Wafer-bonded avalanche photodiodes (APDs) combining InGaAs for the absorption layer and silicon for the multiplication layer have been fabricated. The reported APDs have a very low room-temperature dark current density of only 0.7 mA/cm<SUP>2</SUP> at a gain of 10. The dark current level is as low as that of conventional InGaAs/InP APDs. High avalanche gains in excess of 100 are presented. The photodiode responsivity at a wavelength of 1.31 micrometers is 0.64 A/W, achieved without the use of an anti-reflection coating. The RC-limited bandwidth is 1.45 GHz and the gain-bandwidth product is 290 GHz. The excess noise factor F is much lower than that of conventional InP-based APDs, with values of 2.2 at a gain of 10 and 2.3 at a gain of 20. This corresponds to an effective ionization rate ratio k<SUB>eff</SUB> as low as 0.02. The expected receiver sensitivity for 2.5 Gb/s operation at (lambda) = 1.31 um using our InGaAs/silicon APD is -41 dBm at an optimal gain of M = 80.
A broad-band infrared detector, sensitive over a 10 - 16 micrometer spectral range, based on GaAs/Al<SUB>x</SUB>Ga<SUB>1-x</SUB>As quantum wells grown by molecular beam epitaxy, has been demonstrated. Wavelength broadening of (Delta) (lambda) /(lambda) <SUB>p</SUB> approximately 42% is observed to be about a 400% increase compared to a typical bound-to- quasibound quantum well infrared photodetector (QWIP). In this device structure, which is different from typical QWIP device structures, two different gain mechanisms associated with photocurrent electrons and dark current electrons were observed and explained. Even with broader response, D* approximately 1 X 10<SUP>10</SUP> cm(root)Hz/W at T equals 55 K is comparable to regular QWIPs with similar cutoff wavelengths.
We have successfully fabricated intersubband GaAs/AlGaAs quantum well infrared photodetectors grown on GaAs-on-Si substrate and evaluated their structural, electrical, and optical characteristics. We have found that the performance is comparable to a similar detector structure grown on a semi- insulating GaAs substrate. The results are promising for applications in the important 8 - 12 micrometer atmospheric window.
Diffractive optical elements (microlenses) for quantum well infrared photodetectors (QWIPs) were fabricated by two techniques: (1) standard lithography of a binary optical structure and (2) PMMA pattern transfer for an analog diffractive optic structure. The binary lenses were fabricated by sequential contact lithography and etching using two binary masks. The analog diffractive lenses were fabricated in PMMA by direct-write e-beam lithography followed by acetone development. The resulting PMMA surface relief profile was transferred into the GaAs by dry etching. Both types of lenses were etching into GaAs using an electron cyclotron resonance (ECR) microwave plasma etching system. Although the lenses were fabricated accurately, the performance of the QWIPs was not improved as much as expected due to the angle-of-incidence sensitivity of the QWIP light-coupling grating. The lenses would have likely improved the performance of detectors capable of absorbing normally incident light.
One of the simplest device realizations of the classic particle-in-the-box problem of basic quantum mechanics is the Quantum Well Infrared Photodetector (QWIP). In this paper we discuss the optimization of the detector design, material growth and processing that has culminated in realization of 15 micron cutoff 128 X 128 QWIP focal plane array camera, hand-held and palmsize 256 X 256 long-wavelength QWIP cameras and 648 X 480 long-wavelength camera, holding forth great promise for myriad applications in 6 - 25 micron wavelength range in science, medicine, defense and industry. In addition, we present the recent developments in broadband QWIPs, mid-wavelength/long-wavelength dualband QWIPs, long- wavelength/very long-wavelength dualband QWIPs, and high quantum efficiency QWIPs for low background applications in 4 - 26 micrometer wavelength region for NASA and DOD applications.
One of the simplest device realizations of the classic particle-in-the-box problem of basic quantum mechanics is the quantum well IR photodetector (QWIP). In this paper we discuss the optimization of the detector design, material growth and processing that has culminated in realization of 15 micron cutoff 128 X 128 QWIP focal plane array camera, hand-held and palmsize 256 X 256 long wavelength QWIP cameras and 648 X 480 long-wavelength cameras, holding forth great promise for myriad applications in 6-25 micron wavelength range in science, medicine, defense and industry. In addition, we present the recent developments in broadband QWIPs and mid-wave long-wave dualband QWIPs at Jet Propulsion Lab for various NASA and DOD applications.
Intermixing of the well and barrier layers in quantum well infrared photodetectors (QWIPs) can be used to realize a broadened spectral response as well as multiple color detectors. We describe die experimental results of both rapid thermal annealing (RTA) and laser annealing (LA) QWIPs operating in the 8-12µm regime. The peak spectral response of the annealed detectors was shifted to longer wavelength as compared to die as-grown detectors. In general, a decrease in detector performance after annealing is also observed which may be attributable to a change in the absorption coefficient caused by the out-diffiision of dopants during annealing. Recent advances in growth technology, complimented by innovative structures should offset any degredation in performance. Thus, the post-growth control of the composition profiles by annealing offers opportunities to fine tune various aspects of a QWIP’s response.