Pd/Ge contacts to n-type GaAs

Sintered metal-semiconductor contacts, formed by thin, evaporated layers of Pd and Ge on n-type GaAs, were studied using Auger electron spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, current-voltage measurements, and capacitance-voltage measurements. Prior to sintering, the as-deposited Pd/Ge/GaAs contacts were rectifying and exhibited a reproducible Schottky barrier energy φ_Bn of 0.67±0.02 eV. Auger analysis indicated the initial behavior of the contact structure, upon sintering, to be an interdiffusion and reaction of Pd and Ge on a non-reacting GaAs substrate. Two germanide phases, Pd₂Ge and PdGe, were identified using X-ray diffraction and Auger analysis. The intervening Ge layer prevented the reaction of Pd with the GaAs substrate at low temperatures. Because of the PdGe reaction, φ_Bn increased to approximately 0.85 eV. Sintering at higher temperatures (i.e. between 300 and 400°C) produced additional reactions between Pd and the GaAs substrate. The electrical properties of the contact remained rectifying and φ_Bn exhibited little change from the value of 0.85 eV with the interdiffusion of Pd, Ga, and As. Sintering above 400°C resulted in the formation of ohmic contacts. The diffusion of Ge to the GaAs interface was found to correlate with the onset of ohmic behavior. Current conduction in the contact was best described by thermionic-field emission theory, and a specific contact resistance of 3.5 × 10^?4Ω-cm² was obtained after sintering above 550°C, independent of the initial impurity concentration in the substrate. Over the entire range of sintering temperatures (i.e. at or below 600°C), the interaction between the thin-film layers appeared to be governed by diffusion-controlled, solid-phase processes with no evidence of the formation of a liquid phase. As a result, the surface of the contact structure remained smooth and uniform during sintering.     

Alloyed tin-gold ohmic contacts to n-type indium phosphide

The formation of alloyd ohmic contacts on n-InP using sequentially deposited Sn plus Au films was investigated. The specific contact resistance for metallizations with a Sn content of 5 at. % was determined for annealing temperatures between 250 and 500°C. The minimum specific contact resistance, r_c = (1.8±0.9) × 10^?6 ohm-cm² occurred for a narrow range of annealing temperatures between 380 and 410°C on substrates with n = 3 × 10¹⁸/cm³. For annealing temperatures 350°C the contacts were non-Ohmic and above 420°C the resistance increased dramatically. Contact morphology and metallurgy were studied by optical and scanning electron microscopy, X-ray diffraction, Auger electron spectroscopy and Rutherford backscattering. Films annealed above 320°C contained several phases, mainly Au₄In, AuSn and polycrystalline InP. The contacts annealed at temperatures above 410°C were composed predominantly of the single phase Au₃In₂.     

Temperature behavior and modeling of ohmic contacts to Si implanted n-type GaN

The behavior of an ohmic contact to an implanted Si GaN n-well in the temperature range of 25-300 °C has been investigated. This is the sort of contact one would expect in many GaN based devices such as (source/drain) in a metal-oxide-semiconductor transistor. A low resistivity ohmic contact was achieved using the metal combination of Ti (350 Å)/Al (1150 Å) on a protected (SiO₂ cap) and unprotected samples during the post implantation annealing. Sheet resistance of the implanted layer and metal-semiconductor contact resistance to N⁺ GaN have been extracted at different temperatures. Both, the experimental sheet resistance and the contact resistance decrease with the temperature and their characteristics are fitted by means of physical based models.     

Low noise “ohmic” contacts on n-type silicon

A technique is given for preparation of low noise ohmic contacts on n-type silicon using silver-silicon eutectic. It allows reaching electric fields up to 20 kV cm¹ in a wide range of resistivities and lattice temperatures (300–6°K). Interface chemical composition and structure is studied using scanning electron microscopy and X-ray analysis. Contact resistivity ?_c is studied versus bulk resistivity ? and lattice temperature T. High field conductivity shows that ?_c is not sufficient to characterize ohmic contacts. It is shown that the contact noise (j²/f) law is valid up to 500 MHz and that noise measurements are much more sensitive to contact behavior than first order coefficients. Contact noise may be important at intermediate bias but is proved to be negligible both at very low and very high bias. Moreover it has been possible to define a contact quality factor which provides a quantitative characterization of contact behavior.     

Hall effect study of p-type high resistivity layers on n-type silicon substrates

Analytical expressions for the longitudinal electric field and Hall voltage in p-type high resistivity Halltrons on silicon substrate with opposite conductivity are derived. The effect of Joule heating on the current and Hall voltage has been studied, taking into account the temperature dependence of carrier mobility. It is shown that for p-diffused high resistivity Halltrons on an n-silicon substrate, the theoretical and experimental results are in close agreement.     

Damaged-induced isolation in n-type InP by light-ion implantation

A detailed study of the insulating properties of ion-implantation induced damage in InP has been carried out for H, He, B and Be implantation. For each ion, there was found to be an optimal implantation fluence for the formation of resistive layers. At this fluence, a maximum resistivity of 10³ to 10⁴Ω·cm was observed. Lower resistivities were observed for higher and lower implantation fluences. The primary anneal stage for the maximum resistivity layers was between 250 and 300°C.Anomalous results were observed for H implantation in that the resistivity observed depends on the test structure geometry. Measurements carried out by contacting the front and back of the damage layer gave resistivity values two orders of magnitude greater than those measured by contacting adjacent points on an epitaxial structure. For all other ions, the results obtained for the two geometries were in good agreement. It has been shown that a conductive layer produced by the proton bombardment of the underlying Fe-doped substrate gives rise to a low resistance shunt in the epitaxial study.     

Capacitance voltage measurements on n-type InAs MOS diodes

The electrical properties of the interface between pyrolytically deposited SiO₂ and InAs have been investigated by measuring the admittance of Al-SiO₂-InAs MOS diodes at room temperature and 77°K. The room temperature measurements yield a rather high surface state density of 2·5 × 10¹² states/eV/cm². The surface state density undergoes an anomalous decrease to 2·8 × 10¹¹ states/eV/cm² at 77°K. A charge which is linearly dependent on the voltage across the oxide is trapped in the oxide. Conduction through the oxide causes the diodes to enter a deep depletion condition at 77°K.     

Temperature coefficient of resistance for p- and n-type silicon

The temperature coefficient of resistance for n- and p-type silicon has been calculated between ?50 and 125°C for a wide range of concentrations and levels of compensation. These results provide a useful guide for the design of silicon integrated resistors.     

Electrical field effects in n-type MOSFET and metal-nonmetal transition

The effects of disorder, correlation, external electric field, impurity concentration, and impurity location near and at the Si/SiO₂ interface of a metal-oxide semiconductor field effect transistor (MOSFET), have been investigated. The binding and correlation energies are strongly dependent on the electric field and impurity location. The Hubbard-Mott like model provides further evidence of a MNM transition in agreement with recent experimental findings.     

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.     

Energy levels and degeneracy ratios for magnesium in n-type silicon

The energy levels and degeneracy ratios of magnesium in n-type silicon have been determined by Hall effect measurements with the least square method. Magnesium ions appear to occupy two different sites and show different electrical properties. The first is amphoteric and exhibits an acceptor level at E_c ? 0.115 eV (±0.002 eV), degeneracy ratio γ_I^? = 2.5 as well as a donor level at E_c ? 0.40 eV (±0.01 eV), γ_III⁺ = 1. The second exhibits a donor level at E_c ? 0.227 eV (±0.004 eV), degeneracy ratio $\text{γ}{}_{\mathrm{II}}{}^{\mathrm{+}}\text{=}\text{1}\text{2.5}$. The physical nature of these Mg associated site is unknown.     

Studies of n-type GaAs material properties by anodic current behaviour

The carrier concentrations of n-type GaAs material have been measured by a technique using anodisation in the dark. The results are compared with a theoretical model and with other results based on Schottky barrier reverse breakdown. The technique has also been extended to include surfaces containing more than one type of material and n-type materials of different carrier concentrations. Additionally the technique has been used to make an assessment of the surface quality of the material.     

Carrier compensation in O+ implanted n-type GaAs

Results of electrical measurements on 1 MeV O⁺ implanted n-type GaAs are reported. After annealing the implanted material it is found that the free carrier compenasation rate (k), defined by Favennec[1] as the number of carriers removed per oxygen atom, can be dependent upon both the starting material and the implanted dose.     

Effect of heat treatment on electron traps in n-type GaAs

Annealing of bulk and vapor phase epitaxial (VPE) n-type GaAs at about 600°C for one hour in an apparatus of fused quartz (GE 204 or vitrosil) is seen to cause considerable reduction in the free electron concentration (N_S) and the commonly observed 0.85 eV electron traps (N_T) in these materials. Heating at elevated temperatures in the same environment causes an n-type sample to convert to p-type. The reduction in N_S and N_T is found to be considerably smaller for comparable heat treatments in a vacuum system free from quartz. It is suggested that the observed reduction is caused by in-diffusion of impurities like Cu and Li which are present in the types of quartz used in the experiments.     

Conductivity of complementary error function n-type diffused layers in gallium arsenide

The average conductivity of n-type erfc diffused layers in Gallium Arsenide have been calculated for background doping levels of 10¹⁵ cm^?3, 10¹⁶ cm^?3 and 10¹⁷ cm^?3. The curves are confined to surface concentrations below 10¹⁹ cm^?3 because of the strong dependence of diffusion coefficient upon concentration at higher concentrations.     

Characteristics of metal/sputtered CdTe/n-GaAs diode structures

Incorporation of a thin layer of r.f. sputtered CdTe between the metal and n-GaAs, has resulted in diode structures with MIS and Schottky barrier types C/V characteristics and low-current forward and reverse I/V characteristics. These structures have the potential to be useful in improving the performance of GaAs FET's for microwave and high speed applications.     

Low-resistance,long-life contacts by laser-annealing of silver-implanted p-type PbTe

Various procedures have been investigated for fabricating to p-type PbTe metal contacts which have a low resistance that remains low with age. Polishing and etching procedures for achieving satisfactory simple platinum-plated contacts are described. Significant improvement, yielding lower resistance and longer life contacts, has been effected by a procedure involving implantation of silver at three ion energies followed by a single pulse anneal with ruby laser radiation. Also described is a copper-gallium alloy which allowed convenient and easy bonding, at temperatures just above room temperature, of PbTe diode lasers to heat sinks.     

A reproducible ohmic contact to n-type GaAs0.6P0.4

A study is reported on several alternate metal systems based upon AuGeNiCr, which provide reproducibly $\text{～10}{}^{\mathrm{?4}}\text{Ω-}\text{cm}{}^2$ specific contact resistance to n-type GaAs_0.6P_0.4, employing the conventional open-tube sintering furnace. A simple metallurgical model is presented to explain the success of this technique. It is found that a thin chromium-oxide film is formed during the heat treatment, which is thought to prevent metal pads from balling-up, and volatile components of the semiconductor from evaporating. In this way, the metal-semiconductor interface re-growth process is optimized to consistently provide low resistance ohmic contacts.     

Metallurgical and electrical properties of alloyed Ni/AuGe films on n-type GaAs

An experimental study of the alloying characteristics of a composite thin-film structure which is often used as an ohmic contact to GaAs is presented. A AuGe layer of eutectic composition covered by a thin-film of Ni and deposited on n-type epitaxial GaAs is investigated in order to better understand the relationship between the alloying behavior and the electrical properties of the contact. The barrier energy ?_Bn and the specific contact resistance of the Ni/AuGe/GaAs system is measured for a wide range of alloy temperatures and times. The metallurgical properties of the Ni/AuGe/GaAs system are obtained with Auger electron spectroscopy and scanning electron microscopy. Auger spectroscopy combined with in situ sputter etching is used to determine depth-composition profiles for all constituents of both as-deposited and alloyed Ni/AuGe/GaAs contacts. In samples heat-treated below the AuGe eutectic temperature, Ni is found to move rapidly through the intervening AuGe layer to collect at the GaAs interface, and the effective value of ?_Bn rises to the value characteristic of Ni/GaAs Schottky diodes. For heat-treatment above the AuGe eutectic temperature, ohmic contact behavior is observed, and uniform alloyed contact surfaces are found to result from the presence of Ni at the GaAs interface. Ga outdiffusion and surface accumulation resulting from GaAs dissociation occurs for all alloy temperatures and times. The Ga outdiffusion appears to be characterized by a very low activation energy.     

Very low resistance NiAuGeNi contacts to n-GaAs

The contact resistance of alloyed NiAuGeNi contacts has been measured on n-GaAs. The quality of the contacts was strongly influenced by the pre-evaporation cleaning of the semiconductor surface. Compared to chemical etching, sputter cleaning considerably reduced the contact resistance and improved reproducibility. Optimization of the alloying cycle led to contact resistances lower than the state of the art.