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Translucent
Group IV Materials Translucent Inc. Group IV Materials Key Concepts Discussed: •
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SiGeSn direct band gap material Tuning bandgap voltage for applications Fully lattice matched, strain free films MOCVD compatibility Page 1 952 Commercial St., Palo Alto, CA 94303 • For sales, contact Translucent Inc., Tel. (+1) 650 213 9311, Fax (+1) 650 213 9511 or info@translucentinc.com Translucent
Group IV Materials Introduction / background Ge templates Bulk Ge wafers serve as template for fabrication of multi-‐junction solar cells for space and concentrating photovoltaic (CPV) applications. They are also used as substrates for III-‐V arsenide and phosphide based optoelectronic devices – lasers, detectors, modulators, etc. However, the bulk Ge platform is expensive and an alternative is to transfer the III-‐V material growth to cheaper Si wafers, which also provide known base for signal, power and control circuitry. In addition, thermal properties of silicon are better (higher thermal conductivity) for high thermal dissipation requirements of solar cell technologies. Translucent has developed techniques for the integration of the Ge template onto the Si wafers. This is allowed via our unique low temperature CVD technology. Direct growth of Ge on Si is difficult and requires time consuming high temperature processes. Our approach uses a unique strain relieving effect of Sn atoms in the Ge lattice. Small amounts of Sn allow growth of smooth and flat layers of Ge directly on Si at low temperatures SiGeSn materials The technology implemented at Translucent has an advantage of flexibility of being extended to higher Sn content in Ge. Since Sn is a semimetal with the same crystal structure as Ge, the alloying of the two elements generates alloys with bandgaps lower than that of Ge. Also at certain concentration of Sn (approx. 6-‐ 10%) the alloy becomes direct bandgap semiconductor, which is beneficial for higher light to electricity conversion efficiencies in photovoltaic and higher light emission efficiencies for photonic devices. The alloying also results in expansion of the lattice parameter, which might lead to undesired strain effects. With addition of Si, the strain issues can be eliminated and the bandgap value range can be extended to the values above 0.8 eV. In a limited case, by careful control of compositions, the lattice parameter of ternary alloys containing Si, Ge and Sn can be fixed to a specified value (typically equal to that of Ge or GaAs). Even at this constraint, the bandgap can be tuned within a large range without any lattice mismatch issues. The lattice parameter (strain) vs bandgap tunability allows design of multijunction solar cells and photonic devices, which are lattice matched to the desired platform. For example, on a Ge wafer we can integrate a 1 eV p-‐n junction formed of Si-‐Ge-‐Sn, higher bandgap window and a tunnel junction made from the same elements, while keeping the entire device devoid of misfit defects. In another case, higher bandgap Si-‐Ge-‐Sn can serve as a cladding layer for infrared laser devices integrated directly with a group IV platform. In multijunction solar cell applications the Si-‐Ge-‐Sn alloys enable design and fabrication of fully lattice matched, relaxed junctions fully compatible with the higher bandgap arsenide and phosphide based top layers. Page 2 952 Commercial St., Palo Alto, CA 94303 • For sales, contact Translucent Inc., Tel. (+1) 650 213 9311, Fax (+1) 650 213 9511 or info@translucentinc.com Translucent
Group IV Materials Process Translucent’s CVD process uses a commercially available precursor SnD4,. The process proceeds by the following reaction: x Ge2H6 + (1-‐x) SnD4 -‐> GexSn1-‐x+H2. The amount of Sn in the resulting layer is much smaller than 1%, typically 0.01% of Sn is sufficient to obtain flat layer of Ge on Si. At this amount the deposited layer is essentially pure Ge with all properties matching pure Ge. The grown layers are routinely analyzed by X-‐ray diffraction, spectroscopic ellipsometry and atomic force microscopy. We collect data about the layer thickness, composition, optical absorption and surface roughness. This technology produces a broad range of composition either in the binary Ge-‐Sn system or ternary Si-‐Ge-‐Sn alloys. Practical limit for the binary alloys was 6%, but with compromising the growth rates, we are able to achieve 10% and higher Sn content in germanium. For the ternary system, the focus has been compositions, which are lattice matched to bulk Ge, and centered on the requirements of the CPV industry. Epitaxial results for group IV alloys A: GeSn on Si The solar cell industry is not the only one which can benefit from the advantages of Ge and Ge-‐Sn materials. The “on Silicon” solutions are highly desirable for integration of III-‐V based photonic devices with the Si driving logic circuits. Our technology allows growth of III-‐V materials (arsenides and phosphides) on Silicon at the same conditions as these would be grown on bulk Ge substrates. This is briefly demonstrated in Figure 1, where we show reflectivity spectrum of a DBR structure (base part of a laser cavity) grown on Si wafer via Tranlsucent’s virtual Ge template technology. The spectrum was compared to that of the same structure grown on bulk Ge and is essentially the same except a small shift in the wavelength, which can be compensated by layer structure design. Page 3 952 Commercial St., Palo Alto, CA 94303 • For sales, contact Translucent Inc., Tel. (+1) 650 213 9311, Fax (+1) 650 213 9511 or info@translucentinc.com Group IV Materials Translucent
Figure 1: Comparison of reflectivities for same DBR structure grown by MOCVD on bulk Ge and virtual Ge/Si template. Furthermore the Ge-‐Sn alloys with higher Sn content are of interest for optoelectronic and photonic applications. The increased Sn content lowers the bandgap in Ge and at certain concentration it converts the indirect bandgap nature of Germanium to the direct bandgap type of semiconductor. Such materials can be used as core components for silicon based lasers in conjunction of the ternary Si-‐
Ge-‐Sn materials. We have investigated this opportunity and in the Figure 2 we show several photoluminescence spectra of Ge-‐Sn materials with increasing Sn content from 0.5 to 4% in Ge. The luminescence peak maximum shifts to the higher wavelengths (lower energies) with increasing Sn content in the alloy. The peak shift corresponds to the lowering the bandgap of Germanium and at the same time it is a good indicator of Ge-‐Sn material quality. This data clearly demonstrates the ability to produce these materials using the Translucent’s technology and potential for further engineering of the process to develop more complex device layer structures. Figure 2: Shift of photoluminescence spectra in Ge-‐SN layers with increasing Sn content. Page 4 952 Commercial St., Palo Alto, CA 94303 • For sales, contact Translucent Inc., Tel. (+1) 650 213 9311, Fax (+1) 650 213 9511 or info@translucentinc.com Group IV Materials Translucent
B: SiGeSn on Si Incorporation of both Sn and Si elements into the Ge matrix is demonstrated in a Figure 3, which shows an RBS spectrum of Si-‐Ge-‐Sn layer grown on Si substrate. The Sn content is 1.7%, Si 11% and Ge is 86.3%. The composition is tuned so that it possesses lattice parameter close to that of Ge and a bandgap in the range of 1.0 eV. The uniform spectrum profile indicates compositional uniformity throughout the entire thickness of the layer, which is about 600 nm. On the right side is shown the X-‐ray diffraction spectrum of the same sample (red trace) with only single peak corresponding to the alloy suggesting no segregation of the constituents. Spectrum obtained from a Ge layer (blue trace) is overlaid for reference purposes. Figure 3: RBS spectrum of Si-‐Ge-‐Sn film grown on Si wafer (left), XRD spectrum of the same sample showing SiGeSn material with the same lattice constant as pure Ge. C: SiGeSn on Ge The lattice matching to the bulk Ge substrate is demonstrated in a Figure 4. The Figure shows X-‐ray rocking curves from three different Si-‐Ge-‐Sn layers in which the Sn concentration is increased (blue-‐red-‐green trace) to finally match the substrate. The data also demonstrates high crystalline quality of the material as evidenced from the FWHM of the peaks (54 arcsec for SiGeSn vs 36 arcsec for Ge). This data also shows high level of growth process control. Figure 4: XRD spectrum of SiGeSn films gronw on bulk Ge (left); TEM image showing quality of SiGeSn layer on Ge. Page 5 952 Commercial St., Palo Alto, CA 94303 • For sales, contact Translucent Inc., Tel. (+1) 650 213 9311, Fax (+1) 650 213 9511 or info@translucentinc.com Translucent
Group IV Materials The crystalline quality and uniformity of the material is also shown in the TEM images in the Figure 4. The low resolution image shows a barely discernable interface between the layer and substrate as expected. The high resolution image of the top surface is shown in the inset. Figure 5: Optical image of the SiGeSn surface (left) without any surface defects, (right) AFM image of the same surface with low RMS roughness of 0.2 nm. The surface of the SiGeSn layers is shown in the Figure 5. On the left side there is shown an optical microscope image showing flat featureless layer surface. The right side shows and AMF image in a height mode where we can observe terracing originating from the 6 degree miscut of the Ge substrate. The miscut is deliberately chosen in order to promote defect free nucleation of subsequent III-‐V layers. The RMS roughness of the film surface is about 0.17 nm. The optical absorption related to the bandgap of the SiGeSn alloys with various compositions is measured using spectroscopic ellipsometry and shown in the Figure 6. The plot shows the extinction coefficient, which is proportional to the absorption coefficient, in relation to the bulk Ge values (dark blue trace). All measured alloys shown in this plot are lattice matched to the Ge lattice parameter. With increasing amount of Sn and Si at the same time, the absorption edge shifts to the higher energy values indicating increase of the band edge from 0.8 eV of Ge to 1.1 eV for the highest Si and Sn content obtained. This data demonstrates the bandgap engineering capability of Translucent’s technology. Figure 6: Extinction coefficient (optical absorption) spectra of SiGeSn layers with increasing Si and Sn content. Page 6 952 Commercial St., Palo Alto, CA 94303 • For sales, contact Translucent Inc., Tel. (+1) 650 213 9311, Fax (+1) 650 213 9511 or info@translucentinc.com Translucent
Group IV Materials An important property for solar applicable materials is the minority carrier lifetime, which was measured on SiGeSn layers using non-‐contact method. The recorded values were in the range of 100 ns, which is comparable to the lifetime measured in MOCVD grown III-‐V material on Ge substrate. For a reference the lifetime in bulk Ge is about 4 microseconds. MOCVD Compatibility The Ge on Si virtual substrate technology has been validated by partners with MOCVD growth capabilities. Multiple wafers have been subjected to the nucleation and growth of InGaAs alloys lattice matched to Ge. In these initial experiments we have mainly investigated the stability of Ge-‐Sn alloys in the MOCVD process. Initial concerns centered on the thermodynamic stability of the Ge-‐Sn materials given they are grown at low temperatures. Results have now proven that in fact the virtual Ge templates and Ge-‐Sn layers withstand the MOCVD growth temperatures without impacting their quality. This has been tested by both TEM and X-‐ray diffraction. Moreover, we found no cross contamination of the MOCVD growth systems by Sn or the Ge-‐Sn layers by the III-‐V constituents. An example of the MOCVD grown InGaAs layer on virtual Ge/Si template is shown in the Figure 7. The nucleation temperatures of InGaAs had to be adjusted for the thickness and properties of the silicon wafer, but these do not deviate significantly. Figure 7: Crystalline quality of InGaAs layers grown on virtual Ge/Si template demonstrated by X-‐ray diffraction (left) and TEM (right). Device Validation of SiGeSn epiwafers Currently the SiGeSn epilayers are being fabricated into 1J and 3J solar cells. We anticipate publishing a white paper on this in January 2015 Page 7 952 Commercial St., Palo Alto, CA 94303 • For sales, contact Translucent Inc., Tel. (+1) 650 213 9311, Fax (+1) 650 213 9511 or info@translucentinc.com Group IV Materials Translucent
Summary Translucent has established the Si-‐Ge-‐Sn based technology using a manufacturable process. This has been demonstrated by production of both Ge/Si templates and by the development of lattice matched Si-‐Ge-‐Sn alloys on Ge. The templates have been integrated into the standard upstream MOCVD process and proven to be fully compatible with existing multi-‐junction solar cell production schemes. The SiGeSn alloys enable Translucent to provide materials around the 1.0eV bandgap which has high commercial interest in the photovoltaic industry. The continued focus on increasing the cell efficiency rather than switching to process cost reduction aligns well with Translucent’s Si-‐Ge-‐Sn development. Additionally the Sn based group IV alloys can offer advantages to the broad field of silicon photonics, providing templates / materials for light detection, modulation and generation as utilized in on chip communications. The broad range of bandgaps and compositions available from Si-‐Ge-‐Sn (including GeSn) also enable mid-‐infrared photonic components which find applications in a variety of bio-‐medical, gas detection and defense applications. Page 8 952 Commercial St., Palo Alto, CA 94303 • For sales, contact Translucent Inc., Tel. (+1) 650 213 9311, Fax (+1) 650 213 9511 or info@translucentinc.com