Tzu-Min Hsu

Bio: Tzu-Min Hsu is an academic researcher from National Central University. The author has contributed to research in topics: Quantum dot & Photonic crystal. The author has an hindex of 18, co-authored 49 publications receiving 1022 citations.

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

Abstract: 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.

Photocurrent studies of the carrier escape process from InAs self-assembled quantum dots

Abstract: We present a temperature- and bias-dependent photocurrent study of the excitonic interband transitions of InAs self-assembled quantum dots (QD's). It was found that the carrier escape process from QD's is dominated by hole escape processes. The main path for this hole escape process was found to be thermal-assisted hole tunneling, from the dot level to the GaAs barrier via the wetting layer as an intermediate state. Energy-dependent carrier tunneling from the QD's to the barrier was observed at low temperatures. Energy shifts due to the size-selective tunneling effect and the quantum-confined Stark effect are discussed and compared with the carrier redistribution effect in photoluminescence measurements.

Hole emission processes in InAs/GaAs self-assembled quantum dots

Abstract: We present a study of the hole emission processes in InAs/GaAs quantum dots using capacitance and admittance spectroscopies. From the conductance mapping, the hole levels show a quasicontinuous distribution, instead of the clear shell structures that have been observed in electron systems. According to a comparative analysis of the capacitance and admittance spectroscopies, the hole emission process is identified to be via thermally activated tunneling through the wetting layer as an intermediate state. An energy level diagram of the quantum dot is also constructed, which shows the hole in our quantum dots to be more weakly confined. We propose a general thermally activated tunneling model to explain our results and those in other works. The conclusion is that both the localization energy and the electric field are important for the carrier emission processes. This model is further extended to predict which carrier type (ι.e., electron or hole) will be more relevant during the exciton dissociation processes in quantum dots.

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

Abstract: 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

Abstract: 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.

When group-III nitrides go infrared: New properties and perspectives

Abstract: Wide-band-gap GaN and Ga-rich InGaN alloys, with energy gaps covering the blue and near-ultraviolet parts of the electromagnetic spectrum, are one group of the dominant materials for solid state lighting and lasing technologies and consequently, have been studied very well. Much less effort has been devoted to InN and In-rich InGaN alloys. A major breakthrough in 2002, stemming from much improved quality of InN films grown using molecular beam epitaxy, resulted in the bandgap of InN being revised from 1.9 eV to a much narrower value of 0.64 eV. This finding triggered a worldwide research thrust into the area of narrow-band-gap group-III nitrides. The low value of the InN bandgap provides a basis for a consistent description of the electronic structure of InGaN and InAlN alloys with all compositions. It extends the fundamental bandgap of the group III-nitride alloy system over a wider spectral region, ranging from the near infrared at ∼1.9 μm (0.64 eV for InN) to the ultraviolet at ∼0.36 μm (3.4 eV for GaN...

Theory of enhancement of thermoelectric properties of materials with nanoinclusions.

Abstract: Based on the concept of band bending at metal/semiconductor interfaces as an energy filter for electrons, we present a theory for the enhancement of the thermoelectric properties of semiconductor materials with metallic nanoinclusions. We show that the Seebeck coefficient can be significantly increased due to a strongly energy-dependent electronic scattering time. By including phonon scattering, we find that the enhancement of $ZT$ due to electron scattering is important for high doping, while at low doping it is primarily due to a decrease in the phonon thermal conductivity.

Modulation spectroscopy of semiconductors: bulk/thin film, microstructures, surfaces/interfaces and devices

Abstract: Modulation spectroscopy is a powerful method for the study and characterization of a large number of semiconductor configurations, including bulk/thin film, microstructures (heterojunctions, quantum wells, superlattices, quantum dots), surfaces/interfaces and actual device structures in addition to semiconductor growth/processing. Furthermore, the influence of external perturbations such as temperature, electric fields, hydrostatic pressure, uniaxial stress, etc. can be investigated. This optical technique utilizes a very general principle of experimental physics, in which a periodically applied perturbation (either to the sample or probe) leads to sharp, derivative-like spectral features in the optical response of the system. Because of the richness of the derivative-like spectra, the information in the lineshape fits, room temperature performance and relative simplicity of operation this method is becoming increasingly more important as a tool to study these materials and structures. This article will review developments in the field during the last decade.

Franz–Keldysh oscillations in modulation spectroscopy

Abstract: In the presence of an electric field, the dielectric constant of a semiconductor exhibits Franz–Keldysh oscillations (FKO), which can be detected by modulated reflectance. Although it could be a powerful and simple method to study the electric fields/charge distributions in various semiconductor structures, in the past it has proven to be more complex. This is due to nonuniform fields and impurity induced broadening, which reduce the number of detectible Franz–Keldysh oscillations, and introduce uncertainties into the measurement. In 1989, a new structure, surface–undoped–doped (s‐i‐n+/s‐i‐p+) was developed, which allows the observation of a large number of FKOs and, hence, permitting accurate determination of electric fields. We present a review of the work on measuring electric fields in semiconductors with a particular emphasis on microstructures using the specialized layer sequence. We first discuss the general theory of modulation techniques dwelling on the approximations and their relevance. The cas...