GaAs Heteroepitaxy on Fluoride by Electron Beam Induced Surface Modification
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GaAs Heteroepitaxy on Fluoride by Electron Beam Induced Surface Modification Tobias Fritz, Markus Haiml, Silke Schön and Ursula Keller Swiss Federal Institute of Technology (ETH) Physics Department / Institute of Quantum Electronics ETH Zurich Hoenggerberg – HPT, CH-8093 Zurich, Switzerland ABSTRACT GaAs saturable absorber materials are used in ultrafast nonlinear optics to obtain all-optical switching in optoelectronic devices. They are introduced to semiconductor saturable absorber mirrors (SESAMs) for the generation of ultrashort laser pulses. GaAs is grown on CaF2 by molecular beam epitaxy to fabricate devices, which provide a large high reflection bandwidth. The CaF2 surface was exposed to high- and low-energy electron irradiation before and during growth to increase the surface free energy for the subsequent GaAs overgrowth. A three-layer GaAs/ fluoride device was designed and fabricated to study the impact of the electron exposure on the growth mode and the surface roughness. The surface morphology of an optoelectronic device can cause nonsaturable losses, which degrade the device performance. Therefore, the effect of the electron exposure of the CaF2 layer on the surface roughness of the device was studied by atomic force microscopy. Measurements of the scattered light from the device surface allowed for a quantitative analysis of the nonsaturable losses attributed to the surface morphology of the device. INTRODUCTION Semiconductor saturable absorber mirrors (SESAMs) are used for ultrashort pulse generation [1]. SESAMs combine the nonlinear optical properties of a semiconductor saturable absorber with those of a high reflection Bragg mirror in one device. Since conventional AlGaAs/ AlAs SESAMs limit pulse shortening due to their small high reflection bandwidth [2], novel devices combining materials with a large difference in their refractive indices are needed to be developed [3]. Recently, we reported on the successful growth and operation of an ultrabroadband III-V/fluoride SESAM with a GaAs saturable absorber layer for sub-10 fs pulse generation [4, 5]. Saturable absorber layers are required to show high absorption modulation with fast recovery times and low nonsaturable losses. Nonsaturable losses can be caused by additional absorption from defect states or by scattered light, which originates from the surface and interfaces of the device. Therefore, the surface morphology is an important factor for the device performance. The growth of GaAs saturable absorber layers on CaF2 proceeds by island formation due to the low surface free energy of CaF2 and a low As sticking coefficient [6]. To improve the wettability of the fluoride, the CaF2 surface is altered by electrons [7]. In this paper, we report on the growth of GaAs on CaF2 surfaces exposed to high- and low-energy electrons before and during growth. A three-layer saturable absorber device is analyzed with respect to the nonsaturable losses attributed to the surface morphology. Theoretical simulations of the total scatter ratio using a Green’s-tensor ap
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