Si-doped Layers Research Articles

Luminescence properties of highly Si-doped AlN

We report on the optical characterization of highly Si-doped AlN layers grown by plasma-assisted molecular-beam epitaxy. Cathodoluminescence spectra reveal a sharp band-edge line and two broadbands around 4.4eV and 3.6eV. Near-band-edge luminescence remains intense at room temperature, and shows a systematic redshift with increasing Si concentration. Regarding the low-energy bands, we observe a redshift of the emission around 3.6eV with increasing Si concentration accompanied by an increase in relative intensity compared to the 4.4eV band.

Growth and fabrication of AlGaN-based ultraviolet light emitting diodes on 6H-SiC(0 0 0 1) substrates and the effect of carrier-blocking layers on their emission characteristics

Growth, fabrication, and the electrical and optical characterization of ultraviolet light emitting diodes and their components, including Al x Ga 1− x N films, quantum wells (QWs), and ohmic contacts, and the problems encountered in the process integration of these components have been investigated. Ni/Au ohmic contacts with specific contact resistivities of 2.2 × 10 −4 and 2.0 × 10 −2 Ω cm 2 were achieved on annealed, Mg-doped ([Mg] ∼ 5 × 10 19 cm −3), p-type GaN layers that had been cleaned in HCl at 85 °C and on the backside of the SiC substrates after annealing in nitrogen, respectively. The emission intensity of the diodes increased with an increase in the number of Al 0.06Ga 0.94N/Al 0.10Ga 0.90N QWs and with the use of Si-doped n-type barrier layers. The highest intensities of the principle emission at 353 nm were measured at all values of the injection current in the device with a p-type carrier-blocking layer at the top of the QWs; this device also exhibited the highest values of light output power. Growth of an n-type carrier-blocking layer at the bottom of the QWs had an adverse effect on their characteristics. A broad peak centered at ∼540 nm exhibited yellow luminescence and was present in the spectra acquired from all the devices. This peak is attributed to absorption of the ultraviolet emission by and re-emission from the p-GaN and/or to the luminescence from the AlGaN within QWs by current injection.

Activation energy of n-GaN epitaxial layers grown on vicinal-cut sapphire substrates

Si-doped GaN epitaxial layers were grown on 0° and 1° tilted sapphire substrates by metalorganic chemical vapor deposition (MOVCD). It was found that GaN epitaxial layers on tilted substrates were grown with step-flow mode while those on untilted substrates were grown with spiral growth mode. It was also found that number of compensating acceptor, N COM, equals 1.5×10 17 and 8.7×10 16 cm −3 for GaN epitaxial layers grown on 0° and 1° tilted sapphire substrates, respectively. Furthermore, it was found that there existed two values of activation energy of 15.2 and 23 meV for the GaN epitaxial layer grown on untilted sapphire substrate. In contrast, only the activation energy of 15.2 meV was found from the GaN epitaxial layer grown on 1° tilted sapphire substrate.

Effects of doping type and concentration on precipitation of nanometer arsenic clusters in low-temperature-grown GaAs

In this study, the effects of doping type and concentration on arsenic precipitation in low-temperature-grown GaAs upon postgrowth annealing at 600, 700, and 800 °C were investigated. Three undoped/Si-doped/undoped (i-n-i) regions and three undoped/Be-doped/undoped (i-p-i) regions were grown by low-temperature molecular beam epitaxy. The results show that arsenic precipitation is dependent on doping type and doping concentration. Arsenic depletion was observed in all Be-doped layers for all annealing temperatures. However, a “dual” arsenic precipitation behavior was observed in Si-doped layers: As accumulates in [Si]=2×1018cm−3 doped layers, while it depletes in [Si]=2×1016 and 2×1017cm−3 doped layers. We attribute this “dual” As precipitation phenomenon in Si-doped layers to the different depletion depths.

Effect of Substrate Orientation on Arsenic Precipitation in Low-Temperature-Grown GaAs

Arsenic precipitation in “superlattice” structures of alternately undoped and [Si]=3 ×1018 cm-3-doped GaAs grown at 250°C on (100), (311)A, and (311)B GaAs substrates has been studied using transmission electron microscopy. It is found that upon postgrowth annealing at 800°C, As precipitates are nearly confined in the Si-doped regions, forming two-dimensional cluster arrays located approximately at the center of each Si-doped layer. The results also show that the As precipitates in the (311)B substrate are slightly denser and larger than those in the (311)A substrate and both are markedly denser and larger than those in the (100) substrate. This can be attributed to the varying excess arsenic incorporations in differently orientated substrates.

Friction behaviour of Si-DLC/DLC multi layer films on steel substrate in water environment

The tribological properties of diamond-like carbon (DLC) films over-coated by various thicknesses of Si-doped layers were investigated in a water environment. The multi-layer DLC films were deposited on a stainless steel substrate by pulsed-bias CVD technique. A dependence of friction coefficients on thickness of the over-coated layer was not observed and the friction coefficients of the films showed a stable and low value of approximately less than 0.1. On the other hand, specific wear rates of the films and mating balls depended on the thickness of the layers. Furthermore, the films with thin layers (less than 0.1 μm) and a thick layer (over 0.34 μm) showed the rupture on wear tracks. It was concluded that the optimized thickness of the layer (0.16 μm) improved the rupture strength of film, and showed a low specific wear rate of film and mating ball. It was found that the optimization of the thickness of the over-coated layer was important in achieving good tribological properties of Si-DLC/DLC multi-layer films.

Optimization of AlGaN∕GaN current aperture vertical electron transistor (CAVET) fabricated by photoelectrochemical wet etching

AlGaN ∕ GaN current aperture vertical electron transistor (CAVET) was fabricated and optimized for band gap selective photoelectrochemical wet etching. The large polarization induced voltage offset (around 2.5–4eV) observed in the first generation CAVET was reduced to 0.7V in this structure by employing a δ Si doping layer buried 60Å below the In0.03Ga0.97N (60nm thick) and bottom GaN interface to screen the polarization fields. Other sample structures were studied to achieve an aperture with both good undercut etching and a small voltage offset. It was clearly demonstrated that etch selectivity in the GaN∕InGaN∕GaN undercut structures was influenced by hole confinement and the chemical activity of the N-face GaN.

Structure optimization of InGaN–GaN ultraviolet light-emitting diode with a low-energy electron injection mechanism

We propose a way of increasing overall efficiency of InGaN–GaN ultraviolet light-emitting diodes (UV LEDs) by placing a thin Si-doped AlGaN layer beneath the multiple quantum well (MQW). The AlGaN thin layer plays a role of tunneling barrier for the electrons, making a low-energy electron injection possible. The effect of the AlGaN thin film is verified by the simulation regarding the carrier distributions. By using this idea, the overall efficiency of the UV LED is enhanced by 13.5% over that of the conventional UV LEDs.

Growth and design of deep-UV (240–290 nm) light emitting diodes using AlGaN alloys

Solid-state light sources emitting at wavelengths less than 300 nm would enable technological advances in many areas such as fluorescence-based biological agent detection, non-line-of-sight communications, water purification, and industrial processing including ink drying and epoxy curing. In this paper, we present our recent progress in the development of LEDs with emission between 237 and 297 nm.We will discuss growth and design issues of deep-UV LEDs, including transport in Si-doped AlGaN layers. The LEDs are designed for bottom emission so that improved heat sinking and light extraction can be achieved by flip chipping. To date, we have demonstrated 2.25 mW of output power at 295 nm from 1 mm×1 mm LEDs operated at 500 mA. Shorter wavelength LEDs emitting at 276 nm have achieved an output power of 1.3 mW at 400 mA. The heterostructure designs that we have employed have suppressed deep level emission to intensities that are up to 330×lower than the primary quantum well emission.

Effects of Si-doped GaAs layer on optical properties of InAs quantum dots

We have investigated the optical properties of InAs self-assembled quantum dots (SAQDs) with the Si-doped GaAs barrier layer. Two types of samples are fabricated according to the position of the Si-doped GaAs layer. For type A samples the Si-doped GaAs layer is grown below the QDs, whereas for type B samples the Si-doped GaAs layer is grown above the QDs. For both types of samples the excited-state emissions caused by state filling are observed in photoluminescence (PL) spectra at high excitation power densities. The bandgap renormalization of QDs can be found from the shift of the PL peak energy. Particularly, for type A samples the Si atoms act as nucleation centers during the growth of InAs QDs on the Si-doped GaAs layer and affect the density and the size of the QDs. The Si-doped GaAs layer in type A samples has more effects on the properties of QDs, such as state filling and bandgap renormalization than those of type B samples.

Growth and characterisation of GaN with reduced dislocation density

Two GaN MOVPE growth methods to reduce the threading dislocation (TD) density have been explored. The combined effects of (1) in situ SiN x masking of the sapphire substrate and (2) starting the epitaxial growth at low V-to-III ratio on the GaN film quality were studied by atomic force microscopy, transmission electron microscopy and high-resolution X-ray diffraction. It was found that the annealing condition of the low-temperature nucleation layer after in situ SiN x masking is critical in order to decrease the density of nucleation sites and hence increase the average grain size to about 5 μm. However, the coalescence of large grains with vertical side facets results in the formation of dense bundles of TDs at the grain boundaries combined with large numbers of basal-plane dislocation loops throughout the film. The formation of these dislocations can be prevented by starting the epilayer growth at low V-to-III ratio, resulting in the formation of grains with inclined side facets. The interaction of the TDs with the inclined side facets causes the dislocations to bend 90 ∘ as the grains grow in size and coalesce. GaN films with dislocation densities as low as 1×10 8 cm −2, giving full-width at half-maximum values of 180 and 220 arcsec for respectively (002) and (302) omega scans, were achieved by the combination of in situ masking and low V–III ratio epilayer growth. Hall carrier mobility values in excess of 900 cm 2 V −1 s −1 were deduced for Si-doped layers.

Comparative analysis of the DC performance of DG MOSFETs on highly-doped and near-intrinsic silicon layers

A comparison of dc characteristics of fully depleted double-gate (DG) MOSFETs with respect to low-power circuit applications and device scaling has been performed by two-dimensional device simulation. Three different DG MOSFET structures including a conventional N+ polysilicon gate device with highly doped Si layer, an asymmetrical P+/N+ polysilicon gate device with low doped Si layer and a mid-gap metal gate device with low doped Si layer have been analysed. It was found that DG MOSFET with mid-gap metal gates yields the best dc parameters for given off-state drain leakage current and highest immunity to the variation of technology parameters (gate length, gate oxide thickness and Si layer thickness). It is also found that an asymmetrical P+/N+ polysilicon gate DG MOSFET design offers comparable dc characteristics, but better parameter immunity to technology tolerances than a conventional DG MOSFET.

Excimer laser thermal processing of ultra-shallow junction: laser pulse duration

According to the International Technology Roadmap for Semiconductors (ITRS), source and drain extensions thickness for 65 nm and below technology nodes MOSFET lead to a major challenge. Rapid thermal processing (RTP) tools reach the limit of their physical abilities in term of temperature cycle and pulsed laser thermal processing (LTP) tool arise as a major potential candidates to solve the fundamental problem of ultra-shallow junction (USJ) activation. LTP experiments have been performed with two different XeCl excimer lasers (λ=308 nm) with different pulse characteristics (20 and 200 ns). We examine the influence of the pulse duration on LTP of B+ (with and without Ge+ pre-amorphization) implanted silicon samples on the basis of real-time optical monitoring of the laser induced process, four-point probe resistivity measurements and secondary ion mass spectroscopy (SIMS) depth profiles. Experimental results are compared to model calculations for both laser pulses. The activated dopant dose, junction depth and sheet resistance, as a function of the laser fluence and shot number for both lasers, confirm the relevance and efficiency of laser processing to realize ultra-shallow and highly doped junctions required for the future CMOS generations. Influence of the pulse duration on the USJ formation process is also discussed. In particular, we show the capabilities of the 200 ns pulse to activate B in Si without melting the Si doped layer.

Anodic Oxidation of GaN in N-methylacetamide for Electrical Profiling of Doped Layers in GaN

The anodic oxidation of gallium nitride (GaN) in pH-adjusted N-methylacetamide (NMA) solutions has been carried out by a constant current method. Uniform oxide layers are reproducibly grown in an NMA electrolyte with a pH of 8.0, containing 1% at a current density of Under optimum conditions, a linear relationship with a slope of 0.67 nm/V, is obtained between the thickness of the GaN layer consumed in the oxide and forming voltage. Electrical carrier concentration and mobility profiles for Si-doped epitaxial GaN layers are examined by successive Hall effect and sheet resistivity measurements combined with an anodic oxide growth stripping process, where anodization is carried out under optimumly defined conditions. © 2004 The Electrochemical Society. All rights reserved.

Hole trapping time measurement in low-temperature-grown gallium arsenide

We study hole dynamics in GaAs layers grown by molecular-beam epitaxy at 270 °C by two-color pump-and-probe experiments employing femtosecond 800-nm-wavelength pulses for sample’s excitation and 9-μm-wavelength pulses for probing the induced intervalence band absorption. Hole trapping time in as-grown, undoped layer is equal to 2 ps; it increases after thermal annealing or Be doping, and decreases in Si-doped layer. The mechanism of the hole trapping is discussed; it is shown that experimental observations are consistent with the hole trapping at neutral arsenic antisites model.

Optically detected magnetic resonance of (effective-mass) shallow acceptors in Si-doped GaN homoepitaxial layers

Optically detected magnetic resonance (ODMR) has been performed on Si-doped GaN homoepitaxial layers grown by organometallic chemical vapor deposition on free-standing GaN templates. In addition to intense excitonic bandedge emission with narrow linewidths $(<0.4\mathrm{meV}),$ these films exhibit strong shallow donor--shallow acceptor recombination at 3.27 eV. Most notably, ODMR on this photoluminescence band reveals a highly anisotropic resonance with ${g}_{\ensuremath{\parallel}}=2.193\ifmmode\pm\else\textpm\fi{}0.001$ and ${g}_{\ensuremath{\perp}}\ensuremath{\sim}0$ as expected for effective-mass shallow acceptors in wurtzitic GaN from k\ensuremath{\cdot}p theory. This previously elusive result is attributed to the much reduced dislocation density and impurity levels compared to those typically found in the widely investigated Mg-doped GaN heteroepitaxial layers. The possible chemical origin of the shallow acceptors in these homoepitaxial films will be discussed.

High electron concentrations in Si-doped AlN/AlGaN superlattices with high average Al content of 80%

Uniformly Si-doped AlN/Al0.5Ga0.5N superlattices were shown to have a high electron concentration. Even with high average Al content of approximately 80%, the high electron concentration reached 3.2 × 1018 cm−3, which is about eight times higher than that of a bulk Si-doped AlGaN layer with the same Al content. In the AlN/AlGaN system, the conduction band offset is larger than the ionization energy of the Si donor in AlN. Therefore, the Si donors in the AlN barriers are fully activated and the corresponding electrons are transferred to the AlGaN wells. In addtion, the large band bending caused by the strong strain-induced piezoelectric and spontaneous polarization increases the activation of the donors in the AlGaN wells. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Flicker noise of GaN-based heterostructure field-effect transistors with Si-doped AlGaN carrier injection layer

Aluminum gallium nitride/gallium nitride (AlGaN/GaN) heterostructure field effect transistors (HFETs) with and without Si-doped AlGaN layer were fabricated and investigated. HFETs with the Si-doped AlGaN carrier-injection layer show better DC performance, and the transconductance is 150 mS/mm. However, the HFETs with Si-doped AlGaN layer present the deviation from the 1/f noise at low frequency. The Lorentz shape was observed in the noise spectrum. It suggests that traps might be more pronounced in this kind of structure. Therefore, the DC characteristics of HFETs can be improved by the insertion of Si-doped AlGaN layer, but it can result in more low-frequency noise with the carrier-injection layer.

Current crowding effect on thermal characteristics of Ni/doped-Si contacts

Continuous scaling of integrated circuit elements results in an increase of current density and associated Joule heating in the interconnects. The self-heating effect that leads to a temperature rise at interconnects may become a source of thermal reliability problems. This work reports an experimental and computational investigation of the current crowding effect on thermal characteristics of metal/doped-Si contacts. The temperature rise at the contacts is determined from the Seebeck potential measured in a microfabricated test structure. It is found that a nonuniform current distribution introduces a much higher heating power density than a uniform one at the contact window. The increase of the contact temperature is proportional to the power density at the Ni/doped-Si contact. The dependence of sheet resistance of the doped-Si layer, contact resistivity, and contact size on the contact power density is also discussed.

Properties of InGaP/GaAs Grown by Solid-Source Molecular BeamEpitaxy with a GaP Decomposition Source

Lattice-matched InGaP on GaAs (001) was successfully grown bysolid-source molecular beam epitaxy with a GaP decomposition source. A0.5-µm-thick InGaP epilayer shows photoluminescence peak energy aslarge as 1.991 eV at 15 K, the full width at half maximum as small as 9.4 meVand x-ray diffraction rocking curve linewidth as narrow as 25 arcsec.The electron mobilities of undoped and Si-doped InGaP layers obtainedby Hall measurements are comparable to similar InGaP/GaAsheterojunction grown by solid-source molecular beam epitaxy with othersources or other growth techniques. The results reveal that theInGaP/GaAs heterojunction grown by the present growth way have greatpotential applications for semiconductor devices.