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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.

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 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.

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.

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.

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.

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.

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 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 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.

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 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.