Armin Dadgar

Metalorganic Chemical Vapor Phase Epitaxy of Crack-Free GaN on Si (111) Exceeding 1 µm in Thickness

We present a simple method for the elimination of cracks in GaN layers grown on Si (111). Cracking of GaN on Si usually occurs due to large lattice and thermal mismatch of GaN and Si when layer thicknesses exceeds approximately 1 µm. By introducing thin, low-temperature AlN interlayers, we could significantly reduce the crack density of the GaN layer. The crack density is practically reduced to zero from an original crack density of 240 mm-2 corresponding to crack-free regions of 3×10-3 mm2. Additionally for the GaN layer with low temperature interlayers, the full width at half maximum X-ray (2024) rocking curve is improved from approximately 270 to 65 arcsec.

GaN-based optoelectronics on silicon substrates

Abstract Cracking of GaN on Si usually occurs due to the large thermal mismatch of GaN and Si when layer thicknesses exceed approximately 1 μm in metalorganic chemical vapor deposition (MOCVD) preventing the realization of device-quality material. The thermal stress can be reduced significantly by a combination of different concepts such as the insertion of low-temperature AlN interlayers, introducing multiple AlGaN/GaN interlayers, and growing on prepatterned substrates. The growth of crack-free GaN-based light emitting diodes (LEDs) on silicon on patterned Si(111) with areas of 100 μm×100 μm is reported

Thick, crack-free blue light-emitting diodes on Si(111) using low-temperature AlN interlayers and in situ SixNy masking

Thick, entirely crack-free GaN-based light-emitting diode structures on 2 in. Si(111) substrates were grown by metalorganic chemical-vapor deposition. The ∼2.8-μm-thick diode structure was grown using a low-temperature AlN:Si seed layer and two low-temperature AlN:Si interlayers for stress reduction. In current–voltage measurements, low turn-on voltages and a series resistance of 55 Ω were observed for a vertically contacted diode. By in situ insertion of a SixNy mask, the luminescence intensity is significantly enhanced. A light output power of 152 μW at a current of 20 mA and a wavelength of 455 nm is achieved.

GaN‐Based Devices on Si

Nowadays, GaN-based devices are usually grown on sapphire or silicon-carbide substrates. These are either insulating or very expensive and not available in large diameter. A well-conducting low-cost alternative is silicon also enabling the integration of optoelectronics or high-power electronics with Si-based electronics. The main problem limiting a fast progress of GaN growth on silicon is the thermal mismatch of GaN and Si leading to cracks even below device-relevant layer thicknesses. In the last few years, since the first demonstration of a molecular beam epitaxy grown GaN-based light emitting diode on Si in 1998 the activities in research of GaN on Si increased dramatically. Meanwhile, several concepts to lower stress, avoid cracks, and improve the material quality exist. Meanwhile the material quality has improved significantly and is about as good as on sapphire so that it is only a question of time until GaN-based devices on Si come into market. This article gives a review on the latest developments in group-III nitride growth on Si by metal organic vapor phase epitaxy.

MOVPE growth of GaN on Si(1 1 1) substrates

Metalorganic chemical vapor phase deposition of thick, crack-free GaN on Si can be performed either by patterning of the substrate and selective growth or by low-temperature (LT) AIN interlayers enabling very thick GaN layers. A reduction in dislocation density from 10 10 to 10 9 cm -2 is observed for LT-AIN interlayers which can be further improved using monolayer thick Si x N y in situ masking and subsequent lateral overgrowth. Crack-free AlGaN/GaN transistor structures show high room temperature mobilities of 1590 cm 2 /V s at 6.7×10 12 cm -2 sheet carrier concentration. Thick crack-free light emitters have a maximum output power of 0.42 mW at 498 nm and 20mA.

Arrays of vertically aligned and hexagonally arranged ZnO nanowires: a new template-directed approach

A new template-directed method for large-scale fabrication of hexagonally patterned and vertically aligned ZnO nanowires is demonstrated. The process involves a novel type of metal membrane, gold catalyst templates produced using the membrane as the deposition mask, and catalyst-guided vapour-phase growth of ZnO nanowires. The metal membranes, composed of hexagonal nanotube arrays, are electrochemically replicated from ordered porous alumina. The obtained ZnO nanowires are uniformly aligned perpendicular to the GaN surface and have a distribution according to the pattern defined by the nanotube membrane. We also demonstrate that by modifying the electrochemical parameters and growth conditions, the diameter of the nanowires can be varied in the range 30?110?nm.

Epitaxy of GaN on silicon?impact of symmetry and surface reconstruction

GaN-on-silicon is a low-cost alternative to growth on sapphire or SiC. Today epitaxial growth is usually performed on Si(111), which has a threefold symmetry. The growth of single crystalline GaN on Si(001), the material of the complementary metal oxide semiconductor (CMOS) industry, is more difficult due to the fourfold symmetry of this Si surface leading to two differently aligned domains. We show that breaking the symmetry to achieve single crystalline growth can be performed, e.g. by off-oriented substrates to achieve single crystalline device quality GaN layers. Furthermore, an exotic Si orientation for GaN growth is Si(110), which we show is even better suited as compared to Si(111) for the growth of high quality GaN-on-silicon with a nearly threefold reduction in the full width at half maximum (FWHM) of the -scan. It is found that a twofold surface symmetry is in principal suitable for the growth of single crystalline GaN on Si.

Recording of cell action potentials with AlGaN∕GaN field-effect transistors

An AlGaN∕GaN electrolyte gate field-effect transistor array for the detection of electrical cell signals has been realized. The low-frequency noise power spectral density of these devices exhibits a 1∕f characteristic with a dimensionless Hooge parameter of 5×10−3. The equivalent gate-input noise under operation conditions has a peak-to-peak amplitude of 15μV, one order of magnitude smaller than for common silicon-based devices used for extracellular recordings. Extracellular action potentials from a confluent layer of rat heart muscle cells cultivated directly on the nonmetallized gate surface were recorded with a signal amplitude of 75μV and a signal-to-noise ratio of 5:1.

High-sheet-charge–carrier-density AlInN∕GaN field-effect transistors on Si(111)

AlInN∕GaN heterostructures have been proposed to possess advantageous properties for field-effect transistors (FETs) over AlGaN∕GaN [Kuzmik, IEEE Electron Device Lett. 22, 501 (2001); Yamaguchi et al., Phys. Status Solidi A 188, 895 (2001)]. A major advantage of such structures is that AlInN can be grown lattice-matched to GaN while still inducing high charge carrier densities at the heterointerface of around 2.7×1013cm−3 by the differences in spontaneous polarization. Additionally, it offers a higher band offset to GaN than AlGaN. We grew AlInN FET structures on Si(111) substrates by metalorganic chemical vapor phase epitaxy with In concentrations ranging from 9.5% to 24%. Nearly lattice-matched structures show sheet carrier densities of 3.2×1013cm−2 and mobilities of ∼406cm2∕Vs. Such Al0.84In0.16N FETs have maximum dc currents of 1.33A∕mm for devices with 1μm gate length.

The origin of stress reduction by low-temperature AlN interlayers

Thin low-temperature AlN interlayers can be applied to reduce stress to grow thick crack-free AlGaN layers on GaN buffer layers on sapphire and thick crack-free GaN layers on Si. The mechanism leading to stress reduction is investigated by high resolution x-ray diffractometry measurements on metalorganic chemical vapor phase epitaxy grown samples on Si(111) with different interlayer deposition temperatures. A decrease of tensile stress with decreasing interlayer growth temperature is observed. From reciprocal space maps we conclude that interlayers grown at high temperatures are pseudomorphic, while grown at lower temperatures they are relaxed. Therefore, AlGaN or GaN layers grown on a low temperature AlN interlayer grow under compressive interlayer-induced strain. The stress in the GaN layer depends on the growth temperature that likely controls the amount of AlN interlayer relaxation.