Deep UV photoluminescence spectroscopy has been employed to study the optical properties of AlxGa1−xN alloys (0⩽x⩽1). The emission intensity with polarization of E⊥c and the degree of polarization were found to decrease with increasing x. This is a consequence of the fact that the dominant band edge emission in GaN (AlN) is with polarization of E⊥c(E∥c). Our experimental results suggest that the decreased emission efficiency in AlxGa1−xN alloys and related UV emitters could also be related with their unique polarization property, i.e., the intensity of light emission with polarization of E⊥c decreases with x. It is thus concluded that UV emitters with AlGaN alloys as active layers have very different properties from InGaN and other semiconductor emitters.

With a recently developed unique deep ultraviolet picoseconds time-resolved photoluminescence (PL) spectroscopy system and improved growth technique, we are able to determine the detailed band structure near the Γ point of wurtzite (WZ) AlN with a direct band gap of 6.12 eV. Combined with first-principles band structure calculations we show that the fundamental optical properties of AlN differ drastically from that of GaN and other WZ semiconductors. The discrepancy in energy band gap values of AlN obtained previously by different methods is explained in terms of the optical selection rules in AlN and is confirmed by measurement of the polarization dependence of the excitonic PL spectra.

Mg-doped AlN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrates. Deep UV picosecond time-resolved photoluminescence ͑PL͒ spectroscopy has been employed to study the optical transitions in Mg-doped AlN epilayers. From PL emission spectra and the temperature dependence of the PL emission intensity, a binding energy of 0.51 eV for Mg acceptor in AlN was determined. Together with previous experimental results, the Mg acceptor activation energy in Al x Ga 1Ϫx N as a function of the Al content (x) was extrapolated for the entire AlN composition range. The average hole effective mass in AlN was also deduced to be about 2.7 m 0 from the experimental value of the Mg binding energy together with the use of the effective mass theory.

Deep ultraviolet ͑UV͒ photoluminescence ͑PL͒ spectroscopy has been employed to study deep impurity transitions in Al x Ga 1−x N ͑0 ഛ x ഛ 1͒ epilayers. Two groups of deep impurity transitions were observed, which are assigned to the recombination between shallow donors and two different deep level acceptors involving cation vacancies ͑V cation ͒ and V cation complexes in Al x Ga 1−x N alloys. These acceptor levels are pinned to two different energy levels common to Al x Ga 1−x N alloys ͑0 ഛ x ഛ 1͒. The deep impurity transitions related with V cation complexes were observed in Al x Ga 1−x N alloys between x = 0 and 1, while those related with V cation were only observed in Al x Ga 1−x N alloys between x = 0.58 and 1. This points out to the fact that the formation of V cation is more favorable in Al-rich AlGaN alloys, while V cation complexes can be formed in the whole range of x between 0 and 1. The implications of our findings to the UV optoelectronic devices using AlGaN alloys are also discussed.

AlN epilayers with high optical qualities have been obtained by metalorganic chemical vapor deposition on sapphire substrates. Deep UV picosecond time-resolved photoluminescence (PL) spectroscopy has been employed to study the optical transitions in AlN epilayers. Two PL emission lines associated with the donor bound exciton (D0X, or I2) and free exciton (FX) transitions have been observed, from which the binding energy of the donor bound excitons in AlN epilayers was determined to be around 16 meV. Time-resolved PL measurements revealed that the recombination lifetimes of the I2 and free exciton transitions in AlN epilayers were around 80 and 50 ps, respectively. The temperature dependencies of the free exciton radiative decay lifetime and emission intensity were investigated, from which a value of about 80 meV for the free exciton binding energy in AlN epilayer was deduced. This value is believed to be the largest free exciton binding energy ever reported in semiconductors, implying excitons in AlN are an extremely robust system that would survive well above room temperature. This together with other well-known physical properties of AlN may considerably expand future prospects for the application of III-nitride materials.