Spontaneous formation of nanospiked microstructures in germanium by femtosecond laser irradiation

We report a novel phenomenon of spontaneous formation of nearly regular arrays of nanospikes atop conical microstructures by exposing a germanium surface to femtosecond laser pulses in an environment of SF6. Silicon laser texturing has been reported, but no information has been published on laser microtexturing of Ge and in particular the observation of nanospikes atop conical microstructures. The nanospikes are around 2 μm high having a tip radius of 100 nm and are formed atop conical microstructures that are around 5 μm wide and 10 μm tall. The tip radius could be sharpened down to ∼10 nm by brief chemical etching. A higher laser fluence and fewer shots favour the formation of nanospikes. By increasing the number of shots at higher fluence the surface morphology changes from conical microstructures to tall straight-walled pillar-like structures and the nanospikes disappear. The surface morphology of germanium has been compared with silicon. The germanium samples turn completely black after laser processing, i.e. they exhibit greatly reduced reflectivity throughout the visible spectrum. This paper reports a novel phenomenon of spontaneous formation of nanospiked nearly periodic conical microstructures by exposing a germanium surface to femtosecond laser pulses in an environment of sulfur hexafluoride (SF6). While microstructuring of silicon by laser etching has been extensively reported [1–8] over the past decade, to our knowledge, there has been no report on laser etching of germanium and in particular the observation of nanospikes atop nearly regular conical microstructures. The conical microstructure that is formed in germanium is different in terms of aspect ratio and nanospike formation compared to silicon. Since reducing the light reflection and increasing the absorption is a key issue in improving optoelectronic device performance, structure formation of this type in germanium could lead to the fabrication of highly responsive infrared photodetectors and/or solar cells. In addition, these textured surfaces might find other potential applications in the fabrication of biomedical devices and sensors. Undoped Ge(100) wafers are cleaved into small chips and ultrasonically cleaned with acetone and methanol. One 3 Author to whom any correspondence should be addressed. or more chips is put on a stage inside a vacuum chamber (with base pressure ∼ 1 mbar) mounted on a high precision computer controlled X–Y stage. The chamber is rinsed with SF6 at least twice and then backfilled with 400 mbar SF6. The samples are exposed to 1.4 mJ pulses of 800 nm wavelength and 130 fs pulse duration at a repetition rate of 1 kHz from a regeneratively amplified Spectra Physics Ti–sapphire laser system. The laser beam is focused along the normal onto the sample surface by a 1 m focal length coated lens and the laser fluence is adjusted by using a Glan calcite polarizer. The spatial profile of the laser pulse is nearly Gaussian, though elongated in one axis compared to the other creating an elliptical profile, and the fluence is calculated using the spot size determined by exposing a point on the sample surface to thousands of shots. In order to scan an area bigger than the laser spot size, the samples are translated using a motorized X–Y stage. Scanning also assists to make more uniform surface structures by smoothing out any shot-to-shot irregularities in the beam profile. By varying the scanning speed of the X–Y stage, the number of laser pulses impinging on the sample 0957-4484/07/195302+04$30.00 1 © 2007 IOP Publishing Ltd Printed in the UK Nanotechnology 18 (2007) 195302 B K Nayak et al Figure 1. (a) SEM image of conical microstructures on undoped Ge(100) surface produced by 400 laser pulses of 130 fs duration, 0.6 J cm−2 fluence in SF6 at a pressure of 400 mbar viewed 45◦ from the surface normal. (b) Higher magnification image of (a) showing nanospikes formed on each Ge conical microstructure. (c) A single nanospike of as prepared sample with tip radius 100 nm and height ∼1.5 μm. (d) A nanotip formed after brief chemical etching. surface at a particular spot is controlled. The spot size is 0.3 mm along the minor axis and 0.6 mm along the major axis. Scanning is performed parallel to the minor axis. Samples are produced either with isolated single line scans or with large areas created by overlapping several line scans. The step size between scan lines is chosen to be sufficiently small (generally 0.38 mm) such that successive lines overlap substantially. This further improves texture homogeneity. Homogeneity is further enhanced by performing two overlapping scans in orthogonal directions rather than one overlapping scan with an exposure of the same total number of shots. After laser processing, the samples are analysed with a scanning electron microscope (Zeiss SUPRA 40). Figure 1(a) shows an SEM image of a Ge surface after exposure to femtosecond laser pulses viewed at an angle of 45◦ from the normal. These sharp conical microstructures with nanospikes are formed when the sample is scanned under the laser beam (fluence ∼ 0.6 J cm−2) in a pressure of 400 mbar SF6. As is evident from the image, these structures are almost regular in position, 10–15 μm tall, and have a base diameter of around 5 μm tapering down to ∼100 nm near the tip. The areal density of pillars in figure 1(a) is 0.027 μm−2, which corresponds to a mean spacing of roughly 6 μm between pillars. The sizes of the conical microstructures vary across the scanned line, indicating a response to the spatial profile of the laser pulse. In the region of low fluence, towards the edge of the irradiated line, the structures are of smaller height and are more densely packed than in the centre. From figure 1(b), taken at higher magnification, it is clear that these structures have two distinct features: (a) a conical shape (b) crowned with a very sharp nanospike of radius ∼ 400 nm (∼100 nm at the tip) and up to ∼2 μm in length (see figure 1(c)). The tips can be sharpened further by a brief Figure 2. Comparison of Ge(100) surfaces irradiated in the presence of SF6 at 400 mbar with average laser shots of 400 at fluences of (a) 0.66 J cm−2 (b) 0.56 J cm−2 (c) 0.3 J cm−2 and (d) 0.2 J cm−2 viewed at an angle 45◦ from the surface normal. The samples are created by single line scans. chemical etching (100 ml H2O2 (10 vol%), 8 g NaOH) for 10 s at room temperature. Nanoclusters, formed during laser ablation and deposited on the surface, are also evident from figure 1(b). Figure 2 shows the effect of laser fluence on the surface texturing of germanium keeping the average number of laser shots at each point on the sample surface to 400. It is clear from the picture that texturing is observed at fluences as low as 0.2 J cm−2 (figure 2(d)). However, for higher fluence conditions (figures 2(a) and (b)) the microstructures are well developed, sharper, more conical, and in addition, nanospike formation takes place atop the conical microstructures. It is interesting to note that nanospikes are formed atop almost all of the conical microstructures (figure 1(b)). This nanospike formation is observed to be dependent on the laser fluence and number of shots. Increasing the laser shots from 400 to 600 and keeping the fluence almost the same wipes out all of the nanospikes (figures 4(a) and (b)). On the other hand, by decreasing the fluence from 0.6 to 0.3 J cm−2 or 0.2 J cm−2, we still observe surface texturing, but the features are smaller in width and height, less regular, less smooth and densely populated with very tiny or no nanospike formation (figure 2(c) or (d)). Ultrafast laser induced structure formation for Si in an SF6 atmosphere is a mixed process of laser ablation and chemical reactions at the surface [2, 5]. We anticipate that an analogous mechanism holds for restructuring of germanium. Recently, Mills and Kolasinski observed nanospikes with lengths of several micrometres forming atop silicon pillars only when SF6 was diluted with He in a 1:4 ratio [9]. They proposed the nanospike formation as a solidification driven extrusion taking place with laser melting of silicon and suggested that any material that expands upon freezing should exhibit nanoscale spikes provided that the material is first shaped into hillocks. Germanium also expands upon freezing. These authors suggested that for Si, SF6 needed to be diluted with He because this reduces the chemical etching that results from the interaction of fluoride with the Si surface. If etching is too