Dr. Joel Ager's research interests include:
- Isotopically pure single crystal silicon.
High purity single crystals enriched in all three stable isotopes of
silicon and pulled from polysilicon produced by an LBNL reactor (unique
in world and designed by employee) have been used in collaborative
studies involving research groups around the world. In the particular
case of the >90% 29Si and 30Si enriched material, PL studies have
shown world record narrow linewidths for Si enriched in these two
minority isotopes. Precise photomodulated transmission and
photoluminescence measurements performed by Purdue on a set of
isotopically controlled LBNL crystals were used to determine the
excitonic indirect bandgap of Si with its anharmonicity and
electron-phonon renormalization components removed. This could be
considered the most fundamental and precise measurement of the Si
bandgap ever. Pulsed electron spin resonance measurements perfo rmed by
Princeton University on a 28Si-enriched crystal found that the key
figure of merit for a quantum computing schemes, namely, the electron
spin decoherence time (T2) of electrons bound to phosphorus, is the
longest ever measured in Si (5 ms, more tha n a factor of two longer
than the previous record and long enough to meet the requirement for
quantum error correction).
- InN and In-rich InGaN-based radiation hard solar cell.
Resonance Raman measurements were used to probe the electron-phonon interaction, which appears to be weak for
electrons low in the conduction band. Electron, proton, and alpha
particle irradiation was used to demonstrate In-rich InGaN is at least
3 orders of magnitude more resistant to radiation damage than materials
used now in multijunction solar cells for space applications. The
presence of bulk p-type activity in Mg-doped InN was established for
the first time using electrochemical CV measurements combined with Hall
and PL measurements.
- “Highly mismatched” II-VI semiconductor alloys
“Highly mismatched” II-VI semiconductor alloys(e.g. ZnTeO with O
incorporated at 1-2%, 100x the equilibrium solubility limit in ZnTe)
are being developed as potential multiband (intermediate band) solar
cell materials. A large number of experimental techniques (modulated
photoreflectance, photoconductivity, resonance Raman scattering, and
standard electrical measurements) have been used to investigate the
properties of the two electronic bands formed by the interaction of O
with the conduction band and to design a working solar cell based on
this material.
- Ge nanocrystals
The evolution and relaxation of hydrostatic stress in Ge nanocrystals
grown by ion implantation into silica was studied. Stressed
nanocrystals can be relaxed in a diffusional process governed by the
migration of matrix atoms. Embedded Ge nanocrystals remain solid at up
to 200 °C above the bulk Ge melting point; this contrasts with the
melting point depression usually found in nanocrystalline systems. This
unusual behavior appears to be due to the large difference between the
surface energies of solid silica and liquid Ge.
- UV-Raman spectroscopy
Deep UV-Raman spectroscopy was used to study mineralized mammalian
tissues (bone and teeth) for the first time. UV excitation eliminates
the fluorescence interference found with visible and near-IR excitation
and also enhances the contribution from the organic (collagen)
fraction of the tissue. Spectral changes have been investigated
as a function of age in human bone and teeth and as a function
of solvent in elephant dentin. The spectra are sensitive to collagen
crosslinking and can be correlated with specific aspects of the
fracture properties.
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