Nikolaus Dietz

The growth and optical properties of large, high-quality AlN single crystals

The effect of impurities and defects on the optical properties of AlN was investigated. High-quality AlN single crystals of more than 20mm2 size were examined. Different crucible materials and growth procedures were applied to the growth of bulk AlN by physical vapor transport method to vary the defect and the impurity concentrations. The crystalline orientation was investigated by Raman spectroscopy. Glow discharge mass spectrometry was used to determine the trace concentration of the incorporated impurities such as oxygen and carbon. The photoluminescence emission and absorption properties of the crystals revealed bands around 3.5 and 4.3eV at room temperature. Absorption edges ranging between 4.1 and 5.95eV were observed. Since no straight correlation of the oxygen concentration was obtained, a major contribution of oxygen or oxygen-related impurities was ruled out to generate the observed emission and absorption bands in the Ultraviolet spectral range. The carbon-related impurities and intrinsic defec...

GaN∕AlGaN ultraviolet/infrared dual-band detector

Group III-V wide band gap materials are widely used in developing solar blind, radiation-hard, high speed optoelectronic devices. A device detecting both ultraviolet (UV) and infrared (IR) simultaneously will be an important tool in fire fighting and for military and other applications. Here a heterojunction UV/IR dual-band detector, where the UV/IR detection is due to interband/intraband transitions in the Al0.026Ga0.974N barrier and GaN emitter, respectively, is reported. The UV threshold observed at 360nm corresponds to the band gap of the Al0.026Ga0.974N barrier, and the IR response obtained in the range of 8–14μm is in good agreement with the free carrier absorption model.

Sellmeier parameters for ZnGaP2 and GaP

Sellmeier parameters are determined for the birefringent material ZnGaP2 and the singly refractive material GaP by the minimization of chi-square employing the Levenberg–Marquardt method. The distinguishing feature of the present work is that all five Sellmeier parameters are treated as adjustable. In previous work the Sellmeier parameter related to the restrahlen frequency was fixed and set equal for both ordinary and extraordinary waves in the birefringent material. The fitted results show there is approximately an 8% difference between the two. Taking this parameter as adjustable allows for a better fit on all other parameters.

Characterization of InN layers grown by high-pressure chemical vapor deposition

Structural and optical properties of indium nitride (InN) layers grown by high-pressure chemical vapor deposition (HPCVD) on sapphire and GaN epilayers have been studied. HPCVD extends processing parameters beyond those accessible by molecular beam epitaxy and metal organic chemical vapor deposition, enabling the growth of epitaxial InN layers at temperatures as high as 1150K for reactor pressures around 15bars, leading to vastly improved material properties. InN layers grown on GaN(0002) epilayers exhibit single-phase InN(0002) x-ray diffraction peaks with full width at half maximum (FWHM) around 430arcsec. Optical characterization of the InN layers by infrared (IR) reflectance reveals free carrier concentrations in the low to mid-10+19-cm−3 and optical dielectric function e∞=5.8. The optical properties in the visible and near IR spectral ranges were analyzed by transmission spectroscopy, showing an absorption edge around 1.5eV. The shift of the absorption edge correlates with deviations in the InN stoic...

Magnetic and optical properties of Ga1−xMnxN grown by metalorganic chemical vapour deposition

Epitaxial layers of Ga1−xMnxN with concentrations of up to x = 0.015 have been grown on c-sapphire substrates by metalorganic chemical vapour deposition. No ferromagnetic second phases were detected via high-resolution x-ray diffraction. Crystalline quality and surface structure were measured by x-ray diffraction and atomic force microscopy, respectively. No significant deterioration in crystal quality and no increase in surface roughness with the incorporation of Mn were detected. Optical measurements show a broad emission band attributed to a Mn-related transition at 3.0 eV that is not seen in the underlying GaN virtual substrate layers. Room temperature ferromagnetic hysteresis has been observed in these samples, which may be due to either Mn-clustering on the atomic scale or the Ga1−xMnxN bulk alloy.

Antiphase boundaries in GaP layers grown on (001) Si by chemical beam epitaxy

Abstract We have investigated the origin of contrast features observed in coalesced GaP islands, deposited by chemical beam epitaxy on (001) Si, by high resolution transmission electron microscopy and conventional dark field electron microscopy. Our results indicate that these features are antiphase boundaries (APBs) lying on {110} planes. Image simulations have been performed to show that APBs can only be seen under specific defocus conditions in high resolution lattice images. The observed contrast is attributed to the presence of Ga–Ga and P–P wrong bonds at APBs. A model is proposed to show that the coalescence of GaP islands on the same Si terrace may not produce APBs, and the formation of such boundaries may require the presence of monoatomic steps, separating the coalescing islands.

P-polarized reflectance spectroscopy: A highly sensitive real-time monitoring technique to study surface kinetics under steady state epitaxial deposition conditions

This paper describes the results of real-time optical monitoring of epitaxial growth processes by p-polarized reflectance spectroscopy (PRS) using a single wavelength application under pulsed chemical beam epitaxy (PCBE) condition. The high surface sensitivity of PRS allows the monitoring of submonolayer precursor coverage on the surface as shown for GaP homoepitaxy and GaP on Si heteroepitaxy as examples. In the case of heteroepitaxy, the growth rate and optical properties are revealed by PRS using interference oscillations as they occur during growth. Superimposed on these interference oscillations, the PRS signal exhibits a fine structure caused by the periodic alteration of the surface chemistry by the pulsed supply of chemical precursors. This fine structure is modeled under conditions where the surface chemistry cycles between phosphorus supersaturated and phosphorus depleted surfaces. The mathematical model describes the fine structure using a surface layer that increases during the tertiarybutyl phosphine (TBP) supply and decreases during and after the triethylgallium (TEG) pulse, which increases the growing GaP film thickness. The imaginary part of the dielectric function of the surface layer is revealed from the turning points in the fine structure, where the optical response to the first precursor pulse in the cycle sequence changes sign. The amplitude of the fine structure is determinated by the surface layer thickness and the complex dielectric functions for the surface layer with the underlying bulk film. Surface kinetic data can be obtained by analyzing the rise and decay transients of the fine structure.

NATIVE DEFECT RELATED OPTICAL PROPERTIES OF ZNGEP2

We present photoluminescence, photoconductivity, and optical absorption spectra for ZnGeP2 crystals grown from the melt by gradient freezing and from the vapor phase by high pressure physical vapor transport (HPVT). A model of donor and acceptor related subbands in the energy gap of ZnGeP2 is introduced that explains the experimental results. The emission with peak position at 1.2 eV is attributed to residual disorder on the cation sublattice. The lower absorption upon annealing is interpreted in terms of both the reduction of the disorder on the cation sublattice and changes in the Fermi level position. The n‐type conductivity of ZnGeP2 crystals grown under Ge‐deficient conditions by the HPVT is related to the presence of additional donor states.

Heteroepitaxy of lattice‐matched compound semiconductors on silicon

The heteroepitaxial overgrowth of silicon by nearly lattice‐matched compound semiconductors is reviewed in the context of the separation of the chemical problems associated with the initial sealing of the silicon surface by a contiguous epitaxial compound film from the problems associated with the generation of strain during heteroepitaxial growth. Of the mixed compound systems available dilute solid solutions of composition AlxGa1−xNyP1−y and ZnSySe1−y as well as ZnSixGe1−xP are suitable candidates for the exactly lattice‐matched epitaxial overgrowth of silicon. Real‐time process monitoring by nonintrusive methods is important for gaining an understanding of the epitaxial overgrowth mechanism and for controlling the film properties. A new method, p‐polarized reflectance spectroscopy is introduced that provides detailed information about the growth rate per cycle, the bulk optical properties of the film and its topography. Submonolayer resolution is accomplished for thousands of A of film growth by pulsed...

Orientation mediated self-assembled gallium phosphide islands grown on silicon

Abstract Evolution of gallium phosphide epitaxial islands, grown on the (001), (111), (110) and (113) surfaces of Si by chemical beam epitaxy, has been investigated by p-polarized reflectance spectroscopy, transmission electron microscopy and atomic force microscopy. The growth nucleates as faceted three-dimensional islands on the (001) and (111) Si surfaces because of the polar nature of the heterointerface which increases the interfacial energy. A more two-dimensional-like growth mode is seen on the (110) and (113) surfaces which is attributed to the absence of charge build up at the GaP—Si heterointerface for these orientations, thereby reducing the interface energy. Islands grown on (001) Si become more faceted and larger in size with increase in growth temperature. This is due to a lower incubation time and enhanced atomic mobility at high temperatures. Wurtzite GaP has been observed to coexist with the zincblende polytype in some of the islands grown on (111) Si at 560°C. Arguments have been developed to rationalize these observations.