Table of contents

The low-temperature mobility of holes in an SiGe quantum well is analysed using a theory that goes beyond lowest order in the scattering by treating multiple scattering in a self-consistent fashion. At small carrier concentrations the mobility is reduced by multiple scattering compared with lowest-order theory, a precursor to strong localization. The validity of using lowest-order theory to extract technologically important material parameters is discussed.

The spin relaxation times of electrons in InGaAs(P)/InGaAsP multiple quantum wells have been directly measured as a function of well width using an ultrashort pulse colour centre laser. The times measured range from 20 to 5 ps, considerably faster than previously determined in GaAs quantum wells. In addition, a fast, all-optical polarization switch based on a spin-sensitive optical nonlinearity is demonstrated at a wavelength of . This operating wavelength and the observed switch recovery time of 5.5 ps make it a potentially useful element for future all-optical soliton communication systems.

In this work, we have numerically integrated in space and time the effective-mass Schrödinger equation for an electron wave packet in a strongly coupled double quantum well system. Considering an electronic Hartree interaction and a high magnetic field parallel to the growth plane, we have obtained a dynamical evolution for the electronic wave function. In our double quantum well system, we clearly show the existence of resonant tunnelling oscillations as a function of the electron wave number value in the growth plane. It is also found that the tunnelling time values between both quantum wells are decreased if the magnetic field is increased.

In this article we calculate the effect of the native interface asymmetry on the band structure of `no common atom' quantum wells such as (InGa)As-InP. We show that it leads to a new contribution to the spin-splitting in both the valence and conduction subbands. The splitting appears to be anisotropic and at least one order of magnitude larger in the valence subbands than in the conduction subbands.

We report the negative magnetoresistance measured in GaAs-AlAs short-period superlattices. The samples were doped with silicon so that the electrical conductivity was controlled by the DX silicon donor. We take advantage of the metastable character of the DX states at low temperature to tune the conductivity by increasing the photoionization of the ground state in the persistent photoconductivity regime. The magnetic correction is generally interpreted in terms of weak localization and electron-electron interaction according to the Kawabata and Altshuler theories of the magnetoresistance at low magnetic field in semiconductors. However, we find that these models, previously developed in pure 3D or 2D systems, cannot be strictly applied in superlattices showing anisotropic diffusion and low mobility. Conversely, we show that the self-consistent approach to electron localization recently proposed by Bryksin and Kleinert (BK) in anisotropic systems accounts properly for the experimental results.

This paper reports polarization and temperature dependent contactless electroreflectance (CER) and photoluminescence (PL) study of strain compensated short period superlattices on GaAs(001) substrates which show self-organized lateral composition modulation (LCM) in the form of alternate In/Ga rich regions along the [110] direction in the (001) plane. The LCM related PL peak position is at a lower energy as compared to the corresponding feature in the CER spectrum and the difference is found to be the same as the activation energy for thermal quenching of the PL signal. These results have been explained by suggesting that the PL signal arises mainly from recombination of carriers localized at potential fluctuations (In rich regions with excessively high In concentration) within the lateral superlattice (LS) structure that is formed due to the LCM while the CER feature arises from transitions between the minibands of the average LS structure. The PL signal is quenched due to thermal excitation of the carriers from these excessively In rich regions into the minibands. The LS band gap energy is accurately determined and its implication for the random alloy model used to describe the electronic band structure of such systems is discussed. The temperature dependences of critical point energies have also been measured and analysed.

A model for the incorporation of electrically active Fe in LEC InP is presented. The concentration of in a given cross section of the ingot is expressed as a function of the corresponding grown fraction and the period of time between the solidification of that particular cross section and the end of the pulling. Implicitly, this means that the Fe activity depends on the thermal history and the concentration of In vacancies of that cross section. The final relationship accounts well for experimental results such as (i) an Fe activation higher in the crystal top with respect to the crystal tail and (ii) an Fe activation slightly higher in crystals pulled at lower speed.

The configuration transformation properties of two previously observed metastable defects in 6H-SiC have been studied using various junction space-charge techniques and simulation. The studied configuration transformation processes are mainly governed by the energy released during the electron capturing process. An electron capturing and configuration transformation model is suggested and a transformation probability between the configurations has been defined and measured. Finally, the obtained defect parameters and the configuration model were used to simulate the transformation behaviour.

Selective epitaxial growth of Ge dot structures is investigated to obtain highly regular dot arrays with excellent size uniformity in the nanoscale. The dot structures are grown in patterned fine windows ranging in diameter from 650 to 90 nm on Si(100) substrates covered with masks. The dimensions and number of the dots grown in a window significantly change depending on window size, growth temperature and time and the thickness of the Si buffer layer. These growth characteristics are considered to be induced by the Stranski-Krastanov growth mode and migration mechanism of Ge atoms on Si substrates. It is noted that Ge dots whose diameter is much smaller than the pattern size are formed with high uniformity and that the position is precisely controlled by SEG. The Ge dot structures are found to give rise to prominent luminescence with well separated phonon replicas and the energy position systematically changes when the growth conditions are varied.

In this work, we have studied the dominant transport mechanism in an Au/InSb/InP diode. The InP (n) substrate is restructured with some monolayers of an InSb thin film. The structures have been characterized electrically and the C(V) and I(V) curves have been plotted. The saturation current , the serial resistance and the mean ideality factor n were calculated using the I(V) characteristics and are respectively equal to , and 1.7. Thus, the doping level and diffusion voltage were deduced from the C(V) curves and are evaluated to and .

The analysis of I(V) and C(V) characteristics allows us to determine that the mean interfacial density of states and the transmission coefficient are respectively equal to and .

The presence of deep discrete donor levels in the semiconductor bulk and/or the distributed density of states is responsible for the nonlinearity of the characteristic because the C(V) curve is controlled by two donor levels located at 0.76 eV and 1.02 eV relative to the conduction band edge .

Defects in as-grown and ion-bombarded p-InP crystals have been investigated by positron lifetime measurements. The result showed that indium vacancies existed in as-grown p-InP. In addition, Zn impurities were also observed, which are negative and act as shallow trapping centres at low temperatures. Ion bombardment at lower dose introduced some monovacancies, and with increasing ion dose, divacancies were also observed, which coexisted with monovacancies. The temperature experiment showed that antisite defects were also produced upon bombardment, which we attribute to the negatively charged antisites.

Single-crystalline freestanding GaN, having a current maximum size of and a thickness of 350 , was fabricated by hydride vapour phase epitaxy (HVPE) growth of thick-film GaN on a sapphire substrate and subsequent removal of the host sapphire substrate. We also observed the room-temperature optically pumped stimulated emission (SE) from a 1 mm width cleaved cavity prepared from freestanding GaN. At a maximum power density of 2 MW , the peak energy and FWHM of SE were 3.318 eV and 8 meV, respectively. The threshold pumping power density for the one set of SE was 0.37 MW , and the output becomes highly TE-polarized.

Diode lasers operating at room temperature with an emission wavelength of have been fabricated from compressively strained multiple-quantum-well structures grown by molecular beam epitaxy. Ridge waveguide lasers long exhibited an RT current threshold of 500 mA, optical power efficiency of /facet and a differential quantum efficiency of 14.5%. A pulsed threshold current density less than with a characteristic temperature have been achieved for broad mesa devices. The laser structure, which has a type-II band alignment at the well-barrier interface, showed an internal efficiency , controlled by the electron-hole wavefunction overlap at the interface.

We propose a new method to obtain well defined nanostructures in situ during MBE growth. The method is based on the MBE growth over UHV compatible shadow edges. These edges define a geometrical shadow region which can be modified by tilting the sample with respect to the direction of the molecular beams, so that layers with different lateral extension can be combined to form a buried junction of two quantum wells in the form a wire with lateral confinement. We show for ZnSe quantum wells in ZnMgSSe that the formation of facets at the shadow edge favours the formation of geometrically well defined structures, and quantum well junctions as narrow as 150 nm have been realized. The wires emit pronounced blue cathodoluminescence at room temperature, proving that even for simple structures the confinement of carriers in the resulting wire is on the order of kT at room temperature.

The steady-state equilibrium of photoexcited carrier population in quantum wells (QWs) in GaAs/GaAlAs graded index (GRIN) structures was studied by polarization dependent photocurrent analysis. The carrier escape energy was determined separately for polarized excitation parallel (TE) and perpendicular (TM) to the epitaxial sequence from an Arrhenius plot of the photocurrent magnitude. For that purpose either the spectrum was recorded for each temperature or the spectral feature related to the process of interest was tracked while the temperature was varied. Carrier escape of holes through the light hole band dominates the escape. Nonradiative pair recombination within the well and carrier escape by tunnelling are further loss processes.

A new analytical approach to the low frequency noise analysis in MOSFETs has been developed. Local levels contributing to generation-recombination noise are classified into three groups. For deep levels located near the midgap the approach of zero free carrier concentration inside the space charge region can be used. Simple analytical expressions and conditions of their applicability are derived. Parameters of levels located near the conduction and valence bands can be determined analytically. Analytical expressions for such a situation are also derived. For the intermediate case the level parameters can be only determined numerically. The low frequency noise in n- and p-MOSFETs has been studied experimentally at 220 K < T < 425 K over frequency range 2 Hz < f < Hz. Experimental noise spectra are analysed on the basis of developed theoretical model.

The unit of the dispersion in figure 7 should be: m s^-1 kHz^-1.

The last sentence of the first paragraph on page 98 should read:

The experience shows a SAW dispersion not higher than on the ground surface of iron that is only plastically deformed by surface machining and does not have any microcracks compared with the brittle GaAs.