Photoelectric properties of p +-n junctions based on 4H-SiC ion-implanted with aluminum

The photoelectric properties of p ⁺-n junctions that were based on 4H-SiC ion-implanted with aluminum and were formed in lightly doped n-type epitaxial layers grown by chemical vapor deposition were studied. It is shown that such photodetectors combine in full measure the advantages of photostructures formed on the basis of Schottky barriers and epitaxial p-n junctions. The results of the theoretical calculation of spectral characteristics of ion-implanted photodetectors are in good agreement with experimental data. The structures feature an efficiency of collection of nonequilibrium charge carriers close to 100% in the spectral range of the photon energies of 3.5–4.25 eV.     

A comparative study and performance characteristics of ion-implanted and heterojunction short-wave infrared HgCdTe focal-plane arrays

Short-wave infrared (SWIR) HgCdTe focal-plane arrays (FPAs) with a cutoff wavelength of 2.5 μm have been produced using both planar ion-implanted and heterojunction-mesa device structures. The two-dimesnional FPAs are comprised of a 320×256 format with 30-μm pixel pitch and are cooled by a multistage thermo-electric (TE) cooler. Measured R₀A values of the two types of device structures show similar results below about 130 K because of the performance-limiting effect of the surface passivation of the heterojunction. However, a substantial difference is seen above 130 K and up to 300 K between the two structures types, with the heterojunction-mesa p-on-n device having an order of magnitude higher R₀A value than the planar ion-implanted n-on-p configuration. The difference in the R₀A values is reflected in the FPA images of the two different device types, where at 200 K, both FPAs display a clear picture with the n-on-p implanted device having a somewhat lesser resolution. However, no image can be seen from the planar-implanted FPA at 300 K, whereas the heterojunction-mesa FPA still exhibits a notable image at this temperature. These differences are examined and are attributed largely to higher diffusion and generation-recombination (g-r) currents that are thought to be prevalent in the ion-implanted n-on-p device structure. Yet, baking studies carried out show the ion-implanted diodes to be slightly more robust, as experiments reveal that they tend to survive a 120°C heat treatment longer than the mesa devices, which tend to degrade after a certain period of time. The nature of n-type donors in ion-implanted diodes is discussed, and a new theory based on Te antisites is proposed to explain recent experimental findings.     

Silicon carbide detectors of high-energy particles

The results of studying 4H-SiC p ⁺-n junctions ion-implanted with aluminum as detectors of high-energy particles are reported. The junctions were formed in SiC epitaxial films grown by chemical vapor deposition. The concentration of uncompensated donors was (3–5)×10¹⁵ cm⁻³, and the charge-carrier diffusion length was L _p=2.5 μm. The detectors were irradiated with 4.8–5.5-MeV alpha particles at 20°C. The efficiency of collection of the induced charge was as high as 0.35. The possibilities of operating SiC detectors at elevated temperatures (∼500°C) are analyzed. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 6, 2002, pp. 750–753. Original Russian Text Copyright ? 2002 by Violina, Kalinina, Kholujanov, Kossov, Yafaev, Hallén, Konstantinov.     

Optical and electrical properties of 4<Emphasis Type="Italic">H</Emphasis>-SiC irradiated with fast neutrons and high-energy heavy ions

Photoluminescence and deep-level transient spectroscopy are used to study the effect of irradiation with fast neutrons and high-energy Kr (235 MeV) and Bi (710 MeV) ions on the optical and electrical properties of high-resistivity high-purity n-type 4H-SiC epitaxial layers grown by chemical vapor deposition. Electrical characteristics were studied using the barrier structures based on these epitaxial layers: Schottky barriers with Al and Cr contacts and p⁺-n-n⁺ diodes fabricated by Al ion implantation. According to the experimental data obtained, neutrons and high-energy ions give rise to the same defect-related centers. The results show that, even for the extremely high ionization density (34 keV/nm) characteristic of Bi ions, the formation of the defect structure in SiC single crystals is governed by energy losses of particles due to elastic collisions.     

The characteristics of an enhancement mode VFET

This paper describes a junction FET fabricated by adjusting the depth of a V-Groove anisotropically etched in a thick epitaxial silicon layer to obtain an enhancement mode device. A simple theory and experimental characteristics of the device are presented. The technique can be extended to the fabrication of Schottky barrier enhancement MESFETs for micropower digital applications.     

The use of a four-point probe for profiling sub-micron layers

A number of cautionary measures have been found necessary in applying the layer stripping/four-point probe method to profiling thin epitaxial, ion-implanted and diffused layers. To minimize penetration of the layer by the probe, and carrier injection effects, low probe pressure must be used in conjunction with checks on the voltages developed between the current (outer) probes and the potential (inner) probes. Surface depletion in n-layers results in an apparent translation of the measured profile towards the surface, and for ion-implanted layers further results in a systematic distortion of the profile. These comments apply equally to layer stripping/Hall-effect profiling techniques.     

‘Modulation effect by intense hole injection’ in epitaxial silicon Schottky-barrier-diodes

The forward characteristics of different epitazial n-n⁺ silicon Schottky barrier diodes have been studied up to high current densities. Modulation has been observed in these experiments, that means an increase in charge carrier density in the series resistance region, i.e. in the epitaxial n-layer. By appropriate analysis of the forward characteristics the carrier density can be determined as a function of the current density. In accordance with the predictions of Scharfetter we find that the modulation increases with the barrier height but decreases with rising donor density. Furthermore it is shown how modulation is affected by recombination centres. The modulation effect is attenuated by irradiation with 1·5 MeV electrons and in a similar manner by doping the epitaxial layer with gold.     

Special features of electrical activation of 28Si in single-crystal and epitaxial GaAs subjected to rapid thermal annealing

Concentration profiles of ²⁸Si implanted in single-crystal and epitaxial GaAs were determined by measuring the C-V characteristics after the postimplantation rapid thermal annealings for 12 s at T=825, 870, and 905°C. The temperature dependence of Hall mobility of electrons in the Si-implanted layers subjected to the same annealings was determined by the Van der Pauw method within the range of 70–400 K. As distinct from conventional thermal annealing (for 30 min at 800°C), the rapid thermal annealing brings about a diffusive redistribution of silicon to deeper layers of GaAs for the materials of both types, with the diffusivity of silicon being twice as high in single-crystal GaAs as that in GaAs epitaxial layers. Analysis of temperature dependence of electron mobility in ion-implanted layers following a rapid thermal annealing indicates a significantly lower concentration of the defects limiting the mobility as compared to the case of a conventional thermal annealing for 30 min.     

Noise in two-tier matrix amplifiers

A noise theory for the two-tier matrix amplifier is developed that permits the computation of the amplifier's noise figure as a function of the active device and circuit parameters. The computed results based on the noise parameters of a GaAs MESFET with gate dimensions 0.25 μm×200 μm are discussed. In addition, a comparative study is done on the performance data from a 2×4 matrix amplifier and its equivalent two-stage distributed amplifier. Finally, the noise characteristics of two 2×4 matrix amplifiers incorporating GaAs MESFETs processed on either ion-implanted or VPE (vapor-phase epitaxial) substrate material are compared with those measured on actual amplifiers     

Poly I2L with deposited polysilicon collector

A new high speed high density poly I²L structure with deposited polysilicon collector is analyzed and modeled. The switching speed of the proposed poly I²L structure is 4 times higher than that of the conventional structures and the packing density is improved by a factor of 2.The proposed poly I²L structure is gnvestigated using a developed computer simulation model. Parameters sensitivity analysis of the structure is given. The minimum gate delay decreases as the intrinsic base sheet resistivity is increased and as the thin epitaxial layer under the base is decreased. Down scaling effects are discussed. It is shown that a lateral shift of PDP curves along the current axis is proportional to the change in the device area and the IR drop in npn base is proportional to the scaling factor.A structure with technology linewidth L = 2.5 μm exhibits minimum gate delay of 0.6 ns at 150 μA for fan-out F = 3, and a power-delay product of 30 fJ at low current levels. Simulation results are compared with experimental measurements performed on a given poly I²L structure and good agreement has been observed.     

SiGe drift base bipolar transistor with self-aligned selectiveCVD-tungsten electrodes

A new device and process technology is developed for high-speed SiGe epitaxial base transistors. A 60-nm SiGe epitaxial base and the selectively ion-implanted collector (SIC) structure enhance the cutoff frequency to about 40 GHz. Base resistance is minimized to 165 Ω (emitter area: 0.2×3 μm²), and an f_MAX of 37.1 GHz is achieved by employing 0.2-μm EB lithography for the emitter window, selective CVD tungsten for the base electrode and a self-aligned oxide side wall for the emitter-to-base separation. Circuit simulations predict that this device could reduce the ECL gate delay to below 20 ps     

SRAM devices and circuits optimization toward energy efficiency in multi-Vth CMOS

Minimum-energy-driven circuit design is highly required in numerous emerging applications such as mobile electronics, wireless sensor nodes, implantable biomedical devices, etc. Due to high computing capability requirements in such applications, SRAMs play a critical role in energy consumption. This paper presents SRAM energy analysis utilizing multi-threshold (multi-V_th) voltage devices and various circuit techniques for power reduction and performance improvement, and suggests optimal device combinations for energy efficiency improvement. In general, higher-V_th devices are preferred in the cross-coupled latches and the write access transistors for reducing leakage current while lower-V_th devices are desired in the read port for implementing higher performance. However, excessively raised V_th in the write paths, i.e. the cross-coupled latches and the write access transistors, leads to slower write speed than read, quickly nullifying improved energy efficiency. In this work, the energy efficiency improvement of 6.24× is achieved only through an optimal device combination in a commercial 65 nm CMOS technology. Employing power reduction and performance boosting techniques together with the optimal device combination enhances the energy efficiency further up to 33×.     

Spectrometric properties of SiC detectors based on ion-implanted p +-n junctions

Results of spectrometric studies of nuclear radiation detectors based on p ⁺-n junctions formed in 4H-SiC films are presented for the first time. The junctions were fabricated by ion implantation of aluminum into 26-μm-thick CVD-grown epitaxial 4H-SiC layers with an uncompensated donor concentration of (3–5) × 10¹⁵ cm^?3. The detector characteristics were measured in testing with natural-decay alpha particles with energies of 3.35 and 5.4 MeV. The collection efficiency of charge generated by 3.35 MeV alpha particles was as high as 100% at an energy resolution of ? 2%.     

<Emphasis Type="Italic">In Situ</Emphasis> Stress Measurements During GaN Growth on Ion-Implanted AlN/Si Substrates

In situ wafer curvature measurements were used in combination with postgrowth structural characterization to study the evolution of film stress and microstructure in GaN layers grown by metalorganic chemical vapor deposition on N⁺ ion-implanted AlN/Si (111) substrates. The results were compared with growth on identical unimplanted substrates. In situ stress measurements revealed that, for the unimplanted sample, the GaN initiated growth under compressive stress of −1.41 GPa which arose due to lattice mismatch with the AlN buffer layer. In contrast, GaN growth on the ion-implanted sample began at lower compressive stress of −0.84 GPa, suggesting a reduction in epitaxial stress. In both cases, the compressive growth stress was fully relaxed after ~0.7 μm and minimal tensile stress was generated during growth. During post-growth cooling, tensile stress was introduced in the GaN layer of both samples due to thermal expansion mismatch. Post-growth optical microscopy characterization, however, demonstrated that the ion-implanted sample had lower density of channeling cracks compared with the unimplanted sample. Cross-sectional transmission electron microscopy images of the sample grown on ion-implanted Si with no post-implantation nitrogen annealing revealed the formation of horizontal cracks in the implanted region beneath the AlN buffer layer. The weakened layer acts to decouple the GaN film from the Si substrate and thereby reduces the density of channeling cracks in the film after growth.     

Higher Dislocation Density of Arsenic-Doped HgCdTe Material

There is a well-known direct negative correlation between dislocation density and optoelectronic device performance. Reduction in detector noise associated with dislocations is an important target for improvement of mercury cadmium telluride (Hg_1?x Cd _x Te)-based material in order to broaden its use in the very long-wavelength infrared (VLWIR) regime. The lattice mismatch and differences in physical properties between substrates and the epitaxial Hg_1?x Cd _x Te layers cause an increased threading dislocation density. As demonstrated in this work, the presence of arsenic impurities via p-type doping in molecular beam epitaxy (MBE)-grown epitaxial crystal structure increases the etch pit density (EPD) of Hg_1?x Cd _x Te grown on Si substrates but not on CdZnTe substrates. This EPD increase is not observed in indium n-type-doped Hg_1?x Cd _x Te grown on either Si or CdZnTe substrates. This trend is also seen in layers with different cadmium compositions. All of the EPD variations of the structures studied here are shown to be independent of the MBE machine used to grow the structure. The fundamentals of this higher EPD are not yet completely understood.     

Gain improvement in operational transconductance amplifiers using Graded-Channel SOI nMOSFETS

This paper studies the performance of operational transconductance amplifiers (OTAs) fabricated with Graded-Channel (GC) SOI nMOSFETs and designed to provide high open-loop voltage gain or high gain-bandwidth characteristics. Different design targets were taken in account such as similar power dissipation, transconductance over drain current ratio and die area. Comparisons with OTAs made with conventional SOI nMOSFETs, are performed showing that the GC OTAs presents larger open-loop voltage gain without degrading the phase margin, unit gain frequency and slew rate simultaneously with a significant required die area reduction depending on L_LD/L ratio used. Circuit simulations and experimental results are used to qualify the analysis.     

The influence of ion-implanted profiles on the performance of GaAsMESFET's and MMIC amplifiers

The RF small-signal performance of GaAs MESFETs and MMIC amplifiers as a function of various ion-implanted profiles is theoretically and experimentally investigated. Implantation energy, dose, and recess-depth influence are theoretically analyzed with the help of a novel device simulator. The performance of MMIC amplifiers processed with various energies, doses, recess depths, and bias conditions is discussed and compared to experimental characteristics. Some criteria are proposed for the choice of implantation conditions and process in order to optimize the characteristics of ion-implanted FETs and to realize process-tolerant MMIC amplifiers     

A vacancy model of the heteropolytype epitaxy of SiC

A model for the transformation of SiC polytypes occurring during the growth of an epitaxial layer is suggested that is based on the variation over time of the concentration of carbon vacancies in a transition layer. Experimental data are analyzed in terms of this model. It is shown that the parameter η=Gτ/L _T (L _T is the thickness of the transition layer, G is the film growth rate, and τ is the lifetime of a vacancy in the transition layer) is invariant with respect to the method and temperature of the growth of the epitaxial layer. This parameter is determined only by the concentration of carbon vacancies in the substrate and in the film.     

Radiation hardness of SiC ion detectors under relativistic protons

Schottky diodes based on 6H-SiC epitaxial films exposed to 1000 MeV protons at a dose of 3×10¹⁴ cm^?2 have been studied by precision alpha spectrometry. Parameters of deep levels introduced by protons were determined by deep-level transient spectroscopy. The number of vacancies generated in proton tracks was found using TRIM software. The width of the space charge region and the hole diffusion length before and after irradiation were obtained by processing the alpha-spectrometry and capacitance measurements. Minor variations in the charge transport properties of epitaxial 6H-SiC detectors were observed.     

Double-polysilicon SiGe HBT architecture with lateral base link

We present an analysis of a modified double-polysilicon SiGe:C HBT module showing a CML ring oscillator gate delay τ_D of 2.5 ps, and f_T/f_max/BV_CEo values of 300 GHz/350 GHz/1.85 V (Fox et al., 2008) [1]. A key feature of the HBT module is a connection of the extrinsic and intrinsic base regions by lateral epitaxial overgrowth, which aims to overcome the limits of the conventional double-polysilicon architecture in simultaneously reducing R_B and C_BC. Potential benefits and barriers of the proposed device structure on the way to higher performance are reviewed with regard to the recently demonstrated performance gain of the classical double-polysilicon approach. The paper addresses technological challenges one is faced when the here presented device structure is scaled to minimum device dimensions.