Active Region Of Device Research Articles

Measuring Electrical Resistivity at the Nanoscale in Phase-Change Materials.

Electrical resistivity is the key parameter in the active regions of many current nanoscale devices, from memristors to resistive random-access memory and phase-change memories. The local resistivity of the materials is engineered on the nanoscale to fit the performance requirements. Phase-change memories, for example, rely on materials whose electrical resistance increases dramatically with a change from a crystalline to an amorphous phase. Electrical characterization methods have been developed to measure the response of individual devices, but they cannot map the local resistance across the active area. Here, we propose a method based on operando electron holography to determine the local resistance within working devices. Upon switching the device, we show that electrical resistance is inhomogeneous on the scale of only a few nanometers.

OUTER CYLINDRICAL SPACES OF A HOLLOW CYLINDER OF FINITE LENGTH

Introduction. The solution of the Laplace equation by the method of separation of Fourier variables for cylindrical domains makes it possible to study electromechanical devices in 3D format.
 
 The purpose of the study is to obtain analytical expressions for magnetic potentials and inductions of external three-dimensional spaces of electromechanical devices in a cylindrical coordinate system.
 
 Materials and methods. The unknown constants in the expressions for the regions under consideration are found from the boundary conditions of the magnetic field at the common boundaries of the electromechanical device with its outer space: the scalar magnetic potentials and magnetic inductions are the same, the current sheets experience a jump. When obtaining analytical expressions, the methods of mathematical physics were used.
 
 Research results. To find the Fourier constants, the outer three-dimensional spaces of classical electromechanical devices (generators, motors) are considered, which in a cylindrical coordinate system are represented by both hollow and solid cylinders joining the active regions of the devices with their cylindrical or end surfaces. General expressions for magnetic potentials and inductions of external spaces are obtained based on the solution of the Laplace equation as the Sturm–Liouville problem by the Fourier method of separation of variables. The magnetic fields of the outer spaces of the original hollow cylinder are analyzed: an outer cylinder with an infinitely large radius; internal solid cylinder; end cylinders of finite length. These data make it possible to increase the required number of equations to find the required number of Fourier constants in the analytical calculation of electromechanical devices by the method of separation of variables.
 
 Findings. Analytical expressions for the boundary values of magnetic potentials and inductions of the external space adjacent to the active regions of electromechanical devices with air gaps of both radial and axial form are obtained.

NiO junction termination extension for high-voltage (>3 kV) Ga2O3 devices

Edge termination is the enabling building block of power devices to exploit the high breakdown field of wide bandgap (WBG) and ultra-wide bandgap (UWBG) semiconductors. This work presents a heterogeneous junction termination extension (JTE) based on p-type nickel oxide (NiO) for gallium oxide (Ga2O3) devices. Distinct from prior JTEs usually made by implantation or etch, this NiO JTE is deposited on the surface of Ga2O3 by magnetron sputtering. The JTE consists of multiple NiO layers with various lengths to allow for a graded decrease in effective charge density away from the device active region. Moreover, this surface JTE has broad design window and process latitude, and its efficiency is drift-layer agnostic. The physics of this NiO JTE is validated by experimental applications into NiO/Ga2O3 p–n diodes fabricated on two Ga2O3 wafers with different doping concentrations. The JTE enables a breakdown voltage over 3.2 kV and a consistent parallel-plate junction field of 4.2 MV/cm in both devices, rendering a power figure of merit of 2.5–2.7 GW/cm2. These results show the great promise of the deposited JTE as a flexible, near ideal edge termination for WBG and UWBG devices, particularly those lacking high-quality homojunctions.

Mid-infrared rainbow light-emitting diodes

We demonstrate a room-temperature all-epitaxial guided-mode resonance light-emitting diode operating in the mid-wave infrared. The device comprises a dielectric waveguide with an AlGaAsSb p−i−n diode core, below a layer of grating-patterned GaSb and above a highly doped, and thus, low index, InAsSb layer. Light emitted from the device active region into propagating modes in the waveguide scatters into free space via the GaSb grating, giving rise to spectrally narrow features that shift with emission angle across much of the mid-wave infrared. For collection angles approaching 0°, we are able to obtain linewidths of ∼2.4 meV across the spectral/angular emission of the LED, corresponding to λ/Δλ∼570. Fine control of emission wavelength can be achieved by tuning the applied current, which causes a redshift of approximately 20 nm due to the thermo-optic effect. The presented device has the potential for use in compact, high bandwidth, and low-cost mid-wave infrared sensing applications requiring spectral discrimination.

Characterization of epitaxial layers grown on 4H-SiC (0 0 0 −1) substrates

SiC is an excellent semiconductor material that can be used in power semiconductor devices. It has the advantages of high thermal conductivity, stable chemical properties, and high critical breakdown field strength. The active region of a SiC device is usually in the SiC epitaxial layer, so the growth of high-quality SiC epitaxial layers is extremely valuable. In this paper, we investigate the epitaxial growth of SiC on n-type 4° off-axis SiC (0 0 0–1) substrates. We used a horizontal hot-wall chemical vapor deposition apparatus for rapid epitaxy on SiC substrates, using C2H4, and SiHCl3 as precursors and H2 as carrier gas, respectively, and growing at 1350 °C,1400 °C,1450 °C,1500 °C, 1550 °C, 1600 °C for half an hour to get epitaxial layer. The effect of growth temperature on the surface morphology of SiC (0 0 0 –1) epitaxial layer was analyzed and epitaxial layers is characterized by optical microscope, scanning electron microscope, atomic force microscope and Raman scattering instrument, and the growth rate of epitaxial layer was obtained. It is found that the higher growth temperature is helpful to obtain the epitaxial layer with smooth surface and higher content of 4H-SiC under certain pressure, C/Si molar ratio, and following rate of precursor and carrier gas.

(Invited) Enhanced Efficiency of AlInN Nanowire Ultraviolet Light-Emitting Diodes Using Photonic Crystal Structures

High efficiency ultraviolet (UV) light-emitting diodes (LEDs) are in high demand for a wide range of applications [1-3]. Due to its wide bandgap, covering from deep UV to near visible, AlGaN semiconductor has been intensively studied for high efficiency UV LEDs. However, the performance of AlGaN UV LEDs is still limited, especially in the deep UV wavelength region, due to large dislocation density and extremely inefficient p-type doping in AlGaN film. Therefore, AlGaN UV LEDs have been suffering from low internal quantum efficiency (IQE), poor light extraction efficiency (LEE), and low external quantum efficiency (EQE). Alternatively, AlInN alloy is relatively unexplored for light-emitters even though it holds great potential application in UV and visible light-emitting devices. Recent studies have reported that InAlN has a large optical gain for deep UV LEDs. In this context, we have successfully developed AlInN nanowire LEDs with strong emission in the UV wavelength regime. The AlInN nanowires achieved a high IQE of >50% and significantly strong Transverse Magnetic (TM) polarized emission at 299nm.The AlInN nanowire LED structures were spontaneously grown on n-Si (111) substrates under nitrogen rich conditions by a Veeco GEN II molecular beam epitaxy (MBE) system. The AlInN nanowire was grown on GaN nanowire template. The device active region consists of ~ 100 nm n-Al0.78In0.22N, 40 nm undoped Al0.75In0.25N well and 100 nm p-Al0.78In0.22N layers. The GaN nanowire segments were grown at ~ 800 °C, nitrogen flow rate of 0.5-1.0 sccm, and forward plasma power of ~ 400 W. However, the growth temperature of the active region was at ~ 670 ̶ 700 °C to increase the In incorporation in AlInN segments and the nitrogen flow rate was kept at 2.5 sccm. The MBE grown AlInN under nitrogen rich condition offers an effective approach to eliminate the composition inhomogeneity in the AlInN. The nanowires are vertically aligned to the Si substrate with quite uniform dimension, as illustrated in Figure 1(a). Figure 1(b) presents the structure of a single AlInN nanowire LED. The photoluminescence (PL) spectra of AlInN nanowires were measured using a 266 nm diode-pumped solid-state laser as the excitation source. The peak emissions vary from 280nm to 365nm by varying the Al/In composition in the AlInN layers. The peak emission is shifted to shorter wavelength when the substrate temperature gets increase which is attributed to the increased In adatom desorption at higher growth temperature, resulted in the reduced In composition in the AlInN segment. The AlInN/GaN nanowire exhibits relatively high IQE which is estimated of ~ 52% at room temperature [3, 4]. Illustrated in Figure 1(c), the UV nanowire LED holds strong emission spectra at ~299 nm with a negligible shift in the peak wavelength, attributing to the high crystalline quality and reduced quantum-confined Stark effect in the AlInN nanowires. The LED has a low turn-on voltage of ~5V and low reverse leakage current less, i.e., ~0.5 µA at -8.6V. Moreover, as shown in Figure 1(d), we have demonstrated that the LEE of the AlInN nanowire LEDs could reach ~ 63% using hexagonal photonic crystal nanowire structures which is significantly higher compared to that of the random nanowire arrays [5]. This study paves the way for the development of high-power ultraviolet light-emitters using AlInN nanowire arrays.

Si epitaxy on Cl-Si(100)

Current atomically-precise fabrication methods in Si utilize a scanning tunneling microscope (STM) to pattern a monatomic resist adsorbed on Si(100). Recent interest in the use of Cl-based resists has necessitated further investigations on the impact of the tightly bound Cl to the overall device fabrication process. In particular, impacts to the crystallinity of the Si capping layer used to encapsulate the active device region can have dramatic effects on the functionality of atomic-scale quantum devices. Here, the epitaxial growth of Si films on Cl-Si(100) was studied to identify and characterize the Si thin film growth process. Adatoms diffusion on the surface was simulated based on the experimental results of the number density and distribution of Si adatom chains after <0.10 ML of Si deposition at temperatures ranging from 300–850 K. A diffusion barrier within 0.66–0.88 eV for Si adatoms on Cl-Si(100) was found. SIMS depth profiling of ∼25 nm thick Si films verified that Cl segregated towards the growth surface during Si deposition and was effectively removed below SIMS detection limits from both the buried interface and Si film at 850 K. TEM imaging confirmed good film crystallinity was achieved at temperatures as low as 600 K.

Characterization of AlGaN/GaN high electron mobility transistors on GaN substrates with different thicknesses of GaN channel and buffer layers using side-gate modulation

AlGaN/GaN high electron mobility transistors (HEMTs) on GaN substrates with different thicknesses of GaN channel and C-doped buffer layers were fabricated and characterized with conventional DC and side-gate (SG) measurements. In SG measurement, drain current (I D) was measured while SG bias (V SG) was applied through a separate SG contact that surrounds the device active region. Whereas all HEMTs have comparable DC measurement results (∼500 mA mm−1 I D, −2 V threshold voltage and ∼130 mS mm−1 transconductance), SG measurements show drastically different performances among the samples. Comparing HEMTs with and without C-doped buffer layer, results demonstrate that HEMT with doped buffer was stable against SG modulation until −15 V V SG, whereas the HEMT without doped buffer was modulated near 0 V, and hence unstable against SG bias. Comparing HEMTs different channel thicknesses, the HEMT with a thicker 900 nm channel was more resistant to SG modulation than the HEMT with a thinner 100 nm channel. Therefore, these results highlight the importance of buffer doping and channel thickness to buffer stability.

(Invited) Understanding the Electroluminescence Signature of High-Voltage Vertical GaN Pin Diodes with Different Edge Termination Designs

GaN has superior properties, such as a wide band gap and high critical electric field that make it promising material for future power semiconductor devices. Edge termination for GaN power devices remains a subject of significant interest. In this work, different edge termination designs were systematically evaluated and compared by utilizing broadband electroluminescence technique under various reverse bias conditions. Edge termination is a crucial step in power devices fabrication process, using nitrogen implant is a common step in GaN PiN diode. In this work, Nitrogen ion implantation was used to form junction termination extension (JTE) and Guard Rings (GR). A full 2” wafer was fabricated in a foundry process to build a high current 1.2kV class GaN vertical diodes on non-homogenous GaN substrate. In silicon power devices, a smooth tapered implanted doping profiles are often achieved through a controlled diffusion of the implanted dopant island during activation. Activation and diffusion of acceptor dopants via ion implantation is neither well controlled nor understood in GaN. In this work, based JTE terminations, GR termination and a combination of both were compared. We are comparing several devices on the same wafer type and same fabrication process but with different edge termination designs; and spatially resolving their spectrum, by utilizing the electroluminescence mapping technique to acquire a series of broadband images under high voltage reverse bias. There is a clear difference in the electroluminescence signature. This is the first time we show the impact of edge termination on the evolution of these defects and the transport path of the leakage current. The leakage current, and hence the electroluminescence signature is changing as well with the termination design. All the termination and guard ring designs were based on Nitrogen implant. The nitrogen implant doses and energy are chosen to create enough damage in the p-type region. Stress testing is in-progress to evaluate the impact of these defects on failure mechanisms. JTE only design show a leakage current as a bright ring on the outer perimeter of the device active region closer to trench isolation while a GR only design brings the leakage current near the anode. A combination of both termination designs results in scattered leakage points on the outer perimeter of the device. Simulation results of the filed profiles of the different structures and hyperspectral EL and Raman spectroscopy are in progress along with temperature stressing to understand the nitrogen diffusion mechanisms in GaN. This study will have an impact on the future of GaN power devices and mature its process to compete with SiC technology. Figure 1

Recombination and Charge Collection at Nickel Silicide Precipitates in Silicon Studied by Electron Beam‐Induced Current

Nickel disilicide () has detrimental effects on many devices such as transistors or solar cells if it comes as a precipitate in the active device region. As thin‐film electrical contacts, however, it is frequently incorporated into devices due to its low resistivity and the almost‐perfect lattice match with silicon, allowing for epitaxial growth. Herein, electron beam‐induced current, EBIC, is used to study excess carrier recombination and charge collection at platelets with diameters of several 10 μm and thicknesses of only a few (111) silicide lattice planes. To arrive at such structures, a special preparation protocol is developed consisting of a two‐step cooling procedure and electrical contacting using in situ EBIC/focused ion beam (FIB) on precipitates structurally characterized by transmission electron microscopy. The experimental results provide evidence that the atomically thin silicide layers embedded in silicon have properties similar to bulk in terms of the Schottky barrier height and electric conductivity.

In-silico Investigation of Cyl. Gate all Around (GAA) Tunnel Field Effect Transistor (TFET) Biosensor

For some time now, the advancement of low power and high sensitivity biosensors has been the center of attention for in-situ detection and monitoring which form an integral part of portable health monitoring systems. This paper elucidates the design-optimization of a cylindrical (cyl.) gate all around (GAA) tunnel field effect transistor (TFET) biosensor with retrograde doping using numerical modeling. The device consists of n+ heavily doped SiGe substrate and two insulated gates i.e. primary gate (PrG) and biasing gate (BG) with suitable work functions. Sensitivity of the biosensor is investigated by varying dielectric constant (k) and charge density (ρ) in the active region of device. TFET biosensor design and simulation is performed using TCAD Synopsys software. Computations are carried out for various conditions of dielectric constant (k) and charge density (ρ) for analyzing the sensor sensitivity. Simulation results show that for k=10 and ρ = 3.0x1012cm−2, there is low leakage current (IOFF) = 1.0x10−16A/µm, and high ON current (ION) = 1.0x10−6 A/µm. Results obtained in this work as useful as it will act as a design guideline for developing TFET biosensors for various biological applications.

Formation of a gate dielectric of nanometer thickness by rapid thermal treatment

Investigations of the thickness and optical characteristics of thin SiO2 films obtained by one-, two-, or three-stage rapid thermal processing (RTP) at atmospheric pressure, pulses of 6, 12, and 20 s duration have been carried out. To obtain thin SiO2 films by the RTP method, N-type:Ph 4.5 Оhm/□ (100) silicon wafers were used as initial samples. The samples were preliminarily oxidized at 1000 °C of the obtained wet oxygen (SiO2 d = 100 nm), then the silicon oxide was completely removed in a solution of hydrofluoric acid, after which the wafers were subjected to chemical cleaning using the Radio Corporation of America (RCA) technology. Oxidation in a stationary oxygen atmosphere was carried out in one or two stages by heating the plates with a light pulse of different power up to maximum temperatures of 1035 – 1250 °C, as well as a three-stage process, where the final stage was annealing in a nitrogen atmosphere or in a forming gas (N2 97% + H2 3%). The characteristics of SiO2-Si barrier structures nitrided in N2, obtained by the RTP process by light fluxes with pulses of a second duration, were studied to improve the electrophysical parameters of gate oxides by the RTP method. It is of interest for integrated circuits (ICS) with a high density of the active regions of devices.

Electric field mapping of wide-bandgap semiconductor devices at a submicrometre resolution

Electric fields drive the degradation of wide-bandgap semiconductor devices. However, directly mapping the electric field inside an active device region remains challenging. Here we show that electric-field-induced second harmonic generation can be used to map the electric field in the device channel of GaN-based high-electron-mobility transistors at submicrometre resolution. To illustrate the capabilities of the approach, we use it to examine the impact of carbon impurities in the epitaxial buffer layer of a device. Carbon is a p dopant in GaN, and small changes in its concentration can dramatically change the bulk Fermi level, sometimes resulting in a floating buffer that is ‘short-circuited’ to the device channel via dislocations. Our measurements show that very different electric field distributions can occur in devices with different carbon concentrations, despite them having similar device terminal characteristics. We also show that dislocation-related leakage paths can lead to inhomogeneity in the electric field. Electric-field-induced second harmonic generation can be used to measure the in-plane electric field in gallium nitride high-electron-mobility transistors and probe how dopants influence the electric field distribution.

Characterization of Otto Chips by Particle Swarm Optimization

Abstract Recently a surface plasmon resonance (SPR) optical sensor, based on the Otto configuration — the Otto chip — has been developed. One essential step in the quality control of the fabrication process is characterization of the active region of several devices in a batch. Characterization is done by measuring the angular spectrum of the optical re ectance on several points across the active region of the device, and determining parameters by regression analysis of the data. Traditional gradient methods used in the regression process are extremely dependent on an initial guess and are not very efficient for batch analysis of curves, when those include poorly defined SPR spectra, where an initial guess may be hard to infer. An alternative approach for the regression problem is to model the analysis as an optimization problem and using an efficient stochastic algorithm. In this paper one discusses the use of Particle Swarm Optimization (PSO) for characterization of Otto chip devices. From comparative studies carried out in an existing Otto chip, it is observed that PSO can be a very efficient approach for batch analysis and yields better results when compared with the traditional gradient-based regression method.

Energy band engineering toward hardened electronics in ionizing radiation environments via quantum gettering

Ionizing radiation has the potential to cause operational disruptions and destroy microelectronic devices. This paper introduces and demonstrates a method of hardening microelectronic devices for sustained use in applications where exposure to ionizing radiation exists. By incorporating quantum structures below active regions of devices, gettering of charges created by ionizing radiation becomes possible. The gettering of electrons and holes forces recombination of carriers, thus eliminating photocurrent surges and trap filling which would otherwise disrupt device operation. Experimental results discussed here show a reduction in photocurrent of over two orders of magnitude when utilizing energy band engineering to create quantum structures for charge gettering. In this work, a nitride-based high electron mobility two-dimensional electron gas demonstrates the method. However, the theory utilized pertains not only to nitride-based devices, but transfers to other materials as well.

(Invited) Defect and Impurity Characterization in Wide Bandgap Materials and Interfaces: Impact on Thermal Transport

Wide bandgap materials are suitable for high power and high temperature applications. However, these materials, and metal layers that are part of device structures must effectively transport heat from the active device regions. Structural defects, chemical defects, and interface properties can degrade thermal transport, but relatively little attention has been paid to the precise nature of these defects and how they impact thermal transport. Electron microscopy and x-ray scattering based techniques are employed to provide insight into the defects present in these materials. When these techniques are integrated with thermal transport measurements and theoretical insights gained from molecular dynamics simulations, a much better understanding of thermal transport in wide bandgap materials can be realized. Examples of how defects impact transport for diamond boundary interfaces, AlN layers, and GaN heterostructures will be presented.

Ultraviolet Light-Emitting Diodes Using Aluminium Indium Nitride Nanowire Structures

Ultraviolet (UV) light-emitting diodes (LEDs) have various applications in the field of biochemistry, medicine, water/air purification, etc. To achieve efficient UV LEDs, researchers have essentially focused on the use of AlGaN alloys in the active region. However, the practical applications of AlGaN based UV LEDs are severely limited due to poor external quantum efficiency (EQE). Some reasons behind the low EQE in AlGaN based UV LEDs are efficiency droop, mostly due to electron leakage into the p-region, poor light extraction efficiency (LEE) which is mainly due to the total internal reflection and the absorption of UV light in the p-GaN contact layer. Identifying and developing the potential of alternative UV materials will be critical to make further progress in the development of deep UV emitters. In this regard, AlxIn1-xN alloy has not been widely studied even though it holds great potential applications in UV and visible light-emitting devices [1]. In this study, we have performed the epitaxial growth, electrical and optical properties of spontaneously grown AlInN nanowires on Si (111) substrate by molecular beam epitaxy (MBE) and demonstrated a high crystalline quality of AlInN nanowire LED structures with stable and strong EL emission at ~299 nm peak wavelength.The AlInN nanowire LED structure, shown in Fig. 1(a), was spontaneously grown on n-Si (111) substrates under nitrogen rich conditions by a Veeco GEN II MBE system equipped with a RF plasma-assisted nitrogen source. The growth conditions for GaN nanowire segments include a growth temperature of ~ 770 °C, nitrogen flow rate of 1 sccm, and forward plasma power of ~ 400 W. The device active region consists of ~ 100 nm n-Al0.78In0.22N, 40 nm undoped Al0.75In0.25N well and 100 nm p-Al0.78In0.22N layers. Growth temperature of the active region was at ~ 670 ̶ 700 °C to increase the In incorporation in AlInN segments and the nitrogen flow rate was kept at 2.5 sccm, while the plasma power was fixed at 400 W. After being grown by MBE, the nanowire LED samples were fabricated using standard lithography method as reported elsewhere [2]. The device characterization was performed on LED with an area size of 500×500 µm2. Photoluminescence (PL) spectrum of the grown nanowire LEDs exhibits strong emission in the UV wavelength range as shown in Fig. 1(b). The peak emission at ~ 299 nm and ~ 280 nm is responsible for the emission from the AlInN quantum well and barriers, respectively. The AlInN nanowire UV LEDs have strong EL intensity with the peak wavelength at ~ 299 nm, as shown in Fig. 1(c). This work is being developed toward the achievement of high power AlInN nanowire UV LEDs, and the result will be presented. Reference R. T. Velpula, B. Jain, M. R. Philip et al., Sci. Rep., 10(1) 1-10 (2020).R. T. Velpula, B. Jain, H. Q. T. Bui et al., Optical Materials Express, 10(2) 472-483 (2020) Figure 1

Near-infrared photoresponse in Ge/Si quantum dots enhanced by localized surface plasmons supported by aluminum nanodisks

An array of plasmonic nanoparticles can sustain surface plasmon modes from visible to infrared spectral range and thus offers effective surface light trapping, enhancement of local fields, and interaction with the thin active regions of optical devices. We report the fabrication and optical characterization of a planar Ge/Si quantum dot (QD) detector grown on silicon-on-insulator (SOI) substrate for photodetection in the near-infrared telecommunication wavelength range. The multilayer Ge/Si QD heterostructures are near-field coupled to the adjacent layers of aluminum nanodisks on the detector top. The periodic Al disk arrays have the square lattice symmetry with a lattice constant of 400 nm and the disk diameter varying from 150 to 225 nm. A significant enhancement in the room-temperature detector sensitivity is achieved due to the excitation of localized surface plasmons supported by the metallic disks and radiative coupling to the SOI waveguide modes. Through extinction spectroscopy and numerical modeling, we confirm the emergence of nanoparticle-induced plasmon resonances near the Si–Al interface. We demonstrate that an appropriate choice of the array periodicity and the size of the metal disks is able to increase the photodetector's efficiency by ∼ 40× at λ=1.2μm and by 15× at λ≈1.55μm relative to a bare detector with no plasmonic structure. These outcomes pave the way toward the use of Al as a low-cost plasmonic material with potential applications in infrared photodetection similar to those of the noble metals.

Noise and charge discreteness as ultimate limit for the THz operation of ultra-small electronic devices

To manufacture faster electron devices, the industry has entered into the nanoscale dimensions and Terahertz (THz) working frequencies. The discrete nature of the few electrons present simultaneously in the active region of ultra-small devices generate unavoidable fluctuations of the current at THz frequencies. The consequences of this noise remain unnoticed in the scientific community because its accurate understanding requires dealing with consecutive multi-time quantum measurements. Here, a modeling of the quantum measurement of the current at THz frequencies is introduced in terms of quantum (Bohmian) trajectories. With this new understanding, we develop an analytic model for THz noise as a function of the electron transit time and the sampling integration time, which finally determine the maximum device working frequency for digital applications. The model is confirmed by either semi-classical or full- quantum time-dependent Monte Carlo simulations. All these results show that intrinsic THz noise increases unlimitedly when the volume of the active region decreases. All attempts to minimize the low signal-to-noise ratio of these ultra-small devices to get effective THz working frequencies are incompatible with the basic elements of the scaling strategy. One can develop THz electron devices, but they cannot have ultra-small dimensions. Or, one can fabricate ultra-small electron devices, but they cannot be used for THz working frequencies.

Dual-Wavelength Y-Branch DBR Lasers With 100 mW of CW Power Near 2 μm

The interband GaSb-based diode lasers emitting simultaneously in two narrow bands separated by either ~1.6 or ~3.3 THz were designed, fabricated and characterized. The device active region contained one asymmetric tunnel-coupled double quantum well with separation between two lowest electron subbands controlled by thickness of the tunnel barrier. The Y-branch 6th order distributed Bragg reflector devices have been fabricated with either deep or shallow etched ridge waveguides. The increase of the deeply etched ridge waveguide width from 10 to $20~\mu \text{m}$ improved laser threshold and efficiency thanks to reduction of the relative role of the sidewall defect recombination. Further improvement of the device performance parameter was achieved by shallow etching. The shallow etched lasers with stable dual-wavelength emission spectrum generated 100 mW of continuous wave output power at 20 °C.