F. Bertram

Optical and structural analysis of ZnCdO layers grown by metalorganic vapor-phase epitaxy

The development of ZnO-based semiconductor devices requires band gap engineering. Ternary Zn1−xCdxO allows reduction of the band gap relative to ZnO, which would be necessary for devices emitting visible light. We have analyzed the structural and optical properties of Zn1−xCdxO layers grown by metalorganic vapor-phase epitaxy. A narrowing of the fundamental band gap of up to 300 meV has been observed, while introducing a lattice mismatch of only 0.5% with respect to binary ZnO. Photoluminescence, high-resolution x-ray diffraction, and spatially resolved cathodoluminescence measurements revealed a lateral distribution of two different cadmium concentrations within the Zn1−xCdxO layers.

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.

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.

Optical investigations of AlGaN on GaN epitaxial films

We investigated coherently strained AlxGa1−xN/GaN heterostructures (0<x<0.22) grown by metalorganic vapor phase epitaxy on sapphire with photoluminescence (PL), reflexion and cathodoluminescence experiments. The energetic positions of the free A exciton as a function of the alloy compositions are deduced from temperature dependent PL and from reflexion measurements. We obtain a small bowing parameter and no evidence for a Stokes shift between absorption and emission. Compositional inhomogeneities are present, but the fluctuations are too small to be important for carrier localization. The broadening of the luminescence linewidth in the alloys can be described by statistical disorder of a random alloy.

Vertical strain and doping gradients in thick GaN layers

We report on spatially-resolved low-temperature luminescence and Raman experiments on a 220-μm-thick GaN layer grown on sapphire by hydride vapor phase epitaxy. Our measurements reveal that the peak position of the near-band-gap luminescence strongly depends on the distance to the substrate interface. The luminescence shifts continuously to lower energies with decreasing distance but a strong blue shift occurs directly at the interface. We correlate these effects with the inhomogeneous free-carrier distribution and the strain gradient found by our Raman experiments.

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.

An improved carrier rate model to evaluate internal quantum efficiency and analyze efficiency droop origin of InGaN based light-emitting diodes

A carrier rate model taking carrier delocalization into account is presented to analyze current dependent internal quantum efficiency of InGaN based light-emitting diodes (LEDs). By fitting normalized experimental internal quantum efficiency-current curves, both injection efficiency and radiative recombination efficiency depending on current can be obtained. Based on the fitting results from two LED samples with and without the InGaN interlayer beneath the active regions of 5 InGaN quantum wells (QWs), carrier delocalization and carrier leakage are believed to lead to the efficiency droop effect under considerable and even larger injection, respectively. By investigating two LED samples with 8 and 10 QWs, it is found that the 8-QWs LED has the highest radiative recombination efficiency over 80% and the 10-QWs one has the highest injection efficiency over 50% under 120 A/cm2. This means that increasing QW number is an effective method to suppress droop effect.

Comprehensive comparison of various techniques for the analysis of elemental distributions in thin films

In a recent publication by Abou-Ras et al., various techniques for the analysis of elemental distribution in thin films were compared, using the example of a 2-µm thick Cu(In,Ga)Se2 thin film applied as an absorber material in a solar cell. The authors of this work found that similar relative Ga distributions perpendicular to the substrate across the Cu(In,Ga)Se2 thin film were determined by 18 different techniques, applied on samples from the same identical deposition run. Their spatial and depth resolutions, their measuring speeds, their availabilities, as well as their detection limits were discussed. The present work adds two further techniques to this comparison: laser-induced breakdown spectroscopy and grazing-incidence X-ray fluorescence analysis.The present work shows results on elemental distribution analyses in Cu(In,Ga)Se2 thin films for solar cells performed by use of wavelength-dispersive and energy-dispersive X-ray spectrometry (EDX) in a scanning electron microscope, EDX in a transmission electron microscope, X-ray photoelectron, angle-dependent soft X-ray emission, secondary ion-mass (SIMS), time-of-flight SIMS, sputtered neutral mass, glow-discharge optical emission and glow-discharge mass, Auger electron, and Rutherford backscattering spectrometry, by use of scanning Auger electron microscopy, Raman depth profiling, and Raman mapping, as well as by use of elastic recoil detection analysis, grazing-incidence X-ray and electron backscatter diffraction, and grazing-incidence X-ray fluorescence analysis. The Cu(In,Ga)Se2 thin films used for the present comparison were produced during the same identical deposition run and exhibit thicknesses of about 2 μm. The analysis techniques were compared with respect to their spatial and depth resolutions, measuring speeds, availabilities, and detection limits.

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.

Direct imaging of phase separation in ZnCdO layers

A direct correlation of structural and optical properties of MOCVD-grown ZnCdO-layers with a systematic variation of Cd-content has been achieved on a microscopic scale using highly spatially and spectrally resolved cathodoluminescence. The ZnCdO layer luminescence measured in cathodoluminescence wavelength images reveals strong lateral fluctuations directly visualizing local band gap fluctuation as a consequence of different local Cd incorporation. We give direct evidence for a chemical phase separation into Cd-rich and Cd-poor nanodomains in ZnCdO.