Undoped Semi-insulating InP Research Articles

Investigation of compensation defect centres in semi-insulating InP crystals

Laplace transform photoinduced transient spectroscopy (LTPITS) has been applied to study defect, centres in Fe-doped and undoped semi-insulating (SI) InP. A high resistivity (∼ 2 x 10 7 Ω cm) of the latter wins achieved by annealing at 950 °C for 40 h under a phosphorus overpressure. It is shown that shallow donors in this material have an activation energy of 10 meV and are mainly compensated with deep acceptors characterised by activation energies of 350 and 470 meV with respect to the bottom of the conduction band. In the Fe-doped material, the shallow donors are compensated with Fe-related deep acceptors having activation energies of 590 and 640 meV.

Annealing and activation of silicon implanted in semi-insulating InP substrates

We have investigated the annealing and activation of silicon implanted in both as-grown Fe-doped semi-insulating (SI) InP substrate and undoped SI InP substrate obtained by annealing high purity conductive InP wafer (wafer-annealed). Si implantations were performed at an energy of 500 keV and a dose of 1×10 15 cm −2. Following the implantations, rapid thermal annealing (RTA) cycles were carried out for 30 s at different temperatures. The results of Raman measurements show that for 700°C/30 s RTA, the two Si-implanted SI InP substrates have acquired a high degree of lattice recovery and electrical activation. However, further Hall measurements indicate that the carrier concentration of the wafer-annealed SI InP substrate is about three times higher than that of the as-grown Fe-doped SI InP substrate. The difference can be ascribed to the low Fe concentration of the wafer-annealed SI InP substrate. These experimental data imply that the use of the wafer-annealed SI InP substrate can be conducive to the improvement of InP-based device performances.

Influence of semi-insulating InP substrates on InAlAs epilayers grown by molecular beam epitaxy

Tensile-strained InAlAs layers have been grown by solid-source molecular beam epitaxy on as-grown Fe-doped semi-insulating (SI) InP substrates and undoped SI InP substrates obtained by annealing undoped conductive InP wafers (wafer-annealed InP). The effect of the two substrates on InAlAs epilayers and InAlAs/InP type II heterostructures has been studied by using a variety of characterization techniques. Our calculation data proved that the out-diffusion of Fe atoms in InP substrate may not take place due to their low diffusion coefficient. Double-crystal X-ray diffraction measurements show that the lattice mismatch between the InAlAs layers and the two substrates is different, which is originated from their different Fe concentrations. Furthermore, photoluminescence results indicate that the type II heterostructure grown on the wafer-annealed InP substrate exhibits better optical and interface properties than that grown on the as-grown Fe-doped substrate. We have also given a physically coherent explanation on the basis of these investigations.

Photoluminescence assessment of undoped semi-insulating InP wafers obtained by annealing in iron phosphide vapour

We have investigated the photoluminescence mapping characteristics of semi-insulating (SI) InP wafers obtained by annealing in iron phosphide ambience (FeP2-annealed). Compared with as-grown Fe-doped and undoped SI InP wafers prepared by annealing in pure phosphorus vapour (P-annealed), the FeP2-annealed SI InP wafer has been found to exhibit a better photoluminescence uniformity. Radial Hall measurements also show that there is a better resistivity uniformity on the FeP2-annealed SI InP wafer. When comparing the distribution of deep levels between the annealed wafers measured by optical transient current spectroscopy, we find that the incorporation of iron atoms into the SI InP suppresses the formation of a few defects. The correlation observed in this study implies that annealing in iron phosphorus ambience makes Fe atoms diffuse uniformly and occupy the indium site in the SI InP lattice. As it stands, we believe that annealing undoped conductive InP in iron phosphide vapour is an effective means to obtain semi-insulating InP wafers with superior uniformity.

Creation and suppression of point defects through a kick-out substitution process of Fe in InP

Indium antisite defect InP-related photoluminescence has been observed in Fe-diffused semi-insulating (SI) InP. Compared to annealed undoped or Fe-predoped SI InP, there are fewer defects in SI InP obtained by long-duration, high-temperature Fe diffusion. The suppression of the formation of point defects in Fe-diffused SI InP can be explained in terms of the complete occupation by Fe at indium vacancy. The InP defect is enhanced by the indium interstitial that is caused by the kick out of In and the substitution at the indium site of Fe in the diffusion process. Through these Fe-diffusion results, the nature of the defects in annealed undoped SI InP is better understood.

Characterization of defects and whole wafer uniformity of annealed undoped semi-insulating InP wafers

Semi-insulating (SI) InP wafers of 2 and 3 in. diameters have been prepared by annealing undoped LEC InP at 930 °C for 80 h under pure phosphorus ambient (PP) and iron phosphide ambient (IP). The electrical uniformity of annealed undoped SI wafers, along with a Fe-doped as-grown SI LEC InP wafer, has been characterized by whole wafer PL mapping and radial Hall measurements. Defects in these wafers have been detected by photo-induced current transient spectroscopy (PICTS). The results indicated that the uniformity of IP wafer is much better than that of PP wafer and as-grown Fe-doped SI InP wafer. There are fewer traps in undoped SI InP IP wafer than in as grown Fe-doped and undoped SI InP PP wafer, as evidenced by PICTS. The good uniformity of the IP wafer is related to the nonexistence of high concentration of thermally induced defects. The mechanism for this phenomenon is discussed based on the results.

Fabrication of undoped semi-insulating InP by multiple-step wafer annealing

Recently, it was found that undoped semi-insulating InP can be reproducibly obtained by wafer annealing at 950°C for 40 h under phosphorus vapor pressure of 1 atm. Resistivity variation across the 50 mm diameter wafer after this annealing process, however, was in the range of 22.5–53.7%. In order to realize the fabrication of undoped semi-insulating (SI) InP with uniform electrical properties, multiple-step wafer annealing (MWA) procedure has been applied for the first time. It was found that two-step wafer annealing at 950°C for 40 h under phosphorus vapor pressure of 1 atm and at 807°C for 40 h under phosphorus vapor pressure ranging from 30 to 50 atm, was effective in improvement of the uniformity of the electrical properties of undoped SI InP. By the present MWA, the resistivity variation of 8–12% and the mobility variation of 2–4% could be obtained for 50 mm diameter wafers.

Positron-lifetime study of compensation defects in undoped semi-insulating InP

Positron-lifetime and infrared-absorption spectroscopies have been used to investigate the compensation defects that render undoped n-type liquid encapsulated Czochralski-grown InP semi-insulating under high-temperature annealing. The positron measurements, carried out over the temperature range of 25--300 K, reveal in the as-grown material a positron lifetime of $282\ifmmode\pm\else\textpm\fi{}5 \mathrm{ps}$ which we associate with either the isolated indium vacancy ${V}_{\mathrm{In}}^{3\ensuremath{-}}$ or related hydrogen complexes. The shallow donor complex ${V}_{\mathrm{In}}{\mathrm{H}}_{4},$ responsible for much of the n-type conductivity and the strong infrared absorption signal at 4320 nm, is ruled out as a significant trapping site on the grounds that its neutral state is present at too low a concentration. After annealing at $950\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ in conjunction with the disappearance of the ${V}_{\mathrm{In}}{\mathrm{H}}_{4}$ infrared-absorption signal, trapping into ${V}_{\mathrm{In}}$-related centers is observed to increase slightly, and an additional positron trapping defect having a lifetime of 330 ps appears at a concentration of $\ensuremath{\sim}{10}^{16} {\mathrm{cm}}^{\ensuremath{-}3},$ indicating divacancy trapping. These results support the recent suggestion that the ${V}_{\mathrm{In}}{\mathrm{H}}_{4}$ complex present in as-grown InP dissociates during annealing, forming ${V}_{\mathrm{In}}{\mathrm{H}}_{n}^{(3\ensuremath{-}n)\ensuremath{-}}(0l~nl~3)$ complexes and that the recombination of ${V}_{\mathrm{In}}$ with a phosphorus atom results in the formation of EL2-like deep donor ${\mathrm{P}}_{\mathrm{In}}$ antisite defect, which compensates the material. It is suggested that the divacancy formed on annealing is ${V}_{\mathrm{In}}{V}_{\mathrm{P}},$ and that this defect is probably a by-product of the ${\mathrm{P}}_{\mathrm{In}}$ antisite formation.

Deep centers in undoped semi-insulating InP

Undoped semi-insulating (SI) InP samples, subjected to one-step and multi-step wafer annealing, and lightly and normally Fe-doped SI InP samples without annealing have been characterized by thermally stimulated current (TSC) spectroscopy. A dominant deep center at 0.63 eV is found in all samples and is undoubtedly due to iron. Two prominent TSC traps, Tb (0.44 eV) and Td (0.33 eV), found in undoped SI InP, are thought to be related to the phosphorus antisite PIn, and traps at low temperatures, like Te* (0.19 eV), to the phosphrus vacancy VP.

Traps in undoped semi-insulating InP obtained by high temperature annealing

The presence and evolution of traps in undoped semi-insulating (SI) InP obtained by high temperature annealing (900 °C for 90 h) in poor or rich phosphorus atmosphere has been studied by means of photoinduced current transient spectroscopy. Six traps named A1 to A6 having activation energies ranging from 0.2 to 0.6 eV have been detected in three samples submitted to the same annealing process. The samples differ in the Fe concentration of the starting material and the applied phosphorus pressure in the annealing process. A comparison of the corresponding photoinduced current transient spectroscopy spectra shows that among the observed traps, the 0.2 eV one can be related to a phosphorus deficiency, and the 0.3 eV and 0.4 eV traps could be due to an excess of phosphorus during annealing. Moreover, the trap corresponding to iron (0.6 eV) has been observed in all the studied samples.

Effect of annealing conditions on the uniformity of undoped Semi-Insulating InP

Undoped semi-insulating InP can be obtained by high-pressure annealing of high purity materials. The reproducibility was however not satisfactory. In the present work, we found that not only Fe concentrations but also Cr and Ni concentrations in annealed wafers were slightly increased. In order to reduce the contamination, conductive InP with a trace amount of Fe was annealed under low phosphorus vapor pressure. The minimum Fe concentration for realizing semi-insulating InP was found to be 1/spl times/19/sup 15/ cm/sup -3/. It was also found that annealing conditions largely affect the resistivity uniformity of undoped semi-insulating InP.

Observation of In antisite and Fe-related defects in 30-MeV electron-irradiated Fe-doped semi-insulating InP.

Photoluminescence (PL) spectra are reported and assigned to the ${\mathrm{In}}_{\mathrm{P}}$ antisite in undoped and Fe-doped semi-insulating InP which was irradiated by 30-MeV electrons. In Fe-doped samples, the two PL emissions are at 30 and 117 meV above the valence band, and annihilate in an annealing stage from 300 to 350 \ifmmode^\circ\else\textdegree\fi{}C. ${\mathrm{In}}_{\mathrm{P}}$ defects in undoped samples are \ensuremath{\sim}${10}^{2}$ times smaller than those produced in Fe-doped InP. In Fe-doped samples, an emission is also observed near an emission relating to the deep ${\mathrm{In}}_{\mathrm{P}}$ level. The produced defect is suggested to be a complex defect consisting of an iron impurity and an electron-irradiation-induced phosphorous vacancy.

Preparation and characterization of semi-insulating 2 in InP wafers having a low Fe content by wafer annealing

The preparation of nominally undoped semi-insulating (s.i.) InP by high-temperature annealing under a controlled phosphorus atmosphere provides an effective means to improve InP substrate quality. As reproducible results are restricted to the use of high-purity starting material which cannot be produced with satisfactory yields, the process was applied to InP predoped with a low Fe concentration ([ Fe] ≈ 4 × 10 15 cm −3) . Since the net donors are only partly compensated, this starting material shows normal semiconducting properties. Wafers (2 in) of this material with a low Fe content were converted to the s.i. state by annealing under a P pressure of 1 atm. The lateral uniformity was analysed by mapping the resistivity distribution and room-temperature photoluminescence. The results show that annealed s.i. 2 in wafers provide a lateral uniformity that could be achieved up to now in as-grown s.i. InP:Fe only by considerably higher Fe doping.

Spectroscopic investigation of deep levels related to the compensation mechanism of nominally undoped semi-insulating InP

Measurements were performed by deep level transient spectroscopy (DLTS) and by the thermally stimulated current (TSC) technique on annealed InP samples to investigate the compensation mechanism which causes the semi-insulating (s.i.) behaviour of nominally undoped annealed InP. The results show that the electron traps detected by DLTS in conducting InP can also be found in TSC spectra of s.i. annealed samples. Additionally another deep level is detected, which can then be assigned to a hole trap. The currently used evaluation schemes of TSC measurements had to be reconsidered in favour of a variation of the heating rate to eliminate inherent errors.

Development of high quality InP bulk crystals

InP is becoming an indispensable material not only for long wavelength opto-electronic devices but also for high frequency devices. Promoted by this requirement, InP bulk crystal technology has been rapidly developed. In the present paper, this development is reviewed in the areas of purification, reduction of dislocation density, semi-insulating crystal growth technology and wafer polishing. The possibility of undoped semi-insulating InP, a highlight topic of recent developments, is also discussed on the basis of experiments.

Compensation mechanisms in nominally undoped semi-insulating InP and comparison with undoped InP grown under stoichiometry control

Nominally undoped semi-insulating InP can be prepared reproducibly by annealing under controlled phosphorus pressure. We present the electrical properties of a large series of undoped InP samples before and after annealing. Spectroscopic investigations show that the specimens are contaminated by iron during annealing, but native defects have to be taken into consideration in order to explain the electrical data. Annealed InP specimens are characterized by electrical and optical measurements and the results are compared to those obtained from InP crystals grown under stoichiometry control by the horizontal gradient freeze technique.

Undoped semi-insulating InP by high-pressure annealing

Undoped semi-insulating (SI) InP wafers were obtained by high-temperature annealing under high phosphorus over pressure. These wafers show resistivities higher than 107 Ω cm, with mobilities greater than 4000 cm2 V−1 s−1. The SI properties could be held even after cap annealing with SiNx films at 700 °C for 15 min. The activation energy of deep levels causing the semi-insulation was estimated as 0.64 eV. Photoluminescence measurements made on undoped SI InP show hitherto unknown peaks in the long wavelength region between 1000 and 1400 nm.