Muhammad B. Haider

Metal/semiconductor phase transition in chromium nitride(001) grown by rf-plasma-assisted molecular-beam epitaxy

Structural and electronic properties of stoichiometric single-phase CrN(001) thin films grown on MgO(001) substrates by radio-frequency N plasma-assisted molecular-beam epitaxy, are investigated. In situ room-temperature scanning tunneling microscopy clearly shows the 1×1 atomic periodicity of the crystal structure as well as long-range topographic distortions which are characteristic of a semiconductor surface. This semiconductor behavior is consistent with ex situ resistivity measurements over the range 285 K and higher, whereas below 260 K, metallic behavior is observed. The resistivity-derived band gap for the high-temperature region, 71 meV, is consistent with the tunneling spectroscopy results. The observed electronic (semiconductor/metal) transition temperature coincides with the temperature of the known coincident magnetic (para-antiferro) and structural (cubic-orthorhombic) phase transitions.

Composition-dependent structural properties in ScGaN alloy films: A combined experimental and theoretical study

Experimental and theoretical results are presented regarding the incorporation of scandium into wurtzite GaN. Variation of the a and c lattice constants with Sc fraction in the low Sc concentration regime (0%–17%) are found that can be well explained by the predictions of first-principles theory. The calculations allow a statistical analysis of the variations of the bond lengths and bond angles as functions of Sc concentration. The results are compared to predictions from both a prior experimental study [Constantin et al., Phys. Rev. B 70, 193309 (2004)] and a prior theoretical study [Farrer and Bellaiche et al. Phys. Rev. B 66, 201203(R) (2002)]. It is found that the ScGaN lattice can be very well modeled as being wurtzitelike but with local lattice distortions arising from the incorporation of the Sc atoms. Effects of the addition of Sc on the stacking order for a large Sc fraction is also studied by high resolution transmission electron microscopy. The results show the existence of stacking faults, and...

ScGaN alloy growth by molecular beam epitaxy: Evidence for a metastable layered hexagonal phase

Alloy formation in ScGaN is explored using rf molecular beam epitaxy over the Sc fraction range x=0-100%. Optical and structural analysis show separate regimes of growth, namely (I) wurtzitelike but having local lattice distortions in the vicinity of the Sc{sub Ga} substitutions for small x (x{ =}0.54). In regimes I and III, the direct optical transition decreases approximately linearly with increasing x but with an offset over region II. Importantly, it is found that for regime I, an anisotropic lattice expansion occurs with increasing x in which a increases much more than c. These observations support the prediction of Farrer and Bellaiche [Phys. Rev. B 66, 201203-1 (2002)] of a metastable layered hexagonal phase of ScN, denoted h-ScN.

Ga/N flux ratio influence on Mn incorporation, surface morphology, and lattice polarity during radio frequency molecular beam epitaxy of (Ga,Mn)N

The effect of the Ga/N flux ratio on the Mn incorporation, surface morphology, and lattice polarity during growth by rf molecular beam epitaxy of (Ga,Mn)N at a sample temperature of 550 °C is presented. Three regimes of growth, N-rich, metal-rich, and Ga-rich, are clearly distinguished by reflection high-energy electron diffraction and atomic force microscopy. Using energy dispersive x-ray spectroscopy, it is found that Mn incorporation occurs only for N-rich and metal-rich conditions. For these conditions, although x-ray diffraction in third order does not reveal any significant peak splitting or broadening, Rutherford backscattering clearly shows that Mn is not only incorporated but also substitutional on the Ga sites. Hence, we conclude that a MnxGa1−xN alloy is formed (in this case x∼5%), but there is no observable change in the c-axis lattice constant. We also find that the surface morphology is dramatically improved when growth is just slightly metal rich. When growth is highly metal-rich, but not G...

Heteroepitaxial growth and surface structure of L10-MnGa(111) ultra-thin films on GaN(0001)

L10-structured MnGa(111) ultra-thin films were heteroepitaxially grown on GaN(0001) under lightly Mn-rich conditions using molecular beam epitaxy. Room-temperature scanning tunneling microscopy (STM) investigations reveal smooth terraces and angular step edges, with the surface structure consisting primarily of a 2 × 2 reconstruction along with small patches of 1 × 2. Theoretical calculations were carried out using density functional theory, and the simulated STM images were calculated using the Tersoff-Hamman approximation, revealing that a stoichiometric 1 × 2 and a Mn-rich 2 × 2 surface structure give the best agreement with the observed experimental images.

Mixing Rocksalt and Wurtzite Structure Binary Nitrides to Form Novel Ternary Alloys: ScGaN and MnGaN

Combination of tetrahedral and octahedral based nitrides are explored. The two cases of MnGaN and ScGaN with low Mn and Sc fractions are examined. It is found that for the MnGaN case, the Mn is incorporated under N rich conditions with little lattice change. However, for the ScGaN case, the Sc is incorporated onto the Ga sites but with a local bond angle distortion.

Surface reconstructions of cubic gallium nitride (001) grown by radio frequency nitrogen plasma molecular beam epitaxy under gallium-rich conditions

Cubic GaN has been grown under gallium (Ga)-rich growth conditions using radio frequency nitrogen plasma molecular beam epitaxy on MgO(001) substrates. Reflection high energy electron diffraction patterns indicate the smoothness of the c-GaN surface and show 2× and even 8× periodicities after the growth at sample temperature Ts<200°C and 1×1 at higher temperatures. Scanning tunneling microscopy images reveal a sequence of variant surface reconstructions including c(4×12), 4×7, c(4×16), 4×9, c(4×20), and 4×11. These variant reconstructions correspond to slightly different Ga adatom coverages all less than 1∕4 ML, with 4×11 having the highest, and c(4×12) the lowest Ga coverage. The electronic properties of these six variant reconstructions are investigated, and they are found to have a metallic nature.

Scanning Tunneling Microscopy Study of Cr-doped GaN Surface Grown by RF Plasma Molecular Beam Epitaxy

Abstract: Cr doped GaN was grown by rf N-plasma molecular beam epitaxy on sapphire (0001) at a sample temperature of 700 o C. Cr/Ga flux ratio was set to a value from 5% to 20%. Subsequently, scanning tunneling microscopy was performed on these surfaces. Cr incorporates on the GaN surface at 700 o C at a Cr concentration of 5% and less. By increasing the Cr/Ga flux ratio to 20% in CrGaN, linear nano structures were formed on the surface, which were not observed on the bare GaN surface. The RHEED and STM studies reveal that Cr atoms form 3×3 reconstruction when 0.1 ML of Cr was deposited at room temperature on 1×1 adlayer of Ga on GaN (000-1). Cr substitutes Ga on the surface when deposited at 700 o C on the MBE grown GaN (000-1) surface for all the experiments, which we have performed, provided the Cr concentration is low (~5%). PACS: 75.50.Pp, 81.15.Hi, 61.10.Nz, 61.14.Hg Introduction For last two decades, spintronics has become a field of wide interest, where not only the charge but also spin degree of freedom of the charge carriers will be manipulated. Successful spin injection into existing semiconductor-based devices at room temperature is still being investigated. Many scientists have predicted that some Nitride Dilute Magnetic Semiconductors (NDMS) can be used as spin injectors at room temperature. According to Sato