Doped SbTe phase change material in memory cells

Phase Change Random Access Memory (PCRAM) is investigated as replacement for Flash. The memory concept is based on switching a chalcogenide from the crystalline (low ohmic) to the amorphous (high ohmic) state and vice versa. Basically two memory cell concepts exist: the Ovonic Unified Memory (OUM) and the line cell. Switching to the high ohmic or low ohmic state is done using Joule heating. A relatively short (~ns) electrical pulse with large amplitude is used to heat the crystalline phase to melt and quench into the amorphous state (RESET). A pulse with smaller amplitude heats the amorphous region above its crystallization temperature (lower than the melting temperature) and the material returns into the crystalline state (SET). In the OUM cell this will be at the electrode-phase change material contact, whereas for the line cell this will be at the position where the current density is the highest

Barrier Height Variation in Ni-Based AlGaN/GaN Schottky Diodes

In this paper, we have investigated Ni-based AlGaN/GaN Schottky diodes comprising capping layers with silicon-Technology-compatible metals such as TiN, TiW, TiWN, and combinations thereof. The observed change in Schottky barrier height of a Ni and Ni/TiW/TiWN/TiW contact can be explained by stress effects induced by the TiW/TiWN/TiW capping layer, rather than by chemical reactions at the metal-semiconductor interface. Secondary-ion mass spectroscopy and transmission electron microscopy techniques, for samples with and without a TiW/TiWN/TiW cap, have been used to show that no chemical reactions take place. In addition, electrical characterization of dedicated samples revealed that the barrier height of Ni/TiW/TiWN/TiW contacts increases after stepwise selective removal of the TiW/TiWN/TiW cap, thus demonstrating the impact of strain

Deposition of thin layers containing Ga, C and N by sequential pulses of Trimethylgallium and Ammonia

Gallium nitride (GaN) is a semiconductor with broad applications in the (opto-)electronic industry. State-of-the-art fabrication of GaN demands a nanometer-level control over layer thickness, which can be achieved with atomic layer deposition (ALD). Introducing carbon (C) into GaN layers, similar to introducing C into BN [1] or as a dopant in GaN [2], can facilitate control over material properties such as the band-gap and resistivity, respectively. In this work, we report on our results obtained from thermal deposition of layers, containing varying concentration of gallium (Ga), carbon (C) and nitrogen (N), from trimethylgallium (TMG) and ammonia (NH3) precursors. The precursors were sequentially introduced in a pulsed mode, i.e., without mixing them

Interface Characterization of Metals and Metal-nitrides to Phase Change Materials

We have investigated the interfacial contact properties of the CMOS compatible electrode materials W, TiW, Ta, TaN and TiN to doped-Sb2Te phase change material (PCM). This interface is characterized both in the amorphous and in the crystalline state of the doped-Sb2Te. The electrical nature of the interface is characterized by contact resistance measurements and is expressed in terms of specific interfacial contact resistance (ϿC). These measurements are performed on four-terminal Kelvin Resistor test structures. Knowledge of the ϿC is useful for selection of the electrode in the integration and optimization of the phase change memory cells

Contact resistance of TiW to ultra-thin phase change material layers

In this article we report on the change in contact resistance of TiW to doped-Sb2Te in the 5nm-50nm thickness range of the PCM layer. This interface is characterized both in the amorphous and in crystalline state of doped-Sb2Te. The nature of the interface is characterized by electrical contact resistance measurements and is expressed in terms of specific contact resistance, ϿC. Results from the measurements on these structures with illumination indicated the existence of a space-charge region at the metal amorphous doped-Sb2Te interface

Thermal Atomic Layer Deposition of Polycrystalline Gallium Nitride

We report the successful preparation of polycrystalline gallium nitride (poly-GaN) layers by thermal atomic layer deposition (ALD) at low temperatures (375–425 °C) from trimethylgallium (TMG) and ammonia (NH3) precursors. The growth per cycle (GPC) is found to be strongly dependent on the NH3 pulse duration and the NH3 partial pressure. The pressure dependence makes the ALD atypical. We propose that the ALD involves (i) the reversible formation of the hitherto-unreported TMG:NH3 surface adduct, resulting from NH3 physisorbing on a TMG surface site and (ii) the irreversible conversion of neighboring surface adducts to Ga–NH2–Ga linkages. The pressure dependence arises from the presumed reversible nature of the adduct formation on the surface, equivalent to the known reversible nature of its formation in the gas phase in metal organic chemical vapor deposition reactions. Using in situ spectroscopic ellipsometry (SE), the GPC monitored as a function of several ALD parameters is as high as 0.1 nm/cycle at 60 s NH3 pulse and 1.3 mbar NH3 partial pressure. The changes in the growth pattern (as monitored by SE) caused by changes in the ALD parameters support the proposed growth model. Ex situ characterization reveals that the layer is carbon-free, has a polycystalline wurtzitic structure, and shows a decent conformaility over Si trenches. Tuning the ALD recipe allows us to vary the layer composition from Ga-rich to stoichiometric GaN. The Ga richness is attributed to the simultaneous TMG dissociation at the deposition temperatures. This work is the first full-scale report on low temperature thermal ALD of poly-GaN from industrial precursors, occurring via a novel chemical pathway and not requiring any radical assistance (such as plasma) as used before

Scaling properties of doped Sb2Te phase change line cells

For phase change random access memory applications, the scaling perspective of the 3 main programming parameters is essential. The programming time will largely determine the obtainable data rate. The required programming current will largely determine the transistor size and hence the obtainable memory density. Finally, the programming voltage should preferably not exceed the transistor driving voltage. In this paper, the scaling perspective for these 3 main programming parameters is investigated for doped Sb2Te PCRAM line cells

Composite GaN-C-Ga ("GaCN") Layers with Tunable Refractive Index

This article describes novel composite thin films consisting of GaN, C, and Ga (termed "GaCN", as an analogue to BCN and other carbonitrides) as a prospective material for future optical applications. This is due to their tunable refractive index that depends on the carbon content. The composites are prepared by introducing alternating pulses of trimethylgallium (TMG) and ammonia (NH 3 ) on silicon substrates to mimic an atomic layer deposition process. Because the GaCN material is hardly reported to the best of our knowledge, a comprehensive characterization is performed to investigate into its chemical nature, primarily to determine whether or not it exists as a single-phase material. It is revealed that GaCN is a composite, consisting of phase-segregated, nanoscale clusters of wurtzitic GaN polycrystals, in addition to inclusions of carbon, nitrogen, and gallium, which are chemically bonded into several forms, but not belonging to the GaN crystals itself. By varying the deposition temperature between 400 and 600 °C and the NH 3 partial pressure between 0.7 × 10 -3 and 7.25 mbar, layers with a wide compositional range of Ga, C, and N are prepared. The role of carbon on the GaCN optical properties is significant: an increase of the refractive index from 2.19 at 1500 nm (for carbon-free polycrystalline GaN) to 2.46 (for GaCN) is achieved by merely 10 at. % of carbon addition. The presence of sp 2 -hybridized C=N clusters and carbon at the interface of the GaN polycrystals are proposed to determine their optical properties. Furthermore, the formation of the GaN polycrystals in the composite occurs through a TMG:NH 3 surface-adduct assisted pathway, whereas the inclusions of carbon, nitrogen, and gallium are formed by the thermal decomposition of the chemisorbed TMG species