Significant contribution from impurity-band transport to the room temperature conductivity of silicon-doped AlGaN

Abstract

Silicon-doped n-type (0 0 0 1) AlGaN materials with 60% and 85% AlN content were studied close to the doping condition that gives the lowest resistivity (Si/III ratios in the ranges 2.8-34 × 10 −5 and 1.3-6.6 × 10 −5 , respectively). Temperature-dependent conductivity and Hall-effect measurements showed that, apart from the diffusion-like transport in the conduction band, a significant amount of the conductivity was due to phonon-assisted hopping among localized states in the impurity band, which became almost completely degenerate in the most doped sample of the Al 0.6 Ga 0.4 N series. In the doping range explored, impurityband transport was not only dominant at low temperature, but also significant at roomtemperature, with contributions to the total conductivity up to 46% for the most conductive sample. We show that, as a consequence of this fact, the measurements of Hall carrier concentration and Hall mobility using the usual single-channel approach are not reliable, even at high temperatures. We propose a simple method to separate the contributions of the two channels. Our model, although only approximate, can be used to gain insight into the doping mechanism: particularly it shows that the room-temperature free-electron concentration in the conduction band of the Al 0.6 Ga 0.4 N material reaches its maximum at about 1.6 × 10 18 cm −3 , well below the value that would have been obtained with the standard single-channel analysis of the data. This maximum is already achieved at dopant concentrations lower than the one that gives the best conductivity. However, further increase of the doping levels are required to enhance the impurity-band channel, with concentrations of the carriers participating in this type of transport that increase from 2.1 × 10 18 cm −3 up to 4.3 × 10 18 cm −3 . For the Al 0.85 Ga 0.15 N, even though it was not possible to estimate the actual carrier concentrations, our measurements suggest that a significant impurity-band channel is present also in this material.

Low resistivity of highly Si-doped n-type Al0.62Ga0.38N layer by suppressing self-compensation

“…1. [9][10][11][12][13][14][28][29][30][31][32][33][34][35][36] The lowest resistance of n-AlGaN with an AlN mole fraction of 62% was achieved in this study. The lowest resistivity value of the Al 0.62 Ga 0.38 N layer reached 6.6 × 10 −3 Ω cm at the Si concentration of 3.2 × 10 19 cm −3 .…”

“…(Color online) Resistivity values as a function of the AlN mole fraction for n-AlGaN layers. The data reported from other studies are indicated as open symbols,[9][10][11][12][13][14][28][29][30][31][32][33][34][35][36] while our results are shown as red closed symbols.…”

“…The reduction in the last-mentioned series resistance of n-AlGaN layers has been studied for application to UV LEDs with Al-rich n-AlGaN cladding layers. [9][10][11][12][13][14] There are two possible sources for increasing the resistance of an n-AlGaN layer. First, in the high AlN mole fraction n-AlGaN, a silicon (Si) atom no longer plays as a shallow donor due to the emergence of a DX state with the increase of the AlN mole fraction, resulting in increasing ionization energies.…”

Quantification of Trace-Level Silicon Doping in AlxGa1–xN Films Using Wavelength-Dispersive X-Ray Microanalysis

“…Figure 8 plots the calculated Si concentrations against the disilane to group III ratios (Si 2 H 6 /III ratios) (Dinh et al, 2016 a , 2016 b ; Pampili et al, 2018). In MOCVD growth, the group III atoms are introduced via metal precursors in the form of organic compounds such as trimethylgallium and trimethylaluminium, while ammonia is the nitrogen source.…”

“…These Al x Ga 1- x N samples included polar-oriented [0001] (samples AP, BP, CP) and semipolar-oriented layers [112-2] (samples ASP, CSP). More details on the growth of the Al x Ga 1– x N:Si layers from the Tyndall Institute are given elsewhere (Li et al, 2013; Dinh et al, 2016 a , 2016 b ; Pampili et al, 2018; Spasevski et al, 2021). Samples labeled TS are Al x Ga 1– x N layers with various AlN contents and crystal polarities grown at Technische Universität Berlin (Knauer et al, 2013; Kusch et al, 2014; Mehnke et al, 2016; Foronda et al, 2020).…”