A technique to enhance the hole concentration in activated Mg-doped p-type GaN epitaxial layers is described. The method consists of depositing a porous plasma-enhanced chemical vapor deposited SiOx layer on top of p-GaN after which the sample is heated to 950 degrees C in nitrogen ambient for 1 min followed by the removal of the SiOx layer in a buffered HF solution. A significant improvement of the conductivity of the p-GaN layer has been obtained. (c) 2005 The Electrochemical Society.

The paper reports on the investigation an improvement of the p-type conductivity of Mg-doped GaN by creating Ga-vacancies in the lattice.

We present a systematic approach to reduce the resistance of ohmic contacts on AlGaN/GaN FET structures. We have optimised the Ti/Al/Ni/Au contact with respect to the metal composition and annealing conditions. Our optimised contact has a very low contact resistance of 0.2Ωmm (7.3×10−7Ωcm2), a good reproducibility and an excellent line definition, making this contact very suitable for use in AlGaN/GaN FETs with short gate-source distances. This contact resistance on FET material is among the best values reported. The approach presented here can be applied to other metal schemes like Ti/Al/Pt/Au or Ti/Al/Ti/Au.

Ohmic contacts and Schottky contacts were made on an undoped AlGaN/GaN FET structure. Despite the high Al content (33%), we were still able to obtain a contact resistance of 0.3 ωmm. Pulsed measurements showed the large effect of self-heating even for circular contacts with a radius of 50 μm. The behavior ofthe Ni/Au Schottky contacts is according to the charge control model; the reverse current and capacitanceonly scale with the area of the diode. Tests with polygon type diodes showed no dependence of the reverse current on the number of polygon corners. The reverse current decreased when the devices were aged at 400°C for 30 hrs. Coplanar Waveguide discontinuities were realized on AlN substrates. A scalable lumped element model was derived from measurements for T-junctions, transmission lines, bends and crosses.

We have investigated Coplanar Waveguide (CPW) elements on AlN for use in future AlGaN/GaN based power amplifiers. This technology becomes crucial if a via-hole technology is not available. Lines, discontinuities, metal-insulator-metal (MIM) capacitors and resistors were measured and modelled. These elements are embedded between two adaptors for RF probing. A technique was developed to de-embed the adaptors from the overall measurement and hence correctly determine the properties Of the element itself. Measurements on elements containing multiple ports with right angles can best be carried out using standard calibration techniques followed by carefully reorienting the probes. It is shown that for accurate design of matching networks operating at 10 GHz each element has to be carefully modelled. The method presented in this paper can be a useful contribution tackling some of the problems related to the design of these networks.

P-contacts on MOVPE-grown Mg-doped GaN were investigated as function of various pretreatments in order to enhance the hole concentration of the starting material. After a Mgactivation at 950°C for 30 s in N2-ambient the samples show a typical hole concentration of ~1017 cm-3. In the investigations a metallisation of Ni-Au (20/100 nm) was mostly used which, after a thermal annealing, shows a slight Schottky behaviour. One of the pre-treatments induces a current increase at constant voltage of about 20%.

In this report two different aspects in the development of AlGaN/GaN power amplifiers will be discussed. In the first part of this paper we report on the optimization of the Ti/Al/Ni/Au metallization scheme on a doped AlGaN/GaN FET structure. By a systematic investigation we were able to reduce the contact resistance to 0.2 ωmm (7.3×10×7ωcm2). The Al/Ti thickness ratio for this contact was 6, which according to the Al-Ti binary phase diagram, does not result in excess Ti which should react with nitrogen in the AlGaN layer to render the surface heavily doped. Preliminary results on Schottky contacts indicate an improvement in the reverse leakage current if a RIE oxygen plasma in combination with a NH4OH dip is performed prior to metallization. Coplanar waveguides on AlN are discussed in the second part of this paper. These transmission lines can be used in AlGaN/GaN power amplifiers if no via-hole technology is available or if a hybrid solution is pursued. The signal line should have a large metal cross- sectional area (> 5 × 50 [.proportional]m2) in order to carry enough current in the output stage of an amplifier. It is shown that CPWs with large dimensions show non-quasi TEM behavior related to propagation of parallel plate modes.

A highly chemical reactive ion etching process has been developed for MOVPE-grown GaN on sapphire. The key element for the enhancement of the chemical property during etching is the use of a fluorine containing gas in a chlorine based chemistry. In the perspective of using GaN substrates for homo-epitaxy of high quality GaN/AlGaN structures we have used the above described RIE process to smoothen Ga-polar GaN substrates. The RMS value, measured by AFM, went from 20 Å (after mechanical polishing) down to 4 Å after 6 minutes of RIE. Etching N-polar GaN resulted in a higher etch rate than Ga-polar materials (165 vs. 110 nm/min) but the resulting surface was quite rough and suffers from instability problems. Heat treatment and HCl dip showed a partial recovery of Schottky characteristics after RIE.

Reactive ion etching of {0001} oriented plate-like GaN single crystals has been investigated using SiCl4:Ar:SF6 chemistry. The reactive ion etching process is highly chemical. Large anisotropy of the etching rate and of the morphology has been established on (000\(\bar 1\)) N-polar and (0001) Ga-polar sides of the GaN crystals, with remarkably higher rate on the N-polar side. Atomic force microscopy measurements have shown smooth surface and good polishing effect obtained on Ga-polar side, while N-polar surface exhibits an increased roughness of a factor of 10 after RIE.

Reactive Ion Etching (RIE) of GaN epitaxially grown on (0001) sapphire substrate has been investigated using various chemistries based on SiCl4, Ar and SF6. Plasma deposited SiNx is used for masking. We studied the influence of gas flow, pressure and RF-power on etch rate and morphology. High etch rates up to 150 nm/min can be obtained when using SIC4:Ar:SF6 (10:10:2 sccm) at an acceleration voltage of 370 Volts. Very smooth surfaces and good etch rates (±100 nm/min) were obtained using the same chemistry at a lower RF-power of 105 Watts (DC-bias of ±290 V)

This paper presents a discussion on the processing of AlGaN/GaN HEMTs with different gate widths but all with an optically defined 1 um gate length. Scalability of transistor parameters will be discussed and a small-signal equivalent circuit is extracted which shows the influence of the processing of the Schottky and ohmic contacts.

Reactive ion etching of GaN metallorganic vapor-phase epitaxy grown on a (0001) sapphire substrate has been investigated using various chemistries based on SiCl4. The influence of gas combinations, gas flow, pressure, and radio-frequency (rf) power on etch rate and morphology was studied. Very high etch rates (160 nm/min) were obtained by adding SF6 to SiCl4:Ar. Smooth surfaces and high etch rates (+/-100 nm/min) were achieved at an rf power of 105 W (dc bias of -290 V). (C) 1999 The Electrochemical Society. S1099-0062(98)11-049-0. All rights reserved.

physica status solidi (a)Volume 176, Issue 1 p. 755-758 Original Paper Chemical and Complementary Role of Fluorine in a Chlorine-Based Reactive Ion Etching of GaN F. Karouta, F. Karouta [email protected] Search for more papers by this authorB. Jacobs, B. Jacobs COBRA Inter-University Research Institute on Communication Technology, Eindhoven University of Technology, Department of Electrical Engineering, P.O. Box 513, NL-5600 MB Eindhoven, The NetherlandsSearch for more papers by this authorO. Schoen, O. Schoen AIXTRON AG, Kackertstr. 15—17, D-52072 Aachen, GermanySearch for more papers by this authorM. Heuken, M. Heuken AIXTRON AG, Kackertstr. 15—17, D-52072 Aachen, GermanySearch for more papers by this author F. Karouta, F. Karouta [email protected] Search for more papers by this authorB. Jacobs, B. Jacobs COBRA Inter-University Research Institute on Communication Technology, Eindhoven University of Technology, Department of Electrical Engineering, P.O. Box 513, NL-5600 MB Eindhoven, The NetherlandsSearch for more papers by this authorO. Schoen, O. Schoen AIXTRON AG, Kackertstr. 15—17, D-52072 Aachen, GermanySearch for more papers by this authorM. Heuken, M. Heuken AIXTRON AG, Kackertstr. 15—17, D-52072 Aachen, GermanySearch for more papers by this author First published: 22 November 1999 https://doi.org/10.1002/(SICI)1521-396X(199911)176:1<755::AID-PSSA755>3.0.CO;2-2Citations: 3AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat Abstract Conventional reactive ion etching of epitaxially grown GaN on (0001) sapphire substrate has been investigated using a basic chemistry of SiCl4, Ar and SF6. Photoresist or plasma-deposited SiNx were used for masking. The influence of gas flow, pressure and rf-power on the etch rate and morphology were studied. A maximum etch rate of 430 nm/min was obtained at an rf power of 300 W. Very smooth surfaces and reasonable etch rates (±100 nm/min) were obtained using the same chemistry at a lower rf-power of 105 W (dc bias of ±290 V). The chemical and complementary roles of chlorine and fluorine will be demonstrated. References 1 C. B. Vartuli, S. J. Pearton, J. W. Lee, J. Hong, J. D. Mackenzie, C. R. Abernathy, and S. J. Shul, Appl. Phys. Lett. 69, 1426 (1996). 10.1063/1.117603 CASWeb of Science®Google Scholar 2 M. S. Feng, J. D. Guo, Y. M. Lu, and E. Y. Chang, Mater. Chem. Phys. 45, 80 (1996). 10.1016/0254-0584(96)80053-8 CASWeb of Science®Google Scholar 3 F. Karouta, B. Jacobs, P. Vreugdewater, N. G. H. v. Melick, O. Schoen, H. Protmann, and M. Heuken, Electrochem. Solid State Lett. 2, 241 (1999). 10.1149/1.1390797 Web of Science®Google Scholar 4 I. Adesida, A. Mahajan, and E. Andideh, Appl. Phys. Lett. 63, 2777 (1993). 10.1063/1.110331 CASWeb of Science®Google Scholar Citing Literature Volume176, Issue1November 1999Pages 755-758 ReferencesRelatedInformation

We have investigated coplanar waveguide (CPW) passive components on ceramic AlN substrates for application in hybrid integrated X-band GaN-based high- power amplifiers. CPW technology becomes crucial if a via-hole technology is not available. A set of processed transmission lines (TLs), discontinuities, metal-insulator- metal (MIM) capacitors and resistors has been used for the extraction of scalable models. These elements are embedded between two adaptors for RF probing. We have developed a technique to de-embed the adaptors from the overall measurement in order to correctly determine the properties of the element itself. Measurements on elements containing multiple ports with right angles can best be carried out using standard calibration techniques followed by carefully re-orienting the probes. It is shown that for accurate design of matching networks operating at 10 GHz each element has to be carefully modeled. An inaccuracy of less than 5% between measured and modeled S-parameters at X-band for a fabricated CPW demonstrator circuit proves the validity of the scalable models for CPW passive circuit design in this frequency band.