Monitoring and Controlling of Strain During MOCVD of AlGaN for UV Optoelectronics

Abstract

The grown-in tensile strain, due to a lattice mismatch between AlGaN and GaN, is responsible for the observed cracking that seriously limits the feasibility of nitride-based ultraviolet (UV) emitters. We report in-situ monitoring of strain/stress during MOCVD of AlGaN based on a wafer-curvature measurement technique. The strain/stress measurement confirms the presence of tensile strain during growth of AlGaN pseudomorphically on a thick GaN layer. Further growth leads to the onset of stress relief through crack generation. We find that the growth of AlGaN directly on low-temperature (LT) GaN or AlN buffer layers results in a reduced and possibly controllable strain.

Control and elimination of cracking of AlGaN using low-temperature AlGaN interlayers

“…2 We have shown that relaxation of tensilely stressed AIGaN (on GaN) proceeds through the propagation of incipient cracks, which in turn activates the slip systems for misfit dislocations around the crack tips.3 Even though III-nitride devices are known for their relative insensitivity to a high density of dislocations,4 cracking due to the tensile mismatch between AIGaN and the GaN pseudo-substrate tends to severely deteriorate device performance. 5 Various types of intdayers, generally referred to as a low-temperature (LT) grown layer inserted between two layers grown at regular, high temperatures (HT), have been reported to influence the structural properties of the III-Nitrides. Table I.…”

Control of Dislocations and Stress in AlGaN on Sapphire Using a Low Temperature Interlayer

“…At the initial stage of the growth, HT-GaN is under compressive stress. After growing HT-GaN less than one micron thick, it changes to tensile stress [13]. This process is repeated during the series of the LT-interlayer/HT growth, therefore the final stress is the same for all the GaN layers.…”

High Al-Content AlInGaN Devices for Next Generation Electronic and Optoelectronic Applications

“…The concern was reiterated by Peng et al [63] in stating, based on optical absorption measurements of sputtered polycrystalline films, that AlInN and AlGaInN do not seem to provide good confinement (to GaInN) due to a very strong bowing effect. We have shown from in-situ stress measurement that the use of high Al-fraction (or thick) AlGaN barriers on GaN templates, required for electrical and optical confinement in UV devices, leads to a buildup of tensile stress and subsequently the occurrence of cracking [44]. As discussed in this section, introducing indium into GaN greatly enhances the optical efficiency but at the cost of an increase in emission wavelength.…”

“…Furthermore, mismatch-induced tensile stress was observed during growth of AlGaN heterostructures on thick GaN templates for UV devices [44], leading to relaxation through crack generation. Quaternary AlGaInN compound semiconductors are expected to enclose a finite (non-zero) area on the plot of energy gap versus lattice constant.…”