G. Pozina

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

Disorder-induced exciton localization in a fractional monolayer ZnSe/CdSe superlattice

Abstract Low-temperature spectra of excitonic emission and recombination dynamics have been studied in ZnSe/CdSe fractional monolayer (ML) superlattices (SLs). Two intrinsic emission bands are observed in SLs with the CdSe layer fraction ranging between 0.5 and 0.7 ML, reflecting inhomogeneity of the SL structure. The emission energies, line widths and decay times are interpreted in terms of random coupling of 1 ML thick extended (with respect to the exciton Bohr radius) CdSe islands in different layers of a disordered SL.

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.

Optical properties of nanostructures self-organized in CdSe/ZnSe fractional monolayer superlattices

We report on optical studies of excitonic transfer and localization in a CdSe/ZnSe fractional monolayer superlattice with an embedded deeper ZnCdSe quantum well (QW). Time-resolved selective excitation photoluminescence measurements reveal a two-step nature of the energy relaxation process. The first stage includes hot-exciton cascade relaxation assisted by emission of ZnSe LO phonons, which results in the formation of a narrow nonthermal distribution of free excitons. Different mechanisms compete to destroy the distribution, including optical- and acoustic-phonon-assisted exciton localization by fluctuations of the random potential, as well as tunneling escape of hot free excitons towards the embedded deeper QW in the sample.

Characterization of strained Si/Si1−yCy structures prepared by molecular beam epitaxy

Various structures containing Si1−yCy alloy layers have been prepared and characterized by x-ray diffraction, cross-sectional transmission electron microscopy, photoluminescence (PL), Fourier transform infrared spectroscopy, and spectroscopic ellipsometry. A band gap reduction equal to 63 meV/% C has been estimated from PL when taking into account the quantum well (QW) confinement shift using an effective mass calculation. The QW-related emission observed from a multiple QW structure has a temperature quenching behavior with an activation energy equal to 8 meV. Carbon outdiffusion from the QWs has been evidenced by a blueshift of the PL peak and changes in the x-ray diffraction data after furnace annealing at 800 and 850 °C.

Influence of Si-donor doping on the exciton localization in modulation-doped GaN/Al/sub 0.07/Ga/sub 0.93/N multiple quantum well

We have studied the effects of Si doping on the recombination dynamics and exciton localization in modulation-doped GaN/Al/sub 0.07/Ga/sub 0.93/N multiple-quantum-well structures by means of photoluminescence (PL) and time-resolved PL measurements. The PL peak position shows a blue shift as the Si doping in the barriers is increased (up to 4.2/spl times/10/sup 19/ cm/sup -3/). For even higher doping levels a red shift of the PL emission is observed. The decay time of an undoped sample shows nonexponential behavior, while the Si doped samples show mono-exponential behavior. Surprisingly, the PL decay time at 2 K is found to be nearly constant for all doping levels, in these samples.

Optical properties of InGaN/GaN and AlGaN/GaN multiple quantum well structures

We report on low temperature photoluminescence (PL) in InxGa1-xN multiple qunatum wells (MQWs) with x in the range 0.1 and highly Si doped barriers of In0.01Ga0.99N. One sample with 3 QWs of width 3.5 nm and barriers of width 10.5 nm had the MQW in the depletion region of the outer surface. Two PL peaks were observed, one QW exciton from the QW closest to the GaN buffer, one lower energy peak related to a 2DEG at the interface to the GaN buffer layer. In a second similar sample 5 QWs of width 3 nm and with 6 nm highly Si doped In0.01Ga0.99N barriers the MQW was placed in the n-side depletion region of a pn-junction. At low temperatures the PL and electroluminescence (EL) spectra are quite different at no, low, or reverse bias, the PL appearing at higher energy. At high forward bias a spectral component at the EL position appears. This proves a strong influence of the depletion field on the optical spectra. Preliminary results are also reported for n-doped Al0.07Ga0.93N/GaN structures, with near surface MQWs.

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.

Incorporation and luminescence properties of Er2O3 and ErF3 doped Si layers grown by molecular beam epitaxy

Abstract Er doped Si layer structures have been grown using Er2O3 or ErF3 as evaporation source materials during Si molecular beam epitaxy. By using a low temperature growth process, an Er doping level of ~5×1019/cm3 has been achieved without precipitation. Structural and luminescence properties of these Er/O and Er/F doped Si samples have been studied. Electroluminescence (1.54 μm) has been observed from Er/O doped Si layers at room temperature through hot electron impact excitation. Comparison of the photo- and electroluminescence of these Er-doped Si structures has been made. The major thermal quenching behaviors of both luminescence emissions are characterized by the same activation energy value of ~160 meV, but different on-set temperatures.

On the improvement in thermal quenching of luminescence in SiGe/Si structures grown by molecular beam epitaxy

Thermal quenching of photoluminescence (PL) from SiGe/Si quantum well (QW) structures grown by molecular beam epitaxy is shown to be more severe when grown at a lower temperature. The mechanism responsible for the thermal quenching of PL is discussed as being due to thermally activated nonradiative recombination channels, related to defects in both Si barriers and SiGe QW. Nonradiative defects in Si can be rather efficiently deactivated by post-growth treatments such as hydrogenation and thermal annealing, leading to a significant improvement in the thermal quenching behavior of PL from single QW structures. Nonradiative defects in SiGe are found to be thermally stable, on the other hand, evident from the experimentally observed minor role played by post-growth thermal annealing in the thermal quenching of PL from multiple QW structures.