Sandeep Gaan | Carnegie Mellon University (original) (raw)

Papers by Sandeep Gaan

Research paper thumbnail of (Invited) PMD and STI Gap-Fill Challenges for Advanced Technology of Logic and eNVM

ECS Transactions, 2013

With a gap-fill friendly "V'" profile, HARP can address the stringent gap fill requ... more With a gap-fill friendly "V'" profile, HARP can address the stringent gap fill requirements from 20nm STI and 32nm/28nm PMD with gate first HKMG with not much limitation to spacing CD. The weakest line in the STI trench can be addressed by adding steam into deposition steps or post-dep steam annealing. Dep-etch-dep approach would help to extend the gap fill application of HARP process. But in FinFET STI or PMD with RMG integration where "V" shape profile is difficult to be realized due to their unique integration flows, a FCVD process is needed to fulfill the gap-fill requirement. Excellent FCVD gap-fill has been demonstrated in high AR ratio FinFET STI and PMD even with straight vertical or reentrant profile. Some integration challenges like poor wet resistance and FCVD induced structured bending are addressed.

Research paper thumbnail of Shallow Trench Isolation Integration Methods and Devices Formed Thereby

Research paper thumbnail of Scanning tunneling spectroscopy of molecular thin films and semiconductor nanostructures

Work presented in this thesis mostly deals with nano-scale study of electronic properties of orga... more Work presented in this thesis mostly deals with nano-scale study of electronic properties of organic semiconducting molecules using pentacene (Pn) as a model system and compared with various SiC surfaces to gain more insight into physical processes at nano-scale. In addition, InAs quantum dots (QDs) in a GaAs matrix are studied to probe electronic states of individual QDs. Scanning tunneling microscopy (STM) and spectroscopy (STS) are the primary experimental techniques used to probe local electronic properties on the nano-scale. Vacuum sublimated Pn thin films were deposited onto SiC substrates for STM/STS experiments. STM studies show high quality ordered Pn films. Atomic force microscopy (AFM) images reveal dendritic growth pattern of these films. Local density of states (LDOS) measurements using STS reveals a HOMO-LUMO bandgap. In order to study charge transport properties of Pn films, different amount of charge were injected into the sample by systematically changing the tip-sample separation. Saturation of the tunnel current was observed at positive sample voltages (LUMO states). This effect was attributed to a transport/space charge limitation in tunnel current by treating it as a situation analogous to charge injection into insulators which gives rise to space charge limited current (also previously observed in the case of organic semiconductors). Using a simple model we were able to derive a hopping rate that characterizes nano-scale transport in Pn films at least in the vicinity of the STM probe-tip. We have studied effect of transport limitation in the tunnel current for various semiconductor surfaces. In order to probe surfaces of varying conductivities, we have used Si-rich SiC surfaces such as 3x3 and 3x3 -R30° (both Mott-Hubbard insulators) as well as a highly conducting C-rich graphene surface, and compared those results with the data obtained from Pn. We observe variation of the decay constant kappa (which characterizes the tunneling process) on these surfaces of varying conductivities. The graphene surface shows no transport limitation in the tunnel current, as evidenced by only small changes in kappa as a function of tunnel current for these surfaces. This result is in sharp contrast to the case of Pn where kappa rapidly decays to zero with increasing tunnel current due to transport/space charge limited effects in the semiconductor. Thus, the change is kappa value in STM experiments is reflective of non-ideal behavior of the tunneling. As a specific case of transport limitation on the nano-scale we have also studied InAs QDs grown in a GaAs matrix. We observe that the occupation of discrete quantized states in the dots with electrons has a significant effect on tunneling spectra. When the QD state is occupied by an electron the potential in the dot is modified such that this state does not contribute to the tunnel current. The state then remains "invisible" in the tunneling spectra. Only in presence of transport channels in the vicinity of the dots can the electron localized in the QD state leak out to the substrate, and only then does the state appears in the spectrum. In our experiments these transport channels arise from steps which form as a result of in situ cleaving process for cross-sectional STM (XSTM) measurements.

Research paper thumbnail of Methods for fabricating integrated circuits utilizing silicon nitride layers

Research paper thumbnail of A Comparison of Various Surface Finishes and the Effects on the Early Stages of Pore Formation during High Field Etching of SiC

Materials Science Forum, 2006

Research paper thumbnail of Structure and electronic spectroscopy of steps on GaAs(110) surfaces

Research paper thumbnail of Low-temperature tunneling spectroscopy of Ge(111)c(2×8) surfaces

Physical Review B, 2005

ABSTRACT

Research paper thumbnail of Quantitative Determination of Nanoscale Electronic Properties of Semiconductor Surfaces by Scanning Tunnelling Spectroscopy

Journal of Physics: Conference Series, 2011

Simulation of tunnelling spectra obtained from semiconductor surfaces permits quantitative evalua... more Simulation of tunnelling spectra obtained from semiconductor surfaces permits quantitative evaluation of nanoscale electronic properties of the surface. Band offsets associated with quantum wells or quantum dots can thus be evaluated, as can be electronic properties associated with particular point defects within the material. An overview of the methods employed for the analysis is given, emphasizing the critical requirements of both the experiment and theory that must be fulfilled for a realistic determination of electronic properties.

Research paper thumbnail of Size, shape, composition, and electronic properties of InAs/GaAs quantum dots by scanning tunneling microscopy and spectroscopy

Journal of Applied Physics, 2010

Research paper thumbnail of Electronic states of InAs/GaAs quantum dots by scanning tunneling spectroscopy

Applied Physics Letters, 2010

Research paper thumbnail of Scanning tunneling spectroscopy of molecular thin films and semiconductor nanostructures

Work presented in this thesis mostly deals with nano-scale study of electronic properties of orga... more Work presented in this thesis mostly deals with nano-scale study of electronic properties of organic semiconducting molecules using pentacene (Pn) as a model system and compared with various SiC surfaces to gain more insight into physical processes at nano-scale. In addition, InAs quantum dots (QDs) in a GaAs matrix are studied to probe electronic states of individual QDs. Scanning tunneling microscopy (STM) and spectroscopy (STS) are the primary experimental techniques used to probe local electronic properties on the nano-scale. Vacuum sublimated Pn thin films were deposited onto SiC substrates for STM/STS experiments. STM studies show high quality ordered Pn films. Atomic force microscopy (AFM) images reveal dendritic growth pattern of these films. Local density of states (LDOS) measurements using STS reveals a HOMO-LUMO bandgap. In order to study charge transport properties of Pn films, different amount of charge were injected into the sample by systematically changing the tip-sample separation. Saturation of the tunnel current was observed at positive sample voltages (LUMO states). This effect was attributed to a transport/space charge limitation in tunnel current by treating it as a situation analogous to charge injection into insulators which gives rise to space charge limited current (also previously observed in the case of organic semiconductors). Using a simple model we were able to derive a hopping rate that characterizes nano-scale transport in Pn films at least in the vicinity of the STM probe-tip. We have studied effect of transport limitation in the tunnel current for various semiconductor surfaces. In order to probe surfaces of varying conductivities, we have used Si-rich SiC surfaces such as 3x3 and 3x3 -R30° (both Mott-Hubbard insulators) as well as a highly conducting C-rich graphene surface, and compared those results with the data obtained from Pn. We observe variation of the decay constant kappa (which characterizes the tunneling process) on these surfaces of varying conductivities. The graphene surface shows no transport limitation in the tunnel current, as evidenced by only small changes in kappa as a function of tunnel current for these surfaces. This result is in sharp contrast to the case of Pn where kappa rapidly decays to zero with increasing tunnel current due to transport/space charge limited effects in the semiconductor. Thus, the change is kappa value in STM experiments is reflective of non-ideal behavior of the tunneling. As a specific case of transport limitation on the nano-scale we have also studied InAs QDs grown in a GaAs matrix. We observe that the occupation of discrete quantized states in the dots with electrons has a significant effect on tunneling spectra. When the QD state is occupied by an electron the potential in the dot is modified such that this state does not contribute to the tunnel current. The state then remains "invisible" in the tunneling spectra. Only in presence of transport channels in the vicinity of the dots can the electron localized in the QD state leak out to the substrate, and only then does the state appears in the spectrum. In our experiments these transport channels arise from steps which form as a result of in situ cleaving process for cross-sectional STM (XSTM) measurements.

Research paper thumbnail of Scanning Tunneling Microscopy and Spectroscopy of Pentacene films Deposited on SiC

Among various organic semiconductors, pentacene (Pn) has attracted much attention because of its ... more Among various organic semiconductors, pentacene (Pn) has attracted much attention because of its ability of form ordered structures and its relatively high electron and hole mobilities. We have used SiC surfaces etched at 1600 C in 1 atm of hydrogen to form atomically flat substrates for Pn deposition. Oxidizing these substrates prior to Pn deposition electronically decouples the molecular films from the substrate. Scanning tunneling microscopy (STM) and spectroscopy (STS) was performed at room temperature on in-situ deposited Pn films. STM reveals a dendritic morphology of the films, consistent with prior reports [1]. We find a step height of 1.43±0.10 nm indicating that the Pn molecules are standing up, confirming the relatively weak interaction between the substrate and the film. STS reveals a band gap of about 2.0 eV, which is attributed to the edges of HOMO and LUMO bands of the molecules. Measurements over a wide range of tunnel currents are in progress, in an effort to deduce any transport limitations in the films. Supported by NSF. [1] F.-J. Meyer zu Heringdorf et al., Nature 412, 517 (2001)

Research paper thumbnail of (Invited) PMD and STI Gap-Fill Challenges for Advanced Technology of Logic and eNVM

ECS Transactions, 2013

With a gap-fill friendly "V'" profile, HARP can address the stringent gap fill requ... more With a gap-fill friendly "V'" profile, HARP can address the stringent gap fill requirements from 20nm STI and 32nm/28nm PMD with gate first HKMG with not much limitation to spacing CD. The weakest line in the STI trench can be addressed by adding steam into deposition steps or post-dep steam annealing. Dep-etch-dep approach would help to extend the gap fill application of HARP process. But in FinFET STI or PMD with RMG integration where "V" shape profile is difficult to be realized due to their unique integration flows, a FCVD process is needed to fulfill the gap-fill requirement. Excellent FCVD gap-fill has been demonstrated in high AR ratio FinFET STI and PMD even with straight vertical or reentrant profile. Some integration challenges like poor wet resistance and FCVD induced structured bending are addressed.

Research paper thumbnail of Shallow Trench Isolation Integration Methods and Devices Formed Thereby

Research paper thumbnail of Scanning tunneling spectroscopy of molecular thin films and semiconductor nanostructures

Work presented in this thesis mostly deals with nano-scale study of electronic properties of orga... more Work presented in this thesis mostly deals with nano-scale study of electronic properties of organic semiconducting molecules using pentacene (Pn) as a model system and compared with various SiC surfaces to gain more insight into physical processes at nano-scale. In addition, InAs quantum dots (QDs) in a GaAs matrix are studied to probe electronic states of individual QDs. Scanning tunneling microscopy (STM) and spectroscopy (STS) are the primary experimental techniques used to probe local electronic properties on the nano-scale. Vacuum sublimated Pn thin films were deposited onto SiC substrates for STM/STS experiments. STM studies show high quality ordered Pn films. Atomic force microscopy (AFM) images reveal dendritic growth pattern of these films. Local density of states (LDOS) measurements using STS reveals a HOMO-LUMO bandgap. In order to study charge transport properties of Pn films, different amount of charge were injected into the sample by systematically changing the tip-sample separation. Saturation of the tunnel current was observed at positive sample voltages (LUMO states). This effect was attributed to a transport/space charge limitation in tunnel current by treating it as a situation analogous to charge injection into insulators which gives rise to space charge limited current (also previously observed in the case of organic semiconductors). Using a simple model we were able to derive a hopping rate that characterizes nano-scale transport in Pn films at least in the vicinity of the STM probe-tip. We have studied effect of transport limitation in the tunnel current for various semiconductor surfaces. In order to probe surfaces of varying conductivities, we have used Si-rich SiC surfaces such as 3x3 and 3x3 -R30° (both Mott-Hubbard insulators) as well as a highly conducting C-rich graphene surface, and compared those results with the data obtained from Pn. We observe variation of the decay constant kappa (which characterizes the tunneling process) on these surfaces of varying conductivities. The graphene surface shows no transport limitation in the tunnel current, as evidenced by only small changes in kappa as a function of tunnel current for these surfaces. This result is in sharp contrast to the case of Pn where kappa rapidly decays to zero with increasing tunnel current due to transport/space charge limited effects in the semiconductor. Thus, the change is kappa value in STM experiments is reflective of non-ideal behavior of the tunneling. As a specific case of transport limitation on the nano-scale we have also studied InAs QDs grown in a GaAs matrix. We observe that the occupation of discrete quantized states in the dots with electrons has a significant effect on tunneling spectra. When the QD state is occupied by an electron the potential in the dot is modified such that this state does not contribute to the tunnel current. The state then remains "invisible" in the tunneling spectra. Only in presence of transport channels in the vicinity of the dots can the electron localized in the QD state leak out to the substrate, and only then does the state appears in the spectrum. In our experiments these transport channels arise from steps which form as a result of in situ cleaving process for cross-sectional STM (XSTM) measurements.

Research paper thumbnail of Methods for fabricating integrated circuits utilizing silicon nitride layers

Research paper thumbnail of A Comparison of Various Surface Finishes and the Effects on the Early Stages of Pore Formation during High Field Etching of SiC

Materials Science Forum, 2006

Research paper thumbnail of Structure and electronic spectroscopy of steps on GaAs(110) surfaces

Research paper thumbnail of Low-temperature tunneling spectroscopy of Ge(111)c(2×8) surfaces

Physical Review B, 2005

ABSTRACT

Research paper thumbnail of Quantitative Determination of Nanoscale Electronic Properties of Semiconductor Surfaces by Scanning Tunnelling Spectroscopy

Journal of Physics: Conference Series, 2011

Simulation of tunnelling spectra obtained from semiconductor surfaces permits quantitative evalua... more Simulation of tunnelling spectra obtained from semiconductor surfaces permits quantitative evaluation of nanoscale electronic properties of the surface. Band offsets associated with quantum wells or quantum dots can thus be evaluated, as can be electronic properties associated with particular point defects within the material. An overview of the methods employed for the analysis is given, emphasizing the critical requirements of both the experiment and theory that must be fulfilled for a realistic determination of electronic properties.

Research paper thumbnail of Size, shape, composition, and electronic properties of InAs/GaAs quantum dots by scanning tunneling microscopy and spectroscopy

Journal of Applied Physics, 2010

Research paper thumbnail of Electronic states of InAs/GaAs quantum dots by scanning tunneling spectroscopy

Applied Physics Letters, 2010

Research paper thumbnail of Scanning tunneling spectroscopy of molecular thin films and semiconductor nanostructures

Work presented in this thesis mostly deals with nano-scale study of electronic properties of orga... more Work presented in this thesis mostly deals with nano-scale study of electronic properties of organic semiconducting molecules using pentacene (Pn) as a model system and compared with various SiC surfaces to gain more insight into physical processes at nano-scale. In addition, InAs quantum dots (QDs) in a GaAs matrix are studied to probe electronic states of individual QDs. Scanning tunneling microscopy (STM) and spectroscopy (STS) are the primary experimental techniques used to probe local electronic properties on the nano-scale. Vacuum sublimated Pn thin films were deposited onto SiC substrates for STM/STS experiments. STM studies show high quality ordered Pn films. Atomic force microscopy (AFM) images reveal dendritic growth pattern of these films. Local density of states (LDOS) measurements using STS reveals a HOMO-LUMO bandgap. In order to study charge transport properties of Pn films, different amount of charge were injected into the sample by systematically changing the tip-sample separation. Saturation of the tunnel current was observed at positive sample voltages (LUMO states). This effect was attributed to a transport/space charge limitation in tunnel current by treating it as a situation analogous to charge injection into insulators which gives rise to space charge limited current (also previously observed in the case of organic semiconductors). Using a simple model we were able to derive a hopping rate that characterizes nano-scale transport in Pn films at least in the vicinity of the STM probe-tip. We have studied effect of transport limitation in the tunnel current for various semiconductor surfaces. In order to probe surfaces of varying conductivities, we have used Si-rich SiC surfaces such as 3x3 and 3x3 -R30° (both Mott-Hubbard insulators) as well as a highly conducting C-rich graphene surface, and compared those results with the data obtained from Pn. We observe variation of the decay constant kappa (which characterizes the tunneling process) on these surfaces of varying conductivities. The graphene surface shows no transport limitation in the tunnel current, as evidenced by only small changes in kappa as a function of tunnel current for these surfaces. This result is in sharp contrast to the case of Pn where kappa rapidly decays to zero with increasing tunnel current due to transport/space charge limited effects in the semiconductor. Thus, the change is kappa value in STM experiments is reflective of non-ideal behavior of the tunneling. As a specific case of transport limitation on the nano-scale we have also studied InAs QDs grown in a GaAs matrix. We observe that the occupation of discrete quantized states in the dots with electrons has a significant effect on tunneling spectra. When the QD state is occupied by an electron the potential in the dot is modified such that this state does not contribute to the tunnel current. The state then remains "invisible" in the tunneling spectra. Only in presence of transport channels in the vicinity of the dots can the electron localized in the QD state leak out to the substrate, and only then does the state appears in the spectrum. In our experiments these transport channels arise from steps which form as a result of in situ cleaving process for cross-sectional STM (XSTM) measurements.

Research paper thumbnail of Scanning Tunneling Microscopy and Spectroscopy of Pentacene films Deposited on SiC

Among various organic semiconductors, pentacene (Pn) has attracted much attention because of its ... more Among various organic semiconductors, pentacene (Pn) has attracted much attention because of its ability of form ordered structures and its relatively high electron and hole mobilities. We have used SiC surfaces etched at 1600 C in 1 atm of hydrogen to form atomically flat substrates for Pn deposition. Oxidizing these substrates prior to Pn deposition electronically decouples the molecular films from the substrate. Scanning tunneling microscopy (STM) and spectroscopy (STS) was performed at room temperature on in-situ deposited Pn films. STM reveals a dendritic morphology of the films, consistent with prior reports [1]. We find a step height of 1.43±0.10 nm indicating that the Pn molecules are standing up, confirming the relatively weak interaction between the substrate and the film. STS reveals a band gap of about 2.0 eV, which is attributed to the edges of HOMO and LUMO bands of the molecules. Measurements over a wide range of tunnel currents are in progress, in an effort to deduce any transport limitations in the films. Supported by NSF. [1] F.-J. Meyer zu Heringdorf et al., Nature 412, 517 (2001)