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Polarization-induced carriers play an important role in achieving high electrical conductivity in ultrawide bandgap semiconductor AlGaN, which is essential for various applications ranging from radio frequency and power electronics to deep UV photonics. Despite significant scientific and technological interest, studies on polarization-induced carriers in N-polar AlGaN are rare. We report the observation and properties of polarization-induced two-dimensional electron gases (2DEGs) in N-polar AlGaN/AlN heterostructures on single-crystal AlN substrates by systematically varying the Al content in the 8 nm top layers from x = 0 to x = 0.6, spanning energy bandgaps from 3.56 to 4.77 eV. The 2DEG density drops monotonically with increasing Al content, from 3.8 × 1013/cm2 in the GaN channel, down to no measurable conductivity for x = 0.6. Alloy scattering limits the 2DEG mobility to below 50 cm2/V s for x = 0.49. These results provide valuable insights for designing N-polar AlGaN channel high electron mobility transistors on AlN for extreme electronics at high voltages and high temperatures, and for UV photonic devices.

Resonant tunneling transport in polar heterostructures is intimately connected to the polarization fields emerging from the geometric Berry-phase. In these structures, quantum confinement results not only in a discrete electronic spectrum, but also in built-in polarization charges exhibiting a broken inversion symmetry along the transport direction. Thus, electrons undergo highly asymmetric quantum interference effects with respect to the direction of current flow. By employing doping to counter the broken symmetry, we deterministically control the resonant transmission through GaN/AlN resonant tunneling diodes and experimentally demonstrate the recovery of symmetric resonant tunneling injection across the noncentrosymmetric double-barrier potential., 6 pages, 4 figures

The phenomenon of resonant tunneling transport through polar double-barrier heterostructures is systematically investigated using a combined experimental and theoretical approach. On the experimental side, GaN/AlN RTDs are grown by MBE. In-situ electron diffraction is employed to monitor the number of monolayers incorporated into each tunneling barrier. Using this precise epitaxial control at the monolayer level, we demonstrate exponential modulation of the resonant tunneling current as a function of barrier thickness. Both the peak voltage and characteristic threshold bias exhibit a dependence on barrier thickness as a result of the intense electric fields present in the polar heterostructures. To get further insight into the asymmetric tunneling injection, we present an analytical theory for tunneling transport across polar heterostructures. A general expression for the resonant tunneling current with contributions from coherent and sequential tunneling processes is introduced. After applying this theory to the case of GaN/AlN RTDs, their experimental current-voltage characteristics are reproduced over both bias polarities, with tunneling currents spanning several orders of magnitude. This agreement allows us to elucidate the role played by the internal polarization fields on the magnitude of the tunneling current and broadening of the resonant line shape. Under reverse bias, we identify new tunneling features originating from highly attenuated resonant tunneling phenomena, which are completely captured by our model. Our analytical model, provides a simple expression which reveals the connection between the polar RTD design parameters and its current-voltage characteristics. This new theory paves the way for the design of polar resonant tunneling devices exhibiting efficient resonant current injection and enhanced tunneling dynamics, as required in various practical applications., 13 pages, 3 figures

The recent demonstration of resonant tunneling transport in nitride semiconductors has led to an invigorated effort to harness this quantum transport regime for practical applications. In polar semiconductors, however, the interplay between fixed polarization charges and mobile free carriers leads to asymmetric transport characteristics. Here, we investigate the possibility of using degenerately doped contact layers to screen the built-in polarization fields and recover symmetric resonant injection. Thanks to a high doping density, negative differential conductance is observed under both bias polarities of GaN/AlN resonant tunneling diodes (RTDs). Moreover, our analytical model reveals a lower bound for the minimum resonant-tunneling voltage achieved via uniform doping, owing to the dopant solubility limit. Charge storage dynamics is also studied by impedance measurements, showing that at close-to-equilibrium conditions, polar RTDs behave effectively as parallel-plate capacitors. These mechanisms are completely reproduced by our analytical model, providing a theoretical framework useful in the design and analysis of polar resonant-tunneling devices., Comment: 9 pages, 4 figures

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.

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.

The physics of the bottom tunnel junction (BTJ) and its improvement over standard p-up geometry in InGaN blue LEDs is quantified through pulsed power measurements. It is found that the peak external quantum efficiency (EQE) and wall-plug efficiency (WPE) for a p-down BTJ LED is about threefold that of its counterpart, the p-up top tunnel junction (TTJ) LED. This is contributed to increased radiative recombination and reduced electron overflow. Further, the peaks occur at lower current densities for the BTJ device, suggesting earlier saturation of Shockley-Read-Hall traps. In the droop regime, where electron overflow, device heating, and 3-particle interactions are significant, the performance of the BTJ is found to be consistently better than that of the TTJ, converging at large current densities where the polarization fields are screened.

Crystalline oxides spanning very broad 5.4- to 8.6-eV bandgaps are successfully grown on sapphire by MBE., 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 β-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.

Polarization-induced (Pi) distributed or bulk doping in GaN, with a zero dopant ionization energy, can reduce temperature or frequency dispersions in impurity-doped p–n junctions caused by the deep-acceptor-nature of Mg, thus offering GaN power devices promising prospects. Before comprehensively assessing the benefits of Pi-doping, ideal junction behaviors and high-voltage capabilities should be confirmed. In this work, we demonstrate near-ideal forward and reverse I–V characteristics in Pi-doped GaN power p–n diodes, which incorporates linearly graded, coherently strained AlGaN layers. Hall measurements show a net increase in the hole concentration of 8.9 × 1016 cm−3 in the p-layer as a result of the polarization charge. In the Pi-doped n-layer, a record-low electron concentration of 2.5 × 1016 cm−3 is realized due to the gradual grading of Al0-0.72GaN over 1 μm. The Pi-doped p–n diodes have an ideality factor as low as 1.1 and a 0.10 V higher turn-on voltage than the impurity-doped p–n diodes due to the increase in the bandgap at the junction edge. A differential specific on-resistance of 0.1 mΩ cm2 is extracted from the Pi-doped p–n diodes, similar with the impurity-doped counterpart. The Pi-doped diodes show an avalanche breakdown voltage of ∼1.25 kV, indicating a high reverse blocking capability even without an ideal edge-termination. This work confirms that distributed Pi-doping can be incorporated in high-voltage GaN power devices to increase hole concentrations while maintaining excellent junction properties.

Harnessing resonant tunneling transport in III-nitride semiconductors to boost the operating frequencies of electronic and photonic devices, requires a thorough understanding of the mechanisms that limit coherent tunneling injection. Towards this goal, we present a concerted experimental and theoretical study that elucidates the impact of the collector doping setback on the quantum transport characteristics of GaN/AlN resonant tunneling diodes (RTDs). Employing our analytical model for polar RTDs, we quantify the width of the resonant-tunneling line shape, demonstrating that the setback helps preserve coherent injection. This design results in consistently higher peak-to-valley-current ratios (PVCRs), obtaining a maximum PVCR = 2.01 at cryogenic temperatures.

Here, we have explored the thermal stability of α-(Al,Ga)2O3 grown by the molecular-beam epitaxy on m-plane sapphire under high-temperature annealing conditions for various Al compositions (i.e., 0%, 46%, and 100%). Though uncapped α-Ga2O3 undergoes a structural phase transition to the thermodynamically stable β-phase at high temperatures, we find that an aluminum oxide cap grown by atomic layer deposition preserves the α-phase. Unlike uncapped α-Ga2O3, uncapped α-(Al,Ga)2O3 at 46% and 100% Al content remain stable at high temperatures. We quantify the evolution of the structural properties of α-Ga2O3, α-(Al,Ga)2O3, and α-Al2O3 and the energy bandgap of α-Ga2O3 up to 900 °C. Throughout the anneals, the α-Ga2O3 capped with aluminum oxide retains its high crystal quality, with no substantial roughening.

β-Ga2O3 is a promising ultra-wide bandgap semiconductor whose properties can be further enhanced by alloying with Al. Here, using atomic-resolution scanning transmission electron microscopy, we find the thermodynamically unstable γ-phase is a ubiquitous structural defect in both β-(AlxGa1−x)2O3 films and doped β-Ga2O3 films grown by molecular beam epitaxy. For undoped β-(AlxGa1−x)2O3 films, we observe γ-phase inclusions between nucleating islands of the β-phase at lower growth temperatures (∼500–600 °C). In doped β-Ga2O3, a thin layer of the γ-phase is observed on the surfaces of films grown with a wide range of n-type dopants and dopant concentrations. The thickness of the γ-phase layer was most strongly correlated with the growth temperature, peaking at about 600 °C. Ga interstitials are observed in the β-phase, especially near the interface with the γ-phase. By imaging the same region of the surface of a Sn-doped β-(AlxGa1−x)2O3 after ex situ heating up to 400 °C, a γ-phase region is observed to grow above the initial surface, accompanied by a decrease in Ga interstitials in the β-phase. This suggests that the diffusion of Ga interstitials toward the surface is likely the mechanism for growth of the surface γ-phase and more generally that the more-open γ-phase may offer diffusion pathways to be a kinetically favored and early forming phase in the growth of Ga2O3. However, more modeling and simulation of the γ-phase and the interstitials are needed to understand the energetics and kinetics, the impact on electronic properties, and how to control them.

Deep ultraviolet light-emitting diodes (200–280 nm) have many potential applications in diagnostics, therapeutics, security, and tanning. But, the state-of-the-art LEDs suffer from low external quantum efficiency (< 20%). The external quantum efficiency is composed of internal quantum efficiency, injection efficiency, and the light extraction efficiency. These components are limited due to fundamental material and physics-based challenges. In this chapter, a set of novel heterostructure designs are presented to improve the individual efficiency components that compose the external quantum efficiency. Theoretical analysis followed by crystal growth and experimental data are presented showing enhancement of each efficiency components. Inclusion of such new design techniques will enhance the external quantum efficiency of deep-UV light emitters.

Single-crystal Aluminum Nitride (AlN) crystals enable the epitaxial growth of ultrawide bandgap Al(Ga)N alloys with drastically lower extended defect densities. Here, we report the plasma-MBE growth conditions for high Al-composition AlGaN alloys on single-crystal AlN substrates. An AlGaN growth guideline map is developed, leading to pseudomorphic AlxGa1−xN epitaxial layers with x ∼0.6–1.0 Al contents at a growth rate of ∼0.3 μm/h. These epitaxial layers exhibit atomic steps, indicating step flow epitaxial growth, and room-temperature band edge emission from ∼4.5 to 5.9 eV. Growth conditions are identified in which the background impurity concentrations of O, C, Si, and H in the MBE layers are found to be very near or below detection limits. An interesting Si segregation and gettering behavior is observed at the epitaxial AlGaN/AlN heterojunction with significant implications for the formation and transport of 2D electron or hole gases. Well-controlled intentional Si doping ranging from ∼2 × 1017 to 3 × 1019 atoms/cm3 is obtained, with sharp dopant density transition profiles. In Si-doped Al0.6Ga0.4N epilayers, a room-temperature free electron concentration of ∼3 × 1019/cm3, an electron mobility of ∼27 cm2/V s, and an n-type resistivity of ∼7.5 m Ω cm are obtained. The implications of these findings on electronic and photonic devices on single-crystal AlN substrates are discussed.

Interband Zener tunneling of electrons has been recently used in III-nitride semiconductor based light emitters to efficiently inject holes into p-cladding layers. Zener tunneling probabilities can be significantly enhanced if crystal symmetry-induced internal polarization fields assist the dopant-induced built-in electric fields of tunnel junctions because of the large reduction of the tunneling distance. In a metal-polar buried tunnel junction geometry, such electric field alignment needs an AlN interlayer at the tunnel junction. Because AlN is a larger bandgap semiconductor than GaN, it is not clear a priori if the net tunneling probability is reduced or enhanced compared to a homojunction. By combining theoretical modeling with experimental blue light emitting diodes, we find that the large tunneling enhancement due to the polarization field and band realignment overcome the reduction in tunneling due to the larger bandgap of AlN. Compared to a homojunction tunnel-junction, the inclusion of AlN in the tunnel junction is found to lower the turn-on and operating voltages and increase the wall-plug efficiency. This proves that polarization-induced AlN tunnel junctions are superior to homojunctions at low injection currents, resulting in higher optical emission intensity and superior uniformity.

A recent thrust toward efficient modulated light emitters for use in Li-Fi communications has sparked renewed interest in visible III-N InGaN light-emitting diodes (LEDs). With their high external quantum efficiencies, blue InGaN LEDs are ideal components for such devices. We report a method for achieving voltage-controlled gate-modulated light emission using monolithic integration of fin- and nanowire-n–i–n vertical FETs with bottom-tunnel junction planar blue InGaN LEDs. This method takes advantage of the improved performance of bottom-tunnel junction LEDs over their top-tunnel junction counterparts, while allowing for strong gate control on a low-cross-sectional area fin or wire without sacrificing the LED active area as in lateral integration designs. Electrical modulation of five orders and an order of magnitude of optical modulation are achieved in the device.

We report optically and electrically pumped ∼ 280 n m deep ultraviolet (DUV) light emitting diodes (LEDs) with ultra-thin GaN/AlN quantum disks (QDs) inserted into AlGaN nanorods by selective epitaxial regrowth using molecular beam epitaxy. The GaN/AlN QD LED has shown strong DUV emission distribution on the ordered nanorods and high internal quantum efficiency of 81.2%, as a result of strain release and reduced density of threading dislocations revealed by transmission electron microscopy. Nanorod assembly suppresses the lateral guiding mode of light, and light extraction efficiency can be increased from 14.9% for planar DUV LEDs to 49.6% for nanorod DUV LEDs estimated by finite difference time domain simulations. Presented results offer the potential to solve the issue of external quantum efficiency limitation of DUV LED devices.

This work studies the spontaneous self-assembly of Ge QDs on AlAs, GaAs, and AlGaAs by high temperature in-situ annealing in molecular beam epitaxy (MBE). The morphology of Ge dots formed on AlAs are observed by atom probe tomography, which revealed nearly spherical QDs with diameters approaching 10 nm and confirmed the complete absence of a wetting layer. Reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) of Ge annealed under similar conditions on GaAs and Al0.3Ga0.7As surfaces reveal the gradual suppression of QD formation with decreasing Al-content of the buffer. To investigate the prospects of using encapsulated Ge dots for upconverting photovoltaics, in which photocurrent can still be generated from photons with energy less than the host bandgap, Ge QDs are embedded into the active region of III-V PIN diodes by MBE. It is observed that orders of magnitude higher short-circuit current is obtained at photon energies below the GaAs bandgap compared with a reference PIN diode without Ge QDs. These results demonstrate the promise of Ge QDs for upconverting solar cells and the realization of device-quality integration of group IV and III-V semiconductors.

For the past two decades, repeatable resonant tunneling transport of electrons in III-nitride double barrier heterostructures has remained elusive at room temperature. In this work we theoretically and experimentally study III-nitride double-barrier resonant tunneling diodes (RTDs), the quantum transport characteristics of which exhibit new features that are unexplainable using existing semiconductor theory. The repeatable and robust resonant transport in our devices enables us to track the origin of these features to the broken inversion symmetry in the uniaxial crystal structure, which generates built-in spontaneous and piezoelectric polarization fields. Resonant tunneling transport enabled by the ground state as well as by the first excited state is demonstrated for the first time over a wide temperature window in planar III-nitride RTDs. An analytical transport model for polar resonant tunneling heterostructures is introduced for the first time, showing a good quantitative agreement with experimental data. From this model we realize that tunneling transport is an extremely sensitive measure of the built-in polarization fields. Since such electric fields play a crucial role in the design of electronic and photonic devices, but are difficult to measure, our work provides a completely new method to accurately determine their magnitude for the entire class of polar heterostructures.

Deep UV (DUV) light-emitting diodes (LEDs) are finding increased application in many areas including water purification and sterilization. Sub-270 nm emission is ideal for these applications since bacterial DNA absorbs strongly in this wavelength regime. To extract high energy photons (∼5 eV), the LED cladding regions must be transparent and therefore consist of high Aluminum content (>60%) n- and p-AlGaN. Ensuring high electrical conductivity in such cladding regions becomes increasingly difficult with increasing Al content due to the large dopant activation energies, especially for the acceptor dopant, Magnesium (Mg) [1]. To maximize Mg incorporation by MBE, a commonly utilized approach is to grow the p-cladding region at a low temperature (∼630 °C [2]). A low growth temperature causes defects and undesired compositional inhomogeneity, leading to inferior vertical hole transport (Fig. 1). To address these issues, a combination of high temperature MBE growth of the p-AlGaN cladding and polarization-induced doping is employed. While high growth temperature ensures good crystal quality and compositional uniformity for the Al-rich p-AlGaN, the polarization-induced doping compensates for the reduced Mg incorporation at high temperature. A sub-250 nm DUV LED is demonstrated for the first time using MBE-grown p-AlGaN layers at a high temperature of 730 °C. SIMS analysis (Fig. 2) showed that Mg incorporation decreases by ∼5 times over the temperature range from 650–730°C for 75% AlGaN, and that Mg incorporation decreases at high Al content, consistent with thermodynamic principles [3]. Based on the observations from Fig. 1 and 2, a substrate temperature of 730 °C was chosen for the p-cladding layer for a sub-250 nm DUV LED structure.

We demonstrate a 243 nm deep-ultraviolet (deep-UV) GaN/AlN heterostructure light-emitting diode with and without a tunnel-junction p-contact. Optical emission occurs from 2 monolayer thick GaN quantum structure active regions. Use of a tunnel-junction enhances the current density under forward bias.

Oxide passivation is an active field of investigation for reducing surface state densities on Ge surfaces, namely below high-K gate stacks. However, the quality of Ge oxide passivation has not been as widely investigated in the context of performance in minority carrier photonic devices. p-Ge substrates with a foundry-applied oxide passivation are compared with oxide-stripped substrates passivated by dry thermal oxides grown between 400 ◦ C-650 ◦ Ci n O2. By using p-Ge, only the behavior of minority conduction electrons and their associated traps become the relevant focus of this study. Passivation quality, as evaluated by photoluminescence (PL),isfoundtovaryasafunctionoftemperaturebetween400‐650 ◦ C.ChangesinPLintensityareattributedtodifferencesinsurface terminationofthep-Gesubstrate.X-rayphotoelectronspectroscopy(XPS)studiesfindthatthepresenceofsuboxides(GeOx)increase with oxidation temperature, with GeO2 dominant at 400 ◦ C and mixed suboxides at higher temperatures. Nonuniform desorption of Ge oxide is confirmed above 550 ◦ C. Thermal oxidation is also shown to improve the quality of ion-damaged Ge surfaces. We conclude that suboxides confer better passivation than a GeO2 interface, as determined by PL intensity.

Deep ultraviolet light-emitting diodes (LEDs) composed of III-Nitride semiconductors need layers of heavy doping (>1 × 1019 cm−3) to overcome large dopant activation energies and maintain high electrical conductivity. This work reports that at doping densities of [Si] ∼1.5 × 1019/cm3 for n-Al0.7Ga0.3N, Burstein-Moss and bandgap renormalization effects result in a net reduction of the bandgap of ∼70 meV. At these doping levels, a transition to a metallic conductivity state is observed, with a vanishing of the effective dopant activation energy. The sheet and contact resistivities of Rsh,n = 0.045 Ω cm and ρc,n = 1.13 × 10−6 Ω cm2 are achieved, with uniform conductivity in the vertical direction. The results show that when heavily doped n-AlGaN cladding regions are used for high efficiency deep-UV LEDs or lasers, the accompanying bandgap narrowing reduces the window of optical transparency at the lowest wavelengths that can take advantage of high conductivity.

Using MBE to grow extreme quantum confined heterostructures of 1-2 monolayers GaN/AlN as the active region on bulk AlN substrates, a DUV LED with room temperature electroluminescence at 250 nm MBE grown deep UV LED is demonstrated. LEDs on high quality bulk AlN substrates show significantly lower leakage current compared to those grown on AlN-on-Sapphire templates. Tunneling through the thin AlN barriers and polarization doping for the p-graded AlGaN is used to enhance the efficiency of the devices. Significant decrease in leakage is observed on bulk AlN, with a sharp 13 nm linewidth at 250 nm emisson.

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 WS$_2$ and MoS$_2$ 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 epitaxial growth of GaN on CVD MoS$_2$. Our results show that the transition-metal dichalcogenides can serve as novel near-lattice-matched substrates for nitride growth., Comment: main manuscript: page 1-15, 5 figures; supplementary: page 16-23

A 250 nm deep UV LED is realized on the bulk AlN substrate. Leakage current is reduced significantly. Polarization doping is used for the p-region and GaN active region to enhance the light extraction.

We report GaN p-n diodes on free-standing GaN substrates: a record high Baliga's figure-of-merit (VB2/ Ron) of 12.8 GW/cm2 is achieved with a 32 μm drift layer and a diode diameter of 107 μm exhibiting a BV > 3.4 kV and a Ron < 1 mΩ-cm2. The leakage current density is low: 10−3 − 10−4 A/cm2 at 3 kV. A record low ideality factor of 1.1–1.3 is signature of high GaN quality. These are among the best-reported GaN p-n diodes.

Semiconductor nanowires have received increasing focus from researchers due to their one dimensional characteristics, which offer new horizons for device designs. Nanowire growth has been shown to yield crystalline material on non-lattice matched substrates. Concerning gallium nitride nanowires, common growth substrates have been silicon (100) and (111) substrates. This manuscript discusses the successful molecular beam epitaxial growth of gallium nitride nanowires on amorphous aluminum oxide thin films, grown by atomic layer deposition. Results indicate that regardless of the amorphous nature of the oxide films, the gallium nitride nanowires grown are crystalline and free of defects even at the interface with the oxide. The results offer a method of realizing promising frontiers for device design in which a high quality crystalline material is desired on an amorphous substrate.

We report the generation of room temperature microwave oscillations from GaN/AlN resonant tunneling diodes, which exhibit record-high peak current densities. The tunneling heterostructure grown by molecular beam epitaxy on freestanding GaN substrates comprises a thin GaN quantum well embedded between two AlN tunneling barriers. The room temperature current-voltage characteristics exhibit a record-high maximum peak current density of ∼220 kA/cm2. When biased within the negative differential conductance region, microwave oscillations are measured with a fundamental frequency of ∼0.94 GHz, generating an output power of ∼3.0 μW. Both the fundamental frequency and the output power of the oscillator are limited by the external biasing circuit. Using a small-signal equivalent circuit model, the maximum intrinsic frequency of oscillation for these diodes is predicted to be ∼200 GHz. This work represents a significant step towards microwave power generation enabled by resonant tunneling transport, an ultra-fast pr...

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.

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 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.

The band gaps of rare-earth titanates are commonly reported to be 0.2--0.7 eV. These values are based on optical reflectivity measurements, from which the onset of optical absorption is derived. Here we report experimental and theoretical results on ${\mathrm{GdTiO}}_{3}$ (GTO) indicating that the gap is significantly larger. Photoluminescence (PL) measurements show a strong peak near 1.8 eV, consistent with an observed onset in PL excitation (PLE) at about the same energy. First-principles calculations, based either on density-functional theory (DFT) with a hybrid functional or on DFT+$U$, consistently show that the gap is close to 2 eV. We also propose an interpretation of the previously reported optical absorption spectra. Given the similarities in electronic structure between the rare-earth titanates, our results for GTO have repercussions for the other members of the series. The results also affect the design of complex-oxide heterostructures involving these materials.

Van der Waals (vdW) heterojunctions composed of 2-dimensional (2D) layered materials are emerging as a solid-state materials family that exhibit novel physics phenomena that can power high performance electronic and photonic applications. Here, we present the first demonstration of an important building block in vdW solids: room temperature (RT) 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 widen the potential applications base of 2D layered materials., Comment: 27 pages, 11 figures

Traditional impurity doping schemes are difficult for extreme-gap semiconductors, such as high Al content AlGaN alloys due to the large activation energies for donors and acceptors. Polarization-induced (or Pi-) doping, since its first demonstration [1], is being gradually implemented as an attractive alternative for effective p-type doping in large-bandgap nitride semiconductors both for power electronics, and for deep-UV photonics. Because mobile carriers are field-ionized instead of thermal ionization, they do not freeze out even at cryogenic temperatures. In this work we demonstrate polarization-induced doping deep-UV LEDs emitting in the 235–260 nm (5.25 to 4.75 eV) spectral range at both room temperature and at cryogenic temperatures. The effectiveness of polarization-induced doping is highlighted by a ∼300% increase in the deep-UV EL intensity at 4K compared to the value at 300K.

This work shows that the combination of ultrathin highly strained GaN quantum wells embedded in an AlN matrix, with controlled isotopic concentrations of Nitrogen enables a dual marker method for Raman spectroscopy. By combining these techniques, we demonstrate the effectiveness in studying strain in the vertical direction. This technique will enable the precise probing of properties of buried active layers in heterostructures, and can be extended in the future to vertical devices such as those used for optical emitters, and for power electronics., 18 pages, 4 figures, published in Applied Physics Letters

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.

The synthesis, characterization, and optical properties of high quality straight and branched CdTe nanowires (NWs) are described. A solution-based (solution−liquid−solid) approach is used to synthesize the NWs by employing a low melting bimetallic nanoparticle catalyst to induce one-dimensional (1D) growth. This leverages advances in the development of high quality colloidal quantum dots (QDs) with emerging techniques for manufacturing 1D nanomaterials. Resulting straight and branched CdTe NWs have diameters below twice the corresponding bulk exciton Bohr radius and, as a consequence, exhibit confinement effects in their linear absorption. Size distributions range from 15% to 20%, with NW lengths commonly exceeding 10 μm. Intrawire diameter variations are on the order of 5%. High-resolution transmission electron microscopy (TEM) images reveal that the wires are crystalline and grow exclusively along the and directions of the corresponding zincblende and wurtzite phases. Branched NW morphologi...

Deep ultraviolet (UV) optical emission below 250 nm (∼5 eV) in semiconductors is traditionally obtained from high aluminum containing AlGaN alloy quantum wells. It is shown here that high-quality epitaxial ultrathin binary GaN quantum disks embedded in an AlN matrix can produce efficient optical emission in the 219–235 nm (∼5.7–5.3 eV) spectral range, far above the bulk bandgap (3.4 eV) of GaN. The quantum confinement energy in these heterostructures is larger than the bandgaps of traditional semiconductors, made possible by the large band offsets. These molecular beam epitaxy-grown extreme quantum-confinement GaN/AlN heterostructures exhibit an internal quantum efficiency of 40% at wavelengths as short as 219 nm. These observations together with the ability to engineer the interband optical matrix elements to control the direction of photon emission in such binary quantum disk active regions offer unique advantages over alloy AlGaN quantum well counterparts for the realization of deep-UV light-emitting d...

Tensile-strained Ge is a possible laser material for Si integrated circuits, but reports of lasers using tensile Ge show high threshold current densities and short lifetimes. To study the origins of these shortcomings, Ge ridge waveguides with tensile strain in three dimensions were fabricated using compressive silicon nitride (SiNx) films with up to 2 GPa stress as stress liners. A Raman peak shift of up to 11 cm−1 was observed, corresponding to 3.6% hydrostatic tensile strain for waveguides with a triangular cross-section. Real time degradation in tensile-strained Ge was observed and studied under transmission electron microscopy (TEM). A network of defects, resembling dark line defects, was observed to form and propagate with a speed and density strongly correlated with the local strain extracted from both modeled and measured strain profiles. This degradation suggests highly tensile-strained Ge lasers are likely to have significantly shorter lifetime than similar GaAs or InGaAs quantum well lasers.

Electrically injected deep ultra-violet (UV) emission is obtained using monolayer (ML) thin GaN/AlN quantum structures as active regions. The emission wavelength is tuned by controlling the thickness of ultrathin GaN layers with monolayer precision using plasma assisted molecular beam epitaxy (PAMBE). Single peaked emission spectra is achieved with narrow full width at half maximum (FWHM) for three different light emitting diodes (LEDs) operating at 232 nm, 246 nm and 270 nm. 232 nm (5.34 eV) is the shortest EL emission wavelength reported so far using GaN as the light emitting material and employing polarization-induced doping., Comment: 5 pages, 3 figures

GaN/AlN quantum dots (QDs) have been observed to emit in deep ultra violet (UV) regime. The emission wavelength can be tuned from 270 nm to 238 nm using GaN growth time and Ga flux. In this work, tunnel injection GaN/AlN QD UV LEDs have been fabricated utilizing polarization doped p-n junctions grown on AlN templates on sapphire. The QD EL emission is obtained at 250 nm whereas a second peak emission is observed at 290 nm from the p-type AlGaN. However, the enhanced doping and carrier injection in polarization doped structure boosts the deep-UV emission intensity by 26 times compared to non-polarization doped UV LED.

GaN/AlN structures are utilized to achieve deep-UV emission. By reducing thickenss of GaN QW to 1 ML, 224nm emission is achieved. A further shift to 219 nm is gettable as GaN islands are introduced.

Tremendous progress has been made in the field of III-nitride light-emitting diodes from Schottky diodes to p-n junctions to heterostructures and structures based on quantum wells and quantum dots. The built-in polarization-induced electric field along {0001} reduces the oscillator strength in quantum well structures. However, this has been used to improve p-type doping in AlGaN alloys. InGaN-based high-efficiency blue LEDs have been developed and efforts are under way to fill the green gap in visible light emission from LEDs. On the other hand, wide bandgap III-nitrides are being developed for high-efficiency ultraviolet LEDs. Novel structures incorporating nanowires and photonic crystals are also being investigated.