Optical identification of silicon as a shallow donor in MOVPE grown homoepitaxial AlN

Abstract

Aluminum nitride (AlN) layers doped intentionally with different concentrations of silicon atoms acting as shallow donors were grown by MOVPE on bulk c-plane AlN to minimize dislocations in the doped layers; typical values for the dislocation density in the bulk AlN substrates are less than 10 4 cm À2. The actual silicon concentration was confirmed by secondary ion mass spectroscopy (SIMS) analysis and ranging from 10 17 to 10 19 cm À3. In highly resolved low temperature photoluminescence (PL) investigations, we found dominating bandgap-related sharp emission lines with linewidth lower than 500 meV for undoped samples. We tentatively assign silicon as the shallow donor causing the bound exciton line with 28.5 meV exciton localization energy, for which we found an intensity ratio relative to the free exciton line being linearly dependent on the silicon concentration. For increasing Si content, the respective emission band also broadened asymmetrically and shifted to lower energies as expected from the analogy with other semiconductor systems.

Negative spin-exchange splitting in the exciton fine structure of AlN

“…The temperature dependence of the C 1 exciton as obtained from ellipsometry is fitted by P€ assler's model 23 and yields the parameters Eð0Þ ¼ 6:032 eV; a ¼ 0:77 meV=K; H ¼ 642 K, and D ¼ 0:30, in good agreement with earlier studies. 12 We find that the observed splitting can be best represented by an exciton ground state energy (for j ¼ 0) of 6.038 eV and a spin-exchange interaction constant of j ¼ À4 meV. The splitting between C 1 and C 5 states is 2 j, while the energy distance of the two exciton states is DEðC 1 Þ ¼ þ3j=2 and DEðC 5 Þ ¼ Àj=2 in the present case of negligible mixing with further valence bands.…”

“…On high quality (0001) c-oriented homoepitaxial layers, an energy of 6.040 eV is reported at low temperature for the lowest free exciton transition, [11][12][13] while on off-oriented a)…”

Anisotropic absorption and emission of bulk(11¯00)AlN

“…[4][5][6][7] These applications in turn triggered improvements in the growth of high-quality AlN, [8][9][10] only recently allowing us to investigate the fundamental properties of this semiconductor in unpreceded quality. For example, the full width at half maximum of donor bound-exciton recombinations in AlN could be shown to be below 500 μeV, allowing for unambiguous identification of these lines, 11,12 or the optical properties of nonpolar quantum wells in AlN barriers could be understood. 13 The behavior around the fundamental band gap is strongly influenced by the valence band (VB) order at the point of the Brillouin zone (BZ).…”

Direct determination of the silicon donor ionization energy in homoepitaxial AlN from photoluminescence two-electron transitions

“…Only recently, homoepitaxial AlN layers of good crystal quality became available allowing the proper identification of single bound excitonic emission bands. 12,13 However, for the mostly present donor silicon (Si), there is no agreement on the ionization energy. There is an ongoing discussion, whether Si Al undergoes a strong lattice relaxation and forms a deep DX center, where the dopant atom captures a second electron.…”

“…27 From the first excited free exciton state X n¼2 A , the free exciton binding energy of 51.7 meV can be derived in agreement with earlier results. 12 Two donor bound exciton emission bands show up 13.4 meV and 28.6 meV below the X n¼1 A emission, where the deeper one is labeled Si o X and arises from excitons bound to neutral silicon donors. 12 They dominate the emission spectrum, although the sample is not intentionally doped.…”