Impact of Temperature on the Properties of AlN Materials and Devices +

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AlN, with its wide bandgap of 6.1eV, high thermal conductivity, and high breakdown electric field, is an ideal candidate for extreme-temperature applications. However, the practical application of AlN faces two core challenges: (i) residual stress arising from heteroepitaxy (e.g., on sapphire substrates) due to lattice mismatch (13.2%) and thermal expansion coefficient mismatch (45.4%); and (ii) low n-type doping efficiency limited by the self-compensation effect (DX centers) of Si donors. Both issues are closely related to temperature—temperature affects the release and evolution of stress, and determines the donor ionization rate and carrier transport behavior.

In recent years, Wei et al. conducted in-depth analyses of the stress evolution and optical property changes of MOCVD-grown AlN films over 80–850K, revealing the positive role of nano-patterned substrates in thermal stress release. Meanwhile, Okumura et al. systematically investigated the electrical properties of Si-implanted AlN layers over the temperature range of 300–1150K, successfully demonstrating the operation of AlN Schottky barrier diodes and MESFETs at temperatures exceeding 1000K.

1. Influence of Temperature on Fundamental Properties of AlN Materials

1.1 Temperature Evolution of Residual Stress

Wei et al. grew AlN epitaxial layers on conventional flat sapphire substrates (CSS) and nano-patterned sapphire substrates (NPSS) by MOCVD (referred to as Sample A and Sample B, respectively), and studied the temperature dependence of residual stress via temperature-dependent Raman spectroscopy (80–300K). The Raman shift of the E₂(high) phonon mode exhibits a red shift with increasing temperature, which includes contributions from both lattice constant changes due to thermal expansion and anharmonic effects.

Using the stress–Raman shift relationship σ = (ω – ω₀)/κ (where κ = 4.04cm⁻¹/GPa, ω₀ = 657.4cm⁻¹), the biaxial stress as a function of temperature was calculated. As shown in the figure, the compressive stress in the AlN sample on CSS is as high as 1.59GPa at 80K, decreasing to 0.60GPa as the temperature rises to 300K. For the AlN sample on NPSS, the compressive stress decreases from 0.90GPa at 80K to 0.38GPa at 300K. This indicates that with increasing temperature, thermal expansion partially compensates for the compressive stress accumulated during growth, and the nano-patterned structure provides additional stress release channels through nanopores.

Fig. 1 Raman spectra of AlN/sapphire films in the temperature range of 80–300K, showing red shift and broadening of the E₂(high) peak with increasing temperature

Fig. 2 Temperature dependence of biaxial stress in AlN on conventional flat substrate (sample A) and patterned substrate (sample B)

X-ray diffraction analysis further confirmed the above trend. The (0002) diffraction peak of AlN on CSS is located at 35.9810°, while that on NPSS is at 35.9931°, closer to the theoretical value for strain-free AlN. The temperature dependence of the c-axis residual strain is consistent with the Raman results.

Fig. 3 X-ray diffraction analysis of AlN samples on sapphire substrates

1.2 Temperature Dependence of Optical Constants and Bandgap

Wei et al. investigated the temperature dependence of the optical constants and bandgap of AlN films using temperature-dependent spectroscopic ellipsometry (300–850K). The results show that the refractive index n increases slightly with temperature in the low-energy region, while its peak undergoes a red shift in the high-energy region; the absorption edge of the extinction coefficient k shifts toward lower energies, directly reflecting bandgap narrowing.

The temperature dependence of the bandgap was obtained by linear extrapolation of (αhν)² versus hν. The bandgap of the CSS sample decreases from 6.16eV at 300K to 5.73eV at 850K, while that of the NPSS sample decreases from 6.06eV to 5.64eV. This variation can be accurately described by the Varshni formula Eg(T)=E0-αT²/(T+b). The fitted parameters are: for the CSS sample, E0=6.24eV, α=8.73×10⁻⁴eV/K, b=857K; for the NPSS sample, E0=6.19eV, α=7.84×10⁻⁴eV/K, b=672K. The physical mechanisms of bandgap narrowing include the increase in interatomic spacing due to thermal expansion and electron–phonon interactions.

Fig. 4 (a)(b) Ellipsometric parameters Δ of CSS and NPSS samples at different temperatures; (c)(d) Temperature dependence of refractive index n and extinction coefficient k for the CSS sample; (e) (αhν)² versus hν plot for bandgap extraction; (f) Varshni fits of bandgap as a function of temperature

The Urbach energy EU is a key parameter characterizing the degree of crystal disorder and defect density; its variation with temperature reflects the thermal stability of crystal quality. The EU value is 0.27eV for the CSS sample and 0.23eV for the NPSS sample. The lower EU indicates a lower density of band tail states in the NPSS sample, which is related to its more complete stress release. Transmission spectra show a local transmittance minimum in the 220–245nm range for the NPSS sample, attributed to light scattering by nanopores.

Fig. 5 Plot of ln(α) versus hν; a smaller slope indicates lower crystal disorder

2. Influence of Temperature on Electrical Properties of AlN and Device Performance

2.1 Carrier Transport and Contact Characteristics of Si-Implanted AlN Layers

Okumura et al. achieved Si doping in AlN layers by ion implantation ([Si]=2×10¹⁹cm⁻³), followed by annealing at 1500°C for 30 minutes. Hall effect measurements revealed that as the temperature increases from 500K to 900K, the electron concentration increases significantly from 2.5×10¹⁶cm⁻³ to 1.9×10¹⁸cm⁻³, attributed to thermal excitation promoting donor ionization; the electron mobility decreases from 19cm² V⁻¹ s⁻¹ to 3cm² V⁻¹ s⁻¹, mainly due to enhanced phonon scattering at high temperatures. Even at 900K, the electron concentration remains one order of magnitude lower than the implanted Si concentration. Okumura et al. attributed this to the formation of DX⁻ centers in AlN by Si, with an ionization energy of 320meV. Fitting the charge neutrality condition yields a donor activation rate of only 10% and a compensation ratio of 5.

The sheet resistance and specific contact resistivity measured by the transmission line method are shown in the following figure. Over the range of 500–1100K, the sheet resistance decreases from approximately 10⁶Ω/sq to 1×10⁵Ω/sq, and the specific contact resistivity decreases from about 10⁻¹Ω·cm² to 4.0×10⁻³Ω·cm². This decrease is attributed to enhanced thermionic emission and thermionic field emission mechanisms. Notably, irreversible degradation of the Ti/Al/Ti/Au electrodes occurs at 1150K, indicating that existing metallization schemes face stability challenges above 1100K.

Fig. 6 (a) Temperature dependence of sheet resistance and specific contact resistivity of Si-implanted AlN layers (500–1100K); (b) Temperature dependence of electron concentration and electron mobility (500–900K)

2.2 High-Temperature Rectification Characteristics of AlN Schottky Barrier Diodes

Okumura et al. fabricated AlN Schottky barrier diodes with Ni/Au anodes and Ti/Al/Ti/Au cathodes, and tested their current density–voltage characteristics over 300–1100K. The devices exhibit clear rectification characteristics at all tested temperatures. The turn-on voltage decreases from 1.47V at 500K to 1.16V at 1000K, primarily due to bandgap narrowing of AlN with increasing temperature.

Fig. 7 (a) Current density–voltage characteristics of AlN Schottky barrier diodes; (b) Temperature dependence of ideality factor and apparent barrier height

The ideality factor n decreases from values much larger than 1 at 300K to 1.01 at 1000K and 1100K, indicating that the Ni/AlN interface remains stable at high temperatures. The apparent barrier height increases from 1.31eV at 300K to about 3.5eV at elevated temperatures, attributed to inhomogeneous distribution of the interfacial barrier height. Fitting with a Gaussian distribution model yields a mean zero-bias barrier height φB0 = 3.5eV. The Schottky barrier diode exhibits a breakdown voltage of 610V at 300K, and the on/off ratio remains on the order of ~10⁵ over the range of 600–1000K. Calculations show that the intrinsic carrier concentration of AlN at 1100K is only 6×10⁸cm⁻³, far lower than the electron concentration, which is the fundamental reason for maintaining a high on/off ratio.

2.3 High-Temperature Operation of AlN MESFETs

Okumura et al. fabricated AlN MESFETs with a gate length of 2μm and gate width of 100μm. The devices exhibit good saturation characteristics and gate voltage modulation capability at 1000K. The maximum drain current is 2.2mA/mm (at Vgs = +20V), the on/off ratio exceeds 10², and the three-terminal breakdown voltage is 176V (at Vgs = -34V).

Fig. 8 (a) Cross-sectional schematic of the AlN MESFET and (b) output characteristics at 1000K

Fig. 9 (a) Transfer characteristics at different temperatures and (b) temperature dependence of maximum transconductance and threshold voltage

Analysis of the transfer characteristics shows that the subthreshold swing degrades from 0.42V/decade at 300K to 15V/decade at 1000K, mainly due to increased leakage through the unintentionally doped bottom AlN layer with rising temperature. The maximum transconductance g_m,max increases from 7.5×10⁻⁹S/mm at 300K to 2.8×10⁻⁵S/mm at 1000K, reflecting that the temperature-induced increase in electron concentration enhances the current more than the negative impact of reduced mobility. The threshold voltage shifts from -3.2V (300K) to -34V (1000K), consistent with theoretical calculations.

In summary, increasing temperature significantly releases compressive stress in AlN epitaxial layers and narrows its bandgap. Moreover, elevated temperature exponentially increases the electron concentration in Si-implanted AlN layers, allowing AlN Schottky barrier diodes to maintain an on/off ratio of about 10⁴ and a high barrier height of 3.5eV at 1100K, and enabling stable gate voltage modulation of AlN-based MESFETs at 1000K.

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References:

1. Wei, W., Peng, Y., Wang, J., Farooq Saleem, M., Wang, W., Li, L., … & Sun, W. (2021). Temperature dependence of stress and optical properties in AlN films grown by MOCVD. Nanomaterials, 11(3), 698.
2. Okumura, H., Watanabe, Y., & Shibata, T. (2023). Temperature dependence of electrical characteristics of Si-implanted AlN layers on sapphire substrates. Applied Physics Express, 16(6), 064005.