J. Stemmer

InN thin films grown by metalorganic molecular beam epitaxy on sapphire substrates

In this priority communication it will be shown that InN thin films can be successfully grown using the metalorganic molecular beam epitaxy (MOMBE) method. For the first time the proper choice of growth conditions allows to obtain good quality InN thin films with a charge carrier concentration as low as 8.8×1018 cm−3.

Point defects in gamma-irradiated n-GaN

Radiation-induced point defects and their annealing in silicon-doped n-GaN have been investigated by means of Hall effect measurements and Raman spectroscopy. Correlated compensation effects due to simultaneous introduction of donor and acceptor centres are observed in irradiated n-GaN. The defect production rate is dependent on the dopant concentration. This means that the model of all native defects immobile at room temperature is not true. The behaviour of radiation-induced defects upon heating is complicated, exhibiting two prominent stages of reverse annealing. The presence of radiation defects is still observable after annealing to T = 750 °C.

Smooth GaN surfaces by photoinduced electro-chemical etching

We have etched n-GaN grown both by plasma assisted MBE and MOCVD in 0.5 M KOH under HeCd-laser illumination (110 mW cm -2 ) and controlled the photocurrent by an external voltage source. The reproducible etch depths were linear with respect to the charge. The examination of the dissolution potential revealed strong dependence on illumination and current. Thereby, regimes to obtain smoothly etched surfaces were found at low dissolution potentials. Demonstration of the possibilities in photoelectrochemical (PEC) etching are given.

Current Controlled Photoelectrochemical Etching of Gan Leaving Smooth Surfaces

We have etched GaN grown by plasma source MBE in aqueous solutions of KOH in an electrochemical cell under HeCd laser illumination and additional current control.The etch rate was dramatically enhanced up to 8 μm/h by an applied current density of 6.4 mAcm -2 . Photocurrent control leads to etched GaN surfaces exhibiting mirror-like appearance with uniform interference color. According to mechanical profilometry, they have a roughness of less than 3.5 nm after etching of several hundred nanometers, which is comparable to the roughness prior to etching. This etching process allows in situ control via photocurrent and induced yellow luminescence.

Raman spectroscopy of disorder effects in AlxGa1−xN solid solutions

Abstract The results of Raman spectroscopic studies of the disorder effects in hexagonal Al x Ga 1− x N epitaxial layers grown by MBE and HVPE on different substrates for a large range of Al concentrations are presented. The width of the nonpolar phonon line with E 2 symmetry results from the inhomogeneous broadening due to spatial fluctuations in the Al content. The abnormally small broadening of the A 1 (TO) polar phonon mode for x or (1− x )≪1 and the large broadening for x ≅0.5–0.7 are attributed to the specific frequency dependence of the density of states for the branch with the directional dispersion in pure crystals. Thus the Raman spectrum is found to be highly sensitive to the composition of Al x Ga 1− x N epitaxial layers and its inhomogeneity. It is shown that in the estimation of the crystal composition, on the basis of Raman data, the influence of the homogeneous strain effects could be excluded via measuring a linear combination of two Raman line frequencies.

Structural characterization of high temperature AlN intermediate layer in GaN grown by molecular beam epitaxy

Transmission electron microscopy has been used to study the structural quality of GaN grown on sapphire by plasma assisted molecular beam epitaxy using high temperature AlN intermediate layers with different thicknesses. The introduction of an AlN intermediate layer with an optimum thickness is observed to minimize the density of dislocations reaching the overgrown GaN surface. In this sample, the measured threading dislocation density reaching the surface of 1 x 10 10 cm -2 resulted to be seven times lower than that of a reference sample, without any AlN interlayer. The bending at the GaN/AlN interface and following interactions between dislocations have been observed in cross-sectional transmission electron micrographs. This fact explains the decrease of dislocation density reaching the GaN surface.

Annealing behaviour of electrically active point defects in gamma-irradiated n-GaN films

Abstract Formation processes and annealing of defects in gamma-irradiated n-GaN are investigated for the first time using electrical measurements and Raman spectroscopy. The production rate of defects turned out to be dependent on the dopant concentration. This suggests that at least one kind of native defect is involved in impurity–defect interactions. Two prominent stages of defect annealing are revealed. The annealing processes at T ⩾100°C are associated with mobile native defects. A considerable fraction of radiation defects is still present in the material after an annealing step at T ⩾750°C.

Electron beam pumping in nitride vertical cavities with GaN/Al0.38Ga0.62N Bragg reflectors

Electron beam pumped surface emitting lasers are of great interest for a variety of applications, such as Laser Cathode Ray Tubes (LCRT) in projection display technology and high power UV light sources for photolithography as an application of nitride emitters. Two distributed Bragg reflector (DBR) samples were grown by plasma assisted molecular beam epitaxy (PAMBE). The active regions of the samples are a GaN bulk layer and multihetero (MH) structure, respectively. Also, a separately grown single DBR stack was studied to find optical transmission and reflection properties which were compared to transfer matrix simulations. Scanning electron beam pumping at 80 K was performed on the two vertical cavity structures with an excitation energy of 40 keV in order to characterize the influence of the distributed Bragg reflectors on the resonator properties. Surface emission spectra measured for various electron beam currents revealed luminescence emission maxima located at about 3.45 eV at 80 K for the sample with the MH structure active region. Optical modes appeared for excitation powers greater than 0.85 MW/cm2. Further increasing the excitation power density the number of modes increased and a broadening and redshift of the luminescence spectrum could be observed. Based on our experimental results, we discuss the dependence of optical parameters of the nitride vertical cavity and sample surface reactions on primary electron beam power.

Electronic and structural properties of InN thin films grown by MOMBE on sapphire substrates

The development of high quality semiconductor thin films for different applications is a demanding problem in material science. InN has not been an intensively studied as AlN and GaN. There is relatively little information on the fundamental optical properties, charge carrier transport, and the properties and behavior of electrically active defects in the material. The absence of good-quality material lead even to conflicting data reported in the literature concerning the optical gap and band structure. In this publication it will be shown that InN thin films can be successfully grown using the MO MBE method. For the first time the proper choice of growth conditions allows to obtain good quality InN thin films with a charge carrier concentration as low as 8.8 X 1018 cm-3.

Filtering Study of Threading Dislocations in AlN Buffered MBE GaN/Sapphire Using Single and Multiple High Temperature AlN Intermediate Layers

Conventional and high resolution transmission electron microscopy have been used to characterise GaN/AlN systems grown on sapphire (0001) by plasma assisted molecular beam epitaxy. We analyse the filtering of threading dislocations using single and multiple thin high temperature AlN interlayers between high temperature GaN layers. A reduction of threading dislocation density is obtained comparing the measured values in samples with one and with three AlN interlayers. The dislocation interaction likelihood increases when the GaN layer between AlN interlayers is thicker.