Underlying GaN Layer Research Articles

Applications of TEM imaging, analysis and electron holography to III-nitride HEMT devices

This paper reviews some of our recent studies of III-nitride high electron mobility transistor (HEMT) devices using advanced electron microscopy methods. Sample preparation protocols have been developed that can routinely provide thin, electron-transparent specimen regions of uniform thickness extending across entire HEMT devices. The use of focused-ion-beam (FIB) thinning facilitated access to specific device regions, although structural damage and imaging artifacts can result unless suitable precautions are taken during milling to minimize ion-beam damage. The extent of gallium-ion sidewall implantation during FIB milling, and surface damage caused by deposition of Pt protective layers, have been assessed. As-processed device structures have been examined by conventional diffraction contrast imaging as well as high-resolution phase contrast imaging, while nanospectroscopy and nanoscale elemental mapping have been used to measure local variations in chemical composition. Annealing of Ti/Al/Ni/Au ohmic contacts for AlInN/AlN/GaN devices lead to the formation of TiN contact inclusions that are invariably located at mixed-type threading dislocations originating from the underlying GaN layers. Some preliminary observations of device structures after extended periods of operation and after device failure have also been made. The technique of off-axis electron holography has been used to quantify two-dimensional electrostatic fields within cross-sectioned devices with nanometer-scale resolution. Polarization fields of 6.9 MV/cm and a two-dimensional electron gas of ∼2.1 × 10 13/cm 2 have been measured for an AlInN/AlN/GaN heterostructure. Methods suitable for in situ biasing of HEMT samples during electron holography observations have also been explored.

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)

Dislocation Filtering in GaN Nanostructures

Dislocation filtering in GaN by selective area growth through a nanoporous template is examined both by transmission electron microscopy and numerical modeling. These nanorods grow epitaxially from the (0001)-oriented GaN underlayer through the approximately 100 nm thick template and naturally terminate with hexagonal pyramid-shaped caps. It is demonstrated that for a certain window of geometric parameters a threading dislocation growing within a GaN nanorod is likely to be excluded by the strong image forces of the nearby free surfaces. Approximately 3000 nanorods were examined in cross-section, including growth through 50 and 80 nm diameter pores. The very few threading dislocations not filtered by the template turn toward a free surface within the nanorod, exiting less than 50 nm past the base of the template. The potential active region for light-emitting diode devices based on these nanorods would have been entirely free of threading dislocations for all samples examined. A greater than 2 orders of magnitude reduction in threading dislocation density can be surmised from a data set of this size. A finite element-based implementation of the eigenstrain model was employed to corroborate the experimentally observed data and examine a larger range of potential nanorod geometries, providing a simple map of the different regimes of dislocation filtering for this class of GaN nanorods. These results indicate that nanostructured semiconductor materials are effective at eliminating deleterious extended defects, as necessary to enhance the optoelectronic performance and device lifetimes compared to conventional planar heterostructures.

Effect of Al0.06Ga0.94N/GaN Strained-Layer Superlattices Cladding Underlayer to InGaN-Based Multi-Quantum Well Grown on Si(111) Substrate with AlN/GaN Intermediate Layer

We report effect of the insertion of Al0.06Ga0.94N/GaN strained-layer superlattices (SLSs) cladding underlayer to InGaN-based multi-quantum well (MQW) structure grown on Si(111) substrate with AlN/GaN intermediate layer. The Al0.06Ga0.94N/GaN SLS underlayer improves emission wavelength uniformity and shows a narrower emission full-width at half-maximum (FWHM) than that of conventional GaN underlayer. A Gaussian fitting was performed to photoluminescence (PL) spectra to obtain emission wavelength behavior and integrated intensity of the peak energy. A higher MQW internal quantum efficiency (ηiqe) of 29.4% is obtained in sample with Al0.06Ga0.94N/GaN SLS underlayer than that of 20.6% in GaN underlayer. Transmission electron microscopy (TEM) analysis indicates that the Al0.06Ga0.94N/GaN SLS layer abruptly bends threading dislocation lines thus reduces threading dislocation density to the MQW. Reciprocal space mapping (RSM) suggests that compressive strain in the GaN portion of Al0.06Ga0.94N/GaN SLS forces the abrupt inclination of the threading dislocations.

Photoluminescence of cubic InN films on MgO (001) substrates

AbstractWe have studied photoluminescence from cubic InN films grown on MgO substrates with a cubic GaN underlayer by RF N2 plasma molecular beam epitaxy. A single PL peak was observed at 0.47 eV. By analyzing the reflectance spectra of cubic InN films, we could derive the refractive index and extinction coefficient, and found the band gap energy of cubic InN is 0.48 eV, indicating that the PL peak observed at 0.47 eV is due to the interband transition of cubic InN. The difference in the PL peak energy between hexagonal and cubic InN is in good agreement with that predicted by ab‐initio calculations. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence-excitation spectroscopy as a highly sensitive probe for carrier transport processes affected by surface damages in AlxGa1−xN/GaN heterostructures

We demonstrate that photoluminescence-excitation (PLE) spectroscopy can probe with high sensitivity the effects of plasma-induced surface damages on photogenerated-carrier-transport processes in AlxGa1−xN/GaN heterostructures, on the basis of systematic optical and structural characterization results for the as-grown reference sample and the plasma-exposed sample. It is found from the structural characterizations with atomic force microscopy that the plasma exposure remarkably modifies the atomic step boundaries and the pits on the AlxGa1−xN surface, which leads to a remarkable difference between the PLE spectra of the bound exciton photoluminescence from the underlying GaN layer in the two samples. The PLE spectrum of the reference sample shows a step rising from the AlxGa1−xN fundamental transition energy toward the high energy side, whereas the rising step disappears in the PLE spectrum of the plasma-exposed sample. In contrast, the reflectance characteristics are the same in the two samples; i.e., the excitonic transition itself is not influenced by the plasma exposure. The present findings indicate that the PLE spectral profile is sensitive to the change in efficiency of the photogenerated carrier injection from the AlxGa1−xN layer to the GaN layer. Thus, it is concluded that the PLE characterization is effective to probe the photogenerated-carrier transport in heterostructures.

Al composition dependent structural and electrical properties of InAlGaN/GaN heterostructures

InAlGaN/GaN heterostructures with various Al compositions have been grown on sapphire substrate using the metal organic chemical vapour deposition technique. The solid-to-gas phase ratio indicates a high Al incorporation efficiency. Atomic force microscopy reveals a smooth surface with the formation of hexagonal pits. The size and the density of the hexagonal pits increase with increasing Al mole fraction. The Hall effect and the capacitance–voltage (C–V) studies show the formation of a two-dimensional electron gas (2DEG) at the InAlGaN/GaN interface. A relatively higher Hall sheet carrier density compared with the 2DEG density estimated from the C–V profile indicates parallel conduction via the underlying GaN layer. It is observed that the 2DEG density decreases as a function of the Al composition and these results are discussed based on the increasing depth of the hexagonal pit and the background donor density.

Observation of ultrahigh density InGaN quantum dots

High density InGaN quantum dots (QDs) grown on an underlying GaN layer that was partially masked with SiNx nanocrystals were investigated by cross-sectional high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy, and energy dispersive x-ray spectroscopy. The layer of SiNx masks appeared as a dark line in the HAADF-STEM images, and from the thickness of that dark line, the height of the masks was roughly estimated to be less than 2nm. The InGaN QDs appeared as bright triangles in the HAADF-STEM images. The QDs can be regarded as nanosized island crystals consisting of {101¯1} sidewalls, with a height of several nanometers. The lattices in the InGaN crystals were strained as compared to the underlying and the capping GaN lattices and contacted them coherently.

Scattering analysis of 2DEG carrier extracted by QMSA in undoped Al0.25Ga0.75N/GaN heterostructures

Hall effect measurements on undoped Al0.25Ga0.75N/GaN heterostructures grown by a metalorganic chemical vapour deposition (MOCVD) technique have been carried out as a function of temperature (20–350 K) and magnetic field (0–1.5 T). Magnetic field dependent Hall data were analysed using the quantitative mobility spectrum analysis (QMSA) technique. The mobility and density within the two-dimensional electron gas (2DEG) at the Al0.25Ga0.75N/GaN interface and within the underlying GaN layer were successfully separated by QMSA. Mobility analysis has been carried out using both the measured Hall data at a single field and the extracted data from QMSA. Analysis of the temperature-dependent mobility of 2DEG extracted from QMSA indicates that the interface roughness and alloy disorder scattering mechanisms are the dominant scattering mechanisms at low temperatures while at high temperatures only polar optical phonon scattering is the dominant mechanism. Al0.25Ga0.75N/GaN interface related parameters such as well width, deformation potential constant and correlation length were also accurately obtained from the fits of the simple analytical expressions of scattering mechanisms to the 2DEG mobility.

The growth of high-Al-content InAlGaN quaternary alloys by RF-MBE

High-Al-content InxAlyGa1−x−yN (x = 1–10%, y = 34–45%) quaternary alloys were grown on sapphire by radio-frequency plasma-excited molecular beam epitaxy. Rutherford back-scattering spectrometry, high resolution x-ray diffraction and cathodoluminescence were used to characterize the InAlGaN alloys. The experimental results show that InAlGaN with an appropriate Al/In ratio (near 4.7, which is a lattice-match to the GaN under-layer) has better crystal and optical quality than the InAlGaN alloys whose Al/In ratios are far from 4.7. Some cracks and V-defects occur in high-Al/In-ratio InAlGaN alloys. In the CL image, the cracks and V-defect regions are the emission-enhanced regions.

RF-MBE growth of a-plane InN on r-plane sapphire with a GaN underlayer

a-Plane InN films were grown on r-plane sapphire substrates with GaN underlayers by RF-N 2 plasma molecular beam epitaxy (MBE). X-ray diffraction (XRD) measurements and reflection high-energy electron diffraction (RHEED) observation revealed that the InN films were grown with InN (1 1 2¯ 0)∥GaN (1 1 2¯ 0)∥sapphire (2 2¯ 0 4). We have carried out micro-Raman scattering measurements for a-plane InN film. Raman peaks were observed at 448, 490 and 598 cm −1, which can be identified as the A 1 transversal optical (TO), E 2 (high) and E 1 longitudinal optical (LO) mode phonon, respectively. We also carried out photoluminescence (PL) measurements for a-plane InN film. Strong PL was observed between 0.62 and 0.65 eV, which is the lowest ever reported for InN. It is suggested from the excitation intensity and temperature dependence of the PL spectra that the red shift of the PL peak is due to the Franz–Keldysh effect.

Photoluminescence and persistent photoconductivity of AlxGa1-xN/GaN heterostructures

We have investigated the optical properties of AlxGa1-xN/GaN heterostructures (x=0.08, 0.15, 0.33) grown by metal organic chemical vapor deposition on sapphire using photoluminescence (PL) and persistent photoconductivity (PPC) measurements. For the AlxGa1-xN/GaN heterostructures (HS) containing high Al composition, we observed an anomalous temperature-dependent photoluminescence and persistent photoconductivity effects. These results show a strong dependence of the physical properties of AlxGa1-xN/GaN HS on the Al content and layer thickness. The anomalous temperature-dependent PL is usually attributed to the presence of carrier localization states. These phenomena are explained based on the alloy compositional fluctuations in the AlxGa1-xN/GaN HS. From the PPC measurements, the photocurrent (PC) quenching was observed for AlxGa1-xN/GaN HS and it is explained by the metastable states formed in the underlying GaN layer. Also, the mechanisms behind the PC quenching and PPC phenomena are explained in detail.

Fabrication of GaN nanowire arrays by confined epitaxy

The authors report the fabrication of GaN nanowire arrays inside a thick SiNx, selective growth mask that was patterned by interferometric lithography and dry etching. The GaN nanowires are molded by the apertures in the selective growth mask and the growth is epitaxial with respect to the underlying GaN layer. The precise location and diameter of each nanowire in the array are controlled by the growth mask patterning, and the resulting array has a long-range order that is compatible with photonic crystal applications. This process uses conventional metal organic precursors and does not require any additional metal catalysts.

Hydrostatic pressure dependence of polarization-induced interface charge in AlGaN∕GaN heterostructures determined by means of capacitance-voltage characterization

The hydrostatic pressure dependence of the polarization-induced interface charge in an AlGaN∕GaN heterostructure is investigated via capacitance-voltage (C-V) characterization of the two-dimensional electron gas (2DEG) at the heterojunction. The linear pressure coefficient of (3.3±0.4)×1011cm−2GPa−1 of the polarization-induced interface charge as obtained via C-V measurements of the investigated (28nmAl0.26Ga0.74N)∕GaN heterostructure agrees well with existing theoretical and experimental data (obtained via Hall-effect measurements). It is demonstrated that C-V characterization can be applied even when Hall-effect measurements of the 2DEG are inconclusive because of parallel conduction via the underlying GaN layer. In addition, we discuss the influence on the 2DEG concentration of interface trap states, which introduce a dependence of the measured 2DEG concentration on C-V test frequency. It is shown, however, that this effect does not influence the value of the pressure coefficient of the polarization-induced interface charge as determined via C-V characterization. Collaterally, the applicability of capacitance measurements of AlGaN∕GaN heterostructures for pressure sensing applications is confirmed.

Optical and structural properties of GaN nanopillar and nanostripe arrays with embedded InGaN∕GaN multi-quantum wells

GaN nanopillar and nanostripe arrays with embedded InGaN∕GaN multi-quantum wells (MQWs) were fabricated by holographic lithography and subsequent reactive ion etching. Etch related damage of the nanostructures was successfully healed through annealing in NH3∕N2 mixtures under optimized conditions. The nanopatterned samples exhibited enhanced luminescence in comparison to the planar wafers. X-ray reciprocal space maps recorded around the asymmetric (101¯5) reflection revealed that the MQWs in both nanopillars and nanostripes relaxed after nanopatterning and adopted a larger in-plane lattice constant than the underlying GaN layer. The pillar relaxation process had no measurable effect on the Stokes shift typically observed in MQWs on c-plane GaN, as evaluated by excitation power dependent photoluminescence (PL) measurements. Angular-resolved PL measurements revealed the extraction of guided modes from the nanopillar arrays.

Improvement in the characteristics of GaN-based light-emitting diodes by inserting AlGaN-GaN short-period superlattices in GaN underlayers

We report the influence of short-period superlattice (SPSL)-inserted structures in the underlying undoped GaN on the characteristics of GaN-based light-emitting diodes (LEDs). The measurements of current-voltage (I-V) curves indicate that GaN-based LEDs having pseudomorphic Al/sub 0.3/Ga/sub 0.7/N(2 nm)-GaN(2 nm) SPSL-inserted structures exhibit improvements in device characteristics with the best LED being inserted with two sets of five-pair Al/sub 0.3/Ga/sub 0.7/N(2 nm)-GaN(2 nm) SPSL structure. Based upon the results of etch pit counts, double-crystal X-ray diffraction measurements and transmission electron microscopic observations of the GaN-based LEDs, it was found that the Al/sub 0.3/Ga/sub 0.7/N(2 nm)-GaN(2 nm) SPSL-inserted structures tended to serve as threading dislocation filters in the LEDs so that the improved I-V characteristics were achieved.

RF‐MBE growth of cubic InN films on MgO (001) substrates

AbstractCubic InN films have been grown on MgO substrates with cubic GaN underlayers by RF‐N2 plasma MBE. By changing the growth conditions, we clarified the growth temperature and In flux dependence of the quality of cubic InN films. It was found the surface of cubic InN films grown at relatively higher temperatures was smooth and that the hexagonal phase content in the InN films decreased with increasing In flux. Based on the findings, we have successfully obtained a c‐InN film with high phase‐purity and a smooth surface. We have carried out micro Raman scattering measurements for the high‐quality cubic InN film. Raman peaks were clearly observed at 596 cm–1 and 467 cm–1, which are attributed to longitudinal optical and transverse optical phonon modes of cubic InN, respectively. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

X-ray diffraction reciprocal lattice space mapping ofa-plane AlGaN on GaN

In this study, the anisotropic strain in a-plane AlGaN on GaN was investigated by X-ray diffraction (XRD) analysis using AlGaN/GaN heterostructure grown on r-plane sapphire. An a-plane GaN layer is compressively strained, particularly in the m-axis direction. According to XRD reciprocal lattice space mapping, the AlGaN layer was strained under tensile stress and grown almost coherently to the underlying GaN layer. The tensile stress in the AlGaN layer in the c-axis direction is approximately 1.7 times larger than that in the m-axis direction. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Anisotropically Biaxial Strain in a-Plane AlGaN on GaN Grown on r-Plane Sapphire

In this study, the anisotropically biaxial strain in a-plane AlGaN on GaN is investigated by X-ray diffraction (XRD) analysis using an AlGaN/GaN heterostructure grown on r-plane sapphire. In accordance with XRD reciprocal lattice space mapping, when the AlN molar fraction x in the AlGaN layer is 0.18, the AlGaN layer is fully strained under tensile stress and grows coherently on the underlying GaN layer. However, when x is as large as 0.31, partial relaxation is observed only in the c-axis direction. The tensile stress in the AlGaN layer is calculated taking the actual in-plane lattice constants of the underlying GaN layer into account, and it was found that the stress in the a-plane AlGaN layer in the c-axis direction is approximately 1.7 times larger than that in the m-axis direction.

Fine-structure N-polarity InN∕InGaN multiple quantum wells grown on GaN underlayer by molecular-beam epitaxy

We have succeeded in the growth of very-fine-structure InN∕InGaN (3–16nm∕9nm) multi-quantum wells (MQWs) on GaN underlayer and characterized them by transmission electron microscopy (TEM), high-resolution x-ray diffraction (XRD), and photoluminescence (PL) at 13K. Clear satellite diffraction peaks and sharp heterointerfaces were observed by XRD and TEM, respectively. A single PL-emission peak at 1.75μm was observed in the ten-periods InN(16.1nm)∕In0.67Ga0.33N(9.2nm) MQWs. The use of the InGaN as a barrier layer instead of GaN resulted in remarkable reduction of lattice mismatch between the well and barrier, which was essential for the fabrication of MQWs with superior interface quality. This successful growth of fine-structure InN∕InGaN MQWs would be an important step for the application of InN in optical communication devices.