S. B. Fleischer

Effective band gap inhomogeneity and piezoelectric field in InGaN/GaN multiquantum well structures

The emission mechanisms of strained InxGa1−xN quantum wells (QWs) were shown to vary depending on the well thickness, L, and x. The absorption edge was modulated by the quantum confined Stark effect and quantum confined Franz-Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of the piezoelectric field, FPZ, and L exceed the valence band discontinuity, ΔEV. In this case, holes are confined in the triangular potential well formed at one side of the well producing the apparent Stokes-like shift. Under the condition that FPZ×L exceeds the conduction band discontinuity ΔEC, the electron-hole pair is confined at opposite sides of the well. The QCFK further modulated the emission energy for the wells with L greater than the three dimensional free exciton Bohr radius aB. On the other hand, effective in-plane localization of carriers in quantum disk size potential minima, which are produced by nonrandom alloy compositional fluctuation enhanced by the large bowing parameter and...

High-performance (Al,Ga)N-based solar-blind ultraviolet p–i–n detectors on laterally epitaxially overgrown GaN

Solar-blind ultraviolet photodiodes with a band-edge wavelength of 285 nm were fabricated on laterally epitaxially overgrown GaN grown by metalorganic chemical vapor deposition. Current–voltage measurements of the diodes exhibited dark current densities as low as 10 nA/cm2 at −5 V. Spectral response measurements revealed peak responsivities of up to 0.05 A/W. Response times for these diodes were measured to be as low as 4.5 ns for 90%-to-10% fall time. For comparison, diodes were fabricated using the same p–i–n structure deposited on dislocated GaN. These diodes had dark current densities many orders of magnitude higher, as well as a less sharp cutoff, and a significant slow tail under impulse excitation.

Emission mechanisms of bulk GaN and InGaN quantum wells prepared by lateral epitaxial overgrowth

The emission mechanisms of bulk GaN and InGaN quantum wells (QWs) were studied by comparing their optical properties as a function of threading dislocation (TD) density, which was controlled by lateral epitaxial overgrowth. Slightly improved excitonic photoluminescence (PL) intensity was recognized by reducing TD density from 1010 cm−2 to less than 106 cm−2. However, the major PL decay time was independent of the TD density, but was rather sensitive to the interface quality or material purity. These results suggest that TDs simply reduce the net volume of light-emitting area. This effect is less pronounced in InGaN QWs where carriers are effectively localized at certain quantum disk size potential minima to form quantized excitons before being trapped in nonradiative pathways, resulting in a slow decay time. The absence of any change in the optical properties due to reduction of TD density suggested that the effective band gap fluctuation in InGaN QWs is not related to TDs.

SELF-ASSEMBLED ERAS ISLANDS IN GAAS: GROWTH AND SUBPICOSECOND CARRIER DYNAMICS

We report the growth of self-assembled ErAs islands embedded in GaAs by molecular beam epitaxy. The nucleation of ErAs on GaAs occurs in an island growth mode leading to spontaneous formation of nanometer-sized islands. Several layers of ErAs islands separated by GaAs can be stacked on top of each other to form a superlattice. X-ray diffraction shows superlattice fringes from such samples. Pump–probe measurements indicate carrier capture times as short as 120 fs. These capture times are strongly correlated with the period of the superlattice.

CHARACTERIZATION OF HIGH-QUALITY INGAN/GAN MULTIQUANTUM WELLS WITH TIME-RESOLVED PHOTOLUMINESCENCE

Recombination in single quantum well and multiquantum well InGaN/GaN structures is studied using time-resolved photoluminescence and pulsed photoluminescence measurements. Room-temperature measurements show a rapid lifetime (0.06 ns) for a single quantum well structure, while an increasingly long decay lifetime is measured for multiquantum wells as more quantum wells are incorporated into the structure. Temperature-dependent lifetime measurements show that a nonradiative recombination mechanism activates above 45 K in the single quantum well but is less important in the multiquantum wells.

EFFECTS OF SI-DOPING IN THE BARRIERS OF INGAN MULTIQUANTUM WELL PURPLISH-BLUE LASER DIODES

Optical gain spectra of InGaN multiquantum well laser diode wafers having Si-doped or undoped InGaN barriers were compared. Although evidence for effective band-gap inhomogeneity was found in both structures, the wells with the Si-doped barriers exhibited a smaller Stokes-like shift. Si doping suppressed emergence of a secondary amplified spontaneous emission peak at 3.05 eV, which was uncoupled with the primary one at 2.93 eV. Furthermore Si doping reduced the threshold power density required to obtain the stimulated emission.

High-speed low-temperature-grown GaAs p-i-n traveling-wave photodetector

We report a novel type of p-i-n traveling-wave photodetector utilizing low-temperature-grown GaAs (LTG-GaAs). The devices show a record impulse response time (530-fs fullwidth at half-maximum, /spl sim/560 GHz -3-dB bandwidth) which agrees with theoretical estimates. The effects of various limiting factors on the device performance were analyzed theoretically and compared with measurements obtained by electrooptic characterization of our devices. Calculations indicate that the device speed is dominated by the short carrier lifetime. DC external quantum efficiencies as high as 8% were obtained.

Ultrafast (370 GHz bandwidth) p-i-n traveling wave photodetector using low-temperature-grown GaAs

The authors demonstrate p-i-n traveling wave photodetectors utilizing low-temperature-grown GaAs as the absorption layer. The electro-optically measured impulse response was found to exhibit a pulsewidth of 1.1 ps full width at half maximum, corresponding to a −3 dB bandwidth of 370 GHz with an external quantum efficiency of 8% at 800 nm.

Optical Properties of InGaN/GaN Quantum Wells with Si Doped Barriers

Optical properties of InGaN/GaN multi-quantum wells with Si doped barriers are studied using cw and time resolved photoluminescence (TRPL) and photoluminescence excitation (PLE) spectroscopy. The room temperature carrier lifetime depends strongly on the Si doping level in the quantum well barriers, decreasing from 10 ns to 1 ns as the doping level is increased from unintentionally doped to 5×1018 cm-3 (Si:GaN). The shift between the absorption edge and emission peak decreases from 220 meV to 110 meV as the doping is increased. Temperature dependent photoluminescence measurements indicate a higher density of non-radiative centers in the undoped structures.

Subpicosecond carrier dynamics in low-temperature grown GaAs on Si substrates

This letter describes time-resolved differential reflection measurements on low-temperature grown GaAs on (100) Si substrates. The carrier recombination depends sensitively on growth and anneal conditions. The differential reflectance signals of samples annealed at 600 °C are dominated by an exponential subpicosecond transient, which can be as short as 370 fs. Optical microscopy and atomic force microscopy show that the films are comparably smooth or smoother than other GaAs material grown on Si. X-ray diffraction indicates tensile strain in the films, which is explained by the different thermal expansion coefficients of GaAs and Si.