S. G. Bishop

Investigation of the optical and electronic properties of Ge2Sb2Te5 phase change material in its amorphous, cubic, and hexagonal phases

Ge–Sb–Te alloys are widely used for data recording based on the rapid and reversible amorphous-to-crystalline phase transformation that is accompanied by increases in the optical reflectivity and the electrical conductivity. However, uncertainties about the optical band gaps and electronic transport properties of these phases have persisted because of inappropriate interpretation of reported data and the lack of definitive analytical studies. In this paper we characterize the most widely used composition, Ge2Sb2Te5, in its amorphous, face-centered-cubic, and hexagonal phases, and explain the origins of inconsistent or unphysical results in previous reports. The optical absorption in all of these phases follows the relationship αhν∝(hν−Egopt)2, which corresponds to the optical transitions in most amorphous semiconductors as proposed by Tauc, Grigorovici, and Vancu [Tauc et al., Phys. Status Solidi 15, 627 (1966)], and to those in indirect-gap crystalline semiconductors. The optical band gaps of the amorpho...

Thermal conductivity of phase-change material Ge2Sb2Te5

The thermal conductivity of thin films of the phase-change material Ge2Sb2Te5 is measured in the temperature range of 27°C<T<400°C using time-domain thermoreflectance. From the low thermal conductivity of amorphous phase, the conductivity increases irreversibly with increasing temperature and undergoes large changes with phase transformations. Thermal transport in the amorphous and early cubic phases can be described by a random walk of vibrational energy, i.e., the minimum thermal conductivity. In the hexagonal phase, the electronic contribution to the thermal conductivity is larger than the lattice contribution. Crystallization by laser processing produces a cubic phase with a lower thermal conductivity than cubic phases produced by thermal annealing; the authors attribute this difference in conductivity to a larger degree of atomic-scale disorder in films that are crystallized on short time scales.

Observation of the Role of Subcritical Nuclei in Crystallization of a Glassy Solid

Catching Glassy Crystallization The initial steps for crystallization are difficult to study in atomic materials because they occur on very small length- and time scales. Measurements can be made on surfaces, or by using colloids as analogs, but ideally one would like to observe the ordering phenomena in atomic solids in bulk. Lee et al. (p. 980; see the Perspective by Gibson) use fluctuation transmission electron microscopy to explore the role of nucleation in propagating crystallization and discovered that formation of subcritical nuclei strongly influences the crystallization process. Fluctuation transmission electron microscopy images nanoscale nuclei and their influence on subsequent crystallization. Phase transformation generally begins with nucleation, in which a small aggregate of atoms organizes into a different structural symmetry. The thermodynamic driving forces and kinetic rates have been predicted by classical nucleation theory, but observation of nanometer-scale nuclei has not been possible, except on exposed surfaces. We used a statistical technique called fluctuation transmission electron microscopy to detect nuclei embedded in a glassy solid, and we used a laser pump-probe technique to determine the role of these nuclei in crystallization. This study provides a convincing proof of the time- and temperature-dependent development of nuclei, information that will play a critical role in the development of advanced materials for phase-change memories.

Trap-mediated excitation of Er3+ photoluminescence in Er-implanted GaN

Site-selective photoluminescence (PL) spectra obtained at 6 K from the 1540 nm 4I13/2→4I15/2 emissions characteristic of four distinct Er3+ centers in Er-implanted films of GaN are compared with the Er3+ PL excited by 325 nm above-gap pump light. Two of the site-selective 1540 nm Er3+ PL spectra pumped by below-gap, trap-mediated excitation bands dominate the Er3+ PL spectrum excited by above-gap light. A third broad band-excited spectrum and a fourth spectrum pumped by direct Er3+ 4f-band absorption are apparently not strongly excited by above-gap light. These results indicate that trap-mediated excitation dominates above-gap pumping of Er3+ emission in GaN:Er, and suggest an explanation for the reduced thermal quenching of Er3+ emission in GaN.

Observation of multiple Er3+ sites in Er-implanted GaN by site-selective photoluminescence excitation spectroscopy

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies have been carried out at 6 K on the 1540 nm 4I13/2→4I15/2 emission of Er3+ in Er-implanted films of GaN grown by metalorganic chemical vapor deposition. The PLE spectra exhibit several broad below-gap absorption bands, which excite three distinct site-selective Er3+ PL spectra. The excitation of two of the site-selective Er PL bands involves optical absorption by defects or background impurities, rather than direct intra-f shell absorption, with subsequent nonradiative transfer of the energy to nearby Er3+ luminescence centers. The characteristics of the PLE spectrum of the third site-selective PL band suggest that an exciton bound at an Er-related trap is involved in the excitation mechanism.

The incorporation of arsenic in GaN by metalorganic chemical vapor deposition

We report on the successful incorporation of arsenic (As) in GaN during metalorganic chemical vapor deposition (MOCVD). A characteristic room-temperature luminescence band centered around 2.6 eV (480 nm), similar to the peak position of the As ion-implanted GaN, is found to be related to the As impurity in the MOCVD grown GaN:As films. The arsenic incorporation efficiency as a function of experimental conditions and structure is presented. Temperature- and power-dependent cathodoluminescence measurements have been performed to help establish the nature of the As-related peak.

Low loss photoinduced waveguides in rapid thermally annealed films of chalcogenide glasses

Rapid thermal annealing of sputter-deposited chalcogenide glasses is demonstrated to yield superior quality films which are used to fabricate low loss (<0.3 dB/cm) photoinduced channel waveguides in chalcogenide glasses. This process also enhances the photosensitivity and the magnitude of photoinduced index changes possible (Δn∼5%) in these glass systems.

Selective enhancement of 1540 nm Er3+ emission centers in Er-implanted GaN by Mg codoping

The ∼1540 nm 4I13/2 to 4I15/2 Er3+ photoluminescence (PL) and photoluminescence excitation (PLE) spectra of Er-implanted Mg-doped GaN reveal a selective enhancement of one of the nine different Er3+ centers observed previously in PL and PLE studies of Er-implanted undoped GaN. These Er3+ PL spectra are excited selectively by pump wavelengths that correspond to broadband, below-gap absorption bands associated with different Er3+ centers. In the Er-implanted, Mg-doped GaN, both the 1540 nm PL spectrum characteristic of the so-called violet-pumped Er3+ center and the ∼2.8–3.4 eV (violet) PLE band that enables its selective excitation are significantly enhanced by Mg doping. In addition, the violet-pumped PL center dominates the above-gap-excited Er3+ PL spectrum of Er-implanted Mg-doped GaN, whereas it was nearly unobserveable under above-gap excitation in Er-implanted undoped GaN. These results confirm our hypothesis that appropriate codopants can increase the efficiency of trap-mediated above-gap excitatio...

Optical characteristics of p-type GaN films grown by plasma-assisted molecular beam epitaxy

Using a molecular beam epitaxy system equipped with an inductively coupled radio frequency nitrogen plasma source, p‐type GaN films were grown on sapphire substrates with no postgrowth treatment. Uniformity of the surface morphology and spatial homogeneity of the luminescence of the films were investigated using scanning electron microscopy and cathodoluminescence (CL) imaging, respectively. By examining the dependence of photoluminescence on the excitation laser power density at 6 and 300 K, three different emissions having different origins were identified. A blue emission at ∼3.25 eV is associated with shallow Mg impurities, while two different lower‐energy emissions at ∼2.43 and ∼2.87 eV are associated with deep Mg complexes. The spatial distributions of the shallow and deep Mg impurities dominating the optical properties of the p‐type GaN films were also examined along the growth direction by low‐ and room‐temperature CL using an electron beam with a range of penetration depths

Band gap bowing in GaP1−xNx alloys

The interband optical absorption edge of GaP1−xNx alloys grown by molecular beam epitaxy has been investigated by photoluminescence and photoluminescence excitation (PLE) spectroscopy. The results demonstrate the transition from nitrogen‐bound exciton luminescence indicative of isoelectronic traps in lightly doped films of GaP:N to the formation of indirect band gap alloys in the heavily N‐doped GaP films. A large red shift or reduction in band gap energy with increasing N concentration which is observed in PLE spectroscopy identifies the GaP1−xNx alloy system as one of the limited number of alloy systems whose intermediate compositions can have band gaps smaller than those of either of the constituent binary compounds.