Session Chair: Juergen Christen, Otto-von-Guericke-University Magdeburg Session Chair: Sudhiranjan Tripathy, Institute of Materials Research & Engineering, A*STAR
1:30pm - 2:00pm Invited
Growth and Prospects for Bulk GaN Crystals
Institute of High Pressure Physics PAS Unipress, Poland
Despite an enormous development of GaN based semiconductors that has been awarded by Nobel Prize in Physics in 2014, some fundamental problems of GaN, InN and their ternaries remains unsolved. This is due to severe pressure requirements for stability of these compounds at high temperatures. Basic physical phenomena for GaN, like melting, solubility or liquid phase structure are still not determined in an undisputable way. For instance, it was shown that there are principal discrepancies in the reported data on both the melting p-T conditions and the sign of dT/dp derivative.
Development of an efficient method for growth of bulk high quality crystals is also an “open question” in GaN physics. Due to extreme melting conditions, GaN cannot be grown from its stoichiometric melt by Czochralski or Bridgman methods. The only method supplying GaN substrates for industry is Hydride Vapor Phase Deposition (HVPE). Its main advantage is high growth rate exceeding 100 micrometer/h. Real bulk GaN crystals of very high structural quality are grown by ammonothermal method at moderate pressures of 0.1-0.3GPa and low temperatures of about 400-600oC. Development of this technology is limited by discouragingly low grow rate of about 1 micrometer/h. A new approach to GaN bulk crystallization based on growth by HVPE on Ammono-GaN seeds will be presented. It will be shown that thick (d>2mm) GaN crystals with structural quality as high as the quality of the seeds can be grown with a rate exceeding 200 micrometer/h. Moreover these new crystals are of the highest purity, comparable to epitaxial material. The optical and electrical properties of the GaN crystals will be discussed. These studies are crucial for establishing physical limitations of real bulk GaN crystallization by HVPE. New directions in development of the bulk GaN crystal growth by both low and high pressure methods will be presented.
2:00pm - 2:30pm Invited
MOCVD of GaN-based Layers on AlN templates
Yilmaz DIKME1, Vitaly ZUBIALEVICH2, Peter PARBROOK2,3, Volker SINHOFF1
1AIXaTECH GmbH, Germany; 2Tyndall National Institute, Ireland; 3School of Engineering, University College Cork, Ireland
The epitaxial growth of InGaAl-N-based layers by metal-organic chemical vapor deposition (MOCVD) is counted as a mature technology. Due to the lack of cheap and large area native substrates, there are still many challenges in the heteroepitaxial process mainly in the starting of the growth process to ensure reproducibility, high quality and a better production yield. In this work, we present experimental data of GaN-based material growth by MOCVD on templates consisting of a thin AlN layer grown by a novel low-temperature epitaxial process (LTEP).
The deposition of the AlN layer was enabled by LTEP, which consists of a mix of physical vapor deposition and chemical vapor deposition processing. At a substrate temperature below 200°C single crystal AlN is grown on sapphire. The single crystal and epitaxial growth was verified by X-ray diffraction (XRD) of the phi-scan around the (101) AlN peak. Only allowed peaks could be observed and also the AFM revealed a smooth layer with a rms roughness value of 0.2-0.3 nm. The thickness of this AlN layer was varied between 40 to 100 nm, resulting in a XRD full width of half maximum (FWHM) of around 200 arcsec for the (002) peak, 470 arcsec for the (105) peak and 615 arcsec for the (101) peak.
The templates have been overgrown in MOCVD with AlN and GaN bulk layers and with LED and HEMT structures. Depending on the initial growth in MOCVD processes, GaN buffer layers with a (102) FWHM of 200-250 arc sec can be achieved. For further improvements, the growth of the AlN templates were varied and conditions of the MOCVD starting layer like growth pressure, growth rate, surface temperature and V/III ratio have been investigated. In addition, applicability of such a low-temperature AlN on silicon substrates will be presented.
2:30pm - 2:45pm Oral
Accessing Optical and Compositional Properties of InGaN/GaN Core-Shell Nanorods at the Nanometer Scale
Marcus MUELLER1, Sebastian METZNER1, Peter VEIT1, Frank BERTRAM1, Florian KRAUSE2, Thorsten MERTENS2, Knut MÜLLER-CASPARY2, Andreas ROSENAUER2, Tilman SCHIMPKE3, Adrian AVRAMESCU3, Martin STRASSBURG3, Jürgen CHRISTEN1
1Otto-von-Guericke University Magdeburg, Germany; 2University of Bremen, Germany; 3OSRAM Opto Semiconductors GmbH, Germany
The high crystal quality and the increased effective light emitting area of three-dimensional GaN based nanorods (NRs) in comparison to conventional planar heterostructures makes them promising candidates to achieve highly efficient optoelectronic devices.
In the present work, we demonstrate a nano-scale correlation of the structural, optical, and compositional properties of InGaN/GaN core-shell nanorod LEDs using highly spatially resolved cathodoluminescence spectroscopy (CL) and scanning transmission electron microscopy (STEM).
The core-shell microrods were fabricated by metal organic chemical vapor phase epitaxy (MOVPE) on c-plane GaN/sapphire templates covered with a SiO2-mask. The MOVPE process results in the homogeneous selective area growth of n-doped GaN microrods out of the mask openings. On top of the n-GaN core, an InGaN single quantum well (SQW) encased by GaN barriers was deposited as active region. TEM analyses of single NRs reveal a high crystal quality and the absence of extended defects.
Using highly spatially resolved CL mapping of single NRs performed in cross-section, we give a direct insight into the optical properties of the individual core–shell layers. Going from the bottom part towards the tip of the NR, the SQW luminescence increases in intensity. Simultaneously, the InGaN SQW red shifts from 410 nm to 475 nm along the side-wall. Quantitative STEM analysis of the active region reveals an increasing thickness of the SQW from 6 nm to 13 nm, accompanied by a slight increase of the indium concentration along the non-polar side wall from 11 % to 13 %. Furthermore, compositional mappings of the InGaN quantum well reveal the formation of locally indium rich clusters with several nanometers in size, leading to potential fluctuations in the InGaN SQW energy landscape. This is directly evidenced by nanometer-scale resolved CL mappings which show strong localization effects of the excitonic SQW emission.
2:45pm - 3:00pm Oral
Carbon Doping Effects on GaN Buffer Grown on SiC by MOCVD
1Temasek Laboratories @ Nanyang Technological University, Singapore; 2Centre for Micro/Nano Electronics (NOVITAS), School of Electrical & Electronic Engineering, Nanyang Technological University, Singapore
The growth of AlGaN/GaN HEMT on SiC substrate is preferred because of its smaller lattice and thermal mismatches to GaN and higher thermal conductivity. The growth of high resistive GaN buffer layer is critical for better device performance. Resistive GaN buffer can be achieved either by doping with iron or carbon. Iron is not widely used as a dopant since it has memory effect. However, carbon doping can be achieved by adjusting the growth parameters such as reactor pressure and V/III ratio. Thus, carbon doping of GaN has been adopted to achieve high resistive buffer. In this study, AlGaN/GaN HEMT structures were grown on SiC substrate with different reactor pressures and V/III ratios. The GaN buffer with lower yellow luminescence (YL) had a buffer leakage current in the order of 10-3 mA/mm. Moreover, the measured buffer leakage current at -30V showed higher values than +30V. On the other hand, GaN buffer with higher YL had a high resistive GaN buffer with the leakage current in the order of 10-6 mA/mm. The measured buffer leakage current showed a symmetrical response for both positive and negative bias upto ±30V. Secondary ion mass spectrometry analysis shows that the sample with lower buffer leakage had carbon concentration in order of 1017 cm-3 whereas the carbon concentration is below the detection limit (<1016 cm-3) for the sample with higher buffer leakage. An inverse relationship is observed between the intensity of the YL peak and buffer leakage current. Thus, the incorporation of carbon is found to be critical in achieving high resistive GaN buffer layer. The optimized GaN HEMT on SiC showed an average mobility and sheet carrier density of 1509 cm2/V.s and 1.17 × 1013 cm-2, respectively, with an Idmax of 1100 mA/mm and gmmax of 330 mS/mm for 0.25 mm Lg devices.
3:00pm - 3:15pm Oral
Anisotropic Optical Properties of A-Plane (Al,Ga)N and (Al,In)N Alloys
Nirupam HATUI1, A Azizur RAHMAN1, Ashish ARORA2, Carina B MALIAKKAL1, Arnab BHATTACHARYA1
1Department of Condensed Matter Physics, Tata Institute of Fundamental Research, India; 2Physikalisches Institut, University of Münster, Germany
In group III-nitrides, the valence band has three closely-spaced sub-bands of different symmetries. The ordering of these sub-bands is different for AlN when compared to GaN or InN, which results in different optical transition selection rules for the interaction of linearly polarized light with these materials. The presence of anisotropic in-plane strain in the epitaxially grown films of a-plane nitrides on r-plane sapphire leads to two orthogonally-polarized excitonic transitions, the energy of which depends on the alloy composition. We have used k⋅p perturbation theory to calculate these excitonic transition energies for a-plane Al(x)Ga(1-x)N or Al(x)In(1-x)N alloys and compared the predicted results with experimental polarized transmission spectroscopy measurements on a-plane Al(x)Ga(1-x)N epilayers grown by MOVPE. Similar work for Al(x)In(1-x)N alloys is under progress, and detailed results of the modelling of strain-dependent band structure and the optical measurements will be presented.