Characterization of GaxIn1−xAs grown with TMIn

High quality Ga_xIn_1−xAs, lattice matched to InP, has been reproducibly grown by organometallic vapor phase epitaxy using trimethylgallium (TMGa), trimethylindium (TMIn), and AsH₃ in an atmospheric pressure reactor with no observable adduct formation. For the first time, using TMIn, room temperature electron mobilities of 10⁴ cm²/Vs and 77 K mobilities greater than 4 × 10⁴ cm²/Vs have beep obtained. Residual donor doping densities in the low 10¹⁵ cm⁻³ range have been routinely obtained. Material with excellent morphology has been grown from 540 to 670 C with the highest quality material being obtained near 650 C. The 4 K photoluminescence (PL) peak due to carbon is not seen in the material grown at higher temperatures; however, it increases dramatically as the growth temperature is lowered. This increased carbon incorporation leads to a sharp drop in the electron mobility, which exhibits a T^−0.5 behavior between 77 and 300 K. With optimum growth conditions, 4 K PL halfwidths of 4–5 meV are commonly observed. This high quality material is characterized by x-ray diffraction, PL, and Hall mobility measurements. Carbon and other impurity incorporation as a function of the growth parameters will be described.     

The influence of OMVPE growth pressure on the morphology, compensation, and doping of GaN and related alloys

Growth pressure has a dramatic influence on the grain size, transport characteristics, optical recombination processes, and alloy composition of GaN and AlGaN films. We report on systematic studies which have been performed in a close spaced showerhead reactor and a vertical quartz tube reactor, which demonstrate increased grain size with increased growth pressure. Data suggesting the compensating nature of grain boundaries in GaN films is presented, and the impact of grain size on high mobility silicon-doped GaN and highly resistive unintentionally doped GaN films is discussed. We detail the influence of pressure on AlGaN film growth, and show how AlGaN must be grown at pressures which are lower than those used for the growth of optimized GaN films. By controlling growth pressure, we have grown high electron mobility transistor (HEMT) device structures having highly resistive (10⁵ Ω-cm) isolation layers, room temperature sheet carrier concentrations of 1.2×10¹³ cm⁻² and mobilities of 1500 cm²/Vs, and reduced trapping effects in fabricated devices.     

Status of the MBE technology at leti LIR for the manufacturing of HgCdTe focal plane arrays

This paper presents recent developments that have been made in Leti Infrared Laboratory in the field of molecular beam epitaxy (MBE) growth and fabrication of medium wavelength and long wavelength infrared (MWIR and LWIR) HgCdTe devices. The techniques that lead to growth temperature and flux control are presented. Run to run composition reproducibility is investigated on runs of more than 15 consecutively grown layers. Etch pit density in the low 10⁵ cm⁻² and void density lower than 10³ cm⁻² are obtained routinely on CdZnTe substrates. The samples exhibit low n-type carrier concentration in the 10¹⁴ to 10¹⁵ cm⁻³ range and mobility in excess of 10⁵ cm²/Vs at 77 K for epilayers with 9.5 μm cut-off wavelength. LWIR diodes, fabricated with an-on-p homojunction process present dynamic resistance area products which reach values of 8 10³ Ωcm² for a biased voltage of −50 mV and a cutoff wavelength of 9.5 μm at 77 K. A 320 × 240 plane array with a 30 μm pitch operating at 77 K in the MWIR range has been developed using HgCdTe and CdTe layers MBE grown on a Germanium substrate. Mean NEDT value of 8.8 mK together with an operability of 99.94% is obtained. We fabricated MWIR two-color detectors by the superposition of layers of HgCdTe with different compositions and a mixed MESA and planar technology. These detectors are spatially coherent and can be independently addressed. Current voltage curves of 60 × 60 μm² photodiodes have breakdown voltage exceeding 800 mV for each diode. The cutoff wavelength at 77 K is 3.1 μm for the MWIR-1 and 5 μm for the MWIR-2.     

Transport properties of undoped Cd0.9Zn0.1Te grown by high pressure bridgman technique

The electron drift mobility of undoped Cd_0.9Zn_0.1Te grown by high-pressure Bridgman method is measured by a time-of-flight technique. The sample shows a room temperature mobility and mobility lifetime product of 950 cm²/Vs and 1.6 × 10⁻⁴cm²/V, respectively. The mobility increases monotonically with decreasing temperature to 3000 cm²/Vs at 100 K. The dominant scattering mechanism for the electron transport is discussed by comparing with the theoretical mobility obtained by iterative solution of the Boltzmann equation.     

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).     

On the intrinsic limits of pentacene field-effect transistors

Performance limits for pentacene based field-effect transistors are investigated using single- and polycrystalline devices. Whereas the charge transport in single crystalline devices is band-like with mobilities up to 10⁵ cm²/V s at low temperatures, temperature-independent or thermally activated charge transport can be observed in polycrystalline thin film transistors depending on the growth conditions. Trapping and grain boundary effects significantly influence the temperature dependence of the field-effect mobility. Furthermore, the device performance of p-channel transistors (mobility, on/off ratio, sub-threshold swing) decreases slightly with increasing trap densities. However, the formation of an electron accumulation layer (n-channel) is significantly stronger affected by trapping processes in the thin film devices. Single crystalline p-channel devices exhibit at room temperature mobilities as high as 3.2 cm²/V s, on/off-ratios exceeding 10⁹, and sub-threshold swings as low as 60 mV/decade. Slightly diminished values are obtained for transistors working as n-channel devices (2 cm²/V s, 10⁸, and 150 mV/decade).     

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.     

Characterization of Cd1−xZnxTe crystals grown from a modified vertical bridgman technique

Cd_1−xZn_xTe (CZT) crystals grown from a modified vertical Bridgman technique were characterized by means of an optical polarized transmission technique using the Pockels effect, low-temperature direct current (DC) photo-conductivity technique, low-temperature photoluminescence (PL) spectroscopy, room-temperature PL mapping technique, and detector performance measurements. Electric field mapping indicates that an approximation of a uniform electric field distribution approximation is generally satisfied for CZT detectors operated at room temperature under typical working conditions. A nonuniform electric field distribution is observed under intense infrared (IR) light illumination, and a model is proposed based on charge generation of defects, trapping, and space-charge effects. The largest hole mobility-lifetime product (μτ)_h of the CZT detector measured by DC photoconductivity is 7.0 × 10⁻⁴ cm²/V. The detector treated with 2% bromine in methanol chemical etch has a relatively small surface recombination velocity at room temperature, which was obtained from DC photocurrent and detector performance tests, as measured by irradiation of 5.5-MeV α particles and 59.6-keV γ-rays, respectively. We have clearly shown the equivalence of charge collection efficiency results measured by both DC photocurrent and α particle response. Low-temperature DC photocurrent measurements show that surface recombination velocity increases significantly with decreasing temperature from 300 K to 250 K. The effective electron mobility-lifetime product—combination effects of bulk and surface of CZT crystal—increases with increment of temperature. Room-temperature PL mapping measurements indicate uniformity of zinc concentration within CZT crystals. Low-temperature PL spectroscopy shows that the dominant emission peaks are excitons, which are bound to either shallow neutral donors (D⁰, X) or neutral acceptors (A⁰, X), depending on the temperature, concentration of donors and acceptors, and the incident light intensity. It was found that the luminescence of (D⁰, X) depends linearly on the incident laser intensity, while (A⁰, X) has a nonlinear dependence.     

Comparison of equilibrium and nonequilibrium charge carrier mobilities in polycrystalline synthetic diamond and amorphous diamond-like carbon films

Nonequilibrium charge carrier mobilities of 10⁻³ and 3×10⁻⁸ cm²/V · s in films of polycrystalline diamond and diamondlike carbon, respectively, are determined from the time of flight of injected carriers through the films. In polycrystalline diamond the nonequilibrium hole mobility is 3 orders of magnitude lower than the equilibrium mobility (0.1–1 cm²/V · s) obtained from the dc electrical conductivity of the films. This can be explained by the effect of trapping of nonequilibrium carriers on their transport. Fiz. Tekh. Poluprovodn. 31, 1142–1145 (September 1997)     

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.     

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.     

Material inhomogeneities in Cd1-xZnxTe and their effects on large volume gamma-ray detectors

Cadmium zinc telluride (Cd_1−x Zn_xTe or CZT) has shown great promise as a material for room temperature x-ray and gamma-ray detectors. In particular, polycrystalline material grown by the high pressure Bridgman method with nominal Zn fraction (x) from 0.1 to 0.2 has been used to fabricate high resolution gamma-ray spectrometers with resolution approaching that of cooled high-purity Ge. For increased sensitivity, large areas (> 1 cm²) are required, and for good sensitivity to high energy gamma photons, thick detectors (on the order of 1 cm) are required. Thus, there has been a push for the development of CZT detectors with a volume greater than 1 cm³. However, nonuniformities in the material over this scale degrade the performance of the detectors. Variations in the zinc fraction, and thus the bandgap, and changes in the impurity distributions, both of which arise from the selective segregation of elements during crystal growth, result in spectral distortions. In this work, several materials characterization techniques were combined with detector evaluations to determine the materials properties limiting detector performance. Materials measurements were performed on detectors found to have differing performance. Measurements conducted include infrared transmission, particle induced x-ray emission, photoluminescence, and triaxial x-ray diffraction. To varying degrees, these measurements reveal that “poor-performance” detectors exhibit higher nonuniformities than “spectrometer-grade” detectors. This is reasonable, as regions of CZT material with different properties will give different localized spectral responses, which combine to result in a degraded spectrum for the total device.     

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.     

High-Mobility Ga0.47ln0.53As/lnP heterostructure by atmospheric-pressure MOVPE using cyclopentadienyl indium

Lattice-matched Ga_0.47ln_0.53As/InP heterostructure was grown by atmosphericpressure metalorganic vapor phase epitaxy reaction system using monovalent cyclopentadienyl indium. The lattice-matched heterostructure showed electron mobilities ofμ_300K= 12700 cm²/Vs at n₈= 4.2 x 10¹¹ cm^-2 and μ_77K= 108000 cm²/Vs at n₈ = 3.9 x 10¹¹ cm^-2. The uniformity in electrical properties was measured by Hall element array with 400 μm pitch. Coefficient of variation in electron mobility was 0.18%.     

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     

Effect of copper and chlorine on the properties of SiO2 encapsulated polycrystalline CdSe films

The effects of different copper doping concentrations on the properties of SiO₂ encapsulated CdSe films have been investigated. Two methods were used to dope the films with copper: ion implantation and diffusion from a surface layer. The room temperature dark resistivity of films annealed in oxygen at 450°C was found to increase as the copper concentration was increased until a maximum resistivity of 10⁸ ohm cm occurred at a copper concentration of 10²⁰ atoms cm⁻³. The room temperature resistivity in the light was found to be independent of the copper concentration and whether the films were annealed in argon or oxygen. During annealing the grains grew from 0.03 μm to 0.3 μm and this growth was independent of the doping or the annealing ambient. The energy levels, carrier mobilities, and microstructure of the annealed films were dependent on the method of doping. The ion implanted films had an additional energy level at 0.33 eV and their mobility was a factor of 4 smaller than films doped by the surface diffusion method, whose mobilities were 20 to 35 cm²V⁻¹ s⁻¹. The addition of chlorine to copper doped films had no effect on either the resistivity or photosensitivity but slowed the response times of the photocurrent by a factor of 10. No energy levels were observed which could be associated with the copper nor was the copper found to affect the density of the observed intrinsic levels at 0.65 and 1.1 eV.     

Epitaxial growth and characterization of DyP/GaAs, DyAs/GaAs, and GaAs/DyP/GaAs heterostructures

There is a significant interest in the area of improving high temperature stable contacts to III-V semiconductors. Two attractive material systems that offer promise in this area are dysprosium phosphide/gallium arsenide (DyP/GaAs) and dysprosium arsenide/gallium arsenide (DyAs/GaAs). Details of epitaxial growth of DyP/GaAs and DyAs/GaAs by molecular beam epitaxy (MBE), and their characterization by x-ray diffraction, transmission electron microscopy, atomic force microscopy, Auger electron spectroscopy, Hall measurements, and high temperature current-voltage measurements is reported. DyP is lattice matched to GaAs, with a room temperature mismatch of less than 0.01% and is stable in air with no sign of oxidation, even after months of ambient exposure. Both DyP and DyAs have been grown by solid source MBE using custom designed group V thermal cracker cells and group III high temperature effusion cells. High quality DyP and DyAs epilayer were consistently obtained for growth temperatures ranging from 500 to 600°C with growth rates between 0.5 and 0.7 μm/h. DyP epilayers are n-type with electron concentrations of 3 × 10²⁰ to 4 × 10²⁰ cm⁻³, room temperature mobilities of 250 to 300 cm²/V·s, and a barrier height of 0.81 eV to GaAs. DyAs epilayers are also n-type with carrier concentrations of 1 × 10²¹ to 2 × 10²¹ cm⁻³, and mobilities between 25 and 40 cm²/V·s.     

OMVPE growth of InP and Ga0.47ln0.53as using ethyldimethylindium

We report the organometallic vapor phase epitaxial (OMVPE) growth of InP and Ga_0.47In_0.53As using a new organometallic indium source, ethyldimethylindium (EDMIn), rather than the traditional sources triethylindium (TEIn) or trimethylindium (TMIn). EDMIn is a liquid at room temperature and its vapor pressure at 17° C was found to be 0.85 Torr using thermal decomposition experiments. The growth results using EDMIn were compared to those using TMIn in the same atmospheric pressure reactor. For InP, use of EDMIn resulted in a high growth efficiency of 1.3 × 10⁴ μm/ mole, which was independent of the growth temperature and comparable to the growth efficiency obtained with TMIn. The high growth efficiency is consistent with the observation of no visible parasitic gas phase reactions upstream of the substrate. The 4K photoluminescence (PL) spectra consist of a peak due to bound excitons and an impurity related peak 38 meV lower in energy. This impurity peak is ascribed to conduction band to acceptor transitions from carbon, due to the decreasing relative intensity of this peak with increasing V/III ratio. The relative intensity of the C impurity peak decreases by five times when the growth temperature is increased from 575 to 675° C, with a corresponding increase in the room temperature electron mobility from 725 to 3875 cm²/ Vs. For GalnAs lattice-matched to InP, use of EDMIn also resulted in a temperatureindependent high growth efficiency of 1.0 x 10⁴ μm/mole, indicating negligible parasitic reactions with AsH₃. The In distribution coefficient was nearly constant at a value of 0.9, however the run to run composition variation was slightly higher for EDMIn than for TMIn. The 4K PL showed donor-acceptor pair transitions due to C and Zn. The C impurity peak intensity decreased dramatically with increasing growth temperature, accompanied by an increase in the room temperature electron mobility to 5200 cm²/Vs. Overall, the growth of both InP and GalnAs using EDMIn was qualitatively similar to that using TMIn, although the room temperature electron mobilities were lower for the new source than for our highest purity bottle of TMIn.     

High purity InxGa1−xSb single crystals with cutoff wavelength of 7–8 µm grown by melt epitaxy

For the first time, InGaSb single crystals with a cutoff wavelength of 7–8 μm were successfully grown on GaAs substrates by a new growth technique named melt epitaxy. The band gap of InGaSb layers obviously narrowed compared with those with the same compositions grown by ordinary methods and the longest cutoff wavelength reached 8.3 μm. High electron mobility of 8.05×10⁴ cm²/Vs and low carrier density of 1×10¹⁵ cm⁻³ at 77 K were obtained indicating high purity of InGaSb epilayers.     

Development of nuclear radiation detectors based on epitaxially grown thick CdTe layers on n<Superscript>+</Superscript>-GaAs substrates

A CdTe/n⁺-GaAs heterojunction diode for a room-temperature nuclear radiation detector has been developed and demonstrated. The heterojunction diode was fabricated by growing a 2–5-μm-thick iodine-doped n-CdTe buffer layer on the n⁺-GaAs substrates, followed by about 100-μm-thick undoped p-like single crystalline CdTe layer using metalorganic vapor-phase epitaxy. The n-type buffer layer was found to be essential to improve the junction property of the diode detector. The diode detectors exhibited good rectification property and had the reverse leakage currents typically from 1 μA/cm² to 5 μA/cm² at 40 V bias. The detector clearly demonstrated its energy resolution capability by resolving the 59.54-keV gamma peak from the ²⁴¹Am radioisotope during the radiation detection test.