Δ-doped Layer Research Articles

Plasma Excitations in SiGe/Si Quantum Wells

Plasma and magnetoplasma excitations in high-quality undoped two-dimensional electron systems based on SiGe/Si quantum wells are studied in detail. A two-dimensional electron system is formed by applying a vo-ltage to the top gate, which is partially transparent to subterahertz radiation in the frequency range of 20‒160GHz. The results for SiGe/Si quantum wells with a Sb δ-doping layer are also presented for comparison. The transport and quantum scattering times for both structures are directly determined. It has been found that the effective electron mass is almost independent of the two-dimensional electron density in a wide density range.

Gate-readout and a 3D rectification effect for giant responsivity enhancement of asymmetric dual-grating-gate plasmonic terahertz detectors

Abstract We experimentally investigated the asymmetric dual-grating-gate plasmonic terahertz (THz) detector based on an InGaAs-channel high-electron-mobility transistor (HEMT) in the gate-readout configuration. Throughout the THz pulse detection measurement on the fabricated device, we discovered a new detection mechanism called the “3D rectification effect” at the positive gate bias application, which is a cooperative effect of the plasmonic nonlinearities in the channel with the diode nonlinearity in the heterobarrier between the InGaAs channel layer and the InAlAs spacer/carrier-supply/barrier layers, resulting in a giant enhancement of the detector responsivity. We also found that an undesired long-tail waveform observed on the temporal pulse photoresponse of the device is due to trapping of carriers to the donor levels in the silicon δ-doped carrier-supply layer when they tunnel through the barrier to the gate and can be eliminated completely by introducing the so-called inverted-HEMT structure. The internal current responsivity and noise-equivalent power are estimated to be 0.49 A/W (with the equivalent voltage responsivity of 4.9 kV/W with a high output impedance of 10 kΩ) and 196 pW/√Hz at 0.8 THz. These results pave the way towards the application of the plasmonic THz detectors to beyond-5G THz wireless communication systems.

Electronic structure of boron and aluminum δ-doped layers in silicon

Recent work on atomic-precision dopant incorporation technologies has led to the creation of both boron and aluminum δ-doped layers in silicon with densities above the solid solubility limit. We use density functional theory to predict the band structure and effective mass values of such δ layers, first modeling them as ordered supercells. Structural relaxation is found to have a significant impact on the impurity band energies and effective masses of the boron layers, but not the aluminum layers. However, disorder in the δ layers is found to lead to a significant flattening of the bands in both cases. We calculate the local density of states and doping potential for these δ-doped layers, demonstrating that their influence is highly localized with spatial extents at most 4 nm. We conclude that acceptor δ-doped layers exhibit different electronic structure features dependent on both the dopant atom and spatial ordering. This suggests prospects for controlling the electronic properties of these layers if the local details of the incorporation chemistry can be fine-tuned.

Plasma Excitations in SiGe/Si Quantum Wells

Plasma and magnetoplasma excitations in high-quality undoped two-dimensional electron systems based on SiGe/Si quantum wells are studied in detail. A two-dimensional electron system is formed by applying a vo-ltage to the top gate, which is partially transparent to subterahertz radiation in the frequency range of 20‒160 GHz. The results for SiGe/Si quantum wells with a Sb δ-doping layer are also presented for comparison. The transport and quantum scattering times for both structures are directly determined. It has been found that the effective electron mass is almost independent of the two-dimensional electron density in a wide density range.

Notch-δ-doped InP Gunn diodes for low-THz band applications

The viability of the indium phosphide (InP) Gunn diode as a source for low-THz band applications is analyzed based on a notch-δ-doped structure using the Monte Carlo modeling. The presence of the δ-doped layer could enhance the current harmonic amplitude (A0) and the fundamental operating frequency (f0) of the InP Gunn diode beyond 300 ​GHz as compared with the conventional notch-doped structure for a 600-nm length device. With its superior electron transport properties, the notch-δ-doped InP Gunn diodes outperform the corresponding gallium arsenide (GaAs) diodes with up to 1.35 times higher in f0 and 2.4 times larger in A0 under DC biases. An optimized InP notch-δ-doped structure is estimated to be capable of generating 0.32-W radio-frequency (RF) power at 361 ​GHz. The Monte Carlo simulations predict a reduction of 44% in RF power, when the device temperature is increased from 300 ​K to 500 ​K; however, its operating frequency lies at 280 ​GHz which is within the low-THz band. This shows that the notch-δ-doped InP Gunn diode is a highly promising signal source for low-THz sensors, which are in a high demand in the autonomous vehicle industry.

Analysis of notch-δ-doped GaAs-based Gunn diodes

The performances of GaAs-based Gunn diodes with notch-δ-doped structures are studied in this work. The δ-doped effect is analysed using Monte Carlo modelling in terms of temporal evolution of current density, electric field profile, electron energy, mean velocity and occupancy in the Γ and higher valleys. The presence of a δ-doped layer after the notch causes a significant increase in the harmonic current amplitude of the device, where the growth of the high field domain can be attributed to a slow electron track due to the well-known Gunn effect and an additional fast electron track which appears over a short time window when the domain is reaching the anode. An optimised GaAs notch-δ-doped structure with a 700 nm device length including a 100 nm notch and a 5 nm δ-doped layer can generate signals at a fundamental frequency of 262 GHz with a current harmonic amplitude of 29.4 × 107 A m−2, which is almost twice of that without a δ-doped layer. Its second and third harmonic signals are found to be substantial, reaching into the terahertz range of 512 GHz and 769 GHz.

Atomically Thin Delta-Doping of Self-Assembled Molecular Monolayers by Flash Lamp Annealing for Si-Based Deep UV Photodiodes.

Delta doping (δ-doping) can find a wide range of applications in advanced metal oxide semiconductor field effect transistors, deep UV photodetectors, quantum devices, and others. In this work, we formed a δ-doping layer in silicon by employing flash lamp annealing to treat the PCl3 monolayers grafted on silicon surfaces. The δ-doping layer is atomically thin (<1 nm). Low-temperature Hall measurements show that the δ-doping layer is in a metallic state and exhibits a weak localization phenomenon, implying that a two-dimensional electron gas is formed. When we form such an n-type δ-doping layer on a highly doped p-type Si substrate, a highly sensitive solar-blind UV photodetector is created, which traditionally was only possible by using wide band gap semiconductors such as gallium nitride (GaN) or silicon carbide (SiC).

Activation of two dopants, Bi and Er in δ-doped layer in Si crystal

Conventional doping processes are no longer viable for realizing extreme structures, such as a δ-doped layer with multiple elements, such as the heavy Bi, within the silicon crystal. Here, we demonstrate the formation of (Bi + Er)-δ-doped layer based on surface nanostructures, i.e. Bi nanolines, as the dopant source by molecular beam epitaxy. The concentration of both Er and Bi dopants is controlled by adjusting the amount of deposited Er atoms, the growth temperature during Si capping and surfactant techniques. Subsequent post-annealing processing is essential in this doping technique to obtain activated dopants in the δ-doped layer. Electric transport measurement and photoluminescence study revealed that both Bi and Er dopants were activated after post-annealing at moderate temperature.

Thermal Stability at the Interface between Ferromagnetic Alloys and Germanium for Semiconductor Spintronics Devices

Spin MOSFET has drawn much attention as an attractive candidate for future semiconductor devices because of its nonvolatility and reconfigurability [1]. For realization of the spin MOSFET, highly efficient spin injection from ferromagnets to semiconductors with low parasitic resistance is one of the most important key technologies. Recently, we demonstrated room-temperature spin transport in germanium (Ge) based lateral spin valve devices with Schottky-tunnel contact, where the interface resistance at the ferromagnet/Ge is much smaller than that with conventional insulating tunnel barriers [2, 3]. However, when the thermal annealing was conducted to fabricate gate-stack structures for the spin MOSFET, the spin injection/detection efficiency was degraded due to the formation of reaction layers at the ferromagnetic alloys/Ge interface [4]. In this study, we explore the enhancement in the thermal stability of the ferromagnet/Ge interface and demonstrate room-temperature spin signals even for the lateral spin device annealed at 250 oC by an insertion of five Fe atomic layers between the ferromagnetic alloys and Ge.We fabricated lateral spin valve devices with Co2FeAl0.5Si0.5(CFAS)/Fe (5 atomic-layers)/Ge Schottky tunnel junction [Fig. 1(a)]. A 70-nm-thick phosphorus (P) doped Ge layer was grown on Ge/Si(111) virtual substrate at 350 oC as a spin transport layer by molecular beam epitaxy (MBE). On the P doped Ge layer, five Fe atomic layers and an epitaxial CFAS layer were formed using low temperature MBE. Beneath the ferromagnetic contacts, the P δ-doping with a thin Si layer was inserted to promote the tunnel conduction. The CFAS/Fe and δ-doped Ge layers were patterned into the contacts with 0.4 × 5.0 μm2 and 1.0 × 5.0 μm2, where the edge-to-edge distances between the CFAS/n+-Ge contacts are designed to be 0.45 μm. As a reference sample, a device without the Fe atomic layers was also fabricated. The devices with and without the Fe atomic layers were annealed at 250 oC in N2 atmosphere.Figures 1(b) and (c) show the nonlocal spin signals at 77 K for devices annealed at 250 oC without and with an Fe insertion, respectively, together with schematics of the CFAS/Ge and CFAS/Fe/Ge interfaces after the annealing. For the device without an Fe insertion [Fig. 1(b)], the spin signal is significantly degraded (~97 %) compared with the as prepared device, where the Ge out-diffusion causes the degradation of the spin injection/detection efficiency [4]. On the other hand, even after the annealing, clear hysteretic curves related to the magnetization states of the ferromagnetic injector and detector contacts can be observed for the device with an insertion of Fe atomic layers [Fig. 1(c)]. Although there is ~50 % decrease of the spin signal by the annealing, the magnitude of nonlocal spin signals is ~ 140 mΩ due to the significant improvement of the spin injection efficiency [5] and of the thermal stability of the CFAS/Fe/Ge interface. As shown in schematics of Figs 1(b) and 1(c), the inserted Fe atomic layers can suppress the Ge out-diffusion into CFAS and maintain the highly efficient spin injection. For the device with an Fe atomic insertion annealed at 250 oC, a spin signal of ~7 mΩ can be obtained even at room temperature.This work was partly supported by Grants-in-Aid for Scientific Research (A) (No. 16H02333) and (S) (No. 17H06120 and 19H05616) from the Japan Society for the Promotion of Science (JSPS). M.Y. acknowledges JSPS Research Fellowships for Young Scientists (No.18J00502).[1] S. Sugahara and M. Tanaka, Appl. Phys. Lett. 84, 2307 (2004).[2] M. Yamada et al., Appl. Phys. Express 10, 093001 (2017).[3] M. Tsukahara et al., Appl. Phys. Express 12, 033002 (2019).[4] B. Kuerbanjiang et al., Phys. Rev. B 98, 115304 (2018).[5] M. Yamada et al., (submitted). Figure 1

Temperature-dependent charge-carrier transport between Si-\u03b4-doped layers and AlGaAs/InGaAs/AlGaAs quantum well with various space layer thicknesses measured by Hall-effect analysis

Temperature (T = 40 ~ 300 K) dependence of Hall-effect analysis on the dual Si-δ-doped AlGaAs/InGaAs/AlGaAs quantum-well (QW) structures with various space layer thicknesses (tS = 5, 10 and 15 nm) was performed. An interesting hysteresis behavior of electron sheet concentration [n2D(T)] was observed for tS = 10 and 15 nm but not for tS = 5 nm. A model involving two different activation barriers encountered respectively by electrons in the active QW and by electrons in the δ-doped layers is proposed to account for the hysteresis behavior. However, for small enough tS (= 5 nm ≤ 2.5 s, where s = 2.0 nm is the standard deviation of the Gaussian fit to the Si-δ-doped profile), the distribution of Si dopants near active QW acted as a specific form of “modulation doping” and can not be regarded as an ideal δ-doping. These Si dopants nearby the active QW effectively increase the magnitude of n2D, and hence no hysteresis curve was observed. Finally, effects from tS on the T-dependence of electron mobility in active QW channel are also discussed.

Quantum Dots-Sensitized High Electron Mobility Transistor (HEMT) for Room-Temperature Gas Detection

Introduction Nowadays there is an increasing demand for low power and low-cost gas sensors for a wide range of applications. With the advantages of miniaturization, low power consumption and good compatibility with the standard CMOS circuit, gas sensing field effect transistors (FETs) are the most attractive approaches.Since the first Pd gate field effect transistor gas sensor came into our insight [1], researchers have done a lot of work to materialize the device potential, starting with the choice of the sensing materials and sensor structure [2]. Due to the proximity of two-dimensional electron gas (2DEG) to the sensing surface and high sheet carrier concentration in the order of 1012~1013 cm−2, high electron mobility transistors (HEMTs) are extremely sensitive to the surface charge and variation in the environmental conditions, thereby enabling the sensing of the gases at low concentrations.In this paper, we demonstrate ppm level NO2 detection using AlGaAs/GaAs HEMT-based gas sensor with controlling gate and a PbS quantum dots sensitization layer. Experiment The HEMT structures in this paper were grown by molecular beam epitaxy (MBE) on GaAs substrate [3]. It was composed of an undoped 300 nm thick GaAs buffer layer, 15 nm thick In0.2Ga0.8As channel layer, 4 nm thick Al0.3Ga0.7As spacer layer, Si δ-doping layer, 25 nm thick Al0.3Ga0.7As barrier layer, and 30 nm thick GaAs cap layer. The sheet electron concentration of the HEMT was 2.3 × 1012 cm−2 with a mobility of 5.0 × 103 cm2V−1s−1 at 300 K.The sensing materials, PbS CQDs, were prepared by wet chemical synthesis technique according to previously paper [4], and the PbS quantum dots sensitization layer was formed as a sensing receptor on the exposed Au gate electrode using an electrohydrodynamic jet printer (HWK Co. Ltd., China).The CQDs capped with abundant oleic acid and oleylamine have stable dispersibility, a certain viscosity in the solvent and excellent solution processability, which makes it readily capable of being integrated with the electrohydrodynamic jet printing process at room temperature. The principle is to apply a high voltage power supply between the conductive nozzle (first electrode) and the conductive substrate (second electrode) and use a strong electric field force between nozzle and the substrate to pull the liquid out of the nozzle to form a droplet.Gas sensing characteristics were measured in a vacuum chamber probe station. The gas mixed in the mixing chamber flowed through the inlet of the chamber and out through the outlet. The flow rates of the gases were controlled by mass flow controllers (MFCs). Gas pressure and humidity in the probe station chamber remained constant for stable gas measurement. A series of electrical measurements of the fabricated sensors were carried out using the probe station and a semiconductor parameter analyzer (B1500A, Agilent Technologies). And the sensor response was defined as R=|Ig-I0|/I0, the I0 and Ig is the source to drain current in air and target gas atmosphere, respectively. Results and Conclusions The HEMT-type sensor showed high responsibility and excellent repeatability towards ppm level NO2. In this paper, extremely low concentration of the NO2 (0~25 ppm) was introduced, which shows a high response of 36.1% towards 25 ppm NO2 at room temperature, much higher than reported results [5-7], which even operated at a high temperature over 300 oC.In summary, through gate sensitization, the HEMT shows high sensitivity for ppm level NO2 detection at room temperature, the sensing performance can be both attributed to the receptor function of PbS quantum dots and the transducer function of HEMT. And the results of this paper will pave our future way for multiple target gas detection with in one chip.

Sharp Drop in the Mobility of Holes with a Decrease in Their Two-Dimensional Concentration by an External Voltage in Boron δ-Doped Diamond Layers

It is shown, both analytically and numerically, that the hole mobility in boron δ-doped (i.e., with a thickness on the order of several lattice constants) diamond layers drops with a decrease in their two-dimensional concentration during depletion of the δ-doped layer by an external voltage. This drop is most pronounced for the maximum initial two-dimensional hole concentrations ~ 3 × 1013 cm–2 (limited from above by the condition for the possibility of their significant reduction without the electrical breakdown of diamond). This is explained by a decrease in the degree of screening of scattering Coulomb potentials of ionized boron atoms and an increase in the scattering efficiency of degenerate holes from these atoms due to a drop in the kinetic energy of holes. The corresponding calculations of the transport scattering cross section of holes are performed beyond the Born approximation (i.e., perturbation theory), which is inapplicable for boron δ-doped diamond layers. The predicted effect can be used to increase the efficiency of source-to-drain current modulation by a gate voltage in diamond field-effect transistors (FETs) with δ-doped conduction channels.

Optical absorption coefficient in n-type double δ-doped layers GaAs quantum wells

The superposition principle is one of the cornerstones of physics. In low-dimensional systems, it is routinely used to model the potential profile. That is the case of coupled [Formula: see text]-doped quantum wells, for which, several works have studied the transport and optoelectronic properties. However, the Poisson equation determines the potential profile is not linear, and the superposition principle is not at all valid. The aim of this work is to correct some of the inconsistencies of the mentioned models for coupled [Formula: see text]-doped quantum wells. In the framework of Thomas–Fermi approximation, we calculated the potential profile, the wave functions, the energy values and the relative absorption coefficient for the double system compared to an isolated delta system in terms of impurity density and distance between [Formula: see text]-wells. We found a red shifting in the absorption coefficient when the interlayer distances increase, in addition, an enhancement in the absorption coefficient is detected for a specific separation distance. Our results agree with ab-initio calculations reported for the electronic structure.

Confined polaronic transport in (LaFeO3)n/(SrFeO3)1 superlattices

Functional oxide superlattices offer new and exciting possibilities for the exploration of emergent properties at the nanoscale. While the behavior of La1−xSrxFeO3 films has been extensively investigated at low temperatures, few studies have been carried out at high temperatures, particularly for LaFeO3/SrFeO3 superlattice systems. Here, we investigate the transport behavior and optical properties of (LaFeO3)n/(SrFeO3)1 superlattices at 373 K and above. Using optical spectroscopy, we observe a low energy excitation at ∼1 eV, attributable to charge transfer between the O 2p and Fe 3d states of the δ-doped single SrFeO3 layer. From in-plane conductivity measurements on the superlattices, we determine activation energies that are much lower than those of alloyed samples and vary with the total number of SrFeO3 layers. This suggests that polaronic transport is confined near the SrFeO3 regions, permitting mobilities significantly enhanced over those in alloyed thin films.

Electrostatic and radio frequency performance investigation of δ-doped In0.53Ga0.47As/InP stepped poly gate metal oxide semiconductor field effect transistor

A δ-doped stepped poly gate heterostructure metal oxide semiconductor field effect transistor (HMOSFET) has been analysed. The insertion of a δ-doped layer within the channel region of the proposed In0.53Ga0.47As/InP HMOSFET results in the reduction of short channel effects and the junction capacitance. In the proposed device, the split gate with dissimilar oxide thickness from the source end to drain end have p+ poly in first and third region whereas, n+ poly is in the second region. Further, the δ-doped layer within the channel in stepped poly gate HMOSFET significantly enhances the electrical characteristics of the device. Simulation results reveal that in comparison with stepped poly gate and conventional HMOSFET, δ-doped stepped poly gate HMOSFET gives improved electron transport phenomena and analog characteristics in terms of electron mobility, electric field, electron velocity, transconductance, On resistance, transconductance generation factor and switching ratio. Further, reduction in junction capacitance improves the radio frequency performance metrics in terms of cut-off frequency, maximum frequency and gain bandwidth product.

High Channel Conductivity, Breakdown Field Strength, and Low Current Collapse in AlGaN/GaN/Si &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$\delta$ &lt;/tex-math&gt; &lt;/inline-formula&gt;-Doped AlGaN/GaN:C HEMTs

This paper reports the AlGaN/GaN/ Si δ-doped AlGaN/GaN:C HEMT device on silicon with high channel conductivity, high breakdown field (E-field) strength, and low current collapse by using the Si-doped AlGaN back barriers. The Si δ-doped AlGaN back barrier was used to compensate for the reduction of channel conductivity as a result of a carbon-doped semiinsulating GaN buffer layer. The maximum drain current increases from 412 to 720 mA/mm, and peak extrinsic transconductance is improved from 103 to 210 mS/mm. Due to the reduction of electric field between the gate and drain along the GaN channel by inserting the Si δ-doped AlGaN back barrier layer, it can effectively suppress the capture of electrons in channel by carbon-induced accepted traps in the GaN:C buffer. Combined with the high conductivity of Si δ-doped AlGaN back barrier and high resistance of GaN:C buffer, the device showed the high breakdown E-field strength and the low specific on-resistance. Our proposed device is observed to hold a gate-drain voltage of 769 Vat 10 μA/mm (7-μm gate-drain spacing) and 0.53 mQ · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and the gateto-drain electric field corresponds to 1.1 MV/cm.

Photoluminescence characterization of nonradiative recombination centers in MOVPE grown GaAs:N δ-doped superlattice structure

Nonradiative recombination (NRR) centers in GaAs:N δ-doped superlattice (SL) structure grown by metal organic vapor phase epitaxy (MOVPE) are studied by two-wavelength-excited photoluminescence (PL) method. The PL intensity quenching due to the superposition of below-gap excitation (BGE) light at energies of 0.75, 0.80, 0.92, and 0.95 eV and an above-gap excitation (AGE) light either for the GaAs conduction band (1.69 eV) or E− band excitation (1.45 eV), respectively indicates the existence of NRR centers inside the GaAs:N δ-doped SL and GaAs layers. The AGE density and BGE density dependence of the PL intensity have been studied for both excitation schemes. Recombination model based on two levels has been proposed to interpret the experimental results and energy distribution of NRR centers inside the GaAs:N δ-doped SLs and GaAs layers. The defect-related parameters are estimated systematically by solving the rate equations and fitting the results with the experimental data that give a phenomenological insight into the sample.

Fe delta-doped (In,Fe)Sb ferromagnetic semiconductor thin films for magnetic-field sensors with ultrahigh Hall sensitivity

We have grown Fe δ-doped (In,Fe)Sb thin films by low-temperature molecular beam epitaxy for magnetic-field sensors with ultrahigh Hall sensitivity. Magnetic circular dichroism spectroscopy measurements indicate intrinsic ferromagnetism in the Fe δ-doped (In,Fe)Sb layers. Although the Fe δ-doped (In,Fe)Sb thin films have lower Curie temperature than the Fe uniformly-doped (ln,Fe)Sb with the same average Fe concentration, their anomalous Hall effect is much stronger. The Hall sensitivity of the Fe δ-doped (In1−〈x〉,Fe〈x〉)Sb sample with an average Fe concentration 〈x〉 = 17% reaches 1522 Ω/T at room temperature, which is much higher than those of Fe uniformly-doped (In,Fe)Sb, (Ga,Fe)Sb, and commercial InSb Hall sensors.

The nature of transport and ferromagnetic properties of the GaAs structures with the Mn δ-doped layer

We reveal the nature of transport and ferromagnetic properties of the GaAs structure with the single Mn {\delta}-doped layer. To modify the properties of the structure the electrically active radiation defects are created by irradiation with helium ions. The investigations show that the transport properties of the structure are determined by two parallel conduction channels (the channel associated with hole transport in the valence band and the channel associated with electron transport in the Mn impurity band) and that the ferromagnetic properties are determined by the electrons localized at the allowed states within the Mn impurity band. The results also help to understand the features of structures with the Mn {\delta}-layer nearby the quantum well.

Polarization engineered N-polar Cs-free GaN photocathodes

We report on holistic and systemic approach of development of Cs-free GaN photocathode structures which utilize polarization band engineering in order to allow for air stable operation and eliminate the need for cesium-based surface treatments. Physics-based simulation of band structure and Monte Carlo simulation of electron transport and emission were used to guide experimental development of photocathode structures. By using an N-polar device, the polarization charge allows for the creation of large surface band bending without the need for δ-doped capping layers. The insertion of a thin AlN interlayer allows for the creation of a quasi-band offset and additional beneficial polarization charge to create a desirable band profile. Samples of both polarities were grown and subjected to chemical surface treatments in order to account for differences in native oxide formation on Ga- and N-polar surfaces. Measured photoemission spectra show quantum efficiencies as high as 23% for a HCl-treated Cs-free N-polar photocathode, which is comparable to cesiated devices.