Quantum dot infrared photodetectors suffer from strain in their nanostructures that culminates in various performance-degrading defects. However, many of these defects can be avoided by turning to a novel, strain-free growth method based on the deposition of droplets, says Jiang Wu from University of Arkansas Fayetteville.

Infrared photodetectors continue to attract a great deal of interest thanks to the numerous applications that they can serve. These detectors can be used for night vision, optical communication, target identification, fire fighting, medical diagnostics and surveillance.

The first infrared photodetectors that appeared on the market were fabricated from materials with a narrow bandgap, such as lead salt, InAs1–xSbx, and Hg1–xCdxTe (MCT). Detectors fabricated from these alloys have experienced a great deal of success and they are still selling today. However, they are plagued with growth-related issues, which has stimulated the development of intersubband quantum infrared photodetectors. During the last two decades, infrared photodetectors based on quantum wells and quantum dots have undergone dramatic development. Of the two types, the quantum dot infrared photodetector is the more promising due to intrinsic advantages associated with three-dimensional confinement. These include sensitivity to normal incidence radiation and high temperature operation.

One exciting aspect of the quantum dot infrared photodetector (QDIPs) is its potential to combine high resolution with multicolor detection capability. Traditionally these types of detector are fabricated from either InAs or InGaAs quantum dots. Coherent nanoscale islands are generally formed when a certain amount of In(Ga)As is deposited on the (Al)GaAs surface. However, other lattice mismatched materials have been investigated as well.

Quantum dots are formed by a growth procedure known as Stranski–Krastanov (S-K) growth. Transformation from a two-dimensional growth mode to a three-dimensional one depends on the strain of deposited materials. The inevitable strain arising in S-K quantum dots introduces various defects, including long stacking faults, short stacking faults and dislocations. These defects impair the optical and electronic properties of QDIPs and are one of the biggest factors behind their low quantum efficiency.

At the University of Arkansas Fayetteville we employ a novel growth process for produing strain-free dots: droplet epitaxy. This approach separately supplies source elements. Generally growth begins by forming nanosize droplets of group V materials. These structures are crystallized by group III vapor transforming droplets to yield a process that creates semiconductor nanostructures.

One of the great strengths of the droplet epitaxy approach is its versatility. It can construct quantum dot pairs, quantum molecules, quantum rings and nanoholes. In all cases, these tiny structures are free from strain, which is promising for the fabrication of high-performance devices. The QDIPs that we are developing feature strain-free 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.

A 0.5 μm n-type GaAs layer with silicon doped to 2 x 1018 cm-3 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 Al0.3Ga0.7As window layer that is silicon-doped to 3 x 1018 cm-3, followed by a 5 nm silicondoped 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.



Figure 1. Atomic force microscopy image of quantum dot pairs grown by high temperature droplet epitaxy

 

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.



Figure 2: Probing with a green laser source unveils some of the characteristics of the quantum dots

 

Time-resolved PL reveals two components to electron 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.

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.

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 multicolour 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.

Further reading

J. Wu et al. Nano Lett. 10 1512 (2010)

A. Rogalski, Progress in Quantum Electronics, 27, 59-210, (2003)