PhD theses

Physics, MOVPE growth, and investigation of m-plane GaN films and InGaN/GaN quantum wells on γ-LiAlO₂ substrates (2012)

• Dr. Christof Mauder (Summary)

  LED emit light efficiently and the color of the light can be adjusted well during the LED production process. In most cases, III-V semiconductors are used for the active, light-emitting layer. Group III nitrides (e.g. GaN/InGaN) are used for blue LED or laser diodes. Group III nitrides form a hexagonal crystal structure. This structure does not have an inversion center in the direction in which the layers grow (c-plane), which means that light generation is interfered by spontaneous and piezoelectric polarizations (“quantum confined Stark effect”, in short “QCSE”). To reduce the impact of polarization, in this work the crystalline layers were grown not along the c-axis, but along the m-axis (the lateral plane of the hexagon). This way, polarization is vertical to the light diode’s electrical current. To facilitate crystal growth along the m-axis, γ-LiAlO₂ was chosen as the substrate. Here, the lattice constants of the substrate hardly deviate from those of the epitaxial layer. This promotes growth in the direction of the m-axis.
  In this work, among other things, a nitridation step before epitaxy was introduced, which enabled high-quality growth. This led to relatively few threading dislocations and basal plane stacking faults (crystal defects). Furthermore, m-plane InGaN/GaN MQW structures were grown with different InN contents (5%-30%). High InN contents (>16%) broadened the PL spectra, which might indicate InN clusters. Low InN contents significantly weakened the QCSE. Ultimately, it was possible to fabricate m-plane InGaN/GaN-LED with improved electrical and optical properties.

   

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Comprehensive study on MOVPE of InAlN/GaN HEMT structures and GaN nanowires (2013)

• Dr. Hannes Behmenburg (Summary)

  GaN-based high electron mobility transistors (HEMT) have superb properties in high-frequency ranges. Among other features, these facilitate faster mobile networks. Due to its high breakdown field strength, electron mobility and saturation velocity, GaN is better suited for high-frequency applications than other materials (Si, GaAs, etc.). These kinds of HEMT make it possible to achieve high frequencies and high power densities. This work investigated the use of InAlN as a barrier layer material instead of the usual AlGaN barrier. This material facilitates even greater power densities given the higher carrier densities and higher operation frequency resulting from the thinner barrier layer. Growth conditions for the respective HEMT layers were optimized and a new SiN passivation was analyzed for the barrier layer. State-of-the-art insulating resistances, lateral breakdown field strengths and power densities (at 18 and 40 GHz) were achieved.
  Furthermore, semiconductor nanowires were fabricated in the vapor-liquid-solid process. These nanostructures were investigated with regard to their application in future component concepts. The work succeeded in achieving high growth densities for straight GaN nanowires showing no threading dislocations (a type of crystal defect).

   

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Understanding and Optimization of InN and high Indium containing InGaN alloys by Metal Organic Chemical Vapor Deposition (2013)

• Dr. Öcal Tuna (Summary)

  Light emitting diodes (LED) emit light efficiently and are mostly based on III-V semiconductors. Group III arsenides and group III phosphides (such as GaAs or AlP) are suitable for red to yellow light. For green or blue LED, however, group III nitrides (such as GaN) are required, as these have larger bandgaps. Using ternary compounds (e.g. InGaN), the whole visible range of light can be covered depending on the InN content, as InN has a comparatively small bandgap. Furthermore, InN has high electron mobility, which makes it interesting for electronics applications with high currents or frequencies. Unfortunately, depositing InGaN is not an easy process, as InN decomposes at comparatively low temperatures already, not least as InN and GaN have strongly deviating lattice constants and the vapor pressures also differ greatly. In this work, InN was nevertheless deposited and investigated. High V/III concentrations and low growth temperatures led to improved crystal quality, which was documented using Raman measurements. Hall measurements were deemed unsuitable as numerous surface charges are accumulated for InN, thus distorting the measurement. To deposit InGaN layers of high quality, an InN interlayer was grown before the InGaN layer. By the migration of InN to the InGaN layer, the InN content in the InGaN layer could be increased from 74% to 85%. This way, high-quality InGaN layers with InN contents of between 40% and 85% could be successfully grown. However, InGaN layers with medium InN contents showed a high degree of disorder, a factor which impacted negatively on crystal quality. Furthermore, high p-doped (Mg) InGaN layers were investigated. In the case of over-doping, the InGaN layers changed their conductivity to n-type due to Mg clusters with N vacancies.

   

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Investigation of HCl-assisted MOVPE of group-III-nitrides in a planetary hot-wall system (2014)

• Dr. Dirk Fahle (Summary)

  Efficient light emitting diodes (LED) on group III nitride basis can cover the entire spectrum of visible light as well as the UV range. To assert themselves over other technologies, the production price will have to be reduced significantly. The price equivalent to the the number of LED that can be grown in one reactor (larger reactors) and the faster these can be grown (higher growth rate). Both factors are limited by parasitic vapor phase reactions of the precursor. The objective of this work was to suppress this kind of parasitic vapor phase reaction using hydrogen chloride (HCl) and to raise growth rates. To prevent parasitic deposition of NH₄Cl (NH₃ + HCl), a hot-wall reactor with a three-level injector was used. For GaN, comparatively low, but pressure-dependent ratios of HCl to TMGa were sufficient to prevent parasitic vapor phase reactions. This enabled high growth rates of 16 µm/h to be achieved with the same mobility and morphology, as HCl etched particularly in the gas phase and hardly the layer on the substrate. GaCl did not contribute to growth. In the case of AlN, significantly higher HCl to TMAI ratios were necessary, making it difficult to use HCl in ternary compounds (AlGaN). Building on these findings, the etching process for HCI on GaN and AlN was investigated more closely. Among other factors, the use of pulsed HCl introduction increased the GaN etching rate while drastically reducing HCl consumption. For AlN, higher temperatures were required and the results were poorer. It was apparent that Cl2 was more suitable for in-situ cleaning, as it also etches AlN well. Furthermore, AlN buffer layers were grown on α-Al₂O₃ for UV-LED applications. This approach serves as an alternative to AlN substrates for UV-LEDs. AlN substrates are currently limited in volume and small (<2 inch), making them unsuitable for industrial applications. However, the starting conditions (reactor coating by previous runs) impacted negatively on reproducibility and crystal quality. Performing previous cleaning and using an AlN interlayer significantly improved the crystal quality (defect density reduced from 2.3*10¹⁰cm^-2 to 1.4*10¹⁰cm^-2).

   

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MOVPE growth and characterization of GaN/InGaN nanowires and microrods for next generation solid-state-lightning applications (2016)

• Dr. Bartosz Foltyński (Summary)

  The monolithic integration of optical components made from III-V semiconductors (e.g. LED) with advanced silicon technology is difficult, as the lattice constants of the semiconductors vary widely. III-nitrides even have hexagonal crystal lattices, which do not match the cubic silicon crystal structure. Nanowires and micorods might nevertheless make it possible to achieve high crystal qualities of the grown layer. In this work, vertical nano-columns were grown on Si(111) substrates using MOCVD. This involved using three different growth techniques. The work began by investigating the vapor-liquid-solid method (using gold droplets). The high reactivity between gold and silicon prevented direct growth on silicon. For that reason, sapphire substrates were initially used as an alternative. The NW (nanowires) now grown showed inhomogeneous In contents along the c axis. Particularly at the base of the NW, the coaxial InGaN MQW did not form completely as growth mainly occurred in the gold droplets. Nevertheless, no gold was built into the NW. Sophisticated characterization methods showed that the GaN core was stress-free. The second technique used was “selective area growth” (SAG) on Si(111). This involved applying a mask with different sizes and shapes of openings on the substrate. By working with adjusted growth parameters and silane (SiH₄) gas in the reactor, vertical GaN columns grew in the cavities. Excessively long growth periods, however, resulted in unwanted growth on the mask. The crystal quality was similarly high to that of stress-free bulk crystals. The final technique used was “self-organized growth”. With the assistance of an AlN buffer layer on the Si(111) substrate, it was possible to apply NW, with some of these at an angle. The silane disrupted the MQW formation at the base of the NW, meaning that the light emitted at the bottom was rather red than blue.

   

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Master theses

Growth and Characterization of AlGaN for UV emitting LED (2017)

• Summary

  UV LED are being used in ever more applications, ranging from water sterilization to bank bill identification. UV LED are based on group-III-nitride semiconductors such as AlGaN, as these have large bandgaps. For the UVB and UVC ranges, AlGaN LEDs with very high Al contents are required. Unfortunately, UV LED still have comparatively low efficiency levels, as numerous impurities serving as non-radiative recombination centers are built into the compound semiconductors. AlN substrates are currently too expensive for commercial use, as a result of which sapphire is used as an alternative. This work investigated AlN growth with the aim of minimizing defects in deposition. The morphology of the layers grown was temperature-dependent. The best results were achieved between 1,200°C and 1,350°C, as lower temperatures led to island growth and higher temperatures resulted in etching processes and side reactions with the SiC susceptor. Furthermore, the use of N₂ and H₂ as carrier gases was investigated. Using H₂ led to desorption processes at high temperatures, reducing the growth rate. This was not observable for inert N₂. At lower temperatures, N₂ nevertheless reduced the diffusion coefficients for organometallic molecules significantly more than H₂, thus significantly reducing the growth rate achieved with N₂. Moreover, a linear correlation was mostly identified between the TMAI flow rate and the growth rate, with sublinearities indicating parasitic reactions at high temperatures or base material flows. Finally, it was found that a high NH₃ flow hindered the mobility of the Al ad atoms, impacting negatively on the growth rate.

Cryogenic characterization of 4H-SiC high power MOSFET (2017)

• Summary

  Silicon carbide (SiC) is a compound semiconductor with a large bandgap and, in theory, good electron mobility. SiC is thus particularly well-suited for high-power elements such as MOSFET. At present, however, the mobilities achieved fall short of expectations, as they are disrupted by electron traps at the SiO₂/SiC boundaries. This work investigated a post-oxidation anneal (POA) using NO instead of the usual N₂ to reduce impurities. The MOSFET realized in this work using NO POA showed significantly better mobilities and on-resistances than the MOSFET produced with N₂ POA. The energetic positions of the impurities was determined using cryogenic characterization methods on E_C-0,13eV (interface) and E_C-0,3eV (near interface). Changing the NO flow by ±30% during the POA did not show any noticeable change in the density of the electron traps. The boundary electron traps (and two hole traps) were reduced with NO POA, with the traps close to the interface remaining virtually unaffected. It was suspected that the interface traps involved impurities resulting from the ion implantation of the p-well, which probably reduced the mobility.