Discrete wavelengths observed in electroluminescence originating from Al_1/2Ga_1/2N and Al_1/3Ga_2/3N created in nonflat AlGaN quantum wells

Abstract

When nonflat Al x Ga1−x N quantum wells (QWs) for producing 285 nm light emitting diodes (LEDs) were fabricated on n-AlGaN on AlN templates with dense macrosteps on c(0001) sapphire substrates with a 1.0° miscut relative to the m[1–100] axis, composite electroluminescence (EL) spectra from both inclined and terrace zones in Al x Ga1−x N QWs (x∼ 1/3) were generated owing to compositional and thickness modulations. The shoulder or main peaks in composite EL spectra tended to locate at fixed discrete wavelengths of ∼287, ∼292, and ∼296 nm from 12 nonuniform 285 nm LED wafers that were involved in nonnegligible run-to-run drift, even though these wafers were fabricated using the same source gas flow parameters for metal-organic vapor phase epitaxy. The discrete wavelengths of ∼287, ∼292, and ∼296 nm were attributed to EL from Al1/3Ga2/3N QWs with thicknesses of 8, 9, and 10 monolayers (ML), respectively, by referring to the results of cathodoluminescence (CL) analysis. Also, when nonflat Al x Ga1−x N QWs (x∼ 1/2) for 265 nm LEDs were grown, single-peak-like EL spectra were mainly generated from the inclined zones in nonflat QWs. The EL spectra taken from four nonuniform 265 nm LED wafers tended to show weak structures or main peaks at ∼257, ∼261, ∼266, and ∼271 nm, which were also attributed to emissions from Al1/2Ga1/2N QWs with thicknesses of 5, 6, 7, and 8 ML, respectively, by referring to CL analysis results. The creation of Al1/3Ga2/3N and Al1/2Ga1/2N in nonflat QWs in this work was in agreement with the results of our previous studies that indicated the creation of metastable Al n /12Ga1−n/12N (n: consecutive natural numbers). Furthermore, QW thicknesses of consecutive n ML may imply that Al1/3Ga2/3N and Al1/2Ga1/2N have 1 ML configurations of Al and Ga atoms on a c(0001) plane.

Metastable atomic-ordered configurations for Al1/2Ga1/2N predicted by Monte-Carlo method based on first-principles calculations

“…Previous studies on the ordered structure of Al 1/2 Ga 1/2 N using FPCs used small calculation cell sizes to reduce computational cost [12,26,27]. This limitation resulted to significant differences between theoretical simulations and experimental results obtained by TEM, XRD, SAED, and EL [15][16][17][18][19][20][21][22][23][24][25][32][33][34].…”

“…Our recent analytical studies on AlGaN layers with dense macrosteps revealed the generation of metastable Al n/12 Ga 1−n/12 N (n is an integer from 2 to 10) along macrostep edgelines. Results from our previous works indicated that the atomic configurations of Al n/12 Ga 1−n/12 N should have 2 or fewer monolayer (ML) configurations [31][32][33][34][35][36][37][38]. Hereafter, n ML, represents the periodicity of the structure in the c[0001] direction, where n is the number of atomic layers in a unit cell.…”

“…In such representation, 1 ML means having the same c(0001) in-plane configuration for all atomic layers and 2 ML for every 2 atomic layers. On the other hand, analyses of electroluminescence (EL) indicated a 1 ML configurations along the c(0001) plane for Al 1/3 Ga 2/3 N and Al 1/2 Ga 1/2 N [33]. Other research groups reported that a SAED pattern obtained from Al 0.56 Ga 0.44 N indicated the generation of alternatively stacked AlN and GaN layers in the [ 1 101 ] direction, which disappeared after annealing [22].…”

Weak metastability of Al x Ga1−x N (x = 13/24, 15/24, 17/24) shown by analyzing AlGaN grown on AlN with dense macrosteps

“…ur recent studies on the compositional modulation in nonflat Al x Ga 1−x N (x ∼ 0.7, 0.63, 0.55, and 0.43) layers and Al x Ga 1−x N (x ∼ 0.2) quantum wells (QWs) revealed that metastable Al n/12 Ga 1−n/12 N (n: integer = 2-9) was generated in the Al-rich and Ga-rich zones. [1][2][3][4][5] Photoluminescence (PL) also indicated Al 9/12 Ga 3/12 N and Al 10/12 Ga 2/12 N in a nonflat Al 0.81 Ga 0.19 N on a 0.4°-miscut AlN substrate. 6) Furthermore, cathodoluminescence (CL) with scanning electron microscopy (SEM) from nonflat AlGaN layers (>1 μm) on the AlN by other research groups well agreed with Al n/12 Ga 1−n/12 N metastability (n: integer).…”

“…A comparable accuracy could be expanded to x Al = n/12 (n = 2-10). 4,5) Note that this small deviation is considered to be possible when analyzing metastable AlGaN compositions, because metastability might restrict luminescence centers by reducing compositional fluctuations. The NBE wavelength interval for Δx Al = 1/24 is ∼6-7 nm, indicating the possibility of point use of CL for detecting Al (2m+1)/24 Ga 1−(2m+1)/24 N. Indeed, a 1% difference in x Al is the criterion, and the reproducibility of signals will be discussed.…”