OPINION STANDARDS
What this means is that efforts at determining the composition are closer to resembling the matching of paint colour cards than the making a scientific measurement. In practice, engineers tend to add a little bit of this and a little bit of that until they get a layer that matches the one they grew last month. Several reasons can account for the persistence of this inefficient situation. Habit probably tops this list. Take a compound semiconductor alloy system with a long history of technological importance: Alx
Ga1 - x As.
This alloy has the extremely useful property that its lattice parameter has almost no dependence on the aluminium mole fraction. Thanks to this very favourable attribute, hetero-junctions can easily be grown without introducing strain relaxation from defects. This has led to widespread determination of the aluminium mole fraction with an X-ray diffraction (XRD) rocking curve that can uncover the small elastic strain in AlGaAs. But this method is not flawless: It can be fooled by changes in the substrate lattice parameter or by a doping-induced expansion or contraction of the lattice.
Fortunately, this is not the only approach to determining AlGaAs composition. If the aluminium content is low, its mole fraction can be uncovered by measuring the band gap energy with photoluminescence. This is a high precision method, but it is
again subject to systematic errors from sample heating, impurity and doping shifts of the apparent band edge, and excitation intensity shifts in band-edge transitions.
If you have learnt to determine AlGaAs composition of an epilayer, what method do you follow? Chances are that it is one of the two listed above – measuring strain relative to the substrate with XRD, or determining the bandgap with photoluminescence. And it’s probable that you convert this number to a composition with an equation given to you by your thesis advisor, an older graduate student, or your supervisor.
If
you changed institutions, you might have taken your equation with you, or maybe you accepted a new one. The local nature of these conversion factors is often an impediment to the absolute accuracy of any composition measurement.
Up until now, we’ve only discussed the problems associated with ternary alloys. These are magnified to an entirely new level with quaternary compounds like InGaAsP. When the National Institute of Standards and Technology (NIST) sponsored a round-robin comparison of this class of material in 2002, this institute found substantial variations in XRD and photoluminescence data on identical samples examined in nine different laboratories. Don’t put down these variations to laboratory-dependent calibration factors – they persist even when these factors are removed from the data analysis.
Fortunately, it is not all doom and gloom: We have the technology to remedy this situation. Starting in 1997, NIST began a programme to standardise the measurements of compound semiconductor composition, starting with AlGaAs. Some of the impetus for the work came from the Optoelectronic Industry Development Association (OIDA), and NIST sought input through venues such as CS-MAX and SEMI committees.
Methods for determining composition analysis were examined during this programme, with several papers published that quantified the measurement uncertainty for those methods and outlined best practices. In 2006, the programme culminated with the production of Standard Reference Materials (SRMs) for an aluminium mole fraction in AlGaAs near 0.20 (SRM 2841) and near 0.30 (SRM 2842). Each standard consists of a layer of AlGaAs about 3 µm-thick on a GaAs substrate. The AlGaAs layer has been certified to have a stated aluminium mole fraction with a typical absolute uncertainty of 0.002 (2 σ).
Another highlight of this programme is that it led to a refinement for the correction factors for aluminium, gallium, and arsenic in the CITZAF method for the accurate interpretation of data collected in electron microprobe X-ray analysis.
More recently, the range of materials has increased, with NIST producing SiGe composition reference materials (RMs) in response to industry requests. (Without getting into too much detail, the RMs differ from the SRMs in that they are stated to be suitable for their intended purpose but not directly traceable to the mole.)
X-ray diffraction is widely used to determine the composition of aluminium in AlGaAs. Care is needed, however, because interpretation of the data must account for the substrate lattice parameter and doping-induced expansion or contraction of the lattice. Photo by James Burrus, NIST
54
www.compoundsemiconductor.net October 2013
One of the benefits of having reference materials available is that they can be used to improve accuracy of other compositional analysis methods, such as Auger spectroscopy, X-ray photoemission spectroscopy, and SIMS, provided that any necessary corrections for sampling depth considerations are included. The Fundamental Parameters projects – efforts by
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