Research Review: Nanostructures enhance IR detectors

The desire for detectors that are fast, efficient and free from cooling has fuelled the development of several new classes of infrared (IR) device. Three alternatives capable of two-color operation were highlighted at this year s North American Molecular Beam Epitaxy Conference, which was held on 23–26 September in Albuquerque, NM.

These devices can be employed for various civilian and military applications, including night vision, medical diagnostics and gas detection for environmental monitoring and astronomical observations. For many tasks there is a choice of detector, but each type has its own weaknesses. Microbolometric detectors have a slow response time but can operate at room temperature, while HgCdTe, bulk InSb and quantum-well IR photodetectors are faster but need cryogenic cooling.

Quantum-dot IR detectors are one promising type of device, according to Keh-Yung "Norman" Cheng from the University of Illinois at Urbana Champaign (UIUC). He believes that these low-dimensional structures have the potential to combine speed with freedom from cooling, while offering the advantage over their quantum-well-based equivalents of a strong photoresponse to normal-incident radiation. However, individual quantum-dot layers have low detection efficiency, and addressing this weakness simply by growing more of them is not easy because it tends to produce strain dislocations in the epitaxial layers.

Cheng has been able to avoid these strain issues by fabricating quantum-wire IR photodetectors through a process called strain-induced lateral-layer ordering (SILO). Wires are formed by growing short-period superlattices, which contain alternating layers of InAs and GaAs, with sufficient strain to drive an in situ lateral compositional modulation. These wires are typically 12.5 nm high, 10 nm wide and have a density of 106 cm–1.

Cheng s research team at UIUC has produced several different devices based on the SILO technology, including a detector that consisted of 20 regions of short-period superlattices, which each contain 10 pairs of InAs/GaAs. Devices with a 150 µm × 150 µm detecting area were formed from epiwafers using standard photolithography and wet chemical etching.

This device produced spectral responses at 6.3 and 4.1 µm, which were the results of a bound-to-bound transition and a bound-to-continuum transition, respectively. The strength of the response at each wavelength depended on the device s bias and peaked at 3 mA/W. "This device offers the possibility for integrating two or three detector wavelengths in one structure," remarked Cheng.

Another class of IR photodetector is the quantum-dot-in-a-well (DWELL) design, which contains quantum dots embedded in a quantum well. Such a device also delivers a two-color response, which can be tuned by biasing. However, this detector suffers from low quantum efficiency, according to Rajeev Shenoi, a member of Sanjay Krishna s group at the University of New Mexico.

Shenoi explained that he and his co-workers had managed to boost efficiency by increasing the number of active periods within the device. This required a new DWELL design that minimizes epilayer strain through a reduction in indium content and involved optimization of the dot, well width and capping layers. At the heart of the improved structure are layers of InAs quantum dots, which were grown with 2.4 monolayer coverage, embedded in a quantum well made from 1 nm of InGaAs and 6.85 nm of GaAs. These layers are surrounded by 50 nm thick AlGaAs barriers.

The researchers produced photodetectors with 410 µm × 410 µm mesa structures containing circular apertures ranging from 25 to 300 µm in diameter and an active region consisting of 30 stacks of the optimized DWELL structure. These devices featured a range of InGaAs cap thicknesses and were grown on GaAs substrates in a VG Semicon V80H MBE reactor.

As expected, these detectors produced a two-color response, with sensitivities at 8.9 and 10.5 µm, associated with the electron transition from the dot to the AlGaAs barrier and from the dot to the GaAs shoulder, respectively. The addition of a 1 nm thick InGaAs cap boosted the shorter wavelength response, says Shenoi, but did not influence the longer wavelength. With this design, the detector delivered a response at 8.9 µm of 7.54 A/W under 4 V bias when it was cooled to 77 K.

Shenoi says that his future plans include incorporating the device into focal plane arrays and optimizing the barrier width.

Another promising class of IR detector is based on the InAs/GaSb superlattice. These devices could offer high-temperature operation, thanks to the large electron-effective mass in the superlattice, which cuts tunneling currents and Auger recombination rates.

According to Elena Plis, who is a member of Krishna s group, the standard superlattice detector is based on a photodiode design that is hampered by Shockey–Read–Hall centers and surface states – both significant sources of noise. Plis says that these problems can be avoided by turning to an nBn structure, which features a thin n-type narrow-bandgap contact layer, a 50–100 nm thick wide-bandgap electron barrier and a thick n-type narrow-bandgap absorbing layer.

The inventors of the nBn structure – Shimon Maimon and Gary Wicks from the University of Rochester, NY – had significantly improved the operating temperature of their InAs detector. Plis and co-workers have attempted to transfer this benefit to InAs/GaSb superlattice structures.

The researchers from New Mexico University initially fabricated nBn structures on tellurium-doped GaSb substrates, which comprised a 500 nm n-type superlattice, a 100 nm undoped Al0.4Ga0.6Sb barrier, a 2.5 µm undoped superlattice and an n-type superlattice cap. "The room temperature performance of our nBn detector is comparable to state-of-the-art pin detectors," remarked Plis.

Plis and colleagues went on to build dual-color detectors with similar structures, which feature InAs/GaSb and InAs/InGaSb superlattices and had cut-off wavelengths of 4.5 and 8 µm, respectively. When a bias is applied, the relative response from each of the two superlattices is altered significantly and a dual-color response can be produced at 77 K by adjusting the bias from 0 to 100 mV.