Al/Poly Si specific contact resistivity

Experimental results on the specific contact resistivity of Al/polysilicon are given for Al/1.5-percent Si contacting poly Si implanted with boron or phosphorus, annealed to surface concentrations from 3E18 to 4E20 cm^-3. Specific contact resistivities of the interfaces involved were determined using an extrapolation method. Measurements were taken at room temperature, and were conducted both before and after an anneal cycle of 20 min at 450°C in forming gas. Results provide a useful parameter which enables modeling of Al/poly Si contacts.     

Specific contact resistivity of indium contacts to n-type CdTe

Indium alloyed to n-type CdTe of about 10¹⁶ cm^-3 electron concentration provides a contact resistivity of about 7 x 10^-3 ohm cm². This is achieved by alloying for 10 minutes at 150-450‡C in a sealed ampoule with an overpressure of cadmium. If the alloying is done in an open tube H₂ flow without a Cd vapor overpressure, alloying temperatures above 250‡C cause the contact resistance to rise as cadmium vacancies increase the compensation in the CdTe. Further improvement of the contact resistivity to 1 x 10^-3 ohm cm is obtained by a 900‡C diffusion of In into the n-CdTe (electron concentration 10¹⁶ cm^-3 before the diffusion).     

Allowing for current spreading in semiconductors during measurements of the contact resistivity of ohmic contacts

A modification of the contact-area pattern with radial geometry, which has certain advantages in determining the contact resistivity of ohmic contacts (ρ _c) fabricated on substrates and low-resistance semiconductor layers, is proposed. Different variants of its application for both the transmission line method (TLM) and methods based on a numerical calculation of the resistance of the semiconductor with allowance for current spreading are considered. It is shown that the transmission line method makes it possible to obtain an upper estimate of the contact resistivity on substrates. The errors of such estimates are also calculated as a function of the parameters of the semiconductor and the contact. The TLM estimate is a good first approximation for determining the exact value of ρ _c by numerically calculating the resistance of the semiconductor. The results obtained are used to study the contact resistivity of Ni-based ohmic contacts on n-6H-SiC substrates. Fiz. Tekh. Poluprovodn. 32, 832–837 (July 1998)     

Determining specific contact resistivity from contact end resistance measurements

A method is described to determine specific contact resistivity from contact end resistance measurements using a transmission line model. A test pattern is described which minimizes the effect of current fringing around contact corners and yields an accurate determination of contact width. With this pattern, the specific contact resistivities measured on 1.3 wt % Si/Al contacts to n+ silicon junctions with different dopings show very consistent values and are independent of contact geometries. The dependence of measured specific contact resistivities on doping concentration is also in good agreement with the predictions of tunneling theory. Surprisingly, the dependence on surface concentration extends well beyond the usual range of electrically active solid solubility.     

Calculations and measurements of contact resistance of semi-transparent Ni/Pd contacts to p-GaN

Calculations of specific contact resistance as a function of doping and barrier height were performed for p-type GaN. These calculations took into account two valence bands, each with different effective masses, and show that at low doping, the heavy hole band accounts for most of the conduction, whereas at heavier doping, the light hole band dominates conduction. These calculations also indicate the barrier height for typical contacts to p-GaN is between 0.75 eV and 1 eV. Specific contact resistance measurements were made for oxidized Ni/Au, Pd, and oxidized Ni/Pd ohmic contact metal schemes to p-GaN. The Ni/Pd contact had the lowest specific contact resistance, 6×10⁻⁴ Ω cm². Auger sputter depth profile analysis showed some Ni diffused away from the GaN surface to the contact surface with the bulk of the Pd located in between two areas of Ni. Both Ni and Pd interdiffused with the GaN at the semiconductor surface. The majority of the oxygen observed was with the Ni as NiO. Angle-resolved-x-ray photoelectron spectroscopy (AR-XPS) analyses showed the formation of predominantly NiO and PdO species, with higher Ni and Pd oxides at the contact surface.     

Domain epitaxial growth of TiN/Si(001), TiN/GaAs(001), and Si/TiN/Si(001) heterostructures by laser physical vapor deposition: Theory and experiment

We have successfully deposited epitaxial titanium nitride films on (001) silicon and (001) gallium arsenide substrates and multilayer Si/TiN/Si(001) epitaxial heterostructures using pulsed laser (KrF: λ = 248 nm, τ = 25 ns) physical vapor deposition. The deposition of TiN was carried out at a substrate temperature of 600°C on Si(001) and 400°C on GaAs(00l). The interfaces were sharp without any indication of interfacial reaction. The epitaxial relationships were found to be <001> TiN ‖<001> Si on the silicon substrate, <001> Si ‖<001> TiN |<001> Si on the heterostructure, and [1-10] TiN‖[110] GaAs and [001] TiN ‖[110] GaAs on the GaAs substrate. The growth in these large-mismatch systems is modeled and the various energy terms contributing to the growth of these films are determined. The domain matching epitaxy provides a mechanism of epitaxial growth in systems with large lattice mismatch.The epitaxial growth is characterized by domain epitaxial orientation relationships with m lattice constants of epilayer matching with n of the substrate and with a small residual domain mismatch present in the epilayer. This residual mismatch is responsible for a coherent strain energy. The magnitude of compression of Ti-N bond in the first atomic layer, contributing to the chemical free energy of the interface during the initial stages of growth, is found to be a very important factor in determining the orientation relationship. This result was used to explain the differences in the orientaion relationships between TiN/Si and TiN/GaAs systems. The various energy terms associated with the domain epitaxial growth are evaluated to illustrate that the domain epitaxial growth is energetically favorable compared to the lattice mismatched epitaxial growth. The results of this analysis illustrate that the observed variations in the epitaxial growth are consistent with the minimum energy configurations associated with the domain epitaxial growth.     

Low resistivity as-deposited ohmic contacts to 3C-SiC

The contact resistivities of Al and Ti ohmic contacts to n-type 3C-SiC were measured using the circular TLM method. The surface doping concentration under the contact was increased by ion-implantation of nitrogen into SiC. The contact resistivity was observed to decrease with increasing surface doping concentration for both Al and Ti contacts. The minimum value for the contact resistivities for Aland Ti contacts was 1.4x 10^-5and 1.5 x 10^-5 ω cm², respectively, at the surface doping concentration of 3 x 10²⁰ cm^-3 without any annealing of the contacts. These values are an order of magnitude lower than previously reported minimum values for as-deposited ohmic contacts on n-type 3C-SiC.     

Alloyed Si/Al-based ohmic contacts to AlGaN/GaN nitride heterostructures

For the first time in Russia, the Si/Al/Ti/Au alloyed contact composition is investigated for the formation of ohmic contacts to AlGaN/GaN heterostructures using thermal annealing. The obtained results are compared with those for conventional Ti/Al/Ni/Au ohmic contacts. Use of the composition under investigation makes it possible to decrease the annealing temperature to 675–700°C, which results in improvement in the morphology of alloyed ohmic contacts in comparison with conventional contacts. The value of the contact resistance using the Si/Al-based composition to the AlGaN/GaN heterostructure is obtained in relation to the temperature and annealing duration. It is shown that no qualitative change in the resistance occurs at an annealing duration of several minutes in the temperature range of 700–750°C. In the temperature range of 675–700°C, there is an asymptotic decrease in the contact resistance with increasing annealing duration. The smallest value of the contact resistance amounts to 0.41 Ω mm.     

Study of contact resistivity, mechanical integrity, and thermal stability of Ti/Al and Ta/Al ohmic contacts to n-type GaN

The annealing conditions and contact resistivities of Ta/Al ohmic contacts to n-type GaN are reported for the first time. The high temperature stability and mechanical integrity of Ti/Al and Ta/Al contacts have been investigated. Ta/Al (35 nm/115 nm) contacts to n-type GaN became ohmic after annealing for 3 min at 500°C or for 15 s at 600°C. A minimum contact resistivity of 5×10⁻⁶Ω cm² was measured after contacts were repatterned with an Al layer to reduce the effect of a high metal sheet resistance. Ti/Al and Ta/Al contacts encapsulated under vacuum in quartz tubes showed a significant increase in contact resistivity after aging for five days at 600°C. Cross section transmission electron microscopy micrographs and electrical measurements of aged samples indicate that the increased contact resistivity is primarily the result of degradation of the metal layers. Minimal reactions at the metal/GaN interface of aged samples were observed.     

A further comment on "Determining specific contact resistivity from contact end resistance measurements"

The purpose of this comment is to contribute to a better understanding of the influence of lateral current crowding, sheet resistance, and interface pitting in the determination of the interface contact resistivity in four terminal resistor test patterns, for two different metallization schemes, i.e., 1Al/n⁺Si and (Al + 1.5-percent Si)/n⁺Si.     

Current crowding effects and determination of specific contact resistivity from contact end resistance (CER) measurements

Current crowding effects on Contact End Resistance (CER) test structures due to the finite diffusion overlap of the contact window are studied by experiment and numerical simulation. This finite overlap adds a parasitic resistance component not accounted for by the standard one-dimensional theory, and if uncorrected, this parasitic resistance may lead to gross overestimation of the true specific contact resistivity ρc. The overestimate increases with increasing diffusion sheet resistance and large contact size. Excellent agreement between experiment and simulations has allowed this effect to be modeled. Accurate values of ρcin the range of 5 × 10^-8to 2 × 10^-5Ω cm²are extracted using CER structures.     

Electrical characterization of annealed Ti/TiN/Pt contacts onN-type 6H-SiC epilayer

We report results of the electrical characteristics of in vacuo deposited Ti/TiN/Pt contact metallization on n-type 6H-SiC epilayer as function of impurity concentration in the range of 3.3×10¹⁷ cm^-3 to 1.9×10¹⁹ cm^-3. The as-deposited contacts are rectifying, except for the highly doped sample. Only the lesser doped remains rectifying after samples are annealed at 1000°C between 0.5 and 1 min in argon. Bulk contact resistance ranging from factors of 10^-5 to 10^-4 Ω-cm² and Schottky barrier height in the range of 0.54-0.84 eV are obtained. Adhesion problems associated with metal deposition on pre-processed titanium is not observed, leading to excellent mechanical stability. Auger electron spectroscopy (AES) reveals the out diffusion of Ti-Si and agglomeration of Ti-C species at the epilayer surface. The contact resistance remains appreciably stable after treatment in air at 650°C for 65 h. The drop in SBH and the resulting stable contact resistance is proposed to be associated with the thermal activation of TiC diffusion barrier layer on the 6H-SiC epilayer during annealing     

Integrity of shallow junction CMOS structures with Ti/TiN/Al---Si---Cu and Ti/TiN/Al---Cu contact metallization

Shallow junction complementary metal oxide semiconductor (CMOS) structures (0.25 and 0.35 μm depth) were studied using sputter deposited Ti/TiN/Al---Si---Cu and Ti/TiN/Al---Cu films for contact metallization. Single contact Van de Pauw patterns (to measure the breakdown voltage) as well as large junction area structures with multiple contact windows were used for electrical measurements. An increase in the RTA temperature used to silicide the contacts increased the Si consumption in the junctions and resulted in degradation of junctions yields. The thickness of the Ti layer had a larger influence on the stability of the junction than the thickness of the TiN layer (in the range of thicknesses studied). Al---Si(0.75 wt%)---Cu(0.5 wt%) films are more stable than Al---Cu(0.5 wt%) films for junction spiking. The Al---Cu films are more reactive, and the interdiffusion of Ti into the Al---Cu films makes the junctions less stable. The annealing temperature and post wafer fabrication is critical in maintaining stability of junctions. The junction depths, and dopants (BF₂-p- and As n-implanted) used in forming the junctions affect the breakdown voltages and junction yields. The BF₂ implanted junctions are more stable than arsenic implanted junctions.     

Specific contact resistivity of InGaAs/InP p-isotype heterojunctions

The specific contact resistivity of lattice matched InGaAs/InP p-isotype heterojunction has been measured through the use of an interface transmission line model structure. The measured resistance values are comparable to or greater than those of the metal/semiconductor interface and depend heavily on the doping and the abrupt or graded nature of the interface.<>     

Application of electron holography to the determination of contact potential difference in an AlGaN/AlN/Si heterostructure

Electron holography was applied to determine the contact potential differences in an AlGaN/AlN/Si heterostructure formed by metallorganic vapor phase epitaxy. Since mean inner potentials are generally different for different materials, their values before and after forming the junction were evaluated first, then the contact potential difference was obtained by subtracting the difference of the mean inner potentials before forming the junction from the corresponding difference after forming the junction. The contact potential differences thus obtained were consistent with a reported asymmetric nonlinear behavior in the current-voltage characteristics measured for a similar heterojunction diode.     

Low-resistance bandgap-engineeredW/Si1-xGex/Si contacts

Bandgap-engineered W/Si_1-xGe_x/Si junctions (p⁺ and n⁺) with ultra-low contact resistivity and low leakage have been fabricated and characterized. The junctions are formed via outdiffusion from a selectively deposited Si_0.7Ge _0.3 layer which is implanted and annealed using RTA. The Si _1-xGe_x layer can then be selectively thinned using NH₄OH/H₂O₂/H₂O at 75°C with little change in characteristics or left as-deposited. Leakage currents were better than 1.6×10^-9 A/cm² (areal), 7.45×10^-12 A/cm (peripheral) for p⁺/n and 3.5×10^-10 A/cm² (peripheral) for n⁺/p. W contacts were formed using selective LPCVD on Si_1-xGe_x. A specific contact resistivity of better than 3.2×10^-8 Ω cm² for p ⁺/n and 2.2×10^-8 Ω cm² for n⁺/p is demonstrated-an order of magnitude n⁺ better than current TiSi₂ technology. W/Si_1-xGe _x/Si junctions show great potential for ULSI applications     

Determination of contact parameters of Ni/n-GaP Schottky contacts

The electrical analysis of Ni/n-GaP structure has been investigated by means of current–voltage (I–V), capacitance–voltage (C–V) and capacitance–frequency (C–f) measurements in the temperature range of 120–320 K in dark conditions. The forward bias I–V characteristics have been analyzed on the basis of standard thermionic emission (TE) theory and the characteristic parameters of the Schottky contacts (SCs) such as Schottky barrier height (SBH), ideality factor (n) and series resistance (R_s) have been determined from the I–V measurements. The experimental values of SBH and n for the device ranged from 1.01 eV and 1.27 (at 320 K) to 0.38 eV and 5.93 (at 120 K) for Ni/n-GaP diode, respectively. The interface states in the semiconductor bandgap and their relaxation time have been determined from the C–f characteristics. The interface state density N_ss has ranged from 2.08 × 10¹⁵ (eV^?1 m^?2) at 120 K to 2.7 × 10¹⁵ (eV^?1 m^?2) at 320 K. C_ss has increased with increasing temperature. The relaxation time has ranged from 4.7 × 10^?7 s at 120 K to 5.15 × 10^?7 s at 320 K.     

The electrical characteristics of Sn/methyl-red/p-type Si/Al contacts

The junction characteristics of the organic compound methyl-red film (2-[4-(dimethylamino)phenylazo]benzoic acid) on a p-type Si substrate have been studied. The current-voltage characteristics of the device have rectifying behavior with a potential barrier formed at the interface. The barrier height and ideality factor values of 0.73 eV and 3.22 for the structure have been obtained from the forward bias current-voltage (I-V) characteristics. The interface state energy distribution and their relaxation time have ranged from 1.68 × 10¹² cm⁻² eV⁻¹ and 1.68 × 10⁻³ s in (0.73-E_v) eV to 1.80 × 10¹² cm⁻² eV⁻¹ and 5.29 × 10⁻⁵ s in (0.43-E_v) eV, respectively, from the forward bias capacitance-frequency and conductance-frequency characteristics. Furthermore, the relaxation time of the interface states shows an exponential rise with bias from (0.43-E_v) eV towards (0.73-E_v) eV.