The indirect nature of the fundamental energy gap in the elemental semiconductors Si and Ge prevents the use of these materials and their alloys in laser devices. The objective of this work is to fabricate Sn-based IV-IV compounds for finding direct bandgap material. This work focuses on the material system GeSn/Ge, from the growth, characterization, and optical measurement. Experimental evidences on pseudomorphic growth of thick Ge1-xSnx film for Sn composition up to 10% and the direct optical transition are presented.

Fourier transform infrared spectroscopy (FTIR) is used for probing the bandgap of the GeSn Film. The measurement is arranged in the setup with multi-reflection.
In the conventional approach, Sn-based IV-IV compounds are grown by the technique of Chemical Vapor Deposition (CVD). Different from the conventional approach, in this project, the Sn-based IVIV compounds are fabricated by Molecular Beam Epitaxy (MBE). The technique affords flexibility in the growth of the materials system, whereby conditions such as growth temperature can be varied over a wide range. Different Sn-based IV-IV compounds are fabricated such as GeSn on Ge wafer, SiGeSn on Si wafer, etc.

By employing several proposed techniques, key issues affecting the growth of the alloy and its heterostructure have been resolved (-Sn segregates during the growth of GeSn film and the misfit dislocations develop at GeSn/Ge interface due to the large lattice mismatch between Sn and the Ge wafer).

A series of Ge1-xSnx films with various Sn compositions up to 30% and thickness of 30 nm was grown. From the analysis of cross-sectional transmission electron microscope (XTEM) and Energy- Dispersive x-ray (EDS), for Sn composition 10%, it shows that: (1) the Ge1-xSnx films are misfit dislocation-free in the XTEM, and (2) Sn is uniformly distributed in the Ge1-xSnx films. For Sn composition 10%, the Ge1-xSnx film is not alloying with defects. All these films are above the predicted critical thickness required for the laser structure.

Various measurements have been performed to characterize these samples, including XTEM, EDS, high-resolution Xray, Raman spectroscopy, etc. Fourier transform infrared spectroscopy (FTIR) is employed for probing the bandgap of the GeSn film. The measurement is arranged in the setup with multi-reflection.

In comparison with bulk Ge, the absorption edge of the Ge0.98Sn0.02 film shifts to lower energy. The spectrum is characterized by two features that locate at around 0.68 eV and 0.55 eV associated with the absorption originated from indirect and direct optical transitions. The value of the absorption is roughly the same as the theoretical value of 0.69 and 0.56 eV.

Two lattice-matched laser structures were proposed using GeSn as active layers and GeSiSn as barriers. One has a double heterostructure (DH) and the other has a multiple quantum well (MQW) structure. The DH laser design is relatively easy to grow, but only operates at low temperatures. The MQW design, on the other hand, can potentially operate up to room temperature. Lasing wavelength can be tuned from near- to mid-infrared with the Sn composition. On the implementation side, growth of GeSn alloys was begun with various compositions on Si substrates using MBE.

Several cycles of laser designs, structure growth, device processing, and characterization will be necessary given the degree of complexity of this project. Laser designs will be refined with the material parameters obtained from the characterization of the actual structures grown on Si substrates, which in turn will guide the material and structure development and subsequent device processing.

This work was done by H. H. Cheng of National Taiwan University, G. Sun of the University of Massachusetts, and R. A. Soref of the Air Force Research Laboratory. AFRL-0195


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Tin-Based IV-IV Heterostructures Fabricated Using Molecular Beam Epitaxy

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This article first appeared in the June, 2013 issue of Defense Tech Briefs Magazine.

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