Properties of p- and n-Type PbTe Microwires for Thermoelectric Devices

In thermopower measurements, microwires fabricated from as-purchased bulk PbTe exhibits p-type behavior between room temperature and ～600 K. At higher temperatures, it undergoes majority carrier inversion and exhibits n-type behavior. We report on the preparation and properties of potassium oxide and Zn-doped PbTe microwires, which exhibit stable p- and n-type behavior, respectively, between room temperature and 725 K. Thermoelectric figures of merit (ZT) are reported for device components prepared from bundles of such p- and n-type microwires in a glass matrix.     

A structure-oriented model for determining the substrate spreading resistance in bulk CMOS latch-up paths and its application in holding current prediction

A structure-oriented model has been developed to simulate the actual distribution of majority-carrier current flow paths in the substrate when the parasitic p-n-p-n structure with long-stripe geometry in a CMOS (complementary metal—oxide-semiconductor) circuit is at the latch-up state. Based on this structure-oriented model, the voltage drop across the latch-up path in the substrate can be calculated directly from the structure data. Therefore, the equivalent emitter-base shunting resistance in the substrate can be easily obtained and used to accurately predict the holding current. The two-dimensional numerical simulations have been carried out, based on this structure-oriented model, to obtain the emitter-base shunting resistance associated with the parasitic lateral bipolar transistor in the substrate. The computed substrate shunting resistance and the well emitter-base shunting resistance have been used to calculate the holding current with the help of the measured peak parasitic transistor gains. The predicted holding currents have been found to be in good agreement with the experimental data measured from several p-n-p-n structures, including normal and reversed layouts which are all designed by using the long-stripe geometries. Furthermore, the numerical simulations have been extended to predict the effects of the layout changes of the p-n-p-n structures on the latch-up susceptibility.     

Simultaneous Formation of Ohmic Contacts on p +- and n +-4H-SiC Using a Ti/Ni Bilayer

In this work, Ti/Ni bilayer contacts were fabricated on both p ⁺- and n ⁺-4H-SiC formed by ion implantation, and the effects of the Ti interlayer on the contact resistance and interfacial microstructure were studied. Adoption of a thin (10 nm) Ti interlayer resulted in specific contact resistance of 4.8 μΩ cm² and 1.3 mΩ cm² on n ⁺- and p ⁺-4H-SiC, respectively, comparable to the values for contacts using only Ni. Moreover, contacts using Ti/Ni provide a flat and uniform interface between Ni₂Si and SiC, whereas discontinuous, agglomerated Ni₂Si islands are formed without the use of a Ti interlayer. In addition, the Ti interlayer was demonstrated to effectively dissociate the thin oxide film on SiC, which is advantageous for low-resistance, reliable ohmic contact formation. In summary, use of a Ti/Ni bilayer is a promising solution for one-step formation of ohmic contacts on both p ⁺- and n ⁺-4H-SiC, being especially suitable for SiC n-channel metal-oxide-semiconductor field-effect transistor (nMOSFET) fabrication.     

Electrical characteristics of r.f.-sputtered CdTe thin-films for photovoltaic applications

A method of preparing self-doped p- and n-type and In-doped n-type CdTe thin-films for photovoltaic applications has been developed using r.f. sputtering. Ohmic contacts to n-type films with contact resistivity less than 10^?2 Ω — cm² have been obtained. Schottky barrier diode test devices, formed by evaporation of various metals including Au on n-CdTe films, have been examined for electrical and photovoltaic evaluation of the sputtered films. Although S.B. diodes based on In doped films, prepared under Cd overpressure, show promising electrical and photovoltaic performance (V_oc ～ 315 mV, I_sc ～ 4.6mA/cm²), much improvement remains to be made by further control of dopant concentration and structural details of films.     

A comparison of Pd Schottky contacts on InP,GaAs and Si

The Schottky-barrier energy φ_B of Pd contacts on InP, GaAs and Si were measured and the metallurgical behavior of the contact structures were studied using Auger electron spectroscopy. A carefully processed set of samples were used to show conclusively that φ_B is greater on p-InP that on n-InP, unlike the behavior of GaAs and Si Schottky diodes fabricated at the same time with similar processing steps.     

Silicon solar cells using natural inversion layers found in thermally-oxidized p-silicon

Inversion layer silicon solar cells are described which employ the natural inversion layer occurring at the surface of thermally-oxidized p-type silicon as one side of an induced n-p junction. Very shallow junctions are predicted theoretically with high electric fields in a direction to aid the collection of carriers generated by light of ultra-violet wavelengths. Collection efficiency calculations show the inversion layer cell to be less sensitive to lifetime and surface recombination velocity variations than diffused junction cells. Experimental 2 cm × 2 cm cells have been fabricated with the inversion layer contacted via a fine diffused n⁺ grid overlaid with a Ni-Cu-Au contact. The contact grid, specially designed to minimize the effect of the high inversion layer sheet resistance, produced a total shading of 16%. Illuminated I-V measurements confirm the induced junction to be near ideal, with an ideality factor $\text{A ? 1.05}$ and a reverse saturation current approaching that predicted theoretically. Conversion efficiencies of ? 8% have been obtained, with no special precautions being taken to reduce the series resistance of the back contact, or reflections at the front surface.     

Degradation mechanism for silicon p+-n junctions under forward bias

Leakage current degradation has been observed during forward bias stressing of silicon integrated p⁺-n junctions. Detailed characterization results of the anomalous leakage behavior are discussed in this paper. From these results an electric field-enhanced impurity diffusion mechanism has been proposed to explain both the strong temperature and forward bias dependencies on leakage current time-to-saturation. An activation energy has been determined for this mechanism (0.48±0.04 eV) and is in good agreement with that previously determined for diffusion of interstitial copper in p-type silicon. Subsequent Secondary Ion Mass Spectrometer elemental analysis has confirmed the presence of copper near the surface of the epitaxial layer containing the p⁺-n device.     

Effect of annealing on the spectra of nuclear quadrupole resonance in gallium-indium selenides and characteristics of structures based on these materials

The results of studying the effect of low-temperature annealing (at temperatures no higher than 250°C) on the spectra of nuclear quadrupole resonance in layered GaSe and InSe single-crystal semiconductors are reported. The electrical and photoelectric characteristics of the p-GaSe-n-InSe structures have also been studied. It is shown that, as the annealing temperature is lowered to room temperature, the quality of the samples improves due to a decrease in the defect concentration in the crystals and due to ordering in a system of polytypes. The temperature conditions for the heat treatments, at which the main parameters of the heterojunction are improved, are determined. Mechanisms affecting the behavior of the p-GaSe-n-InSe structure during the course of annealing are discussed.     

Properties of the contact on ion cleaned n and p type silicon surfaces

Schottky contacts were produced by silver evaporation on Si(100) surfaces cleaned by ion sputtering and partial annealing. The samples work function were measured before and after metal deposition with the Kelvin method, in an experimental set-up which allowed a topografical study and direct comparison between n and p types. Clean surfaces with and without a residual layer of oxide was achieved and controlled by AES. It was found that the Fermi level of all the surfaces was pinned by donor states created by the bombardment and that there was no barrier on n type and an important surface barrier on p type. The diodes we obtained presented no barrier on n type and a rectifying contact on p type. So we deduced that the Schottky barrier is already fully formed before metal contact is achieved. Furthermore study of the electrical properties of the diodes had shown that the bombardment creates donor states responsible for the barrier and a perturbated layer with deep acceptor traps responsible for the current flow mechanism. A residual layer of oxide and a post annealing of the device did not noticeably change the Schottky barrier in the diodes achieved on p type but led to clearly differenciated performances for the diodes achieved on both p and n type substrate. So we therefore concluded that the characteristics of the deep acceptor traps of the superfacial layer are modified by the oxide and annealing, both of which on the other hand having no effect on the surface donor states.     

CdZnTe Radiation Detectors with HgTe/HgCdTe Superlattice Contacts for Leakage Current Reduction

CdZnTe-based heterojunction p?Ci?Cn or M?C???Cn detectors using HgTe/HgCdTe superlattice contacts are modeled and designed to reduce leakage currents under high electric fields and thereby improve x-ray and ??-ray detector performance. The employment of an n-type HgTe/HgCdTe superlattice as a contact layer can theoretically result in significantly less leakage current compared with a contact layer using either bulk semiconductor or metal contacts. The benefits arise from the ability to design HgTe/HgCdTe superlattices to have large carrier effective masses in the electric field direction, which results in low carrier velocities. Nevertheless the density of states is lower than that of a comparable bulk semiconductor, which results in low carrier concentrations.     

Low resistance ohmic contacts containing Sb to GaP

A comparison has been made of the contact resistances of AuZn and AuZnSb alloyed contacts to p-type GaP and of AuGeNi and AuGeNiSb alloyed contacts to n-type GaP. The addition of antimony contact materials reduces the contact resistance and this reduction is greatest when the process of contacting is carried out at temperatures below 500°C.     

Depletion layer formation,space-charge injection and current-voltage characteristics for the silicon p-n-p (n-p-n) structure

Depletion layer formation and current-voltage characteristics are described for the general semiconductor p-n-p (n-p-n) structure in which the impurity or defect centre is able to communicate with both sets of transport levels. All possibilities for current lie within the region bounded on one side by the essentially vertical Shockley-Prim punch-through characteristic and on the other side by the square-law Mott-Gurney space-charge-limited characteristic. If the impurity levels lie near the mid-gap position a variety of characteristics within this region can be expected. Representative current-voltage characteristics have been computed and are described for a typical silicon structure.     

Simultaneous Formation of Ni/Al Ohmic Contacts to Both n- and p-Type 4H-SiC

The fabrication procedure for silicon carbide power metal oxide semiconductor field-effect transistors can be improved through simultaneous formation (i.e., using the same contact materials and a one-step annealing process) of ohmic contacts on both the n-source and p-well regions. We have succeeded in the simultaneous formation of Ni/Al ohmic contacts to n- and p-type SiC after annealing at 1000°C for 5 min in an ultrahigh vacuum. Ohmic contacts to n-type SiC were found when the Al-layer thickness was less than about 6 nm, while ohmic contacts to p-type SiC were observed for an Al-layer thickness greater than about 5 nm. Only the contacts with an Al-layer thickness in the range of 5 nm to 6 nm exhibited ohmic behavior to both n- and p-type SiC, with a specific contact resistance of 1.8 × 10⁻⁴ Ω cm² and 1.2 × 10⁻² Ω cm² for n- and p-type SiC, respectively. An about 100-nm-thick contact layer was uniformly formed on the SiC substrate, and polycrystalline δ-Ni₂Si(Al) grains were formed at the contact/SiC interface. In the samples that exhibited ohmic behavior to both n- and p-type SiC, the distribution of the Al/Ni ratios in the δ-Ni₂Si(Al) grains was larger than that observed for any of the samples that showed ohmic behavior to either n- or p-type SiC. Furthermore, the grain size of the δ-Ni₂Si(Al) grains in the samples showing ohmic behavior to both n- and p-type SiC was smaller than the grains in any of the samples that showed ohmic behavior to either n- or p-type SiC. Thus, the large distribution in the Al/Ni ratios and a fine microstructure were found to be characteristic of the ohmic contacts to both n- and p-type SiC. Grains with a low Al concentration correspond to ohmic contacts to n-type SiC, while grains with a high Al concentration correspond to ohmic contacts to p-type SiC.     

Aluminum-silicon ohmic contact on “shallow” n+p junctions

In this work various technologies for contacts of Al on n⁺Si have been experimentally investigated, particularly in view of their suitability to very shallow $\text{n}\text{p}$ junctions.Special test-patterns have been used to measure the contact resistivity, while diodes reverse current density has been checked to evaluate the junction leakage induced by the aluminum-silicon interaction during sintering.Best results are obtained by depositing a thin polysilicon layer on the front surface before the doping process and the Al evaporation. In this case both the requirements of low contact resistivity (< 10^?4 ohm · cm²) and low junction leakage current are satisfied. Comparison with the conventional Al/Si and AgTi/Si ohmic contacts has been performed.     

Electrical characteristics of laser-contacted diodes

We have fabricated p⁺-n and Schottky diodes with contacts made of laser-formed palladium-silicide. The electrical characteristics of these diodes are presented. The reverse currents and breakdown voltages are comparable to conventionally contacted p⁺-n diodes. The barrier height of laser-formed Schottky diodes agrees well with published values for Pd₂Si. The promising results point out the potential applications of contact formation by laser irradiation in device manufacture.     

A technique for modelling p-n junction depletion capacitance of multiple doping regions in integrated circuits

The continuing advancements in integrated circuit technology have placed new burdons on the circuit design engineer, who must rely extensively upon computer simulation to correctly predict circuit behavior. One challenge is to develop better modelling techniques to more accurately deal with complex p-n junction structures often used in modern VLSI designs. This paper presents an easily implemented method for deriving parameters which accurately model the behavior of MOS VLSI structures containing complex p-n junction capacitance components. The methodology is applicable to both planar and laterally diffused junctions, whether formed by direct ion implantation or by diffusion from a finite or infinite source. The theories behind the equations used and results of the application of this new technique are discussed. A flow chart for a fitter program based on the new method is presented and described. The corresponding program written for the TI-59 scientific programmable calculator is available. Final model parameters are given and are shown to produce a numerical capacitance model which is accurate to within 2%.     

Finite metal-sheet-resistance in contact resistivity measurements: Application to Si/TiN contacts

The standard transmission line model cannot be applied to evaluate the contact resistivity of thin TiN layers on highly doped p⁺ and n⁺ substrates because the finite sheet resistance of the TiN must be accounted for. We present two ways to include this effect using existing analytical models. The results are shown to agree with measurements where the effect of the finite sheet resistance of TiN is eliminated with a metallic overlayer. With the help of these evaluation techniques, it is shown that the contact resistivity of TiN changes in opposite ways for p⁺ and n⁺Si after vacuum annealing at 600°C for 15 min. This result is consistent with an increase of the barrier height φ_Bn of the contact by ?0.1 V to near midgap value.     

Numerical Estimations of Carrier Generation–Recombination Processes and the Photon Recycling Effect in HgCdTe Heterostructure Photodiodes

An enhanced computer program has been applied to explain in detail the photon recycling effect which drastically limits the influence of radiative recombination on the performance of p-on-n HgCdTe heterostructure photodiodes. The computer program is based on a solution of the carrier transport equations, as well as the photon transport equations for semiconductor heterostructures. We distinguish photons in two energy ranges according to p ⁺ and n region with unequal band gaps. As a result, both the distribution of thermal carrier generation and recombination rates and spatial photon density distribution in photodiode structures have been obtained. The general conclusion, similar to our earlier work concerning 3-μm n-on-p HgCdTe heterostructure photodiodes, confirms the previous assertion by Humphreys that radiative recombination does not limit HgCdTe photodiode performance.     

Controlling of electronic parameters of GaAs Schottky diode by poly(3,4-ethylenedioxithiophene)-block-poly(ethylene glycol) organic interlayer

The electronic properties of metal-organic semiconductor-inorganic semiconductor structure between GaAs and poly(3,4-ethylenedioxithiophene)-block-poly(ethylene glycol) organic film have been investigated via current-voltage and capacitance-voltage methods. The Au/PEDOT/n-GaAs contact exhibits a rectification behavior with the barrier height of 0.69 eV and ideality factor value of 3.94. The barrier height of the studied diode (0.67 eV) is lower than that of Ni/n-GaAs/In (0.85 eV) and Au/n-GaAs/In Schottky diodes. The decrease in barrier height of Au/n-GaAs/In Schottky diode is likely to be due to the variation in the space charge region in the GaAs. The obtained results indicate that control of the interfacial potential barrier for metal/n-GaAs diode was achieved using thin interlayer of the poly(3,4-ethylenedioxithiophene)-block-poly(ethylene glycol).     

Low resistance ohmic contacts to n- and p-InP

The contact properties of various metal combinations, deposited by vacuum evaporation on InP, were studied. Among these metal combinations, Au/Ge + Ni and Au/Zn proved to be most suitable. The former on n-InP (n = 8 × 10¹⁷/cm³) and the latter on p-InP (p = 9 × 10¹⁷/cm³) exhibited specific contact resistances as low as 1.2 × 10^?6 and 1.1 × 10^?4 Ωcm², respectively. The specific contact resistances were analyzed using a four-point method which also accounts for the spreading resistance. Furthermore, the resistances of metal contacts to InP were calculated as a function of doping concentration and were compared with the experimental results. The described contacting technique was successfully applied to the preparation of quaternary lasers.