Abstracts for Connie J. Chang-Hasnain

The EECS Research Summary for 2003


Buried Selectively-Oxidized AlGaAs Structures Grown on Nonplanar Substrates

P. C. Ku
(Professor Connie J. Chang-Hasnain)

High aluminum (Al) content AlxGa1-xAs is known to be controllably oxidizable under elevated temperature in the presence of water vapor. Oxidized AlxGa1-xAs exhibits properties that make it desirable for device applications, such as being highly selective on aluminum composition, being electrically insulating, and having a low refractive index. Such characteristics make AlOx a desirable material choice for making efficient grating structures.

We proposed a novel buried AlOx/semiconductor horizontal cavity grating structure. Defect-free high Al content AlxGa1-xAs grown on nonplanar substrates with 0.9 < x < 1 was achieved using low-pressure MOCVD [1]. After oxidation, we obtained a saw-tooth pattern oxide front with a period half of the original lithography defined pitch. An RCE (resonant cavity enhanced) photodetector structure was designed and fabricated using this AlOx grating as its DBR mirrors. This structure can find applications for making microcavity single-wavelength lasers and optoelectronic devices.


Figure 1: SEM cross-sectional views of four different samples, each with four pairs of GaAs/AlGaAs. Darker regions are AlGaAs layers. Samples (a) and (b) have trenches along [01-1] directions, but were grown at two different temperatures, 600°C for (a) and 640°C for (b). The thickness enhancement ratios between the sidewall and the planar region for samples (a), (b), and (d) are 2.20, 1.32, and 1.17, respectively. Sample (c) shows the convergence of two sidewalls. Sample (d) has the trench aligned along [011] direction. The no-growth plane (111)B is circled in (d), too. The nominal aluminum for all these samples is 0.92.

Figure 2: Oxide front pattern of AlxGa1-xAs on nonplanar substrates with trenches aligned in the [01-1] direction. (a) A schematic shows the nonplanar substrate . (b) Oxide front pattern bright field image. (c) Oxide front pattern top view from the SEM with the backscattered detector.

[1]
P. C. Ku, J. A. Hernandez, and C. J. Chang-Hasnain, "Buried Selectively-Oxidized AlGaAs Structures Grown on Nonplanar Substrates," IEEE Photon. Technology Lett., Vol. 15, No. 1, January 2003.

Send mail to the author : (peicheng@eecs.berkeley.edu)

Semiconductor All-Optical Buffers

P. C. Ku
(Professor Connie J. Chang-Hasnain)

A controllable variable optical buffer is one of the most critically sought after components in optical communications and signal processing. In such a buffer, optical data would be kept in optical format throughout the storage time without being converted into electronic format. The buffer must be able to turn on to store and off to release optical data at a very rapid rate by an external command. It is generally believed that an optical buffer would enable many new optical systems such as packet-switched networks [1-4], optical signal processing, phase-arrayed antennas, and nonlinear optics.

An ideal optical buffer needs to be compact, have low-power consumption, have variable buffering time, and most importantly, provide enough buffering capacity and bandwidth. We propose the first semiconductor all-optical buffer [5] based on the electromagnetically-induced transparency (EIT) effect [6] in QDs (quantum dots). We establish the conditions and formulation necessary to achieve a large slow-down factor. The light pulses can slow down significantly with negligible dispersion, making it desirable for making optical buffers with an adjustable storage. Narrow linewidth QD fabrication is found to be critical to the overall device performance. Figure 1 shows the schematics of the device and Figure 2 shows the simulation results of signal packet buffering.


Figure 1: Schematics of a semiconductor all-optical buffer based on quantum dot waveguide structures. Signal and pump copropagate (or counterpropagate) in the same waveguide. Signal slow down is induced by the pump via electromagnetically-induced transparency (EIT).

Figure 2: (a) Signal propagation through a quantum dot waveguide with the length of 1 cm. (b) Control of the slow-down factor with the pump power density.

[1]
D. K. Hunter et al., J. Lightwave Technology, Vol. 16, 1998.
[2]
K. Jinguji, J. Lightwave Technology, Vol. 14, 1996.
[3]
J. L. Corral, IEEE Photon. Technology Lett., Vol. 9, 1997.
[4]
K. B. Khurgin, Physical Review A, Vol. 62, 2000.
[5]
P. C. Ku, C. J. Chang-Hasnain, and S. L. Chuang, "A Proposal of Variable Semiconductor All-Optical Buffer," Electronics Lett., Vol. 38, No. 24, November 2002.
[6]
S. E. Harris, "Electromagnetically-Induced Transparency," Physics Today, July 1997.

Send mail to the author : (peicheng@eecs.berkeley.edu)

Thermal Oxidation of AlGaAs: Modeling and Process Control

P. C. Ku
(Professor Connie J. Chang-Hasnain)

We have developed a model [1] for thermal oxidation of AlGaAs using the continuity equation based on the principle of oxidant mass conservation (Figure 1). Oxidant transport is found to be related to the structure-dependent oxidation process (Figure 2). The model takes into account several processing control parameters all in one single equation and therefore can be easily applied to the process control of device fabrication. The relevant control parameters in the device processing include layer thickness, oxidation temperature, oxidation time, spacing between two devices, Al composition, and mesa geometry. Theoretical calculations agree well with reported experimental values. We also apply this model to study the batch process control of VCSEL fabrication. It is found that, in the order of importance, the following parameters will contribute to the fluctuation in the final aperture size: (1) Al composition (can be completely eliminated if AlAs is used); (2) oxidation temperature; (3) initial mesa size; (4) oxidation time; (5) AlGaAs layer thickness; and (6) spacing between two devices.


Figure 1: Master equation used to model the thermal oxidation of AlGaAs. Rho is the oxidant concentration, D is the diffusion coefficient, and v results from the oxidant transport blockade via several possible mechanisms including the following: the pressure difference built up by oxidation reaction by-product out-diffusion or external forces such as surface tension at the oxide/AlGaAs interface, the internal stress in the oxide, and the oxidant diffusion path termination due to formation of porous AlOxHy and the stuffing of pores in the oxide with As containing reaction by-products.

Figure 2: Schematics of thermal oxidation of AlGaAs. (a) Cross-sectional view. (b) Top view for lateral oxidation in a straight mesa configuration. (c) Top view lateral oxidation in a circular mesa configuration.

[1]
P. C. Ku and C. J. Chang-Hasnain, "Thermal Oxidation of AlGaAs: Modeling and Process Control," IEEE J. Quantum Electron. (submitted).

Send mail to the author : (peicheng@eecs.berkeley.edu)