Der-Yuh Lin

High-efficiency InGaN-based light-emitting diodes with nanoporous GaN:Mg structure

In this research nanoporous structures on p-type GaN:Mg and n-type GaN:Si surfaces were fabricated through a photoelectrochemical (PEC) oxidation and an oxide-removing process. The photoluminescence (PL) intensities of GaN and InGaN∕GaN multi-quantum-well (MQW) structures were enhanced by forming this nanoporous structure to increase light extraction efficiency. The PL emission peaks of an MQW active layer have a blueshift phenomenon from 465.5nm (standard) to 456.0nm (nanoporous) measured at 300K which was caused by partially releasing the compressive strain from the top GaN:Mg layers. The internal quantum efficiency could be increased by a partial strain release that induces a lower piezoelectric field in the active layer. The thermal activation energy of a nanoporous structure (85meV) is higher than the standard one (33meV) from a temperature dependent PL measurement. The internal quantum efficiency and light extraction efficiency of an InGaN∕GaN MQW active layer are significantly enhanced by this nano...

Room-temperature photoreflectance and photoluminescence characterization of the AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor structures with varied quantum well compositional profiles

Using room-temperature photoreflectance (PR) and photoluminescence (PL) we have characterized four pseudomorphic AlGaAs/InGaAs/GaAs high electron mobility transistor structures with varied quantum well compositional profiles. Several features from the InGaAs modulation doped quantum well portion of the samples have been observed in addition to signals from the AlGaAs, GaAs, and GaAs/AlGaAs superlattice (SL) buffer layer. The PR spectra from the InGaAs quantum well channel can be accounted for by a line shape function which is the first derivative of a step-like two-dimensional density of states and a Fermi level filling factor. A detailed line shape fit makes it possible to evaluate the Fermi energy, and hence the concentration of two-dimensional electron gas in addition to the energies of the intersubband transitions. The lowest lying intersubband transition has been confirmed by a comparison of the PR and PL spectra. From the difference of intersubband transition energies, the surface segregation effect...

Contactless electroreflectance and piezoreflectance of a two-dimensional electron gas at a GaNAlGaN heterointerface

Abstract We have measured the contactless electroreflectance and piezoreflectance spectra at 300 and 20 K from a GaN AlGaN heterojunction structure grown by metalorganic chemical vapor deposition on 6 H -SiC substrate. The features related to a two-dimensional electron gas confined at the GaN AlGaN heterojunction are observed and the origin and nature of these features are discussed.

Above-barrier states in GaAs–AlGaAs superlattices studied by photoconductivity and photoreflectance

We report that the quasibound states at the above-barrier region in AlGaAs–GaAs superlattices can be clearly observed at room temperature by photoconductivity as well as photoreflectance measurements. We provide concrete evidence to confirm that free-carrier confinement at barrier layer does exist. It is also found that the barrier-width dependence of the above-barrier transition energies can be described quite well by the modified Messiah’s calculation. However, the simple calculation using the constructive interference condition can only explain the transitions at lower energies, and fails with increasing transition energy.

Piezoreflectance and photoreflectance study of GaAs/AlGaAs digital alloy compositional graded structures

We have studied the piezoreflectance (PzR) spectra at 300 and 80 K related to the intersubband transitions from two different (001) GaAs/AlGaAs structures, an asymmetric triangular quantum well and a rectangular quantum well, fabricated by molecular beam epitaxy using the digital alloy compositional grading (DACG) method. A comparison of the relative intensity of heavy‐ and light‐hole related features in the PzR spectra and those in the photoreflectance emphasizes the contribution of the strain dependence of the energies of the confined states which allows us to identify the features associated with the heavy‐ and light‐hole valence bands unambiguously. Comparison of the observed intersubband transitions with the envelope function calculations provides a self‐consistent verification that the DACG method generated the desired potential profiles. Furthermore, the temperature dependence of both the energy position and broadening parameter of the fundamental conduction to heavy‐hole (11H) and light‐hole (11L)...

Temperature-dependent contactless electroreflectance and photoluminescence study of GaAlAs/InGaAs/GaAs pseudomorphic high electron mobility transistor structures

Temperature-dependent contactless electroreflectance (CER) and photoluminescence (PL) measurements in the range of 30 K<T<300 K on two GaAlAs/InGaAs/GaAs pseudomorphic high electron mobility transistor structures with different well widths fabricated by molecular-beam epitaxy on (100) GaAs substrates are presented. For the CER measurement, the 11H transition is completely screened out by the two-dimensional electron gas and the prominent feature related to the Fermi energy edge singularity transition showed a Stokes shift to higher energy with respect to the 21H transition of the PL measurements at low temperature. From the Stokes shifts, the Fermi energy level of the system is evaluated, and hence, the density of the two-dimensional electron gas. The temperature-dependent PL measurements revealed two features, identified to be the 11H and 21H transitions. The relative intensities of the 11H and 21H transitions were analyzed taking into account the effect of the subband filling and the wave-function overlap factors. A good agreement is found between experimental data and theoretical calculation results.

Room-temperature phototransmittance and photoluminescence characterization of the AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor structures with varied quantum well compositional profiles

We have studied the effects of the two-dimensional electron gas (2DEG) and indium surface segregation in four pseudomorphic AlGaAs/InGaAs/GaAs high electron mobility transistor structures using room-temperature phototransmittance (PT) and photoluminescence (PL) measurements. The PT spectra from the InGaAs modulation-doped quantum well channel can be accounted for by a lineshape function which is the first-derivative of a step-like two-dimensional density of states and a Fermi level filling factor. A detailed lineshape fit makes it possible to evaluate the Fermi energy, and hence the concentration of 2DEG in addition to the energies of the intersubband transitions. The lowest-lying intersubband transition has been confirmed by a comparison of the PT and PL spectra. From the difference of intersubband transition energies, the surface segregation effects of indium atoms are demonstrated.

Contactless electroreflectance study of strained Zn0.79Cd0.21Se/ZnSe double quantum wells

We have studied various excitonic transitions of strained Zn0.79Cd0.21Se/ZnSe double quantum wells, grown by molecular beam epitaxy on (100) GaAs substrates, using contactless electroreflectance (CER) at 15 and 300 K. A number of intersub-band transitions in the CER spectra from the sample have been observed. An analysis of the CER spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state and the nth heavy (light)-hole band state. The conduction-band offset Qc is used as an adjustable parameter to study the band offset in the strained Zn0.79Cd0.21Se/ZnSe system. The value of Qc is determined to be 0.67±0.03.

Piezoreflectance study of InP near the absorption edge

Piezoreflectance measurements were carried out on InP samples with different doping concentrations near the absorption edge in the temperature range between 20 and 300 K. We show that the large features often found below the bandgap in piezomodulated spectra of InP samples are related not only to the impurity states but also to the back-surface reflection effects. Their proximity to the band edge depends on temperature, the impurity species and the geometry of the species in which the multiple reflections occur. At the band edge, the Burstein - Moss shift for the heavily S-doped InP and a pronounced excitonic effect for the Fe-doped semi-insulating InP substrates are observed. In addition, the temperature dependence of the energy position and broadening parameter of the excitonic and direct-bandgap transition features are also evaluated. The expression eV is proposed for the temperature dependence of the semi-insulating InP bandgap over the temperature range 20 - 300 K.

Practical photoluminescence and photoreflectance spectroscopic system for optical characterization of semiconductor devices

We present a practical experimental design for performing photoluminescence (PL) and photoreflectance (PR) measurements of semiconductors with only one PL spectroscopic system. The measurement setup is more cost efficient than typical PL-plus-PR systems. The design of the experimental setup of the PL-PR system is described in detail. Measurements of two actual device structures, a high-electron-mobility transistor (HEMT) and a double heterojunction-bipolar transistor (DHBT), are carried out by using this design. The experimental PL and PR spectra of the HEMT device, as well as polarized-photoreflectance (PPR) spectra of the DHBT structure, are analyzed in detailed and discussed. The experimental analyses demonstrate the well-behaved performance of this PL-PR design.