Doctorate in Physics XXXI Cycle, First Year, January 2017
Project: Hydrogenation of dilute nitrides for single photon emitters in photonic crystals
PhD Student: Saeed Younis Supervisors: Prof. Antonio Polimeni, Dr. Marco Felici
Introduction Dilute nitride semiconductors are an example of the failure of “linear” models in predicting the properties of solids: the addition of a few percent of N to GaAs causes changes in the physical properties that are sometimes even opposite to those expected from simply interpolating the GaAs-‐to-‐GaN ratio [1]. Furthermore, some effects of nitrogen incorporation can be eliminated, or greatly reduced, by exposing the material to hydrogen flux [2][3]. The strong N-‐H interaction in dilute nitrides turns H diffusion into a multiple trapping process, resulting in the formation of extremely sharp heterointerfaces between the H-‐free and the H-‐containing regions [4]. This ability, together with the striking effects of hydrogen incorporation on the opto-‐electronic properties of dilute nitrides, facilitate the fabrication of a novel class of site-‐controlled nanostructures, whose properties meet a wide range of technological applications. Objective and methodology The aim of this project is the realization of single photon sources and their integration in photonic crystal (PhC) microcavities operating at 1.31 and 1.55 μm, for the purpose of integration within telecom devices. Together with our Marie Curie ITN partners at the universities of Marburg, Lancaster and Cadiz, InGaAsN/GaAs quantum wells will be grown by MOVPE and by MBE and will be characterized by photoluminescence(PL)-‐based techniques and by transmission electron microscopy. The effects of hydrogen irradiation, with the aim of passivating nitrogen in these wells will then be studied. Fabrication of Quantum dots will afterwards be carried out using:
• E-‐Beam lithography, to create site-‐ and size-‐controlled hydrogen-‐opaque masks on the surface of the samples
• Hydrogen irradiation of the exposed regions, to alter the materials opto-‐electronic properties around the desired area, leaving a nanometric sized, quantumly confined structure, namely a quantum dot, emitting at the desired wavelength
The process is illustrated in figure 1 below. Once such dots are fabricated, and using AlGaAs as a sacrificial layer buried beneath the structure, a second lithography process will be used to create a floating photonic crystal around the active quantum dot. The photonic crystal will in addition have a microcavity centered precisely around the quantum dot area. Mode matching between the photonic crystal microcavity and the emission from the quantum dot will further sharpen the emission profile and enhance its intensity.
Figure 1: simulated illustration of the various steps that are utilized to
fabricate the quantum dot. (a) H-‐opaque mask lithographically patterned on top of a structure containing an active quantum well sandwiched between a buffer and cap GaAs layers. (b) illustration of the sharp diffusion profile of H. (c) realization of a nanometric sized structure of H-‐free material embedded within an H-‐containing
surroundings Together with partners at Tyndall-‐UCC, energy profile calculations will be carried out and utilized to drive the sample design in order to reach the targeted energy level and the highest possible degree of quantum confinement. Results Samples containing a single InGaAsN quantum well either 5 or 10 nm thick, with a nominal composition of 2.5% N and 21% In, stacked between GaAs buffer and cap layers were prepared using MOVPE by our
partners at University of Marburg. Some samples were received as-‐grown, some underwent mild annealing (at 600°C for 10 sec) and some underwent a much stronger annealing process (at 650°C for 30 sec). These samples were irradiated with hydrogen using our custom built hydrogenation chamber with H doses ranging from 5x1017 to 2x1018 ions/cm2. Their normalized PL spectra, taken at room temperature, are shown in figure 2 below. The first conclusion that stems out of the data is that both annealing processes resulted in a dramatic increase in the PL intensity of non-‐hydrogenated (virgin, with H=0) samples of up to 190 times relative to the as-‐grown samples. This increase in intensity is accompanied by a blue-‐shift towards high energies of up to 80 meV. Neither the intensity increase or the blue shift are surprising as the first is expected due to achieving better crystalline quality upon annealing and the latter is attributed to indium rearrangement around nitrogen atoms resulting in the effective cancellation of the N-‐contribution to the bandgap energy and thus elevating it.
figure 2: Room temperature PL spectra of hydrogenated InGaAsN quantum wells. The hydrogenation processes were performed at 300C. H=� indicates the H dose of each subset (ions/cm2). please note that the scale of each hydrogenation subset has
been factorized relative to that of H=0 (lowermost).
The above data also indicate that the optimum H dose for achieving maximal passivation of N is around 1018 ions/cm2. This information is essential for the next step of fabricating the quantum dots. Summary The above data basically concludes phase 1 of the project which was to establish the best optimal conditions for the fabrication and passivation of InGaAsN quantum wells. Based on that, mildly annealed samples measuring 1cm x 1 cm, thus suitable for subsequent lithography, have been fabricated at UMR. [1] – I. A. Buyanova and W. M. Chen (2004). Physics and Applications of Dilute Nitrides (Taylor and Francis, New York). [2] – A. Polimeni et al (2001). Effect of hydrogen on the electronic properties of InxGa1-‐xAs1-‐yNy/GaAs quantum wells, Phys. Rev. B, 63, 201304R. [3] – A. Polimeni et al (2002). Role of hydrogen in III-‐N-‐V Compound Semiconductors, Semicond. Sci. Technol., 17, 797. [4] – M. Felici et al (2006). In-‐plane bandgap engineering by modulated hydrogenation of dilute nitride semiconductors, Adv. Mater., 18, 1993.