Search

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.

Pulsed sputter deposition has been demonstrated to be a viable process for the growth of high quality GaN and InGaN/GaN LEDs. It enables the fabrication of nitride LEDs with a red emission wavelength at large areas. In this study, we explore details on the epitaxial sputter deposition of GaN with a particular emphasis on ion damage. By changing the argon to nitrogen ratio, we adjust the growth mode from island to layer growth. TEM revealed speckles in the epitaxial GaN, which could be identified as isolated basal stacking faults, acting as non-radiative recombination centers. Using Monte Carlo methods, we modeled the energies of backscattered and sputtered atoms in order to get information on the ion damage mechanisms. Considering the collisions on the way from the target to the substrate, we found energetic nitrogen to induce the speckles. A shielding mechanism based on metallic gallium has been identified, leading to the strongly increased luminescence quality in comparison to the non-shielded material.

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.

We studied the near-band-edge emission (NBE) and yellow defect luminescence (YL) of GaN under below bandgap excitation with 40-fs laser pulses at a wavelength of 775 nm. Even though in this case three-photon absorption processes are required for the excitation of band-to-band transitions, fourth-order (INBE ∝ Iexc4) and sub-linear (IYL ∝ Iexc0.5) dependencies were observed for the intensity of the NBE and YL as a function of the excitation intensity, respectively. Modelling the carrier dynamics with a few-level rate-equation model revealed that, for high excitation irradiances, the electron-hole population generated by three-photon absorption is such that the NBE recombination rate is intensified (exponent > 3) and, at the same time, the competition between the electron capturing and the defect-level emission rate suppresses the YL (exponent 3) and, at the same time, the competition between the electron capturing and the defect-level emission rate suppresses the YL (exponent

Gallium and nitrogen ions have been implanted into ZnO crystals and metal organic vapor phase epitaxy grown ZnO layers. Postimplantation annealing behavior in the temperature range between 200 and 900 °C has been studied by means of Raman scattering and low-temperature photoluminescence. The temperature for healing of the implantation-induced defects was found to be 800 °C. Implanted gallium acts as donor with a donor binding energy ED of 53 meV, thus allowing the control of n-type doping in ZnO. From photoluminescence measurements of the donor-acceptor pair transition of a series of nitrogen-implanted ZnO samples we estimate the binding energy EA of the nitrogen acceptor between 163 and 196 meV. Electrical characterization of nitrogen-implanted samples shows a behavior ranging from low n-type to highly compensated. But no unambiguous and reproducible type conversion could be achieved.

Properties of interfaces with no-common atom in type-II ZnSe/BeTe heterostructures are studied with polarization spectroscopy. Structures with four possible configurations of normal and inverted interfaces have been investigated. Radiative recombination and its polarization anisotropy have been found to depend crucially on the interface configuration and excitation power. The comparison of interfaces formed by the growth on anion (Se, Te) and cation (Zn, Be) terminated layers has shown that the latter demonstrate a significantly higher nonradiative recombination rate. In agreement with the quantum well (QW) symmetry, the photoluminescence (PL) of the structures with nonequivalent normal and inverted interfaces is highly linearly polarized both at low and high excitation densities. Unexpectedly, a similarly strong PL polarization has been found for structures with equivalent interfaces up to carrier densities of 1012 cm−2 per QW. The polarization is explained by a built-in electric field, it decreases with...

Three dimensional GaN structures with different crystal facets and doping types have been investigated employing the surface photo-voltage (SPV) method to monitor illumination-induced surface charge behavior using Kelvin probe force microscopy. Various photon energies near and below the GaN bandgap were used to modify the generation of electron–hole pairs and their motion under the influence of the electric field near the GaN surface. Fast and slow processes for Ga-polar c-planes on both Si-doped n-type as well as Mg-doped p-type GaN truncated pyramid micro-structures were found and their origin is discussed. The immediate positive (for n-type) and negative (for p-type) SPV response dominates at band-to-band and near-bandgap excitation, while only the slow process is present at sub-bandgap excitation. The SPV behavior for the semi-polar facets of the p-type GaN truncated pyramids has a similar characteristic to that on its c-plane, which indicates that it has a comparable band bending and no strong influence of the polarity-induced charges is detectable. The SPV behavior of the non-polar m-facets of the Si-doped n-type part of a transferred GaN column is similar to that of a clean c-plane GaN surface during illumination. However, the SPV is smaller in magnitude, which is attributed to intrinsic surface states of m-plane surfaces and their influence on the band bending. The SPV behavior of the non-polar m-facet of the slightly Mg-doped part of this GaN column is found to behave differently. Compared to c- and r-facets of p-type surfaces of GaN-light–emitting diode micro-structures, the m-plane is more chemically stable.

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.

The Be-enhanced formation of CdSe quantum dots in CdSe/ZnSe heterostructures grown by migration enhanced epitaxy on (001)GaAs substrates has been investigated using photoluminescence spectroscopy, x-ray techniques (diffraction and reflectometry), and transmission electron microscopy. Coverage of the ZnSe starting surface with a fractional monolayer of beryllium selenide leads to enhanced island formation well below the CdSe thickness of 0.6 monolayer corresponding to the onset of the CdSe-rich island formation in the Be-free structures. The effect of the fractional Be coverage is demonstrated by observation of sharp lines in the photoluminescence signal from patterned mesas with dimensions down to 60 nm, which is due to the emission from individual exciton localization sites attributed to quantum dots. X-ray diffraction and reflectometry measurements on CdSe/ZnSe short-period superlattices with the submonolayer CdSe insertions confirm an enhanced roughening of the CdSe layer morphology in the case of bery...

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.

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.

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.

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.

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...

We report here on the growth and the characterization of p‐type CdTe grown by molecular beam epitaxy on (001) Cd0.96Zn0.04Te substrates. Nitrogen has been used as a dopant, which is activated in an electron cyclotron resonance plasma source. The carrier concentration was determined using a C/V profiler. Nitrogen has been successfully incorporated substitutionally and hole densities up to 2.6×1017 cm−3 have been achieved. In addition we present data from x‐ray diffraction and photoluminescence, which demonstrate the effect of self‐compensation on the nitrogen‐doped CdTe layers.

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.

We present a comparative study of two different spectroscopic techniques in order to determine the valence band offset in CdTe/Cd1−xMnxTe quantum wells. The energy difference between heavy‐ and light‐hole excitons as a function of the heavy‐hole transition energy is known as a sensitive tool for the determination of the valence band potential height. In the present study we have employed this technique to CdTe/Cd1−xMnxTe quantum wells. A valence band offset around Qv=0.30 is determined, which is found to be valid in the whole range of investigated Mn contents up to x=0.27. In semimagnetic quantum wells the tuning of potential heights in external magnetic fields offers the possibility to evaluate the valence band offset. This technique has been widely employed to CdTe/Cd1−xMnxTe, but no consensus has been reached yet. We have analyzed the Zeeman splitting of the heavy‐hole exciton in CdTe/Cd1−xMnxTe quantum wells with different Mn contents. Using the valence band offset as an adjustable parameter, a smalle...

CdMgTe/CdTe superlattices with a high degree of structural perfection were grown on CdTe and Cd0.975Zn0.025Te substrates by molecular‐beam epitaxy. The structural properties of the superlattices, i.e., the morphology of the layers and the state of strain relaxation, were examined by transmission electron microscopy and x‐ray diffractometry. High‐resolution transmission electron microscopy (HRTEM) reveals the structural quality of the superlattices on an atomic scale. The width of the chemical transition between the CdTe and CdMgTe layers was determined by HRTEM using chemically sensitive imaging conditions. Two different mechanisms of misfit dislocation generation could be observed in situ in the electron microscope studying a cross‐section specimen of a superlattice which was originally fully strained.

We report on the n‐type doping of the wide gap II‐VI semiconductor CdMgTe. Bromine and chlorine have been used as dopants during molecular beam epitaxy. For the CdTe base material both bromine and chlorine give shallow donors, and free carrier concentrations of up to 2.8×1018 cm−3 have been reached. For increasing Mg concentration, however, deep donors are created, limiting the free carrier concentration at room temperature. This is demonstrated by Hall effect measurements at different temperature. The deep donors can be converted to metastable shallow donors by light illumination at temperatures below about 180 K, which results in persistent photoconductivity. We interpret our results in terms of a DX‐like state, which is introduced by both the chlorine and bromine doping, and which moves into the band gap with increasing Mg concentration. We conclude that the limit in the n‐type doping of CdMgTe, the band gap of which can easily be tuned through the whole visible range, is an intrinsic property of the dopants used, and not due to self‐compensation caused by nonideal growth conditions.

We have investigated oxygen on CdTe substrates by means of x-ray photoelectron spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED). A Te oxide layer that was at least 15 A thick was found on the surface of as-delivered CdTe substrates that were mechanically polished. This oxide is not easily evaporated at temperatures lower than 350°C. Furthermore, heating in air, which further oxidizes the CdTe layer, should be avoided. Etching with HCI acid (15% HCl) for at least 20 s and then rinsing with de-ionized water reduces the Te oxide layer on the surface down to 4% of a monoatomic layer. However, according to XPS measurements of the 0 Is peak, 20%-30% of a monoatomic layer of oxygen remains on the surface, which can be eliminated by heating at temperatures ranging between 300 and 340 cC. The RHEED patterns for a molecular beam epitaxially (MBE)-grown CdTe film on a (lOO) CdTe substrate with approximately one monoatomic layer of oxidized Te on the surface lose the characteristics of the normal RHEED pattems for a MBE-grown CdTe film on an oxygen-free CdTe substrate.

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.

Electron spin polarization is studied on n-type modulation doped Cd1−xMnxTe/Cd1−x−yMnxMgyTe single quantum wells. The influence of the electron–electron exchange interaction on the electron spin polarization is discussed. In magnetic fields, the electronic states undergo an initial giant splitting due to the s–d and p–d exchange interactions with localized magnetic moments. Such a giant Zeeman splitting creates an initial spin polarization which will be further enlarged due to the many-body effects of the electron self-energy corrections.

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.

X‐ray photoelectron spectroscopy studies of CdTe‐GaAs interfaces are reported. The growth start of CdTe on GaAs can be nearly stoichiometric if convenient growth parameters are chosen. The valence‐band offset between these two materials is found to be large (470 meV). Cd‐Te‐metal‐GaAs multilayers have been grown with very thin metal films. The CdTe‐GaAs band offset is not influenced by such intermediary metal layers. The experimentally obtained value for the valence‐band offset is compared with recent theoretical calculations taking into account interface dipoles.