Ultraviolet-visible metal-semiconductor-metal photodetectors fabricated from InxGa1−xN (0≤x≤0.13)

Metal-semiconductor-metal (MSM) photodetectors have been fabricated on In_xGa_1−xN epitaxial films grown by metalorganic chemical vapor deposition within the composition range 0≤x≤0.13. The dark current and spectral response were measured for devices with a varying In mole fraction x. The devices, which had nominal finger widths and finger spacing of 5 μm, were biased with modest voltages in the range 2≤V_bias≤5 V. In general, turn-on wave-length and dark current increased with increasing x. Turn-on wavelengths ranged from λ=370 nm to 430 nm and dark current densities ranged from I_dark=2×10⁻² A/cm² (V_bias=5 V, x≈0.05) to 9×10⁴ A/cm² (V_bias=2 V, x≈0.13) depending on the In content, x, of the device active area.     

P-N junction observations on Pb-salt diode lasers using an ebic-mode SEM

A scanning electron microscope (SEM) operating in the elec-tron- beam induced current (EBIC) mode has been used to determine the depths and uniformities of p- n junctions in a variety of Pb- salt diode lasers. Pb- salt materials with 20K band gap values ranging from 44 meV to 450 meV have been investigated. This includes the ternary alloy systems Pb_1−xCd_xS(with 0 ≤ x ≤ 0.048), PbS_1-xSe_x (with 0 ≤ x ≤ l), Pb_1−xSn_xSe (with O ≤ x ≤ O.10),and Pb_1-xSn_xTe (with x = 0.25). With typical carrier concentrations in the 10¹⁸ – 10¹⁹ cm^{− 3} range, the junction depths were found to be independent of temperature between 77K and 300K. Small band gap devices exhibited EBIC signals strongly dependent on T, while devices with band gap values above 130 meV at 77K exhibited relatively little such temperature dependence to 300K. This non- destructive technique is capable of providing junction depth and uniformity information of all the Pb-salt materials, and is useful for estimating minority car-rier diffusion lengths at various temperatures. The method is also useful for investigating some of the more complex confinement heterostructures in these materials.     

Damage constant and deep-level transient spectroscopy in neutron irradiated GaAsP alloys

Damage produced in VPE GaAs_{1− x P x} . alloys by fast neutron irradiation at room temperature was studied, in light emitting diodes, through the evolution of device carrier lifetime, photoluminescence, electroluminescence and transient capacitance spectroscopy characteristics. Neutron fluxes were in the 10¹³−10¹⁴ neutrons/cm² range so as not to heavily damage the devices. Damage constants are 10⁻⁵ to 10⁻⁶ cm²/s for 0.3 ≤x ≤ 1. The carrier removal rate was ≈; 10 cm⁻¹. Deep-level transient spectroscopy in n-type layers revealed that fast neutrons created a broad center atE _c − 0.7 eV, and at a ≈;1 cm⁻¹ generation rate. For thex ≥ 0.4 composition range studied, trap characteristics and introduction rates were rather independent ofx. From photocapacitance quenching measurements it is suggested that the neutron generated centers are EL2-related.     

Nonalloyed Al ohmic contacts to MgxZn1−xO

Al nonalloyed ohmic contacts were fabricated and characterized on Mg_xZn_1−xO (0≤×≤0.34) epilayers, which were grown on R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). Specific contact resistances were evaluated by the transmission line method (TLM). A specific contact resistance of 2.5×10⁻⁵ Ωcm² was obtained for Al contact to ZnO with an electron concentration of 1.6×10¹⁷ cm⁻³. The current flow mechanism was studied by investigating the dependence of specific contact resistances on electron concentration and on temperature. For Al contact to Mg_0.34Zn_0.66O, specific contact resistance values are two orders of magnitude larger than that of Al ohmic contacts to ZnO.     

Molecular beam epitaxial HgCdTe material characteristics and device performance: Reproducibility status

Extensive material, device, and focal plane array (FPA) reproducibility data are presented to demonstrate significant advances made in the molecular beam epitaxial (MBE) HgCdTe technology. Excellent control of the composition, growth rate, layer thickness, doping concentration, dislocation density, and transport characteristics has been demonstrated. A change in the bandgap is readily achieved by adjusting the beam fluxes, demonstrating the flexibility of MBE in responding to the needs of infrared detection applications in various spectral bands. High performance of photodiodes fabricated on MBE HgCdTe layers reflects on the overall quality of the grown material. The photodiodes were planar p-on-n junctions fabricated by As ion-implantation into indium doped, n-type, in situ grown double layer heterostructures. At 77K, diodes fabricated on MBE Hg_1−xCd_xTe with x ≈ 0.30 (λ_co ≈ 5.6 μm), x ≈ 0.26 (λ_co ≈ 7 μm), x ≈ 0.23 (λ_co ≈ 10 μm) show R₀A products in excess of 1 x 10⁶ ohm-cm², 7 x 10⁵ ohm-cm², and 3 x 10² ohm-cm², respectively. These devices also show high quantum efficiency. As a means to assess the uniformity of the MBE HgCdTe material, two-dimensional 64 x 64 and 128 x 128 mosaic detector arrays were hybridized to Si multiplexers. These focal plane arrays show an operability as high as 97% at 77K for the x ≈ 0.23 spectral band and 93% at 77K for the x ≈ 0.26 spectral band. The operability is limited partly by the density of void-type defects that are present in the MBE grown layers and are easily identified under an optical microscope.     

Characterization of Hg0.7Cd0.3Te n- on p-type structures obtained by reactive ion etching induced p- to n conversion

This paper presents transport measurements on both vacancy doped and gold doped Hg_0.7Cd_0.3Te p-type epilayers grown by liquid phase epitaxy (LPE), with N_A=2×10¹⁶ cm⁻³, in which a thin 2 μm surface layer has been converted to n-type by a short reactive ion etching (RIE) process. Hall and resistivity measurements were performed on the n-on-p structures in van der Pauw configuration for the temperature range from 30 K to 400 K and magnetic field range up to 12 T. The experimental Hall coefficient and resistivity data has been analyzed using the quantitative mobility spectrum analysis procedure to extract the transport properties of each individual carrier contributing to the total conduction process. In both samples three distinct carrier species have been identified. For 77 K, the individual carrier species exhibited the following properties for the vacancy and Au-doped samples, respectively, holes associated with the unconverted p-type epilayer with p ≈ 2 × 10¹⁶ cm⁻³, μ ≈ 350 cm²V⁻¹s⁻¹, and p ≈ 6 × 10¹⁵ cm⁻³, μ ≈ 400 cm²V⁻¹s⁻¹; bulk electrons associated with the RIE converted region with n ≈ 3 × 10¹⁵cm⁻³, μ ≈ 4 × 10⁴ cm²V⁻¹s⁻¹, and n ≈ 1.5 × 10¹⁵ cm⁻³, μ ≈ 6 × 10⁴ cm²V⁻¹s⁻¹; and surface electrons (2D concentration) n ≈ 9 × 10¹² cm⁻² and n ≈ 1 × 10¹³ cm⁻², with mobility in the range 1.5 × 10³ cm²V⁻¹s⁻¹ to 1.5 × 10⁴ cm²V⁻¹s⁻¹ in both samples. The high mobility of bulk electrons in the RIE converted n-layer indicates that a diffusion process rather than damage induced conversion is responsible for the p-to-n conversion deep in the bulk. On the other hand, these results indicate that the surface electron mobility is affected by RIE induced damage in a very thin layer at the HgCdTe surface.     

Magnetoresistance and magnetization studies of the layered magnanite system La<Subscript>1.2</Subscript>Ca<Subscript>1.8</Subscript>Mn<Subscript>2−x</Subscript>Ru<Subscript>x</Subscript>O<Subscript>7</Subscript> (0 ≤ x ≤ 1)

We report here the results of magnetotransport and electrical resistivity (ρ) measurements in the temperature range of 4.2–320 K and in the presence of magnetic fields up to 10 T on the Ru-doped, bilayered manganite system, La_1.2Ca_1.8Mn_2−xRu_xO₇ (0≤x≤1). We find that the Ru doping affects the magnetotransport properties considerably. The ρ versus H data were analyzed by fitting the data to the power-law equation, ρ = ρ₀ − αHⁿ. The isothermal magnetoresistance (MR) versus H curves taken up to ± 10 T are highly symmetrical, and their curvature changes from concave up to concave down as the temperature increases. The MR, defined as [ρ(H) − ρ(0)]/ρ(0), is found to increase with Ru doping from 58% to 64% up to x=0.1 and to decrease to 45% for the x=1 sample at 10 K. Analysis of the ρ-T data below 30 K shows that, at low temperature, the system behaves like a disordered metal.     

Molecular beam epitaxy of CdSe and the derivative alloys Zn1−x Cd x Se and Cd1−x M x Se

We report the epitaxial growth of CdSe, Zn_1−x Cd _x Se (0 ≤x ≤ 1) and Cd_1−x Mn _x Se (0 ≤x ≤ 0.8) on (100) GaAs. X-ray diffraction (XRD), electron diffraction and transmission electron microscopy (TEM) indicate that all the epilayers have the cubic (zinc-blende) structure of the GaAs substrate. The energy gaps of these materials were measured using reflectivity measurements. We also report the growth of ZnSe/Zn_1−x Cd _x Se superlattices. TEM and XRD measurements show that high quality modulated structures with sharp interfaces are possible.     

Molecular beam epitaxy and characterization of Al x Ga1-x As y Sb1-y (0.0 ≤x ≤ 1.0) lattice matched to InAs substrates

We report the molecular beam epitaxial growth of Al _x Ga_1-x As _y Sb_1-y (0.0 ≤ x ≤ 1.0) on undoped, liquid-encapsulated Czochralski, (100) oriented InAs substrates. The degree of lattice mismatch was determined by x-ray diffraction. The lattice matched materials (y ≈ 0.08 + 0.08x) were characterized by low temperature photoluminescence, electro-reflectance, and capacitance-voltage measurements. The experimental bandgap energies agree with earlier experimental results for Al _x Ga_1-x Sb, and also with a self-consistent first principles pseudopotential model. The capacitance-voltage measurements indicate background acceptor concentrations for the unintentionally-doped epitaxial layers of about 2 × 10¹⁵ cm^-3 atx = 1.0 to 5 × 10¹⁶ cm^-3 atx ≈ 0.0.     

An optical alternative to the hall test

Effective mass ratios, m*, of electrons in n-type InSb, GaAs, and near intrinsic and n-type Hg_1−xCd_xTe for 0.20 < × < 0.30 over the temperature range 77K < T < 296K were measured using Faraday rotation spectroscopy. m* ranged from 0.0186 to 0.0357 for InSb with carrier concentrations, N, in the range 1.76 < N < 110 × 10¹⁶ cm⁻³ at 296K, in good agreement with available values in the literature. Effective masses of HgCdTe were found to be about twice as large at room temperature as band edge effective mass, m*_be calculations. These calculations can be corrected for thermal excitation by adding a factor, m**, to the band edge calculation: m* = m** m*_be, where m** was found empirically to be m** = 4.52 × 10₋₃T + 0.78. The electron’s mobility is proportional to the ratio of the electron’s Faraday rotation to its absorption; that is, the absorption due to the intraband transitions of the electron itself, not the sample’s total absorption, which may include holes, interband transitions, and the like. The constant of proportionality, or the “mobility constant”, was measured in n-type GaAs and InSb doped above 18 × 10¹⁶ cm⁻³ using absorption directly. Both HgCdTe and InSb have large intrinsic carrier concentrations, on the order of 10¹⁶ cm⁻³. Hole absorption is the majority component of the sample’s absorption at lower n-type dopant concentrations. In these cases, the mobility constant was determined using an absorption cross section.     

Microstructural analysis of a Au/Pt/Pd/Zn ohmic contact to an AlGaAs/GaAs heterojunction bipolar transistor

Gold-based ohmic contacts, incorporating Pt, Pd, and Zn layers, to AIGaAs/GaAs heterojunction bipolar transistors (HBTs) have been characterized using transmission electron microscopy (TEM). The metallization was deposited onto a 30 nm graded emitter layer of n-type Al_xGa_1−xAs, which was on a 30 nm emitter layer of n-type Al_0.3Ga_0.7As, with the aim of contacting the underlying 80 nm thick graded base layer of p-type Al_xGa_1−xAs. Metal layers were deposited sequentially using electron beam evaporation and the resultant metallizations were annealed at temperatures ranging from 250-500°C for up to several minutes. A minimum contact resistance of ≈8.5 × 10⁻⁷ Ω-cm² was achieved, which corresponded to the decomposition of ternary phases at the metallization/semiconductor interface, to binary phases, i.e., PdGa and PtAs₂. Long term stability tests were done on the optimum contacts. Anneals at 270°C for up to four weeks in duration produced virtually no change in microstructure, with the exception of some outward diffusion of Ga and As.     

Electrical properties of indium-doped LPE layers of Pb1−xSnx Te

Liquid-phase epitaxial (LPE) layers of Pb_1−xSn_xTe with an alloy composition 0≤×≤0.25 were doped n-type by adding from 0.002 to 10 at.% indium to the growth solution. Doping characteristics of indium and electrical properties of the epilayers at 77 and 4.2K were studied by Hall and resistivity measurements made directly on the grown layers. Electron concentration and mobility at 77 and 4.2K are presented as a function of indium doping for various x values. Doping coefficients of ~0.05 and ~0.03 are found for PbTe and Pb_0.8Sn_0.2Te, respectively, grown at ~450°C. For medium to high indium doping, the electron concentration saturates to a constant value independent of doping and LPE growth temperature. The saturation values decrease substantially with increasing x and increase with a decrease in sample temperature. Bulklike mobilities practically independent of doping are recorded up to an indium concentration N_ln~0.3 at.%, above which the mobility decreases with increasing indium concentration. The data shows that indium is a suitable donor in liquid-phase epitaxial layers of Pb_l-XSn_xTe.     

Electron mobility in the X conduction band minima of Ga1−xAlxAs alloys

Hall Mobility of the electrons in the indirect energy X conduction band minima of Ga_{1− x}Al_xAs alloys (0.23≤ x≤ 0.78) has been measured as a function of temperature (T≤ 300K). For alloys (0.23≤ x≤ 0.32) investigated with the direct energy band gap, the band structure has been inverted with the application of the hydrostatic pressure to make the X states considerably lower in energy than the next higher energy minima. For indirect energy- gap alloys (0.61≤ x ≤ 0.78) investigated, the mobility has been measured at atmospheric pressure. The acoustic deformation potential, longitudinal optical phenon temperature and the inter-valley deformation potential field for the X minima have been derived from the experimental results, and are found to increase with the alloy composition. A single valley density of states mass with best value of 0.35 m_o with three equivalent X minima situated at the zone edge, are required to fit the data. It has been concluded that the various parameters involved in the alloy and space charge scatterings in the alloys, except the electros mass, do not change with pressure and that the intervalley scatter-ing plays an important role in limiting the electron mobility in. the alloys at a pressure and composition near the Γ- L cross- over.     

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.     

Dependence of photoluminescence spectra of epitaxial Pb1 ? xEuxTe (0 ≤ x ≤ 0.1) alloy layers on conditions of growth

The photoluminescence spectra of the Pb_{1 − x} Eu _x Te (0 ≤ x ≤ 0.1) alloy are studied at low temperatures. Epitaxial layers were grown by molecular-beam epitaxy at different temperatures. Along with the basic line corresponding to interband radiative recombination, additional emission lines are observed in the low-energy region of the spectra. Some nonuniformities are observed at the sample surface (within an area smaller than 1% of the total surface area). The concentration and size (1–10 μm) of the nonuniformities decrease with increasing temperature of growth. The additional emission lines are attributed to local nonuniformities in the layer. The dependence of the band gap of the Pb_1−x Eu _x Te (0 ≤ x ≤ 0.1) alloy on the composition parameter x is determined at 77.4 K. The dependence is nonlinear and adequately described by the relation E _g [eV] = 0.213 + 4.8x − 18.4x ².     

Characterization of Zn1?xMgxO Films Prepared by the Sol–Gel Process and Their Application for Thin-Film Transistors

Transparent semiconductor thin films of Zn_1−x Mg _x O (0 ≤ x ≤ 0.36) were prepared using a sol–gel process; the crystallinity levels, microstructures, and optical properties affected by Mg content were studied. The experimental results showed that addition of Mg species in ZnO films markedly decreased the surface roughness and improved transparency in the visible range. A Zn_1−x Mg _x O film with an x-value of 0.2 exhibited the best average transmittance, namely 93.7%, and a root-mean-square (RMS) roughness of 1.63 nm. Therefore, thin-film transistors (TFTs) with a Zn_0.8Mg_0.2O active channel layer were fabricated and found to have n-type enhancement mode. The Zn_0.8Mg_0.2O TFT had a field-effect mobility of 0.1 cm²/V s, threshold voltage of 6.0 V, and drain current on/off ratio of more than 10⁷.     

Acceptor and donor doping of AlxGa1−xN thin film alloys grown on 6H-SiC(0001) substrates via metalorganic vapor phase epitaxy

Thin films of Si-doped Al_xGa_1−xN (0.03≤x≤0.58) having smooth surfaces and strong near-band edge cathodoluminescence were deposited at 0.35–0.5 μm/h on on-axis 6H-SiC(0001) substrates at 1100°C using a 0.1 μm AlN buffer layer for electrical isolation. Alloy films having the compositions of Al_0.08Ga_0.92N and Al_0.48Ga_0.52N exhibited mobilities of 110 and 14 cm²/V·s at carrier concentrations of 9.6×10¹⁸ and 5.0×10¹⁷ cm⁻³, respectively. This marked change was due primarily to charge scattering as a result of the increasing Al concentration in these random alloys. Comparably doped GaN films grown under similar conditions had mobilities between 170 and ∼350 cm²/V·s. Acceptor doping of Al_xGa_1−xN for x≤0.13 was achieved for films deposited at 1100°C. No correlation between the O concentration and p-type electrical behavior was observed.     

Variable temperature hall effect on p-Hg1−xCdxTe grown on CdTe and sapphire substrates by liquid phase epitaxy

Variable temperature Hall effect measurements have been made down to 9–10K on p-type Hg_1−xCd_xTe grown by liquid phase epitaxy on both CdTe and sapphire substrates. Carrier freeze-out was usually observed throughout the measured temperature range. For most samples, the hole mobility was well-behaved and exhibited a maximum at ˜ 35K. Values of acceptor ionization energy E_A and donor concentration N_D were estimated from the data, using a model assuming significant compensation, which provided a good fit to the low temperature data. In addition, values of N_D were also estimated from an analysis of the low temperature mobility using the hole effective mass as a parameter to provide reasonable agreement between the N_D values calculated from the Hall coefficient and mobility data. The measured carrier concentration is a result of close compensation between stoichiometric acceptors and donors, with N_D usually in the low-10¹⁷ cm⁻³ range. Average values of E_A for as-grown, undoped x = 0.32 layers on CdTe and sapphire substrates are 7.4 and 6.6 meV, respectively. An activation energy of 0.84 meV was determined for a Cu-doped x = 0.32 layer that was annealed in Hg vapor to reduce the number of Hg vacancies. The average E_A for undoped Hg-annealed x = 0.22 layers on CdTe substrates is 2.35 meV. Layers with x = 0.32 grown on sapphire substrates have average carrier concentrations of 2.92 (σ = 0.54) × 10¹⁶ cm⁻³, compared with 4.64 (θ = 1.26) × 10¹⁶ cm⁻³ for the same composition layers grown on CdTe substrates.     

Growth and characterization of OMVPE grown (In,Mn)As diluted magnetic semiconductor

In_1−xMn_xAs diluted magnetic semiconductor (DMS) thin films with x 0.14 have been grown using organometallic vapor phase epitaxy. Tricarbonyl-(methylcyclopentadienyl)manganese was successfully used as the Mn source. Single phase, epitaxial films were achieved for compositions as high as x=0.14 using growth temperatures ≥475°C. For lower growth temperatures or x>0.14, nanometer scale MnAs precipitates were observed within the In_1−xMn_xAs matrix. Transport properties were investigated using the Hall effect. All Mn doped films were p-type with single phase films exhibiting hole concentrations 2≤×10¹⁹ cm⁻³. Magnetization was measured as a function of temperature and applied field for a single phase film with x=0.1. Ferromagnetic ordering was observed at 5 K with a saturation magnetization of M_s=68 emu/cm³, a remnant magnetization, M_r=10 emu/cm³, and a coercive field H_c=400 Oe.     

Growth of (GaAs)1_x(Ge2)x by metalorganic chemical vapor deposition

We report deposition of (GaAs)_{1_x}(Ge₂)_x on GaAs substrates over the entire alloy range. Growth was performed by metalorganic chemical vapor deposition at temperatures of 675 to 750°C, at 50 and 760 Torr, using trimethylgallium, arsine, and germane at rates of 2–10 μ/h. Extrinsic doping was achieved using silane and dimethylzinc in hydrogen. Characterization methods include double-crystal x-ray rocking curve analysis, Auger electron spectroscopy, 5K photoluminescence, optical transmission spectra, Hall-effect, and Polaron profiling. Results achieved include an x-ray rocking curve full-width at half maximum as narrow as 12 arc-s, Auger compositions spanning the alloy range from x = 0.03 to x = 0.94, specular surface morphologies, and 5K photoluminescence to wavelengths as long as 1620 nm. Undoped films are n type, with n ≈ 1 × 10¹⁷ cm⁻³. Extrinsic doping with silane and dimethylzinc have resulted in films which are n type (10¹⁷ to 10¹⁸ cnr⁻³) or p type (5 × 10¹⁸ to 1 × 10²⁰ cm⁻³). Mobilities are generally ≈ 50 cm²/V-s and 500 cm²/V-s, for p and n films, respectively.