L. Lahourcade

Plasma-assisted molecular-beam epitaxy of AlN(112¯2) on m sapphire

The authors report on the plasma-assisted molecular-beam epitaxy of semipolar AlN(112¯2) films on (11¯00) m-plane sapphire. AlN deposited on m sapphire settles into two main crystalline orientation domains, AlN(112¯2) and AlN(101¯0), whose ratio depends on the III/V ratio. The in-plane epitaxial relationships of AlN(112¯2) on m-plane sapphire are [112¯3¯]AlN‖[0001]sapphire and [11¯00]AlN‖[112¯0]sapphire. In the case of AlN(101¯0), the in-plane epitaxial relationships were [12¯10]AlN‖[0001]sapphire and [0001]AlN‖[112¯0]sapphire. Growth under moderate nitrogen-rich conditions enables them to isolate the (112¯2) orientation and to improve the surface morphology of the layers.

Ga kinetics in plasma-assisted molecular-beam epitaxy of GaN(112¯2): Effect on the structural and optical properties

GaN directly deposited on m-sapphire by plasma-assisted molecular-beam epitaxy settles into two main crystalline orientation domains: GaN﴾112¯2﴿ and GaN{101¯3}. The dominant phase is GaN﴾112¯2﴿ with GaN║ sapphire and GaN║ sapphire in-plane epitaxial relationships. Deposition of GaN on top of an AlN﴾112¯2﴿ buffer layer and growth under slightly Ga-rich conditions reduce GaN{101¯3} precipitates below the detection limits. Studies of Ga adsorption demonstrate that it is possible to stabilize up to one Ga monolayer on the GaN﴾112¯2﴿ surface. The presence of this monolayer of Ga excess on the growth front reduces the ﴾112¯2﴿ surface energy and hence minimizes the surface roughness. Photoluminescence from two-dimensional GaN﴾112¯2﴿ layers is dominated by a near-band-edge emission, which is assigned to excitons bound to stacking faults, present with a density around 3x105 cm−1.

Interband and intersubband optical characterization of semipolar (112¯2)-oriented GaN/AlN multiple-quantum-well structures

We report on semipolar GaN/AlN multiple-quantum-well structures grown on m-plane sapphire by plasma-assisted molecular-beam epitaxy. Optical investigation confirms a significant reduction in the quantum-confined Stark effect, in agreement with theoretical calculations, which predict an internal electric field between 0.6 and −0.55 MV/cm in the quantum wells, depending on the strain state. With respect to polar materials, the reduction in the internal electric field results in a substantial redshift of the intersubband energy.

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.

Morphology and strain of self-assembled semipolar GaN quantum dots in (112¯2) AlN

GaN quantum dots (QDs) grown in semipolar (112¯2) AlN by plasma-assisted molecular-beam epitaxy were studied by transmission electron microscopy (TEM) and scanning transmission electron microscopy techniques. The embedded (112¯2)-grown QDs exhibited pyramidal or truncated-pyramidal morphology consistent with the symmetry of the nucleating plane, and were delimited by nonpolar and semipolar nanofacets. It was also found that, in addition to the (112¯2) surface, QDs nucleated at depressions comprising {101¯1} facets. This was justified by ab initio density functional theory calculations showing that such GaN/AlN facets are of lower energy compared to (112¯2). Based on quantitative high-resolution TEM strain measurements, the three-dimensional QD strain state was analyzed using finite-element simulations. The internal electrostatic field was then estimated, showing small potential drop along the growth direction, and limited localization at most QD interfaces.

Mg doping and its effect on the semipolar GaN(112¯2) growth kinetics

We report the effect of Mg doping on the growth kinetics of semipolar GaN(112¯2) synthesized by plasma-assisted molecular-beam epitaxy. Mg tends to segregate on the surface, inhibiting the formation of the self-regulated Ga film which is used as a surfactant for the growth of undoped and Si-doped GaN(112¯2). We observe an enhancement of Mg incorporation in GaN(112¯2) compared to GaN(0001). Typical structural defects or polarity inversion domains found in Mg-doped GaN(0001) were not observed for the semipolar films investigated in the present study.

Stranski-Krastanow growth of "112 ¯ 2…-oriented GaN/AlN quantum dots

Semipolar GaN(112¯2) deposited on AlN(112¯2) by plasma-assisted molecular-beam epitaxy can follow the Frank–Van der Merwe or the Stranski–Krastanow growth mode as a function of the Ga/N ratio. N-rich grown GaN relaxes elastically at a critical thickness but the resulting GaN islands present multiple crystallographic orientations. In contrast, after deposition of a few two-dimensional GaN monolayers under Ga-rich conditions, a growth interruption in vacuum induces (112¯2)-oriented islanding. Applying this latter procedure, we have synthesized GaN/AlN quantum dot superlattices with reduced internal electric field.

Extended Crystallographic Defects in Gallium Nitride

In the fabrication GaN-based devices, several growth orientations are currently under investigation in order to exploit material properties which are inaccessible using layers grown along the c-axis. However, such procedures rely on foreign substrates with large misfits. Therefore, complex crystallographic defects form in the epitaxial layers and have been the subject of extensive studies. They include threading dislocations and stacking faults, which can be within basal or prismatic planes. Out of the c-axis, depending on the growth orientation, the glide planes of perfect dislocations may be no longer available, complicating the relaxation processes.

Polar and semipolar III-nitrides for long wavelength intersubband devices

Extending the intersubband transitions in III-nitride nanostructures from near-infrared to longer wavelengths might have significant consequences for critical applications like imaging, remote sensing and mine detection. In this work, we analyze the potential of polar and semipolar AlGaN/GaN technologies for this relevant spectral range.

Extended defects in semipolar (11-22) gallium nitride

Semipolar (11-22 ) GaN layers grown by metalorganic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) are studied by transmission electron microscopy (TEM). The layers exhibit numerous defects with different geometries in comparison to the growth along the c-axis, they are identified as mostly partial dislocations, basal and prismatic stacking faults. The dislocations density is in the order of 5x109 cm-2, the corresponding Burger vectors are b = 1/6 <20-23>, b= 1/3 <10-10> and a small fraction of perfect a type dislocations with b = 1/3 <11-20> has been observed. The basal stacking fault density is in the order of 1x106 cm-1. In an attempt to reduce the defect density, SixNy interlayers have been used as nanomasks for epitaxial lateral overgrowth, our analysis shows that this leads to a quite small reduction of the defects as compared to the starting layer.