O. Semchinova

First AlGaN/GaN MOSFET with photoanodic gate dielectric

Abstract We present the first GaN based MOSFET with wet chemical processed gate oxide. The oxide was grown photoelectrochemically (PEC) in KOH based aqueous solutions and was determined to be Al x Ga 2− x O 3 . This process offers low surface damage. The gate contact for our created PEC-MOSHFET (metal oxide semiconductor heterostructure field effect transistor) was fabricated by e-beam evaporation of tungsten on the Al x Ga 2− x O 3 layer, followed by a lithographic step and wet etch by H 2 O 2 . Source and drain contacts were placed by the liftoff technique using Ti/Al. Peak values for the mutual conductance ( g m ) are 64 mSxa0mm −1 for MOVPE (metalorganic vapour pressure epitaxy) structures with 2DEG mobility of 190 cm 2 xa0V −1 xa0s −1 . We achieve a maximum drain current I Dmax of 540 mAxa0mm −1 for the PEC-MOSHFET. The results obtained for transistor operation are compared to other gate dielectrics such as SiO 2 with different pre-treatments and to a conventional HFET with a Ni/Au Schottky gate. Depletion starts at threshold voltages V th of −4 V in the case of the PEC-MOSHFET, for the conventional HFET structure V th is about −9 V and for the SiO 2 -MOSHFETs it varies between −11.5 and −14 V depending on the wet chemical pre-treatment. Leakage currents depend on device isolation and on gate currents, which are lowest for the SiO 2 -MOSHFETs (∼2 pA) and several orders of magnitude bigger for the HFET (∼4 μA). Gate currents for the PEC-MOSHFET depend on the oxide growth and vary between microamperes and a few picoamperes.

Investigation of Ni/Au-contacts on p-GaN annealed in different atmospheres

Abstract We investigated the effect of different annealing atmospheres on contact behaviour of Ni/Au contacts on moderately doped p-GaN layers. We used the annealing gases N 2 , O 2 , Ar, and forming gas (N 2 /H 2 ) at varying annealing temperatures from 350°C to 650°C in steps of 50°C. The p-GaN samples were either metalorganic chemical vapor deposition or molecular beam epitaxy grown. Contact characterization was done after each annealing step by using the circular transmission line model. Specific contact resistances were determined to be in the low 10 −4 xa0Ωxa0cm 2 range for oxidized contacts. Accompanying chemical analysis using depth resolved Auger electron spectroscopy revealed that NiO was formed and Au diffused towards the interface, whereas annealing in forming gas prevented oxidation and did not lead to Ohmic behaviour.

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.

Electrical properties of photoanodically generated thin oxide films on n-GaN

Abstract The characteristics of photoelectrochemically (PEC) generated gallium oxide films on n -GaN using an 0.002xa0M KOH electrolyte are described. The chemical composition of the resistive layers was analyzed by Auger electron spectroscopy. The DC and HF characteristics of Al/Ti/PEC-Ga 2 O 3 (gallium sesquioxide)/GaN structures were studied with current–voltage and capacitance–voltage measurements, respectively. Under reverse bias we found extremely low leakage currents ( −8 xa0Acm −2 at −15xa0V) and a very low interface state density; high-temperature operation (up to 166°C tested) motivates the integration of the described dielectric layer forming technique into GaN based device process schemes. Our method may also be employed as gate recess technology.

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.

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.

High temperature AlN intermediate layer in GaN grown by molecular beam epitaxy

Abstract High-temperature AlN intermediate layers with different thicknesses were deposited during the growth of wurtzite GaN on (0xa00xa00xa01) sapphire substrates by plasma-assisted molecular beam epitaxy. When using a 3.5xa0nm AlN intermediate layer temperature-dependent Van-der-Pauw Hall measurements revealed a mobility enhancement by a factor of 2.5 at room temperature and by a factor of 32 at 30xa0K. Transmission electron microscopy confirmed that the better material quality was due to a reduction of dislocation density by about one order of magnitude. Photoluminescence measurements indicate a decrease of full-width at half-maximum of the main emission peak for GaN samples with AlN intermediate layer. Photoluminescence peak position and Raman shift of the E 2 mode hint at increasing biaxial compressive strain with increasing AlN intermediate layer thickness.

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.

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.