Mg-doped AlN Research Articles

(Invited) AlGaN Nanowire Deep Ultraviolet Photonics

AlGaN-based deep ultraviolet (UV) light emitting diodes (LEDs) and lasers are important for a broad range of applications, including water purification, disinfection, surface treatment, and chemical and biochemical sensing. To date, the performance of AlGaN quantum well UV LEDs and laser diodes has been limited by the presence of large densities of defects and dislocations, large polarization field, and extremely inefficient p-type doping. In this context, we have developed nearly defect-free AlGaN nanowire heterostructures. We have achieved highly efficient p-type conduction of Mg-doped AlN and Al-rich AlGaN nanowires. With the use of AlGaN nanowires, we have further demonstrated, for the first time, electrically pumped semiconductor lasers operating in the UV-B and UV-C bands. The AlGaN nanowires are grown on Si substrate under N-rich conditions by plasma-assisted molecular beam epitaxy. Detailed structural characterization suggests the presence of strong compositional modulation. The Ga-rich AlGaN clusters have dimensions of 0.25 to 2 nm along the growth direction and of 2 to 10 nm along the lateral direction. The localized variations in the Ga-concentration are estimated to be ~10%, thereby providing strong quantum-confinement of charge carriers. By changing alloy composition, the emission can be widely tuned in the UV-C band from 207 nm to 280 nm. We have demonstrated AlN nanowire LEDs with a small turn on voltage of ~6 V, which is significantly smaller than previously reported AlN epilayers LEDs (> 20 V). This excellent electrical performance is largely ascribed to the significantly enhanced Mg-dopant incorporation in nanowire structures and the resulting efficient impurity band conduction. For ternary AlGaN nanowire UV LEDs, the device structure consists of n+-GaN/Al/p+-AlGaN tunnel junction, which can remove the resistive p-contact layer and improve hole transport. Compared to devices without tunnel junction, the output power is improved by 2 orders of magnitude. For the device operating at 274 nm, the maximum output power is > 20 mW. It is further envisioned that, with the use of AlGaN nanowires, UV-C LEDs with efficiency >70% can be potentially achieved. By further exploiting Anderson localization of light in such nearly defect-free AlGaN nanowires, we have demonstrated, for the first time, electrically injected lasers operating in the UV-B and UV-C bands. For lasers operating at 239 nm, the threshold current is ~0.3 mA at room-temperature. These studies provide a viable path to achieve high efficiency semiconductor laser diodes operating in the UV-C band that was not previously possible.

(Invited) Progress and Challenges of AlGaN Schottky Diodes Grown on AlN Substrates

Lateral and vertical power switching devices being intensively developed in GaN and SiC materials are beginning to reach a state of maturity and to displace traditional Si devices in certain applications. The relative advantage of these wide bandgap materials, which stems from intrinsic materials properties, can be compared in terms of an applicable figure of merit (FOM), indicating theoretical limits [1]. The most commonly discussed FOM for power applications is the unipolar FOM [2], based on reduction of conduction losses in the on-state and high voltage blocking in the off-state of vertical devices, VB 2/Ron,sp = εμnEc 3/4, where VB is the off-state avalanche breakdown voltage, Ron,sp is the specific on-state resistance, ε is the semiconductor permittivity, μn is the electron mobility, and Ec is the critical electric field. The unipolar FOM scales as the cube of Ec , which is itself a strong function of the bandgap [3], leading to the key advantage for wide bandgap materials. Possessing bandgaps wider than GaN (3.4 eV), AlGaN alloys are expected to provide improved performance. We have investigated quasi-vertical Schottky barrier diodes, consisting of nominal Al0.8Ga0.2N drift layers grown on high quality AlN single crystal substrates. HexaTech has developed proprietary seeded AlN crystal growth technology based on physical vapor transport (PVT), allowing iterative expansion of single crystal size, while maintaining low dislocation density (<103 cm-2), as determined by x-ray diffraction and defect-selective etching. AlN boules were oriented, sliced, lapped, and chemo-mechanically polished (CMP) to produce epi-ready c-plane substrates. Due to the highly insulating nature of the Al-face AlN substrates (resistivity >1013 Ωcm at room temperature), quasi-vertical Schottky diode structures were grown by metal-organic chemical vapor deposition (MOCVD). The device structure consisted of an AlN homoepitaxial layer, followed by a 0.6 µm thick pseudomorphic Si-doped Al0.8Ga0.2N contact layer ([Si] ~2x1019 cm-3), a 1 µm thick low-doped Al0.8Ga0.2N drift layer, and a 0.3 µm Mg-doped AlN cap layer ([Mg] ~2x1018 cm-3), serving as a junction termination extension (JTE). Despite the high activation energy of the Mg dopant in AlN, the Mg acceptors are expected to be ionized under high reverse bias, due to the strong band bending. Following epitaxial growth and dopant activation, circular device mesas were defined by dry etching down to the n-AlGaN contact layer, the JTE region atop the mesa was defined into a series of lateral steps and the Schottky contact area was etched down to the drift layer, Ohmic contacts were deposited and annealed, circular Schottky contacts were deposited, and a blanket polyimide passivation layer was deposited and patterned. Schottky diodes with diameters ranging from 0.3 to 1.0 mm were fabricated. Capacitance-voltage measurements were used to determine the free carrier concentration of the drift region, 2.0x1017 cm-3. The forward and reverse current-voltage characteristics were measured as a function of temperature. The specific on-state resistance was 125 mΩcm2 at room temperature and 190 mΩcm2 at 150 °C, which is partly attributed to high contact resistance. Diodes exhibited a current rectification ratio of ~106. The off-state breakdown voltage was up to 500 V. Breakdown tended to be destructive, likely limited by the polyimide passivation, suggesting improved blocking performance could be achieved with a better dielectric and more developed edge termination strategy. Field calculations based on the measured carrier concentration and breakdown voltage indicated a trapezoidal electric field in the drift region with a 7 MV/cm field at the Schottky junction. This talk will review growth and expansion of high quality AlN single crystals by PVT, and fabrication of epi-ready AlN substrates. Results for high Al composition Schottky diodes grown on these substrates will be presented. Challenges for this emerging technology, including doping limitations and compensation in AlGaN alloys, Ohmic contacts, and device termination will be discussed. [1] T. P. Chow, I. Omura, M. Higashiwaki, H. Kawarada, and V. Pala, IEEE Trans. Electron Devices, 64, 856 (2017). [2] K. Shenai, R. S. Scott, and B. J. Baliga, IEEE Trans. Electron Devices, 36, 1811 (1989). [3] J. L. Hudgins, G. S. Simin, E. Santi, and M. A. Khan, IEEE Trans. Power Electron., 18, 907 (2003).

AlN/h-BN Heterostructures for Mg Dopant-Free Deep Ultraviolet Photonics

Aluminum-rich AlGaN is the ideal material system for emerging solid-state deep-ultraviolet (DUV) light sources. Devices operating in the near-UV spectral range have been realized; to date, however, the achievement of high-efficiency light-emitting diodes (LEDs) operating in the UV-C band (200-280 nm specifically) has been hindered by the extremely inefficient p-type conduction in AlGaN and the lack of DUV-transparent conductive electrodes. Here, we show that these critical challenges can be addressed by Mg dopant-free Al(Ga)N/h-BN nanowire heterostructures. By exploiting the acceptor-like boron vacancy formation, we have demonstrated that h-BN can function as a highly conductive, DUV-transparent electrode; the hole concentration is ∼1020 cm-3 at room temperature, which is 10 orders of magnitude higher than that previously measured for Mg-doped AlN epilayers. We have further demonstrated the first Al(Ga)N/h-BN LED, which exhibits strong emission at ∼210 nm. This work also reports the first achievement of Mg-free III-nitride LEDs that can exhibit high electrical efficiency (80% at 20 A/cm2).

On the mechanism of highly efficient p-type conduction of Mg-doped ultra-wide-bandgap AlN nanostructures

Free hole concentrations up to ∼6 × 1017 cm−3 were measured in Mg-doped AlN nanowires at room-temperature, which is several orders of magnitude larger than that of previously reported AlN epilayers. Detailed studies suggest that such unusually high carrier concentrations stem from the efficient hole hopping conduction in the Mg impurity band, driven by the significantly enhanced Mg-dopant incorporation in nearly defect-free AlN nanostructures. Distinct signatures of hole hopping conduction in the Mg impurity band are observed experimentally, including a relatively small activation energy for electrical conductivity and an increase in hole mobility with increasing temperature.

Ferromagnetism of wide-bandgap semiconductor surfaces: Mg-doped AlN

The surface magnetism of Mg-doped AlN is investigated by the first-principles method. All studied surfaces, whether they are non-polar, polar or semi-polar, favor a ferromagnetic ground state, which is stabilized by the virtual charge hopping between partially filled states of MgN clusters. However, the stability of ferromagnetism varies from surface to surface owing to the interplay among the localization of magnetic moments, energy level splitting, and sp–p interaction between MgN clusters. Among the various surfaces being studied, the Mg-doped AlN surface is expected to be ferromagnetic at room temperature as a result of its strong localization of defect states.

Optical and electrical properties of Mg-doped AlN nanowires grown by molecular beam epitaxy

In this paper, the optical and electrical properties of Mg-doped AlN nanowires are discussed. At room temperature, with the increase of Mg-doping concentration, the Mg-acceptor energy level related optical transition can be clearly measured, which is separated about 0.6 eV from the band-edge transition, consistent with the Mg activation energy in AlN. The electrical conduction measurements indicate an activation energy of 23 meV at 300 K–450 K temperature range, which is significantly smaller than the Mg-ionization energy in AlN, suggesting the p-type conduction being mostly related to hopping conduction. The free hole concentration of AlN:Mg nanowires is estimated to be on the order of 1016 cm−3, or higher.

Tuning the field emission properties of AlN nanocones by doping

Patterned arrays of Si- or Mg-doped AlN nanocones were synthesized, and the field emission properties of the AlN nanocones were enhanced with Si-doping whereas they deteriorated with Mg-doping.

Effect of Mg doping on growth and photoluminescence of AlN hexagonal nanorods

Pure and Mg-doped AlN hexagonal nanorods (HNRs) were synthesized by a simple thermal chemical vapor deposition process without structure-directing templates or catalysts. Mg incorporation promoted not only axial but also radial growth rate of AlN HNRs compared to undoped sample. The corresponding optical properties of the products were investigated by room-temperature photoluminescence (PL) measurement. Quite broad ultraviolet emission bands centered at 280nm and 330nm for AlN HNRs and AlN:Mg HNRs, respectively, were observed. Multi-peak fitting analysis shows that various defects conduced to the broad emission bands of the two samples, and Mg doping has increased the proportion of VN which changed the relative intensity of peaks, instead of forming Mg impurity energy levels that directly affect luminescence.

Investigation of blue luminescence in Mg doped AlN films

The Al1−xMgxN thin films were deposited on (100) silicon substrates by magnetron sputtering. The structural and photoluminescence properties of the films with varying Mg concentrations were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman spectra and photoluminescence (PL), respectively. The results clearly showed that the Mg atoms successfully incorporated into AlN, while the crystal structure of the films was maintained. The Raman spectra of Al1−xMgxN films reveals the enhancement of A1 (TO) mode, a slightly blue-shift and an augment in FWHM for E2 (high) phonon mode with increasing Mg content, which can be associated with the deterioration of (002) orientation and the appearance of (100) orientation. A broad blue band centered at 420nm and 440nm was observed in Mg-doped AlN films. It was suggested that the transitions from the shallow donor level not only to the ground state but also to the excited states of the deep level was responsible for the broad blue emission band. This work indicates the AlN film for the application in lighting emission devices.

Surface magnetism of Mg doped AlN: a first principle study

Using first-principles calculations, we have systematically investigated the magnetic properties of Mg-doped AlN and surfaces. Both the polar and non-polar surfaces are found to be magnetic and the magnetic moments are mainly due to spin polarized 2p orbitals of surface N atoms surrounding Mg. The splitting of energy levels in both cases favours charge hopping between the minority spin states of N 2p orbitals, which leads to a stable ferromagnetic ground state. However, the range of magnetic coupling and the stability of the ferromagnetic state differ between the polar and non-polar surfaces and are dependent on the nature of localization of the defect states. The ferromagnetic state in a Mg-doped reconstructed surface is more stable than in a Mg-doped AlN surface.

Tunable electronic structures of p-type Mg doping in AlN nanosheet

The p-type impurity properties are investigated in the Mg-doped AlN nanosheet by means of first-principles calculations. Numerical results show that the transition energy levels reduce monotonously with the increase in Mg doping concentration in the Mg-doped AlN nanosheet systems, and are lower than that of the Mg-doped bulk AlN case for the cases with larger doping concentration. Moreover, Mg substituting Al atom is energy favorably under N-rich growth experimental conditions. These results are new and interesting to further improve p-type doping efficiency in the AlN nanostructures.

Room temperature ferromagnetism in Mg-doped AlN semiconductor films

AlN films doped with 1.2–7.9at% Mg were deposited by magnetron sputtering. Ferromagnetism is observed in the film with Tc over 380K, and it strongly depends on the Mg concentration. The observed magnetic anisotropy and exclusion of ferromagnetic contamination enforce the assumption for the origin of the intrinsic property in Mg-doped AlN films. The results confirmed that the room temperature ferromagnetism in diluted magnetic semiconductor can be realized by doping the nonmagnetic element Mg.

Room temperature ferromagnetism in new diluted magnetic semiconductor AlN:Mg nanowires

Room-temperature ferromagnetism has been observed in Mg-doped AlN (AlN:Mg) nanowires. The saturation magnetization and the coercivity of the AlN:Mg nanowires are about 0.051 emu g−1 and 127 Oe, respectively. The Al vacancy and substitutional Mg could play very important roles in room temperature ferromagnetism. These findings confirmed the room temperature ferromagnetism in diluted magnetic semiconductor AlN:Mg nanowires by doping with the nonmagnetic element Mg.

Epitaxially grown semiconducting hexagonal boron nitride as a deep ultraviolet photonic material

Hexagonal boron nitride (hBN) has emerged as an important material for various device applications and as a template for graphene electronics. Low-dimensional hBN is expected to possess rich physical properties, similar to graphene. The synthesis of wafer-scale semiconducting hBN epitaxial layers with high crystalline quality and electrical conductivity control has not been achieved but is highly desirable. Large area hBN epitaxial layers (up to 2 in. in diameter) were synthesized by metal organic chemical vapor deposition. P-type conductivity control was attained by in situ Mg doping. Compared to Mg-doped wurtzite AlN, which possesses a comparable energy band gap (∼6 eV), dramatic reductions in Mg acceptor energy level and P-type resistivity (by about six to seven orders of magnitude) have been realized in hBN epilayers. The ability of conductivity control and wafer-scale production of hBN opens up tremendous opportunities for emerging applications, ranging from revolutionizing p-layer approach in III-nitride deep ultraviolet optoelectronics to graphene electronics.

Tunable p-Type Conductivity and Transport Properties of AlN Nanowires via Mg Doping

Arrays of well-aligned AlN nanowires (NWs) with tunable p-type conductivity were synthesized on Si(111) substrates using bis(cyclopentadienyl)magnesium (Cp(2)Mg) vapor as a doping source by chemical vapor deposition. The Mg-doped AlN NWs are single-crystalline and grow along the [001] direction. Gate-voltage-dependent transport measurements on field-effect transistors constructed from individual NWs revealed the transition from n-type conductivity in the undoped AlN NWs to p-type conductivity in the Mg-doped NWs. By adjusting the doping gas flow rate (0-10 sccm), the conductivity of AlN NWs can be tuned over 7 orders of magnitude from (3.8-8.5) × 10(-6) Ω(-1) cm(-1) for the undoped sample to 15.6-24.4 Ω(-1) cm(-1) for the Mg-doped AlN NWs. Hole concentration as high as 4.7 × 10(19) cm(-3) was achieved for the heaviest doping. In addition, the maximum hole mobility (∼6.4 cm(2)/V s) in p-type AlN NWs is much higher than that of Mg-doped AlN films (∼1.0 cm(2)/V s). (2) The realization of p-type AlN NWs with tunable electrical transport properties may open great potential in developing practical nanodevices such as deep-UV light-emitting diodes and photodetectors.

Beryllium acceptor binding energy in AlN

The acceptor binding energy of an alternative dopant, Be, in AlN epilayers has been probed by time-resolved photoluminescence (PL) spectroscopy. The binding energy of excitons bound to Be acceptors in AlN is determined to be about 33meV, which implies that the Be acceptor binding energy in AlN is about 0.33eV in accordance with Haynes’ rule. The measured PL decay lifetimes of the acceptor-bound exciton transitions in Be- and Mg-doped AlN (93 and 119ps, respectively) also indicate that the binding energy of Be acceptor is smaller than that of the most common acceptor dopant in AlN, namely, Mg. The smaller activation energy of Be in AlN has the potential to partly address the critical p-type doping issue in AlN- and Al-rich AlGaN by increasing the room temperature free hole concentration by ∼103 compared to the case of Mg doping.

Electron irradiation-induced increase of minority carrier diffusion length, mobility, and lifetime in Mg-doped AlN∕AlGaN short period superlattice

Minority carrier diffusion length in a p-type Mg-doped AlN∕Al0.08Ga0.92N short period superlattice was shown to undergo a multifold and persistent (for at least 1week) increase under continuous irradiation by low-energy beam of a scanning electron microscope. Since neither the diffusion length itself nor the rate of its increase exhibited any measurable temperature dependence, it is concluded that this phenomenon is attributable to the increase in mobility of minority electrons in the two-dimensional electron gas, which in turn is limited by defect scattering. Cathodoluminescence spectroscopy revealed ∼40% growth of carrier lifetime under irradiation with an activation energy of 240meV.

Growth and photoluminescence studies of Zn-doped AlN epilayers

Deep ultraviolet time-resolved photoluminescence (PL) spectroscopy has been employed to study Zn-doped AlN epilayers grown by metal-organic chemical vapor deposition. The PL spectra of Zn-doped AlN epilayer exhibited two impurity emission lines at 5.40 and 4.50eV, which were absent in undoped epilayers and assigned to the transitions of free electrons and electrons bound to nitrogen vacancies with three positive charges (0.90eV deep) to the Zn0 acceptors. By comparing PL spectra of Zn- and Mg-doped AlN epilayers with undoped epilayers, it was deduced that Zn energy level is about 0.74eV, which is about 0.23eV deeper than the Mg energy level (0.51eV) in AlN. It is thus concluded that contrary to theoretical prediction, Zn would not be a better candidate than Mg as an acceptor dopant in AlN.

Correlation between optical and electrical properties of Mg-doped AlN epilayers

Deep UV photoluminescence and Hall-effect measurements were employed to characterize Mg-doped AlN grown by metal organic chemical vapor deposition. A strong correlation between the optical and electrical properties was identified and utilized for material and p-type conductivity optimization. An impurity emission peak at 4.7eV, attributed to the transition of electrons bound to triply charged nitrogen vacancies to neutral magnesium impurities, was observed in highly resistive epilayers. Improved conductivity was obtained by suppressing the intensity of the 4.7eV emission line. Mg-doped AlN epilayers with improved conductivities predominantly emit the acceptor-bound exciton transition at 5.94eV. From the Hall-effect measurements performed at elevated temperatures, the activation energy of Mg in AlN was measured to be about 0.5eV, which is consistent with the value obtained from previous optical measurements. Energy levels of nitrogen vacancies and Mg acceptors in Mg-doped AlN have been constructed.

Ferromagnetism in Mg-doped AlN from ab initio study

Ab initio calculations based on spin density functional theory were carried out to investigate Mg-doped AlN as a possible dilute magnetic semiconductor. It was found that both Al vacancy and substitutional Mg impurity in AlN lead to spin-polarized ground states. However, sufficient Al vacancy concentration may be difficult to achieve under thermal equilibrium because of the high formation energy of Al vacancy. On the other hand, formation energy of Mg defect is fairly low and the authors’ calculations predict a ferromagnetic coupling among MgN4 tetrahedra. Based on the analysis on Cu-doped ZnO [L. H. Ye et al., Phys. Rev. B 73, 033203 (2006)], room temperature ferromagnetism can be expected in AlN doped with 7% of Mg which can be incorporated at a growth temperature of 2000K under N-rich condition.