+44 (0)24 7671 8970
More publications     •     Advertise with us     •     Contact us
 
Technical Insight

Boosting triple-junction yield with in-situ monitoring

In-situ monitoring provides a great deal of valuable information for developers and producers of multi-junction solar cells. It can determine interface quality; the thickness, doping level and composition of every layer; and wafer bow, says Oliver Schulz from LayTec.




Chipmakers strive for perfection, and that includes trying to produce in-spec devices from everywhere on every epiwafer. This equates to a 100 percent yield target across every wafer in every run, regardless of the material backbone of the product.

To succeed, all the layers within the device have to have a high degree of homogeneity in terms of layer thickness, ternary or quaternary composition, doping, and interface and surface morphology. In the last ten years, variations in these parameters have reduced significantly, thanks to widespread adoption of in-situ metrology – measurements of surface temperature, in particular have spurred epiwafer uniformity to a new level.

At LayTec of Berlin, Germany, we have been a major player in this revolution in in-situ monitoring through our sales of tools for aiding the growth of epiwafers. Many of these have been used to monitor the deposition of stacks of nitride films, which form the foundation for white and blue LEDs. Sales of these solid-state emitters are booming, leading many to overlook advances in the production of conventional III-V chips. Nevertheless, in-situ monitoring has achieved tremendous success in the production and development of such devices, including: InP- and GaAs-based RF electronics, such as HEMTs and HBTs; optoelectronic components, such as laser diodes, VCSELs, photo receivers, semiconductor optical amplifiers and modulators; and concentrating photovoltaic (CPV) cells.

In the lab

The latter class of chip is being grown at Ioffe Physical Technical Institute in St. Petersburg, Russia, using a reactor equipped with an

EpiRAS TT system. This monitors the growth of these multi-junction heterostructures, which are based on III-Vs and germanium. At Ioffe, small chips are mounted into photovoltaic modules that focus the sunlight onto cells with Fresnel lenses.

One major challenge associated with the production of triple-junction cells is the deposition of a low-defect III-V heterostructure on a covalent, group IV germanium substrate. This goal is easier to fulfil using an EpiRAS TT: It helps to avoid the formation of anti-phase boundaries at the interface between the III-Vs and germanium; it can determine composition and growth rate via wide-range spectral reflection measurements; it can reveal doping levels, using reflectance anisotropy data; and it can offer an accurate measurement of wafer temperature, through emissivity-corrected pyrometry.

Researchers at Ioffe have used the reflectance transient at 2.1 eV for growth rate measurements during deposition of the triple-junction heterostructure (Figure 1a, red). It is possible to extract more information from this data, with the amplitude of the Fabry-Pérot oscillations providing information on the refractive index, a material parameter that directly correlates to the composition of the ternary alloys. With the help of the EpiRAS software database and appropriate fitting algorithms, it is even possible to determine the composition of very thin layers – such as the tunnel junction – in real time.

Meanwhile, short-wavelength reflectance anisotropy can determine doping levels. At Ioffe, researchers correlated the reflectance anisotropy signal at 3.8 eV with doping measured ex-situ, to establish their own process control database (see Figure 1a, blue trace; and Figure 1b). Armed with this information, it is possible to estimate changes in doping at homo-interfaces during epiwafer growth.

Analysing the reflectance anisotropy signal during the growth of the first III-V monolayers on germanium provides a wonderful insight into the quality of this epilayer. If the reflectance anisotropy signal is high, this interface is rough and germanium auto-doping can arise (this occurs when dopant atoms evaporate from a germanium substrate to the surface during high temperature treatments, and are then reintroduced into the epiwafer, causing undesired variations in dopant concentration at its surface). It is possible to avoid auto-doping by optimising the growth conditions, a step taken by the researchers at Ioffe. Thanks to insights offered by the reflectance anisotropy signal, this team quickly established a growth recipe for realising high-quality window layers and GaInP buffer layers on germanium substrates (see Figure 1c – red).

Figure 1. Complete growth of a triple-junction GaInP/GaInAs/germanium solar cell structure monitored by EpiRAS TT at Ioffe PTI. Figure 1a: red - reflectance transient at 2.1 eV, blue – reflectance anisotropy measurement at 3.8 eV. The reflectance signal is used for growth rate and ternary composition determination. 

Figure 1b: n-type GaInAs buffer growth: blue – reflectance anisotropy signal at 3.8 eV helps estimate the doping concentration during growth. 

Figure 1c: n-type GaInP on p-type germanium hetero-growth. Reflectance anisotropy signal at 3.8 eV: blue –standard growth, red – improved growth. The quality of the interface can be determined during growth.

In the fab

In-depth analysis of in-situ data is a major aid to a research and development project, but in a high volume fab there is not the time to do this (it also requires expertise that takes time for the operator to acquire). In that environment, what is needed is a more robust analysis of the growth, combined with real-time statistical process control.

Our EpiTT and EpiCurveTT product families, working in unison with the EpiGuard software package, meet this need. These in-situ systems offer a comprehensive, robust monitoring of growth parameters. Measuring the wafer’s temperature with emissivity corrected pyrometry is incredibly insightful, because it is this temperature that governs crystalline quality, composition and doping level. This temperature can be determined on various substrates, including those made from III-Vs, silicon and germanium. For the growth of high-quality epiwafers for efficient triple-junction cells, it is critical to deposit the optimal thickness for every layer. Multi-wavelength reflectance measurements can help (see Figure 2, which shows a screenshot of simultaneous reflectance measurement at 405 nm, 633 nm and 950 nm). Since the frequency of the Fabry-Pérot oscillations decreases with increasing wavelength, the 950 nm measurement is most appropriate for thick layers deposited with a high growth rate. Meanwhile, the signal generated by reflectance from the 633 nm source is more suitable for scrutinising thinner layers. 405 nm emission is absorbed in III-V materials, making this reflectance measurement highly surface sensitive. It offers insights into surface morphology and interface quality, and can determine tunnel junction thickness.


Figure 2. Reflectance measurement of a multi-junction solar cell at three wavelengths: 950 nm, 633 nm and 450 nm (courtesy of Fraunhofer ISE Freiburg, Germany)

Keeping it flat

If a fab is to have a high yield, it must produce wafers with minimal bow, both during and after growth. Historically, curvature measurements have focused on nitride films grown on foreign substrates, due to the high degree of strain in these materials – meanwhile, measurements on lattice-matched III-Vs have been neglected. But even with the latter material system, wafer bow and warp can occur during cooling, due to differences in the thermal mismatch of the substrate and the deposited layers. And when it comes to triple-junction structures, strain engineering becomes essential, due to the growth of pseudomorphic and metamorphic structures.

At Fraunhofer Institute for Solar Energy Systems, scientists employ the EpiCurve TT in the production of multi-junction solar cells, where it is used for managing the strain and optimising the process. The wafer bows significantly when intentionally lattice-mismatched III-V layers are deposited on germanium, and its curvature is strongly aspheric after intentional buffer relaxation for metamorphic growth (see Figure 3 a).With LayTec’s advanced resolution curvature technology, it is possible to distinguish between spherical curvature and asphericity (Figures 3b and 3c).  The resulting signal helps a process engineer to optimise the growth of the buffer and its relaxation at early, decisive stages of the epitaxial process.


Figure 3 (a,) Bow profile (ex-situ): aspherical bow perpendicular to the step edges of the off-cut germanium substrate.

(b, top) in-situ wafer curvature measurements: strained InGaAs middle cell compensates for metamorphic buffer bow. (c, bottom) wafer asphericity measurements by advanced resolution in-situ curvature measurements: asphericity starts with first pseudomorphic buffer relaxation

Researchers are striving to increase the efficiency of multi-junction cells, because this should make CPV technology more competitive. New designs are being pursued, which are often more complex than their predecessors, due to the addition of a fourth junction or the use of an inverted architecture. We can help these ambitious efforts – we can provide the tools for in-situ monitoring, and thanks to our close collaboration with leading researchers in academia, we can offer advice to industry customers from our resulting extensive know-how in analysing and understanding in-situ data associated with epitaxial processes. Our hope is that we will play a key role in driving up the yield and quality of multi-junction solar cells, and ultimately the rapid growth of this industry.



At Ioffe Physical Technical Institute, researchers produce multi-junction solar cells using an Aixtron MOCVD tool equipped with a LayTec’s EpiRAS TT system. These cells have been deployed in a concentrating photovoltaic system


Further reading
J.F. Geisz et.al. J. of Cryst. Growth 310 2339-2344 (2008)
W. Guter et.al. Appl. Phys. Lett. 94 223504 (2009)
N.A. Kalyuzhnyy et.al. Proceedings of the 24th European Photovoltaic Solar Energy Conference (2009)
×
Search the news archive

To close this popup you can press escape or click the close icon.
×
Logo
×
Register - Step 1

You may choose to subscribe to the Compound Semiconductor Magazine, the Compound Semiconductor Newsletter, or both. You may also request additional information if required, before submitting your application.


Please subscribe me to:

 

You chose the industry type of "Other"

Please enter the industry that you work in:
Please enter the industry that you work in: