We report on the enhanced incorporation efficiency of magnesium dopants into facets of hexagonal hillock structures in N-polar GaN, studied by comparative analysis of GaN:Mg films grown by MOCVD on high and low hillock density GaN template layers. Total magnesium concentration in planar regions surrounding a hillock structure is comparable to that within hillock sidewall facets measured at 1.3 × 1019 cm−3 by atom probe tomography, and clustering of Mg atoms is seen in all regions of the film. Within individual hillock structures a decreased Mg cluster density is observed within hillock structures as opposed to the planar regions surrounding a hillock. Additionally, the Mg cluster radius is decreased within the hillock sidewall. The favorable incorporation of Mg is attributed to Mg dopants incorporating substitutionally for Ga during growth of semi-polar facets of the hillock structures. Enhanced p-type conductivity of GaN:Mg films grown on high hillock density template layers is verified by optical and electrical measurement.
We report on holistic and systemic approach of development of Cs-free GaN photocathode structures which utilize polarization band engineering in order to allow for air stable operation and eliminate the need for cesium-based surface treatments. Physics-based simulation of band structure and Monte Carlo simulation of electron transport and emission were used to guide experimental development of photocathode structures. By using an N-polar device, the polarization charge allows for the creation of large surface band bending without the need for δ-doped capping layers. The insertion of a thin AlN interlayer allows for the creation of a quasi-band offset and additional beneficial polarization charge to create a desirable band profile. Samples of both polarities were grown and subjected to chemical surface treatments in order to account for differences in native oxide formation on Ga- and N-polar surfaces. Measured photoemission spectra show quantum efficiencies as high as 23% for a HCl-treated Cs-free N-polar photocathode, which is comparable to cesiated devices.
A systematic photoluminescence study of Be‐doped GaN grown by metal‐organic chemical vapor deposition is presented. All Be‐doped samples show the ultraviolet luminescence (UVLBe) band with a maximum at 3.38 eV and the yellow luminescence (YLBe) band with a maximum at ≈2.15 eV in GaN:Be having various concentrations of Be. The UVLBe band is attributed to the shallow state of the BeGa acceptor with a delocalized hole. The YLBe band is caused by a Be‐related defect, possibly the polaronic state of the BeGa acceptor with the charge transition level at 0.3 eV above the valence band. This broad band exhibits unusual properties. In particular, it always shows two steps in its thermal quenching. The second step at T ≈ 200 K is attributed to the emission of holes from the 0.3 eV level to the conduction band. The origin of the first step remains unexplained.
We demonstrate p-type activation of GaN doped by Mg ion implantation, and in situ during metalorganic chemical vapor deposition through sequential short-duration gyrotron microwave heating cycles at temperatures of 1200–1350 °C. GaN is implanted with 1019 cm−3 Mg ions, capped with AlN, and annealed under 3 MPa N2 overpressure in 5 s heating cycles for less than 60 s total using a high-power gyrotron microwave heating source. Through I–V characterization, photoluminescence spectroscopy, and Raman spectroscopy, we study the evolution of electrical properties, optically active point defects, and material strain in response to implantation and annealing. For Mg-implanted samples, increasing annealing temperature is characterized by an increase in the PL substitutional Mg-related peak (UVL) relative to the shallow-donor vacancy-related peak (GL2). Through comparison of implanted and in situ doped samples, it is demonstrated that the origin of compensating VN lies primarily in implantation rather than degradation from the annealing process. Transmission line measurements and diode I–V measurements show a sheet resistance of 1083 kΩ/□ and a hole concentration of 1.23 × 1015 cm−3, respectively, in the Mg-implanted material annealed at 1350 °C. We conclude that temperature-cycled gyrotron annealing at 1350 °C decreases implant-induced compensating point defects and activates Mg to obtain selective p-type conduction.
Co-implantation of Mg with N has been shown to improve p-type conductivity in Mg-implanted GaN. Achievement of p-type material still requires temperatures beyond the thermodynamic stability of GaN, however. In this study, we present results of implantation and anneal activation of GaN, co-implanted with Mg and N or Mg only by repeated, short thermal cycles of 1350 °C using a high-power gyrotron microwave source with a quasi-gaussian intensity profile. Spatial variations in optical and electrical properties of the resulting films are characterized by photoluminescence and diode I–V and C–V measurements. Resistive Mg/N co-implanted and annealed material shows dominant luminescence of the VN-related green luminescence (GL2) band at 2.37 eV and relatively lower intensity acceptor-related ultraviolet luminescence (UVL) at 3.27 eV. However, a material showing p–n diode behavior shows higher-intensity UVL luminescence and suppression of the GL2 band, permitting observation of the yellow luminescence (YL) present in the as-grown GaN. The YL is attributed to unintentionally introduced CN–ON complexes and is commonly observed in GaN grown by metalorganic chemical vapor deposition but is typically absent in implanted/annealed GaN. Co-implanted material is compared to material implanted only with Mg and annealed under the same conditions, which shows p-type activation, but contains persistent GL2 luminescence post-anneal and lowers maximum hole concentration.
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