Phase equilibria in transition metal Al-Ga-N systems and thermal stability of contacts to AlGaN

Abstract

As a first step in predicting the phase equilibria in TM-Al-Ga-N systems, where TM denotes a transition metal from the first three transition series, phase equilibria are calculated in the TM-Al-N systems, and the condensed phases in equilibrium with both AlN and N 2 gas are discussed with respect to the position of the transition metal in the periodic table, temperature, and nitrogen pressure. Possible phases for use as stable electrical contacts to AlGaN are identified using these predictions, similar previous predictions for the TM-Ga-N systems and experimental findings. fraction increases, the electronic properties of the existing contacts change due to an increasing band gap and decreasing electron affinity of AlGaN, and new metallization schemes may have to be developed to accomplish good contact formation to Al x Ga 1-x N with x > 35%. New metallization schemes may also turn out to yield contacts to Al x Ga 1-x N with x ≤ 35% with superior properties to the existing contacts.The phase equilibria in the TM-Ga-N systems, where TM denotes a transition metal from the first three transition series, have previously been estimated. 30 As a first step in understanding the TM-AlGa-N systems, this article discusses the phase equilibria in the TM-Al-N systems. There are fortunately more experimentally available phase diagrams for these systems 31-42 than for the TM-Ga-N system; 30,43-47 however, many of the diagrams are not available for the conditions of most interest for processing contacts and devices. Thus, calculated ternary phase diagrams have to be used in many cases. Such an approach has been taken for systems composed of transition metals and Al-As, 48,50 In-P, 51,52 and In-As; 48,49 however, the same simple approach that has been used to calculate the ternary phase diagrams is not possible for the TM-Al-Ga-N systems due to lack of information about quarternary phases and the predominance of solution phases in the TM-Al-Ga and AlGa-N systems.This paper presents simple ternary phase diagram calculations of the TM-Al-N systems. These calcula-

Ohmic contact formation mechanism of Ta∕Al∕Mo∕Au and Ti∕Al∕Mo∕Au metallizations on AlGaN∕GaN HEMTs

“…1 Several metallization schemes have been reported in which surface treatments, wet etching, and plasma-based dry etching have been used to lower Ohmic contact resistances. 8 We have previously carried out a comparative study of X / Al/ Mo/ Au, where X represents Ti, Mo, and V metallizations, 9 and demonstrated that Ti/ Al/ Mo/ Au gives the best performance in terms of contact resistance, thermal stability, and surface morphology. 7 In this case, nitride-forming metals are of particular interest, for they directly introduce vacancies as a result of their reaction with nitrogen atoms from the surface of the heterostructure.…”

Condensed phase equilibria in transition metal-Ga-Sb systems and predictions for thermally stable contacts to GaSb

“…Interestingly, both Re and W are also expected to be stable on other III-V semiconductors, i.e., InAs, 15 AlAs, 15 GaAs, 14 and AlN. 18 Besides W and Re, Os is also calculated to be in thermodynamic equilibrium with GaSb, as shown in Fig. 1c.…”

“…Furthermore, they showed that the condensed phase equilibria often do not change dramatically over a significant temperature range in these systems, and this situation often allows an isothermal section determined at one temperature to provide a reasonable estimate of the phase equilibria at another temperature. Similar approaches have been taken for systems of transition metals with In-P, 17 Al-N, 18 and Ga-N. 19 In this work, we describe calculations of transition metal-Ga-Sb phase diagrams using an approach similar to that of previous studies 14,15,17 and discussed in more detail later. More so than for many of the other metal-III-V systems, thermodynamic data were not known well enough to make good predictions about some of the tie-lines in some of the metal-Ga-Sb phase diagrams we evaluated.…”

Intermixing and chemical structure at the interface between n-GaN and V-based contacts

“…Both species (I' and II) in the Ga 2p 3/2 spectra of the RTA-treated samples show an energy shift compared to species I in the n-GaN spectrum. This energetic shift can be explained by a change in the Ga chemical environment 5 (from GaN to Ga 2 O 3 ) after contact formation, as will be discussed below. Also, an interfaceinduced band bending due to the formation of a metal/semiconductor interface could be present.…”

“…Since Galesic and Kolbesen [4] demonstrated the "nitridation" of metallic vanadium films (i.e., the formation of VN) by rapid thermal annealing (RTA) in N 2 atmosphere, it has been hypothesized that VN is also formed at the interface between V-based contacts and n-Al x Ga 1-x N after RTA treatment [3]. VN is stable [5] and has a low work function [6], thus it is suitable to form Ohmic contacts to nGaN (whose electron affinity is about 4.0 eV) and n-Al x Ga 1-x N (whose electron affinity is less than 4.0 eV) [7]. It was found that Al x Ga 1-x N samples need higher RTA temperatures than pure GaN samples for optimal contact resistance [3].…”