G. Hasnain

High sensitivity low dark current 10 μm GaAs quantum well infrared photodetectors

By increasing the quantum well barrier width, we have dramatically reduced the tunneling dark current by an order of magnitude and thereby significantly increased the blackbody detectivity D*BB. For a GaAs quantum well infrared detector having a cutoff wavelength of λc=10.7 μm, we have achieved D*BB =1.0×1010 cm (Hz)1/2/W at T=68 K, a temperature which is readily achievable with a cryogenic cooler.

Performance of gain-guided surface emitting lasers with semiconductor distributed Bragg reflectors

The performance limitations of gain-guided vertical cavity surface emitting lasers (VCSELs) which use epitaxially grown semiconductor distributed Bragg reflectors (DBRs) are discussed. The light-current (L-I) characteristics and emission wavelength of such lasers are examined as a function of temperature and time under continuous wave (CW) and pulsed operation. The authors observed a sharp roll-over in the CW L-I characteristics which limits the maximum output power. The threshold current under CW operation is found to be lower than that obtained under pulsed conditions. Several microseconds long delay in lasing turn-on is also observed. It is shown quantitatively that these anomalies are a consequence of severe heating effects. It is shown that reduction of the series resistance and threshold current density can lead to significant improvements in the power performance of VCSELs. >

High‐detectivity D*=1.0×1010 cm √H̄z̄/W GaAs/AlGaAs multiquantum well λ=8.3 μm infrared detector

We report the first high‐detectivity (D*=1.0×1010 cmu2009(Hz)1/2/W), high‐responsivity (Rv =30u2009000 V/W) GaAs/AlxGa1−xAs multiquantum well detector, sensitive in the long‐wavelength infrared band at λ=8.3 μm (operating at a temperature of T= 77 K). Because of the mature GaAs growth and processing technologies as well as the potential for monolithic integration with high‐speed GaAs field‐effect transistors, large focal plane arrays of these detectors should be possible.

GaAs/AlGaAs multiquantum well infrared detector arrays using etched gratings

Efficient coupling of long‐wavelength infrared (LWIR) radiation to a two‐dimensional (2‐D) array of GaAs/AlGaAs multiple quantum well detectors is achieved by illumination through chemically etched diffraction gratings. Gratings were fabricated on the back surface of the GaAs substrate as well as selectively on the top contact of the detector mesas. Both top and bottom illumination schemes were employed. In all cases, high coupling efficiency (>90%) of the gratings was observed as measured by comparing the responsivity to that of an identical detector illuminated through an angle‐polished facet. The results demonstrate the feasibility of high‐sensitivity GaAs LWIR imagers.

Broadband 8–12 μm high‐sensitivity GaAs quantum well infrared photodetector

Broadband GaAs/AlxGa1−xAs quantum well infrared detectors grown by molecular beam epitaxy have been demonstrated which are sensitive over the λ=8–12 μm atmospheric window spectral region. These are the first high‐detectivity bound state to extended state quantum well detectors which are peaked at λ=10 μm. The spectral bandwidth (Δν/ν) of these devices is three times larger than our earlier λ=8 μm device. The detectivity D* is background limited at T=50 K with D*=1×1010 cm (Hz)1/2 /W, and a noise equivalent temperature change of NEΔT=0.01 K.

Large photoconductive gain in quantum well infrared photodetectors

We demonstrate for the first time that a quantum well infrared photodetector based on bound‐to‐continuum state intersubband transitions can have a photoconductive gain much greater than unity similar to extrinsic photoconductors. An optical gain g=8.1 was determined by comparing the optical absorption, responsivity, and noise characteristics of two multiquantum well detectors which were identical in every respect except for having a different number of periods (2 and 20). The results suggest that since the photocarrier lifetime τL is not transit time limited, detector optimization to increase τL or optical coupling can be expected to lead to an improved detectivity D*.

Mid‐infrared detectors in the 3–5 μm band using bound to continuum state absorption in InGaAs/InAlAs multiquantum well structures

Medium wavelength infrared (MWIR) detectors in the 3–5 μm band are demonstrated using bound to continuum state intersubband absorption in lattice‐matched InGaAs/InAlAs multiquantum well (MQW) structures. Photodetectors with responsivity peaked at 4 μm wavelength showed low dark current giving a background‐limited detectivity D*BL =2.3×1010 cm(Hz)1/2 /W at temperatures up to 120 K.

Low threshold buried heterostructure vertical cavity surface emitting laser

We report the first low threshold, buried heterostructure (BH) vertical cavity surface emitting laser (VCSEL) using organometallic chemical vapor deposition regrowth. Very low threshold current of 0.8 mA and threshold current density of 490 A/cm2 are achieved with 8 and 32 μm diam BH VCSELs, respectively. Both 8 and 16 μm diam BH VCSELs emit a single TEM00 fundamental mode for current levels many times thresholds. Most single‐mode emissions are linearly polarized with a predetermined direction in the [011] crystal orientation.

GaAs vertical-cavity surface emitting lasers fabricated by reactive ion etching

GaAs quantum well vertical-cavity surface emitting lasers fabricated using low damage reactive ion etching are discussed. Lasers which are partially and completely etched through their structure are compared. The surface recombination velocity of exposed GaAs is not exacerbated in deep etched lasers; other loss mechanisms in shallow etched lasers have comparable impact on laser performance. Etched lasers exhibit low voltage and small differential series resistance at threshold, while devices fabricated by a combination of etching and ion implantation possess lower threshold current. It is found that reactive ion etching has little additional effect on laser operation, whereas the different device structures considered do influence laser performance.<<ETX>>

Monolithically Peltier‐cooled vertical‐cavity surface‐emitting lasers

We report the first tunable monolithically integrated thermoelectric controlled GaAs/AlGaAs vertical‐cavity surface‐emitting laser diode. The thermoelectric element is the n+‐GaAs substrate based on the Peltier effect. A variation of active region temperature of ±7.5u2009°C has been achieved using ±100 mA of thermoelectric cooler current. The observed wavelength tuning associated with this temperature shift is ±6 A. The device is useful for applications that require a high degree of frequency stability or small frequency tuning. Some examples of potential applications are in high data rate lightwave transmission, self‐electro‐optic device switches, and spectroscopy.