Mg-doped AlN epilayers grown by metalorganic chemical vapor deposition have been studied by deep UV time-resolved photoluminescence (PL) spectroscopy. A PL emission line at 6.02 eV has been observed at 10 K in Mgdoped AlN, which is about 40 meV below the free-exciton (FX) transition in undoped AlN epilayer. Temperature dependent measurement of the PL intensity of this emission line also reveals a binding energy of 40 meV. This transition line is believed to be due to the recombination of an exciton bound to neutral Mg acceptor (I<sub>1</sub>) with a binding energy, E<sub>bx</sub> of 40 meV. The recombination lifetime of the I<sub>1</sub> transition in Mg doped AlN have been measured to be 130 ps, which is close to the expected value. Excitation intensity dependence of time-resolved PL for Mg-doped AlN epilayer is also measured to understand carrier and exciton dynamics.
Mg doped Al-rich AlGaN epilayers with Al content as high as 0.7 is needed for obtaining deep UV LEDs with wavelengths shorter than 300 nm. This is one of the most crucial layers in deep UV LEDs and plays an important role for electron blocking and affects the hole injection into the active layer. Not only is this layer critical for the efficiency of deep UV LEDs, it could also introduce long wavelength emission components in UV LEDs. However, it is difficult to obtain high quality Mg doped Al-rich AlGaN epilayers and the resistivity of the grown films is usually extremely high. We report here on the growth, optical and electrical properties of Mg doped Al<sub>0.7</sub>Ga<sub>0.3</sub>N epilayers. Mg doped Al<sub>0.7</sub>Ga<sub>0.3</sub>N epilayers of high crystalline and optical qualities have been achieved after optimizing MOCVD growth conditions. Moreover, we have obtained a resistivity around 12,000 Ω cm (near the theoretical limit with Mg doping) at room temperature and confirmed p-type conduction at elevated temperatures for optimized Mg-doped Al<sub>0.7</sub>Ga<sub>0.3</sub>N epilayers. The growth conditions of the optimized epilayer have been incorporated into deep UV LEDs with wavelength shorter than 300 nm. A significant enhancement in power output with a reduction in forward voltage, V<sub>f</sub>, was obtained by employing this optimized Mg doped Al<sub>0.7</sub>Ga<sub>0.3</sub>N epilayer as an electron blocking layer. The long wavelength emission components in deep UV LEDs were also significantly suppressed. The fundamental limit for achieving p-type Al-rich AlGaN alloys is also discussed.
Si and Mg-doped AlN epilayers were grown by metal-organic chemical vapor deposition (MOCVD) on sapphire substrates. Deep ultraviolet (UV) picosecond time-resolved photoluminescence (PL) spectroscopy has been employed to study the optical transitions in the grown epilayers. The donor bound exciton (or I<sub>2</sub>) transition was found to be the dominant recombination line in Si-doped AlN epilayers at 10 K and its emission intensity decreases with increasing Si dopant concentration. Doping induced band-gap renormalization effect has also been observed. Time-resolved PL results on Si-doped AlN revealed a linear decrease of PL decay lifetime with increasing Si dopant concentration, which was believed to be a direct consequence of the doping enhanced nonradiative recombination rates and corroborated the PL intensity results. For Mg-doped AlN epilayers, two emission lines at 4.70 and 5.54 eV have been observed at 10 K, which were assigned to donor-acceptor pair transitions involving Mg acceptor and two different donors (one deep and one shallow). 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 AlGaN as a function of the Al content for the entire AlN composition range was obtained. The average hole effective mass in AlN was also deduced to be about 2.7 m<sub>0</sub> from the experimental value of Mg binding energy together with the effective mass theory. Although Mg acceptors are an effective mass state in ultra-large bandgap AlN, as a consequence of this large acceptor binding energy of 0.51 eV, only a very small fraction (about 10<sup>-9</sup>) of Mg dopants can be activated at room temperature in Mg-doped AlN. Decay lifetimes of these emission lines are also measured as functions of emission energy, temperature, and excitation intensity. The implications of our finding on the applications of AlN epilayers for many novel devices will also be discussed.
AlN epilayers with high optical qualities have been grown on sapphire substrates by metal organic chemical vapor deposition (MOCVD). Deep ultraviolet (UV) photoluminescence (PL) spectroscopy has been employed to probe the optical quality as well as optical transitions in the grown 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 ps 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. The PL emission properties of AlN have been compared with those of GaN. It was shown that the optical quality as well as quantum efficiency of AlN epilayers is as good as that of GaN. It was shown that the thermal quenching of PL emission intensity is greatly reduced in AlN over GaN, which suggests that the detrimental effect of impurities and dislocations or non-radiative recombination channels in A1N is much less severe than in GaN. The observed physical properties of AlN may considerably expand future prospects for the application of III nitride materials.
Si-doped n-type Al<SUB>x</SUB>Ga<SUB>1MINx</SUB>N alloys with x up to 0.5 and Mg-doped p-type Al<SUB>x</SUB>Ga<SUB>1-x</SUB>N alloys with x up to 0.27 were grown by metal-organic chemical vapor deposition (MOCVD) on sapphire substrates. For the n-type Al<SUB>x</SUB>Ga<SUB>1-x</SUB>N, we achieved highly conductive alloys for x up to 0.5. An electron concentration as high as 1x10<SUP>18</SUP>cm<SUP>-3</SUP> was obtained in Si-doped Al<SUB>0.5</SUB>Ga<SUB>0.5</SUB>N alloys with an electron mobility of 16 cm$_2)Vs at room temperature, as confirmed by Hall-effect measurements. Our results also revealed that the conductivity of Al<SUB>x</SUB>Ga<SUB>1-x</SUB>N alloys continuously increases with an increase of Si doping level for a fixed value of Al content (X<0.5), the conductivities of Al<SUB>x</SUB>Ga<SUB>1-x</SUB>N alloys decrease with increasing Al content for a given doping level; the critical Si-doping concentration needed to convert insulating Al<SUB>x</SUB>Ga$1-x)N with high Al contents (X>=0.4) to n- type conductivity is about 1 x 10<SUP>18</SUP>cm<SUP>-3</SUP>. Time- resolved photoluminescence studies carried out on these materials have shown that Si-doping reduces the effect of carrier localization in Al<SUB>x</SUB>Ga<SUB>1-x</SUB>N alloys and a sharp drop in carrier localization energy occurs when the Si doping concentration increases above 1x10<SUP>18</SUP>cm<SUP>-3</SUP>, which directly correlates with the observed electrical properties. For the Mg-doped Al<SUB>x</SUB>Ga<SUB>1-x</SUB>N alloys, p-type conduction was achieved for x up to 0.27, as confirmed by variable temperature Hall measurements. Emission lines of band-to-impurity transitions of free electrons with neutral Mg acceptors as well as localized excitons have been observed in the p-type Al<SUB>x</SUB>Ga<SUB>1-x</SUB>N alloys. The Mg acceptor activation energies E<SUB>A</SUB> were deduces from photoluminescence spectra and were found to increase with Al content and agreed very well with those obtained by Hall measurements. From the measured activation energy as a function of Al content, E<SUB>A</SUB> versus x, the resistivity of Mg-doped Al<SUB>x</SUB>Ga<SUB>1-x</SUB> with high Al contents can be deduced. Our results have also shown that PL measurements provide direct means of obtaining E<SUB>A</SUB> especially where this cannot be obtained accurately by electrical methods due to high resistance of p-type Al<SUB>x</SUB>Ga<SUB>1-x</SUB>N with high Al content.