Carbon pair defects in aluminum nitride

Abstract

AlN bulk single crystals grown by the physical vapor transport method may be beneficially applied as substrates for deep ultraviolet light emitting devices or as a basic material for piezoelectric resonators operating at high temperatures. Identification of point defects which deteriorate the optical, electrical, and electromechanical properties of AlN crystals for such applications is the subject of the present work. Using Raman spectroscopy, two local vibrational modes (LVMs) were discovered at wave numbers of 1189 cm−1 and 1148 cm−1. By analyzing an AlN crystal intentionally enriched with the carbon isotope 13C, it is unambiguously shown that the two LVMs originate from two different, but in each case carbon-related defects. Furthermore, it is evidenced that the defect underlying the LVM at 1189 cm−1 contains exactly two carbon atoms. The tricarbon defect-related LVM reported earlier in an infrared absorption study is found to be Raman active at 1772 cm−1. The Raman scattering intensity of all three LVMs strongly depends on the photon energy of the exciting light what is interpreted as a resonance Raman effect. This allows linking the identified defects with their contribution to the strong, carbon-related ultraviolet absorption around 4.7 eV and proves that these defects introduce optically and electrically active deep levels in the bandgap of AlN.

Photochromism and influence of point defect charge states on optical absorption in aluminum nitride (AlN)

“…59,60 Also, the V N defect is expected as a pronounced compensating donor defect in p-type material. 15,16,61 Carbon pairs and tri-carbon defects exist in the bulk AlN as was found experimentally 30,31 .…”

“…The band peaking at 4.7 eV, usually referred to as "carbon related", was observed in areas B1 and B2; this was discussed previous studies also from our group. 15,[29][30][31] . For these two areas, a clear correlation between the carbon concentration [C] and α at 4.7 eV is observed: for B1…”

“…4 (d). A defect with suitable properties would be C Al (i.e., in DX2 ground state configuration, ≈2.7 eV) 38 or the carbon pair defect (absorption between 3.8 -5.1 eV is expected) 30 . Different charge states of C Al are expected in the range 2 -2.7 eV below CB, 38,55,57 which would fit well to the 4.0 eV induced band and also to the 4.5 eV band generally observed in carbon-dominating samples (E Vis (D2) and E UV (D2) transitions in Fig.…”

“…[5][6][7] Optical and electrical properties of the material are determined by intrinsic defects, such as aluminum and nitrogen vacancies (V Al , [8][9][10][11][12][13][14] V N 15,16 ), and extrinsic defects from unintentional background doping by silicon, oxygen, and carbon. These three impurities provide not only isolated substitutional defects (i.e., oxygen and carbon substituting nitrogen, and silicon substituting aluminum: O N, C N, Si Al ), [17][18][19][20][21][22] but are also assumed to form complexes, such as V Al -nSi Al , 23 V Al -nO N (n=1…4 is a number of Si or O atoms), 13,14,16,[23][24][25][26][27][28] C N -Si Al , 29 carbon pairs, 30 and tri-carbon complexes. 31 Usually, defects can have several allowed charge states in the bandgap, 32,33 including DXstates, [34][35][36][37][38] resulting in the polymorphism of defects properties.…”

“…Unfortunately, the absorption spectra above 4.3 eV for area B1 could not be measured even after the thinning down of the sample; thus, the DOP curves are also not available. That's why we included DOP data from another AlN crystal grown in our laboratory with [C]=5.8×10 18 cm -3 , [O]= 2.4×10 18 cm -3 , and [Si]= 3×10 16 cm -3 , α ≈ 1600 cm -1 at 4.7 eV,30 thus similar to area B1. Positions and magnitudes of the minimum (at 4.3-4.4 eV) and the maximum (at 4.9 eV) coincide for both DOP curves, despite of the clearly different impurity concentrations in the samples.…”