This paper demonstrates a laser direct-write method to form single crystal semiconductor ferroelectric SbSI features on chalcogenide glasses for integration into infrared devices. The method overcomes a major limitation of thin-film deposition techniques, viz. the uncontrolled stoichiometry of SbSI due to very different vapor pressure of its constituents. It promises advantages of selective single-crystal formation and control on the morphology of the crystal. Mechanism of and control parameters for laser crystallization are explored.
© 2011 OSA
Ferroelectric semiconductor antimony sulpho-iodide (SbSI) chalcogenide  shows excellent piezoelectric, photoelectric, pyroelectric and pyro-optic properties [1–3] and is thus a promising material system for applications in infrared detectors, actuators, memory [4,5], etc.
Thus far, thin films of SbSI have been prepared by e-beam evaporation, flash evaporation, thermal evaporation, physical vapor transport and pulsed laser deposition [6–8]. Gerzanich et al.  have reviewed in detail the preparation of bulk and thin film SbSI. Typically, thin film deposition techniques result in amorphous films that may be heat-treated to form polycrystalline films. Unfortunately, the usefulness of such films has been limited by their chemical inhomogeneity and variable orientation. The growth of good quality SbSI thin films has remained a challenge for two reasons: (i) its relatively inflexible high growth rate along the c-axis  (50 times larger than in a- and b-directions), and (ii) large differences in the vapor pressure of antimony, sulfur and iodine.
For integrated electronic and optical devices such as infrared detector arrays or ferroelectric memory, simply the formation of stoichiometric polycrystalline SbSI is not sufficient. A precise control on crystal orientation and growth parameters and ability to selectively form SbSI single crystals at desired location are also crucial. For example, making an uncooled pyroelectric detector  from SbSI would require a large array of their nanometer size single crystal pixels integrated into CMOS circuitry.
In the present article, we describe a laser direct-write method for controlled single crystal formation of glass in pre-selected regions of the sample. Honma et al. and the present authors [11–15] have demonstrated single crystal formation in various glasses via CW laser irradiation. Their method requires fine tuning of various irradiation parameters including laser power, laser scanning speed, laser spot diameter and the focus of the laser spot. In addition to optimizing laser parameters, right glass composition needs to be chosen that precipitates only the desired crystalline phase.
2. Experimental procedure
Four glass compositions Sb37S43.3I19.7, Sb35.7S39.3I25, 10GeS2.90SbSI (Ge10) and 20GeS2.80SbSI (Ge20) [16,17] were prepared directly from elements Ge, Sb, S, and I (>5N purity). An elemental powder mixture totaling about 5-8 g was sealed in an evacuated 12 mm inner diameter and 4” long quartz tube (ampoule) under 10−2 torr (1.3 Pa) pressure. To prevent explosion, ampoules were slowly heated at 1-2 °C rate to 200 °C, 350 °C and 550 °C and kept for 1 hrs each and then heated to final step at 700 °C for 12 hrs. Next, the ampoules were quenched in cold water to form glass and immediately transferred to boiling water to relieve stresses. Boiling water was used for annealing instead of heated air, since the former transferred heat to ampoule more effectively than the latter and thus, prevented cracking of the chalcogenide glass sample. Ampoules were slowly allowed to cool to room temperature before being cut open to obtain solid glass pieces that were sliced into 1–2 mm thin discs. Sample discs were polished using corundum powder to an optical finish.
2.1 Setup for laser crystallization
The experimental setup for laser writing was assembled in-house. It consisted of two main parts ( Fig. 1 ): (1) a laser irradiation assembly and (2) an optical microscopy assembly. The former consisted of a CW 488 nm Ar laser source, a focusing lens (Numerical Aperture 0.75 and 0.3), and a translation stage. The chosen laser wavelength was efficiently absorbed by the glass, thereby enabling localized heating. The objective lens focused the laser beam on the sample while the translation stage moved the sample relative to the laser beam for creating tailored architectures on the glass sample. The stage moved in fine steps that were kept to the minimum allowed value of 0.1 µm for a smooth translation. To vary the speed of translation, time per step was changed. The crystal laser writing process was monitored by an optical transmission microscope assembly which included polarization control.
3. Results and discussion
3.1 Selection of glass composition
For forming stoichiometric SbSI, it is important to choose a composition that satisfies following conditions: (1) forms into glass easily, (2) crystallizes only into SbSI phase and (3) has sufficient strength for normal handling. Since stoichiometric SbSI composition requires impractical high cooling rates >200 °C per second  and other compositions from Sb-S-I ternary system such as Sb37S43.3I19.7 and Sb35.7S39.3I25 precipitate multiple crystalline phases as evident from Differential Scanning Calorimetric (DSC) curves in Fig. 2 ., compositions containing GeS2 and SbSI (viz. Ge10 and Ge20) were tried that are known to precipitate only the SbSI phase . The DSC data for Ge10 and Ge20 (Fig. 2) shows only one crystalline peak. To identify this crystalline phase, X-Ray Diffraction (XRD) was performed on a fully laser crystallized surface of Ge10 sample (Fig. 3 ). The XRD pattern from sample is in excellent agreement with the standard powder diffraction pattern of SbSI (JCPDS PDF # 00-021-0050), shown superimposed on the sample pattern.
Note that an exothermic hump indicated by ‘*’ on the Ge10 DSC curve indicates the presence of an additional crystalline phase. This phase appears to be a precursor phase for the formation of SbSI crystalline phase, since a crystalline phase in a region few microns away from the laser spot has been observed during laser crystallization that disappears when laser is focused directly on it.
In addition to ensuring SbSI crystalline phase formation, GeS2 in the starting glass composition provides a way to influence the crystallization kinetics. The glass transition and crystallization temperature increase with GeS2 addition, presumably because increased connectivity of glass network increases its viscosity.
Based on above observations, Ge10 and Ge20 samples were found suitable for laser crystallization experiments. Since the purpose of GeS2 was only to impart glass forming ability, composition with lesser GeS2 viz. Ge10 was chosen for detailed investigations.
3.2 Mechanism of crystallization
A mechanism of laser-induced crystallization may be proposed for the present chalcogenide system, which is based on the observations made previously with oxide glasses [13,14], since laser induced temperature rise appears to be the underlying cause in both cases. Ge10 and Ge20 glasses show strong absorption of 488 nm light due to electronic transition from the valence band to conduction band . The excited electron then relaxes non-radiatively and leads to the heating of the material, which is sufficient to cause crystallization.
To make crystalline lines, the laser is focused on a spot, which readily transforms glass to a polycrystalline region. Appearance of facets in this region confirms the formation of crystallites. They serve as seeds for drawing crystal lines. Next, the laser is moved relative to the sample, away from the polycrystalline spot. The motion of the laser-heated spot creates a temperature gradient on the sample. When the scanning speed is kept in the range of typical crystal growth speeds (tens of micrometers per second), the temperature gradient induces one or several of the crystals in the original polycrystalline spot to grow in the direction of the motion of laser beam or at an angle to it. The laser melts the glass at the front end of the spot and the crystal grows at the tail-end of the viscous melt. The growth of SbSI crystal is very rapid (e.g. at a growth rate of 100 μm/s at 10 mW power and 0.3 numerical aperture objective). Bigger laser spot and higher laser power result in heating a bigger region and thus, more crystallites form and follow the laser spot as multiple crystals.
3.3 Single crystal formation
Figure 4 shows polarized optical micrograph of 100 µm long lines ‘written’ at varying speeds ranging from 20 to 100 µm/s, laser power of 10 mW and 0.3 numerical aperture of the objective. The lines are estimated to be 1-2 micron in thickness. The right side of Fig. 4 shows Kikuchi diffraction patterns from different spots on a line identified by spot numbers.
Figure 5 is a scanning electron micrograph of a line formed using 1 mW laser power, 0.75 numerical aperture objective and scanning speed of 10 µm/s. The inset of Fig. 5 shows Kikuchi diffraction patterns using EBSD from a number of spots on the line and glass as indicated in the diagram. As expected, no bands appear for the glassy region while the clear EBSD diffraction (Kikuchi) patterns are found for the lines (Fig. 4 and Fig. 5), demonstrating their crystalline nature. Through the geometry of the bands, i.e. the spacing and the angles, the orientation of the crystallites can be deduced. For example, diffraction pattern from a polycrystalline line in Fig. 4 shows variation in the Kikuchi patterns because of different orientation of crystallites. In contrast, all the diffraction patterns from the line in Fig. 5 are identical indicating that the whole line is a single crystal.
Ge10 glasses are found to be very sensitive to the focus point of the laser with reference to the surface. Single crystal is formed only when the focal point is very close to the surface. All attempts to grow crystals deeper than a couple of microns into the bulk resulted in ablated channels. When the same beam was focused much deeper into the glass, no change was observed. This complex dependence of crystallization on the position of focal point can be a result of the temperature profile that is determined by laser absorption and thermal dissipation, and various photo-thermal and photo-structural processes. For example, chalcogenide glasses often exhibit photo-induced expansion/contraction, and in the present composition Ge must diffuse out to allow the formation of SbSI crystal, before the material starts ablating.
The maximum length of the line that can be drawn as a single crystal depends on the surface flatness of the sample. Undulations of the surface affected the focal point of the laser spot and led to either ablation or discontinuation of growth. Polycrystalline lines are more tolerant to surface undulations and consequently, millimeter long polycrystalline features can be made easily. On the other hand, the longest single crystal line is only 15 micron long, as it stopped growing from slight changes in the position of the focal point of the laser. It should be noted that in principle indefinitely long single crystal lines and other architectures can be made, if better control of laser and sample parameters is achieved, as already demonstrated for oxide glasses .
In our setup, the presence of the crystallites is detected visually from the transmitted light image, and as a result crystals are not detected until they grow to the size of a few microns. In other words, the current setup cannot ascertain the beginning of crystallization that would have allowed determining the size of the smallest crystal made using the technique. Future development of the technique will include a simultaneous detection technique for the formation of crystal.
Semiconducting, ferroelectric SbSI single crystalline features, which are useful for infrared integrated optical devices, have been grown on a chalcogenide glass via direct laser writing. EBSD and XRD results confirm that the laser written lines are stoichiometric single crystals. The glass composition and laser parameters may be further optimized to allow for a more precise control required to fabricate optical circuits and active structures on glass.
The authors thank the Basic Energy Sciences Division, Department of Energy for supporting this project (DE-FG02-10ER46698) and Prof. Yong Choi, Ms. Lihua Ding and Dr. Gregory Stone for useful discussion and advice.
References and links
1. E. I. Gerzanich, V. A. Lyakhovitskaya, V. M. Fridkin, and B. A. Popovkin, in Current Topics in Materials Science, E. Kaldis, ed. (North-Holland, Amsterdam 10, 55–190, (1982)).
2. L. E. Cross, A. S. Bhalla, F. Ainger, and D. Damjanovic, “Pyro-optic detector and imager,” U. S. Patent 4994672, Feb. 19, 1991.
3. M. Nowak, P. Szperlich, A. Kidawa, M. Kepinska, P. Gorczycki, and B. Kauch, “Optical and photoelectrical properties of SbSI,” Proc. SPIE 5136, 172–177 (2003). [CrossRef]
4. S. Surthi, S. Kotru, and R. K. Pandey, “SbSI films for ferroelectric memory applications,” Integr. Ferroelectr. 48(1), 263–269 (2002). [CrossRef]
5. M. Nowak, P. Mroczek, P. Duka, A. Kidawa, P. Szperlich, A. Grabowski, J. Szala, and G. Moskal, “Using of textured polycrystalline SbSI in actuators’,” Sens. Actuators A Phys. 150(2), 251–256 (2009). [CrossRef]
6. A. Mansingh and T. Sudersena Rao, “Growth and characterization of flash-evaporated sulphoiodide thin films,” J. Appl. Phys. 58(9), 3530–3535 (1985). [CrossRef]
7. P. K. Ghosh, A. S. Bhalla, and L. E. Cross, “Preparation and electrical properties of thin films of antimony sulphur iodide (SbSI),” Ferroelectrics 51(1), 29–33 (1983). [CrossRef]
8. S. Surthi, S. Kotru, and R. K. Pandey, “Preparation and electrical properties of ferroelectric SbSI films by pulsed laser deposition,” J. Mater. Sci. Lett. 22(8), 591–593 (2003). [CrossRef]
9. K. Nassau, J. W. Shiever, and M. Kowalchik, “The growth of large SbSI crystals: control of needle morphology,” J. Cryst. Growth 7(2), 237–245 (1970). [CrossRef]
10. P. Muralt, “Micromachined infrared detectors based on pyroelectric thin films,” Rep. Prog. Phys. 64(10), 1339–1388 (2001). [CrossRef]
11. T. Honma, “Laser-induced crystal growth of nonlinear optical crystal on glass surface,” J. Ceram. Soc. Jpn. 118(1374), 71–76 (2010). [CrossRef]
13. P. Gupta, H. Jain, D. B. Williams, T. Honma, Y. Benino, and T. Komatsu, “Creation of ferroelectric, single-crystal architecture in Sm0.5La0.5BGeO5 glass,” J. Am. Ceram. Soc. 91(1), 110–114 (2008). [CrossRef]
14. A. Stone, M. Sakakura, Y. Shimotsuma, G. Stone, P. Gupta, K. Miura, K. Hirao, V. Dierolf, and H. Jain, “Directionally controlled 3D ferroelectric single crystal growth in LaBGeO5 glass by femtosecond laser irradiation,” Opt. Express 17(25), 23284–23289 (2009). [CrossRef] [PubMed]
15. P. Gupta, H. Jain, D. B. Williams, J. Toulouse, and I. Veltchev, “Creation of tailored features by laser heating of Nd0.2La0.8BGeO5 glass,” Opt. Mater. 29(4), 355–359 (2006). [CrossRef]
16. L. Ding, D. Zhao, H. Jain, Y. Xu, S. Wang, and G. Chen, “Structure of GeS2-SbSI glasses by Raman spectroscopy,” J. Am. Ceram. Soc. 93(10), 2932–2934 (2010). [CrossRef]
17. V. M. Rubish, M. Y. Rigan, S. M. Gasinets, O. V. Gorina, D. I. Kaynts, and V. V. Tovt, “Obtaining and crystallization pecularities of antimony containing chalcohalogenide glasses,” Ferroelectrics 372(1), 87–92 (2008). [CrossRef]
18. V. M. Rubish, “Thermostimulated relaxation of SbSI glass structure,” J. Optoelectron. Adv. Mater. 3(4), 941–944 (2001).
19. V. M. Rubish, M. Y. Rigan, S. M. Gasinets, O. V. Gorina, D. I. Kaynts, and V. V. Tovt, “Obtaining and Crystallization Peculiarities of Antimony Containing Chalcohalogenide Glasses,” Ferroelectrics 372(1), 87–89 (2008). [CrossRef]
20. S. R. Lukic, D. M. Petrović, I. O. Guth, and M. I. Avramov, “Complex non-crystalline chalcogenides: technology of preparation and spectral characteristics,” J. Res. Phys. 30(2), 111–130 (2006).