F. Schulze

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.

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.

Metalorganic vapor phase epitaxy grown InGaN∕GaN light-emitting diodes on Si(001) substrate

We present GaN-based light emitting diode structures on a Si(001) substrate. The 2.3μm thick, crack-free layers were grown by metalorganic vapor phase epitaxy using a high-temperature AlN seed layer and 4° off-oriented substrates. This allows us to grow a flat, fully coalesced, and single crystalline GaN layer on Si(001). For preventing crack formation, four AlN interlayers were inserted in the buffer structure. The optically active layers consist of five-fold InGaN∕GaN multiple quantum wells showing a bright electroluminescence at 490nm at room temperature. The crystallographic structure was analyzed by x-ray diffraction measurements and the optical properties were determined by photo- and electroluminescence.

Influence of buffer layers on metalorganic vapor phase epitaxy grown GaN on Si(001)

GaN layers grown by metalorganic vapor phase epitaxy on Si(001) substrates were investigated by x-ray analysis and scanning electron microscopy. Several sample series were grown changing the AlN/GaN buffer layer deposition temperature, sequence, and thickness. By variation of the buffer layer structure, two different growth orientations could be realized. First, GaN grows c-axis oriented on the Si(001) substrates with two rotational alignments. Second, the r plane (1012) of the hexagonal GaN-structure is oriented parallel to the surface. In the latter case, four rotational in-plane alignments are observed. By a miscut (2°–6° off) of the Si substrates one of these alignments is preferred.

Bright, Crack-Free InGaN/GaN Light Emitters on Si(111)

Crack-free, up to 2.8 μm thick GaN-based light emitting diodes were grown by metalorganic chemical vapor deposition on 2-inch Si(111) substrates. Elimination of cracks was achieved by using two ∼12 nm thick low-temperature AlN: Si interlayers for stress reduction. A significant enhancement in optical output power was obtained by an in situ insertion of a Si x N y mask. Transmission electron microscopy measurements showed a tenfold reduction in dislocation density to ∼10 9 cm -2 by the low-temperature AlN and Si x N y interlayers, resulting in a significant increase in luminescence intensity. Vertically contacted diodes showed a light output power of 152 μW at a current of 20 mA and a wavelength of 455 nm. Turn-on voltages around 2.8 V and series resistances of 55 Ω were obtained.

Unstrained InAlN/GaN HEMT structure

InAlN has been investigated as barrier layer material for GaN-HEMT structures, potentially offering higher sheet charge densities (Kuzmik, 2002) and higher breakdown fields (Kuzmik, 2001). Lattice matched growth of the barrier layer can be achieved with 17% in content, avoiding piezo polarization. In this configuration the sheet charge density is only induced by spontaneous polarization. First experimental results of unpassivated undoped samples realized on 111-Si substrate exceed a DC output current density of 1.8 A/mm for a gate length of 0.5 /spl mu/m. Small signal measurements yield a f/sub t/ = 26 GHz and f/sub max/ = 14 GHz, still limited by the residual conductivity of the Si-substrate. A saturated output power at 2 GHz in class A bias point yielded a density of 4.1W/mm at V/sub DS/ = 24 V.

Epitaxy of GaN LEDs on large substrates: Si or sapphire?

We present first results on the limits of GaN growth on large diameter sapphire and the challenges that have to be solved for a successful growth of high power LEDs on silicon substrates. Up to 5.4 μm thick crack-free GaN on Si(111) LED structures were grown by metalorganic chemical vapor phase epitaxy. The FWHM of the GaN (0002) ω scan in x-ray diffraction amounts to 380 arcsec. On Si substrates, we achieve low curvatures with radii > 50 m, which is important for a successful processing of the samples on large diameter substrates. Additionally, a low curvature during InGaN multi-quantum-well growth is achieved and enables the growth of homogenous InGaN layers. The main difficulty for GaN-on-Si is light extraction, which leads to an approximately three- to four-fold reduction in direct comparison with GaN LEDs on sapphire.

Bright blue to orange photoluminescence emission from high-quality InGaN/GaN multiple-quantum-wells on Si(111) substrates

Metalorganic-chemical-vapor-phase-deposition-grown InGaN/GaN multiple-quantum-wells on Si(111) substrates were studied by high-resolution x-ray diffractometry (HRXRD) and photoluminescence (PL). By varying the quantum well deposition parameters, growth time, growth temperature, and In flow rate, systematic changes of the quantum well PL were found. The luminescence peak wavelengths and the corresponding intensities depend monotonically on each of these varied growth parameters. A considerable shift of the PL peak wavelength from blue (442 nm) to orange emission (649 nm) was achieved by decreasing the InGaN deposition temperature from 790 to 720 °C. HRXRD analysis shows changes in structural quality with InGaN growth temperature.

Strains and Stresses in GaN Heteroepitaxy - Sources and Control

We present a study of the sources of strain in GaN heteroepitaxy by in- and ex-situ measurement techniques. With an in-situ curvature measurement technique the strain development can be directly correlated to the different layers and doping in simple and device structures. We show several solutions for strain reduction and control. High-quality devices grown on Si are demonstrated.

Nearly strain-free AlGaN on (0 0 0 1) sapphire : X-ray measurements and a new crystallographic growth model

Two micrometer thick Al x Ga 1-x N layers with 0 < x < 0.4 were grown by low-pressure metal organic chemical vapour deposition on sapphire (0001) substrates. For Al concentrations 0.18 < x < 0.25 the layers are found to be nearly strain-free as determined by high resolution X-ray reciprocal space mapping around the (0002), (20 - 24), and (20 - 20) Bragg reflections in conventional and grazing incidence geometry, respectively. The in-plane lattice parameter a of layers grown in this composition regime coincides with that of (2/3)a (sapphire) . Their rotational and tilting disorder shows a minimum as compared to layers grown outside this regime. (GaN/Al x Ga 1 -x N) multi-quantum well structures on top of such buffer layers are fully pseudomorphic having lowest interface disorder and best surface morphology as evaluated by specular and diffuse X-ray reflectivity measurements. The findings are explained by the assumption of a 2D coincidence site lattice for the epitaxial growth of AlGaN on sapphire. The coincidence site lattice has hexagonal symmetry with the lattice parameter three times a(A 0.22 Ga 0.78 N) equals two times a(sapphire).