Measurement of carrier lifetime in micron-scaled materials using resonant microwave circuits
Sukrith DevYinan WangKyounghwan KimMarziyeh ZamiriClark N. KadlecMichael GoldflamSamuel D. HawkinsEric A. ShanerJin Ryoun KimSanjay KrishnaMonica AllenJeffery AllenEmanuel TutucDaniel Wasserman
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Abstract The measurement of minority carrier lifetimes is vital to determining the material quality and operational bandwidth of a broad range of optoelectronic devices. Typically, these measurements are made by recording the temporal decay of a carrier-concentration-dependent material property following pulsed optical excitation. Such approaches require some combination of efficient emission from the material under test, specialized collection optics, large sample areas, spatially uniform excitation, and/or the fabrication of ohmic contacts, depending on the technique used. In contrast, here we introduce a technique that provides electrical readout of minority carrier lifetimes using a passive microwave resonator circuit. We demonstrate >10 5 improvement in sensitivity, compared with traditional photoemission decay experiments and the ability to measure carrier dynamics in micron-scale volumes, much smaller than is possible with other techniques. The approach presented is applicable to a wide range of 2D, micro-, or nano-scaled materials, as well as weak emitters or non-radiative materials.Keywords:
Carrier lifetime
Ohmic contact
A study of NiAuGe contacts to InP has been undertaken. Various cleaning procedures, metalisation combinations and heat treatments have been investigated. During electrical optimisation and subsequent physical analysis the importance of Ni in the NiAuGe contact to InP became apparent. Major differences between the GaAs/NiAuGe and InP/NiAuGe contacts are suggested. TEM micrographs, for the InP system, show an ordered layer structure of constituent phases. These observations are confirmed by RBS results and also agree with other investigations. The most important observations are those associated with nickel and its role during the formation of ohmic contacts to InP. Nickel was found to form ohmic contacts in a similar manner to the ohmic metalisation NiAuGe. As the alloying temperature was increased to the optimum of 450°C, the effective barrier height was initially observed to increase, then above 350°C to fall as the contact became ohmic. NixPy phases are responsible for ohmic behaviour similar to those observered in the NiAuGe ohmic contact.
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Indium phosphide
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Phosphide
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Ohmic contact is one of the fundamental requirements for GaN detectors with high performance.But Ohmic contacts on p-type GaN with low specific contact resistance are more difficult to realize than those on n-type GaN.In this paper,research about Ohmic contacts on p-type GaN is reported.With Au and Cr/Au deposited on p-type GaN materials and appropriate annealing process,good Ohmic contacts have been obtained.Specific contact resistances of the Au/p-GaN and Cr/Au/p-type GaN structures are 7.88×10~(-2)Ω·cm~2 and 1.94×10~(-2)Ω·cm~2 respectively.
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A novel stacked Ti/Al based Ti/Al/Ti/Al/Ti/Al/Ti/Al/Ni/Au Ohmic contact structure is optimized. Compared with the conventional alloyed Ti/Al/Ni/Au Ohmic contact structure, the novel Ohmic contact structure can obtain much lower contact resistance and specific contact resistivity. Through analysis of x-ray diffraction spectra, cross-section transmission electron microscopy images and corresponding electron dispersive x-ray spectroscopy spectra in the novel stacked Ti/Al based Ohmic structure, the reactions between metals and the AlGaN layer were proven to be stable, uniform and continuous, which produced smooth contact interface. In addition, the top Au layer was prevented from diffusing downwards to the metal/AlGaN interface, which degraded the Ohmic performance.
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The effects of recess etching of the Ohmic contact area on the performance of Ti/Al/Mo/Au metallization and its interfacial reactions with AlGaN/AlN/GaN epilayers were investigated. The best Ohmic contact performances of 0.31, 0.41, and 0.26 Ω mm for three epilayers from two different sources were obtained only when the two-dimensional electron gas (2DEG) channels were completely removed under the Ohmic contact metallization. This is due to the direct sideway contact made by the electrode to the 2DEG around the edges of the active-layer mesas or pads; this is believed to be a more efficient carrier transport mechanism than tunneling through the AlGaN barrier.
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We attempted a new ohmic contact process for two-step metallization, and demonstrated improvement of the ohmic properties and device characteristics. The contact resistance of the two-step ohmic process was improved from 4.2 Ωmm obtained in a conventional contact to 2.6 Ωmm. The specific contact resistivity of the transmission line method ohmic contacts improved by one order of magnitude, from 10-4 Ω-cm2 in the conventional contact sample to 10-5 Ω-cm2 in the two-step contact sample treated with Cl2 plasma. The dc measurements of the HFETs showed improved current-voltage curves in the two-step contact samples. The values of Ron of the AlGaN/GaN HFET with the two-step contact process and the conventional HFET were calculated to be 0.23 mΩ-cm2 and 0.28 mΩ-cm2 in the linear (ohmic) region, respectively.
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Abstract P‐type ohmic contact between Au nanoparticles/ITO and p‐GaN during heat treatment is reported. Optimal conditions are selected to minimize the lowest specific contact resistance to 1.66 × 10 –3 Ω cm 2 , as determined by the transmission line model (TLM) after heat treatment process at an alloying temperature of 600 °C for 10 min in air. Au nanoparticles/ITO bi‐layers are also applied to GaN‐based LEDs to form an electrode with a p‐type ohmic contact. Typical I–V characteristics of the GaN‐based LEDs with an ohmic contact layer of Au nanoparticles/ITO exhibit a forward‐bias voltage of 3.43 V at an injection current of 20 mA. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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Widespread Ni/SiC contact after annealing at high temperatures provides good ohmic contact. We managed to prepare Si/SiC annealed ohmic contacts with comparable qualities as Ni/SiC ones. However, the main benefit of these Si/SiC ohmic contacts lies in that they retain their ohmic properties even after they are etched off which brings grounds for promising secondary contacts. Etched contacts were analyzed by XPS and surface composition alteration was observed.
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Ohmic contacts on p-doped 4H-SiC are essential for the fabrication of a wide range of power electron devices. Despite the fact that Ti/Al based ohmic contacts are routinely used for ohmic contacts on p-doped 4H-SiC, the underlying contact formation mechanisms are still not fully understood. TLM structures were fabricated, measured and analyzed to get a better understanding of the formation mechanism. SIMS analyses at the Ti3SiC2-SiC interface have shown a significant increase of the surface near Al concentration. By using numerical simulation it is shown that this additional surface near Al concentration is essential for the ohmic contact formation.
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Non-recessed ohmic contact resistance (Rc) on ultrathin-barrier (UTB) AlGaN(<6 nm)/GaN heterostructure was effectively reduced to a low value of 0.16 Ω·mm. The method called the ‘ohmic-before-passivation’ process was adopted to eliminate the effects of fluorine plasma etching, in which an alloyed Ti/Al/Ni/Au ohmic metal stack was formed prior to passivation. The recovery of 2-D Electron Gas (2DEG) adjacent to the ohmic contact was enhanced by composite double-layer dielectric with AlN/SiNx passivation. It is found that the separation between the recovered 2DEG and the ohmic contacting edge can be remarkably reduced, contributing to a reduced transfer length (LT) and low Rc, as compared to that of ohmic contact to the AlGaN(~20 nm)/GaN heterostructure with a pre-ohmic recess process. Thermionic field emission is verified to be the dominant ohmic contact mechanism by temperature-dependent current-voltage measurements. The low on-resistance of 3.9 Ω·mm and the maximum current density of 750 mA/mm with Vg = 3 V were achieved on the devices with the optimized ohmic contact. The non-recessed ohmic contact with the ‘ohmic-before-passivation’ process is a promising strategy to optimize the performance of low-voltage GaN-based power devices.
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