Faisal F. Sudradjat

Direct-bandgap light-emitting germanium in tensilely strained nanomembranes

Silicon, germanium, and related alloys, which provide the leading materials platform of electronics, are extremely inefficient light emitters because of the indirect nature of their fundamental energy bandgap. This basic materials property has so far hindered the development of group-IV photonic active devices, including diode lasers, thereby significantly limiting our ability to integrate electronic and photonic functionalities at the chip level. Here we show that Ge nanomembranes (i.e., single-crystal sheets no more than a few tens of nanometers thick) can be used to overcome this materials limitation. Theoretical studies have predicted that tensile strain in Ge lowers the direct energy bandgap relative to the indirect one. We demonstrate that mechanically stressed nanomembranes allow for the introduction of sufficient biaxial tensile strain to transform Ge into a direct-bandgap material with strongly enhanced light-emission efficiency, capable of supporting population inversion as required for providing optical gain.

Tensilely strained germanium nanomembranes as infrared optical gain media.

The use of tensilely strained Ge nanomembranes as mid-infrared optical gain media is investigated. Biaxial tensile strain in Ge has the effect of lowering the direct energy bandgap relative to the fundamental indirect one, thereby increasing the internal quantum efficiency for light emission and allowing for the formation of population inversion, until at a strain of about 1.9% Ge is even converted into a direct-bandgap material. Gain calculations are presented showing that, already at strain levels of about 1.4% and above, Ge films can provide optical gain in the technologically important 2.1-2.5 μm spectral region, with transparency carrier densities that can be readily achieved under realistic pumping conditions. Mechanically stressed Ge nanomembranes capable of accommodating the required strain levels are developed and used to demonstrate strong strain-enhanced photoluminescence. A detailed analysis of the high-strain emission spectra also demonstrates that the nanomembranes can be pumped above transparency, and confirms the prediction that biaxial-strain levels in excess of only 1.4% are required to obtain significant population inversion.

Far-infrared intersubband photodetectors based on double-step III-nitride quantum wells

Far-infrared photoconductive detectors based on intersubband transitions in III-nitride semiconductor quantum wells are demonstrated. The device active material is based on a double-step quantum-well design, where two different (Al)GaN compositions are used both in the wells and in the barriers. With this approach, one can create a virtually flat multiple-quantum-well potential energy profile, where the deleterious effects of the intrinsic spontaneous and piezoelectric fields of nitride heterostructures are almost completely eliminated. Photocurrent spectra centered at a wavelength of 23 μm (13 THz frequency) are resolved up to 50 K, with responsivity of approximately 7 mA/W.

Sequential tunneling transport characteristics of GaN/AlGaN coupled-quantum-well structures

Vertical electronic transport in periodic GaN/AlGaN multiple-quantum-well structures grown on free-standing GaN substrates is investigated. Highly nonlinear current-voltage characteristics are measured, displaying a clear transition from a high-resistance state near zero applied bias to a low-resistance state as the voltage is increased. The measurement results, including their temperature dependence and the variations in turn-on voltage with subband structure and bias polarity are in full agreement with a picture of sequential tunneling through the ground-state subbands of adjacent coupled quantum wells. Scattering-assisted tunneling due to interface roughness or structural defects appears to be the dominant transport mechanism. The potential role of photon-assisted tunneling is also investigated.

Strain engineered SiGe multiple-quantum-well nanomembranes for far-infrared intersubband device applications.

SiGe/Si quantum wells are of great interest for the development of Group-IV THz quantum cascade lasers. The main advantage of Group-IV over III-V materials such as GaAs is that, in the former, polar phonon scattering, which significantly diminishes the efficiency of intersubband light emission, is absent. However, for SiGe/Si multiple-quantum-well structures grown on bulk Si, the lattice mismatch between Si and Ge limits the critical thickness for dislocation formation and thus the number of periods that can be grown. Similarly, the use of composition-graded SiGe films as a lattice-matched substrate leads to the transfer of dislocations from the graded buffer substrate into the quantum wells, with a consequent decrease in light emission efficiency. Here we instead employ nanomembrane strain engineering to fabricate dislocation-free strain relaxed substrates, with lattice constants that match the average lattice constants of the quantum wells. This procedure allows for the growth of many periods with excellent structural properties. The samples in this work were grown by low-pressure chemical vapor deposition and characterized via high-resolution X-ray diffraction and far-infrared transmission spectroscopy, showing narrow intersubband absorption features indicative of high crystalline quality.

III-nitride terahertz photodetectors for the Reststrahlen gap of intersubband optoelectronics

We report the development of terahertz intersubband photodetectors based on GaN/AlGaN quantum wells, covering the frequency range that is fundamentally inaccessible to existing III-V semiconductor devices due to Reststrahlen absorption. Two different approaches have been employed to mitigate the deleterious effects of the intrinsic polarization fields of nitride heterostructures: the use of suitably designed double-step quantum wells, and epitaxial growth on semipolar GaN substrates. Promising results are obtained with both approaches, which could be extended to other device applications as a way to utilize the intrinsic advantages of nitride semiconductors for THz intersubband optoelectronics.

SiGe Nanomembrane Active Materials for Far-Infrared Intersubband Devices

SiGe quantum-well nanomembranes are developed where the internal stress due to lattice mismatch is relaxed via elastic strain sharing without plastic deformation. Record narrow far-infrared intersubband absorption spectra are measured.

Strain-engineered SiGe quantum-well nanomembranes for far-infrared intersubband device applications

SiGe/Si quantum-well nanomembranes, where stress due to lattice mismatch is relaxed via elastic strain sharing rather than defect formation, are developed and their potential for far-infrared intersubband device applications is demonstrated.

Grating-coupled strain-enhanced light emission from mechanically stressed germanium nanomembranes

Direct-bandgap light emission from Ge nanomembranes is strongly enhanced and red-shifted through the application of tensile strain, combined with the use of a periodic array of amorphous-Si nanopillars to outcouple the strain-enhanced luminescence.

Direct-bandgap germanium active layers pumped above transparency based on tensilely strained nanomembranes

We show that mechanically stressed nanomembranes can be used to introduce sufficient tensile strain in Ge to transform it into a direct-bandgap, efficient light-emitting material that can support population inversion and thus provide optical gain.