| dc.description.abstract | GeTe and Ge2Sb2Te5 alloys are potential candidates for non-volatile phase change
random access memories (PCRAM). For electrical data storage applications the materials should have stable amorphous and crystalline phases, fast crystallization time, low power to switch, and high crystallization activation energy (to be stable at normal operating
temperatures). Phase change memories can be tuned through compositional variations to
achieve sufficient phase change contrast and thermal stability for data retention. Selenium is one of the attractive choices to use as an additive material owing to its flexible amorphous structure and a variety of possible applications in optoelectronics and solar cells. GeSb2Te3Se alloy, in which 25 at.% of Se substituted for Te, show a higher room temperature resistance with respect to parent GeSb2Te4 alloy, but the transition
temperature is lowered which will affect the thermal stability. The RESET current
observed for Sb65Se35 alloys were reduced and the crystallization speed increased 25 %
faster with respect to Ge2Sb2Te5. Alloys of Ga-Sb-Se possess advantages such as higher
crystallization temperatures, better data retention, higher switching speed, lower thermal conductivity and lower melting point than the GST, but the resistance ratio is limited to about two orders of magnitude. This affects the resistance contrast and data readability.
It is with this background a study has been carried out in GeTe and GeSbTe
system with Se doping. Studies on structural, thermal and optical properties of these
materials all through the phase transition temperatures would be helpful to explore the
feasibility of phase change memory uses. Thin films along with their bulk counterparts
such as (GeTe)1-x Sex ( 0 < x ≤ 0.50) and (GST)1-xSex (0 < x ≤ 0.50), including GeTe and GST alloys, have been prepared. The results are presented in four chapters apart from the Introduction and Experimental techniques chapters. The final chapter summarizes the results.
Chapter 1 provides an introduction to chalcogenide glasses, phase change memory materials and their applications. The fundamental properties of amorphous
solids, basic phase change properties of Ge2Sb2Te5 and GeTe alloys and their applications are presented in detail. Various doping studies on GeTe and Ge2Sb2Te5
reported in literatures are reviewed. The limitations, challenges, future and scope of the present work are presented.
In chapter 2, the experimental techniques used for thin film preparation, electrical
characterizations, optical characterization and surface characterizations etc. are
explained.
Chapter 3 deals entirely on Ge2Sb2Te5 films studied throughout the phase transition, by annealing at different temperatures. Changes in sheet resistance, optical transmission, morphology and surface bonding characteristics are analyzed. The
crystallization leads to an increase of roughness and the resistance changes to three orders of magnitude at 125 oC. Optical studies show distinct changes in transmittance during phase transitions and the optical parameters are calculated. Band gap contrast and disorder variation with annealing temperatures are explained. The surface bonding characteristics studied by XPS show Ge-Te, Sb-Te bonds are present in both amorphous and crystalline phases. The temperature dependent modifications of the band structure of amorphous GST films at low temperatures have been little explored. The band gap increment of around 0.2 eV is observed at low temperature (4.2 K) compared to room temperature 300 K. Other optical parameters like Urbach energy and B1/2 are studied at different temperatures and are evaluated. The observed changes in optical band gap (Eopt) are fitted to Fan’s one phonon approximation, from which a phonon energy (ћω) corresponding to a frequency of 3.59 THz resulted. The frequency of 3.66 THz optical phonons has already been reported by coherent phonon spectroscopy experiment in
amorphous GST. This opens up an indirect method of calculating the phonon frequency
of the amorphous phase change materials.
Chapter 4 constitutes comparison of optical, electrical and structural investigation
of GST and (GST)1-xSex films. It is well known that GST alloys have vacancy in their
structure, which leads to the possibility of switching between the amorphous and
crystalline states with minimum damage. Added Se may occupy the vacancy or change
the bonding characteristics which intern may manifest in the possibility of change in
optical and electrical parameters. The structural studies show a direct amorphous to
hexagonal transition in (GST)1-xSex, where x ≥ 0.10 at.%. Raman spectra of the as
deposited and annealed (GST)1-xSex films show structural modifications. The infrared
transmission spectra indicate a shift in absorption edges from low to high photon energy when Se concentration increases in GST. Band gap values calculated from Tauc plot show the band gap increment with Se doping. It is noted that a small amount of Se doping increases the resistance of the amorphous and crystalline phases and maintains the same orders of resistance contrast. This will be beneficial as it improves the thermal stability
and reduces the write current in a device. Switching studies show an increasing threshold voltage as the Se doping concentration increases.
Chapter 5 comprises compositional dependent investigations of the bulk GeTe
chalcogenides alloys added with different selenium concentrations. The XRD
investigations on bulk (GeTe)1-xSex (x = 0.0, 0.02, 0.10, 0.20 and 0.50 at.%) alloys show
that the crystalline structure of GeTe alloys does not affect ≤ 0.20 at.% of Se
concentration. With increasing amount of Se concentration the alloys gets modified in to
a homogeneous amorphous structure. This result has been verified from the XRD,
Raman, XPS, SEM and DSC measurements. The possibility that Se occupying the Ge
vacancy sites in GeTe structure is explained. Since Se is an easy glass former, the
amorphousness increases in the alloys due to new amorphous phases formed by the Se
with other elements. It is shown from Raman and XPS analysis that the Ge-Te bonds
exists up to Se 0.20 at.% alloys. Ge-Se and GeTe2 bonds are increasing with increasing
Se at.%. Melting temperature has found decreases and the reduction in melting point may
reduces the RESET current. Further studies on switching behavior may bring out its
usefulness.
Chapter 6 deals with studies on (GeTe)1-xSex films for phase change memory applications based on the insight received from their bulk study. Even at low at.% addition of Se makes the as prepared (GeTe)1-xSex film amorphous. At 200 oC, GeTe crystalline structure is evolved and the intensity of the peaks reduces in the alloys with increase of Se content. At 300 oC, more evolved GeTe crystalline structure is seen compared to 200 oC annealed films whereas 0.20 at.% Se alloy remain amorphous.
Resistance and thermal studies shows increase in crystallization temperature. It is
expected that Se sits in the vacancies of the GeTe crystalline structural formation. This
may also account for the increased threshold voltages with increasing Se doping. The
band gap increase with increase of Se at.% signifying the possibility of band gap tuning
in the material. Possible explanation for the increased order in GeTe due to Se doping is
presented. The modifications in the alloy with Se addition can be explained with the help of chemical bond energy approach. Those bonds having higher energy leads to increased
average bond energy of the system and hence the band gap. The XPS core level spectra
and Raman spectra investigation clearly shows the GeTe bonds are replaced by Ge-Se
bonds and GeTe2 bonds. The 0.10 at.% Se alloy is found to have a higher thermal stability in the amorphous state and maintains a gigantic resistance contrast compared to
other Se concentration alloys. This alloy can be considered as an ideal candidate for
multilevel PCM applications.
Chapter 7 summarizes the major findings from this work and the scope for future
work. | en_US |
