M. Niehus

Pulsed sub-band-gap photoexcitation of AlN

Since AlN is a wide band-gap semiconductor it is usually not possible to induce photocurrents with light below 6.2 eV of energy. However, employing high power density laser pulses of 266 nm (4.66 eV) and 532 nm (2.33 eV), we succeeded to monitor the transient photocurrent (TPC) in transverse and parallel configuration using metal-insulator-metal (MIM) structures and single films, respectively. Absorption is possible due to excitation from deep defects, which are indicated by the power-law decays of the photocurrents with exponents below unity. The thickness dependence of the magnitude of the photocurrent and RBS profiling lead us to believe that photocurrents originate mainly from an oxygen-rich transition layer. Films down to a thickness of 5 A can still be characterized by TPC measurements.

Transient thermal gratings and carrier-induced gratings in diffusion experiments

Abstract Complications can arise during measurements of the ambipolar diffusion coefficient, Damb, of charged carriers by the transient grating method (TG) due to the concomitant decay of a transient thermal grating. The thermal diffusion coefficient, Dth, of hydrogenated amorphous silicon (a-Si:H) is of the same order of magnitude as Damb, about 10−2–1 cm2/s. For calibration purposes we measured the thermal change of refractive index, dn/dT, by optical reflection. We monitored the temperature decay after pulsed laser excitation using transient photoreflectance (TPR). A thermal diffusion process is confirmed by a near square-root time dependence. In grating experiments, the initial temperature and its time dependence can be monitored by TPR. In order to determine Damb a good thermal connection to the substrate and a high quality material is needed.

Layer-by-Layer Growth of GaN on Sapphire by Low Temperature Cyclic Pulsed Laser Deposition / Nitrogen RF Plasma

Recently we have proposed a new layer-by-layer method for deposition of group-III nitrides from elemental precursors (Ga, N2) [1,2]. This technique is based on a two-step cyclic process, which alternates Pulsed Laser Deposition (PLD), of a liquid gallium target and nitrogen plasma treatment. In this work, we proceed on the development of this flexible cyclic deposition technique and study the influence of the power and time duration of the 1 mbar nitrogen RF plasma on the GaN thin films. The layers are deposited on pre-nitridated sapphire (0001) substrates at low deposition temperature (600° C) to minimise reevaporation. The cyclic GaN thin films thus obtained are compared in terms of crystal alignment and nitrogen incorporation. X-ray diffraction and optical transmission spectra are the selected tools used to characterise and compare the deposited films.

Lifetime Regime in the Electrically-Detected Transient Grating Method Applied to Amorphous and Microcrystalline Silicon Films

The minority-carrier diffusion length in thin silicon films can be extracted from the electrically-detected transient grating method, EDTG, by a simple ambipolar analysis only in the case of lifetime dominated carrier transport. If the dielectric relaxation time, τ diel , is larger than the photocarrier response time, τ R , then unexpected negative transient signals can appear in the EDTG result. Thin silicon films deposited by hot-wire chemical vapor deposition (HWCVD) near the amorphous-to-microcrystalline transition, where τ R varies over a large range, appeared to be ideal candidates to study the interplay between carrier recombination and dielectric response. By modifying the ambipolar description to allow for a time-dependent carrier grating build-up and decay we can obtain a good agreement between analytical calculation and experimental results.

Non-Stationary Photoconductivity of GaN Nanocomposites In Artificial Opal Matrix

It was recently proposed to use synthetic opals as a host matrix for obtaining 3D arrays of electronic nanodevices [1]. In the present work the opal matrices were infiltrated with GaN. We study electronic properties of opal-GaN, by means of transient photoconductivity (TPC) measurements using 5 ns laser pulses at wavelengths above (266 nm) and below (532 nm) the GaN bandgap (3.4 eV). A broad plateau is observed in the photocurrent decay covering several orders of magnitude. We compare the results with measurements in conventional GaN.

Transient Photoresponse from Co Schottky Barriers on AlGaN

Co on AlGaN is expected to form a large barrier Schottky contact due to its high work function. We have used this material combination with 18 % of Al in AlxGaN for the study of transient photoresponse in the photovoltaic mode and in secondary photocurrent measurements after pulsed laser excitation. In reverse bias and in short- circuit mode a fast decay with a characteristic time of a few microseconds is dominant at room temperature. This mode is appropriate for UV detector operation. At elevated temperature, a much slower tail extending to several milliseconds is also observed. In forward bias operation the slow tail is dominating at any temperature. We discuss this asymmetry with respect to fast minority carrier collection within the space charge region for primary photocurrents and the slower majority carrier transport in forward bias.