Stressed and unstressed Ge:Ga detector arrays for airborne astronomy

We have constructed and used two dimensional arrays of both unstressed and stressed Ge:Ga photoconductive detectors for far-infrared astronomy from the Kuiper Airborne Observatory (KAO). The 25 element (5×5) arrays are designed for a new cryogenically cooled spectrometer, the MPE/UCB Far-Infrared Imaging Fabry-Perot Interferometer (FIFI). All of the pixels for the stressed array performed well on the first flights with FIFI; 25% of the detectors in the array are more sensitive than our best single element detector, with background limited noise equivalent powers (NEPs)?3.0×10^?15 W Hz^?1/2 at 158 μm and 40 km s^?1 spectral resolution. The average array element performs within±15% of this value. With a bias field of 0.1 V/cm, the average detector response is 20±6 Amp/Watt at 158 μm. The cutoff wavelength and response also compare well with our single element detectors. The unstressed array delivers significantly better performance than our single element detector due to the lower thermal background in the new spectrometer. The average background limited NEp at 88 μm and 35 km s^?1 spectral resolution is ～7×10^?15 W Hz^?1/2. The least sensitive pixel is only 40% less sensitive. The unstressed array response at 88 μm with a bias field of 1 V/cm is 5±1 Amp/Watt. Twenty four of the 25 elements worked on the first flights-on subsequent flights all channels have worked. Some of the exciting new science possible with far-infrared detector arrays is also discussed.     

Shallow and Undoped Germanium Quantum Wells: A Playground for Spin and Hybrid Quantum Technology

Buried‐channel semiconductor heterostructures are an archetype material platform for the fabrication of gated semiconductor quantum devices. Sharp confinement potential is obtained by positioning the channel near the surface; however, nearby surface states degrade the electrical properties of the starting material. Here, a 2D hole gas of high mobility (5 × 10⁵ cm² V^?1 s^?1) is demonstrated in a very shallow strained germanium (Ge) channel, which is located only 22 nm below the surface. The top‐gate of a dopant‐less field effect transistor controls the channel carrier density confined in an undoped Ge/SiGe heterostructure with reduced background contamination, sharp interfaces, and high uniformity. The high mobility leads to mean free paths ≈ 6 µm, setting new benchmarks for holes in shallow field effect transistors. The high mobility, along with a percolation density of 1.2 × 10¹¹cm^?2, light effective mass (0.09m_e), and high effective g‐factor (up to 9.2) highlight the potential of undoped Ge/SiGe as a low‐disorder material platform for hybrid quantum technologies.     

Ge/Si photodiodes with embedded arrays of Ge quantum dots forthe near infrared (1.3–1.5 µm) region

A method has been devised for MBE fabrication of p-i-n photodiodes for the spectral range of 1.3–1.5 µm, based on multilayer Ge/Si heterostructures with Ge quantum dots (QDs) on a Si substrate. The sheet density of QDs is 1.2×10¹² cm^?2, and their lateral size is ～8 nm. The lowest room-temperature dark current reported hitherto for Ge/Si photodetectors is achieved (2×10^?5 A/cm² at 1 V reverse bias). A quantum efficiency of 3% at 1.3 µm wavelength is obtained.     

Ohmic contact to p-type GaP

A study was made of the contact properties of a AuBe eutectic and a AuBeNi alloy on p-type GaP. The specific contact resistance varied from 1 × 10^?3 to 7.5 × 10^?5 ω cm² in the acceptor concentration range of 9 × 10¹⁶ to 2 × 10¹⁸ cm^?3. In the sintering temperature range resulting in good ohmic behaviour and low contact resistance the AuBe contacts do not form drops, whereas the AuBeNi contacts became molten; even after melting they wetted the surface of the GaP well. At the temperature of sintering Be diffuses from the contact into the GaP. The diffusion of Be gives rise to an additional acceptor concentration of 5 × 10¹⁸ to 1 × 10¹⁹ cm^?3 beneath the contact surface. Taking this into consideration the concentration-specific contact resistance relationship appears to support a field emission FE conduction mechanism.     

Study on antireflective coatings of PbTe/PbSnTe heterojunction infrared detectors

ZnS antireflective coatings and passivation layer are developed on self-made PbTe/PbSnTe heterojunction infrared detectors and following experiments have been finished: WaterProof properties of ZnS coatings; Anti-reflective properties of PbSnTe materials and their detectors with ZnS coatings, respectively; ageing and stability tests of the PbSnTe detectors with ZnS coatings. All experimental results are excellent: The typical detectivity (D^*) of PbSnTe detector is 2.83×10¹⁰ cmHz^1/2W^?1. (with peak wavelength λp=9.8 μm and cut-off wavelength λc=11.7 μm). Average detectivity of the PbSnTe detector with ZnS anti-reflective coatings is increased by 45%. Ageing tests indicated that the PbSnTe detectors with ZnS coatings have still high stabilities after several years. They are used successfully in medical infrared imaging systems and other applications.     

The effect of irradiation with high-energy protons on 4<Emphasis Type="Italic">H</Emphasis>-SiC detectors

The effect of irradiation of 4H-SiC ionizing-radiation detectors with various doses (as high as 10¹⁶ cm^?2) of 24-GeV protons is studied. Isotopes of B, Be, Li, He, and H were produced in the nuclear spallation reactions of protons with carbon. Isotopes of Al, Mg, Na, Ne, F, O, and N were produced in the reactions of protons with silicon. The total amount of the produced stable isotopes varied in proportion with the radiation dose from 1.2 × 10¹¹ to 5.9 × 10¹³ cm^?2. It is shown that, at high radiation doses, the contact characteristics of the detectors change appreciably. The potential-barrier height increased from the initial value of 0.7–0.75 eV to 0.85 eV; the rectifying characteristics of the Schottky contacts deteriorated appreciably. These effects are attributed to the formation of a disordered structure of the material as a result of irradiation.     

Heteroepitaxy of HgCdTe(112) infrared detector structures on Si(112) substrates by molecular-beam epitaxy

High-quality, single-crystal epitaxial films of CdTe(112)B and HgCdTe(112)B have been grown directly on Si(112) substrates without the need for GaAs interfacial layers. The CdTe and HgCdTe films have been characterized with optical microscopy, x-ray diffraction, wet chemical defect etching, and secondary ion mass spectrometry. HgCdTe/Si infrared detectors have also been fabricated and tested. The CdTe(112)B films are highly specular, twin-free, and have x-ray rocking curves as narrow as 72 arc-sec and near-surface etch pit density (EPD) of 2 × 10⁶ cm⁻² for 8 μm thick films. HgCdTe(112)B films deposited on Si substrates have x-ray rocking curve FWHM as low as 76 arc-sec and EPD of 3-22 × 10⁶ cm⁻². These MBE-grown epitaxial structures have been used to fabricate the first high-performance HgCdTe IR detectors grown directly on Si without use of an intermediate GaAs buffer layer. HgCdTe/Si infrared detectors have been fabricated with 40% quantum efficiency and R₀A = 1.64 × 10⁴ Ωm₂ (0 FOV) for devices with 7.8 μm cutoff wavelength at 78Kto demonstrate the capability of MBE for growth of large-area HgCdTe arrays on Si.     

Solubility,diffusion and electrical activity of Na in bulk Ge crystals

By studying the drift of Na⁺ ions in the firstly grown Na-doped bulk Ge crystals as well as by analyzing optical and some other characteristics of this material, the following conclusions are made, many of which are different from the commonly accepted statements: (1) Ge can be uniformly doped with Na during the bulk Ge crystals growth from the melt; (2) maximum solubility at room temperature and distribution coefficient of Na in Ge are (0.3–1)×10¹⁵ cm⁻³ and (0.7–2.3)×10⁻⁷, respectively; (3) Na is a donor impurity in bulk Ge, and Na atoms introduced during the crystal growth are predominantly electrically active; (4) the evaluated values of diffusion parameters of Na in Ge are as follows: the diffusion coefficient D=3.6×10⁻⁷ cm²/s, pre-exponential factor D₀=0.13 cm²/s, the activation energy for diffusion Q=0.33 eV; (5) Na is an interstitial impurity in Ge and rather rapidly drifts in an electric field, most likely, via interstitial sites; (6) the resistance distribution along the crystal length may be changed by DC electric field application and remain stable at the long-term crystal storage. The stability in the Ge:Na properties opens the possibility for using Ge:Na crystals not only for creating passive optical elements of infrared imaging technique, as we are doing now, but also for the electrical appliances, in particular for the substitution of the thermally unstable Li for Na in germanium detectors of γ-radiation.     

Instability of characteristics of SiC detectors subjected to extreme fluence of nuclear particles

The operation of detectors irradiated with 8-MeV protons at a fluence of 3 × 10¹⁴ cm^?2 has been studied. The detectors were based on modern CVD-grown n-4H-SiC films with a concentration of uncompensated donors equal to ～2 × 10¹⁴ cm^?3 and a thickness of 55 μm. The high concentration of primary radiation defects (～2 × 10¹⁷ cm^?3) determined the deep compensation of the films. The basic characteristics of the detectors—pulse amplitude and resolution—exhibited temporal instability. This effect is due to prolonged capture of nonequilibrium carriers by radiation centers and the resulting appearance of a polarization voltage in the bulk of the detector. The kinetics of attainment of steady values by the quantities specified above was analyzed.     

Schottky-barrier capacitance measurements for deep level impurity determination

The time dependence of a Schottky-barrier capacitance due to thermal excitation of trapping centres has been studied. An expression for the junction capacitance is derived which is not restricted to any special range of reverse bias nor to a special relation between shallow and deep impurity concentration. The concentration ratio of shallow to deep centres is calculated from the values of the capacitance at zero and infinite time. From a capacitance vs. time plot the trap emission rate for electrons e_n is obtained. Their energetic level within the forbidden band-gap is determined from the temperature dependence of e_n as well as from the capacitance-time variation. Experimental studies which do confirm the calculations were carried out on gold contacts on oxygen-doped n-type GaAs. Representative results of the investigated samples were: shallow donor density 3 × 10¹⁵ cm^?3, trap density 9·8 × 10¹⁵ cm^?3, electron emission rate 6 × 10^?2 sec^?1, energetic level 0·68 eV and capture cross section 7 × 10^?16 cm².     

Ultrasensitive Room‐Temperature Terahertz Direct Detection Based on a Bismuth Selenide Topological Insulator

Despite their huge application capabilities, millimeter‐ and terahertz‐wave photodetectors still face challenges in the detection scheme. Topological insulators (TIs) are predicted to be promising candidates for long‐wavelength photodetection, due to the presence of Dirac fermions in their topologically protected surface states. However, photodetection based on TIs is usually hindered by the large dark current, originating from the mixing of bulk states with topological surface states (TSSs) in most realistic samples of TIs. Here millimeter and terahertz detectors based on a subwavelength metal–TI–metal (MTM) heterostructure are demonstrated. The achieved photoresponse stems from the asymmetric scattering of TSS, driven by the localized surface plasmon‐induced terahertz field, which ultimately produces direct photocarriers beyond the interband limit. The device enables high responsivity in both the self‐powered and bias modes even at room temperature. The achieved responsivity is over 75 A/W, with response time shorter than 60 ms in the self‐powered mode. Remarkably, the responsivity increases by several orders of magnitude in the biased configuration, with the noise‐equivalent power (NEP) of 3.6 × 10^?13 W Hz^?1/2 and a detectivity of 2.17 × 10¹¹ cm Hz^?1/2 W^?1 at room temperature. The detection performances open a way toward realistic exploitation of TIs for large‐area, real‐time imaging within long‐wavelength optoelectronics.     

The physical properties of CdTe doped with V and Ge

CdTe:(V, Ge) single crystals are grown using the Bridgman-Stockbarger method. The impurity concentrations in the melt are N_V = 1 × 10¹⁹ cm^?3 and N_Ge = 5 × 10¹⁸ and 1 × 10¹⁹ cm^?3. Electrical and galvanomagnetic characteristics are studied in the temperature range 300–400 K. It is found that the equilibrium characteristics are governed by deep levels (ΔE = 0.75–0.95 eV) located close to the midgap. Low-temperature optical absorption spectra indicate that the impurity levels of V and Ge ions in the low-energy region are in different charge states. In addition, the samples are annealed in Cd vapor and then rapidly cooled. This annealing causes the decomposition of various complexes formed during the crystal growth and an increase in both electrical conductivity and charge carrier concentration.     

Radiation effects on a GE:GA photoconductive detector

We have investigated radiation effects on a Ge:Ga photoconductive infrared detector at low photon background level of 4×10⁸ ph cm^?2s^?1, using Cobalt 60 as a gamma ray source. The irradiation immediately induced spike noises which degrded NEP (short term effect), while it gradually increased responsivity (long term effect). After the removal of the gamma ray source, the spikes disappeared while the responsivity still stayed in a higher level and gradually decreased with a time scale of several hours. The responsivity-change-rate before and after the irradiation is smaller for a higher bias voltage. Finally we have made the first trial to cure the long term effect, using a flashing procedure and have found out its effectiveness.     

Effect of extreme radiation fluences on parameters of SiC nuclear particle detectors

Detectors based on modern CVD-grown films were irradiated with 8 MeV protons at a fluence of 3 × 10¹⁴ cm^?2. The concentration of primary radiation defects was ～10¹⁷ cm^?3, which is three orders of magnitude higher than the concentration of the initially present uncompensated donors. The resulting deep compensation of SiC enabled measurements of detector parameters in two modes: under reverse and forward bias. The basic parameters of the detectors degraded by no more than a factor of 1.7, compared with the fluence of 1 × 10¹⁴ cm^?2. However, there appeared a polarization voltage, which indicates that a space charge is accumulated by radiation defects.     

The electrical characteristics of InP implanted with the column IV elements

A study of the electrical characteristics of InP implanted with C, Si, Ge and Sn demonstrates that all of these column IV elements are donors, although the net electrical activation achieved with the light ion C was only about 5%. Samples implanted at temperatures of 150–200°C generally had lower sheet resistivities, higher mobilities and except for high doses, higher sheet carrier concentrations than those done at room temperatures. Implants at 150–200°C with 1 × 10¹⁴cm^?2 of the heavier ions, Si, Ge or Sn, resulted in layers with sheet carrier concentrations of 7.8 × 10¹³, 5.6 × 10¹³ and 4.7 × 10¹³cm^?2, respectively. Carrier concentration profiles of samples implanted at 200°C with 1 × 10¹⁴cm^?2 of Si agreed reasonably well with LSS theory. Higher doses gave rise to substantial diffusion of the implanted Si, whereas room temperature implants showed poor activation near the surface.     

Defect Characterization in Ge/(001)Si Epitaxial Films Grown by Reduced-Pressure Chemical Vapor Deposition

We studied the microstructural characteristics and electrical properties of epitaxial Ge films grown on Si(001) substrates by x-ray diffraction, atomic force microscopy, and transmission electron microscopy. The films were grown using a two-step technique by reduced-pressure chemical vapor deposition, where the first step promotes two-dimensional growth at a lower substrate temperature. We observed a decrease in defect density with increasing film thickness. Ge films with thickness of 3.5 μm exhibited threading dislocation densities of 5 × 10⁶ cm^?2, which yielded devices with dark current density of 5 mA cm^?2 (1 V reverse bias). We also noted the presence of stacking faults in the form of lines in the films and establish that this is an important defect for Ge films grown by this deposition technique.     

Autodoping effects of Ge in vapor-grown GaAsl-xPx layers on the Ge substrates

Epitaxial growth of GaAs_1-xP_x layer on the Ge substrate has been investigated under the optimized growth conditions for reducing vapor etching of the substrate, using a Ga-PCl₃-AsH₃-H₂ system. The free carrier concentration, ?N_D⁺?N_A^??, and the electroluminescent properties of GaAs_1-xP_x layers with $\text{x ? 0·4}$ are studied, and are correlated with the Ge concentration involved. In the lightly doped region below 1×10¹⁷ atoms/cm³, bright electroluminescence is observed at room temperature from forward-biased p-n junctions fabricated by a zinc-diffusion technique. However, in the narrow region of 1×10¹⁷?4×10¹⁷ atoms/cm³, the enhanced amphoteric behavior of Ge leads to concentration quenching of visible-light emission. The ?N_D⁺?N_A^?? reaches its maximum at ～ 1×10¹⁷ atoms/cm³. Nearly complete self-compensation is observed above 4×10¹⁷ atoms/cm³ due to the increase of the concentration of deep-lying Ge acceptors.     

Growth of fully doped Hg1−xCdxTe heterostructures using a novel iodine doping source to achieve improved device performance at elevated temperatures

Band gap engineered Hg_1−xCd_xTe (MCT) heterostructures should lead to detectors with improved electro-optic and radiometric performance at elevated operating temperatures. Growth of such structures was accomplished using metalorganic vapor phase epitaxy (MOVPE). Acceptor doping with arsenic (As), using phenylarsine (PhAsH₂), demonstrated 100% activation and reproducible control over a wide range of concentrations (1 × 10¹⁵ to 3.5 × 10¹⁷ cm⁻³). Although vapor from elemental iodine showed the suitability of iodine as a donor in MC.T, problems arose while controlling low donor concentrations. Initial studies using ethyliodide (EtI) demonstrated that this source could be used successfully to dope MCT, yielding the properties required for stable heterostructure devices, i.e. ≈100% activation, no memory problems and low diffusion coefficient. Cryogenic alkyl cooling or very high dilution factors were required to achieve the concentrations needed for donor doping below ≈10¹⁶cm⁻³ due to the high vapor pressure of the alkyl. A study of an alternative organic iodide source, 2-methylpropyliodide (2 MePrI), which has a much lower vapor pressure, improved control of low donor concentrations. 2 MePrI demonstrated the same donor source suitability as EtI and was used to control iodine concentrations from ≈ 1 × 10¹⁵ to 5 × 10¹⁷cm⁻³. The iodine from both sources only incorporated during the CdTe cycles of the interdiffused multilayer process (IMP) in a similar manner to both elemental iodine and As from PhAsH₂. High resolution secondary ion mass spectroscopy analysis showed that IMP scale modulations can still be identified after growth. The magnitude of these oscillations is consistent with a diffusion coefficient of≈7 × 10⁻¹⁶cm²s⁻¹ for iodine in MC.T at 365°C. Extrinsically doped device heterostructures, grown using 2 MePrI, have been intended to operate at elevated temperatures either for long wavelength (8–12 smm) equilibrium operation at 145K or nonequilibrium operation at 190 and 295K in both the 3–5 μ and 8–12 μ wavelength ranges. Characterization of such device structures will be discussed. Linear arrays of mesa devices have been fabricated in these layers. Medium wave nonequilibrium device structures have demonstrated high quantum efficiencies and R₀A = 37 Ωcm² for λ_co = 4.9 μ at 190K.     

Electrical properties and low-temperature photolumincesence of Si-doped CdTe crystals

The CdTe:Si single crystals with Si concentration in the range of C _Si ⁰ =2×10¹⁸–5×10¹⁹ cm^?3 are grown by the Bridgman-Stockbarger method. The samples were of the n-and p-type with electrical conductivity σ=2×10^?1–8×10^?9 Ω^?1 cm^?1. Being heated in the temperature range 300–440 K, the p-CdTe crystals were annealed, and their conductivity decreased. The shape of the low-temperature (5–20 K) photoluminescence spectra of the samples are indicative of their high structural quality. The specific feature of the emission of the CdTe:Si crystals is its decrease in the intensity of all lines induced by donors as the samples are cut progressively closer to the ingot top. The results obtained indicate that the Si impurity, in contrast with Ge, Sn, and Pb, does not exhibit the compensating and stabilizing effect in CdTe.     

Analysis of thermal emission processes of electrons from arrays of InAs quantum dots in the space charge region of GaAs matrix

We thoroughly analyze admittance spectroscopy data on the temperature dependence of the rate of electron emission from the ground state of InAs quantum dots in the space-charge layer of a Schottky barrier on an n-GaAs matrix. The experimental results are described using a one-dimensional model of thermally activated tunneling with the involvement of virtual states. The shape of the potential barrier to be overcome by emitted electrons is selected by introducing the effective concentration of shallow donors such that the electron binding energies in the quantum dots were similar to those determined from the measured capacitance-voltage characteristics of the investigated structures. The obtained electron-capture cross sections increase with the ground-state binding energy (quantum dot size). The capture cross-section values for InAs quantum dots with average lateral sizes of 9 and 20 nm lie in the ranges 1 × 10^?14?2 × 10^?13 and 4 × 10^?12?2 × 10^?11 cm².