E. A. Berkman

Effect of doping on the magnetic properties of GaMnN: Fermi level engineering

GaMnN dilute magnetic semiconductor samples, prepared by metalorganic chemical vapor deposition, are shown to exhibit ferromagnetism or even paramagnetism depending upon the type and concentration of extrinsic impurity present in the film. In addition, GaMnN deposited using growth parameters normally yielding a nonferromagnetic film becomes strongly ferromagnetic with the addition of magnesium, an acceptor dopant. Based upon these observations, it seems that ferromagnetism in this material system depends on the relative position of the Mn energy band and the Fermi level within the GaMnN band gap. Only when the Fermi level closely coincides with the Mn-energy level is ferromagnetism achieved. By actively engineering the Fermi energy to be within or near the Mn energy band, room temperature ferromagnetism is realized.

Dependence of ferromagnetic properties on carrier transfer at GaMnN∕GaN:Mg interface

We report on the dependence of ferromagnetic properties of metalorganic chemical vapor deposition grown GaMnN films on carrier transfer across adjacent layers. We found that the magnetic properties of GaMnN, as a part of GaMnN∕GaN:Mg heterostructures, depend on the thickness of both the GaMnN film and the adjacent GaN:Mg layer and on the presence of a wide band gap barrier at this interface. These results are explained based on the occupancy of the Mn energy band and how the occupancy can be altered due to carrier transfer at the GaMnN∕GaN:Mg interfaces.

Nearly lattice-matched n, i, and p layers for InGaN p-i-n photodiodes in the 365–500nm spectral range

We report on nearly lattice-matched grown InGaN based p-i-n photodiodes detecting in the 365–500nm range with tunable peak responsivity tailored by the i-layer properties. The growth of lattice matched i- and n-InGaN layer leads to improvement in the device performance. This approach produced photodiodes with zero-bias responsivities up to 0.037A∕W at 426nm, corresponding to 15.5% internal quantum efficiency. The peak responsivity wavelength ranged between 416 and 466nm, the longest reported for III-N photodiodes. The effects of InN content and i-layer thickness on photodiode properties and performance are discussed.

Magnetic and magnetotransport properties of (AlGaN∕GaN):Mg∕(GaMnN) heterostructures at room temperature

Dilute magnetic semiconductor films (GaMnN) are highly resistive, making transport measurements difficult to achieve. However, when GaMnN films are sandwiched between p-type doped (AlGaN∕GaN) strained-layer superlattices, holes from the superlattice interact with the Mn3+∕2+ ions and transport measurements were realized. The authors have found also that the ferromagnetic properties of GaMnN critically depend on the level of p-type doping in the superlattice. They report anomalous Hall effect measurements in this (AlGaN∕GaN):Mg∕(GaMnN) multilayered structure. The current results also demonstrate the role of carriers, especially holes, in mediating the ferromagnetic properties of GaMnN dilute magnetic semiconductor films.

The Effect of Mn Concentration on Curie Temperature and Magnetic Behavior of MOCVD Grown GaMnN Films

We report on the growth and characterization of dilute magnetic semiconductor GaMnN showing ferromagnetism behavior above room temperature. GaMnN films were grown by MOCVD using (EtCp 2 )Mn as the precursor for in-situ Mn doping. Structural characterization of the GaMnN films was achieved by XRD, SIMS and TEM measurements. XRD and TEM confirmed that the films were single crystal solid solutions without the presence of secondary phases. SIMS analysis verified that Mn was incorporated homogeneously throughout the GaMnN layer which was ∼0.7μm thick. Ferromagnetic behavior for these films was observed along the c-direction (out of plane orientation) in a Mn concentration range of 0.025–2%. The saturation magnetization ranged from 0.18–1.05 emu/cc for different growth conditions. Curie temperatures of the GaMnN films were determined to be from 270 to above 400K depending on the Mn concentration. This dependence of Curie temperature on concentration of Mn in GaMnN indicates that the grown films are random solid solutions . SQUID measurements ruled out the possibility of spin-glass and superparamagnetism in these MOCVD grown GaMnN films. The grown films were electrically semi-insulating; however PL measurements showed that the films were still optically active after Mn doping. This study showed that the growth of III-Nitride films doped with Mn requires a small window of growth conditions that depend on growth temperature and (EtCp) 2 Mn flux to achieve ferromagnetism above room temperature, and the magnetic response of the film depends on the Fermi level position. We suggest that ferromagnetism occurs when the Fermi level lies within the Mn energy level which is 1.4 eV above the GaN valence band.

Magnetic properties of Mn-doped GaN, InGaN, and AlGaN

We report on the growth and magnetic properties of single crystal Mn-doped GaN, InGaN, and AlGaN films. The III-Nitride films were grown by MOCVD, while the Mn doping was performed by solid-state diffusion of a surface Mn layer deposited by pulsed laser ablation. Mn-doped In x Ga1-x N films were grown with x < 0.15, where the easy axis of magnetization rotates from in-plane to out-of-plane by changing the In x Ga1-x N thickness/strain-state of the film from compressively strained to relaxed. Mn-doped Al x Ga1-x N films were grown with x < 0.40 showing ferromagnetic behavior above room temperature. SQUID measurements ruled out superparamagnetism within these films. By optimizing the growth and annealing conditions of Mn-doped III-Nitrides, we have achieved Curie temperatures in the range of 228 to 500K. These ferromagnetic Mn-doped III-Nitride films exhibit hysteresis with a coercivity of 100–500 Oe. TEM analysis showed no secondary phases within these films.

Growth of GaN from elemental Gallium and Ammonia via Modified Sandwich Growth Technique

Gallium nitride (GaN) thin films were grown on (0001) sapphire substrates at 1050°C by controlled evaporation of gallium (Ga) metal and reaction with ammonia NH 3. The feasibility of the growth process was demonstrated and discussed. One of the biggest challenges of working in the Ga–NH 3 system was the instability of molten Ga under NH 3 atmosphere at elevated temperatures, especially between 1100–1200°C. In the first part of the study, transport of Ga species from the source-to-substrate during the GaN growth process and the influence of ammonia–liquid Ga reaction on Ga transport were investigated. Experimental results under different conditions were studied and compared to theoretical predictions to quantify the mechanism of transport in the vapor growth technique. In presence of NH 3 , Ga transport far exceeded the predicted upper limit for the vapor phase transport. Visual observations confirmed that a significant amount of Ga left the source in a cluster rather than atomic form. A novel Ga source design was employed in an effort to obtain a stable and high vapor phase transport of Ga species at moderate temperatures. In this design, pure N 2 was flowed directly above the molten Ga source. This flow prevented the direct contact and reaction between the molten Ga and NH 3 and prevented Ga spattering and GaN crust formation on the source surface. At the same time, it significantly enhanced Ga evaporation rate and enabled control of Ga transport and V/III ratio in the system. Growth characteristics were described by a mass transport model based on process parameters and experimentally verified. The results showed that the process was mass transport limited and the maximum growth rate was controlled by transport of both Ga and reactive ammonia species to the substrate surface. A growth rate of 1.4 µm/h was obtained at 1050°C, 800 Torr, 3 slm of ammonia flow rate, and 1250°C Ga source temperature at a 24 mm source-to-substrate distance. It was found that the process required a more effective supply of active NH 3 to the substrate in order to increase the crystal quality and growth rate. The surface morphology of the deposited layers was examined by optical and scanning electron microscopies. XRD analysis was used to determine the crystallinity of deposited films and revealed a full-width at half-maximum (FWHM) of 0.6 deg. for the (0002) GaN peak. EDX analysis was employed for the chemical characterization of the samples and …