AIN on sapphire

We can grow thin films of AIN on sapphire by applying an electric field to a sample heated in a nitrogen-rich atmosphere with an extremely low oxygen partial pres-... 

H. Hirayama, S. Fujikawa, N. Noguchi, J. Norimatsu, T. Takano, K. Tsubald, N. Kamata, 222-282 nm AlGaN /and InAlGaN-based deep-UV LEDs fabricated on high-quality AIN on sapphire. Phys. Status SoUdi A206, 1176-1182 (2009)... 

Hossain et al [15] measured die cathodoluminescence in undoped aluminium nitride (AIN) thin films at 300, 77 and 4.2 K. These films were grown on sapphire and SiC substrates by LP-MOCVD at 1473 K. As shown in FIGURE 1, two distinct peaks were observed at about 6.1 and 5.9 eV, respectively. The energy position of these peaks increases and the linewidth becomes narrower, as the temperature is decreased. They believed that these two peaks are due to exciton recombination. Recently, MacMillan et al measured cathodoluminescence in AlN-GaN superlattices [16],... 

In this formula, Eg, x and b are the optical bandgap, AIN molar fraction and bowing parameter, respectively. In order to determine this relationship, not only precise characterisation of Eg by the optical method but also precise determination of alloy composition x is quite important. The optical bandgap of AlGaN on sapphire in the whole compositional range was first reported by Yoshida et al [2] in 1982. They used optical absorption edge for determination of Eg, and x was determined by electron... 

Sputtered ZnO films were used to improve nucleation of GaN HVPE thick films on sapphire [2], It is not clear whether the ZnO is really a buffer layer or just a surfactant in this process. Subsequently, ZnO thin films were proposed as buffer layers for the growth of GaN by MBE [20], In practice, low-temperature AIN or GaN buffer layers have out-performed the ZnO-bufifer-layer approach... 

FIGURE 1 SEM images of MOVPE-grown GaN hexagonal pyramids on a Si02 mask patterned on GaN/LT AIN buffer/sapphire at the growth temperatures of (a) 1010, (b) 1025, and (c) 1050°C [5J. 

Figure 5.17 Electron mobility at room temperature of n-type Si-doped a-plane GaN grown on r-plane sapphire. Planar GaN grown on AlGaN/AiN intermediate layer is indicated by Template (circles). SELO indicates GaN grown with the SELO process (squares). That of c-plane GaN grown on sapphire using a low-temperature buffer layer is also shown for comparison (cf Color Plate XXVI).

I Akasaki, H Amano, Y Koide, K Hiramatsu, N Sawaki. Effects of AIN buffer layer on crystallographic structure and on electrical and optical properties of GaN and GaAlN films grown on sapphire substrate by MOVPE. J Cryst Growth 98 209, 1989. 

The dislocation density within the first 0.5 xm of the GaN film on the vicinal 6H-SiC(0001)si substrate was approximately 1x10 cm", as determined from initial plan view TEM analysis by counting the number of dislocations per unit area. This value is approximately an order of magnitude lower than that reported (43) for thicker GaN films deposited on sapphire(OOOl) substrates using low-temperature buffer layers. The dislocation density of the GaN film deposited on the vicinal 6H-SiC(0001)si substrate decreased rapidly as a function of thickness. In contrast, the on-axis wafers had less step and terrace features thus, the HT-AIN buffer layers on these substrates were of higher microstructural quality with smoother surfaces and fewer inversion domain boundaries. Consequently, the microstructural quality of the GaN films were better for on-axis growth as shown by the DCXRC data noted below. 

A large lattice mismatch between sapphire and nitrides (about 13% to AIN, 16% to GaN and 29% to InN) makes even very thin layers fully relaxed at the growth temperature. When the samples are cooled down after growth, a thermal strain is created. Such strain occurs for other materials, for example for GaAs on Si [14], and corresponds to a difference in thermal expansion between the layer and the substrate. Using thermal expansion coefficients for GaN and sapphire one can estimate that the compressive thermal strain Aa/a, which should be generated for MOCVD grown GaN on (00.1)... 

Rutz [13] reported a broad near-UV band in the electroluminescence spectra, extending from 215 nm into the blue aid of the visible range. Moiita et al [14] observed the cathodoluminescence with peaks or humps in epitaxial AIN films on (0001) sapphire substrates at about 2.71, 2.88, 3.12, 3.19, 3.33 and 3.53 eV. They insisted that the last two peaks were due to nitrogen vacancies or interstitial Al impurities, and that the first two peaks were attributed to oxygen impurities or defects induced by the increase in oxygen concentration. 

AIN buffer layer on (0001) sapphire. Composition was determined by help of X-ray diffraction. A continuous increase of the mode energy with x was observed. An AlxGai.xN/GaN/sapphire heterostructure grown with the AIN buffer layer technique was studied in infrared reflection and Raman spectroscopy by Wetzel et al [2] (FIGURE 2). From an X-ray analysis of the c-axis an AlN-ftaction of x = 0.15 was derived. Recently, however, it was shown that AIN layers in heterostructures with GaN are coherently strained up to a thickness of at least 350 nm. This leads to misinterpretation of the AIN fraction [8], Including the deformation of the unit cell in the pseudomorphic structure above, a value 50% smaller is concluded (x = 0.08). In backscattering off the c-plane the Ai(LO) mode was determined at 752 cm 1 (square with cross symbol) in excellent agreement with the infrared reflection data [2],... 

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