Dr. Joel Ager

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.