Delta-doped Structure Research Articles

Investigation on Effect of Doped InP Subchannel Thickness and Delta-Doped InP Layer of Composite Channel HEMT

DC performances of a depletion-mode 100-nm In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As/InP composite channel InP-based high-electron-mobility transistor (HEMT) is investigated by using numerical simulation. The transfer, output, and breakdown characteristics of different doped InP subchannel thickness In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As/InP composite channel HEMT are compared. Decreasing the thickness of the doped InP subchannel from 10 to 5 nm results in a decline of the output current. However, the threshold voltage is increased from −0.77 to −0.6 V and the breakdown voltage is improved from 1.99 to 5.69 V at bias of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{{\mathrm {gs}}}-{V}_{{\mathrm {t}}} = -0.5$ </tex-math></inline-formula> V. To enhance the breakdown characteristic of In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.53</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.47</sub> As/InP composite channel HEMT, we minimize the thickness of the doped InP layer and replace the doped InP layer with the delta-doped layer. The structure is compared with the conventional double delta-doped structure in the performance of transfer, output, and breakdown characteristics. At ON-state, the electrons of two kinds of structure concentrate in the InGaAs channel. The output current and the threshold voltage of the composite channel structure are at the same level compared with the conventional double delta-doped structure. However, at OFF-state, because of the transfer of electrons into the InP subchannel, part of electric potential will not be concentrated at the drain side of the gate region but distributed in other area of the channel. A higher drain voltage is required to achieve electric field strength enough to breakdown and the breakdown voltage is improved obviously.

Realization of pressure induced emission enhancement for rare earth luminescent materials: Adopting delta-doped structure

Design and enhancing the luminescence of rare earth materials under pressure have been important topics in the past few years. Here we report a new strategy for realizing pressure induced emission enhancement of rare earth element thulium (Tm) by confining the dopant luminescentions in a two-dimensional hexagonal NaYF4 planes, NaYF4@β-NaYF4:Yb0.2,Tm0.005@NaYF4 delta-doped structure. Upon compression, NaYF4@β-NaYF4:Yb0.2,Tm0.005@NaYF4 exhibits a 2 fold increase in emissions, obviously higher than that of corresponding homogeneous doped samples. When Tm3+ was overdoped, this pressure induced emissions enhancement would absent in the whole pressure region due to concentration quenching. Moreover, both NaYF4@β-NaYF4:Yb0.2,Tm0.005@NaYF4 and corresponding homogeneous doped samples undergo structure transition under pressure. Raman study and calculations show that the delta-doping induced phonon peak red shift and reduced defects density increasing rate could play major roles for the pressure-induced emission enhancement of NaYF4@β-NaYF4:Yb0.2,Tm0.005@NaYF4, and the deformation of the Y-F9 polyhedrons at vertices leading to the structure transitions of both delta and homogeneous doped samples. These findings suggest that the pressure induced emission enhancement of rare earth materials can be realized by passivating inner and surface defects using a delta-doped structure.

Influence of Silicon-Doping in n-AlGaN Layer on the Optical and Electrical Performance of Deep Ultraviolet Light-Emitting Diodes

The influence of two different methods of silicon doping in AlGaN layer, that is, modulation-doping (MD) and delta-doping (DD), on the optical and electrical performance of deep ultraviolet light-emitting diodes (DUV-LEDs) has been investigated. Both the photoluminescence and electroluminescence intensities in the Si-DD structure are stronger than those obtained by the Si-MD method, while the forward voltage and reverse leakage current are slightly smaller in the DD structure than that in the MD structure. Compared with the MD structure, the DD structure shows higher capacitance-voltage characteristics. This study suggests that the DD method can improve the optical and electrical performance of DUV-LEDs.

Delta-doped SrTiO3 top-gated field effect transistor

Oxide heterostructures are an attractive platform for incorporation in field-effect transistors (FETs) due to their diverse physical properties which can be tuned by electrostatic gating. We report a top-gated FET based on a SrTiO3 delta-doped structure, which operates down to cryogenic temperatures. The device shows excellent DC characteristics with an on/off ratio greater than 104 and field effect mobility estimated to be 2125 cm2/V s at 2 K. The high field effect mobility was consistent with the Hall mobility and is attributed to the formation of a two-dimensional electron system in the delta-doped layer: two-dimensional gate-tunable Shubnikov-de Haas oscillations confirm this. The achievement of an electron density of 3 × 1012 cm−2 in a gate-tunable geometry allows for the exploration of the interplay between magnetic, ferroelectric, and superconducting properties of SrTiO3 in the quantum limit.

Delta-doped quantum wire tunnel junction for highly concentrated solar cells

We propose a novel structure for tunnel junction based on delta-doped AlGaAs/GaAs quantum wires. Higher spatial confinement of quantum wires alongside the increased effective doping concentration in the delta-doped regions extremely increase the peak tunneling current and enhance the performance of tunnel junction. The proposed structure can be used as tunnel junction in the multijunction solar cells under the highest possible thermodynamically limited solar concentration. The combination of the quantum wire with the delta-doped structure can be of benefit to the solar cells’ advantages including higher number of sub-bands and high degeneracy. Simulation results show a voltage drop of 40 mV due to the proposed tunnel junction used in a multijunction solar cell which presents an extremely low resistance to the achieved peak tunneling current.

(Invited) Toward GaN-Based Red LEDs: The Influence of Local and Extended Defect Environments on the Optical and Material Properties of GaN:Eu

The doping of Europium (Eu) into gallium nitride (GaN) has a promising future for solid state lighting applications.1 It has been shown that the Eu ions incorporate themselves into the GaN host in a variety of different defect environments.2 Locally, the defect environments around the Eu ions play a large role in the optical and magnetic properties of the ions themselves. Two of the main centers have been associated with the common vacancy defects in GaN, the gallium vacancy (VGa) and the nitrogen vacancy (VN).1 Once this association has been made, it becomes necessary to consider the charge state of the local vacancy defect. It now appears that one Eu center is actually a charged derivative of another center that was associated with a VGa. The two centers exhibit a temperature dependent meta-stability. Furthermore, the effective g-factor of the 7F2 multiplet for the “charged” center is far above the theoretical expectation for an isolated Eu ion,3 and more than two times larger than that of the other Eu centers. This is thought to be related to the delocalization of a captured carrier over the Eu-VGa complex. On the macroscale, the overall concentration of oxygen, which can be detrimental to device performance, was shown to influence the incorporation of the Eu ions.4 The oxygen was introduced into the samples due its presence in the standard Eu precursor Eu(DPM)3; therefore, a new liquid Eu source, which does not contain oxygen in its molecular structure, was developed. It was discovered that the removal of oxygen led to the broadening of the photoluminescence emission spectra, and the appearance of Eu-N surface precipitation. This was due to poor incorporation of the Eu ions onto Ga sites in the GaN, which was confirmed by Rutherford backscattering/channeling measurements. By intentionally reintroducing oxygen during growth, this issue was resolved. However, this issue can be subverted altogether by using a delta-doping growth structure, with alternating layers of GaN and GaN:Eu. The mechanism for this will be discussed. Moreover, it was recently found that the incorporation of Eu into the GaN matrix can drastically influence the material properties, even at dilute levels (< 1%). By using the delta-doping structure and lowering the growth temperature, the surface morphology, as well as the size and concentration of threading dislocations can be controlled. This control had a large influence on the optical and electrical properties of LEDs grown using these conditions. Not only was the output power per Eu layer thickness significantly enhanced, but an injection current of 20 mA was achieved under an applied voltage of <5V, which is the lowest value reported for this material.5 Lastly, the brightest LED had an output power of ~375µW, and an external quantum of efficiency of ~4.6%, which are the highest values reported for this system. [1] Y. Fujiwara and V. Dierolf, Jpn. J. Appl. Phys. 53, 05FA13 (2014). [2] N. Woodward et al., Opt. Mater. 33, 1050 (2011). [3] N. Woodward, et al., MRS Proceedings, 1342, mrss11-1342-v05-06 (2010). [4] B. Mitchell et al., Scientific Reports 6,18808 (2016). [5] A. Nishikawa et al., Appl. Phys. Lett. 97, 051113 (2010).

Normally off AlGaN/GaN HEMT on Si substrate with selectively dry-etched recessed gate and polarization-charge-compensation δ-doped GaN cap layer

We demonstrate a recessed-gate normally off AlGaN/GaN high-electron-mobility transistor (HEMT) on a silicon substrate that provides a precisely controllable threshold voltage (Vth). To ensure Vth uniformity, dry-etching of GaN with high etching selectivity between GaN and AlGaN is developed. Furthermore, to introduce selective dry-etching in the HEMT fabrication process, we propose a delta-doped GaN cap structure that enables negative polarization charges between the GaN cap and the AlGaN barrier to be compensated. Combining these two technologies, we fabricate recessed-gate normally off metal–insulator–semiconductor HEMTs with a subthreshold slope of 130 mV/dec and an on–off drain current ratio exceeding 107.

Fabrication and characterization of tensile In0.3Al0.7As barrier and compressive In0.7Ga0.3As channel pHEMTs having extremely low gate leakage for low-noise applications

This study focuses on the area of the epitaxial design, fabrication and characterization of a 1 µm gate-length InP-based pseudomorphic high electron mobility transistor (pHEMT) using InGaAs–InAlAs material systems. The advanced epitaxial layer design incorporates a highly strained aluminum-rich Schottky contact barrier, an indium-rich channel and a double delta-doped structure, which significantly improves upon the conventional low-noise pHEMT which suffers from high gate current leakage and low breakdown voltage. The outstanding achievements of the new design approach are 99% less gate current leakage and a 73% increase in breakdown voltage, compared with the conventional design. Furthermore, no degradation in RF performance is observed in terms of the cut-off frequency in this new highly tensile strained design. The remarkable performance of this advanced pHEMT design facilitates the implementation of outstanding low-noise devices.

Magnetism in Cr doped ZnO: Density-functional theory studies

We investigate the magnetism in wurtzite and zinc-blende Zn 0.875Cr 0.125O (wz- and zb-Zn 0.875Cr 0.125O) using the density-functional theory calculations. We demonstrate that the delta-doping structures are more energetically favorable than the homogeneous-doping ones. The delta-doping structure of zb-Zn 0.875Cr 0.125O shows robust ground state ferromagnetism (FM) with ferromagnetic stabilization energy of 575 meV/Cr–Cr pair, while delta-doping structure of wz-Zn 0.875Cr 0.125O shows weak ground state anti-FM. We discuss the magnetic coupling in Zn 1− x Cr x O, and the origin of FM in zb-Zn 1− x Cr x O using a crystal field model. Finally, we anticipate the potential spintronics applications of the zb-Zn 1− x Cr x O.

Magnetism in Cr-doped ZnS: density-functional theory studies

We investigated the magnetism and aggregation trends in cubic Zn1−xCrxS using density-functional theory calculations. We demonstrate that all studied configurations show ground state half-metallic ferromagnetism (HMF), and Cr impurities are energetically favourable for planar clustering into delta-doping structures. The single-layer delta-doping structures of Zn0.75Cr0.25S and Zn0.875Cr0.125S show ferromagnetic stabilization energies (ΔEAF) of 0.551 eV and 0.561 eV per Cr–Cr pair, respectively. The half-layer delta-doping structure of Zn0.875Cr0.125S and the double-layer delta-doping structure of Zn0.75Cr0.25S show ΔEAF of 0.394 eV and 0.166 eV per Cr–Cr pair, respectively. Furthermore, our studies indicate that the cubic ZnS/CrS heterostructure, one extreme situation of the delta-doping structure, also shows ground state HMF. The origin of HMF is discussed using a simple crystal field model. Finally, we anticipate the potential spintronics application of Zn1−xCrxS.

Wide frequency band detection on laterally constricted GaAs structures having nanometric heavily doped layers

Experimental results of planar asymmetrically and symmetrically shaped hot carrier microwave detectors having five delta-doped GaAs layers are presented. The results are compared with the detectors having the same geometrical shape but only homogeneously doped GaAs layer of submicrometric thickness. DC measurements showed that the structure with delta-doped layers had slightly lower resistance than the detector with homogeneously doped layer due to higher carrier mobility in the delta-doped structure. Increase in carrier mobility corresponded to higher voltage sensitivity, which is essential parameter of a microwave detector. Voltage power characteristics of the detectors of both types were linear up to 10 mW of microwave power, whereas at higher powers they became super-linear or sub-linear with the following change in polarity of the detected voltage. Distinctive dependence of the detected voltage on microwave power at strong electric field is attributed to intervalley transfer of charge carriers. Experimental results of voltage sensitivity in microwave frequency range showed that drop-off of sensitivity going from Ka range to W range for both types of detectors is not larger than 3 dB, suggesting their use for wide frequency detection.

An analytical model for discretized doped InAlAs/InGaAs heterojunction HEMT for higher cut-off frequency and reliability

In the proposed work the model has been formulated for discretized doped HEMT, where the conventional uniformly doped, pulsed doped and delta doped structure are the special cases. An expression for sheet carrier density has been formulated considering the effect of doping-thickness product and has been extended to calculate drain current, transconductance, capacitance and cut-off frequency of the device. The model also takes into account the non-linear relationship between sheet carrier density and quasi Fermi energy level to validate it from subthreshold region to high conduction region. The results so obtained have been compared with pulsed doped structure to validate the model. The analysis concentrates on the distance of doping from the heterojunction and gate electrode. Different design criteria have been given to dope the carriers (amount and distance) in different regions to optimize the performance for higher sheet carrier density/parallel conduction voltage/effective parallel conduction voltage (Vc−Voff) to increase the transconductance, cut-off frequency and reliability of the device.

Effect of thin emitter set-back layer on GaAs delta-doped emitter bipolar junction transistor

GaAs delta-doped emitter bipolar junction transistors (δ-BJT) with different emitter set-back layer thicknesses of 10to50nm were fabricated to study the emitter set-back layer thickness effect on device dc performance. We found that the current gain decreases following decrease in the emitter set-back layer thickness. A detailed analysis was performed to explain this phenomenon, which is believed to be caused by reduction of the effective barrier height in the δ-BJT. This is due to change in the electric-field distribution in the delta-doped structure caused by the built-in potential of the base-emitter (B-E) junction. Considering the recombination and barrier height reduction effects, the thickness of the emitter set-back layer should be designed according to the B-E junction depletion width with a tolerance of ±5nm. The dc performance of a δ-BJT designed based on this criteria is compared to that of a Al0.25Ga0.75As∕GaAs heterojunction bipolar transistor (HBT). Both devices employed base doping of 2×1019cm−3 and base-to-emitter doping ratio of 40. Large emitter area (AE≈1.6×10−5cm−2) and small emitter area (AE≈1.35×10−6cm−2) device current gains of 40 and 20, respectively, were obtained in both types of transistors passivated by (NH4)2S treatment. The measured current gain of the GaAs δ-BJT is the highest reported for a homojunction device with such high base-to-emitter doping ratio normally used in HBT devices.

Diamond power devices. Concepts and limits

The prospects and limits of diamond power devices incorporating boron as dopant are discussed using the results of a theoretical/empirical analysis coupled to 2D-numerical simulation. To obtain full activation of the boron acceptor at room temperature, the concept of delta-doping is introduced and used as basic element. Two device concepts were chosen for this analysis: (i) RF power FET incorporating a monolayer boron delta-doped channel and gate recess and gate filed plate structure; and (ii) various types of vertical power diodes including a novel stacked delta-doped structure, which is proposed here for the first time. The analysis was performed considering the limiting material parameters known for single crystal diamond including recent data on the carrier mobility, the phenomena related to thermal activation of boron acceptor and carrier scattering along and across delta-doped layers, thermal and electrical breakdown limits. The results of this study reveal that the optimised delta-FET with the field plate of 1 μm may be capable to produce approximately 34 W/mm continues-wave RF power at 20 °C operation temperature. The projected RF power of a similar FET structure with the field plate of 2 μm is about 75 W/mm, which may be beyond the thermal limit. The proposed stacked delta-doped diode could have the best on-resistance to blocking voltage ratio than any other type of diamond and n-type SiC-based diodes up to 20 kV blocking voltage. Below 3 kV the projected characteristics of the stacked delta diode on diamond are also favourable in comparison to those of an ideal n-type GaN diode. These examples demonstrate that boron delta-doped layer is a very promising configuration for various diamond-based power electronic devices.

Nitrogen delta doping in 6H silicon carbide layers

Nitrogen delta-doped silicon carbide (SiC) layers were grown by a pulse doping method in a chemical vapor deposition. Doping distribution with high peak concentration (1×1018 cm−3) and narrow distribution width (12 nm) was fabricated in the nitrogen delta-doped structure of SiC. Mobility enhancement due to spatial separation of electrons and their ionized parent donors was observed for the delta-doped structure. Metal-semiconductor field-effect transistors with a nitrogen delta-doped channel and a recess gate structure were fabricated. The devices had large source-drain (200 V) and source-gate (140 V) breakdown voltages, high drain current capability and easy control of the threshold voltage with a good pinch-off characteristics.

Low-energy grazing-angle argon-ion irradiation of silicon: A viable option for cleaning?

In recent publications, it has been suggested that atomically clean, flat, crystalline silicon surfaces can be obtained by low-energy (0.1–1 keV) oblique-angle (⩾45° off-normal) argon-ion bombardment at mildly elevated target temperatures (∼500 °C). Here, this procedure has been applied to a multiple boron delta-doped Si structure. It leads to a massive relocation of subsurface doping atoms because of the accompanying injection of point defects into the bulk. This greatly affects the usefulness of the proposed cleaning method and shows that it is hazardous to base claims of quality solely on results obtained with surface-sensitive (∼1 nm) analytical techniques.

Power PHEMT with compact device layout forlow voltage CDMA application

A high efficiency low voltage operation dual delta-doped AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor (PHEMT) for low voltage code division multiple access (CDMA) application has been developed. When tested at 2.4 V and 1.9 GHz under IS-95 CDMA modulation, the 20.16 mm PHEMT device was found to have a linear output power of 28 dBm with a power added efficiency of 30.2%. The device also has a saturation power of 30.0 dBm with a power added efficiency of 61.5%. The high efficiency and linearity of the PHEMT at low bias voltage is attributed to the use of the dual delta-doped PHEMT structure and to the reduction of the size of the device layout. The device is suitable for low voltage CDMA applications.

Electronic Properties of Nitrogen Delta-Doped Silicon Carbide Layers

ABSTRACTNitrogen delta-doped silicon carbide (SiC) layers were grown by a new pulse doping method in a chemical vapor deposition. Doping distribution with high peak concentration (1.×1018 cm−3) and narrow distribution width (12 nm) was fabricated in the nitrogen delta-doped structure of SiC. Mobility enhancement due to spatial separation of electrons and their ionized parent donors was observed for the delta-doped structure. Metal-semiconductor field-effect transistors with a nitrogen delta-doped channel and a recess gate structure were fabricated. The devices had large source-drain breakdown voltages, high drain current capability and easy control of the threshold voltage with a good pinch-off characteristics.

High-performance InGaP/InxGa/sub 1-x/As HEMT with an inverted delta-doped V-shaped channel structure

This letter reports a new and high-performance InGaP/In/sub x/Ga/sub 1-x/As high electron mobility transistor (HEMT) with an inverted delta-doped V-shaped channel. Due to the presence of V-shaped inverted delta-doped InGaP/In/sub x/Ga/sub 1-x/As structure, good carrier confinement and a flat and wide transconductance operation regime are expected. Experimentally, the fabricated device (1×100 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) shows a high gate-to-drain breakdown voltage of 30 V and a high output drain saturation current density of 826 mA/mm at V/sub GS/=2.5 V. The high transconductance expands over a very broad operation range with the maximum value of 201 mS/mm at 300 K. Meanwhile, the studied device exhibits a good microwave frequency linearity.

Extremely Narrow Sb Delta-Doped Epitaxial Layer Characterized by X-Ray Reflectivity

An Sb delta doping layer in silicon is grown at the temperature of 300°C by silicon molecular beam epitaxy and characterized by the small angle x-ray reflectivity measurement using synchrotron radiation beam. The oscillations of the reflectivity caused by dopant Sb at Q as large as 14.5 are detected. Simulation of this curve as a whole shows that the total amount of dopant Sb is 0.15 monolayer and is restricted to two atomic layers. An extremely narrow Sb delta doping structure without Sb segregation is thus obtained at the growth temperature of 300°C as verified by the experiment.