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Improvement of the ohmic characteristics of Pd contacts to p-type GaN using an Ag interlayer

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2006 Semicond. Sci. Technol. 21 L7 (http://iopscience.iop.org/0268-1242/21/2/L01) View the table of contents for this issue, or go to the journal homepage for more

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INSTITUTE OF PHYSICS PUBLISHING

SEMICONDUCTOR SCIENCE AND TECHNOLOGY

doi:10.1088/0268-1242/21/2/L01

Semicond. Sci. Technol. 21 (2006) L7–L10

LETTER TO THE EDITOR

Improvement of the ohmic characteristics of Pd contacts to p-type GaN using an Ag interlayer June-O Song1, J S Kwak2 and Tae-Yeon Seong3 1 School of Electric and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0250, USA 2 Department of Materials Science and Metallurgical Engineering, Sunchon National University, Chonnam 540-742, Korea 3 Division of Materials Science and Engineering, Korea University, Seoul 136-701, Korea

E-mail: [email protected]

Received 5 October 2005, in final form 27 November 2005 Published 16 December 2005 Online at stacks.iop.org/SST/21/L7 Abstract We have investigated the addition effect of an Ag interlayer (2 nm) at the Pd and GaN interface on the ohmic behaviour of a single Pd contact (110 nm). The Ag layer is broken up into nano-dots (11–22 nm in size) when annealed at temperatures of 330–530 ◦ C. It is shown that the use of the Ag interlayer is effective in widening the temperature range for the ohmic formation of the Pd contact and improving the adhesion of the Pd contact to GaN. The improved ohmic behaviours are attributed to the reduction of Schottky barrier heights due to the shift of the surface Fermi level towards the valence-band edge and the formation of Ag nano-dots at the Pd/GaN interfaces. (Some figures in this article are in colour only in the electronic version)

1. Introduction GaN-based laser diodes (LDs) in the blue and ultraviolet wavelengths are of significant technological importance for their applications in data storage devices. The performance of LDs is considerably affected by the electrical properties of ohmic contacts to p-type GaN [1]. However, the large band gap of p-GaN (as wide as about 3.4 eV) makes it difficult to form good ohmic contacts, which have low contact resistivity (10−5 cm2) and good thermal stability. It is noted that the high specific contact resistance and high operating voltage of GaN-based LDs result in a large amount of heat, leading to eventual device failure. Thus, to realize high-performance GaN-based blue LDs having a long lifetime, the development of reliable ohmic contacts is essential. So far, extensive studies have been performed to realize reliable ohmic contacts. For example, for ohmic contacts to p-GaN, a wide variety of metal schemes such as Ni/Au [2, 3], 0268-1242/06/020007+04$30.00

Pd/Au [4], Ta/Ti [5], Au/Ni/Au [6], Pd/Pt/Au [7], Ni/Pt/Au [8], Pt/Ni/Au [9] and Ti/Pt/Au [10] have been investigated. Among these schemes, Ni- and Pd-based ohmic contacts are currently being used as p-type ohmic electrodes for GaN-based LDs because of their reasonable contact resistivity and ease in a device etching process. In particular, Pd-based multilayer contacts were shown to produce ohmic behaviour with low specific contact resistivity [7, 11]. However, the Pd-based contacts suffer from several drawbacks. In other words, for the Pd-based contacts, it is difficult to reproduce the same electrical results at all times. Furthermore, the Pd-based contacts have poor adhesion to p-GaN when the Pd layer thickness exceeds a critical value (e.g., about 50 nm). Thus, in this work, to widen a processing window for forming the best electrical property of Pd contacts and to improve their adhesion to p-GaN, an Ag interlayer (2 nm) is brought in at the Pd/GaN interface. It is shown that the Pd contacts with the Ag interlayers produce specific contact resistances of

© 2006 IOP Publishing Ltd Printed in the UK

L7

Letter to the Editor

∼10−5 cm2 when annealed at temperatures in the range of 330–530 ◦ C. It is further shown that the Pd contact with the Ag interlayer produces much better ohmic behaviour compared with single Pd contacts.

(a )

2. Experimental details 1.5 µm thick p-type GaN:Mg layers (Na = 5 × 1017 cm−3) grown by metalorganic chemical vapour deposition were ultrasonically degreased using trichloroethylene, acetone, methanol and DI water for 5 min in each step, followed by N2 blowing. The GaN wafers were then treated with a buffered oxide etch (BOE) solution for 20 min and rinsed in DI water. After BOE cleaning, Ag layers (2 nm) were deposited on the GaN by an electron-beam evaporation, on which Pd layers (110 nm) were deposited. For comparison, a single Pd layer (110 nm) was also deposited on the BOE-treated GaN sample. Some of the samples were annealed at temperatures of 330– 550 ◦ C for 1 min in air. Circular transfer line method (CTLM) patterns defined by the standard photolithography and liftoff technique were performed for measuring specific contact resistance [12]. The outer dot radius was 75 µm and the spacing between the inner and the outer radii varied from 4 to 25 µm. Current–voltage (I–V) measurements were carried out using a parameter analyser (HP 4155A). X-ray photoemission spectroscopy (XPS, PHI 5200 model) was performed using an Al Kα x-ray source in an ultrahigh vacuum system with a chamber base pressure of ∼10−10 Torr. Auger electron spectroscopy (AES) was performed using a PHI 670 Auger microscope with an electron beam of 5 keV and 0.0236 µA.

(b )

Figure 1. The I–V characteristics of Pd (110 nm) contacts (a) without and (b) with the inserted Ag layers, measured on the 8 µm spaced pads.

(a )

3. Results and discussion Figure 1 shows the I–V characteristics of Pd (110 nm) contacts with and without the inserted Ag layers, measured on the 8 µm spaced pads. Both the as-deposited samples reveal nonlinear I–V behaviours. For the single Pd contacts (figure 1(a)), as the annealing temperature increases, the I–V behaviour reaches the best at 330 ◦ C and then becomes continuously degraded. The specific contact resistance was measured to be 9.63 × 10−5 cm2 for the sample annealed at 330 ◦ C. For the Ag/Pd contacts (figure 1(b)), the I–V behaviours are improved with increasing temperature. Measurements showed that the specific contact resistances are 6.62 × 10−5, 6.02 × 10−5 and 4.35 × 10−5 cm2 for the samples annealed at 330, 430 and 530 ◦ C, respectively. It is worth noting that unlike the single Pd contacts, all the annealed Ag/Pd contacts produce somewhat similar low resistivities. The results show that the use of the Ag interlayer plays a crucial role in widening a processing window. From the viewpoint of device processing, it is very important to have a wider window for good ohmic formation. To characterize interfacial reactions between the metals and the p-GaN, AES examination was made of the Ag/Pd samples before and after annealing at 530 ◦ C. It is evident that for the as-deposited sample, an Ag layer is present at the Pd/GaN interface, figure 2(a). For the annealed sample, figure 2(b), most of Ag outdiffused towards the sample surface region through the Pd layer. It is, however, noted that there is still a small amount of Ag (arrowed) at the Pd/GaN interface. In addition, some amount of Ga outdiffused into the metal L8

] (b )

] Figure 2. AES depth profiles for the Ag/Pd samples (a) before and (b) after annealing at 530 ◦ C.

layer. This is indicative of possible reactions between the Pd and the GaN, resulting in the formation of interfacial gallide [13, 14].


Figure 5. Calculated electric field distribution for p-GaN as a function of the radius of the circular patch (size) of Ag nano-dots and the distance from the p-GaN surface using equation (1). Figure 3. A STEM Z-contrast image from the Ag/Pd sample annealed at 530 ◦ C, showing the presence of an Ag nano-dot (13 nm in size) (as indicated by the arrow).

Figure 4. The Ga 3d core level for the metal layers/GaN interface region before and after annealing at 530 ◦ C.

To investigate interface structures, scanning transmission electron microscopy (STEM) Z-contrast and energy dispersive spectrometry (EDS) examinations were performed on the Ag/Pd contacts annealed at 530 ◦ C. STEM results revealed the formation of Ag nano-dots at the Pd/GaN interface, as expected from the AES result (figure 2(b)). The Ag nanodots were measured to be 11–22 nm in size. For example, a STEM Z-contrast image (figure 3) exhibits the presence of an Ag nano-dot, which was confirmed by the EDS results. (The open circles indicate the regions where EDS analyses were made.) In addition, high-resolution TEM results (not shown) exhibited that thin Pd–Ga–N phase is formed at the Pd/GaN interface, as confirmed by electron energy loss spectroscopy (EELS) and EDS results. This is consistent with the AES results (figure 2). Furthermore, the STEM results show that the Pd layer (110 nm thick) remains stable across the whole GaN substrate. This indicates that the introduction of the Ag interlayer improves the adhesion of the Pd layer to GaN. To characterize the chemical bonding state of Ga, XPS examination was performed on the Ag/Pd contacts on p-GaN before and after annealing at 530 ◦ C. Before commencing analyses, the metal layers were sputtered using Ar+ ions to expose the interface region between the metals and the GaN. Figure 4 shows the Ga 3d core level for the metal layers/GaN interface regions before and after annealing. It is evident that

the Ga 3d core level for the annealed sample shifts towards the low-energy side by 0.51 eV as compared with the as-deposited sample. This indicates that annealing causes a shift of the surface Fermi level towards the valence-band edge [13–16]. The ohmic properties of the Pd contacts were significantly improved when an Ag layer was introduced at the Pd/GaN interface. The improvement could be related to the reduction of Schottky barrier heights (SBHs) due to the shift of the surface Fermi level towards the valence-band edge, as noted from the shift of the Ga 3d core level, figure 4 [13, 17, 18]. The Fermi level shift could be brought about by an increase in carrier concentration. The AES and EDS results indicated that interfacial gallide phases are formed upon annealing. The formation of gallide phases causes Ga vacancies to be generated near the GaN surface, which serve as deep acceptors [14]. In addition, the presence of Ag nano-dots at the Pd/GaN interface (figure 3) seems to contribute to the reduction of SBHs. According to the electronic transport theory at the metal/semiconductor (MS) interface with inhomogeneous SBHs, the electric filed for circular patch geometry (nano-dot) at the MS interface is given as [19] 2z 2 − 2 − φPd−Ag E(z) = Vbi w w 1 z2 (1) × 1/2 − 3/2 , z2 + Ro2 z2 + Ro2 where Vbi is the band bending, z is the distance from the surface of the semiconductor, w is the depletion width, Ro is the radius of the circular patch (Ag nano-dot) and φ Pd−Ag is the SBH difference resulting from the different work functions of Pd and Ag. The first term on the right-hand side of equation (1) indicates the electric field due to a uniform SBH and the second term shows the variation of the electric field due to the presence of nano-dots. Figure 5 shows the calculated electric field distribution versus distance (z) from the metal/semiconductor (MS) interface to the inside of the semiconductor for the samples as a function of the size of nano-dots. The values used in this calculation are obtained from references [20] and [21]. It is worth noting that the electric field at the MS interface increases exponentially with decreasing the size of the Ag nano-dots. Equation (1) and figure 5 show that the difference of the SBHs between Pd and Ag, and the presence of the Ag nano-dots at the MS interface can lead to an increase in the L9


electric field at the MS interface. The increase of the electric field was shown to cause the lowering of a barrier height [22– 24]. Equation (1) also shows that the presence of the Ag nano-dots could lead to the reduction of the SBHs.

4. Summary We investigated the Ag/Pd scheme for producing lowresistance ohmic contacts to GaN for laser diodes. The Ag/Pd contacts produced better contact resistivity (4.35 × 10−5 cm2) than the single Pd contacts; unlike the single Pd contacts, the Ag/Pd contacts became ohmic at the wider annealing temperature range (330–530 ◦ C). It was also shown that the use of the Ag layer effectively improves the adhesion of the Pd layer to GaN. This indicates that the Ag/Pd contact could represent a promising scheme for the fabrication of highperformance laser diodes.

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