Optical properties of transition-metal-doped GaN and ZnO for spintronics applications

University of Technology, Sydney. Faculty of Science.Spin-based devices have the potential to take modern electronics and optoelectronics to the next level. So-called ‘spintronics’ exploit both the charge and the spin of an electron for data processing, transport and storage. A significant step towards the realisation of such devices would be to achieve room temperature ferromagnetic semiconductors. Theoretical works predict the possibility of room temperature ferromagnetism in the wide bandgap semiconductors GaN and ZnO doped with transition metals. The present models of spin-coupling in such dilute magnetic semiconductors require input in form of quantitative information on electronic states that arise from the introduction of transition metal ions into the host lattice. This work focuses on the detailed experimental investigation of such states in GaN and ZnO doped with different transition metals. A large array of Fe, Mn and Ni doped GaN and ZnO samples with different doping levels and n-type and p-type co-doping were intensively studied by a wide range of experimental techniques. The investigation of Fe doped GaP, GaAs and InP provided valuable insights into the transient shallow acceptor state constituted by a hole bound to Fe2+. The most significant results are summarised in the following: A comprehensive literature review is presented on the Fe centre in III-V and II- VI semiconductors. Experimental and theoretical data that have been obtained over a few decades were reviewed thoroughly unveiling common phenomena that can be generalised to other TMs. The positions of established Fe3+/2+ and Fe2+/1+ levels were summarised allowing for predictions on the positions of further charge transfer levels based on the internal reference rule. The Fe3+/4+ level has not been identified unambiguously in any of the studied materials. Detailed term schemes of the observed charge states in tetrahedral and trigonal crystal field symmetry are presented including fine structure, isotope effects and a dynamic Jahn-Teller effect. By means of cathodoluminescence experiments Ni and Fe doping of HVPE-grown GaN was found to promote the formation of inhomogeneous regions with increased donor density and enhanced luminescence efficiency. In these regions richly structured cathodoluminescence patterns are observed at the surface. By means of optical studies on high quality Fe doped GaN samples the electronic structure of Fe3+ and Fe2+ was established in great detail. The effects of spin-orbit interaction, of the axial distortion of the crystal held in hexagonal GaN and of the Jahn-Teller coupling were successfully investigated. Both the Fe3+ centre and the Fe2+ centre were found to be stabilised against a dynamic Jahn Teller effect by the trigonal symmetry of the wurtzite lattice. A bound state with a binding energy of 50±10 meV was identified as a hydrogenic state consisting of a hole localised at an Fe2+ centre. This [Fe2,h] state represents a transient shallow acceptor state. It could be described by effective-mass-theory revealing an effective Bohr radius of 1.5 nm which may enable a long-range spin interaction via overlapping wavefunctions at relatively low Fe doping. The position of the Fe3+/2+ acceptor level could be narrowed down to 2.863±0.005 eV above the valence band maximum. Acting as a deep acceptor Fe incorporation was shown to quench the intrinsic yellow luminescence of GaN by lowering the Fermi level and passivating native donor states. Implications concerning the internal reference rule are discussed. A deep understanding of the effective-mass-like state [Fe2+,h] could be obtained by temperature and stress dependent measurements on Fe doped GaP, GaAs and InP. Besides the ground state, the hole was observed in several excited hydrogenic states each involving different Fe2+ fine structure states. Particularly for the hydrogenic ground state, a weak exchange interaction was found between the hole Fe2+ core states. Due to finite p-d hybridisation of Fe orbitals with the valence band, a weaker binding energy was observed for the ground state than predicted by effective mass theory. Finally, with regard to the Fe3+ ground state, 6A1(S), in GaP and InP, the hyperfine structure level T8 was found to be above the T7 level. ZnO:Fe samples were prepared by Fe coating ZnO crystals, which were grown from the gas phase, and subsequent annealing under varying atmospheres. In these samples the internal Fe2+(5E—5T2) transition was observed for the first time at 395.7 meV by means of Fourier transform infrared transmission spectroscopy. This value is in good agreement with the general trend in III-V and II-VI materials that the (5E—5T2) energy rises with an increasing degree of ionicity and decreasing lattice constant. No axial symmetry was found for the Fe2+ centre which is unusual for wurtzite ZnO. Possible reasons are discussed taking into account a strong Jahn- Teller effect, the non-constant c/a-ratio of ZnO and a high concentration of defects. Moreover, Fe-defect complexes and local vibrational modes could be identified. A large array of GaN samples with varying Mn concentrations and n-type and p-type co-doping allowed for a systematic charge state tuning by shifting the Fermi level providing access to the oxidation states Mn2+, Mn3+ and Mn4+. The respective electronic structures were investigated by means of optical and magnetic techniques. The Mn3+ centre and Mn4+ centre showed clear effects of degradation of crystal quality as a result of Mn, Si and Mg doping. A strong tendency was demonstrated for the formation of Mn-Mg complexes. A photoluminescence structure found around 1 eV in Mg co-doped GaN:Mn samples was proven to originate from Mn4+ involved in such complexes. A resonant Stokes process by secondary excitation and stimulated hole transfer was established in these Mn-Mg complexes. The Mn3+/4+ donor and Mn3+/2+ acceptor levels were found 1.15 eV and 1.65 eV above the VB maximum, respectively, compensating n-type and p-type doping. As a consequence, there is no reasonable chance to achieve high carrier concentrations in GaN:Mn, a precondition for free-carrier-mediated spin-coupling. The results presented in this thesis contribute to the general understanding of transition-metal-related electronic states in III-V and II-VI semiconductors, particularly in GaN and ZnO. These new insights are valuable contributions to a targeted design of dilute magnetic semiconductors that will help to, one day, realise next- generation spintronic devices

Structural and optical inhomogeneities of Fe doped GaN grown by hydride vapor phase epitaxy

We present the results of cathodoluminescence experiments on a set of Fe doped GaN samples with Fe concentrations of 5?1017, 1?1018, 1?1019, and 2?1020 cm-3. These specimens were grown by hydride vapor phase epitaxy with different concentrations of Fe. The introduction of Fe is found to promote the formation of structurally inhomogeneous regions of increased donor concentration. We detect a tendency of these regions to form hexagonal pits at the surface. The locally increased carrier concentration leads to enhanced emission from the band edge and the internal 4T1(G)?6A1(S) transition of Fe3+. In these areas, the luminescence forms a finely structured highly symmetric pattern, which is attributed to defect migration along strain-field lines. Fe doping is found to quench the yellow defect luminescence band and to enhance the blue luminescence band due to the lowering of the Fermi level and the formation of point defects, respectivel

Optical properties of Mn-doped GaN

Molecular beam epitaxy-grown GaN with different Mn concentrations (5-23×1019 cm-3) and codoped with Si were investigated by cathodoluminescence (CL) spectroscopy and optical transmission measurements. In the GaN:Mn, an intense absorption peak at 1.414 +/- 0.002 eV was observed. This peak was attributed to an internal 5T 2→ 5E transition of the deep neutral Mn3+ state since its intensity scaled with the Mn3+ concentration. The CL measurements showed that Mn-doping concentrations around 1020 cm -3 had three effects on the emission spectrum: (i) the donor bound exciton at 3.460 eV was reduced by more than one order of magnitude, (ii) the donor-acceptor-pair band at 3.27 eV was completely quenched and (iii) the yellow luminescence centered at 2.2 eV was the strongly decreased. The latter two effects were attributed to a reduced concentration of VGa. In the infrared spectral range, three broad, Mn-doping related CL emission bands centered at 1.01 ± 0.02 eV, 1.09 ± 0.02 eV and 1.25 ± 0.03 eV were observed. These bands might be related to deep donor complexes, which are generated as a result of the heavy Mn-doping, rather than internal transitions at the Mn atom

Experimental probing of exchange interactions between localized spins in the dilute magnetic insulator (Ga,Mn)N

The sign, magnitude, and range of the exchange couplings between pairs of Mn ions is determined for (Ga,Mn)N and (Ga,Mn)N:Si with x < 3%. The samples have been grown by metalorganic vapor phase epitaxy and characterized by secondary-ion mass spectroscopy; high-resolution transmission electron microscopy with capabilities allowing for chemical analysis, including the annular dark-field mode and electron energy loss spectroscopy; high-resolution and synchrotron x-ray diffraction; synchrotron extended x-ray absorption fine-structure; synchrotron x-ray absorption near-edge structure; infra-red optics and electron spin resonance. The results of high resolution magnetic measurements and their quantitative interpretation have allowed to verify a series of ab initio predictions on the possibility of ferromagnetism in dilute magnetic insulators and to demonstrate that the interaction changes from ferromagnetic to antiferromagnetic when the charge state of the Mn ions is reduced from 3+ to 2+.Comment: 12 pages, 14 figures; This version contains the detailed characterization of the crystal structure as well as of the Mn distribution and charge stat

Examination of a Standardized Test for Evaluating the Degree of Cure of EVA Encapsulation: Preprint

The curing of cross-linkable encapsulation is a critical consideration for photovoltaic (PV) modules manufactured using a lamination process. Concerns related to ethylene-co-vinyl acetate (EVA) include the quality (e.g., expiration and uniformity) of the films or completion (duration) of the cross-linking of the EVA within a laminator. Because these issues are important to both EVA and module manufacturers, an international standard has recently been proposed by the Encapsulation Task-Group within the Working Group 2 (WG2) of the International Electrotechnical Commission (IEC) Technical Committee 82 (TC82) for the quantification of the degree of cure for EVA encapsulation. The present draft of the standard calls for the use of differential scanning calorimetry (DSC) as the rapid, enabling secondary (test) method. Both the residual enthalpy- and melt/freeze-DSC methods are identified. The DSC methods are calibrated against the gel content test, the primary (reference) method. Aspects of other established methods, including indentation and rotor cure metering, were considered by the group. Key details of the test procedure will be described

Grosse Energie aus Nanokügelchen hocheffziente Solarzellen aus Silizium Nanopartikeln

Der Preis für Solarstrom hängt neben den Produktionskosten für Solarzellen vor al lem vom Wirkungsgrad ab, also dem Ver hältnis von eingestrahlter Sonnenenergie zu erzeugter elektrischer Energie. Den grö ten Marktanteil stellen zurzeit Solar zellen aus kristallinen Siliziumscheiben. Aus physikalischen Gründen kann der Wirkungsgrad dieser konventionellen So larzellen, selbst bei perfekter technologi scher Ausführung, die sogenannte Shock ley Queisser Grenze von 33 Prozent nicht übersteigen. Will man beim etablierten, günstigen und ungiftigen Material Silizium bleiben, sind neue Konzepte gefragt Durch die Ver kleinerung von Siliziumstrukturen auf wenige Nanometer kommen Effekte der Quantenphysik zum Tragen. Diese Effek te bergen das Potenzial einer Wirkungs gradsteigerung über die Shockley Queis ser Grenze hinaus. Ein weiterer Vorteil solch kleiner Siliziumstrukturen ist die enorme Reduktion des Materialver brauchs im Vergleich zu Siliziumschei ben. Zurzeit machen die Materialkosten rund ein Drittel der Herstellungskosten klassischer Silizium Solarzellen aus. Durch dieses Potenzial motiviert, stellen Forscher des Helmholtz Zentrums Berlin HZB im Rahmen des vom BMBF geför derten Projektes SINOVA Silizium Na nopartikel her und setzen sie in Solarzel len der nächsten Generation ein. Für die Synthese solch kleiner Partikel auf gro fächigen Solarzellen erforscht das HZB zwei Methoden der Selbstorganisation Erstens die Entnetzung eines dünnen Siliziumflms. Vergleichbar mit Wasser, welches auf einer Glasscheibe Tröpfchen bildet, bildet auch Silizium auf Glas unter bestimmten Bedingungen kleine Kugeln mit wenigen Nanometern Durchmesser. Diese Methode bietet eine gute Kontrolle über Grö e und Abstand der Nanokügel chen. Eine alternative Methode basiert auf der Entmischung von Siliziumoxid. Die ses SiO bildet während eines Heizprozes ses Silizium Nanopartikel eingebettet in SiO2, also Glas. Diese Methode lässt sich mittels üblicher Abscheideanlagen für Si lizium durchführen, bietet aber weniger Kontrolle über Grö e und Abstand der Silizium Nanopartikel. Derzeit arbeitet das HZB daran, Fremd atome in die Nanopartikel einzubringen. Dies ist eine wichtige Voraussetzung da für, dass die Absorption des Sonnenlichts eine elektrische Spannung hervorruf. Sobald auch diese Hürde genommen ist, kann die Nanotechnologie mithilfe von Quantenefekten den Wirkungsgrad von Silizium Solarzellen über die Shockley Queisser Grenze hinaus erhöhe

Fe in III-V and II-VI semiconductors

Many theoretical and experimental studies deal with the realization of room-temperature ferromagnetism in dilute magnetic semiconductors (DMS). However, a detailed quantitative understanding of the electronic properties of transition metal doped semiconductors has often been neglected. This article points out which issues concerning electronic states and charge transfers need to be considered using Fe as an example. Methods to address these issues are outlined, and a wealth of data on the electronic properties of Fe doped III-V and II-VI compound semiconductors that have been obtained over a few decades is reviewed thoroughly. The review is complemented by new results on the effective-mass-like state consisting of a hole bound to Fe2+ forming a shallow acceptor state. The positions of established Fe3+/2+ and Fe2+/1+ charge transfer levels are summarized and predictions on the positions of further charge transfer levels are made based on the internal reference rule. The Fe3+/4+ level has not been identified unambiguously in any of the studied materials. Detailed term schemes of the observed charge states in tetrahedral and trigonal crystal field symmetry are presented including hyperfine structure, isotope effects and Jahn-Teller effect. Particularly, the radiative transitions Fe3+(4T 1 → 1A1) and Fe2+(5E → 5T2) are analyzed in great detail. An effective-mass-like state [Fe2+, h] consisting of a hole bound to Fe2+ is of great significance for a potential realization of spin-coupling in a DMS. New insights on this shallow acceptor state could be obtained by means of stress dependent and temperature dependent absorption experiments in the mK range. The binding energy and effective Bohr radius were determined for GaN, GaP, InP and GaAs and a weak exchange interaction between the hole and the Fe2+ center was detected. With regard to the Fe 3+ ground state, 6A1, in GaP and InP, the hyperfine structure level Γ8 was found to be above the Γ7 level. All results are discussed with respect to a potential realization of a ferromagnetic spin-coupling in DMSs. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA

Development of ultra thin tunneling oxides and Si SiO2 nanostructures for the application in silicon solar cells

The synthesis of Si SiO2 nanostructures for the application as hetero emitter and passivation layer in high efficiency solar cells is explored with the long term perspective of exploiting quantum size effects for next generation photovoltaics. Ultra thin oxides are developed by means of oxidizing crystalline silicon using neutral oxygen atoms supplied by a RF plasma source. These oxides are characterized by an abrupt Si SiO2 junction and good passivation of Si dangling bonds at the interface, a precondition for the implementation of Si SiO2 nanostructures in photovoltaic devices. Another decisive issue is carrier transport across the SiO2 barrier which is demonstrated in form of a tunneling current in I V measurements. Such tunneling oxides on Si 111 wafers are used as substrates for the subsequent deposition of Si nanodots. Nanodot synthesis is accomplished by first depositing a few nanometer thick layer of amorphous Si. During a subsequent recrystallization step at T amp; 8201; gt; amp; 8201;600 amp; 8201; C, the surface tension between c Si and SiO2 causes a dewetting and, thus, the formation of crystalline Si nanodots, the size of which can be controlled by the thickness of the initial amorphous Si layer. Finally the dots are cladded with a shell of tunneling oxide. A close packing of spheres with dots stacked on top of each other is approximated by a repetition of this proces

Structural and electronic properties of Fe3+ and Fe2+ centers in GaN from optical and EPR experiments

This work provides a consistent picture of the structural, optical, and electronic properties of Fe-doped GaN. A set of high-quality GaN crystals doped with Fe at concentrations ranging from 5× 1017 cm-3 to 2× 1020 cm-3 is systematically investigated by means of electron paramagnetic resonance and various optical techniques. Fe3+ is shown to be a stable charge state at concentrations from 1× 1018 cm-3. The fine structure of its midgap states is successfully established, including an effective-mass-like state consisting of a hole bound to Fe2+ with a binding energy of 50±10 meV. A major excitation mechanism of the Fe3+ (T14 → A16) luminescence is identified to be the capture of free holes by Fe2+ centers. The holes are generated in a two-step process via the intrinsic defects involved in the yellow luminescence. The Fe3+/2+ charge-transfer level is found 2.863±0.005 eV above the valence band, suggesting that the internal reference rule does not hold for the prediction of band offsets of heterojunctions between GaN and other III-V materials. The Fe2+ (E5 → T25) transition is observed around 390 meV at any studied Fe concentration by means of Fourier transform infrared spectroscopy. Charge-transfer processes and the effective-mass-like state involving both Fe2+ states are observed. At Fe concentrations from 1× 1019 cm-3, additional lines occur in electron paramagnetic resonance and photoluminescence spectra which are attributed to defect complexes involving Fe3+. With increasing Fe concentration, the Fermi level is shown to move from near the conduction band to the Fe3+/2+ charge-transfer level, where it stays pinned for concentrations from 1× 1019 cm-3. Contrary to cubic II-VI and III-V materials, both electronic states are effected by only a weak Jahn-Teller interaction. © 2006 The American Physical Society