Non-etched Samples Research Articles

Electron and Hole Adducts Formed in Illuminated InP Colloidal Quantum Dots Studied by Electron Paramagnetic Resonance

An electron paramagnetic resonance (EPR) study of photoexcited colloidal InP quantum dots (QD) shows the formation of electron and hole adducts. An EPR signal at g = 0.58 is assigned to a nonradiative hole trap that does not form immediately upon illumination, but forms only after the illuminated sample ages and becomes stabilized at room temperature; it then becomes permanent at the InP QD surface. This signal completely disappears upon electron injection into the QD from a reducing agent (sodium biphenyl). Light immediately quenches the signal at g = 0.58, and it re-forms reversibly when the light is turned off. A signal at g = 2.055 is assigned to electron surface traps, and it appears in nonetched QD samples; it completely disappears after etching with HF. A signal at g = 2.001 has a very narrow line width and is assigned to delocalized mobile holes that are located in the QD core. A defect model for InP QDs is proposed based on the EPR results reported here plus results from optically detected magnetic resonance experiments reported separately.

Continuous-Wave and Time-Resolved Optically Detected Magnetic Resonance Studies of Nonetched/Etched InP Nanocrystals

The low temperature photoluminescence spectrum of InP nanocrystals (NCs) is composed of a near band-edge exciton and a defect emission band at lower energies. The present study characterized the defect emission utilizing an optically detected magnetic resonance spectroscopy (ODMR) both in continuous-wave and time-resolved modes. The continuous-wave ODMR spectra were measured as a function of the laser power, the microwave power, the microwave modulation frequencies, and of the light polarization. The results showed that the defect emission originated from the trap-to-band luminescence of a weakly coupled electron−hole pair, when the electron is trapped at a phosphorus vacancy, Vp (with angular momention of F = 1/2), while the hole is located at the valence band (with F = 3/2). Spin Hamiltonian simulations of the ODMR lineshape, including hyperfine interactions, revealed that the nonetched samples are dominated by Vp at the surface with two adjacent indium atoms. While treatment with hydrogen floride acid eliminates the surface defects, it leaves behind a small percent of Vp at the core of the NCs, with four indium atoms next neighbors at the vertexes of a tetrahedral site. The time-resolved ODMR measurements further distinguished between radiative and nonradiative processes and followed the spin dynamics at the excited state. Kinetic simulations of the PL intensity response to a microwave square wave modulation indicated that the spin−lattice relaxation time and the radiative lifetime of the weakly coupled electron−hole pair are in the microseconds regime.

Continuous and Time Resolved Optically Detected Magnetic Resonance Studies of InP Nanoparticles

AbstractCarriers in small colloidal InP nanoparticles are in strong quantum confinement regime. The low temperature photoluminescence spectrum of InP nanoparticles is composed of an excitonic luminescence at high energies and a non-excitonic defect emission band at lower energies. HF etching of the nanoparticles reduces the defect emission and enhances the exciton process.In this work we apply optically detected magnetic resonance spectroscopy (ODMR) both in continuous wave and time resolved mode (TR-ODMR) to study the defect luminescence in InP nanoparticles. The results show that the defect luminescence originates from weakly coupled electron-hole pair, where the electron is trapped at the surface by phosphorous vacancy, Vp, and the hole is located at the valence band. Additionally, the results suggest that the non-etched samples are dominated by Vp at the surface. Those are mainly eliminated upon HF treatment, leaving behind small percent of Vp in the core of the nanoparticle. We also find the electron-hole exchange interaction from circular polarized ODMR measurements. The TR-ODMR measurement further clarifies the spin dynamics and characteristic of the magnetic sites. Fitting these measurements to the simulated response of the PL intensity to the square wave modulated microwave power revealed that the spin relaxation time and radiative lifetime of electron-hole pair in the nanoparticles are in the microseconds regime.

Characterization of in situ etched and molecular beam epitaxy regrown GaAs interfaces using capacitance–voltage measurements, far infrared spectroscopy, and magnetotransport measurements

In this study, we characterize GaAs interfaces processed by ion beam etching (IBE), chemical assisted ion beam etching (CAIBE), and chemical gas etching (CGE). The etched interfaces are overgrown with GaAs or AlGaAs using molecular beam epitaxy. Interfaces in Si-doped GaAs are characterized with capacitance–voltage (C–V) profiling. Furthermore, we use a two-dimensional electron gas (2DEG), located directly at the etched interface in modulation doped AlGaAs/GaAs heterostructures, to probe the interface quality with far infrared (FIR) transmission spectroscopy and magnetotransport measurements. Applying a CGE process with suitable parameters we can remove more than 100 nm GaAs and generate clean surfaces with 2DEGs similar to those of nonetched reference samples. In addition we can reduce the interface state density of an IBE or CAIBE etched surface significantly applying a subsequent CGE process, which removes the ion damaged layer. However the two-dimensional electron systems at such interfaces show characteristic FIR absorption of dot-like potentials, which is a clear sign of a contaminated or disturbed surface.

Atomic layer in situ etching and MBE regrowth

We report on atomic layer precise etching in MBE with AsBr 3 as the etching species. Selective etching of SiO 2 masked GaAs (1 0 0) substrates allows in situ preparation of extremely sharp V-grooves with almost perfect planar {1 1 0} side facets. By MBE regrowth of V-grooves we can prepare buried layers and completely embedded wires. The electrical properties of etched interfaces in modulation doped GaAs/AlGaAs heterostructures is investigated by Hall-effect measurements. The heterostructures with an etching step introduced at the GaAs/AlGaAs interface where the electron gas is located demonstrate that there is no carrier density reduction. The mobility is reduced only at low temperatures to about 40,000 cm 2/V s at 77 K compared to 100,000 cm 2/V s of non-etched reference samples. This result together with the PL line width increase indicate an enhanced interface roughness for etched samples. Atomic layer precise etching does not provide the same surface quality as epitaxial growth. A problem of the in situ etching technique is the insufficient removal of dopants as well as C and O surface contamination as determined by SIMS studies.

IMPROVED SURFACE FLATNESS OF A Bi-SUBSTITUTED Dy IRON GARNET FILM FOR MAGNETO-OPTICAL RECORDING BY SPUTTER-ETCHING

Bi which precipitates near the surface of as-deposited film is the origin of surface roughness of Bi-substituted garnet film. The Bi-rich thin layer can be effectively removed from the film surface by successively sputter-etching a film after deposition for a period equal to 1/12 of sputtering time. The averaged surface roughness of garnet film crystallized from this state is improved by up to 1/4 compared that of the non-etched sample. Regular shaped bits can be recorded on the garnet bi-layered medium each layer of which was processed using this method.

Influence of CH4/H2 Reactive Ion Etching on Deep Levels in Si-Doped AlxGa1−xAs (x=0.25)

The effect of CH4/H2 reactive ion etching (RIE) on Si-doped AlxGa1−xAs (x=0.25) is studied by deep level transient spectroscopy (DLTS) and Hall measurements. After RIE exposure, the samples were annealed between 250 and 500°C in order to study the recovery kinetics of deep and shallow levels. Non-etched reference samples showed broad DLTS spectra which were deconvoluted in two different emission peaks. We assigned them to DX1 and DX2 centers. The different deep levels are characierized by different aluminium configurations, one or two aluminium atoms, surrounding the silicon donor which are responsible for the DX centers. After RIE exposure and subsequent thermal annealing, a third emission peak is observed. This emission is attributed to the DX3 center, which is characterized by three aluminium atoms neighbouring the silicon donor. The recovery activation energy is calculated based on first-order kinetics. The activation energies are found to be around 1.9 eV in all cases.Complementary Hall measurements as a function of temperature (4-300 K) were used to characterize the electron capture of deep levels in Si-doped AlGaAs exposed to CH4/H2 RIE. We observed that the samples exposed to RIE and annealed at temperatures higher than 400°C exhibit electron capture in the 120-150 K temperature range. On the other hand, samples annealed at lower temperatures, showed additional capture features between 200 and 230 K.

Detailed structural analysis of GaAs grown on patterned Si

The growth of GaAs on patterned Si substrates is essential for the integration of GaAs and Si devices. Moreover this growth may have to be done in wells to planarize the surfaces of the Si and GaAs devices for their interconnection. In this study, GaAs is grown by MBE on such patterned Si substrates, with window width down to 3 μm, and a complete structural characterization of the material is made by electron microscopy. In every case, SEM observations show a very good definition of the pattern by the contrast of the flat surface of the monocrystalline GaAs compared to the very rough surface of the polycrystalline GaAs. Cross-sectional observations by STEM or TEM on non-etched samples reveal that the quality of the monocrystalline GaAs is at least as good in terms of defect density as that of the standard GaAs on Si. On samples etched with a plasma to produce wells, grains often form against the Si sidewall. Chemical etching with lateral etching avoids contact of the growing GaAs with the Si sidewall and subsequent grain formation. The crystalline quality obtained on etched samples is not as good as on non etched samples. A way of preparing the wells to improve this crystalline quality is proposed.

Permeability of Etching Agent Constituents through Dentin

The in vitro dentin permeability of the constituents of two types of conventional phosphoric acid etching agents which had different viscosities, and their pH changes after being permeated dentin were investigated. It was recognized that, even though the etching was for a short time (one minute) followed by rinsing, the etching agents were still detected in the dentinal tubules which had permeated through the dentin. The results of a qualitative and quantitative analysis of the constituents which had permeated through the dentin showed that, after one minute of etching, the concentrations of calcium, magnesium, zinc and phosphorus increased as compared with the non-etched control samples.

Secondary Electron Emission from Insulators by Ion Bombardment

The characteristics of secondary electron emission from a glass surface by Ar/sup +/ ions were investigated experimertally. The yields from glass and quartz microsheets were larger than those from stainless steel. It was first observed that field-enhanced emission and seif-sustained emission were caused by the surface charge. Because increasing of the yield has technological demand, HF etching and sandblast were attempted as one of the means of increasing yield. Then the yields of etched and sandblasted samples were about two times larger than that of nonetched or nonsandblasted sample. Field-enhanced emission and seif-sustained emission cannot be irterpreted only with kinetic ejection or potertial emission theory. A simple model is proposed in which the work function decreases by the band bending with the accumulation of positive charge near the surface. (auth)