Aggregated compound semiconductor single photon avalanche diode (SPAD) arrays are emerging as a viable alternative to the silicon photomultiplier (SiPM). Compound semiconductors have the potential to surpass SiPM performance, potentially achieving orders of magnitude lower dark count rates and improved radiation hardness. New planar processing techniques have been developed to enable compound semiconductor SPAD devices to be produced with pixel pitches of 11 – 25 microns, with thousands of SPADs per array.
Single photon avalanche diodes (SPADs) are revolutionizing ultra-sensitive photodetection applications, providing single photon sensitivity, high quantum efficiency and low dark noise at or near room temperature. When aggregated into arrays, these devices have demonstrated the ability to operate as photon number resolving detectors with wide dynamic range, or as single-photon imaging detectors. SPAD array performance has reached a point where replacing vacuum tube based MCP and PMT photodetectors for most applications is inevitable. Compound semiconductor SPAD arrays offer the unique proposition to tailor performance to match application specific wavelength, speed and radiation hardness requirements. We present a theoretical framework describing performance limits to compound semiconductor SPAD arrays and our latest experimental results detailing the performance of GaAs SPAD arrays. These devices achieve nanosecond rise and fall times, excellent photon number resolving capability, and low dark count rates. Single photon number resolving is demonstrated with 4% single photon detection efficiency at room temperature with dark count rates below 7 Mcps/mm2. Compound semiconductor SPAD arrays have the opportunity to provide orders of magnitude improvement in dark count rate and radiation hardness over silicon SPAD arrays, as well as the ability to detect wavelengths where silicon is blind.
Long-wave infrared (LWIR) detector technologies with the ability to operate at or near room temperature are very
important for many civil and military applications including chemical identification, surveillance, defense and medical
diagnostics. Eliminating the need for cryogenics in a detector system can reduce cost, weight and power consumption;
simplify the detection system design and allow for widespread usage. In recent years, infrared (IR) detectors based on
uni-polar barrier designs have gained interest for their ability to lower dark current and increase a detector's operating
temperature.
Our group is currently investigating nBn and pBp detectors with InAs/GaSb strain layer superlattice (SLS)
absorbers (n) and contacts (n), and AlGaSb and InAs/AlSb superlattice electron and hole barriers (B) respectively. For
the case of the nBn structure, the wide-band-gap barrier material (AlGaSb) exhibits a large conduction band offset and a
small valence band offset with the narrow-band-gap absorber material. For the pBp structure (InAs/AlSb superlattice
barrier), the converse is true with a large valence band offset between the barrier and absorber and a small or zero
conduction band offset. Like the built-in barrier in a p-n junction, the heterojunction barrier blocks the majority carriers
allowing free movement of photogenerated minority carriers. However, the barrier in an nBn or pBp detector, in contrast
with a p-n junction depletion layer, does not contribute to generation-recombination (G-R) current.
In this report we aim to investigate and contrast the performance characteristics of an SLS nBn detector with that of
and SLS pBp detector.
We present our efforts on development of high performance low noise, long-wave infrared (LWIR)
and multicolor detectors based on the InAs/GaSb strained layer material (SLS) material system. The LWIR
SLS detector with PbIbN architecture showed improved performance over the conventional PIN design due to
unipolar current blocking layers. At 77K and Vb=-0.25V, a responsivity of 1.8 A/W, dark current density of
1.2 mA/cm2, quantum efficiency of 23% and shot noise limited detectivity (D*) of 8.7×1010 Jones (λc = 10.8
μm) has been observed. Dual band response was registered with 50% cut-off wavelengths of 5μm and 10μm
from an SLS detector with the pBp design. The responsivity equal to 1.6 A/W (at λ = 5 μm and Vb = +0.4 V)
and 1.8 A/W (at λ = 9 μm and Vb = -0.7 V) for MWIR and LWIR absorbers was achieved with corresponding
values of specific detectivity 5 x 1011 Jones and 2.6 x 1010 Jones, respectively. The maximum values of
quantum efficiency were estimated to 41% (MWIR) and 25% (LWIR) at Vb = +0.4V and Vb = -0.7V.
Moreover, the diffusion-limited behavior of dark current at higher temperatures was observed for the MWIR
absorber for pBp detector. Finally, three-color response was registered from three contact device with nBn
architecture for SWIR and MWIR and heterojunction PIbN architecture for LWIR detection (NbNbiP). At
77K, the cut-off wavelength for SWIR, MWIR and LWIR regions have been observed as 3.0 μm, 4.7 μm, and
10.1 μm respectively. At the same temperature, D* of 1.4 × 1010 Jones, 1.8 × 1010 Jones and 1.5 × 109 Jones
for SWIR, MWIR and LWIR signals has been observed.
Relationship between V/III beam equivalent pressure (BEP) flux ratios during the molecular beam epitaxial
(MBE) growth of long-wave infrared InAs/GaSb strained layer superlattice (SLS) material, crystalline quality of asgrown
material, and devices' signal (responsivity) and noise (dark current) characteristics was investigated. It was found
that the V/III ratio is a critical factor affecting the dark current, cut off wavelength and the responsivity of the device.
Modest change of As/In BEP flux ratio (from 5.5 to 7) resulted in red-shift of cut-off wavelength by 0.6 μm.
Temperature-dependent dark current measurements revealed more than two orders of magnitude difference in dark
current densities of detectors grown with different As/In BEP flux ratios. The highest responsivity and QE values, equal
to 0.75 A/W and 10% (74K, 9 μm, -0.4V), were demonstrated by the device with highest dark current density and notoptimal
structural properties. The observed dependences of devices' signal (responsivity) and noise (dark current)
characteristics in conjunction with the structural properties and the growth conditions of SLS material suggest that the
good structural properties of grown detector material as well as low noise would not necessary result in improved device
performance.
Next generation infrared photodetector technology will require focal plane array (FPA) systems that have multi-spectral
imaging capabilities. One proposed approach to realizing these multicolor devices is to use plasmonic resonators.
However, device development and characterization are commonly addressed with large front side illuminated single
pixel detectors on a supporting epitaxial substrate. The focal plane arrays on the other hand are backside illuminated.
Moreover, in a front side illuminated device, there is significant substrate scattering of the incident light. Here, we
propose a method for the accurate measurement of device performance by using a hybridized chip design (hybrid chip)
that is similar to the fabrication of an FPA system, with the substrate completely removed through a combination of
mechanical polishing and subsequent wet etching techniques. The hybrid chip was also designed to precisely
characterize the effects of varying mesa size by incorporating square mesa structures that range from 25 to 200 μm in
width. This approach offers an advantage over conventional device characterization because it incorporates mesas that
are on the same scale as those normally used in FPA systems, which should therefore provide a fast transition of new
photodetector technology into camera based systems. The photodetector technology chosen for this work is a multi-stack
quantum dots-in-a-well (DWELL) structure designed to absorb electromagnetic radiation in the mid-infrared spectral
range.
Silicon-on-Insulator (SOI) is an attractive platform for fabrication of photonic bandgap devices. The large refractive index step between the silicon waveguide and the SiO2 lower cladding layer permits realization of periodic waveguides with very large index modulation. The large refractive index modulation is obtained by deep anisotropic etching into the silicon guide region, and makes it possible to obtain strong resonances in compact periodic structures with only a few periods. In addition, the processing of these structures is highly compatible with standard silicon CMOS processing. Hence this technology is attractive for low cost, highly integrated photonic and optoelectronic circuits.
We report on a new type of a high quantum efficiency diode photodetector based on ultra-thin low temperature-grown InGaAs-on-Si heteroepitaxial layer. The device was characterized optically and electrically using low- temperature photoluminescence, capacitance-voltage, current- voltage, DLTS and spectral responsivity measurements. We study characteristics of the photodetectors with various In contents in InGaAs film. Photosensitivity spectral characteristics shift to the longer wavelengths with In/Ga flux ratio increase during the ultra-thing film growth. This dependence proves that photosensitivity of the InGaAs/Si heterostructure is a result of photons absorption and carrier separation in thin epitaxial layer of the polar semiconductor (2 - 20 nm). DLTS measurements underscore the role of a trap with activation energy of 0.59 eV in the photodetector properties. Using capacity-voltage measurements we demonstrate that the trap has delta-like spatial distribution and is localized at InGaAs/Si interface. According to our estimations the photosensitivity of the photodetectors exceeds 1.5 A/W at wavelength of 632.8 nm. We also reveal that our InGaAs-on-Si photodetectors have high external quantum efficiency in ultraviolet region, which exceeds one in the visible region. The high responsivity of the photodetector is explained by effective carriers separation in InGaAs layer and the carrier multiplication effect at InGaAs/Si interface.
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