P. J. Parbrook

Gate leakage effects and breakdown voltage in metalorganic vapor phase epitaxy AlGaN/GaN heterostructure field-effect transistors

The gate leakage behavior in AlGaN/GaN heterostructure field-effect transistors was studied as a function of applied bias, temperature, and surface periphery. A surface hopping conduction mechanism with an activation energy of 0.21 eV is proposed for the gate–drain leakage for voltages that exceed pinchoff. The reverse breakdown voltage of the device exhibited a negative temperature coefficient of −0.11 V K−1, suggesting that a breakdown mechanism other than impact ionization, such as thermal runaway, may be responsible.

The MOCVD growth without prereaction of ZnSe and ZnS layers

Abstract It is shown that the liquid adduct dimethylzinc-triethylamine (Me 2 Zn(NEt 3 ) 2 ) can be used as the zinc source to grow at low temperatures single crystal layers, principally of ZnSe but also of ZnS, by MOCVD without any significant prereaction of the constituent reactants. The high quality and uniformity of the ZnSe layers produced is demonstrated by a low residual carrier concentration, high mobility, an absence of deep-centre photoluminescence and a narrow width for X-ray rocking curves.

Comparison of different surface passivation dielectrics in AlGaN/GaN heterostructure field-effect transistors

Different dielectrics were used for post-processing surface passivation of AlGaN/GaN heterostructure field-effect transistors (HFETs) and the resulting electrical characteristics examined. An increase in the maximum drain current of approximately 25% was observed after Si3N4 and SiO2 deposition and ~15% for annealed SiO on AlGaN/GaN HFETs. In all cases, the passivation was found to increase the gate leakage current with an observed reduction in the leakage activation energy. However, the rise in gate leakage current was least for SiO. The plasma enhanced chemical vapour deposition method was found not to contribute to the passivation mechanism, whilst the presence of Si appears to be an important factor.

Electron-beam-induced segregation in InGaN/GaN multiple-quantum wells

We report a study of the morphology and composition of InxGa1−xN/GaN multiple-quantum-well structures and their sensitivity to electron-beam damage. We have employed high-resolution transmission electron microscopy, energy dispersive x-ray analysis, and scanning transmission electron microscopy. Microstructural analysis was performed to investigate the dynamical effects of electron-beam irradiation on the relative indium distribution within the quantum wells. Exposure to relatively low incident beam illumination, corresponding to current densities at the specimen of ∼100 pA/cm2, was found to induce significant nanoclustering of indium within the multiple-quantum wells. These findings highlight the need for caution when reporting the presence of indium-rich clusters within InGaN/GaN multiple-quantum wells studied in the transmission electron microscope.

IN SITU MONITORING OF THE SURFACE RECONSTRUCTIONS ON INP(001) PREPARED BY MOLECULAR BEAM EPITAXY

Reflection anisotropy spectroscopy (RAS) and reflection high-energy electron diffraction (RHEED) were applied to study clean InP(001) surfaces prepared by molecular beam epitaxy (MBE). At phosphorus beam equivalent pressures (BEPs) between 3.5×10−7 and 3.5×10−6 mbar and substrate temperature (Ts) falling from 590 to 150 °C, (2×4), (2×1), (2×2), and c(4×4) RHEED patterns are observed. The main RAS features, observed at 1.7–1.9 and 2.6–2.9 eV are assigned to In and P dimers, respectively. The above reconstruction sequence is associated closely with transformations identified in RAS signatures that are induced by progressively increasing the P surface coverage. The RAS results also imply the existence of (2×4)α and (2×4)β phases. A surface-phase diagram for MBE-grown (001) InP, in the whole range of Ts and phosphorus BEPs is proposed.

MOCVD Layer growth of ZnSe and ZnS / ZnSe multiple layers using nitrogen containing adducts of dimethylzinc

Abstract It is shown that adducts between dimethylzinc and the nitrogen donor ligands triethylamine and triazine can be used successfully as the zinc sources to grow ZnSe and related compounds by MOCVD. High quality single crystal layers of ZnSe have been grown without any significant pre-reaction of the constituent reactants. The high quality character of the layers is demonstrated by the presence of free exciton dominated photoluminescence. Some general points concerning the use of zinc adducts are also reviewed and discussed. In particular, it is shown that one reason for the improved purity of layers grown using adducts compared to dimethylzinc alone is the removal of iodine, an n-type donor in ZnSe, by the adducting process. The relationship between the nature of the adducting species and the control of prereaction is also discussed briefly.

Air-bridged lateral growth of an Al0.98Ga0.02N layer by introduction of porosity in an AlN buffer

We demonstrated air-bridged lateral growth of an Al0.98Ga0.02N layer with significant dislocation reduction by introduction of a porous AlN buffer underneath via metalorganic chemical vapor deposition. By modifying growth conditions, a porous AlN layer and an atomically flat AlN layer have been obtained for comparison, confirmed by atomic force microscopy. An Al0.98Ga0.02N layer was subsequently grown on both the porous AlN layer and the atomically flat AlN layer under identical conditions. Significant dislocation reduction was achieved for the Al0.98Ga0.02N layer grown on the porous AlN buffer layer, compared to the layer grown on the atomically flat AlN layer, as observed by transmission electron microscopy. Clear bubbles from the layer grown on the porous AlN buffer layer have been observed, while in contrast, there was not any bubble from the layer on the flat AlN buffer, confirming the mechanism of lateral growth for dislocation reduction. Asymmetric x-ray diffraction studies also indicated that the ...

Photoluminescence of wide bandgap II–VI superlattices

Abstract Photoluminescence (PL) and PL decay measurements are used to characterize typical samples of wide gap II–VI strained layer superlattices (SLSs). The results show that good quality material is obtained for SLSs of ZnSe/ZnS, CdSe/CdS (hex), ZnSe/CdSe and ZnS/CdS using atmospheric pressure MOCVD. Quantum confinement in SLSs is confirmed by the peak shift, temperature dependence and decay characteristics of the exciton luminescence. The PL intensity at low temperatures (

Control of prereaction in the metalorganic chemical vapour deposition of zinc- and cadmium-based chalcogenides

Abstract It is shown that pyridine, when introduced into the gas phase, is effective in controlling the prereaction between the group II alkyl precursur and the group VI hydride during the metalorganic chemical vapour deposition (MOCVD) of ZnSe. During the corresponding growth of CdSe, pyridine induces a greatly reduced effect. Possible reaction mechanisms to account for prereaction control are proposed.

Critical thickness of common-anion II–VI strained layer superlattices (SLSs)

Abstract The critical thicknesses of quantum wells in the common-anion superlattices ZnSe-CdSe and ZnS-CdS have been determined experimentally for comparison with calculations. Photoluminescence measurements reveal a large drop in luminescence efficiency when the thickness of CdSe (CdS) wells in a ZnSe (ZnS) lattice exceeds 1.3 nm. Annealing a ZnSe-CdSe SLS in which the well width is slightly in excess of the critical thickness (1.3 nm) causes an increase in luminescence efficiency as the ZnSe and CdSe layers interdiffuse thereby reducing the inter-layer strain: in consequence, misfit dislocations shrink and eventually disappear. These results directly confirm that a dislocation mechanism is responsible for the variation of photoluminescence intensity with well width in ZnSe-CdSe and ZnS-CdS SLSs.