Scanning probe microscopy studies on the growth of palladium and nickel on GaN(0001)

Abstract

Abstract. The performance of GaN-based devices, such as blue LEDs and laser diodes, relies on good metal-GaN contacts. However, little is known about the formation and properties of these contacts on the atomic scale. We have studied the initial stages of growth of Pd and Ni on the GaN(0001) surface using variable temperature UHV-STM. The atomic-scale STM images have been compared with tapping-mode AFM images and point I-V measurements using conductive AFM in order to correlate performance on nano-scale and micro-scale. The growth mode of the metals and formation of alloys and interfacial compounds have important consequences for the electrical contact behaviour. Systematic control of the deposition conditions is therefore crucial for the performance of the contacts. IntroductionOver the past decade, gallium nitride (GaN) technology and its device applications have developed rapidly. The performance of these devices (e.g. LEDs, laser diodes and high-power transistors) relies on high-quality metal-to-GaN contacts. Therefore, ohmic contacts and Schottky barriers to GaN have been studied extensively. Previous studies have focused on the improvement of ohmic contacts, such as Ti/Al and Pd/Al on n-type GaN, or Ni/Au, Ni/Pd/Au, Pd/Au on p-type GaN [1, 2]. Low-resistance, thermally stable ohmic contacts on p-type GaN have been reported using Pd/Ni and Pd/NiO based contacts [3,4]. However, little is known about the formation and properties of these contacts on the atomic scale. So far, atomic-scale studies of metals on GaN(0001) using scanning tunnelling microscopy (STM) have only been reported for silver [5] and gold [6].The formation of alloys and interfacial compounds between GaN and a metal contact influences the stability and electrical properties of the contact. For example, the formation of Pd gallides (Ga 2 Pd 5 and Ga 5 Pd) during annealing of Pd contacts on p-GaN results in enhanced thermal stability and reduced contact resistivity [3]. Control of the metal deposition conditions as well as the effective preparation of clean GaN surfaces is therefore crucial for the performance of the contacts. In a previous study, we have identified the effect of different surface treatments on surface morphology and stoichiometry of the GaN(0001) surface [7], whereas other authors have investigated the effect of specific surface treatments on the electrical properties of ohmic contacts to n-and p-type GaN [1,8,9].Here, we have focused on atomic-scale studies of the initial stages of growth of palladium and nickel on the GaN(0001) surface using in-situ variable-temperature STM. Complementary meso-scale tapping-mode AFM imaging was carried out in order to characterize the density and distribution of structures as a function of processing parameters. Point I-V analysis in the contact mode (SECM) with a conductive Au-coated probe was also undertaken in order to elucidate the resultant point-contact electrical characteristics.

Properties of Bare and Thin-Film-Covered GaN(0001) Surfaces

“…It can be applied as a single element layer or a multilayer with other elements, mainly from the nickel family, gold, and silver [26][27][28]31]. Structural and topographical analysis of Ni on GaN performed by STM was reported in [97,98]. The former work concerns the growth mode and interfacial compound formation.…”

“…Investigations of thin Pd films on GaN(0001) using XPS, UPS, and LEED techniques are summarized in the author's two publications [34,101]. Such systems have been previously studied with STM [97]. In both works, Pd films were deposited by the PVD method.…”

Theoretical and experimental investigations of nano-Schottky contacts

“…Some experimental and theoretical analyses have been made to investigate the nano M-S contacts. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The analyses have addressed the effect of the nano contact size on the depletion width and on other interface parameters. It was reported that I-V characteristics showed an enhanced current in the reverse bias, which can be attributed to the enhancement of the electric field at the nano M-S interface due to the confined electric charge on a very small metal surface area.…”

Properties of Thin Film-Covered GaN(0001) Surfaces

“…[26][27][28]31]. Structural and topographical analysis of Ni on GaN performed by STM was reported in [97,98]. The first work concerns the growth mode and the interfacial compounds formation.…”