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Photoluminescence properties of undoped wurtzite GaN epilayers grown on sapphire substrates with different buffer layer treatment conditions in metalorganic chemical vapor deposition (MOCVD) growth have been studied as a function of temperature. At low temperatures, very well resolved spectral features associated with the GaN band structure were observed. From the photoluminescence (PL) data for free excitons, an accurate value of the A exciton binding energy was found. The localization energies of the excitons bound to neutral acceptor are found to agree with Haynes’ rule with the proportionality factor close to 0.1. The longitudinal optical (LO) phonon assisted photoluminescence associated with both the bound and free excitons has been observed. The characteristics of free excitons and their LO phonon replica have been studied in detail with the temperature variation and related to the point defects. The behavior of the peak energy and the full width at half maximum of the exciton band as a function of ...

We investigated the effects of grain boundary diffusion (GBD) at elevated temperatures on Auger electron spectroscopy sputter depth profiles of Co-Ag bilayers by using the modified surface accumulation method which is proposed for the first time to our knowledge. The results agree well with our previous ones done by the surface accumulation method. The GBD parameters for Ag in Co along the grain boundaries were found to be as follows: the activation energy and pre-exponential factor for GBD are 0.425±0.025 eV and ∼1×10−7cm2, respectively. These indicate that the modified method is an effective alternative in studying the effects of GBD, and may be applied to complicated systems for which the previous method does not hold.

Photoluminescence of AlxGa1−xAs/AlyGa1−yAs heterostructures has been performed in presence of high magnetic fields. Under the magnetic field the broad carbon related peak becomes resolved into several narrow peaks. Experimental results and a theoretical analysis based on the single band model suggest that these peaks are due to the recombination of free electrons with holes bound to neutral carbon atoms distributed in the several atomic layers in the well side of the interfaces of the heterostructures. Magnetic fields reduce the effects of interfacial roughness and make it possible to resolve the carbon related peak into several peaks.

The presented theoretical analysis of random telegraph signal (RTS) and 1/f noise data provides consistent interpretation of the measurement results allowing trap characteristics to be extracted and the atomic structure of oxide traps to be identified. We emphasize the critical role of the lattice structural relaxation associated with charge trapping/detrapping, which represents one of the major factors controlling electron capture/emission times.

A method of evaluating interface states for coupled superlattices has been developed. Explicit solutions for these states have been obtained within the framework of the Kronig–Penney model for the case of two semiconductor superlattices coupled by a tunnel barrier. The interface states are shown to be largely dependent on the coupling strength if the constituent superlattices have identical structure parameters, while insensitive to the tunnel barrier thickness if their structure parameters differ. In the latter case, electrons behave as if the system were composed of two decoupled semi‐infinite superlattices.

A two-band model involving the heavy- and light-hole bands was adopted to analyze the temperature-dependent Hall effect measured on Mg-doped p-type GaN epilayers. At 300 K, the hole concentration was determined to be nearly twice the Hall concentration estimated from the measured Hall coefficient, meanwhile the Hall mobility of heavy hole turned out to be only half of the measured one. It is shown that the scattering by space charge and acoustic deformation potential is anomalously enhanced in Mg-doped GaN, and that the light hole affects conspicuously the observed transport parameters.

The temperature dependence of the Hall plateaus at microwave frequencies (∼ 30 GHz) and their disappearance at 160 GHz suggest a unified phenomenological model of the integral quantum Hall effect at zero and high frequencies. In laterally structured samples depolarization effects as well as the Hall plateaus are observed in the Hall conductivity.

We studied the influence of the sample structure on the photoluminescence intensity in terms of carrier diffusion and transfer phenomena in InxGa1−xAs/GaAs multi‐quantum‐well structures. Carrier injection from the barrier, cap, or buffer layers dominates the generation in the well when the well thicknesses are small and the excitation energy is larger than the band gap of the barrier layer. The carrier transport between wells also plays an important role in the photoluminescence particularly in the shallow wells and can be accounted for by phenomenologically introduced hopping time between wells. The hopping time varies from a few tens of picoseconds to a few hundreds of nanoseconds depending on the well composition and width. The strain relaxation of these strained layer quantum‐well structures also leads to the intensity variation as well as the change in the peak position.

The width of the QHE plateaus ΔB is investigated in GaAs/-AlGaAs using DC and microwave (MW) techniques. ΔB is narrower at MW frequencies than at DC and saturates below 2.4K. No fractional plateaus were observed at MW frequencies where they appeared at DC.

In the past, the electronic transport in quantum wires has mostly been studied using d.c. or low-frequency measuring techniques (Washburn and Webb, 1986; Heinrich et al., 1988). The use of high-frequency electric fields, however, is of particular interest if the product ωτϕ ∼ 1, where 1/τϕ is the phase-breaking rate which determines the phase coherence of the wave function (Altshuler et al., 1981; Khmelnitskii, 1988). In quantum wires, high-frequency effects were studied using narrow silicon MOSFETs and GaAs epitaxial layers (Bykov et al., 1989) and metal films (Liu et al., 1988; Lindelof et al., 1987). In the first experiments on heterostructure quantum wires the influence of a microwave field on the weak electron localization (WEL) and the universal conductance fluctuations (UCF) was studied (Kuchar et al., 1989, 1990).

Dry etching of undoped, n- and p-type GaN films was carried out in Cl2-based inductively coupled plasmas (ICPs) using different rf excitation frequencies of 100 kHz and 13.56 MHz, in which the rf chuck power source operates. The etch rates with lower frequency of 100 kHz are somewhat greater than those with a higher frequency of 13.56 MHz due to higher ion bombarding energy with lower frequency. The highest etch rates with the 100 kHz frequency were obtained at moderately high ICP power of 700 W: ∼9300 A/min of n-GaN, ∼5300 A/min of p-GaN, and ∼7100 A/min of undoped GaN. The 13.56 MHz frequency of rf chuck power source produced maximum etch rates of ∼7900 A/min of n-GaN, ∼5800 A/min of p-GaN, and 6100 A/min of undoped GaN at 20 mTorr, 700 W ICP, and 150 W rf power. The surface roughness was relatively independent of the chuck power up to 150 W in 13.56 MHz and showed fairly smooth morphology (rms 1.1–1.3 nm), while etching at higher rf power (>200 W) produced rougher surface.

Quantized values of the Hall conductivity exist in the two-dimensional electron and hole gases of GaAs/AlGaAs heterostructures at frequencies as high as 30 GHz. This is a proof that a low-frequency breakdown does not occur and that the intrinsic plateau width of σxy is non-zero. The microwave measurement also allows to separate effects connected with the boundary of the 2D carrier gas; no significant contribution was observed.

Microwave experiments with crossed waveguides allow measurement of the Hall conductivity Oxy of a two-dimensional electron gas. The quantized behavior of σxy is equivalent to that of πxy measured with DC. In this contribution we report on new results regarding phenomena not accessible to DC techniques.

The observation of integer quantum Hall, effect (IQHE) behavior of σxv at microwave frequencies (~30 GHz) is reported. This clears the discrepancy concerning the socalled low-frequency breakdown of the IQHE, It was observed in a.c, experiments in the MHz range whereas 100 ns pulse experiment did not show a deviation from the d.c. behavior. In this investigation, for the observation of σxy at microwave frequencies, a crossed waveguide arrangement is used. The common part of the cross-sections of the two waveguides is much smaller than the sample area. This is expected to minimize edge effects. From the observed quantized behavior of σxy we conclude that the mechanism of an effective delocalization at low frequencies has to be ruled out.

Quantum Hall effect (QHE) behavior of the Hall conductivity σxy is observed in multiple (MHS) and single heterostructures (SHS) of GaAs/AlxGa1-x As at microwave frequencies (Ka band). Integer plateaus with even filling factor i are observed in σxy of the MHS as well as of the SHS with 2D electron and hole gas layers. In the case of the 2DEG in a SHS also the i=3 plateau is resolved. With this SHS it is clearly shown that the σxy plateaus occur equidistant on a 1/B scale and that the σxy values on the plateaus are multiples of a fundamental value.