Deep ultraviolet (UV) photoluminescence (PL) spectroscopy has been employed to study the optical properties and carrier dynamics in AlN and GaN epilayers at temperatures from 10 to 800 K. The parameters that describe the temperature variation of the energy bandgap (α and β, or a<sub>B</sub> and θ) and linewidth broadening have been obtained and are compared with the previously reported values in AlN and GaN obtained by different measurement methods in narrower temperature ranges. Our experimental results demonstrate that the broader temperature range of measurements is necessary to obtain accurate values of these parameters, particularly for AlN. The phonon-carrier interactions were also investigated in both AlN and GaN epilayers. At low temperatures, the linewidth of PL emission lines increases with temperature due to the electron-acoustic phonon interaction. The electron-LO phonon interaction becomes important above 200 K and eventually dominant at high temperatures in both AlN and GaN. The temperature dependencies of the decay lifetimes were investigated up to 500 K, from which free excitons and free carriers interactions are discussed for AlN and GaN epilayers. The implications of our findings to the optoelectronic and electronic device applications at elevated temperatures are 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.
The propagation properties of light in AlGaN/GaN multiple- quantum-well (MQW) waveguides have been studied by time- resolved photoluminescence (PL) spectroscopy. The waveguides were patterned with fixed width of 0.5micrometers but orientations varying from -30 degree(s) to 60 degree(s) relative to the a-axis of GaN by electron-beam lithography and inductively-coupled plasma (ICP) dry etching. The peak position and line-width of the emission peak were found to vary systematically with orientations of the waveguides and followed the six-fold symmetry of the wurtzite structure. This is explained in terms of anisotropy of the exciton/carrier diffusion coefficient along the different crystal orientations of the semiconductor materials. We also observed a remarkable decrease in the PL intensity as well as increase in time delay of the temporal response as the location of the laser excitation spot on the waveguide is varied. These observations can be understood in terms of exciton- polarization propagation in the waveguides. The speed of generated polaritons with energy corresponding to the well transitions in the waveguides was determined from the time delay of the temporal response to be approximately (1.26+/- 0.16 x 10<SUP>7</SUP> m/sec. The optical loss in the waveguides was determined to be about 5-8 cm<SUP>-1</SUP> for different excitation intensities. The implications of these results to waveguiding in optical devices based on the III- nitride semiconductors are discussed.
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.
In<SUB>x</SUB>Al<SUB>y</SUB>Ga<SUB>1-x</SUB>N quaternary alloys with different In and Al composites were grown on sapphire substrates with GaN buffer by metal-organic chemical vapor deposition. Optical properties of these quaternary alloys were studied by picosecond time-resolved photoluminescence. Our studies have revealed that In<SUB>x</SUB>Al<SUB>y</SUB>Ga<SUB>1-x</SUB>N quaternary alloys with lattice matched with GaN (y approximately 4.7x) have the highest optical quality. More importantly, we can achieve not only higher emission energies but also higher emission intensities (or quantum efficiencies) in In<SUB>x</SUB>Al<SUB>y</SUB>Ga<SUB>1-x-y</SUB>N quaternary alloys than that of GaN.
We present the results of picosecond time-resolved photoluminescence (PL) measurements for GaN/Al<SUB>x</SUB>Ga<SUB>1-x</SUB>N MQWs with varying structural parameters, grown by metalorganic chemical vapor deposition under the optimal GaN-like growth conditions. We have shown that the optimal GaN/AlGaN (x approximately 0.2) MQW structures for UV light emitter applications are those with well widths ranging from 12 and 42 angstroms and barrier widths ranging from 40 to 80 angstroms. The decreased quantum efficiency in GaN/Al<SUB>x</SUB>Ga<SUB>1-x</SUB>N MQWs with well width L<SUB>W</SUB> < 12 angstroms is due to the enhanced carrier leakage to the underlying GaN epilayers, while the decreased quantum efficiency in MQWs with well width L<SUB>W</SUB> > 42 angstroms is associated with an increased nonradiative recombination rate as L<SUB>W</SUB> approaching the critical thickness of MQWs. For the barrier width dependence, when the barrier width is below the critical thickness, the nonradiative recombination rate increases with a decrease of the barrier width due to the enhanced possibilities of the electron and hole wavefunctions at the interfaces as well as in the AlGaN barriers. On the other hand, the misfit dislocation density increases as the barrier width approaches the critical thickness, which can result in an enhanced nonradiative interface recombination rate. Our optimized GaN/Al<SUB>x</SUB>Ga<SUB>1-x</SUB>N MQW structures exhibited extremely high quantum efficiencies as well as a ratio of well emission intensity to barrier emission intensity exceeding 10<SUP>4</SUP>.
The optical properties of Al<SUB>x</SUB>Ga<SUB>1-x</SUB>N alloys with x varied from 0 to 0.35 have been investigated by picosecond time-resolved photoluminescence (PL) spectroscopy. Our results revealed that while the PL intensity decreases with an increase of Al-content, the low temperature PL decay lifetime increases with Al-content. These results can be understood in terms of the effects of tail states in the density of states due to alloy fluctuation in the Al<SUB>x</SUB>Ga<SUB>1-x</SUB>N alloys. The Al content dependence of the energy tail state distribution parameter, E<SUB>0</SUB>, which is an important parameter for determining optical and electrical properties of the AlGaN alloys, has been obtained experimentally. The PL decay lifetime increases with the localization energy and consequently increases with Al content. The implications of our findings to III-nitride optoelectronic device applications are also discussed.