KEYWORDS: Mercury cadmium telluride, Dark current, Signal to noise ratio, Avalanche photodetectors, Passivation, Fabrication, Etching, Design and modelling, Mid-IR
Mid-Wavelength Infrared (MWIR) HgCdTe electron-initiated avalanche photodiodes (e-APDs) have presented excellent performances to resolve and count photons. Aiming at low flux, the Readout Integrated Circuit (ROIC) noise can be extremely reduced by certain device gain, and very low excess noise of HgCdTe e-APDs makes opportunity for noise equivalent photon (NEPh) to be one. Therefore, the main issue for Signal-to-Noise Ratio (SNR) of HgCdTe APD is Gain Normalized Dark Current Density (GNDCD) at high reverse bias. In this work, the electric field distribution is optimized by designing the mesa device structure to suppress the tunneling current at high operating voltage. Furthermore, new combining dry/wet etching technology and passivation technology based on Plasma Atomic Enhanced Layer Deposition (PEALD) were used to reduce surface leakage current. Finally, 20 μm pitch 128×128 array HgCdTe APDs for cut-off wavelengths 4.32 μm @80K corresponding to compositions xcd 0.326 were fabricated, the measured GNDCD of test unit is about 1.5~50×10-9 A/cm2 at 0~13V
HgCdTe electron avalanche photodiodes (e-APDs) with single-carrier multiplication hold great promise for weak signal detection. This work investigates the key metrics that affect the signal-to-noise ratio of mid-wavelength infrared (MWIR) Hg0.7Cd0.3Te e-APD: current, gain, and excess noise factor. The gain is over 1000 at -10 V, but the maximum useful gain is limited by the generation of band-to-band tunneling current at higher bias voltages. The gain dispersion obtained by characterizing the focal plane array is 4.7% at -7 V, indicating a relatively homogeneous gain from pixel to pixel. The excess noise factors evaluated by the noise power spectral density and the gain fluctuation methods both range from 1 to 1.32 at gains below 400, demonstrating the property of near excess-noise-free amplification of HgCdTe e-APDs.
Hg1-xCdxTe is considered as the preferred material for high performance infrared photodetectors and imaging focal plane array (FPA) detectors. One of the technical challenges of multi-dimensional integrated HgCdTe epitaxy by molecular beam epitaxy (MBE) lies in the in-situ extraction, characterization and precisely control of a series of parameters such as alloy composition, surface roughness, substrate temperature and film thickness at a relatively low substrate temperature of about 180°C. Therefore, an in-situ, nondestructive spectroscopic ellipsometry (SE) method is needed to characterize the performance of HgCdTe films. In this paper, real time optical property characterization of short-wave Hg1-xCdxTe epitaxial grown by MBE is reported. Run to run feasibility and stability of in-situ SE is confirmed by buffer layer thickness verification in multiple growth runs. Lorentz oscillator parametric model provides a new approach to describe optical dispersion property of HgCdTe over spectral range of 1.5-4.1 eV. The absorption peaks show blue shift with the increase of HgCdTe Cd composition (x). Under this circumstance, the longitudinal x value for HgCdTe during epitaxy process can be obtained in real time without any surface damage by successfully building a composition-dependent optical constant library, with routine run-to-run reproducibility measurement accuracy Δx of ~ 0.0015. This work will facilitate the fabrication of HgCdTe heterojunctions with complex component distribution and doping profiles.
HgCdTe avalanche photodiodes (APD) have been demonstrated to be one of the most promising paths for low flux and high speed applications. The bandwidth of HgCdTe e-APD has been theoretically predicted to be independent of the gain, owed to its strongly dominant electron multiplication. However, when the photocurrent is high, a large number of electrons exists in the depletion region, and the electrical field in the depletion region might collapse due to the space charge effect, thus limiting the increase of the gain-bandwidth product. In this work, the structure of the device was optimized by simulation, and the effect of the light injection dose on the electric field and bandwidth of the device was studied. Finally, a mid-wavelength infrared HgCdTe e-APD device whose bandwidth almost doesn't decrease with the increase of gain is fabricated. The response bandwidth of the APD is about 480MHz @ gain=625, corresponding to a gain-bandwidth product of 300GHz.
HgCdTe has been shown to be the first semiconductor exhibiting single-carrier multiplication in short-wavelength, medium-wavelength, long-wavelength avalanche photodiodes detectors for cut-off wavelengths from 1.3 µm to 11 µm corresponding to compositions xcd from 0.7 to 0.2, which has the remarkable characteristics of high gain, high bandwidth and almost no excess noise. These results have opened a new horizon in photon starved and high-speed applications, such as active imaging and free space optical communications. In this paper, we report the latest results at SITP of HgCdTe eFAPDs using LPE-grown absorption layers in the SW and MW wavelength bands. The gain of single element short-wavelength HgCdTe APD for 2.57 μm cut-off wavelength is about 100 at 25V reverse bias, and GNDCD is about 1.47×10- 7A/cm2 at gain of 100 at 130K. For MW HgCdTe APDs, increase the P region doping concentration will reduce the overall dark current density and eliminate sudden rise of dark current at large bias and high temperature, and lower Cd composition could be a trade-off way for GNDCD suppression. 50 μm pitch 128×128 array HgCdTe APDs for cut-off wavelengths 4.88 µm corresponding to compositions xcd 0.307 were fabricated, whose GNDCD is less than 1×10-7A/cm2 at 8V reverse bias, gain is over 1000 at 11V reverse bias. A 50 μm pitch 128×128 array HgCdTe APDs with xcd=0.29 was manufactured, whose gain reaches 1570 at 9.8V reverse bias, the average excess noise factor is 1.25 at average gain of 133, noise equivalent photon is about 12 at average gain of 113. By thinning the absorption region thickness, the response bandwidth of Hg0.79Cd0.31Te APD reaches 635MHz under 1V reverse bias. Moreover, the medium-wavelength focal plane of 320×256 array is demonstrated the imaging, and the low noise, high sensitivity and fast imaging characteristics of HgCdTe APDs under linear avalanche gain are verified.
HgCdTe has been shown to be the first semiconductor exhibiting single-carrier multiplication in avalanche photodiodes (APDs) up to gain values larger than 1000 and with close to zero excess noise. These results have opened a new windows for low-flux and versatile imaging. In this paper, we report the latest results on a 50μm pitch 128×128 array HgCdTe APDs with xcd=0.307 manufactured at SITP. Through optimizing the implantation parameters and annihilation parameters, the designed PIN junction structure could be obtained, and then the performance of APD device was improved. The APDs display a gain of 728 around 10V reverse bias, and the standard deviation of the gain was 18.3% of the mean gain of 113 at 7.8V. The GNDC is less than 100nA/cm2 at the bias<9V, but the dark current starts increasing significantly faster than the gain at high bias, and then the device becomes dark current noise limited. The excess noise factor F is less than 1.8 up to gain of 700, and the F factor of 94.75% pixels is less than 1.4 at gain of 126. The Noise Equivalent Photon (NEPh) is 16 photons at gain of 500, and a demonstration imaging was shown.
The HgCdTe avalanche photodiode (APD) with built-in gain mechanism has great application prospects in the field of weak light signal detection. Any dark current will be converted into noise affecting the work efficiency of photodetectors. Therefore, the study of dark current mechanism is an important way to obtain high performance HgCdTe APD. In this paper, the photoelectric detection mechanism of planar junction electron injection HgCdTe APD is systematically studied, focusing on two aspects of structural optimization design and performance improvement. The dependence of device performance parameters on structure parameters is obtained by comparing the simulation results and experimental results. The lower the trap concentration, the smaller the dark current under small reverse bias voltage is, and the higher the operation voltage is. The dark current density of HgCdTe APD array is about 10-8 A/cm2 under small bias voltage. In addition to the material parameters, the dark current is mainly related to the internal electric field, which is affected by the width and doping concentration of the depletion region. The results show that device structure is the major determinant for the performance difference of HgCdTe APD. The structural optimization direction of high-performance HgCdTe APD is to further suppress the local electric field and reduce the dark current by adjusting the process.
HgCdTe has been shown to be the first semiconductor exhibiting single-carrier multiplication in avalanche photodiodes (APDs) up to gain values larger than 1000 and with close to zero excess noise. These results have opened a new windows for low-flux and versatile imaging. In this paper, we report the latest results on MWIR HgCdTe APDs manufactured at SITP. These APDs display a gain of 1000 around 10V reverse bias. The excess noise factor is between 1.2 to 1.45 up to gain of 100, and the quantum efficiency is more than 60% from 1μm wavelength to peak wavelength 4.2 μm. These results show that the technological processes used at SITP are well adapted to APD manufacturing. However, at present, the dark current starts increasing significantly faster than the gain at high bias, and then the device becomes dark current noise limited. APD gain performance was successfully modeled by the simulation of electrical characteristics used Synopsys Sentaurus based on Okuto-Crowell ionizaition coefficient model. Therefore, Sentaurus would be used as a powerful predictive tool for SITP technology and stress its reproducibility and optimize the devices .
The CdZnTe-based and GaAs-based HgCdTe epilayers were grown by liquid phase epitaxy and molecular beam epitaxy, respectively, and then coated by CdTe layers as barrier cap layers for ion implantation. Subsequently, arsenic ions were implanted into the samples at different implant energies, and the two-step high temperature annealing under Hg overpressure was operated on as-implanted samples to eliminate induced damages and activate arsenic ions. After thinning the as-implanted and annealed samples by ion milling, the microstructure of lattice defects in arsenic-implanted and annealed HgCdTe was characterized by high resolution transmission electron microscopy (HRTEM), while the arsenic profiles were measured by secondary ion mass spectroscopy (SIMS). By X-ray diffraction (XRD), the influences of pre-annealing, ion implantation and post-annealing on lattice structure were studied. The experimental results indicate that the implant induced defects underneath the amorphized layer contain dislocation clusters and dislocation lines. For the implant energy of 450keV, a residual point defect belt was observed around the previous amorphous/crystal (a/c) interface in the as-implanted sample after annealing, implying that the recrystallization occurs from surface towards a/c interface. The HRTEM observation of the point defect shows that the defect is a cluster of vacancies in fact. Also, the ion implantation not only broadens the XRD peak, but also makes the peak deviation and split. It indicates that the introduction of atomic stress changes the lattice constant, thereby leading to the peak deviation.
This paper reports the development of 2000×256 format SWIR HgCdTe/Si FPA with multiple-choice gain (i.e. multiple-choice charge handling capacity) for hyperspectral detection. The spectral resolution is about 8nm. To meet the demands of variable low flux detection within each spectral band in the short wave infrared range, low dark current, low noise, variable conversion gains and high SNR (Signal to Noise Ratio) of FPA are needed. In this paper, we fabricate 512×512 pixel 30μm pitch SWIR HgCdTe diode array on Si by using a novel stress-release construction of HgCdTe chip on Si. Moreover, we design low noise, variable conversion gain and large dynamic range read-out integrated circuit (ROIC) and hybridized the ROIC on the HgCdTe diode array on Si substrate. There are 8-choice gains which can be selected locally according to the incident flux to meet high SNR detection demand. By high-accuracy splicing 4 512×512 HgCdTe/Si FPA we get mosaic 2000×512 FPA, and characterizations have been carried out and reveal that the array dark current densities on an order of 10-10A/cm2, quantum efficiency exceeding 70%, and the operability of 99.5% at operating temperature of around 110K. The SNR of this FPA achieved 120 when illuminated under 5×104photons/pixel.
The barrier cap layer (BCL) is considered to be able to absorb partially implant induced damages during ion implantation, thus its structure and property could impact the result of ion implantation. In this paper, for As ion implantation in HgCdTe, the different BCLs were deposited on the CdZnTe-based (LPE) and GaAs-based (MBE) HgCdTe epilayers, respectively. Then, the influences of thicknesses and structures of these BCLs on dopant profiles and implant damages were investigated. The as-grown BCLs include thermally evaporated (TE) ZnS, TE CdTe, electron beam evaporated (EBE) CdTe and in-situ CdTe/ZnTe grown by MBE. The SIMS profiles and TEM characterization indicate: For TE ZnS BCLs, there exists an optimized thickness to obtain the deepest As indiffusion after high temperature annealing, and the end-of-range (EOR) depth is linearly proportional to the thickness ratio of a-MCT layer/damage layer. For TE CdTe BCLs, the barrier layer induced channeling effect (BLICE) occurs to the thin BCL samples, while this effect is suppressed in the thick BCL samples. The phenomenon might be due to that the blocking effect of the layered structure inside each crystal column becomes dominate in the thick BCL samples. Additionally, the EBE CdTe BCL with layered structure can suppress effectively the BLICE effect; in the in-situ CdTe/ZnTe BCL, the short defect layer generated in the CdTe buffer layer and the amorphization of the ZnTe layer during ion implantation also play a significant role in suppressing the BLICE effect.
Detection in the very long wave infrared range (LWIR, 12-15µm) using third-generation infrared focal plane array (FPAs) is essential for remote atmosphere sounding. Indeed, these wavelengths are particularly rich in information about humidity and CO2 levels and provide additional information about cloud structure and temperature profile across the atmosphere. However, the dark current characteristic and associated noise behavior of the HgCdTe photodiode in the wavelength range of 12-15µm, operating at ~77K, are very sensitive to surface passivation techniques as well as to surface material treatments. For current HgCdTe material and device technology, detection of LWIR and VLWIR energy is the subject of current research. Within this range of shrinking band-gaps in detector material, precise control of the quality of the surface passivation and treatment is of great importance. The underlying physics of dark current mechanism is theoretically investigated by using a previously developed simultaneous current extraction approach and numerical simulations.
In addition, HgCdTe electron avalanche photodiodes (e-APD) have been widely used for low-flux and high-speed application. To better understand the dark current transport and electron-avalanche mechanism of the devices and optimize the structures, we perform accurate numerical simulations of the current-voltage characteristics and multiplication factor in planar and mesa homojunction (p-i-n) HgCdTe electron-avalanche photodiodes.
The strain and stress distribution in HgCdTe/CdTe/Si heterostructure (Si 500μm, CdTe 10μm, HgCdTe 10μm) were described by theoretical calculation. The results showed that the strain and stress profiles and curvature radius of HgCdTe/CdTe/Si oriented in asymmetry [211] direction, are asymmetric along in-plane direction along [1-1-1] and [01-1]. The strain of epilayer and substrate are both negative at 77K. The stress at the interface is the largest in this heterostructure. The stress in epilayer is tensile while in substrate it is compressive on the side of interface and tensile on the other side. And a quantitative reference of the Si substrate thickness for a hybrid infrared focal plane arrays was provided.
The recent research results on molecular beam epitaxy of HgCdTe on CdZnTe were presented. The CdZnTe substrates mounting process, under protective atmosphere, was essential to avoid substrate oxidation. It was demonstrated by in-situ reflection high-energy electron diffraction (RHEED), during deoxide process. Concerning the poor thermal conductivity of CdZnTe, good uniformity of HgCdTe on CdZnTe is hard to obtained, compared with the epitaxy on GaAs or Si. It was found that the crystal quality of HgCdTe/CdZnTe was strongly temperature dependent. According to good morphology, crystal quality and maintenance efficiency, the proper growth temperature range of HgCdTe/CdZnTe is 191~193°C. By the enhanced thermal contact sticking during substrate mounting, the uniformity of HgCdTe on CdZnTe was improved, including Cd composition, morphology, as well as crystal quality. In addition, proper in-suit high temperature anneal can reduce dislocation density of HgCdTe epilayers about half order of magnitude. High quality uniform HgCdTe epilayers on 30mm×30mm CdZnTe Substrates were obtained under the optimized growth condition. The X-ray double-crystal rocking curve (XDRC) full-width at half-maximum (FWHM) values vary in a range of 20~30 arcsec. EPD values are bellow 2×105 cm-2, with the best result of 3×104 cm-2.
Degenerate pump-probe experiments have been performed with HgCdTe and GaInNAs thin films. The differential
transmission versus probe delay time shows a negative value for both films, indicating photoinduced absorption from the
trap states. After the negative minimum the differential transmission resumes to zero with long time constants. A rate
equation formalism has been employed to model the carrier dynamics. The calculations fit the experimental differential
transmission very well. The extracted time constants show that the carriers in the trap states of GaInNAs decay to the
equilibrium state with a single time constant of 1.2 ns, while those in HgCdTe shows two time constants of 0.9 ns and 13
ps, respectively. This implies that there exist two types of deep level traps, fast and slow, in HgCdTe thin films.
Surface defects of molecular beam epitaxially grown HgCdTe are the major concern in developing large format
infrared focal plane arrays. Voids were usually observed on the HgCdTe surfaces as previously reported, they were
originated either from the improper substrates preparation or from the growth condition. However, the defects formation
with impurities has not been addressed. This paper presents our recent observation on defects induced by the impurities
involved in the mercury beam fluxes. These defects can be craters or bumps, having a spatially clustering feature. To
identify the origin of these kinds of defects, experiments were performed on HgCdTe as well as CdTe with mercury flux,
and the defects were observed and analyzed by using SEM and EDAX. The result, for the first time, confirmed that
impurities in the mercury beam were responsible to the formation of surface defects.
Dual color detection is a major concept of the third generation infrared focal plane arrays sensors (FPAs) for increasing
the demand of target identification. The performance of these detectors are largely relied on the growth capability of
HgCdTe multilayered structure. This paper presents our preliminary results on growth of MW/LW two-color structure by
using molecular beam epitaxy. The detector had NPpn architecture, with indium doped n-type bottom (window) layer
and Hg-vacancy doped MW and LW p-type layers. The top n-type layer was ion implanted by using B+. The
compositions (mole fraction x) of each layers and its gradient at the interfaces were measured by infrared transmission,
SIMS and SEM. The In doping layer was analyzed by SIMS. The electrical properties of In doping layer were measured
by Hall effects measurements. It was found that the structure obtained agreed well with the growth design. MW/LW two
color detectors of a 64×64 format were fabricated by mesa delineation, and the optimum structure was also discussed.
Some results on the arsenic incorporation in MBE-grown HgCdTe are described. It was found that arsenic surface sticking coefficient during HgCdTe growth was very low, ~1x10-4 at 170°C being very sensitive to the growth temperature. The annealing experiments for activation of arsenic were performed, and the importance of the ambient mercury for the arsenic site transfer was confirmed. It was found that the arsenic dopants could be fully activated as acceptors by anneals at 285°C under the mercury-saturated pressure. The activation energy for the isolated arsenic acceptors in HgCdTe (x=0.27~0.33) was determined as to be 19.5 meV, which decreases with (Na-Nd)1/3 at a slop of 3.1x10-5 meV cm . The diffusion coefficient of arsenic in HgCdTe of 1.0±0.9x10-16 cm2/sec, 8±3x10-15 cm2/sec and 1.5±0.9x10-13 cm2/sec were obtained at annealing temperatures of 240°C, 380°C and 440°C under mercury saturated pressure, respectively.
CdTe growth on Si is the major challenge for HgCdTe. The recent results on MBE growth of 3-in CdTe(211)B/Si are reported. The Si substrates were (211) orientated, and a low temperature surface cleaning process was employed. To obtain twin-free CdTe(211)B, nucleation process of ZnTe on Si was studied at different conditions. Under the optimal growth condition, the average FWHM value less than 120 arc sec of twin-free CdTe(211)B films for 10-12μm was obtained. The lowest FWHM value of 100 arc sec was achieved.
This paper describes some recent results on surface defects, uniformity, dislocation density as well as device applications of MBE growth of HgCdTe at the research center of advanced materials and devices. The features of different surface defects and their origins were studied by using SEM/EDX observations on HgCdTe epilayers with different growth conditions. A variety of surface defects was observed and the formation mechanism was discussed. A good uniformity was observed over 3-in HgCdTe wafers, the Stddev/mean in x and thickness were 1.2%, and 2.7%, respectively. It was found that the dislocation density was sensitive to growth parameters and the composition. The ZnCdTe substrates with 4% mole fraction were found to be suitable for LW HgCdTe, however, for the HgCdTe of shorter wavelengths different Zn composition is required. An average value of EPD of 4.2×105cm−2 was obtained for LW samples. The MBE grown HgCdTe were incorporated into some preliminary FPA devices.
The recent progress in MBE growth of HgCdTe at the Research Center for Advanced Materials and Devices, and the National Laboratory for Infrared Physics is reported. It is found that the excellent compositional uniformity and reproducibility of HgCdTe can be archived by MBE technique. The results of surface morphology, dislocation density, electrical properties and focal plane array detectors are described in the paper.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.