Highly efficient p-type doping of GaN under nitrogen-rich and low-temperature conditions by plasma-assisted molecular beam epitaxy

H Tang,W Jiang,S M Sadaf,X Wu

doi:10.1063/1.5089658

Abstract

Highly efficient and reproducible p-type doping of GaN under nitrogen-rich and low-growth-temperature conditions was demonstrated with the plasma-assisted molecular beam epitaxy technique. The low-temperature range is approximately below 650 °C and refers to growth temperatures at which the thermal desorption of any excess Ga is negligibly slow. The Mg and hole concentrations obtained with the N-rich condition were more than one order of magnitude higher than those obtained with the Ga-rich condition while keeping all other conditions identical. The Mg doping under such N-rich conditions was also found to show Mg-mediated suppression of background impurities, good epitaxy quality on GaN templates, and relatively low surface roughness. Over the investigated growth temperature range from 580 °C to 650 °C, the Mg incorporation efficiency under the N-rich condition was found to be close to unity (70%-80%) and independent of the growth temperature. High hole concentrations of up to 2×1019 cm-3 and activation efficiencies of up to 16.6% were obtained. The result rules out the Mg surface sticking probability as the limiting mechanism for Mg incorporation in this temperature range, as it would be temperature dependent. Instead, the Mg incorporation rate was more likely governed by the availability of substitutional sites for Mg on the surface, which should be abundant under the N-rich growth conditions. Excellent diode characteristics and electroluminescence results were observed when this p-type doping method was employed in the growth of full device structures.

Highlights

The efficient p-type doping in plasma-assisted molecular beam epitaxy (PA-MBE) makes this growth technique a viable alternative to the metal-organic vapor phase epitaxy technique for a wide range of optoelectronic devices including lasers.1,2 In addition, the Mg doping by MBE does not require post-growth activation, which is unique advantage for growth of tunnel-junction devices.3–8 successful p-type doping has been reported for the plasma MBE method under various conditions, the delineation and understanding of the different growth regimes is still not definitive or complete.9 The III/V ratio has profound impact on the growth by plasma MBE
It is worth mentioning that the Mg/Ga flux ratio is somewhat higher for the N-rich growth which may partially contribute to the higher Mg concentration
The p-type doping under such conditions showed Mg-mediated suppression of background impurities, good epitaxy quality on GaN templates, and relatively low surface roughness

Summary

INTRODUCTION

The efficient p-type doping in plasma-assisted molecular beam epitaxy (PA-MBE) makes this growth technique a viable alternative to the metal-organic vapor phase epitaxy technique for a wide range of optoelectronic devices including lasers. In addition, the Mg doping by MBE does not require post-growth activation, which is unique advantage for growth of tunnel-junction devices. successful p-type doping has been reported for the plasma MBE method under various conditions, the delineation and understanding of the different growth regimes is still not definitive or complete. The III/V ratio has profound impact on the growth by plasma MBE. In the growth of undoped GaN, it has been well established that the Ga-rich and Ga bilayer growth condition is critically important for high epitaxy quality especially in the low temperature growth regime.. Successful p-type doping under N-rich conditions was only reported at high growth temperatures (around 740 ○C).22 In such high-temperature regime, the effect of the III/V ratio is less profound, because under both N-rich and Ga-rich conditions, the Ga bilayer could not form due to much accelerated thermal desorption. The N-rich condition was first explicitly suggested to be favorable or necessary for efficient Mg incorporation at low growth temperatures by a series of studies of p-type doping using the metalmodulated-epitaxy (MME) technique.. Highly efficient and reproducible p-type doping under constant N-rich and low-growth-temperature conditions was observed and confirmed through detailed characterization of a series of comparative samples. Excellent device characteristics were obtained when this p-type doping method was applied in the growth of full device structures

EXPERIMENTAL

RESULTS AND DISCUSSION

CONCLUSIONS