M. Heuken

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.

Determination of channel temperature in AlGaN/GaN HEMTs grown on sapphire and silicon substrates using DC characterization method

Self-heating effects and temperature rise in AlGaN/GaN HEMTs grown on silicon and sapphire substrates are studied, exploiting transistor DC characterization methods. A negative differential output resistance is observed for high dissipated power levels. An analytical formula for a source-drain current drop as a function of parasitic source resistance and threshold voltage changes is proposed to explain this behavior. The transistor source resistance and threshold voltage is determined experimentally at different elevated temperatures to construct channel temperature versus dissipated power transfer characteristic. It is found that the HEMT channel temperature increases rapidly with dissipated power and at 6 W/mm reaches values of /spl sim/320/spl deg/C for sapphire and /spl sim/95/spl deg/C for silicon substrate, respectively.

Bright blue electroluminescence from an InGaN/GaN multiquantum-well diode on Si(111): Impact of an AlGaN/GaN multilayer

We present an electroluminescence test structure which consists of an InGaN/GaN multiquantum well as active region on the top of an AlGaN/GaN multilayer grown by metalorganic vapor phase epitaxy on Si(111) substrate. The integral room-temperature electroluminescence spectrum reveals a peak emission wavelength of 467 nm and a significantly higher brightness than an identical reference structure on sapphire substrate. In microelectroluminescence imaging, two emission peaks at 465 and 476 nm can be separated originating from locally different areas of the diode. Cathodoluminescence measurements in cross section and high-resolution x-ray diffraction measurements show that the structure is less strained than a sample without the AlGaN/GaN multilayer. The AlGaN/GaN multiple layer sequence which has a total thickness of 1.5 μm causes lattice relaxation during growth after a thickness of around 0.9 μm as directly visualized by cathodoluminescence line scans across the diode.

Structural and optical properties of epitaxial and bulk ZnO

In this letter, we compare the properties of bulk and epitaxial ZnO. The ZnO thin films were grown on GaN templates and on ZnO single crystals by vapor phase deposition using Zn and NO2 as precursors. We use high-resolution x-ray diffraction to resolve the structural properties. The rocking curves of the bulk crystal are extremely broad caused by a mosaic structure of the substrate. The homoepitaxial ZnO film mimics the properties of the ZnO substrate whereas ZnO films on GaN templates showed superior rocking curve half width as small as 230 arcsec. The optical properties are investigated by temperature-dependent photoluminescence. Different donor and acceptor bound excitons can be distinguished for a half width of the recombination lines less than 1 meV. Free exciton emission is already detectable at liquid-He temperatures proofing the high quality of the epitaxial films.

AlGaN/GaN HEMTs on (111) silicon substrates

AlGaN/GaN HEMTs on silicon substrates have been fabricated and their static and small-signal RF characteristics investigated. The AlGaN/GaN material structures were grown on (111) p-Si by LP-MOVPE. Devices exhibit a saturation current of 0.91 A/mm, a good pinchoff and a peak extrinsic transconductance of 122 mS/mm. A unity current gain frequency of 12.5 GHz and f/sub max//f/sub T/=0.83 were obtained. The highest saturation current reported so far, static output characteristics of up to 20 V and breakdown voltage at pinchoff higher than 40 V demonstrate that the devices are capable of handling /spl sim/16 W/mm static heat dissipation.

Crack‐Free InGaN/GaN Light Emitters on Si(111)

We present MOCVD-grown crack-free GaN based light emitters on Si with a layer thickness of 3.6 μm. The crack free layer is grown on a thin predeposited GaN layer with fields defined by a Si x N y mask. In X-ray diffraction measurements a reduction in stress is observed for the patterned sample as compared to a similar unstructured sample. Lateral growth occurs at the edges of the fields with strongly differing growth rates for perpendicular directions. The impact of the facet type on the growth rate and impurity incorporation is observed by scanning electron microscopy and cathodoluminescence measurements. In electroluminescence the diode shows a bright blue emission at 421 nm.

Photoluminescence and Raman study of compensation effects in Mg-doped GaN epilayers

The compensation of Mg-doped GaN is systematically studied by low-temperature photoluminescence and Raman spectroscopy using a series of samples with different Mg concentrations. Strongly doped samples are found to be highly compensated in electrical measurements. The compensation mechanism is directly related to the incorporation of Mg. Three different deep donor levels are found at 240±30, 350±30, and 850±30 meV from the conduction band, each giving rise to deep unstructured donor-acceptor pair emission.

Dielectric function and optical properties of Al-rich AlInN alloys pseudomorphically grown on GaN

A detailed discussion of the optical properties of Al-rich Al1−xInxN alloy films is presented. The (0 0 0 1)-oriented layers with In contents between x = 0.143 and x = 0.242 were grown by metal-organic vapour phase epitaxy on thick GaN buffers. Sapphire or Si(1 1 1) served as the substrate. High-resolution x-ray diffraction revealed pseudomorphic growth of the nearly lattice-matched alloys; the data analysis yielded the composition as well as the in-plain strain. The complex dielectric function (DF) between 1 and 10 eV was determined from spectroscopic ellipsometry measurements. The sharp onset of the imaginary part of the DF defines the direct absorption edge, while clearly visible features in the high-photon energy range of the DF, attributed to critical points (CPs) of the band structure, indicate promising crystalline quality of the AlInN layers. It is demonstrated that the experimental data can be well reproduced by an analytical DF model. The extracted characteristic transition energies are used to determine the bowing parameters for all CPs of the band structure. In particular, strain and the high exciton binding energies for the Al-rich alloys are taken into account in order to assess the splitting between the valence band with symmetry and the conduction band at the centre of the Brillouin zone. Finally, the compositional dependence of the high-frequency dielectric constants is reported.

Study on quaternary AlInGaN/GaN HFETs grown on sapphire substrates

We report on AlInGaN/GaN heterostructure field effect transistors (HFETs) and the effect of different barrier material compositions. The analytical model for the interface charge in quaternary nitride heterostructures is described in detail and is applied in the calculation of the expected sheet carrier density. Experimental results from different lattice-matched AlInGaN/GaN heterostructures are presented and compared with the analytical predictions. Three heterostructures with AlInGaN barriers grown on sapphire substrates were processed and have been investigated. Each barrier layer was lattice-matched to GaN and the gallium content was 0.1, 0.15 and 0.2 at a barrier thickness of 13.5, 12.8 and 11.3 nm, respectively. Additionally, from these experiments, the basic trends for quaternary nitride Schottky barrier contacts are discussed. Finally, comprehensive dc characterizations have been performed. All devices had a gate length of 1 µm and exhibited a good transconductance of around 260 mS mm−1 at nearly the same current density level. An increase in threshold voltage as well as a decrease in gate leakage current for increasing GaN content has been observed. The nearly constant electron mobility in the range of 1700 cm2 V−1 s−1 at room temperature is within the best reported so far for HFETs with InN-containing barriers.

Lattice parameters of GaN layers grown on a-plane sapphire: Effect of in-plane strain anisotropy

We have studied GaN films grown on a-plane sapphire by hydride vapor phase epitaxy and metalorganic vapor phase epitaxy. The in-plane lattice parameter was determined from sets of equivalent interplanar distances measured for six different directions in order to examine the effect of strain anisotropy. It is found that, in both types of films, the obtained six values of the in-plane lattice parameter can be grouped around two values. The strain anisotropy is estimated to have different value in the films grown by the two techniques and possible explanations are suggested.