Si-doped AlGaN Research Articles

N-type conducting AlInN/GaN distributed Bragg reflectors with AlGaN graded layers

We obtained a 40-pair Si-doped n-type conducting AlInN/GaN distributed Bragg reflector (DBR) with a low surface pit density, 3.0 × 106 cm−2, by introducing 5 nm Si-doped Al0.39Ga0.61N graded layers grown at high temperature, 1150 °C. A combination of a 0.6 nm GaN cap layer on AlInN and a subsequent thermal cleaning during a temperature increase process up to 1150 °C for the following AlGaN graded layer growth was effective for a suppression of pit/threading dislocation generations at the interfaces of the AlInN layers and the AlGaN graded layers in the DBRs without any additional cleaning processes. We also found that an initial AlN mole fraction of 0.39 in the graded AlGaN layers provided the lowest vertical resistance of the Si-doped AlInN/GaN DBRs with the Si-doped AlGaN graded layers, suggesting that Al0.39Ga0.61N provides the lowest potential spike in the conduction band of the interface with Al0.82In0.18N among AlGaN alloys.

Improvements in characteristics of N-polar Si-doped AlGaN epi-layer grown on mis-oriented c-plane sapphire substrate

The N-polar Si-doped n-type AlGaN epi-layer with an Al composition up to 54% was successfully grown on vicinal c-plane sapphire substrates by metal-organic chemical vapor deposition technology. High-resolution X-ray diffraction, atomic force microscopy, and Hall effect measurement were used to evaluate the influence of the use of vicinal sapphire substrate on the crystalline quality, the surface morphology, and the electrical property of the N-polar Si-doped AlGaN epi-layer, respectively. The characterization results reveal that the use of an optimized vicinal sapphire substrate plays a vital role in the improvements in the crystalline quality, the surface morphology, and the electrical property for the N-polar Si-doped n-type AlGaN epi-layers. In fact, a relatively smooth surface morphology with a root-mean-square value as small as 17.3 nm, an electron concentration of 1.9 × 1019 cm−3, and an electron mobility of 23.5 cm2 V−1 S−1 were achieved with the N-polar Si-doped n-type AlGaN epi-layer sample grown on a 1° mis-oriented c-plane sapphire substrate in this study.

Correlation between electrical properties and growth dynamics for Si-doped Al-rich AlGaN grown by metal-organic chemical vapor deposition

Correlation between electrical properties and growth dynamics for Si-doped AlGaN with Al mole fraction above 60% has been investigated. It is found that the electron concentration decreases significantly when decreasing the growth rate, while the electron mobility experiences a non-monotonic process of increasing at first and then decreasing. Combination of secondary ion mass spectroscopy and panchromatic cathodoluminescence results, reveals that the evolution of electrical properties mainly originates from compensation of III vacancy (VIII) to Si dopant, making VIII-nSi complexes, i.e., the concentrations of VIII-nSi complexes increase with decreasing the growth rate, implying high growth rate principle is vital for n-AlGaN.

Improvements in Characteristics of N-Polar Si-Doped Algan Epi-Layer Grown on Mis-Oriented C-Plane Sapphire Substrate

Improvements in Characteristics of N-Polar Si-Doped Algan Epi-Layer Grown on Mis-Oriented C-Plane Sapphire Substrate

Improvements in Characteristics of N-Polar Si-Doped Algan Epi-Layer Grown on Mis-Oriented C-Plane Sapphire Substrate

Improvements in Characteristics of N-Polar Si-Doped Algan Epi-Layer Grown on Mis-Oriented C-Plane Sapphire Substrate

Study on avalanche breakdown and Poole–Frenkel emission in Al-rich AlGaN grown on single crystal AlN

We demonstrate that theoretical breakdown fields can be realized in practically dislocation free Al-rich AlGaN p-n junctions grown on AlN single crystal substrates. Furthermore, we also demonstrate a leakage current density in AlGaN that is independent of the device area, indicating a bulk leakage phenomenon and not surface or mesa-edge related. Accordingly, we identified the Poole–Frenkel emission from two types of point-defect traps in AlGaN as the primary source of reverse leakage before breakdown. Mg-doped AlGaN exhibited leakage currents due to a shallow trap at ∼0.16 eV in contrast with leakage currents observed in Si-doped AlGaN due to a deep trap at ∼1.8 eV.

Defect controls by silicon doping in non-polar a-plane AlGaN epi-layers

Deposition of high-quality Si-doped crystalline AlGaN layers, especially non-polar-grown AlGaN layers, is critical and remains difficult in preparing AlGaN-based light-emitting diodes (LEDs), as the Si-doping-induced variations of crystalline structures are still under exploration. In this work, structural characterizations of Si-doped AlxGa1−xN layers were carried out by associating with examination of their carrier recombination behaviors in photoluminescence (PL) processes, to clarify the physical mechanism on how Si doping controls the formation of structural defects in AlGaN alloy. The obtained results showed that Si doping induced extrinsic shallow donor states and increased the densities of point defects like cation vacancies. On the contrary, Si doping suppressed formation of line defects like dislocations and planar defects like stacking faults with suitable doping concentration. These results may guide further improvement of UV-LEDs based on AlGaN alloy.

Realization low resistivity of high AlN mole fraction Si-doped AlGaN by suppressing the formation native vacancies

The influence of resistivity for n-AlGaN is investigated by changing Si concentration, growth temperature and growth rate. It is found that the resistivity strongly depended on the growth conditions and it reaches to a small value of 5 × 10−3 Ω cm at the growth temperature of 1040 °C. In addition, it is found that there is a strong correlation between the resistivity and the PL emission intensity of group-III-vacancy–Si (VIII-Si) complexes. It indicates that defect compensation can play a key role in high resistivity of n-AlGaN films.

Self-compensation in heavily Ge doped AlGaN: A comparison to Si doping

Self-compensation in Ge- and Si-doped Al0.3Ga0.7N has been investigated in terms of the formation of III vacancy and donor-vacancy complexes. Both Ge- and Si-doped AlGaN layers showed a compensation knee behavior with impurity compensation (low doping regime), compensation plateau (medium doping regime), and self-compensation (high doping regime). A maximum free carrier concentration of 4–5 × 1019 cm−3 was obtained by Ge doping, whereas Si doping resulted in only half of that value, ∼2 × 1019 cm−3. A DFT calculation with the grand canonical thermodynamics model was developed to support the hypothesis that the difference in self-compensation arises from the difference in the formation energies of the VIII-n•donor complexes relative to their onsite configurations. The model suggested that the VIII-2•donor and VIII-3•donor complexes were responsible for self-compensation for both Ge- and Si-doped AlGaN. However, a lower free carrier concentration in Si-doped samples was due to a high VIII-3•Si concentration, resulting from a lower energy of formation of VIII-3•Si.

A systematic comparison of polar and semipolar Si-doped AlGaN alloys with high AlN content

With a view to supporting the development of ultra-violet light-emitting diodes and related devices, the compositional, emission and morphology properties of Si-doped n-type Al x Ga1-x N alloys are extensively compared. This study has been designed to determine how the different Al x Ga1-x N crystal orientations (polar (0001) and semipolar (11–22)) affect group-III composition and Si incorporation. Wavelength dispersive x-ray (WDX) spectroscopy was used to determine the AlN mole fraction (x ≈ 0.57–0.85) and dopant concentration (3 × 1018–1 × 1019 cm−3) in various series of Al x Ga1-x N layers grown on (0001) and (11–22) AlN/sapphire templates by metalorganic chemical vapor deposition. The polar samples exhibit hexagonal surface features with Ga-rich boundaries confirmed by WDX mapping. Surface morphology was examined by atomic force microscopy for samples grown with different disilane flow rates and the semipolar samples were shown to have smoother surfaces than their polar counterparts, with an approximate 15% reduction in roughness. Optical characterization using cathodoluminescence (CL) spectroscopy allowed analysis of near-band edge emission in the range 4.0–5.4 eV as well as various deep impurity transition peaks in the range 2.7–4.8 eV. The combination of spatially-resolved characterization techniques, including CL and WDX, has provided detailed information on how the crystal growth direction affects the alloy and dopant concentrations.

Electrical compensation and cation vacancies in Al rich Si-doped AlGaN

We report positron annihilation results on vacancy defects in Si-doped Al0.90Ga0.10N alloys grown by metalorganic vapor phase epitaxy. By combining room temperature and temperature-dependent Doppler broadening measurements, we identify negatively charged in-grown cation vacancies in the concentration range from below 1×1016 cm−3 to 2×1018 cm−3 in samples with a high C content, strongly correlated with the Si doping level in the samples ranging from 1×1017 cm−3 to 7×1018 cm−3. On the other hand, we find predominantly neutral cation vacancies with concentrations above 5×1018 cm−3 in samples with a low C content. The cation vacancies are important as compensating centers only in material with a high C content at high Si doping levels.

Improvement in structural and electrical characteristics of nonpolar a-plane Si-doped n-AlGaN

Nonpolar Si-doped AlGaN with high electron concentration (EC) and superior surface morphology (SM) layers were successfully grown on r-plane sapphire substrates for the first time with metal organic chemical vapor deposition technology. X-ray diffraction, cross-sectional scanning electron microscope, ultraviolet–visible absorption spectroscopy, atomic force microscopy, and Hall measurements were used to examine the influence of indium surfactant and an extra AlGaN buffer on crystalline quality (CQ), SM, and electrical characteristics of the nonpolar Si-doped n-AlGaN, respectively. The characterization results revealed that the anisotropy in CQ of the nonpolar Si-doped n-AlGaN epi-layers was suppressed effectively, and the SM, as well as the electrical properties including EC, electron mobility (EM), and electron resistivity (ER), had been significantly improved by using indium surfactant and an extra AlGaN buffer layer. In fact, an EC as high as 4.8 × 1017 cm−3, an EM up to 3.42 cm2/V s, an ER down to 4.78 $$\Omega \bullet $$cm , and a root-mean-square value as small as 8.2 nm for surface roughness was achieved for the nonpolar Si-doped n-AlGaN with a relatively high Al composition of 50%.

The role of chemical potential in compensation control in Si:AlGaN

Reduction in compensation in Si-doped Al-rich AlGaN is demonstrated via chemical potential control (CPC). The chemical potentials and the resulting formation energies of carbon on the nitrogen site (CN) and cation vacancy complex with Si (VIII + nSiIII) were related to growth variables through a thermodynamic supersaturation model, which quantitatively predicted the incorporation of CN and the generation of the VIII + nSiIII complex. The compensation “knee” behavior, i.e., decreasing conductivity with increasing Si incorporation beyond a certain concentration, was successfully controlled. The maximum free carrier concentration was improved by impeding the formation of VIII + nSiIII complexes under III-richer conditions, while the impurity compensation by CN was reduced by making the growth environment N-richer. The results of Hall effect measurement and photoluminescence agreed well with quantitative theoretical predictions of the CPC model. Based on the developed model, the highest conductivity of 160 Ω−1 cm−1 with free carrier concentration of 3 × 1019 cm−3 in Al0.7Ga0.3N ever reported was achieved on single crystal AlN substrates. The demonstrated predictive power of the CPC model should greatly reduce the empirical analysis or iterative experimentation that would otherwise be necessary.

Thermionic emission and conversion properties of n-type AlGaN thin film cathodes grown on 6H–SiC substrates

We observed the thermionic emission (TE) and conversion (TC) characteristics of Si-doped AlGaN thin film cathodes grown on n-type 6H-SiC substrates with Cs adsorption on the surfaces. The threshold temperatures of TE decreased as the AlN mole fraction was increased, with threshold temperatures of around 205 °C for Si–doped Al0.5Ga0.5N. TE was detected even when the applied external voltage was 0 V at 600 °C, which means that the cathodes had TC characteristics at a temperature dramatically lower than that of conventional systems with metal-based cathodes. In addition, we observed a notable enhancement of the emission current when the cathodes were irradiated with white light having energy lower than that of the bandgap of the AlGaN films at low temperatures. We suggest that the enhancement was caused by an internal photoelectron effect due to the absorption of the light in the n–type 6H–SiC substrate.

High Channel Conductivity, Breakdown Field Strength, and Low Current Collapse in AlGaN/GaN/Si <inline-formula> <tex-math notation="LaTeX">$\delta$ </tex-math> </inline-formula>-Doped AlGaN/GaN:C HEMTs

This paper reports the AlGaN/GaN/ Si δ-doped AlGaN/GaN:C HEMT device on silicon with high channel conductivity, high breakdown field (E-field) strength, and low current collapse by using the Si-doped AlGaN back barriers. The Si δ-doped AlGaN back barrier was used to compensate for the reduction of channel conductivity as a result of a carbon-doped semiinsulating GaN buffer layer. The maximum drain current increases from 412 to 720 mA/mm, and peak extrinsic transconductance is improved from 103 to 210 mS/mm. Due to the reduction of electric field between the gate and drain along the GaN channel by inserting the Si δ-doped AlGaN back barrier layer, it can effectively suppress the capture of electrons in channel by carbon-induced accepted traps in the GaN:C buffer. Combined with the high conductivity of Si δ-doped AlGaN back barrier and high resistance of GaN:C buffer, the device showed the high breakdown E-field strength and the low specific on-resistance. Our proposed device is observed to hold a gate-drain voltage of 769 Vat 10 μA/mm (7-μm gate-drain spacing) and 0.53 mQ · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and the gateto-drain electric field corresponds to 1.1 MV/cm.

Suppression of unintentional carbon incorporation in AlGaN-based near-ultraviolet light-emitting diode grown on Si

AlGaN-based near-ultraviolet light-emitting diodes (NUV-LEDs) emitting at 370 nm were grown on Si(111) substrates by metal-organic chemical vapor deposition. The effect of growth parameters of Si-doped n-type AlGaN thick layer on the material quality and optical performance was studied. Photoluminescence measurements showed that the near-band-edge emission of n-AlGaN was greatly increased and the yellow luminescence (YL) was substantially reduced, when the n-AlGaN layer was grown at a high temperature, a high chamber pressure, and a low growth rate. It was found that the reduced unintentional carbon incorporation in the n-AlGaN layer under those growth conditions was responsible for the improved optical property. The NUV-LED employing the optimized growth parameters of n-AlGaN showed an enhanced light output power and a suppressed YL emission, as well as a better color purity, as compared with the reference one. The results indicate that performance of NUV-LED can be significantly improved by suppressing unintentional carbon incorporation and the defects-related absorption/re-emission in the n-AlGaN.

Silicon doping of semipolar [formula omitted

The effect of silicon doping on the growth and properties of ∼1.0μm-thickAlxGa1-xN(0.50≤x≤0.55) layers grown on semipolar (112¯2)AlN templates by metalorganic vapour phase epitaxy was studied. The layers were grown with different disilane/group-III precursors ratios that varied from 2.8×10−5 to 3.4×10−4. The surface morphology of the Si-doped (112¯2)AlGaN layers showed undulations along [11¯00]AlGaN,AlN with a root-mean square roughness of about 4.0nm within a scan range of 20×20μm2. Different photoluminescence peaks have been linked to negatively charged cation vacancies (VIII3−) and their complexes with impurities such as VIII3−-3ON1+, (VIII complex)1−, and (VIII complex)2−. The optimised AlGaN:Si layer exhibited a carrier concentration of ∼1.2×1019cm−3, a carrier mobility of 30.7cm2/Vs, and a resistivity of 0.018Ωcm, as determined by Hall-effect measurements at room temperature. A correlation between the resistivity and luminescence emission intensities of AlGaN near-band-edge and impurity-related complexes was found.

Electrical properties of n-type AlGaN with high Si concentration

The electrical properties of Si-doped AlGaN layers (AlN molar fractions: 0.03–0.06) with the donor concentrations (ND) from 8.8 × 1017 to 4.5 × 1020 cm−3 were investigated by variable-temperature Hall effect measurement using the van der Pauw method. A minimum resistivity of 3.6 × 10−4 Ω cm was obtained for Si-doped AlGaN with a smooth surface at room temperature. We found that the activation energy of the Si donor is affected by the Coulomb interaction in the AlGaN layer with ND values from 8.8 × 1017 to 2.5 × 1020 cm−3. In several AlGaN layers, the free-electron concentration did not vary with sample temperature, as expected in the case of degeneracy. The localization of GaN in the AlGaN layer was speculated as a cause of degeneracy of samples.

Microscopic potential fluctuations in Si-doped AlGaN epitaxial layers with various AlN molar fractions and Si concentrations

Nanoscopic potential fluctuations of Si-doped AlGaN epitaxial layers with the AlN molar fraction varying from 0.42 to 0.95 and Si-doped Al0.61Ga0.39N epitaxial layers with Si concentrations of 3.0–37 × 1017 cm−3 were investigated by cathodoluminescence (CL) imaging combined with scanning electron microscopy. The spot CL linewidths of AlGaN epitaxial layers broadened as the AlN molar fraction was increased to 0.7, and then narrowed at higher AlN molar fractions. The experimental linewidths were compared with the theoretical prediction from the alloy broadening model. The trends displayed by our spot CL linewidths were consistent with calculated results at AlN molar fractions of less than about 0.60, but the spot CL linewidths were markedly broader than the calculated linewidths at higher AlN molar fractions. The dependence of the difference between the spot CL linewidth and calculated line broadening on AlN molar fraction was found to be similar to the dependence of reported S values, indicating that the vacancy clusters acted as the origin of additional line broadening at high AlN molar fractions. The spot CL linewidths of Al0.61Ga0.39N epitaxial layers with the same Al concentration and different Si concentrations were nearly constant in the entire Si concentration range tested. From the comparison of reported S values, the increase of VAl did not contribute to the linewidth broadening, unlike the case of the VAl clusters.

Effect of Capping on Electrical and Optical Properties of GaN Layers Grown by HVPE

Gallium nitride, grown by hydride vapor phase epitaxy and capped with a thin AlGaN layer, was studied by photoluminescence (PL) methods. The concentration of free electrons in GaN was found from the time-resolved PL data, and the concentrations of point defects were estimated from the steady-state PL measurements. The intensity of PL from GaN decreases moderately after capping it with Si-doped AlGaN, and it decreases dramatically after capping with Mg-doped AlGaN. At the same time, the concentration of free electrons and the concentrations of main radiative defects in GaN are not affected by the AlGaN capping. We demonstrate that PL is a powerful tool for nondestructive characterization of semiconductor layers buried under overlying device structures.