Jyh-Young Chen

Electrical and optical changes in the near surface of reactively ion etched n-GaN

Abstract Photoluminescence (PL) and ohmic contact resistance measurements were used to characterize the n-GaN surface treated with reactive ion etching (RIE). The n-GaN film was grown on a sapphire substrate by a metal organic chemical vapor deposition process. Subsequently, the GaN film was etched with BCl 3 or Ar gas prior to the PL measurement and contact metal Ti/Al deposition. In PL spectra, we observed the peak shift of the impurity-related emission (yellow luminescence (YL)) for both BCl 3 and Ar etched n-GaN films. The amount of this wavelength shift increases with the RIE rf power increase and depends on the species of the etching gases. The shift of the YL peak decreased after thermal annealing at 500°C for 30 s. The YL peak shift may be attributed to the RIE induced surface point defects. In the ohmic contact resistance measurements, the transmission line model technique was used. The contact resistance of the RIE etched surface is around 5.4×10 −4 Ω cm 2 . However, a lower contact resistance of 5.7×10 −6 Ω cm 2 was obtained for a native surface of n-GaN film. No obvious distinction between the roughness of etched and native GaN surfaces was observed by the atomic force microscope measurements. In an X-ray grazing diffraction measurement, the crystalline lattice orientation and the coherent length of native n-GaN layer were better than the etched n-GaN according to the results of the rocking curve and Θ –2 Θ scan.

Residual strain in ZnO nanowires grown by catalyst-free chemical vapor deposition on GaN/sapphire (0001)

ZnO nanowires were grown on 2-μm-thick GaN templates by chemical vapor deposition without employing any metal catalysts. The GaN template was deposited by metal-organic chemical vapor deposition on a c-plane sapphire substrate. The diameters of the resulting nanowires were in the range of 40–250nm depending on growth time. The ZnO nanowires were vertically well aligned with uniform length, diameter, and distribution density as revealed by electron microscopy. X-ray diffraction spectra showed that ZnO grew in single c-axis orientation with the c axis normal to the GaN basal plane, indicating a heteroepitaxial relationship of (0002)ZnO‖(0002)GaN. The lattice constant of the c axis of the ZnO nanowires with diameter of 40nm was 5.211A, which is larger than that of bulk ZnO (5.207A). The ZnO nanowires exhibit a residual tensile strain along the c axis, which decreases with increasing diameter.

Si diffusion in p-GaN

The characteristics of p-type Mg-doped GaN films diffused with Si are studied. N-type conductivity is achieved, and the carrier mobility of diffused GaN is 90–150 cm2 V−1 s−1, higher than that of p-GaN but less than that of epitaxially grown n-GaN. The Mg acceptor states could become deep compensating defects, and the compensation ratio NA/ND is 0.3, 0.45, 0.6, and 0.75 for 800, 900, 1000, and 1100 °C diffused GaN, respectively. The carrier transport may be dominated by electron hopping through these deep compensating centers or through diffusion. The results of temperature-dependent carrier concentration indicate that thermal annealing may induce defects at the surface, leading to an additional activation energy Ed∼10 meV in the 200–500 K region in diffused GaN.

Hydrogenated amorphous silicon carbide double graded-gap p-i-n thin-film light-emitting diodes

Hydrogenated amorphous silicon carbide (a-SiC:H) p-i-n thin-film light-emitting diodes (TFLEDs) with graded p/sup +/-i and i-n/sup +/ junctions have been proposed and fabricated successfully on an indium-tin-oxide (ITO)-coated glass. An orange TFLED reveals a brightness of 207 cd/m/sup 2/ at an injection current density of 500 mA/cm/sup 2/. This significant increase of brightness could be ascribed to the combined effect of reduced interface states by using the graded-gap junctions, lower contact resistance due to post-metallization annealing, and higher optical gaps of the doped layers.<<ETX>>

Microstructure of InN quantum dots grown on AlN buffer layers by metal organic vapor phase epitaxy

InN quantum dots (QDs) were grown over 2in. Si (1 1 1) wafers with a 300nm thick AlN buffer layer by atmospheric-pressure metal organic vapor phase epitaxy. When the growth temperature increased from 450to625°C, the corresponding InN QDs height increased from 16to108nm while the density of the InN QDs decreased from 1.6×109cm−2to3.3×108cm−2. Transmission electron microscopy showed the presence of a 2nm thick wetting layer between the AlN buffer layer and InN QDs. The growth mechanism was determined to be the Stranski–Krastanov mode. The presence of misfit dislocations in the QDs indicated that residual strain was introduced during InN QDs formation. From x-ray diffraction analysis, when the height of the InN QDs increased from 16to62nm, the residual strain in InN QDs reduced from 0.45% to 0.22%. The residual strain remained at 0.22% for larger heights most likely due to plastic relaxation in the QDs. The critical height of the InN QDs for releasing the strain was determined to be 62nm.

ELECTROLUMINESCENCE OF A-SIC-H P-I-N THIN-FILM LIGHT-EMITTING-DIODES WITH QUANTUM-WELL-INJECTION STRUCTURES

Abstract In order to improve the electroluminescence (EL) characteristics of hydrogenated amorphous silicon carbide (a-SiC:H) p-i-n thin-film light-emitting diodes (TFLEDs), the quantum-well-injection (QWI) structures have been incorporated into their intrinsic (i-) layer. Two types of TFLED were fabricated to study the effect of the incorporated QWI structures on their EL characteristics: the device I contains a step-gap QWI structure of barrier (15 A)/well (45 A)/barrier (15 A) inserted at both the p-i and i-n interfaces, and the device II has only one graded-gap QWI structure of barrier (10 A)/well (10 A)/barrier (10 A) inserted at the p-i interface. The obtained brightness of device I was about 10 cd/m2 at an injection current density of 1 A/cm2. The emission light of device I was yellow-like as detected by human eyes. Whereas, for device II, the brightness was about 256 cd/m2 at 800 mA/cm2 and an orange light emission was observed.