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Various structures of self-assembled Ge/Si quantum dot infrared photodetectors were implemented and investigated. The electronic structure of the QDIPs was studied by electrical and optical techniques including I–V characteristics, dark current, photoconductivity, photoluminescence, and photo-induced infrared absorption. The photoconductive spectra consist of a broad multi-peak, composed of peaks ranging from 70 to 220 meV. Their relative intensity changes with bias. Comparative dark current measurements were performed. Dark current limits the performance of this first generation of Ge/Si QDIPs. It is plausible that direct doping in the dot layer is a viable way of reducing the dark current. � 2003 Elsevier B.V. All rights reserved.

The composition dependence of the fundamental bandgap of thin, pseudomorphic GaAs1−xNx layers (0⩽x⩽5%) on GaAs substrates is studied by optical transmission measurements and high resolution x-ray diffraction. We present a very large set of consistent experimental data. An empirical double exponential composition dependence of the bowing parameter is obtained. This expression accurately describes the measured bandgaps, and allows estimations for higher N incorporations. The bowing parameter reaches 40 eV for very low N incorporations (x≈0.1%), and strongly decreases with increasing N molar fraction. We estimate that the bowing parameter would reach a constant value of 7.5 eV for x≳8%, as in an alloy. This bowing would not be sufficient to close the bandgap for higher N incorporations.

Responsivity and thermionic dark current are studied in asymmetric stepped GaAs/AlGaAs multiquantum well infrared detectors with a single bound state. Although attributed to different transport properties, both the photoexcited and the thermally emitted carriers exhibit asymmetric transport properties with respect to the voltage polarity. The experimental characteristics are explained by the minibands-quasi continuum energy structure above the barrier.

Deep level transient spectroscopy (DLTS) can be used to characterize deep levels in Hg1-xCdxTe if the method is adapted to the narrow bandgap semiconductor case. The presence of the free carrier tail and the Poole-Frenkel effect may significantly influence the emission rate, and must be considered. The trap energy level extracted using DLTS is its entropy, which is equal to the trap activation energy only if the trap level follows the majority carrier band edge. If the trap capture cross section is measured as a function of temperature, the temperature dependence of the trap energy may be found. An n-type MIS capacitor sample was characterized as an example. The emission rate for the electron trap measured showed strong free carrier tail and Poole-Frenkel effects. The true trap energy was found to be a linear function of temperature.

We report on a comparison between the midinfrared absorption and the photocurrent response of n-doped InAs/GaAs self-assembled quantum dots. The absorption, resonant at 160 meV, is polarized along the z growth axis of the dots. The photocurrent is dominated by a z-polarized resonance around 220 meV (5.6 μm wavelength). A weaker component of the photocurrent is observed for an in-plane polarized excitation. The photoresponse can be measured for a 0 V applied bias. The photoresponsivity is investigated as a function of the applied bias. The responsivity and the dark current exhibit an asymmetric profile versus the external bias. This asymmetry is correlated to the structural asymmetry of the quantum dot layers.

The authors propose simple, analytic approximations that describe properties of semiconductors with nonparabolic energy bands. The approach is based on the two-level kp model of the semiconductor statistics which includes effects of band nonparabolicity and carrier degeneracy. The new expressions are in the form of a correction to the classical, Boltzmann approximation of the semiconductor statistics and do not introduce any new adjustable parameters. These relations can be applied to both narrow- and wide-bandgap nonparabolic semiconductors. The authors calculate the low-frequency capacitance of a nonparabolic MIS structure and compare the results with an accurate numerical model and experimental measurements of a HgCdTe capacitor. The solution includes the contributions of both types of carriers and the effect of impurity freezeout. The new approximations should be useful for characterization and modeling of semiconductor devices with nonparabolic energy bands. >

The transport of hot electrons in the AlxGa1−xAs barriers above the wells in a multiple quantum well (MQW) structure is investigated. The structures that are studied are asymmetric quantum well infrared detectors. The transport of the hot electrons normal to the layers is strongly dependent on both voltage and well shape. It is suggested that the key parameter which affects the transport properties is the dwell time of the electrons in the continuum, above the well region. This is most readily seen in asymmetric MQW structures, in which the dwell time under an applied bias depends very strongly on bias polarity. Calculations of electron transmission coefficient and dwell time show that the electron mean free path in asymmetric wells is much larger in positive bias than in a negative one. Employing this model, we achieve a very good fit to experimental data.

We have investigated the midinfrared photoconductivity of Ge/Si self-assembled quantum dots. The self-assembled quantum dots were grown by ultra-high-vacuum chemical vapor deposition on Si(001). The photoresponse of the p-type device exhibits resonances in the midinfrared around 10 μm wavelength. The resonance of the photocurrent shifts to lower energy as the applied bias increases. The photocurrent is weakly dependent on the incoming polarization of the infrared light. The photocurrent is analyzed in terms of bound-to-bound and bound-to-continuum transitions in the valence band. The photocurrent peaks are correlated to the photoluminescence of the device.

Photocurrent spectra due to interlevel transitions of holes in Ge/Si quantum dots show several peaks in the range of 60–300 meV, which superlinearly increase with bias, indicating release of carriers by tunneling. The relative peak intensity drastically changes with applied voltage, its polarity, and the measurement system. Lower energy peaks, at 69 and 86 meV, are observed only with a Fourier transform IR (FTIR) spectrometer. The 69 and 86 meV transitions excite holes into intermediate levels from which they are re-excited to shallow levels in a two-photon process. This is observed with FTIR only where the sample is simultaneously exposed to a wide range of energies. Direct band-to-band excitation at 1.25 eV increases the midinfrared signals by orders of magnitude by pumping the intermediate levels. Placing dopants in the barrier greatly increases photocurrent intensity and reduces noise. One-dimensional and three-dimensional numerical analyses confirm our findings.

InAs/InAlAs-on-InP quantum dots were implemented for infrared photodetection. The photoconductive spectra were measured as a function of polarization angle and bias. Normal incidence configuration was employed, to emphasize the contrast to the response in quantum well infrared photodetectors (QWIPs). Several peaks were observed, ranging from 90 to 420 meV. The strongest peak, at around 95 meV, is highly polarized. The superlinear increase of the intensity with bias, due to bound-to-bound transition followed by tunneling, was modeled successfully by WKB approximation. I–V characteristics show no improvement over QWIP performance.

Wave functions and oscillator strengths for interlevel transitions in InAs dots embedded in an InAlAs matrix, calculated using an eight-band strain-dependent k.p Hamiltonian are compared to measured photoconductive spectra. Several polarized peaks are present in the 80-400-meV range. A large peak is present around 160 meV for side illumination, in spite of small oscillator strength, while it is absent for normal incidence, and as a result of absorption. The high carrier concentrations of the contact layers produce a unique cavity. As the plasma frequency is approached, the real part of the refractive index drops. Radiation entering from the side is totally internally reflected, while front illumination is completely reflected. This effect of the plasma-confined micro-cavity was not reported previously, to the best of our knowledge.

The authors propose simple, explicit expressions that describe properties of semiconductor devices with nonparabolic energy bands. The new model includes effects of carrier degeneracy and impurity freeze-out. The approach is based on a recently proposed approximation for the semiconductor statistics derived from Kane's k.p model. The authors use that approximation to calculate intrinsic properties of semiconductors, n.p product, and the Einstein relation. They then solve a one-dimensional Poisson's equation and obtain a relationship between the total semiconductor charge and the surface potential. This method can be applied to heterojunction and field effect devices with wide and narrow band gaps. They use the new approach to calculate the incremental capacitance of an MIS structure and compare the results with an accurate numerical model and experimental measurements of an HgCdTe capacitor. >

A new type of asymmetric stepped GaAs/AlGaAs multiquantum well infrared detector is reported. These asymmetric detectors utilize the usual bound‐to‐continuum transition. The current responsivity is remarkably asymmetric with regard to the voltage polarity. In contrast with rectangular wells, in which responsivity is saturated in both bias polarities, these wells exhibit saturation only for negative bias. The responsivity increases monotonously with positive electric field. This is attributed to changes induced by the field on the transport properties of the excited electrons. In particular, the bias affects the dwell time spent by the carrier wave packet in the well region. Employing this model, we achieve a very good fit with experimental data.

Energy band nonparabolicity is present in the majority of materials used in heterojunctions, quantum wells, and superlattices. In the present work we derive a simplified dispersion relation for electrons using the k⋅p model. We demonstrate that this dispersion relation can account for nonparabolicity in both narrow and wide gap isotropic semiconductors. We then calculate the carrier effective mass and densities of states for both two‐ and three‐dimensional electron systems. Finally, we derive a simple analytical relation between the carrier concentration and the Fermi energy in a nonparabolic two‐dimensional electron gas. Agreement with a numerical model is demonstrated, while the traditional, parabolic approximation results in a large error. The simplicity of the new approximation allows an intuitive understanding of the nonparabolicity effect in two‐dimensional systems. Therefore, the new approach should be useful for design, characterization, and modeling of quantum semiconductor devices with nonparabolic energy bands.

Infrared detectors were implemented on InAs self-assembled quantum dots fabricated using Stranski–Krastanov growth mode on InAlAs matrix, lattice matched to InP (0 0 1) substrates. These dots grow with a shape of small elongated boxes, with their long axis along the [ 1 1 0] direction, and with a high concentration of 7×10 10 cm −2 . Photoconductive measurements were performed in all three polarizations. Rich spectra in the range of 50–500 meV, with different polarization selection rules were observed. The bias dependence of peak intensity of the intraband transitions serves as an additional tool to identify their origin. Some of the peaks, which increase linearly with bias, are attributed to bound-to-continuum transitions. Others, which appear only at larger biases, and increase superlinearly, are due to bound-to-bound transitions. The magnitude of detector responsivity at normal-incidence is similar to that obtained for polarization normal to the layers, and is comparable to that achieved in QWIPs. BLIP conditions prevail at 77 K for integral photocurrent response at F#1. The effect of unintentional doping is discussed. It is shown that this doping can be destructive for detector operation unless the density of dots is large.

A novel Bragg confining structure is proposed and analyzed. The combination of a shallow quantum well with superlattice‐Bragg reflectors makes it possible to tailor separately the eigen‐energy, and the wave function of the Bragg confined state. Longer lifetimes and higher effective barriers than in previously proposed Bragg confining structures is possible with the present scheme. Photoluminescence at energies close to the highest direct bandgap of the material system in use is expected from the proposed structure.

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.

InAs nanostructures on InAlAs / InP (0 0 1) have been fabricated using Stranski–Krastanov growth mode. Depending on growth conditions, a full coverage of the InAlAs surface by either InAs strings of elongated dots or isolated dots can be achieved. Giant intraband absorptions are observed at mid-infrared wavelengths in both nanostructures. The intraband resonances are strongly polarized in the layer plane as a consequence of the quantum confinement along either [1 1 0] and [1 −1 0] directions.

Photoconductive spectra of InAs quantum dots embedded within InAlAs barriers show several peaks, ranging from 4 to 12 μm . The nature of these peaks differs, as indicated by their dependence on bias and on temperature. The long wavelength peak drops rapidly with temperature, while the first peak hardly changes up to 60 K . While the wide peak around 4 μm depends linearly on bias, indicating a direct excitation into the quasi-continuum, the peak at 12 μm increases super-linearly since the photocarriers are generated by a two-step process, excitation into a bound level followed by tunneling to the continuum. This explains the drop with temperature, since as the temperature increases more carriers relax from the excited state back to the ground level rather than tunneling into the continuum.

The issue of thermal imaging systems exposed to large photon fluxes is of great practical importance, as it is encountered while observing a scenery which includes ‘‘hot’’ sources. High illumination level phenomena appear when excess carrier density is comparable or larger than majority carrier density in the bulk. This state can be reached quite easily in n‐type HgCdTe detectors due to the moderate doping levels (typically ≤1015 cm−3). A modified expression for the current responsivity is derived, suitable for use at both high and low illumination levels. The quasineutrality assumption is verified; thus the ambipolar equation can be applied to describe high illumination behavior. Photoconductive detectors with and without covered electrodes are measured and analyzed. It is shown that under high illumination levels (1) the dominating recombination mechanism is the phonon assisted Auger process, and (2) the relative photoresponse for a covered electrode structure decreases monotonously with increasing cove...

Various surface passivations of p-type Hg1-xCdxTe were studied to understand their interface properties and and their potential for device technology. Anodic oxide forms an inverted layer near the interface. This n-type skin layer exhibits extremely good n-type properties, which equal, and even surpass, bulk properties. The high electron mobility may be explained by quantization of the electron levels in the space-charge region, and the formation of a two dimensional electron gas near the interface. Thick (approx 500 A) anodic sulfide generates a similar inversion layer. The charge density is proportional to the sulfide thickness. Carefully prepared thin (approx 100 A) anodic sulfide films as well as ZnS coating on freshly etched sur-faces, form nearly Hatband conditions which are suitable for n+ on p diode technology. The surface recombination velocity, determined for these two passivations using the photoelectromagnetic effect, is shown to be similar at low temperatures, increasing with decreasing temperatures. The dominant trapping mechanism at the surface is similar to that in the bulk, and is probably mostly due to vacancies.

For heterodyning two optical beams it is necessary to obtain two almost coherent waves. Rather than using an extremely stable laser source, it is possible to split the beam from a single source and use a modulator to shift the frequency of one of the waves. The heterodyne signal generated is at the frequency of modulation, thus, it is susceptible to rf interference by the high modulation voltage. Using two electro‐optical modulators, one can utilize the signal at the sum or difference frequency at which no high voltage is present. It is shown that using a single source and two modulators one obtains a signal without any interference leakage. Thus it is possible to measure easily the frequency response of photodetectors up to high frequencies. This configuration also doubles the frequency range obtainable from a single modulator. The theoretical expressions for the intensities at the various frequencies are presented. Noise equivalent power (NEP) measurements of heterodyne signals performed using this setu...

Room‐temperature photoconductivity measurements were carried out on metalorganic chemical vapor deposition grown CdTe‐ZnTe superlattices with a periodicity of 30–80 A. The threshold of the photoconductivity measurements varied with the size of the well. Transition of the electron to the n=1 level in the conduction band was observed in all samples; the transition to the n=2 level was found for the superlattices with well width exceeding 25 A. These findings agree very well with room‐temperature absorption measurements performed on the same superlattices and with model calculations which were based on the Kronig–Penney model with Bastard’s boundary conditions.

The validity of the assumption that the contribution of the n region to the current of Hg1-xCdxTe diodes is negligible was examined. By placing such diodes in a magnetic field we were able to separate between the current component originating in the p -type substrate, and that from the graded n region. Experimental results show that the ratio between these currents is 0.5 - 3, depending on temperature. The theoretical analysis reveals the influence of the electric field present outside the depletion region on the current generated by the graded region. This field not only produces a drift component, which drives the minority carriers into the junction, it also greatly modifies the excess carrier distribution, thereby changing diffusion part of the current. The analysis shows the importance of the lifetime profile in the graded region, which is a function of the specific recombination mechanism and its dependence on the local dopant concentration. The effect of parameters such as substrate concentration, surface concentration, and junction depth on this current is discussed.