A pH adjusted acidic solution of thioacetamide (TAM) was used as a sulfidizing agent to treat long wavelength infrared
(LWIR) superlattice surface for the first time. The results were compared against those for ammonium sulfide [(NH4)2S]
which have been used earlier for the same purpose. X-ray photoelectron spectroscopy (XPS) results revealed that TAM
treatment attains a much pronounced degree of sulfidization on superlattice surface. Electrical measurements on mesa-etched
diodes exhibited maximum zero bias dynamic resistance times area (R0A) value of 590 Ω-cm2, approximately a
four times improvement compared to (NH4)2S treated diodes. XPS studies revealed the reappearance of detrimental
oxides on the TAM treated surface after long term air exposure asserting the need for a suitable capping layer to preserve
the quality of the surface. Atomic layer deposition (ALD) was used to cap the TAM treated surface with zinc sulfide
(ZnS). Precise deposition of few monolayers of ZnS on TAM treated surface was further studied using XPS to
understand the evolution of bond formations at the semiconductor-dielectric interface.
A novel heterostructured dual carrier multiplication extremely high gain MWIR InAs/InGaSb Type II strained layer
superlattice (T2SLS) impact ionization engineered (I2E) APD was designed and simulated. Spatially separated T2SLS
electron and hole multiplication regions are designed using 14 band k.p bandstructure modeling. In the novel dual carrier
device, the I2E T2SLS electron and hole multiplication regions are placed right next to each other. This allows for a
carrier feedback between the electron and hole multiplication regions. This feedback between the electron and hole
multiplication regions allows for extremely high gain values for the overall device. While the individual gain of the
electron and hole multiplication regions can be kept extremely low, the overall gain can be >103. This can be achieved at
a reverse bias of 3.5V. The effective k is designed to be approximately .07. Such low bias operation of the MWIR APD
allows for active operation and passive mode operation on the same pixel using standard ROIC and this opens up
possibility of large format dual mode imaging arrays.
Photodetectors with high bandwidth and internal gain are required to detect highly attenuated optical signals for defense
application and long distance communication. IR avalanche photodiodes (APDs) are best suited for this purpose due to
their internal gain-bandwidth characteristics coupled with long range data transmission capability. For the past two
decades, HgCdTe has been the most successful material for infrared photodetector applications. Recent advances in
epitaxial growth techniques made possible the growth of advanced HgCdTe APD structures, but to the best of our
knowledge all are grown on expensive substrates (e.g. CdZnTe, CdTe). We report for the first time HgCdTe-based
MWIR (4.5 μm) p-i-n APD grown on Si substrate by molecular beam epitaxy (MBE). The devices were fabricated by
365nm UV photolithography and wet-etching technique. The diode had a junction area of 300μm diameter. The R0A of
the diode was 3 x 106 Ω-cm2 at 77K. Multiplication gains of 800 were measured at a reverse bias of 10 V in the linear
operation regime. The gain increased exponentially as the reverse bias was increased, indicating that only one carrier is
responsible for the impact ionization. Temperature dependence of the multiplication gain and of the breakdown voltage
further confirms that avalanche multiplication dominates high reverse bias I-V characteristics.
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