Very high purity In0.53Ga0.47As grown by molecular beam epitaxy

Very high purity In0_0.53Ga_0.47As layers were grown by molecular beam epitaxy (MBE). Origins ofn-type impurities in undoped In_0.53Ga_0.47As grown on an InP:Fe substrate were systematically examined. The most possible origins were impurities diffusing from the InP:Fe substrate and those contained in As molecular beam. These impurities were dramatically reduced by using an InAlAs buffer layer and a growth condition of high substrate temperature and low As pressure. The lowest electron concentration of the In0_0.53Ga_0.47As layer wasn = 1.8 × 10¹³ cm_-3 with mobilitiesμ = 15200 cm²/Vs at 300 K andμ = 104000 cm²/Vs at 77 K.     

Investigation of iodine as a donor in MBE grown Hg1−xCdxTe

N-type Hg_1−xCd_xTe layers with x values of 0.3 and 0.7 have been grown by molecular beam epitaxy using iodine in the form of CdI₂ as a dopant. Carrier concentrations up to 1.1 × 10¹⁸ cm⁻³ have been achieved for x = 0.7 and up to 7.6 × 10¹⁷ cm⁻³ for x=0.3. The best low temperature mobilities are 460 cm²/(Vs) and 1.2 × 10⁵ cm²/(Vs) for x=0.7 and x=0.3, respectively. Using CdI₂ as the dopant modulation doped HgTe quantum well structures have been grown. These structures display very pronounced Shubnikov-de Haas oscillations and quantum Hall plateaus. Electron densities in the 2D electron gas in the HgTe quantum well could be varied from 1.9 × 10¹¹ cm⁻² up to 1.4 × 10¹² cm⁻² by adjusting the thicknesses of the spacer and doped layer. Typical mobilities of the 2D electron gas are of the order of 5.0 × 10⁴ cm²/(Vs) with the highest value being 7.8 × 10⁴ cm²/(Vs).     

Galvanomagnetic properties of two-dimensional electron gases in strain relaxed InxGa1−xAs(x<0.40) heterostructures grown by molecular beam epitaxy on GaAs substrates

We have investigated, as a function of indium content x, the galvanomagnetic and Shubnikov de Haas (SdH) properties of two-dimensional electron gases (2DEG) formed at lattice matched, strain relaxed InAlAs/InGaAs heterojunctions. These were grown by molecular beam epitaxy on GaAs misoriented substrates with a two degree offcut toward the nearest (110) plane. Variable temperature resistivity and Hall measurements indicate an increase in the electron sheet density n_s from 0.78×10¹²cm⁻² for x=0.15 to 1.80×10¹² cm⁻² for x=0.40 at 300K, and from 0.75×10¹²cm⁻² to 1.67×10¹²cm⁻² at T=1.6K. The room temperature electron mobility, measured along the in plane [110], direction is independent of indium content and equals approximately 9500 cm²/Vs. For T<50K, the mobility is independent of temperature decreasing with increasing x from 82000 cm²/Vs for x=0.15 to 33000 cm²/Vs for x=0.40. The ratios (τ_t/τ_q) at 1.6K between the electron relaxation time τ_t and the single particle relaxation time τq, for the strain relaxed specimens, as well as for pseudomorphically strained Al_0.35Ga_0.65As/In_0.15Ga_0.85As structures grown on GaAs substrates, and In_0.52Al_0.48As/In_0.53Ga_0.47As heterostructures grown lattice matched on InP substrates. Such a study indicates the presence of inhomogeneities in the 2DEGs of the strain relaxed specimens which appear to be related to the process of strain relaxation. Such inhomogeneities, however, have little effect on the electron relaxation time τ_t which, at low temperatures, is limited principally by alloy scattering.     

Doping andp-n junction formation in InAs1-xSbx/lnSb SLS’s by MOCVD

Diethylzinc, dimethylcadmium, hydrogen selenide, silane, dimethyltellurium, and di-ethyltellurium were investigated as dopants for InSb and InAs_1-xSb_x. Carrier concentrations between 5 x 10³ and 5 x 10¹⁹ cm^-3 have been achieved for bothn- and p-type dopants by using dilute mixtures, 10 to 50 ppm, of dimethylcadmium and dimethyltellurium in hydrogen. The 77 K Hall mobilities of p-type InSb ranged from 7000 to 200 cm²/Vs and of n-type from 55,000 to 1700 cm²/Vs. An InAs_0.17Sbo_0.83/InSb SLS infrared photodiode has been fabricated with a wavelength response cutoff of 10.2 μm at 77 K. The zero bias, external current responsivity and infrared absorption of this device were measured. The predicted optical transitions using a type II heterojunction band offset closely match the observed absorption. The minority carrier diffusion length, perpendicular to the growth planes, is approximately 0.5 μm     

Effect of Sputtered ZnO Layers on Behavior of Thin-Film Transistors Deposited at Room Temperature in a Nonreactive Atmosphere

In this work we present the electrical characterization of ZnO-based thin-film transistors fabricated at room temperature. The ZnO films were deposited by radiofrequency magnetron sputtering at variable argon pressure (3 mTorr to 10 mTorr) at room temperature. The sputtered ZnO films were polycrystalline with hexagonal structure and electrical resistivity ranging from 10¹ Ω cm to 10⁸ Ω cm for films deposited from 3 mTorr to 10 mTorr. The trend in the electrical behavior of the devices was found to be due to the variation of the electron concentration of the ZnO films. The devices with better performance showed a field-effect mobility of 2.9 cm²/Vs, threshold voltage of 20 V, I _on/I _off ≈ 10⁶, and electrical resistivity of ~10⁸ Ω cm. In addition, linear behavior of I _on/I _off with deposition pressure was observed. The lowest I _on/I _off ratio (~2) was calculated for devices with ZnO layers deposited at 3 mTorr, and the highest ratio (~10⁶) for devices processed at 10 mTorr. Hall-effect measurements were performed on ZnO films showing the lowest resistivity. The layer grown at 3 mTorr showed a Hall mobility of μ _H = 8.9 cm²/Vs and carrier concentration of n = 4.2 × 10¹⁶ cm⁻³ with resistivity of ρ = 31.8 Ω cm. For films deposited at 5 mTorr, the Hall mobility, carrier concentration, and resistivity were μ _H = 7.9 cm²/Vs, n = 3.4 × 10¹⁶ cm⁻³, and ρ = 38.4 Ω cm, respectively. Films deposited at 8 mTorr and 10 mTorr could not be measured due to their high resistance.     

N-Type Hg1−xCdxTe: Undoped x = 0.3 LPE material for sprite IR detectors

The requirement for two color Sprite detectors, with elements sensitive in the ranges 3-5 μn (MW) and 8-14 μn (LW) at 77K, is met using Hg_1−xCd_xTe elements of composition x = 0.3 and x = 0.2, respectively. The need for low defect levels for increased performance indicates the use of liquid phase epitaxy (LPE). While LW material is fairly well characterized, the growth and conversion to n-type of MW LPE has proved more difficult. Reported work shows limited data and limited success in converting MW LPE to n-type, and this primarily in donor-doped material. This paper describes the growth, annealing to n-type and characterization of Hg_0.7Cd_0.3Te. High n-type conversion yields were obtained, with low donor levels (mid-10¹³ to mid-10¹⁴ cm⁻³), high mobility (>10⁴ cm² (Vs)⁻¹) and long minority carrier lifetime (>10 us).     

Growth of InAs-AlSb quantum wells having both high mobilities and high concentrations

Low-temperature mobilities in InAs-AlSb quantum wells depend sensitively on the buffer layer structures. Reflection high energy electron diffraction and x-ray diffraction show that the highest crystalline quality and best InAs transport properties are obtained by a buffer layer sequence GaAs → AlAs → AlSb → GaSb, with a final GaSb layer thickness of at least 1 μm. Using the improved buffer scheme, mobilities exceeding 600,000 cm²/Vs at 10 K are routinely obtained. Modulation δ-doping with tellurium has yielded electron sheet concentrations up to 8 × 10¹² cm⁻² while maintaining mobilities approaching 100,000 cm²/Vs at low temperatures.     

Growth and characterization of GaSb bulk crystals with low acceptor concentration

GaSb bulk single crystals with low acceptor concentration were grown from a bismuth solution by the traveling heater method. The result is isoelectronic doping by Bi which gives a variation of the opto-electronic properties as a function of grown length as well as a pronounced microscopic segregation. Photoluminescence spectra at 4K show a decrease of the natural acceptor during growth, which is confirmed by Hall measurements. The electrical properties of this isoelectronic doped GaSb are hole concentrations and mobilities of N_A − N_D = 1.7 × 10¹⁶ cm⁻³ and μ = 870 cm²Vs at room temperature and N_A-N_D = 1 × 10¹⁶ cm⁻³ and μ = 4900 cm²/Vs at 77K, respectively. The lowest p-type carrier concentration measured at 300K is N_A − N_D = 3.3 × 10¹⁵ cm⁻³     

Electrical and photo-luminescence properties of ZnSe epitaxial layers grown on GaAs (100) by atmospheric pressure MOVPE: The role of substrate temperature

Epitaxial layers of ZnSe ranging in thickness from 5μm to 30 μm have been grown on GaAs (100) substrates over the temperature range 240° C to 340° C by atmospheric pressure MOVPE employing dimethylzinc and hydrogen selenide. An optimum growth temperature of 280 ± 5° C has been identified and when grown at this temperature the ZnSe epitaxial layers exhibit low resistivity (ρ_{298 K} ≤ 10 ohm · cm), a low compensation ratio (θ_{298 K} = 0.27), a carrier mobility (μ_{298 K}) of 250 ±10 cm²V^-1s^-1) and aren-type (n _{298 K} = 8.0 × 10¹⁴ cm^-3). The ratio of photoluminescence intensity measured at 298K and at 12 K is high (10⁴) and is dominated by a sharp emission due to excitons bound to neutral donors at 2.7956 eV. Mass spectrometric investigations of the chemical reactions occurring inside the reactor in the presence of the GaAs substrate indicate significant surface-controlled reactivity in the region of 280° C.     

Characterization of very high purity InAs grown using trimethylindium and tertiarybutylarsine

The growth of high purity InAs by metalorganic chemical vapor deposition is reported using tertiarybutylarsine and trimethylindiμm. Specular surfaces were obtained for bulk 5-10 μm thick InAs growth on GaAs substrates over a wide range of growth conditions by using a two-step growth method involving a low temperature nucleation layer of InAs. Structural characterization was performed using atomic force microscopy and x-ray diffractometry. The transport data are complicated by a competition between bulk conduction and conduction due to a surface accumulation layer with roughly 2–4 × 10¹² cm⁻² carriers. This is clearly demonstrated by the temperature dependent Hall data. Average Hall mobilities as high as 1.2 x 10⁵ cm²/Vs at 50K are observed in a 10 μm sample grown at 540°C. Field-dependent Hall measurements indicate that the fitted bulk mobility is much higher for this sample, approximately 1.8 × 10⁵ cm²/Vs. Samples grown on InAs substrates were measured using high resolution Fourier transform photoluminescence spectroscopy and reveal new excitonic and impurity band emissions in InAs including acceptor bound exciton “two hole transitions.” Two distinct shallow acceptor species of unknown chemical identity have been observed.     

Growth and characterization of undoped and N-type (Te) doped MOVPE grown gallium antimonide

The growth of GaSb by MOVPE and itsn-type doping using a dimethyltellurium dopant source are investigated. The results of growth rate, morphology and Te incorporation as a function of growth parameters are given. Increasing growth temperature and V/III reactant ratio were found to reduce the Te incorporation. The lowest Hall carrier concentrations obtained at room-temperature, onp-type andn-type MOVPE GaSb are respectively:p _H= 2.2 × 10¹⁶cm⁻³ with a Hall mobility ofμ _H= 860 cm²/V.s andn _H= 8.5 × 10¹⁵cm⁻³ withμ _H= 3860 cm²/V.s. Furthermore, Hall mobilities as high as 5000 cm²/V.s were measured onn-type GaSb samples.     

Solution Growth and Thermoelectric Properties of Single-Phase MnSi<Subscript>1.75−<Emphasis Type="Italic">x</Emphasis></Subscript>

We have investigated the crystal growth of single-phase MnSi_1.75−x by a temperature gradient solution growth (TGSG) method using Ga and Sn as solvents and MnSi_1.7 alloy as the solute, and measured the thermoelectric properties of the resulting crystals. Single-phase Mn₁₁Si₁₉ and Mn₄Si₇ crystals were grown successfully using Ga and Sn as solvents, respectively. The typical size of a grown ingot of Mn₁₁Si₁₉ was 2 mm to 4 mm in thickness and 12 mm in diameter, whereas Mn₄Si₇ had polyhedral shape with dimensions in the range of several millimeters. The single-phase Mn₁₁Si₁₉ has good electrical conduction (ρ = 0.89 × 10⁻³ Ω cm to 1.09 × 10⁻³ Ω cm) compared with melt-grown multiphase higher-manganese silicide (HMS) crystals. The Seebeck coefficient, power factor, and thermal conductivity were 77 μV K⁻¹ to 85 μV K⁻¹, 6.7 μW cm⁻¹ K⁻² to 7.2 μW cm⁻¹ K⁻², and 0.032 W cm⁻¹ K⁻¹, respectively, at 300 K.     

Shockley–Haynes Characterization of Minority-Carrier Drift Velocity,Diffusion Coefficient,and Lifetime in HgCdTe Avalanche Photodiodes

A combined study of the avalanche gain characteristics of HgCdTe electron-avalanche photodiodes (e-APDs) and of the minority electron properties in the p-type absorber using Shockley–Haynes (SH) measurements is presented for various Cd compositions x _Cd. Ideal gain performance associated with a low excess noise factor F = 1.2 have been measured at T = 80 K down to cutoff wavelengths of λ _c = 2.9 μm. The observation of both a record high, exponentially increasing gain of M = 600 in short-wave e-APDs and a low excess noise factor proved that the exclusive electron multiplication is stable down to x _Cd = 0.4. Zero-flux measurements at 80 K confirmed that the dark current tends to decrease at constant gain as x _Cd increases. Measurements using a readout integrated circuit allowed us to establish a new record in sensitivity for APDs: I _{eq_in} = 2 aA, corresponding to 12 e/s at gain of M = 24 in an e-APD with λ _c = 2.9 μm. SH measurements enabled direct estimation of the electron diffusion coefficient, drift velocity, and lifetime in the p-type absorber of the e-APDs as a function of electric field at temperatures between 80 K and 200 K. Measurements at 80 K yielded lifetimes consistent with the values expected for the nominal doping of the samples. The low-field electron drift mobility, estimated from the drift velocity, was found to be a factor of 0.4 to 0.5 lower than the mobility in n-type material with the same composition. In mid-wave (MW) infrared samples with λ _c = 5.3 μm, the mobility was observed to be μ _ep = 15 kcm²/Vs to 20 kcm²/Vs, being less than μ _en ≈ 40 kcm²/Vs to 50 kcm²/Vs. The reduction in mobility can, in part, be attributed to scattering by ionized acceptors and heavy holes. The diffusion mobility, estimated from the diffusion coefficient, was systematically higher than the drift mobility, indicating diffusion of hot electrons with a temperature higher than that of the lattice. The saturation velocity, v _{sat_ep} = 2 × 10⁶ cm/s to 6 × 10⁶ cm/s, did not correlate with the Cd composition in the samples. The measured saturation velocities made it possible to estimate the timing jitter in p-type absorbers with a built-in electric field. Jitter below 100 ps was estimated for SW and MW APDs with absorbing layer thicknesses up to 4 μm.     

Electrical properties of InAlAs/InAsxP1-x/InP composite-channel modulation-doped structures grown by solid source molecular beam epitaxy

We report on the electrical characteristics of the two-dimensional electron gas (2DEG) formed in an InAlAs/InAs_xP_1-x/InP pseudomorphic composite-channel modulation-doped (MD) structure grown by solid source (arsenic and phosphorus) molecular beam epitaxy (SSMBE). The As composition, x, of strained InAs_xP_1-x was determined by x-ray diffraction analysis of InP/InAs_xP_1-x/InP multi-quantum wells (MQWs) with compositions of x=0.14 to x=0.72. As the As composition increases, the room temperature sheet resistance of InAlAs/InAs_xP_1-x/InP composite-channel MD structures grown over a range of As compositions decreased from 510 to 250 Ω/cm², resulting from the greater 2DEG confinement and lower electron effective mass in the InAs_xP_1-x channel as x increases. The influence of growth conditions and epitaxial layer designs on the 2DEG mobility and concentration were investigated using 300 K and 77 K Hall measurements. As the exposure time of the As₄ flux on the growth front of InAs_xP_1-x increased during growth interruptions, the 2DEG mobility, in particular the 77K mobility, was considerably degraded due to increased roughness at the InAlAs/InAs_xP_1-x interface. For the InAlAs/InAs_0.6P_0.4/InP composite-channel MD structure with a spacer thickness of 8 nm, the room temperature 2DEG mobility and density were 7200 cm²/Vs and 2.5 × 10¹² cm⁻², respectively. These results show the great potential of the InAlAs/InAs_xP_1-x/InP pseudomorphic composite-channel MD heterostructure for high frequency, power device applications.     

The temperature and pressure dependence of the electron and hole mobilities in GaxIn1-xAsyP1-y alloys

A wide range of samples of both n-type and p-type Ga_xIn_1-xAs_yP_1-y on InP has been grown by LPE with carrier concentrations in the low 10¹⁶cm⁻³range. The electron mobility (μ_e) at room temperature decreased from about 4000 cm²V⁻¹s⁻¹ at y = 0 and passed through a shallow minimum near y = 0.25. At high y values, μ_e rose steeply, reaching 11 000 cm²V⁻¹s⁻¹ at the ternary boundary. In the p-type material the hole mobility (μ_p) varied from 140 cm V⁻¹s⁻¹ in InP, passed through a minimum of about 70 cm²V⁻¹s⁻¹ near y = 0.5 and then increased swiftly towards the ternary boundary. The temperature dependence of both μ_e and μ_p suggested the presence of alloy or space-charge scattering. In order to distinguish between these two mechanisms the pressure coefficient of the direct band-gap dE_o/dP was measured as a function of y by observing the movement with pressure of the photoconductive edge. From dE_o/dP the pressure variation of the effective mass was deduced. By measuring the change in electron and hole mobilities with pressure, it was then possible to establish that alloy scattering rather than space-charge scattering was occurring. From the composition dependence of the alloy scattering potentials for electrons and holes predictions have been made of the variation of μ_e and μ_P with temperature, pressure and dopant Presently a Nuffield Science Fellow concentration. At room temperature a maximum electron mobility of about 11,200 cm²V⁻¹ s⁻¹ is indicated. Presently a Nuffield Science Fellow     

Scattering and electron mobility in combination-doped HFET-structures AlGaAs/InGaAs/AlGaAs with high electron density

Molecular-beam epitaxy is used for growing structures differing in doping technique and doping level and having a high two-dimensional-electron concentration n _s in the quantum well. The effect of doping combining uniform and δ doping on the electron-transport properties of heterostructures is investigated. A new type of structure with a two-sided silicon δ doping of GaAs transition layers located on the quantum-well boundaries is proposed. The largest value of electron mobility μ_H = 1520 cm²/(V s) is obtained simultaneously with a high electron density n _s = 1.37 × 10¹³ cm⁻² at 300 K with such a doping. It is associated with decreasing electron scattering by an ionized impurity, which is confirmed by the carried out calculations.     

Lpe growth of Hgl−xCdxTe using conventional slider boat and effects of annealing on properties of the epilayers

The epitaxial layers of Hg_1−xCd_xTe (0.17≦×≦0.3) were grown by liquid phase epitaxy on CdTe (111)A substrates using a conventional slider boat in the open tube H₂ flow system. The as-grown layers have hole concentrations in the 10¹⁷− 10¹⁸ cm⁻³ range and Hall mobilities in the 100−500 cm²/Vs range for the x=0.2 layers. The surfaces of the layers are mirror-like and EMPA data of the layers show sharp compositional transition at the interface between the epitaxial layer and the substrate. The effects of annealing in Hg over-pressure on the properties of the as-grown layers were also investigated in the temperature range of 250−400 °C. By annealing at the temperature of 400 °C, a compositional change near the interface is observed. Contrary to this, without apparent compositional change, well-behaved n-type layers are obtained by annealing in the 250−300 °C temperature range. Sequential growth of double heterostructure, Hg_l−xCd_xTe/Hg_l−yCd_yTe on a CdTe (111)A substrate was also demonstrated.     

Time of flight experimental studies of CdZnTe radiation detectors

A time of flight technique was used to study the carrier trapping time, τ, and mobility, μ, in CdZnTe (CZT) and CdTe radiation detectors. Carriers were generated near the surface of the detector by a nitrogen-pumped pulsed dye laser with wavelength ∼500 nm. Signals from generated electrons or holes were measured by a fast oscilloscope and analyzed to determine the trapping time and mobility of carriers. Electron mobility was observed to change with temperature from 1200 cm²/Vs to 2400 cm²/Vs between 293 K and 138 K, respectively. Electron mobilities were observed between 900 cm²/Vs and 1350 cm²/Vs at room temperature for various CZT detectors. Electron mobilities in various CdTe detectors at room temperature were observed between 740 cm²/Vs and 1260 cm²/Vs. Average electron mobility was calculated to be 1120 cm²/Vs and 945 cm²/Vs for CZT and CdTe, respectively. Hole mobilities in both CZT and CdTe were found to vary between 27 cm²/Vs and 66 cm²/Vs. Electron trapping times in CZT at room temperature varied from 1.60 μs to 4.18 μs with an average value of about 2.5 μs. Electron trapping time in CdTe at room temperature varied between 1.7 μs and 4.15 μs with an average value of about 3.1 μs.     

Depth Profiling of Electronic Transport Parameters in n-on-p Boron-Ion-Implanted Vacancy-Doped HgCdTe

We report results of a detailed study of electronic transport in n-on-p junctions formed by 150-keV boron-ion implantation in vacancy-doped p-type Hg_0.769Cd_0.231Te without postimplantation thermal annealing. A mobility spectrum analysis methodology in conjunction with a wet chemical etching-based surface removal approach has been employed to depth profile the transport characteristics of the samples. In the as-implanted samples, three distinct electron species were detected which are shown to be associated with (a) low-mobility electrons in the top 220-nm surface-damaged layer (E ₁: μ _80K = 2940 cm²/Vs), (b) the B-ion implantation region in the top 500-nm region (E ₂: μ _80K = 7490 cm²/Vs), and (c) high-mobility electrons in the n-to-p transition region at a depth of 600 nm to 700 nm (E ₃: μ _80K = 25,640 cm²/Vs). Due to the maximum magnetic field employed (2 T), hole carriers from the underlying vacancy-doped p-type region were detected only after the removal of the top 220 nm of the profiled sample (μ _80K = 126 cm²/Vs), revealing fully p-type character 800 nm below the original sample surface. A comparison of the extracted E ₂ electron concentration and calculated B-impurity profile suggests that the n-type region is due primarily to near-surface implantation-induced lattice damage.     

A controlled radiation source and electron/X-ray flux ratios in an electron beam metal evaporator

The use of ionizing radiation (X-rays, electrons and ions) in semiconductor processes is becoming more pervasive as device dimensions decrease. One such source of ionizing radiation is an electron beam (EB) metal evaporator. It has, in fact, been used earlier as a mixed X-ray/electron source to simulate ionizing radiation processes in device fab-rication sequences. In those studies, it was not known, however, what fraction of the energy striking a specimen was due to electrons, and what fraction was due to X-rays. In the present paper, application of an electron beam evaporator as a controlled, essen-tially monochromatic ionizing radiation source is described. Using a 0.5 mil thick Be foil, and knowing its mass absorption coefficient for X-rays at the wavelengths involved, the percentage electron and X-ray fluxes as a function of hearth beam current for a set of accelerating voltages was estimated. In addition, the absorption coefficient of an in-expensive, expendable, polymeric foil (pellicle) used in place of Be for actual experi-mental studies was evaluated. The 2.85 μm thick pellicle was found to transmit 87% of the incident Al K_α radiation, and to exhibit a mass absorption coefficient of 303 cm²/ g. The electron flux percentage from an aluminum hearth at a distance of 205 mm, was found to be 26% for a range of hearth electron beam currents between 2.5 x 10^-2A and 7.5 x 10^-2A, at an accelerating voltage of 6 kV. For a 10 kV accelerating voltage the electron percentage was found to be 35% between 2.5 x 10^-2A and 7.5 x 10^-2A. X-ray fractions were 74% and 65%, respectively. The radiation system can be used to pro-vide exposures in the 5 x 10⁴ rad(SiO₂) to the 2 x 10⁸ rad(SiO₂) range for Insulated Gate Field Effect transistors, in about an hour-long experiment.