Monitoring and Controlling of Strain During MOCVD of AlGaN for UV Optoelectronics
- PDF / 1,120,474 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 74 Downloads / 189 Views
Jung Han, M. H. Crawford, R. J. Shul, S. J. Hearne, E. Chason J. J. Figiel, and M. Banas Sandia National Laboratories, Albuquerque, NM 87185 Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G7.7 (1999) 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 AIGaN 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 AIN buffer layers results in a reduced and possibly controllable strain. INTRODUCTION Thus far the optoelectronic effort of the III-nitride community has focused primarily on InGaN-based visible light emitting devices for display and data storage applications [I]. Most of these devices were grown on sapphire substrates with thick GaN layers of 2 to 4 .tms inserted for improved structural and morphological quality. (Thick n-GaN layers are also required for lowresistive electrical injection.) The active region typically consists of (higher fraction) InGaNbased quantum wells (QWs) and (lower fraction) InGaN barriers for electrical confinement. Further electrical and optical confinement is attained through the use of wide bandgap AlGaN layers (Figure la). Substantial lattice mismatches, however, exist among the III-nitrides; the mismatches (in the in-plane lattice constant) of InN (a - 0.354 nm) and AIN (a = 0.3112 nm) to the thick and presumably relaxed GaN (a = 0.3188 nm) layers are 11% compression and 2.4% tension, respectively [2]. So far most of the strain-related studies have focused on the optical [3] and structural [4] properties of thick GaN epilayers on sapphire or SiC substrates. A simple analysis of the state of strain energy, denoted here as strain-thickness product in Figure Ic, reveals the benefit of the alternating A1GaN/InGaN heterolayers (Figure la) in balancing the tensile and compressive components to avoid excessive strains and to maintain a pseudomorphic growth (dashed line in Figure Ic). Recently we have reported the growth and device operation of an AlGaN/GaN QW-based UV LED on a thick GaN layer [5]. The use of various AlGaN confinement layers, in the absence of any InGaN layers (Figure l b), results in a steep accumulation of grown-in tensile strain (solid line in Figure Ic). Indeed cracking was * Corresponding author. Present address: Sandia National Laboratories, MS-0601, P.O. Box 5800, Albuquerque, New Mexico 87185-0601, Fax: 505-844-3211, email: jhan @sandia.gov G 7.7 Mat. Res. Soc. Symp. Proc. Vol. 537 ©1999 Materials Research Society
observed during fabrication of AIGaN/GaN UV LEDs with thick AlGaN barriers (Figure 2a). The presence of cracking causes a significant variati
Data Loading...
