High temperature behavior of Pt and Pd on GaN

Abstract

We report on the thermal stability of Pt and Pd thin films on GaN up to 800 °C. We have found that for Pt thin films submicron size metal spheres form at temperatures as low as 600 °C and the film turns into discontinuous islands at 725 °C and above. Pd begins to form islands above 700 °C and areas of the film begin to delaminate due to thermally generated compressive stress. The general behavior of these films is that found for most metal films on ceramics, suggesting that metals on GaN do not react as they typically do on common elemental and compound semiconductors. This ceramic-like behavior must be considered when determining which metals may act as good electrical contacts for GaN, especially for high temperature applications or alloyed contacts.

Thermal reaction of Pt film with 〈110〉 GaN epilayer

“…31 Island formation upon annealing of thin metal films on GaN is reported in the literature with Ni above 800°C, 32 Pt above 725°C, and Pd above 700°C, and is explained with the difference of surface energies between thin metal films and ceramics that causes dewetting. 17 Moreover, differences in the thermal expansion coefficients can generate compressive stress in the metal film and deformation of the film, as observed in our study, or delamination, as it is noted with Pd. 17 The linear coefficients of thermal expansion of materials involved here are ͑in units of ϫ10 6 /°C): Pt:9; Ga:18;…”

“…19 These authors further report the appearance of bubbles on the Pt surface after 2 h. Duxstad et al report on small particles of Pt on the surface of the Pt film after thermal treatment in flowing N 2 at 600°C for 30 min. 17 We observe the formation of blisters after vacuum annealing at 550°C for 30 min, and more so at 650°C ͑see also Fig. 2͒.…”

“…13 Schottky contacts on n-type GaN have not received much attention yet. Au ͑as-deposited͒, 3,14 Pt and Pd, 1,[15][16][17] Ni, 13,18 and PtSi 19 were reported to have rectifying characteristics. Platinum with its high work function of 5.65 eV 20 exhibits ideal Schottky behavior on n-type GaN with a high barrier height.…”

Testing the Temperature Limits of GaN-Based HEMT Devices

“…However, it has exhibited only limited stability under accelerated lifetime tests. The interface and bulk of the passivation materials are usually the location of the surface donor in AlGaN/GaN HEMTs [4], [5]. This surface donor is the state of the counter charge of the polarization discontinuity at the GaN interface, forming up to 40% of the 2DEG channel charge density, depending on the Al content of the barrier layer.…”

“…• C [3], [4]. AlGaN/GaN heterojunctions used in HEMT design are highly strained, and stress may therefore also influence the chemical/thermal stability of the films and interfaces, particularly in the presence of high temperatures and high electrical fields.…”