Growth condition of iodine-doped n+-CdTe layers in metal-organic vapor phase epitaxy

Iodine doping of CdTe layers grown on (100) GaAs by metal-organic vapor phase epitaxy (MOVPE) was studied using diethyltelluride (DETe) and diisopropyltelluride (DiPTe) as tellurium precursors and ethyliodine (EI) as a dopant. Electron densities of doped layers increased gradually with decreasing the growth temperature from 425°C to 325°C. Doped layers grown with DETe had higher electron densities than those grown with DiPTe. When the hot-wall temperature was increased from 200°C to 250°C at the growth temperature of 325°C, doped layers grown with DETe showed an increase of the electron density from 3.7×10¹⁶ cm⁻³ to 2.6×10¹⁸ cm⁻³. On the other hand, such an increase of the electron density was not observed for layers grown with DiPTe. The mechanisms for different doping properties for DETe and DiPTe were studied on the basis of the growth characteristics for these precursors. Higher thermal stability of DETe than that of DiPTe was considered to cause the difference of doping properties. With increasing the hot-wall temperature from 200°C to 250°C, the effective ratio of Cd to Te species on the growth surface became larger for layers grown with DETe than those grown with DiPTe. This was considered to decrease the compensation of doped iodine and to increase the electron density of layers grown with DETe. The effective ratio of Cd to Te species on the growth surface also increased with decreasing growth temperature. This was considered to increase the electron density with decreasing growth temperature.     

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.     

A comparison of ethyl iodide and hydrogen chloride for doping ZnSe grown by photoassisted MOVPE

We have conducted a study of the electrical and photoluminescence properties of ZnSe films grown by photoassisted metalorganic vapor phase epitaxy (MOVPE) (250 Torr, 400°C) with ethyl iodide and hydrogen chloride as n-type dopant sources. A higher peak electron concentration and a lower minimum resistivity were observed using hydrogen chloride (5.4 × 10¹⁸ cm⁻³, and .0070 ohm-cm, respectively), as opposed to ethyl iodide (1.55 × 10¹⁷ cm ⁻³, and 0.067 ohm-cm, respectively). We show that the higher electron concentrations observed in the chlorine doped layers are due to a higher incorporation of chlorine atoms than that of iodine atoms, and that this may be a result of the different tetrahedral misfit factors for these atoms. Our photoluminescence and 77K Hall effect data support this conclusion. Growth rate depression was observed to be more severe for iodine doped layers than for chlorine doped layers. Thus, it appears that hydrogen chloride is a superior dopant source for low-temperature photoassisted MOVPE ZnSe growth of n-type layers for blue-green laser diodes in the pressure-temperature regime investigated.     

Properties and use of cycled grown OMVPE GaAs:Zn,GaAs:Se,and GaAs:Si layers for high-conductance GaAs tunnel junctions

Heavily doped GaAs layers for high conductance GaAs tunnel junctions have been grown by atmospheric pressure organometallic vapor phase epitaxy (OMVPE) using Zn as the dopant for thep ⁺ regions and either Se or Si as the dopant for then ⁺ regions. At a growth temperature of 700° C using a “cycled” growth technique for the Zn-dopedp ⁺⁺-GaAs layer, both the conductance and the peak current density of the tunnel diode has been increased by a factor of ∼65 compared to a tunnel junction with a continuously grown Zn-doped p⁺-GaAs. The conductance of the tunnel junction, which is maximized at a growth temperature of 650° C using cycled growth, is comparable to the best reported values for tunnel junctions grown by molecular beam epitaxy. Cycled growths forn ⁺ Se-doped regions are found to reduce the conductance of a tunnel junction by more than two orders of magnitude. However, cycled growth for the n⁺-GaAs regions with Si doping show no conductance degradation. A model based on incorporation sites of these dopants during OMVPE growth of GaAs is presented to account for the experimental observations.     

Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te Alloys: Application Considerations

The search for alternative energy sources is at the forefront of applied research. In this context, thermoelectricity, i.e., direct conversion of thermal into electrical energy, plays an important role, particularly for exploitation of waste heat. Materials for such applications should exhibit thermoelectric potential and mechanical stability. PbTe-based compounds include well-known n-type and p-type compounds for thermoelectric applications in the 50°C to 600°C temperature range. This paper is concerned with the mechanical and transport properties of p-type Pb_0.5Sn_0.5Te:Te and PbTe<Na> samples, both of which have a hole concentration of ∼1 × 10²⁰ cm⁻³. The ZT values of PbTe<Na> were found to be higher than those of Pb_0.5Sn_0.5Te:Te, and they exhibited a maximal value of 0.8 compared with 0.5 for Pb_0.5Sn_0.5Te:Te at 450°C. However, the microhardness value of 49 H_V found for Pb_0.5Sn_0.5Te:Te was closer to that of the mechanically stable n-type PbTe (30 H_V) than to that of PbTe<Na> (71 H_V). Thus, although lower ZT values were obtained, from a mechanical point of view Pb_0.5Sn_0.5Te:Te is preferable over PbTe<Na> for practical applications.     

Properties of In-Doped ZnO Films Grown by Metalorganic Chemical Vapor Deposition on GaN(0001) Templates

We investigated the properties of indium-doped zinc oxide layers grown by metalorganic chemical vapor deposition on semi-insulating GaN(0001) templates. Specular and transparent films were grown with n-type carrier concentrations up to 1.82 × 10¹⁹ cm⁻³ as determined by Hall measurements, and all In-doped films had carrier concentrations significantly higher than that of a comparable undoped film. For low In flows, the carrier concentration increased accordingly with trimethyl-indium (TMIn) flow until a maximum carrier concentration of 1.82 × 10¹⁹ cm⁻³ was realized. For higher In flows, the carrier concentration decreased with increasing TMIn flow rate. Sheet resistance as low as 185 Ω/sq was achieved for the In-doped films, which is a significant decrease from that of a comparable undoped ZnO film. Our n-type doping studies show that In is an effective dopant for controlling the n-type conductivity of ZnO.     

Iodine Doping of CdTe and CdMgTe for Photovoltaic Applications

Iodine-doped CdTe and Cd_1?x Mg _x Te layers were grown by molecular beam epitaxy. Secondary ion mass spectrometry characterization was used to measure dopant concentration, while Hall measurement was used for determining carrier concentration. Photoluminescence intensity and time-resolved photoluminescence techniques were used for optical characterization. Maximum n-type carrier concentrations of 7.4 × 10¹⁸ cm^?3 for CdTe and 3 × 10¹⁷ cm^?3 for Cd_0.65Mg_0.35Te were achieved. Studies suggest that electrically active doping with iodine is limited with dopant concentration much above these values. Dopant activation of about 80% was observed in most of the CdTe samples. The estimated activation energy is about 6 meV for CdTe and the value for Cd_0.65Mg_0.35Te is about 58 meV. Iodine-doped samples exhibit long lifetimes with no evidence of photoluminescence degradation with doping as high as 2 × 10¹⁸ cm^?3, while indium shows substantial non-radiative recombination at carrier concentrations above 5 × 10¹⁶ cm^?3. Iodine was shown to be thermally stable in CdTe at temperatures up to 600°C. Results suggest iodine may be a preferred n-type dopant compared to indium in achieving heavily doped n-type CdTe.     

Electrodeposition and Thermoelectric Characteristics of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> and Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Films for Thermopile Sensor Applications

A thermopile sensor was processed on a glass substrate by electrodeposition of n-type bismuth telluride (Bi-Te) and p-type antimony telluride (Sb-Te) films. The n-type Bi-Te film electrodeposited at −50 mV in a 50 mM electrolyte with a Bi/(Bi + Te) mole ratio of 0.5 exhibited a Seebeck coefficient of −51.6 μV/K and a power factor of 7.1 × 10⁻⁴ W/K² · m. The p-type Sb-Te film electroplated at 20 mV in a 70 mM solution with an Sb/(Sb + Te) mole ratio of 0.9 exhibited a Seebeck coefficient of 52.1 μV/K and a power factor of 1.7 × 10⁻⁴ W/K² · m. A thermopile sensor composed of 196 pairs of the p-type Sb-Te and the n-type Bi-Te thin-film legs exhibited sensitivity of 7.3 mV/K.     

Flexibility of p–n Junction Formation from SWIR to LWIR Using MBE-Grown Hg(1–x)CdxTe on Si Substrates

In this paper, we show the versatility of using molecular-beam epitaxy (MBE) for the growth of the mercury cadmium telluride (HgCdTe) system. Abrupt composition profiles, changes in doping levels or switching doping types are easily performed. It is shown that high-quality material is achieved with Hg_(1–x)Cd _x Te grown by MBE from a cadmium mole fraction of x = 0.15 to x = 0.72. Doping elements incorporation as low as 10¹⁵ cm⁻³ for both n-type and p-type material as well as high incorporation levels >10¹⁸ cm⁻³ for both carrier types were achieved. X-ray curves, secondary-ion mass spectrometry (SIMS) data, Hall data, the influence of doping incorporation with cadmium content and growth rate, etch pit density (EPD), composition uniformity determined from Fourier-transform infrared (FTIR) transmission spectro- scopy, and surface defect maps from low to high x values are presented to illustrate the versatility and quality of HgCdTe material grown by MBE. All data presented in this work are from layers grown on silicon (112) substrate.     

Characterization of vapor phase epitaxial ZnSe films

Properties of ZnSe films doped with donor impurities were investigated. The ZnSe films were grown at 350°C by using metallic zinc and selenium as the source materials; their vapors were transported separately by H₂ gas under atmospheric pressure. Iodine-doped ZnSe films were grown using CH₃I (1000 ppm, diluted in helium) as a dopant source. However, it was necessary to stop this dopant flow during the film growth to obtain epitaxial films. HC1 gas etching and evacuation of the reaction apparatus before the film growth began were employed to obtain epitaxial films and to avoid redistribution of impurities without heat-treatment at higher temperature. Secondary ion mass spectroscopy analysis indicated that both chlorine and gallium were included in the layers, as well as iodine, because of residual HC1 gas. Optically high-quality and rather highly conductive n-type ZnSe films were obtained. Maximum electron concentration was 3.3 × 10¹⁷ cm⁻³.     

Determination of the Origin of Crystal Orientation for Nanocrystalline Bismuth Telluride-Based Thin Films Prepared by Use of the Flash Evaporation Method

We have investigated the origin of crystal orientation for nanocrystalline bismuth telluride-based thin films. Thin films of p-type bismuth telluride antimony (Bi–Te–Sb) and n-type bismuth telluride selenide (Bi–Te–Se) were fabricated by a flash evaporation method, with exactly the same deposition conditions except for the elemental composition of the starting powders. For p-type Bi–Te–Sb thin films the main x-ray diffraction (XRD) peaks were from the c-axis (Σ{00l}/Σ{hkl} = 0.88) whereas n-type Bi–Te–Se thin films were randomly oriented (Σ{00l}/Σ{hkl} = 0.40). Crystal orientation, crystallinity, and crystallite size were improved for both types of thin film by sintering. For p-type Bi–Te–Sb thin films, especially, high-quality structures were obtained compared with those of n-type Bi–Te–Se thin films. We also estimated the thermoelectric properties of the as-grown and sintered thin films. The power factor was enhanced by sintering; maximum values were 34.9 μW/cm K² for p-type Bi–Te–Sb thin films at a sintering temperature of 300°C and 23.9 μW/cm K² for n-type Bi–Te–Se thin films at a sintering temperature of 350°C. The exact mechanisms of film growth are not yet clear but we deduce the crystal orientation originates from the size of nano-clusters generated on the tungsten boat during flash evaporation.     

The use of secondary ion mass spectrometry in the study of matrix atom diffusion in epitaxial CdTe

The self-diffusion of Cd and Te in liquid phase epitaxially grown CdTe layers has been studied using stable isotopes of Cd¹⁰⁸ and Te¹²². A two layer structure was grown from In solutions at 300–250° C, with the first layer being normal CdTe and the second enriched in Cd₁₀₈ and Te¹²². Secondary ion mass spectrometry was used to measure the concentration of the Cd and Te species as a function of depth in the as-grown structure and after heat treatment at 500° C. The main result was that after heating, no movement of Te could be detected. However, the Cd had diffused rapidly both at the solidsolid epilayer interface and at the solid-vapour interface. The self-diffusion coefficient for Cd in CdTe calculated from this data was D^500° _cd ˜ 3 ± 1 × 10¹³cm²s^-1 in reasonable agreement with published data.     

Simultaneous Formation of Ni/Al Ohmic Contacts to Both n- and p-Type 4H-SiC

The fabrication procedure for silicon carbide power metal oxide semiconductor field-effect transistors can be improved through simultaneous formation (i.e., using the same contact materials and a one-step annealing process) of ohmic contacts on both the n-source and p-well regions. We have succeeded in the simultaneous formation of Ni/Al ohmic contacts to n- and p-type SiC after annealing at 1000°C for 5 min in an ultrahigh vacuum. Ohmic contacts to n-type SiC were found when the Al-layer thickness was less than about 6 nm, while ohmic contacts to p-type SiC were observed for an Al-layer thickness greater than about 5 nm. Only the contacts with an Al-layer thickness in the range of 5 nm to 6 nm exhibited ohmic behavior to both n- and p-type SiC, with a specific contact resistance of 1.8 × 10⁻⁴ Ω cm² and 1.2 × 10⁻² Ω cm² for n- and p-type SiC, respectively. An about 100-nm-thick contact layer was uniformly formed on the SiC substrate, and polycrystalline δ-Ni₂Si(Al) grains were formed at the contact/SiC interface. In the samples that exhibited ohmic behavior to both n- and p-type SiC, the distribution of the Al/Ni ratios in the δ-Ni₂Si(Al) grains was larger than that observed for any of the samples that showed ohmic behavior to either n- or p-type SiC. Furthermore, the grain size of the δ-Ni₂Si(Al) grains in the samples showing ohmic behavior to both n- and p-type SiC was smaller than the grains in any of the samples that showed ohmic behavior to either n- or p-type SiC. Thus, the large distribution in the Al/Ni ratios and a fine microstructure were found to be characteristic of the ohmic contacts to both n- and p-type SiC. Grains with a low Al concentration correspond to ohmic contacts to n-type SiC, while grains with a high Al concentration correspond to ohmic contacts to p-type SiC.     

Microstructural Characterization of CdTe Surface Passivation Layers

The microstructure of CdTe (CT) surface passivation layers deposited on HgCdTe (MCT) heterostructures has been evaluated using transmission electron microscopy (TEM). The MCT heterostructures were grown by liquid-phase epitaxy and consisted of thick (approximately 10 μm to 20 μm) n-type MCT layers and thin (approximately 1 μm to 3 μm) p-type MCT layers. The final CT (approximately 0.3 μm to 0.6 μm) capping layers were grown either by hot-wall epitaxy (HWE) or molecular-beam epitaxy (MBE). One of the wafers with the CT layer grown by MBE was also annealed in Hg atmosphere at 250°C for 96 h. The as-deposited CT passivation layers were polycrystalline and columnar. The CT grains were larger and more irregular when deposited by HWE, whereas those deposited by MBE were generally well textured with mostly vertical grain boundaries. Observations and measurements with several TEM abrupt structurally after annealing techniques showed that the CT/MCT interface became considerably more abrupt structurally after annealing, and the crystallinity of the CT layer was also improved.     

Electrical activity,mode of incorporation and distribution coefficient of group V elements in Hg1−xCdxTe grown from tellurium rich liquid phase epitxial growth solutions

Hg_1−xCd_xTe films were grown liquid phase epitaxially from tellurium rich solutions containing up to 10 at. % of the group V elements P, As, Sb, and Bi. Chemical analysis of the Te growth solutions and the films was carried out in conjunction with extensive Hall effect measurements on the films subsequent to various annealing treatments under Hg rich and Te rich conditions. Despite the presence of a large concentration of the group V elements in the Te source solution, the maximum concentration of these elements incorporated into the liquid phase epitaxially grown Hg_1-xCd_xTe appears to vary from <10¹⁵cm⁻³ for Bi up to 10¹⁷cm⁻³ for phosphorus and As implying a distribution coefficient varying from <10⁻⁵ for Bi up to 10⁻³ for P at growth temperature of ∼500° C. This low value of the distribution coefficient for group V elements for growths from Te rich solutions contrasts with the moderately high values reported in the literature to date for growth from Hg rich solutions as well as pseudobinary solutions (Bridgman growth). The widely differing distribution coefficients and hence the solubility of the group V elements for Hg rich and Te rich liquid phase epitaxial solutions is explained on the basis that the activity coefficient of the group V elements in Te rich solutions is probably orders of magnitude lower than it is in Hg rich solutions. Finally, the results of the anneals at 200° C under Hg saturated conditions with and without a 500° C Hg saturated preanneal have indicatedn top conversion in many of the films attesting to the amphoteric behavior of the group V elements in LPE grown Hg_1−xCd_xTe(s) similar to the previously reported behavior of P in bulk grown Hg_0.8Cd_0.2Te.     

Growth of n-Si layers by molecular-beam epitaxy on the substrates heavily doped with boron

Epitaxial n-Si layers doped with phosphorus or erbium have been grown by sublimation molecularbeam epitaxy at 500°C on heavily boron-doped p ⁺-type substrates with resistivity ρ = 0.005 Ω cm. Distribution profiles of the B, Er, and O impurity concentrations in the samples were determined by secondary-ion mass spectrometry. A thermal annealing of the substrate in vacuum at 1300°C for 10 min and growth at a very low substrate temperature made it possible to obtain an extremely abrupt profile for doping impurities at the layer-substrate interface. This method for growth of n-p ⁺ junctions considerably improves their electrical and luminescent characteristics.     

Carrier Doping in the Type VIII Clathrate Ba<Subscript>8</Subscript>Ga<Subscript>16</Subscript>Sn<Subscript>30</Subscript> Through Sb Substitution

The type VIII clathrate Ba₈Ga₁₆Sn₃₀ has a relatively high figure of merit (ZT) from 200°C to 400°C. Our previous calculations showed that the optimum carrier concentration for high ZT is on the order of 10²⁰/cm³ for both p- and n-type samples. The ZT value exceeds unity for the n-type material. However, actual carrier concentrations for synthesized samples were on the order of 10¹⁹/cm³. With the aim of increasing the carrier concentration, we have synthesized single crystals of Sb-doped Ba₈Ga₁₆Sn₃₀. Contrary to our expectation, the Ga content in the crystal increased with the increase in Sb content. In both p- and n-type samples, the carrier concentration was increased, and the power factor for the p-type samples was improved to 1.4 × 10⁻² W/mK². These results are discussed in relation to the change in the band structure.     

Microstructural Characterization of CdTe(211)B/ZnTe/Si(211) Heterostructures Grown by Molecular Beam Epitaxy

Transmission electron microscopy and small-probe microanalysis have been used to investigate the microstructure and compositional profiles of CdTe(211)B/ZnTe/Si(211) heterostructures. Thin ZnTe buffer layers and subsequent thick CdTe layers were grown on Si(211) substrates using molecular beam epitaxy. Many {111}-type stacking faults were found to be present throughout the entire ZnTe layer, terminating near the point of initiation of CdTe growth. A rotation angle of about 3.5° was observed between lattice planes of the Si substrate and the final CdTe epilayer. Local lattice parameter measurement and elemental profiles indicated that some intermixing of Zn and Cd had taken place. The average widths of the ZnTe layer and the (Cd,Zn)Te transition region were found to be roughly 6.5 nm and 3.5 nm, respectively.     

Annealing experiments in heavily arsenic-doped (Hg,Cd)Te

Arsenic doped molecular beam epitaxy (MBE) (Hg,Cd)Te films were grown on (Cd,Zn)Te substrates. The concentration of arsenic was varied from 5 x 10¹⁸ cm^-3 to 1 x 10²⁰ cm^-3. After the growth, the epitaxial layers were annealed at various partial pressures of Hg within the existence region of (Hg,Cd)Te at temperatures ranging from 400 to 500°C. Hall effect and resistivity measurements were carried out subsequent to the anneals. 77K hole concentration measurements indicate that for concentrations of arsenic <10¹⁹ cm⁻³, most of the arsenic is electrically active acting as acceptors interstitially and/or occupying Te lattice sites at the highest Hg pressures. At lower Hg pressures, particularly at annealing temperatures of 450°C and higher, compensation by arsenic centers acting as donors appears to set in and the hole concentration decreases with decrease in Hg pressure. These results indicate the amphoteric behavior of arsenic and its similarity to the behavior of phosphorus in (Hg,Cd)Te previously inferred by us. A qualitative model which requires the presence of arsenic occupying both interstitial and Te lattice sites along with formation of pairs of arsenic centers is conjectured.     

Effect of surface stoichiometry on Be doping in GaAs by intermittent supply of AsH3 and TEG in an ultra-high vacuum

The effects of surface stoichiometry on Be doping in GaAs grown by molecular layer epitaxy have experimentally been investigated. Be-doped p⁺-GaAs layers were grown on (0 0 1)-oriented GaAs substrates by intermittent supply of AsH₃ and triethylgallium (TEG) in an ultra-high vacuum. Be(MeCp)₂ was used as a p-type dopant gas. The surface stoichiometry before introducing the dopant gas was controlled by changing the AsH₃ and TEG injection sequence and supply time. The doping characteristics were evaluated by secondary ion mass spectroscopy analysis. It was found that doping characteristics of Be-doped GaAs are strongly dependent on the doping sequence and surface stoichiometry. This experimental result and the Be doping mechanism are discussed on the basis of rate law of the surface chemical reaction.