Undoped Spacer Layer Research Articles

Long-Wavelength InAs/GaSb Superlattice Detectors on InAs Substrates With n-on-p Polarity

We report the realization of high-performance long-wavelength infrared detectors with n-on-p polarity based on InAs/GaSb type-II superlattices grown by metalorganic chemical vapor deposition (MOCVD) on InAs substrate. In a normal n-on-p device with PNn heterostructure, Zinc (Zn) is found to diffuse into the critical n-type barrier layer and leads to a high leakage current. By inserting a 200 nm undoped spacer layer after the p-type region is grown, Zn diffusion is completely confined in the spacer layer. The n-on-p device with the spacer shows performances equivalent to those of a p-on-n counterpart, with a dark current density of $6\times 10^{-4}$ A/cm2 at −0.1 V at 80 K, a peak quantum efficiency of ~24%, and 100% cut-off wavelength of $\sim 12~\mu \text{m}$ .

Heavily Doped Semiconductor Metamaterials for Mid‐Infrared Multispectral Perfect Absorption and Thermal Emission

AbstractA metamaterial perfect absorber based on layered, doped and undoped semiconductors is experimentally and theoretically investigated. Design rules are given to control the multispectral, narrow, strong absorption features (>98% absorption) in the mid‐IR spectrum. The proposed sub‐wavelength grating structures support localized surface plasmons and photonic resonances associated to the quarter wavelength optical thickness of the undoped spacer layer. The resonances hybridize depending on the geometric setup of the metamaterial and the material properties of the heavily doped semiconductor. The angular response of the resonances is studied to determine the acceptance angle. While the absorption is best at near‐normal incidence, the spectral position of the absorption bands is weakly dependent on the angular incidence up to angles of 45°. Complementary to the polarization‐dependent grating design of the metamaterial, an alternative based on a crossed grating nanopattern is also investigated for polarization insensitivity. Furthermore, it is demonstrated that the metamaterial perfect absorber provides a tailored thermal emission spectrum. Based on the heavily doped InAsSb/GaSb metamaterial platform, integrable and miniaturized perfect absorbers and thermal sources can be built.

Long range energy transfer in graphene hybrid structures

In this work we quantify the distance dependence for the extraction of energy from excited chromophores by a single layer graphene flake over a large separation range. To this end hybrid structures were prepared, consisting of a thin (2 nm) layer of a polymer matrix doped with a well chosen strongly fluorescent organic molecule, followed by an un-doped spacer layer of well-defined thicknesses made of the same polymer material and an underlying single layer of pristine, undoped graphene. The coupling strength is assessed through the variation of the fluorescence decay kinetics as a function of distance between the graphene and the excited chromophore molecules. Non-radiative energy transfer to the graphene was observed at distances of up to 60 nm; a range much greater than typical energy transfer distances observed in molecular systems.

Influence of Physical Parameters on Microwave Noise Characteristics of Al0.3Ga0.7N/Al0.05Ga0.95N/GaN Composite-Channel HEMTs

THE Al0.3Ga0.7N/Al0.05Ga0.95N/GaN CC-HEMT structure is shown in [12]. It consists of 2 µm sapphire substrates, a 2.5 µm GaN undoped minor channel layer, a 6 nm Al0.05Ga0.95N undoped major channel layer, a 3 nm Al0.3Ga0.7N undoped spacer layer, a 21 nm doped(1e18) carrier supplier layer and a 2nm undoped cap layer. The devices have a source-gate spacing of Lsg=0.5 µm, gate-drain spacing of Lgd=1 µm, a 1 µm -gate-length and a gate width of 1 µm [12]. The band gap (Eg), affinity and electron mobility of layers of this device are shown in Table I. The polarization charge density at the interface of Al0.3Ga0.7N/Al0.05Ga0.95N and Al0.05Ga0.95N/GaN is calculated. The polarization charge density is 1.12e13 cm -2

An InP/InGaAs metamorphic δ-doped heterojunction bipolar transistor with high current gain and low offset voltage

Excellent dc performance of an InP/InGaAs metamorphic δ-doped heterojunction bipolar transistor (HBT) using a linearly graded InxGa1−xP metamorphic buffer layer grown on GaAs substrate is demonstrated. The employment of a δ-doped sheet between two undoped spacer layers could eliminate the potential spike at base–emitter junction and increase the effective barrier, which could prevent the hole injection from base into emitter. A maximum current gain of 255, a low offset voltage of 105mV, and a small second (third) harmonic distortions of 0.545 (−0.05) at VCE=2.5V are obtained. The current gain and offset voltage are the best values than that of the previous InP/InGaAs metamorphic HBT.

Short-period (Ga,Mn)As/(Al,Ga)As multilayer structures studied by cross-sectional scanning tunneling microscopy

(Ga${}_{1\ensuremath{-}x}$,Mn${}_{x}$)As/GaAs and (Ga${}_{1\ensuremath{-}x}$,Mn${}_{x}$)As/(Al${}_{0.2}$,Ga${}_{0.8}$)As multilayer structures grown by molecular beam epitaxy have been studied by cross-sectional scanning tunneling microscopy. These dilute magnetic semiconductor multilayer structures have been predicted to have a strong giant magnetoresistance effect and enhanced Curie temperature. However, a sharp and short-period digital doping profile of the Mn acceptors is essential to achieve this, and therefore the studied samples were grown at a low growth temperature ($250{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$). Cross-sectional scanning tunneling microscopy measurements show that the overall quality of the structure is good but many As antisites are present due to the low growth temperature. The observed Mn profile showed that, despite the low growth temperature, about 20% of the Mn acceptors from the doped layers (eight monolayers thick) end up in the nominally undoped spacer layers (four monolayers thick). This segregation puts serious constraints on the creation and application of short-period dilute magnetic superlattices because of the magnetic shortcut caused by the Mn acceptors in the spacer layer.

Monolithic white light emitting diodes with a broad emission spectrum

Monolithic white light emitting diode (LEDs) grown by molecular beam epitaxy (MBE) is an original approach to realize white LEDs. Using a Ga0.8In0.2N/GaN multi-quantum well (QW) structure with different QW thicknesses in the active region of a GaN p-n junction leads to white light emission provided that the different QW emission wavelengths and relative intensities are properly adjusted. However, the color rendering index (CRI) and the output power are still a limitation for the development of these LEDs. In this study, we propose to use a broad emission spectrum – from blue to red – with the objective of improving the CRI. Carrier recombination mechanisms in the LEDs are studied by electroluminescence (EL) measurements at room temperature and discussed as a function of the structure design. Results show a strong modification of the EL spectrum as a function of the applied voltage and an enhancement of the red emission component with the insertion of a GaN undoped spacer layer between the active region and the p-type layers. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Weak localization and spin‐orbit interaction in self‐organized In 0.2 Ga 0.8 As quantum wires on GaAs (221)A substrates

Anisotropic negative magnetoresistance (MR) parallel and perpendicular to the wires in the regime of quantum interference of hole gas has been investigated in self-organized In0.2Ga0.8As quantum wires (QWRs) grown on GaAs (221)A substrates by MBE and separated from p-type Si-doped Al0.35Ga0.75As by an undoped spacer layer of it (thickness Ls) in order to address issues concerning the dimensionality in weak localization (WL) as well as spin-orbit (SO) interaction. For Ls = 3 nm, the best fits of the parallel MR to the 2D WL theory have been obtained with τso ∼ 24 ps, being well described by the model of anisotropic 2D WL for strong lateral coupling of the wires. For Ls = 20 nm, on the other hand, the best fits to 1D WL theory have been obtained with τso ∼ 150 ps assuming diffusive boundary scattering and the data cannot be fit to 2D WL, being well described by the model of quasi-1D WL. In addition, the data for Ls = 10 nm show a featureof the crossover between 2D WL and 1D WL, indicating the intermediate lateral coupling between the wires. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

S-Band Operation of SiC Power MESFET With 20 W (4.4 W/mm) Output Power and 60% PAE

Previous efforts have revealed instabilities in standard SiC MESFET device electrical characteristics, which have been attributed to charged surface states. This work describes the use of an undoped spacer layer on top of a SiC MESFET to form a structure where the active current carrying channel is removed from the surface. By using this approach, the induced surface traps are physically removed from the channel region, such that the depletion depth caused by the unneutralized surface states cannot reach the conductive channel. This results in minimal RF dispersion (gate lag) and, thus, improved RF performance. Furthermore, the buried-channel approach provides for a relatively broad and uniform transconductance (G/sub m/) with gate bias (V/sub gs/), resulting in higher efficiency MESFETs with improved linearity and lower signal distortion. SiC MESFETs having 4.8-mm gate periphery were fabricated using this buried-channel structure and were measured to have an output power of 21 W (P/sub out//spl sim/4.4 W/mm), 62% power added efficiency, and 10.6 dB power gain at 3 GHz under pulse operation. When operated at continuous wave, similar 4.8-mm gate periphery SiC MESFETs produced 9.2 W output power (P/sub out//spl sim/2 W/mm), 40% PAE, and /spl sim/7 dB associated gain at 3 GHz.

High performances of InP/InGaAs heterojunction bipolar transistors with a δ-doped sheet between two spacer layers

New InP/InGaAs δ-doped heterojunction bipolar transistors have been successfully fabricated and demonstrated. The use of a δ-doped sheet between two undoped spacer layers more effectively eliminates the potential spike at the base–emitter junction and lowers the collector–emitter offset voltage. The maximum current gain and offset voltage of 455 (533) and 55 mV (33 mV) are observed, respectively, for the unpassivated (passivated) emitter device at room temperature. The base surface recombination current of the studied devices is also investigated in this paper. The studied devices are therefore promising for practical circuit applications.

Bias-Tunable Multiple-Transconductance with Improved Transport Characteristics of δ-doped In0.28Ga0.72As/GaAs/In0.24Ga0.76As/GaAs High Electron Mobility Transistor Using a Graded Superlattice Spacer

A δ-doped In0.28Ga0.72As/GaAs/In0.24Ga0.76As/GaAs high electron mobility transistor (HEMT) with a thickness-graded superlattice spacer exhibiting bias-tunable multiple-transconductance performance has been successfully fabricated by molecular beam epitaxy (MBE). As compared to a conventional HEMT structure with a single undoped spacer layer, the present superlattice spacer, which accommodates the lattice-mismatch-induced strain, provides improvement of 31% and 43% on electron mobility and the two-dimensional electron gas (2DEG) concentration-mobility product, respectively. In addition, the device demonstrates bias-tunable multiple-transconductance plateaus due to modulation of the 2DEG concentration and mobility product by real-space transfer at high channel voltage. The distinguished device characteristics of the present structure are promising for a high-speed analog-to-digital converter (ADC) or multiple-state quantizer applications.

Structure and doping optimization of SiGe heterojunction internal photoemission detectors for mid-infrared applications

We present full-scale calculations of highly p-type doped siGe heterojunction internal photoemission (Hip) detectors that operate in the mid-infrared (3- to 5-?m) range of wavelength. We explore the effects of including undoped spacer layers within the highly doped siGe wells, for which a systematic body of experimental data is available to us, and which demonstrates a substantial reduction in the dark current produced by these devices. We model the doping explicitly by means of a screened Coulomb potential, leading to a full description of the resultant impurity band, and compare our calculated optical line shapes with recent state-of-the-art molecular-beam epitaxy experiments. The variation of the optical response with well width, germanium concentration, spacer width and position, and doping concentration are all considered.

Epitaxial Si-based tunnel diodes

Tunneling devices in combination with transistors offer a way to extend the performance of existing technologies by increasing circuit speed and decreasing static power dissipation. We have investigated Si-based tunnel diodes grown using molecular beam epitaxy (MBE). The basic structure is a p + layer formed by B delta doping, an undoped spacer layer, and an n + layer formed by Sb delta doping. In the n-on-p configuration, low temperature epitaxy (300–370°C) was used to minimize the effect of dopant segregation and diffusion. In the p-on-n configuration, a combination of growth temperatures from 320 to 550°C was used to exploit the Sb segregation to obtain a low Sb concentration in the B-doped layer. Post-growth rapid thermal anneals for 1 min in the temperature interval between 600 and 825°C were required to optimize the device characteristics. J p, the peak current density, and the peak-to-valley current ratio (PVCR), were measured at room temperature. An n-on-p diode having a spacer layer composed of 4 nm Si 0.6Ge 0.4, bounded on either side by 1 nm Si, had a J p=2.3 kA/cm 2 and PVCR=2.05. A p-on-n tunnel diode with an 8 nm Si spacer (5 nm grown at 320°C, 3 nm grown at 550°C) had a J p=2.6 kA/cm 2 and PVCR=1.7.

MOCVD‐Grown InGa/GaAs Emitter Delta Doping Heterojunction Bipolar Transistors

The influence of delta doping sheet at base‐emitter (BE) junction for an InGaP/GaAs heterojunction bipolar transistor (HBT) with a 75Å undoped spacer layer is investigated. A common emitter current gain of 235, an offset voltage as small as 50mV and an Ic ideal factor of 1.01 are obtained, respectively. The use of delta doping sheet at BE junction results in a high gain and low offset voltage HBT. The improvement of current gain and offset voltage may be attributed to the reduction of BE potential spike by introducing a delta doping layer even without the BE junction passivation.

Metalorganic chemical vapor deposition of high mobility AlGaN/GaN heterostructures

In this article, we discuss parameters influencing (a) the properties of thin AlxGa1−xN layers grown by metalorganic chemical vapor deposition and (b) the electrical properties of the two-dimensional electron gas (2DEG) forming at the AlxGa1−xN/GaN heterojunction. For xAl>0.3, the AlxGa1−xN layers showed a strong tendency towards defect formation and transition into an island growth mode. Atomically smooth, coherently strained AlxGa1−xN layers were obtained under conditions that ensured a high surface mobility of adsorbed metal species during growth. The electron mobility of the 2DEG formed at the AlxGa1−xN/GaN interface strongly decreased with increasing aluminum mole fraction in the AlxGa1−xN layer and increasing interface roughness, as evaluated by atomic force microscopy of the surfaces prior to AlxGa1−xN deposition. In the case of modulation doped structures (GaN/AlxGa1−xN/AlxGa1−xN:Si/AlxGa1−xN), the electron mobility decreased with decreasing thickness of the undoped spacer layer and increasing silicon doping. The electron mobility was only moderately affected by the dislocation density in the films and independent of the growth temperature of the AlxGa1−xN layers at xAl=0.3. For Al0.3Ga0.7N/GaN heterojunctions, electron mobility values up to 1650 and 4400 cm2/V s were measured at 300 and 15 K, respectively.

Influence of the undoped spacer layer thickness on the DC characteristics of n-type GaAs/AlAs MESFETs

We have investigated the influence of the thickness of an undoped spacer layer under the channel on the current-voltage characteristics of GaAs MESFETs. To that purpose we have grown the active layers of the MESFETs on top of a 50 nm AlAs layer to separate the device effectively from the undoped GaAs substrate. Substrate leakage currents are of the order of A at 30 V source-substrate bias. DC characteristics are measured for four MESFETs with identical layer structure and doping but different spacer layer thicknesses. Comparison shows that MESFETs with thin spacer layers can be useful for low-noise applications but lose in transconductance and mobility values.

Extremely high-mobility two dimensional electron gas: Evaluation of scattering mechanisms

We report on the characterization of selectively doped GaAs/AlGaAs heterostructures, grown by an extremely clean molecular beam epitaxy system, which exhibit a Hall mobility of a two dimensional electron gas exceeding 10×106 cm2/Vs for a wide range of undoped spacer layer thickness (50–100 nm). A maximum electron mobility of 14.4×106 cm2/Vs was measured at 0.1 K in a structure with a 68 nm spacer thickness and an areal carrier density of 2.4×1011 cm−2. This is the highest electron mobility ever reported, leading to a momentum relaxation mean-free path of ∼120 μm. We present experiments that enable us to distinguish between the main scattering mechanisms. We find that scattering due to background impurities limits electron mobility in our best samples, suggesting that further improvement in structure quality is possible.

Photo-Assisted Resonant Tunneling Through Localized States in AlAs/GaAs Double-Barrier Structure With Undoped Spacer Layers

AbstractWe report on the evidence of photo-assisted resonant tunneling through localized states in AlAs/GaAs double-barrier structures (DBSs) with undoped GaAs spacers. In the photocurrent measurement, additional peaks were observed at voltages lower than that of resonance and were enhanced with the laser power. This behavior was more pronounced as the thickness of spacer layers in the DBS increased. These results are attributed to the resonant tunneling of electrons through the localized states, they are induced in the neighboring barriers, by the photoexcited carriers in spacers. We discuss the localization effect of photoexcited carriers on the resonant tunneling.

Growth of Si and C δ-doped nipi doping superlattices in GaAs by metal organic vapor phase epitaxy

We have optimized growth conditions of metal organic vapor phase epitaxy (MOVPE) in order to grow Si and C δ-doped nipi doping superlattices in GaAs. Trimethylaluminium (TMAl) and silane (SiH4) were used as p-type and n-type doping precursors, respectively. We report that at 630 °C, full compensation of free electrons and holes can be obtained in the MOVPE-grown Si and C δ-doped nipi doping superlattices over a very wide range of the sheet carrier densities (1012–1013 cm−2) by choosing proper TMAl flow rate and Si δ-doping time or SiH4 flow rate. The experimental results on electrical and optical characterization of Si and C δ-doped nipi doping superlattices in GaAs with 150 Å thick undoped spacer layers are presented.

Tunnel Current Features Caused by Defect Assisted Process in Resonant-Tunnelling Structures

Practically, all resonant-tunnelling devices contain as their important part an undoped spacer layer separating a heavily-doped layer from the barrier. Including the spacer layer into the structure has several good points. It permits, for instance, to decrease the device capacitance, to prevent impurity diffusion into the active part of the device, to eliminate an effect of the long-range Coulomb potential on the tunnelling, and thus, to improve the current-voltage characteristics of the device. however, a thick spacer layer is also a drawback which can essentially affect the transport properties in the injector. Actually, owing to a light doping in a close vicinity of the emitter barrier, an applied bias voltage creates an accumulation layer. Generally, the presence of the accumulation layer makes the tunnelling dynamics worse, because the recharging processes which occur in this layer are slow as compared with the tunnelling time thus giving rise to an extra frequency cut-off. The physical processes occurring in the injector part of device are still of interest as being of a large significance for the device operation. In this work we grew a structure whose composition was chosen so as to obtain the flat band regime in the emitter spacer under resonance condition. High