Effects of Potassium Adsorption and PotassiumâWater Coadsorption

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Effects of Potassium Adsorption and Potassium-Water Coadsorption on the Chemical and Electronic Properties of n-type GaN(0001) Surfaces Vladimir Irkha, Anja Himmerlich, Stephanie Reiß, Stefan Krischok, and Marcel Himmerlich J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09512 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Effects of Potassium Adsorption and Potassium-Water Coadsorption on the Chemical and Electronic Properties of n-type GaN(0001) Surfaces Vladimir Irkha, Anja Himmerlich, Stephanie Reiß, Stefan Krischok, and Marcel Himmerlich∗ Institut für Physik and Institut für Mikro- und Nanotechnologien MacroNano, Technische Universität Ilmenau, PF 100565, 98684 Ilmenau, Germany E-mail: [email protected] Phone: +49 (0)3677 693417. Fax: +49 (0)3677 693365

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The interaction of n-type GaN(0001) surfaces with potassium and water is investigated using photoelectron spectroscopy with special focus on adsorbate-substrate charge transfer processes and water dissociation. Potassium atoms adsorb at the surface forming a distinct surface dipole layer. For very low K coverage, the attached ionized K adsorbates result in a drop of the work function and the released electrons induce a reduction of the initial upward band bending. After stabilization of both quantities in the submonolayer regime, a reverse effect is observed for higher K coverage up to one monolayer (ML), exceeding the upward band bending of the clean surface. If the K-covered surface is exposed to water, hydroxyl groups are formed, while during long K and H2 O coadsorption a potassium hydroxide film grows. In both cases a further reduction of the work function and an abrupt change in the surface depletion layer is recorded. For the coadsorption, initially an electron accumulation layer forms at the surface, approaching flat band conditions for higher KOH thickness. Overall the surface band bending can be drastically modified in the range between +0.5 and -0.6 eV. These observations clearly show that the electron density at the GaN(0001) surface can be reversibly tuned by alkali-based adsorbates. Different reactions are observed, which are directly linked to the charge transfer processes and chemical reactions induced by the K4s electrons.

Introduction Gallium nitride (GaN) is a III-V-semiconductor whose electronic properties can be significantly influenced and manipulated by interaction with ambient species. Together with its high physical and chemical stability 1,2 this makes GaN a promising material for the use in chemical or biological sensor devices. 3,4 One prominent example is the open gate AlGaN/GaN high-electron mobility transistor (HEMT) structure, 5 which is for example used for sensing the ion concentration or pH value 6 in water based solutions or concentrations in other liquid environments 7 or, after surface functionalization with organic linker molecules, as selective 2


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sensors for enzymes, proteins or DNA. 8 For better chemical and electrical stability of the sensor devices, the AlGaN/GaN active region is normally capped by a few nanometer of GaN 9 in order to prevent oxidation of the incorporated aluminum content. Other sensing concepts are based on GaN nanowire structures which enhance the specific surface of the material 10,11 or systems consisting of ZnO nanowire gated AlGaN/GaN HEMTs. 12 For sensor device characterization and optimization as well as clarification of device degradation mechanisms, a profound understanding of the interaction between the GaN surface and species in the surrounding ambient, such as gaseous molecules or ions dissolved in liquid environment, is of great interest. These aspects include phenomena such as surface reactions or in-diffusion processes in the environment of sensor operation such as e.g. surface oxidation or the interaction with different adsorbates from liquid, humid or gaseous surroundings. As a consequence, surface processes can result in activation/passivation of electronic defects, that might alter the sensor characteristics. Based on surface adsorption, dipole formation and charge transfer processes the electronic properties of the surface can be significantly modified. Several studies have experimentally investigated adsorption processes of gaseous molecules at polar GaN surfaces, such as oxygen, 13–16 water 17,18 or ammonia 19 interaction. For H2 O exposure a dissociative adsorption process and the formation of OH 17 or O groups 18 was concluded including saturation of surface states and a reduction of upward band bending. Theoretical investigations of water adsorption on polar GaN 20 and nonpolar GaN 21–23 surfaces support the energetically favored dissociation of water molecules during adsorption. The spontaneous dissociation of water at nonpolar surfaces results in a high photoelectric activity of this GaN configuration. 22,23 The surface properties of polar and nonpolar GaN surfaces as well as adsorption and interaction processes have been studied to a relatively broad extent. 24 However, there are still unanswered or only recently addressed scientific questions related to surface adsorption kinetics 25–27 and how they can be used for generation of novel functionality 28 or for the manipulation of the electronic properties. 29,30 As one example, the adsorption of alkali and

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alkaline earth metal atoms at polar GaN surfaces has also been studied for the cases of caesium (Cs) and barium (Ba). 31–38 For both alkali metals a deposition time dependent modification of the surface electronic properties was observed with a strong reduction of the work function induced by formation of an adatom induced surface dipole, which is lowering the surface barrier for electron emission. At the same time, the adatoms induce changes in surface band bending and surface/adatom states within the fundamental band gap of GaN. Both combined effects induce a negative electron affinity for the case of p-type GaN covered by Cs. 34,35 However, the reported influences of Cs and Ba adatoms on the resulting electronic structure of the surface and the related surface band alignment include partially contradicting results ranging from formation of a surface that is depleted from electrons 34 to the formation of an electron accumulation layer. 36,38 These discrepancies require an extension of the experimental efforts of alkali adsorption at GaN surfaces to pinpoint the origin of the different observations and to explore possible determining factors. We investigate the interaction of n-type GaN(0001) surfaces with potassium and water, motivated by K species in aqueous environment being especially important for biosensor applications due to their participation in fundamental cell metabolism processes, which can be characterized and monitored by GaN capped open gate HEMT sensors. 39 Changes in the chemical and surface electronic properties during the adsorption and coadsorption of K and water on epitactic GaN surfaces were characterized in-situ by photoelectron spectroscopy.

Experimental details and data analysis GaN(0001) films were grown by plasma-assisted molecular beam epitaxy on N-doped 6HSiC(0001) substrates as described in detail in Ref. 40. The growth was controlled by reflection high energy electron diffraction (RHEED) measurements, which confirm the formation of a 2 × 2 surface reconstruction after growth consisting of nitrogen atoms. 41 The GaN films are unintentionally n-type doped with the bulk Fermi level 0.1 eV below the conduction

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band minimum (CBM). 40 The clean samples were transferred to the analysis chamber under ultra-high vacuum (UHV) conditions (base pressure of the system < 2×10-10 mbar) and characterized by X-ray and UV photoelectron spectroscopy (XPS, UPS) in order to investigate the chemical and electronic properties of the as grown surfaces as well as changes in the surface properties due to adsorption/coadsorption of potassium and water. The measurements were performed in normal emission using a hemispherical electron analyzer and monochromated AlKα (1486.7 eV), He I (21.2 eV) or He II (40.8 eV) radiation for electron excitation. The description of the employed experimental conditions and parameters for PES can be found in Ref. 42. From the XPS valence band spectra the distance between valence band maximum (VBM) and Fermi-level EF at the surface was determined by linear extrapolation of the VB trailing edge to zero, leading to a value of 2.9 eV and therefore a surface upward band bending of 0.35 eV for as grown n-type GaN(0001). 41 From UPS He I spectra, measured by applying a sample bias of up to - 8 V, the work function Φ was determined by linear extrapolation of the onset of secondary electron emission to zero. As grown GaN(0001)–2×2 surfaces typically exhibit a work function of 4.2 eV and were found to be free of surface impurities within the detection limit of the performed XPS measurements (∼0.1 at.%). The variation of electronic properties of the GaN(0001) surface was monitored by UPS and XPS during continuous exposure to potassium atoms and/or water molecules. All surface interaction experiments were performed at room temperature. For potassium adsorption, a self-built evaporator equipped with a conventional alkali dispenser from SAES getters (Italy) was directly mounted to the analysis chamber facing the sample surface during real time experiments. The potassium atom flux (∼ 5 –7 ×1013 cm−2 ·s−1 ) was controlled via the applied filament current which was kept constant at 5.2 A. The samples were exposed to water vapor by backfilling the UHV chamber through a leak valve. To ensure a maximum degree of purity, bidestilled water was used and cleaned by repeated freeze-pump-thaw-cycles of the water reservoir and gas lines prior the experiments. Furthermore, quadrupole mass spectroscopy (QMS) was used to control the purity of the water vapor. The partial H2 O

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pressure was monitored by a Bayard-Alpert ionization gauge without any correction related to gas sensitivity and adjusted to 2 × 10−9 mbar. During the experiments, a partial activation of the offered H2 O molecules cannot be ruled out due to the unavoidable presence of hot filaments inside the analysis chamber. The following different reaction sequences were performed at the GaN(0001)–2×2 surface: • 75 min K adsorption (K) • 43 min K adsorption followed by 57 min H2 O exposure (K → H2 O) • 50 min K & H2 O coadsorption (K & H2 O) • 17 min H2 O exposure followed by 57 min K & H2 O coadsorption (H2 O → K & H2 O)

Results and discussion XPS analysis In Fig. 1, XPS spectra of the O1s and K2p regions measured after growth and after each individual adsorption experiment are illustrated in order to characterize the chemical nature and the quantity of the adsorbed species. For the GaN surface after epitaxy (as grown) neither an oxygen nor a potassium signal was detected. Potassium adsorption (K) as well as potassium and water coadsorption (K & H2 O and H2 O → K & H2 O) lead to the detection of a K2p signal with the K2p3/2 state at a binding energy of 294.7 eV. Furthermore, in the latter case an O1s state at ∼ 532.3 eV was measured. Such an oxygen state is typically observed at GaN surfaces exposed to molecular oxygen or water. 13,18 For experiments of potassium adsorption followed by interaction with water (K → H2 O) comparable K2p and O1s states were detected with the peaks being shifted to higher binding energies by 0.6 eV and 0.5 eV, respectively. It is further important to mention that no carbon signal was detected for all performed measurements within the sensitivity of XPS (∼ 0.1 at. %).

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XPS

0.5 eV

0.6 eV

H2O

1486.7 eV

intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


K & H2O

(a)

(b)

K & H2O

K

K2p

H2O

K as grown 120207002 120208002 120124003 120124002 120124001

O1s

536

534

532

530

528 300

298

296

294

292

binding energy (eV) Figure 1: (a) O1s and (b) K2p XPS core level spectra of the GaN(0001) surface after growth (black) and after different adsorption experiments. The corresponding Ga2p3/2 , N1s and Ga3d core level as well as valence band (VB) spectra are illustrated in Fig. 2. An overview of the determined core level binding energies and VB maxima (VBM) is given in Tab. 1. The measurements illustrate that the interaction of potassium (K) with the reconstructed GaN(0001) surface results in lower binding energy values compared to as grown GaN, while all other performed interaction experiments ended with binding energy values about 0.4 – 0.5 eV larger than the potassium-covered GaN surface. The shift of the GaN related binding energies (e.g. Ga2p3/2 and N1s states) can be explained by changes in the surface band bending induced by adsorbate-substrate interaction. These aspects will be discussed in detail below. On the other hand, several phenomena contribute the absolute binding energy (BE) values of the adsorbate electron states (e.g. O1s, K2p3/2 ) such as adsorbate induced band bending change at the GaN surface and chemical shifts due to different bonding configurations of the formed adsorbate species. Therefore, it is useful to not only consider the absolute BE of adsorbate states, but also the energy differences between substrate & adsorbate states as well as different adsorbate states. The numbers for the energy differences of the Ga2p3/2

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b in d in g e n e r g y ( e V ) 1 1 2 2

1 1 2 0

1 1 1 8

X P S

1 1 1 6

1 1 1 4 4 0 2

(a )

0 .5 e V

1 4 8 6 .7 e V

1 2 0 1 2 0 1 2 0 1 2 0 1 2 0

4 0 0 1 2 4 1 2 4 2 0 8 1 2 4 2 0 7

3 9 8

3 9 6

3 9 4

3 9 2

0 .4 e V

0 0 2 0 0 1 0 0 2 0 0 3 0 0 2

3 9 0

(b )

G a 2 p

3 /2

in te n s ity ( n o r m a liz e d )

→→

N 1 s + G a (L M M )

(c )

0 .4 e V

K

(d ) 0 .4 e V

K & H H 2

O

2

in tr a - g a p s ta te s

a s g ro w n K H 2O O

K & H

2

O

N 2 s

K 3 p

G a 3 d

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2 4

2 2

2 0

V B 1 8

1 6

1 2

1 0

8

6

4

2

0

b in d in g e n e r g y ( e V )

Figure 2: (a) Ga2p3/2 , (b) N1s, (c) Ga3d core level and (d) VB spectra of the GaN(0001) surface after growth (black) and after different adsorption experiments. and K2p3/2 as well as the O1s and K2p3/2 states are also given in Tab. 1. Consequently a difference exists between the characteristics between the experiment of subsequent water exposure (K → H2 O) compared to the two codeposition experiments. The energy differences of the K2p3/2 and O1s states are equal, indicating a comparable chemical bond between K atoms and the oxygen-containing bond partner. However, while the energy offset between the Ga2p and K2p states is the same for the surface directly after K adsorption and after additional water exposure, the relative values after the codeposition experiments are actu-

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Table 1: Ga2p3/2 , O1s, N1s, K2p3/2 and Ga3d core level binding energies, energy differences and valence band maxima based on XPS analysis of the differently prepared GaN(0001) surfaces. All values are given in eV. preparation as grown K K → H2 O K & H2 O H2 O → K & H2 O

Ga2p3/2 1118.3 1118.1 1118.6 1118.6 1118.6

O1s – – 532.8 532.3 532.3

N1s 398.0 397.8 398.2 398.15 398.2

K2p3/2 – 294.7 295.3 294.7 294.7

Ga2p3/2 – K2p3/2 – 823.4 823.3 823.9 823.9

O1s – K2p3/2 – – 237.5 237.6 237.6

Ga3d 20.5 20.3 20.7 20.65 20.7

VBM 3.0 2.8 3.3 3.3 3.3

ally 0.5 – 0.6 eV higher. Reasons for this effect could be the different chemical nature and geometric microstructure within the adsorbate layer and possible further internal dipoles and polarization fields that cause variations for the two cases. As will be clarified below, the water molecules dissociate and hydroxyl groups attach at the surface. In the experiment K → H2 O the chemical nature of the adsorbates could be interpreted in terms of partial saturation of the K-covered GaN surface by hydroxyl groups. When the GaN surface is simultaneously exposed to K and H2 O (codeposition), the adsorbate signals (K2p & O1s) exhibit much higher intensity, indicating the formation of a thicker potassium hydroxide layer with bulk-like KOH atomic arrangement. The adsorbate thicknesses of the K or KOH adlayers were determined using the two-layer (GaN substrate & adsorbate layer) model described in the supporting information. For the data analysis we also considered the experimental results of the measured Ga3s and Ga3p spectra (not shown), since the Ga3d semi-core level is partially superimposed/hybridized with the N2s states. 43 Fig. 3 illustrates the experimentally determined XPS core level area ratios (Ga2p, K2p, K3p and O1s level signals divided by the Ga3p reference signal) as datapoints together with the results of the coverage-dependent adlayer model as solid or dashed lines. Obviously, such calculations include several assumptions that might not cope with the real adatom structure. However, the comparison for different core levels, either by considering the attenuation of the substrate signal or the emission from the adsorbate, reveal a fairly good agreement in the resulting adlayer thickness (compare abscissa value for each experimental datapoint in Fig. 3 given by different symbols (∗, +, ×, ?)). For the K adsorption experiment (∗) we estimate a layer thickness slightly below one monolayer. The subsequent 9



1 0

X P S c o r e le v e l a r e a r a tio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

tw o -la y K K K K K K K 1

e r m o G : K : K O H : O O H : K O H : K O H : G :

d e a 2 2 p 3 p 1 s 3 p 2 p a 2

l p / / G / G / G / G / G p /

G a a 3 a 3 a 3 a 3 a 3 G a

e x p e r im e K a d K a d K & H 2O

3 p p p p p p

n ta l s o rp s o rp H 2O a d s

d a tio tio c o o rp

ta n

n

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→→

H 2 O a d s o r p tio n a d s o r p tio n tio n K & H 2 O c o a d s o r p tio n

3 p

0 .1

0 .0 1 0 .0

0 .5

1 .0

1 .5

2 .0

2 .5

3 .0

3 .5

4 .0

a d la y e r c o v e r a g e ( m o n o la y e r s - M L )

Figure 3: Theoretical Ga2p, K2p, K3p, and O1s core level intensities normalized to the Ga3p intensity in dependence of adsorbate layer thickness based on a two-layer model consisting of a K or KOH adlayer on top of the GaN substrate. The experimentally determined numbers are given by symbols to estimate the adlayer thickness by comparison with the x-axis. H2 O exposure (+) does not significantly change the situation inducing adsorption of oxygencontaining species (hydroxyl groups) at the outermost surface. In contrast, for the two K & H2 O coadsorption experiments (×, ?) the resulting KOH layer thickness is ∼ 3.0 ML for the sequence without previous water saturation and ∼ 3.5 ML for the measurement that started with an initial H2 O exposure. This difference correlates well with the slightly different codeposition time for the two experiments, indicating that under these conditions a monotonic increase of the KOH adlayer thickness occurs.

UPS analysis In order to further analyze the chemical nature of the forming adlayers as well as their electronic properties, UPS measurements have been performed. A comparison of He II spectra is shown in Fig. 4 for different surface preparations. Since both coadsorption experiments 10


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resulted in identical spectra, only one example is shown, while an additional spectrum after saturation of the GaN(0001) surface with water is added for comparison. On the left side

s u rfa c e s ta te

N 2 s

G a 3 d ( H e I I β)

2 .0 5 e V

a s g ro w n

G a 3 d

G a 3 d

5 /2

3 /2

0 7 0 7 0 3 0 0 5 .u p s

1 2 0 1 2 4 0 0 6 .u p s

E F

× 1 0


K 3 p

K

in tr a - g a p s ta te s

in te n s ity ( a r b . u n its )

0 7 1 0 3 1 0 0 4 .u p s

HH 22 O G a 3 d ( H e I I β)

→

5 .1 e V

O 1

O 2

1 2 0 3 1 4 0 1 0 .u p s

K 3 p ( H e I I β)

HH 22 O


K

4 .0 e V

1 πO H

3 σO H

1 2 0 2 0 7 0 0 8 .u p s

K 3 p ( H e I I β)

KK && HH 22 O G a 3 d ( H e I I β)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 .0 e V

1 πO H

3 σO H

2 2

2 0

1 8

1 6

1 4

1 2

1 0

8

6

4

2

0

b in d in g e n e r g y ( e V )

Figure 4: Comparison of the He II UPS spectra of the GaN(0001)–2×2 surface prior and after different surface interaction experiments (K adsorption, H2 O exposure, K → H2 O, K & H2 O coadsorption). Left: Ga3d, K3p and N2s region after subtraction of a Tougaardtype background and contributions from intra-gap surface states of the He I excitation, right: valence band region after subtraction of spectral contributions from He IIβ,γ satellite lines above the VBM.

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of the spectra, the high energy region between 23 and 14.5 eV is shown after removal of the signal background and fitting the spectral distributions. The Ga3d semi-core level with its maximum at ∼ 20.3 eV for the as grown sample is composed of the Ga3d3/2 and Ga3d5/2 states with a spin orbit splitting of 0.45 eV. These states are partially hybridized with the N2s state, 43 which is mainly located at ∼ 16.9 eV. After depositing K at the surface, a third feature arises around 19 eV, which is due to emission from the K3p state. The relative intensities of these electron states confirm the results of the film thickness calculations above and the GaN electron states shift in the same direction and by the same quantity as found in the XPS spectra. The valence band region is plotted on the right side of Fig. 4 and allows analyzing characteristic adsorbate features, GaN electron states, as well as important electronic features at the surface. The VB spectrum of the GaN(0001)–2×2 surface consists of electron states of the GaN VB structure and a well pronounced occupied surface state at 2.05 eV caused by the nitrogen adatom structure. 41 After adsorption of ∼ 1 ML of K on top, the electron binding energies of the VB signatures and the Ga3d states are reduced by 0.15 eV, the K3p state emission is clearly detected close to the Ga3d level and the VB spectrum includes only minor, but important changes. Comparable with the situation for K/InN(0001) 44 we observe that the surface state emission is quickly diminished and subsequently a broad distribution of intra-gap states above the VBM develops that extents to ∼ 100 meV below the Fermi level (EF ). They are visible in UPS (Fig. 4) and XPS (Fig. 2 (d)) valence band spectra after K deposition. Exposure of the as grown surface to water shifts the valence band electron states and the Ga3d states to ∼ 0.4 eV higher binding energies. Furthermore, an additional contribution between the Ga3d and N2s states is detected and two new features are found in the VB region at 10.8 and 5.7 eV, while the initial surface state emission disappeared. These two signatures (O1 and O2) with an energy separation of 5.1 eV were assigned to oxygen-based adsorbate states from the O2p level that are induced by a dissociative adsorption of water molecules at the bare GaN surface. 18 Their spectral fingerprint differs from the valence band spectra

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for the K → H2 O and the K & H2 O codeposition experiments. After these processes, two distinct states at ∼ 10.0 eV and ∼ 6.0 eV are found in the valence band region. The energy offset of 4.0 eV and the relative peak areas are direct hints for the existence of 3σ and 1π OH states 45,46 proving the formation of hydroxyl species at the surface. A further important observation is that in this case no occupied states above the VBM are detected by UPS, which differs from the scenarios after growth or K adsorption. In addition, consistently with the observation in the XPS spectra, a difference in the relative binding energies between the GaN electron states and the adsorbate states is found, if the influence of different band alignment is considered in the analysis. For the GaN(0001) surface after K adsorption (K) and after subsequent water adsorption (K → H2 O), the binding energy difference between the Ga3d and K3p states is 1.3 eV, indicating a comparable chemical bond configuration between substrate and K adatoms, while the energetic separation is considerably larger (1.9 eV) for the coadsorption processes, representing the same offset of 0.6 eV between substrate and adsorbate core levels as in the XPS measurements. Summarizing all observations discussed to this point, there is clear evidence that for each adsorption experiment the resulting adsorbate layer exhibits a unique chemical nature. For single potassium exposure, up to 1 ML of K can be deposited onto the GaN(0001) surface at room temperature. A subsequent exposure to H2 O molecules leads to their dissociation and adsorption of OH groups until the surface is saturated, while codeposition of K & H2 O at room temperature leads to the formation of a continuously growing KOH-like film.

Work function and surface band bending analysis The UPS measurements were additionally utilized to analyze the work function Φ and the resulting numbers are included in Tab. 2. The GaN(0001)–2×2 surface directly after growth exhibits a work function of 4.2 eV, which is drastically reduced to around 1.8 eV for all considered adsorption sequences involving potassium. Obviously, a strong tendency of 4s – electron transfer from the K atoms towards the surface exists, as typically found for K adsorption 13



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on metal and semiconductor surfaces. 47,48 Keeping the strong work function variation in mind, we now reconsider the changes in substrate core level binding energies, which are a direct measure of surface band alignment modifications. For all experiments we observe a comparable shift of the core levels and VBMs in the UPS spectra as for the XPS measurements (compare to Fig. 2 and Tab. 1). Since the UPS measurements allow a better energy resolution we use the shift of the Ga3d state after peak fitting as well as the VB states of the He II spectra for calculation of the changes and absolute numbers of band bending (∆Vbb & Vbb ). The values are listed in Tab. 2. The as grown surface typically exhibits an upward band bending of 0.35 eV. 40,41 After adsorption of ∼ 1 ML potassium the core level binding energies (referred to EF ) were reduced by 0.15 eV, equivalent to a further increase of the upward band bending to 0.5 eV. All other experiments of adsorption and coadsorption (K → H2 O, K & H2 O, H2 O → K & H2 O) show an opposite trend: an increase in binding energy by ∼ 0.4 – 0.5 eV leading to a flattening of the surface band curvature, i.e. removal of the surface/interface depletion layer. Based on the experimentally determined values of the variation in band bending ∆Vbb and work function ∆Φ, it is possible to calculate changes in the effective surface dipole potential ∆Φdip = ∆Φ – ∆Vbb (dependent on the chosen sign convention for up and downward band bending) during adlayer growth (see Tab. 2). The largest surface dipole induced energy change is found for pure potassium adsorption (∆Φdip = -2.5 eV). In this case, the adsorbed K atoms are partially ionized having transferred a portion of their 4s electron density towards Table 2: Experimentally determined values of work function Φ and variation of the surface band alignment ∆Vbb and the calculated surface dipole ∆Φdip of the differently prepared GaN(0001) surfaces based on analysis of the UPS measurements. All values are given in eV. preparation as grown K K → H2 O K & H2 O H2 O → K & H2 O

Φ 4.20 1.85 1.83 1.79 1.75

∆Φ – -2.35 -2.37 -2.41 -2.45

14

∆Vbb – 0.15 -0.38 -0.44 -0.50


Vbb 0.35 0.50 -0.03 -0.09 -0.15

∆Φdip – -2.50 -1.99 -1.97 -1.95

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the GaN substrate. The resulting electrical dipole field between positively charged K species and the outermost GaN layer lower the energy barrier for electron emission. After OH formation on top of the K adlayer or deposition of a thicker KOH film on the GaN surface, in both cases the resulting surface dipole potential is lower (∆Φdip ∼ -2.0 eV).

Coverage dependence of electronic properties The already discussed results illustrate that charge transfer processes occur during K adsorption or K & H2 O coadsorption, but they only give a snapshot of the properties after finalization of the film deposition experiments. Continuous spectral monitoring of the interaction processes allows the characterization of the reaction and charge exchange dynamics. The evolution of the Ga3d and K3p spectral region in the He II spectra as well as the valence band spectra excited by He I and He II radiation for all 4 model adsorption experiments are presented in detail in Figs. S1 – S4 of the supporting information. We have analyzed these data sets, that were continuously acquired during all performed adsorption experiments, by background subtraction, peak fitting and linear extrapolation of emission onsets as described above. Special care was taken to differentiate between He I excited intra-gap states formed during K adsorption that superimpose the high binding energy region of the He II spectra and hence appear as side feature of the Ga3d state (see detailed information in the supporting information). These analyzes allowed obtaining coverage-dependent information on the surface band bending – being clearly indicated by the synchronous energy shifts of the semi-core levels and valence band signatures – and work function changes, and to extract qualitative and semi-quantitative information on the adsorption processes and chemical surface properties. The binding energy shifts of the Ga3d state (excited by He IIα as well as He IIβ radiation) and the VB states during the adsorption experiments are used to determine ∆Vbb during continuous adsorbate interaction. Furthermore, the temporal dependence of the work function was evaluated. Both quantities ∆Vbb (t) and Φ(t) are plotted in Fig. 5 together with the 15



calculated value for the change in surface dipole potential ∆Φdip (t). Remarkable variations in the surface electronic properties are identified that are worth to be analyzed in detail. For better visualization of the changes in Vbb and Φ, the parameter variation ranges during each individual adsorption experiment are highlighted as band diagrams in Fig. 6. Especially the possibility to tune the surface bands from 0.5 eV upward (electron depletion) to -0.6 eV downward bending (electron accumulation) is an important result of these experiments. Let us first consider the changes induced by pure K interaction. Directly after starting the deposition, Φ drops drastically from 4.2 eV striding a minimum of 1.75 eV after ∼ 13 min before slightly increasing again to a plateau value of 1.95 eV after ∼ 22 min. For longer K exposure a further slight reduction by 0.1 eV is observed. Such a progression is known from studies of alkali atoms at metal 47 and semiconductor 48 surfaces and is a result of a coverage dependent charge transfer from the alkali atoms to the substrate. The first K atoms attaching to the surface completely ionize and donate their 4s electron to the substrate forming a surface dipole. For higher coverage, the ionized adatoms interact with each other resulting in depolarization effects that lower the effective charge that is transferred per adatom. 48 At this point, which is typically found at a coverage around half a monolayer, the increased interaction and decreased distance between K atoms force the electrons at the interface to redistribute to achieve equilibrium. Some electrons return to K adatoms resulting in partially occupied 4s states, which decreases the local dipole moments between K atoms and the GaN(0001) surface. These processes result in a slight re-increase of the work function while the accumulated surface dipole potential remains constant, as previously discussed for Cs/GaN by Du et al. 49 In their study, the authors calculated the Cs coverage for minimum work function and found it to be 0.5 – 0.75 ML in dependence of the considered adsorption site. At room temperature, the interaction of K adatoms within the polarization fields at the surface lead to a limitation of the maximum adlayer thickness to one monolayer, in agreement with our thickness analysis. Such a deposition dependence of the work function was also reported experimentally for Cs alkali metal adsorption at GaN surfaces. 33,34

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25 min after deposition stop

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Figure 5: Temporal evolution of the surface band bending Vbb (top), work function Φ (middle) and change in surface dipole potential ∆Φdip (bottom) of the GaN(0001) surface during the different adsorption experiments. The arrows and numbers indicate changes in the surface interaction process: (1) start of K deposition, (2) start of H2 O exposure and (3) stop of K deposition followed by start of the H2 O exposure. The dotted data points on the right correspond to values extracted from measurements performed with better statistics, which represent the surface properties in UHV ∼ 25 min after finalization of the respective surface adsorption experiment.

17



Initially, the upward band bending of the clean surface is reduced to a minimum of Vbb slightly above flat band conditions (0.1 – 0.15 eV) close to the stage of the adsorption experiment (i.e. the same K coverage) at which the work function minimum is observed. For further increasing K coverage, the tendency is inverted and Vbb gradually re-increases to a saturation value of ∼ 0.5 eV as the K adatom density is slowly enhanced to the saturation coverage of ∼ 1 ML at the end of the experiment (Fig. 5). It is important to highlight that there is a temporal correlation between the changes of these two independent experimental parameters – work function and band bending. The effective surface dipole potential is plotted at the bottom of Fig. 5. For K adsorption, two regimes with almost linear variation and a transitional phase are identified. For low K coverage, the initial steep linear decrease indicates that each K atom transfers its 4s electron to the surface and is therefore fully ionized, forming a positively charged adatom layer and a strong surface dipole. In the regime of higher K coverage, the slope of ∆Φdip (t) is ∼ 40 times smaller, clearly indicating that for further K uptake, the transferred effective electron charge per atom is significantly lower due to the reasons discussed above. After deposition of ∼ 1 ML K the strongest effective dipole energy is measured (∆Φdip = -2.5 eV). We want to point out that the observed coverage-dependent variation during K adsorption differs from other reported experiments of alkali adatoms on n-type GaN surfaces. For Cs adsorption at 150 K, Eyckeler et al. also report upward band bending but found the opposite trend of initial increase in electron depletion followed by a reversal to the initial value for higher coverage. 34 In contrast to this early study and our results, Benemanskaya et al. 36,38 indirectly concluded the formation of an electron accumulation layer and strong downward band bending during the adsorption of Ba or Cs from photoemission threshold measurements without analysis of the surface chemistry. For the current results related to K adsorption, it is remarkable that the upward band bending for higher potassium coverage (0.5 eV) exceeds the initial value of the clean surface (0.35 eV). In other words, although K adatoms donate electron charge to the surface, the depletion of electrons is enhanced for higher K coverage, while in the initial

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E Evac 4.2 Dfdip = - (2.2 - 2.5) eV

f (eV)

Dfdip ~ - (2.0 - 2.3) eV Dfdip = - (1.8 - 2.0) eV

1.75 - 1.95 1.5 - 1.8 ~1.6

CBM

EF= 0 eV Egap = 3.4 eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


n-type GaN(0001)-2×2 +K + K H2O + K & H2O

VBM

bulk

surface

VBB 0.1 - 0.5 eV 0.35 eV -0.05 - 0 eV -0.6 - 0 eV

Figure 6: Schematic diagram including the variation range of surface band alignment and work function for as grown GaN(0001) (black) during K adsorption (red), subsequent water interaction (blue) or K & H2 O coadsorption (green). Please note that the extremum of downward band bending for the coadsorption experiment (-0.6 eV) is observed at the initial stage of the experimental sequence (compare to Fig. 5). stage of K adsorption, the donor-like behavior and electron transfer towards the surface is reflected in a reduction of the surface electron depletion. This tendency is complex and cannot be understood by only considering core level shifts. Therefore, in the next step we concentrate on changes in intra-gap surface and adsorbate states which might give a hint for the origin of the change in surface Fermi energy and related band bending variations. For Cs and Ba adsorption Benemanskaya et al. point out that the interaction between adatoms and Ga-dangling bonds are important aspects that lead to gradual filling of empty surface states and a transformation of empty states to filled 19



ones, which affects the surface band alignment. This effect was also predicted by density functional theory calculations for Cs/GaN(0001). 49 In our case, the band bending at the GaN(0001)–2×2 surface after growth is mainly determinded by occupied surface states that are located 0.95 eV above the VBM as well as unoccupied surface dangling bond states below the CBM. 41 Together with unoccupied defect states, also located near the CBM, they are responsible for the electron depletion and the interconnected surface upward band bending observed at clean GaN(0001)–2×2 surfaces. After the start of K deposition, the occupied surface states are found to quickly disappear in the same period as Vbb is being reduced. We anticipate that by adsorption of K atoms, a saturation and filling of the unoccupied surface dangling bond and defect states occurs. Consequently, the electron depletion is repealed during the initial stage of K adsorption and the bands tend towards flat band conditions. At the stage when the initially unoccupied surface states are completely filled, their influence on the electronic surface properties is neutralized and potassium adatominduced electron states shape the electronic structure of the surface. The re-increase in Vbb occurs at that stage of K deposition, at which the additional attaching K adatoms do not significantly further alter the surface dipole potential, i.e. when the effective charge transfer from the 4s electrons towards the surface per additional adatom is strongly reduced. These remaining filled K4s electron states induce electron density close to the Fermi level which can directly interact with the GaN surface band structure forming adsorbate-induced electron states within the fundamental band gap. In consistence, the formation of K adatominduced occupied electron states above the VBM are detected (see Fig. S4 of the supporting information for spectral details), which initially form a well pronounced peak around 1.3 eV and subsequently spread into the gap region for ongoing K deposition. After adsorption of ∼ 1 ML of K, these electron states range from the VBM to 100 meV below the Fermi energy (see "intra-gap states" in Figs. 2 (d) and 4). Similar K-induced intra-gap states were already detected for the K/InN(0001) interface. 44 An important indicator is that these features do not reach up to EF , which could elucidate reasons for the re-increase of upward band bending

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at higher K coverage. If we consider occupied and unoccupied intra-gap states induced by K adsorption, they might change their energy within the band gap in dependence of K adatom density. The possible formation of new K adatom states at a certain coverage could induce transfer of GaN conduction band electrons into these states, again causing a depletion of electrons in the GaN near-surface region. This trend might proceed to the point where the empty states are filled up to the Fermi energy resulting in a metal-like behavior of the surface. As expected, for the K → H2 O experiment the progressions of ∆Φ(t), ∆Vbb (t) and ∆Φdip (t) are identical to those of the K adsorption experiment up to the point where the K evaporation is stopped and H2 O is offered. At this particular time the upward band bending as well as Φ both drop quickly by 0.4 – 0.5 eV (Fig. 5). While the former value stays constant slightly below Vbb = 0 eV for longer H2 O exposure, indicating nearly flat band conditions, the dip in Φ is followed by an asymptotic re-increase to ∼1.8 eV. The surface dipole change ∆Φdip follows this trend and increases nonlinearly to -2 eV, which is the quantity that is also found after K and H2 O coadsorption. As will be discussed in detail in the next section, the adsorption of H2 O at the ∼ 1 ML K/GaN(0001) surface is characterized by the immediate formation of passivating hydroxyl groups at the surface. At the same time, the K adlayer induced intra-gap states immediately disappear (see Fig. S4 of the supporting information). We anticipate that the residual quasi-free 4s electrons of the K adatoms force dissociation of the H2 O molecules and formation of OH− groups at the surface. Consequently, due to the formation of K–OH surface bonds, Φdip is smaller compared to the pristine K adlayer and the GaN bands underneath tend towards flat band conditions to balance out the charge transfer processes at the surface. For discussion of the variation during coadsorption, it is best to first consider the effect of bare H2 O interaction. In general, the adsorption process is characterized by dissociative water interaction, comparable to earlier studies. 17,18,20 As visible in Fig. 5, if the GaN(0001)– 2×2 surface is initially interacting with water (first 17 min of H2 O → K & H2 O experiment),

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the work function change and the surface dipole follow an opposite trend compared to K interaction. Both values (∆Φ and ∆Φdip ) have positive sign due to dissociation of water and adsorption of negatively charged oxygen species. At the same time, the surface states of the GaN(0001)–2×2 structure are saturated and Vbb is reduced by 0.45 eV (Fig. 4) and stabilized slightly below flat band conditions. 18,50 This effect is finalized after exposure of ∼0.6 L H2 O (7 min at 2 × 10−9 mbar) resulting in constant conditions for further interaction experiments. If potassium atoms are added to the surface reaction (beyond 17 min of the H2 O → K & H2 O experiment), the bands rapidly bend further downward by 0.5 eV in the initial period of K & H2 O codeposition. The direct coadsorption of potassium and water on the as grown GaN surface (K & H2 O) shows an identical trend and results in a strong decrease of Vbb for low coverage with a maximum change of ∆Vbb ∼ -0.95 eV. This implies a severe shift of the surface Fermi level for low K & H2 O exposure, i.e. a transition from electron depletion to strong electron accumulation at the GaN surface with a downward band bending of about Vbb ∼ -0.6 eV for this stage of very low coverage. After passing this sharp minimum ∆Vbb and thus Vbb both follow an asymptotic re-increase (∆Vbb = -0.5 – -0.4 eV in both experiments), i.e. slowly approaching flat band conditions but remaining in the slight electron accumulation range. In both coadsorption sequences, Φdip as well as Φ drop immediately after starting the adsorbate exposure and they stabilize after a period of 8 min at values of -2 eV and 1.6 eV, respectively. Compared to the pure K adsorption, the effective surface dipole potential is reduced (-2.0 eV for K & H2 O vs. -2.5 eV for K) while the absolute work function is lowest in the case of coadsorption. A similar observation of further work function reduction was reported in studies of Cs and Cs/O adsorption on GaN, where the addition of oxygen resulted in a decrease of the barrier for electron emission. 35 This tendency was recently utilized to increase the emission current of GaN photocathode devices. 51 Based on first principles calculations the authors of this study find a superposition of dipole moments that lower the work function, if O is incorporated into the Cs layer. A similar scenario could explain the

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observation of lowered work function for the K & H2 O codeposition experiments as well as the drop of Φ for water adsorption at the K-covered surface. For both codeposition experiments (K & H2 O, H2 O → K & H2 O), we observe a saturation of the occupied surface states at 2.05 eV within the very initial stage of the experiment. In other words, the band gap region between the GaN VBM and EF is cleared out from electron states during interaction with pure water or during K & H2 O codeposition. The same tendency is observed for the K-induced intra-gap states if H2 O is added to the ∼ 1 ML thick potassium layer on the GaN(0001) surface. At the same time, the substrate band alignment tends towards flat band conditions for all these cases of hydroxyl formation at relatively high coverage: the adsorbed adlayers induce a balancing of the Fermi level across the heterostructure and the elimination of space charge layers at the interface. However, this is not the case for the very initial stages of K and H2 O coadsorption. At this period of the experiments we do not find any indications of electron states within the band gap that could explain the observed generation of the surface accumulation layer, which is a clear surprise.

Coverage dependence of chemical properties Considering chemical changes upon K adsorption, the K3p state in the He II spectra can be analyzed with high surface sensitivity. In addition, if hydroxyl groups are formed at the surface, they can be sensitively identified by the occurrence of their spectral fingerprint signatures (3σ and 1π OH states – compare to Fig. 4) in the UPS spectra. Based on the measured He II semi-core level and valence band spectra sequences (Figs. S1 & S3), a quantitative analysis of the adsorbate and GaN substrate signals was performed based on peak fitting. The resulting K3p / Ga3d and 1π OH / Ga3d peak area ratios are presented in Fig. 7. For potassium deposition (K), it is found that the uptake of K is nonlinear. The quick increase in the K3p / Ga3d intensity ratio within the first 10 minutes is decelerated beyond this point and only a slight further asymptotic increase of the K adatom density to ∼ 1ML is observed (see quantification above). The sticking coefficient of alkali atoms is initially almost 23


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25 min after deposition stop

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

adsorbate (K3p or 1π OH) / Ga3d signal area ratio


K3p / Ga3d

1π OH / Ga3d K K → H2O K & H2O H2O → K & H2O 0

10

20

30

40

50

60

70

80

90

100

Figure 7: Deposition time dependent variation of the K3p / Ga3d (solid lines) and 1π OH / Ga3d (dashed-dotted lines) signal ratios for the performed adsorption experiments (from analysis of the spectra shown in Fig. S1 of the supporting information; please note the logarithmic ordinate scale). The dotted data points on the right represent values extracted from measurements performed with better statistics (see Fig. 4), which represent the surface properties in UHV ∼ 25 min after finalization of the respective surface adsorption (indicated by the triangles). equal to one due to the high reactivity of the impinging K atoms. It reduces drastically at that moment when the K coverage exceeds the value at which the ionized K species at the surface start to interact with each other, since at the same time the adsorption energy of the alkali adatoms is typically lowered 52 and changes in the electronic properties are induced. If the K-covered surface is subsequently exposed to H2 O molecules (K → H2 O), a further increment in the K3p / Ga3d signal ratio is observed, which is due to a further attenuation of the Ga3d signal combined with an increase in the K3p emission intensity (Fig. S1). This trend should not be interpreted as an addition of potassium from the K dispenser, since the source was switched off at this point. In addition, the progress of the 1π OH / Ga3d peak area ratio (Fig. 7 blue dashed-dotted line) clearly indicates that water dissociation and 24


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saturation of the surface by hydroxyl group adsorption instantaneously occurs at the ∼ 1 ML K covered surface upon begin of H2 O exposure. Several possible mechanisms, which cannot be clarified by the performed experiments, can contribute to the observed increment of the K3p/Ga3d signal ratio: although the K adsorption seems to be saturated at a coverage of ∼ 1 ML, (i) the addition of OH groups at the surface might induce a redistribution of nonhomogeneously adsorbed K atoms and/or (ii) the additional adspecies inherently further cover and saturate surface sites. (iii) It might also be possible that K species have diffused into the subsurface region of the GaN layer and occupied interstitial sites. The changes induced by H2 O exposure could have induced a stimulus to diffuse back to the surface and combine with the forming hydroxyl groups. Upon start of the K & H2 O experiment, the shape of the He II VB spectra directly transfers to the signature of a water pre-exposed GaN surface with O1 and O2 states of the dissociation products (see Figs. S2 and S3 and discussion of Fig. 4), indicating water-rich adsorption conditions. Therefore, we conclude that dissociation products of water molecules at the GaN surface together with the impinging K atoms induce a collaborative effect with drastic changes of the electronic properties as discussed in the previous section. Consequently, in the very beginning of the coadsorption process, the initial K atoms fully ionize by transferring the electrons to the GaN film and the additional exposure to water seems to enhance the effect of electron transfer towards the GaN substrate, leading to the strong downward bending of the valence and conduction bands in the accumulation regime and the even more rapid decrease of Φ and Φdip compared to K deposition within the first 5 min of K & H2 O codeposition, which can be interpreted in terms of a higher effective flux of reactive species (K and H2 O). However, a microscopic model of adsorption on atomic scale cannot be developed solely based on the spectroscopic results. The interaction processes with alkali atoms and water have been studied in very detail for titanium dioxide surfaces. 53,54 In these earlier works, it was observed that hydroxyl formation only occurs if the residual 4s electron density of the alkali atoms can be used for dissociation

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of water molecules. For both coadsorption experiments on GaN (K & H2 O and H2 O → K & H2 O) a continuous uptake of potassium is observed. Although the surface is also exposed to water molecules from the very beginning of these coadsorption experiments, the formation of hydroxyl (OH) groups starts slightly delayed with respect to the start of K adsorption (see Figs. S2 and S3 of the supporting information). The onset of a detectable OH signal coincidences with stage of the coadsorption experiment at which Φ and Φdip stabilize at their lower values and the band bending reached its extremum in the accumulation regime (compare to Fig. 5). This might be interpreted in a way that water dissociation starts when the 4s electron density is no longer completely transferred to the GaN substrate, comparable to the observations for TiO2 surfaces. Beyond this point, both the K3p and 1π OH signals increase consistently indicating the reaction of K with dissociating water molecules that result in the ongoing formation of a thin KOH layer of a few monolayers. The growth rate is found comparable and almost constant in both experiments, suggesting continuous film growth.

Conclusions At room temperature, potassium adsorption and K & H2 O coadsorption at the clean GaN(0001) surface initially lead to the saturation of surface dangling bond states accompanied with drastic changes in the interface properties. The chemical reactions and charge transfer processes are induced by the strong affinity of the GaN surface to assimilate electrons from surrounding species or adsorbates and by the tendency of K atoms to donate their weakly bound 4s electron. Electron transfer towards the highly reactive GaN surface is initially preferred at low K coverage, even if the GaN surface was pre-covered by the typical dissociation products of water interaction. At this stage of the reaction, the K adatoms and the dissociating H2 O molecules act independently (not combining to KOH) but collectively (intensifying the effect on the electronic properties) as adspecies at the semiconductor surface. At the character-

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istic K coverage of ∼ 0.5 ML, the electron charge transfer process towards the substrate is drastically diminished. The remaining quasi-free electrons are in a subsequent period either accumulated as intra-gap states or, if water is involved, they prefer to induce the formation of hydroxyl groups. The K adlayer thickness is limited to one monolayer, which is saturated by OH groups during subsequent H2 O interaction. During K & H2 O coadsorption, a potassium hydroxide overlayer continuously grows to thicknesses beyond the monolayer regime. The electronic properties of the n-type GaN(0001) are strongly influenced by potassium, hydroxyl adsorbates and potassium hydroxide that induce charge transfer processes at the interface between substrate and adsorbate. The surface band alignment can be varied within a range of 1.1 eV, i.e. from 0.5 eV upward band bending for ∼ 1 ML K, exceeding the value for the clean surface, to -0.6 eV downward bending for the early stages of K and H2 O coadsorption. In both cases, a strong surface dipole is formed, lowering the barrier for electron emission. These results provide possible routines for specific manipulation of GaN(0001) surface properties by potassium based reactions.

Supporting Information Sequences of He I and He II UPS spectra acquired during the four model adsorption experiments K, K → H2 O, K & H2 O and H2 O → K & H2 O & mathematical model for the determination of the adsorbate layer thickness from quantitative XPS core level analysis (GaN_K_and_H2O_SI.pdf ).

Acknowledgement We are grateful for financial support by the Carl-Zeiss-Stiftung and the German Academic Exchange Service (DAAD).

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TOC Graphic H 2O

K

-

+

++

-

-

+ ++ GaN(0001)

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Effects of Potassium Adsorption and PotassiumâWater Coadsorption

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Effects of Potassium Adsorption and PotassiumâWater Coadsorption