Doped Contact Layers Research Articles

Continuous-wave GaAs/AlGaAs quantum cascade laser at 5.7 THz.

Design strategies for improving terahertz (THz) quantum cascade lasers (QCLs) in the 5-6 THz range are investigated numerically and experimentally, with the goal of overcoming the degradation in performance that occurs as the laser frequency approaches the Reststrahlen band. Two designs aimed at 5.4 THz were selected: one optimized for lower power dissipation and one optimized for better temperature performance. The active regions exhibited broadband gain, with the strongest modes lasing in the 5.3-5.6 THz range, but with other various modes observed ranging from 4.76 to 6.03 THz. Pulsed and continuous-wave (cw) operation is observed up to temperatures of 117 K and 68 K, respectively. In cw mode, the ridge laser has modes up to 5.71 THz - the highest reported frequency for a THz QCL in cw mode. The waveguide loss associated with the doped contact layers and metallization is identified as a critical limitation to performance above 5 THz.

Characteristics of Resonant Tunneling in Nanostructures with Spacer Layers

The effect of spacer layers on electron transport through two-barrier nanostructures was studied using the numerical solution of the time-dependent Schrodinger–Poisson equations with exact discrete open boundary conditions. The formulation of the problem took into account both the active region consisting of a quantum well and barriers, as well as the presence of highly doped contact layers and spacer layers. The use of the time formulation of the problem avoids the divergence of the numerical solution, which is usually observed when solving a stationary system of the Schrodinger–Poisson equations at small sizes of spacer layers. It is shown that an increase in the thickness of the emitter spacer leads to a decrease in the peak current through the resonant tunneling nanostructures. This is due to the charge accumulation effects, which, in particular, lead to a change in the potential in an additional quantum well formed in the emitter spacer region when a constant electric field is applied. The valley current also decreases as the thickness of the emitter spacer increases. The peak current and valley current are weakly dependent on the thickness of the collector spacer. The collector spacer thickness has a strong effect on the applied peak and valley voltages. The above features are valid for all three different resonant tunneling nanostructures considered in this study. For the RTD structures based on Al0.3Ga0.7As/GaAs, the optimized peak current value Ipmax = 5.6 × 109 A/m2 and the corresponding applied voltage Vp = 0.44 V. For the RTD structures based on AlAs/In0.8Ga0.2As, Ipmax = 14.5 × 109 A/m2 (Vp = 0.54 V); for RTD structures based on AlAs/In0.53Ga0.47As, Ipmax = 45.5 × 109 A/m2 (Vp = 1.75 V).

Microcavity enhanced Stark optical modulator for long-wave infrared

A voltage-tunable microcavity modulator is designed, fabricated and characterized. The device active area comprises a stack of type II metamorphic quantum wells (QW) sandwiched between doped contact layers that also serve as optical mirrors. The device operates in the spectral region of 8–16 μm. The optical transmission of the cavity can be modulated in a range of ∼2% at a wavelength of 9.5 μm by application of a bias voltage with an amplitude of 4 V. The sign of the transmission change depends on the voltage polarity as expected from the first order Stark effect in the polar type-II QWs. This results in a two times higher transmission modulation amplitude at a given voltage compared with type I quantum wells and bulk materials.

Fighting Broken Symmetry with Doping: Toward Polar Resonant Tunneling Diodes with Symmetric Characteristics

The recent demonstration of resonant tunneling transport in nitride semiconductors has led to an invigorated effort to harness this quantum transport regime for practical applications. In polar semiconductors, however, the interplay between fixed polarization charges and mobile free carriers leads to asymmetric transport characteristics. Here, we investigate the possibility of using degenerately doped contact layers to screen the built-in polarization fields and recover symmetric resonant injection. Thanks to a high doping density, negative differential conductance is observed under both bias polarities of GaN/AlN resonant tunneling diodes (RTDs). Moreover, our analytical model reveals a lower bound for the minimum resonant-tunneling voltage achieved via uniform doping, owing to the dopant solubility limit. Charge storage dynamics is also studied by impedance measurements, showing that at close-to-equilibrium conditions, polar RTDs behave effectively as parallel-plate capacitors. These mechanisms are completely reproduced by our analytical model, providing a theoretical framework useful in the design and analysis of polar resonant-tunneling devices.

Long-Term Stability of Oxide Nanowire Sensors via Heavily Doped Oxide Contact.

Long-term stability of a chemical sensor is an essential quality for long-term collection of data related to exhaled breath, environmental air, and other sources in the Internet of things (IoT) era. Although an oxide nanowire sensor has shown great potential as a chemical sensor, the long-term stability of sensitivity has not been realized yet due to electrical degradation under harsh sensing conditions. Here, we report a rational concept to accomplish long-term electrical stability of metal oxide nanowire sensors via introduction of a heavily doped metal oxide contact layer. Antimony-doped SnO2 (ATO) contacts on SnO2 nanowires show much more stable and lower electrical contact resistance than conventional Ti contacts for high temperature (200 °C) conditions, which are required to operate chemical sensors. The stable and low contact resistance of ATO was confirmed for at least 1960 h under 200 °C in open air. This heavily doped oxide contact enables us to realize the long-term stability of SnO2 nanowire sensors while maintaining the sensitivity for both NO2 gas and light (photo) detections. The applicability of our method is confirmed for sensors on a flexible polyethylene naphthalate (PEN) substrate. Since the proposed fundamental concept can be applied to various oxide nanostructures, it will give a foundation for designing long-term stable oxide nanomaterial-based IoT sensors.

Investigation of the High Electron Affinity Molecular Dopant F6‐TCNNQ for Hole‐Transport Materials

Abstract2,2′‐(perfluoronaphthalene‐2,6‐diylidene)dimalononitrile (F6‐TCNNQ) is investigated as a molecular p‐type dopant in two hole‐transport materials, 2,2′,7,7′‐tetrakis(N,N‐diphenylamino)‐9,9‐spirobifluorene (Spiro‐TAD) and tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA). The electron affinity of F6‐TCNNQ is determined to be 5.60 eV, one of the strongest organic molecular oxidizing agents used to date in organic electronics. p‐Doping is found to be effective in Spiro‐TAD (ionization energy = 5.46 eV) but not in TCTA (ionization energy = 5.85 eV). Optical absorption measurements demonstrate that charge transfer is the predominant doping mechanism in Spiro‐TAD:F6‐TCNNQ. The host–dopant interaction also leads to a significant alteration of the host film morphology. Finally, transport measurements done on Spiro‐TAD:F6‐TCNNQ as a function of dopant concentration and temperature, and using a highly doped contact layer to ensure negligible hole injection barrier, lead to an accurate measurement of the film conductivity and hole‐hopping activation energy.

MOCVD Growth of High Quality InGaAs HEMT Layers on Large Scale Si Wafers for Heterogeneous Integration With Si CMOS

We report on the growth of In0.30Ga0.70As channel high electron mobility transistor (HEMT) epi-layers on a 200-mm Si substrate by metal-organic-chemical-vapor-deposition. The HEMT layers were grown on the Si substrate by using a ~3- ${\mu }\text{m}$ thick epitaxial buffer composing of a Ge layer, a GaAs layer, and a compositionally graded and strain relaxed InAlAs layer. The optimized epitaxy has a threading dislocation density of less than $2 {\times } 10^{{7}}$ cm−2 and a root mean square surface roughness of ~6.7 nm. The device active layers include a ${ {\delta } }$ -doped InAlAs bottom barrier, a ~15-nm thick InGaAs channel, a ~8-nm InGaP top barrier layer and a heavily doped InGaAs contact layer. MOSHEMTs with channel length down to 130 nm were fabricated. The devices achieve a peak transconductance of ${\sim }450 ~{\mu }\text{S}/ {\mu }\text{m}$ at ${V} _{ D}$ of 0.5 V. The peak effective mobility ( ${\mu }_{\text {eff}}$ ) in a device with a channel length of 20 $ {\mu }\text{m}$ device channel was ~3700 cm2/ $\text{V} {\cdot }\text{s}$ .

Improvement of High-Frequency Characteristics of InGaAsSb-Base Double Heterojunction Bipolar Transistors by Inserting a Highly Doped GaAsSb Base Contact Layer

This letter presents InP/GaAsSb/InGaAsSb/InP double heterojunction bipolar transistors (DHBTs) with a highly doped base contact layer. In order to reduce the base contact resistivity, a 3-nm-thick highly doped GaAsSb contact layer is inserted between the InP emitter and 17-nm-thick composition- and doping-graded InGaAsSb base. Fabricated DHBTs with a 0.25-μm emitter show a current gain of 32 and a high open-base breakdown voltage BVCEO of 5.2 V. The DHBTs also exhibit fT/f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> = 513/637 GHz at a collector current density of 9.5 mA/μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and VCE = 1 V. The f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> is higher by 124 GHz than that for InP/InGaAsSb DHBTs without the GaAsSb contact layer. These results indicate that the use of the GaAsSb/InGaAsSb base structure is very effective in improving f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> .

Non-destructive mapping of doping and structural composition of MOVPE-grown high current density resonant tunnelling diodes through photoluminescence spectroscopy

We report on photoluminescence (PL) characterisation of metal-organic vapour phase epitaxy (MOVPE) grown high current density (~700kA/cm2) InGaAs/AlAs/InP based resonant tunnelling diodes (RTDs) for terahertz emission. The PL mapping we describe allows important information about doping level and uniformity, ternary alloy composition and uniformity, and uniformity of quantum well thickness to be deduced. PL as a function of doping concentration is studied for InGaAs test layers at low temperatures and correlated to secondary-ion mass spectroscopy (SIMS) and electrochemical capacitance–voltage (eCV) profiling to provide non-destructive mapping of doping over the wafer. For the RTD structures, we utilise eCV as a selective etch tool to identify the origin of low temperature PL emission from the quantum well (QW) and the highly doped contact layers. PL mapping of the RTD wafer at low temperatures is shown to allow the assessment of variations in InGaAs alloy composition and QW thickness. Details of the growth process are discussed and confirmed using high resolution X-ray diffraction (HRXRD) crystallography. The rapid non-destructive characterisation and wafer mapping of these structures promises a route to future growth optimisation of such structures.

InGaAs axial-junction nanowire-array solar cells

Axial p–i–n junction nanowire (NW) solar cells (SCs) with a position-controlled GaAs-based NW array were fabricated by selective-area metal organic vapor phase epitaxy (SA-MOVPE). The measured electron-beam-induced current (EBIC) signals showed the formation of an axial p–i–n junction, which confirms power generation under sunlight illumination. The series resistance of the NW SCs is much higher than that of conventional planar SCs based on Si or other III–V compound semiconductors. The main difficulty concerning the fabrication of these NW SCs is the degradation of series resistance between the GaAs-based NWs and the indium–tin oxide (ITO) deposited as a transparent electrode. The series resistance of the fabricated GaAs-based NW SCs was reduced by introducing a tin doping contact layer between the ITO and the NW array, which is formed by pulse doping. As a result of this improved structure, the fabricated SCs exhibited an open-circuit voltage of 0.544 V, a short-circuit current of 18.2 mA/cm2, and a fill factor of 0.721 for an overall conversion efficiency of 7.14% under AM1.5G illumination. The series resistance of the SCs could be decreased to 0.132 Ω·cm2, which is one order of magnitude lower than that of the SC without a highly doped contact layer. This reduced series resistance indicates that nanostructure SCs with transparent electrodes and multijunction NW SCs with high efficiencies can be fabricated on a commercial basis in the near future.

Получение двухсторонних высоковольтных эпитак­сиальных кремниевых p—i—n-структур методом ЖФЭ

Silicon p—i—n-structures are usually obtained using conventional diffusion method or liquid phase epitaxy (LPE). In both cases, the formation of p- and n-layers occurs in two stages. This technological approach is quite complex. Moreover, when forming bilateral high-voltage epitaxial layers, their parameters significantly deteriorate as a result of prolonged heat treatment of active high-resistivity layer. Besides, when using diffusion method, it is impossible to provide good reproducibility of the process. In this paper a technique of growing bilateral high-voltage silicon p—i—n-structures by LPE in a single process is proposed. The authors have obtained the optimum compounds of silicon-undersaturated molten solutions for highly doped (5•1018 cm–3) contact layers: 0.4—0.8 at. % aluminum in gallium melt for growing p-Si-layers and 0.03—0.15 at. % ytterbium in tin melt for n-Si-layers. Parameters of such structures provide for manufacturing of high-voltage diodes on their basis. Such diodes can be used in navigational equipment, communication systems for household and special purposes, on-board power supply systems, radar systems, medical equipment, etc.

Improving the ohmic properties of contacts to P–GaN by adding p–type dopants into the metallization layer

The work investigates an increase of the density of free charge carriers in the sub-surface region of p-GaN by adding p-type dopants into the Ni-O layer of an Au/Ni-O metallization structure. We have examined electrical properties and concentration depth profiles of contact structures Au/Ni-Mg-O/p-GaN and Au/Ni-Zn-O/p-GaN, thus with magnesium and zinc as p-type dopants. The metallization layers were deposited on p-GaN by DC reactive magnetron sputtering in an atmosphere with a low concentration of oxygen (0.2 at%). The contacts were annealed in N2 . We have found that the structures containing magnesium or zinc exhibit lower values of contact resistivity in comparison with otherwise identical contacts without Mg or Zn dopants. In our opinion, the lower values of contact resistivity of the structures containing of Mg or Zn are caused by an increased density of holes in the sub-surface region of p-GaN due to diffusion of Mg or Zn from the deposited doped contact layers.

GaN photovoltaic leakage current and correlation to grain size

GaN p-i-n solar PV structures grown by rf plasma assisted molecular beam epitaxy (MBE) produce high performance IV characteristics with a leakage current density of less than 1×10−4 mA cm−2 at 0.1 V forward bias and an on-resistance of 0.039 Ω cm2. Leakage current measurements taken for different size diodes processed on the same sample containing the solar cells reveal that current density increases with diode area, indicating that leakage is not a large function of surface leakage along the mesa. Nonannealed Pt/Au Ohmic p-contacts produce a contact resistivity of 4.91×10−4 Ω cm−2 for thin Mg doped contact layers with sheet resistivity of 62196 Ω/◻. Under concentrated sunlight the cells produce an open-circuit voltage of 2.5 V and short circuit currents as high as 30 mA cm−2. Multiple growths comprised the study and on each wafer the IV curves representing several diodes showed considerable variation in parasitic leakage current density at low voltages on some wafers and practically no variation on others. It appears that a smaller grain size within the GaN thin film accounts for higher levels of dark current.

Challenges to III-V Epi Growth for Increased Performance and Functionality Required in Advanced Wireless Devices

InGaP/GaAs HBTs have been achieved via base doping and base compositional grades focused adjacent to the collector. This has been done while avoiding unacceptable increases in base sheet resistances which are typically seen with plain linear grades. Additionally, semi-ballistic launching of carriers into the base layer is shown to further enhance base electron velocity and device gain. These base grading and launching techniques enable enhanced dc gain which, among other benefits, can offset the gain killing effects of very heavily doped contact layers for advanced wireless devices.

Spectroscopic determination of the doping and mobility of terahertz quantum cascade structures

Terahertz time-domain spectroscopy is shown to provide a convenient and rapid means to measure the conductivity of individual layers in semiconductor heterostructures such as terahertz quantum cascade lasers. By modeling the complex transmission at terahertz frequencies, the electron density and the in-plane momentum scattering time of the active regions and doped contact layers were determined for both GaAs/AlGaAs and InGaAs/InAlAs epilayers. The measured temperature dependence of the electron scattering rate revealed the significance of impurity and LO phonon scattering. The implications for laser operation at room temperature are discussed by considering the changes in absorption and resonant tunneling current with temperature.

Oxide-apertured microcavity single-photon emitting diode

The authors have developed a microcavity single-photon source based on a single quantum dot within a planar cavity in which wet oxidation of a high-aluminium content layer provides lateral confinement of both the photonic mode and the injection current. Lateral confinement of the optical mode in optically pumped structures produces a strong enhancement of the radiative decay rate. Using microcavity structures with doped contact layers, they demonstrate a single-photon emitting diode where current may be injected into a single dot.

Quantum structures for multiband photon detection

AbstractThe work describes multiband photon detectors based on semiconductor micro-and nano-structures. The devices considered include quantum dot, homojunction, and heterojunction structures. In the quantum dot structures, transitions are from one state to another, while free carrier absorption and internal photoemission play the dominant role in homo or heterojunction detectors. Quantum dots-in-a-well (DWELL) detectors can tailor the response wavelength by varying the size of the well. A tunnelling quantum dot infrared photodetector (T-QDIP) could operate at room temperature by blocking the dark current except in the case of resonance. Photoexcited carriers are selectively collected from InGaAs quantum dots by resonant tunnelling, while the dark current is blocked by AlGaAs/InGaAs tunnelling barriers placed in the structure. A two-colour infrared detector with photoresponse peaks at ∼6 and ∼17 μm at room temperature will be discussed. A homojunction or heterojunction interfacial workfunction internal photoemission (HIWIP or HEIWIP) infrared detector, formed by a doped emitter layer, and an intrinsic layer acting as the barrier followed by another highly doped contact layer, can detect near infrared (NIR) photons due to interband transitions and mid/far infrared (MIR/FIR) radiation due to intraband transitions. The threshold wavelength of the interband response depends on the band gap of the barrier material, and the MIR/FIR response due to intraband transitions can be tailored by adjusting the band offset between the emitter and the barrier. GaAs/AlGaAs will provide NIR and MIR/FIR dual band response, and with GaN/AlGaN structures the detection capability can be extended into the ultraviolet region. These detectors are useful in numerous applications such as environmental monitoring, medical diagnosis, battlefield-imaging, space astronomy applications, mine detection, and remote-sensing.

Compositionally graded contact layers in quantum-well infrared photodetectors

We have observed a threshold voltage, below which no photocurrent flows, in a class of midwave quantum-well infrared photodetectors using (In,Ga)As/(In,Al)As lattice matched to InP substrates. We propose that this threshold voltage is the result of internal fields established at the interface between doped contact layers and the doped quantum wells in the detector. We show that compositionally graded contact layers can be used to eliminate the threshold voltage and to enhance the low-bias performance of the detectors.

Recent progress of high power GaN‐based violet laser diodes

High performance laser diodes were fabricated on lateral epitaxial overgrown GaN layers on sapphire substrates. The threshold current of the LDs was strongly dependent on the dislocation density. A low threshold voltage was obtained using highly Mg doped contact layers. The lifetime of LDs was also influenced by the operation voltage. Threshold current density and threshold voltage were 3.74 kA/cm2 and 4.8 V, respectively. The LDs showed a lifetime of 1,000 hs at 50 °C under automatic power controlled conditions of 30 mW. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Evaluation of MESFET structures from temperature-dependent Hall effect measurements

We report on temperature-dependent Hall effect measurements performed on a multilayer, epitaxial, 4H–SiC MESFET structure. On the lightly doped (1.5 × 1017 cm–3) active layer activation energies of Edh = 62 meV and Edk = 88 meV have been obtained by fitting the Hall mobility and carrier concentration vs. temperature. On the more heavily doped contact layer, we show that the experimental results have to be discussed in the framework of a two-layers system. Different approaches are comparatively evaluated to extract the mobility and carrier concentration vs. temperature from the experimental data.