Research on the Stability of Different Polar Surfaces in Aluminum Nitride Single Crystals

Abstract

Wurtzite aluminum nitride (AlN) crystal has a non-centrosymmetric crystal structure with only a single axis of symmetry. In an AlN crystal, the electronegativity difference between the Al atom and N atom leads to a distortion of electron cloud distribution outside the nucleus and a spontaneous polarization (SP) along the c-axis direction. The N-polar surface along the directions of [000-1] has higher surface energy than the Al-polar surface along the directions of [0001]. Due to the different atomic arrangement, Al atoms on the Al-polar surface bond with O and OH⁻ in the environment to generate Al₂O₃·xH₂O, which prevents the reaction from occurring inside the crystal. After the Al₂O₃·xH₂O dissolve in an alkaline environment, N atoms have three dangling bonds exposed on the surface, which can also protect OH⁻ from destroying the internal Al-N bonds, so the Al-polar surface is more stable than the N-polar surface.

1. Introduction

As a kind of third-generation semiconductor material, aluminum nitride (AlN) has outstanding properties, including large band gap width, high breakdown field strength, good thermal stability and chemical stability, etc. Hence, AlN is under intense investigation for its application in deep ultraviolet light-emitting diodes (LED), laser diodes (LD), gas sensors, high-temperature electronic devices, second-harmonic emitters, and surface acoustic devices due to their promising prospect in satellite communication, precision guidance, environmental monitoring, food processing, etc. [1,2,3,4,5,6]. As a substrate material, AlN has a smaller lattice and thermal mismatch with the AlGaInN epilayer than the sapphire and SiC [7], which is suitable for a high-quality AlGaInN thin film epitaxy as well as for the construction of AlGaInN-based devices with an improved performance [8,9].

AlN belongs to the III–V semiconductor materials family, and depending on the growth conditions, it can crystalize in three kinds of structures: wurtzite, sphalerite, and halite [10]. Among them, wurtzite structure is the most thermodynamically stable structure of AlN crystal and is also the focus of this paper. AlN wurtzite structure is a non-centrosymmetric crystal, which has only an axis of symmetry. Because of the lack of a center of symmetry, when external stress is present, the positive and negative charge centers are separated because of the lattice deformation, and the dipole moment is formed, which shows the piezoelectric polarization. Meanwhile, due to the great electronegativity difference between N and Al atoms, the distribution of the electron cloud outside the nucleus is distorted in an AlN cell. Resultingly, the positive center produced by the nucleus and the negative center by the electron cloud do not overlap and form an electric moment. This polarization effect is called spontaneous polarization (SP).

In the AlN lattice, one Al(N) atom is bonded to four N(Al) atoms to form a tetrahedron. In the c-axis direction, one Al(N) atom forms a long bond (0.1917 nm) with one N(Al) atom, while the other three N(Al) atoms form three short bonds (0.1885 nm) with the Al(N) atom in the other direction. The atomic stacking sequence in wurtzite AlN crystal is ABabABab, etc. Therefore, the polarities are different along the [0001] and [000-1] directions, and the surface termination of the AlN crystal in this case will be an Al-polar surface or a N-polar surface. Therein, the Al atom occupies the top of the diatomic layer in the Al-polar surface, and the N atom occupies the top of the diatomic layer in the N-polar surface. For the Al surface, the c-axis direction of the crystal is consistent with the built-in electric field, which points from the substrate to the surface, while the polarization direction is opposite. The polarization-induced fixed lattice charge is positive at the substrate interface and negative at the crystal surface. The situation is completely reversed for the N-polar surface, as shown in Figure 1a,b.

The influence of the SP on the charge injection barrier in the interface between epitaxial layers was researched in GaN [11,12]. As a comparison, there are few studies on the polarity of AlN, according to our best knowledge. Yoshitaka Taniyasu et al. in Japan found that AlN has an anisotropic emission pattern; i.e., light is barely emitted from the c-plane but is emitted preferentially from the a-plane. Taking advantage of this characteristic, the LED prepared by a-plane AlN has a high luminescence efficiency at a deep ultraviolet luminescence wavelength (210 nm) [13]. As a suitable substrate material for a high-quality AlGaInN thin film, a very smooth AlN substrate with roughness below 0.1 nm is necessary, which makes the polishing processing particularly important. The influence of the polarity difference caused by the different atomic arrangements on the polishing of the AlN polar surface came under observation. Defect-selective etching is widely used in defect characterization of III–V nitride semiconductor single crystal materials because of its low cost and simple operation, whose theory is that the electrically active non-homogeneities in bulk nitride are detected as areas of different etching rates, thus forming visible corrosion marks in the metal polar and non-polar surface. It has a strong corrosive effect on the strain zone or chemical inhomogeneity of the surface, where defects such as dislocations and impurity streaks exist, with a great difference in the corrosion rate between the defective and non-defective areas due to the exchange of the valence electrons between surface bonds of semiconductor; the etchant is very sensitive to the local deformation of the nitride lattice [14]. Hence, the defects can be marked by corrosion. In this study, properties of different polar surfaces with different SP in AlN were studied through corrosion and polishing experiments and analysis from the point of view of atomic arrangement, with the first-principles calculation results of surface energy and adsorption energy as auxiliary, aiming to clarify the rigidity of the crystal surfaces.

2. Experiment

The AlN single crystal used in the experiment was grown by physical vapor transport, the growth equipment is a self-built tungsten metal growth system, and the growth temperature, pressure, and duration were 2260–2290 °C, (5–7) × 10⁴ Pa, and 34 h, respectively. The growth material was AlN powder after several rounds of sintering [15]. A UNIPOL-1203 chemical mechanical polishing machine was used to polish the crystals. A SPI 3800N SPA300HV scanning probe microscope was used to characterize crystal surface roughness. A JEOL JSM-5910LV scanning electron microscope was used to observe the surface morphology of the crystals after corrosion, and a ROYAL PHILIPS X PertPro high-resolution X-ray diffractometer was used to characterize the quality of the sample.

The AlN sample used in the experiment is single crystal without grain boundary, which has a roughness Ra of 21.9 nm and a dislocation density of about 80/cm², and the impurities concentration of Si, C, and O measured by SIMS was less than 10¹⁹ cm⁻³ and as low as 10¹⁷ cm⁻³, which was been reported in detail in our previous work [16] on the growth and characterization of AlN crystals. The sample was also characterized by XRD, the results of which are shown in Figure 1c; compared with the standard XRD peak card of AlN, there was no significant difference between the two, and there were no other obvious impurity peaks in the test results.

2.1. Polishing of AlN Crystal

AlN [0001] and [000-1] surfaces were ground and polished on a certain condition of the grinding disc under abrasion and pressure application. There were fewer scratches on the Al-polar surface than the N-polar surface. The scanning electron microscope (SEM) images of the processed Al-polar surface and the N-polar surface are shown in Figure 2a,b, indicating that it is more difficult to destroy the Al-polar surface under the action of external forces. As shown in Figure 2c, the principle of polishing is to use abrasive particles to roll and micro-cut the crystal surface, remove the convex surface through the grinding abrasion, and reduce the roughness. In addition, a suitable chemical composition is generally added to the liquid in chemical polishing to make it react on the surface to generate an easily removed substance attached to the surface, and the surface is smoothed through physical grinding and regular dissolution. Due to the microscopic inconsistencies of the crystal surface, the surface micro bumps were preferentially removed. We used different particle-size diamond polishing liquids; iron, copper, and tin polishing discs; and a polyurethane polishing pad. In order to protect the crystal from cracking, the pressure applied did not exceed 10⁵ Pa. The increased difficulty in breaking the surface led directly to an increase in processing time. The time required for polishing the Al-polar surface to a roughness of 3 nm is much longer than that of the N-polar surface, as shown in Figure 2d–f. This means that the Al-polar surface has better stability than the N-polar surface.

2.2. Corrosion Experiment of AlN Crystal

AlN wurtzite crystal is not easily able to react with acidic or alkaline solutions due to its strong chemical stability, especially for the Al-polar surface. The corrosion occurs only at dislocations in alkali melt or hot, strong phosphoric acid (SPA), with other areas unchanged [17]. In this paper, we mixed solid KOH and NaOH at a ratio of 3:1 and heated to a molten state. The AlN crystals were immersed in the molten solution and corroded for 1 or 3 min. In addition, uncorroded AlN crystals were set as the control group. The surface morphologies of the AlN crystals with different corrosion conditions were characterized scanning electron microscope and are shown in Figure 3.

According to the contrast principle of the secondary electron morphology, the brightest part indicates a steep slope, the gray part indicates a flat or gentle slope, and the black part indicates the existence of a hole or groove where the secondary electron cannot be collected. Regular hexagonal corrosion pits were formed on the Al-polar surface because of the corrosion of the strong base. Corrosion pit formation is associated with dislocation. The existence of the screw dislocation on the surface formed a spiral ladder. It is easy to make it bigger through alkali corrosion to form a α-type corrosion pit in the shape of a six-frustum pyramid. Edge dislocation in the vertical direction along the dislocation line formed a β-type corrosion pit in the shape of six pyramids. The mixed-type dislocations with screw and edge components formed γ-type pits, which are the combination of α- and β-type pits.

Figure 3e shows that the corrosion depth of the N-polar surface is larger than that of the Al-polar surface, forming mound-like structures with different sizes and disordered arrangement on the surface. As the corrosion time increased from 1 min to 3 min, the corrosion pits on the Al-polar surface were still regular hexagons with larger sizes. The adjacent corrosion pits merged, and the corrosion pit depth increased slightly. However, on the N-polar surface, corrosion continued along with the increase of time. As can be seen from Figure 3e, continuous random corrosion resulted in larger island structures with uneven surfaces and irregular mound-like structures.

From the above comparison, corrosion on the Al-polar surface is selective by dislocation, and the traces caused by corrosion are basically regular hexagonal pits, whose size and depth increase slightly with the increase of corrosion time. However, random corrosion occurs on the N-polar surface, and the longer the corrosion time, the deeper the trace.

An additional 5 min corrosion experiment was carried out, with the results shown in Figure 4. As the concentration of alkali melt decreased, the Al-polar surface had a similar surface appearance as the control group without corrosion, while the N-polarity surface demonstrated extremely significant corrosion marks. In other words, when the alkali concentration is not enough to corrode the Al-polar surface, corrosion can still happen on the N-polar surface. Therefore, we can draw the conclusion that the chemical stability of the Al-polar surface is higher than that of the N-polar surface.

3. Discussion

3.1. Differences in the Oxidation of the Polar Surface of AlN Crystal

AlN has a strong affinity for oxygen in the air. It slowly reacts with oxygen to form a thin layer of aluminum oxide (Al₂O₃), along with the release of a small amount of ammonia [18]. As the oxide layer thickens, the oxidation reaction slows down gradually until it stops completely. When the temperature rises to 700–800 °C, the oxidation reaction occurs again, and the relationship between the oxidation rate and temperature changes from linear to parabolic [19]. The oxidation of AlN powders and ceramics has been reported in detail before [20,21]. For AlN crystals, some oxygen atoms can dissolve into AlN wurtzite to replace N atoms, forming O_N defects. Thus, AlN crystals placed in the air can produce a layer of oxide film of about 2 nm on the surface in a short time. However, the oxidation process of the AlN single crystal in different polar surfaces has been seldom mentioned.

The adsorption energy of oxygen on AlN surfaces can be used as a powerful tool to quantify the difficulty of the oxidation reaction. During the adsorption process, the movement speed of O changes from fast to slow and eventually stops on the surface of AlN, so some energy will be released due to the decrease in velocity. The higher the adsorption energy, the lower and more stable the energy of the system formed after adsorption, and the easier it is for the adsorption process to occur. In other words, to change the instability of the surface, nearby atoms or micro molecules are adsorbed to the surface to minimize energy and make the system more stable. The energy generated during the adsorption process is called adsorption energy, which can be calculated by the following [22]:

$$E_{ads}=\frac{\left(E_{surf}+n_x{\mu }_x-E_{tot}\right)}{n_x},$$

E_ads is the adsorption energy; E_surf is the total energy of the surface model before adsorption; n_x and μ_x are the number and chemical potential of the adsorbed atoms (or micro molecules), respectively; and E_tot is the total system energy after adsorption.

Honggang Ye et al. calculated the adsorption energy of AlN for oxygen on different polar surfaces by using the above method [21]. The adsorption energy of oxygen on the (000-1)-Al_top surface is nearly 2 eV/atom larger than that of the (0001) surface under the same coverage conditions, which means the (000-1)-Al_top surface is more likely to be oxidized. This is because the surface of (000-1)-Al_top, which is covered with a layer of Al atoms, has more dangling bonds to saturate.

As mentioned in Section 1, the Al-N bond along the c-axis has a longer bond length and a smaller bond energy than the Al-N bonds in other directions and thus is easier to break. Therefore, it is more likely that the Al-polar surface is a layer of Al atoms, while the N-polar surface is a layer of N atoms. In this case, the dangling bond densities of Al and N atoms on the surface can be regarded as the same. When AlN crystals are placed in the air, oxygen atoms adsorbed at active sites are more likely to bond with Al atoms on the polar surface of Al because the Al-O bond energy is 512 kJ /mol larger than the N-O bond energy of 230 kJ /mol.

The resulting free N atoms, along with free N atoms formed due to the rupture of Al-N bonds, could either be released from the crystal solid as N₂ into the atmosphere or bond with oxygen atoms due to the adsorption of oxygen atoms to become N-O. In this case, an amorphous Al-O-N oxide structure will be formed on the surface, as shown in Figure 5a. We used X-ray photoelectron spectroscopy to calculate the ratio of Al and N elements on the N-polar surface to be about 1:1. However, the proportion of O on the Al-polar surface reaches 68.0%, and the content ratio of Al and N elements reached 25:7, which verifies the hypothesis that an Al-O diatomic layer is formed on the surface of Al. This is completely consistent with the conclusion that the Al-polar surface more easily adsorbs oxygen and forms an oxide layer.

With the gradual increasing of Al-O-N oxide layer, it will continue to generate Al₂O₃ in the following reaction:

$$4AlN+3O_2=2{Al}_2{O}_3+2N_2;$$

Amorphous Al₂O₃ and θ-Al₂O₃ would form successively, and the final product, α -Al₂O₃, is formed by further crystal transformation. When this oxide film completely covers the surface of the AlN crystal, forming a structure similar to the Al-O diatomic layer shown in Figure 5b, the oxidation reaction will no longer take place. On the other hand, the total bond energy of the AlN tetrahedral structure is 11.52 eV [23]. Although the two types of bonds have different energies, it is easier to break the Al-N bond, with an average energy of 276 kJ/mol (2.88 eV), than the Al-O bond. Therefore, this oxide film, which forms more easily on the Al-polar surface, protects the AlN crystal, thus making the Al-polar surface more stable than the N-polar surface in some aspects.

3.2. Hydrolyzation Differences of the Polar Surfaces of AlN Crystal

In addition to being oxidized to form Al₂O₃, AlN crystal may also hydrolyze with ambient water molecules to form Al(OH)₃ [17]. The hydrolysis of AlN powder has been studied in detail [24,25].

Since wurtzite AlN crystal has a high stability, a weak hydrolyzation can only occur on its surface. Theoretically, the ease of hydrolysis also can be quantified by the adsorption energy of water. At present, there have been studies on the adsorption of GaN wurtzite (0001) on water molecules, and it is generally considered that the adsorption of water can be divided into chemical adsorption and physical adsorption [26]. For the chemisorption of AlN to water, water molecules undergo dissociated adsorption (decomposed into OH and H or O and 2H) at low coverage (<1/2 ML), as shown in Figure 6a, and undergo molecular adsorption while at high coverage, as shown in Figure 6b. At the same time, adsorption energy decreases with the increase of coverage [27], which is similar to oxygen adsorption, due to the repulsion of adsorption molecules or atom increases.

The adsorption analysis of water molecules on the surface of AlN is complicated, and it is difficult to analyze its chemical stability by comparing the adsorption energy of water molecules on different polar surfaces. However, it was found that in both dissociated and molecular adsorption, the adsorption of AlN to water is bonded with oxygen. Therefore, we chose to analyze the difference of chemical stability by surface atomic arrangement, referring to the first-principles calculation of adsorption energy to water molecules and oxygen on polar surface.

The hydrolysis of AlN produces Al(OH)₃ or meta-aluminic acid (AlOOH), while AlOOH would be converted to Al(OH)₃ under certain conditions due to its poor stability. The above reaction can be expressed as follows:

$$AlN+3H_{2}O=Al{\left(OH\right)}_3+N{H}_3\uparrow$$

$$AlN+2H_{2}O={AlOOH}_{amorph}+N{H}_3\uparrow$$

As mentioned above, the adsorption of AlN to water is similar to that of O; here, Al(OH)₃ and AlOOH can be written as Al₂O₃·3H₂O and Al₂O₃·H₂O, respectively. It has been proven that in the temperature range of 303–373 K, the Gibbs free energy values of AlN hydrolysis are all negative, according to the Gibbs free energy calculation formula:

$$\mathsf{\Delta }G=\mathsf{\Delta }{G}_f\left({Al}_2{O}_3·x{H}_{2}O\right)+2RT\mathit{ln}\left(p_{{NH}_3}\right)-2\mathsf{\Delta }{G}_f\left(AlN\right)-n\mathsf{\Delta }{G}_f\left(H_{2}O\right)$$

The resulting p_NH³ is much smaller than the critical condition when the Gibbs free energy is negative, indicating that the hydrolysis of AlN can proceed spontaneously under standard conditions [24]. Hydrolysis is an exothermic reaction, and the generated NH₃ could dissolve in water, producing NH₄⁺ and OH⁻. Therefore, with the occurrence of the reaction, the temperature and pH value of the environment would increase. The products of hydrolysis are also related to the ambient temperature, as AlOOH tends to be generated when the temperature is greater than 351 K.

The adsorbed H₂O will decompose into OH⁻ and H⁺ on the surface, which occurs spontaneously without any energy barrier, but a further dissociation reaction of OH⁻ into O^2- and H⁺ has an energy barrier of 22.046 kcal/mol [21]. In the case of low coverage, Al atoms on the Al-polar surface of AlN crystals can react with OH⁻, forming a layer of Al(OH)₃ or transition product AlOOH on the surface. On the one hand, the bond energy of the newly formed Al-O bond is greater than the Al-N bond and thus plays a protective role for the surface. On the other hand, Al(OH)₃ is soluble in water, and there are three dangling bonds in the outermost layer of N atoms exposed on the surface at this time, as shown in Figure 6c. The repulsion to OH⁻ also prevents the hydrolysis from moving further into the crystal. However, the outermost layer of N atom on the N-polar surface has only one dangling bond, which has a small repulsion effect on OH⁻. Therefore, when the N atoms on the surface are destroyed, or the atoms are disordered at the dislocation area, the hydrolysis reaction is more likely to occur.

According to the different hydrolysis on the different polar surfaces of AlN crystals, the Al-polar surface is more stable than that of the N-polar surface. In the polishing process, AlN crystals are inevitably in contact with water and oxygen. The Al₂O₃ layer, which is easier to form on the Al-polar surface through oxidation, and the dangling bonds of N atoms exposed on the Al-polar surface after hydrolysis, both play a protective role for the interior crystal. Therefore, it is difficult for external ploughing to destroy the interior Al-N bond in the polishing process, which is reflected in the fact that the polishing process of the Al-polar surface is more difficult and the speed is lower than that of the N surface.

3.3. Differences in Corrosion of AlN Crystal Polar Surface in an Alkaline Environment

In the discussion of AlN crystals hydrolysis, we found that the OH⁻ plays a crucial role in the destruction of the AlN crystal structure. As the AlN crystal hydrolysis is too weak to be observed clearly, alkaline melt was used as an additive in the corrosion experiment to increase the concentration of OH⁻, aiming to make the destruction of the crystal structure macroscopically observable.

OH⁻ can break the Al-N bond on the Al-polar surface, leaving on the exposed surface a layer of N atoms. As mentioned in Section 3.2, the outermost layer of N atoms has three dangling bonds, which have a very large repulsion to OH⁻. In addition, the negative polarized charge distributed on the Al-polar surface due to SP also has a repulsive effect on OH⁻. Therefore, it is difficult for OH⁻ to break the N-Al bond below the N atom, as shown in Figure 7a. With these atomic structures, the N atom can protect the Al atom effectively. OH⁻ can only corrode the crystal along the dislocation line where atoms are disordered, so the dislocation pit is exposed through corrosion. The area would remain intact elsewhere.

However, the outermost layer of the N atom on the N-polar surface has only one dangling bond, which has weak repulsion to OH⁻. Likewise, due to SP, the positive charge distributed on the N-polar surface also attracts OH⁻. OH⁻ can break the N-Al bond on the surface and adsorb on the surface of AlN crystal. The reaction between OH⁻ and AlN would occur and generate Al₂O₃, Al(OH)₃, and other products that dissolve in the alkaline melt. These reactions can generate NH₃ at the same time.

The adsorption and dissolution process are shown in Figure 7b–d. After the above adsorption and dissolution process, OH⁻ would continue to destroy the N-Al bond of the next layer downward, making the corrosion process proceed downward layer by layer. In this case, the crystal of the N⁻ surface is rapidly corroded. In summary, this is the reason why the surface morphology of the Al- and N-polar surfaces is completely different after corrosion.

3.4. Surface Energy Difference of Polar Surfaces in AlN Crystal

First-principles calculation of the surface energy on different polar surfaces is a powerful tool for comparing their stability. In the case of AlN, the atoms on the Al- and N-polar surfaces have one less bond than the inner atoms. The surface dangling bonds, both anion and cation, are partially occupied, which does not conform to the electron-counting rule in the inner atoms. As a result, the surface has higher energy than the interior. Usually, higher energy means more instability [28].

In general, it is well known that the surface energy can be thought of as the extra energy of the surface relative to the interior of the crystal, which can be calculated by the following [29]:

$$\sigma =\frac{\left(E_{total}-E_{bulk}\right)}{2A_{surf}},$$

where E_total is the total energy of the surface slab model, E_bulk is the energy of bulk phase system, and A_surf is the area of surface. However, if the upper and lower surfaces are different, like the (0001) and (000-1) surfaces of wurtzite structure AlN, the absolute value of the surface energy cannot be obtained through the above formula but rather only the average surface energy of the two surfaces, i.e., the splitting energy. C. E. Dreyer et al. assumed that the energy of the passive N-polar surface of the wurtzite structure GaN is approximately equal to the energy of the (111) or (-1-1-1) side of sphalerite structure; therefore, the absolute surface energy of wurtzite GaN on different polar surfaces was calculated by using the wedge structure of sphalerite GaN [30]. In addition, absolute surface energy can also be calculated by using an asymmetric model in which Al and N atoms do not conform to stoichiometric ratios. At this time, the calculation formula of surface energy is usually as follows [21]:

$$\sigma =\frac{\left(E_{surf}-N_{Al}{\mu }_{Al}-N_N{\mu }_N\right)}{2A_{surf}},$$

where N_Al and N_N are atomic numbers of Al and N, respectively, and μ_Al and μ_N are their corresponding chemical potentials.

In order to calculate the surface energy of the polar surface of AlN wurtzite crystal, S. Sun et al. have established the Al terminal and N terminal surface models whose Al and N atoms do not combine in a stoichiometric ratio [31]. The surface energy of the Al and the N-polar surface of AlN crystal were calculated by this method to be 316 meV/Ų and 350 meV/Ų, respectively. indicating the smaller Al-polar surface energy than the N-polar surface [29,31], as well as a better stability of the Al-polar surface than the N-polar surface.

4. Conclusions

A layer of Al atoms on the Al-polar surface is more likely to react with O in the ambient environment. The generated oxide layer protects the oxidation reaction from proceeding into the interior of the crystal and increases the difficulty of grinding on the Al-polar surface in physical polishing. After the Al(OH)₃ generated by the reaction between water molecules in the environment and Al atoms on the Al-polar surface is dissolved, the repulsion to OH⁻ by the three dangling bonds of the N atom exposed outside also plays a protective role for the crystal. Therefore, the Al-polar surface is more strongly protected in the alkaline environment of chemical polishing and corrosion. In addition, numerical calculation revealed that the surface energy of the N-polar surface is larger than that of the Al-polar surface. This indicates that the energy of atoms on the N-polar surface is higher than inside the crystal. This higher energy means that the surface atoms are unstable. In summary, it can be concluded that the stability of the Al-polar surface is stronger than that of the N-polar surface in AlN wurtzite crystal.