Cheng-Der Chiang

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.

256 x 256 InSb focal plane arrays

A 256x256 backside illuminated photovoltaic indium antimonide (InSb) focal plane arrays having spectral response in the medium wavelength infrared (3 to 5 (mu) m) was designed and developed at Chung Shang Institute of Science and Technology for use in a variety of military and commercial applications. Operating at 77 degree(s)K, the arrays had a mean laboratory detectivity of 3.12x1011cmHz½/W with f/3 optics. The responsitivity non-uniformity was 3%, and the operable yield exceeded 99%. The FPA achieves an Noise Equivalent Temperature Difference (NETD) is less than 0.025 degree(s)K at 300 degree(s)K background with f/3 optics.

Photo-enhanced native oxidation process for Hg/sub 0.8/Cd/sub 0.2/Te photoconductors

Proposes an easy and reproducible vapor-phase photo surface treatment method to improve the device performance of the Hg/sub 0.8/Cd/sub 0.2/Te photoconductive detector. We explore the effect of surface passivation on the electrical and optical properties of the HgCdTe photoconductor. Experimental results, including surface mobility, surface carrier concentration, metal-insulator-semiconductor leakage current, 1/f noise voltage spectrum, the 1/f knee frequency, responsivity R/sub /spl lambda//, and specific detectivity D* for stacked photo surface treatment and ZnS or CdTe passivation layers are presented. These data are all directly related to the quality of the interface between the passivation layer and the HgCdTe substrate. We found that, by inserting a photo native oxide layer, we can shift the 1/f knee frequency, reduce the noise power spectrum, and achieve a lower surface recombination velocity S. A higher D* can also be achieved. It was also found that HgCdTe photoconductors passivated with stacked layers show improved interface properties compared to the photoconductors passivated only with a single ZnS or CdTe layer.

Dark Currents in HgCdTe Photodiodes Passivated with ZnS/CdS

Experimental and theoretical results are presented for current-voltage and dynamic resistance-voltage characteristics of Hg 1-x Cd x Te ion-implanted p-n junction photodiodes with x 0.22 passivated with ZnS/CdS layers. By measuring the temperature dependence of the dc characteristics in the temperature range 25-140 K, the dark current mechanisms are studied and the validity of the modeling is confirmed. It was found that the dark currents can be represented with three current components over a broad range of voltage and temperature. At high temperature (>90 K) and in low reverse bias region, the diffusion current dominates. On the other hand, at medium temperature (40-80 K) and medium reverse bias (< -0.15 V), trap-assisted tunneling plays an important role. At low temperature (<40 K) and in the medium reverse bias region (< -0.15 V), band-to-band tunneling is the key leakage current source. However, when the temperature is further lowered to 25 K and the applied reverse bias is very small (-0.15 to 0 V), the band-to-band tunneling current will be ruled out and the trap-assisted tunneling mechanism dominates again.

1/f noise and specific detectivity of HgCdTe photodiodes passivated with ZnS-CdS films

1/f noise in HgCdTe photodiodes has been measured as a function of temperature, diode bias, and dark current. The dependence of 1/f noise on dark current was measured over a wide temperature range. At low temperatures, where surface generation and leakage current were predominant, a linear relationship between 1/f noise and dark current was observed. At higher temperatures, where diffusion current is predominant, the correlation no longer holds. The temperature dependence of 1/f noise was also determined. The temperature dependence of the 1/f noise was found to be the same as that for the surface generation and leakage currents. All the data obtained in these experiments could be fit with theoretical predictions by a simple relationship between 1/f noise and dark current. The 1/f noise in the HgCdTe photodiode varies with diode bias, temperature, and dark current only through the dependence of the surface current on these devices. The maximum specific detectivity (D*) value and the maximum signal-to noise ratio are approximately 3.51/spl times/10/sup 10/ cm/spl middot/Hz/sup 1/2//W and 5096 at 50 mV reverse bias, respectively.

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.

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.

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.

Simulations for Vertically Coupled Wave-Functions of Electrons on the Multiple Lens-Shaped InAs/In(Ga)As Quantum Dot Layers with Dependences of GaAs Spacing Layer

Electronic and optical properties of ideal and realistic 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. This paper is mainly investigated for 3-dimensional (3D) electrical wave-functions and eigen-levels of multiple InAs/In(Ga)As self-assembled QDs matrix embedded in GaAs spacing layer incorporated with the three-dimensional (3D) Schrodinger equation and solved by using numerical finite element method (FEM) and PC-based dual-core CPU hardware. In this study, we will compare the samples with free and GaAs spacing layers of different thickness to investigate the influences on vertically coupled probability of density wave- functions. The spatially 3-dimensional ground-state wave- functions (|Psi|2) of 3-stacked InAs/InGaAs QD matrix with free GaAs spacing layer show the vertically strong coupled wave- functions are clarified to be demonstrated which will enhance the electrical propagated probability between vertically adjacent QD layers. And the vertically decoupled wave-functions (|Psi|2) are observed until the thickness of GaAs spacing layers are thicker than 2 nm.