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...

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

Molecular beam epitaxy growth of GaN, AlN and InN

Spectrally resolved photoresistance investigations of charge storage effects in self-assembled InAs quantum dots (QDs) are reported. Resonant optical excitation of the QDs produces a strong increase of the lateral resistance of a spatially separated electron channel (ΔR) which reflects the stored charge density. This photoresponse is persistent for many hours at 145 K and can be controllably reversed electrically. Pronounced oscillations observed in the spectral variation ΔR are shown to reflect the excitation spectrum of the QD ensemble showing resonances that arise from both direct and phonon-assisted absorption processes.

Electrical detection of optically induced charge storage in self-assembled InAs quantum dots

Prior experimental work had found that the Fermi level at InN growth surfaces is pinned well above the conduction band edge, leading to strong surface band bending and electron accumulation. Using cross-sectional scanning photoelectron microscopy and spectroscopy, we show definitive evidence of unpinned Fermi level for in situ cleaved a-plane InN surfaces. To confirm the presence or absence of band bending, the surface Fermi level relative to the valence band edge was precisely measured by using both the Fermi edge of Au reference sample and the core level of ultrathin Au overlayer. It is confirmed that flat surface bands only occur at cleaved nonpolar surfaces, consistent with the recent theoretical predictions.

Absence of Fermi-Level Pinning at Cleaved Nonpolar InN Surfaces

We demonstrate that vertically aligned InN nanorods can be grown on Si(111) by plasma-assisted molecular-beam epitaxy. Detailed structural characterization indicates that individual nanorods are wurtzite InN single crystals with the growth direction along the c axis. Near-infrared photoluminescence (PL) from InN nanorods can be clearly observed at room temperature. However, in comparison to the InN epitaxial films, the PL efficiency is significantly lower. Moreover, the variable-temperature PL measurements of InN nanorods exhibit anomalous temperature effects. We propose that these unusual PL properties are results of considerable structural disorder (especially for the low-temperature grown InN nanorods) and strong surface electron accumulation effects.

Near-infrared photoluminescence from vertical InN nanorod arrays grown on silicon: Effects of surface electron accumulation layer

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.

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

The carrier distribution and defects have been investigated in InAs/GaAs quantum dots by cross-sectional transmission electron microscopy (XTEM), capacitance–voltage, and deep level transient spectroscopy. Carrier confinement is found for 1.1- and 2.3-monolayer-(ML)-thick InAs samples. For 2.3 ML sample, XTEM images show the presence of defect-free self-assembled quantum dots. With further increase of the InAs thickness to 3.4 ML, significant carrier depletion caused by the relaxation is observed. In contrast to 1.1 and 2.3 ML samples in which no traps are detected, two broad traps and three discrete traps at 0.54, 0.40, and 0.34 eV are observed in 3.4 ML sample. The traps at 0.54 and 0.34 eV are found to be similar to the traps observed in relaxed In0.2Ga0.8As/GaAs single quantum well structures. By comparing with the XTEM images, the trap at 0.54 eV is identified to be the relaxation-induced dislocation trap in the GaAs layer.

https://ir.nctu.edu.tw:443/bitstream/11536/30136/1/000165069500029.pdf

Carrier distribution and relaxation-induced defects of InAs/GaAs quantum dots

The optoelectronic characteristics of self-assembled InAs quantum dots (QDs) with strain-reduced layers (SRLs) were investigated using photoluminescence (PL) spectroscopy. Various SRLs that combine In0.14Al0.86As and In0.14Ga0.86As with the same total thickness were examined to ascertain their confining effect on carriers in InAs QDs. The emission wavelength is blueshifted as the thickness of InAlAs is increased. The energy separation between the ground state and the first excited state of QDs with InAlAs SRLs greatly exceeds that of QDs with InGaAs SRLs. Atomic force microscopic images and PL spectra of the QD samples demonstrated that high-quality InAs QDs with long emission wavelengths and a large energy separation can be generated by growing a low-temperature, thin InAlAs SRL onto self-assembled QDs.

https://ir.nctu.edu.tw:443/bitstream/11536/13811/1/000228729500016.pdf

Characterization of self-assembled InAs quantum dots with InAlAs∕InGaAs strain-reduced layers by photoluminescence spectroscopy

We present detailed studies of the onset of strain relaxation in InAs∕InGaAs quantum dots. We show that the ground-state photoluminescence (PL) emission redshifts with increasing the InAs coverage before relaxation and blueshifts when relaxation occurs. PL spectra of the relaxed samples show two predominant families of dots with very different temperature-dependent efficiency. By comparison we show that the dots emitting at long wavelength are degraded by relaxation while the dots emitting at short wavelength remain coherently strained. Consequently, the PL spectra are dominated by the dots emitting at short wavelength, leading to the observed blueshift. This result suggests that the relaxation does not occur uniformly. In addition, we show that the relaxation occurs in the dot bottom interface.

https://ir.nctu.edu.tw:443/bitstream/11536/13170/1/000232225700021.pdf

Strain relaxation in InAs∕InGaAs quantum dots investigated by photoluminescence and capacitance-voltage profiling

Multilayer self-assembled InAs∕GaAs quantum dot (QD) structures with varying GaAs spacer layer (SL) thickness are systematically investigated using surface photovoltage spectroscopy (SPS) and photoluminescence (PL). The optical transitions are more clearly visible in the room-temperature SPS spectra and exhibit more features in comparison to PL. The enhanced electronic coupling effect for thinner SL results in the appearance of an additional excited state lying higher than the second excited quantum dot state. A peculiar feature denoted as QDX below the fundamental transition is tentatively attributed to the optical absorption from uncoupled dots of which the density is lower than that of vertically coupled ones. The transition blueshifted with a decrease of the SL thickness indicates that the materials intermixing between InAs QDs and GaAs SL are strongly driven by strain. The intensity ratio between the ground state and QDX in the temperature dependent PL spectra is found to be associated with the proce...

J. S. Wang

Papers

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

Carrier distribution and relaxation-induced defects of InAs/GaAs quantum dots

Characterization of self-assembled InAs quantum dots with InAlAs∕InGaAs strain-reduced layers by photoluminescence spectroscopy

Strain relaxation in InAs∕InGaAs quantum dots investigated by photoluminescence and capacitance-voltage profiling

Surface photovoltage spectroscopy and photoluminescence study of vertically coupled self-assembled InAs∕GaAs quantum dot structures