Electrically injected photonic crystal light emitters with spatially localized gain

Electrical injection for photonic crystal light emitters is desirable because diode lasers are intrinsically more efficient and practical. However, photonic crystal defect lasers to date have been nearly exclusively demonstrated by photo-pumping. Large area band-edge photonic crystal [1], edge emitting quantum dot photonic crystal [2], and air suspended photonic crystal with a central post [3] have been operated by electrical current injection. In this work, we demonstrate the first electrically injected photonic crystal light emitters with spatially localized gain. Localization of the quantum well active layer to the defect region of the photonic crystal allows for efficient coupling of the optical mode to the gain medium, and reduces non-radiative recombination at the semiconductor air interfaces formed by the photonic crystal air holes. High Al-composition AlGaAs layer is incorporated under the photonic crystal membrane as shown in Fig. 1. It can be laterally oxidized to form the oxide, which acts as an electrical insulator and thermal conductor. An unoxidized region at the center of the photonic crystal defect provides a current channel as used in conventional oxide VCSELs. To reduce the electrical resistance, the top layer is n-type with a p-type substrate. We also achieve an accurate alignment between a photonic crystal defect and a sub-micron unoxidized region. First we define electron beam lithography alignment marks on the epitaxially grown base structure, which includes an InGaAs quantum well and 10 nm thick GaAs cap layer. The first electron beam lithography step creates the etch mask for selective removal of the quantum well outside of the photonic crystal defect. A 30 second etch with 1:40 = Hydrogen Peroxide: Citric acid solution is used to remove the quantum well and GaAs cap layer as shown in Fig. 2. The upper half of the GaAs membrane including a Si-doped 40 nm top contact layer is regrown on the patterned base structure. The total thickness of the membrane is 220 nm. Photonic crystal patterns and oxidation trenches are simultaneously defined by the second electron beam lithography step. The patterned quantum well must be placed within the photonic crystal defect. The first reactive ion etch forms trenches, and then selective wet oxidation at 370°C is carried out. The second etch produces the photonic crystal air holes into the membrane. Finally, top and bottom metal contacts are deposited. Fig. 3 shows the near-field image of an operating device with only the defect region emitting light, and the SEM image of the H4 defect cavity. Fig. 4 shows the significant enhancement of electroluminescence signal intensity resulting from the localization of the quantum well to the photonic crystal defect. The oxide aperture of the patterned quantum well device is approximately 400 nm, which is smaller than the H4 defect size. This shows that the quantum well patterning can eliminate non-radiative recombination at the photonic crystal air holes. Electroluminescence spectra in Fig. 5 demonstrate the effect of the photonic crystal nanocavity. Each spectrum is measured continuous-wave at 15 μA current. The spectral tuning of over 20 nm is observed by changing the photonic crystal period with the fixed ratio of the air hole radius to the period. Increasing the period of the photonic crystal shifts the cavity mode toward longer wavelengths as expected. Ongoing efforts are focused on reduction of the electrical resistance, and integration of multiple quantum wells to increase the material gain. We anticipate that the resultant increase in available gain will allow stimulated emission to occur in this device structure.