Low background doping in AlInN grown on GaN via metalorganic vapor phase epitaxy

Highlights

• Nearly lattice-matched AlInN films with low background doping on top of GaN/sapphire templates.

• The optical constants (n_r & k) determined via spectroscopic ellipsometry.

• Temperature-dependent Hall measurements show n ~ 3 × 10¹⁷ cm⁻³ and μ_e ~ 320 cm²/V∙s at RT.

• TCAD simulations of AlInN/GaN interface show negligible 2DEG contribution to Hall measurement.

Abstract

Nearly lattice-matched and unintentionally doped AlInN films with low background doping grown via metalorganic vapor phase epitaxy on GaN/sapphire are investigated. The lattice-matched condition is verified with x-ray diffraction (XRD), and the films exhibit typical morphological characteristics for AlInN. The optical constants (n_r & k) and thicknesses of the AlInN films are determined via spectroscopic ellipsometry, finding an n_r ~ 2.2 at 500 nm and a bandgap of ~4.366 eV. Temperature-dependent Hall measurements in the Van der Pauw configuration are performed for temperatures from 80 K up to 350 K, and a low background doping concentration (n ~ 3 × 10¹⁷ cm⁻³) and high electron mobility (μ_e ~ 320 cm²/V∙s) are found at room temperature. Simulations are performed to determine the influence of the 2-D electron gas (2DEG) caused by polarization fields from the GaN/AlInN interface and validate the Hall measurements. Thus, this work shows the potential of achieving high-quality AlInN films with low background doping densities for use in power electronic devices and deep-ultraviolet light-emitting diodes.

Introduction

III-nitride semiconductors are a leading material platform for next generation of devices because of their ability to tune the energy band gap from 0.7 eV (InN) up to 6.2 eV (AlN), along with their high thermal, chemical and mechanical stability [1], [2], [3]. Its largest impact is in the field of solid-state lighting, where the Nobel Prize in Physics was awarded in 2014 for advances in blue light-emitting diodes [4]. Recent advances have also demonstrated, with one more recent development of using GaN to replace Si-based power devices [5]. In addition to GaN for power devices, other ultrawide bandgap (UWBG) semiconductors like AlGaN, BN and β-Ga₂O₃ are sought out due to their ability of achieving high electric fields before impact ionization, leading to a high-power figure-of-merit (FOM). UWBG semiconductors do have challenges though, such as the difficulty of achieving p-type doping in β-Ga₂O₃ [6] and no suitable lattice-matched substrates for high Al-content AlGaN [5].

The ternary Al_xIn_1-xN alloys are also part of the UWBG family with a similar power FOM to β-Ga₂O₃ and AlGaN [19]. The strain state (tensile or compressive) can be controlled when grown on GaN substrates, and when x ~ 0.83, it is lattice-matched to GaN. Lattice matched growth of AlInN to GaN has led to high-quality distributed Bragg reflectors (DBRs) [7], cladding layers in laser diodes [8], photodetectors [9], high-mobility field-effect transistors [10], [11], electron barrier layers in InGaN-based light-emitting diodes [12], and thermoelectricity applications [13], [14]. In addition, AlInN films can be p- and n-type doped [15], [16], can be oxidized [17], have high thermal stability [18] and possess a large energy band gap (~4.4 eV) when grown lattice-matched to GaN [7]. We recently proposed the use of AlInN in vertical power devices [19]. Therefore, the advantageous material properties of AlInN coupled with the potential innovative device design warrant further investigations.

Despite extensive research efforts devoted to understanding the growth mechanisms of high-quality “thick” (>300 nm) AlInN, achieving low background doping densities is a significant challenge [14], [15], [20], [21]. AlInN films grown via metalorganic vapor phase epitaxy (MOVPE) require a relatively low growth temperature (~800 °C) in order to achieve the proper In-content when lattice matched to GaN. Growth at these temperatures can result in the incorporation of carbon and oxygen impurities into the film [14], [15], [22] and have high background doping concentrations on the order of ~10¹⁸–10¹⁹ cm⁻³ [14], [15], [23]. Thus, reducing the background doping concentration and realizing thick AlInN layers are exciting challenges that will enable the use of AlInN as p-type layers and drift layers in power electronics devices.

In this work, we report on the recently improved growth conditions of unintentionally doped, nearly lattice-matched Al_xIn_1-xN (x ~ 0.82) films grown via MOVPE on top of GaN on sapphire templates. The structural and morphological characteristics of the Al_0.82In_0.18N film is determined by x-ray diffraction (XRD) and atomic force microscopy (AFM). Spectroscopic ellipsometry (SE) measurements are employed to estimate the thickness of the AlInN film at ~275 nm, and evaluate its optical constants (refractive indices, n_r, and extinction coefficients, k). Temperature-dependent Hall measurements for temperatures from 80 K up to 350 K are performed in the Van der Pauw configuration to obtain the electron concentration (n) and mobility (μ_n) of the AlInN film. The Hall measurements at room temperature reveal that n ~ 3 × 10¹⁷/cm³ and μ_n ~ 320 cm²/V∙s. In addition, simulations are performed to determine the influence of the 2D electron gas (2DEG) on the electron concentration arising from the polarization-induced electric field at the AlInN/GaN interface and validate the Hall measurements. Finally, impurities such as oxygen and carbon that effect the overall electron concentration are determined from secondary-ion mass spectroscopy (SIMS). This data suggests the promising potential of lowering the background doping density of nearly lattice-matched AlInN to GaN, which has profound impact on the potential integration of AlInN films in III-nitride-based power electronic devices and laser diodes.