Plasma-Assisted Molecular Beam Epitaxy Growth of Mg3N2 and Zn3N2 Thin Films

Summary

This article describes the growth of epitaxial films of Mg₃N₂ and Zn₃N₂ on MgO substrates by plasma-assisted molecular beam epitaxy with N₂ gas as the nitrogen source and optical growth monitoring.

Abstract

This article describes a procedure for growing Mg₃N₂ and Zn₃N₂ films by plasma-assisted molecular beam epitaxy (MBE). The films are grown on 100 oriented MgO substrates with N₂ gas as the nitrogen source. The method for preparing the substrates and the MBE growth process are described. The orientation and crystalline order of the substrate and film surface are monitored by the reflection high energy electron diffraction (RHEED) before and during growth. The specular reflectivity of the sample surface is measured during growth with an Ar-ion laser with a wavelength of 488 nm. By fitting the time dependence of the reflectivity to a mathematical model, the refractive index, optical extinction coefficient, and growth rate of the film are determined. The metal fluxes are measured independently as a function of the effusion cell temperatures using a quartz crystal monitor. Typical growth rates are 0.028 nm/s at growth temperatures of 150 °C and 330 °C for Mg₃N₂ and Zn₃N₂ films, respectively.

Introduction

The II₃-V₂ materials are a class of semiconductors that have received relatively little attention from the semiconductor research community compared to III-V and II-VI semiconductors¹. The Mg and Zn nitrides, Mg₃N₂ and Zn₃N₂, are attractive for consumer applications because they are composed of abundant and non-toxic elements, making them inexpensive and easy to recycle unlike most III-V and II-VI compound semiconductors. They display an anti-bixbyite crystal structure similar to the CaF₂ structure, with one of the interpenetrating fcc F-sublattices being half-occupied....

Protocol

1. MgO substrate preparation

NOTE: Commercial one-side epi-polished (100) oriented single crystal MgO square substrates (1 cm x 1 cm) were employed for the X₃N₂ (X = Zn and Mg) thin film growth.

1. High temperature annealing
   1. Place the MgO on a clean sapphire wafer sample carrier with the polished side facing upwards in a furnace and anneal for 9 h at 1,000 °C. Raise the temperature to 1000 °C over a 10 min period.
      NOTE: High temperature annealing removes carbon from the surface and reconstructs the surface crystal structure of the MgO single crystal substrates.
   ....

Results

The black object in the inset in Figure 5B is a photograph of an as-grown 200 nm Zn₃N₂ thin film. Similarly, the yellow object in the inset in Figure 5C is an as-grown 220 nm Mg₃N₂ thin film. The yellow film is transparent to the extent that it is easy-to-read text placed behind the film¹⁰.

The surface .......

Discussion

A variety of considerations is involved in the choice of substrates and establishing the growth conditions that optimize the structural and electronic properties of the films. The MgO substrates are heated at high temperature in air (1000 °C) to remove carbon contamination from the surface and improve the crystalline order in the substrate surface. Ultrasonic cleaning in acetone is a good alternative method to clean the MgO substrates.

The (400) X-ray diffraction peak for the Zn₃

Materials

Name	Company	Catalog Number	Comments
(100) MgO	University Wafer	214018	one side epi-polished
Acetone	Fisher Chemical	170239	99.8%
Argon laser	Lexel Laser	00-137-124	488 nm visible wavelength, 350 mW output power
Chopper	Stanford Research system	SR540	Max. Frequency: 3.7 kHz
Lock-in amplifier	Stanford Research system	37909	DSP SR810, Max. Frequency: 100 kHz
Magnesium	UMC	MG6P5	99.9999%
MBE system	VG Semicon	V80H0016-2 SHT 1	V80H-10
Methanol	Alfa Aesar	L30U027	Semi-grade 99.9%
Nitrogen	Praxair	402219501	99.998%
Oxygen	Linde Gas	200-14-00067	> 99.9999%
Plasma source	SVT Associates	SVTA-RF-4.5PBN	PBN, 0.11" Aperture, Specify Length: 12" – 20"
Si photodiode	Newport	2718	818-UV Enhanced, 200 - 1100 nm
Zinc	Alfa Aesar	7440-66-6	99.9999%