Quantum Well Intersubband Photodetector Research Articles

Mid-Infrared Intersubband Absorption from p-Ge Quantum Wells Grown on Si Substrates

There are many applications in the mid-infrared part of the electromagnetic spectrum which include thermal imaging [1] and the unique identification of molecules through absorption spectroscopy [2]. For all these applications, sources of mid-infrared light and photodetectors are key to enable any application. III-V and II-VI semiconductor materials have dominated over the last decades with both interband and intersubband emission and/or absorption devices [1-3] but there is now significant interest in developing technology on silicon substrates to enable far cheaper systems for mass market applications in environmental sensing, personalised healthcare and security. SiGe quantum well (QW) intersubband photodetectors (QWIPs) have previously been demonstrated [4] but the number of QWs was limited by the SiGe critical thickness thereby limiting performance. Now Ge QWs have the potential to improve this performance significantly in the mid-infrared and the number of QWs can be increased using strain symmetrisation of the QWs and barriers to allow improved absorption in the longer wavelength mid-infrared. Here we demonstrate strain symmetrized growth of 500 periods of Ge QWs with Si0.5Ge0.5 barriers on Si0.2Ge0.8 virtual substrates grown on top of (001) Si. The samples were grown by low energy plasma enhanced chemical vapour deposition. Transmission electron microscopy, Raman spectroscopy and x-ray diffraction were used to characterize the heterolayer thicknesses, Ge contents, strain and defects. The layers demonstrate excellent quality with low defect densities. Three samples with QW thicknesses (barrier thicknesses) of 5.4 ± 0.4 nm (3.5 ± 0.4 nm), 8.1 ± 0.5 nm (5.2 ± 0.6 nm) and 9.2 ± 0.6 nm (6.1 ± 0.6 nm) have been fully characterized by Fourier transform infrared spectroscopy (FTIR) to understand the potential for mid-infrared QWIPs. An 8-band k.p modelling tool has been used to understand the results. For the lowest QW width, only z-polarised absorption is observed at ~ 7 µm wavelength from the light hole 1 (LH1) to LH2 transition. Despite surface-normal illumination, the z-polarized transition is observed since the heterolayers and substrate will scatter some of the light into the z-polarized mode. For the thicker QWs, only xy-polarized (surface normal) absorption is measured with absorption peaks centred at 7.5 µm and 7.8 µm corresponding to the heavy hole 1 (HH1) to LH2 transition. Temperature dependent FTIR measurements demonstrate virtually no shift in the wavelength of absorption with temperature as is typical for intersubband transitions. Photoluminescence (PL) demonstrates a completely different behavior to the absorption due to the population of states being different through the heating of the samples by the 532 nm PL laser. We will demonstrate how to design QWIPs using a HH1 to LH bound-to-continuum transitions that will enable surface-normal QWIPs to be designed with large pixel numbers and produced cheaply on a silicon substrate using any silicon foundry with SiGe growth capability. The research leading to these results has received funding from the European Unions Seventh Framework Programme under grant agreement no. 613055 and U.K. EPSRC (Project no. EP/N003225/1). [1] A. Rogalski, J. Appl. Phys. 93, 4355 (2003) [2] L. Baldassarre, E. Sakat, J. Frigerio, A. Samarelli, K. Gallacher, E. Calandrini, G. Isella, D. J. Paul, M. Ortolani, and P. Biagioni, Nano Lett. 15, 7225 (2015) [3] A. Rogalski, Infrared Phys. Technol. 38, 295 (1997). [4] R. P. G. Karunasiri, J. S. Park, Y. J. Mii, and K. L. Wang, Appl. Phys. Lett. 57, 2585 (1990) Figure 1

Modeling noise in superlattice quantum-well infrared photodetectors

We propose a model that realistically describes the experimental noise curves of GaAs/AlGaAs quantum-well intersubband photodetectors and that, additionally, allows the determination of some important transport parameters of these devices. The analysis was based on the quantum mechanical calculation of the capture lifetime in a quantum well and on the assumption that the capture probability can be explicitly expressed as a function of the electronic thermal velocity at zero bias and of the saturation velocity at large bias. Our experimental and theoretical results point out that the plateau observed in the noise measurements at low temperature and low electric field is due to an extra conduction path through the miniband of the superlattice formed by the ground states of all the wells.

Non-bandgap defect modes in the photoresponse of mid-IR photonic crystal detectors

Non-bandgap defect modes, defect modes showing up in photonic crystal (PhC) structureswhich do not exhibit a complete photonic bandgap, are investigated experimentally bymeans of a QWIP (quantum well intersubband photodetector) incorporated within atwo-dimensional (2D) triangular photonic crystal of air holes. The modes appear as a localenhancement in the photocurrent spectrum due to an increased in-coupling of surfaceincident light when a defect mode is present. Such delocalized non-bandgap defect modesand the influence of the entire PhC on their energetic positions are experimentallydetermined and show good agreement with (2D as well as three-dimensional (3D))simulations.

Carrier scattering approach to the origins of dark current in mid- and far-infrared (terahertz) quantum-well intersubband photodetectors (QWLPs)

A carrier scattering approach is taken in an analysis of the affect on the dark current of extending the operating wavelength of conventional bound to continuum quantum-well intersubband photodetectors. It is found that both the sequential tunneling and the thermionic emission contributions to the dark current increase as the wavelength of the detector is extended from the mid- to far-infrared. Dark current designs rules are derived.

Growth and characterization of n-type InP/InGaAs quantum well infrared photodetectors for response at 8.93 µm

We present experimental results on the growth and characterization of n-type InGaAs/InP quantum-well intersubband photodetectors for use at 8.93 µm. High-quality InGaAs/InP multiple quantum wells were grown by gas source molecular beam expitaxy, and then characterized by double-crystal x-ray diffraction and cross-sectional transmission electron microscopy. Based upon the structural parameters determined by these methods, the photocurrent response spectra were simulated using a six-band effective bond-orbital model. The theoretical results are in excellent agreement with experimental data. Additional important device characteristics such as dark current, spectral response, and absolute responsivity are also presented.

Molecular beam epitaxy of strain-compensated quantum-well intersubband photodetectors

We report on the growth of quantum well intersubband photodetectors (QWIPs) with InGaAs wells grown on GaAs. Arsenic species and substrate temperature were varied to investigate their effects on the photoluminescence (PL) and lattice relaxation of 40 period InGaAsGaAs QWIP samples. Both PL intensity and lattice relaxation decreased with decreasing substrate temperature - the PL dropping steeply below 500°C and the relaxation dropping gradually below 470°C. In all cases examined, however, the relaxation was at least 33% as measured by asymmetric (1 1 5) five crystal X-ray scans. Introducing a small percentage of phosphorous into the barriers to partially strain-compensate the structure was found to dramatically reduce the lattice relaxation while maintaining the highest PL intensity. Comparison of this strain-compensated InGaAsGaAsP QWIP with the optimized InGaAsGaAs QWIP revealed that the dark current was reduced by a factor of 4 while the photocurrent remained virtually unchanged resulting in an increase in the background limited operating temperature by approximately 3 K.

Thermally activated current–voltage asymmetry in quantum-well inter-subband photodetectors

Continuing research interest in quantum-well inter-subband-based devices can be associated with its prospects for numerous optoelectronic applications in the long wavelength infrared region. This paper presents experimentally measured field dependence of the thermally activated effective-barrier lowering in quantum-well inter-subband photodetectors (QWIPs). This barrier lowering is considered to be the main cause of the commonly observed asymmetry in the current–voltage characteristics of QWIPs. The research results presented here are important for understanding the factors determining the dark-current mechanisms that are crucial for further improvement in the characteristics of these devices. The study of current-carrier transport phenomena in a quantum well is also of interest for developing quantum-well lasers and avalanche photodetectors based on intraband processes, and also transistors based on ballistic or hot carrier transport phenomena.

P-type InGaAs/InP quantum well infrared photodetector with peak response at 4.55 μm

Lattice-matched InGaAs/InP quantum well intersubband photodetectors (QWIPs) have been grown on an InP substrate by gas source molecular beam epitaxy. Detection at 4.55 μm was observed for a narrow well p-type InGaAs QWIP which, when complimented by a high responsivity 8.93 μm n-type InGaAs/InP QWIP, demonstrates the possibility of dual band, monolithically integrated QWIPs on the same InP substrate. Theoretical calculations of the photocurrent spectra are in excellent agreement with the experimental data.

Capture dynamics and far-infrared response in photovoltaic quantum well intersubband photodetectors

Recent progress in photovoltaic quantum well intersubband photodetectors (QWIP) makes these devices suitable for high-performance imaging and heterodyne detection. We report on investigations concerning the basic physics of the transport mechanism, the dynamical behavior, and the further optimization of these structures. GaAs/AlAs/AlGaAs double-barrier QWIPS designed for 3-5 μm wavelength operation provide an interesting model system to study the dynamical aspects of the photovoltaic response. We find strong evidence that carrier transfer across the AlAs barriers mainly occurs due to Γ-X intervalley scattering, even for AlAs layer thicknesses of 1-2 nm. We also discuss experimental results on photovoltaic QWIPs operating in the 8-12 μm regime. In these structures, photovoltaic operation is achieved using a combination of single-barrier quantum wells with built-in space-charge fields. We report on a photovoltaic QWIP with a cutoff energy of 118 meV and a zero-bias detectivity of 2.5×10 9 cm√Hz/W, which is only three times less than the detectivity of a photoconductive QWIP with the same cutoff energy.

Voltage-tunable two-color detection by interband and intersubband transitions in a p-i-n-i-n structure

We report on a novel class of voltage-tunable two-color detectors, based on a quantum-well intersubband photodetector (QWIP) and a p-i-n photodiode in a p-i-n-i-n configuration. In this approach, the two constituting detector structures can be optimized independently because of their different conductance characteristics. The detection concept is demonstrated for the case of an 8-10μm GaAs/AlGaAs QWIP and a 850 nm p-i-n diode containing In0.08Ga0.92As quantum wells.

Integrated quantum well intersub-band photodetector and light emitting diode

The authors propose and demonstrate the integration of a quantum well intersub-band photodetector (QWIP) and a light emitting diode (LED) for making large two-dimensional focal plane arrays for thermal imaging applications. The newly developed QWIP technology is combined with the well established LED technology both based on GaAs and related epitaxially grown alloys, such as AlGaAs and InGaAs.