Multiple Quantum Well Region Research Articles

Effects of quantum well number on spectral response of InGaN/GaN multiple quantum well solar cells

Effects of quantum well number on spectral response of InGaN/GaN multiple quantum well (MQW) solar cells are investigated. It is found that the strain relaxation by forming defects occurs when the number of quantum wells (QWs) increases to 30. It results in the low luminescence intensity and external quantum efficiency (EQE) due to the poor crystal quality of MQW region and the high non-radiative recombination rate in MQW region. However, the EQE of 25-period InGaN/GaN MQW solar cell with relatively high crystal quality of MQW region does not increase at all wavelengths compared with 15-period one. This may be attributed to the lower carrier collection efficiency resulting from the lower electric field of i-MQW region.

Effects of polarization and p-type GaN resistivity on the spectral response of InGaN/GaN multiple quantum well solar cells

Effects of polarization and p-type GaN resistivity on the spectral response of InGaN/GaN multiple quantum well (MQW) solar cells are investigated. It is found that due to the reduction of piezoelectric polarization and the enhancement of tunneling transport of photo-generated carriers in MQWs, the external quantum efficiency (EQE) of the solar cells increases in a low energy spectral range (λ > 370 nm) when the barrier thickness value decreases from 15 nm to 7.5 nm. But the EQE decreases abruptly when the barrier thickness value decreases down to 3.75 nm. The reasons for these experimental results are analyzed. We are aware that the reduction of depletion width in MQW region, caused by the high resistivity of the p-type GaN layer may be the main reason for the abnormally low EQE value at long wavelengths (λ > 370 nm).

Influence of barrier thickness on the structural and optical properties of InGaN/GaN multiple quantum wells

The structural and optical properties of InGaN/GaN multiple quantum wells (MQWs) with different barrier thicknesses are studied by means of high resolution X-ray diffraction (HRXRD), a cross-sectional transmission electron microscope (TEM), and temperature-dependent photoluminescence (PL) measurements. HRXRD and cross-sectional TEM measurements show that the interfaces between wells and barriers are abrupt and the entire MQW region has good periodicity for all three samples. As the barrier thickness is increased, the temperature of the turning point from blueshift to redshift of the S-shaped temperature-dependent PL peak energy increases monotonously, which indicates that the localization potentials due to In-rich clusters is deeper. From the Arrhenius plot of the normalized integrated PL intensity, it is found that there are two kinds of nonradiative recombination processes accounting for the thermal quenching of photoluminescence, and the corresponding activation energy (or the localization potential) increases with the increase of the barrier thickness. The dependence on barrier thickness is attributed to the redistribution of In-rich clusters during the growth of barrier layers, i.e., clusters with lower In contents aggregate into clusters with higher In contents.

Compensation doping in InGaAs / GaAsP multiple quantum well solar cells for efficient carrier transport and improved cell performance

A major challenge for multiple quantum well (MQW) solar cells is to extract sufficient photo-excited carriers to an external circuit through the MQW region under forward bias. The present study reports the effectiveness of compensation doping in the i-region, which includes MQWs, for more efficient transport of both electrons and holes. Unintentional p-type background doping occurs in GaAs by inevitable carbon incorporation during metal-organic vapor phase epitaxy, causing undesirable bending of the band lineup in the i-region of p-on-n devices. By cancelling this out by sulfur compensation doping to obtain a uniform electric field distribution, we achieved much a high carrier collection efficiency (CCE) >90% at the operating bias voltage regardless of the excitation wavelength, compared to < 50% without compensation doping. Consequently, cell performance was greatly improved, in particular showing an enhancement of the fill factor from 0.54 to 0.77, and degradation-free quantum efficiency within the GaAs absorption wavelength range. The photoluminescence (PL) intensity from the MQW increased as the CCE decreased at a large forward bias, and radiative recombination loss was significantly suppressed by compensation doping. Furthermore, time-resolved PL measurements indicated a much higher speed of carrier escape from the wells, showing a quicker PL decay time of 7 ns at 0.6 V, compared to 18–51 ns without compensation doping.

Improved Hole Transport by ${\rm p}\hbox{-}{\rm In}_{x}{\rm Ga}_{1-x}{\rm N}$ Layer in Multiple Quantum Wells of Visible LEDs

Studied is the effect of indium (In) mole fraction in p-In <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sup> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> N:Mg layers with 0 ≤ x <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In</sub> ≤ 0.035 on hole injection and transport behaviors in InGaN/GaN multiple quantum wells (MQWs) using dual-wavelength and triple-wavelength active regions. Electro-optical characteristics of light-emitting diodes containing p-layers with different In content and with silicon doping in selected QW barriers (QWBs) are compared to evaluate hole transport in the active region. The results show that enhanced hole transport and corresponding more uniform distribution of holes across the MQW region are achieved by increasing x <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In</sub> in the p-In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> N:Mg layer, possibly due to modification in energy of holes by a potential barrier between the p-InGaN and GaN QWB.

High-Aspect Ratio Structures for Efficient Light Absorption and Carrier Transport in InGaAs/GaAsP Multiple Quantum-Well Solar Cells

The high aspect ratio (HAR) quantum well was proposed as a general design principle to overcome the tradeoff problem between light absorption and carrier collection in multiple quantum-well (MQW) solar cells. An HAR-MQW structure consists of thin wells and barriers, and its fundamental strategies are 1) thinner wells to enhance the light absorption for 1HH transition and make it possible to absorb the same amount of light with a thinner MQW region; 2) thinner barriers to allow the photogenerated carriers to be extracted by means of tunneling transport; and 3) deeper wells to obtain the same effective bandgap as thicker wells because of the stronger confinement. The enhanced absorption coefficient for HAR-MQW was proved by the measurement of both photoabsorption and the quantum efficiency at a sufficiently large reverse bias. Stronger photon absorption via 1HH transition was achieved with a smaller total thickness of the wells area. In the HAR-MQW cell, although the transport of the heavy holes was found to still be dominated by thermionic processes due to its large effective mass, tunneling of the electrons was clearly observed, and the extraction efficiency of photoexcited electrons remained much higher than that of a normal MQW cell at forward biases.

Photoreflectance Spectroscopy Characterization of Ge/Si0.16Ge0.84Multiple Quantum Wells on Ge Virtual Substrate

We report a detailed characterization of a Ge/Si0.16Ge0.84multiple quantum well (MQW) structure on Ge-on-Si virtual substrate (VS) grown by ultrahigh vacuum chemical vapor deposition by using temperature-dependent photoreflectance (PR) in the temperature range from 10 to 300 K. The PR spectra revealed a wide range of optical transitions from the MQW region as well as transitions corresponding to the light-hole and heavy-hole splitting energies of Ge-on-Si VS. A detailed comparison of PR spectral line shape fits and theoretical calculation led to the identification of various quantum-confined interband transitions. The temperature-dependent PR spectra of Ge/Si0.16Ge0.84MQW were analyzed using Varshni and Bose-Einstein expressions. The parameters that describe the temperature variations of various quantum-confined interband transition energies were evaluated and discussed.

Effects of Background Zn Doping on the Performance of InGaAs/GaAsP Multiple Quantum Well Solar Cells Grown by a Planetary Metal Organic Vapor Phase Epitaxy Reactor

The effects of background Zn doping on the performance of p–i–n GaAs solar cells with InGaAs/GaAsP multiple quantum wells (MQWs) in i-GaAs layer have been studied. The crystal growth was done by a planetary metalorganic vapor phase epitaxy (MOVPE) reactor. The background Zn doping, in an order of 1017 cm-3, degraded the solar cell efficiency by modifying the energy band diagram in a way that obstructed carrier transports. It was shown by calculation that the carrier transports across the MQWs region suffered from decrease in built-in electric field in absorber layers, leading to an efficiency loss by radiative and nonradiative recombinations. Consequently, the external quantum efficiency and the current density of a Zn-contaminated MQW solar cell were terribly poor. Reactor baking at 850 °C for 20 min seems to remove Zn residues effectively without noticeable effects on the succeeding growth of MQW solar cells. The InGaAs/GaAsP MQWs fabricated in the thermally cleaned reactor have shown a potential to extend the absorption edge of GaAs solar cells and to improve the efficiency of multi-junction solar cells by current matching. Therefore, the growth of InGaAs/GaAsP MQWs by planetary MOVPE reactors requires a careful treatment regarding the background doping issue.

Effects of Background Zn Doping on the Performance of InGaAs/GaAsP Multiple Quantum Well Solar Cells Grown by a Planetary Metal Organic Vapor Phase Epitaxy Reactor

The effects of background Zn doping on the performance of p–i–n GaAs solar cells with InGaAs/GaAsP multiple quantum wells (MQWs) in i-GaAs layer have been studied. The crystal growth was done by a planetary metalorganic vapor phase epitaxy (MOVPE) reactor. The background Zn doping, in an order of 1017 cm-3, degraded the solar cell efficiency by modifying the energy band diagram in a way that obstructed carrier transports. It was shown by calculation that the carrier transports across the MQWs region suffered from decrease in built-in electric field in absorber layers, leading to an efficiency loss by radiative and nonradiative recombinations. Consequently, the external quantum efficiency and the current density of a Zn-contaminated MQW solar cell were terribly poor. Reactor baking at 850 °C for 20 min seems to remove Zn residues effectively without noticeable effects on the succeeding growth of MQW solar cells. The InGaAs/GaAsP MQWs fabricated in the thermally cleaned reactor have shown a potential to extend the absorption edge of GaAs solar cells and to improve the efficiency of multi-junction solar cells by current matching. Therefore, the growth of InGaAs/GaAsP MQWs by planetary MOVPE reactors requires a careful treatment regarding the background doping issue.

Effects of an Asymmetric Barrier Layer on the Structural and Optical Properties of InGaN LEDs

InGaN/GaN multiple-quantum-wells (MQWs) with three thinner barriers near the n-GaN layers were grown on c-plane sapphire substrates using metallorganic vapor phase deposition. For LEDs with an asymmetric barrier layer (ABL), high-resolution transmission electron microscopy analysis shows crystalline structure and interface quality are largely unchanged in modified MQWs composed of two kinds of quantum wells with different periods. It was also found that the dependency of strain relaxation on the degree of the exciton localization effect is low due to the InGaN MQWs being pseudomorphically grown on GaN templates. Therefore, the increase of indium segregation could be attributed to variations in the strain status within the MQW region. Experimentally, a large blueshift in the electroluminescence spectra under low level injection conditions provides evidence of the increased biaxial strain in the ABL–containing MQWs. The increase of biaxial strain may be partly accompanied by an increase in the quantum confined Stark effect. However, radiative recombination of the injection carriers in a narrow well will be important for determining the emission properties of the fabricated LEDs. Finally, the proposed LEDs exhibit a 10.8% increase in light output power, presumably because of better carrier distribution and the enhanced localization effect in the InGaN MQWs.

Optical properties of Si 0.8Ge 0.2/Si multiple quantum wells

The optical property was studied on the Si 0.8Ge 0.2/Si strained multiple quantum well (MQW) structure grown using ultra-high vacuum chemical vapor deposition (UHV-CVD). Three peaks are observed in Raman spectrum, which are located at about 510, 410, and 300 cm −1, corresponding to the vibration of Si–Si, Si–Ge, and Ge–Ge phonons, respectively. The photoluminescence (PL) spectrum originates from the radiative recombinations both from the Si substrate and the Si 0.8Ge 0.2/Si MQW. For Si 0.8Ge 0.2/Si strained MQW, the transition peaks related to the MQW region observed in the photocurrent (PC) spectrum were preliminarily assigned to electron–heavy hole (e–hh) and electron–light hole (e–lh) fundamental excitonic transitions.

Luminescence Dynamics and Structural Investigation of InGaN/GaN Multiple Quantum Well Light Emitting Diodes

Luminescence properties of blue emission InGaN/GaN multiple quantum well (MQW) have been studied by temperature dependent photoluminescence (PL), photoluminescence excitation (PLE) and time-resolved photoluminescence (TRPL) spectroscopic techniques. Two typical samples are studied, both consisting of five periods of InGaN wells with different indium compositions of 21% and 24%, respectively. According to the PL and PLE measurement results, large values of activation energy and Stokes’ shift are obtained. This indicates that higher Indium composition results in an increase of composition fluctuation in the InGaN MQW region, showing the stronger carrier localization effect. The lifetime at the low-energy side of the InGaN peaks is longer for higher indium composition, as expected from the larger Stokes shift.

Characteristics of efficiency droop in GaN-based light emitting diodes with an insertion layer between the multiple quantum wells and n-GaN layer

We have studied the characteristics of efficiency droop in GaN-based light emitting diodes (LEDs) with different kinds of insertion layers (ILs) between the multiple quantum wells (MQWs) layer and n-GaN layer. By using low-temperature (LT) (780 °C) n-GaN as IL, the efficiency droop behavior can be alleviated from 54% in reference LED to 36% from the maximum value at low injection current to 200 mA, which is much smaller than that of 49% in LED with InGaN/GaN short-period superlattices layer. The polarization field in MQWs is found to be smallest in LED with InGaN/GaN SPS layer. However, the V-shape defect density, about 5.3×108 cm−2, in its MQWs region is much higher than that value of 2.9×108 cm−2 in LED with LT n-GaN layer, which will lead to higher defect-related tunneling leakage of carriers. Therefore, we can mainly assign this alleviation of efficiency droop to the reduction of dislocation density in MQWs region rather than the decrease of polarization field.

In$_x$Ga$_{1-x}$N–GaN-Based Solar Cells With a Multiple-Quantum-Well Structure on SiCN–Si(111) Substrates

In this letter, the author describes an effective method of obtaining a high photovoltaic efficiency of In <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> N-GaN-based solar cells with a multiple-quantum-well (MQW) structure using a SiCN-Si (111) substrate. The MQW region had a higher solar cell absorption than the non-MQW In <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> N-GaN sample. Under an air mass 1.5 global solar spectrum, a maximum photovoltaic efficiency of 5.43% in the MQW sample over the control sample was observed. It was found that the device and fabrication technology developed in this study are applicable to the realization of solar cells with high open-circuit voltages ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> ) of 2.86-2.96 V, high short-circuit current densities ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> ) of 2.40-2.45 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and high fill factors of 74.8%-76.5%.

Temperature-Dependent Electroluminescence Efficiency in Blue InGaN–GaN Light-Emitting Diodes With Different Well Widths

Temperature dependence of electroluminescence (EL) efficiency in blue InGaN-GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with different well widths is systematically investigated. The EL efficiency at 300 K shows a maximum at the input current of 4, 10, and 60 mA for the LEDs with 1.5-, 2.0-, and 2.5-nm QWs, respectively. Nevertheless, the droop behavior at 80 K is mainly dominated by the low hole mobility and near independence on the QW thickness. According to the simulation results, it is found that the distinct efficiency droop behavior for the LEDs with different well widths at high and low temperature is strongly dependent on the effects of electron overflow and nonuniform hole distribution within the MQW region.

High Photovoltaic Efficiency of InxGa1-xN/GaN-Based Solar Cells with a Multiple-Quantum-Well Structure on SiCN/Si(111) Substrates

In this paper, I describe an effective method of obtaining a high photovoltaic efficiency of InxGa1-xN/GaN-based solar cells with a multiple-quantum-well (MQW) structure using a SiCN/Si (111) substrate. The MQW region had a higher solar cell absorption than the non-MQW InxGa1-xN/GaN sample. Under an air mass 1.5 global solar spectrum, an average increase of 38.5% in the photovoltaic efficiency of the MQW sample over the control sample was observed. It was found that the device and fabrication technology developed in this study are applicable to the realization of solar cells with high open-circuit voltages (Voc) of 2.92 to 3.16 V, high short-circuit current densities (Jsc) of 55.1 to 56.5 mA/cm2, and high fill factors of 89.5 to 91.8%.

Asymmetric external field effects on photoluminescence efficiency in a blue (In,Ga)N quantum‐well diode with an additional n ‐type electron reservoir layer

AbstractExternal field effects on photoluminescence (PL) spectra of a blue In0.3Ga0.7N multiple‐quantum‐well (MQW) diode with an additional n+‐doped In0.18Ga0.82N electron reservoir layer (ERL) have been investigated at 20 K as a function of bias voltage under direct excitation conditions, where only the MQW and ERL regions are photoexcited. When the reverse bias is increased, the PL emission of MQW is strongly quenched, accompanying a blue‐shift due to compensation of the internal piezo‐fields, while the ERL emission is stable. However, the PL intensity of ERL is significantly decreased with increasing the forward field, showing asymmetric behaviors on the PL efficiency with respect to the field direction. These results indicate that field‐induced escape processes of photogenerated electrons and holes from the excited regions play an important role for determination of the PL efficiency. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Optical properties studies in InGaN/GaN multiple-quantum well

A comparative study on temperature-dependent photoluminescence (PL) of InGaN/GaN multiple-quantum-well (MQW) violet-blue light-emitting diodes (LEDs) is presented. For the violet-blue LEDs, the peak energy exhibits a well-established S-shaped temperature behavior. The redshift at low temperatures is explained by carrier relaxation into lower energy states, which leads to dominant radiative recombination occurring mostly at localized states. The temperature-dependent PL was attributed to the localization effects in the MQW region of the samples. Up to three phonon replicas were also observed in the side-band of the quantum well luminescence with an energy separation similar to the GaN longitudinal-optical (LO)-phonon energy (∼91 meV). The properties of LO-phonon satellites were investigated as a function of the indium fraction and the well-width in a active layer at low temperatures.

Carrier localization degree of In0.2Ga0.8N/GaN multiple quantum wells grown on vicinal sapphire substrates

In this work, we have grown In0.2Ga0.8N/GaN multiple quantum well (MQWs) epitaxial structure on vicinal sapphire substrates by low pressure metal-organic chemical vapor deposition and investigated the relationship between carrier localization degree and vicinal angles of sapphire substrates. The optical analysis confirmed that the In0.2Ga0.8N/GaN MQWs grown on 0.2°-off sapphire substrate exhibited the smallest carrier localization degree and more ordered In0.2Ga0.8N/GaN MQW structure. In addition, mechanisms for carrier localization in In0.2Ga0.8N/GaN MQWs grown on vicinal substrate were discussed based on the results obtained from the power and temperature dependent photoluminescence measurements. The Raman spectrum showing the in-plane compressive stress of the GaN epitaxial structures grown on vicinal sapphire substrates revealed the relation between the dislocation density and the carrier localization degree in MQWs. From transmission electron microscopy images, the threading dislocation density (TDD) of In0.2Ga0.8N/GaN MQWs grown on 0.2° vicinal sapphire substrate at the bottom of n-GaN layer was about 9.4×108 cm−2 and reduced to 3.0×108 cm−2 at the top of n-GaN layer. We also obtained the TDD of 5.6×107 cm−2 in the MQW region and only 1.0×107 cm−2 in the p-GaN region. Based on the results mentioned above, 0.2°-off substrate can offer In0.2Ga0.8N/GaN MQW blue light-emitting diode structures with benefits, such as high crystal quality, low defects, and small carrier localization degree.

Photocurrent of SiGe/Si Strained Multiple Quantum-Wells Grown by UHV-CVD

The optical property was studied for the Si0.8Ge0.2/Si strained multiple quantum-well (MQW) structure grown by using ultra high vacuum chemical vapor deposition (UHV-CVD). The structural properties of the Si0.8Ge0.2/Si strained MQW were investigated using high-resolution X-ray diffraction (HR-XRD). The photocurrent spectrum of a Si0.8Ge0.2/Si strained MQW was measured at the room-temperature and the liquid-nitrogen temperature. For Si0.8Ge0.2/Si strained MQW, the transition peaks related to the MQW region observed in the photocurrent spectrum were preliminarily assigned to electron-heavy hole (e-hh) and electron-light hole (e-lh) fundamental excitionic transitions.