Session Chair: André Strittmatter, Otto von Guericke University Session Chair: Yilmaz Dikme, AIXaTECH GmbH
1:30pm - 2:00pm Invited
Nonpolar and Semipolar GaN for Solid-State Lighting: Will They Ever Become Real?
Yale University, United States
Non-polar and semipolar orientations of gallium nitride (GaN) have the possibility to address long-standing problems in III-Nitride light emitting diodes (LEDs). They have drawn substantial attentions in MOCVD community since 2000. In the past several years, high output power green and blue LEDs and laser diodes (LDs) have been demonstrated on (202-1), (20-2-1), and several other oriented GaN. However, all these high-brightness semipolar GaN devices have only been produced on bulk GaN substrates. These bulk GaN substrates are produced by cross-slicing GaN crystal which is neither a manufactuable nor a cost-effective approach.
In this talk we will review the effort in heteroepitaxy of nonpolar and semipolar GaN on large-area, commercially-available substrates. This field has gone through several generations of development from the original planar epitaxy, to ELO growth, and to patterned substrates. Focus will be given to the understanding and control of stacking faults in semipolar GaN, which has been a most challenging issue due to the formation of nitrogen-polar (000-1) facet in heteroepitaxy. There has been recent progress at Yale in reducing the density of stacking faults in heteroepitaxy epitaxy. The availability of high-quality semipolar GaN-on-sapphire templates will enable the production of high-performance, next generation GaN LEDs with economic feasibility.
This work is partially supported by Saphlux Inc (http://www.saphlux.com/). JH is a cofounder of Saphlux and acknowledges that he has a significant financial interest with Saphlux.
2:00pm - 2:30pm Invited
Gaining Insight into Performance- and Reliability-Limiting Phenomena in GaN-Based Heterostructure Field-Effect Transistors by Means of Combined Experimental/Simulation Analysis
Giovanni VERZELLESI1, Alessandro CHINI2, Carlo DE SANTI3, Gaudenzio MENEGHESSO3, Matteo MENEGHINI3, Isabella ROSSETTO3, Enrico ZANONI3
1Department of Sciences and Methods for Engineering (DISMI), Università di Modena e Reggio Emilia, Italy; 2DIEF, Università di Modena e Reggio Emilia, Italy; 3DEI, Università di Padova, Italy
In this talk, we review a set of recent results of ours originated by studies of the parasitic phenomena limiting the performance and the high-field reliability of GaN heterostructure field-effect transistors. In all presented cases, combining experiments with numerical device simulations has proven instrumental to achieving a better overall comprehension of underlying physical mechanisms and of the impact of growth and processing parameters. Technologies include Schottky- and insulated-gate HEMTs for either RF and switching power applications. Investigated phenomena comprise trap-related dispersion effects, RF power gain collapse, threshold-voltage instabilities, high-electric-field degradation effects, lateral and vertical breakdown.
2:30pm - 3:00pm Invited
Fabrication and Characterisation of Novel Hybrid rGO/GaN Self-Powered Photodetector
1Council of Scientific and Industrial Research (CSIR) - National Physical Laboratory, India; 2Academy of Scientific and Innovative Research, Council of Scientific and Industrial Research (CSIR) - National Physical Laboratory, India
Fabrication of typical high performance photodetectors (such as with III-N) is quite challenging (technologically) and extremely costly. It is thus very essential to develop simple and cost effective fabrication routes to facilitate their mass scale deployment. With our proposed technique of drop-casting, we have successfully demonstrated non-cleanroom fabrication of a hybrid device that works in the self-powered (photovoltaic) mode. The device exhibits high photosensitivity (85%) and fast photoresponse (τrise~60 ms) and recovery times (τfall~267 ms) and an ultrasensitive behaviour at low light intensity over the UV region. The results suggest that r-GO integrated with GaN technology is a promising candidate for self-powered UV PD applications. With further optimisation of involved process, the scheme has a potential to be scaled up for mass production of optoelectronic devices based on similar material systems.
3:00pm - 3:15pm Oral
Absence of Electron Accumulation on Non-Polar InN Surfaces
Holger EISELE1, Michael SCHNEDLER2, Andrea LENZ1, Verena PORTZ2, Christian NENSTIEL1, Axel HOFFMANN1, Philipp EBERT2
1Technical University of Berlin, Germany; 2Forschungszentrum Jülich GmbH, Germany
In recent years an intrinsic electron accumulation at different InN surfaces is broadly discussed as a general aspect of this material. Such an electron accumulation would be hindering any p-doping in InN—or at least making the formation of sharp interfaces with p-doped InN difficult. Hence, also photovoltaic applications would not be able to benefit from InN and its material properties, especially not from its useful bandgap in the near infrared wavelength region.
In this contribution new experiments will be presented, showing no electron accumulation at non-polar InN surfaces, neither at the m-plane nor at the a-plane. Here, the non-polar InN surfaces were prepared from high-quality grown thick layers on different substrate materials by in situ cleavage under UHV conditions. This preparation method leads to clean, as-cut stoichiometric m- and a-planes. On these surfaces, the Fermi level is always found to be within the fundamental band gap. No intrinsic surface states are found within the band gap. Only extrinsic defect states are pinning the Fermi energy at approximately midgap position. Hence, no electron accumulation was found as long as the surfaces remained clean.
From this result, an intrinsic electron accumulation can be negated and p-doping, in general, should be possible. In contrast to UHV-cleavage preparation, as-grown non-polar surfaces show electron accumulation. On the one hand, this leads to the question how to improve or modify growth conditions in a way to receive also stoichiometric clean surfaces without electron accumulation. On the other hand, it demonstrates that electron accumulation is not an intrinsic property of InN material, but rather more a question of surface preparation.
3:15pm - 3:30pm Oral
Gain Mechanism and Carrier Transport in Highest Responsivity AlGaN-based Solar Blind Metal Semiconductor Metal Photodetectors
Center for Nano Science and Engineering (CeNSE), Indian Institute of Science, India
We report on the gain and carrier transport mechanisms in highest responsivity AlGaN Metal Semiconductor Metal (MSM) solar-blind photodetectors on sapphire. Devices on MOCVD-grown unintentionally-doped AlGaN samples exhibited sharp absorption cut-off in the range 245-290 nm. Very high responsivity > 5 A/W at 10 V bias was measured with visible rejection ratio > 103 for front illumination. Carrier transport was studied through temperature-dependent current-voltage analysis. The studies revealed that the reverse-bias leakage current (dark current) across the Schottky contacts made to these samples was dominated by thermionic field emission at low biases and Poole-Frenkel emission from a deep trap level (0.7 eV from the conduction band edge for Al0.50Ga0.50N) at high biases. The high responsivity values and their exponential dependence on bias is attributed to an internal gain mechanism operating in these devices owing to a photo-induced lowering of the barrier at the metal-semiconductor interface. The detector performance parameters viz. dark current, photocurrent, responsivity and gain were found to depend on the crystal quality of the AlGaN absorber layer, especially the screw dislocation density.
This work is funded by Joint Advanced Technology Program (JATP), grant number JATPO152, and by the Department of Science and Technology (DST) under its Water Technology Initiative (WTI), grant number DST01519.