Growth and applications of GeSn-related group-IV semiconductor materials Ashok Kumar Pasumarthi Department of Engineering Technology, Middle Tennessee State University Abstract In this paper the epitaxial crystal growth of Ge1-xSnx group-IV semiconductors is reviewed. There are difficulties in crystalline growth of Ge1-xSnx on any substrate because of precipitation of Sn at higher temperatures. The molecular beam epitaxy technique is adopted to hinder the Sn precipitation and to produce novel materials with good crystalline properties like thermal stability, superior crystallinity and unique optical and strain properties like strain structure and energy band engineering. The integration of Si with Ge1-xSnx gives the required properties which has wide applications in optoelectronics. I. Introduction The properties like over speed, power consumption and information band width, strain engineering, band gap engineering need to be improved to integrate optoelectronics with Si Ultra-large integrated circuits(ULSI). Since in metal oxide semiconductor field transistors development there are some pre-scaling issues, hence there is a need for new materials such that they show high mobility channel, high dielectric insulators, metal gates for controlling the function and low resistance metal contact1.
Si nanoelectronics is the latest technology that shows all the listed properties and gives high performance. Germanium tin Ge1-xSnx has overwhelming interest due to the potential applications in high channel mobility of MOSFETS and silicon based infrared photonic devices. Moreover, some experiments in recent times has shown that Ge1-xSnx becomes a direct bandgap material at 10% of Sn and provide route for realizing direct transition semiconductor like III-V group semiconductors. The applications of these high mobility MOSFETS include tunnel field effect transistor(TFETS), IR-waveguides, IR-photodetectors, Lasers, Solar cells.
II. Difficulties in crystalline growth Ge1-xSnx is a eutectic alloy and the thermal equilibrium temperature of Sn in Ge-matrix is very low. Hence it is very difficult to increase the Tin content in Ge1-xSnx alloy. Hence the epitaxial crystalline growth is done at low temperature, which is a non-equilibrium process. III. Molecular Beam Epitaxy growth of Ge1-xSnx epitaxial layer MBE is one of the crystal growing methods for high quality Ge1-xSnx layers on Si, Ge and other substrates. MBE growth of Ge1-xSnx is done in an ultra-high vacuum chamber.
The calculated vapor pressure is high while melting point of Sn is low. Hence the crystalline growth process is done in a crucible at 800° C. Figure 1. The substrate temperature which plays a vial role is held at 400° C for Ge epitaxy slight higher temperature is for Ge1-xSnx growth. The temperature is kept low since the precipitation of Sn takes place at high temperature. Figure 1(a) and (b) shows the typical X-ray diffraction two -dimensional reciprocal space maps(XRD-2DRSM) around Ge224 reciprocal lattice point of a virtual Ge(v-Ge) substrate after growth and post deposit annealing(PDA). The diagonal line indicates strain relaxed epitaxial layer.
PDA introduces misfit dislocations. Figure1(c)and 1(d) indicates cross-sectional transmission microscopy(TEM) images of Ge1-xSnx layers on v-Ge after and before PDA. Figure 2. The threading dislocations indicate the strain relaxed Ge-layer, but after PDA there is contrast in the texture, indicating propagation of misfit dislocations. Figure 2. shows a summary of XRD-2DRSM results with various Sn contents at different temperatures. The Sn contents of 6.8 to 7% were available at 100 to 150° C, showing that precipitation of Sn can be suppressed by lowering the growth temperature.
There is an issue of faults and twin-defects with the growth at lower temperatures, which is hindered using Sn and there by developing the epitaxial layer on Ge. The higher the Sn content the more would be the crystalline properties, which is possible to raise Sn content to 27%. We use InP(Indium phosphide) as substrate and temperature would be low as 50°, to get 25.4% of Sn.
IV. Applications of Ge1-xSnx related materials Optoelectronics devices like lasers, photodiodes, lases solar cells use Ge1-xSnx semiconductors for energy band engineering. With high Sn content up to 8% materials are used to improve photoabsorbance and photo response. Energy band Engineering with ternary alloy of group-IV materials is promising for solar applications. High performance MOSFETS but lower power consumption devices are designed with Sn-related alloys. High speed migration of Sn electrons makes the MOSFETS more reliable and superfast in performance. V. Conclusion: The paper gives clear understanding of molecular beam epitaxy growth of Ge1-xSnx related materials.
Crystalline properties that are obtained in this process and the importance of Sn content in semiconductor alloys is clearly explained. VI. References Growth and applications of GeSn-related group-IV semiconductor materials click here Growth and applications of GeSn-related group-IV semiconductor materials click here Digital Etch Technique for Forming Ultra-Scaled Germanium-Tin (Ge1?xSnx) Fin Structure click here