Markus Maier

Reduced nonthermal rollover of wide-well GaInN light-emitting diodes

Nonthermal rollover (or efficiency droop) of the electroluminescence (EL) efficiency has been investigated for near-UV-emitting (AlGaIn)N single-well light-emitting diodes (LED) with varying GaInN well widths grown on substrates with different dislocation densities (DDs). For each DD the well width of the mesa-LEDs has been optimized for maximum EL efficiency at high operating currents. LEDs on freestanding GaN (DDu20024×107u2002cm−2) with an 18 nm thick GaInN wide-well active region show the highest efficiency, and the output power-versus-current characteristic remains linear up to the highest pulsed current density of 750u2002A/cm2. In contrast, LEDs on sapphire grown with conventional low-temperature nucleation (DDu2002109u2002cm−2) exhibit the optimum well width at 3 nm and show significant nonthermal rollover.

Preferential formation of Al-N bonds in low N-content AlGaAsN

The bonding of nitrogen in low N-content AlxGa1−xAs1−yNy with x⩽0.05 and y⩽0.04 has been studied by Raman spectroscopy. Upon the addition of Al to GaAsN, additional vibrational modes are observed at around 450 cm−1, which is below the GaN-like longitudinal optical (LO) phonon mode centered at 470 cm−1. These modes are attributed to the formation of Al and N containing complexes with Al-to-N bonding. With increasing Al content the Al–N related modes gain intensity at the expense of the GaN-like mode, and they become the dominant N-related feature for an Al-content of 5% at a fixed N content of 1%. On the other hand, increasing the N content from 0% up to 4% at a constant Al concentration of 5% results first in the appearance and eventual saturation in intensity of the AlN-like modes, accompanied by a steep increase in intensity and eventual dominance of the GaN-like vibrational mode. Simultaneously the AlAs-like LO2 phonon mode shows a drastic decrease in intensity for N contents exceeding 2%. All these ob...

Quaternary GaInAsN with high In content: Dependence of band gap energy on N content

Quaternary pseudomorphically strained GaInAsN films and double-quantum wells were grown by plasma assisted molecular-beam epitaxy on an InP substrate. The In content ranged from 53% to 70% while the N content was varied between 0% and 2.4%. A reduction of compressive strain and a low-energy shift of photoluminescence (PL) peak position was observed with increasing N concentration, accompanied by a reduction in PL peak intensity and increase in linewidth. The net effect of N incorporation on the GaInAsN band gap energy was calculated from the measured PL peak energies. The thus obtained composition dependent GaInAsN band gap energy was fitted using the band anticrossing model, yielding values for the interaction parameter CMN for high In-containing GaInAsN being only slightly smaller than that reported for low In-content GaInAsN on GaAs.

Quantitative assessment of Al-to-N bonding in dilute Al0.33Ga0.67As1−yNy

A quantitative assessment of the group III–nitrogen bonding in low N-content Al0.33Ga0.67As1−yNy with y⩽0.04 has been performed, using vibrational mode Raman spectroscopy for the quantitative analysis of local bond formation in combination with energy dispersive x-ray analysis and secondary ion mass spectrometry for chemical analysis. Clear evidence is obtained for the preferential bonding of nitrogen to Al with one nitrogen atom being coordinated to, at the average, 3.4 Al neighbors. This strong preference for Al-to-N bond formation can be understood in terms of the much larger cohesive energy of the Al–N bond compared to the Ga–N chemical bond. In spite of this phase-separation-like formation of local Al–N complexes, the fundamental band gap and the E1/E1+Δ1 band gaps show a continuous low-energy and high-energy shift, respectively, upon the addition of nitrogen as already known from dilute GaAsN.

Bonding of nitrogen in dilute InAsN and high In-content GaInAsN

Dilute InAs1−yNy and high In-content Ga1−xInxAs1−yNy layers with y⩽0.012 and x⩾0.92 were grown by rf-nitrogen plasma source molecular-beam epitaxy on InP substrates using a metamorphic GaInAs buffer layer. The bonding of nitrogen in these alloys was analyzed by Raman spectroscopy, showing that nitrogen is incorporated in dilute InAsN as isolated NAs for a nitrogen content of y=0.005; two additional nitrogen-related modes were found to appear at higher nitrogen contents (y=0.012), possibly due to the formation of higher-order di-nitrogen In–N complexes. The addition of a small amount of Ga to the InAsN ([Ga]⩽8%) was found to lead to an almost complete change from pure In–N bonding to a preferential bonding of the substitutional nitrogen to at least one Ga neighbor. Further, the effect of nitrogen incorporation on the higher-lying E1 and E1+Δ1 interband transitions of InAsN has been studied by spectroscopic ellipsometry, revealing a high-energy shift of both interband transitions with increasing nitrogen co...

Optical investigation of delta -doped In0.1Ga0.9As:Si/GaAs strained quantum wells

Photoluminescence and Raman spectroscopy have been used to study a delta -doped In0.1Ga0.9As:Si/GaAs strained quantum well with the doping spike placed at the centre of the 30 nm wide well. In this structure the potential confining the electron gas is given by a superposition of the V-shaped potential well induced by the doping spike and barriers introduced by the heterointerfaces. Both luminescence and intersubband Raman spectra provide information on the energy spacing and population of the electron subbands. A comparison with results of self-consistent subband calculations is made. Raman spectra excited in resonance with the E0+ Delta 0 bandgap of the In0.1Ga0.9As quantum well show intense single-particle intersubband transitions, which dominate over the signal from collective spin-density and charge-density excitations. Possible mechanisms for the occurrence of such strong single-particle intersubband Raman signals are discussed.

Reduced non-thermal roll-over in violet-emitting GaInN wide-well LEDs grown on low-dislocation-density substrates

Near-UV LEDs emitting at around 400 nm can be used e.g. as pump light source in tri-phosphor RGB white luminescence-conversion LEDs with high color rendering.1 Although non-thermal roll-over decreases towards shorter emission wavelengths in GaInN-based LEDs, this effect still limits the efficiency of 400 nm emitting LEDs at current densities above 50 A/cm2. One way to overcome non-thermal roll-over is to combine a GaInN wide-well active region with the growth on low dislocation density (DD) substrates. Single-well LEDs with GaInN layer widths between 3 nm and 18 nm were grown (a) directly on sapphire substrates with a resulting DD of 109 cm-2, (b) on low DD GaN templates on sapphire (DD of 108 cm-2), and (c) on freestanding GaN substrates (FS-GaN, DD of 4×107 cm-2). At low current densities (pulsed mode operation) the LEDs with a 3 nm GaInN QW active region showed the highest efficiency, irrespective of the substrate. However, the electroluminescence (EL) efficiency peaks at around 50 A/cm2 and shows a clear non-thermal roll-over towards higher current densities. The efficiency of LEDs with well widths >3 nm grown on sapphire decreases with increasing well width over the whole range of current densities (≤300 A/cm2). However, when grown on low DD GaN templates or FS-GaN, the efficiency of the LEDs with 11 and 18 nm wide GaInN layers surpasses that of the conventional LEDs (well widths ≤6 nm) for current densities ≥250 A/cm2, yielding the highest EL efficiency of all LED-structures.

High In-content InP-substrate based GaInAsN and GaInAsN QW diode lasers emitting in the 2.2 to 2.3 /spl mu/m wavelength range

We report on the growth and characterization of high In-content quaternary Ga/sub 1-x/In/sub x/As/sub 1-y/N/sub y/ (0.78 < x < 1,y < 0.02), grown by plasma assisted molecular beam epitaxy on InP-substrates. First, the incorporation of nitrogen in high In-content GaInAsN was analyzed by Raman spectroscopy, revealing that already a small amount of Ga in the GaInAsN alloy (x>0.92) leads to an almost complete change from pure In-N bonding to nitrogen atoms bonded to at least on Ga-neighbor. Next, strained Ga/sub 0.22/In/sub 0.78/As/sub 0.99/N/sub 0.01//Al/sub 0.48/In/sub 0./

Enhancement of (AlGaIn)N near‐UV LED efficiency using freestanding GaN substrate

d5/sub 2/As QWs were optimized for long-wavelength emission. In this way room-temperature photoluminescence could be achieved at a wavelength of 2.3 /spl mu/m for 11 nm wide QWs. Finally, Ga/sub 0.22/In/sub 0.78/As/sub 0.99/N/sub 0.01/ QW-diode lasers were realized, employing lattice matched Al/sub 0.15/Ga/sub 0.32/In/sub 0.53/As as the material for the barriers as well as for the separate confinement layers. InP was used for the cladding layers. Ridge waveguide lasers were fabricated without any post-growth thermal annealing. These devices showed pulsed lasing up to a heat sink temperature of 190 K, for which lasing was at 2.23 /spl mu/m. Higher operating temperatures as well as cw-operation were inhibited by the high threshold current density of almost 2 kA/cm/sup 2/ at 190 K, but room-temperature electroluminescence could be observed at wavelengths up to 2.4 /spl mu/m. As annealing studies on GaInAsN QW-test structures indicate, post-growth annealing of the laser material will lead to a significant improvement in laser performance.