External Quantum Efficiency Research Papers (original) (raw)

The LWIR and longer wavelength regions are of particular interest for new developments and new approaches to realizing long-wavelength infrared (LWIR) photodetectors with high detectivity and high responsivity. These photodetectors are highly desirable for applications such as infrared earth science and astronomy, remote sensing, optical communication, and thermal and medical imaging. Here, we report the design, growth, and characterization of a high-gain band-structure-engineered LWIR heterojunction phototransistor based on type-II superlattices. The 1/e cutoff wavelength of the device is 8.0 µm. At 77 K, unity optical gain occurs at a 90 mV applied bias with a dark current density of 3.2 × 10 −7 A/cm 2. The optical gain of the device at 77 K saturates at a value of 276 at an applied bias of 220 mV. This saturation corresponds to a responsivity of 1284 A/W and a specific detectivity of 2.34 × 10 13 cm Hz 1/2 /W at a peak detection wavelength of~6.8 µm. The type-II superlattice-based high-gain LWIR device shows the possibility of designing the high-performance gain-based LWIR photodetectors by implementing the band structure engineering approach. Recent advances in semiconductor materials and devices have led to much progress in the development of photodetectors for numerous applications across a variety of fields. Photodetectors are now able to broadly cover wavelengths from deep UV, visible, and near-infrared spectra all the way up to long-wavelength infrared (LWIR) and even terahertz spectral bands 1-8. As the prospects for conventional pin detectors begin to saturate, there is a need to develop new designs, such as barrier photo-detectors and ultra-sensitive devices with internal/intrin-sic gain, that can yield better detectivity. The current state-of-the-art LWIR detection technology is based on mercury cadmium telluride (HgCdTe) materials , which can achieve excellent sensitivity and speed. This material has been used to realize LWIR avalanche photodiodes (APDs) based on the gain from impact ionization mechanisms 9,10 ; however, in general, APD structures suffer from low photocurrent gain, which requires high bias voltages and suffer from excess noise associated with the avalanche multiplication process. Additionally, the Hg x Cd 1−x Te compositions needed for LWIR detectors are difficult to grow consistently, which leads to poor spatial uniformity. This complex material growth, as well as the additional challenges associated with processing II-VI materials, reduces device fabrication yield and significantly increases production costs 11. Alternate commercial technologies in the LWIR band include photodetectors based on vanadium oxide or amorphous silicon (α-Si), which offer several benefits such as high-temperature operation, compatibility with complementary metal-oxide-semiconductor technology, and low fabrication costs. However, applications for these photodetectors are limited by shortcomings such as limited tunability of the detection wavelength, poor sensitivity , and slow response speed 12. LWIR photodetectors based on graphene, or another material in combination with graphene, are being developed as a potential solution for future high-performance photodetectors 13-15. However, the application of these graphene-based devices is limited by the vanishing bandgap and poor light absorption of thin graphene layers , which in turn increases the dark current and overall noise level. Although approaches are being developed to