Reduced Workfunction Intermetallic Seed Layers Allow Growth of Porous n-GaN and Low Resistivity, Ohmic Electron Transport

Abstract

Porous GaN crystals have been successfully grown and electrically contacted simultaneously on Pt- and Au-coated silicon substrates as porous crystals and as porous layers. By the direct reaction of metallic Ga and NH(3) gas through chemical vapor deposition, intermetallic metal-Ga alloys form at the GaN-metal interface, allowing vapor-solid-solid seeding and subsequent growth of porous GaN. Current-voltage and capacitance-voltage measurements confirm that the intermetallic seed layers prevent interface oxidation and give a high-quality reduced workfunction contact that allows exceptionally low contact resistivities. Additionally, the simultaneous formation of a lower workfunction intermetallic permits ohmic electron transport to n-type GaN grown using high workfunction metals that best catalyze the formation of porous GaN layers and may be employed to seed and ohmically contact a range of III-N compounds and alloys for broadband absorption and emission.

Fabrication of p-type porous GaN on silicon and epitaxial GaN

“…While a small in-built potential also exists for the Au-contacted sample, the symmetry in the response at negative bias confirms that the presence of the intermetallic Ga 2 Au at the GaN-Si interface prevents a rectifying barrier. Also, as with unintentionally n-type GaN growth we investigated previously, 14 the Pt-Ga contact to the GaN at the silicon interface results in an order of magnitude higher current, but in the case of p-GaN, the transport mechanism gives a non-linear, Schottky-type response.…”

“…For porous GaN specifically, the thickness is determined by the size of the crystals and their assembly on the surface, which results in a "rough" topology, with a high density of grain boundary scattering centers. For electrical measurements, particle distributions 14 confirm that both nearest neighbour distances and overall distribution of various nonspherical catalyst material prevent the formation of a percolating conduction layer beneath the GaN; the electrical characterization is of the porous GaN polycrystalline layer. At room temperature, the effective linearity of the IÀV response measured for porous GaN to high work function metals, with an intermetallic seed layer between the metal and GaN, is maintained at high bias.…”

“…13 The ability to form relatively uniform, crystalline porous GaN layers over large areas without unwanted surface damage through complex etching without photolithographic masking or processing is challenging. Previously, we reported the bottom-up growth of lowresistivity, Ohmically contacted porous n-GaN in a single growth step 14 without the need for any secondary etching or chemical treatment after growth to induce porosity. 15,16 Mg is the dopant most commonly used to generate p-type conductivity in GaN.…”

“…Further details can be found elsewhere. 14,15 Mg doped samples were annealed in atmosphere of N 2 for 20 min in order to break Mg-H complexes and activate the p-type conductivity of porous GaN. The epitaxial GaN substrates had n-type (doped with Si) conductivity.…”

Doping controlled roughness and defined mesoporosity in chemically etched silicon nanowires with tunable conductivity

“…The contact were tested in sandwich geometries on bulk, unetched substrates to confidently test double-junction ohmicity variation due to porosification and size reduction of the Si on one side. In addition, an In-Ga eutectic was also employed so that electrical contact 63 could be made to a copper current collector. Figure 10 below shows the corresponding I-V curves for a series of metal contact sandwiches to unetched Si.…”