The role of liquid phase epitaxy during growth of AlGaN by MBE

Abstract

We propose that the growth of III‐Nitride semiconductors by plasma‐assisted MBE under Ga‐rich conditions takes place though the saturation of the liquid Ga covering the surface of the growing film with nitrogen, alloy constituents (aluminum and indium) and impurities. Thus, the proposed growth mode is a liquid phase epitaxy (LPE) rather than physical vapor phase epitaxy. While in traditional LPE growth the driving force is the gradient between the liquid and seed, in the proposed mode of MBE growth of nitrides the driving force is the concentration gradient due to the constant supply of active nitrogen, alloy components and impurities. The proposed LPE growth mechanism explains the relative temperature stability of GaN growth, the efficient doping with silicon and magnesium, and the selective desorption of oxygen and other impurities. Finally, the proposed LPE growth model can also account for the introduction of band structure potential fluctuations in AlGaN alloys, which improves their emission properties and leads to efficient deep UV‐LEDs. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultraviolet optoelectronic devices based on AIGaN alloys grown by molecular beam epitaxy

“…[47] Another source of the poor IQE of deep UV-LEDs is the incorporation of oxygen in AlGaN due to the high chemical affinity of aluminum for oxygen. [48][49][50] While oxygen is a shallow donor in GaN and InGaN alloys, it is known to form DX-like centers in AlGaN alloys with high AlN mole fraction. [51] Other potential problems are the poor doping efficiency of n-and particularly p-AlGaN with high AlN mole fraction, which is responsible for the poor carrier injection in the active region of the device.…”

“…[50,61,62] Specifically, we have proposed that under Ga-rich conditions the GaN growth by PAMBE is a liquid phase epitaxy (LPE) rather than physical vapor deposition. In other words growth takes place through the saturation with active nitrogen of the metallic Ga at the growing surface, followed by subsequent crystallization from the melt onto the GaN seed.…”

“…We have reported previously that the concentration of impurities such as O, H, and C is in the 10 19 cm −3 range when the GaN films are grown under nitrogen-rich conditions, while they are two to three orders of magnitude less when the films are grown under Ga-rich conditions. [50,62] The reduction in oxygen incorporation can be accounted for by the formation of volatile gallium oxides, which would then desorb. Similarly hydrogen may form volatile gallium hydrides.…”

Plasma‐assisted electroepitaxy of GaN layers

“…The RF and ECR plasma sources used in PA-MBE produce a nitrogen flux consisting of a mixture of neutral molecular nitrogen, charged molecular nitrogen, ionic and neutral atomic nitrogen together with free electrons [4,5]. Which type of nitrogen species is responsible for the growth process during PA-MBE of GaN is still under discussion, but the main contenders are atomic nitrogen and metastable molecular nitrogen [4][5][6]. Whilst the solubility of molecular nitrogen in Ga under normal growth conditions is very low, because the process is dissociative with a high energy barrier; the solubility of activated nitrogen species in liquid Ga is expected to be relatively high [6,7].…”

“…Which type of nitrogen species is responsible for the growth process during PA-MBE of GaN is still under discussion, but the main contenders are atomic nitrogen and metastable molecular nitrogen [4][5][6]. Whilst the solubility of molecular nitrogen in Ga under normal growth conditions is very low, because the process is dissociative with a high energy barrier; the solubility of activated nitrogen species in liquid Ga is expected to be relatively high [6,7]. If this model is correct, the growth process of GaN in PA-MBE is more like liquid phase epitaxy (LPE) [6,7].…”