A. Schremer

ALGAN/GAN HETEROSTRUCTURES ON INSULATING ALGAN NUCLEATION LAYERS

A single temperature process using AlGaN nucleation layers has been developed that produces device-quality, GaN-based materials with bilayer step surfaces. The AlGaN nucleation layer is deposited by flow modulation organometallic vapor phase epitaxy at temperatures in excess of 1000 °C, where GaN and AlGaN films can be subsequently grown. We have optimized this process on both sapphire and SiC substrates, where the conditions for nucleation are found to be quite different. For growth on SiC, aluminum mole fractions ranging from 6% to 35% result in featureless surfaces. Optimizing the alloy composition and thickness of the nucleation layer on SiC allows the deposition of GaN buffer layers exceeding 5 μm without the formation of cracks. A minimum of 15% aluminum in the nucleation layer is required for smooth growth on sapphire substrates. High room temperature two-dimensional electron gas mobilities of 1575 and 1505 cm2/Vs with sheet charge densities of 1.0×1013 and 1.4×1013 cm−2 are observed in undoped AlG...

Bistability in two‐mode semiconductor lasers via gain saturation

The conditions required to achieve bistability in two‐mode semiconductor lasers via the nonlinearity associated with gain saturation are discussed. The laser can be switched between the bistable states through coherent or incoherent optical control. Wavelength bistability in such a laser is demonstrated experimentally.

High electron mobility AlGaN/GaN heterostructure on (111) Si

Room-temperature Hall mobilities exceeding 900 cm2/V s are obtained for AlGaN/GaN heterostructures on (111) Si by single-temperature flow modulation organometallic vapor phase epitaxy. Thin pseudomorphic AlGaN top layers exhibit a 1.5 nm surface roughness and induces a two-dimensional electron gas sheet carrier concentration of 1.0×1013 cm−2. The GaN buffer layer has a background carrier concentration of 1.0×1015 cm−3, 130 arcsec x-ray diffraction full width at half maximum, and a low-temperature photoluminescence linewidth of 10 meV. An AlN nucleation layer provides static electrical isolation between the AlGaN/GaN and the conducting Si substrate. Large crack-free areas of high-crystalline-quality epitaxial material are obtained and have been successfully used for transistor fabrication.

Structural properties of AlGaN/GaN heterostructures on Si(111) substrates suitable for high-electron mobility transistors

Transmission electron microscopy (TEM) investigations of metal organic vapor phase deposition grown AlxGa1−xN/GaN heterostructures on Si(111) containing an AlN high-temperature buffer layer have been carried out. The structural properties at the interface and in the epilayer as well as the electronic properties suitable for a high electron mobility transistor (HEMT) were analyzed and compared with systems grown on Al2O3(0001). High resolution TEM (HRTEM) at the AlN/Si(111) interface reveals a 1.5–2.7 nm thick amorphous SiNx layer due to the high growth temperature of TAlN=1040 °C. Therefore, a grain-like GaN/AlN region extending 40–60 nm appears and it is subsequently overgrown with (0001) orientated GaN material because of geometrical selection. The residual strain at the AlN/Si(111) interface is estimated to be er=0.3±0.6% by Fourier filtering of HRTEM images and a moire fringe analysis. This indicates almost complete relaxation of the large mismatch f(AlN/Si)=+23.4% which seems to be supported by the S...

Nonlinear luminescence and time‐resolved diffusion profiles of photoexcited carriers in semiconductors

We describe a new and general optical technique for studying carrier dynamics in semiconductors which is based on the luminescence nonlinearity resulting from the bimolecular recombination of the carriers. The technique has been used to obtain time‐resolved diffusion profiles and the hole mobility in photoexcited GaAs.

Spatial beam switching and bistability in a diode ring laser

We report bistable switching of the output direction in certain triangular diode ring lasers. These lasers operate unidirectionally at large drive currents, with up to 98% of the output power emerging from either the clockwise or the counterclockwise direction. The preferred direction can switch back and forth, with a large hysteresis loop, as the drive current is varied. Up to 20 mW of optical power changes direction at each switch. We propose an explanation based on the theory of two-mode competition via gain saturation.

Single-frequency tunable external-cavity semiconductor laser using an electro-optic birefringent modulator

An external‐cavity semiconductor laser employing a birefringent electro‐optic modulator for wavelength tuning is described. Wide‐band tuning over 70 A in a single frequency to residual diode modes and narrow‐band tuning over 0.42 A to many longitudinal modes of the external cavity is demonstrated using different modulator configurations. Threshold current and quantum efficiency are nearly as good as that of the solitary laser diode.

Polarization bistability in external cavity semiconductor lasers

Polarization bistability between the transverse electric (TE) and transverse magnetic (TM) modes is observed in an external cavity semiconductor laser. Hysteresis in the polarization‐resolved output power is obtained by controlling the optical feedback power of the TE mode via an intracavity electro‐optic modulator. Transitions between single external‐cavity frequencies of TE and TM modes are also demonstrated. Nonlinear gain saturation is the origin of this bistability.

External-cavity semiconductor laser with 1000 GHz continuous piezoelectric tuning range

A 1000-GHz continuous tuning range is achieved using a single piezoelectric transducer to orient a diffraction grating in an external-cavity semiconductor laser. The laser output power ripple versus wavelength is 15%, and the linewidth is less than 500 kHz. Large-amplitude piezoelectrically controlled wavelength modulation can be done at modulation frequencies on the order of 100 Hz. The maximum modulation frequency, continuous electrical tuning range, and linewidth are limited by the mechanical design of the external cavity. This device should have wide application in spectroscopy, metrology, and remote sensing.<<ETX>>

Single step process for epitaxial lateral overgrowth of GaN on SiC and sapphire substrates

Using flow modulation organometallic vapor phase epitaxy, a process has been developed which produces epitaxial lateral overgrowth of GaN-base materials directly on SiC and sapphire substrates patterned with silicon nitride. The key feature of this single step process is the use of a high temperature AlGaN nucleation layer which wets the exposed substrate surface, without significant nucleation on the mask. This eliminates the need for regrowth while producing smooth growth surfaces in the window opening as well as over the mask. Subsequent GaN deposition results in relatively defect free materials grown laterally over the mask. Using arrays of stripe windows aligned parallel to the 〈11_00〉 crystal direction, the epitaxial films completely planarize after roughly 5 microns of growth. Defect densities estimated from atomic force micrographs indicate a reduction from mid 108 to 105 cm−2 in regions over the window and over the mask, respectively. This process represents a significant simplification over curr...