Samuel D. Hawkins

Time-resolved optical measurements of minority carrier recombination in a mid-wave infrared InAsSb alloy and InAs/InAsSb superlattice

Measurements of carrier recombination rates using time-resolved differential transmission are reported for an unintentionally doped mid-wave infrared InAsSb alloy and InAs/InAsSb superlattice. Measurements at 77 K yield minority carrier lifetimes of 3 μs and 9 μs for the InAsSb alloy and InAs/InAsSb superlattice, respectively. The un-optimized InAsSb-based materials also exhibit long lifetimes (>850 ns) at temperatures up to 250 K, indicating the potential use for these materials as mid-wave infrared photodetectors with improved performance over current type-II superlattice photodetectors at both cryogenic and near-ambient operating temperatures.

Identification of dominant recombination mechanisms in narrow-bandgap InAs/InAsSb type-II superlattices and InAsSb alloys

Minority carrier lifetimes in doped and undoped mid-wave infrared InAs/InAsSb type-II superlattices (T2SLs) and InAsSb alloys were measured from 77–300 K. The lifetimes were analyzed using Shockley-Read-Hall (SRH), radiative, and Auger recombination, allowing the contributions of the various recombination mechanisms to be distinguished and the dominant mechanisms identified. For the T2SLs, SRH recombination is the dominant mechanism. Defect levels with energies of 130 meV and 70 meV are determined for the undoped and doped T2SLs, respectively. The alloy lifetimes are limited by radiative and Auger recombination through the entire temperature range, with SRH not making a significant contribution.

Effects of layer thickness and alloy composition on carrier lifetimes in mid-wave infrared InAs/InAsSb superlattices

Measurements of carrier recombination rates using a time-resolved pump-probe technique are reported for mid-wave infrared InAs/InAs1−xSbx type-2 superlattices (T2SLs). By engineering the layer widths and alloy compositions, a 16 K band-gap of ≃235 ± 10 meV was achieved for all five unintentionally doped T2SLs. Carrier lifetimes were determined by fitting a rate equation model to the density dependent data. Minority carrier lifetimes as long as 10 μs were measured. On the other hand, the Auger rates for all the InAs/InAsSb T2SLs were significantly larger than those previously measured for InAs/GaSb T2SLs. The minority carrier and Auger lifetimes were observed to generally increase with increasing antimony content and decreasing layer thickness.

Comparison of nBn and nBp mid-wave barrier infrared photodetectors

We have fabricated mid-wave infrared photodetectors containing InAsSb absorber regions and AlAsSb barriers in n-barrier-n (nBn) and n-barrier-p (nBp) configurations, and characterized them by current-voltage, photocurrent, and capacitance-voltage measurements in the 100-200 K temperature range. Efficient collection of photocurrent in the nBn structure requires application of a small reverse bias resulting in a minimum dark current, while the nBp devices have high responsivity at zero bias. When biasing both types of devices for equal dark currents, the nBn structure exhibits a differential resistance significantly higher than the nBp, although the nBp device may be biased for arbitrarily low dark current at the expense of much lower dynamic resistance. Capacitance-voltage measurements allow determination of the electron concentration in the unintentionally-doped absorber material, and demonstrate the existence of an electron accumulation layer at the absorber/barrier interface in the nBn device. Numerical simulations of idealized nBn devices demonstrate that photocurrent collection is possible under conditions of minimal absorber region depletion, thereby strongly suppressing depletion region Shockley-Read-Hall generation.

Mesa-isolated InGaAs photodetectors with low dark current

We demonstrate InGaAs photodiodes with an epitaxial heterostructure that allows simple mesa isolation of individual devices with low dark current and high responsivity. An undoped InAlAs barrier and passivation layer enables isolation of detectors without exposing the InGaAs active region, while simultaneously reducing electron diffusion current. Photodetectors with mesa sizes as small as 25×25 μm2 exhibit dark current densities of 10 nA/cm2 at 295 K and responsivities of 0.62 A/W at 1550 nm.

Auger recombination in long-wave infrared InAs/InAsSb type-II superlattices

The Auger lifetime is a critical intrinsic parameter for infrared photodetectors as it determines the longest potential minority carrier lifetime and consequently the fundamental limitations to their performance. Here, Auger recombination is characterized in a long-wave infrared InAs/InAsSb type-II superlattice. Auger coefficients as small as 7.1×10−26 cm6/s are experimentally measured using carrier lifetime data at temperatures in the range of 20 K–80 K. The data are compared to Auger-1 coefficients predicted using a 14-band K·p electronic structure model and to coefficients calculated for HgCdTe of the same bandgap. The experimental superlattice Auger coefficients are found to be an order-of-magnitude smaller than HgCdTe.

Temperature-dependent optical measurements of the dominant recombination mechanisms in InAs/InAsSb type-2 superlattices

Temperature-dependent measurements of carrier recombination rates using a time-resolved optical pump-probe technique are reported for mid-wave infrared InAs/InAs1−xSbx type-2 superlattices (T2SLs). By engineering the layer widths and alloy compositions, a 16 K band-gap of ∼235 ± 10 meV was achieved for five unintentionally and four intentionally doped T2SLs. Carrier lifetimes were determined by fitting lifetime models based on Shockley-Read-Hall (SRH), radiative, and Auger recombination processes to the temperature and excess carrier density dependent data. The minority carrier (MC), radiative, and Auger lifetimes were observed to generally increase with increasing antimony content and decreasing layer thickness for the unintentionally doped T2SLs. The MC lifetime is limited by SRH processes at temperatures below 200 K in the unintentionally doped T2SLs. The extracted SRH defect energy levels were found to be near mid-bandgap. Also, it is observed that the MC lifetime is limited by Auger recombination in ...

Minority carrier lifetime and dark current measurements in mid-wavelength infrared InAs0.91Sb0.09 alloy nBn photodetectors

Carrier lifetime and dark current measurements are reported for a mid-wavelength infrared InAs0.91Sb0.09 alloy nBn photodetector. Minority carrier lifetimes are measured using a non-contact time-resolved microwave technique on unprocessed portions of the nBn wafer and the Auger recombination Bloch function parameter is determined to be |F1F2|=0.292. The measured lifetimes are also used to calculate the expected diffusion dark current of the nBn devices and are compared with the experimental dark current measured in processed photodetector pixels from the same wafer. Excellent agreement is found between the two, highlighting the important relationship between lifetimes and diffusion currents in nBn photodetectors.

Optical and electrical properties of narrow-bandgap infrared W-structure superlattices incorporating AlAs/AlSb/AlAs barrier layers

Optical and electrical properties of nBn photodetectors using InAs/AlAs/AlSb/AlAs/InAs/InAs0.61Sb0.39W-structure superlattice (W-SL) absorbers are reported. Minority carrier lifetimes of 500 ± 50 ns and 400 ± 30 ns, and Auger coefficients of 2.1 × 10−26 cm6/s and 1.6 × 10−25 cm6/s, for samples with bandgap energies of 5.3 μm (W-SL A) and 7.5 μm (W-SL B) are reported at 100 K, respectively. Shockley–Read–Hall defect states are identified at 65 meV and 45 meV above the W-SL valence band edges for W-SLs A and B, respectively. Dark currents are also reported and compared with diffusion currents calculated using the carrier lifetime data, suggesting low vertical heavy hole diffusivity.

GaSb-based infrared detectors utilizing InAsPSb absorbers

InPSb and InAsPSb have been investigated for use as absorber materials in GaSb-based n-type/barrier/n-type (nBn) detectors with cutoff wavelengths shorter than 4.2 μm. The growth temperature window for high-quality InPSb lattice-matched to GaSb by molecular beam epitaxy is approximately 440–460 °C. InPSb films with thicknesses greater than approximately 1 μm or films grown outside this temperature window have high densities of large defects, with films grown at lower temperatures exhibiting evidence of significant phase separation. In contrast, InAsPSb films can be grown with excellent surface morphologies and no apparent phase separation over a wide temperature range. InAsPSb samples with low-temperature photoluminescence between 3.0 and 3.4 μm and lattice mismatch of less than 1 × 10−3 have been grown, although both photoluminescence and x-ray diffraction data exhibit peak splitting indicative of compositional nonuniformity. AlAsSb-barrier nBn detectors with InPSb and InAsPSb absorbers have been fabrica...