J. Christen

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.

Scanning cathodoluminescence microscopy: A unique approach to atomic‐scale characterization of heterointerfaces and imaging of semiconductor inhomogeneities

Luminescence experiments provide a powerful and nondestructive approach to the ex situ investigation of semiconductor heterointerfaces which might be buried up to several μm below the surface in a given complex sample structure. Combined with the ability of taking images simply by scanning the exciting focused electron beam across the area under investigation, lateral fluctuations of electronic properties like the variation of the fundamental band gap Eg(x,y) can be directly visualized by scanning cathodoluminescence (CL). The novel experimental approach, cathodoluminescence wavelength imaging (CLWI), which involves recording of a complete CL spectrum at every scanning position (x,y), yields direct 3D images of the atomic‐scale morphology of quantum wells (QWs) as sensed by the QW exciton: similar to the tip of a scanning tunneling microscope, the exciton samples the local fluctuations of QW thickness Lz and transforms this structural information Lz(x,y) into a spectral one, the lateral variation of band ...

Strain relaxation and strong impurity incorporation in epitaxial laterally overgrown GaN: Direct imaging of different growth domains by cathodoluminescence microscopy and micro-Raman spectroscopy

Epitaxial lateral overgrowth GaN structures oriented along the 〈112_0〉 direction were comprehensively characterized by cathodoluminescence (CL) microscopy and micro-Raman spectroscopy. CL microscopy directly visualizes the significant differences between the overgrown areas on top of the SiO2 mask and the coherently grown regions between the SiO2 stripes in quantitative correlation with micro-Raman spectroscopy mapping of the local strain and free carrier concentration. The overgrown GaN shows a partial strain relaxation and a high carrier concentration that strongly broadens the luminescence. A strong impurity incorporation is evidenced in the coalescence regions. In contrast, the local luminescence from the areas of coherent (0001) growth is dominated by narrow excitonic emission, demonstrating the superior crystalline quality.

Cathodoluminescence atomic scale images of monolayer islands at GaAs/GaAlAs interfaces

Direct images of growth islands differing by 2.8 A [1 monolayer (ML)] height at GaAs/AlGaAs heterointerfaces and of the columnar structure of quantum wells are reported for the first time. The structures are grown by molecular‐beam epitaxy (MBE) with interruptions of the growth of ≊2 min at the interfaces. The method used to obtain these images is scanning cathodoluminescence. The dependence of the lateral extension of these islands on growth conditions is investigated. For fixed growth rate rs≊0.5 ML/s the mean island size decreases from 6–7 μm to 2 μm upon an increase of growth temperature from Tg=600 to 660 °C. Apparently the growth process changes from a planar to a three‐dimensional one. For low‐growth temperature and rate the lateral extension of such islands can be larger than the carrier diffusion length. Under these conditions interisland thermalization of carriers is largely suppressed. Quantitative information on the reduction of roughness of the quantum well interfaces with increasing growth i...

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.

Carrier capture and quantum confinement in GaAs/AlGaAs quantum wire lasers grown on V-grooved substrates

Carrier capture mechanisms in semiconductor quantum wire (QWR) lasers grown by organometallic chemical vapor deposition on nonplanar substrates were investigated by cathodoluminescence and photoluminescence (PL) spectroscopy. Efficient carrier capture into the QWRs via adjacent quantum wells is manifested by a complete transfer of luminescence intensity from the well‐ to the wire‐spectral lines at temperatures above ∼100 K. In addition, higher QWR subbands separated by 19 meV are observed in the PL spectra, in agreement with the calculated subband spacing. The quantum well assisted carrier capture in these wires is important for the efficient room temperature operation of lasers and other optoelectronic devices based on QWRs.

Maskless epitaxial lateral overgrowth of GaN layers on structured Si(111) substrates

GaN layers are laterally overgrown by metalorganic chemical vapor deposition on structured Si(111) substrates in a single growth process. The substrates are structured with parallel grooves along the Si 〈1–10〉 or perpendicular to the Si 〈1–10〉 direction by standard photolithography and subsequent dry etching. Due to the anisotropic chemical dry etch process, the remaining Si ridges are underetched. The GaN layer grows nearly exclusively on the bottom of the grooves and on the top of the ridges between the grooves. These two growth fronts are completely separated from each other. As a consequence, the GaN layer growing from the ridge area between grooves can extend over the grooves. This process is similar to the so called pendeo-epitaxy process, but is completely mask free during growth and does not require any growth interruption. The improvement of the crystalline and the optical quality of the GaN layer is demonstrated by atomic force microscopy and cathodoluminescence spectroscopy.

Low-pressure metal organic chemical vapor deposition of GaN on silicon(111) substrates using an AlAs nucleation layer

We have investigated the growth of GaN on silicon by low-pressure metal organic chemical vapor deposition. Good quality GaN layers are grown on silicon(111) using an AlAs nucleation layer. AlAs is thermally stable even at 1050 °C and, unlike GaN and AlN buffer layers, the formation of SiNx on the Si surface is prevented. Single crystalline GaN films are obtained by introducing a thin low-temperature GaN buffer layer grown on the AlAs nucleation layer. The GaN layers are characterized by x-ray diffraction, atomic force microscopy, secondary ion mass spectroscopy, photoluminescence, and cathodoluminescence.

Self organization phenomena of InGaAs/GaAs quantum dots grown by metalorganic chemical vapour deposition

We have systematically investigated the influence of MOCVD growth parameters on structural and optical properties of InxGa]_xAs/GaAs quantum dots (QDs) formed in the Stranski-Krastanow growth mode. The influence of growth interruption time, V/III ratio, In/Ga flux ratio and growth temperature was examined by photoluminescence (PL), transmission electron microscopy (TEM) and atomic force microscopy (AFM). For samples with high dot densities (up to 8 × 10 I° cm -2) the dots surprisingly show a preferential alignment along the (110) orientations differing from predictions of strain relaxation equilibrium theory and results for MBE grown InAs/GaAs dots. The square base shape of the quantum dots is oriented along (100) in agreement with MBE results and theoretical predictions.

Optical microscopy of electronic and structural properties of epitaxial laterally overgrown GaN

Local strain relaxation as well as inhomogeneous impurity incorporation in epitaxial laterally overgrown GaN (ELOG) structures is microscopically characterized using spectrally resolved scanning cathodoluminescence (CL) and micro-Raman spectroscopy. We correlate the different CL emission spectra with results of spatially resolved Raman-scattering experiments sensing the local strain and free-carrier concentration.