T. Hempel

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.

Metal-organic vapor phase epitaxy and properties of AlInN in the whole compositional range

The authors present a detailed study of Al1−xInxN layers covering the whole composition range of 0.09<x<1. All layers were grown on GaN on Si(111) templates using metal-organic vapor phase epitaxy. For 0.13<x<0.32 samples grow fully strained and without phase separation. At higher In concentrations, the crystalline quality starts to deteriorate and a transition to three-dimensional growth is observed. A comparison of their experimental data with theoretically predicted phase diagrams reveals that biaxial strain increases the stability of the alloy.

Reduction of stress at the initial stages of GaN growth on Si(111)

GaN growth on heterosubstrates usually leads to an initially high dislocation density at the substrate/seed layer interface. Due to the initial growth from small crystallites, tensile stress is generated at the coalescence boundaries during GaN growth. In addition, with tensile thermal stress this leads to cracking of GaN on Si and SiC substrates when cooling to room temperature. By partially masking the typically applied AlN seed layer on Si(111) with an in situ deposited SiN mask a reduction in tensile stress can be achieved for the subsequently grown GaN layer. Additionally, the 6 K GaN band edge photoluminescence is increased by about an order of magnitude and shifts by 21 meV, which can be attributed to a change in tensile stress of ∼0.8 GPa, in good agreement with x-ray diffractometry measurements. This improvement in material properties can be attributed to a reduction of grain boundaries by the growth of larger sized crystallites and lateral overgrowth of less defective GaN.

Fabry-Perot effects in InGaN∕GaN heterostructures on Si-substrate

A strong intensity modulation is found in spatially and angular resolved photoluminescence spectra of InGaN∕GaN heterostructures and quantum wells epitaxially grown on Si(111) substrates. This Fabry-Perot effect results from the high refractive index contrasts at the GaN∕Si and the Air/InGaN interfaces. It can be used for a wavelength stabilization of the sample upon temperature change and, e.g., in the case of light emitting diodes, to additionally reduce the blueshift at increasing injection currents. A simple geometric approach has been chosen to calculate the influence of layer thickness, absorption and refractive indices, as well as detection angle. The cavity can be described quantitatively by a simple three layer Fabry-Perot model. An analytical expression is derived for the external luminescence line shape. Microphotoluminescence measurements at samples with the silicon substrate locally removed corroborate the model.

Formation of Si quantum dots in nanocrystalline silicon

Amorphous Si (a-Si) thin films deposited on Si(110) substrates were crystallized by using rapid thermal annealing. From structural investigations using X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM) we find that this crystallization is a two step process. In a first step single crystalline needles having a thickness of 1–2 nm grow in 〈111〉 direction towards the thin film surface starting from the a-SiSi interface. In a second process these needles induce growth of rectangular shaped Si crystallites (Si quantum dots) with 2–3 nm length and 3–5 nm width in the space between the needles. This process is driven by the difference in density between the a-Si regions and the Si- needles (ϱ(a-Si) > ϱ(Si)) and consequently leads to stretching of the a-Si regions. Strain relief is carried out consecutively by transition of these amorphous regions into the nanocrystalline phase. Room-temperature photoluminescence (PL) using the 337 nm line of an N2 laser for excitation shows intense blue light emission from the nanocrystalline thin films. The luminescence band between 2.6 and 3.2 eV consists of distinct peaks. Time resolved PL yields decay time constants τ1 = 170–250 ps and τ2 = 500–800 ps depending on the spatial positions of PL excitation across the sample surface. The blue light emission from the nanocrystalline thin films is explained by quantum size effects in the Si nanocrystallites which actually are Si quantum dots.

Evolution of stress In GaN heteroepitaxy on AlN/Si(111): From hydrostatic compressive to biaxial tensile

The initial steps of GaN growth on an AlN buffer layer on Si(111) substrates by metalorganic vapor phase epitaxy were investigated using field emission scanning electron microscopy, micro-photoluminescence, as well as by conventional and grazing incidence x-ray diffraction. A series of GaN layers was grown for various times ranging from 7.5 s to several minutes, doubling the growth time for each step. The AlN buffer layer is noncontinuous and consists of (0001)-oriented AlN islands with a mean diameter of about 50 nm. On top of these nucleation centers three-dimensional growth of GaN was observed. With increasing growth times up to 30 s these islands further expanded and their distribution became more homogeneous. At 60 s coalescence started with homogeneously distributed islands, and after 120 s the layer was fully coalesced. The layers grown for 7.5 and 15 s are under a high compressive hydrostatic pressure, which might be enhanced by the lattice mismatch between AlN and GaN. For longer growth times a b...

Optical and structural microanalysis of GaN grown on SiN submonolayers

Lateral overgrowth techniques have demonstrated their ability to strongly reduce the dislocation density in GaN grown on a variety of foreign substrates. The in situ deposition of SiN during metal-organic chemical-vapor phase epitaxy (MOVPE) leads to the formation of a randomly distributed mask layer and induces lateral overgrowth similar to conventional epitaxial lateral overgrowth of GaN. Specifically for GaN on silicon substrate, the insertion of SiN submonolayers is a promising method to reduce not only the dislocation density but also the tensile stress upon Si doping. Besides the advantage of uncomplicated in situ mask formation, it allows complete coalescence and planarization of the overgrown GaN within a layer thickness of about 500 nm depending on the mask thickness, thus reducing the liability to cracking. However, the insertion of ultrathin SiN interlayers and, for thicker GaN stacks, additional stress-compensating low-temperature AIN (LT-AIN) leads to a complicated interplay of stress and dis...

Analysis of point defects in AlN epilayers by cathodoluminescence spectroscopy

We present a systematic cathodoluminescence study yielding a clear correlation between the different growth conditions and the appearance and strength of the characteristic luminescence fingerprints of the individual point defects in AlN. In particular, the incorporation of oxygen and the formation of oxygen-related and probably silicon-related DX centers as well as the native Al and N vacancies are still a problem. The thermal activation of the deep defect centers is investigated by temperature dependent cathodoluminescence spectroscopy.

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.

Strain evaluation in AlInN/GaN Bragg mirrors by in situ curvature measurements and ex situ x-ray grazing incidence and transmission scattering

Strain in lattice matched and mismatched AlInN/GaN Bragg mirror structures were studied by in situ curvature and various ex situ x-ray measurements. In the case of lattice mismatched structures considerable deviations of the in-plane lattice parameters were evidenced near the surface region as well as in depth using x-ray grazing incidence and x-ray transmission scattering in Laue geometry. The experimental findings are explained in terms of partial stress relaxation of the AlInN/GaN Bragg layer stack with respect to the underlying GaN buffer and a mutual tensioning of the GaN and AlInN layers with respect to each other.