F. Fedler

High resolution x-ray analysis of pseudomorphic InGaN/GaN multiple quantum wells: Influence of Si doping concentration

We report the structural properties of InGaN/GaN/AlGaN multiple quantum wells (MQWs) by means of two-dimensional reciprocal space mapping (RSM) of high resolution x-ray diffraction. The influence of Si doping in GaN barriers on the characteristics has been studied for 12-period MQWs grown by metalorganic chemical vapor deposition, which have different Si doping concentrations in the GaN barriers ranging from 1×1017 to 3×1019 cm−3. Information on the structural quality of these MQWs was extracted from the linewidth broadening of the higher-order superlattice satellite peaks, as well as from the presence of Pendellosung oscillations. The measured diffraction curves were modeled using kinematic diffraction theory. From the symmetric and asymmetric RSMs around (0002), (0004) and (1124) reflections, we found that the InGaN/GaN/AlGaN MQWs are grown coherently on the GaN base layer. Better interface properties are achieved with Si doping. Our results indicate that Si doping in the GaN barriers affects the inter...

Strain, morphological, and growth-mode changes in AlGaN single layers at high AlN mole fraction

We report on morphological and residual-strain characteristics of high-AlN-mole-fraction N-polar AlxGa1−xN epilayers on sapphire. Nominally relaxed, thick single-alloy layers in the compositional range 0.4<x<1 were grown by plasma-assisted molecular beam epitaxy (PA-MBE) and characterized structurally and optically. High-resolution X-ray diffraction using an extension of the Bond method was employed to examine residual strains while film morphologies were examined directly with the use of atomic force microscopy. Under nominally identical PA-MBE growth conditions apart from Al flux, a continuous change in growth characteristics as a function of Al flux (alloy composition) is observed. For one particular value of Al flux (corresponding to x≅0.65), a maximum in growth rate and minimum in surface roughness are found whereas at a somewhat greater flux value (corresponding to x=0.86) a minimum in stress (lateral and vertical) is obtained. The observed growth-mode phenomenology suggests an approach for improving PA-MBE growth of high-AlN-mole-fraction layers of certain AlGaN/GaN structures such as the distributed Bragg reflector. Finally, optical transmission experiments lead to a bandgap bowing parameter in the large-x region of AlxGa1−xN of b≅0.75.

Effect of substrate temperature and V/III flux ratio on In incorporation for InGaN/GaN heterostructures grown by plasma-assisted molecular-beam epitaxy

Reflection high-energy electron diffraction (RHEED) and laterally spatially resolved high resolution x-ray diffraction (HRXRD) have been used to identify and characterize rf plasma-assisted molecular-beam epitaxial growth factors which strongly affect the efficiency of In incorporation into InxGa1−xN epitaxial materials. HRXRD results for InxGa1−xN/GaN superlattices reveal a particularly strong dependence of average alloy composition x upon both substrate growth temperature and incident V/III flux ratio. For fixed flux ratio, results reveal a strong thermally activated behavior, with over an order-of-magnitude decrease in x with increasing growth temperature within the narrow range 590–670 °C. Within this same range, a further strong dependence upon V/III flux ratio is observed. The decreased In incorporation at elevated substrate temperatures is tentatively attributed to In surface-segregation and desorption processes. RHEED observations support this segregation/desorption interpretation to account for...

Electron Beam Pumped Nitride Vertical Cavity Surface Emitting Structures with AlGaN/AlN DBR Mirrors

Vertical cavity surface emitting structures designed for electron beam (EB) pumping have been grown by plasma assisted molecular beam epitaxy (PA-MBE). For the first time, AlGaN/AlN DBRs in conjunction with dielectric SiO 2 /HfO 2 reflectors are utilized to embed GaN/AlGaN MQWs. PL spectra measured under weak photoexcitation reveal an enhancement of spontaneous emission reaching a Q factor of 425. One hybrid structure has been successfully pumped by pulsed EB and laser excitation showing an onset of stimulated emission.

HRTEM study of AlxGa1−xN/AlN DBR mirrors

Abstract In the present work semiconductor quarter wavelength distributed Bragg reflector (DBR) mirrors have been studied by high resolution transmission electron microscopy (HRTEM). The mirrors have been fabricated monolithically by plasma assisted molecular beam epitaxy (MBE) on sapphire (0001) substrates. The samples are conformed of a large number of Al x Ga 1− x N/AlN layers with 5.5 and 20.5 periods, both with different aluminium concentration. The samples have been designed utilising spectroscopic ellipsometry (SE) dispersion spectra of previously fabricated single layers. The aim of this work was to determine the distortion of lattice parameters of Al x Ga 1− x N/AlN epilayers, since this is important for the later production of vertical cavity surface emitting lasers (VCSELs). Distortions of half periods layers were determined from HRTEM techniques and are compared with the distortion determination using an equilibrium theory and high resolution X-ray diffraction (XRD) measurements.

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.

Effect of high temperature single and multiple AlN intermediate layers on N-polar and Ga-polar GaN grown by molecular beam epitaxy

Wurtzite GaN samples containing one, three and five 4nm thick high temperature (HT) AlN Interlayers (IL) have been grown on (0001) sapphire substrates by plasma-assisted molecular beam epitaxy (PAMBE). N-polar as well as Ga-polar thin films have been characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), and electrical measurements. All samples under consideration show excellent AFM rms surface roughness below 1nm. Previously, we published a reduction of the threading dislocation (TD) density by a factor of seven due to the introduction of one AlN-IL. When introducing multiple AlN-IL a reduction by a factor of 5.2 is achieved. Hall measurements show a rise in electron mobility due to possible 2DEG formation at the interface between GaN and the AlN-ILs. Significant growth mode differences between Ga-polar and N-polar samples result in drastically higher electron mobility values for N-polar material. For N-polar samples the exceptional mobility increase from 68 (no AlN-IL) to 707 cm 2 /Vs (one AlN-IL) as well as the extremely low intrinsic carrier density of 1 x 10 17 cm -3 prove the applicability of AlN barriers in inverted FET devices.

AlGaN/GaN-based MOSHFETs with different gate dielectrics and treatments

AlGaN/GaN based hetero field effect transistors (HFETs) were capped with different dielectrics, characterized, and tested for DC performance. As dielectrics we use SiO 2 and photoelectrochemical (PEC) grown Al x Ga 2-x O 3 . Combination of this two dielectrics show best performance with respect to gate leakage current and controllability of the drain current ID. TheMOSHEFTs work also at positive gate voltages in accumulation, which is also demonstrated in a broad transconductance peak. The PEC oxidation shows low density of interface states D it and the insulating properties depend strongly on the PEC conditions. Pre-treatments before the SiO 2 deposition result in varying threshold voltages |Vth| and it seems that (NH 4 )Sx pre-treatment leaves the surface in best conditions. Comparison of Ti/Al and Ti/Al/Ti/Au as source/drain contacts for AlGaN/GaN HFETs are done and the annealing behavior of Ti/Al/Ti/Au is displayed resulting in contact resistance as low as RC = 2 &mm after annealing at 850°C in N 2 .

Characterization of Al x Ga 1-x N/ AlyGa1-yN Distributed Bragg Reflectors Grown by Plasma Assisted Molecular Beam Epitaxy

Al x Ga 1-x N/ AlyGa1-yN Distributed Bragg Reflectors (DBRs) with up to 45 periods have been grown on (0001) sapphire substrates by r.f. plasma-assisted molecular beam epitaxy (PAMBE) with the aid of two Al effusion cells. Several samples were grown with an Al mole fraction varying between 0.38 ≤ x ≤ 1 (0 ≤ y ≤ 0.4) at temperatures of up to 890°C. In all samples, an Al x Ga 1-x N buffer layer was used to prevent cracking of the quarter wave stack and improving surface morphology by choosing the Al content so that strain energy in the DBR structure would be compensated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) investigations were performed to determine the thickness of the quarter wave layer periods and the Al mole fraction of corresponding Al x Ga 1-x N single layers. Room-temperature calibrated reflection and transmission (R&T) measurements were performed. Thus stray and self-absorption of the DBRs were extracted from reflectance and transmittance. The thickness of the quarter wave layers was designed such that the measured peak reflectances appeared between 346 nm to 421 nm. The dispersion data, including refractive indices and absorption coefficients, used in the calculation were extracted from R&T measurements done on the above mentioned Al x Ga 1-x N single layers.