Interfacial Polarization of Thin Alq3, Gaq3, and Erq3 Films on GaN(0001)

This report presents results of research on electronic structure of three interfaces composed of organic layers of Alq3, Gaq3, or Erq3 deposited on GaN semiconductor. The formation of the interfaces and their characterization have been performed in situ under ultrahigh vacuum conditions. Thin layers have been vapor-deposited onto p-type GaN(0001) surfaces. Ultraviolet photoelectron spectroscopy (UPS) assisted by X-ray photoelectron spectroscopy (XPS) has been employed to construct the band energy diagrams of the substrate and interfaces. The highest occupied molecular orbitals (HOMOs) are found to be at 1.2, 1.7, and 2.2 eV for Alq3, Gaq3, and Erq3 layers, respectively. Alq3 layer does not change the position of the vacuum level of the substrate, in contrast to the other layers, which lower it by 0.8 eV (Gaq3) and 1.3 eV (Erq3). Interface dipoles at the phase boundaries are found to be −0.2, −0.9, −1.2 eV, respectively, for Alq3, Gaq3, Erq3 layers on GaN(0001) surfaces.


Introduction
Gallium nitride (GaN) is a very attractive semiconductor for applications in optoelectronics and photovoltaics. It is also used for creating high-power and -frequency devices [1,2]. GaN is one of the most commonly used materials for fabricating devices in the mentioned electronic areas. This occurs due to its good physicochemical properties, such as a direct and wide band gap, high thermal conductivity, and thermal stability. The III-nitrides semiconductors also have a potential in creating three-dimensional hybrid organic/inorganic electronic devices, such as organic light-emitting diodes (LEDs), organic field-effect transistors (OFETs), or biosensors [3,4]. The use of organic semiconductors from the Mq 3 chelate group (M-trivalent metal, q-8-hydroxyquinoline) as an active element of electronic devices has been known for years. It began with Tang's report, which showed potential possibilities of Alq 3 [5]. In recent times, Mq 3 molecules seem to have been great candidates for applying to a light-emitting and/or electron-transporting material in hybrid technology. This is because of the merging of the high charge carrier mobility and efficient charge injection of inorganic semiconductors with the strong light-matter coupling and large chemical composition diversity of organic semiconductors [6][7][8]. Mq 3 complexes are promising materials for sensor and biosensor applications, the main reason is their ability to interact with a wide range of analytes, such as p-nitroaniline, NO 2 , ethanol, and methanol [9]. Another relevant application is the incorporation of Mq 3 compounds to improve OLED's device design [10]. Lately, Erq 3 was used to exceed the limitation of the exaction production efficiency of NIR OLED's over the theoretical limit of 100%, which can lead to light sources exceeding the intensity of the OLEDs produced in current technology [11]. The use of Gaq 3 allowed for increased efficiency of the solar cells [12] and OLED's [13] and application of Alq 3 as an acceptor material in the UV-photodetector [14]. With the advancement of technology and making it possible to produce layers with better properties, there has been rediscovered interest in the organic/inorganic hybrid structures in search of new functionalities in various fields of study. Organic materials are projected to enter the GaN-based hybrid device field [15]. The interfacial polarization of inorganic-organic heterojunction is important because it brings steep shifts in electronic band structure across interfaces and thus effectively tunes charge carrier transport. One of the possible ways to modify charge injection behavior in inorganic-organic heterojunction devices is to make use of interfacial polarization caused by the partial alignment of the permanent dipole moments of polar organic molecules [16,17]. Mq 3 molecules have a large electric dipole moment (~4 D) [18,19]. Mq 3 molecules have been widely studied for their potential applications, inter alia, in organic solar cells, light emission diodes, and data storage and communication devices [20][21][22][23]. Regarding this, the molecule/GaN systems are attractive for both industry and academic research. H. Kim et al. in work [24] proposed using an Alq 3 layer in GaN-based heterostructures. Apart from Alq 3 , the Gaq 3 , and Erq 3 appear to be new candidates for applying to such structures. An important issue in the context of such systems is the electronic structure of the interface, in particular the interfacial polarization or the position of the highest occupied molecular orbital (HOMO) level of molecules relative to the valence band maximum (VBM) of the substrate. So far, this information has been omitted in reports on Mq 3 films on GaN(0001) surface. The interfacial polarization has an impact on the band offset at the interface and this, in turn, has a bearing on the current-voltage characteristics of inorganic-organic devices. The tuning effect of central atom M in Mq 3 molecules on the band offset is of application importance.
This report presents a basic study of Alq 3 , Gaq 3 , and Erq 3 layers on GaN(0001) surfaces. The research focused on the electronic properties of the resulting interfaces and was carried out using ultraviolet photoelectron spectroscopy (UPS) assisted by X-ray photoelectron spectroscopy (XPS). The main goal was to check the capability of used Mq 3 to tune the position of HOMO and vacuum levels for the systems formed with p-GaN(0001) surfaces.

Materials and Methods
In this experiment, gallium nitride p-type, (0001)-oriented, on which Mq 3 films were deposited, was used as a substrate. Mg dopant concentration was ∼1 × 10 18 cm −3 . The GaN(0001) samples around 5 × 10 mm 2 in size were cut from one wafer grown by metalorganic chemical vapor deposition. Initial bare surfaces with a trace of residual oxygen were achieved by degassing GaN samples mounted on Mo plates. The samples were thermally annealed up to 500 • C. A radiation heater in an ultrahigh vacuum (UHV) chamber with a base pressure lower than 1 × 10 −10 Torr was utilized. The temperature was monitored by a pyrometer. The three Mq 3 /GaN(0001) systems were grown in situ by evaporation of molecules from quartz crucibles heated with thermal radiation. The calibration of the sources was done by means of XPS [25][26][27][28]. The 1.5 nm attenuation length of electrons with a kinetic energy of ∼370 eV in organic layers was used to evaluate the film thicknesses and thus growth rates. The parameter was calculated based on NIST Standard Reference Database [29]. The films were deposited step by step up to 15 nm. Adsorbate dosages were established from the evaporation time after the source temperature had stabilized. A surface-analysis system (Specs) was employed for in situ characterization. The main technique used was UPS, and the second was XPS. The XPS data collected in this experiment suggest the growth mode of the Mq 3 layers on the substrate, although the study has not focused on determining it. The decrease in the intensity of the Ga 2p core-level line with the increase in the Mq 3 thickness shows that the data are closest to the theoretical prediction of the Volmer-Weber growth mode. It indicates 3D growth mode for all three Mq 3 films. The photoemission experiments were carried out using a hemispherical electron energy analyzer (Phoibos 100) and a UPS source with He I (21.2 eV) excitation line, and two X-ray non-monochromatic radiation sources, i.e., Mg Kα (1253.6 eV) and Al Kα (1486.6 eV). Photoelectrons were collected in the CAE mode with a pass energy of 2 or 10 eV and a step size of 0.025 or 0.1 eV, respectively, for UPS and XPS measurements. During measurement, the optical axis of the analyzer entrance was normal to the substrate surface. Binding energy values refer to the Fermi level (E F ) of the electron analyzer, the position of which was determined using an argon ion cleaned Ag sample. No charging effect was observed during the photoelectron experiments. CasaXPS software was used to analyze XPS and UPS spectra. Gaussian and Lorentzian line shapes with Shirley-type backgrounds were applied. All measurements were made at room temperature.

Results
Herein, the results are presented for 7 nm thick organic films, where electronic states of molecules are stabilized and are not affected by phase boundary electron transfer effects. Therefore, the position of the states is stabilized and does not depend on further thicknesses. The valence band of bare GaN(0001) surface and covered with Alq 3 , Gaq 3 , Erq 3 is presented in Figure 1. The spectrum of the bare substrate reveals the valence band maximum (VBM) located at 2.6 eV below the E F . Separate deposition of Alq 3 , Gaq 3 , Erq 3 molecules onto the bare GaN(0001) surfaces changes the shape of the valence band. When the surface is completely covered with the molecules, the appearance of an additional peak in the vicinity of the Fermi level is clearly visible. These electron states are recognized as the highest occupied molecular orbitals (HOMOs), their onsets are located at 1.2, 1.7, and 2.2 eV below the E F , respectively, for Alq 3 , Gaq 3 , and Erq 3 layers. In the case of Alq 3 molecules, the HOMO level is located in the same position for various coverages. The Gaq 3 HOMO is clearly visible at the lowest coverage at 1.6 eV and shifts by 0.5 eV towards a higher binding energy with increasing film thickness and remains constant for coverages ≥7 nm, while the HOMO of Erq 3 molecules behaves similar to Alq 3 . The positions were determined from the intersection of an extrapolated line fitted to the leading edge of the spectrum and its background. In the photoelectron energy distribution curves, other characteristic features are also visible. The maxima are recognized as deeper electron states of the molecules, i.e., HOMO-1, HOMO-2. The vacuum level (E VAC ) of the bare substrate was located 4.3 eV above the E F , calculated from the equation E VAC = hv − E cutoff , where hv = 21.2 eV is photon energy of He I line and E cutoff is a cut-off of UPS spectrum.
An electron affinity can be calculated from the equation χ = E VAC − E g − E VBM , where E g is a band gap width and E VBM is a position of VBM. For the substrate, the electron affinity equals 4.3 eV. The UPS data allow constructing a sketch of energy bands for the initial GaN(0001) surfaces used in this experiment, as shown in Figure 2. The band bending of the bare substrates, induced by the Fermi level pinning at surface states, is in evidence. Assuming that the bulk Fermi level of the substrate is located 0.1 eV above the valence band maximum, the band banding is equal to 2.5 eV. Even though the substrates are p-type, the surface Fermi level is closer to the conduction band minimum than to the valence band maximum.
This result is in contrast to that in Refs. [30,31], which is most likely due to the fact that the initial surface of the substrates used in this report is depleted of holes. On GaN(0001) close to the conduction band minimum, there is a surface state which derives from Ga dangling bonds [32,33], thus, in the case of p-type GaN, the Fermi level pinning to this state leads to a strong band banding, which is the common observation [34][35][36][37]. Giving that the substrate is Mg-doped, the formation of depletion region is shown. The magnitude of band bending at the substrate needs to be included when trying to analyze the current-voltage characteristics of the device based on the inorganic-organic interface. As is shown further in the text, the magnitude can be changed after the phase boundary formation.
Different termination of the substrate surface generally leads to a vacuum level change. It is not the case for the GaN covered with Alq 3 layer where the E VAC does not alter, thus the work function change relative to the bare GaN(0001) equals zero (∆φ = 0). The same vacuum level was observed for various Alq 3 coverages. For Gaq 3 the vacuum level systematically decreases with increasing film thickness. Finally, for coverages ≥7 nm it is located 3.5 eV above the E F , giving the work function change ∆φ = −0.8 eV. The highest change of the E VAC was noted after deposition of Erq 3 , for which vacuum level was located 3.0 eV above the E F , giving ∆φ = −1.3 eV. The Erq 3 vacuum level decreases with film thickness, similar to Gaq 3 molecules. However, in order to reproduce the true change of work function ∆φ D at the resulting inorganic-organic phase boundary, i.e., to determine the interface dipole, it is necessary to know whether there is an electron transfer at Mq 3 /GaN interface or not. When charging of the interface states occurs, the band bending of the substrate changes. The change leads to a shift of the E VAC level for the substrate covered with molecules, the shift magnitude should be the same as the magnitude of the band bending change. To determine whether an electron transfer has occurred at the interface, it must be specified if the VBM position of the substrate has changed. Unfortunately, when the GaN(0001) surface is covered with an Mq 3 layer, an additional density of states resulting from an overlapping of the ad-molecules' orbitals with the GaN valence band prevents direct determination of the VBM from UPS measurements. Nevertheless, this measurement can be done indirectly using XPS. The core level lines for the substrate covered with the molecules are still visible since the mean free path for electrons from them is longer than for the valence band electrons. The positions of the VBM of the substrate for the three interfaces can be estimated based on the core level lines of the GaN substrate displacements, e.g., the Ga 3d or N 1s core level lines, after the molecule depositions considering the fact that the positions of the peaks relative to the VBM remain constant after ad-layer deposition. It is due to the fact the XPS results do not show indications of meaningful chemical interaction between the substrate and the adsorbed molecules. Figure 3 shows changes in the Ga 3d and N 1s peak positions caused by the presence of Alq 3 , Gaq 3 , Erq 3 layers. One can see that the shifts of the peaks are the same, even in the case of Gaq 3 molecules, where the Ga 3d state is derived from two sources (the substrate and the adsorbate). So the Ga 3d peak for the bare GaN(0001) lies 20.4 eV above the E F and is located 17.8 eV above the VBM (see Figure 2). The latter value is constant and in line with other data [38][39][40].  Knowing the changes of the work function ∆φ and band bending ∆φ BB , we can express the interface dipole as their sum, i.e., ∆φ D = ∆φ + ∆φ BB . The values of interfacial polarization for the three Mq 3 /GaN systems are presented in Table 1.
The electron affinities for organic layers are 2.8, 2.4, 2.3 eV, respectively, for the Alq 3 , Gaq 3 , Erq 3 molecules (assuming their band gaps are 2.7, 2.8, 2.9 eV [41][42][43]). The data allow constructing band diagrams for the three Mq 3 /GaN interfaces, as shown in Figure 4. The interfacial polarization ∆φ D has the smallest value for the Alq 3 film and the largest for the Erq 3 film. This means that the higher is the number of electron shells of the central metal ion in the organic molecule Mq 3 , the higher is ∆φ D (in absolute value).
When the Alq 3 and Gaq 3 molecules reduce the band bending of the inorganic substrate, the Erq 3 molecules slightly increase it (relative to the bare substrate surface). The presence of organic molecules on the GaN(0001) nominally enables the work function reduction up to 1.2 eV. Knowing the electron affinity of the GaN substrate and the adsorbers as well as the interface dipole ∆φ D of the systems unoccupied band offsets at the organic-inorganic interface can be expressed as: To calculate the occupied band offsets, the band gaps of the semiconductors need to be included The band offsets between the conduction band and LUMO of the molecules are 0.5, 0.2, and 0 eV, respectively, for Alq 3 , Gaq 3 , Erq 3 layers on GaN. Obtained data in this research are summarized in Table 2. The band offsets between the valence band and HOMO levels of the molecules Mq 3 are 1.2, 0.8, 0.5 eV, for M = Al, Ga, Er, respectively. It means that that the higher is the number of electron shells of the central metal ion in the organic molecule Mq 3 , the lower is the distance between occupied bands of the inorganic and organic semiconductors. The above analysis shows that tuning of the vacuum levels, HOMO levels, and band offsets at the interfaces is possible by changing the central M atom of the molecule Mq 3 .

Conclusions
UPS assisted by XPS was used to investigate the electronic properties of the three Mq 3 /GaN(0001) interfaces. The electron affinity of the clean GaN(0001) surface was found to be 3.5 eV and the VBM position was measured to be 2.6 eV below the E F . HOMO levels were determined to be at 1.2, 1.7, 2.2 eV for the Alq 3 , Gaq 3 , Erq 3 layers. The interface dipoles at the phase boundaries were amounted to be −0.2, −0.9, and −1.2 eV. The band offsets between the VBM of GaN(0001) and the HOMO level of the Alq 3 , Gaq 3 , Erq 3 molecules amounted to 1.