NAMBE 2023 Session NM-MoM2: Nitrides

Transition metal nitrides have exceptional properties and are used in a wide variety of electrochemical, structural, photochemical, and plasmonic applications. Among these compounds Mn- and Cr- nitrides have shown exceptional potential for magnetic sensing and spintronics. The Mn_xN_y system is complex with several different metastable phases both predicted and experimentally realized. Cr_xN_y has two primary phases, cubic (CrN) and hexagonal (Cr₂N), which exhibit desirable mechanical, thermal, wear, anti-corrosion, thermoelectric properties. Recent studies have provided valuable insights into the growth and formation of phases of both materials using various vapor deposition techniques. However, there are conflicting reports on the electrical and magnetic properties of Cr_xN_y which could be attributed to impurities, nitrogen vacancies, substrate effects, and strain. This controversy calls for a more detailed study and preparation of high-quality monocrystalline CrN to investigate he intrinsic physical properties. This study uses molecular beam epitaxy to synthesize epitaxial thin films of different Mn-N and Cr-N phases. The electrical and magnetic properties of these films are investigated with the rocksalt MnN and CrN both showing metallic behavior, with the latter showing a magnetic transition ~280K. However, when combining these materials at similar growth conditions, instead of maintaining the rocksalt structure, a new ternary cubic phase of Mn_xCr_yN is obtained which shows narrow-gap semiconducting behavior. This work presents an avenue for the epitaxial integration of metallic, magnetic, and semiconductor materials.

Cubic boron nitride (c-BN) shares several properties with diamond, including high mechanical hardness; high thermal conductivity, second only to diamond; and an ultra-wide band gap (E_g ~ 6.2 eV, indirect). In addition, c-BN can be doped both n- and p-type, and the lattice mismatch between c-BN and diamond is only ~1.3%. These similarities suggest the potential for novel electronic devices based on c-BN/diamond heterostructures for high temperature and high power applications. However, the growth of device-quality layers of c-BN is challenging: boron nitride occurs in multiple phases; the desired cubic phase is metastable at pressures and temperatures typical of vapor-phase growth; and the absence of large-area bulk c-BN crystals necessitates heteroepitaxial growth on non-native substrates.

Single crystal epitaxial cubic boron nitride films were grown on (100) oriented IIa diamond substrates by ion beam-assisted molecular-beam epitaxy (MBE) in a custom MBE system equipped with an Ar ion source, a N₂ plasma source, and an electron beam evaporator for supplying elemental boron. The films are fully cubic, as indicated by Fourier transform infrared spectroscopy and corroborated by x-ray photoelectron spectroscopy. Transmission electron microscopy reveals an epitaxial c-BN film with the presence of isolated misfit dislocations but no indication of h-BN. The interface between the c-BN layer and the diamond substrate is structurally abrupt, and no interlayer between the c-BN film and diamond substrate is seen. It was found that trace amounts of impurities, such has Mg, Be, and Si, facilitate the growth of c-BN on diamond by ion-assisted MBE.

ScAlN thin films have attracted significant attention due to their factor of five increase in piezoresponse over AlN for Sc_0.43Al_0.57N. Integration of metallic epitaxial NbN with ScAlN using molecular beam epitaxy (MBE) enables a pathway towards a highly conductive lower electrode while preserving high crystal quality even in relatively thin ScAlN films suitable for use at or above X-band frequencies. Maintaining phase-pure and high crystal quality Sc_xAl_1-xN at high x is critical to improve resonator bandwidth and reduce insertion loss.

In this work, we show the importance of layer nucleation—both an AlN interlayer, and the initial ScAlN layer—to the final crystal quality of MBE-grown ScAlN films on SiC and NbN/SiC. With the inclusion of a 5-nm AlN interlayer, the Sc_0.32Al_0.68N XRD FWHM decreases from 2.0° to 1.13°. An AlN interlayer is also critical to growth of ScAlN on NbN thin films. A two-step AlN growth process can effectively encapsulate the NbN layer while providing a smooth surface on which to nucleate ScAlN growth, and is critical to maintaining high crystal quality for ScAlN grown on NbN.

The ScAlN initiation steps also have a strong impact on the final quality of the film. Instead of a two-step ScAlN growth we previously demonstrated, initiation using a linear composition grade from Sc_0.32Al_0.68N to Sc_0.40Al_0.60N over 100 nm leads to further improvements in the RHEED pattern, including a narrowing of the spots early in the growth, as well as elimination of remaining ring-like character in the final RHEED pattern after an additional 40 nm of growth, and a XRD FWHM as low as 1.22° for ScAlN films grown on SiC. Transmission electron micrographs show near elimination of cubic grains that otherwise form in the initial layers of the Sc_0.40Al_0.60N. The graded sample has the same average ScN fraction and thickness as the two-step sample. The grade thickness can be reduced to 25 nm (with the remaining 125 nm Sc_0.40Al_0.60N) without degrading the XRD FWHM or RHEED pattern, increasing the average ScN fraction from 0.373 to 0.393. Finally, a 500-nm-total-thickness sample (100 nm Sc_0.32Al_0.68N → Sc_0.40Al_0.60N, 400 nm Sc_0.40Al_0.60N) was grown to show the impact of defect annihilation in thicker films, resulting in a reduction of XRD FWHM to 0.89°. Employing the same 25-nm grade followed by 125-nm of Sc_0.40Al_0.60N grown on AlN/NbN/SiC results in a FWHM of 1.97°. The improved layer initiation shows that more gradual changes in surface energy and strain reduces the nucleation of undesirable cubic grains, and may point to a general strategy for elimination of anomalous grains in high ScN fraction ScAlN.

High efficiency light emitting diodes (LEDs) with characteristic length scales on the order of microns or less, also known as µLEDs, have been under intense investigations for their immense promise in various display and communications scenarios. Among the many material systems investigated for µLEDs, the III-nitride family possesses many desirable material properties such as comparatively low surface recombination velocities and excellent wavelength tunability. To date, however, it has remained a challenge to achieve efficient green and red emitting µLEDs, largely due to the enhanced surface recombination and poor p-type doping related to the top-down etching. Moreover, it has remained difficult to achieve the high levels of indium incorporation required for a red emitting indium gallium nitride (InGaN) active region. Here we demonstrate that the efficiency bottleneck of µLEDs can be fundamentally addressed by utilizing bottom-up III-nitride nanostructures. We report on the demonstration of micrometer scale green and red LEDs with an external quantum efficiency of 25% and 8%, respectively, which are the highest values ever reported to the best of our knowledge. We employ selective area plasma-assisted molecular beam epitaxy as the material synthesis platform. Due to efficient strain relaxation, such bottom-up nanostructures are largely free of dislocations. By exploiting the large exciton binding energy and oscillator strength of quantum-confined InGaN nanostructures, we show that the external quantum efficiency of a green-emitting micrometer scale LED can be dramatically improved from ~4% to >25%. The dramatically improved efficiency is attributed to the utilization of semipolar planes in strain-relaxed nanostructures to minimize polarization and quantum-confined Stark effect and the formation of nanoscale quantum-confinement to enhance electron-hole wavefunction overlap. We have further developed a new approach that included an InGaN/GaN short period superlattice together with an InGaN quantum dot active region to achieve high efficiency red emission. A maximum quantum efficiency of >7% was measured. Our studies offer a viable path to achieve high efficiency micrometer scale LEDs for a broad range of applications including mobile displays, virtual/augmented reality, biomedical sensing, and high-speed optical interconnects, that were difficult for conventional quantum well based LEDs.

In this work we present a method for creating AlN/Al_xGa_1-xN short period superlattices (SPSL) with individual layer thicknesses down to 3-4 ML. By taking advantage of the oscillatory nature of metal modulated epitaxy (MME), the formation of alternating AlN and Al_xGa_1-xN layers can be achieved through the introduction of a constant Ga overpressure during the MME growth of AlN. However a typical MME process results in SPSLs with periods between 5 and 6 nm. In order to reduce the thickness of the AlN and Al_xGa_1-xN layers, the shutter timings are reduced in order to limit the total amount of Al deposited on the surface. Initial results with this shortened shutter timing showed 0^th order SL peak with Pendellosung fringes in an XRD coupled scan. Such behavior is typically indicative of a high quality SPSL. However, TEM imaging showed a strong intermixing of between the AlN and Al_xGa_1-xN layers, leading to a structure that more resembles a random Al_xGa_1-xN alloy than a well ordered SPSL. In order to prevent intermixing at the interface layers, 3 altered growth processes were investigated: alternating Al and Ga shutters, inclusion of a N plasma only exposure step and reduction of the Ga flux. The goal with each of these changes was to limit the interaction of the Ga and Al during the final stages of the AlN layer formation. TEM examination of each growth’s layer structures showed a significant reduction in layer intermixing for all samples. Alternating the Al and Ga shutter resulted in a SPSL with high order in its layer structure, with AlN and Al_xGa_1-xN thicknesses of 4 ML and 3 ML respectively. Introduction of the plasma only step resulted similarly improved interfaces however the thicknesses of AlN and Al_xGa_1-xN layers had changed to be 3 ML and 4 ML respectively. Finally, reducing the Ga flux led to a layer structure almost identical to what was obtained using alternating shutters. XRD coupled scans each sample showed Pendellosung fringes centered on a 0^th order peak. Fitting of the peaks enabled determination of the average Al composition for the SPSL, which ranged from a low of 84% for alternating shutters to a high of 90% for lower Ga flux. Because these 2 samples showed the same layer thicknesses in TEM, reducing the Ga flux resulted in higher Al content Al_xGa_1-xN layers. A coupled scan of the N pause sample showed a broad peak to the left of the main alloy peak. The cause of this is a section of growth where the layered structure had non uniformity which can be seen in TEM indicating further optimization is needed. Based on these results, MME shows promise as method for forming digital alloys.