AlInN Layers Research Articles

Microstructure and field mapping of AlInN‐based heterostructures and devices

AbstractThe microstructure, composition and electrostatic fields of near‐lattice‐matched Al1‐xInxN/AlN/GaN heterostructures and devices have been characterized using a variety of electron microscopy techniques. A thin parastic Ga‐rich layer was often observed immediately above the AlN spacer and was attributed to GaN nucleation during specimen cooling. Mapping of electrostatic potential profiles across a GaN/Al0.85In0.15N/AlN/ GaN heterostructure using off‐axis electron holography showed polarization‐induced fields of 7.5MV/cm within the AlN layer, and 2.2MV/cm within the Ga‐rich layer. A two‐dimensional electron gas with a density of ∼9.8×1012 cm–2 was observed in the underlying GaN layer located very close to the AlN/GaN interface. Contact inclusions were observed extending into the AlInN and GaN layers along mixed‐type threading dislocations under the source and drain regions of HFET devices. The density and size of the contact inclusions was determined by the annealing temperature (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural properties of MBE AlInN and AlGaInN barrier layers for GaN‐HEMT structures

AbstractA high‐resolution X‐ray diffraction and X‐ray reflectivity study of the structural properties of AlInN/GaN and AlGaInN/GaN high electron mobility transistor structures deposited by molecular beam epitaxy on metal organic chemical vapor deposition GaN/Al2O3 and GaN/SiC templates is presented. A new AlN/GaN/AlN triple‐interlayer is implemented to improve the interface properties between barrier layer and GaN buffer for a higher mobility of the polarization induced two‐dimensional electron gas. Layer properties and structural parameters like concentration, interface quality, layer thickness, strain and crystalline perfection are analyzed. Best structural properties are achieved for an AlGaInN layer with AlN/GaN/AlN interlayer deposited on a GaN/4H‐SiC (00.1) template.

Strain profiling of AlInN/GaN distributed Bragg reflectors using in situ curvature measurements and ex situ X-ray diffraction

In this work, we show the use of in situ curvature and X-ray measurements to evaluate the metal organic vapor phase growth process of distributed Bragg reflectors consisting of an AlInN/GaN system. At first, it is investigated how the initial wafer bow and the heating of the bare substrates influences the curvature measurements. The strain induced by thermal mismatch and lattice mismatch is discussed at the example of multilayer AlInN/GaN Bragg mirror structures on sapphire substrate. We will demonstrate that thereby an estimation of the composition in the AlInN layers is possible. Additionally the samples were examined with multiple X-ray diffraction techniques, as symmetrical and asymmetrical reciprocal space maps, grazing-incidence in-plane diffraction and X-ray reflectivity. These results are compared with the in situ measurements.

Ti/Al/Ni/Au Ohmic contacts for AlInN/AlN/GaN-based heterojunction field-effect transistors

The microstructure of AuNiAlTi/Al0.84In0.16N/AlN/GaN Ohmic contacts annealed from 700 to 900 °C has been determined using transmission electron microscopy and associated analytical techniques. Intermixing and phase separation of the metal contact layers was observed to degrade the surface roughness. An optimal contact performance was obtained for contacts annealed at 800 °C and was attributed to the formation of TiN contact inclusions that had penetrated through the AlInN layers into the GaN layers underneath. These TiN contact inclusions had an inverted mushroom shape with a density of ∼108 cm−2, and they were invariably located at the positions of mixed-type threading dislocations. These inclusion defects would act as a conduction path between the metal contacts and the two-dimensional electron gas of heterojunction field-effect transistor devices. The AlInN layer remained intact in dislocation-free areas of all samples.

Fabrication and characterisation of microscale air bridges in conductive gallium nitride

Fabrication and electrical characterisation of mi- croscale air bridges consisting of GaN heavily doped with silicon is described. These were made from GaN-AlInN- GaN epitaxial trilayers on sapphire substrates, in which the AlInN was close to the composition lattice matched to GaN at ∼17% InN fraction. The start of the fabrica- tion sequence used inductively coupled plasma etching with chlorine chemistry to define mesas. In situ monitoring by laser reflectometry indicated an AlInN vertical etch rate of 400 nm/minute, ∼70% of the etch rate of GaN. Process- ing was completed by lateral wet etching of the AlInN in hot nitric acid to leave GaN microbridges supported be- tween anchor posts at both ends. Deposition of Ti-Au con- tact pads onto the anchor posts allowed study of the elec- trical characteristics. At low applied voltages, vertical con- duction through the undoped AlInN layers was minimal in comparison with the current path through the Si:GaN bridges. Typical structures showed highly linear current- voltage characteristics at low applied voltages, and had re- sistances of 1050 � . The observed resistance values are compared with the predicted value based on materials para- meters and an idealised geometry. The microbridges showed damage from Joule heating only at current densities above 2 × 10 5 Ac m −2 .

Relaxation of compressively strained AlInN on GaN

Epitaxial layers of wurtzite-phase Al 1− x In x N, ∼120 nm thick with (0 0 0 1) orientation, were grown by metal organic chemical vapour deposition on GaN buffer layers at setpoint temperatures between 760 and 840 °C. For growth temperatures ⩾800 °C, the AlInN layers grew with uniform composition, pseudomorphic with the underlying GaN buffer layer. In the temperature range studied, the InN fractions are a linear function of the setpoint temperature and straddle the near-lattice-match composition around Al 0.83In 0.17N. Lowering the growth temperature to 760 °C caused a compositional grading, a marked change in surface morphology, and a reduction in AlInN crystal quality. The resulting AlInN layer consists of a compressively strained interfacial layer with a composition of Al 0.76In 0.24N, and a mostly relaxed near-surface layer with a composition of Al 0.81In 0.19N. Atomic force microscopy suggests that a transition to a three-dimensional growth mode accompanies the structural relaxation and change in composition.

Mechanisms of AlInN growth by MOVPE: modeling and experimental study

AbstractFactors affecting the AlInN layer composition have been studied, using both modelling and experimental approaches. The experiments have been performed in a single‐wafer Aixtron AIX 200/4 RF‐S reactor at setpoint temperatures ranging from 760 to 840 °C, which corresponds to the InN content variation in a wide interval from 13 to about 24%, if other process parameters remain unchanged. The computations fit well the compositional experimental data, and can be used to identify the mechanisms governing the incorporation of the indium atoms into MOVPE grown AlInN layers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and Luminescence Properties of Subsequently Grown AlInN Layers on AlN Homoepitaxial Layers by Ammonia Gas Source Molecular Beam Epitaxy

We performed the epitaxial growth of Al-rich Al1-xInxN (x<0.02) ternary alloys that have subsequently been grown on AlN epilayers by ammonia gas source molecular beam epitaxy (GS-MBE). The In composition x of Al1-xInxN ternary alloys was controlled by changing In/(In+Al) flux ratio without phase separation. The optical properties of these alloys were investigated using cathodoluminescence measurements as a function of the In composition x of Al1-xInxN thin films. Near-band-edge emissions in a deep ultraviolet region (5.87–6.04 eV) were clearly observed in high-quality Al1-xInxN epilayers at 77 K.

Thermal stability of metal organic vapor phase epitaxy grown AlInN

AlInN layers with a thickness of 100nm were grown by metal organic vapor phase epitaxy on GaN buffer layers on Si(111) substrates. By varying the growth temperature, In and NH3 flows, and reactor pressure, three series with different In contents were produced and thermally treated in the temperature range from 30to960°C. The as grown and annealed layers were investigated by x-ray diffraction in standard and grazing incidence geometry. Nearly lattice matched samples with an indium concentration of 17%–18% show long time stability at annealing temperatures as high as 960°C. At higher temperatures, the onset of severe Ga–Si meltback etching prevents further measurements. Nonlattice matched samples consist of pseudomorphic and relaxed parts. In the latter, a redistribution and loss of indium is observed upon annealing.

Selective wet etching of lattice-matched AlInN–GaN heterostructures

Wet etching of AlInN–GaN epitaxial heterostructures, containing AlInN layers with InN mole fractions close to 0.17 has been studied. One molar aqueous solution of the chelating amine 1,2-diaminoethane (DAE) proved to selectively etch the AlInN layers, without the need for heating above room temperature, or photo-assistance. In experiments with a (0 0 0 1)-oriented AlInN-on-GaN bilayer, the mode of removal of the AlInN layer was predominantly lateral etching, initiated from the sidewalls of pit defects in the AlInN layer. The lateral etch rate was estimated at ∼60 nm/h. The GaN buffer layer surface was roughened concurrently with etching of the AlInN, although the DAE solution has no effect on as-grown GaN (0 0 0 1) surfaces. The roughening of the GaN surface is tentatively attributed to the charge accumulation layer expected at the AlInN–GaN heterointerface. The DAE etchant also proved effective at removing buried AlInN layers from trilayer and more complex multilayer structures, leading to the prospect of epitaxial lift-off processes, and the fabrication of three-dimensional engineered microstructures. These capabilities were demonstrated by the production of suspended microdisk structures from a GaN–AlInN–GaN trilayer, using a combination of dry and wet etching.

Cathodoluminescence of rare earth implanted AlInN

AlInN layers implanted with europium and erbium ions are systematically studied and compared with similarly implanted GaN. Cathodoluminescence from four series of annealed samples shows that the Eu∕Er emissions from AlInN are considerably broader than those from GaN, while the peak positions only change slightly. The rate of increase of cathodoluminescence intensity with annealing temperature, up to 1300°C, is analyzed for all four series. For Eu the increase exceeds 10× in both hosts. Although some decomposition is observed for annealing at 1200°C, well above the growth temperature, AlInN is shown to be a surprisingly robust host for rare earth ions.

Use of AlInN layers in optical monitoring of growth of GaN-based structures on free-standing GaN substrates

When lattice matched to GaN, the AlInN ternary alloy has a refractive index ∼7% lower than that of GaN. This characteristic can be exploited to perform in situ reflectometry during epitaxial growth of GaN-based multilayer structures on free-standing GaN substrates, by insertion of a suitable Al0.82In0.18N layer. The real-time information on growth rates and cumulative layer thicknesses thus obtainable is particularly valuable in the growth of optical resonant cavity structures. We illustrate this capability with reference to the growth of InGaN∕GaN multiple quantum-well structures, including a doubly periodic structure with relatively thick GaN spacer layers between groups of wells. Al0.82In0.18N insertion layers can also assist in the fabrication of resonant cavity structures in postgrowth processing, for example, acting as sacrificial layers in a lift-off process exploiting etch selectivity between Al0.82In0.18N and GaN.

Selective oxidation of AlInN layers for current confinement in III–nitride devices

Highly selective oxidation of an AlInN interlayer buried in a GaN matrix is demonstrated. This technique was successfully applied to form current apertures in III-nitride light-emitting diodes (LEDs). GaN LEDs were grown by metal-organic vapor phase epitaxy with a lattice-matched AlInN layer inserted in the n-doped region of the device. Mesas were etched by Cl2∕Ar reactive ion etching to give access to the AlInN sidewalls. The sample was then oxidized anodically in a nitrilotriacetic acid solution. Using this technique, the AlInN layer was oxidized laterally up to 22μm deep while the surrounding GaN layers were kept unaffected. It was subsequently demonstrated that the oxidized AlInN layers are insulating and are therefore suitable for lateral current confinement in optoelectronic devices.

High‐current AlInN/GaN field effect transistors

AbstractWe present a study on AlInN/GaN field effect transistors (FETs) grown by metalorganic chemical vapor phase epitaxy. AlInN can be grown lattice‐matched to GaN with an In concentration of 18%. In this study samples with In concentrations ranging from 9.5 to 24%, covering a range from tensely to compressively strained AlInN layers, were grown on GaN layers on Si(111). From Hall effect and capacitance‐voltage measurements we find high sheet carrier densities for most of the samples indicating a high electron density at the AlInN/GaN heterointerface. This is also reflected in the behavior of processed FETs. Nearly lattice‐matched structures show sheet carrier densities of 3.2 × 1013 cm–2 and mobilities up to ∼406 cm2/Vs. Such Al0.84In0.16N FETs have maximum DC currents of 1.33 A/mm for devices with 1 µm gate length and 100 µm gate width and an output power of 2.5 W/mm at 2 GHz. The best devices with In concentrations of 19% show maximum output powers of 4.1 W/mm at 2 GHz. In contrast to that a compressively strained AlInN layer with an In concentration of 24% leads to a decreased polarization charge at the heterointerface and a low DC current of ∼70 mA/mm. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Growth and characterization of AlInN on AlN template

AlInN layers were directly grown on AlN/α-Al 2O 3 (0 0 0 1) epitaxial templates by metalorganic vapor phase epitaxy. Indium incorporation coefficient was decreased with increasing the growth temperature. The crystalline quality and the composition of the grown films were strongly affected with the growth temperature and large compositional splitting was observed at 750 °C. Both photoluminescence and cathodoluminescence showed very complicated spectra in the photon-energy range of 2.4–4.3 eV, and the origin of these emissions were caused by a potential minima formed by compositional inhomogeneity. Absorption edge for the Al 0.91In 0.09N was larger than 4.3 eV.

Growth of high quality GaN epilayer on AlInN/GaN/AlInN/GaN multilayer buffer and its device characteristics

The role of AlInN 1st /GaN/AlInN 2nd /GaN multi-layer buffer (MLB) on the growth of the high quality GaN epilayers was demonstrated by atomic force microscope (AFM), X-ray diffraction (XRD), photoluminescence, and Hall measurement. The surface morphology and crystalline quality of GaN epilayers were considerably dependent on AlInN layers thicknesses rather than those of GaN inter layers. With optimal thickness of 2 nd AlInN layer, the pit density of GaN epilayers was substantially reduced. Also, the RMS roughness of the well ordered terraces generated on the GaN surface was 1.8 A at 5 x 5 μm 2 . The omega-rocking width of GaN (0002) Bragg peak and Hall mobility of GaN epilayers grown on AlInN 1st /GaN/AlInN 2nd /GaN MLB were 190 arcsec and 500 cm 2 /Vs, while those values of GaN epilayers on single GaN buffer layer were 250 arcsec and 250 cm 2 /Vs, respectively. Especially, the light output power and operating voltage of the fabricated light emitting diodes with this new buffer layer was about 5 mW and 3.1 V (dominant luminous wavelength -460 nm) at 20 mA, respectively.