B. Jacobs

High etch rate and smooth morphology using a novel chemistry in reactive ion etching of GaN

GaN-based materials are commonly considered as potential candidates for light sources in the green and blue spectral range. The bandgap energy of AlGaInN varies between 1.95 and 6.2 eV. Significant research has been performed on high-brightness blue light emitting diodes (LEDs) and laser diodes (LDs). For example, the advantages of using LEDs over conventional electric incandescent lighting are faster response time, longer life, lower energy consumption, and higher luminescence efficiency. Also GaN, a wide-gap material, presents a potential use for high-temperature and high-power microelectronic applications. The high electron drift velocity, low thermal-generation rate, and high breakdown field make GaN and its related materials the most technologically interesting electronic materials for field-effect transistor applications. However, the intrinsic characteristics of this material system and, more specifically, its inertness, due to an atomic gallium-nitrogen bond energy of 8.92 eV/atom, make processing possible only with dry etching techniques. Dry etching of GaN has been extensively investigated using a wide range of plasma-based machines and sources. Most authors use electron cyclotron resonance (ECR) plasma based on CH 4 -H 2 -Ar, Cl 2 -H 2 , or CH 4 -H 2 -Ar-Cl 2 1-3 or chemically assisted ion beam etching (CAIBE).4,5 Only a few reports describe dry etching of GaN using various chemistries in a more conventional reactive ion etching (RIE) and the number of papers dealing with a SiCl 4 -based chemistry is even lower.6 Experimental In this paper, we present the results of RIE etching of metallorganic vapor-phase epitaxy (MOVPE) grown GaN on a (0001) sapphire substrate. The experiments were performed in a conventional parallel plate and load-locked Oxford Plasmalab System 100. Various combinations have been investigated using SiCl 4 :Ar, and eventually adding SF 6 . The mask used in these experiments is a plasma-deposited SiN x layer ~300 nm thick, annealed at 450°C for 10 min. Patterning the mask was carried out either by wet etching in a buffered HF solution or by using a dry etching process with SF 6 :Ar [10:10 standard cubic centimeters per minute (sccm)] at 150 W and 40 mTorr. The etch rate of the SiN x mask was ~200 nm/min. The influence of gas flow, pressure, and radio-frequency (rf) power were investigated. The basic gas flow rate used was SiCl 4 :Ar (10:10) sccm. Most gas combinations were investigated at three rf power values (70, 105, and 140 W) and at three pressure values (20, 40, and 60 mTorr). These parameters, combined together, determine the dc voltage which is related to the acceleration energy to the active species.

Chemical and Complementary Role of Fluorine in a Chlorine‐Based Reactive Ion Etching of GaN

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

Metal-Semiconductor Contacts and CPW MMIC Issues for AlGaN/GaN FETs

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 ×7 ωcm 2 ). 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 NH 4 OH 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]m 2 ) 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.

Enhancement of p-GaN conductivity using PECVD SiOx

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-nhancedchemical vapor deposited SiOx layer on top of p-GaN after which the sample is heated to 950°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.

Post-growth enhancement of p-GaN conductivity

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

Passive components on AlN for application in AlGaN/GaN power amplifiers

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.

Progress in reactive ion etching of epitaxial GaN

Reactive Ion Etching (RIE) of GaN epitaxially grown on (0001) sapphire substrate has been investigated using various chemistries based on SiCl/sub 4/, Ar and SF/sub 6/. Plasma deposited SiN/sub x/ 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 SIC/sub 4/:Ar:SF/sub 6/ (10:10:2 sccm) at an acceleration voltage of 370 Volts. Very smooth surfaces and good etch rates (/spl plusmn/100 nm/min) were obtained using the same chemistry at a lower RF-power of 105 Watts (DC-bias of /spl plusmn/290 V).