Properties of conducting zinc oxide films prepared by R.F. Sputtering

An effective method of dopant incorporation in rf sputtered ZnO film is reported. The electrical, optical and structural properties of zinc doped ZnO films are investigated. Electron mobility of∼10 cm² /V-sec and electron concentration of∼10¹⁹ cm⁻³ have been measured at room temperature. X-ray diffraction data obtained on films prepared on Corning 7059 glass show (002) peak, dominating. The high electrical conductivity and transmission makes ZnO films very attractive as a component for heterojunction solar cells.     

P- and N-type doping of GaN and AlGaN epitaxial layers grown by metalorganic chemical vapor deposition

Mg- and Si-doped GaN and AlGaN films were grown by metalorganic chemical vapor deposition and characterized by room-temperature photoluminescence and Hall-effect measurements. We show that the p-type carrier concentration resulting from Mg incorporation in GaN:Mg films exhibits a nonlinear dependence both on growth temperature and growth pressure. For GaN and AlGaN, n-type doping due to Si incorporation was found to be a linear function of the silane molar flow. Mg-doped GaN layers with 300K hole concentrations p ∼2×10¹⁸ cm⁻³ and Si-doped GaN films with electron concentrations n∼1×10¹⁹ cm⁻³ have been grown. N-type Al_0.10Ga_0.90N:Si films with resistivities as low as p ∼6.6×10⁻³ Ω-cm have been measured.     

Molecular beam epitaxial growth of BGaAs ternary compounds

B_xGa_1−xAs ternary compounds with boron compositions varying up to x=1% have been grown by molecular beam epitaxy. Reflection high energy electron diffraction and double crystal x-ray diffraction measurements show that grown layers are single crystal with boron composition up to 0.25% and exhibit specular surface morphology. Photoluminescence measurements indicated a monotonic increase in energy bandgap with boron composition up to 0.25%. The layers showed p-type conductivity with hole concentration reaching the low 10¹⁹ cm⁻³ range. Increasing boron concentrations leads to rough surface morphology and reduction in photoluminescence intensity. Initial results indicate that lower growth temperature may be useful for increasing boron incorporation in BGaAs compounds.     

Selective doping of N-type ZnSe layers with chlorine grown by molecular beam epitaxy

N-type ZnSe with electron concentration up to 3 × 10²⁰ cm⁻³ and low resistivity down to 1 × 10⁻⁴ ohm-cm, has been grown using a selective doping technique with chlorine during molecular beam epitaxy. The photoluminescence evaluation shows that the selectively doped ZnSe layers are superior to uniformly doped ones, especially for the case of high-concentration chlorine doping. The in-depth profile of chlorine concentration in a selectively doped sample was measured with secondary-ion mass spectroscopy (SIMS). The SIMS analysis shows only slight diffusion of the incorporated chlorine atoms even in highly doped samples.     

Low temperature growth and planar doping of ZnSe in a plasma-stimulated LP-MOVPE system

In a low-pressure metalorganic vapor phase epitaxy process, we used dc-plasma activated nitrogen to dope ZnSe, grown with ditertiarybutylselenide and dimethylzinc-triethylamine. The nitrogen concentration of up to 2 × 10¹⁸ cm⁻³ in the doped layers can be adjusted by the growth temperature, the dc-plasma power, and the N₂ dopant flow. Due to the high n-type background carrier concentration of the order of 10¹⁷ cm⁻³ in undoped samples, the doped layers show n-type conductivity or were semi-insulating because of an additional compensation by hydrogen incorporated with a concentration of the order of 10¹⁸ cm⁻³. A planar doping scheme was applied to reduce this hydrogen incorporation by one order of magnitude, although H₂ was used as carrier gas.     

Luminescence study of ZnTe:Cr epilayers grown by molecular-beam epitaxy

Incorporation of Cr into ZnTe epilayers grown by molecular-beam epitaxy (MBE) is reported. Photoluminescence (PL) using both continuous wave (CW) and pulsed-excitation sources is used to characterize the radiative efficiency of doped layers in the infrared region. The Cr²⁺ ions produce a broad emission band peaking in the 2–3 μm range, which is of potential use in tunable-laser devices. The optimum Cr concentration for achieving bright, room-temperature infrared emission was found to be in the range from low- to mid-10¹⁸ cm⁻³. Temperature-dependent luminescence studies were performed to determine thermal-quenching activation energies. Using a pulsed-laser operating at 1.9 μm, an investigation of emission lifetimes was made. The emission-decay curves for the Cr²⁺ recombination in ZnTe:Cr films could be described by a single exponential and were nearly independent of temperature from 80 K to 300 K. A room-temperature lifetime of ∼2.5 μsec in a ZnTe:Cr layer with [Cr] ∼1.4 × 10¹⁸ cm⁻³ compares favorably with values reported for bulk ZnTe:Cr.     

Molecular beam epitaxial growth of P-ZnSe:N using a novel plasma source

We have studied the p-type doping in ZnSe molecular beam epitaxial growth using a novel high-power (5 kW) radio frequency (rf) plasma source. The effect of growth conditions such as the rf power, the Se/Zn flux ratio and the growth temperature on p-ZnSe:N was investigated. The net acceptor concentration (N_A—N_D) of around 1 × 10¹⁸ cm⁻³ was reproducibly achieved. The activation ratio ((N_A—N_D)/[N]) of p-ZnSe:N with N_A—N_D of 1.2 × 10¹⁸ cm⁻³ was found to be as high as 60%, which is the highest value so far obtained for N_A—N_D ∼ 10¹⁸ cm⁻³. The 4.2K photoluminescence spectra of p-ZnSe:N grown under the optimized growth condition showed well-resolved deep donor-acceptor pair emissions even with high N_A—N_D. On leave from Sumitomo Electric Industry Ltd. On leave from Sony Corp.     

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 of Zn-doped P-type In0.53Ga0.47as grown by vapor phase epitaxy (VPE) on InP substrates

Electrical properties of Zn-doped, p-type In_0.53Ga_0.47As grown by the vapor phase epitaxy (VPE) technique are presented. High (p ∼ 4.0 × 10¹⁹ cm⁻³) p-type doping and low resistivity (ρ ∼ 2.8 × 10⁻³ Ωsu−cm) was obtained. These propertie's are useful in the formation of ohmic contacts in laser diodes and photodiodes fabricated from the quaternary and ternary alloy systems. A calibration curve for the non-destructive determination of carrier concentration from photoluminescence linewidths has been obtained.     

Correlation of arsenic incorporation and its electrical activation in MBE HgCdTe

The behavior of arsenic for p-type doping of MBE HgCdTe layers has been studied for various annealing temperatures and arsenic doping concentrations. We have demonstrated that arsenic is in-situ incorporated into HgCdTe layers during MBE growth. The carrier concentration has been measured by the Van der Pauw technique, and the total arsenic concentration has been determined by secondary ion mass spectroscopy. After annealing at 250°C under an Hg over pressure, As-doped HgCdTe layers show highly compensated n-type properties and the carrier concentration is approximately constant (∼mid 10¹⁵ cm⁻³) until the total arsenic concentration in the HgCdTe layers approach mid 10¹⁷ cm⁻³. The source of n-type behavior does not appear to be associated with arsenic dopants, such as arsenic atoms occupying Hg vacancy sites, but rather unidentified structural defects acting as donors. When the total arsenic concentration is above mid 10¹⁷ cm⁻³, the carrier concentration shows a dependence on the arsenic concentration while remaining n-type. We conjecture that the increase in n-type behavior may be due to donor arsenic tetramers or donor tetramer clusters. Above a total arsenic concentration of 1∼2×10¹⁸ cm⁻³, after annealing at 300°C, the arsenic acceptor activation ratio rapidly decreases below 100% with increasing arsenic concentration and is smaller than that after annealing at 450°C. The electrically inactive arsenic is inferred to be in the form of neutral arsenic tetramer clusters incorporated during the MBE growth. Annealing at 450°C appears to supply enough thermal energy to break some of the bonds of neutral arsenic tetramer clusters so that the separated arsenic atoms could occupy Te sites and behave as acceptors. However, the number of arsenic atoms on Te sites is saturated at ∼2×10¹⁸ cm⁻³, possibly due to a limitation of its solid solubility in HgCdTe.     

Boron diffusion into 6H-SiC through graphite mask

Experimental results are reported of selective diffusion of boron in 6H−SiC. Photoluminescence spectroscopy scanning electron microscopy cathodoluminescence imaging, secondary-ion mass spectroscopy optical microscopy, and stain-groove technique were used to characterize the selectively doped regions fabricated by diffusion from the vapor phase through a graphite mask. Local p-doped regions of dimensions down to ∼20 μm in diameter were formed on an n-type substrate using the graphite mask. Maximum concentration of boron atoms at the surface, obtained by SIMS, varied from 3×10¹⁹ cm⁻³ to 6×10¹⁹ cm⁻³, depending upon the temperature of diffusion, while the p-n junction depth measured by the stain-groove technique varied from 0.5 μm to 1.2 μm. Planar p-n junction diodes fabricated on the diffused regions exhibited good rectification characteristics with a breakdown voltage of about 1000 V.     

Absorption and photoluminescence spectroscopy of diffusion-doped ZnSe:Cr2+

ZnSe:Cr²⁺ is an attractive candidate as a room-temperature tunable solid-state laser with output in the 2–3 μm range. Passive absorption losses in this emission range currently limit laser performance. In this study, we use absorption and photoluminescence spectroscopies at 5 and 296K to address the origin of these optical losses. A series of diffusion-doped ZnSe:Cr single-crystal samples with Cr²⁺ concentrations in the range from 2×10¹⁷ cm⁻³ to 9×10¹⁹ cm⁻³ were obtained using CrSe powder as the dopant source. We find that trace amounts of Fe²⁺ produce absorption in the 2–3 μm range. Also, we have obtained data on a 680 nm absorption band observed in ZnSe:Cr which has been assigned to an internal transition of Cr²⁺. In our series of samples, the relative intensities of the 680 nm absorption band do not track the relative intensities of the 1.8 μm band (known to be due to Cr²⁺), although excitation near 680 nm does produce weak Cr²⁺ luminescence. Our absorption data do not support the current assignment of the 680 nm absorption as being an internal transition of the Cr²⁺ ion.     

MBE growth and characterization of in situ arsenic doped HgCdTe

We report the results of in situ arsenic doping by molecular beam epitaxy using an elemental arsenic source. Single Hg_1−xCd_xTe layers of x ∼0.3 were grown at a lower growth temperature of 175°C to increase the arsenic incorporation into the layers. Layers grown at 175°C have shown typical etch pit densities of 2E6 with achievable densities as low as 7E4cm⁻². Void defect densities can routinely be achieved at levels below 1000 cm⁻². Double crystal x-ray diffraction rocking curves exhibit typical full width at half-maximum values of 23 arcsec indicating high structural quality. Arsenic incorporation into the HgCdTe layers was confirmed using secondary ion mass spectrometry. Isothermal annealing of HgCdTe:As layers at temperatures of either 436 or 300°C results in activation of the arsenic at concentrations ranging from 2E16 to 2E18 cm⁻³. Theoretical fits to variable temperature Hall measurements indicate that layers are not compensated, with near 100% activation after isothermal anneals at 436 or 300°C. Arsenic activation energies and 77K minority carrier lifetime measurements are consistent with published literature values. SIMS analyses of annealed arsenic doping profiles confirm a low arsenic diffusion coefficient.     

The kinetics of the growth of nitrogen-doped ZnSe grown by photo-assisted MOVPE

Detailed growth kinetic studies of the photo-assisted growth of ZnSe from the precursors dimethylzinc triethylamine adduct (DMZn.TEN) with either dimethylselenium (DMSe) or diethylselenium (DESe) have shown that there are two regimes of growth temperature, low temperature growth dominated by site blocking from unreacted precursors and a high temperature regime dominated by precursor desorption. The proposed growth mechanism is based on a surface bimolecular reaction mediated by hydrogen radicals which is initiated by the decomposition of surface DMZn. The same mechanism has been used to explain the residual hydrogen concentration in the DMSe grown layers of 1 × 10¹⁷ cm⁻³ and the enhancement in hydrogen incorporation that occurs with DESe by deviating from 1:1 precursor ratio. Nitrogen doping using trimethylsilylazide (TMSiN₃) in combination with DESe has achieved nitrogen incorporation up to 1 × 10²⁰ cm⁻³ but the incorporation of hydrogen is also observed to increase and the growth rate decrease. This has been explained using a Langmuir Hinshelwood model causing a disruption of the surface bimolecular reaction and bonding hydrogen radicals to surface nitrogen.     

Low oxygen and carbon incorporation in AIGaAs using tritertiarybutylaluminum in organometallic vapor phase epitaxy

High-quality AIGaAs epilayers have been grown by low pressure organometallic vapor phase epitaxy with a new aluminum precursor tritertiarybutylaluminum (TTBAl). Layers grown at 650°C have a featureless mirror surface morphology and strong room temperature photoluminescence. Carbon was not detectable in chemical analysis by secondary ion mass spectroscopy, nor in low temperature (4K) photoluminescence spectra. Oxygen concentration in Al_0.25Ga_0.75As is as low as ∼2−3 × 10¹⁷ cm⁻³. Nominally undoped AIGaAs layers exhibit n-type conductiv-ity with electron concentrations at ∼ 1−1.5 × 10¹⁶ cm⁻³. A high degree of compo-sitional uniformity over 5 cm diam substrates (0.268 ±0.001) was obtained. These results indicate the potential for TTBA1 as an aluminum precursor for low temperature growth of Al-containing III-V alloys.     

Thermomigration of tellurium precipitates in CdZnTe crystals grown by vertical bridgman method

Te precipitates in CdZnTe have been characterized by x-ray diffraction at room and higher temperatures. From the x-ray results at room temperature, it has been confirmed that Te precipitates in CdZnTe have the same structural phase as observed in elemental Te under high pressure. The x-ray results at higher temperature indicate that Te precipitates melt around 440°C. CdZnTe samples containing Te precipitates have been annealed at temperatures below and above 440°C with thermal gradient of ∼70°C/cm. Results of the observation with infrared microscope before and after the annealings indicate distinct occurrence of thermomigration of Te precipitates in samples annealed at temperature above 440°C compared with ones annealed at temperature below 440°C. Thermomigration velocity obtained from these results is ∼50 μm/h. The average value for the effective diffusion coefficient of the metallic atoms in Te precipitates calculated by using the thermomigration velocity is ∼3 x 10⁻⁵ cm²/s.     

P-type carbon doping of GaSb

The growth of carbon-doped GaSb by MOVPE has never been reported to our knowledge, despite increasing interest in carbon-doped GaAsSb alloys for heterojunction bipolar transistor applications. In this work, we report the use of carbon tetrachloride (CCl₄) in conjunction with triethylgallium (TEGa) and trimethylantimony (TMSb) to achieve p-type doping levels in GaSb from 5 10¹⁶ cm⁻³ to greater than 10¹⁹ cm⁻³. High resolution x-ray diffraction measurements confirm that the effect of carbon on the lattice parameter is significant for hole concentrations above 1 10¹⁹ cm⁻³ as in the case of GaAs. By introducing controlled low doping levels of carbon into thick homoepitaxial samples, we have succeeded in identifying a carbon-related low temperature photoluminescence band at 795 meV, which we ascribe to band-to-acceptor transitions of carbon acceptors. Temperature-dependent Hall measurements on lightly carbon-doped samples yield somewhat lower binding energies than the spectroscopic data due to impurity banding in the acceptor excited states.     

Influence of growth conditions and surface reaction byproducts on GaN grown via metal organic molecular beam epitaxy: Toward an understanding of surface reaction chemistry

The surface reaction byproducts during the growth of GaN films via metal organic molecular beam epitaxy (MOMBE) were investigated as a means to optimize material properties. Ethylene and ethane were identified as the dominant surface reaction hydrocarbon byproducts, averaging 27.63% and 7.15% of the total gas content present during growth. Intense ultraviolet (UV) photoexcitation during growth was found to significantly increase the abundance of ethylene and ethane while reducing the presence of H₂ and N₂. At 920°C, UV excitation was shown to enhance growth rate and crystalline quality while reducing carbon incorporation. Over a limited growth condition range, a 4.5×10¹⁹−3.4×10²⁰ cm⁻³ variation in carbon incorporation was achieved at constant high vacuum. Coupled with growth rate gains, UV excitation yielded films with ∼58% less integrated carbon content. Structural material property variations are reported for various ammonia flows and growth temperatures. The results suggest that high carbon incorporation can be achieved and regulated during MOMBE growth and that in-situ optimization through hydrocarbon analysis may provide further enhancement in the allowable carbon concentration range.     

Controlling the U −-center density in Se-As chalcogenide-glass semiconductors by doping with metals and halogens

Temperature dependences of the drift mobilities of electrons and holes are investigated in chalcogenide-glass semiconductors with composition Se₉₅As₅, both without impurities and with the impurities Ag and Br. The data obtained indicate that the localized states that control the transport of charge carriers are U ⁻-centers, and that the change in the magnitude of the drift mobility after doping is caused by a change in the concentration of these centers. Estimates of the concentrations of positive and negatively charged intrinsic defects show that their values are similar, equalling ∼10¹⁶ cm⁻³ in impurity-free glasses with the composition Se₉₅As₅ and lying in the range 10¹³–10¹⁷ cm⁻³ when these glasses are doped with Ag, Br, and Cl. It is established that halogen impurities change the concentration of U ⁻-centers most strongly (by two to three orders of magnitude). Analysis of the data obtained shows that the percentage of electrically active Br and Cl impurity atoms is 1%, while for Ag atoms it is 10⁻²%. Fiz. Tekh. Poluprovodn. 33, 866–869 (July 1999)     

Defect structure of Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Te films grown by liquid-phase epitaxy,studied by means of low-energy ion treatment

Treatment with low-energy ions and measurements of electrical parameters of samples have been used to study the defect structure of Cd _x Hg_{1 − x} Te films grown by liquid-phase epitaxy. The films contain neutral defects supposedly associated with tellurium nanoinclusions. Ion treatment electrically activates these defects, with a high concentration of donor centers (∼10¹⁷ cm⁻³) created in the films. These defects decompose in ∼10³ min of aging at room temperature. Then the properties of the material are determined by the concentration of residual donors, which is found to be very low (down to ∼10¹⁴ cm⁻³) for the films under study.