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article i nfo Available online 19 March 2013 Gallium focused-ion beam milling is a commonly used technique for sculpting diamond at the nano and micro scale. However even at fluences insufficient to cause sputtering, implanted gallium causes modification of the optical properties of the diamond substrate. We implanted 30 keV gallium ions at fluences from 1×1 0 13 to 5 × 10 14 Ga/cm 2 and studied the effect on the optical properties via spectroscopic ellipsometry (SE), from 0.6 to 6.5 eV, obtaining the changes in refractive index and extinction coefficient in the implanted layer as a function of operating wavelength. Here, we report the first observation of decreased refractive index for a wide spectral range in low fluence implanted diamond. In addition, we observe non-monotonic response of the refractive index with fluence, which is in disagreement with proton studies, but accords with other heavy-ion implantation reports. Such discrepancies suggest that there are different mechanisms for refractive index modification for different species and that resulting optical properties are not solely a function of damage. Further comparisons with the white light reflectance and near infra-red transmittance measurements support the SE data.

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.

Transport properties of many semiconductor devices are dominated by the scattering of unscreened ionized impurities. Unscreened ions can be found either in depletion regions, low-doped bulk material, or in barriers of quantum well infrared photodetectors (QWIP). In QWIPs the electrons are concentrated in the well regions, leaving the ions in the barriers unscreened. Scattering due to unscreened ions is usually investigated using the Conwell‐Weisskopf (CW) model . . This model, originally introduced for interpretation of nuclear processes, uses classical physics by applying the Rutherford formula for particle scattering along with the Born approximation. The divergence of the relaxation time and momentum relaxation time in this model is avoided by neglecting scattering at small angles. This assumption is not valid for quantum particles which are scattered at all angles. In this work we present a different approach to prevent the relaxation divergence without avoiding smallangle scatterings. We show that this approach may result in scattering times and mobilities which are different by an order of magnitude than those derived from the CW model for electrons with kinetic energy below 0.1 eV, resulting in a more realistic simulation of transport properties. This model was applied to transport of optically excited electrons in QWIP structures. The concentration of dopants derived from the model, agree well with known figures.

Raman spectroscopy has been used to study the bond structure of Si 1− x − y Ge x C y alloys. An anharmonic model for the force constants is applied to interpret the results. It is well established that the Ge and C concentrations in these alloys can be chosen so that their average lattice constants may be equal to, or even smaller than that of the Si lattice. Our Raman results show that the C local mode vibrational energy increases rapidly with increasing C concentration, at a rate of 3.2 cm −1 /at%. Additional fine structure is observed near the Si–C main peak in samples grown by rapid thermal chemical vapor deposition (RTCVD). This fine structure is absent in samples grown by pulsed laser induced epitaxy (PLIE). In agreement with previous studies, the satellite structure of the Si–C peak is interpreted as an indication for short-range order. Our analysis shows that pronounced differences exist in the microscopic structures of SiGeC alloys grown by different growth techniques.

In this paper we present an investigation of pulsed laser induced epitaxy (PLIE) to obtain epilayers of Si1 − x − yGexCy on top of Si substrate by annealing non-crystalline SiGeC layers. Si 1 − x − y Ge x C y Si heterostructures were formed from C+ implanted pseudomorphic Si0.84Ge0.16 films and from hydrogenated amorphous SiGeC films deposited on Si(100). All samples were annealed in vacuum by XeCl excimer laser pulses. An analysis of the annealing process is presented, based on a detailed Raman study of the laser treated samples. The interpretation is corroborated by additional techniques: Rutherford backscattering spectroscopy, X-ray diffraction, and secondary ion mass spectroscopy (SIMS). Raman scattering is shown to be a relatively simple, non-destructive technique, to analyze the growth process, i.e. parameters such as crystallization, SiGeC layer depth, Ge content, and C incorporation in substitutional sites.

Si 1− x−y Ge x C y films have been grown by using a rapid thermal chemical deposition reactor. The growth temperatures were 650°C and 550°C and lead to Ge-contents of about 16% and 10%, respectively. The epilayers were characterized by infrared spectroscopy and Raman spectroscopy. The C-fraction varied from 0 to 1.25%. The shifts of Raman frequencies with increasing C-content cannot be explained in terms of change in strain. Electrical characterizations of tungsten contacts to n- and p-type films were achieved through current-voltage measurements. The Schottky barrier on n-type does not depend on the composition. This result indicates that the Fermi level at the interface with W is pinned relative to the conduction band. The Schottky barrier on p-type increased with the C-fraction. The increased magnitude seems to be too large to be explained in terms of variation of the band-gap-energy. The barrier height, for a given C-fraction, is larger for the films prepared at 550°C. This result can be ascribed to the lower Ge-incorporation at low temperature. Some samples were annealed at 1000°C. As expected, substitutional C-atoms precipitate to form silicon carbide. This formation of β-SiC precipitates results in detrimental effects on Schottky diode properties.

We have studied the optical properties and growth by rapid thermal chemical vapour deposition of multi-quantum well heterostructures based on the group IV elements Si, Ge and C. Si 1 − x Ge x Si , Si 1 − x − y Ge x C y Si , Si 1 − x Ge x Si 1 − y C y and Si 1 − y C y Si heterostructures were investigated by X-ray diffraction (XRD), secondary ion mass spectroscopy (SIMS), photoluminescence and Raman spectroscopy. Different features are observed depending on the growth temperature. At 575 °C, carbon and germanium are incorporated as expected, whereas at lower temperatures (550 °C), strong segregation occurs, as indicated by the decrease in the number of periods of the heterostructure observed by SIMS and XRD. Photoluminescence identifies a defect line associated with carbon and oxygen. Raman spectroscopy shows that, although carbon compensates for the external strain, the energy of the Si-Ge vibration mode is only weakly modified.

We have investigated optical properties of SiGe: C layers deposited on Si(100). Bulk and multi-quantum well heterostructures were grown by rapid thermal chemical vapor deposition using methylsilane as carbon precursor. The optical properties are analyzed as a function of the structural properties of the alloys. The photoluminescence of these structures exhibits two features: deep level photoluminescence associated with localized states and band-edge recombination which gives an indication of the band gap variation due to carbon incorporation in substitutional sites. In the case of multi-quantum well heterostructures, we report on an enhancement of the radiative recombination associated with silicon at room temperature in presence of Si 1 − x C x layers. The strain compensation induced by carbon in SiGe, which is evidenced by X-ray diffraction, is compared to the Raman spectroscopy of the vibration modes of the alloy.

Si 1-x-y Ge x C y /Si heterostructures are realized by pulsed laser induced epitaxy (PLIE) from C + implanted pseudomorphic Si 0.84 Ge 0.16 films and from hydrogenated amorphous SiGeC films deposited on Si(100). The laser treated samples are examined by electron channelling, energy dispersive X-ray analysis, Rutherford backscattering spectroscopy and ion channelling, X-ray diffraction, secondary ion mass spectrometry, and infrared and Raman spectroscopy. We show that PLIE occurs when the laser fluence exceeds a threshold for which the liquid-solid interface reaches the crystalline substrate at each laser pulse. Above this threshold, germanium and carbon atoms diffuse inside the melted layer, and carbon incorporation in substitutional sites increases with the laser fluence and the number of pulses. The resulting SiGeC layers are pseudomorphic.

The shadow masked growth technique is presented as a tool to achieve thickness and bandgap variations laterally over the substrate during metalorganic vapor phase epitaxy. Lateral thickness and bandgap variations are very important for the fabrication of photonic integrated circuits, where several passive and active optical components need to be integrated on the same substrate. Several aspects of the shadow masked growth are characterized for InP based materials as well as for GaAs based materials. Thickness reductions are studied as a function of the mask dimensions, the reactor pressure, the orientation of the masked channels and the undercutting of the mask. The thickness reduction is strongly influenced by the mask dimensions and the reactor pressure, while the influence of the orientation of the channels and the amount of undercutting is only significant for narrow mask windows. During shadow masked growth, there are not only thickness variations but also compositional variations. Therefore, we studied the changes in In/Ga and As/P ratios for InGaAs and InGaAsP layers. It appears that mainly the In/Ga-ratio is responsible for compositional changes and that the As/P-ratio remains unchanged during shadow masked growth.

We report on InP/InGaAs quantum-well infrared photodetectors (QWIPs) that cover the spectral range from 8 to 11 μm. The only previously reported operating wavelength for QWIPs of the same material system was around 8 μm. The higher operating wavelengths were achieved by including the strain as an additional band gap engineering parameter. According to our calculations, the InP/InGaAs lattice-mismatched multiple quantum well structures are suitable for design QWIPs covering the 5.5–11 μm range. We demonstrate five different QWIP structures with bound-to-bound and bound-to-continuum transitions for photodetection in the 8–12 μm atmospheric window. The calculations are in very good agreement with the experimental data. We found that the InP/InGaAs material system is more flexible than GaAs/AlGaAs because it has an additional degree of freedom—the strain for band gap engineering design.

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.

The lifetime of excited carriers in $\mathrm{InAs}∕\mathrm{InAlAs}$-on-InP quantum dots is investigated using intraband photoconductive (PC) spectra. A method is presented, based on the analysis of temperature dependence and line shape of PC signals, by which the dynamic properties are derived. Detailed analysis of PC measurements is a powerful method of studying the recombination dynamics. Temperature and energy dependencies are governed by strong electron-phonon interaction. The drop in PC signal with temperature is attributed to increased polaron relaxation due to the increased LA phonon population. Recombination at large detuning from LO phonon energy, as opposed to phonon bottleneck, renders an asymmetric line shape. The peak PC signal is redshifted due to an increased lifetime with detuning.

Intersubband transitions indirect in both real and momentum spaces were observed in GaAs/AlAs type II short period superlattices. Significant absorption of normal incident radiation, with ‘‘forbidden’’ polarization was measured, in addition to absorption in the ‘‘allowed’’ configuration. The transition energy shows a strong temperature dependence. This absorption is attributed to X–Γ transition. Both doped and undoped samples were investigated. Normal incidence absorption is stronger for the doped superlattices. Simulations using a two band model show good agreement to experimental data.

International audience; Low-temperature time-resolved photoluminescence (TR-PL) experiments were used to study the dependence on nitrogen composition of the nature, the energy and the dynamics of radiative carrier recombinations in GaAs1-xNx alloys. Epitaxial layers were grown on [001] GaAs substrates by molecular beam epitaxy using solid sources for group-III and As elements, and a radiofrequency plasma source for N. The nitrogen content, measured by secondary ion mass spectroscopy, was in the range 5 × 10-5 - 7 × 10-2. PL spectra are dominated by the emission from the discrete states of nitrogen-based excitonic complexes for the very low x regime (around 5 × 10-5), showing exponential time decays. For x larger than 5 × 10-4, PL spectra are rather dominated by emission from a continuum of states originating from nitrogen clusters. In this case, nonexponential time decays are obtained. It is shown that these PL decays involve complex carrier transfers between the various available states. Changing the nitrogen content changes the recombination mechanism by changing the distance between the clusters.

We have investigated optical properties of high-density InAs self-assembled quantum dots (QDs) in an ${\mathrm{In}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}\mathrm{As}$ matrix, lattice matched to an InP (001) substrate. The weak lattice mismatch $(\ensuremath{\sim}3%)$ results in a 90% coverage of the ${\mathrm{In}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}\mathrm{As}$ surface with InAs QDs. By means of interband and intraband spectroscopies crossed with atomic force microscopy (AFM) measurements, we have determined that the InAs QDs optical properties depend on the deposited amount of InAs. Photoinduced absorption spectroscopy has been used to investigate midinfrared intraband absorptions. For three monolayers (ML) InAs deposit thickness, just above two-dimensional (2D)/3D growth mode transition (2.5 ML), the islands form as isolated elliptical dots elongated along the $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ direction and exhibit intraband resonances polarized either along the [110] or the $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ direction. For thicker deposition $(g3\mathrm{ML}),$ InAs islands form chains of elliptical dots along the $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ direction where the quantum confinement is lost, resulting in a quantum-wire-like behavior. In this paper, we also report on photoluminescence and photocurrent spectroscopies, in order to get insight into the ${\mathrm{I}\mathrm{n}\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}\mathrm{As}$ island band structure. These experimental results are in good agreement with that of a multiband $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ model.

The observation of intersubband (e1–e2) absorption induced by current injection in GaAs/GaAlAs multiquantum wells is reported. The new technique enables sensitive measurement of intersubband transitions in undoped quantum wells. The carrier concentration in the wells is easily controlled by varying the current density. The technique is demonstrated on undoped multiquantum well layers sandwiched in a p‐i‐n laser‐diodelike structure. The lowest confined level was populated by biasing the junction and hence injecting carriers into the quantum wells. The spectra were taken by a Fourier transform infrared (FTIR) spectrometer, modulating the current and using the double modulation method. The dependence of the absorption on current intensity was examined and excess carrier concentration and lifetime were calculated. The induced absorption saturates at current densities higher than 100 mA/cm2.

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 8×10 9 cmHz 1/2 /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*10 10 cmHz 1/2 /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.

In this work we demonstrated multicolor quantum dot IR photodetectors with normal incidence background limited performance at 70K. The devices, which were studied in this work, were composed of self-assembled InAs dots grown by MBE on InAlAs barrier lattice matched to InP substrate. Normal incidence photocurrent spectra reveal several polarized and unpolarized peaks in the range 100-350 meV due to the intersubband transitions. The detector resonsivity at normal incidence is similar to that obtained for polarization normal to the layers, and is comparable to that achieved in quatnum well IR photodetectors.

A comparison between the optical transmission and the photoconductive response of p‐type quantum well infrared photodetectors is demonstrated. The dependence on polarization was found to be different for these two processes. Light polarized perpendicular to the growth layers was more efficiently absorbed by the quantum wells, while the photoconductive response was stronger for the parallel polarized light. The difference is attributed to the different vertical transport behavior of light and heavy holes and the mixed nature of hole subbands.

Selectively excited room‐temperature photoluminescence spectra of InGaAs‐GaAs‐AlGaAs quantum well heterostructures reveal a broad line at midgap energies originating in the Si doped AlGaAs. When carriers are photoexcited directly in the wells, this line is dramatically enhanced at the expense of the quantum well line, indicating carriers escape from the wells. The broad emission and its enhancement can be explained in terms of recombination between electrons trapped at DX centers in AlGaAs and holes transferred into AlGaAs from the neighboring wells. We also observe the broad emission by direct over‐the‐gap photoexcitation of very highly doped Si:AlGaAs. The doping dependence consistently correlates this line to DX centers.

Multicolor quantum dot infrared photodetectors with normal incidence background limited performance (BLIP) at 77 K were implemented. The devices are composed of self-assembled InAs dots grown on InAlAs barrier layers, lattice matched to InP substrate. These dots are formed in a shape of flattened parallelepipeds. The photoconductive spectra were observed at all polarizations. Normal incidence photocurrent spectra reveal several polarized peaks in the range of 100--400 meV due to intersubband transitions. The detector responsivity at normal incidence is similar to that obtained for polarization normal to the layers, and is comparable to that achieved in quantum well infrared photodetectors (QWIP's). The transition energies, the polarization selection rules, and relative intensities of the peaks were tentatively interpreted in terms of a three-dimensional separation of variables.

The ion implantation of phosphorus into spin‐on‐glass (SOG) SiO2 thin‐film films modified the infrared transmission spectrum of the films. Two SOG types, polysiloxane and silicate, were ion implanted with doses in the 1×1014–1×1015 cm−2 range and an energy of 40 keV. The Fourier‐transform infrared spectrum of such films on silicon substrates was measured and the results are presented as a function of the implanted dose. The effect of the ion implantation on the silicate SOG was minute while significant changes were observed in the polysiloxane SOG. The major absorption peaks in the transmission spectrum were numerically analyzed and fitted to a set of Lorentzian functions. The peak heights, width, and area were measured. The ion implantation reduces the number of CH3 groups while the location of the Si‐O absorption peak is shifted towards a shorter wavelength, i.e., a denser material. A physical interpretation of the absorption peak dependence on the ion‐implanted dose is presented.

We propose a simple approximation relating the Fermi energy to carrier concentration in both parabolic and nonparabolic semiconductors. The solution is in the form of a polynomial correction to Boltzmann’s approximation of semiconductor statistics. The method is similar to the Joyce–Dixon series approximation, but uses polynomial regression to obtain series coefficients which extends the range of the model’s validity. For nonparabolic semiconductors, polynomial coefficients are calculated using Kane’s k■p model for the density of states. The new approximation demonstrates an acceptable accuracy for band gaps larger than 2 kT and for the Fermi energy up to 10 kT. The expression is simple and should be useful in the modeling of advanced semiconductor devices.

Laser induced structural or compositional modifications of Si or IV-IV surface:planarization, pulsed laser induced epitaxy, carbon incorporation, chemical etchingJ. Boulmer' ,B. Dragnea3, C. Guedj', D. Debarre', A. Bosseboeuf', E. Finkman2, B. Bourguignon3I Institut d'Electronique Fondamentale, associe au CNRS, Bat. 220, Universite Paris—Sud, 91405, Orsay, France2 Department of Electrical Engineering and Solid State Institute, Technion, Haifa, 32000 Israel3 Laboratoire de Photophysique Moleculaire du CNRS, Bat. 213, Universite Paris—Sud, 91405, Orsay, FranceABSTRACTWe examine experimentally the modifications induced at the surface of silicon and IV-IV alloys by an excimer laserabove the melting threshold fluence. Laser irradiation takes place under vacuum, or in the presence of a gas. The resultingprocesses are respectively: laser induced planarization, pulsed laser induced epitaxy (PLIE), incorporation of atoms from thegas phase, and laser chemical etching. In turn, the laser induced surface modifications and the presence of adsorbates on thesurface cause important changes in the melting I solidification cycle. We describe a model calculation which takes intoaccount non-equilibrium heat diffusion, phase change, atom diffusion, segregation and desorption. The model is applied to thelaser chemical etching process, and its results are compared to the experimental data. This simulation brings information onthe segregation ofchlorine and on the dynamics of desorption.Keywords: laser processing, Si, SiGeC, IV-IV alloy, laser induced processes, chemical etching, planarization, epitaxy, Cincorporation, desorption, segregation, diffusion, melting.I. INTRODUCTIONThe modifications induced at the surface of an irradiated material are of major importance in many laser processingtechniques which are useful for the fabrication of electronic circuits, micro-electro-mechanical or micro-optical systems, andfor the thin film deposition or the spectroscopic analysis techniques based on laser ablation. In the present paper, we reviewour experimental observations of the modifications induced on Si or IV-lV (SiGeC) samples by a pulsed laser involvingrepeated melting /solidification cycles. Our experiments are performed with an XeC1 excimer laser (X 308 nm, pulseduration 20 ns) operated in vacuum or in the presence of a gas. The laser fluence ranges from 0.3 to 1 .5

3 páginas, 3 figuras.-- PACS: 85.60.Gz, 85.35.Be, 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., This research was partially supported by the Israeli Ministry of Science and Technology. Two of the authors (P.P. and J.G.) received financial support from QUEST, an NSF Science and Technology Center (DMR 11-20007)

We have investigated structural and optical properties of SiGeC layers. Bulk and multiquantum wells heterostructures were grown by rapid thermal chemical vapor deposition. The photoluminescence of these structures exhibits two distinct features : deep level photoluminescence associated with localized states and bandedge photoluminescence which gives an indication of the band gap variation due to carbon incorporation in substitutional sites. In the case of multiquantum wells heterostructures, a strong segregation is reported as the growth temperature is decreased. An alternate technique to incorporate carbon in SiGe layers, the pulsed laser induced epitaxy with an excimer laser, is presented. The incorporation of carbon in substitutional sites is evidenced by Raman spectroscopy. As the laser flux is increased, new Raman lines associated to carbon-induced disorder are observed in the spectra.

We study the effect of 30 keV gallium ion implantation on the optical properties of diamond, as determined using spectroscopic ellipsometry. We find that the refractive index of the implanted layer can be either lower, or higher, than that of pristine diamond, depending on the implantation dose. This observation provides a new route to optical device fabrication in diamond using focused ion beam methods. In particular, in the low dose regime, lowering of the refractive index would allow for core-cladding type structures to be defined where the core has not interacted with the beam, and is hence undamaged by the implantation.

There are several responsivity and gain models adequate to MQW detectors. All are related to two fundamental properties: The escape efficiency of a photo-excited carrier out of the well boundary (0) and the capture (or crossing) probability (P or P =1-Pa). In this work we present a rigorous calculation of the escape and capture probabilities based on a Quantum Mechanical (QM) approach. In the Vicinity of the well the electrons are treated as Gaussian Wave-Packets. Thesimulation calculates the quantum mechanical reflections encountered by the packets at the interfaces as a function ofkinetic energy and applied electric field. The relaxation processes into the bound sub-level are taken into account byintroducing an imaginary potentialThe simulations are carried out on both stepped and rectangular structures. The step reduces the dwell time above the well and hence the capture probability. This result is in good qualitative agreement with responsivity measurements on such structures.The effects of interface charge and non-uniform charge distribution are also considered. We assume an accumulationof negative sheet charge on the "inverted" interface facing the substrate. This excess charge, introduced during thegrowth sequence, induces structural asymmetry. The modified well potential is calculated self-consistently andresults in an increased barrier height for carriers propagating towards the substrate. Those results are consistent withthe asymmetrical responsivity and noise characteristics measured by various groups.Key Words: Wave-Packet, Escape Efficiency, Capture Probability, Dwell time

A new type of asymmetric stepped GaAs/AlGaAs multi quantum well infrared detectors 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. The difference between polarities for the noise is much smaller at low temperatures. As a result, the highest D * in positive polarity is much higher than in the negative one. 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. The transport asymmetry is further studied using identical asymmetric wells which were grown in opposite sequences. It is shown that the effect of the asymmetry in the interfaces is of the same order of magnitude as the structural asymmetry.

The stimulated emission spectra of photoexcited quantum-well structures were measured at room temperature, 77 and 1.8K as a function of the illuminated stripe length and excitation intensity. Three samples are examined: a multiple quantum well, and two superlattices (one of them is type-Il). Based on these data, the gain spectra and gain saturation behavior were obtained using Agrawal's model for the analysis. Both the spontaneous photoluminescence spectra and the stimulated emission spectra are used in order to analyze the spectral shifts. Kronig-Penney model was used to approximate the energy level diagram of our samples.

Experimental results show that the contribution of the graded region to the current of Hg(1-x)Cd(x)Te diodes is not negligible, as compared to that of the p type bulk. The theoretical analysis reveals the influence of the electric field present outside the depletion region on the current generated by the graded region. 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.

Electrical measurements of gold‐doped and undoped p‐type Hg1−xCdxTe with x≂0.22 are reported and two models based on two shallow acceptor levels for the analysis of the Hall data are developed. One model assumes two independent acceptors and the other assumes one divalent acceptor. The models yield ionization energies of 12.5±2 and 2–5 meV. The concentration of the compensating donors is of the order of the hole concentration. Arguments are given in order to assign these levels to established point defects.

The lifetime and carrier‐concentration profile of Hg1−xCdxTe photodiodes (x=0.2–0.3) formed by implantation of B+ ions (100 keV, 1013–1014 cm−2) into a p‐type substrate was investigated. Using graded C‐V measurements, concentration of carriers around the n+p junction was obtained. It was found that the junction is located at a depth of 0.7–1.5 μm, depending on the bulk hole concentration. The technique enables profiling of the same sample after annealing. The annealing process was shown to make the electron concentration shallower and thus to bring the junction closer to the surface. Various techniques were used to measure excess carrier lifetimes. All indicate that the extent of degradation of the excess electron lifetime in the p region is very limited. For all tested samples, an etch of 2 μm was enough to recover the bulk lifetime. These results contradict the general assumption of deep damages in the p‐type substrate. Thus, the contribution of the n+ side to the saturation current and to the R0A produ...

The temperature dependence of the carrier concentration of Hg1−xCdx Te at elevated temperatures is used to determine its band parameters near the bandedge. The electron concentration is calculated by using the Kane model (k⋅p method), and is fit to observed values derived from measurements of the Hall coefficient in the near‐intrinsic region. This procedure results in expressions for the following parameters: the band‐gap Eg and its dependence on temperature and composition; Kane’s interband‐coupling matrix element and the heavy‐hole effective mass ratio. Modified values of the intrinsic carrier concentration are calculated using these parameters.

The results of galvanomagnetic measurements in N‐type Hg1−xCdxTe, as a function of magnetic field for the compositions x≃0.215 and x≃0.29 and over the temperature range 15⩽T⩽300 K are reported. A unified model is proposed which fits the magnetic field dependence of both the conductivity and the Hall coefficient. The most significant feature of the model is that two types of electrons are contributing to the transport properties and are responsible for the magnetic field dependence of the conductivity and the Hall coefficient. The two sets of electrons have mobilities which at the lowest measured temperature (15 K) differ by up to two orders of magnitude, and have concentrations which are of the same order of magnitude.

We have studied six GaAs/AlAs superlattices with periods ranging from 18 to 60 A and different average aluminum composition. Three of these samples are shown to be direct bandgap materials whose band structure differs strongly from that of the corresponding alloy, but is correctly described by an envelope function calculation. The three remaining samples are shown to be indirect both in real and reciprocal space. The lowest energy transitions are found to arise from an exciton involving a heavy hole state mostly confined in the GaAs layer and at the Brillouin zone center (Λ), and an electronic state of X character confined in the AlAs layers. Analysis of the time decay of the luminescence shows that this is a momentum-forbidden exciton made allowed by disorder scattering, which leads to a luminescence efficiency comparable to that of the direct bandgap samples. Partial lifting of the degeneracy of the three X orbitals by the superlattice potential is also observed. Finally, we take advantage of the strong dependence of these indirect transition energies on the band discontinuities to estimate the valence band offset to be about 550 meV in this system.

The recombination mechanisms in HgCdTe are analyzed. Detailed expressions for the radiative lifetime are presented, taking into account recent measurements of the absorption coefficient. In p‐type material, carrier freezeout is shown to increase the lifetime exponentially for the radiative and the Auger processes at low temperatures. The effect of background flux is introduced, taking into account its variation with temperature due to the change in energy gap. Lifetime measurements on p‐type samples are in good agreement with combined Auger 7 and radiative mechanisms, where the Auger process is more effective for low x values. Highly compensated materials are dominated by the Shockley–Read recombination. For all samples, the intrinsic region is controlled entirely by the Auger process.

The surface recombination velocity s has been determined for Hg1−xCdxTe (x∼0.2) for two different surface passivations: (i) anodic sulfide with an overcoating of ZnS and (ii) ZnS coating on freshly etched samples. The method used was photoelectromagnetic effect, in which a magnetic field is applied perpendicular to the diffusion current of optically generated injected carriers. Analysis of the magnetic field dependence of the resulting current can yield s, as well as carrier mobility, lifetime, etc. The temperature dependence of s is very similar for the two passivations at temperatures higher than 50 K, and is increasing with decreasing temperature. At low temperatures s continues to rise for ZnS passivation and stays flat for the native anodic sulfide. Two activation energies are determined: 12.5±1.5 meV at temperatures higher than 60 K, and 2.3±0.2 meV at temperatures lower than 30 K. The high‐temperature activation energy is identical for both passivations. It is concluded that the same surface traps control the surface recombination rate for the two passivations, and their concentration is equal in both cases. The traps are apparently ‘‘intrinsic,’’ and can be related to lattice defects, most probably vacancies. Their concentration at the surface is similar to that in the bulk.

A new method for measuring the ambipolar mobility is described. It is based on the determination of the travel time of excess carriers between two probes. Maintaining the device under a constant voltage, the travel time is depicted as the separation between two zeros on the output trace. Using this technique the ambipolar mobility of electrons in p‐type Hg 0.71Cd0.29Te was measured for the temperature range of 78–256 K.

The intrinsic carrier concentration ni is measured for Hg1−xCdxTe with 0.205⩽x⩽0.22 and x=0.29 and 150

We have used photoreflectance and low temperature photoluminescence to study the optical properties of short period AlAs/GaAs superlattices with periods ranging from 18 to 60 A and varying AlAs to GaAs layer thickness ratios. For superlattices with AlAs to GaAs thickness ratios significantly less than one, the lowest energy transitions are direct in character, that is associated with the Γ state in the GaAs well; while for those with AlAs to GaAs ratios of approximately one the lowest energy transitions are indirect in character. These results are interpreted in the light of recent tight binding calculations which predict that for these indirect superlattices the lowest state for electrons is associated with the X-state in the AlAs barrier, while holes remain predominantly localized in the GaAs layers.

The performance of Hg1−xCdxTe diodes has been thus far investigated assuming it is controlled only by the p side of the junction. Therefore it has been analyzed using a simplistic one‐side diffusion model. In order to examine whether this assumption is valid, we placed Hg1−xCdxTe diodes under a magnetic field parallel to the junction plane, and tested its effect on the saturation current. Due to the high mobility ratio, the magnetic field supresses the electron component of the saturation current, while that of the holes is hardly affected. Therefore, it enables a clear separation between the two components. The results prove that the contribution of the n+ region is comparable to that of the other side and should not be disregarded. We analyze the effect of carrier concentration gradient on the R0A product, generating a drift term and affecting the lifetime. The expressions obtained for the saturation current indicate a considerable drift term; however, it still is inversely proportional to the square of...

The absorption near the fundamental absorption edge of Hg1−xCdxTe was measured over the composition range 0.205⩽x⩽0.220 at temperatures from 80 to 300 K. The dispersion of the index of refraction of Hg1−xCdxTe was obtained from the interference pattern. It was found that the absorption tail obeys a modified Urbach’s rule and is expressed by α=α0 exp[σ (E−E0)/(T+T0)] for 20⩽α (cm−1) ⩽1000. The fitting parameters α0, σ, T0, and E0 vary regularly with x. The expression is used to obtain the absorption coefficient and the temperature coefficient of the gap as a function of x and T. Evidence is presented to show that these parameters may be extrapolated to calculate the absorption beyond the measured composition range.

The growth of epitaxial layers of mercury-cadmium-telluride (Hg1-xCdxTe) with relatively low x (0.2-0.3) from Te-rich solutions in an open tube sliding system is studied. The development of a semiclosed slider system with unique features permits the growth of low x material at atmospheric pressure. The quality of the films is improved by the use of Cd1-yZyTe and Hg1-xCdxTe substrates instead of CdTe. The substrate effects and the growth procedure are discussed and a solidus line at a relatively low temperature is reported. The asgrown epitaxial layers are p-type with hole concentration of the order of 1·1017 cm−3, hole mobility of about 300 cm2·V−1 sec−1 and excess minority carrier life-time of 3 nsec, at 77 K.

We report measurements of the radiative lifetime and optical absorption of resonantly excited bound excitons in gallium arsenide at low temperature. We find that the lifetime of the acceptor bound exciton and the absorption cross section of the donor bound exciton are in good agreement with the theoretically predicted values, but that the lifetime of the donor bound exciton is longer than expected by a factor of about 20. This discrepancy is attributed to the existence of a much larger number of low-lying rotational states of the donor bound exciton than had previously been suspected.