An atomic-level insight into thestep-flow growth of GaN

K. M. Bui, K. Shiraishi, A. Oshiyama
Nagoya University, Japan1

Introduction: why GaN
SiGaN

6H-SiC

Bandgap (eV) @300oC

1.1

3.4

2.9

Electron mobility (cm2/V∙s), RT

1400

1000 (bulk)

600

Saturation velocity (cm/s), 107

12.5

2

Breakdown field (V/cm), 106

0.3>54Melting temperature (K)

1690

>1700

>2100

Higher quality thin films aredemanding.
2

Step edge in MOVPE
Furnace structures& specifications

3GaH surface

NH3 decomposition on Ga rich surface

TMGNH3GaNH
 MOVPE: TOF experiment: NH3 not decomposed in gas phase[1] =>What is the fate of NH3?
 H2 carrier gas, growing surface is 3GaH [2]Step-flow growth schematic

 N2 carrier gas, growing surface is Ga rich [2]. NH3 isdecomposed on surface with barrier of 0.6 eV, NH is foundto intervened in the Ga-Ga weak bond [3] Diffusion, adsorption/desorption near step edge is important[1] Nagamatsu, K. et. al., Physica Status Solidi (b) 254, 1600737 (2017).
[2] Kusaba et. al., Jpn. J. Appl. Phys. 56 (2017) 7.
[3] K. M. Bui et. al., J. Phys. Chem. C 122, 24665 (2018)

Purpose Explore the step edge structures of GaN (0001) andclarify the formation of GaN crystalline near the stepedge using first principle calculation (Density functionaltheory)
Real space DFT (RSDFT)
Exchange correlation type: GGA
Kpoint: 1×3×1~ 400 atoms

Step configuration
Ga2

N2

Ga3N1GaNTopview
GaN(0001)[112� 0][11� 00]
Along [112� 0]: GN step
Along [11� 00] direction:- Ga2: 2 Ga dangling bonds- N2: 2 N dangling bonds- Ga3: 3 Ga dangling bonds- N1: 1 N dangling bonds- Ga2N1- Ga2Ga3- N2N1- N2Ga35

Result: Ga rich GaN surface

K. M Bui, K. Shiraishi, A. Oshiyama, Appl. Surf. Sci. 557, 149542

6

Formation energy
Formation energies of the mono-bilayer atomic steps
on GaN (0001) surface

K. M Bui, K. Shiraishi, A. Oshiyama, Appl. Surf. Sci. 557, 149542

7

Most stable structure of Ga rich GaN (0001) step
Ga2N1-Diff00

MOST STABLE: Ga2 side: 2 Ga (Upper Terrace) – Ga (Lower Terrace) 2.48 ÅGa-Ga weak bonds
Calculated electron densities near the GN step (a) andb) the Ga2 step Ga-Ga bonds appear at the stepedges. => these Ga-Ga weak bond isthe candidate place for the atomic
reactions which promote the epitaxial
growth of GaN

K. M Bui, K. Shiraishi, A. Oshiyama, Appl. Surf. Sci. 557, 149542

9

NH diffusion on the terrace
Diffusion pathway and the corresponding
energy barrier of an NH unit on the terraceof GaN(0001) surface.

 Intervening NH unit was assumed to beubiquitous [1]. Intervening NH unit is found tomove along the almost shortest path and ﬁnallyreaches

theadjacentGa-adatom

site,

intervening in the Ga-Ga bond. Energy cost:
0.59 eV. => NH unit wanders on the terrace
and occasionally approaches the step edges.
[1] K. M. Bui et. al., J. Phys. Chem. C 122, 24665 (2018)10
K. M Bui, K. Shiraishi, A. Oshiyama, Appl. Surf. Sci. 557, 149542

NH unit intervening in Ga-Ga bond at step-edge(a) GN step-edge structure with an NH unit intervening inthe Ga-Ga bond. (b) GN step-edge structure with two NH
units intervening in the Ga-Ga bonds.

 Most stable structure appears at GN step . One
more Ga-Ga weak bond remain => another NH is
expected to come to the edge. Calculated totalenergies reveal that the reaction: 2NH on terrace→ an NH pair at the GN step edge, is exothermicwith the energy gain of 0.45 eV => NH units arelikely to gather at the step edges, cleaving theGa-Ga weak bonds.
 This NH structure is a precursor of the step-ﬂow
growth of GaN.
K. M Bui, K. Shiraishi, A. Oshiyama, Appl. Surf.
Sci.
11557, 149542

Step-flow growth•

Initial structure (Fig.a): Additional Ga is expected to come tothe step edge where 2 NH units bridge the upper-and lowerterrace Ga atoms.

Elementary process in step-ﬂow growth•

Final structure (Fig. b, c): new Ga-N network in which
additional Ga, along with the Ga adatom, forms bonds withthe N atoms at the step edge with the H2 being desorbed.

•

Total-energy proﬁle (Fig. d). Energy costs: 1.82 eV. In reality,the H2 gains the free-energy F of - 2.02 eV, (T = 1300K, p = 0.5atm).

 This elementary process for the epitaxial growth is certainlyplausible.
12Step-flow growth13Result: 3GaH surface
GaLower terrace

Sideview

0101-0101 Topview

Upper terrace

Lower terrace

NGa upper Hterrace

3GaH surface: Formation energy••Formation of the GN step toward [112� 0] is
generally lower
Additional H added to N2N1 And Ga2Ga3 tosatisfy electron counting rule (ECR).

• Most stable configuration is Ga2N1• Reconstruction:- Ga2, Ga3 side: Ga-Ga weak bond formationbetween Ga atom on upper terrace - lowerterrace
N rich- N2 side: N-N dimer formation
Ga rich- N1 side: almost no construction

15

Structure of Ga2N1: the most stable step pair
GaLower terrace

0101-0101 Topview

Upper terrace

Lower terrace

Ga upper HterraceSideview- Ga2 : 1 Ga atom on upper terrace form Ga-Ga bond (2.55 Å) with Ga atom on lower terrace- N1: only one N dangling bond, almost no reconstructionNH configuration (4x4 supercell)
HomogeneousHoleHalf-halfGaNH
H configuration

TunnelEnergy difference(Vs hole) (eV)

Homogeneous

0.345

Half-half0.302

HoleTunnel00NH3 -> NH reaction on Hole - surface
InitialIntermediate

Final

Top-view

Side-view

NH3@T1 => NH2@T1 + H =>NH@H3 + H2 (gas phase)
Ea ~ 0.72 eV
NH2 is abundant on the surface

NH2 diffusion on Half-half surface
InitialIntermediate

Final- NH2 easily diffuses from surface to the step edges withenergy barrier 0.27eV on half-half surface => the surface
where H atoms align along layers is preferable for NHxdiffusion

Reaction at the step?
Step flow
growthmechanismof Ga rich

NH2Growthmechanismof 3GaH?

Summary• We have performed the density-functional-theory-based calculations that elucidate atomic
structures of the surface steps and their energetics of both Ga rich GaN and 3GaH GaN• We have found that additional Ga atoms on the Ga-rich surface under the epitaxial growth approach
the NH decorated step edges and are incorporated at the step edges, forming a new GaN crystalline
units. The present results demonstrate a microscopic mechanisms of complex epitaxial growthphenomena by the quantum-theory-based computations.
• We find that NHx diffusion will be enhanced on the surface where H atoms align along layers. NH2 isfound to be abundant on the surface and easily diffuse from surface to the step edges with energy
barrier 0.27eV. This NH2 is then expected to decomposed and formed a new GaN crystalline units
near the step edges.

Thank you for your attention