Thickness Of Active Region Research Articles

Temperature Effects on Power Characteristics of Vertical-Cavity Surface-Emitting Lasers

This paper presents a numerical study based on temperature dependency on Auger process, examining the impact of temperature change and laser cavity active region parameters on the output power characteristics (Pout) of vertical-cavity surface-emitting lasers (VCSELs). The active region parameters include active region diameter, effective length, active region thickness, injection current, and mirror reflectivity. With increasing temperature, Pout reduction rate rises in proportion to active region diameter growth as opposed to effective length or active region thickness growth. It was determined that Pout thermal effects change minimally with injection current variation but can be reduced or completely mitigated by increasing the mirror reflectivity. This study provides notable insights in the design of VCSEL with consideration of temperature dependency on the Auger process.

Optimization of threshold characteristics of Hg0·8Cd0·2Te -based laser under interband breakdown in a lateral electric field

HgCdTe is promising as a material to solve a problem of development of semiconductor sources in mid- and far-infrared range due to small direct band gap. Since the compound materials HgTe and CdTe are closely lattice matched, it is possible to design a wide variety of multilayer structures based on HgCdTe. In this paper we calculate and optimize the parameters of the Hg0·8Cd0·2Te/Cd0·7Hg0·3Te separate-confinement double heterostructure which are required for lasing. It is shown that lasing at a wavelength of 10 μm for the optimal active region thickness (∼90 nm) and resonator length (∼2 mm) occurs when a lateral electric field of more than 0.96 kV/cm is applied at a temperature of 77 K. This corresponds to a small voltage of ∼10 V at a distance between metal contacts of 100 μm and the threshold current of ∼30 A.

Low-efficiency-droop c-plane InGaN/GaN light-emitting diodes through the use of thick single quantum wells and doped barriers

Efficiency droop at high current densities is a problem for InGaN-based light-emitting diodes (LEDs), especially for conventional c-plane devices. The large internal electric fields in c-plane quantum wells (QWs) lead to an increase in the active region carrier density (n), causing the electrical efficiency droop onset to occur at low current densities. Here, we present an approach to reduce the internal electric fields (Eint) in c-plane QWs by placing doped p-type and n-type GaN barriers close to the QW. The reduced Eint also allows a thick QW active region design, which helps to lower n to further reduce the droop. The concept of using doped barriers to control Eint is explained using theory and device simulations. Following that, multiple series of thick single QW (SQW) LEDs were grown and characterized. Key parameters in the epitaxial design such as the doping levels and the relative position of the doped barriers were systematically studied and optimized. By using optimized doped barriers and a thick SQW, c-plane LEDs with a low-efficiency droop of 14% at 300 A/cm2 [with respect to the peak external quantum efficiency (EQE)] and a high peak EQE of 49% were demonstrated.

Enhanced absorption of infrared light for quantum wells in coupled pillar-cavity arrays.

Periodic pillars of semiconductor in sub-wavelength size can serve multiple roles as diffracting, trapping and absorbing light for effective photoelectric conversion which has been intensively studied in the visible range. Here, we design and fabricate the micro-pillar arrays of AlGaAs/GaAs multi quantum wells(QWs) for high performance detection of long wavelength infrared light. Compared to its planar counterpart, the array offers 5.1 times intensified absorption at peak wavelength of 8.7 µm with 4 times shrinked electrical area. It's illustrated by simulation that the normal incident light is guided in the pillars by HE11 resonant cavity mode to form strengthened Ez electrical field, which enables the inter-subband transition of n-type QWs. Moreover, the thick active region of dielectric cavity that contains 50 periods of QWs with fairly low doping concentration will be beneficial to the optical and electrical merits of the detectors. This study demonstrates an inclusive scheme to substantially raise the signal to ratio of infrared detection with all-semiconductor photonic structures.

Study of critical optical confinement factor for GeSn-based multiple quantum well lasers

SiGeSn material is of great interest for the development of all-group-IV lasers on a Si substrate. While GeSn-based lasers have been reported worldwide, probing the fundamental limit to lase is highly desirable to reveal the material capability as a gain medium. In this work, three GeSn-based multiple quantum well lasers, with four wells, six wells, and ten wells, were characterized. The four-well device cannot achieve lasing due to the thin active region (thickness of 81 nm), resulting in insufficient optical confinement factor. With thicker active region, both six-well (92 nm) and ten-well (136 nm) devices show clear lasing output. The ten-well sample exhibits a higher lasing temperature of 90 K. The finding of this work reveals the fundamental limit of the required optical confinement factor to achieve lasing for a Fabry–Pérot cavity edge emitting laser devices.

Temperature Dependence Study of Electrical and Electro-Optical Performances of Midwave Infrared Ga-Free T2SL Barrier Photodetector

In this paper, we report on temperature dependence performances of a midwave infrared (MWIR) Ga-free InAs/InAsSb type-II superlattice (T2SL) barrier (XBn) photodetector grown by molecular beam epitaxy on n-type GaSb substrate. The T2SL structure, with a 3 µm thick active region, was processed in a mesa device in order to perform dark current measurements and spectral photoresponse as a function of temperature. Analyses of these temperature dependence characterizations help us to improve the design of Ga-free T2SL MWIR XBn detectors.

A 5.7 THz GaN/AlGaN quantum cascade detector based on polar step quantum wells

We report on a GaN/AlGaN quantum cascade detector operating in the terahertz spectral range. The device was grown by metal organic chemical vapor deposition on a c-sapphire substrate and relies on polar GaN/AlGaN step quantum wells. The active region thickness is in micrometer range. The structural, electrical, and optical investigations attest of high structural quality of the synthetized nitride material. The detector exhibits a peak photocurrent at 5.7 THz (23.6 meV) with a responsivity of 0.1 mA/W at 10 K under surface normal irradiation through a 10 μm period grating. The photocurrent persists up to 20 K.

Understanding photosynthetic biofilm productivity and structure through 2D simulation.

We present a spatial model describing the growth of a photosynthetic microalgae biofilm. In this 2D-model we consider photosynthesis, cell carbon accumulation, extracellular matrix excretion, and mortality. The rate of each of these mechanisms is given by kinetic laws regulated by light, nitrate, oxygen and inorganic carbon. The model is based on mixture theory and the behaviour of each component is defined on one hand by mass conservation, which takes into account biological features of the system, and on the other hand by conservation of momentum, which expresses the physical properties of the components. The model simulates the biofilm structural dynamics following an initial colonization phase. It shows that a 75 μm thick active region drives the biofilm development. We then determine the optimal harvesting period and biofilm height which maximize productivity. Finally, different harvesting patterns are tested and their effect on biofilm structure are discussed. The optimal strategy differs whether the objective is to recover the total biofilm or just the algal biomass.

Люминесцентный контроль светодиодных гетероструктур, выращенных методом металлоорганической газофазной эпитаксии на сапфире

The paper presents the results of spectral and cardinal characteristics of AlGaN/GaN and InGaN/GaN LED heterostructure pulsed cathodes and photoluminescence grown on sapphire by metal organic vapor phase epitaxy are introduced. The effects of high current electron beam energy density on the luminescence spectra and amplitude characteristics of heterostructures are studied. The spatial distribution of fluorescence characteristics on the wafer surface was studied. It is found that in InGaN/GaN heterostructures, the maxim shifts of stimulated cathodoluminescence spectra measured at different points of the sample. This result is due to the changes in the composition and thickness of the quantum dimensional active region, and this change is not due to the ideality of the plate growth process.

Light-harvesting enhancement in ultrathin film silicon solar cells using photonic and plasmonic nanostructures

Past few decades, nanostructures are useful in the active region and light-harvesting layers in thin film solar cells. In this work, the high reflectance alternative Si/SiO2 distributed Bragg reflectors (DBRs), and aluminum (Al) thin film layers were integrated as a backside reflectors in ultrathin film silicon solar cells to improve the light trapping mechanism using finite-difference time-domain (FDTD) method. DBR is known as one-dimensional photonic crystal (1DPC) used as backside reflectors with the addition of SiO2 buffer layer. Further, the transverse electric (TE) and magnetic (TM) field distribution shown the strong light scattering in the silicon active layer region by integration of top (ITO) and bottom (Al) nanogratings. Both polarization conditions yielded the highest current density (Jsc) of 22.03 (TE) and 30.42 mA/cm2 (TM) achieved within 50 nm active region thickness.

Квантово-каскадный лазер на 3.3 ТГц на основе активного модуля из трех квантовых ям GaAs/AlGaAs с рабочей температурой >120 K

In this work, we have optimized the THz QCL design with an active module based on three GaAs/Al0.18Ga0.82As quantum wells for high-temperature generation at a frequency of about 3.3 THz. A heterostructure based on the developed design with an active region thickness of 10 μm was grown by molecular beam epitaxy with a deviation of the active module thickness from the nominal less than 1%. The fabricated THz QCLs with a double metal waveguide demonstrate lasing up to a temperature of 125 K. Investigations of the I–V characteristics, the dependences of the integrated radiation on the current and the lasing spectra showed good agreement with the calculated characteristics.

Investigation of characteristics of GaAs/AlGaAs p-i-n connecting tunnel diodes

Current-voltage and capacitance-voltage characteristics of tunnel diode structures with GaAs-n++/i-GaAs/i-Al0,2Ga0,8As/Al0,2Ga0,8As-p++ active region and i-layer thicknesses of 7.5 nm and 10 nm and epitaxial growth temperature of 450 °C and 500 °C have been studied. The tunnel diode structures grown by the molecular beam epitaxy technique are intended for application as connecting elements in multijunction photovoltaic converters of high-power monochromatic optical radiation. According to the results obtained, the peak tunnel current density of up to 200 A/cm2 is achieved. Forward current-voltage and capacitance-voltage characteristics of tunnel p-i-n diode structures were measured at room temperature. The capacitance-voltage characteristics of the tunnel diodes being tested were obtained experimentally with the frequency dependence of S-parameters simulated within 10-1000 MHz at bias voltages of 0.4–1 V.

A fast avalanche Si diode with a 517 μ m low-doped region

A silicon-avalanche shaper/sharpener is a fast-closing semiconductor switch. For positive voltages, it is activated by a high-voltage pulse at its cathode, and, when turned on, the current through the device rises rapidly. Using Synopsys TCAD software, a p+−n−n+ diode is numerically studied. It was shown that for the case of a high-doped active n region, 1014 cm−3, the breakdown process exhibits a fast electric field propagation, as expected. For a low doped active n region, <1011 cm−3, the electric field spreads uniformly along the structure. For this case, we show that the rise time, of the order of 100 ps, is not limited by the active region thickness, allowing the use of a thicker substrate in order to increase the operating voltage. A p+−n−n+ diode was fabricated on a thick, 525 μm, float-zone n-type Si (100) substrate, with a resistivity of 104 Ω cm. The active region, n<1012 cm−3, was 517 μm. When a stack of five, 8 mm2, diodes was driven by an ∼100 kV, 2.26 ns rise time pulse, the output voltage was 46 kV with the rise time and rise rate per diode of 215 ps and 38.4 kV/ns, respectively. When a single, 4 mm2, diode was driven by a 14 kV, 1 ns rise time pulse, the output on a 50 Ω load was around 8 kV, 100 ps, with a rise rate of 57 kV/ns. These results exceed the present state-of-the-art diodes. Furthermore, the thick active region eliminates current fabrication process difficulties such as deep diffusion or thick epitaxial layers.

Mitigating offset frequency drift in frequency combs using a customized power law dispersion.

We introduce a new, to the best of our knowledge, passive technique of mitigating the phase noise in optical frequency combs (FC) by reducing the drift of offset frequency. This can be achieved by customizing the dispersion to attain a power law dependence of the wave vector on frequency, k(ω)∼ωα, ensuring a constant ratio between group and phase velocities. When this condition is maintained, the drift offset frequency is passively mitigated, and phase noise is reduced. Using quantum cascade laser (QCL) FCs as an example, we demonstrate, analytically and numerically, that the desired dispersion can be easily engineered by properly adjusting the thickness of the QCL active region and that stable offset frequency can be combined with low residual group dispersion.

Optical Properties of InGaN/GaN Multiple Quantum Well Structures Grown on GaN and Sapphire Substrates

A set of samples based on InGaN/GaN multiple quantum well (QW) structures with an extremely thick active region and high QW numbers was prepared on different substrates by metal organic vapor phase epitaxy. Their morphology was studied by SEM images, and their optical properties, including photoluminescence (PL) excitation, PL emission spectra, and PL decay curves, were measured by means of time-resolved luminescence spectroscopy. The obtained spectral features were tentatively explained by using the band scheme, which is commonly used for the description of InGaN/GaN multiple QW (MQW) structures. The application potential in the field of nanoscintillators used for fast timing of studied structures is discussed as well.

Design and simulation of losses in Ge/SiGe terahertz quantum cascade laser waveguides.

The waveguide losses from a range of surface plasmon and double metal waveguides for Ge/Si1-xGex THz quantum cascade laser gain media are investigated at 4.79 THz (62.6 μm wavelength). Double metal waveguides demonstrate lower losses than surface plasmonic guiding with minimum losses for a 10 μm thick active gain region with silver metal of 21 cm-1 at 300 K reducing to 14.5 cm-1 at 10 K. Losses for silicon foundry compatible metals including Al and Cu are also provided for comparison and to provide a guide for gain requirements to enable lasers to be fabricated in commercial silicon foundries. To allow these losses to be calculated for a range of designs, the complex refractive index of a range of nominally undoped Si1-xGex with x = 0.7, 0.8 and 0.9 and doped Ge heterolayers were extracted from Fourier transform infrared spectroscopy measurements between 0.1 and 10 THz and from 300 K down to 10 K. The results demonstrate losses comparable to similar designs of GaAs/AlGaAs quantum cascade laser plasmon waveguides indicating that a gain threshold of 15.1 cm-1 and 23.8 cm-1 are required to produce a 4.79 THz Ge/SiGe THz laser at 10 K and 300 K, respectively, for 2 mm long double metal waveguide quantum cascade lasers with facet coatings.

A Novel Memristor-Based Gas Cumulative Flow Sensor

In order to overcome the shortcomings of the measurement methods for gas cumulative flow, especially in the long-term continuous measurement, a novel memristor-based gas cumulative flow sensor is proposed in this paper. The sensor is mainly composed of a hot-wire resistor, the memristor module, a feedback compensation network and some basic circuit components. According to the thermal balance equation and the memory characteristics of memristor, the sensor can measure the gas cumulative flow by combining the memristor resistance with the cross-sectional area of the pipe. In order to verify the design of memristor-based gas accumulation flow sensor, experiments are carried out under a constant gas flow velocity, a gradually increasing flow velocity, and a randomly varying flow velocity. The active region thickness D of the selected TiO2/TiO2-n memristor is 40 nm. The hot-wire used is a tungsten wire having a radius of 10 μ m and a length of 2 mm. The gas measured is air at a temperature of 300 K in the paper. The experimental results show that the sensor achieves the measurement of gas cumulative flow. When gas flow velocity at the center of the pipe is 5 m/s, the measuring range is approximately 0–20 m3 and the sensitivity within the effective range is approximately 28.44 Ω/m3. When the gas flow velocity, the memristor pairs increase and the gain of the amplifier changes, the measuring range, and the sensitivity of the sensor will changes accordingly.

Spontaneous Emission Rate and Radiative Current Density in p-GaAs/i-GaNAsBi/n-GaAs Quantum Well Lasers

We have theoretically investigated the 1.55 μm emission of p–i–n GaNAsBi-based quantum wells (QWs) using a self-consistent calculation combined with (16×16) BAC model. Their performances are evaluated in terms of spontaneous emission rate Rsp and radiative current density Jrad. We have found that Jrad increases as function of the injected carrier density as well as the doping density. The quantum confined Stark effect on radiative current density in ideal lasers is also discussed. The radiative current density versus well width for simple and double p-GaAs/i-GaN·58yAs1-1·58yBiy/n-GaAs quantum well structures is also examined. We have obtained that Jrad increases with increase average thickness of GaNAsBi active region. The optimization of well parameters can be used as a basis for GaNAsBi-based lasers intended for optical fiber telecommunication wavelength.

Electrodeposited MoS2 and WS2 Thin Films for Pseudocapacitance Studies

Cathodic electrodeposition of MoS2 (WS2) atop glassy carbon (ITO) was recently reported utilizing a single-source precursor MoS4 2- (WS4 2-) from an aqueous (acetonitrile) electrolyte. Since MoS2 and WS2 are hydrodesulfurization (HDS) catalysts for sulfur removal from hydrocarbons, they immediately react with and desulfurize the electrolytes from which they are deposited, yielding amorphous deposits that contain excess sulfur. However, the desired stoichiometry MX2 can be recovered by high temperature annealing, which removes excess sulfur and forms moderately crystallized and partially oriented thin films, as shown by x-ray diffraction (XRD) studies. This two-step process for deposition of MoS2 and WS2 thin films of controlled thickness facilitates scientific studies of the nature of pseudocapacitance in these materials. Most reports of MoS2 and WS2 pseudocapacitance study nanocomposites with carbon in order to maximize the specific capacitance, but fundamental electrochemical studies are challenging due to the complex topography and surface heterogeneity. The specific capacitance of MoS2 and WS2 thin films with a thickness ranging from 10 nm to 5μm were studied by cyclic voltammetry and galvanostatic charge discharge in several electrolytes, including 1.0 M aqueous Na2SO4, 1.0 M aqueous LiClO4, and 1.0 M LiClO4 in acetonitrile. For both MoS2 and WS2 thin films, the specific capacitance of stoichiometric films obtained by high temperature annealing is 1-2 orders of magnitude higher than the as-deposited amorphous films. The maximum specific capacitance of 1980 F/g is obtained for 10 nm thick WS2 films in 1.0 M aqueous LiClO4, although this is of course arises from the small mass in the denominator. The capacitance per unit area for WS2 films in 1.0 M aqueous Na2SO4 increases with increasing film thickness, and then reaches a plateau value at a thickness of 30-50 nm. This suggests that the active region thickness for charge storage in WS2 is 30-50 nm, which is close to values previously reported for charge storage in RuO2. Since the cation radius of Li+ (0.90 nm) is smaller than that of Na+ (1.16 nm), higher capacitance per unit area is obtained in LiClO4- than in Na2SO4-containing electrolytes. In addition, the capacitance per unit area in LiClO4-containing electrolytes does not reach a maximum plateau value until a much higher WS2 film thickness. Cathodic electrodeposition of Cu-doped MoS2 atop glassy carbon can also be accomplished from an aqueous electrolyte containing 10.0 mM (NH4)2MoS4, 5.0 mM CuSO4, 0.1 M KCl, and 0.50 M KSCN at pH 6.95. Without the presence of thiocyanate (SCN-) as a complexing agent, the cathodic potentials for electrodeposition of Cu and MoS2 are separated by ~700 mV. However, the addition of SCN- shifts the Cu deposition potential in the cathodic direction towards the deposition potential of MoS2. In addition, a cathodic shoulder is observed during cyclic voltammetry experiments that may indicate an induced co-deposition effect. Electrodeposition at the potential of this cathodic shoulder yields MoS2 thin films that contain 1-4 atom% Cu, as determined by energy dispersive x-ray spectroscopy (EDX) and Rutherford backscattering (RBS). As observed for un-doped MoS2 thin films, high temperature annealing is required to obtain moderately recrystallized and partially oriented films. Four-point probe measurements demonstrate that Cu doping reduces the MoS2 resistivity by 10.3x. The capacitance obtained by cyclic voltammetry and galvanostatic charge discharge for Cu-doped MoS2 thin films is 2.5-3.5x higher than that obtained for MoS2 films of the same thickness.

Threshold Current Density of Al0.1Ga0.9N/GaN Triple Quantum Well Laser

Semiconductor laser is used in processing many issues related to the scientific, military, medical, industrial and agricultural fields due to its unique properties such as coherence and high strength where GaN-based components are the most efficient in this field. Current technological developments mention to the strong connection of GaN with sustainable electronic and optoelectronic devices which have high-efficiency. The threshold current density of Al0.1Ga0.9N/GaN triple quantum well laser structure was investigated to determine best values of the parameters affecting the threshold current density that are well width, average thickness of active region, cavity length, reflectivity of cavity mirrors and optical confinement factor. The optimum value of the threshold current density is 2670 A/cm2 was obtained when the well width (w= 2.5 nm), reflectivity of cavity mirrors (R1=0.75, R2=0.9), cavity length (L=2mm), average thickness of active region (d= 11.5 nm), and optical confinement factor ( Γ= 0.034) at room temperature.