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We present an extensive investigation of the charge-trapping processes in vertical GaN nanowire FETs with a gate-all-around structure. Two sets of devices were investigated: Gen1 samples have unipolar (n-type) epitaxy, whereas Gen2 samples have a p-doped channel and an n-p-n gate stack. From experimental results, we demonstrate the superior performance of the transistor structure with a p-GaN channel/Al2O3 gate insulator in terms of dc performance. In addition, we demonstrate that Gen2 devices have highly stable threshold voltage, thus representing ideal devices for power electronic applications. Insight into the trapping processes in the two generations of devices was obtained by modeling the threshold voltage variations via differential rate equations.

The Zn acceptor level position in GaN:Zn was studied on specially designed GaN(undoped)/GaN:Zn([Zn] = 5 × 1016 cm−3) structures by contactless electroreflectance. A well-defined Franz-Keldysh oscillation is present in optical spectra which results from the built-in electric field present in the near-surface layer. The obtained results show that a significant band bending in the GaN:Zn layer occurs lowering the field in the top layer that is in line with predictions for a semiconductor doped with a high ionization energy acceptor. The acceptor level responsible for the Fermi level position in GaN:Zn is found to be at ∼2.3 eV below the conduction band minimum which coincides with levels introduced by Zn atoms substituting nitrogen in the lattice. At the same time, the surface Fermi level of structures is located in the middle of the bandgap, which is in the lower maximum of (0001) GaN surface density of states.

Lattice-polarity dependence of InN surface photovoltage has been identified by an anomalous surface potential behavior observed via photoassisted Kelvin probe force microscopy. Upon above bandgap light illumination in the ambient atmosphere, the surface photovoltage of the In-polar InN shows a pronounced decrease, while that of the N-polar one keeps almost constant. Those different behaviors between N-polar and In-polar surfaces are attributed to a polarity-related surface reactivity, which is found not to be influenced by Mg-doping. These findings provide a simple and non-destructive approach to determine the lattice polarity and allow us to suggest that the In-polar InN, especially that with buried p-type conduction, should be chosen for sensing application.

We have investigated the properties of ZnMgO epilayers and ZnO–ZnMgO quantum well structures grown by metalorganic vapor-phase epitaxy. A well-controlled incorporation of magnesium, x⩽0.10, could be confirmed resulting in a blueshift of the photoluminescence emission wavelength of the Zn1−xMgxO layers up to 200meV. Using ZnMgO as barrier material, ZnO–ZnMgO quantum well structures with different well widths have then been fabricated. The confinement effect in the ZnO quantum wells leads to the expected increase of the corresponding quantum well emission energy with decreasing well width. A comparison to calculations also suggests a further enhancement of the exciton binding energy in the quantum wells of up to 90meV.

The effects of hydrogen and deuterium on ferromagnetic GaAs doped with high concentrations of Mn (≈1021 cm−3) are studied. Secondary ion mass spectroscopy depth profiles show that D is incorporated in the same concentration as Mn. The epilayers change from metallic to semiconducting behavior upon hydrogenation. Fourier transform infrared absorption measurements show the As–H and As–D local vibrational modes characteristic for the complexes of hydrogen with group-II acceptors in GaAs.

The demonstration of vertical GaN wrap-around gated field-effect transistors using GaN nanowires is reported. The nanowires with smooth a-plane sidewalls have hexagonal geometry made by top-down etching. A 7-nanowire transistor exhibits enhancement mode operation with threshold voltage of 1.2 V, on/off current ratio as high as 108, and subthreshold slope as small as 68 mV/dec. Although there is space charge limited current behavior at small source-drain voltages (Vds), the drain current (Id) and transconductance (gm) reach up to 314 mA/mm and 125 mS/mm, respectively, when normalized with hexagonal nanowire circumference. The measured breakdown voltage is around 140 V. This vertical approach provides a way to next-generation GaN-based power devices.

Three-dimensional III-nitride micro-structures are being developed as a promising candidate for the future opto-electrical devices. In this study, we demonstrate a quick and straight-forward method to locally evaluate free-carrier concentrations and a crystalline quality in individual GaN:Si micro-rods. By employing micro-Raman mapping and analyzing lower frequency branch of A1(LO)- and E1(LO)-phonon-plasmon-coupled modes (LPP–), the free carrier concentrations are determined in axial and planar configurations, respectively. Due to a gradual doping profile along the micro-rods, a highly spatially resolved mapping on the sidewall is exploited to reconstruct free carrier concentration profile along the GaN:Si micro-rods. Despite remarkably high free carrier concentrations above 1 × 1020 cm−3, the micro-rods reveal an excellent crystalline quality, without a doping-induced stress.

We report on detailed investigations of the electronic and magnetic properties of ferromagnetic GaMnAs layers, which have been fabricated by low-temperature molecular-beam epitaxy. Superconducting quantum interference device measurements reveal a decrease of the Curie temperature from the surface to the GaMnAs/GaAs interface. While high resolution x-ray diffraction clearly shows a homogeneous Mn distribution, a pronounced decrease of the carrier concentration from the surface towards the GaMnAs/GaAs interface has been found by Raman spectroscopy as well as electrochemical capacitance-voltage profiling. The gradient in Curie temperature seems to be a general feature of GaMnAs layers grown at low-temperature. Possible explanations are discussed.

We report ferromagnetic resonance experiments on Ga1−xMnxAs thin films. For the dc magnetic field perpendicular to the sample plane, we observe up to eight distinct resonances, which we attribute to spin wave modes. To account for the spacing of the resonances, we infer a linear gradient in the magnetic properties, which is ascribed to a linear variation of the uniaxial magnetic anisotropy with film thickness. Values of D=(1±0.4)×10−9 Oe cm2 for the spin stiffness and JMnMn≈1 meV for the exchange integral between Mn spins are obtained.

Zn(Mn)Se/Be(Mn)Te semiconductor heterostructures with a type-II band alignment have been fabricated by molecular-beam epitaxy. Giant Zeeman splitting of the band states, caused by their interaction with the localized magnetic moments of Mn ions, has been observed for the spatially direct and indirect optical transitions. Strong p–d exchange interaction for the valence band states in (Be,Mn)Te has been demonstrated experimentally.

One major challenge for the development of spintronic devices is the control of the spin polarization of an electron current. We propose and demonstrate the use of a BeTe/Zn1−xSe/BeTe double barrier resonant tunneling diode for the injection of a spin-polarized electron current into GaAs and the manipulation of the spin orientation of the injected carriers via an external voltage. A spin polarization of up to 80% can be observed with a semimagnetic layer of only 3.5 nm thickness. By changing the resonance condition via the external voltage, the degree of spin polarization can be varied, though a complete spin switching has not yet been accomplished.

Bulk BeCdSe layer lattice-matched to a GaAs substrate, as well as a BeCdSe/ZnSe quantum well (QW) structure have been grown using the submonolayer digital alloying mode of molecular beam epitaxy. The structures have demonstrated bright photoluminescence up to room temperature and good structural quality. Stimulated emission under optical pumping has been obtained for a 2 nm BeCdSe/ZnSe multiple QW structure at 80 K. The bowing parameter of the energy gap of this ternary alloy has been estimated as about 4.5 eV.

Atomic force microscopy (AFM) of cleaved facets of ZnSe-based lasers with various active region designs is reported. Different AFM probe friction on the materials forming the laser structures are exploited for imaging their basic layers. Unlike ZnMgSSe-based lasers, the cleaved surface of cladding layers in BeMgZnSe-based structures is atomically flat, which is attributed to hardening of the II–VI materials by Be incorporation. Nanometer-high steps and undulations are observed at the laser heterointerfaces on cleaved facets. The shape and height of such topographic singularities located in the vicinity of a (Zn,Cd)Se quantum well active region depend on the strain distribution in the laser waveguide.

In this letter we report on optical degradation studies on BeMgZnSe separate confinement quantum well laser structures for the blue-green spectral region. The wavelength of the incident light has been tuned from 3.81 down to 2.10 eV, corresponding to an energy range from above the band gap of the cladding layers down to below the band gap of the quantum well. The dominant degradation mechanism is initiated when electron hole pairs are created in the quantum well. Absorption of light in deep defect bands, e.g., of the p-type cladding material is negligible in these structures. The strain state of the quantum well is one possible driving force for the degradation. In this context, point defect propagation as well as a structural phase transition of the ZnCdSe quantum well are discussed.

We report results of a detailed investigation of type-II superlattices under high density photoexcitation. A strong spectral shift (≈0.5 eV) of the recombination band corresponding to the indirect transition from the ZnSe conduction band to the BeTe valence band in ZnSe/BeTe superlattices with increasing carrier density has been found at T=300 K. The dynamical characteristics of this transition are studied by time-resolved spectroscopy. A model which accounts for the dependence of band bending and lifetimes of spatially separated electrons and holes on the concentration of the photoexcited carriers is developed. Numerical simulations of the photoluminescence kinetics are in very good agreement with experimental results. It turns out that despite the huge band offsets involved, the radiative recombination under high excitation conditions can be nearly as fast as in spatially direct quantum wells.

This letter reports on the self-organized growth of nanoscale dot-like CdSe-based islands during molecular beam epitaxy of CdSe/ZnSe nanostructures with a CdSe thickness between 0.75 and 3.0 monolayers. An increase in the nominal CdSe thickness results in a higher density of islands (up to 2×1010 cm−2) and is accompanied by dramatic enhancement of the photoluminescence efficiency. The density of large relaxed islands appears to saturate at a value of (3–4)×109 cm−2. Room temperature (Zn, Mg)(S, Se)-based optically pumped lasers with an extremely low threshold (less than 4 kW/cm2), as well as (Be, Mg, Zn)Se-based injection laser diodes using a single (2.5–2.8) monolayer thick CdSe active region, both demonstrating significantly enhanced degradation stability, have been fabricated and studied.

We report the successful application of alternatively strained short-period superlattices for the waveguide region of optically pumped and injection room-temperature ZnSe-based lasers operating within the 470–523 nm spectral range. The design of optically pumped ZnMgSSe/ZnSSe/ZnCdSe lasers provides extremely low threshold power densities due to the enhanced electronic and optical confinement. Room-temperature BeMgZnSe/ZnCdSe injection lasers with threshold current density of about 750 A/cm2 and characteristic temperature as high as 366 K are demonstrated. The peculiarities of carrier transport across the short-period superlattices are explained by a thermally activated mechanism.

A microstructure has been designed in which an electron gas is created optically with variable density. ZnSe and BeTe quantum wells are separated by a BeZnMgSe alloy with band gap 3 eV. When electron-hole pairs are created by ultraviolet light in the barriers, electrons accumulate in the ZnSe quantum well, and holes in the BeTe quantum well. This is because the BeTe valence band lies 900 meV higher than that of ZnSe. Reflectivity and luminescence spectra of the ZnSe well show the destabilization of excitons by an electron gas and formation of trions. The metastable charge-separated state has a very long lifetime (2 s at 2 K).

We report on resonant electron tunneling in ZnSe/BeTe double-barrier, single-quantum-well heterostructures under high magnetic fields. Current–voltage characteristics have been investigated in the presence of a magnetic field B perpendicular to the tunneling current (B⊥I). The effect of the field on the main resonance position as well as two additional features due to optical phonon assisted tunneling into the quantum well has been studied and analyzed. The peak bias voltage exhibits a quadratic shift with increasing magnetic field due to a Lorentz force acting on the tunneling electrons. The phonon peaks do not shift, because the transverse momentum is no longer conserved during this process.

Beryllium chalcogenides have a much higher degree of covalency than other II–VI compounds. Be containing ZnSe based mixed crystals show a significant lattice hardening effect. In addition, they introduce substantial additional degrees of freedom for the design of wide gap II–VI heterostructures due to their band gaps, lattice constants, and doping behavior. Therefore, these compounds seem to be very interesting materials for short wavelength laser diodes. Here, we report on the first fabrication of laser diodes based on the wide band gap II–VI semiconductor compound BeMgZnSe. The laser diodes emit at a wavelength of 507 nm under pulsed current injection at 77 K, with a threshold current of 80 mA, corresponding to 240 A/cm2.

Optically active semiconductor wires with lateral extensions between 60 nm and 5 μm based on CdTe/Cd1−xMgxTe quantum well structures have been fabricated by electron beam lithography and subsequent pattern transfer using a wet chemical etching process. Changes of the relative photoluminescence efficiency have been studied by photoluminescence spectroscopy as a function of the wire widths. For narrow wires nonradiative recombination at the wire sidewalls becomes the major recombination mechanism, strongly decreasing the photoluminescence efficiency. We discuss the influence of the exciton diffusion on the photoluminescence efficiency by investigating the wire width dependence of the photoluminescence intensity as a function of temperature and quantum well thickness. High photoluminescence efficiencies are obtained in the case of small diffusion lengths as for example at low temperatures and narrow quantum wells.

The influence of ion implantation on the interdiffusion of Cd and Mg in CdTe/CdMgTe single quantum wells was studied. The samples were implanted with 150 keV Ar+ ions and subsequently annealed for 1 min by rapid thermal annealing at temperatures between 400 and 480 °C. Ion implantation and annealing lead to an intermixing of barrier and well material, causing a blue shift of the characteristic emission spectrum. The dependence of the energy shift on the ion dose and the annealing temperature was studied. For an ion dose of 1×1014 cm−2 and an annealing temperature of 400 °C a blueshift of 80 meV compared to a nonimplanted reference sample indicates a significant enhancement of the interdiffusion of Cd and Mg by the implantation.

The results of optically detected cyclotron resonance and photoluminescence kinetics of CdTe/CdMnTe multi‐quantum‐well structures are presented. These experiments indicate that for nominally undoped wide quantum wells ionized shallow donor impurities limit carrier mobilities at low temperature.

Thermally induced interdiffusion in CdTe/CdMnTe quantum wells was investigated by photoluminescence spectroscopy. The single quantum well structures were grown by molecular beam epitaxy annealed by rapid thermal annealing for 1 min at temperatures between 380 and 520 °C. A blue shift close to the barrier energy was observed indicating an almost perfect interdiffusion between the well and the barrier material. We derive an activation energy of 2.8 eV for the interdiffusion process from a Fickian diffusion model applied to our experiments.

We present optical investigations on CdTe/(CdMg)Te single quantum wells (QWs) and demonstrate the high structural quality of the pseudomorphic grown QWs structure which shows high photoluminescence efficiency up to room temperature. Due to the large band‐gap difference between CdTe and Cd0.51Mg0.49Te of more than 0.8 eV remarkable strong confinement effects are observable. A strong enhancement of the exciton binding energies is found by decreasing well width. In the 50‐A‐wide QW the binding energy is more than two times larger compared with that of bulk CdTe. In addition, a strong functional dependence of the localization energy of donor bound excitons on the well thickness is found. A valence‐band offset of 30% in the strain‐free limit is determined from the energy difference between heavy‐ and light‐hole excitons which is consistent with the strong blue shift of exciton energies by decreasing well widths and the observed effective electron‐hole confinement.

For the first time clear evidence for two‐dimensional Shubnikov–de Haas oscillations in modulation‐doped CdTe/CdMnTe quantum‐well structures is reported. The structures were grown by molecular‐beam epitaxy using ZnBr2 as a novel source material for the n‐type doping of II‐VI epitaxial layers. From an analysis of the Shubnikov–de Haas oscillations a carrier density of 9×1011 cm−2 and an effective mass of 0.1 m0 could be deduced. Due to band filling the Fermi energy in the subbands is shifted above the conduction‐band edge. This can be detected as a Stokes shift of absorption compared to photoluminescence recombination. From the Fermi energy shift the carrier concentration can be estimated, which agrees well with values determined by Hall‐effect measurements.

The lifetime of excitons in CdTe/Cd0.75Mn0.25Te single quantum wells (SQW) has been measured by time‐resolved photoluminescence for quantum wells with thicknesses from 34 to 300 A and in the temperature range from 1.6 to 300 K. The exciton lifetimes are of the order of 150 ps at the lowest temperatures, where radiative recombination dominates. The lifetimes are only weakly dependent on well thickness and decrease from 170 to 140 ps for quantum wells with thickness decreasing from 300 to 34 A. A comparison with a high quality GaAs/AlGaAs SQW yields comparable luminescence efficiencies at low temperatures, yet the lifetimes are considerably shorter in the CdTe/Cd0.75Mn0.25Te quantum wells due to the higher exciton oscillator strength.

We report for the first time on successful n‐type doping of CdTe epilayers by the use of chlorine. The free‐carrier concentration in the layers, grown by molecular beam epitaxy (MBE) and photoassisted MBE, can be varied easily over three orders of magnitude by changing the ZnCl2 oven temperature. Peak mobilities between 4700 cm2/V s (n=1.8×1016 cm−3) and 480 cm2/V s (n=1.6×1018 cm−3) are obtained. The resistivity of highly Cl‐doped layers is as low as 8.5×10−3 Ω cm. Compared with In, which is commonly used for obtaining n‐type CdTe epilayers, higher free‐carrier concentrations and higher peak mobilities are obtained for compatible growth procedures by the use of Cl.

We report the growth and optical characterization of CdTe/CdMnTe single quantum wells with well thicknesses ranging from 60 down to 6 A. The single quantum wells were grown by standard molecular beam epitaxy without growth interruption and investigated by reflection, photoluminescence (PL), and excitation PL. All structures including the 6‐A‐thick quantum well exhibit extraordinarily narrow photoluminescence lines. From an analysis of linewidth and Stokes shift of the photoluminescence lines informations on the structure of the CdTe/CdMnTe interfaces are derived. The good quality of those structures made it possible to identify for the first time recombination of two‐dimensional free exciton magnetic polarons.

We report the fact that n‐type conducting CdTe can be grown by molecular beam epitaxy (MBE) with excess Cd flux and show for the first time details of the CdTe/Cd surface phase reconstruction diagram.The surface phase diagram has been mapped over the substrate temperature range 180–375 °C using Cd/CdTe ratios between 0.0 and 1.0. At low Cd/CdTe ratios and high temperatures a (2×1) reconstruction is observed and at high Cd/CdTe flux ratios we observe a c(2×2) reconstruction. We have also observed a strong influence of substrate illumination on the surface reconstruction.

Homogeneous nitrogen-polar GaN core-shell light emitting diode (LED) arrays were fabricated by selective area growth on patterned substrates. Transmission electron microscopy measurements prove the core-shell structure of the rod LEDs. Depending on the growth facets, the InGaN/GaN multi-quantum wells (MQWs) show different dimensions and morphology. Cathodoluminescence (CL) measurements reveal a MQWs emission centered at about 415 nm on sidewalls and another emission at 460 nm from top surfaces. CL line scans on cleaved rod also indicate the core-shell morphology. Finally, an internal quantum efficiency of about 28% at room temperature was determined by an all-optical method on a LED array.

We report on the fabrication of GaN co-doped with silicon and zinc by metalorganic vapor phase epitaxy and a detailed study of photoluminescence in this material. We observe an exceptionally high absolute internal quantum efficiency of blue photoluminescence in GaN:Zn,Si. The value of 0.93±0.04 has been obtained from several approaches based on rate equations.

The behavior of GaN surfaces during photoassisted Kelvin probe force microscopy is demonstrated to be strongly dependant on surface polarity. The surface photovoltage of GaN surfaces illuminated with above-band gap light is analyzed as a function of time and light intensity. Distinct differences between Ga-polar and N-polar surfaces could be identified, attributed to photoinduced chemisorption of oxygen during illumination. These differences can be used for a contactless, nondestructive, and easy-performable analysis of the polarity of GaN surfaces.