Shiang-Feng Tang

Near-room-temperature operation of an InAs/GaAs quantum-dot infrared photodetector

A ten-stacked self-assembled InAs/GaAs quantum-dot infrared photodetector operated in the 2.5–7 μm range by photovoltaic and photoconductive mixed-mode near-room-temperature operation (⩾250 K) was demonstrated. The specific peak detectivity D* is 2.4×108 cm Hz1/2/W at 250 K. The use of high-band-gap Al0.3Ga0.7As barriers at both sides of the InAs quantum-dot structure and the long carrier recombination time are the key factors responsible for its near-room-temperature operation.

High-temperature operation normal incident 256/spl times/256 InAs-GaAs quantum-dot infrared photodetector focal plane array

In this letter, a 256/spl times/256 midwavelength infrared focal plane array (FPA) based on 30-period InAs-GaAs quantum-dot infrared photodetectors (QDIPs) is fabricated. The demonstrated original real-time nonuniformity corrected thermal images of hot soldering iron head with 30-Hz frame rate for the FPA are observed. Without additional light-coupling scheme, the QDIP FPA module is first operated at temperatures higher than 135 K under normal-incident condition with a 30/spl deg/ field of view and f/2 optics. For single device performances, a similar QDIP device with a 30-period InAs-GaAs QD structure is fabricated under the same processing procedure. High specific detectivity D/sup */ 1.5/spl times/10/sup 10/ cm/spl middot/Hz/sup 1/2//W and low noise current density 5.3/spl times/10/sup -13/ A/Hz/sup 1/2/ at applied voltage 0.3 V are observed.

Electronic characteristics of doped InAs/GaAs quantum dot photodetector: temperature dependent dark current and noise density

The noise characteristics associated with dark current, photoconductive gain (PC), capture probability in doped InAs dots embedded in In0.1Ga0.9As/GaAs spacer layer have been proposed. The photoconductive and photovoltaic behaviors of the InAs/GaAs quantum dot infrared photodetector (QDIP) from the intersubband transition measurements are also clearly observed. Through noise measurement in dynamic signal analyzer (HP35670A) 1, the electronic bandpass filter frequencies are set up ranging from 3 to 10 KHz in a low noise current preamplifier (SR570) 2. The lock-in amplifier (SR830) 3 can be also used to measure and calibrate the noise density by means of the mean average deviation (MAD) contrast with noise spectra from HP35670A. The InAs/GaAs QDIP studied in this work belongs to n+-n-n+ structure with the top and free blocking barrier layers. It is observed that the owing blocking layer of QDIP not only suppress dark current successfully but also probably reduce the photocurrent 4-6. By systematically optoelectronic measurements and simulations, the modified model of noise current, photoconductive gain, and capture probability in the quantum devices have been proposed. It is shown that photoconductive gain is almost independent of bias under the lower bias, then increasing exponentially under higher bias and below the temperature of 80K. In contrast to quantum well infrared photodetector (QWIP), a higher photoconductive gain of the quantum dot infrared photodetector has been demonstrated and attributed to the longer lifetimes of excited carriers in quantum dots 7-10. At 80K, a photoconductive gain of tens of thousand is shown in the regions of higher biases. It is clear to note that the highest detectivity of the QDIP surprisingly approach to 3.0×1012 cmHz1/2/W at -0.6V under measured temperature 20 K. Under 80K, the average D* is obtained ~1010 cmHz1/2/W. To our knowledge, this is the one of highest D* data in the world.

InAs Quantum Dots On (001) GaAs Substrate with two Groups of Different Sizes Under Arsenic Shutter Closed Condition

The effect of temperature on the self-assembled InAs quantum dots (QDs) grown on GaAs substrate under arsenic shutter closed condition has been studied. From atomic force microscopy (AFM), it was found that the size of InAs dots exhibited a transition from single-sized uniformly distributed quantum dot (QD) at a growth temperature of 490°C to two groups of different sizes QDs at 510°C. Since the desorption rate of In atoms from the substrate surface is very high at 510°C, a growth model is proposed that attributes the larger sized QDs to the enhanced capture of desorbed In atoms by a local random protrusion which initiates a regenerative capture and growth process and leads to explosive growth.

Dual-band infrared imaging analyses for 256 × 256 InAs/GaAs quantum dot infrared photodetector focal plane array

In this work, the 30 stacked InAs/GaAs quantum dot infrared photodetector (QDIP) structure was grown by solid-source molecular beam epitaxy technique and demonstrated with dual-band mid- (2.7~5.6μm) and long- (7.5~13.5μm) wavelength normal-incident detections without grating and passivated process for 256×256 FPA. The 256 ×256 QDIP FPA hybridized with snapshot-mode ROIC was mounted in a 68 pin leadless ceramic chip carrier which was put in the testing dewar with IR optical cold spectral filters of the 2.9~5.5μm and 6.5~14.5 μm for the dual-band IR detections, respectively. The testing scheme for thermal imaging uniformity of the InAs/GaAs QDIP focal plane array (FPA) has been proposed and calibrated using a plane-typed blackbody source of a high temperature of 373±1K and lower ambient temperature for the two-point temperature correction. The averaged of specific detectivity (D*) and operability of the QDIP FPA have reached 1.5×1010cm-Hz1/2/W and 99% at 80K, respectively. The dominant noise equivalent temperature differences (NEDT) of typical figure of merit for QDIP thermal imaging module operated under the temperature of 80K, device biases of -0.7 V and integration time of 32ms with infrared optics and two-point temperature correction (TL =R.T. and TH= 200 °C) are 1.065 K (mid-wavelength IR) and 131mK (long-wavelength IR), respectively. Meanwhile, it is worth to note that these are the first confirmation for dual-band detections of FPA from direct InAs quantum dots matrix embedded in GaAs heterostructure. In the future, the dual-band IR QDIP FPA will become one of the important candidates for hyper-spectral detection and thermal imaging fusion application.

Temperature dependent VCSEL optical characteristics based on graded Al x Ga 1-x As/GaAs distributed Bragg reflectors: reflectivity and beam profile analyses

The vertical cavity surface emitting laser (VCSEL) based on graded distributed Bragg reflectors (DBR) consisted of a top mirror of 20 pairs of AlxGa1-xAs/AlyGa1-yAs (x=0~0.9, y=0~0.12) quarter-wave stacks and a bottom mirror of 34 pairs of AlyGa1-yAs/AlxGa1-xAs quarter-wave stacks has been demonstrated. Using two proposed transfer matrix methods, the simulation of DBR reflectivity depending on temperature refractive index of AlxGa1-xAs and AlyGa1-yAs are discussed and investigated. The simulation results could be achieved to well predicted the DBR performance under operating temperature variances, i.e., the temperature on varying reflectivity and full width half maximum (FWHM), wavelength stop-band shifts of the laser reflector, where using the multi-layer films evolution software of essential Macleod and modified transfer matrix method, respectively. Under our simulation, assuming the physically VCSEL device feature such as the linear grading DBR structure sandwiched with a n-type GaAs substrate and air films, we have systematically studied the temperature effects on the key parameters of top and bottom DBR schemes. In contrast with the temperature dependent DBR on the 850nm-VCSEL characteristics simulated with the above two transfer methods, the temperature varying spectra of VCSEL are agreed with the our simulated results presented in this paper. Also the temperature dependent model of DBR based on refractive index of graded multi-AlxGa1-xAs/ AlyGa1-yAs has been proposed. So, a series of optoelectronic measurements experimentally confirm our results again. The maximum reflectivity of the top and bottom DBRs are 96.4 and 99.98%, respectively. The central wavelengths of the bandwidth spectra in the top and bottom DBR are same. i.e., 840nm. These results can be obtained the criteria for the high performance VCSEL design. The far-field patterns of transverse electromagnetic fields confined in <15μm active-layer aperture of selectively oxidized VCSEL have been observed.

Integral and fractional charge filling in a InAs/GaAs quantum dot p–i–n diode by capacitance–voltage measurement

The temperature-dependent capacitance–voltage (C–V) characteristics of two stacked InAs/GaAs quantum dot p–i–n diode were investigated. The capacitance discontinuities observed are attributed to charge storage in the InAs quantum dots. The average storage electrons at each InAs quantum dot thus obtained are two and three electrons at room temperature and at temperature below 100 K, respectively. In the intermediate temperature range from 100 to 250 K, fractional charge occupation is observed in each dot. When the C–V measurement frequency is lowered from 800 to 80 kHz, the capacitance turns into a negative value under low biases which indicates the dominance of the inductance at lower frequency.

High temperature operated (/spl sim/250 K) photovoltaic-photoconductive (PV-PC) mixed-mode InAs/GaAs quantum dot infrared photodetector

The 10 stacked self-assembled InAs/GaAs quantum dot infrared photodetector (QDIP) operated in 2.5 to 7 /spl mu/m range by photovoltaic (PV) and photoconductive (PC) mixed-mode near room temperature (/spl sim/250 K) was demonstrated. The specific peak detectivity D* is 2.4/spl times/10/sup 8/ cm-Hz/sup 1/2//W at 250 K. The confining Al/sub x/Ga/sub 1-x/As barrier layers on both sides of stacked QD structure are the key to the high temperature operation.

Numerical simulation of temperature-dependence on distributed Bragg reflector (DBR) and performance analyses for proton-implant/oxide confined VCSEL: comparison with transmission matrix, matrix calculating methods, and Macleod model

This paper mainly focuses on the simulation for temperature-dependent Distributed Bragg Reflector (DBR) of 850nm vertical cavity surface emitting laser (VCSEL) with Transmission Matrix (TMM), Matrix Calculating Methods (MCM) and Macleod Model and performance for comparison with proton-implant/oxide confined process on VCSEL. Using well-developed temperature-dependent DBR-reflectivity solver with Mathcad simulator, we have successfully compared the Macleod Model simulator with theoretical self-developed solution based on the Transmission Matrix (TMM), Matrix Calculating Methods (MCM) and find very good agreement with previous results while accounting for influences of conjugated part of refractive index and graded Al compositions of DBR materials. Moreover, optoelectronic performance of Proton-Implant/Oxide Confined 850nm VCSEL have been demonstrated on this paper using temperature-dependent power output, voltage/injection current, transverse operating wavelengths, optical spectral characteristics, slope efficiency and transverse optical modes with an approximated Marcatilis method extracted and measurement from systematically measuring experiments. Through adequate and precise LD device design and processes, we have proposed the high performance single-mode proton implanted in contrast to the oxide confined 850 nm VCSEL. Under nominal temperature-variety and keeping operating temperature of 30°C,the threshold voltage, injecting current, peak-wavelength and differential resistance of the proton implanted VCSEL with the optical aperture in the dimension of 10 &mgr;m are 1.8 V, 3.2 mA, 851 nm and 36.8 ohm, respectively.

Calculations of bandstructures on the lens and pyramid-shaped InAs quantum dot for confirming the photoluminescence and photoresponse

Electronic and optical properties of ideal and real quantum dots (QDs) are extensively studied and derived for the recent decade. Strain caused by the differences of the lattice constants of dot and wetting, barrier materials are decisive for both the self-assembly mechanisms and the electro-optical properties. The research is mainly investigated for realizing the strain effects on the optical properties of InAs/GaAs self-assembled QDs embedded in GaAs barrier layer incorporated with the three-dimensional (3D) Schördinger equation and solved by using finite element method (FEM). From 3D QD geometrical profiles establishing by the spatially geometric equations, the confined electron and hole bandstructures on altering sized lens and pyramidal shape-like QDs with numerical calculations and strained heterostructure of the finite element approximations have been proposed. Applying the fast FEM models, it is demonstrated that the correspondence of ground, excited eigenstates, the probability of density function (|Ψ|2) of the confined levels from the single InAs QD to a matrix of nine QDs to obtain the transition energy and coordinated absorption wavelength to be predicted and summarized clearly. Through calculating energy levels within the conduction and valence band edge confinement on the InAs/GaAs heterostructure with FEM to contradistinguish with corrected to optical transitions and linear absorption spectra can be achieved for verifying to being the specific wavelengths from photoluminescence (PL) and photoresponse (PR) measurements for quantum dot infrared photodetector. By fitting the energy differences among the subbands, the geometrical shape and size of QD can be predicted. Inducting the tendency from single QD to the matrix of 9 QDs, the step-wise bands have been obtained being some regularity clearly. And from the transmission electron microscope (TEM) measurement, the dominant sizes of QDs in the really grown wafer remain the consistent with the numerical analyses applied in 3D QD profile that is interpreted using spatially geometric equations.