J. Dalfors

OPTICAL PROPERTIES OF DOPED INGAN/GAN MULTIQUANTUM-WELL STRUCTURES

A systematic investigation of the photoluminescence spectra from InGaN/GaN multiquantum-well structures with different levels of Si doping in the quantum well has been carried out, in order to study the screening of the strain induced piezoelectric field and potential fluctuations. It is found that the emission energy strongly depends on the carrier concentration, originating from the doping or photogeneration. The observed strong shift with Si doping can only partly be explained by the screening of the piezoelectric field. The main shift is suggested to be related to the screening of the localization potentials.

Radiative recombination in In 0.15 Ga 0.85 N/GaN multiple quantum well structures

We present a study of the radiative recombination in In 0.15 Ga 0.85 N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied in terms of growth temperature ( 18 cm −3 in the well), with excitation intensity and with delay time after pulsed excitation (also shifts up to 0.2 eV). We suggest a two-dimensional model for electron- and donor screening in this case, which is in reasonable agreement with the observed data, if rather strong localization potentials of short range (of the order 100 A) are present. The possibility that excitons as well as shallow donors are impact ionized by electrons in the rather strong lateral potential fluctuations present at this In composition is discussed

Photoluminescence measurements on GaN/AlGaN modulation doped quantum wells

Photoluminescence spectra were measured for 100 A wurtzite GaN AlGaN modulation doped quantum wells. Three well-resolved peaks originate from the quantum well. The theoretically calculated confinement energies have been compared to the experimental energy positions and found to be in good agreement with the data, assuming that the piezoelectric field is largely screened by the electron gas. The highest energy transition may originate from the Fermi edge, consistent with the temperature dependence of the photoluminescence. Decay times for the different transitions indicate that the photoexcited holes are localized.

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.

Carrier and exciton dynamics in In 0.15 Ga 0.85 NGaN multiple quantum well structures

We present a study of the radiative recombination in In0.15Ga0.85N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied in terms of growth temperature and donor doping. The photoluminescence peak position varies strongly (over a range as large as 0.3 eV) with excitation intensity, with donor doping as well as with delay time after pulsed excitation. The peak position is mainly determined by the Stark effect induced by the piezoelectric field. In addition potential fluctuations play an important role, and largely determine the width of the emission. These potential fluctuations may be as large as 0.2 eV in the present samples. Screening effects from donor electrons and excited electron-hole pairs are important, and account for a large part of the spectral shift with donor doping, with excitation intensity and with delay time after pulsed excitation (shifts up to 0.2 eV). We suggest a dominant role of 2D electron- and donor screening in this case, predicting that rather strong localization potentials of short range (of the order 100 angstroms) are present. The possibility that excitons as well as shallow donors are impact ionized by electrons in these rather strong lateral potential fluctuations present at this In composition is discussed in connection with the long decay times observed at all temperatures.

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.

Mechanism for Radiative Recombination in In0.15Ga0.85N/GaN Multiple Quantum Well Structures

We present a study of the radiative recombination in In 0.15Ga0.85N/GaN multiple quantum well samples, where the conditions of growth of the InGaN quantum layers were varied. The piezoelectric field as well as short range potential fluctuations are screened via different mechanisms by donor electrons and excited electron-hole pairs. These effects account for a large part of the spectral shift with donor doping (an upward shift of the photoluminescence (PL) peak up to 0.2 eV is observed for a Si donor density of 2 x 10 18 cm in the well), with excitation intensity and with delay time after pulsed excitation (also shifts up to 0.2 eV). It appears like 2dimensional screening of short range potential fluctuations is needed to fully explain the data. We suggest that excitons as well as shallow donors are at least partly impact ionized by electrons in the rather strong lateral potential fluctuations.