Michael Heuken

Composition Fluctuations in InGaN Analyzed by Transmission Electron Microscopy

A series of GaN/InxGa1—xN/GaN quantum well structures was investigated by transmission electron microscopy (TEM) and photoluminescence spectroscopy (PL). The structures were grown by metal organic chemical vapor deposition on Al2O3(0001) substrates varying only the substrate temperature between 800 and 860 °C for the InGaN growth under otherwise unaltered growth conditions. A strong shift of the PL emission wavelength from 390 to 480 nm is observed. Composition analyses were carried out by measuring local lattice parameters, which are directly related to the local In concentration, from high-resolution TEM lattice fringe images. An increase of the average In concentration from 9% to 18% is found as the growth temperature is lowered. A laterally inhomogeneous In concentration is observed in all samples with inclusions of high In content embedded in a quantum well with lower In concentration indicating that phase separation occurs even at a low average In concentration of 9%.

Metalorganic vapor phase epitaxy of wide-gap II–VI semiconductors for optoelectronic applications: current status and future trends

The current status and future trends to overcome the major problems of wide-gap II-VI compounds grown by metalorganic vapor phase epitaxy (MOVPE) which are p-type doping and the understanding of interface properties of ZnSSe/ZnSe-based heterostructures will be discussed. Since a low growth temperature is required to reduce defects and impurities and to increase the sticking coefficient of dopant atoms, a matched precursor combination of zinc and selenium compounds or an additional growth assistance (e.g. plasma stimulation) must be employed. The optical and electrical properties of ZnSe doped with nitrogen will be discussed. Emphasis will be put on the fact that most of the MOVPE grown ZnSe:N layers remain highly resistive or that they show only low free hole concentrations. Occurring compensation mechanisms such as parasitic compensating donors associated with nitrogen or compensating nitrogen-hydrogen complexes may be the reason. The ability of MOVPE to handle high vapor pressure elements such as sulphur favours this technology for the growth of sophisticated quantum wells and superlattices to achieve electrical and optical confinement in laser structures and to push the emission wavelength further into the blue. Scanning transmission electron microscope, photoluminescence (PL) and X-ray measurements were used for the analysis of the interface properties. Growth optimization of ZnSSe/ZnSe interfaces results in monolayer fluctuations at the interfaces. High excitation PL experiments show that room temperature stimulated emission is possible with this kind of structures. To realize high bit rate data transmission in the blue spectral range at 2.7 eV the physical properties of optoelectronic modulators based on ZnSSe/ZnSe superlattices were examined

InGaN composition and growth rate during the early stages of metalorganic chemical vapor deposition

Transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy were applied to study the metalorganic chemical vapor deposition of InGaN and the correlation between the structural properties and luminescence of GaN/InxGa1−xN-quantum well structures. A series of samples was grown varying only the growth duration for the InGaN under otherwise unaltered growth conditions. Composition analyses were carried out by measuring local lattice parameters from TEM images, which are directly related to the local In concentration. A rising average In concentration from 6.5% to 15.4% and a decreasing growth rate are observed with increasing growth duration. All samples show an inhomogeneous In distribution containing In-rich agglomerates with a size of only a few nanometers and less pronounced composition fluctuations on a scale of some 10 nm. The redshift of the PL peak energy with increasing quantum well thickness indicates that the luminescence is predominantly determined by the piezoelectric field.

Highly efficient yellow organic light emitting diode based on a layer-cross faded emission layer allowing easy color tuning

An easy way to adjust the color of yellow organic light emitting diodes (OLED) is realized by basing the emission layer on a cross-fading zone of two unipolar-conducting host materials doping parts of it either with a red or green phosphorescent emitter at varying thickness ratios. At color coordinates of 0.47/0.50, a current efficacy of 42.2 cd/A (16.2% external quantum efficiency) and a power efficacy of 32.9 lm/W (1000u2002cd/m2) are measured without light extraction enhancement. Mixed-host emission layer OLED without cross-fading are processed for comparison. Exciton distributions are studied. The concept is suggested to be useful for white OLED.

Employing exciton transfer molecules to increase the lifetime of phosphorescent red organic light emitting diodes

The lifetime of phosphorescent red organic light emitting diodes (OLEDs) is investigated employing either N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (NPB), TMM117, or 4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA) as hole-conducting host material (mixed with an electron conductor). All OLED (organic vapor phase deposition-processed) show similar efficiencies around 30 lm/W but strongly different lifetimes. Quickly degrading OLED based on TCTA can be stabilized by doping exciton transfer molecules [tris-(phenyl-pyridyl)-Ir (Ir(ppy)3)] to the emission layer. At a current density of 50u2002mA/cm2 (12u2009800u2002cd/m2), a lifetime of 387 h can be achieved. Employing exciton transfer molecules is suggested to prevent the degradation of the red emission layer in phosphorescent white OLED.

Hybrid white organic light-emitting diode with a mixed-host interlayer processed by organic vapor phase deposition

Abstract. Organic light-emitting diodes (OLEDs) are a key technology in solid state lighting.Withoutalong-livedphosphorescentblueemitter,ahybridconceptbasedonphosphorescentredandgreenemittersandafluorescentblueemitterinawhiteOLEDstackisapromisingapproachfor pure-white emission. Several challenges such as exciton recombination on all emitters andtripletdiffusion,aswellasquenching,havetobeovercome.Toaddresstheseissues,amixed-hostphosphorescent emission layer is employed. The mixture ratio is locally varied in the emissionlayer. An interlayer separates the phosphorescent and fluorescent emission layer. Strategies totune the color coordinates are presented. The lifetime and color stability versus luminance areinvestigated. At Commission Internationale de I’Eclairage color coordinates of 0.44/0.44, acurrent efficacy of 28.0 cd/A (at 1000 cd/m 2 ), and a luminous efficacy of 20.6 lm/W can bemeasured. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE) . [DOI: 10.1117/1.3545966]

Influence of carrier conductivity and injection on efficiency and chromaticity in small-molecule white organic light-emitting diodes based on 4,4'-bis(2,2'-diphenylvinyl)-1,1'-spirobiphenyl and rubrene

Organic light-emitting devices (OLEDs) employing yellow-emitting 5,6,11,12-tetraphenylnaphthacene (rubrene) and blue-emitting 4,4′-bis(2,2′-diphenylvinyl)-1,1′-spirobiphenyl are optimized using a vacuum thermal evaporator. The influence of various hole injection/hole transport stacks and electron transport materials on the device performance and the electroluminescence spectra are discussed. Device characteristics are explained by the charge carrier distribution among the organic layers. OLEDs with warm-white emission with color coordinates of x=0.43 and y=0.42 were produced with power and current efficiencies of 5lm∕W and 10.9cd∕A, respectively, at a luminance of 1000cd∕m2. The maximum external quantum efficiency at a current density of 20mA∕cm2 was 4.6%.

Controlling the interface charge density in GaN-based metal-oxide-semiconductor heterostructures by plasma oxidation of metal layers

In recent years, investigating and engineering the oxide-semiconductor interface in GaN-based devices has come into focus. This has been driven by a large effort to increase the gate robustness and to obtain enhancement mode transistors. Since it has been shown that deep interface states act as fixed interface charge in the typical transistor operating regime, it appears desirable to intentionally incorporate negative interface charge, and thus, to allow for a positive shift in threshold voltage of transistors to realise enhancement mode behaviour. A rather new approach to obtain such negative charge is the plasma-oxidation of thin metal layers. In this study, we present transmission electron microscopy and energy dispersive X-ray spectroscopy analysis as well as electrical data for Al-, Ti-, and Zr-based thin oxide films on a GaN-based heterostructure. It is shown that the plasma-oxidised layers have a polycrystalline morphology. An interfacial amorphous oxide layer is only detectable in the case of Zr. ...

MOVPE growth of high quality III-nitride material for light emitting device applications in a multiwafer system

Abstract Processes for mass production of GaN and its related alloys, InGaN and AlGaN, have been optimized to achieve high device yield and low cost of ownership. Here we present some of the latest results obtained from AIX 2000HT Planetary Reactors ® in a configuration of 7×2″ which provides unique uniformity capabilities due to the twofold rotation of the substrates. GaN single layers with background electron concentrations below 5×10 16 xa0cm −3 and intended doping levels up to 10 18 xa0cm −3 p-type and 10 20 xa0cm −3 n-type with state of the art homogeneities have been achieved. Thickness homogeneities have been shown to be better than 1% standard deviation on full 2″ wafers, while composition uniformity of ternary material is determined by room temperature photoluminescence mappings. Low-temperature photoluminescence and reflectance spectra of single-layer GaN revealed free-exciton transitions

Heterostructure field effect transistors grown by MOVPE

Abstract Metalorganic vapor phase epitaxy (MOVPE) has received much attention as an important technique for the growth of III–V compound semiconductor layers for electronic and optoelectronic applications. The move of the AlGaAs/GaAs heterostructure field effect transistor (HFET) technology from research to production will be discussed in this paper. In addition developments such as undoped AlGaAs/GaAs heterostructure insulated gate FET (HIGFET) for digital applications will be reported. These FET offer the possibility of easy production of both n- and p-type channel devices on the same layer structure. The improvements in the performance of electronic devices such as latticematched InP-based n-channel HFET grown by MOVPE will be shown. Especially for p-type structures a reduction in the effective hole mass due to strain causes improvements in the operation of p-type pseudomorphic devices. These InP HFET grown by MOVPE are favorable candidates for the application in high-performance optoelectronic integrated circuits in the 1.1–1.65 μm region.