Microscopy Probes Electron Behavior in Lasers
Daniel S. Burgess
Using variants of atomic force microscopy, a group of scientists from the University of Toronto and from Nortel Networks Optical Components in Ottawa has directly investigated the electronic characteristics of a semiconductor laser in operation. The approach promises a quantitative means of diagnosing the internal performance of the lasers and of identifying directions that designers should pursue to improve device efficiency.
Researchers in Canada have used variants of atomic force microscopy to investigate the behavior of electrons in ridge waveguide lasers. The inset displays the flat-band energy diagram of a device operating above threshold, calculated based on scanning spreading resistance microscopy data. Ev is the valence band, Ec is the conduction band, and Efv and Efc are the Fermi levels of the valence and conduction bands, respectively. Images courtesy of E.H. Sargent, University of Toronto.
Edward H. Sargent, who holds the Nortel Networks-Canada research chair in emerging technologies at the university, explained that current methods characterize semiconductor lasers from the outside by comparing their output with the injected current or voltage. The new technique takes an electron's-eye view. "Our research looks inside the operating laser and reveals internal mechanisms: how biasing the laser gives rise to an electric potential profile that guides -- or, in worse lasers, does not effectively guide -- electrons into the light-producing active region," he said.
The researchers examined InAlGaAs/InP multiple quantum well ridge waveguide lasers with a Dimension 3100 system from Digital Instruments that they fitted with a scanning spreading resistance microscopy module. In this mode, the system uses an electrically conductive atomic force microscope probe to detect variations in the local resistivity of a sample.
Scanning voltage microscopy measurements offer a picture of the local voltage drop in the lasers. The inset displays cross-sectional voltage profiles for three layers in a device. Based on such data, the researchers estimated that 40 percent of the current injected into the active region was wasted.
To perform the scanning voltage microscopy measurements, they added a Keithley high-impedance voltmeter to the setup, enabling them to produce two-dimensional, 20-nm-resolution voltage profiles of various layers in the devices by monitoring the local potential on the probe tip.
During these latter studies, the samples operated under a forward current of 100 mA, or 40 mA above their lasing threshold, and produced 10 mW of output. By examining how the voltage dropped across the cross-sectional layers in the functioning device, the researchers visualized how the injected current flowed into the active region, Sargent said, and estimated that approximately 40 percent was being wasted.
Diagnosis: Better devices
The ability to directly measure the internal efficiency of semiconductor lasers offers engineers a new way to assess their devices and to determine promising avenues of development. Sargent said that the team was able to envisage from the voltage data how the choices of layers and the design of the laser ridge determined the final performance of the lasers.
"The results suggest strategies for improving laser performance and also tell us the bounds on improvements to be had through different design changes," he said.
He is confident that the quantitative nature of the new technique will secure its place in the test-and-measurement toolbox and that it will enable further progress in semiconductor technology.
"To me, the best analogy is with medicine," Sargent said. "Diagnosis based on symptoms is useful, but diagnosis based on the real-time functional imaging of live patients is tremendously powerful. We have imaged nanoscale devices while they are producing intense beams of light. The work is key to tracing how physics is transformed into useful functions -- the grand theme that underlies transforming nanoscience into usable nanotechnology."
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