Hommel, Detlef
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Hommel, Detlef
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Hommel, Detlef
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Hommel, D.
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Item-typ:Veröffentlichung, Heteroepitaxy of Group-III-Nitrides for the Application in Laser Diodes(2002-04-22); ; ; The scope of this thesis is the epitaxy of group-III nitride heterostructures on (0001) sapphire substrates for the fabrication of semiconductor laser diodes. MOVPE as well as MBE were used to deposit the structures. The growth of the epilayers was studied with regard to the threading dislocation density, the strain state and the thermodynamic stability. In general, a low threading dislocation density is obtained for growth processes showing an elongated island growth mode, as those grow free of dislocations. According experiments are discussed on the background of a thermodynamic description of the decomposition reaction. The threading dislocation density was estimated by x-ray diffraction, using a model based on the concept of mosaic crystals. It describes the microstructure of the layer as many dislocation-free, but tilted and twisted crystals. It was shown by plan-view TEM images, that the edge- and screw-type threading dislocation densities can be determined within reasonable limits of accuracy. The strain state of the studied GaN layers is found to correlate with the island diameter at coalescence. The thermal compressive strain at room temperature is partly compensated by an intrinsic, tensile strain, which forms during growth and depends on the average island size. It might be a consequence of the gap closure process during island coalescence. The photoluminescence of InGaN quantum wells was studied for different well thicknesses and Si doping levels of the GaN barriers. The structures were simulated numerically, taking piezoelectric fields into account. The results point towards a light emission mechanism governed by well thickness fluctuations superimposed by the piezoelectric field. The increased electron density induced by the Si doping is proposed to block the localized states of the threading dislocations, acting as nonradiative centers. The optimized structures allowed to realize the first GaN laser grown in Bremen.Dissertation435 223 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Elektrisch betriebene oberflächen-emittierende Bauelemente auf Basis von II-VI Halbleiter(2009-09-22); ; ; The key technology towards electrically pumped vertical-cavity surface-emitting lasers (VCSELs) are high reflectivity Bragg-mirrors with high conductivity in vertical direction. A Bragg pair is made of a high and a low refractive index layer with an optical thickness of a quater wavelength each. The ZnSe-based material system is the only one to realize epitaxial high reflectivity distributed Bragg reflectors (DBRs) for monolithic devices lattice matched to GaAs in the blue-green spectral region. The use of II-VI materials enables an emission in the green spectral region which is of high interrest for many applications. With green being a primary color it can be used together with red andblue emitters for display technologies. Another application is the use in an optical network using plastic optical fibre (POF) with a absorption minima at 560 nm. A green-emitting VCSEL will enhance the performance of these networks.In literature approaches of II-VI-DBRs are reported using ZnSe/ZnS or ZnSe/ZnTe-DBR structures. DBRs wtih ZnSe/MgSSLsas low index material have been reported but with ZnSe ashigh index material and fewer SL-periods. These structures reach reflectivities around 93%. Also nitride-based structures with high reflectivities have been reported. But up to now these structures are not conductive.Previous investigations showed that the doping does not affect the optical properties within the accuracy of measurement and p-type doped DBRs have a much lower conductivity than n-type doped DBRs. Therefore only n-doped DBRs seem to be suitable for the use in electrically pumped devices. This work is concentrating on the optimaziation of the design of n-type DBRs, suitable metal contacts and the incorporation of such DBRs into devices. In a first step metal contacts for n- and p-type doped semiconductorswere inverstigated. For the n-type structures Aluminium is asiutable contact material. Current densities of over 160 A/cm2 at 3 volts have been achieved. Concerning the p-type contacts Pd/Au is the best material. The introduction of a epitactically grown ZnTe/ZnSe contact layer leeds to the best results but also contact without that layer are applicable having the benefit of a better crystal quality of the top mirror. The second step was then to optimize the conductivity of the Bragg-mirrors. To achieve this the MgS layer thickness in the low index super lattice was reduced. This leeds also to a reduction of reflectivity. But even after compensation the latter effct with more periodes a conductivity increase of almost a factor of two can beachieved.In the third and final step the above shown concepts were incorporated into electrically driven surface-emitting devices. Here a RCLED demonstrated the feasibillity of such devices. Different approaches for the contacting are displayed, i. e. intra-cavity contacts and dielectric mirrors.Dissertation288 140 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, GaN-basierte Laserdioden : Epitaxie und Simulation(2003-07-15); ; ; The demand for higher recording densities in optical storage devices requires the development of semiconductor lasers with short wavelengths. This thesis deals with the realisation and simulation of GaN-based laser diodes with an emission wavelength of 400 nm. One of the key issues is the reduction of the resistivity of the Magnesium doped epitaxial layers in order to achieve lasing in such devices. The p-type doping of GaN utilizing both metalorganic vapor phase deposition (MOVPE) and molecular beam epitaxy (MBE) is investigated. High room temperature hole concentrations of 4y1018 1/cm3 have been achieved in samples grown by MBE. Compared to published data, these data belong to the best. The incorporation of Magnesium in these samples is limited by the desorption of Magnesium during growth. Therefore the doping is sensitive to the growth temperature. It could be shown that the incorporation of Magnesium can be enhanced by the use of a Hydrogen-Nitrogen mixed plasma. Nevertheless, these layers are compensated due to the formation of both Magnesium related planary defects and Hydrogen induced point defects. Therefore these layers exhibited high resistivities of 130 z cm. The formation of pyramidal defects was realised to be the cruicial factor in the limitation of p-type doping in MOVPE grown samples. The segregation of Magnesium on the growth surface is identified by transmission electron microscopy to be the driving force of defect formation. A rate equation model is employed and a defect formation criterion is established. Gain guided GaN-based laser diodes were produced on sapphire substrates with a Magnesium doping level just below defect formation. These laser structures showed a light output power of up to 263 mW in pulsed operation mode. However, the driving parameters are limited due to the generation of joule heat to pulse lengths and duty cycles of 100 ns and 2 %, respectively. The heat dissipation in the devices was simulated. These simulations are in good agreement with time dependent electroluminescence data collected in pulsed operation. The decay of luminescence intensity during the pulses is identified as a thermal activation process. Different laser structures were simulated and it was found, that the thickness of both pcontact metallization and substrate has a major influence on the thermal resistivity of the laser diodes. The heat dissipation model has been extended by an electrical model based on the laplace equation. The influence of current spreading on the current densities in the active region of gain guided laser structures is investigated. The activation of carriers in the p-type layers and the reduction of the conductivity by phonon scattering in the n-type layers at elevated temperatures during operation has been identified to be the main reason for high threshold current densities of the gain-guided laser diodes.Dissertation363 612 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Strain and crystalline defects in epitaxial GaN layers studied by high-resolution X-ray diffraction(2007-10-16); ; ; The scope of this thesis is to study the strain state, dislocation densities and other microstructuralfeatures of GaN-based layers grown by metalorganic vapor phase epitaxy(MOVPE) on (0001) sapphire and (0001) 6H-SiC substrates using x-ray techniques.Dissertation340 632 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Strukturelle Charakterisierung von Gruppe-III-Nitriden mit Hilfe der hochaufloesenden Roentgenbeugung(2000-06-15); ; ; The investigation of typical defects in GaN layers is based on both X-ray results and transmission electron microscopy investigations from H. Selke. The defect structure in GaN layers grown on GaN nucleation layers is dominated by edge type threading dislocations and inversion domains. Furthermore, possible ion induced damages during molecular beam epitaxy were investigated, where acivated nitrogen for the GaN growth is delivered by a plasma discharge. By applying a substrate bias, nitrogen ions can be attracted or rejected during growth. Remarkable ion induced damages were found in GaN layers grown with an electron cyclotron resonance plasma source at negativ bias. The damages are comparable to those induced by ion implantation though the energies are much lower. The correlation of different kinds of defects with electrical and optical properties of the layers are presented in the last section of the work. It was found that not only the threading edge type dislocations can influence the Hall mobility and the full width at half maximum of the (D^0,X) emission but also the screw dislocations and stacking faults if the density of edge dislocations is approximately constant. Layers grown by molecular beam epitaxy on a GaN nucleation layer show an unknown emission around 3.440 eV.Dissertation287 203 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Interfacial issues in wide-bandgap II-VI materials and related multiple quantum well structures(2001-01-31); ; ; This thesis is devoted to exploring two main processes potentially relevant to the physics and technology of wide-bandgap II-VI materials: :br:i) fabrication of Zn1-yCdySe quantum wells on lattice-matched substrates to implement strain-free blue-green emitters; :br:ii) optimization of the properties of metal/II-VI contacts through modification of the local interface environment. In the first area, we showed that graded composition InxGa1-xAs buffer layers on GaAs (001) wafers can be used to match the Zn1-yCdySe quantum well lattice parameter. Detailed studies of the electronic and structural properties of Zn1-zMgzSe alloys and Zn1 yCdySe/Zn1-zMgzSe heterojunction interfaces allowed us to design and fabricate strain-free Zn1 yCdySe/Zn1-zMgzSe multiple quantum well structures on the novel InxGa1-xAs substrates. We found that the surface corrugations of the III-V buffer do not hinder the structural quality of the quantum wells, and give only rise to long-period coherent undulations of quantum well and barrier layers. In the area of metal/II-VI semiconductor junctions, our experimental studies of Zn/ZnSe (001) interfaces stimulated new theoretical calculations of interface formation energies and electronic structure. Both experiment and theory indicate that this interface represents a unique example of ideally unreactive metal/ZnSe contact. Conversely, we found that Au/ZnSe contacts show evidence of interface reactions, in contrast with most earlier reports. Significant lateral inhomogeneities in the interface parameters are likely related to preferential reaction at steps and kinks of the ZnSe(001) surface. By inserting ultrathin layers of Zn at the interface between Au and ZnSe(001), we demonstrated the possibility of inducing large modifications in the Schottky barrier height, together with an improved lateral homogeneity and ideality of the resulting metal/ZnSe contacts.Dissertation259 87 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Anwendung der Roentgenbeugung zur Analyse der strukturellen Eigenschaften von homo- und heteroepitaktischen Halbleiterschichten auf ZnSe-Basis(2000-06-09); ; ; An asymmetric relaxation was observed for the azimuth directions [110] and [1-10]. The dislocation density before and after heating was estimated from the diffuse scattering. Values of 3.3 *10^3 /cm and 13 *10^3 /cm were derived for the [110]-azimuth direction. The residual strain of single ZnSe layers on GaAs substrates due to work hardening was found to be 2.0 *10^(-4) in agreement with theoretical predictions. A critical layer thickness for work hardening of about 2.0 µm was derived. Comparing the thermal expansion of homo- and heteroepitaxially grown device relevant MgZnSSe layers showed nearly equal thermal expansion coefficients of both layer types. These measurements could be utilized to extrapolate the thermal expansion coefficient of MgSe in the zincblende structure. A value of 4.2 * 10^(-6) /K was derived for the temperature range from 300 K to 500 K. In the second part of the work, the influence of two different growth start procedures on the structural perfection of homoepitaxially grown ZnSe-based layers is discussed. A drastic increase in crystalline quality of the epitaxial layers resulted from various changes in growth procedure. The most important was the introduction of an {\em in situ} hydrogen plasma cleaning of the ZnSe substrates prior to the ZnSe growth. It provides a substrate surface free from contamination enabling to reduce the defect densities in the epitaxial layers by two orders of magnitude.Dissertation309 113 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Phasenseparation von dünnen InGaN-Schichten in der metallorganischen Gasphasenepitaxie - Realisierung und Anwendung von InGaN Quantenpunkten(2011-07-06); ; ; This work is devoted to the understanding and realization of the InGaN quantum dot formation process. The growth is performed on an n-doped GaN layer in a metal organic vapor phase epitaxy reactor. Evidence has been found that spinodal and binodal decomposition are driving a separation process of a thin InGaN layer into two phases with different InN concentrations. The region with low InN content is forming the quantum dots on the surface of a GaN layer. The spinodal phase diagram has been calculated for the case of a strained InGaN layer on GaN. The accordance between theoretical and experimental results is shown. For device application it is necessary to cap these structures with a p-doped GaN layer. The influence of a GaN capping on the phase separated InGaN structures is discussed. Finally, the applications of InGaN quantum dots in LEDs, laser structures and microcavities is presented.Dissertation335 212 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Grüne oberflächenemittierende Halbleiterlaser (VCSEL) auf Basis von II-VI-Verbindungen(2004-03-15); ; ; Semiconductor-based laser diodes represent a key technology, which is used e.g. for optical data storage, data transmission and metrology purposes. However, the usual edge-emitting device design has some drawbacks concerning the properties of the emitted laser beam. This can be overcome by a more sophisticated approach called vertical-cavity surface emitting laser (VCSEL). The aim of the research within this thesis was the realization of a green fully-epitaxial VCSEL based on the II-VI material system deposited on a GaAs substrate. The structures are prepared by molecular beam epitaxy (MBE). The different building blocks for such a device like high- reflectivity distributed Bragg reflectors (DBRs) and monolithic microcavities with a sufficent optical quality (Q) factor were successfully developed. The main challenge was the achievement of a large difference delta n between high and low refractive index of the DBR layers in combination with a pseudomorphic epitaxial growth in zincblende crystal structure. A value of delta n = 0.6 has been reached for a DBR with the reflectivity maximum centered at a wavelength of 520 nm. This was possible by a special approach for the low index material, which consist of a short-period superlattice (SL) made from thin MgS and ZnCdSe layers. As a prerequisite, the growth of MgS in zincblende structure was investigated in detail in order to find suitable growth conditions. This was one crucial part of the work since the MgS compound naturally forms a rocksalt crystal structure. By choosing sulphur-rich conditions and taking advantage of a sulphur cracker evaporation cell, single zincblende MgS layers of up to 15 nm thickness were realized in good structural quality. However, the MgS thickness within the low-index SLs of the DBRs is typically much lower, i.e. in the range of 1 nm, which allows the effective stabilization in zincblende structure also for complex mulitilayer microcavity structures. Complete VCSEL structures with Q factors up to 3,000 were realized, showing lasing at a wavelength of 511 nm and a threshold excitation density of 21 kW/cm-2 under optical pumping. Starting from these planar cavity structures, micropillars have been prepared by focussed ion beam in order to create a three-dimensional confinement of the optical wave within the cavity. In addition, also first investigations with regard to an electrically-pumped VCSEL device had been performed. Since the conductivity of a p-type doped DBR is expected to be low due to the wide band-gap materials used, a reversed biased tunnel-junction light-emitting diode was realized. This injection scheme seems to be the most promising approach besides the preparation of intra-cavity contacts, which require complex etching and processing steps. Furthermore, the emission properties of CdSe quantum dots (QDs) were improved by embedding them between thin MgS barriers. These structures show a bright photo- luminescence of the QD ensemble up to room temperature, making them interesting for the realization of single photon sources as required for applications concerning quantum information science. In conclusion, within the scope of this thesis high-quality wide band-gap monolithic microcavities emitting in the green spectral range were developed, which might be used for practical applications as well as basic experimental investigations concerning quantum electrodynamics, e.g. the Purcell effect or cavity polaritons, in the future.Dissertation334 503 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Analyse struktureller Eigenschaften von GaN mittels hochauflösender Röntgenbeugung bei variabler Meßtemperatur(2007-02-26); ; ; The objective of this work is the investigation of the strain and the stress in GaN-based semiconductor layers by high-resolution x-ray diffraction (HRXRD) at variable temperatures. Due to the lack of native substrates GaN-based layers are typically grown on foreign substrates, which results in strained layers with a complex strain configuration. Moreover, heterostructures of group-III nitrides grown along the [0001] direction suffer from strong polarization fields perpendicular to their interfaces, which are undesirable for light-emitting devices whose active regions consist of quantum wells. Therefore, group-III nitride layers have been recently grown on non-polar planes, such as the a-plane. To account for the various questions regarding strain and stress in group-III-nitrides, the spectrum of different GaN layers, which were studied in this thesis, spans from bulk-like free-standing layers over thick c-plane layers to laterally overgrown structures and non-polar a-plane layers.Dissertation406 215