Introduction

Molecular beam epitaxy, also known as MBE is a highly controllable technique of growing epitaxial layers of semiconductor crystals that was invented in the 70’s to manufacture compound semiconductors and since then it has been widely used both in research and industry to for example grow single-crystal thin films and other nanostructures. Some of its main benefits are high purity levels achieved due to ultrahigh vacuum conditions and its capability to grow superlattices. Due to being an extremely slow process, great dimensional control is achievable. ^[1][2]

Equipment

MBE is performed in pressures as low as 1 * 10^-10 Pa. The material is sublimated or evaporated in several source ovens equipped with shutters to form molecular beams that are at an angle compared to the substrate. The substrate is placed on a heater that can be rotated. The chamber is surrounded by a cooling panel that makes sure that the beams can make only a singular pass and background pressure will be eliminated to ensure the purity of the film. Reflection high-energy electron diffraction (RHEED) gun and screen are installed at a specific angle to the substrate surface.^{[1]  [3]} Figure 1 depicts a MBE chamber.

Figure 1. A simple MBE chamber. Created by Nikhil P at Wikipedia. License: CC BY 3.0. 

Process

The most typical source materials used in MBE are gallium and arsenic due to their ability to form GaAs and act as a semiconductor material. As the temperature in the source oven rises, gallium atoms or As₄ molecules effuse and constitute a beam that has a large mean free path compared to the distance between the hole of the oven and the substrate wafer. If the diameter of the oven’s hole is small compared to the mean free path of the components inside the oven, the flow of either Ga or As₄ on the wafer depends on the partial pressure in the oven, the distance from the hole of the oven to the substrate, molecular weight of the species, the area of the hole and the temperature, which can be modified to to control the beam intensity.

The deposition process is strongly affected by the substrate temperature since it controls the rate of the reaction and thus affects the composition of the film produced. Naturally, too low of a substrate temperature leads to the atoms not being able to find the lowest energy site on the surface and compromises the structure of the film. With a temperature too high, the atoms start to form islands or not react at all. Thermodynamics, surface kinetics and flux ratio of the components also play a role in the resulting morphology of the surface.^[4] The morphology of the surface can be examined with the RHEED.  ^_[3]                     

Surface morphology and impurity profiling

An unique feature in MBE is that the composition of the crystal surface can be examined trough the growth process trough diffraction using RHEED. RHEED deploys an up to 20 KeV electron beam to the sample’s surface at a few degrees angle and creates a diffraction pattern on the opposite fluorescent screen. Due to the electron beam’s small angle, the beam is scattered just in the first atomic layers of the surface and thus the surface morphology can be studied using RHEED. It is also important to note that the electron beam doesn’t interfere with the molecular beam which allows the use of the RHEED technique applicable during the growth process. The rougher the surface is, the more the pattern resembles a 3D rather than 2D and vice versa. ^[3]

Applications and uses

Initially MBE is used to manufacture structures such as heterostructures and superlattices, which are widely used in electronics and optoelectronics devices, such as transistors, lasers, and solar cells. During the last decade, one of the significant fields requiring MBE was wireless communications and mobile devices due to the use of MBE in switches and power amplifier devices. Currently, one of the promising uses for MBE is high efficiency multi-junction solar cells.^[5]MBE  However, MBE is not only needed to produce thin films and superlattices used in practical applications and devices made for the market, but to also study many physical phenomena like the Hall effect or axion-electromagnetic coupling. ^[4]

Heterostructures are widely used in electronics and optoelectronics devices, such as transistors, lasers, and solar cells.

Advantages and limitations

References