Seite - 188 - in Photovoltaic Materials and Electronic Devices

deposited by MBE on nanostructured Si substrate for solar cell applications. The resultsmayopenupnewperspectivesonthedevelopmentoflowcost,highefficiency III-V-basedsolarcellsonSi substrate. 2. ResultsandDiscussion 2.1.GrowthProcess InAs/GaAs QDs with In0.13Ga0.87As strain reducing layer were incorporated within the intrinsic regionofapin-GaAs/n+-SiusingStranski-Kranstanowgrowth mode by molecular beam epitaxy. The use of InGaAs as a strain reducing layer is believed both to reduce the compressive stress acting on the InAs QDs by the GaAsmatrix,andtoreduce indiumout-diffusionfromtheInAsQDs[22–24]. The choice of relatively low indium composition is expected to avoid excessive In-Ga phase separation that alters the optical and structural properties of the InAs QDs and the surrounding material [25]. The number of absorbed photons is proportional to the number of effective QDs in the solar cell active region. For the InAs/GaAs system, theQDs’aerialdensity inasingle layer isaround1010 dots/cm2 onGaAs substrate[26]. Suchavalueistoosmalltoaccountfortheimprovementofthespectral response. Increasing the effective number of QDs is possible by vertical stacking of QD layers. However, the total number of vertically stacked layers is limited by the onset required for the relaxation of the accumulated strain by generation of stacking faults and dislocations. In the present study, we have employed 40 QD layersensuringacompromisebetweenthe increaseof theeffectivenumberofQDs and the overall sample’s structural properties. The pin diode structure has been directly fabricatedonnanostructuredn+-Si substrate. Thepreparationof thenanostructuredn+-typesilicon’ssubstratesurfacehas beenperformedatroomtemperaturebythe formationandsubsequentdissolution ofaporous layer. Theelectrolyteusedto fabricatea5µm-thickporoussilicon layer consistsofamixtureofhydrofluoricacidHF([HF]=36%)andethanol (HF:C2H5OH) inavolumetricproportionof1:1. Theporous layerwas formedbyanodizingtheSi substrate in this electrolyteunder acurrent density of3 mA¨cm´2. The samplewas thenetchedinNaOHsolutiontobreakuptheporoussilicon layerandproduce the structuration of the surface. Indeed, after the chemical dissolution of silicon skeleton, theruggedsurfacewillbeexposedtobeamepitaxy. Additionaldetailsconcerning theprocessaswellas themorphologicalpropertiesof thenanostructuredSisurface andits impactonthequalityofGaAsmaterialgrownonsuchSisurfacecanbefound elsewhere [27]. After surface preparation, a cleaning and out gassing process of the silicon substratewasperformed undervacuumcondition inan introductory chamberwith a rest pressure of 10´9 Torr at high temperature (760˝C), to remove the native oxide 188