Adsorption and desorption of hydrogen at nonpolar $\mathrm{GaN}\left(1\overline{1}00\right)$ surfaces: Kinetics and impact on surface vibrational and electronic properties

The adsorption of hydrogen at nonpolar $\mathrm{GaN}\left(1\overline{1}00\right)$ surfaces and its impact on the electronic and vibrational properties is investigated using surface electron spectroscopy in combination with density functional theory (DFT) calculations. For the surface mediated dissociation of ${\mathrm{H}}_2$ and the subsequent adsorption of H, an energy barrier of 0.55 eV has to be overcome. The calculated kinetic surface phase diagram indicates that the reaction is kinetically hindered at low pressures and low temperatures. At higher temperatures ab initio thermodynamics show, that the H-free surface is energetically favored. To validate these theoretical predictions experiments at room temperature and under ultrahigh vacuum conditions were performed. They reveal that molecular hydrogen does not dissociatively adsorb at the $\mathrm{GaN}\left(1\overline{1}00\right)$ surface. Only activated atomic hydrogen atoms attach to the surface. At temperatures above 820 K, the attached hydrogen gets desorbed. The adsorbed hydrogen atoms saturate the dangling bonds of the gallium and nitrogen surface atoms and result in an inversion of the Ga–N surface dimer buckling. The signatures of the Ga–H and N–H vibrational modes on the H-covered surface have experimentally been identified and are in good agreement with the DFT calculations of the surface phonon modes. Both theory and experiment show that H adsorption results in a removal of occupied and unoccupied intragap electron states of the clean $\mathrm{GaN}\left(1\overline{1}00\right)$ surface and a reduction of the surface upward band bending by 0.4 eV. The latter mechanism largely reduces surface electron depletion.

Figure 1

Changes in the $\mathrm{GaN}\left(1\overline{1}00\right)$ surface electronic properties during continuous adsorption of atomic hydrogen (H) produced by a hot filament in the presence of ${\mathrm{H}}_2$: (a) UPS (He I) valence-band spectra revealing a shift of the occupied states and a reduction of electron emission from the surface state at 3.1 eV. (b) Change of work function and (c) reduction in surface band bending in dependence upon hydrogen exposure.

Figure 2

Comparison of the $\mathrm{GaN}\left(1\overline{1}00\right)$ surface properties after growth, atomic hydrogen adsorption and subsequent annealing at $820\pm 50$ K. (a)–(c) X-ray photoelectron spectra of the Ga $2p_{3/2}$ and Ga $3d$ states as well as the N1s core level including contributions from the Ga(LMM) Auger transition. The individual core level spectra were normalized with respect to their maximum peak height. (d) Valence-band structure measured by UPS using He I radiation. A subtraction of contributions from He I satellite lines was applied. (e) Electronic properties including work function and surface band alignment of the m-plane GaN surface with and without adsorbed hydrogen determined based on the results of the photoelectron spectroscopy measurements.

Figure 3

(a) Specular EEL spectra (circles) of clean $\mathrm{GaN}\left(1\overline{1}00\right)$. The energy of incident electrons was set to 20 eV. The full line depicts calculated results. The prominent loss features are due to single and multiple electron scattering from the Fuchs-Kliewer phonon at 88 meV. (b) Surface band alignment and electron density profile resulting from modeling the experimental EEL spectra measured using different primary electron energies.

Figure 4

(a) Specular EEL spectrum of clean $\mathrm{GaN}\left(1\overline{1}00\right)$ with $\mathrm{FK}n$ indicating loss features that result from single ($n=1$) and multiple ($n\ge 2$) electron scattering from the Fuchs-Kliewer (FK) phonon at 88 meV. The incident electron energy was set to 5 eV. (b) As (a) for H-covered $\mathrm{GaN}\left(1\overline{1}00\right)$. (c) Calculated phonon density of states (DOS) of clean (red) and H-covered (blue) $\mathrm{GaN}\left(1\overline{1}00\right)$ surfaces. The gray shaded area depicts the projected bulk phonon DOS. Additional loss features in (b) and vibrational modes in (c) are due to Ga–H and N–H bending modes (${\delta }_{\text{Ga–H}}$ and ${\delta }_{\text{N–H}}$) with calculated energies around 116 meV. Ga–H and N–H stretching vibrations exhibit experimental (calculated) vibrational energies of ${\nu }_{\text{Ga–H}}=233$ ($\sim 231$) meV and ${\nu }_{\text{N–H}}=403$ (419) meV, respectively. The O–H bending mode appears at ${\delta }_{\text{O–H}}=206$ meV and the detected O–H stretch mode has an energy of ${\nu }_{\text{O–H}}=453$ meV. The surface excitations are indicated and related FK combination losses are marked using dotted lines of the same color.

Figure 5

Band structure of the (a) clean and (b) hydrogen-covered $\mathrm{GaN}\left(1\overline{1}00\right)$ surfaces. The gray shaded areas indicate the projected bulk band structure. Insets: Ball and stick models of the corresponding surfaces in side view. The buckling angles of the Ga–N bonds $\omega$ are indicated. In (a), the displacements $\mathrm{\Delta }x$ and $\mathrm{\Delta }z$ of the Ga surface atoms from the bulklike positions are schematically shown.

Figure 6

Difference in the Gibbs free energy $\mathrm{\Delta }G$ [Eq. (1)] of the hydrogen-covered and clean $\mathrm{GaN}\left(1\overline{1}00\right)$ surfaces as a function of hydrogen pressure and temperature. The thick contour line indicates the range of pressures and temperatures where both systems are in equilibrium. Blue (red) colors indicate smaller (larger) values. Each contour line corresponds to an energy difference of 0.1 eV per $1\times 1$ surface cell area. In the region to the left of the equilibrium line (thick black line), the hydrogen-covered surface is thermodynamically favored.

Figure 7

(Top) Energy change $\mathrm{\Delta }E$ and H–H interatomic distance $d_{\mathrm{H}–\mathrm{H}}$ along the minimum energy path for ${\mathrm{H}}_2$ adsorption at the clean $\mathrm{GaN}\left(1\overline{1}00\right)$ surface. The distance of the ${\mathrm{H}}_2$ center of mass from the surface $\mathrm{\Delta }z$ is used to represent the reaction coordinate. At $\mathrm{\Delta }z=0$, the ${\mathrm{H}}_2$ is adsorbed at the surface and the total energy at this site is used as reference for $\mathrm{\Delta }E$. (Bottom) Schematic representation in side view along $\left[11\overline{2}0\right]$ direction of ${\mathrm{H}}_2$ (i) bound to the surface, (ii) at the transition state, (iii) and in the vacuum. Large green and smaller blue balls indicate Ga and N atoms, respectively. The H atoms are denoted by the smallest red spheres.

Figure 8

Kinetic surface phase diagram of molecular hydrogen adsorption and desorption on the $m$-plane GaN surface. The hydrogen surface coverage is given as a function of time for different ${\mathrm{H}}_2$ pressures and temperatures. (I) and (II) denote clean and fully covered surface initial conditions, respectively.