Unintentional Background Doping Research Articles

Comparison and analysis of Al0.7InAsSb avalanche photodiodes with different background doping polarities

Background doping polarity is a key parameter in the design of numerous electrical and optoelectronic devices. It is especially critical for avalanche photodiodes (APDs). Recently, high-performance APDs have been demonstrated based on the AlInAsSb digital alloy materials system. A critical element of this work was the determination of the background doping polarity of the molecular beam epitaxial grown wafers. In this work, we determine the unintentional background doping polarity of Al0.7InAsSb using the double mesa capacitance-voltage technique. We fabricated two p-i-n Al0.7InAsSb structures: one with p-type background polarity and the other with n-type. The measurements indicate that devices with different background doping polarities show different capacitance relations to the mesa diameters; moreover, the relationship reverses at high voltage in a p-type background device. Subsequent simulations reveal that this reversal is caused by electrical field confinement after the depletion reaches the smaller top mesa. These findings are consistent with reports of reduced surface leakage current in double and triple mesa structures.

Over 1 kV Vertical GaN-on-GaN p-n Diodes With Low On-Resistance Using Ammonia Molecular Beam Epitaxy

Vertical GaN-on-GaN p-n diodes grown by ammonia molecular beam epitaxy (NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> MBE) are reported in this letter for prospective high-power application devices. The diodes, consisting of only 4 μm drift layer with low unintentionalbackgrounddoping of ~ 3×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , showed a maximum breakdown voltage of >1.0 kV and a low differential specific on-resistance of 0.28 mΩ-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Moreover, these diodes exhibited an excellent rectifying behavior with a low minimum ideality factor of 1.36 and a punch-through electric field comparable to the state-of-the-art vertical GaN-on-GaN p-n diodes grown by metalorganic chemical vapor deposition (MOCVD). These results suggest that all-MBE vertical GaN-on-GaN p-n diodes grown by NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> MBE can be promising for small-size high-power electronic devices.

Quantum Well Solar Cells: Principles, Recent Progress, and Potential

Quantum well solar cells, as a promising approach for next-generation photovoltaic technology, have received great attention in the last few years. Recent developments in materials growth and device structures of quantum wells have opened up new avenues for the incorporation of quantum well structures in next-generation III/V multi-junction solar cells. In this paper, the advantages and challenges of growing quantum wells in the unintentionally doped (i) region of p-i-n solar cells are reviewed. We focus on the recent progress in 1.1–1.3 eV strain-balanced InGaAs/GaAsP, 1.6–1.8 eV strain-balanced and lattice-matched InGaAsP/InGaP, and >2.1 eV strained InGaN/GaN quantum well solar cells, including optimization of the quantum well growth conditions and improving the solar cell structure. For each material system, the challenges associated with materials growth and device performance such as critical layer thickness constraints, strain balance, bandgap tunability, and carrier transport limitations, are discussed. The performance of each quantum well solar cell is compared with bulk absorber operating in the same bandgap range, with the advantages of each being highlighted. The effect of the unintentional background doping on carrier collection (by drift) is presented through modeling and recent experimental results. The recent strategies to enhance the electric field distribution across the quantum well region are reviewed. The potential of incorporating quantum well structures in next-generation multi-junction devices is also discussed.

Investigation of Si and O Donor Impurities in Unintentionally Doped MBE-Grown GaN on SiC(0001) Substrate

We have investigated the unintentional n-type background doping in GaN(0001) layers grown on semi-insulating 4H-SiC(0001) substrate by plasma-assisted molecular beam epitaxy under Ga-rich conditions at growth temperatures from 780°C and 900°C. All layers exhibited very smooth surface morphology with monolayer steps as revealed by atomic force microscopy. Hall-effect measurements showed that the sample grown at 900°C had carrier concentration of 9.8 × 1017 cm−3 while the sample grown at 780°C had resistivity too high to obtain reliable measurements. Secondary-ion mass spectroscopy revealed O and Si concentrations of <1017 cm−3 in the sample grown at 900°C but >1017 cm−3 in the sample grown at 780°C. The trend for the atomic concentrations of O and Si, which are common donor impurities in GaN, was thus contrary to the trend of the carrier concentration. The full-width at half-maximum for x-ray rocking curves obtained across the GaN(0002) and GaN(10 bar{1} 5) reflections for the sample grown at 900°C was 62 arcsec and 587 arcsec, respectively. The half-width increased with decreasing growth temperature. The atomic concentrations of O and Si are too low to account for the unintentional background doping levels. A possible explanation proposed in early reports for the background doping is N-vacancies.

Electrode-stress-induced nanoscale disorder in Si quantum electronic devices

Disorder in the potential-energy landscape presents a major obstacle to the more rapid development of semiconductor quantum device technologies. We report a large-magnitude source of disorder, beyond commonly considered unintentional background doping or fixed charge in oxide layers: nanoscale strain fields induced by residual stresses in nanopatterned metal gates. Quantitative analysis of synchrotron coherent hard x-ray nanobeam diffraction patterns reveals gate-induced curvature and strains up to 0.03% in a buried Si quantum well within a Si/SiGe heterostructure. Electrode stress presents both challenges to the design of devices and opportunities associated with the lateral manipulation of electronic energy levels.

Impact of doping on InAs/GaAs quantum-dot solar cells: A numerical study on photovoltaic and photoluminescence behavior

We investigate the effect of doping on quantum dot (QD) solar cells by analysing their behavior in terms of photovoltaic characteristic, external quantum efficiency, and photoluminescence (PL) at room temperature. The analysis addresses the two most widespread methods for QD selective doping, namely modulation and direct doping, to gain a comprehensive device-level assessment of the impact of doping profile and density on the solar cell behavior. Devices are simulated using a physics-based model that accurately describes QD carrier dynamics within a semi-classical drift-diffusion-Poisson model. Different scenarios in terms of crystal quality are considered: in the high-quality material, close to radiative limit, large open circuit voltage recovery is predicted, due to the suppression of radiative recombination through QD ground state. In the defective material, significant photovoltage recovery is also attained owing to the suppression of both nonradiative and QD ground state radiative recombination. In both cases, PL emission from extended wetting layer states becomes dominant at high doping density. The interplay between nonradiative and QD radiative recombination channels, and how their interaction is modified by doping, are analyzed in detail. Strong influence on the cell behavior of unintentional background doping of interdot layers and markedly nonlinear behavior of open circuit PL with respect to excitation intensity are demonstrated. The resulting picture provides new insight on the experimental results in literature.

Meaning of reaction orders in polymer:fullerene solar cells

Nongeminate recombination in polymer:fullerene solar cells is frequently characterized using transient optoelectronic measurements that allow the determination of recombination rates, charge carrier lifetimes, and average charge carrier concentrations as a function of voltage. These data are often interpreted in terms of an empirical reaction order defining how recombination depends on measured charge density. In polymer:fullerene solar cells, the empirical reaction orders are often considerably larger than 2, which had previously been explained in terms of the nonlinear relationship between mobile and trapped charge carriers in the presence of an exponential tail of localized states. Here, we show that experimentally determined reaction orders depend not only on the shape of the density of states but also on the spatial distribution of carriers. In particular, in solar cells with small depletion regions due to small active layer thicknesses or due to large unintentional background doping of the polymers, the reaction order can assume values that are much larger than the value expected from the shape of the density of states alone.

Integration of InGaAs Channel n-MOS Devices on 200mm Si Wafers Using the Aspect-Ratio-Trapping Technique

We report on the fabrication on InGaAs/InP implant free quantum well (IFQW) n-MOSFET devices on 200mm wafers in a Si CMOS processing environment. The starting virtual InP substrates were prepared by means of the aspect-ratio-trapping technique. Post CMP these substrate resulted in a planar substrate with a rms roughness of 0.32 nm. After channel and gate processing source drain regions were formed by the selective epitaxial growth of Si doped InGaAs. Contact to the source/drain regions was made by a standard W-plug/metal 1 process. The contact resistance was estimated to be on the order of 7x10-7 Ω.cm2. Fully processed devices clearly showed gate modulation albeit on top of high levels of source to drain leakage. The source of this leakage was determined to be the result of the unintentional background doping of the InP buffer layer. Simulations show that the inclusion of the p-InAlAs between the InP and InGaAs can effectively suppress this leakage. This work is a significant step towards the integration of InGaAs based devices on a standard CMOS platform.

Properties of Si-doped GaN and AlGaN/GaN heterostructures grown by RF-MBE on high resistivity Fe-doped GaN

Abstract Silicon-doped GaN epilayers and AlGaN/GaN heterostructures were developed by nitrogen plasma-assisted molecular beam epitaxy on high resistivity iron-doped GaN (0001) templates and their properties were investigated by atomic force microscopy, x-ray diffraction and Hall effect measurements. In the case of high electron mobility transistors heterostructures, the AlN mole fraction and the thickness of the AlGaN barrier employed were in the range of from 0.17 to 0.36 and from 7.5 to 30 nm, respectively. All structures were capped with a 2 nm GaN layer. Despite the absence of Ga droplets formation on the surface, growth of both GaN and AlGaN by RF-MBE on the GaN (0001) surfaces followed a step-flow growth mode resulting in low surface roughness and very abrupt heterointerfaces, as revealed by XRD. Reciprocal space maps around the ( 10 1 5 ) reciprocal space point reveal that the AlGaN barriers are fully coherent with the GaN layer. GaN layers, n -doped with silicon in the range from 1015 to 1019 cm−3 exhibited state of the art electrical properties, consistent with a low unintentional background doping level and low compensation ratio. The carrier concentration versus silicon cell temperatures followed an Arhenius behaviour in the whole investigated doping range. The degenerate 2DEG, at the AlGaN/GaN heteroiterfaces, exhibited high Hall mobilities reaching 1860 cm2/V s at 300 K and 10 220 cm2/V s at 77 K for a sheet carrier density of 9.6E12 cm−2. The two dimensional degenerate electron gas concentration in the GaN capped AlGaN/GaN structures was also calculated by self-consistent solving the Schrodinger–Poisson equations. Comparison with the experimental measured values reveals a Fermi level pinning of the GaN (0001) surface at about 0.8 eV below the GaN conduction band.

The influence of composition and unintentional doping on the two-dimensional electron gas density in AlGaN/GaN heterostructures

We have studied the influence of Al content, AlGaN layer thickness, and unintentional background doping by oxygen on the two-dimensional electron gas (2DEG) density in AlGaN/GaN heterostructures. Hall measurements were made on samples grown with molecular beam epitaxy. The 2DEG densities in the range 2–3×1013 cm−2 were measured. A one-dimensional Schrodinger-Poisson model was used to describe the heterostructure. The calculations gave two-dimensional electron densities in accordance with measured values. The electron density is very sensitive to the Al concentration in the AlGaN layer, whereas the sensitivity to layer thickness is small. Our simulations also showed that the two-dimensional concentration increased 50% when the free-carrier concentration changed from 1015 cm−3 to 1018 cm−3. The relation between donor concentration and free-carrier concentration was found to agree when using oxygen ionization energy as a parameter.

Effect of quantum well location on single quantum well p-i-n photodiode dark currents

The photocurrent available from a p-i-n solar cell can be increased by the addition of quantum wells (QWs) to the undoped region. At the same time the QWs reduce the open-circuit voltage by introducing areas of lower band gap where recombination is enhanced. This increase in recombination should be as small as possible for the most favorable effect on the photovoltaic efficiency of the device. Theoretical considerations indicate that nonradiative recombination, which is the dominant loss mechanism in AlxGa1−xAs/GaAs QW structures, may be reduced by positioning the QWs away from the point where the electron-hole product is a maximum. For p-i-n diodes, where recombination is greatest at or near the center of the space charge region, this means locating the QWs closer to the doped regions. Spectral response should not be affected so long as the QWs are still located within the field bearing region. Thus, improved photovoltaic performance may be expected through strategic location of the QWs. We report on measurements on a series of Al0.36Ga0.64As p-i-n photodiodes, three of which contained a single 87 Å GaAs QW within the i region, and one which was a control sample with no QW. The three QW samples were grown with the QW located nearer to the p-doped layer, centrally, and nearer to the n-doped layer, respectively. Spectral response measurements confirm that for good quality samples photocurrent is independent of QW location within the depleted region. Contrary to expectations, the dark current is highest for the sample with the QW located closer to the n region. We analyze these results in terms of structure and doping profile, and compare them with the predictions of a self-consistent model. The observed behavior is attributed to a relatively high unintentional background doping in the intrinsic region.

Epitaxial growth of SiC in a new multi-wafer VPE reactor

SiC epitaxial layers have been grown in a commercial multi-wafer reactor. Results from the initial growth runs are presented. The reactor is vertical and has a high speed rotating susceptor that can support up to six 50 mm diameter wafers. The surface morphology of the grown layers are specular and show no indication of step-bunching. The unintentional background doping is p-type in the low 10 15 cm −3 range, consisting mainly of Al. Both N and Al have been used for doped layers showing wide doping range capability and sharp transients. The best uniformity in thickness and doping achieved so far on the same 35 mm wafer are ±7% and ±10%, respectively.

Cathodoluminescence Studies of Bound Excitons and Near Band Gap Emission Lines in Boron- and Phosphorus-Doped CVD-Diamonds

AbstractCathodoluminescence measurements at cryogenic temperatures are reported on boron- and phosphorus-doped CVD-diamond films grown on silicon substrates. Boron and phosphorus concentrations were determined by SIMS measurements; for boron, they reached from unintentional background doping levels up to 3500 ppm. At increasing boron concentrations, the radiative recombination of boron bound excitons (BEto) at 5.22 eV photon energy systematically broadens and shifts down to 4.99 eV whereas the free exciton emission (FEto) disappears for 40 ppm and higher. In the phosphorus-doped films we observe new lines at 5.16 eV and 4.99 eV which we ascribe to TO- and (TO+Or)-phonon assisted transitions of an exciton bound to a shallow impurity other than boron, possibly phosphorus or a phosphorus-related shallow complex.

Electric-field-dependent electroreflectance spectra of visible-band-gap (InAlGa)P quantum-well structures

We present results from the first studies of electric-field effects on optical transitions in visible-band-gap InGaP/InAlGaP multiple-quantum-well (MQW) structures. These structures, grown at 775 °C by metalorganic vapor phase epitaxy on (100) GaAs substrates misoriented 6° towards P(111〉〈111〉A, consist of nominally undoped MQWs surrounded by doped In0.49Al0.51P cladding layers to form p-i-n diodes. The Stark shifts of various allowed and forbidden quantum-well transitions were observed in bias-dependent electroreflectance spectra of In0.49Ga0.51P/In0.49(Al0.5Ga0.5)0.51P MQW samples with 10-nm-thick layers. We find the magnitude of these shifts to depend on the details of the Mg doping profile, confirming the importance of Mg diffusion and unintentional background doping in these materials. Our results show that (InAlGa)P materials are promising for visible-wavelength electro-optic modulator applications.

Electromodulation spectroscopy of metalorganic-vapor-phase-epitaxy-grown GaAs/AlxGa1-xAs multiple quantum wells.

This paper describes 17-K, electric-field-dependent electroreflectance and electrotransmission measurements on two GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As multiple-quantum-well (MQW) samples, grown by metalorganic vapor-phase epitaxy (MOVPE). Photocurrent and transmission spectra are also taken at different biases for comparison. It is shown how each of these four spectroscopies provide complementary information and how this allows clear deductions to be made about the microscopic structure of the MQW stack. In particular, each technique shows splittings of the E1H1 and E1L1 features, which it is argued derive from sudden changes in the well width at certain depths in the stack. Several measurements demonstrate that the region with wider wells lies deeper in the structure. Under flatband conditions, these monolayer splittings would be smaller than the optical linewidth of the individual quantum wells and would thus be unresolvable. However, the unintentional background doping of the MQW's results in the field being larger at the back of the stack than that at the front, and this enhances the splitting. Hence we have been able to observe monolayer splittings in wider wells (87 and 158 \AA{}) than previously reported. The size of the splittings for both samples, the relative height of the split features in the different spectroscopies, and their dependence on the applied bias are all in agreement with our model for the structure of the stack. The imperfections of the stack have important consequences for MOVPE-grown optoelectronic devices that incorporate MQW's.

Characterization of high purity GaAs films grown by molecular-beam epitaxy from a solid As cracker

Two series of high-quality, unintentionally doped GaAs films were grown to evaluate a solid As cracker source with a graphite crucible. In the first series, material properties comparisons were made between a film grown with the As cracker in the dimer mode, a film from the same cracker source but in the tetramer mode, and lastly, a film from a conventional As cell. The film grown with the dimers had a n-type unintentional background doping level of 5×1014 cm−3 and a 104 000 cm2/V s mobility at 77 K. The films grown with As tetramers gave low p-type unintentional background doping levels of less than 1.1×1014 cm−3, respectively. Photoluminescence spectra taken of these films at 4 K showed various excitonic transitions and comparable C incorporation. In the second series, films grown at As2/Ga incorporation rate ratios of 1.1, 1.5, and 2.0 had layers that were p-type, resistive, and n type, respectively. Samples taken from the film grown at the 2.0 ratio had 77 K mobilities as high as 195 000 cm2/V s, with a corresponding net carrier concentration of 3.8×1014 cm−3. These mobility values are the highest reported for GaAs grown with a solid As dimer source. Results from a variable temperature van der Pauw measurements of another sample cut from this film gave a donor ionization energy of 2.6 meV.