A Chaotic Potential of Charged Dislocations in Group III-Nitride Heterojunctions

Abstract: The electronic properties of inversion and accumulation layers at semiconductor-insulator interfaces and of other systems that exhibit two-dimensional or quasi-two-dimensional behavior, such as electrons in semiconductor heterojunctions and superlattices and on liquid helium, are reviewed. Energy levels, transport properties, and optical properties are considered in some detail, especially for electrons at the (100) silicon-silicon dioxide interface. Other systems are discussed more briefly.

Abstract: A model to explain the observed low transverse mobility in GaN by scattering of electrons at charged dislocation lines is proposed. Filled traps along threading dislocation lines act as Coulomb scattering centers. The statistics of trap occupancy at different doping levels are investigated. The theoretical transverse mobility from Coulomb scattering at charged traps is compared to experimental data. Due to the repulsive potential around the charged dislocation lines, electron transport parallel to the dislocations is unaffected by the scattering at charged dislocation lines.

Abstract: Electron mobilities in GaN and InN are calculated, by variational principle, as a function of temperature for carrier concentrations of 1016, 1017, and 1018 cm−3 with compensation ratio as a parameter. Both GaN and InN have maximum mobilities between 100 and 200 K, depending on the electron density and compensation ratio, with lower electron density peaking at lower temperature. This is due to the interplay of piezoelectric acoustic phonon scattering at low carrier concentrations and ionized impurity scattering at higher carrier concentrations. Above 200 K, polar mode optical phonon scattering is the mobility limiting process. The 300 and 77 K electron and Hall mobilities as functions of carrier concentration in the range of 1016–1020 cm−3 and compensation ratio are also calculated. The theoretical maximum mobilities in GaN and InN at 300 K are about 1000 and 4400 cm2 V−1 s−1, respectively, while at 77 K the limits are beyond 6000 and 30 000 cm2 V−1 s−1, respectively. We compare the results with experimental data and find reasonable correlation, but with evidence that structural imperfection and heavy compensation play important roles in the material presently available. Only phonon limited scattering processes are considered in the calculation of the mobility in AlN since it is an insulator of extremely low carrier concentration. We find a phonon limited electron mobility of about 300 cm2 V−1 s−1 at 300 K.

Abstract: A theory of scattering by charged dislocation lines in a two-dimensional electron gas (2DEG) is developed. The theory is directed towards understanding transport in AlGaN/GaN high-electron-mobility transistors which have a large number of line dislocations piercing through the 2DEG. The scattering time due to dislocations is derived for a 2DEG in closed form. This work identifies dislocation scattering as a mobility-limiting scattering mechanism in 2DEGs with high dislocation densities. The insensitivity of the 2DEG (as compared to bulk) to dislocation scattering is explained by the theory.

Abstract: The temperature and concentration dependences of electron mobility in AlGaN/GaN hetero-structures are studied. The mobility for the samples under study at T = 300 K lies in the range of 450–1740 cm2/(V s). It is established that scattering at charged centers is dominant for samples with low mobility (lower than 1000 cm2/(V s) right up to room temperature. These centers are associated with a disordered piezoelectric charge at the heterointerface because of its roughness or with a piezoelectric charge similarly to the Al-GaN barrier because of alloy disorder, as well as with the deformation field around dislocations. Scattering at optical phonons is dominant for samples with mobility exceeding 1000 cm2/(V s) at T = 300 K. Scattering at alloy disorders, heterointerface roughness, and dislocations are dominant at temperatures lower than 200 K. A decrease in the influence of scattering at roughness with improvement of the heterointerface morphology increases room-temperature mobility from 1400 cm2/(V s) to 1700 cm2/(V s).