Pn Homojunction Research Articles

A conjugated polymer pn junction.

Dopant counterion diffusion has made the conjugated polymer pn homojunction a challenging target for decades. We report the electrochemical fabrication of a polyacetylene pn homojunction based on internally compensated forms where the dopant counterions are covalently bound to the polymer backbone. After drying under vacuum, the pn junction exhibits diode behavior with the ratio of the forward to reverse current at 2 V being 7. Despite such modest diode behavior, the fabricated pn junction is significant because it demonstrates the utility of internal compensation in the fabrication of metastable interfaces between dissimilarly doped conjugated polymers.

Electroluminescence and lasing properties of highly Bi-doped PbTe epitaxial layers grown by temperature difference method under controlled vapor pressure

Electroluminescence (EL) from PbTe pn homojunctions with a highly Bi-doped n-type emission layer with a concentration of NBi > 1019 cm−3, grown by the temperature difference method (TDM) under controlled-Te vapor pressure has shown a positive shift of the peak-photon energy, which coincides with the model that Bi atoms act as both donors and acceptors, and they make the nearest lattice-site or very close donor-acceptor (DA) pairs. Broad-contact pn junctions with highly Bi-doped layers easily cause laser emission compared to the difficulty in the lasing operation of undoped pn junctions, which suggests that the nearest lattice-site Bi-Bi DA pairs act as strong radiative centers in PbTe.

InAs-based heterojunction bipolar transistors

The first InAlAs/InAs heterojunction bipolar transistors are reported. A current gain of 100 at room temperature is measured. The base-collector junction is an InAs pn homojunction, which is optimised to have low reverse leakage at room temperature. The emitter is pseudomorphic AlInAs, linearly graded to increase the barrier which inhibits hole injection into the emitter.

Diffusion Behavior and Microstructural Properties of the CBD-ZnS/CIGS Interface Boundary

ABSTRACTThe diffusion behavior and microstructural properties of the CBD-ZnS/Cu(In,Ga)Se2 (CIGS) interface of high-efficiency Cd-free CIGS thin film solar cells with efficiencies exceeding 17 % have been investigated using energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM). CBD-ZnS thin layers are found to grow with small polycrystalline grains that consist of sphalerite and wurtzite phases. This is in sharp contrast to CBD-CdS layers which grow epitaxially on CIGS grains. Micro-EDX analysis revealed Zn indiffusion into the CIGS thin layer a distance of approximately 40 nm from the CBD-ZnS/CIGS interface boundary after air-annealing at 200°C, whereas no diffusion of Zn was observed before annealing. These results suggest that the main role of the air-annealing process is not substitution of Se vacancies by oxygen atoms but the diffusion of Zn into CIGS which may result in the formation of a pn homojunction. On the other hand, negligible Zn-diffusion into a CuGaSe2 thin film was found at the CBD-ZnS/CuGaSe2 interface boundary even after air-annealing. The cell performance of CBD-ZnS/CIGS thin film solar cells is discussed in connection with the Ga/(In+Ga) atomic ratio, conduction band-offset and Zn-diffusion.

Formation of pn Homojunction in Cu(InGa)Se2 Thin Film Solar Cells by Zn Doping

In this study, a pn homojunction was intentionally fabricated in the Cu(InGa)Se2 (CIGS) layer by Zn doping. For Zn doping of the CIGS layer, Zn was evaporated after CIGS formation, and a potential improvement in cell performance was confirmed by this technique. Furthermore, Zn diffusion into the CIGS film was investigated by secondary ion mass spectroscopy (SIMS). A conductivity-type conversion from p-type to n-type was studied by the measurement of the cross-sectional electron beam-induced current (junction EBIC: JEBIC) and the spectral response of solar cells. A conversion efficiency of 11.5% has been achieved using the Zn-doped CIGS layer without a buffer layer and by the formation of a pn homojunction in the CIGS absorber.

GaN/sub 0.011/P/sub 0.989/-GaP double-heterostructure red light-emitting diodes directly grown on GaP substrates

Novel red light-emitting diodes (LEDs) based on GaN/sub 0.011/P/sub 0.989/-GaP double-heterostructure (DH) directly grown on [100] GaP substrates have been fabricated for the first time. The samples were grown by gas-source molecular beam epitaxy with an RF nitrogen radical beam source to incorporate N in GaP. Compared to conventional GaAs-based AlGaInP red LEDs, this novel LED structure eliminates the complicated steps of etching the light-absorption GaAs substrate and wafer-bonding to a transparent GaP substrate. Based on the uncoated devices made with the heterojunction bipolar transistor masks, the emission efficiency of the DH LEDs is 20 times stronger than that of a GaNP pn homojunction diode.

Scanning capacitance microscopy measurements and modeling: Progress towards dopant profiling of silicon

A scanning capacitance microscope (SCM) has been implemented by interfacing a commercial contact-mode atomic force microscope with a high-sensitivity capacitance sensor. The SCM has promise as a next-generation dopant-profiling technique because the measurement is inherently two dimensional, has a potential spatial resolution limited by tip diameter of at least 20 nm, and requires no current carrying metal–semiconductor contact. Differential capacitance images have been made with the SCM of a variety of bulk-doped samples and in the vicinity of pn junctions and homojunctions. Also, a computer code has been written that can numerically solve Poisson’s equation for a model SCM geometry by using the method of collocation of Gaussian points. Measured data and model output for similar structures are presented. How data and model output can be combined to achieve an experimental determination of dopant profile is discussed.

Effects of interface and bulk recombination on switching characteristics of AlGaAs/GaAs pnpn bistable device

Device modeling is used to analyze the steady-state current voltage characteristics of double heterostructure AlGaAs/GaAs pnpn switches in the presence of interface and bulk recombination mechanisms. Simulation results show that the holding current increases significantly by shortening the carrier lifetime at heterojunction interface, while the holding voltage and breakover point remain relatively constant. Results also indicate that shortening the carrier lifetime in the inner pn homojunction region only increases the off-state leakage current. These results are in agreement with experimental data obtained by others, and may be used to design a device with optimum switching performance.

NUMERICAL INVESTIGATION OF CARRIER TRANSPORT IN DOUBLE HETEROSTRUCTURE

Numerical device simulation is developed to study the steady‐state and transient current‐voltage characteristics of double heterostructure AlGaAs/GaAs PNPN electro‐photonic device when its performance is influenced by the presence of interface and bulk recombination mechanism. The simulation results show that the holding current and voltage and the breakover point are strongly affected by varying the minority carrier lifetime at outer heterojunctions. Numerical results also indicate that shortening the minority carrier lifetime in the inner PN homojunction region only increases the OFF‐state current. These results are in agreement with experimental data on AlGaAs/GaAs PNPN devices. The numerical modelling approach taken in this study is shown to be essential in the design and optimization of PNPN switch.

A silicon homojunction infrared detector having an active metal film on an n/sup ++/ layer

A silicon n/sup ++/pn homojunction infrared detector, in which a degenerate n/sup ++/ layer is backed by a metal film forming an ohmic contact, has been proposed and studied. The metal film is a photoelectric conversion region along with the n/sup ++/ layer. Although, for an n/sup ++/pn detector without the metal film, very poor rectifying properties are observed when the n/sup ++/ layer thickness is extremely reduced, the new detector, employing a thin PtSi film as the metal film, shows normal diode I-V characteristics, since the PtSi film provides increased surface conductivity. The new detector has achieved an increase in operatable temperature, or an extension of cutoff wavelength, and operated with cutoff wavelengths of 11.9 /spl mu/m, 18.7 /spl mu/m and about 30 /spl mu/m at 70 K, 50 K, and 30 K, respectively, because the saturation current density for the new detector has been reduced to about one tenth that for the previously reported n/sup ++/pn detector. The responsivity for the new detector has increased to 1.1-3.8 times as large as that for the previously reported n/sup ++/pn detector, when both detectors have the same cutoff wavelength. >

Study of novel chemical surface passivation techniques on GaAs p n junction solar cells

Novel methods of GaAs surface passivation are investigated. Passivation is acheived by simple chemical treatments using aqueous solutions of Na2S, KOH, RuCl3, and K2Se. GaAs pn homojunction solar cells are used to evaluate the effectiveness of these passivation techniques. A significant reduction in minority-carrier surface recombination velocity is demonstrated. In the best case, the surface recombination velocity decreased from 5×106 cm/s (untreated surface) to 103 cm/s. In addition, we observe improvements in solar cell photogenerated current, short wavelength spectral response, open-circuit voltage, and junction ‘‘dark’’ current.

Low-threshold GaInAsP bridge-contacted planar buried-ridge structure lasers

A new type of GaInAsP/InP buried-heterostructure laser, the bridge-contacted planar buried-ridge structure (BC-PBRS) laser, is reported. Current confinement is obtained by reducing the effective area of the forward-biased pn homojunctions at both sides of the active region. Owing to the self-aligning process technique, the lateral InP confining layer widths are highly controllable and reproducible. Threshold currents of 10 mA and maximum operation temperatures exceeding 100°C are achieved in the 1.3μm wavelength region.

Effects of Na2S and (NH4)2S edge passivation treatments on the dark current-voltage characteristics of GaAs p n diodes

We have investigated the dark current-voltage characteristics of GaAs pn homojunctions whose surfaces have been passivated with Na2S and (NH4)2S chemical treatments. Reductions in 2kT perimeter recombination currents by a factor of 3.2 were obtained for the two treatments. A shunt leakage, observed at low forward bias for the Na2S treated devices, is virtually eliminated with the (NH4)2S treatment. It is also shown that even the high quality, large area (0.25 cm2) pn diodes used in this study are dominated by 2kT edge currents before passivation.

Fabrication of an NpM GaInAs/InP bipolar transistor by a two-step epitaxial process

A two-step liquid-phase-epitaxial process has been used to fabricate widegap-emitter Schottky collector transistors. After the growth of the first two epitaxial layers the sample has been structured and then overgrown in a second run. In that way a pn homojunction in the widegap material (InP) underneath the extrinsic base-emitter region and a pn heterojunction under the collector have been formed. First transistors fabricated show a current gain of 10 in the common-emitter configuration.

Fabrication, characterization, and analysis of mass-transported GaInAsP/InP buried-heterostructure lasers

The mass-transported buried-heterostructure lasers have been investigated in detail. Techniques have been developed for fabrication control and wafer characterization. High yield of low-threshold lasers was obtained with the lowest threshold current being 5.5 mA. Differential quantum efficiencies as high as 28 percent per facet and smooth far-field patterns were also obtained. Deep Zn-diffusion and higher p-doping have been used to improve the linearity of the light-current characteristics. Linear light output to greater than 13 mW per facet and well-behaved threshold-temperature dependence were achieved. A conformal mapping technique has been used to calculate the voltage distribution in the laser mesa. The forward-bias voltage thus obtained for the InP pn homojunctions in the transported regions is shown to be capable of resulting in the sublinear dependence of light output on current observed in lasers with lightly p-doped cap layers. The current limit within which the laser can be operated without leakage through the homojunctions has been calculated for various device geometries and doping levels.

Boundary conditions for pn heterojunctions

Boundary conditions for the excess minority carrier concentrations at the space-charge region (SCR) edges are developed and used to evaluate the current-voltage characteristics of pn heterojunctions. Carrier transport by drift and diffusion within the SCR, by thermionic emission across the interface, and by minority carrier diffusion within the quasi-neutral bulk are included. When transport across the interface is slower than that across the SCR, the results reduce to the emission-diffusion model of Perlman and Feucht. Alternatively, when carrier transport within the SCR is the limiting process, a result analogous to the diffusion theory of metal-semiconductor diodes results. Finally, if the current is limited by minority carrier diffusion within the quasi-neutral bulk, the quasi-Fermi levels are constant within the SCR and the results are similar to those for pn homojunctions.

Integrated tandem solar cells

The performance of integrated tandem solar cell (ITSC) devices made from pn homojunctions has been investigated. The intrinsic electrical interconnection present in an ITSC device places important restrictions on the choice of the energy gaps of the semiconductors to be used. Total conversion efficiencies are calculated and they are compared both with single solar cells and with independent tandem solar cell systems. It is concluded that ITSC devices are attractive solar energy converters provided that proper care is taken in the choice of materials used.

V-1 a single longitudinal and fundamental transverse mode oscillation in AlGaAs transverse junction stripe lasers

(TSS) lasers.',* However these excellent mode properties of TJS lasers have not yet been accounted for sufficiently in the p~evious paper.2 We have investigated particularly the spectral behavior of these lasers in detail to throw light upon the mechanisms responsible for the single mode oscillation. The TJS laser studied has a GaAs Zn diffused pipn homojunction structure sandwiched between two AlGa 4s layers, in which the AlGaAs-GaAs-AlGaAs double heterostructure forms the stripe geometry, and carriers are injecled laterally through the GaAs pn junction perpendicular to [he double heterojunction planes.' The carrier concentration i.n the third layer, in which the active region formed, is chosen to be 2~10'~ ~m-~. The TJS laser has a defined three dimonsional optical waveguide in which the active region is Ihe GaAs p-region surrounded by two heterojunctions, a [1-'p boundary and a pn homojunction. The dimensions of 7'JS lasers are approximately 0.35 pm in thickness, 2.5 pm in width and 300 pm in cavity length. Typical thresh3ld currents are 40 mA at 300°K. The following results were obtained in this stu3y. Firstly, a strong saturation of the spontaneous emiss on above threshold has been first observed at room temperat Ire with cw operation in these lasers. This could be explained loy the fact that the transverse mode spreads outside the gain region owing to the weak optical confinement present in these homojunction lasers with a thin heterojunction stripe. This saturation of the spontaneous emission makes it possible to suppress the oscillation of higher order transverse rnoJes and to oscillate in the fundamental transverse mode for a wide range of the current more than three times threshokl at which catastrophic damage of the mirror occurs. Secondly, a single longitudinal mode oscillation accompanied with a strong suppression of oscillation for the ccmpeting adjacent modes occurs slightly above threshold. -i'his is attributed to a mode competition mechanism which is! an inherent property in these lasers. In well-Ilehaved lasers, the single mode oscillation accompanied with complete supp '8ssion of oscillation for the adjacent modes at threshold have been observed, which shows that the TJS laser is a halnogeneously broadened laser.