Te Photodiodes Research Articles

Large area CdxHg1−xTe photodiode with picosecond response time τRC

The design and manufacturing technology of high-quality CdxHg1−xTe n+−n−p−p+-type mesaphotodiodes ∅ 300 μm for the spectral range of 3–5 μm are presented. The features of the design and technology as well as the main characteristics of the manufactured photodiodes are presented. It is shown that the use of a semitransparent nickel layer 5 nm thick deposited on the surface of the n+ layer makes it possible to reduce the series resistance Rser to 1–2 Ohm and the response time to 10−11 s.

Bias dependence of photo-response in HgCdTe photodiodes due to series resistance

In this paper, we present the experimental results of photo-response measurements performed on Hg 0.7Cd 0.3Te photodiodes and report the dependence of measured quantum efficiency on the applied bias. Because of non-zero series resistance of a photodiode, the apparent photo-response may lower down at near zero bias region of operation or near the bias of peak dynamic resistance, which is the preferred bias. By increasing the reverse bias we may compensate this reduction but only up to a certain reverse voltage. Beyond this limit, the dynamic resistance of the photodiode reduces due to tunneling current, resulting in the reduction in apparent photo-response. Due to this behavior a peak appears in the measured quantum efficiency when plotted against the applied reverse bias voltage in contrast to the view that quantum efficiency is independent of bias.

Franz-Keldysh effect in two-photon absorption

The Franz-Keldysh effect in the two-photon absorption process in the space charge region of an Hg0.695Cd0.305Te photodiode is demonstrated. By employing as the exciting light source a pulsed laser beam at λ0=7.92μm from an optical parametric generator and difference frequency generator pumped by a pico-second Nd:YAG laser, the photo-response has been measured as a function of the excitation intensity. The peak intensity of the pulsed photo-response shows a quadratic dependence on the incident intensity. A relationship between the pulsed photo-voltage and the incident optical intensity has been established with an equivalent RC circuit model to derive the degenerate single beam two-photon absorption coefficient. The results show that the two-photon absorption coefficient within the space charge region of the photodiode is about a factor of 2.7 higher than that outside the space charge region, implying an electric field induced enhancement of the two-photon absorption. This indicates that the Franz-Keldysh effect exists not only in the one-photon absorption process but also in the two-photon absorption process.

Investigation of 1/f Noise Mechanisms in Midwave Infrared HgCdTe Gated Photodiodes

In this work, gated midwave infrared (MWIR) Hg1–xCdx Te photodiodes are used to investigate the physical origin of 1/f noise generation. Gated photodiodes were fabricated on liquid-phase epitaxy p-type HgCdTe MWIR material with a vacancy-doped concentration of 1.6 × 1016 cm−3 and x = 0.31. CdTe was thermally deposited and used as both a passivant and a mask for the plasma-based type conversion, and ZnS was used as an insulator. Fabricated devices show a R0A of 1–5 × 104 Ωcm2 with zero gate bias. Application of 2 V to the gate improves the R0A by more than two orders of magnitude to 6.0 × 106 Ωcm2, which corresponds to the p-type surface being at transition between depletion and weak inversion. Trap-assisted tunneling (TAT) current was observed at negative gate biases and reverse junction biases. For gate biases greater than 3 V, a field-induced junction breakdown was observed. An In = αIβf −0.5 trend was observed above 200 pA reverse bias dark current, with α = 3.5 × 10−5 and β = 0.82, which corresponds to the TAT dominated region. Below 200 pA, junction generation-recombination (GR) current starts to dominate and this previously mentioned trend is no longer observed. Junction GR current was not seen to be correlated with 1/f noise in these photodiodes.

A detailed theoretical and experimental noise study in n-on-p Hg 0.68Cd 0.32Te photodiodes

Both the method enabling numerical modeling of fluctuation phenomena in semiconductor devices as well as experimental results of noise current in Hg 0.68Cd 0.32Te n-on-p photodiodes are presented. While the concept of the applied numerical method was derived earlier [J. Appl. Phys. 90 (3) (2001) 1318], it has been enhanced in this paper by taking into account the influence of temperature fluctuations. The method is based on the assumption that the fluctuations of the generation–recombination processes, carrier mobility, heat generation and heat transfer result in the fluctuations of carrier and ionized impurity concentrations as well as fluctuations of carrier motion and temperature. These, in turn, may be expressed by the fluctuations of electrostatic potential, quasi-Fermi levels and temperature. The fluctuations of the electrostatic potential, quasi-Fermi level fluctuations and temperature fluctuations were calculated by solving the set of “transport equations for fluctuations” in which the fluctuations of generation–recombination processes, heat transfer and mobility act as random source terms. The method was applied to a theoretical analysis of the noise current in n-on-p Hg 0.68Cd 0.32 photodiodes. The noise spectrum in selected HgCdTe photodiodes is calculated and the contribution of different noise sources is determined. Theoretical results are compared with the experimental data. The planar homojunction devices were formed on LPE grown vacancy doped HgCdTe using a reactive ion etching plasma induced conversion process [J. Electron. Mater. 29 (6) (2000) 841].

Enhanced numerical modelling of non-cooled long-wavelength multi-junction (Cd,Hg)Te photodiodes

The theoretical analysis of long wavelength uncooled photovoltaic devices based on complex two-dimensional Hg1−xCdxTe heterostructures is presented. An enhanced computer program has been developed to solve the system of nonlinear continuity equations and the Poisson equation. All physical quantities of semiconductor structure are expressed as functions of electric potential and Fermi quasi-levels. The noise analysis is based on the set of ‘transport equations for fluctuations’ that enables calculations of spatial distribution of electrical potential and Fermi quasi-level fluctuations. Both generation–recombination noise and 1/f noise caused by mobility fluctuations were taken into account. The results of calculations are presented as maps illustrating spatial distributions of current densities, electrical gain, and fluctuations of selected physical quantities. Detectivity of 4×107 cmHz1/2W−1 is predicted for a 10.6 μm unbiased multiple heterojunction photovoltaic device with 20 μm period. The theoretical predictions have been compared with performance of practical devices.

Electronic transport properties of HgMnTe n +–p junctions

Transport properties of Hg 1 x Mn x Te photodiodes for 8–14 μm spectral region are investigated. It is shown that the distributions of space charge density, electric field strength and potential in the depletion layer are significantly affected by both majority and minority free carriers. This leads to changes in the bias voltage dependences of the recombination, diffusion and tunneling currents as well as the depletion-layer capacitance. The calculated results account rather well for the experimental data obtained on Hg 1 x Mn x Te ( E g≈0.1 eV) n +–p junctions.

Computer simulation of non-cooled long-wavelength multi-junction (Cd,Hg)Te photodiodes

A numerical analysis of long-wavelength multi-junction photodiodes constructed from Cd x Hg 1− x Te heterostructures is performed. The standard Newton iterative scheme is applied to solve the non-linear system of continuity equations and the Poisson equation. All quantities are expressed as a function of electric potential and quasi-Fermi levels. Results of computations are presented in the form of maps and plots illustrating spatial distributions of electrical potential and responsivity. Such an illustration enables us to explain the effect of reverse sign of the photovoltage occurring in photoelectric devices being manufactured.

Study of the effect of graded gap epilayers on the performance of CdxHg1−x Te photodiodes

The characteristics of the photodiodes based on CdxHg1−xTe solid solutions with graded gap layers were calculated in the context of a one-dimensional diffusion-drift model. The parameters of the photodiodes were shown to be improved when the p-n junction was located in the near-surface graded gap region rather than in the central homogeneous section of the structure. The photodiodes with the n-type layer adjacent to the substrate were found to offer an advantage over the diodes with the p-layer close to the substrate in the case of illumination from the substrate side.

Carrier freezeout in n-on-p Hg 0.7 Cd 0.3 Te photodiode and acceptor level determination

Cd0.3Te photodiodes at temperatures lower than 25 K. The freezeout was seen under the conditions that interband tunneling dominates the current in the photodiode and that avalanche ionization does not take place. These conditions are fulfilled at temperatures lower than 25 K and reverse bias voltages around 3 V. An acceptor level of about 4 meV was determined for p-type Hg0.7Cd0.3Te with NA-ND∼1016 cm-3 from the temperature dependence of the photodiode resistance governed by carrier freezeout. The difficulties in observation of carrier freezeout in Hg1-xCdxTe with a larger composition ratio x or higher doping concentration are discussed.

Modeling of junction formation and drive-in in ion implanted HgCdTe

N-on-p junction formation and drive-in in ion implanted Hg0.8Cd0.2Te photodiodes have been studied. A model of the junction formation and drive-in processes has been developed that accounts for the variations in injected Hg interstitial concentration, background point defect and extrinsic doping levels, sample geometry, and annealing conditions. The limiting mechanisms controlling junction drive-in were investigated using the model. Experimental data showed the junction drive-in rate was proportional to the square root of time, indicating a diffusion limited process. The diffusion limited process is the result of a solubility limit for the Hg interstitial concentration. This limit is approximately the same value as that obtained for Hg interstitials in Hg saturated Hg0.8Cd0.2Te in type conversion and self-diffusion experiments (DICI = 1.43 × 1013exp(−.457 eV/ kT)*PHg).

Acceptor level determination by carrier freezeout in reverse-biased Hg0.8Cd0.2Te photodiodes

Carrier freezeout in reverse-biased Hg0.8Cd0.2Te photodiodes has been investigated to determine the relevant acceptor level. At temperatures between 12 and 21 K hole freezeout in the p region governs the interband tunneling, which dominates the current across the reverse-biased photodiode. The acceptor level was found to be ∼4 meV in the p-type Hg0.8Cd0.2Te substrate with NA−ND∼1016 cm −3. In comparison with Hall measurements, this new method avoids the uncertainty due to contact caused by surface inversion on p-Hg0.8Cd0.2Te.

Evidence of coherent quantum 1/f noise in long n+p diffusion current dominated (Hg,Cd)Te photodiodes

The 1/ f fluctuations of dark-current- and photocurrent-induced noise in implanted n + p Hg 0.5 Cd 0.5 Te photodiodes are analyzed within the diffusion-current-dominated bias region at 300 K. The lifetime of the electrons in the p -type region has been determined experimentally. Therefore, the Hooge parameter a H is well defined by the 1/ f noise spectrum. The a H value, calculated from dark-current-induced 1/ f noise, is in close agreement with the result of Handel's coherent state 1/ f -noise theory, which yields a H = 4.6 x 10 -3 . The 1/ f noise of the photoinduced current without applied bias can be described with the same a H value. These results indicate coherent state quantum 1/ f noise generated by dark current as well as photoinduced current as a result of mobility fluctuations. If both independent types of currents are generated in the photodiode, the resulting 1/ f noise can be sufficiently described only within the concept of coherent state quantum 1/ f noise.

Thermomigration of Te precipitates and improvement of (Cd,Zn)Te substrate characteristics for the fabrication of LWIR (Hg, Cd)Te photodiodes

(Cd,Zn)Te wafers containing Te precipitates have been annealed under well defined thermodynamic conditions at temperatures below and above the melting of Te. Results of the examination of the wafers with infrared microscopy before and after the anneals indicate a substantial reduction of the Te precipitates in wafers annealed at temperatures in excess of the melting point of Te compared with those annealed at temperatures below the melting point of Te. These results confirm the thermomigration of liquid Te precipitates to be the principally operative mechanism during annealing in the elimination of these precipitates in (Cd,Zn)Te wafers. The occurrence of Te precipitates in (Hg,Cd)Te epitaxial layers grown on (Cd,Zn)Te substrates containing Te precipitates is also explained on the basis of thermomigration of these precipitates during LPE growth from the substrates to the epilayers. Absence of occurrence of Te precipitates in (Hg,Cd)Te epilayers grown on annealed (Cd,Zn)Te substrates with negligible Te precipitates is also confirmed. Usefulness of annealing (Cd,Zn)Te substrates—to eliminate Te precipitates—prior to epilayer growth is confirmed via demonstration of improved long wavelength infrared (Hg,Cd)Te device array performance uniformity in epitaxial layers grown on (Cd,Zn)Te substrates with negligible Te precipitates after annealing.

High detectivity Pb 0.8Sn 0.2Te-PbSe 0.08Te 0.92 heterostructure photodiodes

The performance of Pb 0.8Sn 0.2Te photodiodes manufactured by various methods have been reported for the temperature region 60–80 K. In this paper the performance of Pb 0.8Sn 0.2Te photodiodes fabricated by liquid-phase epitaxy (LPE) and by molecular-beam epitaxy (MBE) in the temperature interval 10–80 K are given. At T < 30 K the incremental resistance area ( R 0 A) product was limited by tunnelling through defects in the depletion region and D ∗ λp ~ 10 13 cm H 1 2 2 W −1 has been achieved.

High performance p− n junctions in the LPE Mn xCd yHg 1−x−y Te layers

The electric properties of n +− p Mn x Cd y Hg, 1− x− y Te photodiodes with a cut-off wavelength of λ co ∼ 3.3–3.8 μm at 77 K have been studied. The p− n junctions were created using ion implantation of boron into the p-type LPE epitaxial layers. Zero bias resistance-area product at T = 85 K is equal to 1.7 × 10 7 Ω ·. cm 2 for the best sample (λ co = 3.8 μm). For the material under study, this surpasses the values which have been reported up until now for a given spectral region, and is comparable with those of the best Cd x Hg 1− x Te diodes with similar energy gap values. Mechanisms of current flow through the p− n junctions at 77 < T < 200 K without and under background illumination are discussed.

Diffusion limited dark current in as-implanted (Hg, Cd)Te photodiodes

High detector performance of (Hg, Cd)Te photodiodes requires small dark currents in the structures. For the first time we have observed as-implanted (Hg, Cd)Te photodiodes offering r 0 A products near to theoretical limits. The pn-junction was obtained by ion implantation through a Al 2O 3 passivation layer.

Contribution of field-enhanced generation-recombination to Hg 1− xCd xTe photodiode characteristics

Field-enhanced generation-recombination, due to thermally assisted deep level-to-band tunneling, has been investigated for Hg 1− x Cd x Te photodiodes. A theoretical model is used for calculating the field-enhanced capture constants of carriers and the field-enhanced g-r current. It is shown that this mechanism can give rise to a large excess current, tending to smear out the hump in the p- n junction forward I- V characteristic induced by deep level-assisted tunneling, resulting in the soft reverse breakdown, and causing the temperature dependence of g-r limited R 0 A product to deviate from the classical g-r line.

Tunneling current calculated for Hg 0.8Cd 0.2Te diodes

The influence of the doping profile on the soft breakdown in Hg 0.8Cd 0.2Te photodiodes is investigated theoretically. The abrupt, linear, n +p -p and n +n -p models are compared one with another using an improved formulae for the interband tunneling generation rate, valid for both strong and inhomogeneous fields. The stationary semiconductor device equations are solved numerically including degeneracy and KANE band structure. The I- U characteristics of the various models are discussed for the different operating modes of the diode.

The performance of Hg 1− xMn xTe photodiodes

The intrinsic carrier concentration and carrier lifetime in Hg 1− x Mn xTe are calculated for the temperature range 77–300 K and the compositional range 0.07 ⩽ x ⩽ 0.20. The influence of different junction current components on the R 0A product of p +− n photodiodes at 77 K is analysed. The highest observed R 0A values are determined by the generation-recombination current in the depletion layer. The comparison between results of calculations and the experimental data shows potential possibilities for constructing higher quality Hg 1− x Mn x Te photodiodes.