D. Gershoni

Formation of a high quality two‐dimensional electron gas on cleaved GaAs

We have succeeded in fabricating a two-dimensional electron gas (2DEG) on the cleaved (110) edge of a GaAs wafer by molecular beam epitaxy (MBE). A (100) wafer previously prepared by MBE growth is reinstalled in the MBE chamber so that an in situ cleave exposes a fresh (110) GaAs edge for further MBE overgrowth. A sequence of Si-doped AlGaAs layers completes the modulation-doped structure at the cleaved edge. Mobilities as high as 6.1×10^5 cm^2/V s are measured in the 2DEG at the cleaved interface.

Optical pumping of the electronic and nuclear spin of single charge-tunable quantum dots.

We present a comprehensive examination of optical pumping of spins in individual GaAs quantum dots as we change the net charge from positive to neutral to negative with a charge-tunable heterostructure. Negative photoluminescence polarization memory is enhanced by optical pumping of ground state electron spins, which we prove with the first measurements of the Hanle effect on an individual quantum dot. We use the Overhauser effect in a high longitudinal magnetic field to demonstrate efficient optical pumping of nuclear spins for all three charge states of the quantum dot.

Calculating the optical properties of multidimensional heterostructures: Application to the modeling of quaternary quantum well lasers

A method for calculating the electronic states and optical properties of multidimensional semiconductor quantum structures is described. The method is applicable to heterostructures with confinement in any number of dimensions: e.g. bulk, quantum wells, quantum wires and quantum dots. It is applied here to model bulk and multiquantum well (MQW) InGaAsP active layer quaternary lasers. The band parameters of the quaternary system required for the modeling are interpolated from the available literature. We compare bulk versus MQW performance, the effects of compressive and tensile strain, room temperature versus high temperature operation and 1.3 versus 1.55 pm wavelength operation. Our model shows that: compressive strain improves MQW laser performance. MQW lasers have higher amplification per carrier and higher differential gain than bulk lasers, however, MQW performance is far from ideal because of occupation of non-lasing minibands. This results in higher carrier densities at threshold than in bulk lasers, and may nullify the advantage of MQW lasers over bulk devices for high temperature operation. >

Effects of two‐dimensional confinement on the optical properties of InGaAs/InP quantum wire structures

We describe fabrication and photoluminescence excitation of InGaAs/InP quantum wires with a lateral dimension of ∼350 A. Transverse confinement results in the splitting of the n=1 heavy hole‐electron transition. Three of these levels are observed in the excitation spectrum. The exciton energies agree with the theoretical predictions based on a new method of solving the two‐dimensional effective mass Schrodinger equation.

Critical layer thickness in strained Ga1−xInxAs/InP quantum wells

We use a combination of electrical, optical, and structural characterization techniques to determine the critical layer thickness of strained Ga1−xInxAs/InP quantum wells. Well compositions covering the entire range of strain available, from −3.8% (GaAs) to +3.2% (InAs), were investigated. We find that the critical layer thickness in this material system is unambiguously described by the classical Matthews and Blakeslee force balance model [J. Cryst. Growth 27, 118 (1974)]. Reverse leakage current of strained‐well samples grown in a p‐i‐n configuration is shown to be the most direct and reliable measure of the pseudomorphic limit.

Entanglement on demand through time reordering

Entangled photons can be generated on demand in a novel scheme involving unitary time reordering of the photons emitted in a radiative decay. This scheme can be applied to the biexciton cascade in quantum dots.

Admittance spectroscopy measurement of band offsets in strained layers of InxGa1−xAs grown on InP

We report measurements of the conduction‐band offset in strained‐layer superlattices of InxGa1−xAs/InP. Admittance spectroscopy was used to measure the activation energy for thermionic emission of electrons over InP barriers in n‐type superlattices. Superlattice dimensions and x values were obtained from high‐resolution x‐ray diffraction and transmission electron microscopy studies. For x=0.37, 0.53, and 0.69, the values obtained for the conduction‐band offset are 175±25 meV, 210±20 meV, and 315±25 meV, respectively.

Metalorganic molecular beam epitaxy of InP, Ga0.47In0.53As, and GaAs with tertiarybutylarsine and tertiarybutylphosphine

Thermally cracked tertiarybutylarsine and tertiarybutylphosphine were used to replace AsH3 and PH3 for the growth of Ga0.47In0.53As and InP by metalorganic molecular beam epitaxy. For both materials, carrier concentrations n=(1–2)×1015 cm−3 were obtained at 300 and 77 K, with 77 K mobilities of 29 000 and 31 000 cm2 V−1 s−1. The GaAs was p‐type with p=4×1015 cm−3 at both temperatures and a 77 K mobility of 2200 cm2 V−1 s−1. The lifetimes for carriers in 14–60 A thick quantum wells were 3±1 ns. The reacting arsenic species for epitaxy were As2 and As4. The reacting phosphorus species were PH2 and possibly PH.

Polarized fine structure in the photoluminescence excitation spectrum of a negatively charged quantum dot.

We report polarized photoluminescence excitation spectroscopy of the negative trion in single charge-tunable quantum dots. The spectrum exhibits a p-shell resonance with polarized fine structure arising from the direct excitation of the electron spin triplet states. The energy splitting arises from the axially symmetric electron-hole exchange interaction. The magnitude and sign of the polarization are understood from the spin character of the triplet states and a small amount of quantum dot asymmetry, which mixes the wave functions through asymmetric e-e and e-h exchange interactions.

Near‐field optical spectroscopy of single quantum wires

Low temperature near‐field scanning optical microscopy is used for spectroscopic studies of single, nanometer dimension, cleaved edge overgrown quantum wires. A direct experimental comparison between a two dimensional system and a single genuinely one dimensional quantum wire system, inaccessible to conventional far field optical spectroscopy, is enabled by the enhanced spatial resolution. We show that the photoluminescence of a single quantum wire is easily distinguished from that of the surrounding quantum well. Emission from localized centers is shown to dominate the photoluminescence from both wires and wells at low temperatures. A factor of 3 absorption enhancement for these wires compared to the wells is concluded from the photoluminescence excitation data.