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The temperature dependences of the specific contact resistance of silicon ρc with a doping step are measured experimentally and described theoretically. The measurements are performed in the temperature range from 4.2 to 380 K. It is established that the contacts of the studied Au–Ti–Pd–n+–n-Si structures are ohmic. It is shown that minimal ρc is implemented at T = 75 K. Its value rises both with a decrease in temperature (due to the freezing effect) and with an increase in temperature (due to the electron-enriched layer at the boundary with the bulk material). It is established that the bulk electron concentration strongly decreases in the near-contact region in a layer with a thickness on the order of one micron due to the compensation of silicon by deep acceptors appearing because of the formation of a rather high vacancy concentration during stress relaxation and the appearance of a high dislocation density, as well as due to their diffusion from the contact after heating to 450°C. The data on the occurrence of vacancy-type defects are confirmed by X-ray measurements. The dislocation density in the studied structures is also estimated from X-ray measurements.

Date of Conference: 1-6 February 2020 Conference Name: SPIE OPTO, 2020: Gallium Nitride Materials and Devices XV 2020 In this work, the effect of introducing a photonic crystal network of silicon nitride (SiN) micro-domes on the backside of silver coated gallium nitride (GaN) based light emitting diodes (LEDs) was studied. First, sapphire side of the top emitting LEDs, which is the bottom surface of the LEDs, is coated with silver (Ag). Light emitted towards the sapphire substrate is reflected upwards to the top surface and the amount of light extracted from the LED is expected to increase. In an alternative approach, SiN micro-domes forming a two dimensional photonic crystal, 2 μm in diameter and 80 nm in height in average, are deposited on the light emitting surface of the device with a period of 2 μm. Coating the backside with Ag has increased the efficiency of a top emitting LED by 11%. By introducing the SiN photonic crystal onto the Ag backside coated sample, total internal reflection is reduced via scattering and the amount of light emitted has been increased by 30% at 5·104 mA/cm2. Integration of SiN micro-domes with Ag coating has significantly impacted light extraction which has been shown to increase the efficiency of GaN based LEDs. Fabrication process and the results are discussed in detail.

WOS: 000463693300002, This paper presents high performance, GaN/InGaN-based light emitting diodes (LEDs) in three different device configurations, namely Top Emitting (TE) LED, conventional Flip Chip (FC) and Distributed Contact (DC) FC. Series resistances as low as 1.1 Omega have been obtained from FC device configurations with a back reflecting ohmic contact of Ni/Au/RTA/Ni/Ag metal stack. A small shift has been observed between electroluminescence (EL) emissions of TE LED and the FC LEDs. In addition, FWHM value of the EL emission of DCFC LED has shown the minimum value of 160 meV (26.9 nm). Furthermore, DCFC LED configuration has shown the highest quantum efficiency and power output, with 330 mW at 500 mA current injection, compared to that of traditional wire bonded TE LEDs and the conventional FC LEDs., Scientific and Technical Research Council of Turkey, (TUBITAK) [113G042, 115E109], We are grateful to H. Morkoc, U Ozgur, V. Avrutin of Virginia Commonwealth University for many fruitful discussions. This work is supported by Scientific and Technical Research Council of Turkey, (TUBITAK Grant no: 113G042 and 115E109).

WOS: 000430037600030, We investigate the effect of InGaN stress compensation layer on the properties of light emitting diodes based on InGaN/GaN multiple quantum well (MQW) structures with step graded electron injectors. Insertion of an InGaN stress compensation layer between n-GaN and the step graded electron injector provides, among others, strain reduction in the MQW region and as a result improves epitaxial quality that can be observed by 15-fold decrease of V-pit density. We observed more uniform distribution of In between quantum wells in MQW region from results of electro- and photoluminescence measurement. These structural improvements lead to increasing of radiant intensity by a factor of 1.7-2.0 and enhancement of LED efficiency by 40%. (C) 2018 Elsevier Ltd. All rights reserved., Scientific and Technological Research Council of Turkey [113G042]; TUBITAK, Financial support for this work is provided by the Scientific and Technological Research Council of Turkey (Grant No: 113G042). Ismail Altuntas acknowledges the Ph.D. Grant support from TUBITAK. We thank UNAM-National Nanotechnology Research Center at Bilkent University for access to fabrication and characterization equipment.

WOS: 000425566100069, Enhancement of InGaN/GaN based light emitting diode performance with step graded electron injectors through a two-step passivation is reported. Perimeter passivation of LED dies with SiO2 immediately following ICP mesa etch in addition to conventional Si3N4 dielectric surface passivation leads to decrease in the reverse bias leakage current by a factor of two as well as a decrease in the shunt current under forward bias by an order of magnitude. Mitigation of the leakage currents owing to the two-step passivation leads to significant increase in the radiant intensity of LEDs by more than a factor of two compared to the conventional single step surface passivation. Further, micro-dome patterned surface of Si3N4 passivation layer allow enhanced light extraction from LEDs. (C) 2017 Elsevier Ltd. All rights reserved., Scientific and Technological Research Council of Turkey (TUBITAK) [113G042]; TUBITAK, Financial support for this project was provided by the Scientific and Technological Research Council of Turkey (TUBITAK) [Grant No: 113G042]. Ismail Altuntas, acknowledges the Ph.D. Grant support from TUBITAK. We thank to UNAM-National Nanotechnology Research Center at Bilkent University for the use of fabrication and characterization equipment.

The temperature dependences of the contact resistivity (ρ c ) of ohmic contacts based on the Au-Ti-Pd-InN system are measured at an InN doping level of 2 × 1018 cm−3 in the temperature range of 4.2–300 K. At temperatures T > 150 K, linearly increasing dependences ρ c (T) are obtained. The dependences are explained within the mechanism of thermionic current flow through metal shunts associated with dislocations. Good agreement between theoretical and experimental dependences is achieved assuming that the flowing current is limited by the total resistance of the metal shunts, and the density of conductive dislocations is ∼5 × 109 cm−2. Using the X-ray diffraction method, the density of screw and edge dislocations in the structure under study is measured: their total density exceeds 1010 cm−2.

The temperature dependence of the contact resistivity ρ c (T) of Au-Ti-Al-Ti-n +-GaN ohmic contacts is studied experimentally and substantiated theoretically in the temperature range T = 4.2–300 K. It is shown that the saturation portion of ρ c (T) is observed in the low-temperature measurement region (4.2–50 K). As the temperature increases, ρ c decreases by the exponential law. The experimental and calculated dependences ρ c (T) are in agreement. The obtained results make it possible to conclude the field nature of the current transfer for the saturation region of ρ c (T) and the thermal-field one, for the exponential region.

Resistivity ρc of InP- and GaAs-based ohmic contacts has been measured in a temperature range of 4.2–300 K. Both temperature dependences are nonmonotonic and exhibit minima at T = 50 K for InP and T = 150 K for GaAs. The nonmonotonic ρc(T) curves for GaAs contacts have been observed for the first time. The obtained experimental temperature dependences of ρc can be explained in the framework of the mechanism of current passage via metal shunts incorporated into semiconductor with allowance for electrons freezing out at liquid-helium temperatures. The ohmic character of contacts is ensured due to limitation of the electron current by diffusion supply in the presence of band bending at the semiconductor–metal interface near the shunt edge.

WOS: 000415768800128, The effect of a transparent ITO current spreading layer on electrical and light output properties of blue InGaN/GaN light emitting diodes (LEDs) is discussed. When finite conductivity of ITO is taken into account, unlike in previous models, the topology of LED die and contacts are shown to significantly affect current spreading and light output characteristics in top emitting devices. We propose an approach for calculating the current transfer length describing current spreading. We show that an inter-digitated electrode configuration with distance between the contact pad and the edge of p-n junction equal to transfer length in the current spreading ITO layer allows one to increase the optical area of LED chip, as compared to the physical area of the die, light output power, and therefore, the LED efficiency for a given current density. A detailed study of unpassivated LEDs also shows that current transfer lengths longer than the distance between the contact pad and the edge of p-n junction leads to increasing surface leakage that can only be remedied with proper passivation. (C) 2017 Elsevier Ltd. All rights reserved., TUBITAK [113G042]; Scientific and Technological Research Council of Turkey (TUBITAK), This work has been performed with the support of TUBITAK (Grant no: 113G042). Ismail Altuntas, acknowledges the Ph.D. Grant support from the Scientific and Technological Research Council of Turkey (TUBITAK).

It is experimentally found that an ohmic contact based on Au-Pt-Ti-Pd-n + �Si metallization is formed due to nanoscale metal shunts containing Si, Au, and Pt in the region of the interface with n + �Si, which appears during heat treatment at T = 450°C for 10 min in a vacuum chamber with a residual pressure of 10 -6 Torr. The high density of shunts adjoining dislocations and other imperfections is confirmed by the temperature dependence of the specific contact resistance ρ c (T). The density of conductive dislocations, cal� culated from the temperature dependence of ρc is ~5 × 10 9 cm -2 which correlates with the density of struc� tural defects, determined by the etch pits after removal of the metallization layers.

The paper considers ohmic contacts of Au-TiB x -Al-Ti-n-GaN, Au-Pd-Ti-Pd-n-AlN and Au-Pd-Ti-n-C to the promising for use in microelectronics wide-gap semiconductors. Ohmic contact formation takes place after sequential layering of metal with further fast thermal processing, which leads to solid-phase reactions between the semiconductor and metal. It is shown that the use of X-ray amorphous TiB x layer in ohmic contact as the diffusion barrier allows for creating thermal stability contacts up to T = 900 °C. Current flow in the considered ohmic contacts is described using a model with current flow along metal shunts considering diffusion limitation on the charge carrier supply.

The temperature dependences of the contact resistance ρ c (T) of ohmic Pd-Ti-Pd-Au contacts to n-GaN and n-AlN wide-gap semiconductors with a high dislocation density are studied. The dependences ρ c (T) for both contacts contain portions of exponential decrease ρ c (T) and very weak dependence ρ c (T) at higher temperatures. Furthermore, a plateau portion ρ c (T) is observed in the low-temperature region for the Au-Pd-Ti-Pd-n-GaN contact. This portion appears only after rapid thermal annealing (RTA). In principle, the appearance of the plateau portion can be associated with preliminary heavy doping of the near-contact region with a shallow donor impurity and with doping during contact fabrication as a result of RTA, if the contact-forming layer contains a material that is a shallow donor in III–N. The dependences obtained are not explained by existing charge-transport mechanisms. Probable mechanisms explaining the experimental dependences ρ c (T) for ohmic contacts to n-GaN and n-AlN are proposed.

Anomalous temperature dependences of the specific contact resistance ρc(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 ρc(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.

Temperature dependences of the contact resistivity ρ c of Au-TiB x -Ge-Au-n-n +-n ++(GaAs)-InP ohmic contacts before and after short-term (10 s) microwave treatment have been studied both experimentally and theoretically. It is shown that ρ c can decrease after microwave treatment in the entire temperature range of ρ c measurements (100–400 K). Good agreement between the theoretical and experimental ρ c (T) curves is attained and interpreted on the assumption that the dislocation density in the semiconductor near-surface region is varied as a result of microwave radiation.

Based on a theoretical analysis of the temperature dependence of the contact resistance Rc for an Au-Ti-Pd2Si-n+-Si ohmic contact, a current-transfer mechanism explaining the experimentally observed increase in Rc in the temperature range 100–380 K is proposed. It is shown that microwave treatment of such contacts results in a decrease in the spread of Rc over the wafer and a decrease in the value of Rc whilst retaining an increase in Rc in the temperatures range 100–380 K.

A new mechanism describing the rise in the contact resistance ρ c of ohmic contacts to n-n +-n ++-GaAs(GaP, GaN, InP) structures with increasing measurement temperature T, experimentally observed in the temperature range 100–400 K, is suggested on the basis of a theoretical analysis of the temperature dependence of ρ c . Good agreement between the experimental and theoretical ρ c (T) dependences is obtained and explained for a case where there is a high density of dislocations (on which metallic shunts are localized) in the near-contact region of the semiconductor.

The radiation resistance of Au-Pd-Ti-Pd-n ++-InP ohmic contacts and Au-TiB x -n-n +-n ++-InP barrier contacts—both initial and subjected to a rapid thermal annealing and irradiated with 60Co γ-ray photons with doses as high as 109 R—has been studied. Before and after external effects, the electrical characteristics of the barrier and ohmic contacts, distribution profiles for components, and phase composition in the metallization layers have been measured. In ohmic Pd-Ti-Pd-Au contacts subjected to rapid thermal annealing and irradiation, a significant distortion of the layered structure of metallization occurs; this distortion is caused by the thermal and irradiation-stimulated transport of Pd over the grain boundaries in polycrystalline Ti and Au films. However, the specific contact resistance ρ c does not change appreciably, which is related to a comparatively unvaried composition of the contact-forming layer at the Pd-n +-InP interface. In the initial sample and the sample subjected to the rapid thermal annealing at T = 400°C with the Au-TiB x -n-n +-n ++-InP barrier contacts and irradiated with the dose as high as 2 × 108 R, a layered structure of metallization is retained. After irradiation with the dose as high as 109 R, in the samples subjected to a rapid thermal annealing at T = 400°C, the layered structure of metallization becomes completely distorted; however, this structure is retained in the initial sample. The electrical properties of the contact structure appreciably degrade only after irradiation of the sample preliminarily subjected to a rapid thermal annealing at T = 400°C.

The temperature dependences of the contact resistivity ρc of Au-TiBx Al-Ti-n+-n-n+-GaN-Al2O3 ohmic contacts have been studied before and after microwave treatment followed by nine-nonth room-temperature sample storage. The temperature dependences of ρc of initial samples were measured twice. The first measurement showed the temperature dependence typical of ohmic contacts; the repeated measurement in the temperature region above 270 K showed a ρc increase caused by metallic conductivity. After microwave treatment, the metallic conductivity in the ohmic contact is not observed. This is presumably associated with local heating of metal Ga inclusions under microwave irradiation and the formation, due to high chemical activity of liquid gallium, of compounds of it with other metallization components. In this case, the temperature dependence of ρc is controlled by ordinary charge transport mechanisms. After nine-nonth room-temperature storage, the temperature dependence of ?c is described by the tunneling mechanism of charge transport.

The effect of ionizing radiation of 60Co γ-ray photons in the dose range 104–2 × 109 rad on metal-semiconductor Au-ZrB x -AlGaN/GaN and Au-TiB x -Al-Ti-n-GaN contacts and Au-ZrB x -n-GaN Schottky diodes is examined. The contacts with the TiB x and ZrB x diffusion barriers do not degrade under the effect of ionizing radiation if the dose does not exceed 108 rad. The Au-ZrB x -n-GaN Schottky diodes remain stable in the dose range 104–106 rad. As the radiation dose is increased to ≳108 rad, the damage to the contact metallization increases and is accompanied by formation of through pores, which is conducive to accumulation of oxygen at the Au-ZrB x (TiB x ) interfaces and to an increase in mass transport of atoms in contact-forming layers. In this case, irradiation-caused degradation of the Schottky diodes is observed. Possible mechanisms of radiation damage of contact structures with diffusion barriers are analyzed.

Modern methods of measuring resistance of ohmic contacts are considered in the review: Cocks-Strack, transmission line, Kelvin, boundary probing. Examples of their application are provided and errors of measuring the contact resistivity are calculated. The most accurate measuring methods are the Kelvin method and the boundary probing method, while the least accurate one is the Cocks-Strack method. Requirements and possibilities of their application are specified.

The effect of rapid thermal annealing on the structural and physical properties of Au-(Ti, Zr)Bx- GaN(SiC) contacts and diode structures on their basis is investigated. The Xraydiffraction investigations and the layerbylayer Auger analysis showed that th e phase composition and structure of the GaN and SiC contacts are retained to the temperatures as high as 900 and 1000°C, respectively. The stability of interphase boundaries is confirmed by almost constant physical properties of contacts before and after the rapid thermal treatments. The Schottkybarrier height ϕb amounts to 0.89-0.9 eV for the contacts with GaN and 0.79- 0.83 eV for SiC; the ideality factor n of the I-V characteristic amounts to n = 1.2 for the Au-TiBx(ZrBx)- nGaN(SiC) contacts and n ≈ 1.12 for the Au-ZrB x -nn + �4 HSiC contacts. The structural investigations indicate that there are glassforming boron and metal oxides at an interphase boundary, which form a thin amorphous vitreous layer resistant against rapid thermal annealing and represent the diffusion barrier for the interphase transport.

Radiation effects in the Au-Ti-Al-Ti-n-GaN multilayer metallization subjected to irradiation with 60Co γ-ray photons in the dose range 4 × 106−2 × 107 Gy are considered, and the effect of radiation on the initial contact structures and the structures subjected to a rapid thermal annealing (RTA) at high-temperature in the nitrogen atmosphere is studied. Irradiation does not significantly affect the properties of structures that were not subjected to the heat treatment. An RTA at 700°C brings about a deterioration of the contact-layer morphology. The morphological and structural transformations in the contact metallization due to the RTA are enhanced by irradiation with γ-ray photons. The combined radiation-thermal treatment is conducive to the mass transfer between contacting layers. In addition, after γ-ray irradiation with the dose of 2 × 107 Gy, the oxygen-impurity atoms appear over the entire contact’s structure and are observed in a large amount in the near-contact GaN region.

We studied the heat resistance of AuTiBx (ZrBx) barrier contacts to n-SiC 6H and n-GaN. The Schottky barrier diode (SBD) parameters, the concentration depth profiles for contact structure components and the phase composition of contact metallization were measured both before and after rapid thermal annealing (RTA) at temperatures up to 900 °С (1000 °С) for contacts to GaN (SiC 6H). It is shown that the layered structure of metallization and electrophysical properties of Schottky barriers (SBs) remain stable after RTA, thus indicating their heat resistance. The ideality factor n of the I-V characteristic of SBDs after RTA was 1.2, while the SB height φВ was ~0.9 eV (~0.8 eV) for the gallium nitride (silicon carbide) barrier structures.

A mechanism of charge transport in Au-TiB x -n-GaN Schottky diodes with a space charge region considerably exceeding the de Broglie wavelength in GaN is studied. Analysis of temperature dependences of current-voltage (I–V) characteristics of forward-biased Schottky barriers showed that, in the temperature range 80–380 K, the charge transport is performed by tunneling along dislocations intersecting the space charge region. Estimation of dislocation density ρ by the I–V characteristics, in accordance with a model of tunneling along the dislocation line, gives the value ρ ≈ 1.7 × 107 cm−2, which is close in magnitude to the dislocation density measured by X-ray diffractometry.

We studied Au-Pd-Ti-Pd ohmic contacts made by thermal evaporation of metals in vacuum onto n -GaN (n -AlN) with high dislocation density heated to a temperature of 350 °C. Temperature dependence of ohmic contacts resistivity may be described by the mechanism of current flow through the metal shunts associated with dislocations, with allowance made for current limitation by diffusion supply of electrons to the contact. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We consider a new mechanism of current flow in a metal-semiconductor contact that describes the growth of contact resistivity ρc with temperature T. The theory developed allows us to explain our experimental results showing the anomalous increase in the temperature dependence of contact resistivity, ρc(T), as well as results obtained previously in ref. [1-5] on the behavior of ohmic contacts to III-V compounds and Si.

A new mechanism of contact resistance formation in ohmic contacts with high dislocation density is proposed. Its specific feature is the appearance of a characteristic region where the contact resistance increases with temperature. According to the mechanism revealed, the current flowing through the metal shunts associated with dislocations is determined by electron diffusion. It is shown that current flows through the semiconductor near-surface regions where electrons accumulate. A feature of the mechanism is the realization of ohmic contact irrespective of the relation between the contact and bulk resistances. The theory is proved for contacts formed to III-V semiconductor materials as well as silicon-based materials. A reasonable agreement between theory and experimental results is obtained. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3702850]

The temperature dependence of contact resistivity rho(c) in lapped silicon specimens with donor concentrations of 5 x 10(16), 3 x 10(17), and 8 x 10(17) cm(-3) was studied experimentally. We found that, after decreasing part of the rho(c)(T) curve in the low temperature range, an increasing part is registered with increasing temperature T. It is demonstrated that the formation of contact to a lapped Si wafer results in the generation of high dislocation density in the near-surface region of the semiconductor and also in ohmic contact behavior. In this case, current flows through the metal shunts associated with dislocations. The theory developed is in good agreement with experimental results. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4752715]