Molecular beam homoepitaxy of N-polar AlN: Enabling role of aluminum-assisted surface cleaning.

N-polar aluminum nitride (AlN) is an important building block for next-generation high-power radio frequency electronics. We report successful homoepitaxial growth of N-polar AlN by molecular beam epitaxy (MBE) on large-area, cost-effective N-polar AlN templates. Direct growth without any in situ surface cleaning leads to films with inverted Al polarity. It is found that Al-assisted cleaning before growth enables the epitaxial film to maintain N-polarity. The grown N-polar AlN epilayer with its smooth, pit-free surface duplicates the structural quality of the substrate, as evidenced by a clean and smooth growth interface with no noticeable extended defects generation. Near band-edge photoluminescence peaks are observed at room temperature on samples with MBE-grown layers but not on the bare AlN templates, implying the suppression of nonradiative recombination centers in the epitaxial N-polar AlN.

Crystal orientation dictated epitaxy of ultrawide-bandgap 5.4- to 8.6-eV α-(AlGa)2O3 on m-plane sapphire.

Ultrawide-bandgap semiconductors are ushering in the next generation of high-power electronics. The correct crystal orientation can make or break successful epitaxy of such semiconductors. Here, it is found that single-crystalline layers of α-(AlGa)2O3 alloys spanning bandgaps of 5.4 to 8.6 eV can be grown by molecular beam epitaxy. The key step is found to be the use of m-plane sapphire crystal. The phase transition of the epitaxial layers from the α- to the narrower bandgap ß-phase is catalyzed by the c-plane of the crystal. Because the c-plane is orthogonal to the growth front of the m-plane surface of the crystal, the narrower bandgap pathways are eliminated, revealing a route to much wider bandgap materials with structural purity. The resulting energy bandgaps of the epitaxial layers span a broad range, heralding the successful epitaxial stabilization of the largest bandgap materials family to date.

Inactivation of Listeria and E. coli by Deep-UV LED: effect of substrate conditions on inactivation kinetics.

Irradiation with deep-ultraviolet light-emitting diodes (DUV LEDs) is emerging as a low energy, chemical-free approach to mitigate microbial contamination, but the effect of surface conditions on treatment effectiveness is not well understood. Here, inactivation of L. innocua and E. coli ATCC25922, as examples of Gram-positive and Gram-negative bacteria, respectively, by DUV LED of 280 nm wavelength was studied. Surface scenarios commonly encountered in environmental, clinical or food processing environments were used: nutrient rich surfaces, thin liquid films (TLF), and stainless steel surfaces (SS). DUV LED exposure achieved 5-log reduction for both strains within 10 min in most scenarios, except for TLF thicker than 0.6 mm. Inactivation kinetics in TLF and on dry SS followed the Weibull model (0.96 ≤ R2 ≤ 0.99), but the model overestimated inactivation by small-dose DUV on wet SS. Confocal microscopy revealed in situ that bacteria formed a dense outer layer at the liquid-air interface of the liquid droplet, protecting the cells inside the droplet from the bactericidal DUV. This resulted in lower than anticipated inactivation on wet SS at small DUV doses, and deviation from the Weibull model. These findings can be used to design effective DUV LED disinfection strategies for various surface conditions and applications.

Layered transition metal dichalcogenides: promising near-lattice-matched substrates for GaN growth.

Most III-nitride semiconductors are grown on non-lattice-matched substrates like sapphire or silicon due to the extreme difficulty of obtaining a native GaN substrate. We show that several layered transition-metal dichalcogenides are closely lattice-matched to GaN and report the growth of GaN on a range of such layered materials. We report detailed studies of the growth of GaN on mechanically-exfoliated flakes WS2 and MoS2 by metalorganic vapour phase epitaxy. Structural and optical characterization show that strain-free, single-crystal islands of GaN are obtained on the underlying chalcogenide flakes. We obtain strong near-band-edge emission from these layers, and analyse their temperature-dependent photoluminescence properties. We also report a proof-of-concept demonstration of large-area growth of GaN on CVD MoS2. Our results show that the transition-metal dichalcogenides can serve as novel near-lattice-matched substrates for nitride growth.

Esaki Diodes in van der Waals Heterojunctions with Broken-Gap Energy Band Alignment.

van der Waals (vdW) heterojunctions composed of two-dimensional (2D) layered materials are emerging as a solid-state materials family that exhibits novel physics phenomena that can power a range of electronic and photonic applications. Here, we present the first demonstration of an important building block in vdW solids: room temperature Esaki tunnel diodes. The Esaki diodes were realized in vdW heterostructures made of black phosphorus (BP) and tin diselenide (SnSe2), two layered semiconductors that possess a broken-gap energy band offset. The presence of a thin insulating barrier between BP and SnSe2 enabled the observation of a prominent negative differential resistance (NDR) region in the forward-bias current-voltage characteristics, with a peak to valley ratio of 1.8 at 300 K and 2.8 at 80 K. A weak temperature dependence of the NDR indicates electron tunneling being the dominant transport mechanism, and a theoretical model shows excellent agreement with the experimental results. Furthermore, the broken-gap band alignment is confirmed by the junction photoresponse, and the phosphorus double planes in a single layer of BP are resolved in transmission electron microscopy (TEM) for the first time. Our results represent a significant advance in the fundamental understanding of vdW heterojunctions and broaden the potential applications of 2D layered materials.

Terahertz imaging employing graphene modulator arrays.

In this paper we propose and experimentally demonstrate arrays of graphene electro-absorption modulators as electrically reconfigurable patterns for terahertz cameras. The active element of these modulators consists of only single-atom-thick graphene, achieving a modulation of the THz wave reflectance > 50% with a potential modulation depth approaching 100%. Although the prototype presented here only contains 4x4 pixels, it reveals the possibility of developing reliable low-cost video-rate THz imaging systems employing single detector.

Extraordinary control of terahertz beam reflectance in graphene electro-absorption modulators.

We demonstrate a graphene-based electro-absorption modulator achieving extraordinary control of terahertz reflectance. By concentrating the electric field intensity in an active layer of graphene, an extraordinary modulation depth of 64% is achieved while simultaneously exhibiting low insertion loss (∼2 dB), which is remarkable since the active region of the device is atomically thin. This modulator performance, among the best reported to date, indicates the enormous potential of graphene for terahertz reconfigurable optoelectronic devices.

Polarization-induced hole doping in wide-band-gap uniaxial semiconductor heterostructures.

Impurity-based p-type doping in wide-band-gap semiconductors is inefficient at room temperature for applications such as lasers because the positive-charge carriers (holes) have a large thermal activation energy. We demonstrate high-efficiency p-type doping by ionizing acceptor dopants using the built-in electronic polarization in bulk uniaxial semiconductor crystals. Because the mobile hole gases are field-ionized, they are robust to thermal freezeout effects and lead to major improvements in p-type electrical conductivity. The new doping technique results in improved optical emission efficiency in prototype ultraviolet light-emitting-diode structures. Polarization-induced doping provides an attractive solution to both p- and n-type doping problems in wide-band-gap semiconductors and offers an unconventional path for the development of solid-state deep-ultraviolet optoelectronic devices and wide-band-gap bipolar electronic devices of the future.

Wavelength Sensitivity of Single Nanowire Excitation Polarization Anisotropies Explained through a Generalized Treatment of Their Linear Absorption.

We investigate the excitation polarization anisotropy of individual semiconductor nanowires (NWs) by monitoring their band edge emission above 680 nm in order to clarify the origin of their strong polarization response. Samples studied include both CdSe and CdSe/CdS core/shell nanowires grown using solution chemistry as well as analogous wires made via chemical-vapor-deposition (CVD). In the limit of optically thick wires, with radii above ∼25 nm, we find NW optical responses consistent with the interaction between strong dielectric contrast influences and the onset of bulk-like behavior. Namely, a sizable wavelength dependence of the excitation polarization anisotropy (ρ(exc)) exists when NW diameters become comparable to the wavelength of light inside the wire. As a consequence, pronounced ρ(exc) rolloffs occur at short wavelengths. By contrast, thinner wires do not exhibit such wavelength dependencies, in agreement with earlier studies. We quantitatively explain observed wavelength sensitivities by modeling the NW as an absorbing dielectric cylinder under plane wave excitation. A comparison of predicted ρ(exc)-values to experimental numbers shows good agreement and confirms the existence of wavelength-dependent ρ(exc)-values in optically thick wires. Additional results of the model include generalized expressions for NW linear absorption cross-sections under parallel, perpendicular, and circularly polarized excitation. This study therefore adds to a growing body of knowledge about NW polarization anisotropies, specifically, their response in a size regime where dielectric contrast effects compete with the onset of bulk-like behavior.

Photocurrent polarization anisotropy of randomly oriented nanowire networks.

While the polarization sensitivity of single or aligned NW ensembles is well-known, this article reports on the existence of residual photocurrent polarization sensitivities in random NW networks. In these studies, CdSe and CdTe NWs were deposited onto glass substrates and contacted with Au electrodes separated by 30-110 microm gaps. SEM and AFM images of resulting devices show isotropically distributed NWs between the electrodes. Complementary high resolution TEM micrographs reveal component NWs to be highly crystalline with diameters between 10 and 20 nm and with lengths ranging from 1 to 10 microm. When illuminated with visible (linearly polarized) light, such random NW networks exhibit significant photocurrent anisotropies rho = 0.25 (sigma = 0.04) [rho = 0.22 (sigma = 0.04)] for CdSe (CdTe) NWs. Corresponding bandwidth measurements yield device polarization sensitivities up to 100 Hz. Additional studies have investigated the effects of varying the electrode potential, gap width, and spatial excitation profile. These experiments suggest electrode orientation as the determining factor behind the polarization sensitivity of NW devices. A simple geometric model has been developed to qualitatively explain the phenomenon. The main conclusion from these studies, however, is that polarization sensitive devices can be made from random NW networks without the need to align component wires.

Spatial and intensity modulation of nanowire emission induced by mobile charges.

Single-molecule optical experiments carried out in conjunction with externally applied electric fields show deliberate spatial and intensity control over CdSe nanowire (NW) emission. In particular, by applying external fields to electrically isolated (single) NWs, their emission can be localized in areas of the wire closest to the positive electrode. In a few cases, the resulting emission intensity increases over the corresponding zero-field value by nearly an order of magnitude. More often than not, factors of 2-3 are seen. Reversing the field polarity causes the emission to localize in opposite regions of the wire. Emission from individual NWs can therefore be modulated. Complementary ac electric field measurements show that the effect persists up to 500 kHz. To explain the phenomenon, the effective passivation of surface trap states by mobile carriers is speculated. This, in turn, causes local changes in the NW emission quantum yield (QY). To verify the presence of such mobile charges, both ensemble and single NW bundle electrophoresis experiments are conducted. By investigating subsequent NW rotational and translational dynamics, an estimate for the number of mobile carriers is determined. A lower limit (best case) linear charge density of approximately 0.45-1.2 mobile electrons per micrometer of the wire is obtained. Apart from self-consistently explaining the field-induced NW emission modulation, the resulting data and subsequent analysis also suggests that the same mobile carriers may be the root cause of NW emission intermittency. Furthermore, given the ubiquity of stray charges, the resulting hypothesis may have additional applicability toward explaining blinking in other systems, a problem of current interest especially within the context of colloidal QDs.

Polarization-sensitive nanowire photodetectors based on solution-synthesized CdSe quantum-wire solids.

Polarization-sensitive photodetectors are demonstrated using solution-synthesized CdSe nanowire (NW) solids. Photocurrent action spectra taken with a tunable white light source match the solution linear absorption spectra of the NWs, showing that the NW network is responsible for the device photoconductivity. Temperature-dependent transport measurements reveal that carriers responsible for the dark current through the nanowire solids are thermally excited across CdSe band gap. The NWs are aligned using dielectrophoresis between prepatterned electrodes using conventional optical photolithography. The photocurrent through the NW solid is found to be polarization-sensitive, consistent with complementary absorption (emission) measurements of both single wires and their ensembles. The range of solution-processed semiconducting NW materials, their facile synthesis, ease of device fabrication, and compatibility with a variety of substrates make them attractive for potential nanoscale polarization-sensitive photodetectors.

Experimental determination of the absorption cross-section and molar extinction coefficient of CdSe and CdTe nanowires.

Absorption cross-sections and corresponding molar extinction coefficients of solution-based CdSe and CdTe nanowires (NWs) are determined. Chemically grown semiconductor NWs are made via a recently developed solution-liquid-solid (SLS) synthesis, employing low melting Au/Bi bimetallic nanoparticle "catalysts" to induce one-dimensional (1D) growth. Resulting wires are highly crystalline and have diameters between 5 and 12 nm as well as lengths exceeding 10 microm. Narrow diameters, below twice the corresponding bulk exciton Bohr radius of each material, place CdSe and CdTe NWs within their respective intermediate to weak confinement regimes. Supporting this are solution linear absorption spectra of NW ensembles showing blue shifts relative to the bulk band gap as well as structure at higher energies. In the case of CdSe, the wires exhibit band edge emission as well as strong absorption/emission polarization anisotropies at the ensemble and single-wire levels. Analogous photocurrent polarization anisotropies have been measured in recently developed CdSe NW photodetectors. To further support fundamental NW optical/electrical studies as well as to promote their use in device applications, experimental absorption cross-sections are determined using correlated transmission electron microscopy, UV/visible extinction spectroscopy, and inductively coupled plasma atomic emission spectroscopy. Measured CdSe NW cross-sections for 1 microm long wires (diameters, 6-42 nm) range from 6.93 x 10(-13) to 3.91 x 10(-11) cm2 at the band edge (692-715 nm, 1.73-1.79 eV) and between 3.38 x 10(-12) and 5.50 x 10(-11) cm2 at 488 nm (2.54 eV). Similar values are obtained for 1 microm long CdTe NWs (diameters, 7.5-11.5 nm) ranging from 4.32 x 10(-13) to 5.10 x 10(-12) cm2 at the band edge (689-752 nm, 1.65-1.80 eV) and between 1.80 x 10(-12) and 1.99 x 10(-11) cm2 at 2.54 eV. These numbers compare well with previous theoretical estimates of CdSe/CdTe NW cross-sections far to the blue of the band edge, having order of magnitude values of 1.0 x 10(-11) cm2 at 488 nm. In all cases, experimental NW absorption cross-sections are 4-5 orders of magnitude larger than those for corresponding colloidal CdSe and CdTe quantum dots. Even when volume differences are accounted for, band edge NW cross-sections are larger by up to a factor of 8. When considered along with their intrinsic polarization sensitivity, obtained NW cross-sections illustrate fundamental and potentially exploitable differences between 0D and 1D materials.