Study of microrelief influence on optical output coefficient of GaN-based LED

Transcription

1 Study of microrelief influence on optical output coefficient of GaN-based LED Danilina T.I., Cistoyedova I.A. and Popov A.A. Tomsk State University of Control Systems and Radioelectronics, Lenina prospect 40, Tomsk, Russia Corresponding Autor: Cistoyedova I.A., Abstract. Tis work is devoted to te investigation of te influence on te ligt output coefficient of antireflective coatings by different configurations of te micro-relief formed on te ligt output surface of a GaN-based LED. Te tecnology of te micro-relief fabrication in SiO -based antireflective coatings was developed wit te use of electron-beam litograpy (EBL) and contact potolitograpy. Simulation of te influence of te microrelief of various proportions and configurations on te optical output coefficient was implemented. It was discovered tat te micro-relief made wit electron-beam litograpy and contact potolitograpy increased te optical output coefficient. 1. Introduction One of te primary goals in te researc of semiconductor ligt-emitting diodes based on gallium nitride and its solid solutions is to increase te external quantum efficiency of te ligt-emitting diode. Tere are several approaces to improving te ion of ligt emitted from te semiconductor material o te surrounding media. It was discovered tat combination of semiconductor wafer and transparent substrate by bonding tecnology can increase te ligt ion [1] wit te use of sopisticated geometries []. Anoter approac to enance te ligt ion is te surface rougening by wet etcing process [3]. Te external quantum efficiency SiO of a ligt-emitting diode crystal is defined by two key parameters: ernal quantum efficiency and optical output coefficient : ext = (1) Te effect of total ernal reflection, consisting in localization of te ligt inside te structure of a ligt-emitting diode, reduces te probability of te escape of potons from a semiconductor. Terefore, a coefficient of optical output of radiation is roduced, defined as te relation of te number of te potons radiated by a ligt-emitting diode to te number of te potons formed in te active area in a unit of time [4]. Te optical output coefficient can be estimated as follows: ( n ) ( n ) P =, () P were P - power of te optical radiation escaping from a ligt-emitting diode; P - power of te optical radiation from te active area; n - energy of a poton. 1

2 Tus, te effect of total ernal reflection on te border of a material wit a ig optical density (semiconductor) and a material wit a low optical density (a sappire substrate or air) is te key factor limiting te efficiency of te output of ligt. For te ligt-emitting diode crystals based on InGaN/GaN-eterostructures, te critical angle is ~ 3 (te refractive indexes of GaN and sappire are.5 and 1.6). Tus, only a small part of te falling of potons on te section border at an angle witin te range of 0 3 can leave a crystal. An increase of te external quantum efficiency is possible due to te creation of ligt-dispersing surfaces and use of te antireflective coatings. Te aim of te work is te development of a tecnology for formation of a microrelief in te antireflective coatings and simulation of te optical output coefficient.. Materials and metods. Tecnology for formation of a microrelief by means of electron-beam litograpy Te formation of microrelief surfaces in te antireflective coatings was developed on te example of SiO films wit te use of electron-beam litograpy and contact potolitograpy. For te creation of a microrelief by means of electron-beam litograpy, a SiO film wit a tickness of 80 nm was deposited on a semi-conductor substrate by means of plasma-cemical deposition (PCD). A resistive mask was formed in te layer of te PММА 950 positive resist. two Exposure and combination were carried out on a Rait 150 electronic litograper wit an accelerating voltage of 30kV and exposure dose of 450 µ mc / cm. Te exposed areas of te resist were developed in a mix of organic solvents of metyl isobutyl ketone and isopropyl alcool. Te time of development was determined by te quality of te windows opened in te resist. Troug a resistive mask wit a diameter of its windows of 0.5µ m and distance between tem of 0.5µ m, an isotropic etcing of te SiO layer was done. Te image control in SiO was carried out by means of a two Rait 150 electronic microscope. Te image of te microrelief was defined by te etcing time. Figure 1 presents an image of te microrelief received in te SiO layer by electron-beam litograpy. Figure 1. Image of te microrelief received in layer by te metod of electron-beam litograpy (te time of etcing of te sample was 40 s ). From Figure 1 it is visible tat after etcing for a duration of 40 s, a regular structure appears in te form of a round aperture in te SiO layer wit a dept of 70 nm, diameter of 460 nm and 8 distance between tem of 130 nm. Te density of te apertures was.8 10 pieces / cm. Between te deepenings in te SiO, layer a microrelief was formed in te form of pyramids, located on a continuous layer of SiO wit tickness of 10 nm. If te etcing time was increased up to85 s, a microrelief was formed in te SiO layer in te form of microedges wit regular structure. Tus,

3 during te electron-beam litograpy, due to te cange of te time of etcing of SiO layer, it became possible to control te configuration of te microrelief. 3. Formation of microrelief by contact potolitograpy For formation of te microedges by te direct and reverse ("explosive") contact potolitograpy, a two-layer mask based on ERP-40 FP-051Su-0.5 resists was used. Exposure was done troug a potomask wit aperture diameters of 1.31µ m and wit a distance between te windows of 1.43 µ m. Due to te reverse litograpy in te SiO layer, microedges were obtained in te form of truncated trapeziums wit a eigt of 439 nm, te size of te top base of µ m, and te bottom base of µ m (Figure ). Te density of te edges was equal to.5 10 pieces / сm. Figure. Image of te microrelief received in te SiO layer by te metod of reverse potolitograpy Te microrelief obtained by te direct potolitograpy was a set of ordered microedges locally connected between temselves in te remaining SiO layer. Te eigt of te pos was 439 nm, te distance between tem was 1.5 µ m, and te diameter of te bottom basis was 0.43 µ m. Te density 8 of te pos was pieces / cm. 4. Results of modeling of optical output coefficient For te researc of te influence of a microrelief surface on te optical output coefficient, simulation was done by means of NEMO LED software developed in te Cair of Pysical Electronics of Tomsk State University of Control Systems and Radioelectronics. Te given product allows us to model te propagation of a ligt beam in te multilayered structures wit different refraction indexes of te layers and to investigate te influence of te microrelief antireflective coatings on te relative number of te quanta of ligt, wic escaped from te crystal (optical output coefficient). For te researc of te influence of te microrelief in te antireflective coatings from SiO and ITO wit optical tickness of l / in NEMO LED software, a model was developed, in wic te lateral and bottom faces of te crystal of gallium nitride (refraction index n GaN =.5 ) were covered by a reflecting material. On te top ligt-ing face on te crystal side, a tin film was deposited wit a refraction index smaller tan tat of gallium nitride ( nsio = 1.43, n 1.9 ITO = ), and a microrelief of various configurations was formed in it (Figure 3). Radiation in te structure arose in te layer of gallium nitride. Figure. Image of te microrelief received in SiO layer, received by te metod of reverse potolitograpy 3

4 Te given model allows us to investigate te efficiency of te output of ligt from te top face because it takes o account te quanta, wic as no reflection during te passage of te border between te active area and te ligt-ing layer (GaN). By defining te relative number of te quanta, wic left te top face at various configurations of te microrelief surface, it is possible to estimate te influence of a microrelief on te coefficient of optical output of a ligt-emitting diode. 5. Discussion Analysis of te received results sows tat for a structure witout an antireflective coating, te optical output coefficient is = Hence, te crystal was left wit 37 % of te potons, wic came o te ligt-ing layer of GaN from te active area. Deposition of te antireflective coatings increased tis coefficient: for SiO film, we received = 0.41, and for ITO film = Formation of a microrelief surface in an antireflective coating from SiO in te form of pyramids, obtained by electron-beam litograpy, resulted in an increase of te optical output coefficient up to = Similar results were received for a microrelief formed by reverse contact potolitograpy. It was establised tat reduction of te distance between te pyramids resulted in te furter increase of te number of te quanta of ligt, wic escaped from te crystal, wit preservation of te correlation of te sizes of te bottom basis of a pyramid to te top one of 3:1, wic corresponds to te angle at te basis During simulation of a microrelief in te form of microedges, te correlation between te widt and eigt of te microedge b/ canged, as did te distance between te pos. It was establised tat te greatest optical output coefficient was in te structures wit a microrelief surface, were te basis of te edge was commensurable wit its eigt, and te microrelief elements were located densely to eac oter (Figure 4). Figure 4. Dependence of te optical output coefficient on te widt/eigt correlation of a microedge. For te structures wit te correlation of b/ = 4/3, te maximal value of te optical output coefficient was received:» 51 % for te ITO antireflective coating and» 45.6 % for te SiO film. 4

5 6. Conclusion It was establised tat presence of te antireflective coatings and formation of a microrelief in tem increases te optical output coefficient. Te tecnologies developed for te formation of a microrelief in te antireflective coatings allow us to ensure a regular structure wit a ig density of edges pieces / cm. Te greatest optical output coefficient can be reaced at te demanded correlation of te geometrical sizes and configuration of te microrelief, wic ensures a microrelief angle at te basis of about References [1] Kis F A et al 1994 Appl. Pys. Lett [] Krames M R et al 1999 Appl. Pys. Lett [3] Liu Zike et al 010 J. Semicond [4] Scubert E F 006 Ligt-Emitting Diodes (Cambridge: Cambridge University Press) 5