J. Rothman

Development of HgCdTe single-element APDs based detectors for low flux short wave infrared applications

The remarkable properties (internal gain larger than 100 and close to unity excess noise factor) of Short Wave Infrared (SWIR) HgCdTe electron-initiated Avalanche Photodiodes (e-APDs) are put to good use to demanding applications, i.e. spectroscopy and LIDAR. Knowing the requirements of both situations, we have designed specific models based on highly sensitive single elements APDs and adapted proximity electronics. On one hand, we use the e-APDs low noise equivalent power (NEP) at 180K (few fW/Hz1/2). We simultaneously designed a specific Transimpedance Amplifier (TIA) which allows us to take advantage of the low APD NEP. The combination of both elements along with a dedicated cryostat enables direct LIDAR detection at moderate bandwidth (BW = 20 MHz) without the need for long time averaging, which is crucial in far field (≥ 5 km) analysis. One the other hand, we have optimized a low-noise and low-frequency LN2 cooled prototype operating with an external commercial amplifier. It allows us to observe the photoluminescence of Ge nanostructures in the range 1.5-2.5 μm with a significantly increased SNR along with a reduce pump laser power. The possibility to use these detectors in the photon counting limit will be discussed in light of our recent results. In parallel, we present preliminary time response measurements performed on SWIR APD suggesting that a higher GHz BW could be reached with this type of detector. This is however subjected to optical optimization at the moment.

Raman spectral shift versus strain and composition in GeSn layers with 6%-15% Sn content

GeSn alloys are the subject of intense research activities as these group IV semiconductors present direct bandgap behaviors for high Sn contents. Today, the control of strain becomes an important challenge to improve GeSn devices. Strain micro-measurements are usually performed by Raman spectroscopy. However, different relationships linking the Raman spectral shifts to the built-in strain can be found in the literature. They were deduced from studies on low Sn content GeSn layers (i.e., xSn < 8%) or on GeSiSn layers. In this work, we have calibrated the GeSn Raman relationship for really high Sn content GeSn binaries (6 < xSn < 15%). We have used fully strained GeSn layers and fully relaxed GeSn under-etched microstructures to clearly differentiate the contributions of strain and chemical composition on the Ge-Ge Raman spectral shift. We have shown that the GeSn Raman-strain coefficient for high Sn contents is higher compared with that for pure Ge.

Gamma bandgap determination in pseudomorphic GeSn layers grown on Ge with up to 15% Sn content

Adding Tin (Sn) to Germanium (Ge) can turn it into a direct bandgap group IV semiconductor emitting in the mid-infrared wavelength range. Several approaches are currently being investigated to improve the GeSn devices. It has been theoretically predicted that the strain can improve their optical properties. However, the impact of strain on band parameters has not yet been measured for really high Sn contents (i.e., above 11%). In this work, we have used the photocurrent and photoluminescence spectroscopy to measure the gamma bandgap in compressively strained GeSn layers grown on Ge buffers. A good agreement is found with the modeling and the literature. We show here that the conventional GeSn deformation potentials used in the literature for smaller Sn contents can be applied up to 15% Sn. This gives a better understanding of strained-GeSn for future laser designs.

Inductively coupled plasma etching of germanium tin for the fabrication of photonic components

The demonstration of a CMOS compatible laser working at room temperature has been eagerly sought since the beginning of silicon photonics. Although bulk Germanium (Ge) is an indirect bandgap material, Tin (Sn) can be incorporated into it to turn the resulting alloy into a direct band-gap semiconductor. Recently, lasing was demonstrated at cryogenic temperatures using thick GeSn layers with Sn contents of 8.5% and above. Optical micro-cavities were later added to reduce the laser threshold. Here, an under-etching of thick GeSn layers selectively with regard to Ge confines optical modes and relaxes the compressive strain built inside the layers, resulting in more direct band-gaps behavior. Such photonic components rely on technological processes dedicated to GeSn. In this paper, we present our recent developments on (i) anisotropic etching of GeSn and (ii) isotropic etching of Ge selective with regard to GeSn. Even for GeSn with a Sn content as low as 6%, the etching selectivity is of 57. For 8% Sn content, the selectivity reaches 433. We used these processes to fabricate micro-disk optical cavities in thick GeSn layers. Under continuous wave pumping, optical modes were detected from photoluminescence spectra.

High-quality and homogeneous 200-mm GeOI wafers processed for high strain induction in Ge

The realization of efficient laser sources compatible with the microelectronics industry is currently one of the main challenges for silicon photonics. As Ge is CMOS compatible, the interest of using tensile strain or n-type doping to improve its light emission properties has significantly increased over the last few years. Theoretically, it has been predicted that the Ge bandgap becomes direct at around 4% strain for uniaxial tensile stress or 2% strain for bi-axial tensile stress. Several methods to induce such extreme levels of strain are currently investigated. The highest value of strain has been reached with Ge micro-bridges fabricated from Ge-On-Insulator (GeOI) substrates in a controllable and reproducible way. In this work we have first of all investigated the material properties of 200-mm GeOI wafers. Very high crystallographic quality is demonstrated at the micron-scale using Raman spectroscopy and synchrotron based Laue micro-diffraction performed at BM32-ESRF. We give then optimized designs of micro-bridge by comparing suspended and landed micro-bridges on different materials. We theoretically show that the thermal management is strongly improved in landed micro-bridges. Finally, we have developed specific processing for landing Ge micro-bridges on Si or SiO2, the photoluminescence measurements performed on landed micro-bridges shows an improvement of the Ge light emission with strain.

HgCdTe detectors for space and science imaging in France: general issues and latest achievements

HgCdTe is very unique material system for infrared (IR) detection. In combination with its lattice matched native substrate CdZnTe, this semiconductor alloy allows to address the whole infrared (IR) band, from the near IR (NIR, 2?m cutoff) to the middle wave IR (MWIR, 5μm cutoff), the long wave IR (LWIR, 10μm cutoff), up to the very long wave IR (VLWIR, cutoffs larger than 14μm).

Non-linear bandgap strain dependence in highly strained germanium using strain redistribution in 200 mm GeOI wafers for laser applications

Summary form only given. Applying a large tensile strain of several percent in a Ge layer is promising in order to improve its optical properties and possibly turn germanium into an efficient CMOS compatible light emitter. Several approaches are currently being explored for high strain induction into Ge. Since biaxial or uniaxial stress inductions are interesting, we have studied both approachs using tensile strain redistribution in 200 mm GeOI wafers. In this work, we compare simulations with experimental results in order to accurately investigate the bandgap-strain dependence in highly strained Ge devices. In order to carefully measure the bandgap-strain dependence, photoluminescence and electro-absorption measurement were performed and compared to strain characterizations by micro-Raman spectroscopy and synchrotron based micro-diffraction at the BM32 beamline of ESRF Grenoble. Due to the high crystalline and electronic quality of GeOI substrates, unprecedented strain amplitudes were achieved in 350 nm thick Ge layers: 1.9 % (8.1 cm-1) for biaxial strain, 4.9 % (9.9 cm-1) for uniaxial stress along <;100> and 3.8 % (14 cm-1) for uniaxial stress along <;110>. Two types of nonlinear strain dependences have been theoretically and experimentally demonstrated for uniaxial stress: the Raman-strain and the gamma bandgap-strain relations. We will discuss the consequences of the updated relationships to obtain the building blocks needed to fabricate an efficient laser based on highly strained Ge material.

Highly strained direct bandgap Germanium cavities for a monolithic laser on Si

Cavity enhanced photoluminescence at a wavelength as long as 5 μm is obtained in uniaxial tensile strained GeOI micro-bridges. We show, using temperature dependent photoluminescence spectroscopy, a crossover to fundamental direct bandgap and reveal from a mode analysis the free carrier induced loss increase.