The mechanism of current flow in an alloyed In-GaN ohmic contact

The resistance of alloyed In-GaN ohmic contact is studied experimentally. In the temperature range 180–320 K, the resistance per unit area increases with temperature, which is typical of metallic conduction and disagrees with current flow mechanisms associated with thermionic, field-effect, or thermal field emission. It is assumed that In-GaN ohmic contact is formed by conducting shunts arising due to precipitation of In atoms on dislocations. As determined from the temperature dependence of the contact resistance, the number of shunts per unit contact area is ～(10⁷–10⁸) cm^?2, which is close to the dislocation density of 10⁸ cm^?2 measured in the initial material.     

Interface state density in Au-nGaAs Schottky diodes

A method for determining the surface state density in Schottky diodes taking into account both I–V and C–V data while considering the presence of a deep donor level is presented. The model assumes that the barrier height is controlled by the energy distribution of surface states in equilibrium with the metal and the applied potential and does not include, explicitly, an interfacial layer. The model was applied to extract interface state densities of Au-nGaAs guarded Schottky diodes fabricated from bulk and VPE (100) GaAs with carrier conentrations between 3 × 10¹⁵ and 8 × 10¹⁶ cm^?3. These diodes exhibited ideality (n) factors of approximately 1.02 and room temperature saturation current densities ～10^?8 A/cm². This model is in substantial agreement with forward bias measurements over the 77–360°K temperature range investigated, in that a temperature-independent energy distribution of interface states was obtained. In reverse bias the interface state model is most valid with the higher carrier concentration material and at high temperature and low bias voltage. Typical interface state densities from 0.07 eV above the zero bias Fermi level to 0.01 eV below the Fermi level were 2 × 10¹³ cm^?2 eV^?1. The validity of the model under reverse bias is restricted by a non-thermionic reverse current, thought to be enhance field emission from traps.     

Specific resistivity of a metal-n GaAs OHMIC contact with an intermediate N+ layer

The specific contact resistivity ρ_c of a metal-n GaAs structure, over a wide range of carrier concentrations in the intermediate layer (5 × 10¹⁸–5 × 10²⁰ cm^?3) and in the substrate (10¹⁵?10¹⁷ cm^?3) is calculated. The results which are presented graphically demonstrate the dependence of ρ_c on metal-semiconductor barrier height from 0.2 eV–0.8 eV. A comparison with the measured data of an alloyed AuGeNi-n GaAs system, suggests that the calculations corresponding to a concentration of 5 × 10¹⁸ cm^?3 in the layer and barrier height of 0.4 eV give the best fit based on the experimental observations. It is hoped that the results can be used as a guideline in developing ohmic contacts of ultra low resistance values.     

Microwave Annealing of High Dose Al<Superscript>+</Superscript>-implanted 4H-SiC: Towards Device Fabrication

High-purity semi-insulating 8° off-axis 〈0001〉 4H-SiC was implanted with Al⁺ at different doses and energies to obtain a dopant concentration in the range of 5 × 10¹⁹–5 × 10²⁰ cm^?3. A custom-made microwave heating system was employed for post-implantation annealing at 2,000 °C for 30 s. Sheet resistance and Hall-effect measurements were performed in the temperature range of 150–700 K. At room temperature, for the highest Al concentration, a minimum resistivity of 3 × 10^?2 Ω cm was obtained, whereas for the lowest Al concentration, the measured resistivity value was 4 × 10^?1 Ω cm. The onset of impurity band conduction was observed at around room temperature for the samples implanted with Al concentrations ≥3 × 10²⁰ cm^?3. Vertical p ⁺-i-n diodes whose anodes were made by 1.5 × 10²⁰ cm^?3 Al⁺ implantation and 2,000 °C/30 s microwave annealing showed exponential forward current–voltage characteristics with two different ideality factors under low current injection. A crossover point of the temperature coefficient of the diode resistance, from negative to positive values, was observed when the forward current entered the ohmic regime.     

The mechanism of contact-resistance formation on lapped n-Si surfaces

Anomalous temperature dependences of the specific contact resistance ρ _c (T) of Pd₂Si-Ti-Au ohmic contacts to lapped n-Si with dopant concentrations of 5 × 10¹⁶, 3 × 10¹⁷, and 8 × 10¹⁷ cm^?3 have been obtained. The anomalous dependences of ρ _c (T) have been accounted for under the assumption that the current flows along nanodimensional metallic shunts, which are combined with dislocations with a diffusionrelated limit in the supply of charge carriers taken into account. The densities of conducting and scattering dislocations in the surface region of the semiconductor are determined.     

Electrical Characteristics of Ti/Al Ohmic Contacts to Molecular Beam Epitaxy-Grown N-polar n-type GaN for Vertical-Structure Light-Emitting Diodes

We investigated the electrical properties of Ti(30?nm)/Al(200?nm) contacts to molecular beam epitaxy-grown N-polar n-GaN with different carrier concentrations. Samples with carrier concentration of 1.2?×?10¹⁸?cm^?3 showed nonohmic behaviors when annealed at 300°C, but ohmic at 500°C and 700°C. All samples with carrier concentration of 2.0?×?10¹⁹?cm^?3 exhibited ohmic behavior. x-Ray photoemission spectroscopy (XPS) results showed that, for samples with carrier concentration of 1.2?×?10¹⁸?cm^?3, the Ga 2p core levels shift to lower or higher binding energy upon annealing at 300°C or above 500°C, respectively. Scanning transmission electron microscopy (STEM) results showed that, for samples with carrier concentration of 1.2?×?10¹⁸?cm^?3, a wurtzite AlN layer (??2?nm thick) formed at the metal/GaN interface when the samples were annealed at 500°C. An interfacial wurtzite AlN layer also formed upon annealing at 700°C, but its thickness was ??4?nm. Based on the XPS and STEM results, the ohmic contact formation and degradation mechanisms are described and discussed.     

Rapid thermal alloyed ohmic contact on inp

The results of a study of the electrical and metallurgical properties of thin metallic layers deposited on InP for use as an ohmic contact are presented. A rapid thermal annealing system was used to alloy AuGe/Ni/Au contacts ton-type ion implanted InP. Rutherford backscattering and contact resistivity measurement were used to evaluate the structural and electrical characteristics of these rapid thermal alloyed thin films. Varying degrees of mixing between the metals and the semiconductor were found depending on the temperature and temperature-time cycle. These results were compared to furnace and graphite strip-heater alloying techniques. A correlation between the interface structure and the contact resistance was found. Temperatures between 430 and 450° C and alloying time of 2 sec have produced the best electrical results, with specific contact resistance as low as 2*10^?7 Ω cm² on semi-insulating InP which was Siimplanted with a peak concentration about 2*10¹⁸ cm^?3. The optimum alloy temperature is marked by the onset of substantial wrinkling of the contact surface, whereas essentially smooth surfaces are obtained at temperatures below optimum. The depth of the alloyed ohmic contact is controlled by the time of heating and could be less than 1000Å.     

Influence of an increase in the implantation dose of erbium ions and annealing temperature on photoluminescence in AlGaN/GaN superlattices and GaN epitaxial layers

Room-temperature photoluminescence (PL) has been studied in AlGaN/GaN superlattices and GaN epitaxial layers implanted with 1-MeV erbium at a dose of 3 × 10¹⁵ cm^?2 and annealed in argon. The intensity of PL from Er³⁺ ions in the superlattices exceeds that for the epitaxial layers at annealing temperatures of 700–1000°C. The strongest difference (by a factor of ～2.8) in PL intensity between the epitaxial layers and the superlattices and the highest PL intensity for the superlattices are observed upon annealing at 900°C. On raising the annealing temperature to 1050°C, the intensity of the erbium emission from the superlattices decreases substantially. This circumstance may be due to their thermal destruction.     

Ohmic contact to p-type GaP

A study was made of the contact properties of a AuBe eutectic and a AuBeNi alloy on p-type GaP. The specific contact resistance varied from 1 × 10^?3 to 7.5 × 10^?5 ω cm² in the acceptor concentration range of 9 × 10¹⁶ to 2 × 10¹⁸ cm^?3. In the sintering temperature range resulting in good ohmic behaviour and low contact resistance the AuBe contacts do not form drops, whereas the AuBeNi contacts became molten; even after melting they wetted the surface of the GaP well. At the temperature of sintering Be diffuses from the contact into the GaP. The diffusion of Be gives rise to an additional acceptor concentration of 5 × 10¹⁸ to 1 × 10¹⁹ cm^?3 beneath the contact surface. Taking this into consideration the concentration-specific contact resistance relationship appears to support a field emission FE conduction mechanism.     

A method for the determination of electron capture cross-section at imperfection centers in gallium arsenide of electroluminescent diodes

A simple method is described for the determination of the electron capture cross-section at the imperfection centers with a dominant deep level in semiconductor electroluminescent diodes. In this method, the electron capture cross-section is determined through simultaneous measurements of the temperature dependence of the minority carrier lifetime and the external quantum efficiency of the EL diodes. When the method is applied to Zn-diffused GaAs EL diodes, the average electron capture cross-section is found to be 10^?16 cm² at a level either 0·1 or 0·2 eV away from the mid-gap.     

The ohmic contact formation mechanism of Au/Pt/Pd ohmic contact to p-ZnTe

The ohmic contact formation mechanism and the role of Pt layer of Au(500Å) Pt(500Å)/Pd(100Å) ohmic contact to p-ZnTe were investigated. The specific contact resistance of Au/Pt/Pd contact depended strongly on the annealing temperature. As the annealing temperature increased, the specific contact resistance decreased and reached a minimum value of 6×10^?6 Θcm² at 200°C. From the Hall measurement, the hole concentration increased with the annealing temperature and reached a maximum value of 2.3×10¹⁹ cm^?3 at 300°C. The Schottky barrier height decreased with the increase of annealing temperature and reached a minimum value of 0.34 eV at 200°C and it was due to the interfacial reaction of Pd and ZnTe. Therefore, the decrease of contact resistance was due to the increase of doping concentration as well as the decrease of Schottky barrier height by the interfacial reaction of Pd ZnTe. The specific contact resistances of Au Pd, Au/Pt/Pd and Au/Mo/Pd as a function of annealing time was investigated to clarify the role of Pt layer.     

Effect of the annealing temperature on erbium ion electroluminescence in Si:(Er,O) diodes with (111) substrate orientation

The influence of the temperature of secondary annealing, stimulating the formation of optically and electrically active centers, on the erbium ion electroluminescence (EL) at λ≈1.54 μm in (111) Si:(Er,O) diodes has been studied. The diodes were fabricated by the implantation of 2.0 and 1.6 MeV erbium ions at doses of 3×10¹⁴ cm⁻² and oxygen ions (0.28 and 0.22 MeV, 3×10¹⁵ cm⁻²). At room temperature, the EL intensity in the breakdown mode grows with the annealing temperature increasing from 700 to 950°C. At annealing temperatures of 975–1100°C, no erbium EL is observed in the breakdown mode owing to the formation of microplasmas. The intensity of the injection EL at 80 K decreases with the annealing temperature increasing from 700 to 1100°C. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 10, 2001, pp. 1224–1227. Original Russian Text Copyright ? 2001 by Sobolev, Emel’yanov, Nikolaev.     

Heavily doped GaAs:Te layers grown by MOVPE using diisopropyl telluride as a source

The capabilities of GaAs epitaxial layers extremely heavily doped with tellurium by metal-organic vapor-phase epitaxy using diisopropyl telluride as a source are studied. It is shown that tellurium incorporation into GaAs occurs to an atomic concentration of 10²¹ cm^–3 without appreciable diffusion and segregation effects. Good carrier concentrations (2 × 10¹⁹ cm^–3) and specific contact resistances of non-alloyed ohmic contacts (1.7 × 10^–6 Ω cm²) give grounds to use such layers to create non-alloyed ohmic contacts in electronic devices. A sharp decrease in the electrical activity of Te atoms, a decrease in the electron mobility, and an increase in the contact resistance at atomic concentrations above 2 × 10²⁰ cm^–3 are detected.     

High-efficiency (49%) and high-power photovoltaic cells based on gallium antimonide

High-efficiency GaSb-based photovoltaic cells designed for conversion of high-power laser radiation and infrared radiation of emitters heated by concentrated solar radiation are fabricated and studied. The maximum efficiency of conversion of the radiation with λ = 1680 nm was 49% at the photocurrent density of 50–100 A/cm² for the fabricated photovoltaic cells. The methods for reducing the losses at ohmic contacts to p-and n-GaSb are investigated. The minimum values of the specific resistance, (1–3) × 10^?6 Θ cm², of contact to p-GaSb with the doping level of 10²⁰ cm^?3 were obtained using the Ti/Pt/Au contact system. The minimum values of the specific contact resistance were (1–3) × 10^?6 Θ cm² in the case of n-GaSb with the doping level of 2 × 10¹⁸ cm^?3 if the Au(Ge)/Ni/Au and Au/Ni/Au contact systems are used.     

Cyano‐Substituted Oligo(p‐phenylene vinylene) Single Crystals: A Promising Laser Material

Organic crystals that combine high charge‐carrier mobility and excellent light‐emission characteristics are expected to be of interest for light‐emitting transistors and diodes, and may offer renewed hope for electrically pumped laser action. High‐luminescence‐efficiency cyano‐substituted oligo(p‐ phenylene vinylene) (CN‐DPDSB) crystals (η ≈ 95%) grown by the physical vapor transport method is reported here, with high mobilities (at ≈10^?2 cm² V^?1 s^?1 order of magnitude) as measured by time‐of‐flight. The CN‐DPDSB crystals have well‐balanced bipolar carrier‐transport characteristics (μ_hole≈ 2.5–5.5 × 10^?2 cm² V^?1 s^?1; μ_electron ≈ 0.9–1.3 × 10^?2 cm² V^?1 s^?1) and excellent optically pumped laser properties. The threshold for amplified spontaneous emission (ASE) is about 4.6 μJ per pulse (23 KW cm^?2), while the gain coefficient at the peak wavelength of ASE and the loss coefficient caused by scattering are ≈35 and ≈1.7 cm^?1, respectively. This indicates that CN‐DPDSB crystals are promising candidates for organic laser diodes.     

Subnanosecond 4<Emphasis Type="Italic">H</Emphasis>-SiC diode current breakers

Mesa epitaxial 4H-SiC-based p ⁺-p-n ₀-n ⁺ diodes have been fabricated and their reverse recovery characteristics have been measured in modes typical of fast semiconductor current breakers, drift step recovery diodes, and SOS diodes. It has been found that, after the short (～10 ns) pulsed injection of nonequilibrium carriers by a forward current with a density of 200–400 A cm^?2 and the subsequent application of a reverse voltage pulse (with a rise time of 2 ns), diodes can break a reverse current with a density of 5–40 kA cm^?2 in a time of about (or less than) 0.3 ns. A possible mechanism for ultrafast current breaking is discussed.     

Low frequency noise measurements on silicon-on-sapphire (SOS) MOS transistors

Measurements of the equivalent input noise are performed on P-channel silicon-on-saphire, metal-oxide-semiconductor (SOSMOS) transistors as a function of the silicon layer bias voltage with respect to the source. Devices are operated in the ohmic region of the characteristics so that the results are easier to analyse. The results show a high degree of dispersion in the noise characteristics for the different tested devices all originated from the same silicon on sapphire wafer. We conclude that a dispersion of the silicon-sapphire interface properties, such as SRH centers density or fixed positive charge, exists on the wafer. From one specific device, we calculate a fixed positive silicon-sapphire interface charge (about 4 × 10¹¹ cm^?2) and a high density (about 5 × 10¹¹ cm^?2) SRH centers localized near the silicon-sapphire interface.     

Dependence of the mechanism of current flow in the in-<Emphasis Type="Italic">n</Emphasis>-GaN alloyed ohmic contact on the majority carrier concentration

Based on the study of the temperature dependence of resistance of the In-n-GaN alloyed ohmic contacts, it is found that the mechanism of current flow in them substantially depends on the concentration N of uncompensated donors in GaN. At N = 5 × 10¹⁶ ? 1 × 10¹⁸ cm^?3, current mainly flows along the metallic shunts, and at N ? 8 × 10¹⁸ cm^?3 it flows by tunneling.     

Electrical properties and luminescence spectra of light-emitting diodes based on InGaN/GaN heterostructures with modulation-doped quantum wells

The distribution of charged centers N(w), quantum efficiency, and electroluminescence spectra of blue and green light-emitting diodes (LED) based on InGaN/AlGaN/GaN p-n heterostructures were investigated. Multiple InGaN/GaN quantum wells (QW) were modulation-doped with Si donors in GaN barriers. Acceptor and donor concentrations near the p-n junction were determined by the heterodyne method of dynamic capacitance to be about N _A ≥ 1 × 10¹⁹ cm^?3 ? N _D ≥ 1 × 10¹⁸ cm^?3. The N(w) functions exhibited maxima and minima with a period of 11–18 (±2–3 nm) nm. The energy diagram of the structures has been constructed. The shifts of spectral peaks with variation of current (J=10^?6–3×10^?2 A) are smaller (13–12 meV for blue and 20–50 meV for green LEDs) than the corresponding values for the diodes with undoped barriers (up to 150 meV). This effect is due to the screening of piezoelectric fields in QWs by electrons. The dependence of quantum efficiency on current correlates with the charge distribution and specific features in the current-voltage characteristics.     

Ohmic contacts on n- and p-layers of GaAs using laser-induced diffusion

Thin surface layers of GaAs have been heavily doped by laser-assisted diffusion from the gas phase using H₂Se and Diethylzinc (DEZ) in H₂. The dependence of sheet carrier concentration and contact resistance of evaporated metal contacts on laser pulse energy, duration and substrate temperature is given. Specific contact resistances of 3·10^?5 ω cm² and 5·10^?5 ω cm² for nonalloyed n- and p-contacts, respectively, are obtained. After alloying they improve to values of 3·10^?6 ω cm². This process can be used to produce local ohmic contacts at low temperature.