T. M. Hsu

Heteroepitaxial growth of wurtzite InN films on Si(111) exhibiting strong near-infrared photoluminescence at room temperature

High-quality InN epitaxial films have been grown by nitrogen-plasma-assisted molecular-beam epitaxy on Si(111) substrates using a double-buffer technique. Growth of a (0001)-oriented single crystalline wurtzite–InN layer was confirmed by reflection high-energy electron diffraction, x-ray diffraction, and Raman scattering. At room temperature, these films exhibited strong near-infrared (0.6–0.9 eV) photoluminescence (PL). In addition to the optical absorption measurement of absorption edge and direct band nature, the PL signal was found to depend linearly on the excitation laser intensity over a wide intensity range. These results indicate that the observed PL is due to the emission of direct band-to-band recombination rather than the band-to-defect (or impurity) deep emission.

Tuning the energy levels of self-assembled InAs quantum dots by rapid thermal annealing

We studied the photoluminescence spectra of rapid-thermal-annealed self-assembled InAs quantum dots at 10 K. For annealing temperatures ranging from 700 to 950 °C, we observed a blueshift in the interband transition energies, a decrease in the intersublevel spacing energies, and a narrowing of photoluminescence linewidths. In this letter, we demonstrate that the tuning of the InAs quantum dots interband transition and intersublevel spacing energies can be achieved by 30 s of rapid thermal annealing. The relation between interband transition energy changes and the intersublevel spacing energies is found to be linear, with a slope close to the ratio of the dots’ height to their diameter.

Matrix dependence of strain-induced wavelength shift in self-assembled InAs quantum-dot heterostructures

We report on the matrix-dependent strain effect in self-assembled InAs quantum-dot heterostructures using photoluminescence measurements. A series of samples were prepared to examine the effect of quantum dot position with respect to the so-called strain-reducing layer (SRL). Since the SRL reduces the residual hydrostatic strain in the quantum dots, long emission wavelength of 1.34 μm is observed for the InAs quantum dots with an In0.16Ga0.84As SRL. The dependence of the emission wavelength on the thickness of the cap layer on SRL also indicates the importance of the role of matrix in the strain relaxation process of the dots. Using In0.16Al0.84As instead of In0.16Ga0.84As as the SRL, a blueshift in wavelength is observed because the elastic stiffness of In0.16Al0.84As is higher than that of In0.16Ga0.84As and less strain is removed from the dots with In0.16Al0.84As SRL.

Direct measurement of piezoelectric field in In0.23Ga0.77N/GaN multiple quantum wells by electrotransmission spectroscopy

In this communication, we present experimental evidence of the piezoelectric-field-induced quantum-confined Stark effect on In0.23Ga0.77N/GaN multiple quantum wells. The optical transitions in In0.23Ga0.77N/GaN p-i-n multiple quantum wells were studied by using electrotransmission (ET) at room temperature. Quantum-well-related signals are well resolved in our ET spectra. Since the strong internal electric field breaks the symmetry of the quantum wells, both the allowed and the forbidden transitions are observed. Clear energy blueshifts in accordance with increasing reversed bias are observed in ET spectra. The strength of piezoelectric field is found to be 1.7–1.9 MV/cm in the In0.23Ga0.77N strain quantum well layer, which is comparable with the measurement reported in the literature. We have shown experimentally how the piezoelectric field affects the energy shift for the strained multiple quantum wells.

Room-temperature electroluminescence at 1.3 and 1.5 μm from Ge/Si self-assembled quantum dots

Room-temperature electroluminescence at 1.3 and 1.5 μm from Ge/Si quantum-dot light-emitting diodes is reported. The devices were fabricated in a mesa-type structure, with a silicon oxide layer on the top for surface/sidewall passivation. Different passivation processes were employed. We found that the integrated electroluminescence intensities were relatively less sensitive to temperature, persisting at nearly the same intensity up to RT. The fabricated device shows an internal quantum efficiency of about 0.015% at RT. The improved emission property is attributed to the reduced nonradiative recombination centers due to the surface passivation and thermal treatment.

Franz–Keldysh oscillations of δ‐doped GaAs

Si‐δ‐doped GaAs (N2D ≊ 1011 cm−2) samples grown by molecular‐beam epitaxy are investigated by using photoreflectance spectroscopy. The oscillations observed above the GaAs fundamental band gap are attributed to the Franz–Keldysh effect in the region between the δ‐doped layer and the crystal surface. This ascription is confirmed by detailed studies through varying the cap thickness (250–2500 A), temperature (10–450 K), and laser pump power (0.05–7 mW/cm2). The surface potential deduced from the Franz–Keldysh oscillations is found to be temperature and laser pump power dependent, which is explained by taking the surface photovoltaic effect into account. The surface Fermi level has been measured by this method and is found to have the value 0.73±0.02 V.

Quantum-confined Stark shift in electroreflectance of InAs/InxGa1−xAs self-assembled quantum dots

Electroreflectance was employed to study the electric-field effect on the interband transitions of InAs quantum dots embedded in an In0.16Ga0.84As matrix. The electric field caused an asymmetric quantum-confined Stark shift, which revealed a nonzero built-in dipole moment in the quantum dots. We found the ground-state and excited-state dipole moments to be in the same direction. The electron wave functions are distributed near the base of the quantum dot, with their centers located below the hole wave functions. We also observed a symmetric Stark shift in the wetting-layer transitions. This implies that the wetting-layer potential is symmetric, despite its being capped with quantum dots.

Photoreflectance of sulfur‐annealed copper indium disulfide

The temperature dependence of the energy gaps for sulfur‐annealed copper indium disulfide has been studied by photoreflectance in the temperature range of 10–300 K. The sulfur‐annealed sample has been found to have larger transition energies, smaller positive temperature coefficients of energy gaps, and larger spin‐orbit splitting energy than the as‐grown sample. This can be explained by the reduction of d‐level contributions in the upper valence band probably caused by the variation of lattice distance due to native defects.

Oxygen doping in close‐spaced‐sublimed CdTe thin films for photovoltaic cells

Efficient CdTe/CdS thin film solar cells have been the recent focus, in which the CdTe layers were reported by close‐spaced sublimation, and oxygen was used to control the  p‐type conductivity of the deposited films. Both the fundamental gap and the impurity level were determined by the wavelength modulation reflectance spectroscopy, which demonstrates that while oxygen atoms have an ionization energy of about 0.1 eV, they do not behave as a simple shallow acceptor. This finding is supported by the electrical characterization. The oxygen concentration incorporated in the CdTe thin films were found to be in the range of 1019–1020 cm−3 by the IR measurements, while a carrier concentration between 1010 and 1012 cm−3 was obtained by Hall measurements.

Self-assembled free-standing colloidal crystals

We propose a novel technique to fabricate a free-standing three-dimensional colloidal crystal by self-assembling the colloidal microspheres with controllable thickness from the air–liquid interface. Highly ordered three-dimensional colloidal crystals are formed by polymethylmethacrylate or polystyrene monodisperse microspheres. We also demonstrate the fabrication technique of the free-standing inversed opals by removing the microspheres using calcination. The free-standing colloidal crystal structures can be used for nano-photonic circuits, white-light LEDs or as a photocatalyst.