In order to improve the crystal quality of GaN-based light emitting devices, photoluminescence (PL) characterization of below-gap states in plasma assisted MBE-grown GaN/AlGaN quantum well (QW) structures has been done by utilizing a below-gap excitation (BGE) light in addition to an above-gap excitation light. The decrease of the band-edge PL intensity clue to the addition of the BGE of 1.17 eV indicates the presence of an energy-matched below-gap state in the two-wavelength excited PL. In continuation to our previous efficiency improvement by applying modulation-doping to GaAs/AlGaAs QW's, we focused on several undoped and Si-doped GaN/AlGaN QW's. Experimental results showed that Si modulation-doping reduces the density of below-gap states in the QW region, hence it is promising for increasing internal quantum efficiency of GaN-based QW's.

Below-gap states in plasma assisted MBE-grown GaN/AlGaN quantum well (QW) structures have been studied by two-wavelength excited photoluminescence (PL). The decrease of the PL intensity with the addition of a below-gap excitation light source of 1.17 eV implies the presence of an energy-matched below-gap state. We have studied several QW structures grown on AlGaN or AlN buffer layers on sapphire substrates. We found improved internal quantum efficiency with increasing the number of QW's and with providing a sufficiently thick AlGaN or AlN buffer layer. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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 AlxGa1−xN/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 AlxGa1−xN/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.

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.

AlxGa1-xN/ 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 AlxGa1-xN 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 AlxGa1-xN 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 AlxGa1-xN single layers.

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 cm2/Vs (one A1N-IL) as well as the extremely low intrinsic carrier density of 1 × 1017 cm−3 prove the applicability of AlN barriers in inverted FET devices.

High reflectivity (>90%) distributed Bragg reflectors (DBR) have been successfully produced utilizing the AlGaN/AlN material system. We present reflectivity and XRD data of Ga-polar AlxGa1—xN/AlN Bragg reflectors grown on sapphire. High peak reflectivities between 54% (5.5 period mirror) and 97% (25.5 period mirror) combined with large reflectivity FWHM of 30 nm have been found. All reflectors have been designed by ex-situ spectroscopic ellipsometry (SE) data of respective reference samples.

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 AlxGa2−xO3. 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 AlxGa2−xO3 layer, followed by a lithographic step and wet etch by H2O2. Source and drain contacts were placed by the liftoff technique using Ti/Al. Peak values for the mutual conductance (gm) are 64 mSmm−1 for MOVPE (metalorganic vapour pressure epitaxy) structures with 2DEG mobility of 190 cm2V−1s−1. We achieve a maximum drain current IDmax of 540 mAmm−1 for the PEC-MOSHFET. The results obtained for transistor operation are compared to other gate dielectrics such as SiO2 with different pre-treatments and to a conventional HFET with a Ni/Au Schottky gate. Depletion starts at threshold voltages Vth of −4 V in the case of the PEC-MOSHFET, for the conventional HFET structure Vth is about −9 V and for the SiO2-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 SiO2-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.

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 NH4Sx 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 NH4Sx-treated, 850 mA for the untreated, and 1.43 A mm(-1) for the one with a 0.2 mum 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 NH4Sx 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.

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 congruent to 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 congruent to 0.75. (C) 2002 Published by Elsevier Science B.V.

In this paper we report on results obtained for different AlGaN/GaN heterostructure field effect transistors (HFET) applying a recently developed photooxidation technique. We have processed and characterized MOS-HFETs in which a photoanodic oxide, a PECVD deposited SiO2 layer, or a combined stack of the photoanodic oxide followed by SiO2 (tandem-approach) is used as gate dielectric. All tandem-MOS-HFET devices have in common a large gate voltage swing and a broad transconductance peak. Additionally, the leakage and gate currents are several orders of magnitude smaller than for the Schottky gate HFETs and are in the range of pA/mm. Tandem MOSHFET devices achieved values of 465 mA/mm in enhancement mode and 46 mS/mm for the drain current IDmax and transconductance gm, respectively. These values are about 50% (IDmax) and 15% (gm) higher than for the HFETs processed on the same wafer. Thermal oxidation at 900 °C for 15 min before SiO2 deposition leads to a normally off transistor type.

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 N2, O2, Ar, and forming gas (N2/H2) 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 10−4Ωcm2 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.

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 8.8×1018cm−3.

Photoanodically grown Ga 2 O 3 layers were characterized with respect to their suitability as gate dielectrics for GaN based MOSFET Device applications. The Ga 2 O 3 layers were produced in a photoelectrochemical cell using aqueous solutions of KOH. IV characterization of MOS structures show insulating behavior of the oxide layers and CV measurements indicate a small density of states at the oxide/GaN interface. Integrating the wet chemical oxide growth in a MOSFET device fabricating process includes tungsten as gate metal together with H 2 O 2 as etching solution for the gate metal. Source/drain areas were made free of oxide by the alkaline developer of a conventional lithographic step and metallization was done by using the liftoff technique. MOS structures show no inversion mode but strong depletion in reverse biasing mode.

The DC characteristics of an AlGaN/GaN metal oxide semiconductor heterostructure field effect transistor are presented. The unique feature of this device is its oxide, which is formed photoelectrochemically at room temperature. For a device with a gate length of 2 /spl mu/m state-of-the-art values of 540 mA/mm and 62 mS/mm were obtained for the drain current and transconductance, respectively.

AlGaN/GaN based hetero field effect transistors (HFETs) were capped with different dielectrics, characterized, and tested for DC performance. As dielectrics we use SiO2 and photoelectrochemical (PEC) grown AlxGa2-xO3. Combination of this two dielectrics show best performance with respect to gate leakage current and controllability of the drain current ID. The MOSHEFTs 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 Dit and the insulating properties depend strongly on the PEC conditions. Pre-treatments before the SiO2 deposition result in varying threshold voltages |Vth| and it seems that (NH4)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 Ωmm after annealing at 850°C in N2.

The characteristics of photoelectrochemically (PEC) generated gallium oxide films on n-GaN using an 0.002 M 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-Ga2O3 (gallium sesquioxide)/GaN structures were studied with current-voltage and capacitance-voltage measurements, respectively. Under reverse bias we found extremely low leakage currents (< 10(-8) Acm(-2) at -15 V) and a very low interface state density: high-temperature operation (up to 166 degreesC 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. (C) 2001 Elsevier Science B.V. All rights reserved.

We report experimental results of two wavelength excited photoluminescence (TWEPL) applied to GaN/Al 0.2 Ga 0.8 N multiple quantum well structures grown by plasma assisted molecular beam epitaxy. An increase or a decrease in photoluminescence (PL) indicate the presence of trap centers with dissimilar characteristics for each case. In these samples, a spatial inhomogeneity of trap distribution originating from the growth process was detected due to different change in PL for distinct regions. Also a time-dependent phenomenon in PL was observed: after a continuous midterm irradiation with UV light (4.12 eV), the amount of change in PL, which is characteristic of TWEPL, became nil. We argue this phenomenon is a complex issue that can be due to migration of native defects or a kind of saturation process involving trap centers.

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 or high power UV light sources for photolithography.Two distributed Bragg reflector (DBR) samples were grown by plasma assisted molecular beam epitaxy (PAMBE). The active regions of the samples are a GaN:Si bulk layer and a 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.