Dual-band infrared photodetectors with a modified pBp design have been demonstrated. The modified pBp structure consisted of a p-type InAs/GaSb superlattice for long-wavelength (LW) detection and a p-type InAs/GaSb/AlSb based superlattice for mid-wavelength (MW) detection, which were separated by a hole barrier consisting of an InAs/AlSb superlattice. Our pBp device showed that dual-band detection was possible by changing the bias polarity of the applied voltage. By using an InAs/GaSb/AlSb based superlattice as an MW absorber for a pBp photodetector, a 100 % cutoff wavelength was blue-shifted from 8 μm to 7 μm compared with a conventional InAs/GaSb superlattice, while maintaining the same 50 % cutoff wavelength of around 6.4 μm. Quantum efficiency per period of the modified MW absorber was comparable with that of a conventional MW absorber. These results indicate that our modified pBp structure is expected to be a promising candidate for dual-band infrared photodetectors.
To suppress the surface leakage current of InAs/GaSb Type-II superlattice (T2SL) infrared photodetectors, atomic layer deposited (ALD)-Al2O3 passivation effects have been investigated. By using the ALD-Al2O3 passivation layers, surface leakage current was more effectively suppressed than by using chemical vapor deposited-SiO2 passivation layers. The deposition temperature of ALD Al2O3 played an important role in minimizing the surface leakage current of T2SL infrared photodetectors. We found that the dark current density of mid-wavelength (MW) p-i-n structures was limited by their bulk components with Al2O3 passivation layers deposited at or below 200 °C, while the dark current density increased with the surface leakage when the layers were deposited at 260 °C. From the capacitance–voltage analysis, it was found that the deposition at 260 °C led to a large interface trap density at the Al2O3/GaSb interface. The results of X-ray photoelectron (XPS) spectroscopy show that the spectra of Sb2O3 decreases while that of Ga2O3 increases when the deposition temperature increases from 200 to 260 °C. This indicates that the reaction of Sb2O3 with GaSb is thermally enhanced. Based on these results, we conclude that Ga2O3 and/or elemental Sb may lead to an additional leakage path. Hence, suppression of the thermal decomposition of Sb-related oxides during Al2O3 deposition is required to obtain good passivation effects.
The authors summarize the past 40-years history on the development of HgCdTe infrared detectors in Japan. At the
early stage of development of material growth, high-quality HgCdTe layers were obtained by liquid phase epitaxy
technique, owing to lattice-matched CdZnTe substrates. Hetero-epitaxial growth techniques of HgCdTe were also
successfully developed to obtain epilayers on much larger and cheaper substrates such as GaAs and Si, using methods of
metal-organic chemical vapor deposition and molecular beam epitaxy, where key issues were controlling surface
orientation, surface polarity and so forth. Fabrication process of p-on-n junction photodiodes was developed with various
improvements on ion implantation and surface passivation. On the basis of technologies mentioned above, large-scale
infrared focal plane arrays were realized with superior thermal images.
Recently, quantum dot infrared photodetectors (QDIP) have been intensively investigated because they can be
fabricated by conventional matured GaAs processing. QDIP can detect normal incident light in contrast to quantum well
infrared photodetectors (QWIP) which need optical grating or reflector. Also QDIPs operate at higher temperature,
taking advantage of their lower dark current theoretically than that of QWIPs.
In this report, we describe our effort to realize long-wavelength infrared (LWIR) QDIP infrared focal plane array
(IRFPA), which uses molecular beam epitaxially grown self-assembled quantum dot (SAQD) multilayers. We have
successfully "engineered" the transition levels of SAQDs to LWIR (8-12 μm) energy region, where relatively lower
quantum levels were pushed up near the conduction band edge of AlGaAs intermediate layers. In addition, these SAQD
multilayers bring QDIP responsivity enhancement due to their higher dot density.
We applied this structure to 256×256 pixel LWIR QDIP IRFPA. As a result, we realized the response peak
wavelength of 10 μm and noise equivalent temperature difference of our newly developed QDIP was 87 mK at 80 K, 120
Hz frame rate with f/2.5 optics. We obtained the excellent quality of IR image and confirmed our QDIP's high sensitivity
and high speed operation.
A novel quantum-well infrared photodetector (QWIP) with peak responsivity in the mid-wavelength (MW) range was characterized, and the performance of a focal-plane array (FPA) based on the MW-QWIP was investigated. InGaAs/AlGaAs quantum wells were used for the QWIP, resulting in a peak absorption wavelength in the range of 4.5~5.0 μm. The doping concentration and In composition of the well layers were varied to improve the photocurrent of the QWIP. The relationship between the noise of the QWIP and the number of multi quantum well (MQW) layers was also investigated, and the optical gain g was estimated. The noise-equivalent temperature differences (NETDs) of QWIPs with various numbers of MQW layers were calculated, and the optimum number of MQW layers was evaluated. It was found that controlling the In composition of the wells was very effective for improving the photocurrent. As a result, a MW-QWIP FPA with a NETD of 21 mK at an operation temperature of 80 K, an integration time of 16 ms, and F2.0 optics was fabricated.
We investigated the mechanism of the photocurrent transmission in mid-wavelength quantum-well infrared photodetectors that were made using InGaAs/AlGaAs quantum wells so that their peak absorption would be at a wavelength near 5 μm. Analyzing the bias-voltage dependence of the photocurrent for the samples with different well layer thicknesses, we found that the photocurrent transmission could be accounted for by taking into account the tunneling process via the triangular barrier, the effect of the intrinsic electric field due to the unintentional impurities, and the effect of the drift velocity.
We investigated GaAs/AlGaAs QWIP devices that had TiW/Au reflector electrodes and found that the thermal stability, and as a result, the uniformity of the IV characteristics were dramatically improved by the insertion of a TiW diffusion barrier. A secondary ion mass spectroscopy analysis showed that with the TiW insertion, the Au in-diffusion during the thermal process at around 400°C was completely suppressed. A reflectivity measurement of the GaAs/Au and GaAs/TiW/Au structures revealed that the reflectivities were almost the same, indicating the TiW/Au structure was a good candidate for use as a reflector electrode. A comparison of the fabricated GaAs/AlGaAs QWIP devices that had conventional Au with our new TiW/Au reflector electrodes showed that our devices exhibits a performance comparable with that of the conventional devices because of their similar reflectivity. Furthermore, the uniformity of the IV characteristics of more than 50 devices was greatly improved, especially in the reverse-biased region where the upper AlGaAs barrier (nearest to the reflector) served as an electron-emitter.
We investigated the behavior of the dark current (Id) in quantum well infrared photodetectors (QWIPs) in which the barrier layers were selectively doped instead of the well layers. Because the selective doping bends the conduction band (CB) edge in the portion of the barrier near the interface, the mechanism by which carriers in the wells can be emitted over the barriers, i.e. thermal emission and tunneling through this portion of the barrier, could be emphasized. We first confirmed that selectively doping the barrier layers clearly affects the Id-V characteristics. Then, by evaluating the activation energy obtained from the temperature dependence of Id, we found that the Poole-Frenkel emission (PFE) mechanism and the thermal-assisted tunneling (TAT)-like mechanism are dominant in the lower bias and higher bias regions, respectively.
We fabricated the GaAs/AlGaAs Quantum Well Infrared Photo detector (QWIP) focal plane array with selectively re-grown N- GaAs interconnection plugs and demonstrated its device operation, in order to establish the technology to obtain both complex device functions and device manufacturability. MBE (Molecular Beam Epitaxy) grown QWIP MQW wafers were covered with SiON and SiNx mask films to obtain selectivity of the re-growth process. N-GaAs plugs were re-grown selectively with low-pressure MOCVD (Metal-Organic Chemical Vapor Deposition) with AsH3 and Dimethylgalliumchloride as precursors, only on the bottom surfaces of the holes for the interconnection to extract the electrodes from the underlying epilayer. Cross- sectional SEM observation revealed that the feature of the re- grown N-GaAs plugs was triangular, rather than rectangular as expected. The reason for this discrepancy is not yet clear. The electrical contact between the epilayer and re-grown N- GaAs plug was 'ohmic-like,' without any trace of interfacial barrier. The Current-Voltage characteristics of the fabricated QWIP device showed no tangible leakage current between the N- GaAs plug and device structure, indicating that electrical insulation between the N-GaAs plugs and device structure was sufficient. Fabricated devices were successfully operated as a hybrid focal plane array, indicating the selective re-growth was a promising technique to realize complex QWIP based devices.
We developed a very simple and useful method for observing the optical absorption due to intersubband transition in quantum wells. This new technique based on attenuated total reflection (ATR) is applied to evaluate epitaxial wafers containing multi quantum wells (MQW), used for quantum well infrared photodetectors (QWIP). For the intersubband transition, normal incidence produces only weak absorption because of the quantum selection rules on polarization. ATR has been used to emphasize the intersubband absorption, however, the GaAs substrate including MQW had to be made into a prism. We observed intersubband absorption without such preparation of GaAs substrate by just attaching a Ge prism to it. Attached prism in ATR is normally used for collecting the absorption near the sample surface by utilizing the total reflection at the prism-sample interface. However, we adjusted the angle of incidence to propagate the refracted light into the sample because absorption occurred inside epitaxial layers in the QWIP structure. Using this modified prism attached ATR, we were able to measure the intersubband absorption more clearly than in the case of conventional prism attached ATR. Due to avoid the complicated sample preparation, we could easily compare the absorption of MQW with QWIP device characteristics.
We investigated the operation temperature dependence of the characteristics of quantum-well infrared photodetector focal plane arrays (QWIP-FPAs) for the 8 - 10 micrometer wavelength region from 65 K to 80 K. We found that a proposed simple circuit model explains the temperature dependence of the DC output and signal intensity of the QWIP-FPA. In this model, we used empirical current-voltage (I-V) characteristics of the QWIP, which was not hybridized with the readout integrated circuit (called 'QWIP itself'), measured at various temperatures and a simplified equivalent circuit model. The signal intensity of the QWIP-FPAs decreases as the temperature increases, while the photo-current of the QWIP itself increases slightly as the temperature increases. The difference between these behaviors is because the bias applied to QWIP in QWIP-FPA varies during the integration cycle and the bias applied to QWIP itself is constant. The noise equivalent temperature difference (NETD) increases from 0.10 K to 0.20 K as the operation temperature increases from 65 K to 80 K, since the signal intensity decreases and the shot noise increases with increasing the dark current.
We investigated the sensitivity enhancement of quantum well IR photodetectors (QWIP) with the pseudo-random optical coupler. We fabricated QWIP focal plane arrays (FPA) containing AlGaAs/GaAs multi-quantum wells (MQW) detecting 7-8 micrometers IR regions. After QWIP-FPA and read-out integrated circuits were hybridized, GaAs substrates attached to the back of the FPA were removed. We compared the sensitivity of a QWIP-FPA without a GaAs substrate with that of a QWIP-FPA with the substrate. When the input IR beams illuminated only some pixels in the QWIP-FPA, the output signal from the FPA without GaAs substrate was about ten times higher than that from the FPA with a substrate. The output signals were almost same for both QWIP-FPAs where all pixels were uniformly illuminated by the IR inputs. These result indicate that the reflected beams from the GaAs back surface illuminate another pixels far away from the pixels which input beams strike first, when the FPA has a sufficiently thick GaAs substrate. We confirmed that the pseudo-random grating coupler for QWIP enhanced IR absorption by confining multi internal total reflections in one pixel, when thick substrates were removed.
Antiphase- and twinning-free (111)B HgCdTe layers were directly grown on (100) Si substrates by metalorganic vapor phase epitaxy (MOVPE). The quality of the HgCdTe layers was evaluated for long-wavelength infrared (LWIR) photodiodes. Direct growth of (111)B CdTe on (100) Si tended to contain antiphase and twinning due to a lack of polarity in the Si crystal structure. To polarize the nonpolar Si surface, we adsorbed polar molecules on Si surface with metalorganic tellurium (Te). A metalorganic tellurium adsorption and annealing technique is effective for growing high quality CdTe buffer layers on Si substrates. This technique eliminates antiphase domains and prevents twinning. The crystallinity of the Hg1-xCdxTe (x equals 0.22 to 0.24) layers grown on Si was evaluated. We have achieved 119 arc sec. full width at half maximum (FWHM) by x-ray analysis and 1.5 multiplied by 106 cm-2 etch pit density (EPD) for a 17-micrometer-thick layer. LWIR photodiodes were fabricated from the p-type (111)B HgCdTe layers on (100) Si substrates using planer technology. The n-type regions, formed by boron ion implantation, were 50 micrometer by 75 micrometer by design. A quantum efficiency of 42% was obtained at a cutoff wavelength of 9.0 micrometer at 78 K. The zero bias resistance-area product (R0A) was 8.9 multiplied by 101 (Omega) cm2. We were able to increase the R0A and quantum efficiency with MOVPE grown HgCdTe/Si wafers by 50% of those obtained with liquid phase epitaxy (LPE) grown HgCdTe/CdZnTe.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.