M. L. Nakarmi

Band structure and fundamental optical transitions in wurtzite AlN

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.

Unique optical properties of AlGaN alloys and related ultraviolet emitters

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.

Mg acceptor level in AlN probed by deep ultraviolet photoluminescence

Mg-doped AlN epilayers were grown 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 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 AlxGa1−xN 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 m0 from the experimental value of the Mg binding energy together with the use of the effective mass theory.

200nm deep ultraviolet photodetectors based on AlN

High quality AlN epilayers were grown on sapphire substrates by metal organic vapor deposition and exploited as active deep ultraviolet (DUV) optoelectronic materials through the demonstration of AlN metal-semiconductor-metal (MSM) photodetectors. DUV photodetectors with peak responsivity at 200nm with a very sharp cutoff wavelength at 207nm have been attained. The AlN MSM photodetectors are shown to possess outstanding features that are direct attributes of the fundamental properties of AlN, including extremely low dark current, high breakdown voltage, and high DUV to visible rejection ratio and high responsivity. The results demonstrate the high promise of AlN as an active material for DUV device applications.

Deep impurity transitions involving cation vacancies and complexes in AlGaN alloys

Deep ultraviolet (UV) photoluminescence (PL) spectroscopy has been employed to study deep impurity transitions in AlxGa1−xN (0⩽x⩽1) 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 (Vcation) and Vcation complexes in AlxGa1−xN alloys. These acceptor levels are pinned to two different energy levels common to AlxGa1−xN alloys (0⩽x⩽1). The deep impurity transitions related with Vcation complexes were observed in AlxGa1−xN alloys between x=0 and 1, while those related with Vcation were only observed in AlxGa1−xN alloys between x=0.58 and 1. This points out to the fact that the formation of Vcation is more favorable in Al-rich AlGaN alloys, while Vcation 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.

Temperature and compositional dependence of the energy band gap of AlGaN alloys

Deep-ultraviolet photoluminescence spectroscopy has been employed to study the temperature and compositional dependence of the band gap of AlxGa1−xN alloys in the temperature range between 10 and 800 K. Band-edge emission peaks in AlxGa1−xN alloys were fitted by the Varshni equation to obtain Varshni coefficients, which increase nonlinearly with x. The values of Varshni coefficients obtained for GaN and AlN binary compounds in the present study are in good agreement with the previously reported values. Based on the experimental data, the compositional and temperature dependence of the band gap of AlxGa1−xN alloy has been empirically determined for the entire alloy range.

Optical and electrical properties of Mg-doped p-type AlxGa1−xN

Mg-doped AlxGa1−xN epilayers with Al content up to 0.27 were grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD). p-type conduction in these alloys has been achieved, as confirmed by variable temperature Hall-effect 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 AlxGa1−xN alloys. The Mg acceptor activation energies EA were deduced 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 the Al content, EA versus x the resistivity of AlxGa1−xN alloys with high Al contents can be deduced. Our results thus indicated that alternative methods for acceptor activation in AlGaN alloys with high Al contents must be developed. Our results have also shown that PL measurements provide direct means of obtaining EA, especially where this canno...

Deep ultraviolet picosecond time-resolved photoluminescence studies of AlN epilayers

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 ...

Correlation between optoelectronic and structural properties and epilayer thickness of AlN

AlN epilayers were grown by metal organic chemical vapor deposition on sapphire substrates. X-ray diffraction measurements revealed that the threading dislocation (TD) density, in particular, the edge TD density, decreases considerably with increasing the epilayer thickness. Photoluminescence results showed that the intensity ratio of the band edge emission to the defect related emission increases linearly with increasing the epilayer thickness. Moreover, the dark current of the fabricated AlN metal-semiconductor-metal deep ultraviolet (DUV) photodetectors decreases drastically with the AlN epilayer thickness. The results suggested that one effective way for attaining DUV optoelectronic devices with improved performance is to increase the thickness of the AlN epilayer template, which results in the reduction of the TD density.

Photoluminescence Studies of Impurity Transitions in AlGaN Alloys

Deep ultraviolet photoluminescence (PL) spectroscopy has been employed to investigate impurity transitions in Si doped Al-rich AlGaN alloys. In addition to the previously reported donor compensating centers—isolated cation vacancy with three negative charges (VIII)3− and cation vacancy complex with two-negative charges (VIIIcomplex)2−—a group of impurity transitions with higher emission energies has been observed in AlGaN alloys grown under different conditions, which has been assigned to the recombination between shallow donors and cation vacancy complexes with one-negative charge (VIIIcomplex)−1. Similar to (VIII)3− and (VIIIcomplex)2−, the energy levels of (VIIIcomplex)1− deep acceptors in AlxGa1−xN (0⩽x⩽1) alloys are pinned to a common energy level in vacuum. A strong correlation between the resistivity and PL emission intensities of the impurity transitions associated with cation vacancies (and complexes) was found.