Si-doped AlN Research Articles

Fully vertical AlN-on-SiC Schottky barrier diodes

We demonstrated fully vertical Schottky barrier diodes (SBDs) that have a Si-doped AlN drift layer directly grown on an n-type 4H-SiC substrate by metal-organic CVD. The AlN SBD with a Ni anode showed a clear rectifying characteristic at 300–500 K and a rectification ratio of about 10–2. We found that the leakage current of the vertical AlN-on-SiC devices is affected by defects in the AlN drift layer and Schottky interface.

Demonstration of near-ideal Schottky contacts to Si-doped AlN

Near-ideal behavior in Schottky contacts to Si-doped AlN was observed as evidenced by a low ideality factor of 1.5 at room temperature. A temperature-independent Schottky barrier height of 1.9 eV was extracted from temperature-dependent I–V measurements. An activation energy of ∼300 meV was observed in the series resistance, which corresponded to the ionization energy of the deep Si donor state. Both Ohmic and Schottky contacts were stable up to 650 °C, with around four orders of magnitude rectification at this elevated temperature. These results demonstrate the potential of AlN as a platform for power devices capable of operating in extreme environments.

High electron mobility in AlN:Si by point and extended defect management

High room temperature n-type mobility, exceeding 300 cm2/Vs, was demonstrated in Si-doped AlN. Dislocations and CN−1 were identified as the main compensators for AlN grown on sapphire and AlN single crystalline substrates, respectively, limiting the lower doping limit and mobility. Once the dislocation density was reduced by the growth on AlN wafers, C-related compensation could be reduced by controlling the process supersaturation and Fermi level during growth. While the growth on sapphire substrates supported only high doping ([Si] > 5 × 1018 cm−3) and low mobility (∼20 cm2/Vs), growth on AlN with proper compensation management enabled controlled doping at two orders of magnitude lower dopant concentrations. This work is of crucial technological importance because it enables the growth of drift layers for AlN-based power devices.

The optimal threading dislocation density of AlN template for micrometer-thick Al0.63Ga0.37N heteroepitaxy

Growth of Si-doped AlN and of Al0.63Ga0.37N on high quality AlN templates grown by HVPE has been investigated. The strain state of Si-doped AlN is thickness-dependent due to the surface-mediated dislocation tilt which induces a tensile strain component. At very low dislocation density in the AlN buffer strain relaxation in micrometer-thick Al0.63Ga0.37N layers is dominated by generation of additional dislocations. As a result, the final threading dislocation density and the surface roughness increase significantly. For a 1.6 μm Al0.63Ga0.37N on 3.4 μm Si-doped AlN bi-layer structure, the optimal threading dislocation density (TDD) of the AlN:Si buffer is estimated to be 7 × 108 cm−2, where the low TDD can be still transferred from the buffer to the thick AlGaN heterostructure without generation of many new dislocations.

The Theoretical Study of Unexpected Magnetism in 2D Si-Doped AlN

In this study, the structural and magnetic properties of Si-doped bulk and 2D AlN were systematically investigated by first-principles calculations. Si atoms prefer to substitute Al atoms in both bulk and 2D AlN under N-rich growth conditions. In bulk AlN, Si dopants exhibit a non-magnetic state, uniform distribution, and a strong anisotropic diffusion energy barrier. In contrast to that, Si dopants prefer to form a buckling structure and exhibit a magnetic moment of 1 μB in 2D AlN. At a low Si concentration, Si atoms tend to get together with antiferromagnetic coupling between each other. However, the magnetic coupling among Si atoms changes to ferromagnetic coupling as Si concentration increases, due to the enhanced exchange splitting and delocalized impurity states. At the extreme doping limit, monolayer SiN, along with its analogs GeN and SnN, is a ferromagnetic semiconductor with a large band gap and high Curie temperature. These results indicate that 2D AlN doped by group IV atoms has potential applications in spintronic devices.

Impact of Si doping on dislocation behavior in MOVPE-grown AlN on high-temperature annealed AlN buffer layers

In this work, we compare the defect structure in unintentionally doped and Si-doped AlN layers grown by metalorganic vapor phase epitaxy (MOVPE) on high-temperature annealed (HTA) sputtered AlN templates on sapphire substrates. Since the HTA process leads to a reduction of the in-plane lattice constant of the AlN layers, further homoepitaxial overgrowth results in compressively strained AlN layers. With increasing MOVPE-AlN layer thickness, strain relaxation takes place mostly by formation of dislocation half-loops of an irregular shape, which accumulate at the homoepitaxial MOVPE-AlN/HTA-AlN interface. We suggest that these dislocations nucleate at the layer surface and move down to the homoepitaxial interface at high temperatures. The formation of these irregular and hardly controllable defects can be avoided by introduction of Si-doping into the MOVPE-AlN layers. Si-doping enlarges the inclination of threading dislocation lines stemming from the HTA-AlN template, producing an alternative mechanism for strain relaxation.

Growth and characterization of PALE Si-doped AlN on sapphire substrate by MOVPE

In this study, we report different SiH4 flow condition effects on crystal, surface, optical, and electrical characteristics of heteroepitaxial Metal-Organic Vapor Phase Epitaxy (MOVPE) grown AlN layers on sapphire substrates. Adjustment of growth kinetics is very important to control the doping. Therefore, pulsed atomic layer epitaxy (PALE) was used to control the growth kinetics and reduce parasitic reactions that inevitably caused adverse impact on the properties of the epitaxial AlN films. As a result of HRXRD (high resolution x-ray diffraction) analysis, the (002) ω FWHM decreased significantly with the PALE method, while the increase occurred due to the development of V defects for the (102) ω scan. Atomic force microscopy (AFM) analyzes showed that SiH4 led to a 3D-like growth mode. It was demonstrated that the increased SiH4 flow increased Si incorporation into the Si-doped AlN layer while increased the sheet resistance due to the self-compensating effect obtained from secondary ion mass spectroscopy (SIMS) and I–V measurement results.

High-quality AlGaN epitaxy on lattice-engineerable AlN template for high-power UVC light-emitting diodes

AlGaN-based UVC light-emitting diodes (LED) were fabricated on high-quality AlN templates with an engineerable in-plane lattice constant. The controllability of the in-plane strain originated from the vacancy formation in Si-doped AlN (AlN:Si) and their interaction with edge dislocations. The strain state of the Si:AlN top interface could be well depicted by a dislocation-tilt model depending on the buffer strain state, threading dislocation density (TDD), and regrown Si:AlN thickness. The validity of the model was verified by cross-sectional TEM analysis. With a gradually widened lattice constant of regrown Si:AlN layer, strain-induced defects of subsequently grown n-AlGaN was suppressed. Therefore, growing a current spreading layer which possesses a moderate Al content (<65%), decent thickness (>1.5 µm), and a low TDD (<1.0 × 109 cm−2) simultaneously becomes possible. Additionally, the idea of an optimal edge TDD (ρe,opt) in the AlN buffer was revealed for growing high-quality n-AlGaN layers with a targeted thickness. After a deliberate strain-TDD engineering for Si:AlN and n-AlGaN, high-power UVC LEDs (λ = 275 nm, P > 200 mW) with a low forward voltage (Vf = 5.7 volt) were demonstrated at I = 1.35 A. The low forward voltage under high current injection density was attributed to the success in preparation of a low series resistance and high-quality n-AlGaN current spreading layer.

Exploring the electronic properties of Si-doped AlN (0001) surface with Cs adsorption layer for solar cell anode applications

The first-principles calculations are utilized to systematically investigate the adsorption behaviors of Cs on Si-doped AlN(0001) surfaces and its potential applications as anode material with low work function. Five Si substitution doping models and eight Cs adsorption surface models are established, respectively. Results show that Si atom tends to replace surface Al atom to form an n-type doping structure. Based on the most stable Si-doped surface, different Cs adsorption configurations with different adsorption sites and Cs coverage are analyzed. During Cs adsorption process, Cs acts as the electron donor and AlN surface is the electron acceptor. The large charge transfer at Cs adsorption interface induces a dipole moment pointing from Cs to AlN surface, which can effectively cut down the surface work function. Specifically, the 0.75 ML Cs-adsorbed surface shows the lowest work function of 0.62 eV, which has great potential application as the anode material of PETE devices. The findings in this work are generally applicable, which may shed new light on future exploration and development of low work function anode materials of PETE devices.

High-Quality A1GaN Epitaxy on Lattice-Engineerable AlN Template for High-Power UVC Light-Emitting Diodes

AlGaN-based UVC light-emitting diodes (LED) were fabricated on high-quality AlN templates with an engineerable in-plane lattice constant. The controllability of the in-plane strain originated from the vacancy formation in Si-doped AlN (AlN:Si) and their interaction with edge dislocations. The strain state of the AlN:Si top interface could be well depicted by a dislocation-tilt model depending on the buffer strain state, threading dislocation density (TDD), and regrown AlN:Si thickness. The validity of the model was verified by cross-sectional TEM analysis. With a gradually widened lattice constant of regrown AlN:Si layer, strain-induced defects of subsequently grown n-AlGaN was suppressed. Therefore, growing a current spreading layer which possesses a moderate Al content ( 1.5 μm), and a low TDD ( 200 mW) with a low forward voltage (V f =5.7 volt) were demonstrated at I=1.35 A. The low forward voltage under high current injection density was attributed to the success in preparation of a low series resistance and high-quality n-AlGaN current spreading layer.

Poisonous Vapor Adsorption on Pure and Modified Aluminum Nitride Nanosheet for Environmental Safety: A DFT Exploration

Through Density Functional Theory (DFT), we have unveiled the atomic structures, adsorption characteristics and electronic structures of the poisonous and explosive vapor, m-dinitrobenzene (m-DNB), on pure, defective and various doped AlN nanosheets from a physical perspective. It is found that the adsorption energy, band gap change and sensitivity to the vapor are significantly increased through atomic-scale modification of the nanosheet. The AlN monolayer with Al-N divacancy has the largest adsorption energy and has potential to be utilized as adsorption or filtration materials for m-DNB vapor. The Si-doped AlN nanosheet possesses a much larger band gap change (−0.691 eV) than the pure nanosheet (−0.092 eV) after adsorption and has a moderate adsorption energy, which could be candidate material for explosive vapor sensing. This theoretical work is proposed to provide guidance and clue for experimentalists to develop more effective two-dimensional materials for environmental safety and sustainability.

Hydride vapor phase epitaxy of Si-doped AlN layers using SiCl4 as a doping gas

Growth of Si-doped AlN layers by hydride vapor phase epitaxy on AlN/sapphire templates prepared by metalorganic vapor phase epitaxy and bulk AlN substrates prepared by physical vapor transport was investigated using silicon tetrachloride (SiCl4) as a doping gas. On the AlN/sapphire templates, when the SiCl4 supply was low, the incorporation ratio of Si decreased due to the influence of O impurities resulting from the decomposition of the sapphire. In addition, pits were formed on the surfaces, and the pit density increased significantly at Si concentrations exceeding 2 × 1019 cm−3. In contrast, on the bulk AlN substrates, the Si concentration increased linearly with increasing SiCl4 supply, and a pit- and stress-free layer could be grown with a Si concentration as high as 6.5 × 1019 cm−3. Hall effect measurements revealed that this layer exhibited n-type conductivity with a donor activation energy of 253 meV. However, the carrier density at room temperature (RT) was as low as 3.6 × 1013 cm−3 because of the high compensation ratio due to the presence of acceptors. A broad peak centered at 3.3 eV was observed in the RT photoluminescence spectra of the Si-doped AlN layers grown on the bulk AlN substrates, indicating that Al vacancies formed by the Fermi level effect due to Si doping acted as acceptors that compensated for carriers.

Ohmic contact to AlN:Si using graded AlGaN contact layer

We formed a graded-AlGaN contact layer to improve the Ohmic characteristics of Si-doped AlN. Linear I-V characteristics were obtained for AlN with the graded-AlGaN layer, and the current was three orders of magnitude larger than that for AlN without the one. The specific contact resistivity decreased with the increasing thickness of the graded-AlGaN layer. This was probably due to a reduction in the three-dimensional negative charge density induced by the polarization charge in the graded AlGaN layer. A minimum contact resistivity of 1.4 ×10−2 Ω cm2 was obtained for a 330-nm-thick graded-AlGaN layer. To obtain the Ohmic contact, the Si-dopant concentration (NSi) should be larger than the negative fixed charge density (ρπ) induced by the polarization charge. However, the heavily doped graded-AlGaN layer (NSi=2.4×1019 cm−3) became semi-insulating due to self-compensation. The results indicated that reducing ρπ by relaxing the compositional slope in the graded layer can improve the Ohmic characteristics.

Growth of Si-doped AlN on sapphire (0001) via pulsed sputtering

The Si-doped AlN films were grown on sapphire (0001) by pulsed sputtering deposition (PSD), and their structural and electrical properties were investigated. A combination of PSD and high-temperature annealing process enabled the growth of high-quality AlN (0001) epitaxial films on sapphire (0001) substrates with atomically flat stepped and terraced surfaces. The transmission electron microscopy observations revealed that the majority of the threading dislocations in AlN belonged to the mixed- or edge-type, with densities of 2.8 × 108 and 4.4 × 109 cm−2, respectively. The Si-doping of AlN by PSD yielded a clear n-type conductivity with a maximum electron mobility of 44 cm2 V−1 s−1, which was the highest value reported for AlN that was grown on sapphire. These results clearly demonstrated the strong potential of the PSD technique for growing high-quality conductive n-type AlN on sapphire.

On compensation in Si-doped AlN

Controllable n-type doping over wide ranges of carrier concentrations in AlN, or Al-rich AlGaN, is critical to realizing next-generation applications in high-power electronics and deep UV light sources. Silicon is not a hydrogenic donor in AlN as it is in GaN; despite this, the carrier concentration should be controllable, albeit less efficiently, by increasing the donor concentration during growth. At low doping levels, an increase in the Si content leads to a commensurate increase in free electrons. Problematically, this trend does not persist to higher doping levels. In fact, a further increase in the Si concentration leads to a decrease in free electron concentration; this is commonly referred to as the compensation knee. While the nature of this decrease has been attributed to a variety of compensating defects, the mechanism and identity of the predominant defects associated with the knee have not been conclusively determined. Density functional theory calculations using hybrid exchange-correlation functionals have identified VAl+nSiAl complexes as central to mechanistically understanding compensation in the high Si limit in AlN, while secondary impurities and vacancies tend to dominate compensation in the low Si limit. The formation energies and optical signatures of these defects in AlN are calculated and utilized in a grand canonical charge balance solver to identify carrier concentrations as a function of Si content. The results were found to qualitatively reproduce the experimentally observed compensation knee. Furthermore, these calculations predict a shift in the optical emissions present in the high and low doping limits, which is confirmed with detailed photoluminescence measurements.

A computational study on the electronic and field emission properties of Mg and Si doped AlN nanocones

Following an experimental work, we explored the effect of replacing an Al atom of an AlN nanocone by Si or Mg atom on its electronic and field emission properties using density functional theory calculations. We found that both Si-doping and Mg-doping increase the electrical conductivity of AlN nanocone, but their influences on the filed emission properties are significantly different. The Si-doping increases the electron concentration of AlN nanocone and results in a large electron mobility and a low work function, whereas Mg-doping leads to a high hole concentration below the conduction level and increases the work function in agreement with the experimental results. It is predicted that Si-doped AlN nanocones show excellent filed emission performance with higher emitted electron current density compared to the pristine AlN nanocone. But the Mg-doping meaningfully decreases the emitted electron current density from the surface of AlN nanocone. The Mg-doping can increase the work function about 41.9% and the Si-doping can decrease it about 6.3%. The Mg-doping and Si-doping convert the AlN nanocone to a p-type and n-type semiconductors, respectively. Our results explain in a molecular level what observed in the experiment.

Excimer laser annealing: An alternative route and its optimisation to effectively activate Si dopants in AlN films grown by plasma assisted molecular beam epitaxy

Si doped AlN films were grown on GaN/Sapphire templates by plasma assisted molecular beam epitaxy (PAMBE) technique. We show that employing an excimer laser annealing with optimized power and frequency rather than the conventional thermal annealing could be a potential alternative route towards improving the structural and electrical properties of AlN layers. Upon optimized laser annealing of the Si-doped AlN layer, the electron concentration was achieved to be as high as ∼4.9×1018cm−3 which was measured by Raman spectroscopy measurement and was further cross checked by standard Hall measurement which estimated the same as ∼7.4×1018cm−3 with a mobility of 109cm2/V-sec. The improvement of free carrier concentration was leveraged by improvement of structural properties. The r.m.s surface roughness of the Si-doped AlN layers measured by atomic force microscopy was reduced to 0.76nm and corresponding residual stress estimated by high resolution XRD and Raman measurement was found to be less than half compared to the in-situ Si-doped sample. Thus laser annealing is proposed to be a suitable method to achieve high electron concentration in Si doped AlN films without compromising the structural quality.

Effects of Si-doping on field emission characteristics of AlN films grown on n-type 6H-SiC by MOCVD

The field-emission (FE) characteristics of 200 nm-thick Si-doped AlN films with different concentration of Si-dopant is investigated. It is found that the AlN film with lower concentration of Si-dopant has a smaller turn-on voltage, larger maximum current density and more stable FE current. Accompanying with atomic force surface micro-images, it is suggested that the stable current is attributed to a uniform surface, which is related a uniform local field enhancement factors and thus a stable FE current without discontinuous fluctuations. Furthermore, the analysis of scanning electron microscopy indicates that a smaller turn-on voltage and larger maximum current density may result from the electron transport channel of V-defects.

Stable ferromagnetic state in Si-doped AlN with cation vacancies: Ab-initio study

The magnetic property of Si-doped AlN with Al-vacancy is studied using first principles calculations based on spin polarized density functional theory. The Si dopant alone does not introduce the magnetic moment in AlN. However, the doping of Si in AlN reduces the formation energy caused by Al-vacancy, and stabilizes the spin polarized state. The magnetic moments are mainly localized on N atoms surrounding the defect. The strong ferromagnetic state is obtained in AlN due to the combined role of Al-vacancy and Si-dopant.

Observation of positive and small electron affinity of Si-doped AlN films grown by metalorganic chemical vapor deposition on n-type 6H–SiC**Project supported by the National Natural Science Foundation of China (Grant Nos. 61574135, 61574134, 61474142, 61474110, 61377020, 61376089, 61223005, and 61321063), the One Hundred Person Project of the Chinese Academy of Sciences, and the Basic Research Project of Jiangsu Province, China

We have investigated the electron affinity of Si-doped AlN films (NSi = 1.0 × 1018–1.0 × 1019 cm−3) with thicknesses of 50, 200, and 400 nm, synthesized by metalorganic chemical vapor deposition (MOCVD) under low pressure on the n-type (001)6H–SiC substrates. The positive and small electron affinity of AlN films was observed through the ultraviolet photoelectron spectroscopy (UPS) analysis, where an increase in electron affinity appears with the thickness of AlN films increasing, i.e., 0.36 eV for the 50-nm-thick one, 0.58 eV for the 200-nm-thick one, and 0.97 eV for the 400-nm-thick one. Accompanying the x-ray photoelectron spectroscopy (XPS) analysis on the surface contaminations, it suggests that the difference of electron affinity between our three samples may result from the discrepancy of surface impurity contaminations.