B. V. Shanabrook

Homogeneous Linewidths in the Optical Spectrum of a Single Gallium Arsenide Quantum Dot

The homogeneous linewidths in the photoluminescence excitation spectrum of a single, naturally formed gallium arsenide (GaAs) quantum dot have been measured with high spatial and spectral resolution. The energies and linewidths of the homogeneous spectrum provide a new perspective on the dephasing dynamics of the exciton in a quantum-confined, solid-state system. The origins of the linewidths are discussed in terms of the dynamics of the exciton in zero dimensions, in particular, in terms of lifetime broadening through the emission or absorption of phonons and photons.

Photoreflectance characterization of interband transitions in GaAs/AlGaAs multiple quantum wells and modulation‐doped heterojunctions

The optical modulation technique of photoreflectance (PR) has been applied to characterize the interband transitions in GaAs/AlGaAs multiple quantum wells (MQW) and modulation‐doped heterojunctions at room temperature. The spectra of the MQW show ‘‘derivativelike’’ reflectance features due to allowed interband transitions from heavy and light hole subbands to conduction subbands, and the E0(Γ8,v→Γ6,c) transitions of the AlGaAs layers. Our data are consistent with a square well calculation using a conduction‐band offset of 60% of the band‐gap discontinuity. For modulation‐doped heterojunctions, a correlation is observed between a PR feature approximately 18 meV above the GaAs fundamental gap and the presence of a two‐dimensional electron gas.

Lifetime-limiting defects in n− 4H-SiC epilayers

Low-injection minority carrier lifetimes (MCLs) and deep trap spectra have been investigated in n− 4H-SiC epilayers of varying layer thicknesses, in order to enable the separation of bulk lifetimes from surface recombination effects. From the linear dependence of the inverse bulk MCL on the concentration of Z1∕Z2 defects and from the behavior of the deep trap spectra in 4H-SiC p-i-n diodes under forward bias, we conclude that it is Z1∕Z2 alone that controls the MCL in this material.

Mobility enhancement in strained p-InGaSb quantum wells

Quantum wells of InGaSb clad by AlGaSb were grown by molecular beam epitaxy. The InGaSb is in compressive strain, resulting in a splitting of the heavy- and light-hole valence bands and an enhancement of the mobility. The mobility was found to increase with increasing InSb mole fraction for values of strain up to 2%. Room-temperature mobilities as high as 1500cm2∕Vs were reached for 7.5nm channels of In0.40Ga0.60Sb. These results are an important step toward the goal of high-performance p-channel field-effect transistors for complementary circuits operating at extremely low power.

Above-room-temperature optically pumped midinfrared W lasers

We report temperature-dependent pulsed lasing performance, internal losses, and Auger coefficients for optically pumped type-II W lasers with wavelengths in the range of 3.08–4.03 μm at room temperature. All lased to at least 360 K, and produced 1.5–5 W peak power at 300 K. Internal losses at 100 K were as low as 10 cm−1, but increased to 90–360 cm−1 at 300 K. Room temperature Auger coefficients varied from 5×10−28u200acm6/s at the shortest wavelength to 3×10−27u200acm6/s at the longest.

Strain‐induced two‐dimensional electron gas in [111] growth‐axis strained‐layer structures

It is demonstrated that strain‐induced electric fields in [111] growth‐axis strained‐layer structures can be used to induce a two‐dimensional electron gas without the necessity of modulation doping. A simple analytic expression is derived for the density of the two‐dimensional electron gas. The density has a simple linear relationship to the strain‐induced electric fields. The calculation predicts that two‐dimensional densities in the range 1011–1012 cm−2 should be easily realized in strained‐layer structures. Results are calculated for a variety of material systems.

Control of interface stoichiometry in InAs/GaSb superlattices grown by molecular beam epitaxy

The InAs/GaSb materials system, with different species for both cations and anions, allows one to envision the construction of heterojunctions with either InSb‐ or GaAs‐like interfaces. As a result, this system provides a unique opportunity to explore the limits of interfacial control that can be achieved at the monolayer level by vapor phase growth techniques. Using migration‐enhanced epitaxial techniques, we have prepared a series of InAs/GaSb superlattices with both types of interfaces. The large differences in bond lengths and vibrational properties of InSb and GaAs interfaces allow x‐ray diffraction and Raman spectroscopy to be sensitive probes of interfacial structure. The x‐ray and Raman measurements reveal that it is possible to grow superlattices with almost pure InSb‐like or GaAs‐like interfaces.

Large temperature changes induced by molecular beam epitaxial growth on radiatively heated substrates

We have performed optical transmission measurements on radiatively heated GaAs substrates as a function of molecular beam epitaxial growth of InAs, GaSb, AlSb, and GaAs films. The energy gap of the GaAs substrate is observed to decrease strongly in energy when materials with band gaps smaller than GaAs are deposited. This decrease in energy gap is a consequence of a substantial increase in growth temperature induced by the deposition of the film. We have observed increases in temperature of over 150u2009°C from the temperature measured before film growth. Because the thermocouple is weakly coupled to the radiatively heated substrate, conventional temperature controllers are ineffective at measuring or accounting for this change in temperature.

Chapter 4 Photoreflectance Spectroscopy of Microstructures

Publisher Summary This chapter describes the application and flexibility of modulation spectroscopy, particularly electroreflectance (ER) and photoreflectance (PR), to study artificially structured materials (ASMs). The chapter explains the way measurements on microstructures have helped understand the properties of ASM and of the modulation mechanisms involved in electromodulation spectroscopy. The types of physical systems to be studied are of a multilayer variety whose building block is usually composed of two materials having different band gaps. This difference in band gaps can lead to the confinement of carriers in one of the two materials (that is, the carriers can become confined to a quantum well). With modulation spectroscopy, the derivative of the absorptivity or reflectivity with respect to some parameter is evaluated. This spectroscopy is sensitive to critical-point transitions in the Brillouin zone, while the resulting spectrum has sharp derivative-like features and a little, if any, featureless background.

Photoreflectance of GaAs/GaAlAs multiple quantum wells: Topographical variations in barrier height and well width

Using photoreflectance at room temperature we have evaluated the topographical variations in quantum level transitions of a GaAs/GaAlAs multiple quantum well (220 A/150 A) due to changes in barrier height and quantum well width. The spatial resolution of the measurement was about 100 μm. A key feature of our analysis is the ability to fit the electromodulation spectra by a third‐derivative functional form line shape factor. We can detect barrier height changes of several millielectron volts and variations in well width as small as 2 A.