Ion bombardment induced formation of self-organized wafer-scale GaInP nanopillar assemblies Cite as: J. Vac. Sci. Technol. B 38, 012801 (2020); https://doi.org/10.1116/1.5127265 Submitted: 10 September 2019 . Accepted: 06 December 2019 . Published Online: 19 December 2019 Dennis Visser, Juliana Jaramillo-Fernandez, Gabriel Haddad, Clivia M. Sotomayor Torres, and Srinivasan Anand COLLECTIONS This paper was selected as Featured ARTICLES YOU MAY BE INTERESTED IN High-brightness source of energetic He atoms Journal of Vacuum Science & Technology B 38, 012601 (2020); https:// doi.org/10.1116/1.5124838 Electrical and optical properties of copper oxide thin films prepared by DC magnetron sputtering Journal of Vacuum Science & Technology B 38, 012803 (2020); https://doi.org/10.1116/1.5131518 Bismuth vanadate photoanodes for water splitting deposited by radio frequency plasma reactive co-sputtering Journal of Vacuum Science & Technology B 38, 012203 (2020); https:// doi.org/10.1116/1.5129612 J. Vac. Sci. Technol. B 38, 012801 (2020); https://doi.org/10.1116/1.5127265 38, 012801 © 2019 Author(s). ARTICLE avs.scitation.org/journal/jvb Ion bombardment induced formation of self-organized wafer-scale GaInP nanopillar assemblies Cite as: J. Vac. Sci. Technol. B 38, 012801 (2020); doi: 10.1116/1.5127265 Submitted: 10 September 2019 · Accepted: 6 December 2019 · View Online Export Citation CrossMark Published Online: 19 December 2019 Dennis Visser,1,a) Juliana Jaramillo-Fernandez,1 Gabriel Haddad,1 Clivia M. Sotomayor Torres,1,2,3 and Srinivasan Anand1 AFFILIATIONS 1 Department of Applied Physics, KTH Royal Institute of Technology, Electrum 229, SE-164 40 Kista, Sweden 2 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, 08193 Bellaterra, Barcelona, Spain 3 ICREA, Pg. Lluis Companys 23, 08010, Barcelona, Spain a) Electronic mail: dvisser@kth.se ABSTRACT Ion sputtering assisted formation of nanopillars is demonstrated as a wafer-scale, lithography-free fabrication method to obtain high optical quality gallium indium phosphide (GaInP) nanopillars. Compared to binary materials, little has been reported on the formation of self-orga- nized ternary nanostructures. Epitaxial (100) Ga0.51In0.49P layers lattice matched to GaAs were sputtered by nitrogen (N2) ions with relatively low ion beam energies (∼400 eV) to reduce ion bombardment induced damage. The influence of process parameters such as temperature, sputter duration, ion beam energy, and ion beam incidence angle on the pillar formation is investigated. The fabricated GaInP nanopillars have average diameters of ∼75–100 nm, height of ∼220 nm, and average density of ∼2–4 × 108 pillars/cm2. The authors show that the ion beam inci- dence angle plays an important role in pillar formation and can be used to tune the pillar shape, diameter, and spatial density. Specifically, tapered to near cylindrical pillar profiles together with a reduction in their average diameters are obtained by varying the ion beam incidence angle from 0° to 20°. A tentative model for the GaInP nanopillar formation is proposed based on transmission electron microscopy and chemi- cal mapping analysis. μ-Photoluminescence and μ-Raman measurements indicate a high optical quality of the c-GaInP nanopillars. © 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http:// creativecommons.org/licenses/by/4.0/). https://doi.org/10.1116/1.5127265 I. INTRODUCTION Bottom-up9–11 and top-down12–14 approaches are commonly used for the fabrication of III–V nanopillars, each with their own Semiconductor nanopillars/wires can be used for a wide variety of applications in the fields of, e.g., optoelectronics and advantages. The bottom-up approach relies mostly on a growth cat- sensing.1–4 They offer unique features for obtaining specific optical alyst to initiate the nanopillar growth. To obtain dense/ordered functions, e.g., light extraction, increased absorption, and optical nanopillar structures, these catalyst (e.g., gold) particles need to be nonlinearity. Nanopillar-based solar cells are of interest due to patterned on the surface to enable a controlled growth of nano- their omnidirectional broadband antireflection and light trapping wires. Top-down approaches typically use surface patterning by abilities to achieve cheaper and more efficient solar cells.5–8 III–V lithography and subsequent pattern transfer into the III–V layers semiconductors, e.g., indium phosphide (InP) and gallium arsenide by etching. A number of lithography methods are commonly used, (GaAs), have the benefit of a direct bandgap, large absorption e.g., electron beam lithography, nanoimprint lithography, focused coefficients, and high conversion efficiencies. Nanostructuring of ion beam, and nanosphere lithography.15,16 For large-scale fabrica- these III–V materials paves the way for implementing light manip- tion, some important factors are the scalability of the method, cost, ulation functions, e.g., trapping, guiding, and antireflection. reduced process complexity, and process time. J. Vac. Sci. Technol. B 38(1) Jan/Feb 2020; doi: 10.1116/1.5127265 38, 012801-1 © Author(s) 2019 ARTICLE avs.scitation.org/journal/jvb Recently, self-organized surface nanopatterning by ion beam low energy IBS, where the main focus is on the self-organized for- sputtering (IBS) at moderate (1–10 keV) and high (10–100 keV) mation of nanopillars. Although no actual application is shown in energies has emerged.17 The production of nanoripple18–21 and this work, direct bandgap GaInP materials are interesting for nanodot22–28 ( periodic) nanostructures using IBS has been several visible range applications such as solar cells54–56 and reported, with structure dimensions ranging from a few up to hun- window layers for (GaAs-based) solar cells.57–59 The influence of dreds of nanometers. The advantage is that these structures can be several important process parameters for the IBS process is investi- fabricated on different materials (crystalline or amorphous) using gated: ion beam energy, substrate temperature, processing time, short processing times and obtain large area coverages. Similar and ion beam incidence angle. Scanning electron microscopy morphologies obtained for different materials indicate the univer- (SEM), transmission electron microscopy (TEM), energy-dispersive sality of the process. A control of the fabricated nanostructures can x-ray spectroscopy (EDS), μ-photoluminescence (PL), and be obtained by engineering the process parameters, e.g., ion energy, μ-Raman measurements were used to characterize the fabricated substrate temperature, ion density, and ion incidence angle. GaInP nanopillars. Sigmund29 provided the first theoretical background for understanding IBS nanostructuring, where he showed that local surface minima erode at a faster rate than local maxima leading to a surface instability; thus, the origin of the nanostructuring is based II. EXPERIMENT on surface-curvature-depended sputtering rates. Followed by this A three grid ion beam etching system from Oxford work, Bradley and Harper30 reported a continuum model regarding Instruments Plasma Technology (Ionfab 300 Plus) was used to fab- ripple formation, and over the years, other models have been pro- ricate the self-organized GaInP nanopillars. A schematic of the posed taking into account the different experimental behaviors.31–33 configuration of the IBS process is shown in Fig. 1(a). The three In 1999, Facsko et al.22 reported the formation of gallium antimon- grid ion source allows energies close to the sputtering threshold ite (GaSb) nanodot patterns by IBS. Since then, nanodot produc- (∼100 eV), which has been shown to be effective for nanopattering tion has been reported in materials such as InP,23 indium arsenide of GaSb.60 Here, a directional ion beam consisting of nitrogen (N2) (InAs),24 indium antimonite (InSb),25 GaAs,26 silicon (Si),27 and ions was used to physically etch the substrate. In previous reported germanium (Ge).28 Due to this, new theoretical descriptions have works, heavier ions with higher ion energies (in the keV range) been reported.34–40 Madi et al.41 and Castro et al.42 changed the were used for the formation of nanostructures by ion beam theoretical view and proposed a mechanism in which the sputtering etching, leading to the degradation of the optical properties.18–28 pattern topographical instability is based on mass transport rather However, nanostructures with good crystalline cores have been than erosion. In 2009, a self-masking process was reported by Le reported with keV argon (Ar) ions.22,23,51 Here, nitrogen ions with Roy et al.,43 where an investigation of GaSb showed that one energies of only a few hundred eV were used to minimize the detri- species segregates and functions as a continuously supplied etch mental influence of the ion beam induced damage on the optical mask. Recent works44–46 investigated the species that should be properties. The nitrogen beam consists of N+2 and N+ species,61 present on the nanostructure top for binary compounds (based on where the singly ionized process gas molecules (N+2 ) are the domi- sputtering yield and diffusivity), where Shipman and Bradley44 nating ions (∼90%); therefore, the final energy per atom is half that developed a theory for ordered nanodot formation based on of the beam voltage. By using an off-normal incidence beam angle, normal incidence ion bombardment. These works indicate that tilted nanopillars can be fabricated [see Fig. 1(b)]. But by varying element segregation may occur after ion beam processing and that the off-normal incidence beam angle (by tilting the sample stage) ex situ characterization may provide distorted information. while rotating the sample [see Fig. 1(c)], vertical nanopillars can be Recently, the fabrication of wafer-scale InP nanopillar arrays using obtained where the geometry (diameter) of the nanopillars can be low energy (400 eV) IBS was reported where broadband antireflec- tuned. This work focuses on the latter configuration. tion and solar cell application were demonstrated.8 However, com- A 1 μm (100) Ga0.51In0.49P (here on referred to as GaInP) pared to binary materials, little has been reported on the lattice matched epitaxial layer was grown on GaAs by a metalor- IBS-assisted nanopatterning of ternary structures. Examples of ganic chemical vapor deposition method (obtained from ENT S.A., reported ternary structures are CdZnTe by Gago et al.47 and Warsaw, Poland). Prior to IBS experiments, a 50 nm GaAs cap HgCdTe by Smirnov et al.;48 however, no results are reported for layer was removed by wet etching (H3PO4:H2O2:H2O = 3:1:25). ternary III–V semiconductor materials. IBS pattern formation of Subsequently, the sample surface was cleaned by sonication with III–V semiconductor materials is interesting due to their rapid for- acetone and isopropanol, rinsed in de-ionized water, and blow mation rate, high aspect ratio structures due to low saturation, the dried by N2 flow. The sample stage/platen can be heated up to occurrence of dense patterns over large surface areas, and enhanced 300 °C. The samples were preheated to the desired process tempera- growth rate for increasing temperatures.43,49–51 Trynkiewicz ture prior to sputtering. The substrate temperature is a crucial et al.52,53 discussed the mechanism of nanopillar formation in III– factor for the formation of nanopillars. Therefore, the influence of V semiconductor materials with special emphasis on the influence the processing temperature was investigated with regard to the of the target temperature. However, there is still a lack of clear com- ability to form group III-rich clusters during ion bombardment, prehension of the process of spontaneous ion-induced pattern for- which will function as an etch mask during sputtering. A diluted mation for III–V semiconductor materials. hydrogen fluoride [HF (50%)] treatment was used to remove the Here, we report on the wafer-scale, self-organized fabrication outer layers of the as-formed nanopillars, thereby obtaining crystal- of gallium indium phosphide (GaInP) nanopillar assemblies using line GaInP nanopillars. J. Vac. Sci. Technol. B 38(1) Jan/Feb 2020; doi: 10.1116/1.5127265 38, 012801-2 © Author(s) 2019 ARTICLE avs.scitation.org/journal/jvb FIG. 1. (a) Schematic sketch showing the ion beam sput- tering (IBS) configuration. Schematic illustration of the etch profiles obtained for a nonvertical ion beam inci- dence angle (b) without and (c) with sample/stage rotation. In this work, the influence of the sample temperature nanopillar (also indicated by the white dashed region). The sup- (Ts: 150–270 °C), processing time (tp: 10–40 min), ion beam porting EDS results for regions I–III are shown in Fig. 2(b). voltage/energy (VB: 200–600 V), and ion beam incidence angle A more detailed element analysis is included in the supplementary (0°–20°; with rotation) has been investigated with regard to GaInP material (Fig. S1).70 Figure 2(c) shows a TEM image of a c-GaInP nanopillar formation. The other parameters that were not varied nanopillar after removal of the Ga-rich a-GaInP shell layer and include the N2 flow of 13 SCCM, beam current of 100 mA In-rich top by HF treatment. The formation of the shell layer (0.6 mA/cm2), accelerator voltage of 500 V, platen rotation speed during IBS is beneficial since this layer protects the c-GaInP core of of 20 revolutions per minute, temperature of the chiller for platen the pillar from damage from the ion bombardment. For any device cooling of 15 °C, and pressure of the helium (He) backside application, it is important to only use the c-GaInP nanopillar part cooling at 15 Torr. by removing the shell layer and In-rich top. Representative high- resolution TEM images for the as-formed pillars are shown in III. RESULTS AND DISCUSSION Figs. 2(d)–2(f ), where Fig. 2(d) represents the red dashed region (region I), Fig. 2(e) represents the green dashed region (region II), A. Configuration of the as-formed nanopillars and Fig. 2(f) represents the blue dashed region (region III) in The formation of InP nanopillars by ion sputtering is known Fig. 2(a), respectively. Figure 2(g) shows a representative high- to arise from preferential sputtering of phosphorus (P) that makes resolution TEM image for the c-GaInP nanopillar for the orange the surface indium (In)-rich.8 The excess In atoms diffuse and dashed region indicated in Fig. 2(c). A factor that should be consid- form clusters that provide self-masking. The surface diffusion of ered related to the use of nitrogen ions is chemical modification the In atoms is not only thermally driven but is also assisted by ion (nitride formation). Similar nanopatterns have been produced on sputtering. In the case of GaInP also, one might expect similar InP surfaces with O+2 ions,62 where the formation of oxides was mechanisms to be responsible for pillar formation. However, the found to be relevant. Carlström and Anand63 reported on the diffusion and sputter yields of gallium (Ga) and In could be damage of InP dry etching using nitrogen sputtering, where 75 eV different. This, in turn, might also result in different elemental dis- nitrogen milling resulted in a nitrogen penetration depth of tribution in the formed nanopillars. Such information is also neces- <100 nm. It was shown that an additional annealing step can be sary for further processing (for example, to obtain crystalline used to remove the nitrogen content. The EDS results in Fig. 2(b) nanopillars) as well as to interpret the geometrical features and indicate nitrogen incorporation in the as-formed structures; optical characteristics of the nanopillars. Therefore, we first discuss however, nitrogen is predominantly present in the outer “jacket” the TEM and EDS results for the as-formed GaInP nanopillars. layer (regions I and II). The nanopillars were fabricated using the following parameters: ion beam incidence angle of 15°, Ts of 270 °C, tp of 10 min, and VB of 400 V. Figure 2(a) shows a representative TEM image of the B. Influence of sample temperature, processing time, and ion beam energy on nanopillar formation as-formed GaInP nanopillar. For TEM analysis, the pillars were scraped from the substrate using a blade and deposited on a TEM Figure 3 shows SEM images indicating the influence of the grid. Three typical regions can be distinguished along the sample temperature (Ts = 150–270 °C), processing time (tp = 10– as-formed pillar. Region I represents an In-rich top of the pillar, 40 min), and ion beam energy (VB = 200–600 V) for a 0° ion beam region II represents a Ga-rich amorphous GaInP (a-GaInP) incidence angle. Figures 3(a)–3(c) show the influence of Ts where “jacket,” and region III represents a crystalline GaInP (c-GaInP) representative SEM images are shown for Ts taken as 190, 230, and J. Vac. Sci. Technol. B 38(1) Jan/Feb 2020; doi: 10.1116/1.5127265 38, 012801-3 © Author(s) 2019 ARTICLE avs.scitation.org/journal/jvb FIG. 2. Transmission electron microscopy (TEM) images of a single GaInP nanopillar for the as-formed and the resulting c-GaInP pillar fabricated with a 15° ion beam incidence angle (combined with sample rotation). (a) Shows a TEM image of the as-formed GaInP nanopillar. The white dashed region indicates the c-GaInP nanopillar. (b) Shows the energy-dispersive x-ray spectroscopy (EDS) spectra for regions I, II, and III indicated in (a). Region I shows to be In-rich, region II shows Ga-rich a-GaInP, and region III shows c-GaInP. (c) Shows a TEM image of the obtained c-GaInP nanopillar after HF treat- ment. (d)–(f ) Shows high-resolution TEM images of the as-formed GaInP nanopillars, showing representative images for the In-rich top [red dashed region depicted in (a)], the Ga-rich a-GaInP side wall [green dashed region depicted in (a)], and the crystalline GaInP bottom part [blue dashed region depicted in (a)]. (g) Shows the crys- tallinity of the GaInP structure obtained after HF treat- ment; the representative image is related to the orange dashed region depicted in (c). 270 °C, respectively, with tp = 10 min and VB = 400 V. For this ion to the temperature limit of the ion beam system. The results regard- energy and sputter duration, the results show that for Ts < 190 °C ing Ts indicate that the height and density of the as-formed GaInP In clusters are not formed at the surface. For Ts = 190 °C, In clus- nanopillars can be tuned by controlling Ts. The optimized value of ters are formed but only small GaInP structures are shaped under- Ts is taken further on as 270 °C. neath. When increasing Ts further to 230 °C, proper as-formed Figures 3(d)–3(f ) show representative SEM images regarding GaInP pillars are obtained, where for a higher temperature of the influence of tp taken as 10, 20, and 40 min, respectively, with 270 °C even longer are obtained. Ts was not further increased due Ts = 270 °C and VB = 400 V. For longer tp, the as-formed GaInP FIG. 3. Scanning electron microscopy (SEM) tilted view images showing the influence of the process parameters: (a)–(c) temperature (190–270 °C), (d)–(f) processing time (10–40 min), and (g)–(i) ion beam voltage (200–600 V). The insets in (g)–(i) show the respective cross sections of the as-formed nanostructures. J. Vac. Sci. Technol. B 38(1) Jan/Feb 2020; doi: 10.1116/1.5127265 38, 012801-4 © Author(s) 2019 ARTICLE avs.scitation.org/journal/jvb FIG. 4. Scanning electron microscopy (SEM) images of the obtained GaInP nanopillars (as-formed and after HF treatment) for an ion beam incidence angle of (a)–(d) 0°, (e)–(h) 5°, and (i)–(l) 15°, where the top view [(a), (e), and (i)], tilted view [(b), (f ), and ( j)], and cross section [(c), (g), and (k)] are shown for the as-formed GaInP nanopil- lar assemblies. Additionally, tilted view images [(d), (h), and (l)] of the c-GaInP nanopillar assemblies (obtained after HF treatment) are shown. pillars become longer in height. Additionally, the density of the were used. Figure 4 shows representative SEM images for three of as-formed pillars increases due to the formation of new In clusters those angles (0°, 5°, and 15°); where for the as-formed pillars top between the original clusters (for shorter tp) and the apparent view, tilted view and cross section images are included and for broadening of the as-formed pillar diameter, where the broadening the resulting c-GaInP nanopillars (obtained after HF treatment) of the “jacket” of the pillar is most likely due to the diffusion of Ga only tilted view images. For some of the as-formed pillars, a pro- (and In) atoms to the side walls and redeposition of P. The value trusion was observed on the side wall of the pillars. EDS analysis for tp is further on taken as 10 min. of this protrusion indicates that this consists of Ga-rich a-GaInP; Figures 3(g)–3(i) show representative SEM images for the representative data for this are included in the supplementary influence of VB taken as 200, 400, and 600 V, respectively, with material (Fig. S2).70 However, this protrusion is removed during Ts = 270° and tp = 10 min. For VB = 200 V, relatively low density the HF treatment, and therefore, no further investigations were In cluster sites are formed, where barely a shape is formed below done regarding its occurrence. The resulting pillar ensembles the In mask [see Fig. 3(g)]. For VB = 400 V, pillar shapes are show that the ion beam incidence angle influences the density formed, and VB = 600 V results in even taller as-formed pillars and shape of the obtained GaInP nanopillars (before and after and slightly higher pillar density. Although larger and higher HF treatment). Figure 5 summarizes the results regarding the density as-formed nanopillars are obtained for VB = 600 V, it is influence on the nanopillar height, average density, and diameter best to limit the ion beam energy bombardment to prevent due to the ion beam incidence angle based on SEM images analy- degrading of optical properties. Due to this, the optimal value for sis. As can be seen in Fig. 5(a), the height of the as-formed VB is taken as 400 V. pillars changes with the applied incidence angle, varying between ∼550 and 800 nm. However, the height of the obtained c-GaInP (after HF treatment) shows to be similar for all investigated inci- C. Influence of the ion beam incidence angle on the dence angles and results in a height of ∼220 nm. As can be seen geometry and spatial arrangement of the (c-GaInP) in Fig. 5(b), the average density also fluctuates with regard to the nanopillars incidence angle, varying between ∼2 and 4 × 108 pillars/cm2. An The influence of the ion beam incidence angle has been interesting observation for the as-formed nanopillars is that the investigated for angles varying between 0° and 20°, with a step incidence angle dependence of the height of the pillars is size of 5°. For this, the previously mentioned process parameters inversely related to the density, where pillars with a larger height J. Vac. Sci. Technol. B 38(1) Jan/Feb 2020; doi: 10.1116/1.5127265 38, 012801-5 © Author(s) 2019 ARTICLE avs.scitation.org/journal/jvb FIG. 5. Graphs showing the influence of the ion beam incidence angle (angles of 0°–20°) on the GaInP nano- pillar (a) height, (b) average density, and the pillar diameter of the (c) as-formed and (d) c-GaInP nanopillars (after HF treatment), respectively. In (c) and (d), the nanopillar diameters were measured as indicated in the insets of both figures. show a lower density. Figures 5(c) and 5(d) show the diameter temperature and increases when the temperature increases. The dependence for the as-formed and c-GaInP nanopillars with continuous ion bombardment additionally contributes to pro- regard to the incidence angle. This has been done at three viding extra energy to the migrating In atoms and is generally height positions of the pillar to indicate the pillar shape: seen as lowering of the energy barrier for diffusion. Newly bottom (I), middle (II), and top (III) of the pillar [see for formed In atoms preferably diffuse to already formed In clusters further clarification the inset in Figs. 5(c) and 5(d)]. The diam- within its diffusion length and thereby sustain the etch mask eter of the as-formed pillars varies between ∼90 and 150 nm, during sputtering as the surface topography evolves. This where the top and middle diameters increase for increasing process is continuous until the length of the formed pillars is incidence angle and the bottom diameter decreases. The longer than the diffusion length of the In atoms, resulting in average diameter of the c-GaInP nanopillars is ∼75–100 nm that the sputtering of the In mask/clusters dominates due to the and changes with the ion beam incidence angle. The shape of lack of additional In atoms provided to the In mask. When the the pillar changes from a pillar to a wire shape when increasing length of the nanopillars reaches within the diffusion length, the incidence angle from 0° to 20°. additional In atoms can reach the top of the pillars and again form the etch mask. This results in that a maximum nanopillar length can be obtained, which is related to the diffusion length D. Tentative model for nanopillar formation of the In atoms. Furthermore, a shadowing effect should be Figure 6 shows a tentative model that is based on TEM, taken into account for off-normal ion beam incidence angles. In EDS, and SEM results in combination with measured chemical addition to In, Ga atoms also diffuse along the surface. mapping plots that are included in the supplementary material However, their diffusion properties could be different. The (Figs. S3 and S4).70 The formation of the GaInP nanopillars is obtained results indicate that the Ga atoms play a minor role in due to the preferential sputtering of P compared to Ga and In, the formation of the top etch mask but accumulate at the sides where Ga is preferentially sputtered compared to In. The In is of the as-formed pillars, where redeposition of the sputtered P partially sputtered away but additionally excess In forms at the atoms also occurs. The dynamics of the surface conditions and surface leading to In enrichment. Diffusion of this excess In evolution during IBS of GaInP is a complex process involving takes place and results in In-rich clusters on the surface within three species of atoms, modification of the surface morphology, its diffusion length, eventually acting as an etch mask. The changes of the chemical composition, and surface diffusion of diffusion of the excess atoms is related to the processing atoms influenced by thermal/ion-assisted factors. A number of J. Vac. Sci. Technol. B 38(1) Jan/Feb 2020; doi: 10.1116/1.5127265 38, 012801-6 © Author(s) 2019 ARTICLE avs.scitation.org/journal/jvb FIG. 6. Tentative model for the as-formed fabricated GaInP nanopillars at (a) normal (0°) and (b) tilted (20°) ion beam incidence angles (with sample/stage rotation). In the model, the In-rich and Ga-rich amorphous GaInP (a-GaInP) regions are indicated with an arrow. The gray color of the structure indicates the crystalline GaInP (c-GaInP). process parameters influence the GaInP nanopillar formation, E. Photoluminescence and Raman characterization e.g., ion species, ion energy (VB), Ts, and ion beam incidence μ-PL and μ-Raman measurements were performed (source angle. A detailed description of the mechanism of the GaInP wavelength of 514 nm) to characterize the optical properties of the nanopillar formation with regard to the geometry, spatial fabricated GaInP nanopillars (see Fig. 7). The laser power and the density, elemental distribution, and material quality is beyond spot size were ∼8 μW and ∼30 μm, respectively, for μ-PL and the scope of this work and requires an extensive study of the ∼3.6 mW and ∼2 μm for μ-Raman, respectively. μ-Raman measure- role of various parameters involved. In this work, preliminary ments were done on the as-formed as well as on the c-GaInP nano- investigations were done for some of these process parameters. pillars and μ-PL measurements only on the c-GaInP nanopillars. FIG. 7. Measured μ-PL and μ-Raman characteristics for the GaInP nanopillars fabricated at ion beam incidence angles varying between 0° and 20° for the cases: as-formed and c-GaInP (after HF treatment) nanopillar assemblies. (a) Shows schematics for the measured structures both for μ-PL and μ-Raman measurements, where a laser source at 514 nm was used. (b) Shows the μ-PL results for the c-GaInP nanopillars. (c) Shows the μ-Raman characteristics for the as-formed and (d) for the c-GaInP nanopillar assemblies. J. Vac. Sci. Technol. B 38(1) Jan/Feb 2020; doi: 10.1116/1.5127265 38, 012801-7 © Author(s) 2019 ARTICLE avs.scitation.org/journal/jvb The as-formed pillars are covered by metallic and amorphous ACKNOWLEDGMENTS layers and, hence, not investigated by PL. Figure 7(b) shows that The authors would like to acknowledge the support from the the μ-PL results where both the peaks related to GaInP (at Linné Center for Advanced Optics and Photonics (ADOPT; Grant ∼657 nm) and GaAs (at ∼870 nm) are indicated with a vertical No. 349-2007-8664), the Swedish Research Council (VR), and the dashed line. Data are included for “bulk” GaInP with a layer thick- Swedish Energy Agency (Energimynigheten; Grant Nos. 45199-1 ness of ∼600 nm. The μ-PL spectra show similar results for almost and 42028-1). The authors thank Frederik Gustavsson for the TEM all incidence angles. For the incidence angle of 0°, a higher peak is and EDS measurements. observed for both GaInP and GaAs peaks, though the direct com- parison of PL intensities is often difficult due to the fact that it is a REFERENCES local and spacially dependent measurement; yet an increase in peak 1 value might suggest higher light extraction. Figures 7(c) and 7(d) H. J. Joyce et al., Prog. 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