0) projection, indicating the inequivalent (0001) and (000
) directions. Cross-sectional view of a wurtzite GaN crystal in a (110) projection, indicating the inequivalent (0001) and (000) directions.
STM images of the GaN(000) surface displaying (a) 1x1, (b) 3x3, (c) 6x6, and (d) c(6x12) reconstructions. Sample bias voltages are -0.75, -0.1, +1.5, and +1.0 V, respectively. Tunnel currents are in the range 0.03 - 0.11 nA. Gray scale ranges are 0.17, 0.88, 1.33, and 1.11 Å respectively. Unit cells are indicated with edges along <110> directions. (From [30]).
(a) The relative energies calculated for possible models of the GaN(0001) surface are shown as a function of the Ga chemical potential. (b) Relative energies for GaN(000) surfaces. The zeroes of energy in (a) and (b) are not related. (From [30]).
Schematic view of structures determined for the (a) 1x1 Ga adlayer and (b) 3x3 adatom-on-adlayer reconstructions of GaN(000). For the 3x3 structure, the lateral (in-plane) displacement of the adlayer atoms bonded to the Ga adatom is 0.51 Å away from the adatom. All other lateral or vertical displacements of the adlayer atoms are less than 0.1 Å. (From [30]).
Simultaneously-acquired dual bias images of the 5x5 reconstruction. Sample biases are +1.0 V and -1.0 V with gray scale ranges of 0.5 Å and 0.9 Å for (a) and (b), respectively. (From [36]).
Structural model for the 5x5 reconstruction. Ga-adatoms in T4 sites and N-adatoms in H3 sites are shown by large black and large grey circles respectively. The small open circles in the diagram represent the Ga rest atoms in the 2nd layer. In the locations where the small open circles are missing, Ga vacancies occur. The N atoms in the 3rd layer are not shown. The light grey circle labelled DB (dangling bond) is a particular Ga rest atom site. In an alternative model, this site could conceivably contain another adatom in a nearby T4 or H3 site. (From [36]).
Dual bias images of the 5x5 and 6x4 reconstructions. The average height difference between the two reconstructions is 0.3 Å for empty states (+1.0 V sample voltage) shown in (a) and 0.4 Å for filled states (-1.0 V sample voltage) shown in (b), with the 5x5 being higher in each case. In both images the total gray scale range is about 1.3 Å. (From [36]).
1x1 RHEED pattern for GaN(0001) during growth, (a), and after cooling to below 350°C, (b), where it converts to a 1+1/6 pattern. The 1+1/6 LEED pattern (Einc=100 eV) is shown in (c). For most "1x1" surfaces (see text), a 1+1/12 pattern is observed below 200°C, as shown in (d) (Einc=40 eV). LEED in vicinity of (0,1) spot (Einc=40 eV) at various temperatures: (e) RT-100°C, (f) 100-150°C, (g) 150-200°C, and (h) above 350°C. (From [33]).
Side view of a possible structural model for the "1x1" surface (at a given instant in time) consisting of 2.7 ML of Ga sitting on top of the Ga-terminated bilayer. The empty circles represent the various possible positions of first-layer Ga atoms plotted with respect to each of several GaN unit cells, illustrate the time-averaged height of the first layer Ga atoms and thus the 1x1 contour which the STM tip will follow. At a given instant in time, however, this incommensurate structure will manifest itself in diffraction as satellites surrounding the integral order peaks. (From [33]).
A schematic representation of a laterally contracted Ga bilayer above a Ga-terminated (0001) substrate. The average separations between layers are z12 = 2.54 Å and z01 = 2.37 Å. The filled and open circles in layer 0 represent a time averaged image of the Ga atoms. The filled circles in layer 0 correspond to the positions at a particular time. The time averaged vertical corrugation of layer 0 is approximately 0.16 Å. Note: In this projection the laterally contracted monolayer (layer 0) has been rotated by 30° for ease of viewing. (From [46]).
Projection on the c-plane of the top two layers for the registry B structure discussed in the text. Open circles represent a 1x1 adlayer of atoms located in T1 sites. (From [46]).
Projection on the c-plane of the top two layers for registry A. The heights (h) of the Ga atoms in layer 0, relative to that of atom 1 in registry B are listed. (From [46]).
Relative energies of the surfaces are plotted as a function of the Ga chemical potential. For Ga-rich conditions the most stable structure is the laterally contracted Ga bilayer. (From [46]).
STM image of the InGaN(000) surface, obtained from a sample with negligible indium incorporation in the bulk. The image was acquired with sample bias voltage of -0.5 V and tunnel current of 0.075 nA. The grey scale range is 0.6 Å for each terrace. A line cut, taken at the position of the arrows in the figure, is shown on the right side of the figure. (From [50]).
Schematic view of the InGaN surface in an (100) projection, showing theoretical results for atomic positions. The surface adlayer consists of 75% In plus 25% Ga, in a 2x2 arrangement. (From [50]).
STM images of InGaN(0001) surface: (a) surface containing 0.9±0.2 ML of indium. Image was acquired at a sample voltage of +1.0 V. Grey scale range is 0.6 Å. Different regions of the sample are labeled according to their In occupation, layer 1 or layer 1+2, as pictured below on the image on the right- and left-hand sides respectively. (b) and (c) Surface containing 1.4±0.2 ML of indium. Images were acquired at sample voltages of (a) -2.0 V and (b) +1.5 V. The entire surface consists of the vacancy island structure. Grey scale range is 0.6 Å for (b) and (c). Tunnel current is 0.075 nA for all images. (From [51]).
(a) Top view of the 7/4 structure comprised of 7/4 ML of In. In this 2x2 structure there is 3/4 ML of In and 1/4 ML Ga in layer 2. Layer 1 (not shown) contains 1 ML of In atoms. (b) Top view of the 7/4 + N-vacancy structure, obtained from the 7/4 structure by removing N atoms from layer 3. (c) Side view of a trench created by removing three rows of N atoms (layer 3) and two rows of In atoms (layer 1). Formation of this trench leads to substantial lateral displacements of the atoms in layers 2 and 3, shown in Å. Indium atoms in sites 1 and 2 are bonded to 1 and 2 N-atoms respectively. (From [51]).
Smooth/rough transition on (a) (000) face (N-polar) and (b) (0001) face (Ga-polar). Nitrogen flux was fixed in both experiments. Substrate temperature was 600°C. Experimental data is shown with dots, each with an error bar. A dashed line is shown in each figure for comparison denoting the line with constant total metal flux. To the right of the transition lines (solid lines) the growth is smooth. (From [52]).
RHEED patterns seen after cooling of GaN films. Non-inverted surface: (a)"1x1" and (b) 2x2. After Mg exposure, inverted surface: (c) 3x3 and (d) 6x6. (From [60]).
(a) Bright field TEM image in (0002) two-beam condition of a MBE-grown GaN film grown on SiC showing inversion boundary labeled with solid arrowheads and growth interrupts labeled with hollow arrowheads. (b) High-resolution image of the same sample as in (a). Brackets indicate the region of the inversion boundary. (c) Bright field (0002) two-beam TEM image of a MOCVD-grown GaN film showing inversion domains (ID). Regrowth interface is indicated by arrows. (From [60]).
(a) Structural model of a c-plane inversion domain boundary (IDB) induced by Mg atoms. (b) Noninverted structure. Both models are shown in a (110) projection. Height of the Mg layer above the underlying atomic plane is 1.29 Å and 2.43 Å for inverted and noninverted structures respectively. (From [60]).
(a)-(c) RHEED patterns of Ga-polar films during growth without any arsenic, as a function of decreasing Ga flux. (d)-(f) A similar progression of RHEED patterns in the presence of arsenic with a BEP of 1x10-9 Torr. (From [19]).
STM images of GaN(0001) surface exposed to 
0.5 ML of silicon. (a) Surface region showing Si-induced 2x2 reconstruction and the 5x5 reconstruction of the bare surface. (b) Two different types of domains (seen on the left and right sides of the image) of the 2x2 structures. Images were acquired with sample bias voltages of -2.5 V and -2.0 V, respectively, and are shown with gray-scale ranges of 1.3 and 1.0 Å, respectively. (From [65]).
STM image of a GaN(0001) surface following 1 ML silicon exposure. (a) Large scale image displaying terraces of "1x1" reconstruction with 4x4 structure seen at the terrace edges. (b) High resolution view of 4x4 structure near a terrace edge. Images were both acquired with a sample bias voltages of +2.0 V, and are shown with gray-scale ranges of 13 and 2.1 Å, respectively. (From [65]).
Phase diagram showing the energetically stable structures determined from first-principles calculations as a function of both Si (µSi) and N (µN) chemical potentials. The atomic geometry of these structures is shown in Figure 24. The limit at N-rich conditions is µSi = 1/3 
Hf(Si3N4) and at Ga-rich conditions is µSi = 1/3 Hf(Si3N4) - 4/3 Hf(GaN). Hf(Si3N4) = -3.32 eV and Hf(GaN) = -1.24 eV are the calculated formation enthalpies of Si3N4 and GaN bulk, respectively. The shaded area shows the region where all structures are unstable against the formation of Si3N4. (From [66]).
Schematic sideview of the energetically favorable structures for bare and Si-covered GaN(0001) surfaces. (a) N terminated with a Si subsurface, (b) Ga bilayer, (c) Ga bilayer with a Si subsurface, (d) Ga adatom, (e) N adatom, (f) Ga adatom with a Si subsurface, and (g) 2 ML of Si. See, also, Figure 23. (From [66]).
(a) Phase diagram for the GaN(0001) surface in the presence of H, as a function of µGa and µH. µH = 0 corresponds to H molecules at T=0; µGa = 0 corresponds to bulk Ga. Dots indicate experimental data from Ref. [14]; within the error bars, these data agree with the calculated NH3 + 3Ga-H / 3Ga-H phase boundary highlighted by the thicker line. Note that the VGa + 6H structure is stoichiometrically and energetically equivalent to NH3 + 3Ga-H. (b) temperature dependence of µH for two different pressures. (From [80]).
Schematic top view of prevalent 2x2 reconstructions for GaN(0001) surfaces. Large open circles represent Ga atoms, solid circles N, and small open circles H. (From [80]).
AFM images of GaN films grown under (a) Ga-rich conditions, and (b) N-rich conditions. The insets shows the corresponding RHEED patterns with the electron beam along (110). (From [61]).
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