technology QDIPs
Figure 2:
Probing with a green laser source unveils some of the characteristics of the quantum dots
relaxation: a slow decay time typically ranging from 0.3- 0.7 ns; and a fast decay time of around ~100 ps. The relatively long PL decay time indicates that the quantum dots have good optical properties.
To probe the energy levels, we performed PL excitation and visible-near IR photoconductivity measurements. PL excitation revealed multiple peaks and confirmed the excited states observed from PL spectra. Meanwhile, the photoconductivity spectra uncovered possible energy level transitions. Under different bias voltages, the optoelectronic transition could be tuned, thanks to state- filling taking place as electron injection changes.
GaAs/AlGaAs quantum dot pairs, which are grown on a (100) semi-insulating GaAs substrate by MBE. Typically the sample structure is a n-i-n photoconductor.
is grown at 580 °C as the bottom contact layer. On top of this we deposit an active region containing 10 periods of GaAs/AlGaAs quantum dot pairs. Due to the strain-free property in future we could incorporate more periods to improve absorption and in turn increase the responsivity of our detectors. After the active region we deposit a 300 nm n-type Al0.3 silicon-doped to 3 x 1018
Ga0.7 cm-3 , followed by a 5 nm silicon-
doped GaAs layer for making the top ohmic contact. The thin cap also prevents oxidation of AlGaAs.
Our efforts involve droplet epitaxy at temperatures in the region of 550 °C. Droplet epitaxy is normally performed at low temperatures, but higher temperatures cut defects, leading to higher quality materials.
We have studied the morphology of our samples with an atomic force microscope (see Figure 1). The images that were acquired reveal that the two quantum dots are laterally aligned along the [011] direction. This is because of the anisotropic surface diffusion coefficient of gallium adatoms. The density of the quantum dot pair is 1.3 x 108
cm-2 , their average height is about 9 nm, and their base diameters range from 150 nm to 200 nm.
Further reading J. Wu et al. Nano Lett. 10 1512 (2010) A. Rogalski, Progress in Quantum Electronics, 27, 59-210, (2003)
To understand our sample’s optical properties we have investigated its photoluminescence (PL) using 532 nm excitation from a Nd: YAG laser. Time-resolved PL has been performed by combining this excitation with that from a 750 nm mode-locked Ti: sapphire laser producing 2 ps pulses at an optical pulse train of 76 MHz (see Figure 2). PL spectra are taken at a low temperature, typically 10 K. For the continuous wave PL, various excitation powers were applied and ground and excited energy levels identified from PL spectra. PL decay transients were measured for detection wavelengths of 810 nm.
Time-resolved PL reveals two components to electron 52
www.compoundsemiconductor.net November / December 2010
A 0.5 µm n-type GaAs layer with silicon doped to 2 x 1018 cm-3
We have used photolithography to fabricate photodetectors from these quantum-dot-pair samples. The area of a single pixel is 500 µm x 500 µm. The pixels exhibit dark current densities of 5.6 x 10-8
A/cm2 at 80 K and 5.76 x 10-5 A/cm2
at 300 K. These low values are a highly desired attribute in high performance infrared photodetectors.
As window layer that is
Further insights into the optical characteristics of our samples have been garnered by studying their mid- infrared (MIR) photoresponse spectra with an FTIR spectrometer, using a normal incidence configuration and a MIR source. These measurements reveal a broadband mid infrared photoresponse spanning 3.0 – 8.0 µm. This wavelength range is of great interest due to the transmission window of the atmosphere. The main photoresponse intensity peak is measured at 5.5 µm (225 meV), corresponding to intersubband transitions in quantum dot pairs. These measurements also reveal a large full width at half maximum (FWHM) in the photoresponse spectrum. This is about 2.1 µm when the detector is biased at 0.4 V.
Due to a large spectral width and relatively large energy separation, the photoresponse includes a contribution from bound-to-continuum transitions. Due to the easy tuning of nanostructures by droplet epitaxy, a multicolor detector can be achieved in a single device. For example, dual sized quantum dot pairs can be employed to detect two distinct wavelengths. Despite the very low density of quantum dot pairs in the device, there is a MIR photoresponse at 80 K. Simply increasing the nanostructure density can dramatically increase this response. The density of quantum dot pairs incorporated in our device is about two orders of magnitude lower than the typical density of In(Ga)As quantum dot detectors.
Given a higher density of quantum dots, this type of detector is expected to achieve state-of-the-art performance. Also, thanks to the flexibility of the growth technique, the energy levels can be easily engineered to detect long-wave infrared light, far infrared light, and even terahertz light besides MIR light. These strain-free nanostructures may also find application in other optoelectronic devices such as lasers and solar cells.
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