Ohmic contact on low-doping-density p-type GaN with nitrogen-annealed Mg

Abstract

We have demonstrated a fabrication process for the Ohmic contact on low-doping-density p-type GaN with nitrogen-annealed Mg. An Ohmic contact with a contact resistance of 0.158 Ω cm2 is realized on p−-GaN ([Mg] = 1.3 × 1017 cm−3). The contact resistance of p-type GaN with higher Mg concentration ([Mg]=1.0 × 1019 cm−3) can also be reduced to 2.8 × 10−5 Ω cm2. A localized contact layer is realized without any etching or regrowth damage. The mechanism underlying this reduced contact resistance is studied by scanning transmission electron microscopy with energy dispersive x-ray spectroscopy and secondary ion mass spectrometry, representing a mutual diffusion of Ga and Mg atoms on the interface. Reductions in the barrier height and surface depletion width with the nitrogen-annealed Mg layer are confirmed by XPS and Hall effect measurements qualitatively.

Wet Chemical Treatment and Mg Doping of p‐InP Surfaces for Ohmic Low‐Resistive Metal Contacts

“…Before the Ni deposition by sputtering, the Mg/p‐InP samples were still etched in hydrofluoric acid (HF) solution (3 m or 5%) to remove an extra unreacted Mg from the surface (Figure S3, Supporting Information). [ 49 ]…”

“…The postheating of Mg/p-InP at 350 °C for S3, Supporting Information). [49] Table 1 shows that incorporating 3 or 4 nm-thick Mg layer into the interface (Sample C or D, respectively) decreases the as-ready contact resistivity as compared to pure Ni/p-InP. However, the 250 °C postheating did not decrease the resistivity as much as it did for the reference Sample A.…”

“…The high postheating temperature around 800 °C, at which InP degrades strongly, was needed for Mg/p-GaN. [49] In this work, we demonstrate that an effective Mg surface doping of InP can be done at 350 °C or lower when a proper hydrochloric acid (HCl)-based solution treatment is combined by engineering the Mg-covered InP surface in ultrahigh-vacuum (UHV) conditions. First, a proper HCl þ isopropanol (IPA) immersion is shown to cause an unusual InP(100) surface reconstruction without any postheating or treatment in UHV.…”

“…Because the Schottky-type band bending is common at a metal/semiconductor interface due to a high density of interface defects, increasing the doping concentration at the semiconductor surface is used to decrease the width of a possible depletion region (Figure 1) and make the carrier tunneling process probable. [13,40,44,49] One way to increase the doping level at semiconductor surfaces before the metal film growth is to deposit an intermediate layer of the metal, which can act as a p-type (or n-type) element, between the semiconductor and the main metal. During the postheating step(s), the metal/ semiconductor elements can diffuse over the interface providing an interface structure where the doping metal atoms replace the host semiconductor element in proper manner, increasing finally the effective doping density.…”

“…This strategy has been recently used successfully for p‐type GaN which has also suffered from the high contact resistivity. [ 49 ] Magnesium (Mg) has been the traditional p‐type doping element for the nitride semiconductors, while Zn and Be are often incorporated into other p‐III–V's to replace the III lattice sites. The high postheating temperature around 800 °C, at which InP degrades strongly, was needed for Mg/p‐GaN.…”