B. Kuhn

Optical phonons and free-carrier effects in MOVPE grown Al xGa 1-xN measured by infrared ellipsometry

We report on the application of infrared spectroscopic ellipsometry (IR-SE) for wavenumbers from 333cm −1 to 1200cm −1 as a novel approach to non-destructive optical characterization of free-carrier and optical phonon properties of group III-nitride heterostructures. Undoped α -GaN, α -AlN, α -Al x Ga 1− x N ( x = 0.17, 0.28, 0.5), and n -type silicon (Si) doped α -GaN layers were grown by metal-organic vapor phase epitaxy (MOVPE) on c -plane sapphire ( α -Al 2 O 3 ). The four-parameter semi-quantum (FPSQ) dielectric lattice-dispersion model and the Drude model for free-carrier response are employed for analysis of the IR-SE data. Model calculations for the ordinary ( ∈ ⊥ ) and extraordinary ( ∈ || ) dielectric functions of the heterostructure components provide sensitivity to IR-active phonon frequencies and free-carrier parameters. We observe that the α -Al x Ga 1− x N layers are unintentionally doped with a back ground free-carrier concentration of 1–4 10 18 cm −3 . The ternary compounds reveal a two-mode behavior in ∈ ⊥ , whereas a one-mode behavior is sufficient to explain the optical response for ∈ || . We further provide a precise set of model parameters for calculation of the sapphire infrared dielectric functions which are prerequisites for analysis of infrared spectra of III-nitride heterostructures grown on α -Al 2 O 3 .

Disorder-activated infrared modes and surface depletion layer in highly Si-doped hexagonal GaN

Three infrared-active low-polar modes are reported for highly Si-doped hexagonal (α-) GaN. The 0.8–1.6 μm thick films, grown by metal organic vapor phase epitaxy or molecular beam epitaxy on (0001) sapphire substrates, were studied by infrared spectroscopic ellipsometry. For GaN epilayers with free-electron concentration N⩾8×1018 cm−3 we observe, besides the usual GaN transverse-optical lattice modes and coupled longitudinal-optical phonon-plasmon modes, a band of additional modes at 567.4±2.5, 752.5±0.9, and 855.0±0.9 cm−1. We tentatively assign the first one to the disorder-activated high E2 GaN mode and the third mode to an acoustic-optical combination band, whereas the origin of the second mode remains unclear. Furthermore, the ellipsometric spectra of highly n-conductive Si-doped GaN reveal thin carrier-depleted regions at the sample surface.

Short-channel effects in AlGAN/GAN HEMTs

We report on our progress on the fabrication of AlGaN/GaN high electron mobility transistors (HEMTs) with extremely short gate length. AlGaN/GaN HEMTs with different gate length from 6 μm down to 60 nm were fabricated to investigate DC- and high frequency behavior as well as short channel effects. We have found that the transistors with gates in the 100 nm range can be improved in the device performance with respect to transconductance and high frequency but also shows short channel effects as the loss of saturation in the output characteristics and a strong dependency of the threshold voltage on the gate length.

Comparison of the Morphology and In Distribution of Capped and Uncapped InGaN Layers by Transmission Electron Microscopy

We have compared and analysed the morphology of capped and uncapped thin InGaN layers by transmission electron microscopy. The samples were grown under the same conditions by metal-organic chemical vapour deposition. The capped layer appears to be homogenous in thickness whereas the uncapped one is characterized by the presence of large islands. Moreover, the In distribution was determined in both samples by quantitative high-resolution transmission electron microscopy. The average indium concentration was found to be comparable. Fluctuations of the In concentration on a large scale occur in both cases whereas fluctuations on a scale of a few nanometers are more pronounced in the capped layer.

Infrared Ellipsometry – a Novel Tool for Characterization of Group‐III Nitride Heterostructures for Optoelectronic Device Applications

We demonstrate the application of spectroscopic infrared ellipsometry to determine nondestructively the free-carrier distribution in group-III nitride heterostructures, such as for optoelectronic and electronic device applications. Results are shown for a blue-light emitting diode structure based on wurtzite III-N materials grown on (0001) sapphire by metal-organic vapor phase epitaxy.

Systematics of Optical Gain in GaInN/GaN Laser Structures

Optical gain and absorption of different laser-structures were measured and calculated in order to clarify the influence of internal fields on these properties. For the calculations a model is used that is based on the assumption of band-to-band transitions and includes the effect of piezoelectric fields. The experimentally found energy shifts between absorption and the onset of gain could be explained only through the presence of an internal field. The minimal threshold current densities for a double quantum well laser and for a well thickness of 2 nm were confirmed.

Analysis of the threshold current in nitride-based lasers

Using optical gain measurements and calculated optical gain spectra we analyse the various contributions to the threshold current observed for laser diodes. Our model is based on band-to-band transitions and includes internal polarization fields as well as multiple quantum wells. Besides good agreement between experiment and theory, our model explains the characteristic dependence of the threshold current on emission wavelength and well number.

FREE-CARRIER RESPONSE AND LATTICE MODES OF GROUP III-NITRIDE HETEROSTRUCTURES MEASURED BY INFRARED ELLIPSOMETRY

We report on the application of infrared spectroscopic ellipsometry (IR-SE) for wavelengths from 3 to 30 μm as a novel approach for nondestructive optical characterization of free-carrier and optical-phonon properties of group III-nitride heterostructures grown by metal-organic vapor phase epitaxy on sapphire. Model calculations for the ordinary (ϵ⊥) and extraordinary (ϵ∥) dielectric functions of the heterostructure components provide sensitivity to the IR-active phonon frequencies and anisotropy of the free-carrier response.

Effective carrier mass and mobility versus carrier concentration in p- and n-type α-GaN determined by infrared ellipsometry and Hall resistivity measurements

Abstract We use infrared spectroscopic ellipsometry (IRSE) to obtain optical free-carrier parameters in p - and n -type hexagonal ( α -) GaN films. Si- and Mg-doped films (40 nm to 4 μm) were grown on sapphire by MOCVD and MBE, respectively. Results from electrical (Hall) measurements are combined with those obtained from the Drude model by lineshape analysis of the IRSE data. We derive effective electron ( m e ) and hole ( m h ) mass parameters for GaN, where m h depends on the free-hole concentration N e . Undoped but n -type conductive films grown under similar conditions with different thicknesses, d , show a double-logarithmic dependence over two orders of magnitude between N e and d : log( N e )∝ c e log( d ) with c e α -GaN. Inhomogeneously activated donors or acceptors due to a decrease in misfit dislocations along the growth direction could explain the thickness dependence. A low electron mobility and large lattice mode broadening were observed at the layer–substrate interface in n -type GaN films. For high N e values, the films reveal surface carrier depletion layers with thickness d SL . We find that d SL increases with decreasing N e .

Infrared spectroscopic ellipsometry for nondestructive characterization of free-carrier and crystal-structure properties of group-III-nitride semiconductor device heterostructures

Infrared Spectroscopic Ellipsometry is presented as a feasible and novel technique for contactless and nondestructive measurement of free-carrier and crystal-structure properties in the characterization of complex semiconductor heterostructures for device applications. Infrared-active lattice modes and coupling of free-carrier plasmons to longitudinal-optical lattice phonon modes strongly affect the infrared-optical response of semiconductor materials. Analysis of ellipsometry data from 2 micrometers to 100 micrometers can provide precise information on phonon mode frequencies and broadening parameters, static dielectric constants, free-carrier concentration, and free-carrier mobility at optical frequencies of III-V compound semiconductors, even for films with thicknesses only a fraction of the probing wavelengths. Alloy composition, strain, crystal quality, and free-carrier properties of constituent layers in thin-film structures, designed for optoelectronic or electronic device applications, can be derived. We demonstrate the characterization of coherent and incoherent light emitter structures based on group-III-nitride materials, where information such as concentration and mobility of free carriers in n- and p-type regions, thickness, composition, and quality of device constituents are accessible.