Bilayer Cr/Au contacts on n-GaN

Cr/Au (40/65 nm) metal layers have been deposited by thermal evaporation onto n-GaN epitaxial layers grown by metal-organic chemical-vapour deposition (MOCVD) on a sapphire substrate. The samples have been annealed at 400, 700 and 900 °C for 10 min in vacuum. Techniques of TEM, EDS, HRTEM, FESEM, XRD and I-V characteristics have been used to characterize the micro-, and nanostructure, morphology, composition and electrical properties of the contacts before and after annealing. A binary phase of Cr₃Ga₄ and Au₇Ga₂ were identified at the interface of the n-GaN/Cr/Au contacts after annealing in vacuum at 700 and 900 °C. Current-voltage characterizations showed that the as-deposited and annealed Cr/Au contacts are rectifying up to 600 °C. After heat treatment in vacuum at 700 °C and 900 °C the Cr/Au contacts were linear.     

Performance of Pd–Ge based ohmic contacts to n-type GaAs

The performance of Pd–Ge based ohmic contacts, with and without Ti–Pt or Ti–Pt–Au capping layers, has been investigated. The contacts were deposited by electron beam evaporation, then characterized electrically using a modified transmission line method (TLM) and structurally using both cross-section and plan-view transmission electron microscopy (TEM). Although both capped and non-capped contact structures underwent the same phase transformations during annealing, capped contacts had significantly better contact resistances (a minimum value of 4×10-7 Ω cm² was achieved) – almost three orders of magnitude better. The superior performance is attributed to the capping layers providing protection for the Pd–Ge layers during contact processing, where the metallization was exposed to a CF₄–O₂ plasma, oxyen descumming, organic solvents and deionized water. Non-capped contacts exhibited PdGe decomposition and oxidation of exposed Ge. Long-term reliability testing of capped contacts showed virtually no change in contact resistance at 235°C (1350 h) and a sevenfold increase after ageing at 290°C for 370 h. There were no phase changes during ageing; the increase in contact resistance was attributed to interdiffusion between Ge and GaAs. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermally stable Pd/Sn and Pd/Sn/Au ohmic contacts to n-type GaAs

Thermal stability of novel Pd/Sn and Pd/Sn/Au Ohmic contacts to n-GaAs has been investigated and compared to the non-alloyed Pd/Ge and alloyed Au–Ge/Ni metallizations. Metallization samples are furnace annealed at various temperatures and systematically characterized utilizing Scanning Electron Microscopy (SEM) and current–voltage (I–V) measurements. Contact resistivities, ρ_c, of the proposed metallization are measured using a conventional Transmission Line Model (cTLM) method. The Pd/Sn Ohmic contacts display superior thermal stability at 410°C when compared to the Pd/Ge contacts. After annealing at 410°C for 4 h, ρ_c of the Pd(50 nm)/Sn(125 nm) metallization remains in the low 10⁻⁵ Ω cm² range, whereas ρ_c values increase to 10⁻⁴ Ω cm² for the Pd(50 nm)/Ge(126 nm) contacts. At 410°C, the Pd/Sn/Au metallizations also display better thermal stability than that of non-alloyed Pd/Ge and alloyed Au–Ge/Ni metallizations. The long-term stability at 300°C of the Pd/Sn and Pd/Sn/Au Ohmic contacts is also reported.     

Native oxide consumption during the atomic layer deposition of TiO2 films on GaAs (100) surfaces

The consumption of the surface native oxides is studied during the atomic layer deposition of TiO₂ films on GaAs (100) surfaces. Films are deposited at 200 °C from tetrakis dimethyl amido titanium and H₂O. Transmission electron microscopy data show that the starting surface consists of ~2.6 nm of native oxide and X-ray photoelectron spectroscopy indicates a gradual reduction in the thickness of the oxide layer as the thickness of the TiO₂ film increases. Approximately 0.1-0.2 nm of arsenic and gallium suboxide is detected at the interface after 250 process cycles. For depositions on etched GaAs surfaces no interfacial oxidation is observed.     

Degradation and breakdown characteristics of Al/HfYOx/GaAs capacitors

Effects of surface passivation and the interfacial layer on the reliability characteristics of Al/HfYO_x/GaAs metal-oxide-semiconductor capacitor structures are reported. Stress-induced leakage current mechanism, critical for understanding the degradation and breakdown in Al/HfYO_x/GaAs capacitors, has been studied in detail. While the devices fabricated with (NH₄)₂S-passivated GaAs substrates show both the soft and hard breakdown failure modes, capacitors with ultrathin interfacial layer (Ge or Si) show only hard breakdown. It is shown that the degradation dynamics follows more closely the logistic power-law relationship rather than the conventional power-law model, frequently used to describe leakage current conduction in high-k gate dielectrics.     

Point defects and diffusion mechanisms pertinent to the Ga sublattice of GaAs

For the Ga sublattice of GaAs, the recent understanding of the impurity and self-diffusion mechanisms and the nature of the point defects responsible are discussed. Analyses of doping enhanced AlAs/GaAs superlattice disordering data and impurity diffusion data have led to the conclusion that, under thermal equilibrium and intrinsic conditions, the triply-negatively-charged Ga vacancy ((V_Ga³⁻) governs Ga self-diffusion and Al---Ga interdiffusion in As-rich crystals, while the doubly-positively-charged Ga self-interstitial (I_Ga²⁺) dominates in Ga-rich crystals. When doped sufficiently, dominates in n-type crystals, while I_Ga²⁺ dominates in p-type crystals, irrespective of the crystal composition. The V_Ga³⁻ species also contributes to the diffusion of the main donor species Si, while I_Ga²⁺ also governs the diffusion of the main acceptor species Zn and Be via the kick-out mechanism. The thermal equilibrium concentration of V_Ga³⁻ (C_VGa³⁻) has been found to exhibit a temperature independence or even a small negative temperature dependence in that, when the temperature is lowered, C_VGa³⁻; is either unchanged or even slightly increases. This C_VGa³⁻ behavior is consistent with many outstanding experimental results.     

Structural characterization of GaSb-capped InAs/GaAs quantum dots with a GaAs intermediate layer

GaSb incorporation to InAs/GaAs quantum dots is considered for improving the opto-electronic properties of the systems. In order to optimize these properties, the introduction of an intermediate GaAs layer is considered a good approach. In this work, we study the effect of the introduction of a GaAs intermediate layer between InAs quantum dots and a GaSb capping layer on structural and crystalline quality of these heterostructures. As the thickness of the GaAs intermediate layer increases, a reduction of defect density has been observed as well as changes of quantum dots sizes. This approach suggests a promising method to improve the incorporation of Sb to InAs heterostructures.     

Study of GaAs spacer layers in InAs/GaAs vertically aligned quantum dot structures

We investigated vertically aligned InAs/GaAs QD structures, grown by atomic layer molecular beam epitaxy, with a number N of layers and with spacer thicknesses d. QD alignment and structure quality were checked by transmission electron microscopy. The dependencies of carrier capture, decay dynamics and existence of quenching channels on the design parameters N and d were studied by time resolved photoluminescence (PL), PL excitation (PLE) and PL temperature-dependent measurements. Our results show that the carrier capture and the radiative efficiency of the QDs are negatively affected by increasing the number of QD layers and by reducing the spacer thicknesses; this effect is likely to be related to the increase of defect concentrations in GaAs spacers, due to relaxation of an increasingly large strain.     

Growth and defects of GaAs and InGaAs films on porous GaAs substrates

We investigated the opportunities to increase the electric uniformity of GaAs and InGaAs films grown by molecular-beam epitaxy (MBE) technique on monocrystalline (single crystal) GaAs: both on porous and conventional so-called “monolithic” (without pores) GaAs (100) substrates. The basic attention was given to study the electrically active defects in films by using scanning electron microscope (SEM) with new technique which is called “Rau-detector” [E.I. Rau, A.N. Zhukov and E.B. Yakimov, Solid-State Phenomena, 1998, v. 53-54, 327.]. We compared the main properties of epitaxial GaAs and InGaAs films grown on above mentioned substrates. The films grown on porous substrates had higher structural perfection including the following advantages: (a) smoother surface due to lateral growth mechanism; (b) less density of structural defects (without dislocation walls), the density of pyramidal defects was ∼ 2 × 10⁵ cm^− 2 as compared with the density 2 × 10⁷ cm^− 2 in the films grown on monolithic substrates; (c) less electrical activity of various structural defects and increased electric uniformity of grown films. The electrical activity of defects in films grown on porous substrates was essentially lowered due to gettering properties of porous substrate.     

Dielectric properties and model of hopping conductivity of GaAs irradiated by H ions

Experimental results on frequency and temperature dependences of conductivity of GaAs layers compensated by polyenergetic H⁺ implantation are presented. A model of hopping conductivity related to amphoteric defects is proposed.     

Temperature-dependent Hall analysis of carbon-doped GaAs

The temperature dependence of the electrical properties, such as hole concentration, Hall mobility and resistivity of carbon-doped GaAs epilayers over a wide range of doping levels has been investigated. The carbon-doped GaAs epilayers have been grown by low pressure metalorganic chemical vapor deposition. The electrical properties have been obtained by Hall measurements. Experimental data on the carrier mobility, Hall effect, and resistivity over a wide temperature range have been analyzed and possible scattering mechanisms have been explained. Our experimental data show that the ionized impurity scattering tend to be dominant at temperatures below 100 K, while the lattice scattering as well as the ionized impurity scattering plays an important role at temperatures above 100 K. The dependence of the electrical on the doping levels has also been studied. In the case of heavily C-doped GaAs, the mobility curves are nearly flat at temperatures below 100 K and the mobility decreases as temperature increases above 100 K. The reason is that the degenerate conduction occurs at high doping level. The degenerate conduction begins at the hole concentration of about 2 × 10¹⁸ cm⁻³ at 77 K and at room temperature.     

Reliability problems in state-of-the-art GaAs devices and circuits

A review of the reliability status of GaAs discrete devices and integrated circuits is given. In the present survey of new devices and circuits it is shown that a significant number of reliability problems continue to persist.     

GaAs wafer mapping by microwave-detected photoconductivity

An improved highly sensitive method of detecting photo conductivity by microwave absorption (MDP) is described. The method is applicable to semi-insulating and low resistivity marterial as well. It is non-destructive and can be applied to epi-layers. MDP topograms are compared with those for photo luminescence, point contact, and EL2 using the same sample. The MDP-contrast is mainly due to a so far unknown recombination centre determining the carrier lifetime under non-equilibrium. The annealing behaviour of MDP is very similar to that of the point-contact method. In general, MDP has the potential to be used as a new tool for material quality inspection with direct evidence for device properties.     

Low-temperature annealed ohmic contacts to Si-doped GaAs and contact formation mechanisms

Using nonferromagnetic contact materials, Au(x nm)/Ge(y nm)/Pd(z nm) structures (where x, y, and z are the thicknesses of Au, Ge and Pd layers, respectively) are fabricated on Si-doped GaAs and studied as a function of x, y and z and n-type substrate doping density and annealing temperature to characterise them as ohmic contacts. The study shows that the structure with x = 100, y = 40 and z = 10, annealed at 180 °C for 1 h, contacts n-type GaAs more reliably with the low contact resistance. Using Rutherford backscattering spectrometry, contact formation mechanisms are also studied.     

Growth and characterization of copper nanoclusters embedded in SiC matrix

Nanocrystalline copper clusters embedded in silicon carbide were made by island growth during sputter deposition. The distribution and morphology of metal clusters were observed by high-resolution transmission electron microscopy. To investigate chemical bonding at the copper-silicon carbide interface, we studied the electronic states of copper and silicon using X-ray photoelectron spectroscopy (XPS). It was found that the formation of copper silicide was suppressed in this system and that small shifts in binding energy were observed for different sizes of clusters, which was different from the chemical shift for copper silicide formation.     

Microstructure and composition of MgF2 optical coatings grown on Si substrate by PVD and IBS processes

MgF₂ is a current material for the optical applications in the UV and deep UV range. Nevertheless, modern applications still require improvement of the optical and structural quality of the deposited layers. In the present work, the composition and microstructure of MgF₂ single layers grown on Si [100] substrate by physical vapour deposition (PVD) and ion beam sputtering (IBS) processes, were analyzed and compared. Experiments were carried out using X-ray photoelectron spectroscopy (XPS) in depth profile, grazing angle X-ray diffraction (XRD) and transmission electron microscopy (TEM). Both layers exhibited a good stoichiometry and a low level of contamination. The sample grown by IBS revealed a more homogeneous and regular columnar microstructure than the other one.     

Al and Ti/Al contacts on n-GaN

Al(60 nm) and Ti(40 nm)/Al(160 nm) metal layers have been deposited by thermal evaporation onto n-GaN epitaxial layers grown by metal organic chemical vapour deposition (MOCVD) on a c-plane sapphire substrate. The samples have been annealed at 300, 400, 700 or 900 °C for 10 min in vacuum. The microstructural and electrical properties of the contacts have been investigated by electron microscopy, X-ray diffraction and by current-voltage measurements. As-deposited Al and Ti/Al contacts were rectifying with Schottky barrier heights below 0.35 eV and 0.38 eV, respectively. After heat treatment at 300 °C and 400 °C both contacts exhibited linear current-voltage characteristics. After annealing at 700 °C Al contacts became rectifying with a barrier height of 0.42 eV, while Ti/Al contacts remained nearly linear at the same temperature. The electrical characteristics and XRD analysis indicated that the upper metal in Ti/Al contact diffused in the Ti layer already during deposition. Cross-sectional transmission electron microscopy revealed that in the case of Ti/Al contacts, the continuity of the Ti layers ceased when annealing above 700 °C. X-ray diffractions showed, that a Ti₂N interface phase formed in Ti/Al contacts at 700 and 900 °C, and an AlN interface phase developed in the same contact at 900 °C.     

TEM study of the indentation behaviour of thin Au film on GaAs

Au films of 8.9 nm thickness have been sputter deposited onto a (001) GaAs substrate at room temperature. An average grain size of 10 nm and no texture were obtained. Subsequent, nanoindentation tests were performed on the coated specimens and the mechanical response was compared to that of a bulk GaAs sample with the same crystallographic orientation. Furthermore, the loading–unloading curves were analysed in view of transmission electron microscopy plan-view images obtained on the deformed substrate–film specimens and compared to results previously reported in the literature for bulk sample. Constrained plasticity of the films was observed to occur for residual depth to thickness ratio below 0.67. Further, plastic deformation of the substrate happened on coated specimens at loads less than those required to plastically deform bare substrate.