Amorphous thin films of Ge2Sb2Te5, sputter-deposited on a thin-film gold electrode, are investigated for the purpose of understanding the local electrical conductivity of recorded marks under the influence of focused laser beam. Being amorphous, the as-deposited chalcogenide films have negligible electrical conductivity. With the aid of a focused laser beam, however, we have written on these films micron-sized crystalline marks, ablated holes surrounded by crystalline rings, and other multi-ring structures containing both amorphous and crystalline zones. Within these structures, nano-scale regions of superior local conductivity have been mapped and probed using our high-resolution, high-sensitivity conductive-tip atomic force microscope (C-AFM). Scanning electron microscopy and energy-dispersive spectrometry have also been used to clarify the origins of high conductivity in and around the recorded marks. When the Ge2Sb2Te5 layer is sufficiently thin, and when laser crystallization/ablation is used to define long isolated crystalline stripes on the samples, we find the C-AFM-based method of extracting information from the recorded marks to be superior to other forms of microscopy for this particular class of materials. Given the tremendous potential of chalcogenides as the leading media candidates for high-density memories, local electrical characterization of marks recorded on as-deposited amorphous Ge2Sb2Te5 films provides useful information for furthering research and development efforts in this important area of modern technology.
© 2011 OSA
Optical and thermal properties of GeSbTe-based phase-change alloys have been used in the past two decades for write-once as well as rewritable optical disk data storage [1–16]. In more recent years, research and development efforts have been directed toward phase-change electronic memories, which exploit the reversible electrical properties of the same class of materials [17–30]. In addition, multi-level optical reflectivity as well as electrical conductivity of various GeSbTe materials have been investigated [31–33]. In the context of electronic memories, understanding and optimizing the material properties at nano-scale require tools and techniques that are sensitive to changes in the electrical conductivity of the material on the scale of individual crystalline grains, typically only a few nanometers in diameter. Conventional methods of atomic force microscopy (AFM), combined with scanning and transmission electron microscopy, provide valuable information about the morphology, crystal structure and orientation, and local composition of the materials. However, local electrical conductivity of the material on the nano-scale can only be monitored with scanning probe microscopy using a conductive tip [34–41].
In this paper, we report the results of electrical conductivity measurements of Ge2Sb2Te5 recorded marks under a broad range of laser irradiation conditions. The conductive-tip atomic force microscope (C-AFM) is used to explore the local electrical conductivity of various zones within the recorded marks, as well as their dependence on the recording conditions. Also discussed are the proportions of crystalline to amorphous material within a given region that give rise to different levels of electrical resistance.
Two sets of thin film stacks on glass substrates, one having the structure ZnS-SiO2(130nm)/Ge2Sb2Te5(10nm), the other being ZnS-SiO2(130nm)/Au(5nm)/Ge2Sb2Te5 (10nm), were fabricated using conventional magnetron sputtering equipment (Shibaura Co., Japan). The as-deposited Ge2Sb2Te5 (GST) films were in the amorphous state; hereinafter, we shall refer to this material as the recording layer. The ZnS-SiO2 layer, used in the phase-change optical disk industry as the standard under-layer (and also over-layer), is valued for its optical and thermal properties, as well as for its protection of the GST film from undesirable environmental effects. We use the gold (Au) film primarily as an electrically conductive electrode, placing it in direct contact with the GST film. Despite its extreme thinness, the high-thermal-conductivity gold layer will influence, perhaps to a small extent, the formation and properties of the marks written onto the adjacent GST layer. Of the two aforementioned stacks, the one without a gold layer will be our benchmark sample, used to demonstrate that no electrical signals can be detected in the absence of the gold electrode. The sample with 5nm of gold directly beneath the 10 nm GST layer will be seen to be the one that yields high-contrast C-AFM images of the recorded marks. We mention in passing that we have also experimented with thicker GST films (50 nm and 20 nm) in conjunction with both thin and thick gold films; however, the C-AFM images of recorded marks on these thicker samples were unsatisfactory in all cases. We are thus lead to believe that the combination of a 10nm Ge2Sb2Te5 film and a 5nm gold electrode is nearly ideal for such studies as reported in the present paper.
The laser irradiation of the as-deposited amorphous GST films was carried out using an optical pump-probe system (Static Media Tester, TOPTICA Co., Munich, Germany). This system focuses a red laser beam (λ o = 658 nm) through a high-numerical-aperture objective lens (NA = 0.65) onto the GST film that is the top layer of our sample. The laser power was controlled in the range from 2.0 mW to 20 mW, while the laser pulse duration could be varied from 100 ns to 1500 ns. When the local temperature of the amorphous GST film exceeds its crystallization threshold, which is estimated to be around 400°C for pulses lasting tens or hundreds of nanoseconds, the “rapidly annealed” material under the focused laser beam becomes polycrystalline. (The melting point of Ge2Sb2Te5 is just above 600°C).
We examined the morphology as well as the electrical conductivity of recorded marks simultaneously using C-AFM (MFP-3DTM, Asylum Research). The C-AFM cantilever probe is coated with PtIr5 and is connected to the virtual ground, as shown in Fig. 1 . A 50 mV bias voltage was applied to the gold film through the application of silver paste and conductive carbon tape to a region of the sample that was about 10 mm away from the area on which various laser-written marks were located.
We also used a scanning electron microscope (SEM, S-4800, Hitachi) equipped with an energy dispersive spectrometer (EDS) to analyze the morphology of the recorded marks as well as the composition ratio of Au within and in the immediate neighborhood of these marks.
3. Result and discussion
Figure 2(a) shows a photo-micrograph of an array of laser-recorded marks on the benchmark sample. The various laser powers (2.0 mW-20 mW) and pulse durations (100 ns-1500 ns) used in writing these marks are indicated on the vertical and horizontal axes, respectively. It is seen that, at higher laser powers and/or longer pulse durations, the center of the mark will be ablated. The edge of each sample is covered with silver paste, forming the contact pad for the battery during C-AFM measurements. Figure 2(b) is an AFM image of marks recorded on the benchmark sample. The range of laser powers and pulse durations used in this case is 6 mW-20 mW and 100 ns-1500 ns, respectively. The ablated holes are seen to be ~10 nm deep, which is the thickness of the GST layer. These holes are surrounded by a ring whose height is nearly 20 nm above the surface of the sample. Figure 2(c) shows the C-AFM image of the same region of the sample as depicted in Fig. 2(b), with the current gain factor set to 20 nA/V. Hardly any current flows through the sample in this case and, therefore, no images of the recorded marks have been captured.
A close-up view of the mark recorded on the benchmark sample with an 18mW-1300ns laser pulse is shown in Fig. 3 . The AFM image in Fig. 3(a) and its corresponding cross-sectional view show the raised boundary, the ablated ring immediately inside the raised boundary, and accumulated debris near the center of the mark. The C-AFM image of the same mark shown in Fig. 3(b) is obtained with the current gain factor set to 20 nA/V. A weak current (peak value ~0.7nA) is seen to flow through the boundary region of the recorded mark.
Figure 3(c) depicts a mark recorded with a 4.0 mW-1300 ns laser pulse. There is no ablation in this case, and the center of the mark is seen to have risen by about 6.0 nm above the surface of the sample, while the region surrounding the central bump is depressed by about 1.0nm. As before, the C-AFM signal shown in Fig. 3(d), with its extremely weak electric current peaking at ~0.5 nA, is probably due to noise and cross-talk.
The AFM image of recorded marks on a sample of glass_substrate/ZnS-SiO2 (130 nm)/ Au (5 nm)/GST (10 nm) with different laser powers and pulse durations (6-20 mW, 100-1500 ns) is shown in Fig. 4(a) . As before, the marks recorded with a high laser power and/or with long pulses are ablated at the center and exhibit a raised ring at their periphery. The C-AFM image of the same region of the sample, monitored with a current gain of 2.0 nA/V, is shown in Fig. 4(b). The recorded marks are now clearly visible in the C-AFM image, especially within the ring regions of the high-power/long-pulse marks.
The AFM close-up image of a mark recorded with an 18 mW-1300 ns laser pulse, depicted in Fig. 4(c), shows that the ablated region is nearly 15 nm deep, that is, both GST and Au have evaporated out of this region. The bright spot at the center of the ablated pit is probably a mixture of GST and Au, but because it is disconnected from the rest of the gold film, it cannot produce any C-AFM signal. The raised boundary of the mark is 15 nm above the surface on the left edge of the ablated hole, and 10 nm on the right edge. The C-AFM image of this mark and its corresponding cross-sectional profile, shown in Fig. 4(d), were obtained with the current gain factor set to 2.0 nA/V. Certain spots within the raised boundary of the mark conduct electricity, with a current that is as large as 10 nA in some regions. Two possibilities exist for the occurrence of electrical conductivity within the raised ring: (i) the GST film has crystallized and the crystalline grains extend through the thickness of the ring and make contact with the underlying gold film; (ii) ablated gold from the interior regions of the mark is mixed in with GST, producing in certain spots an electrical pathway from the top of the ring to the underlying gold electrode.
Figure 4(e) is the AFM image and its corresponding cross-sectional profile for the mark recorded with a 6 mW-1300 ns laser pulse within the 10 nm-thick GST film that is in direct contact with the 5 nm-thick gold electrode. The image indicates that the mark consists of a raised core and a slightly depressed boundary. No ablation has taken place, and we suspect the gold film to have remained intact. In all likelihood, the raised core of the mark in the GST layer is detached from the underlying gold film. The slightly depressed, darker ring surrounding the core of the mark is expected to be partially crystalline. The C-AFM image of this mark, shown together with its cross-sectional profile in Fig. 4(f), were obtained with the current gain factor set to 2.0nA/V. Only the boundary of the recorded mark is seen to be electrically conductive at several spots, with the current being as large as 2.0 nA in some regions. These spots are expected to be crystallites that span the thickness of the GST film and make contact with the Au layer.
Figure 5(a) is a photo-micrograph of marks recorded with different laser powers and pulse durations (2-20 mW, 100-1500 ns) on the 10 nm-thick GST film deposited atop a 5nm gold layer. Also recorded on this sample – in the continuous illumination mode – are parallel straight lines written with a cw laser power of 10 mW and a stage velocity of 100 μm/s. The ablated straight lines are made long enough to contact the silver paste, which makes contact with the gold underlayer and creates a return path for the electric current. The gold has been evaporated from the bottom of the ablated grooves. A scanning electron microscope (SEM) image of the same sample is shown in Fig. 5(b). Marks written at low laser power and with short pulse durations are hardly visible in either image.
Figure 5(c) shows an AFM image of recorded marks as well as straight lines in a region of the sample also depicted in Fig. 5(a). The ranges of the laser pulse power and duration are 6.0-20 mW and 100-1500 ns, respectively. Note that the boundaries of the ablated marks as well as those of the straight lines are raised above the background surface by several nanometers. The C-AFM image of the same region of the sample, acquired with the current gain factor set to 1.0 nA/V, is shown in Fig. 5(d). The background (white area) is poorly conductive (current ~65 pA), whereas the boundaries of the ablated lines are highly conductive (current ~10 nA), as are those of the ablated marks recorded at high power and/or with long pulses. Also conductive are marks recorded at low laser power with short laser pulses. Marks that are nearly impossible to find in the optical, AFM, and SEM images, are clearly visible in this C-AFM image.
Figure 6(a) is the close-up AFM image and its corresponding cross-sectional profile of a mark recorded with an 18.0 mW-1300 ns laser pulse on an isolated stripe created by the laser-ablated straight lines depicted in Fig. 5. The boundary of the mark is seen to have risen by as much as 10 nm above the background surface. The interior of the mark is ablated, with both the GST and the gold layers removed from the top of the dielectric under-layer. In the C-AFM image of the same mark shown in Fig. 6(b), a fairly strong electric current flows through the raised ring surrounding the ablated hole – the current is as large as 9 nA in some regions. Note that the background material, which lacked electrical conductivity in previous examples, is now showing spotty conductivity in what appears to be random locations. The striping of the GST film in the present case must have created alternative paths for current flow which were not previously available.
Figure 6(c) is the AFM image of a mark recorded with a 4.0 mW-1300 ns laser pulse on an isolated stripe created by the laser-ablated straight lines depicted in Fig. 5. The mark is depressed by about 1.5 nm below the surface, a sign that the recorded mark is crystalline. The C-AFM image in Fig. 6(d) shows a spotty current profile in the depressed (crystalline) region and its immediate surroundings, with a current that is often as high as 10 nA. In the optical, SEM, and AFM images, this mark is barely visible, but the C-AFM image is clear and has a good contrast. Note on the right-hand side of Fig. 6(d) how the boundary of the straight-line adjacent to the mark is coming into view.
In Fig. 6(e), the mark recorded with a 4.0mW-300ns laser pulse on an isolated stripe of the 10 nm-thick GST film is not visible in the AFM image, but can be seen clearly as an aggregate of dark spots in the C-AFM image depicted in Fig. 6(f). Similarly, the mark recorded with a 6.0 mW-100 ns laser pulse on the same (striped) GST film is not visible in the AFM image of Fig. 6(g), but can be recognized, albeit faintly, in the collection of dark spots in the C-AFM image of Fig. 6(h).
Figure 7 shows C-AFM images of four recorded marks whose boundaries have been demarcated by red lines. Conductivities of the regions recorded under different laser powers and pulse durations may be divided into four categories, which include the cases of 6 mW-100 ns pulse, 4 mW-300 ns pulse, 4 mW-1300 ns pulse, and 18 mW-1300 ns pulse. The measured values of the average current in each case are listed in Table 1 .
Figure 8 shows SEM images of three marks and a section from a straight line, all recorded on a 10 nm-thick GST film in contact with a 5 nm-thick gold electrode. From top to bottom, the laser pulse power/duration used to record these features were: (a) 18 mW-1300 ns, (b) 4.0 mW-1300 ns, (c) 6.0 mW-100 ns, and (d) 10 mW focused cw beam scanned at 100 μm/s. Localized EDS analysis revealed the composition ratio of Au in different regions of the sample in each case. In Fig. 8, the various probing locations of the EDS are marked on the SEM image, and the corresponding Au composition ratios thus obtained are listed in a table on the right-hand side of each micrograph.
In the case of the ablated mark of Fig. 8(a), the background sample (region 1) shows the atomic percentage of Au at 0.69%. Considering that the gold layer in this region is buried beneath the GST film, this number indicates the fraction of gold that is visible to the micro-probe upon penetrating through the GST layer. Regions 2 and 3 contain nearly four times as much gold as the background region, an indication that the gold at the center of the mark has ablated out of the hole and deposited over the GST ring, or perhaps mixed in with the GST material in this region. In region 4, the boundary zone near the ablated hole, gold concentration is low, and inside the hole there is no gold at all. High levels of electrical conductivity in regions 2 and 3 may thus be associated with an abundance of gold in these areas. This observation may explain, for example, the high electrical conductivity of the raised ring in Fig. 4(d) and also that in Fig. 6(b), which, from previous studies , we know to be mostly amorphous.
The case of Fig. 8(b), corresponding to a mark recorded with a 4.0 mW-1300 ns pulse is somewhat puzzling. From Fig. 6(c), we expect the mark to be a small crystalline island, yet Fig. 8(b) indicates that there is nearly 3 times as much gold in the outer boundary of the mark as there is in the background region, and nearly 5 times as much in the core region. Perhaps the presence of large crystallites in the core and in some exterior regions of this crystalline island facilitates the passage of the electron probe beam through the GST material, causing it to register a larger signal from the buried Au layer than it would have seen otherwise. There is also the possibility that some of the Au may have diffused into the crystalline GST mark and migrated toward the surface during the relatively long (1300 ns) annealing process. In any event, it is unlikely that the large conductivities observed in Fig. 6(d) are primarily gold-related. Rather, the main reason for the observed conductivity of this mark should be the presence of a connected path through the thickness of the GST layer, formed by one or more GST crystallites.
In Fig. 8(c), the mark recorded with a 6.0 mW-100 ns pulse is more or less uniform in its Au content, indicating that the gold electrode has not been altered in any significant way by the laser-marking process. The C-AFM image of this mark, shown in Fig. 6(h), is thus due to crystalline GST grains forming connected paths through the thickness of the GST film.
Finally, Fig. 8(d) shows the violent breakup of both the GST and gold layers in the process of writing the straight lines that create isolated stripes within the GST layer. Since the gold particles formed by laser ablation do not produce a continuous path along the length of the line, it is tempting to conclude that the high conductivity responsible for the C-AFM images of the lines in Fig. 5(d) is rooted in a network of GST crystallites formed at the edges of the ablated lines. The SEM and C-AFM images, of course, share common information, as they are both dependent on the conductivity of the sample. The SEM images of Fig. 8 (and the Au concentrations measured therein) indicate better conductivities where Au concentration is higher; see the white areas in these SEM images. We also note that the straight lines in Fig. 5(d) are discontinuous, a fact that is better appreciated when the images are enlarged. It is therefore possible that both increased Au concentration and partial crystallization of the GST material play roles in the observed results. In any event, the high-contrast images of the lines depicted in Fig. 5(d) require electrical connectivity only from the top of the GST film to the gold underlayer, which could be provided either by an excess of gold concentration or by a network of GST crystallites in the direction perpendicular to the plane of the sample.
In summary, the conductivity of GST thin films recorded under various illumination conditions were investigated using C-AFM. We obtained high-contrast images of laser-written marks on a 10nm-thick film of amorphous GST that has been sputter-deposited on a 5nm-thick gold film acting as a second electrode for examining the conductivity of the layered structure. The image quality and signal-to-noise ratio improved when isolated, 10 μm-wide stripes were created on the GST film by laser ablation of parallel straight lines on the sample. The reasons for this improvement are not entirely clear, but we suspect the laser annealing of the GST stripes to have initiated the formation of crystalline nuclei, which subsequently facilitate the formation and growth of crystalline networks within the thickness of the GST layer. By and large, the observed correlation between laser illumination conditions and the conductivities of various types of GST marks may be attributed to the differing partial crystallization densities of these marks. The results of this work should be helpful in understanding the interaction between phase-change materials and the applied thermal energy fluence, and may find applications in modern optical and electronic memory technologies.
The authors acknowledge with gratitude the financial support for their research program from the National Science Council under grant numbers 98-2120-M-002-004-, 97-2112-M-002-023-MY2, 96-2923-M-002-002-MY3, 99-2811-M-002-003, 98-EC-17-A-09-S1-019, 99-2120-M-002-012 and 99-2911-I-002-127.
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