G. Franssen

Role of the electron blocking layer in the low-temperature collapse of electroluminescence in nitride light-emitting diodes

The low-temperature breakdown of the electroluminescence intensity (ELI) of blue/violet InGaN-based light-emitting diodes (LEDs) is shown to be independent of the structural details of the LED active region. Instead, the presence of an electron blocking layer (EBL) plays a decisive role. The authors attribute the ELI collapse to the low-temperature hole-blocking properties of the EBL. However, removing the EBL leads to a much reduced ELI because of a disproportional increase of electron overflow processes, which shows that the presence of an EBL in blue/violet InGaN-based LEDs is still essential. Optimization of the EBL by means of Mg doping is discussed.

Bowing of the band gap pressure coefficient in InxGa1-xN alloys

The hydrostatic pressure dependence of photoluminescence, dEPL/dp, of InxGa1−xN epilayers has been measured in the full composition range 0 0.4 and a relatively steep dependence for x<0.4. On the basis of the agreement of the observed PL pressure coefficient with our calculations, we confirm that band-to-band recombination processes are responsible for PL emission and that no localized states are involved. Moreover, the good agreement between the experimentally determined dEPL/dp and the theoretical curve of dEG/dp indicates that the hydrostatic pressure dependence of PL measurements can be used to quantify changes of the band gap of the InGaN ternary alloy under pressure, demonstrating that the disorder-re...

Fully-screened polarization-induced electric fields in blue∕violet InGaN/GaN light-emitting devices grown on bulk GaN

Photocurrent spectroscopy and hydrostatic-pressure-dependent electroluminescence are used to show that heavy 1×1019cm−3 Si doping of quantum barriers is sufficient to achieve full screening of polarization-induced electric fields (PIEFs) in nitride light emitting diodes (LEDs) and laser diodes (LDs) with InGaN quantum wells. Furthermore, it is shown that at currents close to lasing threshold in nitride LDs injected charge alone is sufficient to achieve full screening of PIEFs. In contrast, full screening at low currents can only be accomplished via Si doping of quantum barriers.

Correlation between luminescence and compositional striations in InGaN layers grown on miscut GaN substrates

The influence of the miscut angle of GaN substrate on compositional and optical properties of InxGa1−xN epilayers (0.05<x<0.1) was examined using x-ray diffraction, photoluminescence (PL), cathodoluminescence, and Z-contrast scanning electron microscopy. We show that single atomic steps bunch during growth of InGaN and form macrosteps. Indium is incorporated differently at treads and risers of these macrosteps, which causes the layer to decompose and induces the formation of compositional growth striations. Since the growth step density increases with growing miscut angle of the substrate, the average indium concentration decreases and the average PL peak energy blueshifts and broadens with increasing miscut angle. The presented work enables understanding on microscopic scale effects related to the inhomogeneous distribution of indium in InGaN layers on miscut substrates, which is significant from the point of view of optoelectronic applications.

Optical and electrical properties of homoepitaxially grown multiquantum well InGaN/GaN light-emitting diodes

We have studied the electrical and optical properties of homoepitaxially grown InGaN/GaN multiquantum-well light-emitting diodes over a wide range of temperatures and currents. We found thermionic emission rather than tunneling transport to be the most probable mechanism responsible for radiative recombination processes. As we point out, the thermal activation energies of Si donors in the n-type GaN and Mg acceptors in the p-type GaN can explain the temperature dependence of the device series resistance. Furthermore, the presence of strong localization effects in the active InGaN layers can be deduced from the observed considerable blueshifts of the electroluminescence emission peak with increasing temperature and current. We found the values of the internal and external quantum efficiencies of homoepitaxially grown light emitting diodes to be 38% and 1.9%, respectively.

A pressure-tuned blue-violet InGaN/GaN laser diode grown on bulk GaN crystal

We demonstrate efficient wavelength tuning by means of hydrostatic pressure of an InGaN/GaN laser diode grown on bulk GaN crystal. Energy shifts of the emitted light with pressure have been found to be about 36 meV/GPa, which are high magnitudes for nitride-based device structures. This result is interpreted as being indicative of efficient screening of built-in electric fields in the studied device. Furthermore, the threshold current of the laser diode was found to be independent of applied pressure. The high magnitude of the pressure coefficient allowed for the achievement of a laser tuning range of up to 10 nm in the blue/violet region, using compact pressure equipment.

Efficient radiative recombination and potential profile fluctuations in low-dislocation InGaN∕GaN multiple quantum wells on bulk GaN substrates

We investigated the relation between structural properties and carrier recombination processes in InGaN∕GaN multiple quantum well (MQW) structures with quantum well widths of 3 and 9nm, grown by metal-organic chemical-vapor deposition on bulk GaN crystals. Quantum barriers of the samples are heavily n-type doped in order to effectively screen the large polarization-induced electric fields which commonly occur in hexagonal InGaN∕GaN quantum structures. High thermal stability in these structures, reflected by strong photoluminescence (PL) even above 400K, is attributed to a combination of low-dislocation densities and potential profile fluctuations in the InGaN∕GaN quantum wells. The role of potential profile fluctuations is further investigated by time-resolved photoluminescence and cathodoluminescence (CL) mapping. Comparison of both samples shows that the sample with 3‐nm-wide QWs exhibits (i) a larger width of the PL peak in the temperature range of 8–420K, (ii) a higher amplitude of potential profile f...

Substrate misorientation induced strong increase in the hole concentration in Mg doped GaN grown by metalorganic vapor phase epitaxy

We demonstrate that relatively small GaN substrate misorientation can strongly change hole carrier concentration in Mg doped GaN layers grown by metalorganic vapor phase epitaxy. In this work intentionally misoriented GaN substrates (up to 2° with respect to ideal ⟨0001⟩ plane) were employed. An increase in the hole carrier concentration to the level above 1018 cm−3 and a decrease in GaN:Mg resistivity below 1 Ω cm were achieved. Using secondary ion mass spectroscopy we found that Mg incorporation does not change with varying misorientation angle. This finding suggests that the compensation rate, i.e., a decrease in unintentional donor density, is responsible for the observed increase in the hole concentration. Analysis of the temperature dependence of electrical transport confirms this interpretation.

Anomalous temperature characteristics of single wide quantum well InGaN laser diode

By using an atypically wide quantum well (95A) in the active layer of InGaN violet light emitting laser diode, we managed to fabricate a device characterized by very high thermal stability of the threshold current. The characteristic T0 temperature was measured to be 302K, which is the highest reported value up to date. After thermal cycling of the device, T0 drops down to the lower value of 220K. The very high value of T0 in our devices is accompanied by anomalous temperature behavior of the device slope efficiency. The slope efficiency improves with increasing temperature, reaches a maximum and then gradually decreases. This behavior we interpret as the competition between a regular increase of the thermal carrier escape and an improvement of carrier capture efficiency with an opposite temperature dependence. The latter mechanism we tentatively attribute to the temperature quenching of the ballistic transport related carrier leakage from the active region of the laser diode.

Different pressure behavior of GaN/AlGaN quantum structures grown along polar and nonpolar crystallographic directions

High quality GaN/AlGaN multiquantum well (QW) structures were grown by ammonia molecular beam epitaxy along the (0001) polar and (11 20) nonpolar directions. Each sample contains three QWs with thicknesses of 2, 3, and 4 nm as well as 10 nm Al0.30Ga0.70N barriers. The measured photoluminescence (PL) spectrum consists of three peaks originating from the radiative recombination of excitons in individual QWs. In the nonpolar sample, the energy positions (EPL) of the observed peaks are separated because of the quantum confinement effect, whereas in the polar sample an additional redshift is induced by the quantum confined Stark effect. The dependence of EEPL on QW width was used to estimate the built-in electric field magnitude in the latter sample to be about 2 MV/cm. Hydrostatic pressure studies of the PL in both samples gave qualitatively different results. In the polar sample, the pressure shift of EPL, dEPL/dp decreases significantly with QW width. The important finding is derived from the observation of a QW width independent dEPL/dp in the nonpolar sample. It shows that for GaN/Al0.30Ga0.70N, the quantum confinement remains practically independent of the applied hydrostatic pressure. This result reveals that in the polar sample, the variation in dEPL /dp with the QW width is due to the pressure-induced increase in the built-in electric field Fint. Thus, a more quantitative analysis of the latter effect becomes justified. We found that the Fint increases with pressure with a rate of about 80 kV (cm GPa)-1.