|
Reported is a detailed analysis of the dark current versus voltage versus temperature data
of planar hetero-structure P+n mid wavelength infrared MWIR photodiodes with band
gap energy Eg(78K) = 0.243 eV, λg= 5.1 μm and long wavelength infrared LWIR
photodiodes with Eg(78K) = 0.115 eV, λg= 10.8 μm. The purpose of the investigations is
to identify the dominant carrier recombination mechanisms and in particular to determine
at what temperature and voltage is the onset of Shockley Read Hall (SRH) space charge
currents. The important finding is that the currents can mostly be explained by a
combination of Auger (e-e) and radiative carrier recombination processes with no
evidence of SRH recombination through near mid-gap states; a lower bound estimate of
the SRH lifetime for LWIR photodiode is 100 μs. Intrinsic radiative recombination is
found to be the dominant carrier recombination mechanisms for the MWIR photodiode
with a carrier concentration Nd=1015 cm-3, and Auger (e-e) being dominant for the LWIR
photodiode. The LWIR Auger (e-e) lifetime data is well fitted with the Beattie,
Landsberg and Blakemore (BLB) formulas with a constant overlap integral F1F2= 0.15,
which is in accord with recent electronic band structure calculations. From the analysis
of variable area LWIR photodiodes the minority carrier conductivity mobility and
diffusion length at 80K are calculated to be 350 cm2/V-s and 23 μm respectively. The
LWIR lifetime measured by the photoconductive decay method is in agreement with the
expected intrinsic Auger (e-e) lifetime ≈ 2 μs at 80K and with the lifetimes obtained from
device analysis. For T ≤ 40K, trap assisted tunneling is the dominant current in reversed
bias LWIR photodiodes; forward bias currents are dominated by diffusion currents of
origin in the n- layer. For the MWIR photodiode deviation from diffusion limited
behavior to G-R is observed at T < 80K and, the SRH lifetimes ιn0 and ιp0 are estimated
to be 50 ms. Measured and calculated external quantum efficiencies at the peak
responsivity wavelength λpk for both MWIR and LWIR photodiodes are ≈ 70% at 78K.
For imaging in the 3-5 μm spectral band scene temperature 300K, F/3 optics, the noise
equivalent temperature difference NE▵T of MWIR photodiodes is calculated to be near
background limited performance BLIP =12.4 mK for detector temperatures Td ≤ 150K.
|
