Zinc-doping of InP during chemical beam epitaxy using diethylzinc

Diethylzinc was used as ap-type dopant source during InP growth by chemical beam epitaxy. In InP, electrically activated Zn saturated at a concentration of ∼2.0 × 10¹⁸ cm⁻³ for epilayers grown at 540‡ C. Higher role concentrations were obtained by lowering the growth temperature. However, measurements with SIMS indicated that very serious Zn diffusion occurred when the Zn concentration appeared to reduce the pyrolysis efficiency of trimethylindium. This caused a reduction in the InP growth rate and InAs mole fraction in InGaAs epilayers. No Zn “memory effect≓ was detected in our system. Undoped InP epilayers maintained an n-type background of ∼5 × 10¹⁵ cm⁻³.     

Zn-doping in OMVPE Grown lnP:Zn/lnGaAs/lnPp-i-n double heterojunctlons with lnGaAs:Zn contacting layers

We report on the control of zinc in organometallic vapor phase epitaxial (OMVPE) grown InP:Zn/InGaAs/InPp- i- n double heterojunctions with InGaAs:Zn contacting layers. As a function of diethylzinc (DEZn) flow, we measure net acceptor concentrations for the InP:Zn p-layer in the range 2 × 10¹⁷≤N _a−N _d≤ 9 × 10¹⁷ cm⁻³. A 435°C post-growth anneal for 300 sec increases the net acceptor concentrations by a factor of 3.6 − to 6 × 10¹⁷≤N _a−N _d≤ 3 × 10¹⁸ cm⁻³. When the annealed value ofN _a − N_din the InP:Zn layer is 6 × 10¹⁷ cm⁻³ , secondary ion mass spectrometry (SIMS) measurements show abrupt Zn-doping transitions at the heterojunction interfaces. In contrast, when the annealed value ofN _a − N_din the InP:Zn layer is near the saturation value of 3 × 10¹⁸ cm⁻³, SIMS measurements show significant movement of Zn into the nominally undoped InGaAs instrinsic layer. Increasedp-i-n diode capacitance is associated with the Zn movement.     

Metalorganic vapor phase epitaxial growth,characterization and annealing experiments on InP doping superlattices

InP doping superlattices (DSLs) were grown by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE) and their stability was examined by annealing at high temperatures. Diethylzinc (DEZ) and H₂S were used asp- andn-type doping sources, respectively. Photoluminescence (PL) measurements performed on as grown layers show a shift of the main emission peak with increasing excitation power in very good agreement with theoretical models. A comparison of the PL results between these structures and the annealed samples show that even at very high temperatures (up to 850° C) the tunability of the effective bandgap of the annealed superlattices is possible, although less pronounced than for the as grown layers. This is due to diffusion of the dopants, into adjacent layers and partial compensation of each other. Secondary ion mass spectrometry (SIMS) done on the as grown and annealed samples shows that only the Zn atoms diffuse. Diffusion coefficients obtained from the SIMS profiles give values in the range 1 × 10⁻¹⁴ <D < 9 × 10⁻¹⁴ cm²/s, still smaller than other published values estimated on layers, which did not suffer any treatment. This shows the high quality and stability of our layers even at high temperatures.     

Effects of trimethylindium on the purity of In0.5Al0.5P and In0.5Al0.5as epilayers grown by metalorganic chemical vapor deposition

In_0.5Al_0.5P lattice-matched to GaAs and In_0.5A1_0.5As lattice-matched to InP epilayers were grown by atmospheric pressure metalorganic chemical vapor deposition (AP-MOCVD). The effect of trimethylindium on the purity of the as-grown layers was systematically studied using secondary ion mass spectroscopy (SIMS), deep level transient spectroscopy (DLTS), and capacitance-voltage (C-V) measurements. The SIMS results showed that oxygen is the main impurity in all layers and the oxygen concentration in InAlP was approximately one to four orders of magnitude higher than the oxygen concentration found in InALAs when the same indium source was used, indicating that more oxygen was introduced by the phosphine source than by the arsine source. Two electron traps in the InAlP epilayers and four electron traps in the InALAs epilayers were observed in this study. When a high-purity indium source was used, the best InAlP epilayer showed only one deep electron trap at 0.50 eV while the best InALAs epilayer showed no deep levels measured by DLTS. In addition, we also found that a high concentration of oxygen is related to the high resistivity in both material systems; this suggests that semi-insulating (SI) materials can be achieved by oxygen doping and high quality conducting materials can only be obtained through the reduction of oxygen. The oxygen concentration measured by SIMS in the best InALAs epilayer was as low as 3 × 10¹⁷ cm⁻³.     

Dopant diffusion and current-voltage studies on epitaxial InP codoped with Ru and Fe

Semi-insulating Fe and Ru codoped InP epitaxial layers grown by low-pressure hydride vapor phase epitaxy have been investigated. InP∶Ru and InP∶Fe, Ru layers were grown on p-InP∶Zn andn-InP∶S, substrates, in order to study dopant diffusion and electrical characteristics. Dopant diffusion profiles of Ru, Fe and Zn were measured by secondary ion mass spectroscopy. A small but noteworthy diffusion front is observed when InP∶Ru is adjacent to InP∶Zn, but not when adjacent to n-InP. For InP∶Fe codoped with Ru a pronounced interdiffusion of Fe and Zn is observed for Ru concentrations less than 2 10¹⁷ cm⁻³, but for a higher Ru concentration the interdiffusion is clearly suppressed. Moreover, when InP is codoped with Fe and Ru, the small diffusion tail of Ru in InP∶Zn vanishes. Unlike InP∶Fe, resistivities above 1 10⁸ cm are measured for both electron and hole-current injection in InP∶Fe,Ru.     

Advances in LEC growth of InP crystals

By reducing the temperature gradients in the vicinity of the crystal-melt interface, 35-mm-diameter InP boules with much reduced dislocation densities have been grown by the liquid-encapsulated Czochralski technique. A reduction in the residual donor concentration of InP grown by this technique has been achieved by using In-rich charges prepared by adding elemental In to polycrystalline InP ingot material. Nominally undoped crystals with carrier concentrations as low as 1–2 x₄ 10¹⁵ Cm − 1 and 77 K mobilities as high as 7.0 × 10 cm² V⁻¹ s⁻¹ have been obtained. By growing doped crystals at increased seed or crucible rotation rates, short-range longitudinal variations in dopant concentration have been reduced to a few per cent, as determined by optical absorption measurements with a scanning CO₂ laser.     

Photon assisted growth of nitrogen-doped CdTe and the effects of hydrogen incorporation during growth

Nitrogen doping in CdTe epilayers grown by photo-assisted molecular beam epitaxy was demonstrated using an rf plasma source. The effect of the presence of atomic hydrogen during growth of undoped and nitrogen-doped CdTe was investigated. The layers were characterized using photoluminescence spectros-copy (PL), Hall effect, secondary ion mass spectroscopy (SIMS), Fourier transform infrared spectroscopy, and atomic force microscopy. PL confirmed the incorporation of nitrogen as acceptors. While p-type carrier concentrations greater than 10¹⁸ cm⁻³ were easily obtained, SIMS measurements indicated that nitrogen was concentrated near the undoped-doped and epilayer-substrate interfaces which complicates interpretation of activation efficiency. Hydrogen incorporation was found to be enhanced by the presence of nitrogen. Infrared absorption measurements strongly suggested the formation of N-H complexes. Hall measurements indicated that complexes are formed which are donor-like in nature. The presence of atomic hydrogen during growth radically changed the low temperature photoluminescence in both undoped and nitrogen-doped layers. Exciton-related luminescence was quenched at low temperature. Nitrogenrelated donor-acceptor pair luminescence was also absent from the N-doped hydrogenated layers, consistent with complex formation. Copper (a cation-site acceptor) donor-acceptor pair luminescence appeared to be enhanced by hydrogenation.     

Zn doping of InP, InAsP/InP, and InAsP/InGaAs heterostructures through metalorganic vapor phase diffusion (MOVPD)

Zinc incorporation by post-growth metalorganic vapor phase diffusion (MOVPD) is used to achieve high p-doping, which is desirable for the fabrication of photodiodes. Diethylzinc (DEZ) is used as precursor and Zn is diffused into InP and InAs_0.6P epitaxial layers grown by low pressure metalorganic vapor phase epitaxy (MOVPE) on different substrate orientations, enabling the investigation of the dislocation density on the Zn incorporation. Diffusion depths are measured using cleave-and-stain techniques, resistivity measurements, electrochemical profiling, and secondary ion mass spectroscopy. High hole concentrations of, respectively, 1.7 10¹⁹ and 6 10¹⁸ cm⁻³, are obtained for, respectively, InAs_0.60P and InP. The diffusion coefficients are derived and the Zn diffusion is used for the fabrication of lattice-mismatched planar PIN InAsP/InGaAs photodiodes.     

Structural and electrical properties of unintentionally doped 4H-SiC epitaxial layers—Grown by hot-wall CVD

This paper focuses on growth of 4H−SiC epitaxial layers using the hot-wall CVD technique. The relation between the growth regime like total flow, system pressure, C/Si ratio and growth temperature and the characteristics of nominally undoped epilayers, such as thickness uniformity and background doping concentration have been investigated. The epitaxial layers were investigated by optical microscopy, capacitance-voltage measurements, x-ray rocking curve maps, electron channelling patterns and secondary ion mass spectroscopy. Layers up to 40 μm in thickness with a variation of about ±4% and with residual n-type doping levels in the low 10¹⁴ cm⁻³ ranges have been obtained on Si faces wafers. SIMS measurements have shown that the impurity concentration of acceptors like B and Al is below 2×10¹⁴ cm⁻³.     

Dislocations in vapor phase epitaxial GaP

Dislocations in VPE GaP grown on (100) oriented LEC GaP substrates have been characterized, and their origins and effects on LED performance have been investigated. In non-nitrogen doped epilayers, the dislocations are found to originate in the substrate and propagate through the epilayers in straight lines in [100] and <211> directions. The dislocation density of the epilayer is found to be nearly equal to that of the substrate. Introduction of nitrogen during growth of the epilayer has been observed to bend these so-called “inclined≓ dislocations propagating through the layer into [0−1 1] directions in the (100) plane and thus produces segments of [0 −1 1] dislocations to relieve the lattice parameter mismatch due to N. The mismatch dislocation density is observed to be proportional to the N doping level. At very high N doping levels, > 10¹⁹ cm^-3, a large number of new inclined dislocations are observed, which may be in part due to GaN precipitation. The effects of dislocations on LED properties were investigated by measuring dislocation densities in the individual diodes using the electron beam induced current mode of the SEM and comparing this with the spot brightness and luminous flux. The dislocations were observed to produce dark spots in the EL emission in many cases. For a series of runs where all growth and processing parameters were fixed, a good correlation between B/J and dislocation density was observed with B/J decreasing with increasing dislocation density in the range < 1 × 10⁴ cm⁻² to 1 × 10⁶ cm⁻².     

Dislocation-free undoped semi-insulating GaAs epilayers prepared by chloride chemical vapor deposition and successive wafer annealing

Dislocation-free (DF) undoped semi-insulating GaAs epilayers have been realized by chloride chemical vapor deposition and successive wafer annealing. It was found that undoped conductive DF GaAs epilayers grown on Si-doped n-type DF GaAs substrates can be converted to semi-insulating by wafer annealing at temperatures higher than 950°C. The resistivity of these semi-insulating epilayers was higher than 10⁷ Ωcm. The outdiffusion of Si from the substrate to the epilayer was analyzed by secondary ion mass spectrometry and it was found that the thickness of the outdiffusion region was only 1μm.     

The growth of GaAs,AIGaAs, InP and InGaAs by chemical beam epitaxy using group III and V alkyls

The growth kinetics of chemical beam epitaxy (CBE) were investigated with the growth of GaAs, AIGaAs, InP, and InGaAs. Results obtained with epilayers grown by using trimethylarsine (TMAs) and triethylphosphine (TEP) instead of arsine (AsH3) and phosphine (PH₃) were reviewed with some additional results. The CBE grown epilayers have similar optical quality to those grown by molecular beam epitaxy (MBE). Superlattices of GaAs/AlGaAs with abrupt interfaces have been prepared. Since trimethylindium (TMIn) and triethylgallium (TEGa) used in the growth of InGaAs emerged as a single mixed beam, spatial composition uniformity was automatically achieved without the need of substrate rotation in the InGaAs epilayers grown. Lattice-mismatch Δα/α< 1 x 10^-3 have been reproducibly obtained. For epilayers grown with high purity TMAs source, room-temperature electron mobility as high as 9000 cm²/V sec and concentrations of ˜7 x 10¹⁵ cm^-3 were produced. In general, the electron mobilities were as good as those obtained from low-pressure metalorganic chemical vapor deposition. (MO-CVD). Unlike MBE, since the In and Ga were derived by the pyrolysis of TMIn and TEGa molecules at the heated substrate surface, respectively, oval defects observed in MBE grown epilayers due to Ga splitting from Ga melt were not present in CBE grown epilayers. This is important for integrated circuit applications. Unlike MO-CVD, the beam nature of CBE allows for selective area growth of epilayers with well-defined smooth edges using mask shadowing techniques. Typically, growth rates of 2-5μm/h for InP, 2-6μm/h for GaAs and AIGaAs, and 2-5μm/h for InGaAs were used.     

Epitaxial Growth and Characterization of p-Type ZnO

N-doped p-type ZnO thin films were grown on c-sapphire substrates, semi-insulating GaN templates, and n-type ZnO substrates by metal organic chemical vapor deposition (MOCVD). Diethylzinc and oxygen were used as precursors for Zn and O, respectively, while ammonia (NH₃) and nitrous oxide (N₂O) were employed as the nitrogen dopant sources. X-ray diffraction (XRD) studies depicted highly oriented N-doped ZnO thin films. Photoluminescence (PL) measurements showed a main emission line around 380 nm, corresponding to an energy gap of 3.26 eV. Nitrogen concentration in the grown films was analyzed by secondary ion mass spectrometry (SIMS) and was found to be on the order of 10¹⁸ cm⁻³. Electrical properties of N-doped ZnO epilayers grown on semi-insulating GaN:Mg templates were measured by the Hall effect and the results indicated p-type with carrier concentration on the order of 10¹⁷ cm⁻³.     

Hydrogenation of HgCdTe epilayers on Si substrates using glow discharge plasma

Preliminary results of a study of the hydrogenation of HgCdTe epilayers grown by molecular beam epitaxy on Si substrates using a glow-discharge plasma are presented. The aim of the program is to employ H to passivate the detrimental opto-electronic effects of threading dislocations present in the HgCdTe epilayers. Secondary ion mass spectroscopy depth profiling has been performed to characterize ¹H and ²H incorporation. It has been found that H can be controllably incorporated in HgCdTe epilayers to levels in the 10¹⁴ cm⁻³ to 10¹⁸ cm⁻³ range while maintaining the sample at temperatures lower than 60°C. Profiles indicate that H accumulates in regions of known high defect density or in highly strained regions. Analysis of the H depth profile data indicates that the current density-time product is a good figure of merit to predict the H levels in the HgCdTe epilayer. There are progressive differences in the ¹H and ²H uptake efficiencies as a function of depth. Magneto-Hall measurements show consistently higher mobilities at low temperatures for majority carriers in hydrogenated samples.     

The growth of low dislocation densityp-lnP single crystals

High qualityp-type InP is critical for devices ranging from high power injection lasers to space-based solar cells. The growth of 50 mm diameter, low defect density,p-type, Zn:InP substrates has been achieved for the first time at doping levels below 10¹⁸ cm⁻³. The 600 gram 〈111〉 B-seeded crystals were grown by the vertical dynamic gradient freeze technique. Dislocation densities are more than an order of magnitude below those achieved in comparable LEC-growth material. These range from 300 cm⁻² at the seed end to 1200 cm⁻² in the 50 mm diameter portion of the crystal. Single crystals were grown with carrier concentrations ranging from 1–5 × 10¹⁷ cm⁻³ as determined by Hall measurements. Hole mobilities as high as 100 cm² volt⁻¹ sec⁻¹ were achieved. The in-corporation of the zinc dopant follows normal freezing and a distribution coefficient of 0.67 ± .09 was determined. Infrared transmission imaging shows a lower level of stria-tion contrast relative to that observed for sulfur doped InP.     

The lattice position of diffused Zn in InP

We have measured the substitutional fraction (f_s) for Zn atoms diffused into InP crystals using the proton-induced x-ray excitation (PIXE) technique. Diffusion times ranged from 15–60 min at 425–650° C. For several samples with diffusion depths in the range 0.75-3.7 μm (as determined by SIMS analysis), we find that the Zn impurity atoms reside almost totally on lattice sites: f_s = 0.9 ± 0.1. Contrary to results of an earlier study, we find no evidence for precipitates in the diffused layers. However, only ∼10^-3-10^-1 of the Zn is electrically active, consistent with Tuck and Hooper’s suggestion of neutral V_p Zn_In V_p complexes.     

Low pressure MOCVD growth of buried heterostructure laser wafers using high quality semi-insulating InP

Semi-insulating Fe doped InP has been grown by low pressure MOCVD at 100 mbar and 630° C. Complete activation of Fe below the solubility limit of 5 × 10₁₆ cm^-3 has been achieved by reducing the PH₃ concentration during crystal growth to the lowest value required to maintain good surface morphology of the layer. Diffusion of the Fe dopant and dopant spikes at the interface between the substrate and grown layer can be minimized by ensuring that the total Fe concentration in the layer does not exceed the diffusion threshold of 2 × 10¹⁷ cm⁻³. Growth of Fe doped InP around a double heterostructure mesa formed by reactive ion etching produces a structure without either growth of InP on the mesa or notches at the mesa sidewalls, even with minimal overhang of the dielectric mask. Examination of regrown heterostructures shows no evidence of interdiffusion of Fe and Zn, indicating that Fe diffusion has been successfully prevented. Completed lasers have threshold current densities of 2.5 kA/cm² at 20° C and initial aging results which indicate that these devices have good lasing characteristics and potentially high reliability.     

High-efficiency,thin-film InP concentrator solar cells

High-efficiency, thin-film InP solar cells grown heteroepitaxially on GaAs and Si single-crystal bulk substrates are being developed as a means of eliminating the problems associated with using single-crystal InP substrates (e.g., high cost, fragility, high mass density and low thermal conductivity). A novel device structure employing a compositionally graded Ga _x In_1−x As layer (∼8 μm thick) between the bulk substrate and the InP cell layers is used to reduce the dislocation density and improve the minority carrier properties in the InP. The structures are grown in a continuous sequence of steps using computer-controlled atmospheric-pressure metalorganic vapor-phase epitaxy (AP-MOVPE). Dislocation densities as low as 3×10⁷ cm⁻² and minority carrier lifetimes as high as 3.3 ns are achieved in the InP layers with this method using both GaAs or Si substrates. Structures prepared in this fashion are also completely free of microcracks. These results represent a substantial improvement in InP layer quality when compared to heteroepitaxial InP prepared using conventional techniques such as thermally cycled growth and post-growth annealing. The present work is concerned with the fabrication and characterization of thin-film InP solar cells designed for operation at high solar concentration (∼100 suns) which have been prepared from similar device structures grown on GaAs substrates. The cell performance is characterized as a function of the air mass zero (AM0) solar concentration ratio (1–100 suns) and operating temperature (25°–80° C). From these data, the temperature coefficients of the cell performance parameters are derived as a function of the concentration ratio. Under concentration, the cells exhibit a dramatic increase in efficiency and an improved temperature coefficient of efficiency. At 25° C, a peak conversion efficiency of 18.9% (71.8 suns, AM0 spectrum) is reported. At 80° C, the peak AM0 efficiency is 15.7% at 75.6 suns. These are the highest efficiencies yet reported for InP heteroepitaxial cells. Approaches for further improving the cell performance are discussed.     

Maskless selective area growth of InP on Sub-μm V-groove patterned Si(001)

Low pressure metalorganic chemical vapor deposition of InP onexactly oriented Si(OOl) substrates with a periodic V-groove pattern of periodicity ≤1.2 μm using a two temperature growth sequence (400 and 640°C) is reported. Planar InP layers with extremely low defect density of 7 × 10⁴ cm⁻² are obtained. For InP on V-grooves of width g ≤1.0μm, a planar surface is formed after less than 1 μm of growth. Formation or suppression of antiphase domains (APDs) is a function of the widths of the (OOl)-oriented ridges. For s ≤1 μm, epilayers are single domain and the direction is oriented parallel to the grooves. At 400°C, nucleation starts homogeneously on {111}-sidewallsand (001)-facets. While heating up to 640°C, InP migrates into the grooves, depleting almost completely the (001)-facets. During growth of the main layer, first the V-grooves are filled up. Subsequently (001)-ridges are overgrown laterally or voids are formed on top of them. This mechanism is responsible for both planarization and APD-suppression. The surface migration length of InP on Si(001) at 640°C is estimated to be ≈0.5 μm.     

InP growth and properties

Bulk polycrystalline InP is synthesized from the elements via a gradient freeze process. Hall data for a typical boule are N_d-N_a= 4.7 × 10¹⁵/cm³ and Μ77 = 28,000 cm²/V-sec. Photoluminescence data indicate that zinc is present as an acceptor impurity in the polycrystalline InP and in nominally undoped LEC single crystals grown using the synthesized InP as charge material. A series of doping experiments have determined the effective segregation coefficient to be 1.6 × 10⁻³ for Fe in InP. Semi-insulating InP crystals with resistivity > 10⁷ ohm—cm have been grown consistently from melts doped with 150 ppm Fe.