Molecular beam epitaxy of strained heterostructures and their application to optoelectronic devices.

Abstract

The purpose of this study is to analyze molecular beam epitaxial growth modes of strained layers and molecular beam epitaxial regrowth of III-V semiconductors as techniques for realization of novel and conventional monolithically integrated optoelectronic devices. Molecular beam epitaxial (MBE) growth was studied and shown to be controlled by adatom surface kinetics which are controlled by substrate temperature, arsenic flux (surface reconstruction) and the constituent adatom bond strengths. Interface quality can be improved by growth interruption which allows the growth front to smoothen by minimizing its free energy. Strained epitaxy of InGaAs on GaAs using MBE is shown to significantly alter the growth process. Under strain the reflection high energy electron diffraction (RHEED) oscillations are severely dampened, indicating a roughened surface. Growth interruption has little improvement on smoothness of highly strained layers. A 3-D island growth mode is shown to be energetically favored under conditions of high strain ($f\sb0\geq$ 2%). Molecular beam epitaxial regrowth was studied and shown to create a disordered metamorphic region. Recombination velocity at the interface is $\sim$10$\sp5$ cm/sec. Best results are obtained from ion milling followed by rapid thermal annealing of the sample. Strained layers and MBE regrowth are applied to realize a low-loss In-doped GaAs waveguide integrated with a In$\sb{x}$Ga$\sb{1-x}$As/GaAs strained layer superlattice (SLS) PIN diode which is used as an integrated electroabsorption modulator with a 0.9 dB insertion loss and a 20% modulation depth, and as an integrated photodetector with a temporal response of 500 psec. Response time is expected to be limited by the non-ideal interface quality due to regrowth and strain. A monolithically integrated photoreceiver using an In$\sb{0.53}$Ga$\sb{0.47}$As PIN photodiode and an In$\sb{0.53}$Ga$\sb{0.47}$As/In$\sb{0.52}$Al$\sb{0.48}$As modulation doped field effect transistor (MODFET) is demonstrated using MBE regrowth. Isolated PIN photodiodes have impulse response times of 400-500 psec limited by parasitics. Regrown MODFETs exhibit peak DC transconductance of 380 mS/mm, nearly half of normally grown MODFETs of 590 mS/mm. Cut-off frequencies of both MODFETs are comparable, ($f\sb{t}$ and $f\sb{max}$ are 30 GHz and 45 GHz, respectively, for regrown 1$\mu$m $\times$ 200$\mu$m MODFETs, and 25 GHz and 36 GHz for similar normal MODFETs). Modeling of the photoreceiver circuit yields a 3-dB cut-off frequency of 2.4 GHz. An all-optical switch using a vertical AlGaAs waveguide directional coupler with a GaAs/AlGaAs multiple quantum well (MQW) nonlinear coupling medium is demonstrated. This device can switch from a crossed over state at low power to a straight-through condition at high power. A theoretical analysis which describes the switching behavior is developed, and a nonlinear switch was realized which exhibited switching below bandedge of the MQW nonlinear medium.