of fig. 1-a, but n-type doped such that a large built-in electric field is created across the QWs region. One can see that this changes completely the shape of the PL lines. The reason for this change can be attributed to the quantum confined Stark effect (QCSE), as shown in [4]. Interestingly, as seen on fig. 1-b, the effect of the interaction with the etching ions is significantly more pronounced than for the undoped sample. We have shown that this is caused by a specific channeling interaction involving ions (probably Cl- ions) which strongly screen the QCSE. Using plasmas containing N2 (which effect is to reduce the etch rate), very significant blue shifts of the PL lines are seen (up to more than 7 nm). These were associated with the incorporation of N deep in the sample, occupying interstitial positions, thus creating local compressive lattice deformation that leads to the blue shifts [3]. All these effects will be discussed in the context of a model that we established to describe the modifications of the luminescence properties of the QW structures under the effect of low-energy ion bombardment. We will also describe experiments on samples where we take benefit of the plasma etching to define nanoscale structures and examine the spatial variation of the PL/CL spectra after interaction with the ion beams. [1] D. L. Green, E. L. Hu, P. M. Petroff, V. Liberman, M. Nooney, and R. Martin, J. Vac. Sci. Technol., B 11, 2249 (1993). [2] R. Germann, A. Forchel, M. Bresch, and H. P. Meier, J. Vac. Sci. Technol., B 7, 1475 (1989). [3] J.P. Landesman, J. Jiménez, C. Levallois, F. Pommereau, C. Frigeri, A. Torres, Y. Léger, A. Beck, and A. Rhallabi, J. Vac. Sci. Technol., A 34, 041304-1 (2016). [4] L. Viña, E. E. Mendez, W. I. Wang, L. L. Chang and L. Esaki, J. Phys. C: Solid State 20, 2803 (1987). Figure 1

We report the structural and optical properties of Al x Ga 1-x N/Al y Ga 1-y N quantum wells (QWs) structures grown by gas source molecular beam epitaxy with ammonia on sapphire (0001) substrates. QWs structures consist of five pairs of Al y Ga 1-y N, 0.3 x Ga 1-x N, 0.55<x<1, barriers (nominally 5 nm thick). All the structures were completed with a 10 nm thick cap layer of AlN. We observed a significant enhancement in the cathodoluminescence intensities and longer photoluminescence lifetimes for QW structures grown in the 3D mode, as confirmed by spotty reflection high energy electron diffraction patterns. These effects are attributed to the formation of AlGaN quantum dots in the well materials.

We report the effects of n-type doping by Si and p-type doping by Zn in the GaAs substrate on the thermal intermixing of undoped GaAs/Al/sub 0.24/Ga/sub 0.76/As single quantum well (QW) structures. For comparison, semi-insulating GaAs substrate was also used. The samples with Si-doped GaAs substrate and semi-insulating GaAs substrate show blue shifts that are in agreement with thermal stability studies by other workers. The sample with Zn-doped GaAs substrate, however, shows clear evidence of suppressed intermixing by Zn diffusion from the substrate into the QW structure. The enhanced thermal stability of the GaAs/Al/sub 0.24/Ga/sub 0.76/As QW structure is attributed to a Zn-diffusion induced reduction in the number of column-III vacancies in the QW structure.

Low dimensional structures & devices: Part 1

Comparison of optical properties of polarization-matched c-plane and lattice-matched a-plane BInGaN/GaN quantum well structures

To further enhance the color conversion from a quantum-well (QW) structure into a color-converting colloidal quantum dot (QD) through Förster resonance energy transfer (FRET), we designed and implemented a device structure with QDs inserted into a GaN nano-porous structure near the QWs to gain the advantageous nanoscale-cavity effect. Additionally, surface Ag nanoparticles were deposited for inducing surface plasmon (SP) coupling with the QW structure. Based on the measurements of time-resolved and continuous-wave photoluminescence spectroscopies, the FRET efficiency from QW into QD is enhanced through the SP coupling. In particular, performance in the polarization perpendicular to the essentially extended direction of the fabricated pores in the nano-porous structure is more strongly enhanced when compared with the other linear polarization. A numerical simulation study was undertaken, and showed consistent results with the experimental observations.

The plasmonic Dicke effect means a cooperative emission mechanism of multiple light emitters when they are simultaneously coupled with the same surface plasmon (SP) mode of a metal nanostructure to achieve a higher collective emission efficiency. Here, we compare the enhancements of emission efficiency among a series of SP-coupled InGaN/GaN quantum-well (QW) structures of different QW period numbers to show an emission behavior consistent with the plasmonic Dicke effect. The relative enhancement of overall emission efficiency increases with QW period number until it reaches a critical value, beyond which the enhancement starts to decrease. This critical QW period number corresponds to the effective depth range of the plasmonic Dicke effect in a multiple-QW system. It also represents an optimized QW structure for maximizing the SP coupling effect. Internal quantum efficiency and time-resolved photoluminescence are measured for comparing the enhanced emission efficiencies of blue and green QW structures with different QW period numbers through SP coupling induced by surface Ag nanoparticles.

Strong suppression of In desorption from InGaN QW by improved technology of upper InGaN/GaN QW interface

Intersubband absorption coefficients of GaN/AlN and strain-compensated InGaN/InAlN quantum well structures

In the present study we investigate the effect ofcomponent interdiffusion across the interfaces on theelectronic structure of a quantum well embedded in ashort-period superlattice. We calculate the energies of thebound states for an 18 monolayer (5 nm) thick GaAs quantum wellembedded in an (AlAs)4/(GaAs)8 superlattice incomparison with two AlxGa1-xAs/GaAs(x = 1 and 0.33) single quantum wells with the samethickness. The calculations are made within the framework of theenvelope-function approximation. The behaviour of the lowestelectron and hole bound states is studied for values of thediffusion length from 0 to 4 monolayers. The contribution of theX-point conduction band minima to the electronic structure isdiscussed. The calculated transition energies are shown to be inagreement with photoluminescence spectra of such structures. Thepresented approach can be used to assess the effect of thecomponent intermixing on the electronic structure of variouscomplex multilayered systems.

Higher speed short-wavelength (850 nm) VCSELs are required for future high-capacity, short-reach data communication links. The modulation bandwidth of such devices is intrinsically limited by the differential gain of the quantum wells (QWs) used in the active region. We present gain calculations using an 8-band k.p Hamiltonian which show that the incorporation of 10% In in an InGaAs/AlGaAs QW structure can approximately double the differential gain compared to a GaAs/AlGaAs QW structure, with little additional improvement achieved by further increasing the In composition in the QW. This improvement is confirmed by extracting the differential gain value from measurements of the modulation response of VCSELs with optimized InGaAs/AlGaAs QW and conventional GaAs/AlGaAs QW active regions. Excellent agreement is obtained between the theoretically and experimentally determined values of the differential gain, confirming the benefits of strained InGaAs QW structures for high-speed 850-nm VCSEL applications.

Electronic transport properties of as-grown and thermally annealed n- and p-type modulation-doped GaAsBi/AlGaAs quantum well (QW) structures were investigated. Hall mobility of as-grown, n- and p-type modulation doped QW structures are found from raw experimental data as ∼1414 and 95 cm2 Vs−1 at room temperature. A comparison between reported two-dimensional (2D) electron density determined from the analyses of Shubnikov de Haas oscillations and the 2D Hall electron density indicates a presence of parallel conduction in barrier layer (AlGaAs) and QW layer (GaAsBi) in n-type samples, therefore a parallel channel conduction theory is used to separate the electron mobility in the QW and the barrier layers in n-type modulation doped GaAsBi/AlGaAs QW structure. The extracted electron mobility of the as-grown n-type GaAsBi/AlGaAs QW sample is determined as ∼5975 cm2 Vs−1 at 4.2 K, which is closer to the electron mobility in GaAs. It is found that thermal annealing at lower temperature than growth temperature increases electron mobility of 2D electron gas, while annealing at higher temperature than growth temperature decreases electron mobility. The temperature dependence of the extracted electron mobility using parallel conduction approximation is analytically calculated by considering possible scattering mechanisms. Analysis of temperature-dependent electron mobility shows that the dominant scattering mechanisms are interface roughness (IFR), acoustic, and alloy-potential scatterings at low and intermediate temperature range, and IFR and optical phonon scattering at the high-temperature range in n-type modulation doped GaAsBi/AlGaAs QW structures.

Optical gain characteristics of type-II InGaN/GaNSb quantum well (QW) structure are investigated by using the multiband effective mass theory. These results are compared with those of conventional InGaN/GaN QW structures. The transition wavelength rapidly increases with increasing the Sb composition in GaNSb layer while it is less sensitive to the In composition in InGaN layer. Hence, longer wavelength QW structures with a relatively lower In composition can be easily obtained by controlling Sb composition, compared to the conventional type-I InGaN/GaN QW structures. The optical gain and the differential gain (dg/dn) of a type-II QW structure are shown to be much larger than that of a conventional QW structure in an investigated range of carrier densities. This is due to the reduction in the effective well width, in addition to the increase in the optical matrix element.

Characterization of GaP / InGaP and GaP / GaAsP strained-layer quantum wells grown by metalorganic chemical vapor deposition

In this paper, a model for hot-electron transport and capture phenomena in semiconductor heterostructures with multiple uncoupled quantum wells (QW) excited by infrared radiation is presented. The model takes into account the nonlocal character of the electron transport and capture of mobile electrons propagating over the barriers. It includes the Poisson equation and balance equations for electrons and their energy. The model is used for the calculation of steady-state spatial distributions of the electric-field and the average electron energy in multiple QW structures. The conditions of the formation of periodic electric-field domains revealed recently in ensemble Monte Carlo particle simulations are found. The obtained results are compared with the previous analytical calculations based on a simplified model. The relationships between phenomenological parameters of the latter and QW structure parameters are obtained. The developed analytical model highlights the origin of periodic electric-field domains in multiple QW structures and explains some features of such domain behavior.