Yen-Lin Lai

Study of the dominant luminescence mechanism in InGaN∕GaN multiple quantum wells comprised of ultrasmall InGaN quasiquantum dots

High quality green (508nm) and blue (424nm) light emitting diodes (LEDs) from InGaN∕GaN multiple quantum wells with stable ultrasmall indium-rich clusters of 2nm and 3nm from two different nominal indium contents have been grown by metalorganic chemical vapor deposition. Comprehensive calculations including polarization, piezoelectric field, and size effect help derive an indium composition of 59% and 31% for the In-rich clusters of 2nm and 3nm, which agrees amazingly well with the asymmetric phase diagram for phase separation. From this model, we can further demonstrate that the dominant emitting mechanism for green LED is the polarization field, however, for blue LED, both the size effect and polarization effect are equally important.

Effects of the material polarity on the green emission properties of InGaN∕GaN multiple quantum wells

Green-light-emission InGaN∕GaN multiple quantum wells (MQWs) with different polarities were grown by metal organic chemical vapor deposition. A clear phase separation was observed both in the Ga- and N-polarity samples by high resolution transmission electron microscopy, corresponding to two InGaN-related emissions (In-rich dots and an InGaN matrix) seen in photoluminescence spectra. The dot-related emission in the Ga-polarity MQWs shows stronger carrier localization, as well as a weak influence of defects and temperature insensitivity, when compared to the N-polarity MQWs. In addition, efficient carrier transport, from the low-indium InGaN matrix to high-indium In-rich dots, was observed in the Ga-polarity structure, enhancing the function of quantum-dot structures with Ga polarity, and resulting in a high quantum yield of green light emission.

Correlation of In–Ga intermixing with band-tail states in InAs∕GaAs quantum dots

We have investigated the shape and composition profiles of buried and surface InAs∕GaAs Stranski–Krastanov quantum dots (QDs) by using the spectrum-imaging (SI) method with energy-filtered transmission electron microscopy (EFTEM). Indium maps from EFTEM SI reveal lens and truncated pyramid shapes for the surface and buried QDs, with an increase in composition variations for the buried QDs. Photoluminescence measurements reveal an emission at 1.075eV, associated with confined states in the buried QDs, along with a high energy shoulder, associated with band-tail states due to In–Ga intermixing in the vicinity of the buried QDs.

Structural and optical properties of cubic-InN quantum dots prepared by ion implantation in Si (100) substrate

The authors have synthesized InN quantum dots by ion implantation into a Si (100) substrate followed by a postannealing process. X-ray photoemission spectroscopy data verified the formation of In–N bonding in both as-implanted and postannealed samples. Diffraction patterns from transmission electron microscopy (TEM) confirm that the dots are of cubic crystal (zinc-blende phase) with no presence of wurtzite InN. The silicon matrix provides a constraint for the formation of the InN cubic metastable phase. However, dislocations were revealed by high resolution TEM at the interfaces between the dots and the silicon. In addition, the authors found that as the annealing temperature or time increases, dot size increases and dot density decreases. Furthermore, they demonstrate that the main emission energy of zinc-blende InN dots is about 0.736eV.

Comparison of the Microstructure and Optical Properties of InAs Quantum Dots Grown with/without an AlAs Insertion Layer

Be microstructure and optical properties of InAs quantum dots (QDs) grown on a GaAs buffer with a 30 nm thick AlAs insertion layer are investigated and compared with those grown on a plain GaAs buffer by using transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The former InAs QDs exhibit larger dot sizes of 20 nm and higher aspect ratios of 0.4, compared to 15 nm and 0.2, respectively, for the latter. Temperature-dependent PL spectra of the larger dots show that the main emission is dominated by band-tail state transitions at low temperatures and ground state transitions at high temperatures. The ground state transition energy in such quantum dots is significantly red-shifted compared to the smaller InAs QDs. Lower thermal activation energy is also observed for the larger QDs with an AlAs layer. All of the phenomena are caused by different In-Ga intermixing behavior occurring during capping, which is discussed in detail.