A. C. Abare

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...

Structural origin of V-defects and correlation with localized excitonic centers in InGaN/GaN multiple quantum wells

In the growth of InGaN/GaN multiple quantum well (MQW) structures, a novel defect (called the “V-defect”) initiates at threading dislocations in one of the first quantum wells in a MQW stack. This defect is common to almost all InGaN MQW heterostructures. The nature of the V-defect was evaluated using transmission electron microscopy (TEM), scanning TEM (STEM), and low-temperature cathodoluminescence (CL) on a series of In0.20Ga0.80N/GaN MQW samples. The structure of the V-defect includes buried side-wall quantum wells (on the {1011} planes) and an open hexagonal inverted pyramid which is defined by the six {1011} planes. Thus, in cross section this defect appears as an open “V”. The formation of the V-defect is kinetically controlled by reduced Ga incorporation on the pyramid walls ({1011} planes). The V-defect is correlated with the localized excitonic recombination centers that give rise to a long-wavelength shoulder in photoluminescence (PL) and CL spectra. This long-wavelength shoulder has the fol...

Indium tin oxide contacts to gallium nitride optoelectronic devices

We have fabricated GaN-based light-emitting diodes using transparent indium tin oxide (ITO) p contacts. ITO-contacted devices required an additional 2 V to drive 10 mA, as compared to similar devices with metal contacts. However, ITO has lower optical absorption at 420 nm (α=664 cm−1) than commonly used thin metal films (α=3×105 cm−1). Uniform luminescence was observed in ITO-contacted devices, indicating effective hole injection and current spreading.

Growth and characterization of bulk InGaN films and quantum wells

InGaN bulk layers and single quantum wells were grown by atmospheric pressure metalorganic chemical vapor deposition on c‐plane sapphire. We have found that the incorporation efficiency of indium into InGaN epitaxial layers is strongly dependent on the growth rate of the films. Narrow and bright band edge related luminescence was observed for InGaN films up to an indium content of 20% grown at 700 °C. In0.16Ga0.84N single quantum wells with graded InxGa1−xN barriers showed intense luminescence, with an energy shift towards shorter wavelength with decreasing quantum well thickness. The photoluminescence full width at half‐maximum of the 50 A thick well was as low as 7.9 nm (59 meV) at 300 K.

Cleaved and etched facet nitride laser diodes

Room-temperature (RT) pulsed operation of blue (420 nm) nitride-based multiquantum-well laser diodes grown on a-plane and c-plane sapphire substrates has been demonstrated. Structures investigated include etched and cleaved facets as well as doped and undoped quantum wells. A combination of atmospheric and low-pressure metal organic chemical vapor deposition using a modified two-flow horizontal reactor was employed. Threshold current densities as low as 12.6 kA/cm/sup 2/ were observed for 10/spl times/1200 /spl mu/m lasers with uncoated reactive ion etched facets on c-plane sapphire. Cleaved facet lasers were also demonstrated with similar performance on a-plane sapphire. Laser diodes tested under pulsed conditions operated up to 6 h at RT. Lasing was achieved up to 95/spl deg/C and up to a 150-ns pulselength (RT). Threshold current increased with temperature with a characteristic temperature T/sub 0/ of 114 K.

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.

Improved characteristics of InGaN multiple-quantum-well laser diodes grown on laterally epitaxially overgrown GaN on sapphire

InGaN multiple-quantum-well laser diodes have been fabricated on fully coalesced laterally epitaxially overgrown (LEO) GaN on sapphire. The laterally overgrown “wing” regions as well as the coalescence fronts contained few or no threading dislocations. Laser diodes fabricated on these low-dislocation-density regions showed a reduction in threshold current density from 10 to 4.8 kA/cm2 compared to those on conventional planar GaN on sapphire. The internal quantum efficiency also improved from 3% for laser diodes on conventional GaN on sapphire to 22% for laser diodes on LEO GaN on sapphire.

GAIN SPECTROSCOPY ON INGAN/GAN QUANTUM WELL DIODES

We have investigated spectroscopically the emergence of gain in InGaN/GaN quantum well diodes under high current injection (>kA/cm2). The spectral characteristics suggest that the electronic states responsible for blue laser action in this material are strongly influenced by the presence of microscopic crystalline disorder.

Large coherent acoustic-phonon oscillation observed in InGaN/GaN multiple-quantum wells

We have observed coherent acoustic-phonon oscillation in InGaN/GaN multiple-quantum wells. With femtosecond pulse excitation, photogenerated carriers screen out the strained-induced piezoelectric field and initiate the coherent acoustic-phonon oscillation. The resulted modulation of the piezoelectric field will then cause large optical property oscillation through the quantum-confined Stark effect.