Stress-assisted Copper-induced Lateral Growth of Polycrystalline Germanium
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Stress-assisted Copper-induced Lateral Growth of Polycrystalline Germanium B. Hekmatshoar, D. Shahrjerdi, S. Mohajerzadeh, A. Khakifirooz1, M. Robertson2 and A. Afzali-Kusha Department of Electrical and Computer Engineering, Thin Film laboratory, University of Tehran, Tehran, Iran, [email protected]. 1 Microsystems Technology Laboratories, Massachusetts Institute of Technology, Cambridge, MA 02139, [email protected]. 2 Department of Physics, Acadia University, Wolfville, NS B4P 2R6, Canada ABSTRACT Lateral growth of poly-Ge at temperatures as low as 150˚C is reported. External mechanical stress has been properly manipulated to drive the low temperature Cu-induced crystallization of poly-Ge wherever Cu is deposited to form the crystallization seed for lateral growth. Uniaxial compressive stress has been externally applied to the Ge layer by bending the flexible PET substrate inward. A 10-hour period thermo-mechanical post-treatment in the presence of 0.05% equivalent compressive strain leads to a growth rate of 2.5 µm/hour in the direction of the applied stress and 1.8 µm/hour in the perpendicular direction, as confirmed by SEM analysis. We believe that partial growth of the Cu-seeded poly-Ge region in the form of tetragonal structures is the key feature which leads to the lateral growth of the pure-Ge strip. Elimination of the compressive stress hinders the lateral growth completely, even at reasonably high temperatures. INTRODUCTION Significant attention has been paid to low-temperature crystallization of Si and Ge in today’s semiconductor technology. This is due to the general trend aimed at fabricating reasonably highquality electronic devices on low-cost, low-temperature substrates such as metal or plastic foils. Furthermore, such low-temperature process steps are fundamental in some applications such as the flexible display technology [1] or the reel-to-reel process methodology [2] employed for fabrication of solar cells or thin-film transistors (TFTs) on flexible substrates. As a well-known approach, metal-induced crystallization (MIC) has been widely manipulated to lower the crystallization temperature by alloying the semiconductor with metal or forming intimate contacts of metal-semiconductor single or multi-layers before annealing [3]. Al-induced crystallization of Ge has been widely investigated by forming multi-layers of Al/Ge [4-6] as well as the Cu-induced crystallization of Ge [7,8]. Although MIC with Al may reduce the crystallization temperature considerably, the semiconductor properties of Ge become severely degraded by the incorporated metal contamination. On the other hand, MIC with Cu needs temperatures as high as 400°C that is still high for flexible plastic substrates. We have recently developed a stress-assisted, Cu-induced crystallization technique, which reduces the crystallization temperature of Ge to as low as 130°C, by means of the mechanical compressive stress externally applied to the flexible substrate during annealing [9]. Proper patterning of the amorphous Ge layer b
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