Abstract book

A.D. Utrilla,¹ J. M. Ulloa,¹ L. Domínguez,² D. F. Reyes,² D. González,² A. Guzman¹ and A. Hierro¹

¹Institute for Systems based on Optoelectronics and Microtechnology (ISOM) and Dpto. Ingeniería Electrónica, Universidad Politecnica de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain

²Departamento de Ciencia de los Materiales e IM y QI, Universidad de Cádiz, 11510 Puerto Real (Cádiz), Spain
The use of the quaternary GaAsSbN as a capping layer (CL) on InAs/GaAs quantum dots has been proposed as a strategy to tune independently the electron and hole confinement potentials and to lengthen the peak emission wavelengths while keeping a type-I band alignment. However, to date, no room temperature emission has been reported in such a system. Aiming this target, an extensive study has been carried out in MBE grown samples in order to find the optimum growth conditions for the CL. After optimization, intense room temperature photoluminescence around 1.3 μm is obtained. Using high growth rates, the GaAsSbN CL provides an enhanced integrated intensity and longer peak wavelengths than the ternary GaAsSb or GaAsN CLs, revealing a significant improvement by the simultaneous presence of Sb and N in the CL. Moreover, the best luminescence properties are obtained by using a GaAsSb/GaAsN superlattice-like structure as a CL, which also allows extending the peak wavelength beyond 1.4 μm. The influence of the growth conditions on the structural properties of the samples was investigated by transmission electron microscopy.
Hydrogen irradiation of In(AsN)

S. Birindelli¹, L. Qi², M. Kesaria², Q.D. Zhuang², A. Krier², A. Patanè³, A. Polimeni¹, M. Capizzi¹

¹ Dipartmento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy

² Department of Physics, Lancaster University, Lancaster LA1 4YB, United Kingdom

³ School of Physics and Astronomy, Nottingham University, Nottingham NG7 2RD, United Kingdom

Small amounts of nitrogen atoms into III-V semiconductors ─such as GaAs and InAs─ greatly affect the physical properties of these materials, whose bandgap energy decreases because of a large optical bowing. Recently, a bright photoluminescence (PL) emission ─up to 4.5 µm─ reported in In(AsN) alloys has raised a considerable interest because of its potential for the realization of optical devices operating in the mid infrared range [1].

Here, we report the effects that low energy (≈ 100 eV) hydrogen irradiation has on the optical properties of In(AsN) epilayers ([N] up to ≈2%) grown by nitrogen plasma source molecular beam epitaxy. PL emission intensity may increase by more than one order of magnitude upon low temperature (250 °C) hydrogenation. A passivation of surface defects for hydrogen doses decreasing with increasing N content is inferred. Measurements of the In(AsN) energy gap have been performed easily up to room temperature. Preliminary results of hydrogenation at 300 °C show, instead, increasing band-filling effects, which supports the formation of bulk donors upon deep hydrogenation we previously demonstrated by transport measurements in these sample samples.
[1] M. de la Mare et al., J. Phys. D: Appl. Phys. 45, 395103 (2012) and references therein
Ga_0.35In_0.65N_0.02As_0.08/GaAs Bi-directional Light Emitting and Absorbing Heterojunction operating at 1.3 μm

F.A.I. Chaqmaqchee^1* and Naci Balkan<sup>2

1</sup>University of Koya, Faculty of Science and Health, Koya Kurdistan Region, KO50 1001, IRAQ

²University of Essex, School of Computer Science and Electronic Engineering Colchester, CO43SQ UK ^*[e-mail]
Top-Hat Hot Electron Light Emission and Lasing in Semiconductor Heterostructure Vertical Cavity Semiconductor Optical Amplifier (THH-VCSOA) is a bi-directional light emitting and absorbing heterojunction device.

The device contains 11 Ga_0.35In_0.65N_0.02As_0.08/GaAs MQWs in its intrinsic active region which is enclosed between 6-pairs of AlAs/GaAs top DBR and 20.5-pairs of AlAs/GaAs bottom DBR mirrors. The THH-VCSOA is fabricated using a four-contact-configuration. The wavelength conversion with amplification is achieved by the appropriate biasing of the absorption and emission regions within the device. Absorption and emission regions may be reversed by changing the polarity of the applied voltage. Emission wavelength is about 1300 nm and a maximum gain at this wavelength is around 5 dB at T=300 K.

GaInNAsSb Solar Cells Grown by MBE
A. Aho, A. Tukiainen, V. Polojärvi, W. Zhang, J. Salmi, M. Guina
Optoelectronics Research Centre, Tampere University of Technology, FI-33101 Tampere, Finland
Multijunction solar cells (MJSC) are excellent devices for concentrated and space photovoltaics. The driving force for the material and technological development of these multilayer structures is the high efficiency, which is further improved under highly concentrated solar illumination. Currently up to 44% efficiency triple junction solar cell has been demonstrated and efficiencies of more than 50% are predicted when using more junctions [1, 2] .We studied the molecular beam epitaxy growth of GaInNAsSb solar cells with N and Sb contents ranging from 0 to 6% and 0 to 4%, respectively. Without Sb, the short circuit current densities of solar cells were improved up to 4% N. [3] N contents higher than 4% lead to phase separation during growth. The best solar cells without Sb showed short circuit current density of 39 mA/cm² (real sun illumination, 1000 W/m²), which is among the highest reported values. [4] With addition of Sb the current density improves 7.5%. [5] Also the band gap of GaInNAsSb lattice matched to GaAs can be pushed down below 0.85 eV which helps absorption of long wavelength photons. This however comes with a cost of bandgap open circuit voltage offset, which is lower for solar cells without Sb. Finally, we present the performance of GaInP/GaAs/GaInNAs(Sb) triple junction solar cells.

*[e-mail]

[1] M. A. Green et al. Progress in Photovoltaics: Research and Applications, 21, pp 1–11, (2013)

[2] R. R. King et al., Advances in OptoElectronics, 2007, 2007

[3] A. Aho et al., 26^th EU PVSEC 58-61 (2011), doi:10.4229/26thEUPVSEC2011-1AO.8.3

[4] A. Aho et al., Proc. SPIE 8620, (2013); doi:10.1117/12.2002972

[5] A. Aho et al., AIP Conf. Proc. 1477, pp. 49-52; (2012) doi:http://dx.doi.org/10.1063/1.4753831

Strain-Engineered InAs/Ga(In)NAs/GaAs Quantum Dot Solar Cells
E. Pavelescu ¹, V. Polojärvi ², A. Aho ², A. Tukiainen ², A. Schramm ², W. Zhang ², J. Salmi ², M. Guina ^2
1 National Institute for Research and Development in Microtechnologies, Erou Iancu Nicolae 126A, 077190

Bucharest, Romania

² Optoelectronics Research Centre, Tampere University of Technology, P.O.Box 692, 33101 Tampere, Finland

III-V quantum dots (QD) are interesting candidates for intermediate band in semiconductor solar cells. In order to form an efficient intermediate band and increase its absorbtion several layers of, for example, InAs QDs, need to be stacked and ordered in all three dimensions. Also, the shape, size and composition of the QDs should be uniform. It is known, that stacks with more than ~ 10 layers of InAs/GaAs QDs with lattice mismatch of ~7% easily generate defects due to accumulated strain. Therefore, strain compensation layers, such as GaNAs with about 1% of N, have been previously proposed to compensate the induced strain in QD solar cell structures. Here we present a study of Ga(In)NAs strain compensation and mediation layers inserted into InAs QD solar cells. The effect of strain-engineering dilute nitride layers on QD photoluminescence (PL) emission, dots morphology as well as spectral response at near-infrared part of the spectrum was investigated. It was found that the insertion of the GaInNAs strain-mediating into the close vicinity of the strain-compesated InAs/GaNAs QDs improves the optical quality and significantly red shifts the absorption edge of the solar cells structures.

SESSION III - InGaN & GaN

24 SEPTEMBER 2013 TUESDAY

17:00-18:00
Molecular Beam Epitaxy of Single Phase InGaN Films in the Entire Alloy Composition Range for Photovoltaic Applications

E. Papadomanolaki¹, M. Androulidaki², K. Tsagaraki², C. Bazioti³, Th. Kehagias³, G. Dimitrakopulos³ and E. Iliopoulos^{1,2 *}

¹ Physics Dept., University of Crete, P.O.Box 2208, 71003 Heraklion, Greece

² Micro/Nano Electronics Research Group, IESL-FORTH, P.O.Box 1385, 71110 Heraklion, Greece

³ Physics Dept., Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
^* [e-mail]
Indium gallium nitride (InGaN) alloys is a family of wurtzite compound semiconductors ideally suited for photovoltaic (PV) applications[1]. Their direct bandgaps almost spans the entire solar spectrum, ranging from 3.4 eV (GaN) to 0.65 eV (InN). Furthermore, their internal polarization fields enhance electron-hole separation. However, for successful application of these alloys, in PV devices, two major problems need to be effectively addressed: (a) alloy phase separation phenomena[2,3], present due to the large miscibility gap in the corresponding quasi-binary alloy phase diagram, which are highly detrimental for PV devices since they promote carrier recombination and (b) extended structural defects in epitaxial layers, caused by the large difference in alloy endpoints lattice constants, which may detriment material optoelectronic properties. In this work, plasma assisted molecular beam epitaxy (RF-MBE) on GaN(0001) substrates was employed, to develop thick (300-600 nm) single phase epitaxial In_xGa_1-xN layers, in the entire composition range (x=0 to 1) and their structural and optoelectronic properties were studied by high-resolution x-ray diffraction (HR-XRD), high resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), variable angle spectroscopic ellipsometry (VASE), photoluminescence (PL) and Hall effect measurements.

The kinetic mechanisms involved in the RF-MBE growth of metal-polar InGaN alloys have been studied as a function of substrate temperature, ratio of group-III atoms to active nitrogen species arrival rate (III/V flux ratio) and nominal alloy composition. In lower substrate temperatures, indium incorporation is dictated solely by the available N^* arrival flux, similarly to the case of AlGaN RF-MBE growth. In higher substrate temperatures, indium incorporation is dictated by the interplay of indium adatoms desorption and InGaN decomposition. In the latter case, an arrival III/V flux ration independent growth mode is established, and the layer composition is controlled solely by the substrate temperature.

Phase separation was not observed in as-grown InGaN(0001) films, neither by x-ray diffraction or transmission electron microscopy studies. The absence of phase separation, despite growth temperatures well within the miscibility gap of the alloy phase diagram, is attributed to the far-from-equilibrium character of the RF-MBE growth. Rarely, under specific growth conditions, secondary compositions peaks are observed in the XRD spectra, as would be expected for phase separated alloy films. However, in those cases, these peaks are attributed to unstable growth conditions driven, rather than thermodynamics-driven, composition modulations.

Structural studies by TEM and HR-TEM confirm that the principal threading dislocations (TDs) were of a-type. Introduction of misfit dislocations at the InGaN/GaN(0001) interface was attributed to in-plane sources. Depending on the growth conditions, discontinuous or continuous strain relaxation was observed. The former comprised a strained InGaN interfacial layer followed by introduction of TDs from an extended stacking fault (SF). In the case of continuous relaxation, TDs were gradually introduced with increasing thickness. The optoelectronic properties of the InGaN epitaxial layers have been studied in details and correlated to growth conditions and structural defects.

[1] E. Trybus et al., J. Crys. Growth 288, 218 (2006)

[2] R. Singh et al., Appl. Phys. Lett. 70, 1089 (1997)

[3] I. Ho et al., Appl. Phys. Lett. 69, 2701 (1998)

[3] E. Iliopoulos et al., Appl. Phys. Lett. 81, 295 (2002)

Acknowledgement This research has been co-financed by the European Union (European Social Fund – ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: THALES.
InGaN MQW photoluminescence enhancement by localized surface plasmons in isolated Ag nanoparticles

G. Tamulaitis¹, D. Dobrovolskas¹, J. Mickevičius¹, C.-W. Huang², C.-Y. Chen², C.-H. Liao², C. Hsieh², Y.-L. Jung², and C.C. Yang<sup>2

1</sup> Semiconductor Physics Department and Institute of Applied Research,Vilnius University, Sauletekio 9-III, LT-10222 Vilnius, Lithuania

² Institute of Photonics and Optoelectronics, National Taiwan University, 1, Roosevelt Road, Section 4, Taipei, 10617 Taiwan

InGaN-based light emitting diodes (LEDs) efficiency can be enhanced by increasing radiative recombination rate by coupling emitted photons with plasmons in metal layers or nanoparticles in close proximity of the active layer of an LED.

In this work we report on spatially resolved photoluminescence (PL) study of InGaN multiple quantum wells (MQW) with silver nanoparticles (NPs) deposited on top. The localized surface plasmon resonance (LSP) wavelength was tuned to match PL emission by controlling the size of the silver NPs. Near-field scanning microscopy and confocal spectroscopy were employed to study the spatial distribution of PL. Atomic force microscopy was used to match the location of the nanoparticles with the image of PL spatial distribution in confocal experiments. A reference sample without NPs was also studied.
PL intensity mapping images of sample covered with Ag NPs reveal areas, where PL intensity exceeds the average value by a factor up to 4. The effect is especially strong in the areas close to assemblies of several metal NPs. It was found that the enhancement was stronger in the microscopic sample areas emitting at wavelengths better matched with LSP resonance.
Chemical Synthesis and Characterization of GaN Quantum Dots

Incorporated in Simple Photonic Devices


M. Vasileiadis¹, I. Koutselas¹, D. Alexandropoulos¹, N. Kehagias³, K. Dimos², M. Karakassides², L’uboš Jankovič⁴, R. Zbořil⁵, Peter Komadel⁴, and N. Vainos<sup>1

1</sup> Department of Materials Science, University of Patras, 26504 Patras, Greece

2 Department of Materials Science and Engineering, University of Ioannina, 45110 Ioannina, Greece

3 Catalan Institute of Nanoscience and Nanotechnology (ICN2),Campus de la UAB, 08193 Bellaterra, Spain

4 Inst.of Inorg. Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 84536 Bratislava, Slovakia

5 Regional Centre of Advanced Technologies and Materials, Faculty of Science, Department of Physical Chemistry, Palacky University in Olomouc, 77146 Olomouc, Czech Republic

In this paper we discuss a novel route for low-temperature (650oC) synthesis of gallium nitride (GaN) quantum dots (QDs) and there implementation in photonic devices. Simple solid state reaction is shown to produce GaN QDs, exhibiting blue shifted photoluminescence in the UV region compared to bulk GaN. For the synthesis no organogallium precursor was used. The growth of the QDs was assisted and controlled by a host matrix of an ordered mesoporous silica MCM-41. The QDs were successfully extracted by destruction of the host matrix and transferred via solvent exchange methods to polypropylene glycol methyl ether acetate (PGMEA) in order to facilitate the mix with polymers for the fabrication of photonic structures. The fabrication of our final photonic devices was performed by means of soft nanolithography and excimer laser micro fabrication techniques.

Research co-financed by the European Union and Greece through the Operational Program “Heracleitus II”.


SESSION IV - III-V on Silicon & Bismide alloys I
25 SEPTEMBER 2013 WEDNESDAY

09:00-10:20
3D heteroepitaxy of Ge and GaAs on patterned Si substrates: a new monolithic integration strategy
S. Sanguinetti¹, R. Bergamaschini¹, S. Bietti¹, F. Isa², G. Isella², A. Marzegalli¹, F. Montalenti¹, F. Pezzoli¹, A. Scaccabarozzi¹, C. V. Falub³, H. von Känel³ and L. Miglio^1
1 L-NESS and Dip. di Scienza dei Materiali, Università di Milano Bicocca, I-20125Milano, Italy

²L-NESS and Dipartimento di Fisica, Politecnico di Milano, I-22100 Como, Italy

³ Laboratory for Solid State Physics, ETH Zurich, CH-8093 Zurich,Switzerland

We have recently shown that crystal defects and internal stresses commonly hampering Ge heteroepitaxy on Si can be greatly reduced by promoting the nucleation of micro-crystals at the top of Si pillars deeply patterned on the substrate [1]. Ge-on-Si epitaxy by low-energy plasma-enhanced CVD (LEPECVD), allows for controlling the microcrystal faceting, in turn affecting the evolution of threading dislocations and providing complete expulsion of threading arms to the microcrystal sidewalls [2]. In addition, the microcrystals are self-assembled in dense arrays, with a spacing ranging from tens to one hundred nanometers. The growth mechanism of the micro-crystals is an intriguing “nano” effect, which has been interpreted in terms of independent facet growth. This effect is not a unique feature of Ge deposition by LEPECVD, but a more general growth mode, which can be extended to other materials systems and deposition techniques. GaAs microcrystals deposited by MBE on patterned Si substrates show as well the complete tessellation of the surface by closely separated microcrystals. The complete elimination of thermal strain and reduction of threading dislocations is confirmed by the high PL yield.

[1] C.V. Falub et al., Science, 335, 1330-1334, 2012;
[2] A. Marzegalli et al., Advanced Materials, 2013

Bi-assisted nucleation of GaAs grown on silicon by molecular beam epitaxy

P. Boonpeng^1,2, H. Makhloufi^1,2, G. Lacoste^1,2, A.Arnoult^1,2,C.Fontaine^1,2

¹ CNRS, LAAS, 7 avenue du colonel Roche, F-31400 Toulouse, France

² Université de Toulouse, Université Paul Sabatier, LAAS, F-31400 Toulouse, France

GaAs Nucleation on (001) silicon is known to lead to a high density of defects: stacking faults, microtwins, dislocations and antiphase defects. One of the origin of these defects is related to 3D islanding occurring upon GaAs/Si nucleation. We have investigated on the surfactant effect of Bi to improve this situation. The results that we have obtained for molecular beam growth in a 32P RIBER system

of thin (<100nm) GaAs layers on 5°off (011) silicon substrates will be presented. The substrates were ex situ treated in a diluted HF2 : H2O solution and then were annealed in the growth chamber. We will discuss how the Si surface evolves by means of reflection high

energy diffraction (RHEED). We will then present our study of the early stage of GaAs(Bi) growth on these silicon surfaces by using different analyses during GaAs(Bi)/Si nucleation: RHEED iffraction

during growth and, ex-situ, atomic force analyses, X-Ray diffraction.

We will discuss the low temperature emission properties of GaInAs quantum wells grown at different distances from the GaAs/Si interface. Finally, rapid thermal annealing has been applied to these heteroepitaxial structures. Its influence on the quantum well mission

will be shown.
Valence band structure of dilute bismide alloys for optoelectronic device applications

E.P. O’Reilly^1,2, C. Broderick^1,2, P. Harnedy^1,2 , M. Usman<sup>1

1</sup> Tyndall National Institute, Lee Maltings, Cork, Ireland

² Department of Physics, University College Cork, Ireland

GaBi_xAs_1-x has several novel electronic properties, including a rapid reduction in energy gap and a strong increase in spin-orbit-splitting energy with increasing Bi composition, x. This makes dilute bismide alloys a strong candidate system for the realisation of highly efficient and temperature stable GaAs-based photonic devices operating at telecom and longer wavelengths.

We present tight-binding calculations which confirm that the observed variation of the band gap and spin-orbit-splitting energy are well described by a band-anticrossing interaction between the GaAs valence band edge states and localized Bi impurity states in the valence band. We then derive an accurate 12-band k.p model for GaBiAs heterostructures, which we use to analyse the electronic structure and gain characteristics of GaBi_xAs_1-x lasers grown at Marburg (x~2-4%). Photovoltage measurements support a weak type-I conduction band offset for a GaBi_0.02As_0.98/GaAs quantum well (QW) structure. We show that this weak confinement can be overcome by using AlGaAs barriers, and report self-consistent gain calculations to identify an Al barrier composition which delivers the maximum modal gain. We then consider higher Bi composition GaBiAs/GaAs and GaBiNAs/GaAs QW structures, and present calculations to illustrate the wide flexibility which such alloys offer for the design and implementation of high efficiency photonic devices.

SESSION V- BISMIDE ALLOYS II
25 SEPTEMBER 2013 WEDNESDAY

11:00-12:30