Abderraouf Boucherif

Multijunction Solar Cell Designs Using Silicon Bottom Subcell and Porous Silicon Compliant Membrane

A novel approach to the design of multijunction solar cells on silicon substrates for 1-sun applications is described. Models for device simulation, including porous silicon layers, are presented. A silicon bottom subcell is formed by diffusion of dopants into a silicon wafer. The top of the wafer is porosified to create a compliant layer, and a III-V buffer layer is then grown epitaxially, followed by middle and top subcells. Because of the resistivity of the porous material, these designs are best suited to high-efficiency 1-sun applications. Numerical simulations of a multijunction solar cell that incorporates a porous silicon-compliant membrane indicate an efficiency of 30.7% under AM1.5G, 1-sun for low-threading dislocation density, decreasing to 23.7% for a TDD of 107 cm-2.

Mesoporous germanium morphology transformation for lift-off process and substrate re-use

The morphology of electrochemically formed mesoporous Ge double-layer and its transformations during ultra-high-vacuum annealing at 600–700 °C are investigated by scanning electron microscopy. It was found that the transformation occurs via mass transport at constant volume. The process transforms the pores into faceted spherical voids. These findings determine the optimal conditions for the transformation of the mesoporous Ge into a useful structure, which consists of a 1.8 μm thick monocrystalline Ge film with buried lateral cavities allowing for subsequent lift-off. The monocrystalline nature of the film and its suitability as a seed layer for GaAs epitaxy are demonstrated by X-ray diffraction.

Chemical Composition of Nanoporous Layer Formed by Electrochemical Etching of p-Type GaAs.

We have performed a detailed characterization study of electrochemically etched p-type GaAs in a hydrofluoric acid-based electrolyte. The samples were investigated and characterized through cathodoluminescence (CL), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). It was found that after electrochemical etching, the porous layer showed a major decrease in the CL intensity and a change in chemical composition and in the crystalline phase. Contrary to previous reports on p-GaAs porosification, which stated that the formed layer is composed of porous GaAs, we report evidence that the porous layer is in fact mainly constituted of porous As2O3. Finally, a qualitative model is proposed to explain the porous As2O3 layer formation on p-GaAs substrate.

Growth optimization and optical properties of AlGaNAs alloys

The effect of Al on the surface morphology of chemical beam epitaxy grown AlGaNAs alloys is studied. Pits attributed to N clustering appearing on the dilute nitride surface become smaller, denser, and more uniformly distributed in the presence of Al. This reveals that the introduction of Al results in more homogenous N atoms spatial distribution. A growth temperature study reveals the formation of 3D structures at high temperature due to phase separation. The density of these structures decreases, while their diameter and height increase when the temperature is raised from 380 °C to 565 °C. At growth temperatures in the 380–420 °C range, the phase separation is suppressed and the growth mode is 2D. At 420 °C, the N incorporation is also maximized, making it the optimum temperature. The absorption coefficient and the bandgap of AlGaNAs alloys are extracted from transmittance measurement. A good agreement is obtained between the experimentally measured bandgap and the theoretical values calculated using the band anticrossing model. A bandgap as low as 1.22 eV was reached using Al and N concentrations of ∼15% and ∼3.4%, respectively.

Composition variation in Al-based dilute nitride alloys using apertureless scanning near-field optical microscopy

We use apertureless scanning near-field optical microscopy to study the phase separation in chemical beam epitaxy grown Al0.1Ga0.9NxAs1-x alloys. Pits attributed to nitrogen-clustering observed on the Al0.1Ga0.9NxAs1-x surface grown at 420 °C become larger at higher growth temperatures, and 3D islands appear on the surface at 565 °C. Atomic force microscopy phase measurements reveal a composition difference between the islands and the pits, whereas the sample grown at 420 °C appears to be homogeneous. Confocal Raman spectra show that all the N atoms are bonded to Al instead of Ga. Using apertureless scanning near-field optical microscopy, the luminescence of a gold tip is mapped over the surface of the sample grown at 565 °C. We extract the shift of the tips surface plasmon resonance and determine the variation in the refractive index between the islands and the pits to be close to 0.2. Numerical simulations of the tip luminescence while in contact with the sample predict a similar variation of ∼0.3 in the refractive indices between AlGaAs islands and AlN pits, a substantially smaller value than the difference in the bulk refractive indices of the two media (∼1.8), which we attribute to a convolution of material distribution in an uneven topography. The excellent agreement between simulation and experiments supports the hypothesis of nitrogen-clustering in the pits.

Near-infrared emission from mesoporous crystalline germanium

Mesoporous crystalline germanium was fabricated by bipolar electrochemical etching of Ge wafer in HF-based electrolyte. It yields uniform mesoporous germanium layers composed of high density of crystallites with an average size 5-7 nm. Subsequent extended chemical etching allows tuning of crystallites size while preserving the same chemical composition. This highly controllable nanostructure exhibits photoluminescence emission above the bulk Ge bandgap, in the near-infrared range (1095-1360nm) with strong evidence of quantum confinement within the crystallites.

Tunable conductivity in mesoporous germanium

Germanium-based nanostructures have attracted increasing attention due to favourable electrical and optical properties, which are tunable on the nanoscale. High densities of germanium nanocrystals are synthesized via electrochemical etching, making porous germanium an appealing nanostructured material for a variety of applications. In this work, we have demonstrated highly tunable electrical conductivity in mesoporous germanium layers by conducting a systematic study varying crystallite size using thermal annealing, with experimental conductivities ranging from 0.6 to 33 (×10-3) Ω-1 cm-1. The conductivity of as-prepared mesoporous germanium with 70% porosity and crystallite size between 4 and 10 nm is shown to be ∼0.9 × 10-3 Ω-1 cm-1, 5 orders of magnitude smaller than that of bulk p-type germanium. Thermal annealing for 10 min at 400 °C further reduced the conductivity; however, annealing at 450 °C caused a morphological transformation from columnar crystallites to interconnecting granular crystallites and an increase in conductivity by two orders of magnitude relative to as-prepared mesoporous germanium caused by reduced influence of surface states. We developed an electrostatic model relating the carrier concentration and mobility of p-type mesoporous germanium to the nanoscale morphology. Correlation within an order of magnitude was found between modelled and experimental conductivities, limited by variation in sample uniformity and uncertainty in void size and fraction after annealing. Furthermore, theoretical results suggest that mesoporous germanium conductivity could be tuned over four orders of magnitude, leading to optimized hybrid devices.

Graphene-Mesoporous Si Nanocomposite as a Compliant Substrate for Heteroepitaxy

The ultimate performance of a solid state device is limited by the restricted number of crystalline substrates that are available for epitaxial growth. As a result, only a small fraction of semiconductors are usable. This study describes a novel concept for a tunable compliant substrate for epitaxy, based on a graphene-porous silicon nanocomposite, which extends the range of available lattice constants for epitaxial semiconductor alloys. The presence of graphene and its effect on the strain of the porous layer lattice parameter are discussed in detail and new remarkable properties are demonstrated. These include thermal stability up to 900 °C, lattice tuning up to 0.9 % mismatch, and compliance under stress for virtual substrate thicknesses of several micrometers. A theoretical model is proposed to define the compliant substrate design rules. These advances lay the foundation for the fabrication of a compliant substrate that could unlock the lattice constant restrictions for defect-free new epitaxial semiconductor alloys and devices.

Chemical beam epitaxy growth of AlGaAs/GaAs tunnel junctions using trimethyl aluminium for multijunction solar cells

AlGaAs/GaAs tunnel junctions for use in high concentration multijunction solar cells were designed and grown by chemical beam epitaxy (CBE) using trimethyl aluminium (TMA) as the p-dopant source for the AlGaAs active layer. Controlled hole concentration up to 4⋅1020 cm−3 was achieved through variation in growth parameters. Fabricated tunnel junctions have a peak tunneling current up to 6140 A/cm2. These are suitable for high concentration use and outperform GaAs/GaAs tunnel junctions.

Quasi-monocrystalline Ge as an interface layer for multi-junction solar cells on Si substrates: Electrical resistivity and device modelling

Quasi-monocrystalline germanium (QMC Ge) is investigated for potential applications in high-efficiency, low-cost multi-junction solar cell design. The morphology and electrical resistivity of QMC Ge is characterized and computer simulations of MJSCs on Si substrates using QMC Ge interface layer are developed.