S. Maimon

Intersublevel transitions in InAs/GaAs quantum dots infrared photodetectors

Thermal generation rate in quantum dots (QD) can be significantly smaller than in quantum wells, rendering a much improved signal to noise ratio. QDs infrared photodetectors were implemented, composed of ten layers of self-assembled InAs dots grown on GaAs substrate. Low temperature spectral response shows two peaks at low bias, and three at a high one, polarized differently. The electronic level structure is determined, based on polarization, bias, and temperature dependence of the transitions. Although absorbance was not observed, a photoconductive signal was recorded. This may be attributed to a large photoconductive gain due to a relatively long lifetime, which indicates, in turn, a reduced generation rate.Thermal generation rate in quantum dots (QD) can be significantly smaller than in quantum wells, rendering a much improved signal to noise ratio. QDs infrared photodetectors were implemented, composed of ten layers of self-assembled InAs dots grown on GaAs substrate. Low temperature spectral response shows two peaks at low bias, and three at a high one, polarized differently. The electronic level structure is determined, based on polarization, bias, and temperature dependence of the transitions. Although absorbance was not observed, a photoconductive signal was recorded. This may be attributed to a large photoconductive gain due to a relatively long lifetime, which indicates, in turn, a reduced generation rate.

Anophthalmia-plus syndrome: a clinical report and review of the literature.

We describe a term male infant of healthy non‐consanguineous parents, born with congenital malformations, including bilateral cleft palate and lip, mild microphthalmia with iris coloboma and glaucoma of the right eye, and blepharophimosis with severe microphthalmia of the left eye. Spine radiograph and MRI showed first sacral hemivertebra with spina bifida, and agenesis of the 2nd, 3rd, 4th, and 5th sacral vertebrae and coccyx. Spine MRI showed caudal tethering of spinal cord at L3 level, filum terminalis lipoma and a syringomyelia. Brain ultrasound and MRI showed hypoplasia of corpus callosum with mild dilatation of the lateral ventricles. Orbital MRI showed bilateral microphthalmia—distorted small left eyeball with posteriorly located lens, and a split vitreous body in the right eye, suggestive of primary hyperplastic vitreous. The karyotype was normal. Summary of the findings in nine cases (our case and eight published cases) support the notion that anophthalmia‐plus syndrome (APS) is a distinct syndrome. Gene locus of APS is yet to be identified.

Strain compensated InGaAs/InGaP quantum well infrared detector for midwavelength band detection

A high detectivity multiquantum well midinfrared photodetector is reported. It is based on a strain compensated InGaAs/InGaP on InP structure, using bound-to-continuum intersubband absorption, with λP=4.9 μm and ∼0.5 μm full width at half maximum. This design is unique by enabling a large critical thickness, thus increasing the quantum efficiency. Photodetectors with background-limited performance (BLIP) with detectivity of Dλ*(BLIP)=3.2×1010 cmHz/W up to 110 K, with only ten quantum well periods were implemented. Arguments are given to predict an optimized background-limited performance for this design up to 135 K.

Measurement of electron capture probability in quantum well

A new method for measuring the capture probability of excited electrons into quantum wells is presented. Unlike previous techniques, this method is direct, and enables the determination of capture probability as a function of injected electron energy above the well. It is based on an HBT-like structure, with a quantum well as a base. The capture probability is the ratio between base and emitter currents. Two types of bases were studied, a highly p-doped and a low n-doped. The capture by the p+ well is much larger than by the n well, probably since the dominant capture mechanism is by electron–hole and electron–hole–plasmon in the former, while in the n doped well the capture is due to a polar optical phonon interaction.

Vertical drift mobility of excited carriers in multi quantum well structures

Measurements of the vertical mobility of photo‐excited carriers in quantum well infrared detectors are reported. The low temperature mobility is extracted by means of a photo geometrical magnetoresistance technique. This approach is most suitable for the multi quantum well structure geometry. Both rectangular and stepped well structures were studied. They exhibit asymmetry with regard to bias polarity for both gain and mobility. This asymmetry is shown to be contributed by both well structure, and growth induced inequivalent interfaces. The low field mobility is governed mainly by unscreened ionized impurity scattering in the barriers. The well interfaces act as additional anisotropic scattering centers. We observe clear correlation between the extracted drift velocity and the optical gain. It is postulated that anisotropic well recapture causes polarity dependent energy distribution above the barriers. In addition, we conclude that intervalley scattering of the hot electrons has a substantial influence o...

Analysis of transients in semiconductor/semi‐insulator junctions

A junction between an epitaxial semiconductor and a semi‐insulating substrate serves as a tool for analyzing transient behavior in semi‐insulators (SI). At equilibrium a narrow region which is fully depleted from electrons exists in the SI. The application of a reverse bias results in an additional partially depleted region, initially depleted just from free carriers, adjacent to the fully depleted one. As the transient progresses electrons are released from the deep traps in the SI. The partially depleted region collapses into a wider fully depleted region. This process is manifested by a substantial current transient through the SI. The charges emitted recombine in the epitaxial layer leading to a decrease in its conductance due to a narrowing conducting path. Thus it is possible to characterize this process by measuring the (large) current through the epilayer rather than the (small) current through the SI. These transients are a function of the emission coefficient. Their analysis provides data also o...

Characterization of deep traps in semi‐insulators by current transients

Deep traps in semi‐insulators (SI) are characterized using a junction composed of an epitaxial p‐type layer grown on SI n‐type layer. At a reverse bias electrons are released from the traps resulting in a current transient through the substrate. Simultaneously the depletion region in the epilayer expands until the entire layer is depleted leading to a decaying epitaxial current. The analysis of these transients renders the electron emission and capture coefficients and lifetime, and the energy location of the traps. The long current decay are accelerated by illuminating the sample with photons of energy below the band gap, as long as their energy is larger than the difference between trap energy and the bottom of the conduction band. Thus we determined directly this energy difference.

InP-based QWIPs for long- and mid-wavelength band detection

In this work we reporrt on strained controlled InP/InGaAs and strained compensated InP/InGaAs/InGaP quantum well IR photodetectors (QWIPS) that cover the spectral bands 8-11 μm and 4-6μm respectively. Two different approaches were demonstrated in this work. i) We explore the effect of introducing non lattice-matched InGaAs well in InP/InGaAs QWIP structures. We show that this enables to extend the QWIP operating wavelength up to 11.5 μm taking advantage of strain as a bandgap engineering design parameter. State of art QWIPs with peak detectivity at 8.5 μm of 8×109 cmHz1/2/W at 80K with 2pi FOV was demonstrated. ii) A high detectivity mid-IR QWIPs were demonstrated. These devices are based on strain compensated INGaAs/InP/InGaP multiquantum well structures, where the InGaP conduction band offset extends the wavelength detection range ino the mid-IR. Photodetectors with background-limited performance with detectivity of D*λ (BLIP)=3.2*1010 cmHz1/2/W up to 110 K, were implemented. We conclude that InP/InGaAs material system is suitable fo multicolro QWIPs applications, particularly for two color QWIPs operating in the 8-12 and 3-5 microns range simultaneously.

Quantum Dots Infrared Photodetectors (QDIP).

Theoretical predictions show that thermal generation rate in quantum dots can be significantly smaller than in quantum wells, rendering a much improved signal to noise ratio. Quantum Dots Infrared Photodetectors (QDIP) were implemented in a GaAs/InAs material system. The device is composed of 10 layers of self assembled InAs dots grown on GaAs substrate. The illumination was introduced through a wedge in order to investigate polarization effects. A clear spectral response was obtained at 13K, with two peaks. The first, at 6.5 μm, is polarized parallel to the growth axis, and is attributed to excitation from the first confined level to the continuum. The second, centered around 15 μm, is unpolarized, and is due to excitation from the second confined level to the continuum. The fact that a photoconductive signal was recorded even though no absorbance was observed, indicates a very large gain due to a very long lifetime, consistent with theoretical predictions. It is estimated that once a device with quantum efficiency comparable to that of Quantum Well Infrared Photodetectors (QWIPs) is realized, it will be possible to operate QDIPs at temperatures higher than QWIPs.

Photoresponse of semiconductor on semi‐insulator structure

A new type of photoconductivity is demonstrated in a p‐type GaAs epitaxial layer grown on a semi‐insulating substrate. Rather than increasing the hole concentration in the epilayer, the photons absorbed in the substrate reduce the junction depletion region, thus increasing the thickness of the conducting path in the epilayer. Small photon fluxes increase drastically the conductance of the structure. A flux of 3×1013 cm−2 s−1 is sufficient to double the conductance at room temperature. Voltage responsivities of the order of 5×106 V/W were obtained at a wavelength of 0.84 μm at room temperature. Hall measurements indicate that the total hole concentration increases slowly at low fluxes (below 1015 cm−2 s−1) and steeply at higher ones. These two regions correspond to two processes contributing to the increased population. Photons absorbed in the substrate, originally n type, turn it into p type in the vicinity of the junction, since the lifetime of holes, dominated by recombination with EL2 traps, is four or...