Photonic crystals in biology - NanoTR-VI

Poster Session, Thursday, June 17Theme F686 - N1123Graded-Index Antireflection Coatings for Nanostructured Photovoltaics, Light Emission andPhotodetectionAbstract— The problem of increasing Fresnel reflections around the resonance wavelength of quantum dots, wells and wiresis proposed to be greatly reduced by introducing and optimizing antireflection coatings for visible ( = 400-700nm), nearinfrared(telecommunication wavelengths, = 750-1400nm) and long wave infrared (microbolometer wavelengths, = 8-12m) given an upper limit for the coating thickness. Optimized simulation results show that reflection losses can be reducedbelow 0.1% over wide angular and spectral ranges.After the development of imaging, manipulation,deposition, etching and probing tools for the nanostructure ofmaterials, a vast range of nanomaterials have been introduced.The possibility of engineering the electronic band structure ofthe materials through changing the size of the material at thenanoscale enables researchers achieve novel optical,mechanical, electronic and thermal properties that areunavailable in nature, tunable over a wide range and highlystable. Applications that grew out of this have been the use ofquantum wells, quantum wires and quantum dots forenhancing photovoltaics[1], light emitting diodes[2], lasers[3]and photodetectors [4]. The ability to tune the luminescenceand absorption properties of nanoparticles embedded inside adielectric host comes with its own fundamental drawback:Fresnel reflections between air and the outermost layerincreases significantly around the resonance wavelength of thenanoparticle.Fresnel reflections increase as a function of refractiveindex mismatch between the outermost layer and air. TheFresnel reflection loss of bare silicon is about R=(3.6-1) 2 /(3.6+1) 2 =31.9%. The absorption spectra of nanoparticleshave Lorentzian resonance peaks and by Kramers-Krönigrelations and according to our ellipsometry measurements ofGe quantum dot layers embedded in Silicon (GeNC-in-Si),effective refractive index of the GeNC-in-Si layers increasesfrom n Si = 3.6 upto n effective = 5.1 due to the existence of thedots. Around the resonance wavelength of the nanomaterial,the refractive index mismatch increases and this causesreflection loss to increase beyond that of bare silicon, i.e. R =(5.1-1) 2 / (5.1+1) 2 =45.2%. The increase of index mismatcharound the resonance limits the full utilization of quantum dotsand this increases (i) internal reflections for LED’s, (ii)surface reflections of solar cells, (iii) pumping thresholdcurrent for lasers using nanomaterials as gain materials.For efficient light absorption in solar cells, we haveepitaxially grown Germanium quantum dot layers on siliconwafer. The deposition conditions were chosen deliberately toachieve very large quantum dot size dispersion and thusbroadband absorption enhancement of the whole sample. InFig. 1a, the effective refractive indices of the multilayerGermanium quantum dots in Silicon are compared with that ofbare silicon.In order to reduce surface reflections, producing refractiveindex gradient between air and the layers have been proposed[5, 6]. The layers can be fabricated using the technique calledoblique angle deposition [7]. In our study, we compared andoptimized exponential, linear, sinusoidal and polynomiallygraded refractive index profiles for the coatings.Figure1: (a) Ellipsometric measured refractive index spectra for bare silicon andthe quantum dot embedded sample. (b) Optimized % Reflection for visible,given 1 m total thickness limit. (c) Optimized % Reflection for near infrared,given 1 m total thickness limit. (d) Optimized index profile for long waveinfrared, given 10 m total thickness limit. (e) Optimized % Reflection for longwave infrared (= 8-12m). (f) Angular % Reflection for (e)Because of significant size dispersion of the nanocrystals,there is multitude of resonance wavelengths and this increasesthe sample’s refractive index broadband (Fig.1a). As asolution, graded index antireflection coatings (GIAR) forvisible have been optimized (Reflection spectrum shown inFig.1b). For near-infrared photodetection, GIAR wereoptimized with 1 micron total thickness limit (Fig.1c). Therefractive index profile of the optimized coating for nearinfraredis in Fig.1d.This work was supported by TUBITAK 108E163, 109E044,EU FP7 PIOS.[1] Conibeer, G. Materials Today, Vol. 10, No. 11, 4250[2] Nizamoglu, S., Demir, H. V., Nanotechnology 18 (2007) 405702[3] Strauf, S. et al. Physical Review Letters 96, 127404 (2006).[4] Wang, J. et. al., Science, Vol. 293. no. 5534, pp. 1455 - 1457[5] XI, J.Q. et.al., Nature Photonics, Vol. 1, March 2007, 176179[6] Chhajed, S. et. al. Applied Physics Letters, 93, 251108 (2008)[7] Kennedy, S. R., et. al. , Applied Optics, Vol. 42, No. 22, 4573 45796th Nanoscience and Nanotechnology Conference, zmir, 2010 631