InGaN-based Devices Research Articles

Suppressing the zincblende-phase inclusions in nitrogen-polar wurtzite-phase InGaN films by pulsed metalorganic chemical vapor deposition

Nitrogen-polar (N-polar) wurtzite (WZ)-phase InGaN films suffer from severe zincblende (ZB)-phase inclusions and thus present a rough surface, which severely restricts its application in GaN-based optoelectronic devices. In this paper, several different pulsed modes were adopted to grow N-polar InGaN films with the purpose of suppressing the ZB-phase inclusions, and their effects on the surface morphology and crystal quality of the InGaN films were studied in detail. Our results demonstrate that the pulsed mode of continuously feeding In source while alternately injecting N and Ga sources can effectively suppress the formation of ZB-phase inclusions, due to the increased mobility of group-III adatoms on the N-polar surface. Benefiting from the suppression of the ZB-phase inclusions, the surface roughness of InGaN films is significantly reduced. Meanwhile, the screw dislocation density of the InGaN decreases from 1.4 ​× ​109 to 6.0 ​× ​108 ​cm−2. This work provides an approach to the growth of high-quality N-polar InGaN films free of ZB-phase inclusions for N-polar InGaN-based devices.

Stacking faults in plastically relaxed InGaN epilayers

Relaxed InGaN layers have recently gained a lot of interest as potential pseudo-substrates for InGaN-based devices. In this work we study PAMBE grown thick In0.2Ga0.8N layers relaxed through the formation of (a+c)-type misfit dislocations. We show that either Ga/N flux ratio (ΦGa/ΦN) below 0.6 or high nitrogen flux (ΦN) equal to 1.4 μm h−1 results in basal stacking fault (BSF) formation. Structural analysis has shown that stacking faults are I1-type in I3 and I4 configurations. We discuss the interactions between (a+c)-type dislocations and BSFs which depend on stacking fault configuration. Probably due to the lack of defect character of BSFs in I3 configuration, dislocations can cross them. Whereas BSFs in I4 configuration stop the dislocations a few nanometers above them, possibly due to the repulsive interactions between the descending dislocation and the Shockley partial dislocations surrounding the BSFs.

Growth and Characterization of High In-content InGaN grown by MBE using Metal Modulated Epitaxy Technique (MME)

We investigated the growth and characterization of InGaN films with high In content up to 51.9% obtained by MBE MME-growth. Comparing the structural and optical properties of InGaN films grown using common growth method and MME technique, we could acquire the good quality of InGaN sample by MME-growth method with lower dislocation density, better composition uniformity and smoother surface. These experimental results suggest that MME has an enormous potential to promote the quality of high In-content InGaN materials and the development of InGaN-based devices. Besides, further study shows the process of MME growth, the parameters of material growth have opposite effects on the background carrier concentration and carrier mobility.

Auger recombination in AlGaN quantum wells for UV light-emitting diodes

We show that the often observed efficiency droop in AlGaN quantum well heterostructures is an internal carrier loss process, analogous to the InGaN system. We attribute this loss process to Auger recombination, with C = 2.3 × 10−30 cm6 s−1; a similar value found commonly in InGaN-based devices. As a result, the peak internal quantum efficiency (IQE) of our structures is limited to 66%. These values were obtained by resonant excitation (time-resolved) photoluminescence (PL), avoiding common error sources in IQE measurements. The existence of strong Auger recombination implies that simple methods employed for IQE determination, such as temperature-dependent PL, may lead to erroneous values. Auger losses will have to be considered once the challenges regarding carrier injection are solved.

Dislocations in AlGaN: Core Structure, Atom Segregation, and Optical Properties.

We conducted a comprehensive investigation of dislocations in Al0.46Ga0.54N. Using aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray spectroscopy, the atomic structure and atom distribution at the dislocation core have been examined. We report that the core configuration of dislocations in AlGaN is consistent with that of other materials in the III-Nitride system. However, we observed that the dissociation of mixed-type dislocations is impeded by alloying GaN with AlN, which is confirmed by our experimental observation of Ga and Al atom segregation in the tensile and compressive parts of the dislocations, respectively. Investigation of the optical properties of the dislocations shows that the atom segregation at dislocations has no significant effect on the intensity recorded by cathodoluminescence in the vicinity of the dislocations. These results are in contrast with the case of dislocations in In0.09Ga0.91N where segregation of In and Ga atoms also occurs but results in carrier localization limiting non-radiative recombination at the dislocation. This study therefore sheds light on why InGaN-based devices are generally more resilient to dislocations than their AlGaN-based counterparts.

Degradation of InGaN-based MQW solar cells under 405 nm laser excitation

Within this paper we analyze the reliability of 25×multi quantum well InGaN-based devices, designed to be used as high power photodetectors or in multi-junction solar cells. Under stress with monochromatic excitation at 405nm by means of a laser diode, we detect degradation at optical power levels significantly above the common AM1.5 spectrum; the main degradation modes are a reduction in short circuit current and in open circuit voltage, and an improvement in fill factor. The analysis of the wavelength-dependent EQE highlights, as a consequence of stress, the increase in concentration of a deep level compatible with the yellow luminescence in gallium nitride, suggesting that gallium vacancies may play a role in the degradation of the detectors.

Carrier localization in the vicinity of dislocations in InGaN

We present a multi-microscopy study of dislocations in InGaN, whereby the same threading dislocation was observed under several microscopes (atomic force microscopy, scanning electron microscopy, cathodoluminescence imaging and spectroscopy, transmission electron microscopy), and its morphological optical and structural properties directly correlated. We achieved this across an ensemble of defects large enough to be statistically significant. Our results provide evidence that carrier localization occurs in the direct vicinity of the dislocation through the enhanced formation of In-N chains and atomic condensates, thus limiting non-radiative recombination of carriers at the dislocation core. We highlight that the localization properties in the vicinity of threading dislocations arise as a consequence of the strain field of the individual dislocation and the additional strain field building between interacting neighboring dislocations. Our study therefore suggests that careful strain and dislocation distribution engineering may further improve the resilience of InGaN-based devices to threading dislocations. Besides providing a new understanding of dislocations in InGaN, this paper presents a proof-of-concept for a methodology which is relevant to many problems in materials science.

Electronic properties of substitutional impurities in InGaN monolayer quantum wells

InGaN alloys and, in particular, InGaN monolayer quantum wells (MLQWs) are attracting an increasing amount of interest for opto-electronic applications. Impurities, incorporated during growth, can introduce electronic states that can degrade the performance of such devices. For this reason, we present a density functional and group theoretical study of the electronic properties of C, H, or O impurities in an InGaN MLQW. Analysis of the formation energy and symmetry reveals that these impurities are mostly donors and can be held accountable for the reported degradation of InGaN-based devices.

Indium nanowires in thick (InGaN) layers as determined by x-ray analysis

300-nm-thick InGaN layers with In concentrations up to 21% were grown by low-pressure metalorganic chemical-vapor deposition. Besides the InGaN and GaN Bragg peaks, the symmetric (0002) x-ray spectra of strain-relaxed samples show an additional signal which could be identified stemming from metallic tetragonal indium. The indium is incorporated in InGaN with its pseudohexagonal (101) plane parallel to the InGaN(0001) plane in a sixfold configuration. From the widths and intensities of the asymmetric In(211) and the symmetric In(h0h) diffraction peaks, the lateral and perpendicular crystallite sizes of the In inclusions are estimated to be ∼30 and ∼300 nm, respectively, i.e., the In is incorporated in a wire-like manner in the growth direction. In InGaN-based devices such indium wires could act as highly conducting channels detrimental for electronic and optoelectronic applications.

Effects of alloy potential fluctuations in InGaN epitaxial films

Results of photoluminescence and photoconductivity measurements in epitaxial films are presented. The photoluminescence peak energy and intensity show several anomalous behaviours. The peak energy changes with temperature exhibiting an inverted S-shape dependence, where it decreases, then increases with increasing temperature in the range 40-100 K and finally decreases with increasing temperature. The intensity shows a temperature dependence similar to that of amorphous semiconductors and disordered superlattices. A blue shift of the photoluminescence energy with increasing excitation intensity is observed. A large Stokes shift between the photoluminescence peak position and the band edge transition energy is found; it decreases with decreasing indium content. A persistent photoconductivity effect has been detected up to room temperature with a stretched-exponential function for its decay rate. All these observations can be explained in a consistent way by alloy potential fluctuations, and these clearly indicate the existence of compositional fluctuations. These two related effects thus appear to constitute the mechanism for the widely observed localized excitons in InGaN-based devices.