G. Nataf

Temperature quenching of photoluminescence intensities in undoped and doped GaN

This work discusses the temperature behavior of the various photoluminescence (PL) transitions observed in undoped, n- and p-doped GaN in the 9–300 K range. Samples grown using different techniques have been assessed. When possible, simple rate equations are used to describe the quenching of the transitions observed, in order to get a better insight on the mechanism involved. In undoped GaN, the temperature dependence of band edge excitonic lines is well described by assuming that the A exciton population is the leading term in the 50–300 K range. The activation energy for free exciton luminescence quenching is of the order of the A rydberg, suggesting that free hole release leads to nonradiative recombination. In slightly p-doped samples, the luminescence is dominated by acceptor related transitions, whose intensity is shown to be governed by free hole release. For high Mg doping, the luminescence at room temperature is dominated by blue PL in the 2.8–2.9 eV range, whose quenching activation energy is in...

Growth of high quality crack-free AlGaN films on GaN templates using plastic relaxation through buried cracks

A method is presented to achieve thick high quality crack-free AlGaN layers on GaN. This method uses jointly plastic relaxation and lateral growth. In a first step, plastic relaxation by cracking and misfit dislocation introduction is realized. Then the cracks are overgrown to obtain a smooth surface. By this reproducible technique, we grew smooth metal-organic chemical vapor deposition Al0.2Ga0.8N films with a threading dislocation density as low as 5×108 cm−2. This result is the best ever reported for crack-free AlGaN growth over a large area. The control of the complete plastic relaxation opens up perspectives for the realization of high performance devices. In order to explain the mechanisms involved in the full relaxation of the AlGaN/GaN heterostructure, we propose a relaxation scheme and discuss its different steps.

Semipolar GaN films on patterned r-plane sapphire obtained by wet chemical etching

It is shown that (112¯2)-oriented GaN films can be achieved from r-sapphire patterned by chemical etching. Growth first occurs selectively from the inclined c-facet of sapphire, leading finally to a fully coalesced layer with (112¯2) orientation. The structural and optical quality of these layers was assessed by x-ray diffraction, cathodoluminescence and photoluminescence measurements. The results clearly show that the quality of (112¯2) GaN on patterned r-sapphire is markedly improved in comparison with (112¯2) GaN on m-sapphire.

Improved semipolar (112¯2) GaN quality using asymmetric lateral epitaxy

Semipolar (112¯2) GaN films were obtained by epitaxial lateral overgrowth from (112¯2) GaN templates patterned with SiO2 stripes 7 μm wide with 3 μm spacing, oriented along the [11¯00] GaN in-plane direction. The growth conditions were optimized in order to promote a fast growth rate along the +c [0001] direction. The crystal expands both laterally and vertically until a situation where it overgrows the adjacent crystal, thus stopping the propagation of stacking faults and threading dislocations. The growth anisotropy and filtering of defects is observed by cross-sectional scanning electron microscopy and cathodoluminescence. The lowering of defect density is confirmed by x-ray diffraction measurements. The photoluminescence spectrum of the coalesced epitaxial lateral overgrowth of the (112¯2) epilayers exhibits a strong band edge emission and a low emission band at 3.41 eV, assigned to the remaining stacking faults.

Defect structure in heteroepitaxial semipolar (11\bar {2} 2 ) (Ga, Al)N

The defect structures in semipolar (1122)-GaN, AlN layers grown on m-sapphire by metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) are characterized by transmission electron microscopy. The epitaxial relationships are identified as [1010](GaN) || [1120]sap and [1213](GaN) || [0001]sap. Defects are identified as mostly partial dislocations, I1-basal and prismatic stacking faults. The density of dislocations is of the order of 5.5 × 10(9) cm(-2). They are Frank-Shockley partial dislocations with b = 1/6<2023> (90%), Shockley partial dislocations with b = 1/3<1010> (8%) and perfect dislocations of a-type with b = 1/3<1120> (2%). This is in contrast with the growth in c- or a-orientations, where the large majority of extended defects consists of perfect dislocations. Upon MBE regrowth of GaN on MOVPE GaN, no additional defects are generated, although the defects in the substrate propagate through the overgrown layer. However, in the case of MBE deposition of AlN on MOVPE GaN, new threading dislocations of the type b = 1/3<1123> are generated taking stepped and curved structures along their lines.

Low temperature photoluminescence of n‐type GaInAsP layers grown on InP by liquid phase epitaxy

Layers of the quaternary alloy GaxIn1−xAsyP1−y (x∼0.1, y∼0.3) were grown on InP (100) substrates by super‐cooling liquid phase epitaxy. Tellurium was used for n‐type doping in the range 8×1017–5×1019 cm−3. A good homogeneity of the composition of the layers was obtained at the surface. No microprecipitates of tellurium were detected by scanning electron microscope. Systematic studies of luminescence at low temperature were used to characterize the samples, with a particular attention focused on the degeneracy of the conduction band. Calculations of the line shapes were performed based on a recombination mechanism involving acceptor‐like states of the valence band and conduction band filling effects. Good agreement was obtained between calculated and experimental line shapes. The Fermi level in quaternary alloys is found to be lower than predicted by theoretical calculations using band structure properties developed in the past for definite III‐V compounds such as GaAs and InP. It is suggested that this co...

Systematic study of near-infrared intersubband absorption of polar and semipolar GaN/AlN quantum wells

We report on the observation of intersubband absorption in GaN/AlN quantum well superlattices grown on (112¯2)-oriented GaN. The absorption is tuned in the 1.5–4.5 μm wavelength range by adjusting the well thickness. The semipolar samples are compared with polar samples with identical well thickness grown during the same run. The intersubband absorption of semipolar samples shows a significant red shift with respect to the polar ones due to the reduction of the internal electric field in the quantum wells. The experimental results are compared with simulations and confirm the reduction of the polarization discontinuity along the growth axis in the semipolar case. The absorption spectral shape depends on the sample growth direction: for polar quantum wells the intersubband spectrum is a sum of Lorentzian resonances, whereas a Gaussian shape is observed in the semipolar case. This dissimilarity is explained by different carrier localization in these two cases.

Efficient blocking of planar defects by prismatic stacking faults in semipolar (112¯2)-GaN layers on m-sapphire by epitaxial lateral overgrowth

For the next-generation solid state lighting, the production of high quality semipolar (112¯2) GaN layers on sapphire obtained using asymmetric epitaxial lateral overgrowth (ELO) method has been investigated. This type of ELO leads to efficient blocking of the basal stacking faults (BSFs) in the bulk, and enables the formation of nondefective layers at the surface. The BSFs terminate due to generation of prismatic stacking faults along a well defined boundary. The corresponding intensity of GaN band edge photoluminescence emission is increased by more than four orders of magnitude in comparison to that from semipolar templates.

Epitaxial growth of InAs1−xSbx alloys by MOCVD

InAs1−xSbx is one of the most promising alloys for achieving high performance III–V photodetectors. MOVPE of this solid solution from TEI, TMSb and AsH3 was successful both on (0001) Al2O3 and (111) InAs substrates (with respectively x < 0.1 and x ⩽ 0.23) from 520 to 580°C. Intermediate layers permit accomodation of the lattice mismatch between the substrate and the ternary alloy.

High-Efficiency Al0.2Ga0.8As/Si Stacked Tandem Solar Cells Using Epitaxial Lift-Off

In this paper, the potential of high conversion efficiency of solar energy through the combination of Al0.2Ga0.8As and Si cells is illustrated by a mechanically stacked tandem using, for the first time, the epitaxial lift-off (ELO) technique to remove the top cell from its substrate. The selective etching of the GaAs substrate provides efficient light transmission to the bottom cell. Photoluminescence decay experiments show that substrate removal also enhances photon recycling effects. The measured efficiency of the Al0.2Ga0.8As/Si tandem reaches 21% AM1.5.