Microlens-Enhanced Substrate Patterning and MBE Growth of GaP Nanowires

Abstract: In this paper we demonstrate the results on selective area growth of GaP nanowires via self-catalyzed growth method using molecular beam epitaxy (MBE) technique on patterned Si(111) substrates. The pattern fabrication method on a base of the photolithography process over an array of microspherical lenses has been studied theoretically and then optimized in order to obtain the nanostructures with controlled morphology. It was found that the positive resist thickness corresponding to the best achievable resolution in the subwavelength region is 250 nm in case of 1.5 μm silica spheres and excitation with 365 nm LED. The silica growth mask for selective epitaxy was fabricated. The ordered array of GaP nanowires was synthesized with MBE. Large scale ordering and selectivity of the growth technique is demonstrated.

Gallium phosphide is the semiconductor material having low lattice mismatch of 0.37% with silicon [1, 2]. Diluted solutions of this material with N and As provide direct band gap and ability to control its value in a wide range of 1.5-2 eV [3] and lattice-matching with Si which makes it a perfect candidate for integration of optical A3B5 components on Si platform. Lately it has been demonstrated experimentally that metastable wurtzite phase of GaP possesses the direct band gap [2]. One of the promising ways to stabilize the metastable phase is the growth of the nanostructures, in particular, the nanowires (NWs), i.e. wutzite GaAs and GaP.

Development of the semiconductor NWs growth methods is actual today due to intriguing possibilities of these nanostructures implementation as basic elements for nanophotonics and nanoelectronics [4-6]. Fast development of the nanostructure-based devices [7-9] for nanoelectronics stimulates the studies in the field of ordered nanostructures synthesis. Typical methods of the ordered NWs growth include fabrication of the patterned mask on the substrate surface. High potential of the NWs synthesis study is also dictated by several perspective applications [10].

In this work, we propose and study the sphere projection photolithography method allowing one to decrease the mask fabrication costs. Variation of the photolithography process parameters can be used to control diameter of the nanostructures, while period of the fabricated patterned resist layer can be controlled via variation of the sphere diameter and their arrangement.

The sphere photolithography uses the effect of the light focusing with the microspheres allowing one to fabricate the nanoscaled pattern in the photoresist. To obtain proper light focusing in order to fabricate the pattern having the smallest possible holes for further NWs growth we carried out the numerical modeling. The calculation of the light propagation based on solution of Maxwell equations was carried out with the use of CST Microwave Studio package.

The proposed approach considers fabrication of the growth mask on Si(111) substrate using silica. The deposition of the mask material is carried out prior to the deposition of the photoresist. The spheres are then deposited over the mask material and photolithography takes place leading to the formation of the pattern, which is then used for etching of the mask material. In this case the positive photoresist have to be preferentially used.

In order to obtain proper light focusing in our modeling we considered different photoresist layer thicknesses. The diameter of the silica spheres was set to 1.5 μm. The choice of the silica spheres diameter is dictated by the commercial availability. The arrangement of the spheres ordering was considered to be the hexagonal close-packing. The excitation wavelength was 365 nm corresponding to light-emitting diode used in the projection photolithography in our experiments. The optical parameters of the photoresist corresponded to particular AZ1505 resist nature.

Our aim was achievement of the smallest possible cross-section of the focused radiation in the photoresist layer providing the smallest diameter of the pattern holes. To reach this goal, we have calculated the effective diameter of the radiation at its thickest cross-section in the resist layer. The effective diameter corresponds to the full width at half maximum of the light intensity at the thickest cross-section. Figure 1 demonstrates the dependence of the radiation focusing on the photoresist thickness. It was found that the theoretical curve possess one deep minimum corresponding to the 250 nm resist layer thickness. Very thin layer leads to broadening of the focused radiation area, while larger thicknesses correspond both to elevation of the area and to ineffective light propagation into the resist further leading to the insufficient exposition and partial removal of the resist in the exposed areas after the development.

Dependence of the light focusing effective area diameter on the photoresist layer thickness.

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In the second (experimental) part of our work we have fabricated the selective area growth masks using the results of the numerical calculation discussed in the previous section. In our experiments (111) oriented silicon wafers were used as substrates. First, chemical treatment of the wafer with Shiraki cleaning procedure was carried out. With our first approach of silica mask growth we deposited a 20 nm thick SiO2 layer with plasma-enhanced chemical vapor deposition (PECVD) at 350oC using SiH4 and N2O as precursors. It was found experimentally that AZ1505 photoresist provides poor adhesion to the silica. In order to improve the adhesion, the nitrous oxide source was turned off during the last period of the mask layer deposition in order to obtain a few nanometers thick amorphous silicon layer. Next, the substrate was spin coated with the photoresist.

We then worked out the microspheres deposition to obtain the process parameters corresponding to dense monolayer arrays. In our study highly monodisperse 1.5 μm SiO2 spheres in 5% aqueous solution produced by Micro particles GmbH were used. The spin coating was performed in a wide range of the rotation speeds—250-3000 min-1 and process time between 10 and 20 s. The obtained optimum parameters of the process are 2000 min-1 and 10 s (Fig. 2a). In case of lower rotation speed the centrifugal force is insufficient to overcome the interaction between the microspheres in the multilayer arrays obtained after the deposition of the spheres solution. While at higher rotation speeds centrifugal force becomes larger than the van der Waals interaction between the spheres and the resist surface leading to formation of bald spots uncovered with the spheres (Fig. 2b).

SEM images of the microspheres deposited with spin-coating (a) highly ordered array obtained at 2000 min-1, (b) array obtained at 3000 min-1.

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After deposition of the spheres, the sample was exposed under 3W LED illumination with 365 nm wavelength for 25 s. The spheres were then removed and development of the resist with AZ MIF 726 developer solution took place. Image of the obtained resist pattern with 300 nm ordered holes is presented in Fig. 3a.

SEM images of the openings in the phototresist (a) and in the silica layer (b).

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On the next step, the silica mask layer was selectively etched with inductively coupled SF6 plasma through the obtained resist pattern. The resist was then removed. To insure the resist and organics removal we chemically treated the sample with NH4OH:H2O2 (1:1) boiling solution for 10 min. SEM image of the fabricated growth mask is presented in Fig. 3b.

The growth experiments were carried out in Veeco MBE GEN III machine. Prior to the substrate introduction in the molecular beam epitaxy (MBE) growth chamber, it was treated with the buffered HF solution to remove the native oxide in the mask openings and passivate the Si surface. The substrate was then annealed in UHV at 750°C for 10 minutes. The temperature was then lowered to 660oC and deposition of Ga layer with 0.6 ML effective thickness was carried out leading to the formation of Ga nanodroplets in the hollows of the fabricated SiO2 mask [11]. We then deposited Ga and P simultaneously and formation of the ordered array of the self-catalyzed [12] GaP NWs was observed.

SEM images of the fabricated ordered array of GaP NWs are presented in Fig. 4. We intentionally left part of the substrate surface covered with non-etched silica layer to demonstrate the selectivity of the growth technique that is clearly seen in Fig. 4a. We have also demonstrated the large scale ordering of the obtained arrays on mm2 area.

SEM images of the obtained ordered GaP NWs array.

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Close up SEM image of the NWs is presented in Fig. 4b. First, we note that Ga droplet is settling at the NW top facet corresponding to the self-catalyzed growth mechanism. NWs possess vertical orientation. Surprisingly, not all of the mask openings were the nucleation sites for the NWs growth. We assume that this effect relates to insufficient quantity of Ga deposited prior to the NWs growth. Nevertheless, GaP nanostructures of irregular shape were formed in the mask openings where the NWs do not nucleate. Potentially, at the lower growth times these nanoislands can adopt the disc shape of the mask opening.

Bottom sides of both NWs and nanoislands are laterally extended and their diameters surpass the diameter of the mask opening. To check whether this phenomenon is not related to the extension of the mask opening due to its etching with metallic Ga we selectively etched the NWs and studied the substrate surface with AFM. It was found, that no lateral extension of the openings happened. Conventionally, lateral extension of the nanostructures during MBE growth can be tailored with growth parameters variation, namely the growth temperature and fluxes, thus further improvement of the seeding layer deposition and the growth regime should be carried out aimed at nucleation of the NWs in every mask opening and suppression of their lateral extension. Potentially the developed technique provides new method for growth of different morphology nanostructures [13].

In this work we have studied the nanosphere photolithography as a tool for fast fabrication of the growth masks that can be used in the epitaxial synthesis for the large-scale ordered arrays of the nanostructures.

In the first part of our investigation we carried out the numerical modeling of the light propagation into the photoresist layer. It was found that with 1.5 μm silica spheres and 365 nm excitation wavelength the best resolution corresponds to 250 nm thick AZ1505 photoresist layer.

Using the proposed approach we have fabricated first, the resist pattern and second, the silica mask layer on Si(111) substrate. The obtained diameter of the mask openings was 300 nm demonstrating the subwavelength resolution of the method. No extension of the mask openings compare to the openings in the resist pattern was observed during the etching.

The ordered array of self-catalyzed vertical GaP NWs was synthesized with MBE. Large scale and low time consumption capabilities of the method are demonstrated. The technique has great potential for fabrication of the GaP(N,As)-based and other III-V material functional devices.

The article is published in the original.

A.M.M. and I.S.M. thanks for support of the MBE growth processess the government of the Russian Federation (grants 3.9796.2017/8.9 and 16.2593.2017/4.6). L.N.D. thanks for support of the numerical modeling the Russian Foundation for Basic Research (grant no. 18-32-00899). V.V.F. thanks for support of the AFM and SEM studies on nanostructure morphology and growth mask topography the Russian Science Foundation (grant no. 18-72-00219).