Measurement Offers Insight into Diodes

Paul Mortensen

Unamplified spontaneous emission and gain spectra offer insight into the behavior of semiconductor laser materials and structures. But the analytical methods that use them require complex materials processing to enable the measurements in different directions that are necessary to avoid unwanted amplification of the spontaneous emission. Now a group of researchers from Cardiff University in the UK have developed a simpler method that measures from the end of a segmented-contact device to determine the complete emission spectrum of a laser diode.

In a new method for measuring the true spontaneous emission spectrum of a laser diode, researchers analyze the radiation from the end of the device, which is divided into segments (1-5) that can be driven independently, correcting for amplification or absorption in the structure. Courtesy of Peter Blood, Cardiff University.

The method interrogates the spontaneous emission spectrum in both the transverse electric (TE) and transverse magnetic (TM) polarizations. From these, the researchers also can determine the internal quantum efficiency.

Until now, direct measurement of the spontaneous emission spectrum has been a complex process. Previous investigators employed a geometry that avoided amplification or absorption by observing the emission via a top-contact window in the substrate or through the side of a narrow, buried heterostructure or mesa device. They measured gain, however, by analyzing the amplified spectra from the end of the device; i.e., this "window" method measured gain and spontaneous emission from different regions of the structure, making it difficult to convert the data into real units, such as photons per second. In the new method, both are deduced from the emission at the end of the laser, so no special processing is needed to produce a window. A structure with a segmented contact, however, is required.

The data derived from the amplified spectra emission reveal energy distributions of the carriers. If there are more electrons in the excited than in the ground states, the researchers can convert the spectra into real units.

"We have the actual number of photons emitted per second per unit area at each wavelength," said Peter Blood, a professor at the university and one of the researchers on the project. "By adding these over the whole spectrum, we have the total number of photons emitted per second in a given polarization."

The team recently measured and added the spectra for both the TE- and TM-polarized radiation to obtain the total number of photons emitted per second, he said. The "window" method, in contrast, measures only one polarization. "We can work out the total number of electrons flowing through the laser per second."

If a diode were 100 percent efficient, the number of photons emitted per second would be equal to the number of electrons flowing per second. "Of course, this is not the case, and the ratio of number of photons to number of electrons gives us the internal efficiency," which had been difficult to measure properly, Blood said.

The team is applying the method to quantum dot lasers. It also will investigate the gain characteristics and the gain-generating processes in blue-emitting nitride structures.