The reflectivity of Mo/Ag/Au ohmic contacts on p-type GaN for flip-chip light-emitting diode (FCLED) applications

The Mo/Ag/Au contact for flip-chip light-emitting diode (FCLED) applications is examined on its contact resistance and light reflectance. A high reflectance of 90% is achieved in un-annealed contact, but a strong inter-diffusion of ohmic metals and GaN during the annealing process is found to result in poor reflectance (55% at the wavelength of 465 nm). The secondary ion mass spectrometry (SIMS) depth profiles indicate that a wide inter-diffusion region existed in the annealed contacts; thus the low reflectivity of the Mo/Ag/Au-annealed contacts can be attributed to the strong inter-diffusion of Au and Ag.     

Application of the modified electrostatic model to the impurity diffusion in nickel

The modified electrostatic model (Neumann and Tölle 1995) is applied to the impurity diffusion in nickel.Z ₀ = 0.4 is used for the effective charge of the nickel ion.The comparison of calculated and experimental diffusion parameters reveals that the sign of J Q, the difference between impurity diffusion and self-diffusion energy, and the sign of the difference between impurity diffusion and self-diffusion coefficient is correctly predicted in all cases. On the other hand the comparison exhibits some systematic deviations for 5p impurities, which cannot be explained in terms of the current impurity diffusion models.     

Analysis of temperature and enhanced diffusion effects in sputtering of CrSi2

Previously reported experimental results on sputtering and enhanced diffusion processes in CrSi₂ during 100 keVXe⁺ bombardment at different temperatures have been quantitatively analyzed.The framework for the analysis is a simple theoretical model in which the Si atoms are considered mobile in a matrix of Cr atoms whose density is assumed constant and diffusivity is considered zero everywhere. Erosion velocity of the matrix (due to the sputtering of Cr atoms), sputtering and enhanced diffusion processes of Si atoms are taken into account in the mathematical model.In our analysis we show that the time evolution of the total number of sputtered atoms in binary solids cannot yield an unambigous conclusion as to the existence of preferential sputtering.Further, it is found that in the case of CrSi₂ the preferential sputtering of Si atoms depends on the suicide temperature.     

Thermal equilibrium concentrations and effects of negatively charged Ga vacancies in n-type GaAs

We have calculated the thermal equilibrium concentrations of the various negatively charged Ga vacancy species in GaAs. The triply-negatively-charged Ga vacancy, V _Ga ^3– , has been emphasized, since it dominates Ga self-diffusion and Ga-Al interdiffusion under intrinsic and n-doping conditions, as well as the diffusion of Si donor atoms occupying Ga sites. Under strong n-doping conditions, the thermal equilibrium V _Ga ^3– concentration, , has been found to exhibit a temperature independence or a negative temperature dependence, i.e., the value is either unchanged or increases as the temperature is lowered. This is quite contrary to the normal point defect behavior for which the point defect thermal equilibrium concentration decreases as the temperature is lowered. This property provides explanations to a number of outstanding experimental results, either requiring the interpretation that V _Ga ^3– has attained its thermal equilibrium concentration at the onset of each experiment, or requiring mechanisms involving point defect non-equilibrium phenomena.     

Oxygen diffusion in C60 films

We have performed detailed resistivity measurements as a function of temperature in the range from 12 to 300 K on oxygen loaded C₆₀ films. We observe that two ordering phase transitions (i.e.,T ₀=260 K andT _g =90 K) are present in (T), which, in addition, strongly depends on the oxygen content. We find a decrease of both ordering temperatures with increasing oxygen concentrations. The mechanisms of oxygen diffusion are greatly enhanced in the ordered phase on heating. Finally, the transition to a glassy state atT _g is detected as a point of reversibility of the resistivity curve as a function of temperature.     

Advanced diffusion studies with isotopically controlled materials

The use of enriched stable isotopes combined with modern epitaxial deposition and depth profiling techniques enables the preparation of material heterostructures, highly appropriate for self- and foreign-atom diffusion experiments. Over the past decade we have performed diffusion studies with isotopically enriched elemental and compound semiconductors. In the present paper, we highlight our recent results and demonstrate that the use of isotopically enriched materials ushered in a new era in the study of diffusion in solids, which yields greater insight into the properties of native defects and their roles in diffusion. Our approach of studying atomic diffusion is not limited to semiconductors and can be applied also to other material systems. Current areas of our research concern the diffusion in the silicon-germanium alloys and glassy materials such as silicon dioxide and ion conducting silicate glasses.     

Mechanisms of ion-beam-enhanced diffusion in amorphous silicon

We have investigated ion-beam-enhanced diffusion of Au in undoped and B doped amorphous Si. The diffusion coefficients depend linearly on ion flux and exibit an Arrhenius-like temperature dependence with an activation energy of 0.37 eV in the temperature range 200–350° C. Moreover the diffusivity is enhanced by a factor of 5 by B-doping at a concentration of 1×10²⁰ atoms/cm³. A similar enhancement is observed in thermal diffusion of Au which has an activation energy of 1.5 eV. On the basis of these results a model for the ion-beam-enhanced diffusion of Au is proposed where the high density of defects present in amorphous Si act as traps for the fast moving interstitial Au atoms. The effectiveness of this trapping process can be changed by the high concentration of mobile defects generated by the beam and also by a change in the charge state of the traps induced by the presence of B.     

Volume and grain boundary diffusion of implanted 113Sn in aluminium

Volume and grain boundary diffusion of ¹¹³Sn in aluminium was investigated with the radiotracer method. The implantation technique was used for tracer deposition to avoid problems of tracer hold-up caused by the oxide layer always present on aluminium. The diffusion penetration was chosen large enough to permit serial sectioning of samples with the aid of a microtome.The temperature dependence of the volume diffusivity was determined as D(T)=4.54×10^–5×exp[–(114.5±1.2)kJmol^–1/RT] m ² s ^–1. This confirms previous measurements from our group which already showed that Sn is the fastest foreign metal diffusor so far investigated in aluminium.Grain boundary diffusion of ¹¹³Sn in Al polycrystals was measured in the type-B kinetic regime. The grain boundary diffusion product P=sD _gb (s=segregation factor, =grain boundary width, D _gb=grain boundary diffusivity) was found to be strongly affected by the impurity content of aluminium. For Al polycrystals of 99.9992% nominal purity we obtained P _5N(T)=1.08×10^–8exp [–(96.9±7.5) kJ mol^–1/RT] m³ s^–1 and for less pure Al polycrystals of 99.99% nominal purity P _4N(T)=3.0×10^–10 exp [–(90.1±4.2) kJ mol^–1/RT] m³ s^–1 was determined. The grain boundary diffusion product in the purer material is more than one order of magnitude higher than in the less pure material. Very likely this is an effect of co-segregation of non-diffusant impurities into the grain boundaries.     

Oxygen diffusion barriers using a new sacrificial design concept for future high-density memory devices

We emphasize the importance of the new design concept for diffusion barriers in high-density memory capacitors. RuTiN and RuTiO films are proposed as sacrificial oxygen diffusion barriers. They showed much lower sheet resistance up to 800 °C than various barriers including binary and ternary nitrides, reported by others. The contact resistance for both the Pt/RuTiN/TiSi_x/n⁺⁺poly-plug/n⁺channel layer/Si and the Pt/RuTiO/RuTiN/TiSi_x/n⁺⁺poly-plug/n⁺channel layer/Si contact structures, the most important electrical parameter for the diffusion barrier in the bottom-electrode structure of capacitors, exhibited values as low as 5 kΩ, even after annealing up to 750 °C. When each RuTiN and TiN film is inserted as a glue layer between the bottom electrode Pt layer in the CVD–BST simple stack-type structure, the thermal stability of the RuTiN glue layer is observed to be 150 °C higher than that of the TiN glue layer. Moreover, the capacitance of the PVD–BST simple stack-type structure with a TiN glue layer initially degrades after annealing at 500 °C, and thereafter failed completely. In the case of RuTiN and the RuTiO/RuTiN glue layers, however, the capacitance continuously increased up to 550 °C. These new experimental results accommodate the introduction of the sacrificial design concept of diffusion barriers against oxygen in high-density memory capacitors. Received: 6 February 2002 / Accepted: 4 March 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax: +82-31/360-4545, E-mail: dongsoo.yoon@hynix.com     

Hydrogen implantation and diffusion in silicon and silicon dioxide

Depth profiles of hydrogen implanted into crystalline silicon in random direction at different fluences have been measured by the¹⁵N technique and by SIMS. Whereas hydrogen implanted at a fluence of 10¹⁵ ions/cm² shows some limited mobility, no such mobility is observed for higher implantation fluences. In these cases, ballistic computer codes describe the depth distributions well, within the ranges of both experimental and theoretical accuracy. Annealing up to 510 K does not change the hydrogen distributions.Furthermore, high-fluence hydrogen implantation into silicon dioxide has been examined. There is some indication for radiation-enhanced diffusion during the implantation process. Upon subsequent thermal annealing, the hydrogen is found to diffuse, probably via a trapping/detrapping mechanism associated with an OH/H₂ transformation of the hydrogen bonding.     

Determination of the copper diffusion coefficient in silicon from transient ion-drift

We use the transient ion drift in a depletion region of a Schottky barrier to determine ion diffusivities at moderate temperatures. The pulsed reverse bias leads to temperature dependent capacitance transients similar to deep level carrier emission transients. A simple theoretical model together with classical transient signal analysis provide the means to extract the ion diffusion constant. When applied to copper in silicon, diffusion data are obtained in a not yet investigated temperature range (280–400 K) which agree well with both low and high temperature diffusion data.     

Evidence of very strong inter-epitaxial-layer diffusion in Zn-doped GaInPAs/InP structures

Zn-doped InP and GaInPAs layers were grown by OrganoMetallic Vapor-Phase Epitaxy (OMVPE). The epitaxial films consist of a primary GaInPAs/InP epitaxial layer and a secondary InP/GaInAs epitaxial layer. We present evidence that the redistribution of Zn acceptors in the primary epitaxial layer is strongly influenced by the Zn doping concentration in the secondary epitaxial layer. Rapid redistribution of Zn acceptors in the primary epitaxial layer occurs if the Zn doping concentration in the secondary epitaxial layer exceeds a critical concentration ofN _Zn3×10¹⁸cm^–3. The influence of the growth temperature on this effect is also presented.     

Approximation to the two-step diffusion profile by a series of gaussian functions

The two-step diffusion profile is approximated by a series of gaussian functions. Its accuracy is dependent upon the number of terms used in the series, but independent of the number and the spacing of exact data points.     

Oxygen diffusion through thin Pt films on Si(100)

We have investigated the diffusion of oxygen through evaporated platinum films on Si(100) upon exposure to air using substrates covered with Pt films of spatially and continuously varying thickness (0–500 Å). Film compositions and morphologies before and after silicidation were characterized by modified crater edge profiling using scanning Auger microscopy, energy-dispersive X-ray microanalysis, scanning tunneling microscopy, and transmission electron microscopy. We find that oxygen diffuses through a Pt layer of up to 170 Å forming an oxide at the interface. In this thickness range, silicide formation during annealing is inhibited and is eventually stopped by the development of a continuous oxide layer. Since the platinum film consists of a continuous layer of nanometer-size crystallites, grain boundary diffusion of oxygen is the most probable way for oxygen incorporation. The diffusion constant is of the order of 10^–19 cm²/s with the precise value depending on the film morphology.     

Carbon precipitation and diffusion in SiGeC alloys under silicon self-interstitial injection

In this work we investigate the diffusion and precipitation of supersaturated substitutional carbon in 200-nm-thick SiGeC layers buried under a silicon cap layer of 40 nm. The samples were annealed in either inert (N₂) or oxidizing (O₂) ambient at 850 °C for times ranging from 2 to 10 h. The silicon self-interstitial (I) flux coming from the surface under oxidation enhances the C diffusion with respect to the N₂-annealed samples. In the early stages of the oxidation process, the loss of C from the SiGeC layer by diffusion across the layer/cap interface dominates. This phenomenon saturates after an initial period (2–4 h), which depends on the C concentration. This saturation is due to the formation and growth of C-containing precipitates that are promoted by the I injection and act as a sink for mobile C atoms. The influence of carbon concentration on the competition between precipitation and diffusion is discussed. Received: 19 October 2001 / Accepted: 19 December 2001 / Published online: 20 March 2002 / Published online: 20 March 2002     

Diffusion of Al in ion-implanted Pd and Pt

The diffusion coefficients of aluminium have been measured in polycrystalline fcc Pd and Pt. The Al-implanted palladium and platinum samples were annealed at 400°–800 °C and 450°–900 °C, respectively. The aluminium profiles were probed using the nuclear resonance broadening (NRB) technique. Values of (1.41±0.09) and (1.38±0.09) eV for the activation energy and (1.5 _–1.0 ⁺⁵ )×10^–6 and (4 _–3 ⁺¹⁰ )×10^–7cm²/s for the frequency factor were obtained for Al in Pd and Pt, respectively. These anomalous results, compared to the normal impurity diffusion, were checked using also Al-evaporated samples.     

Diffusion of nitrogen in ion-implanted chromium and tungsten

The diffusion of N in the group VI B metals Cr and W has been studied in the previously uninvestigated temperature ranges 300°–550 °C (Cr) and 600°–800 °C (W) using ion-beam techniques. Diffusion couples were created by ion-implantation. The timedependent diffusion profiles were monitored by the use of the Nuclear Resonance Broadening (NRB) technique. The linear Arrhenius plots extracted from the measured diffusivities indicate that the diffusivity of implanted N in Cr and W can be described by the activation energyQ=1.39±0.06 eV and 2.32±0.16 eV and the pre-exponential factorD ₀=(7.0±7.2)×10^–4cm^2/s and 4.3±8.3cm^2/s, respectively. The solubilities of N in Cr and W from the implanted distributions were found to deviate from those obtained using conventional metallographical methods.     

Diffusion of Ta in α-Ti

The diffusion of Ta in the hcp (α) phase of high-purity Ti (99.99%) was studied at different temperatures from 911 K up to 1123 K. The Rutherford Backscattering Spectrometry (RBS) and Heavy Ion RBS (HIRBS) techniques were used to obtain the penetration profiles. The evolution of the diffusion coefficient, D, as a function of temperature follows prediction of the Arrhenius law. The activation energy of the diffusion process is (318±7)kJ/mol, similar to that corresponding to self-diffusion in α-Ti. On the other hand, the measured values of D are systematically lower than those corresponding to self-diffusion by a factor of approximately 5. This reduction could be explained by taking into account the mass difference between Ta and Ti. An increase of the diffusion coefficient was measured when the diffusion proceeds on a less pure Ti (99.9%) matrix. This increment is higher at lower temperatures. Received: 12 November 2001 / Accepted: 12 March 2002 / Published online: 5 July 2002 RID="*" ID="*" RID="*" ID="*" RID="**" ID="**"Corresponding author. Fax: +54-11/6772-7362, E-mail: dyment@cnea.gov.ar RID="*" ID="*"Members of the Carrera del Investigador Científico del Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina     

Diffusion of Al in ion-implanted α-Zr and α-Hf

The diffusion of Al in the group IVa metals Zr and Hf has been studied for the first time in the temperature ranges 600°–800°C (Zr) and 750°–900°C (Hf) using ion-beam techniques. Diffusion couples were created by ion-implantation. The time-dependent diffusion profiles were monitored by the use of the Nuclear Resonance Broadening (NRB) technique. The linear Arrhenius plots extracted from the measured diffusivities indicate that the diffusivity of implanted Al in Zr and Hf can be described by the activation energyQ=2.9±0.2eV and 3.7±0.3eV and the pre-exponential factorD ₀=17±42cm²/s and 170±600cm²/s, respectively.     

Field-enhanced neutralization of electrically active boron in hydrogen implanted Schottky diodes

We have observed that hydrogen implantation in p-type boron-doped silicon material induces a neutralization of boron in a 10 m deep region after the Schottky diodes have been heated at a 90° C temperature under reverse-biasing. The profile of neutralized acceptors can be reversibly shaped by successively applying different reverse biases.