Eun-joo Shin

Magnesium acceptor levels in GaN studied by photoluminescence

Magnesium doped GaN epitaxial layers were grown by metal-organic chemical vapor deposition on sapphire substrate. Energy levels of these acceptors were investigated by systematic photoluminescence measurements in the temperature range of 12–300 K. Magnesium concentration was varied from <1×1019 to higher than 5×1019 cm−3. Photoluminescence measurements were made on the as-grown and annealed samples. We have observed various transitions related to donor to acceptor and their phonon replicas, conduction band to acceptors, and free excitons. Their dependence on temperature, concentration of the magnesium impurity and annealing conditions was discussed. In our study, two important observations were made. First, very deep level luminescence was not observed even in the highly magnesium doped as-grown samples. Second, free exciton transitions including valence band splittings were observed for the first time in the Mg-doped materials, demonstrating the high quality of the samples.

Stress relaxation in Si-doped GaN studied by Raman spectroscopy

We report the Si-doping-induced relaxation of residual stress in GaN epitaxial layers grown on (0001) sapphire substrate by the metalorganic vapor phase epitaxy technique. Micro-Raman spectroscopy is used to assess stress situation in the films with systematically modulated doping concentration from 4.0×1017 up to 1.6×1019 cm−3. As the Si-doping concentration increases, a monotonic decrease of the E2 phonon frequency is observed, which signifies gradual relaxation of the stress in the film. The layers are fully relaxed when electron concentration exceeds 1.6×1019 cm−3. The linear coefficient of shift in Raman frequency (ω) induced by the in-plane biaxial compressive stress (σ∥) is estimated to be Δω/Δσ∥=7.7 cm−1/GPa. We suggest that Si doping increases density of misfit dislocation, judging from linewidth of x-ray rocking curve.

Temperature-dependent photoluminescence study of C60 and C70

Abstract The photoluminescence (PL) spectra of C60 and C70 in toluene, as films on silicon and as KBr pellet, were obtained as a function of temperature. Different emission profiles are observed between the fullerenes in solution and in the solid form. The PL spectra of the C60 solids are similar regardless of their substrates, while those of C70 show different emission profiles between the films on silicon and the pellet, which is believed to be due to the substrate-dependent stacking disorder resulting from the nonspherical shape of the molecule. The changes in emission intensity and profile upon temperature are observed in the PL spectra of solid C60 and C70, suggesting the strong influence of rotational diffusion. The vibrational mode progressions are identified from the low-temperature photoluminescence spectra of C60 and C70.

MANY-BODY EFFECTS ON MODULATION-DOPED INAS/GAAS QUANTUM DOTS

The excitation intensity dependent photoluminescence spectra of various modulation-doped InAs/GaAs quantum dots exhibit the band filling and band-gap renormalization. From the time-resolved photoluminescence spectra, in addition to the interaction of the carriers in quantum dots with the remote ionized impurities in a GaAs barrier, the screening due to the two-dimensional charges is found to mainly affect the carrier lifetime in the modulation-doped quantum dots.

Effects of Si-doping in the barriers on the recombination dynamics in In0.15Ga0.85N/In0.015Ga0.985N quantum wells

A systematic study of time-resolved photoluminescence spectra from In0.15Ga0.85N/In0.015Ga0.985N quantum wells (QWs) with different Si-doping concentration in the barriers has been carried out. The 10 K recombination lifetimes in the QWs depend strongly on the Si-doping in the barriers, decreasing from ∼80 to ∼20 ns as the Si-doping level increases from 2×1018 to 1×1019 cm−3. The separation between the spontaneous and stimulated emission (SE) peaks and a shift of the emission peak to longer wavelengths with delay time after a pulsed excitation decrease with increasing Si doping. The increased recombination rate, the decrease of peak shift with delay time, and the reduced separation between the spontaneous and SE peaks with increasing Si doping are attributed to the screening of piezoelectric field by carriers originated from Si-doped barriers.

A compact extended-cavity diode laser with a Littman configuration

We have constructed a compact extended-cavity diode laser (ECDL) that is based on a Littman configuration with a grating and a reflector. The whole structure is installed in a 2-inch kinematic mount. ECDLs operating at 852 nm (Cs D/sub 2/ line), 894 nm (Cs D/sub 1/ line), 780 nm (Rb D/sub 2/ line), and 794 nm (Rb D/sub 1/ line) were fabricated and tested. As a result of the performance test, up to 9 GHz continuous tuning without mode hopping could be obtained by tuning with a piezoelectric transducer only. The linewidth from the beat-note spectrum of two ECDLs was about 200 kHz.

Going beyond the mean-field approximations of alloys and alloy superlattices: a few puzzles solved?

We discuss a few examples of cases in which the widely used mean-field approaches to alloys and alloy superlattices may not give complete solutions. These examples include the anomalously large Stokes and anti-Stokes real space-charge transfer over thick alloy barriers and the spatial extent of optical phonons in alloys and alloy superlattices, which have remained unsolved or controversial. We argue, both theoretically and experimentally, that approaches that fully account for inhomogeneities, partial ordering, and disorder effects in the alloys as well as the proper understanding of coupled quantum-mechanical systems do give answers to these important puzzles.

Photoluminescence decay measurements of ZnSxSe1−x (0 < x < 0.12) epilayers on GaAs substrate grown by molecular beam epitaxy

The steady-state and time-resolved photoluminescence (PL) studies of ZnSxSe1−x epilayers on GaAs substrate grown by molecular beam epitaxy around the lattice matching composition (0 < x < 0.12) are discussed. We have investigated the PL decay dynamics of ZnSxSe1−x epilayers and found that the decay time of the ZnSxSe1−x epilayer with sulfur composition closely lattice-matched with the substrate is longer than that of any other sample. This finding is interpreted as indicating that the defects induced by lattice mismatch act as nonradiative recombination centers and consequently reduce the PL lifetimes of the epilayers. These studies suggest that the lattice mismatch has a strong correlation with PL lifetimes of the ZnSxSe1−x epilayers.

Time-resolved spectroscopy of InAs quantum dots using one-side modulation-doping technique: renormalization and screening

We present the optical properties of modulation-doped InAs/GaAs quantum dots (QDs). Bandgap renormalization is clearly observed from a red shift of the emission peaks on the QDs with various modulation-doping concentrations, demonstrating that exchange interactions of electrons within single states have no effect on renormalization. Screening due to the electrons at modulation-doped layer as well as the photo-generated carriers in GaAs matrix plays an important role in reducing the radiative recombination rate.

A compact external-cavity diode laser with a littman configuration

We have constructed a compact extemal-cavity diode laser (ECDL) based on a Limnan configuration with a robust structure of cavity. The laser system offers up to 9-GHz continuous tuning using PZT only. The linewidth of 200 kHz is obtained from beat-note spectrum of two ECDLs.