On the step-graded doped-channel (SGDC) field-effect transistor

An i-InGaP/n-In_xGa_1 − xAs/i- GaAs step-graded doped-channel field-effect transistor (SGDCFET) has been fabricated and studied. Due to the existence of a V-shaped energy band formed by the step-graded structure, a large output current density, a large gate voltage swing with high average transconductance, and a high breakdown voltage can be expected. In this study, first, a theoretical model and a transfer matrix technique are employed to analyze the energy states and wavefunctions in the step-graded quantum wells. Experimentally, for a 1  ×  80 μm²gate dimension device, a maximum drain saturation current density of 830 mAmm ^− 1, a maximum transconductance of 188mSmm ^− 1 , a high gate breakdown voltage of 34 V, and a large gate voltage swing 3.3 V with transconductance larger than 150 mSmm ^− 1are achieved. These performances show that the device studied has a good potentiality for high-speed, high-power, and large input signal circuit applications.     

Electrical characteristics and efficiency of organic light-emitting diodes depending on hole-injection layer

In a device structure of ITO/hole-injection layer/N,N′-biphenyl-N,N′-bis-(1-naphenyl)-[1,1′-biphthyl]4,4′-diamine(NPB)/tris(8-hydroxyquinoline)aluminum(Alq₃)/Al, we investigated the effect of the hole-injection layer on the electrical characteristics and external quantum efficiency of organic light-emitting diodes. Thermal evaporation was performed to make a thickness of NPB layer with a rate of 0.5–1.0 Å/s at a base pressure of 5 × 10⁻⁶ Torr. We measured current–voltage characteristics and external quantum efficiency with a thickness variation of the hole-injection layer. CuPc and PVK buffer layers improve the performance of the device in several aspects, such as good mechanical junction, reducing the operating voltage, and energy band adjustment. Compared with devices without a hole-injection layer, we found that the optimal thickness of NPB was 20 nm in the device structure of ITO/NPB/Alq₃/Al. By using a CuPc or PVK buffer layer, the external quantum efficiencies of the devices were improved by 28.9% and 51.3%, respectively.     

Conductance of quantum interference transistors in parallel and in series

We theoretically study the electronic conductance G and the current–voltage characteristics of two quantum interference transistors in parallel and in series. We use two different definitions of conductance,G ∼ T and G ∼ T / R. Neither can reproduce the classical additivity law in the case of coherent transport due to quantum interference for the elements in series and quasibound states when elements are in parallel. In the case of two transistors in series, we find that the quantityT / R only qualitatively better represents the additivity law, which is probably expected because this model avoids counting the contact resistance twice. However, for the parallel configuration of transistors, the conductance is almost additive for the majority of energies when G ∼ T, except for the single-mode regime. Possible use of these configurations in digital electronics for basic logic functions is discussed.     

Molecular Beam Epitaxy growth and characterization of silicon – Doped InAs dot in a well quantum dot infrared photo detector (DWELL-QDIP)

We report the growth by Molecular Beam Epitaxy (MBE), fabrication and characterization of silicon doped 20 layer InAs dot in a well quantum dot infrared photo detector (DWELL-QDIP) device structures. Two structures with InAs dots of vertical heights of 50 Å and 40 Å were compared. A 2–8 μm band normal incidence photo response of the detector with polarization and bias dependence was obtained at 77 K. The specific peak detectivity D1 be 0.8 × 10⁹ Jones for one of the detectors.     

Optical nutation in a magnetized semiconductor quantum well structure

Using the semiclassical coherent radiation—semiconductor interaction model, optical nutation has been analysed in aGaAs / Al_xGa_1 − xAs quantum well structure (QWS) assumed to be immersed in a moderately strong magnetic field and irradiated by a not-too-strong near band gap resonant femtosecond pulsed Ti–sapphire laser. The finite potential well depth of the QWS and the Wannier–Mott excitonic structure of the crystal absorption edge is taken into account. The excitation intensity is assumed to be below the Mott transition where the various many-body effects have been neglected with adequate reasoning. Numerical analysis made for a GaAs quantum well of thickness  ∼  100 Åand the confining layers ofAl_xGa_1 − xAs withx =  0.3 at intensity I ≤  5  ×  10⁶Wcm ^− 2reveals that the real and imaginary parts of the transient complex-induced polarization are enhanced with an increase in the magnetic field and their ringing behaviour confirms the occurrence of optical nutation in the QWS.     

Thermal annealing of GaSb quantum dots in GaAs formed by droplet epitaxy

We investigate effects of annealing on GaSb quantum dots (QDs) formed by droplet epitaxy. Ga droplets grown on GaAs are exposed to Sb molecular beam and then annealed at T_a=340–450 °C for 1 min to form GaSb QDs. An atomic force microscope study shows that with the increase of T_a, the average diameter of dots increases by about 60%, while their density decreases to about 1/3. The photoluminescence (PL) of GaSb QDs is observed at around 1 eV only for those samples annealed above T_a=380 °C, which indicates that the annealing process plays an important role in forming high quality GaSb QDs.     

Classical and quantum hall effect measurements in GaInNAs/GaAs quantum wells

We have performed magneto-transport experiments in modulation-doped Ga_0.7In_0.3N_yAs_1−y/GaAs quantum wells with nitrogen mole fractions 0.4%, 1.0% and 1.5%. Classical magnetotransport (resistivity and low-field Hall effect) measurements have been performed in the temperatures between 1.8 and 275 K, while quantum Hall effect measurements in the temperatures between 1.8 and 47 K and magnetic fields up to 11 T.The variations of Hall mobility and Hall carrier density with nitrogen mole fractions and temperature have been obtained from the classical magnetotransport measurements. The results are used to investigate the scattering mechanisms of electrons in the modulation-doped Ga_0.7In_0.3N_yAs_1−y/GaAs quantum wells. It is shown that the alloy disorder scattering is the major scattering mechanism at investigated temperatures.The quantum oscillations in Hall resistance have been used to determine the carrier density, effective mass, transport mobility, quantum mobility and Fermi energy of two-dimensional (2D) electrons in the modulation-doped Ga_0.7In_0.3N_yAs_1−y/GaAs quantum wells. The carrier density, in-plane effective mass and Fermi energy of the 2D electrons increases when the nitrogen mole fraction is increased from y=0.004 to 0.015. The results found for these parameters are in good agreement with those determined from the Shubnikov-de Haas effect in magnetoresistance.     

Off-state breakdown characteristics of InGaP-based high-barrier gate heterostructure field-effect transistors

In this work, the off-state breakdown characteristics of two different types InGaP-based high-barrier gate heterostructure field-effect transistors are studied and demonstrated. These devices have different high-barrier gate structures, e.g. the i-InGaP layer for device A and n  ⁺ - GaAs/p ⁺  -InGaP/n-GaAs camel-like structure for device B. The wide-gap InGaP layer is used to improve the breakdown characteristics. Experimentally, the studied devices show high off-state breakdown characteristics even at high temperature operation regime. This indicates that the studied devices are suitable for high-power and high-temperature applications. In addition, the off-state breakdown mechanisms are different for device A and B. For device A, off-state breakdown characteristics is only gate dominated at the temperature regime from 30 to 180  ^∘ C. For device B, off-state breakdown characteristics are gate and channel dominated at 30  ^∘ C and only gate dominated within 150 to 210  ^∘ C.     

Binding energies of excitons in symmetric and asymmetric quantum wells in a magnetic field

The binding energy of the exciton in the symmetric and asymmetric GaAs/Ga_1 − xAl_xAs quantum wells is calculated with the use of a variational approach. Results have been obtained as a function of the potential symmetry, and the size of the quantum well in the presence of an arbitrary magnetic field. The applied magnetic field is taken to be parallel to the axis of growth of the quantum well structure. The role of the asymmetric barriers, magnetic field, and well width in the excitonic binding is discussed as the tunability parameters of the GaAs/Ga_1 − xAl_xAs system.     

High-efficiency organic light-emitting diodes (OLEDs) with a mixed layer structure

Improved performance of organic light-emitting diodes (OLEDs) as obtained by a mixed layer was investigated. The OLEDs with a mixed layer which were composed of N,N′-diphenyl-N,N′-bis(1-napthyl-phenyl)-1,1′-biphenyl-4,4′-diamine (NPB), tris-(8-hydroxyquinolato) aluminum (Alq₃) and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) showed the highest brightness and efficiency, which reached 19048 cd/m² at 17 V and 4.3 cd/A at 10 mA/cm², respectively. The turn-on voltage of the device is 2.6 V. Its Commission Internationale del’Eclairage (CIE) coordinate is (0.497, 0.456) at 17 V, and the CIE coordinates of the device are largely insensitive to the driving voltages, which depicts stabilized yellow color.     

Third-order photonic-crystal distributed-feedback quantum cascade lasers

We demonstrate room-temperature operation of broad-area edge-emitting photonic-crystal distributed-feedback quantum cascade lasers at λ ～ 4.6 μm. The lasers use a weak-index perturbed third-order photonic-crystal lattice to control the optical mode in the wafer plane. Utilizing this coupling mechanism, the near-diffraction-limited beam quality with a far-field profile normal to the facet can be obtained. Single-mode operation with a signal-to-noise ratio of about 20 dB is achieved in the temperature range of 85–290 K. The single-facet output power is above 1 W for a 55 μm × 2.5 mm laser bar at 85 K in pulsed mode.     

Optical properties of GaAs/Ga 1-xAlxAs ridge quantum wire: Third-harmonic generation

In this paper, the third-harmonic generation (THG) in GaAs/Ga_1 ? xAl_xAs ridge quantum wires is studied in detail. An analytic expression for the THG is obtained using a compact density matrix approach and an iterative procedure. Numerical calculations are performed for the typical GaAs/Ga_1 ? xAl_xAs ridge quantum wire. The results show that the maximum THG over 10^? 9_? m²_?/V²_? can be obtained. Another important point is that the structural parameters have great influence on the THG in this system.     

Defects related emission and nanosecond optical power limiting in CuS quantum dots

We report optical and nonlinear optical properties of CuS quantum dots and nanoparticles prepared through a nontoxic, green, one-pot synthesis method. The presence of surface states and defects in the quantum dots are evident from the luminescent behavior and enhanced nonlinear optical properties measured using the open aperture Z-scan, employing 5 ns laser pulses at 532 nm. The quantum dots exhibit large effective third order nonlinear optical coefficients with a relatively lower optical limiting threshold of 2.3 J cm⁻², and the optical nonlinearity arises largely from absorption saturation and excited state absorption. Results suggest that these materials are potential candidates for designing efficient optical limiters with applications in laser safety devices.     

An organic electroluminescent device made from a gadolinium complex

A gadolinium ternary complex, tris(1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone) (phenanthroline) gadolinium [Gd(PMIP)₃(Phen)] was synthesized and used as a light emitting material in the organic electroluminescent (EL) devices. The triple layer device with a structure of indium tin oxide (ITO)/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) (20 nm)/Gd(PMIP)₃(Phen) (80 nm)/2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (bathocuproine or BCP) (20 nm)/Mg: Ag(200 nm)/Ag(100 nm) exhibited green emission peaking at 535 nm. A maximum luminance of 230 cd/m² at 17 V and a peak power efficiency of 0.02 lm/w at 9 V were obtained.     

Spatially varying effective mass effects on energy level populations in semiconductor quantum devices

For the analysis and design of semiconductor intersubband devices, accurate values for the Fermi energy and the subband electron population are needed. The effect of the position-dependent electron effective massm ^*  (z) is commonly neglected in the determination of these two intersubband device parameters. This approach is nearly valid for single-well devices in the GaAs/Al _xGa_1 − x As material system. However, in multiple-coupled-well devices, the variable nature of the effective mass must be taken into account. In material systems other than the ones based on GaAs, failure to include the position-dependence of the electron mass may give rise to significant errors in the values of the Fermi energy and other device parameters such as the intersubband absorption coefficient. In this paper, the effects ofm ^*  (z) on the Fermi level and the intersubband charge distribution are explored and quantified.Theoretical formulation for the intersubband Fermi energy and the subband electron distribution, with the inclusion of position-dependent electron mass, is presented. The eigenenergies of the intersubband structures are obtained by solving the single-band effective-mass Schroedinger equation using the argument principle method. The electron distribution and the Fermi energy are calculated using both the approximate method ( m₀ ^* ) and the rigorous formulation [ m ^* (z)], and the relative differences in the corresponding values are presented. It is demonstrated that these differences are small in the GaAs/Al _xGa_1 − x As material system, but can become very significant in other materials. In addition, the variable nature of carrier effective mass plays an important role in other types of devices such as interband quantum well photodetectors and lasers that employ optical transitions between the valence and the conduction bands. Electronic devices such as the resonant tunneling diode are also affected by the position-dependence of carrier mass and thus the results are applicable to both optoelectronic and electronic quantum devices.     

Raman scattering from confined phonons in GaAs/AlGaAs quantum wires

We report on photoluminescence and Raman scattering performed at low temperature (T =  10 K) on GaAs/Al_0.3Ga_0.7As quantum-well wires with effective wire widths ofL =  100.0 and 10.9 nm prepared by molecular beam epitaxial growth followed by holographic patterning, reactive ion etching, and anodic thinning. We find evidence for the existence of longitudinal optical phonon modes confined to the GaAs quantum wire. The observed frequency at ο_L10 =  285.6 cm⁻¹forL =  11.0 nm is in good agreement with that calculated on the basis of the dispersive dielectric continuum theory of Enderleinas applied to the GaAs/Al_0.3Ga_0.7As system. Our results indicate the high crystalline quality of the quantum-well wires fabricated using these techniques.     

Surface modified polymeric gate insulators for pentacene organic thin-film transistors

This paper demonstrates effects of a surface modification of polymeric gate insulators on a performance of organic thin-film transistor (OTFT). Pentacene OTFTs were fabricated with three types of polymer gate insulators—poly(4-vinylphenol) (PVP, G1) with comparably high dielectric constant, polyimide (PI, G2) with n-octadecyl (C₁₈) side chain, which resulted in hydrophobicity and low dielectric constant, and surface modified PVP(G3). The G3 was prepared by a spin-coating the solution of G2 onto the G1 film. We found that the n-octadecyl group of the G3 protruded from the surface and made the PVP surface more hydrophobic. The less polar surface strongly improved the device performance. Subthreshold slope (s.s.) of the OTFT with G3 as the gate insulator decreased significantly to 2.7 V/dec, which was much smaller than that of OTFTs fabricated with G1 (4.0 V/dec). That is, thin layer with fewer C₁₈ group in contact with pentacene induced a good electrical property like lower s.s. Further the higher dielectric constant of the underlying layer resulted in higher mobility of the device. The mobility (0.50 cm² V⁻¹ s⁻¹) of the OTFT with G3 as the gate insulator showed a higher value compared to that (0.25 cm² V⁻¹ s⁻¹) of the OTFT with G2.     

Room temperature phosphorescence lifetime and quantum yield of erythrosine B and rose bengal in aerobic alkaline aqueous solution

The room-temperature phosphorescence behavior of erythrosine B (ER) and rose bengal (RB) in aerobic aqueous solution at pH 10 (10^?4 M NaOH) is investigated. The samples were excited with sliced second harmonic pulses of a Q-switched Nd:glass laser. A gated photomultiplier tube was used for instantaneous fluorescence signal discrimination and a digital oscilloscope was used for signal recording. For phosphorescence lifetime measurement the oscilloscope response time was adjusted to appropriate time resolution and sensitivity by the ohmic input resistance. In the case of phosphorescence quantum yield determination the gated photomultiplier – oscilloscope arrangement was operated in integration mode using 10 MΩ input resistance. Phosphorescence quantum yield calibration was achieved with erythrosine B and rose bengal doped starch films of known quantum yields. The determined phosphorescence lifetimes (quantum yields) of ER and RB in 0.1 mM NaOH are τ_P=1.92±0.1 μs (?_P=(1.5±0.3)×10^?5) and 2.40±0.1 μs ((5.7±0.9)×10^?5), respectively. The results are discussed in terms of triplet state deactivation by dissolved molecular oxygen.     

Demonstration of a bias tunable quantum dots-in-a-well focal plane array

Infrared detectors based on quantum wells and quantum dots have attracted a lot of attention in the past few years. Our previous research has reported on the development of the first generation of quantum dots-in-a-well (DWELL) focal plane arrays, which are based on InAs quantum dots embedded in an InGaAs well having GaAs barriers. This focal plane array has successfully generated a two-color imagery in the mid-wave infrared (i.e. 3–5 μm) and the long-wave infrared (i.e. 8–12 μm) at a fixed bias voltage. Recently, the DWELL device has been further modified by embedding InAs quantum dots in InGaAs and GaAs double wells with AlGaAs barriers, leading to a less strained InAs/InGaAs/GaAs/AlGaAs heterostructure. This is expected to improve the operating temperature while maintaining a low dark current level. This paper examines 320 × 256 double DWELL based focal plane arrays that have been fabricated and hybridized with an Indigo 9705 read-out integrated circuit using Indium-bump (flip-chip) technology. The spectral tunability is quantified by examining images and determining the transmittance ratio (equivalent to the photocurrent ratio) between mid-wave and long-way infrared filter targets. Calculations were performed for a bias range from 0.3 to 1.0 V. The results demonstrate that the mid-wave transmittance dominates at these low bias voltages, and the transmittance ratio continuously varies over different applied biases. Additionally, radiometric characterization, including array uniformity and measured noise equivalent temperature difference for the double DWELL devices is computed and compared to the same results from the original first generation DWELL. Finally, higher temperature operation is explored. Overall, the double DWELL devices had lower noise equivalent temperature difference and higher uniformity, and worked at higher temperature (70 K and 80 K) than the first generation DWELL device.     

Highly efficient flexible organic light-emitting devices utilizing F4-TCNQ/m-MTDATA multiple quantum well structures

Flexible organic light-emitting devices (FOLEDs) based on multiple quantum well (MQW) structures, which consist of alternate layers of 2,3,5,6-Tetrafluoro-7,7,8,8,-tetracyano-quinodimethane (F4-TCNQ) and 4,4′,4″-tris-(3-methylphenylphe-nylamino)tripheny-lamine (m-MTDATA) have been fabricated. The Alq₃-based device with double quantum well (DQW) structure exhibits the remarkable electroluminescent (EL) performances for the brightness of 23,500 cd/m² at 14 V and the maximum current efficiency of 7.0 cd/A at 300.3 mA/cm², respectively, which are greatly improved by 114% and 56% compared with the brightness of 10,958 cd/m² at 14 V and the maximum current efficiency of 4.5 cd/A at 174.0 mA/cm² for the conventional device without MQW structures. These results demonstrate that the EL performances of FOLEDs could be greatly improved by utilizing the novel MQW structures, and the reason for this improvement has also been explained by the effect of interfacial dipole and interfacial doping between F4-TCNQ and m-MTDATA in this article.