Sheet resistance measurement on AlGaN/GaN wafers and dispersion study J. Lehmann a,b,⇑ , C. Leroux a , M. Charles a , A. Torres a , E. Morvan a , D. Blachier a , G. Ghibaudo b , E. Bano b , G. Reimbold a a CEA-LETI Minatec Campus, 17 rue des Martyrs, 38000 Grenoble, France b IMEP-LAHC, 3 Parvis Louis Néel, 38016 Grenoble, France article info Article history: Available online 6 April 2013 Keywords: AlGaN/GaN HeMT Sheet resistance Light Trapping abstract An important dispersion in the Rsheet measurement of AlGaN/GaN wafers was found on different sam- ples, with or without SiN passivation. The dispersion is due to a drift of the Rsheet appearing when the sample is placed in the dark. This drift is different for each sample and it is caused by a trapping mech- anism. A constant measurement of the Rsheet can be obtained under illumination. Thus, two parameters deserve measurement for an efficient screening of a technology: Rsheet under light and DRsheet in obscurity. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction AlGaN/GaN HEMTs are very promising transistors for power applications [1]. However, in order to obtain good devices, it is important to be able to measure precisely the sheet resistance of the 2DEG AlGaN/GaN substrate. In this work, we have studied the measurement of the Rsheet of AlGaN/GaN wafers, the measure- ment dispersion and the influence of the light thereon. Light studies on GaN HEMTs have already been made [2,3] but, to our knowledge, such systematic studies of Rsheet measurement dispersion and the effect of light on these measurements have not been conducted. 2. Experimental The study was carried out on Van Der Pauw structures (Fig. 1(a)) [4], comparing three different AlGaN/GaN wafers, as shown in Fig. 1(b). The first sample was grown on Si, the second on SiC and the third on Sapphire. The vertical structure for sample 1 consists of a GaN buffer and a 22 nm AlGaN layer with 25% alu- minium. For samples 2 and 3, the thickness was 24 nm with 22% and 22.5% aluminium respectively. The ohmic contacts are based on Ti/Al for sample 1 and Ti/Al/Au for samples 2 and 3. A 500 nm SiN passivation layer was deposited on samples 2 and 3. The struc- tures were isolated by ionic implantation of Argon. 3. Results 3.1. Evidence of dispersion On the first sample, eight different Van Der Pauw structures were measured four times on two different days (Fig. 2). The devices are represented on the X axis where each bar represents an Rsheet measurement. We can see a large dispersion in the mea- surement, up to 2.4% of the Rsheet value. Moreover, this phenom- enon does not seem repeatable, as we obtain different results on each day. The Rsheet of a single Van Der Pauw structure was measured during more than sixty hours, as shown in Fig. 3 where we observe a drift of about 6% of the Rsheet value. 3.2. Influence of illumination A light/dark/light measurement was conducted on one of the Van Der Pauw structures with the Rsheet shown as a function of time in Fig. 4. At 550 s, the sample is placed in the dark. At 1300 s, the sample is again placed under illumination. In the dark the Rsheet starts drifting immediately whereas under illumination, we obtain a stable Rsheet measurement. Another measurement a day later (Fig. 4, Day 2) shows a perfect reproducibility of the phenomenon. In Fig. 5 we shows DRsheet against time for a large set of de- vices on the second sample. (DRsheet is Rsheet minus Rsheet un- der illumination). The shift is comparable across the entire sample but varies with the sample technology (Table 1). On all three samples, we have performed seven measure- ments on eight different devices in darkness and under illumina- 0167-9317/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mee.2013.03.161 ⇑ Corresponding author at: CEA-LETI Minatec Campus, 17 rue des Martyrs, 38000 Grenoble, France. Tel.: +33 438 786 262. E-mail address: jonathan.lehmann@cea.fr (J. Lehmann). Microelectronic Engineering 109 (2013) 334–337 Contents lists available at SciVerse ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee