J. Graul

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.

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.

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.

Formation and Characterization of Oxides on GaN surfaces

We characterized oxides formed directly on n -GaN surfaces. The methods used for oxide layer formation were both photoanodic oxidation and thermal oxidation. The photoanodic oxidation took place in aqueous solutions of potassium hydroxide with pH values lower than 13. Homogenous oxide films were obtained in the voltage range from -0.6 V to 0.4 V vs the saturated calomel electrode (SCE). The characterization of the oxide layers was performed primarily by Auger electron spectroscopy (AES). First the surface chemistry was determined, proving that Ga-oxide is formed with an attributed stoichiometry of Ga 2 O 3 . Secondly, depth profiling shows the oxide thickness to be dependent on the photoanodic voltage and oxidation time. Complementary X-ray diffraction (XRD) studies suggest an amorphous state of the formed layers. Annealing GaN in O 2 -atmospheres above 900°C also lead to surfaces fully covered with gallium oxide. We found that N-polar surfaces oxidize faster than Ga-polar surfaces, which is in agreement to the theoretical work of Zywietz et al [1]. Furthermore, we report on the electrical properties of the anodized oxide layers by analyzing MOS structures.

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.

Influence of Process Technology on DC-Performance of GaN-Based HFETs

This work reports on the influence of the surface and the gate length on the performance of AlGaN/ GaN based Hetero Field Effect Transistors (HFETs). Differently NH 4 S x treated surfaces result in variation of the drain current I Dmax of more then 100%. Gate recessing by photoelectrochemical treatment changes the threshold voltage V th but affects the drain current little. Next, the reduction of the gate length increases the I Dmax further by more than 60%. The I Dmax values for the transistors are 350 mA mm -1 for the NH 4 S x -treated, 850 mA for the untreated, and 1.43 A mm -1 for the one with a 0.2 μm gate length. The corresponding transconductances g m are 66, 150, and 280 mS mm -1 , respectively. Surface analysis with Auger Electron Spectroscopy (AES) and contact characterization (TLM) reveals, that the NH 4 S x treatment removes the native oxide and increases the contact resistance as well. Therefore we attribute the increase of I Dmax and g m mainly to a beneficial behavior of gallium-oxide at the surface on the sheet carrier density n s of the 2DEG at the heterointerface.

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.

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.