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Intersubband transitions in Si-doped molecular beam epitaxy grown GaN/AlGaN multiple quantum wells on c-plane sapphire were investigated using the Fourier-transform infrared optical absorption technique. Several GaN quantum well samples were grown with either AlGaN bulk or GaN/AlGaN short period superlattice barriers. The measurements were made in a waveguide configuration utilizing a facet polished at 45° to the c plane. The integrated area of the intersubband transitions in several waveguides cut from different location of the wafer was measured, from which we estimated the two-dimensional electron gas density (σ). The measured values of σ are about two orders of magnitude larger than the Si doping level of ∼8×1017 cm−3, which is consistent with the polarization effects, particularly considering the large number of GaN/AlGaN interfaces. The internal quantum efficiency of the intersubband transitions was estimated to be on the order of 40% for samples with superlattice barriers.

The time invariant behaviors of topological insulators are expected to be changed with magnetic doping, which motivate the present study. Here, we show that for Bi2� xCrxSe3 (0.01 � x � 0.3) thin films grown on Si, the non-trivial topological surface state is weakened by the Cr dopants. The band gap of surface is opened and monotonically increased with Cr concentration up to � 100meV at 10K. Meanwhile, the semiconducting behavior is well-maintained in the bulk owing to the reduction of background doping by means of a modified growth strategy and an in situ passivation method. Besides, we also observe the existence of unconventional ferromagnetic ordering below 35K, for which the Curie-Weiss Law and conventional/modified Arrott equations do not apply. These observations may further help us investigate extraordinary magneto-electric effect in topological insulators, and the result will also pave the way for realizing the quantized anomalous Hall effect. V C 2012 American Institute of Physics .[ http://dx.doi.org/10.1063/1.4754452]

As state of the art flash memory technologies scale down to sub 30 nm node, conventional floating gate flash memory approaches its physical scaling limit mainly because of the high gate coupling ratio (GCR) requirement to secure proper memory window. Here, we report a novel flash memory device called Cr metal thin film memory (MTFM) that can circumvent the GCR issue and extend flash memory scalability by employing Cr thin film as a storage layer. Cr metal thin film memory devices with simple and low temperature processes produced a wide memory window of 10 V at the ±18 V voltage sweep with GCR of only 0.3. Such a large window can be adopted for multi-level cell operations, which can further increase the memory density. Also, retention measurement shows more than 10 years retention time due to higher energy barrier between Cr metal and tunnel oxide than conventional poly silicon and tunnel oxide. Cross section transmission electron microscope (TEM) images showed the structure and accurate dimensions of the Cr MTFM device with continuous Cr film and sharp interfaces. As for material characterizations, an amorphous like Cr phase was observed through TEM and x-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) confirmed the Cr-Cr bond and Cr-O bond near the Cr surface after evaporation and rapid thermal annealing. This metal thin film memory may open a new route to achieve the terabit level flash memory.

In this paper, we report the epitaxial growth of Bi2Se3 thin films on Si (111) substrate, using molecular beam epitaxy (MBE). We show that the as-grown samples have good crystalline quality, and their surfaces exhibit terracelike quintuple layers. Angel-resolved photoemission experiments demonstrate single-Dirac-conelike surface states. These results combined with the temperature- and thickness-dependent magneto-transport measurements, suggest the presence of a shallow impurity band. Below a critical temperature of ∼100K, the surface states of a 7 nm thick film contribute up to 50% of the total conduction.

Sb-doped p-type ZnO film was grown on Si (100) substrate by molecular-beam epitaxy. Al/Ti metal was evaporated on the ZnO film to form contacts. As-deposited contacts were Schottky with a barrier height of 0.8 eV. Ohmic conduction was achieved after thermal annealing. The different combinations of Ohmic and Schottky contacts on Sb-doped ZnO layer led to metal-semiconductor-metal (MSM), Schottky, and photoconductive devices. Ohmic contacts on Sb-doped p-type ZnO and backside of n-type Si substrate formed a heterojunction diode. MSM, Schottky, and photoconductor devices exhibited typical electrical characteristics, however, inverted rectification was observed for heterojunction diodes. All devices exhibited ultraviolet (UV) photoresponse. Secondary ion mass spectroscopy measurements were performed on the Ohmic and Schottky contacts on Sb-doped ZnO film to trace the metal profiles before and after annealing. Mechanisms of the formation of Schottky and Ohmic contacts to Sb-doped p-type ZnO and their device op...

Minority electron diffusion length was measured in p-type, Sb-doped ZnO as a function of temperature using the electron beam induced current technique. A thermally induced increase of electron diffusion length was determined to have an activation energy of 184±10meV. Irradiation with a low energy (5kV) electron beam also resulted in an increase of diffusion length with a similar activation energy (219±8meV). Both phenomena are suggested to involve a SbZn–2VZn acceptor complex. Saturation and relaxation dynamics of minority carrier diffusion length are explored. Details of a possible mechanism for diffusion length increase are presented.