Effect of threading dislocations on carrier mobility in AlGaN/GaN quantum wells

Abstract

The various scattering mechanisms induced by dislocations have been reviewed and adapted to the case of threading dislocations in AlGaN/GaN quantum wells. These scattering mechanisms can be classified into two categories, the first one issuing straightforwardly from the dislocation strain field, the other one being due to the Coulomb potential created by electrons trapped on the energy states that dislocations may create in the GaN band gap. For the first category of mechanisms (strain field effects), we indicate that edge dislocations can only be connected with the so-called deformation potential, the piezoelectric coupling being ruled out because of the particular geometry of the threading dislocation strain fields. We show that the dislocation deformation potential can only be responsible for a very weak, even negligible, effect on the carrier mobility. Then, after a survey of the various results found in the literature concerning the possible existence of dislocation energy states we conclude that dislocations are responsible for the existence of shallow acceptor states below the conduction band and propose a model for describing the potential associated with such states when filled by electrons. More particularly, we show that the linear dislocation charge density resulting from the trapped carriers at dislocation states can not be uniform, as it is systematically assumed in the literature, and we propose a description of this linear charge density as a function of the dislocation energy state position and of the various features characterizing the quantum well. Using the scattering potential induced by such a spatially-dependent dislocation charge density together with the usual scattering mechanisms allows us to give an estimation of their effect on the free carriers’ mobility. We particularly show that at low carrier density (∼1012 cm−2) the mobility is mainly determined by the combination of dislocation scattering mechanisms and intrinsic scattering mechanisms. Finally we suggest that our model could be employed for determining the position of the dislocation energy level in the gap.

Barrier Strain and Carbon Incorporation‐Engineered Performance Improvements for AlGaN/GaN High Electron Mobility Transistors**

“…Firstly, phonon and carrier‐carrier scattering mechanisms result in strong mobility decay for very high carrier density. Therefore, sample D with a high carrier density in the 2DEG channel (1.45 × 10 13 cm −2 ) shows low 2DEG mobility (1300 cm 2 V −1 s −1 ) although the XRD results demonstrate a high material crystalline quality, and secondly, due to large screening effects in the high carrier density regime (2DEG) . Although the densities of dislocations in sample C increase slightly, the 2DEG mobility increases considerably (1900 cm 2 V −1 s −1 ).…”

Controlling a three dimensional electron slab of graded AlxGa1−xN

“…According to theoretical models, the main contribution to electron scattering due to dislocations in wz-III-nitride systems, originates from a-edge dislocations and from the edge component of (a + c) mixed dislocations. The contribution from c-screw dislocations was shown to have a negligible effect on the electron scattering (and consequently on the electron mobilities) in Al x Ga 1−x N/GaN based structures [42]. From TEM measurements it is seen that the edge dislocation density in sample S4 is ∼1.55 times greater than in S1, but magnetotransport measurements reveal a Hall mobility of ∼ 210 cm 2 /Vs in S4, much greater than the 15 cm 2 /Vs of S1.…”

Mobility Limitations due to Dislocations and Interface Roughness in AlGaN/AlN/GaN Heterostructure

“…The areal density of dislocations in the AlGaN layer is estimated to be 1 × 10 9 cm −2 by counting dislocations from plan-view TEM images (see, e.g., Figure 2 Dislocations can affect the 2DEG mobility in two ways: first, they induce local deformation of the crystal structure; secondly, the accumulated charges along the dislocation line can scatter electrons via Coulomb interactions. The effect of the local strain field on electron mobility is negligibly small compared with that of the Coulomb interactions [5]; therefore, only the Coulomb interactions are taken into account in the dislocation scattering in this study. Coulomb interactions are isotropic; thus, the calculations do not require determining the dislocation types.…”

“…The scattering theories of the 2DEG system have been well developed by several authors [5,[13][14][15][16][17][18][19]. We give below a brief description of the analytical expressions for the momentum relaxation time ( ) due to the important scattering mechanisms.…”

“…Great efforts have been made to improve the crystal quality of GaN films by preparing a nucleation layer, optimizing growth parameters, and so on, but GaN still has a dislocation density up to 10 8 ∼ 10 10 cm −2 [3]. Although highdensity dislocations do not significantly affect the efficiencies of GaN-based light-emitting diodes (LEDs) and laser diodes (LDs) [4], they may dominate the electron mobility in AlGaN/GaN heterostructures [5,6].…”