Polarity Control Technology for AlN Thin Films: Principles, Methods, and Device Applications +

PAM-XIAMEN focuses on the research and supply of AlN on sapphire and AlN on silicon templates. For more information, please consult [email protected].

Aluminum nitride (AlN), with its excellent properties such as a wide bandgap (~6.2 eV), high thermal conductivity, high breakdown field strength, and strong polarization effects, is an ideal material for fabricating deep ultraviolet optoelectronic devices, as well as high-frequency and high-power electronic devices. In heterostructure devices, polarity directly influences the formation of two-dimensional electron gas (2DEG) or two-dimensional hole gas (2DHG), quantum confinement effects, and device characteristics like voltage withstand capability and frequency performance. For instance, N-polar AlN can serve as a back barrier layer in high electron mobility transistors (HEMTs), enhancing breakdown voltage and thermal management capability. Conversely, Al-polar AlN offers unique advantages in p-type layer engineering and hole gas induction. Therefore, achieving controllable growth of AlN polarity is a core prerequisite for advancing its device applications.

1. Fundamental Principles of AlN Polarity Control

The polarity of AlN is determined by the stacking sequence of atoms at the initial growth interface. On Si(111), Al atoms first form a eutectic alloy (Al–Si eutectic) with Si, promoting Al termination at the interface and thus inducing N-polar growth. If this eutectic layer is consumed or modulated by impurities, polarity inversion to Al-polarity may occur. On sapphire (0001), high-temperature nitridation leads to the formation of a nitrided layer on the surface, which typically promotes N-polar growth. Factors during growth such as the V/III ratio, temperature, and impurity incorporation can modulate the formation energy of polarity, enabling polarity selection or inversion.

2. Polarity Control and Crystalline Quality Optimization of AlN on Si(111) Substrates

Fan et al. achieved the growth of high-quality N-polar and Al-polar AlN thin films on Si(111) substrates via plasma-assisted molecular beam epitaxy (PA-MBE). The study shows that under continuous Al overflux conditions (Al/N flux ratio=1.2), N-polar AlN with a root mean square (RMS) surface roughness of 0.30nm and full width at half maximum (FWHM) values of 475arcsec and 1177arcsec for the XRD (002) and (102) rocking curves, respectively, can be obtained (Sample C2). The polarity was verified by wet chemical etching using tetramethylammonium hydroxide (TMAH) and by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). TMAH selectively etches N-polar AlN, forming hexagonal pyramids, while the Al-polar surface remains smooth.

Fig. 1 Comparison of AlN thin films grown on Si(111)

Fig. 2 Surface morphology and polarity verification of AlN thin films grown on Si(111): (a, e) Cross-sectional SEM images of AlN films grown under continuous growth (Al/N=1.2) and MME growth (Al/N=1.3) conditions, respectively; (b, f) Corresponding 3×3μm² AFM images; (c, g) RHEED patterns along the [1120] and [1100] directions during growth; (d, h) 75°-tilted view SEM images of the as-grown AlN films before (top) and after (bottom) TMAH wet etching.

Fig. 3 HAADF-STEM atomic structure characterization of the N-polar AlN thin film: (a) Bright-field cross-sectional TEM image of the N-polar AlN film (Sample C2); (b-d) Atomic-resolution HAADF-STEM images taken at the AlN/Si interface (green box), middle of the film (red box), and top surface (cyan box), respectively. The overlaid atomic models show the stacking sequence of N-polar AlN (N atoms located below Al atoms).

To achieve polarity inversion, researchers employed the metal-flux modulation epitaxy (MME) strategy. By using periodic Al flux modulation (20s ON/10s OFF), complete inversion from N- to Al-polarity was accomplished within approximately 50nm. HAADF-STEM revealed that anti-phase boundaries (APBs) formed on the (2201) planes and subsequently annihilated, leading to a purely Al-polar surface. The resulting Al-polar AlN exhibited (002) and (102) FWHMs of 1505 and 2380arcsec, respectively, and an RMS roughness of 1.41nm. Strain analysis indicated a relatively uniform tensile strain distribution (0.113%–1.160%) for the N-polar AlN, whereas the Al-polar AlN showed a broader strain range (0.482%–2.406%) due to strain relaxation during the polarity inversion process. This study provides an important pathway for achieving device-grade polarity engineering on Si.

Fig. 4 HAADF-STEM analysis of Al-polarity inversion induced by MME growth: (a) Dark-field cross-sectional image of the Al-polar AlN film (Sample C5), showing APBs near the interface; (b) Atomic-resolution image of the film surface confirming Al-polarity; (c) Process of APBs originating and annihilating within a thickness of ~50nm; (d) Al-polar region at 60nm from the interface; (e) Atomic image of the interface region showing APBs lying on the (2201) crystalline planes; (f–h) Zoomed-in images of the region above, at the intersection of, and below APBs, respectively.

3. Growth Optimization and Polarity Control of N-polar AlN on Sapphire Substrates

Growing smooth N-polar AlN on sapphire has long been challenging. Pampili and Pristovsek achieved N-polar AlN templates with sub-nanometer RMS roughness on vicinal sapphire substrates via MOVPE. The study identified the V/III ratio as a critical factor controlling polarity and surface morphology: on 4° offcut sapphire, step-flow growth and avoidance of hexagonal hillocks were only possible with V/III ratios below 2. The optimized growth window was narrow, with a temperature deviation exceeding 20°C leading to surface roughening. Furthermore, the nitridation step required strict control of duration (15s) to minimize defects induced by substrate damage.

Fig. 5 Influence of V/III ratio on the surface morphology of AlN grown on 0.2° (top row) and 4° (bottom row) offcut sapphire substrates

Fig. 6 Influence of different growth temperatures on the surface morphology of N-polar AlN: (a) 1260°C, (b) 1280°C, (c) 1300°C

Fig. 7 Influence of nitridation time on surface defect density: (a, b) 150 s, (c) 15s

The study also found that the growth rate influenced step-bunching behavior: a smooth surface could be maintained at rates below 0.93μm/h, while rates above 1.4μm/h led to a transition to columnar growth. By finely tuning the V/III ratio to the ammonia-limited regime of 0.67–0.78, a fully Al-covered surface could be achieved, resulting in high-quality N-polar AlN with an RMS roughness as low as 0.33nm. This research provides a detailed process window and mechanistic explanation for the controllable preparation of N-polar AlN on sapphire.

4. Fabrication and Application Prospects of AlN Lateral Polarity Structures (LPS)

Tanigawa et al. successfully fabricated AlN lateral polarity structures with a period of 2μm by regrowing N-polar AlN on patterned Al-polar AlN templates via MOVPE. The study noted that Al-polar AlN could be grown at a high V/III ratio (~75), while N-polar AlN required a low V/III ratio (~5) for growth. Furthermore, a high carbon impurity concentration might suppress the formation of polarity inversion layers (e.g., AlON), thereby stabilizing N-polar growth.

During fabrication, inclined sidewalls formed by reactive ion etching (RIE) led to void formation in regrowth. Treatment with KOH wet etching significantly improved sidewall verticality, enabling the successful fabrication of void-free AlN LPS. Sidewall verticality and the low V/III ratio regrowth condition were key to achieving high-quality polarity interfaces. This type of LPS can be used to fabricate quasi-phase matching (QPM) second-harmonic generators in the ultraviolet wavelength range, opening new pathways for AlN applications in nonlinear optical devices.

Fig. 8 Improvement of sidewall verticality by KOH wet etching

Fig. 9 Fabrication and verification of AlN lateral polarity structures: (a) Void-free LPS obtained after regrowing N-polar AlN on a patterned Al-polar AlN template with vertical sidewalls; (b) Schematic diagram of the growth mechanism

Polarity control is the cornerstone for the functionalization and device application of AlN. The MME strategy enables controllable preparation and high-quality growth of N-polar AlN on Si(111). Smooth N-polar AlN epitaxy on sapphire can be achieved through low V/III ratios, precise temperature control, and short-time nitridation. The successful fabrication of lateral polarity structures further demonstrates the potential of polarity engineering in integrated optical devices.

Whether you need AlN templates for research or for industrial applications, please contact us email at [email protected] and [email protected].

 

References:

1. Fan, S., Yin, Y., Liu, R., Zhao, H., Liu, Z., Sun, Q., & Yang, H. (2024). Polarity control and crystalline quality improvement of AlN thin films grown on Si (111) substrates by molecular beam epitaxy. Journal of Applied Physics, 136(14).
2. Pampili, P., & Pristovsek, M. (2024). Nitrogen-polar growth of AlN on vicinal (0001) sapphire by MOVPE. Journal of Applied Physics, 135(19).
3. Tanigawa, S., Sakoyama, T., Kurai, S., Okada, N., & Yamada, Y. (2023). Control of Polarity of AlN Grown on Sapphire Substrate and Growth with Both Al‐and N‐Polarities. physica status solidi (b), 260(8), 2200576.