Electrical and Computer Engineering ETDs

Ram Attaluri

Publication Date

9-9-2007

Abstract

Quantum dot infrared photodetectors (QDIPs) have been shown to be a key technology in mid and long wavelength (3-14 μm) infrared detection due to their potential for normal incidence operation and low dark current. In our research group, we have been investigating infrared detectors based on intersubband transitions in a novel InAs/In0.15Ga0.85As quantum dots-in-well (DWELL) heterostructure. In the DWELL structure, the InAs quantum dots are placed in an In0.15Ga0.85As well, which in turn is placed in a GaAs matrix. Due to the large band offset between the ground electronic state of the InAs quantum dot and conduction band edge of the GaAs barrier, thermionic emission and dark current are significantly reduced in the DWELL structure. The DWELL design also offers other advantages such as better control over the operating peak response wavelength and bias dependent tunable spectral response based on the quantum confined stark effect (QCSE). We have recently fabricated the first long wavelength quantum dot infrared photodetector (QDIP) focal plane array based on this system and for the first time collobarators at Jet Propulsion Laboratory (JPL) have shown that QDIP performance has surpassed that of Quantum Well Infrared Photodetector (QWIP). In this work, we will investigate various methods we implemented in improving the performance of the DWELL photodetectors. Although QDIPs based on intersubband transitions have been investigated before, there has been no careful study on the effects of Si-doping on the performance of these detectors. A careful study has been done to determine the optimal doping of the InAs/In0.15Ga0.85As/GaAs DWELL detectors. It has been found that 3 x 1010 cm-2 is the optimal doping for the DWELL detectors. It has been observed that the spectral response, photocurrent, dark current, responsivity and detectivity (D*) increased with the amount of doping in the InAs QDs. In addition, the background limited infrared photodetector (BLIP) temperature (91K) is the highest for one electron per dot sample. In our standard QDIPs there is only a single pass of incident light through the active region. The development of a mechanism for multiple light passes through the active region should result in a significant responsivity enhancement of QDIP detectors. One such method to create multiple light passes is to add a mirror that reflects the light back into the active region effectively developing an optically resonant cavity. In this work, we have epitaxially inserted a DBR below the QDIP device that has a broad reflectivity spectrum (i.e. 8-11μm) and designed the resonant cavity for 9.5 μm wavelength. We have observed an increase in the responsivity of the device (0.76A/W at 1.4V) relative to devices with the same active region and no mirror or cavity. Hence, we believe that the QDIP with resonant cavity and distributed Bragg reflector has improved the performance of the device. The D* increased by a factor of three compared to the standard DWELL at a bias of 1.2 V and 77 K. In the standard QDIP the average compressive strain in the DWELL is about 1.35% and, therefore, more number of DWELLs cannot be grown without introducing defects or dislocations. Ideally, more number of DWELLs mean more absorption, which translates to increased quantum efficiency and performance of the device. A low strain alternative design InAs/GaAs/Al0.1Ga0.9As DWELL structure is developed which maintains approximately the same band offset between the singly degenerate ground state of the dot and the conduction band edge of the barrier. This alternative design has only (~0.35%) compressive strain in the DWELL, which allows incorporation of more DWELL layers in the active region. We observed spectrally tunable response with bias and long wave IR response at 6.2 μm and 8.4 μm. This design was also tech transferred to JPL who demonstrated a 640 x 512 infrared camera with 40 mK NEDT at 60 K. Further work is being done to fabricate FPA based on this device and compare it with the standard DWELL design.

Keywords

Infrared detectors, Optical detectors, Quantum dots

Sponsors

National Science Foundation, Defense Intelligence Agency, Air Force Office of Scientific Research

Document Type

Dissertation

Language

English

Degree Name

Electrical Engineering

Level of Degree

Doctoral

Department Name

Electrical and Computer Engineering

First Committee Member (Chair)

Krishna, Sanjay

Second Committee Member

Hayat, Majeed

Third Committee Member

Han, Sang

Fourth Committee Member

Stintz, Andreas

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