Ferdynand P. Dabkowski

Disordering of the ordered structure in metalorganic chemical vapor deposition grown Ga0.5In0.5P on (001) GaAs substrates by zinc diffusion

Transmission electron microscopy is used to study sublattice atomic ordering in as‐grown and Zn‐diffused epitaxial layers of Ga0.5In0.5P that are grown lattice matched to GaAs by low‐pressure metalorganic chemical vapor deposition. The as‐grown Ga0.5In0.5P layers exhibit an ordered trigonal structure with In‐Ga ordering occurring on only two sets of {111} planes. After a Zndiffusion is performed at 650 °C, the ordered structure is no longer observed in selected area diffraction patterns. Simultaneously, the room‐temperature photoluminescence peak shifts by ≊90 meV to higher energy, as compared to the undiffused samples. These data provide direct experimental evidence that Ga0.5In0.5P with an ordered distribution of Ga and In atoms on the column III sublattice can be converted to a random alloy by Zndiffusion.

Temperature profile along the cavity axis of high power quantum well lasers during operation

Measurements of the temperature distribution along the cavity of 0.5 W AlGaAs quantum well lasers are presented. The spatial distribution of temperature was determined by measuring the spectral shift of electroluminescence emitted through a window in the GaAs substrate metallization. The average temperature of the active layer is 10–15 K higher than the heatsink temperature at 0.5 W output. Facet temperatures can exceed the average active layer temperature by over 100 K. Data are also presented illustrating the temperature profile at different drive currents between threshold and the maximum operating current. A temperature profile of a laser with a damaged front facet is presented, showing a hot region that is twice the size of the defect.

Micro-Raman imaging of GaN hexagonal island structures

We use Raman scattering to obtain a stress map of lateral epitaxy overgrown GaN. Isolated hexagonal islands are grown by selective area overgrowth without a seed layer. Stress mapping is obtained from shifts in the E2 phonon. GaN in the aperture area has the greatest biaxial compressive stress, ≈0.18 GPa. The overgrowth region is under slightly smaller stress, about 0.15 GPa. We attribute marked variations in the A1(LO) phonon intensity to spatial variations in the free carrier concentration. This is found to be small in the aperture region and high in the lateral overgrowth. The position-dependent presence of the lower coupled plasmon–phonon band is consistent with this interpretation.

Photoluminescence measurement of the facet temperature of 1 W gain‐guided AlGaAs/GaAs laser diodes

The output facet temperature of high‐power AlGaAs/GaAs single quantum well (SQW) laser diodes was measured during operation. The front output facets were passivated with Al2O3 coatings. The spectral shift of photoluminescence from the cladding layers was used to determine the temperature rise at the front facet with increasing output power. The spatial resolution of the technique allowed to look at each cladding layer individually and to study the correlation between the near‐field pattern and the temperature profile along the active layer. The local temperature on the facet at 1 W total optical power (corresponding to an average linear power density of 10 mW/μm) was found to vary between 25 and 45 K above the average active layer temperature and to exceed the heat‐sink temperature by up to 70 K. This represents a significant reduction of facet temperature in comparison to earlier reports and can be attributed to high‐quality passivation coatings.

Selective Area Growth of GaN Directly on (0001) Sapphire by the HVPE Technique

In this paper, we report on the selective area growth (SAG) of GaN directly on patterned c-plane sapphire substrates by hydride vapor phase epitaxy (HVPE). A number of researchers have reported that the HVPE growth technique, unlike the MBE and MOCVD methods, is capable of producing device quality GaN films without the need for any low temperature nucleation/buffer layers. The density of edge dislocations in these HVPE films decreases dramatically as the film thickness is increased, and the dislocation density values for thick films (> 10μm) are comparable to those reported for the best GaN films grown by other methods on c-sapphire. These advantages of the HVPE growth technique makes it possible to achieve high quality selective area growth of GaN directly on c-sapphire substrates. C-plane sapphire substrates were coated with PECVD SiO 2 and photolithographically patterned with different size and shape openings. Subsequently, these patterned substrates were introduced in a horizontal, hot-wall quartz reactor for the GaN growth. It was observed that single crystal GaN growth was preferentially initiated in the openings in the oxide layer. This selective area growth was followed by epitaxial lateral overgrowth (ELO), leading to the formation of hexagonal GaN prisms terminated in smooth, vertical (1 00) facets. We have been successful in shearing these pyramid structures from the sapphire substrates as individual devices, which do not require any post-growth etching for feature definition. This procedure allows for the dramatic reduction of the process complexity and the duration and expense for GaN growth for device applications. Stimulated emission results on these self-formed optical cavities are also presented.

High-power, reliable operation of 730 nm AlGaAs laser diodes

High-power operation of AlGaAs multi-quantum-well laser diodes emitting near 730 nm is reported. 1000 h, reliable operation at a power of 1.0 W for 100 μm emission aperture and 500 μm cavity length devices has been demonstrated at room temperature. These devices have threshold current densities of 770 A/cm2 with the characteristic temperature coefficients of threshold current, T0, and external differential quantum efficiency, T1, of 152 and 167 K, respectively.

Zn disordering of a Ga0.5In0.5P‐(AlxGa1−x)0.5In0.5P quantum well heterostructure grown by metalorganic chemical vapor deposition

It is well established by now that epitaxial layers of (AlxGa1−x)0.5In0.5P and Ga0.5In0.5P grown on (001) GaAs substrates by metalorganic chemical vapor deposition at temperatures below 700 °C show an ordered arrangement of the group III atoms on the column III sublattice, resulting in a shift of the band gap to lower energies by ≊90 meV. In this letter we show that an (AlxGa1−x)0.5In0.5P‐Ga0.5In0.5P quantum well heterostructure containing the ordered phase can be converted to random alloy by a relatively short sealed‐tube zinc diffusion at a temperature of 600 °C, without affecting the dimensional or compositional stability of the quantum well. Complete intermixing of the quantum well with the cladding layers occurs at diffusion times longer than that required to disorder the column III ordered structure.

Zinc diffusion in III‐V semiconductors using a cubic‐zirconia protection layer

Zinc was diffused into GaAs, Al0.38Ga0.62As, and GaAs0.6P0.4 using a 2000–3000‐A‐thick yttria‐stabilized cubic‐zirconia (YSZ) protection layer to produce planar p‐n junctions. The YSZ layer greatly reduced thermal decomposition of the semiconductor while allowing zinc to diffuse into the III‐V semiconductors. The diffusion depth showed a square‐root‐of‐time dependence for all samples protected with YSZ. Characterization of the GaAs diffused with the YSZ protection process indicates that the YSZ layer has virtually no effect on either the carrier concentration profile or activation energy. No thermal decomposition was observed by visual inspection following a diffusion using only elemental Zn on the GaAs, Al0.38Ga0.62As, or GaAs0.6P0.4 samples protected with a YSZ layer. Photoluminescence analysis shows that the YSZ‐protected GaAs samples have higher luminescence efficiency relative to unprotected samples. Device fabrication has been demonstrated by combining the YSZ passivation process with selective area...

Description and applications of high-brightness multi-laser-diode system

This paper describes a novel, high-brightness, multi-laser- diode system that provides great flexibility for use in a wide array of applications. The system consists of eight individual, field-replaceable laser diodes, whose outputs are optically combined to provide a collimated beam. Field replaceability of the diodes and mechanical robustness of this system make it particularly suitable for highly demanding environments. CW optical power greater than 90 Watts at 915 nm was focused to a spot size of 140 X 130 micrometer and a numerical aperture of 0.22 NA. This high CW power density (approximately 5 X 105 W/cm2) was achieved by polarization coupling of two multi-laser-diode systems. Optical power in excess of 52 W was obtained from a single-end pumped, grating stabilized Yb:fiber laser at 1100 nm. This paper will also present results on digital printing, CD-RW disk initialization and solid-state laser pumping. A unique feature of this system is the ability for direct-diode coupling to fiber, eliminating any splicing or connector- related losses.

Evaluations of As-fabricated GaN-based light-emitting diodes

We have assessed the structural perfection of as-fabricated light emitting devices using cross-sectional transmission electron microscopy and high resolution transmission electron microscopy. We have used the electron beam induced current and cathodoluminescence techniques to evaluate the electrical activity of the structural defects. Results indicate that the density of threading dislocations in the device structure is about 1010 cm-2, and they are of edge character. We argue that this high density is a consequence of the coalescence of the AlN islands that form on the SiC substrates. We also discuss the replication behavior of the threading dislocations during subsequency growth. Both the electron beam induced current and cathodoluminescence images show non-radiative recombination regions. We attribute them to the presence of the threading dislocations.