Electronic Structures of Wide Band-Gap (AlN) m (GaN) n [001] Superlattices
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ABSTRACT Wide bandgap III-V nitrides, such as GaN and AIN, have become topical in the nearterm technology of blue lasers. We report detailed electronic band-structure calculations for (AIN)m(GaN),, [001] zinc-blende superlattices (SL), with m + n < 12, using the all-electron full-potential linear-muffin-tin-orbital method. For n > 3, the SL are found to have a direct band gap. For n < 2 and m > 3, all the band gaps are indirect. In ultrathin SL, m < 3 and n < 2, only (m, n) = (3, 1) is found to have an indirect gap. The band offsets are estimated by calculating the core-level shifts of nitrogen atoms in the central planes of the GaN and AIN layers. The calculated densities of states, electron- and hole- effective masses (m*), etc., as a function of m and n, are reported; a remarkable dependence of m* on the number of layers is revealed.
INTRODUCTION The wide-band-gap nitrides have gained attention recently because of their potential in the technology of blue and blue-green lasers [1]. High-quality GaN and AIN layers can now be grown on sapphire and intentionally n- and p-type doped layers and structures can be produced. AIN and GaN usually crystallize in the wurtzite (WR) form, but can also be grown in the zinc-blende (ZB) structure under appropriate conditions [1, 2]. The large band-gap difference between the two materials [E. = 6.2 (5.11) eV for AIN in the WR
(ZB) structure, and 3.5 (3.2) eV for GaN], but nearly identical lattice constants, makes it possible to fabricate a class of A1N/GaN alloys and SL with widely tunable wavelengths [3]. The increasing number of theoretical studies of bulk AIN and GaN [4] reflects the growing technological interest. However, very few studies have been performed for the A1N/GaN superlattices (SL). The electronic structure of short-period A1N/GaN SL in the WR structure has been examined using first-principles methods [5]. Quasiparticle band-structures were calculated in the WR [001] and ZB [111] directions for the 1 xl, 1 x2, and 2 x1 configurations [6]. In these studies, the WR SL are found to have direct band gaps, whereas a transition from direct to indirect is found for ZB SL. To our knowledge, there is only one local-density-approximation (LDA) band-structure (BS) calculation of nitride SL in the ZB [001] direction [7]; however, the calculated quantities have not been reported in detail. In this paper, we study the (AIN),,.(GaN), family of ZB SL, with (m + n) _ 2, all band gaps are found to be direct. For n < 2 and m > 3, all the gaps are indirect. The CBM of the indirect gap is located at the M point when m + n is even, and at X when m + n is odd (cf. Table I). In the ultrathin SL, i.e., m < 3 and n < 2), all band gaps are found to be direct except for (in, n) = (3,1). Roughly speaking, the band gap decreases with the Ga concentration. In the case of direct-band-gap SL, the CBM is found at the ZB F-folded state; it exhibits mainly s-type character and is therefore highly isotropic. This can be seen from the dispersion curves of Fig. 1, as well as from the effective masses,
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