Hybrid Back Surface Reflector GaInAsSb Thermophotovoltaic Devices Page: 4 of 9
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this level of reflectivity is expected to be sufficient to promote the effects of photon
recycling in device performance.
Masks for both the frontside and backside processing of reference and BSR TPV
cells were designed. The frontside mask pattern consisted of a two- level mask, involving
a mesa-etch mask and an ohmic-contact mask. Designed for processing one quarter of a
2-inch wafer, six 0.5 cm by 1 cm cells and two 0.25 by 0.5 cm cells can be
accommodated in one fabrication chip. The size of the two smaller cells was chosen
from previous investigations of the size dependence on TPV performance. Mesa etching
was selected instead of cleaving or saw cutting, as a means of electrical isolation of cells
in the design to minimize sidewall damage [11], [13]. For TPV cell separation, the mask
is designed to accommodate either cleaving or saw cutting.
The hybrid backside ohmic contact and BSR is constructed with a backside
fabrication process. The open areas between the contacts are for the BSR, which consists
of sputtered A1203/Ti/Au. The coverage of the BSR on the backside is approximately
90% by area. The mask is designed for a liftoff of the BSR, and subsequent evaporation
of the n-type ohmic contact between the mirror segments.
The specific contact resisitivity of the n-GaSb ohmic contacts, consisting of
Au/Sn/Ti/Pt/Au, has been measured by the Cox-Strack technique to be approximately 2.3
x 10-6 S2-cm2. The resulting contribution from the hybrid BSR/contact to the diode series
resistance is expected to be negligible compared with the typical 1-2 mQ series resistance
of these diodes.
After fabrication of both reference and BSR cells, the devices are packaged onto
copper submounts with indium solder for improved thermal management and to facilitate
testing (particularly for the 100- m-thick BSR cells, which are fragile). A metallized
ceramic flat is used as a contact pad for the device busbar, and multiple wirebond
connections are made from the contact pad to the busbar to ensure electrical current
uniformity.
Device Measurements
Illuminated I-V measurements are performed with the packaged device bolted to a
copper heatsink on top of a thermoelectric cooler. In figure 3, illuminated I-V
characteristics of a BSR cell (size approximately 0.7 cm by 0.5 cm) at a heatsink
temperature of T=10 C, fabricated from wafer 01-499 are shown. Excellent TPV diode
performance is apparent from these multiple intensity characteristics. The extracted Jsc
versus Voc is shown in figure 4 for the BSR and reference TPV cells at T=10 C, and in
figure 5 for the BSR and reference TPV cells at T=25 C. At a fixed Jsc=2.2 A/cm2, at
T=100 C, Voc=341 mV for the BSR cell as compared with Voc=336 mV for the reference
cell. At this same current density at T=250 C, Voc=317 mV for the BSR cell as compared
with Voc=312 mV for the reference cell. The improvement in Voc is about 4 to 5 mV at
both temperatures, as expected from the calculations discussed earlier. Moreover, the fill
factor of the BSR cells was better than that of the reference cell at both temperatures. At3
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Huang, R. K.; Wang, C. A.; Connors, M. K.; Turner, G. W. & Dashiell, M. Hybrid Back Surface Reflector GaInAsSb Thermophotovoltaic Devices, report, May 11, 2004; United States. (https://digital.library.unt.edu/ark:/67531/metadc783043/m1/4/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.