The effect of the diameters of the nanowires on the reflection spectrum

Transcription

1 The effect of the diameters of the nanowires on the reflection spectrum Bekmurat Dalelkhan Lund University Course: FFF042 Physics of low-dimensional structures and quantum devices 1. Introduction Vertical III-V nanowire arrays have recently drawn most attraction for using photo devices and photo detectors. The nanowire geometry has significant advantages in material science compare to the conventional thin-film geometry. due to NW s free surface they have efficient defect-free strain relaxation as compared to the thin films of the same materials[1]. Furthermore, III-V materials can be realized on a lattice mismatched substrate like silicon which leads to much reduce the costs. Most III-V nanowires have direct band gap that could give them ability to interact strongly with light and in result they can be considered the most promising material for using to photo devises. Among the III-V materials, InP to be found as the best candidate application on solar cells because of it s high band gap as well as large absorption coefficient. Improvements of the efficiency of the InP solar cells needs to further study the way of increasing light absorption or equivalently decreasing light reflectance from the nanowires. NW arrays geometry plays key rule in both absorption and reflectance of light on the arrays. In this project work, we have studied the reflectance of light with on arrays vertical oriented InP nanowires in experimentally by varying nanowire diameters. We performed reflectance measurements in the visible wavelength region of the light. 2. Theory Light propagates in the form of plane waves in bulk samples. In contrast, when the structure is very approach at the wave length of light in size that prediction of propagating in bulk sample must be modified strongly and so that tunable propagation of light should be expected to be seen in a such small structure.

2 If one predicts that the absorption A, transmission T and reflectance R when light is being shone on the sample, then the reflectance of light can be written as R=1-A-T (1) A schematic of a nanowires arrays of period p, diameter D and length L is given in figure1.[3] Figure1: the schematic of a nanowire arrays The transmission T in equation 1 depends on the fraction of the substrate f that defined as D 2 2 f = π [( )] p (2) Where, D is diameter of nanowires, p is the periodicity of the array[3]. It is obvious that in equation 2, if f is kept constant, the reflectivity depends on the diameter of nanowires. Besides dependency of f, the reflectivity is also governed by the amount of absorbing material given by f*l where L is the length of nanowires.[2] The optical response of absorbing nanowire arrays could be determined in extensive arranges by the relation between the diameter of nanaowires and the wave length of light. If D>>λ, the interaction between the nanowirse and light can be described by geometrical optics, if D<<λ, the interaction of light with nanowires must be described by electrostatics[3]. Thus the dependence of the reflectance on the diameter is strongly related to the dependence of the absorption on the electric

3 field E inside the nanowire. The electric field also depends on diameter in response. Therefore, we can write relations between absorption, electric field and absorption coefficient α by following expression[3]: A~ E 2 α (3) It is possible to get different absorption/reflectance specter by changing the geometry of the nanowire arrays(diameter, length, array periodicity). 3. Experiments We used two different samples in the measurements. In this project work, we examined only the dependence of reflectance on diameters of nanowires, because both length of nanowires and the periodicity of arrays were kept constant. To investigate more accurately, we used two InP substrates each has twenty area in which InP nanowires were grown at different diameters. The nanowires were grown by metal-organic vapour phase epitaxy by assisted gold particle as catalyst which were deposited on each area of substrate with various diameter by nanoimprint lithography. Both the schematic arrangement of areas and nanowires are given in figure2 a and 2b. Figure 2. the schematics of substrate and nanowire areas[2] On the both samples the nanowire diameter varied from 30nm to 80nm. The length of nanowires were 1,1µm for both samples. The measurements were done by using a Filmetrix F40 Thin Film Analyzer. By shining a light beam of varying wave length on the sample we measured the reflected light intensity.

4 4. Results and Discussions The reflectance specters of twenty nanowire arrays in which nanowires placed with different diameters compare to each array are given in fig.3 Figure3: Reflectance of InP nanowire arrays with varing diameter from 30nm to 80nm. Minimum reflection is shifting toward the short wave lengths when decreasing the diameters. In order to see changing in reflectance more clearly as diameters of nanowires are increased, we represented some specific measurements in fig4 which were separated from fig3 that can provide us more precisely assumptions related to dependency of reflectance on nanowire diameters. Figure4: Reflection in short wavelength is shifting toward short wavelengths when decreasing D, and the reflection of nanowire is increasing

5 It can be seen clearly from fig4 that reflectance of light falls down as diameters of nanowires are increasing. Further more, the minimum of reflectance moves towards larger wavelengths with increasing of nanowire diameters. The changes in reflectance of light more efficiently in the range of nm wavelengths. Out of that range reflectance still decreases with increasing of nanowire diameters but not so strong. Decreasing of reflectance with increasing of nanowire diameters can be interpreted by two reasons. Increasing in nanowire diameters causes to increase in the fraction of substrate f which leads to get more light transmission through the arrays. From equation 1, it is obvious that reflectance should be reduce when transmission of the light increases. It can be also explained in terms of the absorptions of light as we discussed in the theory part of this project. The coupling of the electric field of the light with the electric field inside the nanowires leads to increase the absorption of the light which also strongly depends on nanaowire diameters. Conclusion In this project work we studied the effect of nanowire diameters on the reflectance of the light. The reflectance of the light reduces as the diameters of nanowires are increased. Although we did not consider the effect of lengths in this study, which has remarkable effect on reflectance of the light from nanowire arrays. Thus, for further investigations, it would be interesting to measure reflectance of light on both length of nanowire and the periodicity of arrays. References 1. Nicklas Anttu, Alireza Abrand, Damir Asoli, Magnus Heurlin, Ingvar Åberg, Lars Samuleson, and Magnus Borgström. Absorption of light in inp nanowire arrays. Nano Research 2014, 7(6): Andreas Johansson and Markus Hellenbrand. The effect of nanowire geometry on their reflectivity spectrum 3. Nicklas Anttu. Nanophotonics in absorbing III-V nanowire arrays. Doctoral thesis, Lund University 2013.