Leo J. Schowalter

Some effects of oxygen impurities on AlN and GaN

Oxygen is a common substitutional impurity in both AlN and GaN crystals. In the wurtzite 2H phase it can be present in AlN up to concentrations of 1 x 10 21 /cm 3 while in GaN it can reach concentrations of 3 × 10 22 /cm 3 . These high concentrations of oxygen affect the luminescence, the optical absorption, the thermal conductivity, and the crystal perfection. The effects are somewhat similar in AlN and GaN. Representative experimental data will be presented to demonstrate the similarities, and to show how the oxygen content may be estimated from these property measurements.

Crystallographic tilting of heteroepitaxial layers

Abstract Mismatched heteroepitaxial layers generally exhibit a crystallographic tilt with respect to their vicinal substrates, but this phenomenon is poorly understood. Two models have been proposed for tilted epitaxy previously. The Nagai model considers the mismatch at surface steps on vicinal substrates and the misfit dislocation theory considers misfit dislocations with inclined Burgers vectors as the primary cause of tilted epitaxy. Neither of these theories applies in the general case, however. The misfit dislocation theory does not consider the mechanism of the preferential introduction of dislocations with a particular orientation, and therefore does not predict the direction of tilted epitaxy. Here a general theory for tilted epitaxy is proposed, including both contributions cited above. We propose that the preferential glide of certain Burgers orientations is the result of vicinal substrates, commonly used in heteroepitaxy, which create an assymetry in the stresses for the various active slip systems. If the substrate inclination is about an axis of symmetry, then the tilt of the epitaxial growth is about the same axis, and its sign is determined by the nature of the mismatch: the relief of compressive strain results in negative tilt whereas the relief of tensile strain results in positive tilt. The exact amount of tilt will depend on the dynamics of strain relaxation. Upper and lower limits for the tilt have been derived. The observed tilt angle contains information on how the relaxation process took place.

Growth and characterization of single crystal insulators on silicon

Abstract Epitaxial insulators have a number of potential applications in the semiconductor industry. These include semiconductor-on-insulator (SOI) structures, three-dimensional (3-D) and/or high-density integrated circuits, improved gate insulators, and optoelectronic applications such as integrated wave guides. Because of the potential for such applications, there are a number of approaches to epitaxial insulators that are being pursued at different laboratories. Much of this activity has centered on the growth of epitaxial Group-I1 fluorides by molecular beam epitaxy (MBE). However, alternatives such as vapor phase epitaxy (VPE) of spinel are also being pursued. In any case, the mechanisms for good heteroepitaxy must be understood in order to grow good material. Applications in the semiconductor industry will also require that the electrical properties, such as dielectric breakdown, current transport across the interface, and current transport along the interface, be understood. These topics are review...

Ultraviolet semiconductor laser diodes on bulk AlN

Current-injection ultraviolet lasers are demonstrated on low-dislocation-density bulk AlN substrates. The AlGaInN heterostructures were grown by metalorganic chemical vapor deposition. Requisite smooth surface morphologies were obtained by growing on near-c-plane AlN substrates, with a nominal off-axis orientation of less than 0.5°. Lasing was obtained from gain-guided laser diodes with uncoated facets and cavity lengths ranging from 200 to 1500 μm. Threshold current densities as low as 13 kA/cm2 were achieved for laser emission wavelengths as short as 368 nm, under pulsed operation. The maximum light output power was near 300 mW with a differential quantum efficiency of 6.7%. This (first) demonstration of nitride laser diodes on bulk AlN substrates suggests the feasibility of using such substrates to realize nitride laser diodes emitting from the near to deep ultraviolet spectral regions.

High Output Power from 260 nm Pseudomorphic Ultraviolet Light-Emitting Diodes with Improved Thermal Performance

This letter reports on the improved performance of a pseudomorphic ultraviolet light-emitting diode (LED). At 100 mA input current, 9.2 mW of quasi-CW output power was measured in a calibrated integrating sphere. The addition of a heat sink, required for CW and higher power operation, introduced a numerical aperture of 0.86, and 72 mW was measured in pulsed mode at 1.7 A, indicating that the total output power exceeds 100 mW when corrected by the coupling factor. The high characteristic temperature of 983 K was instrumental in achieving these record output powers for an LED with wavelength shorter than 265 nm.

270 nm Pseudomorphic Ultraviolet Light-Emitting Diodes with Over 60 mW Continuous Wave Output Power

In this letter, the achievement of over 60 mW output power from pseudomorphic ultraviolet light-emitting diodes in continuous wave operation is reported. Die thinning and encapsulation improved the photon extraction efficiency to over 15%. Improved thermal management and a high characteristic temperature resulted in a low thermal rolloff up to 300 mA injection current with an output power of 67 mW, an external quantum efficiency (EQE) of 4.9%, and a wall plug efficiency (WPE) of 2.5% for a single-chip device emitting at 271 nm in continuous wave operation.

Near-bandedge cathodoluminescence of an AlN homoepitaxial film

Cathodoluminescence experiments were performed on a high-quality AlN epitaxial film grown by organometallic vapor phase epitaxy on a large single crystal AlN substrate. The low-temperature near-bandedge spectra clearly show six very narrow lines. The thermal quenching behavior of these emission lines provides insight on how to assign them to free and bound exciton recombination processes. The binding energy for the free-exciton-A in AlN was found to be nearly twice that in GaN. The observation of the free-exciton-A first excited state permitted us to estimate its reduced effective mass and, by using recent reported values for the hole effective mass in Mg-doped AlN, the electron effective mass in AlN has been deduced.

Electromechanical coupling coefficient for surface acoustic waves in single-crystal bulk aluminum nitride

The electromechanical coupling coefficient K2 for surface acoustic waves propagating on c and a surfaces of bulk AlN single crystals has been measured using the S11-parameter method in the frequency range of 160–360 MHz. The extracted values of K2 are 0.11% and 0.47% for the c and a surfaces, respectively. By fitting our experimental data to our numerical simulation results, we have estimated piezoelectric constants, which are in a reasonable agreement with literature data. Our results are consistent with the negative sign of the e15 constant.

Properties of Mid-Ultraviolet Light Emitting Diodes Fabricated from Pseudomorphic Layers on Bulk Aluminum Nitride Substrates

High quality bulk aluminum nitride substrates were used to obtain pseudomorphic AlxGa1-xN layers with low dislocation density, smooth surfaces, and high conductivity. These layers were fabricated into mid-ultraviolet light emitting diodes with peak wavelengths in the range of 240–260 nm. The low dislocation density of the pseudomorphic quantum wells resulted in improved performance over previously published data. The output powers of the on-wafer measurements were greater than 5 mW in continuous wave operation, and 16 mW in pulsed operation. This was achieved utilizing single die (with an active area of 1×10-3 cm2).

Surface acoustic wave velocity in single-crystal AlN substrates

The surface acoustic wave velocity has been measured on a-plane (c-propagation) and c-plane oriented bulk aluminum nitride (AlN) single crystals using the S/sub 11/-parameter method in the frequency range 160-360 MHz. The SAW velocity is 5760 m/s for both orientations. From comparison of this value with the simulations using various elastic constants of AlN available in literature, we estimated the elastic constant C/sub 44/ to be 122 /spl plusmn/ 1 GPa.