Spectral utilization in thermophotovoltaic devices Page: 6 of 9
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- Sample 5
- S le 2 Sample 3
ample 1Sample 4
0 5 10 15 20
Figure 2. Reflectance vs. Wavelength for a series of n-In0.S3Gao.47As layers grown epitaxially onto
InP. Free-carrier absorption bands can be seen at 7, 10, 12.5 and 7 in Samples 1, 2, 3 and 5,
---respectively..Absorption-in-the-InP-substate-is seen in Sample 4 because the lowly-doped
LCL does not exhibit plasma reflectance at these wavelengths.
Plasma reflection in highly n-doped layers results in the reflection of low-energy photons
(approaching 98% reflectance for Sample 1). With decreasing dopant density, the plasma reflection is
attenuated and the resonance absorption band broadens. Note that layers doped below -3 x 1018 cni3
exhibit no free carrier absorption band and reveal the absorption characteristics of the InP substrate that
were previously masked by the plasma reflection (Sample 4 in Figure 2).
The importance of layer thickness is evident upon comparison of the reflectances of Samples 1 and
5. Although doped at comparable levels, the plasma absorption is greatly attenuated in Sample 5 which
is -1/3 as thick as Sample 1. This behavior is expected from the exponential dependence of absorption
on thickness. Also, although each of the layers were grown as the n-Ino.53Gao.47As (Xg=1.70 pm)
composition, the Burstein-Moss shift decreases the bandgap with increasing dopant density.3
The second major source of optical loss within TPV devices is absorption by p-type layers. Here,
the emitter dominates the other p-type layers in the device due to its dopant density and its thickness. In
order to evaluate the emitter's contribution to the spectral utilization, a series of p-Ino.s3Gao.47As layers
were grown epitaxially onto semi-insulating InP. Additionally, an.InP layer was grown to quantitate the
parasitic absorption in the top window layer. The pertinent properties of these layers are summarized in
Table 3. Reflectance vs. wavelength data acquired for the four samples is shown in Figure 3.
Table 2. Physical Properties of p-Ino.53Gao.47As layers on InP.
Sample Carrier Density Carrier Mobility Thickness % Power Absorbed (PA) X > 2 pm*
(x1019 cm-3) (cmn/Vs) (Pum) 1750*F Radiator 2250*F Radiator
6 1.2 70 0.6 28.2 26.8
7 1.2 70 0.3 22.2 20.7
8 1.2 70 0.1 8.9 8.0
p+lnP 0.2 81 0.1 7.1 6.7
Although p-type layers do not display resonance absorption bands such as those caused by free-
carriers in n-type materials, broadband absorption in p-type materials is equally detrimental to device
efficiency. This fact is evident through comparison of the percent power absorbed in the various layers
listed in Tables 2 and 3. In fact, p-type materials that are 10% as thick as comparably doped n-type
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Clevenger, M.B. & Murray, C.S. Spectral utilization in thermophotovoltaic devices, article, December 31, 1997; United States. (https://digital.library.unt.edu/ark:/67531/metadc708642/m1/6/: accessed March 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.