C. P. Lee

High quantum efficiency dots-in-a-well quantum dot infrared photodetectors with AlGaAs confinement enhancing layer

We demonstrate the high quantum efficiency InAs∕In0.15Ga0.85As dots-in-a-well (DWELL) quantum dot infrared photodetectors (QDIPs). A thin Al0.3Ga0.7As layer was inserted on top of the InAs quantum dots (QDs) to enhance the confinement of QD states in the DWELL structure. The better confinement of the electronic states increases the oscillation strength of the infrared absorption. The higher excited state energy also improves the escape probability of the photoelectrons. Compared with the conventional DWELL QDIPs, the quantum efficiency increases more than 20 times and the detectivity is about an order of magnitude higher at 77K.

Magnetic properties of parabolic quantum dots in the presence of the spin–orbit interaction

We present a theoretical study of the effect of the spin–orbit interaction on the electron magnetization and magnetic susceptibility of small semiconductor quantum dots. Those characteristics demonstrate quite interesting behavior at low temperature. The abrupt changes of the magnetization and susceptibility at low magnetic fields are attributed to the alternative crossing between the spin–split electron levels in the energy spectrum, essentially due to the spin–orbit interaction (an analog of the general Paschen–Back effect). Detailed calculation using parameters of InAs semiconductor quantum dot demonstrates an enhancement of paramagnetism of the dots. There is an additional possibility to control the effect by external electric fields or the dot design.

Multicolor infrared detection realized with two distinct superlattices separated by a blocking barrier

A multicolor infrared photodetector was realized with two superlattices separated by a blocking barrier. The photoresponse is switchable between 7.5–12 and 6–8.5 μm by the bias polarity, and is also tunable by the bias magnitude in each wavelength regime. In addition, our detector exhibits advantages including little temperature dependence of the spectral response and the same order of responsivity in the two wavelength regimes. The measured peak responsivities in the two regimes are 117 mA/W at 9.8 μm under 1 V and 129 mA/V at 7.4 μm under −0.8 V, respectively. Also, the detectivities are comparable with the conventional multistack detector. The zero background peak detectivities are 2.3×1010u2009cmu200aHz0.5/W at 50 K and 9.8 μm under 0.7 V, and 8.7×1010u2009cmu200aHz0.5/W at 70 K and 7.4 μm under −0.7 V.

Ultrafast carrier capture and relaxation in modulation-doped InAs quantum dots

We report investigations on carrier capture and relaxation processes in undoped and modulation-doped InAs∕GaAs self-assembled quantum dots (QDs) by using time-resolved spectroscopy technique with a time resolution of ∼200fs. We find that carrier capture and relaxation in the ground state of the charged QD are faster compared to the undoped dots even at an excitation level as low as 1×1010cm−2. It is attributed to the triggering of the vibrating polarization field induced by the presence of cold carriers in the doped dots. The rate of an electron been captured by a positively charged QD is also calculated based on our proposed model.

Carrier capture and relaxation in InAs quantum dots

We have investigated the carrier capture and relaxation processes in InAs/GaAs self-assembled quantum dots at room temperature by time-resolved photoluminescence techniques with a high time resolution of ∼200 fs. Following the initial fast relaxation in GaAs barriers, we have observed rising processes in time-resolved PL intensity at the energies of quantum dot confined states and the wetting layer. The rising processes are assigned to the carrier capture from the barriers into the wetting layer and confined states in InAs dots and subsequent relaxation in each detected energy level. We found that the carrier capture rate is faster than the intra-dot relaxation within the range of excitation densities that we investigated. Under high excitation intensity, the electronic states in the dots were populated mainly by carriers directly captured from the barrier. However, at low excitation densities, the PL rise times were influenced by the carrier diffusion.

Selective growth of single InAs quantum dots using strain engineering

A method to achieve ordering and selective positioning of single InAs self-assembled quantum dots (QDs) has been developed. The selective growth was achieved by manipulating the strain distribution on the sample surface. The QDs are formed on predesigned mesas with added strain. Single dots were obtained on small mesas. Using this technique, two-dimensional single QD arrays have been achieved.

2–3 μm mid infrared light sources using InGaAs/GaAsSb “W” type quantum wells on InP substrates

The structure of a “W” type QW used in this work is composed of symmetric InGaAs/GaAsSb/InGaAs layers, which are sandwiched between two InAlAs barrier layers, lattice matched to InP. The barrier layers provide a strong quantum confinement to enhance the electron-hole wave function overlap and hence the optical matrix element. The band alignment of a represented In0.53Ga0.47As latticematched to InP/GaAs0.4Sb0.6 0.7% compressive strain/In0.52Al0.48As “W” structure is shown in Fig. 1a .I n

Spin-dependent delay time in electronic resonant tunneling at zero magnetic field

Abstract The dependence of the phase tunneling time on electronic spin polarization in symmetric and asymmetric double-barrier semiconductor heterostructures is studied theoretically. The effective one-band Hamiltonian approximation and spin-dependent boundary conditions are used for theoretical investigation of the electron spin influence on the delay time in tunneling processes. It is shown that the spin–orbit splitting in the dispersion relation for the electrons can provide a dependence of the delay time on the electron spin polarization without additional magnetic field. This dependence can be controlled by an external electric field and can be very pronounced for realistic double-barrier semiconductor heterostructures.

Self-assembled GaAs antiwires in In0.53Ga0.47As matrix on (100) InP substrates

The growth of GaAs antiwires in the In0.53Ga0.47As matrix on InP substrate has been investigated. The periodic, wire-like structure was obtained when a proper amount of GaAs was deposited. The grown antiwires have a height about 1.2–2.0 nm and a period about 23 nm. Using an In0.53Ga0.47As/In0.52Al0.48As modulation-doped structure, the effect of the GaAs antiwires on the two-dimensional electron gas mobility was investigated. For the sample with antiwires near the two-dimensional channel, a significant anisotropy in low temperature mobility was observed.

Electron delocalization of tensily strained GaAs quantum dots in GaSb matrix

The magneto-optical response of type-II tensily strained GaAs self-assembled quantum dots in GaSb was investigated in magnetic fields up to 14 T. By depositing different GaAs amount, the dot sizes and the corresponding emission energies were varied. We analyzed the carrier wave function extent of different dots using the diamagnetic shift results. It was found that, with the increase in the energy (the reduction in the dot size), the diamagnetic coefficient first rises quickly and then saturates at around 21u2002μeV/T2. Based on a simple calculation model, this unusual tendency is attributed to the electrons gradually spilling out of the quantum dot to the wetting layer as the dots get smaller. This delocalization effect is enhanced in this material system due to the tensile strain relaxation within the dots, which raises the conduction band edge over that in the wetting layer.