T. Lundström

Photoluminescence related to the two‐dimensional electron gas at a GaN/AlGaN heterointerface

We report photoluminescence (PL) spectra related to a two‐dimensional electron gas confined at a GaN/AlGaN heterointerface. The recombination between electrons confined in the bottom of the interface potential and photoexcited holes causes a broad PL emission about 50 meV below the GaN exciton emissions. A second emission, attributed to the recombination of electrons in the first excited level at the interface, is also observed close to the excitonic band gap in GaN. The data agree with a self‐consistent calculation of the energy levels and electron concentration at the interface.

Optical characterisation of GaN and related materials

Abstract Recent experimental results on optical properties of GaN and related materials are discussed. Photoluminescence data of free excitons for sufficiently pure GaN samples demonstrate the dominance of excitonic recombination well above room temperature. Transient PL data give a radiative lifetime of about 200 ps for the A-exciton at 2 K in strain-free samples. A corresponding value of about 2 ns at room temperature is extrapolated. Radiative lifetimes for bound excitons are measured as about 250 ps for shallow donors and about 1.5 ns for shallow acceptors. Photoluminescence spectra from the 2D electron gas at a GaN AlGaN heterointerface are also demonstrated.

The effective masses in strained InGaAs/InP quantum wells deduced from magnetoexcitation spectroscopy

The reduced effective masses in InxGa1−xAs/InP quantum wells have been determined as a function of strain (x value) and well width by means of magneto-optical methods. Magnetoexcitons have been observed in photoluminescence excitation spectra in the presence of a magnetic field. At higher magnetic fields, the observed magnetoexcitons will asymptotically approach the free Landau levels. From a least square fit, the dependence of the reduced effective masses on strain and well width has been deduced. Also, the reduced effective mass including the light hole state has been determined for the tensile strained quantum well structure.

Strain effects on the intervalence‐subband normal‐incidence absorption in a p‐type InGaAs/InP quantum well

The lattice mismatch (strain) effects on the normal‐incidence infrared absorption in In1−xGaxAs/InP quantum wells is investigated systematically, both tensile (x≳0.47) and compressive (x<0.47) cases being considered. The difference of the valence‐band parameters in the well and barrier materials is taken into account in the dipole matrix element calculations. For a constant hole sheet density, the compressive stress is found to enhance the infrared absorption substantially in the frequency range around 100 meV, corresponding to the H1–H3 type transitions, and the tensile stress is shown to decrease the normal‐incidence intervalence‐subband absorption.The lattice mismatch (strain) effects on the normal‐incidence infrared absorption in In1−xGaxAs/InP quantum wells is investigated systematically, both tensile (x≳0.47) and compressive (x<0.47) cases being considered. The difference of the valence‐band parameters in the well and barrier materials is taken into account in the dipole matrix element calculations. For a constant hole sheet density, the compressive stress is found to enhance the infrared absorption substantially in the frequency range around 100 meV, corresponding to the H1–H3 type transitions, and the tensile stress is shown to decrease the normal‐incidence intervalence‐subband absorption.

The electronic structure of InGaAs/InP quantum wells measured by Fourier transform photoluminescence excitation spectroscopy

We report on novel results from a systematic study of excitonic transitions in high quality metalorganic vapor phase epitaxy grown InxGa1−xAs/InP quantum wells (QWs). The electronic structure of the QWs has been studied as a function of QW width as well as the built‐in strain. The characterization has been performed by means of a combined Fourier transform photoluminescence (FTPL) and FTPL excitation study of the InxGa1−xAs/InP QWs. Detailed information on the energy positions for the excitons associated with various subbands (for the electrons, heavy and light holes) up to n=5 have been obtained. The experimentally determined energy positions have been compared with theoretical predictions based on an effective mass model and bulk deformation potential theory.

Optical intervalence‐subband transitions in strained p‐type In1−xGaxAs/InP quantum wells

The lattice mismatch (strain) effects on the infrared absorption in p‐type In1−xGaxAs/InP quantum wells are investigated systematically for both tensile (x≳0.47) and compressive (x<0.47) strains. The mismatch of the valence‐band parameters in the well and barrier materials is taken into account in the optical matrix element calculations. We find that normal incidence optical matrix elements substantially increase in the case of the compressive strain (the ground state is heavy hole) and decreases in the case of the tensile strain (the ground state is light hole). The peak of the normal incidence absorption in the compressively strained QW is shown to reach a considerable value of 5000–6000 cm−1 for a sheet hole concentration of 1012 cm−2. For the z‐polarization of the light we found a substantial enhancement of the optical matrix elements in the case of tensile strain (i.e., for a light‐hole ground state).

Bistable electroluminescence in p-i-n light-emitting tunnel-diodes enhanced by aperiodic-superlattice injectors

A p-i-n resonant tunnel diode is designed and investigated using photoluminescence (PL) spectroscopy. The device is based on an Al0.4Ga0.6As/GaAs graded-index waveguide heterostructure enhanced by aperiodic-superlattice injectors for simultaneous resonant injection of electrons and heavy holes. The bias-dependent study of photocurrent, electroluminescence (EL) and PL show strong resonance behavior in the optical intensity confirming the field-dependent resonant injection of the excited states in the emission layers. Pronounced voltage-current bistability due to injection efficiency leads to multiple-wavelength EL and lasing action.

Electron gas in modulation doped GaN/AlGaN structures

Abstract We report low temperature photoluminescence (PL), time resolved PL and electrical transport measurements related to the two-dimensional electron gas (2DEG) in GaN/AlGaN heterostructures and quantum wells grown by MOVPE on sapphire. Electrical measurements of the heterostructure samples show a room temperature mobility of up to 1300 cm2 V−1 s−1 and carrier concentration of 1013 cm−2. The PL spectrum at low temperatures shows a broad emission about 50 meV below the bulk exciton emission, attributed to recombination involving electrons from the lowest subband of the 2DEG at the GaN/AlGaN heterointerface and photoexcited holes in the GaN layer. The data agrees with a self consistent calculation of the energy levels and the electron concentration at the interface. The modulation doped GaN/AlGaN quantum well had an electron concentration of 3.0 × 1012 cm−3 and a mobility of 850 cm2 V−1 s−1 at 300 K. In the low temperature PL spectra we observed three well defined peaks, at 3.53, 3.58 and 3.62 eV, which we attribute to recombination processes in the quantum well (QW).

Radiative recombination in modulation-doped GaAs/AlGaAs heterostructures in the presence of an electric field

The radiative recombination processes involving two dimensional (2D) carriers from the notch potential formed at the interface of modulation doped GaAs/AlGaAs heterostructures have been studied by means of photoluminescence (PL) and photoluminescence excitation spectroscopy in the presence of an external electric field applied perpendicular to the layers via a gate electrode. Two PL bands related to the 2D electron gas are interpreted as the radiative recombination between 2D electrons and holes from the valence band (HB1) and from residual acceptors (HB2), respectively. The band bending in the active layer, which determines the energy positions of these H-bands, can be controlled by applying an external electric field. However, also the separation between the Fermi edge, EF, and the second 2D electron subband is deliberately varied by applying an electric field. At a sufficiently small separation, an efficient scattering path near k=0 is available for electrons at the Fermi energy. This can be observed in the PL spectra as a striking enhancement of the many-body excitonic transition, usually referred to as the Fermi edge singularity (FES). The enhancement of the FES is usually explained in terms of an efficient scattering for electrons at the Fermi edge via the nearly resonant adjacent subband. The efficiency of this process is dependent on the separation between the Fermi edge, EF, and the next subband, which can be controlled via the applied field in our experiments.

Strain and quantum confinement energies in n‐type modulation‐doped lattice‐mismatched InAsP quantum‐well wires

Low‐temperature luminescence and magnetoluminescence experiments have been performed on n‐type modulation‐doped lattice‐mismatched InAsxP1−x/InP quantum‐well wires. From these experiments we can obtain information about the conduction‐band subband structure, the electron effective mass, and consequently the conduction‐band density of states. The doping level is high enough to populate several subbands in the conduction band which become observable in the luminescence spectra. The low‐temperature luminescence spectra contain a distinct signature of the Fermi level at the high‐energy slope. The zero‐field wire luminescence exhibits an energy blue shift due to lateral quantum confinement within the wire and strain energy enlargement of the optical band gap. We have determined the separate energy contributions to the blue shift by high‐field magnetoluminescence experiments. We have also calculated the (nonuniform) strain distribution and the strain‐induced band shift within the wires. The theoretical results ...