Low-Cost,Uncooled, NIR and SWIR FPA Using SiGeSn on Silicon

Overview of technical approach involving SWIR Ge1-ySny on Si technology The objective of the project is to fabricate photodetectors comprising Ge1-ySny on Si devices. This technology will be used to build a lower cost focal plane array (FPA) imager that operates at room temperature. The wavelengths of interest for SWIR imaging range from 1.6 up to 3 microns at the onset of the mid IR. Ultimately we will plan to pursue backside illuminated 1-m (Si) to a cutoff wavelength of 3m for Phase I through II of the program. Our prior work has utilized Ge1-ySny with tunable direct gaps to demonstrate fabrication and testing of high performance photodiodes through primarily front side illumination. These devices were mostly based on p-i-n diode structures for which it is desirable that the direct band gap of the doped layers be higher than that of the intrinsic layers. The device stacks with Ge1-ySny intrinsic layers are designed to operate in the spectral range of 1.6-3 m. Ge1-ySny alloys with concentrations in the range of y=1-14 % exhibit the target band gaps within the above wavelength window. These materials have been routinely synthesized upon large area Si wafers (4 in diameter) by CVD at CMOS compatible conditions and applied to create working detectors at ASU. Considerable experience has been developed in doping and processing these materials into working diodes. Specially designed in situ doping protocols are adapted to these materials to generate the n- and p-type layers. Diborane B2H6 is used for p-type doping and P(GeH3)3 for n-type doping. The doped cladding layers may consist of Ge1-ySny materials with slightly lower Sn concentration than the intrinsic layer, ternary Ge1-x-ySixSny alloys with larger bandgaps, or even elemental Ge. In the latter case the p-i-n stack may be grown on an n-type Ge buffer layer that can serve as the bottom contact. This layer also facilitates strain relaxation and mitigate defects. Alloys with Sn compositions spanning the SWIR at 1.6 m to the MWIR at 4 m range have been demonstrated on Ge-buffered Si and on bare Si wafers. The latter samples will provide an ideal starting point for producing back side illumination compatible detectors. The fabricated samples in this work are characterized using a combination of Rutherford backscattering (RBS), high resolution X-ray diffraction (HRXRD), spectroscopic ellipsometry, and cross sectional transmission electron microscopy (XTEM). SIMS measurements will be employed to determine the dopant atom concentrations. Temperature dependent Hall experiments may also be utilized to evaluate the electrical properties as a figure of merit to judge basic materials performance for subsequent application in devices. Basic materials properties of Ge1-ySny on Si en-route to devices The specific samples targeted in this work with compositions in the in the 1-14 % range will be synthesized by CVD using stoichiometric reactions of commonly available starting materials Ge3H8 and SnD4. The growth rate is in the range ~ 7-10 nm per minute, yielding films with large thicknesses up to a micron and higher. A typical example is illustrated in Figure 1 which shows a cross sectional TEM image of a Ge0.938Sn0.062 (6.2 %Sn) film, the underlying Ge buffer and the Si(100) wafer. The film thickness is estimated to be ~ 700 nm. The atomic composition was measured by RBS to be 6.2 % Sn corresponding to a direct band gap of 2.1 microns. XRD indicated that the films were nearly relaxed as expected due to the large thickness, with some evidence of a small residual strain that can be removed by post growth thermal processing. Figure 1 also illustrates the typical morphology and microstructure of the target sample revealing excellent crystallinity. The bulk material appears to be free of threading defects and other types of structural imperfections within the field of view. The top surface is flat and the interfaces are uniform and well defined. High resolution image of the top interface (see inset) reveals full epitaxial alignment between the epi