InAs quantum dots embedded in InGaAs quantum wells with InAlAs barriers on InP substrate grown by
metalorganic chemical vapor deposition are utilized for high operating temperature detectors and focal plane arrays
in the middle wavelength infrared. This dot-well combination is unique because the small band offset between the
InAs dots and the InGaAs well leads to weak dot confinement of carriers. As a result, the device behavior differs
significantly from that in the more common dot systems that have stronger confinement. Here, we present energy
level modeling of our QD-QW system and apply these results to interpret the detector behavior. Detectors showed
high performance with D* over 10<sup>10</sup> cmHz<sup>1/2</sup>/W at 150 K operating temperature and with high quantum efficiency
over 50%. Focal plane arrays have been demonstrated operating at high temperature due to the low dark current
observed in these devices.
Here we report the first demonstrations of infrared focal plane array (FPA) based on GaAs and InP based quantum dot infrared photodetectors (QDIPs). QDIPs are extension of quantum well infrared photodetectors (QWIPs) and are predicted to outperform QWIPs due to their potential advantages including normally incident absorption, higher responsivity and high temperature operation. Two material systems have been studied: InGaAs/InGaP QDIPs on GaAs substrates and InAs QDIP on InP substrates.
An InGaAs/InGaP QDIP has been grown on GaAs substrate by LP-MOCVD. Photoresponse was observed at temperatures up to 200 K with a peak wavelength of 4.7 μm and cutoff wavelength of 5.2 μm. A detectivity of 1.2x10<sup>11</sup> cmHz<sup>1/2</sup>/W was obtained at T=77 K and bias of -0.9 V, which is the highest for QDIPs grown by MOCVD.
An InAs QDIP structure has also been grown on InP substrate by LP-MOCVD. Photoresponse of normal incidence was observed at temperature up to 160K with a peak wavelength of 6.4 μm and cutoff wavelength of 6.6 μm. A detectivity of 1.0x10<sup>10</sup> cmHz<sup>1/2</sup>/W was obtained at 77K at biases of -1.1 V, which is the first and highest detectivity reported for QDIP on InP substrate.
256×256 detector arrays were fabricated first time in the world for both the GaAs and InP based QDIPs. Dry etching and indium bump bonding were used to hybridize the arrays to a Litton readout integrated circuit. For the InGaAs/InGaP QDIP FPA, thermal imaging was achieved at temperatures up to 120 K. At T=77K, the noise equivalent temperature difference (NEDT) was measured as 0.509K with a 300K background and f/2.3 optics. For the InP based QDIPs, thermal imaging was achieved at 77 K.
We report an InGaAs/InGaP/GaAs quantum dot infrared photodetector grown by metalorganic chemical vapor deposition with detectivity of 1.3x10<sup>11</sup> cmHz1/2/W at 77K and 1.2x10<sup>10</sup> cmHz<sup>1/2</sup>/W at 120K. Modeling of the Quantum dot energy levels showed us that increased photoresponse could be obtained by doping the quantum dots to 4 electrons per dot instead of the usual 2 electrons per dot. This happens because the primary photocurrent transition is from the first excited state to a higher excited state. Increasing the quantum doping in our device yielded significant responsivity improvement and much higher detectivity as a result. This paper discusses the performance of this higher doping device and compares it to our previously reported device with lower doping.
Inter-subband detectors such as quantum well infrared photodetectors (QWIP) have been widely used in infrared detection. Quantum dot infrared photodetectors (QDIPs) have been predicted to have better performance than QWIPs including higher operation temperature and normal incidence detection. Here we report our recent results of InAs QDIP grown on InP substrate by low-pressure metalorganic chemical vapor deposition (MOCVD). The device structures consist of multiple stacks of InAs quantum dots with InP barriers. High detectivities in the range of 10<sup>10</sup>cmHz<sup>1/2</sup>/W were obtained at 77K. The measurements at higher temperatures show better temperature dependent performance than QWIP. However, the performances of QDIPs are still far from the expected. One of the reasons is the low quantum efficiency due to the low fill factor of quantum dots layer. Resonant cavity enhanced QDIP has been studied to increase the quantum efficiency. Different schemes of mirrors using free carrier plasma and distributed Bragg reflector are discussed.
Inter-subband detectors such as quantum well infrared photodetectors (QWIP) have been widely used in infrared remote sensing. Quantum dot infrared photodetectors (QDIPS) have been predicted to have better performanc than QWIPs due to the novel properties of quantum dots caused by the extra confinement. Here we report our recent results of InAs QDIP grown on InP substrate by low-pressure metalorganic chemical vapor deposition. The device structure consists of multiple stacks of InAs quantum dots with GaAs/AlInAs/InP barrier. 400μx400μm test mesas were fabricated for device characterizations. Photoresponse was observed with a peak wavelength of 6.4 μm and a cutoff wavelength of 6.6 μm at both 77K and 100K. A detectivity of 1.0x10<sup>10</sup> cmHz<sup>1/2</sup>/W was obtained at 77K at a bias of -1.1. V. To the best of our knowledge, this is the highest detectivity reported for InAs QDIP grown on InP substrate. At 100K, the detectivity only drops to 2.3x10<sup>9</sup>cmHz<sup>1/2</sup>/W.
A light valve is a key component for optical signal processing. A liquid crystal layer is activated by light due to a proximate photoconducting layer. Contemporary commercial light valves are made with an amorphous silicon photoconducting layer which offers an impedance change between light and dark states of up to three orders of magnitude. One drawback of using amorphous silicon is that the resolution of the valve is limited by lateral photocharge spreading. We hope to overcome this by using thin organic photoconducting layers.
The recently published data on phototransit signals in smectic and discotic liquid crystals, have led us to reconsider the old problem of the weak temperature dependence of the mobility in ordered narrow band systems and inthe liquid crystalline phases. We argue that one has to distinguish between currents which are due to light-generated carriers and currents due to band conduction in equilibrium and which can be described using the Kubo-Greenwood formula. We use a first principle band model in an electric field and show how a T-independent mobility can be derived for a single particle which obeys band transport, includes joule energy relaxation, elastic disorder, and agrees with the few carrrier limit of the Kubo formula. The result is essentially a generalized Drude velocity applicable to describe "single particle currents". We also discuss alternative explanations for the observed temperature independent mobilities which are based on hopping with weak disorder and polaron theories.