The ordinary complex dielectric function (DF) of Al x Ga 1-x N alloys with 0 ≤ x ≤ 0.53 is determined by fitting spectroscopic ellipsometry data from the infrared to the vacuum ultraviolet spectral region (0.74 eV ≤ E ≤ 9.8 eV). The dispersion of the real part of the DF below the band gap is found to be in excellent agreement with previously published data. The obtained band gap energies are verified by photoluminescence and photoreflectance spectroscopy. In the high-energy range, three critical points of the band structure are clearly resolved. By applying a third-derivative based DF line shape analysis, the corresponding transition energies are determined. Their compositional dependences can be described on the basis of small bowing parameters.
A detailed analysis of the piezoelectric response of (GaN/)AlGaN/GaN heterostructures is reported. The electromechanical properties of two types of heterostructures with an Al content of 31% are compared. Only a single two-dimensional electron gas (2DEG) is formed for samples with thin GaN cap layers, while both a 2DEG and a two-dimensional hole gas coexist in the case of thick GaN caps. The lower GaN layer represents the mechanically supporting layer, while the AlGaN film, and in some cases an additional GaN cap layer, serves as the piezoelectrically active layers for actuation. The 2DEG (at the lower AlGaN/GaN interface) provides the conducting channel which was used as back electrode for the applied external voltage. Electroreflectance spectroscopy is applied in order to determine the electric field distribution across the whole structure as a function of the applied voltage. It is found that only a part of the modulation voltage drops across the active region. Piezoelectric force microscopy yields the field (voltage)-dependent actuation of the layers. By correlating the results of the two experimental techniques we are able to determine the piezoelectric modulus d33 with considerably improved reliability. A value for Al0.31Ga0.69N of 5 pm/V is found which is higher than an estimation based on previously reported data for GaN and AlN
We systematically investigate Al(0.22)Ga(0.78)N/GaN high electron mobility transistors with GaN cap layer thicknesses of 0, 1, and 3 nm. All samples have electron mobilities around 1700 cm2/Vs and sheet carrier concentrations around 8x10(exp 12) cm-2 as determined by Hall effect measurements. From photoreflectance measurements we conclude that the electric field strength within the AlGaN barrier increases with GaN cap layer thickness leading to a broadening of the transition peaks as determined by spectroscopic ellipsometry. The surface potential as determined by photoreflectance varies in the range between 0.585 and 0.249 eV dependent on the thickness of the GaN cap. Device results show a significant decrease in Ohmic contact resistance, an increase in ideality factor, a decrease in gate and drain leakage currents, an increase in gain, and an increase in power added efficiency with increasing cap layer thickness. Finally, devices with GaN cap show an improved direct current reliability compared to their counterparts without GaN cap
IntroductionThe development of nitride-based photonic devices requires detailed knowledge of the optical constants over an extended photon energy range. They are usually described in terms of the complex dielectric function (DF) or the complex index of refraction. Minor attention has been devoted so far to their reliable determination. A recent review showed that only the ordinary DF of wurtzite (hexagonal) GaN was known very precisely [1] until 2002, while for all other compounds pseudodielectric functions were reported. Those data do not provide a satisfactory basis for developing analytical DF models [2]. However, the situation has been improved considerably within the last few years. We succeed in determining the anisotropic DF of InN (with a band gap of 0.68 eV) [3,4], the ordinary DFs of In-rich InAlN [5] and InGaN [6] alloys as well as of AlGaN [7] compounds.In this chapter, these latest experimental data are summarized. The fundamental properties influencing the dispersion of the DF are discussed in detail. Then a DF model will be presented which allows the description of the optical constants of the binary nitrides over a large photon energy range and the calculation of DFs for alloys. Finally, the influence of strong electric fields on the DF will be briefly addressed. Dielectric Function and Band Structure Fundamental RelationsStrain-free nitrides with wurtzite structure belong to the P6 3 mc(C 4 6v ) space group and are optically uniaxial materials. It is typical for most epitaxially grown films that the optic axis (c-axis) is oriented normal to the surface (here
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