J. Walker

Hydrogen passivation of Mg acceptors in GaN grown by metalorganic chemical vapor deposition

The effects of the deliberate hydrogenation of GaN were investigated for heteroepitaxial layers grown by metalorganic chemical vapor deposition. The GaN layers were either Mg‐doped, p‐type after thermal activation, or Si‐doped, n type. Elemental depth profiles from secondary ion mass spectroscopy reveal a striking contrast after a deuteration at 600u2009°C: the deuterium concentration in Mg‐doped GaN is ∼1019 cm−3 while there is no detectable deuterium incorporation in the n‐type material. Variable temperature Hall effect measurements provide the most direct evidence to date for Mg–H complex formation with the decrease in the hole concentration upon hydrogenation accompanied by an increase in the hole Hall mobility.

Characterization of AlGaInN diode lasers with mirrors from chemically assisted ion beam etching

Current-injection InGaAlN heterostructure laser diodes grown by metalorganic chemical vapor deposition on sapphire substrates are demonstrated with mirrors fabricated by chemically assisted ion beam etching. Due to the independent control of physical and chemical etching, smooth vertical sidewalls with a root-mean-squared roughness of 4–6 nm have been achieved. The diodes lased under pulsed current-injection conditions at wavelengths in the range from 419 to 423 nm. The lowest threshold current density was 25 kA/cm2. Lasing was observed in both gain-guided and ridge-waveguide test diodes, with cavity lengths from 300 to 1000 μm; and output powers of 10–20 mW were achieved. Laser performance is illustrated with light output-current and current–voltage characteristics and with a high-resolution optical spectrum.

Dry-etching and characterization of mirrors on III-nitride laser diodes from chemically assisted ion beam etching

Abstract Vertical mirrors have been fabricated with chemically assisted ion beam etching (CAIBE) on OMVPE grown InGaN/AlGaN laser diode structures. AFM measurements show that smooth vertical sidewalls are obtained which exhibit a root mean squared (rms) roughness of only 40–60xa0A. The inclination angle of the etched mirrors is within ±2° of vertical, as SEM studies indicate. Photopumping measurements reveal that the reflectivity of the etched mirrors corresponds to 60–70% of the value for an ideal GaN/air interface. The reduced reflectivity may be due to surface roughness, a slight tilt in the facet angle, and the excitation of higher-order transverse waveguide modes in the laser structure.

Hydrogen passivation in nitrogen and chlorine‐doped ZnSe films grown by gas source molecular beam epitaxy

The incorporation of hydrogen in ZnSe:N and ZnSe:Cl films grown by gas source molecular beam epitaxy (GSMBE) using elemental Zn and H2Se as source material has been investigated. The hydrogenation behavior was found to be significantly enhanced when nitrogen was used as a dopant, and typically resulted in highly resistive films. On the other hand, Cl‐doped ZnSe films showed a hydrogen concentration at or near the background levels independent of the Cl concentration. ZnSe was also grown by conventional molecular beam epitaxy with intentionally introduced H2 in order to clarify the source of the hydrogen. Significant hydrogen incorporation was observed in the MBE‐grown ZnSe:N layers only when hydrogen gas was introduced. Injection of hydrogen in excess of the amount generated during typical GSMBE experiments was found to give rise to an unambiguous increase in the hydrogen concentration, but with a hydrogen:nitrogen ratio less than that measured in GSMBE films.

Observations on the limits to p‐type doping in ZnSe

In this letter secondary ion mass spectroscopy measurements of nitrogen concentrations in p‐type ZnSe and ZnTe, doped using a nitrogen plasma source with molecular beam epitaxy, are correlated with transport data from the temperature‐dependent Hall effect measurements. The results suggest that, at least for the growth conditions employed in this study, the nitrogen acceptor solubility is the controlling factor in determining that the acceptor concentration in ZnTe exceeds that in ZnSe by about one order of magnitude despite the similar growth conditions.

Hydrogen incorporation in undoped microcrystalline silicon

Hydrogen in undoped, unalloyed microcrystalline silicon (μc‐Si:H) has been investigated with secondary‐ion mass spectrometry (SIMS), Raman spectroscopy, infrared absorption spectroscopy, and nuclear magnetic resonance (NMR). The samples were grown by plasma‐enhanced chemical vapor deposition with hydrogen to silane dilution ratios (H2:SiH4) ranging from 0:1 to 98:1. Microcrystallinity is obtained for dilution ratios of 20:1 and greater. The hydrogen concentration is shown to depend nonmonotonically on the degree of hydrogen dilution. The H concentration in the films decreases with dilution for ratios from 0:1 to 10:1 and then increases with greater dilution. This dependence on dilution is established with both NMR and SIMS and suggests the existence of competing processes in the incorporation of hydrogen during deposition. It is further observed that the formation of microcrystallites is accompanied by the appearance of both higher order silicon hydrides and large concentrations of unbound molecular hydrogen.

Hydrogenation of wide‐band‐gap II‐VI semiconductors

The incorporation of plasma‐generated monatomic hydrogen into a wide‐band‐gap II‐VI zincblende semiconductor (e.g. ZnSe) has been achieved by using a deposited layer of silicon dioxide as a hydrogen‐permeable encapsulation to prevent decomposition (etching) of the semiconductor during hydrogenation. Depth profiles from secondary‐ion mass spectrometry demonstrate deuterium penetration into ZnSe layers epitaxially grown on GaAs. Low‐temperature photoluminescence reveals that the commonly observed donor‐acceptor pair transitions are selectively removed by hydrogenation.

The study of nitrogen doping in ZnSe and ZnSe:Te

A comparative study of nitrogen doping in ZnSe and ZnTe has been performed and the results suggest that dopant solubility seems to be the limiting factor, at least under our growth conditions, in obstructing degenerate p-type doping of ZnSe. In an effort to increase the nitrogen acceptor concentration in ZnSe, we have investigated the effects of Te isoelectronic impurity on the nitrogen doping concentration in ZnSe. It was found that the total nitrogen concentration and the nitrogen acceptor concentration are indeed increased, but the room temperature free hole concentration actually drops slightly. Temperature dependent transport measurements were performed and the results show a large increase in compensation ratio as well as a dramatical reduction in hole mobility. The latter is attributed to the tendency for hole localization at the isoelectronic impurity.

p-Type and n-type doping of ZnSe: Effects of hydrogen incorporation

The hydrogenation behavior of p- and n-type ZnSe grown on GaAs by gas source molecular beam epitaxy (GSMBE) is presented. Recent advances in p-type doping, using a radio frequency (RF) plasma source with nitrogen, have led to the successful fabrication of blue /green light emitters based on the (Zn,Mg)(S,Se) material system grown by molecular beam epitaxy (MBE). GSMBE replaces the high vapor pressure group VI elements with hydride gases which are amenable to regulation using precision mass flow controllers, and has the potential to deliver improved compositional control and reproducibility. We have found that the presence of hydrogen does not affect the electrical conductivity of ZnSe:Cl grown by GSMBE. In contrast, nitrogen-doped ZnSe is speculated to be electrically passivated by hydrogen for certain growth conditions as evidenced by: (1) coherent tracking of the hydrogen concentration with variations in the nitrogen concentration, which is measured by secondary ion mass spectrometry (SIMS), and (2) indications of high resistivity determined by capacitance-voltage (C-V) measurements. Conventional and rapid thermal annealing (RTE) have been investigated to modify the degree of hydrogen passivation

Solid-source doping of hydrogenated amorphous silicon

Abstract Arsenic doping of hydrogenated amorphous silicon (a-Si: H) from a solid source in a remote hydrogen plasma reactor is reported. Arsenic incorporation in the deposited films was directly demonstrated with secondary-ion mass spectrometry, and doping efficiency was determined from d.c. conductivity. The electronic properties of the doped films were indistinguishable from those of conventional r.f. glow discharge deposited a-Si:H. The technique should be applicable to other dopant and alloying elements