| 1. |
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| 2. |
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| 3. |
- Danielsson, Örjan, et al.
(författare)
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Investigation of the temperature profile in a hot-wall SiC chemical vapour deposition reactor
- 2002
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Ingår i: Journal of Crystal Growth. - : ScienceDirect. - 0022-0248 .- 1873-5002. ; 235:1-4, s. 352-364
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Tidskriftsartikel (refereegranskat)abstract
- The chemical vapor deposition (CVD) technique is widely used to grow epitaxial layers of silicon carbide. To meet the demands for high quality epitaxial layers, which have good morphology and a minimum variation of the doping and thickness, a good knowledge of the CVD process is essential. The present work uses a simulation tool to investigate several parameters influencing the heating of a hot-wall CVD reactor. The simulations are set up as 2D axisymmetric problems and validation is made in a 2D horizontal hot-wall CVD reactor. By applying the knowledge achieved from the simulations, the temperature profile is optimized to give as large area as possible with homogeneous temperature. New susceptor and coil designs are tested. A very good agreement between the simulated and the measured results is obtained. The new design has a temperature variation of less than 0.5% over more than 70% of the total susceptor length at an operating temperature of 1650°C. In addition, the power input needed to reach the operating temperature is decreased by 15% compared to the original design. 3D simulations are performed to show that the changes made in the 2D case give similar results for the real 3D case.
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| 4. |
- Forsberg, Urban, 1971-, et al.
(författare)
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Aluminum doping of epitaxial Silicon Carbide
- 2003
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Ingår i: Journal of Crystal Growth. - : ScienceDirect. - 0022-0248 .- 1873-5002. ; 253:1-4, s. 340-350
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Tidskriftsartikel (refereegranskat)abstract
- Intentional doping of aluminum in 4H and 6H SiC has been performed using a hot-wall CVD reactor. The dependence of aluminum incorporation on temperature, pressure, C/Si ratio, growth rate, and TMA flow has been investigated. The aluminum incorporation showed to be polarity dependent. The high aluminum incorporation on the Si-face is closely related to the carbon coverage on the SiC surface. Changes in process parameters changes the effective C/Si ratio close to the SiC surface. Increased growth rate and C/Si ratio increases the aluminum incorporation on the Si-face. Diffusion limited incorporation occurs at high growth rate. Reduced pressure increases the effective C/Si ratio, and at low growth rate, the aluminum incorporation increases initially, levels off at a critical pressure, and continues to decrease below the critical pressure. The aluminum incorporation showed to be constant in a temperature range of 50°C. The highest atomic concentration of aluminum observed in this study was 3×1017 and 8×1018 cm−3 in Si and C-face, respectively.
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| 5. |
- Forsberg, Urban, 1971-
(författare)
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CVD Growth of Silicon Carbide for High Frequency Applications
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Silicon Carbide (SiC) is an important wide band gap semiconductor with outstanding electronic properties. With figures of merit far better than silicon, SiC is believed to replace and outcompete silicon in many applications using high frequencies, high voltage and high temperatures. With the introduction of seeded sublimation technique, a realisation of substrates with large diameter and high quality became possible. Recent progress in the bulk growth using high temperature chemical vapour deposition (HTCVD) has shown excellent results with high purity substrates with semi insulating (SI) properties. The availability of high quality SI substrates allows the fabrication of microwave devices with low rf losses such as the Metal Schottky Field Effect Transistor (MESFET). With the introduction of the hot-wall CVD technique, thick low doped n-type epitaxial layers have been grown for high power devices (> 4 kV) such as the PiN diode.The main contribution of the present work relates to the investigation of growth of MESFET structures. The goal has been to demonstrate the ability to grow MESFET structures using the hot-wall CVD technique. The challenge with abrupt interfaces and controlled doping has been investigated. A comprehensive investigation has been made on how nitrogen and aluminum dopant atoms incorporate into the SiC lattice using the hot-wall CVD technique. Fundamental research of MESFET structures has been combined with growth of device structures for both Swedish and European groups as well as industries. The research has been focused towards the understanding of dopant incorporation, characterization of doped epitaxial layers, the growth of device structures, the modelling of temperature distribution in a hot-wall susceptor and the development of growth systems for future up scaling.In paper 1 we present how the nitrogen dopant is incorporated into the SiC lattice. The influence of several different growth parameters on the nitrogen incorporation is presented. Equilibrium thermodynamical calculations have been performed to give a further insight into the incorporation mechanism. The investigation shows that the N2 molecule itself does not contribute directly to the nitrogen incorporation, however, molecules like the HCN and HNC are more likely.In paper 2 the incorporation of the aluminum dopant into the SiC lattice is investigated in a similar way as the nitrogen incorporation in paper 1. The results show that the aluminum incorporation in SiC is mainly controlled by the carbon coverage on the SiC surface. The investigation shows that it is difficult to obtain high aluminum doping on carbon face whereas the silicon face is sensitive to changes of the growth parameters. High growth rate resulted in a diffusion controlled incorporation.In Paper 3 we present the results from the growth of MESFET structures as well as characterization of the structures and final device properties. Knowledge taken from paper 1 and 2 was used to improve the abruptness of the grown structures.Paper 4 presents the results obtained by low temperature photoluminescence (LTPL) on separately grown 4H-SiC epitaxial layers. Doping calibration curves for nitrogen in the doping range from 1⋅1014 to 2⋅1019 cm-3 are presented. A discussion concerning the Mott transition is also presented.Paper 5 presents the results of the use of simulation to investigate the heating of a hot-wall CVD reactor. New susceptor and coil design are tested. The simulation has been verified with experimental heating tests which show excellent agreement. The new design has a temperature variation of less than 0.5 % over more than 70% of the total susceptor length in addition to a decreased power input of 15 %.In the final two papers, paper 6 and 7, we present work of growth of AlN on SiC. Thin films were grown and characterized with different techniques concerning crystal quality and thickness. The use of infrared reflectance and the features of the AlN reststrahl reflectance band allowed us to determine the thickness of AlN films as thin as 250 Å.
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| 6. |
- Forsberg, Urban, 1971-, et al.
(författare)
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Growth and characterisation 4H-SiC MESFET structures grown by Hot-Wall CVD
- 2001
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Ingår i: Proc. of the MRS 2000 Fall Meeting. ; , s. H2.3.2-
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Konferensbidrag (refereegranskat)abstract
- Metal semiconductor field effect transistor structures have been grown in a hot-wall CVD reactor. Using trimethylaluminium and nitrogen, p- and n-type epitaxial layers were grown on semi insulating substrates. A comprehensive characterization study of thickness and doping of these multi structures has been performed by using scanning electron microscopy , secondary ion mass spectrometry, capacitance-voltage and low temperature photoluminescence. Optimisation of growth parameters has resulted in very abrupt doping profiles. The grown metal semiconductor field effect transistor structures have been processed and parts of the transistor properties are presented.
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| 7. |
- Forsberg, Urban, 1971-, et al.
(författare)
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Growth of high quality AlN Epitaxial Films by Hot-Wall Chemical Vapour Deposition
- 1998
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Ingår i: Proceedings of the International Conference on Silicon Carbide, III-Nitrides and Related Materials, 1997. ; , s. 1133-1136
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Konferensbidrag (refereegranskat)abstract
- Epitaxial films of high quality AlN have been grown on SiC substrates at 1200 °C and 1450 °C, using a hot-wall CVD reactor. The thickness of the epitaxial layers were measured using room temperature infrared reflectance. To verify the crystal quality, X-ray diffraction (XRD) rocking curves of the ALN 0002 peak were measured. A 250 Å thick film grown at 1450°C had a full width half maximum (FWHM) of 42 arcsec, whereas a 1000 Å thick film grown at 1200 °C had a FWHM of 100 arcsec. A TEM image of the sample grown at the lower temperature showed thickness of around 950 Å, thereby verifying the infrared reflectance measurements. We conclude that the higher temperature the better the crystal quality we obtain.
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| 8. |
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| 9. |
- Forsberg, Urban, 1971-, et al.
(författare)
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Nitrogen doping of epitaxial Silicon Carbide
- 2002
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Ingår i: Journal of Crystal Growth. - : ScienceDirect. - 0022-0248 .- 1873-5002. ; 236:1-3, s. 101-112
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Tidskriftsartikel (refereegranskat)abstract
- Intentional doping with nitrogen of 4H- and 6H-SiC has been performed using a hot-wall CVD reactor. The nitrogen doping dependence on the temperature, pressure, C/Si ratio, growth rate and nitrogen flow has been investigated. The nitrogen incorporation for C-face material showed to be C/Si ratio independent, whereas the doping decreased with increasing C/Si ratio for the Si-face material in accordance with the “site-competition” model. The nitrogen incorporation was constant in a temperature “window” of 75°C on Si-face material indicating a mass transport limited incorporation. Increasing the growth rate resulted in a decrease of nitrogen incorporation on Si-face but an increase on C-face material. Finally, a comparison between previously published results on cold-wall CVD-grown material and the present hot-wall-grown material is presented.
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| 10. |
- Gunnarsson, Svante, 1959-, et al.
(författare)
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Reflections about reflections
- 2023
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Ingår i: Proceedings of the 19th CDIO International Conference. - : NTNU SEED. - 9788230361863 ; , s. 56-66
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Konferensbidrag (refereegranskat)abstract
- A case study of the use of reflections within the Applied physics and electrical engineering program at Linköping University is presented. Reflections have been used for several years and they are done at four stages in the program, in terms of reflections at the end of the Introductory course in year one, design-implement experiences in year three and five, and a reflection document that is the last component of the Master’s thesis. In the first three stages a project model is used to support the planning and execution of the project, and in the project model the project work ends with a reflection. In the reflection document connected to the Master’s thesis the student reflects upon both the thesis work itself and the entire education program, according to the sections and subsections of the CDIO Syllabus. The paper describes how the reflections are integrated in the program. Experiences from student perspective are collected in a small-scale study via interviews with students from year one and year five.
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| 11. |
- Henry, Anne, et al.
(författare)
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Determination of nitrogen doping concentration in doped 4H-SiC epilayers by low temperature photoluminescence
- 2005
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Ingår i: Physica Scripta. - 0031-8949 .- 1402-4896. ; 72:2-3, s. 254-257
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Tidskriftsartikel (refereegranskat)abstract
- A complete calibration of nitrogen concentration in doped 4H-SiC material is presented. This is done in the very large range of doping available today, i.e. from low 1014 to 1019 cm-3. The samples are 4H-SiC films fabricated by hot-wall chemical vapour deposition. Low temperature photoluminescence is used as the experimental tool. For doping concentrations less than 8 × 1017 cm-3 comparison between the intensity of various luminescence lines is used, whereas for doping higher than 3 × 1018 cm-3 the energy position of an observed broad band allows the determination of the doping level.
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| 12. |
- Huang, Jing-Jia, 1990-, et al.
(författare)
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Growth of silicon carbide multilayers with varying preferred growth orientation
- 2022
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Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 447
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Tidskriftsartikel (refereegranskat)abstract
- SiC multilayer coatings were deposited via thermal chemical vapor deposition (CVD) using silicon tetrachloride (SiCl4) and various hydrocarbons under identical growth conditions, i.e. at 1100 °C and 10 kPa. The coatings consisted of layers whose preferred growth orientation alternated between random and highly 〈111〉-oriented. The randomly oriented layers were prepared with either methane (CH4) or ethylene (C2H4) as carbon precursor, whereas the highly 〈111〉-oriented layers were grown utilizing toluene (C7H8) as carbon precursor. In this work, we demonstrated how to fabricate multilayer coatings with different growth orientations by merely switching between hydrocarbons. Moreover, the success in depositing multilayer coatings on both flat and structured graphite substrates has strengthened the assumption proposed in our previous study that the growth of highly 〈111〉-oriented SiC coatings using C7H8 was primarily driven by chemical surface reactions.
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| 13. |
- Huang, Jing-Jia, 1990-
(författare)
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Surface-Controlled Chemical Vapor Deposition of Silicon Carbide
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polycrystalline cubic silicon carbide, 3C-SiC, has long been investigated in the field of hard coating materials. The typical synthesis method for 3C-SiC coatings is thermal chemical vapor deposition (CVD) using either multicomponent precursors, e.g. methyltrichlorosilane, or a combination of single component precursors, e.g. silane and propane. In this thesis, the fabrication of polycrystalline SiC coatings has been explored from the new aspects on the basis of thermal CVD utilizing silicon tetrachloride (SiCl4) and various hydrocarbons, i.e. toluene (C7H8), methane (CH4) and ethylene (C2H4) as the precursors. The goal of this thesis is to control the surface chemistry in the SiCl4-based SiC CVD and has been accomplished by the following three different approaches: In the first approach to control the surface chemistry of SiC CVD, the difference in the adsorption energy of aromatic and aliphatic hydrocarbons on different SiC crystal planes was utilized. Under identical deposition conditions, a highly <111>-oriented 3C-SiC coating was deposited using C7H8 as the carbon precursor, whereas using CH4 resulted in a randomly oriented 3C-SiC. The results from quantum chemical calculation showed that the active film forming carbon species, i.e. C6H6 in the C7H8 process and CH3 in both C7H8 and CH4 processes, behaved differently when they adsorbed on the 3C-SiC (111) and (110) planes. CH3 is strongly chemisorbed on both planes, while C6H6 is chemisorbed on the (111) plane, but only physiosorbed on the other. The significant difference in the adsorption energy of CH3 and C6H6 on the (111) and (110) planes therefore explains the resulting highly <111>-oriented 3C-SiC from the C7H8 process. Furthermore, the ability to deposit 3C-SiC coatings with alternating highly <111>- and randomly oriented layers by merely switching the carbon precursor between C7H8 and CH4 or C2H4 in a single CVD deposition has further proven that the effect of aromatic hydrocarbons on the preferred growth orientation of 3C-SiC was controlled primarily by the surface chemistry. The second approach to the surface-controlled SiC CVD was based on the reduction of surface reaction probability (β) for conformal film growth via low-temperature, low-pressure CVD, which was originally proposed by Abelson and Girolami. Their strategies in reducing β, including lowering the temperature and increasing the precursor partial pressure, were successfully adapted to the SiC CVD growth using SiCl4 and C2H4 as the precursors in this thesis, where an elevated temperature and a moderate pressure were used. Moreover, the addition of Cl species as a growth inhibitor to the process further reduced the β, leading to a superconformal SiC growth. The third approach employed in this thesis for the SiC growth was pulsed CVD. Instead of a continuous and simultaneous SiCl4 and C2H4 flow, the precursors were pulsed alternately into the chamber with each precursor pulse being separated by a H2 purge. In this precursor delivery mode, the gas phase reactions between SiCl4 and C2H4 were avoided and hence the SiC growth was mostly controlled by the surface chemistry. Altering the pulse durations of the precursors led to a variation of growth per cycle (GPC), which was explained by a two-step mechanism. During the SiCl4 pulse, a thin layer of Si is deposited, which is carburized by carbon species produced during the C2H4 pulse. Additionally, the separation of precursor pulses should lead to a large increase in the surface coverage of Cl species, further enhancing the inhibition effect and resulting in a superconformal SiC growth. By using this approach, superconformal SiC coatings were achieved at temperatures where conventional CVD only yielded nonconformal SiC coatings. The observed decline in coating conformality with an elongated purge implied that more surface Cl species were replaced by H during the H2 purge and consequently the inhibition effect was diminished.
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| 14. |
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| 15. |
- Kakanakova-Georgieva, Anelia, 1970-, et al.
(författare)
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Uniform hot-wall MOCVD epitaxial growth of 2 inch AlGaN/GaN HEMT structures
- 2007
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Ingår i: Journal of Crystal Growth, Vol. 300. - : Elsevier BV. - 0022-0248. ; , s. 100-103
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Konferensbidrag (refereegranskat)abstract
- The hot-wall metalorganic chemical vapor deposition (MOCVD) concept has been applied to the growth of AlxGa1-xN/GaN high electron mobility transistor (HEMT) device heterostructures on 2 inch 4H-SiC wafers. Due to the small vertical and horizontal temperature gradients inherent to the hot-wall MOCVD concept the variations of all properties of a typical HEMT heterostructure are very small over the wafer: GaN buffer layer thickness of 1.83 μm±1%, Al content of the AlxGa1-xN barrier of 27.7±0.1%, AlxGa1-xN barrier thickness of 25 nm±4%, sheet carrier density of 1.05×1013 cm-2±4%, pinch-off voltage of -5.3 V±3%, and sheet resistance of 449 Ω±1%.
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| 16. |
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| 17. |
- MacMillan, Mike F., et al.
(författare)
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Infrared Reflectance of Extremely Thin AlN Epi Films Deposited on SiC Substrates
- 1998
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Ingår i: Materials Science Forum Vols. 264-268. ; , s. 649-652
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Konferensbidrag (refereegranskat)abstract
- The room temperature reflectance of thin (£ 1000Å) AlN epi-films deposited on n type 6H SiC has been measure. These epi-films are too thin to produce interference fringes, from which epi-films thickness is often extracted, within the measured spectral region. However, features from the AlN reststrahl reflectance band can be clearly seen for AlN epi-films as thin as 250Å. Thicknesses are extracted from the measured spectra by comparing them directly to calculated spectra with the epi-film thickness being the only fitting parameter. The accuracy of these thickness determinations is confirmed by comparing them to thickness measured on samples studied by cross sectional TEM.
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| 18. |
- Mpofu, Pamburayi, et al.
(författare)
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Thermal atomic layer deposition of In2O3 thin films using a homoleptic indium triazenide precursor and water
- 2022
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Ingår i: Dalton Transactions. - : Royal Society of Chemistry. - 1477-9226 .- 1477-9234. ; 51:12, s. 4712-4719
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Tidskriftsartikel (refereegranskat)abstract
- Indium oxide (In2O3) is an important transparent conducting material widely used in optoelectronic applications. Herein, we study the deposition of In2O3 by thermal atomic layer deposition (ALD) using our recently reported indium(iii) triazenide precursor and H2O. A temperature interval with self-limiting growth was found between similar to 270 and 385 degrees C with a growth per cycle of similar to 1.0 angstrom. The deposited films were polycrystalline cubic In2O3 with In : O ratios of 1 : 1.2, and low levels of C and no detectable N impurities. The transmittance of the films was found to be >70% in visible light and the resistivity was found to be 0.2 m omega cm. The high growth rates, low impurities, high optical transmittance, and low resistivity of these films give promise to this process being used for ALD of In2O3 films for future microelectronic displays.
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| 19. |
- Palisaitis, Justinas, et al.
(författare)
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Characterization of InGaN/GaN quantum well growth using monochromated valence electron energy loss spectroscopy
- 2014
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 115:3, s. 034302-
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Tidskriftsartikel (refereegranskat)abstract
- The early stages of InGaN/GaN quantum wells growth for In reduced conditions have been investigated for varying thickness and composition of the wells. The structures were studied by monochromated STEM–VEELS spectrum imaging at high spatial resolution. It is found that beyond a critical well thickness and composition, quantum dots (>20 nm) are formed inside the well. These are buried by compositionally graded InGaN, which is formed as GaN is grown while residual In is incorporated into the growing structure. It is proposed that these dots may act as carrier localization centers inside the quantum wells.
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| 20. |
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| 21. |
- Rouf, Polla, 1993-
(författare)
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Time-resolved CVD of Group 13-Nitrides
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Group 13 nitrides (AlN, GaN and InN) and their alloys are semiconductor materials with a wide bandgap span covering from UV down to IR range. Their excellent electronic properties make them extremely attractive materials for light emitting diodes (LEDs) and different kind of transistor structures, especially high electron mobility transistors (HEMTs). These materials are routinely deposited by chemical vapor deposition (CVD) at high temperatures. The most sought-after material among the group 13 nitrides is InN due to its high electron mobility making it extremely useful in transistor structures. InN needs to be deposited at low temperatures as it decomposes at high temperatures. This does not only limit the deposition temperature for InN growth but also for all the other materials that will be deposited on top of InN. In this thesis the deposition of group 13 nitrides is investigated by low temperature atomic layer deposition (ALD) via both a thermal and plasma route. This was conducted by both process development and by improving the deposition chemistry by developing new precursors. Carbon impurities is one of the greater challenges when using the standard aluminum precursor trimethylaluminum (TMA) in ALD due to the strong Al–C bonds in the molecule. An in-situ removal of carbon impurities was investigated by introducing a cleaning pulse, after the TMA pulse. The cleaning pulse consisted of an H2, N2 or Ar gas pulse perpendicular to the surface. The introduction of the cleaning pulse reduced the carbon impurity in the AlN film from 3 at% down to under 1 at%. This made it possible to deposit AlN at higher temperature to obtain better crystalline quality and on the same time reduce the impurity levels. Kinetic simulations showed that the cleaning pulse cleans the surface from desorbed methyl groups resulting in a suppressed reabsorption pathway. To further reduce carbon impurities, the strong M–C bonded precursors was replaced with a M–N bonded one. The precursor used were tris(dimethylamido)gallium together with ammonia (NH3) plasma to deposit GaN. The precursor showed ALD behavior and the resulting GaN film possessed significantly lower carbon impurities compared to M-C bonded precursor at low deposition temperatures. This precursor could also produce epitaxial GaN directly on 4H-SiC without a need of a seed layer. To further investigate the precursor impact on deposition chemistry and ultimately the film quality, three indium precursors were evaluated, indium(III)guanidinate, indium(III)amidinate and indium(III)formamidinate. All three precursors have more or less the same structure, only difference being the size of the substituent on the endocyclic carbon position (-NMe2, -Me and -H respectively). Experimental results showed that smaller groups on the endocyclic carbon position improved the InN film quality in terms of crystallinity, morphology, stoichiometry and optical properties. Density functional theory (DFT) calculations showed that smaller moieties on the endocyclic position will lead to less surface and steric repulsion with the exocyclic position. As the size is decreased the exocyclic groups can fold up closer towards the endocyclic position leading to elongated metal-ligand bonds which will result in easier removal of the ligand for the upcoming NH3 plasma pulse. From these results a new ligand was developed to further improve the deposition chemistry where the endocyclic carbon atom in the ligand backbone of the foramidinate ligand was replaced by a N atom to form a triazenide ligand (iPr–N–N=N–iPr). The triazenide ligand possess no moiety on the endocyclic position compared to the ligands used previously and hence should result in improved material quality if extrapolated from our previous study. The ligand was placed on indium and gallium forming In(III)triazenide and Ga(III)triazenide respectively. Both precursors showed excellent thermal properties making them good ALD precursors. Their use for depositing InN and GaN was investigated with NH3plasma. The resulting films showed excellent quality where no carbon could be detected for either InN nor GaN using XPS and ERDA. Both InN and GaN showed epitaxial growth behavior on 4H-SiC at deposition temperature of 350 °C, a factor of three lower deposition temperature compared to CVD. Interestingly, several linear growth regimes (ALD windows) upon changing the temperature were observed, two and three for InN and GaN respectively. This indicated that the precursors decomposed upon increasing the temperature to form smaller fragments which increased the growth rate but on the same time the smaller precursor fragments saturated the surface. This was further confirmed by DFT calculations. The In(III)triazenide and Ga(III)triazenide was further used to deposit the ternary InGaN phase. A new method was developed where both precursors were mixed in the bubbler and co-sublimed into the reactor via a single pulse. The composition of the films could be tuned via bubbler temperature, deposition temperature and premixed ratio of the precursors in the bubbler. Near In0.5Ga0.5N could be obtained at low deposition temperatures confirmed by both XPS, ERDA and bandgap measurement. Deposition at 350 °C on 4H-SiC resulted in epitaxial In1-xGaxN without a need of a seed layer.
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| 22. |
- Syväjärvi, Mikael, 1968-, et al.
(författare)
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A surface study of wet etched AlGaN epilayers grown by hot-wall MOCVD
- 2007
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Ingår i: Journal of Crystal Growth, Vol. 300. - : Elsevier BV. - 0022-0248. ; , s. 242-245
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Konferensbidrag (refereegranskat)abstract
- Epitaxial layers of AlGaN were grown by hot-wall MOCVD and their surfaces wet chemically etched with phosphorous acid. The as-grown surfaces and the development of the etched surfaces after 10 and 20 min of etching were studied with atomic force microscopy (AFM) and CL. In the as-grown layers growth features may be resolved while the RMS is as low as 1.4 Å in a scan area of 2×2 μm. Surfaces etched for 10 min had developed etch pits and a low RMS roughness of 7 Å indicating a uniform quality of the layers. Micrometer scale hexagonal features were observed after 20 min of etching. In some cases a deep hexagonal etch pit is observed in the centre of the hexagonal feature with a 30° rotation to each other, suggesting that the origin is substrate-induced defects. © 2006 Elsevier B.V. All rights reserved.
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