Band Gap Discontinuity Research Articles

Buffer Layers for Narrow Bandgap A-SIGE Solar Cells

AbstractIn high efficiency narrow bandgap (NBG) a-SiGe solar cells, thin buffer layers of unalloyed hydrogenated amorphous silicon (a-Si) are usually used at the interfaces between the a-SiGe intrinsic layer and the doped layers. We investigated the effect of inserting additional a-SiGe interface layers between these a-Si buffer layers and the a-SiGe absorber layer. We found that such additional interface layers increase solar cell VOC and FF sizably, most likely due to the reduction or elimination of the abrupt bandgap discontinuity between the a-SiGe absorber layer and the a-Si buffer layers. With these improved narrow bandgap solar cells incorporated into the fabrication of triple-junction a-Si based solar cells, we obtained triple cells with initial efficiency of 10.6%.

Role of Bandgap Grading for the Performance of Microcrystalline Silicon Germanium Solar Cells

AbstractThe optical absorption of microcrystalline silicon germanium alloys (μc-Si1-xGex:H) increases in the near infrared region with increasing germanium content. Therefore, these alloys are promising candidates for the application as intrinsic absorber material in thin film solar cells with tandem and triple structure or IR-detectors. The material properties for a germanium content (x) up to 0.6 and the performance of solar cells based on this material were investigated. Graded bandgap structures are used to optimize the cell performance. For cells with x > 0.3 a continuously graded bandgap in the rear 20 nm of the i-layer (at the i/n interface) results in an enhancement of the open circuit voltage by 40-80 mV compared to an abrupt bandgap discontinuity. When the design of the p/i interface region is changed in a similar way no effect on Voc is observed.

Enhanced Thermionic Emission Cooling in High Barrier Superlattice Heterostructures

AbstractThermionic emission current in heterostructures can be used to enhance thermoelectric properties beyond what can be achieved with conventional bulk materials. The Bandgap discontinuity at the junction between two materials is used to selectively emit hot electrons over a barrier layer from cathode to anode. This evaporative cooling can be optimized at various temperatures by adjusting the barrier height and thickness. Theoretical and experimental results for nonisothermal thermionic emission in heterostructures are presented. Single stage InGaAsP-based heterostructure integrated thermionic (HIT) coolers are fabricated and characterized. Cooling on the order of a degree over one micron thick barriers has been observed. Nonisothermal transport in highly doped tall barrier superlattices is also investigated. An order of magnitude improvement in cooling efficiency is predicted for InAlAs/InP superlattices.

Effect of bandgap discontinuity on the cut-off frequency, base transit time and junction capacitance of npn AlGaAs/GaAs heterojunction bipolar transistors

The effect of the quasi-Fermi level discontinuity and the electrostatic potential nonlinearity on the transconductance, junction capacitance, base transit time and the emitter charge time of heterojunction bipolar transistors (HBT) are investigated. The analysis takes into account the effect of the thermionic emission boundary condition at the heterojunction interface on the base transit time, the transconductance and the junction capacitance. It is evident from the analysis that the base transit time and the transconductance are functions of the electrons injected from the emitter. Calculation shows that the effect of a high level of electron injection to the base on the base transit time is less effective for an abrupt bandgap emitter than that for a graded-gap emitter HBT.

20 GHz high performance planar Si/InGaAs p-i-n photodetector

Planar Si/InGaAs wafer fused p-i-n photodetectors were fabricated and measured. They show high internal quantum efficiency, high speed, record low dark current, and no evidence of charge trapping, recombination centers, or a bandgap discontinuity at the heterointerface.

Determining the band discontinuities of ZnSe/GaAs and ZnMgSSe/GaAs heterojunctions using free electron laser

The conduction band discontinuities of ZnSe/GaAs and ZnMgSSe/GaAs heterojunctions were investigated using the free electron laser (FEL) internal photoemission technique. This technique is based on the photocurrent spectroscopy utilizing the tunability and intense peak power of the FEL operative in the infrared range. We found the conduction band discontinuities of 113 (ZnSe/GaAs) and 180 meV (ZnMgSSe/GaAs). It is suggested that the band gap discontinuity between ZnSe and ZnMgSSe is δEc (conduction band): δEv (valence band)=0.45:0.55 at 77 K.

Exciton magnetic polarons in CdTe/Cd 1−xMn xTe quantum wells with high manganese contents

We report on optical studies of exciton magnetic polarons in CdTe/Cd 1−xMn xTe quantum well structures with Mn contents 0.4 ≤ x ≤ 0.8. The magnetic polaron energies and formation times are measured by cw- and time-resolved photoluminescence under selective excitation. We find an overall increase of the polaron energy with increasing Mn-concentration in the whole range of Mn contents studied and attribute it to diffusion induced changes of the Mn profile at the interfaces between nonmagnetic and semimagnetic layers. In addition, we find that in these quantum structures the formation time of the polaron is predominantly determined by the Mn content and not by quantum confinement effects. The large band gap discontinuity (up to 1.2 eV) causes large splittings between heavy- and light-hole-states and leads to a strong anisotropy in the suppression of the magnetic polaron formation by magnetic fields applied parallel and perpendicular to the structure growth axis. For both orientations of magnetic field the polaron suppression is qualitatively described by model calculations.

Theoretical investigation of minority carrier leakages of high-power 0.8 μm InGaAsP/InGaP/GaAs laser diodes

We report a theoretical model that accurately describes the effects of minority carrier leakage from the InGaAsP waveguide into InGaP cladding layers in high-power aluminum-free 0.8 μm InGaAsP/InGaP/GaAs separate confinement heterostructure lasers. Current leakage due to the relatively low band-gap discontinuity between the active region and the InGaP barrier can be eliminated by employing laser diodes with cavity length longer than 500 μm. Experimental results for lasers grown by low-pressure metalorganic chemical vapor deposition are in excellent agreement with the theoretical model.

Energy band lineup at the porous-silicon/silicon heterointerface measured by electron spectroscopy

The energy band gap of light-emitting porous silicon is determined by high-resolution electron energy loss spectroscopy, and the valence band edge of porous silicon with respect to its Fermi level is measured by ultraviolet photoelectron spectroscopy. By combining the results with that measured from clean Si, a picture of band lineup at the porous-silicon/p-Si heterointerface is proposed, in which 70% of the total band gap discontinuity occurs at the valence band edge.

Direct determination of the band discontinuities in InxGa1-xP/InyAl1-yP multiple quantum wells.

The band structure of ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$P/${\mathrm{In}}_{0.50}$${\mathrm{Al}}_{0.50}$P multiple quantum wells grown by molecular-beam epitaxy has been determined from pressure-dependent-photoluminescence measurements at low temperature. The photoluminescence signals from the direct-gap well and the indirect barrier were monitored as a function of pressure up to 4 GPa. High pressure transformed the multiple quantum well from a type I to a staggered aligned type II at 1.1 GPa. This transition was evidenced by the appearance of a photoluminescence signal due to the recombination of carriers separated in momentum and space. The simultaneous detection of this transition and that of the barrier material, allowed the direct determination of a valence-band offset energy of (0.24\ifmmode\pm\else\textpm\fi{}0.05) eV, without requiring any information on parameters of the bulk materials. Considering that the total band-gap discontinuity for this heterostructure system is 0.50 eV at 20 K, an approximate band-gap splitting of 52:48 is determined to be the band lineup at the InGaP/InAlP interface. Variations in the pressure coefficients of the indirect transitions in the barrier indicated that the valence band alignment changes with pressure at a rate of \ensuremath{\approxeq}18 meV/GPa, due to shifting of the heavy- and light-hole states with biaxial strain generated by applying pressure in the epilayers.

Band-gap discontinuity control for InGaAs/InGaAsP multiquantum-well structures by tensile-strained barriers

The changes in InGaAsP conduction- and valence-band edge energies due to tensile strain, have been measured by optical methods at 77 K. The measured increases in the conduction- and valence-band edge energies for the 0.5% tensile-strained InGaAsP, compared to the unstrained InGaAsP (1.2 μm band-gap wavelength), are 70 and 38 meV, respectively. The experimentally obtained values are in accordance with calculated values. The results show that the ratio of the conduction- and valence-band discontinuities in InGaAs/InGaAsP multiquantum-well structures can be controlled by the tensile-strained barrier.

Simulation and modeling of p-n-p-n optical switches

A one-dimensional semiconductor device simulation program (FARDEV) with current boundary conditions is developed to study the steady-state and the transient characteristics of heterostructure four-layer p-n-p-n optical switches. Avalanche effect, radiative recombination, and band-gap discontinuities are included in the model. To demonstrate the use of the simulator, the modeling results are studied for both InP-In/sub 0.53/Ga/sub 0.47/As and Al/sub 0.3/Ga/sub 0.7/As-GaAs devices, and compared with experimental results in the latter case. The effects of optical generation and carrier lifetime on electrical characteristics of p-n-p-n switches are investigated. Simulation and experiment agree on holding current and voltage and on high-current on-state characteristics, while the limitation and simplification of the numerical model leads to discrepancy in the low-current off-state. The simulator is shown to be useful in evaluating the effects of device geometry, material parameters, avalanche mechanism, heterostructure spacing, and light generation on key switching parameters of four-layer p-n-p-n switches. >

Trap behavior in nonintentionally doped AlGaAs/GaAs single quantum well structures

The GaAs/AlGaAs single quantum well (SQW) samples with nonintentionally doped confining layers were studied using deep level transient spectroscopy (DLTS) and capacitance-voltage-temperature. A sizeable DLTS signal was observed and believed to be from the thermal emission of the well electrons. However, it was found that the major signal peak was accompanied by two subpeaks and thus the QW must be a multilevel trap state. Different combinations of reverse voltage and fill pulse height allowed a DLTS study of the region before, within, and beyond the well location. Such an observation, in conjunction with the use of undoped AlGaAs barrier layers, proved that the DLTS signal is indeed from the well because it was only significant when probed within the well region and the assumption of the DX centers in some previous studies can be excluded. The fact that classically derived activation energy is close to the estimated band-gap discontinuity value and the carrier distribution centered at the geometric QW at room temperature revealed that the quantization effect was of second order. However, the detected activation energy depends on the testing conditions that precludes the determination of the band offset using the DLTS technique.

Engineering the future of electronics

The development of epitaxial growth techniques, such as molecular beam epitaxy and special kinds of chemical vapour phase epitaxy, allows us to control the growth of individual semiconductor layers on an atomic scale. These achievements provide a strong basis for designing semiconductor materials with tailored band structures and, consequently, tailored electronic and optical properties. The combination of different composites in the form of heterostructures and superlattices (periodic sequences of ultra-thin layers of different semiconducting materials) has opened up exciting possibilities for novel devices. It has also increased the basic understanding of electronic behaviour in reduced dimensions. The important parameters in this “band structure engineering” are the microscopic nature of the interfaces; the bandgap discontinuities; the doping profiles; and the related space-charge layers.

Electronic structure and total energy of diamond/BeO interfaces

Electronic structure calculations are used to study the bonding at diamond/BeO interfaces. The {110} interface between zinc blende BeO and diamond is used as a representative model for general reconstructed interfaces characterized by an equal amount of Be–C and O–C bonds. The interface energy is calculated to be 2 J/m2 and combined with the estimated free surface energies to obtain an estimate of the adhesion energy. It is found to be close to the adhesion of BeO to itself, but somewhat lower than that of diamond to itself. The effects of the 7% lattice mismatch on the total energy and the band structure for a biaxially strained pseudomorphic diamond film are investigated. The effect of misfit dislocations, expected to occur for thicker films, on the adhesion energy is estimated to be lower than 10%. The bulk properties, such as equilibrium lattice constant, bulk modulus, cohesive energy, and band gap of BeO are shown to be in good agreement with experimental values and previous calculations. The valence-band offset is calculated to be 3.9 eV and found to take up most of the large band gap discontinuity. The nature of the bonding is discussed in terms of the local densities of states near the interface. The interface localized features are identified in terms of Be–C and O–C bonding and antibonding states.

Empirical fit to band discontinuities and barrier heights in III–V alloy systems

We present a figure summarizing the variation of conduction band discontinuity, valence band discontinuity, and gold Schottky barrier height for binary and ternary III–V semiconductors. This figure, which applies to unstrained material, makes use of the property of transitivity in band alignments, and the observed experimental correlation between barrier heights and band gap discontinuities, to consolidate a wide range of data. The figure should be very useful in the design of novel heterostructure electronic and optical devices.

Excited exciton states in CdTe/(CdMn)Te quantum wells

We report low temperature photoluminescence excitation measurements on CdTe/(CdMn)Te single quantum wells in magnetic fields up to 7.5 T. From the analysis of the 2s state of the heavy-hole exciton, we deduce the binding energy as a function of the well width. By comparison of the asymmetric Zeeman splitting of the heavy-hole exciton with results of variational calculations for the exciton energies in an external magnetic field, we determine the valence band offset to 20%±10% of the bandgap discontinuity. This non-vanishing valence band offset allows us to study exciton transitions from excited subbands.

Admittance spectroscopy measurements of band offsets in Si/Si1−xGex/Si heterostructures

Admittance spectroscopy has been used to measure conduction- and valence-band discontinuities in Si/Si1−xGex heterojunctions (0<x<0.45). Most of the band-gap discontinuity was in the valence band. The measured valence-band offset increased with increasing Ge concentration in the strained Si1−xGex films, and it decreased when the Si1−xGex layers started to relax. These results indicate that admittance spectroscopy can be used to monitor the electronic properties of transistorlike Si/Si1−xGex/Si heterostructures.

Optical investigation of a strain-induced mixed type-I-type-II superlattice system: CdTe/Cd1-xZnxTe.

We present a systematic optical study of strained, CdTe/${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Te (x\ensuremath{\approxeq}0.1) superlattices grown by molecular-beam epitaxy. We have observed the intrinsic heavy- and light-hole exciton transitions of these superlattices, as well as the excited states (2s) of the heavy-hole exciton. By studying the energy variation of these transitions as a function of the period, we point out the mixed nature of the superlattice band structure (type I or II for heavy- or light-hole exciton transitions, respectively) due to the opposite strain experienced by the two kinds of layer (CdTe and ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Te); this is revealed by the relative variation of the light-hole exciton binding energy as a function of the superlattice period compared with that of the heavy hole. All these data provide a determination of the partition of the band-gap discontinuities between the valence and the conduction bands, found to lie between 1/9 and -1/11. Because the valence-band configuration is essentially influenced by the strain, we change the respective energy positions of the direct and indirect exciton transitions just by changing the average strain in the superlattice (namely, by growing the structures on buffer layers of different zinc concentrations); therefore we observe a strain-mediated type-I--type-II transition.

Characterization and determination of the band-gap discontinuity of the InxGa1−xAs/GaAs pseudomorphic quantum well

Single and multiple quantum well samples have been grown by atmospheric pressure metalorganic chemical vapor deposition at In compositions from 9 to 28% and layer thicknesses ranging from 15 to 140 Å, depending upon the composition. Selected samples containing three quantum wells of a given composition but with different thicknesses were characterized by x-ray double-crystal diffractometry, low-temperature photoluminescence, and transmission electron microscopy (TEM). Using a simulation technique based on the dynamical theory of x-ray diffraction in concert with TEM measurements, the In composition in the quantum well as well as the thicknesses can be directly extracted. The peak positions of the photoluminescence are used to determine the strained and unstrained energy gap and the conduction band offsets associated with InxGa1−xAs of a given composition. We have found the discontinuities to be 60% of the difference in the energy gap of GaAs and strained InxGa1−xAs.