Molecular Beam Epitaxy Growth and Characerization of ZnO-based layers and Heterostructures

,

• Publisher: Cuvillier Verlag
• Year: 2008
• Print ISBN: 9783867277013
• eBook ISBN: 9783736927018
• Edition: 1

In semiconductor research a reliable epitaxial growth technique for growing high quality thinfilms and heterostructures is necessary. In the case of ZnO one of the main difficulties is theabsence of suitable substrate material for ZnO epitaxial growth. Although special oxidematerial (for example ScAlMgO4) and ZnO bulk crystal can serve as lattice matchedsubstrates, the quality of the substrates themselves, the size of the available wafer, and theexpense do not encourage to use these lattice matched substrates for ZnO epitaxial growth. Inthe current research, a widely used low cost commercial substrate sapphire was employed todevelop a reliable epitaxial growth technique and growth process for ZnO. The versatileepitaxial growth technique, molecular beam epitaxy (MBE) equipped with a rf-plasma sourcewas developed for growth and various characterizations methods were conducted to obtain afundamental understanding in both the epitaxial processes and material properties of ZnO thinfilms and heterostructures.Employing a thin HT MgO buffer layer prior to ZnO growth is the key to overcome the verylarge mismatches between c-Al2O3 substrate. Wetting the surface of Al2O3 substrate with afew MgO monolayers, lowed the surface energy, so that the lateral growth of ZnO ispromoted at the initial growth stage. MgO can be grown in the same chamber as ZnO withoutany contamination problem. These advantages make the growth procedure of a HT MgObuffer fast and easy. The growth temperature and the growth rate of MgO buffer are found tobe important to improve the ZnO heteroepitaxy. An intermediate spinel layer in epitaxialrelation with the sapphire substrate as well as with the HT MgO buffer layer is formed in theearly stage of growth during the deposition of the MgO at 700°C. It was found that thecombination of these two layers is useful for the progressive reduction of the ZnO overgrownwith the sapphire substrate.Annealing experiments reveal that as soon as the spinel layer isformed at about 700°C, it remains stable at least up to 1000°C, and even it is extended inthickness.By recording and analyzing RHEED intensity oscillations, the growth kinetics has beeninvestigated. Flat surface morphology and layer-by-layer growth has been achieved. Thestoichiometry has been deduced by analyzing the growth rate as a function of Zn and O fluxesfor various growth temperatures. It is found that the sticking coefficient of oxygen radicals isless dependent on the substrate temperature than that of Zn. The stoichiometric conditionshifts to larger Zn flux at higher growth temperature. The kink rZnO values determine theactivated O-flux supplied by the RF plasma source at TS=500°C, 400W and a given O2-flowrate. It equals 0.5±0.05 Ås-1 per sccm. Absolute ?Zn values versus TS, defined as?Zn=rZnO(T)/rZnO(max), where rZnO(max) is recalculated from the Zn flux measured by aquartz monitor, using Zn/ZnO molar mass and density ratios.Ex-situ characterization of the grown ZnO layers indicate that the surface morphology andcrystal quality of the ZnO films grown on sapphire by MBE using either oxygen plasma cellor H2O2 as an oxidant can be extensively improved by using an HT MgO buffer.ZnO layers reveal strong variation of surface morphology versus the O/Zn flux ratio. Themost flat surface morphology of ZnO is obtained when the ratio is within the 0.7-1 range. Thegrowth under O-rich conditions leads to formation of hexagonal pyramids and at higher O/Znratios to a 3D growth with the top layer formed by perfectly c-oriented columnar structures of50-100 nm in a diameter. It was also possible to recover the initial 3D growth mode to the 2Done by employing the Zn-rich growth conditions at O/Zn=0.4-0.6.Structural characterizations by high resolution X-ray diffraction (HR-XRD) and transmissionelectron microscopy (TEM) indicate a dramatic reduction in defect density in the ZnOepilayers grown with an HT MgO buffer. By using TEM, it was found that the dominantextending defects are edge, screw and mixed-type dislocations along c-axis. The main defectswere threading dislocations. This is resulted from the well controlled layer-by-layer growth, since only the edge-type dislocation is able to accommodate the lattice mismatch, while thescrew type dislocation forms much related to the initial nucleation environment.Themicrostructure of ZnO epilayers has been studied by HR-XRD. The full width at halfmaximum of the (0002) reflection, 0.007 degree, is much smaller than that of the (10-10)reflection, 0.27 degree revealing the micro-twist dominates the mosaicity, while micro-tilt ismuch less important.This pronounced difference of the rocking curve widths between the(0002) and (1010) reflections strongly indicates that the density of pure edge threadingdislocations is greater than that of pure screw dislocations.Optical characterizations reveal that exciton plays an important role in ZnO. At roomtemperature free exciton recombinations dominate the photoluminescence. The ZnO epilayersreveal well resolved low temperature PL excitonic spectra with a dominant bound exciton line (3.355 eV) possessing a ~2 meV half-width and a peak of free A exciton at 3.374eV. The low-energy tail extending from the excitonic emission peaks due to the latticedeformation is significantly reduced, which allows the observation of two electron satellitesand LO-phonons replicas of free and bound excitons. Variation of growth stoichiometry fromO-rich to Zn-rich results in the pronounced quench of the acceptor-bound part of the excitonicband, as well as the strong intensity redistribution of donor-bound lines which seems to beattributed to a change in the point defect density. Temperature dependence of PL spectrabetween 6K and room temperature every 30 K under the same excitation conditions wasperformed. Slowly decreases coming at 300K to about one third of the intensity at 6K. Thiscorresponds to the activation of non-radiative channels in the capturing and recombinationprocesses. This result was confirm by decay time measurements. PL mapping of 2 inch ZnOepilayer shows high lateral homogeneity from PL intensity distribution and PL FWHMdistribution.Hall-effect measurements and Electrochemical profiling (ECV) were used to characterize theelectrical properties of ZnO samples. Hall-effect measurements indicated n-type behaviorwith carrier concentration of 2.0x1016 cm-3 and mobility of approximately 96 cm2/Vs. ECVprofile versus depth measured for the top 2.5 ?m thick sample gives surface carrierconcentration is 2.0x1016 cm-3 increasing to a maximum value of 1.0x1018 cm-3 thesemiconductor/substrate interface.P-n heterojuntions and mesa structures comprising MBE n-ZnO layers and CVD p-4H-SiClaser were manufactured and investigated. Electrical properties of the mesa diodes have beenstudied with Hall measurements, and current-voltage measurements (I-V). I-V measurementsof the device show good rectifying behavior, from which a turn-on voltage of about 2 V wasobtained.With the excitation of O and N gas mixture in a single plasma cell, followed by the sampleannealing procedure. P-type ZnO: N layers with a net hole concentration 3x1017 cm-3 usingwas measured. The combination of low growth temperature, slightly O-rich conditions andpost-growth annealing is shown to be effective way to obtain p-doping. Further efforts arenecessary to improve structural quality of the low-temperature p-type ZnO: N films.Optical properties of ZnO based II-VI heterostructures and quantum structures have also beenstudied. The surface roughness of ZnxMg1?xO was as low as 0.7 nm. The optical band gapand photoluminescence peak can be turned to larger energy with the same high crystallinityand without significant change in the lattice constant. The prominent PL peaks related to theSQW show a systematic blueshift with decreasing well width, which is consistent with thequantum size effect. The SQW-related emission peaks exhibit an S-shaped (redshift-blueshiftredshift)behaviour with increasing temperature, which is in contrast with that ascribed toband gap shrinkage (redshift). The observed behavior is discussed in terms of localization atlateral interface potential fluctuations. For T >70 K the integrated PL intensity is thermallyactivated with activation energies much less than the band offsets. It is argued that thedominant mechanism leading to the quenching of the ZnO SQW-related PL is due to thethermionic emission of excitons out of the lateral potential minima caused by potentialfluctuations, such as interface fluctuations by 1 ML. Stimulated emission has been achieved atroom temperature in a separate confinement double heterostructure having a 3 nm wide SQWas an active region. It has been found that a critical parameter for the lasing is theinhomogeneous broadening of both QW and barrier emission bands.MBE process for ZnO has been developed where high quality ZnO epilayers andheterostructures can be grown by molecular beam epitaxy on sapphire substrate. For nitrogendoping of ZnO, Oxygen and nitrogen were activated in the single plasma cell. Noreproducible and reliable experimental results on the achievement of p-type conductivityachieved. Stimulated emission has been achieved at room temperature.