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After a brief discussion of the present state of development of blue GaN- and green ZnSe lasers an overview is given of efforts to improve the lifetime of ZnSe-based laser diodes by incorporation of beryllium. Be-chalcogenides have a relatively large strength due to their bonding properties. Because BeTe and ZnSe are almost lattice matched to GaAs, the growth of type II superlattices of high perfection is possible. This allows to produce graded BeTe/ZnSe low resistance contacts to ZnSe, which have performed very satisfactorily. It can be anticipated that by making use of the full potential of Be-chalcogenides in the design of II–VI laser diodes emitting in the green spectral range, the lifetime of such devices can be extended significantly. This would allow numerous technical applications.

High-quality ZnSe-based heterostructures are grown by uninterrupted molecular beam epitaxy using the concept of strain compensation and alternating-strain multilayers. To verify the advantages of this technique, optically pumped ZnSSe/ZnCdSe laser structures containing short-period superlattices or multiple quantum wells have been grown and studied. A room-temperature injection laser diode with a BeZnSe/ZnSe superlattice waveguide is described.

A number of publications devoted to studies of optical anisotropy of heterostructures with no-common atom at interfaces (InAs/GaP, etc.) has been published recently [1-3]. This effect is attributed to a low local syinmetry of a single interface between two different zinc-blende type structures. In heterostructures CA/C'A' (001) with no-common cations (C ≠ C') and anions (A ≠ A'), an interface consists of the two atomic planes with A—C' or A'—C bonds. In this case there exist four different types of quantum-well structures (QWs) and superlattices (SLs). The structures with similar interfaces have the D2d point symmetry and are optically isotropic in the plane of interfaces. If the bonds at the left and right interfaces are different, an ideal structure with no-common atom is characterized by the in-plane anisotropy with the principal axes [110] and [110]. The microscopical reason for this in-plane anisotropy is a mixing of heavy hole and light hole states at the interfaces [3, 4]. In this paper we present an experimental study of the in-plane optical anisotropy in novel ZnSe/BeTe type-II quantum-well structures. Samples were grown by MBE on (100)-oriented GaAs substrates with controlling the type of interfaces. Three types of ZnSe/BeTe (100/50 Α) x 20 samples were grown: with BeSe...BeSe, ZnTe. . .ZnTe and BeSe...ZnTe interfaces. Basic optical properties of excitonic transitions in the ZnSe/BeTe quantum well structures were reported recently in Refs. [5, 6]. The observed two photoluminescence lines with energy positions at ,^s 2.8 eV and 2.0 eV were related to a

We study the Landau level fan diagram for charged and uncharged states in n-type modulation doped quantum wells with photoluminescence (PL) and photoluminescence excitation (PLE) measurements. In magnetic fields, the adjacent peak distance is different for PL and PLE. The peak energy for PL oscillates periodically in inverse magnetic fields while the peak energy for PLE does not oscillate. Time resolved PL shows that the difference of adjacent peak distance in PL and PLE is due to the hole relaxation and breaking of the transition selection rule. Within the experimental resolution, we attribute the different oscillatory behavior of the PL and PLE peak energies to different self-energy corrections for charged and uncharged states.

Using photoelectron spectroscopy, we have investigated the band alignment at the interface of pseudomorphic BeTe/ZnSe(100) heterojunctions for different interface terminations. The heterostructures of high structural quality have been produced by molecular beam epitaxy; the interface termination was adjusted by variation of the growth parameters between the growth process of ZnSe and BeTe. The valence band offset for a Zn-rich BeTe/ZnSe interface is determined to be 1.26±0.15 eV, for the Se-rich BeTe/ZnSe interface a value of 0.46±0.15 eV is obtained. Our results show that the band alignment can be modified by the interface composition even for isovalent heterostructures.

An additional, yet untried approach to further improve the reliability of ZnSe-based devices is to use beryllium containing II–VI compounds. BeS, BeSe and BeTe are characterized by a considerable amount of covalent bonding and a high bond energy. This distinguishes these materials from the conventional ionic wide gap II–VI semiconductors like ZnSe, ZnTe or CdTe. Recently, thin film structures using Be-compounds have been fabricated and characterized. It became clear that — besides the application aspects — these materials are also very interesting from a more fundamental point of view. Using Be-containing II–VI compounds, ionic and covalent lattice matched II–VI materials can be combined in quantum well structures. The type II band alignment of BeTe and ZnSe gives additional freedom in the band gap engineering, and it is possible to grow lattice matched quaternaries of low polarity onto silicon. Here, basic properties of Be containing II–VI compounds will be described, and the potential of these novel materials will be discussed.

We present an optical study of novel ZnSe/BeTe heterostructures with a type-II band alignment based on beryllium chalcogenides. A strong exciton transition involving a confined electron and a quasibound hole state (both in the ZnSe layer) is observed in photoluminescence, photoluminescence excitation and reflectivity spectra. This exciton state is drastically broadened by increasing temperature due to enhanced exciton—acoustic phonon interaction. Pronounced features related to the quasibound hole states in the ZnSe layer (upto n = 4) are detected.

BeTe is a new material within the class of II–VI semiconductors with novel and interesting properties. It is well lattice matched to GaAs as well as ZnSe and therefore can be combined in superlattices with these materials. Using photoelectron spectroscopy we have investigated the valence-band offset (Δ E v ) of BeTe/ZnSe and BeTe/GaAs heterojunctions, grown by molecular-beam epitaxy (MBE). For the BeTe/ZnSe heterostructure with a Zn-rich interface, we measured a valence-band offset of 1.26±0.15 eV and for the heterovalent BeTe/GaAs interface we determined a far smaller valence-band offset of −0.25 ± 0.15 eV. From UPS measurements for increasing layer thickness of BeTe on ZnSe we calculated the accumulation layer of BeTe with a Debye-length of 6 nm and a defect concentration of 4.10 17 cm −3 .

The theory of resonant optical reflection from long-period multiple quantum-well structures has been extended to take into account the difference between the dielectric constant of the barrier, e b , and the nonresonant contribution, e a , to the dielectric function in the wells. The reflection spectra taken from CdTe/CdMgTe quasi-Bragg quantum wells reveal, in addition to sharp excitonic features, smooth interference fringes determined by the mismatch e a ≠ ϵ b . Implications arising due to the dielectric constant mismatch as well as due to detuning from the resonant Bragg condition are analyzed and discussed.

In this contribution we report on resonant tunneling in ZnSe/BeTe double-barrier structures. Vertical transport across the resonant-tunneling structure has been analyzed in detail as a function of temperature and applied magnetic field.

We present an investigation of the vibrational properties, the elastic constant c11 and the infrared dielectric constant e∞ of BeTe. For this purpose we applied far-infrared reflectance and Raman spectroscopy to single BeTe layers and BeTe/ZnSe superlattices. The experimental data are used to access the bond ionicity in the framework of an empirical tight binding approach. A low bond polarity of 0.61 is determined for BeTe, which corresponds to a high lattice stability. Furthermore, the elastic constant c11 is determined as to be 8.9 ± 0.9.1010 N/m2 from the frequency positions of folded acoustic phonon (FAP) modes in BeTe/ZnSe superlattices. The experimental results are correlated to data obtained by first-principles local density approximation (LDA) calculations.

An investigation of strongly polarized lines in the spectra of epitaxially grown ZnSe films is presented. From the observed dependence of the polarization with the sample thickness, 〈1 1 0〉 aligned extended defects are identified. For the thinnest ZnSe films the defects associated with the Y line form first in the [ 1 1 0] direction. From this observation, we suggest that these defects are directly correlated with misfit dislocations. However, an asymmetry in the residual strain is not detected in these samples. In 2 μm thick nitrogen-doped ZnSe films, a considerable polarization of all lines associated with the nitrogen acceptor is observed while the Y line is not polarized. Linearly polarized luminescence is thus a sensitive optical tool to characterize structural defects in ZnSe films.

We report on the molecular beam epitaxy and the properties of Be-chalcogenides. Especially the interface between ZnSe and BeTe and BeTe/ZnSe superlattices have been studied by means of HRXRD (high resolution X-ray diffraction), HRTEM (high resolution transmission-electron-microscopy), RHEED (reflection-high-energy-electron-diffraction), photo-electron-spectroscopy (PES) and photo-luminescence (PL). A strong dependence of the structural and electronic properties of these superlattices (SL) on the interface configuration has been found. A high valence band offset of about 1 eV between BeTe and ZnSe has been determined by means of photo-luminescence and photo-electron-spectroscopy.

A combined exciton–cyclotron resonance is found in the photoluminescence excitation and reflectivity spectra of semiconductor quantum wells with an electron gas of low density. In external magnetic fields an incident photon creates an exciton in the ground state and simultaneously excites an electron between Landau levels. A theoretical model is developed and suggests the dominating contribution of the exchange exciton–electron interaction.

A combined exciton-cyclotron resonance is found in photoluminescence excitation and reflectivity spectra of semiconductor quantum wells containing an electron gas of low density. In external magnetic fields, an incident photon creates an exciton in the ground state and simultaneously excites one of the resident electrons from the lowest to one of the upper Landau levels. A theoretical model is developed, which gives a good quantitative description of the energy position and the intensity of the combined exciton-cyclotron resonance.

An experimental and theoretical study of the magnetic polaron states of two-dimensional excitons in quantum wells based on semimagnetic semiconductors (Cd,Mn)Te is reported. It is shown that magnetic-polaron formation in in-plane magnetic fields leads to a lowering of the system symmetry, provided the fields are not too strong. The magnetic moment of the polaron thus formed is not parallel to the external magnetic field and contains a component normal to the quantum-well plane. This spontaneous lowering of the symmetry results in a change of the polarization characteristics of the luminescence from magnetic polaron states and in a weakening (compared to the three-dimensional case) in the efficiency of magnetic field-induced polaron suppression.

The spin-glass transition in Cd1-xMnxTe epitaxial 1ayers and bulk samples with 0.24 < x < 0.43 and in quantum well structures on the basis of Cd1-xMnxΤe were investigated by means of optical spectroscopy. Reduction of dimensionality of Cd1-xMnxTe 1ayers down to the quasi-two-dimensional case realized in Cd1-x Mnx Te/Cd1y Mn y Τe heterostructures frustrates the spin-glass formation, which is in agreement with theoretical predictions. The spin-glass formation is also frustrated in the vicinity of interfaces between semimagnetic and nonmagnetic semiconductors in CdTe/Cd1xMnx Te quantum wells. .

Theory of specular light reflection from long-period quantum-well structures taking into account the exciton contribution to dielectric polarization has been developed for an arbitrary relation between the background refractive index in the well, n a, and barrier-material refractive index nb. General expressions for the optical reflection and transmission coefficients for a structure with N equidistant quantum wells are derived with the use of the Green’s function and transfer matrix methods. Normal and oblique light reflectance spectra from II-VI-based heterostructures were found to reveal a bright interference pattern caused by the difference between n a and n b. A comparison of the theory with experiment has yielded the dispersion of n a and n b within a broad wavelength range and the parameters of the quasi-two-dimensional heavyhole exciton (e1-hh1), namely, the resonant frequency and the radiative and nonradiative damping rates. Reflectance spectra from resonant Bragg and quasi-Bragg structures with real exciton parameters are calculated, and the effect on these spectra of the refractive-index difference and the deviation from the Bragg condition is analyzed.

Spin relaxation of Mn ions in a Cd0.97Mn0.03Te/Cd0.75Mg0.25Te quantum well with photogenerated quasi-two-dimensional electron-hole plasma at liquid helium temperatures in an external magnetic field has been investigated. Heating of Mn ions by photogenerated carriers due to spin and energy exchange between the hot electron-hole plasma and Mn ions through direct sd-interaction between electron and Mn spins has been detected. This process has a short characteristic time of about 4 ns, which leads to appreciable heating of the Mn spin subsystem in about 0.5 ns. Even under uniform excitation of a dense electron-hole plasma, the Mn heating is spatially nonuniform, and leads to formation of spin domains in the quantum well magnetic subsystem. The relaxation time of spin domains after pulsed excitation is measured to be about 70 ns. Energy relaxation of excitons in the random exchange potential due to spin domains results from exciton diffusion in magnetic field B=14 T with a characteristic time of 1 to 4 ns. The relaxation time decreases with decreasing optical pump power, which indicates smaller dimensions of spin domains. In weak magnetic fields (B=2 T) a slow down in the exciton diffusion to 15 ns has been detected. This slow down is due to exciton binding to neutral donors (formation of bound excitons) and smaller spin domain amplitudes in low magnetic fields. The optically determined spin-lattice relaxation time of Mn ions in a magnetic field of 14 T is 270±10 and 16±7 ns for Mn concentrations of 3% and 12%, respectively.

A giant blue shift (≈0.5 eV) and a large decrease in the emission time of a spectral band corresponding to radiative recombination of spatially separated electrons and holes are observed in ZnSe/BeTe superlattices at high laser excitation levels. On the basis of numerical calculations, the observed defects are attributed to band bending arising in type-II structures at high carrier density.

We report resonant spin-flip Raman scattering measurements in CdTe/Cd${}_{1\ensuremath{-}x}$Mg${}_{x}$Te single quantum wells with $x=0.15, 0.26$, and $0.5$ as well as in Cd${}_{1\ensuremath{-}x}$Mg${}_{x}$Te alloys with $x=0, 0.15$, and $0.5$. Effective $g$ factors of electrons and heavy holes are measured as a function of the quantum well width $(18\ensuremath{-}100$ \AA{}) and the angle of the magnetic field with respect to the growth axis. The electron $g$ factors in quantum wells are between the values for bulk CdTe ${(g}^{e}=\ensuremath{-}1.644\ifmmode\pm\else\textpm\fi{}0.005)$ and Cd${}_{0.85}$Mg${}_{0.15}$Te ${(g}^{e}=\ensuremath{-}0.802\ifmmode\pm\else\textpm\fi{}0.005)$. We show that the change in the energy gap induced by the quantum confinement of the carriers provides the dominant contribution to the electron $g$ factor in quantum wells. A large $g$ factor anisotropy for conduction electrons is observed. It is shown that this anisotropy depends on the splitting between light- and heavy-hole states. The experimentally determined $g$ factors are in good agreement with theoretical predictions.

We report on the spin-flip Raman scattering of electrons bound to shallow donors in single quantum wells of ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te between ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{y}}$${\mathrm{Mg}}_{\mathrm{y}}$Te barriers. An enhancement of the scattering intensity of the bound magnetic polaron was found for narrow quantum wells. A variational treatment of the quantum well exciton in combination with the polaron model developed for bulk material is applied to interpret the experimental observations. The reduced symmetry of the quantum well states manifests itself through the observation of new resonances in the Raman scattering cross section. A simple model taking into account the anisotropy of the Zeeman effect of the valence band can reproduce the observations.

We have grown Be-chalcogenides for the first time by molecular beam epitaxy (MBE) on GaAs substrates. The lattice constants of both BeTe and BeSe are smaller than that of GaAs, therefore Be containing compounds such as (BeMg)Te or (BeZn)Se and (BeMgZn)Se are ideal candidates to replace ZnTe or Zn(SSe) and (ZnMg)(SSe) ternaries and quaternaries, respectively. In this contribution we present first results on the p-type doping of such layers using a modified nitrogen plasma source. The p-type dopability of BeTe as determined by van-der-Pauw measurements is comparable to that of ZnTe, free-hole concentrations of more than 1 × 1020cm−3 have been reached. Heavily p-doped ternary (BeMg)Te or (BeMn)Te layers are obtained easily which are fully lattice-matched to GaAs. On the other hand, (BeZn)Se and (BeMgZn)Se ternaries and quaternaries seem to behave similar to their sulphur-containing counterparts, an increase of the band gap leads to a decrease of the free-hole concentration. Based on these results, we propose an alternative for the p-type doping of wide gap II–VI semiconductors. The small lattice mismatch of BeTe to GaAs allows to grow BeTe ZnSe superlattices with excellent structural quality. We have calculated possible superlattice structures which should have an effective band gap comparable to or higher than ZnSe and at the same time are highly p-doped and almost lattice-matched to the GaAs substrate.

Beryllium chalcogenides are semiconductors with a large band gap. They have a higher-degree of covalent bonding than all of the other II–VI compounds. The bond energies of BeTe, BeSe and BeS are comparable to those of GaN. Due to their band gaps and their lattice constants they can be incorporated in quaternary mixed crystals which are lattice matched to GaAs. The hardness of II–VI materials containing beryllium offers new possibilities to enlarge the lifetime of laser diodes, emitting in the green and blue spectral range. First light-emitting diodes (LEDs) with beryllium as a constituent have been produced on GaAs substrates by molecular beam epitaxy (MBE). Double heterostructure devices were realized with quantum wells either of BeZnSeTe, ZnSe or BeZnSe. Special ohmic contacts consisting of BeTeZnSe graded superlattices were employed. Although for the first devices the growth conditions were not optimized, the lifetime at typical operating currents was very promising.

Light-emitting diodes with BeMgZnSe cladding layers and an active region formed by a ZnSe, BeZnSe or BeZnSeTe single quantum well are characterized by means of current-voltage measurements and electroluminescence at room temperature. A longterm stability exceeding 4000 h has been estimated in first experiments at low current densities (15 A cm−2). The emission wavelength was tuned between 452 and 518 nm by varying the composition of the quantum well material. Additionally, first results on the n-type doping of BeMgZnSe during molecular beam epitaxy have been obtained. The properties of ohmic contacts to n-type epilayers are discussed.

In this contribution, we present first results on the p-type doping of beryllium-containing II–VI compounds such as BeTe, (BeMg)Te, (BeZn)Se and (BeMgZn)Se grown by molecular beam epitaxy (MBE) using a nitrogen plasma source. These materials are a very promising alternative to ZnSSe and MgZnSSe ternaries and quaternaries, respectively. The p-type dopability of (BeMg)Te:N as determined by means of van-der-Pauw measurements and far-infrared reflectivity (FIR) spectroscopy almost exceeds that of ZnTe, and free hole concentrations NA - ND of more than 1 × 1020 cm−3 have been reached. Heavily p-doped ternary (BeMg)Te:N layers are obtained easily which are lattice matched to GaAs and have a high structural quality. The sheet and contact resistance of such layers have been below 5 × 10−3 Ωcm and 7 × 10−3 Ωcm2, respectively. Therefore, this material is the ideal replacement for ZnTe:N, which is used nowadays as top contact layer in ZnSe-based blue-green laser devices. (BeZn)Se and (BeMgZn)Se ternaries and quaternaries seem to behave similarly to their sulfur-containing counterparts; an increase of the band gap leads to a decrease of the free hole concentration. Several (BeMgZn)Se/(BeZn)Se single quantum well (SQW) and multiple quantum well (MQW) light-emitting devices (LEDs) have been fabricated which all show a bright, deep blue emission at low turn-on voltages. One reason for the low operating voltage is that the valence band of BeTe is aligned rather well with that of GaAs, and together with the high p-type dopability of BeTe, a smooth transition from the p-GaAs substrate to p-ZnSe via a BeTe/ZnSe pseudograding is possible.

A method has been developed for recording and analyzing the differential magnetoreflection (magnetotransmission) spectra of semiconductor structures with quantum wells. The method was used to determine the exciton g-factor in semimagnetic CdTe/(Cd, Mn)Te heterostructures with quantum wells. In nonmagnetic structures with quantum wells containing a two-dimensional electron gas, the excitonic damping depends on the spin state of the exciton. This effect is explained by the exchange contribution to exciton-electron scattering.

We have investigated optical properties of ZnTe epilayers, Zn1−xMgxSeyTe1−y epilayers, and ZnTe/Zn1−xMgxSeyTe1−y quantum wells (QWs) grown on (100)-InAs substrates by molecular beam epitaxy. We observed several sharp photoluminescence lines close to the excitonic position and no detectable luminescence from deep levels in ZnTe epilayers. Bright luminescence has been obtained from Zn1−xMgxSeyTe1−y epilayers which are lattice matched with InAs. The band alignment of ZnTe/Zn1−xMgxTe QWs was found to be type I. The reduction of the band gap energy of the ZnTe layer due to a tensile strain was confirmed in this structure. Nearly lattice-matched ZnTe/Zn1−xMgxSeyTe1−y QWs have been fabricated. A type II band alignment was observed for many of these QWs. We estimated bowing parameters not only of the band gap but also of the valence band for ZnSeyTe1−y.

Beryllium-containing ZnSe-based compound semiconductors introduce substantial additional degrees of freedom for the design of wide gap II–VI heterostructures. An overview of the advantages of beryllium chalcogenides is given here. A variety of BeTe-ZnSe and BeMgZnSe-ZnSe structures has been fabricated by molecular beam epitaxy, and their structural, optical and electrical properties have been investigated. The main aspects are the lattice matching of BeTe with its high lying valence band and high p-type dopability, the possibility to grade the valence band of GaAs down to the one of ZnSe by using a lattice matched ZnSe-BeTe pseudograding, the use of beryllium instead of sulphur for high band gap claddings, as well as the expected strengthening of beryllium compounds as compared to the standard materials used today on the basis of ZnMgSSe.

This article will give a short overview about some aspects of the molecular beam epitaxy (MBE) of Be-containing II–VI compounds on GaAs substrates. BeTe, BeMgTe, BeMgZnSe, BeZnSeTe, BeZnCdSe layers and ZnSe-BeTe superlattices have been produced. The growth regime and some properties of these new materials will be presented here.

The development of low-cost and low-power MEMS-based cantilever sensors for possible application in hand-held airborne ultrafine particle monitors is described in this work. The proposed resonant sensors are realized by silicon bulk micromachining technology with electrothermal excitation, piezoresistive frequency readout, and electrostatic particle collection elements integrated and constructed in the same sensor fabrication process step of boron diffusion. Built-in heating resistor and full Wheatstone bridge are set close to the cantilever clamp end for effective excitation and sensing, respectively, of beam deflection. Meanwhile, the particle collection electrode is located at the cantilever free end. A 300 μm-thick, phosphorus-doped silicon bulk wafer is used instead of silicon-on-insulator (SOI) as the starting material for the sensors to reduce the fabrication costs. To etch and release the cantilevers from the substrate, inductively coupled plasma (ICP) cryogenic dry etching is utilized. By controlling the etching parameters (e.g., temperature, oxygen content, and duration), cantilever structures with thicknesses down to 10 - 20 μm are yielded. In the sensor characterization, the heating resistor is heated and generating thermal waves which induce thermal expansion and further cause mechanical bending strain in the out-of-plane direction. A resonant frequency of 114.08 ± 0.04 kHz and a quality factor of 1302 ± 267 are measured in air for a fabricated rectangular cantilever (500x100x13.5 μm3). Owing to its low power consumption of a few milliwatts, this electrothermal cantilever is suitable for replacing the current external piezoelectric stack actuator in the next generation of the miniaturized cantilever-based nanoparticle detector (CANTOR).

A closed loop circuit capable of tracking resonant frequencies for MEMS-based piezoresistive cantilever resonators is developed in this work. The proposed closed-loop system is mainly based on a phase locked loop (PLL) circuit. In order to lock onto the resonant frequency of the resonator, an actuation signal generated from a voltage-controlled oscillator (VCO) is locked to the phase of the input reference signal of the cantilever sensor. In addition to the PLL component, an instrumentation amplifier and an active low pass filter (LPF) are connected to the system for gaining the amplitude and reducing the noise of the cantilever output signals. The LPF can transform a rectangular signal into a sinusoidal signal with voltage amplitudes ranging from 5 to 10 V which are sufficient for a piezoactuator input (i.e., maintaining a large output signal of the cantilever sensor). To demonstrate the functionality of the system, a self-sensing silicon cantilever resonator with a built-in piezoresistive Wheatstone bridge is fabricated and integrated with the circuit. A piezoactuator is utilized for actuating the cantilever into resonance. Implementation of this closed loop system is used to track the resonant frequency of a silicon cantilever-based sensor resonating at 9.4 kHz under a cross-sensitivity test of ambient temperature. The changes of the resonant frequency are interpreted using a frequency counter connected to the system. From the experimental results, the temperature sensitivity and coefficient of the employed sensor are 0.3 Hz/°C and 32.8 ppm/°C, respectively. The frequency stability of the system can reach up to 0.08 Hz. The development of this system will enable real-time nanoparticle monitoring systems and provide a miniaturization of the instrumentation modules for cantilever-based nanoparticle detectors.

Vertical silicon nanowire (SiNW) resonators are designed and fabricated in order to assess exposure to aerosol nanoparticles (NPs). To realize SiNW arrays, nanolithography and inductively coupled plasma (ICP) deep reactive ion etching (DRIE) at cryogenic temperature are utilized in a top-down fabrication of SiNW arrays which have high aspect ratios (i.e., up to 34). For nanolithography process, a resist film thickness of 350 nm is applied in a vacuum contact mode to serve as a mask. A pattern including various diameters and distances for creating pillars is used (i.e., 400 nm up to 5 μm). In dry etching process, the etch rate is set high of 1.5 μm/min to avoid underetching. The etch profiles of Si wires can be controlled aiming to have either perpendicularly, negatively or positively profiled sidewalls by adjusting the etching parameters (e.g., temperature and oxygen content). Moreover, to further miniaturize the wire, multiple sacrificial thermal oxidations and subsequent oxide stripping are used yielding SiNW arrays of 650 nm in diameter and 40 μm in length. In the resonant frequency test, a piezoelectric shear actuator is integrated with the SiNWs inside a scanning electron microscope (SEM) chamber. The observation of the SiNW deflections are performed and viewed from the topside of the SiNWs to reduce the measurement redundancy. Having a high deflection of ~10 μm during its resonant frequency of 452 kHz and a low mass of 31 pg, the proposed SiNW is potential for assisting the development of a portable aerosol resonant sensor.

A modeling approach for standing spin-wave resonances based on a finite-difference formulation of the Landau-Lifshitz-Gilbert equation is presented. In contrast to a previous study [Bihler et al., Phys. Rev. B 79, 045205 (2009)], this formalism accounts for elliptical magnetization precession and magnetic properties arbitrarily varying across the layer thickness, including the magnetic anisotropy parameters, the exchange stiffness, the Gilbert damping, and the saturation magnetization. To demonstrate the usefulness of our modeling approach, we experimentally study a set of (Ga,Mn)As samples grown by low-temperature molecular-beam epitaxy by means of electrochemical capacitance-voltage measurements and angle-dependent standing spin-wave resonance spectroscopy. By applying our modeling approach, the angle dependence of the spin-wave resonance data can be reproduced in a simulation with one set of simulation parameters for all external field orientations. We find that the approximately linear gradient in the out-of-plane magnetic anisotropy is related to a linear gradient in the hole concentrations of the samples., 14 pages, 6 figures

In this article, We focus on silicon nanowire (SiNW)-based resonators that were fabricated and employed to sense aerosol nanoparticles (NPs) by measuring resonant frequency shifts induced by the mass of stuck NPs. The fabrication of SiNW arrays was performed using inductively coupled plasma (ICP) cryogenic dry etching and multiple thermal oxidations. The SiNWs were coated with gold (Au) for contacting to the homebuilt electrostatic NP sampler to collect the flowing NPs. A piezoelectric shear actuator mounted in the frequency measurement system was used to excite the SiNW sensors into resonance. Tested in a titanium dioxide (TiO2) aerosol sampling with a total concentration of ~8,500 NPs/cm3, the sensor displayed its feasibility as a nanobalance to detect aerosol NPs in the femtogram scale with a mass sensitivity of 7.1 Hz/fg and a mass resolution of 31.6 fg. To extend the operating life of the sensor, an ultrasonic removal method was used to detach the adhered NPs.

A silicon resonant cantilever sensor is developed for detection of airborne nanoparticles (NPs) by monitoring the change in resonant frequency induced by an additional mass of trapped NPs. A piezoelectric stack actuator and a piezoresistive strain gauge are involved in the sensor system in order to actuate and detect the oscillation of cantilever sensor, respectively. An electrostatic precipitator is employed to trap the NPs on the cantilever surface. The proposed sensor reveals a mass sensitivity of 10 Hz/ng and a quality factor of 1206 while operated in the fundamental flexural mode. As necessary for an application under workplace conditions the limitations of the sensor sensitivity imposed by the environment are investigated, i.e., the influences of temperature, relative humidity, and pressure on the sensor are measured.

We report on resonant tunneling through ZnSe/BeTe double‐barrier, single‐quantum‐well structures. Negative differential resistance has been observed in the current–voltage characteristics up to room temperature. Due to a conduction‐band offset of more than 2 eV, four resonances with negative differential resistance could be detected for this semiconductor material combination at liquid‐helium temperature. The structures exhibit a peak‐to‐valley ratio up to 6:1 at 4.2 K. Current–voltage characteristics as a function of temperature have been studied and analyzed.

We have investigated the Raman signals of vibrational modes and the phonon sidebands of resonantly excited donor - acceptor-pair recombination in p-doped CdTe and ZnSe. Under resonant excitation of donor - acceptor-pair states we observe an enhanced sensitivity of the optical spectroscopy to the spectrum of local vibrational excitations. The measured signals are interpreted as local vibrational modes of the dopants or as phonons with wavevectors at distinguished points of the Brillouin zone others than the centre point . These latter signals correspond either to the maxima of the phonon density of states of the host crystal or to longitudinal optical phonons with finite but discrete wavevectors defined by the donor - acceptor-pair separations. The results can contribute to the characterization of the materials as well as to the investigation of the processes of the acceptor compensation.

Magnetoluminescence studies of highly excited undoped CdTe/${\mathrm{Cd}}_{0.88}$${\mathrm{Mn}}_{0.12}$Te and ${\mathrm{Cd}}_{0.97}$${\mathrm{Mn}}_{0.03}$Te/${\mathrm{Cd}}_{0.75}$${\mathrm{Mg}}_{0.25}$Te quantum wells (QW's) have been carried out in magnetic fields up to 14 T at 4.2 K. Qualitatively different excitonic systems, spin aligned in (Cd,Mn)Te QW and spin depolarized in CdTe QW, have been generated. The interaction of spin-aligned excitons are found to be weakly repulsive. In contrast, an additional well pronounced emission line M has been found to appear at approximately 5 meV below the excitonic line in the spectra of the spin-degenerate system. This line is assigned to the emission of excitonic molecules consisting of excitons with different spins. Finally, we have shown that the renormalization of Landau-level transitions in dense electron-hole magnetoplasmas with several occupied Landau levels depends on their spin state, it is markedly weaker in the spin-polarized plasmas. \textcopyright{} 1996 The American Physical Society.

A variety of BeMgZnSe–ZnSe‐ as well as BeTe‐based quantum‐well structures has been fabri‐ cated and investigated. BeTe buffer layers improve the growth start on GaAs substrates drasti‐ cally compared to ZnSe/GaAs. The valence‐band offset between BeTe and ZnSe has been determined to be 0.9 eV (type II). Due to the high‐lying valence band of BeTe, a BeTe–ZnSe pseudograding can be used for an efficient electrical contact between p‐ZnSe and p‐GaAs. BeMgZnSe quaternary thin‐film structures have reproducibly been grown with high struc‐ tural quality, and rocking curve widths below 20 arcsec could be reached. Quantum‐well structures show a high photoluminescence intensity even at room temperature.

We have investigated g-factor oscillations of electrons as well as of photoexcited holes in a quasi-two dimensional electron gas (2DEG) of CdTe/(CdMg)Te modulation-doped single quantum wells by means of photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. The oscillation amplitude is found to be related to Landau level numbers and filling factors as, predicted by the theory. The exchange constants A NN , of the g-factor enlargement have been obtained experimentally for the first time. The observed enlargement of the g factor in CdTe/(CdMnMgTe) heterostructures, where the sign of the intrinsic g factor is reversed by the sp-d exchange interaction, confirms the results obtained for nonmagnetic CdTe/(Cdmg)Te heterostructures.

We report the observation of resonant tunneling through CdTe/${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mg}}_{\mathit{x}}$Te double barrier, single-quantum-well heterostructures. Pronounced negative differential resistance has been observed in the current-voltage characteristics with peak-to-valley ratios up to 8:1 at 4.2 K. Vertical transport across the double barrier has been analyzed in detail as a function of temperature and applied magnetic fields. \textcopyright{} 1996 The American Physical Society.

In zinc‐blende II–VI alloys the thermal‐expansion coefficient for low temperatures is negative and becomes positive at higher temperatures. We investigated the luminescence properties of molecular‐beam‐epitaxy‐grown (CdMnMg)Te layers in the temperature range from 2 up to 200 K and show that the anomalous temperature dependence of the lattice constant is reflected in the luminescence properties of the excitonic recombination and the internal transition of manganese (ITM). The temperature behavior of the ITM energy is nonmonotonic and the existence of a minimum in the photon energy (at the temperature TMn) can be correlated to the change of sign of the thermal‐expansion coefficient. The decay constants of the ITM begins to decrease drastically at TMn, too. Considering a lattice constant dependent energy transfer rate to the infrared emitting state (1.2 eV) of the manganese ion the variation of the lifetimes can also be explained by the temperature dependence of the lattice constant. Furthermore, we have mea...

The internal site symmetry of exciton binding traps and the local structure are investigated revealing cadmium vacancy related defect complexes in aluminum-, chlorine-, iodine- and indium-doped CdTe films. The Zeeman experiments on the low temperature photoluminescence are discussed in terms of a modified effective Hamiltonian. The corresponding g-factor of the electron and the hole parameters K and L have been determined. From the studies of line intensities associated with the defect complexes as a function of the MBE growth conditions, cadmium vacancies are identified as constituents of these complex defects. The proposed microscopic models are compared with the observed properties of the bound exciton recombinations. Pronounced acoustical phonon replica and luminescence polarized parallel to a [110] axis are obviously evidences for a preferential alignment of the Cd vacancy related complex centers in the cubic CdTe lattice. A degradation of the luminescence intensity of complex associated lines with time has been detected.

The origin of a compensation mechanism in highly halogen doped CdTe grown by molecular beam epitaxy is investigated by means of electrical transport, photoluminescence and positron annihilation measurements. From the data one can identify vacancies to play an important part in the compensation mechanism at high doping levels. The behaviour of highly doped compensated CdTe layers in the electrical transport measurements as well as in photoluminescence turned out to be dominated by potential fluctuations, which are probably created due to the random distribution of donor impurities and compensating defects.